Cu 2 ZnSnSe 4 thin films prepared by selenization of precursor evaporated from Cu 2 ZnSnSe 4 and Na 2 Se
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Cu2ZnSnSe4 thin films prepared by selenization of precursor evaporated from Cu2ZnSnSe4 and Na2Se
Mitsuki Nakashima1, Toshiyuki Yamaguchi1 and Masanobu Izaki2 1 Wakayama National College of Technology, 77 Noshima, Nada, Gobo 644-0023, Japan 2 Toyohashi University of Technology, 1-1 Hibarigaoka Tenpaku-cho, Toyohashi 441-8580, Japan
ABSTRACT Cu2ZnSnSe4 thin films were prepared by using the synthesized Cu2ZnSnSe4 ingot and Na2Se powder at various Na2Se/Cu2ZnSnSe4 mole ratio as evaporation materials for selenization process. From EPMA analysis, the composition was approximately constant even if the Na2Se/Cu2ZnSnSe4 mole ratio increased. X-ray diffraction studies revealed that the thin films had a kesterite Cu2ZnSnSe4 structure and the foreign phases disappeared with increasing the Na2Se/Cu2ZnSnSe4 mole ratio. The Na2Se addition enhanced to grow thin films having a close-packed structure and columnar grains. The values of Voc and Isc in Cu2ZnSnSe4 thin film solar cells increased with increasing the Na2Se/Cu2ZnSnSe4 mole ratio.
INTRODUCTION Solar cell is one of the promising technologies for the clean and safety energy source to solve the energy problem. In recent years, thin film solar cell based on Cu(In,Ga)Se2 has achieved a conversion efficiency of 20.3% [1]. However, Cu(In,Ga)Se2 thin film production is limited by the availability of the rare elements In and Ga, which are expected to increase significantly in cost. Cu2ZnSn(S,Se)4, Cu2ZnSnS4 and Cu2ZnSnSe4 thin film are considerd to be promising matelials for low cost absorber layers with no rare elements. Solution-based deposition techniques have received a great deal of attention because of a record over 10% conversion efficiency for the mixed sulfide-selenide Cu2ZnSn(S,Se)4 materials [2]. Up to date, record efficiencies of 11.1% for Cu2ZnSn(S,Se)4 thin film solar cells by the hydrazine-based solution deposition technique [3]. However, the hydrazine is highly toxic and very unstable, which requires extreme caution during handling. Zhang et al. reported a relatively safe process to fabricate the Cu2ZnSn(S,Se)4 thin film solar cells with 2.2% efficiency using a low temperature
colloid technique to synthesize Cu2ZnSn(S,Se)4 nanoparticles [4]. Otherwise, many deposition techniques have been investigated for fabrication of Cu2ZnSnS4-based, Cu2ZnSnSe4-based thin films and solar cells including vacuum-based methods and non-vacuum approaches [5-12]. Ito et al. reported the Cu2ZnSnS4 thin film by atom beam sputtering and the photovoltaic effect for the first time [5]. After Katagiri et al. reported many researches for the fabrication and characterization of Cu2ZnSnS4 thin films[6-8], and their solar cell showed the conversion efficiency over 6.7% in 2008 [8]. Recently Shin et al. have achieved the conversion efficiency of 8.4% for Cu2ZnSnS4 thin film solar cells using a low temperature vacuum evaporation process with elemental sources [9]. The Cu2ZnSnSe4-based thin films have been prepared using selenization of spputered precursors [10] and electrodeposited metal stack
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