Electronic structure of Alq(3)/LiF/Al interfaces studied by UV photoemission

The electronic structure of tris (8-hydroxyquinolino) aluminum (Alq(3))/LiF /Al system was studied in relation to the enhancement of electron-injection efficiency by the insertion of LiF insulating layer at Alq(3)/Al contact, using UV photoemission spectroscopy (UPS) Xray photoelectron spectroscopy ( XPS), and metastable atom electron spectroscopy (MAES). The observed energy separation between the HOMO of Ale and the Fermi level of Al substrate inc reased from 2.7eV to 3.0eV by inserting 0.5nm thick LiF layer. This result indicates that the LiF layer induces the decrease of the electron injection barrier. We also found extra states probably caused by the interaction at the Alq(3)/Al interface. The spectral intensity of this extra state decreas ed with increasing LiF thickness, and vanished at 0.5nm.