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Cu2ZnSnS4 thin films: Facile and cost-effective preparation by RF-magnetron sputtering and texture control
Journal of Alloys and Compounds 552 (2013) 418–422
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Cu2ZnSnS4 thin films: Facile and cost-effective preparation by RF-magnetron sputtering and texture control Jiansheng Wang, Song Li, Jiajia Cai, Bo Shen, Yuping Ren, Gaowu Qin ⇑ Key Lab for Anisotropy and Texture of Materials (MoE), Northeastern University, Shenyang 110819, China
a r t i c l e
i n f o
Article history: Received 10 August 2012 Received in revised form 10 November 2012 Accepted 15 November 2012 Available online 22 November 2012 Keywords: Cu2ZnSnS4 (CZTS) RF magnetron sputtering Photovoltaic semiconductor Texture control
a b s t r a c t Cu2ZnSnS4 photovoltaic semiconductor films (CZTS) were deposited by RF magnetron sputtering using a home-made Cu2S–ZnS–SnS2–S mixed powder target. The phase information of the CZTS films was studied via X-ray diffraction and Raman scattering techniques. The films prepared at substrate temperatures between 150 °C and 200 °C exhibited strong preferential orientation of grains along <1 1 2>. Higher substrate temperature (over 200 °C) induced a flower-like structure of CZTS films without preferred orientation and the light absorption coefficients were largely enhanced. The energy band gaps of CZTS films deposited at different substrate temperatures were approximate 1.7 eV. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Quaternary Cu2ZnSnS4 (CZTS) is the most promising photovoltaic absorber layer to replace Cu(In, Ga)Se2 (CIGS) because the CZTS is comprised of abundant and inexpensive elements and in addition has a high absorption coefficient of 104 cm1 [1]. Theoretical solar conversion efficiency of CZTS-based thin films solar cells is estimated as 32.2% [2]. Therefore, much effort has been devoted to the fabrication and chemical composition control of CZTS thin films in the past several years, in which liquid chemical routes and sulfuration of metallic films are typical ways for a majority of reported fabrication methods [3–7]. However, these methods are generally processing complex and pollution present, especially for liquid chemical methods. As a film deposition method, magnetron sputtering have advantages such as simplicity and precise in chemical composition control, matching with tradition solar cell production line, as well as easy scale-up. However, little work has been reported so far to prepare CZTS thin films by single step sputtering. For solar cell applications, controlling the texture or preferred orientation of absorber layer is helpful for optimizing the performance from two aspects. Firstly, proper crystalline orientation may decrease recombination of charge carriers [8], thus the con-
⇑ Corresponding author. Tel: +86 24 8369 1565, fax: +86 24 83686455. E-mail address: [email protected] (G. Qin). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.082
version efficiency can be enhanced [9]. For instance, Hanna [8] investigated differently textured Cu(In, Ga)Se2 absorber films and found that a preferred (220/204) surface orientation lead to higher efficiencies. The enhancement was assigned to specific grain boundaries with negative or neutral charges which were favorable for reducing the electronic activity and thus the recombination of charge carriers [8,10]. Secondly, depending on their structures, semiconductors demonstrate anisotropic light absorption [11] which has been used to enhance the photovoltaic and/or photoelectrochemical performance of semiconducting devices [12,13]. In this study, RF magnetron sputtering was used to prepare quaternary CZTS films on glass substrates by using a homemade target. The growth of the films and their structures and textures, morphologies, optical properties were investigated in detail. 2. Experimental Cu2ZnSnS4 films were deposited on corning 7059 glass substrates by RF magnetron sputtering. The CZTS compound target was prepared by sintering at 750 °C under high purity argon atmosphere, using solid-state reaction of Cu2S, ZnS, SnS2 and S fine powders. Prior to film deposition, the sputter vacuum chamber was evacuated to less than 5 105 Pa. High purity Ar was introduced as working gas with a flow rate of 30 sccm, and the sputtering pressure was maintained at 0.6 Pa. The structure of the CZTS thin films was analyzed by X-ray diffraction (XRD, PW3040/60 X’Pert PRO, Cu Ka). The surface morphologies of the CZTS thin films were investigated by field-emission scanning electron microscopy (FE-SEM, JEOL JEM 7001F) and atomic force microscopy (AFM). The sample compositions were determined using attached energy dispersive X-ray analysis (EDS) system. The optical property of the films was measured using a UV–VIS-NIR Spectrophotometer (Lambda-650, PerkinElmer). The film thickness was measured by surface profiler (Dektak 150).
