A giant surface potential (GSP) has been observed on a
tris-(8-hydroxyquinolate) aluminum (Alq<sub>3</sub>) film deposited on a glass or a metal substrate under dark conditions. However, the effects of a GSP on the device properties of Alq<sub>3</sub>-based organic light-emitting diodes (OLEDs) has not been considered. In this paper, we report on the effects of ambient light during the fabrication of an Alq<sub>3</sub>-based OLED on the device properties by displacement current measurement. We found that the light irradiation significantly reduces the density of charge existing at the
4,4'-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl/Alq<sub>3</sub> interface and results in the formation of charge traps in the Alq<sub>3</sub> layer. Considering the similarities between the GSP and the interfacial charge, they can be attributed to the same origin; the orientaion polarization of Alq<sub>3</sub> film.
We examined the driving mechanism of indium-tin oxide (ITO)/4,4-bis[<i>N</i>-(1-naphthyl)-N-phenyl-amino]biphenyl
(α-NPD)/Tris-(8-hydroxiquinolate) aluminum (Alq<sub>3</sub>)/cathode type organic light-emitting diode (OLED) by using a displacement current measurement (DCM). The DCM enables us to directly calculate the amount of accumulated charge.
There exists a maximum value in the amounts of the blocked holes at α-NPD/Alq<sub>3</sub> interface. The maximum value was
about 120 nC/cm<sup>2</sup>, this value is consistent with the density of the fixed interfacial charge proposed by Brütting et al. By using hole-only device with Au cathode, we also investigated the hole blocking and the subsequent overflow of hole
current beyond the interface. The observed feature can be explained by the hole blocking due to the interfacial charge
rather than by that due to the HOMO mismatch at the interface.