Revealing superoxide-induced degradation in lead-free tin perovskite solar cells

Abstract

The poor stability of lead (Pb)-free tin (Sn)-based perovskites under only oxygen (O₂) exposure has attracted extensive research, while their stability under simultaneous light and O₂ (light/O₂) exposure is unexplored. Herein, we found that the (NH₂)₂CHSnI₃ (FASnI₃) perovskite degrades more severely when exposed to light/O₂ than only O₂, which we attribute to the superoxide generated via the reaction between O₂ and photoexcited electrons. We propose the superoxide-induced degradation routes of FASnI₃. Fluorescent molecular probe results indicate higher yield of superoxide in FASnI₃ than in FAPbI₃. Density functional theory simulation results rationalize that the formation energy of the most preferred sites (iodine vacancy, V_I) for superoxide formation in FASnI₃ is much lower than that in FAPbI₃; meanwhile, superoxide formation at V_I is more energetically favorable in FASnI₃. A targeted strategy by incorporating halide-containing additives is applied to fill the V_I in FASnI₃ to suppress superoxide formation. Combined with superoxide yield measurement, capacitance-dominated characterization can systematically differentiate the reduction of surface and interior V_I after modification. Understanding the degradation mechanism of FASnI₃ upon light/O₂ exposure manifests the fatal effect of superoxide on the stability of Sn-based perovskite solar cells.