

###The Sobering Reality of Perovskite/Si Tandem Solar Cells under Realistic Operating Conditions||Moritz H. Futscher,Bruno Ehrler###

The Sobering Reality of Perovskite/Si Tandem Solar Cells under Realistic Operating Conditions. Perovskite/Si tandem solar cells have the potential to considerably
out-perform conventional solar cells. Under standard test conditions,
perovskite/Si tandem solar cells already outperform the Si single junction.
Under realistic conditions, however, as we show, those tandem solar cells are
hardly more efficient than the Si cell alone. We model the performance of
realistic perovskite/Si tandem solar cells under real-world climate conditions,
by incorporating parasitic cell resistances, non-radiative recombination, and
optical losses into the detailed-balance limit. We show quantitatively that
optimizing these parameters in the perovskite top cell, perovskite/Si tandem
solar cells reach an efficiency advantage of up to 14% absolute, even while
leaving the Si cell untouched. Despite the rapid efficiency increase of
perovskite solar cells, our results emphasize the need for further material
development, careful device design, and light management strategies, all
necessary for highly efficient perovskite/Si tandem solar cells.

###Highly efficient light management for perovskite solar cells||Dong-Lin Wang,Hui-Juan Cui,Guo-Jiao Hou,Zhen-Gang Zhu,Qing-Bo Yan,Gang Su###

Highly efficient light management for perovskite solar cells. Organic-inorganic halide perovskite solar cells have enormous potential to
impact the existing photovoltaic industry. As realizing a higher conversion
efficiency of the solar cell is still the most crucial task, a great number of
schemes were proposed to minimize the carrier loss by optimizing the electrical
properties of the perovskite solar cells. Here, we focus on another significant
aspect that is to minimize the light loss by optimizing the light management to
gain a high efficiency for perovskite solar cells. In our scheme, the slotted
and inverted prism structured SiO2 layers are adopted to trap more light into
the solar cells, and a better transparent conducting oxide layer is employed to
reduce the parasitic absorption. For such an implementation, the efficiency and
the serviceable angle of the perovskite solar cell can be promoted
impressively. This proposal would shed new light on developing the
high-performance perovskite solar cells.

###Moon's Radiation Environment and Expected Performance of Solar Cells during Future Lunar Missions||T. E Girish,S Aranya###

Moon's Radiation Environment and Expected Performance of Solar Cells during Future Lunar Missions. Several lunar missions are planned ahead and there is an increasing demand
for efficient photovoltaic power generation in the moon. The knowledge of solar
cell operation in the lunar surface obtained during early seventies need to be
updated considering current views on solar variability and emerging space solar
cell technologies. In this paper some aspects of the solar cell performance
expected under variable lunar radiation environment during future space
missions to moon are addressed. We have calculated relative power expected from
different types of solar cells under extreme solar proton irradiation
conditions and high lunar daytime temperature. It is also estimated that 2-3 %
of annual solar cell degradation is most probable during the future lunar
missions. We have also discussed photovoltaic power generation in long term
lunar bases emphasizing technological needs such as sunlight concentration,
solar cell cooling and magnetic shielding of radiation for improving the
efficiency of solar cells in the lunar environment.

###Assessing Material Qualities and Efficiency Limits of III-V on Silicon Solar Cells Using External Radiative Efficiency||Kan-Hua Lee,Kenji Araki,Li Wang,Nobuaki Kojima,Yoshio Ohshita,Masafumi Yamaguchi###

Assessing Material Qualities and Efficiency Limits of III-V on Silicon Solar Cells Using External Radiative Efficiency. The paper presents a quantitative approach to the investigation and
comparison of the material qualities of III-V on silicon (III-V/Si) solar cells
by using external radiative efficiencies. We use this analysis to predict the
limiting efficiencies and evaluate the criteria of material quality in order to
achieve high efficiency III-V/Si solar cells. This result yields several
implications for the design of high efficiency III-V/Si solar cells.

###Two-Terminal Tandem Solar Cells based on Perovskite and Transition Metal Dichalcogenides||Harishankar Suman,Avijit Kumar###

Two-Terminal Tandem Solar Cells based on Perovskite and Transition Metal Dichalcogenides. Perovskite solar cells have shown power conversion efficiencies (PCE)
comparable to cystalline silicon solar cell despite involving low-temperature,
solution based synthesis processes outside clean room environment. As the
theoretical PCE of a perovskite solar cell with band gap 1.55 eV is capped to
33 % due to Shockley-Queisser limit, tandem configurations are being
investigated to go beyond this limit. Here, we propose a two-terminal (2T)
tandem solar cell structure consisting of perovskite and multilayer transition
metal dichalcogenide as the absorber layers of the top and the bottom subcells,
respectively and investigate their performance parameters using Solar Cell
Capacitance Simulator-1 Dimension (SCAPS-1D) software package. We demonstrate
that the 2T tandem solar cell consisting of CH3NH3PbI3 with band gap 1.55 eV
and MoTe2 with bandgap 1.1 eV shows PCE of maximum 35.3 % under AM 1.5 G
illumination. This work motivates experimental realization of such solar cells
for further investigation.

###Double-absorber thin-film solar cell with 34% efficiency||Faiz Ahamd,Akhlesh Lakhtakia,Peter B. Monk###

Double-absorber thin-film solar cell with 34% efficiency. Power-conversion efficiency is a critical factor for the wider adoption of
solar-cell modules. Thin-film solar cells are cheap and easy to manufacture,
but their efficiencies are low compared to crystalline-silicon solar cells and
need to be improved. A thin-film solar cell with two absorber layers (instead
of only one), with bandgap energy graded in both, can capture solar photons in
a wider spectral range. With a 300-nm-thick CIGS~absorber layer and an
870-nm-thick CZTSSe~absorber layer, an efficiency of $34.45\%$ is predicted by
a detailed optoelectronic model, provided that the grading of bandgap energy is
optimal in both absorber layers.

###Electromagnetic approach to ultrathin solar cell efficiencies||A. Niv,M. Gharghi,Z. R. Abrams,C. Gladden,X. Zhang###

Electromagnetic approach to ultrathin solar cell efficiencies. Current methods for evaluating solar cell efficiencies cannot be applied to
extremely thin cells where phenomena from the realm of near field optics
prevail. We overcome this problem by offering a rigorous electromagnetic
calculation of solar cell efficiencies based on the fluctuation dissipation
theorem. Our approach is demonstrated by calculating the efficiency of a GaAs
solar cell with an Au back reflector for thicknesses well below the typical
wavelength of the solar flux. It is shown that near field optics affect the
performance of low dimensional solar cells.

###Temperature coefficient of Silicon based carrier selective solar cells||Nithin Chatterji,Aldrin Antony,Pradeep R. Nair###

Temperature coefficient of Silicon based carrier selective solar cells. Carrier Selective (CS) Silicon solar cells are increasingly explored as a low
cost alternative to PN junction Silicon solar cells. While the recent trends on
power conversion efficiency are encouraging, the temperature coefficient and
hence the power output under elevated temperatures are not well explored for
such solar cells. Here, we address this issue through detailed numerical
simulations to explore the influence of interface and material parameters on
the temperature coefficient. Our results indicate that irrespective of the
interface quality, the temperature coefficient of CS solar cells improves with
an increase in band discontinuities. Interestingly, contrary to the trends
related to efficiency, our results indicate that the temperature coefficient of
CS solar cells is more critically affected by the interface quality of the
minority carrier extraction layer than the majority carrier extraction layer.
These insights have important implications towards the choice of optimal
material and processing conditions for Si based CS solar cells.

###Axial vs. Radial Junction Nanowire Solar Cell||Vidur Raj,Hark Hoe Tan,Chennupati Jagadish###

Axial vs. Radial Junction Nanowire Solar Cell. Both axial and radial junction nanowire solar cells have their challenges and
advantages. However, so far, there is no review that explicitly provides a
detailed comparative analysis of both axial and radial junction solar cells.
This article reviews some of the recent results on axial and radial junction
nanowire solar cells with an attempt to perform a comparative study between the
optical and device behavior of these cells. In particular, we start by
reviewing different results on how the absorption can be tuned in axial and
radial junction solar cells. We also discuss results on some of the critical
device concepts that are required to achieve high efficiency in axial and
radial junction solar cells. We include a section on new device concepts that
can be realized in nanowire structures. Finally, we conclude this review by
discussing a few of the standing challenges of nanowire solar cells.

###High-Temperature Annealing of TiO2 Nanotube Membranes for Efficient Dye-Sensitized Solar Cells||Fatemeh Mohammadpour,Marco Altomare,Seulgi So,Kiyoung Lee,Mohamed Mokhtar,Abdelmohsen Alshehri,Shaeel A. Al-Thabaiti,Patrik Schmuki###

High-Temperature Annealing of TiO2 Nanotube Membranes for Efficient Dye-Sensitized Solar Cells. We fabricate photo-anodes by transferring anodic TiO2 nanotube membranes in
tube-top-down configuration on FTO glass, and use them for constructing
frontside illuminated dye-sensitized solar cells. Prior to solar cell
construction, the tube-based photo-anodes are crystallized at different
temperatures (400-800{\deg}C), and the effects of tube electron transport
properties on the photovoltaic performance of the solar cells are investigated.
We show that improved solar cell efficiencies (up to ca. 8.0%) can be reached
by high-temperature treatment of the tube membranes. Consistently with electron
transport time measurements, remarkably enhanced electron mobility is enabled
when tube membranes are crystallized at 600{\deg}C.

###On the Nature of Localization in Ti doped Si||Yi Zhang,R. Nelson,K. -M. Tam,W. Ku,U. Yu,N. S. Vidhyadhiraja,H. Terletska,J. Moreno,M. Jarrell,T. Berlijn###

On the Nature of Localization in Ti doped Si. Intermediate band semiconductors hold the promise to significantly improve
the efficiency of solar cells, but only if the intermediate impurity band is
metallic. We apply a recently developed first principles method to investigate
the origin of electron localization in Ti doped Si, a promising candidate for
intermediate band solar cells. Although Anderson localization is often
overlooked in the context of intermediate band solar cells, our results show
that in Ti doped Si it plays a more important role in the metal insulator
transition than Mott localization. Implications for the theory of intermediate
band solar cells are discussed.

###Analysis Of SnS2 Buffer Layer And SnS Back Surface Layer Based CZTS Solar Cells Using SCAPS||Atul Kumar,Ajay D. Thakur###

Analysis Of SnS2 Buffer Layer And SnS Back Surface Layer Based CZTS Solar Cells Using SCAPS. A Copper-Zinc-Tin-Sulphide (CZTS)based solar cell with a modified ce3ll
configuration of Mo/SnS/CZTS/SnS2/ZnO is simulated using SCAPS. An SnS2 buffer
layer is used in simulation instead of the standard CdS layer. An additional
back surface passivation layer of SnS is added in the modified cell
configuration. An improvement in the solar cell efficiency compared to the
standard CdS buffer based solar cell configuration Mo/CZTS/CdS/ZnO is found.
The observations suggest the possibility of using SnS2 as a potential
replacement of CdS. In addition, the use of a back surface passivation layer
leads to improved solar cell performance.

###Effect of the shell material and confinement type on the conversion efficiency of the core/shell quantum dot nanocrystal solar cells||Mehmet Sahin###

Effect of the shell material and confinement type on the conversion efficiency of the core/shell quantum dot nanocrystal solar cells. In this study, effects of the shell material and confinement type on the
conversion efficiency of the core/shell quantum dot nanocrystal (QDNC) solar
cells have been investigated in a detail manner. For this purpose, the
conventional, i.e original, detailed balance model, developed by Shockley and
Queisser to calculate an upper limit for conversion efficiency of silicon p-n
junction solar cells, is modified in a simple and an effective way and
calculated the conversion efficiency of core/shell QDNC solar cells. Since the
existing model relies on the gap energy ($E_g$) of the solar cell, it does not
make an estimation about the effect of QDNC materials on the efficiency of the
solar cells and gives the same efficiency values for several QDNC solar cells
with the same $E_g$. The proposed modification, however, estimates a conversion
efficiency in relation to the material properties and also confinement type of
the QDNCs. The results of the modified model show that, in contrast to the
original one, the conversion efficiencies of different QDNC solar cells, even
if they have the same $E_g$, become different depending upon the confinement
type and shell material of the core/shell QDNCs and this is crucial in design
and fabrication of the new generation solar cells to predict the confinement
type and also appropriate QDNC materials for better efficiency.

###Computational design of organic solar cell active layer through genetic algorithm||Caine Ardayfio###

Computational design of organic solar cell active layer through genetic algorithm. The active layer microstructure of organic solar cells is critical to
efficiency. By studying the photovoltaic properties of organic solar cell's
microstructure, it is possible to increase the efficiency of the solar cell. A
graph-based microstructure model was employed to approximate the efficiency,
measured as short circuit current, of a solar cell given a microstructure.
Through probabilistic graph-based optimization, a class of microstructures were
found with an efficiency surpassing that of more conventional morphologies.
These optimized solar cells surpass the efficiency of more conventional
photovoltaic devices as they better facilitate charge transport, generation,
and dissociation. A device was designed with a 40.29% increase in short circuit
current from the solar cells with the currently believed optimal morphology.
The designed morphologies feature two dendritic clusters of the donor material
poly(3-hexylthiophene-2,5-diyl) (P3HT) and the acceptor material
phenyl-C61-Butyric-Acid-Methyl Ester (PCBM). The designed microstructure's
increase in performance contrasts with more conventional structures featuring
interdigitated or bilayer strands of P3HT and PCBM. The change of
microstructure morphology through graph-based evolution obtains an organic
solar cell with an efficiency significantly greater than conventional organic
solar cells, proves the validity of graph-based microstructure models for
simulation in materials science, and advances the vision of an inexpensive,
efficient form of renewable energy.

###III-V Solar Cells||James P. Connolly,Denis Mencaraglia###

III-V Solar Cells. III-V materials show a wide range of gaps making them ideal for the design of
high efficiency solar cells. This chapter reviews relevant growth methods and
material properties of these materials and discusses methods of combining
heterogeneous materials, demonstrating the flexibility of design possible with
these materials. The fundamental loss mechanisms of solar cells are analysed
and quantified as a prelude to analysing high efficiency cell designs in
single, tandem, and triple junction solar cells. The detailed analysis of loss
mechanisms is used to obtain understanding of the limiting behaviour of these
designs, and show that bulk cells remain dominated by non-radiative losses
despite unity ideality factors. To conclude, this is contrasted with the
operating regime of nanostructured solar cells which can be shown to operate in
a radiatively dominated mode, and which therefore approach ideal solar cell
efficiency limits.

###Efficient indoor p-i-n hybrid perovskite solar cells using low temperature solution processed NiO as hole extraction layers||Lethy Krishnan Jagadamma,Oskar Blaszczyk,Muhammad T. Sajjad,Arvydas Ruseckas,Ifor D. W. Samuel###

Efficient indoor p-i-n hybrid perovskite solar cells using low temperature solution processed NiO as hole extraction layers. Hybrid perovskites have received tremendous attention due to their
exceptional photovoltaic and optoelectronic properties. Among the two widely
used perovskite solar cell device architectures of n-ip and p-i-n, the latter
is interesting in terms of its simplicity of fabrication and lower energy
input. However this structure mostly uses PEDOT:PSS as a hole transporting
layer which can accelerate the perovskite solar cell degradation. Hence the
development of stable, inorganic hole extraction layers (HEL), without
compromising the simplicity of device fabrication is crucial in this
fast-growing photovoltaic field. Here we demonstrate a low temperature (~100
oC) solution - processed and ultrathin (~ 6 nm) NiO nanoparticle thin films as
an efficient HEL for CH3NH3PbI3 based perovskite solar cells. We measure a
power conversion efficiency (PCE) of 13.3 % on rigid glass substrates and 8.5 %
on flexible substrates. A comparison with PEDOT:PSS based MAPbI3 solar cells
(PCE ~ 7.9 %) shows that NiO based solar cells have higher short circuit
current density and improved open circuit voltage (1.03V). Apart from the
photovoltaic performance under 1 Sun, the efficient hole extraction property of
NiO is demonstrated for indoor lighting as well with a PCE of 23.0 % for NiO
based CH3NH3PbI2.9Cl0.1 p-i-n solar cells under compact fluorescent lighting.
Compared to the perovskite solar cells fabricated on PEDOT:PSS HEL, better
shelf-life stability is observed for perovskite solar cells fabricated on NiO
HEL. Detailed microstructural and photophysical investigations imply uniform
morphology, lower recombination losses, and improved charge transfer properties
for CH3NH3PbI3 grown on NiO HEL.

###Efficient color coatings for single junction and multijunction colored solar cells||Farid Elsehrawy,Konrad Klockars,Orlando J. Rojas,Janne Halme###

Efficient color coatings for single junction and multijunction colored solar cells. Colored solar cells suffer from lower efficiency due to reflection and
absorption losses by coatings. By studying different types of coatings on solar
cells, the spectrum parameters impacting the solar cell efficiency were
identified. A collection of color coatings was fabricated and characterized
using spectrophotometry and colorimetric photography. The coatings include
commercial absorption filters, commercial distributed bragg reflectors,
lab-made interference coatings, and cellulose nanocrystal coatings. Commercial
single junction and multijunction solar cells were used to measure the impact
of the color coatings on solar cell performance. Structural colors were found
to result in the highest brightness to color loss ratio. Structural colors do
not fade provided that the structure is not compromised. Color loss in single
junction solar cells is a result of the proportion of reflected light in the
absorption range of the material. Multijunction solar cells were strongly
affected by current mismatch losses, where narrow reflection peaks resulted in
high efficiency losses. Structural color coatings made using cellulose
nanocrystals exhibited low and broad reflection peaks that produced bright
colors and maintained high performance of single junction and multijunction
solar cells, retaining up to 83 % of the reference power obtained in the
absence of a color coating.

###High performance solar cells based on graphene-GaAs heterostructures||Xiaoqiang Li,Shengjiao Zhang,Peng Wang,Huikai Zhong,Zhiqian Wu,Hongshen Chen,Cheng Liu,Shisheng Lin###

High performance solar cells based on graphene-GaAs heterostructures. The honeycomb connection of carbon atoms by covalent bonds in a macroscopic
two-dimensional scale leads to fascinating graphene and solar cell based on
graphene/silicon Schottky diode has been widely studied. For solar cell
applications, GaAs is superior to silicon as it has a direct band gap of 1.42
eV and its electron mobility is six times of that of silicon. However,
graphene/GaAs solar cell has been rarely explored. Herein, we report
graphene/GaAs solar cells with conversion efficiency (Eta) of 10.4% and 15.5%
without and with anti-reflection layer on graphene, respectively. The Eta of
15.5% is higher than the state of art efficiency for graphene/Si system
(14.5%). Furthermore, our calculation points out Eta of 25.8% can be reached by
reasonably optimizing the open circuit voltage, junction ideality factor,
resistance of graphene and metal/graphene contact. This research strongly
support graphene/GaAs hetero-structure solar cell have great potential for
practical applications.

###Two-Photon Photocurrent in InGaN/GaN Nanowire Intermediate Band Solar Cells||Ross Cheriton,Sharif M. Sadaf,Luc Robichaud,Jacob J. Krich,Zetian Mi,Karin Hinzer###

Two-Photon Photocurrent in InGaN/GaN Nanowire Intermediate Band Solar Cells. Intermediate band solar cells hold the promise of ultrahigh power conversion
efficiencies using a single semiconductor junction. Many current
implementations use materials with bandgaps too small to achieve maximum
efficiency or use cost-prohibitive substrates. Here we demonstrate a material
system for intermediate band solar cells using InGaN/GaN
quantum-dot-in-nanowire heterostructures grown directly on silicon to provide a
lower cost, large-bandgap intermediate band solar cell platform. We demonstrate
sequential two-photon current generation with sub-bandgap photons, the hallmark
of intermediate band solar cell operation, through vertically stacked quantum
dots in the nanowires. Near-infrared light biasing with an 850 nm laser
intensity up to 200 W/cm2 increases the photocurrent above and below the
bandgap by up to 19% at 78 K, and 44% at room temperature. The nanostructured
III-nitride strategy provides a route towards realistic room temperature
intermediate band solar cells while leveraging the cost benefits of silicon
substrates.

###Carrier loss mechanisms in textured crystalline Si-based solar cells||Akihiro Nakane,Shohei Fujimoto,Hiroyuki Fujiwara###

Carrier loss mechanisms in textured crystalline Si-based solar cells. A quite general device analysis method that allows the direct evaluation of
optical and recombination losses in crystalline silicon (c-Si)-based solar
cells has been developed. By applying this technique, the optical and physical
limiting factors of the state-of-the-art solar cells with ~20% efficiencies
have been revealed. In the established method, the carrier loss mechanisms are
characterized from the external quantum efficiency (EQE) analysis with very low
computational cost. In particular, the EQE analyses of textured c-Si solar
cells are implemented by employing the experimental reflectance spectra
obtained directly from the actual devices while using flat optical models
without any fitting parameters. We find that the developed method provides
almost perfect fitting to EQE spectra reported for various textured c-Si solar
cells, including c-Si heterojunction solar cells, a dopant-free c-Si solar cell
with a MoOx layer, and an n-type passivated emitter with rear locally diffused
(PERL) solar cell. The modeling of the recombination loss further allows the
extraction of the minority carrier diffusion length and surface recombination
velocity from the EQE analysis. Based on the EQE analysis results, the carrier
loss mechanisms in different types of c-Si solar cells are discussed.

###Reversible electron-hole separation in a hot carrier solar cell||Steven Limpert,Stephen Bremner,Heiner Linke###

Reversible electron-hole separation in a hot carrier solar cell. Hot-carrier solar cells are envisioned to utilize energy filtering to extract
power from photogenerated electron-hole pairs before they thermalize with the
lattice, and thus potentially offer higher power conversion efficiency compared
to conventional, single absorber solar cells. The efficiency of hot-carrier
solar cells can be expected to strongly depend on the details of the energy
filtering process, a relationship which to date has not been satisfactorily
explored. Here, we establish the conditions under which electron-hole
separation in hot-carrier solar cells can occur reversibly, that is, at maximum
energy conversion efficiency. We thus focus our analysis on the internal
operation of the hot-carrier solar cell itself, and in this work do not
consider the photon-mediated coupling to the sun. After deriving an expression
for the voltage of a hot-carrier solar cell valid under conditions of both
reversible and irreversible electrical operation, we identify separate
contributions to the voltage from the thermoelectric effect and the
photovoltaic effect. We find that, under specific conditions, the energy
conversion efficiency of a hot-carrier solar cell can exceed the Carnot limit
set by the intra-device temperature gradient alone, due to the additional
contribution of the quasi-Fermi level splitting in the absorber. We also
establish that the open-circuit voltage of a hot-carrier solar cell is not
limited by the band gap of the absorber, due to the additional thermoelectric
contribution to the voltage. Additionally, we find that a hot-carrier solar
cell can be operated in reverse as a thermally driven solid-state light
emitter. Our results help explore the fundamental limitations of hot-carrier
solar cells, and provide a first step towards providing experimentalists with a
guide to the optimal configuration of devices.

###The Effects of Geometry on a-Si:H Solar Cell Performance||T. Kirkpatrick,M. J. Burns,M. J. Naughton###

The Effects of Geometry on a-Si:H Solar Cell Performance. We present a model for simulating performance of 3D nano -coaxial and
-hemispherical thin film solar cells. The material system considered in these
simulations is hydrogenated amorphous silicon (a-Si:H), with solar cells
fabricated in an n-i-p stacking architecture. Simulations for the performance
of the planar a-Si:H device are compared against simulations performed using
SCAPS-1D and found to be in close agreement. Electrical and optical properties
of devices are discussed for the respective geometries. Maximum power point
efficiencies are plotted as a function of i-layer thickness for insight into
optimizing spatial parameters. Simulation results show that while geometrical
changes in the energy band diagram impact charge carrier collection, a-Si:H
solar cell performance is most significantly impacted by light absorption
properties associated with nanoscopic arrays of non-planar structures. We
compare our simulations to results of fabricated nanocoaxial a-Si:H solar cells
and infer the mechanisms of enhanced absorption observed experimentally in such
solar cells.

###Significant efficiency enhancement in thin film solar cells using laser beam-induced graphene transparent conductive electrodes||L. V. Thekkekara,Bouyan Cai###

Significant efficiency enhancement in thin film solar cells using laser beam-induced graphene transparent conductive electrodes. Thin film solar cells have been attractive for decades in advanced green
technology platforms due to its possibilities to be integrated with buildings
and on-chip applications. However, the bottleneck issues involved to consider
the current solar cells as a major electricity source includes the lower
efficiencies and cost-effectiveness. We numerically demonstrate the concept of
the absorption enhancement in thin-film amorphous silicon solar cells using the
laser beam-induced graphene material based on the insensitive polarization
space-filling fractal design as transparent conductive electrodes. With the
optimization of parameters such as thickness, width, and period of fractals, an
enhancement of photocurrent generation of solar cells by a factor of 24.5% is
achieved compared to reference solar cell with a traditional ITO.

###Plasmonic Metamaterial Perovskite Solar Cells: Fundamental Tradeoffs, Limitations, and Opportunities||Kwangjin Kim,Seungwoo Lee###

Plasmonic Metamaterial Perovskite Solar Cells: Fundamental Tradeoffs, Limitations, and Opportunities. Whether dispersal of plasmonic nanoparticles (NPs) within a perovskite active
layer can increase the efficiency of solar cells is a long-standing question.
It is well known that inclusion of metallic NPs in an active layer can boost
the surrounding near-field intensity around them owing to the dipolar localized
surface plasmon resonance (LSPR, also called antenna effect), which can
increase light absorption by solar cells. However, the use of plasmonic NPs in
perovskite solar cells has been barely reported, and it is not known whether
inserting plasmonic NPs into a perovskite active layer produces any performance
advantage compared with a pure perovskite counterpart. We explore the
fundamental and practical limits of plasmonic metamaterial perovskite solar
cells by applying effective medium theory and a detailed balance analysis. Our
results indicate that an increase in effective refractive index of perovskite
through dispersed plasmonic NPs can in principle enhance the performance of
solar cells.

###Simulation of the Efficiency of CdS/CIGS Tandem Multi-Junction Solar Cells Using AMPS-1D||Ashrafalsadat S. Mirkamali,Khikmat Kh. Muminov###

Simulation of the Efficiency of CdS/CIGS Tandem Multi-Junction Solar Cells Using AMPS-1D. In this paper we conduct numerical simulation of CdS/CIGS solar cells by use
of the AMPS-1D software aiming to formulate the optimal design of the new
multi-junction tandem solar cell providing its most efficient operation. We
start with the numerical simulation of single-junction CdS/CIGS solar cells,
which shows that its highest efficiency of 17.3% could be achieved by the
thickness of CIGS p-layer of 200 nm. This result is in a good agreement with
experimental data where the highest efficiency was 17.1% with the solar cell
thickness of 1 micron. By use of the results of the numerical simulation of the
single-junction solar cells we developed the design and conducted optimization
of the new multi-junction tandem CdS/CIGS solar cell structure. Numerical
simulation shows that the maximum efficiency of this solar cell is equal to
48.3%, which could be obtained with the thickness of the CIGS p-layer of 600 nm
at a standard illumination of AM 1.5.

###The Potential of Singlet Fission Photon Multipliers as an Alternative to Silicon-based Tandem Solar Cells||Moritz H. Futscher,Akshay Rao,Bruno Ehrler###

The Potential of Singlet Fission Photon Multipliers as an Alternative to Silicon-based Tandem Solar Cells. Singlet fission, an exciton multiplication process in organic semiconductors
which converts one singlet exciton into two triplet excitons is a promising way
to reduce thermalization losses in conventional solar cells. One way to harvest
triplet excitons is to transfer their energy into quantum dots, which then emit
photons into an underlying solar cell. We simulate the performance potential of
such a singlet fission photon multiplier combined with a silicon base cell and
compare it to a silicon-based tandem solar cell. We calculate the influence of
various loss-mechanisms on the performance potential under real-world operation
conditions using a variety of silicon base cells with different efficiencies.
We find that the photon multiplier is more stable against changes in the solar
spectrum than two-terminal tandem solar cells. We furthermore find that, as the
efficiency of the silicon solar cell increases, the efficiency of the photon
multiplier increases at a higher rate than the tandem solar cell. For current
record silicon solar cells, the photon multiplier has the potential to increase
the efficiency by up to 4.2% absolute.

###A Silicon-Singlet Fission Parallel Tandem Solar Cell Exceeding 100 % External Quantum Efficiency||Luis M. Pazos,Ju Min Lee,Anton Kirch,Maxim Tabachnyk,Richard H. Friend,Bruno Ehrler###

A Silicon-Singlet Fission Parallel Tandem Solar Cell Exceeding 100 % External Quantum Efficiency. Silicon solar cells dominate the solar cell market with record lab
efficiencies reaching almost 26%. However, after 60 years of research, this
efficiency saturated close to the theoretical limit for silicon, and radically
new approaches are needed to further improve the efficiency. Here we present
parallel-connected tandem solar cells based on down-conversion via singlet
fission. This design allows raising the theoretical power conversion efficiency
limit to 45% with far superior stability under changing sunlight conditions in
comparison to traditional series tandems. We experimentally demonstrate a
silicon/pentacene parallel tandem solar cell that exceeds 100% external quantum
efficiency at the main absorption peak of pentacene, showing efficient
photocurrent addition and proving this design as a realistic prospect for
real-world applications.

###Nanocarbon-Based photovoltaics||Marco Bernardi,Jessica Lohrman,Priyank V. Kumar,Alec Kirkeminde,Nicola Ferralis,Jeffrey C. Grossman,Shenqiang Ren###

Nanocarbon-Based photovoltaics. Carbon materials are excellent candidates for photovoltaic solar cells: they
are Earth-abundant, possess high optical absorption, and superior thermal and
photostability. Here we report on solar cells with active layers made solely of
carbon nanomaterials that present the same advantages of conjugated
polymer-based solar cells - namely solution processable, potentially flexible,
and chemically tunable - but with significantly increased photostability and
the possibility to revert photodegradation. The device active layer composition
is optimized using ab-initio density functional theory calculations to predict
type-II band alignment and Schottky barrier formation. The best device
fabricated is composed of PC70BM fullerene, semiconducting single-walled carbon
nanotubes and reduced graphene oxide. It achieves a power conversion efficiency
of 1.3% - a record for solar cells based on carbon as the active material - and
shows significantly improved lifetime than a polymer-based device. We calculate
efficiency limits of up to 13% for the devices fabricated in this work,
comparable to those predicted for polymer solar cells. There is great promise
for improving carbon-based solar cells considering the novelty of this type of
device, the superior photostability, and the availability of a large number of
carbon materials with yet untapped potential for photovoltaics. Our results
indicate a new strategy for efficient carbon-based, solution-processable, thin
film, photostable solar cells.

###Quantitative determination of optical and recombination losses in thin-film photovoltaic devices based on external quantum efficiency analysis||Akihiro Nakane,Hitoshi Tampo,Masato Tamakoshi,Shohei Fujimoto,Kang Min Kim,Shinho Kim,Hajime Shibata,Shigeru Niki,Hiroyuki Fujiwara###

Quantitative determination of optical and recombination losses in thin-film photovoltaic devices based on external quantum efficiency analysis. In developing photovoltaic devices with high efficiencies, quantitative
determination of the carrier loss is crucial. In conventional solar-cell
characterization techniques, however, photocurrent reduction originating from
parasitic light absorption and carrier recombination within the light absorber
cannot be assessed easily. Here, we develop a general analysis scheme in which
the optical and recombination losses in submicron-textured solar cells are
evaluated systematically from external quantum efficiency (EQE) spectra. In
this method, the optical absorption in solar cells is first deduced by imposing
the anti-reflection condition in the calculation of the absorptance spectrum,
and the carrier extraction from the light absorber layer is then modeled by
considering a carrier collection length from the absorber interface. Our
analysis method is appropriate for a wide variety of photovoltaic devices,
including kesterite solar cells [Cu2ZnSnSe4, Cu2ZnSnS4, and Cu2ZnSn(S,Se)4],
zincblende CdTe solar cells, and hybrid perovskite (CH3NH3PbI3) solar cells,
and provides excellent fitting to numerous EQE spectra reported earlier. Based
on the results obtained from our EQE analyses, we discuss the effects of
parasitic absorption and carrier recombination in different types of solar
cells.

###Design guidelines for a highly efficient high-purity Germanium (HPGe)-based double-heterojunction solar cell||Jaker Hossain,Md. Mahabub Alam Moon,Bipanko Kumar Mondal,Mohammad Abdul Halim###

Design guidelines for a highly efficient high-purity Germanium (HPGe)-based double-heterojunction solar cell. In spite of having higher carrier mobilities and absorption coefficients of
germanium (Ge) than those of silicon (Si), there has been less focus on
Ge-based solar cells due to the low bandgap and high-cost of Ge wafer as well
as requirement of its high-purity level. Currently, availability of high-purity
Ge (HPGe), the low-cost wafer slicing method and proper design guidelines make
it possible to design HPGe-based solar cells. Accordingly, in this article, we
have designed and simulated a novel n-CdS/p-HPGe/p+-BaSi2 based npp+
double-heterojunction solar cell, where HPGe, cadmium sulfide (CdS) and
orthorhombic barium disilicide (beta-BaSi2) have been used as the absorber,
window and back-surface field (BSF) layers, respectively. Using the solar cell
capacitance simulator (SCAPS-1D), the effects of different physical parameters
such as the thickness, doping and defect densities, band offsets and
temperature on the photovoltaic (PV) parameters of the designed solar cells
have been investigated systematically. This article renders the optimized PV
parameters to improve the device performance with the highest power conversion
efficiency (PCE) of ~45.65% with a high open-circuit voltage of 1.16 V owing to
the high built-in voltage of 1.7 V for the n-CdS/p-HPGe/p+-BaSi2 solar cells.
This efficiency is almost consistent with the detailed-balance limit for double
heterojunction solar cell.

###Modeling of heterojunction photovoltaic cells based on ZnO nanowires array and earth-abundant cuprous oxide absorbers||Qilin Gu###

Modeling of heterojunction photovoltaic cells based on ZnO nanowires array and earth-abundant cuprous oxide absorbers. As a potential solution for low-cost efficient solar cells, radial junctions
consisting of ZnO nanowires arrays embedded in Cu2O thin films have been
theoretically modeled. Calculations have been performed to explore the
geometric dependence of performance of such wire-based solar cells. By properly
setting material properties and cell dimensions, a reasonable power conversion
efficiency of 19.7% can be expected in a material with 2 {\mu}m minority
carrier diffusion length. The detrimental effects of bulk, interface and
contact-related states on solar cell performance have also been studied, from
which the efficiencies between ~22% and ~12% for a series of materials, ranging
from optimal to seriously poor-quality, are extracted. The findings suggest
that rational device design plays a crucial role in implementing efficient
Cu2O/ZnO wire radial junction solar cells.

###Potential of the three-terminal heterojunction bipolar transistor solar cell for space applications||Antonio Martí,Pablo García-Linares,Marius Zehender,Simon A. Svatek,Irene Artacho,Ana Belén Cristóbal,José R. González,Carsten Baur,Iñigo Ramiro,Federica Cappelluti,Elisa Antolín###

Potential of the three-terminal heterojunction bipolar transistor solar cell for space applications. Multi-terminal multi-junction solar cells (MJSC) offer higher efficiency
potential than series connected (two-terminal) ones. In addition, for
terrestrial applications, the efficiency of multi-terminal solar cells is less
sensitive to solar spectral variations than the two-terminal series-connected
one. In space, generally, cells are always illuminated with AM0 spectrum and no
impact is expected from spectral variations. Still, in space, the
multi-terminal approach offers some advantages in comparison with the
series-connected architecture approach derived from a higher end of life (EOL)
efficiency. In this work we review the potential of multi-terminal solar cells
for achieving extended EOL efficiencies with emphasis in the potential of the
three-terminal heterojunction bipolar transistor solar cell, a novel
multi-terminal MJSC architecture with a simplified structure not requiring, for
example, tunnel junctions.

###Direct measurements of band gap grading in polycrystalline CIGS solar cells||M. P. Heinrich,Z-H. Zhang,Y. Zhang,O. Kiowski,M. Powalla,U. Lemmer,A. Slobodskyy###

Direct measurements of band gap grading in polycrystalline CIGS solar cells. We present direct measurements of depth-resolved band gap variations of
CuIn(1-x)Ga(x)Se2 thin-film solar cell absorbers. A new measurement technique
combining parallel measurements of local thin-film interference and spectral
photoluminescence was developed for this purpose. We find sample-dependent
correlation parameters between measured band gap depth and composition
profiles, and emphasize the importance of direct measurements. These results
bring a quantitative insight into the electronic properties of the solar cells
and open a new way to analyze parameters that determine the efficiency of solar
cells.

###Structural and electrical properties of electrodeposited single junction of cuprous (I) oxide copper||Edward Rówiński,Mateusz Pławecki###

Structural and electrical properties of electrodeposited single junction of cuprous (I) oxide copper. Cuprous (I) oxide (Cu_{2}O) based solar cells were fabricated with the use of
the electrodeposition technique at nanometre scale, and the structural,
morphological and electrical properties were investigated. The Cu_{2}O layers
were electrodeposited on crystalline and polycrystalline copper substrates. To
complete the Cu_{2}O/Cu(100) and Cu_{2}O/Cu interfaces as the solar cells the
top electrodes of silver paste were painted on the rear of Cu_{2}O. The
theoretical analysis of the current voltage curve was provided to determine the
values of electrical parameters of the most efficient solar cell of
Ag/Cu_{2}O/Cu(100) and clearly indicate presence of two Schottky barriers at
interfaces.

###Inhomogeneous light photovoltaic effect in neighboring quantum dots||Wenxi Lai###

Inhomogeneous light photovoltaic effect in neighboring quantum dots. Photovoltaic effect of double quantum dots under nonuniform light field
intensity has been studied theoretically. Comparing with the traditional p-n
type photovoltaic effect, the inhomogeneous light field provides asymmetric
potential creating polarization of electron number distribution in the
neighboring quantum dots and furthermore gives rise to net current. Current
density and efficiency of such kind solar cells are estimated to be comparable
to the traditional p-n type material based solar cells. Motion of electron is
described using quantum master equation around room temperature. The
inhomogeneous light photovoltaic effect has potential applications for the gain
of more economical solar cells.

###Modelling Grain Boundaries in Polycrystalline Halide Perovskite Solar Cells||Ji-Sang Park,Aron Walsh###

Modelling Grain Boundaries in Polycrystalline Halide Perovskite Solar Cells. Solar cells are semiconductor devices that generate electricity through
charge generation upon illumination. For optimal device efficiency, the
photo-generated carriers must reach the electrical contact layers before they
recombine. A deep understanding of the recombination process and transport
behavior is essential to design better devices. Halide perovskite solar cells
are commonly made of a polycrystalline absorber layer, but there is no
consensus on the nature and role of grain boundaries. This review paper
concerns theoretical approaches for the investigation of extended defects. We
introduce recent computational studies on grain boundaries, and their influence
on point defect distributions, in halide perovskite solar cells. We conclude
the paper with discussion of future research directions.

###Hot Carrier Extraction Using Energy Selective Contacts and Its Impact on the Limiting Efficiency of a Hot Carrier Solar Cell||Steven Limpert,Stephen Bremner###

Hot Carrier Extraction Using Energy Selective Contacts and Its Impact on the Limiting Efficiency of a Hot Carrier Solar Cell. Extraction of charge carriers from a hot carrier solar cell using energy
selective contacts, and the impact on limiting power conversion efficiency is
analyzed. It is shown that assuming isentropic conversion of carrier heat into
voltage implies zero power output at all operating points. Under conditions of
power output, lower voltages than in the isentropic case are obtained due to
the irreversible entropy increase associated with carrier flow. This lowers the
limiting power conversion efficiency of a hot carrier solar cell.

###Nano-patterned back-reflector with engineered near-field/far-field light scattering for enhanced light trapping in silicon-based multi-junction solar cells||Andrea Cordaro,Ralph Müller,Stefan Tabernig,Nico Tucher,Patrick Schygulla,Oliver Höhn,Benedikt Bläsi,Albert Polman###

Nano-patterned back-reflector with engineered near-field/far-field light scattering for enhanced light trapping in silicon-based multi-junction solar cells. Multi-junction solar cells provide a path to overcome the efficiency limits
of standard silicon solar cells by harvesting more efficiently a broader range
of the solar spectrum. However, Si-based multi-junction architectures are
hindered by incomplete harvesting in the near-infrared (near-IR) spectral
range, as Si sub-cells have weak absorption close to the band gap. Here, we
introduce an integrated near-field/far-field light trapping scheme to enhance
the efficiency of silicon-based multi-junction solar cells in the near-IR
range. To achieve this, we design a nanopatterned diffractive silver
back-reflector featuring a scattering matrix that optimizes trapping of
multiply-scattered light into a range of diffraction angles. We minimize
reflection to the 0th-order and parasitic plasmonic absorption in the silver by
engineering destructive interference in the patterned back contact. Numerical
and experimental assessment of the optimal design on the performance of
single-junction Si TOPCon solar cells highlights an improved external quantum
efficiency (EQE) over a planar back-reflector (+1.52 mA/cm2). Nanopatterned
metagrating back-reflectors are fabricated on GaInP/GaInAsP//Si two-terminal
triple-junction solar cells via Substrate Conformal Imprint Lithography (SCIL)
and characterized optically and electronically, demonstrating a power
conversion efficiency improvement of +0.9%abs over the planar reference.
Overall, our work demonstrates the potential of nanophotonic light trapping for
enhancing the efficiency of silicon-based multi-junction solar cells, paving
the way for more efficient and sustainable solar energy technologies.

###New strategy to promote conversion efficiency using high-index nanostructures in thin-film solar cells||DongLin Wang,Gang Su###

New strategy to promote conversion efficiency using high-index nanostructures in thin-film solar cells. Nano-scaled metallic or dielectric structures may provide various ways to
trap light into thin-film solar cells for improving the conversion efficiency.
In most schemes, the textured active layers are involved into light trapping
structures that can provide perfect optical benefits but also bring undesirable
degradation of electrical performance. Here we propose a novel approach to
design high-performance thin-film solar cells. In our strategy, a flat active
layer is adopted for avoiding electrical degradation, and an optimization
algorithm is applied to seek for an optimized light trapping structure for the
best optical benefit. As an example, we show that the efficiency of a flat
a-Si:H thin-film solar cell can be promoted close to the certified highest
value. It is also pointed out that, by choosing appropriate dielectric
materials with high refractive index (>3) and high transmissivity in wavelength
region of 350nm-800nm, the conversion efficiency of solar cells can be further
enhanced.

###Numerical simulation of InGaN Schottky solar cell||Sidi Ould Saad Hamady,Adaine Abdoulwahab,Nicolas Fressengeas###

Numerical simulation of InGaN Schottky solar cell. The Indium Gallium Nitride (InGaN) III-Nitride ternary alloy has the
potentiality to allow achieving high efficiency solar cells through the tuning
of its band gap by changing the Indium composition. It also counts among its
advantages a relatively low effective mass, high carriers\^a mobility, a high
absorption coefficient along with good radiation tolerance.However, the main
drawback of InGaN is linked to its p-type doping, which is difficult to grow in
good quality and on which ohmic contacts are difficult to realize. The Schottky
solar cell is a good alternative to avoid the p-type doping of InGaN. In this
report, a comprehensive numerical simulation, using mathematically rigorous
optimization approach based on state-of-the-art optimization algorithms, is
used to find the optimum geometrical and physical parameters that yield the
best efficiency of a Schottky solar cell within the achievable device
fabrication range. A 18.2% efficiency is predicted for this new InGaN solar
cell design.

###Optimizations of GaAs Nanowire Solar Cells||Anna H. Trojnar,Christopher E. Valdivia,Ray R. LaPierre,Karin Hinzer,Jacob J. Krich###

Optimizations of GaAs Nanowire Solar Cells. The efficiency of GaAs nanowire solar cells can be significantly improved
without any new processing steps or material requirements. We report coupled
optoelectronic simulations of a GaAs nanowire (NW) solar cell with vertical
p-i-n junction and high band gap AlInP passivating shell. Our
frequency-dependent model facilitates calculation of quantum efficiency for the
first time in NW solar cells. For passivated NWs, we find that short-wavelength
photons can be most effectively harnessed by using a thin emitter while
long-wavelength photons are best utilized by extending the intrinsic region to
the nanowire/substrate interface, and using the substrate as a base. These two
easily implemented changes, coupled with the increase of NW height to 3.5 um
with realistic surface recombination in the presence of a passivation shell,
result in a NW solar cell with greater than 19% efficiency.

###High current, high efficiency graded band gap perovskite solar cells||Onur Ergen,S. Matt Gilbert,Thang Pham,Sally J. Turner,Mark Tian Zhi Tan,Marcus A. Worsley,Alex Zettl###

High current, high efficiency graded band gap perovskite solar cells. Organic-inorganic halide perovskite materials have emerged as attractive
alternatives to conventional solar cell building blocks. Their high light
absorption coefficients and long diffusion lengths suggest high power
conversion efficiencies (PCE),1-5 and indeed perovskite-based single band gap
and tandem solar cell designs have yielded impressive performances.1-16 One
approach to further enhance solar spectrum utilization is the graded band gap,
but this has not been previously achieved for perovskites. In this study, we
demonstrate graded band gap perovskite solar cells with steady-state conversion
efficiencies averaging 18.4%, with a best of 21.7%, all without reflective
coatings. An analysis of the experimental data yields high fill factors of ~75%
and high short circuit current densities up to 42.1 mA/cm2. These cells, which
are based on a novel architecture of two perovskite layers (MASnI3 and
MAPbI3-xBrx), incorporating GaN, monolayer hexagonal boron nitride, and
graphene aerogel, display the highest efficiency ever reported for perovskite
solar cells.

###Pinhole induced efficiency variation in perovskite solar cells||Sumanshu Agarwal,Pradeep R. Nair###

Pinhole induced efficiency variation in perovskite solar cells. Process induced efficiency variation is a major concern for all thin film
solar cells, including the emerging perovskite based solar cells. In this
manuscript, we address the effect of pinholes or process induced surface
coverage aspects on the efficiency of such solar cells through detailed
numerical simulations. Interestingly, we find the pinhole size distribution
affects the short circuit current and open circuit voltage in contrasting
manners. Specifically, while the Jsc is heavily dependent on the pinhole size
distribution, surprisingly, the Voc seems to be only nominally affected by it.
Further, our simulations also indicate that, with appropriate interface
engineering, it is indeed possible to design a nanostructured device with
efficiencies comparable to that of ideal planar structures. Additionally, we
propose a simple technique based on terminal IV characteristics to estimate the
surface coverage in perovskite solar cells.

###Efficiency limits of electronically-coupled upconverter and quantum ratchet solar cells using detailed balance||Emily Z. Zhang,Jacob J. Krich###

Efficiency limits of electronically-coupled upconverter and quantum ratchet solar cells using detailed balance. The intermediate band solar cell (IBSC) and quantum ratchet solar cell (QRSC)
have the potential to surpass the efficiency of standard single-junction solar
cells by allowing sub-gap photon absorption through states deep inside the band
gap. High efficiency IBSC and QRSC devices have not yet been achieved, however,
since introducing mid-gap states also increases recombination, which can harm
the device. We consider the electronically coupled upconverter (ECUC) solar
cell and show that it can achieve the same efficiencies as the QRSC. Although
they are equivalent in the detailed balance limit, the ECUC is less sensitive
to nonradiative processes, which makes it a more practical implementation for
IB devices. We perform a case study of crystalline-silicon based ECUC cells,
focusing on hydrogenated amorphous silicon as the upconverter material and
highlighting potential dopants for the ECUC. These results illustrate a new
path for the development of IB-based devices.

###Efficiency Limit Of AlxGa1-xAs Solar Cell Modified By AlyGa1-ySb Quantum Dot Intermediate Band Embedded Outside Of The Depletion Region||A. Kechiantz,A. Afanasev,J. -L. Lazzari,A. Bhouri,Y. Cuminal,P. Christol###

Efficiency Limit Of AlxGa1-xAs Solar Cell Modified By AlyGa1-ySb Quantum Dot Intermediate Band Embedded Outside Of The Depletion Region. Recombination through quantum dots (QDs) is a major factor that limits
efficiency of QD intermediate-band (IB) solar cells. Our proposal for a new IB
solar cell based on type-II GaSb QDs located outside the depletion region of a
GaAs p-n-junction aims to solve this problem. The important advantage of
proposed heterostructure appears due to the outside location of IB. Such IB
does not assist generation of additional leakage current flow through the
depletion region. Carriers cannot escape from outside QDs through the buffer
layer and the depletion region into GaAs substrate by tunneling because QDs are
far from the depletion layer. Only solar photon or thermal assistance may
enable electron escape from QDs. Such type-II QD IB solar cell concept promises
an efficiency enhancement relative to that of GaAs solar cells.

###Enhanced Efficiency of Light-Trapping Nanoantenna Arrays for Thin Film Solar Cells||Constantin R. Simovski,Dmitry K. Morits,Pavel M. Voroshilov,Michael E. Guzhva,Pavel A. Belov,Yuri S. Kivshar###

Enhanced Efficiency of Light-Trapping Nanoantenna Arrays for Thin Film Solar Cells. We suggest a novel concept of efficient light-trapping structures for
thin-film solar cells based on arrays of planar nanoantennas operating far from
plasmonic resonances. The operation principle of our structures relies on the
excitation of chessboard-like collective modes of the nanoantenna arrays with
the field localized between the neighboring metal elements. We demonstrated
theoretically substantial enhancement of solar-cell short-circuit current by
the designed light-trapping structure in the whole spectrum range of the
solar-cell operation compared to conventional structures employing
anti-reflecting coating. Our approach provides a general background for a
design of different types of efficient broadband light-trapping structures for
thin-film solar-cell technologically compatible with large-area thin-film
fabrication techniques.

###High Open Circuit voltage solar cells based on bright mixed halide CsPbBrI2 perovskite nanocrystals synthesized under ambient air conditions||Sotirios Christodoulou,Francesco Di Stasio,Santanu Pradhan,Alexandros Stavrinadis,Gerasimos Konstantatos###

High Open Circuit voltage solar cells based on bright mixed halide CsPbBrI2 perovskite nanocrystals synthesized under ambient air conditions. Lead halide perovskite nanocrystals (NCs) are currently emerging as one of
the most interesting solution processed semiconductors since they possess high
photoluminescence quantum yield (PLQY), and colour tunability through anion
exchange reactions or quantum confinement. Here, we show efficient solar cells
based on mixed halide (CsPbBrI2) NCs obtained via anion exchange reactions in
ambient conditions. We performed anion exchange reactions in concentrated NC
solutions with I-, thus inducing a PL red-shift up to 676 nm, and obtaining a
high PLQY in film (65%). Solar cell devices operating in the wavelength range
350-660 nm were fabricated in air with two different deposition methods. The
solar cells display a photo-conversion efficiency of 5.3% and open circuit
voltage (Voc) up to 1.31V, among the highest reported for perovskite based
solar cells with band gap below 2eV, clearly demonstrating the potential of
this material.

###Coupled Optoelectronic Simulation and Optimization of Thin-Film Photovoltaic Solar Cells||Tom H. Anderson,Benjamin J. Civiletti,Peter Monk,Akhlesh Lakhtakia###

Coupled Optoelectronic Simulation and Optimization of Thin-Film Photovoltaic Solar Cells. A design tool was formulated for optimizing the efficiency of inorganic,
thin-film, photovoltaic solar cells. The solar cell can have multiple
semiconductor layers in addition to antireflection coatings, passivation
layers, and buffer layers. The solar cell is backed by a metallic grating which
is periodic along a fixed direction. The rigorous coupled-wave approach is used
to calculate the electron-hole-pair generation rate. The hybridizable
discontinuous Galerkin method is used to solve the drift-diffusion equations
that govern charge-carrier transport in the semiconductor layers. The chief
output is the solar-cell efficiency which is maximized using the differential
evolution algorithm to determine the optimal dimensions and bandgaps of the
semiconductor layers.

###Coordination Engineering of Cu-Zn-Sn-S Aqueous Precursor for Efficient Kesterite Solar Cells||Linbao Guo,Jiangjian Shi,Qing Yu,Biwen Duan,Xiao Xu,Jiazheng Zhou,Jionghua Wu,Yusheng Li,Dongmei Li,Huijue Wu,Yanhong Luo,Qingbo Meng###

Coordination Engineering of Cu-Zn-Sn-S Aqueous Precursor for Efficient Kesterite Solar Cells. Aqueous precursors provide an alluring approach for low-cost and
environmentally friendly production of earth-abundant Cu2ZnSn(S,Se)4 (CZTSSe)
solar cells. The key is to find an appropriate molecular agent to prepare a
stable solution and optimize the coordination structure to facilitate the
subsequent crystallization process. Herein, we introduce thioglycolic acid,
which possesses strong coordination (-SH) and hydrophilic (-COOH) groups, as
the agent and use deprotonation to regulate the coordination competition within
the aqueous solution. Ultimately, metal cations are adequately coordinated with
thiolate anions, and carboxylate anions are released to become hydrated to form
an ultrastable aqueous solution. These factors have contributed to achieving
CZTSSe solar cells with efficiency of as high as 12.2% (a certified efficiency
of 12.0%) and providing an extremely wide time window for precursor storage and
usage. This work represents significant progress in the non-toxic solution
fabrication of CZTSSe solar cells and holds great potential for the development
of CZTSSe and other metal sulfide solar cells.

###Efficiency limits of Perovskite Solar Cells with Transition Metal Oxides as Hole Transport Layers||Dhyana Sivadas,Swasti Bhatia,Pradeep Nair###

Efficiency limits of Perovskite Solar Cells with Transition Metal Oxides as Hole Transport Layers. Transition metal oxides (TMOs) like MoOx are increasingly explored as hole
transport layers for perovskite-based solar cells. Due to their large work
function, the hole collection mechanism of such solar cells are fundamentally
different from other materials like PEDOT: PSS, and the associated device
optimizations are not well elucidated. In addition, the prospects of such
architectures against the challenges posed by ion migration are yet to be
explored - which we critically examine in this contribution through detailed
numerical simulations. Curiously, we find that, for similar ion densities and
interface recombination velocities, ion migration is more detrimental for
Perovskite solar cells with TMO contact layers with much lower achievable
efficiency limits (21%). The insights shared by this work should be of broad
interest to the community in terms of long-term stability, efficiency
degradation and hence could help critically evaluate the promises and prospects
of TMOs as hole contact layers for perovskite solar cells.

###Influence of Light Soaking on Silicon Heterojunction Solar Cells With Various Architectures||Jean Cattin,Laurie-Lou Senaud,Jan Haschke,Bertrand Paviet-Salomon,Matthieu Despeisse,Christophe Ballif,Mathieu Boccard###

Influence of Light Soaking on Silicon Heterojunction Solar Cells With Various Architectures. In this article, we investigate the effect of prolonged light exposure on
silicon heterojunction solar cells. We show that, although light exposure
systematicallyimproves solar cell efficiency in the case of devices using
intrinsic and p-type layers with optimal thickness, this treatment leads to
performance degradation for devices with an insufficiently thick (p) layer on
the light-incoming side. Our results indicate that this degradation is caused
by a diminution of the (i/p)-layer stack hole-selectivity because of light
exposure. Degradation is avoided when a sufficiently thick (p) layer is used,
or when exposure of the (p) layer to UV light is avoided, as is the case of the
rear-junction configuration, commonly used in the industry. Additionally,
applying a forward bias current or an infrared light exposure results in an
efficiency increase for all investigated solar cells, independently of the
(p)-layer thickness, confirming the beneficial influence of recombination on
the performance of silicon heterojunction solar cells.

###Modeling of V graded In(x)Ga(1-x)N solar cells: comparison of strained and relaxed features||Mirsaeid Sarollahi,Mohammad Zamani Alavijeh,Rohith Allaparthi,Reem Alhelais,Malak A. Refaei,Md Helal Uddin Maruf,Morgan E. Ware###

Modeling of V graded In(x)Ga(1-x)N solar cells: comparison of strained and relaxed features. The optical properties of V graded InGaN solar cells are studied. Graded
InGaN well structures with the indium composition increasing then decreasing in
a V shaped pattern have been designed. Through polarization doping, this
naturally creates alternating p-type and n-type regions. Separate structures
are designed by varying the indium alloy profile from GaN to maximum indium
concentrations ranging from 20% to 80%, while maintaining a constant overall
structure thicknesses of 100 nm. The solar cell parameters under fully strained
and relaxed conditions are considered. The results show that a maximum
efficiency of 5.5%, under fully strained condition occurs at x=60%. Solar cell
efficiency under relaxed conditions increases to a maximum of 8.3% at 90%.
While Vegards law predicts the bandgap under relaxed conditions, a Vegard like
law is empirically determined from the output of Nextnano for varying In
compositions in order to calculate solar cell parameters under strain.

###Simulation of the Efficiency of a-SiC:H/a-Si:H Tandem Multilayer Solar Cells||Khikmat Kh. Muminov,Ashrafalsadat S. Mirkamali###

Simulation of the Efficiency of a-SiC:H/a-Si:H Tandem Multilayer Solar Cells. In this paper we carried out theoretical study of the general issues related
to the efficiency of SiC:H/a-Si:H single- and multi-junction tandem solar
cells. Implementation of numerical simulations by the use of AMPS-1D program of
one-dimensional analysis of microelectronic and photonic structures for the
analysis of hydrogenated silicon solar cells allowed us to formulate the
optimal design of new kind of multi-junction tandem solar cells, providing its
most efficient operation. The numerical analysis of SiC:H/a-Si:H
single-junction solar cell whith doped i-layer used as the intermediate
absorbing layer (a -Si: H) placed between layers of p-type (a-SiC: H) and
n-type (a-Si: H) has been conducted. It has been established that after
optimizing the solar cell parameters its highest efficiency of 19.62% is
achieved at 500 nm thickness of i-layer. The optimization of the newly
developed multi-junction structure of a-SiC:H/a-Si:H tandem solar cell has been
conducted. It has been shown numerically that its highest efficiency of 22.6%
is achieved at the thickness of 270 nm of intermediate i-layer.

###CH3NH3PbI3/GeSe bilayer heterojunction solar cell with high performance||Guo-Jiao Hou,Dong-Lin Wang,Roshan Ali,Yu-Rong Zhou,Zhen-Gang Zhu,Gang Su###

CH3NH3PbI3/GeSe bilayer heterojunction solar cell with high performance. Perovskite (CH3NH3PbI3) solar cells have made significant advances recently.
In this paper, we propose a bilayer heterojunction solar cell comprised of a
perovskite layer combining with a IV-VI group semiconductor layer, which can
give a conversion efficiency even higher than the conventional perovskite solar
cell. Such a scheme uses a property that the semiconductor layer with a direct
band gap can be better in absorption of long wavelength light and is
complementary to the perovskite layer. We studied the semiconducting layers
such as GeSe, SnSe, GeS, and SnS, respectively, and found that GeSe is the
best, where the optical absorption efficiency in the perovskite/GeSe solar cell
is dramatically increased. It turns out that the short circuit current density
is enhanced 100% and the power conversion efficiency is promoted 42.7% (to a
high value of 23.77%) larger than that in a solar cell with only single
perovskite layer. The power conversion efficiency can be further promoted so
long as the fill factor and open-circuit voltage are improved. This strategy
opens a new way on developing the solar cells with high performance and
practical applications.

###Efficiency Limit of Intermediate Band AlxGa1-xAs Solar Cell Based on AlyGa1-ySb Type-II Quantum Dots Embedded Outside of the Depletion Region||A. Kechiantz,A. Afanasev,J. -L. Lazzari###

Efficiency Limit of Intermediate Band AlxGa1-xAs Solar Cell Based on AlyGa1-ySb Type-II Quantum Dots Embedded Outside of the Depletion Region. The intermediate band (IB) cell is a concept of highly efficient solar cells
proposed by Luque and Marti in 1997. The IB concept uses nonlinear effect of
two photon absorption enforced with concentration of such photons for
generation of additional photocurrent in single p-n-junction cells. In this
theoretical work we demonstrate an important role of self-organized strained
type-II AlyGa1-ySb quantum dots for operation in IB GaAs solar cells.

###Optimal quantum dot size for photovoltaics with fusion||Benedicta Sherrie,Alison M. Funston,Laszlo Frazer###

Optimal quantum dot size for photovoltaics with fusion. Light fusion increases the efficiency of solar cells by converting photons
with lower energy than the bandgap into higher energy photons. The solar cell
converts the product photons to current. We use Monte Carlo simulation to
predict that lead sulfide (PbS) quantum dot sensitizers will enable fusion with
a figure of merit on the mA cm$^{-2}$ scale, exceeding current records, while
enabling silicon cell compatibility. Performance is highly sensitive to quantum
dot size, on the order of mA cm$^{-2}$ nm$^{-1}$.

###On the Use of Graphene to Improve the Performance of Concentrator III-V Multijunction Solar Cells||Laura Barrutia,IvÁn Lombardero1,Mario Ochoa,Mercedes GabÁs,IvÁn GarcÍa,TomÁs Palacios,Andrew Johnson,Ignacio Rey-Stolle,Carlos Algora###

On the Use of Graphene to Improve the Performance of Concentrator III-V Multijunction Solar Cells. Graphene has been intensively studied in photovoltaics focusing on emerging
solar cells based on thin films, dye-sensitized, quantum dots, nanowires, etc.
However, the typical efficiency of these solar cells incorporating graphene are
below 16%. Therefore, the photovoltaic potential of graphene has not been
already shown. In this work the use of graphene for concentration applications
on III-V multijunction solar cells, which indeed are the solar cells with the
highest efficiency, is demonstrated. Firstly, a wide optoelectronic
characterization of graphene layers is carried out. Then, the graphene layer is
incorporated onto triple-junction solar cells, which decreases their series
resistance by 35% (relative), leading to an increase in Fill Factor of 4%
(absolute) at concentrations of 1,000 suns. Simultaneously, the optical
absorption of graphene produces a relative short circuit current density
decrease in the range of 0-1.8%. As a result, an absolute efficiency
improvement close to 1% at concentrations of 1,000 suns was achieved with
respect to triple junction solar cells without graphene. The impact of
incorporating one and two graphene monolayers is also evaluated.

###Practical development of efficient thermoelectric-photovoltaic hybrid systems based on wide-gap solar cells||Bruno Lorenzi,Paolo Mariani,Andrea Reale,Aldo Di Carlo,Gang Chen,Dario Narducci###

Practical development of efficient thermoelectric-photovoltaic hybrid systems based on wide-gap solar cells. The decrease of solar cell efficiency with temperature is a known problem for
photovoltaics (PV). Temperature sensitivity can lead to a considerable amount
of energy losses over the lifetime of solar panels. In this perspective Hybrid
Thermoelectric-Photovoltaic (HTEPV) systems, which recover solar cell heat
losses to produce an additional power output, can be a suitable option. However
only hybridization of wide-gap solar cells is convenient in terms of efficiency
gains and deserves investigation to evaluate HTEPV devices effectiveness. In
this work we report the modeling and the development of customized bismuth
telluride thermoelectric generators, optimized to be hybridized with amorphous
silicon (aSi), Gallium Indium Phosphide (GaInP) or Perovskites solar cells. The
model results showed in all three cases efficiency gains with a maximum of
+3.1% for Perovskites (from 16.4% to 19.5%). These enhancements were then
experimentally validated for the case of Perovskites solar cells, for which
maximum gains were found to occur at typical operating temperatures of
conventional PVs. This experimental evaluation demonstrated in an accurate
fashion the real potential of thermoelectric hybridization of solar cells.

###Does Singlet Fission Enhance the Performance of Organic Solar Cells?||K. Aryanpour,J. A. Muñoz,S. Mazumdar###

Does Singlet Fission Enhance the Performance of Organic Solar Cells?. Singlet fission, in which the optical spin-singlet exciton dissociates into
two low energy triplet excitons, has been proposed as a viable approach to
enhance the quantum efficiency of organic solar cells. We show that even when
singlet fission is occurring in the donor molecule, the electronic structure at
the donor$-$acceptor interface must satisfy specific requirements for the solar
cell performance to be enhanced by this process. We focus on the
pentacene$-$C$_{60}$ solar cell, and on the basis of our calculations and
available experimental data, we conclude that there is not enough evidence that
these requirements are met by the donor$-$acceptor interface here. We propose
experiments that can determine whether the minimal requirement for enhanced
performance driven by singlet fission is met in this and other solar cells.

###Low cost and high performance light trapping structure for thin-film solar cells||DongLin Wang,Huijuan Cui,Gang Su###

Low cost and high performance light trapping structure for thin-film solar cells. Nano-scaled dielectric and metallic structures are popular light tapping
structures in thin-film solar cells. However, a large parasitic absorption in
those structures is unavoidable. Most schemes based on such structures also
involve the textured active layers that may bring undesirable degradation of
the material quality. Here we propose a novel and cheap light trapping
structure based on the prism structured SiO2 for thin-film solar cells, and a
flat active layer is introduced purposefully. Such a light trapping structure
is imposed by the geometrical shape optimization to gain the best optical
benefit. By examining our scheme, it is disclosed that the conversion
efficiency of the flat a-Si:H thin-film solar cell can be promoted to exceed
the currently certified highest value. As the cost of SiO2-based light trapping
structure is much cheaper and easier to fabricate than other materials, this
proposal would have essential impact and wide applications in thin-film solar
cells.

###A Physics-based Analytical Model for Perovskite Solar Cells||Xingshu Sun,Reza Asadpour,Wanyi Nie,Aditya D. Mohite,Muhammad A. Alam###

A Physics-based Analytical Model for Perovskite Solar Cells. Perovskites are promising next-generation absorber materials for low-cost and
high-efficiency solar cells. Although perovskite cells are configured similar
to the classical solar cells, their operation is unique and requires
development of a new physical model for characterization, optimization of the
cells, and prediction of the panel performance. In this paper, we develop such
a physics-based analytical model to describe the operation of different types
of perovskite solar cells, explicitly accounting non-uniform generation,
carrier selective transport layers, and voltage-dependent carrier collection.
The model would allow experimentalists to characterize key parameters of
existing cells, understand performance bottlenecks, and predict performance of
perovskite-based solar panel - the obvious next step to the evolution of
perovskite solar cell technology.

###New Design of Potentially Low-cost Solar Cells Using TiO2/Graphite Composite as Photon Absorber||Dui Yanto Rahman,Mamat Rokhmat,Elfi Yuliza,Euis Sustini,Mikrajuddin Abdullah###

New Design of Potentially Low-cost Solar Cells Using TiO2/Graphite Composite as Photon Absorber. A solar cell design using the combination of titanium dioxide and graphite as
active photon absorbing materials were proposed. The titanium dioxide absorbs
photons of nearly ultraviolet wavelengths to produce electron hole pairs, while
graphite is expected to absorb photons of longer wavelengths. Although many
authors have claimed that graphite is not a semiconductor, we observed that a
model of a solar cell containing titanium dioxide only as the active material
behaves exactly the same as a model containing graphite only as the active
material. Additionally, we observed that a model of a solar cell made using a
composite of titanium dioxide and graphite as the active material had much
higher efficiency than solar cells made using titanium dioxide only or graphite
only active materials.

###Alternative Pathways for Multiple Exciton Generation Solar Cells by Tandem Configurations||Jongwon Lee###

Alternative Pathways for Multiple Exciton Generation Solar Cells by Tandem Configurations. Multiple exciton generation solar cells (MEGSCs) undergo low efficiency due
to material imperfections such as nonradiative recombination This paper
introduces alternative approaches for realizing photovoltaic (PV) devices
similar to MEGSCs. Furthermore, we reorganize the detailed balance equation of
MEGSCs such that it is similar to that of independent connection tandem solar
cells. This is possible because of the spectral dependence of the ideal QY.
Finally, we compare these two similar equations and propose alternative
approaches for realizing MEGSC-like tandem solar cells. We explain the
difficulty in fabricating MEGSCs, which arises from the high rate of
non-idealities. In this regard, the deconstruction of the detailed balance
equation of MEGSCs can reveal alternative paths for replacing MEGSCs with
tandem solar cell configurations.

###Interfaces of high efficient kesterite Cu2ZnSnS(e)4 thin film solar cells||Shoushuai Gao,Zhenwu Jiang,Li Wu,Jianping Ao,Yu Zeng,Yun Sun,Yi Zhang###

Interfaces of high efficient kesterite Cu2ZnSnS(e)4 thin film solar cells. Cu2ZnSnS(e)4 (CZTS(e)) solar cells have attracted much attention due to the
elemental abundance and the non-toxicity. However, the record efficiency of
12.6% for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is much lower than that of
Cu(In,Ga)Se2 (CIGS) solar cells. One crucial reason is the recombination at
interfaces. In recent years, large amount investigations have been done to
analyze the interfacial problems and improve the interfacial properties via a
variety of methods. This paper gives a review of progresses on interfaces of
CZTS(e) solar cells, including: (1) the band alignment optimization at
buffer/CZTS(e) interface, (2) tailoring the thickness of MoS(e)2 interfacial
layers between CZTS(e) absorber and Mo back contact, (3) the passivation of
rear interface, (4) the passivation of front interface, and (5) the etching of
secondary phases.

###Solution-processed ZnO as the efficient passivation and electron selective layer of silicon solar cells||Jiangnan Ding,Yurong Zhou,Gangqiang Dong,Ming Liu,Donghong Yu,Fengzhen Liu###

Solution-processed ZnO as the efficient passivation and electron selective layer of silicon solar cells. Solution-processed intrinsic ZnO and Al doped ZnO (ZnO:Al) were spin coated
on textured n-type c-Si wafer to replace the phosphorus doped amorphous silicon
as the electron selective transport layer (ESTL) of the Si heterojunction (SHJ)
solar cells. Besides the function of electron selective transportation, the
non-doped ZnO was found to possess certain passivation effect on c-Si wafer.
The SHJ solar cells with different combinations of passivation layer (intrinsic
a-Si:H, SiOx and non-doped ZnO) and electron transport layer (non-doped ZnO and
ZnO:Al ) were fabricated and compared. An efficiency up to 18.46% was achieved
on a SHJ solar cell with an a-Si:H/ZnO:Al double layer back structure. And, the
all solution-processed non-doped ZnO/ZnO:Al combination layer presents fairly
good electron selective transportation property for SHJ solar cell, resulting
in an efficiency of 17.13%. The carrier transport based on energy band diagrams
of the rear side of the solar cells has been discussed related to the
performance of the SHJ solar cells.

###Optoelectronic Reciprocity in Hot Carrier Solar Cells with Ideal Energy Selective Contacts||Andreas Pusch,Milos Dubajic,Michael P. Nielsen,Gavin J. Conibeer,Stephen P. Bremner,Nicholas J. Ekins-Daukes###

Optoelectronic Reciprocity in Hot Carrier Solar Cells with Ideal Energy Selective Contacts. Hot carrier solar cells promise theoretical power conversion efficiencies far
beyond the single junction limit. However, practical implementations of hot
carrier solar cells have lagged far behind those theoretical predictions.
Reciprocity relations for electro-luminescence from conventional single
junction solar cells have been extremely successful in driving their efficiency
ever closer to the theoretical limits. In this work, we discuss how the
signatures of a functioning hot carrier device should manifest experimentally
in electro-luminescence and dark $I-V$ characteristics. Hot carrier properties
lead to deviations from the Shockley diode equation that is typical for
conventional single junction solar cells. These deviations are directly linked
to an increase in temperature of the carriers and therefore the temperature
measured from electro-luminescence spectra. We also elucidate how the behaviour
of hot carrier solar cells in the dark depends on whether Auger processes play
a significant role, revealing a stark contrast between the regime of negligible
Auger recombination (carrier conservation model) and dominant Auger
recombination (Impact Ionization model) for hot carrier solar cells.

###Cataloguing MoSi$_2$N$_4$ and WSi$_2$N$_4$ van der Waals Heterostructures: An Exceptional Material Platform for Excitonic Solar Cell Applications||Che Chen Tho,Chenjiang Yu,Qin Tang,Qianqian Wang,Tong Su,Zhuoer Feng,Qingyun Wu,C. V. Nguyen,Wee-Liat Ong,Shi-Jun Liang,San-Dong Guo,Liemao Cao,Shengli Zhang,Shengyuan A. Yang,Lay Kee Ang,Guangzhao Wang,Yee Sin Ang###

Cataloguing MoSi$_2$N$_4$ and WSi$_2$N$_4$ van der Waals Heterostructures: An Exceptional Material Platform for Excitonic Solar Cell Applications. Two-dimensional (2D) materials van der Waals heterostructures (vdWHs)
provides a revolutionary route towards high-performance solar energy conversion
devices beyond the conventional silicon-based pn junction solar cells. Despite
tremendous research progress accomplished in recent years, the searches of
vdWHs with exceptional excitonic solar cell conversion efficiency and optical
properties remain an open theoretical and experimental quest. Here we show that
the vdWH family composed of MoSi$_2$N$_4$ and WSi$_2$N$_4$ monolayers provides
a compelling material platform for developing high-performance ultrathin
excitonic solar cells and photonics devices. Using first-principle
calculations, we construct and classify 51 types of MoSi$_2$N$_4$ and
WSi$_2$N$_4$-based [(Mo,W)Si$_2$N$_4$] vdWHs composed of various metallic,
semimetallic, semiconducting, insulating and topological 2D materials.
Intriguingly, MoSi$_2$N$_4$/(InSe, WSe$_2$) are identified as Type-II vdWHs
with exceptional excitonic solar cell power conversion efficiency reaching well
over 20%, which are competitive to state-of-art silicon solar cells. The
(Mo,W)Si$_2$N$_4$ vdWH family exhibits strong optical absorption in both the
visible and ultraviolet regimes. Exceedingly large peak ultraviolet absorptions
over 40%, approaching the maximum absorption limit of a free-standing 2D
material, can be achieved in (Mo,W)Si$_2$N$_4$/$\alpha_2$-(Mo,W)Ge$_2$P$_4$
vdWHs. Our findings unravel the enormous potential of (Mo,W)Si$_2$N$_4$ vdWHs
in designing ultimately compact excitonic solar cell device technology.

###Numerical modeling of CuSbSe2-based dual-heterojunction thin film solar cell with CGS back surface layer||Bipin Saha,Bipanko Kumar Mondal,Shaikh Khaled Mostaque,Mainul Hossain,Jaker Hossain###

Numerical modeling of CuSbSe2-based dual-heterojunction thin film solar cell with CGS back surface layer. Ternary chalcostibite copper antimony selenide (CuSbSe2) is a promising
absorber material for next generation thin film solar cells due to the
non-toxic nature, earth-abundance, low-cost fabrication technique, optimum
bandgap and high optical absorption coefficient of CuSbSe2. Conventional single
heterojunction CuSbSe2 solar cells suffer from high recombination rate at the
interfaces and the presence of a Schottky barrier at the back contact, which
limit their power conversion efficiencies (PCEs). In this study, we propose a
dual-heterojunction n-ZnSe/p-CuSbSe2/p+-CGS solar cell, having copper gallium
selenide (CGS) as the back surface field (BSF) layer. The BSF layer absorbs
longer wavelength photons through a tail-states-assisted (TSA) two-step
upconversion process, leading to enhanced conversion efficiency. Numerical
simulations were carried out using SCAPS-1D to investigate the performance of
the proposed solar cell with respect to absorber layer thickness, doping
concentrations and defect densities. The simulation results exhibit PCE as high
as 43.77% for the dual-heterojunction solar cell as compared to 27.74% for the
single heterojunction n-ZnSe/p-CuSbSe2 counterpart. The dual-heterojunction
structure has, therefore, the potential to approach the Shockley-Queisser (SQ)
detailed balance limit and can lead to extremely high PCEs in emerging thin
film solar cells.

###Machine learning for accelerating the discovery of high performance low-cost solar cells: a systematic review||Satyam Bhatti,Habib Ullah Manzoor,Bruno Michel,Ruy Sebastian Bonilla,Richard Abrams,Ahmed Zoha,Sajjad Hussain,Rami Ghannam###

Machine learning for accelerating the discovery of high performance low-cost solar cells: a systematic review. Solar photovoltaic (PV) technology has merged as an efficient and versatile
method for converting the Sun's vast energy into electricity. Innovation in
developing new materials and solar cell architectures is required to ensure
lightweight, portable, and flexible miniaturized electronic devices operate for
long periods with reduced battery demand. Recent advances in biomedical
implantable and wearable devices have coincided with a growing interest in
efficient energy-harvesting solutions. Such devices primarily rely on
rechargeable batteries to satisfy their energy needs. Moreover, Artificial
Intelligence (AI) and Machine Learning (ML) techniques are touted as game
changers in energy harvesting, especially in solar energy materials. In this
article, we systematically review a range of ML techniques for optimizing the
performance of low-cost solar cells for miniaturized electronic devices. Our
systematic review reveals that these ML techniques can expedite the discovery
of new solar cell materials and architectures. In particular, this review
covers a broad range of ML techniques targeted at producing low-cost solar
cells. Moreover, we present a new method of classifying the literature
according to data synthesis, ML algorithms, optimization, and fabrication
process. In addition, our review reveals that the Gaussian Process Regression
(GPR) ML technique with Bayesian Optimization (BO) enables the design of the
most promising low-solar cell architecture. Therefore, our review is a critical
evaluation of existing ML techniques and is presented to guide researchers in
discovering the next generation of low-cost solar cells using ML techniques.

###Analytical Device-Physics Framework for Non-Planar Solar Cells||T. Kirkpatrick,M. J. Burns,M. J. Naughton###

Analytical Device-Physics Framework for Non-Planar Solar Cells. Non-planar solar-cell devices have been promoted as a means to enhance
current collection in absorber materials with charge-transport limitations.
This work presents an analytical framework for assessing the ultimate
performance of non-planar solar-cells based on materials and geometry. Herein,
the physics of the p-n junction is analyzed for low-injection conditions, when
the junction can be considered spatially separable into quasi-neutral and
space-charge regions. For the conventional planar solar cell architecture,
previously established one-dimensional expressions governing charge carrier
transport are recovered from the framework established herein. Space-charge
region recombination statistics are compared for planar and non-planar
geometries, showing variations in recombination current produced from the
space-charge region. In addition, planar and non-planar solar cell performance
are simulated, based on a semi-empirical expression for short-circuit current,
detailing variations in charge carrier transport and efficiency as a function
of geometry, thereby yielding insights into design criteria for solar cell
architectures. For the conditions considered here, the expressions for
generation rate and total current are shown to universally govern any solar
cell geometry, while recombination within the space-charge region is shown to
be directly dependent on the geometrical orientation of the p-n junction.

###Molybdenum oxide hole selective transport layer by hot wire oxidation-sublimation deposition for silicon heterojunction solar cells||Fengchao Li,Yurong Zhou,Ming Liu,Gangqiang Dong,Fengzhen Liu,Wenjing Wang,Donghong Yu###

Molybdenum oxide hole selective transport layer by hot wire oxidation-sublimation deposition for silicon heterojunction solar cells. In this article, a novel hot wire oxidation-sublimation deposition (HWOSD)
technique was developed to prepare molybdenum oxide (MoOx) thin films with high
quality. Silicon heterojunction (SHJ) solar cells with the HWOSD MoOx as a hole
selective transport layer (HSL) were fabricated. Thickness of the MoOx layer
and annealing process of the solar cells were studied and optimized. A power
conversion efficiency up to 21.10% was achieved on a SHJ solar cell using a
14nm MoOx layer as the HSL. Dark current density-voltage-temperature (J-V-T)
characteristics of the SHJ solar cell were measured at the temperatures from
200K to 380K. Transport processes including thermionic emission of electrons
over the potential barrier and quantum assisted tunneling of holes through the
gap states in the MoOx layer were proposed for the MoOx/n c-Si heterojunction.
The investigation of the transport mechanisms provides us a better
understanding of the characteristics of the novel SHJ solar cells and it is
helpful for us to fully demonstrate the potential of such kind of solar cells
in the future.

###Antimony Chalcogenide-based Solid State Sensitizers for Solar Cells: A Forgotten Hero or Low Potential Candidate||Sumanshu Agarwal,Harekrishna Yadav,Kundan Kumar###

Antimony Chalcogenide-based Solid State Sensitizers for Solar Cells: A Forgotten Hero or Low Potential Candidate. The use of stibnite (Sb2S3) as sensitizers in the solid-state sensitized
solar cells received considerable research interest during the transition of
the millennium. However, the use of perovskite diminished the research in the
field and the potential of antimony chalcogenide (Sb2(S,Se)3) was not explored
thoroughly. Although these materials also provide bandgap tuning like
perovskite by varying the composition of S and Se, it is not as popular as
perovskite mainly because of the low efficiency of the solar cells based on it.
In this paper, we present a landscape of the functional role of various device
parameters on the performance of Sb2(S,Se)3 based solar cells. For the purpose,
we first calibrate the optoelectronic model used for the simulation with the
experimental results from the literature. The model is then subjected to
parametric variations to explore the performance metrics for this class of
solar cells. Our results show that despite the belief that open circuit voltage
is independent of contact layers doping in proper band aligned sensitized solar
cells, here we observe otherwise and the open circuit voltage is indeed
dependent on the doping density of the contact layers. Using the detailed
numerical simulation and analytical model we further identify the performance
optimization map of Sb2(S,Se)3 based sensitized solar cells.

###III-V-on-silicon triple-junction based on the heterojunction bipolar transistor solar cell concept||E. Antolin,M. H. Zehender,S. A. Svatek,P. G. Linares,A. Marti###

III-V-on-silicon triple-junction based on the heterojunction bipolar transistor solar cell concept. We propose a new triple-junction solar cell structure composed of a III-V
heterojunction bipolar transistor solar cell (HBTSC) stacked on top of, and
series-connected to, a Si solar cell (III-V-HBTSC-on-Si). The HBTSC is a novel
three-terminal device, whose viability has been recently experimentally
demonstrated. It has the theoretical efficiency limit of an
independently-connected double-junction solar cell. Here, we perform detailed
balance efficiency limit calculations under one-sun illumination that show that
the absolute efficiency limit of a III-V-HBTSC-on-Si device is the same as for
the conventional current-matched III-V-on-Si triple-junction (47% assuming
black-body spectrum, 49% with AM1.5G). However, the range of band-gap energies
for which the efficiency limit is above 40% is much wider in the
III-V-HBTSC-on-Si stack case. From a technological point of view, the
lattice-matched GaInP/GaAs combination is particularly interesting, which has
an AM1.5G efficiency limit of 47% with the HBTSC-on-Si structure and 39% if the
current-matched III-V-on-Si triple junction is considered. Moreover, we show
that interconnecting the terminals of the HBTSC to achieve a two-terminal
GaInP/GaAs-HBTSC-on-Si device only reduces the efficiency limit by three
points, to 43%. As a result, the GaInP/GaAs-HBTSC-on-Si solar cell becomes a
promising device for two-terminal, high-efficiency one-sun operation. For it to
also be cost-effective, low-cost technologies must be applied to the III-V
material growth, such as high-throughput epitaxy or sequential growth.

###Towards the efficiency limits of silicon solar cells: how thin is too thin?||Piotr Kowalczewski,Lucio Claudio Andreani###

Towards the efficiency limits of silicon solar cells: how thin is too thin?. It is currently possible to fabricate crystalline silicon solar cells with
the absorber thickness ranging from a few hundreds of micrometers (conventional
wafer-based cells) to devices as thin as $1\,\mu\mathrm{m}$. In this work, we
use a model single-junction solar cell to calculate the limits of energy
conversion efficiency and estimate the optimal absorber thickness. The limiting
efficiency for cells in the thickness range between 40 and $500\,\mu\mathrm{m}$
is very similar and close to 29%. In this regard, we argue that decreasing the
thickness below around $40\,\mu\mathrm{m}$ is counter-productive, as it
significantly reduces the maximum achievable efficiency, even when optimal
light trapping is implemented. We analyse the roles of incomplete light
trapping and extrinsic (bulk and surface) recombination mechanisms. For a
reasonably high material quality, consistent with present-day fabrication
techniques, the optimal thickness is always higher than a few tens of
micrometers. We identify incomplete light trapping and parasitic losses as a
major roadblock in improving the efficiency upon the current record of 25.6%
for silicon solar cells. Finally, considering the main parameters that impact
solar cell performance, we quantify the constraints and requirements for
achieving a specified energy conversion efficiency, which is important for a
proper design strategy of high efficiency silicon solar cells.

###Maximum efficiencies and performance limiting factors of inorganic and hybrid perovskite solar cells||Yoshitsune Kato,Shohei Fujimoto,Masayuki Kozawa,Hiroyuki Fujiwara###

Maximum efficiencies and performance limiting factors of inorganic and hybrid perovskite solar cells. The Shockley and Queisser limit, a well-known efficiency limit for a solar
cell, is based on unrealistic physical assumptions and its maximum limit is
seriously overestimated. To understand the power loss mechanisms of
record-efficiency cells, a more rigorous approach is necessary. Here, we have
established a new formalism that can accurately predict absolute performance
limits of solar cells in conventional thin film form. In particular, we have
estimated the maximum efficiencies of 13 well-studied solar cell materials
[GaAs, InP, CdTe, a-Si:H, CuInSe2, CuGaSe2, CuInGaSe2, Cu2ZnSnSe4, Cu2ZnSnS4,
Cu2ZnSn(S,Se)4, Cu2ZnGeSe4, CH3NH3PbI3, HC(NH2)2PbI3] in a 1-um-thick physical
limit. Our calculation shows that over 30% efficiencies can be achieved for
absorber layers with sharp absorption edges (GaAs, InP, CdTe, CuInGaSe2,
Cu2ZnGeSe4). Nevertheless, many record-efficiency polycrystalline solar cells,
including hybrid perovskites, are limited by open-circuit voltage and
fill-factor losses. We show that the maximum conversion efficiencies described
here present new alternative limits that can predict the power generation of
real-world solar cells.

###Impact of Spatial Separation of Type-II GaSb Quantum Dots from the Depletion Region on the Conversion Efficiency Limit of GaAs Solar Cells||A. Kechiantz,A. Afanasev,J. -L. Lazzari###

Impact of Spatial Separation of Type-II GaSb Quantum Dots from the Depletion Region on the Conversion Efficiency Limit of GaAs Solar Cells. The purpose of this work is to look for a practical structure for application
of quantum dots (QD) in solar cells in order to enhance sub-band gap photon
absorption. We focuse on a stack of strain-compensated GaSb/GaAs type-II QDs.
We propose a novel structure with GaSb/GaAs type-II QD absorber embedded in the
p-doped region of ideal solar cell, but spatially separated from the depletion
region. We developed the model and used the detailed balance principle along
with Poisson and continuity equations for calculating of the energy band
bending along with the photocurrent and the dark current, and the conversion
efficiency of the cell. Our model takes into account both single-photon and
double-photon absorption as well as non-radiative processes in QDs and predicts
that oncentration from 1-sun to 500-sun raises the efficiency from 30% to 50%.
We showed that accumulation of charge in the QD absorber is the clue to
understanding of potentially superior performance of the proposed solar cell.
An attractive feature of the proposed solar cell is that QDs do not reduce the
open circuit voltage but facilitate generation of the additional photocurrent
to the extent that photovoltaic characteristics reduce to that of ideal IB
solar cell while the efficiency meets the Luque-Marti limit. It should be noted
that, although non-radiative processes like relaxation in QDs and recombination
through QDs degrade photovoltaic characteristics of the proposed solar cell,
its conversion efficiency is still predicted to be above the Shockley-Queisser
limit by 5% to 10%. This study is an important step toward producing practical
solar cells that benefit from additional photocurrent generated by sub-band gap
photons.

###Ultra-Efficient Thermophotovoltaics Exploiting Spectral Filtering by the Photovoltaic Band-Edge||Vidya Ganapati,T. Patrick Xiao,Eli Yablonovitch###

Ultra-Efficient Thermophotovoltaics Exploiting Spectral Filtering by the Photovoltaic Band-Edge. Thermophotovotaics convert thermal radiation from local heat sources to
electricity. A new breakthrough in creating highly efficient thin-film solar
cells can potentially enable thermophotovoltaic systems with unprecedented high
efficiency. The current 28.8% single-junction solar efficiency record, by Alta
Devices, was achieved by recognizing that a good solar cell needs to reflect
infrared band-edge radiation at the back surface, to effectively recycle
infrared luminescent photons. The effort to reflect band-edge luminescence in
solar cells has serendipitously created the technology to reflect all infrared
wavelengths, which can revolutionize thermophotovoltaics. We have never before
had such high back reflectivity for sub-bandgap radiation, permitting
step-function spectral control for the first time. Thus, contemporary
efficiency advances in solar photovoltaic cells create the possibility of
realizing a $>50\%$ efficient thermophotovoltaic system.

###Efficient charge separation in organic photovoltaics through incoherent hopping||Stavros Athanasopoulos,Steffen Tscheuschner,Heinz Bässler,Anna Köhler###

Efficient charge separation in organic photovoltaics through incoherent hopping. We demonstrate that efficient and nearly field-independent charge separation
in organic planar heterojunction solar cells can be described by an incoherent
hopping mechanism. We model the separation efficiency of electron-hole pairs
created at donor-acceptor organic interfaces. By using kinetic Monte Carlo
simulations that include the effect of on-chain delocalization we show that
efficient charge extraction to the electrodes requires bipolar transport and
increased dimensionality. This model explains experimental results of almost
field independent charge separation in some molecular systems and provides
important guidelines at the molecular level for maximizing the efficiencies of
organic solar cells.

###Simulation of the Efficiency of CdS/CdTe Tandem Multi-Junction Solar Cells||Ashrafalsadat S. Mirkamali,Khikmat Kh. Muminov###

Simulation of the Efficiency of CdS/CdTe Tandem Multi-Junction Solar Cells. In this paper we study CdS/CdTe solar cells by means of AMPS-1D software.
First we study the effect of thickness of semiconductor layers on the output
parameters of the CdS/CdTe solar cell, such as density of short-circuit
current, open circuit voltage, fill factor and efficiency. Numerical simulation
shows that the highest efficiency of single-junction CdS/CdTe solar cell equal
to 18.3% is achieved when the CdTe layer thickness is 1000 nm and a CdS layer
is 60 nm. Then, in order to obtain the maximal value of the efficiency, new
tandem multi-junction structure consisting of layers of two solar cells
connected with each other back to back are designed and engineered taking into
account the results obtained for the single-junction solar cells. Numerical
simulations show that its highest efficiency in 31.8% can be obtained when the
thickness of CdS p-layer is equal to 50 nm, and the thickness of the CdS
n-layer is equal to 200 nm, while thicknesses of the CdTe n-layer and CdTe
p-layer are kept fixed and equal to 3000 nm and 1000 nm, respectively.

###Relating Band Edge DOS Occupancy Statistics Associated Excited State Electrons Entropy Generation to Free Energy Loss and Intrinsic Voc Deficit of Solar Cells||Like Huang###

Relating Band Edge DOS Occupancy Statistics Associated Excited State Electrons Entropy Generation to Free Energy Loss and Intrinsic Voc Deficit of Solar Cells. Ever science the invention of solar cells, thermodynamics has been used to
assess their performance limits, guiding advances in materials science and
photovoltaic technology to reduce the gap between the practical efficiencies
and the thermodynamic limits to photovoltaic energy conversion. By
systematically addressing the thermodynamic efficiency losses in current
photovoltaic, ultrahigh efficiency photovoltaic can be expected. Currently, the
non-radiative recombination of some ultrahigh efficient solar cells is almost
completely suppressed, and the radiative recombination loss is then the key to
restrict the further improvement of device performance. This work relates the
energy band edge electronic density of states (DOS) of semiconductor absorber
and transport layer, excited/transfer state electronic entropy to
thermodynamically inevitable energy loss during photoelectric conversion in
solar cells. On accounts of the basic limitations of thermodynamic laws on the
energy conversion process, this work reveals a hidden variable that affects the
photovoltaic performance and puts forward the band edge DOS engineering as a
new dimension in performance optimization of solar cell apart from the
traditional material and defect passivation engineering, etc. This work
highlights the great importance of DOS engineering for further improving the
performance of any solar cell devices.

###Semiconducting Monolayer Materials as a Tunable Platform for Excitonic Solar Cells||Marco Bernardi,Maurizia Palummo,Jeffrey C. Grossman###

Semiconducting Monolayer Materials as a Tunable Platform for Excitonic Solar Cells. The recent advent of two-dimensional monolayer materials with tunable
optoelectronic properties and high carrier mobility offers renewed
opportunities for efficient, ultra-thin excitonic solar cells alternative to
those based on conjugated polymer and small molecule donors. Using
first-principles density functional theory and many-body calculations, we
demonstrate that monolayers of hexagonal BN and graphene (CBN) combined with
commonly used acceptors such as PCBM fullerene or semiconducting carbon
nanotubes can provide excitonic solar cells with tunable absorber gap,
donor-acceptor interface band alignment, and power conversion efficiency, as
well as novel device architectures. For the case of CBN-PCBM devices, we
predict the limit of power conversion efficiencies to be in the 10 - 20% range
depending on the CBN monolayer structure. Our results demonstrate the
possibility of using monolayer materials in tunable, efficient, polymer-free
thin-film solar cells in which unexplored exciton and carrier transport regimes
are at play.

###Computer simulation of a-Si/c-Si heterojunction solar cell with high conversion efficiency||Chen AQing,Zhu KaiGui###

Computer simulation of a-Si/c-Si heterojunction solar cell with high conversion efficiency. The p-type amorphous/ n-type crystalline silicon (P+ a-Si/N+ c-Si)
heterojunction was simulated for developing the solar cells with high
conversion efficiency and low cost. The characteristic of such cells with
different work function of transparent conductive oxide (TCO) were calculated.
The energy band structure, quantum efficiency and electric field are analyzed
in detail to understand the mechanism of the heterojunction cell. Our results
show that the a-Si/c-Si heterojunction is hypersensitive to the TCO work
function, and the TCO work function should be large enough in order to achieve
high conversion efficiency for P+ a-Si:H/N+ c-Si solar cell. With the optimized
parameters set, the P+ a-Si:H/N+ c-Si solar cell reaches a high efficiency
({\eta}) up to 21.849% (FF: 0.866, VOC: 0.861 V, JSC: 29.32 mA/cm2).

###The Shockley-Queisser limit for nanostructured solar cells||Yunlu Xu,Tao Gong,Jeremy N. Munday###

The Shockley-Queisser limit for nanostructured solar cells. The Shockley-Queisser limit describes the maximum solar energy conversion
efficiency achievable for a particular material and is the standard by which
new photovoltaic technologies are compared. This limit is based on the
principle of detailed balance, which equates the photon flux into a device to
the particle flux (photons or electrons) out of that device. Nanostructured
solar cells represent a new class of photovoltaic devices, and questions have
been raised about whether or not they can exceed the Shockley-Queisser limit.
Here we show that single-junction nanostructured solar cells have a theoretical
maximum efficiency of 42% under AM 1.5 solar illumination. While this exceeds
the efficiency of a non- concentrating planar device, it does not exceed the
Shockley-Queisser limit for a planar device with optical concentration. We
conclude that nanostructured solar cells offer an important route towards
higher efficiency photovoltaic devices through a built-in optical
concentration.

###The fundamental problem of treating light incoherence in photovoltaics and its practical consequences||Aline Herman,Michael Sarrazin,Olivier Deparis###

The fundamental problem of treating light incoherence in photovoltaics and its practical consequences. The incoherence of sunlight has long been suspected to have an impact on
solar cell energy conversion efficiency, although the extent of this is
unclear. Existing computational methods used to optimize solar cell efficiency
under incoherent light are based on multiple time-consuming runs and
statistical averaging. These indirect methods show limitations related to the
complexity of the solar cell structure. As a consequence, complex corrugated
cells, which exploit light trapping for enhancing the efficiency, have not yet
been accessible for optimization under incoherent light. To overcome this
bottleneck, we developed an original direct method which has the key advantage
that the treatment of incoherence can be totally decoupled from the complexity
of the cell. As an illustration, surface corrugated GaAs and c-Si thin-films
are considered. The spectrally integrated absorption in these devices is found
to depend strongly on the degree of light coherence and, accordingly, the
maximum achievable photocurrent can be higher under incoherent light than under
coherent light. These results show the importance of taking into account
sunlight incoherence in solar cell optimization and point out the ability of
our direct method in dealing with complex solar cells structures.

###Single-nanowire, low-bandgap hot carrier solar cells with tunable open-circuit voltage||Steven Limpert,Adam Burke,I-Ju Chen,Nicklas Anttu,Sebastian Lehmann,Sofia Fahlvik,Stephen Bremner,Gavin Conibeer,Claes Thelander,Mats-Erik Pistol,Heiner Linke###

Single-nanowire, low-bandgap hot carrier solar cells with tunable open-circuit voltage. Compared to traditional pn-junction photovoltaics, hot carrier solar cells
offer potentially higher efficiency by extracting work from the kinetic energy
of photogenerated "hot carriers" before they cool to the lattice temperature.
Hot carrier solar cells have been demonstrated in high-bandgap ferroelectric
insulators and GaAs/AlGaAs heterostructures, but so far not in low-bandgap
materials, where the potential efficiency gain is highest. Recently, a high
open-circuit voltage was demonstrated in an illuminated wurtzite InAs nanowire
with a low bandgap of 0.39 eV, and was interpreted in terms of a
photothermoelectric effect. Here, we point out that this device is a hot
carrier solar cell and discuss its performance in those terms. In the
demonstrated devices, InP heterostructures are used as energy filters in order
to thermoelectrically harvest the energy of hot electrons photogenerated in
InAs absorber segments. The obtained photovoltage depends on the
heterostructure design of the energy filter and is therefore tunable. By using
a high-resistance, thermionic barrier an open-circuit voltage is obtained that
is in excess of the Shockley-Queisser limit. These results provide
generalizable insight into how to realize high voltage hot carrier solar cells
in low-bandgap materials, and therefore are a step towards the demonstration of
higher efficiency hot carrier solar cells.

###Combined DFT, SCAPS-1D, and wxAMPS frameworks for design optimization of efficient Cs2BiAgI6-based perovskite solar cells with different charge transport layers||M. Khalid Hossain,A. A. Arnab,Ranjit C. Das,K. M. Hossain,M. H. K. Rubel,Md. Ferdous Rahman,H. Bencherif,M. E. Emetere,Mustafa K. A. Mohammed,Rahul Pandey###

Combined DFT, SCAPS-1D, and wxAMPS frameworks for design optimization of efficient Cs2BiAgI6-based perovskite solar cells with different charge transport layers. In this study, combined DFT, SCAPS-1D, and wxAMPS frameworks are used to
investigate the optimized designs of Cs2BiAgI6 double perovskite-based solar
cells. The first-principle calculation is employed to investigate the
structural stability, optical responses, and electronic contribution of the
constituent elements in Cs2BiAgI6 absorber material, where SCAPS-1D and wxAMPS
simulators are used to scrutinize different configurations of Cs2BiAgI6 solar
cells. Here, PCBM, ZnO, TiO2, C60, IGZO, SnO2, WS2, and CeO2 are used as ETL,
and Cu2O, CuSCN, CuSbS2, NiO, P3HT, PEDOT: PSS, Spiro-MeOTAD, CuI, CuO, V2O5,
CBTS, CFTS are used as HTL, and Au is used as a back contact. About ninety-six
combinations of Cs2BiAgI6-based solar cell structures are investigated, in
which eight sets of solar cell structures are identified as the most efficient
structures. Besides, holistic investigation on the effect of different factors
such as the thickness of different layers, series and shunt resistances,
temperature, capacitance, Mott-Schottky and generation-recombination rates, and
J-V (current-voltage density) and QE (quantum efficiency) characteristics is
performed. The results show CBTS as the best HTL for Cs2BiAgI6 with all eight
ETLs used in this work, resulting in a power conversion efficiency (PCE) of
19.99%, 21.55%, 21.59%, 17.47%, 20.42%, 21.52%, 14.44%, 21.43% with PCBM, TiO2,
ZnO, C60, IGZO, SnO2, CeO2, WS2, respectively. The proposed strategy may pave
the way for further design optimization of lead-free double perovskite solar
cells.

###Intrinsic femtosecond charge generation dynamics in a single crystal organometal halide perovskite||David A. Valverde-Chávez,Carlito Ponseca Jr.,Constantinos Stoumpos,Arkady Yartsev,Mercouri G. Kanatzidis,Villy Sundström,David G. Cooke###

Intrinsic femtosecond charge generation dynamics in a single crystal organometal halide perovskite. Hybrid metal-organic perovskite solar cells have astounded the solar cell
community with their rapid rise in efficiency over the past three years.
Despite this success, the basic processes governing the photogeneration of free
charges, particularly their dynamics and efficiency, remain unknown. Here we
use ultrabroadband pulses of THz frequency light to see the intrinsic
photophysical properties of single crystal lead halide perovskite just
femtoseconds after a photon is first absorbed. Our spectra reveal the dynamics
and efficiencies of free charge creation, the remarkable ease in which they
move through the lattice and the complicated interplay between free and bound
charges in these materials.

###Spin-enhanced organic bulk heterojunction photovoltaic solar cells||Ye Zhang,Tek P. Basel,Bhoj R. Gautam,Xiaomei Yang,Debra J. Mascaro,Feng Liu,Z. Valy Vardeny###

Spin-enhanced organic bulk heterojunction photovoltaic solar cells. Recently, much effort has been devoted to improve the efficiency of organic
photovoltaic solar cells based on blends of donors and acceptors molecules in
bulk heterojunction architecture. One of the major losses in organic
photovoltaic devices has been recombination of polaron pairs at the
donor/acceptor domain interfaces. Here, we present a novel method to suppress
polaron pair recombination at the donor/acceptor domain interfaces and thus
improve the organic photovoltaic solar cell efficiency, by doping the device
active layer with spin 1/2 radical galvinoxyl. At an optimal doping level of 3
wt%, the efficiency of a standard
poly(3-hexylthiophene)/1-(3-(methoxycarbonyl)propyl)-1-1-phenyl)(6,6)C61 solar
cell improves by 18%. A spin-flip mechanism is proposed and supported by
magneto-photocurrent measurements, as well as by density functional theory
calculations in which polaron pair recombination rate is suppressed by resonant
exchange interaction between the spin 1/2 radicals and charged acceptors, which
convert the polaron pair spin state from singlet to triplet.

###The Efficiency Limit of CH3NH3PbI3 Perovskite Solar Cells||Wei E. I. Sha,Xingang Ren,Luzhou Chen,Wallace C. H. Choy###

The Efficiency Limit of CH3NH3PbI3 Perovskite Solar Cells. With the consideration of photon recycling effect, the efficiency limit of
methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells is predicted by
a detailed balance model. To obtain convincing predictions, both AM 1.5
spectrum of Sun and experimentally measured complex refractive index of
perovskite material are employed in the detailed balance model. The roles of
light trapping and angular restriction in improving the maximal output power of
thin-film perovskite solar cells are also clarified. The efficiency limit of
perovskite cells (without the angular restriction) is about 31%, which
approaches to Shockley-Queisser limit (33%) achievable by gallium arsenide
(GaAs) cells. Moreover, the Shockley-Queisser limit could be reached with a 200
nm-thick perovskite solar cell, through integrating a wavelength-dependent
angular-restriction design with a textured light-trapping structure.
Additionally, the influence of the trap-assisted nonradiative recombination on
the device efficiency is investigated. The work is fundamentally important to
high-performance perovskite photovoltaics.

###Designing Few-layer Graphene Schottky Contact Solar Cell: Theoretical Efficiency Limits and Parametric Optimization||Xin Zhang,Jicheng Wang,Yee Sin Ang,Juncheng Guo###

Designing Few-layer Graphene Schottky Contact Solar Cell: Theoretical Efficiency Limits and Parametric Optimization. We theoretically study the efficiency limits and performance characteristics
of few-layer graphene-semiconductor solar cells (FGSCs) based on a Schottky
contact device structure. We model and compare the energy conversion efficiency
of various configurations by explicitly considering the non-Richardson
thermionic emission across few-layer graphene/semiconductor Schottky
heterostructures. The calculations reveal that ABA-stacked trilayer
graphene-silicon solar cell exhibits a maximal conversion efficiency exceeding
28\% due to a lower reversed saturation current when compared to that of the
ABC-stacking configuration. The thermal coefficients of PCE for ABA and ABC
stacking FGSCs are -0.064\%/K and -0.049\%/K, respectively. Our work offers
insights for optimal designs of graphene-based solar cells, thus paving a route
towards the design of high-performance FGSC for future nanoscale energy
converters.

###Material and Process Tolerant High Efficiency Solar Cells with Dynamic Recovery of Performance||Nithin Chatterji,Swasti Bhatia,Anil Kumar,Aldrin Antony,Pradeep R. Nair###

Material and Process Tolerant High Efficiency Solar Cells with Dynamic Recovery of Performance. Low cost, highly efficient, and stable solar cells demand low temperature
processing, less stringent criteria on materials, and possibility of dynamic
recovery from long term degradation: a combination of features unachievable
from the perspectives of current cSi technology. To this end, here we propose a
novel solar cell architecture with an additional control gate. Our simulation
results indicate that the proposed device can achieve excellent efficiency even
if the back-surface passivation is sub-optimal; thus allowing exploration of a
wide variety of materials and low temperature fabrication processes.
Importantly, such solar cells can dynamically offset efficiency loss due to
elevated temperature and interface degradation associated with long term field
operation and hence could be of broad interest to the PV community.

###A precisely regulating phase evolution strategy for highly efficient kesterite solar cells||Jiazheng Zhou,Xiao Xu,Huijue Wu,Jinlin Wang,Licheng Lou,Kang Yin,Yuancai Gong,Jiangjian Shi,Yanhong Luo,Dongmei Li,Hao Xin,Qingbo Meng###

A precisely regulating phase evolution strategy for highly efficient kesterite solar cells. Phase evolution during the selenization is crucial for high-quality kesterite
Cu2ZnSn(S, Se)4 (CZTSSe) absorbers and efficient solar cells. Herein, we
regulate kinetic process of phase evolution from Cu+-Sn4+-MOE (MOE:
2-methoxyethanol) system by precisely controlling positive chamber pressure. We
found that, at the heating-up stage, Se vapor concentration is intentionally
suppressed in low-temperature region, which effectively reduces collision
probability between the CZTS and Se atoms, thus remarkably inhibiting formation
of secondary phases on the surface and multiple-step phase evolution processes.
This strategy enables the phase evolution to start at relatively higher
temperature and thereby leading to high crystalline quality CZTSSe absorber
with fewer defects, and corresponding CZTSSe solar cell can present 14.1%
efficiency (total area), which is the highest result so far. This work provides
important insights into selenization mechanism of CZTSSe absorbers and explores
a new way of kinetic regulation strategy to simplify the phase evolution path
to efficient CZTSSe solar cells.

###Nonequilibrium theory of the conversion-efficiency limit of solar cells including thermalization and extraction of carriers||Kenji Kamide,Toshimitsu Mochizuki,Hidefumi Akiyama,Hidetaka Takato###

Nonequilibrium theory of the conversion-efficiency limit of solar cells including thermalization and extraction of carriers. The ideal solar cell conversion efficiency limit known as the
Shockley-Queisser (SQ) limit, which is based on a detailed balance between
absorption and radiation, has long been a target for solar cell researchers.
While the theory for this limit uses several assumptions, the requirements in
real devices have not been discussed fully. Given the current situation in
which research-level cell efficiencies are approaching the SQ limit, a
quantitative argument with regard to these requirements is worthwhile in terms
of understanding of the remaining loss mechanisms in current devices and the
device characteristics of solar cells that are operating outside the detailed
balance conditions. Here we examine two basic assumptions: (1) that the
photo-generated carriers lose their kinetic energy via phonon emission in a
moment (fast thermalization), and (2) that the photo-generated carriers are
extracted into carrier reservoirs in a moment (fast extraction). Using a model
that accounts for the carrier relaxation and extraction dynamics, we
reformulate the nonequilibrium theory for solar cells in a manner that covers
both the equilibrium and nonequilibrium regimes. Using a simple planar solar
cell as an example, we address the parameter regime in terms of the carrier
extraction time and then consider where the conventional SQ theory applies and
what could happen outside the applicable range.

###Low Frequency Carrier Kinetics in Perovskite Solar Cells||Vinod K. Sangwan,Menghua Zhu,Sarah Clark,Kyle A. Luck,Tobin J. Marks,Mercouri G. Kanatzidis,Mark C. Hersam###

Low Frequency Carrier Kinetics in Perovskite Solar Cells. Hybrid organic-inorganic halide perovskite solar cells have emerged as
leading candidates for third-generation photovoltaic technology. Despite the
rapid improvement in power conversion efficiency (PCE) for perovskite solar
cells in recent years, the low-frequency carrier kinetics that underlie
practical roadblocks such as hysteresis and degradation remain relatively
poorly understood. In an effort to bridge this knowledge gap, we perform here
correlated low-frequency noise (LFN) and impedance spectroscopy (IS)
characterization that elucidates carrier kinetics in operating perovskite solar
cells. Specifically, we focus on planar cell geometries with a SnO2 electron
transport layer and two different hole transport layers, namely,
poly(triarylamine) (PTAA) and Spiro-OMeTAD. PTAA and Sprio-OMeTAD cells with
moderate PCEs of 5 to 12 percent possess a Lorentzian feature at 200 Hz in LFN
measurements that corresponds to a crossover from electrode to dielectric
polarization. In comparison, Spiro-OMeTAD cells with high PCEs (15 percent)
show four orders of magnitude lower LFN amplitude and are accompanied by a
cyclostationary process. Through a systematic study of more than a dozen solar
cells, we establish a correlation with noise amplitude, power conversion
efficiency, and fill factor. Overall, this work establishes correlated LFN and
IS as an effective methodology for quantifying low frequency carrier kinetics
in perovskite solar cells, thereby providing new physical insights that can
rationally guide ongoing efforts to improve device performance,
reproducibility, and stability.

###New Strategies for Solar Cells Beyond the Visible Spectral Range||Fabio Marangi,Matteo Lombardo,Andrea Villa,Francesco Scotognella###

New Strategies for Solar Cells Beyond the Visible Spectral Range. The endeavor of the scientific community to maximize the possibility to
harvest Sun irradiation for energy production is mainly devoted to the
improvement of the power conversion efficiency of devices and to the extension
of the spectral range in which solar devices operate. Considering that a
significant portion of the Sun irradiation at the ground level is in the
infrared, the research on materials and systems that operate in such region is
gaining increasing attention. In this review, we will report recent
advancements in multijunction solar cells, inorganic-organic perovskite solar
cells, organic solar cells, colloidal quantum dot solar cells focusing on the
absorption of such devices in the infrared. In addition, the use of
upconverting nanostructures will be introduced as a way to indirectly exploit
infrared radiation to increase power conversion efficiency of photovoltaic
devices. Moreover, we will describe plasmon induced hot electron extraction
based solar cells, that are particularly promising in absorbing the infrared
portion of the Sun irradiation when the active materials are doped
semiconductors, which show intense plasmonic resonances in the infrared. The
review includes the optical spectroscopy tools to study the hot electron
extraction from doped semiconductor-based heterojunctions.

###Exploring co-sputtering of ZnO:Al and SiO2 for efficient electron-selective contacts on silicon solar cells||Sihua Zhong,Monica Morales-Masis,Mathias Mews,Lars Korte,Quentin Jeangros,Weiliang Wu,Mathieu Boccard,Christophe Ballif###

Exploring co-sputtering of ZnO:Al and SiO2 for efficient electron-selective contacts on silicon solar cells. In recent years, considerable efforts have been devoted to developing novel
electron-selective materials for crystalline Si (c-Si) solar cells with the
attempts to simplify the fabrication process and improve efficiency. In this
study, ZnO:Al (AZO) is co-sputtered with SiO2 to form AZO:SiO2 films with
different SiO2 content. These nanometer-scale films, deposited on top of thin
intrinsic hydrogenated amorphous silicon films and capped with
low-work-function metal (such as Al and Mg), are demonstrated to function
effectively as electron-selective contacts in c-Si solar cells. On the one
hand, AZO:SiO2 plays an important role in such electron-selective contact and
its thickness is a critical parameter, thickness of 2 nm showing the best. On
the other hand, at the optimal thickness of AZO:SiO2, the open circuit voltage
(VOC) of the solar cells is found to be relatively insensitive to either the
work function or the band gap of AZO:SiO2. Whereas, regarding the fill factor
(FF), AZO without SiO2 content exhibits to be the optimal choice. By using
AZO/Al as electron-selective contact, we successfully realize a 19.5%-efficient
solar cell with VOC over 700 mV and FF around 75%, which is the best result
among c-Si solar cells using ZnO as electron-selective contact. Also, this work
implies that efficient carrier-selective film can be made by magnetron
sputtering method.

###Performance limitation of Si Nanowire solar cells: Effects of nanowire length and surface defects||Deepika Bora,Shrestha Bhattacharya,Nitin Kumar,Aishik Basu Mallick,Avriti Srivastava,Mrinal Dutta,Sanjay K. Srivastava,P. Prathap,C. M. S. Rauthan###

Performance limitation of Si Nanowire solar cells: Effects of nanowire length and surface defects. In Si nanowire (SiNW) solar cells enhanced light confinement property in
addition to decoupling of charge carrier collection and light absorption
directions plays a significant role to resolve the draw backs of bulk Si solar
cells. In this report we have studied the dependence of the phovoltaic
properties of Si NW array solar cells on the SiNW length and enhanced surface
defect states as a result of enhanced surface area of the NWs. The SiNW arrays
have been fabricated using metal catalyzed electroless etching (MCEE)
technique. p-n junction has been produced by spin-on-dopant technique followed
by thermal diffusion process. Front and rear electrodes have been deposited by
e-beam evaporation techniques. SiNW lengths have been controlled from ~ 320 nm
to 6.4 micro meter by controlling the parameters of MCEE technique.
Photovoltaic properties of the solar cells have been characterized by measuring
quantum efficiency and photocurrent density vs. voltage characteristics.
Morphological studies have been carried out by using scanning electron
microscopy. Reduction in light trapping capability comes at the benefit of
reduced surface defects. The reduction of surface defects has been proved to be
more advantageous in comparison to the decrement of light trapping capability.
The major contribution to the changes in cell efficiency comes from the
enhancement of short circuit current density with a very weak dependence on
open circuit voltage. This work is beneficial for the production commercial Si
solar cell where SiNW arrays could be used as a antireflection coating instead
of using separate antireflection layers and thus could reduced the production
cost.

###Photonically-confined solar cells: prospects for exceeding the Shockley-Queisser limit||Qian Zhou,Arfa Karani,Yaxiao Lian,Baodan Zhao,Richard H. Friend,Dawei Di###

Photonically-confined solar cells: prospects for exceeding the Shockley-Queisser limit. The Shockley-Queisser (SQ) limit, introduced by W. Shockley and H. J.
Queisser in 1961, is the most well-established fundamental efficiency limit for
single-junction photovoltaic solar cells. For widely-studied semiconductors
such as Si, GaAs and lead-halide perovskite, the SQ limits under standard solar
illumination (1-sun) are 32.7%, 32.5% and 31% for bandgaps of 1.12 eV, 1.43 eV
and 1.55 eV, respectively. Here, we propose that the fundamental efficiency
limits for single-junction solar cells may be surpassed via photon confinement,
substantially raising the theoretical limits to 49%, 45.2% and 42.1% for Si,
GaAs and methylammonium lead iodide (MAPbI3) perovskite cells under 1-sun. Such
enhancement is possible through the containment of luminescent photons within
the solar cell, allowing the suppression of both non-radiative and radiative
recombination losses, which were considered inevitable in the classical SQ
model. Importantly, restricting photon emission from the solar cells raises the
open-circuit voltage (VOC) to values approaching the semiconductor bandgaps,
surpassing the theoretical VOC values predicted by the SQ model. The fill
factors of the cells are expected to increase substantially, resulting in
current-voltage characteristics with very-high squareness for ideal diode
operation. Our work introduces a new framework for improving solar cell
performance beyond the conventional limits.

###Quantum Dot Solar cells||Husien Salama###

Quantum Dot Solar cells. There remains wide interest in solar cells being made using inexpensive
materials and simple device manufacturing techniques to harvest ever-increasing
amounts of energy. New semiconductor materials and new quantum nanostructures
are exploited to fabricate high-efficiency next-generation solar cells. Quantum
dots have offered an attractive option for photovoltaics. A single photon
absorbed by a quantum dot produces more than one bound electron-hole pair, or
exciton, thereby doubling normal conversion efficiency numbers seen in
single-junction silicon cells. One potential is to use the solar cell
configuration which incorporates. Quantum Dots Super Lattice (QDSL). QDSL
provides a mechanism for the enhancement of solar sales due to their higher
band gap and absorption coefficient when compared to their bulk material
counterpart. The mini bands in the conduction, as well as the valence band of a
QDSL, play an important role in solar cells because the photogenerated carriers
are collected via transport using mini bands. In recent years, a lot of
research has been done on crystal growth, structural, electrical, and optical
properties of thin films and nanostructures as well as fabrication processes
and characterization of photovoltaic devices. In this paper, all the recent
developments in future generation quantum dot solar cells like a tandem,
intermediate band, and solution-processed band alignment engineering from
several research works will be presented.

###Photonic Design: From Fundamental Solar Cell Physics to Computational Inverse Design||Owen D. Miller###

Photonic Design: From Fundamental Solar Cell Physics to Computational Inverse Design. Photonic innovation is becoming ever more important in the modern world.
Optical systems are dominating shorter and shorter communications distances,
LED's are rapidly emerging for a variety of applications, and solar cells show
potential to be a mainstream technology in the energy space. The need for
novel, energy-efficient photonic and optoelectronic devices will only increase.
This work unites fundamental physics and a novel computational inverse design
approach towards such innovation.
  The first half of the dissertation is devoted to the physics of
high-efficiency solar cells. As solar cells approach fundamental efficiency
limits, their internal physics transforms. Photonic considerations, instead of
electronic ones, are the key to reaching the highest voltages and efficiencies.
Proper photon management led to Alta Device's recent dramatic increase of the
solar cell efficiency record to 28.3%. Moreover, approaching the
Shockley-Queisser limit for any solar cell technology will require light
extraction to become a part of all future designs.
  The second half of the dissertation introduces inverse design as a new
computational paradigm in photonics. An assortment of techniques (FDTD, FEM,
etc.) have enabled quick and accurate simulation of the "forward problem" of
finding fields for a given geometry. However, scientists and engineers are
typically more interested in the inverse problem: for a desired functionality,
what geometry is needed? Answering this question breaks from the emphasis on
the forward problem and forges a new path in computational photonics. The
framework of shape calculus enables one to quickly find superior, non-intuitive
designs. Novel designs for optical cloaking and sub-wavelength solar cell
applications are presented.

###Single nanowire solar cells beyond the Shockley-Queisser limit||Peter Krogstrup,Henrik Ingerslev Jørgensen,Martin Heiss,Olivier Demichel,Jeppe V. Holm,Martin Aagesen,Jesper Nygard,Anna Fontcuberta i Morral###

Single nanowire solar cells beyond the Shockley-Queisser limit. Light management is of great importance to photovoltaic cells, as it
determines the fraction of incident light entering the device. An optimal
pn-junction combined with an optimal light absorption can lead to a solar cell
efficiency above the Shockley-Queisser limit. Here, we show how this is
possible by studying photocurrent generation for a single core-shell p-i-n
junction GaAs nanowire solar cell grown on a silicon substrate. At one sun
illumination a short circuit current of 180 mA/cm^2 is obtained, which is more
than one order of magnitude higher than what would be predicted from
Lambert-Beer law. The enhanced light absorption is shown to be due to a light
concentrating property of the standing nanowire as shown by photocurrent maps
of the device. The results imply new limits for the maximum efficiency
obtainable with III-V based nanowire solar cells under one sun illumination.

###Short circuit current enhancement in GaAs/AlGaAs MQW solar cells||James P. Connolly,Keith W. J. Barnham,Jenny Nelson,Christine Roberts,Malcolm Pate,John S. Roberts###

Short circuit current enhancement in GaAs/AlGaAs MQW solar cells. The GaAs/AlGaAs quantum well solar cell (QWSC) shows promise as a novel
approach to higher efficiency solar cells but suffers from a poor short circuit
current Jsc. We report on efforts to reduce this problem with the use of
compositional grading and back surface mirroring. We present experimental
quantum efficiency (QE) data on a range of compositionally graded QWSCs and
devices in which the back surface of the cell is coated with a mirror,
increasing the optical thickness of the quantum well layer in the long
wavelength range. The experimental QE spectra are reproduced by a model which
deals with arbitrary compositional profiles and optical cavities formed in the
mirrored cells. The model is used to design an optimised QWSC, and projected
Jsc values given. Applications including II-VI and tandem solar cells are
considered.

###IBIC analysis of CdTe/CdS solar cells||E. Colombo,A. Bosio,S. Calusi,L. Giuntini,A. Lo Giudice,C. Manfredotti,M. Massi,P. Olivero,A. Romeo,N. Romeo,E. Vittone###

IBIC analysis of CdTe/CdS solar cells. This paper reports on the investigation of the electronic properties of a
thin film CdS/CdTe solar cell with the Ion Beam Induced Charge (IBIC)
technique. The device under test is a thin film (total thickness around 10 um)
multilayer heterojunction solar cell, displaying an efficiency of 14% under
AM1.5 illumination conditions. The IBIC measurements were carried out using
focused 3.150 MeV He ions raster scanned onto the surface of the back
electrode. The charge collection efficiency (CCE) maps show inhomogeneous
response of the cell to be attributed to the polycrystalline nature of the CdTe
bulk material. Finally, the evolution of the IBIC signal vs. the ion fluence
was studied in order to evaluate the radiation hardness of the CdS/CdTe solar
cells in view of their use in solar modules for space applications.

###Application of compact TiO$_2$ layer fabricated by pulsed laser deposition in organometal trihalide perovskite solar cells||Hao Zhang,Hong Wang,Meiyang Ma,Yu Wu,Shuai Dong,Qingyu Xu###

Application of compact TiO$_2$ layer fabricated by pulsed laser deposition in organometal trihalide perovskite solar cells. Organometal trihalide perovskite solar cells have been rapidly developed and
attracted much attention in recent years due to their high photoelectric
conversion efficiency and low cost. Pulsed laser deposition (PLD) is a widely
adopted technology which is used in the preparation of thin films, especially
oxide thin films. With this technology, the thickness and composition of films
can be conveniently and accurately controlled. In the structure of perovskite
solar cells, TiO$_2$ layer working as the n-type semiconductor is used to block
holes and transport electrons into electrode, which is crucial for the
performance of whole devices. We introduced the PLD technique into preparation
of TiO$_2$ layer. In comparison with common spin coating method, TiO$_2$ layer
prepared by this technique is ultrathin and more compact. Compact TiO$_2$
(c-TiO$_2$) layers with optimized thickness of 32 nm have been prepared by the
PLD method and the highest efficiency of 13.95 % for the MAPbI$_3$-based solar
cell devices has been achieved.

###A microstructural analysis of 2D halide perovskites: Stability and functionality||Susmita Bhattacharya,Goutam Kumar Chandra,P. Predeep###

A microstructural analysis of 2D halide perovskites: Stability and functionality. Recent observations indicated that the photoelectric conversion properties of
perovskite materials are intimately related to the presence of superlattice
structures and other unusual nanoscale features in them. The low dimensional or
mixed dimensional halide perovskite family are found to be more efficient
materials for device application compared to 3-dimensional halide perovskites.
The emergence of perovskite solar cell has revolutionized the solar cell
industry because of their flexible architecture and rapidly increased
efficiency. Tuning the dielectric constant, charge separation are the main
objective in designing a photovoltaic device that can be explored using
2-dimensional perovskite family. Thus, revisiting the fundamental properties of
perovskite crystals could reveal further possibilities for recognizing these
improvements towards device functionality. In this context, this review
discusses the material properties of 2-dimensional halide perovskite and
related optoelectronic devices aiming particularly for solar cell application.

###Are Mobilities in Hybrid Organic-Inorganic Halide Perovskites Actually 'High'?||Thomas M. Brenner,David A. Egger,Andrew M. Rappe,Leeor Kronik,Gary Hodes,David Cahen###

Are Mobilities in Hybrid Organic-Inorganic Halide Perovskites Actually 'High'?. We present an experimental and theoretical viewpoint on the electronic
carrier mobilities of typical hybrid organic-inorganic perovskites (HOIPs).
While these mobilities are often quoted as high, a review of them shows that
although otherwise the semiconducting properties of HOIPs are impressively
good, mobilities of HOIPs used in most solar cells are actually not that high.
This is especially apparent if they are compared to those of inorganic
semiconductors used in other high efficiency solar cells. We critically examine
possible causes and focus on electron-lattice coupling mechanisms that are
active at room temperature, and can lead to carrier scattering. From this, we
propose scattering due to acoustic phonons or polarons as possible causes, but
also point out the difficulties with each of these in view of additional
experimental and theoretical findings in the literature. Further research in
this direction will contribute to making HOIP solar cells even more efficient
than they already are.

###Large diameter TiO$_2$ nanotubes enable integration of conformed hierarchical and blocking layers for enhanced dye-sensitized solar cell efficiency||Abdelhamid Elzarka,Ning Liu,Imgon Hwang,Mustafa Kamal,Patrik Schmuki###

Large diameter TiO$_2$ nanotubes enable integration of conformed hierarchical and blocking layers for enhanced dye-sensitized solar cell efficiency. In the present work we grow anodic TiO$_2$ nanotube layer with tube diameter
~ 500 nm and an open tube mouth. We use this morphology in dye-sensitized solar
cells (DSSCs) and show that these tubes allow the construction of hybrid
hierarchical photoanode structures of nanotubes with a defined and
wall-conformance TiO2 nanoparticles decoration. At the same time, the large
diameter allows the successful establishment of an additional (insulating)
blocking layer of SiO$_2$ or Al$_2$O$_3$. We show that this combination of
hierarchical structure and blocking layer significantly enhances the solar cell
efficiency by suppressing recombination reactions. In such a DSSC structure,
the solar cell efficiency under back side illumination with AM1.5 illumination
is enhanced from 5% neat tube to 7 %.

###Impact of the III-V/Ge nucleation routine on the performance of high efficiency multijunction solar cells||Laura Barrutia,Ivan García,Enrique Barrigon,Mario Ochoa,Carlos Algora,Ignacio Rey-Stolle###

Impact of the III-V/Ge nucleation routine on the performance of high efficiency multijunction solar cells. This paper addresses the influence of III-V nucleation routines on Ge
substrates for the growth of high efficiency multijunction solar cells. Three
exemplary nucleation routines with differences in thickness and temperature
were evaluated. The resulting open circuit voltage of triple-junction solar
cells with these designs is significantly affected (up to 50 mV for the best
optimization routine), whereas minimal differences in short circuit current are
observed. Electroluminescence measurements show that both the Ge bottom cell
and the Ga(In)As middle cell present a VOC gain of 25 mV each. This result
indicates that the first stages of the growth not only affect the Ge subcell
itself but also to subsequent subcells. This study highlights the impact of the
nucleation routine design in the performance of high efficiency multijunction
solar cell based on Ge substrates.

###Material selection method for a perovskite solar cell design based on the genetic algorithm||Eungkyun Kim,Indranil Bhattacharya###

Material selection method for a perovskite solar cell design based on the genetic algorithm. In this work, we propose a method of selecting the most desirable
combinations of material for a perovskite solar cell design utilizing the
genetic algorithm. Solar cells based on the methylammonium lead halide,
CH3NH3PbX3, attract researchers due to the benefits of their high absorption
coefficient and sharp Urbach tail, long diffusion length and carrier lifetime,
and high carrier mobility. However, their poor stability under exposure to
moisture still poses a challenge. In our work, we assigned stability index,
power conversion efficiency index, and cost-effectiveness index for each
material based on the available experimental data in the literature, and our
algorithm determined the TiO2/CH3NH3PbI2.1Br0.9/Spiro-OMeTAD as the most well
balanced solution in terms of cost, efficiency, and stability. The proposed
method can be extended further to aid the material selection in all-perovskite
multijunction solar cell design as more data on perovskite materials become
available in the future.

###Lithography free method to synthesize the ultra-low reflection inverted-pyramid arrays for ultra-thin silicon solar cell||Anil Kumar,Divya Rani,Anjali Sain,Neeraj Joshi,Ravi Kumar Varma,Mrinal Dutta,Arup Samanta###

Lithography free method to synthesize the ultra-low reflection inverted-pyramid arrays for ultra-thin silicon solar cell. Silicon inverted pyramids arrays have been suggested as one of the most
promising structure for high-efficient ultrathin solar cells due to their
ability of superior light absorption and low enhancement of surface area.
However, the existing techniques for such fabrication are either expensive or
not able to create appropriate structure. Here, we present a lithography free
method for the fabrication of inverted pyramid arrays by using a modified metal
assisted chemical etching (MACE) method. The size and inter-inverted pyramids
spacing can also be controlled through this method. We used an isotropic
chemical etching technique for this process to control the angle of etching,
which leads to ultra-low reflection, even < 0.5%, of this nanostructure. Using
this specification, we have predicted the expected solar cell parameters, which
exceeds the Lambertian limit. This report provides a new pathway to improve the
efficiency of the ultrathin silicon solar cells at lower cost.

###Solar cell efficiency, diode factor and interface recombination: insights from photoluminescence||T. Wang,F. Ehre,T. P. Weiss,B. Veith-Wolf,V. Titova,N. Valle,M. Melchiorre,J. Schmidt,S. Siebentritt###

Solar cell efficiency, diode factor and interface recombination: insights from photoluminescence. Metastable defects can decisively influence the diode factor and thus the
efficiency of a solar cell. The diode factor is also influenced by the doping
level and the recombination mechanisms in the solar cell. Here we quantify how
the various parameters change the diode factor by photoluminescence
measurements and simulations. In addition, we show that backside recombination
reduces the open circuit voltage in CuInSe2 solar cells by more than 40 mV.
Passivation by a Ga gradient is shown to be as efficient as a passivation by
dielectric layers. Increased backside recombination reduces the diode factor,
not because of less metastable defect transformation but because of a sublinear
increase in photo generated carriers with excitation. This reduction in diode
factor is unwanted, since the increased recombination reduces the voltage. A
higher doping level, on the other hand, reduces the diode factor, thereby
increasing the fill factor, and at the same time increases the voltage.

###A First-principles study on ABBr3 (A = Cs, Rb, K, Na; B = Ge, Sn) halide perovskites for photovoltaic applications||Dibyajyoti Saikia,Mahfooz Alam,Jayanta Bera,Atanu Betal,Appala Naidu Gandhi,Satyajit Sahu###

A First-principles study on ABBr3 (A = Cs, Rb, K, Na; B = Ge, Sn) halide perovskites for photovoltaic applications. In recent years, halide perovskite-based solar cells have received intensive
attention, and demonstrated power conversion efficiency as high as 25.8%. With
regard to the toxicity of Pb and the instability of organic elements, all
inorganic lead-free perovskites (ILPs) have been extensively studied to achieve
comparable or greater photovoltaic performance. In order to develop ILPs as an
alternative for solar cell applications, we performed first-principles
calculations of ABBr3 perovskites (A = Cs, Rb, K, and Na, and B = Sn, and Ge).
Structural, electronic, and optical properties were systematically studied to
probe the potentiality in photovoltaic applications. All these ILPs exhibited a
direct bandgap in the range of 1.10 to 1.97 eV, highly beneficial for absorbing
solar energy. Furthermore, these ILPs demonstrated significant optical
absorption (over 105 cm-1) in the whole UV-Vis spectrum. These results will be
helpful for designing highly efficient lead-free perovskite solar cells.

###Direct generation of charge carriers in c-Si solar cells due to embedded nanoparticles||Martin Kirkengen,Joakim Bergli,Yuri M. Galperin###

Direct generation of charge carriers in c-Si solar cells due to embedded nanoparticles. It is known that silicon is an indirect band gap material, reducing its
efficiency in photovoltaic applications. Using surface plasmons in metallic
nanoparticles embedded in a solar cell has recently been proposed as a way to
increase the efficiency of thin film silicon solar cells. The dipole mode that
dominates the plasmons in small particles produces an electric field having
Fourier components with all wave numbers. In this work, we show that such a
field creates electron-hole-pairs without phonon assistance, and discuss the
importance of this effect compared to radiation from the particle and losses
due to heating.

###Indium-Gallium Segregation in CuIn$_{x}$Ga$_{1-x}$Se$_2$: An ab initio based Monte Carlo Study||Christian D. R. Ludwig,Thomas Gruhn,Claudia Felser,Tanja Schilling,Johannes Windeln,Peter Kratzer###

Indium-Gallium Segregation in CuIn$_{x}$Ga$_{1-x}$Se$_2$: An ab initio based Monte Carlo Study. Thin-film solar cells with CuIn$_x$Ga$_{1-x}$Se$_2$ (CIGS) absorber are still
far below their efficiency limit, although lab cells reach already 19.9%. One
important aspect is the homogeneity of the alloy. Large-scale simulations
combining Monte Carlo and density functional calculations show that two phases
coexist in thermal equilibrium below room temperature. Only at higher
temperatures, CIGS becomes more and more a homogeneous alloy. A larger degree
of inhomogeneity for Ga-rich CIGS persists over a wide temperature range, which
may contribute to the low observed efficiency of Ga-rich CIGS solar cells.

###Mode Splitting for Efficient Plasmoinc Thin-film Solar Cell||Tong Li,Lei Dai,Chun Jiang###

Mode Splitting for Efficient Plasmoinc Thin-film Solar Cell. We propose an efficient plasmonic structure consisting of metal strips and
thin-film silicon for solar energy absorption. We numerically demonstrate the
absorption enhancement in symmetrical structure based on the mode coupling
between the localized plasmonic mode in Ag strip pair and the excited waveguide
mode in silicon slab. Then we explore the method of symmetry-breaking to excite
the dark modes that can further enhance the absorption ability. We compare our
structure with bare thin-film Si solar cell, and results show that the
integrated quantum efficiency is improved by nearly 90% in such thin geometry.
It is a promising way for the solar cell.

###Oxide Heterostructures for Efficient Solar Cells||Elias Assmann,Peter Blaha,Robert Laskowski,Karsten Held,Satoshi Okamoto,Giorgio Sangiovanni###

Oxide Heterostructures for Efficient Solar Cells. We propose an unexplored class of absorbing materials for high-efficiency
solar cells: heterostructures of transition-metal oxides. In particular, LaVO_3
grown on SrTiO_3 has a direct band gap ~1.1 eV in the optimal range as well as
an internal potential gradient, which can greatly help to separate the
photo-generated electron-hole pairs. Furthermore, oxide heterostructures afford
the flexibility to combine LaVO_3 with other materials such as LaFeO_3 in order
to achieve even higher efficiencies with band-gap graded solar cells. We use
density-functional theory to demonstrate these features.

###Realization of Carrier Tunneling from InAlAs Quantum Dots to AlAs||Masataka Koyama,Dai Suzuki,Xiangmeng Lu,Yoshiaki Nakata,Shunichi Muto###

Realization of Carrier Tunneling from InAlAs Quantum Dots to AlAs. With the aim of improving solar cell efficiency, a structure for realizing
electron tunneling from In0.6Al0.4As quantum dots (QDs) through an Al0.4Ga0.6As
barrier to AlAs has been grown using molecular beam epitaxy. The
photoluminescence decay time decreased from 1.1 ns to 390 ps as the barrier
thickness decreased from 4 to 2 nm, which indicates that the photo-excited
carriers tunneled from the QDs to the AlAs X energy level for a barrier
thickness 2 nm in 0.6 ns, which is significantly longer than the tunneling time
of GaAs and InAlAs quantum wells. We expect that this structure will assist in
developing high-efficiency QD sensitized solar cells.

###Semiconductor quantum dots enhanced graphene/CdTe heterostructure solar cells by photo-induced doping||Xiaoqiang Li,Shengjiao Zhang,Peng Wang,Zhijuan Xu,Huikai Zhong,Zhiqian Wu,Shisheng Lin###

Semiconductor quantum dots enhanced graphene/CdTe heterostructure solar cells by photo-induced doping. We report a type of solar cells based on graphene/CdTe Schottky
heterostructure, which can be improved by surface engineering as graphene is
one-atomic thin. By coating a layer of ultrathin CdSe quantum dots onto
graphene/CdTe heterostructure, the power conversion efficiency is increased
from 2.08% to 3.1%. Photo-induced doping is mainly accounted for this
enhancement, as evidenced by transport, photoluminescence and quantum
efficiency measurements. This work demonstrates a feasible way of designing
solar cells with incorporating one dimensional and two dimensional materials.

###Understanding effects of TCO work function on the performance of organic solar cells by numerical simulation||Aqing Chen,Kaigui Zhu,Qingyi Shao,Zhenguo Ji###

Understanding effects of TCO work function on the performance of organic solar cells by numerical simulation. The influences of work function of transparent conducting oxides (TCO) on the
per-formance of organic solar cells, including open circuit voltage, conversion
efficiency and fill factor, has been simulated. It is obtained that for
non-Ohmic contact the open circuit voltage and conversion efficiency increase
monotonically with the TCO work function but keep constant for Ohmic contact.
Fill factor decreases and increases with the electrode work function when the
electrode work function is below and above a critical value (4.2 eV for TCO and
4.5 eV for back-contact), respectively. The results of this simulation are
significant in the choice of TCO contacts to optimize organic planar
heterojunction solar cells.

###Enhanced Performance of Dye-Sensitized Solar Cells based on TiO2 Nanotube Membranes using Optimized Annealing Profile||F. Mohammadpour,M. Moradi,K. Lee,G. Cha,S. So,A. Kahnt,D. M. Guldi,M. Altomare,P. Schmuki###

Enhanced Performance of Dye-Sensitized Solar Cells based on TiO2 Nanotube Membranes using Optimized Annealing Profile. We use free-standing TiO2 nanotube membranes that are transferred onto FTO
slides in front-side illuminated dye-sensitized solar cells (DSSCs). We
investigate the key parameters for solar cell arrangement of self-ordered
anodic TiO2 nanotube layers on the FTO substrate and namely the influence of
the annealing procedure on the DSSC light conversion efficiency. The results
show that using an optimal temperature annealing profile can significantly
enhance the DSSC efficiency (in our case 9.8 %), as it leads to a markedly
lower density of trapping states in the tube oxide, and thus to strongly
improved electron transport properties.

###Efficient long-distance energy transport in molecular systems through adiabatic passage||Arend G. Dijkstra,Almut Beige###

Efficient long-distance energy transport in molecular systems through adiabatic passage. The efficiencies of light-harvesting complexes in biological systems can be
much higher than the current efficiencies of artificial solar cells. In this
paper, we therefore propose and analyse an energy transport mechanism which
employs adiabatic passages between the states of an artificially designed
antenna molecular system to significantly enhance the conversion of incoming
light into internal energy. It is shown that the proposed transport mechanism
is relatively robust against spontaneous emission and de-phasing, while also
being able to take advantage of collective effects. Our aim is to provide new
insight into the energy transport in molecular complexes and to improve the
design of solar cells.

###Towards the maximum efficiency design of a perovskite solar cell by material properties tuning: A multidimensional approach||Manfred Georg Kratzenberg,Ricardo Ruther,Carlos Renato Rambo###

Towards the maximum efficiency design of a perovskite solar cell by material properties tuning: A multidimensional approach. To obtain significant increases in the Power Conversion Efficiency (PCE) of
solar cells, future cell research and development should be based on the
concomitant improvement of multiple material properties, rather than on the
state-of-the-art one or two-dimensional improvements. In this context,
researchers should know, which combined material properties and cell design
parameters lead to the highest efficiency increase. For the same objective, it
should also be known which relationships in-between these variables have to be
adjusted. Such knowledge becomes available by simulation and numerical
optimization, which we present for a Perovskite Solar Cell(PSC)in a hypercube
space of variables.

###Simulation approach to reach the SQ limit in CIGS-based dual-heterojunction solar cell||Shaikh Khaled Mostaque,Bipanko Kumar Mondal,Jaker Hossain###

Simulation approach to reach the SQ limit in CIGS-based dual-heterojunction solar cell. In this article, we demonstrate the design and simulation of a
highly-efficient n-CdS/p-CIGS/p+-CGS dual heterojunction solar cell. The
simulation was performed using SCAPS-1D software with reported experimental
physical parameters. The simulation performance of our proposed design arises
47% with Voc=0.98 V, Jsc=59.94 mA/cm2 and FF=80.07%, respectively. The high
short circuit current and hence the high efficiency is predominantly originated
from the longer wavelength absorption of photon through a tail-states-assisted
two-step upconversion in dual heterojunction (DH) and thus reaches the SQ
detailed balance limit of DH solar cell.

###Whats special about Y6; the working mechanism of neat Y6 organic solar cell||Elifnaz Saglamkaya,Artem Musiienko,Mohammad Saeed Shadabroo,Bowen Sun,Sreelakshmi Chandrabose,Giulia Lo Gerfo M,Niek F van Hulst,Dieter Neher,Safa Shoaee###

Whats special about Y6; the working mechanism of neat Y6 organic solar cell. Non-fullerene acceptors (NFA) have delivered advance in bulk heterojunction
organic solar cell efficiencies, with the significant milestone of 20% now in
sight. However, these materials challenge the accepted wisdom of how organic
solar cells work. In this work we present neat Y6 device with efficiency above
4.5%. We thoroughly investigate mechanisms of charge generation and
recombination as well as transport in order to understand what is special about
Y6. Our data suggest Y6 generates bulk free charges, with ambipolar mobility,
which can be extracted in the presence of transport layers

###Nanophotonic Light Management for Perovskite-Silicon Tandem Solar Cells||D. Chen,P. Manley,P. Tockhorn,D. Eisenhauer,G. Köppel,M. Hammerschmidt,S. Burger,S. Albrecht,C. Becker,K. Jäger###

Nanophotonic Light Management for Perovskite-Silicon Tandem Solar Cells. Perovskite-silicon tandem solar cells are currently one of the most
investigated concepts to overcome the theoretical limit for the power
conversion efficiency of silicon solar cells. For monolithic tandem solar cells
the available light must be distributed equally between the two subcells, which
is known as current matching. For a planar device design, a global optimization
of the layer thicknesses in the perovskite top cell allows current matching to
be reached and reflective losses of the solar cell to be minimized at the same
time. However, even after this optimization reflection and parasitic absorption
losses occur, which add up to 7 mA/cm$^2$.
  In this contribution we use numerical simulations to study, how well
hexagonal sinusoidal nanotextures in the perovskite top-cell can reduce the
reflective losses of the combined tandem device. We investigate three
configurations. The current density utilization can be increased from 91% for
the optimized planar reference to 98% for the best nanotextured device (period
500 nm and peak-to-valley height 500 nm), where 100% refers to the
Tiedje-Yablonovitch limit. In a first attempt to experimentally realize such
nanophotonically structured perovskite solar cells for monolithic tandems, we
investigate the morphology of perovskite layers, which are deposited onto
sinusoidally structured substrates.

###Resonant Silicon Nanoparticles for Enhanced Light Harvesting in Halide Perovskite Solar Cells||A. D. Furasova,E. Calabró,E. Lamanna,E. Y. Tiguntseva,E. Ushakova,E. V. Ubyivovk,V. Y. Mikhailovskii,A. A. Zakhidov,S. V. Makarov,A. Di Carlo###

Resonant Silicon Nanoparticles for Enhanced Light Harvesting in Halide Perovskite Solar Cells. Implementation of resonant colloidal nanoparticles for improving performance
of organometal halide perovskites solar cells is highly prospective approach,
because it is compatible with the solution processing techniques used for any
organic materials. Previously, resonant metallic nanoparticles have been
incorporated into perovskite solar cells for better light absorption and charge
separation. However, high inherent optical losses and high reactivity of noble
metals with halides in perovskites are main limiting factors for this approach.
In turn, low-loss and chemically inert resonant silicon nanoparticles allow for
light trapping and enhancement at nanoscale, being suitable for thin film
photovoltaics. Here photocurrent and fill-factor enhancements in
meso-superstructured organometal halide perovskite solar cells, incorporating
resonant silicon nanoparticles between mesoporous TiO2 transport and active
layers, are demonstrated. This results in a boost of the device efficiency up
to 18.8\% and fill factor up to 79\%, being a record among the previously
reported values on nanoparticles incorporation into CH3NH3PbI3 (MAPbI3)
perovskites based solar cells. Theoretical modeling and optical
characterization reveal the significant role of Si nanoparticles for increased
light absorption in the active layer rather than for better charge separation.
The proposed strategy is universal and can be applied in perovskite solar cells
with various compositions, as well as in other optoelectronic devices.

###Solar Cell Surface Defect Inspection Based on Multispectral Convolutional Neural Network||Haiyong Chen,Yue Pang,Qidi Hu,Kun Liu###

Solar Cell Surface Defect Inspection Based on Multispectral Convolutional Neural Network. Similar and indeterminate defect detection of solar cell surface with
heterogeneous texture and complex background is a challenge of solar cell
manufacturing. The traditional manufacturing process relies on human eye
detection which requires a large number of workers without a stable and good
detection effect. In order to solve the problem, a visual defect detection
method based on multi-spectral deep convolutional neural network (CNN) is
designed in this paper. Firstly, a selected CNN model is established. By
adjusting the depth and width of the model, the influence of model depth and
kernel size on the recognition result is evaluated. The optimal CNN model
structure is selected. Secondly, the light spectrum features of solar cell
color image are analyzed. It is found that a variety of defects exhibited
different distinguishable characteristics in different spectral bands. Thus, a
multi-spectral CNN model is constructed to enhance the discrimination ability
of the model to distinguish between complex texture background features and
defect features. Finally, some experimental results and K-fold cross validation
show that the multi-spectral deep CNN model can effectively detect the solar
cell surface defects with higher accuracy and greater adaptability. The
accuracy of defect recognition reaches 94.30%. Applying such an algorithm can
increase the efficiency of solar cell manufacturing and make the manufacturing
process smarter.

###Theoretical insight into the enhancement of longer-wavelength light absorption in silicon solar cell with multilevel impurities||Shaikh Khaled Mostaque,Bipanko Kumar Mondal,Jaker Hossain###

Theoretical insight into the enhancement of longer-wavelength light absorption in silicon solar cell with multilevel impurities. In this article, we theoretically demonstrate multilevel impurity
photovoltaic effect in an efficient silicon dual-homojunction solar cell that
ensures an extended absorption of longer wavelength light. Along with suitable
contact work functions (Ni and Ta as anode and cathode, respectively), three
impurity energy levels from acceptor type impurities (One from Tl and two from
Zn) have been introduced in the energy gap of the absorber layer in the solar
cell. The pristine Si solar cell shows a PCE of 25.4% with JSC= 37.99 mA/cm2,
VOC=0.780V and FF=85.76%, respectively. The incorporation of Tl impurity level
alone provides a PCE of 33.4%, with JSC= 51.56 mA/cm2, VOC=0.789 V and
FF=82.03%, respectively. The PCE of the solar cell further enhances to 35.4%
with a further enhancement of the short circuit current by 3.76 mA/cm2 due to
the inclusion of Zn impurity into the optimized structure. This enhancement of
the JSC and hence PCE is resulted from the longer wavelength light absorption
due to impurity-assisted two-step photon upconversion in the solar cell.

###Fill Factor Losses and Deviations from the Superposition Principle in Lead-Halide Perovskite Solar Cells||David Grabowski,Zhifa Liu,Gunnar Schöpe,Uwe Rau,Thomas Kirchartz###

Fill Factor Losses and Deviations from the Superposition Principle in Lead-Halide Perovskite Solar Cells. The enhancement of the fill factor in the current generation of perovskite
solar cells is the key for further efficiency improvement. Thus, methods to
quantify the fill factor losses are urgently needed. A classical method to
quantify Ohmic and non-Ohmic resistive losses in solar cells is based on the
comparison between the voltage in the dark and under illumination analysed at
equal recombination current density. Applied to perovskite solar cells, we
observe a combination of an Ohmic series resistance with a voltage-dependent
resistance that is most prominent at short circuit and low forward bias. The
latter is most likely caused by the poor transport properties of the electron
and/or hole transport layers. By measuring the photoluminescence of perovskite
solar cells as a function of applied voltage, we provide direct evidence for a
high quasi-Fermi level splitting at low and moderate forward bias that
substantially exceeds the externally applied voltage. This quasi-Fermi level
splitting causes recombination losses and, thus, reduces both the short-circuit
current and the fill factor of the solar cell.

###InGaN Metal-IN Solar Cell: optimized efficiency and fabrication tolerance||Abdoulwahab Adaine,Sidi Ould Saad Hamady,Nicolas Fressengeas###

InGaN Metal-IN Solar Cell: optimized efficiency and fabrication tolerance. Choosing the Indium Gallium Nitride (InGaN) ternary alloy for thin films
solar cells might yield high benefits concerning efficiency and reliability,
because its bandgap can be tuned through the Indium composition and radiations
have little destructive effect on it. It may also reveal challenges because
good quality p-doped InGaN layers are difficult to elaborate. In this letter, a
new design for an InGaN thin film solar cell is optimized, where the player of
a PIN structure is replaced by a Schottky contact, leading to a Metal-IN (MIN)
structure. With a simulated efficiency of 19.8%, the MIN structure performs
better than the previously studied Schottky structure, while increasing its
fabrication tolerance and thus functional reliability a. Owing to its good
tolerance to radiations [1], its high light absorption [2, 3] and its
Indium-composition-tuned bandgap [4, 5], the Indium Gallium Nitride (InGaN)
ternary alloy is a good candidate for high-efficiency-high-reliability solar
cells able to operate in harsh environments. Unfortunately, InGaN p-doping is
still a challenge, owing to InGaN residual n-doping [6], the lack of dedicated
ac-ceptors [7] and the complex fabrication process itself [8, 9]. To these
drawbacks can be added the uneasy fabrication of ohmic contacts [4] and the
difficulty to grow the high-quality-high-Indium-content thin films [10] which
would be needed to cover the whole solar spectrum. These drawbacks still
prevent InGaN solar cells to be competitive with other well established III-V
and silicon technologies [11]. In this letter, is proposed a new Metal-IN (MIN)
InGaN solar cell structure where the InGaN p-doped layer is removed and
replaced by a Schottky contact, lifting one of the above mentioned drawbacks. A
set of realistic physical models based on actual measurements is used to
simulate and optimize its behavior and performance using mathematically
rigorous multi-criteria optimization methods, aiming to show that both
efficiency and fabrication tolerances are better than the previously described
simple InGaN Schottky solar cell [12].

