Vibrational Energy Mediates Charge Separation in Organic Photovoltaic Materials

Charge separation and charge trapping following photoinduced electron transfer from the conjugated polymer, poly (2-methoxy-5-ethylhexyloxy-1,4-phenylenecyanovinylene (CN-MEH-PPV), to the electron accepting functionalized fullerene, [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM), is directly measured using ultrafast vibrational spectroscopy. Our group previously demonstrated that the vibrational frequency of the carbonyl (C=O) 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 of PCBM molecules to the interfaces enables the time evolution of the frequency of the bleach peak to be interpreted in terms of dissociation of charge transfer states into charge separated states. Temperature-dependent measurements of the rate of this charge separation process indicate that excess vibrational energy in “hot” charge transfer states resulting from the electron transfer reaction enables electrons to escape their Coulombic potentials at the interfaces on ultrafast timescales. Furthermore, temperature-dependent measurements on longer timescales indicate that after the initial charge separation reaction, electrons enter shallow trap states that, on the basis of the radial variation of vibrational frequencies, lie in the interior of PCBM clusters. The vibrational spectra suggest that these interior regions have higher intermolecular order in comparison to the interfaces.