Bandgap lowering in mixed alloys of Cs3Bi2-xSbxBr9 perovskite powders

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 Cs3Bi2-xSbxBr9 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 Cs3Bi2-xSbxBr9 has been investigated using DFT calculations and the bandgap bowing of Cs3Bi2-xSbxBr9 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.