Cd_1−𝑥Zn_𝑥S Thin Films with Low Zn Content Prepared by Chemical Bath Deposition

Abstract

Chemical bath deposition (CBD) was used for the growth of Cd_1−𝑥Zn_𝑥S thin films with low Zn content. The influence of preparation conditions, such as pH, temperature, and concentration, on film properties was investigated. The chemical growth mechanism of Cd_1−𝑥Zn_𝑥S thin films was analyzed, and optimized growth conditions for the thin films were established. The fill factor and short-circuit current were improved while Cd_1−𝑥Zn_𝑥S was used to replace CdS as the window layer in CdTe solar cells.

1. Introduction

The highest conversion efficiency of CdS/CdTe thin film solar cells till now is 16.7% [1]. However, the band gap of CdS is not high enough so that the short wavelength photons are easy to be absorbed. The band gap of Cd_1−𝑥Zn_𝑥S varies in the range of 2.42 to 3.6 eV as zinc content increases [2]. While Cd_1−𝑥Zn_𝑥S is used to replace CdS as a window layer, not only the absorption of the window layer will be reduced, but also the short-circuit current of the Cd_1−𝑥Zn_𝑥S/CdTe thin film solar cells will be increased [3]. It is worth noting that two issues exist in the CdTe solar cells. One is the lattice parameter varying linearly with zinc content in the compound and thus resulting in a large lattice mismatch between Cd_1−𝑥Zn_𝑥S and CdTe. The other is the electrical resistivity increasing dramatically by the increase of zinc content [4]. Therefore, Cd_1−𝑥Zn_𝑥S at low Zn content is an ideal alternative material for CdTe thin film solar cells as the window layer.

Many ways, such as vacuum evaporation [4], chemical bath deposition [3, 5, 6] and, SILAR [7], have been used to prepare Cd_1−𝑥Zn_𝑥S thin films. Among these techniques, CBD presents many advantages: inexpensive, simple equipment, low temperature (<100°C), and large-area deposition. So it is well suited for the manufacture of photovoltaic devices [3, 5]. Because the solubility products of CdS (𝐾_sp=10⁻²⁸) and ZnS (𝐾_sp=10^−23.8) are different by several orders of magnitude, the complexing agent is essential for codeposition of CdS and ZnS. In this work, NH₃ was used as the complexing agent to prepare Cd_1−𝑥Zn_𝑥S thin films at low Zn content. The influence of growth conditions on the properties of Cd_1−𝑥Zn_𝑥S was investigated to understand the deposition process and thus to improve the characteristics of the films. Subsequently Cd_1−𝑥Zn_𝑥S thin films were used as the window layer for the CdTe solar cells.

2. Experimental Details

In the CBD process, Cd_1−𝑥Zn_𝑥S thin films are prepared by slow release of S²⁻ and effective control of free Zn²⁺ and Cd²⁺ ions in the solution. The main chemical reactions happened in the solution can be described as follows:(1−𝑥)Cd(NH₃)₄²⁺+𝑥Zn(NH₃)₄²⁺+SC(NH₂)₂+2OH⁻⟶Cd_1−𝑥Zn_𝑥S+CH₂N₂+2H₂O+4NH₃ (1)

In this work, a certain a mount of ZnCl₂, CdCl₂, and NH₄Cl were dissolved in 200 mL DI water at room temperature. When the solution was heated to 60°C, a solution of ammonia was added to the solution and stirred for 5 min. Here the ammonia combines Cd²⁺ and Zn²⁺ to form the complex Cd(NH₃)₄²⁺ and Zn(NH₃)₄²⁺ which can constantly release Cd²⁺ and Zn²⁺. Then clean glass substrates or TCO-coated ones were placed vertically in the reaction bath. Finally, a certain a mount of thiourea was added to the solution, which slowly releases S²⁻ as a sulfur source. The rotating speed of the agitator during the deposition process was 30 r/min. The depositions were performed with CdCl₂, ZnCl₂, and SC(NH₂)₂ concentrations varying from 0.001 to 0.06 mol/L, ammonia concentrations in the range of 0.072 to 0.179 mol/L, and pH values between 8.5 and 10.5 to obtain the optimum growth conditions of the Cd_1−𝑥Zn_𝑥S thin films. The values of pH were controlled by adding NH₄Cl to form a buffer solution with the desired pH. After a deposition of 1.5 hour, the substrates were released from the solution. One side was cleaned by HCl for testing convenience and then the substrates were deterged by DI water to remove the contaminant ions.

