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. In this talk, I will first review our theoretical understanding of the photovoltaic properties of possible candidates of Pb-free or less Pb perovskites and related absorber materials. We conclude that fully or partially replacing Pb by Sn is the most promising approach for producing high efficiency Pb-free or less Pb PVSCs. I will then review our latest results on fabrication of efficient Pb-free and mixed Pb-Sn planar Sn-based PVSCs. Our Pb-free PVSCs consist of formamidinium tin triiodide (FASnI3) as the light absorber. Our Pb-free FASnI3 PVSCs have achieved power conversion efficiencies (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, ~1.25 eV, was obtained with x = 0.4. With this Pb content, our mixed Sn-Pb perovskite solar cells have achieved a certified PCE of over 17%. The efficient low-bandgap mixed Pn-Sn PVSCs enabled the fabrication of all-perovskite tandem solar cells. When mechanically stacked with a ~1.58 eV bandgap perovskite top cell, our best all-perovskite 4-terminal tandem solar cell obtains a maximum PCE of 21.2% and a steady-state efficiency of 21.0%.

Pb-free and less Pb perovskite thin-film solar cells: Theory and device

254th National Meeting (2017)

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