Band offsets at the ZnSe / CuInS 2 interface
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Band offsets at the ZnSe / CuInS2 interface O. Papathanasiou1, S. Siebentritta, I. Lauermanna, T. Hahnb, H. Metznerb, M. Ch. Lux-Steinera,c a
Hahn-Meitner-Institut, Glienicker Str. 100, D-14109 Berlin, Germany Friedrich-Schiller-Universität Jena, Institut für Festkörperphysik, Max-Wien-Platz 1, D07743 Jena, Germany c Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany b
ABSTRACT ZnSe has been shown to be a promising alternative buffer in CuInS2 thin film solar cells. Here we present for the first time photoemission measurements to determine the band alignment at the ZnSe/CuInS2 interface. Epitaxial CuInS2 is used as a substrate. ZnSe is deposited in varying thicknesses by MOCVD. X-ray photoelectron spectra are measured with an Mg laboratory source and with synchrotron radiation. A valence band offset of 0.4+/-0.1eV is obtained. INTRODUCTION Conventional CuInS2 solar cells contain a CdS buffer layer, deposited in a chemical bath. However, Cd is a toxic material and the deposition introduces a wet chemistry process step in the otherwise dry production line of the solar cells. In an effort to replace CdS, ZnSe was shown to be a promising alternative. CuInS2 solar cells with ZnSe buffer layers prepared by metal organic chemical vapor deposition (MOCVD) achieved efficiencies above 7%, which was in the range of the CdS-buffered references at that time [1]. It was shown before that CdS (Eg=2.3eV) forms a “cliff” with CuInS2, i.e. a negative conduction band offset from the absorber to the buffer [2]. This configuration favors interface recombination [3]. Due to the higher band gap of ZnSe (Eg=2.7eV) a “spike” structure is expected, i.e. the conduction band offset between absorber and buffer is positive. Such a configuration is favorable for solar cells. Metal organic chemical vapor deposition (MOCVD) of ZnSe is chosen, because it is a well controllable and reproducible dry process which can easily be integrated into an in-line production. The lowest deposition temperatures using organic precursors are possible with ditertiary-butyl-selenide (DtBSe) as a Se source and dimethyl-zinc-triethylamine (DMZnTEN) as a Zn source [4]. Illumination with UV light during growth leads to an increased growth rate at these low temperatures [5] (photo assisted growth, PAG). However, to understand and model solar cells, the band structure in the pn-junction has to be known. Therefore, we report in this paper on photoelectron spectroscopy (PES) of CuInS2 films which were covered with ZnSe of varying thickness. From PES the valence band alignment between the ZnSe buffer and the CuInS2 absorber is determined. SAMPLE PREPARATION The CuInS2 films were grown using three-sources-molecular-beam epitaxy (MBE) [6]. The atomic beams of copper and indium (metal purity 99.9999%) were produced by means of 1
corresponding author: [email protected]
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standard hot-lip evaporation sources (VTS CREATEC). The molecular sulfur beam (sulfur purity 99.9995%) was delivered by a cracker source which has been described elsewhere [6].
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