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

Correlated quantum-chemical techniques are applied to the description of el ectronic excitations in luminescent conjugated polymers. We first address t he role of intermolecular interactions on the emission properties of organi c conjugated materials. The nature of the lowest excited states in molecula r aggregates is discussed and a special emphasis is devoted to the chain-le ngth dependence of the exciton coupling. By applying a molecular orbital pe rturbation approach, we then calculate the formation rates for singlet and triplet molecular excitons associated with intermolecular charge-transfer p rocesses. Application of our approach to a model system for poly(paraphenyl enevinylene) shows that the ratio between the electroluminescence and photo luminescence quantum yields generally exceeds the 25% spin-degeneracy stati stical limit. (C) 2001 Elsevier Science B.V. All rights reserved.