Exploring the structural, phononic, electronic, magnetic, optical, and thermoelectric properties of Pb-free vanadium-based double perovskites using the first-principles approach for optoelectronic and thermoelectric applications

Double perovskites are very famous among the scientific community as non-traditional, eco-friendly, low cost, efficient and stable energy harvesting materials, especially for the perovskite solar cells and thermoelectric generators. These non-traditional energy production schemes are gaining much attention with each passing day since the current energy sources mostly based on traditional fossil fuel methods are not adequate for future generations and also, badly damage the environment. Within the framework of density functional theory and Boltzmann transport theory, we investigated the structural, phononic, electronic, magnetic, optical, and thermoelectric properties of Cs2NaVX6 (X = Cl, Br, I) using generalized gradient approximation (GGA) with the inclusion of on-site Hubbard-U term as an exchange-correlation potential. Goldschmidt's tolerance (tau) and octahedral factor (mu) were used to assess structural stability while thermodynamic stability was confirmed from the existence of all positive frequencies in phonon dispersion curves. The calculated direct band gaps of 2.9 eV, 2.0 eV, and 1.7 eV, high absorption coefficients in the range of 10(6) cm(-1), low reflection (below 20%), high optical conductivity near similar to 10(15) sec(-1)in the visible region makes these compounds very promising for optoelectronic applications. Moreover, large Seebeck coefficients with values 1906.67 mu V/K, 667.17 mu V/K, and 462.98 mu V/K, high power factors i.e. 1.21 x 10(11) W/msK(2), 2.09 x 10(11) W/msK(2) and 4.67 x 10(11) W/msK(2) and figure of merit greater than or close to unity i.e. 1.06, 0.96 and 0.91 for Cs2NaVCl6, Cs2NaVBr6 and Cs2NaVI6 make these compounds very promising for optoelectronic and thermoelectric applications.