| AVS 55th International Symposium & Exhibition | |
| Surface Science | Wednesday Sessions |
| Session SS2-WeA |
| Session: | Electrons and Electronic Spectra at Surfaces |
| Presenter: | C.H. Schwalb, Philipps University Marburg, Germany |
| Authors: | C.H. Schwalb, Philipps University Marburg, Germany M. Marks, Philipps University Marburg, Germany S. Sachs, University Würzburg, Germany A. Schöll, University Würzburg, Germany F. Reinert, University Würzburg, Germany E. Umbach, Forschungszentrum Karlsruhe, Germany U. Höfer, Philipps University Marburg, Germany |
| Correspondent: | Click to Email |
The injection of charge carriers at a molecule-metal interface plays a decisive role in the performance of organic semiconductor devices. Especially new electronic states that may develop due to the interaction of the adsorbed molecules with the metal substrate can alter the injection mechanisms drastically. We present a recent study for a well characterized model system of epitaxial 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA) thin films on a Ag(111) substrate. Two-photon photoemission (2PPE) displays a dispersing unoccupied state between the metallic Fermi level and the lowest unoccupied molecular orbitals (LUMO) of PTCDA with an effective electron mass of 0.39 me at the Γ-point. Its energetic position in the band gaps of both the Ag(111) substrate and the PTCDA overlayer identify it as a genuine interface state, a result that is corroborated by model calculations. Time-resolved measurements show that the lifetime of electrons excited into this interface state is 55 fs. This is a relatively small value for an unoccupied state located only 0.6 eV above the Fermi level and is indicative for a large penetration of the wavefunction into the projected sp-gap of Ag(111). In order to investigate the role of the interface state for carrier transport between the organic semiconductor and the metal we populate the LUMO of PTCDA by absorbing 2.4 eV photons in films of varying thickness up to 100 ML and simultaneously record fluorescence and angle-resolved photoemission spectra. We observe a long lived component in the 2PPE intensity close to the Fermi level which clearly correlates with film thickness and fluorescence lifetime.