CdTe Thin Film Solar Cells: The CdS/SnO 2 Front Contact
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CdTe Thin Film Solar Cells: The CdS/SnO2 Front Contact J. Fritsche, S. Gunst, A. Thißen, R.Gegenwart1, A. Klein, and W. Jaegermann Institute of Materials Science, Darmstadt University of Technology Petersenstrasse 23, 64287 Darmstadt, Germany 1 ANTEC Technology GmbH Arnstädter Str. 22, D-99334 Rudisleben/Thür., Germany ABSTRACT Tin dioxide (SnO2) coated glass is the commonly used substrate for thin film solar cells based on CdTe absorbers. We have investigated the properties of the CdS/SnO2 interface by Xray and ultraviolet photoelectron spectroscopy. SnO2 coated glass substrates as used for solar cell preparation were cleaned by different procedures such as derinsing, sputtering, heating and annealing in oxygen atmosphere. Different surface properties with a strongly dependent number of defects in the SnO2 band gap are identified. CdS films were deposited stepwise by thermal evaporation to determine the electronic interface properties for different surface preparation conditions. Comparative barrier heights at the CdS/SnO2 contact are found for most surface pretreatments. The Fermi level position in these cases is situated in the SnO2 band gap. A different interface behaviour is determined for sputter cleaned SnO2 surfaces, which is attributed to the formation of oxygen vacancies during sputtering and subsequent formation of an interfacial SnOxSy compound. INTRODUCTION CdTe thin film solar cells are prepared by close-spaced-sublimation (CSS) or electrochemical deposition with high reproducibility [1-4]. Large scale production lines have been set up recently years by different companies (see e.g. [5]). In spite of this technological success many material and device properties are not well understood. In addition to electrical and optical properties of the bulk, interfaces play an important role for device performance. They determine the barrier heights and control by the dopant profile also the electric field distribution in the cell. They might also be responsible for carrier recombination due to electronic interface states. A number of interfaces between different materials exist due to the deposition sequence ITO/SnO2/CdS/CdTe/back-contact. Several transparent conductive oxides (TCO) have been tested as a front contact for superstrate CdTe/CdS solar cells. Best energy conversion efficiencies have been obtained with SnO2 contacts so far. In this contribution we will address the interface between SnO2 and CdS, which we have studied by X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS). Photoelectron spectroscopy allows not only the determination of chemical properties of the interface. In addition, and even more important, the electronic potential distribution is probed since the binding energies are measured with respect to the Fermi energy. An ideal front contact for the solar cell should have low resistivity and high transparency. It should be chemically stable in subsequent processing steps and no energy barrier for charge transport should exist. When the TCO-contact is used as the substrate (superstra
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