Computational study of optoelectronic properties of Cu_2-xAg_xZnSnSe₄ (x=0, 0.5, 1, 1.5, 2) compounds

We report a systematic first-principles investigation of the structural, electronic and optical behaviour of the quaternary chalcogenides Cu_2-xAg_xZnSnSe₄ (x = 0, 0.5, 1.0, 1.5, 2.0) crystallizing in the kesterite structure. Structural relaxations were carried out within density functional theory, and electronic properties were refined using the modified Becke-Johnson (mBJ) potential combined with Hubbard U parameter (mBJ + U) to overcome the well-known underestimation of band gaps in semi-local approaches. Ag incorporation induces a progressive lattice expansion and modifies the local bonding environment while maintaining overall structural stability. The calculated band structures reveal direct band gaps in the technologically relevant range (≈1.156-1.591 eV), with the valence-band edge dominated by Cu/Ag-d and Se-p states and the conduction band primarily governed by Sn-s/Se-p antibonding interactions. Optical calculations show pronounced absorption in the visible and near-ultraviolet regions, together with notable dielectric anisotropy and reflectivity behavior consistent with interband transitions. The results demonstrate that controlled Ag substitution offers a practical route for tuning band alignment and enhancing light-harvesting capability without compromising structural integrity. Overall, this study provides mechanistic insight into composition-structure-property relationships in Ag-modified kesterites and highlights their potential as scalable absorber materials for next-generation thin-film photovoltaics.