Regensburg 2013 – scientific programme
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DS: Fachverband Dünne Schichten
DS 31: Organic thin films II
DS 31.3: Talk
Thursday, March 14, 2013, 15:15–15:30, H8
Origin of pinning-levels at molecularly modified electrodes: Mixed monolayer — •Stefanie Winkler1,2, Johannes Frisch2, Raphael Schlesinger2, Martin Oehzelt1,2, Ralph Rieger3, Joachim Räder3, Jürgen P. Rabe2, Klaus Müllen3, and Norbert Koch1,2 — 1Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin — 2Humboldt-Universität zu Berlin, Institut für Physik, 12489 Berlin — 3Max Planck Institute für Polymerforschung, 55128 Mainz
Our photoemission study reveals strong work function (Φ) increases by up to 2.1 eV when strong electron acceptors comprising tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) or hexaazatriphenylene-hexacarbonitrile (HATCN) are deposited on indium tin oxide (ITO) surfaces (pristine ITO Φ=4.2 eV). The evolution of the sample-Φ and the hole injection barrier upon subsequent deposition of the hole transport material N,N’-bis(1-naphthyl)-N,N’-diphenyl-1,1’-biphenyl-4,4’-diamine (α-NPD) on various submonolayer acceptor-coverages is reported for the modified ITO electrodes. We find the highest occupied molecular level of multilayer α-NPD pinned 0.5 eV below the Fermi-level, when employing the acceptor interlayers. Surprisingly pinning is achieved at a 0.4 eV (5.2 eV) higher initial Φ than one would expect according to the ionization energy of α-NPD (IE=5.3 eV). With a simple model including a uniform electrostatic field above the first organic layer (mixture of acceptor and α-NPD) we can assign the origin of the shifted pinning-level to be caused by the pushback effect of α-NPD completing the only partly covered monolayer.