(355b) Enhancing Efficiency and Stability of Triple-Cation, Double-Halide Pb-Sn Alloyed Perovskite Solar Cells

Hybrid organic-inorganic lead halide perovskite solar cells have emerged in the past decade as a promising low-cost thin film photovoltaic device. The power conversion efficiency (PCE) has increased from 3.8% to 22.7%. Despite lead perovskites with triple-cation (cesium (Cs⁺), methylammonium (MA) (CH₃NH₃⁺), and formamidinium (FA) (CH₃(NH₂)₂⁺)) and double-halide (Br^- and I^-) were found to improve the stability and efficiency of perovskite solar cells, their effects on the alloyed Pb-Sn perovskites were unexplored. Here, we successfully made pinhole-free, cubic phase perovskite thin films with the compositions of Cs_x(MA_0.17FA_0.83)_1-xPb_1-ySn_y(I_0.83Br_0.17)₃, where x = 0.05, 0.1, and 0.2 and y = 0, 0.25, 0.50 0.75, and 1, via one-step solution process with anti-solvent washing. The band gap of these perovskites can be widely tuned. Increasing Sn from 0 to 75% lowed the band gap from 1.61-1.64 eV to 1.25-1.31 eV and then increased to 1.42-1.46 eV with 100% Sn. Increasing Cs from 5 to 20% enlarged the band gap slightly for Pb-riched perovskites while narrowed the band gap for Sn-riched perovskites due to the complex effects of lattice contraction and octahedral tilting. We fabricated perovskite solar cells with the inverted device structure of indium tin oxide (ITO)/poly (3,4, -ethylenedioxythiphene): polystyrene sulfonate (PEDOT: PSS)/Perovskite/[6,6]-phenyl-C₆₀-butyric acid methyl ester (PC₆₀BM)/fullerene (C₆₀)/2,9-dimethyl-7,7-diphenyl-1,10-phenanthroline (BCP)/Ag. This device architecture eliminated the need for dopants that introduce instabilities, which commonly occured in the conventional device architecture, and thus decreased hysteresis. Moreover, the devices stabilized at max powers. Due to the high-quality film and ideal single junctionband gap (1.36 eV) of Cs_0.10(MA_0.17FA_0.83)_0.9Pb_0.75Sn_0.25(I_0.83Br_0.17)₃, the derived solar cell reached a maximum PCE of 15.78%. The triple-cation mixture and SnF₂ addition alleviated the oxidation of Sn²⁺ for Sn-rich, lowband gap (1.26 eV) perovskite of Cs_0.10(MA_0.17FA_0.83)_0.9Pb_0.25Sn_0.75(I_0.83Br_0.17)₃, resulting in a record maximum PCE of 9.61%. Moreover, this 75% Sn device can retain 80% of initial PCE after 30 days storage in inert condition followed by over 100 hours in ambient condition. Overall, this study demonstrated the implementation of the triple-cation and double-halide mixtures as an effective strategy for improving phase stability as well as performance and stability of Sn-riched perovskite solar cells, providing a route towards lead-free devices in the future.