Enhanced Solar Cell Performance of Cu2ZnSn(S,Se)4 Thin Films Through Structural Control by Using Multi-Metallic Stacked Nanolayers and Fast Ramping Process for Sulfo-Selenization
Wei-Chao Chen1,2,3*, Cheng-Ying Chen3, Venkatesh Tunuguntla1,3, Shao Hung Lu4, Chaochin Su4, Chih-Hao Lee2, Kuei-Hsien Chen1, Li-Chyong Chen3
1Institute of Atomic and Molecular Sciences, Acdemia Sinica, Taipei, Taiwan
2Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan
3Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
4Department of Molecular Science and Engineering, National Taipei University of Technology, Taipei, Taiwan
* presenting author:Wei-Chao Chen, email:chen.weichao0129@gmail.com
Kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) is a promising material for the next generation low-cost photovoltaic application due to the replacement of valuable In and Ga by earth-abundant Zn and Sn. It has a direct and tunable band gap (1.01–1.5 eV), and an absorption coefficient as high as Cu2InxGa1-XSe2 (CIGS) (~10⁴ cm−1) [1,2]. Recently, the reported power conversion efficiency (PCE) of CZTSSe based solar cells has increased from 0.66% to 12.6%, by optimizing the fabrication process [3–5]. The synthesis of CZTSSe thin films can be done using various vacuum [6,7] and non-vacuum techniques [8,9]. Sulfurization/selenization of CZT metallic or CZTS samples under high sulfur/selenium vapor by using very pure sulfur/selenium pellets is one of the key methods to achieve high quality of CZTSSe absorber layers. Moreover, the stacking order of metallic precursor also has significant impact on the thin film growth, precursor alloy formation, elemental loss, morphology, and secondary phase formation with certain distribution, which gives rise to different device performance

In this paper, Cu2ZnSn(S,Se)4 (CZTSSe) thin-films were prepared by sulfo-selenization of metal precursors in H₂S environment instead of using metal selenides/sulfides as precursors. High quality CZTSSe thin films were obtained using multi-stacking metallic nanolayer precursors undergoing a fast ramping process. For the preparation of metallic stacked nanolayer precursors, we have developed a 9-layer sequential deposition of Sn/Zn/Cu metal stack onto Mo-coated soda lime glass substrate by RF-sputtering. Due to inevitable metal inter-diffusion during the sulfo-selenization, we further studied the effect of the Sn/Zn/Cu metal stacking number (therefore, the layer thickness) on the quality of thin film with respect to its device performance. In the device prepared with conventional 3-layer stack, due to insufficient inter-diffusion of precursors, excessive Cu-rich secondary phase was formed at the back contact region and resulted in poor performance of devices. By using the modified 9 layer stacked precursor and fast ramping heating process the device efficiency can be improved from 4.9% to 7.7% and open circuit voltage from 0.44 to 0.5 V. This improvement can be ascribed to a compact, smooth microstructure, presence of bronze formation and the suppression of Cu-rich bi-layer formation in the 9-layer approach.

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Keywords: CZTSSe, Thin film solar cell, Multi-metallic stacked precursor