Binary ZnS–ZnO films as an alternative buffer layer for solar cell applications

In this research, binary ZnS–ZnO films were fabricated by a two-step process, offering an alternative buffer layer solution for photovoltaic solar cell applications. ZnS films were attained through thermal evaporation, after which they were annealed in air at separate temperatures resulting in films containing both ZnS and ZnO phases. Structural, electrical, ellipsometric, optical, and surface properties were examined in detail to elucidate their applicability as a buffer layer in photovoltaic applications. X-ray diffraction patterns revealed that the films exhibit cubic ZnS and hexagonal ZnO crystal structures, wherein crystallite size is augmented with higher annealing temperatures. ZnS films exhibited a needle-shaped surface morphology, as confirmed through atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) images. Annealing caused noteworthy modifications on the surfaces of the films. Additionally, absorption spectra denote two distinct absorption regions forming as a result of the annealing process, possibly indicating the emergence of ZnS and ZnO phases. Photoluminescence analyses demonstrate that binary ZnS–ZnO films exhibit greater emission intensities than single-phase ZnS films. Additionally, the annealing process caused the electrical resistivity of films to reduce from 1.28 × 105 to 3.84 × 101 Ω cm. These results suggest that binary ZnS–ZnO films produced via annealing can be considered as promising buffer layers in potential photovoltaic solar cell applications.