First principles quantum analysis of structural, electronic, optical and thermoelectric properties of XCu₂GeQ₄ (X = Ba, Sr and Q = S, Se) for energy applications

Cu-based chalcogenide materials have attracted a great deal of attention due to their promising optoelectronic properties. The density functional theory (DFT) framework is used in order to estimate the optical and electronic properties of XCu₂GeQ₄ (X = Ba, Sr and Q = S, Se). We report the optical and electronic properties of Cu-based chalcogenides in this study, which have narrow and direct bandgap materials. The calculated energy bandgap of quaternary chalcogenide materials decreases in the following sequence: SrCu₂GeS₄ (0.697 eV), BaCu₂GeS₄ (0.667 eV), BaCu₂GeSe₄ (0.378 eV), and SrCu₂GeSe₄ (0.195 eV). This reduction in energy bandgaps shows significant effect of changing dopants on electronic and consequently optical properties of XCu₂GeQ₄ (X = Ba, Sr and Q = S, Se). The optical characteristics of these materials are investigated in order to explore their potential for optoelectronic applications. However, other materials are emerging as contenders for solar cells, which operate from UV to infrared regions. Initially in infrared region, we can note a redshift in the maximum absorption of incident photons from ε₂(ω) plots in the following sequence: BaCu₂GeS₄ (1.52 eV), SrCu₂GeS₄ (1.50 eV), BaCu₂GeSe₄ (1.30 eV), and SrCu₂GeSe₄ (0.93 eV). The approximated values of reflectivity, R(ω) are plotted against incident photon energy from 0 to14 eV. Thus, the reflectivity is approximately below 50% before E ≈ 12.0 eV and then increased to 70% reflection at ~ 13.0 eV. Based on calculated thermoelectric properties, these chalcogenides are promising thermoelectric materials. The ZT values of XCu₂GeQ₄ (X = Ba, Sr and Q = S, Se) decreased in the following sequence: SrCu₂GeSe₄ (2.6), BaCu₂GeSe₄ (1.85), SrCu₂GeS₄ (1.01) and BaCu₂GeS₄ (0.94). Hence, we believe our findings propose promising materials for anti-reflecting coating layers in optoelectronic technology.