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

Tin fluoride (SnF2) is an indispensable additive for high-efficiency Pb-Sn perovskite solar cells (PSCs). However, the spatial distribution of SnF2 in the perovskite absorber is seldom investigated while essential for a comprehensive understanding of the exact role of the SnF2 additive. Herein, we revealed the spatial distribution of the SnF2 additive and made structure-optoelectronic properties-flexible photovoltaic performance correlation. We observed the chemical transformation of SnF2 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 % SnF2 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.