Spontaneous buildup of giant surface potential by vacuum deposition of Alq3 and its removal by visible light irradiation

We observed high and persistent spontaneous buildup of the surface potential (SP) upon vacuum deposition of tris(8-hydroxyquinolinato) aluminum(III) (Alq3) on an Au substrate under dark conditions. SP determined by the Kelvin probe method reached 28 V at a thickness of 560 nm and the surface of the Alq3 film was positively charged. We propose a model in which preferential orientation of the dipole moments of Alq3 molecules is the origin of this buildup of the SP. The intensity of second-harmonic generation was also dramatically increased by the deposition of Alq3 under dark conditions, which supports the notion of a buildup of dipole layers. This giant surface potential was almost completely removed by irradiation of Alq3 molecules with visible light, and irradiation during deposition also prevented the buildup of SP.