DFT insights on the opto-electronic and thermoelectric properties of double perovskites K2AgSbX6 (X = Cl, Br) via halides substitutions for solar cell applications

•Physical characteristics of double pérovskites K2AgSbX6 (X = Cl, Br) are investigated by the DFT method.•The compounds are indirect band gap semiconductors.•The mechanical properties reveal the stability of the compounds.•The computed optical properties show the solar cell applications.•The figure of merit for both compounds suggests that they are potential candidates for thermoelectric applications. The structural, mechanical, electrical, optical, and thermoelectric characteristics of double perovskites crystal structure with #225 Fm3m space were studied using the Full Potential Linearized Augmented Plane-wave approach (FP-LAPW), which is based on density functional theory (DFT). The exchange and correlation effects were considered using the Perdew-Burke-Ernzerh generalized gradient plus modified Becke Johnson ((PBE-GGA + mBJ) approximation. The mechanical stability of these materials was confirmed by the estimated elastic constants such as Bulk modulus, Young modulus, Poisson's ratio, and shear anisotropy factor. The indirect electronic bandgap values 2.30 eV and 1.52 eV are justified by calculating electronic properties and maximum absorption by calculating the optical properties. The electronic portion of thermoelectric transport parameters such as Seebeck coefficient, electrical conductivity, thermal conductivity, Power factor, and figure of merits versus temperature, chemical potential, and carriers’ concentrations was computed using Boltzmann transport equations. The 0.78, 1, and 0.95 values of ZT advocate that these materials can be used in real-life solar cells and thermal devices.