We present a study of charge transport in polyfluorene:[6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend films. The electron and hole mobilities, measured via time-of-flight photocurrent experiments, show a strong dependence on the PCBM concentration. As a result, ambipolar transport can be obtained in blends of two otherwise unipolar materials. Both the magnitude of the mobility and the shape of the current transient are strongly affected by the presence of PCBM molecules in the film. Highly dispersive electron and hole transport for the pure polymer becomes nondispersive when PCBM is added.

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