Surface segregation and self-stratification in blends of spin-cast polyfluorene derivatives

We have used helium-3 nuclear reaction analysis to study the morphology of spin-cast blends of poly(9,-dioctylfluorene) (F8) and poly(9,-dioctylfluorene-alt-benzothiadiazole) as a function of composition, casting solvent, and initial solvent concentration. In blends cast from toluene, a surface segregated layer is observed for a broad range of F8 compositions. The surface excess has a maximum of ?nm at an F8 volume fraction = 0.5, dropping down to ?2?nm for = 0.2 and?0.8. The existence of such surface segregated layers could play an important role in charge transport in optoelectronic devices made from blends of conjugated polymers. Films cast from chloroform show negligible surface segregation. Scanning probe microscopy experiments show that films cast from toluene exhibit significant in-plane structure, in contrast to those cast from chloroform, which show minimal lateral structure.