Effect of post-sulfurization on the composition, structure and optical properties of Cu2ZnSnS4 thin films deposited by sputtering from a single quaternary target

doi:10.1016/j.apsusc.2012.09.140

Applied Surface Science

Volume 264, 1 January 2013, Pages 133-138

https://doi.org/10.1016/j.apsusc.2012.09.140 Get rights and content

Abstract

Quaternary Cu₂ZnSnS₄ (CZTS) thin films were deposited on heated glass substrates directly from a non-stoichiometric quaternary CZTS target by radio-frequency (RF) magnetron sputtering process, followed by post-sulfurization in atmosphere of Ar + H₂S(5%). The results of X-ray diffraction (XRD), Raman spectra, and scanning electron microscope (SEM) show that post-annealed process can improve the crystallinity of CZTS thin films. Both XRD and Raman spectra analysis indicate the internal compressive stress relaxes in post-annealed CZTS thin films. Further transmission spectra demonstrate that the band gaps of post-annealed CZTS thin films are smaller than those of as-deposited due to the relaxation of internal compressive stress and the increase of Cu content in the post-annealed CZTS films.

Highlights

► CZTS films were deposited directly from a CZTS target by sputtering process. ► The internal compressive stress relaxes in post-annealed CZTS films. ► The band gaps of post-annealed CZTS films are smaller than those of as-deposited.

Introduction

Cu₂ZnSnS₄ (CZTS) is a promising material for the absorber layer of thin film solar cells. The CZTS compound is the crystal-structural descendant from Cu(In,Ga)Se₂ by substitution of Zn, Sn for In, Ga and S for Se. All constituents in this CZTS film are abundant in the crust of the earth, and are nontoxic. Furthermore, CZTS film has high absorption coefficients (>10⁴ cm⁻¹), an optimal direct band gap [1], [2]. Hence, CZTS has attracted great interest in recent years. Up to now the best power conversion efficiency (PCE) based on CZTS thin film solar cells has been reached 10.1% [3], which was fabricated by chemical solution-based method. In terms of pure CZTS, the best result so far is around 8.4% [4]. This result demonstrates CZTS-based thin films have the great potential as an alternative absorber for CdTe or Cu(In,Ga)(S,Se)₂ thin film solar cells.

Deposition technologies for CZTS which have been investigated by various researchers include thermal evaporation [4], [5], [6], sputtering [7], [8], [9], electrodeposition [10], [11], spray pyrolysis [12], solution-based synthesis [3], [13], Pulsed Laser Deposition (PLD) [14], [15], [16], etc. As for sputtering process, there are three major approaches for depositing the CZTS thin film [7], [8], [9]: (i) co-sputtering. This is by far the most successful sputtering route to prepare CZTS thin film. Katagiri et al. reported a conversion efficiency of 6.77% by RF co-sputtering process [7]. (ii) Sequential sputtering from separate targets. The compositional ratio of CZTS thin films using stacked precursors are controlled easily by adjusting the thickness of each stacking layer. However, major issue for stacked precursor thin film is stacking orders for each element. The characteristics of CZTS thin films using metallic stacked precursors are strongly dependent on the stacking orders in the precursor thin films [8]. (iii) One quaternary target sputtering. Compared to previous two methods, one-target is rarely used due to significant deviation of the elemental composition between targets and thin films [9]. However, one-target sputtering technique offers numerous advantages, including uniform composition of the thin film, smooth surface, and simple process [17], [18]. These advantages will help to enhance the conversion efficiency. Hence, one-target sputtering should be suitable to produce quaternary CZTS thin films if we apply a non-stoichiometric target. Furthermore, post-annealing in argon atmosphere containing appropriate H₂S gas is an important process during CZTS thin film growth. On the one hand, post-annealed process can improve the crystallinity of thin film. On the other hand, H₂S gas is an active gas; using appropriate H₂S gas can increase the compositional ratio of sulfur. Thus, post-sulfurization was conducted to prepare high-quality CZTS thin films.

In this work, we report on the fabrication of kesterite CZTS absorber layers using a single non-stoichiometric quaternary CZTS target. Cu–Zn–Sn–S precursors were prepared by one-step RF magnetron sputtering with varying substrate temperature. The precursors were treated by a post-sulfurization with Ar + H₂S(5%). XRD and Raman results indicate that the as-deposited and post-annealed quaternary films have the kesterite structure. The crystalline quality of the CZTS thin films was significantly improved by the post-sulfurization process.

Section snippets

Experimental

CZTS thin films were deposited on glass substrates by RF magnetron sputtering process from a sintered pellet target (2 inch and 4 mm thick). The non-stoichiometric CZTS pellet was synthesized by the solid state reaction of Cu₂S, ZnS, SnS₂ and S powders mixed at 2:1.5:1:1 mol ratio. The powders were ball-milled in ethanol for 8 h with the speed of 350 rpm, and these mixed powders were dried in a non-vacuum evaporator at 50 °C. The dried powders were hot pressed (20 MPa) into a pellet and sintered at

Composition analysis

In sputtering deposition, the stoichiometry of the thin films is found to be difficult to control when growing quaternary compound films using one-step RF magnetron sputtering process [8], [19]. In order to obtain the stoichiometric ratio of CZTS thin films, we changed the atomic ratio of sputtering target with Cu₂S:ZnS:SnS₂:S = 2:1.5:1:1, which was expected to be capable of growing a stoichiometric CZTS films [9]. As shown in Table 1, the elemental ratios of as-deposited thin films are deduced

Conclusion

Quaternary CZTS thin films were deposited on heated glass substrates directly from a non- stoichiometry quaternary CZTS target by RF magnetron sputtering process. All of as-deposited CZTS thin films are of Cu-poor, Zn-rich, and S-deficient states. To enhance S content, all CZTS thin films were annealed in a furnace in atmosphere of Ar + H₂S(5%) at a temperature of 520 °C. After post-annealed process, the compositional ratio of S and Cu increase slightly for all CZTS thin films. The analysis of XRD

Acknowledgments

The authors are grateful to the measurement of Professor Zhigao Hu Group in Raman scattering spectra and transmission spectra. This project was financed by specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20100076120009), the Science and Technology Commission of Shanghai Municipality Project (Grant Nos. 11ZR1411400, 10DJ1400200, and 10JC1404600), the National Natural Science Foundation of China (61106064, 10874127, 60990312 and 61076060), and Knowledge

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Effect of post-sulfurization on the composition, structure and optical properties of Cu2ZnSnS4 thin films deposited by sputtering from a single quaternary target

Abstract

Highlights

Introduction

Section snippets

Experimental

Composition analysis

Conclusion

Acknowledgments

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Effect of post-sulfurization on the composition, structure and optical properties of Cu₂ZnSnS₄ thin films deposited by sputtering from a single quaternary target