Like the semiconductors silicon and gallium arsenide, graphene absorbs photons, but unlike these, the photon forces multiple electrons from their orbits, instead of just one.
Semiconductor photovoltaic solar cells operate absorbing photons. When a photon is absorbed, it knocks a single electron out of place, which can be collected to generate an electric current.
There is much more energy in a photon than can be carried by this single electron, and so the rest of the energy is lost as heat, which is why silicon and gallium arsenide semiconductor solar cells are currently limited to less than a 30% maximum theoretical efficiency.
Researchers at MIT have recently discovered that graphene has an interesting property that could make for more efficient solar cells. Where semiconductor solar cells have a theoretical maximum efficiency somewhere around 30%, graphene solar cells could be over 60% efficient.
Graphene, as a nanomaterial, is fairly inexpensive when compared to silicon or gallium arsenide, so solar cells made from graphene could be cheaper as well. There’s only one problem though, that is, harnessing the electrons. Making the transition from nanomaterials to macromaterials, such as a copper wire, is a giant leap in technology, so researchers will have to figure this out before such a technology could be commercialized.