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Preparation of Cu2ZnSnS4 thin films by sulfurization of metallic precursors evaporated with a single source
SCT-17635; No of Pages 4 Surface & Coatings Technology xxx (2012) xxx–xxx
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Preparation of Cu2ZnSnS4 thin films by sulfurization of metallic precursors evaporated with a single source Xin Jiang a, b, c, Le-Xi Shao c, Jun Zhang c, Da Li c, Wei Xie c, Chang-Wei Zou c, Jian-Min Chen a,⁎ a b c
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China Graduate University of the Chinese Academy of Sciences, Beijing, 100049, China School of Physical Science and Technology, Zhanjiang Normal University, Zhanjiang 524048, China
a r t i c l e Available online xxxx Keywords: Cu2ZnSnS4 Thin film Solar cell Evaporation Sulfurization
i n f o
a b s t r a c t In this study, Cu2ZnSnS4 (CZTS) thin films were prepared on Mo-coated soda-lime glass substrates by sulfurization of precursors deposited by single-source evaporating the metallic mixture of Cu–Zn–Sn. The influences of the composition ratio of evaporation sources and sulfurization temperature on the properties of CZTS thin films were investigated. The morphology, composition and structure of CZTS thin films were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Raman scattering, respectively. The XRD patterns revealed that CZTS films from both Cu-rich and Cucorrect evaporation sources contained a Cu1.96 S phase, whereas films from Cu-poor evaporation sources showed no secondary phase and exhibited kesterite structure with a (112) plane preferred orientation. This identification was further confirmed by a Raman spectroscope. In addition, the results of XRD patterns and SEM images showed that the crystalline quality and surface morphology of CZTS thin films were greatly improved by increasing the sulfurization temperature. The CZTS thin films possessing a near-stoichiometric composition, homogeneous surface morphology and a single CZTS phase were fabricated using a Cu-poor evaporation source at the sulfurization temperature of 550 °C, and it is suitable for CZTS based high performance solar cells as absorber. © 2012 Published by Elsevier B.V.
1. Introduction At present, thin film solar cells based on Cu(In,Ga)Se2 (CIGS) absorber layer have demonstrated in laboratory efficiency of 20.1% [1]. However, this compound employs expensive and rare metals such as indium and gallium, which affects the large-scale production of these modules at low cost. The need for a conveniently deposited material containing cheap and readily available elements has led to a recent shift of research focus towards the kesterite-related of thin-film chalcogenide materials, such as Cu2ZnSnS4, Cu2ZnSnSe4, Cu2CdSnS4, Cu2ZnGeS4, etc. Among them, Cu2ZnSnS4 (CZTS) is considered to be an ideal alternative for CIGS as the absorber owing to its direct optical band gap of 1.5 eV, high optical absorption coefficient over 10 4 cm − 1 and p-type electrical conductivity [2,3], as well as the abundant and nontoxic constituents. The CZTS film deposition has been successfully achieved through a variety of techniques including atom beam sputtering [2], RF (radio frequency) magnetron sputtering [3], hybrid sputtering [4], ion beam sputtering [5], reactive magnetron sputtering [6], co-evaporation [7], pulsed laser deposition (PLD) [8–10], sulfurization of electron beam evaporated precursors [11],
⁎ Corresponding author. Tel.: + 86 931 4968018; fax: + 86 931 8277088. E-mail address: [email protected] (J.-M. Chen).
electrodeposition [12,13], spray pyrolysis [14], sol–gel sulfurizing method, [15] etc. So far, the highest conversion efficiency for pure sulfide Cu2ZnSnS4 thin film solar cells has been achieved 8.4%, where CZTS layer was produced by vacuum thermal evaporating precursor and subsequent annealing [16]. However, this efficiency was still much lower than that of the CIGS solar cells. The key barriers toward improving efficiency are the complex and incompletely understood basic nature of CZTS materials and poor controllability for chemical composition of CZTS thin films. Therefore, it is very important how to control accurately the atomic ratio of the constituents in the quaternary film, as well as a thorough understanding of the basic structural, optical and electrical properties for the films deposited under various conditions. In this paper, we presented a simple and effective method to deposit metallic precursor. Three metallic elements Cu, Zn and Sn were simultaneously evaporated in a single-source. By adjusting respectively the masses of three metallic elements in evaporation source, the atomic ratio of Cu:Sn:Zn in the precursor can be easily controlled. Compared with the approaches based on multi-source deposition [3,5,7,8], complex system and monitoring equipment are not required in our method. Thus, it is simpler and more economical for large-scale production. The effects of the composition ratios of evaporation sources and sulfurization temperature on the compositional, structural, morphological and optical properties of CZTS thin films were investigated.