J. Wang et al. / Journal of Alloys and Compounds 552 (2013) 418–422
Fig. 1. Chemical composition of as-prepared CZTS thin films as a function of substrate temperature.
3. Results and discussion Fig. 1 presents the composition variation of as-sputtered CZTS thin films prepared at different substrate temperatures. All CZTS thin films deposited at substrate temperatures between 100 °C and 300 °C were close to the stoichiometric one in composition,
Fig. 2. CZTS films deposited at different substrate temperatures, (a) XRD patterns; (b) Raman spectra.
419
in which the copper content increased with the substrate temperature while the Zn content decreased accordingly, probably arising from the evaporating of Zn. Similar correlation between composition of CZTS films and substrate temperature had been observed in other preparation techniques such as chemical evaporation [14,15]. Fig. 2(a) shows XRD patterns of CZTS thin films with different substrate temperatures. No diffraction peak was observed for thin film deposited at room temperature, indicating the amorphous structure. At higher substrate temperature, diffraction peaks that can be indexed to tetragonal Cu2ZnSnS4 (ICDD No. 26-575) appeared on the spectra. Specifically, diffraction patterns of films obtained at substrate temperature of 150 °C and 200 °C show strong preferential orientation of (1 1 2) plane of CZTS parallel to the substrate. The orientated growth of the CZTS films is speculated to result from the surface energy competition at low substrate temperature [16] since the surface energy of (1 1 2) is relatively low in the kesterite structure [17]. When films were deposited at higher temperatures, 250 °C and 300 °C, the orientation preference normal to (1 1 2) vanished and peaks of secondary phases Cu2-xS could be observed (the inset of Fig. 2(a)). The presence of the secondary phase may arise from the excess Cu and poor Zn. Similar result that copper-rich impurity phase also easily occurred in Cu-rich CZTS thin films had been reported by other researchers in the fabrication of CZTS thin films [18,19]. In order to confirm the CZTS phase prepared by sputtering, different from the possible existence of ZnS and Cu2SnS3 phase with similar XRD peaks to that of the CZTS phase, Raman scattering was performed. Raman spectra for CZTS films deposited at different substrate temperatures are depicted in Fig. 2b in which characteristic peak at 338 cm1 belonging to CZTS was obtained [20,21]. And the peak shifted slightly toward low wave number direction with increasing substrate temperature. The peaks at 225 cm1 and 303 cm1 were attributed to impurity phase of Cu2-xS and Cu2SnS3 [22,23], respectively. By combining XRD and Raman analysis, the sputtering deposited films were composed of CZTS in principal, and a very limited amount of impurity phases occurred at a higher substrate temperature larger than 250 °C. Substrate temperature affects the morphology of the deposited films by influencing the growth rates of the nanocrystals. Fig. 3 presents the SEM images of the CZTS films deposited at different temperatures. The films deposited between 100 °C and 200 °C showed compact columnar grain characteristics which resulted from textured growth normal the (1 1 2) plane. The crystal grains have an average diameter below 70 nm. Because the surface of these films was rather smooth, AFM measurements were carried out to detect the planar topology. Fig. 4 shows AFM images of samples grown at 100 °C and 150 °C respectively. As expected, CZTS films grown at 100 °C showed textured surface with uniform island-like topography. When the substrate temperature increases up to 150 °C, the layer was more compact with smaller grain sizes. In contrast, elevating the substrate temperature over 250 °C caused a prompt change in film morphology. As shown in Fig. 3(c) and (d), the sputtered CZTS films displayed a flower-like structure which was built up by nanoflakes with a thickness of about 20 nm, implying the diminishing of preferred orientation normal to (1 1 2). And corresponding XRD results in Fig. 2 gave the proof. For magnetron sputtered films, the morphology mainly depended on surface diffusion rate of atoms. Increasing the substrate temperature can increase the surface diffusion rate and further result in rapid growth of CZTS crystals. Thus the flower-like structure forms. Fig. 5 shows the TEM image of a piece of CZTS nanoflake in Fig. 3(c) with size about 500 nm. The selected area electron diffraction (SAED) pattern in the inset of Fig. 5(a) matched the structure of CZTS, and the diffraction rings can be assigned to (1 1 2), (1 0 3), (2 2 0) and (3 1 2) planes. The EDS mapping in
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J. Wang et al. / Journal of Alloys and Compounds 552 (2013) 418–422
Fig. 3. Plan view FESEM micrographs of CZTS films deposited at different temperatures: (a) 100 °C, inset shows cross-sectional view; (b) 200 °C; (c) 250 °C; and (d) 300 °C. Sputtering was conducted in Ar flow under a pressure of 0.6 Pa and charge power of 75 W.
