Bimolecular recombination kinetics and interfacial electronic structuresof poly[2-methoxy-5-(2-ethyl-hexyloxy)- p -phenylene vinylene]on gold studied using two-photon photoemission spectroscopy

Photoexcitation kinetics and interfacial electronic structures of poly[2-methoxy-5(2-ethyl hexyloxy)- p -phenylene vinylene] (MEH-PPV) film on gold have been investigated using two-photon photoemission spectroscopy (2PPE). The authors directly probed a fixed intermediate state located at 0.95 eV above the Fermi level (or 2.95 eV below the vacuum level), assigned to a charged polaron. Based on the power law slope and the 2PPE spectra with laser intensity, they found that the polaron follows a second order bimolecular annihilation process. The 2PPE yield dramatically increases with increasing photon energy. They attribute this to an enhanced dissociation of hotter excitons at higher excitation levels. The work function of MEH-PPV/Au is measured to be 3.9 eV , 1.2 eV downshift from the clean gold, attributable to interface dipole effects. The energy gap between the intermediate polaron state and the hole polaron level is estimated to be 2.45 eV .