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2. Experimental details The CZTS thin films were prepared by a two-step approach. Firstly, precursors were deposited by the thermal evaporation system with a single-source consisting of Cu patch, Zn and Sn powder. Cu, Zn and Sn were deposited on a Mo-coated soda lime glass substrate (Mo-coated SLG) simultaneously. Then the precursors were sulfurized in the sulfur (S) ambient of the quartz chamber of an electric furnace to obtain CZTS films. Before depositing the precursors, the substrates were thoroughly cleaned with organic solvents and deionized water. The background pressure of the evaporation chamber was evacuated below 1 × 10 − 3 Pa with a rotary mechanical pump and a diffused pump. The metallic mixture of Cu, Sn and Zn (purity 99.99%) was used as the evaporation sources. The pressure was maintained at 2 × 10 − 3 Pa during evaporation. The thicknesses of the precursors could be well adjusted by the mass of evaporation sources. After depositing, the precursors were immediately transferred to the electric furnace for sulfurization. The sulfurization followed the same procedure described in previous works [5]. To reduce the loss of Zn in the precursors during the sulfurization processes, the evaporation sources for all samples were designed to be Zn-rich with a ratio of Zn/Sn = 1.25. To study the effect of the elemental ratio of evaporation source on the formation of CZTS films, the sample Nos. 1 to 3 were fabricated with different ratios of Cu/ (Sn + Zn) of evaporation source keeping sulfurization temperature at 550 °C. The sample Nos. 3 to 5 were prepared with the same evaporation source at varied sulfurization temperatures to optimize the sulfurization conditions. The chemical composition of evaporation sources and sulfurization temperatures were listed in Table 1. In addition, in order to evaluate the optical properties, all samples were also deposited on a soda lime glass substrate (SLG) under the same conditions. The structural properties of the films were characterized by conventional θ–2θ X-ray diffraction analysis using Cu Kα radiation (XRD, Rigaku D/Max-IIIC) and Confocal Raman spectrometer (Renishaw inVia micro-Raman system) excited by Ar + laser with wavelength of 488 nm, respectively. The surface morphology, chemical composition and optical properties of the films were analyzed by scanning electron microscope (SEM Hitachi S4800), energy dispersive spectrometry (EDS, Hitachi S4800) and UV–VIS spectrophotometer (UV-3000), respectively.
3. Results and discussion The dependences of the composition of the resulting films grown at the different temperatures on the compositions of evaporation sources were demonstrated by the EDS measurement and shown in Table 1. Obviously, the ratio of Zn/Sn in all films decreases after sulfurization, while the ratio of Cu/(Zn + Sn) increases. These changes can be attributed to the re-evaporation of zinc from thin film surface during the sulfurization period. It is possible to compensate for Zn loss at the sulfurization stage by using binary compound zinc sulfide
instead of the zinc element as a Zn-source [17]. In sample No. 1, excess Cu was detected as a result of the Cu-rich composition in evaporation source and the re-evaporation of Zn. By gradually decreasing the ratio of Cu in the source, the Cu content of CZTS thin films could be reduced proportionally, as revealed in the sample Nos. 2 and 3. In addition, the composition ratio of sulfur reaches about 50% for all samples after the sulfurization, which indicates that sufficient sulfurization was achieved. No significant influence of different sulfurization temperatures on the elemental ratios can be observed, except a slight decrease in the Zn/Sn ratio at higher temperatures. It can be seen from Table 1 that the sample No. 3 deposited from Cupoor source at the sulfurization temperature of 550 °C was nearly stoichiometric with Cu/(Zn + Sn) ratio of 0.914, Zn/Sn ratio of 1.105 and S/metal ratio of 1.01. Such near-stoichiometric but slightly Cupoor and Zn-rich chemical composition can lead to good optoelectronic properties according to previous research by Katagiri et al. [18]. It is concluded from above results that the atomic ratio of Cu: Sn:Zn in CZTS thin films deposited by sulfurizing single-source evaporated precursor can be effectively controlled by adjusting the constituent ratio of the evaporation sources. Fig. 1 shows X-ray diffraction patterns of CZTS films deposited on Mo/SLG substrates. All films are observed to be grown well with polycrystalline configuration, and the XRD peaks were attributed to the (1 1 2), (2 0 0), (2 2 0) and (3 1 2) planes of CZTS (JCPDS 26-0575) and to Mo. A preferential (112) orientation was observed for all deposited films. As shown in Fig. 1(a) and (b), the sample Nos. 1 and 2 deposited from Cu-rich and Cu-correct sources, respectively, present peaks corresponding to Cu1.96 S (JCPDS 12-0174). This binary sulfides detected in the sample Nos. 1 and 2 originated from the partial reaction of the excessive Cu with S during sulfurization process. Whereas, for the sample No. 3 from Cu-poor sources (see Fig. 1(c)), only peaks corresponding to CZTS are observed indicating that films might be single phase since the pattern does not explicitly show any peak corresponding to secondary phase. Actually, the diffraction pattern of CZTS is very similar to that of β-ZnS due to their similar crystal structure [19]. It is difficult for XRD analysis to make a distinction between CZTS and β-ZnS. Therefore, the identification of the CZTS samples was further conducted on a Raman spectroscope. Fig. 2 illustrates the typical Raman spectrum of the sample No. 3 from Cu-poor evaporation source. From the spectrum, we can obviously observe three peaks at ~ 288, ~ 338, and ~368 cm − 1, which correspond to quaternary CZTS. This result is in good agreement with the Raman spectrum of CZTS film reported by Francesco and Ahmed et al. [20,21]. Moreover, it is apparent that there are no extra peaks related to the presence of other compounds, which means that the single phase CZTS film was obtained from Cu-poor source. With the increase of the Cu/(Zn + Sn) ratio, the intensity of (1 1 2) diffraction peak
Table 1 Compositional ratios of evaporation sources and CZTS films grown at different sulfurization temperatures. Sample
No. No. No. No. No.
