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Preparation and properties of CdS thin films deposited by chemical bath deposition
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Preparation and properties of CdS thin films deposited by chemical bath deposition Jiankang Li Institute of Electronic Information Engineering, Suzhou Vocational University, ZhiNeng Road No. 106, Suzhou, Jiangsu Province 215104, China Received 26 October 2014; accepted 19 March 2015
Abstract In this work, a systematic research on the relationship between the CdS crystalline structures and pH value of the solution and annealing treatments for the CdS thin films were carried out. CdS thin films were deposited on ITO substrates by chemical bath deposition methods at a certain bath temperature, in which the CdAc2 and H2NCSNH2 were used as starting materials. During the reactions, NH3H2O was used to control pH value of the solutions. Morphological, structural and optical properties of the CdS films were investigated using scanning electron microcopy (SEM), X-ray diffraction and UV–vis spectrophotometer, respectively. As the pH value increased up to 10.5, CdS film showed the best crystallinity accordingly. From the diffraction patterns, the crystallite sizes were found to increase with increasing annealing temperature. The obvious blue-shift of those annealed with CdCl2 compared with those directly annealed was conducive to improve the efficiency of CdS/CdTe solar cell. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: CdS thin film; Chemical bath deposition; ITO glass
1. Introductions As the world is suffering from impending death of fossil fuels and serious pollution, solar energy is now regarded as one of the promising solution to the global energy crisis. Solar cells are an effective way to convert solar energy into electrical energy. Of all the solar cells, thin-film polycrystalline CdS/CdTe solar cells are promising due to the low cost, high conversion efficiency and being suitable for mass production. The efficiency of CdTe solar cell has reached 17% in laboratory scale and 15.7% in industrial manufacture [1]. CdS is the important semiconductor thin film material for producing CdS/CdTe solar cells. CdS is a semiconductor of the group II–VI compound, which has a direct band gap of 2.4 eV [2,3], used as a suitable window layer for CdTe based photovoltaic devices. Among various techniques which are used for the preparation of thin CdS, such as physical vapor deposition (PVD), close-spaced-sublimation (CSS) [4,5], E-mail address: [email protected]
screen printing (SP) [6] and MOCVD [7], chemical bath deposition (CBD) is the most widely used for its low cost and simplicity. Also, it is easy to control the growth rate and to produce reproducible, uniform and adherent films [8–10]. In this work, the changes of morphology and optical properties with different pH values and annealing treatments are investigated accordingly. Possible explanations to these changes are discussed in details. 2. Experimental CdS thin films were deposited on ITO glass substrates by using a CBD technique. Substrate surfaces were ultrasonically washed and cleaned in acetone, alcohol and de-ionized water, respectively. Finally, the substrates were dried at 80 1C in a drying oven before deposition. The chemicals used for preparing the thin films were all analytical reagent. The chemical solution contained 0.007 M cadmium acetate (Cd (CH3COO)2 2H2O) and 0.05 M thiourea (H2NCSNH2). Ammonia was used to adjust the pH value of solutions to
http://dx.doi.org/10.1016/j.ceramint.2015.03.160 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Li, Preparation and properties of CdS thin films deposited by chemical bath deposition, Ceramics International (2015), http://dx.doi. org/10.1016/j.ceramint.2015.03.160
J. Li / Ceramics International ] (]]]]) ]]]–]]]
2
