Identification of Impurity Phases in Cu 2 ZnSnS 4 Thin-film Solar Cell Absorber Material by Soft X-ray Absorption Spectr
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Identification of Impurity Phases in Cu2ZnSnS4 Thin-film Solar Cell Absorber Material by Soft X-ray Absorption Spectroscopy M. Bär,1 B.-A. Schubert,1 R.G. Wilks,1 B. Marsen,1 Y. Zhang,2 M. Blum,2,3 S. Krause,2 W. Yang,3 T. Unold,1 L. Weinhardt,4 C. Heske,2 and H.-W. Schock1 1
Solar Energy Research, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB), Hahn-Meitner-Platz 1, D-14109 Berlin, Germany 2 Department of Chemistry, University of Nevada, Las Vegas, 4505 Maryland Pkwy., Las Vegas, NV 89154, U.S.A. 3 Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, U.S.A. 4 Exp. Physik VII, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany ABSTRACT The composition of Cu2ZnSnS4 thin-film solar cell absorbers was varied to induce the formation of secondary impurity phases. For their identification, the samples have been investigated by Cu L3 and S L2,3 soft x-ray absorption (XAS) spectroscopy. We find that Cu L3 XAS is especially sensitive to the presence of copper sulfides as well as copper oxides and/or changes in the electron configuration, suggesting a basis for future studies of the surface, defect, and interface characterization of similar samples. Additionally, it is shown that the S L2,3 absorption data can be used as a very sensitive probe of the variations in the prevalence of S-Zn bonds in the near-surface region of the investigated samples. INTRODUCTION The two most important factors in the commercial development of photovoltaic (PV) technologies are the achieved solar cell device efficiency (on laboratory and large scale) and the costs per power in an industrial mass production. On these grounds, various thin-film PV technologies already successfully compete with the current state-of-the-art Si-wafer based solar cells. Solar cells based on the Cu-chalcopyrite alloy system (Cu(In1-XGaX)(SYSe1-Y)2) reach efficiencies in excess of 20 % [1] in the laboratory, while for CdTe-based thin-film devices (record efficiency: 16.5 % [2]) industrial large-scale manufacturing costs below 0.80 US$/Wp [3] are reported. Thus thin-film PV devices are able to convert sunlight into electrical energy at the same performance level as polycrystalline Si-wafer based devices [4] at potentially lower production costs. Due to the limited availability of some Cu(In1-XGaX)(SYSe1-Y)2 absorber constituents, concern exists that high material expenses will jeopardize the promise of lower industrial-scale production costs for this potentially higher performing thin-film PV technology. As a result, kesterites used as thin-film solar cell absorbers have attracted much attention in the recent past [5], and efficiencies of up to 9.6 % [6] have already been reported for Cu2ZnSn(S,Se)4-based photovoltaic devices. In order to reach the next level of solar cell performance, a detailed insight into the chemical and electronic structure of kesterites is necessary. However, so far the electronic structure of the Cu2ZnSn(S,Se)4 material system has only been thoroughly investigated in th
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