Imbalanced mobilities causing S-shaped IV curves in planar heterojunction organic solar cells

We show that S-kinks in the current voltage characteristics, which decrease the fill factor significantly, can be caused by a strong imbalance of charge carrier mobilities (hole mobility in donor and electron mobility in acceptor) in planar/flat heterojunction organic solar cells. Electrical simulations according to a drift-diffusion model predict the occurrence of an S-kink for a mobility mismatch factor larger than 100. By combining a low-mobility donor material, (1,2,3,4,9,10,11,12-octaphenyl-diindeno[1,2,3-cd: 1',2',3'-lm]perylene), with the acceptors C(60) and N, N'-dimethylperylene-3,4: 9,10-dicarboximide, which show different electron mobilities, we experimentally verify the predictions. Our results demonstrate that not only interface effects but also the photoactive material itself can cause S-kinks. (C) 2011 American Institute of Physics. [doi:10.1063/1.3553764]