the range of 9.5–11 (sample as 9.5, sample as 10, sample as 10.5 and sample as 11). After the solution was prepared, ITO glasses were dipped into the solution separately. The bath temperature was kept at 80 1C for 60 min to finish the deposition. After deposition, the CdS films were taken out from the solution and washed ultrasonically to remove the loosely adhered CdS particles on the surface and finally dried in air. And then, all the films were annealed accordingly, as listed in Table 1. Sample A, sample B, sample C and sample F were coated with a thin CdCl2 layer before annealing, and then annealed in a muffle furnace at 400 1C for 60 min with the heating rate of 2 1C/min. Sample D was annealed directly at 400 1C for 60 min with the heating rate of 2 1C/min without coating CdCl2 layer. Sample E was as-deposited without any treatment. X-ray diffraction measurements were used to index the structure of the CdS films. The surface morphology of the films were observed by Scanning Electron Microscope. UV–vis spectrophotometer was used to study the optical properties of CdS films. 3. Results and discussion 3.1. X-ray diffractions Fig.1 shows XRD patterns of CdS thin films by the CBD technique in different pH value. The identification and assignments of the observed diffraction patterns are indexed using Table 1 The table of sample list. Annealed treatments
Whether coating CdCl2
A B C D E
9.5 10 10.5 11 10.5
Yes Yes Yes Yes No
Yes Yes Yes No No
30
40
50
CdS(112)
CdS(110)
CdS(101)
d
c
c
b
b a 20
CdS(100)
intensity
CdS(112)
CdS(110)
CdS(101)
CdS(100)
intensity
CdS(111)
pH value
CdS(111)
Sample
the JPDS data. For the thin film prepared in pH ¼ 9.5 solution, a small peak located at the angle 26.21 is assigned as the (111) plane of the cubic phase. However, as pH value increases to 10, four diffraction peaks are observed, located at 24.81, 26.21, 28.21and 50.81. Three peaks at 24.81,28.21 and 50.81 are respectively associated to diffraction planes of (100), (101) and (112) of hexagonal phase. When the pH value reaches 10.5, one more diffraction peak appears at 43.71, which belongs to (110) diffraction plane of hexagonal phase. Furthermore, the other peaks become rather sharp, especially for the peaks of (111) and (101), which show good crystallinity of CdS film. These indicate that the structure of CdS film transforms from cubic structure to mixed cubic and hexagonal structure by increasing the pH value of the reaction solution. Hexagonal structure CdS has a higher transmission and good electrical conductivity in CdTe solar cells. Relative to the cubic phase, the hexagonal CdS thin films are more suitable to be n-type window layer for CdTe solar cell [11]. As the pH value increases to 11, the diffraction peaks of (111) and (110) descend slightly. The reason may be the generated large particles of CdS and Cd(OH)2 during the reactions, which will postpone the deposition of CdS on the substrate. Fig. 2 shows XRD patterns of CdS thin films by the CBD technique with different annealing temperatures when the pH value is 10.5. From the figure, the CdS film without annealing has no diffraction peaks, which shows that in this condition CdS cannot crystallize. The film annealed directly has three diffraction peaks, located at 26.21, 28.21and 43.71 respectively. The peak at 24.81 belongs to cubic phase. The other two peaks belong to hexagonal phase. Although the diffraction peaks are not very sharp, this verifies our opinion that annealing treatment is helpful for the crystallization of CdS film. When CdCl2 treatment is carried out, it is observed that CdS film has a better crystallization. Compared with the CdS film annealed directly, it has two more diffraction peaks, located at 24.81 and 50.81, which belong to hexagonal phase. Furthermore, the intensity of the peak increases after CdCl2 treatment. As a consequence, annealing treatment promotes the recrystallization of CdS thin films and results in a good crystal
60
2θ(°) θ(°) Fig. 1. XRD patterns of CdS thin films by the CBD technique in different pH solution, (a) pH¼ 9.5, (b) pH¼ 10, (c) pH¼ 10.5, and (d) pH¼11.
a
20
30
40
50
60
2θ(°) θ(°)
Fig. 2. XRD patterns of CdS thin films by the CBD technique with different annealing temperatures, (a) no annealed, (b) annealed directly, and (c) CdCl2 treated and annealed.
Please cite this article as: J. Li, Preparation and properties of CdS thin films deposited by chemical bath deposition, Ceramics International (2015), http://dx.doi. org/10.1016/j.ceramint.2015.03.160
J. Li / Ceramics International ] (]]]]) ]]]–]]]
3
Fig. 3. SEM of CdS thin films by the CBD technique in various pH solution, (a) pH¼ 9.5, (b) pH¼ 10, (c) pH¼ 10.5, and (d) pH¼11.
