Solar energy research in the College of Engineering at Cornell encompasses inorganic, organic and materials. We are pursuing the development of multijunction solar cells based on lattice mismatched systems. Examples include AlInN and GaInN solar cells that have been discovered at Cornell and shown to be perfectly matched to the solar spectrum. We are also pursuing organic donor/acceptor blends and heterojunctions as low-cost solar cell alternatives. We combine synchrotron x-ray scattering and multiscale modeling to understand the connection between structure and properties in prototypical systems such as pentacene/fullerene junctions. Our efforts in hybrid cells include dye-sensitized cells with self-assembled titania networks, and systems based on semiconductor nanoparticles with tailored energy bands and high multiexciton generation efficiency. Finally solar energy research also involves the production of hydrogen by splitting water directly using wide bandgap semiconductors in both the single crystal and powder form.