First-Principles Study on the Structure, Electronic, and Optical Properties of Cs2AgBiBr6-xClx Mixed-Halide Double Perovskites

Double halide perovskites Cs2AgBiX6 (X = Cl, Br) have been proposed as promising nontoxic materials with enhanced chemical stability for the optoelectronic application. Controllably tuning the band gap by mixing the halide components is a valuable approach for its application and has not yet been explored. Here, we report density functional theory calculations, through which we investigate the crystal structure, electronic, and optical properties of mixed-halide Cs2AgBiBr6-xClx (x = 0, 1, 2, 3, 4, 5, and 6) compounds. Our results show that the crystal structure changes with the x value, and some structural phases with x = 2, 3, and 4 have the same energy indicating very similar structural stability. Band structure calculations show that Cs2AgBiBr6-xClx compounds have similar electronic properties. That is, they all have indirect band gaps with the valence band maximum at the X point and the conduction band minimum at the L point in the Brillouin zone. The band gap increases almost linearly as a function of incorporated, but the slope is different depending on which methods are used in the calculations. Based on the electronic structure, the optical absorptions of Cs2AgBiBr6-xClx are calculated using both the regular semilocal and hybrid functionals. Our results show that the band gap of Cs2AgBiBr6 can be tuned controllably by Cl-doping, which is valuable for its potential photoelectronic application.