Crystallization-Induced 10-nm Structure Formation in P3HT/PCBM Blends

The miscibility and aggregation of PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) in a polymer matrix is of great importance for the development of fullerene-based organic photovoltaic cells (OPVs). In this study we have systematically investigated the loading of PCBM in regioregular and regiorandom P3HT (poly(3-hexylthiophene-2,5-diyl). Using optical microscopy, we demonstrate the partial miscibility of PCBM in thermally annealed P3HT films and relate it to the relative crystallinity of P3HT. The low polydispersity and the nearly 100% regioregularity of a self-synthesized P3HT allowed a detailed X-ray characterization as a function of PCBM content, revealing a superstructure of periodic amorphous and crystalline lamellar domains of fully chain extended polymer chains. PCBM dissolves in the amorphous interlamellar P3HT regions (partially index-matching the X-ray scattering contrast) up to a threshold, above which PCBM aggregates start to form. These results show that crystallization of P3HT into 10-nm-wide lamellar domains sets the main length scale in P3HT/PCBM structure formation. PCBM is displaced into the amorphous intralamellar regions, swelling the lamellar stack. This structure formation by crystallization, which is intrinsic to most semicrystalline polymers, followed by the enrichment, segregation, and crystallization of PCBM provides an interdigitated structure, which is conceptually ideal for excitonic solar cells.