Bandgap lowering in mixed alloys of Cs 3 Bi 2- x Sb x Br 9 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 Cs 3 Bi 2 Br 9 , 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 Cs 3 Bi 2- x Sb x Br 9 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 Cs 3 Bi 2- x Sb x Br 9 has been investigated using DFT calculations and the bandgap bowing of Cs 3 Bi 2- x Sb x Br 9 is deduced to be related to the type-II band alignment between the Cs 3 Bi 2 Br 9 and Cs 3 Sb 2 Br 9 . 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.