Solar Energy Materials & Solar Cells 95 (2011) 856–863
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Effect of different annealing conditions on the properties of chemically deposited ZnS thin films on ITO coated glass substrates Seung Wook Shin a, So Ra Kang b, Jae Ho Yun c, A.V. Moholkar b,d, Jong-Ha Moon b, Jeong Yong Lee a, Jin Hyeok Kim b,n a
Department of Materials Science and Engineering, KAIST, Daejeon 305-701, South Korea Department of Materials Science Engineering, Chonnam National University, Gwangju 500-757, South Korea c Photovoltaic Research Group, Korea Institute of Energy Research, 71-2 Jang-Dong, Yuseong-Gu, Daejeon 305-343, South Korea d Department of Physics, Gopal Krishna Gokhale College, Kolhapur 416 012, M.S., India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 November 2009 Received in revised form 1 November 2010 Accepted 7 November 2010 Available online 3 December 2010
The effects of different annealing conditions such as atmospheres, temperatures and times on the structural, morphological and optical properties of ZnS thin films prepared on ITO coated glass substrates by chemical bath deposition were studied. Aqueous solutions of zinc acetate and thiourea were used as precursors along with stable complexing agents, such as Na2EDTA and Na3-citrate, in an alkaline medium. X-ray diffraction patterns showed that the as-deposited and as-annealed ZnS films had an amorphous structure or poor crystallinity below the optimized annealing conditions of 500 1C and 60 min with the exception of the films annealed in N2 + H2S annealing atmosphere. The ZnS thin films annealed in N2 + H2S atmosphere for 1 h at 500 1C showed three sharp peaks for the (1 1 1), (2 2 0) and (1 1 3) planes of polycrystalline cubic ZnS without any unwanted secondary ZnO phases. X-ray photoelectron spectroscopy revealed Zn–OH and Zn–S bonding in the as-deposited ZnS thin film. However, the ZnS thin films annealed at 500 1C showed Zn–S bonding regardless of the annealing atmosphere. The sharp absorption edge and band gap energy of the as-deposited and as-annealed ZnS thin films varied from 295 to 310 nm and 3.5 to 3.89 eV, respectively. & 2010 Elsevier B.V. All rights reserved.
Keywords: Polycrystalline ZnS films Chemical Bath Deposition (CBD) Annealing atmosphere Annealing temperature Less-toxic complexing agents XPS studies
1. Introduction Recently, Cu(In,Ga)Se2 (CIGS)-based thin film solar cells with high efficiency for laboratory scale and large-area solar cells were reported [1]. Although the theoretical conversion efficiency of CIGS-based thin film solar cell is 30%, the best efficiency achieved thus far has been approximately 20% for small-area solar cells [2]. The low efficiency is strongly dependent on the interfacial properties. This problem is common to many semiconductor devices and is particularly crucial in hetero-junction devices because of the difference in lattice mismatch and band gap energy between the n- and p-type layers [2]. In CIGS-based thin film solar cells, a chemically deposited CdS (CBD-CdS) buffer layer with high resistivity is generally used between the absorber layer and transparent conducting oxide layer with the best efficiencies of 19.9% over a small area of 0.5 cm2 [3]. However, CBD-CdS causes serious environmental problems due to the large amount of Cd-containing waste during deposition process [4–7]. In addition, the efficiency of CBD-CdS thin film solar cells decreases due to the
n
Corresponding author. Tel.: + 82 62 530 1709; fax: +82 62 530 1699. E-mail address:
[email protected] (J.H. Kim).