structure. These CdS films contain a mixture of cubic and hexagonal structures. CdCl2 treatment obviously enhanced the hexagonal phase of CdS thin film. This is because in the process of annealing, hanging with CdCl2 solution, can prevent the volatilization of Cd ion, guarantees the stoichiometric ratio between Cd and S, which benefits the recrystallization of CdS thin film. 3.2. SEM micrographs of the CdS films Fig. 3 shows SEM images of CdS thin films by the CBD technique in various pH solution. Starting from pH ¼ 9.5 solution, it can be seen that the morphology is not smooth. Lots of cracks between grain boundaries are observed. When the concentration of ammonia is low, the decomposition rate of sulfur is slow, which causes the low concentration of S anion. When pH increases to 10, the surface modification is obvious. Cracks and pinholes are fewer than those before. When the pH value is continually increased to 10.5, the surface of the CdS is continuous and compact, which has a good influence on the performance of solar cells. Nevertheless, when pH increases to 11, a few cracks appear again. It is obvious that the best deposition pH is 10.5. Fig. 4 shows SEM of CdS thin films by the CBD technique with different annealing temperatures when the pH value is 10.5. The modification of surface morphologies and roughness
is clearly observed. The grain size of as-deposited CdS is very small, which indicates poor crystallization. However, the surface of CdS film annealed directly is rather rough and not uniform. A few pinholes and cracks appear in the grain boundaries. The size of the grain grows slightly. An obvious evolution of CdS surface appears after CdCl2 treatment and annealed. The particle size obviously increases, which illustrate CdCl2 treatment can promote grain growth. CdS films with large grain size and smooth surface can effectively promote the absorption of light. It is concluded that better crystalline quality of CdS film can be obtained with CdCl2 annealing treatment. 3.3. Optical properties of CBD CdS films Optical absorbance values of CdS thin films by the CBD technique in different pH solution are shown in Fig. 5. Compared with a–d absorption spectra, absorbance peaks appear at wavelength of about 380 nm, which is consistent with results reported by Kodigala [12]. Absorption edge is gentle when pH is 9.5 and the reason is that lots of cracks between CdS grains cause light scattering. When pH increases to 10.5, surface of CdS is decorated and causes this absorbance curve to shift to the long wavelength direction, which indicates that the band gap decreases slightly. Based on literatures, the
Please cite this article as: J. Li, Preparation and properties of CdS thin films deposited by chemical bath deposition, Ceramics International (2015), http://dx.doi. org/10.1016/j.ceramint.2015.03.160
J. Li / Ceramics International ] (]]]]) ]]]–]]]
4
Fig. 4. SEM of CdS thin films by the CBD technique with different annealing temperatures, (a) no annealed, (b) annealed directly, and (c) CdCl2 treated and annealed. c
abs(a.u.)
d
350
b a
400
450
500
550
600
λ(nm)
Fig. 5. Optical absorbance values of CdS thin films by the CBD technique in different pH solution, (a) pH¼ 9.5, (b) pH¼ 10, (c) pH¼ 10.5, and (d) pH¼11.
c
abs(a.u.)
b
350
4. Conclusions
a
400
absorbance curve has a red-shift, the absorption rate is still high after the wavelength 500 nm. Therefore, the transmittance rate is relatively low, which has a bad effect on the performance of solar cells. Fig. 6 shows optical absorbance values of CdS thin films by the CBD technique with different annealing temperature. Compared to curve a, curve b and c appear to shift to the long wavelength direction. The obvious result is that the band gap of the CdS film becomes narrower with annealing treatment. There are a lot of factors that affect the band gap, such as crystalline quality of the film, the volatilization of sulfur and so on [14]. CdS film annealed with CdCl2 becomes sharper than other two curves and the absorption edge blueshift compared with curve b. This indicates that there are fewer defects and impurity energy levels in the CdS film annealed with CdCl2 [14]. Fewer defects may reduce surface scattering and grain boundary scattering, which is conducive in improving the efficiency of the solar cell. Good crystallinity of the CdS thin films under pH value 10.5 and annealed with CdCl2 was the main factor for the optical absorbance properties.
450
500
550
600
λ(nm) Fig. 6. Optical absorbance values of CdS thin films by the CBD technique with different annealing temperatures, (a) no annealed, (b) annealed directly, and (c) CdCl2 treated and annealed.
