To get an efficient organic solar cell, as much light as possible should be absorbed by the photoactive layer; as a consequence, thick layers should be preferable. However, it is often observed that much thinner photoactive layers result in more efficient devices than the corresponding thicker layers absorbing more light. Besides light absorption, other aspects such as efficient exciton dissociation, charge transportation, and charge collection are of crucial importance, and all of them are strongly influenced by the volume morphology of the photoactive layer. In this study of bulk-heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM), we show that the resulting P3HT/PCBM morphology is strongly determined by the layer thickness because the kinetics of solvent evaporation and crystallization is different in films of different thickness. For the preparation conditions chosen in this Study, an optimum morphological organization of the photoactive layer characterized by high crystallinity of P3HT, viz. numerous crystalline P3HT nanowires forming a genuine three-dimensional network, and enrichment of crystalline P3HT closer to the hole collecting electrode can only be achieved for relatively thin (100 nm) P3HT/PCBM layers. Corresponding devices absorb only a limited fraction of all available photons but have the highest efficiency.