1. Introduction

CdS, an N-type direct band-gap semiconductor material with an optical band gap of 2.43 eV and a good transmittance in the visible light range, has been studied as a buffer layer for heterojunction solar cells for many years^[1]. However, due to the narrow optical band gap of CdS, the absorption layer of the short wave is hindered. In addition, cadmium is a toxic substance that is detrimental to human health. An environmentally friendly N-type semiconductor compound ZnS, with an optical band gap of 3.71 eV^[2] and a good transmittance for short wave, has been widely considered to replace CdS. While, copper indium gallium selenide (CIGS) band gap values range from 1.02 to 1.67 eV, there is a big difference between them. Direct contact results in poor lattice matching with the solar cell absorption layer^[3], affecting modules photoelectric conversion efficiency. Studies have found that the band gap of CdS can be adjusted by doping Zn to improve the thin film performance^[4], and sulfur heterojunction materials containing cadmium and zinc can be prepared in the form of Cd_1–xZn_xS. Due to the doping of Zn, the band gap of the thin film varies between 2.43 and 3.71 eV. The photoelectric properties of the thin film can be improved by regulating the proportion of zinc in the thin film properly.

A number of methods have been developed for preparing Cd_1–xZn_xS thin films including spray pyrolysis, successive ion layer absorption and reaction (SILAR), chemical vapor deposition (CVD) and chemical bath deposition (CBD)^[5], etc. The CBD method is adopted in experimental research for its low-cost and relatively reduced damage to the absorption layer. The quality of the thin film prepared by this method is affected by the following factors, for example, concentration of reaction solution, temperature, pH value and the Cd²⁺ concentration^[6]. Olivia et al. concluded that the surface morphology of CdZnS films with low cadmium ion concentration was better than that of films with high cadmium ion concentration^[7]. Zhang used CBD to deposit Cd_0.9Zn_0.1S thin film with the band gap of 2.56 eV, exhibiting high transmittance and dense surface. Offiah et al. thought that higher Cd source concentrations were favorable for single-phase deposition of CdZnS films, and the optical band gap increased with the increase of cadmium source concentration^[8].

Although studies on the effects of cadmium ion concentration on Cd_1–xZn_xS thin films have been extensively reported, the sources of cadmium are mostly cadmium chloride (CdCl₂), cadmium nitrate (Cd(NO₃)₂) or cadmium acetate (Cd(CH₃CO₂)₂), and little research has been conducted on the effects of cadmium sulfate (CdSO₄) on thin films. In this paper, the influence of cadmium sulfate concentration on the morphology structure and optical properties of Cd_1–xZn_xS thin films is studied, aiming to improve the properties of thin films.

2. Experimental

2.1 Cd_1–xZn_xS films deposited by chemical bath deposition (CBD)

Part of the ions of insoluble substances enter the solution and the others deposit on the solid surface. The dissolution equilibrium is reached when the rate is the same in both, and the equilibrium constant is called solubility product^[9]. However, materials prepared by the CBD process generally have a small solubility product, once Cd²⁺, Zn²⁺ and S^2– is presented in the solution, precipitation will be generated rapidly, affecting the growth of thin films. Thus, complexing agents are usually added to reduce a metal ion release rate. Ammonia is generally chosen as the complexing agent and ammonium sulfate as the buffer. NH₄⁺ reacts with Cd²⁺ and Zn²⁺ to form a stable complex of cadmium and zinc for controlling the film deposition rate effectively and improving the film growth quality.

The reagents used under experimental growth conditions of Cd_1–xZn_xS thin films are shown in Table 1. In this experiment, cadmium sulfate and zinc sulfate are used as the source of Cd²⁺ and Zn²⁺ and thiourea as the source of S^2–. Besides, ammonia water is used as the complexing agent and ammonium sulfate as the buffer agent. During the experiment, the 2 × 3 cm² glass substrates that cleaned with deionized water ultrasonic cleaning solution are put into five beakers, followed by the addition of cadmium sulfate, zinc sulfate, ammonium sulfate and an appropriate amount of deionized water. Adding 25% ammonia and 1 M thiourea in turn when the temperature of the reaction liquid water reaches 85 °C, and timing is started. The mixture is stirred every 5 min, and yellow precipitation gradually formed in the beakers. After 30 min of reaction, take out the beakers, wash and dry the samples. The chemical reactions during the deposition of the film are as follows:

$${\rm{N}}{{\rm{H}}_{\rm{3}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{N}}{{\rm{H}}_{\rm{4}}}^+ {\rm{ + O}}{{\rm{H}}^-},$$