0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.11.002
reduced absorption resulting from the CdS buffer layer [8]. The losses introduced by the CdS buffer layer in the UV range limits the performance of solar cells. Therefore, the preparation of a Cd-free buffer layer is one of the major objectives in the field of CIGS-based thin film solar cells. Cd-free buffer materials, such as ZnS, Zn(OH)2, ZnO, ZnSe, In2S3 and InSe, have been investigated as an alternative buffer layer to CdS [9–14]. Unfortunately, the conversion efficiency of Cd-free thin film solar cells fabricated using these materials is inferior to those fabricated using CBD-CdS buffer layers. ZnS is considered the most promising buffer layer because it is less toxic and cheaper. The decreased absorption in the NIR and UV–vis region improves the short circuit current of thin-film-based solar cells [8]. The chemically deposited ZnS (CBD-ZnS) thin films are generally prepared from aqueous solutions of Zn salts, thiourea or thioacetamide (for S source) in a hot basic medium using one or more stable complexing agents that control the Zn2 + ion concentration while maintaining the desired pH during the reaction with thiourea or thioacetamide [5,11,15]. Hydrazine hydrate is commonly used as a complexing agent [16–18]. Although addition of hydrazine hydrate improves the growth rate and crystal quality of ZnS thin films, it is flammable, carcinogenic and toxic [6,19]. Therefore, it is necessary to find out less toxic complexing agents in order to
S.W. Shina et al. / Solar Energy Materials & Solar Cells 95 (2011) 856–863
overcome these difficulties. Tri-sodium citrate (Na3-citrate) and disodiumethylenediamine tetra-acetate (Na2EDTA) are promising candidate materials for less-toxic complexing agents. Johnston et al. [7] and Goudarzi et al. [20] reported the growth of highly transparent polycrystalline ZnS thin films with amorphous or poor crystalline quality with an improved growth rate by the addition of Na3-citrate or Na2EDTA as a complexing agent. While fabricating CIGS-based thin film solar cells, annealing treatments of the chemically deposited buffer layer were carried out to remove the CdOH or ZnOH phases and improve the crystallinity of the buffer layer and adhesion to the absorber layer [1,3]. The structural, chemical and optical properties of ZnS thin films annealed in different atmospheres, such as air, H2, H2S, N2, Ar and vacuum, have been reported [21–24]. However, there are no reports of a direct comparison of the structural, chemical and optical properties of ZnS thin films annealed in different atmospheres, including vacuum, N2 and N2 + H2S. Therefore, this study examined the effect of annealing treatments with different conditions to remove the ZnOH phase and improve the crystallinity of the as-deposited ZnS thin films. In this manuscript, CBD-ZnS thin films were deposited on ITO coated glass substrates by chemical bath deposition (CBD) technique using less-toxic complexing agents, Na3-citrate and Na2EDTA, in an aqueous alkaline medium prepared at 80 1C. The effects of the different annealing conditions, such as annealing temperature, atmosphere and time, on the structural, chemical, morphological and optical properties of ZnS thin films were studied.
2. Experimental details ZnS thin films were deposited on indium–tin-oxide (ITO) coated glass substrates by chemical bath deposition technique using an aqueous solution of zinc acetate, thiourea, Na3-citrate and Na2EDTA in a hot alkaline medium. Na3-citrate and Na2EDTA were used as less-toxic complexing agents. The bath solution was prepared using 40 mL of a 0.2 M zinc acetate dehydrate (Zn(CH3COO)2 2 H2O) solution and a mixture containing 60 mL of 0.2 M Na3-citrate (Na3C6H5O7) and 20 mL of 0.4 M of Na2EDTA (Na2C10H16N2O8). Subsequently, 80 mL of a 0.055M thiourea (SC(NH2)2) solution was added and the pH was adjusted to 10 by adding an ammonia (NH4OH) solution. Finally, the required quantity of deionized water was added to make a 200 mL solution in the reaction bath. The ITO coated glass substrates were cleaned ultrasonically using acetone, methanol, isopropyl alcohol and deionized water for 10 min consecutively, followed by drying in air. The cleaned substrates were placed vertically in a jug that was immersed in a reaction bath at 80 1C. After deposition time of 4 h, the ITO substrates were removed from the reaction bath, rinsed with deionized water, dried in air and preserved in an airtight plastic container. The ZnS thin films were uniform, pinhole-free and well adherent to the substrate. To study the effect of different annealing treatments, the as-deposited ZnS thin films were annealed in different atmospheres including a vacuum, N2 and N2 (95%) + H2S (5%), at different annealing temperatures ranging from 300 to 550 1C. After identifying the optimal annealing atmosphere (N2 + H2S atmosphere) and annealing temperature (500 1C), the annealing time was varied from 30 to 120 min, respectively. The structural properties of the as-deposited and as-annealed ZnS thin films were studied by high-resolution X-ray diffraction (XRD, X’pert PRO, Philips, Eindhoven, Netherlands). The chemical binding energy of the elements in the ZnS films was studied by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000, ThermoVG Scientific, UK) at room temperature. The carbon 1s line corresponding to 285.0 eV was used to calibrate the binding
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energies in the spectrometer. The surface and cross-sectional morphologies of the films were observed by field emission scanning electron microscopy (FE-SEM, Model: JSM-6701F) operated at room temperature. The chemical composition was determined using an energy dispersive spectrometer (EDS) system attached to a FE-SEM (JEOL, JSM-7500F, Japan). The optical transmittance of the thin films was measured at room temperature using a UV–vis spectrophotometer (Cary 100, Varian, Mulgrave, Australia).