decrease of the energy band gap is produced by an increase in the structural disorder induced by the change from cubic to hexagonal phase of CdS [13]. Curve d shows that although
The physical and optical properties of CdS films deposited on ITO substrates by chemical bath deposition were investigated in this work. XRD patterns indicated that with increasing pH value of the solution, the intensity of diffraction peak increases and the film structure changes from cubic to hexagonal structure. The pH value equals 10.5 shows the best crystal structure in the films. Meanwhile, annealing treatment can obviously promote the growth of CdS films, especially for those with CdCl2 annealed treatment. The SEM micrographs of the surfaces of the film indicated that when the pH value increases to 10.5, defects and cracks vanished accordingly. And after annealing treatment, CdS grain grows and the crystallinity of films improved. The films with CdCl2 annealed were symmetric and compact with a bigger grain size and improved crystal condition than others. It was observed that
Please cite this article as: J. Li, Preparation and properties of CdS thin films deposited by chemical bath deposition, Ceramics International (2015), http://dx.doi. org/10.1016/j.ceramint.2015.03.160
J. Li / Ceramics International ] (]]]]) ]]]–]]]
the absorption spectra coefficient of CdS films was steadily increasing when pH changed from 9.5 to 10.5. Absorption spectra coefficient declined when the pH is above 10.5. In addition, the absorption edge near the band gap of the films with CdCl2 annealed had been significantly improved, and the absorption edge showed a blue-shift compared with the direct annealing, which illustrated CdS had a narrower band gap, which was conducive to improve the efficiency of CdS/CdTe solar cell. Conflict of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. References [1] M.A. Green, K. Emery, Prog. Photovolt. Res. Appl. 20 (2012) 606–614. [2] N. Romeo, A. Bosio, V. Canevari, A. Podesta, Sol. Energy 77 (2004) 795–801.
5
[3] M. Bedir, M.O. ztas, H. Kara, J. Mater. Sci. Mater. Electron. 21 (2012) 1065–1069. [4] M.G. Faraj, K. Ibrahim, J. Mater. Sci. Mater. Electron. 23 (2012) 1219–1223. [5] J. Schaffner, E. Feldmeier, A. Swirschuk, Thin Solid Films 51 (2011) 7556–7559. [6] H. Uda, H. Matumoto, Y. Komatsu, A. Nakano, et al., IEEE, New York, 1982, p. 801. [7] S.J.C. Irvine, V. Barrioz, D. Lamb, J. Cryst. Growth 310 (2008) 5198–5203. [8] K.V. Zinoviev, O. Zeleya-Angel, Mater. Chem. Phys. 70 (2001) 100. [9] S. Mahanty, D. Basak, F. Rueda, et al., J. Electron. Mater. 28 (5) (1999) 559. [10] T.D. Dzhafarov, M. Altunbas, A.I. Kopya, et al., J. Phys. D 32 (24) (1999) 125. [11] Y.E.H. Chin-Yu, Z.W. Lu, S. Froyen, et al., Phys. Rev. B 46 (1992) 10086–10097. [12] K.S. Ramaiah, R.D. Pilkington, A.E. Hill, et al., Mater. Chem. Phys. 68 (2001) 22–30. [13] N.A. Shah, A. Ali, S. Hussain, et al., J. Coat. Technol. Res. 7 (2010) 105–110. [14] H. Metin, R. Esen, J. Cryst. Growth 258 (2003) 141–148.
Please cite this article as: J. Li, Preparation and properties of CdS thin films deposited by chemical bath deposition, Ceramics International (2015), http://dx.doi. org/10.1016/j.ceramint.2015.03.160
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Preparation and properties of CdS thin films deposited by chemical bath deposition Jiankang Li Institute of Electronic Information Engineering, Suzhou Vocational University, ZhiNeng Road No. 106, Suzhou, Jiangsu Province 215104, China Received 26 October 2014; accepted 19 March 2015
Abstract In this work, a systematic research on the relationship between the CdS crystalline structures and pH value of the solution and annealing treatments for the CdS thin films were carried out. CdS thin films were deposited on ITO substrates by chemical bath deposition methods at a certain bath temperature, in which the CdAc2 and H2NCSNH2 were used as starting materials. During the reactions, NH3H2O was used to control pH value of the solutions. Morphological, structural and optical properties of the CdS films were investigated using scanning electron microcopy (SEM), X-ray diffraction and UV–vis spectrophotometer, respectively. As the pH value increased up to 10.5, CdS film showed the best crystallinity accordingly. From the diffraction patterns, the crystallite sizes were found to increase with increasing annealing temperature. The obvious blue-shift of those annealed with CdCl2 compared with those directly annealed was conducive to improve the efficiency of CdS/CdTe solar cell. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: CdS thin film; Chemical bath deposition; ITO glass
