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• The calculated band gaps of Cs 2 NaVX 6 (X = Cl, Br, I) are found direct. • High absorption coefficients in the range of 106 cm−1, low reflection i.e. below 20% • High optical conductivity near ∼ 1015 sec-1 in the visible region. • Figure of merit greater than or close to unity. • Compounds are very much promising 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 Cs 2 NaVX 6 (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 (τ) and octahedral factor (μ) 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 106 cm−1, low reflection (below 20%), high optical conductivity near ∼ 1015 sec-1in the visible region makes these compounds very promising for optoelectronic applications. Moreover, large Seebeck coefficients with values 1906.67 µV/K, 667.17 µV/K, and 462.98 µV/K, high power factors i.e. 1.21 × 1011 W/msK2, 2.09 × 1011 W/msK2 and 4.67 × 1011 W/msK2 and figure of merit greater than or close to unity i.e. 1.06, 0.96 and 0.91 for Cs 2 NaVCl 6 , Cs 2 NaVBr 6 and Cs 2 NaVI 6 make these compounds very promising for optoelectronic and thermoelectric applications. [ABSTRACT FROM AUTHOR]