Cu2ZnSnSb(S,Se,Te)4 film formation from selenization of sputtered self-prepared single ceramic target
Introduction
Cu2ZnSnSe4 (CZTSe) and its mixed sulfur counterpart Cu2ZnSn(S,Se)4 (CZTSSe) are promising absorber layer materials for thin-film solar cells, alternatives to Cu-III-VI2 compound of Cu(In,Ga)(S,Se)2 (CIGS) because of the non-toxicity and abundance of the constituent elements [1], [2]. The record efficiency is currently 12.7% for Cu2ZnSn(S,Se)4 (CZTSSe) processed via a hydrazine-based solution approach [3]. Yet, the hazards associated with hydrazine may inhibit the adaptation of this particular method for large-scale industrial production. In this regard, sputtering is one of the promising methods which are viable for large-scale production of CZTSSe thin films with demonstrable productivity and easy adjustment. The best CZTSe-based cell is made by respectively co-evaporation and sputtering processes and reaches an efficiency of 9.15% [4] and 11.0% [5]. All the good performance of CZTSSe solar cells is related to a Cu-deficient and Zn-rich conditions with the Cu/(Zn + Sn) ratio at 0.8–0.95 and Zn/Sn ratio at 1.10–1.25 [6], [7], [8].
Post-selenization of precursor layers is one of the leading methods to fabricate the CIGS, CZTS(Se) absorber layers [9], [10], [11], [12], [13], [14]. The precursor layers are typically prepared by sputtering or evaporation to sequentially deposit metallic elements, the related alloys or compounds, with or without selenium or sulfur, from multi-sputtering or multi-evaporation sources at low temperatures [1], [15]. However, the major issue of these multi-source processes is the absorber composition and chemical of the precursor layers. Moreover, the growth of homogeneous CZTSSe layer is degraded by the high vapor pressures of S(e) and SnS(e) compounds at high growth temperatures [16]. Therefore, it is critical to control the chemical composition of the CZTS or CZTSSe absorber layer to produce high-efficiency solar cells. To adjust the chemical composition of the deposited film, the thickness of each layer must be controlled. Therefore, a single CZTSSe ceramic target is better for the composition control of the absorber layers.
In this paper, we report the growth of polycrystalline Cu2ZnSnSb(S,Se,Te)4 (CZTASSeTe) thin films deposited on Mo-coated glass substrates using a single target one-step process by radio frequency sputtering with post-selenizating treatment. We have investigated the effect of the temperature of selenization on the structure, composition, morphology and electrical properties of CZTASSeTe thin films which was selenized at various temperatures between 400 °C and 600 °C. The effect of substrate temperature in grain growth during sputtering is also examined. The study only dealt with the preparation and characterization of CZTASSeTe thin films without the fabrication of CZTASSeTe solar cells.
Section snippets
Experimental details
Cu-deficient Cu2ZnSnSb(S,Se,Te)4 (CZTASSeTe) films on Mo-coated glass substrate were prepared by selenizing Cu–Zn–Sn–Sb–S–Se–Te ceramic films deposited by radio frequency sputtering of CZTASSeTe ceramic target at the power of 70 W for 90 min. The Cu2ZnSnSb(S,Se,Te)4 target was made by the hot pressing method at 450 °C for 30 min under a pressure of 45 psi in a graphite die from Cu2Se, SnSe2, ZnSe, Sb2S3, and Te powders. Sb2S3 and Te were added for densification purpose. The constituent weights were
Results and discussion
Effects of substrate temperature on the growth of CZTASSeTe thin films were investigated and presented in Fig. 1. Fig. 1(a) displays X-ray diffraction patterns of CZTASSeTe thin films deposited Mo-coated glass substrate at different substrate temperatures (Tsub) for 90 min. At room temperature, the (112)-oriented reflection is suppressed, or very weak. This caused by the amorphous nature of the film from ceramic target at low deposition temperature. It could also be influenced by Mo layer which
Conclusions
Polycrystalline CZTASSeTe thin films have been grown on the heated Mo-coated glass substrates by one-step sputtering from single Cu–Zn–Sn–Sb–S–Se–Te ceramic target. Structural investigation of the grown films revealed single-phase tetragonal structure corresponding to kesterite CZTSSe. Zn-rich and Cu-poor CZTASSeTe film (Zn/Sn = 1.15, Cu/(Zn + Sb + Sn) = 0.89, and metal/(S + Se + Te) = 1.02) was obtained at 600 °C in Se-ambient. The selenization conditions, such as annealing temperature and selenium partial
Acknowledgments
The authors would like to thank the National Taiwan University of Science and Technology (Taiwan, Grant No. NSC 101-2221-E-011-046) and the postgraduate research division of University of the Free State (South Africa, Grant No. NLC-LREIG00-CON-001) for financially supporting this research.
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