3. Results and discussion Fig. 1 shows XRD patterns of the as-deposited and annealed ZnS thin films annealed at temperatures ranging from 300 to 550 1C for 1 h in different atmospheres, including vacuum (a), N2 (b) and N2 +H2S (c) and different annealing times from 30 to 120 min in N2 +H2S atmosphere at 500 1C (d). No peaks corresponding to the cubic phase were observed for the as-deposited film, which confirmed the amorphous nature of the as-deposited ZnS thin film. No characteristic peaks resulting from an impurity phase ZnO were observed in the as-deposited and annealed films. The XRD patterns of the ZnS thin films annealed in N2 + H2S atmosphere at different annealing temperatures (Fig. 1(c)) showed that a broad peak at approximately 28.51 was observed for the ZnS films annealed at 300 and 400 1C, whereas three sharp peaks for the (1 1 1), (2 2 0) and (1 1 3) planes of cubic ZnS phase (JCPDS data file No.: 65-0309 (ZnS/Cub.)) were observed for the ZnS films annealed above 500 1C. The intensity of the ZnS (1 1 1) peaks in the films annealed in N2 + H2S atmosphere increased with increase in annealing temperature to 500 1C, whereas it deteriorated above 500 1C. This suggests that the improved crystallinity of the annealed films is due to thermal energy. However, the ZnS thin films annealed in vacuum and N2 atmospheres at different annealing temperatures are amorphous or poorly crystalline (Fig. 1(a) and (b)). Fig. 1(d) shows that the diffraction peak of the ZnS thin films annealed in N2 +H2S atmosphere becomes sharper and its intensity enhanced up to 60 min with increasing annealing times, whereas it deteriorated over 90 min. In order to directly compare the effects of annealing atmospheres on crystallinty of annealed films, the XRD patterns of films annealed at different annealing atmospheres using optimized values of annealing temperature and time of 500 1C and 60 min, respectively, have been plotted (Fig. 2). The N2 + H2S atmosphere was found to be optimal for obtaining ZnS thin films with good crystallinity. The enhanced X-ray peak intensity of the ZnS thin film annealed in the N2 + H2S atmosphere was attributed to the continuous supply of additional S compared to that of the as-deposited and annealed ZnS thin films in other annealing atmospheres. XPS was performed to determine the chemical bonding characteristics in the ZnS thin films. Fig. 3 shows the XPS spectra of the as-deposited ZnS thin film prepared at 80 1C in 4 h, and the films annealed at 500 1C for 1 h in different annealing atmospheres. The binding energies of Zn 2p (a) and S 2p (b) were observed. The binding energy of Zn 2p for the as-deposited thin film was approximately 1022 and 1022.7 eV resulting from Zn–OH and Zn–S bonding (Fig. 3(a)), respectively, which indicates that the asdeposited ZnS thin film had grown as a mixture of ZnS and Zn(OH)2 phases [25]. However, the binding energy of Zn 2p for the thin films annealed in different atmospheres was observed to be 1022.9 eV due to Zn–S bonding, which shows that the annealed thin films have a ZnS phase [25]. Moreover, the signals for the S 2p binding energy from the as-deposited and annealed ZnS thin films in different atmospheres were observed at approximately 162 and 163 eV, respectively (Fig. 3(b)). The first binding energy was assigned to the Zn–S bond, and the second binding energy was due to surface contamination, including S8 or CS2 [11].
500 °C 400 °C
ITO sub.
ITO sub.
550 °C
Intensity (arb.unit)
550 °C Intensity (arb.unit)
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ITO sub.
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500 °C 400 °C
300 °C 300 °C As.Dep. As.Dep.
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40 2θ (°)
ZnS cub.(113)
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300 °C
20
60
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ZnS cub.(220)
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40 2θ (°)
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ZnS cub.(111) ITO sub.
30
ITO sub.
20
60
ZnS cub.(113)
ZnS cub.(220)
550 °C
50
ITO sub.
Intensity (arb.unit)
40 2θ (°)
ITO sub.
30
ZnS cub.(111) ITO sub.
20
As.Dep.
