Occupied and Unoccupied Levels of Half-Fluorinated and Perfluorinated Rubrene Thin Films Probed by One- and Two-Photon Photoemission

摘要

We measured occupied and unoccupied energy levels of half-fluorinated and perfluorinated rubrenes (C42F14H14, F-14-RUB, and C42F28, PF-RUB) thin films prepared on a highly oriented pyrolytic graphite substrate by a combination of one- and two-photon photoemission spectroscopies. It is revealed that the energy levels near the Fermi level (E-F) decrease when the fluorination degree increases, where the energies o f the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are located at E-F - 1.83 eV and E-F + 0.9 eV for F-14-RUB and E-F - 2.51 eV and E-F + 0.6 eV for PF-RUB, respectively. The HOMO-LUMO gaps corresponding to the energy for the creation o f free photocarriers in molecular films- known as "transport gaps"- are evaluated by 2.7 eV for F-14-RUB and 3.1 eV for PF-RUB.