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Anomalous dark current ideality factor (n > 2) in thin-film solar cells: The role of grain-boundary defects

By comparing simulated and measured dark I-V characteristics of CIGS cells, we investigate the low-voltage anomalous (> 2) ideality factor, and find that it can be explained by large donor trap concentrations at grain boundaries, and SRH recombination therein, with no need of complex models involving tunneling, coupled traps, etc. We studied 3 different samples, in all cases achieving excellent fit of the non-monotonic ideality factor. The illuminated cell performance also matches the experiments very well. The most important parameter determining value and voltage dependence of the anomalous ideality factor is the peak energy of the grain boundary donor distribution.