Bandgap lowering in mixed alloys of Cs2Ag(SbxBi1−x)Br6 double perovskite thin films

Abstract

Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air and long charge-carrier lifetimes. However, most double perovskites, including Cs₂AgBiBr₆, have wide bandgaps, which limits photoconversion efficiencies. The bandgap can be reduced through alloying with Sb³⁺, but Sb-rich alloys are difficult to synthesize due to the high formation energy of Cs₂AgSbBr₆, which itself has a wide bandgap. We develop a solution-based route to synthesize phase-pure Cs₂Ag(Sb_xBi_1−x)Br₆ thin films, with the mixing parameter x continuously varying over the entire composition range. We reveal that the mixed alloys (x between 0.5 and 0.9) demonstrate smaller bandgaps than the pure Sb- and Bi-based compounds. The reduction in the bandgap of Cs₂AgBiBr₆ achieved through alloying (170 meV) is larger than if the mixed alloys had obeyed Vegard's law (70 meV). Through in-depth computations, we propose that bandgap lowering arises from the type II band alignment between Cs₂AgBiBr₆ and Cs₂AgSbBr₆. The energy mismatch between the Bi and Sb s and p atomic orbitals, coupled with their non-linear mixing, results in the alloys adopting a smaller bandgap than the pure compounds. Our work demonstrates an approach to achieve bandgap reduction and highlights that bandgap bowing may be found in other double perovskite alloys by pairing together materials forming a type II band alignment.