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 6 (VBMs) and photoconductivities of tin iodide perovskites (methylammonium cation, MASnI(3); formamidinium cation, FASnI(3)). 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 MASnI(3) and FASnI(3), as revealed by PYS, and to partly mitigate the degradation of MASnI(3), as revealed by TRMC. A low-dimensional perovskite (MA(2)SnI(6)) composed of the oxidized form of Sn(IV) was also evaluated to explain the anomalous TRMC behavior of the air-exposed MASnI(3). Our results provide an important basis for correlation with the degradation and energetics of a device.