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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]

[Display omitted] • New energy harvesting mechanisms as alternatives for traditional energy sources. • Optoelectronics and thermoelectricity have gained much attention. • The future of thermoelectric generators. • High figure of merit promising for thermoelectric applications. Researchers have shown substantial interest in vacancy-ordered halide perovskites for non-traditional energy harvesting applications including solar cells and thermoelectric generators. In this study we performed band gap engineering of vacancy-ordered halide perovskite Cs 2 SnCl 6 from substitutional doping of Pt (Cs 2 Sn (1-x) Pt x Cl 6 where x = 0, 0.25, 0.50, 0.75 and 1.00) and studied its effects on thermoelectric properties using first-principles approach. Thermodynamic stability has been proven by computing the formation energies and the tolerance factors. The band gaps were efficiently reduced to 2.50 eV from 3.61 eV by the band gap engineering approach of Pt doping. Consequently, the materials exhibited improved thermoelectric properties, including low thermal conductivities, high Seebeck coefficients, and high values of ZT. The maximum value of ZT = 2.27 was estimated for x = 1.00. The Pt doping technique can be deemed effective based on the enhanced structural and thermoelectric performance parameters. [ABSTRACT FROM AUTHOR]

Vacancy-ordered halide perovskites have gained considerable attention from researchers regarding non-traditional energy harvesting applications like solar cells and thermoelectric generators. However, in most of the reported cases, band gaps are larger (>3.5eV) consequently the efficiency of solar cells and thermoelectric generators becomes low. Currently, we studied non-toxic and stable vacancy-ordered halide perovskites Cs 2 GeCl 6 and tuned its band gap via substitutional doping of Pt (0, 25, 50, 75, and 100 %) using the first-principles approach. The band gap engineering strategy of Pt doping effectively decreased the band gaps (up to 2.50eV) hence, the more attractive optical and thermoelectric parameters are obtained for instance; high absorption coefficients (∼105 cm−1), low reflectivity (∼0.3–10 %), high optical conductivity (∼1015 sec−1), and large figure of merits (∼1). Based on these enhanced optical and thermoelectric performance parameters, the Pt doping strategy can be taken as an effective practice to tune the band gap significantly in order to stimulate the performance of future solar cells and thermoelectric generators. • New energy harvesting mechanisms as alternatives for traditional energy sources. • Optoelectronics and thermoelectricity have gained much attention. • The future of Perovskite thermoelectric generators. • High absorption coefficient double perovskite with reasonable band gaps. • High figure of merit promising for thermoelectric applications. [ABSTRACT FROM AUTHOR]

Robust and Eco-friendly double perovskites X 2 AgFeBr 6 (X=Na, Li) simulations and exploration in terms of thermoelectric aspects. [Display omitted] • New energy harvesting mechanisms as alternatives for traditional energy sources. • Stable, Efficient, and Eco-friendly double perovskites. • Thermoelectric energy sources have gained much attention. • High figure of merits promising for thermoelectric applications. In this computational work, we simulated the robust and eco-friendly double perovskites X 2 AgFeBr 6 (X = Na, Li) for the first time in detail for green energy harvesting applications involving thermoelectric generators. The structural, mechanical, and thermodynamic stability is assured from the Goldschmidt's tolerance and octahedral parameters, Burn-Haun criterion, formation energies, and phonon spectra respectively. The electronic band structures and density of states informed us the semiconducting nature of the compounds with band gaps of 4.81 eV, and 3.82 eV. The thermoelectric parameters like electrical conductivity close to ∼ 1017(Ωms)-1, Seebeck coefficients up to 531.56 (μV/K), power factor in range of ∼ 1010 W/msk2, and figure of merits close to one are observed at room temperature (T = 300 K). These fascinating results provide a theoretical basis for the synthesis of lead-free double perovskites X 2 AgFeBr 6 (X = Na, Li), which find applications in thermoelectric devices. [ABSTRACT FROM AUTHOR]