Effect of spatial confinement on the morphology evolution of thin poly(p-phenylenevinylene)/methanofullerene composite films

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 T_g 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.