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

Abstract

A previously developed kinetic scheme for charge carrierrecombination in single-layer (SL) organic light emitting diodes (LEDs)[Kalinowski J, Cocchi M, Giro G, Fattori V and Di Marco P 2001J. Phys. D: Appl. Phys. 34 2274] in which, dependent on the applied field, the formationof a correlated carrier pairs (CPs) or their subsequent dissociation into freecarriers becomes a rate-determining process is extended to double-layer (DL)LEDs based on molecularly-doped polymers (MDPs). At high fields thedissociation of CPs becomes progressively important, indicating the Thomsonrather than Langevin recombination to operate within the emission zone. Thecurrent-field characteristics of the DL ITO/MDP/Alq3/Mg/Ag diodes as wellas the field evolution of their light output and quantum yield prove thedevices operate in the injection-controlled electroluminescence (EL) mode. Itis shown that manipulating the molecular composition of MDP-basedhole-transporting layers and relation between component layer thickness allowsone to maximize the quantum EL efficiency of such DL LEDs. The resultsindicate that the internal redistribution of the electric field due to theinterface accumulation of charge does not modify the Schottky-like behaviourof the current but leads to a quantitative difference in its characteristicapparent parameter. The nonlinearly voltage increasing leakage of carriers atthe interface of Alq3 with a multicomponent MDP layer leads to the lightemission from this layer to increase progressively as compared to the ELoutput from Alq3, allowing voltage control of the LED colour. Themicrocavity effects account for a maximum light output and cell conductivityoccurring when the thickness of each of the two constituent layers isapproximately 60 nm.