Investigation on influence of the deposition parameters on the properties of CdS thin film prepared by the chemical bath deposition method

Abstract

In the preparation process of CdS thin film, the influences of the deposition parameters were studied, such as the reactant concentration, deposition temperature and time on the film properties and solar cell performances, in order to apply in the fabricating of large-area solar cells. Here, we have considered the optical, morphological and structural properties of CdS films with the variation of deposition parameters with the theoretical consideration of the reaction mechanism and growth process of thin film. S/Cd ratio had an influence on the morphology and transmission of CdS thin film and solar cell performances, whereas the crystal structure was affected by the concentration of NH₄Cl and NH₄OH. The bath temperature influenced the growth rate, morphology, microstructure, transmittance of thin film, the change of the morphological, optical and electrical properties also depended on the deposition time.

Keywords: CdS thin film, Chemical Bath Deposition, CdS/CdTe solar cell, window layer, deposition parameter, S/Cd ratio.

1. Introduction

Photovoltaic (PV) solar energy conversion is an attractive method for clean energy generation, which comes at the top end of the renewable energy list, and therefore worldwide research is continuing to develop low-cost and high-efficiency solar panels[1]. For several decades, many researchers have been studying how to improve the performance of thin film solar cells based CdTe, CuInSe₂, Cu(In,Ga)Se₂ (CIGS), Cu₂ZnSnS₄(CZTS). CdS is an excellent heterojunction partner for these p-type absorber layers because of its novel properties like wide optical band gap, high transparency, photoconductivity, high index of refraction and its high electron affinity[2]. CdS can also be used in many applications of electronic and optoelectronic devices such as thin films field effect transistor, transducers, laser materials, light detectors, photo-conducting cells, photo-sensors, optical wave guides and non-linear integrated optical devices[3,4].

Several physical and chemical deposition techniques have been used in preparing of CdS thin film including RF sputtering[5], physical vapour deposition (PVD)[6], close space sublimation(CSS)[7], molecular beam epitaxy(MBE)[8], closed space vapor transport(CSVT)[9], electrodeposition(ED)[1], metal organic chemical vapor deposition(MOCVD)[10], chemical bath deposition(CBD), spray pyrolysis(SP)[11], successive ionic layer adsorption and reaction(SILAR)[12].

Among these methods, Chemical bath deposition (CBD) is the most widely used technique in the research area and industry since it has advantages as follows:

-The CBD technique is relatively simple, fast, low cost compared to other methods requiring vacuum environment and capable to yield films with good quality at optimum growth conditions[4,13].

-The CBD method facilitates the better orientation of the crystallites with improved grain structure[14].

-This technique offers an excellent control to deposit uniform thin films[2].

-The highest efficiency was obtained with the use of the CBD technique to deposit thin films of CdS as a window layer for CdTe, CIGS and CZTS solar cells[2].

Many researchers deposited CdS thin film with different starting materials (chloride, sulfate, nitrate, acetate, iodide, etc) and reaction condition, investigated the influence of the reactant concentration(S/Cd ratio [15,16], cadmium salt[17], thiourea[18]), bath temperature [19,20,21], deposition time [22,23] on thin film properties and solar cell performances.

Fouad Ouachtari et al [14] reported the influence of the S/Cd ratio, bath temperature and deposition time on the physical properties and chemical composition of CdS thin films elaborated by CBD method. However, few studies have been published on the influence of the overall deposition parameters in CBD process. Moreover, the results of studies differ from researchers with different experimental conditions and starting materials.

In this paper, we have qualitatively dealt with the influence of deposition parameters such as reactant concentration, bath temperature, deposition time on the CdS thin film properties including film growth, crystalline structure, surface morphology, optical properties etc, summarizing the different results to help future study in this research area.

2. The formation mechanism of CdS thin film

There are two growth mechanism of CdS thin film as follows:

(i) Ion by ion mechanism: the reaction of Cd²⁺and S^2- on the substrate from an aqueous basic medium, containing thiourea and cadmium ions in the form of a complex species [22].

(ii) Cluster by cluster mechanism: Small amounts of Cd(OH)₂ initially form on the substrate and CdS forms when the OH^- groups in Cd(OH)₂ are replaced by S^2- ions[23]

The colloid particles in solution are condensed into nuclei and then abundant ions absorbed on the surface of the nuclei form the continuous film through the reaction. The nucleation on the substrate starts from the selective adsorption of the small amount of ions, which is necessary for the existence of catalyzer (Cd(OH)₂) to make the selective adsorption of Cd²⁺ and S^2- possible[4].

