COLL 328 |
| Organic heterojuction photovoltaic cells rely on the dissociation of excitons at donor-acceptor interfaces. Such interfacial exciton dissociation does not necessarily result in charge carriers. Due to the low dielectric constant of a typical organic semiconductor material, electrostatic interaction between an electron and a hole is sufficiently long ranged, on the order of a few to a few tens of nanometers. This leads to the formation of a weakly bound geminate pair, i.e., a charge transfer(CT) exciton, which keeps the electron and the hole in close proximity and significantly increases the possibility of charge recombination. Although such geminate pairs or CT excitons have been proposed, there is little experimental evidence of these transient species. Here, we report the observation of CT excitons on the surface of a crystalline organic semiconductor, pentacene, from femtosecond time-resolved two-photon photoemission spectroscopy. In this model system, pentacene is the electron donor and vacuum is the “electron acceptor”. The excited electron in a CT exciton state is bound by two contributions: electronic polarization of the pentacene surface and the electrostatic attraction from the hole. The binding energies of the two observed CT excitons are BE = 0.87 (n = 1) and 0.44 eV (n = 2), with transient lifetimes of 70 fs and 130 fs, respectively. The implications of these CT excitons in organic heterojunction photovoltaic process are discussed. |
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Interfacial Electron Transfer and Solar Energy Conversion: From Molecules to Nanomaterials
2:00 PM-5:40 PM, Tuesday, April 8, 2008 Morial Convention Center -- Rm. 226, Oral
Division of Colloid & Surface Chemistry |