Building blocks for organic solar cells are made from P3HT in a P3HT:PCBM solution in toluene and used to tune the morphology of the photoactive layer. The approach presented here decouples the structure and morphology formation, providing precise control over both the structures in solution and the morphology of the photoactive layer. For the characterization of the nanostructures in the organic casting solutions, cryo-TEM was successfully employed, and reveals the P3HT crystals and even the 1.7 nm lamellar stacking, which in combination with cryogenic low dose electron diffraction clearly proves the high crystallinity of P3HT aggregates realized. The photoactive layers made from pre-crystallized solutions show a morphology that is closely related to the structures in solution. A clear trend of decreasing open circuit voltage and increasing short circuit current with increasing order in the casting solutions and the devices was observed, which correlates with the evolution of the morphology from very intermixed with small fibrillar structures to phase-separated with large polymer crystals, as evaluated from representative devices, characterized in 3D with electron tomography.