Dissociation of hot excitons in ladder-type polymer light-emitting diodes

We present photocurrent excitation cross-correlation experiments which provide time-resolved information on the dissociation of photoexcited states in a prototypical π-conjugated polymer. A first pump pulse generates singlet exciton states (S1) which are re-excited to a higher lying state by a second time delayed pulse resonant with a singlet exciton transition (S1→Sn). Dissociation is monitored via the change of photocurrent response in single layer devices. Our experiment probes diffusion driven, ultrafast dissociation of inter-chain-hot excitons during inter-chain-migration to sites lower in energy. Once this process has concluded intrinsic dissociation occurs from a higher lying state reached after the second excitation.