Halide-driven band engineering in double perovskites Cs2LiIrX6 (X = Cl/ Br) for photovoltaic and thermoelectric applications; DFT study.

This study focuses on exploring the multifunctional energy conversion capability of the double perovskite semiconductors Cs2LiIrX6 (X = Cl/Br), driven by the growing need for stable, lead-free materials for next-generation optoelectronic and thermoelectric devices. Employing first-principles calculations with the modified Becke-Johnson potential, Cs2LiIrX6 (X = Cl,Br) are found to have direct band gaps of 1.43 eV and 1.1 eV, respectively, located at the X-high symmetry k-point. Their narrow, direct bandgaps in the IR and near IR region highlight their potential for IR-sensitive technologies such as thermal imaging and optical communication. Thermoelectric energy conversion is another interesting application for modern energy materials. A stable thermoelectric figure of merit between 0.76 and 0.77 is obtained for both compounds over a 200- 800 K temperature range. The thermodynamic stability of the compounds is verified by their calculated Debye temperatures and Gibbs free energy, while their negative formation energies further confirm their structural stability.