Characterization and optimization of ZnS thin film properties synthesis via chemical bath deposition method for solar cell buffer layer

Abstract

Zinc Sulphide is one of most studied semiconductor with wide band gap (3.5–3.9 eV) versatile material due to its physical and chemical properties. ZnS is a non-toxic material and a suitable candidate to be a buffer layer for heterojunction solar cells. In this study, Zinc Sulphide (ZnS) thin films were deposited by chemical bath deposition technique using Zinc Acetate Dihydrate [Zn (CH₃COO)₂. 2H₂O] and Thiourea [CH₄N₂S]. The ZnS thin films samples were characterized by UV-Vis NIR Spectroscopy, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDX), Fourier-Transform Infrared Spectroscopy (FTIR) and Thin-Film Measurement Instrument. FTIR spectra confirmed the presence of ZnS bond in the crystalline thin film. XRD data confirmed the cubic structure of the deposited thin film only when the amount of Thiourea was increased and the complexing agent Hydrazine Hydrate was replaced with Tri-Sodium Citrate. Crystallite size and strain were estimated using Debye-Scherrer model and Williamson-Hall model and lattice constant was estimated using Nelson-Riley plot. Otherwise, XRD showed the amorphous phase. UV-Vis data confirmed ZnS thin films as enough transmittive and it showed higher bandgap. Thin-Film Measurement Instrument was used to measure the thickness of the ZnS thin films. Synthesized ZnS thin films exhibited promising characteristics for using as the buffer layer of the heterojunction solar cells.

Highlights

ZnS thin films were prepared successfully by simple, low cost and environment friendly chemical bath deposition method.

XRD measurement confirmed both Amorphous and Crystalline phase of ZnS thin films.

By changing the precursor only can be achieved crystalline phase from amorphous phase of ZnS thin film.

The amount of precursor and deposition conditions can be optimized to produce crystalline ZnS thin film.

Keywords

Zinc sulphide buffer layer hydrazine hydrate nelson-riley heterojunction

1 Introduction

Zinc sulphide (ZnS) is one of the direct II-VI semiconductor compounds which has a large band gap energy of ∼3.65 eV at room temperature [1]. This material forms two types of crystalline structures which are wurtzite and zincblende (also known as sphalerite). Wurtzite is a hexagonal structure, where zincblende is a cubic structure [2]. Sometimes it shows mixed phase crystalline structure. While cubic ZnS has been reported to have a wide direct band gap of ∼3.65 eV at 300K, hexagonal ZnS has been reported to have a band gap of ∼3.91 eV [3]. Due to its large energy band gap nature it has several applications in different sectors of science and technology. It is a very useful material for optoelectronic applications such as blue light emitting diodes [4], electroluminescent devices and photovoltaic cells [5]. Different techniques have been employed by different researchers to deposit ZnS thin films, such as atomic layer deposition (ALD) [6], pulsed electro chemical deposition (ECD), chemical vapor deposition (CVD) [7], chemical bath deposition (CBD) [8] and spray pyrolysis [9]. Within these methods, CBD method is reliable as it is simple, cheap and can deposit a large area thin film. A very simple balanced chemical reaction is used to prepare a thin film in Chemical bath deposition (CBD) method. This method is also capable to grow thin films with nanocrystal, microcrystal and epitaxial structure under several growth conditions [10]. The crystalline structure, morphological characteristics, elemental analysis, optical properties, vibrational characteristics and film thickness were analyzed by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDX), UV-Vis NIR Spectroscopy, Fourier-Transform Infrared Spectroscopy (FTIR) and Thin-Film Measurement Instrument respectively [11]. In the heterojunction solar cell Cd is used as the buffer layer but Cd is a toxic material and harmful for our environment. Without its toxicity, the energy band gap of CdS is about 2.40–2.50 eV, which results significant photon loss in short-wavelength range which reduces the overall performance of the solar cells. Moreover, it will lead to poor photo stability due to the small conduction band of set between CdSe core and CdS shell. For effective electron confinement, a thin shell of wide band gap ZnS semiconductor was more effective than that of CdSe/CdS. (Optical Characteristics of ZnS Passivated CdSe/CdS Quantum Dots for High Photostability and Lasing). Again, in terms of ZnSe, it is softer and more frangible than ZnS and Se is highly toxic in higher concentration. On the other hand ZnS is a low-cost, non-toxic and earth abundant materials and it has ability to develop the efficiency of chalcogenide and kesterite based photovoltaic due to its wide band gap which results in reducing the absorption loss compared to CdS [12]. Buffer layer should have some properties such as high transparency for incident light, good conduction band lineup with absorber material, low interface recombination, high resistivity and good device stability. ZnS thin film grants all the properties of the buffer layer and so ZnS thin film might be the best replacement of the CdS buffer layer in CIGS solar cell.

