Energy Level Alignment at Alq3/LiF/Al Interfaces Studied by Electron Spectroscopies: Island Growth of LiF and Size-Dependence of the Electronic Structures

Abstract

Various models have been proposed, to date, to explain the improvement in organic electroluminescent devices by the insertion of an insulating layer such as LiF between the light emitting tris-(8-hydroxyquinolinato) aluminum (Alq3) layer and the cathode metal deposited on it. Although the average thickness of the LiF for the optimal performance is typically only 0.5 nm, the layer structure of the inserted LiF has been assumed explicitly or implicitly. In order to clarify the growth mode and electronic structure of LiF in the system, the Alq3–LiF–Al system was investigated by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). We found island formation of LiF on the Al substrate, and also that the Al surface is only partly covered when a 0.5-nm-thick LiF layer is deposited on it. MAES measurements revealed that island-grown LiF has density of states similar to that of a thick LiF layer except for the existence of defect states, which is probably due to Li vacancies near the interface. MAES also suggested a slight difference in the spatial extention of the wave function of LiF between a 0.5-nm-thick island and a 2.0-nm-thick film, while a distinct difference was found between LiF and Alq3 films. These findings are important in order to discuss their attenuation lengths in relationship to electron injection. We have constructed a realistic energy diagram of the interface, taking account of the island nature of the 0.5-nm-thick LiF layer. This energy diagram suggests it is necessary to reexamine the proposed models.