title: Seleno groups control the energy-level alignment between conjugated organic molecules and metals.
abstract: The charge injection from metallic electrodes into hole transporting layers of organic devices often suffers from deviations from vacuum-level alignment at the interface. Even for weakly interacting cases, Pauli repulsion causes an interface dipole between the metal and conjugated organic molecules (COMs) (so called "push-back" or "cushion" effect), which leads notoriously to an increase of the hole injection barrier. On the other hand, for chalcogenol self assembled monolayers (SAMs) on metal surfaces, chemisorption via the formation of chalcogen-metal bonds is commonly observed. In these cases, the energy-level alignment is governed by chalcogen-derived interface states in the vicinity of the metal Fermi-level. In this work, we present X-ray and ultraviolet photoelectron spectroscopy data that demonstrate that the interfacial energy-level alignment mechanism found for chalcogenol SAMs also applies to seleno-functionalized COMs. This can be exploited to mitigate the push-back effect at metal contacts, notably also when COMs with low ionization energies are employed, permitting exceedingly low hole injection barriers, as shown here for the interfaces of tetraseleno-tetracene with Au(111), Ag(111), and Cu(111).
authors.affiliation: Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Niederhausen
authors.affiliation: Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan.
authors.forename: Steffen
authors.initials: S
authors.lastname: Duhm
authors.affiliation: Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany.
authors.forename: Georg
authors.initials: G
authors.lastname: Heimel
authors.affiliation: Institut für Angewandte Physik, Universität Tübingen, 72076 Tübingen, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: Bürker
authors.affiliation: Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan.
authors.forename: Qian
authors.initials: Q
authors.lastname: Xin
authors.affiliation: Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Wilke
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, BESSY II, 12489 Berlin, Germany.
authors.forename: Antje
authors.initials: A
authors.lastname: Vollmer
authors.affiliation: Institut für Angewandte Physik, Universität Tübingen, 72076 Tübingen, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Schreiber
authors.affiliation: Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Kera
authors.affiliation: Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany.
authors.forename: Jürgen P
authors.initials: JP
authors.lastname: Rabe
authors.affiliation: Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan.
authors.forename: Nobuo
authors.initials: N
authors.lastname: Ueno
authors.affiliation: Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany.
authors.forename: Norbert
authors.initials: N
authors.lastname: Koch