Fig. 4. AFM images of the CZTS thin films grown at substrate temperatures of (a) 100 °C; and (b) 150 °C. Scan area is 2 lm 2 lm.
Fig. 5(b) shows that the elements Cu, Zn, Sn, and S distribute uniformly. The obtained films at different substrate temperature were dark-brown and semitransparent. Light transmittance and reflectance data were measured to estimate the light absorption coefficient according to the following equation:
1 t
a ¼ ln
ð1 RÞ2 T
where a is the absorption coefficient, t is the film thickness which was determined from profiler measurement, and R and T are reflectance and transmittance respectively. Fig. 6(a) demonstrates Tauc plots of the CZTS thin films deposited at different substrate temperatures. The direct optical energy band gap of the thin films is determined by extrapolating the straight line of ðahtÞ2 to the intercept of the horizontal axis of the photon energy (eV). The estimated band gaps (Eg) of about 1.7 eV was slightly larger than some reported for CZTS films by other authors [24], which may be due to the insufficient accuracy in measuring film thickness [18], especially for films deposited at higher temperature resulting in large surface roughness. Interestingly, the light absorption coefficients of the flower-like films are much larger than that of the films deposited at temperature below 200 °C. And the light absorption coefficients
were maximized at substrate temperature of 250 °C, implying potential applications in solar cells. The dependence of absorption coefficient on substrate temperature could be explained from two aspects. Firstly, the rough surface due to the flower-like structure enhanced the light trapping, which could be proven by the much lower reflectance especially in the light range with ht above 1.5 eV (Fig. 6(b)). Controlling surface morphology has been shown effective in increasing the light trapping in silicon solar modules [25] and other semiconductor films [26]. Secondly, the anisotropic light absorption of the CZTS crystal structure could not be ruled out because in this work the films deposited at temperature below 200 °C showed strong texture. Few works have been reported on utilizing the anisotropic light absorption of semiconductors [13]. Again, RF magnetron sputtering is a promising, feasible, simple and cost-effective way to prepare CZTS photovoltaic semiconductor thin films, especially simple substrate temperature control induces strong texture, enabling us to investigate the anisotropic photovoltaic effect of CZTS thin films in the near future. 4. Conclusions CZTS thin films with band gaps of 1.7 eV have been successfully grown on glass substrates by RF magnetron sputtering. XRD
J. Wang et al. / Journal of Alloys and Compounds 552 (2013) 418–422
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Fig. 6. Plot of (a) ðahtÞ2 versus photon energy ht; and (b) reflectance of the CZTS films deposited at different substrate temperature. Fig. 5. (a) TEM images of the CZTS thin films grown at 150 °C, inset shows diffraction pattern; (b)element distribution.
References and Raman scattering results reveal that CZTS thin films have strong (1 1 2) texture when substrate temperature varied from 100 °C to 200 °C. A little fraction Cu2-xS secondary phase in the CZTS thin films prepared at 250 °C and 300 °C was likely related to the evaporation of Zn at higher temperature. CZTS layer was composed of a compact film with small grains when the films were deposited at 100–200 °C, in contrast, flower-like structure formed when substrate temperature was over 250 °C, and the (1 1 2) texture disappeared. The highest light absorption was obtained for flower-like samples prepared at substrate temperature of 250 °C.
Acknowledgments This work was supported by the NSFC under Grant No. 51002026 and the Fundamental Research Funds for the Central Universities (Nos. N110410002, N110810001 and N100702001). G. W. Qin appreciates Program for New Century Excellent Talents in University (No. NCET-10-0272). The authors would like to thank the anonymous reviewer for the valuable comments that improved the present manuscript.