CdTe solar cells with a Cd_1−𝑥Zn_𝑥S layer (~180 nm) were fabricated for indicating the application of Cd_1−𝑥Zn_𝑥S thin films. CdTe films followed by the deposition of Cd_1−𝑥Zn_𝑥S thin films were prepared by the close-spaced sublimation (CSS) technique. After CSS deposition of CdTe with thickness of 8 μm, the samples were treated in CdCl₂ vapor at 380°C for 30 min. Then they were etched with Br-Methanol and rinsed in DI water and blown dry in N₂. The back contact layer was formed by applying Sb₂Te₃ to the etched CdTe surface and heat treating it in N₂ at 300°C [8]. Finally, Au was used to complete the fabrication of CdTe solar cells. For reference purpose, CdTe solar cells with a CdS layer (~180 nm) were also fabricated.

The composition of the Cd_1−𝑥Zn_𝑥S thin films was determined by X-ray fluorescence (XRF) measurements. The structural properties of the films were studied by X-ray diffraction (XRD). Measurement of the optical transmittance was made over the wavelength range 300~900 nm with a data interval of 1 nm. Scanning electron microscopy (SEM) was performed to investigate the surface morphology of the films. Electrical resistivity along the lateral direction was measured by depositing Al-stripes on the Cd_1−𝑥Zn_𝑥S thin films and performing two-point probe measurements. The resulting PV devices were characterized using the light 𝐽−𝑉 measurement under simulated AM 1.5 illumination (i.e., 100 mW/cm²) with standard current-voltage measurement equipments.

3. Results and Discussions

3.1. Structural Properties

Figure 1 shows XRD patterns of Cd_1−𝑥Zn_𝑥S thin films prepared at different pH values. It is found that the Cd_1−𝑥Zn_𝑥S thin films are hexagonal with a preferential orientation along the (002) plane in the alkaline environment. When the pH value is 8.5, only a single strong Cd_1−𝑥Zn_𝑥S (002) peak can be detected. However, as the pH value increases, the intensity of the Cd_1−𝑥Zn_𝑥S (002) peak decreases and peaks of ZnS and ZnO also emerge in these patterns. This is probably caused by the excessive ammonia which leads to the reactions of Zn(NH₃)₄²⁺ and OH⁻, and that may be responsible for the accentuation of ZnO peak when the pH value increases to 9.5. When the pH value reaches up to 10.0, on the contrary, all the peaks are weakened.

Figure 1 

XRD patterns of Cd1−𝑥Zn𝑥S thin films prepared at different pH values.

We investigate XRD patterns (not shown there) of Cd_1−𝑥Zn_𝑥S thin films prepared at different temperatures. As the bath temperature increases from 60°C to 90°C, the intensity of Cd_1−𝑥Zn_𝑥S (002) firstly becomes strong and then weakens. Also, peaks other than Cd_1−𝑥Zn_𝑥S cannot be detected in the thin films deposited at 70°C. Therefore, the optimal bath temperature to preparing Cd_1−𝑥Zn_𝑥S thin films by CBD is 70°C.

As the buffering agent, NH₄Cl also affects the hydrolytic equilibrium of NH₄⁺; therefore, it could balance the concentration of NH₃ to control the reaction rate. If no buffering agent is expected in the reactions, the deposition rate would be too fast at the beginning; however, it would be saturated quickly, and this is harmful to fabricate Cd_1−𝑥Zn_𝑥S thin films with good quality. A suitable concentration of NH₄Cl enables the linear increase of deposition rate of Cd_1−𝑥Zn_𝑥S thin films with the time, and that will be beneficial to deposit Cd_1−𝑥Zn_𝑥S thin films [9]. When the concentration of NH₄Cl is 0.04 mol/L, the intensity of Cd_1−𝑥Zn_𝑥S (002) peak is stronger than that of the concentrations of 0.03 and 0.05 mol/L. What is more, no obvious peak of Cd_1−𝑥Zn_𝑥S (002) could be observed from the XRD patterns when the concentration of NH₄Cl changes to 0.025 mol/L or 0.055 mol/L. This is due to the insufficient buffering of NH₃ when the concentration of NH₄Cl is too low.