In this present work we describe the deposition of ZnS thin films by CBD method and compare their structural, optical, morphological and vibrational properties for different growth conditions and also optimization the growth conditions for crystalline ZnS thin film.

2 Experimental

ZnS thin film was deposited by chemical bath deposition (CBD) method using Zinc acetate dihydrate [Zn(CH₃COO)₂.2H₂O] as zinc ion source and thiourea [CH₄N₂S] as sulphide ion source. ZnS thin film was deposited on a glass substrate by decomposition of thiourea [CH₄N₂S] in an alkaline solution including zinc salt and a complexing agent within a heating condition. To deposit ZnS thin films, 30 ml of 0.2 M Zinc acetate dihydrate solution was taken in a pre-cleaned beaker. 30 ml of 0.2 M (for sample 1&2) or 30 ml of 0.6 M (for sample 3&4) thiourea solutions was added with the Zinc acetate dihydrate solution. After that the solution was stirred for 30 minutes. To improve the homogeneity and growth rate of the ZnS thin film 15 ml of 1M Hydrazine Hydrate [N₂H₄.H₂O] (for sample 1, 2 &3) or 15 ml of 1 M Tri-Sodium Citrate Dihydrate [C₆H₅Na₃O₇.2H₂O] (for sample 4) was added with the solution. Then ammonia solution was added drop wise to form the complex and to raise the pH of the solution. The pH value was maintained between 9 and 10. The mixture was vigorously stirred and heated until the temperature rose to 70°C (for sample 1) or 80°C (for sample 2, 3 & 4). Borosilicate base glass substrates were used to deposit the ZnS thin films. Cleaning the substrate is very important in the thin film deposition. Before the deposition process the glass substrates were treated by a mixture of concentrated nitric acid and propan-2 M solution. After that the substrates were washed out with distilled water and heated in an oven at 120°C for drying. After that the substrates were kept vertically inside the solution. The substrates were held vertically with the help of metal holder. To avoid contamination from metal holder substrate part that was not in solution was folded with Teflon tape tightly. After the substrates were dipped into the solution keeping bath temperature maintained for 30–45 minutes with stirring. Then the whole solution was kept undisturbed at room temperature for further deposition of the films. The films were kept deposited for 24 hours. Then the glass slides covered with a white layer were taken out. All samples were dried at 120°C in an oven for 15 minute to remove any water content. Structural properties such as XRD pattern of the film was carried out by XPERT-PRO diffractometer at the Material Science Division of Bangladesh Atomic Energy Centre. Scanning Electron Microscopy (SEM) with an embedded energy dispersive X-ray analyzer (EDX) measurement was performed at Materials Science Division, Bangladesh Atomic Energy Centre. Optical properties such as transmission (T) and absorption (A) were performed by UV-Vis-NIR spectrophotometer (model: UV-3100, Shimadzu, Japan) in the Experimental Physics Division of Bangladesh Atomic Energy Centre. Vibrational characteristics such as Fourier-transform infrared spectroscopy (FTIR) was carried out using a Spectrum Two FTIR Spectrometers by PerkinElmer in the Material Science Division of Bangladesh Atomic Energy Centre.

The different growth conditions for different ZnS thin films are given below in Table 1.

