Electroluminescence of Cu(In,Ga)Se 2 solar cells and modules
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Electroluminescence of Cu(In,Ga)Se2 solar cells and modules U. Rau1, T. C. M. Müller1, T. M. H. Tran1, B. E. Pieters1, and A. Gerber1 1 IEK5-Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany ABSTRACT Fundamental aspects of (electro-)luminescence of Cu(In,Ga)Se2 solar cells and modules are investigated by means of spectrally and spatially resolved measurements. The validity of the reciprocity relation between spectrally resolved electroluminescence emission and photovoltaic quantum efficiency is verified for the case of industrially produced ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells. Further we find that photo- and electroluminescent emission in these devices obey a superposition principle only in a limited range of the applied electrical or illumination bias. This range depends on the light soaking history of the sample and extends up to an injected current density of approximately 15 mAcm-2 after 3 h of light soaking at a temperature of 400 K. In the state prior to light soaking this range is limited to 4 mAcm-2. At higher bias, a characteristic discrepancy between electroluminescence and electro-modulated photoluminescence appears. We attribute this anomaly to a potential barrier behavior close to the CdS/ Cu(In,Ga)Se2 interface. Metastable defect reactions induced by holes injected into the space charge region partly reduce this barrier. We further find that the luminescence efficiency is enhanced by a factor of 3 by light soaking at 400 K. Spatially resolved electroluminescence measurements conducted during application of voltage or current bias at ambient temperature in the dark are qualitatively compatible with the conclusions drawn from the spectrally resolved measurements. INTRODUCTION Electroluminescence (EL) is the complementary physical action to the normal operating mode of a solar cell or module. Therefore, EL imaging [1] is an attractive tool for the characterization of such devices. As a direct semiconductor, Cu(In,Ga)Se2 (CIGS) is especially suitable for this method and EL imaging was used for the analysis of CIGS modules in the past [2-5]. In recent years, EL was widely used to gain quantitative information, e.g., on resistive losses within a solar module or on the fundamental properties of light absorption and emission in a photovoltaic material. Thus, the method is useful in a wide range of materials and length scales. However, the basic principles and the physical preconditions justifying a simple quantitative interpretation of EL measurements are usually not scrutinized. The present paper starts with a rigorous experimental analysis of basic physical constraints on the luminescence emission that have to be met in order to allow for a straightforward interpretation of spectrally and/or spatially resolved EL measurements. We find that CIGS solar cells only half-way meets these requirements and that the conditions under which a simple quantitative analysis is possible changes with the light soaking history of the sample. Therefore, we use luminescence measurements as a tool for a
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