Quantitative insight into morphology evolution of thin PPV/PCBM composite films upon thermal treatment

Atomic force microscopy (AFM) is used to study the morphology evolution upon thermal treatment of a thin composite film typically used for polymer solar cells application. This film consists of poly[2-methoxy-5-(3',7'-dimethyloetyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) with 80 wt % of PCBM in the composite. AFM topographic images clearly demonstrate that PCBM crystal clusters grow up out of the film, and the areas surrounding the crystals become thinner due to the depletion of PCBM material, which causes the phase separation in the composite film upon annealing. Volume quantification analysis on these sequential AFM topographic images in-situ recorded during annealing shows that the volumes of both PCBM crystals and depletion zones are significantly increased during the initial annealing times, followed by a stable period indicating the occurrence of equilibrium state. The different morphology evolution kinetics between the depletion zones and PCBM crystals is mostly due to the asynchronism between the move out of PCBM from its original position and collapse down of the film wherein. The global equilibrium state is finally achieved as most of the PCBM in the whole film has been depleted for crystal growth.