Route to Stable Lead-Free Double Perovskites with the Electronic Structure of CH3NH3PbI3: A Case for Mixed-Cation [Cs/CH3NH3/CH(NH2)2]2InBiBr6

During the past year, halide double perovskites attracted attention as potential lead-free alternatives to Pb-based halide perovskites. However, none of the compounds discovered so far can match the optoelectronic properties of MAPbI(3) (MA = CH3NH3). Here we argue that, from the electronic structure viewpoint, the only option to make Pb-free double perovskites retaining the remarkable properties of MAPbI(3) is to combine In and Bi as B+ and B3+ cations, respectively. While inorganic double perovskites such as Cs2InBiX6 were found to be unstable due to In oxidizing into In3+, we show that the +1 oxidation state of In becomes progressively more stable as the A-site cation changes from K to Cs. Hence, we propose the use of MA and FA [FA = CH(NH2)(2)] to stabilize MA(2)InBiBr(6) double perovskites. We show that the optoelectronic properties of MA(2)InBiBr(6) are remarkably similar to those of MAPbI(3) and explore the mixed-cation (Cs/MA/FA)(2)InBiBr6 halide double perovskites.