1 2 3 4 5
Compositional ratio of sources Cu/(Zn + Sn)
Zn/Sn
1.1 1.0 0.85 0.85 0.85
1.25 1.25 1.25 1.25 1.25
Sulfurization temperature (°C)
Compositional ratio of CZTS Cu/(Zn + Sn)
Zn/Sn
S/Metal
550 550 550 500 450
1.19 1.07 0.93 0.92 0.93
1.10 1.11 1.08 1.10 1.13
1.01 1.03 0.98 1.00 1.01
Fig. 1. XRD spectra of CZTS films. (a)–(e) corresponds to sample Nos. 1–5, respectively.
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Fig. 2. Typical Raman spectra of CZTS film (sample No. 3 from Cu-poor source).
becomes relatively more intense and sharp. It indicates that the crystalline quality of the CZTS films becomes better under Cu-rich growth condition, which is consistent with the result in literature [7]. The XRD patterns of the CZTS films synthesized at various sulfurization temperatures (550 °C, 500 °C, 450 °C) are also shown in Fig. 1(c)–(e), respectively. All the CZTS films exhibit the kesterite structure with preferential orientation along the (1 1 2) direction. For all samples, no additional peaks corresponding to any secondary phase can be seen, which suggested that single-phase CZTS films were prepared at various sulfurization temperatures. With the increasing sulfurization temperature, the intensity and the full width at half maximum (FWHM) of the diffraction peak from the (1 1 2) plane become strong and narrow, respectively, indicating that higher sulfurization temperature is beneficial to the improvement of crystallinity for CZTS films. Fig. 3 shows SEM micrographs of the samples deposited with different composition ratio of evaporation sources. It is found that with the increasing Cu/(Zn + Sn) ratio, the film surface is rougher and more porous although the grain size is increased, which is detrimental to the improvement of conversion efficiency. Moreover, the film surface becomes relatively more uniform, and the grain size becomes
3
larger with the increasing sulfurization temperature (not shown), indicating that the grain growth is enhanced at higher temperature. This trend accords with the results of XRD analysis. From the cross section of the sample No. 3 as shown in Fig. 3(d), it can be seen that the film is densely packed with thickness around 1 μm. There are no voids between the film and the Mo-coated SLG, indicating a good adhesion of the film to the substrate. The sample No. 3 deposited from Cu-poor source at sulfurization temperature of 550 °C shows a relative smooth surface and densely packed grains with the size of about 1 μm, which is very desirable for the fabrication of a high efficiency solar cell. From the above analysis, it is concluded that the surface morphology of CZTS thin film is strongly dependent on the ratio of Cu/(Zn + Sn) as well as sulfurization temperature. Fig. 4 shows an UV–VIS absorption spectrum of the sample No. 3 deposited on a SLG substrate where α is the absorption coefficient and hν is the photon energy. The absorption coefficient in the visible region was larger than 10 4 cm − 1. The direct optical band gap of the CZTS films was determined to be 1.52 eV by extrapolating the linear part of curve in Fig. 4 to (αhν) 2 = 0. These optical properties indicate that the sample No. 3 is suitable for the absorber layer of thin film solar cells.
4. Conclusion We have prepared the CZTS thin films by sulfurization of metallic precursor deposited by single-source evaporating the mixture of Cu– Zn–Sn. It has been found that the constituent ratio of the evaporation source has a significant effect on the composition, microstructure, and morphology of CZTS thin films. The ratio of Zn/Sn in all films decreases after sulfurization, while the ratio of Cu/(Zn + Sn) increases, which is due to the re-evaporation of volatile constituents during the sulfurization period. The CZTS films from both Cu-rich and Cucorrect evaporation sources contained a Cu1.96 S phase, whereas the films from Cu-poor evaporation sources showed no secondary phase and exhibited kesterite structure with a (112) plane preferred orientation. In addition, the crystallinity and grain size of CZTS films were improved at relatively high sulfurization temperature.
