AVS 46th International Symposium
    Organic Electronic Materials Topical Conference Tuesday Sessions
       Session OE+EM+AS-TuM

Paper OE+EM+AS-TuM10
A Photoemission Investigation of Interfaces of poly(2, 5-diheptyl-1,4-phenylene-alt-2,5-thienylene) with an oligomer (p-sexiphenyl) and a Metal - Calcium

Tuesday, October 26, 1999, 11:20 am, Room 616/617

Session: Interfaces and Characterization of Organic Thin Films
Presenter: A. Rajagopal, Univ. Notre-Dame, Belgium
Authors: A. Rajagopal, Univ. Notre-Dame, Belgium
N Koch, Tech. Univ. Graz, Austria
J Ghijsen, Univ. Notre-Dame, Belgium
K. Kaeriyama, Kyoto Inst. of Tech., Japan
R.L Johnson, Univ. Hamburg, Germany
G. Leising, Tech. Univ. Graz, Austria
J.J. Pireaux, Univ. Notre-Dame, Belgium
Correspondent: Click to Email
Poly(2, 5-diheptyl-1,4-phenylene-alt-2,5-thienylene) (PDHPT) is a blue light emitting conjugated polymer with a band gap of ca. 3 eV, and is a promising candidate as an active material in organic light emitting diodes. The interface of this polymer with a) a blue light emitting oligomer (sexiphenyl- 6P), and with b) a low work function cathode material, calcium (Ca) have been investigated using both ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS). Both 6P and PDHPT have a band gap of ca. 3 eV, as well as similar ionization potentials, and this makes the interface an interesting one. A systematic study of the in-situ growth of 6P on PDHPT allows the estimation of the relative positions of the HOMOs and the vacuum levels. We find that the HOMO of PDHPT is below that of 6P, and there is a vacuum level offset of 0.3 eV suggesting charge transfer from 6P to PDHPT. In the case of metal-PDHPT interface, Ca was deposited in extremely small steps in-situ, and both XPS and UPS data were recorded after each subsequent deposition. Contrary to what can be predicted based on calculations and experiments on similar systems, namely phenylenes and thiopehenes, we propose that Ca forms covalent bonds with the polymer. The consequences of these experimental findings will be discussed within the context of LEDs.