Mesoscopic structural length scales in the photoactive layer of organic solar cells affect exciton dissociation into charges and thus device performance. It is currently hypothesized that these length scales are largely affected by processing conditions such as annealing temperature, annealing time, and casting solvent. In our study, we utilized grazing incidence small-angle X-ray scattering and energy-filtered transmission electron microscopy to characterize the in-plane morphology of poly(3-hexylthiophene-2,5-diyl)/[6,6]- phenyl-C61-butyric acid methyl ester mixtures and compared structural data with device performance. We found that the characteristic length scales of the mesostructures are dominated by the crystallization motif of the polymer and do not vary significantly for different processing conditions. For example, thermal annealing of films spun-cast from different solvents yielded similar in-plane morphologies; consequently, device performance was similar once thickness effects were accounted for through optical modeling. © 2013 American Chemical Society.