Metal vs. Polymer Electrodes in Organic Devices: Energy Level Alignment, Hole Injection, and Structure
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Metal vs. Polymer Electrodes in Organic Devices: Energy Level Alignment, Hole Injection, and Structure N. Koch1,2, B. Nickel3, J. Ghijsen4, A. Elschner5, J. Schwartz3, J.-J. Pireaux4, A. Kahn1 1 Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, U.S.A. 2 Institut f. Physik, Humboldt-Universität zu Berlin, D-10115 Berlin, Germany 3 Department of Chemistry, Princeton University, Princeton, NJ 08544, U.S.A. 4 H.C.Starck GmbH, c/o Bayer AG Uerdingen, D-47829 Krefeld, Germany 5 Laboratoire Interdisciplinaire de Spectroscopie Electronique, Facultés Universitaires NotreDame de la Paix, B-5000 Namur, Belgium ABSTRACT We have investigated the electronic, structural, and charge injection properties of interfaces formed between three electroactive conjugated organic materials, i.e., N,N’-bis-(1naphthyl)-N,N’-diphenyl1-1,1-biphenyl1-4,4’-diamine (α-NPD), pentacene, p-sexiphenyl, and two high work function electrode materials, i.e., gold and the conducting polymer poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). Ultraviolet photoelectron spectroscopy shows that the hole injection barrier between the three organic materials and PEDOT:PSS is lower by 0.6-1.0 eV as compared to Au, despite a similar work function of the pristine electrode material surfaces (ca. 5 eV). This very large difference is due to an effective change of the metal work function due to the deposition of organic molecules, i.e., a decrease of the Au surface dipole due to adsorption. Accordingly, model device structures built from α-NPD and pentacene on the two different electrode materials show much higher current densities for hole injection from PEDOT:PSS than from Au. Hole injection from Au for α-NPD devices is independent of deposition sequence and substrate. Pentacene devices exhibit significant asymmetries in that respect, due to a strong dependence of the morphology and preferred molecular orientation of the crystalline material on the substrate, as shown by atomic force microscopy and X-ray diffraction. Consequently, great care must be taken when modeling current-voltage characteristics of devices comprised of crystalline organic solids, especially when the influence of film thickness or different substrate materials is to be studied.
INTRODUCTION The electronic structure of most metal-organic molecular semiconductor interfaces investigated so far departs from the simple Schottky-Mott limit and exhibits a substantial (~0.51.0 eV) interface dipole barrier [1,2]. Recent experimental and theoretical studies have suggested that a significant fraction of the interface dipole barrier at organic-on-metal interfaces corresponds to a lowering of the metal work function (φ) by the adsorbed molecules [3,4]. Indeed, the work function of a metal is comprised of both bulk and surface-dipole contributions [4,5], the latter corresponding to the tail of electrons spilling out from the metal surface into the vacuum. This surface-dipole contribution is substantially modified by the presence of an adsorbate. In the c
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