Michael Chabynic University of California, Santa Barbara (UC Santa Barbara) Solar cells with organic semiconductors have been demonstrated with power conversion efficiencies of ~10%. Despite this progress, there are still significant questions about their detailed operation. In bulk heterojunction (BHJ) organic solar cells, the generation and extraction of carriers depends on the nanoscale phase separation in a blend of an electron donating material, usually a polymer, and electron accepting material, usually a fullerene. Both components are structurally disordered leading to a complex electronic density of states for holes and electrons. The inhomogeneity of BHJs at the nanoscale creates significant uncertainties about the structure of molecular interfaces and how they impact the charge generation and extraction process. We have studied the composition of BHJs using a variety of physical characterization methods and performed detailed studies of the miscibility and mass diffusion of fullerenes in polymers. A strong correlation was found between the power conversion efficiency of BHJs and the miscibility of fullerene acceptors in the amorphous fraction of polymer donors. We will also discuss recent work on non-fullerene acceptors and molecular solar cells that helps to reveal detailed information about interfacial states in BHJs. These studies suggest strategies and opportunities for development of detailed models of the operation of organic solar cells.