Nano-Crystalline Fullerene Phases in Polymer/Fullerene Bulk-Heterojunction Solar Cells: A Transmission Electron Microscopy Study

The nanoscale phase separation in polymer/fullerene bulk heterojunction plastic solar cells requires the use of high resolution techniques for imaging. In this study we used a high-resolution transmission electron microscope (HR-TEM) together with selected area electron diffraction (SAED) to visualize the polymer and fullerene distributions and their amorphous or crystalline organization in the film. While pristine polymer films exhibited no crystalline order, the fullerene organized in nanocrystals. Upon annealing of the blend film, the accompanying phase separation reaches the micron level and fullerene phases aggregate to larger single crystals, as could be seen by SAED.