Chemical and Electronic Properties of Metal/Sb 2 Te 3 /CdTe Contacts for CdTe Thin Film Solar Cells Studied by Photoelec
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Chemical and Electronic Properties of Metal/Sb2Te3/CdTe Contacts for CdTe Thin Film Solar Cells Studied by Photoelectron Spectroscopy D. Kraft, B. Späth, A. Thißen, A. Klein and W. Jaegermann Surface Science Division, Department of Materials Science, Darmstadt University of Technology, Petersenstr. 23, D-64287 Darmstadt, Germany ABSTRACT Formation of low resistance back contacts in CdTe thin film solar cells has been a research issue for many years. Ohmic contacts to the absorber layer are typically prepared using the diffusion of dopant atoms from the back contact material into the CdTe forming a thin space charge layer that can be easily tunnelled. Stable CdTe solar cells with reasonable back contact characteristics have been prepared using metal/Sb2Te3 layer sequences. In this study the chemical and electronic properties of such layer systems have been investigated using photoelectron spectroscopy. The vacuum deposited Sb2Te3 layers do not react with the CdTe substrate. Band alignment does not indicate the formation of a good back contact. By subsequent deposition of metals a chemical reaction is induced forming metal-tellurides, a metal/Sb-alloy and elemental Sb. Although an Sb diffusion into the CdTe absorber is observed, no increase of pdoping in the surface region is evident.
INTRODUCTION The advantage of polycrystalline thin film solar cells, for example based on CdTe, compared to conventional devices is a reasonable conversion efficiency of about 10% and above reached by a low cost production process [1-4]. The commonly used CdTe cell setup is composed of a float-glass substrate with the transparent front contact, usually an ITO layer with a SnO2 diffusion barrier and the energy converting heterocontact CdS/CdTe [4,5]. A subsequent CdCl2activation step is necessary to obtain a good cell performance. Finally the back contact is formed. Main limitation of the conversion efficiency seems to occur due to "non-ohmic" back contacts. Therefore, comparative studies of different preparation procedures and alternative material systems for an ohmic back contact formation have to be carried out. PVD grown CdTe layers show a typical Fermi level position at the surface of EF-EVBM=0.8eV [6]. Despite their high work functions, all contacts to CdTe formed using transition metal dichalcogenides show a large valence band offset and nearly unchanged Fermi level positions in the CdTe interface region resulting in a large barrier for hole transport, regardless, of whether the dichalcogenides are formed by chemical decomposition of CdTe or if CdTe is deposited on single crystalline dichalcogenides [6]. Another way to prepare ohmic contacts to CdTe is the formation of a thin highly p-doped space charge layer acting as tunneling contact for the holes [7]. Cu containing back contact materials may lead to reasonable conversion efficiencies, but due to further diffusion of the Cu into the CdTe absorber a degradation of cell properties may occur [8,9]. Stable CdTe solar cells have been prepared using a Sb2Te3 buffer layer befo
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