Solar Cell Sets World-Record For Efficiency, 37% Increase From Previous Record Holder

By Mo Mozuch on August 2, 2012 5:02 PM EDT

photo:NASA
photo:NASA

Researchers from the University of Toronto and King Abdullah University of Science and Technology have developed a solar cell that set a world record for efficiency. The cell utilizes colloidal quantum dot (CQM) films that blend organic and inorganic chemistry, a breakthrough that allows for low-cost mass production on the level of paint or ink. The discovery could allow for solar cells to be "printed" on flexible materials similar to the way newspapers are manufactured.

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Ted Sargent, an engineering professor at the University of Toronto, said in a press release that "our world urgently needs innovative, cost-effective ways to convert the sun's abundant energy into usable electricity. This work shows that the abundant materials interfaces inside colloidal quantum dots can be mastered in a robust manner, proving that low cost and steadily-improving efficiencies can be combined."

Quantum dots are nanometer-sized semiconductors that capture electrical energy from both visible and invisible solar spectrums. This is an important feature for solar cells, because only half of the sun's energy lies in the visible spectrum. The infrared range contains the other half. Sargent recently published an article in the March 2012 edition of the journal Nature Photonics describing the advantage CQM cells have in harnessing solar energy, as well as the steps that need to be taken to ensure their commercial viability.

"The Sun's visible and infrared radiation can be more efficiently harnessed if a number of different light-absorbing materials are employed in series," Sargent said. "In such a multijunction solar cell, a stack of semiconductors, chirped through the visible to the infrared range, capture not only the abundant energy available in each visible photon, but also the considerable photon current available in the short-wavelength infrared photons, which ordinarily would not be absorbed."

Previous quantum dot solar cells produced low levels of electricity because large, internal surface areas on nanoparticles in the film made it difficult to trap and extract electrons.  Improving the poor surface quality in order to more efficiently trap electrons was the primary challenge for researchers. They developed a process called hybrid passivation which uses organic compounds to increase the electron traps, and in turn, increase efficiency.

"By introducing small chlorine atoms immediately after synthesizing the dots, we're able to patch the previously unreachable nooks and crannies that lead to electron traps," explained doctoral student and lead co-author Alex Ip. "We follow that by using short organic linkers to bind quantum dots in the film closer together."

The new CQM cells have a power-conversion efficiency of seven percent. Most experts set a 10 percent power-conversion efficiency as the bar to meet. Once there, researchers claim, the commercial viability of solar cell technology will increase dramatically.

"If solar energy harvesting is achieved at an installed cost of $1 per watt-peak, then it will produce electricity over its lifetime at an equivalent cost of around $0.05 kWh, which is compellingly competitive with grid prices," said Sargent in the Nature Photonics piece. "This is a purely economic advantage that requires no subsidies to compete in the open market and does not even begin to consider the important environmental benefits of solar power."

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