Excitation transport and charge separation in an organic photovoltaic material: Watching excitations diffuse to interfaces

Photoinduced charge separation and excitation transport to the interfaces of an organic photovoltaic material are examined using ultrafast visible pump-infrared probe spectroscopy. The carbonyl (C = O) stretch of the butyric acid methyl ester group of a functionalized fullerene, PCBM, is probed as a local vibrational reporter of the charge-transfer dynamics in a blend of PCBM with a conjugated polymer, CN-MEH-PPV. Following ultrafast excitation of the polymer, charge transfer to the fullerene occurs at the interfaces of the materials over many time scales ranging from sub- 100 fermoseconds to nanoseconds. A fast charge-transfer component arises from excited states in the polymer that form near inter-faces with PCBM aggregates where little or no excitation transport is required to affect charge separation. A slower component occurs because excited states that are created throughout the polymer phase must diffuse toward the interfaces to affect charge separation, giving rise to an induction period in the charge-transfer dynamics. Polymer segments arc excited at a distribution of distances from the interfaces with a corresponding range of induction periods that collectively cause the slow component to appear with a roughly 3 ns time constant. The time scale and amplitude of the slow component is directly related to the spatial transport of excitations in the polymer, which suggests that this approach may prove useful for studying excitation transport in the complex phase-separated environments of organic solar cells.