Investigation of the surface potential formed in Alq3 films on metal surface by Kelvin probe and nonlinear optical measurement

Polar orientation of Alq3 (Tris(8-quinolinolato)aluminum) evaporated films on metal electrode was investigated by Kelvin probe method and optical second harmonic generation (SHG) measurement. For as-deposited films under the dark condition, a large potential formed across the films was confirmed and it decreased drastically by light irradiation with a wavelength of 400 nm. From a viewpoint of the interfacial electrostatic phenomena, there are two contributions to the surface potential, i.e. space charge effect and the alignment of dipoles. Before irradiation, alignment dipoles are the main contributor to the surface potential, whereas excess charges contribute mainly to the surface potential after irradiation. According to the SHG measurement, decrease of the signal from Alq3 films was observed after light irradiation. It is also found that using the fundamental light with a wavelength of 900 nm, SH intensity decreases steeply during the SH measurement in the absence of 400 nm irradiation possibly due to the two-photon excitation.