###Very high efficiency of low cost graphite-based solar cell by improving the fill factor using optimal ion concentration in polymer electrolyte||Dui Yanto Rahman,Fisca Dian Utami,Asep Ridwan Setiawan,Euis Sustini,Mikrajuddin Abdullah###

Very high efficiency of low cost graphite-based solar cell by improving the fill factor using optimal ion concentration in polymer electrolyte. We report the development of graphite-based solar cells using a simple method
and low cost materials. Suspension of graphite powder in mineral water was
simply dropped onto the surface of fluorine-doped tin oxide glass (FTO) to form
a thick film. Surprisingly, using mineral waters greatly improved the
efficiency of the solar cell to reach the highest efficiency of 6.97%. Due to
some minerals contained, the mineral water induced the development of fibrous
structure between the graphite particles which is assumed to play a role as a
bridge for the photoexcited electrons to quickly move to the electrode and
suppress recombination with holes. This efficiency is very attractive when
considering the materials used to develop the solar cell are all low cost.
Economically this may challenge the present high efficiency semiconductor-based
solar cells. We achieved the high efficiency by manipulating the cell fill
factor through optimizing the ion concentration in PVA.LiOH polymer
electrolyte. We also propose an equation to describe the effect of LiOH
concentration and efficiency and we also provide strong correlation between the
cell efficiency and the polymer conductivity

###Multiple State Representation Scheme for Organic Bulk Heterojunction Solar Cells: A Novel Analysis Perspective||Mario Einax,Marcel Dierl,Philip R. Schiff,Abraham Nitzan###

Multiple State Representation Scheme for Organic Bulk Heterojunction Solar Cells: A Novel Analysis Perspective. The physics of organic bulk heterojunction solar cells is studied within a
six state model, which is used to analyze the factors that affect
current-voltage characteristics, power-voltage properties and efficiency, and
their dependence on nonradiative losses, reorganization of the nuclear
environment, and environmental polarization. Both environmental reorganization
and polarity is explicitly taken into account by incorporating Marcus
heterogeneous and homogeneous electron transfer rates. The environmental
polarity is found to have a nonnegligible influence both on the stationary
current and on the overall solar cell performance. For our organic bulk
heterojunction solar cell operating under steady-state open circuit condition,
we also find that the open circuit voltage logarithmically decreases with
increasing nonradiative electron-hole recombination processes.

###Optimisation of High Efficiency AlGaAs MQW Solar Cells||J. P. Connolly,K. W. J. Barnham,J. Nelson,P. Griffin,G. Haarpaintner,C. Roberts,M. Pate,J. S. Roberts###

Optimisation of High Efficiency AlGaAs MQW Solar Cells. The GaAs/AlGaAs materials system is well suited to multi-bandgap applications
such as the multiple quantum well solar cell. GaAs quantum wells are inserted
in the undoped AlGaAs active region of a pin structure to extend the absorption
range while retaining a higher open circuit voltage than would be provided by a
cell made of the well material alone. Unfortunately aluminium gallium arsenide
(AlGaAs) suffers from poor transport characteristics due to DX centres and
oxygen contamination during growth, which degrade the spectral response. We
investigate three mechanisms for improving the spectral response of the MQW
solar cell while an experimental study of the open circuit voltage examines the
voltage enhancement. An optimised structure for a high efficiency GaAs/AlGaAs
solar cell is proposed.

###Degradation Kinetics of Inverted Perovskite Solar Cells||Mejd Alsari,Andrew J. Pearson,Jacob Tse-Wei Wang,Zhiping Wang,Augusto Montisci,Neil C. Greenham,Henry J. Snaith,Samuele Lilliu,Richard H. Friend###

Degradation Kinetics of Inverted Perovskite Solar Cells. We explore the degradation behaviour under continuous illumination and direct
oxygen exposure of inverted unencapsulated
formamidinium(FA)0.83Cs0.17Pb(I0.8Br0.2)3, CH3NH3PbI3, and CH3NH3PbI3-xClx
perovskite solar cells. We continuously test the devices in-situ and
in-operando with current-voltage sweeps, transient photocurrent, and transient
photovoltage measurements, and find that degradation in the CH3NH3PbI3-xClx
solar cells due to oxygen exposure occurs over shorter timescales than
FA0.83Cs0.17Pb(I0.8Br0.2)3 mixed-cation devices. We attribute these
oxygen-induced losses in the power conversion efficiencies to the formation of
electron traps within the perovskite photoactive layer. Our results highlight
that the formamidinium-caesium mixed-cation perovskites are much less sensitive
to oxygen-induced degradation than the methylammonium-based perovskite cells,
and that further improvements in perovskite solar cell stability should focus
on the mitigation of trap generation during ageing.

###Homogenization of Halide Distribution and Carrier Dynamics in Alloyed Organic-Inorganic Perovskites||Juan-Pablo Correa-Baena,Yanqi Luo,Thomas M. Brenner,Jordan Snaider,Shijing Sun,Xueying Li,Mallory A. Jensen,Lea Nienhaus,Sarah Wieghold,Jeremy R. Poindexter,Shen Wang,Ying Shirley Meng,Ti Wang,Barry Lai,Moungi G. Bawendi,Libai Huang,David P. Fenning,Tonio Buonassisi###

Homogenization of Halide Distribution and Carrier Dynamics in Alloyed Organic-Inorganic Perovskites. Perovskite solar cells have shown remarkable efficiencies beyond 22%, through
organic and inorganic cation alloying. However, the role of alkali-metal
cations is not well-understood. By using synchrotron-based nano-X-ray
fluorescence and complementary measurements, we show that when adding RbI
and/or CsI the halide distribution becomes homogenous. This homogenization
translates into long-lived charge carrier decays, spatially homogenous carrier
dynamics visualized by ultrafast microscopy, as well as improved photovoltaic
device performance. We find that Rb and K phase-segregate in highly
concentrated aggregates. Synchrotron-based X-ray-beam-induced current and
electron-beam-induced current of solar cells show that Rb clusters do not
contribute to the current and are recombination active. Our findings bring
light to the beneficial effects of alkali metal halides in perovskites, and
point at areas of weakness in the elemental composition of these complex
perovskites, paving the way to improved performance in this rapidly growing
family of materials for solar cell applications.

###First-principles prediction into robust high-performance photovoltaic double perovskites A$_{2}$SiI$_{6}$ (A = K, Rb, Cs)||Qiaoqiao Li,Liujiang Zhou,Yanfeng Ge,Yulu Ren,Jiangshan Zhao,Wenhui Wan,Kaicheng Zhang,Yong Liu###

First-principles prediction into robust high-performance photovoltaic double perovskites A$_{2}$SiI$_{6}$ (A = K, Rb, Cs). Despite the exceeding 23\% photovoltaic efficiency achieved in
organic-inorganic hybrid perovskite solar cells obtaining, the stable materials
with desirable band gap are rare and are highly desired. With the aid of
first-principles calculations, we predict a new promising family of nontoxic
inorganic double perovskites (DPs), namely, silicon (Si)-based halides
A$_{2}$SiI$_{6}$ (A = K, Rb, Cs; X = Cl, Br, I). This family containing the
earth-abundant Si could be applied for perovskite solar cells (PSCs).
Particularly A$_{2}$SiI$_{6}$ exhibits superb physical traits, including
suitable band gaps of 0.84-1.15 eV, dispersive lower conduction bands, small
carrier effective masses, wide photon absorption in the visible range.
Importantly, the good stability at high temperature renders them as promising
optical absorbers for solar cells.

###Thinned GaInP/GaInAs/Ge solar cells grown with reduced cracking on Ge|Si virtual substrates||Ivan García,Laura Barrutia,Shabnam Dadgostar,Manuel Hinojosa,Andrew Johnson,Ignacio Rey-Stolle###

Thinned GaInP/GaInAs/Ge solar cells grown with reduced cracking on Ge|Si virtual substrates. Reducing the formation of cracks during growth of GaInP/GaInAs/Ge 3-junction
solar cells on Ge|Si virtual substrates has been attempted by thinning the
structure, namely the Ge bottom cell and the GaInAs middle cell. The
theoretical analysis performed using realistic device parameters indicates that
the GaInAs middle cell can be drastically thinned to 1000 nm while increasing
its In content to 8% with an efficiency loss in the 3-junction cell below 3%.
The experimental results show that the formation of macroscopic cracks is
prevented in thinned GaInAs/Ge 2-junction and GaInP/GaInAs/Ge 3-junction cells.
These prototype crack-free multijunction cells demonstrate the concept and were
used to rule out any possible component integration issue. The performance
metrics are limited by the high threading dislocation density over 2e7cm-2 in
the virtual substrates used, but an almost current matched, crack-free, thinned
3-junction solar cell is demonstrated, and the pathway towards solar cells with
higher voltages identified.

###Characterization and Quantum Efficiency Determination of Monocrystalline Silicon Solar Cells as Sensors for Precise Flux Calibration||Sasha Brownsberger,Lige Zhang,David Andrade,Christopher Stubbs###

Characterization and Quantum Efficiency Determination of Monocrystalline Silicon Solar Cells as Sensors for Precise Flux Calibration. As the precision frontier of astrophysics advances towards the one
millimagnitude level, flux calibration of photometric instrumentation remains
an ongoing challenge. We present the results of a lab-bench assessment of the
viability of monocrystalline silicon solar cells to serve as large-aperture (up
to 125mm diameter), high-precision photodetectors. We measure the electrical
properties, spatial response uniformity, quantum efficiency (QE), and frequency
response of 3$^{rd}$ generation C60 solar cells, manufactured by Sunpower. Our
new results, combined with our previous study of these cells' linearity, dark
current, and noise characteristics, suggest that these devices hold
considerable promise, with QE and linearity that rival those of traditional,
small-aperture photodiodes. We argue that any photocalibration project that
relies on precise knowledge of the intensity of a large-diameter optical beam
should consider using solar cells as calibrating photodetectors.

###Ideal Solar Cell Efficiencies||Tom Markvart###

Ideal Solar Cell Efficiencies. In a recent paper, Guillemoles et al [J.F. Guillemoles, T. Kirchartz, D.
Cahen and U Rau, Guide for the perplexed to the Shockley-Queisser model for
solar cells, Nat. Photonics, 13, 501 (2019)] attempt to clarify and explain the
often cited paper by Shockley and Queisser (SQ) which defines the limits to
photovoltaic conversion by a single-junction solar cell. The SQ paper is not
easy to read and is therefore easily misunderstood. As modern solar cells
approach theoretical efficiency limits, the fundamentals become particularly
important and the effort by Guillemoles et al is therefore to be welcome.
However, in doing so, the authors have fallen into several pitfalls and the aim
of the present note is to clarify a number of misconceptions and correct some
errors in that paper.

###Triple-junction solar cells with 39.5% terrestrial and 34.2% space efficiency enabled by thick quantum well superlattices||Ryan M. France,John F. Geisz,Tao Song,Waldo Olavarria,Michelle Young,Alan Kibbler,Myles A. Steiner###

Triple-junction solar cells with 39.5% terrestrial and 34.2% space efficiency enabled by thick quantum well superlattices. Multijunction solar cell design is guided by both the theoretical optimal
bandgap combination as well as the realistic limitations to materials with
these bandgaps. For instance, triple-junction III-V multijunction solar cells
commonly use GaAs as a middle cell because of its near-perfect material
quality, despite its bandgap being higher than optimal for the global spectrum.
Here, we modify the GaAs bandgap using thick GaInAs/GaAsP strain-balanced
quantum well (QW) solar cells with excellent voltage and absorption. These
high-performance QWs are incorporated into a triple-junction inverted
metamorphic multijunction device consisting of a GaInP top cell, GaInAs/GaAsP
QW middle cell, and lattice-mismatched GaInAs bottom cell, each of which has
been highly optimized. We demonstrate triple-junction efficiencies of 39.5% and
34.2% under the global and space spectra, respectively, which are higher than
previous record six-junction devices.

###On the potential of Hg-Photo-CVD process for the low temperature growth of nano-crystalline silicon (Topical Review)||A. Barhdadi###

On the potential of Hg-Photo-CVD process for the low temperature growth of nano-crystalline silicon (Topical Review). Mercury-Sensitized Photo-Assisted Chemical Vapor Deposition (Hg-Photo-CVD)
technique opens new possibilities for reducing thin film growth temperature and
producing novel semiconductor materials suitable for the future generation of
high efficiency thin film solar cells onto low cost flexible plastic
substrates. This paper provides an overview of this technique, with the
emphasis on its potential in low temperature elaboration of nano-crystalline
silicon for the development of thin films photovoltaic technology.
  Keywords: Photovoltaic, Solar Cell, Thin films, Nano-Crystalline Silicon,
Hydrogenated Amorphous Silicon, Hydrogen Dilution, Low Temperature Growth,
Photo-Chemical Vapor Deposition.

###Optical characterization of a-Si:H thin films grown by Hg-Photo-CVD||A. Barhdadi,S. Karbal,N. Mgafad,A. Benmakhlouf,M. Chafik El Idrissi,B. M. Aka###

Optical characterization of a-Si:H thin films grown by Hg-Photo-CVD. Mercury-Sensitized Photo-Assisted Chemical Vapor Deposition (Hg-Photo-CVD)
technique opens new possibilities for reducing thin film growth temperature and
producing novel semiconductor materials suitable for the future generation of
high efficiency thin film solar cells onto low cost flexible plastic
substrates. This paper provides some experimental data resulting from the
optical characterization of hydrogenated amorphous silicon thin films grown by
this deposition technique. Experiments have been performed on both as-deposited
layers and thermal annealed ones.
  Keywords: Photovoltaic, Solar Cell, Thin films, Nano-Crystalline Silicon,
Hydrogenated Amorphous Silicon, Optical Properties, Thermal Annealing PACS
Numbers: 68.60.Dv, 78.66.Jg, 78.66.-w, 73.50.Pz, 81.15.-z, 84.60.Jt

###Hydrogen-bonded supramolecular assembly of dyes at nanostructured solar cell interfaces||Christopher E. Patrick,Feliciano Giustino###

Hydrogen-bonded supramolecular assembly of dyes at nanostructured solar cell interfaces. We calculate from first principles the O1s core-level shifts for a variety of
atomistic models of the interface between TiO2 and the dye N3 found in
dye-sensitized solar cells. A systematic comparison between our calculations
and published photoemission data shows that only interface models incorporating
hydrogen bonding between the dyes are compatible with experiment. Based on our
analysis we propose that at the TiO2/N3 interface the dyes are arranged in
supramolecular assemblies. Our work opens a new direction in the modeling of
semiconductor/dye interfaces and bears on the design of more efficient
nanostructured solar cells.

###Effects of n-type doping in InAs/GaAs quantum dot layer on current-voltage characteristic of intermediate band solar cells||Yong-Xian Gu,Xiao-Guang Yang,Hai-Ming Ji,Peng-Fei Xu,Tao Yang###

Effects of n-type doping in InAs/GaAs quantum dot layer on current-voltage characteristic of intermediate band solar cells. We investigated the current-voltage characteristic of InAs/GaAs quantum dot
intermediate band solar cells (QD IBSCs) with different n-type doping density
in the QD layer. The n-type doping evidently increases the open circuit
voltage, meanwhile decreases the short circuit current density, and leads to
the conversion efficiency approaching that of the control solar cell, that is
the major role of n-type doping is to suppress the effects of QDs on the
current-voltage characteristic. Our model adopts practical parameters for
simulation rather than those from detailed balanced method, so that the results
in our simulation are not overestimated.

###Limit of light coupling strength in solar cells||A. Naqavi,F. -J. Haug,C. Ballif,T. Scharf,H. P. Herzig###

Limit of light coupling strength in solar cells. We introduce a limit for the strength of coupling light into the modes of
solar cells. This limit depends on both a cell's thickness and its modal
properties. For a cell with refractive index n and thickness d, we obtain a
maximal coupling rate of 2c*sqrt(n^2-1)/d where c is speed of light. Our method
can be used in the design of solar cells and in calculating their efficiency
limits; besides, it can be applied to a broad variety of resonant phenomena and
devices.

###Enhanced electron photoemission by collective lattice resonances in plasmonic nanoparticle-array photodetectors and solar cells||Sergei V. Zhukovsky,Viktoriia E. Babicheva,Alexander V. Uskov,Igor E. Protsenko,Andrei V. Lavrinenko###

Enhanced electron photoemission by collective lattice resonances in plasmonic nanoparticle-array photodetectors and solar cells. We propose to use collective lattice resonances in plasmonic nanoparticle
arrays to enhance photoelectron emission in Schottky-barrier photodetectors and
solar cells. We show that the interaction of lattice resonances (the Rayleigh
anomaly) and individual particle excitations (localized surface plasmon
resonances) leads to stronger local field enhancement and significant increase
of the photocurrent compared to the case when only individual particle
excitations are present. The results can be used to design new photodetectors
with highly selective, tunable spectral response, able to detect photons with
the energy below the semiconductor bandgap, and to develop solar cells with
increased efficiency.

###Fabrication of single-walled carbon nanotube/Si heterojunction solar cell with high photovoltaic conversion efficiency and stability||Feijiu Wang,Daichi Kozawa,Yuhei Miyauchi,Kazushi Hiraoka,Shinichiro Mouri,Yutaka Ohno,Kazunari Matsuda###

Fabrication of single-walled carbon nanotube/Si heterojunction solar cell with high photovoltaic conversion efficiency and stability. The photovoltaic properties of carbon nanotube/Si heterojunction solar cells
were investigated using network films of high quality single-walled carbon
nanotubes (SWNTs) grown by atmospheric-pressure floating-catalyst chemical
vapor deposition. Because of the optimization of the device window size and the
utilization of SWNT thin films with both low resistivity and high transparency,
a high photovoltaic conversion efficiency of greater than 12% was achieved for
SWNTs/Si heterojunction solar cells without any post processing, such as
carrier doping treatment. In addition, the high stability and reproducibility
of the photovoltaic performance of these devices in air was demonstrated.

###Thermodynamic Limits of Solar Cells with Non-ideal Optical Response||M. Ryyan Khan,Peter Bermel,Muhammad A. Alam###

Thermodynamic Limits of Solar Cells with Non-ideal Optical Response. The Shockley-Queisser (S-Q) theory defines the thermodynamic upper limits for
Jsc, Voc, FF, and efficiency of a solar cell. The classical calculation assumes
an abrupt onset of absorption at the band-edge, perfect absorption for all
energies above the bandgap, and absence of non-radiative recombination. These
assumptions are never satisfied for any practical solar cell. In this paper, we
explain how the S-Q limits are redefined in the presence of the non-ideal
optical effects, and we provide closed-form analytical expressions for the new
limits for Jsc, Voc, and FF. Remarkably, these new limits can be achieved to a
very high degree, even with significantly imperfect materials.

###Lithography-free sub-100nm nanocone array antireflection layer for low-cost silicon solar cell||Zhida Xu,Logan Liu###

Lithography-free sub-100nm nanocone array antireflection layer for low-cost silicon solar cell. High density and uniformity sub-100nm surface oxidized silicon nanocone
forest structure is created and integrated onto the existing texturization
microstructures on photovoltaic device surface by a one-step high throughput
plasma enhanced texturization method. We suppressed the broadband optical
reflection on chemically textured grade-B silicon solar cells for up to 70.25%
through this nanomanufacturing method. The performance of the solar cell is
improved with the short circuit current increased by 7.1%, fill factor
increased by 7.0%, conversion efficiency increased by 14.66%. Our method
demonstrates the potential to improve the photovoltaic device performance with
low cost high and throughput nanomanufacturing technology.

###Enhanced Photovoltaic Performances of Graphene/Si Solar Cells by Insertion of an MoS2 Thin Film||Yuka Tsuboi,Feijiu Wang,Daichi Kozawa,Kazuma Funahashi,Shinichiro Mouri,Yuhei Miyauchi,Taishi Takenobu,Kazunari Matsuda###

Enhanced Photovoltaic Performances of Graphene/Si Solar Cells by Insertion of an MoS2 Thin Film. Atomically thin layered materials such as graphene and transition-metal
dichalcogenides exhibit great potential as active materials in optoelectronic
devices because of their high carrier-transporting properties and strong
light-matter interactions. Here, we demonstrated that the photovoltaic
performances of graphene/Si Schottky junction solar cells were significantly
improved by inserting a chemical vapor deposition (CVD)-grown, large MoS2
thin-film layer. This layer functions as an effective passivation and
electron-blocking/hole-transporting layer. We also demonstrated that the
photovoltaic properties are enhanced with increasing number of graphene layers
and decreasing thickness of the MoS2 layer. A high photovoltaic conversion
efficiency of 11.1% was achieved with the optimized trilayer-graphene/MoS2/n-Si
solar cell.

###Utilization of Naturally Occurring Dyes as Sensitizers in Dye Sensitized Solar Cells||Nipun Sawhney,Soumitra Satapathi###

Utilization of Naturally Occurring Dyes as Sensitizers in Dye Sensitized Solar Cells. Dye sensitized Solar cells (DSSCs) were fabricated with four naturally
occurring anthocyanin dyes extracted from naturally found fruits/ juices (viz.
Indian Jamun, Plum, Black Currant and Berries) as sensitizers. Extraction of
anthocyanin was done using acidified ethanol. The highest power conversion
efficiencies ({\eta}) of 0.55% and 0.53% were achieved for the DSSCs fabricated
using anthocyanin extracts of blackcurrant and mixed berry juice. Widespread
availability of these fruits/juices, high concentration of anthocyanins in them
and ease of extraction of anthocyanin dyes from these commonly available fruits
render them novel and inexpensive candidates for solar cell fabrication.

###Effect of the quantum well thickness on the performance of InGaN photovoltaic cells||L. Redaelli,A. Mukhtarova,S. Valdueza-Felip,A. Ajay,C. Bougerol,C. Himwas,J. Faure-Vincent,C. Durand,J. Eymery,E. Monroy###

Effect of the quantum well thickness on the performance of InGaN photovoltaic cells. We report on the influence of the quantum well thickness on the effective
band gap and conversion efficiency of In0.12Ga0.88N/GaN multiple quantum well
solar cells. The band-to-band transition can be redshifted from 395 to 474 nm
by increasing the well thickness from 1.3 to 5.4 nm, as demonstrated by
cathodoluminescence measurements. However, the redshift of the absorption edge
is much less pronounced in absorption: in thicker wells, transitions to higher
energy levels dominate. Besides, partial strain relaxation in thicker wells
leads to the formation of defects, hence degrading the overall solar cell
performance.

###Use of Anodic TiO2 Nanotube Layers as Mesoporous Scaffolds for Fabricating CH3NH3PbI3 Perovskite-based Solid State Solar Cells||Raul Salazar,Marco Altomare,Kiyoung Lee,Jyotsna Tripathy,Robin Kirchgeorg,Nhat Truong Nguyen,Mohamed Mokhtar,Abdelmohsen Alshehri,Shaeel A. Al-Thabaiti,Patrik Schmuki###

Use of Anodic TiO2 Nanotube Layers as Mesoporous Scaffolds for Fabricating CH3NH3PbI3 Perovskite-based Solid State Solar Cells. We optimize the deposition of CH3NH3PbI3 perovskite into mesoporous
electrodes consisting of anodic TiO2 nanotube layers. By a simple spin coating
approach, complete filling of the tube scaffolds is obtained, that leads to
interdigitated perovskite structures in conformal contact with the TiO2 tube
counterparts. Such assemblies can be used as solid state solar cells in
hole-transporting material-free configuration, i.e., the tube scaffold serves
as electron collector and blocking layer, while the perovskite acts as visible
light absorber and hole transporting material. We show that the complete
filling of the tube scaffold with the perovskite is essential to improve the
solar cell efficiency.

###Conical-Shaped Titania Nanotubes for Optimized Light Management in DSSCs Reach Back-side Illumination Efficiencies > 8%||Seulgi So,Arian Kriesch,Ulf Peschel,Patrik Schmuki###

Conical-Shaped Titania Nanotubes for Optimized Light Management in DSSCs Reach Back-side Illumination Efficiencies > 8%. In the present work, we introduce the anodic growth of conical shaped TiO2
nanotube arrays. These titania nanocones provide a scaffold for dye-sensitized
solar cell (DSSC) structures with significantly improved photon management,
providing an optimized absorption profile compared with conventional
cylindrical nanotube arrays. Finite difference time domain (FDTD) modelling
demonstrates a drastically changed power-absorption characteristic over the
tube length. When used in a back-side illumination DSSC configuration, nanocone
structures can reach over 60 % higher solar cell conversion efficiency than
conventional tubes. The resulting {\eta} of ca. 8 % represents one of the
highest reported values for Graetzel type DSSCs used under back-side
illumination.

###Effective parameters of porous layer in silicon cells||Zeinulla Zhanabaev,Kadyrzhan Dikhanbayev###

Effective parameters of porous layer in silicon cells. Theoretically and experimentally we have proved the existence of optimal
thickness of the porous layer used as an anti-reflection coating in solar
cells. We have taken into account the joint mechanisms of generation and
recombination of electrons and holes stimulated by photons. The porous silicon
surface has been obtained by electrochemical etching. Results of scanning
electron microscopy, measuring short-circuit current, open circuit voltage, and
etching time have shown that in case thickness of the porous layer is
approximately 2/3 of electron diffusion length the solar cell efficiency
increases by 30% in comparison with the corresponding value in the absence of
texturing.

###Inverted GaInP/GaAs Three-Terminal Heterojunction Bipolar Transistor Solar Cell||Marius H. Zehender,Simon. A. Svatek,Myles A. Steiner,Iván García,Pablo García Linares,Emily L. Warren,Antonio Martí,Adele. C. Tamboli,Elisa Antolín###

Inverted GaInP/GaAs Three-Terminal Heterojunction Bipolar Transistor Solar Cell. Here we present the experimental results of an inverted three-terminal
heterojunction bipolar transistor solar cell (HBTSC) made of GaInP/GaAs. The
inverted growth and processing enable contacting the intermediate layer (base)
from the bottom, which improves the cell performance by reducing shadow factor
and series resistance at the same time. With this prototype we show that an
inverted processing of a three-terminal solar cell is feasible and pave the way
for the application of epitaxial lift-off, substrate reuse and mechanical
stacking to the HBTSC which can eventually lead to a low-cost high-efficiency
III-V-on-Si HBTSC technology.

###Optimization of nonhomogeneous indium-gallium-nitride Schottky-barrier thin-film solar cells||Tom H. Anderson,Akhlesh Lakhtakia,Peter B. Monk###

Optimization of nonhomogeneous indium-gallium-nitride Schottky-barrier thin-film solar cells. A two-dimensional model was developed to simulate the optoelectronic
characteristics of indium-gallium-nitride (InGaN), thin-film, Schottky-barrier
solar cells. The solar cells comprise a window, designed to reduce the
reflection of incident light, Schottky-barrier and ohmic front electrodes, an
n-doped InGaN wafer, and a metallic periodically corrugated back-reflector
(PCBR). The ratio of indium to gallium in the wafer varies periodically
throughout the thickness of the absorbing layer of the solar cell. Thus, the
resulting InGaN wafer's optical and electrical properties are made to vary
periodically. This material nonhomogeneity could be physically achieved by
varying the fractional composition of indium and gallium during deposition.
Empirical models for indium nitride and gallium nitride were combined using
Vegard's law to determine the optical and electrical constitutive properties of
the alloy. The nonhomogeneity of the electrical properties of the InGaN aids in
the separation of the excited electron-hole pairs, while the periodicities of
optical properties and the back-reflector enable the incident light to couple
to multiple guided wave modes. The profile of the resulting
charge-carrier-generation rate when the solar cell is illuminated by the AM1.5G
spectrum was calculated using the rigorous coupled-wave approach. The
steady-state drift-diffusion equations were solved using COMSOL, which employs
finite-volume methods, to calculate the current density as a function of the
voltage. Mid-band Shockley-Read-Hall, Auger, and radiative recombination rates
were taken to be the dominant methods of recombination. The model was used to
study the effects of the solar-cell geometry and the shape of the periodic
material nonhomogeneity on efficiency. The solar-cell efficiency was optimized
using the differential evolution algorithm.

###First-principles studies of oxygen interstitial dopants in RbPbI$_3$ halide for perovskite solar cells||Chongyao Yang,Wei Wu,Kwang-Leong Choy###

First-principles studies of oxygen interstitial dopants in RbPbI$_3$ halide for perovskite solar cells. Recent research on perovskite solar cells has caught much attention for the
application in renewable energy materials. However, the effect of external
dopants on the performance of perovskite solar cell is yet to be understood
properly. Oxygen atom or molecule is important dopant to influence the
stability of structural, electronic and optical properties as well as the
performance of perovskite solar cells. RbPbX3-type perovskites have fantastic
chemical stability and good power conversion efficiency. Here for the first
time, we have studied the effect of interstitial oxygen atom (O1) and molecule
(O2) on the structural properties, and hence the electronic structure of RbPbI3
from first principles. A significant reduction of the band gap from ~2.6 eV to
~ 1.0 eV, which is close to the optimal band gap, has been predicted when
incorporating oxygen. This could in turn be applied to improve the optical
properties for harvesting light if we can control the oxygen level
appropriately. In addition, an exotic metallic state has been found in our
calculations for interstitial oxygen molecule when there are strong O-O, O-Pb,
and O-I bonds, indicating the complex nature of oxygen-doped perovskite solar
cells. The comparison between oxygen atom and molecules is consistent with the
previous report about oxygen-molecule passivation of perovskite solar cells.
This indicated oxygen incorporation can not only improve efficiency and
stability but also facilitate the optimal band-gap engineering. Our work has
therefore provided an important and timely theoretical insight to the effect of
oxygen dopants in perovskite solar cells. Moreover, these results also provide
theoretical foundation for further simulations such as molecular dynamics.

###New formalism for selfconsistent parameters optimization of highly efficient solar cells||A. V. Sachenko,V. P. Kostylyov,M. R. Kulish,I. O. Sokolovskyi,A. Chkrebtii###

New formalism for selfconsistent parameters optimization of highly efficient solar cells. We analysed self-consistently photoconversion efficiency of direct-gap A3B5
semicon-ductors based solar cells and optimised their main physical
characteristics. Using gallium ar-senide (GaAs) as the example and new
efficient optimization formalism, we demonstrated that commonly accepted light
re-emission and re-absorption in solar cells (SC) in technologically produced
GaAs (in particular, with solid- or liquid-phase epitaxy) are not the main
factors re-sponsible for high photoconversion efficiency. As we proved instead,
the doping level of the base material and its doping type as well as
Shockley-Read-Hall (SRH) and surface recombination velocities are much more
important factors responsible for the photoconversion. We found that the
maximum photoconversion efficiency (about 27% for AM1.5 conditions) in GaAs
with typical parameters of recombination centers can be reached for p-type base
doped at $2 \cdot 10^{17}$ cm$^{-3}$. The open circuit voltage $V_{OC}$
formation features are analyzed. The optimization provides a significant
increase in $V_{OC}$ and the limiting photoconversion efficiency close to 30%.
The approach of this research allows to predict the expected solar cells (for
both direct-gap and indirect-band semiconductor) characteristics if material
parameters are known. Obtained formalism allows to analyze and to optimize mass
production both tandem solar cell (TSC) and one-junction SC parameters.

###Optimization and effect of UV-ozone exposure of electron transport layer on the efficiency of the dye-sensitized solar cells||Chandan Dawo,Mohammad Adil Afroz,Parameswar Krishnan Iyer,Harsh Chaturvedi###

Optimization and effect of UV-ozone exposure of electron transport layer on the efficiency of the dye-sensitized solar cells. The surface states of the active TiO2 layer is crucial while fabricating an
efficient solar cell. This work experimentally analyses the effect of exposing
TiO2 based electron transport layer (ETL) to the ultraviolet-ozone (UV-O3) and
optimizes the exposure time for improving power conversion efficiency (PCE) of
fabricated dye-sensitized solar cells (DSSCs). These results demonstrate that
the performance of DSSCs can be improved significantly by UV-O3 exposure of
sintered TiO2 photoanode surface, with the duration of exposure being a
critical parameter. Fabricated devices show 33.01 % increase in PCE for the
optimum exposure. Nevertheless, overexposure of the sample beyond the optimum
time decreases the efficiency of the fabricated solar cells. The device with
optimum exposure exhibits the highest PCE of 8.34% with short circuit current
density (Jsc) of 15.15 mA/cm2, open circuit voltage (Voc) of 756 mV and Fill
factor (FF) of 71.10%. This increase in efficiency is attributed to the
enhanced crystallization and reduction in the organic contaminants C-C/C-H from
57.90 to 52.40% as shown by the X-ray diffraction (XRD) and X-ray photoelectron
spectroscopy (XPS), respectively. The XPS result further indicates an increase
in oxygen vacancy from 12.40 to 23.40% for O 1s state and from 9.30 to 14.30%
for Ti 2p state of Ti3+. Results from the Atomic Force Microscope (AFM) also
confirms the minimized surface roughness of 16.36 nm for the optimally exposed
TiO2 film, and increase in hydrophilicity leading to improved efficiency of the
solar cells which were optimally exposed to UV-O3.

###All-Inorganic Spin-Cast Nanoparticle Solar Cells with Non-Selective Electrodes||I. E. Anderson,J. D. Olson,L. Yang,S. A. Carter###

All-Inorganic Spin-Cast Nanoparticle Solar Cells with Non-Selective Electrodes. Spin-cast all-inorganic nanoparticle solutions have been used to make a
CdTe/CdSe solar cell with an efficiency of up to 2.8% without alumina or
calcium buffer layers. The type of junction and non-selective nature of the
contacts made to these devices is explored.

###Chemical Doping and Enhanced Solar Energy Conversion of Graphene/Silicon Junctions||Xinming Li,Hongwei Zhu,Kunlin Wang,Jinquan Wei,Guifeng Fan,Xiao Li,Dehai Wu###

Chemical Doping and Enhanced Solar Energy Conversion of Graphene/Silicon Junctions. The effect of chemical doping of graphene films on the photovoltaic
properties of the graphene/silicon Schottky junction solar cells was
investigated. Thionyl chloride modification greatly enhanced the conductivity
of graphene film, resulting in a significant improvement in cell performance
with a 3-fold increase in conversion efficiency (up to 3.9%) and good
short-term stability.

###Quantifying Efficiency Loss of Perovskite Solar Cells by a Modified Detailed Balance Model||Wei E. I. Sha,Hong Zhang,Zi Shuai Wang,Hugh L. Zhu,Xingang Ren,Francis Lin,Alex K. -Y. Jen,Wallace C. H. Choy###

Quantifying Efficiency Loss of Perovskite Solar Cells by a Modified Detailed Balance Model. A modified detailed balance model is built to understand and quantify
efficiency loss of perovskite solar cells. The modified model captures the
light-absorption dependent short-circuit current, contact and transport-layer
modified carrier transport, as well as recombination and photon-recycling
influenced open-circuit voltage. Our theoretical and experimental results show
that for experimentally optimized perovskite solar cells with the power
conversion efficiency of 19%, optical loss of 25%, non-radiative recombination
loss of 35%, and ohmic loss of 35% are the three dominant loss factors for
approaching the 31% efficiency limit of perovskite solar cells. We also find
that the optical loss will climb up to 40% for a thin-active-layer design.
Moreover, a misconfigured transport layer will introduce above 15% of energy
loss. Finally, the perovskite-interface induced surface recombination, ohmic
loss, and current leakage should be further reduced to upgrade device
efficiency and eliminate hysteresis effect. The work contributes to fundamental
understanding of device physics of perovskite solar cells. The developed model
offers a systematic design and analysis tool to photovoltaic science and
technology.

###From planar junction to local junction: A new structure design of amorphous/crystalline silicon heterojunction solar cells for high efficiency and low cost||Haibin Huang,Lang Zhou,Jiren Yuan,Zhijue Quan###

From planar junction to local junction: A new structure design of amorphous/crystalline silicon heterojunction solar cells for high efficiency and low cost. In order to obtain higher conversion efficiency and to reduce production cost
for amorphous silicon/crystalline silicon (a-Si/c-Si) based heterojunction
solar cells, a Heterojunction of Amorphous silicon and Crystalline silicon with
Localized pn structure (HACL) has been designed. The potential performance of
the HACL solar cell has been assessed by ATLAS simulation program. Its
potential performance is compared with that of the Heterojunction with
Intrinsic Thin film (HIT) and Heterojunction of Amorphous silicon and
Crystalline silicon with Diffused junction (HACD) solar cells. The simulated
results indicated that the conversion efficiency and the short-circuit current
density of the HACL cell can reach to 28.18% and 43.06 mA/cm2, respectively,
and are higher than that of the HIT and HACD cells. The main reasons for the
great improvement are (1) to increase the light utilization rate on both sides
of the cell and (2) to enhance the collection efficiency of the photocarriers.
Moreover, the HACL structure can decrease the consumption of rare materials,
such as indium, since the transparent conductive oxide (TCO) can be free in
this structure. It is concluded that the HACL solar cell is a promising
structure for high efficiency and low cost.

###Flexible perovskite/Cu(In,Ga)Se2 monolithic tandem solar cells||Fan Fu,Shiro Nishiwaki,Jeremie Werner,Thomas Feurer,Stefano Pisoni,Quentin Jeangros,Stephan Buecheler,Christophe Ballif,Ayodhya N. Tiwari###

Flexible perovskite/Cu(In,Ga)Se2 monolithic tandem solar cells. We report a proof-of-concept two-terminal perovskite/Cu(In, Ga)Se2 (CIGS)
monolithic thin-film tandem solar cell grown on ultra-thin (30-microns thick),
light-weight, and flexible polyimide foil with a steady-state power conversion
efficiency of 13.2% and a high open-circuit voltage over 1.75 V under standard
test condition.

###Engineering the reciprocal space for ultrathin GaAs solar cells||Jeronimo Buencuerpo,Jose M. Llorens,Jose M. Ripalda,Myles A. Steiner,Adele C. Tamboli###

Engineering the reciprocal space for ultrathin GaAs solar cells. III-V solar cells dominate the high efficiency charts, but with significantly
higher cost than other solar cells. Ultrathin III-V solar cells can exhibit
lower production costs and immunity to short carrier diffusion lengths caused
by radiation damage, dislocations, or native defects. Nevertheless, solving the
incomplete optical absorption of sub-micron layers presents a challenge for
light-trapping structures. Simple photonic crystals have high diffractive
efficiencies, which are excellent for narrow-band applications. Random
structures a broadband response instead but suffer from low diffraction
efficiencies. Quasirandom (hyperuniform) structures lie in between providing
high diffractive efficiency over a target wavelength range, broader than simple
photonic crystals, but narrower than a random structure. In this work, we
present a design method to evolve a simple photonic crystal into a quasirandom
structure by modifying the spatial-Fourier space in a controlled manner. We
apply these structures to an ultrathin GaAs solar cell of only 100 nm. We
predict a photocurrent for the tested quasirandom structure of 25.3 mA/cm$^2$,
while a planar structure would be limited to 16.1 mA/cm$^2$. The modified
spatial-Fourier space in the quasirandom structure increases the amount of
resonances, with a progression from discrete number of peaks to a continuum in
the absorption. The enhancement in photocurrent is stable under angle
variations because of this continuum. We also explore the robustness against
changes in the real-space distribution of the quasirandom structures using
different numerical seeds, simulating variations in a self-assembly method.

###Abnormal Staebler-Wronski effect of amorphous silicon||Wenzhu Liu,Jianhua Shi,Liping Zhang,Anjun Han,Shenglei Huang,Xiaodong Li,Jun Peng,Yuhao Yang,Yajun Gao,Jian Yu,Kai Jiang,Xinbo Yang,Zhenfei Li,Junlin Du,Xin Song,Youlin Yu,Zhixin Ma,Yubo Yao,Haichuan Zhang,Lujia Xu,Jingxuan Kang,Yi Xie,Hanyuan Liu,Fanying Meng,Frédéric Laquai,Zengfeng Di,Zhengxin Liu###

Abnormal Staebler-Wronski effect of amorphous silicon. Great achievements in last five years, such as record-efficient
amorphous/crystalline silicon heterojunction (SHJ) solar cells and cutting-edge
perovskite/SHJ tandem solar cells, place hydrogenated amorphous silicon
(a-Si:H) at the forefront of emerging photovoltaics. Due to the extremely low
doping efficiency of trivalent boron (B) in amorphous tetravalent silicon,
light harvesting of aforementioned devices are limited by their fill factors
(FF), which is a direct metric of the charge carrier transport. It is
challenging but crucial to develop highly conductive doped a-Si:H for
minimizing the FF losses. Here we report intensive light soaking can
efficiently boost the dark conductance of B-doped a-Si:H "thin" films, which is
an abnormal Staebler-Wronski effect. By implementing this abnormal effect to
SHJ solar cells, we achieve a certificated power conversion efficiency (PCE) of
25.18% (26.05% on designated area) with FF of 85.42% on a 244.63-cm2 wafer.
This PCE is one of the highest reported values for total-area "top/rear"
contact silicon solar cells. The FF reaches 98.30 per cent of its
Shockley-Queisser limit.

###Efficient all-perovskite tandem solar cells by dual-interface optimisation of vacuum-deposited wide-bandgap perovskite||Yu-Hsien Chiang,Kyle Frohna,Hayden Salway,Anna Abfalterer,Bart Roose,Miguel Anaya,Samuel D. Stranks###

Efficient all-perovskite tandem solar cells by dual-interface optimisation of vacuum-deposited wide-bandgap perovskite. Tandem perovskite solar cells beckon as lower cost alternatives to
conventional single junction solar cells, with all-perovskite tandem
photovoltaic architectures showing power conversion efficiencies up to 26.4%.
Solution-processing approaches for the perovskite layers have enabled rapid
2optimization of perovskite solar technologies, but new deposition routes are
necessary to enable modularity and scalability, facilitating further efficiency
improvements and technology adoption. Here, we utilise a 4-source vacuum
deposition method to deposit FA$_{0.7}$ Cs$_{0.3}$Pb(I$_x$Br$_{1-x}$)$_3$
perovskite, where the bandgap is widened through fine control over the halide
content. We show how the combined use of a MeO-2PACz self-assembled monolayer
as hole transporting material and passivation of the perovskite absorber with
ethylenediammonium diiodide reduces non-radiative losses, with this
dual-interface treatment resulting in efficiencies of 17.8% in solar cells
based on vacuum deposited perovskites with bandgap of 1.76 eV. By similarly
passivating a narrow bandgap FA$_{0.75}$Cs$_{0.25}$Pb$_{0.5}$Sn$_{0.5}$I$_3$
perovskite and combining it with sub-cells of evaporated
FA$_{0.7}$Cs$_{0.3}$Pb(I$_{0.64}$Br$_{0.36}$)$_3$, we report a 2-terminal
all-perovskite tandem solar cell with champion open circuit voltage and power
conversion efficiency of 2.06 V and 24.1%, respectively. The implementation of
our dry deposition method enables high reproducibility in complex device
architectures, opening avenues for modular, scalable multi-junction devices
where the substrate choice is unrestricted.

###Thermodynamic performance of hot-carrier solar cells: A quantum transport model||Ludovico Tesser,Robert S. Whitney,Janine Splettstoesser###

Thermodynamic performance of hot-carrier solar cells: A quantum transport model. In conventional solar cells, photogenerated carriers lose part of their
energy before they can be extracted to make electricity. The aim of hot-carrier
solar cells is to extract the carriers before this energy loss, thereby turning
more energy into electrical power. This requires extracting the carriers in a
nonequilibrium (nonthermal) energy distribution. Here, we investigate the
performance of hot-carrier solar cells for such nonequilibrium distributions.
We propose a quantum transport model in which each energy-loss process (carrier
thermalization, relaxation, and recombination) is simulated by a B\"uttiker
probe. We study charge and heat transport to analyze the hot-carrier solar
cell's power output and efficiency, introducing partial efficiencies for
different loss processes and the carrier extraction. We show that producing
electrical power from a nonequilibrium distribution has the potential to
improve the output power and efficiency. Furthermore, in the limit where the
distribution is thermal, we prove that a boxcar-shaped transmission for the
carrier extraction maximizes the efficiency at any given output power.

###Improved GaInP/GaAs/GaInAs inverted metamorphic triple-junction solar cells by reduction of Zn diffusion in the top subcell||Manuel Hinojosa,Ivan Lombardero,Carlos Algora,Ivan Garcia###

Improved GaInP/GaAs/GaInAs inverted metamorphic triple-junction solar cells by reduction of Zn diffusion in the top subcell. The growth of heavily doped tunnel junctions in inverted metamorphic
multijunction solar cells induces a strong diffusion of Zn via a
point-defects-assisted mechanism. The redistribution of Zn can compensate the
n-type doping in the emitter of the GaInP top junction, degrading severely the
conductivity of the whole solar cell and its conversion efficiency. This work
evaluates different epitaxial growth strategies to achieve control on the Zn
profile of an inverted metamorphic triple-junction structure, including: the
reduction of the doping concentration in the tunnel junction to minimize the
injection of point defects that trigger the diffusion mechanism; the use of
different barrier layers to keep the injected point defects away from active
layers and, finally, the minimization of Zn concentration in the AlGaInP
back-surface-field layer of the GaInP subcell. This last approach enables a
high-conductivity multijunction solar cell device without redesigning the
tunnel junction as well as a high electronic quality in the GaInP subcell,
which shows a collection efficiency higher than 93% and an open-circuit-voltage
offset of 410 mV at 1 sun irradiance. The characterization of final
triple-junction devices, including quantum efficiency, electroluminescence, and
light current-density-voltage curves at different irradiances, demonstrates a
successful integration of all the subcell and tunnel junction components. This
way, final solar cells with peak efficiencies exceeding 40% at 500 suns are
demonstrated, despite using doping levels in the AlGaInP:Zn back-surface-field
of the GaInP subcell and using non-optimized antireflective coatings.

###Opto-electronic properties and solar cell efficiency modelling of Cu$_2$ZnXS$_4$ (X=Sn,Ge,Si) kesterites||Thomas Ratz,Jean-Yves Raty,Guy Brammertz,Bart Vermang,Ngoc Duy Nguyen###

Opto-electronic properties and solar cell efficiency modelling of Cu$_2$ZnXS$_4$ (X=Sn,Ge,Si) kesterites. In this work, first principle calculations of Cu$_2$ZnSnS$_4$ (CZTS),
Cu$_2$ZnGeS$_4$ (CZGS) and Cu$_2$ZnSiS$_4$ (CZSS) are performed to highlight
the impact of the cationic substitution on the structural, electronic and
optical properties of kesterite compounds. Direct bandgaps are reported with
values of 1.32, 1.89 and 3.06 eV respectively for CZTS, CZGS and CZSS. In
addition, absorption coefficient values of the order of $10^4$ cm$^{-1}$ are
obtained, indicating the applicability of these materials as absorber layer for
solar cell applications. In the second part of this study, ab initio results
are used as input data to model the electrical power conversion efficiency of
kesterite-based solar cell. In that perspective, we used an improved version of
the Shockley-Queisser theoretical model including non-radiative recombination
via an external parameter defined as the internal quantum efficiency. Based on
predicted optimal absorber layer thicknesses, the variation of the solar cell
maximal efficiency is studied as a function of the non-radiative recombination
rate. Maximal efficiencies of 25.88, 19.94 and 3.11% are reported respectively
for CZTS, CZGS and CZSS for vanishing non-radiative recombination rate. Using
an internal quantum efficiency providing $V_{OC}$ values comparable to
experimental measurements, solar cell efficiencies of 15.88, 14.98 and 2.66%
are reported respectively for CZTS, CZGS and CZSS (for an optimal thickness of
1.15 $\mu$m). With this methodology, we confirm the suitability of CZTS in
single junction solar cells, with a possible efficiency improvement of 10%
enabled through the reduction of the non-radiative recombination rate. In
addition, CZGS appears to be an interesting candidate as top cell absorber
layer for tandem approaches whereas CZSS might be interesting for transparent
PV windows.

###Simulation of High Conversion Efficiency and Open-circuit Voltages Of α-si/poly-silicon Solar Cell||AQing Chen,QingYi Shao###

Simulation of High Conversion Efficiency and Open-circuit Voltages Of α-si/poly-silicon Solar Cell. The P+ {\alpha}-Si /N+ polycrystalline solar cell is molded using the AMPS-1D
device simulator to explore the new high efficiency thin film poly-silicon
solar cell. In order to analyze the characteristics of this device and the
thickness of N+ poly-silicon, we consider the impurity concentration in the N+
poly-silicon layer and the work function of transparent conductive oxide (TCO)
in front contact in the calculation. The thickness of N+ poly-silicon has
little impact on the device when the thickness varies from 20 {\mu}m to 300
{\mu}m. The effects of impurity concentration in polycrystalline are analyzed.
The conclusion is drawn that the open-circuit voltage (Voc) of P+ {\alpha}-Si
/N+ polycrystalline solar cell is very high, reaching 752 mV, and the
conversion efficiency reaches 9.44%. Therefore, based on the above optimum
parameters the study on the device formed by P+ {\alpha}-Si/N+ poly-silicon is
significant in exploring the high efficiency poly-silicon solar cell.

###Titanium dioxide hole-blocking layer in ultra-thin-film crystalline silicon solar cells||Yangsen Kang,Huiyang Deng,Yusi Chen,Yijie Huo,Jieyang Jia,Li Zhao,Zain Zaidi,Kai Zang,James S. Harris###

Titanium dioxide hole-blocking layer in ultra-thin-film crystalline silicon solar cells. One of the remaining obstacles to approaching the theoretical efficiency
limit of crystalline silicon (c-Si) solar cells is the exceedingly high
interface recombination loss for minority carriers at the Ohmic contacts. In
ultra-thin-film c-Si solar cells, this contact recombination loss is far more
severe than for traditional thick cells due to the smaller volume and higher
minority carrier concentration of the former. This paper presents a novel
design of an electron passing (Ohmic) contact to n-type Si that is
hole-blocking with significantly reduced hole recombination. This contact is
formed by depositing a thin titanium dioxide (TiO2) layer to form a silicon
metal-insulator-semiconductor (MIS) contact. A 2 {\mu}m thick Si cell with this
TiO2 MIS contact achieved an open circuit voltage (Voc) of 645 mV, which is 10
mV higher than that of an ultra-thin cell with a metal contact. This MIS
contact demonstrates a new path for ultra-thin-film c-Si solar cells to achieve
high efficiencies as high as traditional thick cells, and enables the
fabrication of high-efficiency c-Si solar cells at a lower cost.

###Materials Discovery of Stable and Nontoxic Halide Perovskite Materials for High-Efficiency Solar Cells||Ryan Jacobs,Guangfu Luo,Dane Morgan###

Materials Discovery of Stable and Nontoxic Halide Perovskite Materials for High-Efficiency Solar Cells. Two critical limitations of organic-inorganic lead halide perovskite
materials for solar cells are their poor stability in humid environments and
inclusion of toxic lead. In this study, high-throughput density functional
theory (DFT) methods are used to computationally model and screen 1845 halide
perovskites in search of new materials without these limitations that are
promising for solar cell applications. This study focuses on finding materials
that are comprised of nontoxic elements, stable in a humid operating
environment, and have an optimal bandgap for one of single junction, tandem
Si-perovskite, or quantum dot-based solar cells. Single junction materials are
also screened on predicted single junction photovoltaic (PV) efficiencies
exceeding 22.7%, which is the current highest reported PV efficiency for halide
perovskites. Generally, these methods qualitatively reproduce the properties of
known promising nontoxic halide perovskites that have either been
experimentally evaluated or predicted from theory. From a set of 1845
materials, 15 materials pass all screening criteria for single junction cell
applications, 13 of which have not been previously investigated, such as
(CH3NH3)0.75Cs0.25SnI3, ((NH2)2CH)Ag0.5Sb0.5Br3, CsMn0.875Fe0.125I3,
((CH3)2NH2)Ag0.5Bi0.5I3, and ((NH2)2CH)0.5Rb0.5SnI3. These materials, together
with others predicted in this study, may be promising candidate materials for
stable, highly efficient, and non-toxic perovskite-based solar cells.

###Theoretical Efficiency Comparison between Carrier Multiplication and Down-Conversion 3rd Generation Solar Cell Designs||Z. R. Abrams,A. Niv,C. Gladden,M. Gharghi,X. Zhang###

Theoretical Efficiency Comparison between Carrier Multiplication and Down-Conversion 3rd Generation Solar Cell Designs. Methods of exceeding the detailed balance limit for a single junction solar
cell have included down-converting high energy photons to produce two photons;
and carrier multiplication, whereby high energy photons produce more than one
electron-hole pair. Both of the methods obey the conservation of energy in
similar ways, and effectively produce a higher current in the solar cell. Due
to this similarity, it has been assumed that there is no thermodynamic
difference between the two methods. Here, we compare the two methods using a
generalized approach based on Kirchhoff's law of radiation and develop a new
model for carrier multiplication. We demonstrate that there is an entropic
penalty to be paid for attempting to accomplish all-in-one splitting in carrier
multiplication systems, giving a small thermodynamic - and therefore efficiency
- advantage to spectral splitting prior to reaching the solar cell. We show
this analytically using a derivation of basic thermodynamic identities;
numerically by solving for the maximal efficiency; and generally using
heat-generation arguments. Our result modifies the existing literature on
entropy generation limits in solar cells, and creates a new distinction among
3rd generation photovoltaic technologies.

###Graphene-Enhanced Thermal Interface Materials for Thermal Management of Photovoltaic Solar Cells||M. Saadah,D. Gamalath,E. Hernandez,A. A. Balandin###

Graphene-Enhanced Thermal Interface Materials for Thermal Management of Photovoltaic Solar Cells. The increase in the temperature of photovoltaic (PV) solar cells affects
negatively their power conversion efficiency and decreases their lifetime. The
negative effects are particularly pronounced in concentrator solar cells.
Therefore, it is crucial to limit the PV cell temperature by effectively
removing the excess heat. Conventional thermal phase change materials (PCMs)
and thermal interface materials (TIMs) do not possess the thermal conductivity
values sufficient for thermal management of the next generation of PV cells. In
this paper, we report the results of investigation of the increased efficiency
of PV cells with the use of graphene-enhanced TIMs. Graphene reveals the
highest values of the intrinsic thermal conductivity. It was also shown that
the thermal conductivity of composites can be increased via utilization of
graphene fillers. We prepared TIMs with up to 6% of graphene designed
specifically for PV cell application. The solar cells were tested using the
solar simulation module. It was found that the drop in the output voltage of
the solar panel under two-sun concentrated illumination can be reduced from 19%
to 6% when graphene-enhanced TIMs are used. The proposed method can recover up
to 75% of the power loss in solar cells.

###Potential of PEDOT:PSS as a hole selective front contact for silicon heterojunction solar cells||Sara Jäckle,Martin Liebhaber,Clemens Gersmann,Mathias Mews,Klaus Jäger,Silke Christiansen,Klaus Lips###

Potential of PEDOT:PSS as a hole selective front contact for silicon heterojunction solar cells. We show that the highly conductive polymer
poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) can
successfully be applied as a hole selective front contact in silicon
heterojunction (SHJ) solar cells. In combination with a superior electron
selective heterojunction back contact based on amorphous silicon (a-Si),
mono-crystalline n-type silicon (c-Si) solar cells reach power conversion
efficiencies up to 14.8% and high open-circuit voltages exceeding 660 mV. Since
in the PEDOT:PSS/c-Si/a-Si solar cell the inferior hybrid junction is
determining the electrical device performance we are capable of assessing the
recombination velocity v_I at the PEDOT:PSS/c-Si interface. An estimated v_I of
~ 400 m/s demonstrates, that while PEDOT:PSS shows an excellent selectivity on
n-type c-Si, the passivation quality provided by the formation of a native
oxide at the c-Si surface restricts the performance of the hybrid junction.
Furthermore, by comparing the measured external quantum efficiency with optical
simulations, we quantify the losses due to parasitic absorption of PEDOT:PSS
and reflection of the device layer stack. By pointing out ways to better
passivate the hybrid interface and to increase the photocurrent we discuss the
full potential of PEDOT:PSS as a front contact in SHJ solar cells.

###An Efficient Descriptor Model for Designing Materials for Solar Cells||Fahhad H Alharbi,Sergey N Rashkeev,Fedwa El-Mellouhi,Hans P Lüthi,Nouar Tabet,Sabre Kais###

An Efficient Descriptor Model for Designing Materials for Solar Cells. An efficient descriptor model for fast screening of potential materials for
solar cell applications is presented. It works for both excitonic and
non-excitonic solar cells materials, and in addition to the energy gap it
includes the absorption spectrum ($\alpha(E)$) of the material. The charge
transport properties of the explored materials are modeled using the
characteristic diffusion length ($L_{d}$) determined for the respective family
of compounds. The presented model surpasses the widely used Scharber model
developed for bulk-heterojunction solar cells [Scharber \textit{et al.,
Advanced Materials}, 2006, Vol. 18, 789]. Using published experimental data, we
show that the presented model is more accurate in predicting the achievable
efficiencies. Although the focus of this work is on organic photovoltaics
(OPV), for which the original Scharber model was developed, the model presented
here is applicable also to other solar cell technologies. To model both
excitonic and non-excitonic systems, two different sets of parameters are used
to account for the different modes of operation. The analysis of the presented
descriptor model clearly shows the benefit of including $\alpha(E)$ and $L_{d}$
in view of improved screening results.

###A Design Based on Stair-case Band Alignment of Electron Transport Layer for Improving Performance and Stability in Planar Perovskite Solar Cells||Shang-Hsuan Wu,Ming-Yi Lin,Sheng-Hao Chang,Wei-Chen Tu,Chih-Wei Chu,Yia-Chung Chang###

A Design Based on Stair-case Band Alignment of Electron Transport Layer for Improving Performance and Stability in Planar Perovskite Solar Cells. Among the n-type metal oxide materials used in the planar perovskite solar
cells, zinc oxide (ZnO) is a promising candidate to replace titanium dioxide
(TiO2) due to its relatively high electron mobility, high transparency, and
versatile nanostructures. Here, we present the application of low temperature
solution processed ZnO/Al-doped ZnO (AZO) bilayer thin film as electron
transport layers (ETLs) in the inverted perovskite solar cells, which provide a
stair-case band profile. Experimental results revealed that the power
conversion efficiency (PCE) of perovskite solar cells were significantly
increased from 12.25 to 16.07% by employing the AZO thin film as the buffer
layer. Meanwhile, the short-circuit current density (Jsc), open-circuit voltage
(Voc), and fill factor (FF) were improved to 20.58 mA/cm2, 1.09V, and 71.6%,
respectively. The enhancement in performance is attributed to the modified
interface in ETL with stair-case band alignment of ZnO/AZO/CH3NH3PbI3, which
allows more efficient extraction of photogenerated electrons in the CH3NH3PbI3
active layer. Thus, it is demonstrated that the ZnO/AZO bilayer ETLs would
benefit the electron extraction and contribute in enhancing the performance of
perovskite solar cells.

###Efficient Volumetric Method of Moments for Modeling Plasmonic Thin-Film Solar Cells with Periodic Structures||Zi He,Ji Hong Gu,Wei E. I. Sha,Ru Shan Chen###

Efficient Volumetric Method of Moments for Modeling Plasmonic Thin-Film Solar Cells with Periodic Structures. Metallic nanoparticles (NPs) support localized surface plasmon resonances
(LSPRs), which enable to concentrate sunlight at the active layer of solar
cells. However, full-wave modeling of the plasmonic solar cells faces great
challenges in terms of huge computational workload and bad matrix condition. It
is tremendously difficult to accurately and efficiently simulate near-field
multiple scattering effects from plasmonic NPs embedded into solar cells. In
this work, a preconditioned volume integral equation (VIE) is proposed to model
plasmonic organic solar cells (OSCs). The diagonal block preconditioner is
applied to different material domains of the device structure. As a result,
better convergence and higher computing efficiency are achieved. Moreover, the
calculation is further accelerated by two-dimensional periodic Green's
functions. Using the proposed method, the dependences of optical absorption on
the wavelengths and incident angles are investigated. Angular responses of the
plasmonic OSCs show the super-Lambertian absorption on the plasmon resonance
but near-Lambertian absorption off the plasmon resonance. The volumetric method
of moments and explored physical understanding are of great help to investigate
the optical responses of OSCs.

###Luminescent Waveguides In-situ Integrated with Organic Solar Cells for Internet of Things||Sadra Sadeghi,Mertcan Han,Shashi Bhushan Srivastava,Sedat Nizamoglu###

Luminescent Waveguides In-situ Integrated with Organic Solar Cells for Internet of Things. Transparent electronics offer exciting light harvesting solutions for
generation of electrical power via demonstration of "see-through"
optoelectronic devices. The wireless energy harvesting ability can empower
unplugged and battery-free operations for Internet of Things (IoT) devices. In
this study, we report a transparent, luminescent, and elastomeric optical
waveguide incorporating quantum dots that is in-situ coupled with organic solar
cell array made of P3HT:PC61BM bulk heterojunction. CdSe@ZnS QDs have a
photoluminescence quantum yield (PLQY) of 91% and are synthetically engineered
to match their photoluminescence spectra with the photo-response of P3HT:PC61BM
solar cells for efficient energy harvesting. Integrated devices can generate
sufficient power for the active radio frequency identification (RFID) tags to
send signals in the communication distance of 35 meters at low illumination
level of 0.1-sun and ~0.1 km under 1-sun condition, respectively, which is
sufficient for indoor and outdoor communications. Advantageously, the
combination of organic solar cells with the waveguide during the elastomer
curing leads to the elimination of the undesired post-fabrication processes
such as alignment of the solar cells with waveguide and gluing the separate
parts with curable polymers. This study paves the way toward using luminescence
as transparent, efficient, and configurable energy harvesting solutions for IoT
applications.

###Cross-Linking of Doped Organic Semiconductor Interlayers for Organic Solar Cells: Potential and Challenges||Staffan Dahlström,Sebastian Wilken,Yadong Zhang,Christian Ahläng,Stephen Barlow,Mathias Nyman,Seth R. Marder,Ronald Österbacka###

Cross-Linking of Doped Organic Semiconductor Interlayers for Organic Solar Cells: Potential and Challenges. Solution-processable interlayers are an important building block for the
commercialization of organic electronic devices such as organic solar cells.
Here, the potential of cross-linking to provide an insoluble, stable and
versatile charge transport layer based on soluble organic semiconductors is
studied. For this purpose, a photo-reactive tris-azide cross-linker is
synthesized. The capability of the small molecular cross-linker is illustrated
by applying it to a p-doped polymer used as a hole transport layer in organic
solar cells. High cross-linking efficiency and excellent charge extraction
properties of the cross-linked doped hole transport layer are demonstrated.
However, at high doping levels in the interlayer, the solar cell efficiency is
found to deteriorate. Based on charge extraction measurements and numerical
device simulations, it is shown that this is due to diffusion of dopants into
the active layer of the solar cell. Thus, in the development of future
cross-linker materials, care must be taken to ensure that they immobilize not
only the host, but also the dopants.

###Metastable defects decrease the fill factor of solar cells||Thomas Paul Weiss,Omar Ramírez,Stefan Paetel,Wolfram Witte,Jiro Nishinaga,Thomas Feurer,Susanne Siebentritt###

Metastable defects decrease the fill factor of solar cells. Cu(In,Ga)Se2 based solar cells exceed power conversion efficiencies of 23 %.
Yet, the fill factor of these solar cells, with best values around 80 %, is
relatively low (Si reaches 84.9%) mostly due to diode factors greater than one.
Recently, we proposed metastable defects, a general feature of the Cu(In,Ga)Se2
alloy, to be the origin of the increased diode factor. We measure the diode
factor of the bare absorber layers by excitation-dependent photoluminescence.
For high quality and thus high luminescent polycrystalline absorbers, we
evaluate the diode factor excitation dependence over four orders of magnitude.
Using simulations and the model of metastable defects, we can well describe the
experimental findings on n- and p-type epitaxial films as well as the
polycrystalline absorbers, providing additional evidence for this model. We
find that the diode factors measured optically by photoluminescence impose a
lower limit for the diode factor measured electrically on a finished solar
cell. Interestingly, the lowest diode factor (optical and electrical) and
consequently highest fill factor of 81.0 % is obtained by Ag alloying, i.e. an
(Ag,Cu)(In,Ga)Se2 absorber. This finding hints to a pathway to increase fill
factors and thus efficiencies for Cu(In,Ga)Se2-based solar cells.

###Predicted annual energy yield of III-V/c-Si tandem solar cells: modelling the effect of changing spectrum on current-matching||Ian Mathews,Shenghui Lei,Ronan Frizzell###

Predicted annual energy yield of III-V/c-Si tandem solar cells: modelling the effect of changing spectrum on current-matching. High efficiencies of >30% are predicted for series-connected tandem solar
cells when current-matching is achieved between the wide-bandgap top cell and
silicon bottom cell. Sub-cells are typically optimised for current-matching
based on the standard AM1.5G spectrum, but in practice, the incident radiation
on a solar cell can be very different from this standard due to the effects of
the sun's location in the sky, atmospheric conditions, total diffuse element
etc. The resulting deviations in spectral content from optimum conditions lead
to current mismatch between tandem cell layers that adversely affects the
device's performance. To investigate the impact of this issue the energy yield
(%) of tandem solar cells comprising a III-V wide-bandgap solar cell connected
electrically and optically in series with a silicon bottom cell was simulated
over a full year using measured spectral data from Denver, CO. Top cells with
bandgaps from 1.5-1.9 eV were modelled using an external radiative efficiency
method. The predicted annual energy yields were as high as 31% with an optimum
1.8 eV top cell, only 2.8% lower (absolute) than the AM1.5G predicted
efficiency. The annual energy yield of tandem cells with no current-matching
constraint, i.e. parallel-connected devices, was also simulated. Here the
difference between series and parallel connections were only significant for
non-optimum bandgap combinations. Our results indicate that AM1.5G based
optimization of sub-cells can be effectively employed to achieve high energy
yields of >25% for III-V/Si tandem solar cells in mid-latitude US locations,
despite the continuous variation in spectra throughout a calendar year.

###High Open Circuit Voltages in pin-Type Perovskite Solar Cells through Strontium Addition||Pietro Caprioglio,Fengshuo Zu,Christian M. Wolff,José A. Márquez Prieto,Martin Stolterfoht,Norbert Koch,Thomas Unold,Bernd Rech,Steve Albrecht,Dieter Neher###

High Open Circuit Voltages in pin-Type Perovskite Solar Cells through Strontium Addition. The incorporation of even small amounts of strontium (Sr) into lead-based
quadruple cation hybrid perovskite solar cells results in a systematic increase
of the open circuit voltage (Voc) in pin-type perovskite solar cells. We
demonstrate via transient and absolute photoluminescence (PL) experiments how
the incorporation of Sr significantly reduces the non-radiative recombination
losses in the neat perovskite layer and specifically at the perovskite/C60
interface. We show that Sr segregates at the perovskite surface, where it
induces important changes of morphology and energetics. Notably, the
Sr-enriched surface exhibits a wider band gap and a more n-type character,
accompanied with significantly stronger surface band bending. As a result, we
observe a significant increase of the quasi-Fermi level splitting in the neat
perovskite by reduced surface recombination and more importantly, a strong
reduction of losses attributed to non-radiative recombination at the interface
to the C60 electron-transporting layer. The resulting solar cells exhibited a
Voc of 1.18 V, which could be further improved to nearly 1.23 V through
addition of a thin polymer interlayer, bringing the non-radiative voltage loss
to only 110 meV. Our work shows that simply adding a small amount of Sr to the
precursor solutions induces a beneficial surface modification in the
perovskite, without requiring any post treatment, resulting in high efficiency
solar cells with power conversion efficiency (PCE) up to 20.3%. Our results
demonstrate very high Voc values and efficiencies in Sr-containing quadruple
cation perovskite pin solar cells and highlight the imperative importance of
addressing and minimizing the recombination losses at the interface between
perovskite and charge transporting layer.

###Inverted rear-heterojunction GaInP solar cells using Te memory effect||Manuel Hinojosa,Iván García,Ignacio Rey-Stolle,Carlos Algora###

Inverted rear-heterojunction GaInP solar cells using Te memory effect. Tellurium allows attaining heavy n-type doping levels in GaAs, which is
suited to achieve very low contact resistivities in solar cells. Besides, it
modifies the energy bandgap of MOVPE-grown GaInP by reducing the group-III
sublattice ordering and presents a strong memory effect which induces residual
n-type doping in subsequent layers, potentially detrimental to the performance
of the solar cell. In this work, we present an inverted rear-heterojunction
GaInP solar cell that employs a thick Te-doped GaInP layer as absorber, with a
doping profile obtained exclusively by controlling the memory effect of Te
coming from the preceding growth of a heavily doped GaAs contact layer. In this
way, GaInP is partially disordered with the use of no additional surfactant,
leading to an increase in the solar cell bandgap of around 35 meV as compared
to traditional samples doped with silicon. In the proof-of-concept experimental
devices developed so far, the use of a rear-heterojunction configuration and
the bandgap increase results in a global open-circuit voltage enhancement of
109 mV. The photocurrent decreases by 1.32 mA/cm2, mostly due to the bandgap
blue-shift, with about 0.35 mA/cm2 attributable to lower carrier collection
efficiencies. These preliminary results are discussed by analyzing the I-V
curve parameters and quantum efficiencies of a Te-doped rear-heterojunction, a
Si-doped rear-heterojunction and a Si-doped front-junction solar cell. An
additional advantage is that the emitter sheet resistance is reduced from 551
to 147 ohms/sq, which offers potential for higher efficiencies through lower
front grid shadowing factors, as demonstrated with the concentrator
measurements presented.

###Experimental demonstration of ions induced electric field in perovskite solar cells||Zeguo Tang,Takashi Minemoto###

Experimental demonstration of ions induced electric field in perovskite solar cells. The anomalous hysteresis is reported in photo current density-voltage (J-V)
curves of perovskite solar cells. The origins responsible for hysteresis are
primarily attributed to ferroelectricity, trapping/de-trapping, and ions
migration. Meanwhile, a switchable photovoltaic effect is disclosed in lateral
structure perovskite solar cells, where a p-i-n structure is formed after
poling the device with a reverse bias. Considering the normal sandwiched
structure of perovskite solar cells, i.e. the intrinsic perovskite layer is
situated in an electric field built by p-type (Spiro-OMeTAD) and n-type (TiO2)
layers, the poling process also works in such devices. The migration of ions
will induce a new electric field with opposite direction as inherent built-in
electric field. As a consequence, less collection of photogenerated carriers is
predicted due to the built-in electric field is partially screened. Here, the
ions induced electric field is experimentally demonstrated by exploring the
external quantum efficiency (EQE) spectra under various forward voltage biases.
The anomalous hysteresis observed on J-V curves is explained based on the
insight of ions migrations under bias voltage. The external applied voltage
presents significant influence on the stability of solar cells. Meanwhile, a
novel phenomenon of negative capacitance is initially detected at low
frequency, which is ascribed to the position reversal of negatively and
positively charged ions under forward bias.

###Design of Lead-Free Inorganic Halide Perovskites for Solar Cells via Cation-Transmutation||Xin-Gang Zhao,Ji-Hui Yang,Yuhao Fu,Dongwen Yang,Qiaoling Xu,Liping Yu,Su-Huai Wei,Lijun Zhang###

Design of Lead-Free Inorganic Halide Perovskites for Solar Cells via Cation-Transmutation. Hybrid organic-inorganic halide perovskites with the prototype material of
CH$_{3}$NH$_{3}$PbI$_{3}$ have recently attracted intense interest as low-cost
and high-performance photovoltaic absorbers. Despite the high power conversion
efficiency exceeding 20% achieved by their solar cells, two key issues -- the
poor device stabilities associated with their intrinsic material instability
and the toxicity due to water soluble Pb$^{2+}$ -- need to be resolved before
large-scale commercialization. Here, we address these issues by exploiting the
strategy of cation-transmutation to design stable inorganic Pb-free halide
perovskites for solar cells. The idea is to convert two divalent Pb$^{2+}$ ions
into one monovalent M$^{+}$ and one trivalent M$^{3+}$ ions, forming a rich
class of quaternary halides in double-perovskite structure. We find through
first-principles calculations this class of materials have good phase stability
against decomposition and wide-range tunable optoelectronic properties. With
photovoltaic-functionality-directed materials screening, we identify eleven
optimal materials with intrinsic thermodynamic stability, suitable band gaps,
small carrier effective masses, and low excitons binding energies as promising
candidates to replace Pb-based photovoltaic absorbers in perovskite solar
cells. The chemical trends of phase stabilities and electronic properties are
also established for this class of materials, offering useful guidance for the
development of perovskite solar cells fabricated with them.

###Simulation studies of CZT(S,Se) single and tandem junction solar cells towards possibilities for higher efficiencies up to 22%||Goutam K Gupta,Ambesh Dixit###

Simulation studies of CZT(S,Se) single and tandem junction solar cells towards possibilities for higher efficiencies up to 22%. We simulated photovoltaic characteristics of single heterojunction solar cell
with Cu2ZnSnS4 and Cu2ZnSnSe4 absorber layer numerically using one dimensional
solar cell capacitance simulator (SCAPS-1D). n-CdS/ZnO double buffer layer is
used for hetrostructure interfaces with the absorber layer. The cell
performance is investigated against variation of different material layer
properties such as thickness, carrier concentration, defect density etc. The
performance is optimized first for the single junction solar cell with Mo as
back contact material with work function 5 eV. A double junction CZTS/CZTSe
tandem cell structure is realized keeping the same material properties as is
used in the single CZTS and CZTSe solar cell simulation and considering the
flat band condition at the interface. Tandem cell performance is determined
after matching the current condition for top and bottom sub-cells. The
CZTS/CZTSe short circuit current density is ~ 20.98 mA/cm2 for current matched
211.33 nm thick CZTS top cell in conjunction with 2000 nm bottom cell. The
maximum efficiency obtained under the flat band condition at the contact is
~21.7% with open circuit voltage ~1.324 V.

###Quantification of Ion Migration in CH3NH3PbI3 Perovskite Solar Cells by Transient Capacitance Measurements||Moritz H. Futscher,Ju Min Lee,Lucie McGovern,Loreta A. Muscarella,Tianyi Wang,Muhammad Irfan Haider,Azhar Fakharuddin,Lukas Schmidt-Mende,Bruno Ehrler###

Quantification of Ion Migration in CH3NH3PbI3 Perovskite Solar Cells by Transient Capacitance Measurements. Solar cells based on organic-inorganic metal halide perovskites show
efficiencies close to highly-optimized silicon solar cells. However, ion
migration in the perovskite films leads to device degradation and impedes large
scale commercial applications. We use transient ion-drift measurements to
quantify activation energy, diffusion coefficient, and concentration of mobile
ions in methylammonium lead triiodide (MAPbI3) perovskite solar cells, and find
that their properties change close to the tetragonal-to-orthorhombic phase
transition temperature. We identify three migrating ion species which we
attribute to the migration of iodide (I-) and methylammonium (MA+). We find
that the concentration of mobile MA+ ions is one order of magnitude higher than
the one of mobile I- ions, and that the diffusion coefficient of mobile MA+
ions is three orders of magnitude lower than the one for mobile I- ions. We
furthermore observe that the activation energy of mobile I- ions (0.29 eV) is
highly reproducible for different devices, while the activation energy of
mobile MA+ depends strongly on device fabrication. This quantification of
mobile ions in MAPbI3 will lead to a better understanding of ion migration and
its role in operation and degradation of perovskite solar cells.

###Imaging of bandtail states in silicon heterojunction solar cells||M. Y. Teferi,H. Malissa,A. B. Morales-Vilches,C. T. Trinh,L. Korte,B. Stannowski,C. C. Williams,C. Boehme,K. Lips###

Imaging of bandtail states in silicon heterojunction solar cells. Silicon heterojunction (SHJ) solar cells represent a promising technological
approach towards higher photovoltaics efficiencies and lower fabrication cost.
While the device physics of SHJ solar cells have been studied extensively in
the past, the ways in which nanoscopic electronic processes such as
charge-carrier generation, recombination, trapping, and percolation affect SHJ
device properties macroscopically have yet to be fully understood. We report
the study of atomic scale current percolation at state-of-the-art a-Si:H/c-Si
heterojunction solar cells under ambient operating conditions, revealing the
profound complexity of electronic SHJ interface processes. Using conduction
atomic force microscopy (cAFM), it is shown that the macroscopic
current-voltage characteristics of SHJ solar cells is governed by the average
of local nanometer-sized percolation pathways associated with bandtail states
of the doped a-Si:H selective contact leading to above bandgap open circuit
voltages ($V_{\mbox{OC}}$) as high as 1.2 V ($V_{\mbox{OC}}>e
E_{\mbox{gap}}^{\mbox{Si}}$). This is not in violation of photovoltaic device
physics but a consequence of the nature of nanometer-scale charge percolation
pathways which originate from trap-assisted tunneling causing dark leakage
current. We show that the broad distribution of local photovoltage is a direct
consequence of randomly trapped charges at a-Si:H dangling bond defects which
lead to strong local potential fluctuations and induce random telegraph noise
of the dark current.

###Energetics and Kinetics Requirements for Organic Solar Cells to 2 Break the 20% Power Conversion Efficiency Barrier||Oskar J Sandberg,Ardalan Armin###

Energetics and Kinetics Requirements for Organic Solar Cells to 2 Break the 20% Power Conversion Efficiency Barrier. The thermodynamic limit for the efficiency of solar cells is predominantly
defined by the energy bandgap of the used semiconductor. In case of organic
solar cells both energetics and kinetics of three different species play role:
excitons, charge transfer states and charge separated states. In this work, we
clarify the effect of the relative energetics and kinetics of these species on
the recombination and generation dynamics. Making use of detailed balance, we
develop an analytical framework describing how the intricate interplay between
the different species influence the photocurrent generation, the recombination,
and the open-circuit voltage in organic solar cells. Furthermore, we clarify
the essential requirements for equilibrium between excitons, CT states and
charge carriers to occur. Finally, we find that the photovoltaic parameters are
not only determined by the relative energy level between the different states
but also by the kinetic rate constants. These findings provide vital insights
into the operation of state-of-art non-fullerene organic solar cells with low
offsets.

###Ge virtual substrates for high efficiency III-V solar cells: applications, potential and challenges||Iván García,Manuel Hinojosa,Iván Lombardero,Luis Cifuentes,Ignacio Rey-Stolle,Carlos Algora,Huy Nguyen,Stuart Edwards,Aled Morgan,Andrew Johnson###

Ge virtual substrates for high efficiency III-V solar cells: applications, potential and challenges. Virtual substrates based on thin Ge layers on Si by direct deposition have
achieved high quality recently. Their application to high efficiency III-V
solar cells is analyzed in this work. Replacing traditional Ge substrates with
Ge/Si virtual substrates in standard lattice-matched and upright metamorphic
GaInP/Ga(In)As/Ge solar cells is feasible according to our calculations using
realistic parameters of state-of-the-art Ge solar cells but with thin bases (<
5um). The first experimental steps are tackled by implementing Ge
single-junction and full GaInP/Ga(In)As/Ge triple-junction solar cells on
medium quality Ge/Si virtual substrates with 5um thick Ge layers. The results
show that the photocurrent in the Ge bottom cell is barely enough to achieve
current matching with the upper subcells, but the overall performance is poor
due to low voltages in the junctions. Moreover, observed cracks in the
triple-junction structure point to the need to reduce the thickness of the Ge +
III-V structure or using other advanced approaches to mitigate the thermal
expansion coefficient mismatch effects, such as using embedded porous silicon.
Next experimental work will pursue this objective and use more advanced Ge/Si
virtual substrates available with lower threading dislocation densities and
different Ge thicknesses.

###Glass engineering to enhance Si solar cells: a case study of Pr$^{3+}$-Yb$^{3+}$ codoped tellurite-tungstate as spectral converter||Maiara Mitiko Taniguchi,Vitor Santaella Zanuto,Pablo Portes,Luis Carlos Malacarne,Nelson Guilherme Astrath,Jorge Diego Marconi,Marcos Paulo Belançon###

Glass engineering to enhance Si solar cells: a case study of Pr$^{3+}$-Yb$^{3+}$ codoped tellurite-tungstate as spectral converter. Spectral converters are known to increase photovoltaic energy conversion by
minimizing losses due to fundamental non-absorption and thermalization
processes, and have been suggested to surpass the Shockley-Queisser efficiency
limit in single junction solar cells. Here we present a detailed spectroscopic
study of photoluminescence in tellurite-tungstate glasses doped and codoped
with $Pr^{3+}-Yb^{3+}$ and $Ag$ nanoparticles. The energy transfer mechanisms
between $Pr^{3+}$ and $Yb^{3+}$ are discussed based on the near infrared
emission under excitation at $442$ nm and on the upconversion emission under
excitation at $980$ nm. Fluorescence quenching of $^2 F_{5/2}$ level of
$Yb^{3+}$ is observed by increasing the concentration of $Pr^{3+}$, as well as
by the addition of $Ag$ nanoparticles. In addition, a discussion on the
potential of this glass to increase energy production in spectral converters is
presented. The results suggest that the few undesirable energy transfer
processes occurring in this material are difficult to be controlled or
eliminated properly, resulting in intrinsic losses. This discussion is extended
to the potential of glass science to enhance energy production in solar cells,
showing that newer designs such as bifacial cells may facilitate the
exploration of glasses other than soda-lime for mass production of solar cells.
The focus on extending the lifespan by reducing UV induced degradation seems to
be a more effective approach than the development of spectral converters for Si
solar cells.

###Non-destructive determination of phase, size, and strain of individual grains in polycrystalline photovoltaic materials||Mariana Mar Lucas,Tiago Ramos,Peter S. Jørgensen,Stela Canulescu,Peter Kenesei,Jonathan Wright,Henning F. Poulsen,Jens W. Andreasen###

Non-destructive determination of phase, size, and strain of individual grains in polycrystalline photovoltaic materials. We demonstrate a non-destructive approach to provide structural properties on
the grain level for the absorber layer of kesterite solar cells. Kesterite
solar cells are notoriously difficult to characterize structurally due to the
co-existence of several phases with very similar lattice parameters.
Specifically, we present a comprehensive study of 597 grains in the absorber
layer of a 1.64% efficient Cu2ZnSnS4 (CZTS) thin-film solar cell, from which 15
grains correspond to the secondary phase ZnS. By means of three dimensional
X-ray diffraction (3DXRD), we obtained statistics for the phase, size,
orientation, and strain tensors of the grains, as well as their twin relations.
We observe an average tensile stress in the plane of the film of ~ 70 MPa and a
compressive stress along the normal to the film of ~ 145 MPa. At the grain
level, we derive a 3D stress tensor that deviates from the biaxial model
usually assumed for thin films. 41% of the grains are twins. We calculate the
frequency of the six types of $\Sigma$3 boundaries, revealing that 180{\deg}
rotations along axis <221> is the most frequent. This technique can be applied
to polycrystalline thin film solar cells in general, where strain can influence
the bandgap of the absorber layer material, and twin boundaries play a role in
the charge transport mechanisms.

###Theoretical simulation and design of AlSb thin films solar cells||Huijin Song,Zilong Wang,Jingwen Wang,Qiang Yan,Kai Xia,Xiangfeng Deng,Minqiang Li###

Theoretical simulation and design of AlSb thin films solar cells. The effects of thickness, doping concentration and recombination of AlSb
films on the performance of CdS/AlSb cells are simulated by one dimensional
simulation program called analysis of microelectronic and photonic
structures(AMPS1D) soft ware to understand the influence of material
characteristic (such as carrier concentration and thickness) on the solar
cells. The methods to improve the performance of CdS/AlSb cells by optimizing
the properties of AlSb have been found. The results show that the thicker AlSb
film can improve the long wave response for the higher short-circuit current
density (Jsc ) of CdS/AlSb solar cells and the higher carrier concentration of
the film can improve open-circuit voltage (Voc ) and fill factor (FF), and its
optical thickness for CdS/AlSb solar cells is in the range of 500nm~2000nm. The
conversion efficiency can be improved from 10.6% to15.3% for introducing
AlSb:Te, AlSb:Cu and ZnTe:Cu thin films to CdS/ AlSb structure. Furthermore,
the thicker AlSb:Te film can improve the short wave response for the higher Jsc
of the cells, and its optical thickness CdS/AlSb:Te/AlSb/ZnTe:Cu solar cells is
in the range of 100nm~200nm. And the lower doping concentration can promote Voc
and FF to improve the characteristic of the cells.

###Effect of plasmonic Aluminum nanoparticles shapes on optical absorption enhancement in silicon thin-film solar cells||Maedeh Rassekh,Reza Shirmohammadi,Roghayeh Ghasempour,Fatemeh Razi Astaraei,Saber Farjami Shayesteh###

Effect of plasmonic Aluminum nanoparticles shapes on optical absorption enhancement in silicon thin-film solar cells. Scattering from metal nanoparticles near their localized plasmon resonance;
especially, the resonances of noble metals which are mostly in the visible or
infrared part of the electromagnetic spectrum; is a way of improving light
absorption in thin-film solar cells. The surface plasmon resonance can be
affected by different factors such as the type, size, shape, and dielectric
properties of the surrounding medium. Here we investigate, using the Finite
Difference Time Domain (FDTD) method, how different shapes of aluminum
nanoparticles affect absorption enhancement in silicon thin-film solar cells.
Our results show that using these particles more than 30% conversion efficiency
for plasmonic solar cells can be achieved compared to a cell without particles.
We have also found that although the spherical particles have the highest
absorption peak, optimization of some parameters such as the height of the
cylinder or disk-shaped particles and their distance from the substrate can
increase the absorption. The results can provide more information and insight
to understand and optimize plasmonic particles for solar cell applications.

###Anomaly segmentation model for defects detection in electroluminescence images of heterojunction solar cells||Alexey Korovin,Artem Vasilyev,Fedor Egorov,Dmitry Saykin,Evgeny Terukov,Igor Shakhray,Leonid Zhukov,Semen Budennyy###

Anomaly segmentation model for defects detection in electroluminescence images of heterojunction solar cells. Efficient defect detection in solar cell manufacturing is crucial for stable
green energy technology manufacturing. This paper presents a
deep-learning-based automatic detection model SeMaCNN for classification and
semantic segmentation of electroluminescent images for solar cell quality
evaluation and anomalies detection. The core of the model is an anomaly
detection algorithm based on Mahalanobis distance that can be trained in a
semi-supervised manner on imbalanced data with small number of digital
electroluminescence images with relevant defects. This is particularly valuable
for prompt model integration into the industrial landscape. The model has been
trained with the on-plant collected dataset consisting of 68 748
electroluminescent images of heterojunction solar cells with a busbar grid. Our
model achieves the accuracy of 92.5%, F1 score 95.8%, recall 94.8%, and
precision 96.9% within the validation subset consisting of 1049 manually
annotated images. The model was also tested on the open ELPV dataset and
demonstrates stable performance with accuracy 94.6% and F1 score 91.1%. The
SeMaCNN model demonstrates a good balance between its performance and
computational costs, which make it applicable for integrating into quality
control systems of solar cell manufacturing.

###Pulsed Laser Ejection of Single-Crystalline III-V Solar Cells From GaAs Substrates||Benjamin A. Reeves,Myles A. Steiner,Thomas E. Carver,Ze Zhang,Aaron M. Lindenberg,Bruce M. Clemens###

Pulsed Laser Ejection of Single-Crystalline III-V Solar Cells From GaAs Substrates. Like many optoelectronics, the highest quality III-V solar cells start out as
thin single-crystalline multilayers on GaAs substrates. Separating these device
layers from their growth substrate enables higher performing devices and wafer
reuse, both of which are critical for III-V solar cell viability in a
terrestrial market. Here, we remove rigidly-bonded, lattice-matched, 16 mm$^2$
x 3.5 um thick GaAs devices off a GaAs substrate using a 10 ns, unfocused
Nd:YAG laser pulse. The pulse is selectively absorbed in a lower-bandgap,
lattice-matched, crystalline layer below the device, driving a quasi-two
dimensional ablation event that ejects the crystalline multilayer from the
substrate. After minutes of selective wet-chemical etching and front contact
deposition, our champion 0.1 cm$^2$ device showed a (17.4 +/- 0.5) % power
conversion efficiency and an open-circuit voltage of 1.07 V, using AM1.5 direct
(1000 W m$^{-2}$) with no anti-reflection coating. We show that the performance
is comparable to similar solar cells produced via conventional substrate
dissolution processes. We discuss unique process characteristics and
opportunities, such as the potential to separate wafer-sized thin film solar
cells per laser pulse.

###Cross-sectional profile of photocarrier mobility in thin film solar cell via nongeminate recombination and charge extraction by linearly increasing voltage (cs-p-CELIV)||Noah B. Stocek,Miguel J. Young,Reg Bauld,Tianhao Ouyang,Giovanni Fanchini###

Cross-sectional profile of photocarrier mobility in thin film solar cell via nongeminate recombination and charge extraction by linearly increasing voltage (cs-p-CELIV). The ability to spatially resolve the carrier mobility profile along the cross
section of micrometer-thin solar cells is vital both for fundamental studies in
photovoltaics and as a quality control for reproducibly obtaining high
conversion efficiencies in commercial solar cell modules. Presently, no
technique capable of such an endeavor is available to the best of our
knowledge. Here, we introduce a novel method capable of profiling the carrier
mobility along the z-axis in thin-film photovoltaics. Our setup is based on the
integration of photogenerated charge extraction by linearly increasing voltage
(p-CELIV) with a scanning confocal optical microscope (SCOM) towards a
cross-sectional sensitive p-CELIV (cs-p-CELIV) system. As geminate
recombination of excess carriers is the most frequent radiative pathway for
electrons and holes in solar cells at low power density of illumination, while
nongeminate recombination dominates at high power, enhanced nongeminate
recombination occurs at the SCOM focal plane. Thus, the cs-p-CELIV signal
provides enhanced information on the mobility of all of the cross-sectional
layers, except for the focal plane. By scanning the focal plane along the
z-axis, the mobility profile can be derived. To demonstrate our technique, we
use it to investigate the carrier mobility in a hydrogenated amorphous silicon
(a-Si:H) solar cell. The mobility profile obtained by cs-p-CELIV correlates
well with the H content profile, measured independently, and is in excellent
agreement with models suggesting a critical role of Si-H bonding in locally
determining the carrier mobility in a-Si:H.

###Organic solar cell efficiencies under the aspect of reduced surface recombination velocities||A. Wagenpfahl,C. Deibel,V. Dyakonov###

Organic solar cell efficiencies under the aspect of reduced surface recombination velocities. The charge carrier mobility is a key parameter for the organic bulk
heterojunction solar cell efficiency. It was recently shown that the interplay
charge carrier transport and recombination, both depending on electron and hole
mobilities, leads to a point of maximum power conversion efficiency at a finite
mobility. Changes of bulk and surface recombination rate, however, can strongly
influence this behavior. These processes were previously not considered
adequately, as surface recombination velocities of infinity were implicitly
assumed or bulk recombination parameters not discussed in detail. In this
manuscript, using a macroscopic effective medium simulation, we consider how a
reduced bulk recombination process in combination with finite surface
recombination velocities affect the power conversion efficiency. Instead of a
maximum efficiency at a specific charge carrier mobility, we show that with
realistic assumptions and passivated surfaces the efficiency is increased
further, saturating only at higher mobilities. Thus, a mobility optimisation is
more important for the solar cell performance then previously shown.

###Narrow-bandwidth solar upconversion: design principles, efficiency limits, and case studies||Justin A. Briggs,Ashwin C. Atre,Jennifer A. Dionne###

Narrow-bandwidth solar upconversion: design principles, efficiency limits, and case studies. We employ a detailed balance approach to model a single-junction solar cell
with a realistic narrow-band, non-unity-quantum-yield upconverter. As
upconverter bandwidths are increased from 0 to 0.5 eV, maximum cell
efficiencies increase from the Shockley-Queisser limit of 30.58% to over 43%.
Such efficiency enhancements are calculated for upconverters with near-infrared
spectral absorption bands, readily accessible with existing upconverters. While
our model shows that current bimolecular and lanthanide-based upconverting
materials will improve cell efficiencies by <1%, cell efficiencies can increase
by several absolute percent with increased upconverter quantum yield - even
without an increased absorption bandwidth. By examining the efficiency limits
of a highly realistic solar cell-upconverter system, our model provides a
platform for optimizing future solar upconverter designs.

###Towards highly efficient thin-film solar cells with a graded-bandgap CZTSSe layer||Faiz Ahmad,Akhlesh Lakhtakia,Tom H. Anderson,Peter B. Monk###

Towards highly efficient thin-film solar cells with a graded-bandgap CZTSSe layer. A coupled optoelectronic model was implemented along with the differential
evolution algorithm to assess the efficacy of grading the bandgap of the CZTSSe
layer for enhancing the power conversion efficiency of thin-film CZTSSe solar
cells. Both linearly and sinusoidally graded bandgaps were examined, with the
molybdenum backreflector in the solar cell being either planar or periodically
corrugated. Whereas an optimally graded bandgap can dramatically enhance the
efficiency, the effect of periodically corrugating the backreflector is modest
at best. An efficiency of 21.74% is predicted with sinusoidal grading of a
870-nm-thick CZTSSe layer, in comparison to 12.6% efficiency achieved
experimentally with a 2200-nm-thick homogeneous CZTSSe layer. High
electron-hole-pair generation rates in the narrow-bandgap regions and a high
open-circuit voltage due to a wider bandgap close to the front and rear faces
of the CZTSSe layer are responsible for the high enhancement of efficiency.

###Intense Internal and External Fluorescence as Solar Cells Approach the Shockley-Queisser Efficiency Limit||Owen D. Miller,Eli Yablonovitch,Sarah R. Kurtz###

Intense Internal and External Fluorescence as Solar Cells Approach the Shockley-Queisser Efficiency Limit. Absorbed sunlight in a solar cell produces electrons and holes. But, at the
open circuit condition, the carriers have no place to go. They build up in
density and, ideally, they emit external fluorescence that exactly balances the
incoming sunlight. Any additional non-radiative recombination impairs the
carrier density buildup, limiting the open-circuit voltage. At open-circuit,
efficient external fluorescence is an indicator of low internal optical losses.
Thus efficient external fluorescence is, counter-intuitively, a necessity for
approaching the Shockley-Queisser efficiency limit. A great Solar Cell also
needs to be a great Light Emitting Diode. Owing to the narrow escape cone for
light, efficient external emission requires repeated attempts, and demands an
internal luminescence efficiency >>90%.

###Modeling of high-efficiency silicon solar cells in realistic operating conditions||A. V. Sachenko,A. I. Shkrebtii,R. M. Korkishko,V. P. Kostylyov,N. R. Kulish,I. O. Sokolovskyi###

Modeling of high-efficiency silicon solar cells in realistic operating conditions. The selfconsistent model for the temperature dependence of photoconversion
efficiency $\eta$ for highly efficient silicon solar cells (SCs) is developed.
It is demonstrated that effect of the efficiency decrease due to increasing
temperature is less pronounced in the SCs with lower surface recombination
velocity, thus offering a possibility to improve the cells' performance.
  The photoconversion efficiency of the high efficiency silicon solar cells is
modeled for the realistic ambient conditions. The SC operating temperature is
determined by self-consistently solving the photocurrent, photovoltage, and
energy balance equations, considering both radiative and convective cooling
mechanisms. The SC temperature is shown to be substantially higher than the
ambient temperature even at very high convection coefficients, such as, e.g.,
300 $W / (m^2 \cdot K)$, used in our examples. The photoconversion efficiency
for this case is substantially below the efficiency of thermally stabilized SC,
for which the operating temperature is close to the external temperature.
  The open-circuit voltage and photoconversion efficiency of the high-quality
silicon solar cells under concentrated illumination are also investigated
including the tradeoff between SCs heating and cooling processes.

###Perovskite-perovskite tandem photovoltaics with optimized bandgaps||Giles E. Eperon,Tomas Leijtens,Kevin A. Bush,Rohit Prasanna,Thomas Green,Jacob Tse-Wei Wang,David P. McMeekin,George Volonakis,Rebecca L. Milot,Richard May,Axel Palmstrom,Daniel J. Slotcavage,Rebecca A. Belisle,Jay B. Patel,Elizabeth S. Parrott,Rebecca J. Sutton,Wen Ma,Farhad Moghadam,Bert Conings,Aslihan Babayigit,Hans-Gerd Boyen,Stacey Bent,Feliciano Giustino,Laura M. Herz,Michael B. Johnston,Michael D. McGehee,Henry J. Snaith###

Perovskite-perovskite tandem photovoltaics with optimized bandgaps. We demonstrate four and two-terminal perovskite-perovskite tandem solar cells
with ideally matched bandgaps. We develop an infrared absorbing 1.2eV bandgap
perovskite, $FA_{0.75}Cs_{0.25}Sn_{0.5}Pb_{0.5}I_3$, that can deliver 14.8 %
efficiency. By combining this material with a wider bandgap
$FA_{0.83}Cs_{0.17}Pb(I_{0.5}Br_{0.5})_3$ material, we reach monolithic two
terminal tandem efficiencies of 17.0 % with over 1.65 volts open-circuit
voltage. We also make mechanically stacked four terminal tandem cells and
obtain 20.3 % efficiency. Crucially, we find that our infrared absorbing
perovskite cells exhibit excellent thermal and atmospheric stability,
unprecedented for Sn based perovskites. This device architecture and materials
set will enable 'all perovskite' thin film solar cells to reach the highest
efficiencies in the long term at the lowest costs.

###Optoelectronic optimization of graded-bandgap thin-film AlGaAs solar cells||Faiz Ahmad,Akhlesh Lakhtakia,Peter B. Monk###

Optoelectronic optimization of graded-bandgap thin-film AlGaAs solar cells. An optoelectronic optimization was carried out for an AlGaAs solar cell
containing (i) an n-AlGaAs absorber layer with a graded bandgap and (ii) a
periodically corrugated Ag backreflector combined with localized ohmic Pd-Ge-Au
backcontacts. The bandgap of the absorber layer was varied either sinusoidally
or linearly. An efficiency of 33.1% with the 2000-nm-thick n-AlGaAs absorber
layer is predicted with linearly graded bandgap along with silver backreflector
and localized ohmic backcontacts, in comparison to 27.4% efficiency obtained
with homogeneous bandgap and a continuous ohmic backcontact. Sinusoidal grading
of the bandgap {is predicted to enhance} the maximum efficiency to 34.5%. Thus,
grading the bandgap of the absorber layer, along with a periodically corrugated
Ag backreflector and localized ohmic Pd-Ge-Au backcontacts can help realize
ultrathin and high-efficient AlGaAs solar cells for terrestrial applications.

###Designing III-V Multijunction Solar Cells on Silicon||J. P. Connolly,D. Mencaraglia,C. Renard,D. Bouchier###

Designing III-V Multijunction Solar Cells on Silicon. Single junction Si solar cells dominate photovoltaics but are close to their
efficiency limits. This paper presents ideal limiting efficiencies for tandem
and triple junction multijunction solar cells subject only to the constraint of
the Si bandgap and therefore recommending optimum cell structures departing
from the single junction ideal. The use of III-V materials is considered, using
a novel growth method capable of yielding low defect density III-V layers on
Si. In order to evaluate the real potential of these proposed multijunction
designs, a quantitative model is presented, the strength of which is the joint
modelling of external quantum efficiency and current-voltage characteristics
using the same parameters. The method yields a single parameter fit in terms of
the Shockley-Read-Hall lifetime. This model is validated by fitting
experimental data of external quantum efficiency, dark current, and conversion
efficiency of world record tandem and triple junction cells under terrestrial
solar spectra without concentration. We apply this quantitative model to the
design of tandem and triple junction solar cells, yielding cell designs capable
of reaching efficiencies without concentration of 32% for the best tandem cell
and 36% for the best triple junction cell. This demonstrates that efficiencies
within a few percent of world records are realistically achievable without the
use of concentrating optics, with growth methods being developed for
multijunction cells combining III-V and Si materials.

###On Quantum Coherence Effects in Photo and Solar Cells||Kimberly Chapin,Konstantin Dorfman,Anatoly Svidzinsky,Marlan Scully###

On Quantum Coherence Effects in Photo and Solar Cells. We show that quantum coherence can increase the quantum efficiency of various
thermodynamic systems. For example, we can enhance the quantum efficiency for a
quantum dot photocell, a laser based solar cell and the photo-Carnot quantum
heat engine. Our results are fully consistent with the laws of thermodynamics
contrary to comments found in the paper of A.P. Kirk, Phys. Rev. Lett. 106,
048703 (2011).

###Heterojunction organic photovoltaic cells as molecular heat engines: A simple model for the performance analysis||Mario Einax,Marcel Dierl,Abraham Nitzan###

Heterojunction organic photovoltaic cells as molecular heat engines: A simple model for the performance analysis. Organic heterojunction solar cells are analyzed within a minimal model that
includes the essential physical features of such systems. The dynamical
properties of this model, calculated using a master equation approach, account
for the qualitative behavior of such systems. The model yields explicit results
for current-voltage behavior as well as performance characteristics expressed
in terms of the thermodynamic efficiency as well as the power conversion
efficiency at maximum power, making it possible to evaluate the optimal setup
for this device model.

###Microcavity effects on the generation, fluorescence, and diffusion of excitons in organic solar cells||G. Kozyreff,D. C. Urbanek,L. T. Vuong,O. Nieto-Silleras,J. Martorell###

Microcavity effects on the generation, fluorescence, and diffusion of excitons in organic solar cells. We compute the short-circuit diffusion current of excitons in an organic
solar cell, with special emphasis on fluorescence losses. The exciton diffusion
length is not uniform but varies with its position within the device, even with
moderate fluorescence quantum efficiency. With large quantum efficiencies, the
rate of fluorescence can be strongly reduced with proper choices of the
geometrical and dielectric parameters. In this way, the diffusion length can be
increased and the device performance significantly improved.

###Why Lead Methylammonium tri-IODIDE perovskite-based solar cells requires a mesoporous electron transporting scaffold (but not necessarily a hole conductor)||Eran Edri,Saar Kirmayer,Alex Henning,Sabyasachi Mukhopadhyay,Konstantin Gartsman,Yossi Rosenwaks,Gary Hodes,David Cahen###

Why Lead Methylammonium tri-IODIDE perovskite-based solar cells requires a mesoporous electron transporting scaffold (but not necessarily a hole conductor). CH3NH3PbI3-based solar cells were characterized with electron beam-induced
current (EBIC), and compared to CH3NH3PbI3-xClx ones. A spatial map of charge
separation efficiency in working cells shows p-i-n structures for both thin
film cells. Effective diffusion lengths, LD, (from EBIC profile) show that
holes are extracted significantly more efficiently than electrons in
CH3NH3PbI3, explaining why CH3NH3PbI3-based cells require mesoporous electron
conductors, while CH3NH3PbI3-xClx ones, where LD values are comparable for both
charge types, do not.

###Polarization-Engineered InGaN/GaN Heterojunctions for Photovoltaic Applications||Stylianos A. Kazazis,Elena Papadomanolaki,Eleftherios Iliopoulos###

Polarization-Engineered InGaN/GaN Heterojunctions for Photovoltaic Applications. The photovoltaic properties of (0001) n-InGaN/p-GaN single heterojunctions
were investigated numerically and compared with those of conventional
p-GaN/i-InGaN/n-GaN structures, employing realistic material parameters. This
alternative device architecture exploits the large polarization fields, and
high efficiency modules are achieved for In-rich, partially relaxed and
coherently strained InGaN films. Conversion efficiencies up to 14% under AM1.5G
illumination can be reached, revealing the true potential of InGaN single
junction solar cells with proper design.

###Growth and Photoelectrochemical Study of Germanium Sulphoselenide GeS$_{0.25}$Se$_{0.75}$ (I2) Crystals||Love Trivedi,Sandip Unadkat,Aastha Anish Patel###

Growth and Photoelectrochemical Study of Germanium Sulphoselenide GeS$_{0.25}$Se$_{0.75}$ (I2) Crystals. In the present investigation, the author has employed a Chemical Vapour
Transport (CVT) technique to grow the crystals of GeS$_{0.25}$Se$_{0.75}$ using
iodine as a transporting agent. The grown crystals were then characterized for
a Photoelectrochemical(PEC) study to find out solar parameters e.g. Fill Factor
(FF), Open Circuit Voltage (Voc), Short Circuit Current (Isc), and Efficiency
(n). The found results have been thoroughly described and implications have
been discussed.
  Keywords: Crystal growth, PEC solar cell, fill factor, efficiency, Solar
Energy

###Sn4+ Precursor Enables 12.4% Efficient Kesterite Solar Cell from DMSO Solution with Open Circuit Voltage Deficit Below 0.30 V||Yuancai Gong,Yifan Zhang,Erin Jedlicka,Rajiv Giridharagopal,James A. Clark,Weibo Yan,Chuanyou Niu,Ruichan Qiu,Jingjing Jiang,Shaotang Yu,Sanping Wu,Hugh W. Hillhouse,David S. Ginger,Wei Huang,Hao Xin###

Sn4+ Precursor Enables 12.4% Efficient Kesterite Solar Cell from DMSO Solution with Open Circuit Voltage Deficit Below 0.30 V. The limiting factor preventing kesterite (CZTSSe) thin film solar cell
performance further improvement is the large open-circuit voltage deficit
(Voc,def) issue, which is 0.345V for the current world record device with an
efficiency of 12.6%. In this work, SnCl4 and SnCl2_2H2O are respectively used
as tin precursor to investigate the Voc,def issue of dimethyl sulfoxide (DMSO)
solution processed CZTSSe solar cells. Different complexations of tin compounds
with thiourea and DMSO lead to different reaction pathways from solution to
absorber material and thus dramatic difference in photovoltaic performance. The
coordination of Sn2+ with Tu leads to the formation of SnS and ZnS and Cu2S in
the precursor film, which converted to selenides first and then fused to
CZTSSe, resulting in poor film quality and device performance. The highest
efficiency obtained from this film is 8.84% with a Voc,def of 0.391V. The
coordination of Sn4+ with DMSO facilitates direct formation ofkesterite CZTS
phase in the precursor film which directed converted to CZTSSe during
selenization, resulting in compositional uniform absorber and high device
performance. A device with active area efficiency 12.2% and a Voc,def of 0.344
V was achieved from Sn4+ solution processed absorber. Furthermore, CZTSSe/CdS
heterojunction heat treatment (JHT) significantly improved Sn4+ device
performance but had slightly negative effect on Sn2+ device. A champion CZTSSe
solar cell with a total area efficiency of 12.4% (active are efficiency 13.6%)
and low Voc,def of 0.297 V was achieved from Sn4+ solution. Our results
demonstrate the preformed uniform kesterite phase enabled by Sn4+ precursor is
the key in achieving highly efficient kesterite absorber material. The lowest
Voc-def and high efficiency achieved here shines new light on the future of
kesterite solar cell.

###On the energy conversion efficiency of the bulk photovoltaic effect||Andreas Pusch,Udo Römer,Dimitrie Culcer,Nicholas J. Ekins-Daukes###

On the energy conversion efficiency of the bulk photovoltaic effect. The bulk photovoltaic effect (BPVE) leads to directed photo-currents and
photo-voltages in bulk materials. Unlike photo-voltages in p-n junction solar
cells that are limited by carrier recombination to values below the bandgap
energy of the absorbing material, the BPVE photo-voltages have been shown to
greatly exceed the bandgap energy. Therefore the BPVE is not subject to the
Shockley-Queisser limit for sunlight to electricity conversion in single
junction solar cells and experimental claims of efficiencies beyond this limit
have been made. Here, we show that BPVE energy conversion efficiencies are, in
practice, orders of magnitude below the Shockley-Queisser limit of single
junction solar cells and are subject to different, more stringent limits. The
name BPVE stands for two different fundamental effects, the shift current and
the injection current. In both of these, the voltage bias necessary to produce
electrical energy, accelerates both, intrinsic and photo-generated, carriers.
We discuss how energy conservation alone fundamentally limits the BPVE to a
bandgap-dependent value that exceeds the Shockley Queisser limit only for very
small bandgaps. Yet, small bandgap materials have a large number of intrinsic
carriers, leading to high conductivity which suppresses the photo-voltage. We
discuss further how slightly more stringent fundamental limits for injection
(ballistic) currents may be derived from the trade-off between high
resistivity, needed for a high voltage, and long ballistic transport length,
needed for a high current. We also explain how erroneous experimental and
theoretical claims of high efficiency have arisen. Finally, we calculate the
energy conversion efficiency for an example 2D material that has been suggested
as candidate material for high efficiency BPVE based solar cells and show that
the efficiency is very similar to the efficiency of known 3D materials.

###Influence of Charge Carrier Mobility on the Performance of Organic Solar Cells||C. Deibel,A. Wagenpfahl,V. Dyakonov###

Influence of Charge Carrier Mobility on the Performance of Organic Solar Cells. The power conversion efficiency of organic solar cells based on
donor--acceptor blends is governed by an interplay of polaron pair dissociation
and bimolecular polaron recombination. Both processes are strongly dependent on
the charge carrier mobility, the dissociation increasing with faster charge
transport, with raised recombination losses at the same time. Using a
macroscopic effective medium simulation, we calculate the optimum charge
carrier mobility for the highest power conversion efficiency, for the first
time accounting for injection barriers and a reduced Langevin-type
recombination. An enhancement of the charge carrier mobility from
$10^{-8}$m$^2$/Vs for state of the art polymer:fullerene solar cells to about
$10^{-6}$m$^2$/Vs, which yields the maximum efficiency, corresponds to an
improvement of only about 20% for the given parameter set.

###First-principle calculation of solar cell efficiency under incoherent illumination||Michael Sarrazin,Aline Herman,Olivier Deparis###

First-principle calculation of solar cell efficiency under incoherent illumination. Because of the temporal incoherence of sunlight, solar cells efficiency
should depend on the degree of coherence of the incident light. However,
numerical computation methods, which are used to optimize these devices,
fundamentally consider fully coherent light. Hereafter, we show that the
incoherent efficiency of solar cells can be easily analytically calculated. The
incoherent efficiency is simply derived from the coherent one thanks to a
convolution product with a function characterizing the incoherent light. Our
approach is neither heuristic nor empiric but is deduced from first-principle,
i.e. Maxwell's equations. Usually, in order to reproduce the incoherent
behavior, statistical methods requiring a high number of numerical simulations
are used. With our method, such approaches are not required. Our results are
compared with those from previous works and good agreement is found.

###Device Engineering of Perovskite Solar Cells to Achieve Near Ideal Efficiency||Sumanshu Agarwal,Pradeep R. Nair###

Device Engineering of Perovskite Solar Cells to Achieve Near Ideal Efficiency. Despite the exciting recent research on perovskite based solar cells, the
design space for further optimization and the practical limits of efficiency
are not well known in the community. In this manuscript, we address these
aspects through theoretical calculations and detailed numerical simulations.
Here, we first provide the detailed balance limit efficiency in the presence of
radiative and Auger recombination. Then, using coupled optical and carrier
transport simulations, we identify the physical mechanisms that contribute
towards bias dependent carrier collection, and hence low fill factors of
current perovskite based solar cells. Curiously, we find that while Auger
recombination is not a dominant factor at the detailed balance limit, it plays
a significant role in device level implementations. Surprisingly, our device
designs indicate that it is indeed possible to achieve efficiency and fill
factor greater than 25% and 85%, respectively - even in the presence of Auger
recombination.

###Dipole-field-assisted charge extraction in metal-perovskite-metal back-contact solar cells||Xiongfeng Lin,Askhat N. Jumabekov,Niraj N. Lal,Alexander R. Pascoe,Daniel E. Gomez,Noel W. Duffy,Anthony S. R. Chesman,Kallista Sears,Maxime Fournier,Yupeng Zhang,Qiaoliang Bao,Yibing Cheng,Leone Spiccia,Udo Bach###

Dipole-field-assisted charge extraction in metal-perovskite-metal back-contact solar cells. Hybrid organic-inorganic halide perovskites are low-cost solution-processable
solar cell materials with photovoltaic properties that rival those of
crystalline silicon. The perovskite films are typically sandwiched between thin
layers of hole and electron transport materials, which efficiently extract
photogenerated charges. This affords high-energy conversion efficiencies but
results in significant performance and fabrication challenges. Herein we
present a simple charge transport layer-free perovskite solar cell (PSC),
comprising only a perovskite layer with two interdigitated gold back-contacts.
Charge extraction is achieved via self-assembled molecular monolayers (SAMs)
and their associated dipole fields at the metal/perovskite interface.
Photovoltages of approximately 600 mV generated by SAM-modified PSCs are
equivalent to the built-in potential generated by individual dipole layers.
Efficient charge extraction results in photocurrents of up to 12.1 mA/cm2 under
simulated sunlight, despite a large electrode spacing.

###Semi-Transparent Solar Cell enabled by Frequency Selective Light Trapping||Duncan C. Wheeler,Yichen Shen,Yi Yang,Svetlana V. Boriskina,Yi Huang,Ognjen Ilic,Gang Chen,Marin Soljacic###

Semi-Transparent Solar Cell enabled by Frequency Selective Light Trapping. We propose a frequency selective light trapping scheme that enables the
creation of more visually-transparent and yet simultaneously more efficient
semitransparent solar cells. A nanoparticle scattering layer and photonic stack
back reflector create a selective trapping effect by total internal reflection
within a medium, increasing absorption of IR light. We propose a strong
frequency selective scattering layer using spherical TiO2 nanoparticles with
radius of 255 nm and area density of 1.1% in a medium with index of refraction
of 1.5. Using detailed numerical simulations for this configuration, we find
that it is possible to create a semitransparent silicon solar cell that has a
Shockley Queisser efficiency of 12.0%\pm0.4% with a visible transparency of
60.2%\pm1.3%, 13.3%\pm1.3 more visibly-transparent than a bare silicon cell at
the same efficiency.

###Upper limit to the photovoltaic efficiency of imperfect crystals||Sunghyun Kim,José A. Márquez,Thomas Unold,Aron Walsh###

Upper limit to the photovoltaic efficiency of imperfect crystals. The Shockley-Queisser (SQ) limit provides a convenient metric for predicting
light-to-electricity conversion efficiency of a solar cell based on the band
gap of the light-absorbing layer. In reality, few materials approach this
radiative limit. We develop a formalism and a computational method to predict
the maximum photovoltaic efficiency of imperfect crystals from first
principles. Our scheme includes equilibrium populations of native defects,
their carrier-capture coefficients, and the associated recombination rates.
When applied to kesterite solar cells, we reveal an intrinsic limit of 20% for
$\mathrm{Cu_2ZnSnSe_4}$, which falls far below the SQ limit of 32%. The effects
of atomic substitution and extrinsic doping are studied, leading to pathways
for enhanced efficiency of 31%. This approach can be applied to support
targeted-materials selection for future solar-energy technologies.

###A General Approach to High Efficiency Perovskite Solar Cells by Any Antisolvent||Alexander D. Taylor,Qing Sun,Katelyn P. Goetz,Qingzhi An,Tim Schramm,Yvonne Hofstetter,Maximillian Litterst,Fabian Paulus,Yana Vaynzof###

A General Approach to High Efficiency Perovskite Solar Cells by Any Antisolvent. Deposition of perovskite thin films by antisolvent engineering is one of the
most common methods employed in perovskite photovoltaics research. Herein, we
report on a general method that allows the fabrication of highly efficient
perovskite solar cells by any antisolvent via the manipulation of the
antisolvent application rate. Through a detailed structural, compositional and
microstructural characterization of perovskite layers fabricated by 14
different antisolvents, we identify two key factors that influence the quality
of the perovskite active layer: the solubility of the organic precursors in the
antisolvent and its miscibility with the host solvent(s) of the perovskite
precursor solution. Depending on these two factors, each antisolvent can be
utilized to produce high performance devices reaching power conversion
efficiencies (PCEs) that exceed 21%. Moreover, we demonstrate that by employing
the optimal antisolvent application procedure, highly efficient solar cells can
be fabricated from a broad range of precursor stoichiometries, with either a
significant excess or deficiency of organic iodides.

###Ab initio calculation of the detailed balance limit to the photovoltaic efficiency of single p-n junction kesterite solar cells||Sunghyun Kim,Aron Walsh###

Ab initio calculation of the detailed balance limit to the photovoltaic efficiency of single p-n junction kesterite solar cells. The thermodynamic limit of photovoltaic efficiency for a single-junction
solar cell can be readily predicted using the bandgap of the active light
absorbing material. Such an approach overlooks the energy loss due to
non-radiative electron-hole processes. We propose a practical ab initio
procedure to determine the maximum efficiency of a thin-film solar cell that
takes into account both radiative and non-radiative recombination. The required
input includes the frequency-dependent optical absorption coefficient, as well
as the capture cross-sections and equilibrium populations of point defects. For
kesterite-structured Cu$_2$ZnSnS$_4$, the radiative limit is reached for a film
thickness of around 2.6 micrometer, where the efficiency gain due to light
absorption is counterbalanced by losses due to the increase in recombination
current.

###Detailed study of N,N'-(diisopropylphenyl)- terrylene-3,4:11,12-bis(dicarboximide) as electron acceptor for solar cells application||Julien Gorenflot,Andreas Sperlich,Andreas Baumann,Daniel Rauh,Aleksey Vasilev,Chen Li,Martin Baumgarten,Carsten Deibel,Vladimir Dyakonov###

Detailed study of N,N'-(diisopropylphenyl)- terrylene-3,4:11,12-bis(dicarboximide) as electron acceptor for solar cells application. We report on terrylene-3,4:11,12-bis(dicarboximide) (TDI) as electron
acceptor for bulk-heterojunction solar cells using poly(3-hexyl thiophene)
(P3HT) as complementary donor component. Enhanced absorption was observed in
the blend compared to pure P3HT. As shown by the very efficient
photoluminescence (PL) quenching, the generated excitons are collected at the
interface between the donor and acceptor, where they separate into charges
which we detect by photoinduced absorption and electron-spin resonance (ESR).
Time-of-flight (TOF) photoconductivity measurements reveal a good electron
mobility of 10-3 cm2 V-1 s-1 in the blend. Nevertheless, the photocurrent in
solar cells was found to be surprisingly low. Supported by the external quantum
efficiency (EQE) spectrum as well as morphological studies by way of X-ray
diffraction and atomic force microscopy, we explain our observation by the
formation of a TDI hole blocking layer at the anode interface which prevents
the efficiently generated charges to be extracted.

###Study of simulations of double graded InGaN solar cell structures||Mirsaeid Sarollahi,Manal A. Aldawsari,Rohith Allaparthi,Malak A. Refaei,Reem Alhelais,Md Helal Uddin Maruf,Yuriy Mazur,Morgan E. Ware###

Study of simulations of double graded InGaN solar cell structures. The performances of various configurations of InGaN solar cells are compared
using nextnano software. Here we compare a flat base graded wall GaN/InGaN
structure, with an InxGa1-xN well with sharp GaN contact layers, and an
InxGa1-xN structure with InxGa1-xN contact layers, i.e. a homojunction. The
doping in the graded structures are the result of polarization doping at each
edge (10 nm from each side) due to the graded structure, while the well
structures are intentionally doped at each edge (10 nm from each side) equal to
the doping concentration in the graded structure. The solar cells are
characterized by their open-circuit voltage, V_oc, short circuit current, I_sc,
solar efficiency, and energy band diagram. The results indicate that an
increase in I_sc and efficiency results from increasing both the fixed and the
maximum indium compositions, while the V_oc decreases. The maximum efficiency
is obtained for the InGaN well with 60% In.

###Flexibility-assisted heat removal in thin crystalline silicon solar cells||Seok Jun Han,Pauls Stradins,Sang M. Han,Sang Eon Han###

Flexibility-assisted heat removal in thin crystalline silicon solar cells. Thin crystalline silicon solar photovoltaics holds great potential for
reducing the module price by material saving and increasing the efficiency by
reduced bulk recombination loss. However, the module efficiency decreases
rather sensitively as the module temperature rises under sunlight. Effective,
inexpensive approach to cooling modules would accelerate large-scale market
adoption of thin crystalline silicon photovoltaics. For effective cooling, we
exploit high flexibility of single-crystalline thin silicon films to create
wavy solar cells. These wavy cells possess larger surface area than
conventional flat cells, while occupying the same projected area. We
experimentally demonstrate that the temperature of thin wavy crystalline
silicon solar cells under the sunlight can be significantly reduced by
increased convective cooling due to their large surface area. The substantial
efficiency gain, achieved by the effective heat removal, points to
high-performance thin crystalline silicon photovoltaic systems that are
radically different in configuration from conventional systems.

###Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading||Faiz Ahmad,Tom H. Anderson,Peter B. Monk,Akhlesh Lakhtakia###

Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading. The power conversion efficiency of an ultrathin CIGS solar cell was maximized
using a coupled optoelectronic model to determine the optimal bandgap grading
of the nonhomogeneous CIGS layer in the thickness direction. The bandgap of the
CIGS layer was either sinusoidally or linearly graded, and the solar cell was
modeled to have a metallic backreflector corrugated periodically along a fixed
direction in the plane. The model predicts that specially tailored bandgap
grading can significantly improve the efficiency, with much smaller
improvements due to the periodic corrugations. An efficiency of 27.7% with the
conventional 2200-nm-thick CIGS layer is predicted with sinusoidal bandgap
grading, in comparison to 22% efficiency obtained experimentally with
homogeneous bandgap. Furthermore, the inclusion of sinusoidal grading increases
the predicted efficiency to 22.89% with just a 600-nm-thick CIGS layer. These
high efficiencies arise due to a large electron-hole-pair generation rate in
the narrow-bandgap regions and the elevation of the open-circuit voltage due to
a wider bandgap in the region toward the front surface of the CIGS layer. Thus,
bandgap nonhomogeneity, in conjunction with periodic corrugation of the
backreflector, can be effective in realizing ultrathin CIGS solar cells that
can help overcome the scarcity of indium.

###Designing a Concentrated High-Efficiency Thermionic Solar Cell Enabled by Graphene Collector||Xin Zhang,Xiaohang Chen,Jinchan Chen,Lay Kee Ang,Yee Sin Ang###

Designing a Concentrated High-Efficiency Thermionic Solar Cell Enabled by Graphene Collector. We propose a concentrated thermionic emission solar cell design, which
demonstrates a high solar-to-electricity energy conversion efficiency larger
than 10\% under 600 sun, by harnessing the exceptional electrical, thermal and
radiative properties of the graphene as a collector electrode. By constructing
an analytical model that explicitly takes into account the non-Richardson
behavior of the thermionic emission current from graphene, space charge effect
in vacuum gap, and the various irreversible energy losses within the
subcomponents, we perform a detailed characterization on the conversion
efficiency limit and electrical power output characteristics of the proposed
system. We systematically model and compare the energy conversion efficiency of
various configurations of graphene-graphene and graphene-diamond and
diamond-diamond thermionic emitter, and show that utilizing diamond films as an
emitter and graphene as a collector offers the highest maximum efficiency, thus
revealing the important role of graphene in achieving high-performance
thermionic emission solar cell. A maximum efficiency of 12.8\% under 800 sun
has been revealed, which is significantly higher than several existing
solid-state solar cell designs, such as the solar-driven thermoelectric and
thermophotovoltaic converters. Our work thus opens up new avenues to advance
the efficiency limit of thermionic solar energy conversion and the development
of next-generation novel-nanomaterial-based solar energy harvesting technology.

###Design guidelines for efficient plasmonic solar cells exploiting the trade-off between scattering and metallic absorption||Xiaofeng Li,Nicholas P. Hylton,Vincenzo Giannini,Ned J. Ekins-Daukes,Stefan A. Maier###

Design guidelines for efficient plasmonic solar cells exploiting the trade-off between scattering and metallic absorption. We report on the role of plasmonic resonances in determining the delicate
balance between scattering and absorption of light in nanometric particle
arrays applied to the front surface of solar cells. Strong parasitic absorption
is shown to be dependent upon the excitation of localized surface plasmon
resonances and prohibits efficient scattering into the underlying
semiconductor. Via detailed analytical and numerical investigations we obtain
the dependence of scattering and absorption in nanoparticles upon their complex
refractive index. These results provide an insight into the optimum material
properties required to minimize parasitic optical absorption, while maintaining
high scattering cross-section efficiency, thus providing a general design
guideline for efficient light trapping with scattering nanoparticles. The work
is extended to include comprehensive optoelectronic simulations of plasmonic
solar cells in which the scattering metals are made from either Au, Ag or Al.
We show that Al particles provide the closest approximation to the optimized
particle refractive index and therefore exhibit the smallest parasitic
absorption and correspondingly lead to the greatest solar cell efficiency
enhancements. Indeed, for the Al particles we report a full-band enhancement of
external quantum efficiency over the reference device.

###Solar water splitting: efficiency discussion||Jurga Juodkazyte,Gediminas Seniutinas,Benjaminas Sebeka,Irena Savickaja,Tadas Malinauskas,Kazimieras Badokas,Kestutis Juodkazis,Saulius Juodkazis###

Solar water splitting: efficiency discussion. The current state of the art in direct water splitting in
photo-electrochemical cells (PECs) is presented together with: (i) a case study
of water splitting using a simple solar cell with the most efficient water
splitting electrodes and (ii) a detailed mechanism analysis. Detailed analysis
of the energy balance and efficiency of solar hydrogen production are
presented. The role of hydrogen peroxide formation as an intermediate in oxygen
evolution reaction is newly revealed and explains why an oxygen evolution is
not taking place at the thermodynamically expected 1.23 V potential.
  Solar hydrogen production with electrical-to-hydrogen conversion efficiency
of 52% is demonstrated using a simple ~0.7%-efficient n-Si/Ni Schottky solar
cell connected to a water electrolysis cell. This case study shows that
separation of the processes of solar harvesting and electrolysis avoids
photo-electrode corrosion and utilizes optimal electrodes for hydrogen and
oxygen evolution reactions and achieves ~10% efficiency in light-to-hydrogen
conversion with a standard 18% efficient household roof Si-solar cells.

###Investigating the effect of different quantum dots on the absorption spectrum and characteristics of quantum dot sensitized solar cells||Hossein Vahid Dastjerdi,Hamidreza Fallah,Morteza Hajimahmoodzadeh###

Investigating the effect of different quantum dots on the absorption spectrum and characteristics of quantum dot sensitized solar cells. Quantum dot sensitized solar cells are among the new generations of solar
cells that have attracted much attention. Theoretical and simulation studies
have predicted high efficiency for these cells so that in the future, these
cells could be an excellent alternative to silicon solar cells. Other
advantages of these cells are their ease of fabrication and cheaper
manufacturing methods than existing cells. This paper's main idea is to
simulate the effect of different quantum dots on the optical and electrical
characteristics of these cells and, in particular, the efficiency. We then
simulated the effect of simultaneous sensitizing by different quantum dots, and
we observed that the cell's light absorption and the efficiency in simultaneous
sensitizing, increased. Then we experimentally studied one of the cells that
give the best simulation result (PbS/CdS co-sensitized). We deposited the
quantum dots on transparent TiO2, and we obtained the light absorption,
efficiency, and other characteristics of cells. Further, we investigated the
effect of cobalt sulfide as the counter electrode in PbS/CdS, instead of
platinum and gold, and we found that the efficiency has increased.

###Design and characterization of effective solar cells||Varun Ojha,Giorgio Jansen,Andrea Patane,Antonino La Magna,Vittorio Romano,Giuseppe Nicosia###

Design and characterization of effective solar cells. We propose a two-stage multi-objective optimization framework for full scheme
solar cell structure design and characterization, cost minimization and quantum
efficiency maximization. We evaluated structures of 15 different cell designs
simulated by varying material types and photodiode doping strategies. At first,
non-dominated sorting genetic algorithm~II (NSGA-II) produced
Pareto-optimal-solutions sets for respective cell designs. Then, on
investigating quantum efficiencies of all cell designs produced by NSGA-II, we
applied a new multi-objective optimization algorithm~II (OptIA-II) to discover
the Pareto fronts of select (three) best cell designs. Our designed OptIA-II
algorithm improved the quantum efficiencies of all select cell designs and
reduced their fabrication costs. We observed that the cell design comprising an
optimally doped zinc-oxide-based transparent conductive oxide (TCO) layer and
rough silver back reflector (BR) offered a quantum efficiency ($Q_e$) of
$0.6031.$ Overall, this paper provides a full characterization of cell
structure designs. It derives a relationship between quantum efficiency, $Q_e$
of a cell with its TCO layer's doping methods and TCO and BR layer's material
types. Our solar cells design characterization enables us to perform a
cost-benefit analysis of solar cells usage in real-world applications

###Thermoelectrical Field Effects in Low Dimensional Structure Solar Cells||Stefan Kettemann,Jean-Francois Guillemoles###

Thermoelectrical Field Effects in Low Dimensional Structure Solar Cells. Taking into account the temperature gradients in solar cells, it is shown
that their efficiency can be increased beyond the Shockley-Queisser limit (J.
Appl. Phys. 32 (1961) 510). The driving force for this gain is the temperature
gradient between this region and its surroundings.
  A quantitative theory is given. Though the effect is found to be weak in
conventional solar cells, it is argued that it can be substantially increased
by proper choice of materials and design of the device. In particular, it is
shown that the insertion of a quantum well can enhance the efficiency beyond
one of the single gap cell, due to the presence of temperature jumps at the
heterojunctions.

###Polymer-Fullerene Bulk Heterojunction Solar Cells||Carsten Deibel,Vladimir Dyakonov###

Polymer-Fullerene Bulk Heterojunction Solar Cells. Organic solar cells have the potential to be low-cost and efficient solar
energy converters, with a promising energy balance. They are made from
carbon-based semiconductors, which exhibit favourable light absorption and
charge generation properties, and can be manufactured by low temperature
processes such as printing from solvent-based inks, which are compatible with
flexible plastic substrates or even paper. In this review, we will present an
overview of the physical function of organic solar cells, their
state-of-the-art performance and limitations, as well as novel concepts to
achieve a better material stability and higher power conversion efficiencies.
We will also briefly review processing and cost in view of the market
potential.

###Carrier Multiplication in Graphene||Torben Winzer,Andreas Knorr,Ermin Malic###

Carrier Multiplication in Graphene. Graphene as a zero-bandgap semiconductor is an ideal model structure to study
the carrier relaxation channels, which are inefficient in conventional
semiconductors. In particular, it is of fundamental interest to address the
question whether Auger-type processes significantly influence the carrier
dynamics in graphene. These scattering channels bridge the valence and
conduction band allowing carrier multiplication - a process that generates
multiple charge carriers from the absorption of a single photon. This has been
suggested in literature for improving the efficiency of solar cells. Here we
show, based on microscopic calculations within the density matrix formalism,
that Auger processes do play an unusually strong role for the relaxation
dynamics of photo-excited charge carriers in graphene. We predict that a
considerable carrier multiplication takes place, suggesting graphene as a new
material for high-efficiency solar cells and for high-sensitivity
photodetectors.

###Reduced Coulomb interaction in organic solar cells by the introduction of inorganic high-k nanostructured materials||Miriam Engel,David Schaefer,Daniel Erni,Niels Benson,Roland Schmechel###

Reduced Coulomb interaction in organic solar cells by the introduction of inorganic high-k nanostructured materials. In this article a concept is introduced, which allows for reduced Coulomb
interaction in organic solar cells and as such for enhanced power conversion
efficiencies. The concept is based on the introduction of electrically
insulating, nanostructured high-k materials into the organic matrix, which do
not contribute to the charge transport, however, effectively enhance the
permittivity of the organic active layer and thereby reduce the Coulomb
interaction. Using an analytical model it is demonstrated that even at a
distance of 20 nm to the organic / inorganic interface of the nanostructure,
the Coulomb interaction can be reduced by more than 15 %. The concept is
implemented using P3HT:PCBM solar cells with integrated high-k nanoparticles
(strontium titanate). It could be demonstrated that in comparison to a
reference cell without integrated nanoparticles, the power conversion
efficiencies could be improved by ~20 %.

###Modification of band alignment at interface of AlyGa1-ySb/AlxGa1-xAs type-II quantum dots by concentrated sunlight in intermediate band solar cells with separated absorption and depletion regions||A. Kechiantz,A. Afanasev,J. -L. Lazzari###

Modification of band alignment at interface of AlyGa1-ySb/AlxGa1-xAs type-II quantum dots by concentrated sunlight in intermediate band solar cells with separated absorption and depletion regions. We propose a new intermediate band GaAs solar cell comprising an AlxGa1-xAs
absorber with built-in GaSb type-II quantum dots (QDs) [a gradual AlxGa1-xAs
absorber with built-in AlyGa1-ySb QDs (0<x=y<0.40) as a variant] separated from
the depletion region. We study the modification of the band alignment at
type-II interface by two-photon absorption of concentrated sunlight. Our
calculation shows that photogenerated carriers produce localized exciton-like
electron-hole pairs spatially separated at QDs. Local field of such pairs may
essentially modify potential barrier surrounding QDs, increase recombination
lifetime of mobile carriers and additional photocurrent generated by twophoton
absorption. Concentration of about 300-sun pushes by 15% up the conversion
efficiency as compared to the efficiency of the reference single junction GaAs
solar cell without QDs.

###Study on the Fermi level of microstructured Silicon with impurities introduced by chalcogenides and their affect on solar cell efficiency||Huili He,Changshui Chen,Fang Wang,Songhao Liu###

Study on the Fermi level of microstructured Silicon with impurities introduced by chalcogenides and their affect on solar cell efficiency. Microstructured Silicon, which is obtained by irradiating the surface of a
Silicon wafer with femtosecond laser pulses under certain circumstances, has
unusual optical properties such as the strong absorption of light with
wavelength from 0.25{\mu}m to 17{\mu}m. So it holds great promise in the
intermediate band solar cell (IBSC). Some articles have discussed the
electronic structure associating with simple substitutional impurities in
Silicon introduced by chalcogenides. And on this basis, after high temperature
annealing treatment, we establish the mode of impurity levels of
microstructured Silicon introduced by sulfur and oxygen. Using generalized
statistics of multi-level,we analyze the probability of electronic in all local
energy levels and the relationship among Fermi level, temperature and the
density of impurities. Then the theoretical conversion efficiency of the
corresponding IBSC is discussed with the Detailed Balance Theory. And the issue
of making high efficiency solar cells based on femtosecond laser
microstructured Silicon is discussed in detail.

###Organic-inorganic Copper(II)-based Material: a Low-Toxic, Highly Stable Light Absorber beyond Organolead Perovskites||Xiaolei Li,Xiangli Zhong,Yue Hu,Bochao Li,Yusong Sheng,Yang Zhang,Chao Weng,Ming Feng,Hongwei Han,Jinbin Wang###

Organic-inorganic Copper(II)-based Material: a Low-Toxic, Highly Stable Light Absorber beyond Organolead Perovskites. Lead halide perovskite solar cells have recently emerged as a very promising
photovoltaic technology due to their excellent power conversion efficiencies;
however, the toxicity of lead and the poor stability of perovskite materials
remain two main challenges that need to be addressed. Here, for the first time,
we report a lead-free, highly stable C6H4NH2CuBr2I compound. The C6H4NH2CuBr2I
films exhibit extraordinary hydrophobic behavior with a contact angle of
approximately 90 degree, and their X-ray diffraction patterns remain unchanged
even after four hours of water immersion. UV-Vis absorption spectrum shows that
C6H4NH2CuBr2I compound has an excellent optical absorption over the entire
visible spectrum. We applied this copper-based light absorber in printable
mesoscopic solar cell for the initial trial and achieved a power conversion
efficiency of 0.5%. Our study represents an alternative pathway to develop
low-toxic and highly stable organic-inorganic hybrid materials for photovoltaic
application.

###Exceeding the Shockley-Queisser limit within the detailed balance framework||Marnik Bercx,Rolando Saniz,Bart Partoens,Dirk Lamoen###

Exceeding the Shockley-Queisser limit within the detailed balance framework. The Shockley-Queisser limit is one of the most fundamental results in the
field of photovoltaics. Based on the principle of detailed balance, it defines
an upper limit for a single junction solar cell that uses an absorber material
with a specific band gap. Although methods exist that allow a solar cell to
exceed the Shockley-Queisser limit, here we show that it is possible to exceed
the Shockley-Queisser limit without considering any of these additions. Merely
by introducing an absorptivity that does not assume that every photon with an
energy above the band gap is absorbed, efficiencies above the Shockley-Queisser
limit are obtained. This is related to the fact that assuming optimal
absorption properties also maximizes the recombination current within the
detailed balance approach. We conclude that considering a finite thickness for
the absorber layer allows the efficiency to exceed the Shockley-Queisser limit,
and that this is more likely to occur for materials with small band gaps.

###Direct Observation of Sub-picosecond Hole Injection from Lead Halide Perovskite by Differential Transient Transmission Spectroscopy||Kunie Ishioka,Bobby G. Barker Jr.,Masatoshi Yanagida,Yasuhiro Shirai,Kenjiro Miyano###

Direct Observation of Sub-picosecond Hole Injection from Lead Halide Perovskite by Differential Transient Transmission Spectroscopy. Efficient charge separation at the interfaces between the perovskite and with
the carrier transport layers is crucial for perovskite solar cells to achieve
high power conversion efficiency. We systematically investigate the hole
injection dynamics from MAPbI$_3$ perovskite to three typical hole transport
materials (HTMs) PEDOT:PSS, PTAA and NiO$_x$ by means of pump-probe
transmission measurements. We photoexcite only near the MAPbI$_3$/HTM interface
or near the back surface, and measure the differential transient transmission
between the two excitation configurations to extract the carrier dynamics
directly related to the hole injection. The differential transmission signals
directly monitor the hole injections to PTAA and PEDOT:PSS being complete
within 1 and 2 ps, respectively, and that to NiO$_x$ exhibiting an additional
slow process of 40 ps time scale. The obtained injection dynamics are discussed
in comparison with the device performance of the solar cells containing the
same MAPbI$_3$/HTM interfaces.

###Ultrafast polarization control of zero-bias photocurrent and terahertz emission in hybrid organic perovskites||Petr A. Obraztsov,Dmitry Lyashenko,Pavel A. Chizhov,Kuniaki Konishi,Natsuki Nemoto,Makoto Kuwata-Gonokami,Eric Welch,Alexander N. Obraztsov,Alex Zakhidov###

Ultrafast polarization control of zero-bias photocurrent and terahertz emission in hybrid organic perovskites. Methylammonium lead iodide (MAPI) is a benchmark hybrid organic perovskite
material, which is used for the low-cost, printed solar cells with over 20
percent power conversion efficiency. Yet, the nature of light-matter
interaction in MAPI as well as the exact physical mechanism behind device
operation is currently debated. Here we report room temperature, ultrafast
photocurrent and freespace terahertz (THz) emission generation from unbiased
MAPI induced by 150 fs light pulses. Polarization dependence of the observed
photoresponse is consistent with the Bulk Photovoltaic Effect (BPVE) caused by
a combination of injection and shift currents. We believe that this observation
of can shed light on low recombination, and long carrier diffusion lengths due
to indirect bandgap. Moreover, ballistic by nature shift and injection BPVE
photocurrents may enable third generation perovskite solar cells with
efficiency that exceed the Shockley_Queisser limit. Our observations also open
new venues for perovskite spintronics and tunable THz sources.

###Role of contact work function, back surface field and conduction band offset in CZTS solar cell||Atul Kumar,Ajay D. Thakur###

Role of contact work function, back surface field and conduction band offset in CZTS solar cell. We employ simulation based approach for enhancing the efficiency of Cu2ZnSnS4
(CZTS) based solar cells. Initial benchmarking of simulation with the
experimentally reported solar cell in literature is performed by incorporating
a suitable defect model. We then explore the effects of: (a) conduction band
offset (CBO) at CZTS/CdS junction, (b) back surface field (BSF) due to an
additional layer with higher carrier density, and (c) high work function back
contact. Efficiency is observed to improve by about 70% upon optimizing the
above three parameters. We also observe that utilizing BSF in the configuration
can reduce the high work function requirement of the back contact. A work
function of 5.2 eV (e.g., using Ni), a BSF layer (e.g., using SnS), and a CBO
of 0.1 eV (e.g., using ZnS) constitute an optimal configuration.

###Device physics of van der Waals heterojunction solar cells||Marco M. Furchi,Florian Höller,Lukas Dobusch,Dmitry K. Polyushkin,Simone Schuler,Thomas Mueller###

Device physics of van der Waals heterojunction solar cells. Heterostructures based on atomically thin semiconductors are considered a
promising emerging technology for the realization of ultrathin and ultralight
photovoltaic solar cells on flexible substrates. Much progress has been made in
recent years on a technological level, but a clear picture of the physical
processes that govern the photovoltaic response remains elusive. Here, we
present a device model that is able to fully reproduce the current-voltage
characteristics of type-II van der Waals heterojunctions under optical
illumination, including some peculiar behaviors such as exceedingly high
ideality factors or bias-dependent photocurrents. While we find the spatial
charge transfer across the junction to be very efficient, we also find a
considerable accumulation of photogenerated carriers in the active device
region due to poor electrical transport properties, giving rise to significant
carrier recombination losses. Our results are important to optimize future
device architectures and increase power conversion efficiencies of atomically
thin solar cells.

###Novel high efficiency quadruple junction solar cell with current matching and quantum efficiency simulations||Mohammad Jobayer Hossain,Bibek Tiwari,Indranil Bhattacharya###

Novel high efficiency quadruple junction solar cell with current matching and quantum efficiency simulations. A high theoretical efficiency of 47.2% was achieved by a novel combination of
In0.51Ga0.49P, GaAs, In0.24Ga0.76As and In0.19Ga0.81Sb subcell layers in a
simulated quadruple junction solar cell under 1 sun concentration. The
electronic bandgap of these materials are 1.9 eV, 1.42 eV, 1.08 eV and 0.55 eV
respectively. This unique arrangement enables the cell absorb photons from
ultraviolet to deep infrared wavelengths of the sunlight. Emitter and base
thicknesses of the subcells and doping levels of the materials were optimized
to maintain the same current in all the four junctions and to obtain the
highest conversion efficiency. The short-circuit current density, open circuit
voltage and fill factor of the solar cell are 14.7 mA/cm2, 3.38 V and 0.96
respectively. In our design, we considered 1 sun, AM 1.5 global solar spectrum.

###Diffusive external light-trap for solar cells||Ido Frenkel,Shilpi Shital,Avi Niv###

Diffusive external light-trap for solar cells. The ability to absorb light is indispensable for high efficient solar power
generation. This places conflicting requirements on the structure of a solar
cell: On one hand, it needs to have thick active layers to absorb more of the
available sunlight while on the other it needs thinner ones for better charge
transport. This dilemma stands in the way of any semiconductor from ever
achieving its full potential as a solar cell material. Recently, external
light-traps have emerged as a cost-effective solution for this dilemma by being
able to decouple the optics from the electronic aspects of the power-generating
process in the cell. In this paper, we study the effectiveness of external
light-traps with diffusive inner reflecting walls. Results indicate that given
the correct design, external light-traps may present a genuine opportunity for
high-efficiency cost-effective solar power production.

###Pitfalls and prospects of optical spectroscopy to characterize perovskite-transport layer interfaces||Eline M. Hutter,Thomas Kirchartz,Bruno Ehrler,David Cahen,Elizabeth von Hauff###

Pitfalls and prospects of optical spectroscopy to characterize perovskite-transport layer interfaces. Perovskite photovoltaics has witnessed an unprecedented increase in power
conversion efficiency over the last decade. The choice of transport layers,
through which photo-generated electrons and holes are transported to the
electrodes, is a crucial factor for further improving both the device
performance and stability. In this perspective, we critically examine the
application of optical spectroscopy to characterize the quality of the
transport layer-perovskite interface. We highlight the power of complementary
studies that use both continuous wave (cw) and time-resolved photoluminescence
(PL) to understand non-radiative losses, and additional transient
spectroscopies for characterizing the potential for loss-less carrier
extraction at the solar cell interfaces. Based on this discussion, we make
recommendations on how to extrapolate results from optical measurements to
assess the quality of a transport layer, and its impact on solar cell
efficiency.

###Triplet-sensitization by lead halide perovskite thin films for near-infrared-to-visible upconversion||Lea Nienhaus,Juan-Pablo Correa-Baena,Sarah Wieghold,Markus Einzinger,Ting-An Lin,Katherine E. Shulenberger,Nathan D. Klein,Mengfei Wu,Vladimir Bulovic,Tonio Buonassisi,Marc A. Baldo,Moungi G. Bawendi###

Triplet-sensitization by lead halide perovskite thin films for near-infrared-to-visible upconversion. Lead halide-based perovskite thin films have attracted great attention due to
the explosive increase in perovskite solar cell efficiencies. The same
optoelectronic properties that make perovskites ideal absorber materials in
solar cells are also beneficial in other light-harvesting applications and make
them prime candidates as triplet sensitizers in upconversion via
triplet-triplet annihilation in rubrene. In this contribution, we take
advantage of long carrier lifetimes and carrier diffusion lengths in perovskite
thin films, their high absorption cross sections throughout the visible
spectrum, as well as the strong spin-orbit coupling owing to the abundance of
heavy atoms to sensitize the upconverter rubrene. Employing bulk perovskite
thin films as the absorber layer and spin-mixer in inorganic/organic
heterojunction upconversion devices allows us to forego the additional
tunneling barrier owing from the passivating ligands required for colloidal
sensitizers. Our bilayer device exhibits an upconversion efficiency in excess
of 3% under 785 nm illumination.

###Rationalizing the influence of tunable energy levels on quantum efficiency to design optimal non-fullerene acceptor-based ternary organic solar cells||Safakath Karuthedath,Sri H. K . Paleti,Anirudh Sharma,Hang Yin,Catherine S. P. De Castro,Si Chen,Han Xi,Nisreen Alshehri,Nicolas Ramos,Jafar I. Khan,Jaime Martin,Gang Li,Frédéric Laquai,Derya Baran,Julien Gorenflot###

Rationalizing the influence of tunable energy levels on quantum efficiency to design optimal non-fullerene acceptor-based ternary organic solar cells. Non-fullerene acceptor (NFA)-based ternary bulk heterojunction solar cells
(TSC) are the most efficient organic solar cells (OSCs) today due to their
broader absorption and quantum efficiencies (QE) often surpassing those of
corresponding binary blends. We study how the energetics driving charge
transfer at the electron donor:electron acceptor (D/A) interfaces impact the QE
in blends of PBDB-T-2F donor with several pairs of lower bandgap NFAs. As in
binary blends, the ionization energy offset between donor and acceptor
({\Delta}IE) controls the QE and maximizes for {\Delta}IE > 0.5 eV. However,
{\Delta}IE is not controlled by the individual NFAs IEs but by their average,
weighted for their blending ratio. Using this property, we improved the QE of a
PBDB-T-2F:IEICO binary blend that had an insufficient {\Delta}IE for charge
generation by adding a deep IE third component: IT-4F. Combining two NFAs
enables to optimize the D/A energy alignment and cells' QE without molecular
engineering.

###Chemical design rules for non-fullerene acceptors in organic solar cells||A. Markina,K. -H. Lin,W. Liu,C. Poelking,Y. Firdaus,D. R. Villalva,J. I. Khan,S. H. K. Paleti,G. T. Harrison,J. Gorenflot,W. Zhang,S. De Wolf,I. McCulloch,T. D. Anthopoulos,D. Baran,F. Laquai,D. Andrienko###

Chemical design rules for non-fullerene acceptors in organic solar cells. Efficiencies of organic solar cells have practically doubled since the
development of non-fullerene acceptors (NFAs). However, generic chemical design
rules for donor-NFA combinations are still needed. Such rules are proposed by
analyzing inhomogeneous electrostatic fields at the donor-acceptor interface.
It is shown that an acceptor-donor-acceptor molecular architecture, and
molecular alignment parallel to the interface, results in energy level bending
that destabilizes the charge transfer state, thus promoting its dissociation
into free charges. By analyzing a series of PCE10:NFA solar cells, with NFAs
including Y6, IEICO, and ITIC, as well as their halogenated derivatives, it is
suggested that the molecular quadrupole moment of ca 75 Debye A balances the
losses in the open circuit voltage and gains in charge generation efficiency.

###Revealing the role of tin fluoride additive in narrow bandgap Pb-Sn perovskites for highly efficient flexible all-perovskite tandem cells||Johnpaul K. Pious,Yannick Zwirner,Huagui Lai,Selina Olthof,Quentin Jeangros,Evgeniia Gilshtein,Radha K. Kothandaraman,Kerem Artuk,Philipp Wechsler,Cong Chen,Christian M. Wolff,Dewei Zhao,Ayodhya. N. Tiwari,Fan Fu###

Revealing the role of tin fluoride additive in narrow bandgap Pb-Sn perovskites for highly efficient flexible all-perovskite tandem cells. Tin fluoride (SnF2) is an indispensable additive for high-efficiency Pb-Sn
perovskite solar cells (PSCs). However, the spatial distribution of SnF2 in the
perovskite absorber is seldom investigated while essential for a comprehensive
understanding of the exact role of the SnF2 additive. Herein, we revealed the
spatial distribution of SnF2 additive and made structure-optoelectronic
properties-flexible photovoltaic performance correlation. We observed the
chemical transformation of SnF2 to a fluorinated oxy-phase on the Pb-Sn
perovskite film surface, due to its rapid oxidation. In addition, at the buried
perovskite interface, we detected and visualized the accumulation of F- ions.
We found that the photoluminescence quantum yield of Pb-Sn perovskite reached
the highest value with 10 mol% SnF2 in the precursor solution. When integrating
the optimized absorber in flexible devices, we obtained the flexible Pb-Sn
perovskite narrow bandgap (1.24 eV) solar cells with an efficiency of 18.5% and
demonstrated 23.1%-efficient flexible 4-terminal all-perovskite tandem cells.

###Equivalent Circuit Description of Non-compensated n-p Codoped TiO2 as Intermediate Band Solar Cells||Tian-Li Feng,Guang-Wei Deng,Yi Xia,Feng-Cheng Wu,Ping Cui,Hai-Ping Lan,Zhen-Yu Zhang###

Equivalent Circuit Description of Non-compensated n-p Codoped TiO2 as Intermediate Band Solar Cells. The novel concept of non-compensated n-p codoping has made it possible to
create tunable intermediate bands in the intrinsic band gap of TiO2, making the
codoped TiO2 a promising material for developing intermediate band solar cells
(IBSCs). Here we investigate the quantum efficiency of such IBSCs within two
scenarios - with and without current extracted from the extended intermediate
band. Using the ideal equivalent circuit model, we find that the maximum
efficiency of 57% in the first scenario and 53% in the second are both much
higher than the Shockley-Queisser limit from single gap solar cells. We also
obtain various key quantities of the circuits, a useful step in realistic
development of TiO2 based solar cells invoking device integration. These
equivalent circuit results are also compared with the efficiencies obtained
directly from consideration of electron transition between the energy bands,
and both approaches reveal the intriguing existence of double peaks in the
maximum quantum efficiency as a function of the relative location of IBs.

