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 carriermobilities(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 interfaceeffects but also the photoactive material itself can cause S-kinks.