Fig. 3. SEM images of the CZTS films fabricated using different composition of evaporation sources. (a) Top view for sample No. 1; (b) top view for sample No. 2; (c) top view for sample No. 3; and (d) cross-sectional view for sample No. 3.
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References
Fig. 4. Plot of (αhν)2 vs. hν for the estimation of the band gap energy of sample No. 3 grown on a SLG substrate.
It is concluded that the composition ratio of Cu:Zn:Sn in CZTS thin films can be easily and effectively controlled by adjusting the constituent ratio of the evaporation source. The CZTS thin film fabricated from Cu-poor evaporation source at the sulfurization temperature of 550 °C possesses a near-stoichiometric composition, pure CZTS phase, homogeneous surface morphology and optimum optical band gap of 1.5 eV, and it is suitable to be used as absorber for CZTS based high performance solar cells.
[1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovoltaics Res. Appl. (2011), http://dx.doi.org/10.1002/pip.1078. [2] K. Ito, T. Nakazawa, Jpn. J. Appl. Phys. 27 (1988) 2094. [3] J.S. Seol, S.Y. Lee, J.C. Lee, H.D. Nam, K.H. Kim, Sol. Energy Mater. Sol. Cells 75 (2003) 155. [4] T. Tanaka, T. Nagatomo, D. Kawasaki, M. Nishio, Q. Guo, A. Wakahara, A. Yoshida, H. Ogawa, J. Phys. Chem. Solids 66 (2005) 1978. [5] J. Zhang, L.X. Shao, Sci. Chin. Ser. E: Technol. Sci. 52 (2009) 269. [6] F.Y. Liu, Y. Li, K. Zhang, B. Wang, C. Yan, Y.Q. Lai, Z.A. Zhang, J. Li, Y.X. Liu, Sol. Energy Mater. Sol. Cells 94 (2010) 2431. [7] T. Tanaka, A. Yoshida, D. Saiki, K. Saito, Q. Guo, M. Nishio, T. Yamaguchi, Thin Solid Films 518 (2010) S29. [8] A.V. Moholkar, S.S. Shinde, A.R. Babar, K.U. Sim, Y.B. Kwon, K.Y. Rajpure, P.S. Patil, C.H. Bhosale, J.H. Kim, Sol. Energy 85 (2011) 1354. [9] K. Moriya, K. Tanaka, H. Uchiki, Jpn. J. Appl. Phys. 47 (2008) 602. [10] S.M. Pawar, A.V. Moholkar, I.K. Kim, S.W. Shin, J.H. Moon, J.I. Rhee, J.H. Kim, Curr. Appl. Phys. 10 (2010) 565. [11] H. Katagiri, K. Saitoh, T. Washio, H. Shinohara, T. Kurumadani, S. Miyajima, Sol. Energy Mater. Sol. Cells 65 (2001) 141. [12] C.P. Chan, H. Lam, C. Surya, Sol. Energy Mater. Sol. Cells 94 (2010) 207. [13] X. Zhang, X. Shi, W. Ye, C. Ma, C. Wang, Appl. Phys. A 94 (2009) 381. [14] Y.B. Kishore Kumar, G. Suresh Babu, P. Uday Bhaskar, V. Sundara Raja, Sol. Energy Mater. Sol. Cells 93 (2009) 1230. [15] K. Tanaka, M. Oonuki, N. Moritake, H. Uchiki, Sol. Energy Mater. Sol. Cells 93 (2009) 583. [16] B. Shin, O. Gunawan, Y. Zhu, N.A. Bojarczuk, S. Jay Chey, S. Guha, Prog. Photovoltaics Res. Appl. (2011) 1174. [17] Th.M. Friedlmeier, N. Wieser, Th. Walter, H. Dittrich, H.W. Schock, Proceeding of 14th European PVSEC and Exhibition, 1997, P4B.10. [18] H. Katagiri, Thin Solid Films 480 (2005) 426. [19] P.A. Fernandes, P.M.P. Salomé, S.F. da Cunha, J. Alloys Compd. 509 (2011) 7600. [20] B. Francesco, C. Rosa, V. Matteo, Energy Procedia 10 (2011) 187. [21] S. Ahmed, K.B. Reuter, O. Gunawan, L. Guo, L.T. Romankiw, H. Deligianni, Adv. Energy Mater. 2 (2012) 253.
Acknowledgments The authors thank the National Natural Science Foundation of 435 China (Grant No. 50705093 and No. 50575217), the Innovative Group 436 Foundation from NSFC (Grant No. 50421502) and the National 973 437 Project (No. 2007 CB607601) for financial support.