###Design of n-AlInN on p-silicon heterojunction solar cells||R. Blasco,F. B. Naranjo,S. Valdueza-Felip###

Design of n-AlInN on p-silicon heterojunction solar cells. Aluminum Indium Nitride (AlInN) alloys offer great potential for photovoltaic
devices thanks to their wide direct bandgap energy that covers the solar
spectrum from 0.7 eV (InN) to 6.2 eV (AlN), and their superior resistance to
high temperatures and high-energy particles. In this paper, we report the
design of AlInN on silicon heterojunctions, with the aim to explore their
potential for solar cell devices through the analysis and optimization of the
properties of the AlInN on Si heterojunction. In particular, we study the
influence of the AlInN bandgap energy, AlInN thickness and carrier
concentration, silicon surface recombination, interface defects and Si wafer
quality on the photovoltaic properties (conversion efficiency and external
quantum efficiency) of the AlInN on Si heterojunctions. The effect of
introducing an anti-reflective coating is also studied. Optimized AlInN on Si
heterostructure shows a conversion efficiency of 18% under 1-sun AM1.5G
illumination for low-quality Si wafers, which increases to 23.6% for
high-quality Si wafers and incorporating a properly designed anti-reflective
layer. In comparison with standard Si solar cells without AlInN, the external
quantum efficient of the devices increases for wavelengths below 500 nm, making
them appealing for space applications. These results lead to the AlInN on Si
heterojunction a promising future as a novel technology for solar cell devices.

###Impact of Semiconductor Band Tails and Band Filling on Photovoltaic Efficiency Limits||Joeson Wong,Stefan T. Omelchenko,Harry A. Atwater###

Impact of Semiconductor Band Tails and Band Filling on Photovoltaic Efficiency Limits. The theoretical maximum efficiency of a solar cell is typically characterized
by a detailed balance of optical absorption and emission for a semiconductor in
the limit of unity radiative efficiency and an ideal step-function response for
the density of states and absorbance at the semiconductor band edges, known as
the Shockley-Queisser limit. However, real materials have non-abrupt band
edges, which are typically characterized by an exponential distribution of
states, known as an Urbach tail. We develop here a modified detailed balance
limit of solar cells with imperfect band edges, using optoelectronic
reciprocity relations. We find that for semiconductors whose band edges are
broader than the thermal energy, kT, there is an effective renormalized bandgap
given by the quasi-Fermi level splitting within the solar cell. This
renormalized bandgap creates a Stokes shift between the onset of the absorption
and photoluminescence emission energies, which significantly reduces the
maximum achievable efficiency. The abruptness of the band edge density of
states therefore has important implications for the maximum achievable
photovoltaic efficiency.

###Loss Mechanism Analyses of Perovskite Solar Cells with an Equivalent Circuit Model||Ting Xu,Zi-Shuai Wang,Xuan-Hua Li,Wei E. I. Sha###

Loss Mechanism Analyses of Perovskite Solar Cells with an Equivalent Circuit Model. Understanding and quantifying the main loss factors affecting the power
conversion efficiency of perovskite solar cells are urgently needed. In this
work, based on semiconductor physics, the expressions of bulk and surface
recombination currents are analytically derived. Then taking the optical loss,
series and shunt resistance losses, and bulk and surface recombination losses
into consideration, an equivalent circuit model is proposed to describe the
current density-voltage characteristics of practical perovskite solar cells.
Furthermore, by comparing to the drift-diffusion model, the pre-defined
physical parameters of the drift-diffusion model well agree with the fitting
parameters retrieved by the equivalent circuit model, which verifies the
reliability of the proposed model. Moreover, when the circuit model is applied
to analyze experimental results, the fitting outcomes show favorable
consistency to the physical investigations offered by the experiments. And the
relative fitting errors of the above cases are all less than 2%. Through
employing the model, the dominant recombination type is clearly identified and
split current density-voltage curves characterizing different loss mechanisms
are offered, which intuitively reveals the physical principles of efficiency
loss. Additionally, through calculating the efficiency loss ratios under the
open-circuit voltage condition, quantifying the above-mentioned loss mechanisms
becomes simple and compelling. Consequently, this model offers a guideline to
approach the efficiency limit from a circuit-level perspective. And the model
is a comprehensive simulation and analysis tool for understanding the device
physics of perovskite solar cells.

###Spontaneous Radiative Cooling to Enhance the Operational Stability of Perovskite Solar Cells via a Black-body-like Full Carbon Electrode||Bingcheng Yu,Jiangjian Shi,Yiming Li,Shan Tan,Yuqi Cui,Fanqi Meng,Huijue Wu,Yanhong Luo,Dongmei Li,Qingbo Meng###

Spontaneous Radiative Cooling to Enhance the Operational Stability of Perovskite Solar Cells via a Black-body-like Full Carbon Electrode. Operational stability of perovskite solar cells is remarkably influenced by
the device temperature, therefore, decreasing the interior temperature of the
device is one of the most effective approaches to prolong the service life.
Herein, we introduce the spontaneous radiative cooling effect into the
perovskite solar cell and amplified this effect via functional structure design
of a full-carbon electrode (F-CE). Firstly, with interface engineering, >19%
and >23% power conversion efficiencies of F-CE based inorganic CsPbI3 and
hybrid perovskite solar cells have been achieved, respectively, both of which
are the highest reported efficiencies based on carbon electrode and are
comparative to the results for metal electrodes. Highly efficient thermal
radiation of this F-CE can reduce the temperature of the operating cell by
about 10 {\deg}C. Compared with the conventional metal electrode-based control
cells, the operational stability of the above two types of cells have been
significantly improved due to this cooling effect. Especially, the CsPbI3 PSCs
exhibited no efficiency degradation after 2000 hours of continuous operational
tracking.

###Angular constraint on light-trapping absorption enhancement in solar cells||Zongfu Yu,Shanhui Fan###

Angular constraint on light-trapping absorption enhancement in solar cells. Light trapping for solar cells can reduce production cost and improve energy
conversion efficiency. Understanding some of the basic theoretical constraints
on light trapping is therefore of fundamental importance. Here, we develop a
general angular constraint on the absorption enhancement in light trapping. We
show that there is an upper limit for the angular integration of absorption
enhancement factors. This limit is determined by the number of accessible
resonances supported by an absorber.

###Darker than black: radiation-absorbing metamaterial||E. E. Narimanov,H. Li,Yu. A. Barnakov,T. U. Tumkur,M. A. Noginov###

Darker than black: radiation-absorbing metamaterial. We show that corrugated surfaces of hyperbolic metamaterials scatter light
preferentially inside the media, resulting in a very low reflectance and
ultimate dark appearance in the spectral range of hyperbolic dispersion. This
phenomenon of fundamental importance, demonstrated experimentally in arrays of
silver nanowires grown in alumina membranes, originates from a broad-band
singularity in the density of photonic states. It paves the road to a variety
of applications ranging from the stealth technology to high-efficiency solar
cells and photodetectors.

###Simulation study of a new InGaN p-layer free Schottky based solar cell||Abdoulwahab Adaine,Sidi Ould Saad Hamady,Nicolas Fressengeas###

Simulation study of a new InGaN p-layer free Schottky based solar cell. On the road towards next generation high efficiency solar cells, the ternary
Indium Gallium Nitride (InGaN) alloy is a good passenger since it allows to
cover the whole solar spectrum through the change in its Indium composition.
The choice of the main structure of the InGaN solar cell is however crucial.
Obtaining a high efficiency requires to improve the light absorption and the
photogenerated carriers collection that depend on the layers parameters,
including the Indium composition, p-and n-doping, device geometry.. .
Unfortunately, one of the main drawbacks of InGaN is linked to its p-type
doping, which is very difficult to realize since it involves complex
technological processes that are difficult to master and that highly impact the
layer quality. In this paper, the InGaN p-n junction (PN) and p-in junction
(PIN) based solar cells are numerically studied using the most realistic
models, and optimized through mathematically rigorous multivariate optimization
approaches. This analysis evidences optimal efficiencies of 17.8% and 19.0% for
the PN and PIN structures. It also leads to propose, analyze and optimize
player free InGaN Schottky-Based Solar Cells (SBSC): the Schottky structure and
a new MIN structure for which the optimal efficiencies are shown to be a little
higher than for the conventional structures: respectively 18.2% and 19.8%. The
tolerance that is allowed on each parameter for each of the proposed cells has
been studied. The new MIN structure is shown to exhibit the widest tolerances
on the layers thicknesses and dopings. In addition to its being player free,
this is another advantage of the MIN structure since it implies its better
reliability. Therefore, these new InGaN SBSC are shown to be alternatives to
the conventional structures that allow removing the p-type doping of InGaN
while giving photovoltaic (PV) performances at least comparable to the standard
multilayers PN or PIN structures.

###Measurement of light diffusion in ZnO nanowire forests||Marijn A. M. Versteegh,Ruben E. C. van der Wel,Jaap I. Dijkhuis###

Measurement of light diffusion in ZnO nanowire forests. Optimum design of efficient nanowire solar cells requires better
understanding of light diffusion in a nanowire array. Here we demonstrate that
our recently developed ultrafast all-optical shutter can be used to directly
measure the dwell time of light in a nanowire array. Our measurements on
disordered ZnO nanowire arrays, "nanowire forests," indicate that the photon
mean free path and the dwell time of light can be well predicted from SEM
images.

###Inverted Perovskite Photovoltaics using Flame Spray Pyrolysis Solution based CuAlO2/Cu-O Hole Selective Contact||Achilleas Savva,Ioannis T. Papadas,Dimitris Tsikritzis,Apostolos Ioakeimidis,Fedros Galatopoulos,Konstantinos Kapnisis,Roland Fuhrer,Benjamin Hartmeier,Marek F. Oszajca,Norman A. Luechinger,Stella Kennou,Gerasimos Armatas,Stelios A. Choulis###

Inverted Perovskite Photovoltaics using Flame Spray Pyrolysis Solution based CuAlO2/Cu-O Hole Selective Contact. We present the functionalization process of a conductive and transparent
copper aluminum oxide, copper oxide alloy. The copper aluminum oxide, copper
oxide powders were developed by flame spray pyrolysis and their stabilized
dispersions were treated by sonication and centrifugation methods. We show that
when the supernatant part of the treated copper aluminum oxide, copper oxide
dispersions is used for the development of hole transporting layers the
corresponding inverted perovskite solar cells show improved functionality and
power conversion efficiency with negligible hysteresis effect.

###Nitrobenzene as Additive to Improve Reproducibility and Degradation Resistance of Highly Efficient Methylammonium-Free Inverted Perovskite Solar Cells||Apostolos Ioakeimidis,Stelios. A. Choulis###

Nitrobenzene as Additive to Improve Reproducibility and Degradation Resistance of Highly Efficient Methylammonium-Free Inverted Perovskite Solar Cells. We show that the addition of 1 % (v/v) nitrobenzene within the perovskite
formulation can be used as a method to improve the power conversion efficiency
and reliability performance of methylammonium-free (CsFA) inverted perovskite
solar cells. Addition of nitrobenzene increased PCE due to defect passivation
and provides smoother films resulting in PVSCs with narrower PCE distribution.
Moreover, the nitrobenzene additive methylammonium-free hybrid PVSCs exhibit
prolonged lifetime compare to additive free PVSCs due to enhanced air and
moisture degradation resistance.

###Temperature and intensity dependence of the open-circuit voltage of InGaN/GaN multi-quantum well solar cells||M. Auf der Maur,G. Moses,J. M. Gordon,X. Huang,Y. Zhao,E. A. Katz###

Temperature and intensity dependence of the open-circuit voltage of InGaN/GaN multi-quantum well solar cells. We have analyzed the temperature and intensity dependence of the open-circuit
voltage of InGaN/GaN multi-quantum well solar cells up to 725 K and more than
1000 suns. We show that the simple ABC model routinely used to analyze the
measured quantum efficiency data of InGaN/GaN LEDs can accurately reproduce the
temperature and intensity dependence of the measured open-circuit voltage if a
temperature-dependent Shockley-Read-Hall lifetime is used and device heating is
taken into account.

###Intermediate Mirrors to Reach Theoretical Efficiency Limits of Multi-Bandgap Solar Cells||Vidya Ganapati,Chi-Sing Ho,Eli Yablonovitch###

Intermediate Mirrors to Reach Theoretical Efficiency Limits of Multi-Bandgap Solar Cells. Creating a single bandgap solar cell that approaches the Shockley-Queisser
limit requires a highly reflective rear mirror. This mirror enhances the
voltage of the solar cell by providing photons with multiple opportunities for
escaping out the front surface. Efficient external luminescence is a
pre-requisite for high voltage. Intermediate mirrors in a multijunction solar
cell can enhance the voltage for each cell in the stack. These intermediate
mirrors need to have the added function of transmitting the below bandgap
photons to the next cell in the stack. In this work, we quantitatively
establish the efficiency increase possible with the use of intermediate
selective reflectors between cells in a tandem stack. The absolute efficiency
increase can be up to ~6% in dual bandgap cells with optimal intermediate and
rear mirrors. A practical implementation of an intermediate selective mirror is
an air gap sandwiched by antireflection coatings. The air gap provides perfect
reflection for angles outside the escape cone, and the antireflection coating
transmits angles inside the escape cone. As the incoming sunlight is within the
escape cone, it is transmitted on to the next cell, while most of the
internally trapped luminescence is reflected.

###Role of Carrier Mobility and Band Alignment Engineering on the Efficiency of Colloidal Quantum Dot Solar Cells||Roha Saad,Nauman Z. Butt###

Role of Carrier Mobility and Band Alignment Engineering on the Efficiency of Colloidal Quantum Dot Solar Cells. We investigate physics based design of colloidal quantum dot (CQD) solar
cells using self-consistent computational modeling. The significance of band
alignment engineering and optimized carrier mobility are quantitatively
explored as a function of sub bandgap defect densities (N_t) in the bulk CQD.
For $N_t \leq 10^{15} cm^{-3}$, band alignment engineering near the interface
of CQD and the metal contact could significantly improve open circuit voltage
by suppressing the forward bias dark current. This effect could enhance cell
efficiency up to ~37% for thinner $(< 1 \mu m)$ CQD layers. For thicker $(> 1
\mu m)$ CQD layer, the effect of band engineering is diminished as the forward
bias dark current becomes diffusion-limited and less dependent on the
interfacial band offsets. An optimal carrier mobility in CQD lies in the range
~ 10^{-2} cm^2/Vs - 10^0 cm^2/Vs and shows variation as a function of CQD layer
thickness and the interfacial band offset. For $N_t \approx 10^{14} cm^{-3}$,
an optimally designed cell could provide ~20% efficiency under AM1.5G solar
spectrum without employing advanced structural optimizations such as the
nanostructured electrodes. These physical insights contribute to a better
understanding of quantum dot solar cell design, allowing a step further towards
a highly efficient and a low cost solar cell technology.

###Influence of drying temperature on morphology of MAPbI$_3$ thin films and the performance of solar cells||Hao Zhang,Yalan Wang,Hong Wang,Meryang Ma,Shuai Dong,Qingyu Xu###

Influence of drying temperature on morphology of MAPbI$_3$ thin films and the performance of solar cells. Photoelectric conversion efficiency of organic-inorganic perovskite solar
cells has been rapidly raised and attracted great attention in recent years.
The quality of perovskite films is vital for the performance of devices. We
used the anti-solvent method to prepare CH$_3$NH$_3$PbI$_3$ thin films by spin
coating and dried them at various temperature to transform adduct
MAI.PbI$_2$.DMSO into CH$_3$NH$_3$PbI$_3$. We researched in detail on the
relationship between surface morphology of MAPbI$_3$ thin films fabricated by
the anti-solvent method and various drying temperature. We found that surface
roughness and grain size of CH$_3$NH$_3$PbI$_3$ films together increased with
increasing drying temperature. The larger grain size could efficiently reduce
crystal boundaries which is advantageous for the suppression of photo-induced
charge carrier recombination resulting in increase of FF. However, increase of
surface roughness resulted in larger contact area at interface which might
produce more tarp states and poorer wettability of HTM solution leading in
decrease of Jsc. Surface morphology of MAPbI3 layer on the performance of solar
cell devices is also an important research issue. By optimizing the drying
temperature to 60 oC, the highest efficiency of 14.4% was achieved for the
CH3NH3PbI3-based solar cell devices.

###Concept of round non-flat thin film solar cells and their power conversion efficiency calculation||Jabbar Ganji###

Concept of round non-flat thin film solar cells and their power conversion efficiency calculation. Thin-film solar cells that are considered as the second generation of solar
cells are known for their low cost and acceptable efficiency. In this
technology, semiconductor layers with a thickness of micrometer are deposited
on thick enough substrates to maintain physical consistency. The relatively low
processing temperature helps use substrates of different materials. Compared
with crystalline solar cells, which are mainly made up of rigid flat plates, it
is also possible to make thin-film cells on flexible or non-flat substrates. In
this study, a method was first proposed to calculate the efficiency of such
cells without the need for 3D simulation, and then it is investigated using
non-flat conical and paraboloid substrates as a novel method to enhance the
light trapping. As a result, a significant increase in the efficiency of the
studied non-flat cells was observed and reported in comparison with the flat
cells. In addition, the paraboloid shape shows a better performance than that
of the conical, to use as the cell's substrate.

###Enhancement in Power Conversion Efficiency of CdS Quantum Dot Sensitized Solar Cells Through a Decrease in Light Reflection||Farzaneh Ahangarani Farahani,Atila Poro,Maryam Rezaee,Mehdi Sameni###

Enhancement in Power Conversion Efficiency of CdS Quantum Dot Sensitized Solar Cells Through a Decrease in Light Reflection. In this research, the effect of Magnesium Fluoride (MgF2) Anti-Reflection
(AR) layer was investigated in quantum dot sensitized solar cells (QDSCs). MgF2
nanoparticles with the dominant size of 20 nm were grown by a thermal
evaporation method and a thin layer was formed on the front side of the
fluorine-doped tin oxide (FTO) substrate. In order to study the effect of the
AR layer on the efficiency of solar cells, this substrate was utilized in CdS
QDSCs. In this conventional structure of QDSC, TiO2 nanocrystals (NCs) were
applied on the FTO substrate, and then it was sensitized with CdS quantum dots
(QDs). According to the results, the QDSCs with MgF2 AR layer represented the
maximum Power Conversion Efficiency (PCE) of 3%. This efficiency was increased
by about 47% compared to the reference cell without the AR layer. The reason is
attributed to the presence of the AR layer and the reduction of incident light
reflected from the surface of the solar cell.

###New hybrid organic-inorganic ferrophotovoltaic perovskites nanoparticles with high voltage for indoor and IoT applications||Rémi Ndioukane,Fanta Baldé,Ndéye C. Y. Fall,Diouma Kobor,Laurence Motte###

New hybrid organic-inorganic ferrophotovoltaic perovskites nanoparticles with high voltage for indoor and IoT applications. The ideal band gap for a photovoltaic active layer for the solar spectrum is
around 1.3 eV. However oxides with such values are rare. One of the most
studied oxides to date as a photovoltaic active layer is the cuprous oxide
Cu2O. Its band gap is around 2.1 eV and is therefore not ideal for the solar
spectrum. Power Conversion Efficiency generally do not exceed 4%. In this paper
we propose to study an emerging type of solar cell that is based on
ferroelectricity. In this type of solar cell, a p-n junction is not necessarily
required, unlike conventional solar cells. Interesting conversion efficiencies
are beginning to be obtained with this type of cell, however the mechanisms are
still not well understood and several material and engineering challenges must
be addressed. The objective of this paper is to initiate an innovative
photovoltaic technology based on novel inorganic with suitable bandgap widths
and organic materials (biopolymer). These oxides are more stables. We
synthesized ferroelectric materials that absorb a large part of the solar
spectrum with reduced bandgap widths. PZN-4.5PT nanoparticles were dispersed in
a biopolymer matrix. Hybrid thin films with these inorganic nanoparticles
embedded in a biopolymer have been successfully fabricated by spin coating on
ITO substrate. Structural, morphological and electrical properties were
investigated. The best Power Conversion Efficiencies measure under a light LED
illumination of 3550 lux are respectively 21.83 % and 31.62 % for 15 and 30 min
light exposition with an open-circuit voltage of 5.17 and 5.86 V.

###Hot Carrier Dynamics in InAs-AlAsSb Core-Shell Nanowires||Daniel Sandner,Hamidreza Esmaielpour,Fabio del Giudice,Matthias Nuber,Reinhard Kienberger,Gregor Koblmüller,Hristo Iglev###

Hot Carrier Dynamics in InAs-AlAsSb Core-Shell Nanowires. Semiconductor nanowires (NWs) have shown evidence of robust hot carrier
effects due to their small dimensions. The relaxation dynamics of hot carriers
in these nanostructures, generated by photo-absorption, are of great importance
in optoelectronic devices and high efficiency solar cells, such as hot carrier
solar cells. Among various III-V semiconductors, indium arsenide (InAs) NWs are
promising candidates for their applications in advanced light harvesting
devices due to their high photo-absorptivity and high mobility. Here, we
investigate the hot carrier dynamics in InAs-AlAsSb core-shell NWs, as well as
bare-core InAs NWs, using ultrafast pump-probe spectroscopy with widely tuned
pump and probe energies. We have found a lifetime of 2.3 ps for longitudinal
optical (LO) phonons and hot electron lifetimes of about 3 ps and 30 ps for
carrier-carrier interactions and electron-phonon interactions, respectively. In
addition, we have investigated the electronic states in the AlAsSb-shell and
found that, despite the large band offset of the core-shell design in the
conduction band, excited carriers remain in the shell longer than 100 ps. Our
results indicate evidence of plasmon-tailored core-shell NWs for efficient
light harvesting devices, which could open potential avenues for improving the
efficiency of photovoltaic solar cells.

###Modelling and Optimising GaAs/Al(x)Ga(1-x)As Multiple Quantum Well Solar Cells||James P. Connolly###

Modelling and Optimising GaAs/Al(x)Ga(1-x)As Multiple Quantum Well Solar Cells. The quantum well solar cell (QWSC) is a p - i - n solar cell with quantum
wells in the intrinsic region. Previous work has shown that QWSCs have a
greater open circuit voltage (Voc) than would be provided by a cell with the
quantum well effective bandgap. This suggests that the fundamental efficiency
limits of QWSCs are greater than those of single bandgap solar cells. The
following work investigates QWSCs in the GaAs/AlxGa1-xAs materials system. The
design and optimisation of a QWSC in this system requires studies of the
voltage and current dependencies on the aluminium fraction. QWSCs with
different aluminium fractions have been studied and show an increasing Voc with
increasing barrier aluminium composition. The QE however decreases with
increasing aluminium composition. We develop a model of the QE to test novel
QWSC designs with a view to minimising this problem. This work concentrates on
two design changes. The first deals with com- positionally graded structures in
which the bandgap varies with position. This bandgap variation introduces an
quasi electric field which can be used to increase minority carrier collection
in the low efficiency p and n layers. This technique also increases the light
flux reaching the highly efficient depletion regions. The second design change
consists of coating the back of the cell with a mirror to exploit the portion
of light which is not absorbed on the first pass. A model of the QE of
compositionally graded QWSC solar cells with back surface mirrors is developed
in order to analyse the effect of these design changes. These changes are
implemented separately in a number of QWSC designs and the resulting
experimental data compared with the model. An optimised design is then
presented.

###Monolithic thin-film chalcogenide-silicon tandem solar cells enabled by a diffusion barrier||Alireza Hajijafarassar,Filipe Martinho,Fredrik Stulen,Sigbjørn Grini,Simón López-Mariño,Moises Espíndola-Rodríguez,Max Döbeli,Stela Canulescu,Eugen Stamate,Mungunshagai Gansukh,Sara Engberg,Andrea Crovetto,Lasse Vines,Jørgen Schou,Ole Hansen###

Monolithic thin-film chalcogenide-silicon tandem solar cells enabled by a diffusion barrier. Following the recent success of monolithically integrated Perovskite/Si
tandem solar cells, great interest has been raised in searching for alternative
wide bandgap top-cell materials with prospects of a fully earth-abundant,
stable and efficient tandem solar cell. Thin film chalcogenides (TFCs) such as
the Cu2ZnSnS4 (CZTS) could be suitable top-cell materials. However, TFCs have
the disadvantage that generally at least one high temperature step (>500 C) is
needed during the synthesis, which could contaminate the Si bottom cell. Here,
we systematically investigate the monolithic integration of CZTS on a Si bottom
solar cell. A thermally resilient double-sided Tunnel Oxide Passivated Contact
(TOPCon) structure is used as bottom cell. A thin (<25 nm) TiN layer between
the top and bottom cells, doubles as diffusion barrier and recombination layer.
We show that TiN successfully mitigates in-diffusion of CZTS elements into the
c-Si bulk during the high temperature sulfurization process, and find no
evidence of electrically active deep Si bulk defects in samples protected by
just 10 nm TiN. Post-process minority carrier lifetime in Si exceeded 1.5 ,s.
i.e., a promising implied open-circuit voltage (i-Voc) of 715 mV after the high
temperature sulfurization. Based on these results, we demonstrate a first
proof-of-concept two-terminal CZTS/Si tandem device with an efficiency of 1.1%
and a Voc of 900 mV. A general implication of this study is that the growth of
complex semiconductors on Si using high temperature steps is technically
feasible, and can potentially lead to efficient monolithically integrated
two-terminal tandem solar cells.

###UMG silicon for solar PV: from defects detection to PV module degradation||Eduardo Fornies,Carlos del Canizo,Laura Mendez,Alejandro Souto,Antonio Perez-Vazquez,Daniel Garrain###

UMG silicon for solar PV: from defects detection to PV module degradation. Upgraded metallurgical grade silicon (UMG-Si) for photovoltaic (PV) solar
applications has been manufactured through the metallurgical route by means of
the process developed by Ferrosolar. In an ambitious mass production test,
performed in commercial solar cells and modules production lines, the silicon
was proven to be appropriate for photovoltaics applications (Fornies et al.,
2019 Mass production test of solar cells and modules made of 100% umg silicon.
20.76% record efficiency. Energies 12), reaching, in a conventional production
line, up to 20.76% of solar cell efficiency with multicrystalline cells made of
100% UMG silicon. In this paper we present more results from the mentioned
massive test. Defect engineering is being applied to improve the bulk lifetime
of the UMG wafers and to guide in the identification of the limiting defects in
the material. Moreover, the modules produced with 100% UMG silicon solar cells
were installed together with the modules produced in the same production line
with polysilicon material to assess the degradation of the UMG silicon when
compared to polysilicon. After 24 months of outdoor PV generation, the
degradation, in terms of Performance Ratio at 25C (25PR) diminution, has been
the same for both types of modules. Additionally, a Life Cycle Assessment (LCA)
has been performed for this UMG silicon and state-of-the-art Siemens
polysilicon to compare the environmental impact of both silicon feedstocks. The
results presented in this paper; chemical analysis of wafers, defect
engineering, low degradation, average efficiency and environmental assessment,
lead to a complete study of UMG silicon, confirming its potential to be used as
raw material for PV applications.

###Understanding and Minimizing $V_{OC}$ Losses in All-Perovskite Tandem Photovoltaics||Jarla Thiesbrummel,Francisco Peña-Camargo,Kai Oliver Brinkmann,Emilio Gutierrez-Partida,Fengjiu Yang,Jonathan Warby,Steve Albrecht,Dieter Neher,Thomas Riedl,Henry J. Snaith,Martin Stolterfoht,Felix Lang###

Understanding and Minimizing $V_{OC}$ Losses in All-Perovskite Tandem Photovoltaics. All-perovskite tandem solar cells promise high photovoltaic performance at
low cost. So far however, their efficiencies cannot compete with traditional
inorganic multi-junction solar cells and they generally underperform in
comparison to what is expected from the isolated single junction devices.
Understanding performance losses in all-perovskite tandem solar cells is a
crucial aspect that will accelerate advancement. Here, we perform extensive
selective characterization of the individual sub-cells to disentangle the
different losses and limiting factors in these tandem devices. We find that
non-radiative losses in the high-gap subcell dominate the overall recombination
losses in our baseline system as well as in the majority of literature reports.
We consecutively improve the high-gap perovskite subcell through a
multi-faceted approach, allowing us to enhance the open-circuit voltage
($V_{OC}$) of the subcell by up to 120 mV. Due to the (quasi) lossless indium
oxide interconnect which we employ for the first time in all-perovskite
tandems, the $V_{OC}$ improvements achieved in the high-gap perovskites
translate directly to improved all-perovskite tandem solar cells with a
champion $V_{OC}$ of 2.00 V and a stabilized efficiency of 23.7%. The
efficiency potential of our optimized all-perovskite tandems reaches 25.2% and
27.0% when determined from electro- and photo-luminescence respectively,
indicating significant transport losses as well as imperfect energy-alignment
between the perovskite and the transport layers in the experimental devices.
Further improvements to 28.4% are possible considering the bulk quality of both
absorbers measured using photo-luminescence on isolated perovskite layers. Our
insights therefore not only show an optimization example but a generalizable
evidence-based strategy for optimization utilizing optical sub-cell
characterization.

###Pushing limits of photovoltaics and photodetection using radial junction nanowire devices||Vidur Raj,Yi Zhu,Kaushal Vora,Lan Fu,Hark Hoe Tan,Chennupati Jagadish###

Pushing limits of photovoltaics and photodetection using radial junction nanowire devices. Nanowire devices have long been proposed as an efficient alternative to their
planar counterparts for different optoelectronic applications. Unfortunately,
challenges related to the growth and characterization of doping and p-n
junction formation in nanowire devices (along axial or radial axis) have
significantly impeded their development. The problems are further amplified if
a p-n junction has to be implemented radially. Therefore, even though radial
junction devices are expected to be on par with their axial junction
counterparts, there are minimal reports on high-performance radial junction
nanowire optoelectronic devices. This paper summarizes our recent results on
the simulation and fabrication of radial junction nanowire solar cells and
photodetectors, which have shown unprecedented performance and clearly
demonstrate the importance of radial junction for optoelectronic applications.
Our simulation results show that the proposed radial junction device is both
optically and electrically optimal for solar cell and photodetector
applications, especially if the absorber quality is extremely low. The radial
junction nanowire solar cells could achieve a 17.2% efficiency, whereas the
unbiased radial junction photodetector could show sensitivity down to a single
photon level using an absorber with a lifetime of less than 50 ps. In
comparison, the axial junction planar device made using same substrate as
absorber showed less than 1% solar cell efficiency and almost no photodetection
at 0 V. This study is conclusive experimental proof of the superiority of
radial junction nanowire devices over their thin film or axial junction
counterparts, especially when absorber lifetime is extremely low. The proposed
device holds huge promise for III-V based photovoltaics and photodetectors.

###Fundamental Limit of Nanophotonic Light-trapping in Solar Cells||Zongfu Yu,Aaswath Raman,Shanhui Fan###

Fundamental Limit of Nanophotonic Light-trapping in Solar Cells. Establishing the fundamental limit of nanophotonic light-trapping schemes is
of paramount importance and is becoming increasingly urgent for current solar
cell research. The standard theory of light trapping demonstrated that
absorption enhancement in a medium cannot exceed a factor of 4n^2/
sin^2(\theta), where n is the refractive index of the active layer, and \theta
is the angle of the emission cone in the medium surrounding the cell. This
theory, however, is not applicable in the nanophotonic regime. Here we develop
a statistical temporal coupled-mode theory of light trapping based on a
rigorous electromagnetic approach. Our theory reveals that the standard limit
can be substantially surpassed when optical modes in the active layer are
confined to deep-subwavelength scale, opening new avenues for highly efficient
next-generation solar cells.

###Urchin-inspired zinc oxide as building blocks for nanostructured solar cells||Jamil Elias,Mikhael Bechelany,Ivo Utke,Rolf Erni,Davood Hosseini,Johann Michler,Laetitia Philippe###

Urchin-inspired zinc oxide as building blocks for nanostructured solar cells. High surface area nanowire based architectures have been identified as
important components for future optoelectronic nanodevices, solar cells,
wettability coatings, gas sensors, and biofuel cells. Here we report on a novel
urchin-inspired nanowire architecture: its interwoven three-dimensional,
high-surface-area nanowire arrangement can be precisely controlled by using a
low-cost and scalable synthesis based on a combination of nanosphere
lithography, low-temperature atomic layer deposition, and electrodeposition.
The performance of single-layer arrays of urchin-inspired ZnO nanowire building
blocks competes to that of planar nanowire carpets. We illustrate this
capability by fabricating fully-inorganic extremely thin absorber solar cells
using CdSe as absorber and CuSCN as hole-collector material. The light
diffusion of the urchin-inspired nanowire arrays was varied from 15% to 35%.
Homogenous absorption in the wavelength range of 400-800 nm of up to 90% was
obtained. Solar conversion efficiencies of ~ 1.33% were achieved.

###Effective Absorption Enhancement in Small Molecule Organic Solar Cells by Employing Trapezoid Gratings||Xiang Chun-Ping,Liu Jie-Tao,Jin Yu,Xu Bin-Zong,Wang Wei-Min,Wei Xin,Song Guo-Feng,Xu Yun###

Effective Absorption Enhancement in Small Molecule Organic Solar Cells by Employing Trapezoid Gratings. We demonstrate the optical absorption has been enhanced in the small molecule
organic solar cells by employing trapezoid grating structure. The enhanced
absorption is mainly attributed to both waveguide modes and surface plasmon
modes, which has been simulated by using finite-difference time-domain method.
The simulated results show that the surface plasmon along the semitransparent
metallic Ag anode is excited by introducing the periodical trapezoid gratings,
which induce high intensity field increment in the donor layer. Meanwhile, the
waveguide modes result a high intensity field in acceptor layer. The increment
of field improves the absorption of organic solar cells, significantly, which
has been demonstrated by simulating the electrical properties. The simulated
results exhibiting 31 % increment of the short-circuit current has been
achieved in the optimized device, which is supported by the experimental
measurement. The power conversion efficiency of the grating sample obtained in
experiment exhibits an enhancement of 7.7 %.

###Relativistic Solar Cells||Paolo Umari,Edoardo Mosconi,Filippo De Angelis###

Relativistic Solar Cells. Hybrid AMX3 perovskites (A=Cs, CH3NH3; M=Sn, Pb; X=halide) have
revolutionized the scenario of emerging photovoltaic technologies. Introduced
in 2009 by Kojima et al., a rapid evolution very recently led to 15% efficient
solar cells. CH3NH3PbI3 has so far dominated the field, while the similar
CH3NH3SnI3 has not been explored for photovoltaic applications, despite the
reduced band-gap. Replacement of Pb by the more environment-friendly Sn would
facilitate the large uptake of perovskite-based photovoltaics. Despite the
extremely fast progress, the materials electronic properties which are key to
the photovoltaic performance are relatively little understood. Here we develop
an effective GW method incorporating spin-orbit coupling which allows us to
accurately model the electronic, optical and transport properties of CH3NH3SnI3
and CH3NH3PbI3, opening the way to new materials design. The different
CH3NH3SnI3 and CH3NH3PbI3 properties are discussed in light of their
exploitation for solar cells, and found to be entirely due to relativistic
effects.

###Ferroelectric Materials for Solar Energy Conversion: Photoferroics Revisited||Keith T. Butler,Jarvist M. Frost,Aron Walsh###

Ferroelectric Materials for Solar Energy Conversion: Photoferroics Revisited. The application of ferroelectric materials (i.e. solids that exhibit
spontaneous electric polarisation) in solar cells has a long and controversial
history. This includes the first observations of the anomalous photovoltaic
effect (APE) and the bulk photovoltaic effect (BPE). The recent successful
application of inorganic and hybrid perovskite structured materials (e.g.
BiFeO3, CsSnI3, CH3NH3PbI3) in solar cells emphasises that polar semiconductors
can be used in conventional photovoltaic architectures. We review developments
in this field, with a particular emphasis on the materials known to display the
APE/BPE (e.g. ZnS, CdTe, SbSI), and the theoretical explanation. Critical
analysis is complemented with first-principles calculation of the underlying
electronic structure. In addition to discussing the implications of a
ferroelectric absorber layer, and the solid state theory of polarisation (Berry
phase analysis), design principles and opportunities for high-efficiency
ferroelectric photovoltaics are presented.

###Improved Efficiency of Plasmonic Tin Sulfide Solar Cells||Priyal Jain,P. Arun###

Improved Efficiency of Plasmonic Tin Sulfide Solar Cells. Solar cells with the structure ITO-PEDOT:PSS-Ag:SnS-Al were fabricated with
the active layer of tin sulphide with silver nano-particles (Ag:SnS) grown by
thermal co-evaporation. To understand the influence of the silver nanoparticles
on the energy conversion process, various cells with varying active layer
thicknesses were compared. Results showed that the Ag nanoparticles act as
scattering centers, resulting in longer optical path lengths for incident
light. This in turn results in more charge carriers being generated and thus
enhances the efficiency of the structure as compared to the pristine
ITO-PEDOT:PSS-SnS-Al structure. The plasmonic solar cells of SnS showed an
improvement of more than 40\%. The results are encouraging and suggests more
concerted effort needs to be made on SnS.

###Graphene plasmonics for light trapping and absorption engineering||Jianfa Zhang,Zhihong Zhu,Wei Liu,Xiaodong Yuan,Shiqiao Qin###

Graphene plasmonics for light trapping and absorption engineering. Plasmonics can be used to improve absorption in optoelectronic devices and
has been intensively studied for solar cells and photodetectors. Graphene has
recently emerged as a powerful plasmonic material. It shows significantly less
losses compared to traditional plasmonic materials such as gold and silver and
its plasmons can be tuned by changing the Fermi energy with chemical or
electrical doping. Here we propose the usage of graphene plasmonics for light
trapping in optoelectronic devices and show that the excitation of localized
plasmons in doped, nanostructured graphene can enhance optical absorption in
its surrounding media including both bulky and two-dimensional materials by
tens of times, which may lead to a new generation of highly efficient,
spectrally selective photodetectors in mid-infrared and THz ranges. The
proposed concept could even revolutionize the field of plasmonic solar cells if
graphene plasmons in the visible and near-infrared are realized.

###A concept for Lithography-free patterning of silicon heterojunction back-contacted solar cells by laser processing||Bugra Turan,Kaining Ding,Stefan Haas###

A concept for Lithography-free patterning of silicon heterojunction back-contacted solar cells by laser processing. Silicon heterojunction (SHJ) solar cells with an interdigitated back-contact
(IBC) exhibit high conversion efficiencies of up to 25.6%. However, due to the
sophisticated back-side pattern of the doped layers and electrode structure
many processing and patterning steps are required. A simplification of the
patterning steps could ideally increase the yield and/or lower the production
costs. We propose a patterning approach for IBC SHJ solar cells free of any
photo-lithography with the help of laser-induced forward transfer (LIFT) of the
individual layer stacks to create the required back-contact pattern. The
concept has the potential to lower the number of processing steps significantly
while at the same time giving a large degree of freedom in the processing
conditions optimization of emitter and BSF since deposition of the
intrinsic/doped layers and processing of the wafer are all independent from
each other.

###Enhancement of perovskite solar cells by plasmonic nanoparticles||Mikhail Omelyanovich,Sergey Makarov,Valentin Milichko,Constantin Simovski###

Enhancement of perovskite solar cells by plasmonic nanoparticles. Synthetic perovskites with photovoltaic properties open a new era in solar
photovoltaics. Due to high optical absorption perovskite-based thin-film solar
cells are usually considered as fully absorbing solar radiation on condition of
ideal blooming. However, is it really so? The analysis of the literature data
has shown that the absorbance of all photovoltaic pervoskites has the spectral
hole at infrared frequencies where the solar radiation spectrum has a small
local peak. This absorption dip results in the decrease of the optical
efficiency of thin-film pervoskite solar cells by nearly 3% and close the ways
of utilise them at this range for any other applications. In our work we show
that to cure this shortage is possible complementing the basic structure by an
inexpensive plasmonic array.

###Large diffusion lengths of excitons in perovskite and ${\it TiO_2}$ heterojunction||Zhyrair Gevorkian,Vladimir Gasparian,Yurii Lozovik###

Large diffusion lengths of excitons in perovskite and ${\it TiO_2}$ heterojunction. Solar cells based on organometal halide perovskites have recently become very
promising among other materials because of their cost-effective character and
improvements in efficiency. Such performance is primarily associated with
effective light absorption and large diffusion length of charge carriers. Our
paper is devoted to the explanation of large diffusion lengths in these
systems. The transport mean free path of charged carriers in a perovskite/${\it
TiO_2}$ heterojunction that is an important constituent of the solar cells have
been analyzed. Large transport length is explained by the planar diffusion of
indirect excitons. Diffusion length of the coupled system increases by several
orders compared to single carrier length due to the correlated character of the
effective field acting on the exciton.

###Do ultrafast exciton-polaron decoherence dynamics govern photocarrier generation efficiencies in polymer solar cells?||E. Vella,H. Li,P. Gregoire,Sachetan M. Tuladhar,Michelle S. Vezie,Sheridan Few,Claudia M. Bazan,Jenny Nelson,Carlos Silva-Acuna,Eric R Bittner###

Do ultrafast exciton-polaron decoherence dynamics govern photocarrier generation efficiencies in polymer solar cells?. All-organic-based photovoltaic solar cells have attracted considerable
attention because of their low-cost processing and short energy payback time.
In such systems the primary dissociation of an optical excitation into a pair
of photocarriers has been recently shown to be extremely rapid and efficient,
but the physical reason for this remains unclear. Here, two-dimensional
photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy,
is used to probe the ultrafast coherent decay of photoexcitations into
charge-producing states in a polymer:fullerene based solar cell. The
two-dimensional photocurrent spectra are interpreted by introducing a
theoretical model for the description of the coupling of the electronic states
of the system to an external environment and to the applied laser fields. The
experimental data show no cross-peaks in the two-dimensional photocurrent
spectra, as predicted by the model for coherence times between the exciton and
the photocurrent producing states of 20\,fs or less.

###First-principles analysis of the intermediate band in CuGa$_{1-x}$Fe$_x$S$_2$||J. Koskelo,J. Hashemi,S. Huotari,M. Hakala###

First-principles analysis of the intermediate band in CuGa$_{1-x}$Fe$_x$S$_2$. We present a comprehensive study of the electronic, magnetic, and optical
properties of CuGa$_{1-x}$Fe$_x$S$_2$, as a promising candidate for
intermediate-band (IB) solar cells. We use hybrid exchange-correlation
functional within the density functional theory framework, and show that Fe
doping induces unoccupied states 1.6-1.9 eV above the valence band. The IBs
significantly enhance the optical absorption in lower energy part of the
spectrum. We find that at moderate $n$-type co-doping concentration, the added
charge occupies part of the IB in the gap, but large concentrations lower the
energy of the occupied IB toward the valence band. Moreover, we show that Fe
impurities tend to cluster within the compound and they choose
antiferromagnetic ordering. The findings can have a significant effect in
understanding this material and help to synthesize more efficient IB solar
cells.

###ZnSnS3 : Structure Prediction, Ferroelectricity, and Solar Cell Applications||Radi A. Jishi,Marcus A. Lucas###

ZnSnS3 : Structure Prediction, Ferroelectricity, and Solar Cell Applications. The rapid growth of the solar energy industry has produced a strong demand
for high performance, efficient photoelectric materials. Many ferroelectrics,
composed of earth-abundant elements, are useful for solar cell applications due
to their large internal polarization. However, their wide band gaps prevent
them from absorbing light in the visible to mid-infrared range. Here, we
address the band gap issue by investigating, in particular, the substitution of
sulphur for oxygen in the perovskite structure ZnSnO3 . Using evolutionary
methods we identify the stable and metastable structures of ZnSnS3 and compare
them to those previously characterized for ZnSnO3 . Our results suggest that
ZnSnS3 forms a monoclinic structure followed by metastable ilmenite and
lithium-niobate structures. The latter structure is highly polarized and it
possesses a significantly reduced band gap of 1.28 eV. These desirable
characteristics make it a prime candidate for solar cell applications.

###Suppressing photochemical reactions with quantized light fields||Javier Galego,Francisco J. Garcia-Vidal,Johannes Feist###

Suppressing photochemical reactions with quantized light fields. Photoisomerization, i.e., a change of molecular structure after absorption of
a photon, is one of the most fundamental photochemical processes. It can
perform desirable functionality, e.g., as the primary photochemical event in
human vision, where it stores electronic energy in the molecular structure, or
for possible applications in solar energy storage and as memories, switches,
and actuators; but it can also have detrimental effects, for example as an
important damage pathway under solar irradiation of DNA, or as a limiting
factor for the efficiency of organic solar cells. While photoisomerization can
be avoided by shielding the system from light, this is of course not a viable
pathway for approaches that rely on the interaction with external light (such
as solar cells or solar energy storage). Here, we show that strong coupling of
organic molecules to a confined light mode can be used to strongly suppress
photoisomerization, and thus convert molecules that normally show fast
photodegradation into photostable forms.

###Imaging the Long Transport Lengths of Photo-generated Carriers in Oriented Perovskite Films||Shuhao Liu,Lili Wang,Wei-Chun Lin,Sukrit Sucharitakul,Clemens Burda,Xuan. P. A. Gao###

Imaging the Long Transport Lengths of Photo-generated Carriers in Oriented Perovskite Films. Organometal halide perovskite has emerged as a promising material for solar
cells and optoelectronics. Although the long diffusion length of
photo-generated carriers is believed to be a critical factor responsible for
the material's high efficiency in solar cells, a direct study of carrier
transport over long distances in organometal halide perovskites is still
lacking. We fabricated highly oriented crystalline CH$_3$NH$_3$PbI$_3$
(MAPbI$_3$) thin film lateral transport devices with long channel length (~ 120
$\mu$m). By performing spatially scanned photocurrent imaging measurements with
local illumination, we directly show that the perovskite films prepared here
have very long transport lengths for photo-generated carriers, with a minority
carrier (electron) diffusion length on the order of 10 $\mu$m. Our approach of
applying scanning photocurrent microscopy to organometal halide perovskites may
be further used to elucidate the carrier transport processes and vastly
different carrier diffusion lengths (~ 100 nm to 100 $\mu$m) in different types
of organometal halide perovskites.

###Modeling of novel lateral AlGaAs/GaAs quantum well solar cell||M. Rashidi,Asghar Asgari###

Modeling of novel lateral AlGaAs/GaAs quantum well solar cell. In this paper, a novel lateral quantum well solar cell has been introduced,
and the structural parameters effects of these nano-structures on the
performance of the device have been investigated. For modeling, the continuity
equation has been solved in the quasi neutral regions. However, to analyze the
quantum wells' effects, first the Schrodinger and Poisson equations have been
solved self-consistently. To find the absorption coefficient derived from the
Fermi's golden rule, the obtained Eigen states and energies and also the
effects of multilayers using Transfer Matrix Method have been employed. Then,
to find the solar cell performance parameters, all radiative and non-radiative
recombinations have been accounted. It is found that modifying different
geometrical parameters, including the thickness of the system, the widths of
the wells and barriers, and also some structural parameters such as the
barriers' mole fraction could noticeably influence the characteristics of the
device. So, optimizing these parameters is necessary to obtain a good
efficiency.

###Micro-engineered CH$_3$NH$_3$PbI$_3$ nanowire/graphene phototransistor for low intensity light detection at room temperature||M. Spina,M. Lehmann,B. Náfrádi,L. Bernard,E. Bonvin,R. Gaál,A. Magrez,L. Forró,E. Horváth###

Micro-engineered CH$_3$NH$_3$PbI$_3$ nanowire/graphene phototransistor for low intensity light detection at room temperature. Methylammonium lead iodide perovskite has revolutionized the field of third
generation solid-state solar cells leading to simple solar cell structures1 and
certified efficiencies up to 20.1%. Recently the peculiar light harvesting
properties of organometal halide perovskites have been exploited in
photodetectors where responsivities of ~3.5 A/W and 180 A/W have been
respectively achieved for pure perovskite-based devices and hybrid
nanostructures. Here, we report on the first hybrid phototransistors where the
performance of a network of photoactive Methylammonium Lead Iodide nanowires
(hereafter MAPbI$_3$NW) are enhanced by CVD-grown monolayer graphene. These
devices show responsivities as high as ~2.6x10$^6$ A/W in the visible range
showing potential as room-temperature single-electron detector.

###Modeling based screening for optimal carrier selective material for Si based solar cells||Nithin Chatterji,Aldrin Antony,Pradeep R. Nair###

Modeling based screening for optimal carrier selective material for Si based solar cells. Carrier selective (CS) silicon solar cells are increasingly explored using a
variety of different materials. However, the optimum properties of such CS
materials are not well understood. In this context, through detailed analytical
and numerical modeling, here we provide several interesting insights on the
efficiency tradeoff with CS material properties. First, we show that perfect
band alignment is a desirable feature only if the interface is devoid of any
trap states. Otherwise, a band offset of around 0.2eV-0.4eV provides sufficient
band bending to reduce the effect of interface recombination, thus improving
the performance. Surprisingly, the interface passivation quality for the
minority carrier extraction layer is found to be far less demanding than that
for the majority carrier extraction layer. Additionally, doping density and
dielectric constant of CS layers have a similar effect as band offset on solar
cell performance. Our results have obvious implications toward the selection of
appropriate materials as carrier selective layers and hence are of broad
interest to the community.

###The impact of the halide cage on the electronic properties of fully inorganic caesium lead halide perovskites||Z. Yang,A. Surrente,K. Galkowski,A. Miyata,O. Portugall,R. J. Sutton,A. A. Haghighirad,H. J. Snaith,D. K. Maude,P. Plochocka,R. J. Nicholas###

The impact of the halide cage on the electronic properties of fully inorganic caesium lead halide perovskites. Perovskite solar cells with record power conversion efficiency are fabricated
by alloying both hybrid and fully inorganic compounds. While the basic
electronic properties of the hybrid perovskites are now well understood, key
electronic parameters for solar cell performance, such as the exciton binding
energy of fully inorganic perovskites, are still unknown. By performing magneto
transmission measurements, we determine with high accuracy the exciton binding
energy and reduced mass of fully inorganic CsPbX$_3$ perovskites (X=I, Br, and
an alloy of these). The well behaved (continuous) evolution of the band gap
with temperature in the range $4-270$\,K suggests that fully inorganic
perovskites do not undergo structural phase transitions like their hybrid
counterparts. The experimentally determined dielectric constants indicate that
at low temperature, when the motion of the organic cation is frozen, the
dielectric screening mechanism is essentially the same both for hybrid and
inorganic perovskites, and is dominated by the relative motion of atoms within
the lead-halide cage.

###Magnetic fields: a tool for the study of organic solar cells||S. Oviedo-Casado,A. Urbina,J. Prior###

Magnetic fields: a tool for the study of organic solar cells. Charge transfer in polymer devices represents a crucial, though highly
inaccessible stage of photocurrent generation. In this article we propose
studying the properties and behaviour of organic solar cells through the
modification of photocurrent generation when an external magnetic field is
applied. By allowing the parameters of our theoretical model not to be
constrained to any specific material, we are able to show that not only a
modest external magnetic field leads to a significant increase in photocurrent
intensity, but also how such magnetic field can be used to study in detail the
energy levels and transition rates within the polymer compound. Systematic
exploration of key properties in organic composites thus can lead to highly
optimised devices in which a magnetic field produces an enhancement in the
efficiency of polymer solar cells.

###Opposites Attract, Muons as Direct Probes for Iodide Diffusion in Methyl Ammonium Lead Iodide||D. W. Ferdani,A. L. Johnson,S. E. Lewis,P. J. Baker,P. J. Cameron###

Opposites Attract, Muons as Direct Probes for Iodide Diffusion in Methyl Ammonium Lead Iodide. The volume of research into organo-lead hailde perovskites is increasing
rapidly, with perovskite solar cell efficiencies reaching as high as 22
percent. There is considerable evidence that mobile ions in the perovskite
strongly influence the properties of the solar cell, with the majority of
studies carried out on whole cells under bias. Here we use muon spin relaxation
to directly probe iodide diffusion in methyl ammonium lead iodide (MAPI). This
is the first time that has been used to detect iodide diffusion in any material
and the results provide valuable insight into the movement of ions in lead
halide perovskites. The experiment was carried out in the dark with no external
biases applied and allowed us to calculate a diffusion coefficient of 1.6
x10-14 cm2/s for iodide in MAPI at 300 K.

###Determining the True Optical Gap in a High-Performance Organic Photovoltaic Polymer Using Single-Molecule Spectroscopy||Gordon J. Hedley,Florian Steiner,Jan Vogelsang,John M. Lupton###

Determining the True Optical Gap in a High-Performance Organic Photovoltaic Polymer Using Single-Molecule Spectroscopy. Low-gap conjugated polymers have enabled an impressive increase in the
efficiencies of organic solar cells, primarily due to their red absorption
which allows harvesting of that part of the solar spectrum. Here, we report
that the true optical gap of one prototypical material, PTB7, is in fact at
significantly higher energy than has previously been reported, indicating that
the red absorption utilized in these materials in solar cells is entirely due
to chain aggregation. Using single-molecule spectroscopy we find that PL from
isolated nanoscale aggregates consists of multiple independently emitting
chromophores. At the single-molecule level, however, straight single chains
with a high degree of emission polarization are observed. The PL is found to be
~0.4 eV higher in energy, with a longer lifetime than the red aggregates, and
is attributed to single chromophores. Our findings indicate that the impressive
light-harvesting abilities of PTB7 in the red spectral region arises solely
from chain aggregation.

###Employing surfactant-assisted hydrothermal synthesis to control CuGaO2 nanoparticle formation and improved carrier selectivity of perovskite solar cells||Ioannis T. Papadas,Achilleas Savva,Apostolos Ioakeimidis,Polyvios Eleftheriou,Gerasimos S. Armatas,Stelios A. Choulis###

Employing surfactant-assisted hydrothermal synthesis to control CuGaO2 nanoparticle formation and improved carrier selectivity of perovskite solar cells. Delafossites like CuGaO2 have appeared as promising p-type semiconductor
materials for opto-electronic applications mainly due to their high optical
transparency and electrical conductivity. However, existing synthetic efforts
usually result in particles with large diameter limiting their performance
relevant to functional electronic applications. In this article, we report a
novel surfactant-assisted hydrothermal synthesis method, which allows the
development of ultrafine (~5 nm) monodispersed p-type CuGaO2 nanoparticles
(NPs). We show that DMSO can be used as a ligand and dispersing solvent for
stabilizing the CuGaO2 NPs. The resulting dispersion is used for the
fabrication of dense, compact functional CuGaO2 electronic layer with
properties relevant to advanced optoelectronic applications. As a proof of
concept, the surfactant-assisted hydrothermal synthesized CuGaO2 is
incorporated as a hole transporting layer (HTL) in the inverted p-i-n
perovskite solar cell device architecture providing improved hole carrier
selectivity and power conversion efficiency compared to conventional PEDOT:PSS
HTL based perovskite solar cells.

###Influence of the AlN interlayer thickness on the photovoltaic properties of In-rich AlInN on Si heterojunctions deposited by RF sputtering||S. Valdueza-Felip,A. Núñez-Cascajero,R. Blasco,D. Montero,L. Grenet,M. de la Mata,S. Fernández,L. Rodríguez-De Marcos,S. I. Molina,J. Olea,F. B. Naranjo###

Influence of the AlN interlayer thickness on the photovoltaic properties of In-rich AlInN on Si heterojunctions deposited by RF sputtering. We report the influence of the AlN interlayer thickness (0-15 nm) on the
photovoltaic properties of Al0.37In0.63N on Si heterojunction solar cells
deposited by radio frequency sputtering. The poor junction band alignment and
the presence of a 2-3 nm thick amorphous layer at the interface mitigates the
response in devices fabricated by direct deposition of n-AlInN on p-Si(111).
Adding a 4-nm-thick AlN buffer layer improves the AlInN crystalline quality and
the interface alignment leading to devices with a conversion efficiency of 1.5%
under 1-sun AM1.5G illumination. For thicker buffers the performance lessens
due to inefficient tunnel transport through the AlN. These results demonstrate
the feasibility of using In-rich AlInN alloys deposited by radio frequency
sputtering as novel electron-selective contacts to Si-heterojunction solar
cells.

###Tuning photovoltaic response in Bi2FeCrO6 films by ferroelectric poling||A. Quattropani,A. S. Makhort,M. V. Rastei,G. Versini,G. Schmerber,S. Barre,A. Dinia,A. Slaoui,J. -L. Rehspringer,T. Fix,S. Colis,B. Kundys###

Tuning photovoltaic response in Bi2FeCrO6 films by ferroelectric poling. Ferroelectric materials are interesting candidates for future photovoltaic
applications due to their potential to overcome the fundamental limits of
conventional single bandgap semiconductor-based solar cells. Although a more
efficient charge separation and above bandgap photovoltages are advantageous in
these materials, tailoring their photovoltaic response using ferroelectric
functionalities remains puzzling. Here we address this issue by reporting a
clear hysteretic character of the photovoltaic effect as a function of electric
field and its dependence on the poling history. Furthermore, we obtain insight
into light induced nonequilibrium charge carrier dynamics in Bi2FeCrO6 films
involving not only charge generation, but also recombination processes. At the
ferroelectric remanence, light is able to electrically depolarize the films
with remanent and transient effects as evidenced by electrical and
piezoresponse force microscopy (PFM) measurements. The hysteretic nature of the
photovoltaic response and its nonlinear character at larger light intensities
can be used to optimize the photovoltaic performance of future
ferro-electric-based solar cells.

###Ion Induced Passivation of Grain Boundaries in Perovskite Solar Cells||Vikas Nandal,Pradeep R. Nair###

Ion Induced Passivation of Grain Boundaries in Perovskite Solar Cells. Demonstration of high-efficiency large area cells with excellent stability is
an important requirement towards commercialization of perovskite solar cells
(PSC). With reports of high-quality perovskite grains, it is evident that the
performance of such large area cells will be strongly influenced by phenomena
like carrier recombination and ion migration at grain boundaries (GBs). Here,
we develop a modeling framework to address performance limitation due to GBs in
large area PSCs. Through detailed numerical simulations, we show that
photo-carrier recombination has a non-trivial dependence on the orientation of
GBs. Interestingly, we find that ions at GBs lead to significant performance
recovery through field effect passivation, which is influenced by critical
parameters like density and polarity of ions, and the location of GB. These
results have interesting implications towards long-term stability and hence are
relevant for the performance optimization of large area polycrystalline based
thin film solar cells such as PSCs, CIGS, CZTS, etc.

###Detailed Performance Loss Analysis of Silicon Solar Cells using High-Throughput Metrology Methods||Mohammad Jobayer Hossain,Geoffrey Gregory,Hardik Patel,Siyu Guo,Eric J. Schneller,Andrew M. Gabor,Zhihao Yang,Adrienne L. Blum,Kristopher O. Davis###

Detailed Performance Loss Analysis of Silicon Solar Cells using High-Throughput Metrology Methods. In this work, novel, high-throughput metrology methods are used to perform a
detailed performance loss analysis of approximately 400 industrial crystalline
silicon solar cells, all coming from the same production line. The
characterization sequence includes a non-destructive transfer length method
(TLM) measurement technique featuring circular TLM structures hidden within the
busbar region of the cells. It also includes a very fast external quantum
efficiency and reflectance measurement technique. More traditional
measurements, like illuminated current-voltage, Suns-VOC, and photoluminescence
imaging are also used to carry out the loss analysis. The variance of the
individual loss parameters and their impact on cell performance are
investigated and quantified for this large group of industrial solar cells.
Some important correlations between the measured loss parameters are found. The
nature of these distributions and correlations provide important insights about
loss mechanisms in a cell and help prioritize efforts to optimize the
performance of the production line.

###Advanced material system for the design of an intermediate band solar cell: type-II CdTe quantum dots in a ZnCdSe matrix||V. Deligiannakis,G. Ranepura,I. L. Kuskovsky,M. C. Tamargo###

Advanced material system for the design of an intermediate band solar cell: type-II CdTe quantum dots in a ZnCdSe matrix. We explore CdTe fractional monolayer quantum dots (QDs) in a ZnCdSe host
matrix for potential application in an intermediate band solar cell device.
Careful consideration has been taken during the initiation of the growth
process of QDs by migration enhanced epitaxy, in order to avoid the formation
of undesirable interfacial layers that can form due to the lack of common anion
between the two materials. A superlattice structure of 100 periods of
alternating QD and spacer layers is analyzed by high resolution X-ray
diffraction (XRD) and photoluminescent (PL) spectroscopy. Simple arguments are
used following continuum elastic theory to deduce the size of the dots and the
strain within the superlattice from XRD data. This is further verified using PL
and used in the energy calculations that yield the values of the intermediate
band energy. The results suggest that the optimized materials are highly
suitable for these high efficiency solar cells.

###Novel heterojunction bipolar transistor architectures for the practical implementation of high-efficiency three-terminal solar cells||Pablo G. Linares,Elisa Antolín,Antonio Martí###

Novel heterojunction bipolar transistor architectures for the practical implementation of high-efficiency three-terminal solar cells. Practical device architectures are proposed here for the implementation of
three-terminal heterojunction bipolar transistor solar cells (3T-HBTSCs). These
photovoltaic devices, which have a potential efficiency similar to that of
multijunction cells, exhibit reduced spectral sensitivity compared with
monolithically and series-connected tandem solar cells. In addition, the
simplified n-p-n (or p-n-p) structure does not require the use of tunnel
junctions. In this framework, four architectures are proposed and discussed in
this paper: 1) one in which the top cell is based on silicon and the bottom
cell is based on a heterojunction between silicon and III-V nanomaterials; 2)
one in which the top cell is made of amorphous silicon and the bottom cell is
made of an amorphous silicon-silicon heterojunction; 3) one based on the use of
III-V semiconductors aimed at space applications; and 4) one in which the top
cell is based on a perovskite material and the bottom cell is made of a
perovskite-silicon heterostructure.

###Application of Genetic Algorithm for More Efficient Multi-Layer Thickness Optimization in Solar Cells||Premkumar Vincent,Gwenaelle Cunha Sergio,Jaewon Jang,In Man Kang,Jaehoon Park,Hyeok Kim,Minho Lee,Jin-Hyuk Bae###

Application of Genetic Algorithm for More Efficient Multi-Layer Thickness Optimization in Solar Cells. Thin-film solar cells are predominately designed similar to a stacked
structure. Optimizing the layer thicknesses in this stack structure is crucial
to extract the best efficiency of the solar cell. The commonplace method used
in optimization simulations, such as for optimizing the optical spacer layers'
thicknesses, is the parameter sweep. Our simulation study shows that the
implementation of a meta-heuristic method like the genetic algorithm results in
a significantly faster and accurate search method when compared to the
brute-force parameter sweep method in both single and multi-layer optimization.
While other sweep methods can also outperform the brute-force method, they do
not consistently exhibit $100\%$ accuracy in the optimized results like our
genetic algorithm. We have used a well-studied P3HT-based structure to test our
algorithm. Our best-case scenario was observed to use $60.84\%$ fewer
simulations than the brute-force method.

###Nanoparticulate Metal Oxide Top Electrode Interface Modification Improves the Thermal Stability of Inverted Perovskite Photovoltaics||Ioannis T. Papadas,Fedros Galatopoulos,Gerasimos S. Armatas,Nir Tessler,Stelios A. Choulis###

Nanoparticulate Metal Oxide Top Electrode Interface Modification Improves the Thermal Stability of Inverted Perovskite Photovoltaics. Solution processed {\gamma}-Fe2O3 nanoparticles via the solvothermal
colloidal synthesis in conjunction with ligand-exchange method are used for
interface modification of the top electrode in inverted perovskite solar cells.
In comparison to more conventional top electrodes such as PC(70)BM/Al and
PC(70)BM/AZO/Al, we show that incorporation of a {\gamma}-Fe2O3 provides an
alternative solution processed top electrode (PC(70)BM/{\gamma}-Fe2O3/Al) that
not only results in comparable power conversion efficiencies but also improved
thermal stability of inverted perovskite photovoltaics. The origin of improved
stability of inverted perovskite solar cells incorporating PC(70)BM/
{\gamma}-Fe2O3/Al under accelerated heat lifetime conditions is attributed to
the acidic surface nature of {\gamma}-Fe2O3 and reduced charge trapped density
within PC(70)BM/ {\gamma}-Fe2O3/Al top electrode interfaces.

###Direct evidence of weakly dispersed and strongly anharmonic optical phonons in hybrid perovskites||A. C. Ferreira,S. Paofai,A. Létoublon,J. Ollivier,S. Raymond,B. Hehlen,B. Rufflé,S. Cordier,C. Katan,J. Even,P. Bourges###

Direct evidence of weakly dispersed and strongly anharmonic optical phonons in hybrid perovskites. Hybrid organolead perovskites (HOP) have started to establish themselves in
the field of photovoltaics, mainly due to their great optoelectronic properties
and steadily improving solar cell efficiency. Study of the lattice dynamics is
key in understanding the electron-phonon interactions at play, responsible for
such properties. Here, we investigate, via neutron and Raman spectroscopies,
the optical phonon spectrum of four different HOP single crystals: MAPbBr$_3$,
FAPbBr$_3$, MAPbI$_3$, and $\alpha$-FAPbI$_3$. Low temperature spectra reveal
weakly dispersive optical phonons, at energies as low as 2-5~meV, which seem to
be the origin of the limit of the charge carriers mobilities in these
materials. The temperature dependence of our neutron spectra shows as well a
significant anharmonic behaviour, resulting in optical phonon overdamping at
temperatures as low as 80~K, questionning the validity of the quasi-particle
picture for the low energy optical modes at room temperature where the solar
cells operate.

###Ti-alloying of BaZrS3 chalcogenide perovskite for photovoltaics||Xiucheng Wei,Haolei Hui,Samanthe Perera,Aaron Sheng,David F. Watson,Yi-Yang Sun,Quanxi Jia,Shengbai Zhang,Hao Zeng###

Ti-alloying of BaZrS3 chalcogenide perovskite for photovoltaics. BaZrS3, a prototypical chalcogenide perovskite, has been shown to possess a
direct band gap, an exceptionally strong near band edge light absorption, and
good carrier transport. Coupled with its great stability, non-toxicity with
earth abundant elements, it is thus a promising candidate for thin film solar
cells. However, its reported band gap in the range of 1.7-1.8 eV is larger than
the optimal value required to reach the Shockley-Queisser limit of a single
junction solar cell. Here we report the synthesis of Ba(Zr1-xTix)S3 perovskite
compounds with a reduced band gap. It is found that Ti alloying is extremely
effective in band gap reduction of BaZrS3: a mere 4 at% alloying decreases the
band gap from 1.78 to 1.51 eV, resulting in a theoretical maximum power
conversion efficiency of 32%. Higher Ti-alloying concentration is found to
destabilize the distorted chalcogenide perovskite phase.

###Nitride-based interfacial layers for monolithic tandem integration of new solar energy materials on Si: The case of CZTS||Filipe Martinho,Alireza Hajijafarassar,Simón Lopez-Marino,Moises Espíndola-Rodríguez,Sara Engberg,Mungunshagai Gansukh,Fredrik Stulen,Sigbjørn Grini,Stela Canulescu,Eugen Stamate,Andrea Crovetto,Lasse Vines,Jørgen Schou,Ole Hansen###

Nitride-based interfacial layers for monolithic tandem integration of new solar energy materials on Si: The case of CZTS. The monolithic tandem integration of third-generation solar energy materials
on silicon holds great promise for photoelectrochemistry and photovoltaics.
However, this can be challenging when it involves high-temperature reactive
processes, which would risk damaging the Si bottom cell. One such case is the
high-temperature sulfurization/selenization in thin film chalcogenide solar
cells, of which the kesterite Cu2ZnSnS4 (CZTS) is an example. Here, by using
very thin (<10 nm) TiN-based diffusion barriers at the interface, with
different composition and properties, we demonstrate on a device level that the
protection of the Si bottom cell is largely dependent on the barrier layer
engineering. Several monolithic CZTS/Si tandem solar cells with open-circuit
voltages (Voc) up to 1.06 V and efficiencies up to 3.9% are achieved,
indicating a performance comparable to conventional interfacial layers based on
transparent conductive oxides, and pointing to a promising alternative design
in solar energy conversion devices.

###Computational Simulation and Analysis of Major Control Parameters of Time-Dependent PV/T Collectors||Jimeng Shi,Cheng-Xian Lin###

Computational Simulation and Analysis of Major Control Parameters of Time-Dependent PV/T Collectors. In order to improve performance of photovoltaic/thermal (or PV/T for
simplicity) collectors, this paper firstly validated a previous computational
thermal model and then introduced an improved computational thermal model to
investigate the effects of the major control parameters on the thermal
performance of PV/T collectors, including solar cell temperature, back surface
temperature, and outlet water temperature. Besides, a computational electrical
model of PV/T system was also introduced to elaborate the relationship of
voltage, current and power of a PV module (MSX60 polycrystalline solar cell)
used in an experiment in the literature. Simulation results agree with the
experimental data very well. The effects of the time-steps from 1 hour to
minute, which is closed to the real time, were also reported. At last, several
suggestions to improve the efficiency of PV/T system were illustrated.

###Slow Relaxation of Photogenerated Charge Carriers Boosts Open-Circuit Voltage of Organic Solar Cells||Tanvi Upreti,Sebastian Wilken,Huotian Zhang,Martijn Kemerink###

Slow Relaxation of Photogenerated Charge Carriers Boosts Open-Circuit Voltage of Organic Solar Cells. Among the parameters determining the efficiency of an organic solar cell, the
open-circuit voltage ($V_\text{OC}$) is the one with most room for improvement.
Existing models for the description of $V_\text{OC}$ assume that photogenerated
charge carriers are thermalized. Here, we demonstrate that quasi-equilibrium
concepts cannot fully describe $V_\text{OC}$ of disordered organic devices. For
two representative donor:acceptor blends it is shown that $V_\text{OC}$ is
actually 0.1-0.2 V higher than it would be if the system was in thermodynamic
equilibrium. Extensive numerical modeling reveals that the excess energy is
mainly due to incomplete relaxation in the disorder-broadened density of
states. These findings indicate that organic solar cells work as nonequilibrium
devices, in which part of the photon excess energy is harvested in the form of
an enhanced $V_\text{OC}$.

###Understanding the Effect of Lead Iodide Excess on the Performance of Methylammonium Lead Iodide Perovskite Solar Cells||Zeeshan Ahmad,Rebecca A. Scheidt,Matthew P. Hautzinger,Kai Zhu,Matthew C. Beard,Giulia Galli###

Understanding the Effect of Lead Iodide Excess on the Performance of Methylammonium Lead Iodide Perovskite Solar Cells. The presence of unreacted lead iodide in organic-inorganic lead halide
perovskite solar cells is widely correlated with an increase in power
conversion efficiency. We investigate the mechanism for this increase by
identifying the role of surfaces and interfaces present between methylammonium
lead iodide perovskite films and excess lead iodide. We show how type I and II
band alignments arising under different conditions result in either passivation
of surface defects or hole injection. Through first-principles simulations of
solid-solid interfaces, we find that lead iodide captures holes from
methylammonium lead iodide and modulates the formation of defects in the
perovskite, affecting recombination. Using surface-sensitive optical
spectroscopy techniques, such as transient reflectance and time-resolved
photoluminescence, we show how excess lead iodide affects the diffusion and
surface recombination velocity of charge carriers in methylammonium lead iodide
films. Our coupled experimental and theoretical results elucidate the role of
excess lead iodide in perovskite solar cells.

###Programmable Charge Trap for Junction-less selective extraction of holes in Solar Cells||Swasti Bhatia,Aldrin Antony,Pradeep R. Nair###

Programmable Charge Trap for Junction-less selective extraction of holes in Solar Cells. Selective extraction of photo-generated carriers is a fundamental challenge
in solar cells which is usually achieved through junctions with the associated
doping as well as band offset differences. In this context, here we propose a
new paradigm for selective extraction for majority carriers through novel usage
of the programmable charge trap which comprises of Oxide-Nitride-Oxide (ONO)
stack and has the primary function of holding electrically injected charge.
Through detailed numerical simulations, here we show that such a charge trap
with an additional metal contact can (i) compensate for efficiency loss due to
sub-optimal passivation and sub-optimal hole selectivity in homojunction as
well as transition metal oxide-based heterojunction solar cells and (ii) can
also function as a standalone hole selection scheme. The proposed scheme, with
its easy integration and the capability of programmable compensation of
performance loss, is of interest to the photovoltaic community.

###Quantifying Charge Extraction and Recombination Using the Rise and Decay of the Transient Photovoltage of Perovskite Solar Cells||Lisa Krückemeier,Zhifa Liu,Thomas Kirchartz,Uwe Rau###

Quantifying Charge Extraction and Recombination Using the Rise and Decay of the Transient Photovoltage of Perovskite Solar Cells. The extraction of photogenerated charge carriers and the generation of a
photovoltage belong to the fundamental functionalities of any solar cell. These
processes happen not instantaneously but rather come with finite time
constants, e.g., a time constant related to the rise of the externally measured
open circuit voltage following a short light pulse. The present paper provides
a new method to analyze transient photovoltage measurements at different bias
light intensities combining rise and decay times of the photovoltage. The
approach uses a linearized version of a system of two coupled differential
equations that is solved analytically be determining the eigenvalues of a 2 x 2
matrix. By comparison between the eigenvalues and the measured rise and decay
times during a transient photovoltage measurement, we determine the rates of
carrier recombination and extraction as a function of bias voltage and
establish a simple link between their ratio and the efficiency losses in the
perovskite solar cell.

###Characterizing the Influence of Charge Extraction Layers on the Performance of Triple-Cation Perovskite Solar Cells||Johanna Siekmann,Ashish Kulkarni,Samah Akel,Benjamin Klingebiel,Michael Saliba,Uwe Rau,Thomas Kirchartz###

Characterizing the Influence of Charge Extraction Layers on the Performance of Triple-Cation Perovskite Solar Cells. Selecting suitable charge transport layers and suppressing non-radiative
recombination at interfaces to the absorber layer are vital to maximize the
efficiency of halide perovskite solar cells. In this work, high-quality
perovskite thin films and devices are fabricated with different fullerene-based
electron transport layers and different self-assembled monolayers as hole
transport layers. We then perform a comparative study of a significant variety
of different electrical, optical and photoemission-based characterization
techniques to quantify the properties of the solar cells, the individual layers
and importantly the interfaces between them. In addition, we highlight the
limitations and problems of the different measurements, the insights gained by
combining different methods and the different strategies to extract information
from the experimental raw data.

###Suppression of Edge Recombination in InAs/InGaAs DWELL Solar Cells||Tingyi Gu,Mohamed A. El-Emawy,Kai Yang,Andreas Stintz,Luke F. Lester###

Suppression of Edge Recombination in InAs/InGaAs DWELL Solar Cells. The InAs/InGaAs DWELL solar cell grown by MBE is a standard pin diode
structure with six layers of InAs QDs embedded in InGaAs quantum wells placed
within a 200-nm intrinsic GaAs region. The GaAs control wafer consists of the
same pin configuration but without the DWELL structure. The typical DWELL solar
cell exhibits higher short current density while maintaining nearly the same
open-circuit voltage for different scales, and the advantage of higher short
current density is more obvious in the smaller cells. In contrast, the smaller
size cells, which have a higher perimeter to area ratio, make edge
recombination current dominant in the GaAs control cells, and thus their open
circuit voltage and efficiency severely degrade. The open-circuit voltage and
efficiency under AM1.5G of the GaAs control cell decrease from 0.914V and 8.85%
to 0.834V and 7.41%, respectively, as the size shrinks from 5*5mm2 to 2*2mm2,
compared to the increase from 0.665V and 7.04% to 0.675V and 8.17%,
respectively, in the DWELL solar cells.

###Role of photon recycling in perovskite solar cells||Mohammad Ryyan Khan,Xufeng Wang,Reza Asadpour,Mark Lundstrom,Muhammad A. Alam###

Role of photon recycling in perovskite solar cells. Nearly perfect photon recycling helped GaAs cells achieve the highest
efficiency ever reported for a solar cell. Recent reports of photon recycling
in perovskite solar cells suggest that, once optimized, it may as well achieve
GaAs-like performance. In this paper, we show that GaAs and perovskite cells
recycle photons in different ways. First, although bare-perovskite has been
shown to have lifetimes (~1us) in the radiative limit, non-radiative
recombination at the transport layers restricts the solar cell operation far
below the "photon-recycling" regime. GaAs cells have no such limitation.
Second, even if the transport layers were optically and electrically perfect,
the poor mobility of the perovskite layer would still restrict the optimum
thickness ~1um. Thus, a very high quality mirror (reflectivity >96%) is
required to utilize photon-recycling. The mirror reflectivity restriction was
far more relaxed for the thicker (~2-3um) GaAs cells. Therefore, a nontrivial
co-optimization of device geometry, mirror reflectivity, and material choice is
necessary for achieving highest theoretical efficiency anticipated for
perovskite cells.

###A Microscopic Perspective on Photovoltaic Reciprocity in Ultrathin Solar Cells||Urs Aeberhard,Uwe Rau###

A Microscopic Perspective on Photovoltaic Reciprocity in Ultrathin Solar Cells. The photovoltaic reciprocity theory relates the electroluminescence spectrum
of a solar cell under applied bias to the external photovoltaic quantum
efficiency of the device as measured at short circuit conditions. Its
derivation is based on detailed balance relations between local absorption and
emission rates in optically isotropic media with non-degenerate
quasi-equilibrium carrier distributions. In many cases, the dependence of
density and spatial variation of electronic and optical device states on the
point of operation is modest and the reciprocity relation holds. In
nanostructure-based photovoltaic devices exploiting confined modes, however,
the underlying assumptions are no longer justifiable. In the case of ultrathin
absorber solar cells, the modification of the electronic structure with applied
bias is significant due to the large variation of the built-in field.
Straightforward use of the external quantum efficiency as measured at short
circuit conditions in the photovoltaic reciprocity theory thus fails to
reproduce the electroluminescence spectrum at large forward bias voltage. This
failure is demonstrated here by numerical simulation of both spectral
quantities at normal incidence and emission for an ultrathin GaAs p-i-n solar
cell using an advanced quantum kinetic formalism based on non-equilibrium
Green's functions of coupled photons and charge carriers. While coinciding with
the semiclassical relations under the conditions of their validity, the theory
provides a consistent microscopic relationship between absorption, emission and
charge carrier transport in photovoltaic devices at arbitrary operating
conditions and for any shape of optical and electronic density of states.

###High performance of mixed halide perovskite solar cells: role of halogen atom and plasmonic nanoparticles on the ideal current density of cell||Mohammad Ali Mohebpour,Mohaddeseh Saffari,Hamid Rahimpour Soleimani,Meysam Bagheri Tagani###

High performance of mixed halide perovskite solar cells: role of halogen atom and plasmonic nanoparticles on the ideal current density of cell. To be able to increase the efficiency of perovskite solar cells which is one
of the most substantial challenges ahead in photovoltaic industry, the
structural and optical properties of perovskite CH3NH3PbI3-xBrx for values
x=1-3 have been studied employing density functional theory (DFT). Using the
optical constants extracted from DFT calculations, the amount of light
reflectance and ideal current density of a simulated single-junction perovskite
solar cell have been investigated. The results of DFT calculations indicate
that adding halogen bromide to CH3NH3PbI3 compound causes the relocation of
energy bands in band structure which its consequence is increasing the bandgap.
In addition, the effect of increasing Br in this structure can be seen as a
reduction in lattice constant, refractive index, extinction and absorption
coefficient. As well, results of the simulation suggest a significant current
density enhancement as much as 22% can be achieved by an optimized array of
Platinum nanoparticles that is remarkable. This plan is able to be a prelude
for accomplishment of solar cells with higher energy conversion efficiency.

###Experimental Determination of Power Losses and Heat Generation in Solar Cells for Photovoltaic-Thermal Applications||Bruno Lorenzi,Maurizio Acciarri,Dario Narducci###

Experimental Determination of Power Losses and Heat Generation in Solar Cells for Photovoltaic-Thermal Applications. Solar cell thermal recovery is recently attracting more and more attention in
the research community as a viable solution to increase photovoltaic
efficiency. However the convenience of the implementation of such strategy is
bound to the precise evaluation of the recoverable thermal power, and to a
proper definition of the losses occurring within the solar device. In this work
we establish a framework in which all the solar cell losses are defined and
described. Aim is to determine the components of the thermal fraction. We
therefore describe an experimental method to precisely compute these components
from the measurement of the external quantum efficiency, the current-voltage
characteristics, and the reflectivity of the solar cell. Applying this method
to three different types of devices (bulk, thin film, and multi-junction) we
could exploit the relationships among losses for the main three generations of
PV cells available nowadays. In addition, since the model is explicitly
wavelength-dependent, we could show how thermal losses in all cells occur over
the whole solar spectrum, and not only in the infrared region. This
demonstrates that profitable thermal harvesting technologies should enable heat
recovery over the whole solar spectral range.

###Rashba-Dresselhaus Effect in Inorganic/Organic Lead Iodide Perovskite Interfaces||Chang Woo Myung,Saqib Javaid,Kwang S. Kim,Geunsik Lee###

Rashba-Dresselhaus Effect in Inorganic/Organic Lead Iodide Perovskite Interfaces. Despite the imperative importance in solar-cell efficiency, the intriguing
phenomena at the interface between perovskite solar-cell and adjacent carrier
transfer layers are hardly uncovered. Here we show that PbI$_2$/AI-terminated
lead-iodide-perovskite (APbI$_3$; A=Cs$^+$/ methylammonium(MA)) interfaced with
the charge transport medium of graphene or TiO2 exhibits the sizable/robust
Rashba-Dresselhaus (RD) effect using density-functional-theory and ab initio
molecular dynamics (AIMD) simulations above cubic-phase temperature. At the
PbI$_2$-terminated graphene/CsPbI3(001) interface, ferroelectric distortion
towards graphene facilitates an inversion breaking field. At the MAI-terminated
TiO$_2$/MAPbI$_3$(001) interface, the enrooted alignment of MA$^+$ towards
TiO$_2$ by short-strong hydrogen-bonding and the concomitant PbI$_3$ distortion
preserve the RD interactions even above 330 K. The robust RD effect at the
interface even at high temperatures, unlike in bulk, changes the direct-type
band to the indirect to suppress recombination of electron and hole, thereby
letting these accumulated carriers overcome the potential barrier between
perovskite and charge transfer materials, which promotes the solar-cell
efficiency.

###State-of-the-Art Perovskite Solar Cells Benefit from Photon Recycling at Maximum Power Point||Roberto Brenes,Madeleine Laitz,Joel Jean,Dane W. deQuilettes,Vladimir Bulovic###

State-of-the-Art Perovskite Solar Cells Benefit from Photon Recycling at Maximum Power Point. Photon recycling is required for a solar cell to achieve an open-circuit
voltage ($V_{OC}$) and power conversion efficiency (PCE) approaching the
Shockley-Queisser theoretical limit. In metal halide perovskite solar cells,
the achievable performance gains from photon recycling remain uncertain due to
high variability in perovskite material quality and the non-radiative
recombination rate ($k_{1}$). In this work, we study state-of-the-art
$\textrm{Cs}_{0.05}(\textrm{MA}_{0.17}\textrm{FA}_{0.83})_{0.95}\textrm{Pb}(\textrm{I}_{0.83}\textrm{Br}_{0.17})_{3}$
films and analyze the impact of varying non-radiative recombination rates on
photon recycling and device performance. Importantly, we predict the impact of
photon recycling at the maximum power point (MPP), demonstrating an absolute
PCE increase of up to 2.0% in the radiative limit, primarily due to a 77 mV
increase in $V_{MPP}$. Even with finite non-radiative recombination, benefits
from photon recycling can be achieved when non-radiative lifetimes and external
LED electroluminescence efficiencies measured at open-circuit,
$Q_{e}^{LED}(\textrm{V}_{OC})$, exceed 2 $\mu$s and 10%, respectively. This
analysis clarifies the opportunity to fully exploit photon recycling to push
the real-world performance of perovskite solar cells toward theoretical limits.

###Voltage matching, étendue and ratchet steps in advanced concept solar cells||Andreas Pusch,Nicholas J. Ekins Daukes###

Voltage matching, étendue and ratchet steps in advanced concept solar cells. Many advanced solar cell concepts propose surpassing the Shockley Queisser
(SQ) limit by introducing multiple quasi-Fermi level separations that are
arranged in series and/or in parallel. Exceeding the SQ limit with any parallel
arrangement involves intermediate states that deliver additional charge
carriers at, ideally, the same electro-chemical potential as the other elements
in the parallel network. This can be thought of as voltage matching individual
parallel components and in intermediate band materials is intricately linked to
solar concentration and \'etendue mismatch between absorption and emission.
Generally, to achieve voltage matching under sub-optimal conditions, an
additional degree of freedom in the absorption thresholds of the material
through a carrier relaxation or ratchet step is required. We explain why the
ideal ratchet step decreases with solar concentration and how it depends on
radiative efficiency and emission \'etendue of the individual transitions. For
solar cell concepts that use Auger type carrier-carrier interactions or
molecular triplet states for energetic up- or down-conversion, ideal bandgap
combinations and achievable efficiencies also depend on interaction rates. We
show that Auger assisted solar cells suffer more strongly from finite
interaction rates than carrier multiplication devices.

###Enhancing CdTe Solar Cell Performance by Reducing the "Ideal" Bandgap of CdTe through CdTe1-xSex Alloying||Jingxiu Yang,Su-Huai Wei###

Enhancing CdTe Solar Cell Performance by Reducing the "Ideal" Bandgap of CdTe through CdTe1-xSex Alloying. CdTe is one of the leading materials for low cost, high efficiency thin-film
solar cells, because it has a high absorption coefficient and a nearly ideal
band gap of 1.48 eV for solar cell according to the Shockley-Queisser limit.
However, its solar to electricity power conversion efficiency (PCE) is hindered
by the relatively low open circuit voltage (VOC) due to intrinsic defect
related issues. Here, we propose the strategy of improving CdTe solar cell
performance byr reducing the "ideal" band gap of CdTe to gain more
short-circuit current from long-wavelength absorption without sacrificing much
VOC. Alloying CdTe with CdSe seems to be the most appropriate approach to
reduce the band gap because of the large optical bowing and relatively small
lattice mismatch in this system, even though CdSe has larger band gap than
CdTe. Using the first principle hybrid functional calculation, we find that the
minimum band gap of the CdTe1-xSex alloy can be reduced from 1.48 eV at x=0 to
1.39 eV at x=0.32. We also show that the formation of the alloy can improve the
defect property, for example, p-type doping of CdTe by CuCd can be greatly
enhanced by the alloying effects.

###Contribution to the study of sub-bandgap photon absorption in quantum dot InAs/AlGaAs intermediate band solar cells||Juan Villa,Iñigo Ramiro,José María Ripalda,Ignacio Tobías,Pablo García-Linares,Elisa Antolín,Antonio Martí###

Contribution to the study of sub-bandgap photon absorption in quantum dot InAs/AlGaAs intermediate band solar cells. Intermediate band solar cells (IBSCs) pursue the increase in efficiency by
absorbing below-bandgap energy photons while preserving the output voltage.
Experimental IBSCs based on quantum dots have already demonstrated that both
below-bandgap photon absorption and the output voltage preservation, are
possible. However, the experimental work has also revealed that the
below-bandgap absorption of light is weak and insufficient to boost the
efficiency of the solar cells. The objective of this work is to contribute to
the study of this absorption by manufacturing and characterizing a quantum dot
intermediate band solar cell with a single quantum dot layer with and without
light trapping elements. Using one-dimensional substrate texturing, our results
show a three-fold increase in the absorption of below bandgap energy photons in
the lowest energy region of the spectrum, a region not previously explored
using this approach. Furthermore, we also measure, at 9K, a distinguished split
of quasi-Fermi levels between the conduction and intermediate bands, which is a
necessary condition to preserve the output voltage of the cell.

###Effect of Cd diffusion on the electrical properties of the Cu(In,Ga)Se2 thin-film solar cell||Anna Koprek,Pawel Zabierowski,Marek Pawlowski,Luv Sharma,Christoph Freysoldt,Baptiste Gault,Roland Wuerz,Oana Cojocaru-Miredin###

Effect of Cd diffusion on the electrical properties of the Cu(In,Ga)Se2 thin-film solar cell. Cu(In,Ga)Se2 (CIGSe)-based solar cells are promising candidates for efficient
sunlight harvesting. However, their complex composition and microstructure can
change under operation conditions, for instance heating from sun light
illumination can lead to a degradation in performance. Here, we investigate the
thermally-induced degradation processes in a set of CIGSe-based solar cells
that were annealed at temperatures between 150C and 300C. Using correlative
atom probe tomography (APT)/transmission electron microscope (TEM), we found
that the buffer-absorber interface is not sharp but consists of an interfacial
zone (2 - 6.5 nm wide) where a gradient of constituent elements belonging to
the CdS buffer and CIGSe absorber appears. An enhanced short-range Cd
in-diffusion inside the CIGSe was observed whenever a low Ga/(Ga+In) ratio
occurred at the interface. This might indicate the presence of Ga vacancies as
a channeling defect for Cd in-diffusion inside the CIGSe layer leading to a
buried pn-homojunction. We evidence that a considerable amount of Cd is found
inside the CIGSe layer at annealing temperatures higher than 150C. Further
investigations of the elemental redistribution inside the CIGSe layer combined
with C-V measurements support the formation of CdCu donor like defects deep
inside the p-type CIGSe which lead to a strong compensation of the CIGSe layer
and hence to strong deterioration of cell efficiency at annealing temperatures
higher than 200C. Hence, understanding the degradation processes in
Cu(In,Ga)Se2 (CIGSe)-based solar cells opens new opportunities for further
improvement of the long-term device performance.

###Ultrathin plasma polymer passivation of perovskite solar cells for improved stability and reproducibility||Jose M. Obrero-Perez,Lidia Contreras-Bernal+,Fernando Nunez-Galvez,Javier Castillo-Seoane,Karen Valadez-Villalobos,Francisco J. Aparicio,Juan A. Anta,Ana Borras,Juan R. Sanchez-Valencia,Angel Barranco+###

Ultrathin plasma polymer passivation of perovskite solar cells for improved stability and reproducibility. Despite the youthfulness of hybrid halide perovskite solar cells, their
efficiencies are currently comparable to commercial silicon and have surpassed
quantum-dots solar cells. Yet, the scalability of these devices is a challenge
due to their low reproducibility and stability under environmental conditions.
However, the methods reported to date to tackle such issues recurrently involve
the use of solvent methods that would further complicate their transfer to
industry. Herein we present a reliable alternative relaying in the
implementation of an ultrathin plasma polymer as passivation interface between
the electron transport material and the hybrid perovskite layer. Such
nanoengineering interface provides solar devices with increased long-term
stability under ambient conditions. Thus, without consideringr any additional
encapsulation step, the cells retain more than 80 % of their efficiency after
being exposed to the ambient atmosphere for more than 1000 h. Moreover, this
plasma polymer passivation strategy significantly improves the coverage of the
mesoporous scaffold by the perovskite layer, providing the solar cells with
enhanced performance as well as improved reproducibility.

###Understanding the Role of Non-Fullerene Acceptors Crystallinity on the Charge Transport Properties and Performance of Organic Solar Cells||Pierluigi Mondelli,Pascal Kaienburg,Francesco Silvestri,Rebecca Scatena,Claire Welton,Martine Grandjean,Vincent Lemaur,Eduardo Solano,Mathias Nyman,Peter Horton,Simon Coles,Esther Barrena,Moritz Riede,Paolo Radaelli,David Beljonne,Manjunatha Reddy,Graham Morse###

Understanding the Role of Non-Fullerene Acceptors Crystallinity on the Charge Transport Properties and Performance of Organic Solar Cells. The active layer crystallinity has long been associated with favourable
organic solar cells (OSCs) properties such as high mobility and Fill Factor. In
particular, this applies to acceptor materials such as fullerene-derivatives
and the most recent Non-Fullerene Acceptors (NFAs), which are now surpassing
19% of Power Conversion Efficiency. Despite these advantages are being commonly
attributed to their 3-dimensional crystal packing motif in the single crystal,
the bridge that links the acceptor crystal packing from single crystals to
solar cells has not clearly been shown yet. In this work, we investigate the
molecular organisation of seven NFAs (o-IDTBR, IDIC, ITIC, m-ITIC, 4TIC, 4TICO,
m-4TICO), following the evolution of their packing motif in single-crystals,
powder and thin films made with pure NFAs and donor:NFA blends. In general, we
observed a good correlation between the NFA single crystal packing and their
molecular arrangement in the bulk heterojunction. However, the NFA packing
motif is not directly affecting the device parameters but it provide an impact
on the material propensity to form highly crystalline domain in the blend.
Although that NFA crystallinity is required to obtain high mobility, the domain
purity is more important to limit the bimolecular recombination and to obtain
high efficiency organic solar cells.

###How Good Can 2D Excitonic Solar Cells Be?||Zekun Hu,Da Lin,Jason Lynch,Kevin Xu,Deep Jariwala###

How Good Can 2D Excitonic Solar Cells Be?. Excitonic semiconductors have been a subject of research for photovoltaic
applications for many decades. Among them, the organic polymers and small
molecules based solar cells have now exceeded 19% power conversion efficiency
(PCE). While organic photovoltaics (OPVs) are approaching maturity, the advent
of strongly excitonic inorganic semiconductors such as two-dimensional
transition metal dichalcogenides (TMDCs) has renewed interest in excitonic
solar cells due to their high-optical constants, stable inorganic structure and
sub-nm film thicknesses. While several reports have been published on TMDC
based PVs, achieving power conversion efficiencies higher than 6% under one-sun
AM1.5G illumination has remained challenging. Here, we perform a full optical
and electronic analysis of design, structure and performance of monolayer TMDC
based, single-junction excitonic PVs. Our computational model with optimized
properties predicts a PCE of 9.22% in a superlattice device structure. Our
analysis suggests that, while the PCE for 2D excitonic solar cells may be
limited to < 10%, a specific power > 100 W g-1 may be achieved with our
proposed designs, making them attractive in aerospace, distributed remote
sensing, and wearable electronics.

###Controlling selenization equilibrium enables high-quality Cu2ZnSn(S, Se)4 absorbers for efficient solar cells||Xiao Xu,Jiazheng Zhou,Kang Yin,Jinlin Wang,Licheng Lou,Menghan Jiao,Bowen Zhang,Dongmei Li,Jiangjian Shi,Huijue Wu,Yanhong Luo,Qingbo Meng###

Controlling selenization equilibrium enables high-quality Cu2ZnSn(S, Se)4 absorbers for efficient solar cells. Cu2ZnSn(S, Se)4 (CZTSSe) is one of most competitive photovoltaic materials
for its earth-abundant reserves, environmental friendliness, and high
stability.The quality of CZTSSe absorber determines the power-conversion
efficiency (PCE) of CZTSSe solar cells. The absorber's quality lies on
post-selenization process, which is the reaction of Cu-Zn-Sn precursor and
selenium vapor. And the post-selenization is dependent on various factors (e.g.
temperature, precursor composition, reaction atmosphere, etc).However,
synergistic regulation of these factors cannot be realized under a widely-used
single-temperature zone selenization condition.Here, in our dual-temperature
zone selenization scheme, a solid-liquid and solid-gas (solid precursor and
liquid/gas phase Se) synergistic reaction strategy has been developed to
precisely regulate the selenization. Pre-deposited excess liquid Se provides
high Se chemical potential to drive a direct and fast formation of the CZTSSe
phase, significantly reducing the amount of binary and ternary compounds within
phase evolution. And organics removal can be accomplished via a synergistic
optimization of Se condensation and subsequent volatilization. We achieve a
high-performance CZTSSe solar cell with a remarkable PCE of 13.6%, and the
highest large-area PCE of 12.0% (over 1cm2). Our strategy will provide a new
idea for further improving efficiency of CZTSSe solar cells via phase evolution
regulation, and also for other complicated multi-compound synthesis.

###Heterojunction interface regulation to realize high-performance flexible Kesterite solar cells||Xiao Xu,Jiazheng Zhou,Kang Yin,Jinlin Wang,Licheng Lou,Dongmei Li,Jiangjian Shi,Huijue Wu,Yanhong Luo,Qingbo Meng###

Heterojunction interface regulation to realize high-performance flexible Kesterite solar cells. Flexible Cu2ZnSn(S, Se)4 (CZTSSe) solar cells take the advantages of
environmental friendliness, low cost, and multi-scenario applications, and have
drawn extensive attention in recent years. Compared with rigid devices, the
lack of alkali metal elements in the flexible substrate is the main factor
limiting the performance of flexible CZTSSe solar cells. This work proposes a
Rb ion additive strategy to simultaneously regulate the CZTSSe film surface
properties and the CdS chemical bath deposition (CBD) processes. Material and
chemical characterization reveals that Rb ions can passivate the detrimental
Se0 cluster defect and additionally provide a more active surface for the CdS
epitaxial growth. Furthermore, Rb can also coordinate with thiourea (TU) in the
CBD solution and improve the ion-by-ion deposition of the CdS layer. Finally,
the flexible CZTSSe cell fabricated by this strategy has reached a high
total-area efficiency of 12.63% (active-area efficiency of 13.2%), with its VOC
and FF reaching 538 mV and 0.70, respectively. This work enriches the alkali
metal passivation strategies and provides new ideas for further improving
flexible CZTSSe solar cells in the future.

###Maximising and Stabilising Luminescence in Metal Halide Perovskite Device Structures||Mojtaba Abdi-Jalebi,Zahra Andaji-Garmaroudi,Stefania Cacovich,Camille Stavrakas,Bertrand Philippe,Johannes M. Richter,Mejd Alsari,Edward P. Booker,Eline M. Hutter,Andrew J. Pearson,Samuele Lilliu,Tom J Savenije,Håkan Rensmo,Giorgio Divitini,Caterina Ducati,Richard H. Friend,Samuel D. Stranks###

Maximising and Stabilising Luminescence in Metal Halide Perovskite Device Structures. Metal halide perovskites are attracting tremendous interest for a variety of
optoelectronic applications. The ability to tune the perovskite bandgap by
tweaking the chemical compositions opens up new applications as coloured
emitters and as components of tandem photovoltaics. Nevertheless, non-radiative
losses are still limiting performance, with luminescence yields in
state-of-the-art perovskite solar cells still far from 100% under solar
illumination conditions. Furthermore, in mixed halide perovskite systems
designed for continuous bandgap tunability (bandgaps ~1.7-1.9 eV),
photo-induced ion segregation leads to bandgap instabilities. Here, we
substantially mitigate both non-radiative losses and photo-induced ion
migration in perovskite structures by decorating the surfaces and grain
boundaries with passivating potassium-halide interlayers. We demonstrate
external photo-luminescence quantum yields of 66%, translating to internal
yields exceeding 95%. The high luminescence yields are achieved while
maintaining high mobilities over 40 cm2V-1s-1, giving the elusive combination
of both high luminescence and excellent charge transport. We find that the
external luminescence yield when interfaced with electrodes in a solar cell
device stack, a quantity that must be maximized to approach the efficiency
limits, remains as high as 15%, indicating very clean interfaces. We also
demonstrate the inhibition of photo-induced ion migration processes across a
wide range of mixed halide perovskite bandgaps that otherwise show bandgap
instabilities. We validate these results in full operating solar cells,
highlighting the importance of stabilising luminescence in device structures.
Our work represents a critical breakthrough in the construction of tunable
halide perovskite films and interfaces that can approach the efficiency limits
in tandem solar cells and coloured LEDs.

###Efficient and environmental-friendly perovskite solar cells via embedding plasmonic nanoparticles: an optical simulation study on realistic device architecture||George Perrakis,George Kakavelakis,George Kenanakis,Constantinos Petridis,Emmanuel Stratakis,Maria Kafesaki,Emmanuel Kymakis###

Efficient and environmental-friendly perovskite solar cells via embedding plasmonic nanoparticles: an optical simulation study on realistic device architecture. Solution-processed, lead halide-based perovskite solar cells have overcome
important challenges over the recent years, offering low-cost and high solar
power conversion efficiencies. However, they still undergo unoptimized light
collection due mainly to the thin (~350 nm) polycrystalline absorber layers.
Moreover, their high toxicity (due to the presence of lead in the perovskite
crystalline structure) makes it necessary that the thickness of the absorber
layers to be further reduced, for their future commercialization, without
reducing the device performance. Here we aim to address these issues via
embedding spherical plasmonic nanoparticles of various sizes, composition,
concentrations, and vertical positions, for the first time in realistic
halide-based perovskite solar cells architecture, and to clarify their effect
on the absorption properties and enhancement. We theoretically show that
plasmon-enhanced near-field effects and scattering leads to a device
photocurrent enhancement of up to ~7.3% when silver spheres are embedded inside
the perovskite layer. Interestingly, the combination of silver spheres in
perovskite and aluminum spheres inside the hole transporting layer (PEDOT:PSS)
of the solar cell leads to an even further enhancement, of up to ~12%. This
approach allows the employment of much thinner perovskite layers in PSCs (up to
150 nm) to reach the same photocurrent as the nanoparticles-free device and
reducing thus significantly the toxicity of the device. Providing the
requirements related to the size, shape, position, composition, and
concentration of nanoparticles for the PSCs photocurrent enhancement, our study
establishes guidelines for a future development of highly-efficient,
environmentally friendly and low-cost plasmonic perovskite solar cells.

###Describing transport in defected nanoparticle solids using a new, hierarchical, simulation tool, TRIDENS||Chase Hansen,Davis Unruh,Miguel Alba,Caroline Qian,Alex Abelson,Matt Law,Gergely T. Zimanyi###

Describing transport in defected nanoparticle solids using a new, hierarchical, simulation tool, TRIDENS. The efficiency of nanoparticle (NP) solar cells has grown impressively in
recent years, exceeding 16%. However, the carrier mobility in NP solar cells,
and in other optoelectronic applications remains low, thus critically limiting
their performance. Therefore, carrier transport in NP solids needs to be better
understood to further improve the overall efficiency of NP solar cell
technology. However, it is technically challenging to simulate experimental
scale samples, as physical processes from atomic to mesoscopic scales all
crucially impact transport. To rise to this challenge, here we report the
development of TRIDENS: the Transport in Defected Nanoparticle Solids
Simulator, that adds three more hierarchical layers to our previously developed
HINTS code for nanoparticle solar cells. In TRIDENS, we first introduced planar
defects, such as twin planes and grain boundaries into individual NP SLs that
comprised the order of 10^3 NPs. Then we used HINTS to simulate the transport
across tens of thousands of defected NP SLs, and constructed the distribution
of the NP SL mobilities with planar defects. Second, the defected NP SLs were
assembled into a resistor network with more than 10^4 NP SLs, thus representing
about 10^7 individual NPs. Finally, the TRIDENS results were analyzed by finite
size scaling to explore whether the percolation transition, separating the
phase where the low mobility defected NP SLs percolate, from the phase where
the high mobility undefected NP SLs percolate drives a
low-mobility-to-high-mobility transport crossover that can be extrapolated to
macroscopic length scales. For the theoretical description, we adapted the
Efros-Shklovskii bimodal mobility distribution percolation model. We
demonstrated that the ES bimodal theory's two-variable scaling function is an
effective tool to quantitatively characterize this
low-mobility-to-high-mobility transport crossover.

###Two-dimensional GaAs/AlGaAs superlattice structures for solar cell applications: ultimate efficiency estimation||Jaroslaw Klos,Maciej Krawczyk###

Two-dimensional GaAs/AlGaAs superlattice structures for solar cell applications: ultimate efficiency estimation. We calculate the band structure of a two-dimensional GaAs/AlGaAs superlattice
and estimate the ultimate efficiency of solar cells using this type of
structure for solar energy conversion. The superlattice under consideration
consists of gallium arsenide rods forming a square lattice and embedded in
aluminium gallium arsenide. The ultimate efficiency is determined versus
structural parameters including the filling fraction, the superlattice
constant, the rod geometry and the concentration of Al in the matrix material.
The calculated efficiency of the superlattice proves to exceed the efficiency
of each component material in the monolithic state in a wide range of parameter
values.

###Unraveling the Role of Morphology on Organic Solar Cell Performance||Biswajit Ray,Pradeep R. Nair,Muhammad A. Alam###

Unraveling the Role of Morphology on Organic Solar Cell Performance. Polymer based organic photovoltaic (OPV) technology offers a relatively
inexpensive option for solar energy conversion provided its efficiency
increases beyond the current level (6-7%) along with significant improvements
in operational lifetime. The critical aspect of such solar cells is the complex
morphology of distributed bulk heterojunctions, which plays the central role in
the conversion of photo-generated excitons to electron-hole pairs. However, the
fabrication conditions that can produce the optimal morphology are still
unknown due to the lack of quantitative understanding of the effects of process
variables on the cell morphology. In this article, we develop a unique
process-device co-simulation framework based on phase-field model for phase
separation coupled with self-consistent drift-diffusion transport to
quantitatively explore the effects of the process conditions (e.g., annealing
temperature, mixing ratio, anneal duration) on the organic solar cell
performance. Our results explain experimentally observed trends of open circuit
voltage and short circuit current that would otherwise be deemed anomalous from
the perspective of conventional solar cells. In addition to providing an
optimization framework for OPV technology, our morphology-aware modeling
approach is ideally suited for a wide class of problems involving porous
materials, block co-polymers, polymer colloids, OLED devices etc.

###A Simple and Scalable Graphene Patterning Method and Its Application in CdSe Nanobelt/Graphene Schottky Junction Solar Cells||Yu Ye,Lin Gan,Lun Dai,Yu Dai,Xuefeng Guo,Hu Meng,Bin Yu,Zujin Shi,Guogang Qin###

A Simple and Scalable Graphene Patterning Method and Its Application in CdSe Nanobelt/Graphene Schottky Junction Solar Cells. We develop a simple and scalable graphene patterning method using
electron-beam or ultraviolet lithography followed by a lift-off process. This
method, with the merits of: high pattern resolution and high alignment
accuracy, free from additional etching or harsh process, universal to arbitrary
substrates, compatible to Si microelectronic technology, can be easily applied
to diverse graphene-based devices, especially in array-based applications,
where large-scale graphene patterns are desired. We have applied this method to
fabricate CdSe nanobelt (NB)/graphene Schottky junction solar cells, which have
potential application in integrated nano-optoelectronic systems. Typical
as-fabricated solar cell shows excellent photovoltaic behavior with an
open-circuit voltage of ~ 0.51 V, a short-circuit current density of ~ 5.75
mA/cm2, and an energy conversion efficiency of ~1.25%. We attribute the high
performance of the cell to the as-patterned high-performance graphene, which
can form an ideal Schottky contact with CdSe NB. Our results suggest both the
developed graphene patterning method and the as-fabricated CdSe nanobelt
(NB)/graphene Schottky junction solar cells have reachable application
prospect.

###Influence of Phase Segregation on Recombination Dynamics in Organic Bulk-Heterojunction Solar Cells||Andreas Baumann,Tom J. Savenije,Dharmapura Hanumantharaya K. Murthy,Martin Heeney,Vladimir Dyakonov,Carsten Deibel###

Influence of Phase Segregation on Recombination Dynamics in Organic Bulk-Heterojunction Solar Cells. We studied the recombination dynamics of charge carriers in organic bulk
heterojunction solar cells made of the blend system poly(2,5-bis(3-dodecyl
thiophen-2-yl) thieno[2,3-b]thiophene) (pBTCT-C12):[6,6]-phenyl-C61-butyric
acid methyl ester (PC61BM) with a donor--acceptor ratio of 1:1 and 1:4. The
techniques of charge carrier extraction by linearly increasing voltage
(photo-CELIV) and, as local probe, time-resolved microwave conductivity (TRMC)
were used. We observed a difference in the initially extracted charge carrier
concentration in the photo-CELIV experiment by one order of magnitude, which we
assigned to an enhanced geminate recombination due to a fine interpenetrating
network with isolated phase regions in the 1:1 pBTCT-C12:PC61BM bulk
heterojunction solar cells. In contrast, extensive phase segregation in 1:4
blend devices leads to an efficient polaron generation resulting in an
increased short circuit current density of the solar cell. For both studied
ratios a bimolecular recombination of polarons was found using the
complementary experiments. The charge carrier decay order of above two for
temperatures below 300 K can be explained by a release of trapped charges. This
mechanism leads to a delayed bimolecular recombination processes. The
experimental findings can be generalized to all polymer:fullerene blend systems
allowing for phase segregation.

###Triplet Exciton Generation in Bulk-Heterojunction Solar Cells based on Endohedral Fullerenes||Moritz Liedtke,Andreas Sperlich,Hannes Kraus,Andreas Baumann,Carsten Deibel,Maarten J. M. Wirix,Joachim Loos,Claudia M. Cardona,Vladimir Dyakonov###

Triplet Exciton Generation in Bulk-Heterojunction Solar Cells based on Endohedral Fullerenes. Organic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic
nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy
carbonyl]propyl-1-phenyl-Lu3N@C80 (Lu3N@C80-PCBEH) show an open circuit voltage
(VOC) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid
methyl ester (PC61BM). To fully exploit the potential of this acceptor molecule
with respect to the power conversion efficiency (PCE) of solar cells, the short
circuit current (JSC) should be improved to become competitive with the state
of the art solar cells. Here, we address factors influencing the JSC in blends
containing the high voltage absorber Lu3N@C80-PCBEH in view of both
photogeneration but also transport and extraction of charge carriers. We apply
optical, charge carrier extraction, morphology, and spin-sensitive techniques.
In blends containing Lu3N@C80-PCBEH, we found 2 times weaker photoluminescence
quenching, remainders of interchain excitons, and, most remarkably, triplet
excitons formed on the polymer chain, which were absent in the reference
P3HT:PC61BM blends. We show that electron back transfer to the triplet state
along with the lower exciton dissociation yield due to intramolecular charge
transfer in Lu3N@C80-PCBEH are responsible for the reduced photocurrent.

###Angular behavior of the absorption limit in thin film silicon solar cells||Ali Naqavi,Franz-Josef Haug,Karin Söderström,Corsin Battaglia,Vincent Paeder,Toralf Scharf,Hans Peter Herzig,Christophe Ballif###

Angular behavior of the absorption limit in thin film silicon solar cells. We investigate the angular behavior of the upper bound of absorption provided
by the guided modes in thin film solar cells. We show that the 4n^2 limit can
be potentially exceeded in a wide angular and wavelength range using
two-dimensional periodic thin film structures. Two models are used to estimate
the absorption enhancement; in the first one, we apply the periodicity
condition along the thickness of the thin film structure but in the second one,
we consider imperfect confinement of the wave to the device. To extract the
guided modes, we use an automatized procedure which is established in this
work. Through examples, we show that from the optical point of view, thin film
structures have a high potential to be improved by changing their shape. Also,
we discuss the nature of different optical resonances which can be potentially
used to enhance light trapping in the solar cell. We investigate the two
different polarization directions for one-dimensional gratings and we show that
the transverse magnetic polarization can provide higher values of absorption
enhancement. We also propose a way to reduce the angular dependence of the
solar cell efficiency by the appropriate choice of periodic pattern. Finally,
to get more practical values for the absorption enhancement, we consider the
effect of parasitic loss which can significantly reduce the enhancement factor.

###Enhancing Intrinsic Stability of Hybrid Perovskite Solar Cell by Strong, yet Balanced, Electronic Coupling||Fedwa El-Mellouhi,El Tayeb Bentria,Sergey N Rashkeev,Sabre Kais,Fahhad H Alharbi###

Enhancing Intrinsic Stability of Hybrid Perovskite Solar Cell by Strong, yet Balanced, Electronic Coupling. In the past few years, the meteoric development of hybrid organic--inorganic
perovskite solar cells (PSC) astonished the community. The efficiency has
already reached the level needed for commercialization; however, the
instability hinders its deployment on the market. Here, we report a mechanism
to chemically stabilize PSC absorbers. We propose to replace the widely used
methylammonium cation (\ce{CH3NH3+}) by alternative molecular cations allowing
an enhanced electronic coupling between the cation and the \ce{PbI6} octahedra
while maintaining the band gap energy within the suitable range for solar
cells. The mechanism exploits establishing a balance between the
electronegativity of the materials' constituents and the resulting ionic
electrostatic interactions. The calculations demonstrate the concept of
enhancing the electronic coupling, and hence the stability, by exploring the
stabilizing features of \ce{CH3PH3+}, \ce{CH3SH2+}, and \ce{SH3+} cations,
among several other possible candidates. Chemical stability enhancement hence
results from a strong, yet balanced, electronic coupling between the cation and
the halides in the octahedron. This shall unlock the hindering instability
problem for PSCs and allow them to hit the market as a serious low-cost
competitor to silicon based solar cell technologies.

###Graphene-based electron transport layers in perovskite solar cells: a step-up for an efficient carrier collection||F. Biccari,F. Gabelloni,E. Burzi,M. Gurioli,S. Pescetelli,A. Agresti,A. E. Del Rio Castillo,A. Ansaldo,E. Kymakis,F. Bonaccorso,A. Di Carlo,A. Vinattieri###

Graphene-based electron transport layers in perovskite solar cells: a step-up for an efficient carrier collection. The electron transport layer (ETL) plays a fundamental role in perovskite
solar cells. Recently, graphene-based ETLs have been proved to be good
candidate for scalable fabrication processes and to achieve higher carrier
injection with respect to most commonly used ETLs. In this work we
experimentally study the effects of different graphene-based ETLs in sensitized
MAPI solar cells. By means of time-integrated and picosecond time-resolved
photoluminescence techniques, the carrier recombination dynamics in MAPI films
embedded in different ETLs is investigated. Using graphene doped mesoporous
TiO2 (G+mTiO2) with the addition of a lithium-neutralized graphene oxide
(GO-Li) interlayer as ETL, we find that the carrier collection efficiency is
increased by about a factor two with respect to standard mTiO2. Taking
advantage of the absorption coefficient dispersion, we probe the MAPI layer
morphology, along the thickness, finding that the MAPI embedded in the ETL
composed by G+mTiO2 plus GO-Li brings to a very good crystalline quality of the
MAPI layer with a trap density about one order of magnitude lower than that
found with the other ETLs. In addition, this ETL freezes MAPI at the tetragonal
phase, regardless of the temperature. Graphene-based ETLs can open the way to
significant improvement of perovskite solar cells.

###Interface band gap narrowing behind open circuit voltage losses in Cu$_2$ZnSnS$_4$ solar cells||Andrea Crovetto,Mattias Palsgaard,Tue Gunst,Troels Markussen,Kurt Stokbro,Mads Brandbyge,Ole Hansen###

Interface band gap narrowing behind open circuit voltage losses in Cu$_2$ZnSnS$_4$ solar cells. We present evidence that band gap narrowing at the heterointerface may be a
major cause of the large open circuit voltage deficit of Cu$_2$ZnSnS$_4$/CdS
solar cells. Band gap narrowing is caused by surface states that extend the
Cu$_2$ZnSnS$_4$ valence band into the forbidden gap. Those surface states are
consistently found in Cu$_2$ZnSnS$_4$, but not in Cu$_2$ZnSnSe$_4$, by
first-principles calculations. They do not simply arise from defects at
surfaces but are an intrinsic feature of Cu$_2$ZnSnS$_4$ surfaces. By including
those states in a device model, the outcome of previously published
temperature-dependent open circuit voltage measurements on Cu$_2$ZnSnS$_4$
solar cells can be reproduced quantitatively without necessarily assuming a
cliff-like conduction band offset with the CdS buffer layer. Our
first-principles calculations indicate that Zn-based alternative buffer layers
are advantageous due to the ability of Zn to passivate those surface states.
Focusing future research on Zn-based buffers is expected to significantly
improve the open circuit voltage and efficiency of pure-sulfide Cu$_2$ZnSnS$_4$
solar cells.

###Predictive modeling of ion migration induced degradation in perovskite solar cells||Vikas Nandal,Pradeep R. Nair###

Predictive modeling of ion migration induced degradation in perovskite solar cells. With excellent efficiencies being reported from multiple labs across the
world, device stability and the degradation mechanisms have emerged as the key
aspects that could determine the future prospects of perovskite solar cells.
However, the related experimental efforts remain scattered due to the lack of
any unifying theoretical framework. In this context, here we provide a
comprehensive analysis of ion migration effects in perovskite solar cells.
Specifically, we show, for the first time, that (a) the effect of ionic charges
is almost indistinguishable from that of dopant ions, (b) ion migration could
lead to simultaneous improvement in Voc and degradation in Jsc - a unique
observation which is beyond the realm of mere parametric variation in carrier
mobility and lifetime, (c) champion devices are more resilient towards the ill
effects of ion migration, and finally (d) we propose unique characterization
schemes to determine both magnitude and polarity of ionic species. Our results,
supported by detailed numerical simulations and direct comparison with
experimental data, are of broad interest and provide a much needed predictive
capability towards the research on performance degradation mechanisms in
perovskite solar cells

###Quantum-kinetic perspective on photovoltaic device operation in nanostructure-based solar cells||Urs Aeberhard###

Quantum-kinetic perspective on photovoltaic device operation in nanostructure-based solar cells. The implementation of a wide range of novel concepts for next-generation
high-efficiency solar cells is based on nanostructures with
configuration-tunable optoelectronic properties. On the other hand, effective
nano-optical light-trapping concepts enable the use of ultra-thin absorber
architectures. In both cases, the local density of electronic and optical
states deviates strongly from that in a homogeneous bulk material. At the same
time, non-local and coherent phenomena like tunneling or ballistic transport
become increasingly relevant. As a consequence, the semi-classical, diffusive
bulk picture conventionally assumed may no longer be appropriate to describe
the physical processes of generation, transport, and recombination governing
the photovoltaic operation of such devices. In this review, we provide a
quantum-kinetic perspective on photovoltaic device operation that reaches
beyond the limits of the standard simulation models for bulk solar cells.
Deviations from bulk physics are assessed in ultra-thin film and
nanostructure-based solar cell architectures by comparing the predictions of
the semi-classical models for key physical quantities such as absorption
coefficients, emission spectra, generation and recombination rates as well as
potentials, densities and currents with the corresponding properties as given
by a more fundamental description based on non-equilibrium quantum statistical
mechanics. This advanced approach, while paving the way to a comprehensive
quantum theory of photovoltaics, bridges simulations at microscopic material
and macroscopic device levels by providing the charge carrier dynamics at the
mesoscale.

###Transfer Learning Using Ensemble Neural Networks for Organic Solar Cell Screening||Arindam Paul,Dipendra Jha,Reda Al-Bahrani,Wei-keng Liao,Alok Choudhary,Ankit Agrawal###

Transfer Learning Using Ensemble Neural Networks for Organic Solar Cell Screening. Organic Solar Cells are a promising technology for solving the clean energy
crisis in the world. However, generating candidate chemical compounds for solar
cells is a time-consuming process requiring thousands of hours of laboratory
analysis. For a solar cell, the most important property is the power conversion
efficiency which is dependent on the highest occupied molecular orbitals (HOMO)
values of the donor molecules. Recently, machine learning techniques have
proved to be very useful in building predictive models for HOMO values of donor
structures of Organic Photovoltaic Cells (OPVs). Since experimental datasets
are limited in size, current machine learning models are trained on data
derived from calculations based on density functional theory (DFT). Molecular
line notations such as SMILES or InChI are popular input representations for
describing the molecular structure of donor molecules. The two types of line
representations encode different information, such as SMILES defines the bond
types while InChi defines protonation. In this work, we present an ensemble
deep neural network architecture, called SINet, which harnesses both the SMILES
and InChI molecular representations to predict HOMO values and leverage the
potential of transfer learning from a sizeable DFT-computed dataset- Harvard
CEP to build more robust predictive models for relatively smaller HOPV
datasets. Harvard CEP dataset contains molecular structures and properties for
2.3 million candidate donor structures for OPV while HOPV contains DFT-computed
and experimental values of 350 and 243 molecules respectively. Our results
demonstrate significant performance improvement from the use of transfer
learning and leveraging both molecular representations.

###Semitransparent Polymer-Based Solar Cells with Aluminum-Doped Zinc Oxide Electrodes||Sebastian Wilken,Verena Wilkens,Dorothea Scheunemann,Regina-Elisabeth Nowak,Karsten von Maydell,Jürgen Parisi,Holger Borchert###

Semitransparent Polymer-Based Solar Cells with Aluminum-Doped Zinc Oxide Electrodes. With the usage of two transparent electrodes, organic solar cells are
semitransparent and may be combined to parallel-connected multi-junction
devices or used for innovative applications like power-generating windows. A
challenging issue is the optimization of the electrodes, in order to combine
high transparency with adequate electric properties. In the present work, we
study the potential of sputter-deposited aluminum-doped zinc oxide (AZO) as an
alternative to the widely used but relatively expensive indium tin oxide (ITO)
as cathode material in semitransparent polymer-fullerene solar cells.
Concerning the anode, we utilized an insulator/metal/insulator structure based
on ultra-thin Au films embedded between two evaporated MoO$_3$ layers, with the
outer MoO$_3$ film (capping layer) serving as a light coupling layer. The
performance of the ITO-free semitransparent solar cells is systematically
studied as dependent on the thickness of the capping layer and the active
layer, as well as the illumination direction. These variations are found to
have strong impact on the obtained photocurrent. We performed optical
simulations of the electric field distribution within the devices to analyze
the origin of the current variations and provide deep insight in the device
physics. With the conventional absorber materials studied herein, optimized
ITO-free and semitransparent devices reached 2.0% power conversion efficiency
and a maximum optical transmission of 60%, with the device concept being
potentially transferable to other absorber materials.

###Impact of a doping-induced space-charge region on the collection of photo-generated charge carriers in thin-film solar cells based on low-mobility semiconductors||Oskar J. Sandberg,Staffan Dahlström,Mathias Nyman,Sebastian Wilken,Dorothea Scheunemann,Ronald Österbacka###

Impact of a doping-induced space-charge region on the collection of photo-generated charge carriers in thin-film solar cells based on low-mobility semiconductors. Unintentional doping of the active layer is a source for lowered device
performance in organic solar cells. The effect of doping is to induce a
space-charge region within the active layer, generally resulting in increased
recombination losses. In this work, the impact of a doping-induced space-charge
region on the current-voltage characteristics of low-mobility solar cell
devices has been clarified by means of analytical derivations and numerical
device simulations. It is found that, in case of a doped active layer, the
collection efficiency of photo-generated charge carriers is independent of the
light intensity and exhibits a distinct voltage dependence, resulting in an
apparent electric-field dependence of the photocurrent. Furthermore, an
analytical expression describing the behavior of the photocurrent is derived.
The validity of the analytical model is verified by numerical drift-diffusion
simulations and demonstrated experimentally on solution-processed organic solar
cells. Based on the theoretical results, conditions of how to overcome charge
collection losses caused by doping are discussed. Furthermore, the presented
analytical framework provides tools to distinguish between different mechanisms
leading to voltage dependent photocurrents.

###Impact of $Al_2O_3$ Passivation on the Photovoltaic Performance of Vertical $WSe_2$ Schottky Junction Solar Cells||Elaine McVay,Ahmad Zubair,Yuxuan Lin,Amirhasan Nourbakhsh,Tomás Palacios###

Impact of $Al_2O_3$ Passivation on the Photovoltaic Performance of Vertical $WSe_2$ Schottky Junction Solar Cells. Transition metal dichalcogenide (TMD) materials have emerged as promising
candidates for thin film solar cells due to their wide bandgap range across the
visible wavelengths, high absorption coefficient and ease of integration with
both arbitrary substrates as well as conventional semiconductor technologies.
However, reported TMD-based solar cells suffer from relatively low external
quantum efficiencies (EQE) and low open circuit voltage due to unoptimized
design and device fabrication. This paper studies $Pt/WSe_2$ vertical Schottky
junction solar cells with various $WSe_2$ thicknesses in order to find the
optimum absorber thickness.Also, we show that the photovoltaic performance can
be improved via $Al_2O_3$ passivation which increases the EQE by up to 29.5% at
410 nm wavelength incident light. The overall resulting short circuit current
improves through antireflection coating, surface doping, and surface trap
passivation effects. Thanks to the ${Al_2O_3}$ coating, this work demonstrates
a device with open circuit voltage ($V_{OC}$) of 380 mV and short circuit
current density ($J_{SC}$) of 10.7 $mA/cm^2$. Finally, the impact of Schottky
barrier height inhomogeneity at the $Pt/WSe_2$ contact is investigated as a
source of open circuit voltage lowering in these devices

###Tuning the Electronic Levels of NiO with Alkali Halides Surface Modifiers for Perovskite Solar Cells||Sofia Apergi,Geert Brocks,Shuxia Tao###

Tuning the Electronic Levels of NiO with Alkali Halides Surface Modifiers for Perovskite Solar Cells. Favorable optoelectronic properties and ease of fabrication make NiO a
promising hole transport layer for perovskite solar cells. To achieve maximum
efficiency, the electronic levels of NiO need to be optimally aligned with
those of the perovskite absorber. Applying surface modifiers by adsorbing
species on the NiO surface, is one of the most widespread strategies to tune
its energy levels. Alkali halides are simple inorganic surface modifiers that
have been extensively used in organic optoelectronics, however, rarely studied
in perovskite solar cells. Using density functional theory (DFT) calculations,
we investigate the effect of single layer adsorption of twenty different alkali
halides on the electronic levels of NiO. Our results show that alkali halides
can shift the position of the valence band maximum (VBM) of NiO to a
surprisingly large extend in both directions, from -3:10 eV to +1:59 eV. We
interpret the direction and magnitude of the shift in terms of the surface
dipoles, formed by the adsorbed cations and anions, where the magnitude of the
VBM shift is a monotonic function of the surface coverage. Our results indicate
that with alkali halide surface modifiers, the electronic levels of NiO can be
tuned robustly and potentially match those of many perovskite compositions in
perovskite solar cells.

###Perovskite/silicon tandem solar cells: Effect of luminescent coupling and bifaciality||Klaus Jäger,Peter Tillmann,Eugene A. Katz,Christiane Becker###

Perovskite/silicon tandem solar cells: Effect of luminescent coupling and bifaciality. The power conversion efficiency of the market-dominating silicon
photovoltaics approaches its theoretical limit. Bifacial solar operation with
harvesting additional light impinging on the module back and the
perovskite/silicon tandem device architecture are among the most promising
approaches for further increasing the energy yield from a limited area. Here,
we calculate the energy output of perovskite/silicon tandem solar cells in
monofacial and bifacial operation considering, for the first time, luminescent
coupling between two sub-cells. For energy yield calculations we study
idealized solar cells at both, standard testing as well as realistic weather
conditions in combination with a detailed illumination model for periodic solar
panel arrays. Considering typical, experimental photoluminescent quantum yield
values we find that more than 50% of excess electron-hole pairs in the
perovskite top cell can be utilized by the silicon bottom cell by means of
luminescent coupling. As a result, luminescent coupling strongly relaxes the
constraints on the top-cell bandgap in monolithic tandem devices. In
combination with bifacial operation, the optimum perovskite bandgap shifts from
1.71 eV to the range 1.60-1.65 eV where already high-quality perovskite
materials exist. The results can hence change a paradigm in developing the
optimum perovskite material for tandem solar cells.

###Technology ready use of single layer graphene as a transparent electrode for hybrid photovoltaic devices||Zhibing Wang,Conor P. Puls,Neal E. Staley,Yu Zhang,Aaron Todd,Jian Xu,Casey A. Howsare,Matthew J. Hollander,Joshua A. Robinson,Ying Liu###

Technology ready use of single layer graphene as a transparent electrode for hybrid photovoltaic devices. Graphene has been used recently as a replacement for indium tin oxide (ITO)
for the transparent electrode of an organic photovoltaic device. Due to its
limited supply, ITO is considered as a limiting factor for the
commercialization of organic solar cells. We explored the use of large-area
graphene grown on copper by chemical vapor deposition (CVD) and then
transferred to a glass substrate as an alternative transparent electrode. The
transferred film was shown by scanning Raman spectroscopy measurements to
consist of >90% single layer graphene. Optical spectroscopy measurements showed
that the layer-transferred graphene has an optical absorbance of 1.23% at a
wavelength of 532 nm. We fabricated organic hybrid solar cells utilizing this
material as an electrode and compared their performance with ITO devices
fabricated using the same procedure. We demonstrated power conversion
efficiency up to 3.98%, higher than that of the ITO device (3.86%), showing
that layer-transferred graphene promises to be a high quality, low-cost,
flexible material for transparent electrodes in solar cell technology.

###Effect of Ion Migration Induced Electrode Degradation on the Operational Stability of Perovskite Solar Cells||Boris Rivkin,Paul Fassl,Qing Sun,Alexander D. Taylor,Zhuoying Chen,Yana Vaynzof###

Effect of Ion Migration Induced Electrode Degradation on the Operational Stability of Perovskite Solar Cells. Perovskite-based solar cells are promising due to their rapidly improving
efficiencies, but suffer from instability issues. Recently it has been claimed
that one of the key contributors to the instability of perovskite solar cells
is ion migration induced electrode degradation, which can be avoided by
incorporating inorganic hole blocking layers (HBL) in the device architecture.
In this work, we investigate the operational environmental stability of
methylammonium lead iodide (MAPbI3) perovskite solar cells that contain either
an inorganic or organic HBL, with only the former effectively blocking ions
from migrating to the metal electrode. This is confirmed by X-ray photoemission
spectroscopy measured on electrodes of degraded devices, where only electrodes
of devices with an organic HBL show a significant iodine signal. Despite this,
we show that when these devices are degraded under realistic operational
conditions (i.e. constant illumination in a variety of atmospheric conditions),
both types of devices exhibit nearly identical degradation behavior. These
results demonstrate that contrary to prior suggestions, ion-induced electrode
degradation is not the dominant factor in perovskite environmental instability
under operational conditions.

###Influence of the Cathodes Microstructure on the Stability of Inverted Planar Perovskite Solar Cells||Svetlana Sirotinskaya,Roland Schmechel,Niels Benson###

Influence of the Cathodes Microstructure on the Stability of Inverted Planar Perovskite Solar Cells. One of the main challenges for perovskite solar cells (PSC) is their
stability, due to environment-induced perovskite decomposition. The resulting
decomposition compounds are mobile and may, therefore, react with charge
carrier extraction layers or the contact metallization, in addition to
enhancing the recombination rate in the absorber layer. In this contribution,
the influence of different contact metallization layers, such as aluminum (Al),
silver (Ag), gold (Au) and nickel (Ni) on the storage stability of inverted
planar methylammonium lead iodide (MAPI)-based perovskite solar cells without
encapsulation has been investigated. For this study current-voltage (J-V) and
impedance measurements in combination with scanning electron microscope (SEM)
and Energy-dispersive X-ray spectroscopy (EDX) analysis were used to examine
and correlate structural device information with the development of PSC
electrical properties. While a strong perovskite decomposition and further
iodide diffusion to the contacts were detected for devices using Al, Ag or Au
as cathode electrodes, the microstructure of Ni cathodes inhibits such
decomposition process. This experiment has allowed for the realization of MAPI
based solar cells with Ni contacts, which exhibit no efficiency decrease below
as-fabricated values for up to one month of storage and select AM1.5 testing in
ambient atmosphere.

###Direct quantification of quasi-Fermi level splitting in organic semiconductor devices||Drew B. Riley,Oskar J. Sandberg,Nora M. Wilson,Wei Li,Stefan Zeiske,Nasim Zarrabi,Paul Meredith,Ronald Osterbacka,Ardalan Armin###

Direct quantification of quasi-Fermi level splitting in organic semiconductor devices. Non-radiative losses to the open-circuit voltage are a primary factor in
limiting the power conversion efficiency of organic photovoltaic devices. The
dominant non-radiative loss is intrinsic to the active layer and can be
determined from the quasi-Fermi level splitting (QFLS) and the radiative
thermodynamic limit of the photovoltage. Quantification of the QFLS in thin
film devices with low mobility is challenging due to the excitonic nature of
photoexcitation and additional sources of nonradiative loss associated with the
device structure. This work outlines an experimental approach based on
electro-modulated photoluminescence, which can be used to directly measure the
intrinsic non-radiative loss to the open-circuit voltage; thereby, quantifying
the QFLS. Drift-diffusion simulations are carried out to show that this method
accurately predicts the QFLS in the bulk of the device regardless of
device-related non-radiative losses. State-of-the-art PM6:Y6-based organic
solar cells are used as a model to test the experimental approach, and the QFLS
is quantified and shown to be independent of device architecture. This work
provides a method to quantify the QFLS of organic solar cells under operational
conditions, fully characterizing the different contributions to the
non-radiative losses of the open-circuit voltage. The reported method will be
useful in not only characterizing and understanding losses in organic solar
cells, but also other device platforms such as light-emitting diodes and
photodetectors.

###High-Specific-Power Flexible Transition Metal Dichalcogenide Solar Cells||Koosha Nassiri Nazif,Alwin Daus,Jiho Hong,Nayeun Lee,Sam Vaziri,Aravindh Kumar,Frederick Nitta,Michelle Chen,Siavash Kananian,Raisul Islam,Kwan-Ho Kim,Jin-Hong Park,Ada Poon,Mark L. Brongersma,Eric Pop,Krishna C. Saraswat###

High-Specific-Power Flexible Transition Metal Dichalcogenide Solar Cells. Semiconducting transition metal dichalcogenides (TMDs) are promising for
flexible high-specific-power photovoltaics due to their ultrahigh optical
absorption coefficients, desirable band gaps and self-passivated surfaces.
However, challenges such as Fermi-level pinning at the metal contact-TMD
interface and the inapplicability of traditional doping schemes have prevented
most TMD solar cells from exceeding 2% power conversion efficiency (PCE). In
addition, fabrication on flexible substrates tends to contaminate or damage TMD
interfaces, further reducing performance. Here, we address these fundamental
issues by employing: 1) transparent graphene contacts to mitigate Fermi-level
pinning, 2) $\rm{MoO}_\it{x}$ capping for doping, passivation and
anti-reflection, and 3) a clean, non-damaging direct transfer method to realize
devices on lightweight flexible polyimide substrates. These lead to record PCE
of 5.1% and record specific power of $\rm{4.4\ W\,g^{-1}}$ for flexible TMD
($\rm{WSe_2}$) solar cells, the latter on par with prevailing thin-film solar
technologies cadmium telluride, copper indium gallium selenide, amorphous
silicon and III-Vs. We further project that TMD solar cells could achieve
specific power up to $\rm{46\ W\,g^{-1}}$, creating unprecedented opportunities
in a broad range of industries from aerospace to wearable and implantable
electronics.

###All Inorganic p_n Heterojunction Solar Cells by Solution Combustion Synthesis using n_type FeMnO3 Perovskite Photoactive Layer||Ioannis T. Papadas,Apostolos Ioakeimidis,Ioannis Vamvasakis,Polyvios Eleftheriou,Gerasimos S. Armatas,Stelios A. Choulis###

All Inorganic p_n Heterojunction Solar Cells by Solution Combustion Synthesis using n_type FeMnO3 Perovskite Photoactive Layer. This study outlines the synthesis and physicochemical characteristics of a
solution-processable iron manganite (FeMnO3) nanoparticles via a chemical
combustion method using tartartic acid as a fuel and demonstrates the
performance of this material as a n-type photoactive layer in all-oxide solar
cells. It is shown that the solution combustion synthesis (SCS) method enables
the formation of pure crystal phase FeMnO3 with controllable particle size. XRD
pattern and morphology images from TEM confirm the purity of FeMnO3 phase and
the relative small crystallite size (~13 nm), firstly reported in the
literature. Moreover, to assemble a network of connected FeMnO3 nanoparticles,
\b{eta}-alanine was used as a capping agent and dimethylformamide (DMF) as a
polar aprotic solvent for the colloidal dispersion of FeMnO3 NPs. This
procedure yields a ~500 nm thick photoactive layer. The proposed method is
crucial to obtain functional solution processed NiO/FeMnO3 heterojunction
inorganic photovoltaics. The optoelectronic properties of the heterojunction
were established. These solar cells demonstrate a high open circuit voltage of
1.31 V with sufficient fill factor of 54.3% and low short circuit current of
0.07 mA cm-2 delivering a power conversion efficiency of 0.05% under 100 mW
cm-2 illumination. This work expands on the burgeoning of environmentally
friendly, low-cost, sustainable solar cell material that derive from metal
oxides.

###Post-deposition annealing and interfacial ALD buffer layers of Sb$_2$Se$_3$/CdS stacks for reduced interface recombination and increased open-circuit voltages||Thomas Paul Weiss,Ignacio Minguez-Bacho,Elena Zuccalà,Michele Melchiorre,Nathalie Valle,Brahime El Adib,Tadahiro Yokosawa,Erdmann Spiecker,Julien Bachmann,Phillip J. Dale,Susanne Siebentritt###

Post-deposition annealing and interfacial ALD buffer layers of Sb$_2$Se$_3$/CdS stacks for reduced interface recombination and increased open-circuit voltages. Currently, Sb$_2$Se$_3$ thin films receive considerable research interest as
a solar cell absorber material. When completed into a device stack, the major
bottleneck for further device improvement is the open circuit voltage, which is
the focus of the work presented here. Polycrystalline thin film Sb$_2$Se$_3$
absorbers and solar cells are prepared in substrate configuration and the
dominant recombination path is studied using photoluminescence spectroscopy and
temperature dependent current-voltage characteristics. It is found that a
post-deposition annealing after the CdS buffer layer deposition can effectively
remove interface recombination since the activation energy of the dominant
recombination path becomes equal to the bandgap of the Sb$_2$Se$_3$ absorber.
The increased activation energy is accompanied by an increased
photoluminescence yield, i.e. reduced non-radiative recombination. Finished
Sb$_2$Se$_3$ solar cell devices reach open circuit voltages as high as 485 mV.
Contrarily, the short-circuit current density of these devices is limiting the
efficiency after the post-deposition annealing. It is shown that atomic layer
deposited intermediate buffer layers such as TiO$_2$ or Sb$_2$Se$_3$ can pave
the way for overcoming this limitation.

###Ultra-long charge carrier recombination time in methylammonium lead halide perovskites||A. Bojtor,S. Kollarics,B. G. Markus,A. Sienkiewicz,M. Kollar,L. Forro,F. Simon###

Ultra-long charge carrier recombination time in methylammonium lead halide perovskites. Due to their exceptional photovoltaic properties, metal halide perovskites
(MHPs) are extensively studied for their potential applications in solar cells.
In recent years, the power conversion efficiencies of MHPs-based solar cells
rapidly increased from the initial few \% towards more than 25\,\% for
single-junction devices. Therefore, also taking into account their low costs
and ease of manufacturing, MHPs-based solar cells have become the
fastest-advancing photovoltaic technology. In this regard, much of the recent
work has been dominated by absorber materials based on methylammonium MHPs,
such as MAPbX$_3$, where MA=CH$_3$NH$_3$ and X=Cl, Br and I. Here, we present
the results of contactless time-resolved photoconductivity measurements in an
exceptionally wide range of temperatures of $4$ to $290\ \text{K}$ that were
performed for the various crystalline forms of the three parent MAPbX$_3$,
i.e., MAPbCl$_3$, MAPbBr$_3$ and MAPbI$_3$. This approach was made possible by
the use of a high quality-factor (Q) microwave resonator, which cooperated with
a commercially available microwave bridge equipped with an automatic frequency
control (AFC) and a helium gas-flow cryostat.

###Outdoor Characterization of Solar Cells with Micro-structured Anti-Reflective Coating in a Concentrator Photovoltaic Module||Arnaud J. K. Leoga,Arnaud Ritou,Mathieu Blanchard,Lysandre Dirand,Yanis Prunier,Philippe St-Pierre,David Chuet,Philippe-Olivier Provost,Maite Volatier,Vincent Aimez,Gwenaelle Hamon,Abdelatif Jaouad,Christian Dubuc,Maxime Darnon###

Outdoor Characterization of Solar Cells with Micro-structured Anti-Reflective Coating in a Concentrator Photovoltaic Module. Micro-structured anti reflective coatings (ARC) have been identified as a
promising solution to reduce optical losses in Concentrator Photovoltaics
modules (CPV). We fabricated and tested in field a CPV modules made of 4
sub-modules with a concentration factor of 250x, that embed either solar cells
with micro-structured encapsulating ARC or solar cells with multilayer ARC as a
reference. The micro-structured encapsulating ARC was made of semi-buried
silica beads in polydimethylsiloxane (PDMS). The module was in operation for 1
year in the severe climatic conditions of Sherbrooke, Quebec, Canada, before
extracting the sub-modules performance under Concentrator Standard Operating
Condition (CSOC). An acceptance angle of +/-0.78 degree was determined for all
sub-modules, demonstrating that improving angular collection at the cell level
has no significant impact on the angle of acceptance at the module level. We
report an increase of 12 to 14% of the short-circuit current and of 15 to 19%
of maximum power at CSOC for solar cells with a micro structured encapsulating
ARC compared to the reference. Despite a sub-optimal module design, we report a
sub-module efficiency of 29.7% at CSOC for a cell with micro-structured
encapsulating ARC. This proves the potential of micro-structured encapsulating
ARC to improve CPV system performance and shows promise of reliability for
sumi-buried microbeads in PDMS as encapsulating ARC.

###Nanowire array photovoltaics: Radial disorder versus design for optimal efficiency||Björn C. P. Sturmberg,Kokou B. Dossou,Lindsay C. Botten,Ara A. Asatryan,Christopher G. Poulton,Ross C. McPhedran,C. Martijn de Sterke###

Nanowire array photovoltaics: Radial disorder versus design for optimal efficiency. Solar cell designs based on disordered nanostructures tend to have higher
efficiencies than structures with uniform absorbers, though the reason is
poorly understood. To resolve this, we use a semi-analytic approach to
determine the physical mechanism leading to enhanced efficiency in arrays
containing nanowires with a variety of radii. We use our findings to
systematically design arrays that outperform randomly composed structures. An
ultimate efficiency of 23.75% is achieved with an array containing 30% silicon,
an increase of almost 10% over a homogeneous film of equal thickness.

###Plasmonic multiple exciton generation||Jiantao Kong,Xueyuan Wu,Xin Wang,Michael J Naughton,Krzysztof Kempa###

Plasmonic multiple exciton generation. We show that bi-exciton formation can be highly efficient in a solar cell
with the semiconductor absorber filled with an array of metallic nanoparticles
having plasmonic resonance tuned to the semiconductor gap energy. This process
can be viewed as plasmon-enhanced multiple exciton generation (PMEG), with the
resulting cell efficiency exceeding the Shockley-Queisser limit. We
demonstrate, that efficiency of the PMEG process, increases with decreasing of
the semiconductor gap size, and illustrate that by considering in detail three
systems with gradually decreasing gap size: GaAs, Si and Ge.

###Optical Absorption Characteristics of Silicon Nanowires for Photovoltaic Applications||Vidur Parkash,Anand K. Kulkarni###

Optical Absorption Characteristics of Silicon Nanowires for Photovoltaic Applications. Solar cells have generated a lot of interest as a potential source of clean
renewable energy for the future. However a big bottleneck in wide scale
deployment of these energy sources remain the low efficiency of these
conversion devices. Recently the use of nanostructures and the strategy of
quantum confinement have been as a general approach towards better charge
carrier generation and capture. In this article we have presented calculations
on the optical characteristics of nanowires made out of Silicon. Our
calculations show these nanowires form excellent optoelectronic materials and
may yield efficient photovoltaic devices.

###Hot Carrier Solar Cells : In the Making ?||A. Le Bris,L. Lombez,Jean Francois Guillemoles,R. Esteban,M. Laroche,Jj. Greffet,G. Boissier,P. Christol,S. Collin,Jl. Pelouard,P. Aschehoug,F. Pellé###

Hot Carrier Solar Cells : In the Making ?. Hot carrier solar cells allow potential efficiency close to the
thermodynamical limit in ideal conditions. However, the feasability of such
devices has not been clearly stated so far and only ideal cells were considered
in previous studies. Here we develop a model with realistic energy selective
contacts, carrier thermalization and absorptivity. The requirements in term of
contact selectivity is investigated, showing that semi-selective contacts are
not incompatible with high efficiencies. Candidates for absorbing material were
synthesized and the required thermalization properties are obtained. Specific
structures were designed to enhance absorption of concentrated sunlight in a
small material thickness, allowing high carrier density in the absorber.

###High Efficiency Graphene Solar Cells by Chemical Doping||Xiaochang Miao,Sefaattin Tongay,Maureen K. Petterson,Kara Berke,Andrew G. Rinzler,Bill R. Appleton,Arthur F. Hebard###

High Efficiency Graphene Solar Cells by Chemical Doping. We demonstrate single layer graphene/n-Si Schottky junction solar cells that
under AM1.5 illumination exhibit a power conversion efficiency (PCE) of 8.6%.
This performance, achieved by doping the graphene with
bis(trifluoromethanesulfonyl)amide, exceeds the native(undoped) device
performance by a factor of 4.5 and the best previously reported PCE in similar
devices by a factor of nearly 6. Current-voltage, capacitance-voltage and
external quantum efficiency measurements show the enhancement to be due to the
doping induced shift in the graphene chemical potential which increases the
graphene carrier density (decreasing the cell series resistance) and increases
the built-in potential.

###A single diffractive optical element for implementing spectrum-splitting and beam-concentration functions simultaneously with high diffraction efficiency||Jia-Sheng Ye,Jin-Ze Wang,Qing-Li Huang,Bi-Zhen Dong,Yan Zhang,Guo-Zhen Yang###

A single diffractive optical element for implementing spectrum-splitting and beam-concentration functions simultaneously with high diffraction efficiency. In this paper, a novel method is proposed, and employed to design a single
diffractive optical element (DOE) for implementing spectrum-splitting and
beam-concentration (SSBC) functions simultaneously. We develop an optimization
algorithm, through which the SSBC DOE can be optimized within an arbitrary
thickness range, according to the limitations of modern photolithography
technology. Theoretical simulation results reveal that the designed SSBC DOE
has a high optical focusing efficiency. It is expected that the designed SSBC
DOE should have practical applications in high-efficiency solar cell systems.

###Quantum Wells in Photovoltaic Cells||C Rohr,P Abbott,I M Ballard,D B Bushnell,J P Connolly,N J Ekins- Daukes,K W J Barnham###

Quantum Wells in Photovoltaic Cells. The fundamental efficiency limit of a single bandgap solar cell is about 31%
at one sun with a bandgap of about Eg = 1.35 eV (1), determined by the
trade-off of maximising current with a smaller bandgap and voltage with a
larger bandgap. Multiple bandgaps can be introduced to absorb the broad solar
spectrum more efficiently. This can be realised in multi- junction cells, for
example, where two or more cells are stacked on top of each other either
mechanically or monolithically connected by a tunnel junction. An alternative
or complementary (see section 1.4) approach is the quantum well cell (QWC).

###A Comparison of Anodic TiO2 Nanotube Membranes used for Front-side Illuminated Dye-Sensitized Solar Cells||Fatemeh Mohammadpour,Mahmood Moradi,Gihoon Cha,Seulgi So,Kiyoung Lee,Marco Altomare,Patrik Schmuki###

A Comparison of Anodic TiO2 Nanotube Membranes used for Front-side Illuminated Dye-Sensitized Solar Cells. In the present work we compare TiO2 nanotube lift-off strategies for the
construction of front-side illuminated dye-sensitized solar cells (DSSCs).
Anodic nanotube layers were detached from the metallic back contact by using
different techniques and transferred onto an FTO substrate. We show that if we
use an optimized potential step treatment to fabricate membranes, DSSC cell
efficiencies can be significantly increased (>8%). This improved efficiency is
ascribed to higher specific dye-loading and enhanced electron transport
properties of optimally fabricated TiO2 nanotube membranes.

###Investigation of Carrier Recombination Dynamics of InGaP/InGaAsP Multiple Quantum Wells for Solar Cells via Photoluminescence||K. -H. Lee,K. W. J. Barnham,John S. Roberts,D. Alonso-Alvarez,N. P. Hylton,M. Fuhrer,N. J. Ekins-Daukes###

Investigation of Carrier Recombination Dynamics of InGaP/InGaAsP Multiple Quantum Wells for Solar Cells via Photoluminescence. The carrier recombination dynamics of InGaP/InGaAsP quantum wells are
reported for the first time. By studying the photoluminescence (PL) and
time-resolved PL decay of InGaP/InGaAsP multiple-quantum-well(MQW)
heterostructure samples, it is demonstrated that InGaP/InGaAsP MQWs have very
low non-radiative recombination rate and high radiative efficiency compared to
the control InGaP sample. Along with the analyses of PL emission spectrum and
external quantum efficiencies, it suggests that this is due to small
confinement potentials in the conduction band but high confinement potentials
in the valence band. These results explain several features found in
InGaP/InGaAsP MQW solar cells previously.

###Modeling photoconversion efficiency of perovskite solar cells||A. V. Sachenko,V. P. Kostylyov,A. V. Bobyl,V. M. Vlasyuk,I. O. Sokolovskyi,E. I. Terukov,M. Evstigneev###

Modeling photoconversion efficiency of perovskite solar cells. A theoretical approach to photoconversion efficiency modeling in perovskite
p-i-n structures is developed. The results of this modeling compare favorably
with the experiment and indicate that the surfaces of the perovskite solar
cells (SCs) are naturally textured. It is shown that photoconversion efficiency
in the limiting case of negligible Shockley-Read-Hall and surface recombination
and in the absence of optical losses reaches the value of 29%. In the realistic
case, the current-voltage curve ideality factor equals 2. This value is not due
to recombination in the space-charge region; rather, it can be explained by
taking into account the effect of the rear surface and high excitation level.

###Universal trade-off between power, efficiency, and constancy in steady-state heat engines||Patrick Pietzonka,Udo Seifert###

Universal trade-off between power, efficiency, and constancy in steady-state heat engines. Heat engines should ideally have large power output, operate close to Carnot
efficiency and show constancy, i.e., exhibit only small fluctuations in this
output. For steady-state heat engines, driven by a constant temperature
difference between the two heat baths, we prove that out of these three
requirements only two are compatible. Constancy enters quantitatively the
conventional trade-off between power and efficiency. Thus, we rationalize and
unify recent suggestions for overcoming this simple trade-off. Our universal
bound is illustrated for a paradigmatic model of a quantum dot solar cell and
for a Brownian gyrator delivering mechanical work against an external force.

