Organic Cation‐Dependent Degradation Mechanism of Organotin Halide Perovskites

The applications of organotin halide perovskites are limited because of their chemical instability under ambient conditions. Upon air exposure, Sn 2+ can be rapidly oxidized to Sn 4+ , causing a large variation in the electronic properties. Here, the role of organic cations in degradation is investigated by comparing methylammonium tin iodide (MASnI 3 ) and formamidinium tin iodide (FASnI 3 ). Through chemical analyses and theoretical calculations, it is found that the organic cation strongly influences the oxidation of Sn 2+ and the binding of H 2 O molecules to the perovskite lattice. On the one hand, Sn 2+ can be easily oxidized to Sn 4+ in MASnI 3 , and replacing MA with FA reduces the extent of Sn oxidation; on the other hand, FA forms a stronger hydrogen bond with H 2 O than does MA, leading to partial expansion of the perovskite network. The two processes compete in determining the material's conductivity. It is noted that the oxidation is a difficult process to prevent, while the water effect can be largely suppressed by reducing the moisture level. As a result, FASnI 3 ‐based conductors and photovoltaic cells exhibit much better reproducibility as compared to MASnI 3 ‐based devices. This study sheds light on the development of stable Pb‐free perovskite optoelectronic devices through new material design.