Quantitative Defect Analysis on Solar Cells by Laser Beam Induced Current (LBIC) Measurements and 3D Network Simulations
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Quantitative Defect Analysis on Solar Cells by Laser Beam Induced Current (LBIC) Measurements and 3D Network Simulations Minh Nguyen,1,2 Andreas Schütt,2 Jürgen Carstensen,2 and Helmut Föll2 1 Corporate Research, Robert Bosch GmbH, Stuttgart, Germany 2 Institute for Materials Science, Christian-Albrechts-University, Kiel, Germany ABSTRACT Measurements with the CELLO (solar cell local characterization) technique in the LBIC (laser beam induced current) mode under dark conditions with various constant bias voltages are used to analyze the lateral distribution, and mean values, of photocurrent response maps. Local solar cell defects such as local shunts were found to have a characteristic bias voltage dependence: At negative and small positive voltages a local shunt resistance gives less current response than the adjacent area. Upon applying higher positive voltages, a transition of the mean value to lower current response and an inversion of the local defect characteristics are found. These results were modeled by a newly introduced three dimensional (3D) equivalent circuit model of a solar cell divided into subcells. Measurements and simulations of solar cells with various local defects show our method to be a new powerful tool for the quantitative analysis of local solar cell defects. INTRODUCTION Thin film photovoltaic technologies, like CIGS, thin-film silicon or organic photovoltaics (OPV) offer the potential of low-cost production at relatively high efficiency levels [1]. Scaling up from lab cell sizes to mass production relevant sizes requires production and quality control on the whole area of the solar cell taking into account coating induced defects, edge shunting effects etc. Therefore, imaging methods like electro- and photoluminescence [2], thermography [3, 4] or light beam induced current (LBIC) [5, 6] and its further development CELLO [7] become more and more important. However, a quantitative analysis of local solar cell defects is still difficult as it usually requires a sophisticated calibration of the obtained images [8] or extensive numerical (device) simulations [9]. In the present work, the CELLO technique is used to unambiguously identify local defects as reduced shunt resistances and provides the possibility to quantify them. EXPERIMENT The CELLO technique is applied to microcrystalline thin film silicon (μc-Si) solar cells. The laser induced current responses at different constant bias voltages are measured at switched off global illumination. A scheme of the measurement setup is depicted in Fig. 1. The potentiostat unit together with the feedback control allow for very accurate current measurements while guaranteeing a constant bias voltage.
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SIMULATION For the simulations, a 3D equivalent circuit network is used and shown in Fig. 3. The squared solar cell is modeled with SPICE by a resistance network and by subcells connected in parallel, already described in [10]. The CELLO measurement is modeled by the following procedure: For all subcell positions (i,j) the difference of the t
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