Electroluminescence imaging of Cu(In,Ga)Se 2 thin film solar modules
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Electroluminescence imaging of Cu(In,Ga)Se2 thin film solar modules Uwe Rau1, Thomas Kirchartz1, Anke Helbig2, and Bart E. Pieters1 1 IEF5-Photovoltaik, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany 2 Institut für Physikalische Elektronik, Universität Stuttgart, 70569 Stuttgart, Germany ABSTRACT Electroluminescence images gained from Cu(In,Ga)Se2 mini-modules under different voltage bias conditions are investigated. The mini-modules of area 20 x 20 cm2 with 42 cells exhibit typically 10-20 localized shunts. The consequences of these shunts on the performance of the individual cells and of the entire module are analyzed quantitatively by evaluating the electroluminescence images. Our evaluation method uses the fact that the electroluminescence intensity at each position in each cell within the module depends on the actual voltage drop over the junction at this specific location. Thus, the analysis of the electroluminescence intensity allows us to reconstruct the current/voltage characteristics of all individual cells in the module. In addition, we provide first simulations using a distributed diode network model to quantitatively explain the experimental results. INTRODUCTION Spatially resolved electroluminescence (EL) imaging has attracted much attention [1-3] in recent years, due to its ability to obtain information about the solar cell, solar module or solar system with high resolution in a short time. Up to now, this method has been mostly applied to crystalline silicon solar cells to obtain information about the series resistances [4], the shunted regions in the cell [5], or the diffusion length [6].Up to now, characterization of thin-film Cu(In,Ga)Se2 (CIGS) photovoltaic devices with EL was focused on investigations of the dependence of the global EL emission on temperature and photon energy [7,8]. The present contribution investigates the spatially resolved EL of mini-modules made from CIGS. These CIGS solar modules are fabricated by an industrial in-line co-evaporation process on Mo-covered glass substrate and finished by chemical bath deposition of the CdS layer and by sputtering of the ZnO window layer [9]. The present approach to EL analysis uses the ability of luminescence emission to characterize individual parts of a solar cell or module independently. Thus, the method used in this paper is similar to that recently applied to the EL analysis of GaInP/GaInAs/Ge triple-junction cells, which enables access to the individual current density/voltage (JV) curves of the different subcells in the stack [10]. In the present case of CIGS modules, we extract the JV-curves of the individual subcells of a solar module by analyzing EL images recorded at different injection currents. EXPERIMENTAL RESULTS Figure 1 displays EL images of a CIGS module at two different current densities J = 1.25 mA/cm2 (a) and at J = 50 mA/cm2 (b). The module consists of Nc = 42 cells connected in series where
Figure 1a-b: EL images at a) J = 1.25 mA/cm² and b) J = 50 mA/cm² of the same module. Areas with quenched EL
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