One of the most elegant ways of utilizing solar energy is to transform the energy of photons energy into electricity. This is done using photovoltaic devices or solar cells. I will discuss the principles of the operation and key factors affecting solar cell efficiencies. Our group’s efforts focus on design and synthesis of materials with properties optimized for most efficient light absorption and charge collection. I will present the work on group III-nitride semiconductors for multijunction solar cells and the first demonstration of the intermediate band solar cell that utilizes highly mismatched alloys, a new class of materials discovered in our laboratory. Finally, I will show that the new materials offer a potential for making photoelectrochemical cells that use the solar photon energy to split water molecules into hydrogen and oxygen.