Exciton dissociation at phthalocyanine-C$_{60}$ interfaces

Exciton dissociation and charge transfer processes occurring within 10's of nanometers of donor-acceptor interfaces are critical for the performance of organic photovoltaic (OPV) structures. We investigated fundamental issues of exciton dissociation near prototypical donor-acceptor interface using time-resolved two-photon photoemission (TR-2PPE). Phthalocyanine (Pc)-C$_{60}$ interfaces with known structures were formed using organic molecular beam epitaxy. Pc $\pi \to \pi $* (Q-band) transitions were created by a sub-picosecond pump pulse, producing a population of singlet (S$_{1})$ Pc excitons. The dynamics of this population were then probed via photoemission by a time-delayed UV pulse. For CuPc$\backslash $C$_{60}$ interfaces, the dynamics for excitons created far from the interface were modeled with a combination of vibrational or intraband relaxation plus intersystem crossing (ISC) to triplet levels. Relaxation leads predominantly to triplet (T$_{1})$ exciton levels on timescales of $\approx $ 1-2 ps. The decay dynamics of S$_{1}$ excitons excited in the CuPc layer adjacent to C$_{60}$ were increased due to the additional channel leading to exciton dissociation, occurring with a rate of $\approx $ 7 x 10 $^{12}$ sec$^{-1}$. However, excitons that relax to T$_{1}$ levels at the interface dissociate with a rate $\approx $ 500 to 1000 times slower, providing a picture of the energy dependence of exciton dissociation at this interface. The dependence of exciton dissociation versus Pc thickness at analogous H$_{2}$Pc $\backslash $C$_{60}$ interfaces will also be presented. The results indicate that, for this interface, exciton dissociation is much faster for the interfacial layer with dissociation from the 2$^{\mathrm{nd}}$, and subsequent layers of H$_{2}$Pc, reduced by at least a factor of 10.