The Solar Cell That Gets 2 Electrons For The Price Of 1 Photon

Silicon solar cells are very particular about the photons they can use to generate electricity. The photon hitting the cell needs to have enough energy to knock an electron from the valence band to the conduction band of the semi-conductor. If it doesn’t, then the photon is useless. If the photon has more energy than is needed, the excess energy will be wasted – a red photon might do the job just as well as a blue photon, even though the blue photon has more energy. So the question is, how can we make the most of the photons arriving from the sun?
Researchers at the Massachusetts Institute of Technology’s Center for Excitonics think they have found a way to use an organic dye called pentacene to make high-energy photons propel two current carriers instead of just one – effectively getting 2 electrons from 1 photon. In solar cells, it could potentially double the photocurrent from high-energy photons. Daniel Congreve, Jiye Lee, Nicholas Thompson, Marc Baldo and six others found a way to exploit singlet-exciton fission in pentacene to generate more than one electron for every incident photon within a small portion of the visible spectrum, boosting the internal quantum efficiency to around 160 percent.
The main draw-back at the moment is the pentacene cell only works at this efficiency within an extremely narrow band of the visible spectrum. But with current silicon technology rapidly approaching the theoretical limit of efficiency (set at around 29%), any way to squeeze more electricity from the sun’s photons could have enormous benefits. And with solar farms popping up all over the world, the heat is on to make photovoltaic power stations a viable alternative to their fossil fuel parents. Therefore, even the smallest increase in efficiency would have huge benefits when scaled up. Marc Baldo, a professor of electrical engineering at MIT, believes the key is to combining the pentacene break-through with current silicon technology. “Can we apply this thing as a coating on silicon?” Baldo asks. “If we can do that, it would have a pretty major impact on solar cell technology.”

Sources:
http://www.scientificamerican.com/article.cfm?id=the-solar-cell-that-turns-1-photon-into-2-electrons
http://www.sciencemag.org/content/340/6130/334