[1] H. Katagiri, Thin Solid Films 481 (2005) 426–432. [2] W. Shockley, H.J. Queisser, J. Appl. Phys. 32 (1961) 510–519. [3] N.M. Shinde, D.P. Dubal, D.S. Dhawale, C.D. Lokhande, J.H. Kim, J.H. Moon, Mater. Res. Bull. 47 (2012) 302–307. [4] Z. Su, C. Yan, K. Sun, Z. Han, F. Liu, J. Liu, Y. Lai, J. Li, Y. Liu, Appl. Surf. Sci. 258 (2012) 7678–7682. [5] H. Deligianni, S. Ahmed, L.T. Romankiw, Electrochem. Soc. Interface 20 (2011) 47. [6] T. Kameyama, T. Osaki, K. Okazaki, T. Shibayama, A. Kudo, S. Kuwabata, T. Torimoto, J. Mater. Chem. 20 (2010) 5319–5324. [7] A. Ennaoui, M. Lux-Steiner, A. Weber, D. Abou-Ras, I. Kötschau, H.W. Schock, R. Schurr, A. Hölzing, S. Jost, R. Hock, Thin Solid Films 517 (2009) 2511–2514. [8] G. Hanna, T. Glatzel, S. Sadewasser, N. Ott, H.P. Strunk, U. Rau, J.H. Werner, Appl. Phys. A 82 (2005) 1–7. [9] P.C. Hjemås, O. Lohne, A. Wandera, H. Singh Tathgar, Solid State Phenomena 95 (2003) 217–222. [10] G. Hanna, Determination and influence of Na supply and Se flux during growth of Cu(In, Ga)Se2 thin films, PhD thesis (University of Stuttgart), 2004. [11] A. Shik, H.E. Ruda, Optics of anisotropic nanostructures (nanowires and nanorods), in: R.R.K. Drampyan (Ed.), Photonics and Micro- and NanoStructured Materials 2011, 2012, p. 841408. [12] Z. Hens, I. Moreels, J. Mater. Chem. 22 (2012) 10406–10415. [13] L. Cao, J.S. White, J.-S. Park, J.A. Schuller, B.M. Clemens, M.L. Brongersma, Nat. Mater. 8 (2009) 643–647. [14] R.A. Wibowo, W.S. Kim, E.S. Lee, B. Munir, K.H. Kim, J. Phys. Chem. Solids 68 (2007) 1908–1913. [15] C. Platzer-Björkman, J. Scragg, H. Flammersberger, T. Kubart, M. Edoff, Sol. Energy Mater. Sol. Cells 98 (2012) 110–117. [16] S. Li, G.W. Qin, Y. Ren, L. Zuo, Mater. Lett. 89 (2012) 143–145.
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[17] T. Schlenker, V. Laptev, H.W. Schock, J.H. Werner, Thin Solid Films 480-481 (2005) 29–32. [18] L. Sun, J. He, H. Kong, F. Yue, P. Yang, J. Chu, Sol. Energy Mater. Sol. Cells 95 (2011) 2907–2913. [19] H. Yoo, J. Kim, Thin Solid Films 518 (2010) 6567–6572. [20] P.A. Fernandes, P.M.P. Salomé, A.F. Cunha, Thin Solid Films 517 (2009) 2519– 2523. [21] K. Wang, O. Gunawan, T. Todorov, B. Shin, S.J. Chey, N.A. Bojarczuk, D. Mitzi, S. Guha, Appl. Phys. Lett. 97 (2010) 143508.
