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 (C-60), and on MEH-PPV/C-60 composite films of various concentrations. We find that even in the picosecond regime, charge transfer from the conjugated polymer to C-60 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 approximate to 200 Angstrom) 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-stare decay curves, we estimate the charge-transfer range to be 80 Angstrom and interpret that range as resulting from quantum delocalization of the photoexcitations.