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 Csx(MA0.17FA0.83)1-xPb1-ySny(I0.83Br0.17)3, where x = 0.05, 0.1, and 0.2 and y = 0, 0.25, 0.50 0.75, and 1. 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 and decreased hysteresis, allowing us to achieve stabilize max power scans. Due to the high-quality film and ideal single junction band gap (1.36 eV) of Cs0.10(MA0.17FA0.83)0.9Pb0.75Sn0.25(I0.83Br0.17)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, low band gap (1.26 eV) perovskite of Cs0.10(MA0.17FA0.83)0.9Pb0.25Sn0.75(I0.83Br0.17)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 mixturesas an effective strategy for improving performance and stability of Sn-riched perovskite solar cells, providing a route towards lead-free devices in the future.

Enhancing performance and stability of Pb-Sn alloyed perovskite solar cells via triple-cation and double-halide

256th National Meeting (2018)

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