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Interaction and Energy Level Alignment at Interfaces between Pentacene and Low Work Function Metals

Published online by Cambridge University Press:  15 March 2011

N. Koch,
J. Ghijsen,
R. Ruiz,
J. Pflaum,
R. L. Johnson,
J.-J. Pireaux,
J. Schwartz and
A. Kahn
N. Koch
Affiliation:
Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, U.S.A.
J. Ghijsen
Affiliation:
Laboratoire Interdisciplinaire de Spectroscopie Electronique, Facultés Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium
R. Ruiz
Affiliation:
Vanderbilt University, Department of Physics and Astronomy Nashville, TN 37235, U.S.A.
J. Pflaum
Affiliation:
3. Physikalisches Institut, Universitaet Stuttgart, D-70569 Stuttgart, Germany
R. L. Johnson
Affiliation:
II. Institut für Experimentalphysik, Universität Hamburg, D-22761 Hamburg, Germany
J.-J. Pireaux
Affiliation:
Laboratoire Interdisciplinaire de Spectroscopie Electronique, Facultés Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium
J. Schwartz
Affiliation:
Department of Chemistry, Princeton University, Princeton, NJ 08544, U.S.A.
A. Kahn
Affiliation:
Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, U.S.A.
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Abstract

A number of low workfunction metals (samarium, alkali metals) were deposited onto vacuum sublimed thin films of pentacene. The change in the valence electronic structure of the organic material was studied by in situ ultraviolet photoemission spectroscopy (UPS). Alkali metal intercalation leads to the appearance of a new photoemission feature within the pentacene energy gap, due to a charge transfer from the alkali atoms to the lowest unoccupied molecular orbital (LUMO) of the organic material. The energy spacing between this emission feature and the relaxed highest occupied molecular orbital (HOMO) of the pristine molecule is 1 eV. From X-ray photoemission spectroscopy core level analysis, we estimate a concentration ratio of two alkali metal atoms per pentacene molecule at maximum intercalation level, leading to a complete filling of the LUMO. This is consistent with the results from UPS that the new emission is always observed below the Fermi-level. Samarium is found to exhibit a more subtle interaction with pentacene: the molecular orbitals remain almost unperturbed upon Sm deposition. The resulting energy level alignment at this interface seems to be very favorable for the injection of electrons from Sm into pentacene, as the HOMO-onset is found at 1.8 eV below the metal Fermi edge. This value is close to the 2.2 eV HOMO-LUMO gap of pentacene measured by UPS and inverse photoemission spectroscopy, thus corresponding to a small electron injection barrier.

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