Band gap engineering of novel double perovskites Na2LiYX6 (X=Cl, Br) through halide ion replacement for solar cell and energy harvesting applications

Using the Perdew - Burke - Ernzerhof-generalized-gradient approximation (PBE-GGA) and PBE+SOC exchange - correlation functional, this work investigates the physical properties and energy storage capacity of novel double perovskite Halides Na2LiYX6 (X=Cl and Br) through Density Functional Theory (DFT) and semiclassical transport theory under ideal conditions. Thermal stability has been verified through the computation of thermodynamic parameters and formation energies. Brittle and anisotropic nature has been confirmed by the calculation of elastic and mechanical parameters. With PBE+SOC, the band gaps (E-g) for Na2LiYCl6 and Na2LiYBr6 are 2.90 and 3.76 eV, respectively, whereas with PBE-GGA, they are 3.061 and 3.824 eV, respectively. The analysis of the computed optical characteristics determines whether double perovskite halides are suitable for use in photovoltaic applications. The calculations for the transport properties were carried out using the BoltzTraP code. The effectiveness and transport mechanism for thermal energy conversion have been studied by determining the thermoelectric properties. To assess the stability at high temperatures and suitability for industrial applications, thermal properties such as heat capacity, entropy, Debye temperature, and thermal expansion are investigated. Overall, evaluations of the compounds' thermoelectric characteristics show that they have a high-power factor and remarkably low thermal conductivity, which makes them ideal for energy harvesting devices.