AVS 45th International Symposium
    Organic Electronic Materials Topical Conference Monday Sessions
       Session OE-MoP

Paper OE-MoP1
Photoelectron Spectroscopic Studies of Interfaces Between Metals and CBP

Monday, November 2, 1998, 5:30 pm, Room Hall A

Session: Organic Electronic Materials Poster Session
Presenter: I.G. Hill, Princeton University
Authors: I.G. Hill, Princeton University
A. Kahn, Princeton University
Correspondent: Click to Email
A mixture of Mg and Ag is traditionally used as a low work function electron injecting contact in small molecule organic light emitting devices (OLEDs). Thin cathode interface layers of aromatic diamines, such as 4,4'-N,N'-dicarbazolyl-biphenyl (CBP), have recently been shown to increase the electron injection efficiency of Ag cathodes.@footnote 1@ The resulting devices are more stable than traditional structures utilizing reactive MgAg cathodes, and have comparable efficiencies. The role of the aromatic diamine interface layer is unclear. We have investigated the interfaces between CBP and Mg, Ag and Au using ultraviolet photoemission spectroscopy. Each interface was studied by depositing the organic on the metal, and by depositing the metal on the organic. The two methods produced inequivalent interfaces, except in the case of Au/CBP. We have measured the Fermi level/HOMO offset, and estimated the electron injecting barriers at each of these interfaces. The electron injection barrier was 0.5 eV for Mg, 0.6 eV for Ag and 1.2 eV for Au. This variation with the metal work function is quite strong, when compared to PTCDA and Alq@sub 3@.@footnote 2@ The discontinuity of the vacuum level at the interface, caused by the formation of an interface dipole layer, was also measured. The barriers to electron injection from Mg and Ag are comparable in magnitude, and surprisingly large, considering the performance of devices using these contacts. The evolution of the UPS spectral shape with overlayer thickness indicates a rough morphology in the cases of Ag and Mg on CBP. This qualitatively supports the suggestion that a rough interface may lower the barrier to electron injection and improve contact efficiencies.@footnote 3@ @FootnoteText@ @footnote 1@H. Kanai, S. Ichinosawa and Y. Sato, Synthetic Metals, 91, (1997) 195. @footnote 2@I. G. Hill, A. Rajagopal, A. Kahn and Y. Hu, Submitted to Appl. Phys. Lett. @footnote 3@Y. Yang, E. Westerweele, E. Zhang, P. Smith and A. J. Heeger, J. Appl. Phys., 77, (1995) 694.