Electric field-induced hole injection-enhanced photoluminescence in a N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)-benzidine-based emitter

Non-monotonic, asymmetrical electric field dependence of photoluminescence (PL) intensity is observed in a monolayer sample of tris-(8-hydroxyquinoline) aluminum (AlQ) doped N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)-benzidine (TPD). A possible model is proposed: the charge separation from the dissociated photoexcited excitons causes energy band bending in the organic films and improves the hole injection from the electrode, which brings about the extra fluorescence. This mechanism is further verified by a series of experiments using a series of samples, variously featuring symmetrical electrodes, block layers, and hosts with lower hole mobilities.