###Exploring the Way to Approach the Efficiency Limit of Perovskite Solar Cells by Drift-Diffusion Model||Xingang Ren,Zishuai Wang,Wei E. I. Sha,Wallace C. H. Choy###

Exploring the Way to Approach the Efficiency Limit of Perovskite Solar Cells by Drift-Diffusion Model. Drift-diffusion model is an indispensable modeling tool to understand the
carrier dynamics (transport, recombination, and collection) and simulate
practical-efficiency of solar cells (SCs) through taking into account various
carrier recombination losses existing in multilayered device structures.
Exploring the way to predict and approach the SC efficiency limit by using the
drift-diffusion model will enable us to gain more physical insights and design
guidelines for emerging photovoltaics, particularly perovskite solar cells. Our
work finds out that two procedures are the prerequisites for predicting and
approaching the SC efficiency limit. Firstly, the intrinsic radiative
recombination needs to be corrected after adopting optical designs which will
significantly affect the open-circuit voltage at its Shockley-Queisser limit.
Through considering a detailed balance between emission and absorption of
semiconductor materials at the thermal equilibrium, and the Boltzmann
statistics at the non-equilibrium, we offer a different approach to derive the
accurate expression of intrinsic radiative recombination with the optical
corrections for semiconductor materials. The new expression captures light
trapping of the absorbed photons and angular restriction of the emitted photons
simultaneously, which are ignored in the traditional Roosbroeck-Shockley
expression. Secondly, the contact characteristics of the electrodes need to be
carefully engineered to eliminate the charge accumulation and surface
recombination at the electrodes. The selective contact or blocking layer
incorporated nonselective contact that inhibits the surface recombination at
the electrode is another important prerequisite. With the two procedures, the
accurate prediction of efficiency limit and precise evaluation of efficiency
degradation for perovskite solar cells are attainable by the drift-diffusion
model.

###Spin orbit coupling and Lorentz force enhanced efficiency of TiO2 based dye sensitized solar cells||U. M. Kannan,M. Venkat Narayana,Ganesh Kotnana,Jaipal Kandhadi,L. Giribabu,Surya Prakash Singh,S. Narayana Jammalamadaka###

Spin orbit coupling and Lorentz force enhanced efficiency of TiO2 based dye sensitized solar cells. We report on the effect of the strong spin orbit coupling and the Lorentz
force on the efficiency of TiO2 based dye sensitized solar cells. Upon
inclusion of Ho2O3, due to the strong spin orbit coupling of the rare earth
Ho3+ ion, we do see 13 percent enhancement in the efficiency. We attribute such
an enhancement in power conversion efficiency to the increased lifetime of the
photo-excited excitons. Essentially, a Ho3+ ion accelerates the phenomenon of
the spin rephasing or the intersystem crossing of the excitons in a
photosensitizer. Increase in the absorbance and decrease in the
photoluminescence intensity suggests a decrease in the recombination rate,
hinting an enhanced charge transport and is in accordance with our
electrochemical impedance spectra and the J V characteristics. From the above
we strongly believe that enhanced efficiency of the device is due to increased
intersystem crossing which would accelerate the exciton dissociation. On top of
spin orbit interaction, a configuration where the electric and magnetic fields
are perpendicular to each other helped in enhancing the efficiency by 16
percent, suggesting that the Lorentz force also plays a dominant role in
controlling the charge transport of the photo-generated charge carriers. We
strongly believe that this simple and novel strategy of improving the
efficiency may pave the way for realizing higher efficiency dye sensitized
solar cells.

###Pathways towards 30% efficient single-junction perovskite solar cells and the role of mobile ions||Jonas Diekmann,Pietro Caprioglio,Moritz H. Futscher,Vincent M. Le Corre,Sebastian Reichert,Frank Jaiser,Malavika Arvind,Lorena Perdigon Toro,Emilio Gutierrez-Partida,Francisco Pena-Camargo,Carsten Deibel,Bruno Ehrler,Thomas Unold,Thomas Kirchartz,Dieter Neher,Martin Stolterfoht###

Pathways towards 30% efficient single-junction perovskite solar cells and the role of mobile ions. Perovskite semiconductors have demonstrated outstanding external luminescence
quantum yields, enabling high power conversion efficiencies (PCE). However, the
precise conditions to advance to an efficiency regime above monocrystalline
silicon cells are not well understood. Here, we establish a simulation model
that well describes efficient p-i-n type perovskite solar cells and a range of
different experiments. We then study important device and material parameters
and we find that an efficiency regime of 30% can be unlocked by optimizing the
built-in potential across the perovskite layer by using either highly doped
(10^19 cm-3), thick transport layers (TLs) or ultrathin undoped TLs, e.g.
self-assembled monolayers. Importantly, we only consider parameters that have
been already demonstrated in recent literature, that is a bulk lifetime of 10
us, interfacial recombination velocities of 10 cm/s, a perovskite bandgap of
1.5 eV and an EQE of 95%. A maximum efficiency of 31% is predicted for a
bandgap of 1.4 eV. Finally, we demonstrate that the relatively high mobile ion
density does not represent a significant barrier to reach this efficiency
regime. Thus, the results of this paper promise continuous PCE improvements
until perovskites may become the most efficient single-junction solar cell
technology in the near future.

###Counterbalancing light absorption and ionic transport losses in the electrolyte for integrated solar water splitting with III-V/Si dual-junctions||Moritz Kölbach,Ciler Özen,Oliver Höhn,David Lackner,Markus Feifel,Fatwa F. Abdi,Matthias M. May###

Counterbalancing light absorption and ionic transport losses in the electrolyte for integrated solar water splitting with III-V/Si dual-junctions. Recently, significant progress in the development of III-V/Si dual-junction
solar cells has been achieved. This not only boosts the efficiency of Si-based
photovoltaic solar cells, but also offers the possibility of highly efficient
green hydrogen production via solar water splitting. Using such dual-junction
cells in a highly integrated photoelectrochemical approach and aiming for
upscaled devices with solar-to-hydrogen efficiencies beyond 20\%, however, the
following frequently neglected contrary effects become relevant: (i) light
absorption in the electrolyte layer in front of the top absorber and (ii) the
impact of this layer on the ohmic and transport losses. Here, we initially
model the influence of the electrolyte layer thickness on the maximum
achievable solar-to-hydrogen efficiency of a device with an Si bottom cell and
show how the top absorber bandgap has to be adapted to minimise efficiency
losses. Then, the contrary effects of increasing ohmic and transport losses
with decreasing electrolyte layer thickness are evaluated. This allows us to
estimate an optimum electrolyte layer thickness range that counterbalances the
effects of parasitic absorption and ohmic/transport losses. We show that
fine-tuning of the top absorber bandgap and the water layer thickness can lead
to an STH efficiency increase of up to 1\% absolute. Our results allow us to
propose important design rules for high-efficiency photoelectrochemical devices
based on multi-junction photoabsorbers.

###Design and numerical investigation of cadmium telluride (CdTe) and iron silicide (FeSi2) based double absorber solar cells to enhance power conversion efficiency||Md. Ferdous Rahman,M. J. A. Habib,Md. Hasan Ali,M. H. K. Rubel,M. Rounakul Islam,Abu Bakar Md. Ismail,M. Khalid Hossain###

Design and numerical investigation of cadmium telluride (CdTe) and iron silicide (FeSi2) based double absorber solar cells to enhance power conversion efficiency. Inorganic CdTe and FeSi2-based solar cells have recently drawn a lot of
attention because they offer superior thermal stability and good optoelectronic
properties compared to conventional solar cells. In this work, a unique
alternative technique is presented by using FeSi2 as a secondary absorber layer
and In2S3 as the window layer for improving photovoltaic (PV) performance
parameters. Simulating on SCAPS-1D, the proposed double-absorber
(Cu/FTO/In2S3/CdTe/FeSi2/Ni) structure is thoroughly examined and analyzed. The
window layer thickness, absorber layer thickness, acceptor density (NA), donor
density (ND), defect density (Nt), series resistance (RS), and shunt resistance
(Rsh) were simulated in detail for optimization of the above configuration to
improve PV performance. According to this study, 0.5 um is the optimized
thickness for both the CdTe and FeSi2 absorber layers in order to maximize
efficiency. Here, the value of the optimum window layer thickness is 50 nm. For
using CdTe as a single absorber, the efficiency is achieved by 13.26%. But for
using CdTe and FeSi2 as a dual absorber, the efficiency is enhanced and the
obtaining value is 27.35%. The other parameters are also improved and the
obtaining values for fill factor (FF) are 83.68%, open-circuit voltage (Voc) is
0.6566V, and short circuit current density (JSc) is 49.78 mA/cm2. Furthermore,
the proposed model performs good at 300 K operating temperature. The addition
of the FeSi2 layer to the cell structure has resulted in a significant quantum
efficiency (QE) enhancement because of the rise in solar spectrum absorption at
longer wavelengths. The findings of this work offer a promising approach for
producing high-performance and reasonably priced CdTe-based solar cells.

###Photon Management in Two-Dimensional Disordered Media||Kevin Vynck,Matteo Burresi,Francesco Riboli,Diederik S. Wiersma###

Photon Management in Two-Dimensional Disordered Media. Elaborating reliable and versatile strategies for efficient light coupling
between free space and thin films is of crucial importance for new technologies
in energy efficiency. Nanostructured materials have opened unprecedented
opportunities for light management, notably in thin-film solar cells. Efficient
coherent light trapping has been accomplished through the careful design of
plasmonic nanoparticles and gratings, resonant dielectric particles and
photonic crystals. Alternative approaches have used randomly-textured surfaces
as strong light diffusers to benefit from their broadband and wide-angle
properties. Here, we propose a new strategy for photon management in thin films
that combines both advantages of an efficient trapping due to coherent optical
effects and broadband/wide-angle properties due to disorder. Our approach
consists in the excitation of electromagnetic modes formed by multiple light
scattering and wave interference in two-dimensional random media. We show, by
numerical calculations, that the spectral and angular responses of thin films
containing disordered photonic patterns are intimately related to the in-plane
light transport process and can be tuned through structural correlations. Our
findings, which are applicable to all waves, are particularly suited for
improving the absorption efficiency of thin-film solar cells and can provide a
novel approach for high-extraction efficiency light-emitting diodes.

###Phosphorene and Doped Monolayers Interfaced TiO$_2$ with Type-II Band Alignments: Novel Excitonic Solar Cells||Liujiang Zhou,Jin Zhang,Zhiwen Zhuo,Liangzhi Kou,Wei Ma,Bin Shao,Aijun Du,Sheng Meng,Thomas Frauenheim###

Phosphorene and Doped Monolayers Interfaced TiO$_2$ with Type-II Band Alignments: Novel Excitonic Solar Cells. Phosphorene, a new elemental two dimensional (2D) material recently isolated
by mechanical exfoliation, holds the feature of a direct band gap of around 2.0
eV, overcoming graphene's weaknesses (zero band gap) to realize the potential
application in optoelectronic devices. Constructing van der Waals
heterostructures is an efficient approach to modulate the band structure, to
advance the charge separation efficiency, and thus to optimize the
optoelectronic properties. Here, we theoretically investigated three type-II
heterostructures based on perfect phosphorene and its doped monolayers
interfaced with TiO$_2$(110) surface. Doping in phosphorene has a tunability on
built-in potential, charge transfer, light absorbance, as well as electron
dynamics, which helps to optimize the light absorption efficiency. Three
excitonic solar cells (XSCs) based on the phosphorene$-$TiO$_2$ heterojunctions
have been proposed, which exhibit high power conversion efficiencies dozens of
times higher than conventional solar cells, comparable to MoS$_2$/WS$_2$ XSC.
The nonadiabatic molecular dynamics within the time-dependent density
functional theory framework shows ultrafast electron transfer time of
6.1$-$10.8 fs, and slow electron$-$hole recombination of 0.58$-$1.08 ps,
yielding $>98\%$ quantum efficiency for charge separation, further guaranteeing
the practical power conversion efficiencies in XSC.

###Optimization of broadband omnidirectional antireflection coatings for solar cells||Xia Guo,Qiaoli Liu,Chong Li,Hongyi Zhou,Benshun Lv,Yajie Feng,Huaqiang Wang,Wuming Liu###

Optimization of broadband omnidirectional antireflection coatings for solar cells. Broadband and omnidirectional antireflection coating is a generally effective
way to improve solar cell efficiency, because the destructive interference
between the reflected and input waves could maximize transmission light in the
absorption layer. Several theoretical calculations have been developed to
optimize the anti-reflective coating to maximize the average transmittance.
However, the solar irradiances of the clear sky spectral direct beam on a
receiver plane at different positions and times are variable greatly. Here we
report a new theoretical calculation of anti-reflective coating with incident
quantum efficiency {\eta}in as evaluation function for practical application.
The two-layer and three-layer anti-reflective coatings are optimized over
{\lambda} = [300, 1100] nm and {\theta} = [0{\deg}, 90{\deg}] for cities of
Quito, Beijing and Moscow. The {\eta}in of two-layer anti-reflective coating
increases by 0.26%, 1.37% and 4.24% for these 3 cities, respectively, compared
with that other theoretical calculations due to better match between the local
actual solar spectrum and quantum efficiency spectrum. Our numerical simulation
and comparison data with other optimization methods suggest that this
optimization method combining ant colony algorithm method with SPCTRL2 solar
spectral irradiance can effectively push the efficient solar cell toward higher
quantum efficiency, thus enabling high utilization efficiency of solar
irradiance.

###Lifetime enhancement for multi-photon absorption in intermediate band solar cells||Anibal Thiago Bezerra,Nelson Studart###

Lifetime enhancement for multi-photon absorption in intermediate band solar cells. A semiconductor structure consisting of two coupled quantum wells embedded
into the intrinsic region of a {\it p-i-n} junction is proposed to be
implemented as an intermediate band solar cell with ratchet state. The
localized conduction subband of the right-hand side quantum well is thought as
the intermediated band, while the excited conduction subband of the right-hand
side quantum well, coupled to right-rand side one, is thought to acts as the
ratchet state. The photo-excited electron in the intermediate band can tunnel
out the thin barrier separating the wells and accumulate into ratchet subband.
This might raise the electron probability of being hit by a second photon and
exiting out to the continuum, increasing solar cell current. Is presented a
temporal rate model for describing the charge transport properties of the cell.
Calculations are carried out by solving the time-dependent Schr\"odinger
equation applying the time evolution operator within a pertinent choice of the
non-commuting kinetic and potential operators. The efficiency in the generation
of current is analyzed directly by studying the occupation of the subbands
wells in the p-i-n junction, taking into account the injection and draining
dynamic provided by the electrical contacts connected to the cell. As a result,
the efficiency in the generation of current was found to be directly correlated
to the relationship between optical generation and recombination rates
regarding to the scattering to the ratchet state rate. This suggests that a
good coupling between the intermediate band and the additional band is a key
point to be analyzed when developing an efficient solar cell.

###Hybrid Halide Perovskites: Fundamental Theory and Materials Design||Marina R. Filip,George Volonakis,Feliciano Giustino###

Hybrid Halide Perovskites: Fundamental Theory and Materials Design. Hybrid organic-inorganic halide perovskites have emerged as a disruptive new
class of materials, exhibiting optimum properties for a broad range of
optoelectronic applications, most notably for photovoltaics. The first report
of highly efficient organic-inorganic perovskite solar cells in 2012 marked a
new era for photovoltaics research, reporting a power conversion efficiency of
over 10%. Only five years after this discovery, perovskite photovoltaic devices
have reached a certified efficiency of 22.7%, making them the first solution
processable technology to surpass thin film and multi-crystalline silicon solar
cells. The remarkable development of perovskite solar cells is due to the ideal
optoelectronic properties of organic-inorganic lead-halide perovskites. The
prototypical compound, methylammonium lead iodide, CH3NH3PbI3 is a direct band
gap semiconductor with a band gap in the visible, high charge carrier mobility,
long diffusion length and low excitonic binding energy. Due to these ideal
properties, CH3NH3PbI3 is also drawing interest across many other applications
beyond photovoltaics, such as light emitting devices, lasers, photocatalysts
and transistors. The continued progress of metal-halide perovskite
optoelectronics relies not only on a detailed understanding of the electronic
and optical properties of materials in this class, but also on the development
of practical strategies to tune their properties by controlling parameters such
as chemical composition. In this context, ab initio computational modelling can
play a key role in providing a physical interpretation of experimental
measurements, and guiding the design of novel halide perovskites with tailored
properties. In this chapter we will present an account of the contributions to
this fast developing field of research from our computational modelling group.

###Photochemical dynamics under incoherent illumination: light harvesting in self-assembled molecular J-aggregates||Luis Felipe Morales-Curiel,Roberto de J. León-Montiel###

Photochemical dynamics under incoherent illumination: light harvesting in self-assembled molecular J-aggregates. Transport phenomena in organic, self-assembled molecular J-aggregates have
long attracted a great deal of attention due to its potential role in designing
novel organic photovoltaic devices. A large number of theoretical and
experimental studies have been carried out describing excitonic energy transfer
in J-aggregates under the assumption that excitons are induced by a coherent
laser-light source or initialized by a localized excitation on a particular
chromophore. However, these assumptions may not provide an accurate description
to assess the efficiency of J-aggregates, particularly as building blocks of
organic solar cells. In natural conditions, J-aggregates would be subjected to
an incoherent source of light (as is sunlight), which would illuminate the
whole photosynthetic complex rather than a single molecule. In this work, we
present the first study of the efficiency of photosynthetic energy transport in
self-assembled molecular aggregates under incoherent sunlight illumination. By
making use of a minimalistic model of a cyanine dye J-aggregate, we demonstrate
that long-range transport efficiency is enhanced when exciting the aggregate
with incoherent light. Our results thus support the conclusion that
J-aggregates are indeed excellent candidates for devices where efficient
long-range incoherently-induced exciton transport is desired, such as in highly
efficient organic solar cells.

###Quantum Efficiency of Intermediate-Band Solar Cells Based on Non-Compensated n-p Codoped TiO2||Fengcheng Wu,Haiping Lan,Zhenyu Zhang,Ping Cui###

Quantum Efficiency of Intermediate-Band Solar Cells Based on Non-Compensated n-p Codoped TiO2. As an appealing concept for developing next-generation solar cells,
intermediate-band solar cells (IBSCs) promise to drastically increase the
quantum efficiency of photovoltaic conversion. Yet to date, a standing
challenge lies in the lack of materials suitable for developing IBSCs.
Recently, a new doping approach, termed non-compensated n-p codoping, has been
proposed to construct intermediate bands (IBs) in the intrinsic energy band
gaps of oxide semiconductors such as TiO$_2$. We explore theoretically the
optimal quantum efficiency of IBSCs based on non-compensated n-p codoped
TiO$_2$ under two different design schemes. The first preserves the ideal
condition that no electrical current be extracted from the IB. The
corresponding maximum quantum efficiency for the codoped TiO$_2$ can reach
52.7%. In the second scheme, current is also extracted from the IB, resulting
in a further enhancement in the maximum efficiency to 56.7%. Our findings also
relax the stringent requirement that the IB location be close to the optimum
value, making it more feasible to realize IBSCs with high quantum efficiencies.

###Photophysical Properties of BODIPY-derivatives for the Implementation of Organic Solar Cells: A Computational Approach||Duvalier Madrid-Usuga,Alejandro Ortiz,John H. Reina###

Photophysical Properties of BODIPY-derivatives for the Implementation of Organic Solar Cells: A Computational Approach. Solar cells based on organic compounds are a proven emergent alternative to
conventional electrical energy generation. Here, we provide a computational
study of power conversion efficiency optimization of BODIPY-derivatives by
means of their associated open circuit voltage, short-circuit density, and fill
factor. In so doing, we compute for the derivatives' geometrical structures,
energy levels of frontier molecular orbitals, absorption spectra, light
collection efficiencies, and exciton binding energies, via density functional
theory (DFT) and time dependent (TD)--DFT calculations. We fully-characterize
four D--$\pi$--A (BODIPY) molecular systems of high efficiency and improved
$J_{sc}$ that are well suited for integration into bulk heterojunction (BHJ)
organic solar cells as electron-donor materials in the active layer. Our
results are two-fold: We found that molecular complexes with an structural
isoxazoline ring exhibit a higher power conversion efficiency (PCE), a useful
result for improving the BHJ current, and, on the other hand, by considering
the molecular systems as electron-acceptor materials, with P3HT as the
electron-donor in the active layer, we found a high PCE compound favorability
with a pyrrolidine ring in its structure, in contrast to the molecular systems
built with an isoxazoline ring. The theoretical characterization of the
electronic properties of the BODIPY-derivatives here provided, computed with a
combination of ab-initio methods and quantum models, can be readily applied to
other sets of molecular complexes in order to hierarchize optimal power
conversion efficiency.

###Study of the band--gap shift in CdS films: Influence of thermal annealing in different atmospheres||S. A. Tomas###

Study of the band--gap shift in CdS films: Influence of thermal annealing in different atmospheres. We study by photoacoustic spectroscopy the band--gap shift effect of CdS
films. The CdS films were grown by chemical bath deposition and exposed to
different annealing atmospheres over a range of temperature in which the sample
structure is observed to change. We show the band--gap evolution as a function
of temperature of thermal annealing and determine the process which produces
the best combination of high band--gap energy and low resistivity. It allows us
to know a possible procedure to obtain low--resistivity CdS/CdTe solar cells
with high--quantum efficiency.

###Electronic and hole spectra of layered systems of cylindrical rod arrays: solar cell application||J. W. Klos,M. Krawczyk###

Electronic and hole spectra of layered systems of cylindrical rod arrays: solar cell application. We have computed the electronic and hole spectra of a 3D superlattice
consisting of layers of GaAs rods of finite height arranged in a hexagonal
lattice and embedded in an AlGaAs matrix, alternating with spacer layers of
homogeneous AlAs. The spectra are calculated in the envelope function
approximation, with both light-hole and heavy-hole subbands and hole spin
degeneracy taken into account. The application of thick spacers allows to
investigate the band structure of isolated layers of cylindrical rods. We
estimate the ultimate efficiency of solar energy conversion in a solar cell
based on an array of cylindrical quantum dots versus the dot height, and
determine the optimal value of this parameter.

###Thin Film Absorbers Based on Plasmonic Phase Resonances||Yanxia Cui,Kin Hung Fung,Jun Xu,Sailing He,Nicholas X. Fang###

Thin Film Absorbers Based on Plasmonic Phase Resonances. We demonstrate an efficient double-layer light absorber by exciting plasmonic
phase resonances. We show that the addition of grooves can cause mode splitting
of the plasmonic waveguide cavity modes and all the new resonant modes exhibit
large absorptivity greater than 90%. Some of the generated absorption peaks
have wide-angle characteristics. Furthermore, we find that the proposed
structure is fairly insensitive to the alignment error between different
layers. The proposed plasmonic nano-structure designs may have exciting
potential applications in thin film solar cells, thermal emitters, novel
infrared detectors, and highly sensitive bio-sensors.

###Universality in Intensity Modulated Photocurrent in Bulk-Heterojunction Polymer Solar Cells||Monojit Bag,K. S. Narayan###

Universality in Intensity Modulated Photocurrent in Bulk-Heterojunction Polymer Solar Cells. We observe a universal feature in the frequency dependence of intensity
modulated photocurrent Iph based on studies of a variety of efficient
bulk-heterojunction polymer solar cells (BHJ-PSCs). This feature of Iph appears
in the form of a local maximum in the 5 kHz < frequency < 10 kHz range and is
observed to be largely independent of the external parameters such as modulated
light intensity (Lac), wavelength, temperature (T), and external field (EF)
over a wide range. Simplistic kinetic models involving carrier generation,
recombination and extraction processes are used to interpret the overall
essential features of Iph and correlate it to the device parameters.

###Embedded metal nanopatterns for near-field scattering-enhanced optical absorption||Fan Ye,Michael J. Burns,Michael J. Naughton###

Embedded metal nanopatterns for near-field scattering-enhanced optical absorption. Simulations of metal nanopatterns embedded in a thin photovoltaic absorber
show significantly enhanced absorbance within the semiconductor, with a more
than 300% increase for {\lambda} = 800 nm. Integrating with AM1.5 solar
irradiation, this yields a 70% increase in simulated short circuit current
density in a 60 nm amorphous silicon film. Embedding such metal patterns inside
an absorber maximally utilizes enhanced electric fields that result from
intense, spatially organized, near-field scattering in the vicinity of the
pattern. Appropriately configured (i.e. with a thin insulating coating), this
optical metamedium architecture may be useful for increasing photovoltaic
efficiency in thin film solar cells, including offering prospects for realistic
ultrathin hot electron cells.

###Photonic assisted light trapping integrated in ultrathin crystalline silicon solar cells by nanoimprint lithography||Christos Trompoukis,Ounsi El Daif,Valérie Depauw,Ivan Gordon,Jef Poortmans###

Photonic assisted light trapping integrated in ultrathin crystalline silicon solar cells by nanoimprint lithography. We report on the fabrication of two-dimensional periodic photonic
nanostructures by nanoimprint lithography and dry etching, and their
integration into a 1-{\mu}m-thin mono-crystalline silicon solar cell. Thanks to
the periodic nanopatterning, a better in-coupling and trapping of light is
achieved, resulting in an absorption enhancement. The proposed light trapping
mechanism can be explained as the superposition of a graded index effect and of
the diffraction of light inside the photoactive layer. The absorption
enhancement is translated into a 23% increase in short-circuit current, as
compared to the benchmark cell, resulting in an increase in energy-conversion
efficiency.

###Simulation of nanostructure-based high-efficiency solar cells: challenges, existing approaches and future directions||Urs Aeberhard###

Simulation of nanostructure-based high-efficiency solar cells: challenges, existing approaches and future directions. Many advanced concepts for high-efficiency photovoltaic devices exploit the
peculiar optoelectronic properties of semiconductor nanostructures such as
quantum wells, wires and dots. While the optics of such devices is only
modestly affected due to the small size of the structures, the optical
transitions and electronic transport can strongly deviate from the simple bulk
picture known from conventional solar cell devices. This review article
discusses the challenges for an adequate theoretical description of the
photovoltaic device operation arising from the introduction of nanostructure
absorber and/or conductor components and gives an overview of existing device
simulation approaches.

###Structural and Electronic Properties of Hybrid Perovskites for High-Efficiency Thin-Film Photovoltaics from First-Principles||Federico Brivio,Alison B. Walker,Aron Walsh###

Structural and Electronic Properties of Hybrid Perovskites for High-Efficiency Thin-Film Photovoltaics from First-Principles. The performance of perovskite solar cells recently exceeded 15%
solar-to-electricity conversion efficiency for small-area devices. The
fundamental properties of the active absorber layers, hybrid organic-inorganic
perovskites formed from mixing metal and organic halides [\textit{e.g.}
(NH$_4$)PbI$_3$ and (CH$_3$NH$_3$)PbI$_3$], are largely unknown. The materials
are semiconductors with direct band gaps at the boundary of the first Brillouin
zone. The calculated dielectric response and band gaps show an orientation
dependence, with a low barrier for rotation of the organic cations. Due to the
electric dipole of the methylammonium cation, a photoferroic effect may be
accessible, which could enhance carrier collection.

###Photovoltaic performance of n-type SnS active layer in ITO/PEDOT:PSS/SnS/Al structure||Priyal Jain,P. Arun###

Photovoltaic performance of n-type SnS active layer in ITO/PEDOT:PSS/SnS/Al structure. The present paper discusses the influence of Tin Sulphide's grain size on the
performance of ITO/PEDOT:PSS/SnS/Al structured solar cells fabricated by
thermal evaporation. The grain sizes were maintained in the range of 11-18~nm
by controlling the thickness of SnS films. While the open circuit voltage (Voc)
was found to be a constant for this structure, Parameters such as short circuit
current density (Jsc), series resistance (Rs), parallel resistance (Rp),
ideality factor and the overall efficiency were found to be dependent on the
SnS grain size and incident light intensity. The experimental work directly
reconfirms the theoretical results and ideas raised in literature by early
researchers.

###The effects of cap layer thickness on the performance of InGaN/GaN MQW solar cell||Saina Haghkish,Asghar Asgari###

The effects of cap layer thickness on the performance of InGaN/GaN MQW solar cell. Following letter introduces a theoretical approach to investigate the effect
of two-step GaN barrier layer growth methodology on the performance of
InGaN/GaN MQW solar cell, in which a lower temperature GaN cap layer was grown
on top of each quantum well followed by a higher temperature GaN barrier layer.
Different growth conditions would cause changes in the concentration of trap
level density of states and imperfection sites. The simulation and comparison
of 3 samples each with different cap layer thickness, reveals the fact that
increasing cap layer thickness results in higher quantum efficiency, improved
short circuit density of current and 3.2% increase of the fill factor.

###Search for potential precursors for Si-atomic layer deposition- a quantum chemical study||P. Vajeeston,H. Fjellvåg,a,O. Nilsen###

Search for potential precursors for Si-atomic layer deposition- a quantum chemical study. Thin film of silicon is an interesting material for many technological
applications in electronic industry and in energy harvesting technologies, but
requires a method for controlled growth of thin films. The purpose of this
study is to screen a wide variety of Si content precursors for Si ALD reactions
using state-of-the-art density-functional calculations. Among the studied 85 Si
content precursors we found that C7H12OSi(Methoxy-trivinyl-silane) and C7H9NSi
(Benzyliminosilane) show positive indications for ALD reactivity for Si
deposition. We believe that this finding will be helpful to develop low-cost,
high-energy efficiency thin-film solar cells for future scale up implementation
in photovoltaics.

###A photon ratchet route to high-efficiency hybrid halide perovskite intermediate band solar cells||Jarvist M. Frost,Pooya Azarhoosh,Scott McKechnie,Mark van Schilfgaarde,Aron Walsh###

A photon ratchet route to high-efficiency hybrid halide perovskite intermediate band solar cells. The spin-split indirect bandgap in hybrid-halide perovskites provides a
momentum-space realisation of a photon-ratchet intermediate band. Excited
electrons thermalise to recombination-protected Rashba pockets offset in
momentum space, building up the charge density to have sufficient flux to the
higher lying conduction band. This effect could be used to form an intrinsic
intermediate band solar cell with efficiencies beyond the Shockley-Queisser
limit if a selective low-electron affinity contact can be made to the higher
conduction state. This concept is supported by analysis of the many-body
electronic structure. Production of above-bandgap voltages under illumination
would affirm the physical mechanism proposed here.

###The influence of base thickness on textured silicon solar cells' efficiency||A. V. Sachenko,V. P. Kostylyov,A. V. Bobyl,V. N. Vlasiuk,I. O. Sokolovskyi,V. N. Verbitskiy,E. I. Terukov,M. Z. Shvarts,M. Evstigneev###

The influence of base thickness on textured silicon solar cells' efficiency. The transformation of the long-wavelength edge of the external quantum exit
(EQE) formation mechanisms in textured silicon solar cells (SCs) is revealed,
depending on their thickness. Expressions for experimental EQE dependences on
the long-wavelength absorption edge are obtained for a wide range of base
thicknesses (100-450 {\mu}m). The expressions allow optimal SC base thickness
values calculation from the condition of maximal photoconversion efficiency
taking into account surface recombination velocity. In particular, it was found
that optimal 100-{\mu}m base thickness corresponds to the surface recombination
velocity of about 3 cm/s.

###Exceed Improved Efficient Perovskite Solar Cells Under Dual-Irradiation System||Tao Ye,Xianqiang Li,Shaoyang Ma,Dan Wu,Lei Wei,Xiaohong Tang,Jian Wei Xu,Seeram Ramakrishna,Chellappan Vijila,Xizu Wang###

Exceed Improved Efficient Perovskite Solar Cells Under Dual-Irradiation System. In general, perovskite solar cells (PSC) with a sensitized or thin film
architecture absorb light from a single illumination. This paper reported a PSC
architecture with a semitransparent Au/ITO counter electrode, which allows
light to pass it partially. When the device was illuminated simultaneously from
both the FTO and Au/ITO sides, the PSC has achieved an overall power conversion
efficiency (PCE) as high as 20.1% under high light intensity (1.4 sun), which
is much higher than that of the single-irradiation system.

###Semiconducting Carbon Nanotubes in Photovoltaic Blends: the case of PTB7:PC60BM:(6,5) SWNT||Diana Gisell Figueroa del Valle,Giuseppe M. Paternò,Francesco Scotognella###

Semiconducting Carbon Nanotubes in Photovoltaic Blends: the case of PTB7:PC60BM:(6,5) SWNT. Blends of carbon nanotubes with conjugated polymer and fullerene derivatives
are complex nanocomposite systems, which have recently attracted a great
research interest for their photovoltaic ability. Therefore, gaining a better
understanding of the excitonic dynamics in such materials can be important to
boost the efficiency of excitonic solar cells. Here, we studied the
photophysics of a ternary system in which the polymer PTB7 and the fullerene
derivative PCBM are integrated with (6,5) SWNTs. We highlight the contribution
of SWNTs in the exciton dissociation and in the charge transfer process. These
findings can be useful for the exploitation of these multi-component systems
for organic photovoltaic and, in general, optoelectronic applications.

###Insights into the Unusual Semiconducting Behavior in Low-Dimensional Boron||Shao-Gang Xu,Xiao-Tian Li,Yu-Jun Zhao,Wang-Ping Xu,Ji-Hai Liao,Xiu-Wen Zhang,Hu Xu,Xiao-Bao Yang###

Insights into the Unusual Semiconducting Behavior in Low-Dimensional Boron. Elementary semiconductors are rare and attractive, especially for
low-dimensional materials. Unfortunately, most of boron nanostructures were
found to be metallic, despite of their typical semiconducting bulk structure.
Herein, we propose a general recipe to realize low-dimensional semiconducting
boron. This unusual semiconducting behavior is attributed to charge transfer
and electron localization, induced by the symmetry breaking that divides boron
atoms into cations and anions. In addition, it is feasible to accomplish band
gap engineering by rationally designing various structures. Importantly, the
low-dimensional semiconducting boron are predicted to be an excellent
solar-cell material with the power conversion efficiency of higher than 20%,
paving the way for their promising optoelectronic applications.

###Towards Photoferroic Materials by Design: Recent Progresses and Perspective||Ivano E. Castelli,Thomas Olsen,Yunzhong Chen###

Towards Photoferroic Materials by Design: Recent Progresses and Perspective. The use of photoferroic materials that combine ferroelectric and light
harvesting properties in a photovoltaic device is a promising route to
significantly improve the efficiency of solar cells. These materials do not
require the formation of a p-n junction and can produce photovoltages well
above the value of the band gap, because of the spontaneous intrinsic
polarization and the formation of domain walls. In this perspective, we discuss
the recent experimental progresses and challenges for the synthesis of these
materials and the theoretical discovery of novel photoferroic materials using a
high-throughput approach.

###Electron-Extraction from Excited Quantum Dots with Higher Order Coulomb Scattering||Alex Arash Sand Kalaee,Andreas Wacker###

Electron-Extraction from Excited Quantum Dots with Higher Order Coulomb Scattering. The electron kinetics in nanowire-based hot-carrier solar cells is studied,
where both relaxation and extraction are considered concurrently. Our kinetics
is formulated in the many-particle basis of the interacting system. Detailed
comparison with simplified calculations based on product states shows that this
includes the Coulomb interaction both in lowest and higher orders. While
relaxation rates of 1 ps are obtained, if lowest order processes are available,
timescales of tens of ps arise if these are not allowed for particular designs
and initial conditions. Based on these calculations we quantify the second
order effects and discuss the extraction efficiency, which remains low unless
an energy filter by resonant tunnelling is applied.

###Multiple Exciton Generation in Nanostructures for Advanced Photovoltaic Cells||Nicholas Siemons,Alessio Serafini###

Multiple Exciton Generation in Nanostructures for Advanced Photovoltaic Cells. This paper reviews both experimental and theoretical work on nanostructures
showing high quantum yields due to the phenomenon of multiple exciton
generation. It outlines the aims and barriers to progress in identifying
further such nanostructures, and also includes developments concerning solar
devices where nanostructures act as the light-absorbing component. It reports
on both semiconductor and carbon structures, both monocomposite (of various
dimensionalities) and heterogeneous. Finally, it looks at future directions
that can be taken to push solar cell efficiency above the classic limit set by
Shockley and Queissier in 1961.

###Dual-band Dielectric Light-harvesting Nanoantennae Made by Nature||Julian Juhi-Lian Ting###

Dual-band Dielectric Light-harvesting Nanoantennae Made by Nature. Mechanisms to use nanoparticles to separate sunlight into photovoltaic useful
range and thermally useful range to increase the efficiency of solar cells and
to dissipate heat radiatively are discussed based upon lessons we learnt from
photosynthesis. We show that the dual-band maxima in the absorption spectrum of
bacterial light harvestors not only are due to the bacteriochlorophylls
involved but also come from the geometry of the light harvestor. Being able to
manipulate these two bands arbitrarily enables us to fabricate the
nanoparticles required. Such mechanisms are also useful for the design of
remote power charging and light sensors.

###Breaking the Selection Rules of Spin-Forbidden Molecular Absorption in Plasmonic Nanocavities||Oluwafemi S. Ojambati,William M. Deacon,Rohit Chikkaraddy,Charlie Readman,Qianqi Lin,Zsuzsanna Koczor-Benda,Edina Rosta,Oren A. Scherman,Jeremy J. Baumberg###

Breaking the Selection Rules of Spin-Forbidden Molecular Absorption in Plasmonic Nanocavities. Controlling absorption and emission of organic molecules is crucial for
efficient light-emitting diodes, organic solar cells and single-molecule
spectroscopy. Here, a new molecular absorption is activated inside a gold
plasmonic nanocavity, and found to break selection rules via spin-orbit
coupling. Photoluminescence excitation scans reveal absorption from a normally
spin-forbidden singlet to triplet state transition, while drastically enhancing
the emission rate by several thousand fold. The experimental results are
supported by density functional theory, revealing the manipulation of molecular
absorption by nearby metallic gold atoms.

###Bulk carrier lifetime surpassing 600 us in Upgraded Metallurgical-grade Silicon multicrystalline wafers after Phosphorus Diffusion Gettering||Nerea Dasilva-Villanueva,Sergio Catalán-Gómez,David Fuertes Marrón,Miguel García-Corpas,Carlos del Cañizo###

Bulk carrier lifetime surpassing 600 us in Upgraded Metallurgical-grade Silicon multicrystalline wafers after Phosphorus Diffusion Gettering. Upgraded metallurgical-grade (UMG) Si is obtained via a purification route
alternative to the one used for conventional polysilicon and with significantly
reduced environmental impact. Additionally, despite a lower purity level in the
feedstock than polysilicon, UMG-Si has demonstrated potential for the
fabrication of highly efficient and low-cost solar cells. Low initial bulk
carrier lifetimes recorded in UMG-Si bare wafers can be improved by means of an
adequate Phosphorus Diffusion Gettering (PDG) process to the level of mc-Si. In
this letter, optimized PDG processes for UMG-Si are reported, resulting in
increased values between 20 and 250 times the original carrier lifetimes and
record figures above 645 us.

###Synthesis and optical characterization of perovskite layer for solar cell application||Manoj Pandey,Dipendra Hamal,Bijaya Basnet,Bhim Kafle###

Synthesis and optical characterization of perovskite layer for solar cell application. Solvent engineering offers fine control over the photovoltaic efficiency,
film morphology, and crystallization quality of perovskite films and also
enables to optimize light transmittance and absorbance in solar cell
applications. In the present work, the band gap and reflectance were reduced
through solvent engineering. We found that perovskite thin films produced using
DMF (Dimethyl formamide) solvent had a band gap that was 0.24 eV less than
those produced using IPA (Isopropyl Alcohol) solvent. Perovskite thin films
produced using DMF solvent also exhibited considerably lower solar spectrum
reflectance.

###Exciton dissociation mediated by phonons in organic photovoltaics||Stepan Fomichev,Leonard Ruocco,Alexandra Tully,Mona Berciu###

Exciton dissociation mediated by phonons in organic photovoltaics. It is well known that phonons can overscreen the bare Coulomb
electron-electron repulsion, turning it into the effective attraction that
binds the Cooper pairs responsible for BCS superconductivity. Here, we use a
simple lattice model to prove that the counterpart of this is also possible,
whereby phonons overscreen the bare electron-hole attraction and may turn it
repulsive at short distances, driving exciton dissociation in certain regions
of the parameter space. We argue that this phonon-mediated short-range
screening plays an important role in the physics of organic solar cell
materials (and other materials with strong electron-phonon coupling) and could
point the way to new strategies for optimizing their efficiencies.

###Rigorous Treatment of Photon Recycling in Thermodynamics of Photovoltaics: The Case of Perovskite Thin-Film Solar Cells||Muluneh G. Abebe,Aimi Abass,Guillaume Gomard,Lin Zschiedrich,Uli Lemmer,Bryce S. Richards,Carsten Rockstuhl,Ulrich W. Paetzold###

Rigorous Treatment of Photon Recycling in Thermodynamics of Photovoltaics: The Case of Perovskite Thin-Film Solar Cells. The establishment of a rigorous theory on thermodynamics of light management
in photovoltaics that accommodates various loss mechanisms as well as
wave-optical effects in the absorption and reemission of light is at stake in
this contribution. To this end, we propose a theoretical framework to calculate
the open-circuit voltage enhancement resulting from photon recycling ($\Delta
V^{\mathrm{PR}}_{\mathrm{oc}}$) with rigorous wave-optical treatment. It can be
applied to both planar thin-film and nanostructured single-junction solar
cells. We derive an explicit expression for $\Delta
V^{\mathrm{PR}}_{\mathrm{oc}}$, which reveals its dependence on internal
quantum luminescence efficiency, parasitic reabsorption, and on photon escape
probabilities of reemmited photons. While the internal quantum luminescence
efficiency is an intrinsic material property, both latter quantities can be
determined rigorously for an arbitrary solar cell architecture by
three-dimensional electrodynamic dipole emission calculations. We demonstrate
the strengths and validity of our framework by determining the impact of photon
recycling on the $V_{\mathrm{oc}}$ of a conventional planar organo-metal halide
perovskite thin-film solar cell and compare it to established reference cases
with perfect antireflection and Lambertian light scattering. Our calculations
reveal $\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$ values of up to 80 mV for the
considered device stack in the absence of angular restriction and up to 240 mV
when the escape cone above the cell is restricted to
$\theta_{\mathrm{out}}=2.5^\circ$ around the cell normal. These improvements
impose severe constraints on the parasitic absorption as a parasitic
reabsorption probability of only 2\% reduces the $\Delta
V^{\mathrm{PR}}_{\mathrm{oc}}$ to 100 mV for the same angular restriction. Our
work here can be used to provide design guidelines.

###Emergence of New Materials for Exploiting Highly Efficient Carrier Multiplication in Photovoltaics||Sourav Maiti,Marco van der Laan,Deepika Poonia,Peter Schall,Sachin Kinge,Laurens D. A. Siebbeles###

Emergence of New Materials for Exploiting Highly Efficient Carrier Multiplication in Photovoltaics. In conventional solar cell semiconductor materials (predominantly Si) photons
with energy higher than the band gap initially generate hot electrons and
holes, which subsequently cool down to the band edge by phonon emission. Due to
the latter process, the energy of the charge carriers in excess of the band gap
is lost as heat and does not contribute to the conversion of solar to
electrical power. If the excess energy is more than the band gap it can in
principle be utilized through a process known as carrier multiplication (CM) in
which a single absorbed photon generates two (or more) pairs of electrons and
holes. Thus, through CM the photon energy above twice the band gap enhances the
photocurrent of a solar cell. In this review, we discuss recent progress in CM
research in terms of fundamental understanding, emergence of new materials for
efficient CM, and CM based solar cell applications. Based on our current
understanding, the CM threshold can get close to the minimal value of twice the
band gap in materials where a photon induces an asymmetric electronic
transition from a deeper valence band or to a higher conduction band. In
addition, the material must have a low exciton binding energy and high charge
carrier mobility, so that photoexcitation leads directly to the formation of
free charges that can readily be extracted at external electrodes of a
photovoltaic device. Percolative networks of coupled PbSe quantum dots, Sn/Pb
based halide perovskites, and transition metal dichalcogenides such as MoTe2
fulfill these requirements to a large extent. These findings point towards
promising prospects for further development of new materials for highly
efficient photovoltaics.

###Ab initio study of oxygen segregation in silicon grain boundaries: the role of strain and vacancies||Rita Maji,Eleonora Luppi,Nathalie Capron,Elena Degoli###

Ab initio study of oxygen segregation in silicon grain boundaries: the role of strain and vacancies. Multi-crystalline silicon is widely used for producing low-cost and
high-efficiency solar cells. During crystal growth and device fabrication,
silicon solar cells contain grain boundaries (GBs) which are preferential
segregation sites for atomic impurities such as oxygen atoms. GBs can induce
charge carriers recombination significantly reducing carrier lifetimes and
therefore they can be detrimental for Si device performance. We studied the
correlation between structural, energetic and electronic properties of
{\Sigma}3{111} Si GB in the presence of vacancies, strain and multiple O
segregation. The study of the structural and energetic properties of GBs in the
presence of strain and vacancies gives an accurate description of the complex
mechanisms that control the segregation of oxygen atoms. We analysed tensile
and compressive strain and we obtained that local tensile strain around O
impurities is very effective for segregation. We also studied the role of
multiple O impurities in the presence of Si vacancies finding that the
segregation is favorite for those structures which have restored tetrahedral
covalent bonds. The presence of vacancies attract atomic impurities in order to
restore the electronic stability: the interstitial impurity becomes
substitutional. This analysis was the starting point to correlate the change of
the electronic properties in {\Sigma}3{111}Si GBs with O impurities in the
presence of strain and vacancies. For each structure we analysed the density of
states and its projection on atoms and states, the band gaps, the segregation
energy and their correlation in order to characterise the nature of new energy
levels. Actually, knowing the origin of defined electronic states would allow
the optimization of materials in order to reduce non-radiative electron-hole
recombination avoiding charge and energy losses and therefore improving solar
cell efficiency.

###Broadband Quantum Efficiency Enhancement in High Index Nanowires Resonators||Yiming Yang,Xingyue Peng,Steven Hyatt,Dong Yu###

Broadband Quantum Efficiency Enhancement in High Index Nanowires Resonators. Light trapping in sub-wavelength semiconductor nanowires (NWs) offers a
promising approach to simultaneously reducing material consumption and
enhancing photovoltaic performance. Nevertheless, the absorption efficiency of
a NW, defined by the ratio of optical absorption cross section to the NW
diameter, lingers around 1 in existing NW photonic devices, and the absorption
enhancement suffers from a narrow spectral width. Here, we show that the
absorption efficiency can be significantly improved in NWs with higher
refractive indices, by an experimental observation of up to 350% external
quantum efficiency (EQE) in lead sulfide (PbS) NW resonators, a 3-fold increase
compared to Si NWs. Furthermore, broadband absorption enhancement is achieved
in single tapered NWs, where light of various wavelengths is absorbed at
segments with different diameters analogous to a tandem solar cell. Overall,
the single NW Schottky junction solar cells benefit from optical resonance,
near bandgap open circuit voltage, and long minority carrier diffusion length,
demonstrating power conversion efficiency (PCE) comparable to single Si NW
coaxial p-n junction cells11, but with much simpler fabrication processes.

###Highly Efficient Carrier Multiplication in van der Waals layered Materials||Ji-Hee Kim,Matthew R. Bergren,Jin Cheol Park,Subash Adhikari,Michael Lorke,Thomas Fraunheim,Duk-Hyun Choe,Beom Kim,Hyunyong Choi,Tom Gregorkiewicz,Young Hee Lee###

Highly Efficient Carrier Multiplication in van der Waals layered Materials. Carrier multiplication (CM), a photo-physical process to generate multiple
electron-hole pairs by exploiting excess energy of free carriers, is explored
for efficient photovoltaic conversion of photons from the blue solar band,
predominantly wasted as heat in standard solar cells. Current state-of-the-art
approaches with nanomaterials have demonstrated improved CM but are not
satisfactory due to high energy loss and inherent difficulties with carrier
extraction. Here, we report ultra-efficient CM in van der Waals (vdW) layered
materials that commences at the energy conservation limit and proceeds with
nearly 100% conversion efficiency. A small threshold energy, as low as twice
the bandgap, was achieved, marking an onset of quantum yield with enhanced
carrier generation. Strong Coulomb interactions between electrons confined
within vdW layers allow rapid electron-electron scattering to prevail over
electron-phonon scattering. Additionally, the presence of electron pockets
spread over momentum space could also contribute to the high CM efficiency.
Combining with high conductivity and optimal bandgap, these superior CM
characteristics identify vdW materials for third-generation solar cell.

###Generalized Reciprocity Relations in Solar Cells with Voltage-Dependent Carrier Collection: Application to p-i-n Junction Devices||Kasidit Toprasertpong,Amaury Delamarre,Yoshiaki Nakano,Jean-François Guillemoles,Masakazu Sugiyama###

Generalized Reciprocity Relations in Solar Cells with Voltage-Dependent Carrier Collection: Application to p-i-n Junction Devices. Two reciprocity theorems are important for fundamental understanding of the
solar cell operation and applications to device evaluation: (1) the
carrier-transport reciprocity connecting the dark-carrier injection with the
short-circuit photocarrier collection and (2) the optoelectronic reciprocity
connecting the electroluminescence with the photovoltaic quantum efficiency at
short circuit. These theorems, however, fail in devices with thick depletion
regions such as p-i-n junction solar cells. By properly linearizing the
carrier-transport equation in such devices, we report that the dark-carrier
injection is related to the photocarrier collection efficiency at the operating
voltage, not at short circuit as suggested in the original theorem. This leads
to the general form of the optoelectronic reciprocity relation connecting the
electroluminescence with the voltage-dependent quantum efficiency, providing a
correct interpretation of the optoelectronic properties of p-i-n junction
devices. We also discuss the validity of the well-known relation between the
open-circuit voltage and the external luminescence efficiency. The impact of
illumination intensity and device parameters on the validity of the reciprocity
theorems is quantitatively investigated.

###Colloidal Quantum Dot Tandem Solar Cells Using CVD Graphene as An Atomically Thin Intermediate Recombination Layer||Yu Bi,Santanu Pradhan,Mehmet Zafer Akgul,Shuchi Gupta,Alexandros Stavrinadis,Jianjun Wang,Gerasimos Konstantatos###

Colloidal Quantum Dot Tandem Solar Cells Using CVD Graphene as An Atomically Thin Intermediate Recombination Layer. Two-terminal tandem cell architectures are believed to be an effective way to
further improve the power conversion efficiency in solution processed
photovoltaics. To design an efficient tandem solar cell, two key issues need to
be considered. Firstly, subcells with well-matched currents and complementary
absorption characteristics are a prerequisite for high efficiency. Secondly
identifying the appropriate intermediate layer (IML) to connect the subcells is
necessary to minimize the optical and electronic losses. PbS colloidal quantum
dots (CQDs) are a notable choice for the subcells due to their low cost,
solution processibility and remarkable wide range band gap tunability. Single
layer Graphene (Gr) has been proposed to be a promising IML due to its high
transparency and conductivity. Here, as a proof of concept, we demonstrate a
solution processed two terminal PbS CQDs tandem solar cell employing chemical
vapor deposited Gr as the IML. In doing so, we report a PbS CQD cell comprising
subcells with bandgaps of 1.4 and 0.95 eV that delivers power conversion
efficiency in excess of 7%, substantially higher than previously reported CQD
tandem cells.

###Sequentially Deposited versus Conventional Nonfullerene Organic Solar Cells: Interfacial Trap States, Vertical Stratification, and Exciton Dissociation||Jiangbin Zhang,Moritz H. Futscher,Vincent Lami,Felix U. Kosasih,Changsoon Cho,Qinying Gu,Aditya Sadhanala,Andrew J. Pearson,Bin Kan,Giorgio Divitini,Xiangjian Wan,Daniel Credgington,Neil C. Greenham,Yongsheng Chen,Caterina Ducati,Bruno Ehrler,Yana Vaynzof,Richard H. Friend,Artem A. Bakulin###

Sequentially Deposited versus Conventional Nonfullerene Organic Solar Cells: Interfacial Trap States, Vertical Stratification, and Exciton Dissociation. Bulk-heterojunction (BHJ) non-fullerene organic solar cells prepared from
sequentially deposited donor and acceptor layers (sq-BHJ) have recently been
promising to be highly efficient, environmentally friendly, and compatible with
large area and roll-to-toll fabrication. However, the related photophysics at
donor-acceptor interface and the vertical heterogeneity of donor-acceptor
distribution, critical for exciton dissociation and device performance, are
largely unexplored. Herein, steady-state and time-resolved optical and
electrical techniques are employed to characterize the interfacial trap states.
Correlation with the luminescent efficiency of interfacial states and its
non-radiative recombination, interfacial trap states are characterized to be
about 50% more populated in the sq-BHJ than as-cast BHJ (c-BHJ), which probably
limits the device voltage output. Cross-sectional energy-dispersive X-ray
spectroscopy and ultraviolet photoemission spectroscopy depth profiling
directly vizualize the donor-acceptor vertical stratification with a precision
of 1-2 nm. From the proposed "needle" model, the high exciton dissociation
efficiency is rationalized. Our study highlights the promise of sequential
deposition to fabricate efficient solar cells, and points towards improving the
voltage output and overall device performance via eliminating interfacial trap
states.

###Light management in highly-textured perovskite solar cells: From full-device ellipsometry characterization to optical modelling for quantum efficiency optimization||Chenxi Ma,Daming Zheng,Dominique Demaille,Bruno Gallas,Catherine Schwob,Thierry Pauporté,Laurent Coolen###

Light management in highly-textured perovskite solar cells: From full-device ellipsometry characterization to optical modelling for quantum efficiency optimization. While perovskite solar cells (PSCs) are now reaching high power conversion
efficiencies (PCEs), further performance improvement requires a fine management
and an optimization of the light pathway and harvesting in the cells. These go
through an accurate understanding, characterization and modelling of the
optical processes occurring in these complex, often textured, multi-layered
systems. In the present work, we have considered a typical methylammonium lead
iodide (MAPI) solar cell built on a fluorine-doped tin oxide (FTO) electrode of
high roughness (43 nm RMS). By variable-angle spectroscopic ellipsometry (VASE)
of the full PSC device, we have been able to determine the optical constants of
all the device layers. We have designed a one-dimensional (1D) optical model of
the stacked layers where the rough texture is described as layers of
effective-medium index. We have supported the model using data extracted from
scanning electron microscopy, diffuse spectroscopy and photovoltaic efficiency
measurements. We show that the 1D model, while insufficient to describe
scattering by the FTO plate alone, gives an accurate description of the full
device optical properties. By comparison with the experimental external quantum
efficiency (EQE), we estimate the internal quantum efficiency (IQE) and the
effect of the losses related to electron transfer. Based on this work, we
finally discuss the optical losses mechanisms and the possible strategies that
can be implemented to improve light management within PSC devices and further
increase their performances.

###The role of triplet excitons in enhancing polymer solar cell efficiency: a photo-induced absorption study||K. Yang,U. Scherf,S. Guha###

The role of triplet excitons in enhancing polymer solar cell efficiency: a photo-induced absorption study. Inclusion of heavy metal atoms in a polymer backbone allows transitions
between the singlet and triplet manifolds. Interfacial dissociation of triplet
excitons constitutes a viable mechanism for enhancing photovoltaic (PV)
efficiencies in polymer heterojunction-based solar cells. The PV efficiency
from polymer solar cells utilizing a ladder-type poly (para-phenylene) polymer
(PhLPPP) with trace quantity of Pd atoms and a fullerene derivative (PCBM) is
much higher than its counterpart (MeLPPP) with no Pd atom. Evidence is
presented for the formation of a weak ground-state charge-transfer complex
(CTC) in the blended films of the polymer and PCBM, using photo-induced
absorption (PIA) spectroscopy. The CTC state in MeLPPP:PCBM has a singlet
character to it, resulting in a radiative recombination. In contrast, the CTC
states in PhLPPP:PCBM are more localized with a triplet character. An
absorption peak at 1.65 eV is observed in PhLPPP:PCBM blend in the PIA, which
may be converted to weakly-bound polaron-pairs, contributing to the enhancement
of PV efficiency.

###Effect of doping on performance of organic solar cells||V. A. Trukhanov,V. V. Bruevich,D. Yu. Paraschuk###

Effect of doping on performance of organic solar cells. Conventional models of planar and bulk heterojunction organic solar cells
have been extended by introducing doping in the active layer. We have studied
the performance of organic solar cells as a function of dopant concentration.
For bulk heterojunction cells, the modeling shows that for the most studied
material pair (poly-3-hexylthiophene, P3HT, and phenyl-C61-butyric acid methyl
ester, PCBM) doping decreases the short-circuit current density (JSC), fill
factor (FF) and efficiency. However, if bulk heterojunction cells are not
optimized, namely, at low charge carrier mobilities, unbalanced mobilities or
non-ohmic contacts, the efficiency can be increased by doping. For planar
heterojunction cells, the modeling shows that if the acceptor layer is n doped,
and the donor layer is p doped, the open-circuit voltage, JSC, FF and hence the
efficiency can be increased by doping. Inversely, when the acceptor is p doped,
and the donor is n doped; FF decreases rapidly with increasing dopant
concentrations so that the current-voltage curve becomes S shaped. We also show
that the detrimental effect of nonohmic contacts on the performance of the
planar heterojunction cell can be strongly weakened by doping.

###A Vertical Architecture for Increasing Photogalvanic Solar Cell Efficiency: Theory and Modeling||Mohammad Ali Mahmoudzadeh,John D. W. Madden###

A Vertical Architecture for Increasing Photogalvanic Solar Cell Efficiency: Theory and Modeling. Photogalvanic solar cells, the original dye based solar cell, have yet to
fulfill their promise as a low fabrication cost, scalable energy conversion
system. The efficient performance of photogalvanic cells relies on high dye
solubility and selective electrodes with fast electron transfer kinetics. A new
configuration is proposed for the photogalvanic cells that removes these
impractical requirements. Instead of illuminating the device through the
electrode, as is the conventional approach, a new vertical configuration is
employed with light coming between the two electrodes. This way, the light
absorption and hence electron generation is spread through the depth of the
device. As a result, unreasonably fast electrode kinetics are no longer
required. The depth can be adjusted according to the concentration of the dyes,
and thus deeper cells enable low solubility dyes to be employed. The proposed
configuration is mathematically modeled and the advantages over the
conventional cell are shown. A numerical model is built for more detailed
analysis that gives practical guidelines for working towards device parameters
with high power conversion efficiency. The analysis suggests that upon the
realization of highly selective electrodes and an improved dye/mediator couple,
an efficiency of 13% should be achievable from the new configuration.

###Perovskite Twin Solar Device with Estimated 50% Bifacial PCE Potential and New Solar Material Options||Hans Hermann Otto###

Perovskite Twin Solar Device with Estimated 50% Bifacial PCE Potential and New Solar Material Options. There are recent investigations regarding tandem solar cells with a top
perovskite cell and a bottom silicon one with a potential of > 25% power
conversion efficiency. Because of still high production costs of silicon cells
it is believed that this tandem cell does not satisfy future requirements. Here
the construction of a low-cost FAPbI3 twin solar cell is proposed with assumed
PCE of 30%. With an ingenious rear illumination even 50% bifacial power
conversion efficiency should be feasible. Importantly, a single twin cell can
deliver the minimum potential difference of 1.7 V to conduct water splitting in
practice. Avoiding additional electrodes, the twin cell device may be expanded
by a large band gap sensitizer film at the mirror plan, derived from known
electrically isolating phosphor storage materials that can capture otherwise
wasted high-energetic radiation. Further, the substitution ot TiO2 (rutile) by
ferroelectric and photo-catalytically active Bi2SiO5 with its comparable energy
gap is sugested. CuO1-x could serve as new back electrode material due to its
high electric conductivity. In addition, an environmentally benign
(Cs,FA)2(Na,Cu,Ag)Bi(I,Br)6 elpasolite solar absorber material is discussed.
Alternatively, pyroelectric hexagonal bismuth sulfide iodide with its complex
superstructure may deliver a promising multiple band gap feature with suggested
singlet fission capability as intrinsic property to overcome its lower
efficiency.

###Accelerated Discovery of Efficient Solar-cell Materials using Quantum and Machine-learning Methods||Kamal Choudhary,Marnik Bercx,Jie Jiang,Ruth Pachter,Dirk Lamoen,Francesca Tavazza###

Accelerated Discovery of Efficient Solar-cell Materials using Quantum and Machine-learning Methods. Solar-energy plays an important role in solving serious environmental
problems and meeting high-energy demand. However, the lack of suitable
materials hinders further progress of this technology. Here, we present the
largest inorganic solar-cell material search to date using density functional
theory (DFT) and machine-learning approaches. We calculated the spectroscopic
limited maximum efficiency (SLME) using Tran-Blaha modified Becke-Johnson
potential for 5097 non-metallic materials and identified 1997 candidates with
an SLME higher than 10%, including 934 candidates with suitable convex-hull
stability and effective carrier mass. Screening for 2D-layered cases, we found
58 potential materials and performed G0W0 calculations on a subset to estimate
the prediction-uncertainty. As the above DFT methods are still computationally
expensive, we developed a high accuracy machine learning model to pre-screen
efficient materials and applied it to over a million materials. Our results
provide a general framework and universal strategy for the design of
high-efficiency solar cell materials. The data and tools are publicly
distributed at: https://www.ctcms.nist.gov/~knc6/JVASP.html,
https://www.ctcms.nist.gov/jarvisml/, https://jarvis.nist.gov/ and
https://github.com/usnistgov/jarvis .

###Next Generation Quantum Dots Based Multijunction Photovoltaics||Ankul Prajapati,Bade M H###

Next Generation Quantum Dots Based Multijunction Photovoltaics. Photovoltaic cells (PVc), as an energy provider to the next generation and
the biggest source of renewable energy. Since the last decade improving
efficiency and reducing the cost of PVc has been a subject of active research
among scientists. Promising progress in the field of material science and
manufacturing process at Nano-level played a big role. Still, at present there
are many challenges before photovoltaics for efficient and economic energy.
However, Photovoltaics cell based on p-n type homojunction semiconductors with
different organic and inorganic materials reported thus for generally suffer
from poor performance. According to the available literature, colloidal quantum
dots having immense properties like a wide range of light absorption, easily
charge separation and transport. To utilize the maximum part of the spectrum of
solar energy reaching to the earth and making effective energy production, here
we introduce the complete cell architecture and numerical investigation on
quantum dot based solar cells (QDSCs) with a heterostructure multijunction
approach. Successive ionic layer adsorption at different heterogeneous
interfaces were analyzed. We majorly focused on improving the electrical and
optical properties of the QDSCs achieved by different materials and structural
approaches. Here, we report a heterostructure II-Type of band alignment
engineering strategy for QDSCs interfaces that significantly enhances the
efficiency descriptors. In the context of intermediate band solar cell (IBSC),
we investigated optical properties of QDs and strain effects on multilayer PVc
and we summarize the strain effect in QDs growth and local energy band bending
of conduction band (CB) and valence band (VB).

###Computational design of high performance hybrid perovskite on silicon tandem solar cells||A. Rolland,L. Pedesseau,A. Beck,M. Kepenekian,C. Katan,Y. Huang,S. Wang,C. Cornet,O. Durand,J. Even###

Computational design of high performance hybrid perovskite on silicon tandem solar cells. In this study, the optoelectronic properties of a monolithically integrated
series-connected tandem solar cell are simulated. Following the large success
of hybrid organic-inorganic perovskites, which have recently demonstrated large
efficiencies with low production costs, we examine the possibility of using the
same perovskites as absorbers in a tandem solar cell. The cell consists in a
methylammonium mixed bromide-iodide lead perovskite, CH3NH3PbI3(1-x)Br3x (0 < x
< 1), top sub-cell and a single-crystalline silicon bottom sub-cell. A Si-based
tunnel junction connects the two sub-cells. Numerical simulations are based on
a one-dimensional numerical drift-diffusion model. It is shown that a top cell
absorbing material with 20% of bromide and a thickness in the 300-400 nm range
affords current matching with the silicon bottom cell. Good interconnection
between single cells is ensured by standard n and p doping of the silicon at
5.10^19cm-3 in the tunnel junction. A maximum efficiency of 27% is predicted
for the tandem cell, exceeding the efficiencies of stand-alone silicon (17.3%)
and perovskite cells (17.9%) taken for our simulations, and more importantly,
that of the record crystalline Si cells.

###SiNx:Tb3+--Yb3+, an efficient down-conversion layer compatible with a silicon solar cell process||Lucile Dumont,Julien Cardin,Patrizio Benzo,Marzia Carrada,Christophe Labbe,Andrea L. Richard,David C. Ingram,Wojciech M. Jadwisienczak,Fabrice Gourbilleau###

SiNx:Tb3+--Yb3+, an efficient down-conversion layer compatible with a silicon solar cell process. SiN x : Tb 3+-Yb 3+, an efficient down-conversion layer compatible with
silicon solar cell process Abstract Tb 3+-Yb 3+ co-doped SiN x down-conversion
layers compatible with silicon Photovoltaic Technology were prepared by
reactive magnetron co-sputtering. Efficient sensitization of Tb 3+ ions through
a SiN x host matrix and cooperative energy transfer between Tb 3+ and Yb 3+
ions were evidenced as driving mechanisms of the down-conversion process. In
this paper, the film composition and microstructure are investigated alongside
their optical properties, with the aim of maximizing the rare earth ions
incorporation and emission efficiency. An optimized layer achieving the highest
Yb 3+ emission intensity was obtained by reactive magnetron co-sputtering in a
nitride rich atmosphere for 1.2 W/cm${}^2$ and 0.15 W/cm${}^2$ power density
applied on the Tb and Yb targets, respectively. It was determined that
depositing at 200 {\textdegree}C and annealing at 850 {\textdegree}C leads to
comparable Yb 3+ emission intensity than depositing at 500 {\textdegree}C and
annealing at 600 {\textdegree}C, which is promising for applications toward
silicon solar cells.

###Novel High Efficiency Quadruple Junction Solar Cell with Current Matching and Optimized Quantum Efficiency||Mohammad Jobayer Hossain###

Novel High Efficiency Quadruple Junction Solar Cell with Current Matching and Optimized Quantum Efficiency. A high photon to electricity conversion efficiency of 47.2082% was achieved
by a novel combination of In0.51Ga0.49P, GaAs, In0.24Ga0.76As and
In0.19Ga0.81Sb subcell layers in a quadruple junction solar cell design. The
electronic bandgap of these materials are 1.9 eV, 1.42 eV, 1.08 eV and 0.55 eV
respectively. This novel III-V arrangement enables the cell to absorb photons
from the ultraviolet to deep infrared wavelengths of the solar spectrum. After
careful consideration of important semiconductor parameters such as thicknesses
of emitter and base layers, doping concentrations, diffusion lengths, minority
carrier lifetimes and surface recombination velocities an optimized quadruple
junction design has been suggested. Current matching of the subcell layers was
ensured to obtain maximum efficiency from the proposed design. The
short-circuit current density, open circuit voltage and fill factor of the
solar cell are 14.7 mA/cm2, 3.3731 V and 0.9553 respectively. In the design
process, 1 sun AM1.5 global solar spectrum was considered. The cell performance
was also investigated for extraterrestrial illumination (AM0). A modified
design is proposed for space applications. With a short circuit current density
of 18.5 mA/cm2, open circuit voltage of 3.4104 and the fill factor of 0.9557,
the power conversion efficiency of the modified quadruple junction design is
44.5473% in space.

###Strongly enhanced upconversion in trivalent erbium ions by tailored gold nanostructures: toward high-efficient silicon-based photovoltaics||Jeppe Christiansen,Joakim Vester-Petersen,Søren Roesgaard,Søren H. Møller,Rasmus E. Christiansen,Ole Sigmund,Søren P. Madsen,Peter Balling,Brian Julsgaard###

Strongly enhanced upconversion in trivalent erbium ions by tailored gold nanostructures: toward high-efficient silicon-based photovoltaics. Upconversion of sub-band-gap photons constitutes a promising way for
improving the efficiency of silicon-based solar cells beyond the
Shockley-Queisser limit. 1500 to 980 nm upconversion by trivalent erbium ions
is well-suited for this purpose, but the small absorption cross section hinders
real-world applications. We employ tailored gold nanostructures to vastly
improve the upconversion efficiency in erbium-doped TiO$_2$ thin films. The
nanostructures are found using topology optimization and parameter optimization
and fabricated by electron beam lithography. In qualitative agreement with a
theoretical model, the samples show substantial electric-field enhancements
inside the upconverting films for excitation at 1500 nm for both s- and
p-polarization under a wide range of incidence angles and excitation
intensities. An unprecedented upconversion enhancement of 913(51) is observed
at an excitation intensity of 1.7 Wcm$^{-2}$. We derive a semi-empirical
expression for the photonically enhanced upconversion efficiency, valid for all
excitation intensities. This allows us to determine the upconversion properties
needed to achieve significant improvements in real-world solar-cell devices
through photonic-enhanced upconversion.

###High open-circuit voltage in transition metal dichalcogenide solar cells||Simon A. Svatek,C. Bueno,Der-Yuh Lin,James Kerfoot,Carlos Macías,Marius H. Zehender,Ignacio Tobías,Pablo García-Linares,Takashi Taniguchi,Kenji Watanabe,Peter Beton,Elisa Antolín###

High open-circuit voltage in transition metal dichalcogenide solar cells. The conversion efficiency of ultra-thin solar cells based on layered
materials has been limited by their open-circuit voltage, which is typically
pinned to a value under 0.6 V. Here we report an open-circuit voltage of 1.02 V
in a 120 nm-thick vertically stacked homojunction fabricated with
substitutionally doped MoS2. This high open-circuit voltage is consistent with
the band alignment in the MoS2 homojunction, which is more favourable than in
widely-used TMDC heterostructures. It is also attributed to the high
performance of the substitutionally doped MoS2, in particular the p-type
material doped with Nb, which is demonstrated by the observation of
electroluminescence from tunnelling graphene/BN/MoS2 structures in spite of the
indirect nature of bulk MoS2. We find that illuminating the TMDC/metal contacts
decreases the measured open-circuit voltage in MoS2 van der Waals homojunctions
because they are photoactive, which points to the need of developing
low-resistance, ohmic contacts to doped MoS2 in order to achieve high
efficiency in practical devices. The high open-circuit voltage demonstrated
here confirms the potential of layered transition-metal dichalcogenides for the
development of highly efficient, ultra-thin solar cells.

###Unraveling the varied nature and roles of defects in hybrid halide perovskites with time-resolved photoemission electron microscopy||Sofiia Kosar,Andrew J. Winchester,Tiarnan A. S. Doherty,Stuart Macpherson,Christopher E. Petoukhoff,Kyle Frohna,Miguel Anaya,Nicholas S. Chan,Julien Madéo,Michael K. L. Man,Samuel D. Stranks,Keshav M. Dani###

Unraveling the varied nature and roles of defects in hybrid halide perovskites with time-resolved photoemission electron microscopy. With rapidly growing photoconversion efficiencies, hybrid perovskite solar
cells have emerged as promising contenders for next generation, low-cost
photovoltaic technologies. Yet, the presence of nanoscale defect clusters, that
form during the fabrication process, remains critical to overall device
operation, including efficiency and long-term stability. To successfully deploy
hybrid perovskites, we must understand the nature of the different types of
defects, assess their potentially varied roles in device performance, and
understand how they respond to passivation strategies. Here, by correlating
photoemission and synchrotron-based scanning probe X-ray microscopies, we
unveil three different types of defect clusters in state-of-the-art triple
cation mixed halide perovskite thin films. Incorporating ultrafast
time-resolution into our photoemission measurements, we show that defect
clusters originating at grain boundaries are the most detrimental for
photocarrier trapping, while lead iodide defect clusters are relatively benign.
Hexagonal polytype defect clusters are only mildly detrimental individually,
but can have a significant impact overall if abundant in occurrence. We also
show that passivating defects with oxygen in the presence of light, a
previously used approach to improve efficiency, has a varied impact on the
different types of defects. Even with just mild oxygen treatment, the grain
boundary defects are completely healed, while the lead iodide defects begin to
show signs of chemical alteration. Our findings highlight the need for
multi-pronged strategies tailored to selectively address the detrimental impact
of the different defect types in hybrid perovskite solar cells.

###A DFT computational design and exploration of novel direct band gap silver-thallium double perovskites||Syed Zuhair Abbas Shah,Shanawer Niaz,Tabassum Nasir,James Sifuna###

A DFT computational design and exploration of novel direct band gap silver-thallium double perovskites. Researchers have addressed the non-traditional power generation schemes as
alternatives to the traditional fossil-fuel methods enormously since the
scientific community has serious concerns about shortages of energy on our
planet for future generations. In this scenario, the innovative materials for
photovoltaic and thermoelectric device applications are required by addressing
current issues of instability and efficiency. Perovskites are very popular in
this regard particularly having higher power conversion efficiency of 25.2% in
the case of solar cells. In the current article, we investigated innovative
small direct band gap double perovskites (elapsolite) Cs$_2$AgTlX$_6$ (X= Cl,
Br) with a comprehensive discussion on structural, electronic, optical, and
thermoelectric properties using a first-principles approach. The compounds
under investigation are found stable, efficient, and economical with alluring
optical and thermoelectric properties. The higher absorption peaks in the
visible range, substantial optical conductivities (~10$^{16}$ sec$^{-1}$), and
a lower percentage of reflection in the visible range make these compounds
fascinating for solar cell applications. Whereas large values of Seebeck
coefficients, electrical conductivities, the figure of merits (greater than
unity), and small values of thermal conductivities suggest the applications of
these compounds in thermoelectric generators.

###Nature of excitons in PPDT2FBT: PCBM solar cell: Role played by PCBM||Subhamoy Sahoo,Dhruvajyoti Barah,Dinesh Kumar S,Nithin Xavier,Soumya Dutta,Debdutta Ray,Jayeeta Bhattacharyya###

Nature of excitons in PPDT2FBT: PCBM solar cell: Role played by PCBM. In organic semiconductor based bulk heterojunction solar cells, the presence
of acceptor increases the formation of charge transfer (CT) excitons, thereby
leading to higher exciton dissociation probabilities. In this work we used
steady state EA measurements to probe the change in the nature of excitons as
the blend composition of the solar cell active layer material is varied. We
investigated blends of
poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)]
(PPDT2FBT) and (6,6)-Phenyl C71 butyric acid methyl ester (PCBM). Analysis of
the EA spectra showed that in presence of fullerene based acceptor, like PCBM,
CT characteristics of the excitons were modified, though, no new CT signature
was observed in the blend. Enhancement in the CT characteristic in the blend
was reflected in the photoluminescence (PL) measurements of the blends, where,
PL quenching of $\sim$ 63\% was observed for 1\% PCBM. The quenching reaches
saturation at about 20\% PCBM. However, the maximum efficiency of the devices
was obtained for the blend having 50\% PCBM. Comparing experimental results
with simulations, the variation of the device efficiency with PCBM percentage
was shown to be arising from multiple factors like increase in polarizability
and dipole moment of excitons, and the efficiency of the carrier collection
from the bulk of the active layer.

###Two-Dimensional $β$-PdX$_2$ (X = S, Te) Monolayers for Efficient Solar Energy Conversion Applications||Mukesh Jakhar,Ashok Kumar###

Two-Dimensional $β$-PdX$_2$ (X = S, Te) Monolayers for Efficient Solar Energy Conversion Applications. The search for highly effective and environmentally safe photocatalysts for
water splitting and photovoltaic solar cells is essential for renewable solar
energy conversion and storage. Based on first principles calculations, we show
that novel 2D $\beta$-PdX$_2$ (X = S, Te) monolayer possesses excellent
stabilities and great potentials in solar energy conversion applications.
Comprehensive studies show that the $\beta$-PdX$_2$ monolayer exhibits
semiconductor characteristics with an indirect gap, suitable band alignment,
efficient carrier separation, and high solar to hydrogen (STH) efficiencies,
supporting its good photoelectronic performance. The surface catalytic and
adsorption/intercalation energies calculation reveals that the photogenerated
holes have adequate driving forces to render hydrogen reduction half-reactions
to proceed spontaneously and the ability to cover and incorporate water
molecules on $\beta$-PdX$_2$ monolayer. Besides, the $\beta$-PdX$_2$ monolayer
is promising donor material for excitonic solar cells with high photovoltaic
performance. More importantly, due to suitable donor band gap and small
conduction band offset in the proposed type-II heterostructure, the calculated
power conversion efficiencies (PCE) is calculated up to ~23%
($\beta$-PdX$_2$/WTe$_2$), ~21% ($\beta$-PdX$_2$/ MoTe$_2$) and ~18%
($\beta$-PdTe2/$\beta$-PdX$_2$), making it a promising candidate for solar
energy conversion applications.

###Dynamics of the sub-ambient gelation and shearing of solutions of P3HT incorporated with a non-fullerene acceptor o-IDTBR towards active layer formation in bulk heterojunction organic solar cells||Li Quan,Dongrun Ju,Stephanie Lee,Dilhan M. Kalyon###

Dynamics of the sub-ambient gelation and shearing of solutions of P3HT incorporated with a non-fullerene acceptor o-IDTBR towards active layer formation in bulk heterojunction organic solar cells. Organic solar cells (OSCs) containing an active layer consisting of a
nanostructured blend of a conjugated polymer like poly(3-hexylthiophene) (P3HT)
and an electron acceptor molecule have the potential of competing against
silicon-based photovoltaic panels. However, this potential is unfulfilled
primarily due to interrelated production and stability issues. The generally
employed spin coating process for fabricating organic solar cells cannot be
scaled up. Recently, He et al., have reported that the gelation of P3HT with
[6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) under sub-ambient
conditions can provide a continuous extrusion/coating based route to the
processing of organic solar cells and that increases in power conversion
efficiencies (PCEs) of the P3HT/PC60BM active layer are possible under certain
shearing and thermal histories of the P3HT/PC60BM gels. Here oscillatory and
steady torsional flows were used to investigate the gel formation dynamics of
P3HT with a recently proposed non-fullerene o-IDTBR under sub-ambient
conditions. The gel strengths defined on the basis of linear viscoelastic
material functions as determined via small-amplitude oscillatory shear were
observed to be functions of the P3HT and o-IDTBR concentrations, the solvent
used and the shearing conditions. Overall, the gels which formed upon quenching
to sub-zero temperatures were found to be stable during small-amplitude
oscillatory shear (linear viscoelastic range) but broke down even at the
relatively low shear rates associated with steady torsional flows, suggesting
that the shearing conditions used during the processing of gels of P3HT with
small molecule acceptor blends can alter the gel structure and possibly affect
the resulting active layer performance.

###Single crystal monolithic up-converter solar cell device tandems with integrated optics||Georgios E. Arnaoutakis,Elena Favilla,Mauro Tonelli,Bryce S. Richards###

Single crystal monolithic up-converter solar cell device tandems with integrated optics. Solar photons possessing energy less than the band-gap of a single-junction
solar cell can be utilized via the up-conversion (UC) of two or more photons,
resulting in the emission of a single above-bandgap photon. Due to the
non-linear nature of UC, highly concentrated light is required, which is
typically much greater than the practical concentration limits of a solar cell.
It has been proposed that concentrating up-conversion solar cells (UC-SC) with
optical elements integrated into the device could help realize the high solar
irradiance required. To avoid scattering problems arising from common UC
materials based on micro-crystalline powders, in this work concentrators are
investigated with mono-crystalline up-converters in silicon-based tandem
devices. An external quantum efficiency (EQE) of 6% with 1493 nm infrared
illumination at $876 W/m^2$ was obtained in upconverter device with concave
integrated optics. At an irradiance higher than $90 W/m^2$ (equivalent to 2.95x
in the 1450-1600 nm range), the non-concentrating UC-SC exhibited 1.5x higher
EQE than the UC-SC with CPC, while below $90 W/m^2$ the CPC UC-SC exhibited
1.95x higher EQE than the non-concentrating reference device. Due to the
negligible scattering of the UC layer, the distribution of localized irradiance
is revealed along with its effect on the performance of devices. It is found
that irradiance is accumulated within the first 1 mm of the UC layer with peaks
at variable depths according to the concentrating scheme. These results suggest
ample space for improved up-conversion devices by using integrated optics.

###Density-Functional Theory (DFT) and Time-Dependent DFT Study of the Chemical and Physical Origins of Key Photoproperties of End-Group Derivatives of the Nonfullerene Bulk Heterojunction Organic Solar Cell Acceptor Molecule IDIC||Taouali W,Alimi K,Nangraj A. S.,Casida M. E###

Density-Functional Theory (DFT) and Time-Dependent DFT Study of the Chemical and Physical Origins of Key Photoproperties of End-Group Derivatives of the Nonfullerene Bulk Heterojunction Organic Solar Cell Acceptor Molecule IDIC. As emphasized in a recent review article [Chem. Rev. 122, 14180 (2022)],
organic solar cell (OSC) photoconversion efficiency has been rapidly evolving
with results increasingly comparable to those of traditional inorganic solar
cells. Historically, OSC performance improvement focused first on the
morphology of P3HT:PC61BM solar cells then went through different stages to
shift lately interest towards nonfullerene acceptors (NFAs) as a replacement of
PC61BM acceptor (ACC) molecule. Here, we use density-functional theory (DFT)
and time-dependent (TD) DFT to investigate four novel NFAs of A-D-A
(acceptor-donor-acceptor) form derived from the recently synthesized IDIC-4Cl
[Dyes and Pigments 166, 196 (2019)]. Our level of theory is carefully evaluted
for IDIC-4Cl and then applied to the four novel NFAs in order to understand how
chemical modifications lead to physical changes in cyclic voltammetry (CV)
frontier molecular orbital (FMO) energies and absorption spectra in
solution.Finally we design and apply a new type of Scharber plot for NFAs based
upon some simple but we think reasonable assumptions. Unlike the original
Scharber plots where a larger DON band gap favors a larger PCE, our modified
Scharber plot reflects the fact that a smaller ACC band gap may favor PCE by
filling in gaps in the DON acceptor spectrum. We predict that only the
candidate molecule with the least good acceptor A, with the highest frontier
molecular orbital energies, and one of the larger CV lowest unoccupied
molecular orbital (LUMO) highest unoccupied molecular orbital (HOMO) gaps, will
yield a PM6:ACC PCE exceeding that of the parent IDIC-4Cl ACC. This candidate
also shows the largest oscillator strength for the primary 1 (HOMO,LUMO)
charge-transfer transition and the largest degree of delocalization of charge
transfer of any of the ACC molecules investigated here.

###Efficient Multiple Exciton Generation in Monolayer MoS2||Ashish Soni,Dushyant Kushavah,Li-Syuan Lu,Wen-Hao Chang,Suman Kalyan Pal###

Efficient Multiple Exciton Generation in Monolayer MoS2. Utilizing the excess energy of photoexcitation that is otherwise lost as
thermal effects can improve the efficiency of next-generation light-harvesting
devices. Multiple exciton generation (MEG) in semiconducting materials yields
two or more excitons by absorbing a single high-energy photon, which can break
the Shockley-Queisser limit for the conversion efficiency of photovoltaic
devices. Recently, monolayer transition metal dichalcogenides (TMDs) have
emerged as promising light-harvesting materials because of their high
absorption coefficient. Here, we report efficient MEG with low threshold energy
and high (86%) efficiency in a van der Waals (vdW) layered material, MoS2.
Through different experimental approaches, we demonstrate the signature of
exciton multiplication and discuss the possible origin of decisive MEG in
monolayer MoS2. Our results reveal that vdW-layered materials could be a
potential candidate for developing mechanically flexible and highly efficient
next generation solar cells and photodetectors.

###Effect of Solar-Terrestrial Phenomena on Solar Cell's Efficiency||Kashif Bin Zaheer,Waseem Ahmed Ansari,Syed Mohammad Murshid Raza###

Effect of Solar-Terrestrial Phenomena on Solar Cell's Efficiency. It is assumed that the solar cell efficiency of PV device is closely related
to the solar irradiance, considered the solar parameter Global Solar Irradiance
(G) and the meteorological parameters like daily data of Earth Skin Temperature
(E), Average Temperature (T), Relative Humidity (H) and Dew Frost Point (D),
for the coastal city Karachi and a non-coastal city Jacobabad, K and J is used
as a subscripts for parameters of Karachi and Jacobabad respectively. All
variables used here are dependent on the location (latitude and longitude) of
our stations except G. To employ ARIMA modeling, the first eighteen years data
is used for modeling and forecast is done for the last five years data. In most
cases results show good correlation among monthly actual and monthly forecasted
values of all the predictors. Next, multiple linear regression is employed to
the data obtained by ARIMA modeling and models for mean monthly observed G
values are constructed. For each station, two equations are constructed the R2
values are above 93% for each model, showing adequacy of the fit. Our
computations show that Solar cell efficiency can be increased if better
modeling for meteorological predictors governs the process.

###TASC-1D-cSi: a simulation tool to scrutinize the thermal impacts on the performances of crystalline silicon solar cells||Olivier Dupré,Mohamed Amara,Rodolphe Vaillon###

TASC-1D-cSi: a simulation tool to scrutinize the thermal impacts on the performances of crystalline silicon solar cells. The capabilities of a simulation tool for the in-depth analysis of the
thermal impacts on the performances of solar cells are described. TASC-1D
(Thermal Analysis of Solar Cells - 1D, version cSi) solves the coupled
electrical, radiative and thermal transport problems for a crystalline silicon
cell, as a function of irradiation and thermal conditions. In addition to the
electrical outputs that are obtained with the existing simulation tools, it
provides the cell equilibrium temperature as well as the spatial and spectral
distributions of many relevant quantities. The physical modeling and associated
numerical solution techniques are summarized. Result cases with a prescribed
cell temperature demonstrate that the code performs well, is able to provide
valuable information on the quantum efficiency and power loss mechanisms, and
is capable of handling diffuse irradiations. The intricate correlation between
thermal sources and optical-electrical losses is discussed. The other cases
include the solution of the heat transfer problem and analyses are conducted on
the cell operating temperature and efficiency as a function of the thermal
conditions. This simulation tool is likely to allow new optimizations of
photovoltaic cells that include a thermal criterion in addition to the optical
and electrical criteria.

###Growth Route Toward III-V Multispectral Solar Cells on Silicon||C. Renard,N. Cherkashin,A. Jaffré,T. Molière,L. Vincent,A. Michel,J. Alvarez,J. P. Connolly,J. -P. Kleider,D. Mencaraglia,D. Bouchier###

Growth Route Toward III-V Multispectral Solar Cells on Silicon. To date, high efficiency multijunction solar cells have been developed on Ge
or GaAs substrates for space applications, and terrestrial applications are
hampered by high fabrication costs. In order to reduce this cost, we propose a
breakthrough technique of III-V compound heteroepitaxy on Si substrates without
generation of defects critical to PV applications. With this technique we
expect to achieve perfect integration of heterogeneous Ga1-xInxAs
micro-crystals on Si substrates. In this paper, we show that this is the case
for x=0. GaAs crystals were grown by Epitaxial Lateral Overgrowth on Si (100)
wafers covered with a thin SiO2 nanostructured layer. The cristallographic
structure of these crystals is analysed by MEB and TEM imaging. Micro-Raman and
Micro-Photomuminescence spectra of GaAs crystals grown with different
conditions are compared with those of a reference GaAs wafer in order to have
more insight on eventual local strains and their cristallinity. This work aims
at developping building blocks to further develop a GaAs/Si tandem demonstrator
with a potential conversion efficiency of 29.6% under AM1.5G spectrum without
concentration, as inferred from our realistic modeling. This paper shows that
Epitaxial Lateral Overgrowth has a very interesting potential to develop
multijunction solar cells on silicon approaching the today 30.3% world record
of a GaInP/GaAs tandem cell under the same illumination conditions, but on a
costlier substrate than silicon.

###Molecular-Level Switching of Polymer/Nanocrystal Non-Covalent Interactions and Application in Hybrid Solar Cells||Carlo Giansante,Rosanna Mastria,Giovanni Lerario,Luca Moretti,Ilka Kriegel,Francesco Scotognella,Guglielmo Lanzani,Sonia Carallo,Marco Esposito,Mariano Biasiucci,Aurora Rizzo,Giuseppe Gigli###

Molecular-Level Switching of Polymer/Nanocrystal Non-Covalent Interactions and Application in Hybrid Solar Cells. Hy brid composites obtained upon blending conjugated polymers and colloidal
inorganic semiconductor nanocrystals are regarded as attractive photo-active
materials for optoelectronic applications. Here we demonstrate that tailoring
nanocrystal surface chemistry permits to exert control on non-covalent bonding
and electronic interactions between organic and inorganic components. The
pendant moieties of organic ligands at the nanocrystal surface do not merely
confer colloidal stability while hindering charge separation and transport, but
drastically impact morphology of hybrid composites during formation from blend
solutions. The relevance of our approach to photovoltaic applications is
demonstrated for composites based on poly(3-hexylthiophene) and Pbs
nanocrystals, considered as inadequate before the submission of this
manuscript, which enable the fabrication of hybrid solar cells displaying a
power conversion efficiency that reaches 3 %. Upon (quasi)steady-state and
time-resolved analisys of the photo-induced processes in the nanocomposites and
their organic and inorganic components, we ascertained that electron transfer
occurs at the hybrid interface yielding long-lived separated charge carriers,
whereas interfacial hole transfer appears slow. Here we provide a reliable
alternative aiming at gaining control over macroscopic optoelectronic
properties of polymer/nanocrystal composites by acting at the molecular-level
via ligands' pendant moieties, thus opening new possibilities towards efficient
solution-processed hybrid solar cells.

###Atomistic origins of high-performance in hybrid halide perovskite solar cells||Jarvist M. Frost,Keith T. Butler,Federico Brivio,Christopher H. Hendon,Mark van Schilfgaarde,Aron Walsh###

Atomistic origins of high-performance in hybrid halide perovskite solar cells. The performance of organometallic perovskite solar cells has rapidly
surpassed that of both conventional dye-sensitised and organic photovoltaics.
High power conversion efficiency can be realised in both mesoporous and
thin-film device architectures. We address the origin of this success in the
context of the materials chemistry and physics of the bulk perovskite as
described by electronic structure calculations. In addition to the basic
optoelectronic properties essential for an efficient photovoltaic device
(spectrally suitable band gap, high optical absorption, low carrier effective
masses), the materials are structurally and compositionally flexible. As we
show, hybrid perovskites exhibit spontaneous electric polarisation; we also
suggest ways in which this can be tuned through judicious choice of the organic
cation. The presence of ferroelectric domains will result in internal junctions
that may aid separation of photoexcited electron and hole pairs, and reduction
of recombination through segregation of charge carriers. The combination of
high dielectric constant and low effective mass promotes both Wannier-Mott
exciton separation and effective ionisation of donor and acceptor defects. The
photoferroic effect could be exploited in nanostructured films to generate a
higher open circuit voltage and may contribute to the current-voltage
hysteresis observed in perovskite solar cells.

###Bilayer Phosphorene: Effect of Stacking Order on Bandgap and its Potential Applications in Thin-Film Solar Cells||Jun Dai,Xiao Cheng Zeng###

Bilayer Phosphorene: Effect of Stacking Order on Bandgap and its Potential Applications in Thin-Film Solar Cells. Phosphorene, a monolayer of black phosphorus, is promising for nanoelectronic
applications not only because it is a natural p-type semiconductor but also it
possesses a layer-number dependent direct bandgap (in the range of 0.3 eV~1.5
eV). On basis of the density functional theory calculations, we investigate
electronic properties of the bilayer phosphorene with different stacking
orders. We find that the direct bandgap of the bilayers can vary from 0.78 -
1.04 eV with three different stacking orders. In addition, a vertical electric
field can further reduce the bandgap down to 0.56 eV (at the field strength 0.5
V/{\AA}). More importantly, we find that when a monolayer of MoS_2 is
superimposed with the p-type AA- or AB-stacked bilayer phosphorene, the
combined tri-layer can be an effective solar-cell material with type-II
heterojunction alignment. The power conversion efficiency is predicted to be
~18% or 16% with AA- or AB-stacked bilayer phosphorene, higher than reported
efficiencies of the state-of-the-art trilayer graphene/transition metal
dichalcogenide solar cells.

###Steric engineering of metal-halide perovskites with tunable optical band gaps||Marina R. Filip,Giles E. Eperon,Henry J. Snaith,Feliciano Giustino###

Steric engineering of metal-halide perovskites with tunable optical band gaps. Owing to their high energy-conversion efficiency and inexpensive fabrication
routes, solar cells based on metal-organic halide perovskites have rapidly
gained prominence as a disruptive technology. An attractive feature of
perovskite absorbers is the possibility of tailoring their properties by
changing the elemental composition through the chemical precursors. In this
context, rational in silico design represents a powerful tool for mapping the
vast materials landscape and accelerating discovery. Here we show that the
optical band gap of metal-halide perovskites, a key design parameter for solar
cells, strongly correlates with a simple structural feature, the largest
metal-halide-metal bond angle. Using this descriptor we suggest continuous
tunability of the optical gap from the mid-infrared to the visible. Precise
band gap engineering is achieved by controlling the bond angles through the
steric size of the molecular cation. Based on these design principles we
predict novel low-gap perovskites for optimum photovoltaic efficiency, and we
demonstrate the concept of band gap modulation by synthesising and
characterising novel mixed-cation perovskites.

###Accelerated development of CuSbS2 thin film photovoltaic device prototypes||Adam W. Welch,Lauryn L. Baranowski,Pawel Zawadzki,Clay DeHart,Steve Johnston,Stephan Lany,Colin A. Wolden,Andriy Zakutayev###

Accelerated development of CuSbS2 thin film photovoltaic device prototypes. Development of alternative thin film photovoltaic technologies is an
important research topic due to the potential for low-cost, large-scale
fabrication of high-efficiency solar cells. Despite the large number of
promising alternative absorbers and corresponding contacts, the rate of
progress is limited by complications that arise during solar cell fabrication.
One potential solution to this problem is the high-throughput combinatorial
method, which has been extensively used for research and development of
individual absorber and contact materials. Here, we demonstrate an accelerated
approach to development of thin film photovoltaic device prototypes based on
the novel CuSbS2 absorber, using the device architecture employed for
CuInxGa(1-x)Se2 (CIGS). The newly developed three-stage, self-regulated CuSbS2
growth process enables the study of PV device performance trends as a function
of phase purity, crystallographic orientation, layer thickness of the absorber,
and numerous back contacts. This exploration results in initial CuSbS2 device
prototypes with ~1% conversion efficiency; currently limited by low
short-circuit current due to poor collection of photoexcited electrons, and a
small open-circuit voltage due to a cliff-type conduction band offset at the
CuSbS2/CdS interface (suggested by first-principles calculations). Overall,
these results illustrate the potential of combinatorial methods to accelerate
the development of thin film photovoltaic devices with novel absorbers.

###Silicon solar cells efficiency analysis. Doping type and level optimization||A. V. Sachenko,V. P. Kostylyov,M. V. Gerasymenko,R. M. Korkishko,N. R. Kulish,M. I. Slipchenko,I. O. Sokolovskyi,V. V. Chernenko###

Silicon solar cells efficiency analysis. Doping type and level optimization. The theoretical analysis of photovoltaic conversion efficiency of highly
effective silicon solar cells (SC) is performed for n-type and p-type bases.
The case is considered when the Shockley-Read-Hall recombination in the silicon
bulk is determined by the deep level of Fe. It is shown that due to the
asymmetry of the recombination parameters of this level the photovoltaic
conversion efficiency is increasing in the SC with the n-type base and
decreasing in the SC with the p-type base with the increase in doping. Two
approximations for the band-to-band Auger recombination lifetime dependence on
the base doping level are considered when performing the analysis. The
experimental results are presented for the key characteristics of the solar
cells based on $\alpha-Si:H-n-Si$ heterojunctions with intrinsic thin layer
(HIT). A comparison between the experimental and calculated values of the HIT
cells characteristics is made. The surface recombination velocity and series
resistance are determined from it with a complete coincidence of the
experimental and calculated SC parameters' values.

###Design principles for shift current photovoltaics||Ashley M. Cook,Benjamin M. Fregoso,Fernando de Juan,Sinisa Coh,Joel E. Moore###

Design principles for shift current photovoltaics. While the basic principles and limitations of conventional solar cells are
well understood, relatively little attention has gone toward maximizing the
potential efficiency of photovoltaic devices based on shift currents. In this
work, we outline simple design principles for the optimization of shift
currents for frequencies near the band gap, derived from the analysis of a
general effective model. The use of a novel sum rule allows us to express the
band edge shift current in terms of a few model parameters and to show it
depends explicitly on wavefunctions via Berry connections in addition to
standard band structure. We use our approach to identify two new classes of
shift current photovoltaics, ferroelectric polymer films and single-layer
orthorhombic monochalcogenides such as GeS. We introduce tight-binding models
for these systems, and show that they exhibit the largest shift current
responsivities at the band edge reported so far. Moreover, exploring the
parameter space of these models we find photoresponsivities that can exceed
$100$ mA/W. Our results show how the study of the shift current via effective
models allows one to improve the possible efficiency of devices based on this
mechanism and better grasp their potential to compete with conventional solar
cells.

###Omnidirectional and broadband absorption enhancement from trapezoidal Mie resonators in semiconductor metasurfaces||Ragip A. Pala,Serkan Butun,Koray Aydin,Harry A. Atwater###

Omnidirectional and broadband absorption enhancement from trapezoidal Mie resonators in semiconductor metasurfaces. Light trapping in planar ultrathin-film solar cells is limited due to a small
number of optical modes available in the thin-film slab. A nanostructured
thin-film design could surpass this limit by providing broadband increase in
the local density of states in a subwavelength volume and maintaining efficient
coupling of light. Here we report a broadband metasurface design, enabling
efficient and broadband absorption enhancement by direct coupling of incoming
light to resonant modes of subwavelength-scale Mie nanoresonators defined in
the thin-film active layer. Absorption was investigated both theoretically and
experimentally in prototypes consisting of lithographically patterned,
two-dimensional periodic arrays of silicon nanoresonators on silica substrates.
A crossed trapezoid resonator shape of rectangular cross section is used to
excite broadband Mie resonances across the visible and near-IR spectra. Our
numerical simulations, optical absorption measurements and photocurrent
spectral response measurements demonstrate that crossed trapezoidal Mie
resonant structures enable angle-insensitive, broadband absorption. A short
circuit current density of 12.0 mA/cm2 is achieved in 210 nm thick patterned Si
films, yielding a 4-fold increase compared to planar films of the same
thickness. It is suggested that silicon metasurfaces with Mie resonator arrays
can provide useful insights to guide future ultrathin-film solar cell designs
incorporating nanostructured thin active layers.

###Smooth anti-reflective three-dimensional textures for liquid-phase crystallized silicon thin-film solar cells on glass||David Eisenhauer,Grit Köppel,Klaus Jäger,Duote Chen,Oleksandra Shargaieca,Bernd Rech,Christiane Becker###

Smooth anti-reflective three-dimensional textures for liquid-phase crystallized silicon thin-film solar cells on glass. Recently, liquid-phase crystallization of thin silicon films has emerged as a
promising candidate for thin-film photovoltaics. On 10 \mu m thin absorbers,
wafer-equivalent morphologies and open-circuit voltages were reached, leading
to a record efficiency of 12.1%. However, short-circuit current densities are
still limited, mainly due to optical losses at the glass-silicon interface.
While nano-structures at this interface have been shown to efficiently reduce
reflection, up to now these textures caused an increase in recombination.
Therefore, optical gains were mitigated due to electronic losses. Here, the
SMooth Anti-Reflective Three-dimensional (SMART) texture is introduced in order
to overcome this trade-off. By smoothing nanoimprinted SiOx nano-pillar arrays
with spin-coated TiOx layers, light-trapping properties of laser-crystallized
silicon solar cells could significantly be improved as successfully shown in
three-dimensional simulations and in experiment. At the same time, the smooth
surface morphology of SMART textures allows preserving electronic material
quality equivalent to that of planar reference samples and reaching Voc values
above 640 mV in 8 \mu m thin liquid-phase crystallized silicon solar cells.
Furthermore, the short-circuit current density jsc could be increased from 21.0
mA cm-2 for planar reference cells with already optimized anti-reflective
interlayer stacks to 23.3 mA cm-2 on SMART textures, corresponding to a
relative increase of 11%.

###Mutual Photoluminescence Quenching and Photovoltaic Effect in Large-Area Single-Layer MoS2-Polymer Heterojunctions||Tejas A. Shastry,Itamar Balla,Hadallia Bergeron,Samuel H. Amsterdam,Tobin J. Marks,Mark C. Hersam###

Mutual Photoluminescence Quenching and Photovoltaic Effect in Large-Area Single-Layer MoS2-Polymer Heterojunctions. Two-dimensional transition metal dichalcogenides (TMDCs) have recently
attracted attention due to their superlative optical and electronic properties.
In particular, their extraordinary optical absorption and semiconducting band
gap have enabled demonstrations of photovoltaic response from heterostructures
composed of TMDCs and other organic or inorganic materials. However, these
early studies were limited to devices at the micrometer scale and/or failed to
exploit the unique optical absorption properties of single-layer TMDCs. Here we
present an experimental realization of a large-area type-II photovoltaic
heterojunction using single-layer molybdenum disulfide (MoS2) as the primary
absorber, by coupling it to the organic {\pi}-donor polymer PTB7. This
TMDC-polymer heterojunction exhibits photoluminescence intensity that is
tunable as a function of the thickness of the polymer layer, ultimately
enabling complete quenching of the TMDC photoluminescence. The strong optical
absorption in the TMDC-polymer heterojunction produces an internal quantum
efficiency exceeding 40% for an overall cell thickness of less than 20 nm,
resulting in exceptional current density per absorbing thickness in comparison
to other organic and inorganic solar cells. Furthermore, this work provides new
insight into the recombination processes in type-II TMDC-polymer
heterojunctions and thus provides quantitative guidance to ongoing efforts to
realize efficient TMDC-based solar cells.

###Exciton delocalization incorporated drift-diffusion model for bulk-heterojunction organic solar cells||Zi Shuai Wang,Wei E. I. Sha,Wallace C. H. Choy###

Exciton delocalization incorporated drift-diffusion model for bulk-heterojunction organic solar cells. Modeling the charge-generation process is highly important to understand
device physics and optimize power conversion efficiency of bulk-heterojunction
(BHJ) organic solar cells (OSCs). Free carriers are generated by both ultrafast
exciton delocalization and slow exciton diffusion and dissociation at the
heterojunction interface. In this work, we developed a systematic numerical
simulation to describe the charge-generation process by a modified
drift-diffusion model. The transport, recombination, and collection of free
carriers are incorporated to fully capture the device response. The theoretical
results match well with the state-of-the-art high-performance organic solar
cells. It is demonstrated that the increase of exciton delocalization ratio
reduces the energy loss in the exciton diffusion-dissociation process, and
thus, significantly improves the device efficiency especially for the
short-circuit current. By changing the exciton delocalization ratio, OSC
performances are comprehensively investigated under the conditions of
short-circuit and open-circuit. Particularly, bulk recombination dependent fill
factor saturation is unveiled and understood. As a fundamental electrical
analysis of the delocalization mechanism, our work is important to understand
and optimize the high-performance OSCs.

###How measurement protocols influence the dynamic J-V characteristics of perovskite solar cells: theory and experiment||G. A. Nemnes,Cristina Besleaga,A. G. Tomulescu,Alexandra Palici,L. Pintilie,A. Manolescu,Ioana Pintilie###

How measurement protocols influence the dynamic J-V characteristics of perovskite solar cells: theory and experiment. The dynamic effects observed in the J-V measurements represent one important
hallmark in the behavior of the perovskite solar cells. Proper measurement
protocols (MPs) should be employed for the experimental data reproducibility,
in particular for a reliable evaluation of the power conversion efficiency
(PCE), as well as for a meaningful characterization of the type and magnitude
of the hysteresis. We discuss here several MPs by comparing the experimental
J-V characteristics with simulated ones using the dynamic electrical model
(DEM). Pre-poling conditions and bias scan rate can have a dramatic influence
not only on the apparent solar cell performance, but also on the hysteretic
phenomena. Under certain measurement conditions, a hysteresis-free behavior
with relatively high PCEs may be observed, although the J-V characteristics may
be far away from the stationary case. Furthermore, forward-reverse and
reverse-forward bias scans show qualitatively different behaviors regarding the
type of the hysteresis, normal and inverted, depending on the bias pre-poling.
We emphasize here that correlated forward-reverse or reverse-forward bias scans
are essential for a correct assessment of the dynamic hysteresis. In this
context, we define a hysteresis index which consistently assigns the hysteresis
type and magnitude. Our DEM simulations, supported by experimental data,
provide further guidance for an efficient and accurate determination of the
stationary J-V characteristics, showing that the type and magnitude of the
dynamic hysteresis may be affected by unintentional pre-conditioning in typical
experiments.

###Advances in Modelling and Simulation of Halide Perovskites for Solar Cell Applications||Chol-Jun Yu###

Advances in Modelling and Simulation of Halide Perovskites for Solar Cell Applications. Perovskite solar cells (PSCs) are attracting great attention as the most
promising candidate for the next generation solar cells. This is due to their
low cost and high power conversion efficiency in spite of their relatively
short period of development. Key components of PSCs are a variety of halide
perovskites with ABX3 stoichiometry used as a photoabsorber, which brought the
factual breakthrough in the field of photovoltaic (PV) technology with their
outstanding optoelectronic properties. To commercialize PSCs in the near
future, however, these materials need to be further improved for a better
performance, represented by high efficiency and high stability. As in other
materials development, atomistic modelling and simulation can play a
significant role in finding new functional halide perovskites as well as
revealing the underlying mechanisms of their material processes and properties.
In this sense, computational works for the halide perovskites, mostly focusing
on first-principles works, are reviewed with an eye looking for an answer how
to improve the performance of PSCs. Specific modelling and simulation
techniques to quantify material properties of the halide perovskites are also
presented. Finally, the outlook for the challenges and future research
directions in this field is provided.

###Electrochemical performance of decorated reduced graphene oxide by MoO3 nanoparticles as a counter electrode||Mahyar Servati,Reza Rasuli###

Electrochemical performance of decorated reduced graphene oxide by MoO3 nanoparticles as a counter electrode. We present an efficient electrocatalytic material based on anchored MoO3
nanoparticles on reduced graphene oxide (RGO) nanosheets. After preparation of
graphene oxide (GO), the MoO3 nanoparticles anchored on GO nanosheet by using
the arc-discharge method. X-ray diffraction patterns show that the MoO3
nanoparticles are well crystallized on RGO in the orthorhombic crystalline
phase with a crystallite size of 83 nm. In addition, FT-IR and Raman
spectroscopy results show that during the arc-discharge process, the GO
nanosheets have been reduced and RGO nanosheets are decorated with MoO3
nanoparticles which form a porous structure. The surface energy of the prepared
electrode was measured as 44.56 mJ/m2, which shows the desirable spreading
ability of the electrolyte on the electrode. Finally, electrochemical
performance was measured in the symmetrical dummy cell by impedance
spectroscopy and cyclic voltammogram, and the photochemical test was measured
in the dye-sensitized solar cell by current density measurement. Our results
show that the electrochemical performance of the RGM electrode is better than
the RGO electrode and is comparable with the Platinum electrode and also the
efficiency of RGM electrode used in a dye-sensitized solar cell as a counter
electrode is 5.55% near to Platinum electrode performance.

###Investigation of the Spatially Dependent Charge Collection Probability in CuInS$_2$/ZnO Colloidal Nanocrystal Solar Cells||Dorothea Scheunemann,Sebastian Wilken,Jürgen Parisi,Holger Borchert###

Investigation of the Spatially Dependent Charge Collection Probability in CuInS$_2$/ZnO Colloidal Nanocrystal Solar Cells. Solar cells with a heterojunction between colloidal CuInS$_2$ and ZnO
nanocrystals are an innovative concept in solution-processed photovoltaics, but
the conversion efficiency cannot compete yet with devices employing lead
chalcogenide quantum dots. Here, we present a detailed study on the charge
collection in CuInS$_2$/ZnO solar cells. An inverted device architecture was
utilized, in which the ZnO played an additional role as optical spacer layer.
Variations of the ZnO thickness were exploited to create different charge
generation profiles within the light-harvesting CuInS$_2$ layer, which strongly
affected both the external and internal quantum efficiency. By the
reconstruction of these experimental findings with the help of a purely optical
model, we were able to draw conclusions on the spatial dependency of the charge
collection probability. We provide evidence that only carriers generated within
a narrow zone of circa 40 nm near the CuInS$_2$/ZnO interface contribute to the
external photocurrent. The remaining part of the absorber can be considered as
"dead zone" for charge collection, which reasonably explains the limited device
performance and indicates a direction for future research. From the methodical
point of view, the optical modeling approach developed in the present work has
the advantage that no electrical input parameters are required and is believed
to be easily transferable to other material systems.

###Embedding Physics Domain Knowledge into a Bayesian Network Enables Layer-by-Layer Process Innovation for Photovoltaics||Zekun Ren,Felipe Oviedo,Muang Thway,Siyu I. P. Tian,Yue Wang,Hansong Xue,Jose Dario Perea,Mariya Layurova,Thomas Heumueller,Erik Birgersson,Armin Aberle,Christoph J. Brabec,Rolf Stangl,Shijing Sun,Qianxiao Li,Fen Lin,Ian Marius Peters,Tonio Buonassisi###

Embedding Physics Domain Knowledge into a Bayesian Network Enables Layer-by-Layer Process Innovation for Photovoltaics. Process optimization of photovoltaic devices is a time-intensive, trial and
error endeavor, without full transparency of the underlying physics, and with
user-imposed constraints that may or may not lead to a global optimum. Herein,
we demonstrate that embedding physics domain knowledge into a Bayesian network
enables an optimization approach that identifies the root cause(s) of
underperformance with layer by-layer resolution and reveals alternative optimal
process windows beyond global black-box optimization. Our Bayesian-network
approach links process conditions to materials descriptors (bulk and interface
properties, e.g., bulk lifetime, doping, and surface recombination) and device
performance parameters (e.g., cell efficiency), using a Bayesian inference
framework with an autoencoder-based surrogate device-physics model that is 100x
faster than numerical solvers. With the trained surrogate model, our approach
is robust and reduces significantly the time consuming experimentalist
intervention, even with small numbers of fabricated samples. To demonstrate our
method, we perform layer-by-layer optimization of GaAs solar cells. In a single
cycle of learning, we find an improved growth temperature for the GaAs solar
cells without any secondary measurements, and demonstrate a 6.5% relative
AM1.5G efficiency improvement above baseline and traditional black-box
optimization methods.

###CVD-graphene/graphene flakes dual-films as advanced DSSC counter electrodes||Andrea Capasso,Sebastiano Bellani,Alessandro Lorenzo Palma,Leyla Najafi1,Antonio Esaù Del Rio Castillo,Nicola Curreli1,Lucio Cinà,Vaidotas Miseikis,Camilla Coletti,Giuseppe Calogero,Vittorio Pellegrini,Aldo Di Carlo,Francesco Bonaccorso###

CVD-graphene/graphene flakes dual-films as advanced DSSC counter electrodes. The use of graphene-based electrodes is burgeoning in a wide range of
applications, including solar cells, light emitting diodes, touch screens,
field-effect transistors, photodetectors, sensors and energy storage systems.
The success of such electrodes strongly depends on the implementation of
effective production and processing methods for graphene. In this work, we take
advantage of two different graphene production methods to design an advanced,
conductive oxide- and platinum-free, graphene-based counter electrode for
dye-sensitized solar cells (DSSCs). In particular, we exploit the combination
of a graphene film, produced by chemical vapor deposition (CVD) (CVD-graphene),
with few-layer graphene (FLG) flakes, produced by liquid phase exfoliation. The
CVD-graphene is used as charge collector, while the FLG flakes, deposited atop
by spray coating, act as catalyst for the reduction of the electrolyte redox
couple (i.e., I3-/I-- and Co+2/+3). The as-produced counter electrodes are
tested in both I3-/I-- and Co+2/+3-based semitransparent DSSCs, showing power
conversion efficiencies of 2.1% and 5.09%, respectively, under 1 SUN
illumination. At 0.1 SUN, Co+2/+3-based DSSCs achieve a power conversion
efficiency as high as 6.87%. Our results demonstrate that the electrical,
optical, chemical and catalytic properties of graphene-based dual films,
designed by combining CVD-graphene and FLG flakes, are effective alternatives
to FTO/Pt counter electrodes for DSSCs for both outdoor and indoor
applications.

###High-Throughput Screening for Band gap Engineering by Sublattice Mixing of Cs$_2$AgBiCl$_6$ from First-Principles||Deepika Gill,Preeti Bhumla,Manish Kumar,Saswata Bhattacharya###

High-Throughput Screening for Band gap Engineering by Sublattice Mixing of Cs$_2$AgBiCl$_6$ from First-Principles. The lead-free double perovskite material (viz. Cs$_2$AgBiCl$_6$) has emerged
as an efficient and environmentally friendly alternative to lead halide
perovskites. To make Cs$_2$AgBiCl$_6$ optically active in the visible region of
solar spectrum, band gap engineering approach has been undertaken. Using
Cs$_2$AgBiCl$_6$ as a host, band gap and optical properties of Cs$_2$AgBiCl$_6$
have been modulated by alloying with M(I), M(II), and M(III) cations at
Ag-/Bi-sites. Here, we have employed density functional theory (DFT) with
suitable exchange-correlation functionals in light of spin-orbit coupling (SOC)
to determine the stability, band gap and optical properties of different
compositions, that are obtained on Ag-Cl and Bi-Cl sublattices mixing. On
analyzing the 64 combinations within Cs$_2$AgBiCl$_6$, we have identified 19
promising configurations having band gap sensitive to solar cell applications.
The most suitable configurations with Ge(II) and Sn(II) substitutions have
spectroscopic limited maximum efficiency (SLME) of 32.08% and 30.91%,
respectively, which are apt for solar cell absorber.

###Hexagonal Rare-Earth Manganites as Promising Photovoltaics and Light Polarizers||Xin Huang,Tula R. Paudel,Shuai Dong,Evgeny Y. Tsymbal###

Hexagonal Rare-Earth Manganites as Promising Photovoltaics and Light Polarizers. Ferroelectric materials possess a spontaneous electric polarization and may
be utilized in various technological applications ranging from non-volatile
memories to solar cells and light polarizers. Recently, hexagonal rare-earth
manganites, h-RMnO$_3$ (R is a rare-earth ion) have attracted considerable
interest due to their intricate multiferroic properties and improper
ferroelectricity characterized by a sizable remnant polarization and high Curie
temperature. Here, we demonstrate that these compounds can serve as very
efficient photovoltaic materials and, in addition, possess remarkable optical
anisotropy properties. Using first-principles methods based on
density-functional theory and considering h-TbMnO$_3$ as a representative
manganite, we predict a strong light absorption of this material in the solar
spectrum range, resulting in the maximum light-to-electricity energy conversion
efficiency up to 33%. We also predict an extraordinary optical linear dichroism
and linear birefringence properties of h-TbMnO$_3$ in a broad range of optical
frequencies. These results uncover the unexplored potential of hexagonal
rare-earth manganites to serve as photovoltaics in solar cells and as
absorptive and birefringent light polarizers.

###Efficient and Stable PbS Quantum Dot Solar Cells by Triple-Cation Perovskite Passivation||Miguel Albaladejo-Siguan,David Becker-Koch,Alexander D. Taylor,Qing Sun,Vincent Lami,Pola Goldberg Oppenheimer,Fabian Paulus,Yana Vaynzof###

Efficient and Stable PbS Quantum Dot Solar Cells by Triple-Cation Perovskite Passivation. Solution-processed quantum dots (QDs) have a high potential for fabricating
low cost, flexible and large-scale solar energy harvesting devices. It has
recently been demonstrated that hybrid devices employing a single monovalent
cation perovskite solution for PbS QD surface passivation exhibit enhanced
photovoltaic performance when compared to standard ligand passivation. Herein
we demonstrate that the use of a triple cation
Cs0.05(MA0.17FA0.83)0.95Pb(I0.9Br0.1)3 perovskite composition for surface
passivation of the quantum dots results in highly efficient solar cells, which
maintain 96 % of their initial performance after 1200h shelf storage. We
confirm perovskite shell formation around the PbS nanocrystals by a range of
spectroscopic techniques as well as high-resolution transmission electron
microscopy. We find that the triple cation shell results in a favorable
energetic alignment to the core of the dot, resulting in reduced recombination
due to charge confinement without limiting transport in the active layer.
Consequently, photovoltaic devices fabricated via a single-step film deposition
reached a maximum AM1.5G power conversion efficiency of 11.3 % surpassing most
previous reports of PbS solar cells employing perovskite passivation.

###The role of charge recombination to spin-triplet excitons in non-fullerene acceptor organic solar cells||Alexander J. Gillett,Alberto Privitera,Rishat Dilmurat,Akchheta Karki,Deping Qian,Anton Pershin,Giacomo Londi,William K. Myers,Jaewon Lee,Jun Yuan,Seo-Jin Ko,Moritz K. Riede,Feng Gao,Guillermo C. Bazan,Akshay Rao,Thuc-Quyen Nguyen,David Beljonne,Richard H. Friend###

The role of charge recombination to spin-triplet excitons in non-fullerene acceptor organic solar cells. The power conversion efficiencies (PCEs) of organic solar cells (OSCs) using
non-fullerene acceptors (NFAs) have now reached 18%. However, this is still
lower than inorganic solar cells, for which PCEs >20% are commonplace. A key
reason is that OSCs still show low open-circuit voltages (Voc) relative to
their optical band gaps, attributed to non-radiative recombination. For OSCs to
compete with inorganics in efficiency, all non-radiative loss pathways must be
identified and where possible, removed. Here, we show that in most NFA OSCs,
the majority of charge recombination at open-circuit proceeds via formation of
non-emissive NFA triplet excitons (T1); in the benchmark PM6:Y6 blend, this
fraction reaches 90%, contributing 60 mV to the reduction of Voc. We develop a
new design to prevent recombination via this non-radiative channel through the
engineering of significant hybridisation between the NFA T1 and the
spin-triplet charge transfer exciton (3CTE). We model that the rate of the back
charge transfer from 3CTE to T1 can be reduced by an order of magnitude,
allowing re-dissociation of the 3CTE. We then demonstrate NFA systems where T1
formation is suppressed. This work therefore provides a clear design pathway
for improved OSC performance to 20% PCE and beyond.

###Effect of Different Device Parameters on Tin Based Perovskite Solar Cell Coupled with In2S3 Electron Transport Layer and CuSCN and Spiro-OMeTAD Alternative Hole Transport Layers for High Efficiency Performance||Intekhab Alam,Md Ali Ashraf###

Effect of Different Device Parameters on Tin Based Perovskite Solar Cell Coupled with In2S3 Electron Transport Layer and CuSCN and Spiro-OMeTAD Alternative Hole Transport Layers for High Efficiency Performance. SCAPS 1-D was used for the simulation of lead-free environmentally benign
methylammonium tin-iodide (CH3NH3SnI3) based solar cell. Indium sulphide
(In2S3) was utilized as the electron transport layer (ETL) for its high carrier
mobility and optimized band structure, unlike traditional titanium oxide (TiO2)
ETL. Traditional expensive spiro-OMeTAD (C81H68N4O8) and cheaper cuprous
thiocyanate (CuSCN) were utilized alternatively as hole transport layer (HTL)
to observe the effect of different HTL on cell performance. We investigated the
trend in electrical measurements by altering parameters such as thickness,
defect density, valence band (VB) effective density of state and bandgap of the
absorber layer, interfacial trap densities and defect density of ETL. At
optimum condition, the device revealed the highest efficiency of 18.45% for
CuSCN (HTL) and 19.32% for spiro-OMeTAD (HTL) configuration. The effect of
working temperature, the wavelength of light and band-to-band radiative
recombination rate was also observed for both configurations. All these
simulation results will help to fabricate eco-friendly high-efficiency
perovskite solar cell by replacing the commonly used toxic lead-based
perovskite.

###Structural Stability and Defect-Tolerance of Ionic Spinel Semiconductor for High-Efficiency Solar Cells||Hanzhen Liang,Huiwen Xiang,Rui Zhu,Chengyan Liu,Yu Jia###

Structural Stability and Defect-Tolerance of Ionic Spinel Semiconductor for High-Efficiency Solar Cells. The incompatibility between defect-tolerance and structural stability is a
severe issue hindering the wide application of high-efficiency solar cells.
Usually, covalent/polar semiconductors with prototype of Si/CdTe crystals
exhibit great structural stability owing to their compactly composed
tetrahedral building blocks, but present extremely poor defect-tolerance due to
the similar electronegativity of component elements. On the contrary, ionic
semiconductors, such as perovskite series, always exhibit benign electronic
properties of intrinsic defects owing to the great disparity of
electronegativity between anions and cations, but are structurally unstable
because of the sparsely composed octahedral building blocks supported by large
cations. Combining the stable framework of covalent semiconductors and benign
defects of ionic compounds, we find that HgX2S4 (X=In, Sc and Y) spinel
semiconductors possess both the merits. The tightly combined tetrahedral and
octahedral blocks ensures the structural stability, and the band edge of ionic
characteristic, which is mainly dominated by Hg-6s and S-3p orbitals for
conduction band minimum (CBM) and valence band maximum (VBM), respectively,
makes HgX2S4 defect-tolerant. The prominent downward bending of CBM caused by
spatially spreading Hg-6s spherical orbital not only induces a suitable optical
band gap which is often too large in ionic compounds, but also promotes the
formation and transport of n-type carriers. This study presents that Hg-based
chalcogenide spinels are promising candidates for high-efficiency solar cells,
and suggests that adopting cations with delocalized orbitals under the
framework of spinel crystal is an alternative way for synthesizing the stable
and defect-tolerant photovoltaic materials.

###Influence of local surface defects on the minority-carrier lifetime of passivating-contact solar cells||Jean Cattin,Jan Haschke,Christophe Ballif,Matthieu Boccard###

Influence of local surface defects on the minority-carrier lifetime of passivating-contact solar cells. Unlocking the full potential of passivating contacts, increasingly popular in
the silicon solar cell industry, requires determining the minority carrier
lifetime. Minor passivation drops limit the functioning of solar cells,
however, they are not detected in devices with open-circuit voltages below 700
mV. In this work, simulations and experiments were used to show the effect of
localized surface defects on the overall device performance. Although the
defects did not significantly affect lifetime measurements prior to electrode
deposition or open-circuit voltage measurements at standard-test conditions, it
had a significant impact on the point of operation and, in turn, device
efficiency (up to several percent efficiency drop). Furthermore, this study
demonstrates that localized defects can have a detrimental effect on
well-passivated areas located several centimeters away through electrical
connection by the electrode. This leads to a low-injection lifetime drop after
electrode deposition. Thus, commonly measured lifetime curves before
metallization (and therefore internal voltage) are usually not representative
of their respective values after metallization. The low-injection lifetime drop
often observed after electrode deposition can derive from such local surface
defects, and not from a homogeneous passivation drop.

###Local Nanoscale Defective Phase Impurities Are the Sites of Degradation in Halide Perovskite Devices||Stuart Macpherson,Tiarnan A. S. Doherty,Andrew J. Winchester,Sofiia Kosar,Duncan N. Johnstone,Yu-Hsien Chiang,Krzystof Galkowski,Miguel Anaya,Kyle Frohna,Affan N. Iqbal,Bart Roose,Zahra Andaji-Garmaroudi,Paul A. Midgley,Keshav M. Dani,Samuel D. Stranks###

Local Nanoscale Defective Phase Impurities Are the Sites of Degradation in Halide Perovskite Devices. Halide perovskites excel in the pursuit of highly efficient thin film
photovoltaics, with power conversion efficiencies reaching 25.5% in single
junction and 29.5% in tandem halide perovskite/silicon solar cell
configurations. Operational stability of perovskite solar cells remains a
barrier to their commercialisation, yet a fundamental understanding of
degradation processes, including the specific sites at which failure mechanisms
occur, is lacking. Recently, we reported that performance-limiting deep
sub-bandgap states appear in nanoscale clusters at particular grain boundaries
in state-of-the-art $Cs_{0.05}FA_{0.78}MA_{0.17}Pb(I_{0.83}Br_{0.17})_{3}$
(MA=methylammonium, FA=formamidinium) perovskite films. Here, we combine
multimodal microscopy to show that these very nanoscale defect clusters, which
go otherwise undetected with bulk measurements, are sites at which degradation
seeds. We use photoemission electron microscopy to visualise trap clusters and
observe that these specific sites grow in defect density over time under
illumination, leading to local reductions in performance parameters. Scanning
electron diffraction measurements reveal concomitant structural changes at
phase impurities associated with trap clusters, with rapid conversion to
metallic lead through iodine depletion, eventually resulting in pinhole
formation. By contrast, illumination in the presence of oxygen reduces defect
densities and reverses performance degradation at these local clusters, where
phase impurities instead convert to amorphous and electronically benign lead
oxide. Our work shows that the trapping of charge carriers at sites associated
with phase impurities, itself reducing performance, catalyses redox reactions
that compromise device longevity. Importantly, we reveal that both performance
losses and intrinsic degradation can be mitigated by eliminating these
defective clusters.

###Mixed-anion mixed-cation perovskite (FAPbI$_3$)$_{0.875}$(MAPbBr$_3$)$_{0.125}$: an ab-initio molecular dynamics study||Eduardo Menéndez-Proupin,Shivani Grover,Ana L. Montero-Alejo,Scott D. Midgley,Keith T. Butler,Ricardo Grau-Crespo###

Mixed-anion mixed-cation perovskite (FAPbI$_3$)$_{0.875}$(MAPbBr$_3$)$_{0.125}$: an ab-initio molecular dynamics study. Mixed-anion mixed-cation perovskites with (FAPbI$_3$)$_{1-x}$(MAPbBr$_3$)$_x$
composition have allowed record efficiencies in photovoltaic solar cells, but
their atomic-scale behaviour is not well understood yet, in part because their
theoretical modelling requires consideration of complex and interrelated
dynamic and disordering effects. We present here an ab initio molecular
dynamics investigation of the structural, thermodynamic, and electronic
properties of the (FAPbI$_3$)$_{0.875}$(MAPbBr$_3$)$_{0.125}$ perovskite. A
special quasi-random structure is proposed to mimic the disorder of both the
molecular cations and the halide anions, in a stoichiometry that is close to
that of one of today's most efficient perovskite solar cells. We show that the
rotation of the organic cations is more strongly hindered in the mixed
structure in comparison with the pure compounds. Our analysis suggests that
this mixed perovskite is thermodynamically stable against phase separation
despite the endothermic mixing enthalpy, due to the large configurational
entropy. The electronic properties are investigated by hybrid density
functional calculations including spin-orbit coupling in carefully selected
representative configurations extracted from the molecular dynamics. Our model,
that is validated here against experimental information, provides a more
sophisticated understanding of the interplay between dynamic and disordering
effects in this important family of photovoltaic materials.

###Electrical barriers and their elimination by tuning (Zn,Mg)O composition in Cu(In,Ga)S2: Systematic approach to achieve over 14% power conversion efficiency||Mohit Sood,Poorani Gnanasambandan,Damilola Adeleye,Sudhanshu Shukla,Noureddine Adjeroud,Renaud Leturcq,Susanne Siebentritt###

Electrical barriers and their elimination by tuning (Zn,Mg)O composition in Cu(In,Ga)S2: Systematic approach to achieve over 14% power conversion efficiency. Traditional CdS buffer layer in selenium-free Cu(In,Ga)S2 solar cells leads
to reduced open-circuit voltage because of a negative conduction band offset at
the Cu(In,Ga)S2/CdS interface. Reducing this loss necessitates the substitution
of CdS by an alternative buffer layer. However, the substitute buffer layer may
introduce electrical barriers in the device due to unfavorable band alignment
at the other interfaces such as between buffer/ZnO i-layer. This study aims to
reduce interface recombinations and eliminate electrical barriers in
Cu(In,Ga)S2 solar cells using a combination of Zn1-xMgxO and Al-doped Zn1-xMgxO
buffer and i-layer combination deposited using atomic layer deposition and
magnetron sputtering, respectively. The devices prepared with these layers are
characterized by current-voltage and photoluminescence measurements. Numerical
simulations are performed to comprehend the influence of electrical barriers on
the device characteristics. An optimal composition of Zn1 xMgxO x = 0.27 is
identified for a suitable conduction band alignment with Cu(In,Ga)S2 with a
bandgap of ~1.6 eV, suppressing interface recombination and avoiding barriers.
Optimized buffer composition together with a suitable i-layer led to a device
with 14 % efficiency and an open-circuit voltage of 943 mV. A comparison of
optoelectronic measurements for devices prepared with ZnO and Al:(Zn,Mg)O shows
the necessity to replace the ZnO i-layer with Al:(Zn,Mg)O i-layer for a
high-efficiency device.

###High-Performance Flexible All-Perovskite Tandem Solar Cells with Reduced VOC-Deficit in Wide-Bandgap Subcell||Huagui Lai,Jincheng Luo,Yannick Zwirner,Selina Olthof,Alexander Wieczorek,Fangyuan Ye,Quentin Jeangros,Xinxing Yin,Fatima Akhundova,Tianshu Ma,Rui He,Radha K. Kothandaraman,Xinyu Chin,Evgeniia Gilshtein,André Müller,Changlei Wang,Jarla Thiesbrummel,Sebastian Siol,José Márquez Prieto,Thomas Unold,Martin Stolterfoht,Cong Chen,Ayodhya N. Tiwari,Dewei Zhao,Fan Fu###

High-Performance Flexible All-Perovskite Tandem Solar Cells with Reduced VOC-Deficit in Wide-Bandgap Subcell. Among various types of perovskite-based tandem solar cells (TSCs),
all-perovskite TSCs are of particular attractiveness for building- and
vehicle-integrated photovoltaics, or space energy areas as they can be
fabricated on flexible and lightweight substrates with a very high
power-to-weight ratio. However, the efficiency of flexible all-perovskite
tandems is lagging far behind their rigid counterparts primarily due to the
challenges in developing efficient wide-bandgap (WBG) perovskite solar cells on
the flexible substrates as well as the low open-circuit voltage (VOC) in the
WBG perovskite subcell. Here, we report that the use of self-assembled
monolayers as hole-selective contact effectively suppresses the interfacial
recombination and allows the subsequent uniform growth of a 1.77 eV WBG
perovskite with superior optoelectronic quality. In addition, we employ a
post-deposition treatment with 2-thiopheneethylammonium chloride to further
suppress the bulk and interfacial recombination, boosting the VOC of the WBG
top cell to 1.29 V. Based on this, we present the first proof-of-concept
four-terminal all-perovskite flexible TSC with a PCE of 22.6%. When integrating
into two-terminal flexible tandems, we achieved 23.8% flexible all-perovskite
TSCs with a superior VOC of 2.1 V, which is on par with the VOC reported on the
28% all-perovskite tandems grown on the rigid substrate.

###Radiative coupling the easy way: Using transfer coefficients to model series-connected multi-junction solar cells||Rune Strandberg###

Radiative coupling the easy way: Using transfer coefficients to model series-connected multi-junction solar cells. When the quality of multijunction solar cells becomes sufficiently high,
radiative exchange of photons between cells has to be taken into account to
properly model these devices. In this work it is shown how this radiative
coupling can be accounted for in series connected multi-junction solar cells by
constants called transfer coefficients. Under the assumption that the exchanged
radiation only travels one way, from higher to lower band gaps, the transfer
coefficients allows the relation between the voltage and current of the device
to be expressed by a convenient mathematical expression. Another advantage of
this model is that it allows the short circuit current of radiatively coupled
multijunction cells to be calculated in a straightforward way. Non-ideality may
be included by means of the external radiative efficiency. A number of examples
are discussed to show how the transfer coefficients describes the radiative
coupling. In addition, the model is used to find the efficiency peaks of
radiatively coupled multi-junction cells when illuminated by the AM1.5
spectrum.

###Steric engineering of point defects in lead halide perovskites||Lucy D. Whalley###

Steric engineering of point defects in lead halide perovskites. Due to their high photovoltaic efficiency and low-cost synthesis, lead halide
perovskites have attracted wide interest for application in new solar cell
technologies. The most stable and efficient ABX$_3$ perovskite solar cells
employ mixed A-site cations, however the impact of cation mixing on carrier
trapping and recombination -- key processes that limit photovoltaic performance
-- is not fully understood. Here we analyse non-radiative carrier trapping in
the mixed A-cation hybrid halide perovskite MA$_{1-x}$Cs$_x$PbI$_3$. By using
rigorous first-principles simulations we show that cation mixing leads to a
hole trapping rate at the iodine interstitial that is eight orders of magnitude
greater than in the single cation system. We demonstrate that the same defect
in the same material can display a wide variety of defect activity -- from
electrically inactive to recombination centre -- and, in doing so, resolve
conflicting reports in the literature. Finally, we propose a new mechanism in
which steric effects can be used to determine the rate of carrier trapping;
this is achieved by controlling the phase and dynamical response of the lattice
through the A-site composition. Our findings elucidate crucial links between
chemical composition, defect activity and optoelectronic performance, and
suggest a general approach that can help to rationalise the development of new
crystalline materials with target defect properties.

###Thermal-Carrier-Escape Mitigation in a Quantum-Dot-In-Perovskite Intermediate Band Solar Cell via Bandgap Engineering||Ugur D. Menda,Guilherme Ribeiro,Jonas Deuermeier,Esther López,Daniela Nunes,Santanu Jana,Irene Artacho,Rodrigo Martins,Iván Mora-Seró,Manuel J. Mendes,Iñigo Ramiro###

Thermal-Carrier-Escape Mitigation in a Quantum-Dot-In-Perovskite Intermediate Band Solar Cell via Bandgap Engineering. By harvesting a wider range of the solar spectrum, intermediate band solar
cells (IBSCs) can achieve efficiencies 50% higher than conventional
single-junction solar cells. For this, additional requirements are imposed to
the light-absorbing semiconductor, which must contain a collection of in-gap
levels, called intermediate band (IB), optically coupled to but thermally
decoupled from the valence and conduction bands (VB and CB).
Quantum-dot-in-perovskite (QDiP) solids, where inorganic quantum dots (QDs) are
embedded in a halide perovskite matrix, have been recently suggested as a
promising material platform for developing IBSCs. In this work, QDiP solids
with excellent morphological and structural quality and strong absorption and
emission related to the presence of in-gap QD levels are synthesized. With
them, QDiP-based IBSCs are fabricated and, by means of temperature-dependent
photocurrent measurements, it is shown that the IB is strongly thermally
decoupled from the valence and conduction bands. The activation energy of the
IB$\rightarrow$CB thermal escape of electrons is measured to be 204 meV,
resulting in the mitigation of this detrimental process even under
room-temperature operation, thus fulfilling the first mandatory requisite to
enable high-efficiency IBSCs.

###High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy||R. D. Schaller,V. I. Klimov###

High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy. We demonstrate for the first time that impact ionization (II) [the inverse of
Auger recombination (AR)] occurs with very high efficiency in semiconductor
nanocrystals (NCs). Interband optical excitation of PbSe NCs at low pump
intensities, for which less than one exciton is initially generated per NC on
average, results in the formation of two or more excitons (carrier
multiplication) when pump photon energies are more than three times the NC band
gap energy. Generation of multiexcitons from a single photon absorption event
is observed to take place on an ultrafast (picosecond) timescale and occurs
with up to 100% efficiency depending upon the excess energy of the absorbed
photon. Efficient II in NCs can be used to considerably increase the power
conversion efficiency of NC-based solar cells.

###Van der Waals Materials for Atomically-Thin Photovoltaics: Promise and Outlook||Deep Jariwala,Artur R. Davoyan,Joeson Wong,Harry A. Atwater###

Van der Waals Materials for Atomically-Thin Photovoltaics: Promise and Outlook. Two-dimensional (2D) semiconductors provide a unique opportunity for
optoelectronics due to their layered atomic structure, electronic and optical
properties. To date, a majority of the application-oriented research in this
field has been focused on field-effect electronics as well as photodetectors
and light emitting diodes. Here we present a perspective on the use of 2D
semiconductors for photovoltaic applications. We discuss photonic device
designs that enable light trapping in nanometer-thickness absorber layers, and
we also outline schemes for efficient carrier transport and collection. We
further provide theoretical estimates of efficiency indicating that 2D
semiconductors can indeed be competitive with and complementary to conventional
photovoltaics, based on favorable energy bandgap, absorption, external
radiative efficiency, along with recent experimental demonstrations. Photonic
and electronic design of 2D semiconductor photovoltaics represents a new
direction for realizing ultrathin, efficient solar cells with applications
ranging from conventional power generation to portable and ultralight solar
power.

###An electronic ratchet is required in nanostructured intermediate band solar cells||Amaury Delamarre,Daniel Suchet,Nicolas Cavassilas,Yoshitaka Okada,Masakazu Sugiyama,Jean-François Guillemoles###

An electronic ratchet is required in nanostructured intermediate band solar cells. We investigate in this letter the intrinsic properties that have limited the
efficiency of nanostructured intermediate band solar cells. Those devices take
advantage of intra-band transitions, which occur on narrow energy width, and
present low radiative recombination efficiency. We derive the minimum
requirements in terms of those two characteristics to achieve efficiencies in
excess of the Shockley-Queisser limit, and show that compatible nanostructures
are challenging to obtain. Especially, we evidence that currently
experimentally considered materials cannot overcome the best single junction
cells. In order to solve those issues, we consider devices including an
electronic ratchet mechanism. Firstly, such devices are shown to be much less
sensitive on the limitations of the nanostructures characteristics, so that
requirements for high efficiencies can be met. Secondly, we show that quantum
well devices present advantages over their quantum dots counterparts, although
they have attracted much less interest so far.

###First principles prediction of the solar cell efficiency of chalcopyrite materials AgMX2 (M=In,Al; X=S, Se,Te)||GM Dongho-Nguimdo,Emanuel Igumbor,Serges Zambou,Daniel P. Joubert###

First principles prediction of the solar cell efficiency of chalcopyrite materials AgMX2 (M=In,Al; X=S, Se,Te). Using the Spectroscopic Limited Maximum Efficiency, and Shockley and Queisser
predictor models, we compute the solar efficiency of the chalcopyrites
AgMX2(M=In,Al;X=S,Se,Te). The results presented are based on the estimation of
the electronic and optical properties obtained from first principles density
functional theory as well as the many-body perturbation theory calculations.
The results from this report were consistent with the experimental data. The
optical bandgap was accurately estimated from the absorption spectra, obtained
by solving the Bethe and Salpeter equation. Fitting the Tauc's plot on the
absorption spectra, we also predicted that the materials studied have a direct
allowed optical transition. The theoretical estimations of the solar cell
performance showed that the efficiencies from the Shockley and Queisser model
are higher than those from the spectroscopic limited maximum efficiency model.
This improvement is attributed to the absorption.

###Theoretical Characterization of Photoactive Molecular Systems Based on BODIPY-Derivatives for the Design of Organic Solar Cells||Duvalier Madrid-Úsuga,Ana G. Mora-Leon,Andrea Cabrera-Espinosa,Braulio Insuasty,Alejandro Ortiz###

Theoretical Characterization of Photoactive Molecular Systems Based on BODIPY-Derivatives for the Design of Organic Solar Cells. To search for high-efficiency narrow-band donor materials to improve the
short-circuit current density ($J_{sc}$) of organic solar cells, a series of
small molecules based in Bodipy-Triphenylamine were characterized using density
functional theory (DFT) and time-dependent (TD-DFT) calculations. According to
the energy of the exciton driving force they have the appropriate energy levels
to match \textbf{\textit{PC$_{61}$BM}}. The properties affecting the open
circuit voltage ($V_{oc}$), $J_{sc}$ and the fill factor ($FF$) were
investigated by calculating the geometric structures, the boundary molecular
orbital energy levels, absorption spectra, light collection efficiencies, chare
transfer rates, and exciton binding energies. The results show that the
\textbf{\textit{BTPA~III}} system has a lower LUMO level, high absorption
efficiency, and exction dissociation than other molecular systems, facilitating
the improvement of $V_{oc}$, $J_{sc}$ and $FF$. Finally,
\textbf{\textit{BTPA~III}} would be the most promising of this series of donors
and further increase the efficiency of the device.

###Efficient light-trapping in ultrathin GaAs solar cells using quasi-random photonic crystals||Jeronimo Buencuerpo,Theresa E. Saenz,Mark Steger,Michelle Young,Emily L. Warren,John F. Geisz,Myles A. Steiner,Adele C. Tamboli###

Efficient light-trapping in ultrathin GaAs solar cells using quasi-random photonic crystals. Ultrathin solar cells reduce material usage and allow the use of
lower-quality materials thanks to their one order of magnitude smaller
thickness than their conventional counterparts. However, efficient photonic
light-trapping is required to harvest the incident light efficiently for an
otherwise insufficient absorber thickness. Quasi-random photonic crystals are
predicted to have high efficient light-trapping while being more robust under
angle and thickness variations than simple photonic crystals. Here we
experimentally demonstrate a light-trapping solution based on quasi-random
photonic crystals fabricated by polymer blend lithography. We control the
average lattice parameter by modifying the spin-coating speed. We demonstrate
an ultrathin GaAs cell of 260 nm with a rear quasi-random pattern with
submicron features, and a Jsc =26.4 mA/cm2 and an efficiency of 22.35% under
the global solar spectrum.

###Electron transport in the dye sensitized nanocrystalline cell||A Kambili,A B Walker,F Qiu,A C Fisher,A D Savin,L M Peter###

Electron transport in the dye sensitized nanocrystalline cell. Dye sensitised nanocrystalline solar cells (Gr\"{a}tzel cells) have achieved
solar-to-electrical energy conversion efficiencies of 12% in diffuse daylight.
The cell is based on a thin film of dye-sensitised nanocrystalline TiO$_2$
interpenetrated by a redox electrolyte. The high surface area of the TiO$_2$
and the spectral characteristics of the dye allow the device to harvest 46% of
the solar energy flux. One of the puzzling features of dye-sensitised
nano-crystalline solar cells is the slow electron transport in the titanium
dioxide phase. The available experimental evidence as well as theoretical
considerations suggest that the driving force for electron collection at the
substrate contact arises primarily from the concentration gradient, ie the
contribution of drift is negligible. The transport of electrons has been
characterised by small amplitude pulse or intensity modulated illumination.
Here, we show how the transport of electrons in the Gr\"{a}tzel cell can be
described quantitatively using trap distributions obtained from a novel charge
extraction method with a one-dimensional model based on solving the continuity
equation for the electron density. For the first time in such a model, a back
reaction with the I$_3^-$ ions in the electrolyte that is second order in the
electron density has been included.

###Broad Band Photon Harvesting Biomolecules for Photovoltaics||P. Meredith,B. J. Powell,J. Riesz,R. Vogel,D. Blake,I. Kartini,G. Will,S. Subianto###

Broad Band Photon Harvesting Biomolecules for Photovoltaics. We discuss the key principles of artificial photosynthesis for photovoltaic
energy conversion. We demonstrate these principles by examining the operation
of the so-called "dye sensitized solar cell" (DSSC) - a photoelectrochemical
device which simulates the charge separation process across a nano-structured
membrane that is characteristic of natural systems. These type of devices have
great potential to challenge silicon semiconductor technology in the low cost,
medium efficiency segment of the PV market. Ruthenium charge transfer complexes
are currently used as the photon harvesting components in DSSCs. They produce a
relatively broad band UV and visible response, but have long term stability
problems and are expensive to manufacture. We suggest that a class of
biological macromolecules called the melanins may be suitable replacements for
the ruthenium complexes. They have strong, broad band absorption, are
chemically and photochemically very stable, can be cheaply and easily
synthesized, and are also bio-available and bio-compatible. We demonstrate a
melanin-based regenerative solar cell, and discuss the key properties that are
necessary for an effective broad band photon harvesting system.

###Dramatic reduction of surface recombination by in-situ surface passivation of silicon nanowires||Yaping Dan,Kwanyong Seo,Kuniharu Takei,Jhim H. Meza,Ali Javey,Kenneth B. Crozier###

Dramatic reduction of surface recombination by in-situ surface passivation of silicon nanowires. Nanowires have unique optical properties [1-4] and are considered as
important building blocks for energy harvesting applications such as solar
cells. [2, 5-8] However, due to their large surface-to-volume ratios, the
recombination of charge carriers through surface states reduces the carrier
diffusion lengths in nanowires a few orders of magnitude,[9] often resulting in
the low efficiency (a few percent or less) of nanowire-based solar cells. [7,
8, 10, 11] Reducing the recombination by surface passivation is crucial for the
realization of high performance nanosized optoelectronic devices, but remains
largely unexplored. [7, 12-14] Here we show that a thin layer of amorphous
silicon (a-Si) coated on a single-crystalline silicon nanowire (sc-SiNW),
forming a core-shell structure in-situ in the vapor-liquid-solid (VLS) process,
reduces the surface recombination nearly two orders of magnitude. Under
illumination of modulated light, we measure a greater than 90-fold improvement
in the photosensitivity of individual core-shell nanowires, compared to regular
nanowires without shell. Simulations of the optical absorption of the nanowires
indicate that the strong absorption of the a-Si shell contributes to this
effect, but we conclude that the effect is mainly due to the enhanced carrier
lifetime by surface passivation.

###Charge Trapping Dynamics in PbS Colloidal Quantum Dot Photovoltaic Devices||Artem A. Bakulin,Stefanie Neutzner,Huib J. Bakker,Laurent Ottaviani,Damien Barakel,Zhuoying Chen###

Charge Trapping Dynamics in PbS Colloidal Quantum Dot Photovoltaic Devices. The efficiency of solution-processed colloidal quantum dot (QD) based solar
cells is limited by poor charge transport in the active layer of the device,
which originates from multiple trapping sites provided by QD surface defects.
We apply a recently developed ultrafast electro-optical technique, pump-push
photocurrent spectroscopy, to elucidate the charge trapping dynamics in PbS
colloidal-QD photovoltaic devices at working conditions. We show that IR
photo-induced absorption of QD in the 0.2-0.5 eV region is partly associated
with immobile charges, which can be optically de-trapped in our experiment.
Using this absorption as a probe, we observe that the early trapping dynamics
strongly depend on the nature of the ligands used for QD passivation while it
depends only slightly on the nature of the electron-accepting layer. We find
that weakly bound states, with a photon-activation energy of 0.2 eV, are
populated instantaneously upon photoexcitation. This indicates that the
photogenerated states show an intrinsically bound-state character, arguably
similar to charge-transfer states formation in organic photovoltaic materials.
Sequential population of deeper traps (activation energy 0.3-0.5 eV) is
observed on the ~0.1-10 ns time scales, indicating that most of carrier
trapping occurs only after substantial charge relaxation/transport. The
reported study disentangles fundamentally different contributions to charge
trapping dynamics in the nanocrystal-based optoelectronic devices and can serve
as a useful tool for QD solar cell development.

###Light Generation and Harvesting in a Van der Waals Heterostructure||Oriol Lopez-Sanchez,Esther Alarcon Llado,Volodymyr Koman,Anna Fontcuberta i Morral,Aleksandra Radenovic,Andras Kis###

Light Generation and Harvesting in a Van der Waals Heterostructure. Two-dimensional (2D) materials are a new type of materials under intense
study because of their interesting physical properties and wide range of
potential applications from nanoelectronics to sensing and photonics.
Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have
been proposed as promising channel materials for field-effect transistors
(FETs). Their high mechanical flexibility, stability and quality coupled with
potentially inexpensive production methods offer potential advantages compared
to organic and crystalline bulk semiconductors. Due to quantum mechanical
confinement, the band gap in monolayer MoS2 is direct in nature, leading to a
strong interaction with light that can be exploited for building
phototransistors and ultrasensitive photodetectors. Here, we report on the
realization of light-emitting diodes based on vertical heterojunctions composed
of n-type monolayer MoS2 and p-type silicon. Careful interface engineering
allows us to realize diodes showing rectification and light emission from the
entire surface of the heterojunction. Electroluminescence spectra show clear
signs of direct excitons related to the optical transitions between the
conduction and valence bands. Our pn diodes can also operate as solar cells,
with typical external quantum efficiency exceeding 4%. Our work opens up the
way to more sophisticated optoelectronic devices such as lasers and
heterostructure solar cells based on hybrids of two-dimensional (2D)
semiconductors and silicon.

###Bipolar polaron pair recombination in P3HT/PCBM solar cells||Alexander J. Kupijai,Konstantin M. Behringer,Michael Corazza,Suren A. Gevorgyan,Frederik C. Krebs,Martin Stutzmann,Martin S. Brandt###

Bipolar polaron pair recombination in P3HT/PCBM solar cells. The unique properties of organic semiconductors make them versatile base
materials for many applications ranging from light emitting diodes to
transistors. The low spin-orbit coupling typical for carbon-based materials and
the resulting long spin lifetimes give rise to a large influence of the
electron spin on charge transport which can be exploited in spintronic devices
or to improve solar cell efficiencies. Magnetic resonance techniques are
particularly helpful to elucidate the microscopic structure of paramagnetic
states in semiconductors as well as the transport processes they are involved
in. However, in organic devices the nature of the dominant spin-dependent
processes is still subject to considerable debate. Using multi-frequency pulsed
electrically detected magnetic resonance (pEDMR), we show that the
spin-dependent response of P3HT/PCBM solar cells at low temperatures is
governed by bipolar polaron pair recombination involving the positive and
negative polarons in P3HT and PCBM, respectively, thus excluding a unipolar
bipolaron formation as the main contribution to the spin-dependent charge
transfer in this temperature regime. Moreover the polaron-polaron coupling
strength and the recombination times of polaron pairs with parallel and
antiparallel spins are determined. Our results demonstrate that the pEDMR pulse
sequences recently developed for inorganic semiconductor devices can very
successfully be transferred to the study of spin and charge transport in
organic semiconductors, in particular when the different polarons can be
distinguished spectrally.

###Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH3NH3PbI3||Yue-Yu Zhang,Shiyou Chen,Peng Xu,Hongjun Xiang,Xin-Gao Gong,Aron Walsh,Su-Huai Wei###

Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH3NH3PbI3. The organic-inorganic hybrid perovskite CH3NH3PbI3 has attracted significant
interest for its high performance in converting solar light into electrical
power with an efficiency exceeding 20%. Unfortunately, chemical stability is
one major challenge in the development of the CH3NH3PbI3 solar cells. It was
commonly assumed that moisture or oxygen in the environment causes the poor
stability of hybrid halide perovskites, however, here we show from the
first-principles calculations that the room-temperature tetragonal phase of
CH3NH3PbI3 is thermodynamically unstable with respect to the phase separation
into CH3NH3I + PbI2, i.e., the disproportionation is exothermic, independent of
the humidity or oxygen in the atmosphere. When the structure is distorted to
the low-temperature orthorhombic phase, the energetic cost of separation
increases, but remains small. Contributions from vibrational and
configurational entropy at room temperature have been considered, but the
instability of CH3NH3PbI3 is unchanged. When I is replaced by Br or Cl, Pb by
Sn, or the organic cation CH3NH3 by inorganic Cs, the perovskites become more
stable and do not phase-separate spontaneously. Our study highlights that the
poor chemical stability is intrinsic to CH3NH3PbI3 and suggests that
element-substitution may solve the chemical stability problem in hybrid halide
perovskite solar cells.

