IEEE Spectrum has an article on "Hot carrier" solar cells which could be twice as efficient as today's - Breakthrough in Capturing Lost Energy in Solar Cells.
Material chemists at the University of Texas at Austin and the University of Minnesota say that according to their research, the efficiency of a solar cell may potentially be increased to more than 60 percent, up from what was thought to be a limit of about 30 percent. They report their findings in today’s edition of the journal Science.
From a cost standpoint, boosting efficiency is one of the keys to making electricity from solar cells competitive with fossil-fuel-derived power. “Imagine the practical applications” of a solar cell more than twice as efficient as today’s, says Xiaoyang Zhu, a chemist at the University of Texas. The work he and his colleagues have done, Zhu says, proves that formerly squandered energy “can be taken out and worked with.”
If a photon has about the same energy as the solar cell semiconductor’s band gap, it knocks an electron into the conduction band, where it can flow as current. However, some high-energy photons in sunlight exceed the band-gap energy of the cell. The ejected “hot” electron, however, quickly loses its excess energy, “cooling down” to the bottom of the conduction band within a picosecond. So far it has been impossible to retrieve the lost energy, according to Zhu.
But Zhu and his team discovered that the lost energy can be salvaged and transferred to an adjacent electron-conducting layer. According to their findings, energy transfer is not only possible but also occurs much faster than anyone expected, in less than 50 femtoseconds (quadrillionths of a second).