Organic photovoltaics (OPVs) are promising for developing low cost solar energy conversion technologies. Because light absorption in OPVs initially generates strongly bound excitons, an electron donor/acceptor interface with suitable HOMO/LUMO energy level offsets is necessary for efficient charge photogeneration. Bulk heterojunction polymer solar cells, consisting of a blend of a donor polymer semiconductor and a fullerene acceptor molecule, now have power conversion efficiencies of up to 5-8% depending on the selection of materials and device optimization. Realization of even higher efficiency (>10-15%) polymer solar cells requires advances in the synthesis, processing, tuning of electronic structure, and properties of polymer semiconductors. Towards this end, our laboratory is exploring molecular and nanoscale engineering approaches to tailoring materials and devices for high performance bulk heterojunction (BHJ) polymer solar cells. In this talk I will use several examples to illustrate our recent efforts and progress in this area. These include new small band-gap donor polymers for constructing highly efficient polymer/fullerene and hybrid polymer/inorganic nanocrystal solar cells. Nanostructured assemblies of new block copolymers are found to have enhanced photovoltaic properties compared to the parent homopolymers while self-assembled polymer nanowires (5-30 nm width and 400-900 aspect ratios) are shown to be good building blocks for constructing efficient bulk heterojunction solar cells. To overcome the drawbacks of fullerenes new oligomer and polymer acceptor materials are also being developed for OPVs.