Stability, electronic structure, and optical properties of lead-free perovskite monolayer Cs₃B₂X₉ (B=Sb, Bi; X=Cl, Br, I) and bilayer vertical heterostructure Cs₃B₂X₉/Cs₃B₂'X₉ (B,B'=Sb, Bi; X=Cl, Br, I)

The lead-free perovskites Cs₃B₂X₉ (B=Sb, Bi; X=Cl, Br, I) as the popular photoelectric materials have excellent optical properties with lower toxicity. In this study, we systematically investigate the stable monolayer Cs₃B₂X₉ and bilayer vertical heterostructure Cs₃B₂X₉//Cs₃B₂'X₉ (B, B'=Sb, Bi; X=Cl, Br, I) via first-principles simulations. By exploring the electrical structures and band edge positions, we find the band gap reduction and the band type transition in the heterostructure Cs₃B₂X₉//Cs₃B₂'X₉ due to the charge transfer between layers. Furthermore, the results of optical properties reveal light absorption from the visible light to UV region, especially monolayer Cs₃Sb₂I₉ and heterostructure Cs₃Sb₂I₉/Cs₃Bi₂I₉, which have absorption peaks in the visible light region, leading to the possibility of photocatalytic water splitting. These results provide insights for more two-dimensional semiconductors applied in the optoelectronic and photocatalytic fields.