Bandgap lowering in mixed alloys of Cs3Bi2−xSbxBr9 perovskite powders

Abstract

Lead-free metal halide perovskites have received widespread attention due to their composition of minimal hazardous components, excellent air stability, and long carrier lifetimes. However, the majority of the lead-free metal halide perovskites, such as Cs₃Bi₂Br₉, have wide bandgaps, which limits their photoelectric in solar cells and optoelectronic devices. To address this issue, attempts have been made to adjust the bandgap through material alloying. Based on a solution approach, a pure phase of Cs₃Bi_2−xSb_xBr₉ crystals has been synthesized, with the alloying parameter x changing over the full range of composition. It is found that the mixed alloy has a smaller bandgap than pure Bi-based and Sb-based perovskites, with the smallest bandgap of 2.22 eV near x = 1, and there is a phenomenon of bandgap bowing throughout the alloying process. The electronic structure of Cs₃Bi_2−xSb_xBr₉ has been investigated using DFT calculations and the bandgap bowing of Cs₃Bi_2−xSb_xBr₉ is deduced to be related to the type-II band alignment between the Cs₃Bi₂Br₉ and Cs₃Sb₂Br₉. Owing to the mismatch of s and p orbital energies of Bi and Sb, the mixed alloy has a smaller bandgap. Our work demonstrated a method for achieving bandgap reduction and explained the phenomenon of bandgap bowing by pairing materials into type-II band alignment, which may also be found in other lead-free metal perovskites.