Annealing effect on structural, morphological, optical, and electrical properties of spin coated ZnS thin films for photovoltaic application

The conventional cadmium sulfide (CdS) window/buffer layer in photovoltaic cells is environmentally hazardous because of the poisoning of cadmium (Cd). Alternatively, ZnS is more environmentally friendly than CdS and has a larger band gap, which makes it a potential candidate for window/buffer layers. In this study, ZnS thin films were deposited on glass substrates by a spin coating process and annealed at three (250 °C, 350 °C, and 450 °C) different temperatures. The XRD patterns confirmed that all the spin coated films had mixed wurtzite and cubic structures with a preferred orientation along the (111) plane of the predominant cubic phase. The highest crystallite size and lowest dislocation density were found at 350 °C annealing temperature due to the narrow, sharp and high intensity diffraction peak compared with those at 250 °C and 450 °C annealing temperatures. The SEM results indicate that the surface of the ZnS film annealed at 350 °C has a better surface coverage area with good uniformity, and is more homogeneous with a minimum amount of pinholes, voids and cracks than the other samples annealed at 250 °C, and 450 °C. The estimated optical band gap was determined to be between 3.957 and 3.991 eV. The calculated electrical resistivity values are on the order of \({10}^{4}\) Ω cm. All the findings revealed that the film annealed at 350 °C presented good material properties for utilizing as buffer layer in thin film solar cells.