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 s ingle-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 car rier pairs (CPs) or their subsequent dissociation into free carriers become s 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 Langevi n recombination to operate within the emission zone. The current-field char acteristics of the DL ITO/MDP/Alq(3)/Mg/Ag diodes as well as the field evol ution of their light output and quantum yield prove the devices operate in the injection-controlled electroluminescence (EL) mode. It is shown that ma nipulating the molecular composition of MDP-based hole-transporting layers and relation between component layer thickness allows one to maximize the q uantum EL efficiency of such DL LEDs. The results indicate that the interna l redistribution of the electric field due to the interface accumulation of charge does not modify the Schottky-like behaviour of the current but lead s to a quantitative difference in its characteristic apparent parameter. Th e nonlinearly voltage increasing leakage of carriers at the interface of Al q(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), a llowing voltage control of the LED colour. The microcavity effects account for a maximum light output and cell conductivity occurring when the thickne ss of each of the two constituent layers is approximately 60 nm.