1. Introductions As the world is suffering from impending death of fossil fuels and serious pollution, solar energy is now regarded as one of the promising solution to the global energy crisis. Solar cells are an effective way to convert solar energy into electrical energy. Of all the solar cells, thin-film polycrystalline CdS/CdTe solar cells are promising due to the low cost, high conversion efficiency and being suitable for mass production. The efficiency of CdTe solar cell has reached 17% in laboratory scale and 15.7% in industrial manufacture [1]. CdS is the important semiconductor thin film material for producing CdS/CdTe solar cells. CdS is a semiconductor of the group II–VI compound, which has a direct band gap of 2.4 eV [2,3], used as a suitable window layer for CdTe based photovoltaic devices. Among various techniques which are used for the preparation of thin CdS, such as physical vapor deposition (PVD), close-spaced-sublimation (CSS) [4,5], E-mail address: [email protected]
screen printing (SP) [6] and MOCVD [7], chemical bath deposition (CBD) is the most widely used for its low cost and simplicity. Also, it is easy to control the growth rate and to produce reproducible, uniform and adherent films [8–10]. In this work, the changes of morphology and optical properties with different pH values and annealing treatments are investigated accordingly. Possible explanations to these changes are discussed in details. 2. Experimental CdS thin films were deposited on ITO glass substrates by using a CBD technique. Substrate surfaces were ultrasonically washed and cleaned in acetone, alcohol and de-ionized water, respectively. Finally, the substrates were dried at 80 1C in a drying oven before deposition. The chemicals used for preparing the thin films were all analytical reagent. The chemical solution contained 0.007 M cadmium acetate (Cd (CH3COO)2 2H2O) and 0.05 M thiourea (H2NCSNH2). Ammonia was used to adjust the pH value of solutions to
http://dx.doi.org/10.1016/j.ceramint.2015.03.160 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Li, Preparation and properties of CdS thin films deposited by chemical bath deposition, Ceramics International (2015), http://dx.doi. org/10.1016/j.ceramint.2015.03.160
J. Li / Ceramics International ] (]]]]) ]]]–]]]
2
the range of 9.5–11 (sample as 9.5, sample as 10, sample as 10.5 and sample as 11). After the solution was prepared, ITO glasses were dipped into the solution separately. The bath temperature was kept at 80 1C for 60 min to finish the deposition. After deposition, the CdS films were taken out from the solution and washed ultrasonically to remove the loosely adhered CdS particles on the surface and finally dried in air. And then, all the films were annealed accordingly, as listed in Table 1. Sample A, sample B, sample C and sample F were coated with a thin CdCl2 layer before annealing, and then annealed in a muffle furnace at 400 1C for 60 min with the heating rate of 2 1C/min. Sample D was annealed directly at 400 1C for 60 min with the heating rate of 2 1C/min without coating CdCl2 layer. Sample E was as-deposited without any treatment. X-ray diffraction measurements were used to index the structure of the CdS films. The surface morphology of the films were observed by Scanning Electron Microscope. UV–vis spectrophotometer was used to study the optical properties of CdS films. 3. Results and discussion 3.1. X-ray diffractions Fig.1 shows XRD patterns of CdS thin films by the CBD technique in different pH value. The identification and assignments of the observed diffraction patterns are indexed using Table 1 The table of sample list. Annealed treatments
Whether coating CdCl2
A B C D E
9.5 10 10.5 11 10.5
Yes Yes Yes Yes No
Yes Yes Yes No No
30
40
50
CdS(112)
CdS(110)
CdS(101)
d
c
c
b
b a 20
CdS(100)
intensity
CdS(112)
CdS(110)
CdS(101)
CdS(100)
intensity
CdS(111)
pH value
CdS(111)
Sample
the JPDS data. For the thin film prepared in pH ¼ 9.5 solution, a small peak located at the angle 26.21 is assigned as the (111) plane of the cubic phase. However, as pH value increases to 10, four diffraction peaks are observed, located at 24.81, 26.21, 28.21and 50.81. Three peaks at 24.81,28.21 and 50.81 are respectively associated to diffraction planes of (100), (101) and (112) of hexagonal phase. When the pH value reaches 10.5, one more diffraction peak appears at 43.71, which belongs to (110) diffraction plane of hexagonal phase. Furthermore, the other peaks become rather sharp, especially for the peaks of (111) and (101), which show good crystallinity of CdS film. These indicate that the structure of CdS film transforms from cubic structure to mixed cubic and hexagonal structure by increasing the pH value of the reaction solution. Hexagonal structure CdS has a higher transmission and good electrical conductivity in CdTe solar cells. Relative to the cubic phase, the hexagonal CdS thin films are more suitable to be n-type window layer for CdTe solar cell [11]. As the pH value increases to 11, the diffraction peaks of (111) and (110) descend slightly. The reason may be the generated large particles of CdS and Cd(OH)2 during the