We have investigated the charge generation dynamics in intimately mixed blends of polyfluorene copolymers optimized for photocurrent generation. Using femtosecond transient absorption spectroscopy, we find that the charge generation time is limited by exciton diffusion to the interface. Combined with the kinetics of exciton energy migration, the data reveal the blend morphology on a length scale of sub-10 nm. Furthermore, we demonstrate that excitons are guided efficiently to the interface, which is consistent with an accumulation of low energy sites at the heterojunction.