The morphology evolution of poly(p-phenylenevinylene)/methanofullerene (MDMO-PPV/PCBM) composite films as used for photovoltaic devices has been investigated upon thermal annealing under various spatial confinements. Three types of spatial confinement have been studied: no confinement, which corresponds to free-standing composite films; single-sided confinement, in which the composite films are deposited on a substrate; and double-sided or sandwich-like confinement, in which the deposited composite films are additionally covered by a top layer. For all the confinement types, annealing above the glass transition temperature Tg of bulk MDMO-PPV forces crystallization of PCBM molecules into single crystals from the MDMO-PPV matrix and causes phase separation. The mobility of PCBM molecules in the MDMO-PPV matrix and its crystal growth rates decrease with increased degree of confinement. In the case of free-standing films the diffusion rate of PCBM is so high that the molecule incorporation rate at the growing front of the PCBM crystals determines their growth rate; elongated single crystals are formed due to the anisotropy of the crystal growth in the lateral dimensions. For single- and double-sided confinement, the mobility of PCBM is lower, and in particular for double-sided confinement, diffusion instead of incorporation rate of PCBM molecules dominates crystal growth, which results in less elongated (single-sided confinement) and even circular (double-sided confinement) shapes of the single crystals formed. Therefore, spatial confinement reduces the mobility of PCBM molecules and thus its crystallization kinetics in its thin composite films.
Materials and Interface Chemistry
Facility/equipment: Research lab