Protective Role of Oxides on Pb-Free Halide Perovskite Surfaces: Interfacial Effects and Excitonic Optical Properties from First-Principles

In this work, the impact of a GeO₂ overlayer on the stability and optoelectronic properties of Pb-free Cs(Sn_1-xGe_x)I₃ perovskites is systematically investigated using first-principles calculations. Ge incorporation is found to substantially reduce the exciton binding energy relative to pristine CsSnI₃, thereby promoting more efficient electron-hole separation and enhancing the potential for carrier extraction. Guided by this trend, Cs(Sn_1-xGe_x)I₃ (x = 0.25) slab models exposing the stable (001) surface were constructed, and both CsI- and MI₂-terminated facets (M = Sn, Ge, as well as Ge-only) were examined. The resulting perovskite/GeO₂ heterointerfaces display a pronounced dependence of structural rearrangements and band edge alignment on both composition and termination, allowing the identification of specific configurations that best preserve the desirable optoelectronic response. To better mirror experimental conditions, crystalline interface models were complemented by amorphous GeO₂ structures, thus capturing the structural complexity of realistic germania capping layers. Taken together, these results provide a microscopic picture of how GeO₂ overlayers can stabilize (Sn,Ge)-based halide perovskites while retaining electronic characteristics compatible with high-efficiency photovoltaic operation.