Route to Stable Lead-Free Double Perovskites with the Electronic Structure of CHNHPbI: A Case for Mixed-Cation [Cs/CHNH/CH(NH)]InBiBr.

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 (MA = CHNH). Here we argue that, from the electronic structure viewpoint, the only option to make Pb-free double perovskites retaining the remarkable properties of MAPbI is to combine In and Bi as B and B cations, respectively. While inorganic double perovskites such as CsInBiX were found to be unstable due to In oxidizing into In, 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(NH)] to stabilize AInBiBr double perovskites. We show that the optoelectronic properties of MAInBiBr are remarkably similar to those of MAPbI and explore the mixed-cation (Cs/MA/FA)InBiBr halide double perovskites.