Charge-transfer range for photoexcitations in conjugated polymer/fullerene bilayers and blends

Time-resolved photoinduced absorption was measured on bilayers of poly-[2-methoxy,5-(2^′-ethylhexoxy)-1,4-phenylenevinylene] (MEH-PPV) and fullerene (C₆₀), and on MEH-PPV/C₆₀ composite films of various concentrations. We find that even in the picosecond regime, charge transfer from the conjugated polymer to C₆₀ slows down the decay dynamics relative to the decay in the pure samples. The fact that charge transfer occurs in the picosecond time scale in bilayer structure (thickness≈200 Å) implies that diffusion of localized excitations to the interface is not the dominant mechanism; the charge-transfer range is a significant fraction of the film thickness. From an analysis of the excited-state decay curves, we estimate the charge-transfer range to be 80 Å and interpret that range as resulting from quantum delocalization of the photoexcitations.