24September2017

Materials and Electronics Engineering

Property dependence of CuxZnSnS4 thin films on the Cu composition ratio deposited by a sol–gel method

Kailiang Fua, Ping Liub, Chao Liua, Fang Yanga, Yongsheng Chena,*, Jingxiao Lua, Shi-e Yanga

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aKey Lab of Material Physics, Department of Physics, Zhengzhou University, Zhengzhou 450052, PR China
bSchool of Electric and Information Engineering, Zhongyuan University of Technology, Zhengzhou 450007, PR China

Materials and Electronics Engineering 2014,1:7

Publication Date (Web): December 18, 2014 (Article)

DOI:10.11605/mee-1-7

*Corresponding author. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

 

Abstract 


Figure 8 Optical absorption coefficient at Cu/(Zn+Sn)=0.8 (a) and plots of (αhν)2 at different Cu/(Zn+Sn) ratios (b) versus the incident photon energy.

      We report the relation between secondary-phase formation and Cu composition ratio in a precursor solution for Cu2ZnSnS4 (CZTS) thin films. The films were prepared by a sol–gel method from the methanolic solution of a metal–thiourea complex, followed by annealing processes with elemental sulfur under protective N2 gas. With decreased Cu/(Zn+Sn) in solution, the secondary phase transformed from Cu2SnS3 (CTS) to Sn2S and ZnS, resulting in decreased conductivity and irregularly varied optical bandgap. This phenomenon indicated that decreasing the Cu composition ratio contributed to the elimination of the formation of CTS and Sn2S phases to obtain Zn-rich film. The optimum kesterite structure of CZTS film was synthesized at Cu/(Zn+Sn) = 0.80 and Zn/Sn = 1.2, with the conductivity of 0.02 S/cm and the bandgap of 1.47 eV.


 

Keywords

Cu2ZnSnS4 thin films; Sol–gel method; Kesterite

 

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Introduction

      Cu2ZnSnS4 (CZTS) is one of the most promising novel materials for thin-film solar cells because it is low-cost, non-toxic, and abundant [1–3]. CZTS commonly has kesterite structure and suitable optical bandgap of approximately 1.5eV with large absorption coefficient (>104cm-1) [4–6]. The deposition techniques for CZTS films are classified as thermal co-evaporation[7-9], sputtering[10,11], pulsed laser deposition[12], electroplation[13,14], and solution process[15,16]. With the use of hydrazine-based precursors, Wang et al. [17] obtained the highest efficiency of 12.6% for CZTSSe solar cells. However, the major drawback of their method is the use of large amounts of hydrazine, which is explosive, hepatotoxic, and carcinogenic. Therefore, it is necessary to find an alternative precursors which are highly soluble, inexpensive, commercially available, and non-toxic. Ki et al. [18] used sol–gel spin coating method to form CZTS films from Cu(CH3COO)2•H2O, ZnCl2, SnCl2•2H2O, and thiourea in dimethyl sulfoxide solvent. Spin-coated precursor films were annealed at 580°C to grow polycrystalline CZTS thin films. A solar cell of 4.1% efficiency is obtained after selenization. Schnabel et al. [19] obtained CZTSSe solar cells with efficiency of 7.5% using the same approach. Chaudhuri et al. [20] deposited CZTS films by direct liquid coating from metal–thiourea precursor solution and obtained the bandgap of 1.4eV and the electrical conductivity of 0.5S/cm. To fabricate high-efficiency CZTS solar cells, it is necessary to evaluate the influence of the Cu/(Zn+Sn) and Zn/Sn ratios on the formation of secondary phases in compounds. The quantitative criteria of chemical composition ratios is important in determining the photoelectric properties of the CZTS films. However, few reports are available on such influences, particularly in the case of sol–gel deposited CZTS. In this paper, we investigate the effect of Cu composition ratio on the properties of CZTS films deposited by sol–gel technology.

