First-Principles Study on the Structure, Electronic and Optical Properties of Cs₂AgSb_xBi_1–xCl₆ Double Perovskites

Lead-free halide double perovskites A₂BB′X₆ have been proposed as potential materials for optoelectronic application. Among them, Cs₂AgBiCl₆ exhibits an indirect band gap in the ultraviolet regime, which is not ideal for the applications in thin-film photovoltaics. In this work, we report a strategy to tune the band gap by Sb doping. We perform density functional theory calculations to investigate the crystal structures and electronic and optical properties of mixed halide Cs₂AgSb_xBi_1–xCl₆ (x = 0, 0.25, 0.5, 0.75, and 1) compounds. Our results show that the crystal structure and symmetry of these compounds change with the value of x and that the band gap (HSE) can vary between 2.46 and 3.09 eV depending on the level of Sb doping. We also find that the spin–orbit coupling varies quite significantly as the Sb/Bi ratio changes. These results show that the band gap of Cs₂AgBiCl₆ can be tuned controllably by chemical adjustment, which is valuable for its potential optoelectronic application.