Table 1
Different growth conditions for different ZnS thin films

ZnS thin film Reagent – 01 Reagent – 02 Complexing agent Deposition temperature Deposition time

Sample 01 30 ml 0.2 M Zinc acetate 30 ml 0.2 M Thiourea Hydrazine hydrate 70°C 24 hour

Sample 02 30 ml 0.2 M Zinc acetate 30 ml 0.2 M Thiourea Hydrazine hydrate 80°C 24 hour

Sample 03 30 ml 0.2 M Zinc acetate 30 ml 0.6 M Thiourea Hydrazine hydrate 80°C 24 hour

Sample 04 30 ml 0.2 M Zinc acetate 30 ml 0.6 M Thiourea Tri-sodium citrate dihydrate 80°C 24 hour

ZnS thin film	Reagent – 01	Reagent – 02	Complexing agent	Deposition temperature	Deposition time
Sample 01	30 ml 0.2 M Zinc acetate	30 ml 0.2 M Thiourea	Hydrazine hydrate	70°C	24 hour
Sample 02	30 ml 0.2 M Zinc acetate	30 ml 0.2 M Thiourea	Hydrazine hydrate	80°C	24 hour
Sample 03	30 ml 0.2 M Zinc acetate	30 ml 0.6 M Thiourea	Hydrazine hydrate	80°C	24 hour
Sample 04	30 ml 0.2 M Zinc acetate	30 ml 0.6 M Thiourea	Tri-sodium citrate dihydrate	80°C	24 hour

3 Results and discussion

3.1 Reaction mechanism

Growth of ZnS thin film happens when amount of ionic product of Zn²⁺ and S^2- ions increases more than the solubility product of ZnS. Deposition of ZnS thin film is based on the gradual release of Zn²⁺ and S^2- ions in the chemical solution which condense on the substrate. In aqueous solution, zinc acetate dihydrate dissolves and release Zn²⁺ ions. Again in alkaline medium thiourea provides S^2- ions. Ammonia reacts with water and gives OH ^- ions in the solution. Ammonia solution also reacts with Zn²⁺ ions and form complex Zn(NH₃)₄² ₊. In CBD method complexing agent is used to minimize the metal hydroxide ions concentrations from the solution which in turns repel the rapid bulk precipitation of the desired product. Thus the chemical reaction occurs during the deposition of ZnS thin film can be given as follows [13]: $Zn {(C H_{3} COO)}_{2} . 2 H_{2} O \to Z n^{2 +} + 2 H_{2} O + 2 C H_{3} CO O^{1 -}$ $Z n^{2 +} + 4 N H_{3} \to Zn {(N H_{3})}_{4}^{2 +}$ $SC {(N H_{2})}_{2} + 2 O H^{-} \to {S^{2}}^{-} + C H_{2} N_{2} + 2 H_{2} O$ $Zn {(N H_{3})}_{4}^{2 +} + SC {(N H_{2})}_{2} + 2 O H^{-}$ $\to ZnS + 4 N H_{3}_{+} C H_{2} N_{2}_{+} 2 H_{2} O$

3.2 X-ray diffractograms

Figure 1 shows the XRD spectra of ZnS thin films for different growth condition. The sample 01 ZnS thin film was deposited via chemical bath deposition technique, 30 ml 0.2 M zinc acetate dehydrate was used for Zn²⁺ ion source and 30 ml 0.2 M thiourea used for S^2- ion source. We use Hydrazine Hydrate as the complexing agent and kept the deposition temperature 70°C according to Manash et al. [14]. But in this step we get the amorphous phase of the ZnS thin film. To achieve nanocrystalline ZnS thin film, increased the deposition temperature and set it to 80°C. The physical quality of the thin film became improved, but the crystallinity was not improve i.e. the film remain in the amorphous phase. Further the deposited thin film sample 02 was annealed at 300°C to improve the crystallinity, but there was no change found. Again the quantity of thiourea was increased in the solution according to V.Padmavathy et al. [15]. 30 ml 0.6 M thiourea was given in the solution for sample 03 and took the deposition temperature 80°C. Complexing agent Hydrazine Hydrate was remain the same. But the crystallinity was not change in this step too i.e. the film again remain in the amorphous phase. Finally when the complexing agent was changed, the crystallinity of ZnS thin film was improved. The Tri-Sodium Citrate Dihydrate was used as the complexing agent for this sample. The other chemical and bath property was remain as same as the sample 03. The XRD peaks appear at 33.41°, 46.42° and 68.10° in the pure ZnS film corresponding to the diffraction from (200), (220), (400) planes respectively of the cubic phase of ZnS which is also known as zinc blende or sphalerite. Results are very much closer to the value of JCPDS card file 05-0566 [14]. Tri-Sodium Citrate Dihydrate helps to improve the crystallinity of ZnS thin film and it is also ecofriendly material where hydrazine hydrate is a toxic material [16, 17]. Thus, the deposited film is polycrystalline in nature. From the XRD spectra it is clear that the intensity of (200) plane in the film is higher than that of the other peaks which indicates that the orientation of the grain growth is along (200) plane. The unidentified peaks observed in ZnS thin film were due to the other impurities.

