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A New Alternative for the Low-Workfunction Electrode in Organic Devices

Published online by Cambridge University Press:  21 March 2011

Norbert Koch,
Egbert Zojer,
Aparna Rajagopal,
Jacques Ghijsen,
Robert L. Johnson,
Günther Leising and
Jean-Jacques Pireaux
Norbert Koch
Affiliation:
Institut f. Festkörperphysik, TU-Graz, A-8010 Graz, Austria
Egbert Zojer
Affiliation:
Institut f. Festkörperphysik, TU-Graz, A-8010 Graz, Austria
Aparna Rajagopal
Affiliation:
Laboratoire Interdisciplinaire de Spectroscopie Electronique, Facultés Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium
Jacques Ghijsen
Affiliation:
Laboratoire Interdisciplinaire de Spectroscopie Electronique, Facultés Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium
Robert L. Johnson
Affiliation:
Institut f. Experimentalphysik, Universität Hamburg, D-22761 Hamburg, Germany
Günther Leising
Affiliation:
Institut f. Festkörperphysik, TU-Graz, A-8010 Graz, Austria
Jean-Jacques Pireaux
Affiliation:
Laboratoire Interdisciplinaire de Spectroscopie Electronique, Facultés Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium
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Abstract

The application of ele ctroactive organic materials in devices (light emitting diodes, photovoltaic cells) often requires electrodes with a low workfunction. Commonly, aluminum or alkaline earth metals are used, which usually exhibit strong interaction with the organic material, such as the formation of new covalent bonds or doping. This results in a strong modification of the electronic structure of the organic/metal interface, and in most cases does not yield the energy level alignment expected for the unreacted interface. As a n alternative to the above-mentioned metals we propose the use of samarium, with a workfunction of 2.7 eV, for the following reason: we have studied the interface formation between Sm and p -sexiphenyl (6P, which exhibits intense blue electroluminescence), with ultraviolet photoelectron spectroscopy. Sm was deposited stepwise onto thin films of 6P in ultrahigh vacuum, and the photoelectron spectra were recorded after each step. We did not find any indication for a strong interaction between the two materials. Metallic Sm is formed instantaneously and the valence electronic structure of 6P remains unchanged upon the metal deposition. The weak interaction at this interface allows one to determine the energy level alignment between a metal and an organic material in a direct manner from the photoelectron spectra, without the need for making any assumptions on the workfunction or ionization potential.

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