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

A previously developed kinetic scheme for charge carrier recombination in single-layer (SL) organic light emitting diodes (LEDs) [Kalinowski J, Cocchi M, Giro G, Fattori V and Di Marco P 2001 J. Phys. D: Appl. Phys. 34 2274] in which, dependent on the applied field, the formation of a correlated carrier pairs (CPs) or their subsequent dissociation into free carriers becomes a rate-determining process is extended to double-layer (DL) LEDs based on molecularly-doped polymers (MDPs). At high fields the dissociation of CPs becomes progressively important, indicating the Thomson rather than Langevin recombination to operate within the emission zone. The current-field characteristics of the DL ITO/MDP/Alq(3)/Mg/Ag diodes as well as the field evolution of their light output and quantum yield prove the devices operate in the injection-controlled electroluminescence (EL) mode. It is shown that manipulating the molecular composition of MDP-based hole-transporting layers and relation between component layer thickness allows one to maximize the quantum EL efficiency of such DL LEDs. The results indicate that the internal redistribution of the electric field due to the interface accumulation of charge does not modify the Schottky-like behaviour of the current but leads to a quantitative difference in its characteristic apparent parameter. The nonlinearly voltage increasing leakage of carriers at the interface of Alq(3) with a multicomponent MDP layer leads to the light emission from this layer to increase progressively as compared to the EL output from Alq(3), allowing voltage control of the LED colour. The microcavity effects account for a maximum light output and cell conductivity occurring when the thickness of each of the two constituent layers is approximately 60 nm.