Device Analysis of Cu(In,Ga)Se 2 Heterojunction Solar Cells - Some Open Questions
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Device Analysis of Cu(In,Ga)Se2 Heterojunction Solar Cells - Some Open Questions
U. Rau, K. Weinert, Q. Nguyen, M. Mamor, G. Hanna, A. Jasenek, and H. W. Schock Institute of Physical Electronics, University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany
ABSTRACT The paper discusses the electronic properties of Cu(In,Ga)Se2–based heterojunction solar cells with a special focus on questions which at present are not satisfactorily understood. First, we discuss an apparent quantitative contradiction between measured concentrations of recombination centers in the Cu(In,Ga)Se2 absorber material and the actual recombination rate in the solar cells. We propose, as a possible explanation for that observation, that the defect concentration in Cu(In,Ga)Se2 is spatially inhomogeneous with a systematic increase towards the heterojunction interface. Second, we address the issue of electronic metastabilities in ZnO/CdS/Cu(In,Ga)Se2 heterojunctions and, especially, in devices that use alternative buffer materials instead of CdS. Starting from a brief review of the experimentally observed types of metastabilities, we demonstrate by thermally stimulated capacitance measurements that a specific type of metastability that severely limits the performance of solar cells with non-CdS buffers is present also in high-efficiency standard devices though it has virtually no influence on the output parameters in the latter case. A possible explanation of this type of metastability points to a metastable defect reaction localized in the close to surface region of Cu(In,Ga)Se2. At the moment we cannot propose conclusive models for both open questions. However, we can localize the answers to both problems in the close-to-surface region of the Cu(In,Ga)Se2 absorber. INTRODUCTION The number of possible models to explain the electronic behavior of solar cells is inversely proportional to the degree of device perfection. To explain a device with an efficiency at the theoretical maximum we would need hardly more than the detailed balance considerations of Shockley and Queisser [1]. In contrast, for solar cells with say 1 % efficiency, we would struggle hard to find reproducible experiments that allow for a single unambiguous explanation. Moreover, such a model might not survive the next technological step that leads to an improved device quality. Thus, the number of possible models for low-performance devices is likely to grow to infinity. With these two extremas in mind, device analysis finds itself in a fortunate situation in the case of Cu(In,Ga)Se2 solar cells: The degree of device perfection achieved during recent years is the highest of all thin-film technologies yet challenging multi-crystalline bulk silicon in terms of efficiency. Therefore, from the device analysis point of view, we expect to get reproducible and physically meaningful results. The explanations can be focussed on what is important and they should converge towards a finite number of meaningful models with non-zero lifetime. However, the gap between an efficiency le
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