Charge separation in organic photovoltaic materials mediated by excess vibrational energy

The dynamics of charge separation in photovoltaic polymer blends following photoinduced electron transfer from the conjugated polymer, CN-MEH-PPV, to the electron accepting functionalized fullerene, PCBM, are observed with ultrafast vibrational spectroscopy. The investigators take advantage of a solvatochromic shift of the vibrational frequency of the carbonyl (C=O) stretch of PCBM to directly measure the rate of escape of electrons from their Coulombically bound radical pairs at the interfaces on ultrafast time scales. The data indicate that the rate of free carrier formation is temperature independent from 200 to 350 K indicating that excess vibrational energy resulting from the electron transfer reaction plays an important role in mediating charge separation. These observations suggest that efforts to develop new low band-gap polymers for organic solar cells should target electron donor and acceptor pairs capable of advantageously redistributing excess vibrational energy to efficiently separate charge with minimal donor-acceptor energy level offsets.