Optimizing Zinc Sulphide Thin Films for Solar Cells: Effects of Annealing and Substrate Temperature on Structural and Optical Properties

Using the spray pyrolysis method, Zinc sulphide (ZnS) thin films were deposited on microscope glass slides at different substrate temperatures. Zinc acetate and thiourea were used as the precursor to prepare the ZnS films. The substrate temperature were respectively  set at 300  ^oC and 350 ^oC for the samples under investigation. The annealing temperature for the prepared film is 400 ⁰C. The films range in thickness from 4.42 to 29.7 µm. The impacts of both substrate and annealing temperature on the optical and structural properties of ZnS were investigated via the UV-visible spectroscopy and x-ray diffraction (XRD) analyses respectively. The results revealed that the grain size of ZnS films varied from 20.74 nm to 28.70 nm as substrate temperature increased from 300 ^oC to 350 ^oC. The films also possessed a polycrystalline cubic structure with (111) preferential orientation. The ZnS films showed more than 20 to 25 % transparency in the near-infrared and visible spectrums. The refractive index falls between 3.04 and 3.07. The energy band gaps of prepared films decreases from 3.35 eV to 2.80 eV with increasing substrate temperature, as estimated from the optical analysis for the ZnS. The obtained properties show a suitably fit material as window layer in a thin film solar cell and some other optoelectronic applications.