50
60
20
30
40 2θ (°)
50
60
Fig. 1. XRD patterns of as-deposited and annealed ZnS thin films at 300–550 1C for 1 h in different atmospheres, including vacuum atmosphere (a), N2 atmosphere (b), N2 + H2S atmosphere (c) and different annealing times from 30 to 120 min in the N2 + H2S atmosphere at 500 1C (d).
XPS confirmed that the ZnOH phases in the as-deposited ZnS thin films can be removed by annealing. Fig. 4 shows the composition ratio of the as-deposited and annealed ZnS thin films in different atmospheres at 500 1C for 1 h. The compositional ratio of the as-deposited and annealed ZnS thin films in a vacuum was nearly stoichiometric. However, the ZnS thin films annealed in N2 and N2 +H2S atmospheres were poor in Zn and rich in sulfur. The Zn atomic ratio of the as-deposited and annealed ZnS thin films in vacuum, N2 and N2 + H2S was 50.14%, 49.85%, 46.85% and 42.02%, respectively. The higher S concentrations of the ZnS thin films annealed in different atmospheres were attributed to the evaporated Zn atoms by the thermal energy and additional S from the H2S gas.
Fig. 5 shows the surface and cross-sectional tilted-view FE-SEM images of the ZnS thin films: as-deposited (a and e) and ZnS annealed in different atmospheres, including vacuum (b and f), N2 (c and g) and N2 + H2S (d and h). The ITO layer was clearly observed between the ZnS thin film and glass substrate. The thickness of the as-deposited ZnS thin film was 100 nm. Both as-deposited and annealed ZnS thin films in different atmospheres showed uniform, continuous and dense microstructures consisting of spherical shaped grains of sizes ranging from 30 to 100 nm. The microstructure of the as-deposited film (Fig. 5(a) and (e)) and annealed films (Fig. 5(b)–(d), (f)–(h)) were similar. Fig. 6 shows the transmittance spectra in the wavelength region 300–800 nm of as-deposited and annealed ZnS thin films at
S.W. Shina et al. / Solar Energy Materials & Solar Cells 95 (2011) 856–863
different annealing temperatures from 300 to 550 1C for 1 h in different atmospheres, including vacuum (a), N2 (b) and N2 + H2S (c) and different annealing times from 30 to 120 min in N2 + H2S atmosphere at 500 1C (d). The transmittance in the visible wavelength region from 300 to 800 nm of annealed ZnS thin films ranged from 60% to 70%. All the as-deposited and annealed ZnS thin
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films in different atmospheres showed sharp absorption. This sharp absorption feature was attributed to the good homogeneity in the grain shape and size, and low defect concentration in the film. The values of the absorption edge of the as-deposited and annealed ZnS thin films under different annealing conditions were approximately 290–300 nm with the exception of the ZnS thin film
Compositional ratio (%)
ZnS cub. (113)
ITO sub.
ITO sub.
ZnS cub. (220)
60
ITO sub.
Intensity (arb.unit)
ZnScub. (111)
65
N2+H 2S
N2
Zn S
55
50
45
Vacuum 40
As.Dep. 35
20
30
40 2θ (°)
50
As.dep.
60
N2
Vacuum
N2+H2S
Annealing atmosphere
Fig. 2. XRD patterns of as-deposited and annealed ZnS thin films in different annealing atmospheres at 500 1C for 1 h.
Fig. 4. Elemental analysis of the ZnS thin films annealed in different atmospheres at 500 1C for 1 h.
N2+H2S
Intensity (arb.unit)
N2
Vacuum
Intensity (arb.unit)
N2+H 2S
N2
Vacuum
As.Dep. As.Dep.
1020
1022 1024 Binding Energy (eV)
1026
160
162 164 Binding Energy (eV)
166
Fig. 3. High-resolution XPS spectra of Zn 2p3/2 (a) and S 2p (b) of as-deposited and annealed ZnS thin films in different annealing atmospheres at 500 1C for 1 h.
