Grazing incidence small angle neutron scattering (GISANS) will be used to investigate the lateral phase behavior in thin films of blends of electron-donor benzothiadiazole-dithienopyrrole derivatives and the electron-acceptor material PCBM. These compounds are currently under investigation as model small organic molecule-fullerene bulk heterojunction solar cells. The lateral phase behavior of the film as a function of blend donor/acceptor composition, preparation conditions, and thermal treatment will be evaluated, and compared to device performance measurements and complementary NR measurements of the vertical composition profile of the films. The GISANS will provide essential information that can¿t be obtained by standard SANS or other transmission morphology measurements.