Experimental

        All of the chemicals used in our experiments were analytical grade. The sol–gel solution was prepared by dissolving copper (II) acetate, zinc acetate, tin (II) chloride and thiourea in methanol then was stirring at room temperature for 2 h. The Zn/Sn elemental ratio was fixed at 1.2, and the compositional ratio Cu/(Zn+Sn) varied between 1.00 and 0.75. The precursor was then spin coated onto a soda-lime glass substrate at 3000 rpm for 30 s, followed by drying at 200°C for 10min on a hot plate to obtain the black thin film. This step was repeated by 10 times to obtain thicker film. The sulfurization of precursor samples to synthesize CZTS layers occurred in a tubular furnace at 550°C for 10min under N2 atmosphere. Sulfur powder was compressed into pellets and then was placed near the precursors.The structural properties of the films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive spectrometry (EDS), and Raman spectroscopy. For the Raman spectroscopy, the laser source at 514nm was used as the excitation source, and the laser power level was 5mW. A UV-visible-near-infrared spectrophotometer (UV-3600) was used to measure the transmission and absorption of the films in the wavelength range from 560 nm to 1040 nm. The conductivity was measured with the use of a 6517A electrometer (Keithley 4200-scs/F). All measurements were conducted at room temperature.

Results

Figure 1 XRD patterns of sulfurized films deposited at different Cu/(Zn+Sn) ratios.

Figure 1 XRD patterns of sulfurized films deposited at different Cu/(Zn+Sn) ratios.

      Fig.1 shows the XRD patterns of the sulfurized CZTS films deposited at different Cu/(Zn+Sn) elemental ratios in solutions with stabilized Zn/Sn ratio of 1.2. The peaks of the films were consistent with those of CZTS (JCPDS 26-0575), and (112) peak is observed to be dominant for all of the fabricated films. The kesterite (space group ) and stannite (space group ) structures are often found in CZTS film. In stannite structure, the peaks (220) and (204), as well as (312) and (116), can be distinguished. In the present case, a = 0.541nm and c = 1.074 nm, which were calculated by the Bragg law 2dhkl sinθ = λ and the relation 1/dhkl2 = h2/a2 + k2/b2 + l2/c2. Therefore, we obtained a c/2a ratio of 0.9926. At same time, the doublets (220)/(204) and (312)/(116) were considered, and kesterite structure is confirmed. The theoretical study also indicates that kesterite is more stable and may co-exist with stannite in the synthesized samples [21]. Neutron diffraction showed that the c/2a ratios of CZTS powder are larger than 1 [22,23]. This difference is attributed to the strain and the presence of secondary phases, because CZTS, Cu2SnS3, and ZnS have similar diffraction patterns in the Cu–Zn–Sn–S material system.

Figure 2 Raman spectra of CZTS films deposited at different Cu/(Zn+Sn) ratios.

Figure 2 Raman spectra of CZTS films deposited at different Cu/(Zn+Sn) ratios.

        The corresponding Raman spectra of the samples in Fig.1 are shown in Fig.2. The existence of CZTS is confirmed by peaks at 253, 286, 337, and ~368cm−1. These peaks are consistent with the published data for kesterite phase of CZTS. The full width at half maximum (FWHM) for 337cm− 1 peak is presented in Fig.3. The film deposited at a ratio of Cu/(Zn+Sn) = 1.00 has significantly high FWHM, which is due to the formation of the Cu2SnS3 (CTS) ternary compound associated with the shoulders observed at the left of the main CZTS peak. An additional peak at ~315cm−1 attributed to SnS2 is detected with the decrease in Cu/(Zn+Sn), thereby indicating that the obtained films are mainly CZTS with small amount of SnS2 phase. With further decrease in Cu/(Zn+Sn) in solution, the Raman intensity at 315cm−1 decreases and eventually disappeared, and CZTS film without any secondary phases is obtained at Cu/(Zn+Sn) = 0.8. However, a weak peak is found at the right side of the main CZTS peak at Cu/(Zn+Sn) = 0.75, which indicates the formation of ZnS phase, thereby leading to increased FWHM. This observation is confirmed by the elemental composition of the thin films obtained at different Cu/(Zn+Sn) ratios (Table1). With the decrease in Cu/(Zn+Sn) ratios in solution, Cu and Sn concentrations decrease, whereas the Zn concentration increases. This phenomenon may be due to the volatilization of Zn in the form of zinc acetate (boiling point ~240°C) in the drying process.