[22] P. Fernandes, P. Salomé, A.F. Cunha, J. Phys. D: Appl. Phys. 43 (2010) 215403. [23] P.A. Fernandes, P.M.P. Salome, A.F. Da Cunha, J. Alloys Comp. 509 (2011) 7600– 7606. [24] J.S. Seol, S.Y. Lee, J.C. Lee, H.D. Nam, K.H. Kim, Sol. Energy Mater. Sol. Cells 75 (2003) 155–162. [25] J. Zhao, A. Wang, M.A. Green, F. Ferrazza, Appl. Phys. Lett. 73 (1998) 1991– 1993. [26] S.L. Diedenhofen, O.T.A. Janssen, G. Grzela, E.P.A.M. Bakkers, J. Go´mez Rivas, ACS Nano 5 (2011) 2316–2323.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Cu2ZnSnS4 thin films: Facile and cost-effective preparation by RF-magnetron sputtering and texture control Jiansheng Wang, Song Li, Jiajia Cai, Bo Shen, Yuping Ren, Gaowu Qin ⇑ Key Lab for Anisotropy and Texture of Materials (MoE), Northeastern University, Shenyang 110819, China
a r t i c l e
i n f o
Article history: Received 10 August 2012 Received in revised form 10 November 2012 Accepted 15 November 2012 Available online 22 November 2012 Keywords: Cu2ZnSnS4 (CZTS) RF magnetron sputtering Photovoltaic semiconductor Texture control
a b s t r a c t Cu2ZnSnS4 photovoltaic semiconductor films (CZTS) were deposited by RF magnetron sputtering using a home-made Cu2S–ZnS–SnS2–S mixed powder target. The phase information of the CZTS films was studied via X-ray diffraction and Raman scattering techniques. The films prepared at substrate temperatures between 150 °C and 200 °C exhibited strong preferential orientation of grains along <1 1 2>. Higher substrate temperature (over 200 °C) induced a flower-like structure of CZTS films without preferred orientation and the light absorption coefficients were largely enhanced. The energy band gaps of CZTS films deposited at different substrate temperatures were approximate 1.7 eV. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Quaternary Cu2ZnSnS4 (CZTS) is the most promising photovoltaic absorber layer to replace Cu(In, Ga)Se2 (CIGS) because the CZTS is comprised of abundant and inexpensive elements and in addition has a high absorption coefficient of 104 cm1 [1]. Theoretical solar conversion efficiency of CZTS-based thin films solar cells is estimated as 32.2% [2]. Therefore, much effort has been devoted to the fabrication and chemical composition control of CZTS thin films in the past several years, in which liquid chemical routes and sulfuration of metallic films are typical ways for a majority of reported fabrication methods [3–7]. However, these methods are generally processing complex and pollution present, especially for liquid chemical methods. As a film deposition method, magnetron sputtering have advantages such as simplicity and precise in chemical composition control, matching with tradition solar cell production line, as well as easy scale-up. However, little work has been reported so far to prepare CZTS thin films by single step sputtering. For solar cell applications, controlling the texture or preferred orientation of absorber layer is helpful for optimizing the performance from two aspects. Firstly, proper crystalline orientation may decrease recombination of charge carriers [8], thus the con-
⇑ Corresponding author. Tel: +86 24 8369 1565, fax: +86 24 83686455. E-mail address: [email protected] (G. Qin). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.082
version efficiency can be enhanced [9]. For instance, Hanna [8] investigated differently textured Cu(In, Ga)Se2 absorber films and found that a preferred (220/204) surface orientation lead to higher efficiencies. The enhancement was assigned to specific grain boundaries with negative or neutral charges which were favorable for reducing the electronic activity and thus the recombination of charge carriers [8,10]. Secondly, depending on their structures, semiconductors demonstrate anisotropic light absorption [11] which has been used to enhance the photovoltaic and/or photoelectrochemical performance of semiconducting devices [12,13]. In this study, RF magnetron sputtering was used to prepare quaternary CZTS films on glass substrates by using a homemade target. The growth of the films and their structures and textures, morphologies, optical properties were investigated in detail. 2. Experimental Cu2ZnSnS4 films were deposited on corning 7059 glass substrates by RF magnetron sputtering. The CZTS compound target was prepared by sintering at 750 °C under high purity argon atmosphere, using solid-state reaction of Cu2S, ZnS, SnS2 and S fine powders. Prior to film deposition, the sputter vacuum chamber was evacuated to less than 5 105 Pa. High purity Ar was introduced as working gas with a flow rate of 30 sccm, and the sputtering pressure was maintained at 0.6 Pa. The structure of the CZTS thin films was analyzed by X-ray diffraction (XRD, PW3040/60 X’Pert PRO, Cu Ka). The surface morphologies of the CZTS thin films were investigated by field-emission scanning electron microscopy (FE-SEM, JEOL JEM 7001F) and atomic force microscopy (AFM). The sample compositions were determined using attached energy dispersive X-ray analysis (EDS) system. The optical property of the films was measured using a UV–VIS-NIR Spectrophotometer (Lambda-650, PerkinElmer). The film thickness was measured by surface profiler (Dektak 150).
