title: Understanding energy-level alignment in donor-acceptor/metal interfaces from core-level shifts.
abstract: The molecule/metal interface is the key element in charge injection devices. It can be generally defined by a monolayer-thick blend of donor and/or acceptor molecules in contact with a metal surface. Energy barriers for electron and hole injection are determined by the offset from HOMO (highest occupied) and LUMO (lowest unoccupied) molecular levels of this contact layer with respect to the Fermi level of the metal electrode. However, the HOMO and LUMO alignment is not easy to elucidate in complex multicomponent, molecule/metal systems. We demonstrate that core-level photoemission from donor-acceptor/metal interfaces can be used to straightforwardly and transparently assess molecular-level alignment. Systematic experiments in a variety of systems show characteristic binding energy shifts in core levels as a function of molecular donor/acceptor ratio, irrespective of the molecule or the metal. Such shifts reveal how the level alignment at the molecule/metal interface varies as a function of the donor-acceptor stoichiometry in the contact blend.
authors.affiliation: Departamento Fisica Aplicada I, Universidad del Pais Vasco UPV/EHU, E-20018 San Sebastian, Spain.
authors.forename: Afaf
authors.initials: A
authors.lastname: El-Sayed
authors.forename: Patrizia
authors.initials: P
authors.lastname: Borghetti
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Goiri
authors.forename: Celia
authors.initials: C
authors.lastname: Rogero
authors.forename: Luca
authors.initials: L
authors.lastname: Floreano
authors.forename: Giacomo
authors.initials: G
authors.lastname: Lovat
authors.forename: Duncan John
authors.initials: DJ
authors.lastname: Mowbray
authors.forename: Jose Luis
authors.initials: JL
authors.lastname: Cabellos
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Wakayama
authors.forename: Angel
authors.initials: A
authors.lastname: Rubio
authors.forename: Jose Enrique
authors.initials: JE
authors.lastname: Ortega
authors.forename: Dimas G
authors.initials: DG
authors.lastname: de Oteyza