The reaction process for the formation of CdS may be described by the following steps [22]:

(i) Ammonium ion formation.

NH₃+H₂O↔NH₄⁺+OH^- (1)

(ii) Cadmium salt react with the anions to form the complex compound

Cd²⁺+2OH^-↔Cd(OH)₂ (2)

CdA_C+4NH₃→〔Cd(NH₃)₄〕A_C (3)

(iii) Thiourea is the S^2-source in an alkaline medium, the sulfide ions are released as follows.

(NH₂)₂CS+OH^-→HS^-+CH₂N₂+H₂O (4)

HS^-+OH^-→S^2-+H₂O (5)

(iv) Formation of CdS

Cd (NH₃)₄²⁺+S^2-→CdS+4NH₃ (6)

In the reaction, ammonia is the complexing agent and the hydroxide source. NH₄A_C is used as a buffer.

Cadmium ions and sulfide ions migrate to the substrate surface and form CdS. Urea in the reaction solution can adjust the balance of the hydrolyzation and deposition reactions[17].

The increase of CdA_C concentration leads to the increase of CdS thin film thickness and the turbidity of solution, also promotes both the heterogeneous and homogeneous reaction. The increase of thiourea concentration promotes the homogeneous reaction in solution and restricts the heterogeneous reaction on the substrate. If the homogeneous reaction dominates compare to the heterogeneous one, the formation process of CdS colloid in bath is restricted, consequently finish the growth of film with thin thickness[4]. On the substrate, the internal continuous layer grown by the heterogeneous reaction and the porous coating layer by the adhesion of colloids and granules occurred by the homogeneous reaction are formed. The latter is not adherent, so easily removed by cleaning.

At a given temperature, the rate of formation of CdS is determined by the concentration of Cd²⁺ provided by Cd(NH₃)₄²⁺and the concentration of S^2- from the hydrolysis of (NH₂)₂CS [22].

3. Influence of the solution concentration

When the reactant concentration is too high, the reaction rate becomes fast and the color of solution becomes yellow, resulting in films with the brittle nature and poor uniformity due to the adherence of a great amount of CdS precipitates onto the surface, which is undesirable for solar cell fabrication. On the other hand, low concentration results in the decrease of deposition rate and non-uniformity of thin film. When the reactant concentration is controlled suitably, the residual solution after the reaction can be colorless to obtain the uniform, compact and transparent films[4].

3.1.Effect of ratio CdCl₂ to (NH₂)₂CS

The molar ratio of CdCl₂ and (NH₂)₂CS (named as S/Cd ratio) has a direct influence upon CdS film properties and solar cell performances[4].

The S/Cd ratio changes the physical properties of CdS thin film such as the morphology, thickness, transmittance and electrical conductivity etc.

3.1.1. morphology and thickness

Fouad Ouachtari et al[14] reported that the mechanism of film formation turned from ion by ion deposition to cluster by cluster with the increase of S/Cd ratio, while the size of spherical particles decrease and accumulate to form large clusters. In addition to this, film thickness increase and then decrease.

3.1.2.spectral response and transmission

The transmittance becomes above 70% for wavelengths larger than 550nm, also improved in the blue wavelength region due to the shift of the quantum efficiency to smaller wavelengths in the short wavelength range of the spectral response, corresponding with their respective transmission spectrum[15]. The spectral response is modified significantly only in the short wavelength range, associated to the CdS–CdTe interface. This spectral response variation show that the CdS deposition bath solution affects mostly the characteristics of the interface between CdS and CdTe as well as the absorption coefficient of the CdS layer, but it does not modify the bulk properties of the CdTe layer in the solar cell[15].

As-grown CBD–CdS films usually do not show any luminescence signal at room temperature possibly because of a high density of non-radiative defects with energy levels near the midgap. The sulfur enrichment CdS-CBD films causes a decrease of the carrier trap density at the grain boundaries, which accounts for a decrease of the non-radiative centers giving rise to a enhancement of the PL signal at room temperature[16].

In order to evaluate the sulfur enrichment, the behavior between the low energy band associated to sulfur vacancies(Vs) and the high energy band related to interstitial sulfur(Is) is to consider the relative intensity Vs to Is as a function of S/Cd ratio. According to the experimental results for the photoluminescence measurements performed by O.Vigil.Galan et al[24] and R. Mendoza-Pe´rez et al[25], for the low S/Cd ratio as more sulfur atoms are present in the bath solution more sulfur vacancies are produced in relation to interstitial sulfur, gradually an equilibrium between the introduction of sulfur atoms at interstitial sites and sulfur vacancies is achieved, for the high S/Cd ratio the sulfur atoms prefer to go into interstitial sites[4].

