First‐Principles Study of the Thermoelectric and Optoelectronic Behavior of Lead‐Free Cs ₂ AgSbBr ₆ Halide Double Perovskite

The halide double perovskite (HDP) Cs ₂ AgSbBr ₆ is studied using WIEN2k simulation code based on DFT with LSDA for its structural, optoelectronic, and thermoelectric properties. A lattice constant of 10.955 Å is derived from the optimized structure, and the bulk modulus (33.887 GPa) indicates moderate mechanical stability. The electronic band structure shows an indirect bandgap (L–X) of 1.567 eV, which is in good agreement with available experimental data, thus verifying the calculation methods and their use in optoelectronic and photovoltaic devices. The optical analysis corresponds to a reflectivity R(0) = 11.67%, refractive index n(0) = 2.037, and static dielectric constant ε ₁ (0) = 4.151. Further, the TE figure of merit approaches ∼1 (upper limit) at 300 K and slightly decreases as temperature increases, indicating relatively good TE performance compared to similar lead‐free double perovskites at lower temperatures. These results show that Cs ₂ AgSbBr ₆ is a promising lead‐free multifunctional energy system.