Electric field, light wavelength and excitation intensity characteristics of photoconduction have been measured in an aluminum (Al)/tris-(8-hydroxyquinolinato) aluminum( III) (Alq3)/Al sandwich system. The results show the photocurrent to be underlain by intrinsic dissociation of singlet excited states (Alq3 * ) into a pair of free electrons and holes. However, conventional theories, such as Onsager or Poole-Frenkel models for charge photogeneration cannot account for the experimentally observed characteristics. Excellent agreement with experiment is provided by the 3D-Onsager theory of geminate recombination combined with volume (bimolecular) recombination of the photogenerated space charge. This model predictions are also consistent with independent electric field-induced fluorescence quenching experiments and are able to account for the decrease in the electroluminescence quantum efficiency of Alq3-based organic light-emitting diodes at high electric fields.
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