Electronic structure of Alq3/LiF/Al interfaces studied by UV photoemission

The electronic structure of tris (8-hydroxyquinolino) aluminum (Alq3/LiF/Al system was studied in relation to the enhancement of electron-injection efficiency by the insertion of LiF insulating layer at Alq3/Al contact, using UV photoemission spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). The observed energy separation between the HOMO of Alq3 and the Fermi level of Al substrate increased 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 Alq3/Al interface.The spectral intensity of this extra state decreased with increasing LiF thickness, and vanished at 0.5nm.