Structural and Optical Properties of Cu<sub>2</sub>ZnSn(S<sub>1−x</sub>Se<sub>x</sub>)<sub>4</sub> Nanostructures Thin Film for Photovoltaic Applications

Copper zinc tin sulfide selenide, Cu2ZnSn(S1−xSex)4, absorbers are promising earth-abundant and environmentally benign materials for low-cost photovoltaic applications. This study investigates the structural and optical properties of Cu2ZnSn(S1−xSex)4 nanostructured thin films prepared by pulsed laser deposition using melt-quenched targets with selenium compositions x = 0.0–1.0. X-ray diffraction revealed that films with low selenium content remained amorphous, whereas higher selenium incorporation promoted the formation of polycrystalline kesterite–stannite phases with preferred orientations along (112), (200), (220), and (312). The crystallite size increased from 12.3 to 17.9 nm as selenium reached x = 1.0, indicating enhanced crystal growth. Atomic force microscopy showed composition-dependent surface evolution, where average roughness decreased initially, reached a maximum of 88.29 nm at x = 0.6, and then declined, reflecting structural reorganization during phase transition. Optical characterization by UV–Vis–NIR spectroscopy demonstrated high absorption coefficients exceeding 104 cm−1 in the visible region, confirming strong light-harvesting capability. The direct optical band gap was tunable between 2.00 and 2.30 eV, with the highest value observed at x = 0.6 due to quantum confinement, nanoscale disorder, and compositional effects. The refractive index, extinction coefficient, and dielectric constants decreased with selenium addition up to x = 0.6, then increased at higher selenium contents, indicating a strong correlation between composition and optical response. The combined results highlight selenium control as an effective route for tailoring phase stability, transparency, and photon management for devices.