18 June 2010—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).
Their experiment was carried out using a model system composed of colloidal lead-selenide nanocrystals, or quantum dots, coupled with an electron-conducting titanium dioxide layer. The researchers used lead selenide for practical reasons, but they say the process will work with other quantum-dot materials as well.
Since the 1980s, scientists have been conducting trials on what are known as hot-carrier solar cells with the aim of capitalizing on the extra energy. Within the past 10 years, many studies have confirmed the promise held by quantum dots for slowing down the cooling of hot-charge carriers so they keep their energy longer. Essentially, the small size of the nanocrystal forces a high number of electron-electron interactions. This “quantum confinement effect” maintains electrons at a high level of excitement for up to a nanosecond, potentially enough time for their energy to be put to use.
“There have been other papers that indicate hot-electron transfer within similar systems,” says Arthur Nozik, senior research fellow at the U.S. Department of Energy’s National Renewable Energy Laboratory and an expert on quantum dot chemistry, but “the evidence was indirect.” In particular, Zhu and his colleagues gathered specific information about the timescale on which the transfer occurred. Knowing the timing could help materials scientists create commercial solar cells that could capture the extra energy.
Zhu cautions that it will take a lot of work before the results are applicable outside the laboratory. Ultimately, the value of the findings hinges on scientists’ ability to channel the rescued energy for practical use. As it stands, much of the excess energy captured by the titanium dioxide would be lost as heat in the conducting wires exiting the cell. Scientists and engineers will have to invent a new interface between materials to prevent this from happening.