Lead-Free Hybrid Perovskites: Structural and Electronic Analysis of MA0.5Rb0.5Bi0.5Ge0.25I3 and MA0.5Rb0.5Sb0.5Ge0.25I3

Structural and electronic stability of MA_0.5Rb_0.5Bi_0.5Ge_0.25I₃ and MA_0.5Rb_0.5Sb_0.5Ge_0.25I₃ lead-free hybrid perovskites is addressed. The substitution of Pb in MAPbI₃ perovskite is increasing the scientific attention due to its toxicity as well as stability of interest in the design of environmentally friendly solar cells. MA cation (CH₃NH₃⁺) is replaced by 50% Rb⁺ to improve stability and retain the organic characteristics. The substitution of lead in MA_0.5Rb_0.5PbI₃ structure has been studied up to 100% Ge. Electron localization function (ELF) analysis for structures with 75 and 100% of Ge, show zones with no electron localization that indicates certain degree of structural instability. ELF and density of states (DOS) analysis of the structure MA_0.5Rb_0.5Pb_0.5Ge_0.5I₃ with 50% Ge corroborate stability characteristics of this perovskite. Thus, the MA_0.5Rb_0.5Pb_0.5Ge_0.5I₃ structure has been selected for the substitution of lead by Bi and Sb. ELF and non-covalent index (NCI) analysis indicate Sb structures are slightly more stable than those with Bi. The presence of Bi and Sb drastically decreases the band-gap in the MA_0.5Rb_0.5Bi_0.5Ge_0.25I₃ and MA_0.5Rb_0.5Sb_0.5Ge_0.25I₃ structures, respectively, which makes both structures without lead of interest for use in photovoltaic devices. These findings provide a pathway for designing stable, lead-free perovskites with improved optoelectronic properties for next-generation solar cells.