Injection-controlled electroluminescence in organic light-emitting diodes based on molecularly-doped polymers: I. Single-layer devices

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Abstract
Organic single-layer light-emitting diodes (LEDs) based on molecularly-doped polymers (MDPs) have been characterized via their current-field characteristics and the dependence of light output, quantum efficiency, and spectrum of the electroluminescence (EL) on the applied electric field (F) and molecular composition of the organic layers. The results discussed within the framework of the thermionic carrier injection model prove the LEDs to operate in the injection-controlled EL mode. Analytic considerations are presented relating the light output and quantum EL efficiency to the charge recombination mechanisms. The results indicate that the quantum EL efficiency ( EL) is determined by both the diffusion-controlled formation of correlated electron-hole (e-h) pairs and their fusion (ultimate recombination event) into an emitting molecular state. Thus, the increasing EL(F) at low fields is predicted to follow the Langevin-like recombination formalism, whereas the decreasing function EL(F) would be a consequence of the Thomson-like recombination prevailing at higher fields. These predictions are in good agreement with experiment. Information concerning the binding energy and charge separation in the correlated (e-h) pairs can be inferred from the high-field dependence of the EL(F).