Quantitative evaluation method for electroluminescence images of a-Si: H thin-film solar modules

This work presents a method for extracting the absolute local junction voltage of a-Si:H thin-film solar cells and modules from electroluminescence (EL) images. It is shown that the electroluminescent emission of a-Si:H devices follows a diode law with a radiative ideality factor n(r) larger than one. We introduce an evaluation method that allows us to determine the absolute local junction voltage in cases of n(r) > 1, while existing approaches rely on the assumption of n(r) = 1. Furthermore, we find that the experimentally determined values of n(r) vary from sample to sample. It is also explained why the derived radiative ideality factor is influenced by the spectral sensitivity of the camera system used in the experiment. ((c) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)