###Rotational Dynamics of Organic Cations in CH3NH3PbI3 Perovskite||T. Chen,B. J. Foley,B. Ipek,M. Tyagi,J. R. D. Copley,C. M. Brown,J. J. Choi,S. -H. Lee###

Rotational Dynamics of Organic Cations in CH3NH3PbI3 Perovskite. Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shown
impressive power conversion efficiencies of above 20%. However, the microscopic
mechanism of the high photovoltaic performance is yet to be fully understood.
Particularly, the dynamics of CH3NH3+ cations and their impact on relevant
processes such as charge recombination and exciton dissociation are still
poorly understood. Here, using elastic and quasi-elastic neutron scattering
techniques and group theoretical analysis, we studied rotational modes of the
CH3NH3+ cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327K)
and tetragonal (165K < T < 327K) phases, the CH3NH3+ ions exhibit four-fold
rotational symmetry of the C-N axis (C4) along with three-fold rotation around
the C-N axis (C3), while in orthorhombic phase (T < 165K) only C3 rotation is
present. Around room temperature, the characteristic relaxation times for the
C4 rotation is found to be ps while for the C3 rotation ps. The -dependent
rotational relaxation times were fitted with Arrhenius equations to obtain
activation energies. Our data show a close correlation between the C4
rotational mode and the temperature dependent dielectric permittivity. Our
findings on the rotational dynamics of CH3NH3+ and the associated dipole have
important implications on understanding the low exciton binding energy and slow
charge recombination rate in CH3NH3PbI3 which are directly relevant for the
high solar cell performance.

###Opening the band gap of graphene through silicon doping for improved performance of graphene/GaAs heterojunction solar cells||Shengjiao Zhang,Shisheng Lin,Xiaoqiang Li,Xiaoyi Liu,Hengan Wu,Peng Wang,Zhiqian Wu,Huikai Zhong,Wenli Xu,Zhijuan Xu###

Opening the band gap of graphene through silicon doping for improved performance of graphene/GaAs heterojunction solar cells. Graphene has attracted increasing interests due to its remarkable properties,
however, the zero band gap of monolayer graphene might limit its further
electronic and optoelectronic applications. Herein, we have successfully
synthesized monolayer silicon-doped graphene (SiG) in large area by chemical
vapor deposition method. Raman spectroscopy and X-ray photoelectron
spectroscopy measurements evidence silicon atoms are doped into graphene
lattice with the doping level of 3.4 at%. The electrical measurement based on
field effect transistor indicates that the band gap of graphene has been opened
by silicon doping, which is around 0. 28 eV supported by the first-principle
calculations, and the ultraviolet photoelectron spectroscopy demonstrates the
work function of SiG is 0.13 eV larger than that of graphene. Moreover, the
SiG/GaAs heterostructure solar cells show an improved power conversion
efficiency of 33.7% in average than that of graphene/GaAs solar cells, which
are attributed to the increased barrier height and improved interface quality.
Our results suggest silicon doping can effectively engineer the band gap of
monolayer graphene and SiG has great potential in optoelectronic device
applications.

###Demonstration of a novel dispersive spectral splitting optical element for cost- effective photovoltaic conversion||Carlo Maragliano,Tim Milakovich,Matteo Bronzoni,Stefano Rampino,Eugene A. Fitzgerald,Matteo Chiesa,Marco Stefancich###

Demonstration of a novel dispersive spectral splitting optical element for cost- effective photovoltaic conversion. In this letter we report the preliminary validation of a low-cost paradigm
for photovoltaic power generation that utilizes a prismatic Fresnel-like lens
to simultaneously concentrate and separate sunlight into continuous laterally
spaced spectral bands, which are then fed into spectrally matched
single-junction photovoltaic cells. A prismatic lens was designed using
geometric optics and the dispersive properties of the employed material, and
its performance was simulated with a ray- tracing software. After device
optimization, it was fabricated by injection molding, suitable for large-scale
mass production. We report an average optical transmittance of ~ 90% over the
VNIR range with spectral separation in excellent agreement with our
simulations. Finally, two prototype systems were tested: one with GaAsP and
c-Si photovoltaic devices and one with a pair of copper indium gallium selenide
based solar cells. The systems demonstrated an increase in peak electrical
power output of 51% and 64% respectively under white light illumination. Given
the ease of manufacturability of the proposed device, the reported spectral
splitting approach provides a cost- effective alternative to multi-junction
solar cells for efficient light-to-electricity conversion ready for mass
production.

###General point dipole theory for periodic metasurfaces: magnetoelectric scattering lattices coupled to planar photonic structures||Yuntian Chen,A. Femius Koenderink###

General point dipole theory for periodic metasurfaces: magnetoelectric scattering lattices coupled to planar photonic structures. We study semi-analytically the light emission and absorption properties of
arbitrary stratified photonic structures with embedded two-dimensional
magnetoelectric point scattering lattices, as used in recent plasmon-enhanced
LEDs and solar cells. By employing dyadic Green's function for the layered
structure in combination with Ewald lattice summation to deal with the particle
lattice, we develop an efficient method to study the coupling between planar 2D
scattering lattices of plasmonic, or metamaterial point particles, coupled to
layered structures. Using the `array scanning method' we deal with localized
sources. Firstly, we apply our method to light emission enhancement of dipole
emitters in slab waveguides, mediated by plasmonic lattices. We benchmark the
array scanning method against a reciprocity-based approach to find that the
calculated radiative rate enhancement in k-space below the light cone shows
excellent agreement. Secondly, we apply our method to study
absorption-enhancement in thin-film solar cells mediated by periodic Ag
nanoparticle arrays. Lastly, we study the emission distribution in k-space of a
coupled waveguide-lattice system. In particular, we explore the dark mode
excitation on the plasmonic lattice using the so-called Array Scanning Method.
Our method could be useful for simulating a broad range of complex nanophotonic
structures, i.e., metasurfaces, plasmon-enhanced light emitting systems and
photovoltaics.

###Depletion region surface effects in electron beam induced current measurements||Paul M. Haney,Heayoung P. Yoon,Benoit Gaury,Nikolai B. Zhitenev###

Depletion region surface effects in electron beam induced current measurements. Electron beam induced current (EBIC) is a powerful characterization technique
which offers the high spatial resolution needed to study polycrystalline solar
cells. Current models of EBIC assume that excitations in the $p$-$n$ junction
depletion region result in perfect charge collection efficiency. However we
find that in CdTe and Si samples prepared by focused ion beam (FIB) milling,
there is a reduced and nonuniform EBIC lineshape for excitations in the
depletion region. Motivated by this, we present a model of the EBIC response
for excitations in the depletion region which includes the effects of surface
recombination from both charge-neutral and charged surfaces. For neutral
surfaces we present a simple analytical formula which describes the numerical
data well, while the charged surface response depends qualitatively on the
location of the surface Fermi level relative to the bulk Fermi level. We find
the experimental data on FIB-prepared Si solar cells is most consistent with a
charged surface, and discuss the implications for EBIC experiments on
polycrystalline materials.

###Optical Management Techniques for Organic Solar Cells||Adharsh Rajagopal###

Optical Management Techniques for Organic Solar Cells. In this thesis, two different optical management techniques for organics
based solar cells are explored. The first part is focused on the development of
a textured rear reflector for OPVs. The use of textured reflector (TR)
facilitates an increase in the optical path length along with light trapping
within the active layer. TR was fabricated through a relatively simpler
technique by depositing metal films over a microlens array (MLA). Zinc oxide
nanoparticles were used to minimize the shadowing effect. Using TR,
enhancements in short-circuit current density and power conversion efficiencies
up to 10-25% were demonstrated for a polymer based organic solar cell. The
second part is focused on improving the effectiveness of MLA incorporation in
OPVs. The increase in path length achieved using MLA can be improved by
increasing the refractive index of MLA and incorporating MLA directly on the
transparent electrode instead of glass substrate. This approach could avoid the
optical losses occurring at the interface between MLA and glass substrate and
increase the net path length of light within the active layer. Initial
experimental results are discussed to understand the potential of this
approach. Approximately, ~15% enhancement in short-circuit current density was
achieved by using MLA directly on the transparent electrode in ITO-free
devices.

###Ultra-stable 2D/3D hybrid perovskite photovoltaic module||Giulia Grancini,Cristina Roldán-Carmona,Iwan Zimmermann,David Martineau,Stéphanie Narbey,Frédéric Oswald,Mohammad Khaja Nazeeruddin###

Ultra-stable 2D/3D hybrid perovskite photovoltaic module. Hybrid perovskite solar cells, with their power conversion efficiency (PCE)
exceeding 22%, have been representing a revolutionary concept for future energy
power generation. Although listed among the Top 10 Emerging Technologies of
2016, device longevity is the actual bottleneck for their real uptake in the
market. Here we design an ultra-stable molecular junction of two/three
dimensional (2D/3D) perovskites. It consists of a 2D (HOOC(CH2)2NH3)2PbI4,
anchored at the oxide substrate, that templates the growth of a highly ordered
3D CH3NH3PbI3 perovskite stabilizing in the orthorhombic phase, even at room
temperature. The unique and exceptional 2D/3D structure yields 14.6% PCE in
solar cells with Spiro-OMeTAD and Au, and 12.9% PCE in hole-conductor free
architecture. Aiming at the up-scaling of this technology, we realize 10x10 cm2
large-area photovoltaic modules by a low-cost, fully printable,
industrial-scale process delivering 11.2% PCE. We demonstrate a record
stability in the PCE of 5,000 hours, setting the direction for the new
generation of carbon free energy.

###Improved Performance and Reliability of p-i-n Perovskite Solar Cells via Doped Metal Oxides||Achilleas Savva,Ignasi Burgues-Ceballos,Stelios A. Choulis###

Improved Performance and Reliability of p-i-n Perovskite Solar Cells via Doped Metal Oxides. Perovskite photovoltaics (PVs) have attracted attention because of their
excellent power conversion efficiency (PCE). Critical issues related to large
area PV performance, reliability and lifetime need to be addressed. Here, we
show that doped metal oxides can provide ideal electron selectivity, improved
reliability and stability for perovskite PVs. We report p-i-n perovskite PVs
with device areas ranging from 0.09cm2 to 0.5cm2 incorporating a thick aluminum
doped zinc oxide (AZO) electron selective contact with hysteresis-free PCE of
over 13% and high fill factor values in the range of 80%. AZO provides suitable
energy levels for carrier selectivity, neutralizes the presence of pinholes and
provides intimate interfaces. Devices using AZO exhibit an average PCE increase
of over 20% compared with the devices without AZO and maintain the high PCE for
the larger area devices reported. Furthermore, the device stability of p-i-n
perovskite solar cells under the ISOS-D-1 is enhanced when AZO is used, and
maintains 100% of the initial PCE for over 1000 hours of exposure when AZO/Au
is used as the top electrode. Our results indicate the importance of doped
metal oxides as carrier selective contacts to achieve reliable and high
performance long lived large area perovskite solar cells.

###DFT analysis and FDTD simulation of CH3NH3PbI3-xClx mixed halide perovskite solar cells: role of halide mixing and light trapping technique||Mohaddeseh Saffari,Mohammad Ali Mohebpour,Hamid Rahimpour Soleimani,Meysam Bagheri Tagani###

DFT analysis and FDTD simulation of CH3NH3PbI3-xClx mixed halide perovskite solar cells: role of halide mixing and light trapping technique. Since perovskite solar cells have attracted a lot of attentions over the past
years, the enhancement of their optical absorption and current density are
among the basic coming challenges. For this reason, first, we have studied
structural and optical properties of organic-inorganic hybrid halide perovskite
CH3NH3PbI3 and the compounds doped by chlorine halogen CH3NH3PbI3-xClx in the
cubic phase by using density functional theory (DFT). Then, we investigate the
light absorption efficiency and optical current density of the single-junction
perovskite solar cell CH3NH3PbI3-xClx for the values (x=0,1,2,3) by utilizing
optical constants (n,k) resulted from these calculations. The results suggest
that increasing the amount of chlorine in the CH3NH3PbI3-xClx compound leads an
increase in bandgap energy, as well as a decrease in lattice constants and
optical properties like refractive index and extinction coefficient of the
structure. Also, the results obtained by simulation express that by taking
advantage of light trapping techniques of SiO2, a remarkable increase of light
absorption will be achieved to the magnitude of 83.13%, which is noticeable.

###Investigation and modeling of photocurrent collection process in multiple quantum well solar cells||Kasidit Toprasertpong,Tomoyuki Inoue,Yoshiaki Nakano,Masakazu Sugiyama###

Investigation and modeling of photocurrent collection process in multiple quantum well solar cells. Solar cells employing quantum wells can enhance the light absorption but
suffer from the difficulty in photogenerated carrier extraction. Here, we
analyzed the spectral response and the photocarrier collection mechanism of
p-i-n multiple quantum well (MQW) solar cells using the effective-mobility
model. Both the simulation and experiment results imply that the spatial
profiles of electron and hole densities in MQWs play an important role in the
carrier collection process. By considering the recombination increment under
illumination, our findings suggest that the concept of the majority/minority
carriers is important even in the intrinsic region: photogenerated electrons
and holes only experience significant recombination when passing through the
hole-rich and electron-rich regions, respectively. This can accurately explain
the photocurrent behavior in cells with high background doping, background
illumination, and different MQW positions. Based on the experimental findings,
we derived analytical formulae for carrier collection efficiency, which
directly show the impact of each cell parameter and can be used for the
systematic cell design.

###Comparison of Donor-Acceptor $π$-Conjugated Dyes in Model Solar Cells: A Study of Interfacial Ultrafast Electron Migration||Gunter Hermann,Felix Witte,Jean Christophe Tremblay###

Comparison of Donor-Acceptor $π$-Conjugated Dyes in Model Solar Cells: A Study of Interfacial Ultrafast Electron Migration. Interfacial ultrafast electron migration processes are simulated in finite
cluster models of dye-sensitized solar cells within a single active electron
approach. Initially, three different donor-acceptor $\pi$-conjugated dyes
supported on colloidal titania clusters are compared from the perspective of
their optical and electronic properties. The potential performance of the model
solar cell devices for charge migration processes is predicted from a static
perspective. For this purpose, parameter-free expressions for state-resolved
injection times and currents are established and evaluated for the three
systems. A broadband laser excitation promoting the excited states in the
visible region initiates the electron migration process. The evolution of the
electronic wave packet is analyzed with a density-based toolset including the
electronic yields partitioned for characteristic fragments of the model
complexes and the time-dependent one-electron density for distinctive time
steps in the dynamics. On the one hand, these reveal a microscopic picture for
the mechanistic pathway of the charge migration and on the other hand, they
validate the results from the time-independent analysis concerning the
photovoltaic efficiency.

###Preparation of CH3NH3PbI3 thin films with tens of micrometer scale at high temperature||Hao Zhang,Mian Tao,Baizhi Gao,Wei Chen,Qi Li,Qingyu Xu,Shuai Dong###

Preparation of CH3NH3PbI3 thin films with tens of micrometer scale at high temperature. The fabrication of high-quality organic-inorganic hybrid halide perovskite
layers is the key prerequisite for the realization of high efficient photon
energy harvest and electric energy conversion in their related solar cells. In
this article, we report a novel fabrication technique of CH3NH3PbI3 layer based
on high temperature chemical vapor reaction. CH3NH3PbI3 layers have been
prepared by the reaction of PbI2 films which were deposited by pulsed laser
deposition, with CH3NH3I vapor at various temperatures from 160 oC to 210 oC.
X-ray diffraction patterns confirm the formation of pure phase, and
photoluminescence spectra show the strong peak at around 760 nm. Scanning
electron microscopy images confirm the significantly increased average grain
size from nearly 1 {\mu}m at low reaction temperature of 160 oC to more than 10
{\mu}m at high reaction temperature of 200 oC. The solar cells were fabricated,
and short-circuit current density of 15.75 mA/cm2, open-circuit voltage of 0.49
V and fill factor of 71.66% have been obtained.

###The effect of electric field on multiple exciton generation in lead chalcogenide nanocrystals||Mahdi Gordi,Mohammad Kazem Moravvej-Farshi,Hamidreza Ramezani###

The effect of electric field on multiple exciton generation in lead chalcogenide nanocrystals. Unique properties of lead chalcogenides have enabled multiple exciton
generation (MEG) in their nanocrystals that can be beneficial in enhancing the
efficiency of the third generation solar cells. Although the intrinsic electric
field plays an imperative role in a solar cell, its effect on the multiple
exciton generation (MEG) has been overlooked, so far. Using EOM-CCSD as a
many-body approach, we show that any electric field can affect the absorptivity
spectra of the lead chalcogenide nanocrystals (Pb4Te4, Pb4Se4, and Pb4S4). The
same electric field, however, has insignificant effects on the MEG quantum
probabilities and the thresholds in these nanocrystals. Furthermore,
simulations show that Pb4Te4, among the aforementioned nanocrystals, has the
lowest MEG threshold and the strongest absorptivity peak that is located in the
multi-excitation window, irrespective of the field strength, making it the most
suitable candidate for MEG applications. Simulations also demonstrate that an
electric field affects the MEG characteristics in the Pb4Te4 nanocrystal, in
general, less than it perturbs MEG characteristics in Pb4Se4 and Pb4S4
nanocrystals. Our results can have a great impact in designing optoelectronic
devices whose performance can be significantly influenced by MEG.

###Correlated In-Situ Low-Frequency Noise and Impedance Spectroscopy Reveal Recombination Dynamics in Organic Solar Cells using Fullerene and Non-Fullerene Acceptors||Kyle A. Luck,Vinod K. Sangwan,Patrick E. Hartnett,Heather N. Arnold,Michael R. Wasielewski,Tobin J. Marks,Mark C. Hersam###

Correlated In-Situ Low-Frequency Noise and Impedance Spectroscopy Reveal Recombination Dynamics in Organic Solar Cells using Fullerene and Non-Fullerene Acceptors. Non-fullerene acceptors based on perylenediimides (PDIs) have garnered
significant interest as an alternative to fullerene acceptors in organic
photovoltaics (OPVs), but their charge transport phenomena are not well
understood, especially in bulk heterojunctions (BHJs). Here, we investigate
charge transport and current fluctuations by performing correlated
low-frequency noise and impedance spectroscopy measurements on two BHJ OPV
systems, one employing a fullerene acceptor and the other employing a dimeric
PDI acceptor. In the dark, these measurements reveal that PDI-based OPVs have a
greater degree of recombination in comparison to fullerene-based OPVs.
Furthermore, for the first time in organic solar cells, 1/f noise data are fit
to the Kleinpenning model to reveal underlying current fluctuations in
different transport regimes. Under illumination, 1/f noise increases by
approximately four orders of magnitude for the fullerene-based OPVs and three
orders of magnitude for the PDI-based OPVs. An inverse correlation is also
observed between noise spectral density and power conversion efficiency.
Overall, these results show that low-frequency noise spectroscopy is an
effective in-situ diagnostic tool to assess charge transport in emerging
photovoltaic materials, thereby providing quantitative guidance for the design
of next-generation solar cell materials and technologies.

###Phosphorene-AsP Heterostructure as a Potential Excitonic Solar Cell Material - A First Principles Study||M. R. Ashwin Kishore,P. Ravindran###

Phosphorene-AsP Heterostructure as a Potential Excitonic Solar Cell Material - A First Principles Study. Solar energy conversion to produce electricity using photovoltaics is an
emerging area in alternative energy research. Herein, we report on the basis of
density functional calculations, phosphorene/AsP heterostructure could be a
promising material for excitonic solar cells (XSCs). Our HSE06 functional
calculations show that the band gap of both phosphorene and AsP fall exactly
into the optimum value range according to XSCs requirement. The calculated
effective mass of electrons and holes show anisotropic in nature with effective
masses along ${\Gamma}$-X direction is lower than the ${\Gamma}$-Y direction
and hence the charge transport will be faster along ${\Gamma}$-X direction. The
wide energy range of light absorption confirms the potential use of these
materials for solar cell applications. Interestingly, phosphorene and AsP
monolayer forms a type-II band alignment which will enhance the separation of
photogenerated charge carriers and hence the recombination rate will be lower
which can further improve its photo-conversion efficiency if one use it in
XSCs.

###The Correlated Electronic States of a few Polycyclic Aromatic Hydrocarbons: A Computational Study||Geetanjali Giri,Anusooya Y. Pati,S. Ramasesha###

The Correlated Electronic States of a few Polycyclic Aromatic Hydrocarbons: A Computational Study. In recent years Polycyclic Aromatic Hydrocarbons (PAHs) have been studied for
their electronic properties as they are viewed as nanodots of graphene. They
have also been of interest as functional molecules for applications such as
light emitting diodes and solar cells. Since last few years varying structural
and chemical properties corresponding to the size and geometry of these
molecules have been studied both theoretically and experimentally. In this
paper, we carry out a systematic study of the electronic states of several PAHs
using the Pariser-Parr-Pople model which incorporates long-range electron
correlations. In all the molecules studied by us, we find that the 2A state is
below the 1B state and hence none of them will be fluorescent in the gaseous
phase. The singlet-triplet gap is more than one-half of the singlet-singlet gap
in all cases and hence none of these PAHs can be candidates for improved solar
cell efficiencies in a singlet fission. We discuss in detail the properties of
the electronic states which include bond orders and spin densities (in
triplets) of these systems.

###On the effect of surface recombination in thin film solar cells, light emitting diodes and photodetectors||Oskar J. Sandberg,Ardalan Armin###

On the effect of surface recombination in thin film solar cells, light emitting diodes and photodetectors. Radiative and non-radiative charge carrier recombination in thin-film diodes
plays a key role in determining the efficiency of electronic devices made of
next generation semiconductors such as organic, perovskite and nanocrystals. In
this work, we show that lowering the bulk recombination does not necessarily
result in enhanced performance metrics of electronic devices. From the
perspective of charge carrier extraction and injection, the radiative limit of
the open-circuit voltage of solar cells, noise current of photodetectors and
lasing threshold of injection lasers cannot be improved if the contacts are not
perfectly selective. A numerical drift-diffusion model is used to investigate
the interplay between bulk recombination and surface recombination of minority
carriers at the contacts in bipolar thin diode devices based on low-mobility
semiconductors. The surface recombination becomes prominent in case of reduced
bulk recombination strengths when non-selective contacts, i. e. contacts that
are either metallic or have imperfect charge-selective interlayer, are
employed. Finally, we derive analytical approximations for the case when
diffusion-limited surface recombination of minority carriers at Ohmic contacts
dominates the dark current. These results indicate that having perfectly
selective contacts becomes crucial in systems with suppressed bulk
recombination - a challenging requirement for future state-of-the-art thin-film
solar cells, light-emitting devices and photodetectors made of next generation
semiconductors.

###Inorganic photovoltaic cells based on BiFeO3: spontaneous polarization, lattice matching, light polarization and their relationship to photovoltaic performance||Chao He,Guocai Liu,Huiyan Zhao,Kun Zhao,Zuju Ma,Xingtao An###

Inorganic photovoltaic cells based on BiFeO3: spontaneous polarization, lattice matching, light polarization and their relationship to photovoltaic performance. Inorganic ferroelectric perovskite oxides are more stable than hybrid
perovskites. However, their solar energy harvest efficiency is not so good.
Here, by constructing a series of BiFeO3 based devices (solar cells), we
investigated three factors that influence the photovoltaic performance,
including spontaneous polarization, terminated ions species in the interface
between BiFeO3 and the electrode, and polarized light irradiation. This work
was carried out in the framework of density functional theory combined with
non-equilibrium Green's function theory under built in electric field or finite
bias. The results showed that 1. the photocurrent is larger only under a
suitable electronic band gap rather than larger spontaneous polarization; 2.
the photocurrent reaches the largest in Bi ions terminated interface than in
the case of Fe ion or O ion with SrTiO3 electrode; 3. the photocurrent could be
largely enhanced if the polarized direction of the monochromatic light is
perpendicular to the spontaneous polarization direction. The results would
deepen the understanding of some experimental results of BiFeO3 based solar
cells.

###Fabrication of solar cells based on $Cu_2ZnSnS_4$ prepared from $Cu_2SnS_3$ synthesized using a novel chemical procedure||John M. Correa,Raúul A. Becerra,Asdrubal A. Ramírez,Gerardo Gordillo###

Fabrication of solar cells based on $Cu_2ZnSnS_4$ prepared from $Cu_2SnS_3$ synthesized using a novel chemical procedure. Solar cells based on kesterite-type $Cu_2ZnSnS_4$ (CZTS) thin films were
fabricated using a chemical route to prepare the $CZTS$ films, consisting in
sequential deposition of $Cu_2SnS_3$ (CTS) and $ZnS$ thin films followed by
annealing at $550^\circ C$ in nitrogen atmosphere. The $CTS$ compound was
prepared in a one-step process using a novel chemical procedure consisting of
simultaneous precipitation of $Cu_2S$ and $SnS_2$ performed by diffusion
membranes assisted $CBD$ (chemical bath deposition) technique. Diffusion
membranes were used to optimize the kinetic growth through a moderate control
of release of metal ions into the work solution. As the conditions for the
formation in one step of the $Cu_2SnS_3$ compound have not yet been reported in
literature, special emphasis was put on finding the parameters that allow
growing the $Cu_2SnS_3$ thin films by simultaneous precipitation of $Cu_2S$ and
$SnS_2$. For that, we propose a methodology that includes numerical solution of
the equilibrium equations that were established through a study of the chemical
equilibrium of the system $SnCl_2$, $Na_3C_6H_5O_7\cdot 2H2O$, $CuCl_2$ and
$Na_2S_2O_3\cdot5H_2O$. The formation of thin films of $CTS$ and $CZTS$ free of
secondary phases grown with a stoichiometry close to that corresponding to the
$Cu_2SnS_3$ and $Cu_2ZnSnS_4$ phases, was verified through measurements of
X-ray diffraction $(XRD)$ and Raman spectroscopy. Solar cell with an efficiency
of $4.2%$, short circuit current of $16.2 mA/cm^2$ and open-circuit voltage of
0.49 V was obtained.

###Influence of hydrogen radicals treatment on layers and solar cells made of solution-processed amorphous silicon||Torsten Bronger,Jan Wördenweber,Paul Wöbkenberg,Stefan Muthmann,Odo Wunnicke,Reinhard Carius###

Influence of hydrogen radicals treatment on layers and solar cells made of solution-processed amorphous silicon. Solution-processed amorphous silicon is a promising material for
semiconductor devices. Unfortunately, its manufacturing leaves a high density
of defects in the layer, which can be reduced by a treatment with hydrogen
radicals. Here, we present an optimized hydrogen treatment, which is used for
best performing solar cells made of solution-processed amorphous silicon. We
examine the amount and the nature of hydrogen incorporation using infrared
absorption and hydrogen effusion. The hydrogen treatment not only increases
hydrogen content significantly, it also enlarges the fraction of hydrogen in a
bonding configuration which is known to be advantageous for electronic
properties, albeit only close to the surface. Using electron spin resonance and
and photothermal deflection spectroscopy spectra, we confirm a reduction of
defect density. Regarding the electrical properties, the ratio of photo and
dark conductivity is increased by almost two decades. This leads to a greatly
enhanced performance of solar cell devices which use the material as the
absorber layer. In particular, the efficiency jumps by a factor of three.

###Improved evaluation of deep-level transient spectroscopy on perovskite solar cells reveals ionic defect distribution||Sebastian Reichert,Jens Flemming,Qingzhi An,Yana Vaynzof,Jan-Frederik Pietschmann,Carsten Deibel###

Improved evaluation of deep-level transient spectroscopy on perovskite solar cells reveals ionic defect distribution. One of the key challenges for future development of efficient and stable
metal halide perovskite solar cells is related to the migration of ions in
these materials. Mobile ions have been linked to the observation of hysteresis
in the current--voltage characteristics, shown to reduce device stability
against degradation and act as recombination centers within the band gap of the
active layer. In the literature one finds a broad spread of reported ionic
defect parameters (e.g. activation energies) for seemingly similar perovskite
materials, rendering the identification of the nature of these species
difficult. In this work, we performed temperature dependent deep-level
transient spectroscopy (DLTS) measurements on methylammonium lead iodide
perovskite solar cells and developed a extended regularization algorithm for
inverting the Laplace transform. Our results indicate that mobile ions form a
distribution of emission rates (i.e. a distribution of diffusion constants) for
each observed ionic species, which may be responsible for the differences in
the previously reported defect parameters. Importantly, different DLTS modes
such as optical and current DLTS yield the same defect distributions. Finally
the comparison of our results with conventional boxcar DLTS and impedance
spectroscopy (IS) verifies our evaluation algorithm.

###Photo-effect on ion transport in mixed cation and halide perovskites and implications for photo de-mixing||Gee Yeong Kim,Alessandro Senocrate,Yaru Wang,Davide Moia,Joachim Maier###

Photo-effect on ion transport in mixed cation and halide perovskites and implications for photo de-mixing. Organic-inorganic hybrid perovskites are considered to be most promising
photovoltaic materials. Highest efficiencies of perovskite solar cells have
been achieved by using appropriate cation and anion mixtures. Mixed perovskite
solar cells also show an improved stability. For both performance as well as
stability, experimental information on electronic and ionic charge carriers is
key, an information that so far has only been provided for methylammonium lead
iodide; there we also found that light can enhance not only electronic but also
ionic conductivities by more than one order of magnitude. We also proposed a
mechanism for this surprising photo-ionic effect and explained its impact on
photo-decomposition. Here we quantitatively deconvolute ionic and electronic
transport properties for the practically relevant substitutions and mixtures.
Specifically, we investigate various cation and anion substitutions (Cs; FA;
Br) with a special eye on their photo-ionic effect. The results are not only of
importance for light-induced degradation but also for light-induced demixing.
As far as the photo-ionic effect is concerned, we find that the choice of the
halide is of crucial importance, while the cationic substitutions are less
relevant. The huge ionic conductivity enhancement found for iodide perovskites,
is weakened by bromide substitution and eventually becomes insignificant for
the pure bromide. Based on these experimental results, we provide a rationale
for the experimentally observed photo-demixing.

###Electrical Characterization of CIGS Thin Film Solar Cells by Two and Four-Wires Probe Technique||Amirhosein Mosavi,Beszedes Bertalan,Felde Imre,Laszlo Nadai,Nima E. Gorji###

Electrical Characterization of CIGS Thin Film Solar Cells by Two and Four-Wires Probe Technique. The characterization of thin film solar cells is of huge importance for
obtaining high open circuit voltage and low recombination rates from the
interfaces or within the bulk of the main materials. Among the many electrical
characterization techniques, the two and four wire probe using the Cascade
instrument is of interest since the resistance of the wires, and the electrical
contacts can be excluded by the additional two wires in 4 wire probe
configuration. In this paper, both two and four-point probes configuration are
employed to characterize the CIGS chalcogenide thin film solar cells. The two
wire probe has been used to measure the current-voltage characteristics of the
cell which results in a huge internal resistance. Therefore, the four wire
connection are also used to eliminate the lead resistance to enhance the
characterization accuracy. The load resistance in the twowire probe diminishes
the photogenerated current density at smaller voltage ranges. In contrast, the
proposed four wire probe collects more current at higher voltages due to
enhanced carrier collection efficiency from contact electrodes. The current
conduction mechanism is also identified at every voltage region represented by
the value of the ideality factor of that voltage region.

###Thin Film Growth of Phase-Separating Phthalocyanine-Fullerene Blends: A Combined Experimental and Computational Study||Berthold Reisz,Eelco Empting,Matthias Zwadlo,Martin Hodas,Giuliano Duva,Valentina Belova,Clemens Zeiser,Jan Hagenlocher,Santanu Maiti,Alexander Hinderhofer,Alexander Gerlach,Martin Oettel,Frank Schreiber###

Thin Film Growth of Phase-Separating Phthalocyanine-Fullerene Blends: A Combined Experimental and Computational Study. Blended organic thin films have been studied during the last decades due to
their applicability in organic solar cells. Although their optical and
electronic features have been examined intensively, there is still lack of
detailed knowledge about their growth processes and resulting morphologies,
which play a key role for the efficiency of optoelectronic devices such as
organic solar cells. In this study, pure and blended thin films of copper
phthalocyanine (CuPc) and the Buckminster fullerene (C60) were grown by vacuum
deposition onto a native silicon oxide substrate at two different substrate
temperatures, 310 K and 400 K. The evolution of roughness was followed by
in-situ real-time X-ray reflectivity. Crystal orientation, island densities and
morphology were examined after the growth by X-ray diffraction experiments and
microscopy techniques. The formation of a smooth wetting layer followed by
rapid roughening was found in pure CuPc thin films, whereas C60 shows a fast
formation of distinct islands at a very early stage of growth. The growth of
needle-like CuPc crystals loosing their alignment with the substrate was
identified in co-deposited thin films. Furthermore, the data demonstrates that
structural features become larger and more pronounced and that the island
density decreases by a factor of four when going from 310 K to 400 K. Finally,
the key parameters roughness and island density were well reproduced on a
smaller scale by kinetic Monte-Carlo simulations of a generic, binary lattice
model with simple nearest-neighbor interaction energies.

###Interplay of photons and charge carriers in thin-film devices||Pyry Kivisaari,Mikko Partanen,Toufik Sadi,Jani Oksanen###

Interplay of photons and charge carriers in thin-film devices. Thin films are gaining ground in photonics and optoelectronics, promising
improvements in their efficiency and functionality as well as decreased
material usage as compared to bulk technologies. However, proliferation of thin
films would benefit not only from continuous improvements in their fabrication,
but also from a unified and accurate theoretical framework of the interplay of
photons and charge carriers. In particular, such a framework would need to
account quantitatively and self-consistently for photon recycling and
interference effects. To this end, here we combine the drift-diffusion
formalism of charge carrier dynamics and the fluctuational electrodynamics of
photon transport self-consistently using the recently introduced
interference-extended radiative transfer equations. The resulting equation
system can be solved numerically using standard simulation tools, and as an
example, here we apply it to study well-known GaAs thin-film solar cells. In
addition to obtaining the expected device characteristics, we analyze the
underlying complex photon transport and recombination-generation processes,
demonstrating the physical insight provided for unevenly excited structures
through the direct and self-consistent description of photons and charge
carriers. The methodology proposed in this work is general and can be used to
obtain accurate physical insight into a wide range of planar optoelectronic
devices, of which the thin-film single-junction solar cells studied here are
only one example.

###The Evolution of Materials Acceleration Platforms -- Towards the Laboratory of the Future with AMANDA||Jerrit Wagner,Christian G. Berger,Xiaoyan Du,Tobias Stubhan,Jens A. Hauch,Christoph J. Brabec###

The Evolution of Materials Acceleration Platforms -- Towards the Laboratory of the Future with AMANDA. The development of complex functional materials poses a multi-objective
optimization problem in a large multidimensional parameter space. Solving it
requires reproducible, user independent laboratory work and intelligent
preselection of experiments. However, experimental materials science is a field
where manual routines are still predominant, although other domains like
pharmacy or chemistry have long used robotics and automation. As the number of
publications on Materials Acceleration Platforms (MAPs) increases steadily, we
review selected systems and fit them into the stages of a general material
development process to examine the evolution of MAPs. Subsequently we present
our approach to laboratory automation in materials science. We introduce AMANDA
(Autonomous Materials and Device Application Platform), a generic platform for
distributed materials research comprising a self-developed software backbone
and several MAPs. One of them, LineOne (L1), is specifically designed to
produce and characterize solution processed thin-film devices like organic
solar cells (OSC). It is designed to perform precise closed-loop screenings of
up to 272 device variations per day yet allows further upscaling. Each
individual solar cell is fully characterized and all process steps are
comprehensively documented. We want to demonstrate the capabilities of AMANDA
L1 with OSCs based on PM6:Y6 with 13.7% efficiency when processed in air.
Further we discuss challenges and opportunities of highly automated research
platforms and elaborate on the future integration of additional techniques,
methods and algorithms in order to advance to fully autonomous self-optimizing
systems - a paradigm shift in functional materials development leading to the
laboratory of the future.

###Pressure-induced reversible phase transition and amorphization of CH$_3$NH$_3$PbI$_3$||Kai Wang,Ran Liu,Yuancun Qiao,Jinxing Cui,Bo Song,Bingbing Liu,Bo Zou###

Pressure-induced reversible phase transition and amorphization of CH$_3$NH$_3$PbI$_3$. Recent advances of highly efficient solar cells based on organic-inorganic
halide perovskites have triggered intense research efforts to establish the
fundamental properties of these materials. In this work, we utilized diamond
anvil cell to investigate the pressure-induced structural and electronic
transformations in methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$) up to 7 GPa
at room temperature. The synchrotron X-ray diffraction experiment show that the
sample transformed from tetragonal to orthorhombic phase at 0.3 GPa and
amorphized above 4 GPa. Further high pressure IR spectroscopy experiments
illustrated the high pressure behavior of organic (CH$_3$NH$_3$)$^+$ cations.
We also analyzed the pressure dependence of the band gap energy based on the
optical absorption and photoluminescence (PL) results. Moreover, all the
observed changes were fully reversible when the pressure was completely
released. Our in situ high pressure studies provide essential information for
the intrinsic properties and stability of organic-inorganic halide perovskites,
which significantly affect the performance of perovskite solar cells.

###Novel Physical Vapor Deposition Approach to Hybrid Perovskites: Growth of MAPbI3 Thin Films by RF-Magnetron Sputtering||Sara Bonomi,Daniela Marongiu,Nicola Sestu,Michele Saba,Maddalena Patrini,Giovanni Bongiovanni,Lorenzo Malavasi###

Novel Physical Vapor Deposition Approach to Hybrid Perovskites: Growth of MAPbI3 Thin Films by RF-Magnetron Sputtering. Solution-based methods represent the most widespread approach used to deposit
hybrid organic-inorganic perovskite films for low-cost but efficient solar
cells. However, solution-process techniques offer limited control over film
morphology and crystallinity, and most importantly do not allow sequential film
deposition to produce perovskite-perovskite heterostructures. Here the
successful deposition of CH3NH3PbI3 (MAPI) thin films by RF-magnetron
sputtering is reported, an industry-tested method to grow large area devices
with precisely controlled stoichiometry. MAPI films are grown starting from a
single-target made of CH3NH3I (MAI) and PbI2. Films are single-phase, with a
barely detectable content of unreacted PbI2, full surface coverage and
thickness ranging from less than 200 nm to more than 3 {\mu}m. Light absorption
and emission properties of the deposited films are comparable to as-grown
solution-processed MAPI films. The development of vapor-phase deposition
methods is of interest to advance perovskite photovoltaic devices with the
possibility of fabricating perovskite multijunction solar cells or multicolor
bright light-emitting devices in the whole visible spectrum.

###Giant Photocurrent Enhancement by Coulomb Interaction in a Single Quantum Dot for Energy Harvesting||Kai Peng,Shiyao Wu,Xin Xie,Jingnan Yang,Chenjiang Qian,Feilong Song,Sibai Sun,Jianchen Dang,Yang Yu,Shushu Shi,Jiongji He,Xiulai Xu###

Giant Photocurrent Enhancement by Coulomb Interaction in a Single Quantum Dot for Energy Harvesting. Understanding the carrier excitation and transport processes at the
single-charge level plays a key role in quantum-dot-based solar cells and
photodetectors. Here, we report on Coulomb-induced giant photocurrent
enhancement of positive charged trions (\emph{X$^+$}) in a single
self-assembled InAs/GaAs quantum dot embedded in an \emph{n-i-}Schottky device
by high-resolution photocurrent (PC) spectroscopy. The Coulomb repulsion
between the two holes in the \emph{X$^+$} increases the tunneling rate of the
hole, and the remaining hole can be reused as the initial state to regenerate
\emph{X$^+$} again. This process brings the PC amplitude of \emph{X$^+$} up to
30 times larger than that of the neutral exciton. The analysis of the hole
tunneling time gives the equivalent change of hole tunnel barriers caused by
Coulomb interaction between two holes with a value of 8.05 meV during the
tunneling process. Our work brings a fundamental understanding of energy
conversion for solar cells in nanoscale to improve internal quantum efficiency
for energy harvesting.

###Removing Leakage and Surface Recombination in Planar Perovskite Solar Cells||K. Tvingstedt,L. Gil-Escrig,C. Momblona,P. Rieder,D. Kiermasch,M. Sessolo,A. Baumann,H. J. Bolink,V. Dyakonov###

Removing Leakage and Surface Recombination in Planar Perovskite Solar Cells. Thin-film solar cells suffer from various types of recombination, of which
leakage current usually dominates at lower voltages. Herein, we demonstrate
first a three-order reduction of the shunt loss mechanism in planar
methylammonium lead iodide perovskite solar cells by replacing the commonly
used hole transport layer
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with a
better hole-selective polyarylamine. As a result, these cells exhibit superior
operation under reduced light conditions, which we demonstrate for the extreme
case of moonlight irradiance, at which open-circuit voltages of 530 mV can
still be obtained. By the shunt removal we also observe the VOC to drop to zero
after as long as 2 h after the light has been switched off. Second, at higher
illumination intensities the dominant losses in the PEDOT:PSS-based cell are
ascribed to surface recombination and are also proven to be substantially
minimized by instead employing the polyarylamine. We attribute the reduced
shunt and surface recombination to the far better suited semiconductor
character of the polyarylamine, compared to that of PEDOT:PSS, efficiently
blocking electrons from recombining at this electrode.

###Perovskite PV-powered RFID: enabling low-cost self-powered IoT sensors||Sai Nithin R. Kantareddy,Ian Mathews,Shijing Sun,Mariya Layurova,Janak Thapa,Juan-Pablo Correa-Baena,Rahul Bhattacharyya Tonio Buonassisi,Sanjay E. Sarma,Ian Marius Peters###

Perovskite PV-powered RFID: enabling low-cost self-powered IoT sensors. Photovoltaic (PV) cells have the potential to serve as on-board power sources
for low-power IoT devices. Here, we explore the use of perovskite solar cells
to power Radio Frequency (RF) backscatter-based IoT devices with a few {\mu}W
power demand. Perovskites are suitable for low-cost, high-performance,
low-temperature processing, and flexible light energy harvesting that hold the
possibility to significantly extend the range and lifetime of current
backscatter techniques such as Radio Frequency Identification (RFID). For these
reasons, perovskite solar cells are prominent candidates for future low-power
wireless applications. We report on realizing a functional perovskite-powered
wireless temperature sensor with 4 m communication range. We use a 10.1%
efficient perovskite PV module generating an output voltage of 4.3 V with an
active area of 1.06 cm2 under 1 sun illumination, with AM 1.5G spectrum, to
power a commercial off-the-shelf RFID IC, requiring 10 - 45 {\mu}W of power.
Having an on-board energy harvester provides extra-energy to boost the range of
the sensor (5x) in addition to providing energy to carry out high-volume sensor
measurements (hundreds of measurements per min). Our evaluation of the
prototype suggests that perovskite photovoltaic cells are able to meet the
energy needs to enable fully autonomous low-power RF backscatter applications
of the future. We conclude with an outlook into a range of applications that we
envision to leverage the synergies offered by combining perovskite
photovoltaics and RFID.

###Radiation hardness and post irradiation regeneration behavior of GaInAsP solar cells||R. Lang,J. Schön,J. Lefèvre,B. Boizot,F. Dimroth,D. Lackner###

Radiation hardness and post irradiation regeneration behavior of GaInAsP solar cells. Recent developments have renewed the demand for solar cells with increased
tolerance to radiation damage. To investigate the specific irradiation damage
of 1 MeV electron irradiation in GaInAsP lattice matched to InP for varying In
and P contents, a simulation based analysis is employed: by fitting the quantum
efficiency and open-circuit voltage simultaneously before and after
irradiation, the induced changes in lifetime are detected. Furthermore, the
reduction of irradiation damage during regeneration under typical satellite
operating conditions for GEO missions (60{\deg}C and AM0 illumination) is
investigated. A clear decrease of the radiation damage is observed after post
irradiation regeneration. This regeneration effect is stronger for increasing
InP-fraction. It is demonstrated that the irradiation induced defect
recombination coefficient for irradiation with 1 MeV electrons after
regeneration for 216 hours can be described with a linear function of
InP-fraction between 1*10$^{-5}$ cm$^2$/s for GaAs and 7*10$^{-7}$ cm$^2$/s for
InP. The results show that GaInAsP is a promising material for radiation hard
space solar cells.

###Tetracene ultrathin film growth on silicon||Jens Niederhausen,Hazem Aldahhak,Rowan W. MacQueen,Wolf Gero Schmidt,Uwe Gerstmann,Klaus Lips###

Tetracene ultrathin film growth on silicon. Inorganic-organic interfaces are important for enhancing the power conversion
efficiency of silicon-based solar cells through singlet exciton fission (SF).
We elucidated the structure of the first monolayers of tetracene (Tc), a SF
molecule, on hydrogen-passivated Si(111) [H-Si(111)] and hydrogenated amorphous
Si (a-Si:H) by combining near-edge X-ray absorption fine structure (NEXAFS) and
X-ray photoelectron spectroscopy (XPS) experiments with density functional
theory (DFT) calculations. For samples grown at or below substrate temperatures
of 265 K, the resulting ultrathin Tc films are dominated by almost
upright-standing molecules. The molecular arrangement is very similar to the Tc
bulk phase, with only slightly higher average angle between the conjugated
molecular plane normal and the surface normal ($\alpha$) around 77{\deg}.
Judging from carbon K-edge X-ray absorption spectra, the orientation of the Tc
molecules are almost identical when grown on H-Si(111) and a-Si:H substrates as
well as for (sub)mono- to several-monolayer coverages. Annealing to room
temperature, however, changes the film structure towards a smaller $\alpha$ of
about 63{\deg}. A detailed DFT-assisted analysis suggests that this structural
transition is correlated with a lower packing density and requires a
well-chosen amount of thermal energy. Therefore, we attribute the resulting
structure to a distinct monolayer configuration that features less inclined,
but still well-ordered molecules. The larger overlap with the substrate
wavefunctions makes this arrangement attractive for an optimized interfacial
electron transfer in SF-assisted silicon solar cells.

###Operando direct observation of spin states correlated with device performance in perovskite solar cells||Takahiro Watanabe,Toshihiro Yamanari,Kazuhiro Marumoto###

Operando direct observation of spin states correlated with device performance in perovskite solar cells. Perovskite solar cells are one of the most attracting cells because of
remarkably improved power conversion efficiency (PCE) recently. Toward their
practical application, it is important not only to increase the PCE but also to
elucidate the deterioration mechanism. Here, we present operando direct
observation of spin states in the cells using electron spin resonance (ESR)
spectroscopy in order to investigate the operation and deterioration mechanisms
from a microscopic viewpoint. By simultaneous measurements of solar-cell and
ESR characteristics of the same cell, the spin states in the hole-transport
material (HTM) spiro-OMeTAD are demonstrated to be changed at the molecular
level, which varies the device performance under device operation. These
variations are ascribed to the change of hole transport by charge-carrier
scatterings and filling of deep trapping levels in the HTM, and to interfacial
electric dipole layers formed at the HTM interfaces. In addition, reverse
electron transfer from TiO2 layer to the HTM layer is directly demonstrated at
the molecular level under ultraviolet light irradiation, which causes the
decrease in the HTM doping effect. Thus, conducting such operando microscopic
investigation on the internal states in the cells would be useful to obtain a
new further guideline for improving the device performance and durability.

###Transmission electron microscopy of organic-inorganic hybrid perovskites: myths and truths||Shulin Chen,Ying Zhang,Jinjin Zhao,Zhou Mi,Jingmin Zhang,Jian Cao,Jicai Feng,Guanglei Zhang,Junlei Qi,Jiangyu Li,Peng Gao###

Transmission electron microscopy of organic-inorganic hybrid perovskites: myths and truths. Organic-inorganic hybrid perovskites (OIHPs) have attracted extensive
research interest as a promising candidate for efficient and inexpensive solar
cells. Transmission electron microscopy characterizations that can benefit the
fundamental understanding and the degradation mechanism are widely used for
these materials. However, their sensitivity to the electron beam illumination
and hence structural instabilities usually prevent us from obtaining the
intrinsic information or even lead to significant artifacts. Here, we
systematacially investigate the structural degradation behaviors under
different experimental factors to reveal the optimized conditions for TEM
characterizations of OIHPs by using low-dose electron diffraction and imaging
techniques. We find that a low temperature does not slow down the beam damage
but instead induces a rapid amorphization for OIHPs. Moreover, a less severe
damage is observed at a higher accelerating voltage. The beam-sensitivity is
found to be facet-dependent that a (100) exposed MAPbI3 surface is more stable
than (001) surface. With these guidance, we successfully acquire the atomic
structure of pristine MAPbI3 and identify the characterization window that is
very narrow. These findings are helpful to guide future electron microscopy
characterization of these beam-sensitive materials, which are also useful for
finding strategies to improve the stability and the performance of the
perovskite solar cells.

###Ligand Dependent Oxidation Dictates the Performance Evolution of High Efficiency PbS Quantum Dot Solar Cells||David Becker-Koch,Miguel Albaladejo Siguan,Vincent Lami,Fabian Paulus,Hengyang Xiang,Zhuoying Chen,Yana Vaynzof###

Ligand Dependent Oxidation Dictates the Performance Evolution of High Efficiency PbS Quantum Dot Solar Cells. Lead sulfide (PbS) quantum dot (QD) photovoltaics have reached impressive
efficiencies of 12%, making them particularly promising for future
applications. Like many other types of emerging photovoltaic devices, their
environmental instability remains the Achilles heel of this technology. In this
work, we demonstrate that the degradation processes in PbS QDs which are
exposed to oxygenated environments are tightly related to the choice of
ligands, rather than their intrinsic properties. In particular, we demonstrate
that while 1,2-ethanedithiol (EDT) ligands result in significant oxidation of
PbS, lead iodide/lead bromide (PbX2) coated PbS QDs show no signs of oxidation
or degradation. Consequently, since the former is ubiquitously used as a hole
extraction layer in QD solar cells, it is predominantly responsible for the
device performance evolution. The oxidation of EDT-PbS QDs results in a
significantly reduced effective QD size, which triggers two competing
processes: improved energetic alignment that enhances electron blocking, but
reduced charge transport through the layer. At early times, the former process
dominates, resulting in the commonly reported, but so far not fully explained
initial increase in performance, while the latter governs the onset of
degradation and deterioration of the photovoltaic performance. Our work
highlights that the stability of PbS quantum dot solar cells can be
significantly enhanced by an appropriate choice of ligands for all device
components.

###Computational investigation on non-linear optical properties of hexaphyrin and core modified hexaphyrins||Sumit Naskar,Mousumi Das###

Computational investigation on non-linear optical properties of hexaphyrin and core modified hexaphyrins. Expanded porphyrin-based (Hexaphyrins) sensitizers are promising due to their
excellent light harvesting feature in dye-sensitized solar cell (DSSC). We
calculated the low-lying excitations of expanded porphyrins (EPs) as hexaphyrin
and core modified hexaphyrin structures using Time-Dependent Density Functional
Theory. Our calculation showed the EPs (both hexaphyrin and core modified
hexaphyrin) have broad range of absorption band suitable for harvesting the
visible and near infrared region of solar spectrum. All EPs studied here
satisfy the energy condition of singlet fission (SF). SF is the process in
which the theoretical limit of Shockley-Quiesser (SQ) (33\%) can be overcome in
single junction solar cell. The non-linear optical properties like first hyper
polarizability $\beta$ and second order hyper polarizability $\gamma$ were
calculated using coupled perturbed Hartree-Fock approach. From the second order
NLO properties we carried out degenerate four wave mixing (DFWM) component
($\gamma^{(2)}(-\omega;\omega,\omega,-\omega$)) and finally quadratic non
linear refractive indices of these EPs are calculated. Calculation showed EPs
are promising as organic dye for the opto-electronic applications and useful
for high efficiency DSSC and also useful for potential NLO materials as their
hyper polarizabilities showed higher order non linearities.

###Studies on optical signal due to oxygen effect on hydrogenated amorphous/crystalline silicon thin-films||Meenakshi Rana,Chandan Banerjee,Papia Chowdhury###

Studies on optical signal due to oxygen effect on hydrogenated amorphous/crystalline silicon thin-films. We have studied the effects of oxygen on hydrogenated amorphous/crystalline
silicon films in terms of their structural and optical properties. Different
hydrogenated silicon oxide (SiO:H) and silicon (Si:H) films are fabricated
between microcrystalline and amorphous transition region. X-ray diffraction,
Raman, FTIR and UV-Vis emission spectrometry have been used to characterize
different films. A comparison of the results with those of different types of
films like hydrogenated amorphous silicon oxide (a-SiO:H), hydrogenated
amorphous silicon (a-Si:H) and microcrystalline silicon ($\mu$c-Si:H) films
reveal their superiority as an excellent substance for solar cell. X-ray
diffraction, FTIR and Raman spectral analysis show that difference of the H
dilution effect has a major effect on the structure of the film and the optical
properties. Photoluminescence analysis of amorphous silicon-oxygen and
silicon-hydride alloy films has established their efficient application
appropriate as Si based light emitting devices. A large optical band gap of
1.83 eV and appearance of strong photo luminescence at 2.0 eV validates the
applicability of a-SiO:H film as a better alternative for the solar cells.

###Open-Circuit Voltage Limitation by Surface Recombination in Perovskite Solar Cells||Sebastian Reichert,Katelyn Goetz,Christopher Wöpke,Yana Vaynzof,Carsten Deibel###

Open-Circuit Voltage Limitation by Surface Recombination in Perovskite Solar Cells. Fundamental electronic processes such as charge-carrier transport and
recombination play a critical role in determining the efficiency of hybrid
perovskite solar cells. The presence of mobile ions complicates the development
of a clear understanding of these processes as the ions may introduce
exceptional phenomena such as hysteresis or giant dielectric constants. As a
result, the electronic landscape, including its interaction with mobile ions,
is difficult to access both experimentally and analytically. To address this
challenge, we applied a series of small perturbation techniques including
impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy
(IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) to planar
$\mathrm{MAPbI_3}$ perovskite solar cells. Our measurements indicate that both
electronic as well as ionic responses can be observed in all three methods and
assigned by literature comparison. The results reveal that the dominant
charge-carrier loss mechanism is surface recombination by limitation of the
quasi-Fermi level splitting. The interaction between mobile ions and the
electronic charge carriers leads to a shift of the apparent diode ideality
factor from 0.74 to 1.64 for increasing illumination intensity, despite the
recombination mechanism remaining unchanged.

###Multiple Exciton Generation Solar Cells: Numerical Approach of Quantum Yield Extraction and its Limiting Efficiencies||Jongwon Lee###

Multiple Exciton Generation Solar Cells: Numerical Approach of Quantum Yield Extraction and its Limiting Efficiencies. Multiple exciton generation solar cells exhibit a low power conversion
efficiency owing to nonradiative recombination even if numerous electron and
hole pairs are generated per incident photon. This paper elucidates the
non-idealities of multiple exciton generation solar cells (MEGSCs) and
alternative approaches for realizing photovoltaic (PV) devices similar to
MEGSCs. First, we present mathematical approaches for determining the quantum
yield (QY) to discuss the non-idealities of MEGSCs by adjusting the delta
function. In particular, we employ the Gaussian distribution function to
present the occupancy status of carriers at each energy state by Dirac delta
function. By adjusting the Gaussian distribution function for each energy
state, we obtain the ideal and non-ideal QYs. Through this approach, we discuss
the material imperfections of MEGSCs by analyzing the mathematically obtained
QYs. By calculating the ratio between the radiative and nonradiative
recombination, we can discuss the status of radiative recombination calculate
Furthermore, we apply this approach into the detailed balance limit of MEGSC to
investigate the practical limit of MEGSC.

###Passivating Surface Defects and Reducing Interface Recombination in CuInS2 Solar Cells by a Facile Solution Treatment||Mohit Sood,Alberto Lomuscio,Florian Werner,Aleksandra Nikolaeva,Phillip J. Dale,Michele Melchiorre,Jerome Guillot,Daniel Abou-Ras,Susanne Siebentritt###

Passivating Surface Defects and Reducing Interface Recombination in CuInS2 Solar Cells by a Facile Solution Treatment. Interface recombination at the absorber buffer interface impedes the
efficiency of a solar cell with an otherwise excellent absorber. The internal
voltage or the quasi-Fermi level splitting (qFLs) measures the quality of the
absorber. Interface recombination reduces the open circuit voltage (VOC) with
respect to the qFLs. The present work explores a facile sulfur-based
post-deposition treatment (S-PDT) to passivate the interface of CuInS2 thin
films grown under Cu-rich conditions, which show excellent qFLs values, but
much lower VOCs. The CuInS2 absorbers are treated in three different
S-containing solutions at 80 oC. Absolute calibrated photoluminescence and
current-voltage measurements demonstrate a reduction of the deficit between
qFLs and VOC in the best S-PDT device by almost one third compared to the
untreated device. Analysis of temperature dependence of the open-circuit
voltage shows increased activation energy for the dominant recombination path,
indicating less interface recombination. In addition, capacitance transient
measurements reveal the presence of slow metastable defects in the untreated
solar cell. The slow response is considerably reduced by the S-PDT, suggesting
passivation of these slow metastable defects. The results demonstrate the
effectiveness of solution based S-treatment in passivating defects, presenting
a promising strategy to explore and reduce defect states near the interface of
chalcogenide semiconductors.

###Stability enhancement of ITO-free non-inverted PTB7:PC71BM solar cell using two-step post-treated PEDOT:PSS||Mehrdad Kankanan,Abdolnabi Kosarian,Ebrahim Farshidi###

Stability enhancement of ITO-free non-inverted PTB7:PC71BM solar cell using two-step post-treated PEDOT:PSS. The conductivity and stability of specially treated PEDOT:PSS thin films are
investigated. Based on the proposed treatment method, ITO-free PTB7:PC71BM
organic solar cells are fabricated and the electrical properties of the cells
are analyzed. It is shown that by applying a two-step post-treatment method
using methanol and ethylene glycol, the conductivity of the PEDOT:PSS thin film
increases from 0.9 to 1448 S/cm, and at the same time, a significant
improvement of the stability of the layer over time is achieved. It is shown
that after 30 days of aging under high humidity condition, the conductivity
remains above 70% of its initial value, which is a remarkable result compared
with the results reported in the literature. In this paper, important factors
affecting the conductivity and stability of the treated layer are studied in
detail. In addition, the effect of immersion time in methanol on the
conductivity of the layer is also investigated and it is found that dipping
times less than 3 min have no appreciable effects on the improvement of the
conductivity. An ITO-free non-inverted PTB7:PC71BM solar cell is also
fabricated using the proposed post-treated PEDOT:PSS thin films as the
transparent anode. The power conversion efficiency of the resulting cell is
5.8%. The stability of the fabricated ITO-free cells is considerably better
than the stability of the non-treated ITO-free cells or the cells made using
ITO as anode.

###Impact of excitation energy on hot carrier properties in InGaAs MQW structure||Hamidreza Esmaielpour,Laurent Lombez,Maxime Giteau,Jean-Francois Guillemoles,Daniel Suchet###

Impact of excitation energy on hot carrier properties in InGaAs MQW structure. Hot carrier solar cells aim to overcome the theoretical limit of
single-junction photovoltaic devices by suppressing the thermalization of hot
carriers and extracting them through energy selective contacts. Designing
efficient hot carrier absorbers requires further investigation on hot carrier
properties in materials. Although the thermalization of hot carriers is
responsible for a large portion of energy loss in solar cells, it is still one
of the least understood phenomena in semiconductors. Here, the impact of
excitation energy on the properties of photo-generated hot carriers in an
InGaAs multi-quantum well (MQW) structure at various lattice temperatures and
excitation powers is studied. Photoluminescence (PL) emission of the sample is
detected by a hyperspectral luminescence imager, which creates spectrally and
spatially resolved PL maps. The thermodynamic properties of hot carriers, such
as temperature and quasi-Fermi level splitting, are carefully determined via
applying full PL spectrum fitting, which solves the Fermi-Dirac integral and
considers the band-filling effect in the nanostructured material. In addition,
the impact of thermalized power density and carrier scattering with
longitudinal optical phonons on the spectral linewidth broadening under two
excitation energies is studied.

###n-type electrical conduction in SnS thin films||Issei Suzuki,Sakiko Kawanishi,Sage R. Bauers,Andriy Zakutayev,Zexin Lin,Satoshi Tsukuda,Hiroyuki Shibata,Minseok Kim,Hiroshi Yanagi,Takahisa Omata###

n-type electrical conduction in SnS thin films. Tin monosulfide (SnS) usually exhibits p-type conduction due to the low
formation enthalpy of acceptor-type defects, and as a result n-type SnS thin
films have never been obtained. This study realizes n-type conduction in SnS
thin films for the first time by using RF-magnetron sputtering with Cl doping
and sulfur plasma source during deposition. N-type SnS thin films are obtained
at all the substrate temperatures employed in this study (221-341 C),
exhibiting carrier concentrations and Hall mobilities of ~2 x 10 18 cm-3 and
0.1-1 cm V-1s-1, respectively. The films prepared without sulfur plasma source,
on the other hand, exhibit p-type conduction despite containing a comparable
amount of Cl donors. This is likely due to a significant amount of
acceptor-type defects originating from sulfur deficiency in p-type films, which
appears as a broad optical absorption within the band gap. The demonstration of
n-type SnS thin films in this study is a breakthrough for the realization of
SnS homojunction solar cells, which are expected to have a higher conversion
efficiency than the conventional heterojunction SnS solar cells.

###Implementation of MPPT Technique of Solar Module with Supervised Machine Learning||Ruhi Sharmin,Sayeed Shafayet Chowdhury,Farihal Abedin,Kazi Mujibur Rahman###

Implementation of MPPT Technique of Solar Module with Supervised Machine Learning. In this paper, we proposed a method using supervised ML in solar PV system
for MPPT analysis. For this purpose, an overall schematic diagram of a PV
system is designed and simulated to create a dataset in MATLAB/ Simulink. Thus,
by analyzing the output characteristics of a solar cell, an improved MPPT
algorithm on the basis of neural network (NN) method is put forward to track
the maximum power point (MPP) of solar cell modules. To perform the task,
Bayesian Regularization method was chosen as the training algorithm as it works
best even for smaller data supporting the wide range of the train data set. The
theoretical results show that the improved NN MPPT algorithm has higher
efficiency compared with the Perturb and Observe method in the same
environment, and the PV system can keep working at MPP without oscillation and
probability of any kind of misjudgment. So it can not only reduce misjudgment,
but also avoid power loss around the MPP. Moreover, we implemented the
algorithm in a hardware set-up and verified the theoretical result comparing it
with the empirical data.

###Highly conductive charge transport layers impair charge extraction selectivity in thin-film solar cells||Mathias Nyman,Christian Ahläng,Staffan Dahlström,Manasi Pranav,Johannes Benduhn,Syeda Qudsia,Jan-Henrik Smått,Donato Spoltore,Ronald Österbacka###

Highly conductive charge transport layers impair charge extraction selectivity in thin-film solar cells. Charge selective interlayers are crucial in thin-film photovoltaics, such as
organic and Perovskite solar cells. Charge transporting layers (doped and
undoped) constitute perhaps the most important class of charge selective
interlayers; however, it is not well understood how a charge transporting layer
should be designed in order to ensure efficient extraction of majority carriers
while blocking minority carriers. This work clarifies how well
charge-transporting layers with varying majority carrier conductivities block
minority carriers. We use the Charge Extraction by a Linearly Increasing
Voltage technique to determine the surface recombination velocity of minority
carriers in model system devices with varying majority carrier conductivity in
the transporting layer. Our results show that transporting layers with high
conductivity for majority carriers do not block minority carriers - at least
not at operating voltages close to or above the built-in voltage, due to direct
bi-molecular recombination across the transporting layer-absorber layer
interface. We furthermore discuss and propose design principles to achieve
selective charge extraction in thin film solar cells using charge transporting
layers.

###Eliminating the Perovskite Solar Cell Manufacturing Bottleneck via High-Speed Flexography||Julia E. Huddy,Youxiong Ye,William J. Scheideler###

Eliminating the Perovskite Solar Cell Manufacturing Bottleneck via High-Speed Flexography. Perovskite solar cells have potential to deliver terawatt-scale power via
low-cost manufacturing. However, scaling is limited by slow, high-temperature
annealing of the inorganic transport layers and the lack of reliable,
large-area methods for depositing thin (< 30 nm) charge transport layers
(CTLs). We present a method for scaling ultrathin NiOx hole transport layers
(HTLs) by pairing high-speed (60 m/min) flexographic printing with rapidly
annealed sol-gel inks to achieve the fastest reported process for fabrication
of inorganic CTLs for perovskites. By engineering precursor rheology for rapid
film-leveling, NiOx HTLs were printed with high uniformity and ultralow pinhole
densities resulting in photovoltaic performance exceeding that of spin-coated
devices. Integrating these printed transport layers in planar inverted PSCs
allows rapid fabrication of high efficiency (PCE > 15%) Cs(x)FA(1-x)PbI solar
cells with improved short circuit currents (Jsc) of 22.4 mA/cm2. Rapid
annealing of the HTL accelerates total processing time by 60X, while
maintaining the required balance of optoelectronic properties and the chemical
composition for effective hole collection. These results build an improved
understanding of ultrathin NiOx and reveal opportunities to enhance device
performance via scalable manufacturing of inorganic CTLs.

###Worldwide Energy Harvesting Potential of Hybrid CPV/PV Technology||Juan F. Martínez,Marc Steiner,Maike Wiesenfarth,Henning Helmers,Gerald Siefer,Stefan W. Glunz,Frank Dimroth###

Worldwide Energy Harvesting Potential of Hybrid CPV/PV Technology. Hybridization of multi-junction concentrator photovoltaics with
single-junction flat plate solar cells (CPV/PV) can deliver the highest power
output per module area of any PV technology. Conversion efficiencies up to
34.2% have been published under the AM1.5g spectrum at standard test conditions
for the EyeCon module which combines Fresnel lenses and III-V four-junction
solar cells with bifacial c-Si. We investigate here its energy yield and
compare it to conventional CPV as well as flat plate PV. The advantage of the
hybrid CPV/PV module is that it converts direct sunlight with the most advanced
multi-junction cell technology, while accessing diffuse, lens-scattered and
back side irradiance with a Si cell that also serves as the heat distributor
for the concentrator cells. This article quantifies that hybrid bifacial CPV/PV
modules are expected to generate a 25 - 35% higher energy yield with respect to
their closest competitor in regions with a diffuse irradiance fraction around
50%. Additionally, the relative cost of electricity generated by hybrid CPV/PV
technology was calculated worldwide under certain economic assumptions.
Therefore, this article gives clear guidance towards establishing competitive
business cases for the technology.

###Efficient Passivation of Surface Defects by Lewis Base in Lead-free Tin-based Perovskite Solar Cells||Hejin Yan,Bowen Wang,Xuefei Yan,Qiye Guan,Hongfei Chen,Zheng Shu,Dawei Wen,Yongqing Cai###

Efficient Passivation of Surface Defects by Lewis Base in Lead-free Tin-based Perovskite Solar Cells. Lead-free tin-based perovskites are highly appealing for the next generation
of solar cells due to their intriguing optoelectronic properties. However, the
tendency of Sn2+ oxidation to Sn4+ in the tin-based perovskites induces serious
film degradation and performance deterioration. Herein, we demonstrate, through
the density functional theory based first-principle calculations in a surface
slab model, that the surface defects of the Sn-based perovskite FASnI3 (FA =
NH2CHNH2+) could be effectively passivated by the Lewis base molecules. The
passivation performance of Lewis base molecules in tin-based perovskite is
tightly correlated with their molecular hardness. We reveal that the degree of
hardness of Lewis adsorbate governs the stabilization via dual effects: first,
changing the stubborn spatial distribution of tin vacancy (VSn) by triggering
charge redistribution; second, saturating the dangling states while
simultaneously reducing the amounts of deep band gap states. Specifically, the
hard Lewis base molecules like edamine (N-donor group) and Isatin-Cl (Cl-donor
group) would show a better healing effect than other candidates on the
defects-contained tin-based perovskite surface with a somehow hard Lewis acid
nature. Our research provides a general strategy for additive engineering and
fabricating stable and high-efficiency lead-free Sn-based perovskite solar
cells.

###Optimization of p-i-n GaAs/AlGaAs Heterojunction Nanowire Solar Cell for improved Optical and Electrical Properties||Sambuddha Majumder,Sooraj Ravindran###

Optimization of p-i-n GaAs/AlGaAs Heterojunction Nanowire Solar Cell for improved Optical and Electrical Properties. In this study, we designed and optimized the performance of pin junction
GaAs/AlGaAs heterojunction nanowire solar cell arrays. It is done by performing
coupled optoelectronic simulations to find the optimal doping for the GaAs core
and AlGaAs shell, and to see the influence of GaAs and AlGaAs shell thickness
and junction positions on the solar cell performance. Further, the impact of
different surface effects that exists at the semiconductor interface such as
surface traps, surface recombination velocities, and associated lifetime
degradation are also investigated. It has been observed that a high core and
shell doping is essential to achieve the appropriate band configuration and
carrier extraction. Further, it is observed that having a larger doping density
is more important than having a larger lifetime. The importance of the
thickness and the passivation properties of the radial and axial AlGaAs layer
is also examined and it has been observed that having a thick AlGaAs shell at
the cost of the i-GaAs region can be detrimental to the performance due to
increased local carrier generation and recombination. Finally, the effect of
having different Aluminium compositions (on the shell) on the photogeneration
inside the nanowire is examined and it was observed that having a large
Aluminium composition can confine most of the photogeneration to the inner GaAs
regions, thus potentially allowing for thicker Aluminium shells which can more
efficiently prevent surface recombination.

###Enhancement of Hot Carrier Effects and Signatures of Confinement in Terms of Thermalization Power in Quantum Well Solar Cells||Imam Makhfudz,Nicolas Cavassilas,Maxime Giteau,Hamidreza Esmaielpour,Daniel Suchet,Anne-Marie Daré,Fabienne Michelini###

Enhancement of Hot Carrier Effects and Signatures of Confinement in Terms of Thermalization Power in Quantum Well Solar Cells. A theoretical model using electron-phonon scattering rate equations is
developed for assessing carrier thermalization under steady-state conditions in
two-dimensional systems. The model is applied to investigate the hot carrier
effect in III-V hot-carrier solar cells with a quantum well absorber. The
question underlying the proposed investigation is: what is the power required
to maintain two populations of electron and hole carriers in a
quasi-equilibrium state at fixed temperatures and quasi-Fermi level splitting?
The obtained answer is that the thermalization power density is reduced in
two-dimensional systems compared to their bulk counterpart, which demonstrates
a confinement-induced enhancement of the hot carrier effect in quantum wells.
This power overall increases with the well thickness, and it is moreover shown
that the intra-subband contribution dominates at small thicknesses while the
inter-subband contribution increases with thickness and dominates in the bulk
limit. Finally, the effects of the thermodynamic state of phonons and screening
are clarified. In particular, the two-dimensional thermalization power density
exhibits a non-monotonic dependence on the thickness of the quantum well layer,
when both out-of-equilibrium longitudinal optical phonons and screening effects
are taken into account. Our theoretical and numerical results provide tracks to
interpret intriguing experimental observations in quantum well physics. They
will also offer guidelines to increase the yield of photovoltaic effect based
on the hot carrier effect using quantum well heterostructures, a result
critical to the research toward high-efficiency solar cell devices.

###Simulating multiple quantum well solar cells||James P. Connolly,Jenny Nelson,Keith W. J. Barnham,Ian Ballard,C. Roberts,J. S. Roberts,C. T. Foxon,.###

Simulating multiple quantum well solar cells. The quantum well solar cell (QWSC) has been proposed as a route to higher
efficiency than that attainable by homojunction devices. Previous studies have
established that carriers escape the quantum wells with high efficiency in
forward bias and contribute to the photocurrent. Progress in resolving the
efficiency limits of these cells has been dogged by the lack of a theoretical
model reproducing both the enhanced carrier gen- eration and enhanced
recombination due to the quantum wells. Here we present a model which
calculates the incremental generation and recombination due to the QWs and is
verified by modelling the experimental light and dark current-voltage
characteristics of a range of III-V quantum well structures. We find that
predicted dark currents are significantly greater than experiment if we use
lifetimes derived from homostructure devices. Successful simulation of light
and dark currents can be obtained only by introducing a parameter which
represents a reduction in the quasi-Fermi level separation.

###Transport and Spectroscopic Studies of the Effects of Fullerene Structure on the Efficiency and Lifetime of Polythiophene-based Solar Cells||Emilee L. Sena,Justin H. Peel,Devin Wesenberg,Shreya Nathan,Marianne Wallis,Maxwell J. Giammona,Thorsteinn Adalsteinsson,Brian J. McNelis,Richard P. Barber Jr###

Transport and Spectroscopic Studies of the Effects of Fullerene Structure on the Efficiency and Lifetime of Polythiophene-based Solar Cells. Time-dependent measurements of both power conversion efficiency and
ultraviolet-visible absorption spectroscopy have been observed for solar cell
blends containing the polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) with two
different functionalized C60 electron acceptor molecules: commercially
available [6,6]-phenyl C61 butyric acid methyl ester (PCBM) or [6,6]-phenyl C61
butyric acid octadecyl ester (PCBOD) produced in this laboratory. Efficiency
was found to decay with an exponential time dependence, while spectroscopic
features show saturating exponential behavior. Time constants extracted from
both types of measurements showed reasonable agreement for samples produced
from the same blend. In comparison to the PCBM samples, the stability of the
PCBOD blends was significantly enhanced, while both absorption and power
conversion efficiency were decreased.

###Multijunction solar cells efficiency simulation||A. V. Sachenko,V. P. Kostylyov,N. P. Kulish,I. O. Sokolovskyi,A. I. Shkrebtii###

Multijunction solar cells efficiency simulation. The radiative recombination, Shokley-Read recombination, frontal-surface and
rear-surface recombination and the recombination at the heterojunction
boundaries and the recombination in the space charge region are considered in
the calculation of the multijunction solar cell (MSC) efficiency. The
calculation is performed by a self-consistent solution of the equations for the
photocurrent and photovoltage, as well as the heat balance equation. A cooling
of MSC with the increase of the number of cells n and the improvement in the
heat dissipation is regarded. It was found that, as the number of cells n is
increased, narrowing of spectral range for each cell causes additional
reduction of current. A substantial increase in the MSC efficiency can be
achieved by improving the heat extraction using radiators and increasing
emissivity. A comparison is made between the calculated and experimental
efficiency values. A rather good agreement was found. A comparison between this
calculation and other formalisms is given.

###Network analysis of the performance of organic photovoltaic cells: The open circuit voltage and the zero current efficiency||Mario Einax,Abraham Nitzan###

Network analysis of the performance of organic photovoltaic cells: The open circuit voltage and the zero current efficiency. Photovoltaic energy conversion in photovoltaic cells has been analyzed by the
detailed balance approach or by thermodynamic arguments. Here we introduce a
network representation to analyze the performance of such systems once a
suitable kinetic model (represented by a master equation in the space of the
different system states) has been constructed. Such network representation
allows one to decompose the steady state dynamics into cycles, characterized by
their cycle affinities. The maximum achievable efficiency of the device is
obtained in the zero affinity limit. This method is applied to analyze a
microscopic model for a bulk heterojunction organic solar cell that includes
the essential optical and interfacial electronic processes that characterize
this system, leading to an explicit expression for the theoretical efficiency
limit in such system. In particular, the deviation from Carnot's efficiency
associated with the exciton binding energy is quantified.

###Resistance and lifetime measurements of polymer solar cells using glycerol doped poly[3,4-ethylenedioxythiophene]: poly[styrenesulfonate] hole injection layers||Emma Lewis,Bhaskar Mantha,Richard P. Barber Jr###

Resistance and lifetime measurements of polymer solar cells using glycerol doped poly[3,4-ethylenedioxythiophene]: poly[styrenesulfonate] hole injection layers. We have performed resistivity measurements of
poly[3,4-ethylenedioxythiophene]: poly[styrenesulfonate] (PEDOT:PSS) films with
varying concentrations of glycerol. Resistivity is seen to decrease
exponentially from roughly 3 ohm-cm for pure PEDOT:PSS to 3x10-2 ohm-cm for 35
mg/cm3 glycerol in PEDOT:PSS. Beyond this concentration adding glycerol does
not significantly change resistivity. Bulk heterojunction polymer solar cells
using these variously doped PEDOT:PSS layers as electrodes were studied to
characterize the effects on efficiency and lifetime. Although our data display
significant scatter, lowering the resistance of the PEDOT:PSS layers results in
lower device resistance and higher efficiency as expected. We also note that
the lifetime of the devices tends to be reduced as the glycerol content of
PEDOT:PSS is increased. Many devices show an initial increase in efficiency
followed by a roughly exponential decay. This effect is explained based on
concomitant changes in the zero bias conductance of the samples under dark
conditions.

###Electron beam induced current in photovoltaics with high recombination||Paul M. Haney,Heayoung P. Yoon,Prakash Koirala,Robert W. Collins,Nikolai B. Zhitenev###

Electron beam induced current in photovoltaics with high recombination. Electron beam induced current (EBIC) is a powerful characterization technique
which offers the high spatial resolution needed to study polycrystalline solar
cells. Ideally, an EBIC measurement reflects the spatially resolved quantum
efficiency of the device. In this work, a model for EBIC measurements is
presented which applies when recombination within the depletion region is
substantial. This model is motivated by cross-sectional EBIC experiments on
CdS-CdTe photovoltaic cells which show that the maximum efficiency of carrier
collection is less than 100 \% and varies throughout the depletion region. The
model can reproduce experimental results only if the mobility-lifetime product
$\mu\tau$ is spatially varying within the depletion region. The reduced
collection efficiency is speculated to be related to high-injection effects,
and the resulting increased radiative recombination.

###Optimizing Photovoltaic Charge Generation of Nanowire Arrays: A Simple Semi-Analytic Approach||Björn C. P. Sturmberg,Kokou B. Dossou,Lindsay C. Botten,Ara A. Asatryan,Christopher G. Poulton,Ross C. McPhedran,C. Martijn de Sterke###

Optimizing Photovoltaic Charge Generation of Nanowire Arrays: A Simple Semi-Analytic Approach. Nanowire arrays exhibit efficient light coupling and strong light trapping,
making them well suited to solar cell applications. The processes that
contribute to their absorption are interrelated and highly dispersive, so the
only current method of optimizing the absorption is by intensive numerical
calculations. We present an efficient alternative which depends solely on the
wavelength-dependent refractive indices of the constituent materials. We choose
each array parameter such that the number of modes propagating away from the
absorber is minimized while the number of resonant modes within the absorber is
maximized. From this we develop a semi-analytic method that quantitatively
identifies the small range of parameters where arrays achieve maximum short
circuit currents. This provides a fast route to optimizing NW array cell
efficiencies by greatly reducing the geometries to study with full device
models. Our approach is general and applies to a variety of materials and to a
large range of array thicknesses.

###Performance Analysis and Fault Diagnosis Method for Concentrator Photovoltaic Modules||Harsh G. Kamath,Nicholas J. Ekins-Daukes,Kenji Araki,Sheela K. Ramasesha###

Performance Analysis and Fault Diagnosis Method for Concentrator Photovoltaic Modules. Concentrator Photovoltaic (CPV) systems use high efficiency multi-junction
solar cells with efficiencies >40%, but the module efficiency is often much
lower. The increased complexity of a CPV module, with optics, receiver and the
tracker gives an increased probability that faults will arise during the
operational lifetime. In addition, a location like India has varied atmospheric
conditions that further complicates the diagnosis of faults. It is therefore
important to decouple effects due to the external environment (such as the
atmosphere) from effects due to the degradation of the module. By applying a
computer model to outdoor CPV test data in Bangalore, India we have established
a method to assess the performance of the CPV module and finally we present a
method to diagnose faults in the module.

###High Responsivity and Quantum Efficiency of Graphene / Silicon Photodiodes Achieved by Interdigitating Schottky and Gated Regions||Sarah Riazimehr,Satender Kataria,Jose-Maria González-Medina,Mehrdad Shaygan,Stephan Suckow,Francisco G. Ruiz,Olof Engström,Andres Godoy,Max Christian Lemme###

High Responsivity and Quantum Efficiency of Graphene / Silicon Photodiodes Achieved by Interdigitating Schottky and Gated Regions. Graphene / silicon (G/Si) heterostructures have been studied extensively in
the past years for applications such as photodiodes, photodetectors and solar
cells, with a growing focus on efficiency and performance. Here, a specific
contact pattern scheme with interdigitated Schottky and
graphene/insulator/silicon (GIS) structures is explored to experimentally
demonstrate highly sensitive G/Si photodiodes. With the proposed design, an
external quantum efficiency (EQE) of > 80 % is achieved for wavelengths ranging
from 380 to 930 nm. A maximum EQE of 98% is observed at 850 nm, where the
responsivity peaks to 635 mA/W, surpassing conventional Si p-n photodiodes.
This efficiency is attributed to the highly effective collection of charge
carriers photogenerated in Si under the GIS parts of the diodes. The
experimental data is supported by numerical simulations of the diodes. Based on
these results, a definition for the true active area in G/Si photodiodes is
proposed, which may serve towards standardization of G/Si based optoelectronic
devices.

###Inverted structure perovskite solar cells: A theoretical study||Anurag Sahu,Ambesh Dixit###

Inverted structure perovskite solar cells: A theoretical study. We analysed perovskite CH3NH3PbI3-xClx inverted planner structure solar cell
with nickel oxide (NiO) and spiro-MeOTAD as hole conductors. This structure is
free from electron transport layer. The thickness is optimized for NiO and
spiro-MeOTAD hole conducting materials and the devices do not exhibit any
significant variation for both hole transport materials. The back metal contact
work function is varied for NiO hole conductor and observed that Ni and Co
metals may be suitable back contacts for efficient carrier dynamics. The solar
photovoltaic performance showed a linear decrease in efficiency with increasing
temperature. The electron affinity and band gap of transparent conducting oxide
and NiO layers are varied to understand their impact on conduction and valence
band offsets. A range of suitable band gap and electron affinity values are
found essential for efficient device performance.

###Efficiency Increase in Multijunction Monochromatic Photovoltaic Devices Due to Luminescent Coupling||Daixi Xia,Jacob J. Krich###

Efficiency Increase in Multijunction Monochromatic Photovoltaic Devices Due to Luminescent Coupling. We present a multijunction detailed balance model that includes the effects
of luminescent coupling, light trapping and nonradiative recombination,
suitable for treatment of multijunction solar cells and photonic power
converters -- photovoltaic devices designed to convert narrow-band light. The
model includes both specular and Lambertian reflections using a ray-optic
formalism and treats nonradiative processes using an internal radiative
efficiency. Using this model, we calculate and optimize the efficiency of
multijunction photonic power converters for a range of material qualities and
light-trapping schemes. Multijunction devices allow increased voltage with
lower current, decreasing series resistance losses. We show that efficiency
increases significantly with increased number of junctions, even without series
resistance, when the device has an absorbing substrate. Such an increase does
not occur when the device has a back reflector. We explain this effect using a
simplified model, which illustrates the origin of the decreased radiative
losses in multijunction devices on substrates.

###Carrier Multiplication via Photocurrent Measurements in Dual-Gated MoTe_2||Jun Suk Kim,Minh Dao Tran,Sung-Tae Kim,Daehan Yoo,Sang-Hyun Oh,Ji-Hee Kim,Young Hee Lee###

Carrier Multiplication via Photocurrent Measurements in Dual-Gated MoTe_2. Although van der Waals layered transition metal dichalcogenides from
transient absorption spectroscopy have successfully demonstrated an ideal
carrier multiplication (CM) performance with an onset of nearly
2Eg,interpretation of the CM effect from the optical approach remains
unresolved owing to the complexity of many-body electron-hole pairs. We
demonstrate the CM effect through simple photocurrent measurements by
fabricating the dual-gate P-N junction of a MoTe2 film on a transparent
substrate. Electrons and holes were efficiently extracted by eliminating the
Schottky barriers in the metal contact and minimizing multiple reflections. The
photocurrent was elevated proportionately to the excitation energy. The boosted
quantum efficiency confirms the multiple electron-hole pair generation of >2Eg,
consistent with CM results from an optical approach, pushing the solar cell
efficiency beyond the Shockley-Queisser limit.

###Fullerene-Based Transparent Solar Cells with Average Visible Transmission Exceeding 80%||Ruiqian Meng,Qianqing Jiang,Dianyi Liu###

Fullerene-Based Transparent Solar Cells with Average Visible Transmission Exceeding 80%. Transparent photovoltaic (TPV) devices have the great potential to apply as
smart windows in the construction and agriculture field. The efficiencies of
TPVs are growing up quickly in recent years and the champion efficiency even
exceeds 10%. However, the transparency is still hard to further improved after
the average visible transmission (AVT) achieved 73%. Each component of the TPV
devices will influent the transparency of the TPV. To date, the TPV with the
AVT over 80% has not been reported yet. In this work, we describe the
fullerene-based highly transparent solar cells. The CuSCN/C60 heterojunction is
used as the effective light absorber. By finely optimizing the thickness of
fullerene films and introducing the highly transparent electrodes, the TPV
exhibits the AVT up to 82% while the device efficiency is above 0.3%. This
study affords a new avenue to construct highly transparent TPV device.

###Efficient Extraction of Hot Carriers in Perovskite Quantum Dot through Building State Coupled Complex||Yusheng Li,Junke Jiang,Dandan Wang,Dong Liu,Shota Yajima,Hua Li,Akihito Fuchimoto,Hongshi Li,Guozheng Shi,Shuzi Hayase,Shuxia Tao,Jiangjian Shi,Qingbo Meng,Chao Ding,Qing Shen###

Efficient Extraction of Hot Carriers in Perovskite Quantum Dot through Building State Coupled Complex. Utilizing hot carriers is the crucial approach for solar cell to exceed the
thermodynamic detailed balance limit, yet effective extraction of hot carriers
in absorber materials via most commonly used semiconductor acceptors has been a
challenge in both materials and photophysics research for many years. Herein,
we build series of CsPbI3 quantum dot and fullerene derivative systems to
explore the decisive factors of this process and have for the first time
realized efficient hot carrier extraction in these systems (maximum extraction
efficiency ~ 84%). We find building the systems as state-coupled complexes
creates new carrier transport channels at about 0.22 eV above CsPbI3 quantum
dot bandgap, which facilitates highly efficient HC extraction. Our research
directly visualizes the inner connection of molecule interaction and ultrafast
hot carrier extraction. The knowledge and strategy gained here are of universal
meaning, taking an important step forward true hot carrier photovoltaics.

###Phononic engineering with nanostructures for hot carrier solar cells||Jean Francois Guillemoles,Gavin Conibeer,Martin Green###

Phononic engineering with nanostructures for hot carrier solar cells. Hot Carrier solar cells have long been recognized as an attractive contender
in the search for high efficiency photovoltaic devices but their fabrication
requires solution of two important material challenges: finding materials with
drastically reduced carrier cooling rates and realization of selective energy
contacts to extract the photogenerated carriers. This paper is concerned with
the problem of absorber phononic engineering to reduce carrier cooling rates.
The physics of carrier cooling is explored and experimental data of other
authors are discussed with a view to assessing the potential of state-of-the
art nanostructured materials for PV conversion. A tentative initial calculation
based on the thermalisation in these nanostructures and assuming all other
aspects as ideal, gives 54% efficiency at 2500suns as compared to 54% for no
thermalisation at all. Phononic band gap engineering to further reduce carrier
cooling or bring down the threshold concentration is discussed.

###Optical orientation in bipolar spintronic devices||Jaroslav Fabian,Igor Zutic###

Optical orientation in bipolar spintronic devices. Optical orientation is a highly efficient tool for the generation of
nonequilibrium spin polarization in semiconductors. Combined with
spin-polarized transport it offers new functionalities for conventional
electronic devices, such as pn junction bipolar diodes or transistors. In
nominally nonmagnetic junctions optical orientation can provide a source for
spin capacitance--the bias-dependent nonequilibrium spin accumulation--or for
spin-polarized current in bipolar spin-polarized solar cells. In magnetic
junctions, the nonequilibrium spin polarization generated by spin orientation
in a proximity of an equilibrium magnetization gives rise to the spin-voltaic
effect (a realization of the Silsbee-Johnson coupling), enabling efficient
control of electrical properties such as the I-V characteristics of the
junctions by magnetic and optical fields. This article reviews the main results
of investigations of spin-polarized and magnetic pn junctions, from spin
capacitance to the spin-voltaic effect.

###Influence of the pattern shape on the photonic efficiency of front-side periodically patterned ultrathin crystalline silicon solar cells||Aline Herman,Christos Trompoukis,Valerie Depauw,Ounsi El Daif,Olivier Deparis###

Influence of the pattern shape on the photonic efficiency of front-side periodically patterned ultrathin crystalline silicon solar cells. Patterning the front side of an ultra-thin crystalline silicon (c Si) solar
cell helps keeping the energy conversion efficiency high by compensating for
the light absorption losses. A super-Gaussian mathematical expression was used
in order to encompass a large variety of nanopattern shapes and to study their
influence on the photonic performance. We prove that the enhancement in the
maximum achievable photo-current is due to both impedance matching condition at
short wavelengths and to the wave nature of light at longer wavelengths. We
show that the optimal mathematical shape and parameters of the pattern depend
on the c Si thickness. An optimal shape comes with a broad optimal parameter
zone where fabricating errors would have much less influence on the efficiency.
We prove that cylinders are not the best suited shape. To compare our model
with a real slab, we fabricated a nanopatterned c Si slab via Nano Imprint
Lithography.

###Direct optical measurement of light coupling into planar waveguide by plasmonic nanoparticles||Antti M. Pennanen,J. Jussi Toppari###

Direct optical measurement of light coupling into planar waveguide by plasmonic nanoparticles. Coupling of light into a thin layer of high refractive index material by
plasmonic nanoparticles has been widely studied for application in photovoltaic
devices, such as thin-film solar cells. In numerous studies this coupling has
been investigated through measurement of e.g. quantum efficiency or
photocurrent enhancement. Here we present a direct optical measurement of light
coupling into a waveguide by plasmonic nanoparticles. We investigate the
coupling efficiency into the guided modes within the waveguide by illuminating
the surface of a sample, consisting of a glass slide coated with a high
refractive index planar waveguide and plasmonic nanoparticles, while directly
measuring the intensity of the light emitted out of the waveguide edge. These
experiments were complemented by transmittance and reflectance measurements. We
show that the light coupling is strongly affected by thin-film interference,
localized surface plasmon resonances of the nanoparticles and the illumination
direction (front or rear).

###Influence of morphology on the plasmonic enhancement effect of Au@TiO2 core-shell nanoparticles in dye-sensitized solar cells||Wei-Liang Liu,Fan-Cheng Lin,Yu-Chen Yang,Chen-Hsien Huang,Shangjr Gwo,Michael H. Huang,Jer-Shing Huang###

Influence of morphology on the plasmonic enhancement effect of Au@TiO2 core-shell nanoparticles in dye-sensitized solar cells. Plasmonic core-shell nanoparticles (PCSNPs) can function as nanoantennas and
improve the efficiency of dye-sensitized solar cells (DSSCs). To achieve
maximum enhancement, the morphology of PCSNPs need to be optimized. Here we
precisely control the morphology of Au@TiO2 PCSNPs and systematically study its
influence on the plasmonic enhancement effect. Enhancement mechanism was found
to vary with the thickness of TiO2 shell. PCSNPs with thinner shell enhance the
current due to plasmonic effect, whereas particles with thicker shell improve
the voltage due to increasing semiconducting character. Wavelength-independent
enhancement in the visible range was observed and attributed to plasmonic
heating effect. PCSNPs with 5-nm shell give highest efficiency enhancement of
23%. Our work provides a new synthesis route for well-controlled Au@TiO2
core-shell nanoparticles and gains insight into the plasmonic enhancement in
DSSCs.

###Performance enhancement of TiO2-based dye-sensitized solar cells by carbon nanospheres in photoanode||Elham Bayatloo,Esmaiel Saievar-Iranizad###

Performance enhancement of TiO2-based dye-sensitized solar cells by carbon nanospheres in photoanode. The conversion efficiency of dye-sensitized solar cells (DSSCs) is optimized
by modifying the optical design and improving absorbance within the cell. These
objectives are obtained by creating different sized cavities in TiO2
photoanode. For this purpose, carbon nanospheres with diameters 100-600 nm are
synthesized by hydrothermal method. A paste of TiO2 is mixed with various
amounts of carbon nanospheres. During TiO2 photoanode sintering processes at
500C temperature, the carbon nanospheres are removed. This leads to random
creation of cavities in the DSSCs photoanode. These cavities enhance light
scattering and porosity which improve light absorbance by dye N719 and provide
a larger surface area for dye loading. These consequences enhance performance
of DSSCs. By mixing 3% Wt. carbon nanospheres in the TiO2 pastes, we were able
to increase the short circuit current density and efficiency by 40% (from 12.59
to 17.73 mA/cm2) and 33% (from 5.72% to 7.59%), respectively.

###Hybrid photovoltaic and electron-tunneling converters||Shanhe Su,Jincan Chen,Tien-Mo Shih###

Hybrid photovoltaic and electron-tunneling converters. Photon impingement is capable of liberating electrons in semiconductors. When
the electron transport is primarily governed by temperature gradients, high
irreversibilities will result, thus lowering converters' efficiencies. A
fundamental study in the absence of photovoltaics\cite{1} has achieved the
reduction of these irreversibilities by considering entropy changes due to
electron flows. Here we present an unreported mechanism that integrates
photovoltaic conversion and electron tunneling. Photon-excited electrons that
occupy energy levels beyond windowed limits are first imprisoned inside the
cathode, then given opportunities to rapidly re-thermalize, and eventually
allowed to enter the tunnel. Energies wasted by both the irreversibility and
the recombination are minimized with respect to the transmission energy and the
transmission window that characterize the tunnel. Upon application of this
mechanism to high-concentration solar cells, the proposed hybrid model
outperforms others. It further provides a guide for elevating efficiencies in
future photon-to-electron converters typified by third-generation photovoltaic
systems.

###Optoelectronic excitations and photovoltaic effect in strongly correlated materials||John E. Coulter,Efstratios Manousakis,Adam Gali###

Optoelectronic excitations and photovoltaic effect in strongly correlated materials. Solar cells based on conventional semiconductors have low efficiency in
converting solar energy into electricity because the excess energy beyond the
gap of an incident solar photon is converted into heat by phonons. Here we show
by ab initio methods that the presence of strong Coulomb interactions in
strongly correlated insulators (SCI) causes the highly photo-excited
electron-hole pair to decay fast into multiple electron-hole pairs via impact
ionization (II). We show that the II rate in the insulating $M_1$ phase of
vanadium dioxide (chosen for this study as it is considered a prototypical SCI)
is two orders of magnitude higher than in Si and much higher than the rate of
hot electron/hole decay due to phonons. Our results indicate that a rather
broad class of materials may be harnessed for an efficient solar-to-electrical
energy conversion that has been not considered before.

###Revealing the role of organic cations in hybrid halide perovskites CH3NH3PbI3||Carlo Motta,Fedwa El Mellouhi,Sabre Kais,Nouar Tabet,Fahhad Alharbi,Stefano Sanvito###

Revealing the role of organic cations in hybrid halide perovskites CH3NH3PbI3. The hybrid halide perovskite CH$_{3}$NH$_{3}$PbI$_{3}$ has enabled solar
cells to reach an efficiency of about 18\%, demonstrating a pace for
improvements with no precedents in the solar energy arena. Despite such
explosive progress, the microscopic origin behind the success of such material
is still debated, with the role played by the organic cations in the
light-harvesting process remaining unclear. Here van-der-Waals-corrected
density functional theory calculations reveal that the orientation of the
organic molecules plays a fundamental role in determining the material
electronic properties. For instance, if CH$_{3}$NH$_{3}$ orients along a
(011)-like direction, the PbI$_{6}$ octahedral cage will distort and the band
gap will become indirect. Our results suggest that molecular rotations, with
the consequent dynamical change of the band structure, might be at the origin
of the slow carrier recombination and the superior conversion efficiency of
CH$_{3}$NH$_{3}$PbI$_{3}$.

###On fundamental mechanisms in dye sensitized solar cells through the behaviour of different mesoporous titanium dioxide films||Lidice Vaillant Roca,Elena Vigil,Fresnel Forcade,Thierry Thami,Hania Adnani,Christelle Yacou,André Ayral,Pierre Saint-Grégoire###

On fundamental mechanisms in dye sensitized solar cells through the behaviour of different mesoporous titanium dioxide films. Understanding mechanisms in DSSCs is fundamental for their improvement; this
includes the nanocrystalline semiconducting layer behaviour. Different
mesoporous TiO2 layers are fabricated and analyzed for possible use in DSSC
solar cells. The preparations included the addition of P123 triblock copolymer
as structuring agent to the synthesized anatase sol. This preparation was also
mixed with Degussa P25 nanoparticles in one case and polystyrene latex in
another. Mesoporous mixed TiO2-SiO2 thin layers were also analyzed. The diverse
morphology and features are studied by microscopic techniques and by means of
spectral quantum efficiency of a photoelectrochemical cell (PEC) that uses as
photoelectrode the unsensitized porous TiO\_2 layer. Contact angle measurements
are also performed. We have found that a very high specific area due to very
small nanocrystals and small pores can hinder electrolyte penetration in the
pores formed by TiO\_2 nanograins, affecting photoelectrodes efficiency.

###Photovoltaic effect in individual asymmetrically contacted lead sulfide nanosheets||Sedat Dogan,Thomas Bielewicz,Vera Lebedeva,Christian Klinke###

Photovoltaic effect in individual asymmetrically contacted lead sulfide nanosheets. Solution-processable, two-dimensional semiconductors are promising
optoelectronic materials which could find application in low-cost solar cells.
Lead sulfide nanocrystals raised attention since the effective band gap can be
adapted over a wide range by electronic confinement and observed multi-exciton
generation promises higher efficiencies. We report on the influence of the
contact metal work function on the properties of transistors based on
individual two-dimensional lead sulfide nanosheets. Using palladium we observed
mobilities of up to 31 cm2/Vs. Furthermore, we demonstrate that asymmetrically
contacted nanosheets show photovoltaic effect and that the nanosheets' height
has a decisive impact on the device performance. Nanosheets with a thickness
5.4 nm contacted with platinum and titanium show a power conversion efficiency
of up to 0.94 % (EQE 75.70 %). The results underline the high hopes put on such
materials.

###Integration of a 2D Periodic Nanopattern Into Thin Film Polycrystalline Silicon Solar Cells by Nanoimprint Lithography||Islam Abdo,Christos Trompoukis,Jan Deckers,Valérie Depauw,Loic Tous,Dries Van Gestel,Rafik Guindi,Ivan Gordon,Ounsi El Daif###

Integration of a 2D Periodic Nanopattern Into Thin Film Polycrystalline Silicon Solar Cells by Nanoimprint Lithography. The integration of two-dimensional (2D) periodic nanopattern defined by
nanoimprint lithography and dry etching into aluminum induced crystallization
(AIC) based polycrystalline silicon (Poly-Si) thin film solar cells is
investigated experimentally. Compared to the unpatterned cell an increase of 6%
in the light absorption has been achieved thanks to the nanopattern which, in
turn, increased the short circuit current from 20.6 mA/cm2 to 23.8 mA/cm2. The
efficiency, on the other hand, has limitedly increased from 6.4% to 6.7%. We
show using the transfer length method (TLM) that the surface topography
modification caused by the nanopattern has increased the sheet resistance of
the antireflection coating (ARC) layer as well as the contact resistance
between the ARC layer and the emitter front contacts. This, in turn, resulted
in increased series resistance of the nanopatterned cell which has translated
into a decreased fill factor, explaining the limited increase in efficiency.

###Strain-balanced type-II superlattices for efficient multi-junction solar cells||A. Gonzalo,A. D. Utrilla,D. F. Reyes,V. Braza,J. M. Llorens,D. Fuertes Marron,B. Alen,T. Ben,D. Gonzalez,A. Guzman,A. Hierro,J. M. Ulloa###

Strain-balanced type-II superlattices for efficient multi-junction solar cells. We propose type-II GaAsSb/GaAsN superlattices (SLs) lattice-matched to GaAs
as a novel material for the 1 eV sub-cells present in highly efficient
GaAs/Ge-based multi-junction solar cells. We demonstrate that, among other
benefits, the spatial separation of Sb and N allows a better control over
composition and lattice matching, avoiding the growth problems related to the
concomitant presence of both elements in GaAsSbN layers. This approach not only
reduces clustering and improves crystal quality and interface abruptness, but
also allows for additional control of the effective bandgap in the 1.0-1.15 eV
spectral region through the SL period thickness. The optimized SL structure
exhibits a type-II band alignment and strong electronic coupling at 0 V. Both
effects cooperate to increase the minority carrier collection and leads to a
net strong enhancement of the external quantum efficiency (EQE) under
photovoltaic conditions with respect to bulk layers of equivalent thickness.

###Photocurrent enhancement of spin coated CdS thin films by adding Cu||P. Samarasekara,B. M. M. B. Basnayaka,Sunil Dehipawala###

Photocurrent enhancement of spin coated CdS thin films by adding Cu. Cu added CdS films were synthesized using spin coating technique at different
spin speeds for different time durations. Films were subsequently annealed at
different temperatures for different time periods in air to crystallize the
phase of CdS in thin film form. Films were characterized using XRD, UV- visible
spectrometer and solar simulator. According to XRD patterns, addition of trace
amount of Cu did not change the structure of CdS. However, the optical band gap
gradually decreases with percentage of Cu as expected. As a result, the
photocurrent, fill factor and efficiency measured in KI/I2 electrolyte
gradually increase with the amount of Cu. Photovoltaic properties could be
improved without altering the structure of CdS. Efficiency enhanced CdS films
find potential applications in solar cell industry.

###Time resolved photoemission spectroscopy of electronic cooling and localization in CH$_3$NH$_3$PbI$_3$ crystals||Zhesheng Chen,Min-i Lee,Zailan Zhang,Hiba Diab,Damien Garrot,Ferdinand Lédée,Pierre Fertey,Evangelos Papalazarou,Marino Marsi,Carlito Ponseca,Emmanuelle Deleporte,Antonio Tejeda,Luca Perfetti###

Time resolved photoemission spectroscopy of electronic cooling and localization in CH$_3$NH$_3$PbI$_3$ crystals. We measure the surface of CH$_3$NH$_3$PbI$_3$ single crystals by making use
of two photon photoemission spectroscopy. Our method monitors the electronic
distribution of photoexcited electrons, explicitly discriminating the initial
thermalization from slower dynamical processes. The reported results disclose
the fast dissipation channels of hot carriers (0.25 ps), set a upper bound to
the surface induced recombination velocity ($<4000$ cm/s) and reveal the
dramatic effect of shallow traps on the electrons dynamics. The picosecond
localization of excited electrons in degraded CH$_3$NH$_3$PbI$_3$ samples is
consistent with the progressive reduction of photoconversion efficiency in
operating devices. Minimizing the density of shallow traps and solving the
aging problem may boost the macroscopic efficiency of solar cells to the
theoretical limit.

###Modelling of limitations of bulk heterojunction architecture in organic solar cells||Jacek Wojtkiewicz,Marek Pilch###

Modelling of limitations of bulk heterojunction architecture in organic solar cells. Polymer solar cells are considered as very promising candidates for
development of photovoltaics of the future. They are cheap and easy to
fabricate, however, up to now, they possess fundamental drawback, low
effectiveness. In the most popular BHJ (bulk heterojunction) architecture the
actual record of efficiency is about 13 percent. One ask the question how
fundamental this limitation is. In our paper we propose the simple model which
examines the limitations of efficiency by analysis of geometrical aspects of
the BHJ architecture. In this paper we considered two dimensional model. We
calculated the effective length of the donor-acceptor border in the random
mixture of donor and acceptor nanocrystals and further compared it with an
ideal comb architecture. It turns out that in the BHJ architecture, this
effective length is about 2 times smaller than in the comb architecture.

###Performance Loss Analysis and Design Space Optimization of Perovskite Solar Cells||Sumanshu Agarwal,Pradeep R. Nair###

Performance Loss Analysis and Design Space Optimization of Perovskite Solar Cells. While the performance enhancement witnessed in the field of perovskite solar
cells over the recent years has been impressive, it is now evident that further
optimization beyond the existing literature would require detailed analysis of
various loss mechanisms. Here we address the same through detailed numerical
simulations of optical and electrical characteristics. We quantify the various
losses like optical losses (5-6%), recombination losses (3-4%), and resistive
losses against the Auger limited practical efficiency limits. Moreover, we
illustrate the schemes that result in reduction of these losses and eventual
increase in efficiency. In addition, we extend the analyses to identify the
optimum thickness of perovskite and the factors affecting the optimum thickness
have been discussed in detail.

###The Effects of Interfacial Recombination and Injection Barrier on the Electrical Characteristics of Perovskite Solar Cells||Lin Xing Shi,Zi Shuai Wang,Zengguang Huang,Wei E. I. Sha,Haoran Wang,Zhen Zhou###

The Effects of Interfacial Recombination and Injection Barrier on the Electrical Characteristics of Perovskite Solar Cells. Charge carrier recombination in the perovskite solar cells (PSCs) has a deep
influence on the electrical performance, such as open circuit voltage, short
circuit current, fill factor and ultimately power conversion efficiency. The
impacts of injection barrier, recombination channels, doping properties of
carrier transport layers and light intensity on the performance of PSCs are
theoretically investigated by drift-diffusion model in this work. The results
indicate that due to the injection barrier at the interfaces of perovskite and
carrier transport layer, the accumulated carriers modify the electric field
distribution throughout the PSCs. Thus, a zero electric field is generated at a
specific applied voltage, with greatly increases the interfacial recombination,
resulting in a local kink of current density-voltage (J-V) curve. This work
provides an effective strategy to improve the efficiency of PSCs by pertinently
reducing both the injection barrier and interfacial recombination.

###Efficiency at Maximum Power of Laser Quantum Heat Engine Enhanced by Noise-Induced Coherence||Konstantin E. Dorfman,Dazhi Xu,Jianshu Cao###

Efficiency at Maximum Power of Laser Quantum Heat Engine Enhanced by Noise-Induced Coherence. Quantum coherence has been demonstrated in various systems including organic
solar cells and solid state devices. In this letter, we report the lower and
upper bounds for the performance of quantum heat engines determined by the
efficiency at maximum power. Our prediction based on the canonical 3-level
Scovil and Schulz-Dubois maser model strongly depends on the ratio of
system-bath couplings for the hot and cold baths and recovers the theoretical
bounds established previously for the Carnot engine. Further, introducing a
4-th level to the maser model can enhance the maximal power and its efficiency,
thus demonstrating the importance of quantum coherence in the thermodynamics
and operation of the heat engines beyond the classical limit.

###Low Temperature Combustion Synthesis of a Spinel NiCo2O4 Hole Transport Layer for Perovskite Photovoltaics||Ioannis T. Papadas,Apostolos Ioakeimidis,Gerasimos S. Armatas,Stelios A. Choulis###

Low Temperature Combustion Synthesis of a Spinel NiCo2O4 Hole Transport Layer for Perovskite Photovoltaics. In the present study, we report the synthesis and characterization of a
low-temperature solution-processable monodispersed nickel cobaltite (NiCo2O4)
nanoparticles via a combustion synthesis using tartaric acid as fuel and
demonstrate its performance as hole transport layer (HTL) for Perovskite Solar
Cells (PVSCs). NiCo2O4 is a p-type semiconductor consisting of environmentally
friendly, abundant elements and higher conductivity compared to NiO. We show
that the combustion synthesis of spinel NiCo2O4 using tartaric acid as fuel can
be used to control the NPs size and provide smooth, compact and homogeneous
functional HTLs processed by blade coating. Study of PVSCs with different
NiCo2O4 thickness as HTL reveal a difference on hole extraction efficiency and
for 15 nm optimized thickness enhanced hole carrier collection is achieved. As
a result, p-i-n structure of PVSCs with 15 nm NiCo2O4 HTLs showed reliable
performance and power conversion efficiency values in the range of 15.5 % with
negligible hysteresis.

###Probing near-field light-matter interactions with single-molecule lifetime imaging||Dorian Bouchet,Jules Scholler,Guillaume Blanquer,Yannick De Wilde,Ignacio Izeddin,Valentina Krachmalnicoff###

Probing near-field light-matter interactions with single-molecule lifetime imaging. Nanophotonics offers a promising range of applications spanning from the
development of efficient solar cells to quantum communications and biosensing.
However, the ability to efficiently couple fluorescent emitters with
nanostructured materials requires to probe light-matter interactions at
subwavelength resolution, which remains experimentally challenging. Here, we
introduce an approach to perform super-resolved fluorescence lifetime
measurements on samples that are densely labelled with photo-activatable
fluorescent molecules. The simultaneous measurement of the position and the
decay rate of the molecules provides a direct access to the local density of
states (LDOS) at the nanoscale. We experimentally demonstrate the performance
of the technique by studying the LDOS variations induced in the near field of a
silver nanowire, and we show via a Cram\'er-Rao analysis that the proposed
experimental setup enables a single-molecule localisation precision of 6 nm.

###Transient Analysis during Maximum Power Point Tracking (TrAMPPT) to Assess Dynamic Response of Perovskite Solar Cells||Aniela Czudek,Katrin Hirselandt,Lukas Kegelmann,Amran Al-Ashouri,Marko Jošt,Weiwei Zuo,Antonio Abate,Lars Korte,Steve Albrecht,Janardan Dagar,Eva L. Unger###

Transient Analysis during Maximum Power Point Tracking (TrAMPPT) to Assess Dynamic Response of Perovskite Solar Cells. Determination of the device performance parameters of perovskite solar cells
is far from trivial as transient effects may cause large discrepancies in
current-voltage measurements as a function of scan rate and pre-conditioning.
Maximum power point tracking, MPPT, enables to determine the steady-state
maximum power conversion efficiency. However, the MPPT does not provide any
information on the device performance parameters, which are reliable only if
extracted from current-voltage curves collected under steady-state conditions.
We show that is possible to determine the shorter settling or delay time
suitable to carry out J-V measurements under steady-state conditions by
analysis of the transient device response around the MPP. This procedure proves
to be more time-efficient than measurement J-V measurements at a variety of
scan rates. Furthermore, the generic algorithm presented here can be
implemented to assess changes in the dynamic response of devices during
long-term device ageing.

###Iodide-methylammonium interaction is responsible for ferroelectricity in CH3NH3PbI3||Joachim Breternitz,Frederike Lehmann,Sarah A. Barnett,Harriott Nowell,Susan Schorr###

Iodide-methylammonium interaction is responsible for ferroelectricity in CH3NH3PbI3. Excellent conversion efficiencies of over 20 % and facile cell production
have placed hybrid perovskites at the forefront of novel solar cell materials
with CH3NH3PbI3 being its archetypal compound. The question why CH3NH3PbI3 has
such extraordinary characteristics, particularly a hugely efficient light
absorption, is hotly debated with ferroelectricity being a promising candidate.
This does, however, afford the crystal structure to be non-centrosymmetric and
we herein present crystallographic evidence as to how the symmetry breaking
occurs on a crystallographic, and therefore long-scale, level. While the
molecular cation CH3NH3+ is intrinsically polar, it is heavily disordered and
cannot be the sole reason for ferroelectricity. We show that it, nonetheless,
plays an important role as it distorts the neighboring iodide positions from
their centrosymmetric positions.

###Partially-Bright Triplet Excitons in Perovskite Nanocrystals||Albert Liu,Diogo B. Almeida,Luiz G. Bonato,Gabriel Nagamine,Luiz F. Zagonel,Ana F. Nogueira,Lazaro A. Padilha,Steven T. Cundiff###

Partially-Bright Triplet Excitons in Perovskite Nanocrystals. Advances in opto-electronics require the development of materials with novel
and engineered characteristics. A class of materials that has garnered
tremendous interest is metal-halide perovskites, stimulated by meteoric
increases in photovoltaic efficiencies of perovskite solar cells. In addition,
recent advances have applied perovskite nanocrystals (NCs) in light-emitting
devices. It was discovered recently that, for cesium lead-halide perovskite
NCs, their unusually efficient light-emission may be due to a unique excitonic
fine-structure composed of three bright triplet states that minimally interact
with a proximal dark singlet state. To study this fine-structure without
isolating single NCs, we use multi-dimensional coherent spectroscopy at
cryogenic temperatures to reveal coherences involving triplet states of a
CsPbI$_3$ NC ensemble. Picosecond timescale dephasing times are measured for
both triplet and inter-triplet coherences, from which we infer a unique exciton
fine-structure level-ordering comprised of a dark state energetically
positioned within the bright triplet manifold.

###Inverse Design of Potential Singlet Fission Molecules using a Transfer Learning Based Approach||Akshay Subramanian,Utkarsh Saha,Tejasvini Sharma,Naveen K. Tailor,Soumitra Satapathi###

Inverse Design of Potential Singlet Fission Molecules using a Transfer Learning Based Approach. Singlet fission has emerged as one of the most exciting phenomena known to
improve the efficiencies of different types of solar cells and has found uses
in diverse optoelectronic applications. The range of available singlet fission
molecules is, however, limited as to undergo singlet fission, molecules have to
satisfy certain energy conditions. Recent advances in material search using
inverse design has enabled the prediction of materials for a wide range of
applications and has emerged as one of the most efficient methods in the
discovery of suitable materials. It is particularly helpful in manipulating
large datasets, uncovering hidden information from the molecular dataset and
generating new structures. However, we seldom encounter large datasets in
structure prediction problems in material science. In our work, we put forward
inverse design of possible singlet fission molecules using a transfer learning
based approach where we make use of a much larger ChEMBL dataset of
structurally similar molecules to transfer the learned characteristics to the
singlet fission dataset.

###Triple-cation perovskite solar cells fabricated by hybrid PVD/blade coating process using green solvents||Severin Siegrist,Shih-Chi Yang,Evgeniia Gilshtein,Xiaoxiao Sun,Ayodhya N. Tiwari,Fan Fu###

Triple-cation perovskite solar cells fabricated by hybrid PVD/blade coating process using green solvents. The scalability of highly efficient organic-inorganic perovskite solar cells
(PSCs) is one of the remaining challenges of solar module manufacturing.
Various scalable methods have been explored to strive for uniform perovskite
films of high crystal quality on large-area substrates. However, each of these
methods have individual drawbacks, limiting the successful commercialization of
perovskite photovoltaics. Here, we report a fully scalable hybrid process,
which combines vapor- and solution-based techniques to deposit high quality
uniform perovskite films on large-area substrates. This two-step process does
not use toxic solvents, and it further allows facile implementation of
passivation strategies and additives. We fabricated PSCs based on this process
and used blade coating to deposit both charge transporting layers (SnO2 and
Spiro-OMeTAD) without hazardous solvents in ambient air. The fabricated PSCs
have yielded open-circuit voltage up to 1.16 V and power conversion efficiency
of 18.7 % with good uniformity on 5 cm x 5 cm substrates.

###Localized Surface Plasmon Resonance in SnS:Ag Nano-composite Films||Priyal Jain,P. Arun###

Localized Surface Plasmon Resonance in SnS:Ag Nano-composite Films. Nano-composite films of Tin Sulfide (SnS) and silver (Ag) fabricated by
thermal evaporation showed two prominent peaks in the visible region of their
extinction spectra. Theoretical modeling of the extinction spectra suggest that
these two peaks (500 nm and 580 nm) correspond to the longitudinal mode (LM)
and transverse mode (TM) surface plasmon resonance peaks arising from oblate
silver nano-particles. Using grain size of silver and SnS obtained from
structural and morphological characterizations of the samples and dielectric
constants as per actuals, we have compared the experimental results with those
from theory. The study shows that silver nano-particles efficiently scatters
light and can be used for developing plasmonic based SnS solar cells with
improved efficiencies.

###Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells||Michael Kulbak,Satyajit Gupta,Nir Kedem,Igal Levine,Tatyana Bendikov,Gary Hodes,David Cahen###

Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells. Direct comparison between perovskite-structured hybrid organic-inorganic -
methyl ammonium lead bromide (MAPbBr3) and all-inorganic cesium lead bromide
(CsPbBr3), allows identifying possible fundamental differences in their
structural, thermal and electronic characteristics. Both materials possess a
similar direct optical band-gap, but CsPbBr3 demonstrates a higher thermal
stability than MAPbBr3. In order to compare device properties we fabricated
solar cells, with similarly synthesized MAPbBr3 or CsPbBr3, over mesoporous
titania scaffolds. Both cell types demonstrated comparable photovoltaic
performances under AM1.5 illumination, reaching power conversion efficiencies
of ~6 % with a poly-aryl amine-based derivative as hole transport material.
Further analysis shows that Cs-based devices are as efficient as, and more
stable than methyl ammonium-based ones, after aging (storing the cells for 2
weeks in a dry (relative humidity 15-20%) air atmosphere in the dark) for 2
weeks, under constant illumination (at maximum power), and under electron beam
irradiation.

###Role of EV+0.98 eV trap in light soaking-induced short circuit current instability in CIGS solar cells||P. K. Paul,T. Jarmar,L. Stolt,A. Rockett,A. R. Arehart###

Role of EV+0.98 eV trap in light soaking-induced short circuit current instability in CIGS solar cells. Light-induced instabilities/degradation in Cu(In,Ga)Se2 (CIGS) solar cells
are a prevalent and urgent issue to resolve to improve performance, uniformity,
and reliability. Here, mechanisms contributing to light-induced instabilities
are identified focusing on an observed short circuit current (JSC) reduction.
External quantum efficiency measurements before and after light soaking
identified a reduction in long wavelength photon carrier collection efficiency
in the CIGS absorber layer. Using deep level optical spectroscopy (DLOS), the
concentration of CIGS EV+0.98 eV deep level is correlated with the amount of
JSC degradation, Finally, capacitance voltage (C-V) measurements reveal light
induces a large reduction in the depletion depth and reduction of carrier
collection and are all correlated with the JSC reduction. Finally, the EV+0.53
eV trap concentrations are shown to correlate with VOC instability but not the
JSC reduction confirming that multiple trap-induced mechanism are responsible
for the light-induced instabilities.

###Watching Space Charge Build up in an Organic Solar Cell||Sebastian Wilken,Oskar J. Sandberg,Dorothea Scheunemann,Ronald Österbacka###

Watching Space Charge Build up in an Organic Solar Cell. Space charge effects can significantly degrade charge collection in organic
photovoltaics (OPVs), especially in thick-film devices. The two main causes of
space charge are doping and imbalanced transport. Although these are completely
different phenomena, they lead to the same voltage dependence of the
photocurrent, making them difficult to distinguish. In this work, a method is
introduced how the build-up of space charge due to imbalanced transport can be
monitored in a real operating organic solar cell. The method is based on the
reconstruction of quantum efficiency spectra and requires only optical input
parameters that are straightforward to measure. This makes it suitable for the
screening of new OPV materials. Furthermore, numerical and analytical means are
derived to predict the impact of imbalanced transport on the charge collection.
It is shown that when charge recombination is sufficiently reduced, balanced
transport is not a necessary condition for efficient thick-film OPVs.

###Organic Solar Cells; Fabrication Technique, Operating Principle, Characterization and Improvement||Fahmi F. Muhammadsharif###

Organic Solar Cells; Fabrication Technique, Operating Principle, Characterization and Improvement. Organic solar cells (OSCs) have received a special attention over the past
years due to their solution processability, low cost, flexibility and
capability of role-to-role production. The power conversion efficiency of these
devices has been significantly increased over the past decades from 1% in 1986
to 5% in 2005 and to up to 13% in 2017 thanks to the molecular optimization and
the use of non-fullerene acceptors in their active materials. Despite such
interesting efficiency, their applications remain limited so far because of
instability and short life time of their active layers. It is expected that
these obstacles will be surmounted in a foreseeable future upon rigorous
research studies performed in the field. This paper is devoted to reviewing the
operating principle, characterization parameters and the most important
approaches that are considered aiming at improving the overall performance of
these devices.

###Surface Treatment of Cu:NiOx Hole-Transporting Layer Using \b{eta}-Alanine for Hysteresis-Free and Thermally Stable Inverted Perovskite Solar Cells||Fedros Galatopoulos,Ioannis T. Papadas,Apostolos Ioakeimidis,Polyvios Eleftheriou,Stelios A. Choulis###

Surface Treatment of Cu:NiOx Hole-Transporting Layer Using \b{eta}-Alanine for Hysteresis-Free and Thermally Stable Inverted Perovskite Solar Cells. Inverted perovskite solar cells (PSCs) using a Cu:NiOx hole transporting
layer (HTL) often exhibit stability issues and in some cases J/V hysteresis. In
this work, we developed a \b{eta}-alanine surface treatment process on Cu:NiOx
HTL that provides J/V hysteresis-free, highly efficient, and thermally stable
inverted PSCs. The improved device performance due to \b{eta}-alanine-treated
Cu:NiOx HTL is attributed to the formation of an intimate Cu:NiOx/perovskite
interface and reduced charge trap density in the bulk perovskite active layer.
The \b{eta}-alanine surface treatment process on Cu:NiOx HTL eliminates major
thermal degradation mechanisms, providing 40 times increased lifetime
performance under accelerated heat lifetime conditions. By using the proposed
surface treatment, we report optimized devices with high power conversion
efficiency (PCE) (up to 15.51%) and up to 1000 h lifetime under accelerated
heat lifetime conditions (60 C, N2).

###Effect of the structure of lead iodine perovskites on the photovoltaic efficiencies||Cesar Tablero###

Effect of the structure of lead iodine perovskites on the photovoltaic efficiencies. Methyl-ammonium lead iodide perovskite crystallizes in different structures
depending on the temperature: orthorhombic, tetragonal and cubic. An important
point to be considered is the effect of the microscopic properties of the
different structures on the optical and photovoltaic properties. Using first
principles we obtain the absorption coefficients that will determine the
absorption of solar radiation. In order to analyze the contributions of the
different atoms to the absorption coefficients we split them into a
many-species expansion. Using a similar methodology we also split the
efficiencies as a many-species expansion. It allows the contribution of the
atomic species to be identified and quantified to the absorption coefficients
and to the solar cell efficiency of the different phases. Additionally the
effect of the cell thickness w is quantified.

###dPV: An End-to-End Differentiable Solar-Cell Simulator||Sean Mann,Eric Fadel,Samuel S. Schoenholz,Ekin D. Cubuk,Steven G. Johnson,Giuseppe Romano###

dPV: An End-to-End Differentiable Solar-Cell Simulator. We introduce dPV, an end-to-end differentiable photovoltaic (PV) cell
simulator based on the drift-diffusion model and Beer-Lambert law for optical
absorption. dPV is programmed in Python using JAX, an automatic differentiation
(AD) library for scientific computing. Using AD coupled with the implicit
function theorem, dPV computes the power conversion efficiency (PCE) of an
input PV design as well as the derivative of the PCE with respect to any input
parameters, all within comparable time of solving the forward problem. We show
an example of perovskite solar-cell optimization and multi-parameter discovery,
and compare results with random search and finite differences. The simulator
can be integrated with optimization algorithms and neural networks, opening up
possibilities for data-efficient optimization and parameter discovery.

###Increasing the Efficiency of Photovoltaic Systems by Using Maximum Power Point Tracking (MPPT)||Alireza Tofigh Rihani,Majid Ghandchi###

Increasing the Efficiency of Photovoltaic Systems by Using Maximum Power Point Tracking (MPPT). Using Photovoltaic systems is gradually expanded by increasing energy demand.
Abundance and availability of this energy, has turned to one of the most
important sources of renewable energy. Unfortunately, photovoltaic systems have
two big problems: first, those have very low energy conversion efficiency (in
act between 12 and 42 percent under certain circumstances). Second, the power
produced by the solar cell depends on nonlinear conditions such as solar
radiation, temperature and charge feature. According to this, received power
maximum of photovoltaic cells depends on different non-linear variables, it is
necessary to be continuously traced, as maximum received power of the cell (by
controller). In this research, the increasing efficiency of photovoltaic
systems has been investigated by using Maximum Power Point Tracking (MPPT) in
two different modes contained connected to the Grid and disconnected from the
grid with simulation by MATLAB software. The obtained results showed that the
proposed technique is able to improve the current, voltage and power output of
photovoltaic cells.

###"Forbidden" polarisation and extraordinary piezoelectric effect in organometallic lead halide perovskites||Milica Vasiljevic,Marton Kollar,David Spirito,Lukas Riemer,Laszlo Forro,Endre Horvath,Semen Gorfman,Dragan Damjanovic###

"Forbidden" polarisation and extraordinary piezoelectric effect in organometallic lead halide perovskites. Organometallic lead halide perovskites are highly efficient materials for
solar cells and other optoelectronic applications due to their high quantum
efficiency and exceptional semiconducting properties. A peculiarity of these
perovskites is the substantial ionic motion under external forces. Here, we
reveal that electric field-and light-induced ionic motion in MAPbX3 crystals
(X=Cl, Br, I and MA=CH3NH3) leads to unexpected piezoelectric-like response, an
order of magnitude larger than in ferroelectric perovskite oxides. The nominal
macroscopic symmetry of the crystals is broken by redistribution of ionic
species, which can be controlled deterministically by light and electric field.
The revealed piezoelectric response is possibly present in other materials with
significant ionic activity but the unique feature of organometallic perovskites
is the strong effect on the piezoelectric response of interplay of ionic motion
(MA+ and X-1) and photoelectrons generated with illumination.

###Preconditioning for a Phase-Field Model with Application to Morphology Evolution in Organic Semiconductors||Kai Bergermann,Carsten Deibel,Roland Herzog,Roderick C. I. MacKenzie,Jan-Frederik Pietschmann,Martin Stoll###

Preconditioning for a Phase-Field Model with Application to Morphology Evolution in Organic Semiconductors. The Cahn--Hilliard equations are a versatile model for describing the
evolution of complex morphologies. In this paper we present a computational
pipeline for the numerical solution of a ternary phase-field model for
describing the nanomorphology of donor--acceptor semiconductor blends used in
organic photovoltaic devices. The model consists of two coupled fourth-order
partial differential equations that are discretized using a finite element
approach. In order to solve the resulting large-scale linear systems
efficiently, we propose a preconditioning strategy that is based on efficient
approximations of the Schur-complement of a saddle point system. We show that
this approach performs robustly with respect to variations in the
discretization parameters. Finally, we outline that the computed morphologies
can be used for the computation of charge generation, recombination, and
transport in organic solar cells.

###Ab initio theory of free-carrier absorption in semiconductors||Xiao Zhang,Guangsha Shi,Joshua A. Leveillee,Feliciano Giustino,Emmanouil Kioupakis###

Ab initio theory of free-carrier absorption in semiconductors. The absorption of light by free carriers in semiconductors results in optical
loss for all photon wavelengths. Since free-carrier absorption competes with
optical transitions across the band gap, it also reduces the efficiency of
optoelectronic devices such as solar cells because it does not generate
electron-hole pairs. In this work, we develop a first-principles theory of
free-carrier absorption taking into account both single-particle excitations
and the collective Drude term, and we demonstrate its application to the case
of doped Si. We determine the free-carrier absorption coefficient as a function
of carrier concentration and we obtain excellent agreement with experimental
data. We identify the dominant processes that contribute to free-carrier
absorption at various photon wavelengths, and analyze the results to evaluate
the impact of this loss mechanism on the efficiency of Si-based optoelectronic
devices.

###Enhanced photovoltaic effect in graphene-silicon Schottky junction under mechanical manipulation||Dong Pu,Muhammad Abid Anwar,Jiachao Zhou,Renwei Mao,Xin Pan,Jian Chai,Feng Tian,Hua Wang,Huan Hu,Yang Xu###

Enhanced photovoltaic effect in graphene-silicon Schottky junction under mechanical manipulation. Graphene-silicon Schottky junction (GSJ) which has the potential for
large-scale manufacturing and integration can bring new opportunities to
Schottky solar cells for photovoltaic (PV) power conversion. However, the
essential power conversion limitation for these devices lies in the small
open-circuit voltage ($V_{oc}$), which depends on the Schottky barrier height
(SBH). In this study, we introduce an electromechanical method based on the
flexoelectric effect to enhance the PV efficiency in GSJ. By atomic force
microscope (AFM) tip-based indentation and in situ current measurement, the
current-voltage (I-V) responses under flexoelectric strain gradient are
obtained. The $V_{oc}$ is observed to increase for up to 20$\%$, leading to an
evident improvement of the power conversion efficiency. Our studies suggest
that strain gradient may offer unprecedented opportunities for the development
of GSJ based flexo-photovoltaic applications.

###Optical Signatures of Förster-induced energy transfer in organic/TMD heterostructures||Joshua J. P. Thompson,Marina Gerhard,Gregor Witte,Ermin Malic###

Optical Signatures of Förster-induced energy transfer in organic/TMD heterostructures. Hybrid van der Waals heterostructures of organic semiconductors and
transition metal dichalcogenides (TMDs) are promising candidates for various
optoelectronic devices, such as solar cells and biosensors. Energy-transfer
processes in these materials are crucial for the efficiency of such devices,
yet they are poorly understood. In this work, we develop a fully microscopic
theory describing the effect of the F\"{o}rster interaction on exciton dynamics
and optics in a WSe$_2$/tetracene heterostack. We demonstrate that the
differential absorption and time-resolved photoluminescence can be used to
track the real-time evolution of excitons. We predict a strongly unidirectional
energy transfer from the organic to the TMD layer. Furthermore, we explore the
role temperature has in activating the F\"{o}rster transfer and find a good
agreement to previous experiments. Our results provide a blueprint to tune the
light-harvesting efficiency through temperature, molecular orientation and
interlayer separation in TMD/organic heterostructures.

###Surface and bulk effects of K in highly efficient Cu$_{1-x}$K$_x$InSe$_2$ solar cells||Christopher P. Muzzillo,Jian V. Li,Lorelle M. Mansfield,Kannan Ramanathan,Timothy J. Anderson###

Surface and bulk effects of K in highly efficient Cu$_{1-x}$K$_x$InSe$_2$ solar cells. To advance knowledge of K bonding in Cu(In,Ga)(Se,S)2 (CIGS) photovoltaic
(PV) absorbers, recent Cu-K-In-Se phase growth studies have been extended to PV
performance. First, the effect of distributing K throughout bulk Cu1-xKxInSe2
absorbers at low K/(K+Cu) compositions (0 <= x <= 0.30) was studied.
Efficiency, open-circuit voltage (VOC), and fill factor (FF) were greatly
enhanced for x ~ 0.07, resulting in an officially-measured 15.0%-efficient
solar cell, matching to the world record CuInSe2 efficiency. The improvements
were a result of reduced interface and bulk recombination, relative to CuInSe2
(x ~ 0). However, higher x compositions had reduced efficiency, short-circuit
current density (JSC), and FF due to greatly increased interface recombination,
relative to the x ~ 0 baseline. Next, the effect of confining K at the
absorber/buffer interface at high K/(K+Cu) compositions (0.30 <= x <= 0.92) was
researched. Previous work showed that these surface layer growth conditions
produced CuInSe2 with a large phase fraction of KInSe2. After optimization (75
nm surface layer with x ~ 0.41), these KInSe2 surface samples exhibited
increased efficiency (officially 14.9%), VOC, and FF as a result of decreased
interface recombination. The KInSe2 surfaces had features similar to previous
reports for KF post-deposition treatments (PDTs) used in world record CIGS
solar cells-taken as indirect evidence that KInSe2 can form during these PDTs.
Both the bulk and surface growth processes greatly reduced interface
recombination. However, the KInSe2 surface had higher K levels near the
surface, greater lifetimes, and increased inversion near the buffer interface,
relative to the champion bulk CKIS absorber. These characteristics demonstrate
that K may benefit PV performance by different mechanisms at the surface and in
the absorber bulk.

###Light-trapping enhanced thin-film III-V quantum dot solar cells fabricated by epitaxial lift-off||F Cappelluti,D Kim,M van Eerden,AP Cédola,T Aho,G Bissels,F Elsehrawy,J Wu,H Liu,P Mulder,G Bauhuis,J Schermer,T Niemi,M Guina###

Light-trapping enhanced thin-film III-V quantum dot solar cells fabricated by epitaxial lift-off. We report thin-film InAs/GaAs quantum dot (QD) solar cells with $n-i-p{+}$
deep junction structure and planar back reflector fabricated by epitaxial
lift-off (ELO) of full 3-inch wafers. External quantum efficiency measurements
demonstrate twofold enhancement of the QD photocurrent in the ELO QD cell
compared to the wafer-based QD cell. In the GaAs wavelength range, the ELO QD
cell perfectly preserves the current collection efficiency of the baseline
single-junction ELO cell. We demonstrate by full-wave optical simulations that
integrating a micro-patterned diffraction grating in the ELO cell rearside
provides more than tenfold enhancement of the near-infrared light harvesting by
QDs. Experimental results are thoroughly discussed with the help of
physics-based simulations to single out the impact of QD dynamics and defects
on the cell photovoltaic behavior. It is demonstrated that non radiative
recombination in the QD stack is the bottleneck for the open circuit voltage
($V_{oc}$) of the reported devices. More important, our theoretical
calculations demonstrate that the $V_{oc}$ offest of 0.3 V from the QD ground
state identified by \emph{Tanabe et al., 2012}, from a collection of
experimental data of high quality III-V QD solar cells is a reliable - albeit
conservative - metric to gauge the attainable $V_{oc}$ and to quantify the
scope for improvement by reducing non radiative recombination. Provided that
material quality issues are solved, we demonstrate - by transport and rigorous
electromagnetic simulations - that light-trapping enhanced thin-film cells with
twenty InAs/GaAs QD layers reach efficiency higher than 28\% under
unconcentrated light, ambient temperature. If photon recycling can be fully
exploited, 30\% efficiency is deemed to be feasible.

###A novel graph-based formulation for characterizing morphology with application to organic solar cells||Olga Wodo,Srikanta Tirthapura,Sumit Chaudhary,Baskar Ganapathysubramanian###

A novel graph-based formulation for characterizing morphology with application to organic solar cells. Organic solar cells have the potential for widespread usage due to their
promise of low cost, roll-to-roll manufacturability, and mechanical
flexibility. However, deployment is impeded by their relatively low power
conversion efficiencies. The last decade has seen significant progress in
enhancing the power conversion of these devices through various strategies. One
such approach is based on morphology control. This is because morphology
affects all phenomena involved in solar conversion: light absorption and
electron-hole pair (exciton) generation; exciton diffusion and dissociation
into free charges; and transport of charges to the electrodes. Progress in
experimental characterization and computational modeling now allow
reconstruction and imaging of the thin film morphology. This opens up the
possibility of rationally linking fabrication with morphology, as well as
morphology with performance. In this context, a comprehensive set of
computational tools to rapidly quantify and classify the heterogeneous internal
structure of thin films will be invaluable in linking process, structure and
property. We present a novel graph-based framework to efficiently construct a
broad suite of physically meaningful morphology descriptors. These morphology
descriptors are further classified according to the physical subprocesses
within an organic solar cells. The approach is motivated by the equivalence
between a discretized morphology and a labeled, weighted, undirected graph. We
utilize this approach to pose key questions related to structure
characterization. We subsequently construct estimates and upper bounds of
various efficiencies. The approach is showcased by characterizing the effect of
thermal annealing on time-evolution of a thin film morphology. We conclude by
formulating natural extensions to characterize crystallinity and anisotropy of
the morphology using the framework.

###Multiscale approaches to high efficiency photovoltaics||J. P. Connolly,Lejo J. Koduvelikulathu,D. Mencaraglia,Julio C. Rimada,Ahmed Nejim,G. Sanchez###

Multiscale approaches to high efficiency photovoltaics. While renewable energies are achieving parity around the globe, efforts to
reach higher solar cell efficiencies becomes ever more difficult as they
approach the limiting efficiency. The so-called third generation concepts
attempt to break this limit through a combination of novel physical processes
and new materials and concepts in organic and inorganic systems. Some examples
of semi-empirical modelling in the field are reviewed, in particular for
multispectral solar cells on silicon (french ANR project MULTISOLSI). Their
achievements are outlined, and the limits of these approaches shown. This
introduces the main topic of this contribution, which is the use of multiscale
experimental and theoretical techniques to go beyond the semi-empirical
understanding of these systems. This approach has already led to great advances
at modelling which have led to modelling software which is widely known. Yet a
survey of the topic reveals a fragmentation of efforts across disciplines,
firstly, such as organic and inorganic fields, but also between the high
efficiency concepts such as hot carrier cells and intermediate band concepts.
We show how this obstacle to the resolution of practical research obstacles may
be lifted by inter-disciplinary cooperation across length scales, and across
experimental and theoretical fields, and finally across materials systems. We
present a European COST Action MultiscaleSolar kicking off in early 2015 which
brings together experimental and theoretical partners in order to develop
multiscale research in organic and inorganic materials. The goal of this
defragmentation and interdisciplinary collaboration is to develop understanding
across length scales which will enable the full potential of third generation
concepts to be evaluated in practise, for societal and industrial applications.

###Figure-of-merit for Semi-transparent Solar Cells||Arun Kumar,Sonia Rani,Dhriti Sundar Ghosh###

Figure-of-merit for Semi-transparent Solar Cells. Semi-transparent Solar Cells (ST-SCs) has emerged as one of the most
prominent energy harvesting technology that combines the benefits of light
transparency and light-to-electricity conversion. The biggest opportunities for
such technologies lie in their integration as windows and skylights within
energy-sustainable buildings or combining them with other solar cell
technologies in tandem configuration. The performance of ST-SCs is mainly
determined by the trade-off between the competing parameters of the capability
to convert the incident light into electricity while allowing some parts to
transmit providing transparency through the device. Depending on the target
application, the selection of ST-SCs is a tricky affair as some devices might
offer high efficiency but compromises transparency and vice-versa. On the other
way around, this is again not helped by the fact that due to advancements in
materials engineering, processing, and characterization, a vastly different
combination of efficiency and transparency has been reported by research
groups. So, in order to quantify the performance of ST-SCs, we proposed, a
figure-of-merit (FoM) which can be used as a tool that can help in analysing
and comparing the performance among various ST-SCs. The defined FoM focuses on
the power conversion efficiency of the device, bifaciality factor,
transmittance in the desired region, and that corresponding to 550 nm
wavelength. Additionally, in this work, we have been shown how the proposed FoM
can be correlated for tandem and building-integrated photovoltaics
applications. Based on these resultant parameters, FoM is calculated and
compared for different device architectures available in the literature. The
proposed FoM shall serve as a meaningful guiding path to the researchers for
the development of advanced ST-SCs.

###Sn/Ge substitution in ((C$_\textrm{n}$H$_{2\textrm{n}-1}$NH$_3$)$_2$PbI$_4$; n=3): An emerging 2D layered hybrid perovskites with enhanced optoelectronic properties$^†$||Deepika Gill,Gunjana Yadav,Saswata Bhattacharya###

Sn/Ge substitution in ((C$_\textrm{n}$H$_{2\textrm{n}-1}$NH$_3$)$_2$PbI$_4$; n=3): An emerging 2D layered hybrid perovskites with enhanced optoelectronic properties$^†$. Two-dimensional (2D) perovskites show higher stability in comparison to their
three-dimensional (3D) counterparts. Therefore, 2D perovskites have invoked
remarkable attention in basic understanding of their physical properties and
optoelectronic applications. Here we present a low-dimensional naturally
self-assembled inorganic-organic (IO) hybrid systems based on primary cyclic
ammonium-based (C$_{\textrm{n}}$H$_{2\textrm{n}-1}$NH$_{3}$) semiconductor
series [viz. ((C$_{\textrm{n}}$H$_{2\textrm{n}-1}$ NH$_3$)$_2$PbI$_4$; n=3-6)].
However, the wide bandgap nature and presence of toxicity due to lead (Pb)
prohibit their applications. Therefore, in the present work, we study the role
of Ge/Sn substitution and Pb-vacancy (Pb-$\boxtimes$) to reduce concentration
of Pb and to enhance solar cell efficiency by the formation of mixed perovskite
structures. We have discussed the effect of spin-orbit coupling (SOC) using
state-of-the-art hybrid density functional theory (DFT). We find the mixed
conformers with Pb-$\boxtimes$ do not possess structural stability. Moreover,
they have indirect bandgap, which is not good for solar cell applications. Only
those conformers, which have favourable thermodynamics and structural
stability, are considered for further study of optical properties. Our results
infer that Sn substitution is more favorable than that of Ge in replacing Pb
and enhancing the efficiency. Exciton binding energies calculated using
Wannier-Mott approach for pristine and substituted conformers are larger than
lead halide perovskites, while the electron-phonon coupling is smaller in the
former. From computed spectroscopic limited maximum efficiency (SLME), these 2D
perovskites show enough promise as alternatives to conventional lead halide
perovskites.

###Structure-related bandgap of hybrid lead halide perovskites and close-packed APbX3 family of phases||Ekaterina I. Marchenko,Sergey A. Fateev,Vadim V. Korolev,Vladimir Buchinskii,Eremin N. N.,Eugene A. Goodilin,Alexey B. Tarasov###

Structure-related bandgap of hybrid lead halide perovskites and close-packed APbX3 family of phases. Metal halide perovskites APbX3 (A+ = FA+ (formamidinium), MA+
(methylammonium) or Cs+, X- = I-, Br-) are considered as prominent innovative
components in nowadays perovskite solar cells. Crystallization of these
materials is often complicated by the formation of various phases with the same
stoichiometry but structural types deviating from perovskites such as
well-known the hexagonal delta FAPbI3 polytype. Such phases are rarely placed
in the focus of device engineering due to their unattractive optoelectronic
properties while they are, indeed, highly important because they influence on
the optoelectronic properties and efficiency of final devices. However, the
total number of such phases has not been yet discovered and the complete
configurational space of the polytypes and their band structures have not been
studied systematically. In this work, we predicted and described all possible
hexagonal polytypes of hybrid lead halides with the APbI3 composition using the
group theory approach, also we analyzed theoretically the relationship between
the configuration of close-packed layers in polytypes and their band gap using
DFT calculations. Two main factors affecting the bandgap were found including
the ratio of cubic (c) and hexagonal (h) close-packed layers and the thickness
of blocks of cubic layers in the structures. We also show that the dependence
of the band gap on the ratio of cubic (c) and hexagonal (h) layers in these
structures are non-linear. We believe that the presence of such polytypes in
the perovskite matrix might be a reason for a decrease in the charge carrier
mobility and therefore it would be an obstacle for efficient charge transport
causing negative consequences for the efficiency of solar cell devices.

###Minority Carrier Diffusion Lengths for High Purity Liquid Phase Epitaxial GaAs||D. Alexiev,D. A. Prokopovich,L. Mo###

Minority Carrier Diffusion Lengths for High Purity Liquid Phase Epitaxial GaAs. The diffusion length of minority carriers L p,n is an important
characterisation parameter in semiconductor materials and is of particular
interest when constructing devices such as solar cells (Hovel 1975), double
hetero junction lasers (Casey and Panish 1978) and bipolar transistors. Their
efficiency depends primarily on the ability of minority carriers to diffuse
through neutral material to a p-n junction or Schottky barrier where they
recombine with majority carriers. For this reason diffusion lengths have been
measured in a variety of semiconductor materials. The GaAs material was grown
by liquid phase epitaxy (LPE) at the Australian Nuclear Science and Technology
Organisation. The diffusion lengths measured for high purity ptype and n-type
LPE-GaAs samples were observed to be longer than any previously reported.
Measurements of minority carrier diffusion lengths for p-type and n-type GaAs
were carried out using an electron beam induced current (EBIC) technique.

###Mirrored strain-balanced quantum well concentrator cells in the radiative limit||J. P. Connolly,M. F. Führer,D. C. Johnson,I. M. Ballard,K. W. J. Barnham,M. Mazzer,T. N. D Tibbits,J. S. Roberts,G. Hill,C. Calder###

Mirrored strain-balanced quantum well concentrator cells in the radiative limit. Strain-balanced Quantum Well Solar Cells (SB- QWSCs) are radiatively
dominated at concentrator current levels. Incorporating back surface mirrors on
the back of the doped substrate leads to a reduction in this radiative
recombination current while the non-radiative dark current at lower bias
remains unchanged. This work extends a previous model of radiative dark
currents to cells where the substrate is not a perfectly absorbing sink for
emitted luminescence by including multiple passes of photons emitted by
radiative recombination. The luminescence in such mirrored structures is
reduced and the radiative dark current lower. The structure is suited to
concentrator applications because it corresponds to the high efficiency limit
where losses due to light re-emission are limited to the solid angle for
absorption of solar radiation.

###Recombination in polymer-fullerene bulk heterojunction solar cells||Sarah R. Cowan,Anshuman Roy,Alan J. Heeger###

Recombination in polymer-fullerene bulk heterojunction solar cells. Recombination of photogenerated charge carriers in polymer bulk
heterojunction (BHJ) solar cells reduces the short circuit current (Jsc) and
the fill factor (FF). Identifying the mechanism of recombination is, therefore,
fundamentally important for increasing the power conversion efficiency. Light
intensity and temperature dependent current-voltage measurements on polymer BHJ
cells made from a variety of different semiconducting polymers and fullerenes
show that the recombination kinetics are voltage dependent and evolve from
first order recombination at short circuit to bimolecular recombination at open
circuit as a result of increasing the voltage-dependent charge carrier density
in the cell. The "missing 0.3V" inferred from comparison of the band gaps of
the bulk heterojunction materials and the measured open circuit voltage at room
temperature results from the temperature dependence of the quasi-Fermi-levels
in the polymer and fullerene domains - a conclusion based upon the fundamental
statistics of Fermions.

###Extraordinarily Efficient Conduction in a Redox-Active Ionic Liquid||Verner K. Thorsmølle,Guido Rothenberger,Daniel Topgaard,Jan C. Brauer,Dai-Bin Kuang,Shaik M. Zakeeruddin,Björn Lindman,Michael Grätzel,Jacques-E. Moser###

Extraordinarily Efficient Conduction in a Redox-Active Ionic Liquid. Iodine added to iodide-based ionic liquids leads to extraordinarily efficient
charge transport, vastly exceeding that expected for such viscous systems.
Using terahertz time-domain spectroscopy, in conjunction with dc conductivity,
diffusivity and viscosity measurements we unravel the conductivity pathways in
1-methyl-3-propylimidazolium iodide melts. This study presents evidence of the
Grotthuss mechanism as a significant contributor to the conductivity, and
provides new insights into ion pairing processes as well as the formation of
polyiodides. The terahertz and transport results are reunited in a model
providing a quantitative description of the conduction by physical diffusion
and the Grotthuss bond-exchange process. These novel results are important for
the fundamental understanding of conduction in molten salts and for
applications where ionic liquids are used as charge-transporting media such as
in batteries and dye-sensitized solar cells.

###Thickness dependences of photoelectric characteristics of silicon backside contact solar cells||A. P. Gorban,V. P. Kostylyov,A. V. Sachenko,O. A. Serba,I. O. Sokolovskyi,V. V. Chernenko###

Thickness dependences of photoelectric characteristics of silicon backside contact solar cells. The thickness dependences of the photocurrent quantum yield and photoenergy
parameters of silicon backside contact solar cells (BC SC) are investigated
theoretically and experimentally. The surface recombination rate on the
irradiated surface was minimized by means of creating the layers of microporous
silicon. A method of finding the surface recombination rate and the diffusion
length of minority carriers from the thickness dependences of the photocurrent
quantum yield under conditions of the strong absorption is proposed. The
performed studies allowed us to establish that the thinning of the BC SC
samples in the case of minimizing the surface recombination rate gives a
possibility to achieve rather high efficiencies of photoconversion. It is also
shown that the agreement between the experimental and theoretical spectral
dependences of the photocurrent quantum yield can be reached only with regard
for the coefficient of light reflection from the backside surface.

###Spatial modeling of the 3D morphology of hybrid polymer-ZnO solar cells, based on electron tomography data||O. Stenzel,V. Schmidt,H. Hassfeld,R. Thiedmann,L. J. A. Koster,S. D. Oosterhout,S. S. van Bavel,M. M. Wienk,J. Loos,R. A. J. Janssen###

Spatial modeling of the 3D morphology of hybrid polymer-ZnO solar cells, based on electron tomography data. A spatial stochastic model is developed which describes the 3D nanomorphology
of composite materials, being blends of two different (organic and inorganic)
solid phases. Such materials are used, for example, in photoactive layers of
hybrid polymer zinc oxide solar cells. The model is based on ideas from
stochastic geometry and spatial statistics. Its parameters are fitted to image
data gained by electron tomography (ET), where adaptive thresholding and
stochastic segmentation have been used to represent morphological features of
the considered ET data by unions of overlapping spheres. Their midpoints are
modeled by a stack of 2D point processes with a suitably chosen correlation
structure, whereas a moving-average procedure is used to add the radii of
spheres. The model is validated by comparing physically relevant
characteristics of real and simulated data, like the efficiency of exciton
quenching, which is important for the generation of charges and their transport
toward the electrodes.

###Intrinsic Defects and Dopability of Zinc Phosphide||Steven Demers,Axel van de Walle###

Intrinsic Defects and Dopability of Zinc Phosphide. Zinc Phosphide ($Zn_3P_2$) could be the basis for cheap and highly efficient
solar cells. Its use in this regard is limited by the difficulty in n-type
doping the material. In an effort to understand the mechanism behind this, the
energetics and electronic structure of intrinsic point defects in zinc
phosphide are studied using generalized Kohn-Sham theory and utilizing the
Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional for exchange and
correlation. Novel 'perturbation extrapolation' is utilized to extend the use
of the computationally expensive HSE functional to this large-scale defect
system. According to calculations, the formation energy of charged phosphorus
interstitial defects are very low in n-type $Zn_3P_2$ and act as 'electron
sinks', nullifying the desired doping and lowering the fermi-level back towards
the p-type regime. This is consistent with experimental observations of both
the tendency of conductivity to rise with phosphorus partial pressure, and with
current partial successes in n-type doping in very zinc-rich growth conditions.

