Visibly, all the films prepared by CBD were mirror-like and showed good adhesion to the substrate. The color of films changed partially from yellow to dark yellow by increasing the copper content in solution.

Study of the morphology of films deposited by CBD showed they are polycrystalline nature (Figure 1). System 1 (undoped) shows peaks at 2θ = 26.5887, 30.9108, 44.1861, 52.2420 and 54.4284 (ref: 01-080-0019), which belong to (111), (200), (220), (311) and (222) of cubic CdS phase, respectively. These peaks also appeared in all doped film which indicates that Sn doping did not change the cubic CdS lattice structure. Films with lower Sn doping exhibited strong preferential orientation of (111) plane. The corresponding peak intensities from (111) plane increased with increasing Sn doping, which indicates that Sn atoms of slightly bigger radius are inserted at the smaller S atoms in the cubic lattice. Hence, the Sn ions substitute S in lattice positions in good order instead of filling the free spaces. Therefore, very low concentration of Sn dopants did not change the crystal structure of CdS; however, other peaks related to Sn compounds, mostly the SnS with orthorhombic structure, appeared in all doped films. Increasing Sn doping of CdS was accompanied by decrease in d spacing (see Table 2), indicating smaller cubic structure. The d values are in agreement with JCPDS standard. In the presence of low Sn doping also, the films produced a few additional small peaks at 2θ = 23.2035, 60.867 and 52.716 (ref: 01-079-2193) that belong to (111), (314) and (132) related to SnS with orthorhombic phase. These peaks did not show significant change in agreement with previous works [12,14].

The crystallite sizes, as determined by using Scherrer formula on the XRD patterns [15], increased from 18 nm for undoped CdS to 37 nm for CdS doped with Sn of 0.042 M concentration (Table 2). The small average crystallite size indicates that dominated deposition takes place by ion-by-ion condensation because of proper conditions of deposition and slow growth rate during the whole experiment. As a result, a much smaller, denser grain size and thinner films were achieved [16].

The UV-Vis transmission spectra of all the Sn-doped films exhibited sharp fall at the band edge, which suggests good crystallinity of the thin films (Figure 2). Films doped with Sn showed a sudden increase in transmittance because they are thicker and there is more space inside the lattices in doped films. However, in System 3 onwards, the transmittance spectra showed less transparency with increasing dopant, which agrees well with the earlier report on Copper doping [11]. This may be attributed to more scattering of photons by the introduction of dopant as foreign atoms, which may reduce transmittance. Compared with the constant value of 65% transmittance of undoped film before band edge, the doped films have about 90% transmittance of Incident Solar Spectrum around 1100 nm wavelength region and fall to a minimum of 73% before band edge. This is about a 20% improvement in transparency for the low Sn doped film.

The absorption spectra obtained from the transmission spectra of each sample is shown in Figure 3. Here, α value increases with the increase in Sn ions in growth chemical solution. All doped film spectra have lower α values compared to the undoped films.

A typical plot of (αhυ)² versus photon energy, obtained from Figure 3 is shown in Figure 4 for Systems 1 and 3. For the Systems 1, 2, 3, 4, 5, the corresponding thicknesses are 400, 423, 525, 570, 662 nm, and the corresponding E_g’s are 2.43, 2.33, 2.30, 2.26, 2.24 eV, respectively. The dependency of E_g decreases with film thickness (Figure 5). These phenomena are attributed to the quantum size effect of the grain size that changes the thicknesses of films [17]. No absorption occurs below the band gap (Figure 3). Hence, the increment of extinction coefficient is due to scattering by dopant atoms on the film surface. Urbach energy (E_u), known as band tail width, is due to the disorder in the thin film material. As shown (Figure 5), the variation of Urbach energy is opposite to E_g. In other words, the E_g decreases when there is an increase of band tail and vice versa.

Figure 6 presents FE-SEM micrographs of the film surfaces of System 1, 2 and 5. These images reveal that the films are compact, free of colloidal particles and are in good order. The sizes of grains were in the same range as those that show homogenous distribution of Sn dopant in films. Increasing the dopant led to grain growth. Both the doped films are more compact compared with the undoped film. During the growth of films, the nuclei appeared and grew as discrete surface grain. The surface grain size became larger, with less void area, and led to the formation of high quality film surfaces. The EDAX analysis of the undoped and two doped systems is shown in Figure 7. For undoped film, the maximum amount of Cd and S observed were 31.04% and 10.08%, respectively. For System 2 and 5, the quantities of Cd decreased, S remained almost constant, and Sn increased from (10.09:3.68:6.20) to (9.06:3.0:9.06). In both the systems, at low dopant concentration, the Sn atoms substitute Cd atoms in the lattice. Other systems, such as 3 and 4, showed variations intermediate to Systems 2 and 5.