Efficient Pb-free and Less Pb Perovskite Solar Cells: Fabrication Process and Cell Configuration
Corey Grice a, Changlei Wang a, Yu Yu a, Alexander Cimaroli a, Weiqiang Liao a, Dewei Zhao a, Yanfa Yan a, Ren-Gen Xiong b, Kai Zhu c
a Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, 2801 Bancroft Street, Toledo, 43606, United States
b Ordered Matter Science Research Center, Southeast University, Nanjing 211189
c Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Invited Speaker Session, Yanfa Yan, presentation 089
Publication date: 7th November 2016

Despite the rapid improvement in the device efficiency of organic-inorganic lead (Pb) halide perovskite solar cells (PVSCs) in the past years, the use of toxic Pb may severely limit their broad applications and commercialization.The development of Pb-free or less Pb PVSCs is highly desirable and has drawn extensive attention. Methylammonium tin iodide (MASnI3) based Pb-free PVSCs with PCEs of around 6% have been reported in 2014, which have stimulated considerable follow-up studies on Pb-free Sn-based PVSCs. However, since 2014, no further improvement in the PCE of Pb-free Sn-based PVSCs was reported. We report on the fabrication of efficient Pb-free planar Sn-based PVSCs, which consist of formamidinium tin triiodide (FASnI3) as the light absorber. Along with solvent engineering, the use of tin fluoride additives is critical for synthesizing highly uniform and pinhole-free compact FASnI3 perovskites with a suitable carrier density. Our Pb-free FASnI3 PVSCs have achieved PCEs higher than 6%, exhibiting very little J-V hysteresis and high reproducibility. Furthermore, using a new precursor combining a FASnI3 precursor with a MAPbI3 precursor, we are able to deposit high-quality mixed Sn-Pb thin films, which, in turn, enables us to fabricate efficient low-bandgap mixed Sn-Pb PVSCs. The composition of the thin films is controlled by the molar ratio of MAPbI3 to FASnI3 precursors, giving a formula of (FASnI3)1-x(MAPbI3)x. The lowest bandgap, ~1.2 eV, was obtained with x = 0.4. With this Pb content, our mixed Sn-Pb perovskite solar cells have achieved PCEs over 15%. For both Pb-free and less Pb perovskite solar cells, an inverted structure of glass/ITO/PEDOT:PSS/perovskite/C60/BCP/Ag is used. It was noticed that the lithium (Li) and/orcobalt (Co) salts used as dopants in organic hole selective layers may damage Sn-containingperovskites. The use of the inverted cell structure avoids this problem and palys a key role for successfully fabricating efficient Sn-free and less Pb mixed Sn-Pb perovskite solar cells.

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