Grazing-incidence X-ray diffraction and rocking scans have quantified the structure of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester in the active layers of organic solar cells. Our study reveals that the device JSC correlates with the local structural development of pure PCBM and, to second order, the extent of out-of-plane P3HT Π-stacking.