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

Time-resolved photoinduced absorption was measured on bilayers of poly-[${2-\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{x}\mathrm{y},5-(2}^{\prime }$-ethylhexoxy)-1,4-phenylenevinylene] (MEH-PPV) and fullerene $\left({\mathrm{C}}_{60}\right),$ and on ${\mathrm{M}\mathrm{E}\mathrm{H}-\mathrm{P}\mathrm{P}\mathrm{V}/\mathrm{C}}_{60}$ composite films of various concentrations. We find that even in the picosecond regime, charge transfer from the conjugated polymer to ${\mathrm{C}}_{60}$ 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≈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.