Unique Behavior of Halide Double Perovskites with Mixed Halogens

Engineering halide double perovskite (A2M+M3+XVII6) by mixing elements is a viable way to tune its electronic and optical properties. In spite of many emerging experiments on halide double perovskite alloys, the basic electronic properties of the alloys have not been fully understood. In this work, we chose Cs2AgBiCl6 as an example and systematically studied electronic properties of its different site alloys Cs2NaxAg1–xBiCl6, Cs2AgSbxBi1–xCl6, and Cs2AgBi(BrxCl1–x)6 (x = 0.25, 0.5, 0.75) by first-principles calculations. Interestingly, the halogen site alloy shows opposite behavior to M+ and M3+ cation site alloys; that is, Cs2AgBi(BrxCl1–x)6 displays virtual crystal behavior without substantial broadening, while Cs2NaxAg1–xBiCl6 and Cs2AgSbxBi1–xCl6 show split-band behaviors with substantial broadening, which indicates that lifetimes of electrons and holes in Cs2AgBi(BrxCl1–x)6 would be longer than those in Cs2NaxAg1–xBiCl6 and Cs2AgSbxBi1–xCl6. We further found that long lifetimes of electrons and holes are common for mixed halide perovskites. Moreover, the band alignment is provided to determine the band gap change of alloys and to understand the transport of electrons and holes when these pure compounds form heterostructures. Our systematical studies should be helpful for future optoelectronic applications of halide perovskites.