Probing interfacial charge accumulation in ITO/α-NPD/Alq3/Al diodes under two electroluminescence operational modes by electric-field induced optical second-harmonic generation

By using electric field induced optical second harmonic generation measurement, charge accumulation at the double-layer interface of ITO/alpha-NPD/Alq(3)/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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4762014]