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Abstract:
If the ratio of two open circuit photoluminescence (Voc-PL) images taken at two different light intensities is displayed, some grain boundaries (GBs) may show up as bright lines. This indicates that these special GBs show distinct injection intensity-dependent recombination properties. It will be shown here that this results in an apparent ideality factor of the light emission smaller than unity. The effect is reproduced with numerical device simulations using a usual distribution of defects in the band gap along grain boundaries. Quantitative imaging of this apparent luminescence ideality factor by PL imaging is complicated by the lateral horizontal balancing currents flowing at open circuit. The local voltage response of an inhomogeneous solar cell at different injection levels under open circuit is modelled by Griddler simulations, based on PL investigations of this cell. The evaluation of Voc-PL images at different illumination intensities allows us to conclude that the apparent luminescence ideality factor at the special GBs is about 0.89, whereas in the other regions it is between 0.94 and 0.95. Reverse bias electroluminescence showed no pre-breakdown sites, and hyperspectral PL imaging showed only in one of the investigated GBs particular defect luminescence. TEM investigations of two GBs, one showing distinct injection intensity-dependent recombination and the other one showing none, revealed that the investigated special GB is a large-angle GB whereas the GB not showing this effect is a small-angle GB.
