Effect of molecular packing on the exciton diffusion length in organic solar cells

The efficiency of photocurrent generation in bilayer organic solar cells is shown to increase when molecular order is improved. This effect is studied in cells using pure cis and trans isomers of 3,4,9,10-perylene tetracarboxylic bisbenzimidazole. X-ray diffraction studies show that the π-π stacking direction lies in the substrate plane for both isomers and that the trans isomer exhibits improved molecular order in the out-of-plane direction. The improved stacking leads to an increased exciton diffusion length and increased external quantum and power conversion efficiencies. These results provide insight into the effect of molecular structure and packing on the exciton diffusion length.