Fig. 1

XRD spectra of ZnS thin films deposited by CBD method on glass substrate.

The average crystallite size of ZnS thin film was estimated by measuring the full-width at the half-maximum of the diffraction peak using Debye-Scherrer equation (Cullity, 1978): $D = k λ / β_{hkl} cos θ$ (1) where k is a constant whose value is 0.94, β is the full width at half maximum (FWHM) intensity of the peak in radian, θ is the Bragg’s diffraction angle, λ is the wave length of X-ray (1.5406 Å). Average crystallite size (D) was calculated from the most prominent peak of XRD pattern. Average crystallite size was found to be about 112 nm which is tabulated in Table 2.

Table 2

Structural parameters of deposited ZnS thin film

Sample	2θ	Plane	crystallite	Average Crystallite		Microstrain	Dislocation
	(degree)	(hkl)	size	Size		(ɛ)×10^–3	density
			(nm)	D (nm)			(δ)×10^–3
				Scherre’s	W-H plot		Lines/m²
				formula
ZnS thin	33.41°	200	172	112	57	0.721	0.034
film	46.42°	220	30			2.931	1.110
	68.10°	400	133			0.465	0.056

The strain (ɛ) was calculated for the ZnS thin film sample was calculated by the following formula: $ɛ = β / 4 tan θ$ (2)

These calculated values of the strain along with the intense peak position are tabulated in

Table 2. The dislocation density δ is defined as the length of dislocation lines per unit volume of the crystal. The dislocation density was determined from the Williamson and Smallman’s formula (Girija et al., 2009): $δ = n / D^{2}$ (3)

Where n is a factor which is equal to unity giving minimum dislocation density, usually n is equal to 1 and D is the average crystallite size. Dislocation density for ZnS thin film has been given in Table 2.

3.2.1 Williamson-Hall model

The total broadening of X-ray diffraction peaks arises due to the simultaneous contribution of both particle size and internal strain. Therefore using the Williamson and Hall (W-H) method (Nelson and Riley, 1945) for Cauchy nature of broadened profile, we have the relation (Williamson and Hall, 1953): $β = K λ / D cos θ + 4 ɛ tan θ$ (4) Or, $β cos θ = K λ / D + 4 ɛ sin θ$ (5)

Where ɛ is the average internal strain. For multiple ordered diffraction pattern a plot of βcosθ versus 4sinθ will give a straight line and the inverse of intercept of this plot (W-H plot) i.e. the point where the straight line and the Y axis intercepting with each other will give the value of average crystallite size D and the slope of the straight line will give the value of average internal strain ɛ. The W-H plot for ZnS thin film is shown in Fig. 2. The crystal size of the ZnS thin film was found 57 nm which is tabulated in Table 2. Value of average crystallite size from W-H plot is different from that obtained from Scherrer’s equation. This indicates that the films are under strain. The slope of the fitted trend straight line of the W-H plot (Fig. 2) was negative, indicating the existence of compressive strain in the lattice of ZnS thin film samples [18]. The W-H method is very simple way to separate out size and strain. The main data points in that plot are obtained from the XRD analysis. Due to the instrumental broadening, actual shape information, size distribution or domain shape etc. may affect the calculated data from XRD spectra. Therefore the data points look slightly scattered. However, for strain contained films, this method is very useful to calculate the average crystallite size and microstrain.

Fig. 2

Williamson-Hall plot of ZnS thin film deposited by CBD method on glass substrate.

