Stability, electronic structure, and optical properties of lead-free perovskite monolayer Cs₃ B ₂ X ₉ (B = Sb, Bi; X = Cl, Br, I) and bilayer vertical heterostructure Cs₃ B ₂ X ₉/Cs₃ B2′ X ₉(B,B′ = Sb, Bi; X = Cl, Br, I)

Abstract

The lead-free perovskites Cs 3 B 2 X 9 ( 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 Cs 3 B 2 X 9 and bilayer vertical heterostructure Cs 3 B 2 X 9 /Cs 3 B 2 ′ X 9 ( 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 Cs 3 B 2 X 9 /Cs 3 B 2 ′ X 9 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 Cs 3 Sb 2 I 9 and heterostructure Cs 3 Sb 2 I 9 /Cs 3 Bi 2 I 9 , 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.