(1)

$${\rm{ZnS}}{{\rm{O}}_{\rm{4}}} \to {\rm{Z}}{{\rm{n}}^{{\rm{2+ }}}}{\rm{ + S}}{{\rm{O}}^{{{2- }}}},$$

(2)

$${\rm{Z}}{{\rm{n}}^{{\rm{2+ }}}}{\rm{ + 4N}}{{\rm{H}}_{\rm{3}}} \to {\left[ {{\rm{Zn}}{{\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)}_{\rm{4}}}} \right]^{{\rm{2+ }}}},$$

(3)

$${\rm{CdS}}{{\rm{O}}_{\rm{4}}} \to {\rm{C}}{{\rm{d}}^{{\rm{2+ }}}}{\rm{ + S}}{{\rm{O}}^{{{2- }}}},$$

(4)

$${\rm{C}}{{\rm{d}}^{{\rm{2+ }}}}{\rm{ + 4N}}{{\rm{H}}_{\rm{3}}} \to {\left[ {{\rm{Cd}}{{\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)}_{\rm{4}}}} \right]^{{\rm{2+ }}}},$$

(5)

$${\left( {{\rm{N}}{{\rm{H}}_{\rm{2}}}} \right)_{\rm{2}}}{\rm{CS + 2O}}{{\rm{H}}^-} \to {{\rm{S}}^{{{2- }}}}{\rm{ + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O + C}}{{\rm{H}}_{\rm{2}}}{{\rm{N}}_{\rm{2}}},$$

(6)

$${\left[ {{\rm{Cd}}{{\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)}_{\rm{4}}}} \right]^{{\rm{2+ }}}}{\rm{ + }}{\left[ {{\rm{Zn}}{{\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)}_{\rm{4}}}} \right]^{{\rm{2+ }}}} +\, {{\rm{S}}^{\rm{2}}}^- \to {\rm{C}}{{\rm{d}}_{{{1 - x}}}}{\rm{Z}}{{\rm{n}}_{{x}}}{\rm{S + waste}},$$

(7)

2.2 Characterization of Cd_1–xZn_xS thin films

The surface morphology of the film is studied by scanning electron microscope (SEM 460L03040702) and energy dispersion spectroscopy (EDS) attached to the environment scanning electron microscope with a field emission gun (Quanta-FEG 250) is used to analyse the composition and element content of the thin film. A study is made on the crystal structure of the thin film by using X-ray diffraction (XRD) with a scanning range of 10° to 70°, a step length of 0.02, a voltage of 40 kV and a current of 200 mA. The thin film thickness data is recorded with a step height measuring instrument (Veeco Dektak 150). The optical properties of the thin film are studied by a UV–Vis–NIR spectrophotometer at the wavelength range of 300–800 nm.

3. Results and discussion

3.1 Film thickness

Table 2 reveals the thickness of the film obtained by the step height measurement instrument. With the increase of cadmium sulfate concentration, the thickness of the film first increases, then decreasess and increase again, indicating that the film growth rate changes on the trend of the increase, decrease, and increase. Due to the augment of zinc content hindering the reaction between Cd²⁺ and NH₃ to form the cadmium complex, the release rate of cadmium and zinc ions in the solution is accelerated, resulting in the formation of precipitation rate faster than that of the formation of Cd_1–xZn_xS thin films; the thin film growth rate decreases and the film becomes thinner.

3.2 Surface morphology

Fig. 1 exhibits the surface morphology of Cd_1–xZn_xS thin films deposited under the concentration of 0.003, 0.004, 0.005, 0.006, 0.007 M cadmium sulfate respectively. It can be seen that the Cd_1–xZn_xS thin films with cadmium sulfate concentration of 0.003, 0.004, and 0.007 M present better uniformity. For the reason that there is less Cd²⁺ in the solution. Due to the addition of the complexing agent, the release rate of metal ions slows down, Cd²⁺ react with OH^– to produce less precipitation of Cd(OH)₂, while the complex ions react with S^2– to form Cd_1–xZn_xS. The accelerated growth rate makes the film have fewer pores and better density. With the increase of cadmium sulfate concentration, excessive Cd²⁺ exists in the solution. The precipitation of Cd(OH)₂ and Zn(OH)₂ increases rapidly, hindering the film’s growth and the solution alkalinity increases, leading to more pores. When the concentration continues to increase to 0.007 M, less Cd²⁺ and S^2– is involved in the reaction results in accelerating the film growth rate and improving compactness. Since pores increase the probability of carrier recombination and reduce the photoelectric conversion efficiency, thin films with fewer pores on the surface are more suitable for the buffer layer.

