On the luminescence efficiency of polymer light-emitting diodes:: a quantum-chemical investigation

Correlated quantum-chemical techniques are applied to the description of electronic excitations in luminescent conjugated polymers. We first address the role of intermolecular interactions on the emission properties of organic conjugated materials. The nature of the lowest excited states in molecular aggregates is discussed and a special emphasis is devoted to the chain-length dependence of the exciton coupling. By applying a molecular orbital perturbation approach, we then calculate the formation rates for singlet and triplet molecular excitons associated with intermolecular charge-transfer processes. Application of our approach to a model system for poly(paraphenylenevinylene) shows that the ratio between the electroluminescence and photoluminescence quantum yields generally exceeds the 25% spin-degeneracy statistical limit. (C) 2001 Elsevier Science B.V. All rights reserved.