Charge trapping in organic photovoltaic materials examined with time-resolved vibrational spectroscopy

Bimolecular charge recombination is examined in a polymer blend photovoltaic material using time-resolved vibrational spectroscopy on the 100 fs to millisecond time scales. The carbonyl (C=O) stretch of the functionalized fullerene, PCBM, is probed as a local vibrational reporter of electron transfer and subsequent bimolecular charge recombination in a blend of the conjugated polymer, CN-MEH-PPV, with PCBM. Electron transfer from CN-MEH-PPV to PCBM occurs on the sub-100 fs to 1 ps time scale following ultrafast excitation of the conjugated polymer. Bimolecular charge recombination occurs much more slowly on the 100 μs time scale, in accord with recent optical measurements of charge recombination in similar MDMO-PPV-based polymer blends. However, a new vibrational feature that is assigned to the negative polaron of PCBM appears in the transient spectra on the few microsecond time scale. The appearance of this feature on time scales that are slow in comparison to electron transfer but fast with respect to bimolecular charge recombination suggests that it arises from an intermediate relaxation process, perhaps resulting from the formation of trapped electrons. The time scale for the appearance of the negative polaron absorption coincides with the bimolecular charge recombination lifetime of similar MDMO-PPV-based polymer blends that have been reported in the literature from transient photocurrent measurements.