Impact of post-deposition annealing on the optical, electrical, and structural properties of CdS thin films for solar cell applications

In this study thermally evaporated cadmium Sulfide (CdS) thin films with the thickness of 100 nm were fabricated at 150 degrees C under the pressure of 2 x 10-5 mbar and subjected to annealing treatment at various temperatures. The optical, electrical, and structural properties of CdS thin films, which were influenced by annealing treatment, were then studied using ultraviolet-visible (UV-Vis) spectroscopy, Hall effect measurement system, Xray diffraction (XRD), Raman spectroscopy and Atomic force microscopy (AFM) analysis. The energy band gap was found to decrease from 2.402 eV for the as-grown CdS film to 2.250 eV for the film annealed at 450 degrees C. The electrical parameters of CdS thin films, such as carrier concentration, resistivity and mobility, were then calculated using the Hall effect measurement system; these were improved with the increase of the annealing temperature up to 350 degrees C. The crystallite size of the preferred (111) orientation was in the range of 12-40 nm and increased with annealing temperature. The increase in the intensity of peaks and shifting peaks toward the single-crystal mode was observed in the Raman spectra of the annealed CdS thin films. The AFM images also revealed that the film roughness increased with heat treatment. Further, the obtained SEM images showed the uniform surface of the film. The efficiency of the designed solar cell was in the range of 21.84-22.18 %; it was increased by using the annealed CdS films. The present work, thus, introduced the annealed CdS thin film at 350 degrees C as a promising window layer to fabricate highly efficient CdTe-based solar cells.