First principles insights into the electronic, optical, and thermoelectric properties of Cu2ZnSnS4 for energy conversion applications

• DFT-based FP-LAPW study of kesterite Cu 2 ZnSnS 4 using WIEN2k code. • Optimized structure confirms stable tetragonal symmetry and mixed bonding. • TB-mBJ results show a direct 1.0 eV band gap ideal for solar absorbers. • Strong optical absorption (10 4 –10 5 cm −1 ) supports efficient light harvesting. • Thermodynamic analysis shows stability and predictable heat capacity trend. • BoltzTraP transport data reveal high Seebeck and good electrical conductivity. • CZTS shows potential for photovoltaic and thermoelectric energy conversion. The quaternary chalcogenide Cu 2 ZnSnS 4 (CZTS), a kesterite-type semiconductor, has been comprehensively investigated using density functional theory (DFT) within the FP-LAPW method as implemented in WIEN2k. Structural optimization confirms tetragonal symmetry with stable bonding arising from mixed covalent–ionic interactions between Cu–S, Zn–S, and Sn–S, as revealed by charge density analysis. The electronic band structure, refined using the Tran–Blaha modified Becke–Johnson (TB-mBJ) potential, demonstrates a direct band gap of 1.0 eV at the Γ-point, aligning well with the optimal range for single-junction photovoltaic absorbers. Optical spectra derived from the frequency-dependent dielectric function indicate a static dielectric constant ε 1 (0) ≈ 6–8, strong absorption (10 4 –10 5 cm −1 ) above the band edge, confirming its suitability for efficient light harvesting. Thermodynamic analysis predicts stable behavior with increasing temperature, where Gibbs free energy decreases, heat capacity (Cv) approaches the Dulong–Petit limit, and entropy increases monotonically. Transport calculations based on BoltzTraP reveal favorable thermoelectric parameters, including high Seebeck coefficients and good electrical conductivity, yielding an appreciable thermoelectric figure of merit (ZT). Overall, the synergistic combination of a direct optimal band gap, strong light absorption, stable bonding, and promising thermoelectric response establishes CZTS as a viable candidate for photovoltaic devices and energy conversion applications.