What can chemical bonding tell us about photoinduced self-healing reactions in inorganic semiconductors? Insight from Bismuth-Antimony Selenide

Photo-reactive self-healing semiconductors with suitable bandgaps for solar energy conversion offer an intriguing path to making resilient and low-cost photovoltaic devices through the introduction of a self-recovery path. However, only few inorganic photovoltaic materials have such quality, and the underlying chemical properties that enable it are unknown, which poses a significant limit to our ability to study and discover new self-healing semiconductors. Recently, we have found antimony trichalcogenide (Sb2Se3, Sb2S3) and chalcohalides (e.g., SbSeI) can undergo a reversible photo-induced phase transition (PIPT) in which the structure is restored after photo-induced damage is incurred to the materials. This group of materials offer a unique opportunity for studying PIPT and its limits. In particular, this group of materials facilitates the study of functional permutation to specific crystalline sites, and to finding the limits of PIPT occurrence, which sheds light on the origin of the PIPT and self-recovery of this class of materials. We found that PIPT magnitude decays upon gradual BiSb(1) substitution in a Sb2-xBixSe3 homologous series, until nearly one in five Sb ions is substituted with Bi. Then, PIPT diminishes completely. The homologous series occurs along a transition from a covalent to metavalent chemical bonding. By expanding our search, we find a correlation between bonding type and photoreactivity does exist but is an insufficient condition. Instead we suggest that sufficient bonding states at the bottom of the conduction band are also required. This study pushes the limits of designing self-healing inorganic semiconductors for various applications and provides tools to further expansion.