3.2.2 Lattice constant

The lattice constant for ZnS thin film was determined from the relation: $a = d {(h^{2} + k^{2} + l^{2})}^{1 / 2}$ (6)

Where d is the spacing between the crystal planes. The values of lattice constants evaluated are slightly different for different orientations of the same film. This is due to the divergence of the X-ray beams, refraction and absorption of X-rays by the specimens, etc. These give a number of systematic errors in the measurement of θ and hence in the d values. Therefore the Nelson-Riley (N-R) plots have been drawn (Lifshim, 1999). The Nelson-Riley curve is plotted between the calculated values of lattice constant for different planes and the error function: $(θ) = 1 / 2 ({cos}^{2} θ / sin θ + {cos}^{2} θ / θ$ (7)

The corrected values of lattice constants are estimated from the intercept of the plot for error function fθ = 0. The N-R plot of as prepared ZnS thin film has been shown in Fig. 3. The straight lines have been drawn as best fit regressive line. The calculated and corrected values of lattice constant have been tabulated in Table 3.

Fig. 3

Nelson-Riley plot of ZnS thin film deposited by CBD method on glass substrate.

Table 3

Calculated and corrected lattice constant

Sample	Plane	Calculated	Corrected
	(hkl)	lattice	lattice
		constant	constant
		(Å)	(Å)
ZnS thin film	200	5.52096	5.49956
	220	5.528302
	400	5.50268

3.3 Scanning Electron Microscopy (SEM)

Figure 4 shows surface morphology of the different ZnS thin films analyzed by SEM technique. These images indicate the morphology of ZnS thin film for different condition and properties. The particle size distribution of the different ZnS thin film was measured using the imagej software from the SEM image. The average particle size of sample 01, sample 02, sample 03 and sample 04 ZnS thin film is 8 nm, 36 nm, 5 nm and 55 nm respectively.

Fig. 4

SEM images of ZnS thin films deposited by CBD method on glass substrate.

3.4 Energy dispersive X-ray spectroscopy

Energy-dispersive X-ray spectroscopy is an analytical technique used for the elemental analysis or chemical characterization of a sample. Figure 5 shows EDX spectra of different ZnS thin films. Every sample has an amount of oxygen atom because vacuum process was not used for deposition. The silicon atom came from the glass slide because the silica glass slide was used for deposition. Some impurity showed in the spectrum which came from the reagents which used for creating ZnS thin films. In sample 01 the amount of Zn and S was very small, in sample 02 the amount didn’t change. In sample 03 when the amount of thiourea increased the amount of Zn and S atom increased slightly. But in sample 04 when the complexing agent Hydrazine Hydrate was replaced with Tri-Sodium Citrate Dihydrate the thin film became a Nano crystalline thin film, the amount of Zn and S atom increased and the amount of oxygen and other impurity atom decreased significantly.

Fig. 5

EDX spectra of ZnS thin films deposited by CBD method on glass substrate.

3.5 Optical properties

Optical transmittance and absorption of Zns thin films have been studied in the wave length range 300–2500 nm. Optical absorption studies of semiconducting materials provide some information related to band structure. Optical transmittance and absorption spectra of typical ZnS thin films have been shown in Fig. 6. At higher wavelength i.e. at lower energy side, absorption is low towards visible region when transmittance is high. However, an increase of absorption is seen in lower wave length (higher energy) side for all the thin films where decreasing of transmittance is observed. Higher transmittance has been observed in all ZnS films. Among these films the transmittance of the crystalline ZnS thin film is found slightly smaller than other. This is due to the well arrangement of the atom or the density of the atom is higher than other.

Fig. 6

Transmittance and Absorbance spectra of ZnS Thin films deposited by CBD method on glass substrate.