Please cite this article as: X. Jiang, et al., Surf. Coat. Technol. (2012), doi:10.1016/j.surfcoat.2012.05.057
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Preparation of Cu2ZnSnS4 thin films by sulfurization of metallic precursors evaporated with a single source Xin Jiang a, b, c, Le-Xi Shao c, Jun Zhang c, Da Li c, Wei Xie c, Chang-Wei Zou c, Jian-Min Chen a,⁎ a b c
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China Graduate University of the Chinese Academy of Sciences, Beijing, 100049, China School of Physical Science and Technology, Zhanjiang Normal University, Zhanjiang 524048, China
a r t i c l e Available online xxxx Keywords: Cu2ZnSnS4 Thin film Solar cell Evaporation Sulfurization
i n f o
a b s t r a c t In this study, Cu2ZnSnS4 (CZTS) thin films were prepared on Mo-coated soda-lime glass substrates by sulfurization of precursors deposited by single-source evaporating the metallic mixture of Cu–Zn–Sn. The influences of the composition ratio of evaporation sources and sulfurization temperature on the properties of CZTS thin films were investigated. The morphology, composition and structure of CZTS thin films were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Raman scattering, respectively. The XRD patterns revealed that CZTS films from both Cu-rich and Cucorrect evaporation sources contained a Cu1.96 S phase, whereas films from Cu-poor evaporation sources showed no secondary phase and exhibited kesterite structure with a (112) plane preferred orientation. This identification was further confirmed by a Raman spectroscope. In addition, the results of XRD patterns and SEM images showed that the crystalline quality and surface morphology of CZTS thin films were greatly improved by increasing the sulfurization temperature. The CZTS thin films possessing a near-stoichiometric composition, homogeneous surface morphology and a single CZTS phase were fabricated using a Cu-poor evaporation source at the sulfurization temperature of 550 °C, and it is suitable for CZTS based high performance solar cells as absorber. © 2012 Published by Elsevier B.V.
1. Introduction At present, thin film solar cells based on Cu(In,Ga)Se2 (CIGS) absorber layer have demonstrated in laboratory efficiency of 20.1% [1]. However, this compound employs expensive and rare metals such as indium and gallium, which affects the large-scale production of these modules at low cost. The need for a conveniently deposited material containing cheap and readily available elements has led to a recent shift of research focus towards the kesterite-related of thin-film chalcogenide materials, such as Cu2ZnSnS4, Cu2ZnSnSe4, Cu2CdSnS4, Cu2ZnGeS4, etc. Among them, Cu2ZnSnS4 (CZTS) is considered to be an ideal alternative for CIGS as the absorber owing to its direct optical band gap of 1.5 eV, high optical absorption coefficient over 10 4 cm − 1 and p-type electrical conductivity [2,3], as well as the abundant and nontoxic constituents. The CZTS film deposition has been successfully achieved through a variety of techniques including atom beam sputtering [2], RF (radio frequency) magnetron sputtering [3], hybrid sputtering [4], ion beam sputtering [5], reactive magnetron sputtering [6], co-evaporation [7], pulsed laser deposition (PLD) [8–10], sulfurization of electron beam evaporated precursors [11],
⁎ Corresponding author. Tel.: + 86 931 4968018; fax: + 86 931 8277088. E-mail address: [email protected] (J.-M. Chen).
electrodeposition [12,13], spray pyrolysis [14], sol–gel sulfurizing method, [15] etc. So far, the highest conversion efficiency for pure sulfide Cu2ZnSnS4 thin film solar cells has been achieved 8.4%, where CZTS layer was produced by vacuum thermal evaporating precursor and subsequent annealing [16]. However, this efficiency was still much lower than that of the CIGS solar cells. The key barriers toward improving efficiency are the complex and incompletely understood basic nature of CZTS materials and poor controllability for chemical composition of CZTS thin films. Therefore, it is very important how to control accurately the atomic ratio of the constituents in the quaternary film, as well as a thorough understanding of the basic structural, optical and electrical properties for the films deposited under various conditions. In this paper, we presented a simple and effective method to deposit metallic precursor. Three metallic elements Cu, Zn and Sn were simultaneously evaporated in a single-source. By adjusting respectively the masses of three metallic elements in evaporation source, the atomic ratio of Cu:Sn:Zn in the precursor can be easily controlled. Compared with the approaches based on multi-source deposition [3,5,7,8], complex system and monitoring equipment are not required in our method. Thus, it is simpler and more economical for large-scale production. The effects of the composition ratios of evaporation sources and sulfurization temperature on the compositional, structural, morphological and optical properties of CZTS thin films were investigated.
