Optoelectronic properties of Cu2-xAgxMgSnSe4 (x = 0, 0.5, 1, 1.5, 2) quaternary chalcogenides using mBJ + U potentials

This investigation employs density functional theory (DFT) based computational methods to analyze the structural, electronic, and optical characteristics of quaternary chalcogenides Cu_2-xAg_xMgSnSe2 (x = 0, 0.5, 1, 1.5, 2). The study examines these compounds in two distinct crystalline phases: kesterite (KS) and stannite (ST). To ensure accurate theoretical modelling, multiple exchange-correlation functional were utilized, including the generalized gradient approximation (GGA), the Tran-Blaha modified Becke-Johnson potential (TB-mBJ) for improved band gap predictions, and Hubbard corrections (U) to account for strong electron correlations. Structural optimization was performed, and lattice parameters were calculated for each composition and crystal phase. The results provide insights into how silver (Ag) substitution influences the material's properties for potential photovoltaic applications. Electronic structure analysis reveals direct band gap at the Γ-point, varying from 1.003 eV to 1.584 eV, making them promising candidates for optoelectronic applications. Further examination of the total and partial density of states (TDOS/PDOS) indicates that the valence band maximum is dominated by Cu/Ag d-states and Se p-states, while the conduction band minimum consists mainly of Sn s-states and Se p-states. Optical properties were systematically evaluated, including dielectric function components, refractive index, reflectivity, and extinction coefficient. The absorption coefficients suggest strong light-harvesting capabilities, emphasizing the potential of these materials for solar energy applications. The findings provide critical insights for future research on bandgap engineering, defect tolerance studies, and device optimization of these compounds for solar energy applications.