Device Analysis of Cu(In,Ga)Se2 Heterojunction Solar Cells - Some Open Questions

Abstract

ABSTRACTThe 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.