Vibrational solvatochromism in organic photovoltaic materials: method to distinguish molecules at donor/acceptor interfaces.

We report the observation of vibrational solvatochromism in organic photovoltaic materials. The frequency of the carbonyl (C=O) stretch of the methyl ester group of the functionalized fullerene, PCBM, is sensitive to the local phase separated morphology of polymer blends and bilayers incorporating the fullerene. In particular, PCBM molecules at interfaces with conjugated polymers exhibit higher frequency carbonyl stretch vibrations in comparison to molecules imbedded in the interiors of PCBM clusters/layers. The resulting frequency gradient was recently used to examine the dynamics of charge photogeneration in a blend of the conjugated polymer, CN-MEH-PPV, with PCBM. In this contribution, we explore the origin of the frequency shift and show that it arises from variations in the inhomogeneous solvent environments experienced by PCBM molecules as a result of phase separation in the polymer blend.