3.5.1 Band gap calculation for ZnS thin films

There are many methods to obtain the optical band gap of the thin films. We use the tauc plot method to obtain the band gap of the ZnS thin films. While investigating the optical and electronic properties of amorphous germanium, Tauc proposed and substantiated a method for determining the band gap using optical absorbance data plotted appropriately with respect to energy [19]. This was further developed in Davis and Mott’s more general work on amorphous semiconductors [20, 21]. They show that the optical absorption strength depends on the difference between the photon energy and the band gap as shown in below: ${(α hv)}^{1 / n} = A (hv - Eg)$ (8)

Where h is Planck’s constant, ν is the photon’s frequency, α is the absorption coefficient, Eg is the band gap and A is a proportionality constant. The value of the exponent denotes the nature of the electronic transition, whether allowed or forbidden and whether direct or indirect:

For direct allowed transitions n = 1/2

For direct forbidden transitions n = 3/2

For indirect allowed transitions n = 2

For indirect forbidden transitions n = 3

Typically, the allowed transitions dominate the basic absorption processes, giving either n = 1/2 or n = 2, for direct and indirect transitions, respectively. Thus, the basic procedure for a Tauc analysis is to acquire optical absorbance data for the sample in question that spans a range of energies from below the band gap transition to above it. Plotting the (αhν)^1/ ⁿ versus (hν) is a matter of testing n = 1/2 or n = 2 to compare which provides the better fit and thus identifies the correct transition type [22].

Figure 7 shows the tauc plot for ZnS thin films. From the Tauc plot it is seen that the energy band gap of Sample 01, 02, 03 and 04 is 3.64 eV, 3.62 eV, 3.63 eV and 3.64 eV respectively. The band gap of sample 01 and sample 04 is well matched with the previous report [23].

Fig. 7

Tauc plot from UV-Vis analysis of ZnS thin films deposited by CBD method on glass substrate.

The extinction coefficient or imaginary part of refractive index k was calculated from: $k = α λ / 4 π$ (9)Or, $k = 2.303 λ {log}_{10} (1 / T) / 4 π d$ (10)

Here α is the absorption coefficient; λ is the wavelength; T is the transmittance; d is the thickness of the thin films. The thickness of Sample 01, 02, 03 and 04 is 273.2 nm, 228 nm, 222 nm and 227 nm respectively. Variations of k with wavelength have been plotted in Fig. 8. The k value is observed to decrease slowly up to the wavelength 330 nm for all films, and then increases slightly towards the IR region. From the comparison it is seen that value of extinction coefficient is become higher in crystalline ZnS film which indicates more efficiency of crystal ZnS film in electrical conduction than amorphous ZnS film.

Fig. 8

Plots of k versus λ of ZnS thin films deposited by CBD method on glass substrate.

3.6 Fourier-transform infrared spectroscopy

FTIR is a technique that is used to obtain chemical bonding information in a material. It is used to identify the elemental constituents of a material. The characteristics peaks exhibited by crystalline ZnS thin film as shown in Fig. 9. The strong absorption band observed at 512/cm was assigned to the vibrational characteristics ZnS. The peak at 823/cm was obtained due to the ZnS lattice which generated from the resonance interaction between sulfide ions at vibrational modes in the crystal [24]. The absorption bands which were found at 1390/cm and 1540/cm can be assigned as the stretching vibration of C = S group in the structure of – CSS^–. The absorption peak at 3327/cm was characterized the weak band of C-H [25].

Fig. 9

FTIR spectra of Crystalline ZnS thin films deposited by CBD method on glass substrate.

4 Conclusions

Thin films of ZnS have been prepared by the CBD method. XRD results show that polycrystalline ZnS thin film found when Tri-Sodium citrate used as a complexing agent, the molarity of thiourea should be three times than the molarity of the Zinc Acetate Dihydrate. Different structural parameters such as crystallite size, lattice constant, internal strain and dislocation density were calculated from their XRD spectra. SEM images show the surface morphology of the films. Imagej software was used to calculate the particle size distribution of the thin films from the SEM images. EDX results show the comical compound or elements in the films. Optical studies confirm the direct band gap nature of the films and confirm the higher band gap nature of the ZnS thin films. Higher value of extinction coefficient crystalline ZnS film may enhance the efficiency of Crystal ZnS film in electrical conduction than that of amorphous ZnS film. Optical studies also confirm that ZnS thin films are enough transmittive. FTIR analysis confirms the presence of ZnS bond in the crystalline thin film.

Footnotes

Acknowledgments

Authors thank to Experimental Physics Division and Material Science Division of Bangladesh atomic Energy Centre, Bangladesh Atomic Energy Commission giving instrumental facilities to carry out this work.

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