Determining the Exciton Diffusion Length in a Polyfluorene from Ultrafast Fluorescence Measurements of Polymer/Fullerene Blend Films

Emission quenching is studied in systems composed of [9,9-dioctylfluorene-co-N-(4-butylPhenyl)-diphenylamine] (TFB) and various concentrations of three different types of acceptors: [6,6]-phenyl-C61 butyric acid methyl ester (mono-PCBM) and its multiadduct analogues bis-PCBM and tris-PCBM. We find that the degree of emission quenching for a given fullerene concentration decreases as the PCBM adduct number increases, while the microstructure, as observed with transmission electron microscopy, becomes more coarse. These observations are rationalized in terms of possible differences in the miscibility of fullerenes in the polymer and different excitation dissociation rates. We also extract a value for the exciton diffusion length in TFB of 9.0 +/- 2 nm from ultrafast fluorescence decay measurements. The results have been confirmed with independent measurements.