###Antisite traps and metastable defects in Cu(In,Ga)Se2 thin-film solar cells studied by screened-exchange hybrid density functional theory||Johan Pohl,Thomas Unold,Karsten Albe###

Antisite traps and metastable defects in Cu(In,Ga)Se2 thin-film solar cells studied by screened-exchange hybrid density functional theory. Electronic structure calculations within screened-exchange hybrid density
functional theory show that Cu(In,Ga) antisites in both CuInSe2 and CuGaSe2 are
localized hole traps, which can be attributed to the experimentally observed N2
level. In contrast, GaCu antisites and their defect complexes with copper
vacancies exhibit an electron trap level, which can limit the open-circuit
voltage and efficiency in Ga-rich Cu(In,Ga)Se2 alloys. Low-temperature
photoluminescence measurements in CuGaSe2 thin-film solar cells show a
free-to-bound transition at an energy of 1.48 eV, in very good agreement with
the calculated transition energy for the GaCu antisite. Since the intrinsic DX
center does not exhibit a pinning level within the band gap of CuInSe2,
metastable DX behaviour can only be expected for GaCu antisites.

###Hybrid density functional study of band alignment in ZnO/GaN and ZnO/(Ga1-xZnx)(N1-xOx)/GaN heterostructures||Zhenhai Wang,Mingwen Zhao,Xiaopeng Wang,Yan Xi,Xiujie He,Xiangdong Liu,Shishen Yan###

Hybrid density functional study of band alignment in ZnO/GaN and ZnO/(Ga1-xZnx)(N1-xOx)/GaN heterostructures. The band alignment in ZnO/GaN and related heterostructures are crucial for
the uses in solar harvesting technology. Here, we report our density functional
calculations of the band alignment and optical properties of ZnO/GaN and
ZnO/(Ga1-xZnx)(N1-xOx)/GaN heterostructures using a Heyd-Scuseria-Ernzerhof
(HSE) hybrid functional. We found that the conventional GGA functionals
underestimate not only the band gap but also the band offset of these
heterostructures. Using the hybrid functional calculations, we show that the
(Ga1-xZnx)(N1-xOx) solid solution has a direct band gap of about 2.608 eV, in
good agreement with the experimental data. More importantly, this solid
solution forms type-II band alignment with the host materials. A
GaN/(Ga1-xZnx)(N1-xOx)/ZnO core-shell solar cell model is presented to improve
the visible light adsorption ability and carrier collection efficiency.

###Many-body Green's function study of coumarins for dye-sensitized solar cells||C. Faber,I. Duchemin,T. Deutsch,X. Blase###

Many-body Green's function study of coumarins for dye-sensitized solar cells. We study within the many-body Green's function $GW$ and Bethe-Salpeter
formalisms the excitation energies of several coumarin dyes proposed as an
efficient alternative to ruthenium complexes for dye-sensitized solar cells.
Due to their internal donor-acceptor structure, these chromophores present
low-lying excitations showing a strong intramolecular charge-transfer
character. We show that combining $GW$ and Bethe-Salpeter calculations leads to
charge-transfer excitation energies and oscillator strengths in excellent
agreement with reference range-separated functional studies or coupled-cluster
calculations. The present results confirm the ability of this family of
approaches to describe accurately Frenkel and charge-transfer photo-excitations
in both extended and finite size systems without any system-dependent
adjustable parameter, paving the way to the study of dye-sensitized
semiconducting surfaces.

###Microscopic theory of singlet exciton fission. I. General formulation||Timothy C. Berkelbach,Mark S. Hybertsen,David R. Reichman###

Microscopic theory of singlet exciton fission. I. General formulation. Singlet fission, a spin-allowed energy transfer process generating two
triplet excitons from one singlet exciton, has the potential to dramatically
increase the efficiency of organic solar cells. However, the dynamical
mechanism of this phenomenon is not fully understood and a complete,
microscopic theory of singlet fission is lacking. In this work, we assemble the
components of a comprehensive microscopic theory of singlet fission that
connects excited state quantum chemistry calculations with finite-temperature
quantum relaxation theory. We elaborate on the distinction between localized
diabatic and delocalized adiabatic bases for the interpretation of singlet
fission experiments in both the time and frequency domains. We discuss various
approximations to the exact density matrix dynamics and propose Redfield theory
as an ideal compromise between speed and accuracy for the detailed
investigation of singlet fission in dimers, clusters, and crystals.
Investigations of small model systems based on parameters typical of singlet
fission demonstrate the numerical accuracy and practical utility of this
approach.

###Metal nanoparticle plasmons operating within quantum lifetime||Mehmet Emre Tasgin###

Metal nanoparticle plasmons operating within quantum lifetime. We investigate the dynamics of a plasmonic oscillation over a metal
nanoparticle when it is strongly coupled to a quantum emitter (e.g. quantum
dot, molecule). We simulate the density matrix evolution for a simple model;
coupled classical--quantum oscillators system. We show that lifetime of the
plasmonic oscillations can be increased several orders of magnitude, upto the
decay time of the quantum emitter. This effect shows itself as the narrowing of
the plasmon emission band in the spaser (surface plasmon amplification by
stimulated emission of radiation) experiment [{\it Nature}, {\bf 2009}, 460,
1110], where a gold nanoparticle interacts with the surrounding molecules.
Enhancement of the plasmonic excitation lifetime enables stimulated emission to
overcome the spontaneous one. The enhancement occurs due to the emergence of a
phenomenon analogous to electromagnetically induced transparency (EIT). The
effect can find applications in many areas of nanoscale physics, such as in
quantum information with plasmons and in increasing solar cell efficiency.

###Effect of electrode geometry on photovoltaic performance of polymer solar cells||Meng Li,Heng Ma,Zhaokui Wang,Chuankun Wang,Yurong Jiang,Ning Liu###

Effect of electrode geometry on photovoltaic performance of polymer solar cells. This paper investigates the impact of electrode geometry on the performance
of polymer solar cells (PSCs). Four types of negative electrodes with equal
area (0.09 cm2) but different shape (round, oval, square, and triangular) are
evaluated with respect to short-circuit current density, open-circuit voltage,
fill factor, and power conversion efficiency of PSCs. The results show that the
device with round electrodes gives the best photovoltaic performance; in
contrast, the device with triangular electrodes reveals the worst properties.
Maximum almost twice increase in PCE with round electrode is obtained in the
devices compared with that of the triangular electrode. As a conclusion, the
electrode boundary curvature has a strong influence on the performance of PSCs.
The larger curvature, i.e. the sharper electrodes edge, maybe is a negative
effector on exciton separation and carrier transport in PSC system.

###Multiscale modeling of solar cells with interface phenomena||David H. Foster,Timothy Costa,Malgorzata Peszynska,Guenter Schneider###

Multiscale modeling of solar cells with interface phenomena. We describe a mathematical model for heterojunctions in semiconductors which
can be used, e.g., for modeling higher efficiency solar cells. The continuum
model involves well-known drift-diffusion equations posed away from the
interface. These are coupled with interface conditions with a nonhomogeneous
jump for the potential, and Robin-like interface conditions for carrier
transport. The interface conditions arise from approximating the interface
region by a lower-dimensional manifold. The data for the interface conditions
are calculated by a Density Functional Theory (DFT) model over a few atomic
layers comprising the interface region. We propose a domain decomposition
method (DDM) approach to decouple the continuum model on subdomains which is
implemented in every step of the Gummel iteration. We show results for
CIGS/CdS, Si/ZnS, and Si/GaAs heterojunctions.

###Solar-energy conversion and light emission in an atomic monolayer p-n diode||Andreas Pospischil,Marco M. Furchi,Thomas Mueller###

Solar-energy conversion and light emission in an atomic monolayer p-n diode. Two-dimensional (2D) atomic crystals, such as graphene and atomically thin
transition metal dichalcogenides (TMDCs), are currently receiving a lot of
attention. They are crystalline, and thus of high material quality, even so,
they can be produced in large areas and are bendable, thus providing
opportunities for novel applications. Here, we report a truly 2D p-n junction
diode, based on an electrostatically doped tungsten diselenide (WSe2)
monolayer. As p-n diodes are the basic building block in a wide variety of
optoelectronic devices, our demonstration constitutes an important advance
towards 2D optoelectronics. We present applications as (i) photovoltaic solar
cell, (ii) photodiode, and (iii) light emitting diode. Light power conversion
and electroluminescence efficiencies are ca. 0.5 % and 0.1 %, respectively.
Given the recent advances in large-scale production of 2D crystals, we expect
them to profoundly impact future developments in solar, lighting, and display
technologies.

###Ultrafast Charge Separation and Nongeminate Electron-Hole Recombination in Organic Photovoltaics||Samuel L Smith,Alex W Chin###

Ultrafast Charge Separation and Nongeminate Electron-Hole Recombination in Organic Photovoltaics. The mechanism of electron-hole separation in organic solar cells is currently
hotly debated. Recent experimental work suggests that these charges can
separate on extremely short timescales (<100 fs). This can be understood in
terms of delocalised transport within fullerene aggregates, which is thought to
emerge on short timescales before vibronic relaxation induces polaron
formation. However, in the optimal heterojunction morphology, electrons and
holes will often re-encounter each other before reaching the electrodes. If
such charges trap and cannot separate, then device efficiency will suffer. Here
we extend the theory of ultrafast charge separation to incorporate polaron
formation, and find that the same delocalised transport used to explain
ultrafast charge separation can account for the suppression of nongeminate
recombination in the best devices.

###Charge transfer from an adsorbed ruthenium-based photosensitizer through an ultra-thin aluminium oxide layer and into a metallic substrate||Andrew J. Gibson,Robert H. Temperton,Karsten Handrup,Matthew Weston,Louise C. Mayor,James N. O'Shea###

Charge transfer from an adsorbed ruthenium-based photosensitizer through an ultra-thin aluminium oxide layer and into a metallic substrate. The interaction of the dye molecule N3
(cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4'-dicarboxylato)-ruthenium(II))
with the ultra-thin oxide layer on a AlNi(110) substrate, has been studied
using synchrotron radiation based photoelectron spectroscopy, resonant
photoemission spectroscopy (RPES) and near edge X-ray absorption fine structure
spectroscopy (NEXAFS). Calibrated X-ray absorption and valence band spectra of
the monolayer and multilayer coverages reveal that charge transfer is possible
from the molecule to the AlNi(110) substrate via tunnelling through the
ultra-thin oxide layer and into the conduction band edge of the substrate. This
charge transfer mechanism is possible from the LUMO+2&3 in the excited state
but not from the LUMO, therefore enabling core-hole clock analysis, which gives
an upper limit of $6.0\pm$2.5fs for the transfer time. This indicates that
ultra-thin oxide layers are a viable material for use in dye-sensitized solar
cells (DSSC), which may lead to reduced recombination effects and improved
efficiencies of future devices.

###Plasmonic Light Trapping in an Ultrathin Photovoltaic Layer with Film-Coupled Metamaterial Structures||Hao Wang,Liping Wang###

Plasmonic Light Trapping in an Ultrathin Photovoltaic Layer with Film-Coupled Metamaterial Structures. A film-coupled metamaterial structure is numerically investigated for
enhancing the light absorption in an ultrathin photovoltaic layer of
crystalline gallium arsenide (GaAs). The top subwavelength concave grating and
the bottom metallic film could not only effectively trap light with the help of
wave interference and magnetic resonance effects excited above the bandgap, but
also practically serve as electrical contacts for photon-generated charge
collection. The energy absorbed by the active layer is greatly enhanced in the
film-coupled metamaterial structure, resulting in significant enhancement on
the short-circuit current density by three times over a free-standing GaAs
layer at the same thickness. The results would facilitate the development of
next-generation ultrathin solar cells with lower cost and higher efficiency.

###Impact of nanostructure configuration on the photovoltaic performance of quantum dot arrays||Aude Berbezier,Urs Aeberhard###

Impact of nanostructure configuration on the photovoltaic performance of quantum dot arrays. In this work, a mesoscopic model based on the non-equilibrium Green's
function formalism for a tight-binding-like effective Hamiltonian is used to
investigate a selectively contacted quantum dot array designed for operation as
a single junction quantum dot solar cell. By establishing a direct relation
between nanostructure configuration and optoelectronic properties, the
investigation reveals the influence of inter-dot and dot-contact coupling
strengths on the rates of charge carrier photogeneration, radiative
recombination, and extraction at contacts, and consequently on the ultimate
performance of photovoltaic devices with finite quantum dot arrays as the
active medium. For long carrier lifetimes, the dominant configuration effects
originate in the dependence of the joint density of states on the inter-dot
coupling in terms of band width and effective band gap. In the low carrier
lifetime regime, where recombination competes with carrier extraction, the
extraction efficiency shows a critical dependence on the dot-contact coupling.

###Domain Decomposition for Heterojunction Problems in Semiconductors||Timothy Costa,David Foster,Malgorzata Peszynska###

Domain Decomposition for Heterojunction Problems in Semiconductors. We present a domain decomposition approach for the simulation of charge
transport in heterojunction semiconductors. The problem is characterized by a
large variation of primary variables across an interface region of a size much
smaller than the device scale, and requires a multiscale approach in which that
region is modeled as an internal boundary. The model combines drift diffusion
equations on subdomains coupled by thermionic emission heterojunction model on
the interface which involves a nonhomogeneous jump computed at fine scale with
Density Functional Theory. Our full domain decomposition approach extends our
previous work for the potential equation only, and we present perspectives on
its HPC implementation. The model can be used, e.g., for the design of higher
efficiency solar cells for which experimental results are not available. More
generally, our algorithm is naturally parallelizable and is a new domain
decomposition paradigm for problems with multiscale phenomena associated with
internal interfaces and/or boundary layers.

###Magnetic dipolar interaction between correlated triplets created by singlet fission in tetracene crystals||Rui Wang,Chunfeng Zhang,Bo Zhang,Yunlong Liu,Xiaoyong Wang,Min Xiao###

Magnetic dipolar interaction between correlated triplets created by singlet fission in tetracene crystals. Singlet fission (SF) can potentially break the Shockley-Queisser efficiency
limit in single-junction solar cells by splitting one photo-excited singlet
exciton (S1) into two triplets (2T1) in organic semiconductors. A dark
multi-exciton (ME) state has been proposed as the intermediate connecting S1 to
2T1. However, the exact nature of this ME state, especially how the
doubly-excited triplets interact, remains elusive. Here, we report a
quantitative study on the magnetic dipolar interaction between SF-induced
correlated triplets in tetracene crystals by monitoring quantum beats relevant
to the ME sublevels at room temperature. The resonances of ME sublevels
approached by tuning an external magnetic field are observed to be avoided,
which agrees well with the theoretical predictions considering a magnetic
dipolar interaction of ~ 0.008 GHz. Our work paves a way to quantify the
magnetic dipolar interaction in organic materials and marks an important step
towards understanding the underlying physics of the ME state.

###Domain Walls Conductivity in Hybrid Organometallic Perovskites: The Key of CH3NH3PbI3 Solar Cell High Performance||Sergey N. Rashkeev,Fedwa El-Mellouhi,Sabre Kais,Fahhad H. Alharbi###

Domain Walls Conductivity in Hybrid Organometallic Perovskites: The Key of CH3NH3PbI3 Solar Cell High Performance. The past several years has witnessed a surge of interest in organometallic
trihalide perovskites, which are at the heart of the new generation of
solid-state solar cells. Here, we calculated the static conductivity of charged
domain walls in n- and p- doped organometallic uniaxial ferroelectric
semiconductor perovskite CH3NH3PbI3 using the Landau-Ginzburg-Devonshire (LGD)
theory. We find that due to the charge carrier accumulation, the static
conductivity may drastically increase at the domain wall by 3-4 orders of
magnitude in comparison with conductivity through the bulk of the material.
Also, a two-dimensional degenerated gas of highly mobile charge carriers could
be formed at the wall. The high values of conductivity at domain walls and
interfaces explain high efficiency in organometallic solution-processed
perovskite films which contains lots of different point and extended defects.
These results could suggest new routes to enhance the performance of this
promising class of novel photovoltaic materials.

###Modeling of dual-metal Schottky contacts based silicon micro and nano wire solar cells||M. Golam Rabbani,Amit Verma,Michael M. Adachi,Jency P. Sundararajan,Mahmoud M. Khader,Reza Nekovei,M. P. Anantram###

Modeling of dual-metal Schottky contacts based silicon micro and nano wire solar cells. We study solar cell properties of single silicon wires connected at their
ends to two dissimilar metals of different work functions. Effects of wire
dimensions, the work functions of the metals, and minority carrier lifetimes on
short circuit current as well as open circuit voltage are studied. The most
efficient photovoltaic behavior is found to occur when one metal makes a
Schottky contact with the wire, and the other makes an Ohmic contact. As wire
length increases, both short circuit current and open circuit voltage increase
before saturation occurs. Depending on the work function difference between the
metals and the wire dimensions, the saturation length increases by
approximately an order of magnitude with a two order magnitude increase in
minority carrier length. However current per surface area exposed to light is
found to decrease rapidly with increase in length. The use of a multi-contact
interdigitated design for long wires is investigated to increase the
photovoltaic response of the devices.

###CuSbSe2 photovoltaic devices with 3% efficiency||Adam Welch,Lauryn Baranowski,Pawel Zawadzki,Stephan Lany,Colin Wolden,Andriy Zakutayev###

CuSbSe2 photovoltaic devices with 3% efficiency. Recent technical and commercial successes of existing thin film solar cell
technologies motivates exploration of next-generation photovoltaic (PV)
absorber materials. Of particular scientific interest are compounds like
CuSbSe$_2$, which do not have the conventional tetrahedral semiconductor
bonding. Here, we demonstrate 1.5 {\mu}m thick CuSbSe$_2$ PV prototypes
prepared at 380-410{\deg}C by a self-regulated sputtering process using the
conventional substrate device architecture. The p-type CuSbSe$_2$ absorber has
a 1.1 eV optical absorption onset, ~$10^{5}$ cm$^{-1}$ absorption coefficient
at 0.3 eV above the onset, and a hole concentration of ~10$^{17}$ cm$^{-3}$.
The promising >3% energy conversion efficiency (Jsc = 20 mA/cm$^2$, FF = 0.44,
Voc = 0.35 V) in these initial devices is limited by bulk recombination that
limits photocurrent, device engineering issues that affect fill factor, and a
photovoltage deficit that likely results from the non-ideal CuSbSe2/CdS band
offset.

###Ultra-directional super-scattering of homogenous spherical particles with radial anisotropy||Wei Liu###

Ultra-directional super-scattering of homogenous spherical particles with radial anisotropy. We study the light scattering of homogenous radially-anisotropic spherical
particles. It is shown that radial anisotropy can be employed to tune
effectively the electric resonances, and thus enable flexible overlapping of
electric and magnetic dipoles of various numbers, which leads to unidirectional
forward super-scattering at different spectral positions. We further reveal
that through adjusting the radial anisotropy parameters, electric and magnetic
resonances of higher orders can be also made overlapped, thus further
collimating the forward scattering lobes. The ultra-directional
super-scattering we have obtained with individual homogenous radially
anisotropic spherical particles may shed new light to the design of compact and
efficient nanoantennas, which may find various applications in solar cells,
bio-sensing and many other antenna based researches.

###Selective coupling of optical energy into the fundamental diffusion mode of a scattering medium||Oluwafemi S. Ojambati,Hasan Yilmaz,Ad Lagendijk,Allard P. Mosk,Willem L. Vos###

Selective coupling of optical energy into the fundamental diffusion mode of a scattering medium. We demonstrate experimentally that optical wavefront shaping selectively
couples light into the fundamental diffusion mode of a scattering medium. The
total energy density inside a scattering medium of zinc oxide (ZnO)
nanoparticles was probed by measuring the emitted fluorescent power of spheres
that were randomly positioned inside the medium. The fluorescent power of an
optimized incident wave front is observed to be enhanced compared to a
non-optimized incident front. The observed enhancement increases with sample
thickness. Based on diffusion theory, we derive a model wherein the
distribution of energy density of wavefront-shaped light is described by the
fundamental diffusion mode. The agreement between our model and the data is
striking not in the least since there are no adjustable parameters. Enhanced
total energy density is crucial to increase the efficiency of white LEDs, solar
cells, and of random lasers, as well as to realize controlled illumination in
biomedical optics.

###Degradation of Co-Evaporated Perovskite Thin Film in Air||Congcong Wang,Youzhen Li,Xuemei Xu,Chenggong Wang,Fangyan Xie,Yongli Gao###

Degradation of Co-Evaporated Perovskite Thin Film in Air. Methylammonium lead halide perovskites have been developed as highly
promising materials to fabricate efficient solar cells in the past few years.
The real impact to energy applications relies on the understanding and
controlling of the stability of the material. We investigated the degradation
of CH3NH3PbI3 by air exposure using x-ray diffraction (XRD), x-ray
photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The
stoichiometric sample was grown with co-evaporation of PbI2 and CH3NH3I on a Au
coated Si wafer. It was found that the perovskite thin film gradually turned to
PbI2 in air, accompanied with complete removal of N and substantial reduction
of I. It was also observed that PbI2 crystallization roughened the film and
resulted in a partial exposure of the Au substrate.

###Non-equilibrium quantum heat machines||Rober Alicki,David Gelbwaser-Klimovsky###

Non-equilibrium quantum heat machines. Standard heat machines (engine, heat pump, refrigerator) are composed of a
system ("working fluid") coupled to at least two equilibrium baths at different
temperatures and periodically driven by an external device (piston or rotor)
called sometimes work reservoir. The aim of this paper is to go beyond this
scheme by considering environments which are stationary but cannot be
decomposed into few baths at thermal equilibrium. Such situations are
important, for example in solar cells, chemical machines in biology, various
realizations of laser cooling or nanoscopic machines driven by laser radiation.
We classify non-equilibrium baths depending on their thermodynamic behavior and
show that the efficiency of heat machines operating under their influences is
limited by a generalized Carnot bound.

###An efficient method for performance improvement of organometal halide perovskite solar cell via external electric field||Xiu Gong,Heng Ma,Yu-Rong Jiang,Meng Li,Zhao-Kui Wang,Tetsuo Soga###

An efficient method for performance improvement of organometal halide perovskite solar cell via external electric field. An effective method, performed adding external electric field (EEF) on
CH3NH3PbI3-xClx (OPIC) perovskite layer during the annealing process, is
proposed to improve the performance of the solar cell. By harmonizing EEF
direction with the hole/electron modified layer, a significant improvement on
the short circuit current and fill factor is obtained. Using the simplest
planar device, the largest positive EEF of 2.5*10^6 V/m makes PCE increase from
12.86 to 14.33, whose increment reaches 11.4% compared with non-EFE sample. By
analyzing the best and the statistics data, a fine positive correlation between
EEF and PEC is found. The physical mechanism which a displacement polarization
field induced by the ionic migration enhances the built in field of the
perovskite heterojunction is discussed. The study proposed a physical process
in modifying the cell efficiency and provides a new evidence on current-voltage
hysteresis of OPIC devices.

###Phosphorene and Transition Metal Dichalcogenide 2D Heterojunctions: Application in Excitonic Solar Cells||Vellayappan Dheivanayagam S/O Ganesan,Chun Zhang,Yuan Ping Feng,Lei Shen###

Phosphorene and Transition Metal Dichalcogenide 2D Heterojunctions: Application in Excitonic Solar Cells. Using the first-principles GW-Bethe-Salpeter equation method, here we study
the excited-state properties, including quasi-particle band structures and
optical spectra, of phosphorene, a two-dimensional (2D) atomic layer of black
phosphorus. The quasi-particle band gap of monolayer phosphorene is 2.15 eV and
its optical gap is 1.6 eV, which is suitable for excitonic thin film solar cell
applications. Next, this potential application is analysed by considering
type-II heterostructures with single layered phosphorene and transition metal
dichalcogenides (TMDs). These heterojunctions have a potential maximum power
conversion efficiency of up to 12\%, which can be further enhanced to 20\% by
strain engineering. Our results show that phosphorene is not only a promising
new material for use in nanoscale electronics, but also in optoelectronics.

###Natural Regulation of Energy Flow in a Green Quantum Photocell||Trevor B. Arp,Yafis Barlas,Vivek Aji,Nathaniel M. Gabor###

Natural Regulation of Energy Flow in a Green Quantum Photocell. Manipulating the flow of energy in nanoscale and molecular photonic devices
is of both fundamental interest and central importance for applications in
light harvesting optoelectronics. Under erratic solar irradiance conditions,
unregulated power fluctuations in a light harvesting photocell lead to
inefficient energy storage in conventional solar cells and potentially fatal
oxidative damage in photosynthesis. Here, we show that regulation against these
fluctuations arises naturally within a two-channel quantum heat engine
photocell, thus enabling the efficient conversion of varying incident solar
spectrum at Earth's surface. Remarkably, absorption in the green portion of the
spectrum is avoided, as it provides no inherent regulatory benefit. Our
findings illuminate a quantum structural origin of regulation, provide a novel
optoelectronic design strategy, and may elucidate the link between
photoprotection in photosynthesis and the predominance of green plants on
Earth.

###Separate-path electron and hole transport across pi-stacked ferroelectrics for photovoltaic applications||Malgorzata Wawrzyniak-Adamczewska,Malgorzata Wierzbowska###

Separate-path electron and hole transport across pi-stacked ferroelectrics for photovoltaic applications. Electron and hole separate-path transport is theoretically found in the
pi-stacked organic layers and columns. This effect might be a solution for the
charge recombination problem. The building molecules, named
1,3,5-tricyano-2,4,6-tricarboxy-benzene, contain the mesogenic flat aromatic
part and the terminal dipole groups which make the system ferroelectric. The
diffusion path of the electrons cuts through the aromatic rings, while holes
hop between the dipole groups. The transmission function and the charge
mobilities, especially for the holes, are very sensitive to the distance
between the molecular rings, due to the overlap of the pi-type orbitals. We
verified that the separation of the diffusion paths is not destroyed by the
application of the graphene leads. These features make the system suitable for
the efficient solar cells, with the carrier mobilities higher than these in the
organometal halide perovskites.

###Role of Polar Phonons in the Photo Excited State of Metal Halide Perovskites||Menno Bokdam,Tobias Sander,Alessandro Stroppa,Silvia Picozzi,D. D. Sarma,Cesare Franchini,Georg Kresse###

Role of Polar Phonons in the Photo Excited State of Metal Halide Perovskites. The development of high efficiency perovskite solar cells has sparked a
multitude of measurements on the optical properties of these materials. For the
most studied methylammonium(MA)PbI$_3$ perovskite, a large range (6-55 meV) of
exciton binding energies has been reported by various experiments. The
existence of excitons at room temperature is unclear. For the MAPb$X_3$
perovskites we report on relativistic $GW$-BSE calculations. This method is
capable to directly calculate excitonic properties from first-principles. At
low temperatures it predicts exciton binding energies in agreement with the
reported 'large' values. For MAPbI$_3$, phonon modes present in this frequency
range have a negligible contribution to the ionic screening. By calculating the
polarisation in time from finite temperature molecular dynamics, we show that
at room temperature this does not change. We therefore exclude ionic screening
as an explanation for the experimentally observed reduction of the exciton
binding energy at room temperature.

###Lattice Thermal Conductivity of Organic-Inorganic Hybrid Perovskite CH3NH3PbI3||Xin Qian,Xiaokun Gu,Ronggui Yang###

Lattice Thermal Conductivity of Organic-Inorganic Hybrid Perovskite CH3NH3PbI3. Great success has been achieved in improving the photovoltaic energy
conversion efficiency of the organic-inorganic perovskite-based solar cells,
but with very limited knowledge on the thermal transport in hybrid perovskites,
which would affect the device lifetime and stability. Based on the potential
developed from the density functional theory calculations, we studied the
lattice thermal conductivity of the hybrid halide perovskite CH3NH3PbI3 using
equilibrium molecular dynamics simulations. Temperature-dependent thermal
conductivity is reported from 160 K to 400 K, which covers the tetragonal phase
(160-330 K) and the pseudocubic phase (>330K). A very low thermal conductivity
(0.50 W/mK) is found in the tetragonal phase at room temperature, whereas a
much higher thermal conductivity is found in the pseudocubic phase (1.80 W/mK
at 330 K). The low group velocity of acoustic phonons and the strong
anharmonicity are found responsible for the relatively low thermal conductivity
of the tetragonal CH3NH3PbI3.

###Energy density distribution of shaped waves inside scattering media mapped onto a complete set of diffusion modes||Oluwafemi S. Ojambati,Allard P. Mosk,Ivo M. Vellekoop,Ad Lagendijk,Willem L. Vos###

Energy density distribution of shaped waves inside scattering media mapped onto a complete set of diffusion modes. We show that the spatial distribution of the energy density of optimally
shaped waves inside a scattering medium can be described by considering only a
few of the lowest eigenfunctions of the diffusion equation. Taking into account
only the fundamental eigenfunction, the total internal energy inside the sample
is underestimated by only 2%. The spatial distribution of the shaped energy
density is very similar to the fundamental eigenfunction, up to a cosine
distance of about 0.01. We obtained the energy density inside a quasi-1D
disordered waveguide by numerical calculation of the joined scattering matrix.
Computing the transmission-averaged energy density over all transmission
channels yields the ensemble averaged energy density of shaped waves. From the
averaged energy density obtained, we reconstruct its spatial distribution using
the eigenfunctions of the diffusion equation. The results from our study have
exciting applications in controlled biomedical imaging, efficient light
harvesting in solar cells, enhanced energy conversion in solid-state lighting,
and low threshold random lasers.

###Theoretical study of asymmetric A-π-D-π-D-π-A' tribranched organic sensitizer for Dye-sensitized solar cells||Geon Hyeong Lee,Young Sik Kim###

Theoretical study of asymmetric A-π-D-π-D-π-A' tribranched organic sensitizer for Dye-sensitized solar cells. An asymmetric A-{\pi}-D-{\pi}-D-{\pi}-A' tribranched organic dye (dye1) with
a cyanoacrylic acid and an indolinum carboxyl acid as electron acceptors and a
triphenylamine as an electron donor was designed and theoretically investigated
for dye-sensitized solar cells (DSSCs). Dye1 was compared to reference
well-known dyes with single electron acceptors (D5 and JYL-SQ6). Density
functional theory and time-dependent density functional theory calculations
were used to estimate the photovoltaic properties of the dyes. Due to the
different lowest unoccupied molecular orbital levels of each acceptor and the
energy antenna of the dual electron donor (D-{\pi}-D), the absorption spectrum
of each branch displayed different shapes. Considering the overall properties,
the asymmetric A-{\pi}-D-{\pi}-D-{\pi}-A' tribranched organic dye exhibited
high conversion efficiency performance for DSSCs. The findings of this work
suggest that optimizing the branch of electron donors and acceptors in dye
sensitizers based on asymmetric A-{\pi}-D-{\pi}-D-{\pi}-A' tribranched organic
dye produces good photovoltaic properties for DSSCs.

###Circular Photogalvanic Effect in Organometal Halide Perovskite CH$_3$NH$_3$PbI$_3$||Junwen Li,Paul M. Haney###

Circular Photogalvanic Effect in Organometal Halide Perovskite CH$_3$NH$_3$PbI$_3$. We study the circular photogalvanic effect in the organometal halide
perovskite solar cell absorber CH$_3$NH$_3$PbI$_3$. For crystal structures
which lack inversion symmetry, the calculated photocurrent density is about
$10^{-9}$ A/W, comparable to the previously studied quantum well and bulk
Rashba systems. Because of the dependence of the circular photogalvanic effect
on inversion symmetry breaking, the degree of inversion asymmetry at different
depths from the surface can be probed by tuning the photon energy and
associated penetration depth. We propose that measurements of this effect may
clarify the presence or absence of inversion symmetry, which remains a
controversial issue and has been argued to play an important role in the high
conversion efficiency of this material.

###Room temperature dynamic correlation between methylammonium molecules in lead-iodine based perovskites: An ab-initio molecular dynamics perspective||Jonathan Lahnsteiner,Georg Kresse,Abhinav Kumar,D. D. Sarma,Cesare Franchini,Menno Bokdam###

Room temperature dynamic correlation between methylammonium molecules in lead-iodine based perovskites: An ab-initio molecular dynamics perspective. The high efficiency of lead organo-metal-halide perovskite solar cells has
raised many questions about the role of the methylammonium (MA) molecules in
the Pb-I framework. Experiments indicate that the MA molecules are able to
'freely' spin around at room temperature even though they carry an intrinsic
dipole moment. We have performed large supercell (2592 atoms) finite
temperature ab-initio molecular dynamics calculations to study the correlation
between the molecules in the framework. An underlying long range
anti-ferroelectric ordering of the molecular dipoles is observed. The dynamical
correlation between neighboring molecules shows a maximum around room
temperature in the mid-temperature phase. In this phase, the rotations are slow
enough to (partially) couple to neighbors via the Pb-I cage. This results in a
collective motion of neighboring molecules in which the cage acts as the
mediator. At lower and higher temperatures the motions are less correlated.

###Charge Transport in Dendrimer Melt using Multiscale Modeling Simulation||Saientan Bag,Manish Jain,Prabal K Maiti###

Charge Transport in Dendrimer Melt using Multiscale Modeling Simulation. In this paper we present a theoretical calculation of the charge carrier
mobility in two different dendrimeric melt system (Dendritic phenyl azomethine
with Triphenyl amine core and Dendritic Carbazole with Cyclic Phenylazomethine
as core), which have recently been reported1 to increase the efficiency of
Dye-Sensitized solar cells (DSSCs) by interface modification. Our mobility
calculation, which is a combination of molecular dynamics simulation, first
principles calculation and kinetic Monte Carlo simulation, leads to mobilities
that are in quantitative agreement with available experimental data. We also
show how the mobility depends on the dendrimer generation. Furthermore, we
examine the variation of mobility with external electric field and external
reorganization energy. Physical mechanisms behind observed electric field and
generation dependencies of mobility are also explored.

###Hierarchical DSSC structures based on single walled TiO2 nanotube arrays reach back-side illumination solar light conversion efficiency of 8%||Seulgi So,Imgon Hwang,Patrik Schmuki###

Hierarchical DSSC structures based on single walled TiO2 nanotube arrays reach back-side illumination solar light conversion efficiency of 8%. In the present work we introduce a path to the controlled construction of
DSSCs based on hierarchically structured single walled, self-organized TiO2
layers. In a first step we describe a simple approach to selectively remove the
inner detrimental shell of anodic TiO2 nanotubes (NTs). This then allows
controlled well-defined layer-by-layer decoration of these TiO2-NT walls with
TiO2 nanoparticles (this in contrast to conventional TiO2 nanotubes). We show
that such defined multiple layered decoration can be optimized to build dye
sensitized solar cells that (under back-side illumination conditions) can yield
solar light conversion efficiencies in the range of 8 %. The beneficial effects
observed can be ascribed to a combination of three factors : 1) improved
electronic properties of the single walled tubes themselves, 2) a further
improvement of the electronic properties by the defined TiCl4 treatment, and 3)
a higher specific dye loading that becomes possible for the layer-by-layer
decorated single walled tubes.

###Unraveling Energetic Disorder in Organic Bulk Heterojunction Photovoltaics by Capacitance-Voltage Spectroscopy||Xixiang Zhu,Kai Wang,Changfeng Han,Qin Yang,Xiaojuan Sun,Haomiao Yu,Ming Shao,Fujun Zhang,Bin Hu###

Unraveling Energetic Disorder in Organic Bulk Heterojunction Photovoltaics by Capacitance-Voltage Spectroscopy. Organic semiconductors possess an intrinsic energetic disorder
characteristic, which holds an exceptionally important role for understanding
organic photovoltaic (OPV) operation and future optimization. We performed
illumination intensity dependence of capacitance-voltage (C-V) measurements in
PIDTDTQx:PC70BM based organic bulk heterojunction (BHJ) photovoltaics in
working conditions. Energetic disorder profiles for the photo-active layer,
PIDTDTQx:PC70BM, changed significantly when different interfaces were involved.
The effects of energetic disorder that could be reflected from C-V profiles are
incorporated through an exponential or Gaussian model of density of states
(DOS), or a combination of these two. Results underlie that an identical
organic blend in BHJ solar cells exhibits different energetic disorder when it
interacts with various interfaces. It may, thus, has a certain impact on OPV
performances, such as open-circuit voltage (V_oc ). Our study provides device
physicists a different perspective view for tailoring the organic energetic
disorder parameter via interfaces in order to enhance photo-electron conversion
efficiencies (PCE).

###Large Thermal Motion in Halide Perovskites||T. A. Tyson,W. Gao,Y. -S. Chen,S. Ghose,Y. Yan###

Large Thermal Motion in Halide Perovskites. Solar cells based on hybrid perovskites have shown high efficiency while
possessing simple processing methods. To gain a fundamental understanding of
their properties on an atomic level, we investigate single crystals of
CH3NH3PbI3 with a narrow transition (~5 K) near 327 K. Temperature dependent
structural measurements reveal a persistent tetragonal structure with smooth
changes in the atomic displacement parameters (ADPs) on crossing T*. We show
that the ADPs for I ions yield extended flat regions in the potential wells
consistent with the measured large thermal expansion parameter. Molecular
dynamics simulations reveal that this material exhibits significant high
asymmetries in the Pb-I pair distribution functions. We also show that the
intrinsically enhanced freedom of motion of the iodine atoms enables large
deformations. This flexibility (softness) of the atomic structure results in
highly localized atomic relaxation about defects and hence accounts for both
the high carrier mobility as well as the structural instability.

###Azetidinium Lead Iodide for Perovskite Solar Cells||Samuel R. Pering,Wentao Deng,Joel R. Troughton,Ralf G. Niemann,Federico Brivio,Peter S. Kubiak,Florence E. Jeffrey,Trystan M. Watson,Paul. R. Raithby,Andrew L. Johnson,Simon E. Lewis,PJ. Cameron###

Azetidinium Lead Iodide for Perovskite Solar Cells. Hybrid organic-inorganic perovskites have been established as good candidate
materials for emerging photovoltaics, with device efficiencies of over 22 %
being reported. There are currently only two organic cations, methylammonium
and formamidinium, which produce 3D perovskites with band gaps suitable for
photovoltaic devices. Numerous computational studies have identified
azetidinium as a potential third cation for synthesizing organic-inorganic
perovskites, but to date no experimental reports of azetidinium containing
perovskites have been published. Here we prepare azetidinium lead iodide for
the first time and show that it is a stable, bright orange material that can be
successfully used as the absorber layer in solar cells. We also show that it is
possible to make mixed cation devices by adding the azetidinium cation to
methylammonium lead iodide. Mixed azetidinium-methylammonium cells show
improved performance and reduced hysteresis compared to methylammonium lead
iodide cells.

###Experimental Realization of Deep Subwavelength Confinement in Dielectric Optical Resonators||S. Hu,M. Khater,R. Salas-Montiel,E. Kratschmer,S. Engelmann,W. M. J. Green,S. M. Weiss###

Experimental Realization of Deep Subwavelength Confinement in Dielectric Optical Resonators. The ability to highly localize light with strong electric field enhancement
is critical for enabling higher efficiency solar cells, light sources, and
modulators. While deep subwavelength modes can be realized with plasmonic
resonators, large losses in these metal structures preclude most practical
applications. We developed an alternative approach to achieving subwavelength
confinement that is not accompanied by inhibitive losses. We experimentally
demonstrate a dielectric bowtie photonic crystal structure that supports mode
volumes commensurate with plasmonic elements and quality factors that reveal
ultra-low losses. Our approach opens the door to the extremely strong
light-matter interaction regime with simultaneously both ultra-low mode volume
and ultra-high quality factor that has remained elusive in optical resonators.

###Defect properties of Sn- and Ge-doped ZnTe: Suitability for intermediate-band solar cells||Mauricio A. Flores###

Defect properties of Sn- and Ge-doped ZnTe: Suitability for intermediate-band solar cells. We investigate the electronic structure and defect properties of Sn- and Ge-
doped ZnTe by first-principles calculations within the DFT+$GW$ formalism. We
find that $(\text{Sn}_\text{Zn})$ and $(\text{Ge}_\text{Zn})$ introduce
isolated energy levels deep in the band gap of ZnTe, derived from Sn-5s and
Ge-4s states, respectively. Moreover, the incorporation of Sn and Ge on the Zn
site is favored in p-type ZnTe, in both Zn-rich and Te-rich environments. The
optical absorption spectra obtained by solving the Bethe-Salpeter equation
reveals that sub-bandgap absorptance is greatly enhanced due to the formation
of the intermediate band. Our results suggest that Sn- and Ge-doped ZnTe would
be a suitable material for the development of intermediate-band solar cells,
which have the potential to achieve efficiencies beyond the single-junction
limit.

###Ferroic Domains of Alternating Polar and Nonpolar Orders Regulate Photocurrent in Single Crystalline CH3NH3PbI3 Films Self-grown on FTO/TiO2 Substrate||Boyuan Huang,Guoli Kong,Ehsan Nasr Esfahani,Shulin Chen,Qian Li,Junxi Yu,Ningan Xu,Ying Zhang,Shuhong Xie,Haidan Wen,Peng Gao,Jinjin Zhao,Jiangyu Li###

Ferroic Domains of Alternating Polar and Nonpolar Orders Regulate Photocurrent in Single Crystalline CH3NH3PbI3 Films Self-grown on FTO/TiO2 Substrate. Photovoltaic conversion efficiency (PCE) of halide perovskite solar cells has
risen spectacularly, yet the very crystalline structure of CH3NH3PbI3 remains
ambiguous after extensive researches, and its polar nature remains hotly
debated. Here we present compelling evidences that CH3NH3PbI3 crystals
self-grown on FTO/TiO2 substrate consist of ferroic domains with alternating
polar and nonpolar orders, in contrast to previous experimental and theoretical
expectations, and polar domains possess reduced photocurrent. It is found that
polar and nonpolar orders of CH3NH3PbI3 can be distinguished from their
distinct lateral piezoresponse, energy dissipation, first and second harmonic
electromechanical couplings, and temperature variation, even though their
difference in crystalline lattice is very subtle, and they possess two-way
memory effect through cubic-tetragonal phase transition. These findings resolve
key questions regarding polar nature of CH3NH3PbI3 and its implication on
photovoltaics, reconcile contradictory data widely reported, and point a
direction toward engineering ferroic domains for enhanced PCE.

###Influence of the aggregate state on band structure and optical properties of C60 computed with different methods||Amrita Pal,Saeid Arabnejad,Koichi Yamashita,Sergei Manzhos###

Influence of the aggregate state on band structure and optical properties of C60 computed with different methods. C60 and C60 based molecules are efficient acceptor and electron transport
layers for planar perovskite solar cells. While properties of these molecules
are well studied by ab initiomethods, those of solid C60, specifically its
optical absorption properties, are not. We present a combined Density
Functional Theory - Density Functional Tight Binding study of the effect of
solid state packing on bandstructure and optical absorption of C60. The valence
and conduction band edge energies of solid C60 differ on the order of 0.1 eV
from single molecule frontier orbital energies. We show that calculations of
optical properties using linear response TD-DFT(B) or the imaginary part of the
dielectric constant (dipole approximation) can result in unrealistically large
redshift in the presence of intermolecular interactions compared to available
experimental data. We show that optical spectra computed from the
frequency-dependent real polarizability better reproduce the effect of C60
aggregation on optical absorption and may be more suited to study effects of
molecular aggregation.

###Boron-doping of cubic SiC for intermediate band solar cells: a scanning transmission electron microscopy study||Patricia Almeida Carvalho,Annett Thørgesen,Quanbao Ma,Daniel Nielsen Wright,Spyros Diplas,Augustinas Galeckas,Alexander Azarov,Valdas Jokubavicius,Jianwu Sun,Mikael Syväjärvi,Bengt Gunnar Svensson,Ole Martin Løvvik###

Boron-doping of cubic SiC for intermediate band solar cells: a scanning transmission electron microscopy study. Boron (B) has the potential for generating an intermediate band in cubic
silicon carbide (3C-SiC), turning this material into a highly efficient
absorber for single-junction solar cells. The formation of a delocalized band
demands high concentration of the foreign element, but the precipitation
behavior of B in the 3C polymorph of SiC is not well known. Here,
probe-corrected scanning transmission electron microscopy and secondary-ion
mass spectrometry are used to investigate precipitation mechanisms in
B-implanted 3C-SiC as a function of temperature. Point-defect clustering was
detected after annealing at 1273 K, while stacking faults, B-rich precipitates
and dislocation networks developed in the 1573 - 1773 K range. The precipitates
adopted the rhombohedral B13C2 structure and trapped B up to 1773 K. Above this
temperature, higher solubility reduced precipitation and free B diffused out of
the implantation layer. Dopant concentrations E19 at.cm-3 were achieved at 1873
K.

###Role of electron-phonon coupling and thermal expansion on band gaps, carrier mobility, and interfacial offsets in kesterite thin-film solar cells||Bartomeu Monserrat,Ji-Sang Park,Sunghyun Kim,Aron Walsh###

Role of electron-phonon coupling and thermal expansion on band gaps, carrier mobility, and interfacial offsets in kesterite thin-film solar cells. The efficiencies of solar cells based on kesterite Cu$_2$ZnSnS$_4$ (CZTS) and
Cu$_2$ZnSnSe$_4$ (CZTSe) are limited by a low open-circuit voltage due to high
rates of non-radiative electron-hole recombination. To probe the origin of this
bottleneck, we calculate the band offset of CZTS(Se) with CdS, confirming a
weak spike of 0.1 eV for CZTS/wurtzite-CdS and a strong spike of 0.4 eV for
CZTSe/wurtzite-CdS. We also consider the effects of temperature on the band
alignment, finding that increasing temperature significantly enhances the
spike-type offset. We further resolve an outstanding discrepancy between
measured and calculated phonon frequencies for the kesterites, and use these to
estimate the upper limit of electron and hole mobilities based on optic phonon
Fr\"ohlich scattering, which uncovers an intrinsic asymmetry with faster
(minority carrier) electron mobility.

###Photocarrier extraction in GaAsSb/GaAsN type-II QW superlattice solar cells||Urs Aeberhard,Alicia Gonzalo,Jose María Ulloa###

Photocarrier extraction in GaAsSb/GaAsN type-II QW superlattice solar cells. Photocarrier transport and extraction in GaAsSb/GaAsN type-II quantum well
superlattices are investigated by means of inelastic quantum transport
calculations based on the non-equilibrium Green's function formalism.
Evaluation of the local density of states and of the spectral current flow
enables the identification of different regimes for carrier localization,
transport, and extraction as a function of configurational parameters. These
include the number of periods, the thicknesses of the individual layers in one
period, the built-in electric field, and the temperature of operation. The
results for the carrier extraction efficiency are related to experimental data
for different symmetric GaAsSb/GaAsN type-II quantum well superlattice solar
cell devices and provide a qualitative explanation for the experimentally
observed dependence of photovoltaic device performance on period thickness.

###Inter-Domain Charge Transfer as a Rationale for Superior Photovoltaic Performances of Mixed Halide Lead Perovskites||Marine E. F. Bouduban,Fabrizio Giordano,Arnulf Rosspeintner,Joël Teuscher,Eric Vauthey,Michael Grätzel,Jacques-E. Moser###

Inter-Domain Charge Transfer as a Rationale for Superior Photovoltaic Performances of Mixed Halide Lead Perovskites. Organic-inorganic lead halide perovskites containing a mixture of iodide and
bromide anions consistently perform better in donor-acceptor heterojunction
solar cells than the standard methylammonium lead triiodide material. This
observation is counterintuitive, as it is generally expected for photovoltaic
materials that heterogeneities and compositional disorder cause reduced carrier
diffusion length and conversion efficiency. Here, we combine ultrafast
photoinduced electroabsorption and broadband fluorescence up-conversion
spectroscopy measurements to scrutinize the carrier dynamics in mixed-cations,
mixed-halide lead perovskite thin films. Our results evidence the formation of
charge transfer excitons (CTE) across the boundaries of domains of various
halide compositions. A global analysis of photoinduced transient Stark signals
shows that CTE evolve gradually from Br-rich to I-rich domains over tens to
hundreds of picoseconds. Rather than constituting recombination centres,
boundaries between domains of various halide compositions appear then to favour
charge carrier separation by driving photogenerated holes along channels of
decreasing bromide content.

###Perturbing beyond the shallow amplitude regime: Green's function scattering formalism with Bloch modes||A. Abass,A. Martins,S. Nanz,B. -H. V. Borges,E. R. Martins,C. Rockstuhl###

Perturbing beyond the shallow amplitude regime: Green's function scattering formalism with Bloch modes. We present a Bloch modes' based Green's function scattering formalism for
cost efficient forward modelling of disordered binary surface textures. The
usage of Bloch modes of an unperturbed reference ordered system as ansatz
allows our formalism to address surface scattering beyond the shallow amplitude
regime. The main advantage of our formalism is the possibility to utilize a
small amount of plane waves to represent the assumed Bloch modes thereby
reducing computational costs, while still allowing one to estimate the
scattering response to all channels accessible by the disordered system.
Benchmarking calculations discussed in the paper demonstrate how the usage of
our Bloch modes ansatz provides an excellent estimate of the scattering
response over an important regime of disorder. As an example of our method's
strength, we examine an electrically decoupled binary light trapping texture
and demonstrate how introducing disorder may improve light incoupling into the
considered solar cell structure.

###Optoelectronic and thermoelectric properties of Ba3DN (D = Sb, Bi): A DFT investigation||Enamul Haque,Md. Taslimur Rahman,M. Anwar Hossain###

Optoelectronic and thermoelectric properties of Ba3DN (D = Sb, Bi): A DFT investigation. We have investigated the optoelectronic and thermoelectric properties of
hexagonal antiperovskites Ba$_3$DN (D = Sb, Bi) using DFT calculations. The
calculated equilibrium lattice parameters of both compounds are in good
agreement with the available data. The calculated electronic structures
indicate that they are direct bandgap semiconductors and the values of bandgaps
are 1.35 and 1.33 eV for Ba3SbN and Ba3BiN, respectively. The inclusion of the
spin-orbit effect split the conduction bands and the band gap of Ba$_3$BiN is
much reduced. These two compounds have a high absorption coefficient, notably
higher than that for GaAs and close to that for silicon. The obtained static
refractive index is ~2.8 and 3.26, for Ba$_3$SbN and Ba$_3$BiN, respectively.
We predict that both materials are suitable for a high-efficiency solar cell.
Both compounds exhibit a high Seebeck coefficient and high power factor. Our
analysis predicts that the studied materials are potential candidates in
thermoelectric device applications.

###Finding a junction partner for candidate solar cell absorbers enargite and bournonite from electronic band and lattice matching||Suzanne K. Wallace,Keith T. Butler,Yoyo Hinuma,Aron Walsh###

Finding a junction partner for candidate solar cell absorbers enargite and bournonite from electronic band and lattice matching. An essential step in the development of a new photovoltaic (PV) technology is
choosing appropriate electron and hole extraction layers to make an efficient
device. We recently proposed the minerals enargite (\enargite) and bournonite
(\bournonite) as materials that are chemically stable with desirable
optoelectronic properties for use as the absorber layer in a thin-film PV
device. For these compounds, spontaneous lattice polarization with internal
electric fields --- and potential ferroelectricity --- may allow for enhanced
carrier separation and novel photophysical effects. In this work, we calculate
the ionization potentials for non-polar surface terminations and propose
suitable partners for forming solar cell heterojunctions by matching the
electronic band edges to a set of candidate electrical contact materials. We
then further screen these candidates by matching the lattice constants and
identify those that are likely to minimise strain and achieve epitaxy. This
two-step screening procedure identified a range of unconventional candidate
contact materials including SnS2, ZnTe, WO3, and Bi2O3.

###Crystallization-Arrested Viscoelastic Phase Separation in Semiconducting Polymer Gels||Jing He,Xiaoqing Kong,Yuhao Wang,Michael Delaney,Dilhan M. Kalyon,Stephanie S. Lee###

Crystallization-Arrested Viscoelastic Phase Separation in Semiconducting Polymer Gels. Through a combination of rheological characterization and
temperature-variable imaging methods, a novel gelation pathway in dilute
solutions of a semiconducting polymer to achieve interconnected, crystalline
networks with hierarchical porosity is reported. Upon rapid cooling, solutions
of regioregular poly(3-hexylthiophene) (RR-P3HT) in ortho-dichlorobenzene
formed thermoreversible gels. Temperature-variable confocal microscopy revealed
cooling-induced structural rearrangement to progress through viscoelastic phase
separation. The phase separation process arrested prematurely during the
formation of micron-sized solvent-rich "holes" within the RR-P3HT matrix due to
intrachain crystallization. Cryogen-based scanning electron microscopy of RR
P3HT gels revealed the existence of an interfibrillar network exhibiting
nano-sized pores. Remarkably, these networks formed to equal gel strengths when
a third component, either small molecule phenyl C61 butyric acid methyl ester
(PCBM) or non-crystallizing regiorandom (Rra)-P3HT, was added to the solution.
Organic solar cells in which the active layers were deposited from
phase-separated solutions displayed 45% higher efficiency compared to reference
cells.

###Influence of Carbon Nanomaterial Counter Electrode Composition, Dye Selection, and Photoanode Scaffolding on DSSC Performance||Benjamin K Barnes,Joshua Orebiyi,Kausik S Das###

Influence of Carbon Nanomaterial Counter Electrode Composition, Dye Selection, and Photoanode Scaffolding on DSSC Performance. As technology continues to evolve, the demand for renewable and sustainable
energy continues to grow. As the use of renewable energies, specifically
photovoltaics, is continually being adopted and incorporate into everyday life,
it is evident that a need for an increase in the amount of energy that is
derived from these processes is necessary. In the exploration of dye sensitized
solar cells (DSSC) a recent and increased involvement in the application of
carbon based nanomaterials and the effects of their unique electronic
properties is being investigated. Additionally, as the development of DSSC
continues to break way, different methods of dye selection and photoanode
scaffolding are being researched to ultimately increase the Power Conversion
Efficiency (PCE) of these cells.

###Hot Carrier and Surface Recombination Dynamics in Layered InSe Crystals||Chengmei Zhong,Vinod K. Sangwan,Joohoon Kang,Jan Luxa,Zdeněk Sofer,Mark C. Hersam,Emily A. Weiss###

Hot Carrier and Surface Recombination Dynamics in Layered InSe Crystals. Layered indium selenide (InSe) is a van der Waals solid that has emerged as a
promising material for high-performance ultrathin solar cells. The
optoelectronic parameters that are critical to photoconversion efficiencies,
such as hot carrier lifetime and surface recombination velocity, are however
largely unexplored in InSe. Here, these key photophysical properties of layered
InSe are measured with femtosecond transient reflection spectroscopy. The hot
carrier cooling process is found to occur through phonon scattering. The
surface recombination velocity and ambipolar diffusion coefficient are
extracted from fits to the pump energy-dependent transient reflection kinetics
using a free carrier diffusion model. The extracted surface recombination
velocity is approximately an order of magnitude larger than that for
methylammonium lead-iodide perovskites, suggesting that surface recombination
is a principal source of photocarrier loss in InSe. The extracted ambipolar
diffusion coefficient is consistent with previously reported values of InSe
carrier mobility.

###Efficient Nano Antenna for Photonic Devices||Vishal K. Doltani,Fahim A Umrani,Riaz A. Soomro###

Efficient Nano Antenna for Photonic Devices. This paper presents the efficient Yagi-Uda nanoantenna with the chain of
directors, reflector and a feed element, where these elements have been
optimized to fulfil the requirement of high directivity and the gain of
antenna. The proposed design consists of six core and cladding with silver core
and silicon cladding is used to achieve high directivity. The design is
analyzed by FIT based CST Software package by which Directivity and Gain of the
antenna are computed. The enhancement of this directivity is mainly contributed
by the increase in the number of directors from the four to five and the
optimized length of the directors which is scaled by 0.9. The simulated results
show that the proposed Yagi antenna provides good performance in terms of
directivity. The suggested antenna design shows the improved directivity of
17.62 at 500 nm wavelength. The proposed design has wide range of applications
including as solar cells.

###Exciton propagation and halo formation in two-dimensional materials||Raul Perea-Causin,Samuel Brem,Roberto Rosati,Roland Jago,Marvin Kulig,Jonas D. Ziegler,Jonas Zipfel,Alexey Chernikov,Ermin Malic###

Exciton propagation and halo formation in two-dimensional materials. The interplay of optics, dynamics and transport is crucial for the design of
novel optoelectronic devices, such as photodetectors and solar cells. In this
context, transition metal dichalcogenides (TMDs) have received much attention.
Here, strongly bound excitons dominate optical excitation, carrier dynamics and
diffusion processes. While the first two have been intensively studied, there
is a lack of fundamental understanding of non-equilibrium phenomena associated
with exciton transport that is of central importance e.g. for high efficiency
light harvesting. In this work, we provide microscopic insights into the
interplay of exciton propagation and many-particle interactions in TMDs. Based
on a fully quantum mechanical approach and in excellent agreement with
photoluminescence measurements, we show that Auger recombination and emission
of hot phonons act as a heating mechanism giving rise to strong spatial
gradients in excitonic temperature. The resulting thermal drift leads to an
unconventional exciton diffusion characterized by spatial exciton halos.

###Photochemical Upconversion Theory: Importance of Triplet Energy Levels and Triplet Quenching||David Jefferies,Timothy W. Schmidt,Laszlo Frazer###

Photochemical Upconversion Theory: Importance of Triplet Energy Levels and Triplet Quenching. Photochemical upconversion is a promising way to boost the efficiency of
solar cells using triplet exciton annihilation. Currently, predicting the
performance of photochemical upconversion devices is challenging. We present an
open source software package which takes experimental parameters as inputs and
gives the figure of merit of an upconversion system, enabling theory-driven
design of better solar energy devices. We incorporate the statistical
distribution of triplet excitons between the sensitizer and the emitter. Using
the dynamic quenching effect of the sensitizer on emitter triplet excitons, we
show that the optimal sensitizer concentration can be below the sensitizer
solubility limit in liquid devices. These theoretical contributions can
explain, without use of heavy atom-induced triplet exciton formation or phenyl
group rotation, the experimental failure of zinc octaethylporphyrin to
effectively sensitize diphenylanthracene, where platinum octaethylporphyrin
succeeds. Our predictions indicate a change in direction for device design that
will reduce triplet exciton losses.

###The influence of impurities on the charge carrier mobility of small molecule organic semiconductors||Pascal Friederich,Artem Fediai,Jing Li,Anirban Mondal,Naresh B. Kotadiya,Franz Symalla,Gert-Jan A. H. Wetzelaer,Denis Andrienko,Xavier Blase,David Beljonne,Paul W. M. Blom,Jean-Luc Brédas,Wolfgang Wenzel###

The influence of impurities on the charge carrier mobility of small molecule organic semiconductors. Amorphous organic semiconductors based on small molecules and polymers are
used in many applications, most prominently organic light emitting diodes
(OLEDs) and organic solar cells. Impurities and charge traps are omnipresent in
most currently available organic semiconductors and limit charge transport and
thus device efficiency. The microscopic cause as well as the chemical nature of
these traps are presently not well understood. Using a multiscale model we
characterize the influence of impurities on the density of states and charge
transport in small-molecule amorphous organic semiconductors. We use the model
to quantitatively describe the influence of water molecules and water-oxygen
complexes on the electron and hole mobilities. These species are seen to impact
the shape of the density of states and to act as explicit charge traps within
the energy gap. Our results show that trap states introduced by molecular
oxygen can be deep enough to limit the electron mobility in widely used
materials.

###Quantifying mobile ions in perovskite-based devices with temperature-dependent capacitance measurements: frequency versus time domain||Moritz H. Futscher,Mahesh K. Gangishetty,Daniel N. Congreve,Bruno Ehrler###

Quantifying mobile ions in perovskite-based devices with temperature-dependent capacitance measurements: frequency versus time domain. Perovskites have proven to be a promising candidate for highly-efficient
solar cells, light-emitting diodes, and X-ray detectors, overcoming limitations
of inorganic semiconductors. However, they are notoriously unstable. The main
reason for this instability is the migration of mobile ions through the device
during operation, as they are mixed ionic-electronic conductors. Here we show
how measuring the capacitance in both the frequency and the time domain can be
used to study ionic dynamics within perovskite-based devices, quantifying
activation energy, diffusion coefficient, sign of charge, concentration, and
the length of the ionic double layer in the vicinity of the interfaces.
Measuring the transient of the capacitance furthermore allows for
distinguishing between ionic and electronic effects.

###A highly integrated, stand-alone photoelectrochemical device for large-scale solar hydrogen production||Minoh Lee,Bugra Turan,Jan-Philipp Becker,Katharina Welter,Benjamin Klingebiel,Elmar Neumann,Yoo Jung Sohn,Tsvetelina Merdzhanova,Thomas Kirchartz,Friedhelm Finger,Uwe Rau,Stefan Haas###

A highly integrated, stand-alone photoelectrochemical device for large-scale solar hydrogen production. Although photoelectrochemical water splitting is likely to be an important
and powerful tool to provide environmentally friendly hydrogen, most
developments in this field have been conducted on a laboratory scale so far. In
order for the technology to make a sizeable impact on the energy transition,
scaled up devices made of inexpensive and earth abundant materials must be
developed. In this work, we demonstrate a scalable (64 cm2 aperture area)
artificial photoelectrochemical device composed of triple-junction thin-film
silicon solar cells in conjunction with an electrodeposited bifunctional nickel
iron molybdenum water splitting catalyst. Our device shows a solar to hydrogen
efficiency of up to 4.67% (5.33% active area) without bias assistance and wire
connection. Furthermore, gas separation was enabled by incorporating a membrane
in a 3D printed device frame.

###Microscopic observation of carrier-transport dynamics in quantum-structure solar cells using a time-of-flight technique||Kasidit Toprasertpong,Naofumi Kasamatsu,Hiromasa Fujii,Tomoyuki Kada,Shigeo Asahi,Yunpeng Wang,Kentaroh Watanabe,Masakazu Sugiyama,Takashi Kita,Yoshiaki Nakano###

Microscopic observation of carrier-transport dynamics in quantum-structure solar cells using a time-of-flight technique. In this study, we propose a carrier time-of-flight technique to evaluate the
carrier transport time across a quantum structure in an active region of solar
cells. By observing the time-resolved photoluminescence signal with a
quantum-well probe inserted under the quantum structure at forward bias, the
carrier transport time can be efficiently determined at room temperature. The
averaged drift velocity shows linear dependence on the internal field, allowing
us to estimate the quantum structure as a quasi-bulk material with low
effective mobility containing the information of carrier dynamics. We show that
this direct and real-time observation is more sensitive to carrier transport
than other conventional techniques, providing better insights into microscopic
carrier transport dynamics to overcome a device design difficulty.

###Interface Optimization via Fullerene Blends Enables Open-Circuit Voltages of 1.35 V in CH3NH3Pb(I0.8Br0.2)3 Solar Cells||Zhifa Liu,Johanna Siekmann,Benjamin Klingebiel,Uwe Rau,Thomas Kirchartz###

Interface Optimization via Fullerene Blends Enables Open-Circuit Voltages of 1.35 V in CH3NH3Pb(I0.8Br0.2)3 Solar Cells. Non-radiative recombination processes are the biggest hindrance to
approaching the radiative limit of the open-circuit voltage for wide-band gap
perovskite solar cells. In addition, to high bulk quality, good interfaces and
good energy level alignment for majority carriers at charge transport
layer-absorber interfaces are crucial to minimize non-radiative recombination
pathways. By tuning the lowest-unoccupied molecular-orbital of electron
transport layers via the use of different fullerenes and fullerene blends, we
demonstrate open-circuit voltages exceeding 1.35 V in CH3NH3Pb(I0.8Br0.2)3
device. Further optimization of mobility in binary fullerenes electron
transport layer can boost the power conversion efficiency as high as 18.6%. We
note in particular that the Voc-fill factor product is > 1.085 V, which is the
highest value reported for halide perovskites with this band gap.

###Ion Mobility Independent Large Signal Switching of Perovskite Devices||Saketh Tirupati,Abhimanyu Singareddy,Dhyana Sivadas,Pradeep R. Nair###

Ion Mobility Independent Large Signal Switching of Perovskite Devices. The presence of mobile ions in perovskites is well known to influence the
device electrostatics leading to a wide variety of anomalous characteristics
related to hysteresis, efficiency degradation, low frequency capacitance, large
signal switching, etc. Accordingly, the ion mobility is understood to a have a
critical influence on the associated time constants/delays. Quite contrary to
this broadly accepted thought, here we show that the time delays associated
with large signal switching show a universal behavior dictated by electronic
dipoles, rather than ionic dipoles. Due to the resultant sudden and dramatic
collapse of contact layer depletion region, switching delays are independent of
ion mobilities! Further, our detailed numerical simulations, well supported by
experimental results, indicate that terminal currents show near steady state
behavior well ahead of the relaxation of ionic distributions to their steady
state conditions. These results have interesting implications towards the
understanding and optimization of perovskite based electronic devices,
including solar cells and LEDs.

###Effect of heterostructure engineering on electronic structure and transport properties of two-dimensional halide perovskites||Rahul Singh,Prashant Singh,Ganesh Balasubramania###

Effect of heterostructure engineering on electronic structure and transport properties of two-dimensional halide perovskites. Organic-inorganic halide perovskite solar cells have attracted much attention
due to their low-cost fabrication, flexibility, and high-power conversion
efficiency. More recent efforts show that the reduction from three- to
two-dimensions (2D) of organic-inorganic halide perovskites promises an
exciting opportunity to tune their electronic properties. Here, we explore the
effect of reduced dimensionality and heterostructure engineering on the
intrinsic material properties, such as energy stability, bandgap and transport
properties of 2D hybrid organic-inorganic halide perovskites using
first-principles density functional theory. We show that the energetic
stability is significantly enhanced by engineered perovskite heterostructures
that also possess excellent transport properties similar to their bulk
counterparts. These layered chemistries also demonstrate the advantage of a
broad range of tunable bandgaps and high-absorption coefficient in the visible
spectrum. The proposed 2D heterostructured material holds potential for
nano-optoelectronic devices as well as for effective photovoltaics.

###Van der Waals heterojunction devices based on organohalide perovskites and two-dimensional materials||Hung-Chieh Cheng,Gongming Wang,Dehui Li,Qiyuan He,Anxiang Yin,Yuan Liu,Hao Wu,Mengning Ding,Yu Huang,Xiangfeng Duan###

Van der Waals heterojunction devices based on organohalide perovskites and two-dimensional materials. The recently emerged organohalide perovskites (e.g., CH3NH3PbI3) have drawn
intense attention for high efficiency solar cells. However, with a considerable
solubility in many solvents, these perovskites are not typically compatible
with conventional lithography processes for more complicated device
fabrications that are important for both fundamental studies and technological
applications. Here we report the creation of novel heterojunction devices based
on perovskites and two-dimensional (2D) crystals by taking advantage of the
layered characteristic of lead iodide (PbI2) and vapor phase intercalation. We
show a graphene/perovskite/graphene vertical stack can deliver a highest
photoresponsivity of ~950 A/W and photoconductive gain of ~2200, and a
graphene/WSe2/perovskite/graphene heterojunction can display a high on/off
ratio (~10^6) transistor behavior with distinct gate-tunable diode
characteristics and open-circuit voltages. Such unique perovskite-2D
heterostructures have significant potential for future optoelectronic research
and can enable broad possibilities with compositional tunability of
organohalide perovskites and the versatility offered by diverse 2D materials.

###Transient terahertz photoconductivity measurements of minority-carrier lifetime in tin sulfide thin films: Advanced metrology for an early-stage photovoltaic material||R. Jaramillo,Meng-Ju Sher,Benjamin K. Ofori-Okai,V. Steinmann,Chuanxi Yang,Katy Hartman,Keith A. Nelson,Aaron M. Lindenberg,Roy G. Gordon,T. Buonassisi###

Transient terahertz photoconductivity measurements of minority-carrier lifetime in tin sulfide thin films: Advanced metrology for an early-stage photovoltaic material. Materials research with a focus on enhancing the minority-carrier lifetime of
the light-absorbing semiconductor is key to advancing solar energy technology
for both early-stage and mature material platforms alike. Tin sulfide (SnS) is
an absorber material with several clear advantages for manufacturing and
deployment, but the record power conversion efficiency remains below 5%. We
report measurements of bulk and interface minority-carrier recombination rates
in SnS thin films using optical-pump, terahertz (THz)-probe transient
photoconductivity (TPC) measurements. Post-growth thermal annealing in H_2S gas
increases the minority-carrier lifetime, and oxidation of the surface reduces
the surface recombination velocity. However, the minority-carrier lifetime
remains below 100 ps for all tested combinations of growth technique and
post-growth processing. Significant improvement in SnS solar cell performance
will hinge on finding and mitigating as-yet-unknown recombination-active
defects. We describe in detail our methodology for TPC experiments, and we
share our data analysis routines as freely-available software.

###Promise of Commercialization: Carbon Materials for Low-Cost Perovskite Solar Cells||Yu Cai,Lusheng Liang,Peng Gao###

Promise of Commercialization: Carbon Materials for Low-Cost Perovskite Solar Cells. Perovskite solar cells (PVSCs) have attracted extensive studies due to their
high power conversion efficiency (PCE) with low-cost in both raw material and
processes. However, there remain obstacles that hinder the way to its
commercialization. Among many drawbacks in PVSCs, we note the problems brought
by the use of noble metal counter electrodes (CEs) such as gold (Au) and silver
(Ag). The costly Au and Ag need high energy-consumption thermal evaporation
process which can be made only with expensive evaporation equipment under
vacuum. All the factors elevate the threshold of PVSCs' commercialization.
Carbon material, on the other hand, is a readily available electrode candidate
for the application as CE in the PVSCs. In this review, endeavors on PVSCs with
low-cost carbon materials will be comprehensively discussed based on different
device structures and carbon composition. We believe that the PVSCs with
carbon-based CE hold the promise of commercialization of this new technology.

###Enhancement of photovoltaic efficiency by insertion of a polyoxometalate layer at the anode of an organic solar cell||M. Alaaeddine,Q. Zhu,D. Fichou,G. Izzet,J. E. Rault,N. Barrett,A. Proust,L. Tortech###

Enhancement of photovoltaic efficiency by insertion of a polyoxometalate layer at the anode of an organic solar cell. In this article the Wells-Dawson polyoxometalate K6[P2W18O62] is grown as an
interfacial layer between indium tin oxide and bulk heterojunction of
poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM). The structure of the POM layers depends on the thickness and shows a
highly anisotropic surface organization. The films have been characterized by
atomic force microscopy and X-ray photoelectron spectroscopy (XPS) to gain
insight into their macroscopic organization and better understand their
electronic properties. Then, they were put at the anodic interface of a
P3HT:PCBM organic solar cell and characterized on an optical bench. The
photovoltaic efficiency is discussed in terms of the benefit of the
polyoxometalate at the anodic interface of an organic photovoltaic cell.

###High Throughput Production of Transparent Conductive Single-Walled Carbon Nanotube Films via Advanced Floating Catalyst Chemical Vapor Deposition||Qiang Zhang,Weiya Zhou,Kewei Li,Nan Zhang,Yanchun Wang,Zhuojian Xiao,Qingxia Fan,Sishen Xie###

High Throughput Production of Transparent Conductive Single-Walled Carbon Nanotube Films via Advanced Floating Catalyst Chemical Vapor Deposition. Single-walled carbon nanotube (SWCNT) films are promising materials for
transparent conductive films (TCFs) with potential applications in flexible
displays, touch screens, solar cells and solid-state lighting1,2. However,
further reductions in resistivity and in cost of SWCNT films are necessary for
high quality TCF products3. Here, we report an improved floating catalyst
chemical vapor deposition method to directly and continuously produce ultrathin
and freestanding SWCNT films at the hundred meter-scale. Both carbon conversion
efficiency and SWCNT TCF yield are increased by three orders of magnitude
relative to the conventional floating catalyst chemical vapor deposition. After
doping, the film manifests a sheet resistance of 40 ohm/sq. at 90%
transmittance, representing record performance for large-scale SWCNT films. Our
work provides a new avenue to accelerate the industrialization of SWCNT films
as TCFs.

###Quantifying Charge Carrier Mobilities and Recombination Rates in Metal Halide Perovskites from Time-Resolved Microwave Photo-conductivity Measurements||Tom J. Savenije,Dengyang Guo,Valentina M. Caselli,Eline M. Hutter###

Quantifying Charge Carrier Mobilities and Recombination Rates in Metal Halide Perovskites from Time-Resolved Microwave Photo-conductivity Measurements. The unprecedented rise in power conversion efficiency of solar cells based on
metal halide perovskites (MHPs) has led to enormous research effort to
understand their photo-physical properties. In this paper, we review the
progress in understanding the mobility and recombination of photo-generated
charge carriers from nanosecond to microsecond time scales, monitored using
electrodeless transient photoconductivity techniques. In addition, we present a
kinetic model to obtain rate constants from transient data recorded using a
wide range of laser intensities. For various MHPs the temperature dependence of
the mobilities and recombination rates are evaluated. Furthermore, we show how
these rate constants can be used to predict the upper limit for the
open-circuit voltage Voc of the corresponding device. Finally, we discuss
photo-physical properties of MHPs that are not yet fully understood, and make
recommendations for future research directions.

###Theory of shift current in Anderson insulator||Hiroaki Ishizuka,Naoto Nagaosa###

Theory of shift current in Anderson insulator. Shift current is a photovoltaic current in bulk noncentrosymmetric insulator.
Studies on shift current have so far focused on the extended Bloch waves, such
as in semiconductors and perovskites. In contrast, it is unknown whether the
localized wavefunctions support the dc photocurrent. Here, we show
theoretically that the dc shift current appears in a noncentrosymmetric
disordered one-dimensional insulator with random potential. When the light
illuminates the entire sample, the photocurrent forms in presence of the
electron-phonon coupling. We find this photocurrent remains robustly even when
the energy scale of random potential is larger than the bandwidth. On the other
hand, the photocurrent decays exponentially when the excitation is local, or
the relaxation is only due to the contact with the electrodes. These results
open a route to design high-efficiency solar cells and photodetectors.

###First principles study of optical and tunable electronic properties of crystalline Li2TeO3||Aditya Dey###

First principles study of optical and tunable electronic properties of crystalline Li2TeO3. The optical and electronic properties of crystalline Li2TeO3, which is a
tellurite glass, is studied in the framework of density functional theory (DFT)
implemented software SIESTA. The material has monoclinic symmetrized structure
and the unit or primitive cell of the material, periodic in all directions has
been taken to study the properties. The electronic structures show that it is a
wide-gap semiconductor and the property changes to metallic when subjected to
electric field. This tunable property can be used in various fields of
electronics. The optical properties studied tells that Li2TeO3 can be a
promising material to be used as a hole transport material (HTM) for developing
efficient perovskite solar cell including other applications as well.

###Polarization Origin of Photoconductivity in MAPbI3 Thin Films||Rohit Saraf,Cecile Saguy,Vivek Maheshwari,Hemaprabha Elangovan,Yachin Ivry###

Polarization Origin of Photoconductivity in MAPbI3 Thin Films. Hybrid-halide perovskite (HHP) films exhibit exceptional photo-electric
properties. These materials are utilized for highly efficient solar cells and
photoconductive technologies. Both ion migration and polarization have been
proposed as the source of enhanced photoelectric activity, but the exact origin
of these advantageous device properties has remained elusive. Here, we combined
microscale and device-scale characterization to demonstrate that
polarization-assisted conductivity governs photoconductivity in thin HHP films.
Conductive atomic force microscopy under light and variable temperature
conditions showed that the photocurrent is directional and is suppressed at the
tetragonal-to-cubic transformation. It was revealed that polarization-based
conductivity is enhanced by light, whereas dark conductivity is dominated by
non-directional ion migration, as was confirmed by large-scale device
measurements. Following the non-volatile memory nature of polarization domains,
photoconductive memristive behavior was demonstrated. Understanding the origin
of photoelectric activity in HHP allows designing devices with enhanced
functionality and lays the grounds for photoelectric memristive devices.

###Change in Tetracene Polymorphism Facilitates Triplet Transfer in Singlet Fission-Sensitized Silicon Solar Cells||Benjamin Daiber,Sourav Maiti,Silvia Ferro,Joris Bodin,Alyssa F. J. van den Boom,Stefan L. Luxembourg,Sachin Kinge,Sidharam Pujari,Han Zuilhof,Laurens D. A. Siebbeles,Bruno Ehrler###

Change in Tetracene Polymorphism Facilitates Triplet Transfer in Singlet Fission-Sensitized Silicon Solar Cells. Singlet fission in tetracene generates two triplet excitons per absorbed
photon. If these triplet excitons can be effectively transferred into silicon
(Si) then additional photocurrent can be generated from photons above the
bandgap of Si. This could alleviate the thermalization loss and increase the
efficiency of conventional Si solar cells. Here we show that a change in the
polymorphism of tetracene deposited on Si due to air exposure, facilitates
triplet transfer from tetracene into Si. Magnetic field-dependent photocurrent
measurements confirm that triplet excitons contribute to the photocurrent. The
decay of tetracene delayed photoluminescence was used to determine a triplet
transfer time of 215 ns and a maximum yield of triplet transfer into Si of ~50
%. Our study suggests that control over the morphology of tetracene during
deposition will be of great importance to boost the triplet transfer yield
further.

###Effect of Precursor Stoichiometry on the Performance and Stability of MAPbBr3 Photovoltaic Devices||Lukas M. Falk,Katelyn P. Goetz,Vincent Lami,Qingzhi An,Paul Fassl,Jonas Herkel,Fabian Thome,Alexander D. Taylor,Fabian Paulus,Yana Vaynzof###

Effect of Precursor Stoichiometry on the Performance and Stability of MAPbBr3 Photovoltaic Devices. The wide band gap methylammonium lead bromide perovskite is promising for
applications in tandem solar cells and light-emitting diodes. Despite its
utility, there is only a limited understanding of its reproducibility and
stability. Herein, the dependence of the properties, performance, and shelf
storage of thin films and devices on minute changes to the precursor solution
stoichiometry is examined in detail. Although photovoltaic cells based on these
solution changes exhibit similar initial performance, the shelf-storage depends
strongly on the precursor solution stoichiometry. While all devices exhibit
some degree of healing, the bromide-deficient films show a remarkable
improvement, more than doubling in their photoconversion efficiency.
Photoluminescence spectroscopy experiments performed under different
atmospheres suggest that this increase is due in part to a trap healing
mechanism that occurs upon exposure to the environment. Our results highlight
the importance of understanding and manipulating defects in lead halide
perovskites to produce long-lasting, stable devices.

###Numerical Simulation of Cs2AgBiBr6-based Perovskite Solar Cell with ZnO Nanorod and P3HT as the Charge Transport Layers||Intekhab Alam,Rahat Mollick,Md Ali Ashraf###

Numerical Simulation of Cs2AgBiBr6-based Perovskite Solar Cell with ZnO Nanorod and P3HT as the Charge Transport Layers. We carried out simulative investigations on a non-toxic, lead-free perovskite
solar cell (PSC), where Cs2AgBiBr6, P3HT, ZnO nanorod, and C were utilized as
the absorber layer, hole transport layer, electron transport layer, and back
contact, respectively. At 600 nm optimum absorber thickness, the device
achieved a maximum power conversion efficiency of 4.48%. The PSC operated
optimally when the electron affinities were set at 3.3 eV and 4.6 eV for P3HT
and ZnO nanorod, respectively. Moreover, the hole mobility and acceptor
concentration of P3HT should be weighed during the choosing of appropriate
doping additives and doping levels. Besides, the optimum back contact work
function and absorber defect density were found to be 5.2 eV and 1E15 cm-3,
respectively. We also observed the effect of radiative recombination rates and
different charge transport layers on the device's performance. Overall, this
study's simulation results will provide insightful guidance towards fabricating
an environmentally benign PSC.

###The interesting case of a single-junction solar cell in outer space||Ido Frenkel,Avi Niv###

The interesting case of a single-junction solar cell in outer space. An isolated single-junction solar cell's temperature in outer space depends
only on its radiation exchange with its environment. We consider a cell with
zero emissivity below its bandgap and unity above it -- an idealization so far
considered to define the upper limit of photovoltaic power conversion
efficiency. For this case, we show that the detailed-balance and the energy
conservation laws that govern the cell's temperature and potential, cannot be
mutually solved. However, the cell's temperature and potential can be
determined if a finite amount of sub-bandgap emissivity is included, the exact
amount of which is found by minimizing the process's entropy generation.
Finally, we generalize this result to a photovoltaic system in contact with
some environment, hence for terrestrial conditions. Therefore, a universal
thermodynamic formulation of the photovoltaic effect emerges. Unlike former
attempts to thermodynamically justify the PV effect, our formalism applies to a
work producing system.

###Optoelectronic Properties of Chalcogenide Perovskites by Many-Body Perturbation Theory||Manish Kumar,Arunima Singh,Deepika Gill,Saswata Bhattacharya###

Optoelectronic Properties of Chalcogenide Perovskites by Many-Body Perturbation Theory. Chalcogenide perovskites have emerged as non-toxic and stable photovoltaic
materials, acting as an alternative to lead halide hybrid perovskites having
similar optoelectronic properties. In the present work, we report the
electronic and optical properties of chalcogenide perovskites AZrS$_3$ (A=Ca,
Sr, Ba) by using the density functional theory (DFT) and many-body perturbation
theory (MBPT viz. G$_0$W$_0$ and BSE). This study includes excitonic analysis
for the aforementioned systems. The exciton binding energy (E$_\textrm{B}$) is
found to be larger than that of the halide perovskites, as the ionic
contribution to dielectric screening is negligible in the former. We also
observe a more stable charge-separated polaronic state as compared to that of
the bound exciton. Finally, on the basis of direct gap and absorption
coefficient, the estimated spectroscopic limited maximum efficiency (SLME) of
the solar cells is large and suggests the applicability of these perovskites in
photovoltaics.

###Origin of the electrocatalytic activity in carbon nanotube fiber counter-electrodes for solar-energy conversion||Alba Martínez-Muíño,Moumita Rana,Juan J. Vilatela,Rubén D. Costa###

Origin of the electrocatalytic activity in carbon nanotube fiber counter-electrodes for solar-energy conversion. However, their electrocatalytic activity is still poorly understood. This
work deciphers the origin of the catalytic activity of counter-electrodes
(CEs)/current collectors made of self-standing carbon nanotubes fibers (CNTfs)
using Co$^(+2)$/Co$^(+3)$ redox couple electrolytes. This is based on
comprehensive electrochemical and spectroscopic characterizations of fresh and
used electrodes applied to symmetric electrochemical cells using platinum-based
CEs as a reference. As the most relevant findings, two straight relationships
were established: i) the limiting current and stability increase rapidly with
surface concentration of oxygen-containing functional groups, and ii) the
catalytic potential is inversily related to the amount of residual metallic Fe
catalyst nanoparticles interspersed in the CNTf network. Finally, the fine tune
of the metallic nanoparticle content and the degree of functionalization
enabled fabrication of efficient and stable dye-sensitized solar cells with
cobalt electrolytes and CNTf-CE outperforming those with reference Pt-CEs.

###Surface chemistry models for GaAs epitaxial growth and hydride cracking using reacting flow simulations||Malik Hassanaly,Hariswaran Sitaraman,Kevin L. Schulte,Aaron J. Ptak,John Simon,Kevin Udwary,Jacob H. Leach,Heather Splawn###

Surface chemistry models for GaAs epitaxial growth and hydride cracking using reacting flow simulations. Hydride Vapor Phase Epitaxy (HVPE) is a promising technology that can aid in
the cost reduction of III-V materials and devices manufacturing, particularly
high-efficiency solar cells for space and terrestrial applications. However,
recent demonstrations of ultra fast growth rates ($\sim$ 500 $\mu$m/h) via
uncracked hydrides are not well described by present models for the growth.
Therefore, it is necessary to understand the kinetics of the growth process and
its coupling with transport phenomena, so as to enable fast and uniform
epitaxial growth. In this work, we derive a kinetic model using experimental
data and integrate it into a computational fluid dynamics simulation of an HVPE
growth reactor. We also modify an existing hydride cracking model that we
validate against numerical simulations and experimental data. We show that the
developed growth model and the improved cracking model are able to reproduce
experimental growth measurements of \ce{GaAs} in an existing HVPE system.

###Identifying optimal photovoltaic technologies for underwater applications||Jason A. Röhr,Ed Sartor,Joel N. Duenow,Zilun Qin,Juan Meng,Jason Lipton,Stephen A. Maclean,Udo Römer,Michael P. Nielsen,Suling Zhao,Jaemin Kong,Matthew O. Reese,Myles A. Steiner,N. J. Ekins-Daukes,André D. Taylor###

Identifying optimal photovoltaic technologies for underwater applications. Improving solar energy collection in aquatic environments would allow for
superior environmental monitoring and remote sensing, but the identification of
optimal photovoltaic technologies for such applications is challenging as
evaluation requires either field deployment or access to large water tanks.
Here, we present a simple bench-top characterization technique that does not
require direct access to water and therefore circumvents the need for field
testing during initial trials of development. Employing LEDs to simulate
underwater solar spectra at various depths, we compare Si and CdTe solar cells,
two commercially available technologies, with GaInP cells, a technology with a
wide band gap close to ideal for underwater solar harvesting. We use this
method to show that while Si cells outperform both CdTe and GaInP under
terrestrial AM1.5G solar irradiance, both CdTe and GaInP outperform Si at
depths > 2 m, with GaInP cells operating with underwater efficiencies exceeding
51%.

###Determining ultrafast carrier dynamics of hybrid perovskites at various stages of nucleation and growth kinetics||Bibek S. Dhami,Ravi P. N. Tripathi,David J. Hoxie,Kannatassen Appavoo###

Determining ultrafast carrier dynamics of hybrid perovskites at various stages of nucleation and growth kinetics. With hybrid organic-inorganic perovskites increasing its technological reach,
from photovoltaics solar cells to light-emitting devices, to nanoscale
transistors, it is critical to establish the role of microstructures in
dictating how carrier dynamics dictate device efficiency. Here we report on the
ultrafast dynamics of charge carriers in hybrid perovskites at various stages
of nucleation and growth kinetics. A solution-processed fabrication technique,
with spin-coating conditions optimized to control the nucleation density of an
intermediate phase, converts to hybrid perovskites upon a temperature gradient
annealing. This strategy decouples the nucleation and growth steps that lead
eventually to large-grain thin films, allowing us to probe electronic and
carrier dynamic differences. We find, surprisingly, that the nucleating
microcrystals already display the electronic properties of hybrid perovskites
and share similar femtosecond-to-nanosecond dynamics as large-grain hybrid
perovskite thin films.

###Preserving the Stoichiometry of Triple-Cation Perovskites by Carrier-Gas-Free Antisolvent Spraying||Oscar Telschow,Miguel Albaladejo-Siguan,Lena Merten,Alexander D. Taylor,Katelyn P. Goetz,Tim Schramm,O. V. Konovalov,M. Jankowski,Alexander Hinderhofer,Fabian Paulus,Frank Schreiber,Yana Vaynzof###

Preserving the Stoichiometry of Triple-Cation Perovskites by Carrier-Gas-Free Antisolvent Spraying. The use of antisolvents during the fabrication of solution-processed lead
halide perovskite layers is increasingly common. Usually, the antisolvent is
applied by pipetting during the spin-coating process, which often irreversibly
alters the composition of the perovskite layer, resulting in the formation of
PbI2 at the surface and bulk of the perovskite layer. Here, we demonstrate that
by applying the antisolvent via carrier-gas free spraying, the stoichiometry of
the perovskite layer is far better preserved. Consequently, the photovoltaic
performance of triple cation photovoltaic devices fabricated in an inverted
architecture is enhanced, mainly due to an increase in the open-circuit
voltage. By exploring different volumes of antisolvent, we show that spraying
as little as 60 uL results in devices with power conversion efficiencies as
high as 21%. Moreover, solar cells with sprayed antisolvent are more stable
than those fabricated by pipetting the antisolvent.

###Multiple exciton generation and giant external quantum efficiency in VO$_2$||S. R. Sahu,A. Tripathy,K. Dey,N. Mansuri,V. G. Sathe,D. K. Shukla###

Multiple exciton generation and giant external quantum efficiency in VO$_2$. Multiple exciton generation (MEG) is a widely studied phenomenon in
semiconductor nanocrystals and quantum dots wherein photo-excited carriers
relax by generating additional electron-hole pairs. Here, we present the first
experimental observation of MEG and the same leading to giant external quantum
efficiency (EQE) in VO$_2$, a prototype strongly correlated material. By
employing a photoexcitation (lamda ~ 488 nm) of ~ 4.2 times the bandgap, EQE in
VO$_2$ is enhanced up to ~ 170 % at room temperature. Temperature dependent
experiments exhibit the direct relation between MEG and strength of electron
correlation and suggest that such a phenomenon could be exploited in large
number of strongly correlated materials for high performance solar cell
research in near future.

###ABO3 Perovskites' Formability Prediction and Crystal Structure Classification using Machine Learning||Minhaj Uddin Ahmad,A. Abdur Rahman Akib,Md. Mohsin Sarker Raihan,Abdullah Bin Shams###

ABO3 Perovskites' Formability Prediction and Crystal Structure Classification using Machine Learning. Renewable energy sources are of great interest to combat global warming, yet
promising sources like photovoltaic (PV) cells are not efficient and cheap
enough to act as an alternative to traditional energy sources. Perovskite has
high potential as a PV material but engineering the right material for a
specific application is often a lengthy process. In this paper, ABO3 type
perovskites' formability is predicted and its crystal structure is classified
using machine learning with high accuracy, which provides a fast screening
process. Although the study was done with solar-cell application in mind, the
prediction framework is generic enough to be used for other purposes.
Formability of perovskite is predicted and its crystal structure is classified
with an accuracy of 98.57% and 90.53% respectively using Random Forest after
5-fold cross-validation. Our machine learning model may aid in the accelerated
development of a desired perovskite structure by providing a quick mechanism to
get insight into the material's properties in advance.

###Impact of metastable defect structures on carrier recombination in solar cells||Seán R. Kavanagh,David O. Scanlon,Aron Walsh,Christoph Freysoldt###

Impact of metastable defect structures on carrier recombination in solar cells. The efficiency of a solar cell is often limited by electron-hole
recombination mediated by defect states within the band gap of the photovoltaic
(PV) semiconductor. The Shockley-Read-Hall (SRH) model considers a static trap
that can successively capture electrons and holes. In reality however, true
trap levels vary with both the defect charge state and local structure. Here we
consider the role of metastable structural configurations in capturing
electrons and holes, taking the tellurium interstitial in CdTe as an
illustrative example. Consideration of the defect dynamics, and
symmetry-breaking, changes the qualitative behaviour and activates new pathways
for carrier capture. Our results reveal the potential importance of metastable
defect structures in non-radiative recombination, in particular for
semiconductors with anharmonic/ionic-covalent bonding, multinary compositions,
low crystal symmetries or highly-mobile defects.

###Theoretical proposal of a revolutionary water-splitting photocatalyst: The monolayer of boron phosphide||Tatsuo Suzuki###

Theoretical proposal of a revolutionary water-splitting photocatalyst: The monolayer of boron phosphide. Recently, hydrogen generation by water-splitting photocatalysts is attracting
attention as a sustainable and clean energy resource. Photocatalytic
hydrogen-generation systems are much simpler, cheaper, and easier to scale up
than the coupled systems of electrolysis and solar cells, wind-power
generation, etc. However, photocatalytic hydrogen generation is currently
inefficient. This paper proposes the monolayer of boron phosphide as a stable
highly-efficient water-splitting photocatalyst by high-precision
density-functional theory calculations using a HSE06 functional with a solvent
effect. The monolayer of boron phosphide has a direct allowed energy gap of
about 1.4 eV, and functions as a one-step excitation photocatalyst. It absorbs
sunlight with wavelengths below about 890 nm (ultraviolet, visible, and
near-infrared light) and produces both hydrogen gas and oxygen gas from water
at a suitable pH condition. By calculating the overpotentials of hydrogen and
oxygen evolution reactions, its photocatalytic effectiveness was confirmed. The
monolayers of boron phosphide will realize green hydrogen revolution.

###Dopant size effect on BiFeO$\rm_{3}$ perovskite structure for enhanced photovoltaic activity||Tewodros Eyob,Kenate Nemera,Lemi Demeyu###

Dopant size effect on BiFeO$\rm_{3}$ perovskite structure for enhanced photovoltaic activity. This study is carried out using first principles density functional theory
calculations within gpaw code. Atomic size effect is analyzed and investigated
by doping either Li, Cs or both on Barium doped BiFeO$_3$ (BFO) which belongs
to monoclinic $P2_1/m$ space group. The calculated results reveal that Cs doped
BFO had significantly improved photocurrent density seemingly due to broadened
absorption peaks and biplasmons generation. Co-doping two atoms with large size
difference have a significant effect on plasmon width and peak than doping with
a single and small sized atom. At higher photon energy realms of the order
10eV, the index of refraction reduces to $n(\omega) \to 1$ implying that light
wave can tunnel through the pristine and doped BFO without any phase change,
thus indicating its potential as an efficient candidate for a photonic
application. In addition, doped BFO shows abundant photocurrent generation
properties which would be important in solar cell and photovoltaic
applications.

###Coupling Perovskite Quantum Dot Pairs in Solution using Nanoplasmonic Assembly||Hao Zhang,Parinaz Moazzezi,Juanjuan Ren,Brett Henderson,Cristina Cordoba,Vishal Yeddu,Arthur M. Blackburn,Makhsud I. Saidaminov,Irina Paci,Stephen Hughes,Reuven Gordon###

Coupling Perovskite Quantum Dot Pairs in Solution using Nanoplasmonic Assembly. Perovskite quantum dots (PQDs) provide a robust solution-based approach to
efficient solar cells, bright light-emitting devices, and quantum sources of
light. Quantifying heterogeneity and understanding coupling between dots is
critical for these applications. We use double-nanohole optical trapping to
size individual dots and correlate to emission energy shifts from quantum
confinement. We were able to assemble a second dot in the trap, which allows us
to observe the coupling between dots. We observe a systematic red-shift of 1.1
$\pm$ 0.6 meV in the emission wavelength. Theoretical analysis shows that the
observed shift is consistent with resonant energy transfer and is unusually
large due to moderate-to-large quantum confinement in PQDs. This demonstrates
the promise of PQDs for entanglement in quantum information applications. This
work enables future in situ control of PQD growth as well as studies of the
coupling between small PQD assemblies with quantum information applications in
mind.

###Prospects for Perovskite/Silicon tandem solar cells to outperform c-Silicon solar cells at elevated temperatures||Ganga Vinod Chittiboina,Pradeep R. Nair###

Prospects for Perovskite/Silicon tandem solar cells to outperform c-Silicon solar cells at elevated temperatures. Successful commercialization of Perovskite/Si tandem solar cells (P/Si TSCs)
need a-priori estimation of technological benchmarks to outperform c-Si based
technologies under field conditions. To this end, through detailed numerical
simulations and analytical modeling, here we identify the limits of ion
migration and lifetime degradation till which P/Si TSCs remain competitive. Our
results unravel a unique scaling law for the evolution of the efficiency and
the temperature coefficient of P/Si TSCs which allows us to anticipate the
limiting annual degradation rates. Interestingly, we find that 4T cells are
potentially more immune to the ill effects of ion migration as compared to 2T
cells. These insights are of broad relevance for material/interface engineering
approaches and physics based accelerated tests which are focused towards long
term stability and module reliability.

###Thermal annealing effects on Graphene/n-Si Schottky junction Solar cell: Removal of PMMA residues||Yuzuki Ono,Hojun Im###

Thermal annealing effects on Graphene/n-Si Schottky junction Solar cell: Removal of PMMA residues. Thermal annealing is one of most effective way to improve the efficiency of
graphene/n-Si Schottky junction solar cell. Here, its underlying mechanism has
been investigated by comparative studies in terms of the removal of polymethyl
methacrylate (PMMA) residues, using the J-V characteristics, the transient
photocurrent and photovoltage measurements. Experimental results have revealed
that there are trap states which are originated from the PMMA residues and
cause the large photocurrent leakage as the intensity of the incident light
increases. It is also found that the PMMA residues accelerate deterioration and
rapidly invalidate hole doping effects. Such undesirable PMMA residues were
effectively removed by the thermal annealing treatments, serving to reduce the
photocurrent leakage and to increase the stability.

###(3-Aminopropyl)trimethoxysilane Surface Passivation Improves Perovskite Solar Cell Performance by Reducing Surface Recombination Velocity||Yangwei Shi,Esteban Rojas-Gatjens,Jian Wang,Justin Pothoof,Rajiv Giridharagopal,Kevin Ho,Fangyuan Jiang,Margherita Taddei,Zhaoqing Yang,Carlos Silva-Acuña,David S. Ginger###

(3-Aminopropyl)trimethoxysilane Surface Passivation Improves Perovskite Solar Cell Performance by Reducing Surface Recombination Velocity. We demonstrate reduced surface recombination velocity (SRV) and enhanced
power-conversion efficiency (PCE) in mixed-cation mixed-halide perovskite solar
cells by using (3-aminopropyl)trimethoxysilane (APTMS) as a surface passivator.
We show the APTMS serves to passivate defects at the perovskite surface, while
also decoupling the perovskite from detrimental interactions at the C60
interface. We measure a SRV of ~125 + 14 cm/s, and a concomitant increase of
~100 meV in quasi-Fermi level splitting in passivated devices compared to the
controls. We use time-resolved photoluminescence and excitation-correlation
photoluminescence spectroscopy to show that APTMS passivation effectively
suppresses non-radiative recombination. We show that APTMS improves both the
fill factor and open-circuit voltage (VOC), increasing VOC from 1.03 V for
control devices to 1.09 V for APTMS-passivated devices, which leads to PCE
increasing from 15.90% to 18.03%. We attribute enhanced performance to reduced
defect density or suppressed nonradiative recombination and low SRV at the
perovskite/transporting layers interface.

###Quantum Mechanical Assessment of Optimal Photovoltaic Conditions in Organic Solar Cells||Artur M. Andermann,Luis G. C. Rego###

Quantum Mechanical Assessment of Optimal Photovoltaic Conditions in Organic Solar Cells. Recombination losses contribute to reduce $J_{SC}$, $V_{OC}$ and the fill
factor of organic solar cells. Recent advances in non-fullerene organic
photovoltaics have shown, nonetheless, that efficient charge generation can
occur under small energetic driving forces ($\Delta E_{DA}$) and low
recombination losses. To shed light on this issue, we set up a coarse-grained
open quantum mechanical model for investigating the charge generation dynamics
subject to various energy loss mechanisms. The influence of energetic driving
force, Coulomb interaction, vibrational disorder, geminate recombination,
temperature and external bias are included in the analysis of the optimal
photovoltaic conditions for charge carrier generation. The assessment reveals
that the overall energy losses are not only minimized when $\Delta E_{DA}$
approaches the effective reorganization energy at the interface but also become
insensitive to temperature and electric field variations. It is also observed
that a moderate reverse bias reduces geminate recombination losses
significantly at vanishing driving forces, where the charge generation is
strongly affected by temperature.

###Hot carrier extraction from 2D semiconductor photoelectrodes||Rachelle Austin,Yusef Farah,Thomas Sayer,Brad M. Luther,Andrés Montoya-Castillo,Amber Krummel,Justin Sambur###

Hot carrier extraction from 2D semiconductor photoelectrodes. Hot carrier-based energy conversion systems could double the efficiency of
conventional solar energy technology or drive photochemical reactions that
would not be possible using fully thermalized, ``cool'' carriers, but current
strategies require expensive multi-junction architectures. Using an
unprecedented combination of photoelectrochemical and in situ transient
absorption spectroscopy measurements, we demonstrate ultrafast (<50 fs) hot
exciton and free carrier extraction under applied bias in a proof-of-concept
photoelectrochemical solar cell made from earth-abundant and potentially
inexpensive monolayer (ML) MoS2. Our approach facilitates ultrathin 7\AA charge
transport distances over 1 cm^2 areas by intimately coupling ML-MoS2 to an
electron-selective solid contact and a hole-selective electrolyte contact. Our
theoretical investigations of the spatial distribution of exciton states
suggest greater electronic coupling between hot exciton states located on
peripheral S atoms and neighboring contacts likely facilitates ultrafast charge
transfer. Our work delineates future 2D semiconductor design strategies for
practical implementation in ultrathin photovoltaic and solar fuels
applications.

###Comparative study of the physical properties for the A$_2$TiX$_6$ (A= Cs or NH$_4$ and X= Cl or Br) vacancy-ordered double perovskites||M. Talebi,A. Mokhtari###

Comparative study of the physical properties for the A$_2$TiX$_6$ (A= Cs or NH$_4$ and X= Cl or Br) vacancy-ordered double perovskites. The vacancy-ordered double perovskites (VODP) are emerging materials for the
renewable energy because of their extraordinary stability. In the present work,
we have addressed the structural, electronic and optical properties of the
A$_2$TiX$_6$ (A= Cs or NH$_4$ and X= Cl or Br) organic/inorganic halide VODPs
based on first-principles calculations. Our results demonstrate that these
compounds have several interesting properties, including high stability,
suitable band gap and excellent optical absorption. The band structure
calculations for the Cs$_2$TiBr$_6$ and (NH$_4$)$_2$TiBr$_6$ perovskites reveal
direct band gaps about 1.77 and 1.59 eV respectively, which is predicted these
materials be ideal for application in the solar cells. The investigated
materials possess high absorption coefficients in the order of 105cm$^{-1}$ in
the visible light region. Our research can provide a way to identify stable,
bio-friendly and high-efficiency light absorber material for use in
optoelectronic and photovoltaic devices.

###Comparative Study of MPPT and Parameter Estimation of PV cells||Sahil Kumar,Sahitya Gupta,Vajayant Pratik,Pascal Brunet###

Comparative Study of MPPT and Parameter Estimation of PV cells. The presented work focuses on utilising machine learning techniques to
accurately estimate accurate values for known and unknown parameters of the
PVLIB model for solar cells and photovoltaic modules.Finding accurate model
parameters of circuits for photovoltaic (PV) cells is important for a variety
of tasks. An Artificial Neural Network (ANN) algorithm was employed, which
outperformed other metaheuristic and machine learning algorithms in terms of
computational efficiency. To validate the consistency of the data and output,
the results were compared against other machine learning algorithms based on
irradiance and temperature. A Bland Altman test was conducted that resulted in
more than 95 percent accuracy rate. Upon validation, the ANN algorithm was
utilised to estimate the parameters and their respective values.

###Long-lived exciton coherence in mixed-halide perovskite crystals||Stefan Grisard,Artur V. Trifonov,Ivan A. Solovev,Dmitri R. Yakovlev,Oleh Hordiichuk,Maksym V. Kovalenko,Manfred Bayer,Ilya A. Akimov###

Long-lived exciton coherence in mixed-halide perovskite crystals. Compositional engineering of the optical properties of hybrid
organic-inorganic lead halide perovskites is one of the cornerstones for the
realization of efficient solar cells and tailored light-emitting devices. We
study the effect of compositional disorder on coherent exciton dynamics in a
mixed FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ perovskite crystal using photon
echo spectroscopy. We reveal that the homogeneous linewidth of excitons can be
as narrow as 16$\mu$eV at a temperature of 1.5K. The corresponding exciton
coherence time of $T_2=83$ps is exceptionally long being attributed to the
localization of excitons due to variation of composition at the scale of ten to
hundreds of nanometers. From spectral and temperature dependences of the two-
and three-pulse photon echo decay we conclude that for low-energy excitons,
pure decoherence associated with elastic scattering on phonons is comparable
with the exciton lifetime, while for excitons with higher energies, inelastic
scattering to lower energy states via phonon emission dominates.

###Top-Ranked Cycle Flux Network Analysis of Molecular Photocells||Nikhil Gupt,Shuvadip Ghosh,Arnab Ghosh###

Top-Ranked Cycle Flux Network Analysis of Molecular Photocells. We introduce a top-ranked cycle flux ranking scheme of network analysis to
assess the performance of molecular junction solar cells. By mapping the
Lindblad master equation to the quantum-transition network, we propose a
microscopic Hamiltonian description underpinning the rate equations commonly
used to characterize molecular photocells. Our approach elucidates the
paramount significance of edge flux and unveils two pertinent electron transfer
pathways that play equally important roles in robust photocurrent generation.
Furthermore, we demonstrate that non-radiative loss processes impede the
maximum power efficiency of photocells, which may otherwise be above the
Curzon-Ahlborn limit. These findings shed light on the intricate
functionalities that govern molecular photovoltaics and offer a comprehensive
approach to address them in a systematic way.

###Highly-Efficient Charge Separation and Polaron Delocalization in Polymer-Fullerene Bulk-Heterojunctions: A Comparative Multi-Frequency EPR & DFT Study||Jens Niklas,Kristy L. Mardis,Brian P. Banks,Gregory M. Grooms,Andreas Sperlich,Vladimir Dyakonov,Serge Beaupré,Mario Leclerc,Tao Xu,Luping Yu,Oleg G. Poluektov###

Highly-Efficient Charge Separation and Polaron Delocalization in Polymer-Fullerene Bulk-Heterojunctions: A Comparative Multi-Frequency EPR & DFT Study. The ongoing depletion of fossil fuels has led to an intensive search for
additional renewable energy sources. Solar-based technologies could provide
sufficient energy to satisfy the global economic demands in the near future.
Photovoltaic (PV) cells are the most promising man-made devices for direct
solar energy utilization. Understanding the charge separation and charge
transport in PV materials at a molecular level is crucial for improving the
efficiency of the solar cells. Here, we use light-induced EPR spectroscopy
combined with DFT calculations to study the electronic structure of charge
separated states in blends of polymers (P3HT, PCDTBT, and PTB7) and fullerene
derivatives (C60-PCBM and C70-PCBM). Solar cells made with the same composites
as active layers show power conversion efficiencies of 3.3% (P3HT), 6.1%
(PCDTBT), and 7.3% (PTB7), respectively. Under illumination of these
composites, two paramagnetic species are formed due to photo-induced electron
transfer between the conjugated polymer and the fullerene. They are the
positive, P+, and negative, P-, polarons on the polymer backbone and fullerene
cage, respectively, and correspond to radical cations and radical anions. Using
the high spectral resolution of high-frequency EPR (130 GHz), the EPR spectra
of these species were resolved and principal components of the g-tensors were
assigned. Light-induced pulsed ENDOR spectroscopy allowed the determination of
1H hyperfine coupling constants of photogenerated positive and negative
polarons. The experimental results obtained for the different polymer-fullerene
composites have been compared with DFT calculations, revealing that in all
three systems the positive polaron is distributed over distances of 40-60 A on
the polymer chain. This corresponds to about 15 thiophene units for P3HT,
approximately three units PCDTBT, and about three to four units for PTB7. No
spin density...

###Two-Dimensional Transition Metal Dichalcogenides-Based Counter Electrodes for Dye-Sensitized Solar Cells||Eric Singh,Ki Seok Kim,Geun Young Yeom,Hari Singh Nalwa###

Two-Dimensional Transition Metal Dichalcogenides-Based Counter Electrodes for Dye-Sensitized Solar Cells. Dye-sensitized solar cells (DSSCs) are gaining considerable interest as
alternatives to the semiconductor-based thin film solar cells. The noble metal
platinum (Pt) is conventionally used as counter electrode (CE) material for
fabricating DSSCs. Since Pt is expensive and scarce, new materials have been
explored to develop cost-effective Pt-free counter electrodes for DSSCs.
Two-dimensional (2D) graphene-based counter electrodes have achieved the
highest known power conversion efficiency ({\eta}) of 13%, which has stimulated
research activities in 2D layered transition metal dichalcogenides (TMDs) for
developing Pt-free DSSCs. In this review, progress made on alternative counter
electrodes for fabricating low-cost Pt-free DSSCs, based on earth-abundant 2D
TMDs including MoS2, WS2, TiS2, FeS2, CoS2, NiS2, SnS2, MoSe2, NbSe2, TaSe2,
NiSe2, FeSe2, CoSe2, Bi2Se3 and their based composites, are discussed and
summarized. Also, the considerable progress made on thin films of MoS2 and MoS2
based carbon, graphene, carbon nanotubes (CNTs), carbon nanofibers (CNFs), and
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composites
as efficient counter electrodes (CEs) for DSSCs are discussed, in terms of
their electrochemical and photovoltaic properties. At present, PCE values
higher than that of standard Pt CE have been recorded for a number of TMD-based
CEs, which include MoS2 and MoSe2/thin films deposited on Mo foil, MoS2/CNTs,
MoS2/graphene, MoS2/carbon, MoSe2/PEDOT:PSS, NbSe2, FeS2, FeSe2 nanosheets,
TiS2/graphene, and NiS2/graphene hybrid systems in DSSCs, for the reduction of
triiodide (I3-) to iodide (I-). The highest PCE ({\eta}=10.46%) versus Pt CE
({\eta}=8.25%) at 1 Sun (100 mW/cm2, AM 1.5G) was measured for DSSCs having a
low cost and flexible CoSe2/carbon-nanoclimbing wall counter electrode
deposited on a nickel foam.

###Influence of Environmentally Affected Hole Transport Layers on Spatial Homogeneity and Charge Transport Dynamics of Organic Solar Cells||Huei -Ting Chien,Florian Pilat,Thomas Griesser,Harald Fitzek,Peter Poelt,Bettina Friedel###

Influence of Environmentally Affected Hole Transport Layers on Spatial Homogeneity and Charge Transport Dynamics of Organic Solar Cells. After the efficiency of organic photovoltaic (OPV) cells achieved more than
10%, the control of stability and degradation mechanisms of solar cells became
a prominent task. The improvement of device efficiency due to incorporation of
a hole-transport layer (HTL) in bulk-heterojunction solar cells has been
extensively reported. However, the most widely used HTL material, PEDOT:PSS is
frequently suspected to be the dominating source for devices instability under
environmental conditions. Thereby effects like photooxidation and electrode
corrosion are often reported to shorten device lifetime. However, often in
environmental device studies, the source of degradation, whether being from the
HTL, the active layer or the metal cathode are rather difficult to distinguish,
because the external diffusion of oxygen and water affects all components. In
this study, different HTLs, namely prepared from traditional PEDOT:PSS and also
two types of molybdenum trioxide (MoO3), are exposed to different environments
such as oxygen, light or humidity, prior to device finalization under inert
conditions. This allows investigating any effects within the HTL and from
reactions at its interface to the indium-tin-oxide electrode or the active
layer. The surface and bulk chemistry of the exposed HTL has been monitored and
discussed in context to the observed device physics, dynamic charge transport
and spatial performance homogeneity of the according OPV device. The results
show that merely humidity-exposure of the HTL leads to decreased device
performance for PEDOT:PSS, but also for one type of the tested MoO3. The losses
are related to the amount of absorbed water in the HTL, inducing loss of active
area in terms of interfacial contact. The device with PEDOT:PSS HTL after humid
air exposure showed seriously decreased photocurrent by micro-delamination of
swelling/shrinkage of the hygroscopic layer.

###Photoconversion in the HIT solar cells: Theory vs experiment||A. V. Sachenko,Yu. V. Kryuchenko,V. P. Kostylyov,A. V. Bobyl,E. I. Terukov,S. N. Abolmasov,A. S. Abramov,D. A. Andronikov,M. Z. Shvarts,I. O. Sokolovskyi,M. Evstigneev###

Photoconversion in the HIT solar cells: Theory vs experiment. We obtain theoretical expressions for the photocurrent in the Heterojunction
solar cells with Intrinsic Thin layer (HIT cells). Our calculations take into
account tunneling of electrons and holes through wide-bandgap layers of
$\alpha$-Si:H or $\alpha$-SiC:H. We introduce the criteria, under which
tunneling does not lead to the deterioration of solar cell characteristics, in
particular, to the reduction of the short-circuit current and open-circuit
voltage. We propose an algorithm to compute the photoconversion efficiency of
HIT elements, taking into account the peculiarities of the open-circuit voltage
generation, in particular, its rather high values. We test our theoretical
predictions against the experimental results. For this, we fabricate HIT
elements with the efficiency of about $20\,\%$. We measured the temperature
dependence of the short-circuit current, open-circuit voltage, photoconversion
power, and fill factor of the current-voltage curve of these elements in a wide
temperature range from 80 to 420\,K. In the low-temperature range, the
open-circuit voltage and the photoconversion power decrease on cooling. At $T
\ge 200$\,K, the theoretical expressions and the experimental curves agree
rather well. The behavior of the fill factor and output power at low
temperatures is explained by the increase of the series resistance on cooling.
We discuss the reasons behind the reduction of the power temperature
coefficient in HIT elements. We show that they are related to the low value of
the combined surface and volume recombination rate. Finally, we derive a
theoretical expression for the HIT element's operation temperature under
natural working conditions.

###Structure, Electrical and Optical Properties of ITO Thin Films and their Influence on Performance of CdS/CdTe Thin-Film Solar Cells||Moustafa Ahmed,Ahmed Bakry,Essam R. Shaaban,Hamed Dalir###

Structure, Electrical and Optical Properties of ITO Thin Films and their Influence on Performance of CdS/CdTe Thin-Film Solar Cells. In terms of mixing graded TiO2 and SnO2 powders by solid-state reaction
method, ITO was prepared. Using electron beam gun technology, ITO films with
different thicknesses were prepared. The influence of film thickness on
structure, electrical and optical properties was studied. The XRD patterns were
utilized to determine the structural parameters (lattice strain and crystallite
size) of ITO with different thicknesses. It is observed that the average
crystallite size increases as the film thickness increases, but the lattice
strain decreases. SEM shows that as the film thickness increases, the grain
size of ITO increases and improves. The electrical properties of ITO films with
different thicknesses were measured by the standard four-point probe method. It
can be seen that as the thickness of the ITO film increases from 75 nm to 325
nm, the resistivity decreases from 29x10^-4 Ohm/cm to 1.65x10^-4 Ohm/cm. This
means that ITO films with lower electrical properties will be more suitable for
high-efficiency CdTe solar cells. Three optical layer models (adhesive layer of
the substrate/B-spline layer of ITO film/surface roughness layer) are used to
calculate the film thickness with high-precision ellipsometry. In the higher
T(lambda) and R(lambda) absorption regions, the absorption coefficient is
determined to calculate the optical energy gap, which increases from 3.56 eV to
3.69 eV. Finally, the effects of ITO layers of various thicknesses on the
performance of CdS/CdTe solar cells are also studied. When the thickness of the
ITO window layer is 325 nm, Voc = 0.82 V, Jsc = 17 mA/cm2, and FF = 57.4%, the
highest power conversion efficiency (PCE) is 8.6%.

###Theoretical investigation of the role of the organic cation in methylammonium lead iodide perovskite||Veysel Çelik###

Theoretical investigation of the role of the organic cation in methylammonium lead iodide perovskite. The hybrid halide perovskite CH$_3$NH$_3$PbI$_3$ is easy to manufacture and
inexpensive. Despite these, its efficiency as a solar cell is comparable to
today's efficient solar cells. For these reasons, it is attracting a lot of
attention today. However, the effects of the CH$_3$NH$_3^+$ (MA) molecule in
the perovskite structure on the electronic and structural properties are still
a matter of debate. Previous studies have generally focused on the rotation of
the MA molecule. In this study, from a different perspective, the effects of
the movement of the MA molecule along the C-N axis are investigated. With this
method, the effects of the MA molecule were examined in a more controlled way.
In this study, density functional theory (DFT) that accounts for van der Waals
(vdW) interactions was used in the calculations for the cases. According to the
data obtained, H-I ionic bonds are formed between the MA molecule and the
inorganic framework. Within the structure, the H-I bond length tends to be
preserved, although the position of the MA changes. In this mechanism, the I
ion plays an important role by moving away from its place in the Pb-I-Pb
alignment. The position of the I ion determines the nature of the band gap
transition. Another effect is on the value of the band gap. Depending on the
position of the I ion, the band gap may narrow by about 0.26 eV. The separation
of the I ion from the Pb-I-Pb alignment by the effect of the MA molecule breaks
the inverse symmetry. According to the data obtained from this study, this
mechanism in the band gap is due to the breaking of the inverse symmetry in the
crystal structure.

###Effect of various electron and hole transport layers on the performance of CsPbI3-based perovskite solar cells: A numerical investigation in DFT, SCAPS-1D, and wxAMPS frameworks||M. Khalid Hossain,Mirza Humaun Kabir Rubel,G. F. Ishraque Toki,Intekhab Alam,Md. Ferdous Rahman,H. Bencherif###

Effect of various electron and hole transport layers on the performance of CsPbI3-based perovskite solar cells: A numerical investigation in DFT, SCAPS-1D, and wxAMPS frameworks. CsPbI3 has recently received tremendous attention as a possible absorber of
perovskite solar cells (PSCs). However, CsPbI3-based PSCs have yet to achieve
the high performance of the hybrid PSCs. In this work, we performed a density
functional theory (DFT) study using the Cambridge Serial Total Energy Package
(CASTEP) code for the cubic CsPbI3 absorber to compare and evaluate its
structural, electronic, and optical properties. The calculated electronic band
gap (Eg) using the GGA-PBE approach of CASTEP was 1.483 eV for this CsPbI3
absorber. Moreover, the computed density of states (DOS) exhibited the dominant
contribution from the Pb-5d orbital, and most charge also accumulated for the
Pb atom as seen from the electronic charge density map. Fermi surface
calculation showed multiband character, and optical properties were computed to
investigate the optical response of CsPbI3. Furthermore, we used IGZO, SnO2,
WS2, CeO2, PCBM, TiO2, ZnO, and C60 as the electron transport layers (ETLs),
and Cu2O, CuSCN, CuSbS2, Spiro-MeOTAD, V2O5, CBTS, CFTS, P3HT, PEDOT: PSS, NiO,
CuO, and CuI as the hole transport layers (HTLs) to identify the best
HTL/CsPbI3/ETL combinations using the SCAPS-1D solar cell simulation software.
Among 96 device structures, the best-optimized device structure,
ITO/TiO2/CsPbI3/CBTS/Au was identified, which exhibited an efficiency of 17.9%.
The effect of absorber and ETL thickness, series resistance, shunt resistance,
and operating temperature was also evaluated for the six best devices along
with their corresponding generation rate, recombination rate,
capacitance-voltage, current density-voltage, and quantum efficiency
characteristics. The obtained results from SCAPS-1D were also compared with
wxAMPS simulation software.

###Autonomous Optimization of an Organic Solar Cell in a 4-dimensional Parameter Space||Tobias Osterrieder,Frederik Schmitt,Larry Luer,Jerrit Wagner,Thomas Heumüller,Jens Hauch,Christoph Brabec###

Autonomous Optimization of an Organic Solar Cell in a 4-dimensional Parameter Space. Optimizing solution-processed organic solar cells is a complex task due to
the vast parameter space in organic photovoltaics (OPV). Classical Edisonian or
one-variable-at-a-time (OVAT) optimization approaches are laborious,
time-consuming, and may not find the optimal parameter set in multidimensional
design spaces. To tackle this problem, we demonstrate here for the first time
artificial intelligence (AI) guided closed-loop autonomous optimization for
fully functional organic solar cells. We empower our LineOne, an automated
materials and device acceleration platform with a Bayesian Optimizer (BO) to
enable autonomous operation for solving complex optimization problems without
human interference. The system is able to fabricate and characterize complete
OPV devices and navigate efficiently through the design space spanned by
composition and processing parameters. In addition, a Gaussian Progress
Regression (GPR) based early prediction model is employed to predict the
efficiency of the cells from cheap proxy measurements, in our case, thin film
absorption spectra, which are analyzed using a spectral model based on physical
properties to generate microstructure features as input for the GPR. We
demonstrate our generic and complete autonomous approach by optimizing
composition and processing conditions of a ternary OPV system (PM6:Y12:PC70BM)
in a four-dimensional parameter space. We identify the best parameter set for
our system and obtain a precise objective function over the whole parameter
space with a minimal number of samples. We demonstrate autonomous optimization
of a complex opto-electronic device within 40 samples only, whereas an
Edisonian approach would have required about 1000 samples. This raises an
important discussion on the necessity of autonomous platforms to accelerate
Material science.

###Intrinsic Oxygen Vacancy and Extrinsic Aluminium Dopant Interplay: A Route to the Restoration of Defective TiO$_2$||Conn O'Rourke,David R. Bowler###

Intrinsic Oxygen Vacancy and Extrinsic Aluminium Dopant Interplay: A Route to the Restoration of Defective TiO$_2$. Density functional theory (DFT) and DFT corrected for on-site Coulomb
interactions (DFT+U) calculations are presented on Aluminium doping in bulk
TiO$_2$ and the anatase (101) surface. Particular attention is paid to the
mobility of oxygen vacancies throughout the doped TiO$_2$ lattice, as a means
by which charge compensation of trivalent dopants can occur. The effect that Al
doping of TiO$_2$ electrodes has in dye sensitised solar cells is explained as
a result of this mobility and charge compensation. Substitutional defects in
which one Al3+ replaces one Ti4+ are found to introduce valence band holes,
while intrinsic oxygen vacancies are found to introduce states in the band-gap.
Coupling two of these substitutional defects with an oxygen vacancy results in
exothermic defect formation which maintain charge neutrality. Nudged elastic
band calculations have been performed to investigate the formation of these
clustered defects in the (101) surface by oxygen vacancy diffusion, with the
resulting potential energy surface suggesting energetic gains with small
diffusion barriers. Efficiency in- creases observed in dye sensitised solar
cells as a result of aluminium doping of TiO$_2$ electrodes are investigated by
adsorbing the tetrahydroquinoline C2-1 chromophore on the defective surfaces.
Adsorption on the clustered extrinsic Al3+ and intrinsic oxygen vacancy defects
are found to behave as if adsorbed on a clean surface, with vacancy states not
present, while adsorption on the oxygen vacancy results in a down shift of the
dye localised states within the band-gap and defect states being present below
the conduction band edge. Aluminium doping therefore acts as a benign dopant
for 'cleaning' TiO$_2$ through oxygen vacancy diffusion.

###Optical transitions in hybrid perovskite solar cells: Ellipsometry, density functional theory, and quantum efficiency analyses for CH3NH3PbI3||Masaki Shirayama,Hideyuki Kadowaki,Tetsuhiko Miyadera,Takeshi Sugita,Masato Tamakoshi,Masato Kato,Takemasa Fujiseki,Daisuke Murata,Shota Hara,Takurou N. Murakami,Shohei Fujimoto,Masayuki Chikamatsu,Hiroyuki Fujiwara###

Optical transitions in hybrid perovskite solar cells: Ellipsometry, density functional theory, and quantum efficiency analyses for CH3NH3PbI3. We report artifact-free CH3NH3PbI3 optical constants extracted from
ultra-smooth perovskite layers without air exposure and assign all the optical
transitions in the visible/ultraviolet region unambiguously based on density
functional theory (DFT) analysis that assumes a simple pseudo-cubic crystal
structure. From the self-consistent spectroscopic ellipsometry analysis of the
ultra-smooth CH3NH3PbI3 layers, we find that the absorption coefficients of
CH3NH3PbI3 (alpha = 3.8 x 10^4 cm-1 at 2.0 eV) are comparable to those of
CuInGaSe2 and CdTe, and high alpha values reported in earlier studies are
overestimated seriously by extensive surface roughness of CH3NH3PbI3 layers.
The polarization-dependent DFT calculations show that CH3NH3+ interacts
strongly with the PbI3- cage, modifying the CH3NH3PbI3 dielectric function in
the visible region rather significantly. When the effect of CH3NH3+ on the
optical transition is eliminated in the DFT calculation, CH3NH3PbI3 dielectric
function deduced from DFT shows excellent agreement with the experimental
result. As a result, distinct optical transitions observed at E0 (Eg) = 1.61
eV, E1 = 2.53 eV, and E2 = 3.24 eV in CH3NH3PbI3 are attributed to the direct
semiconductor-type transitions at the R, M, and X points in the pseudo-cubic
Brillouin zone, respectively. We further perform the quantum efficiency (QE)
analysis for a standard hybrid-perovskite solar cell incorporating a mesoporous
TiO2 layer and demonstrate that the QE spectrum can be reproduced almost
perfectly when the revised CH3NH3PbI3 optical constants are employed.
Depth-resolved QE simulations confirm that Jsc is limited by the material's
longer wavelength response and indicate the importance of optical confinement
and long carrier diffusion lengths in hybrid perovskite solar cells.

###General Rules for the Impact of Energetic Disorder and Mobility on Nongeminate Recombination in Phase-Separated Organic Solar Cells||Guangzheng Zuo,Safa Shoaee,Martijn Kemerink,Dieter Neher###

General Rules for the Impact of Energetic Disorder and Mobility on Nongeminate Recombination in Phase-Separated Organic Solar Cells. State of the art organic solar cells exhibit power conversion efficiencies of
18 % and above. These devices benefit from the suppression of free charge
recombination with regard to the Langevin-limit of charge encounter in a
homogeneous medium. It has been recognized that the main cause of suppressed
free charge recombination is the reformation and resplitting of charge transfer
states at the interface between donor and acceptor domains. Here, we use
kinetic Monte Carlo simulations to understand the interplay between free charge
motion and recombination in an energetically-disordered phase-separated
donor-acceptor blend. We identify conditions for encounter-dominated and
resplitting-dominated recombination. In the former regime, recombination is
proportional to mobility for all parameters tested and only slightly reduced
with respect to the Langevin limit. In contrast, mobility is not the decisive
parameter determining the non-geminate recombination coefficient k_2in the
latter case where k_2 is a sole function of the morphology, CT and CS
energetics and CT states decay properties. Our simulations also show that free
charge encounter in the phase-separated disordered blend is determined by the
average mobility of all carriers, while CT reformation and resplitting involves
mostly states near the transport energy. Therefore, charge encounter is more
affected by increased disorder than the resplitting of the CT state. As a
consequence, for a given mobility, larger energetic disorder in combination
with a higher hopping rate is preferred. These findings have important
implications for the understanding of suppressed recombination in solar cells
with non-fullerene acceptors which are known to exhibit lower energetic
disorder than fullerenes.

###Deposition of Reduced Graphene Oxide Thin Film by Spray Pyrolysis Method for Perovskite Solar Cell||Manoj Pandey,Dipendra Hamal,Deepak Subedi,Bijaya Basnet,Rajaram Sah,Santosh K. Tiwari,Bhim Kafle###

Deposition of Reduced Graphene Oxide Thin Film by Spray Pyrolysis Method for Perovskite Solar Cell. The Perovskite absorber layer, the electron transport layer (ETL), the hole
transport layer (HTL), and the transparent conducting oxide layer (TCO) are the
major components that make up a Perovskite solar cell. Between ETL and HTL, the
absorber layer is sandwiched, on which electron-hole pairs are created after
absorption of solar radiation. Despite substantial progress toward efficiency,
long-term stability still remains a serious concern. Present work focuses
toward contributing on the later issue by adopting Titanium dioxide (TiO2) as
ETL and reduced graphene oxide (rGO) as HTL. Specifically, in the present work,
we report our efforts on the preparation of compact titanium dioxide (C-TiO2)
and mesoporous titanium dioxide (M-TiO2) layers as an ETL and a reduced
graphene oxide thin film as a HTL. The C-TiO2 film was spin casted on FTO glass
followed by casting of M-TiO2 film using Doctor Blading technique. Similarly,
the rGO film was produced by spray casting over the glass substrate. The
as-prepared ETL and HTL layers were characterized by measuring their optical
properties (transmittance and reflectance of thin films). Then, the bandgap, Eg
was extracted from reflectance and transmittance curves for ETL and HTL
respectively. In the case of rGO, we found the value of Eg to be 2.1 eV, which
varies between 2.7eV and 0.02eV depending upon its reduction level based on the
previously reported values. Similarly, the bandgap of the C-TiO2 was 4.51 eV
which was reduced to 4.12 eV after the addition of M-TiO2, which are 0.9 to 1.1
eV higher than previously reported values. However, bandgap shows decreasing
trend after employing M- TiO2 over C-TiO2. In a Perovskite solar cell, both ETL
and HTL will be investigated.

###Selfconsistent Model of Photoconversion Efficiency for Multijunction Solar Cells||A. V. Sachenko,A. I. Shkrebtii,V. P. Kostylyov,M. R. Kulish,I. O. Sokolovskyi###

Selfconsistent Model of Photoconversion Efficiency for Multijunction Solar Cells. To accurately calculate efficiencies $\eta$ of experimentally produced
multijunction solar cells (MJSCs) and optimize their parameters, we offer
semi-analytical photoconversion formalism that incorporates radiative
recombination, Shockley-Read-Hall (SRH) recombination, surface recombination at
the front and back surfaces of the cells, recombination in the space charge
region (SCR) and the recombination at the heterojunction boundaries.
Selfconsistent balance between the MJSC temperature and efficiency was imposed
by jointly solving the equations for the photocurrent, photovoltage, and heat
balance. Finally, we incorporate into the formalism the effect of additional
photocurrent decrease with subcell number increase. It is shown that for an
experimentally observed Shockley-Read-Hall lifetimes, the effect of
re-absorption and re-emission of photons on MJSC efficiency can be neglected
for non-concentrated radiation conditions. A significant efficiency $\eta$
increase can be achieved by improving the heat dissipation using radiators and
bringing the MJSC emissivity to unity, that is closer to black body radiation
rather than grey body radiation. Our calculated efficiencies compare well with
other numerical results available and are consistent with the experimentally
achieved efficiencies. The formalism can be used to optimize parameters of
MJSCs for maximum photoconversion efficiency.

###A new approach to simulation of limiting photoconversion efficiency of tandem solar cells||A. V. Sachenko,V. P. Kostylyov,N. P. Kulish,I. O. Sokolovskyi,A. I. Shkrebtii###

A new approach to simulation of limiting photoconversion efficiency of tandem solar cells. We develop a new approach to calculate the obtainable limit of
photoconversion efficiency of tandem solar cells (SCs) and applied it to SCs
with both vertical and lateral designs at AM0 and AM1.5 conditions. To get the
maximum efficiency, only radiative recombination has been considered using
typical radiative recombination parameters of the direct band gap III-V
semiconductors, and explicit energy dependence of light absorption. When
simulating the efficiency, we selfconsistently took into account the fact that
the amount of the heat dissipated by SC decreases as the number of
current-matched sub-cells increases. As the operating SCs temperature decreases
both the open-circuit voltage and the photoconversion efficiency increase. It
is shown that the above effect is especially strong for SCs operating under AM0
conditions. As the number of subcells is increased, narrowing the spectral
range for each subcell, the photocurrent is additionally reduced due to the
energy dependent light absorption, the factor generally ignored in the standard
approaches. Application of our formalism results in a maximum in the
theoretical dependence of the efficiency on the number of subcells, which was
indeed observed experimentally. Besides agreement with experiment, our
theoretical results are also close to other efficiencies calculated using
detailed balance based approaches.

###Modelling and Simulation of of high efficiency GaAs PIN-Solar Cell||Ali Imran,Deborah Eric,Muhammad Noaman Zahid,Muhammad Yousaf###

Modelling and Simulation of of high efficiency GaAs PIN-Solar Cell. Solar energy is the most convenient and reliable energy source among all
renewable energy resources and an efficient photovoltaic device is required to
convert this energy into utilizable energy. Different types of solar cells (SC)
are commercially available. However, various parameters need to be optimized to
get maximum efficiency from a SC. In this study we have presented a SC model in
which dependence of quantum efficiency (QE) on various parameters has been
investigated. The mobility of the carriers has been varied with wide range
along with the carrier life time (LT). Results show that maximum efficiencies
can be achieved up to 11.10% and 10.81% keeping the electron and hole mobility
to be 1500 cm2V-1s-1 and 300 cm2V-1s-1 respectively with electron and hole
carrier LT to be 3ns and 7ns respectively. The effect of surface recombination
velocity (SRV) has also been brought under observation and the maximum
efficiency is found to be 13.75% at electron and hole SRV equal to be 103ms-1.
Results shows that the higher photovoltaic efficiencies can be achieved by
increasing the mobility and carrier LT while decreasing the surface
recombination velocities.

###Bifacial Si Heterojunction-Perovskite Organic-Inorganic Tandem to Produce Highly Efficient Solar Cell||Reza Asadpour,Raghu V. K. Chavali,M. Ryyan Khan,Muhammad A. Alam###

Bifacial Si Heterojunction-Perovskite Organic-Inorganic Tandem to Produce Highly Efficient Solar Cell. As single junction thin-film technologies, both Si heterojunction (HIT) and
Perovskite based solar cells promise high efficiencies at low cost. One expects
that a tandem cell design with these cells connected in series will improve the
efficiency further. Using a self-consistent numerical modeling of optical and
transport characteristics, however, we find that a traditional series connected
tandem design suffers from low Jsc due to band-gap mismatch and current
matching constraints. It requires careful thickness optimization of Perovskite
to achieve any noticeable efficiency gain. Specifically, a traditional tandem
cell with state-of-the-art HIT (24%) and Perovskite (20%) sub-cells provides
only a modest tandem efficiency of ~25%. Instead, we demonstrate that a
bifacial HIT/Perovskite tandem design decouples the optoelectronic constraints
and provides an innovative path for extraordinary efficiencies. In the bifacial
configuration, the same state-of the-art sub-cells achieve a normalized output
of 33%, exceeding the bifacial HIT performance at practical albedo reflections.
Unlike the traditional design, this bifacial design is relatively insensitive
to Perovskite thickness variations, which may translate to simpler manufacture
and higher yield.

###Percolation assisted excitation transport in discrete-time quantum walks||Martin Stefanak,Jaroslav Novotny,Igor Jex###

Percolation assisted excitation transport in discrete-time quantum walks. Coherent transport of excitations along chains of coupled quantum systems
represents an interesting problem with a number of applications ranging from
quantum optics to solar cell technology. A convenient tool for studying such
processes are quantum walks. They allow to determine in a quantitative way all
the process features. We study the survival probability and the transport
efficiency on a simple, highly symmetric graph represented by a ring. The
propagation of excitation is modeled by a discrete-time (coined) quantum walk.
For a two-state quantum walk, where the excitation (walker) has to leave its
actual position to the neighboring sites, the survival probability decays
exponentially and the transport efficiency is unity. The decay rate of the
survival probability can be estimated using the leading eigenvalue of the
evolution operator. However, if the excitation is allowed to stay at its
present position, i.e. the propagation is modeled by a lazy quantum walk, then
part of the wave-packet can be trapped in the vicinity of the origin and never
reaches the sink. In such a case, the survival probability does not vanish and
the excitation transport is not efficient. The dependency of the transport
efficiency on the initial state is determined. Nevertheless, we show that for
some lazy quantum walks dynamical percolations of the ring eliminate the
trapping effect and efficient excitation transport can be achieved.

###First-principles study of the optoelectronic properties and photovoltaic absorber layer efficiency of Cu-based chalcogenides||Nasrin Sarmadian,Rolando Saniz,Bart Partoens,Dirk Lamoen###

First-principles study of the optoelectronic properties and photovoltaic absorber layer efficiency of Cu-based chalcogenides. Cu-based chalcogenides are promising materials for thin-film solar cells with
more than 20% measured cell efficiency. Using first-principles calculations
based on density functional theory, the optoelectronic properties of a group of
Cu-based chalcogenides Cu$_2$-II-IV-VI$_4$ is studied. They are then screened
with the aim of identifying potential absorber materials for photovoltaic
applications. The spectroscopic limited maximum efficiency (SLME) introduced by
Yu and Zunger is used as a metric for the screening. After constructing the
current-voltage curve, the maximum spectroscopy dependent power conversion
efficiency is calculated from the maximum power output. The role of the nature
of the band gap, direct or indirect, and also of the absorptivity of the
studied materials on the maximum theoretical power conversion efficiency is
studied. Our results show that Cu$_2$-II-GeSe$_4$ with II=Cd and Hg, and
Cu$_2$-II-SnS$_4$ with II=Cd and Zn have a higher theoretical efficiency
compared to the materials currently used as absorber layer.

###High-efficiency perovskite-polymer bulk heterostructure light-emitting diodes||Baodan Zhao,Sai Bai,Vincent Kim,Robin Lamboll,Ravichandran Shivanna,Florian Auras,Johannes M. Richter,Le Yang,Linjie Dai,Mejd Alsari,Xiao-Jian She,Lusheng Liang,Jiangbin Zhang,Samuele Lilliu,Peng Gao,Henry J. Snaith,Jianpu Wang,Neil C. Greenham,Richard H. Friend,Dawei Di###

High-efficiency perovskite-polymer bulk heterostructure light-emitting diodes. Perovskite-based optoelectronic devices have gained significant attention due
to their remarkable performance and low processing cost, particularly for solar
cells. However, for perovskite light-emitting diodes (LEDs), non-radiative
charge carrier recombination has limited electroluminescence (EL) efficiency.
Here we demonstrate perovskite-polymer bulk heterostructure LEDs exhibiting
record-high external quantum efficiencies (EQEs) exceeding 20%, and an EL
half-life of 46 hours under continuous operation. This performance is achieved
with an emissive layer comprising quasi-2D and 3D perovskites and an insulating
polymer. Transient optical spectroscopy reveals that photogenerated excitations
at the quasi-2D perovskite component migrate to lower-energy sites within 1 ps.
The dominant component of the photoluminescence (PL) is primarily bimolecular
and is characteristic of the 3D regions. From PL quantum efficiency and
transient kinetics of the emissive layer with/without charge-transport
contacts, we find non-radiative recombination pathways to be effectively
eliminated. Light outcoupling from planar LEDs, as used in OLED displays,
generally limits EQE to 20-30%, and we model our reported EL efficiency of over
20% in the forward direction to indicate the internal quantum efficiency (IQE)
to be close to 100%. Together with the low drive voltages needed to achieve
useful photon fluxes (2-3 V for 0.1-1 mA/cm2), these results establish that
perovskite-based LEDs have significant potential for light-emission
applications.

###Passive radiative cooling impact on commercial crystalline silicon-based photovoltaics||George Perrakis,Anna C. Tasolamprou,George Kenanakis,Eleftherios N. Economou,Stelios Tzortzakis,Maria Kafesaki###

Passive radiative cooling impact on commercial crystalline silicon-based photovoltaics. The radiative cooling of objects during daytime under direct sunlight has
recently been shown to be significantly enhanced by utilizing nanophotonic
coatings. Multilayer thin film stacks, 2D photonic crystals, etc. as coating
structures improved the thermal emission rate of a device in the infrared
atmospheric transparency window reducing considerably devices' temperature. Due
to the increased heating in photovoltaic (PV) devices, that has significant
adverse consequences on both their efficiency and life-time, and inspired by
the recent advances in daytime radiative cooling, we developed a coupled
thermal-electrical modeling to examine the physical mechanisms on how a
radiative cooler affects the overall efficiency of commercial photovoltaic
modules. Employing this modeling, which takes into account all the major
processes affected by the temperature variation in a PV device, we evaluated
the relative impact of the main radiative cooling approaches proposed so far on
the PV efficiency, and we established required conditions for optimized
radiative cooling. Moreover, we identified the validity regimes of the
currently existing PV-cooling models which treat the PV coolers as simple
thermal emitters. Finally, we assessed some realistic photonic coolers from the
literature, compatible with photovoltaics, to implement the radiative cooling
requirements, and demonstrated their associated impact on the temperature
reduction and PV efficiency. Providing the physical mechanisms and requirements
for cooling radiatively solar cells, our study provides guidelines for
utilizing suitable photonic structures as radiative coolers, enhancing the
efficiency and the lifetime of PV devices.

###Optical Refrigeration for Ultra-Efficient Photovoltaics||Assaf Manor,Leopoldo L. Martin,Carmel Rotschild###

Optical Refrigeration for Ultra-Efficient Photovoltaics. Improving the conversion efficiency of solar energy to electricity is most
important to mankind. For single-junction photovoltaic solar-cells, the
Shockley-Queisser thermodynamic efficiency limit is extensively due to the heat
dissipation, inherently accompanying the quantum process of electro-chemical
potential generation. Concepts such as solar thermo-photovoltaics and
thermo-photonics, have been suggested to harness this wasted heat, yet
efficiencies exceeding the Shockley-Queisser limit have not been demonstrated
due to the challenge of operating at high temperatures. Here, we present a
highly efficient converter based on endothermic photoluminescence, which
operates at relative low temperatures. The thermally induced blue-shifted
photoluminescence of a low-bandgap absorber is coupled to a high-bandgap
photovoltaic cell. The high absorber's photo-current and the high cell's
voltage results in 69% maximal theoretical conversion efficiencies. We
experimentally demonstrate tenfold thermal-enhancement of useful radiation for
the high-bandgap cell and 107% enhancement in average photon energy. This paves
the way for introducing disruptive-innovation in photovoltaics.

###Water vapour pressure as determining control parameter to fabricate high efficiency perovskite solar cells at ambient conditions||Lidia Contreras-Bernal,Juan Jesus Gallardo,Javier Navas,Jesus Idigoras,Juan A. Anta###

Water vapour pressure as determining control parameter to fabricate high efficiency perovskite solar cells at ambient conditions. Although perovskite solar cells have demonstrated impressive efficiencies in
research labs (above 23%), there is a need of experimental procedures that
allow their fabrication at ambient conditions, which would decrease
substantially manufacturing costs. However, under ambient conditions, a
delicate control of the moisture level in the atmosphere has to be enforced to
achieve efficient and highly stable devices. In this work, we show that it is
the absolute content of water measured in the form of partial water vapour
pressure (WVP) the only determining control parameter that needs to be
considered during preparation. Following this perspective, MAPbI3 perovskite
films were deposited under different WVP by changing the relative humidity (RH)
and the lab temperature. We found that efficient and reproducible devices can
be obtained at given values of WVP. Furthermore, it is demonstrated that small
temperature changes, at the same value of the RH, result in huge changes in
performance, due to the non-linear dependence of the WVP on temperature. We
have extended the procedure to accomplish high-efficient FA0.83MA0.17PbI3
devices at ambient conditions by adjusting DMSO proportion in precursor
solution as a function of WVP only. As an example of the relevance of this
paramater, a WVP value of around of 1.6 kPa appears to be an upper limit for
safe fabrication of high efficiency devices at ambient conditions, regardless
the RH and lab temperature.

###Modelling of limitations of bulk heterojunction architecture in organic solar cells II: 3d model||Jacek Wojtkiewicz,Marek Pilch###

Modelling of limitations of bulk heterojunction architecture in organic solar cells II: 3d model. Polymer solar cells are considered as very promising candidates for
development of photovoltaics of the future. They are cheap and easy to
fabricate. However, up to now, they possess fundamental drawback: low
effectiveness. In the most popular BHJ (bulk heterojunction) architecture the
actual long-standing top efficiency is about 12\% (recent achievements about
15\%). One ask the question how fundamental this limitation is, as certain
theoretical considerations suggest that it should be about two times higher. In
our paper we analyze the `geometric factor' as one of possible explanation of
relatively low efficiency of BHJ architecture. More precisly, we calculate the
effective area of the donor-acceptor border in the random mixture of donor and
acceptor nanocrystals and further compare it with an ideal 'brush'
architecture. In our previous calculation for the two dimensional model, we
have found that the maximal value of geometric factor was about 40\%. In the
actual three dimensional model, it turned out that both architectures give very
close value of the effective area. So the geometric factor seems to be not
significant as a factor limiting efficiency. Implications of this fact are
discussed: we list two other factors (mentioned but not thoroughly discussed in
literature) which can be responsible for limitations of efficiency of BHJ
architecture. We estimate their scale, and suggest that these limitations are
inevitable, or at least very hard to overcome. We suggest that return to layer
architecture could radically improve efficiency limitations -- however, to make
breakthrough, materials with large exciton diffusion length have to be
invented.

###Detailed-balance efficiency limits of two-terminal perovskite/silicon tandem solar cells with planar and Lambertian spectral splitters||Verena Neder,Stefan W. Tabernig,Albert Polman###

Detailed-balance efficiency limits of two-terminal perovskite/silicon tandem solar cells with planar and Lambertian spectral splitters. We derive the photovoltaic conversion efficiency limit for two-terminal
tandem solar cells with a perovskite top cell and silicon bottom cell with an
embedded spectrum splitter. For large-bandgap top-cells a spectrum splitter
strongly enhances the efficiency because of enhanced light absorption and
trapping. A Lambertian spectral splitter shows a significantly improved effect
compared to a planar splitter: we find an ideal efficiency enhancement in the
thermodynamic limit for a 500 nm thick top cell of 6% absolute for bandgaps
above 1.75 eV. Vice versa, the use of a spectral splitter geometry enables the
use of a thinner top cell. Using experimental parameters for perovskite cells
we show that for a top-cell bandgap of 1.77 eV a 2.8% absolute efficiency can
be gained. The calculations in this work show that integration of a spectral
splitter into perovskite/silicon tandem cells with a top bandgap above 1.7 eV
can lead to a large increase in efficiency, even with realistic experimental
losses and non-unity reflection of the spectral splitter.

###Optimized Design of Silicon Heterojunction Solar Cells for Field Operating Conditions||Jean Cattin,Olivier Dupré,Brahim Aïssa,Jan Haschke,Christophe Ballif,Mathieu Boccard###

Optimized Design of Silicon Heterojunction Solar Cells for Field Operating Conditions. Solar modules are currently characterized at standard test conditions (STC),
defined at 1000W/m2 and 25 {\deg}C. However, solar modules in actual outdoor
operating conditions typically operate at lower illumination and higher
temperature than STC, which significantly affects their performance ratio
(average harvesting efficiency over efficiency in STC). Silicon heterojunction
(SHJ) technology displays both good temperature coefficient and good
low-illumination performances, leading to outstanding performance ratios. We
investigate here SHJ solar cells that use a-SiCx(n) layer as front doped layer
with different carbon contents under different climates conditions. Adding
carbon increases transparency but also resistive losses at room temperature
(compared with carbon-free layers), leading to a significant decrease in
efficiency at STC. We demonstrate that despite this difference at STC, the
difference in energy harvesting efficiency is much smaller in all investigated
climates. Furthermore, we show that a relative gain of 0.4 to 0.8 percent in
harvesting efficiency is possible by adding a certain content of carbon in the
front (n) layer, compared with carbon-free cells optimized for STC.

###Structural order promotes efficient separation of delocalized charges at molecular heterojunctions||Xiangkun Jia,Lorenzo Soprani,Giacomo Londi,Seyed Mehrdad Hosseini,Felix Talnack,Stefan Mannsfeld,Safa Shoaee,Dieter Neher,Sebastian Reineke,Luca Muccioli,Gabriele D'Avino,Koen Vandewal,David Beljonne,Donato Spoltore X. Jia,S. Reineke,L. Soprani,L. Muccioli,G. Londi,D. Beljonne,S. M. Hosseini,S. Shoaee,D. Neher,F. Talnack,S. Mannsfeld,G. D'Avino,K. Vandewal,D. Spoltore###

Structural order promotes efficient separation of delocalized charges at molecular heterojunctions. The energetic landscape at the interface between electron donating and
accepting molecular materials favors efficient conversion of intermolecular
charge-transfer states (CTS) into free charge carriers in high-performance
organic solar cells. Here, we elucidate how interfacial energetics, charge
generation and radiative recombination are affected by structural ordering. We
experimentally determine the CTS binding energy of a series of model, small
molecule donor-acceptor blends, where the used acceptors (B2PYMPM, B3PYMPM and
B4PYMPM) differ only in the nitrogen position of their lateral pyridine rings.
We find that the formation of an ordered, face-on molecular packing in B4PYMPM
is beneficial to efficient, field-independent charge separation, leading to
fill factors over 70% in photovoltaic devices. This is rationalized by a
comprehensive computational protocol showing that, compared to the more
amorphous and isotropically oriented B2PYMPM, the higher order of the B4PYMPM
molecules provides more delocalized CTS. Furthermore, we find no correlation
between the quantum efficiency of radiative free charge carrier recombination
and the bound or unbound nature of the CTS. This work highlights the importance
of structural ordering at donor-acceptor interfaces for efficient free carrier
generation and shows that more ordering and less bound CT states do not
preclude efficient radiative recombination.

###Broadband enhancement of light harvesting in luminescent solar concentrator||Yun-Feng Xiao,Chang-Ling Zou,Yi-Wen Hu,Yan Li,Lixin Xiao,Fang-Wen Sun,Qihuang Gong###

Broadband enhancement of light harvesting in luminescent solar concentrator. Luminescent solar concentrator (LSC) can absorb large-area incident sunlight,
then emit luminescence with high quantum efficiency, which finally be collected
by a small photovoltaic (PV) system. The light-harvesting area of the PV system
is much smaller than that of the LSC system, potentially improving the
efficiency and reducing the cost of solar cells. Here, based on Fermi-golden
rule, we present a theoretical description of the luminescent process in
nanoscale LSCs where the conventional ray-optics model is no longer applicable.
As an example calculated with this new model, we demonstrate that a slot
waveguide consisting of a nanometer-sized low-index slot region sandwiched by
two high-index regions provides a broadband enhancement of light harvesting by
the luminescent centers in the slot region. This is because the slot waveguide
can (1) greatly enhance the spontaneous emission due to the Purcell effect, (2)
dramatically increase the effective absorption cross-section of luminescent
centers, and (3) strongly improve the quantum efficiency of luminescent
centers. It is found that about 80% solar photons can be ultimately converted
to waveguide-coupled luminescent photons even for a low luminescent quantum
efficiency of 0.5. This LSC is potential to construct a tandem structure which
can absorb nearly full-spectrum solar photons, and also may be of special
interest for building integrated nano-PV applications.

###High Performance Inverted Organic Photovoltaics Without Hole Selective Contact||Achilleas Savva,Ignasi Burgues-Ceballos,Giannis Papazoglou,Stelios A. Choulis###

High Performance Inverted Organic Photovoltaics Without Hole Selective Contact. A detailed investigation of the functionality of inverted organic
photovoltaics (OPVs) using bare Ag contacts as top electrode is presented. The
inverted OPVs without hole transporting layer (HTL) exhibit a significant gain
in hole carrier selectivity and power conversion efficiency (PCE) after
exposure in ambient conditions. Inverted OPVs comprised of
ITO/ZnO/poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester
(P3HT:PCBM)/Ag demonstrate over 3.5% power conversion efficiency only if the
devices are exposed in air for over 4 days. As concluded through a series of
measurements, the oxygen presence is essential to obtain fully operational
solar cell devices without HTL. Moreover, accelerated stability tests under
damp heat conditions (RH=85% and T=65oC) performed to non-encapsulated OPVs
demonstrate that HTL-free inverted OPVs exhibit comparable stability to the
reference inverted OPVs. Importantly, it is shown that bare Ag top electrodes
can be efficiently used in inverted OPVs using various high performance
polymer:fullerene bulk heterojunction material systems demonstrating 6.5% power
conversion efficiencies.