This reveals that the sulfur goes into the CdS grain boundaries at the low S/Cd ratios, while for the higher value of S/Cd ratios the amount of interstitial sulfur decreases due to the precipitation of sulfur in the solution.

3.1.3. solar cell performance

High optical transmission and large bandgap values of CdS window layer should improve the short circuit current of the solar cells. On the other hand, better open circuit voltage and Fill Factors are related to the decrease of the minority carrier recombination at the CdS/CdTe interface and at the grain boundaries

From the J –V curves of the CBD–CdS based solar cells, one of the differences comes from the knee of the curves, which is partially associated to a change of the series resistance for the different samples. This feature can also be associated to a larger recombination ratio at the space–charge region of the CdS/CdTe hetero-junction[25]. the highest performance was observed for S/Cd=6 corresponding to the proper amount of sulfur vacancies. This feature can be explained that sulfur enrichment in the CdS layers causes a decrease of carrier trap density at the grain boundaries and larger polycrystalline disorder at the CdS/CdTe interface with higher S/Cd ratio [25].

All these properties seem to improve the solar cell characteristics (better Voc and FF, due to a decrease of the minority carrier recombination at the CdS–CdTe interface and at the grain boundaries; increases in the short circuit current density, due the high values of the band-gap;decreases the bulk resistivity under illumination because the high photoconductive gain and decreases the dark current in the devices due to a larger surface covering fraction)[24]

Therefore, the optimal S/Cd ratio is related to the density of sulfur deficiency which affects the recombination in CdS/CdTe heterojunction, a proper sulfur enrichment decreases the minority carrier recombination level at the CdS/CdTe interface and the grain boundary to enhance the open circuit voltage, fill factor and the efficiency of solar cells.

3.2. Effect of NH₄Cl and NH₄OH

For the low NH₄Cl concentrations, hexagonal phase dominates while mixed phases of cubic and hexagonal structures are found with the increase of NH₄Cl concentration, and then cubic phases appear, as NH₄Cl concentration further increases, are turned to the hexagonal structure again[4].

The variation of crystal phases of CdS thin films with the solution PH can be interpreted as following mechanism [4,26].

Since the crystal structure of metal Cd is a hexagonal system and simple substance S has a orthorhombic structure, the cubic phases are formed when S^2- ion occupies first the surface active sites, whereas the hexagonal phases are formed in case of Cd(NH₃)₄²⁺ .

NH₄Cl is in the dissociation equilibrium state as follows:

NH₄Cl = NH₄⁺ + Cl^- (7)

As NH₄Cl concentration increases, the formation of Cd (NH₃)₄²⁺ is restricted due to the limitation of dissociation of CdCl₂ by the increase of Cl^- ion concentration in solution. Therefore abundant S^2- ions generated from the reaction (4) occupy the surface active sites to form the hexagonal structure.

On the other hand, NH₄⁺ ions generated from reaction (7) reacts with OH^- as follows:

NH₄⁺ + OH^- = NH₃ + H₂O (8)

The consumption of OH^- decreases the solution PH and the production of NH₃ promotes the formation of Cd(NH₃)₄²⁺, which restricts the formation of HS^- and S^2- for the reaction (4) and (5), as a result, the cubic structure is formed.

When NH₄Cl concentration exceeds a threshold value, the action to restrict the generation of HS^- and S^2- dominates, which form the hexagonal structure.

At this time, we can, by adding ammonia, raise the PH and shift the equilibrium of the reaction (8) to the left, which promotes the formation of HS^- and S^2- to form the cubic structure.

The PH depending on ammonia concentration changes the decomposition rate of thiourea and the complexing ability to affect strongly CdS film properties. The reaction takes place through the ion by ion mechanism at low PH and cluster by cluster mechanism at high PH. As the PH increases, the combination probability of Cd²⁺ and OH^- ions becomes higher, the content of Cd(OH)₂ within thin film increases to reduce the transmittance.

4. Influence of bath temperature

The bath temperature affects the growth rate, surface morphology, microstructure and adhesion of CdS thin films.