Figure 3 Variations in FWHM of 337cm− 1 peak for the films deposited at different Cu/(Zn+Sn) ratios.

Figure 3 Variations in FWHM of 337cm− 1 peak for the films deposited at different Cu/(Zn+Sn) ratios.

Table 1. Elemental composition of the films deposited at different Cu/(Zn+Sn) ratios

 

         In this study, the secondary phase transfers from CTS to SnS2 and ZnS increase with the decrease in Cu/(Zn+Sn) ratios in films or in solutions. It provides information on the reaction mechanism of CZTS films. In the drying process (200°C), CuS, SnS, SnS2, and ZnS binary phases are formed because of the reaction of Cu, Sn, and Zn ions with thiourea [24,25]. This phenomenon is consistent with the report in reference [26]. In addition, the uniform mixing of the precursor solution of metal-thiourea complexes contributes to the formation of CZTS and CTS phases. These can be verified by the Raman spectra of the as-deposited films in Fig. 4. The peak at approximately 331cm−1 is related to the presence of high-degree disorder in the cation sublattice in CZTS phase [27] or CZTS nanocrystals [28].

Figure 4 Raman spectra of the as-deposited film.

Figure 4 Raman spectra of the as-deposited film.

      In the annealing process, CuS is transformed into Cu2S under excess S condition (450°C to 500°C) [29,30]. A liquid Cu2S+SnS phase is then formed (eutectic composition from 32% to 68%, eutectic point at 480 °C). CZTS is formed more easily with gaseous sulfur and solid ZnS according to the reaction (1) presented in reference [31]:
      When Zn is insufficient in film, the redundant liquid Cu2S+SnS phase would transform into CTS during cooling as in Reaction (2). This phenomenon agrees with the appearance of direct evidence in the experimental data on the formation of CTS ternary phase for the film deposited at Cu/(Zn+Sn) = 1.00. With the decrease in Cu/(Zn+Sn) in solution, the film is in a Cu-poor and Sn-rich state, thereby leading to the segregation of SnS2 phase in the cooling process. With further decrease Cu/(Zn+Sn), the decomposition of SnS2 or CZTS is enhanced with reaction (3), resulting in the severe loss of Sn [32,33]. Thus, Zn-rich condition is obtained. Thereby it causes the formation of ZnS phase which is consistent with the Raman results.
      The SEM images of the CZTS films deposited with different Cu/(Zn + Sn) ratios are shown in Fig.5. The images show that the CZTS films are composed of spherical grains with size varying from 100nm to 200nm. With the decrease in Cu/(Zn + Sn) ratios, the densities of agglomerated grains and voids are also decreased, thereby resulting in the fabrication of a compact, smooth and uniform film at a ratio of 0.9. However, when Cu/(Zn + Sn) ratio is smaller than 0.9, the agglomeration and voids increase because of the loss of volatile SnS and the formation of ZnS phase during annealing. Fig.6 shows a cross-sectional view of the CZTS film on glass prepared at Cu/(Zn + Sn)=0.8. The SEM image also shows growth of smooth, homogeneous, compact film. Cross sectional view of the coated film shows the thickness of 330 nm. Thus, each coat contributes 33nm to the film growth. No pores, cracks, or voids are observed throughout the film.

Figure 5 SEM images of CZTS films deposited at different Cu/(Zn + Sn) ratios: (a) 1.00; (b) 0.95; (c) 0.90; (d) 0.85; (e) 0.80; and (f) 0.75.

Figure 5 SEM images of CZTS films deposited at different Cu/(Zn + Sn) ratios: (a) 1.00; (b) 0.95; (c) 0.90; (d) 0.85; (e) 0.80; and (f) 0.75.

Figure 6 SEM cross-sectional view of the CZTS film on glass.

Figure 6 SEM cross-sectional view of the CZTS film on glass.

        Fig.7 shows the conductivity of films deposited with different Cu/(Zn+Sn) ratios. The conductivity increases with the increase in Cu/(Zn+Sn) ratios because of the secondary phase transformed from ZnS to SnS2. It has been reported that the formation of CTZ and CuS phases degraded the properties of solar cell [34–36]. Therefore, controlling the Cu amount in film is important to guarantee a low contribution of these phases.

