Exciton Dynamics at CuPc/C60 Interfaces: Energy Dependence of Exciton Dissociation

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 charge-transfer states. Connections to semiclassical models of charge transfer and implications for device performance are discussed.