0257-8972/$ – see front matter © 2012 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2012.05.057
Please cite this article as: X. Jiang, et al., Surf. Coat. Technol. (2012), doi:10.1016/j.surfcoat.2012.05.057
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2. Experimental details The CZTS thin films were prepared by a two-step approach. Firstly, precursors were deposited by the thermal evaporation system with a single-source consisting of Cu patch, Zn and Sn powder. Cu, Zn and Sn were deposited on a Mo-coated soda lime glass substrate (Mo-coated SLG) simultaneously. Then the precursors were sulfurized in the sulfur (S) ambient of the quartz chamber of an electric furnace to obtain CZTS films. Before depositing the precursors, the substrates were thoroughly cleaned with organic solvents and deionized water. The background pressure of the evaporation chamber was evacuated below 1 × 10 − 3 Pa with a rotary mechanical pump and a diffused pump. The metallic mixture of Cu, Sn and Zn (purity 99.99%) was used as the evaporation sources. The pressure was maintained at 2 × 10 − 3 Pa during evaporation. The thicknesses of the precursors could be well adjusted by the mass of evaporation sources. After depositing, the precursors were immediately transferred to the electric furnace for sulfurization. The sulfurization followed the same procedure described in previous works [5]. To reduce the loss of Zn in the precursors during the sulfurization processes, the evaporation sources for all samples were designed to be Zn-rich with a ratio of Zn/Sn = 1.25. To study the effect of the elemental ratio of evaporation source on the formation of CZTS films, the sample Nos. 1 to 3 were fabricated with different ratios of Cu/ (Sn + Zn) of evaporation source keeping sulfurization temperature at 550 °C. The sample Nos. 3 to 5 were prepared with the same evaporation source at varied sulfurization temperatures to optimize the sulfurization conditions. The chemical composition of evaporation sources and sulfurization temperatures were listed in Table 1. In addition, in order to evaluate the optical properties, all samples were also deposited on a soda lime glass substrate (SLG) under the same conditions. The structural properties of the films were characterized by conventional θ–2θ X-ray diffraction analysis using Cu Kα radiation (XRD, Rigaku D/Max-IIIC) and Confocal Raman spectrometer (Renishaw inVia micro-Raman system) excited by Ar + laser with wavelength of 488 nm, respectively. The surface morphology, chemical composition and optical properties of the films were analyzed by scanning electron microscope (SEM Hitachi S4800), energy dispersive spectrometry (EDS, Hitachi S4800) and UV–VIS spectrophotometer (UV-3000), respectively.
3. Results and discussion The dependences of the composition of the resulting films grown at the different temperatures on the compositions of evaporation sources were demonstrated by the EDS measurement and shown in Table 1. Obviously, the ratio of Zn/Sn in all films decreases after sulfurization, while the ratio of Cu/(Zn + Sn) increases. These changes can be attributed to the re-evaporation of zinc from thin film surface during the sulfurization period. It is possible to compensate for Zn loss at the sulfurization stage by using binary compound zinc sulfide
instead of the zinc element as a Zn-source [17]. In sample No. 1, excess Cu was detected as a result of the Cu-rich composition in evaporation source and the re-evaporation of Zn. By gradually decreasing the ratio of Cu in the source, the Cu content of CZTS thin films could be reduced proportionally, as revealed in the sample Nos. 2 and 3. In addition, the composition ratio of sulfur reaches about 50% for all samples after the sulfurization, which indicates that sufficient sulfurization was achieved. No significant influence of different sulfurization temperatures on the elemental ratios can be observed, except a slight decrease in the Zn/Sn ratio at higher temperatures. It can be seen from Table 1 that the sample No. 3 deposited from Cupoor source at the sulfurization temperature of 550 °C was nearly stoichiometric with Cu/(Zn + Sn) ratio of 0.914, Zn/Sn ratio of 1.105 and S/metal ratio of 1.01. Such near-stoichiometric but slightly Cupoor and Zn-rich chemical composition can lead to good optoelectronic properties according to previous research by Katagiri et al. [18]. It is concluded from above results that the atomic ratio of Cu: Sn:Zn in CZTS thin films deposited by sulfurizing single-source evaporated precursor can be effectively controlled by adjusting the constituent ratio of the evaporation sources. Fig. 1 shows X-ray diffraction patterns of CZTS films deposited on Mo/SLG substrates. All films are observed to be grown well with polycrystalline configuration, and the XRD peaks were attributed to the (1 1 2), (2 0 0), (2 2 0) and (3 1 2) planes of CZTS (JCPDS 26-0575) and to Mo. A preferential (112) orientation was observed for all deposited films. As shown in Fig. 1(a) and (b), the sample Nos. 1 and 2 deposited from Cu-rich and Cu-correct sources, respectively, present peaks corresponding to Cu1.96 S (JCPDS 12-0174). This binary sulfides detected in the sample Nos. 1 and 2 originated from the partial reaction of the excessive Cu with S during sulfurization process. Whereas, for the sample No. 3 from Cu-poor sources (see Fig. 1(c)), only peaks corresponding to CZTS are observed indicating that films might be single phase since the pattern does not explicitly show any peak corresponding to secondary phase. Actually, the diffraction pattern of CZTS is very similar to that of β-ZnS due to their similar crystal structure [19]. It is difficult for XRD analysis to make a distinction between CZTS and β-ZnS. Therefore, the identification of the CZTS samples was further conducted on a Raman spectroscope. Fig. 2 illustrates the typical Raman spectrum of the sample No. 3 from Cu-poor evaporation source. From the spectrum, we can obviously observe three peaks at ~ 288, ~ 338, and ~368 cm − 1, which correspond to quaternary CZTS. This result is in good agreement with the Raman spectrum of CZTS film reported by Francesco and Ahmed et al. [20,21]. Moreover, it is apparent that there are no extra peaks related to the presence of other compounds, which means that the single phase CZTS film was obtained from Cu-poor source. With the increase of the Cu/(Zn + Sn) ratio, the intensity of (1 1 2) diffraction peak
Table 1 Compositional ratios of evaporation sources and CZTS films grown at different sulfurization temperatures. Sample
No. No. No. No. No.
