Focused Session FB-6
Perovskite Photovoltaics

Session FB-6.1 - Material Synthesis and Processing
 
FB-6.1:IL01  Efficient Sulfur-based Hole Transporting Materials for Perovskite Solar Cells
N. MARTIN, Departamento de Química Orgánica, Facultad de Química, Universidad Complutense, Madrid, Spain; IMDEA-Nanoscience, Madrid, Spain

Methylammonium (MA) lead halide MAPbX3 perovskites are currently among the most active materials for photovoltaic applications. Actually, the PCE of perovskite-based solar cells (PSCs) has dramatically increased from the initial 3.8% to a recently certified 22.1%. Emerging from this groundbreaking discovery, an unprecedented scientific research has sprouted in the field of photovoltaics due to their exceptional physical properties. Therefore, the development of cost-effective HTMs with high efficiency along with a good stability is an important task to address. Planar and sulfur-rich polycyclic aromatic hydrocarbons bearing arylamine moieties have demonstrated to be a successful approach for designing new highly efficient HTMs for PSCs. Conventionally, the π-extended conjugation associated with the planar and electron-rich structure of the fused heterocycles enable them to show strong stacking through intermolecular interactions (π‒π, S···S), thereby bestowing enhanced hole-carrier mobilities. This behaviour is beatifully exemplified by sulfur-rich HTMs recently reported. The performance of the solar cells employing the novel HTMs were measured under simulated 1 sun irradiation and conversion efficiencies up to 19 % were observed.


FB-6.1:IL03  Molecular Engineering of Hole-transporting Materials for Perovskite Solar Cells
A. MOLINA-ONTORIA¹, I. ZIMMERMANN², J. URIETA³, J. ARAGO⁴, E. ORTI⁴, M.K. NAZEERUDDIN², N. MARTÍN^{1, 3}, ¹IMDEA-Nanoscience, Campus de Cantoblanco, Madrid, Spain; ²Ecole Polytechnique Fédérale de Lausanne, Sion, Switzerland; ³Department of Organic Chemistry, Faculty of Chemistry, University Complutense, Madrid, Spain; ⁴Instituto de Ciencia Molecular, Universidad de Valencia, Paterna, Spain

Polycyclic aromatic hydrocarbons bearing arylamine moieties have demonstrated to be a successful approach for designing new highly efficient HTMs for PSCs. Conventionally, the π-extended conjugation associated with the planar and electron-rich structure of the fused heterocycles enable them to show strong stacking through intermolecular interactions (π‒π, S···S), thereby bestowing enhanced hole-carrier mobilities. This behaviour is beatifully exemplified by anthra[1,2-b:4,3-b′:5,6-b′′:8,7-b′′′]tetrathiophene-based (ATT) HTMs previously reported. On the other hand, three-dimensional (3D) HTMs show excellent solubility in organic solvents, which is beneficial for the formation of the hole transport layer on top of the perovskite layer, leading to remarkable device performance. Here we report a readily available new class of multi-armed hole transporting materials based on 3D extended pi-conjugation of the central cores. Devices were fabricated using state-of-the-art mixed anion mixed halide perovskite composition with the nominal formula [FAPbI3]0.85[MAPbBr3]0.15 (FA = formamidinium, MA = methylammonium). The performance of the solar cells employing the novel HTMs were measured under simulated 1 sun irradiation and conversion efficiencies beyond 18 % were measured.


FB-6.1:L04  Understanding the Effect of Precursor Solution Aging in Triple-cation Lead Perovskite
P. BOONMONGKOLRAS, DAEHAN KIM, BYUNGHA SHIN, Dept. of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon, South Korea

Due to its ease of fabrication and low production cost, solution process has been widely used to prepare halide perovskite absorbers for high efficiency photovoltaic devices. However, insufficient level of reproducibility has been a concern for the solution process. Recently, attention has been paid to understanding chemistry of perovskite precursor solutions. We studied the aging effects of precursor solutions for triple-cation halide perovskite on physical properties of the resultant films as well as on the final device performance. A precursor solution aged for the optimum hours was free of large colloidal aggregates while others (aged shorter or longer than the optimum hours) contained large aggregates on the order of several microns as revealed by Dynamic Light Scattering measurements. The absence of the large aggregates in the solutions appeared to be correlated to the phase purity of the prepared perovskite films, ultimately leading to the best efficiencies close to 17%. Details of further characterization supporting our conclusion will be presented.


