Efficiency of exciton and charge carrier photogeneration in a semiconducting polymer

We determine the efficiencies for the formation of excitons and charge carriers following ultrafast photoexcitation of a semiconducting polymer (MEH-PPV). The simultaneous, quantitative determination of exciton and charge photoyields is achieved through subpicosecond studies of both the real and the imaginary components of the complex conductivity over a wide frequency range. Predominantly excitons, with near-unity quantum efficiency, are generated on excitation, while only a very small fraction (<10^-2) of free charges are initially excited, consistent with rapid (~100 fs) hot exciton dissociation. These initial charges are very short lived, decaying on subpicosecond time scales.