Neutron reflectivity will be used to investigate the composition in the direction perpendicular to the substrate for films of blends of electron-donor benzothiadiazole-dithienopyrrole derivatives and the electron-acceptor material PCBM. These compounds are currently under investigation in bulk heterojunction solar cells. The vertical distribution of the film components as a function of the blend donor/acceptor content, preparation conditions, and thermal treatment will be used, together with film morphology, optical spectroscopy characteristics, and the bulk thermodynamic phase behavior of the donor/acceptor blends, to provide an improved understanding of the performance of solar cells containing such bulk heterojunction films.