AVS 60th International Symposium and Exhibition
    Surface Science Thursday Sessions
       Session SS1-ThA

Paper SS1-ThA8
Donor-to-Acceptor Core-level Shift in Molecular Blend/Metal Interfaces

Thursday, October 31, 2013, 4:20 pm, Room 201 A

Session: Organic Layers on Surfaces
Presenter: P. Borghetti, Material Physics Center (MPC), Spain
Authors: P. Borghetti, Material Physics Center (MPC), Spain
A. El-Sayed, Material Physics Center (MPC), Spain
E. Goiri, Donostia International Physics Center (DIPC), Spain
C. Rogero, MPC, Spain
L. Floreano, CNR-IOM, Lab Nazionale TASC, Italy
D. Mowbray, DIPC, Spain
J.L. Cabellos-Quiroz, DIPC, Spain
Y. Wakayama, International Center for Materials Nanoarchitectonics (WPI-MANA), Japan
A. Rubio, Univ. del País Vasco, Spain
J.E. Ortega, Univ. del Paıs Vasco, Spain
D.G. De Oteyza, DIPC, Spain
Correspondent: Click to Email
Self assembled donor-acceptor complexes on metallic electrodes are at the heart of novel organic optoelectronic applications such as solar cells. In these devices, the organic/metal interface is of fundamental importance, since it defines the charge injection barrier that determines the ultimate device performance. Despite the dramatic progress of the organic electronic field so far, the ability to accurately model and predict the electronic properties at interfaces is still inadequate, thereby hampering the translation of organic thin film growth into an established technology. One of the key issues is how the molecule/metal interaction plus the intermolecular interactions affect the energy-level alignment, i.e., how do the Highest Occupied Molecular Orbital, HOMO, and the Lowest Unoccupied Molecular Orbital, LUMO, align with respect to the metal Fermi energy. However, the HOMO and LUMO alignment is not easy to elucidate in complex multi-component, molecular/metal systems. Here we demonstrate that core-level photoemission from donor-acceptor/metal interfaces can straightforwardly and transparently determine molecular level alignment. We focus on 2D crystalline networks, and carry out a systematic investigation over a number of donor-acceptor/metal systems. In particular, we use Au(111), Cu(111) and Ag(111) as substrates, perfluorinated copper-phthalocyanine (F16CuPc) and perfluoropentacene (PFP) as aromatic acceptors, and copper-phthalocyanine (CuPc) and pentacene (PEN) as electron donors. For each combination, XPS spectra show a characteristic binding energy shift in core-levels as a function of molecular donor/acceptor ratio, irrespectively of the molecule or the metal. Such shift reveals how the level alignment at the molecule/metal interface varies as a function of the donor-acceptor stochiometry in the contact blend. We also show that the energy level alignment is barely affected by donor-to-acceptor charge transfer, and majorly determined by the electron potential created by donor (high attractive potential) or acceptor (low attractive potential) molecules in their vicinity, i.e., by the average change in work function.