Band Gap Engineering of Cs₃Bi₂I₉ Perovskites with Trivalent Atoms Using a Dual Metal Cation

Ternary metal halides (A₃X₂I₉) have attracted considerable interest because they have good stability and reduced toxicity compared with Pb-based halide perovskites. The main issue with A₃X₂I₉ is their band gap, which is relatively large for use in a single junction solar cell (1.9–2.2 eV for the Cs₃Bi₂I₉). This theoretical study found that the band gap of Cs₃Bi₂I₉ can be successfully modulated by using dual metal cations, i.e., by forming Cs₃BiXI₉ (X: trivalent cation). Among the various trivalent atoms investigated, In and Ga showed very promising band gap modulation behaviors. Additionally, the indirect band gap of Cs₃Bi₂I₉ can be changed into a direct band gap.