Session FB-6.2 - Theoretical Modelling of Materials and Devices
 
FB-6.2:IL01  Device Physics of Perovskite Solar Cells
W. TRESS, EPFL, Lausanne, Switzerland

Solar cells based on lead halide perovskites have recently emerged showing a tremendous increase of power-conversion efficiency which exceeded 22%. In this contribution, the device physics of perovskite solar cells is addressed. The focus is on recombination of charge carriers because this process is ultimately limiting the performance. The origin of the open-circuit voltage is discussed based on the reciprocity relation between electroluminescence and photovoltaic quantum efficiency. Sharp absorption onset and high radiative recombination yield due to an extraordinary defect tolerance are identified as reasons for the outstanding optoelectronic properties of perovskites. Furthermore, the role of defect and surface recombination are addressed during light-soaking and degradation. The current-voltage curve of perovskite solar cells yields different results dependent on the initial voltage of the sweep. The resulting hysteresis is discussed is the framework of recombination and mobile ionic defects. An outlook is given on strategies aiming for a further improvement of open-circuit voltage and performance of perovskite solar cells toward their thermodynamic limit.


FB-6.2:IL02  Charge Carrier Diffusion and Trapping Models in Lead Halide Perovskites
HIROKI URATANI, KOICHI YAMASHITA, Department of Chemical System Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo, Japan

The high performance of recently emerged lead halide perovskite-based photovoltaic devices has been attributed to remarkable carrier properties in this kind of material: long carrier diffusion length, long carrier lifetime, and low electron-hole recombination rate. However, the mechanism of the charge separation is still not fully understood. In my talk, it will be demonstrated that the charge separation is induced by the structural fluctuation of the inorganic lattice using first-principles molecular dynamics simulations. The charge separation is attributed to the electrostatic potential fluctuation coupled to the inorganic lattice dynamics based on both simple tight-binding model-based analyses and first-principles calculations. These results suggest that the organic cations, which are often used as components of lead halide perovskites, are unlikely to be essential for the above-mentioned carrier properties. Hence, it is expected that all-inorganic lead halide perovskite-based photovoltaics might be able to rival organic-inorganic lead halide perovsktite-based ones in performance.


Session FB-6.3 - Material and Device Stability
 
FB-6.3:L02  How to Assess Operational Stability of Perovskite Solar Cells with Reversible Degradation?
M.V. KHENKIN¹, K.M. ANOOP¹, I. VISOLY-FISHER^{1, 2}, Y. GALAGAN³, F. DI GIACOMO³, B. RAMESH PATIL⁴, G. SHERAFATIPOUR⁴, V. TURKOVIC⁴, M. MADSEN⁴, T. MERCKX⁵, G. UYTTERHOEVEN⁵, J.P. A. BASTOS^{5, 6}, T. AERNOUTS⁵, F. BRUNETTI⁷, M. LIRA-CANTU⁸, E.A. KATZ^{1, 2}, ¹Dept. of Solar Energy and Environmental Physics, Swiss Inst. for Dryland Environmental and Energy Research, J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, Israel; ²Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beersheva, Israel; ³Holst Centre - Solliance, Eindhoven, the Netherlands; ⁴SDU NanoSYD, Mads Clausen Institute, University of Southern Denmark, Sønderborg, Denmark; ⁵IMEC - a partner in Solliance, Leuven, Belgium; ⁶Department Electrical Engineering, KU Leuven, Leuven, Belgium; ⁷CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of Rome Tor Vergata, Rome, Italy; ⁸Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, Barcelona, Spain

