Flexible chalcogenide perovskite Ba₃Te₂S₇ with high electron mobility and strong optical absorption ability

Inorganic-organic hybrid halide perovskites exhibit outstanding optoelectronic characteristics, rendering them promising materials for photovoltaic applications. Despite achieving an impressive power conversion efficiency of up to 26.1%, their extensive use is hampered by the inherent instability of organic constituents and the toxicity associated with lead. With the motivation of searching for stable and non-toxic perovskite photovoltaic materials, we turned our attention to inorganic chalcogenide perovskites and performed extensive material screening excluding toxic elements. We found a stable and flexible chalcogenide perovskite Ba3Te2S7 with the distorted phase. This material boasts partial covalent bonds between Te4+ and S2- ions, enhancing both its structural stability and charge transfer capabilities. Furthermore, we observed a direct-to-indirect bandgap transition in Ba3Te2S7 with values of 0.39 eV and 0.37 eV from the monolayer to bulk, respectively. Notably, the electron mobility of Ba3Te2S7 can reach 10(4) cm(2) V-1 s(-1), surpassing hole mobility by two orders of magnitude. Moreover, the high optical absorption coefficient (10(5)-10(6) cm(-1)) of bulk Ba3Te2S7 in the visible-light region, owing to the allowed band-edge optical transitions, combined with favorable conduction-band offsets for efficient electron injection from the absorber to the electron transporting layer (TiO2), positions Ba3Te2S7 as a promising material for photovoltaic applications.