Optimization of the properties of CdxZn1-xS films prepared by chemical bath deposition

CdxZn1-xS thin films were deposited on glass substrates by chemical bath deposition (CBD) in solutions containing cadmium sulfate, zinc sulfate, thiourea, ammonia sulfate and ammonia. The effect of low Cd2+ concentration on the film properties was systematically studied by experiments and simulations, while the film properties were greatly improved by optimizing the concentration ratios of different solutions. The SEM results show that the films have optimal homogeneity and denseness at a Cd2+ concentration of 0.007 M. Also, XRD results at this concentration indicate that the films have better crystallinity. The optical characterization results illustrate that the transmittance of the films is up to 90% at 500 nm–800 nm, and the band gap value of the films varies from 3.05 eV to 3.72 eV when the Cd2+ concentration is 0.001–0.009 M.


Introduction
CdS is an n-type direct band gap semiconductor material with a narrow optical band gap of about 2.42 eV [1]. CdS has been studied as a buffer layer for heterojunction solar cells for many years because it exhibits good transmittance, high absorption coefficient and low resistivity in the visible range. However, the absorption layer at short wavelengths is hindered by the narrow optical band gap of CdS [2]. In addition, Cadmium is a toxic substance that can cause harm to the environment and human body. Therefore, another non-toxic and environmentally friendly N-type semiconductor compound, ZnS, has been widely considered to replace CdS, which has an optical band gap of about 3.74 eV [3] and good short-wavelength transmittance. However, the optimal band gap value of copper indium gallium selenide (CIGS) is between 1.3 eV and 1.45 eV, which does not match the lattice of ZnS and greatly affects the efficiency of thin-film solar cells [4]. The researchers found that doping Zn in CdS can not only adjust the band gap of the film to reduce the effect of lattice matching, but also adjust the optical properties of the film to improve the efficiency of the solar cell [5]. Therefore, it is crucial to fabricate ternary Cd x Zn 1-x S thin film semiconductor materials.
Currently, there are many methods to prepare Cd x Zn 1-x S thin films, such as spray pyrolysis [6], thermal evaporation [7], successive ionic layer adsorption and reaction (SILAR) [8], atomic layer deposition [9] , vacuum evaporation [10] and chemical bath deposition (CBD) [11]. CBD is of great interest due to its simple deposition process and low-cost effectiveness. At the same time, the films produced by CBD cause relatively little damage to the absorber layer, and its photovoltaic properties can be easily changed by adjusting the bath conditions.
The effect of Cd 2+ concentration on Cd x Zn 1-x S thin films has been widely reported. Offiah et al [12]. studied its effect on the morphology, structure and electrochemical impedance properties of Cd x Zn 1-x S thin films using different concentrations of Cd (NO 3 ) 2 (0.006-0.06M). Yao et al [13] investigated the effect of different concentrations of CdCl 2 (0.01-0.04M) on the photocurrent response characteristics of Cd x Zn 1-x S films. The effect on the properties of Cd x Zn 1-x S films at different CdCl 2 concentrations (0.1-0.9M) and different Zn 2+ concentrations was investigated using bivariate by Zellagui et al [14]. Nurhafiza [15] studied the effect on the photovoltaic properties of Cd x Zn 1-x S films at different concentrations of CdSO 4 (0.01-0.09M). However, the Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Cd 2+ concentration used by the above researchers to conduct their experiments was high and still posed a significant environmental hazard. Zhang et al [16] studied the effect of different concentrations of cadmium sulfate (0.003-0.007M) on the structure and optical properties of Cd x Zn 1-x S films, but its atomic percentage of Cd started to decrease with the increase of the concentration of CdSO 4 solution, which was not consistent with reality and is problematic, and also the properties of the prepared films were low. In conclusion, most of the cadmium sources used were cadmium nitrate and cadmium chloride at high concentrations, and systematic studies on the preparation of Cd x Zn 1-x S films at extremely low concentrations of cadmium sulfate were lacking.
Under the premise of enhancing the film properties and reducing environmental pollution. In this paper, the effect of low Cd 2+ concentrations (0.001M-0.009M) on the properties of prepared Cd x Zn 1-x S thin films was systematically investigated by optimizing the concentration ratios of all reaction solutions. and the atomic percentage of Cd increased with increasing CdSO 4 solution concentration, contrary to the experimental results performed by Zhang et al [16]. Also, the prepared Cd x Zn 1-x S thin films have better performance at a CdSO 4 concentration of 0.007 M compared with the results of previous studies.

