Band Gap Engineering of Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> Perovskites with Trivalent Atoms Using a Dual Metal Cation

Ternary metal halides (A<sub>3</sub>X<sub>2</sub>I<sub>9</sub>) have attracted considerable interest because they have good stability and reduced toxicity compared with Pb-based halide perovskites. The main issue with A<sub>3</sub>X<sub>2</sub>I<sub>9</sub> is their band gap, which is relatively large for use in a single junction solar cell (1.9–2.2 eV for the Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub>). This theoretical study found that the band gap of Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> can be successfully modulated by using dual metal cations, i.e., by forming Cs<sub>3</sub>BiXI<sub>9</sub> (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 Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> can be changed into a direct band gap.