Interfaces in CdTe Solar Cells: From Idealized Concepts to Technology
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Interfaces in CdTe Solar Cells: From Idealized Concepts to Technology Wolfram Jaegermann, Andreas Klein, Jochen Fritsche, Daniel Kraft, Bettina Späth Darmstadt University of Technology, Institute of Materials Science, Surface Science Division, Petersenstrasse 23, 64287 Darmstadt, Germany
ABSTRACT In thin film solar cells interfaces between lattice mismatched or dissimilar materials are used for the front and the back contact. A p-i-n device structure should be possible as most simple but ideally suited thin film solar cell. In contrast the interfaces in CdTe solar cells are found to be much more complex containing interdiffused phase boundaries at the front as well as at the back contact. By comparison to non-interdiffused interfaces using contact phases of adapted work functions it can be shown that the contact potentials of the front contact but also of the back contact are dominated by Fermi level pinning. The pinning states are evidently due to dislocation defects at the boundary of CdTe to the contact phases. Based on these results it is concluded that interdiffused phase boundaries or appropriate passivation layers are a precondition for efficient solar cells whenever strongly lattice mismatched or dissimilar materials are combined.
INTRODUCTION Thin film solar cells have always been considered as promising alternatives for economic competitiveness of photovoltaic energy converters. But despite of promising scientific results obtained for thin film Si, CIS or CdTe solar cells the photovoltaic market is still dominated by single and polycrystalline bulk Si converters. One of the reasons is evidently due to the fact that the given thin film device structures turn out to be more complicated as may be expected from a naïve point of view and therefore need complicated preparation and processing procedures, which are not available as standard and thus rather cheap technologies. As most simple and also most effective thin film device structure a p-i-n cell may be envisaged, which is composed of a thin film intrinsic absorber layer between p+ and n+ hetero contacts of transparent and highly conductive window materials (see Fig. 1). Such device structures have already been suggested some time ago for CdTe absorber films [1] and later on for FeS2 [2] and layered chalcogenides [3]. In comparison to the dye injection type solar cell a similar working principle is used for energy conversion: after light absorption and electron-hole pair production in the absorber layer the respective charge carriers are injected into the respective contact phases and are transformed there to majority carriers (see also Ref. [4]). As the most consequent transformation of the dye sensitizer concept to solid state devices the eta cell has been suggested [5]. In all these devices high conversion efficiencies are expected, when the diffusion length l=(Dτ)1/2 of both charge carriers (at the maximum power point) exceeds the thickness of the absorber layer d, which on the other hand should exceed 3/α (α: absorption coefficient) for optim
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