p-ZnTe for Back Contacts to CdTe Thin Film Solar Cells
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p-ZnTe for Back Contacts to CdTe Thin Film Solar Cells Bettina Späth, Jochen Fritsche, Andreas Klein and Wolfram Jaegermann Darmstadt University of Technology, Institute of Materials Science, Surface Science Division, Petersenstrasse 23, 64287 Darmstadt, Germany
ABSTRACT CdTe thin film solar cells still suffer from problems related to back contacts which provide stable ohmic contacts without electrical losses. In previous studies metal/CdTe contacts have shown significant limitations. A promising option for the realization of ohmic back contacts is the use of a highly p-doped ZnTe interlayer on top of the CdTe absorber, in particular since metal/ZnTe contacts have shown very good electrical properties. In this work we studied the electronic und chemical properties of nitrogen doped p-ZnTe films. p-ZnTe:N films were prepared by using reactive RF magnetron sputtering with N2/Ar gas mixtures or by thermal evaporation with an additional nitrogen plasma source. Samples and their contacts have been prepared in DAISY-SOL (DArmstadt Integrated SYstem for SOLar energy research) which combines a full vacuum production with an in-situ photoelectron spectroscopy (XPS/UPS) analysis. The results of XPS/UPS investigations and electrical measurements will be discussed in comparison to previous results on metal/CdTe contacts. As ZnTe forms an interlayer in the CdTe thin film solar cell, the ZnTe/CdTe interface properties must be also taken into consideration. Our experiments have shown in agreement to previous studies that the valence band offset is nearly ideal for hole transport across the interface. Also electrical measurements have been carried out to investigate the metal/ZnTe/CdTe layer sequences in their contact properties.
INTRODUCTION CdTe thin film solar cells are now investigated for more than thirty years. The best so far obtained efficiencies of 16.4 % [1, 2] are, however, still far from their potential of at least 25 %. Although the CdTe solar cell is made of a less complex binary compound semiconductor, much less work has been performed on materials and interface related issues concerning solar cell applications compared to the Cu(In,Ga)(S,Se)2 material system. Nevertheless several production lines have been started to fabricate large area (60 cm × 120 cm) CdTe solar cell modules. One of the world’s largest CdTe solar cell production plants with a capacity of ~ 10 MWp has been built by ANTEC Solar in Arnstadt, Germany [3]. The high capacity of the production line, which is able to produce modules with a frequency of 90 seconds, is achieved using very fast deposition techniques as sputtering and closed spaced sublimation (CSS). The modules are prepared on glass substrates starting with a tin dioxide/indium tin oxide (ITO) window layer deposited by magnetron sputtering followed by CdS and CdTe layers, which are deposited by closed spaced sublimation at high substrate and source temperatures [3-5]. A so-called activation step is necessary after CdTe deposition to achieve high conversion efficiencies, typically
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