title: Organic heterojunctions: Contact-induced molecular reorientation, interface states, and charge re-distribution.
abstract: We reveal the rather complex interplay of contact-induced re-orientation and interfacial electronic structure - in the presence of Fermi-level pinning - at prototypical molecular heterojunctions comprising copper phthalocyanine (H16CuPc) and its perfluorinated analogue (F16CuPc), by employing ultraviolet photoelectron and X-ray absorption spectroscopy. For both layer sequences, we find that Fermi-level (EF) pinning of the first layer on the conductive polymer substrate modifies the work function encountered by the second layer such that it also becomes EF-pinned, however, at the interface towards the first molecular layer. This results in a charge transfer accompanied by a sheet charge density at the organic/organic interface. While molecules in the bulk of the films exhibit upright orientation, contact formation at the heterojunction results in an interfacial bilayer with lying and co-facial orientation. This interfacial layer is not EF-pinned, but provides for an additional density of states at the interface that is not present in the bulk. With reliable knowledge of the organic heterojunction's electronic structure we can explain the poor performance of these in photovoltaic cells as well as their valuable function as charge generation layer in electronic devices.
authors.affiliation: Institut für Physik &IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Opitz
authors.affiliation: Institut für Physik &IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Wilke
authors.affiliation: Institut für Physik &IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin, Germany.
authors.forename: Patrick
authors.initials: P
authors.lastname: Amsalem
authors.affiliation: Institut für Physik &IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin, Germany.|Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Bereich Erneuerbare Energien, Berlin, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Oehzelt
authors.affiliation: Institut für Physik &IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin, Germany.
authors.forename: Ralf-Peter
authors.initials: RP
authors.lastname: Blum
authors.affiliation: Institut für Physik &IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin, Germany.
authors.forename: Jürgen P
authors.initials: JP
authors.lastname: Rabe
authors.affiliation: National Institute of Advanced Industrial Science and Technology, Osaka, Japan.
authors.forename: Toshiko
authors.initials: T
authors.lastname: Mizokuro
authors.affiliation: Institute of Physics, University of Augsburg, Augsburg, Germany.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Hörmann
authors.affiliation: Department of Engineering and Physics, Karlstad University, Karlstad, Sweden.
authors.forename: Rickard
authors.initials: R
authors.lastname: Hansson
authors.affiliation: Department of Engineering and Physics, Karlstad University, Karlstad, Sweden.
authors.forename: Ellen
authors.initials: E
authors.lastname: Moons
authors.affiliation: Institut für Physik &IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin, Germany.|Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Bereich Erneuerbare Energien, Berlin, Germany.
authors.forename: Norbert
authors.initials: N
authors.lastname: Koch