Barrierless free carrier formation in an organic photovoltaic material measured with ultrafast vibrational spectroscopy.

The dynamics of free carrier formation following photoinduced electron transfer from the conjugated polymer, CN-MEH-PPV, to the electron-accepting functionalized fullerene, PCBM, are directly measured using ultrafast vibrational spectroscopy. The vibrational frequency of the carbonyl (C horizontal lineO) stretch of PCBM is sensitive to the location of the molecules relative to the interfaces formed between PCBM clusters and CN-MEH-PPV. The correlation between the carbonyl frequency and the proximity to the interfaces provides the ability to directly measure the escape of electrons from their Coulombically bound radical pairs. The data indicate that the rate of free carrier formation is temperature independent from 200 to 350 K suggesting that excess vibrational energy resulting from the electron transfer reaction enables electrons to escape their Coulombic potentials on ultrafast time scales.