4.1. Growth rate

The growth rate of the CdS thin film rises prominently with the increase in bath temperature[19]. The decomposition rate of complex Cd(NH₃)₄²⁺, (NH₂)₂CS changes depending on the deposition temperature as well as the solution concentration. CdS films may grow by the decomposition of a metastable complex, Cd(OH)₂(NH₂)₂SC(NH₂)₂, going with a release of NH₃. The absorbed NH₃ on the substrate would hinder the growth of CdS and cause the island growth or the hole on the films. And it could concluded that desorption of NH₃ is the main factor which affect the uniformity of CdS films[27].

The volatilization of ammonia in the solution accelerated with the increase of the bath temperature which led the complexing ability of the complex agent to reduce. Furthermore, the reaction rate increased at a high temperature and the consumption of OH⁻ ions increased rapidly. To speed up the complex ion [Cd(NH₃)₄]²⁺ decomposition, the concentration of the Cd²⁺ ion in solution was increased[19].

Meanwhile, the speeding decomposition rate of thiourea made the S²⁻ ion concentration increase in the solution. All these factors would promote the homogeneous and heterogeneous depositions of the particle–particle system in the solution. So high temperature could also lead to surface roughness of CdS thin films. Therefore, high quality CdS thin films were difficult to obtain when the bath temperature was too high or too low [19].

4.2. Surface morphology and thickness

At the beginning of deposition, the glasses are isotropic body and random nucleation occurs. At the lower temperature, we can observe many pinholes which cause shunt between the front and rear electrodes to reduce the open circuit voltage of solar cell. As the bath temperature increases, pinholes considerably decrease and the surface morphology becomes more homogeneous[27].

For the lower deposition temperature, due to the low rate of chemical reaction and ion diffusion to the grains, the surface grains grow too slow to coalesce with each other to create a continuous film which resulted in the existence of voids and the smaller grain sizes compared to films deposited at higher temperature, whereas, CdS film deposited at higher temperature displays a rather rough, inhomogeneous surface with overgrowth grains[20].

At lower temperature, an abundance of CdS particles are directly deposited because of lower solubility product of CdS(K_sp), meanwhile, increasing bath temperature yields the films with thin and poor quality due to the increase of K_sp and decrease of the precipitation rate.

However, the surface grains become coarse in exorbitant bath temperatures. The films have poor uniformity and were covered by relatively large particles when the deposition temperature is low[19].

In heterojunction application, any pinhole of the films may affect the electronic characteristic of apparatus by short circuit[27]. The increase of the bath temperature is an effective method to diminish the pinhole on the CdS films.

With the increase of deposition temperature, the density of pinholes increases to reduce the average grain size.

Table 1 shows the thickness and average grain size of CdS thin films deposited with different bath temperatures(parenthesized data is refer to ref[27])

Table 1. Thickness and average grain size of CdS thin films deposited with different bath temperatures[21]([27]).

For the high temperatures, we can observe the number of voids occurred by absorbed NH₃.

4.3. Microstructure

The deposition temperature also affects the crystalline structure of CdS thin film.

CdS films deposited at low temperature include the amorphous phases or mixture of cubic and hexagonal phases with poor crystallinity. With the increase of deposition temperature, the crystallinity of the films be improved, especially become predominantly hexagonal phases which are preferable to be used for the heterojunction solar cells due to its excellent stability[19].

The increase of the bath temperature decreases the grain size and promotes the formation of CdS thin film.As the deposition temperature increases, film structure become predominantly hexagonal and the intensity and sharpening of peak increase, which is caused by improving crystallinity of the films[20,27].

4.4. Transmittance

There is an Eg shift toward the blue wavelength region as temperature decreases [19]. At low temperature the dispersion of ion in solution by thermal movement is reduced, and a preferential columnar growth parallel to the substrate edge appears. The decrease in band gap can be attributed to the improvement of crystallinity. Although the low bath temperature is beneficial for the crystallinity of CdS films, the rough surface contributed to big crystalline size bring the flat absorption edge, and may destroy the electric property for short current[27].

Table 2 shows the average transmittance and band gap of CdS thin films deposited with different bath temperatures

Table 2. Optical properties of CdS thin films deposited with different bath temperatures[20].

The transmittance of the film decreases rapidly with the increase of deposition temperature, which is caused by reducing voids and increasing film thickness mainly. For higher deposition temperature, the transmittance initially increases due to less light scattering by its smoothest surface, and then decreases due to either more light scattering on their rough surfaces or the transition of the CdS phase from the cubic to hexagonal structure. The band gap Eg are found to decrease for hexagonal phase with the increase of deposition temperature, and the sub-band gap assigned to the cubic phase is noticed at low deposition temperature[20].

