Tunable Band Gap and Rhombohedral Distortion in Lead-Free CsSn1–xGexI3 Mixed Perovskites

The absence of lead and also organic molecules in perovskites materials can be a good path to avoid toxicity and increase the stability of these high-potential compounds for photovoltaic applications. Particularly, a CsSnGeI3 alloy appears as a good alternative in this way, with initial evidence of improved properties. We provide a theoretical study that includes local strains, disorder effects, spin–orbit interactions, and approximated quasiparticle corrections via a DFT-1/2 method, delivering energy gaps in excellent agreement with experiments and state-of-art calculations. The alloy is stable in the entire composition range for usual growth temperatures, but a structural transition tendency happens between cubic and rhombohedral structures. Differently from tin–lead perovskite alloys, the fundamental energy band gap varies almost linearly with the composition, with very small bowing, an effect here explained in terms of statistics and average orbital characters. Our results increase the knowledge of the basic properties of this alloy and emphasize it as a promising material for solar-cell technology.