Fabrication, structural, morphological, and optical studies of CdS:Cu nanostructured thin films: effect of Cu concentration

Copper-doped cadmium sulfide (CdS:Cu) nanocrystalline thin films were coated on glass substrates with different Cu concentrations (0 wt.%, 3 wt.%, and 6 wt.%) using the sol–gel spin coating technique. X-ray diffraction (XRD) spectrometry, scanning electron microscope (SEM), and energy-dispersive X-ray (EDX) analysis were used to analyze the average crystallite size, surface morphology, and chemical analysis of prepared thin films in 3 and 6 wt.% of Cu concentrations. According to the XRD results and SEM micrographs, nanocrystalline grains of doped thin films have an almost regular geometric shape, which become slightly larger by increasing Cu concentration from 3 to 6 wt.%. The optical and electrical characteristics of all three films were examined by spectroscopy ellipsometry (SE) technique in the wavelength range between 300 and 900 nm. Based on the SE results, the thin film prepared with 0 wt.% Cu concentration showed a higher refractive index, extinction coefficient, and absorbance compared to the other Cu-doped thin films. The CdS:Cu thin films with 3 and 6 wt.% Cu concentrations demonstrated significantly higher transparency than pure ones (0 wt.% Cu doped), making them particularly suitable for different optoelectronic applications. Moreover, the optical band gap energy (E_g) values were evaluated. It can be concluded that the pure film has the largest band gap energy, and Cu doping reduces the E_g values. The spin-coated 6 wt.% CdS:Cu thin film showed high transparency (67.83%), good homogeneity, and a substantial E_g value (2.6 eV), making it ideal for photocatalytic and photovoltaic applications.