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By using electric field induced optical second harmonic generation measurement, charge accumulation at the double-layer interface of ITO/α-NPD/Alq3/Al diodes was verified under two electroluminescence (EL) operational modes, which were activated in the low and high frequency regions, respectively, with application of large ac square voltage. Results supported our proposed idea [A. Sadakata et al., J. Appl. Phys. 110, 103707 (2011)] that accumulated holes suppress hole injection in the low frequency region and lead to the decrease of the EL intensity activated by the recombination of holes and electrons injected from opposite electrodes. On the one hand, the accumulated holes assist electron injection in the high frequency region and result in the increase of EL intensity activated by the recombination of the interfacial accumulated holes and injected electrons from Al electrode. [ABSTRACT FROM AUTHOR]

Abstract: By using time-resolved electric-field-induced optical second-harmonic generation measurement, we studied the electroluminescence (EL) mechanism which is activated in double-layer organic light-emitting diodes (indium tin oxide/N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD)/tris(8-hydroxy-quinolinato) aluminum (III) (Alq3)/Al) in the high frequency region with application of AC square voltages at various duty-ratios. Results showed that holes were accumulated at the α-NPD/Alq3 interface in proportion to the duty-ratio, and corresponded well to the increasing EL enhancement at the high frequency. Our previous proposed model on the EL enhancement assisted by electron injection from the Al electrode in the presence of the space charge field was well confirmed. [Copyright &y& Elsevier]