Usually, two mechanisms, homogeneous particles formation and heterogeneous surface reaction, are available when thiourea reacts with CdCl₂ and ZnCl₂. On the one hand, [Cd(NH₃)₄]²⁺ and [Zn(NH₃)₄]²⁺ combine with OH⁻ to form complexing ions. These ions are absorbed on the surface of the substrate, and then they react with thiourea to form metastable complex. Subsequently, the metastable complex decomposes to form a new Cd_1−𝑥Zn_𝑥S surface. Therefore, the surface of Cd_1−𝑥Zn_𝑥S thin films is compact and homogeneous (see Figure 2). On the other hand, the S²⁻ released from thiourea through hydrolysis reaction combines with free Cd²⁺ and Zn²⁺ ions to generate Cd_1−𝑥Zn_𝑥S particles [10, 11]. These particles deposit on the surface of the substrate, which results in a rough and porous surface. By carefully controlling the bath concentration of thiourea, the reaction can be driven mainly as the heterogeneous mechanism. Hence, the structural properties of Cd_1−𝑥Zn_𝑥S thin films will be different. When the concentration of thiourea increases from 0.02 to 0.03 mol/L, the intensity of Cd_1−𝑥Zn_𝑥S (002) peak is strengthened; however, as the concentration increases more, the peak height reduces lower and can hardly be differentiated from the background in XRD patterns.

Figure 2 

SEM Image of Cd1−𝑥Zn𝑥S thin films prepared by CBD.

Upon the previous analysis, it is found that the pH value, temperature, the bath concentration of NH₄Cl, or thiourea greatly affect the deposition of Cd_1−𝑥Zn_𝑥S thin films. As far as good crystallinity films are concerned, the standard conditions for growth of Cd_1−𝑥Zn_𝑥S thin films are [ZnCl₂] = 0.001 mol/L, [CdCl₂] = 0.005 mol/L, [NH₄Cl] = 0.04 mol/L, [SC(NH₂)₂] = 0.03 mol/L, T = 70°C, and pH 8.5. In addition, we deposited Cd_1−𝑥Zn_𝑥S thin films under these conditions and then annealed at 350°C in N₂ atmosphere. The XRD patterns of as-deposited and annealed Cd_1−𝑥Zn_𝑥S thin films are shown in Figure 3. The diffraction peak positions do not change dramatically after annealing, but the average grain sizes of the films change from 17.5 to 32.8 nm according to Scherrer’s equation.

Figure 3 

XRD patterns of Cd1−𝑥Zn𝑥S prepared under the optimum conditions before and after annealing.

3.2. Compositional Properties

The X-ray fluorescence analysis using the fundamental parameter (FP) method was used to determine the composition of the Cd_1−𝑥Zn_𝑥S films. Figure 4 gives the evolution of Zn content with the preparation conditions, that is, pH value, and the concentration of thiourea. It is found that, when the pH value increases, Zn content decreases, and the same situation is found when the temperature rises. Similarly, we varied the thiourea concentration, while the rest of the variables were maintained under the standard conditions. The Zinc content reaches up to 20 at% by increasing the thiourea concentration and then decreases. This is ascribed to the different solubility products of CdS and ZnS and the different stability constants for the cadmium and zinc precursors. For the standard conditions, Cd_1−𝑥Zn_𝑥S films at x ~20 at% were obtained.

(a)

(b)

Figure 4 

Plots of Zn content versus different deposition conditions of Cd1−𝑥Zn𝑥S thin films.

3.3. Electrical Properties

Figure 5 shows the plots of conductivity versus temperature for Cd_1−𝑥Zn_𝑥S thin films. The conductivity of Cd_1−𝑥Zn_𝑥S thin films prepared at pH values varied from 8.5 to 9.5 increases monotonically as the temperature increases. The increase of 𝜎 with temperature is ascribed to the growth of large grains, which increases the electron mean free path and reduces the scattering effect. However, for thin films prepared at pH 10, an abnormal behavior occurred in the range of 39–110°C. This phenomenon is also observed in CdS thin films [12, 13]. The relationship between 𝜎 and 𝑇 can be described as𝜎=𝜎₀−𝐸exp^𝑎,𝑘𝑇 (2) where 𝜎₀ is constant, 𝑘 is the Boltzmann constant, and 𝐸_𝑎 is activation energy. The values of 𝐸_𝑎 for thin films except for pH 10 are listed in Table 1. The thin films prepared at pH from 8.5 to 9.5 indicate three linear regions. The first at lower temperatures may be ascribed to the transitions from the impurity levels in the band gap to the bottom of the conduction band or the top of the valence band. The second is attributed to electron or hole transitions to the conduction or valence band. The third at higher temperatures is due to the electron transitions through the extended states in the conduction band [14].

