© 2021In this study, the properties of ZnS films which can be used as a buffer layer in photovoltaic thin film solar cell technologies were tried to be improved by applying post-production heat treatments. ZnS films were subjected to annealing processes in Nitrogen, Argon and Air atmospheres with an atmosphere-controlled heat treatment system and the effects of these atmospheres on the films were investigated. As a result of XRD analysis, it was determined that all films show a preferential orientation towards the (111) ZnS plane. The crystallite size of the ZnS film increased with the applied heat treatment. It was also confirmed by transmittance measurements that the ZnS film annealed in air is partially oxidized and contains the secondary ZnO phase in its structure. Optical band gap values for ZnS films were found to be 3.53–3.56 eV. The refractive index values of the films were determined by spectroscopic ellipsometry. Two-probe technique was used to determine the electrical resistivity and heat treatment caused a decrease in the resistivity values of the films. Surface analyzes of the films were performed with the help of AFM and SEM measurements. As an outcome of this study, it has been revealed that the surfaces of ZnS films can be modified by heat treatment in Argon atmosphere. The production of ZnS films and their optimization with subsequent heat treatments were carried out, and ZnS films with high transmittance and improved morphology were obtained. These materials can be used as buffer layer or window layer in CZTS, CIGS and CdTe based solar cells.