Influence of (Ar+ O2) atmosphere and CdCl2 coating heat treatment on physical properties of CdS thin film for solar cell applications

Cadmium sulphide (CdS) thin films have been grown on corning glass substrate by thermal evaporation technique under base pressure 2  ×  10⁵ mbar. In this study we investigate the influence of Cadmium chloride (CdCl₂) heat treatment and argon and oxygen (Ar  +  O₂) annealing on the structural, morphological optical and electrical properties of CdS thin films to achieve high quality thin films for solar cells applications. X-ray diffraction (XRD), Field Emission scanning electron microscope (FE-SEM) and UV–Vis-NIR spectrophotometer were used to confirm and investigate the different physical properties of CdS films. The quality of CdS thin films crystallinity including the average grain size (D _avg), the average lattice strain (ε_ls) and the dislocation density (ρ _D) were extracted via XRD line broadening analysis. XRD analysis revealed that the samples are polycrystalline in nature with a mixed (hexagonal  +  cubic) structure and the crystallinity of the films increases with both (Ar  +  O₂) annealing and CdCl₂ heat treatment. After (Ar  +  O₂) annealing and CdCl₂ heat treatment processes the mean values of the grain size indicating the grain growth of CdS films. The surface morphology of the CdS films was examined by (FE-SEM) analysis; which confirmed the nano-crystalline nature of the CdS films with uniform coverage of the substrate surface; The optical properties of as-deposited, (Ar  +  O₂) annealed and CdCl₂ treated CdS films were discussed in detail. The optical energy gap was calculated using Tauc plot extrapolation. It was found to be increased from 2.43 eV for as-deposited film to 2.45 for CdCl₂ treated CdS film. Swanepoel method was employed to estimate the thicknesses and extract the refractive index of the CdS films. The Room temperature resistivity decreased from 13.4 Ω cm for as-deposited CdS film to 2 and 0.8 Ω cm for (Ar  +  O₂) annealed and CdCl₂ treated CdS films respectively. The CdCl₂treated CdS film has the highest figure of merit value of 29  ×  10⁻⁴ Ω⁻¹. The obtained results confirm that the CdCl₂ heat treatment process in (Ar  +  O₂) atmosphere can enhance the physical properties of CdS thin films as a promising window layer or buffer layer material for CdS/CdTe and Cu(In,Ga)Se₂ solar cells.