CdS films prepared from a reaction mixture containing cadmium chloride and thiourea for both molar concentrations (A and B) show polycrystalline nature [18,19] as shown in the XRD pattern, Figure 1. The XRD graphs are scaled to a small size and seven graphs are presented in one figure, many of the small peaks are lost in this process. Here, all of the peaks that were observed are described in detail in Table 1. Both samples without any heat treatment are polycrystalline and a mixture of cubic and hexagonal phase of CdS [18–20].

In all samples except A₀ and B₀, two peaks were observed at 2θ = 20.8511° and 21.5199° (ref: 00-036-0898) belonging to (110), (020) orthorhombic CdSO₄. It is also observed that the preferred orientation is (111) which is (111) due to the controlled nucleation process occurring in the growing film. This suggests the slow growth rate of the film deposition [21]. Thickness of the film sample B₀ is 62.7 nm and is further decreased with the increase of the annealing temperature (Figure 2). The thickness is about 64.6 nm for the sample B₃₆₀. It was observed that the thickness of the samples prepared from the solution concentration A is lower than that of the samples prepared from solution concentration B for all annealing temperatures. The minimum thickness 33.6 nm was observed for the sample A₃₂₀. This indicates the thickness of the film is lower when the higher solution concentration is used.

The average crystallite size of the film sample A₀ is 20.7 nm. The crystallite size of the sample A₂₀₀ is 31 nm and is further reduced to 20 nm for the sample A₃₆₀, Figure 3. The average crystallite size of the film sample B₀ is 41.4 nm and is reduced to 7.4 nm for the sample B₃₆₀. This indicates a decrease in crystallite size with the increase of annealing temperature, suggesting an increase in crystallinity with annealing temperature. This situation was not observed for the samples prepared from the solution concentration A, which is higher than that of solution concentration B. This indicates the films developed at higher concentration were more crystalline in nature before annealing.

The transmittance spectra of CdS films were recorded over 350 to 1100 nm (Figures 4 and 5). The spectra showed transmittance dependence of film on the wavelength at a different annealing temperature. The transmittance (T%) is about 89% at wavelength of 550 nm for the sample A₃₆₀ and is increased to 97% at the wavelength 850 nm, Figure 4. It is slightly increased with the further increase of wavelength. The transmittance (T%) is about 98% at wavelength of 550 nm for the sample B₃₆₀ and remains approximately constant with the further increase of the wavelength up to 1100 nm, Figure 5.

It was observed that transmission spectra shift towards the lower wavelength range for the sample B₃₆₀, which indicates the increase in the optical energy band gap. This suggests the higher concentration of the solution (concentration A) has better crystallinity as compared to the samples that are prepared from the lower concentration (concentration B) at the annealing temperature 360 °C. The transmittance is about 80% at wavelength of 550 nm for the sample A₀ and is increased to 91% at the wavelength 850 nm. It is slightly increased with the further increase of wavelength. The transmittance is 78% at wavelength of 550 nm for the sample B₀ and is slightly increased with the further increase of wavelength up to 1100 nm. The transmission spectrum is observed to shift towards the lower wavelength range for the sample B₀. This shift of spectra indicates the increase in the optical energy band gap (E_g), as shown in Figure 8 [22]. This suggests the higher concentration of the solutions (concentration A) has the better crystallinity as compared to the samples which are prepared from the lower concentration solutions (concentration B) for the as-deposited CdS thin films. The variation of the optical absorption coefficient (α) with wavelength for different annealing temperatures and solution concentrations is shown in Figure 6.

The graph (αhυ)² versus photon energy (hυ) is shown in Figure 7. The value of the optical energy band gap (E_g) is 2.22 eV for sample A₀. The value of the optical energy band gap (E_g) is 2.65 eV for sample B₀ Figure 8. It was observed that there is a prominent change in the value of E_g for the samples A₀ and B₀. This suggests that the crystallinity of the films increased with the increase of the solution concentration. It is clear from Figure 2 that the film thickness is also increased with the decrease of solution concentration. Particle size also increased with the decrease of solution concentration. It is supposed that loosely adherent collides are formed with the decrease of the concentration. This may be the reason why the value of E_g is increased. It is also observed that the value of E_g for solution concentration A (higher concentration) increases with the increase of annealing temperature indicating that the crystallinity is enhanced with annealing temperature. The trend in the value of E_g decreases for the samples prepared from solution concentration B (lower concentration). This suggests a decrease in the crystallinity for very low concentrations. E_u is Urbach energy and is shown in Figures 9 and 10. E_u is also known as band tail width and is due to the disorder in the thin film material. Its variation with annealing temperature is shown in Figure 11 for concentration A and B. The variation of bond length and bond angle from their standard value in the crystalline material is called disorder [23,24]. It is clear that the optical band gap is opposite to the disorder. This behavior indicates that the obtained optical band gap is governed with the disorder variation in the CdS films.