Structural, Electronic and Defect Properties of Cu 2 ZnSn(S,Se) 4 Alloys
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Structural, Electronic and Defect Properties of Cu2ZnSn(S,Se)4 Alloys Shiyou Chen1,2, Xin-Gao Gong1, Aron Walsh3, and Su-Huai Wei4 1Key Laboratory for Computational Physical Sciences (MOE) and Surface Physics Laboratory, Fudan University, Shanghai 200433, China 2Key Laboratory of Polar Materials and Devices (MOE), East China Normal University, Shanghai 200241, China 3Department of Chemistry, University College London, London WC1E 6BT, UK 4National Renewable Energy Laboratory, Golden, Colorado 80401, USA ABSTRACT Kesterite Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) compounds are candidate low-cost absorber materials for thin-film solar cells, and a light-to-electricity efficiency as high as ~10% has been achieved in the solar cell based on their alloys, Cu2ZnSn(S,Se)4 (CZTSSe). In this paper, we discuss the crystal and electronic structure of CZTSSe alloys with different composition, showing that the mixed-anion alloys keep the kesterite cation ordering, and are highly miscible with a small band gap bowing parameter. The phase stability of CZTS and CZTSe relative to secondary compounds such as ZnS and Cu2SnS3 has also been studied, showing that chemical potential control is important for growing high-quality crystals, and the coexistence of these secondary compounds is difficult to be excluded using X-ray diffraction technique. Both CZTS and CZTSe are self-doped to p-type by their intrinsic defects, and the acceptor level of the dominant CuZn antisite is deeper than Cu vacancy. Relatively speaking, CZTSe has shallower acceptor level and easier n-type doping than CZTS, which gives an explanation to the high efficiency of CZTSSe based solar cells. INTRODUCTION In the past five years, the study about quaternary chalcogenide semiconductor Cu2ZnSnS4 (CZTS) has intensified, because CZTS is a strong candidate thin-film solar cell absorber material with the optimal band gap 1.5 eV and a high adsorption coefficient 104 cmÃ1. [1-6] Compared to the currently used thin-film solar cell absorbers, e.g. ternary CuInSe2 and binary CdTe, the advantage of CZTS is that all the constituent elements are naturally abundant and nontoxic, thus benefit the future large-area production with low cost of raw materials. CZTS can be derived from CuInSe2 through replacing two In atoms by one Zn and one Sn, thus its crystal and electronic structure inherit the characters of CuInSe2.[7,8] Due to the similarity between CZTS and CuInSe2, CZTS-based solar cells can take the same device structure as Cu(In, Ga)Se2 (CIGS) solar cells,[4] and it is expected that CZTS may substitute CIGS in the future. Recently the alloys of CZTS and its Se counterpart Cu2ZnSnSe4 (CZTSe), which adopts the same crystal structure but has a smaller band gap (1.0 eV),[7] draw more and more attention. Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cell has achieved a light to electricity conversion efficiency as high as 10%,[9] which is currently the highest efficiency of CZTS related solar
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cells. More recently, a group in Purdue University has fabricated CZTSSe nanocrystals based s
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