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Fig. 5. Surface and cross-sectional tilted-view FE-SEM images of the as-deposited ZnS thin films (a) and (e) and ZnS thin films annealed in vacuum (b) and (f), N2 (c) and (g) and N2 + H2S (d) and (h) at 500 1C for 1 h.
annealed in N2 +H2S atmosphere for 60 min (about 310 nm). The transmittance of the films annealed in different annealing atmospheres using the optimal annealing temperature and time of 500 1C and 60 min, respectively, was plotted to compare the effect of annealing atmospheres on the optical properties of annealed films (Fig. 8(a)).From Fig. 8(a), the transmittance in the visible region of the as-deposited and annealed ZnS thin films in vacuum and N2 atmospheres was approximately 70%, whereas that of the ZnS thin film annealed in N2 +H2S atmosphere was approximately 60%. Although the grain size of the film annealed in
the N2 + H2S atmosphere was larger (Fig. 5(d)), the average transmittance was lower (Fig. 8(a)). This characteristic was attributed to many small particles on the surface of the ZnS thin film annealed in N2 +H2S atmosphere, which acts as a scattering center in the film. The transmittance in the visible region of films annealed in a N2 +H2S atmosphere at different annealing temperatures was lower than that of the as-deposited film. Fig. 7 shows that the variation of (ahu)2 vs. hu of the as-deposited and annealed ZnS thin films at annealing temperatures ranging from 300 to 550 1C for 1 h in vacuum (a), N2 (b) and N2 +H2S
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100
80
80 Transmittance (%)
Transmittance (%)
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60 ITO As.Dep. 300 °C 400 °C 500 °C 550 °C
40
20
60 Annealing temperature ITO As.Dep. 300 °C 400 °C 500 °C 550 °C
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20
0
861
0 300 400 500 600 700 800 Wavelength (nm)
300 400 500 600 700 800 Wavelength (nm)
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80 Transmittance (%)
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60
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Annealing temperature ITO As.Dep. 300 °C 400 °C 500 °C 550 °C
20
0 300
400 500 600 700 Wavelength (nm)
60
Annealing time ITO As.Dep. 30 min. 60 min. 90 min. 120 min.
40
20
0 800
300
400 500 600 700 Wavelength (nm)
800
Fig. 6. Visible transmittance spectra from 300 to 800 nm of the as-deposited and annealed ZnS thin films at 300 1C–550 1C for 1 h in different atmospheres, including vacuum (a) N2 atmosphere (b), N2 +H2S atmosphere (c), and different annealing times from 30 to 120 min using N2 +H2S atmosphere at 500 1C (d).
(c) and for 30–120 min in N2 +H2S atmosphere at 500 1C (d). The plots were linear at the absorption edge, which confirms that ZnS is a direct band gap semiconductor. The band gap energy was derived by extrapolating the straight-line portion of the (ahu)2 vs. hu plot to a zero absorption coefficient value. The optical band gap of the as-deposited ZnS thin film was 3.76 eV. However, the optical band gap of the ZnS thin films annealed in vacuum and N2 were similar, approximately 3.8 eV. The optical band gap of the ZnS thin film annealed in N2 +H2S decreased up to 500 1C (3.5 eV), whereas that of the films annealed above 550 1C increased (3.78 eV). The band gap of ZnS thin films annealed in vacuum, N2 and N2 +H2S at 500 1C for 1 h was 3.9, 3.78 and 3.5 eV, respectively (Fig. 8(b)). The higher band gap of the ZnS thin films than the typical value of bulk ZnS (3.6 eV) was attributed to quantum confinement effects due to the small grain size of the polycrystalline ZnS films. A similar behavior was also reported by Sartale et al. [5].
4. Conclusions Good quality films of closely packed nanocrystals of ZnS were prepared using the CBD technique from an aqueous solution of zinc acetate, thiourea, Na3-citrate and EDTA in a hot alkaline medium. The as-deposited ZnS thin film was amorphous while the ZnS thin films annealed in N2 and N2 + H2S atmospheres at higher than 500 1C were polycrystalline with a cubic structure. The ZnS thin films annealed in N2 +H2S at 500 1C for 60 min showed the best crystallinity. The as-deposited thin film contained Zn–OH and Zn–S bonds, whereas the annealed thin films consisted only of Zn–S bonds. The ZnOH phases in the ZnS thin films can be removed by annealing. The grain size of the ZnS thin film annealed in N2 + H2S was larger than that of the as-deposited ZnS thin films and those annealed in vacuum and N2. The as-deposited and annealed films showed good adhesion to the glass substrates with moderate transmittance ( 470%) in the visible region. The direct band gap
S.W. Shina et al. / Solar Energy Materials & Solar Cells 95 (2011) 856–863
11
(αhυ)2 (cm eV )
9.0x10
11
9.0x10
ITO As.Dep. 300 °C 400 °C 500 °C 550 °C
11
6.0x10
(αhυ)2 (cm eV )
862
ITO As.Dep. 300 °C 400 °C 500 °C 550 °C
11
6.0x10
11
11
3.0x10
3.0x10
0.0 3.0
3.5
4.0
3.0
4.5
Photon energy (eV)
4.0
4.5
11
11
9.0x10
9.0x10
ITO As.Dep. 300 °C 400 °C 500 °C 550 °C
11
6.0x10
(αhυ)2 (cm eV )
(αhυ)2 (cm eV )
3.5
Photon energy (eV)
11
ITO As.Dep. 30 min, 60 min. 90 min. 120 min .
