We report an in situ X-ray investigation of the composition dependence on the structural evolution during drying of doctor-bladed blends of poly-(3-hexylthiophene) (P3HT) and [6,6]-phenyl C-61-butyric acid methyl ester (PCBM). This study enables an observation of the microstructure evolution in real time during blend crystallization. P3HT:PCBM blends with ratios of 1:0.5, 1:0.8, and 1:2 exhibit differing structural evolution during the course of solvent evaporation resulting in a different microstructure of the blends upon solidification. Large excess of PCBM over the eutectic composition impedes the pi-pi packing of P3HT chains and leads to a not yet observed diffraction feature with an associated spacing of 12.6 angstrom, which might originate from a disordered phase of intimate mixed P3HT and PCBM molecules. This work provides a microscopic understanding of the composition dependence of the film formation from solution. The structural results are discussed in relation to the composition dependence of photovoltaic performance previously reported.