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

• Published in: Journal of physical chemistry. C Vol. 121; no. 36; pp. 19650 - 19656
• Main Authors: Nishikubo, Ryosuke, Ishida, Naoki, Katsuki, Yukie, Wakamiya, Atsushi, Saeki, Akinori
• Format: Journal Article
• Language: English
• Published: American Chemical Society 14.09.2017
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.7b06294

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 TRMC. A low-dimensional perovskite (MA2SnI6) composed of the oxidized form of Sn­(IV) was also evaluated to explain the anomalous TRMC behavior of the air-exposed MASnI3. Our results provide an important basis for correlation with the degradation and energetics of a device.