Development of hybrid organic-inorganic halide perovskite solar cells (PSCs) combining high performance and operational stability is a key issue for implementation of this technology. Both reversible improvement and reversible degradation of PSC efficiency were previously reported under illumination-darkness cycling. Quantifying the performance and stability of cells exhibiting significant diurnal performance variation is challenging and requires specific protocols. In this article we discuss outdoor stability measurements of two types of devices showing either reversible photo-degradation or pronounced reversible efficiency improvement under sunlight-soaking. Analysis of the results suggests that the figures of merit for photovoltaic performance and stability of such devices should be reconsidered. Instead of the classical approach of reporting the initial (or stabilized) efficiency value and estimation of T80, we propose to use the value of energy output generated during the first day of the exposure (or first illumination period in the light/darkness cycling indoor) and the time needed for reaching its 20% drop, respectively. The latter accounts for both the long-term irreversible degradation and the reversible diurnal efficiency variation and does not depend on the type of processes prevailing in a given perovskite cell.
 

Session FB-6.4 - Design of Lead-free New Materials
 
FB-6.4:IL01  Enhancement of Sn-perovskite Solar Cells from View Point of Hetero-interface Design and Crystal Defect Density
SHUZI HAYASE, Kyushu National Institute of Technology, Kitakyushu, Japan

We have already reported that the interface between TiO2 / perovskite (MAPbI3: Pb-PVK) contains Ti-O-Pb linkages which passivates the hetero-interface traps acting as charge recombination centers (1-5). In addition, we have proved that insertion of thin passivation layer (Trifluoro,ammonium propane cation: (F1)) increased the efficiency of the Pb-PVK solar cells (6). These results were applied for enhancing the efficiency of SnPb mixed metal perovskite solar cells. We have already reported that mixed metal perovskite (MAPbxSnyI3:SnPb-PVK) showed photoconversion properties in IR region (up to 1000 nm) (1). The SnPb PVK has a potential to possess ideal band gap (about 1.4eV), which is better than that of MAPbI3 (around 1.55eV) (1). The short circuit current (Jsc) was high, reaching to 30 mA/cm2 (for comparison, 24mA/cm2 for MAPbI3) because of the wide range of light harvesting properties from visible to IR region. However, the open-circuit voltage (Voc) was lower than 0.3 V. suggesting the presence of high density charge recombination centers. We discuss why the perovskite solar cells consisting of Sn have low efficiency, compared to MAPbI3 from the view point of hetero-interface architecture obtained from Pb-PVK study. We focused on the Voc loss for the evaluation of these hetero-interfaces. In the composition of TCO/c-TiO2/mp-TiO2/SnPb-PVK/SPIRO/Au (A), the Voc loss was about 0.9eV, which is larger than that of conventional MAPbI3 (0.44 eV). We found that Ti-O-Sn linkages are present at the hetero-interface between TiO2 and SnPb PVK and create new traps (charge recombination center). In order to remove the hetero-interface, inversion structure (TCO/PEDOT-PSS/SnPb-PVK/C60/Au)(B) was made. The Voc loss for (B) decreased to 0.5 eV which was lower than 0.9 eV for the common structure (A). In addition, the insertion of F1 was also effective for enhancing the efficiency. Decreasing crystal defect is another issue for enhancing the efficiency. After coping with these issues causing charge recombination, the Voc loss decreased to 0.43 eV and 16% efficiency was obtained. Voc loss for SnPb mixed metal PVK was almost the same as that for Pb-PVK solar cells, leading the conclusion that SnPb PVK has potential to high efficiency solar cell. These directions to enhancing efficiency were applied to increasing the efficiency of Pb-free Sn-perovskite solar cells. Pathway for enhancing the efficiency from 2% to 5.3% efficiency will be discussed.
1. Y. Ogomi, et al., J. Phys. Chem. Lett. 2014, 5, 1004-1011; 2. S. Nakabayashi, et al., J. Photonics for Energy; 2015, 5, 057410; 3. Y. Ogomi, et al., J. Phys. Chem. C, 2014, 118, 16651-16659; 4. Q. Shen, et al., Phys. Chem. Chem. Phys. 2014, 19984-19992; 5. Y. Ogomi, et al., Chem. Phys. Chem. 2014, 15, 1062-1069; 6. H. Moriya, et al., ChemSusChem., 2016, 9, 2634-2639.


