Analysis of Carrier Transients in Double-Layer Organic Light Emitting Diodes by Electric-Field-Induced Second-Harmonic Generation Measurement

By using time-resolved electric-field-induced optical second-harmonic generation (EFISHG) measurements, we directly probed carrier transients in double layer (alpha-NPD/Alq3) organic light emitting diodes (OLED) (alpha-NPD, N,N'-di-[(1-naphthyl)-N,N'-diphenyl]-(1,1'-biphenyl)-4,4'-diamine; Alq3, tris(8-hydroxy-quinolinato)aluminum(III)). Results showed that carrier transients are comprised of two relaxation processes. One is charging on electrodes in a single exponential form exp(-t/tau(RC)) with a time constant of tau(RC) = RsC0 (R-s, connecting lead resistance; C-0, device capacitance), independent of applied voltage. The other one is charging at the alpha-NPD/Alq3 interface in a stretched form exp(-(t/tau)(beta)) (beta = 1.6 +/- 0.1) with a relaxation time, tau, proportional to V-2.1 (OLED with Al cathode) and V-2.6 (LiF/Al cathode). The Maxwell-Wagner model analysis well accounts for the two relaxation processes. We conclude that analysis of the carrier transients, directly probed by EFISHG, is effective for understanding the carrier mechanism leading to electroluminescence in OLEDs.