Spontaneous buildup of giant surface potential by the deposition of Alq3

Large and persistent surface potential (SP) was obtained in the film of tris(8-hydroxyquinolinato) aluminum (III) (Alq 3 ) prepared on a Au substrate by vacuum deposition. Under experimental condition without light irradiation, the SP changed continuously with increasing the thickness of Alq 3 , after abrupt shift at the interface. The value of the SP reached 28 V at the 560 nm-deposition of Alq 3 . This phenomenon was also observed in the Alq 3 film on an Al substrate. Preferential orientation of the Alq 3 molecules with a permanent dipole moment is suggested as an origin of this giant surface potential of the films, which corresponds to the buildup of the dipole layers in the film. Results of the second harmonic generation measurements supported this model. When we deposited Alq, under a usual experimental condition with the irradiation of the visible light, only abrupt vacuum level shift was observed and there was no further change of the SP. The giant surface potential was easily recovered by irradiation of the visible light for 1hour. These results indicate that the light irradiation during the film growth would affect the film structure or performance of organic electronic devices.