AVS 51st International Symposium
    Surface Science Wednesday Sessions
       Session SS+OF-WeA

Paper SS+OF-WeA9
Influence of Electrode Contamination on @alpha@-NPD/Au Hole-injection Barriers

Wednesday, November 17, 2004, 4:40 pm, Room 213B

Session: Contacts to Molecules and Molecular Films
Presenter: A.S. Wan, Princeton University
Authors: A.S. Wan, Princeton University
J.H. Hwang, Princeton University
A. Kahn, Princeton University
Correspondent: Click to Email
Processing for OLEDs and other organic devices often involves exposure of surfaces and interfaces to nominally controlled atmosphere (e.g. N@sub 2@) or even ambient conditions. A model system for studying barriers built under such "practical" conditions, as opposed to barriers built on ultra-clean substrates prepared under ultra-high vacuum (UHV) conditions, is the interface between Au and @alpha@-NPD, a standard hole- transport material for OLEDs. In this work, we investigate the effect of ambient exposure of polycrystalline Au surfaces on the hole-injection barrier using ultraviolet photoemission spectroscopy (UPS) and I-V measurements. We compare @alpha@-NPD layers grown in UHV on: i. Au as-loaded, exposed to air; ii. Au cleaned by Ar@super +@ sputtering; iii. fresh Au deposited in UHV. Conventional wisdom is that the higher the work function of the contact metal, the lower the hole-injection barrier. However, our UPS studies unambiguously show that the injection barrier is 0.5-0.6 eV lower for Au exposed to air, which has a work function of 4.7-4.9 eV, than for clean Au (sputtered or UHV deposited), which has a work function of 5.3-5.4 eV. The vacuum level shift, or dipole barrier, (down from Au to organic) is 0.3 eV for the former and 1.3 eV for the latter. I-V measurements on Au/@alpha@-NPD/Au devices made on clean vs. "dirty" Au show several orders of magnitude increase in injection from "dirty" Au, entirely consistent with the changes in barriers measured in UPS. The large differences in dipole and injection barriers are attributed to the layer of contamination (consisting primarily of carbon) on the air- exposed Au, which lowers the work function of the metal and decouples the interface molecules from the Au atoms. Mechanisms based on disruption of interface bonding and/or metal-induced states will be discussed.