Mixed Halide Lead-free Double Perovskite Alloys for Band Gap Engineering

Lead-free double perovskites are currently among some of the most researched materials for solar cell applications, with the aim to identify promising perovskite-like materials to improve stability and reduce toxicity observed in lead-based perovskites. In this work, we present a theoretical investigation based on density functional theory calculations within semilocal and hybrid exchange-correlation functionals of the structural, energetic, and electronic properties of the inorganic Cs8Ag4Bi4QmQ′n double perovskites, where m + n = 24 and Q and Q′ = Cl, Br, and I. Based on the combination of different halide species and compositions within the crystal structures, we identified a balance between the stability driven by the smaller halide species in Cs8Ag4Bi4QmQ′n compounds and a higher power conversion efficiency (PCE) driven by the larger halide species. From that, we highlighted the perovskite Cs8Ag4Bi4Cl12I12 as an example that was shown to be as stable as the already synthesized Cs2AgBiBr6 according to our metrics, but with a PCE of almost 10%, i.e., 5 times greater than the PCE of the latter. Therefore, our findings can provide a path for further experimental studies, with the aim to validate our results and identify potential lead-free perovskites for solar cell applications via mixed halide alloying.