We report the composition profile in blends of hole-accepting and electron-accepting polyfluorenes. During solvent evaporation to form thin (similar to 150 nm thickness) solid films, demixing of the polymers produces micrometer scale lateral phase separation. We show here that demixing is more complete in regions closer to the interface between the two phases. We propose that carriers transport is easier in these regions than in the bulk. We show how this affects the photovoltaic efficiency of diodes made with these blends, which show a linear dependence of efficiency on the length of the interface between the phases.