Minute-Scale Degradation and Shift of Valence-Band Maxima of (CH₃NH₃)SnI₃ and HC(NH₂)₂SnI₃ Perovskites upon Air Exposure

The development of lead-free, tin-based perovskite solar cells is becoming a pervasive topic; however, the inherent instabilities of such cells have prevented a boost of their power conversion efficiency and a deeper understanding of their fundamental properties. By using the photoelectron yield spectroscopy (PYS) and flash-photolysis time-resolved microwave conductivity (TRMC) techniques, we investigate the effects of air exposure on the valence-band maxima (VBMs) and photoconductivities of tin iodide perovskites (methylammonium cation, MASnI3; formamidinium cation, FASnI3). These perovskites exhibit a shift of the VBM (e.g., from −5.02 eV at 0 min to −5.17 eV at 18 min), deterioration of the PYS profiles, and progressive decrease of the TRMC transients on the minute scale of air exposure. The addition of SnF2 was found to suppress the initial defect-related density of the filled electronic states of MASnI3 and FASnI3, as revealed by PYS, and to partly mitigate the degradation of MASnI3, as revealed by T...