First-Principles Calculations of Novel Lead-Free X2GeSnI6 (X = Rb, Cs) Double Perovskite Compounds for Optoelectronic and Energy Exploitations.

We investigate the structural, optoelectronic, and thermoelectric properties of halide double perovskite X₂GeSnI₆ (X = Rb, Cs) compounds employing the full potential linearized augmented plane wave approach within the framework of density functional theory. The negative formation energies and positive phonon frequencies indicate their structural and dynamical stabilities. The density of states and band structures were estimated while utilizing Tran and Blaha-modified Becke-Johnson (TB-mBJ) potential as exchange–correlation approximation. The electronic structure calculation show direct bandgaps of 0.49 eV for Rb₂GeSnI₆ and 0.57 eV for Cs₂GeSnI₆, suggesting semiconducting behavior. To establish their use in photovoltaic and optoelectronic devices, we compute the optical properties of the compounds. Furthermore, the thermoelectric characteristics such as electrical conductivity, Seebeck coefficient, thermal conductivity, and figure of merit, have been studied in the temperature range of 100 to 800 K. Both materials exhibit positive Seebeck coefficient, indicating the materials to be p-type semiconductors. The examined thermoelectric properties of both the compounds imply their potential use in thermoelectric devices. [ABSTRACT FROM AUTHOR]