Revealing the Role of Tin Fluoride Additive in Narrow Bandgap Pb-Sn Perovskites for Highly Efficient Flexible All-Perovskite Tandem Cells.

Tin fluoride (SnF 2 ) is an indispensable additive for high-efficiency Pb-Sn perovskite solar cells (PSCs). However, the spatial distribution of SnF 2 in the perovskite absorber is seldom investigated while essential for a comprehensive understanding of the exact role of the SnF 2 additive. Herein, we revealed the spatial distribution of the SnF 2 additive and made structure-optoelectronic properties-flexible photovoltaic performance correlation. We observed the chemical transformation of SnF 2 to a fluorinated oxy-phase on the Pb-Sn perovskite film surface due to its rapid oxidation. In addition, at the buried perovskite interface, we detected and visualized the accumulation of F - ions. We found that the photoluminescence quantum yield of Pb-Sn perovskite reached the highest value with 10 mol % SnF 2 in the precursor solution. When integrating the optimized absorber in flexible devices, we obtained the flexible Pb-Sn perovskite narrow bandgap (1.24 eV) solar cells with an efficiency of 18.5% and demonstrated 23.1% efficient flexible four-terminal all-perovskite tandem cells.