###Effect of surface recombination on electroluminescence and photoconversion in a-Si:H/c-Si heterojunction solar cells||A. V. Sachenko,A. V. Bobyl,V. N. Verbitskiy,V. M. Vlasyuk,D. M. Zhigunov,V. P. Kostylyov,I. O. Sokolovskyi,E. I. Terukov,P. A. Forsh,M. Evstigneev###

Effect of surface recombination on electroluminescence and photoconversion in a-Si:H/c-Si heterojunction solar cells. Surface recombination affects both light-to-electricity and
electricity-to-light conversion in solar cells (SCs). Therefore, quantitative
analysis and reduction of surface recombination is an important direction in SC
research. In this work, electroluminescence (EL) intensity and photoconversion
efficiency of a set of 93 large-area (239\,cm$^2$) a-Si:H/c-Si heterojunction
SCs (HJSCs) are measured under AM1.5 conditions at 298 K. The HJSC samples
differed only in surface recombination velocity, $S$, but otherwise were
identical. Variation in $S$ was due to the variation of the chemical conditions
under which the samples were treated. It is established that EL quantum
efficiency, is affected by $S$ much more strongly than photoconversion
efficiency, $\eta$: namely, the reduction of the latter from 20.5\% to 18\% due
to an increase of $S$ is accompanied by a decrease of the former by more than
an order of magnitude. In HJSCs with well passivated surfaces, i.e. low $S$, EL
efficiency reached 2.1\%, which is notably higher than the known values in
silicon homojunction diodes. For temperature-dependent measurements of EL and
dark I-V curves, one of the samples was cut into small-area (1 cm$^2$) pieces.
It was found that EL intensity as a function of temperature develops a maximum
at $T$ = 223 K. At low temperatures, the current at weak bias is shown to be
due to tunneling mechanism. A theoretical model is developed that explains all
these findings quantitatively.

###Highly efficient visible colloidal lead-halide perovskite nanocrystal light-emitting diodes||Fei Yan,Jun Xing,Guichuan Xing,Lina Quan,Swee Tiam Tan,Jiaxin Zhao,Rui Su,Lulu Zhang,Shi Chen,Yawen Zhao,Alfred Huan,Edward H. Sargent,Qihua Xiong,Hilmi Volkan Demir###

Highly efficient visible colloidal lead-halide perovskite nanocrystal light-emitting diodes. Lead-halide perovskites have been attracting attention for potential use in
solid-state lighting. Following the footsteps of solar cells, the field of
perovskite light-emitting diodes (PeLEDs) has been growing rapidly. Their
application prospects in lighting, however, remain still uncertain due to a
variety of shortcomings in device performance including their limited levels of
luminous efficiency achievable thus far. Here we show high-efficiency PeLEDs
based on colloidal perovskite nanocrystals (PeNCs) synthesized at room
temperature possessing dominant first-order excitonic radiation (enabling a
photoluminescence quantum yield of 71% in solid film), unlike in the case of
bulk perovskites with slow electron-hole bimolecular radiative recombination (a
second-order process). In these PeLEDs, by reaching charge balance in the
recombination zone, we find that the Auger nonradiative recombination, with its
significant role in emission quenching, is effectively suppressed in low
driving current density range. In consequence, these devices reach a record
high maximum external quantum efficiency of 12.9% reported to date and an
unprecedentedly high power efficiency of 30.3 lm W-1 at luminance levels above
1000 cd m-2 as required for various applications. These findings suggest that,
with feasible levels of device performance, the PeNCs hold great promise for
their use in LED lighting and displays.

###Bridging the gap between photovoltaics R&D and manufacturing with data-driven optimization||Felipe Oviedo,Zekun Ren,Xue Hansong,Siyu Isaac Parker Tian,Kaicheng Zhang,Mariya Layurova,Thomas Heumueller,Ning Li,Erik Birgersson,Shijing Sun,Benji Mayurama,Ian Marius Peters,Christoph J. Brabec,John Fisher III,Tonio Buonassisi###

Bridging the gap between photovoltaics R&D and manufacturing with data-driven optimization. Novel photovoltaics, such as perovskites and perovskite-inspired materials,
have shown great promise due to high efficiency and potentially low
manufacturing cost. So far, solar cell R&D has mostly focused on achieving
record efficiencies, a process that often results in small batches, large
variance, and limited understanding of the physical causes of underperformance.
This approach is intensive in time and resources, and ignores many relevant
factors for industrial production, particularly the need for high
reproducibility and high manufacturing yield, and the accompanying need of
physical insights. The record-efficiency paradigm is effective in early-stage
R&D, but becomes unsuitable for industrial translation, requiring a repetition
of the optimization procedure in the industrial setting. This mismatch between
optimization objectives, combined with the complexity of physical root-cause
analysis, contributes to decade-long timelines to transfer new technologies
into the market. Based on recent machine learning and technoeconomic advances,
our perspective articulates a data-driven optimization framework to bridge R&D
and manufacturing optimization approaches. We extend the maximum-efficiency
optimization paradigm by considering two additional dimensions: a
technoeconomic figure of merit and scalable physical inference. Our framework
naturally aligns different stages of technology development with shared
optimization objectives, and accelerates the optimization process by providing
physical insights.

###Directing Near-Infrared Photon Transport with Core@Shell Particles||Kevin M. Conley,Vaibhav Thakore,Fahime Seyedheydari,Mikko Karttunen,Tapio Ala-Nissila###

Directing Near-Infrared Photon Transport with Core@Shell Particles. Directing the propagation of near-infrared radiation is a major concern in
improving the efficiency of solar cells and thermal insulators. A facile
approach to scatter light in the near-infrared region without excessive heating
is to embed compact layers with semiconductor particles. The directional
scattering by semiconductor@oxide (core@shell) spherical particles (containing
Si, InP, TiO$_2$, SiO$_2$, or ZrO$_2$) with a total radius varying from 0.1 to
4.0 {\mu}m and in an insulating medium at low volume fraction is investigated
using Lorenz-Mie theory and multiscale modelling. The optical response of each
layers is calculated under irradiation by the sun or a blackbody emitter at
1180 K. Reflectance efficiency factors of up to 83.7% and 63.9% are achieved
for near-infrared solar and blackbody radiation in 200 {\mu}m thick compact
layers with only 1% volume fraction of bare Si particles with a radius of 0.23
{\mu}m and 0.50 {\mu}m, respectively. The maximum solar and blackbody
efficiency factors of layers containing InP particles was slightly less (80.2%
and 60.7% for bare particles with a radius of 0.25 {\mu}m and 0.60 {\mu}m,
respectively). The addition of an oxide coating modifies the surrounding
dielectric environment, which improves the solar reflectance efficiency factor
to over 90% provided it matches the scattering mode energies with the incident
spectral density. The layers are spectrally-sensitive and can be applied as a
back or front reflector for solar devices, high temperature thermal insulators,
and optical filters in Gradient Heat Flux Sensors for fire safety applications.

###Efficiency enhancement of black dye-sensitized solar cell by newly synthesized D-$π$-A coadsorbents: A theoretical study||Yavar T. Azar,Mahmoud Payami###

Efficiency enhancement of black dye-sensitized solar cell by newly synthesized D-$π$-A coadsorbents: A theoretical study. In this work, using the DFT and TDDFT, we have theoretically studied the
electronic and optical properties of the two recently synthesized coadsorbents
Y1 and Y2, which were aimed to enhance the efficiency of the black
dye-sensitized solar cells. To determine the solvatochromic shifts, both the
implicit and mixed implicit-explicit models have been used. The connection
between the solvatochromic shifts and the changes of dipole moments in the
excitation process is discussed. The difference in excitation charge transfer
is utilized to explain the experimentally observed difference in $J_{sc}$ for
Y1 and Y2. Investigating the interactions of I$_2$ molecules in the electrolyte
solution with the coadsorbents showed that with Y1 the recombination loss was
weakened through decreasing the I$_2$ concentration near the TiO$_2$ surface,
whereas with Y2 it was increased. As a result, the higher values of both
$J_{sc}$ and $V_{oc}$ with Y1 coadsorbent explains its experimentally observed
higher efficiency. The present study sheds light on how to design and engineer
newer coadsorbents or organic dyes for higher efficiencies.

###Exact Wave Packet Dynamics of Singlet Fission in Unsubstituted and Substituted Polyene Chains within Long-Range Interacting Models||Suryoday Prodhan,S. Ramasesha###

Exact Wave Packet Dynamics of Singlet Fission in Unsubstituted and Substituted Polyene Chains within Long-Range Interacting Models. Singlet fission (SF) is a potential pathway for significant enhancement of
efficiency in organic solar cells (OSC). In this paper, we study singlet
fission in a pair of polyene molecules in two different stacking arrangements
employing exact many-body wave packet dynamics. In the non-interacting model,
the SF yield is absent. The individual molecules are treated within Hubbard and
Pariser-Parr-Pople (PPP) models and the interaction between them involves
transfer terms, intersite electron repulsions and site-charge--bond-charge
repulsion terms. Initial wave packet is constructed from excited singlet state
of one molecule and ground state of the other. Time development of this wave
packet under the influence of intermolecular interactions is followed within
the Schr\"odinger picture by an efficient predictor-corrector scheme. In
unsubstituted Hubbard and PPP chains, $2{}^1A$ excited singlet state leads to
significant SF yield while the $1{}^1B$ state gives negligible fission yield.
On substitution by donor-acceptor groups of moderate strength, the lowest
excited state will have sufficient $2{}^1A$ character and hence results in
significant SF yield. Because of rapid internal conversion, the nature of the
lowest excited singlet will determine the SF contribution to OSC efficiency.
Furthermore, we find the fission yield depends considerably on the stacking
arrangement of the polyene molecules.

###Surface and Bulk Effects of K in Cu$_{1-x}$K$_x$In$_{1-y}$Ga$_y$Se$_2$ Solar Cells||Christopher P. Muzzillo,Timothy J. Anderson###

Surface and Bulk Effects of K in Cu$_{1-x}$K$_x$In$_{1-y}$Ga$_y$Se$_2$ Solar Cells. Two strategies for enhancing photovoltaic (PV) performance in chalcopyrite
solar cells were investigated: Cu1-xKxIn1-yGaySe2 absorbers with low K content
(K/(K+Cu), or x ~ 0.07) distributed throughout the bulk, and CuIn1-yGaySe2
absorbers with KIn1-yGaySe2 grown on their surfaces. For the Ga-free case,
increased temperature improved PV performance in the KInSe2 surface absorbers,
but not in the bulk x ~ 0.07 absorbers. Growth temperature also increased
KInSe2 phase fraction, relative to Cu1-xKxInSe2 alloys-evidence that surface
KInSe2 improved performance more than bulk KInSe2. Surface KIn1-yGaySe2 and
bulk x ~ 0.07 Cu1-xKxIn1-yGaySe2 films with Ga/(Ga+In), or y of 0.3 and 0.5
also had improved efficiency, open-circuit voltage (VOC), and fill factor (FF),
relative to CuIn1-yGaySe2 baselines. On the other hand, y ~ 1 absorbers did not
benefit from K introduction. Similar to Cu1-xKxInSe2, the formation of
Cu1-xKxGaSe2 alloys was favored at low temperatures and high substrate Na
content, relative to the formation of mixed-phase CuGaSe2 + KGaSe2.
KIn1-yGaySe2 alloys were grown for the first time, as evidenced by X-ray
diffraction and ultraviolet/visible spectroscopy. For all Ga/(Ga+In)
compositions, the surface KIn1-yGaySe2 absorbers had superior PV performance in
buffered and buffer-free devices. However, the bulk x ~ 0.07 absorbers only
outperformed the baselines in buffered devices. The data demonstrate that
KIn1-yGaySe2 passivates the surface of CuIn1-yGaySe2 to increase efficiency,
VOC, and FF, while bulk Cu1-xKxIn1-yGaySe2 absorbers with x ~ 0.07 enhance
efficiency, VOC, and FF by some other mechanism.

###Organic solar cell design as a function of radiative quantum efficiency||Blaise Godefroid,Gregory Kozyreff###

Organic solar cell design as a function of radiative quantum efficiency. We study the radiative decay, or fluorescence, of excitons in organic solar
cells as a function of its geometrical parameters. Contrary to their
non-radiative counterpart, fluorescence losses strongly depend on the
environment. By properly tuning the thicknesses of the buffer layers between
the active regions of the cell and the electrodes, the exciton lifetime and,
hence, the exciton diffusion length can be increased. The importance of this
phenomenon depends on the radiative quantum efficiency, which is the fraction
of the exciton decay that is intrinsically due to fluorescence. Besides this
effect, interferences within the cell control the efficiency of sunlight
injection into the active layers. An optimal cell design must rely on the
consideration of these two aspects. By properly managing fluorescence losses,
one can significantly improve the cell performance. To demonstrate this fact,
we use realistic material parameters inspired from literature data and obtain
an increase of power conversion efficiency from 11.3% to 12.7%. Conversely, not
to take into account the strong dependence of fluorescence on the environment
may lead to a sub-optimal cell design and a degradation of cell performance.
The presence of radiative losses, however small, significantly changes the
optimal thicknesses. We illustrate this latter situation with experimental
material data.

###How Much is the Efficiency of Solar Cells Enhanced by Quantum Coherence?||Sangchul Oh###

How Much is the Efficiency of Solar Cells Enhanced by Quantum Coherence?. We study how much the efficiency of a solar cell as a quantum heat engine
could be enhanced by quantum coherence. In contrast to the conventional
approach that a quantum heat engine is in thermal equilibrium with both hot and
cold reservoirs, we propose a new description that the quantum heat engine is
in the cold reservoir and the thermal radiation from the hot reservoir is
described by the pumping term in the master equation. This pumping term solves
the problem of the incorrect mean photon number of the hot reservoir assumed by
the previous studies. By solving the master equation, we obtain the
current-voltage and the power-voltage curves of the photocell for different
pumping rates. We find that, as the photon flux increases, the power output of
the photocell increases linearly at first and then becomes saturated, but the
efficiency decreases rapidly. It is demonstrated that while the power output is
enhanced significantly by the quantum coherence via the dark state of the
coupled donors, the improvement of the efficiency is not significant.

###Enhancing silicon solar cells with singlet fission: the case for Foerster resonant energy transfer using a quantum dot intermediate||S. W. Tabernig,B. Daiber,T. Wang,B. Ehrler###

Enhancing silicon solar cells with singlet fission: the case for Foerster resonant energy transfer using a quantum dot intermediate. One way for solar cell efficiencies to overcome the Shockley-Queisser limit
is downconversion of high-energy photons using singlet fission (SF) in
polyacenes like tetracene (Tc). SF enables generation of multiple excitons from
the high-energy photons which can be harvested in combination with Si. In this
work we investigate the use of lead sulfide quantum dots (PbS QDs) with a band
gap close to Si as an interlayer that allows Foerster Resonant Energy Transfer
(FRET) from Tc to Si, a process that would be spin-forbidden without the
intermediate QD step. We investigate how the conventional FRET model, most
commonly applied to the description of molecular interactions, can be modified
to describe the geometry of QDs between Tc and Si and how the distance between
QD and Si, and the QD bandgap affects the FRET efficiency. By extending the
acceptor dipole in the FRET model to a 2D plane, and to the bulk, we see a
relaxation of the distance dependence of transfer. Our results indicate that
FRET efficiencies from PbS QDs to Si well above 50 % are be possible at very
short, but possibly realistic distances of around 1 nm, even for quantum dots
with relatively low photoluminescence quantum yield.

###Mixed Halide Perovskite Light Emitting Solar Cell||Dmitry Gets,Arthur Ishteev,Eduard Danilovskiy,Danila Saranin,Ross Haroldson,Sergey Makarov,Anvar Zakhidov###

Mixed Halide Perovskite Light Emitting Solar Cell. Organic-inorganic halide perovskites recently have emerged as a promising
material for highly effective light-emitting diodes (LEDs) and solar cells
(SCs). Despite efficiencies of both perovskite SCs and LEDs are already among
the best, the development of a perovskite dual functional device that is
capable of working in these two regimes with high efficiencies is still
challenging. Here we demonstrate that the dual functional device based on mixed
halide perovskite CH3NH3PbBr2I can be switched from SC to LED with low
threshold voltage Vth < 2 V by exposing to Sun at open circuit Voc or at small
bias voltage of Vpol ~ 1 - 2 V. Such photo-poling creates in-situ p-i-n
junction via methylammonium (CH3NH3+, MA+) and I-/Br- ions migration to
interfaces, lowering charge injection barriers, and self-balancing injection
currents in perovskite LED. We show that before the photo-poling, the
electroluminescence (EL) is highly unstable in LED regime, whereas after the
photo-poling, stabilized EL exhibits unusual dynamics, increasing with time and
poling cycle number, while Vth and injection current decrease with cycling
runs. Additionally, photo-induced and current-induced halide segregation
accumulates with cycling, that is found beneficial for LED, increasing its
efficiency and brightness, but reversibly degrading photovoltaic (PV)
performance, which can be easily recovered.

###Non-equilibrium thermodynamics of charge separation in organic solar cells||Waldemar Kaiser,Veljko Jankovic,Nenad Vukmirovic,Alessio Gagliardi###

Non-equilibrium thermodynamics of charge separation in organic solar cells. This work presents a novel theoretical description of the non-equilibrium
thermodynamics of charge separation process in organic solar cells (OSCs).
Using the theory of stochastic thermodynamics, we connect the phonon-assisted
dynamics and recombination of electron-hole pairs within a photo-excited
organic bilayer with the thermodynamic free energy. We analyze the impact of
energetic disorder and delocalization on the free energy, average energy and
entropy. For high energetic disorder, the site population is well described by
equilibrium. We observe significant deviations from equilibrium for delocalized
electron-hole pairs at small energetic disorder, representing efficient OSCs.
Our results emphasize that both a large Gibbs entropy and large initial
separation are required to achieve efficient charge separation. A decrease in
free energy barrier with increased distances between charges does not
necessarily correlate with the separation yield. Our presented framework can
further shed light on the transient thermodynamic free energy, allowing
previously inaccessible insight into the individual thermodynamic contributions
from energy and entropy on sub-ns timescales. Transient simulations reveal
large Gibbs entropy on ps-timescales for even highest disorder, which may
explain the efficient separation of "hot" CT states.

###Time-resolved imaging of non-diffusive carrier transport in long-lifetime halide perovskite thin films||Aravindan Sridharan,Nakita K. Noel,Hyeon Hwang,Soroush Hafezian,Barry P. Rand,Stéphane Kéna-Cohen###

Time-resolved imaging of non-diffusive carrier transport in long-lifetime halide perovskite thin films. Owing to their exceptional semiconducting properties, hybrid
inorganic-organic perovskites show great promise as photovoltaic absorbers. In
these materials, long-range diffusion of charge carriers allows for most of the
photogenerated carriers to contribute to the photovoltaic efficiency. Here,
time-resolved photoluminescence (PL) microscopy is used to directly probe
ambipolar carrier diffusion and recombination kinetics in hybrid perovskites.
This technique is applied to thin films of methylammonium lead tri-iodide
MAPbI$_3$ obtained with two different fabrication routes, methylammonium lead
tribromide (MAPbBr$_3$), and an alloy of formamidinium lead tri-iodide
(FAPbI$_3$) and methylammonium lead bromide
FA$_{0.85}$MA$_{0.15}$Pb(I$_{0.85}$Br_${0.15}$)$_3$. Average diffusion
coefficients in the films leading to the highest device efficiencies and
longest lifetimes, i.e., in FA$_{0.85}$MA$_{0.15}$Pb(I$_{0.85}$Br$_{0.15}$)$_3$
and acetonitrile-processed MAPbI$_3$, are found to be several orders of
magnitude lower than in the other films. Further examination of the
time-dependence shows strong evidence for non-diffusive transport. In
particular, acetonitrile-processed MAPbI$_3$ shows distinct diffusion regimes
on short and long timescales with an effective diffusion constant varying over
2 orders of magnitude. Our results also highlight the fact that increases in
carrier lifetime in this class of materials are not necessarily concomitant
with increased diffusion lengths and that the PL quantum efficiency under solar
cell operating conditions is a greater indication of material, and ultimately
device, quality.

###On the Absence of Triplet Exciton Loss Pathways in Non-Fullerene Acceptor based Organic Solar Cells||Maria S. Kotova,Giacomo Londi,Johannes Junker,Stefanie Dietz,Alberto Privitera,Kristofer Tvingstedt,David Beljonne,Andreas Sperlich,Vladimir Dyakonov###

On the Absence of Triplet Exciton Loss Pathways in Non-Fullerene Acceptor based Organic Solar Cells. We investigate the viability of highly efficient organic solar cells (OSCs)
based on non-fullerene acceptors (NFA) by taking into consideration efficiency
loss channels and stability issues caused by triplet excitons (TE) formation.
OSCs based on a blend of the conjugated donor polymer PBDB-T and ITIC as
acceptor were fabricated and investigated with electrical, optical and
spin-sensitive methods. The spin-Hamiltonian parameters of molecular TEs and
charge transfer TEs in ITIC e.g., zero-field splitting and charge distribution,
were calculated by Density Functional Theory (DFT) modelling. In addition, the
energetic model describing the photophysical processes in the donor-acceptor
blend was derived. Spin-sensitive photoluminescence measurements prove the
formation of charge transfer (CT) states in the blend and the formation of TEs
in the pure materials and the blend. However, no molecular TE signal is
observed in the completed devices under working conditions by spin-sensitive
electrical measurements. The absence of a molecular triplet state population
allows to eliminate a charge carrier loss channel and irreversible
photooxidation facilitated by long-lived triplet states. These results
correlate well with the high power conversion efficiency of the
PBDB-T:ITIC-based OSCs and their high stability.

###Atomic scale model and electronic structure of Cu$_2$O/CH$_3$NH$_3$PbI$_3$ interfaces in perovskite solar cells||Jesús E. Castellanos-Águila,Lucas Lodeiro,Eduardo Menéndez-Proupin,Ana L. Montero-Alejo,Pablo Palacios,José C. Conesa,Perla Wahnón###

Atomic scale model and electronic structure of Cu$_2$O/CH$_3$NH$_3$PbI$_3$ interfaces in perovskite solar cells. Cuprous oxide has been conceived as a potential alternative to traditional
organic hole transport layers in hybrid halide perovskite-based solar cells.
Device simulations predict record efficiencies using this semiconductor, but
experimental results do not yet show this trend. More detailed knowledge about
the Cu$_2$O/perovskite interface is mandatory to improve the photoconversion
efficiency. Using density functional theory calculations, here we study the
interfaces of CH$_3$NH$_3$PbI$_3$ with Cu$_2$O to assess their influence on
device performance. Several atomistic models of these interfaces are provided
for the first time, considering different compositions of the interface atomic
planes. The interface electronic properties are discussed on the basis of the
optimal theoretical situation, but in connection with the experimental
realizations and device simulations. It is shown that the formation of
vacancies in the Cu$_2$O terminating planes is essential to eliminate dangling
bonds and trap states. The four interface models that fulfill this condition
present a band alignment favorable for photovoltaic conversion. Energy of
adhesion, and charge transfer across the interfaces are also studied. The
termination of CH$_3$NH$_3$PbI$_3$ in PbI$_2$ atomic planes seems optimal to
maximize the photoconversion efficiency.

###A New Perspective on the Role of A-site Cation in Perovskite Solar Cells||Chang Woo Myung,Jeonghun Yun,Geunsik Lee,Kwang S. Kim###

A New Perspective on the Role of A-site Cation in Perovskite Solar Cells. As the race towards higher efficiency for inorganic/organic hybrid perovskite
solar cells (PSCs) is becoming highly competitive, a design scheme to maximize
carrier transport towards higher power efficiency has been urgently demanded.
Here, we unravel a hidden role of A-site cation of PSCs in carrier transport
which has been largely neglected, i.e., tuning the Frohlich electron-phonon
(e-ph) coupling of longitudinal optical (LO) phonon by A-site cations. The key
for steering Frohlich polaron is to control the interaction strength and the
number of proton (or lithium) coordination to halide ion. The coordination to I
alleviates electron-phonon scattering by either decreasing the Born effective
charge or absorbing the LO motion of I. This novel principle discloses lower
electron-phonon coupling by several promising organic cations including
hydroxyl-ammonium cation (NH$_3$OH$^+$) and possibly Li$^+$ solvating
methylamine (Li$^+$NH$_2$CH$_3$) than methyl-ammonium cation. A new perspective
on the role of A-site cation could help in improving power efficiency and
accelerating the application of PSCs.

###Impermeable Inorganic Walls Sandwiching Photoactive Layer toward Inverted Perovskite Solar and Indoor-Photovoltaic Devices||Jie Xu,Jun Xi,Hua Dong,Namyoung Ahn,Zonglong Zhu,Jinbo Chen,Peizhou Li,Xinyi zhu,Jinfei Dai,Ziyang Hu,Bo Jiao,Xun Hou,Jingrui Li,Zhaoxin Wu###

Impermeable Inorganic Walls Sandwiching Photoactive Layer toward Inverted Perovskite Solar and Indoor-Photovoltaic Devices. Interfaces between the perovskite active layer and the charge-transport
layers (CTLs) play a critical role in both efficiency and stability of
halide-perovskite photovoltaics. One of the major concerns is that surface
defects of perovskite could cause detrimental nonradiative recombination and
material degradation. In this work, we addressed this challenging problem by
inserting ultrathin alkali-fluoride (AF) films between the tri-cation
lead-iodide perovskite layer and both CTLs. This bilateral inorganic walls
strategy makes use of both physical-blocking and chemical-anchoring
functionalities of the continuous, uniform and compact AF framework: on the one
hand, the uniformly distributed alkali-iodine coordination at the perovskite-AF
interfaces effectively suppresses the formation of iodine-vacancy defects at
the surfaces and grain boundaries of the whole perovskite film, thus reducing
the trap-assisted recombination at the perovskite-CTL interfaces and therewith
the open-voltage loss; on the other hand, the impermeable AF buffer layers
effectively prevent the bidirectional ion migration at the perovskite-CTLs
interfaces even under harsh working conditions. As a result, a power-conversion
efficiency (PCE) of 22.02% (certified efficiency 20.4%) with low open-voltage
deficit (< 0.4V) was achieved for the low-temperature processed inverted planar
perovskite solar cells. Exceptional operational stability (500 h, ISOS-L-2) and
thermal stability (1000 h, ISOS-D-2) were obtained. Meanwhile, a 35.7% PCE was
obtained under dim-light source (1000 lux white LED light) with the optimized
device, which is among the best records in perovskite indoor photovoltaics.

###Four-terminal perovskite/silicon tandem solar cell with integrated Mie-resonant spectral splitter metagrating||Verena Neder,Dong Zhang,Sjoerd Veenstra,Albert Polman###

Four-terminal perovskite/silicon tandem solar cell with integrated Mie-resonant spectral splitter metagrating. A spectral splitting, light trapping dielectric metasurface is designed,
fabricated and integrated into a four-terminal perovskite/silicon hybrid tandem
solar cell to increase the absorption of light close to the bandgap of the
perovskite top cell, and enhance transmission of the near-infrared spectral
band towards the bottom cell. The metagrating is composed of a hexagonal array
of unit cells of 150-nm-tall hydrogenated amorphous silicon trimer
nanostructures with dielectric Mie resonances in the 600-800 nm perovskite
near-gap region, made using substrate-conformal imprint lithography. By
tailoring the metasurface resonant scattering modes and their interference with
the direct reflection paths we minimize specular reflection and obtain high
diffraction efficiency that leads to improved light trapping in the perovskite
top cell. The measured short-circuit current increase in the perovskite top
cell is 0.5 mA/cm2 corresponding to an estimated efficiency gain of 0.26%
(absolute) for the metasurface-integrated 4T perovskite/silicon tandem cell.
Simulations for a further optimized metasurface spectrum splitter geometry
predict a short-circuit current gain in the perovskite top cell of 1.4 mA/cm2
and an efficiency gain for the 4T tandem cell of 0.4% (absolute). The
metagrating approach for simultaneous spectral splitting, light trapping and
reflectance reduction provides a flexible platform that can be applied to many
tandem cell geometries.

###A Universal Strategy of Perovskite Ink-Substrate Interaction to Overcome the Poor Wettability of a Self-Assembled Monolayer for Reproducible Perovskite Solar Cells||Ashish Kulkarni,Ranjini Sarkar,Samah Akel,Maria Haeser,Benjamin Klingebiel,Matthias Wuttig,Sudip Chakraborty,Michael Saliba,Thomas Kirchartz###

A Universal Strategy of Perovskite Ink-Substrate Interaction to Overcome the Poor Wettability of a Self-Assembled Monolayer for Reproducible Perovskite Solar Cells. Perovskite solar cells employing self assembled monolayers such as Me-4PACz
as hole transport layer has been reported to demonstrate high device
efficiency. However, the poor perovskite wetting on the Me-4PACz caused by poor
perovskite ink interaction with the underlying Me-4PACz presents significant
challenges for fabricating efficient perovskite devices. A triple co-solvent
system comprising of dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and
N-methyl-2-pyrrolidone (NMP) is employed to improve the perovskite
ink-substrate interaction and obtain a uniform perovskite layer. In comparison
to DMF, DMSO-based inks, the inclusion of NMP shows considerably higher binding
energies of the perovskite ink with Me-4PACz as revealed by density-functional
theory calculations. With the optimized triple co-solvent ratio, the perovskite
devices deliver high power conversion efficiencies of >20%, 19.5% and ~18.5%
for active areas of 0.16 cm2, 0.72 cm2 and 1.08 cm2 respectively. Importantly,
this perovskite ink-substrate interaction approach is universal and helps in
obtaining a uniform layer and high photovoltaic device performance for other
perovskite compositions such as MAPbI3, FAMAPbI3-xBrx, and MA-free
FACsPbI3-xBrx.

###Janus $β$-Te$_2$X (X = S, Se) Monolayers for Efficient Excitonic Solar Cells and Photocatalytic Water Splitting||Jaspreet Singh,Ashok Kumar###

Janus $β$-Te$_2$X (X = S, Se) Monolayers for Efficient Excitonic Solar Cells and Photocatalytic Water Splitting. Highly efficient, environmental friendly and renewable sources of energy are
of great need today to combat with increasing energy demands and environmental
pollution. In this work, we have investigated the novel 2D allotropes i.e.,
$\beta$-Te$_2$X (X = S, Se) using first-principles calculations and study their
potential applications in light harvesting devices. Both the monolayers possess
to have the high stability and semiconducting nature with an indirect band gap.
The high carrier mobilities and excellent optical absorption of these
monolayers make them potential candidates for solar conversion applications. We
have proposed the type-II heterojunction solar cells and calculated their power
conversion efficiencies (PCEs). The small conduction band offset and
appropriate band gap of donor material in case of
$\beta$-Te$_2$S(S-Side)/$\alpha$-Te$_2$S(Te-Side) heterojunction results in the
PCE of ~ 21%. In addition to that, the band alignments of these monolayers
properly engulf the redox potentials of the water. The overpotentials required
to trigger the hydrogen reduction (HER) and water oxidation (OER) half
reactions reveal that HER and OER preferred the acidic and neutral mediums,
respectively. The calculated solar-to-hydrogen (STH) efficiencies of
$\beta$-Te$_2$S ($\beta$-Te$_2$Se) monolayers come out to be ~ 13 % (~12 %),
respectively, which implies their practical applications in water splitting.
Thus, our work provides strong evidence regarding the potential applications of
these materials in the field of light harvesting devices.

###Polymeric Squaraine Dyes as Electron Donors in Bulk Heterojunction Solar Cells||Sebastian F. Voelker,Shinobu Uemura,Moritz Limpinsel,Markus Mingebach,Carsten Deibel,Vladimir Dyakonov,Christoph Lambert###

Polymeric Squaraine Dyes as Electron Donors in Bulk Heterojunction Solar Cells. A polysquaraine low band gap polymer was synthesized by Yamamoto coupling of
a monomeric dibromo indolenine squaraine dye. The resulting polymer has a
weight average molar mass in the order of Mw ~30.000-50.000 and a
polydispersity of ca. 1.7 as determined by gel-permeation chromatography (GPC).
The electronic properties of monomer and polymer were investigated by cyclic
voltammetry, absorption and emission spectroscopy. Owing to exciton coupling
the absorption bands of the polymer are red-shifted and strongly broadened
compared to the monomer squaraine dye. Bulk heterojunction solar cells were
prepared from blends of the polysquaraine with the fullerene derivative
[6,6]-phenyl C61-butyric acid methyl ester (PCBM) in different weight ratios
(1:3 to 1:1). The power conversion efficiencies under simulated AM 1.5
conditions yielded 0.45 % for these non-optimized systems. The external quantum
efficiency (EQE) shows that the photoresponse spans the range from 300 to 850
nm, which illustrates the promising properties of this novel organic
semiconductor as a low band gap donor material in organic photovoltaics.

###Modelling Multi Quantum Well Solar Cell Efficiency||James P. Connolly,Jenny Nelson,Ian Ballard,Keith W. J. Barnham,Carsten Rohr,Chris Button,John Roberts,Tom Foxon###

Modelling Multi Quantum Well Solar Cell Efficiency. The spectral response of quantum well solar cells (QWSCs) is well understood.
We describe work on QWSC dark current theory which combined with SR theory
yields a system efficiency. A methodology published for single quantum well
(SQW) systems is extended to MQW systems in the Al(x) Ga(1-x) As and
InGa(0.53x) As(x) P systems. The materials considered are dominated by
Shockley-Read-Hall (SRH) recombination. The SRH formalism expresses the dark
current in terms of carrier recombination through mid-gap traps. The SRH
recombination rate depends on the electron and hole densities of states (DOS)
in the barriers and wells, which are well known, and of carrier non-radiative
lifetimes. These material quality dependent lifetimes are extracted from
analysis of suitable bulk control samples. Consistency over a range of AlGaAs
controls and QWSCs is examined, and the model is applied to QWSCs in InGaAsP on
InP substrates. We find that the dark currents of MQW systems require a
reduction of the quasi Fermi level separation between carrier populations in
the wells relative to barrier material, in line with previous studies.
Consequences for QWSCs are considered suggesting a high efficiency potential.

###Ferroelectric field effect of the bulk heterojunction in polymer solar cells||Meng Li,Heng Ma,Hairui Liu,Yurong Jiang,Heying Niu,Adil Amat###

Ferroelectric field effect of the bulk heterojunction in polymer solar cells. A ferroelectric field effect in the bulk heterojunction was found when an
external electric field (EEF) was applied on the active layer of polymer solar
cells (PSCs) during the annealing process of the active layer spin-coated with
poly (3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM).
For one direction field, the short circuit current density of PSCs was improved
from 7.2 to 8.0 mA/cm2, the power conversion efficiency increased from 2.4 to
2.8%, and the incident photon-to-current conversion efficiency increased from
42 to 49% corresponding to the different EEF magnitude. For an opposite
direction field, the applied EEF brought a minus effect on the performance
mentioned above. EEF treatment can orientate molecular ordering of the polymer,
and change the morphology of the active layer. The authors suggest a
explanation that the ferroelectric field has been built in the active layer,
and therefore it plays a key role in PSCs system. A needle-like surface
morphology of the active film was also discussed.

###Copper and Transparent-Conductor Reflectarray Elements on Thin-Film Solar Cell Panels||Philippe Dreyer,Monica Morales-Masis,Sylvain Nicolay,Christophe Ballif,Julien Perruisseau-Carrier###

Copper and Transparent-Conductor Reflectarray Elements on Thin-Film Solar Cell Panels. This work addresses the integration of reflectarray antennas (RA) on thin
film Solar Cell (SC) panels, as a mean to save real estate, weight, or cost in
platforms such as satellites or transportable autonomous antenna systems. Our
goal is to design a good RA unit cell in terms of phase response and bandwidth,
while simultaneously achieving high optical transparency and low microwave
loss, to preserve good SC and RA energy efficiencies, respectively. Since there
is a trade-off between the optical transparency and microwave surface
conductivity of a conductor, here both standard copper and transparent
conductors are considered. The results obtained at the unit cell level
demonstrates the feasibility of integrating RA on a thin-film SC, preserving
for the first time good performance in terms of both SC and RA efficiency. For
instance, measurement at X-band demonstrate families of cells providing a phase
range larger than 270{\deg} with average microwave loss of -2.45dB (resp.
-0.25dB) and average optical transparency in the visible spectrum of 90% (resp.
85%) using transparent conductive multilayer (resp. a copper layer).

###Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers||Federico Brivio,Keith T. Butler,Aron Walsh,Mark van Schilfgaarde###

Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers. Solar cells based on a light absorbing layer of the organometal halide
perovskite CH$_3$NH$_3$PbI$_3$ have recently reached 15% conversion efficiency,
though how these materials work remains largely unknown. We analyse the
electronic structure and optical properties within the quasiparticle
self-consistent GW approximation. While this compound bears some similarity to
conventional sp semiconductors, it also displays unique features. Quasiparticle
self-consistency is essential for an accurate description of the band
structure: bandgaps are much larger than what is predicted by the local density
approximation (LDA) or GW based on the LDA. Valence band dispersions are
modified in a very unusual manner. In addition, spin orbit coupling strongly
modifies the band structure and gives rise to unconventional dispersion
relations and a Dresselhaus splitting at the band edges. The average hole mass
is small, which accounts for the long diffusion lengths recently observed. The
surface ionisation potential (workfunction) is calculated to be 5.7 eV with
respect to the vacuum level, explaining efficient carrier transfer to TiO$_2$
and Au electrical contacts.

###Coherent control of photocurrent in a strongly scattering photoelectrochemical system||Seng Fatt Liew,Sebastien M. Popoff,Stafford W. Sheehan,Arthur Goetschy,Charles A. Schmuttenmaer,A. Douglas Stone,Hui Cao###

Coherent control of photocurrent in a strongly scattering photoelectrochemical system. A fundamental issue that limits the efficiency of many photoelectrochemical
systems is that the photon absorption length is typically much longer than the
electron diffusion length. Various photon management schemes have been
developed to enhance light absorption; one simple approach is to use randomly
scattering media to enable broadband and wide-angle enhancement. However, such
systems are often opaque, making it difficult to probe photo-induced processes.
Here we use wave interference effects to modify the spatial distribution of
light inside a highly-scattering dye-sensitized solar cell to control photon
absorption in a space-dependent manner. By shaping the incident wavefront of a
laser beam, we enhance or suppress photocurrent by increasing or decreasing
light concentration on the front side of the mesoporous photoanode where the
collection efficiency of photoelectrons is maximal. Enhanced light absorption
is achieved by reducing reflection through the open boundary of the photoanode
via destructive interference, leading to a factor of two increase in
photocurrent. This approach opens the door to probing and manipulating
photoelectrochemical processes in specific regions inside nominally opaque
media.

###Optical Absorption Spectra and Excitons of Dye-Substrate Interfaces: Catechol on TiO$_2$(110)||Duncan John Mowbray,Annapaola Migani###

Optical Absorption Spectra and Excitons of Dye-Substrate Interfaces: Catechol on TiO$_2$(110). Optimizing the photovoltaic efficiency of dye-sensitized solar cells (DSSC)
based on staggered gap heterojunctions requires a detailed understanding of
sub-band gap transitions in the visible from the dye directly to the
substrate's conduction band (CB) (type-II DSSCs). Here, we calculate the
optical absorption spectra and spatial distribution of bright excitons in the
visible region for a prototypical DSSC, catechol on rutile TiO$_2$(110), as a
function of coverage and deprotonation of the OH anchoring groups. This is
accomplished by solving the Bethe-Salpeter equation (BSE) based on hybrid
range-separated exchange and correlation functional (HSE06) density functional
theory (DFT) calculations. Such a treatment is necessary to accurately describe
the interfacial level alignment and the weakly bound charge transfer
transitions that are the dominant absorption mechanism in type-II DSSCs. Our
HSE06 BSE spectra agree semi-quantitatively with spectra measured for catechol
on anatase TiO$_2$ nanoparticles. Our results suggest deprotonation of
catechol's OH anchoring groups, while being nearly isoenergetic at high
coverages, shifts the onset of the absorption spectra to lower energies, with a
concomitant increase in photovoltaic efficiency. Further, the most relevant
bright excitons in the visible region are rather intense charge transfer
transitions with the electron and hole spatially separated in both the [110]
and [001] directions. Such detailed information on the absorption spectra and
excitons is only accessible via periodic models of the combined dye-substrate
interface.

###Influence of the Surface of a Nanocrystal on its Electronic and Phononic Properties||Nuri Yazdani,Deniz Bozyigit,Kantawong Vuttivorakulchai,Mathieu Luisier,Vanessa Wood###

Influence of the Surface of a Nanocrystal on its Electronic and Phononic Properties. Over the past thirty years, it has been consistently observed that surface
engineering of colloidal nanocrystals (NC) is key to their performance
parameters. In the case of lead chalcogenide NCs, for example, replacing thiols
with halide anion surface termination has been shown to increase power
conversion efficiency in NC-based solar cells. To gain insight into the origins
of these improvements, we perform ab initio molecular dynamics (AIMD) on
experimentally-relevant sized lead sulfide (PbS) NCs constructed with thiol or
Cl, Br, and I anion surfaces. The surface of both the thiol- and
halide-terminated NCs exhibit low and high-energy phonon modes with large
thermal displacements not present in bulk PbS; however, halide anion surface
termination reduces the overlap of the electronic wavefunctions with these
vibration modes. These findings suggest that electron-phonon interactions will
be reduced in the halide terminated NCs, a conclusion that is supported by
analyzing the time-dependent evolution of the electronic energies and
wavefunctions extracted from the AIMD. This work explains why electron-phonon
interactions are crucial to charge carrier dynamics in NCs and how surface
engineering can be applied to systematically control their electronic and
phononic properties. Furthermore, we propose that the computationally efficient
approach of gauging electron-phonon interaction implemented here can be used to
guide the design of application-specific surface terminations for arbitrary
nanomaterials.

###Revealing the stability and efficiency enhancement in mixed halide perovskites MAPb(I$_{1-x}$Cl$_x$)$_3$ with ab initio calculations||Un-Gi Jong,Chol-Jun Yu,Yong-Man Jang,Gum-Chol Ri,Song-Nam Hong,Yong-Hyon Pae###

Revealing the stability and efficiency enhancement in mixed halide perovskites MAPb(I$_{1-x}$Cl$_x$)$_3$ with ab initio calculations. A little addition of Cl to \ce{MAPbI3} has been reported to improve the
material stability as well as light harvesting and carrier conducting
properties of organometal trihalide perovskites, the key component of
perovskite solar cell (PSC). However, the mechanism of performance enhancement
of PSC by Cl addition is still unclear. Here, we apply the efficient virtual
crystal approximation method to revealing the effects of Cl addition on the
structural, electronic, optical properties and material stability of
\ce{MAPb(I_{1-x}Cl_x)3}. Our {\it ab initio} calculations present that as the
increase of Cl content cubic lattice constants and static dielectric constants
decrease linearly, while band gaps and exciton binding energies increase
quadratically. Moreover, we find the minimum of exciton binding energy at the
Cl content of 7\%, at which the chemical decomposition reaction changes
coincidentally to be from exothermic to endothermic. Interactions among
constituents of compound and electronic charge transferring during formation
are carefully discussed. This reveals new prospects for understanding and
designing of stable, high efficiency PSCs.

###White light emission from silicon nanoparticles||Chengyun Zhang,Yi Xu,Jin Liu,Juntao Li,Jin Xiang,Hui Li,Jinxiang Li,Qiaofeng Dai,Sheng Lan,Andrey E. Miroshnichenko###

White light emission from silicon nanoparticles. As one of the most important semiconductors, silicon (Si) has been used to
fabricate electronic devices, waveguides, detectors, and solar cells etc.
However, its indirect bandgap hinders the use of Si for making good emitters1.
For integrated photonic circuits, Si-based emitters with sizes in the range of
100-300 nm are highly desirable. Here, we show that efficient white light
emission can be realized in spherical and cylindrical Si nanoparticles with
feature sizes of ~200 nm. The up-converted luminescence appears at the magnetic
and electric multipole resonances when the nanoparticles are resonantly excited
at their magnetic and electric dipole resonances by using femtosecond (fs)
laser pulses with ultralow low energy of ~40 pJ. The lifetime of the white
light is as short as ~52 ps, almost three orders of magnitude smaller than the
state-of-the-art results reported so far for Si (~10 ns). Our finding paves the
way for realizing efficient Si-based emitters compatible with current
semiconductor fabrication technology, which can be integrated to photonic
circuits.

###Technological guidelines for the design of tandem III-V nanowire on Si solar cells from opto-electrical simulations||Vladimir Maryasin,Davide Bucci,Quentin Rafhay,Federico Panicco,Jérôme Michallon,Anne Kaminski-Cachopo###

Technological guidelines for the design of tandem III-V nanowire on Si solar cells from opto-electrical simulations. Effect of geometrical and structural parameters on the efficiency of the
tandem solar cell based on the III-V nanowire array on silicon is studied by
the means of coupled opto-electrical simulations. A close to realistic
structure, consisting of AlGaAs core-shell nanowire array, connected through a
tunnel diode to a Si subcell is modelled, revealing the impact of top contact
layer, growth mask and tunnel junction. Optical simulation of the tandem
structure under current matching condition determine optimal geometrical
parameters of the nanowire array. They are then used in the extensive
electrical optimization of the radial junction in the nanowire subcell. Device
simulations show the necessity of high doping of the junction in order to avoid
full shell depletion. The influence of bulk and surface recombination on the
performance of the top subcell is studied, exposing the importance of the good
surface passivation near the depleted region of the radial p - n junction.
Finally, simulations of the fully optimized tandem structure show that a
promising efficiency of 27.6% with the short-circuit current density of 17.1
mA/cm^2 can be achieved with reasonable bulk and surface carrier lifetime.

###Methylamine Vapor Exposure for Improved Morphology and Stability of Cesium-Methylammonium Lead Halide Perovskite Thin-Films||Akash Singh,Arun Singh Chouhan,Sushobhan Avasthi###

Methylamine Vapor Exposure for Improved Morphology and Stability of Cesium-Methylammonium Lead Halide Perovskite Thin-Films. Mixed-cation Cesium-Methylammonium lead halide perovskite (CsxMA1-xPbI3-xBrx)
thin-films have been used to demonstrate stable and efficient perovskite
devices. However, a systematic study of the Cs incorporation on the properties
of the perovskite films has not been reported. In this report, Impact of Cesium
incorporation on the minority carrier recombination lifetime of
Cesium-Methylammonium lead halide perovskite thin-films is studied. The
lifetime for the as-deposited perovskite films decreases with increasing
concentration of cesium. However, mixed cation perovskite film is more stable,
showing higher lifetime (15-20 micro seconds) after 9 hours of ambient exposure
than just after deposition (6-13 micro seconds). Methylamine Vapor Exposure
(MVE) technique was used to improve the morphology of the as-deposited film.
MVE treated films are more oriented along (110) direction and were even more
stable in ambient, with Cs0.10MA0.90PbI2.90Br0.10 films showing lifetime of
almost 50 micro seconds after 9 hours of ambient exposure, twice the lifetime
of a comparable MAPbI3 film. These results throw light on why mixed-cation
cesium-methylamine lead halide perovskite films are better for highly efficient
and stable perovskite solar cells.

###Optimal Design of Thin-film Plasmonic Solar Cells using Differential Evolution Optimization Algorithms||Ankit Vora,Satyadhar Joshi,Arun Matai,Joshua M. Pearce,Durdu Guney###

Optimal Design of Thin-film Plasmonic Solar Cells using Differential Evolution Optimization Algorithms. An approach using a differential evolution (DE) optimization algorithm is
proposed to optimize design parameters for improving the optical absorption
efficiency of plasmonic solar cells (PSC). This approach is based on
formulating the parameters extraction as a search and optimization process in
order to maximize the optical absorption in the PSC. Determining the physical
parameters of three-dimensional (3-D) PSC is critical for designing and
estimating their performance, however, due to the complex design of the PSC,
parameters extraction is time and calculation intensive. In this paper, this
technique is demonstrated for the case of commercial thin-film hydrogenated
amorphous silicon (a-Si:H) solar photovoltaic cells enhanced through patterned
silver nano-disk plasmonic structures. The DE optimization of PSC structures
was performed to execute a real-time parameter search and optimization. The
predicted optical enhancement (OE) in optical absorption in the active layer of
the PSC for AM-1.5 solar spectrum was found to be over 19.45% higher compared
to the reference cells. The proposed technique offers higher accuracy and
automates the tuning of control parameters of PSC in a time-efficient manner.

###First-principles study on the chemical decomposition of inorganic perovskites \ce{CsPbI3} and \ce{RbPbI3} at finite temperature and pressure||Un-Gi Jong,Chol-Jun Yu,Yun-Hyok Kye,Chol-Ho Kim,Son-Guk Ri,Yue Chen###

First-principles study on the chemical decomposition of inorganic perovskites \ce{CsPbI3} and \ce{RbPbI3} at finite temperature and pressure. Inorganic halide perovskite \ce{Cs(Rb)PbI3} has attracted significant
research interest in the application of light-absorbing material of perovskite
solar cells (PSCs). Although there have been extensive studies on structural
and electronic properties of inorganic halide perovskites, the investigation on
their thermodynamic stability is lack. Thus, we investigate the effect of
substituting Rb for Cs in \ce{CsPbI3} on the chemical decomposition and
thermodynamic stability using first-principles thermodynamics. By calculating
the formation energies of solid solutions \ce{Cs$_{1-x}$Rb$_x$PbI3} from their
ingredients \ce{Cs$_{1-x}$Rb$_x$I} and \ce{PbI2}, we find that the best match
between efficiency and stability can be achieved at the Rb content $x\approx$
0.7. The calculated Helmholtz free energy of solid solutions indicates that
\ce{Cs$_{1-x}$Rb$_x$PbI3} has a good thermodynamic stability at room
temperature due to a good miscibility of \ce{CsPbI3} and \ce{RbPbI3}. Through
lattice-dynamics calculations, we further highlight that \ce{RbPbI3} never
stabilize in cubic phase at any temperature and pressure due to the chemical
decomposition into its ingredients \ce{RbI} and \ce{PbI2}, while \ce{CsPbI3}
can be stabilized in the cubic phase at the temperature range of 0$-$600 K and
the pressure range of 0$-$4 GPa. Our work reasonably explains the experimental
observations, and paves the way for understanding material stability of the
inorganic halide perovskites and designing efficient inorganic halide PSCs.

###Excitonic States in Semiconducting Two-dimensional Perovskites||Alejandro Molina-Sánchez###

Excitonic States in Semiconducting Two-dimensional Perovskites. Hybrid organic/inorganic perovskites have emerged as efficient semiconductor
materials for applications in photovoltaic solar cells with conversion
efficiency above 20 \%. Recent experiments have synthesized ultra-thin
two-dimensional (2D) organic perovskites with optical properties similar to
those of 2D materials like monolayer MoS$_2$: large exciton binding energy and
excitonic effects at room temperature. In addition, 2D perovskites are
synthesized with a simple fabrication process with potential low-cost and
large-scale manufacture.
  Up to now, state-of-the-art simulations of the excitonic states have been
limited to the study of bulk organic perovskites. A large number of atoms in
the unit cell and the complex role of the organic molecules make inefficient
the use of \textit{ab initio} methods. In this work, we define a simplified
crystal structure to calculate the optical properties of 2D perovskites,
replacing the molecular cations with inorganic atoms. We can thus apply
state-of-the-art, parameter-free and predictive \textit{ab initio} methods like
the GW method and the Bethe-Salpeter equation to obtain the excitonic states of
a model 2D perovskite. We find that optical properties of 2D perovskites are
strongly influenced by excitonic effects, with binding energies up to 600 meV.
Moreover, the optical absorption is carried out at the bromine and lead atoms
and therefore the results are useful for a qualitatively understanding of the
optical properties of organic 2D perovskites.

###Nickel oxide-based heterostructures with large band offsets||Robert Karsthof,Holger von Wenckstern,Jesus Zuniga-Perez,Christiane Deparis,Marius Grundmann###

Nickel oxide-based heterostructures with large band offsets. We present research results on the electronic transport in heterostructures
based on p-type nickel oxide (NiO) with the n-type oxide semiconductors zinc
oxide (ZnO) and cadmium oxide (CdO). NiO is a desirable candidate for
application in (opto-)electronic devices. However, because of its small
electron affinity, heterojunctions with most n-type oxide semiconductors
exhibit conduction and valence band offsets at the heterointerface in excess of
1 eV. ZnO/NiO junctions exhibit a so called type-II band alignment, making
electron-hole recombination the only process by which a current can vertically
flow through the structure. These heterojunctions are nevertheless shown to be
of practical use in efficient optoelectronic devices, as exemplified here by
our UV-converting transparent solar cells. These devices, although exhibiting
high conversion efficiencies, suffer from two light-activated recombination
channels connected to the type-II interface, one of which we identify and
analyse in more detail here. Furthermore, CdO/NiO contacts were studied - a
heterostructure with even larger band offsets such that a type-III band
alignment is achieved. This situation theoretically enables the development of
a 2-dimensional electronic system consisting of topologically protected states.
We present experiments demonstrating that the CdO/NiO heterostructure indeed
hosts a conductive layer absent in both materials when studied separately.

###Strong Performance Enhancement in Lead-Halide Perovskite Solar Cells through Rapid, Atmospheric Deposition of n-type Buffer Layer Oxides||Ravi D. Raninga,Robert A. Jagt,Solène Béchu,Tahmida N. Huq,Mark Nikolka,Yen-Hung Lin,Mengyao Sun,Zewei Li,Wen Li,Muriel Bouttemy,Mathieu Frégnaux,Henry J. Snaith,Philip Schulz,Judith L. MacManus-Driscoll,Robert L. Z. Hoye###

Strong Performance Enhancement in Lead-Halide Perovskite Solar Cells through Rapid, Atmospheric Deposition of n-type Buffer Layer Oxides. Thin (approximately 10 nm) oxide buffer layers grown over lead-halide
perovskite device stacks are critical for protecting the perovskite against
mechanical and environmental damage. However, the limited perovskite stability
restricts the processing methods and temperatures (<=110 C) that can be used to
deposit the oxide overlayers, with the latter limiting the electronic
properties of the oxides achievable. In this work, we demonstrate an
alternative to existing methods that can grow pinhole-free TiOx (x =
2.00+/-0.05) films with the requisite thickness in <1 min without vacuum. This
technique is atmospheric pressure chemical vapor deposition (AP-CVD). The rapid
but soft deposition enables growth temperatures of >=180 {\deg}C to be used to
coat the perovskite. This is >=70 {\deg}C higher than achievable by current
methods and results in more conductive TiOx films, boosting solar cell
efficiencies by >2%. Likewise, when AP-CVD SnOx (x ~ 2) is grown on
perovskites, there is also minimal damage to the perovskite beneath. The SnOx
layer is pinhole-free and conformal, which reduces shunting in devices, and
increases steady-state efficiencies from 16.5% (no SnOx) to 19.4% (60 nm SnOx),
with fill factors reaching 84%. This work shows AP-CVD to be a versatile
technique for growing oxides on thermally-sensitive materials.

###Rational Design of Photo-Electrochemical Hybrid Devices based on Graphene and Chlamydomonas reinhardtii Light-Harvesting Proteins||Martha Ortiz-Torres,Miguel Fernández-Niño,Juan C Cruz,Andrea Capasso,Fabio Matteocci,Edgar J. Patiño,Yenny Hernández,Andrés Fernando González Barrios###

Rational Design of Photo-Electrochemical Hybrid Devices based on Graphene and Chlamydomonas reinhardtii Light-Harvesting Proteins. Dye-sensitized solar cells (DSSCs) have been highlighted as the promising
alternative to generate clean energy based on low pay-back time materials.
These devices have been designed to mimic solar energy conversion processes
from photosynthetic organisms (the most efficient energy transduction
phenomenon observed in nature) with the aid of low-cost materials. Recently,
light-harvesting complexes (LHC) have been proposed as potential dyes in DSSCs
based on their higher light-absorption efficiencies as compared to synthetic
dyes. In this work, photo-electrochemical hybrid devices were rationally
designed by adding for the first time Leu and Lys tags to heterologously
expressed light-harvesting proteins from Chlamydomonas reinhardtii, thus
allowing their proper orientation and immobilization on graphene electrodes.
The light-harvesting complex 4 from C. reinhardtii (LHC4) was initially
expressed in Escherichia coli, purified via affinity chromatography and
subsequently immobilized on plasma-treated thin-film graphene electrodes. A
photocurrent density of 40.30 \pm 9.26 \mu A/cm2 was measured on devices using
liquid electrolytes supplemented with a phosphonated viologen to facilitate
charge transfer. Our results suggest that a new family of graphene-based
thin-film photovoltaic devices can be manufactured from rationally tagged LHC
proteins and opens the possibility to further explore fundamental processes of
energy transfer for biological components interfaced with synthetic materials.

###Perovskite-Inspired Materials for Photovoltaics -- From Design to Devices||Yi-Teng Huang,Sean R. Kavanagh,David O. Scanlon,Aron Walsh,Robert L. Z. Hoye###

Perovskite-Inspired Materials for Photovoltaics -- From Design to Devices. Lead-halide perovskites have demonstrated astonishing increases in power
conversion efficiency in photovoltaics over the last decade. The most efficient
perovskite devices now outperform industry-standard multi-crystalline silicon
solar cells, despite the fact that perovskites are typically grown at low
temperature using simple solution-based methods. However, the toxicity of lead
and its ready solubility in water are concerns for widespread implementation.
These challenges, alongside the many successes of the perovskites, have
motivated significant efforts across multiple disciplines to find lead-free and
stable alternatives which could mimic the ability of the perovskites to achieve
high performance with low temperature, facile fabrication methods. This Review
discusses the computational and experimental approaches that have been taken to
discover lead-free perovskite-inspired materials, and the recent successes and
challenges in synthesizing these compounds. The atomistic origins of the
extraordinary performance exhibited by lead-halide perovskites in photovoltaic
devices is discussed, alongside the key challenges in engineering such
high-performance in alternative, next-generation materials. Beyond
photovoltaics, this Review discusses the impact perovskite-inspired materials
have had in spurring efforts to apply new materials in other optoelectronic
applications, namely light-emitting diodes, photocatalysts, radiation
detectors, thin film transistors and memristors. Finally, the prospects and key
challenges faced by the field in advancing the development of
perovskite-inspired materials towards realization in commercial devices is
discussed.

###Atomistic insights into the degradation of halide perovskites: a reactive force field molecular dynamics study||Mike Pols,José Manuel Vicent-Luna,Ivo Filot,Adri C. T. van Duin,Shuxia Tao###

Atomistic insights into the degradation of halide perovskites: a reactive force field molecular dynamics study. Halide perovskites make efficient solar cells due to their exceptional
optoelectronic properties, but suffer from several stability issues. The
characterization of the degradation processes is challenging because of the
limitations in the spatio-temporal resolution in experiments and the absence of
efficient computational methods to study the reactive processes. Here, we
present the first effort in developing reactive force fields for large scale
molecular dynamics simulations of the phase instability and the defect-induced
degradation reactions in inorganic CsPbI$_{3}$. We find that the phase
transitions are driven by a combination of the anharmonicity of the perovskite
lattice with the thermal entropy. At relatively low temperatures, the Cs
cations tend to move away from the preferential positions with good contacts
with the surrounding metal halide framework, potentially causing its conversion
to a non-perovskite phase. Our simulations of defective structures reveal that,
although both iodine vacancies and interstitials are very mobile in the
perovskite lattice, the vacancies have a detrimental effect on the stability,
initiating the decomposition reactions of perovskites to PbI$_{2}$. Our work
puts ReaxFF forward as an effective computational framework to study reactive
processes in halide perovskites.

###Modeling upconversion of erbium doped microcrystals based on experimentally determined Einstein coefficients||S. Fischer,H. Steinkemper,P. Löper,M. Hermle,J. C. Goldschmidt###

Modeling upconversion of erbium doped microcrystals based on experimentally determined Einstein coefficients. Upconversion of infrared photons is a promising possibility to enhance solar
cell efficiency by producing electricity from otherwise unused sub-band-gap
photons. We present a rate equation model, and the relevant processes, in order
to describe upconversion of near-infrared photons. The model considers
stimulated and spontaneous processes, multi-phonon relaxation and energy
transfer between neighboring ions. The input parameters for the model are
experimentally determined for the material system \beta-NaEr0.2Y0.8F4. The
determination of the transition probabilities, also known as the Einstein
coefficients, is in the focus of the parameterization. The influence of
multi-phonon relaxation and energy transfer on the upconversion are evaluated
and discussed in detail. Since upconversion is a non-linear process, the
irradiance dependence of the simulations is investigated and compared to
experimental data of quantum efficiency measurements. The results are very
promising and indicate that upconversion is physically reasonably described by
the rate equations. Therefore, the presented model will be the basis for
further simulations concerning various applications of upconversion, such as in
combination with plasmon resonances in metal nanoparticles.

###Switchable Ferroelectric Photovoltaic Effects in Epitaxial Thin Films of h-RFeO3 having Narrow Optical Band Gaps||Hyeon Han,Donghoon Kim,Ji Hyun Lee,Jucheol Park,Sang Yeol Nam,Mingi Choi,Kijung Yong,Hyun Myung Jang###

Switchable Ferroelectric Photovoltaic Effects in Epitaxial Thin Films of h-RFeO3 having Narrow Optical Band Gaps. Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their
high stability, environmental safety, anomalously high photovoltages, coupled
with reversibly switchable photovoltaic responses. However, FPVs suffer from
extremely low photocurrents, which is primarily due to their wide band gaps.
Here, we present a new class of FPVs by demonstrating switchable ferroelectric
photovoltaic effects using hexagonal ferrite (h-RFeO3) thin films having narrow
band gaps of ~1.2 eV, where R denotes rare-earth ions. FPVs with narrow band
gaps suggests their potential applicability as photovoltaic and optoelectronic
devices. The h-RFeO3 films further exhibit reasonably large ferroelectric
polarizations, which possibly reduces a rapid recombination rate of the
photo-generated electron-hole pairs. The power conversion efficiency (PCE) of
h-RFeO3 thin-film devices is sensitive on the magnitude of polarization. In the
case of h-TmFeO3 (h-TFO) thin film, the measured PCE is twice as large as that
of the BiFeO3 thin film, a prototypic FPV. We have further shown that the
switchable photovoltaic effect dominates over the unswitchable internal field
effect arising from the net built-in potential. This work thus demonstrates a
new class of FPVs towards high-efficiency solar cell and optoelectronic
applications.

###Ti4+ Substituted Magnesium Hydride as Promising Material for Hydrogen Storage and Photovoltaic Applications||R Varunaa,S Kiruthika,P Ravindran###

Ti4+ Substituted Magnesium Hydride as Promising Material for Hydrogen Storage and Photovoltaic Applications. In order to overcome the disadvantages of MgH2 towards its applications in
on-board hydrogen storage, first principle calculations have been performed for
Ti (2+, 3+, and 4+) substituted MgH2. Our calculated enthalpy of formation and
H site energy implies that Ti substitution in Mg site reduces the stability of
MgH2 which improve the hydrogen storage properties and Ti prefers to be in +4
oxidation state in MgH2. The bonding analyses through partial density of
states, electron localization function and Bader charge of these systems
confirm the existence of iono-covalent bonding. Electronic structure obtained
from hybrid functional calculations show that intermediate bands (IB) are
formed in Ti4+ substituted MgH2 which could improve the solar cell efficiencies
due to multiple photon absorption from valence band to conduction band via IBs
and converts low energy photons in the solar spectrum also into electricity.
Further, our calculated carrier effective masses and optical absorption spectra
show that Ti4+ substituted MgH2 is suitable for higher efficiency photovoltaic
applications. Our results suggest that Ti4+ substituted MgH2 can be considered
as a promising material for hydrogen storage as well as photovoltaic
applications.

###Solution Processed Infrared- and Thermo- Photovoltaics based on 0.7 eV Bandgap PbS Colloidal Quantum Dots||Yu Bi,Arnau Bertran,Shuchi Gupta,Iñigo Ramiro,Santanu Pradhan,Sotirios Christodoulou,Shanmukh-Naidu Majji,Mehmet Zafer Akgul,Gerasimos Konstantatos###

Solution Processed Infrared- and Thermo- Photovoltaics based on 0.7 eV Bandgap PbS Colloidal Quantum Dots. Harnessing low energy photons is of paramount importance for multi-junction
high efficiency solar cells as well as for thermo-photovoltaic applications.
However, semiconductor absorbers with bandgap lower than 0.8 eV have been
limited to III-V (InGaAs) or IV (Ge) semiconductors that are characterized by
high manufacturing costs and complicated lattice matching requirements in their
growth and integration with the higher bandgap cells. Here, we have developed
solution processed low bandgap photovoltaic devices based on PbS colloidal
quantum dots (CQDs) with a bandgap of 0.7 eV suited for both
thermo-photovoltaic as well as low energy solar photon harvesting. By matching
the spectral response of those cells to that of the infrared solar spectrum, we
report a record high short circuit current (JSC) of 37 mA/cm2 under full solar
spectrum and 5.5 mA/cm2 when placed at the back of a silicon wafer resulting in
power conversion efficiencies (PCE) of 6.4 % and 0.7 % respectively. Moreover,
the device reached an above bandgap PCE of ~6 % as a thermo-photovoltaic cell
recorded under a 1000 {\deg}C blackbody radiator.

###Enhancement of Photovoltaic Current Generation through Dark States in Donor-Acceptor Pairs of Tungsten-based Transition Metal Di-Chalcogenides (TMDCs)||Sayan Roy,Zixuan Hu,Sabre Kais,Peter Bermel###

Enhancement of Photovoltaic Current Generation through Dark States in Donor-Acceptor Pairs of Tungsten-based Transition Metal Di-Chalcogenides (TMDCs). As several photovoltaic materials experimentally approach the
Shockley-Queisser limit, there has been a growing interest in unconventional
materials and approaches with the potential to cross this efficiency barrier.
One such candidate is dark state protection induced by the dipole-dipole
interaction between molecular excited states. This phenomenon has been shown to
significantly reduce carrier recombination rate and enhance photon-to-current
conversion, in elementary models consisting of few interacting chromophore
centers. Atomically thin 2D transition metal di-chalcogenides (TMDCs) have
shown great potential for use as ultra-thin photovoltaic materials in solar
cells due to their favorable photon absorption and electronic transport
properties. TMDC alloys exhibit tunable direct bandgaps and significant dipole
moments. In this work, we introduce the dark state protection mechanism to a
TMDC based photovoltaic system with pure tungsten diselenide (WSe2) as the
acceptor material and the TMDC alloy tungsten sulfo-selenide (WSeS) as the
donor material. Our numerical model demonstrates the first application of the
dark state protection mechanism to a photovoltaic material with a photon
current enhancement of up to 35% and an ideal photon-to-current efficiency
exceeding the Shockley-Queisser limit.

###Printed high-mobility p-type buffer layers on perovskite photovoltaics for efficient semi-transparent devices||Robert A. Jagt,Tahmida N. Huq,Sam A. Hill,Maung Thway,Tianyuan Liu,Mari Napari,Bart Roose,Krzysztof Gałkowsk,Weiwei Li,Serena Fen Lin,Samuel D. Stranks,Judith L. MacManus-Driscoll,Robert L. Z. Hoye###

Printed high-mobility p-type buffer layers on perovskite photovoltaics for efficient semi-transparent devices. Perovskite solar cells (PSCs) with transparent electrodes can be integrated
with existing solar panels in tandem configurations to increase the power
conversion efficiency. A critical layer in semi-transparent PSCs is the
inorganic buffer layer, which protects the PSC against damage when the
transparent electrode is sputtered on top. The development of n-i-p structured
semi-transparent PSCs has been hampered by the lack of suitable p-type buffer
layers. In this work we develop a p-type CuOx buffer layer, which can be grown
uniformly over the perovskite device without damaging the perovskite or organic
charge transport layers, can be grown using industrially scalable techniques
and has high hole mobility (4.3 +/- 2 cm2 V-1 s-1), high transmittance (>95%),
and a suitable ionisation potential for hole extraction (5.3 +/- 0.2 eV).
Semi-transparent PSCs with efficiencies up to 16.7% are achieved using the CuOx
buffer layer. Our work demonstrates a new approach to integrate PSCs into
tandem configurations, as well as enable the development of other devices that
need high quality p-type layers.

###Efficient simulation of bi-periodic, layered structures based on the T-matrix method||Dominik Beutel,Achim Groner,Carsten Rockstuhl,Ivan Fernandez-Corbaton###

Efficient simulation of bi-periodic, layered structures based on the T-matrix method. Predicting the optical response of macroscopic arrangements of individual
scatterers is a computational challenge, as the problem involves length scales
across multiple orders of magnitude. We present a full-wave optical method to
highly efficiently compute the scattering of light at objects that are arranged
in bi-periodic arrays. Multiple arrays or homogeneous thin-films can be stacked
to build up an entire multicomposite material in the third dimension. The
scattering properties of the individual objects in each array are described by
the T-matrix formalism. Therefore, arbitrarily shaped objects and even
molecules can be the basic constituent of the arrays. Taking the T-matrix of
the individual scatterer as the point of departure allows to explain the
optical properties of the bulk material from the scattering properties of its
constituents. We use solutions of Maxwell's equations with well defined
helicity. Therefore, chiral media are particularly easy to consider as
materials for both scatterers or embedding media. We exemplify the efficiency
of the algorithm with an exhaustive parametric study of anti-reflective
coatings for solar cells made from cylinders with a high degree of helicity
preservation. The example shows a speed-up of about 500 with respect to
finite-element computations. A second example specifically exploits the use
helicity modes to investigate the enhancement of the circular dichroism signal
in a chiral material.

###Charge transfer via deep hole in the J51/N2200 blend||Xiaoyu Xie,Chunfeng Zhang,Haibo Ma###

Charge transfer via deep hole in the J51/N2200 blend. In recently developed non-fullerene acceptor (NFA) based organic solar cells
(OSCs), both the donor and acceptor parts can be excited by absorbing light
photons. Therefore, both electron transfer and hole transfer channels could
occur at the donor/acceptor interface for generating free charge carriers in
NFA based OSCs. However, in many molecular and DNA systems, recent studies
revealed the high charge transfer (CT) efficiency cannot be reasonably
explained by a CT model with only highest occupied molecular orbitals (HOMOs)
and lowest unoccupied molecular orbitals (LUMOs) of donor and acceptor
molecules. In this work, taking an example of a full-polymer blend consisting
of benzodithiophenealt-benzotriazole copolymers (J51) as donor and naphthalene
diimide-bithiophene (N2200) as acceptor, in which the ultrafast hole transfer
has been recently reported, we investigate its CT process and examine the
different roles of various frontier molecular orbitals. Through a joint study
of quantum mechanics electronic structure calculation and nonadiabatic dynamics
simulation, we find the hole transfer between HOMOs of J51 and N2200 can hardly
happen but the hole transfer from HOMO of N2200 to HOMO-1 of J51 is much more
efficient. This points out the underlying importance of deep hole channel in CT
process and indicates that including frontier molecular orbitals (FMOs) other
than HOMOs and LUMOs is highly necessary to build a robust physical model for
studying CT process in molecular optoelectronic materials.

###Strain-induced metallization and defect suppression at zipper-like interdigitated atomically thin interfaces enabling high-efficiency halide perovskite solar cells||Nikolai Tsvetkov,Byeong Cheul Moon,Jeung Ku Kang,Muhammad Ejaz Khan,Yong-Hoon Kim###

Strain-induced metallization and defect suppression at zipper-like interdigitated atomically thin interfaces enabling high-efficiency halide perovskite solar cells. Halide perovskite light absorbers have great advantages for photovoltaics
such as efficient solar energy absorption, but charge accumulation and
recombination at the interface with an electron transport layer (ETL) remains a
major challenge in realizing their full potential. Here we report the
experimental realization of a zipper-like interdigitated interface between a
Pb-based halide perovskite light absorber and an oxide ETL by the PbO capping
of the ETL surface, which produces an atomically thin two-dimensional metallic
layer that can significantly enhance the perovskite/ETL charge extraction
process. As the atomistic origin of the emergent two-dimensional interfacial
metallicity, first-principles calculations performed on the representative
MAPbI$_3$/TiO$_2$ interface identify the interfacial strain induced by the
simultaneous formation of stretched I-substitutional Pb bonds (and thus Pb-I-Pb
bonds bridging MAPbI$_3$ and TiO$_2$) and contracted substitutional Pb-O bonds.
Direct and indirect experimental evidences for the presence of interfacial
metallic states are provided, and a non-conventional defect-passivating nature
of the strained interdigitated perovskite/ETL interface is emphasized. It is
experimentally demonstrated that the PbO capping method is generally applicable
to other ETL materials including ZnO and SrTiO$_3$, and that the zipper-like
interdigitated metallic interface leads to about two-fold increase in charge
extraction rate. Finally, in terms of the photovoltaic efficiency, we observe a
volcano-type behavior with the highest performance achieved at the
monolayer-level PbO capping. The method established here might prove to be a
general interface engineering approach to realize high-performance perovskite
solar cells.

###Thermal Management of Photovoltaics using Porous Nanochannels||Sajag Poudel,An Zou,Shalabh C. Maroo###

Thermal Management of Photovoltaics using Porous Nanochannels. The photoelectric conversion efficiency of a solar cell is dependent on its
temperature. When the solar radiation is incident on the photovoltaics (PV)
panel, a large portion of it is absorbed by the underlying material which
increases its internal energy leading to the generation of heat. An overheated
PV panel results in a decline in its performance which calls for an efficient
cooling mechanism that can offer an optimum output of the electrical power. In
the present numerical work, thermal management with a porous nanochannels
device capable to dissipate high heat flux is employed to regulate the
temperature of a commercial PV panel by integrating the device on the back face
of the panel. The spatial and temporal variation of the PV surface temperature
is obtained by solving the energy balance equation numerically. By evaluating
the steady-state PV surface temperature with and without thermal management,
the extent of cooling and the resulting enhancement in the electrical power
output is studied in detail. The nanochannels device is found to reduce the PV
surface temperature significantly with an average cooling of 31.5 oC.
Additionally, the enhancement in the electrical power output by ~33% and the
reduction in the response time to 1/8th highlight the potential of using porous
nanochannels as a thermal management device. Furthermore, the numerical method
is used to develop a universal curve which can predict the extent of PV cooling
for any generic thermal management device.

###Reduction of Trapping and Recombination in Upgraded Metallurgical Grade Silicon: Impact of Phosphorous Diffusion Gettering||N. Dasilva-Villanueva,S. Catalán-Gómez,D. Fuertes Marrón,J. J. Torres,M. García-Corpas,C. del Cañizo###

Reduction of Trapping and Recombination in Upgraded Metallurgical Grade Silicon: Impact of Phosphorous Diffusion Gettering. Upgraded metallurgical grade (UMG) silicon (Si) has raised interest as an
alternative material for solar cells due to its low cost, low environmental
impact and low CAPEX. Maximum cell efficiencies at the level of those obtained
from high purity poly-Si have been reported. However, a higher defect density
and the compensated doping character result in UMG-based cell efficiencies
varying over wider ranges in frequency distribution charts. In this report we
characterize mc-Si UMG samples with different defect densities, comparing them
with mono-Si UMG and commercial high-performance mc-Si samples, analysing the
impact of carrier trapping by means of photoconductance (PC) decay
measurements, and its evolution after applying a phosphorous diffusion
gettering (PDG) process. When analyzing the decay time constant of the PC
measurements, slow (66.8+-14.3 ms) and fast (16.1+-3.5 ms) traps are found in
mc-Si samples, while no evidence of trapping is found in mono-UMG samples. Slow
traps are effectively removed after the PDG process, while fast traps do
remain. The influence of dislocations clusters and the possible role of oxygen,
as revealed by Fourier-transform infrared spectroscopy (FTIR) is discussed.
Finally, the improvement in minority carrier lifetime due to the PDG treatment
is reported for each sample type, reaching values up to 140 us in mc-Si samples
with no slow traps neither interstitial oxygen FTIR-peaks

###Combining optical and magnetic resonance spectroscopies to probe charge recombination via triplet excitons in organic solar cells||Alberto Privitera,Jeannine Grune,Akchheta Karki,William K. Myers,Vladimir Dyakonov,Thuc-Quyen Nguyen,Moritz K. Riede,Richard H. Friend,Andreas Sperlich,Alexander J. Gillett###

Combining optical and magnetic resonance spectroscopies to probe charge recombination via triplet excitons in organic solar cells. Organic solar cells (OSCs) have recently shown a rapid improvement in their
performance, bringing power conversion efficiencies (PCEs) closer to the point
where commercial applications of the technology become viable. However, the low
open-circuit voltage (Voc) of OSCs relative to their optical gap still limits
PCEs to below 20%. A key factor contributing to the large Voc deficit in OSCs
is non-radiative recombination to spin-triplet excitons, which is widely, but
not universally, observed in blends using both fullerene and non-fullerene
electron acceptors. Here, we present an experimental framework that combines
time resolved optical and magnetic resonance spectroscopies to detect triplet
excitons and identify their formation mechanisms. We apply our methodology to
two well-studied polymer:fullerene systems, PM6:PC60BM and PTB7-Th:PC60BM,
enabling us to selectively investigate distinct triplet formation pathways. In
contrast to the more efficient non-fullerene acceptor systems that show only
triplet states formed via non-geminate recombination, the fullerene systems
also show significant triplet formation via geminate processes. We associate
this with electrons trapped at the isolated fullerenes that sit within the
alkyl sidechains of the donor polymers. Thus, our model study demonstrates how
these complex and overlapping processes can be successfully deconvoluted to
reveal the intricacies of triplet generation dynamics in OSC blends.

###Relevance of Ge incorporation to control the physical behaviour of point defects in kesterite||Thomas Ratz,Ngoc Duy Nguyen,Guy Brammertz,Bart Vermang,Jean-Yves Raty###

Relevance of Ge incorporation to control the physical behaviour of point defects in kesterite. To reduce the prominent VOC-deficit that limits kesterite-based solar cells
efficiencies, Ge has been proposed over the recent years with encouraging
results, as the reduction of the non-radiative recombination rate is considered
as a way to improve the well-known Sn-kesterite world record efficiency. To
gain further insight into this mechanism, we investigate the physical behaviour
of intrinsic point defects both upon Ge doping and alloying of Cu2ZnSnS4
kesterite. Using a first-principles approach, we confirm the p-type
conductivity of both Cu2ZnSnS4 and Cu2ZnGeS4, attributed to the low formation
energies of the VCu and CuZn acceptor defects within the whole stable phase
diagram range. Via doping of the Sn-kesterite matrix, we report the lowest
formation energy for the substitutional defect GeSn. We also confirm the
detrimental role of the substitutional defects XZn (X=Sn,Ge) acting as
recombination centres within the Sn-based, the Ge-doped and the Ge-based
kesterite. Finally, we highlight the reduction of the lattice distortion upon
Ge incorporation resulting in a reduction of the carrier capture cross section
and consequently a decrease of the non-radiative recombination rate within the
bulk material.

###Investigating the theoretical performance of Cs$_2$TiBr$_6$-based perovskite solar cell with La-doped BaSnO$_3$ and CuSbS$_2$ as the charge transport layers||Kumar Shivesh,Intekhab Alam,A. K. Kushwaha,Manish Kumar,S. V. Singh###

Investigating the theoretical performance of Cs$_2$TiBr$_6$-based perovskite solar cell with La-doped BaSnO$_3$ and CuSbS$_2$ as the charge transport layers. A lead-free, completely inorganic, and non-toxic Cs2TiBr6-based double
perovskite solar cell (PSC) was simulated via SCAPS 1-D. La-doped BaSnO3 (LBSO)
was applied as the electron transport layer (ETL) unprecedentedly in the
simulation study of PSCs, while CuSbS2 was utilized as the hole transport layer
(HTL). wxAMPS was used to validate the results of SCAPS simulations. Moreover,
the first-principle density function theory (DFT) calculations were performed
for validating the 1.6 eV bandgap of the Cs2TiBr6 absorber. To enhance the
device performance, we analyzed and optimized various parameters of the PSC
using SCAPS. The optimum thickness, defect density, and bandgap of the absorber
were 1000 nm, 1013 cm-3, and 1.4 eV, respectively. Furthermore, the optimum
thickness, hole mobility, and electron affinity of the HTL were 400 nm, 102
cm2V-1s-1, and 4.1 eV, respectively. However, the ETL thickness had a
negligible effect on the device's efficiency. The optimized values of doping
density for the absorber layer, HTL, and ETL were 1015, 1020, and 1021 cm-3,
respectively. Herein, the effect of different HTLs was analyzed by matching up
the built-in voltage (Vbi) in respect of the open-circuit voltage (VOC). It was
found that the Vbi was directly proportional to the VOC, and CuSbS2 was the
champion in terms of efficiency for the PSC. The optimum work function of metal
contact and temperature of the PSC were 5.9 eV and 300 K, respectively. After
the final optimization, the device achieved an exhilarating PCE of 29.13%.

###The critical role of the donor polymer in the stability of high-performance non-fullerene acceptor organic solar cells||Yiwen Wang,Alberto Privitera,Giacomo Londi,Alexander J. Sneyd,Deping Qian,Yoann Olivier,Lorenzo Sorace,David Beljonne,Zhe Li,Alexander J. Gillett###

The critical role of the donor polymer in the stability of high-performance non-fullerene acceptor organic solar cells. Driven by the rapid development of non-fullerene electron acceptors (NFAs),
the power conversion efficiencies of organic solar cells (OSCs) have reached
levels suitable for commercial applications. However, the poor operational
stability of high-performance NFA OSCs is a remaining fundamental challenge
that must be addressed. Whilst previous studies have primarily focused on the
NFA component, we consider here the degradation pathways of both the donor and
acceptor materials in the benchmark PM6:Y6 blend. Here, we show that light
soaking greatly increases the energetic disorder and trap state density in PM6,
with little effect on Y6. This is corroborated by electron paramagnetic
resonance spectroscopy, which reveals increased recombination via trapped
polarons on PM6 after light soaking. In addition, ultrafast optical
spectroscopy studies on light-soaked samples show that PM6 singlet excitons are
rapidly converted into interchain polaron pairs on sub-100 fs timescales; this
process outcompetes electron transfer to Y6, significantly reducing the charge
generation yield of the blend. We make similar observations in the parent
polymer, PBDB-T, indicating that this class of donor materials, used in most
high-performance OSCs to date, are intrinsically unstable to light soaking.
Thus, we reveal that the donor polymer can be a further critical weak link in
efficient OSC systems, whose degradation mechanism needs to be addressed
collectively with NFAs.

###Beyond Lambertian light trapping for large-area silicon solar cells: fabrication methods||Jovan Maksimovic,Jingwen Hu,Soon Hock Ng,Tomas Katkus,Gediminas Seniutinas,Tatiana Pinedo Rivera,Michael Stuiber,Yoshiaki Nishijima,Sajeev John,Saulius Juodkazis###

Beyond Lambertian light trapping for large-area silicon solar cells: fabrication methods. Light trapping photonic crystal (PhC) patterns on the surface of Si solar
cells provides a novel opportunity to approach the theoretical efficiency limit
of 32.3%, for light-to-electrical power conversion with a single junction cell.
This is beyond the efficiency limit implied by the Lambertian limit of ray
trapping 29%. The interference and slow light effects are harnessed for
collecting light even at the long wavelengths near the Si band-gap. We compare
two different methods for surface patterning, that can be extended to large
area surface patterning: 1) laser direct write and 2) step-&-repeat 5-times
reduction projection lithography. Large area throughput limitations of these
methods are compared with the established electron beam lithography (EBL)
route, which is conventionally utilised but much slower than the presented
methods. Spectral characterisation of the PhC light trapping is compared for
samples fabricated by different methods. Reflectance of Si etched via laser
patterned mask was 7% at visible wavelengths and was comparable with Si
patterned via EBL made mask. The later pattern showed a stronger absorbance
than the Lambertian limit (M.-L. Hsieh et al., Sci. Rep. 10, 11857 (2020)).

###The effect of B-site alloying on the electronic and opto-electronic properties of RbPbI3: A DFT study||Anupriya Nyayban,Subhasis Panda,Avijit Chowdhury###

The effect of B-site alloying on the electronic and opto-electronic properties of RbPbI3: A DFT study. Divalent cations mixed lead halide perovskites with enhanced performances,
high stabilities, and reduced toxicity are requisite to make persistent
progress in perovskite solar cells. However, the mixing strategy is not
reported extensively in search of a lead reduced structure. Herein, we report
the structural, electronic and optical properties of RbPb{1-x}MxI3 (where,
M={Sn,Ge} and x={0.25, 0.50, 0.75}) by alloying the B-site with Sn and Ge,
using the density functional theory. The formation enthalpy is estimated for
all RbPb{1-x}MxI3 (with x= 0.25, 0.50, 0.75), which confirms stability for all
the structures. The energy bandgap and density of states (DOS) have been
thoroughly investigated. The energy bandgap decreases with the increasing Sn/Ge
contents, the lowest bandgap of 1.850 eV is observed at x = 0.50 in the case of
RbPb{1-x}GexI3 systems. Further, the effective masses and the binding energy of
excitons and spectroscopic limited maximum efficiency (SLME) are also estimated
for all the mixed systems. The exciton type is observed to change from
Mott-Wannier to Frenkel type with increasing the contents of both Sn and Ge at
the B-site. The maximum efficiency of 23% is achieved using an active layer
containing an equal admixture of Sn/Ge and Pb. The estimated parameters of both
the mixed systems are consistent with the available literature of similar
types.

###Lithography Free Process for the Fabrication of Periodic Silicon Micro/Nano-Wire Arrays and Its Light-trapping Properties||Divya Rani,Anil Kumar,Anjali Sain,Deepika Singh,Neeraj Joshi,Ravi Kumar Varma,Mrinal Dutta,Arup Samanta###

Lithography Free Process for the Fabrication of Periodic Silicon Micro/Nano-Wire Arrays and Its Light-trapping Properties. Vertically aligned silicon micro/nanowire arrays of different sizes have been
synthesized by combining the modified metal-assisted chemical etching (MACE)
and reactive ion etching (RIE) methods. This is a novel lithography-free method
to fabricate silicon micro/nanowire arrays. The size of micro/nanowire arrays
is controlled by controlling the etching rate and diameter of silica particles.
The silicon micro/nanowire geometry can utilize for efficient collection of
photo-generated charge carriers from impure silicon wafers, which have a short
minority carrier diffusion length also act as a self-antireflection coating
layer. For micro/nanowire having average diameters of 40 nm, 330 nm and 950 nm
and their corresponding average length 1.12 micron, 1.1 micron, and 1 micron,
respectively, the observed average reflectance was 0.22, 0.6 and 0.33 percent
at 45-degree incident angle, while the average reflectance was increased up to
4.2, 9.2, and 11 percent, respectively at 75-degree incident angle in the broad
range of 300 - 1200 nm of the solar spectrum. The measured average reflectance
for these samples is quite low compared to the planar silicon wafer. Thus this
geometry is a promising candidate for fabricating low-cost and highly efficient
radial junction silicon micro/nanowire arrays based solar cells.

###Efficient and ultra-stable perovskite light-emitting diodes||Bingbing Guo,Runchen Lai,Sijie Jiang,Yaxiao Lian,Zhixiang Ren,Puyang Li,Xuhui Cao,Shiyu Xing,Yaxin Wang,Weiwei Li,Chen Zou,Mengyu Chen,Cheng Li,Baodan Zhao,Dawei Di###

Efficient and ultra-stable perovskite light-emitting diodes. Perovskite light-emitting diodes (PeLEDs) have emerged as a strong contender
for next-generation display and information technologies. However, similar to
perovskite solar cells, the poor operational stability remains the main
obstacle toward commercial applications. Here we demonstrate ultra-stable and
efficient PeLEDs with extraordinary operational lifetimes (T50) of 1.0x10^4 h,
2.8x10^4 h, 5.4x10^5 h, and 1.9x10^6 h at initial radiance (or current
densities) of 3.7 W/sr/m2 (~5 mA/cm2), 2.1 W/sr/m2 (~3.2 mA/cm2), 0.42 W/sr/m2
(~1.1 mA/cm2), and 0.21 W/sr/m2 (~0.7 mA/cm2) respectively, and external
quantum efficiencies of up to 22.8%. Key to this breakthrough is the
introduction of a dipolar molecular stabilizer, which serves two critical roles
simultaneously. First, it prevents the detrimental transformation and
decomposition of the alpha-phase FAPbI3 perovskite, by inhibiting the formation
of lead and iodide intermediates. Secondly, hysteresis-free device operation
and microscopic luminescence imaging experiments reveal substantially
suppressed ion migration in the emissive perovskite. The record-long PeLED
lifespans are encouraging, as they now satisfy the stability requirement for
commercial organic LEDs (OLEDs). These results remove the critical concern that
halide perovskite devices may be intrinsically unstable, paving the path toward
industrial applications.

###Triplet Excitons and associated Efficiency-Limiting Pathways in Organic Solar Cell Blends based on (Non-) Halogenated PBDB-T and Y-Series||Jeannine Grüne,Giacomo Londi,Alexander J. Gillett,Basil Stähly,Sebastian Lulei,Maria Kotova,Yoann Olivier,Vladimir Dyakonov,Andreas Sperlich###

Triplet Excitons and associated Efficiency-Limiting Pathways in Organic Solar Cell Blends based on (Non-) Halogenated PBDB-T and Y-Series. The great progress in organic photovoltaics (OPV) over the past few years has
been largely achieved by the development of non-fullerene acceptors (NFAs),
with power conversion efficiencies now approaching 20%. To further improve
device performance, loss mechanisms must be identified and minimized. Triplet
states are known to adversely affect device performance, since they can form
energetically trapped excitons on low-lying states that are responsible for
non-radiative losses or even device degradation. Halogenation of OPV materials
has long been employed to tailor energy levels and to enhance open circuit
voltage. Yet, the influence on recombination to triplet excitons has been
largely unexplored. Using the complementary spin-sensitive methods of
photoluminescence detected magnetic resonance (PLDMR) and transient electron
paramagnetic resonance (trEPR) corroborated by transient absorption and
quantum-chemical calculations, we unravel exciton pathways in OPV blends
employing the polymer donors PBDB-T, PM6 and PM7 together with NFAs Y6 and Y7.
All blends reveal triplet excitons on the NFA populated via non-geminate hole
back transfer and, in blends with halogenated donors, also by spin-orbit
coupling driven intersystem crossing. Identifying these triplet formation
pathways in all tested solar cell absorber films highlights the untapped
potential for improved charge generation to further increase plateauing OPV
efficiencies.

###Comparing Methods of Characterizing Energetic Disorder in Organic Solar Cells||Paula Hartnagel,Sandheep Ravishankar,Benjamin Klingebiel,Oliver Thimm,Thomas Kirchartz###

Comparing Methods of Characterizing Energetic Disorder in Organic Solar Cells. Energetic disorder has been known for decades to limit the performance of
structurally disordered semiconductors such as amorphous silicon and organic
semiconductors. However, in the past years, high performance organic solar
cells have emerged showing a continuously reduced amount of energetic disorder.
While searching for future high efficiency material systems, it is therefore
important to correctly characterize this energetic disorder. While there are
several techniques in literature, the most common approaches to probe the
density of defect states are using optical excitation as in external quantum
efficiency measurements or sequential filling of the tail states by applying an
external voltage as in admittance spectroscopy. A metanalysis of available
literature as well as our experiments using four characterization techniques on
two material systems reveal that electrical, voltage-dependent measurements
frequently yield higher values of energetic disorder than optical measurements.
With drift-diffusion simulations, we demonstrate that the approaches probe
different energy ranges of the subband-gap density of states. We further
explore the limitations of the techniques and find that extraction of
information from a capacitance-voltage curve can be inhibited by an internal
series resistance. Thereby, we explain the discrepancies between measurements
techniques with sensitivity to different energy ranges and electronic
parameters.

###Resolving the Hydrophobicity of Me-4PACz Hole Transport Layer for High-Efficiency Inverted Perovskite Solar Cells||Kashimul Hossain,Ashish Kulkarni,Urvashi Bothra,Benjamin Klingebiel,Thomas Kirchartz,Michael Saliba,Dinesh Kabra###

Resolving the Hydrophobicity of Me-4PACz Hole Transport Layer for High-Efficiency Inverted Perovskite Solar Cells. [4-(3,6-dimethyl-9H-carbazole-9-yl)butyl]phosphonic acid (Me-4PACz)
self-assembled monolayer (SAM) has been employed in perovskite single junction
and tandem devices demonstrating high efficiencies. However, a uniform
perovskite layer does not form due to the hydrophobicity of Me-4PACz. Here, we
tackle this challenge by adding a conjugated polyelectrolyte
poly(9,9-bis(3'-(N,N-dimethyl)-N-ethylammonium-propyl-2,7-fluorene)-alt-2,7-(9,9dioctylfluorene)dibromide
(PFN-Br) to the Me-4PACz in a specific ratio, defines as Pz:PFN. With this
mixing engineering strategy of Pz:PFN, the PFN-Br interacts with the A-site
cation and is confirmed via solution-state nuclear magnetic resonance studies.
The narrow full width at half maximum (FWHM) of diffraction peaks of perovskite
film revealed improved crystallization on the optimal mixing ratio of Pz:PFN.
Interestingly, the mixing of PFN-Br additionally tunes the work function of the
Me-4PACz as revealed by the Kelvin probe force microscopy and built-in-voltage
estimation in solar cells. Devices employing optimized Pz:PFN mixing ratio
deliver open-circuit voltage (Voc)of 1.16 V and efficiency >20% for perovskites
with a bandgap of 1.6 eV with high reproducibility and concomitant stability.
Considering significant research on Me-4PACz SAM, our work highlights the
importance of obtaining a uniform perovskite layer with improved yield and
performance.

###Ultraviolet Photodetectors based on GaN and AlGaN/AlN Nanowire Ensembles: Effects of Planarization with Hydrogen Silsesquioxane and Nanowire Architecture||E. Akar,I. Dimkou,A. Ajay,Martien I. den Hertog,E. Monroy###

Ultraviolet Photodetectors based on GaN and AlGaN/AlN Nanowire Ensembles: Effects of Planarization with Hydrogen Silsesquioxane and Nanowire Architecture. The interest in nanowire photodetectors stems from their potential to improve
the performance of a variety of devices, including solar cells, cameras,
sensors, and communication systems. Implementing devices based on nanowire
ensembles requires a planarization process which must be conceived to preserve
the advantages of the nanowire geometry. This is particularly challenging in
the ultraviolet (UV) range, where spin coating with hydrogen silsesquioxane
(HSQ) appears as an interesting approach in terms of transmittance and
refractive index. Here, we report a comprehensive study on UV photodetectors
based on GaN or AlGaN/AlN nanowire ensembles encapsulated in HSQ. We show that
this material is efficient for passivating the nanowire surface, it introduces
a compressive strain in the nanowires and preserves their radiative efficiency.
We discuss the final performance of planarized UV photodetectors based on three
kinds of nanowire ensembles: (i) non-intentionally-doped (nid) GaN nanowires,
(ii) Ge-doped GaN nanowires, and (iii) nid GaN nanowires terminated with an
AlGaN/AlN superlattice. The incorporation of the superlattice allows tuning the
spectral response with bias, which can enhance the carrier collection from the
AlGaN/AlN superlattice or from the GaN stem. In all the cases, the performance
of the planarized devices remains determined by the nanowire nature, since
their characteristics in terms of linearity and spectral selectivity are closer
to those demonstrated in single nanowires than those of planar devices. Thus,
the visible rejection is several orders of magnitude and there is no indication
of persistent photocurrent, which makes all the samples suitable for
UV-selective photodetection applications.

###A Thermal-Photovoltaic Device Based on Thermally Enhanced Photoluminescence||Assaf Manor,Carmel Rotschild###

A Thermal-Photovoltaic Device Based on Thermally Enhanced Photoluminescence. Single-junction photovoltaic cells are considered to be efficient solar
energy converters, but even ideal cells cannot exceed the their fundamental
thermodynamic efficiency limit, first analysed by Shockley and Queisser (SQ).
For moderated irradiation levels, the efficiency limit ranges between 30%-40%.
The efficiency loss is, to a great extent, due to the inherent heat-dissipation
accompanying the process of electro-chemical potential generation. Concepts
such as solar thermo-photovoltaics (STPV) and thermo-photonics4 aim to harness
this dissipated heat, yet exceeding the SQ limit has not been achieved, mainly
due to the very high operating temperatures needed. Recently, we demonstrated
that in high-temperature endothermic-photoluminescence (PL), the photon rate is
conserved with temperature increase, while each photon is blue shifted. We also
demonstrated how endothermic-PL generates orders of magnitude more
energetic-photons than thermal emission at similar temperatures. These new
findings show that endothermic-PL is an ideal optical heat-pump. Here, we
propose and thermodynamically analyse a novel device based on Thermally
Enhanced Photo Luminescence (TEPL). In such a device, solar radiation is
harvested by a low-bandgap PL material. In addition to the PL excitation, the
otherwise lost heat raises the temperature and allows the TEPL emission to be
coupled to a higher bandgap solar cell. The excessive thermal energy is then
converted to electrical work at high voltage and enhanced efficiency. Our
results show that such a TEPL based device can reach theoretical maximal
efficiencies of 70%, as high as in STPV, while the significantly lowered
operating temperatures are below 1000C. In addition to the theoretical
analysis, we experimentally demonstrated enhanced photo-current in TEPL device
pumped by sub-bandgap radiation. This opens the way for a new direction in
photovoltaics.

###Resonant Energy Transfer from Organics to Quantum Dots and Carrier Multiplication||Vladimir M. Agranovich,Gerard Czajkowski###

Resonant Energy Transfer from Organics to Quantum Dots and Carrier Multiplication. It was shown in the recent experiments that the hybrid organic/inorganic
resonant structures can provide a flexible materials platform aimed at the
design of novel light emitting devices. The applications of hybrid structures
for photovoltaic solar cell can also be useful. We pay attention in this note
that the resonant energy transfer in hybrid structure from the organic thin
layer to the semiconductor nanostructures can drastically increase the
intensity of the free carrier generation. To demonstrate this idea we use the
results of recently published paper by Zhang et al., Nature Nanotechnology 2,
555 (2007), demonstrating the highly efficient resonance energy transfer from
J-aggregates layer to semiconductor nanocrystals. It is known that the
semiconductor nanocrystals with small energy gap represent a promising route to
increased solar conversion in single--junction photovoltaic cells. We argue
that the using of nanocrystals with small energy gap in the hybrid
organic/inorganic structures similar to created by Zhang et al. can increase
tens times the total intensity of carrier multiplication. The organic part in
such hybrid structures will play a role of the peculiar organic concentrator of
the light energy.

###Electronic and hole minibands in quantum wire arrays of different crystallographic structure||M. Krawczyk,J. W. Klos###

Electronic and hole minibands in quantum wire arrays of different crystallographic structure. We consider quantum wire arrays consisting of GaAs rods embedded in
Al$_{x}$Ga$_{1-x}$As and disposed in sites of a square or triangular lattice.
The electronic and hole spectra around the conduction band bottom and the
valence band top are examined versus geometry of the lattice formed by the
rods, concentration of Al in the matrix material, and structural parameters
including the filling fraction and the lattice constant. Our calculations use
the envelope function and are based on the effective-mass approximation. We
show that the electronic and hole spectra resulting from the periodicity of the
heterostructure, depend on the factors considered and that the effect of
lattice geometry varies substantially with lattice constant. For low lattice
constant values the minigaps are significantly wider in the case of triangular
lattice, while for high lattice constant values wider minigaps occur in the
square lattice-based arrays. We analyse the consequences of our findings for
the efficiency of solar cells based on quantum wire arrays.

###Investigation of the Photocurrent in Bulk Heterojunction Solar Cells||M. Limpinsel,A. Wagenpfahl,M. Mingebach,C. Deibel,V. Dyakonov###

Investigation of the Photocurrent in Bulk Heterojunction Solar Cells. We investigated the photocurrent in poly(3-hexylthiophene-2,5-diyl)
(P3HT):[6,6]-phenyl-C$_{61}$ butyric acid methyl ester (PCBM) solar cells by
applying a pulsed measurement technique. For annealed samples, a point of
optimal symmetry (POS) with a corresponding voltage $V_\text{POS}$ of
0.52--0.64 V could be determined. Based on macroscopic simulations and results
from capacitance--voltage measurements, we identify this voltage with flat band
conditions in the bulk of the cell, but not the built-in voltage as proposed by
[Ooi et al., J. Mater. Chem. 18 (2008) 1644]. We calculated the field dependent
polaron pair dissociation after Onsager--Braun and the voltage dependent
extraction of charge carriers after Sokel and Hughes with respect to this point
of symmetry. Our analysis allows to explain the experimental photocurrent in
both forward and reverse directions. Also, we observed a voltage--independent
offset of the photocurrent. As this offset is crucial for the device
performance, we investigated its dependence on cathode material and thermal
treatment. From our considerations we gain new insight into the photocurrent`s
voltage dependence and the limitations of device efficiency.

###Efficiency limits of quantum well solar cells||J. P. Connolly,I. M. Ballard,K. W. J. Barnham,D. B. Bushnell,T. N. D. Tibbits,J. S. Roberts###

Efficiency limits of quantum well solar cells. The quantum well solar cell (QWSC) has been proposed as a flexible means to
ensuring current matching for tandem cells. This paper explores the further
advantage afforded by the indication that QWSCs operate in the radiative limit
because radiative contribution to the dark current is seen to dominate in
experimental data at biases corresponding to operation under concentration. The
dark currents of QWSCs are analysed in terms of a light and dark current model.
The model calculates the spectral response (QE) from field bearing regions and
charge neutral layers and from the quantum wells by calculating the confined
densities of states and absorption coefficient, and solving transport equations
analytically. The total dark current is expressed as the sum of depletion layer
and charge neutral radiative and non radiative currents consistent with
parameter values extracted from QE fits to data. The depletion layer dark
current is a sum of Shockley-Read-Hall non radiative, and radiative
contributions. The charge neutral region contribution is expressed in terms of
the ideal Shockley radiative and non-radiative currents modified to include
surface recombination. This analysis shows that the QWSC is inherently subject
to the fundamental radiative efficiency limit at high currents where the
radiative dark current dominates, whereas good homojunction cells are well
described by the ideal Shockley picture where the limit is determined by
radiative and non radiative recombination in the charge neutral layers of the
cell.

###Excitation Dynamics in Low Band Gap Donor-Acceptor Copolymers and Blends||Björn Gieseking,Berthold Jäck,Eduard Preis,Stefan Jung,Michael Forster,Ullrich Scherf,Carsten Deibel,Vladimir Dyakonov###

Excitation Dynamics in Low Band Gap Donor-Acceptor Copolymers and Blends. Donor-acceptor (D-A) type copolymers show great potential for the application
in the active layer of organic solar cells. Nevertheless the nature of the
excited states, the coupling mechanism and the relaxation pathways following
photoexcitation are yet to be clarified. We carried out comparative
measurements of the steady state absorption and photoluminescence (PL) on the
copolymer poly[N-(1-octylnonyl)-2,7-carbazole] -alt-5,5-[4',7'
-di(thien-2-yl)-2',1',3' -benzothiadiazole] (PCDTBT), its building blocks as
well as on the newly synthesized
N-(1-octylnonyl)-2,7-bis-[(5-phenyl)thien-2-yl)carbazole (BPT-carbazole) (see
Figure 1). The high-energy absorption band (HEB) of PCDTBT was identified with
absorption of carbazoles with adjacent thiophene rings while the low-energy
band (LEB) originates instead from the charge transfer (CT) state delocalized
over the aforementioned unit with adjacent benzothiadiazole group.
Photoexcitation of the HEB is followed by internal relaxation prior the
radiative decay to the ground state. Adding PC70BM results in the efficient PL
quenching within the first 50 ps after excitation. From the PL excitation
experiments no evidence for a direct electron transfer from the HEB of PCDTBT
towards the fullerene acceptor was found, therefore the internal relaxation
mechanisms within PCDTBT can be assumed to precede. Our findings indicate that
effective coupling between copolymer building blocks governs the photovoltaic
performance of the blends.

###Oxidation effects on graded porous silicon anti-reflection coatings||Annett Thøgersen,Josefine H. Selj,Erik S. Marstein###

Oxidation effects on graded porous silicon anti-reflection coatings. Efficient anti-reflection coatings (ARC) improve the light collection and
thereby increase the current output of solar cells. By simple electrochemical
etching of the Si wafer, porous silicon (PS) layers with excellent broadband
anti-reflection properties can be fabricated. In this work, ageing of graded PS
has been studied using Spectroscopic Ellipsometry, Transmission Electron
Microscopy and X-ray Photoelectron Spectroscopy. During oxidation of PS
elements such as pure Si (Si$^0$), Si$_2$O (Si$^+$), SiO (Si$^{2+}$),
Si$_2$O$_3$ (Si$^{3+}$), and SiO$_2$ (Si$^{4+}$) are present. In addition both
hydrogen and carbon is introduced to the PS in the form of Si$_3$SiH and CO.
The oxide grows almost linearly with time when exposed to oxygen, from an
average thickness of 0 - 3.8 nm for the surface PS. The oxidation is then
correlated to the optical stability of multi-layered PS ARCs. It is found that
even after extensive oxidation, the changes in the optical properties of the PS
structures are small.

###Oxidation of GaN: An ab initio thermodynamic approach||Adam J. Jackson,Aron Walsh###

Oxidation of GaN: An ab initio thermodynamic approach. GaN is a wide-bandgap semiconductor used in high-efficiency LEDs and solar
cells. The solid is produced industrially at high chemical purities by
deposition from a vapour phase, and oxygen may be included at this stage.
Oxidation represents a potential path for tuning its properties without
introducing more exotic elements or extreme processing conditions. In this
work, ab initio computational methods are used to examine the energy potentials
and electronic properties of different extents of oxidation in GaN. Solid-state
vibrational properties of Ga, GaN, Ga2O3 and a single substitutional oxygen
defect have been studied using the harmonic approximation with supercells. A
thermodynamic model is outlined which combines the results of ab initio
calculations with data from experimental literature. This model allows free
energies to be predicted for arbitrary reaction conditions within a wide
process envelope. It is shown that complete oxidation is favourable for all
industrially-relevant conditions, while the formation of defects can be opposed
by the use of high temperatures and a high N2:O2 ratio.

###Optoelectronics with electrically tunable PN diodes in a monolayer dichalcogenide||Britton W. H. Baugher,Hugh O. H. Churchill,Yafang Yang,Pablo Jarillo-Herrero###

Optoelectronics with electrically tunable PN diodes in a monolayer dichalcogenide. One of the most fundamental devices for electronics and optoelectronics is
the PN junction, which provides the functional element of diodes, bipolar
transistors, photodetectors, LEDs, and solar cells, among many other devices.
In conventional PN junctions, the adjacent p- and n-type regions of a
semiconductor are formed by chemical doping. Materials with ambipolar
conductance, however, allow for PN junctions to be configured and modified by
electrostatic gating. This electrical control enables a single device to have
multiple functionalities. Here we report ambipolar monolayer WSe2 devices in
which two local gates are used to define a PN junction exclusively within the
sheet of WSe2. With these electrically tunable PN junctions, we demonstrate
both PN and NP diodes with ideality factors better than 2. Under excitation
with light, the diodes show photodetection responsivity of 210 mA/W and
photovoltaic power generation with a peak external quantum efficiency of 0.2%,
promising numbers for a nearly transparent monolayer sheet in a lateral device
geometry. Finally, we demonstrate a light-emitting diode based on monolayer
WSe2. These devices provide a fundamental building block for ubiquitous,
ultra-thin, flexible, and nearly transparent optoelectronic and electronic
applications based on ambipolar dichalcogenide materials.

###Micrometer-Thin Crystalline-Silicon Solar Cells Integrating Numerically Optimized 2-D Photonic Crystals||V. Depauw,X. Meng,O. El Daif,G. Gomard,L. Lalouat,E. Drouard,C. Trompoukis,A. Fave,C. Seassal,I. Gordon###

Micrometer-Thin Crystalline-Silicon Solar Cells Integrating Numerically Optimized 2-D Photonic Crystals. A 2-D photonic crystal was integrated experimentally into a thin-film
crystalline-silicon solar cell of 1-{\mu}m thickness, after numerical
optimization maximizing light absorption in the active material. The photonic
crystal boosted the short-circuit current of the cell, but it also damaged its
open-circuit voltage and fill factor, which led to an overall decrease in
performances. Comparisons between modeled and actual optical behaviors of the
cell, and between ideal and actual morphologies, show the global robustness of
the nanostructure to experimental deviations, but its particular sensitivity to
the conformality of the top coatings and the spread in pattern dimensions,
which should not be neglected in the optical model. As for the electrical
behavior, the measured internal quantum efficiency shows the strong parasitic
absorptions from the transparent conductive oxide and from the back-reflector,
as well as the negative impact of the nanopattern on surface passivation. Our
exemplifying case, thus, illustrates and experimentally confirms two
recommendations for future integration of surface nanostructures for light
trapping purposes: 1) the necessity to optimize absorption not for the total
stack but for the single active material, and 2) the necessity to avoid damage
to the active material by pattern etching.

###Inter-band Coulomb coupling in narrow gap semiconductor nanocrystals: $\mathbf{k}\cdot\mathbf{p}$ theory||Maryam Azizi,Paweł Machnikowski###

Inter-band Coulomb coupling in narrow gap semiconductor nanocrystals: $\mathbf{k}\cdot\mathbf{p}$ theory. We derive the matrix elements of Coulomb interaction between states with
different number of electrons and holes in a semiconductor nanocrystal within
the 8-band $\mathbf{k}\cdot\mathbf{p}$ theory. These matrix elements are
responsible for multiple exciton generation which may contribute to the
enhancement of the efficiency of solar cells. Our calculations are performed
within the multi band envelope function formalism based on the states resulting
from diagonalization of the 8-band $\mathbf{k}\cdot\mathbf{p}$ Hamiltonian. We
study in detail and compare two contributions to the inter-band Coulomb
coupling: the mesoscopic one, which involves only the envelope functions and
relies on band mixing, and the microscopic one, that relies on the Bloch parts
of the wave functions and is non-zero even between single- band states. We show
that these two contributions are of a similar order of magnitude. We study also
the statistical distribution of the magnitudes of the inter-band Coulomb matrix
elements and show that the overall coupling to remote states decays according
to a power law favorable for the convergence of numerical computations.

###First-principles investigation of organic photovoltaic materials C$_{60}$, C$_{70}$, [C$_{60}$]PCBM, and bis-[C$_{60}$]PCBM using a many-body $G_0W_0$-Lanczos approach||Xiaofeng Qian,Paolo Umari,Nicola Marzari###

First-principles investigation of organic photovoltaic materials C$_{60}$, C$_{70}$, [C$_{60}$]PCBM, and bis-[C$_{60}$]PCBM using a many-body $G_0W_0$-Lanczos approach. We present a first-principles investigation of the excited-state properties
of electron acceptors in organic photovoltaics including C$_{60}$, C$_{70}$,
[6,6]-phenyl-C$_{61}$-butyric-acid-methyl-ester ([C$_{60}$]PCBM), and
bis-[C$_{60}$]PCBM using many-body perturbation theory within the Hedin's
$G_0W_0$ approximation and an efficient Lanczos approach. Calculated vertical
ionization potentials (VIP) and vertical electron affinities (VEA) of C$_{60}$
and C$_{70}$ agree very well with experimental values measured in gas phase.
The density of states of all three molecules is also compared to photoemission
and inverse photoemission spectra measured on thin-films, exhibiting a close
agreement - a rigid energy-gap renormalization owing to intermolecular
interactions in the thin-films. In addition, it is shown that the low-lying
unoccupied states of [C$_{60}$]PCBM are all derived from the highest-occupied
molecular orbitals and the lowest-unoccupied molecular orbitals of fullerene
C$_{60}$. The functional side group in [C$_{60}$]PCBM introduces a slight
electron transfer to the fullerene cage, resulting in small decreases of both
VIP and VEA. This small change of VEA provides a solid justification for the
increase of open-circuit voltage when replacing fullerene C$_{60}$ with
[C$_{60}$]PCBM as the electron acceptor in bulk heterojunction polymer solar
cells.

###Plasmonic Graded-Chains as Deep-Subwavelength Light Concentrators||Natalia Esteves-López,Horacio M. Pastawski,Raúl A. Bustos-Marún###

Plasmonic Graded-Chains as Deep-Subwavelength Light Concentrators. We have studied the plasmonic properties of aperiodic arrays of identical
nanoparticles (NPs) formed by two opposite and equal graded-chains (a chain
where interactions change gradually). We found that these arrays concentrate
the external electromagnetic fields even in the long wavelength limit. The
phenomenon was understood by identifying the system with an effective cavity
where plasmonics excitations are trapped between effective band edges,
resulting from the change of passband with NP's position. Dependence of
excitation concentration on several system's parameter was also assessed. This
includes, different gradings as well as NP's couplings, damping, and resonant
frequencies. In the spirit of the scaling laws in condensed matter physics, we
developed a theory that allows us to rationalize all these system's parameters
into universal curves. The theory is quite general and can also be used on many
other situations (different arrays for example). Additionally, we also provided
an analytical solution, in the tight-binding limit, for the plasmonic response
of homogeneous linear chains of NPs illuminated by a plane wave. Our results
can find applications on sensing, near field imaging, plasmon-enhanced
photodetectors, as well as to increase solar cell efficiency.

###Efficiency Enhancement in Organic Solar Cells by Incorporating Silica-coated Gold Nanorods at the Buffer/Active interface||Haoyang Zhao,Fan Yang,Peiqian Tong,Yanxia Cui,Yuying Hao,Qinjun Sun,Fang Shi,Qiuqiang Zhan,Hua Wang,Furong Zhu###

Efficiency Enhancement in Organic Solar Cells by Incorporating Silica-coated Gold Nanorods at the Buffer/Active interface. The performance of organic solar cells (OSCs) can be greatly improved by
incorporating silica-coated gold nanorods (Au@SiO2 NRs) at the interface
between the hole transporting layer and the active layer due to the plasmonic
effect. The silica shell impedes the aggregation effect of the Au NRs in
ethanol solution as well as the server charge recombination on the surface of
the Au NRs otherwise they would bring forward serious reduction in open circuit
voltage when incorporating the Au NRs at the positions in contact with the
active materials. As a result, while the high open circuit voltage being
maintained, the optimized plasmonic OSCs possess an increased short circuit
current, and correspondingly an elevated power conversion efficiency with the
enhancement factor of ~11%. The origin of performance improvement in OSCs with
the Au@SiO2 NRs was analyzed systematically using morphological, electrical,
optical characterizations along with theoretical simulation. It is found that
the broadband enhancement in absorption, which yields the broadband enhancement
in exciton generation in the active layer, is the major factor contributing to
the increase in the short circuit current density. Simulation results suggest
that the excitation of the transverse and longitudinal surface plasmon
resonances of individual NRs as well as their mutual coupling can generate
strong electric field near the vicinity of the NRs, thereby an improved exciton
generation profile in the active layer. The incorporation of Au@SiO2 NRs at the
interface between the hole transporting layer and the active layer also
improves hole extraction in the OSCs.

###A multi-pathway model for Photosynthetic reaction center||M. Qin,H. Z Shen,X. X. Yi###

A multi-pathway model for Photosynthetic reaction center. Charge separation in light-harvesting complexes occurs in a pair of tightly
coupled chlorophylls at the heart of photosynthetic reaction centers of both
plants and bacteria. Recently it has been shown that quantum coherence can, in
principle, enhance the efficiency of a solar cell, working like a quantum heat
engine (QHE). Here, we propose a biological quantum heat engine (BQHE)
motivated by Photosystem {\rm II} reaction center (PS{\rm II} RC) to describe
the charge separation. Our model mainly considers two charge-separation
pathways more than that in the published literature. The two pathways can
interfere via cross-couplings and work together to enhance the
charge-separation yields. We explore how these cross-couplings increase the
current and voltage of the charge separation and discuss the advantages of
multiple pathways in terms of current and power. The robustness of the BQHE
against the charge recombination in natural PS{\rm II} RC and dephasing induced
by environments is also explored, and extension from two pathways to multiple
pathways is made. These results suggest that nature-mimicking architectures
with engineered multiple pathways for charge separations might be better for
artificial solar energy devices.

###Using $G_0W_0$ Level Alignment to Identify Catechol's Structure on TiO$_2$(110)||Duncan J. Mowbray,Annapaola Migani###

Using $G_0W_0$ Level Alignment to Identify Catechol's Structure on TiO$_2$(110). We perform state-of-the-art calculations for a prototypical dye sensitized
solar cell: catechol on rutile TiO$_2$(110). Catechol is often used as an
anchoring group for larger more complex organic and inorganic dyes on TiO$_2$
and forms a type II heterojunctions on TiO$_2$(110). In particular, we compare
quasiparticle (QP) $G_0W_0$ with hybrid exchange correlation functional (HSE)
density functional theory (DFT) calculations for the catechol-rutile
TiO$_2$(110) interface. In so doing, we provide a theoretical interpretation of
ultraviolet photoemission spectroscopy (UPS) and inverse photoemission
spectroscopy (IPES) experiments for this prototypical system. Specifically, we
demonstrate that the position, presence, and intensity of peaks associated with
catechol's HOMO, intermolecular OH$-$O bonds, and interfacial hydrogen bonds to
the surface bridging O atoms (O$_{br}$H$-$C and O$_{br}$H$-$O) may be used to
fingerprint deprotonation of catechol's OH anchoring groups. Furthermore, our
results suggest deprotonation of these groups, while being nearly isoenergetic
at high coverages, may significantly increase the photovoltaic efficiency of
catechol$-$TiO$_2$(110) interfaces.

###Plasmonic and silicon spherical nanoparticle anti-reflective coatings||K. V. Baryshnikova,M. I. Petrov,V. E. Babicheva,P. A. Belov###

Plasmonic and silicon spherical nanoparticle anti-reflective coatings. Over the last decade, plasmonic antireflecting nanostructures have been
extensively studied to be utilized in various optical and optoelectronic
systems such as lenses, solar cells, photodetectors, and others. The growing
interest to all-dielectric photonics as an alternative optical technology along
with plasmonics motivates us to compare antireflection properties of
all-dielectric and plasmonic nanoparticle coatings based on silver and
crystalline silicon. Our results of numerical simulations for periodic arrays
of spherical nanoparticles on top of amorphous silicon show that both silicon
and silver nanoparticle coatings demonstrate strong anti-reflective properties
in the visible spectral range. In this work, we show for the first time that
blooming effect, that is zero reflection from the structure, with silicon
coatings originates from the interference of electric- and magnetic-dipole
responses of nanoparticles with the wave reflected from the substrate, and we
refer to it as substrate-mediated Kerker effect. For the silver coating, our
results agree with previously observed substrate-induced bi-anisotropy and
blooming, caused by substrate-induced magnetic response. Finally, we
numerically show high effectiveness of silicon and silver coatings for the
application in thin-film photovoltaic elements, which is related to the
suppression of reflection from the high-index substrate and increased light
absorbance in the active layer with coating. Depending on the nanoparticle
size, either silicon or silver coating is more efficient, and overall increase
of absorption up to 30% can be achieved.

###Evaluation and determination of seven and five parameters of a photovoltaic generator by an iterative method||Ahmed Yahfdhou,Abdel Kader Mahmoud,Issakha Youm###

Evaluation and determination of seven and five parameters of a photovoltaic generator by an iterative method. The mathematical modeling of solar cells is essential for any optimization
operation of the efficiency or the diagnostics of the photovoltaic generator.
The photovoltaic module is generally represented by an equivalent circuit whose
parameters are experimentally calculated by using the characteristic
current-tension, I-V. The precise determination of these parameters stays a
challenge for the researchers, what led to a big diversification in the models
and the digital methods dedicated to their characterizations. In the present
paper; we are interested in the parametric characterization of a model in both
following cases: with single and two diodes, in order to plan the behavior of
the photovoltaic generator under real functioning conditions. We developed an
identification method of the parameters using Newton Raphson's method by using
the software Matlab/Simulink. This method is a fast technique which allows the
identification of several parameters and can be used in real time applications.
The results of the proposed method show a high agreement between the
experimental and simulated characteristics photovoltaic generator.

###Optical Spintronics in Organic-Inorganic Perovskite Photovoltaics||Junwen Li,Paul M. Haney###

Optical Spintronics in Organic-Inorganic Perovskite Photovoltaics. Organic-inorganic halide CH$_3$NH$_3$PbI$_3$ solar cells have attracted
enormous attention in recent years due to their remarkable power conversion
efficiency. When inversion symmetry is broken, these materials should exhibit
interesting spin-dependent properties as well, owing to their strong spin-orbit
coupling. In this work, we consider the spin-dependent optical response of
CH$_3$NH$_3$PbI$_3$. We first use density functional theory to compute the
ballistic spin current generated by absorption of unpolarized light. We then
consider diffusive transport of photogenerated charge and spin for a thin
CH$_3$NH$_3$PbI$_3$ layer with a passivated surface and an Ohmic, non-selective
contact. The spin density and spin current are evaluated by solving the
drift-diffusion equations for a simplified 3-dimensional Rashba model of the
electronic structure of the valence and conduction bands. We provide analytic
expressions for the photon flux required to induce measurable spin densities,
and propose that these spin densities can provide useful information about the
role of grain boundaries in the photovoltaic behavior of these materials. We
also discuss the prospects for measuring the optically generated spin current
with the inverse spin Hall effect.

###Universal rules for visible-light absorption in hybrid perovskite materials||Masato Kato,Takemasa Fujiseki,Tetsuhiko Miyadera,Takeshi Sugita,Shohei Fujimoto,Masato Tamakoshi,Masayuki Chikamatsu,Hiroyuki Fujiwara###

Universal rules for visible-light absorption in hybrid perovskite materials. A variety of organic-inorganic hybrid perovskites (APbX3) consisting of mixed
center cations [A = CH3NH3+, HC(NH2)2+, Cs+] with different PbX3- cages (X = I,
Br, Cl) have been developed to realize high-efficiency solar cells.
Nevertheless, clear understanding for the effects of A and X on the optical
transition has been lacking. Here, we present universal rules that allow the
unified interpretation of the optical absorption in various hybrid perovskites.
In particular, we find that the influence of the A-site cation on the light
absorption is rather significant and the absorption coefficient (alpha) reduces
to half when CH3NH3+ is replaced with HC(NH2)2+ in the APbI3 system. Our
density functional theory (DFT) calculations reproduce all of the fine
absorption features observed in HC(NH2)2PbI3 and CH3NH3PbBr3, allowing the
unique assignment of the interband transitions in the Brillouin zone. In
contrast to general understanding that the A-site cation involves weakly in the
optical process, our theoretical calculations reveal that the center cation
plays a critical role in the interband transition and the absorption strength
in the visible region is modified by the strong A-X interaction. Furthermore,
our systematic analyses show that the variation of the absorption spectrum with
X can be described simply by the well-known sum rule. The universal rules
established in this study explain the large reduction of alpha in HC(NH2)2PbI3
and predict CsPbI3 as the highest alpha material.

###Controlling the intensity of light in large areas at the interfaces of a scattering medium||Oluwafemi S. Ojambati,John T. Hosmer-Quint,Klaas-Jan Gorter,Allard P. Mosk,Willem L. Vos###

Controlling the intensity of light in large areas at the interfaces of a scattering medium. The recent advent of wave-shaping methods has demonstrated the focusing of
light through and inside even the most strongly scattering materials. Typically
in wavefront shaping, light is focused in an area with the size of one speckle
spot. It has been shown that the intensity is not only increased in the target
speckle spot, but also in an area outside the optimized speckle spot.
Consequently, the total transmission is enhanced, even though only the
intensity in a single speckle spot is controlled. Here, we experimentally study
how the intensity enhancement on both interfaces of a scattering medium depends
on the optimization area on the transmission side. We observe that as the
optimization radius increases, the enhancement of the total transmitted
intensity increases. We find a concomitant decrease of the total reflected
intensity, which implies an energy redistribution between transmission and
reflection channels. In addition, we find a qualitative evidence of a
long-range reflection-transmission correlation. Our result is useful for
efficient light harvesting in solar cells, multi-channel quantum secure
communications, imaging, and complex beam delivery through a scattering medium.

###Simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 using four-wave mixing spectroscopy||Samuel A. March,Charlotte Clegg,Drew B. Riley,Daniel Webber,Ian G. Hill,Kimberley C. Hall###

Simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 using four-wave mixing spectroscopy. Solar cells incorporating organic-inorganic perovskite, which may be
fabricated using low-cost solution-based processing, have witnessed a dramatic
rise in efficiencies yet their fundamental photophysical properties are not
well understood. The exciton binding energy, central to the charge collection
process, has been the subject of considerable controversy due to subtleties in
extracting it from conventional linear spectroscopy techniques due to strong
broadening tied to disorder. Here we report the simultaneous observation of
free and defect-bound excitons in CH3NH3PbI3 films using four-wave mixing (FWM)
spectroscopy. Due to the high sensitivity of FWM to excitons, tied to their
longer coherence decay times than unbound electron-hole pairs, we show that the
exciton resonance energies can be directly observed from the nonlinear optical
spectra. Our results indicate low-temperature binding energies of 13 meV (29
meV) for the free (defect-bound) exciton, with the 16 meV localization energy
for excitons attributed to binding to point defects. Our findings shed light on
the wide range of binding energies (2-55 meV) reported in recent years.

###Carrier Lifetimes in a III-V-N Intermediate Band Semiconductor||J. N. Heyman,A. M. Schwartzberg,K. M. Yu,A. V. Luce,O. D. Dubon,Y. J. Kuang,C. W. Tu,W. Walukiewicz###

Carrier Lifetimes in a III-V-N Intermediate Band Semiconductor. We have used transient absorption spectroscopy to measure carrier lifetimes
in the multiband band semiconductor GaPAsN. These measurements probe the
electron populations in the conduction band, intermediate band and valance band
as a function of time after an excitation pulse. Following photoexcitation of
GaP0.32As0.67N0.01 we find that the electron population in the conduction band
decays exponentially with a time constant 23ps. The electron population in the
intermediate band exhibits bimolecular recombination with recombination
constant r = 2 10^-8 cm-3/s. In our experiment an optical pump pulse excited
electrons from the valance band to the intermediate and conduction bands, and
the change in interband absorption due to absorption saturation and induced
absorption was probed with a delayed white light pulse. We modeled the optical
properties of our samples using the band anti-crossing model to extract carrier
densities as a function of time. These results indicate that the minority
carrier lifetimes are too short for efficient solar power conversion and that
improvements in material quality will be required for practical applications of
GaPAsN based intermediate band solar cells.

###Impact of Grain Boundaries on Efficiency and Stability of Organic-Inorganic Trihalide Perovskites||Zhaodong Chu,Mengjin Yang,Philip Schulz,Di Wu,Xin Ma,Edward Seifert,Liuyang Sun,Kai Zhu,Xiaoqin Li,Keji Lai###

Impact of Grain Boundaries on Efficiency and Stability of Organic-Inorganic Trihalide Perovskites. Organic-inorganic perovskite solar cells have attracted tremendous attention
because of their remarkably high power conversion efficiencies (PCEs). To
further improve the device performance, however, it is imperative to obtain
fundamental understandings on the photo-response and long-term stability down
to the microscopic level. Here, we report the first quantitative nanoscale
photoconductivity imaging on two methylammonium lead triiodide (MAPbI3) thin
films with different PCEs by light-stimulated microwave impedance microscopy.
The intrinsic photo-response is largely uniform across grains and grain
boundaries, which is direct evidence on the inherently benign nature of
microstructures in the perovskite thin films. In contrast, the carrier mobility
and lifetime are strongly affected by bulk properties such as the sample
crystallinity. As visualized by the spatial evolution of local
photoconductivity, the degradation due to water diffusion through the capping
layer begins with the disintegration of large grains rather than the nucleation
and propagation from grain boundaries. Our findings provide new insights to
improve the electro-optical properties of MAPbI3 thin films towards large-scale
commercialization.

###Energetic disorder induced leakage current in organic bulk heterojunction solar cells: comprehending the ultra-high open circuit voltage loss at low temperatures||Wenchao Yang,Yongsong Luo,Pengfei Guo,Haibin Sun,Yao Yao###

Energetic disorder induced leakage current in organic bulk heterojunction solar cells: comprehending the ultra-high open circuit voltage loss at low temperatures. In organic bulk heterojunction solar cells, the open circuit voltage
($V_\mathrm{oc}$) suffers from an ultra-high loss at low temperatures. In this
work we investigate the origin of the loss through calculating the
$V_\mathrm{oc}-T$ plots with the device model method systematically and
comparing it with experimentally observed ones. When the energetic disorder is
incorporated into the model by considering the disorder-suppressed and
temperature-dependent charge carrier mobilities, it is found that for
nonselective contacts the $V_\mathrm{oc}$ reduces drastically under the low
temperature regime, while for selective contacts the $V_\mathrm{oc}$ keeps
increasing with the decreasing temperature. The main reason is revealed that as
the temperature decreases, the reduced mobilities give rise to low charge
extraction efficiency and small bimolecular recombination rate for the
photogenerated charge carriers, so that in the former case they can be
extracted from the wrong electrode to form a leakage current which counteracts
the photocurrent and increases quickly with voltage, leading to the anomalous
reduction of $V_\mathrm{oc}$. In addition, it is revealed that the charge
generation rate is slow-varying with temperature and does not induce
significant $V_\mathrm{oc}$ loss. This work also provides a comprehensive
picture for the $V_\mathrm{oc}$ behavior under varying device working
conditions.

###Functionality-directed Screening of Pb-free Hybrid Organic-inorganic Perovskites with Desired Intrinsic Photovoltaic Functionalities||Dongwen Yang,Jian Lv,Xingang Zhao,Qiaoling Xu,Yuhao Fu,Yiqiang Zhan,Alex Zunger,Lijun Zhang###

Functionality-directed Screening of Pb-free Hybrid Organic-inorganic Perovskites with Desired Intrinsic Photovoltaic Functionalities. The material class of hybrid organic-inorganic perovskites has risen rapidly
from a virtually unknown material in photovoltaic applications a short 7 years
ago into a ~20% efficient thin-film solar cell material. As promising as this
class of materials is, however, there are limitations associated with its poor
long-term stability, non-optimal band gap, presence of environmentally-toxic Pb
element, etc. We herein apply a functionality-directed theoretical materials
selection approach as a filter for initial screening of the compounds that
satisfy the desired intrinsic photovoltaic functionalities and might overcome
the above limitations. First-principles calculations are employed to
systemically study thermodynamic stability and photovoltaic-related properties
of hundred of candidate hybrid perovskites. We have identified in this
materials selection process fourteen Ge and Sn-based materials with potential
superior bulk-material-intrinsic photovoltaic performance. A distinct class of
compounds containing NH3COH$^+$ with the organic molecule derived states
intriguingly emerging at band-edges is found. Comparison of various candidate
materials offers insights on how composition variation and microscopic
structural changes affect key photovoltaic relevant properties in this family
of materials.

###Temperature Dependence of the Energy Levels of Methylammonium Lead Iodide Perovskite from First Principles||Wissam A. Saidi,Samuel Poncé,Bartomeu Monserrat###

Temperature Dependence of the Energy Levels of Methylammonium Lead Iodide Perovskite from First Principles. Environmental effects and intrinsic energy-loss processes lead to
fluctuations in the operational temperature of solar cells, which can
profoundly influence their power conversion efficiency. Here we determine from
first principles the effects of temperature on the band gap and band edges of
the hybrid pervoskite CH$_3$NH$_3$PbI$_3$ by accounting for electron-phonon
coupling and thermal expansion. From $290$ to $380$ K, the computed band gap
change of $40$ meV coincides with the experimental change of $30$-$40$ meV. The
calculation of electron-phonon coupling in CH$_3$NH$_3$PbI$_3$ is particularly
intricate, as the commonly used Allen-Heine-Cardona theory overestimates the
band gap change with temperature, and excellent agreement with experiment is
only obtained when including high-order terms in the electron-phonon
interaction. We also find that spin-orbit coupling enhances the electron-phonon
coupling strength, but that the inclusion of nonlocal correlations using hybrid
functionals has little effect. We reach similar conclusions in the metal-halide
perovskite CsPbI$_3$. Our results unambiguously confirm for the first time the
importance of high-order terms in the electron-phonon coupling by direct
comparison with experiment.

###Warming Up Density Functional Theory||Justin C. Smith,Francisca Sagredo,Kieron Burke###

Warming Up Density Functional Theory. Density functional theory (DFT) has become the most popular approach to
electronic structure across disciplines, especially in material and chemical
sciences. Last year, at least 30,000 papers used DFT to make useful predictions
or give insight into an enormous diversity of scientific problems, ranging from
battery development to solar cell efficiency and far beyond. The success of
this field has been driven by usefully accurate approximations based on known
exact conditions and careful testing and validation. In the last decade,
applications of DFT in a new area, warm dense matter, have exploded. DFT is
revolutionizing simulations of warm dense matter including applications in
controlled fusion, planetary interiors, and other areas of high energy density
physics. Over the past decade or so, molecular dynamics calculations driven by
modern density functional theory have played a crucial role in bringing
chemical realism to these applications, often (but not always) with excellent
agreement with experiment. This chapter summarizes recent work from our group
on density functional theory at non-zero temperatures, which we call thermal
DFT. We explain the relevance of this work in the context of warm dense matter,
and the importance of quantum chemistry to this regime. We illustrate many
basic concepts on a simple model system, the asymmetric Hubbard dimer.

###Local photo-excitation of shift current in noncentrosymmetric systems||Hiroaki Ishizuka,Naoto Nagaosa###

Local photo-excitation of shift current in noncentrosymmetric systems. Photocurrent in solids is an important phenomenon with many applications
including the solar cells. In conventional photoconductors, the electrons and
holes created by light irradiation are separated by the external electric
field, resulting in a current flowing into electrodes. Shift current in
noncentrosymmetric systems is distinct from this conventional photocurrent in
the sense that no external electric field is needed, and, more remarkably, is
driven by the Berry phase inherent to the Bloch wavefunction. It is analogous
to the polarization current in the ground state but is a d.c. current
continuously supported by the nonequilibrium steady state under the pumping by
light. Here we show theoretically, by employing Keldysh-Floquet formalism
applied to a simple one-dimensional model, that the local photo excitation can
induce the shift current which is independent of the position and width of the
excited region and also the length of the system. This feature is in stark
contrast to the conventional photocurrent, which is suppressed when the sample
is excited locally at the middle and increases towards the electrodes. This
finding reveals the unconventional nature of shift current and will pave a way
to design a highly efficient photovoltaic effect in solids.

###Non-reciprocal Light-harvesting Nanoantennae Made by Nature||Julian Juhi-Lian Ting###

Non-reciprocal Light-harvesting Nanoantennae Made by Nature. Most of our current understanding of mechanisms of photosynthesis comes from
spectroscopy. However, classical definition of radio-antenna can be extended to
optical regime to discuss the function of light-harvesting antennae. Further to
our previously proposed model of a loop antenna we provide several more
physical explanations on considering the non-reciprocal properties of the light
harvesters of bacteria. We explained the function of the non-heme iron at the
reaction center, and presented reasons for each module of the light harvester
being composed of one carotenoid, two short $\alpha$-helical polypeptides and
three bacteriochlorophylls; we explained also the toroidal shape of the light
harvester, the upper bound of the characteristic length of the light harvester,
the functional role played by the long-lasting spectrometric signal observed,
and the photon anti-bunching observed. Based on these analyses, two mechanisms
might be used by radiation-durable bacteria, {\it Deinococcus radiodurans}; and
the non-reciprocity of an archaeon, {\it Haloquadratum walsbyi}, are analyzed.
The physical lessons involved are useful for designing artificial light
harvesters, optical sensors, wireless power chargers, passive super-Planckian
heat radiators, photocatalytic hydrogen generators, and radiation protective
cloaks. In particular it can predict what kind of particles should be used to
separate sunlight into a photovoltaically and thermally useful range to enhance
the efficiency of solar cells.

###Localization landscape theory of disorder in semiconductors II: Urbach tails of disordered quantum well layers||Marco Piccardo,Chi-Kang Li,Yuh-Renn Wu,James S. Speck,Bastien Bonef,Robert M. Farrell,Marcel Filoche,Lucio Martinelli,Jacques Peretti,Claude Weisbuch###

Localization landscape theory of disorder in semiconductors II: Urbach tails of disordered quantum well layers. Urbach tails in semiconductors are often associated to effects of
compositional disorder. The Urbach tail observed in InGaN alloy quantum wells
of solar cells and LEDs by biased photocurrent spectroscopy is shown to be
characteristic of the ternary alloy disorder. The broadening of the absorption
edge observed for quantum wells emitting from violet to green (indium content
ranging from 0 to 28\%) corresponds to a typical Urbach energy of 20~meV. A 3D
absorption model is developed based on a recent theory of disorder-induced
localization which provides the effective potential seen by the localized
carriers without having to resort to the solution of the Schr\"odinger equation
in a disordered potential. This model incorporating compositional disorder
accounts well for the experimental broadening of the Urbach tail of the
absorption edge. For energies below the Urbach tail of the InGaN quantum wells,
type-II well-to-barrier transitions are observed and modeled. This contribution
to the below bandgap absorption is particularly efficient in near-UV emitting
quantum wells. When reverse biasing the device, the well-to-barrier below
bandgap absorption exhibits a red shift, while the Urbach tail corresponding to
the absorption within the quantum wells is blue shifted, due to the partial
compensation of the internal piezoelectric fields by the external bias. The
good agreement between the measured Urbach tail and its modeling by the new
localization theory demonstrates the applicability of the latter to
compositional disorder effects in nitride semiconductors.

###Nitrogen-doped Nanoporous Carbon Membranes Functionalized with Co/CoP Janus-type nanocrystals as Hydrogen Evolution Electrode in Both Acid and Alkaline Environment||Hong Wang,Shixiong Min,Qiang Wang,Debao Li,Gilberto Casillas,Chun Ma,Yangyang Li,Zhixiong Li,Lain-Jong Li,Jiayin Yuan,Markus Antonietti,Tom Wu###

Nitrogen-doped Nanoporous Carbon Membranes Functionalized with Co/CoP Janus-type nanocrystals as Hydrogen Evolution Electrode in Both Acid and Alkaline Environment. Self-supported electrocatalysts being generated and employed directly as
electrode for energy conversion has been intensively pursued in the fields of
materials chemistry and energy. Herein, we report a synthetic strategy to
prepare freestanding hierarchically structured, nitrogen-doped nanoporous
graphitic carbon membranes functionalized with Janus-type Co/CoP nanocrystals
(termed as HNDCM-Co/CoP), which were successfully applied as a
highly-efficient, binder-free electrode in hydrogen evolution reaction (HER).
Benefited from multiple structural merits, such as high degree of
graphitization, three-dimensionally interconnected micro-/meso-/macropores,
uniform nitrogen-doping, well-dispersed Co/CoP nanocrystals as well as the
confinement effect of the thin carbon layer on the nanocrystals, HNDCM-Co/CoP
exhibited superior electrocatalytic activity and long-term operation stability
for HER under both acid and alkaline conditions. As a proof-of-concept of
practical usage, a macroscopic piece of HNDCM-Co/CoP of 5.6 cm x 4 cm x 60 um
in size was prepared in our laboratory. Driven by a solar cell,
electroreduction of water in alkaline condition (pH 14) was performed, and H2
has been produced at a rate of 16 ml/min, demonstrating its potential as
real-life energy conversion systems.

###Microscopic structure differences in CZTSe quaternary alloys prepared by different techniques revealed by spatially-resolved laser-induced-modification Raman spectroscopy||Qiong Chen,Sergio Bernardi,Yong Zhang###

Microscopic structure differences in CZTSe quaternary alloys prepared by different techniques revealed by spatially-resolved laser-induced-modification Raman spectroscopy. While producing comparable efficiencies and showing similar properties when
probed by conventional techniques, such as Raman, photoluminescence and X-ray
diffraction, two thin film solar cell materials with complex structures, such
as quaternary compound CZTSe, may in fact differ significantly in their
microscopic structures. In this work, laser induced modification Raman
spectroscopy, coupled with high spatial resolution and high temperature
capability, is demonstrated as an effective tool to obtain important structure
information beyond that the conventional characterization techniques can offer,
and thus to reveal the microscopic scale variations between nominally similar
alloys. Specifically, CZTSe films prepared by sputtering and co-evaporation
methods that exhibited similar Raman and XRD features were found to behave very
differently under high laser power and high temperature Raman probe, because
the differences in their microscopic structures lead to different structure
modifications in response to the external stimuli, such as light illumination
and temperature. They were also shown to undergo different degree of plastic
changes and have different thermal conductivities as revealed by
spatially-resolved Raman spectroscopy.

###New 3,3'-(ethane-1, 2-diylidene)bis(indolin-2-one) (EBI)-based small molecule semiconductors for organic solar cells||Mylene Le Borgne,Jesse Quinn,Jaime Martín,Natalie Stingelin,Yuning Li,Guillaume Wantz###

New 3,3'-(ethane-1, 2-diylidene)bis(indolin-2-one) (EBI)-based small molecule semiconductors for organic solar cells. A series of donor-acceptor-donor (D-A-D) structured small-molecule compounds,
with 3,3'-(ethane-1,2-diylidene)bis(indolin-2-one) (EBI) as a novel electron
acceptor building block coupled with various electron donor end-capping
moieties (thiophene, bithiophene and benzofuran), were synthesized and
characterized. When the fused-ring benzofuran is combined to EBI (EBI-BF), the
molecules displayed a perfectly planar conformation and afforded the best
charge tranport properties among these EBI compounds with a hole mobility of up
to 0.021 cm2 V-1 s-1. All EBI-based small molecules were used as donor material
along with a PC61BM acceptor for the fabrication of solution-processed
bulk-heterojunction (BHJ) solar cells. The best performing photovoltaic devices
are based on the EBI derivative using the bithiophene end-capping moiety
(EBI-2T) with a maximum power conversion efficiency (PCE) of 1.92%, owing to
the broad absorption spectra of EBI-2T and the appropriate morphology of the
BHJ. With the aim of establishing a correlation between the molecular structure
and the thin film morphology, differential scanning calorimetry, atomic force
microscopy and X-ray diffraction analysis were performed on neat and blend
films of each material.

###Influence of water intercalation and hydration on chemical decomposition and ion transport in methylammonium lead halide perovskites||Un-Gi Jong,Chol-Jun Yu,Gum-Chol Ri,Andrew P. McMahon,Nicholas M. Harrison,Piers R. F. Barnes,Aron Walsh###

Influence of water intercalation and hydration on chemical decomposition and ion transport in methylammonium lead halide perovskites. The use of methylammonium (MA) lead halide perovskites \ce{CH3NH3PbX3} (X=I,
Br, Cl) in perovskite solar cells (PSCs) has made great progress in performance
efficiency during recent years. However, the rapid decomposition of \ce{MAPbI3}
in humid environments hinders outdoor application of PSCs, and thus, a
comprehensive understanding of the degradation mechanism is required. To do
this, we investigate the effect of water intercalation and hydration of the
decomposition and ion migration of \ce{CH3NH3PbX3} using first-principles
calculations. We find that water interacts with \ce{PbX6} and MA through
hydrogen bonding, and the former interaction enhances gradually, while the
latter hardly changes when going from X=I to Br and to Cl. Thermodynamic
calculations indicate that water exothermically intercalates into the
perovskite, while the water intercalated and monohydrated compounds are stable
with respect to decomposition. More importantly, the water intercalation
greatly reduces the activation energies for vacancy-mediated ion migration,
which become higher going from X=I to Br and to Cl. Our work indicates that
hydration of halide perovskites must be avoided to prevent the degradation of
PSCs upon moisture exposure.

###Impact of Unintentional Oxygen Doping on Organic Photodetectors||Julie Euvrard,Amelie Revaux,Alexandra Cantarano,Stephanie Jacob,Antoine Kahn,Dominique Vuillaume###

Impact of Unintentional Oxygen Doping on Organic Photodetectors. Oxygen plasma is a widely used treatment to change the surface properties of
organic layers. This treatment is particularly interesting to enable the
deposition from solution of
poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) on top of
the active layer of organic solar cells or photodetectors. However, oxygen is
known to be detrimental to organic devices, as the active layer is very
sensitive to oxygen and photo-oxidation. In this study, we aim to determine the
impact of oxygen plasma surface treatment on the performance of organic
photodetectors (OPD). We show a significant reduction of the sensitivity as
well as a change in the shape of the external quantum efficiency (EQE) of the
device. Using hole density and conductivity measurements, we demonstrate the
p-doping of the active layer induced by oxygen plasma. Admittance spectroscopy
shows the formation of trap states approximately 350 meV above the highest
occupied molecular orbital of the active organic semiconductor layer. Numerical
simulations are carried out to understand the impact of p-doping and traps on
the electrical characteristics and performance of the OPDs.

###Impact of Small Phonon Energies on the Charge-Carrier Lifetimes in Metal-Halide Perovskites||Thomas Kirchartz,Tom Markvart,Uwe Rau,David A. Egger###

Impact of Small Phonon Energies on the Charge-Carrier Lifetimes in Metal-Halide Perovskites. Solar cells based on metal-halide perovskite absorber layers have resulted in
outstanding photovoltaic devices with long non-radiative lifetimes as a crucial
feature enabling high efficiencies. Long non-radiative lifetimes occur if the
transfer of the energy of the electron-hole pair into vibrational energy is
slow, due to, e.g., a low density of defects, weak electron phonon coupling or
the release of a large number of phonons needed for a single transition. Here,
we discuss the implications of the known material properties of metal-halide
perovskites (such as permittivities, phonon energies and effective masses) and
combine those with basic models for electron-phonon coupling and
multiphonon-transition rates in polar semiconductors. We find that the low
phonon energies of MAPbI$_3$ lead to a strong dependence of recombination rates
on trap position, which can be readily deduced from the underlying physical
effects determining non-radiative transitions. Here, we show that this is
important for the non-radiative recombination dynamics of metal-halide
perovskites, as it implies that these systems are rather insensitive to defects
that are not at midgap energy. This can lead to long lifetimes, which indicates
that the low phonon energies are likely an important factor for the high
performance of optoelectronic devices with metal halide perovskites.

###Control of Excitation Energy Transfer in Condensed Phase Molecular Systems by Floquet Engineering||Nguyen Thanh Phuc,Akihito Ishizaki###

Control of Excitation Energy Transfer in Condensed Phase Molecular Systems by Floquet Engineering. Excitation energy transfer (EET) is one of the most important processes in
both natural and artificial chemical systems including, for example,
photosynthetic complexes and organic solar cells. The EET rate, however, is
strongly suppressed when there is a large difference in the excitation energy
between the donor and acceptor molecules. Here, we demonstrate both
analytically and numerically that the EET rate can be greatly enhanced by
periodically modulating the excitation energy difference. The enhancement of
EET by using this Floquet engineering, in which the system's Hamiltonian is
made periodically time-dependent, turns out to be efficient even in the
presence of strong fluctuations and dissipations induced by the coupling with a
huge number of dynamic degrees of freedom in the surrounding molecular
environments. As an effect of the environment on the Floquet engineering of
EET, the optimal driving frequency is found to depend on the relative
magnitudes of the system and environment's characteristic time scales with an
observed frequency shift when moving from the limit of slow environmental
fluctuations (inhomogeneous broadening limit) to that of fast fluctuations
(homogeneous broadening limit).

###CMOS-compatible controlled hyperdoping of silicon nanowires||Yonder Berencén,Slawomir Prucnal,Wolfhard Möller,René Hübner,Lars Rebohle,Roman Böttger,Markus Glaser,Tommy Schönherr,Ye Yuan,Mao Wang,Yordan M. Georgiev,Artur Erbe,Alois Lugstein,Manfred Helm,Shengqiang Zhou,Wolfgang Skorupa###

CMOS-compatible controlled hyperdoping of silicon nanowires. Hyperdoping consists of the intentional introduction of deep-level dopants
into a semiconductor in excess of equilibrium concentrations. This causes a
broadening of dopant energy levels into an intermediate band between the
valence and conduction bands.[1,2] Recently, bulk Si hyperdoped with chalcogens
or transition metals has been demonstrated to be an appropriate
intermediate-band material for Si-based short-wavelength infrared
photodetectors.[3-5] Intermediate-band nanowires could potentially be used
instead of bulk materials to overcome the Shockley-Queisser limit and to
improve efficiency in solar cells,[6-9] but fundamental scientific questions in
hyperdoping Si nanowires require experimental verification. The development of
a method for obtaining controlled hyperdoping levels at the nanoscale
concomitant with the electrical activation of dopants is, therefore, vital to
understanding these issues. Here, we show a CMOS-compatible technique based on
non-equilibrium processing for the controlled doping of Si at the nanoscale
with dopant concentrations several orders of magnitude greater than the
equilibrium solid solubility. Through the nanoscale spatially controlled
implantation of dopants, and a bottom-up template-assisted solid phase
recrystallization of the nanowires with the use of millisecond-flash lamp
annealing, we form Se-hyperdoped Si/SiO2 core/shell nanowires that have a
room-temperature sub-band gap optoelectronic photoresponse when configured as a
photoconductor device.

###Topological insulator materials for advanced optoelectronic devices||Zengji Yue,Xiaolin Wang,Min Gu###

Topological insulator materials for advanced optoelectronic devices. Topological insulators are quantum materials that have an insulating bulk
state and a topologically protected metallic surface state with spin and
momentum helical locking and a Dirac-like band structure. Unique and
fascinating electronic properties, such as the quantum spin Hall effect,
quantum anomalous Hall effect, and topological magnetoelectric effect, as well
as magnetic monopole images and Majorana fermions, have been observed in the
topological insulator materials. With these unique properties, topological
insulator materials have great potential applications in spintronics and
quantum information processing, as well as magnetoelectric devices with higher
efficiency and lower energy consumption. On the other hand, topological
insulator materials also exhibit a number of excellent optical properties,
including Kerr and Faraday rotation, ultrahigh bulk refractive index,
near-infrared frequency transparency, unusual electromagnetic scattering, and
ultra-broadband surface plasmon resonances. Specifically, Dirac plasmon
excitations have been observed in Bi2Se3 micro-ribbon arrays at THz
frequencies. Ultraviolet and visible frequency plasmonics have been observed in
nanoslit and nanocone arrays of Bi1.5Sb0.5Te1.8Se1.2 crystals. High
transparency has been observed in Bi2Se3 nanoplates. An ultrahigh refractive
index has been observed in bulk Bi1.5Sb0.5Te1.8Se1.2 crystals as well as in
Sb2Te3 thin films. These excellent optical properties mean that topological
insulator materials are suitable for various optoelectronic devices, including
plasmonic solar cells, ultrathin holograms, plasmonic and Fresnel lens,
broadband photodetectors, and nanoscale waveguides. In this chapter, we focus
on the excellent electronic and optical properties of topological insulator
materials and their wide applications in advanced optoelectronic devices.