3.3 Thin film composition

The distribution diagrams of Cd, S and Zn in Cd_1–xZn_xS films deposited under the different concentration of cadmium sulfate are showed in Fig. 2, in which yellow represents cadmium, red represents zinc, and green represents sulfur. Figs. 2(c) and 2(d) diagrams show strong red, corresponding to CdSO₄ concentration of 0.005 and 0.006 M, while Figs. 2(a), 2(b) and 2(e) show more green and yellow, corresponding to CdSO₄ concentration of 0.003, 0.004 and 0.007 M. The above phenomenon indicates that zinc content changes on the trend of the decrease, increase and decrease, and it is preliminarily concluded that it is formed due to the augmentation of the cadmium sulfate concentration.

The EDS data given in Table 3 shows that with the increase of CdSO₄ concentration, the percentage of zinc content presents a trend of decrease, increase and decrease.

When the concentration of CdSO₄ increases to 0.004 M, the complexity of cadmium in the solution increases and reacts with S^2– to form Cd_1–xZn_xS. The addition of complexing agent reduces the release rate of metal ions, resulting in less precipitation and slightly increases the film growth rate. EDS results show the percentage of zinc decreases slightly, indicating that it is related to the film growth rate. When the concentration increases to 0.005 M, cadmium and zinc ions react with OH^– to form the precipitation of Cd(OH)₂ and Zn(OH)₂ rapidly, making the film growth rate slow down and the percentage of zinc content increase significantly. The same is true when concentration adds up to 0.006 M. While it continues to increase to 0.007 M, due to the excessive consumption of Cd²⁺ and S^2– involved in the reaction and little precipitation in the solution, the film growth rate accelerates, and the percentage of zinc content decreases rapidly. It is confirmed that the increasing cadmium sulfate concentration results in the changing trend of zinc content in the film, so does the film growth rate. The growth of thin films is inhibited with the increase of zinc content.

3.4 Crystal structure characteristics

The XRD pattern of Cd_1–xZn_xS films prepared under the different CdSO₄ concentrations is shown in Fig. 3. It can be seen visually that diffraction peaks appear at position 2$\theta$ = 27.020 corresponding to the concentration of 0.004 and 0.005 M. This demonstrates that the film crystallinity under this concentration is better than other concentrations. There is a strong preferential orientation on the (002) plane of the hexagonal phase. It can be seen that the (002) peak position of Cd_1–xZn_xS generated at the five cadmium sulfate concentrations is shifted to a higher angle with respect to 2$\theta$ = 26.507 corresponding to the XRD standard diffraction peak (002) of the given CdS. Due to the addition of ZnS, the lattice constant of the hexagonal phase ZnS is smaller than that of hexagonal phase CdS, thus the lattice constant of the Cd_1–xZn_xS thin film is smaller than CdS, making the (002) peak shift to a higher angle. It can be concluded from EDS results that the thin film crystallinity is related to the zinc content. The higher the zinc content, the worse the thin film crystallinity will be. For the reason that the increasement of zinc content hinders the reaction between Cd²⁺ and NH₃ to form a cadmium complex and accelerates the release rate of metal ions, leading to more precipitation in the solution, worse density of the thin film and the lower diffraction peak^[10].

3.5 Optical properties

The optical properties of the thin film are studied by using ultraviolet visible near the infrared spectrophotometer. The absorbance is shown in Fig. 4(a). In the figure, the thin film has a higher absorbance at the wavelength of less than 500 nm than that of more than 500 nm, and the absorbance decreases sharply in the wavelength range of 300–350 nm. The absorbance gradually increases as the cadmium sulfate concentration increases, while it decreases at the concentration of 0.007 M.

The light transmittance is exhibited in Fig. 4(b). The transmittance of the thin film increases sharply with the increase of the wavelength in the range of 300–500 nm, and rises slowly when exceeding 500 nm. The thin films prepared under the five cadmium sulfate concentrations have good transmittance, the average transmittance within the visible light range is over 80%. Especially when concentration is 0.007 M, it exceeds 85%. In addition, the absorbance edge of the film presents significant blue shift as the change of cadmium sulfate concentration^[11].

