Engineering halide composition to control structural and electronic properties in bismuth-based perovskite-inspired...

Recent advances in photovoltaic technologies have established lead halide perovskites as benchmark materials for optoelectronic applications, but serious concerns persist regarding the toxicity of lead, their principal constituent element. In this context, bismuth-based perovskite-inspired materials have emerged as a promising lead-free alternative, offering comparable electronic characteristics. Here, we explore the structural, electronic and transport properties of Cs3Bi2I9 and Cs3Bi2Br9, two perovskite-inspired materials with significant potential for photovoltaic and photocatalytic applications. With state-of-the-art first-principles calculations, we investigate the subtle effects of iodine/bromine (I/Br) mixing on the materials' physico-chemical properties. We predict a change in phase stabilities around 40% Br content: below 30% Br, the iodine-dominant P6₃/mmc phase is stable, while beyond 40% Br, the bromine-dominant P-3m1 phase becomes energetically favorable, consistent with experimental observations. The electronic bandgap increases with Br content, and effective mass calculations indicate that electrons exhibit lower effective masses and higher mobility compared to holes, with hole localization intensifying as the Br content increases. Overall, our findings underscore the critical role of halogen composition in modulating the structural, electronic, and transport properties of these materials, providing valuable insights for optimizing halide contents in perovskite-inspired systems for next-generation optoelectronic applications.