The quest for a better solar cell
UT research team seeks to turn more sunlight into electricity
Sunlight is free. But it’s not cheap or easy to wring abundant amounts of power from it.
Enter some tiny crystals known as quantum dots and a chemistry professor at the University of Texas named Xiaoyang Zhu, or, as his students call him, XYZ.
A research team led by Zhu, who refers to his Center for Materials Chemistry as the XYZ Lab, has shown that it’s possible to convert much more of the sun’s energy to electricity than conventional solar cells are able to gen- erate. The conventional cells, which are made of silicon, turn no more than about 20 percent of the energy into juice, and their maximum theoretical efficiency is only 31 percent because of technical limitations.
By using a compound called lead selenide in the form of quantum dots, also called semiconductor nanocrystals, and by routing electrons stirred up by the sunlight from the lead selenide to another compound called titanium dioxide, the researchers showed that it’s theoretically possible to harvest 66 percent of the energy.
To put it another way, the XYZ group has figured out some of the ABCs of a better solar cell. Any commercial application is years in the future because considerably more scientific and engineering work needs to be done.
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The team’s findings, published recently in the scholarly journal Science, are part of a growing body of research aimed at improving the efficiency — and reducing the cost — of solar cells. The goal is to make solar energy a viable alternative to fossil fuels that contribute to global warming and to dependence on supplies in politically volatile parts of the world.
“I’m hoping by the time I retire, we have solar cells like this on the roof,” said Zhu, 46. “That would be my dream.”
Zhu, who began this research at the University of Minnesota, was recruited to UT in July 2009. His solar cell lab in UT’s Nanoscience and Technology Building doesn’t look like a conventional chemistry lab. There’s not a test tube in sight; rather, the place is jammed with lasers, optical equipment and a vacuum chamber that looks vaguely like a person wrapped in foil.
Researchers use the gear to study the energy, movement and speed of electrons, the negatively charged subatomic particles whose flow constitutes electricity. Liquid helium is used to cool samples to minus 450 degrees Fahrenheit, and experiments take place atop special tables that rest on a thin cushion of air. The temperature and tables minimize unwanted vibrations and movements.
In conventional solar cells made of silicon, much of the incoming sunlight contains energy that is too high for the cells to capture and use. That energy, in the form of high-energy electrons, or hot electrons, is lost as heat. If those hot electrons could be captured and put to work, the efficiency of the solar cell would increase dramatically.
“There are a few steps needed to create what I call this ultimate solar cell,” Zhu said. “First, the cooling rate of hot electrons needs to be slowed down. Second, we need to be able to grab those hot elec- trons and use them quickly before they lose all of their energy.”
Previous research by other scientists has shown that quantum dots can slow the cooling of hot electrons, thanks in part to the dots’ ultra-small size: They measure 3 to 10 nanometers across. By comparison, a sheet of paper is about 100,000 nanometers thick.
Zhu’s team took the research a step further, showing that the hot electrons can be transferred from the quantum dots to an electrical conductor made of titanium dioxide, a mineral used in sunscreen. In fact, treating the surface of the quantum dots with certain chemicals caused the transfer to occur more rapidly than the researchers expected.
Lead selenide quantum dots have other advantages over silicon. They can be grown in the lab from the elements lead and selenium in a process that is easier and cheaper than preparing silicon for solar cells, said Kenrick Williams, a graduate student at UT and a mem- ber of the research team.
The other members of the team were William Tisdale, now at the Massachusetts Institute of Technology, and Brooke Timp, David Norris and Eray Aydil, all of the University of Minnesota.
Tianquan Lian, a chemistry professor at Emory University who was not involved in this research but studies the use of nanoscale materials for solar energy, told Popular Mechanics magazine that the findings, if verified, make highly efficient quantum dot solar cells more realistic.
Zhu, who received $490,000 in funding from the U.S. Department of Energy for the project, recently received an additional $630,000 to continue the research.
The next challenge is to transfer the hot electrons to a conducting wire without losing too much energy as heat.
“We want to capture most of the energy of sunlight,” Zhu said. “That’s the ultimate solar cell.”
Kenrick Williams and other researchers at the University of Texas are breaking ground in trying to create solar cells that are more efficient, cheaper and easier to make than conventional solar collectors.
UT’s Xiaoyang Zhu leads a team that aims to make solar energy a viable alternative to fossil fuels.