Exciton diffusion and dissociation in conjugated polymer fullerene blends and heterostructures

We investigate the exciton dynamics in composite systems of conjugated poly mers and fullerene molecules by photoluminescence (PL) and femtosecond tran sient absorption experiments. In solid mixtures (blends) we find a strong c oncentration-dependent quenching of the polymer FL. This is attributed to a n efficient electron transfer (ET) from the photoexcited conjugated polymer to the fullerene. The ET dynamics is directly monitored by measuring the t ransient stimulated emission of the conjugated polymer. The transfer rate d epends linearly on the C-60 concentration and ranges between (66 ps)(-1) an d (5 ps)(-1) for concentrations from 0.5% to 5%. This dependence is in acco rdance with an exciton diffusion process occurring prior to the ET. The exc iton diffusion length in the conjugated polymer is directly determined by m easuring the PL quenching in well-defined heterostructures comprising a sel f-assembled fullerene monolayer and a thin spin-coated polymer layer of var iable thickness. From these measurements we infer a value of 14 nm for the exciton diffusion length in ladder-type poly (p-phenylene). Our results are of direct relevance for further optimization of polymer photovoltaic devic es. [S0163-1829(99)07923-0].