Fabrication of Cu 2 ZnSn(S,Se) 4 solar cells by printing and high-pressure sintering process
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Fabrication of Cu2ZnSn(S,Se)4 solar cells by printing and high-pressure sintering process Feng Gao, Tsuyoshi Maeda, and Takahiro Wada Department of Materials Chemistry, Ryukoku University, Seta, Otsu 520-2194, Japan ABSTRACT We fabricated Cu2ZnSn(SxSe1-x)4 (CZTSSe) solar cells by a printing and high-pressure sintering (PHS) process. First, the CZTSSe solid solution powders were synthesized by heating the elemental mixtures at 550oC for 5 h in an N2 gas atmosphere. We fabricated CZTSSe films by a printing and high-pressure sintering (PHS) process. The obtained dense CZTSSe film was post-annealed at 550oC for 10 min under an N2 +5% H2S gas atmosphere. We fabricated CZTSSe solar cells with the device structure of Ag/ITO/i-ZnO/CdS/CZTSSe/Mo/soda-lime glass. The CZTSSe solar cell showed an efficiency of 2.1%, with Voc of 272 mV, Jsc of 18.0 mA/cm2 and FF of 0.44. INTRODUCTION After a Cu(In,Ga)Se2 (CIGS) solar cell was achieved with a conversion efficiency of 20.3% [1], an increasing number of companies moved to commercialize CIGS PV modules. Therefore, substituting abundant elements for indium and gallium in CIGS has become an important issue because they are expensive rare metals. Cu2ZnSnS4 (CZTS) is anticipated to be an indium-free absorber material [2]. Katagiri et al. reported a CZTS solar cell with 6.7% efficiency fabricated by a combination of precursor layer sputtering and post-sulfurization [3]. An IBM group fabricated Cu2ZnSn(S,Se)4 (CZTSSe) solar cell with an efficiency of 11.1% by the hybrid coating process [4]. We studied the fabrication of CIGS films by a non-vacuum particulate-based deposition process. In such processes, solid particles are dispersed in a solvent to form an ink that can be coated onto a substrate [5]. We fabricated CIGS films by a combination of mechanochemical and screen printing/sintering processes [6, 7]. CIGS powder suitable for screen printing was prepared using a mechanochemical process (MCP) [8, 9]. First, we fabricated a CIGS solar cell with 2.8% efficiency [6]. Then we fabricated CIGS films by a mechanochemical process, wet bead-milling of CIGS powder, and screen printing and sintering processes, and obtained a CIGS solar cell with 3.1% efficiency [7]. Recently, we fabricated high-density CIGS films by a printing and high-pressure sintering (PHS) process and obtained a CIGS solar cell with 3.2% efficiency [10]. We prepared Cu deficient Cu2(1-x)ZnSnSe4 and characterized their crystal structures by XRD and XAFS [11]. Then, we characterized their optical properties. The band gaps of the CZTSSe solid solutions were determined from diffuse reflectance spectra of the powders and transmittance spectra of the films. The band gap (Eg) of the Cu2ZnSn(SxSe1-x)4 solid solution linearly increases from 1.05 eV for CZTSe (x=0.0) to 1.51 eV for CZTS (x=1.0) [12]. In this study, we fabricated CZTSSe films by PHS process [5] and CZTSSe solar cells with the device structure of Ag/ITO/i-ZnO/CdS/CZTSSe/Mo/soda-lime glass. A CdS buffer layer (100 nm) was formed by a conventional chemical bath deposition
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