A Multidimensional View of Charge Transfer Excitons at Organic Donor-Acceptor Interfaces

How tightly bound charge transfer (CT) excitons dissociate at organic donor-acceptor interfaces has been a long-standing question in the organic photovoltaics community. Recently, it has been proposed that exciton delocalization reduces the exciton binding energy and promotes exciton dissociation. In order to understand this mechanism, it is critical to resolve the evolution of the exciton's binding energy and coherent size with femtosecond time resolution. However, because the coherent size is just a few nanometers, it presents a major experimental challenge to capture the CT process simultaneously in the energy, spatial, and temporal domains. In this work, the challenge is overcome by using time-resolved photoemission spectroscopy. The spatial size and electronic energy of a manifold of CT states are resolved at the zinc phthalocyanine (ZnPc)-fullerene (C₆₀) donor-acceptor interface. It is found that CT at the interface first populates delocalized CT excitons with a coherent size of 4 nm. Then, this delocalized CT exciton relaxes in energy to produce CT states with delocalization sizes in the range of 1-3 nm. While the CT process from ZnPc to C₆₀ occurs in about 150 fs after photoexcitation, the localization and energy relaxation occur in 2 ps. The multidimensional view on how CT excitons evolve in time, space, and energy provides key information to understand the exciton dissociation mechanism and to design nanostructures for effective charge separation.