J. Wang et al. / Journal of Alloys and Compounds 552 (2013) 418–422
Fig. 1. Chemical composition of as-prepared CZTS thin films as a function of substrate temperature.
3. Results and discussion Fig. 1 presents the composition variation of as-sputtered CZTS thin films prepared at different substrate temperatures. All CZTS thin films deposited at substrate temperatures between 100 °C and 300 °C were close to the stoichiometric one in composition,
Fig. 2. CZTS films deposited at different substrate temperatures, (a) XRD patterns; (b) Raman spectra.
419
in which the copper content increased with the substrate temperature while the Zn content decreased accordingly, probably arising from the evaporating of Zn. Similar correlation between composition of CZTS films and substrate temperature had been observed in other preparation techniques such as chemical evaporation [14,15]. Fig. 2(a) shows XRD patterns of CZTS thin films with different substrate temperatures. No diffraction peak was observed for thin film deposited at room temperature, indicating the amorphous structure. At higher substrate temperature, diffraction peaks that can be indexed to tetragonal Cu2ZnSnS4 (ICDD No. 26-575) appeared on the spectra. Specifically, diffraction patterns of films obtained at substrate temperature of 150 °C and 200 °C show strong preferential orientation of (1 1 2) plane of CZTS parallel to the substrate. The orientated growth of the CZTS films is speculated to result from the surface energy competition at low substrate temperature [16] since the surface energy of (1 1 2) is relatively low in the kesterite structure [17]. When films were deposited at higher temperatures, 250 °C and 300 °C, the orientation preference normal to (1 1 2) vanished and peaks of secondary phases Cu2-xS could be observed (the inset of Fig. 2(a)). The presence of the secondary phase may arise from the excess Cu and poor Zn. Similar result that copper-rich impurity phase also easily occurred in Cu-rich CZTS thin films had been reported by other researchers in the fabrication of CZTS thin films [18,19]. In order to confirm the CZTS phase prepared by sputtering, different from the possible existence of ZnS and Cu2SnS3 phase with similar XRD peaks to that of the CZTS phase, Raman scattering was performed. Raman spectra for CZTS films deposited at different substrate temperatures are depicted in Fig. 2b in which characteristic peak at 338 cm1 belonging to CZTS was obtained [20,21]. And the peak shifted slightly toward low wave number direction with increasing substrate temperature. The peaks at 225 cm1 and 303 cm1 were attributed to impurity phase of Cu2-xS and Cu2SnS3 [22,23], respectively. By combining XRD and Raman analysis, the sputtering deposited films were composed of CZTS in principal, and a very limited amount of impurity phases occurred at a higher substrate temperature larger than 250 °C. Substrate temperature affects the morphology of the deposited films by influencing the growth rates of the nanocrystals. Fig. 3 presents the SEM images of the CZTS films deposited at different temperatures. The films deposited between 100 °C and 200 °C showed compact columnar grain characteristics which resulted from textured growth normal the (1 1 2) plane. The crystal grains have an average diameter below 70 nm. Because the surface of these films was rather smooth, AFM measurements were carried out to detect the planar topology. Fig. 4 shows AFM images of samples grown at 100 °C and 150 °C respectively. As expected, CZTS films grown at 100 °C showed textured surface with uniform island-like topography. When the substrate temperature increases up to 150 °C, the layer was more compact with smaller grain sizes. In contrast, elevating the substrate temperature over 250 °C caused a prompt change in film morphology. As shown in Fig. 3(c) and (d), the sputtered CZTS films displayed a flower-like structure which was built up by nanoflakes with a thickness of about 20 nm, implying the diminishing of preferred orientation normal to (1 1 2). And corresponding XRD results in Fig. 2 gave the proof. For magnetron sputtered films, the morphology mainly depended on surface diffusion rate of atoms. Increasing the substrate temperature can increase the surface diffusion rate and further result in rapid growth of CZTS crystals. Thus the flower-like structure forms. Fig. 5 shows the TEM image of a piece of CZTS nanoflake in Fig. 3(c) with size about 500 nm. The selected area electron diffraction (SAED) pattern in the inset of Fig. 5(a) matched the structure of CZTS, and the diffraction rings can be assigned to (1 1 2), (1 0 3), (2 2 0) and (3 1 2) planes. The EDS mapping in
420
J. Wang et al. / Journal of Alloys and Compounds 552 (2013) 418–422
Fig. 3. Plan view FESEM micrographs of CZTS films deposited at different temperatures: (a) 100 °C, inset shows cross-sectional view; (b) 200 °C; (c) 250 °C; and (d) 300 °C. Sputtering was conducted in Ar flow under a pressure of 0.6 Pa and charge power of 75 W.