11
6.0x10
11
3.0x10
3.0x10
0.0 3.0
0.0 3.5
4.0
3.0
4.5
Photon energy (eV)
3.5
4.0
4.5
Photon energy (eV)
Fig. 7. Plots of (ahu)2 vs. hu of as-deposited and annealed ZnS thin films in different atmospheres at 300–550 1C for 1 h in vacuum atmosphere (a), N2 atmosphere (b), N2 + H2S atmosphere (c) and different annealing times from 30 to 120 min in a N2 + H2S atmosphere at 500 1C (d).
100 11
9.0x10
ITO As.Dep. Vacuum N2
2
(cm eV )
N2+H 2S
11
6.0x10
-2
60
20
υ)
2
ITO As.Dep. Vacuum N2
40
(α
Transmittance (%)
80
11
3.0x10
N2+H 2S
0 0.0
300
400 500 600 700 Wavelength (nm)
800
3.0
3.5 4.0 Photon energy (eV)
4.5
Fig. 8. Visible transmittance spectra in the wavelength region from 300 to 800 nm of the as-deposited and annealed ZnS thin films in different atmospheres (a) and plots of (ahu)2 vs. photon energy of the as-deposited and annealed ZnS thin films in different atmospheres (b).
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of the as-deposited films was 3.74eV and those for the annealed films varied from 3.89 to 3.5 eV depending on the annealing conditions.
Acknowledgements This study was supported partially by Energy R&D program (2008N-PV08-P-08) under the Korea Ministry of Knowledge Economy (MKE) and partially by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (no. 20100007691) and partially by the National Research Foundation of Korea (NRF) Grant funded by Korea government (MEST; no. 2008-0062289). References [1] L. Qi, G. Mao, J. Ao, Chemical bath-deposited ZnS thin films: preparation and characterization, Appl. Surf. Sci. 254 (2008) 5711–5714. [2] S.N. Kundu, S. Johnston, L.C. Olsen, Traps identification in copper–indium– gallium–sulfur–selenide solar cells completed with various buffer layers by deep level transient spectroscopy, Thin Solid Films 515 (2006) 2625–2631. [3] I. Repinsl, B. Contreras, C. Egaas, J. Dehart, C. Scharf, B. Perkins, R. Noufi, 19.9%efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor, Prog. Photovoltaics: Res. Appl. 16 (2008) 235–239. [4] T. Nakada, M. Hongo, E. Hayashi, Band offset of high efficiency CBD-ZnS/CIGS thin film solar cells, Thin Solid Films 431–432 (2003) 242–248. [5] S.D. Sartale, B.R. Sankapal, M. Lux-Steiner, A. Ennaoui, Preparation of nanocrystalline ZnS by a new chemical bath deposition route, Thin Solid Films 480– 481 (2005) 168–172. [6] I.O. Oladeji, L. Chow, Synthesis and processing of CdS/ZnS multilayer films for solar cell application, Thin Solid Films 474 (2005) 77–83. [7] D.A. Johnston, M.H. Carletto, K.T.R. Reddy, I. Forbes, R.W. Miles, Chemical bath deposition of zinc sulfide based buffer layers using low toxicity materials, Thin Solid Films 403–404 (2002) 102–106. [8] I.O. Oladeji, L. Chow, J.R. Liu, W.K. Chu, A.N.P. Bustamante, C. Fredricksen, A.F. Schulte, Comparative study of CdS thin films deposited by single, continuous, and multiple dip chemical processes, Thin Solid Films 359 (2000) 154–159. [9] A. Ennaoui, W. Eisele, M. Lux-Steiner, T.P. Niesen, F. Karg, Highly efficient Cu(Ga,In)(S,Se)2 thin film solar cells with zinc-compound buffer layers, Thin Solid Films 431–432 (2003) 335–339.
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