Solar power simply won't be able to supply the terawatts of power we need until we identify better materials for solar cells. Silicon, which is used in most photo­voltaics, is too expensive; the materials used to make cheaper thin-film solar cells, such as cadmium telluride, are rare--and some are toxic. To uncover other options, Berkeley Lab researcher Cyrus Wadia did an economic analysis of materials that have good electrical properties and can efficiently absorb sunlight. His results point to two previously overlooked materials: iron pyrite and copper sulfide. The analysis shows that the costs of extracting these compounds from existing reserves are low: less than .000002 cents per watt for iron pyrite and .0014 cents for copper sulfide. Wadia has since developed ways to synthesize pure nanoparticles of each. He's made functional but, so far, low-efficiency solar cells from the copper sulfide nanoparticles and is working on iron pyrite cells. --Katherine Bourzac