Figure 7 Conductivity of the samples deposited at different Cu/(Zn+Sn) ratios.

Figure 7 Conductivity of the samples deposited at different Cu/(Zn+Sn) ratios.

        To estimate the optical properties of the films, the optical absorption coefficient (α) is obtained from the measured spectral transmittance (Tλ) and reflectance (Rλ) data using the formula (4) presented in reference [37].

 

where t is the thickness of the film. A typical plot of the ln(α) versus hν is shown in Fig.8(a). Given that CZTS is a direct bandgap material, the optical bandgap energies (Eg) of the reach films are obtained from the equation (5).

 

      where A is a constant. Fig.8(b) shows the bandgap values of the CZTS films at different Cu/(Zn+Sn) ratios. The values are 1.45, 1.49, 1.56, 1.52, 1.47, and 1.61eV for the Cu/(Zn + Sn) ratios 1.00, 0.95, 0.90, 0.85, 0.80, and 0.75, respectively.The bandgap energies of ZnS, CTS, and SnS2 are approximately 3.65, 1.0, and 2.6 eV, respectively. The incorporation of these secondary phases in the CZTS films resulted in the relatively narrow or wide optical bandgap energy compared with the ideal bandgap energy. An optimal Eg of 1.47eV is obtained at Cu/(Zn + Sn) =0.80, which is consistent with the data occurred in reference [3,20]. Higher or lower Cu/(Zn+Sn) ratios are unfavorable to the CZTS solar cell.  

Figure 8 Optical absorption coefficient at Cu/(Zn+Sn)=0.8 (a) and plots of (αhν)2 at different Cu/(Zn+Sn) ratios (b) versus the incident photon energy.X

Figure 8 Optical absorption coefficient at Cu/(Zn+Sn)=0.8 (a) and plots of (αhν)2 at different Cu/(Zn+Sn) ratios (b) versus the incident photon energy.

Conclusion

        In summary, CZTS thin films were successfully prepared using sol–gel method followed by annealing process. The evolution of secondary phases presented in the sol–gel CZTS films is related with the elemental composition ratio, and such evolution can be suppressed by changing the Cu composition ratio in the solution. The results also show that the CZTS film synthesized at Cu/(Zn+Sn) = 0.80, Zn/Sn = 1.2 has a kesterite structure, 0.02S/cm conductivity, approximately 1.47eV bandgap , and 104cm−1 absorption coefficient. Therefore, this developed low-cost deposition method may be adopted for the fabrication of CZTS-based solar cell.

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Acknowledgements

        This work is supported by the Educational Commission of Henan Province of China (13A140736) and Key Research Project of Zhengzhou of China (131PPTGG409-4).

