High‐Performance Lead‐Free Cs ₂ XTlCl ₆ (X = Y, Sc) Perovskites for Solar and Thermoelectric Energy Conversion

This study investigates the structural, electronic, mechanical, optical, and thermoelectric properties of the lead‐free double perovskite compounds Cs 2 XTlCl 6 (X = Y, Sc) using density functional theory (DFT) within the WIEN2k framework. Structural optimization and mechanical stability were carried out using the generalized gradient approximation (GGA), while the Tran‐Blaha modified Becke–Johnson (TB‐mBJ) potential was employed to achieve accurate electronic bandgap estimations. Both compounds crystallize in a cubic perovskite structure with space group Fmm, exhibiting direct bandgaps at the L‐point of the Brillouin zone 4.282 eV for Cs 2 YTlCl 6 and 3.566 eV for Cs 2 ScTlCl 6 indicating their wide‐bandgap semiconductor nature. Elastic constant calculations confirm mechanical stability, with both materials displaying brittle and anisotropic behavior. Optical analyses reveal strong absorption in the UV and visible regions, as well as high dielectric responses, supporting their potential in ultraviolet optoelectronics, photodetectors, and wide‐bandgap tandem‐device applications. Thermoelectric evaluations indicate a high power factor and a figure of merit (ZT > 0.7), underscoring their viability for energy conversion applications. Overall, Cs 2 XTlCl 6 (X = Y, Sc) emerges as a lead‐free alternative to conventional Pb‐based perovskites, with promising applications in solar cells, photodetectors, and thermoelectric devices. This work provides a theoretical foundation for future experimental efforts aimed at optimizing these materials for sustainable energy technologies.