Experimental
The main raw materials for the preparation of Cd x Zn 1-x S films include cadmium sulfate, zinc sulfate, ammonium sulfate, thiourea, and ammonia, all the chemical raw materials are analytical pure grade reagents and can be directly added to deionized water to configure the solution. The experimental device and procedure are shown in figure 1.
Firstly, ZnSO 4 (0.015M), (NH 4 ) 2 SO 4 (0.015M) and five concentrations of CdSO 4 (0.001M, 0.003M, 0.005M, 0.007M, 0.009M) were placed in five beakers, and then an appropriate amount of deionized water was added to obtain a stable mixed solution. The cleaned glass substrates (2 cm × 3 cm) were then placed diagonally against the inside of the beakers. Secondly, the beakers were heated in a water bath apparatus and when the water bath was heated to 86°C, SC(NH 2 ) 2 (0.03M) and NH 3 * H 2 O (13.33 M) were added to each of the five beakers. Thirdly, the solution was well stirred with a glass rod, the PH value measured by the PH meter at this time is 10.3. Finally, the deposition time was 35 min, and the solution was stirred every five minutes to make the solution more homogeneous. After 35 min of reaction, the beakers were removed, and the samples were washed and dried. In the reaction, ZnSO 4 provided Zn 2+ , CdSO 4 provided Cd 2+ , and thiourea served as a source of S 2− . In addition, NH 3* H 2 O was used as a complexing agent and acid-base balance regulator; (NH 4 ) 2 SO 4 was used as a buffering agent to reduce the release rate of metal ions and promote the deposition of Cd x Zn 1-x S films.
The structural properties of the samples were characterized using a Rigaku D/max-RB X-ray diffractometer (XRD) with scanning angles from 10°to 70°in steps of 0.01°, voltage of 40 kV and current of 200 mA. The surface morphology and elemental composition of the samples were analyzed using a scanning electron microscope (SEM 460L03040702) with energy dispersion spectroscopy (EDS) for the analysis. The thickness data of the prepared Cd x Zn 1-x S films were recorded with a step height measuring instrument (Veeco Dektak 150). The optical properties of the samples were studied by UV-vis-NIR spectrophotometer in the wavelength range of 300nm to 800nm.

Results and discussion
3.1. Surface morphology Figure 2 shows the SEM images of the Cd x Zn 1-x S films, and the surface morphology of the films grown at different CdSO 4 concentrations has obvious changes. When the concentration of CdSO 4 was 0.005M and 0.007M, the surface of the films was dense and uniform without large pores, and the films had the best surface morphology at 0.007M. As the concentration of CdSO 4 decreased, the surface morphology of the samples started to become rough and loose at 0.003 M, and more holes appeared; at 0.001 M, the surface morphology was extremely uneven and a large number of holes appeared, which was due to the low content of Cd 2+ in the solution, failing to react with ammonia to form more Cd complexes, thus hindering the growth of Cd x Zn 1-x S films. And when the concentration of cadmium sulfate continued to increase to 0.009M, the surface morphology of the samples produced defects and holes appeared, which was due to the excess of Cd ions in the solution, the rapid increase of the precipitation of Cd (OH) 2 and Zn (OH) 2 , which hindered the growth of the film, and the increase of the solution alkalinity, which led to the increase of the pores. These pores increase the compounding of carriers and affect the photoelectric conversion efficiency of the cell.
In addition, at a magnification of 20,000x, the overall morphology of the film can be seen. When at 200,000x magnification, the local features of the film can be seen more visually. It can be seen that the film is very homogeneous and dense when the cadmium concentration is at 0.007M, with varying degrees of defects in the other concentrations. Also, the approximate thickness of the film can be measured from the cross-sectional image of the film. However, due to the limited performance of the experimental equipment, the resolution is not high and the overall effect is mediocre.

EDS and thickness
The atomic percentages of sulfur, zinc and cadmium in Cd x Zn 1-x S films formed at different cadmium concentrations were measured by EDS attached to a scanning electron microscope. In table 1, it can be seen that with the increase of Cd 2+ concentration, the trend of the change of the atomic percentage content of Cd and S increases and then decreases, while the trend of Zn changes in the opposite direction. The reason is that when the concentration of CdSO 4 increased from 0.001 M to 0.007 M, the concentration of Cd 2+ in the reaction solution increased, the Cd 2+ involved in the reaction increased, the deposited CdS colloidal particles increased, and the consumption of Zn 2+ increased, so the atomic percentages of Cd and S in the films increased and the atomic percentages of Zn decreased. When the concentration of CdSO 4 continues to increase to 0.009 M, there is too much Cd 2+ in the solution, which combines with S 2to generate a large amount of CdS precipitate, which hinders the deposition of the film and makes the consumption of Zn 2+ in the solution decrease. Therefore, the atomic percentages of Cd and S in the film decreased and those of Zn increased.
In addition, the cross-sectional image by SEM could only observe the approximate thickness of the film edges, and in order to obtain a more accurate film thickness, it was measured by a step height measuring instrument (Veeco Dektak 150). The results are shown in table 2. As the concentration of Cd increases, the concentration of Cd complexes in solution increases and the reaction moves in the positive direction, so the concentration of S 2in solution increases, leading to an increase in the growth rate of Cd x Zn 1-x S, so as the concentration of Cd 2+ increases, the film becomes thicker first, and when the concentration of Cd 2+ is too high, the large amount of precipitation generated hinders the growth of the film and the thickness becomes thin agan.