1 2 3 4 5
Compositional ratio of sources Cu/(Zn + Sn)
Zn/Sn
1.1 1.0 0.85 0.85 0.85
1.25 1.25 1.25 1.25 1.25
Sulfurization temperature (°C)
Compositional ratio of CZTS Cu/(Zn + Sn)
Zn/Sn
S/Metal
550 550 550 500 450
1.19 1.07 0.93 0.92 0.93
1.10 1.11 1.08 1.10 1.13
1.01 1.03 0.98 1.00 1.01
Fig. 1. XRD spectra of CZTS films. (a)–(e) corresponds to sample Nos. 1–5, respectively.
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Fig. 2. Typical Raman spectra of CZTS film (sample No. 3 from Cu-poor source).
becomes relatively more intense and sharp. It indicates that the crystalline quality of the CZTS films becomes better under Cu-rich growth condition, which is consistent with the result in literature [7]. The XRD patterns of the CZTS films synthesized at various sulfurization temperatures (550 °C, 500 °C, 450 °C) are also shown in Fig. 1(c)–(e), respectively. All the CZTS films exhibit the kesterite structure with preferential orientation along the (1 1 2) direction. For all samples, no additional peaks corresponding to any secondary phase can be seen, which suggested that single-phase CZTS films were prepared at various sulfurization temperatures. With the increasing sulfurization temperature, the intensity and the full width at half maximum (FWHM) of the diffraction peak from the (1 1 2) plane become strong and narrow, respectively, indicating that higher sulfurization temperature is beneficial to the improvement of crystallinity for CZTS films. Fig. 3 shows SEM micrographs of the samples deposited with different composition ratio of evaporation sources. It is found that with the increasing Cu/(Zn + Sn) ratio, the film surface is rougher and more porous although the grain size is increased, which is detrimental to the improvement of conversion efficiency. Moreover, the film surface becomes relatively more uniform, and the grain size becomes
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larger with the increasing sulfurization temperature (not shown), indicating that the grain growth is enhanced at higher temperature. This trend accords with the results of XRD analysis. From the cross section of the sample No. 3 as shown in Fig. 3(d), it can be seen that the film is densely packed with thickness around 1 μm. There are no voids between the film and the Mo-coated SLG, indicating a good adhesion of the film to the substrate. The sample No. 3 deposited from Cu-poor source at sulfurization temperature of 550 °C shows a relative smooth surface and densely packed grains with the size of about 1 μm, which is very desirable for the fabrication of a high efficiency solar cell. From the above analysis, it is concluded that the surface morphology of CZTS thin film is strongly dependent on the ratio of Cu/(Zn + Sn) as well as sulfurization temperature. Fig. 4 shows an UV–VIS absorption spectrum of the sample No. 3 deposited on a SLG substrate where α is the absorption coefficient and hν is the photon energy. The absorption coefficient in the visible region was larger than 10 4 cm − 1. The direct optical band gap of the CZTS films was determined to be 1.52 eV by extrapolating the linear part of curve in Fig. 4 to (αhν) 2 = 0. These optical properties indicate that the sample No. 3 is suitable for the absorber layer of thin film solar cells.
4. Conclusion We have prepared the CZTS thin films by sulfurization of metallic precursor deposited by single-source evaporating the mixture of Cu– Zn–Sn. It has been found that the constituent ratio of the evaporation source has a significant effect on the composition, microstructure, and morphology of CZTS thin films. The ratio of Zn/Sn in all films decreases after sulfurization, while the ratio of Cu/(Zn + Sn) increases, which is due to the re-evaporation of volatile constituents during the sulfurization period. The CZTS films from both Cu-rich and Cucorrect evaporation sources contained a Cu1.96 S phase, whereas the films from Cu-poor evaporation sources showed no secondary phase and exhibited kesterite structure with a (112) plane preferred orientation. In addition, the crystallinity and grain size of CZTS films were improved at relatively high sulfurization temperature.