FB-6.4:L02  Bismuth and Antimony-based Lead Free Double Perovskites in Solar Cells
M. PANTALER, C. FETTKENHAUER, I. ANUSCA, D.C. LUPASCU, Institute for Materials Science, University of Duisburg-Essen, Essen, Germany

Bismuth- or antimony-based lead-free double perovskites have been considered as alternatives to the emerging lead-based perovskites for solar cell applications. Organic–inorganic lead halide perovskite absorbers have an excellent photovoltaic properties, such as suitable bandgap, high optical absorption, and long carrier lifetime. The underlying issues are the presence of toxic lead and their instability under ambient atmosphere (e.g. O2 and H2O). Trivalent cations, such as Bi3+ and Sb3+, along with monovalent cations, such as Ag+, have been concurrently introduced into the B-sites of halide perovskites, leading to B cation double perovskites with the general chemical formula of A2B’B’’X6. These Pb-free double perovskites have been reported to have promising photovoltaic properties, including long carrier recombination lifetime, good stability against air and moisture, and low carrier effective masses. Thus potential alternative to the toxic lead halide perovskites. In our work, we report on different synthesis paths for obtaining a halide double perovskite, A2B’B’’X6 (A=Cs+, MA+, B’=Ag+, B’’=Sb3+, Bi3+, X=Br-, I-). We explore crystallographic and electronic structure, lifetime and mobility, optical absorption coefficient and the potential as absorber material in solar cells.

 
Session FB-6.5 - Scale up, Module Development and Measurement Protocols
 
FB-6.5:IL01  Carbon Perovskite Solar Cells from Laboratory to Factory
T. WATSON, SPECIFIC Swansea University, Swansea, UK

The “hole-conductor free” fully printable perovskite solar cell or triple mesoscopic carbon perovskite solar cell (C-PSC) as it is sometimes known is an exciting leap forward in the pursuit of industrial deployment of perovskite technology with recent reports demonstrating 10,000 hours of continuous light soaking data. This triple layered device structure comprising mesoporous TiO2, ZrO2 and Carbon deposited and sintered sequentially is likely to be one of the forerunners for early commercial adoption because the manufacturing method used, screen printing, requires low capital investment and delivers uniform large area deposition using inexpensive materials. This paper will present the recent challenges overcome in the transition to C-PSC modules; from patterning of the glass to the screen printing of multiple layers to infiltration of the perovskite into the device stack.


FB-6.5:L04  Development of Pb-free Perovskites and their Application for Solar Cells
CHU ZHANG¹, LIGUO GAO², SHUZI HAYASE¹, TINGLI MA^1,2, ¹Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology,Kitakyushu, Fukuoka, Japan; ²School Petroleum and Chemical Engineering, Dalian University of Technology, Panjin Campus, Pan-jin, Liaoning, China

Perovskite solar cells (PSCs) have been attracted much attention due to their high-energy conversion effi-ciency and low production cost. However, the issues of stability and toxicity are still remained. Our group focuses on the fundamental studies of PSCs devices, including development of a series of materials for PSCs. In this work, we will introduce the current progress of several types of Pb-free perovskites solar cells and remaining issues, as well as present the recent results of our group in development of Pb-free double perovskites such as Cs₂NaBX₆ (B = Sb and Bi; X = Cl, Br, I) and (CH₃NH₃)₃Bi₂I₉ (MBI) and their application in solar cells.