Electroluminescence from trap-limited current transport in vacuum deposited organic light emitting devices

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Abstract
We investigate the current transport and electroluminescence mechanisms controlling the efficiency of organic heterojunction, green light emitting devices consisting of tris-(8-hydroxyquinoline) aluminum (Alq) and a hole transport layer (HTL). From the dependence of the current-voltage characteristics and luminescence efficiency on layer thickness and HTL composition, we find that current is controlled by trap-limited transport of electrons injected into Alq. Electroluminescence efficiency, on the other hand, is controlled by hole injection from the HTL into Alq. The luminescence is generated by recombination of electrons localized at a high density trap in the Alq, and holes injected from the HTL. These results provide insight into the fundamental processes which control the operating voltage and luminescence efficiency of vacuum deposited organic light emitters.