Fig. 3. SEM images of the CZTS films fabricated using different composition of evaporation sources. (a) Top view for sample No. 1; (b) top view for sample No. 2; (c) top view for sample No. 3; and (d) cross-sectional view for sample No. 3.
Please cite this article as: X. Jiang, et al., Surf. Coat. Technol. (2012), doi:10.1016/j.surfcoat.2012.05.057
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References
Fig. 4. Plot of (αhν)2 vs. hν for the estimation of the band gap energy of sample No. 3 grown on a SLG substrate.
It is concluded that the composition ratio of Cu:Zn:Sn in CZTS thin films can be easily and effectively controlled by adjusting the constituent ratio of the evaporation source. The CZTS thin film fabricated from Cu-poor evaporation source at the sulfurization temperature of 550 °C possesses a near-stoichiometric composition, pure CZTS phase, homogeneous surface morphology and optimum optical band gap of 1.5 eV, and it is suitable to be used as absorber for CZTS based high performance solar cells.
[1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovoltaics Res. Appl. (2011), http://dx.doi.org/10.1002/pip.1078. [2] K. Ito, T. Nakazawa, Jpn. J. Appl. Phys. 27 (1988) 2094. [3] J.S. Seol, S.Y. Lee, J.C. Lee, H.D. Nam, K.H. Kim, Sol. Energy Mater. Sol. Cells 75 (2003) 155. [4] T. Tanaka, T. Nagatomo, D. Kawasaki, M. Nishio, Q. Guo, A. Wakahara, A. Yoshida, H. Ogawa, J. Phys. Chem. Solids 66 (2005) 1978. [5] J. Zhang, L.X. Shao, Sci. Chin. Ser. E: Technol. Sci. 52 (2009) 269. [6] F.Y. Liu, Y. Li, K. Zhang, B. Wang, C. Yan, Y.Q. Lai, Z.A. Zhang, J. Li, Y.X. Liu, Sol. Energy Mater. Sol. Cells 94 (2010) 2431. [7] T. Tanaka, A. Yoshida, D. Saiki, K. Saito, Q. Guo, M. Nishio, T. Yamaguchi, Thin Solid Films 518 (2010) S29. [8] A.V. Moholkar, S.S. Shinde, A.R. Babar, K.U. Sim, Y.B. Kwon, K.Y. Rajpure, P.S. Patil, C.H. Bhosale, J.H. Kim, Sol. Energy 85 (2011) 1354. [9] K. Moriya, K. Tanaka, H. Uchiki, Jpn. J. Appl. Phys. 47 (2008) 602. [10] S.M. Pawar, A.V. Moholkar, I.K. Kim, S.W. Shin, J.H. Moon, J.I. Rhee, J.H. Kim, Curr. Appl. Phys. 10 (2010) 565. [11] H. Katagiri, K. Saitoh, T. Washio, H. Shinohara, T. Kurumadani, S. Miyajima, Sol. Energy Mater. Sol. Cells 65 (2001) 141. [12] C.P. Chan, H. Lam, C. Surya, Sol. Energy Mater. Sol. Cells 94 (2010) 207. [13] X. Zhang, X. Shi, W. Ye, C. Ma, C. Wang, Appl. Phys. A 94 (2009) 381. [14] Y.B. Kishore Kumar, G. Suresh Babu, P. Uday Bhaskar, V. Sundara Raja, Sol. Energy Mater. Sol. Cells 93 (2009) 1230. [15] K. Tanaka, M. Oonuki, N. Moritake, H. Uchiki, Sol. Energy Mater. Sol. Cells 93 (2009) 583. [16] B. Shin, O. Gunawan, Y. Zhu, N.A. Bojarczuk, S. Jay Chey, S. Guha, Prog. Photovoltaics Res. Appl. (2011) 1174. [17] Th.M. Friedlmeier, N. Wieser, Th. Walter, H. Dittrich, H.W. Schock, Proceeding of 14th European PVSEC and Exhibition, 1997, P4B.10. [18] H. Katagiri, Thin Solid Films 480 (2005) 426. [19] P.A. Fernandes, P.M.P. Salomé, S.F. da Cunha, J. Alloys Compd. 509 (2011) 7600. [20] B. Francesco, C. Rosa, V. Matteo, Energy Procedia 10 (2011) 187. [21] S. Ahmed, K.B. Reuter, O. Gunawan, L. Guo, L.T. Romankiw, H. Deligianni, Adv. Energy Mater. 2 (2012) 253.
Acknowledgments The authors thank the National Natural Science Foundation of 435 China (Grant No. 50705093 and No. 50575217), the Innovative Group 436 Foundation from NSFC (Grant No. 50421502) and the National 973 437 Project (No. 2007 CB607601) for financial support.
Please cite this article as: X. Jiang, et al., Surf. Coat. Technol. (2012), doi:10.1016/j.surfcoat.2012.05.057