  Using transmission electron microscopy (TEM) and Z-contrast imaging we have
demonstrated elongated nanostructure formation of fullerene derivative
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) within an organic host
through annealing. The annealing provides an enhanced mobility of the PCBM
molecules and, with good initial dispersion, allows for the formation of
exaggerated grain growth within the polymer host. We have assembled these
nanostructures within the regioregular conjugated polymer
poly(3-hexylthiophene) (P3HT). This PCBM elongated nanostructure formation
maybe responsible for the very high efficiencies observed, at very low loadings
of PCBM (1:0.6, polymer to PCBM), in annealed photovoltaics. Moreover, our high
resolution TEM and electron energy loss spectroscopy studies clearly show that
the PCBM crystals remain crystalline and are unaffected by the 200-keV electron
beam
