  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 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 4-terminal all-perovskite tandem cells.