reactions, which will postpone the deposition of CdS on the substrate. Fig. 2 shows XRD patterns of CdS thin films by the CBD technique with different annealing temperatures when the pH value is 10.5. From the figure, the CdS film without annealing has no diffraction peaks, which shows that in this condition CdS cannot crystallize. The film annealed directly has three diffraction peaks, located at 26.21, 28.21and 43.71 respectively. The peak at 24.81 belongs to cubic phase. The other two peaks belong to hexagonal phase. Although the diffraction peaks are not very sharp, this verifies our opinion that annealing treatment is helpful for the crystallization of CdS film. When CdCl2 treatment is carried out, it is observed that CdS film has a better crystallization. Compared with the CdS film annealed directly, it has two more diffraction peaks, located at 24.81 and 50.81, which belong to hexagonal phase. Furthermore, the intensity of the peak increases after CdCl2 treatment. As a consequence, annealing treatment promotes the recrystallization of CdS thin films and results in a good crystal
60
2θ(°) θ(°) Fig. 1. XRD patterns of CdS thin films by the CBD technique in different pH solution, (a) pH¼ 9.5, (b) pH¼ 10, (c) pH¼ 10.5, and (d) pH¼11.
a
20
30
40
50
60
2θ(°) θ(°)
Fig. 2. XRD patterns of CdS thin films by the CBD technique with different annealing temperatures, (a) no annealed, (b) annealed directly, and (c) CdCl2 treated and annealed.
Please cite this article as: J. Li, Preparation and properties of CdS thin films deposited by chemical bath deposition, Ceramics International (2015), http://dx.doi. org/10.1016/j.ceramint.2015.03.160
J. Li / Ceramics International ] (]]]]) ]]]–]]]
3
Fig. 3. SEM of CdS thin films by the CBD technique in various pH solution, (a) pH¼ 9.5, (b) pH¼ 10, (c) pH¼ 10.5, and (d) pH¼11.
structure. These CdS films contain a mixture of cubic and hexagonal structures. CdCl2 treatment obviously enhanced the hexagonal phase of CdS thin film. This is because in the process of annealing, hanging with CdCl2 solution, can prevent the volatilization of Cd ion, guarantees the stoichiometric ratio between Cd and S, which benefits the recrystallization of CdS thin film. 3.2. SEM micrographs of the CdS films Fig. 3 shows SEM images of CdS thin films by the CBD technique in various pH solution. Starting from pH ¼ 9.5 solution, it can be seen that the morphology is not smooth. Lots of cracks between grain boundaries are observed. When the concentration of ammonia is low, the decomposition rate of sulfur is slow, which causes the low concentration of S anion. When pH increases to 10, the surface modification is obvious. Cracks and pinholes are fewer than those before. When the pH value is continually increased to 10.5, the surface of the CdS is continuous and compact, which has a good influence on the performance of solar cells. Nevertheless, when pH increases to 11, a few cracks appear again. It is obvious that the best deposition pH is 10.5. Fig. 4 shows SEM of CdS thin films by the CBD technique with different annealing temperatures when the pH value is 10.5. The modification of surface morphologies and roughness
is clearly observed. The grain size of as-deposited CdS is very small, which indicates poor crystallization. However, the surface of CdS film annealed directly is rather rough and not uniform. A few pinholes and cracks appear in the grain boundaries. The size of the grain grows slightly. An obvious evolution of CdS surface appears after CdCl2 treatment and annealed. The particle size obviously increases, which illustrate CdCl2 treatment can promote grain growth. CdS films with large grain size and smooth surface can effectively promote the absorption of light. It is concluded that better crystalline quality of CdS film can be obtained with CdCl2 annealing treatment. 3.3. Optical properties of CBD CdS films Optical absorbance values of CdS thin films by the CBD technique in different pH solution are shown in Fig. 5. Compared with a–d absorption spectra, absorbance peaks appear at wavelength of about 380 nm, which is consistent with results reported by Kodigala [12]. Absorption edge is gentle when pH is 9.5 and the reason is that lots of cracks between CdS grains cause light scattering. When pH increases to 10.5, surface of CdS is decorated and causes this absorbance curve to shift to the long wavelength direction, which indicates that the band gap decreases slightly. Based on literatures, the
Please cite this article as: J. Li, Preparation and properties of CdS thin films deposited by chemical bath deposition, Ceramics International (2015), http://dx.doi. org/10.1016/j.ceramint.2015.03.160
J. Li / Ceramics International ] (]]]]) ]]]–]]]
4
Fig. 4. SEM of CdS thin films by the CBD technique with different annealing temperatures, (a) no annealed, (b) annealed directly, and (c) CdCl2 treated and annealed. c
abs(a.u.)
