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 o f MEH-PPV. For excitation near the absorption edge, the applied field leads only to a decrease of fluorescence Lifetimes, leaving the initial fluoresc ence amplitude unaffected, which indicates direct dissociation of the excit ed states into electron-hole pairs. For higher energy excitation, a small r eduction in the initial fluorescence amplitude is also observed. In this ca se, energy migration from higher to lower energy sites results in a small a mplitude quenching. These experimental results are difficult to reconcile w ith the band model, which suggests direct generation of electron-hole pairs by optical excitation. (C) 1998 Elsevier Science B.V. All rights reserved.