Elucidating the role of lone-pair ns2 electrons in B-site cation-disordering of double halide perovskite Cs2AgBiCl6 and Cs2AgInCl6

Abstract

Double perovskites, such as Cs2AgBiCl6 and Cs2AgInCl6, have been proposed as stable, nontoxic (or low toxic) and inorganic alternatives of lead halide perovskites. While their electronic structures should be modulated to achieve improved performance of corresponding optoelectronic and photovoltaic devices. Herein, we performed first-principles calculations to explore the accessibility of B-site cation disordering in Cs2AgBiCl6 and Cs2AgInCl6 and its effects on the electronic structures of both double perovskites. It is revealed that lower energy cost is required to form cation-disordered Cs2AgBiCl6 than the latter. Lone-pair electrons of Bi3+ makes its electron configuration distinct from Ag+, which results in much higher energy cost for electronic relaxation during cation-disordering of Cs2AgBiCl6. Large ionic size difference between Bi3+ and Ag+ results in significant ionic relaxation during cation-disordering, effectively compensating the energy cost. Furthermore, the resulted remarkable ionic relaxation and change in electron redistribution in cation-disordered Cs2AgBiCl6 effectively protect its electronic structure from being influenced by cation-disordering. Otherwise, cation-disordering shows significant influence on the electronic structure of Cs2AgInCl6. Our results provide in-depth understanding on the lone-pair effect on cation-disordering and electronic properties of double halide perovskites, which may be useful to develop and design high-performance optoelectronic and photovoltaic devices based on double perovskites.