d
350
b a
400
450
500
550
600
λ(nm)
Fig. 5. Optical absorbance values of CdS thin films by the CBD technique in different pH solution, (a) pH¼ 9.5, (b) pH¼ 10, (c) pH¼ 10.5, and (d) pH¼11.
c
abs(a.u.)
b
350
4. Conclusions
a
400
absorbance curve has a red-shift, the absorption rate is still high after the wavelength 500 nm. Therefore, the transmittance rate is relatively low, which has a bad effect on the performance of solar cells. Fig. 6 shows optical absorbance values of CdS thin films by the CBD technique with different annealing temperature. Compared to curve a, curve b and c appear to shift to the long wavelength direction. The obvious result is that the band gap of the CdS film becomes narrower with annealing treatment. There are a lot of factors that affect the band gap, such as crystalline quality of the film, the volatilization of sulfur and so on [14]. CdS film annealed with CdCl2 becomes sharper than other two curves and the absorption edge blueshift compared with curve b. This indicates that there are fewer defects and impurity energy levels in the CdS film annealed with CdCl2 [14]. Fewer defects may reduce surface scattering and grain boundary scattering, which is conducive in improving the efficiency of the solar cell. Good crystallinity of the CdS thin films under pH value 10.5 and annealed with CdCl2 was the main factor for the optical absorbance properties.
450
500
550
600
λ(nm) Fig. 6. Optical absorbance values of CdS thin films by the CBD technique with different annealing temperatures, (a) no annealed, (b) annealed directly, and (c) CdCl2 treated and annealed.
decrease of the energy band gap is produced by an increase in the structural disorder induced by the change from cubic to hexagonal phase of CdS [13]. Curve d shows that although
The physical and optical properties of CdS films deposited on ITO substrates by chemical bath deposition were investigated in this work. XRD patterns indicated that with increasing pH value of the solution, the intensity of diffraction peak increases and the film structure changes from cubic to hexagonal structure. The pH value equals 10.5 shows the best crystal structure in the films. Meanwhile, annealing treatment can obviously promote the growth of CdS films, especially for those with CdCl2 annealed treatment. The SEM micrographs of the surfaces of the film indicated that when the pH value increases to 10.5, defects and cracks vanished accordingly. And after annealing treatment, CdS grain grows and the crystallinity of films improved. The films with CdCl2 annealed were symmetric and compact with a bigger grain size and improved crystal condition than others. It was observed that
Please cite this article as: J. Li, Preparation and properties of CdS thin films deposited by chemical bath deposition, Ceramics International (2015), http://dx.doi. org/10.1016/j.ceramint.2015.03.160
J. Li / Ceramics International ] (]]]]) ]]]–]]]
the absorption spectra coefficient of CdS films was steadily increasing when pH changed from 9.5 to 10.5. Absorption spectra coefficient declined when the pH is above 10.5. In addition, the absorption edge near the band gap of the films with CdCl2 annealed had been significantly improved, and the absorption edge showed a blue-shift compared with the direct annealing, which illustrated CdS had a narrower band gap, which was conducive to improve the efficiency of CdS/CdTe solar cell. Conflict of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. References [1] M.A. Green, K. Emery, Prog. Photovolt. Res. Appl. 20 (2012) 606–614. [2] N. Romeo, A. Bosio, V. Canevari, A. Podesta, Sol. Energy 77 (2004) 795–801.
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Please cite this article as: J. Li, Preparation and properties of CdS thin films deposited by chemical bath deposition, Ceramics International (2015), http://dx.doi. org/10.1016/j.ceramint.2015.03.160