Fig. 4. AFM images of the CZTS thin films grown at substrate temperatures of (a) 100 °C; and (b) 150 °C. Scan area is 2 lm 2 lm.
Fig. 5(b) shows that the elements Cu, Zn, Sn, and S distribute uniformly. The obtained films at different substrate temperature were dark-brown and semitransparent. Light transmittance and reflectance data were measured to estimate the light absorption coefficient according to the following equation:
1 t
a ¼ ln
ð1 RÞ2 T
where a is the absorption coefficient, t is the film thickness which was determined from profiler measurement, and R and T are reflectance and transmittance respectively. Fig. 6(a) demonstrates Tauc plots of the CZTS thin films deposited at different substrate temperatures. The direct optical energy band gap of the thin films is determined by extrapolating the straight line of ðahtÞ2 to the intercept of the horizontal axis of the photon energy (eV). The estimated band gaps (Eg) of about 1.7 eV was slightly larger than some reported for CZTS films by other authors [24], which may be due to the insufficient accuracy in measuring film thickness [18], especially for films deposited at higher temperature resulting in large surface roughness. Interestingly, the light absorption coefficients of the flower-like films are much larger than that of the films deposited at temperature below 200 °C. And the light absorption coefficients
were maximized at substrate temperature of 250 °C, implying potential applications in solar cells. The dependence of absorption coefficient on substrate temperature could be explained from two aspects. Firstly, the rough surface due to the flower-like structure enhanced the light trapping, which could be proven by the much lower reflectance especially in the light range with ht above 1.5 eV (Fig. 6(b)). Controlling surface morphology has been shown effective in increasing the light trapping in silicon solar modules [25] and other semiconductor films [26]. Secondly, the anisotropic light absorption of the CZTS crystal structure could not be ruled out because in this work the films deposited at temperature below 200 °C showed strong texture. Few works have been reported on utilizing the anisotropic light absorption of semiconductors [13]. Again, RF magnetron sputtering is a promising, feasible, simple and cost-effective way to prepare CZTS photovoltaic semiconductor thin films, especially simple substrate temperature control induces strong texture, enabling us to investigate the anisotropic photovoltaic effect of CZTS thin films in the near future. 4. Conclusions CZTS thin films with band gaps of 1.7 eV have been successfully grown on glass substrates by RF magnetron sputtering. XRD
J. Wang et al. / Journal of Alloys and Compounds 552 (2013) 418–422
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Fig. 6. Plot of (a) ðahtÞ2 versus photon energy ht; and (b) reflectance of the CZTS films deposited at different substrate temperature. Fig. 5. (a) TEM images of the CZTS thin films grown at 150 °C, inset shows diffraction pattern; (b)element distribution.
References and Raman scattering results reveal that CZTS thin films have strong (1 1 2) texture when substrate temperature varied from 100 °C to 200 °C. A little fraction Cu2-xS secondary phase in the CZTS thin films prepared at 250 °C and 300 °C was likely related to the evaporation of Zn at higher temperature. CZTS layer was composed of a compact film with small grains when the films were deposited at 100–200 °C, in contrast, flower-like structure formed when substrate temperature was over 250 °C, and the (1 1 2) texture disappeared. The highest light absorption was obtained for flower-like samples prepared at substrate temperature of 250 °C.
Acknowledgments This work was supported by the NSFC under Grant No. 51002026 and the Fundamental Research Funds for the Central Universities (Nos. N110410002, N110810001 and N100702001). G. W. Qin appreciates Program for New Century Excellent Talents in University (No. NCET-10-0272). The authors would like to thank the anonymous reviewer for the valuable comments that improved the present manuscript.
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