Evidence for electric field-assisted dissociation of the excited singlet state into charge carriers in MEH–PPV

Electric field-modulated time-resolved fluorescence was measured in films of MEH–PPV. For excitation near the absorption edge, the applied field leads only to a decrease of fluorescence lifetimes, leaving the initial fluorescence amplitude unaffected, which indicates direct dissociation of the excited states into electron–hole pairs. For higher energy excitation, a small reduction in the initial fluorescence amplitude is also observed. In this case, energy migration from higher to lower energy sites results in a small amplitude quenching. These experimental results are difficult to reconcile with the band model, which suggests direct generation of electron–hole pairs by optical excitation.