Berlin 2012 – wissenschaftliches Programm

Bereiche | Tage | Auswahl | Suche | Aktualisierungen | Downloads | Hilfe

DS: Fachverband Dünne Schichten

DS 34: Organic thin films II: Interface spectroscopy

DS 34.2: Vortrag

Donnerstag, 29. März 2012, 15:15–15:30, H 2032

Origin of pinning-levels at molecularly modified electrodes — •Stefanie Winkler¹, Ralf-Peter Blum², Ralph Rieger³, Joachim Räder³, Jürgen P. Rabe², Klaus Müllen³, and Norbert Koch^1,2 — ¹Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Str.15. 12489 Berlin — ²Humboldt-Universität zu Berlin, Institut für Physik, 12489 Berlin — ³Max Planck Institute für Polymerforschung, Ackermannweg 10, 55128 Mainz

Our photoemission study reveals pronounced work function (φ) increases by up to 2.0 eV when a strong electron acceptor monolayer comprising tetrafluoro-tetracyanoquinodimethane or hexaazatriphenylene-hexacarbonitrile is deposited on indium tin oxide (ITO) surfaces (pristine ITO φ: 4.2 eV). This increase is attributed to a charge-transfer type interaction between ITO and the acceptors, involving partial filling of the LUMO of pristine acceptors. The evolution of sample φ and hole injection barrier upon incremental deposition of the hole transport material N,N’-bis(1-naphthyl)-N,N’-diphenyl- 1,1’-biphenyl-4,4’-diamine (α-NPD) is reported, for both type of acceptor modified ITO electrodes. We find that the onset of the highest molecular level of multilayer α-NPD is pinned 0.5 eV below the Fermi-level. Surprisingly, pinning is achieved at about 0.4 eV higher initial electrode φ (5.2 eV) than would be expected according to the ionization energy of α-NPD (5.3 eV). φ inhomogeneities at sub-monolayer acceptor coverages, resulting in laterally adjacent pinned and unpinned α-NPD on top, can explain this unexpected behaviour, which highlights the importance of knowing sample inhomogeneities on the molecular scale.