Watching excitations diffuse and charges recombine at interfaces in organic photovoltaic materials

Photoinduced charge separation, excitation transport, and charge recombination at the interfaces of polymer blend organic photovoltaic materials are examined using ultrafast visible pump - infrared probe spectroscopy. Native vibrational modes of both the electron donating polymers and electron accepting fullerenes are independently probed as local vibrational reporters of charge transfer yielding detailed dynamical information with structural specificity. Following ultrafast excitation, charge transfer to fullerene molecules occurs over many time scales ranging from sub-100 femtoseconds to nanoseconds. The slower component occurs because excitations that are created throughout the polymer phase must diffuse toward the interfaces to affect charge separation giving rise to an induction period in the charge transfer dynamics. The induction period occurs in the charge transfer kinetics observed in both the donor and acceptor transient vibrational spectra. Charge recombination is found to be composition dependent and occurs on the few microseconds to millisecond time scales.