Exciton diffusion and energy transfer in organic solar cells based on dicyanovinyl-terthiophene

We discuss exciton transport and energy transfer in organic solar cells based on dicyanovinyl-terthiophene (DCV3T). Time-resolved surface luminescence quenching experiments on double layers of DCV3T and zinc-phthalocyanine as luminescence quencher are analyzed in the framework of a three-level luminescence model with an initially absorbing state of short lifetime that relaxes to a longer living emitting state. Luminescence quenching of the emitting state is assigned to Forster-type energy transfer with an apparent Forster radius of 2.1 nm, which is in accordance with the Forster radius obtained from the spectral overlap integral. A diffusion based analysis for the emitting state yields a diffusion length of L-e=6.9 nm. The short living initial state is quenched by diffusion of the excitons to the interface with a diffusion length of L-a=5.3 nm. External quantum efficiency measurements of the photocurrent in a corresponding organic solar cell structure evaluated with a two-level diffusion model give a diffusion length of L-EQE=6.0 nm, whereas a two-level model for stationary luminescence quenching yields a diffusion length of L-cw=12 nm. This suggests that only one type of the excitons contributes to the photocurrent.