Environmentally safe lead-free double perovskites (Cs2/Rb2)LiGaCl6: a DFT-based exploration of their optoelectronic and thermoelectric performance

Abstract

The growing demand for environmentally safe and stable alternatives to lead-based halide perovskites has stimulated extensive research into lead-free double perovskites for energy applications. In this work, we present a comprehensive density functional theory (DFT) investigation of the structural, electronic, optical and thermoelectric properties of (Cs₂/Rb₂)LiGaCl₆. Both compounds crystallize in a stable cubic phase with direct band gaps of 1.647 eV and 1.846 eV, respectively. These values lie close to the Shockley–Queisser limit, making them promising absorbers for solar energy conversion. Density of states and charge density analyses reveal a mixed ionic–covalent bonding character dominated by Ga–Cl interactions. Thermoelectric results show high Seebeck coefficients, favourable power factors and dimensionless figure of merit (ZT) values approaching unity. These findings confirm that these materials are good candidates for thermoelectric applications and indicate their potential for waste heat recovery. Optical properties demonstrate strong absorption across the UV spectrum, a pronounced dielectric response and significant optical conductivity arising from interband transitions. Overall, the findings highlight (Cs₂/Rb₂)LiGaCl₆ as structurally robust, environmentally safe and multifunctional materials suitable for optoelectronic and thermoelectric energy technologies.