The efficiency of bulk-heterojunction polymer:fullerene solar cells critically depends on the dominant length scale of the morphology of the photoactive layer. At present little is known on the sequence of events that control the morphology formation. Optical experiments allow studying the kinetics of film and morphology formation in real time and have been performed on a blend of a diketopyrrolopyrrole-based polymer with a fullerene derivative, deposited from solvents with and without co-solvents. Combined with ex-situ transmission electron microscopy, it is possible to determine the parameters that influence the dominant length scales in the phase separated films. Without co-solvents, large fullerene domains are formed by liquid-liquid phase separation. With co-solvents, on other hand, the dominant length scale is determined by polymer aggregation. The width of the polymer fibers that are formed by the aggregation can be finely controlled by adjusting the nature of the co-solvent mixture and the molecular weight. I will discuss possible mechanisms for this behavior.