Characteristics of Photoexcitations and Interfacial Energy Levels of Regioregular Poly(3-hexythiophene-2,5-diyl) on Gold

We have studied characteristics of photoexcitations and interfacial electronic structures of regioregular poly(3‐hexlythiophene‐2,5‐diyl) (P3HT) on gold using two‐photon photoemission (2PPE) spectroscopy. The vacuum level threshold is decreased by 1.3 eV from that of bare gold, attributable to interface dipole effects. The 2PPE spectral width narrows as the film thickness increases. We tentatively understand that this is due to destabilization of long‐lived localized polaron, attributed to strong interchain interactions. On the basis of the analysis of the 2PPE distribution as a function of photon energy and laser power, the polaron level is located at 3.1 eV below the vacuum level. Using this value and a polaron level of 1.75 eV above the HOMO, we indirectly estimate an ionization potential of 4.85 eV for P3HT. An increase in two‐photon photoemission yield with increasing photon energy is attributed to an enhanced electron‐hole pair dissociation yield at higher photo‐excitation levels. The decrease in power law slope with increasing film thickness is understood by Langevin recombination kinetics and saturation of photoexcitations Polaron destabilization in highly stacked regioregular poly(3‐hexythiophene‐2,5‐diyl) reflects high charge‐carrier mobility. This is indicated by the narrowing of two‐photon photoemission spectroscopy profiles with increasing thickness (see picture).