DFT Insight into Structural, Electronic, Optical and Thermoelectric Properties of Eco-Friendly Double Perovskites Rb2GeSnX6 (X = Cl, Br) for Green Energy Generation

Halide double perovskites in the form of A 2 B I B III X 6 present a rich compositional space that holds promise for discovering novel materials possessing intriguing properties applicable in diverse fields. However, the photovoltaic efficiency of halide double perovskites has been hindered by the large indirect bandgaps in the currently known materials. Nonetheless, the potential applications of halide double perovskites extend beyond outdoor photovoltaics, encompassing memory devices, thermoelectric, light-emitting diodes, X-ray detectors, sensors, and many more. Herein, we report novel metal lead-free double halide perovskites Rb 2 GeSnX 6 (X = Cl, Br) for renewable energy applications via density functional theory calculations. The compounds are found thermodynamically stable based on the negative formation and Gibbs free energies values. The tolerance factor confirms the structural stability in the cubic crystalline form, while the stable phonon dispersion spectrum supports dynamical stability. We observe an increase in the lattice constant and bulk modulus as the halogen anions change (i.e., Cl replacing Br). The calculated direct band gaps, along the Γ symmetry point are found to be 1.296 eV for Rb 2 GeSnCl 6 and 0.799 eV for Rb 2 GeSnBr 6 , using theTran and Blaha modified Becke and Johnson potential. To assess their potential in solar cells and optoelectronic devices, we analyze the dielectric function, optical conductivity, and reflectivity. In addition, we investigate their thermoelectric parameters such as electronic and thermal conductivities, carrier concentration, electrical conductivity, Seebeck coefficient, and figure of merit. These findings validate the potential of these compounds for thermoelectric power generation at high temperatures.