The absorption edge becomes steeper for the films grown at higher temperatures, showing that bath temperature affects the band gap value, which shifts towards blue wavelength region with the decrease of temperature[14].

5. Influence of deposition time

5.1. Process of film growth

Deposition time influences the film thickness with the constant reactant concentration. At the initial stage, the number of voids occurred by CdS colloid particles form the discontinuous film, resulting in the decrease of shunt resistance to affect solar cell performance[4]. After about 30 minutes, the voids disappear to grow the compact and continuous films.

Table 3 shows the thickness and grain size of CdS thin films grown with the various deposition time.

Table 3. Thickness and average grain size of CdS thin films grown at the various deposition time[21].

The growth process can be considered as two stages, that is, the incubation stage [21] that film thickness increases almost linearly with the deposition time and the saturation stage that the growth of thin film becomes slow to reach saturation state[4]. The former corresponds to the nucleation step followed by a subsequent linear growth, and the latter passes through the nucleation, coalescence stage and the subsequent vertical growth[21].

CdS precipitates occur in solution, simultaneously abundant of Cd²⁺ and S^2- are consumed in the deposition process, as a result, ion precursor reach rapidly the equilibrium state [28]. Therefore, it can be concluded that the film growth is almost completed within 60min. As the deposition time increases, the surface of film becomes compact, uniform and continuous[4].

5.2. Optical properties

At the initial stage the absorption edge is gradual due to the existence of disordered interface states. With the increase of deposition time, the absorption edge becomes steeper and transmittance becomes lower in the region of short wavelength, meanwhile, above 70％ in the long wavelength region. The increase of deposition time fulfills the characteristics of CdS thin film as a window layer[4].

The optical gap variation with the deposition time is opposite to the disorder variation,suggesting that the optical gap is controlled by the disorder in the film network[21].

5.3. Electrical properties

Since the voids, disordered states and Cd(OH)₂ are formed at the initial growth stage of thin film, the diffusion length in CdTe absorber layer becomes short and carrier concentration decreases[28]. Photo-carriers are mainly generated in the depletion region of the absorber layer, and they approach near to the grain boundary but not reach to the heterojunction, therefore the recombination probability of photo-carriers becomes larger and the carrier collection decreases, which affect to the efficiency of solar cells[4].

At low deposition time, the obtained films present a relatively high dark conductivity, this may originate from two reasons: (i) At the initial stage of CdS films growth, the dominant mechanism is ion by ion, which yields to a dense and less porous material (ii) The Cd²⁺ ions stick faster than the S²⁻ ones [21],

thereafter the concentration of S vacancy is large in the first deposited layers and then their conductivity is high, since these defects behave as donor defects [26].

The reduction in the dark conductivity with increasing the deposition time is due to the reduction of S vacancy. However, the increase in the conductivity with further increasing deposition time may originate from the enlargement of the grain size.

H. Moualkia reported that the photoconductivity of the deposited films was one to three orders of decade larger than the dark conductivity. The low photoconductivity in film is due to the large recombination velocity of the photocarriers by the structural defects such as the large disorder. The reduction in the photoconductivity in film deposited after 60 min of deposition time can be explained by the optical losses through the light diffraction at the top film surface. The latter looses its smoothness with increasing deposition time, due to the deposition process transition from the heterogeneous(ion-by-ion) to the homogeneous ones (cluster-by-cluster), as generally recognized [21]. Since in the cluster by cluster mechanism the species involved in the growth process have a bigger size than in the ion by ion one.

Same with for dark conductivity, the improvement of the photoconductivity with further increasing deposition time, is associated with the improvement of the crystalline structure as suggested by the increase in the grain size.

6. Conclusion

In this work, we have investigated the influence of deposition parameters such as the reactant concentration, bath temperature and deposition time on the morphological, optical, structural, electrical properties of CdS thin films, deposited by chemical bath deposition (CBD) method. S/Cd ratio mainly influenced on the transmission of CdS thin film and solar cell performances, while the crystal structure was affected by the concentration of NH₄Cl and NH₄OH. The bath temperature affected the growth rate, surface morphology, microstructure, transmittance of thin film. With the various deposition time, the morphological, optical and electrical properties are changed. These data will help future research of this area.

Acknowledgment

Authors would like to thank General Assay Centre, Kim Chaek University of Technology for carrying out the measurements of XRD, SEM and UV-Vis spectrophotometer of our samples.

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