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

Abstract

Organic single-layer light-emitting diodes (LEDs) based onmolecularly-doped polymers (MDPs) have been characterized via theircurrent-field characteristics and the dependence of light output, quantumefficiency, and spectrum of the electroluminescence (EL) on the appliedelectric field (F) and molecular composition of the organic layers. Theresults discussed within the framework of the thermionic carrier injectionmodel prove the LEDs to operate in the injection-controlled EL mode. Analyticconsiderations are presented relating the light output and quantum ELefficiency to the charge recombination mechanisms. The results indicate thatthe quantum EL efficiency (φEL) is determined by both thediffusion-controlled formation of correlated electron-hole (e-h) pairs andtheir fusion (ultimate recombination event) into an emitting molecular state.Thus, the increasing φEL(F) at low fields is predicted to followthe Langevin-like recombination formalism, whereas the decreasing functionφEL(F) would be a consequence of the Thomson-like recombinationprevailing at higher fields. These predictions are in good agreement withexperiment. Information concerning the binding energy and charge separation inthe correlated (e-h) pairs can be inferred from the high-field dependence ofthe φEL(F).