Solvent additives are shown to drastically affect internal ordering of the polymeric component of bulk heterojunction solar cells. Grazing incidence wide angle X-ray scattering measurements reveal that additives affect polymeric crystallite type, perfection, orientation, and population within the film. Improvements in device performance that result from additive processing are well correlated with increases in the population of crystallites within the film of the type capable of pi-pi stacking.