2 February 2001 Recombination limit for quantum efficiency of organic light-emitting diodes
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Abstract
A two-step kinetic model for the recombination process is presented to describe experimentally observed emission characteristics of organic light-emitting diodes (LEDs) including their electric field-dependent quantum efficiency (QE) and field evolution of the electroluminescence (EL) spectra. The model assumes the emitting excited states to be produced through an intermediate charge pair (CP) state whose decay is subject to the competitive processes of which formation of various emissive states, such as localized molecular excitons, locally excited excimers and exciplexes, or electroplexes, and dissociation into two separated carriers are expected to be of crucial importance for the QE of organic LEDs and shape of their EL spectra. Depending on the carrier injection efficiency at the electrodes and the magnitude of applied field the recombination probability can be either governed by the CP formation-to-transmit time ratio (usually at low fields) or the capture-to-dissociation time ratio of CP states (usually at high fields). Combining both an expression for the QE in organic LEDs is proposed. The field and temperature dependence of the QE is discussed, showing that the reciprocal of QE can be approximated by a Poole-Frenkel-type field dependence with a negative gradient at low fields and positive gradient in the upper limit of the accessible field range. The experimental data for single layer (SL) and double layer (DL) LEDs confirm these predictions; as a rule, the QE increases in the low-field, and decreases in the high-field regime of applied fields. Increasing probability of CP dissociation, considered as a part of the overall recombination process, is thus a limiting factor for the QE, unless the increasing field enhances in some way formation of emitting states. Such an effect is observed with some organic DL LEDs, where donor- acceptor species form heteromolecular CPs with their electric dipole moments oriented antiparallel to the applied field.
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Jan Kalinowski, "Recombination limit for quantum efficiency of organic light-emitting diodes", Proc. SPIE 4105, Organic Light-Emitting Materials and Devices IV, (2 February 2001); doi: 10.1117/12.416895; https://doi.org/10.1117/12.416895
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