Structural properties
The structural properties of the Cd x Zn 1-x S films grown at different CdSO 4 concentrations were analyzed by x-ray diffraction (XRD), and the diffraction patterns are shown in figure 3. All samples have a strong preferred orientation along the (002) plane of the hexagonal phase, except for the weak peaks of the diffraction peaks of the films prepared at 0.001M concentration of CdSO 4 solution. The film peaks at about 2θ = 26.7 for CdSO 4 concentration varying between 0.001M and 0.007M and at 2θ = 26.82 for 0.009M. The (002) peak position of Cd x Zn 1-x S shifts to a slightly larger angle relative to the given XRD standard diffraction peak (002) of CdS corresponding to 2θ = 26.507. The Cd x Zn 1-x S films also show small peaks at 43.9•and 52.6•, associated with 110 and 200 planes. The weaker diffraction peak when the concentration of CdSO 4 solution is 0.001M is due to the high content of Zn in the solution which hinders the formation of Cd complexes, resulting in the poor performance of the grown Cd x Zn 1-x S films. When the concentration of CdSO 4 solution is 0.003M-0.009M, the (002) peak is enhanced and then weakened, and the highest intensity of the diffraction peak is at 0.007M. Combined with the results of the atomic percentages measured by EDS in table 1, it can be seen that the crystallinity of the film is related to the content of zinc. The higher the content of zinc, the worse the crystallinity of the film.

Optical properties
The transmittance curves are shown in figure 4(a), and it can be observed that from 500 nm to 800 nm, all samples exhibit high transmittance up to 90% or more. Furthermore, at a CdSO 4 concentration of 0.001 M, the films have a high transmittance between 320-800 nm. This is mainly due to the thin thickness of the Cd x Zn 1-x S  films deposited at this concentration. The general trend of the transmittance of the samples at 0.001M-0.009M decreases with the increase of the atomic percentage of Cd. The relationship between the optical band gap and the absorption coefficient of the Cd x Zn 1-x S film can be found from equations (1) and (2), where α is the absorption coefficient, Eg is the optical band gap, hν is the photon energy, A is a constant, T is the film transmittance, and d is the film thickness [17]. Figure 4(b) shows the (αhν) 2 versus hν curves for the Cd x Zn 1-x S thin film samples, and the intersection of its tangent line with the x-axis is the band gap value of the corresponding sample. The optical band gaps of Cd x Zn 1-x S films were 3.72eV, 3.61eV, 3.46eV, 3.05eV and 3.33eV when the CdSO 4 concentrations were 0.001M, 0.003M, 0.005M, 0.007M and 0.009M, respectively. With the increase of Cd 2+ concentration, the optical band gaps first decreased and then increased, and the band gap trend is consistent with the trend of the atomic percentage content of Zn, indicating that the change of Zn content can regulate the band gap and change the transmittance of the film.

Conclusion
In this paper, Cd x Zn 1-x S thin films were successfully prepared by chemical water bath deposition at low Cd 2+ concentration. With the premise of improving film performance and reducing environmental pollution, the multifaceted properties of the films were enhanced by optimizing different solution concentration ratios. As shown from the numerical simulation, the prepared Cd x Zn 1-x S film is suitable as a buffer layer for CIGS solar cells when the CdSO 4 concentration is 0.007M. SEM and XRD experimental results show that the films at this concentration have good surface morphology and crystallinity. Meanwhile, it is known from the spectrophotometer that the film has a high transmittance with an optical band gap of 3.05 eV at 0.007M Cd 2+ concentration. In short, it is an ideal concentration for the preparation of Cd x Zn 1-x S buffer layer of CIGS solar cells. It not only best preserves the properties of the ternary compound, but also greatly reduces the environmental hazards of Cadmium.