Tunable optoelectronic properties of Cs2GeX6 (X = Cl, Br) mixed-halide double perovskites using the supercell approach

In this study, density functional theory (DFT) is utilised to examine the structural, electronic, and optical properties of non-toxic Cs2GeX6 (X = Cl, Br) perovskites and its mixed halide counterparts Cs2GeClyBr1-y (y = 0.25, 0.5, 0.75) through a supercell method. The examined perovskites showed that the structure of the pristine Cs2GeX6 (X = Cl, Br) perovskites is cubic while the mixed halide variants Cs2GeBryCl1-y are orthorhombic. The lattice constants of the pristine compounds increased at the exchange of Cl with Br due to an increase in the ionic radius. The lattice constants of the pristine compounds matched well with other theoretical results, while the lattice constants of the mixed variants increased due to the use of supercells. The electronic properties of Cs2GeCl6 and Cs2GeBr6 perovskites indicate semiconducting properties. The direct bandgaps of Cs2GeCl6 and Cs2GeBr6 are 2.10 and 0.90 eV, respectively with implications on the optical characteristics. The optical properties calculation shows that the initial peaks of these compounds are in the visible light spectrum, indicating their potential as absorber materials for tandem solar cells.