Local junction voltages and radiative ideality factors of a -Si:H solar modules determined by electroluminescence imagin
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Local junction voltages and radiative ideality factors of a-Si:H solar modules determined by electroluminescence imaging T. M. H. Tran 1, B. E. Pieters1, M. Schneemann1, T.C.M. Müller1, A. Gerber1, T. Kirchartz2, and U. Rau1 1 IEK5-Photovoltaik, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany, email: [email protected] 2 Department of Physics and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, United Kingdom ABSTRACT In this contribution, we show that the dominant electroluminescent emission of hydrogenated amorphous silicon (a-Si:H) thin-film solar cells follows a diode law, whose radiative ideality factor nr is larger than one. This is in contrast to crystalline silicon and Cu(In, Ga)Se2 solar cells for which nr equals one. As a consequence, the existing quantitative analysis for the extraction of the local junction voltage Vj(r) from luminescence images fails for a-Si:H solar cells. We expand the existing analysis method, and include the radiative ideality factor nr into the model. With this modification, we are able to determine the local junction voltage Vj(r) for a-Si:H solar cells and modules. We investigated the local junction voltage Vj(r) and the radiative ideality factor nr for both initial and stabilized a-Si:H solar modules. Furthermore, we show that the apparent radiative ideality factor is affected by the spectral sensitivity of the used camera system. INTRODUCTION Recently, electroluminescence (EL) has received much attention as a fast inline characterization tool for photovoltaic devices [1-3]. Spatially resolved EL imaging is used to derive minority carrier diffusion length [2], local recombination current, and local series resistance of crystalline silicon solar cells and modules [4,5]. Furthermore, EL is widely used for the detection and quantitative analysis of local defects (shunts) in crystalline silicon solar cells and modules [6,7]. Moreover, Helbig et al. applied an EL imaging method to quantitatively analyze local junction voltage differences, sheet resistances of the front- and back contact, and power losses due to series resistances and shunt resistances in Cu(In,Ga)Se2 (CIGS) thin-film solar modules [8]. The determination of local voltage differences is based on the assumption that the emission follows a diode law with a radiative ideality factor of unity, i.e, nr = 1. If we furthermore assume the superposition principle holds [9], absolute values for the junction voltage Vj can be obtained by calibrating the relative local voltage ∆Vj calculated from EL images with the open circuit voltage (Voc). Note that the absolute junction voltage does not include the voltage drop over (internal) series resistances. This method can also be applied directly for crystalline silicon solar cells and modules. However, in case of hydrogenated amorphous silicon (a-Si:H) solar modules, the interpretation of EL images is not as straight-forward because nr is larger than one, and the superposition principle is not held. In this work, we demonstrate that with a modification of
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