AVS 46th International Symposium
    Organic Electronic Materials Topical Conference Monday Sessions
       Session OE-MoP

Paper OE-MoP7
Investigation of the Influence of LiF-Coatings on the Electronic Structure of Organic Semiconductor/Electrode Interfaces by Photoemission Spectroscopy

Monday, October 25, 1999, 5:30 pm, Room 4C

Session: Poster Session
Presenter: P.A. Lee, University of Arizona
Authors: R. Schlaf, State University of New York at Binghamton
P.A. Lee, University of Arizona
K.W. Nebesny, University of Arizona
N.R. Armstrong, University of Arizona
Correspondent: Click to Email
Recently, it was demonstrated that the performance of organic light emitting diodes (OLED) can be considerably improved by inserting thin (~1nm) LiF layers between charge injecting electrodes and organic semiconductor layers. We have determined the electronic structure of typical metal/LiF interfaces with photoemission spectroscopy (PES). In our experiments LiF thin films were grown in multiple steps on sputter-cleaned Al, Pt and ITO substrates. Before growth of the LiF films and after each LiF deposition step the samples were characterized in-situ by combined X-ray and UV photoemission spectroscopy (XPS, UPS) measurements. Our results suggest that the LiF coatings considerably lower the work function of the electrode surfaces, hence resulting in a much reduced electron injection barrier between low work function electrodes and organic electron transport materials. In the case of Al electrodes a surface dipole created by chemisorbed water emitted from the LiF evaporation source is partially responsible for the work function reduction observed after the LiF deposition. Our measurements also demonstrate that even in insulators such as LiF effects similar to band bending in semiconductors may occur. The results suggest, that a redistribution of charged Schottky or Frenkel type defects in the LiF layers, caused by the thermodynamic equilibration between LiF and the contact metal, is possibly responsible for the observed phenomena.