Donor−acceptor interfaces are critical for the operation of organic photovoltaic devices. Exciton dynamics at these interfaces play a significant role in determining efficiency and controlling open circuit voltage (V OC ), short circuit current (J SC ), or fill factor (FF). These fundamental interfacial dynamical processes are dependent on the interfacial electronic and molecular structure. In this report we use time-resolved two-photon photoemission (TR-2PPE) to investigate exciton dissociation, recombination, and relaxation processes occurring at well-characterized prototypical donor−acceptor interfaces of copper phthalocyanine (CuPc) layers on C 60 . S 1 excitons are created by excitation in the CuPc Q-band. The excited S 1 population is probed as a function of time via photoemission with a UV probe pulse. TR-2PPE measurements provide a picture of subpicosecond charge separation and recombination processes as a function of distance from the CuPc/C 60 interface, starting with a CuPc single layer. Analysis via rate equation modeling reveals that the bulk intersystem crossing and intraband relaxation occur on picosecond to subpicosecond time scales, resulting in rapid relaxation of the exciton population. At the interface, these processes compete with electron transfer to C 60 . The rate constant governing exciton dissociation is energy-dependent, decreasing by orders of magnitude for excitons below the energy of interfacial chargetransfer states. Connections to semiclassical models of charge transfer and implications for device performance are discussed.

Electron transfer at organic/metal interfaces is a fundamental issue of interest to a large number of problems in chemistry. We use two-photon photoemission (2PPE) spectroscopy to investigate heterogeneous electron transport in a model system: naphthalene adsorbed on Cu(111). The dependence of 2PPE spectra on photon energy establishes the occurance of photoinduced electron transfer to an unoccupied state at 3.1 eV above the Fermi level (or 1.1 eV below the vacuum level) at one monolayer coverage. Polarization and dispersion measurements reveal the image-like property of this electron-transfer state. The binding energy of this state is dependent on the thickness of the adsorbate layer. This observation is in agreement with simulation based on the dielectric continuum model. The simulation also shows that, while partially distributed in the adsorbate layer at one monolayer coverage, the excited electron is confined within the adsorbate layer at high coverages, an effect which can be attributed to the electron affinity of naphthalene.

Unoccupied electronic states in C 60 thin films on Cu(111) have been measured by laser two-photon photoemission (2PPE) spectroscopy. These measurements allow the quantitative determination of energetic positions for the lowest unoccupied molecular orbital (LUMO), LUMO + 1, LUMO + 2, image potential states, as well as the highest occupied molecular orbital (HOMO). The transiently populated LUMO and LUMO + 1 levels are stabilized by the on-molecule charge correlation energy during the 2PPE process. Compared to previous measurements using one-photon photoemission and inverse photoemission, the HOMO-LUMO gap determined by 2PPE is stabilized by half of the Hubbard U. Both intramolecular excitation and a minor metal-to-molecule electron-transfer excitation channel are observed.

We probe electronic interaction at molecular-solid/metal interfaces in the model system of C60/Cu(111) using femtosecond two-photon photoemission (2PPE) spectroscopy. The second lowest unoccupied molecular orbital (LUMO+1) and the LUMO+2 levels in C60 are transiently populated via the creation of electronic excitons, with lifetimes in the 10-14−10-13 s region, likely due to self-trapping and/or decay into lower-lying exciton states. These lifetimes decrease as film thickness decreases. The effect is seen for films as thick as 50 Å and is attributed to quenching via charge transfer between the Cu substrate and electronic bands in C60. The rates of quenching are found to depend exponentially on film thickness, with a β value of 0.11 ± 0.02 Å-1 for both LUMO+1 and the LUMO+2 levels.

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