Protective Role of Germania on (Sn,Ge)-Mixed Halide Perovskites: Interfacial Effects and Excitonic Optical Properties from First-Principles

In this work, we assess the impact of a GeO2 overlayer on the stability and optoelectronic properties of Pb-free Cs(Sn1−xGex)I3 perovskites. First-principles calculations show that Ge incorporation significantly reduces the exciton binding energy with respect to pristine CsSnI3, indicating enhanced electron–hole separation and improved carrier-extraction potential. Guided by this trend, we investigated Cs(Sn1−xGex)I3 (x=0.25) slab models along the stable (001) orientation and analyzed different surface terminations (CsI and MI2, with M = Sn, Ge and M=Ge only). The resulting perovskite/GeO2 interfaces exhibit composition- and termination-dependent structural rearrangements and band alignments, highlighting specific configurations that better preserve the perovskite’s optoelectronic response. To ensure a realistic comparison with experiments, crystalline models were complemented with amorphous GeO2 structures. Overall, our results clarify how germania capping layers can stabilize (Sn,Ge)-based perovskites while maintaining favorable electronic properties for photovoltaic applications.