References

1. W.X. Huang, Q. Li, Y.H. Chen, Y.D. Xia, H.H. Huang, C.C. Dun, Y. Li, D.L. Carroll, Surface modification enabled carrier mobility adjustment in CZTS nanoparticle thin films. Sol. Energ. Mat. Sol. C. 127, 188 (2014). doi:10.1016/j.solmat.2014.04.027 
2. M.P. Suryawanshi, S.W. Shin, U.V. Ghorpade, K.V. Gurav, G.L. Agawane, Chang Woo Hong, Jae Ho Yun, P.S. Patil, Jin Hyeok Kim, A.V. Moholkar, A chemical approach for synthesis of photoelectrochemically active Cu2ZnSnS4(CZTS) thin films. Sol. Energy. 110, 221 (2014). doi:10.1016/j.solener.2014.09.008 
3. D.B. Mitzi, O. Gunawan, T.K. Todorov, K. Wang, S. Guha, The path towards a high-performance solution-processed kesterite solar cell. Sol. Energ. Mat. Sol. C. 95, 1421 (2011). doi:10.1016/j.solmat.2010.11.028 
4. L. Grenet, S. Bernardi, D. Kohen, C. Lepoittevin, S. Noel, N. Karst, A. Brioude, S. Perraud, H. Mariette, Cu2ZnSn(S1-xSex)4 based solar cell produced by selenization of vacuum deposited precursors. Sol. Energ. Mat. Sol. C. 101, 11 (2012). doi:10.1016/j.solmat.2012.02.016
5. Y.Z. Zhang, C. Liao, K. Zong, H. Wang, J.B. Liu, T. Jiang, J.F. Han, G.Q. Liu, L. Cui, Q.Y. Ye, H. Yan, W.M. Lau, Cu2ZnSnSe4 thin film solar cells prepared by rapid thermal annealing of co-electroplated Cu-Zn-Sn precursors. Sol. Energy 94, 1 (2013). doi:10.1016/j.solener.2013.05.002 
6. C.P. Chan, H. Lam, C. Surya, Preparation of Cu2ZnSnS4 films by electrodeposition using ionic liquids. Sol. Energ. Mat. Sol. C. 94, 207 (2010). doi:10.1016/j.solmat.2009.09.003 
7. K. Wang, B. Shin, K.B. Reuter, T. Todorov, D.B. Mitzi, S. Guha, Structural and elemental characterization of high efficiency Cu2ZnSnS4 solar cells. Appl. Phys. Lett. 98, 051912 (2011). doi:10.1063/1.3543621
8. K. Wang, O. Gunawan, T. Todorov, et al., Thermally evaporated Cu2ZnSnS4 solar cells. Appl. Phys. Lett. 97, 143508 (2010). doi:10.1063/1.3499284 
9. A. Redinger, S. Siebentritt, Coevaporation of Cu2ZnSnS4 thin films. Appl. Phys. Lett. 115, 173502 (2014). doi:10.1063/1.4871664 
10. H. Katagiri, K. Jimbo, S. Yamada, et al., Enhanced conversion efficiencies of Cu2ZnSnS4-based thin film solar cells by using preferential etching technique. Appl. Phys. Express 1, 041201 (2008). doi:10.1143/APEX.1.041201 
11. P.A. Fernandes, P.M.P. Salome, A.F. daCunha, Growth and Raman scattering characterization of Cu2ZnSnS4 thin films. Thin Solid Films 517, 2519 (2009). doi:10.1016/j.tsf.2008.11.031 
12. K. Moriya, K. Tanaka, H. Uchiki, Cu2ZnSnS4 thin films annealed in H2S atmosphere for solar cell absorber prepared by pulsed laser deposition. Jpn. J. Appl. Phys. 47, 602 (2008). doi:10.1143/JJAP.47.602 
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Additional Info

  • Type of Publishing: JOUR - Journal
  • Title:

    Property dependence of CuxZnSnS4 thin films on the Cu composition ratio deposited by a sol–gel method

  • Author: Kailiang Fu, Ping Liu, Chao Liu, Fang Yang, Yongsheng Chen, Jingxiao Lu, Shi-e Yang
  • Year: 2014
  • Volume: 1
  • Issue: 1
  • Journal Name: Materials and Electronics Engineering
  • Publisher: Nicety Press Company Limited
  • ISSN: 2410-1648
  • URL: http://www.meej.org/volume-1/december-2014/item/368-property-dependence-of-cuxznsns4-thin-films-on-the-cu-composition-ratio-deposited-by-a-sol%E2%80%93gel-method
  • Abstract:

    We report the relation between secondary-phase formation and Cu composition ratio in a precursor solution for Cu2ZnSnS4 (CZTS) thin films. The films were prepared by a sol–gel method from the methanolic solution of a metal–thiourea complex, followed by annealing processes with elemental sulfur under protective N2 gas. With decreased Cu/(Zn+Sn) in solution, the secondary phase transformed from Cu2SnS3 (CTS) to Sn2S and ZnS, resulting in decreased conductivity and irregularly varied optical bandgap. This phenomenon indicated that decreasing the Cu composition ratio contributed to the elimination of the formation of CTS and Sn2S phases to obtain Zn-rich film. The optimum kesterite structure of CZTS film was synthesized at Cu/(Zn+Sn) = 0.80 and Zn/Sn = 1.2, with the conductivity of 0.02 S/cm and the bandgap of 1.47 eV.

  • Publish Date: Thursday, 18 December 2014
  • Start Page: 7
  • DOI: 10.11605/mee-1-7