AVS 45th International Symposium
    Organic Electronic Materials Topical Conference Monday Sessions
       Session OE+EM-MoM

Paper OE+EM-MoM10
A Comparison of the Change in the Valence Electronic Structure of p-sexiphenyl Thin Films upon Doping with K and Cs

Monday, November 2, 1998, 11:20 am, Room 327

Session: Organic Thin Film Devices I: Light Emitters
Presenter: J.-J. Pireaux, Facultés Universitaires Notre-Dame de la Paix, Belgium
Authors: N. Koch, Technische Universität Graz, Austria
J.-J. Pireaux, Facultés Universitaires Notre-Dame de la Paix, Belgium
L.M. Yu, Facultés Universitaires Notre-Dame de la Paix, Belgium
R.L. Johnson, Universität Hamburg, Germany
G. Leising, Technische Universität Graz, Austria
Correspondent: Click to Email
Thin films of the electroactive conjugated material para-sexiphenyl (6P) were doped with potassium and cesium under ultra-high vacuum conditions. The changes in the valence electronic structure of 6P upon increasing dopant exposure were followed with synchrotron ultraviolet photoelectron spectroscopy (UPS), and are compared subsequently. For doping with Cs, new occupied electronic states are only found in the energy gap at 5.8 and 3.6 eV (with respect to the vacuum level); they lie well below the Fermi level, and can be interpreted to negative bipolaron states in 6P. The experimental findings are different when doping with K; for very low doping levels, a finite density of valence states (DOVS) is observed at the position of the Fermi-level E@sub F@ . Higher doses of K lead to a shift of E@sub F@ closer to the vacuum level, and out of the DOVS. This observation could be interpreted in terms of a polaron to bipolaron transition of the charged species of 6P with progressive doping. But also a different interpretation will be proposed, in a manner that bipolarons are formed from the beginning of the doping experiment.