Non-thermal free carrier formation in organic photovoltaic polymer blends

Ultrafast visible pump - infrared probe spectroscopy is used to examine the dynamics of free carrier formation following photoinduced electron transfer in an organic photovoltaic polymer blend. The carbonyl (C=O) stretch of the functionalized fullerene, PCBM, is probed as a local vibrational reporter of the dynamics in a blend with a conjugated polymer, CN-MEH-PPV. It has been determined that PCBM molecules at the interfaces of PCBM and polymer phases possess higher frequency carbonyl vibrational modes while molecules in the centers of PCBM domains have lower frequency modes. The shift in frequency of the carbonyl stretching mode is used to directly resolve the dynamics of free carrier formation that occur on the few picosecond timescale. The fast dynamics suggest that the presence of an interfacial dipole causes the charge carriers to experience a smaller Coulombic potential than that which would be predicted from the dielectric properties of the materials. The free carrier formation dynamics are temperature independent indicating that excess vibrational energy remaining for a short period of time after the