3.6 Electrical properties

The optical band gap and absorption coefficient of the Cd_1–xZn_xS thin films satisfy the Tauc equation^[12]:

$${(\alpha {{hv}})^{{{{1}} / {{n}}}}} = {{A}}({{hv}} - {{E_{\rm g}}}),$$

(8)

where $\alpha$ is the absorption coefficient, hv is photon energy, E_g is the optical band gap and A is a constant. Depending on the type of semiconductor, the exponential n is 1/2 for the direct band-gap semiconductor and 2 for the indirect band-gap semiconductor^[13]. Since Cd_1–xZn_xS is a direct band-gap semiconductor, the value of n is 1/2.

While the absorption coefficient $\alpha$ can be calculated by Eq. (9):

$$\alpha = {{ - \ln}}({{T}})/{{d}},$$

(9)

where T is the light transmittance, d is the thin film thickness. $\alpha$ is calculated by Eq. (9), then bring it into Eq. (8) and draw the ($\alpha {{h}}v$)²–hv scatter diagram, which is shown in Fig. 5. In the figure, the straight line is the tangent line of the curve, the extension line intersects with the horizontal axis, and the abscissa of the intersection is the thin film optical band gap. According to this, the thin film band gaps corresponding to the CdSO₄ concentration of 0.003, 0.004, 0.005, 0.006 and 0.007 M are 2.95, 2.93, 3.24, 3.27 and 2.97 eV. It can be observed that with the increase of cadmium sulfate concentration, the band gap changes on the decrease, increase and decrease. The change trend of it is consistent with the percentage of zinc atoms, indicating that the zinc content is closely related to the band gap.

It can be intuitively seen from Fig. 6 that the thin film band gap values varies between 2.43 and 3.71 eV at different cadmium sulfate concentrations. When x = 0, the film is undoped with zinc, the band gap value is 2.43 eV; while x = 1, the band gap value is 3.71 eV. Fig. 6 shows the thin film band gap values change with x, which is nonlinear. Further studies have found that there is a certain functional relationship between the two, which can be expressed by the dielectric model and the pseudopotential model^[14]. They satisfy the following equation:

$$E_{\rm{g}}(x) = kx^2 + (E_{\rm{g},\rm{ZnS}} - E_{\rm{g},\rm{CdS}} - k)x + E_{\rm{g},\rm{CdS}},$$

(10)

where E_g,CdS and E_g,ZnS represent the band gap values of CdS and ZnS in Cd_1–xZn_xS and k represents the bending coefficient, which can be calculated by the following formula^[15]:

$$k = 4[0.5(E_{\rm{g},\rm{CdS}} + E_{\rm{g},\rm{ZnS}}) - E_{\rm{g},0.5}],$$

(11)

E_g,0.5 represents the band gap value when x = 0.5. Through the analysis of the atomic percentage and the film band gap, results are exhibited in Table 4. Eventually, the nonlinear relationship between Cd_1–xZn_xS thin film band gap value E_g and x can be expressed as:

$${E_{\rm{g}}}\left( x \right) = 0.59{x^2} + 0.69x + 2.43.$$

(12)

According to Eq. (12), increasing the zinc content in the thin film will increase the optical band gap. Due to the addition of Zn, the band gap of the thin film varies between 2.43 and 3.71 eV. The proportion of zinc element in the thin film can be adjusted properly to match the energy band between the buffer layer and the absorption layer, increase the thin film absorbance and improve the photoelectric properties.

4. Conclusion

The zinc content affects the compactness and the growth rate of the thin film. The lower the zinc content, the denser the thin film and the faster the growth rate will be. It can be seen from the SEM results that the surface of the thin film prepared under the cadmium sulfate concentration of 0.005 M is relatively dense for the reason that less Cd²⁺ is involved in the reaction and little precipitation in the solution. The changes of cadmium sulfate concentration affect the thin film growth rate. In the process of increasing the concentration, the growth rate changes on the increase, decrease and increase, due to the precipitation in the solution changes from less to more and less. XRD results indicate that the zinc content affects crystallinity and the higher the zinc content, the worse the crystallinity will be. According to UV–Vis–NIR spectrophotometer data, the proportion of zinc x is related to the optical band gap value E_g, which satisfy the equation E_g(x) = 0.59x² + 0.69x + 2.43. Increasing the zinc content will improve the optical band gap, absorbance and transmittance. When the cadmium sulfate concentration is 0.005 M, the thin film has good absorbance, 80% transmittance, and band gap value of 3.24 eV, which is suitable for use as a buffer layer for solar cells.

Acknowledgements

This work was supported by the Tianjin Municipal Education Commission, Horizontal subject (grant number 70304901).