Electric field-assisted dissociation of singlet excitons in tris-(8-hydroxyquinolinato) aluminum (III)

Results based on the electric-field dependence of electro-modulated fluorescence and steady-state photoconduction in films of tris-(8-hydroxyquinolinato) aluminum (III) $({\mathrm{Alq}}_3)$ are consistent with a model where singlet excitons dissociate into electron–hole pairs through charge transfer state. Dissociation into free carriers or predominantly geminate recombination results from this intermediate state. The quantum efficiency of charge separation is in agreement with predictions based on a model using the three-dimensional Onsager theory of geminate recombination combined with volume (bimolecular) recombination of the photogenerated space charge. This model is used to account for the decrease in the electroluminescence quantum efficiency of ${\mathrm{Alq}}_3$ -based organic light-emitting diodes at high electric fields.