Ultrafast IR spectroscopic study of free carrier formation and charge trapping in organic photovoltaic polymer blends

Photoinduced charge separation, charge recombination, and trapping in polymer blend organic photovoltaic materials are examined using ultrafast 2D IR and vis-IR spectroscopy in conjunction with transient photocurrent techniques. Native vibrational modes of the functionalized fullerene, PCBM, are probed as reporters of the charge carrier dynamics. Following a free carrier formation process that occurs on a surprisingly fast picosecond time scale, charge carriers become trapped in poly(p-phenylenevinylene) (PPV)-based OPV materials but are not readily trapped in polythiophene (PT)-based materials. The trapping process results in the formation of distinct vibrational features that are not present in the PT-based materials. Although the origin of the distinct trapping characteristics remains unclear, they help rationalize the qualitatively different bimolecular charge recombination behavior that has been observed in PPV-based versus PT-based OPV materials. 2D IR spectroelectrochemical investigations of trapped charges may provide a means to elucidate the structures of the traps in PPV-based materials.