Structural and Electronic Characteristics of Double and Mixed Cs₂GeX₆ (X= Br, I, Cl) Perovskite from First Principles

Lead-free metal halide perovskites are increasingly receiving attention for the commercialization of optoelectronic devices. In this study, first principles simulation using density functional theory has been used to investigate the structural and electronic properties of non-toxic Cs₂GeX₆ (X= Br, I, Cl) with a mixed halide counterpart Cs₂GeBr_xCl_1-x (x=0.25, 0.5) using the virtual crystal approximation (VCA). The trend in the gradients observed in the lattice constants of the perovskites followed the increase in ionic radius of the halides. The investigation of the electronic properties showed semiconducting properties for Cs₂GeCl₆and Cs₂GeBr₆perovskites, while semiconducting to metallic transitions were observed for the Cs₂GeI₆ and Cs₂GeBr_xCl_1-x perovskites. The values obtained for the direct bandgaps of Cs₂GeCl₆ and Cs₂GeBr₆were 2.28 eV, and 0.93 eV, respectively. The elastic constants calculations showed that the perovskite compounds were mechanically stable. The physics behind the semiconducting to metallic transitions for some of the perovskite compounds studied have been discussed. Our datasets provide useful information for fundamental research on Cs₂GeX₆perovskites and its practical applications in emerging optoelectronics.