Bimolecular crystals and intercalated molecular structures of polymer/fullerene in bulk heterojunction solar cells

The performance of polymer:fullerene solar cells is heavily influenced by the packing and ordering at the molecular level in the blends as the local packing determines the stability, carrier recombination dynamics, and charge transport properties of these blends. We use x-ray diffraction, ambipolar charge transport measurements, and optical microscopy to demonstrate the formation of stable well-ordered bimolecular crystals of fullerene intercalated between the side-chains of the semiconducting polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT). We show that fullerene intercalation is general and is likely to occur in blends with both amorphous and semicrystalline polymers when there is enough free volume between the side-chains to accommodate the fullerene molecule. These findings offer explanations for why luminescence is completely quenched in crystals much larger than exciton diffusion lengths, how the hole mobility of poly(2-methoxy-5-(3′,7′-dimethyloxy)-p-phylene vinylene) (MDMO-PPV) increases by over 2 orders of magnitude when blended with fullerene derivatives, and why large-scale phase separation occurs in some polymer:fullerene blend ratios while thermodynamically stable mixing on the molecular scale occurs for others, and finally what determines the optimum polymer:fullerene blending ratios.