Microscopic inhomogeneity and ultrafast orientational motion in an organic photovoltaic bulk heterojunction thin film studied with 2D IR vibrational spectroscopy

Two-dimensional infrared vibrational spectroscopy is used to examine conformational inhomogeneity and ultrafast orientational motion within local environments of an organic photovoltaic bulk heterojunction thin film. The bulk heterojunction material consists of a mixture of the electron donor poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-(1-cyanovinylene)phenylene] (CN-MEH-PPV) and the electron acceptor [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). PCBM species reside in a distribution of environments within large domains of the molecules that cause their C=O stretch modes to be inhomogeneously broadened. The molecular inhomogeneity also results in frequency dependent vibrational relaxation dynamics. The butyric acid methyl ester group of PCBM undergoes ultrafast wobbling-in-the-cone orientational motion on the 110 fs time scale within a cone semiangle of 29°. The vibrational dynamics are sensitive metrics of molecular order in the material and have implications for charge mobility and degradation phenomena in organic photovoltaic devices. This report represents the first study of organic photovoltaic materials using ultrafast two-dimensional infrared vibrational spectroscopy.