###Experimental demonstration of correlated flux scaling in photoconductivity and photoluminescence of lead-halide perovskites||Hee Taek Yi,Pavel Irkhin,Prakriti P. Joshi,Yuri N. Gartstein,Xiaoyang Zhu,Vitaly Podzorov###

Experimental demonstration of correlated flux scaling in photoconductivity and photoluminescence of lead-halide perovskites. Lead-halide perovskites attracted attention as materials for high-efficiency
solar cells and light emitting applications. Among their attributes are
solution processability, high absorbance in the visible spectral range and
defect tolerance, as manifested in long photocarrier lifetimes and diffusion
lengths. The microscopic origin of photophysical properties of perovskites is,
however, still unclear and under debate. Here, we have observed an interesting
universal scaling behavior in a series of (hybrid and all-inorganic) perovskite
single crystals investigated via simultaneous measurements of the Hall effect,
photoconductivity and photoluminescence. A clear correlation between
photoconductivity and photoluminescence as functions of the incident photon
flux is observed. While photoconductivity exhibits a crossover in the power-law
dependence between power exponents 1 and 1/2, photoluminescence exhibits a
crossover between power exponents 2 and 3/2. This correlation is found in all
the studied compounds irrespective of the cation type (organic or inorganic) or
crystallographic phases. We propose phenomenological microscopic mechanisms
that explain these interesting non-trivial power exponents and crossovers
between them in this broad class of lead-halide perovskites.

###Optoelectronic properties of defective MoS$_2$ and WS$_2$ monolayers||Saboura Salehi,Alireza Saffarzadeh###

Optoelectronic properties of defective MoS$_2$ and WS$_2$ monolayers. We theoretically explore the effect of metal and disulphur vacancies on
electronic and optical properties of MoS$_2$ and WS$_2$ monolayers based on a
Slater-Koster tight-binding model and including the spin-orbit coupling. We
show that the vacancy defects create electronic flat bands by shifting the
Fermi level towards the valence band, indicating that both types of vacancies
may act as acceptor sites. The optical spectra of the pristine monolayers show
step-like features corresponding to the transition from spin split valence band
to the conduction band minimum, whereas the defective monolayers exhibit
additional peaks in their spectra arising from induced midgap states in their
band structures. We find that Mo and W vacancies contribute mostly in the
low-energy optical spectrum, while the S$_2$ vacancies enhance the optical
conductivity mainly in the visible range of the spectrum. This suggests that
depending on the type of vacancy, the atomic defects in MoS$_2$ and WS$_2$
monolayers may increase the efficiency of solar cells used in photovoltaic
systems.

###Enhanced stability of 2D organic-inorganic halide perovskites by doping and heterostructure engineering||Rahul Singh,Prashant Singh,Ganesh Balasubramanian###

Enhanced stability of 2D organic-inorganic halide perovskites by doping and heterostructure engineering. Organic-inorganic halide perovskite solar cells have recently attracted much
attention due to their low-cost fabrication, flexibility, and high-power
conversion efficiency. The reduction from three- to two-dimension (2D) promises
an exciting opportunity to tune the electronic properties of organic-inorganic
halide perovskites. Here, we propose first-principles density-functional theory
based route to study the effect of reduced dimensionality, impurity doping, and
heterostructure engineering on energy stability, band-gap and transport
properties of 2D hybrid organic-inorganic halide perovskites. We show that the
energetic stability of two-dimensional organic-inorganic halide perovskites can
be significantly enhanced by chemically depositing MoS2 monolayer as a
precursor in the system by heterostructure engineering. While on one hand, the
structures have similar and excellent transport properties as their bulk
counterparts, on the other, they possess the advantage of a broad range of
tunable band gaps. Our predictions will expedite future efforts (both
theoretical and experimental) in the synthesis, measurements and applications
of high performance perovskites.

###Acoustic Phonon Lifetimes Limit Thermal Transport in Methylammonium Lead Iodide||Aryeh Gold-Parker,Peter M. Gehring,Jonathan M. Skelton,Ian C. Smith,Dan Parshall,Jarvist M. Frost,Hemamala I. Karunadasa,Aron Walsh,Michael F. Toney###

Acoustic Phonon Lifetimes Limit Thermal Transport in Methylammonium Lead Iodide. Hybrid organic-inorganic perovskites (HOIPs) have become an important class
of semiconductors for solar cells and other optoelectronic applications.
Electron-phonon coupling plays a critical role in all optoelectronic devices,
and although the lattice dynamics and phonon frequencies of HOIPs have been
well studied, little attention has been given to phonon lifetimes. We report
the first high-precision measurements of acoustic phonon lifetimes in the
hybrid perovskite methylammonium lead iodide (MAPI), using inelastic neutron
spectroscopy to provide high energy resolution and fully deuterated single
crystals to reduce incoherent scattering from hydrogen. Our measurements reveal
extremely short lifetimes on the order of picoseconds, corresponding to
nanometer mean free paths and demonstrating that acoustic phonons are unable to
dissipate heat efficiently. Lattice-dynamics calculations using ab-initio
third-order perturbation theory indicate that the short lifetimes stem from
strong three-phonon interactions and a high density of low-energy optical
phonon modes related to the degrees of freedom of the organic cation. Such
short lifetimes have significant implications for electron-phonon coupling in
MAPI and other HOIPs, with direct impacts on optoelectronic devices both in the
cooling of hot carriers and in the transport and recombination of band edge
carriers. These findings illustrate a fundamental difference between HOIPs and
conventional photovoltaic semiconductors and demonstrate the importance of
understanding lattice dynamics in the effort to develop metal halide perovskite
optoelectronic devices.

###The Effect of Hole Transporting Layer in Charge Accumulation Properties of p-i-n Perovskite Solar Cells||Fedros Galatopoulos,Achilleas Savva,Ioannis T. Papadas,Stelios A. Choulis###

The Effect of Hole Transporting Layer in Charge Accumulation Properties of p-i-n Perovskite Solar Cells. The charge accumulation properties of p-i-n perovskite solar cells were
investigated using three representative organic and inorganic hole transporting
layer (HTLs): a) Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS, Al 4083), b) copper-doped nickel oxide (Cu:NiOx) and c) Copper
oxide (CuO). Through impedance spectroscopy analysis and modelling it is shown
that charge accumulation is decreased in the HTL/Perovskite interface, between
PEDOT:PSS to Cu:NiOx and CuO respectively. This was indicative from the
decrease in double layer capacitance (Cdl) and interfacial charge accumulation
capacitance (Cel), resulting in an increase to recombination resistance (Rrec),
thus decreased charge recombination events between the three HTLs. Through AFM
measurements it is also shown that the reduced recombination events (followed
by the increase in Rrec) is also a result of increased grain size between the
three HTLs, thus reduction in the grain boundaries area. These charge
accumulation properties of the three HTLs have resulted in an increase to the
power conversion efficiency between the PEDOT:PSS (8.44%), Cu:NiOx (11.45%) and
CuO (15.3%)-based devices.

###Impact of excess and disordered Sn sites on Cu2ZnSnS4 absorber material and device performance: A 119Sn Mossbauer Study||Goutam Kumar Gupta,V R Reddy,Ambesh Dixit###

Impact of excess and disordered Sn sites on Cu2ZnSnS4 absorber material and device performance: A 119Sn Mossbauer Study. Mossbauer analysis is carried out on CZTS samples, subjected to a low
temperature processing at 3000C (S1) and high temperature processing at 5500C
under sulfur environment (S2). Loss of Sn is observed in sample S2 due to high
temperature thermal treatment.The isomer shifts obtained in the Mossbauer
spectra confirms the existence of Sn at its 4+ valance state in both the
samples. Relatively high quadriple splitting is observed in S1 with respect to
S2, suggesting dislocations and crystal distortion present in S1, which are
reduced drastically by high temperature annealed S2 sample. The fabricated
solar cell with S1 and S2 absorbers showed significant improvement in
efficiency from ~0.145% to ~1%. The presence of excess Sn in S1 allows enhanced
recombination and the diode ideality factor shows larger value of 4.23 compared
to 2.17 in case of S2. The experiments also validate the fact that S1 with Sn
rich configuration shows lower acceptor carrier concentration as compared to S2
because of enhanced compensating defects in S1.

###Nanowire lasers||C. Couteau,A. Larrue,C. Wilhelm,C. Soci###

Nanowire lasers. We review principles and trends in the use of semiconductor nanowires (NWs)
as gain media for stimulated emission and lasing. Semiconductor nanowires have
recently been widely studied for use in integrated optoelectronic devices, such
as LEDs, solar cells, and transistors. Intensive research has also been
conducted on the use of nanowires for sub-wavelength laser systems that take
advantage of their quasi-one-dimensional nature, flexibility in material choice
and combination, and intrinsic optoelectronic properties. First, we provide an
overview on using quasi-one-dimensional nanowire systems to realize
sub-wavelength lasers with efficient, directional, and low-threshold emission.
We then describe the state-of-the-art for nanowire lasers in terms of
materials, geometry, and wavelength tunability. Next, we present the basics of
lasing in semiconductor nanowires, define the key parameters for stimulated
emission, and introduce the properties of nanowires. We then review advanced
nanowire laser designs from the literature. Finally, we present interesting
perspectives for low-threshold nanoscale light sources and optical
interconnects. We intend to illustrate the potential of nanolasers in many
applications, such as nanophotonic devices that integrate electronics and
photonics for next-generation optoelectronic devices. For instance, these
building blocks for nanoscale photonics can be used for data storage and
biomedical applications when coupled to on-chip characterization tools. These
nanoscale monochromatic laser light sources promise breakthroughs in
nanophotonics, as they can operate at room temperature, potentially be
electrically driven, and yield a better understanding of intrinsic nanomaterial
properties and surface state effects in low-dimensional semiconductor systems.

###Atomistic Mechanism of the Nucleation of Methylammonium Lead Iodide Perovskite from Solution||Paramvir Ahlawat,Pablo Piaggi,Michael Graetzel,Michele Parrinello,Ursula Rothlisberger###

Atomistic Mechanism of the Nucleation of Methylammonium Lead Iodide Perovskite from Solution. In the ongoing intense quest to increase the photoconversion efficiencies of
lead halide perovskites, it has become evident that optimizing the morphology
of the material is essential to achieve high peformance. Despite the fact that
nucleation plays a key role in controlling the crystal morphology, very little
is known about the nucleation and crystal growth processes. Here, we perform
metadynamics simulations of nucleation of methylammonium lead triiodide (MAPI)
in order to unravel the atomistic details of perovskite crystallization from a
$\gamma$-butyrolactone solution. The metadynamics trajectories show that the
nucleation process takes place in several stages. Initially, dense amorphous
clusters mainly consisting of lead and iodide appear from the homogeneous
solution. These clusters evolve into lead iodide (PbI$_{2}$) like structures.
Subsequently, methylammonium (MA$^{+}$) ions diffuse into this PbI$_{2}$-like
aggregates triggering the transformation into a perovskite crystal through a
solid-solid transformation. Demonstrating the crucial role of the monovalent
cations in crystallization, our simulations provide key insights into the
evolution of the perovskite microstructure which is essential to make
high-quality perovskite based solar cells and optoelectronics.

###The perovskite/transport layer interfaces dominate non-radiative recombination in efficient perovskite solar cells||Martin Stolterfoht,Pietro Caprioglio,Christian M. Wolff,José A. Márquez,Joleik Nordmann,Shanshan Zhang,Daniel Rothhardt,Ulrich Hörmann,Alex Redinger,Lukas Kegelmann,Steve Albrecht,Thomas Kirchartz,Michael Saliba,Thomas Unold,Dieter Neher###

The perovskite/transport layer interfaces dominate non-radiative recombination in efficient perovskite solar cells. Charge transport layers (CTLs) are key components of diffusion controlled
perovskite solar cells, however, they can induce additional non-radiative
recombination pathways which limit the open circuit voltage (V_OC) of the cell.
In order to realize the full thermodynamic potential of the perovskite
absorber, both the electron and hole transport layer (ETL/HTL) need to be as
selective as possible. By measuring the quasi-Fermi level splitting (QFLS) of
perovskite/CTL heterojunctions, we quantify the non-radiative interfacial
recombination current for a wide range of commonly used CTLs, including various
hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find
that all studied CTLs limit the V_OC by inducing an additional non-radiative
recombination current that is significantly larger than the loss in the neat
perovskite and that the least-selective interface sets the upper limit for the
V_OC of the device. The results also show that the V_OC equals the internal
QFLS in the absorber layer of (pin, nip) cells with selective CTLs and power
conversion efficiencies of up to 21.4%. However, in case of less selective
CTLs, the V_OC is substantially lower than the QFLS which indicates additional
losses at the contacts and/or interfaces. The findings are corroborated by
rigorous device simulations which outline several important considerations to
maximize the V_OC. This work shows that the real challenge to supress
non-radiative recombination losses in perovskite cells on their way to the
radiative limit lies in the suppression of carrier recombination at the
perovskite/CTL interfaces.

###Lone-pair effect on carrier capture in Cu$_2$ZnSnS$_4$ solar cells||Sunghyun Kim,Ji-Sang Park,Samantha N. Hood,Aron Walsh###

Lone-pair effect on carrier capture in Cu$_2$ZnSnS$_4$ solar cells. The performance of kesterite thin-film solar cells is limited by a low
open-circuit voltage due to defect-mediated electron-hole recombination. We
calculate the non-radiative carrier-capture cross sections and
Shockley-Read-Hall recombination coefficients of deep-level point defects in
Cu$_2$ZnSnS$_4$ (CZTS) from first-principles. While the oxidation state of Sn
is +4 in stoichiometric CZTS, inert lone pair (5$s^2$) formation lowers the
oxidation state to +2. The stability of the lone pair suppresses the ionization
of certain point defects, inducing charge transition levels deep in the band
gap. We find large lattice distortions associated with the lone-pair defect
centers due to the difference in ionic radii between Sn(II) and Sn(IV). The
combination of a deep trap level and large lattice distortion facilitates
efficient non-radiative carrier capture, with capture cross-sections exceeding
$10^{-12}$ cm$^2$. The results highlight a connection between redox active
cations and `killer' defect centres that form giant carrier traps. This lone
pair effect will be relevant to other emerging photovoltaic materials
containing n$s^2$ cations.

###Influence of Disorder and Anharmonic Fluctuations on the Dynamical Rashba Effect in Purely Inorganic Lead-Halide Perovskites||Arthur Marronnier,Guido Roma,Marcelo Carignano,Yvan Bonnassieux,Claudine Katan,Jacky Even,Edoardo Mosconi,Filippo De Angelis###

Influence of Disorder and Anharmonic Fluctuations on the Dynamical Rashba Effect in Purely Inorganic Lead-Halide Perovskites. Doping organic metal-halide perovskites with cesium could be the best
solution to stabilize highly-efficient perovskite solar cells. The
understanding of the respective roles of the organic molecule, on one hand, and
the inorganic lattice, on the other, is thus crucial in order to be able to
optimize the physical properties of the mixed-cation structures. In particular,
the study of the recombination mechanisms is thought to be one of the key
challenges towards full comprehension of their working principles. Using
molecular dynamics and frozen phonons, we evidence sub-picosecond anharmonic
fluctuations in the fully inorganic $CsPbI_3$ perovskite. We reveal the effect
of these fluctuations, combined with spin-orbit coupling, on the electronic
band structure, evidencing a dynamical Rashba effect. Our study show that under
certain conditions space disorder can quench the Rashba effect. As for time
disorder, we evidence a dynamical Rashba effect which is similar to what was
found for $MAPbI_3$ and which is still sizable despite temperature disorder,
the large investigated supercell, and the absence of the organic cations'
motion. We show that the spin texture associated to the Rashba splitting cannot
be deemed responsible for a consistent reduction of recombination rates,
although the spin mismatch between valence and conduction band increases with
the ferroelectric distortion causing the Rashba splitting.

###Review: Solid-state physics of halide perovskites||Jarvist Moore Frost###

Review: Solid-state physics of halide perovskites. Halide perovskite solar cells presented a unique opportunity to apply modern
computational materials science techniques to an (initially) poorly understood
new material. In this review, we recount the key understanding developed during
the last five years, through a narrative review of research progress. The
central enigma of the material is how it can be so defective, and yet work so
well as a photovoltaic. The physical properties of the material were understood
through molecular and lattice dynamic calculations, revealing the material to
show large dynamic responses on a wide range of time scales. Longer length
scales in the material was simulated with effective classical potentials,
showing that complex domains can be generated by the interacting molecular
dipoles, generating structured features in the electrostatic potential of the
lattice. Relativistic electronic structure reveals unique features in the
bands, which may explain observed slow recombination, and could be used in high
efficiency photovoltaics. The large dielectric response of the lattice leads to
a strong drive for the formation of polarons, some device physics of which are
discussed. These polarons offer a possible explanation for the observed slow
cooling of photoexcitations in the material.

###Thermodynamic Stability and Structural Insights for CH$_3$NH$_3$Pb$_{1-x}$Si$_x$I$_3$, CH$_3$NH$_3$Pb$_{1-x}$Ge$_x$I$_3$, and CH$_3$NH$_3$Pb$_{1-x}$Sn$_x$I$_3$ Hybrid Perovskite Alloys: A Statistical Approach from First Principles Calculations||Diego Guedes-Sobrinho,Ivan Guilhon,Marcelo Marques,Lara K. Teles###

Thermodynamic Stability and Structural Insights for CH$_3$NH$_3$Pb$_{1-x}$Si$_x$I$_3$, CH$_3$NH$_3$Pb$_{1-x}$Ge$_x$I$_3$, and CH$_3$NH$_3$Pb$_{1-x}$Sn$_x$I$_3$ Hybrid Perovskite Alloys: A Statistical Approach from First Principles Calculations. The recent reaching of 20% of conversion efficiency by solar cells based on
metal hybrid perovskites (MHP), e.g., the methylammonium (MA) lead iodide,
CH3NH3PbI3 (MAPbI3), has excited the scientific community devoted to the
photovoltaics materials. However, the toxicity of Pb is a hindrance for large
scale commercial of MHP and motivates the search of another congener
eco-friendly metal. Here, we employed first-principles calculations via density
functional theory combined with the generalized quasichemical approximation to
investigate the structural, thermodynamic, and ordering properties of
MAPb1-xSixI3, MAPb1-xGexI3, and MAPb1-xSnxI3 alloys as pseudo-cubic structures.
The inclusion of a smaller second metal, as Si and Ge, strongly affects the
structural properties, reducing the cavity volume occupied by the organic
cation and limitating the free orientation under high temperature effects.
Unstable and metaestable phases are observed at room temperature for
MAPb1-xSixI3, whereas MAPb1-xGexI3 is energetically favored for Pb-rich in
ordered phases even at very low temperatures. Conversely, the high miscibility
of Pb and Sn into MAPb1-xSnxI3 yields an alloy energetically favored as a
pseudo-cubic random alloy with tunable properties at room temperature.

###Free-electron effects on optical absorption of hybrid perovskite CH$_3$NH$_3$PbI$_3$ from first principles||Joshua Leveillee,André Schleife###

Free-electron effects on optical absorption of hybrid perovskite CH$_3$NH$_3$PbI$_3$ from first principles. Hybrid organic-inorganic perovskites, such as methyl-ammonium lead tri-iodide
(MAPbI$_3$), are interesting candidates for efficient absorber materials in
next-generation solar cells, partly due to an unusual combination of low
exciton binding energy and strong optical absorption. Excitonic effects in this
material have been subject to debate both for experiment and theory, indicating
a need for better understanding of the screening mechanisms that act upon the
electron-hole interaction. Here we use cutting-edge first-principles
theoretical spectroscopy, based on density-functional and many-body
perturbation theory, to study atomic geometries, electronic structure, and
optical properties of three MAPbI$_3$ polymorphs and find good agreement with
earlier results and experiment. We then study the influence of free electrons
on the electron-hole interaction and show that this explains consistently
smaller exciton binding energies, compared to those in the material without
free electrons. Interestingly, we also find that the absorption line shape
strongly resembles that of the spectrum without free electrons up to high free
electron concentrations. We explain this unexpected behavior by formation of
Mahan excitons that dominate the absorption edge, making it robust against
free-electron induced changes observed in other semiconductors.

###Fast and robust detection of solar modules in electroluminescence images||Mathis Hoffmann,Bernd Doll,Florian Talkenberg,Christoph J. Brabec,Andreas K. Maier,Vincent Christlein###

Fast and robust detection of solar modules in electroluminescence images. Fast, non-destructive and on-site quality control tools, mainly high
sensitive imaging techniques, are important to assess the reliability of
photovoltaic plants. To minimize the risk of further damages and electrical
yield losses, electroluminescence (EL) imaging is used to detect local defects
in an early stage, which might cause future electric losses. For an automated
defect recognition on EL measurements, a robust detection and rectification of
modules, as well as an optional segmentation into cells is required. This paper
introduces a method to detect solar modules and crossing points between solar
cells in EL images. We only require 1-D image statistics for the detection,
resulting in an approach that is computationally efficient. In addition, the
method is able to detect the modules under perspective distortion and in
scenarios, where multiple modules are visible in the image. We compare our
method to the state of the art and show that it is superior in presence of
perspective distortion while the performance on images, where the module is
roughly coplanar to the detector, is similar to the reference method. Finally,
we show that we greatly improve in terms of computational time in comparison to
the reference method.

###Dopant-free molecular hole transport material that mediates a 20% power conversion efficiency in a perovskite solar cell||Yang Cao,Yunlong Li,Thomas Morrissey,Brian Lam,Brian O. Patrick,David J. Dvorak,Zhicheng Xia,Timothy L. Kelly,Curtis P. Berlinguette###

Dopant-free molecular hole transport material that mediates a 20% power conversion efficiency in a perovskite solar cell. Organic molecular hole-transport materials (HTMs) are appealing for the
scalable manufacture of perovskite solar cells (PSCs) because they are easier
to reproducibly prepare in high purity than polymeric and inorganic HTMs. There
is also a need to construct PSCs without dopants and additives to avoid
formidable engineering and stability issues. We report here a power conversion
efficiency (PCE) of 20.6% with a molecular HTM in an inverted (p-i-n) PSC
without any dopants or interlayers. This new benchmark was made possible by the
discovery that annealing a spiro-based dopant-free HTM (denoted DFH) containing
redox-active triphenyl amine (TPA) units undergoes preferential molecular
organization normal to the substrate. This structural order, governed by the
strong intermolecular interactions of the DFH dioxane groups, affords high
intrinsic hole mobility (1x10-3 cm2 V-1 s-1). Annealing films of DFH also
enables the growth of large perovskite grains (up to 2 um) that minimize charge
recombination in the PSC. DFH can also be isolated at a fraction of the cost of
any other organic HTM.

###Charge transport layer dependent electronic band bending in perovskite solar cells and its correlation to device degradation||Junseop Byeon,Jutae Kim,Ji-Young Kim,Gunhee Lee,Kijoon Bang,Namyoung Ahn,Mansoo Choi###

Charge transport layer dependent electronic band bending in perovskite solar cells and its correlation to device degradation. Perovskite solar cells (PSCs) have shown remarkably improved power-conversion
efficiency of around 25%. However, their working principle remains arguable and
the stability issue has not been solved yet. In this report, we revealed that
the working mechanism of PSCs is explained by a dominant pn junction occurring
at the different interface depending on electron transport layer, and charges
are accumulated at the corresponding dominant junction initiating device
degradation. Locations of a dominant pn junction, the electric field, and
carrier-density distribution with respect to electron-transport layers in the
PCS devices were investigated by using the electron-beam-induced current
measurement and Kelvin probe force microscopy. The amount of accumulated
charges in the devices was analyzed using the charge-extraction method and the
degradation process of devices was confirmed by SEM measurements. From these
observations, we identified that the dominant pn junction appears at the
interface where the degree of band bending is higher compared to the other
interface, and charges are accumulated at the corresponding junction where the
device degradation is initiated, which suggests that there exists a strong
correlation between PSC working principle and device degradation. We highlight
that an ideal pin PSC that can minimize the degree of band bending should be
designed for ensuring long-term stability, via using proper selective contacts

###Structural defects in MBE-grown CdTe-based heterojunctions for photovoltaic applications||Karolina Wichrowska,Tadeusz Wosinski,Jaroslaw Z. Domagala,Slawomir Kret,Sergij Chusnutdinow,Grzegorz Karczewski###

Structural defects in MBE-grown CdTe-based heterojunctions for photovoltaic applications. Structural defects in the p-ZnTe/i-CdTe/n-CdTe single-crystalline
heterojunctions designed for photovoltaic applications have been investigated
by transmission electron microscopy (TEM) and deep-level transient spectroscopy
(DLTS). Lattice parameters and misfit strain in the undoped CdTe absorber
layers of the heterojunctions, grown by the molecular-beam epitaxy technique on
two different substrates, GaAs and CdTe, have been determined with
high-resolution X-ray diffractometry. A dense network of misfit dislocations at
the lattice-mismatched CdTe/GaAs and ZnTe/CdTe interfaces and numerous
threading dislocations and stacking faults have been shown by the
cross-sectional TEM imaging of the heterojunctions. The DLTS measurements
revealed five deep-level traps in the heterojunctions grown on the GaAs
substrates and only three of them in the heterojunctions grown on CdTe. One of
the traps, showing the exponential capture kinetics of charge carriers, has
been identified as associated with the double acceptor level of Cd vacancies in
the CdTe absorber layers. All the other traps have been attributed to the
electronic states of extended defects, presumably dislocations, on the grounds
of their logarithmic capture kinetics. Two of these traps, displaying the
largest values of their capture cross-section and the properties characteristic
of bandlike electronic states, have been ascribed to the core states of
dislocations. It is argued that they are most likely responsible for decreased
lifetime of photo-excited carriers resulting in a low energy conversion
efficiency of solar cells based on similarly grown heterojunctions.

###Computational Study of Defect variant Perovskites A2BX6 for Photovoltaic Applications||M. Faizan,K. C. Bhamu,S. H. Khan,G. Murtaza,Xin He###

Computational Study of Defect variant Perovskites A2BX6 for Photovoltaic Applications. A comprehensive study of the structural, electronic, and optical properties
of lead-free perovskites has been carried out by means of first principles
method based on DFT. The calculations are performed for the compound of the
type A2BX6 with A=Rb, and Cs; B=Sn, Pd, and Pt; and X=Cl, Br, and I. The
calculated structural parameters (lattice constants and bond lengths) agree
well with the experiments. The computed band gap reveals a semiconducting
profile for all these compounds showing a decreasing trend of the band gap
energy by changing the halide ions consecutively from Cl to Br and Br to I.
However, for variation in the B-site cation, the band gap increases by changing
the cation from Pd to Pt via Sn. The most likely compounds, Rb2PdBr6 and
Cs2PtI6, exhibit a band gap within the optimal range of 0.9-1.6 eV for
single-junction photovoltaic applications. The optical properties in terms of
the optimal value of the dielectric constant, optical conductivity, and
absorption coefficient are also investigated upto the photon energy of 10 eV.
Our results indicate that upon changing the halogen ions (Cl by Br and Br by I)
the optical properties altered significantly. Maximum dielectric constants and
high optical absorption are found for Rb2PdI6 and Cs2PtI6. The unique
optoelectronic properties such as ideal band gap, high dielectric constants,
and optimum absorption of A2BX6 perovskites could be efficiently utilized in
designing high performance single and multi-junction perovskite solar cells.

###Deducing the key physical properties of a perovskite solar cell from its impedance response: insights from drift-diffusion modelling||Antonio Riquelme,Laurence J. Bennett,Nicola E. Courtier,Matthew J. Wolf,Lidia Contreras-Bernal,Alison Walker,Giles Richardson,Juan A. Anta###

Deducing the key physical properties of a perovskite solar cell from its impedance response: insights from drift-diffusion modelling. Interpreting the impedance response of perovskite solar cells (PSC) is
significantly more challenging than for most other photovoltaics. This is for a
variety of reasons, of which the most significant are the mixed
ionic-electronic conduction properties of metal halide perovskites and the
difficulty in fabricating stable, and reproducible, devices. Experimental
studies, conducted on a variety of PSCs, produce a variety of impedance spectra
shapes. However, they all possess common features, the most noteworthy of which
is that they have at least two signals, at high and low frequency, with
different characteristic responses to temperature, illumination and electrical
bias. It is shown, by a combination of experiment and drift-diffusion modelling
of the ion and charge carrier transport and recombination within the cell, that
these common features are well reproduced by the simulation. In addition, we
show that the high frequency response contains all the key information relating
to the steady-state performance of a PSC, i.e. it is a signature of the
recombination mechanisms and provides a measure of charge collection
efficiency. Moreover, steady-state performance is significantly affected by the
distribution of mobile ionic charge within the perovskite layer. Comparison
between the electrical properties of different devices should therefore be made
using high frequency impedance measurements performed in the steady-state
voltage regime in which the cell is expected to operate.

###Theoretical Study of Ternary CoSP Semiconductor: a Candidate for Photovoltaic Applications||Abdesalem Houari,Fares Benissad###

Theoretical Study of Ternary CoSP Semiconductor: a Candidate for Photovoltaic Applications. The electronic structure of pyrite-type cobalt phosphosulfide (CoSP) has been
studied using density-functional theory. The calculated band structure reveals
the non-magnetic semiconducting character of the compound. The electronic
structure is described through the electronic band structure and the densities
of states. A band gap of 1.14 eV has been computed within standard GGA, a value
which is enhanced using hybrid functional. It separates the upper part of the
valence band dominated by Co-3d-t2g states from the lower part of the
conduction band made exclusively of Co-3d-eg , above of which lie S-3p and P-3p
ones. The obtained values are suitable for applications in solar cells,
according to Shockley-Queisser theory of light to electric conversion
efficiency. The origin of the larger CoSP band gap, with respect to the one of
the promising FeS2 compound, is explained and the chemical bonding properties
are addressed. A comparative picture is established where several similarities
have been found, suggesting that CoSP could be for a great practical interest
in photovoltaics.

###A compact flat solar still with high performance||Guilong Peng,Swellam W. Sharshir,Rencai Ji,Zhixiang Hu,Jianqiang Ma,A. E. Kabeel,Huan Liu,Jianfeng Zang,Nuo Yang###

A compact flat solar still with high performance. Solar still is a convenient off-grid device for desalination, which can
provide fresh water for families, ships, islands and so on. The conventional
inclined solar still (ISS) suffers from low efficiency and low productivity. To
improve the performance of solar still, a flat solar still (FSS) is proposed,
which has a working principle similar to the solar cell. The condensate water
in FSS is collected by the capillary grid attached under the ultra-hydrophilic
glass cover, instead of by gravity. Therefore, FSS avoids the inclined
structure and is much more compact than ISS. The daily productivity of FSS
reaches up to 4.3 kg/m2. Theoretical analysis shows that the enhanced mass
transfer in FSS by the compact structure is an important factor for high
performance. More interestingly, FSS can also be easily extended to more stage
for latent heat recovery. The results show that the daily productivity of a
double-stage FSS reaches up to 7 kg/m2, which is much higher than the
conventional solar still. FSS paves a new way in designing and optimizing of
solar still.

###Gryffin: An algorithm for Bayesian optimization of categorical variables informed by expert knowledge||Florian Häse,Matteo Aldeghi,Riley J. Hickman,Loïc M. Roch,Alán Aspuru-Guzik###

Gryffin: An algorithm for Bayesian optimization of categorical variables informed by expert knowledge. Designing functional molecules and advanced materials requires complex design
choices: tuning continuous process parameters such as temperatures or flow
rates, while simultaneously selecting catalysts or solvents. To date, the
development of data-driven experiment planning strategies for autonomous
experimentation has largely focused on continuous process parameters despite
the urge to devise efficient strategies for the selection of categorical
variables. Here, we introduce Gryffin, a general purpose optimization framework
for the autonomous selection of categorical variables driven by expert
knowledge. Gryffin augments Bayesian optimization based on kernel density
estimation with smooth approximations to categorical distributions. Leveraging
domain knowledge in the form of physicochemical descriptors, Gryffin can
significantly accelerate the search for promising molecules and materials.
Gryffin can further highlight relevant correlations between the provided
descriptors to inspire physical insights and foster scientific intuition. In
addition to comprehensive benchmarks, we demonstrate the capabilities and
performance of Gryffin on three examples in materials science and chemistry:
(i) the discovery of non-fullerene acceptors for organic solar cells, (ii) the
design of hybrid organic-inorganic perovskites for light harvesting, and (iii)
the identification of ligands and process parameters for Suzuki-Miyaura
reactions. Our results suggest that Gryffin, in its simplest form, is
competitive with state-of-the-art categorical optimization algorithms. However,
when leveraging domain knowledge provided via descriptors, Gryffin outperforms
other approaches while simultaneously refining this domain knowledge to promote
scientific understanding.

###Large (bi)polarons for novel energy conversion and superconductivity||David Emin###

Large (bi)polarons for novel energy conversion and superconductivity. Materials containing high densities of exceptionally displaceable ions (e.g.
perovskites) have extremely large ratios of their static to high-frequency
dielectric constants, > 2. Large polarons form in such materials as their
electronic charge carriers self-trap by displacing surrounding ions. Large
polarons are very heavy-massed slow-moving quasi-particles that are very weakly
scattered by ambient phonons. Large-polaron mobilities, e.g. 1 cm2/V-sec at 300
K, are much smaller than the minimum possible for conventional electronic
charge carriers. The minimum mobility for an itinerant charge carrier of
effective mass m, eh/mkT, occurs when its mean-free-path falls to its de
Broglie wavelength, e.g. 300 cm2/V-sec at room temperature for m equaling the
free-electron mass. Distinctively, large-polarons frequency-dependent
conductivities consist of two contributions that separate as the temperature is
reduced. Large polarons Drude-like contributions are relegated to frequencies
below those of characteristic phonons. Contributions from excitations of large
polarons self-trapped electronic carriers occur above those of characteristic
phonons. Oppositely charged large polarons repel one another at short range.
The resulting suppressed recombination facilitates exceptionally efficient
solar cells. Large polarons of the same charge attract one another at short
range to enable their real-space pairing into singlet bipolarons. Additional
attractions between large bipolarons facilitates their condensation into
liquids that can exhibit superconductivity.

###A Universal Urbach Rule for Disordered Organic Semiconductors||Christina Kaiser,Oskar J. Sandberg,Nasim Zarrabi,Wei Li,Paul Meredith,Ardalan Armin###

A Universal Urbach Rule for Disordered Organic Semiconductors. In crystalline semiconductors, absorption onset sharpness is characterized by
temperature dependent Urbach energies. These energies quantify the static,
structural disorder causing localized exponential-tail states, and dynamic
disorder from electron-phonon scattering. Applicability of this
exponential-tail model to disordered solids has been long debated. Nonetheless,
exponential fittings are routinely applied to sub-gap absorption analysis of
organic semiconductors. Herein, we elucidate the sub-gap spectral line-shapes
of organic semiconductors and their blends by temperature-dependent quantum
efficiency measurements. We find that sub-gap absorption due to singlet
excitons is universally dominated by thermal broadening at low photon energies
and the associated Urbach energy equals the thermal energy, regardless of
static disorder. This is consistent with absorptions obtained from a
convolution of Gaussian density of excitonic states weighted by Boltzmann-like
thermally activated optical transitions. A simple model is presented that
explains absorption line-shapes of disordered systems, and we also provide a
strategy to determine the excitonic disorder energy. Our findings elaborate the
meaning of the Urbach energy in molecular solids and relate the photo-physics
to static disorder, crucial for optimizing organic solar cells for which we
present a new radiative open-circuit voltage limit.

###Hybrid design of spectral splitters and concentrators of light for solar cells using iterative search and neural networks||Alim Yolalmaz,Emre Yüce###

Hybrid design of spectral splitters and concentrators of light for solar cells using iterative search and neural networks. The need for optically multi-functional micro- and nano-structures is growing
in various fields. Designing such structures is impeded by the lack of
computationally low-cost algorithms. In this study, we present a hybrid design
scheme, which relies on a deep learning model and the local search optimization
algorithm, to optimize a diffractive optical element that performs spectral
splitting and spatial concentration of broadband light for solar cells. Using
generated data set during optimization of a diffractive optical element, which
is a one-time effort, we design topography of diffractive optical elements by
using a deep learning-based inverse design scheme. We show that further
iterative optimization of the reconstructed diffractive optical elements
increases amount of spatially concentrated and spectrally split light. Our
joint design approach both speeds up optimization of diffractive optical
elements as well as providing better performance at least 57% excess light
concentration with spectral splitting. The algorithm that we develop here will
enable advanced and efficient design of multi-functional phase plates in
various fields besides the application that we target in solar energy. The
algorithm that we develop is openly available to contribute to other
applications that rely on phase plates.

###Hot-carrier optoelectronic devices based on semiconductor nanowires||Jonatan Fast,Urs Aeberhard,Stephen P. Bremner,Heiner Linke###

Hot-carrier optoelectronic devices based on semiconductor nanowires. In optoelectronic devices such as solar cells and photodetectors, a portion
of electron-hole pairs are generated as so called hot carriers with an excess
energy that is typically lost as heat. The long standing aim to harvest this
excess energy to enhance device performance has proven to be very challenging,
largely due to the extremely short-lived nature of hot carriers. Efforts thus
focus on increasing the hot carrier relaxation time, and on tailoring
heterostructures that allow for hot-carrier extraction on short time- and
length-scales. Recently, semiconductor nanowires have emerged as a promising
system to achieve these aims, because they offer unique opportunities for
heterostructure engineering as well as for potentially modified phononic
properties that can lead to increased relaxation times. In this review we
assess the current state of theory and experiments relating to hot-carrier
dynamics in nanowires, with a focus on hot-carrier photovoltaics. To provide a
foundation, we begin with a brief overview of the fundamental processes
involved in hot-carrier relaxation, and how these can be tailored and
characterized in nanowires. We then analyze the advantages offered by nanowires
as a system for hot-carrier devices and review the status of proof-of-principle
experiments related to hot-carrier photovoltaics. To help interpret existing
experiments on photocurrent extraction in nanowires we provide modelling based
on non-equilibrium Green's functions. Finally, we identify open research
questions that need to be answered in order to fully evaluate the potential
nanowires offer towards achieving more efficient, hot-carrier based,
optoelectronic devices.

###Accurate, rapid identification of dislocation lines in coherent diffractive imaging via a min-max optimization formulation||A. Ulvestad,M. Menickelly,S. M. Wild###

Accurate, rapid identification of dislocation lines in coherent diffractive imaging via a min-max optimization formulation. Defects such as dislocations impact materials properties and their response
during external stimuli. Defect engineering has emerged as a possible route to
improving the performance of materials over a wide range of applications,
including batteries, solar cells, and semiconductors. Imaging these defects in
their native operating conditions to establish the structure-function
relationship and, ultimately, to improve performance has remained a
considerable challenge for both electron-based and x-ray-based imaging
techniques. However, the advent of Bragg coherent x-ray diffractive imaging
(BCDI) has made possible the 3D imaging of multiple dislocations in
nanoparticles ranging in size from 100 nm to1000 nm. While the imaging process
succeeds in many cases, nuances in identifying the dislocations has left manual
identification as the preferred method. Derivative-based methods are also used,
but they can be inaccurate and are computationally inefficient. Here we
demonstrate a derivative-free method that is both more accurate and more
computationally efficient than either derivative- or human-based methods for
identifying 3D dislocation lines in nanocrystal images produced by BCDI. We
formulate the problem as a min-max optimization problem and show exceptional
accuracy for experimental images. We demonstrate a 260x speedup for a typical
experimental dataset with higher accuracy over current methods. We discuss the
possibility of using this algorithm as part of a sparsity-based phase retrieval
process. We also provide the MATLAB code for use by other researchers.

###Optical properties of spin coated and sol-gel dip coated cupric oxide thin films||P. Samarasekara,N. G. K. V. M. Premasiri###

Optical properties of spin coated and sol-gel dip coated cupric oxide thin films. Spin coating technique was employed to fabricate CuO films at different spin
speeds for different time duration, and they were annealed at different
temperatures for different time durations in air. These thin films were
characterized using UV Visible spectrometer and solar cell simulator. As the
spinning rate increases, the corresponding band gap energy of the thin film
increases. However, the annealing temperature does not affect the optical band
gap. Moreover, the optical band gap does not depend on the annealing time and
spin time. It was observed that the optical band gap decreases with the number
of CuO layers. The optical band gap values of CuO thin films were between 1.843
and 1.869 eV. Inclusion of the two additives enhances the photocurrent,
photovoltage and efficiency. Photocurrent density can be enhanced from 0.1464
to 1.172 mA per cm2 using the additive ethylene glycol. Properties of CuO films
synthesized using sol gel dip coating technique were compared with those of CuO
films grown using spin coating method.

###Ideal Bandgap in a 2D Ruddlesden-Popper Perovskite Chalcogenide for Single-junction Solar Cells||Shanyuan Niu,Debarghya Sarkar,Kristopher Williams,Yucheng Zhou,Yuwei Li,Elisabeth Bianco,Huaixun Huyan,Stephen B. Cronin,Michael E. McConney,Ralf Haiges,R. Jaramillo,David J. Singh,William A. Tisdale,Rehan Kapadia,Jayakanth Ravichandran###

Ideal Bandgap in a 2D Ruddlesden-Popper Perovskite Chalcogenide for Single-junction Solar Cells. Transition metal perovskite chalcogenides (TMPCs) are explored as stable,
environmentally friendly semiconductors for solar energy conversion. They can
be viewed as the inorganic alternatives to hybrid halide perovskites, and
chalcogenide counterparts of perovskite oxides with desirable optoelectronic
properties in the visible and infrared part of the electromagnetic spectrum.
Past theoretical studies have predicted large absorption coefficient, desirable
defect characteristics, and bulk photovoltaic effect in TMPCs. Despite recent
progresses in polycrystalline synthesis and measurements of their optical
properties, it is necessary to grow these materials in high crystalline quality
to develop a fundamental understanding of their optical properties and evaluate
their suitability for photovoltaic application. Here, we report the growth of
single crystals of a two-dimensional (2D) perovskite chalcogenide, Ba3Zr2S7,
with a natural superlattice-like structure of alternating double-layer
perovskite blocks and single-layer rock salt structure. The material
demonstrated a bright photoluminescence peak at 1.28 eV with a large external
luminescence efficiency of up to 0.15%. We performed time-resolved
photoluminescence spectroscopy on these crystals and obtained an effective
recombination time of ~65 ns. These results clearly show that 2D
Ruddlesden-Popper phases of perovskite chalcogenides are promising materials to
achieve single-junction solar cells.

###Room Temperature Quantum Coherence vs. Electron Transfer in a Rhodanine Derivative Chromophore||Duvalier Madrid-Úsuga,Cristian E. Susa,John H. Reina###

Room Temperature Quantum Coherence vs. Electron Transfer in a Rhodanine Derivative Chromophore. Understanding electron transfer in organic molecules is of great interest in
quantum materials for light harvesting, energy conversion, and integration of
molecules into solar cells. This, however, poses the challenge of designing
specific optimal molecular structure for which the processes of ultrafast
quantum coherence and electron transport are not so well understood. In this
work, we investigate subpicosecond time scale quantum dynamics and electron
transfer in an efficient electron acceptor Rhodanine chromophoric complex. We
consider an open quantum system approach to model the complex-solvent
interaction and compute the crossover from weak to strong dissipation on the
reduced system dynamics for both a polar (Methanol) and a non-polar solvent
(Toluene). We show that the electron transfer rates are enhanced in the strong
chromophore-solvent coupling regime, being the highest transfer rates those
found at room temperature. Even though the computed dynamics are highly
non-Markovian, and they may exhibit a quantum character up to hundreds of
femtoseconds, we show that quantum coherence does not necessarily optimize the
electron transfer in the chromophore.

###Proposed high-power beta cells from MgAlB14-type icosahedral-boron semiconductors||David Emin###

Proposed high-power beta cells from MgAlB14-type icosahedral-boron semiconductors. Beta cells generate electric power as carrier-producing beta irradiation from
incorporated radioisotopes bombard a series of p-n-junctions. However,
radiation damage to the semiconductors commonly used in solar cells limits beta
cells to extremely weak irradiations that generate concomitantly miniscule
electric powers, e.g. micro-Watts. By contrast, beta cells that generate many
orders-of-magnitude larger powers are possible with icosahedral boron-rich
semiconductors since their bombardment-induced atomic displacements
spontaneously self-heal. Furthermore, substitutions for Mg and Al atoms of
icosahedral-boron-rich semiconductors based on the MgAlB14 structure can
produce p-n junctions as electron transfers from doping-induced interstitial
extra-icosahedral atoms convert some normally p-type materials to n-type.
Moreover, electron-phonon interactions of the resulting readily displaceable
interstitial cations with charge carriers foster their forming large polarons.
Oppositely charged polarons repel one another at short range. These repulsions
suppress the recombination of n-type with p-type polarons thereby increasing
the beta-cell efficiency. All told, use of these icosahedral boron-rich
semiconductors could enable beta cells with electric powers that are many
orders of magnitude larger than those of existing beta cells. This development
opens a new avenue for generating electricity from nuclear decays.

###First principle studies on the optoelectronic properties of rubidium lead halides||Anupriya Nyayban,Subhasis Panda,Avijit Chowdhury,B. Indrajit Sharma###

First principle studies on the optoelectronic properties of rubidium lead halides. Entirely inorganic perovskites have attracted enormous attention of late
owing to their outstanding applications in optoelectronics including highly
stable perovskite solar cells. In-depth understanding of the optoelectronic and
transport properties of such materials are vital for practical implementation
of the same. The carrier transport properties of the electronic devices based
on perovskite materials significantly depend on the effective mass of the
respective charge carriers. Here, we have performed first principle
calculations with FP-LAPW method for the orthorhombic rubidium lead halide
structures (\ch{RbPbX_3}, where \ch{X=I,Br,Cl}) to study the optoelectronic and
transport properties. The effective mass of electron (hole) is found to be
minimum for \ch{RbPbBr_3} (\ch{RbPbI_3}), suggesting an efficient transport of
electrons (holes) in the corresponding materials. Our calculated values such as
the dielectric constants, refractive indices, absorption coefficients and
reflectivities show good agreement with reported experimental data. To the best
of our knowledge, ab-initio study of electronic and optical properties of
\ch{RbPbBr_3} \& \ch{RbPbCl_3} in orthorhombic phase (\ch{NH_4CdCl_3} type
structure) is reported for the first time.

###Ideal near-Dirac triple-point semimetal in III-V semiconductor alloys||Zhenyao Fang,Heng Gao,Jörn W. F. Venderbos,Andrew M. Rappe###

Ideal near-Dirac triple-point semimetal in III-V semiconductor alloys. Despite the growing interest in topological materials, the difficulty of
experimentally synthesizing and integrating them with other materials has been
one of the main barriers restricting access to their unique properties. Recent
advances in synthesizing metastable phases of crystalline materials can help to
overcome this barrier and offer new platforms to experimentally study and
manipulate band topology. Because III-V semiconductors have a wide range of
functional material applications (including optoelectronic devices,
light-emitting diodes, and highly efficient solar cells), and because Bi-doped
III-V materials can be synthesized by ion plantation and ion-cutoff methods, we
revisit the effect of bismuth substitution in metastable III-V semiconductors.
Through first-principles calculation methods, we show that in wurtzite
structure III-V materials, Bi substitution can lead to band inversion phenomena
and induce nontrivial topological properties. Specifically, we identify that
GaBi and InBi are Dirac-Weyl semimetals, characterized by the coexistence of
Dirac points and Weyl points, and $\text{GaAs}_{0.5} \text{Bi}_{0.5}$,
$\text{GaSb}_{0.5} \text{Bi}_{0.5}$, $\text{InSb}_{0.5} \text{Bi}_{0.5}$ are
triple-point semimetals, characterized by two sets of "near Dirac" triple
points on the Fermi level. These experimentally-accessible bismuth-based
topological semimetals can be integrated into the large family of functional
III-V materials for experimental studies of heterostructures and future
optoelectronic applications.

###Self-formed 2D/3D Heterostructure on the Edge of 2D Ruddlesden-Popper Hybrid Perovskites Responsible for Intriguing Optoelectronic Properties and Higher Cell Efficiency||Zhaojun Qin,Shenyu Dai,Chalapathi Charan Gajjala,Chong Wang,Viktor G. Hadjiev,Guang Yang,Jiabing Li,Xin Zhong,Zhongjia Tang,Yan Yao,Arnold M. Guloy,Rohith Reddy,David Mayerich,Liangzi Deng,Qingkai Yu,Guoying Feng,Zhiming Wang,Jiming Bao###

Self-formed 2D/3D Heterostructure on the Edge of 2D Ruddlesden-Popper Hybrid Perovskites Responsible for Intriguing Optoelectronic Properties and Higher Cell Efficiency. The observation of low energy edge photoluminescence and its beneficial
effect on the solar cell efficiency of Ruddlesden-Popper perovskites has
unleashed an intensive research effort to reveal its origin. This effort,
however, has been met with more challenges as the underlying material structure
has still not been identified; new modellings and observations also do not seem
to converge. Using 2D (BA)2(MA)2Pb3Br10 as an example, we show that 3D MAPbBr3
is formed due to the loss of BA on the edge. This self-formed MAPbBr3 can
explain the reported edge emission under various conditions, while the reported
intriguing optoelectronic properties such as fast exciton trapping from the
interior 2D perovskite, rapid exciton dissociation and long carrier lifetime
can be understood via the self-formed 2D/3D lateral perovskite heterostructure.
The 3D perovskite is identified by submicron infrared spectroscopy, the
emergence of XRD signature from freezer-milled nanometer-sized 2D perovskite
and its photoluminescence response to external hydrostatic pressure. The
revelation of this edge emission mystery and the identification of a
self-formed 2D/3D heterostructure provide a new approach to engineering 2D
perovskites for high-performance optoelectronic devices.

###Determining the three-dimensional atomic structure of a metallic glass||Yao Yang,Jihan Zhou,Fan Zhu,Yakun Yuan,Dillan Chang,Dennis S. Kim,Minh Pham,Arjun Rana,Xuezeng Tian,Yonggang Yao,Stanley Osher,Andreas K. Schmid,Liangbing Hu,Peter Ercius,Jianwei Miao###

Determining the three-dimensional atomic structure of a metallic glass. Amorphous solids such as glass are ubiquitous in our daily life and have
found broad applications ranging from window glass and solar cells to
telecommunications and transformer cores. However, due to the lack of
long-range order, the three-dimensional (3D) atomic structure of amorphous
solids have thus far defied any direct experimental determination without model
fitting. Here, using a multi-component metallic glass as a proof-of-principle,
we advance atomic electron tomography to determine the 3D atomic positions in
an amorphous solid for the first time. We quantitatively characterize the
short-range order (SRO) and medium-range order (MRO) of the 3D atomic
arrangement. We find that although the 3D atomic packing of the SRO is
geometrically disordered, some SRO connect with each other to form crystal-like
networks and give rise to MRO. We identify four crystal-like MRO networks -
face-centred cubic, hexagonal close-packed, body-centered cubic and simple
cubic - coexisting in the sample, which show translational but no orientational
order. These observations confirm that the 3D atomic structure in some parts of
the sample is consistent with the efficient cluster packing model. Looking
forward, we anticipate this experiment will open the door to determining the 3D
atomic coordinates of various amorphous solids, whose impact on non-crystalline
solids may be comparable to the first 3D crystal structure solved by x-ray
crystallography over a century ago.

###Temperature dependence of the optical properties of silicon nanocrystals||Marios Zacharias,Pantelis C. Kelires###

Temperature dependence of the optical properties of silicon nanocrystals. Silicon nanocrystals (SiNCs) have been under active investigation in the last
decades and have been considered as a promising candidate for many
optoelectronic applications including highly-efficient solar cells. Some of the
fundamental properties of interest in these nanostructures is the temperature
dependence of their optical absorption onset, and how this is controlled by
different passivation regimes. In the present work we employ first-principles
calculations in conjunction with the special displacement method to study the
temperature dependence of the band gap renormalization of free-standing
hydrogen-terminated, and oxidized SiNCs, as well as matrix-embedded SiNCs in
amorphous silica, and we obtain good agreement with experimental
photoluminescence data. We also provide strong evidence that the
electron-phonon interplay at the surface of the nanocrystal is suppressed by
oxidation and the surrounding amorphous matrix. For the matrix-embedded SiNCs,
we show a high correlation between the temperature dependence of the band gap
and the Si-Si strained bonds. This result emphasizes the immanent relationship
of electron-phonon coupling and thermal structural distortions. We also
demonstrate that, apart from quantum confinement, Si- Si strained bonds are the
major cause of zero-phonon quasidirect transitions in matrix-embedded SiNCs. As
a final point, we clarify that, unlike optical absorption in bulk Si,
phonon-assisted electronic transitions play a secondary role in SiNCs.

###Energy Level Alignment in Ternary Organic Solar Cells||Vincent Lami,Yvonne J. Hofstetter,Julian F. Butscher,Yana Vaynzof###

Energy Level Alignment in Ternary Organic Solar Cells. Ternary organic solar cells (TOSC) are currently under intensive
investigation, recently reaching a record efficiency of 17.1%. The origin of
the device open-circuit voltage (VOC), already a multifaceted issue in binary
OSC, is even more complex in TOSCs. Herein, we investigate two ternary systems
with one donor (D) and two acceptor materials (A1, A2) including fullerene and
non-fullerene acceptors. By varying the ratio between the two acceptors, we
find the VOC to be gradually tuned between those of the two binary systems,
D:A1 and D:A2. To investigate the origin of this change, we employ ultra-violet
photoemission spectroscopy (UPS) depth profiling, which is used to estimate the
photovoltaic gap in the ternary systems. Our results reveal an excellent
agreement between the estimated photovoltaic gap and the VOC for all mixing
ratios, suggesting that the energetic alignment between the blend components
varies depending on the ratio D:A1:A2. Furthermore, our results indicate that
the sum of radiative and non-radiative losses in these ternary systems is
independent of the blend composition. Finally, we demonstrate the superiority
of UPS over X-ray photoemission spectroscopy (XPS) depth profiling in resolving
compositional profiles for material combinations with very similar chemical,
but dissimilar electronic structures, as common in TOSCs.

###Texture Formation in Polycrystalline Thin Films of All-Inorganic Lead Halide Perovskite||Julian A. Steele,Eduardo Solano,Handong Jin,Vittal Prakasam,Tom Braeckevelt,Haifeng Yuan,Zhenni Lin,René de Kloe,Qiong Wang,Sven M. J. Rogge,Veronique Van Speybroeck,Dmitry Chernyshov,Johan Hofkens,Maarten B. J. Roeffaers###

Texture Formation in Polycrystalline Thin Films of All-Inorganic Lead Halide Perovskite. Controlling grain orientations within polycrystalline all-inorganic halide
perovskite solar cells can help increase conversion efficiencies toward their
thermodynamic limits, however the forces governing texture formation are
ambiguous. Using synchrotron X-ray diffraction, we report meso-structure
formation within polycrystalline CsPbI2.85Br0.15 powders as they cool from a
high-temperature cubic perovskite ({\alpha}-phase). Tetragonal distortions
(\b{eta}-phase) trigger preferential crystallographic alignment within
polycrystalline ensembles, a feature we suggest is coordinated across multiple
neighboring grains via interfacial forces that select for certain lattice
distortions over others. External anisotropy is then imposed on polycrystalline
thin films of orthorhombic ({\gamma}-phase) CsPbI3-xBrx perovskite via
substrate clamping, revealing two fundamental uniaxial texture formations; (i)
I-rich films possess orthorhombic-like texture (<100> out-of-plane; <010> and
<001> in-plane), while (ii) Br-rich films form tetragonal-like texture (<110>
out-of-plane; <1-10> and <001> in-plane). In contrast to relatively
uninfluential factors like the choice of substrate, film thickness and
annealing temperature, Br incorporation modifies the {\gamma}-CsPbI3-xBrx
crystal structure by reducing the orthorhombic lattice distortion (making it
more tetragonal-like) and governs the formation of the different, energetically
favored textures within polycrystalline thin films.

###Designing intramolecular singlet-fission materials using indeno[1,2-b]fluorene dimers: A DMRG and TDDFT study||Sumit Naskar,Mousumi Das###

Designing intramolecular singlet-fission materials using indeno[1,2-b]fluorene dimers: A DMRG and TDDFT study. Low-lying excited states for indeno[1,2-b]fluorene homo dimers with or
without benzene spacers are calculated using the Density Matrix Renormalization
group (DMRG) approach within Pariser-Parr-Pople (PPP) model Hamiltonian. DMRG
calculations suggest that all the dimers studied here satisfy the essential
energy conditions for SF. SF is a multiexciton generation process. As it is
spin allowed, the process is very fast. By generating multiple exciton at a
time SF underestimate SQ limit to enhance photo-conversion efficiency of single
junction solar cells. Frontier orbital calculation through Density Functional
Theory (DFT) depicts orbital localization of triplets on either side of the
covalent spacers. Which supports the entangled triplet-triplet state $^1(TT)$.
Here the process is intramolecular (iSF), which has many advantages over the
intermolecular (xSF) process, as in intermolecular process the SF process is
highly dependent on the crystal packing, defects, dislocations etc. The
entangled $^1(TT)$ state for xSF is localized on both of the chromophores, thus
the appropriate crystal packing is essential for xSF. However iSF does not
depend on the crystal packing. Our DMRG calculation and TDDFT calculation are
in well agreement with experimental results found in the literature. Thus
indeno[1,2-b]fluorene homo dimers can be applicable in iSF application.

###Implementing strong interference in ultrathin film top absorbers for tandem solar cells||Yifat Piekner,Hen Dotan,Anton Tsyganok,Kirtiman Deo Malviya,Daniel A. Grave,Ofer Kfir,Avner Rothschild###

Implementing strong interference in ultrathin film top absorbers for tandem solar cells. Strong interference in ultrathin film semiconductor absorbers on metallic
back reflectors has been shown to enhance the light harvesting efficiency of
solar cell materials. However, metallic back reflectors are not suitable for
tandem cell configurations because photons cannot be transmitted through the
device. Here, we introduce a method to implement strong interference in
ultrathin film top absorbers in a tandem cell configuration through use of
distributed Bragg reflectors (DBRs). We showcase this by designing and
fabricating a photoelectrochemical-photovoltaic (PEC-PV) stacked tandem cell in
a V-shaped configuration where short wavelength photons are reflected back to
the photoanode material (hematite, Fe2O3), whereas long wavelength photons are
transmitted to the bottom silicon PV cell. We employ optical simulations to
determine the optimal thicknesses of the DBR layers and the V-shape angle to
maximize light absorption in the ultrathin (10 nm thick) hematite film. The DBR
spectral response can be tailored to allow for a more than threefold
enhancement in absorbed photons compared to a layer of the same thickness on
transparent current collectors. Using a DBR to couple a bottom silicon PV cell
with an ultrathin hematite top PEC cell, we demonstrate unassisted solar water
splitting and show that DBRs can be designed to enhance strong interference in
ultrathin films while enabling stacked tandem cell configuration.

###Metal halide perovskite toxicity effects on plants are caused by iodide ions||Eline M. Hutter,Reiny Sangster,Christa Testerink,Bruno Ehrler,Charlotte M. M. Gommers###

Metal halide perovskite toxicity effects on plants are caused by iodide ions. Highly-efficient solar cells containing lead halide perovskites are expected
to revolutionize sustainable energy production in the coming years. Combining
these next-generation solar panels with agriculture, can optimize land-use, but
brings new risks in case of leakage into the soil. Perovskites are generally
assumed to be toxic because of the lead (Pb), but experimental evidence to
support this prediction is scarce. We used Arabidopsis thaliana to test the
toxicity of the lead-based perovskite MAPbI3 (MA = CH3NH3) and several of its
precursors in plants. Our results show that MAPbI3 severely hampers plant
growth at concentrations above 5 microM. Surprisingly, we find that the
precursors MAI is equally toxic, while lead-based precursors without iodide are
only toxic above 500 microM. These observations reveal that perovskite toxicity
at low concentrations is caused by iodide ions specifically, and contrast the
widespread idea that lead is the most harmful component. We calculate that
iodide toxicity thresholds are likely to reach in the soil upon perovskite
leakage, but much less so for lead toxicity thresholds. Hence, this work
stresses the importance to further understand and predict harmful effects of
iodide-containing perovskites in the environment.

###Ab-initio investigations for Structural, Mechanical, Optoelectronic, and Thermoelectric properties of Ba2SbXO6 (X=Nb, Ta) compounds||Hansraj,K. C. Bhamu,Sung Gu Kang,A. K. Kushwaha,D. P. Rai,Subrahmanyam Sappati,J. Sahariya,Amit Soni###

Ab-initio investigations for Structural, Mechanical, Optoelectronic, and Thermoelectric properties of Ba2SbXO6 (X=Nb, Ta) compounds. We report the structural, mechanical, electronic, optical, thermoelectric
properties and spectroscopic limited maximum efficiency (SLME) of oxide double
perovskite structure Ba2SbNbO6 and Ba2SbTaO6 compounds. All the investigations
were performed through the first-principles density functional theory (DFT).
The obtained values for the elastic constants reveal the mechanical stability
of the studied compounds. The calculated data of bulk modulus (B), shear
modulus (G), and Young's modulus (E) for Ba2SbTaO6 are found to be greater than
those of Ba2SbNbO6. The ratio of Bulk to shear ratio (B/G) shows that Ba2SbNbO6
and Ba2SbTaO6 are ductile. The computed electronic band structure reveals the
semiconducting nature of both compounds. We have also studied the electron
relaxation time-dependent thermoelectric properties, such as Seebeck
coefficient, thermal conductivity, electrical conductivity, thermoelectric
power factor, and the figure of merit as a function of chemical potential at
various temperatures for p-type and n-type charge carriers. The high absorption
spectra and good figure of merit (ZT) reveal that both the studied compounds,
Ba2SbXO6 (X = Nb, Ta) are promising materials for photovoltaic and
thermoelectric applications. The calculated SLME of 26.8% reveals that Ba2SNbO6
is an appealing candidate for single-junction solar cells.

###Nongeminate and Geminate Recombination in PTB7:PC$_{71}$BM solar cells||A. Foertig,J. Kniepert,M. Gluecker,T. Brenner,V. Dyakonov,D. Neher,C. Deibel###

Nongeminate and Geminate Recombination in PTB7:PC$_{71}$BM solar cells. A combination of transient photovoltage (TPV), voltage dependent charge
extraction (CE) and time delayed collection field (TDCF) measurements is
applied to
poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]
[3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]]
(PTB7):[6,6]-phenyl-C71-butyric acid (PC$_{71}$BM) bulk heterojunction solar
cells to analyze the limitations of photovoltaic performance. Devices are
processed from pure chlorobenzene (CB) solution and a subset was optimized with
1,8-diiodooctane (DIO) as co-solvent. The dramatic changes in device
performance are discussed with respect to the dominating loss processes. While
in the devices processed from CB solution, severe geminate and nongeminate
recombination is observed, the use of DIO facilitates efficient polaron pair
dissociation and minimizes geminate recombination. Thus, from the determined
charge carrier decay rate under open circuit conditions and the voltage
dependent charge carrier densities $n(V)$, the nongeminate loss current
$j_{loss}$ of the samples with DIO alone enables us to reconstruct the
current/voltage ($j/V$) characteristics across the whole operational voltage
range. Geminate and nongeminate losses are considered to describe the $j/V$
response of cells prepared without additive, but lead to a clearly
overestimated device performance. We attribute the deviation between measured
and reconstructed $j/V$ characteristics to trapped charges in isolated domains
of pure fullerene phases.

###Multi-Pulse Terahertz Spectroscopy Unveils Hot Polaron Photoconductivity Dynamics in Metal-Halide Perovskites||Xijia Zheng,Thomas R. Hopper,Andrei Gorodetsky,Marios Maimaris,Weidong Xu,Bradley A. A. Martin,Jarvist M. Frost,Artem A. Bakulin###

Multi-Pulse Terahertz Spectroscopy Unveils Hot Polaron Photoconductivity Dynamics in Metal-Halide Perovskites. The behavior of hot carriers in metal-halide perovskites (MHPs) present a
valuable foundation for understanding the details of carrier-phonon coupling in
the materials as well as the prospective development of highly efficient hot
carrier and carrier multiplication solar cells. Whilst the carrier population
dynamics during cooling have been intensely studied, the evolution of the hot
carrier properties, namely the hot carrier mobility, remain largely unexplored.
To address this, we introduce a novel ultrafast visible pump - infrared push -
terahertz probe spectroscopy (PPP-THz) to monitor the real-time conductivity
dynamics of cooling carriers in methylammonium lead iodide. We find a decrease
in mobility upon optically depositing energy into the carriers, which is
typical of band-transport. Surprisingly, the conductivity recovery dynamics are
incommensurate with the intraband relaxation measured by an analogous
experiment with an infrared probe (PPP- IR), and exhibit a negligible
dependence on the density of hot carriers. These results and the kinetic
modelling reveal the importance of highly-localized lattice heating on the
mobility of the hot electronic states. This collective polaron-lattice
phenomenon may contribute to the unusual photophysics observed in MHPs and
should be accounted for in devices that utilize hot carriers.

###Recent Progress on Synthesis, Characterization, and Applications of Metal Halide Perovskites@Metal Oxide||Yanyan Duan,De-Yi Wang,Rubén D. Costa###

Recent Progress on Synthesis, Characterization, and Applications of Metal Halide Perovskites@Metal Oxide. Metal halide perovskites (MHPs) have become a promising candidate in a myriad
of applications, such as light-emitting diodes, solar cells, lasing,
photodetectors, photocatalysis, transistors, etc. This is related to the
synergy of their excellent features, including high photoluminescence quantum
yields, narrow and tunable emission, long charge carrier lifetimes, broad
absorption spectrum along with high extinction absorptions coefficients, among
others. However, the main bottleneck is the poor stability of the MHPs under
ambient conditions. This is imposing severe restrictions with respect to their
industrialized applications and commercialization. In this context, metal oxide
(MOx) coatings have recently emerged as an efficient strategy towards
overcoming the stabilities issues as well as retain the excellent properties of
the MHPs, and therefore facilitate the development of the related devices
stabilities and performances.This review provides a summary of the recent
progress on synthetic methods, enhanced features, the techniques to assess the
MHPs-MOxcomposites, and applications of the MHPs@MOx.Specially, novel
approaches to fabricate the composites and new applications of the composites
are also reported in this review for the first time. This is rounded by a
critical outlook about the current MHPs stability issues and the further
direction to ensure a bright future of MHPs@MOx

###Carrier transport and performance limit of semi-transparent photovoltaics: CuIn$_{1-x}$Ga$_x$Se$_2$ as a case study||Eymana Maria,Ajanta Saha,M. Ryyan Khan,Md. Abdullah Zubair,Md. Zunaid Baten,Redwan N. Sajjad###

Carrier transport and performance limit of semi-transparent photovoltaics: CuIn$_{1-x}$Ga$_x$Se$_2$ as a case study. Semi-transparent photovoltaic devices for building integrated applications
have the potential to provide simultaneous power generation and natural light
penetration. CuIn$_{1-x}$Ga$_x$Se$_2$ (CIGS) has been established as a mature
technology for thin-film photovoltaics, however, its potential for
Semi-Transparent Photovoltaics (STPV) is yet to be explored. In this paper, we
present its carrier transport physics explaining the trend seen in recently
published experiments. STPV requires deposition of films of only a few hundred
nanometers to make them transparent and manifests several unique properties
compared to a conventional thin-film solar cell. Our analysis shows that the
short-circuit current, Jsc is dominated by carriers generated in the depletion
region, making it nearly independent of bulk and back-surface recombination.
The bulk recombination, which limits the open-circuit voltage Voc, appears to
be higher than usual attributable to numerous grain boundaries. When the
absorber layer is reduced below 500 nm, grain size reduces resulting in more
grain boundaries and higher resistance. This produces an inverse relationship
between series resistance and absorber thickness. We also present a
thickness-dependent model of shunt resistance showing its impact in these
ultra-thin devices. For various scenarios of bulk and interface recombinations,
shunt and series resistances, AVT and composition of CuIn$_{1-x}$Ga$_x$Se$_2$,
we project the efficiency limit which - for most practical cases - is found to
be $\leq$10% for AVT $\geq$25%.

###Seeing Structural Evolution of Organic Molecular Nano-crystallites Using 4D Scanning Confocal Electron Diffraction||Mingjian Wu,Christina Harreiss,Colin Ophus,Erdmann Spiecker###

Seeing Structural Evolution of Organic Molecular Nano-crystallites Using 4D Scanning Confocal Electron Diffraction. Direct observation of organic molecular nanocrystals and their evolution
using electron microscopy is extremely challenging, due to their radiation
sensitivity and complex structure. Here, we introduce 4D-scanning confocal
electron diffraction (4D-SCED), which enables direct in situ observation of
bulk heterojunction (BHJ) thin films. 4D-SCED combines confocal electron
microscopy with a pixelated detector to record focused spot-like diffraction
patterns with high angular resolution, using an order of magnitude lower dose
than previous methods. We apply it to study an active layer in organic solar
cells, namely DRCN5T:PC$_{71}$BM BHJ thin films. Structural details of DRCN5T
nano-crystallites oriented both in- and out-of-plane are imaged at ~5 nm
resolution and dose budget of ~5 e$^-$/A$^2$. We use in situ annealing to
observe the growth of the donor crystals, evolution of the crystal orientation,
and progressive enrichment of PC$_{71}$BM at interfaces. This highly
dose-efficient method opens new possibilities for studying beam sensitive soft
materials.

###Energetics of the Charge Generation in Organic Donor-Acceptor Interfaces||Artur M. Andermann,Luis G. C. Rego###

Energetics of the Charge Generation in Organic Donor-Acceptor Interfaces. Non-fullerene acceptor (NFA) materials have posed new paradigms for the
design of organic solar cells (OSC), whereby efficient carrier generation is
obtained with small driving forces, in order to maximize the open-circuit
voltage. In this paper we use a coarse-grained mixed quantum-classical method,
that combines Ehrenfest and Redfield theories, to shed light on charge
generation process in small energy offset interfaces. We have investigated the
influence of the energetic driving force as well as the vibronic effects on the
charge generation and photovoltaic energy conversion. By analyzing the effects
of the Holstein and Peierls vibrational couplings, we find that vibrational
couplings produce an overall effect of improving the charge generation.
However, the two vibronic mechanisms play different roles: the Holstein
relaxation mechanism decreases the charge generation whereas the Peierls
mechanism always assists the charge generation. Moreover, by examining the
electron-hole binding energy as a function of time, we evince two distinct
regimes for the charge separation: the temperature independent excitonic spread
on a sub-100 fs timescale and the complete dissociation of the charge-transfer
state that occurs on the timescale of tens to hundreds of picoseconds,
depending on the temperature. The quantum dynamics of the system exhibits the
three regimes of the Marcus electron transfer kinetics as the energy offset of
the interface is varied.

###Cloaking a nanolaser||Sergey Lepeshov,Andrey Vyshnevyy,Alex Krasnok###

Cloaking a nanolaser. Light emitters are bound to strongly interact with light through enhanced
absorption and scattering, which imposes limitations on the design and
performance of photonic devices such as solar cells, nanoantennas, and (nano)
lasers. Overcoming these limitations forces the use of ineffective
nonreciprocity approaches or separation of radiation and scattering in the
frequency or time domain. A design that combines the properties of an efficient
emitter in one state and the property of being invisible in another state is
vital for various applications. In this work, we propose a nanolaser design
based on a semiconductor nanoparticle with gain coated by a phase transition
material (Sb2S3), switchable between lasing and cloaking (nonscattering) states
at the same operating frequency without change in pumping. The operation
characteristics of the nanolaser are rigorously investigated. The designed
nanolaser can operate with optical or electric pumping and possesses attributes
of a thresholdless laser due to the high beta-factor and strong Purcell
enhancement in the strongly confined Mie resonance mode. We design a
reconfigurable metasurface composed of lasing-cloaking metaatoms that can
switch from lasing to a nonscattering state in a reversible manner.

###A study of singlet fission-halide perovskite interfaces||Alan R. Bowman,Samuel D. Stranks,Bartomeu Monserrat###

A study of singlet fission-halide perovskite interfaces. A method for improving the efficiency of solar cells is combining a
low-bandgap semiconductor with a singlet fission material (which converts one
high energy singlet into two low energy triplets following photoexcitation).
Here we present a study of the interface between singlet fission molecules and
low-bandgap halide pervoskites. We briefly show a range of experiments
screening for triplet transfer into a halide perovskite. However, in all cases
triplet transfer was not observed. This motivated us to understand the halide
perovskite/singlet fission interface better by carrying out first-principles
calculations using tetracene and cesium lead iodide. We found that tetracene
molecules/thin films preferentially orient themselves parallel to/perpendicular
to the halide perovskite's surface, in a similar way to on other inorganic
semiconductors. We present formation energies of all interfaces, which are
significantly less favourable than for bulk tetracene, indicative of weak
interaction at the interface. It was not possible to calculate excitonic states
at the full interface due to computational limitations, so we instead present
highly speculative toy interfaces between tetracene and a
halide-perovskite-like structure. In these models we focus on replicating
tetracene's electronic states correctly. We find that tetracene's singlet and
triplet energies are comparable to that of bulk tetracene, and the triplet is
strongly localised on a single tetracene molecule, even at an interface. Our
work provides new understanding of the interface between tetracene and halide
perovskites, explores the potential for modelling excitons at interfaces, and
begins to explain the difficulties in extracting triplets directly into
inorganic semiconductors.

###Thermal analysis of metal organic precursors for functional Cu doped NiOx hole transporting layer in inverted perovskite solar cells the role of solution combustion chemistry in Cu doped NiOx thin films processing||Apostolos Ioakeimidis,Ioannis T. Papadas,Eirini D. Koutsouroubi,Gerasimos S. Armatas,Stelios A. Choulis###

Thermal analysis of metal organic precursors for functional Cu doped NiOx hole transporting layer in inverted perovskite solar cells the role of solution combustion chemistry in Cu doped NiOx thin films processing. Low temperature solution combustion synthesis emerges as a facile method for
synthesis of functional metal oxides thin films for electronic applications. We
study the solution combustion synthesis process of Cu:NiOx using different
molar ratios (w/o, 0.1 and 1.5) of fuel acetylacetone (Acac) to oxidizer (Cu,
Ni Nitrates) as a function of thermal annealing tempera-tures 150, 200 and 300
oC. The solution combustion synthesis process, in both thin films and bulk
Cu:NiOx, is investigated. Thermal analysis studies using TGA and DTA reveal
that the Cu:NiOx thin films show a more gradual mass loss while the bulk
Cu:NiOx exhibits a distinct combustion process. The thin films can crystallize
to Cu:NiOx at annealing temperature of 300 oC irrespective to the Acac/Oxidizer
ratio whereas lower annealing temperatures (150 and 200 oC) produce amorphous
materials. A detail characterization study of solution combustion synthesized
Cu:NiOx including XPS, UV-Vis, AFM and Contact angle measurements is presented.
Finally, 50 nm Cu:NiOx thin films are introduced as HTLs within the inverted
perovskite solar cell device architecture. The Cu:NiOx HTL annealed at 150 and
200 oC provided PVSCs with limited functionality whereas efficient
triple-cation Cs0.04(MA0.17FA0.83)0.96 Pb(I0.83Br0.17)3 based PVSCs achieved
for Cu:NiOx HTLs annealed at temperature 300 oC.

###A topological principle for photovoltaics: Shift current in intrinsically polar insulators||A. Alexandradinata###

A topological principle for photovoltaics: Shift current in intrinsically polar insulators. To realize an efficient solar cell without inhomogeneous doping, one would
like to maximize the shift component of the bulk photovoltaic current, in
noncentric semiconductors with wide band gaps. I achieve this maximization for
a new class of topological insulators whose band topology is only compatible
with a polar crystal class. For such insulators, it is impossible to
continuously tune the $\boldsymbol{k}$-dependent electron-hole dipole moment
(or `shift vector') to zero throughout the Brillouin zone. Averaging the shift
vector over all high-symmetry cross-sections of the Brillouin zone gives
exactly a rational multiple of a Bravais lattice vector, which points parallel
to the polar axis. Even with wide band gaps, the frequency-integrated shift
conductivity of intrinsically polar insulators greatly exceeds $e^3/h^2$, and
is at least three orders of magnitude larger than the conductivity of the
prototypical ferroelectric BaTiO$_3$, challenging a widely-held expectation
that small band gaps are necessary for large shift currents in topological
materials. Close to a topological phase transition, the integrated conductivity
diverges as $|E_g|^{-1/2}$ with $E_g$ the band gap, suggesting an application
to ultrafast infrared detection.

###Grain Boundary Development of Silicon during Directional Solidification: A Phase-Field Study||Chuanqi Zhu,Yuichiro Koizumi,Chunwen Guo###

Grain Boundary Development of Silicon during Directional Solidification: A Phase-Field Study. In order to control the grain structure of multi-crystalline (mc) silicon
during directional solidification, the development process of grain boundaries
(GBs) with respect to the temperature gradient should be understood. A
phase-field model incorporated with anisotropic interface energy and
anisotropic attachment kinetic coefficient has produced the faceted shape of a
growing silicon crystal, which is in agreement with experimental observation.
The growth of coupled silicon grains under various growth velocities has been
simulated to see the morphology of the solid-liquid front and the development
process of the GBs. It has been found that the direction of GB is governed by
either the kinetic rule or the equilibrium rule at the grain groove, depending
on the growth velocity and the orientation relationship between grains on two
sides. The GB beneath a groove with facet-facet surfaces follows the bisector
of the two surfaces, while the direction of a GB stays far from the bisector
when the groove has a rough surface. This research provides a numerical
approach to predicting grain boundary development and gaining insights from
grain structure evolution in mc-silicon, which can be potentially applied for
high-efficiency and low-cost solar cells.

###Drone Delivery Systems and Energy Management: A Review and Future Trends||Mohammad Sadra Rajabi,Pedram Beigi,Sina Aghakhani###

Drone Delivery Systems and Energy Management: A Review and Future Trends. Advanced technological breakthroughs and exceptional levels of innovation are
enhancing the capabilities and potential of autonomous unmanned aerial vehicles
(UAVs or drones), and in so doing attracting the interest of a broader swath of
logistic companies, online retailers, and governmental agencies. These
technology advancements and their impact on regulatory agencies may soon pave
the way for the widespread use of drones for delivery and monitoring purposes.
Moreover, increasingly urgent environmental factors that include CO2 emissions
reductions and other energy-saving approaches are intensifying to need to
reduce vehicular usage and congestion, which could further spur their usage. To
optimize these systems, drones often employ a hybrid power supply system
architecture to boost endurance and performance. Fuel cells, batteries, solar
cells, and supercapacitors are examples of power sources that may be combined
in a hybrid power architecture. To enable todays drones (and those of the
future) to work efficiently, the appropriate energy management system must be
selected based on optimal and accurate modeling techniques This chapter
provides a comprehensive review of drone energy-supply management and strategic
systems to identify their plusses and minuses, as well as suggests
recommendations for future research.

###Long-range electrostatic contribution to the electron-phonon couplings and mobilities of two-dimensional and bulk materials||Samuel Poncé,Miquel Royo,Massimiliano Stengel,Nicola Marzari,Marco Gibertini###

Long-range electrostatic contribution to the electron-phonon couplings and mobilities of two-dimensional and bulk materials. Charge transport plays a crucial role in manifold potential applications of
two-dimensional materials, including field effect transistors, solar cells, and
transparent conductors. At most operating temperatures, charge transport is
hindered by scattering of carriers by lattice vibrations. Assessing the
intrinsic phonon-limited carrier mobility is thus of paramount importance to
identify promising candidates for next-generation devices. Here we provide a
framework to efficiently compute the drift and Hall carrier mobility of
two-dimensional materials through the Boltzmann transport equation by relying
on a Fourier-Wannier interpolation. Building on a recent formulation of
long-range contributions to dynamical matrices and phonon dispersions [Phys.
Rev. X 11, 041027 (2021)], we extend the approach to electron-phonon coupling
including the effect of dynamical dipoles and quadrupoles. We identify an
unprecedented contribution associated with the Berry connection that is crucial
to preserve the Wannier-gauge covariance of the theory. This contribution is
not specific to 2D crystals, but also concerns the 3D case, as we demonstrate
via an application to bulk SrO. We showcase our method on a wide selection of
relevant monolayers ranging from SnS2 to MoS2, graphene, BN, InSe, and
phosphorene. We also discover a non-trivial temperature evolution of the Hall
hole mobility in InSe whereby the mobility increases with temperature above 150
K due to the mexican-hat electronic structure of the InSe valence bands.
Overall, we find that dynamical quadrupoles are essential and can impact the
carrier mobility in excess of 75%.

###Optimization of pin GaAs/AlGaAs Heterojunction Nanocone Array Solar Cell based on its Photovoltaic Properties||Sambuddha Majumder,Sooraj Ravindran###

Optimization of pin GaAs/AlGaAs Heterojunction Nanocone Array Solar Cell based on its Photovoltaic Properties. In this paper, we have designed and investigated the performance of radial
GaAs/AlGaAs pin junction nanocone array solar cells by performing coupled
optoelectronic simulations to obtain the most optimal design configuration
based on its photovoltaic properties. Each model has been compared with its
GaAs shell counterparts for different levels of surface passivations. It has
been observed that the nanocones with the AlGaAs shell has a much better
performance compared to those having GaAs shell. AlGaAs shell acts as a strong
barrier restricting most of the photogeneration to the inner GaAs regions and
it also acts as a strong passivation layer, reducing the recombination losses
due to surface effects. Further, it is observed that the nanocones achieve
their highest photoconversion efficiency when they are sparsely packed, with a
constant i-shell thickness of 7-9 nm and have an angle of tilt of 5 degrees.
This enhanced performance is attributed to a more effective and extended
photogeneration throughout the nanowire length, a strong overlapping built-in
electric field, and lower recombination losses.

###Capacitive and inductive effects in perovskite solar cells: the different roles of ionic current and ionic charge accumulation||Nicolae Filipoiu,Amanda Teodora Preda,Dragos Victor Anghel,Roxana Patru,Rachel Elizabeth Brophy,Movaffaq Kateb,Cristina Besleaga,Andrei Gabriel Tomulescu,Ioana Pintilie,Andrei Manolescu,George Alexandru Nemnes###

Capacitive and inductive effects in perovskite solar cells: the different roles of ionic current and ionic charge accumulation. Dynamic hysteresis effects have been long known to occur in the J-V
characteristics of perovskite solar cells (PSCs), with the ionic migration
being identified as the primary factor. The hysteretic effects impacted early
studies by the uncertainty in the evaluation of the power conversion
efficiency, while currently, potential links to degradation mechanisms are in
the focus. Therefore, understanding ion migration is a central goal, typically
addressed by performing a combined large- and small signal analysis. The
reported large capacitive and inductive effects created controversies with
respect to the underlying mechanisms, yielding essentially two classes of
models, one based on large accumulation capacitances and the other based on
ionic modulation of the collected current. We introduce here an equivalent
circuit model and interpret these phenomena in terms of recombination current
modulation, identifying the distinct contributions from ion current and ionic
charge accumulations. These contributions to the recombination current are
associated with capacitive and inductive effects, respectively, and we
corroborate the numerical simulations with electrochemical impedance
spectroscopy (EIS) measurements. These show the role of the recombination
currents of photogenerated carriers in producing both capacitive and inductive
effects as the illumination is varied. Moreover, we provide a bridging point
between the two classes of models and suggest a framework of investigation of
defect states based on the observed inductive behavior, which would further aid
the mitigation of the degradation effects.

###Structural Dynamics of Polymer:Non-Fullerene Organic Solar Cell Blends: A Neutron Spectroscopy Perspective||Mohamed Zbiri,Peter A. Gilhooly-Finn,Peter Fouquet,Christian B. Nielsen,Anne A. Y. Guilbert###

Structural Dynamics of Polymer:Non-Fullerene Organic Solar Cell Blends: A Neutron Spectroscopy Perspective. Organic solar cells (OSCs) based on ADA-type (acceptor-donor-acceptor)
non-fullerene acceptors (NFAs) exhibit improved power conversion efficiency
(PCE) compared to the conventional fullerene-based analogues. The
optoelectronic properties of OSC active layer blends are correlated to their
underlying structural dynamics and therefore influence the device performance.
Using synergistically different neutron spectroscopy techniques, we studied the
dynamics of binary and ternary blends made of the NFAs O-IDTBR and O-IDFBR and
the regioregular donor polymer P3HT. Deuteration was considered for a contrast
variation purpose. In addition to shedding light on the miscibilty and alloying
characters of the blends, a main outcome of this work is the evidenced similar
dynamical response of the blend components. This finding is in contrast with
our previous neutron spectroscopy and molecular dynamics studies of the
fullerene-based blend P3HT:PCBM, where we highlighted distinct behaviors of
P3HT and PCBM in terms of the vitrification/frustration of P3HT and the
plasticization of PCBM by P3HT upon blending. Alike P3HT vitrification is not
presently observed. The absence or the weak vitrification evidenced here is in
line with recent reports and is likely related to the improved PCE exhibited by
the ADA-type NFA-based OSCs.

###The Interfacial Structure of InP(100) in Contact with HCl and H$_2$SO$_4$ studied by Reflection Anisotropy Spectroscopy||Mario Löw,Margot Guidat,Jongmin Kim,Matthias M. May###

The Interfacial Structure of InP(100) in Contact with HCl and H$_2$SO$_4$ studied by Reflection Anisotropy Spectroscopy. Indium phosphide and derived compound semiconductors are materials often
involved in high-efficiency solar water splitting due to their versatile
opto-electronic properties. Surface corrosion, however, typically deteriorates
the performance of photoelectrochemical solar cells based on this material
class. It has been reported that (photo)electrochemical surface
functionalisation protects the surface by combining etching and controlled
corrosion. Nevertheless, the overall involved process is not fully understood.
Therefore, access to the electrochemical interface structure under operando
conditions is crucial for a more detailed understanding. One approach for
gaining structural insight is the use of operando reflection anisotropy
spectroscopy. This technique allows the time-resolved investigation of the
interfacial structure while applying potentials in the electrolyte. In this
study, p-doped InP(100) surfaces are cycled between anodic and cathodic
potentials in two different electrolytes, hydrochloric acid and sulphuric acid.
For low, 10 mM electrolyte concentrations, we observe a reversible processes
related to the reduction of a surface oxide phase in the cathodic potential
range which is reformed near open-circuit potentials. Higher concentrations of
0.5 N, however, already lead to initial surface corrosion.

###Ethylenediamine Addition Improves Performance and Suppresses Phase Instabilities in Mixed-Halide Perovskites||Margherita Taddei,Joel A. Smith,Benjamin M. Gallant,Suer Zhou,Robert J. E. Westbrook,Yangwei Shi,Jian Wang,James N. Drysdale,Declan P. McCarthy,Stephen Barlow,Seth R. Marder,Henry J. Snaith,David S. Ginger###

Ethylenediamine Addition Improves Performance and Suppresses Phase Instabilities in Mixed-Halide Perovskites. We show that adding ethylenediamine (EDA) to perovskite precursor solution
improves the photovoltaic device performance and material stability of
high-bromide-content, methylammonium-free, formamidinium cesium lead halide
perovskites FA1-xCsxPb(I1-yBry)3 which are currently of interest for
perovskite-on-Si tandem solar cells. Using spectroscopy and hyperspectral
microscopy, we show that the additive improves film homogeneity and suppresses
the phase instability that is ubiquitous in high-Br perovskite formulations,
producing films that remain stable for over 100 days in ambient conditions.
With the addition of 1 mol% EDA we demonstrate 1.69 eV-gap perovskite
single-junction p-i-n devices with a VOC of 1.22 V, and a champion maximum
power point tracked power conversion efficiency of 18.8%, comparable to the
best reported methylammonium-free perovskites. Using nuclear magnetic resonance
(NMR) spectroscopy and X-ray diffraction techniques, we show that EDA reacts
with FA+ in solution, rapidly and quantitatively forming imidazolinium cations.
It is the presence of imidazolinium during crystallization which drives the
improved perovskite thin-film properties.

###Windowed Green Function MoM for Second-Kind Surface Integral Equation Formulations of Layered Media Electromagnetic Scattering Problems||Rodrigo Arrieta,Carlos Pérez-Arancibia###

Windowed Green Function MoM for Second-Kind Surface Integral Equation Formulations of Layered Media Electromagnetic Scattering Problems. This paper presents a second-kind surface integral equation method for the
numerical solution of frequency-domain electromagnetic scattering problems by
locally perturbed layered media in three spatial dimensions. Unlike standard
approaches, the proposed methodology does not involve the use of layer Green
functions. It instead leverages an indirect M\"uller formulation in terms of
free-space Green functions that entails integration over the entire unbounded
penetrable boundary. The integral equation domain is effectively reduced to a
small-area surface by means of the windowed Green function method, which
exhibits high-order convergence as the size of the truncated surface increases.
The resulting (second-kind) windowed integral equation is then numerically
solved by means of the standard Galerkin method of moments (MoM) using RWG
basis functions. The methodology is validated by comparison with Mie-series and
Sommerfeld-integral exact solutions as well as against a layer Green
function-based MoM. Challenging examples including realistic structures
relevant to the design of plasmonic solar cells and all-dielectric
metasurfaces, demonstrate the applicability, efficiency, and accuracy of the
proposed methodology.

###Deep-level transient spectroscopy of the charged defects in p-i-n perovskite solar cells induced by light-soaking||A. A. Vasilev,D. S. Saranin,P. A. Gostishchev,M. P. Tuhova,S. I. Didenko,A. Y. Polyakov,A. Di Carlo###

Deep-level transient spectroscopy of the charged defects in p-i-n perovskite solar cells induced by light-soaking. The long-term stability of halide perovskite solar cells (PSCs) remains the
critical problem of this photovoltaic technology. Different structural defects
formed in the thin-film perovskite films were considered as a main trigger for
the decomposition of the absorber and corrosion of the interfaces in the device
structure. The changes in the stability performance of the PSCs require a
detailed analysis of the defects generated under external stress (light and
heat). Using admittance, deep-level transient spectroscopy (DLTS) and reverse
DLTS we determined the evolution of the defect energy levels in p-i-n PCS under
continuous light soaking stress. We compared the impact of the charged defects
on the performance and long-term stability of the CsFAPbI3 based devices with
and without Cl-doping. Despite the gain in the output performance of the PCSs,
the devices with CsFAPbI3-xClx showed improved light soaking stability. The T80
(time required to reduce initial efficiency by 20%) for Cl-doped PSCs was
1280h, while for pure CsFAPbI3 based devices only 650h. Three different defect
energy levels were determined for different device configurations. We found
that Cl-doping suppressed the formation of the antisite defects (IPb, IFA) and
iodine interstitials (Ii). The changes in the defect's energy levels after
continuous light soaking stress were analyzed and discussed. The present work
provides new insights for the defect behavior of PSCs under continuous external
stress, revealing the physical-chemical impact of the Cl-additive strategy.

###Supramolecular self-assembly as a tool to preserve electronic purity of perylene diimide chromophores||Ina Heckelmann,Zifei Lu,Joseph C. A. Prentice,Florian Auras,Tanya K. Ronson,Richard H. Friend,Jonathan R. Nitschke,Sascha Feldmann###

Supramolecular self-assembly as a tool to preserve electronic purity of perylene diimide chromophores. Small molecule organic semiconductors hold great promise for efficient,
printable, and flexible optoelectronic applications like solar cells and
displays. However, strong excited-state quenching due to uncontrolled
aggregation currently limits their performance and employability in devices.
Here, we report on the self-assembly of a supramolecular pseudo-cube formed
from six modified tetradentate perylene diimides (PDIs). The rigid,
shape-persistent cage sets the distance and orientation of the PDI chromophores
and suppresses intramolecular rotations and vibrations, leading to
non-aggregated, monomer-like electronic properties in solution as well as in
the solid state, in contrast to the fast fluorescence quenching in the free
ligand. The stabilized excited state and electronic purity of the cage enable
the observation of delayed fluorescence due to a bright excited multimer state,
which acts as an excited state reservoir, due to a rare case of benign
inter-chromophore interactions in the cage. Our results suggest that not only
the photophysical properties of the subcomponents but the geometric structure
is crucial for the overall optoelectronic properties of supramolecular systems.
We show that self-assembly provides a powerful tool for retaining and
controlling the electronic properties of well-studied chromophores, providing a
route to bring molecular electronics applications in reach.

###Improvement of both performance and stability of photovoltaic devices by in situ formation of a sulfur-based 2D perovskite||Milon Kundar,Sahil Bhandari,Sein Chung,Kilwon Cho,Satinder K. Sharma,Ranbir Singh,Suman Kalyan Pal###

Improvement of both performance and stability of photovoltaic devices by in situ formation of a sulfur-based 2D perovskite. Perovskite solar cells (PSCs) with superior performance have been recognized
as a potential candidate in photovoltaic technologies. However, the defects in
active perovskite layer induce non-radiative recombination which restricts the
performance and stability of the PSCs. The construction of thiophene-based 2D
structure is one of the significant approaches for surface passivation of
hybrid PSCs that may combine the benefits of the stability of 2D perovskite
with the high performance of 3D perovskite. Here, a sulfur-rich spacer cation
2-thiopheneethylamine iodide (TEAI) is synthesized as a passivation agent for
the construction of three-dimensional/two-dimensional (3D/2D) perovskite
bilayer structure. TEAI-treated PSCs possess a much higher efficiency (20.06%)
compared to the 3D perovskite (MAFAPbI3) devices (17.42%). Time-resolved
photoluminescence (TRPL) and femtosecond transient absorption (TA) spectroscopy
are employed to investigate the effect of surface passivation on the charge
carrier dynamics of the 3D perovskite. Additionally, the stability test of
TEAI-treated perovskite devices reveals significant improvement in humid (RH ~
56%) and thermal stability as the sulfur-based 2D (TEA)2PbI4 material
self-assembles on the 3D surface making the perovskite surface hydrophobic. Our
findings provide a reliable approach to improve device stability and
performance successively, paving the way for industrialization of PSCs.

###On the origin of tail states and VOC losses in Cu(In,Ga)Se2||Omar Ramírez,Jiro Nishinaga,Felix Dingwell,Taowen Wang,Aubin Prot,Max Hilaire Wolter,Vibha Ranjan,Susanne Siebentritt###

On the origin of tail states and VOC losses in Cu(In,Ga)Se2. The detrimental effect of tail states on the radiative and non-radiative
voltage loss has been demonstrated to be a limiting factor for the open circuit
voltage in Cu(In,Ga)Se2 solar cells. A strategy that has proven effective in
reducing tail states is the addition of alkali metals, the effect of which has
been associated with the passivation of charged defects at grain boundaries.
Herein, tail states in Cu(In,Ga)Se2 are revisited by studying the effect of
compositional variations and alkali incorporation into single crystals. The
results demonstrate that alkalis decrease the density of tail states despite
the absence of grain boundaries, suggesting that there is more to alkalis than
just grain boundary effects. Moreover, an increase in doping as a result of
alkali incorporation is shown to contribute to the reduced tail states, which
are demonstrated to arise largely from electrostatic potential fluctuations and
to be determined by grain interior properties. By analyzing the voltage loss in
high-efficiency polycrystalline and single crystalline devices, this work
presents a model that explains the entirety of the voltage loss in Cu(In,Ga)Se2
based on the combined effect of doping on tail states and VOC.

###A Continuum Model for Morphology Formation from Interacting Ternary Mixtures: Simulation Study of the Formation and Growth of Patterns||Rainey Lyons,Stela Andrea Muntean,Emilio N. M. Cirillo,Adrian Muntean###

A Continuum Model for Morphology Formation from Interacting Ternary Mixtures: Simulation Study of the Formation and Growth of Patterns. Our interest lies in exploring the ability of a coupled nonlocal system of
two quasilinear parabolic partial differential equations to produce phase
separation patterns. The obtained patterns are referred here as morphologies.
Our target system is derived in the literature as the rigorous hydrodynamic
limit of a suitably scaled interacting particle system of Blume--Capel--type
driven by Kawasaki dynamics. The system describes in a rather implicit way the
interaction within a ternary mixture that is the macroscopic counterpart of a
mix of two populations of interacting solutes in the presence of a background
solvent. Our discussion is based on the qualitative behavior of numerical
simulations of finite volume approximations of smooth solutions to our system
and their quantitative postprocessing in terms of two indicators (correlation
and structure factor calculations). Our results show many similar features
compared to what one knows at the level of the stochastic Blume--Capel dynamics
with three interacting species. The properties of the obtained morphologies
(shape, connectivity, and so on) can play a key role in, e.g., the design of
the active layer for efficient organic solar cells.

###Understanding the Role of Triplet-triplet Annihilation in Non-fullerene Acceptor Organic Solar Cells||Lucy J. F. Hart,Jeannine Grüne,Wei Liu,Tsz-ki Lau,Joel Luke,Yi-Chun Chin,Xinyu Jiang,Huotian Zhang,Daniel J. C. Sowood,Darcy M. L. Unson,Ji-Seon Kim,Xinhui Lu,Yingping Zou,Feng Gao,Andreas Sperlich,Vladimir Dyakonov,Jun Yuan,Alexander J. Gillett###

Understanding the Role of Triplet-triplet Annihilation in Non-fullerene Acceptor Organic Solar Cells. Non-fullerene acceptors (NFAs) have enabled power conversion efficiencies
exceeding 19% in organic solar cells (OSCs). However, the open-circuit voltage
of OSCs remains low relative to their optical gap due to excessive
non-radiative recombination, and this now limits performance. Here, we consider
an important aspect of OSC design, namely management of the triplet exciton
population formed after non-geminate charge recombination. By comparing the
blends PM6:Y11 and PM6:Y6, we show that the greater crystallinity of the NFA
domains in PM6:Y11 leads to a higher rate of triplet-triplet annihilation
(TTA). We attribute this to the four times larger ground state dipole moment of
Y11 versus Y6, which improves the long range NFA out-of-plane ordering. Since
TTA converts a fraction of the non-emissive triplet states into bright singlet
states, it has the potential to reduce non-radiative voltage losses. Through a
kinetic analysis of the recombination processes under 1-Sun illumination, we
provide a framework for determining the conditions under which TTA may improve
OSC performance. If these could be satisfied, TTA has the potential to reduce
non-radiative voltage losses by up to several tens of mV and could thus improve
OSC performance.

###High-sensitive MIS structures with silicon nanocrystals grown via solid-state dewetting of silicon-on-insulator for solar cell and photodetector applications||Mansour Aouassa,Saud Algarni,Ibrahim Althobaiti,Luc Favre,Isabelle Berbezier###

High-sensitive MIS structures with silicon nanocrystals grown via solid-state dewetting of silicon-on-insulator for solar cell and photodetector applications. This work reports an original method for the fabrication of
Metal-Isulator-Semiconductor (MIS) structures with silicon nanocrystals (Si
NCs) based active layers embedded in the insulating SiO 2 oxide, for high
performance solar cell and photodetector applications. The Si NCs are produced
via the in situ solid-state dewetting of ultra-pure amorphous
silicon-oninsulator (a-SOI) grown by solid source molecular beam epitaxy
(SSMBE). The size and density of Si NCs are precisely tuned by varying the
deposited thickness of silicon. The morphological characterization carried out
by using atomic force microscopy (AFM) and scanning electron microscopy (SEM)
shows that the Si NCs have homogeneous size with welldefined spherical shape
and densities up to ~10 12 /cm 2 (inversely proportional to the square of
nominal a-Si thickness). The structural investigations by high resolution
transmission electron microscopy (HR-TEM) show that the ultra-small Si NCs
(with mean diameter ~7 nm) are monocrystalline and free of structural defects.
The electrical measurements performed by current versus voltage (I-V) and
photocurrent spectroscopies on the Si-NCs based MIS structures prove the
efficiency of Si NCs to enhance the electrical conduction in MIS structures and
to increase (x10 times) the photocurrent (i.e. at bias voltage V =-1 V) via the
photogeneration of additional electron-hole pairs in the MIS structures. These
results evidence that the Si NCs obtained by the combination of MBE growth and
solid-state dewetting are perfectly suitable for the development of novel high
performance optoelectronic devices compatible with the CMOS technology.

###The Thermodynamic Limit of Indoor Photovoltaics Based on Energetically-Disordered Molecular Semiconductors||Austin M. Kay,Maura E. Fitzsimons,Gregory Burwell,Paul Meredith,Ardalan Armin,Oskar J. Sandberg###

The Thermodynamic Limit of Indoor Photovoltaics Based on Energetically-Disordered Molecular Semiconductors. Due to their tailorable optical properties, organic semiconductors show
considerable promise for use in indoor photovoltaics (IPVs), which present a
sustainable route for powering ubiquitous "Internet-of-Things" devices in the
coming decades. However, owing to their excitonic and energetically disordered
nature, organic semiconductors generally display considerable sub-gap
absorption and relatively large nonradiative losses in solar cells. To optimize
organic semiconductor-based photovoltaics, it is therefore vital to understand
how energetic disorder and non-radiative recombination limit the performance of
these devices under indoor light sources. In this work, we explore how
energetic disorder, sub-optical gap absorption, and non-radiative open-circuit
voltage losses detrimentally affect the upper performance limits of organic
semiconductor-based IPVs. Based on these considerations, we provide realistic
upper estimates for the power conversion efficiency. The energetic disorder,
inherently present in molecular semiconductors, is generally found to shift the
optimal optical gap from 1.83 eV to ~1.9 eV for devices operating under LED
spectra. Finally, we also describe a methodology (accompanied by a
computational tool with a graphical user interface) for predicting IPV
performance under arbitrary illumination conditions. Using this methodology, we
estimate the indoor PCEs of several photovoltaic materials, including the
state-of-the-art systems PM6:Y6 and PM6:BTP-eC9.

###Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2||Sikandar Aftab,Muhammad Arslan Shehzad,Muhammad Salman Ajmal,Fahmid Kabir,Muhammad Zahir Iqbal###

Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2. In future solar cell technologies, the thermodynamic Shockley-Queisser limit
for solar-to-current conversion in traditional p-n junctions could potentially
be overcome with a bulk photovoltaic effect by creating an inversion broken
symmetry in piezoelectric or ferroelectric materials. Here, we unveiled
mechanical distortion-induced bulk photovoltaic behavior in a two-dimensional
material (2D), MoTe2, caused by phase transition and broken inversion symmetry
in MoTe2. The phase transition from single-crystalline semiconducting 2H-MoTe2
to semi-metallic 1T-MoTe2 was confirmed using X-ray photoelectron spectroscopy
(XPS). We used a micrometer-scale system to measure the absorption of energy,
which reduced from 800 meV to 63 meV when phase transformation from hexagonal
to distorted octahedral and revealed a smaller bandgap semi-metallic behavior.
Experimentally, a large bulk photovoltaic response is anticipated with the
maximum photovoltage VOC = 16 mV and a positive signal of the ISC = 60 uA (400
nm, 90.4 Wcm-2) in the absence of an external electric field. The maximum
values of both R and EQE were found to be 98 mAW-1 and 30 %, respectively. Our
findings unveil distinctive features of the photocurrent responses caused by
in-plane polarity and its potential from a wide pool of established TMD-based
nanomaterials, and a novel approach to reach high efficiency in converting
photons-to-electricity for power harvesting optoelectronics devices.

###Intermolecular CT excitons enable nanosecond excited-state lifetimes in NIR-absorbing non-fullerene acceptors for efficient organic solar cells||Xian-Kai Chen,Christopher C. S. Chan,Sudhi Mahadevan,Yu Guo,Guichuan Zhang,He Yan,Kam Sing Wong,Hin-Lap Yip,Jean-Luc Bredas,Sai Wing Tsang,Philip C. Y. Chow###

Intermolecular CT excitons enable nanosecond excited-state lifetimes in NIR-absorbing non-fullerene acceptors for efficient organic solar cells. State-of-the-art Y6-type molecular acceptors exhibit nanosecond excited-state
lifetimes despite their low optical gaps (~1.4 eV), thus allowing organic solar
cells (OSCs) to achieve highly efficient charge generation with extended
near-infrared (NIR) absorption range (up to ~1000 nm). However, the precise
molecular-level mechanism that enables low-energy excited states in Y6-type
acceptors to achieve nanosecond lifetimes has remained elusive. Here, we
demonstrate that the distinct packing of Y6 molecules in film leads to a strong
intermolecular charge-transfer (iCT) character of the lowest excited state in
Y6 aggregates, which is absent in other low-gap acceptors such as ITIC. Due to
strong electronic couplings between the adjacent Y6 molecules, the iCT-exciton
energies are greatly reduced by up to ~0.25 eV with respect to excitons formed
in separated molecules. Importantly, despite their low energies, the iCT
excitons have reduced non-adiabatic electron-vibration couplings with the
electronic ground state, thus suppressing non-radiative recombination and
allowing Y6 to overcome the well-known energy gap law. Our results reveal the
fundamental relationship between molecular packing and nanosecond excited-state
lifetimes in NIR-absorbing Y6-type acceptors underlying the outstanding
performance of Y6-based OSCs.

###Powering AI at the Edge: A Robust, Memristor-based Binarized Neural Network with Near-Memory Computing and Miniaturized Solar Cell||Fadi Jebali,Atreya Majumdar,Clément Turck,Kamel-Eddine Harabi,Mathieu-Coumba Faye,Eloi Muhr,Jean-Pierre Walder,Oleksandr Bilousov,Amadeo Michaud,Elisa Vianello,Tifenn Hirtzlin,François Andrieu,Marc Bocquet,Stéphane Collin,Damien Querlioz,Jean-Michel Portal###

Powering AI at the Edge: A Robust, Memristor-based Binarized Neural Network with Near-Memory Computing and Miniaturized Solar Cell. Memristor-based neural networks provide an exceptional energy-efficient
platform for artificial intelligence (AI), presenting the possibility of
self-powered operation when paired with energy harvesters. However, most
memristor-based networks rely on analog in-memory computing, necessitating a
stable and precise power supply, which is incompatible with the inherently
unstable and unreliable energy harvesters. In this work, we fabricated a robust
binarized neural network comprising 32,768 memristors, powered by a miniature
wide-bandgap solar cell optimized for edge applications. Our circuit employs a
resilient digital near-memory computing approach, featuring complementarily
programmed memristors and logic-in-sense-amplifier. This design eliminates the
need for compensation or calibration, operating effectively under diverse
conditions. Under high illumination, the circuit achieves inference performance
comparable to that of a lab bench power supply. In low illumination scenarios,
it remains functional with slightly reduced accuracy, seamlessly transitioning
to an approximate computing mode. Through image classification neural network
simulations, we demonstrate that misclassified images under low illumination
are primarily difficult-to-classify cases. Our approach lays the groundwork for
self-powered AI and the creation of intelligent sensors for various
applications in health, safety, and environment monitoring.

###Generalised Framework for Controlling and Understanding Ion Dynamics with Passivated Lead Halide Perovskites||Tomi K. Baikie,Philip Calado,Krzysztof Galkowski,Zahra Andaji-Garmaroudi,Yi-Chun Chin,Joel Luke,Charlie Henderson,Tom Dunlop,James McGettrick,Ji-Seon Kim,Akshay Rao,Jenny Nelson,Samuel D. Stranks,Piers R. B. Barnes###

Generalised Framework for Controlling and Understanding Ion Dynamics with Passivated Lead Halide Perovskites. Metal halide perovskite solar cells have gained widespread attention due to
their high efficiency and high defect tolerance. The absorbing perovskite layer
is as a mixed electron-ion conductor that supports high rates of ion and charge
transport at room temperature, but the migration of mobile defects can lead to
degradation pathways. We combine experimental observations and drift-diffusion
modelling to demonstrate a new framework to interpret surface photovoltage
(SPV) measurements in perovskite systems and mixed electronic ionic conductors
more generally. We conclude that the SPV in mixed electronic ionic conductors
can be understood in terms of the change in electric potential at the surface
associated with changes in the net charge within the semiconductor system. We
show that by modifying the interfaces of perovskite bilayers, we may control
defect migration behaviour throughout the perovskite bulk. Our new framework
for SPV has broad implications for developing strategies to improve the
stability of perovskite devices by controlling defect accumulation at
interfaces. More generally, in mixed electronic conductors our framework
provides new insights into the behaviour of mobile defects and their
interaction with photoinduced charges, which are foundational to physical
mechanisms in memristivity, logic, impedance, sensors and energy storage.

###Selenium and the role of defects for photovoltaic applications||Hadeel Moustafa,Jiban Kangsabanik,Fabian Bertoldo,Simone Manti,Kristian S. Thygesen,Karsten W. Jacobsen,Thomas Olsen###

Selenium and the role of defects for photovoltaic applications. We present first principles calculations of the electronic properties of
trigonal selenium with emphasis on photovoltaic applications. The band gap and
optical absorption spectrum of pristine selenium is calculated from many-body
perturbation theory yielding excellent agreement with experiments. We then
investigate the role of intrinsic as well as extrinsic defects and estimate the
equilibrium concentrations resulting from realistic synthesis conditions. The
intrinsic defects are dominated by vacancies, which act as acceptor levels and
implies $p$-doping in agreement with previous predictions and measurements, and
we show that these do not give rise to significant non-radiative recombination.
The charge balance remains dominated by vacancies when extrinsic defects are
included, but these may give rise to sizable non-radiative recombination rates,
which could severely limit the performance of selenium based solar cells. Our
results thus imply that the pollution by external elements is a decisive factor
for the photovoltaic efficiency, which will be of crucial importance when
considering synthesis conditions for any type of device engineering.

###Generation and modulation of multiple 2D bulk photovoltaic effects in space-time reversal asymmetric 2H-FeCl2||Liang Liu,Xiaolin Li,Luping Du,Xi Zhang###

Generation and modulation of multiple 2D bulk photovoltaic effects in space-time reversal asymmetric 2H-FeCl2. The two-dimensional (2D) bulk photovoltaic effect (BPVE) is a cornerstone for
future highly efficient 2D solar cells and optoelectronics. The ferromagnetic
semiconductor 2H-FeCl2 is shown to realize a new type of BPVE in which spatial
inversion (P), time reversal (T), and space-time reversal (PT) symmetries are
broken (PT-broken). Using density functional theory and perturbation theory, we
show that 2H-FeCl2 exhibits giant photocurrents, photo-spin-currents, and
photo-orbital-currents under illumination by linearly polarized light. The
injection-like and shift-like photocurrents coexist and propagate in different
directions. The material also demonstrates substantial photoconductance,
photo-spin-conductance, and photo-orbital-conductance, with magnitudes up to
4650 (nm{\cdot}{\mu}A/V2), 4620 (nm{\cdot}{\mu}A/V2 {\hbar}/2e), and 6450
(nm{\cdot}{\mu}A/V2 {\hbar}/e), respectively. Furthermore, the
injection-currents, shift-spin-currents, and shift-orbital-currents can be
readily switched via rotating the magnetizations of 2H-FeCl2. These results
demonstrate the superior performance and intriguing control of a new type of
BPVE in 2H-FeCl2.

###Integrating quantum-dots and dielectric Mie resonators: a hierarchical metamaterial inheriting the best of both||Antonio Capretti,Arnon Lesage,Tom Gregorkiewicz###

Integrating quantum-dots and dielectric Mie resonators: a hierarchical metamaterial inheriting the best of both. Nanoscale dielectric resonators and quantum-confined semiconductors have
enabled unprecedented control over light absorption and excited charges,
respectively. In this work, we embed luminescent silicon nanocrystals (Si-NCs)
into a 2D array of SiO2 nanocylinders, and experimentally prove a powerful
concept: the resulting metamaterial preserves the radiative properties of the
Si-NCs and inherits the spectrally-selective absorption properties of the
nanocylinders. This hierarchical approach provides increased photoluminescence
(PL) intensity obtained without utilizing any lossy plasmonic components. We
perform rigorous calculations and predict that a freestanding metamaterial
enables tunable absorption peaks up to 50% in the visible spectrum, in
correspondence of the nanocylinder Mie resonances and of the grating condition
in the array. We experimentally detect extinction spectral peaks in the
metamaterial, which drive enhanced absorption in the Si-NCs. Consequently, the
metamaterial features increased PL intensity, obtained without affecting the PL
lifetime, angular pattern and extraction efficiency. Remarkably, our
best-performing metamaterial shows +30% PL intensity achieved with a lower
amount of Si-NCs, compared to an equivalent planar film without nanocylinders,
resulting in a 3-fold average PL enhancement per Si-NC. The principle
demonstrated here is general and the Si-NCs can be replaced with other
semiconductor quantum dots, rare-earth ions or organic molecules. Similarly,
the dielectric medium can be adjusted on purpose. This spectral selectivity of
absorption paves the way for an effective light down-conversion scheme to
increase the efficiency of solar cells. We envision the use of this
hierarchical design for other efficient photovoltaic, photo-catalytic and
artificial photosynthetic devices with spectrally-selective absorption and
enhanced efficiency.

###Effects of Defect on Work Function and Energy Alignment of PbI2: Implications for Solar Cell Applications||Hongfei Chen,Hejin Yan,Yongqing Cai###

Effects of Defect on Work Function and Energy Alignment of PbI2: Implications for Solar Cell Applications. Two-dimensional (2D) layered lead iodide (PbI2) is an important precursor and
common residual species during the synthesis of lead-halide perovskites. There
currently exist some debates and uncertainties about the effect of excess PbI2
on the efficiency and stability of the solar cell with respect to its energy
alignment and energetics of defects. Herein, by applying the first-principles
calculations, we investigate the energetics, changes of work function and the
defective levels associated with the iodine vacancy (VI) and interstitial
iodine (II) defects of monolayer PbI2 (ML-PbI2). We find that the PbI2 has a
very low formation energy of VI of 0.77 and 0.19 eV for dilute and high
concentration, respectively, reflecting coalescence tendency of isolated VI,
much lower than that of vacancies in other 2D materials like phosphorene.
Similar to VI, a low formation energy of II of 0.65 eV is found, implying a
high population of such defects. Both defects generate in-gap defective levels
which are mainly due to the unsaturated chemical bonds of p-orbitals of exposed
Pb or inserted I. Such rich defective levels allow the VI and II as the
reservoir or sinks of electron/hole carriers in PbI2. Our results suggest that
the remnant PbI2 in perovskite MAPbI3 (or FAPbI3) would play dual opposite
roles in affecting the efficiency of the perovskite: (1) Forming Schottky-type
interface with MAPbI3 (or FAPbI3) in which the built-in potential would
facilitate the electron-hole separation and prolong the carrier lifetime; (2)
Acting as the recombination centers due to the deep defective levels. To
promote the efficiency by the Schottky effect, our work reveals that the II
defect is favored, and to reduce the recombination centers the VI defect should
be suppressed. Our results shall be beneficial in improving strategies for the
related optoelectronics applications.

###High carrier lifetimes in UMG multicrystalline wafers after P- diffusion compatible with high-efficiency cell structures||Nerea Dasilva-Villanueva,Bülent Arıkan,Hasan Hüseyin Canar,David Fuertes Marrón,Bo-Kyung Hong,Ahmet Emin Keçeci,Sümeyye Koçak Bütüner,Gence Bektaş,Raşit Turan,Carlos del Cañizo###

High carrier lifetimes in UMG multicrystalline wafers after P- diffusion compatible with high-efficiency cell structures. High-quality multicrystalline Upgraded Metallurgical Grade Silicon (UMG-Si)
offers significant advantages over conventional polysilicon-based PV
technology, associated to lower cost, lower energy budget and lower carbon
footprint. The aim of this study is twofold: on the one hand, to ascertain the
efficiency potential of solar cells based on this material in terms of carrier
lifetime; and on the other hand, to explore, as a result of that, the adoption
of high-efficiency cell architectures by establishing an effective rear-side
passivation scheme for the implementation of passivated emitter rear contact
(PERC) devices. The carrier lifetime and the surface passivation efficacy are
investigated for different passivating layer configurations after single and
double P-diffusion gettering processes. Layer stacks consisting of Al2O3,
SiOxNy and a-SiNx:H capping overlayers have been optimized, on industrial size,
saw-damage-etched UMG wafers and results compared to those obtained using
reference iodine-ethanol (IE) passivation. Diagnosis based on minority carrier
lifetime and implied Voc (iVoc) measurements helped monitor the impact of
parameter optimization on wafer quality, particularly after firing processes.
Carrier lifetimes over 600 us at 10^15 cm-3 injection level as well as up to
790 us locally have been measured in UMG-Si wafers passivated with IE after a
Phosphorus Diffusion Gettering (PDG), demonstrating the suitability of the
material for high-efficiency cell architectures. Values higher than 300 us have
been obtained with Al2O3-based passivation layers for gettered UMG wafers, with
implied Voc values up to 710 mV. These record-breaking lifetimes and iVoc
figures obtained with p-type multicrystalline UMG-Si material demonstrate a
significant upgrading of its electronic quality by means of industry-scalable
technical processes.

###Thermodynamics of the mono-energetic energy selective Contacts of the hot carrier solar cell||Antonio Martí,Elisa Antolín,Iñigo Ramiro###

Thermodynamics of the mono-energetic energy selective Contacts of the hot carrier solar cell. The hot carrier solar cell (HCSC) has the potential for converting solar
energy into electrochemical energy with an efficiency of 85.4%. For this, in
addition to an idealized light absorber, the HCSC has to be connected to the
external load by means of the so-called \emph{mono-energetic energy selective
contacts} (ESCs). However, the thermodynamic properties that these types of
contact have to exhibit, such as their electric, thermal conductivity and
Seebeck coefficient, have not been explored. This paper aims to fill this gap.
In this respect, we model electron transport in non-ideal ESCs using the
transport theory proposed by Datta and Landauer which has allowed us to
calculate the value of these parameters as a function of the temperature and
electrochemical potential of operation. Our findings also reveal that, to
preserve the HCSC efficiency above 82%, the ESCs could require in the order of
$3 \times 10^{19}$ cm$^{-3}$ electron states. As the ESCs depart from ideality,
the temperature of the hot carriers at which optimum efficiency is obtained
increases to above 2540 K. The mono-enenergetic selective contact characterized
by the highest energy demands an electric, thermal conductivity and Seebeck
coefficient that, when combined, are characterized by a high thermoelectric
figure of merit $(ZT\approx 8)$. We are not aware of any material exhibiting
this figure of merit which illustrates the difficulty in putting the HCSC
concept into practice. Conversely, our work supports the idea that pursuing
materials capable of transporting electrons ballistically through
mono-energetic electron channels can provide the key for achieving materials
characterized by high $ZT$

###Spectrally-tunable Dielectric Grating-based Metasurface for Broadband Planar Light Concentration||Ameen Elikkottil,Mohammed H Tahersima,MVN Surendra Gupta,Rishi Maiti,Volker J. Sorger,Bala Pesala###

Spectrally-tunable Dielectric Grating-based Metasurface for Broadband Planar Light Concentration. The energy consumption of buildings is increasing at a rapid pace due to
urbanization, while net-zero energy buildings powered by renewable sources
offer a green and sustainable solution. However, the limited rooftop space
available on skyscrapers and multi-story buildings poses a challenge for
large-scale integration of solar photovoltaic modules. Conventional
photovoltaics solutions such as silicon solar panels block the visible light
from entering the building rending them unfeasible to cover all building
surfaces. Here, we demonstrate a novel dielectric metasurface (based-on silicon
nitride grating) acting as a planar light concentrator. We integrate this
functional device onto a window glass transmitting visible light while
simultaneously guiding the near infrared portion (NIR) of sunlight to the edges
of the glass window where it can be converted to electricity by a small PV
module. Utilizing the grating design flexibility, we tune the spectra to enable
guiding of the near NIR sunlight portion and realize polarization independence
demonstrated using finite difference time domain simulations. Experimentally,
we observe about 5.25% of optical guiding efficiency in the NIR region
(700-1000 nm), leaving majority of the visible portion to be transmitted for
natural room lighting. Integrating the solar cell at the window edge, we find a
power conversion efficiency of about 4.2% of NIR light on a prototype of area
25 mm 2. We confirm that the majority of the loss is due to the absorption,
scattering and fabrication non-uniformity over large area which can be further
optimized in future. Such a functional window combining renewable energy
generation, with room lighting, and building-envelope reduction could mitigate
urban heat-islanding issues of modern cities.

###Surface Passivation of III-V GaAs Nanopillars by Low Frequency Plasma Deposition of Silicon Nitride for Active Nanophotonic Devices||Bejoys Jacob,Filipe Camarneiro,Jérôme Borme,Oleksandr Bondarchuk,Jana B. Nieder,Bruno Romeira###

Surface Passivation of III-V GaAs Nanopillars by Low Frequency Plasma Deposition of Silicon Nitride for Active Nanophotonic Devices. Numerous efforts have been devoted to improve the electronic and optical
properties of III-V compound materials via reduction of their nonradiative
states, aiming at highly-efficient III-V sub-micrometer devices. Despite many
advances, there is still a controversial debate on which combination of
chemical treatment and capping dielectric layer can best reproducibly protect
the crystal surface of III-Vs, while being compatible with readily available
plasma deposition methods. This work reports on a systematic experimental study
on the role of sulfide ammonium chemical treatment followed by dielectric
coating in the passivation effect of GaAs/AlGaAs nanopillars. Our results
conclusively show that the best surface passivation is achieved using ammonium
sulfide followed by encapsulation with a thin layer of silicon nitride by low
frequency plasma enhanced chemical deposition. Here, the sulfurized GaAs
surfaces, the high level of hydrogen ions and the low frequency (380 kHz)
excitation plasma that enable intense bombardment of hydrogen, all seem to
provide a combined active role in the passivation mechanism of the pillars. We
observe up to a 29-fold increase of the photoluminescence (PL) integrated
intensity for the best samples as compared to untreated nanopillars. X-ray
photoelectron spectroscopy analysis confirms the best treatments show
remarkable removal of gallium and arsenic native oxides. Time-resolved micro-PL
measurements display nanosecond lifetimes resulting in a record-low surface
recombination velocity for dry etched GaAs nanopillars. We achieve robust,
stable and long-term passivated nanopillar surfaces which creates expectations
for remarkable high internal quantum efficiency (IQE>0.5) in nanoscale
light-emitting diodes. The enhanced performance paves the way to many other
nanostructures and devices such as miniature resonators, lasers, photodetectors
and solar cells.

###First-principles study of optoelectronic and thermoelectronic properties of the ScAgC half-Heusler compound||Vinod Kumar Solet,Shamim Sk,Sudhir K. Pandey###

First-principles study of optoelectronic and thermoelectronic properties of the ScAgC half-Heusler compound. This work presents a theoretical study regarding photovoltaic (PV) &
thermoelectric (TE) applications of ScAgC. The electronic, optical and
thermoelectric properties are investigated using DFT and semi-classical
Boltzmann transport theory. DFT calculates a direct band-gap of $\sim$$0.47$
eV, while $G_{0}W_{0}$ method estimates a band-gap of $\sim1.01$ eV. We used
parabola fitting to estimate effective mass ($m^{*}$) values for bands B1-B4 at
$\Gamma$-point, which are $\sim$ -0.087 (-0.075), $\sim$ -0.17 (-0.27), $\sim$
-0.17 (-0.27), and $\sim$ 0.049 (0.058) along the $\Gamma-X$ ($\Gamma-L$)
direction, respectively. We also examine phonon dispersion and thermal
properties. Furthermore, optoelectronics' properties are calculated within the
energy range of 0 to 10 eV. The optical conductivity $\sigma (\omega)$,
refractive index $\tilde{n}(\omega)$, and dielectric function $\epsilon
(\omega)$ show strong optical transitions in the visible region. The lowest
calculated value of reflectivity r($\omega$) is $\sim$0.24 at $\sim$4.7 eV, and
the highest calculated value of absorption coefficient $\alpha (\omega )$ is
$\sim1.7\times 10^{6} cm^{-1}$ at $\sim$ 8.5 eV. At 300 K, we expect a maximum
solar efficiency (SLME) of $\sim$33% at $\sim$1 $\mu m$. The lattice thermal
conductivity $\kappa_{ph}$ shows maximum value of $\sim$3.8 W$m^{-1}K^{-1}$ at
1200 K. At 1200 K, for electron doping of $\sim$3.9$\times$$10^{21}$$cm^{-3}$,
the maximum value of $S^{2}\sigma /\tau$ is $\sim$145 $\times 10^{14}$ $\mu
WK^{-2}cm^{-1}s^{-1}$, while for hole doping of
$\sim$1.5$\times$$10^{21}$$cm^{-3}$, it is $\sim$123 $\times 10^{14}$ $\mu
WK^{-2}cm^{-1}s^{-1}$. The highest $ZT$ at 1200 K is expected to be $\sim$0.53,
whereas the optimal \%efficiency is predicted as $\sim$8.5% for cold (hot)
temperatures of 300 (1200) K. The results suggest that ScAgC can be a potential
candidate for solar cell and TE applications.

###A simple all-inorganic hole-only device structure for monitoring the trap densities in perovskite solar cells||Atena Mohamadnezhad,Mahmoud Samadpour###

A simple all-inorganic hole-only device structure for monitoring the trap densities in perovskite solar cells. One of the most critical challenges in soaring the performance of perovskite
solar cells is decreasing the density of trap states in the light-absorbing
perovskite layer. These traps cause an increase in the recombination of charge
carriers and decrease the efficiency of devices. One of the methods to study
the trap density is space charge limited current (SCLC) analysis. For this
purpose, some structures are needed with the ability to transport only
electrons or holes. The trap density can be calculated by investigating the
current-voltage diagram and finding the voltage corresponding to the slope
change point. One of the challenges in these structures is using organic
polymers like Spiro-OMeTAD, PEDOT: PSS, and PTAA as hole transport layers. They
have problems like high acidity, lack of stability against moisture, low charge
mobility, low conductivity, and high cost. In this work, a hole-only device
structure is explained, made based on inorganic materials, which possesses high
stability, a simple preparation method, and reasonable cost compared to
conventional hole-only device structures. This structure is built by coating a
nanostructured NiOx layer, perovskite, CIS, and Au on the ITO substrate. To
investigate the performance of this structure, various perovskite layers were
made at different experimental conditions, and their trap density was obtained
by the proposed hole-only device structure. The analysis of the photovoltaic
characteristics of cells revealed a clear correlation between the perovskite
layer's trap density and the cells' performance. Our results show the
introduced structure is a simple and stable structure that can be utilized in
studying the trap density in perovskite layers to make more efficient cells.

###Understanding electric field control of electronic and optical properties of strongly-coupled multi-layer quantum dot molecules||Muhammad Usman###

Understanding electric field control of electronic and optical properties of strongly-coupled multi-layer quantum dot molecules. Strongly-coupled quantum dot molecules (QDMs) are widely deployed in the
design of a variety of optoelectronic, photovoltaic, and quantum information
devices. An efficient and optimized performance of these devices demands
engineering of the electronic and optical properties of the underlying QDMs.
The application of electric fields offers a knob to realise such control over
the QDM characteristics for a desired device operation. We perform
multi-million-atom atomistic tight-binding calculations to study the influence
of electric fields on the electron and hole wave function confinements and
symmetries, the ground-state transition energies, the band-gap wavelengths, and
the optical transition modes. The electrical fields both parallel ($\vec{E_p}$)
and anti-parallel ($\vec{E_a}$) to the growth direction are investigated to
provide a comprehensive guide on the understanding of the electric field
effects. The strain-induced asymmetry of the hybridized electron states is
found to be weak and can be balanced by applying a small $\vec{E_a}$ electric
field, of the order of 1 KV/cm. The strong interdot couplings completely break
down at large electric fields, leading to single QD states confined at the
opposite edges of the QDM. This mimics a transformation from a type-I band
structure to a type-II band structure for the QDMs, which is a critical
requirement for the design of intermediate-band solar cells (IBSC). The
analysis of the field-dependent ground-state transition energies reveal that
the QDM can be operated both as a high dipole moment device by applying large
electric fields and as a high polarizibility device under the application of
small electric field magnitudes. [abstract is truncated to fit the character
count of arXiv]

###Molecular Electronics by Chemical Modification of Semiconductor Surfaces||Ayelet Vilan,David Cahen###

Molecular Electronics by Chemical Modification of Semiconductor Surfaces. Inserting molecular monolayers within metal / semiconductor interfaces
provides one of the most powerful expressions of how minute chemical
modifications can affect electronic devices. This topic also has direct
importance for technology as it can help improve the efficiency of a variety of
electronic devices such as solar cells, LEDs, sensors and possible future
bioelectronic devices, which are based mostly on non-classical semiconducting
materials (section 1). The review covers the main aspects of using chemistry to
- control alignment of energy levels at interfaces (section 2): - passivate
interface states (section 3), - insert molecular dipoles at interfaces (section
4), - induce charge rearrangement at and around interfaces (section 5). After
setting the stage, we consider the unique current-voltage characteristics that
result from transport across metal / molecular monolayer / semiconductor
interfaces. Here we focus on the interplay between the monolayer as tunneling
barrier on the one hand, and the electrostatic barrier within the
semiconductor, due to its space-charge region (section 6), on the other hand,
as well as how different monolayer chemistries control each of the these
barriers. Section 7 provides practical tools to experimentally identify these
two barriers, and distinguish between them, after which section 8 concludes the
story with a summary and a view to the future. While this review is concerned
with hybrid semiconductor / molecular effects (see Refs. 1,2 for earlier
reviews on this topic), issues related to formation of monolayers and contacts,
as well as charge transport that is solely dominated by molecules, have been
reviewed elsewhere[3-6], including by us recently[7].

###MinXSS-1 CubeSat On-Orbit Pointing and Power Performance: The First Flight of the Blue Canyon Technologies XACT 3-axis Attitude Determination and Control System||James Paul Mason,Matt Baumgart,Bryan Rogler,Chloe Downs,Margaret Williams,Thomas N. Woods,Scott Palo,Phillip C. Chamberlin,Stanley Solomon,Andrew Jones,Xinlin Li,Rick Kohnert,Amir Caspi###

MinXSS-1 CubeSat On-Orbit Pointing and Power Performance: The First Flight of the Blue Canyon Technologies XACT 3-axis Attitude Determination and Control System. The Miniature X-ray Solar Spectrometer (MinXSS) is a 3 Unit (3U) CubeSat
designed for a 3-month mission to study solar soft X-ray spectral irradiance.
The first of the two flight models was deployed from the International Space
Station in 2016 May and operated for one year before its natural deorbiting.
This was the first flight of the Blue Canyon Technologies XACT 3-axis attitude
determination and control system -- a commercially available, high-precision
pointing system. We characterized the performance of the pointing system on
orbit including performance at low altitudes where drag torque builds up. We
found that the pointing accuracy was 0.0042\degree\ - 0.0117\degree\ (15$''$ -
42$''$, 3$\sigma$, axis dependent) consistently from 190 km - 410 km, slightly
better than the specification sheet states. Peak-to-peak jitter was estimated
to be 0.0073\degree\ (10 s$^{-1}$) - 0.0183\degree\ (10 s$^{-1}$) (26$''$ (10
s$^{-1}$) - 66$''$ (10 s$^{-1}$), 3$\sigma$). The system was capable of dumping
momentum until an altitude of 185 km. We found small amounts of sensor
degradation in the star tracker and coarse sun sensor. Our mission profile did
not require high-agility maneuvers so we are unable to characterize this
metric. Without a GPS receiver, it was necessary to periodically upload
ephemeris information to update the orbit propagation model and maintain
pointing. At 400 km, these uploads were required once every other week. At
$\sim$270 km, they were required every day. We also characterized the power
performance of our electric power system, which includes a novel pseudo-peak
power tracker -- a resistor that limited the current draw from the battery on
the solar panels. With 19 30\% efficient solar cells and an 8 W system load,
the power balance had 65\% of margin on orbit. We present several
recommendations to other CubeSat programs throughout.

###Achieving balanced open circuit voltage and short circuit current by tuning the interfacial energetics in organic bulk heterojunction solar cells: A drift-diffusion simulation||Wenchao Yang###

Achieving balanced open circuit voltage and short circuit current by tuning the interfacial energetics in organic bulk heterojunction solar cells: A drift-diffusion simulation. In organic bulk heterojunction solar cells, the donor/acceptor interfacial
energy offset ($\Delta E$) is found to provide the driving force for efficient
charge separation which gives rise to high short circuit current density
($J_\mathrm{sc}$), but a high $\Delta E$ inevitably undermines the open circuit
voltage ($V_\mathrm{oc}$). In this paper, employing the device model method we
calculated the steady state current density-voltage ($J-V$) and the
$J_\mathrm{sc}-\Delta E$ curves under two different charge separation
mechanisms to investigate the optimum driving force required for achieving
sizable $V_\mathrm{oc}$ and $J_\mathrm{sc}$ simultaneously. Under the Marcus
charge transfer mechanism, with the increased $\Delta E$ the Jsc increases
rapidly for $\Delta E\leq 0.2$ eV, and then maintains a nearly constant value
before decreasing at the Marcus inverted region, which is due to the
accumulation of undissociated excitons within their lifetime and is beneficial
for obtaining a sizable $J_\mathrm{sc}$ under a $\Delta E$ much smaller than
the reorganization energy $\lambda$. For the coherent charge transfer mechanism
in which the driving force act as the energy window of accessible charge
separated states, with two typical types of density of states for the charge
transfer excitons, it is shown that the highest $J_\mathrm{sc}$ can also be
achieved under a small $\Delta E$ of 0.2\,eV if the high-lying delocalized
states are harvested in high proportion. This work demonstrates the existence
of the optimum driving force of 0.2\,eV and provides some guidelines for
engineering the interfacial energetics to achieve the high balanced
$J_\mathrm{sc}$ and $V_\mathrm{oc}$.

###Deep Levels and Mixed Conductivity in Organometallic Halide Perovskites||Artem Musiienko,Pavel Moravec,Roman Grill,Petr Praus,Igor Vasylchenko,Jakub Pekarek,Jeremy Tisdale,Katarina Ridzonova,Eduard Belas,Lucie Abelova,Bin Hu,Eric Lukosi,Mahshid Ahmadi###

Deep Levels and Mixed Conductivity in Organometallic Halide Perovskites. Understanding the type, formation energy and capture cross section of defects
is one of the challenges in the field of organometallic halide perovskite
(OMHP) devices. Currently, such understanding is limited, restricting the power
conversion efficiencies of OMHPs solar cells from reaching their Shockley
Queisser limit. Here, we report on deep level (DL) defects and their effect on
free charge transport properties of single crystalline methylammonium lead
bromide perovskite (MAPB). In order to determine DL activation energy and
capture cross section we used photo-Hall effect spectroscopy (PHES) with
enhanced illumination in both steady-state and dynamic regimes. This method has
shown to be convenient due to the direct DL visualization by sub-bandgap
photo-excitation of trapped carriers. DLs with activation energies of 1.05 eV,
1.5 eV, and 1.9 eV above valence band were detected. The hole capture cross
section was found using photoconductivity relaxation after sub-bandgap
photo-excitation. We found the DL defects responsible for non-radiative
recombination and its impact on band alignment for the first time.
Additionally, the transport properties of MAPB single crystal is measured by
Time of Flight (ToF) at several biases. The analysis of ToF measurement further
confirms increase of Hall mobility and the enhancement of hole transport
produced by sub-bandgap illumination in MAPB devices. Our studies provide a
strong evidence on deep levels in OMHPs and opens a richer picture of the role
and properties of deep levels in MAPB single crystals as a system model for the
first time. The deeper knowledge of the electrical structure of OMHP could open
further opportunities in the development of more feasible technology.

###MoS2 Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of CH3NH3PbI3 Perovskite Solar Cell with Efficiency over 20%||Leyla Najafi,Babak Taheri,Beatriz Martin-Garcia,Sebastiano Bellani,Diego Di Girolamo,Antonio Agresti,Reinier Oropesa-Nunez,Sara Pescetelli,Luigi Vesce,Emanuele Calabro,Mirko Prato,Antonio E. Del Rio Castillo,Aldo Di Carlo,Francesco Bonaccorso###

MoS2 Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of CH3NH3PbI3 Perovskite Solar Cell with Efficiency over 20%. Interface engineering of organic-inorganic halide perovskite solar cells
(PSCs) plays a pivotal role in achieving high power conversion efficiency
(PCE). Graphene and related two-dimensional materials (GRMs) are promising
candidates to tune on demand the interface properties of PSCs. In this work, we
fully exploit the potential of GRMs by controlling the optoelectronic
properties of hybrids between molybdenum disulfide (MoS2) and reduced graphene
oxide (RGO) as hole transport layer (HTL) and active buffer layer (ABL) in
mesoscopic methylammonium lead iodide (CH3NH3PbI3) perovskite (MAPbI3)-based
PSC. We show that zero-dimensional MoS2 quantum dots (MoS2 QDs), derived by
liquid phase exfoliated MoS2 flakes, provide both hole-extraction and
electron-blocking properties. In fact, on the one hand, intrinsic n-type
doping-induced intra-band gap states effectively extract the holes through an
electron injection mechanism. On the other hand, quantum confinement effects
increase the optical band gap of MoS2 (from 1.4 eV for the flakes to > 3.2 for
QDs), raising the minimum energy of its conduction band (from -4.3 eV for the
flakes to -2.2 eV for QDs) above the one of conduction band of MAPbI3 (between
-3.7 and -4 eV) and hindering electron collection. The van der Waals
hybridization of MoS2 QDs with functionalized reduced graphene oxide (f-RGO),
obtained by chemical silanization-induced linkage between RGO and
(3-mercaptopropyl)trimethoxysilane, is effective to homogenize the deposition
of HTLs or ABLs onto the perovskite film, since the two-dimensional (2D) nature
of RGO effectively plug the pinholes of the MoS2 QDs films. Our graphene
interface engineering (GIE) strategy based on van der Waals MoS2 QD/graphene
hybrids enable MAPbI3-based PSCs to achieve PCE up to 20.12% (average PCE of
18.8%).

###Strain engineering for ultra-coherent nanomechanical oscillators||Amir H. Ghadimi,Sergey A. Fedorov,Nils J. Engelsen,Mohammad J. Bereyhi,Ryan Schilling,Dalziel J. Wilson,Tobias J. Kippenberg###

Strain engineering for ultra-coherent nanomechanical oscillators. Elastic strain engineering utilizes stress to realize unusual material
properties. For instance, strain can be used to enhance the electron mobility
of a semiconductor, enabling more efficient solar cells and smaller, faster
transistors. In the context of nanomechanics, the pursuit of resonators with
ultra-high coherence has led to intense study of a complementary strain
engineering technique, "dissipation dilution", whereby the stiffness of a
material is effectively increased without added loss. Dissipation dilution is
known to underlie the anomalously high Q factor of Si$_3$N$_4$ nanomechanical
resonators, including recently-developed "soft-clamped" resonators; however,
the paradigm has to date relied on weak strain produced during material
synthesis. By contrast, the use of geometric strain engineering techniques --
capable of producing local stresses near the material yield strength -- remains
largely unexplored. Here we show that geometric strain combined with
soft-clamping can produce unprecedentedly high Q nanomechanical resonators.
Specifically, using a spatially non-uniform phononic crystal pattern, we
colocalize the strain and flexural motion of a Si$_3$N$_4$ nanobeam, while
increasing the former to near the yield strength. This combined strategy
produces string-like modes with room-temperature Q$\times$frequency products
approaching $10^{15}$ Hz, an unprecedented value for a mechanical oscillator of
any size. The devices we study can have force sensitivities of aN/rtHz, perform
hundreds of quantum coherent oscillations at room temperature, and attain Q >
400 million at radio frequencies. These results signal a paradigm shift in the
control of nanomechanical dissipation, with impact ranging from precision force
microscopy to tests of quantum gravity. Combining the reported approach with
crystalline or 2D materials may lead to further improvement, of as yet unknown
limitation.

###Ionically gated perovskite solar cell with tunable carbon nanotube interface at thick fullerene electron transporting layer: comparison to gated OPV||D. S. Saranin,D. S. Muratov,R. Haroldson,A. G. Nasibulin,A. R. Ishteev,D. V. Kuznetsov,M. N. Orlova,S. I. Didenko,A. A. Zakhidov###

Ionically gated perovskite solar cell with tunable carbon nanotube interface at thick fullerene electron transporting layer: comparison to gated OPV. We demonstrate an ionically gated planar PS-PV solar cell with ultra-thick
fullerene ETL with a porous CNT electron collector on top of it. Perovskite
photovoltaic devices usually have undoped electron transport layers, usually
thin like C60 due to its high resistance. Metallic low work function cathodes
are extremely unstable in PS-PV due to reaction with halogens I-/Br-, and it
would be desirable to have stable carbon cathodes on top of thick low
resistance ETL for enhancing the stability of PS-PVs. We show that gating such
top CNT cathode in ionic liquid, as part of a supercapacitor charged by Vg
tunes the Fermi level of CNT by EDL charging, and causes lowering of a barrier
at of C60/C70 ETL. Moreover, at higher gating voltage ions further propagates
into fullerene by electrochemical n-doping, which increases dramatically PV
performance by raising mostly two parameters: Isc and FF, resulting in PCE
efficiency raised from 3 % to 11 %. N-doping of ETL strongly enhances charge
collection by ETL and CNT raising Isc and lowering series resistance and thus
increasing strongly PCE. Surprisingly Voc is not sensitive in PS-PV to external
Vg gating, on the contrary, to strongly enhanced Voc in ionically gated organic
PV, where it is the main gating effect. This insensitivity of Voc to lowering
of the work function of Vg gated CNT electrode is a clear indication that Voc
in PS-PV is determined by inner p-i-n junction formation in PS itself, via
accumulation of its intrinsic mobile ionic species halogens and cations and
their vacancies.

###Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication||Filchito Renee G. Bagsican,Michael Wais,Natsumi Komatsu,Weilu Gao,Lincoln W. Weber,Kazunori Serita,Hironaru Murakami,Karsten Held,Frank A. Hegmann,Masayoshi Tonouchi,Junichiro Kono,Iwao Kawayama,Marco Battiato###

Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication. Excitons play major roles in optical processes in modern semiconductors, such
as single-wall carbon nanotubes (SWCNTs), transition metal dichalcogenides, and
2D perovskite quantum wells. They possess extremely large binding energies
(>100~meV), dominating absorption and emission spectra even at high
temperatures. The large binding energies imply that they are stable, that is,
hard to ionize, rendering them seemingly unsuited for optoelectronic devices
that require mobile charge carriers, especially terahertz emitters and solar
cells. Here, we have conducted terahertz emission and photocurrent studies on
films of aligned single-chirality semiconducting SWCNTs and find that excitons
autoionize, i.e., spontaneously dissociate into electrons and holes. This
process naturally occurs ultrafast (<1~ps) while conserving energy and
momentum. The created carriers can then be accelerated to emit a burst of
terahertz radiation when a dc bias is applied, with promising efficiency in
comparison to standard GaAs-based emitters. Furthermore, at high bias, the
accelerated carriers acquire high enough kinetic energy to create secondary
excitons through impact exciton generation, again in a fully energy and
momentum conserving fashion. This exciton multiplication process leads to a
nonlinear photocurrent increase as a function of bias. Our theoretical
simulations based on nonequilibrium Boltzmann transport equations, taking into
account all possible scattering pathways and a realistic band structure,
reproduce all our experimental data semi-quantitatively. These results not only
elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in
SWCNTs but also suggest promising routes toward terahertz excitonics despite
the orders-of-magnitude mismatch between the exciton binding energies and the
terahertz photon energies.

###Antiferromagnetism-induced second-order nonlinear optical responses of centrosymmetric bilayer CrI$_3$||Vijay Kumar Gudelli,Guang-Yu Guo###

Antiferromagnetism-induced second-order nonlinear optical responses of centrosymmetric bilayer CrI$_3$. Antiferromagnetism (AF) in AB'-stacked centrosymmetric bilayer (BL) CrI$_3$
breaks both spatial inversion ($P$) and time-reversal ($T$) symmetries but
maintains the combined $PT$ symmetry, thus inducing novel second-order
nonlinear optical (NLO) responses such as second-harmonic generation (SHG),
linear electric-optic effect (LEO) and bulk photovoltaic effect (BPVE). In this
work, we calculate AF-induced NLO responses of the BL CrI$_3$ based on the
density functional theory with the generalized gradient approximation (GGA)
plus onsite Coulomb correlation (U), i.e., the GGA+U method. Interestingly, we
find that the magnetic SHG, LEO and photocurrent in the AF BL CrI$_3$ are huge,
being comparable or even larger than that of the well-known nonmagnetic
noncentrosymmetric semiconductors. For example, the calculated SHG coefficients
are in the same order of magnitude as that of MoS$_2$ monolayer (ML), the most
promising 2D material for NLO devices. The calculated LEO coefficients are
almost three times larger than that of MoS$_2$ ML. The calculated NLO
photocurrent in the CrI$_3$ BL is among the largest values predicted so far for
the BPVE materials. On the other hand, unlike nonmagnetic semiconductors, the
NLO responses in the AF BL CrI$_3$ are nonreciprocal and also switchable by
rotating magnetization direction. Therefore, our interesting findings indicate
that the AF BL CrI$_3$ will not only provide a valuable platform for exploring
new physics of low-dimensional magnetism but also have promising applications
in magnetic NLO and LEO devices such as frequency conversion, electro-optical
switches, and light signal modulators as well as high energy conversion
efficiency photovoltaic solar cells.

###A Gaussian Approximation Potential for Amorphous Si:H||Davis Unruh,Reza Vatan Meidanshahi,Stephen M. Goodnick,Gábor Csányi,Gergely T. Zimányi###

A Gaussian Approximation Potential for Amorphous Si:H. Hydrogenation of amorphous silicon (a-Si:H) is critical for reducing defect
densities, passivating mid-gap states and surfaces, and improving
photoconductivity in silicon-based electro-optical devices. Modelling the
atomic scale structure of this material is critical to understanding these
processes, which in turn is needed to describe c-Si/a-Si:H heterjunctions that
are at the heart of the modern solar cells with world record efficiency.
Density functional theory (DFT) studies achieve the required high accuracy but
are limited to moderate system sizes a hundred atoms or so by their high
computational cost. Simulations of amorphous materials in particular have been
hindered by this high cost because large structural models are required to
capture the medium range order that is characteristic of such materials.
Empirical potential models are much faster, but their accuracy is not
sufficient to correctly describe the frustrated local structure. Data driven,
"machine learned" interatomic potentials have broken this impasse, and have
been highly successful in describing a variety of amorphous materials in their
elemental phase. Here we extend the Gaussian approximation potential (GAP) for
silicon by incorporating the interaction with hydrogen, thereby significantly
improving the degree of realism with which amorphous silicon can be modelled.
We show that our Si:H GAP enables the simulation of hydrogenated silicon with
an accuracy very close to DFT, but with computational expense and run times
reduced by several orders of magnitude for large structures. We demonstrate the
capabilities of the Si:H GAP by creating models of hydrogenated liquid and
amorphous silicon, and showing that their energies, forces and stresses are in
excellent agreement with DFT results, and their structure as captured by bond
and angle distributions, with both DFT and experiments.

###Intrinsic doping limitations in inorganic lead halide perovskites||Fernando P. Sabino,Alex Zunger,Gustavo M. Dalpian###

Intrinsic doping limitations in inorganic lead halide perovskites. Inorganic Halide perovskites (HP's) of the CsPbX3 (X=I, Br, Cl) type have
reached prominence in photovoltaic solar cell efficiencies. Peculiarly, they
have shown, however, an asymmetry in their ability to be doped by holes rather
than by electrons. Indeed, both structural defect-induced doping as well as
extrinsic impurity-induced doping strangely result in a unipolar doping
(dominantly p-type) with low free carriers concentration. This raises the
question whether such doping limitations presents just a temporary setback due
to insufficient optimization of the doping process, or perhaps this represents
an intrinsic, physically-mandated bottleneck. In this paper we study three
fundamental Design Principles (DP's) for ideal doping, applying them via
density functional doping theory to these HP's, thus identifying the violated
DP that explains the doping limitations and asymmetry in these HP's. Here, the
target DP are: (i) requires that the thermodynamic transition level induced by
the dopants must ideally be energetically shallow both for donors (n-type) or
acceptors (p-type); DP-(ii) requires that the 'Fermi level pinning energies'
for electrons and holes (being the limiting values of the Fermi level before a
structural defect that compensate the doping forms spontaneously) should
ideally be located inside the conduction band for n-type doping and inside the
valence band for p-type doping. DP-(iii) requires that the doping-induced
equilibrium Fermi energy shifts towards the conduction band for n-type doping
(shift towards the valence band, for p-type doping) to be sufficiently large.
We find that, even though in HP's based on Br and Cl there are numerous shallow
level dopants that satisfy DP-(i), in contrast DP-(ii) is satisfied only for
holes and DP-(iii) fail for both holes and electrons, being the ultimate
bottleneck for the n-type doping in Iodine HP's.

###Gettering in polySi/SiOx passivating contacts enables Si-based tandem solar cells with high thermal and contamination resilience||Alireza Assar,Filipe Martinho,Jes Larsen,Nishant Saini,Denver Shearer,Marcos V. Moro,Fredrik Stulen,Sigbjørn Grini,Sara Engberg,Eugen Stamate,Jørgen Schou,Lasse Vines,Stela Canulescu,Charlotte Platzer-Björkman,Ole Hansen###

Gettering in polySi/SiOx passivating contacts enables Si-based tandem solar cells with high thermal and contamination resilience. Multijunction solar cells in a tandem configuration could further lower the
costs of electricity if crystalline Si (c-Si) is used as bottom cell. However,
for direct monolithic integration on c-Si, only a restricted number of top and
bottom cell architectures are compatible, due to either epitaxy or high
temperature constraints, where the interface between subcells is subject to a
trade-off between transmittance, electrical interconnection, and bottom cell
degradation. Using polySi/SiOx passivating contacts for Si, this degradation
can be largely circumvented by tuning the polySi/SiOx stacks to promote
gettering of contaminants admitted into the Si bottom cell during the top cell
synthesis. Applying this concept to the low-cost top cell chalcogenides
Cu2ZnSnS4 (CZTS), CuGaSe2 (CGSe) and AgInGaSe2 (AIGSe), fabricated under harsh
S or Se atmospheres above 550 {\deg}C, we show that increasing the
heavily-doped polySi layer thickness from 40 to up to 400 nm prevents a
reduction in Si carrier lifetime by one order of magnitude, with final
lifetimes above 500 us uniformly across areas up to 20 cm2. In all cases, the
increased resilience was correlated with a 99.9% reduction in contaminant
concentration in the c-Si bulk, provided by the thick polySi layer, which acts
as a buried gettering layer in the tandem structure without compromising the Si
passivation quality. The Si resilience decreased as AIGSe > CGSe > CZTS, in
accordance with the measured Cu contamination profiles and higher annealing
temperatures. An efficiency of up to 7% was achieved for a CZTS/Si tandem,
where the Si bottom cell is no longer the limiting factor.