Table 1 

The values of activation energies for Cd_1−𝑥Zn_𝑥S prepared at different pH values.

Figure 5 

Variation of ln𝜎 versus 1000/T for Cd1−𝑥Zn𝑥S prepared at different pH values.

3.4. Optical Properties

Considering the potential application of Cd_1−𝑥Zn_𝑥S thin film as the window layer in CdTe thin film solar cells, it is necessary to investigate the influence of preparation conditions on the optical properties of Cd_1−𝑥Zn_𝑥S thin films. We varied one of the standard conditions while maintained other rest of variables. Figure 6 shows the transmittance of the Cd_1−𝑥Zn_𝑥S thin films prepared at different pH values. An extended spectral response for all the thin films at short wavelengths is noted. One can also see that the transmittance of the Cd_1−𝑥Zn_𝑥S thin films is higher than 60% when the wavelength is longer than 500 nm, in which the transmittance increases as the pH value decreases. For thin films prepared at pH 8.5 and 9.0, it is much higher than that of the other pH values. The same situation is found while varying the temperature, the concentration of NH₄Cl and thiourea. It seems that pH 8.5 or 9.0 is the optimum pH value. Using the Tauc equation, the band gap of Cd_1−𝑥Zn_𝑥S thin films is calculated by extrapolating the linear part of the curve to the 𝛼ℎ𝑣 axis, and the interception is the value of 𝐸_g. From Figure 6, for thin films prepared at pH 9.5 and 10.0, they exhibit two absorption edges. The value of 𝐸_g can be calculated in the short wavelength regions to be about 3.6 eV. As evidenced by the XRD observations (see Section 3.1), we attribute it to the formation of ZnS or ZnO. The value of 𝐸_g for thin films prepared at pH 8.5 and 9.0 is about 2.56 eV, which is larger than that of CdS (~2.4 eV). That means Cd_1−𝑥Zn_𝑥S thin films prepared on these conditions may be better than CdS as the window layer of CdTe thin film solar cells.

Figure 6 

Transmittance curves of Cd1−𝑥Zn𝑥S thin films prepared at different pH values.

From the properties of Cd_1−𝑥Zn_𝑥S thin films at low Zn content, we find that the standard conditions are the optimum ones for thin film deposition in fact.

3.5. Device Performance

CdTe thin film solar cells with the Cd_1−𝑥Zn_𝑥S window layer were fabricated based on the previous analysis. Table 2 shows the electrical parameters, for example, open circuit 𝑉_oc, short-circuit current density 𝐽_sc, fill factor 𝐹𝐹, and conversion efficiency 𝜂 for CdTe thin film solar cells with a Cd_1−𝑥Zn_𝑥S or CdS widow layer. It is found that open circuit voltage changes very little, but the short circuit current for CdTe thin film solar cells with a Cd_1−𝑥Zn_𝑥S layer is higher than those with a CdS layer.

Table 2 

Parameters for CdTe thin film solar cells.

4. Conclusions

Cd_1−𝑥Zn_𝑥S thin films in the range of 𝑥≤20 at% were prepared by CBD. Under the optimum preparation conditions, the thin films were found to be hexagonal, which show highly preferential orientation. And the value of activation energies attributed to electron or hole transitions to the conduction or valence band is about 1.67 eV. The optical properties also show that the given preparation conditions are proper to deposit Cd_1−𝑥Zn_𝑥S thin films with high transmittance in both long and short wavelength ranges. Cd_1−𝑥Zn_𝑥S is used to replace CdS as the window layer; the fill factor and short-circuit current were improved, which means Cd_1−𝑥Zn_𝑥S thin films are more suitable for the window layer in CdTe solar cells.

Acknowledgments

Financial support from National Basic Research Program of China (Grant no. 2011CBA007008), National Natural Science Foundation of China (Grant no. 61076058), and the Science and Technology Program of Sichuan Province, China, (Grant no. 2008GZ0027) is gratefully acknowledged.