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 (alpha-NPD)/tris(8-hydroxy-quinolinato) aluminum (III) (Alq(3))/Al) in the high frequency region with application of AC square voltages at various duty-ratios. Results showed that holes were accumulated at the alpha-NPD/Alq(3) 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. (C) 2013 Elsevier B.V. All rights reserved.