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, Sn2+ can be rapidly oxidized to Sn4+, 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 Sn2+ and the binding of H2O molecules to the perovskite lattice. On the one hand, Sn2+ can be easily oxidized to Sn4+ 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 H2O 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.