Interface Engineering of Chalcogenide Semiconductors in Thin Film Solar Cells: CdTe as an Example
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Interface Engineering of Chalcogenide Semiconductors in Thin Film Solar Cells: CdTe as an Example J. Fritsche, D. Kraft, A. Thissen, Th. Mayer, A. Klein, W. Jaegermann, Department of Materials Science, Darmstadt University of Technology, Petersenstr. 23, 64287 Darmstadt, Germany ABSTRACT In this paper the electronic properties of the different interfaces of CdTe thin film solar cells will be analysed by using a surface science approach. Experimental basis for the experiments is an integrated UHV systems which allows to prepare and analyse real solar cells as well as appropriate model interfaces. Recently obtained data on the ITO surface, the ITO/SnO2/CdS front contact, the CdS/CdTe heterojunction and the CdTe/Te back contact will be presented. In addition, bulk properties as doping and lateral inhomogeneities will be addressed. For all these interfaces experimentally determined band energy diagrams will be given and discussed in relation to solar cell performance. Finally, the sum of the results will be used to propose a modified band energy diagram of the complete CdTe thin film solar cell and its implication for further cell improvement will be presented. INTRODUCTION Thin film solar cells based on CIGS or CdTe as absorber layer are on the edge to commercialisation with the expectation of high cost reduction potentials. High efficiency laboratory cells with energy conversion efficiencies of about 19% for CIGS [1] and 16% for CdTe [2] have been realised due to intensive research and development in recent years. For large area modules efficiencies in the range of 10% are envisaged from the production lines. These achievements have only been possible by a steady optimisation of empirically developed processing steps. But in many cases the fundamental reasons for obtaining high quality solar cells by using certain „tricks“ are still not very well understood [3]. This is especially true for the electronic properties of the different interfaces in thin film solar cells which are crucial for reaching high conversion yields. The different interfaces which have to be optimised are schematically shown in Fig. 1 together with a SEM schematic cross sectional view of a CdTe solar cell as prepared by ANTEC using CSS [4]. In understanding the device properties it is necessary to analyse the electronic structure of the respective interfaces which usually depend strongly on the used preparation procedure as there are: the ohmic back contact usually to metals, the semiconductor/semiconductor heterointerface, which is often considered to be the energy converting contact determining the diffusion voltage, the transparent and conductive front contact based on oxides, the grain boundaries in the absorber layer. In addressing the different interfaces in solar cell research different levels of understanding must be considered. First and by no means trivial step is a precise determination of the contact properties reached after a certain processing step or sequence of steps. As the electronic response of the device will strongly be influe
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