Solar photovoltaic prices are coming down. Between 1998 and 2010, installed costs have dropped by 43 percent, and that does not include any government tax credits or subsidies. This is good for consumers, but not so good for some solar manufacturers. Energy Secretary Chu says that solar will soon achieve cost parity with fossil fuels. That is great, but even at parity, people will still need incentives to make the investment to change. And with government budgets so tight, continuing those incentives will be an ongoing battle with fiscal conservatives, especially those with allegiances to the fossil fuel industry.
But what if the cost were to drop significantly? Then, there would surely be no excuse not to go solar. The cost of the panels themselves has more or less stabilized, but what if the amount of power that comes out of each panel were to go up by a factor of two? Current technology has a cell efficiency of 31 percent. This is largely limited by the fact that much of the sun’s energy is contained in a part of the spectrum that is “too hot” for existing panels to capture and convert to electricity. If this part of the spectrum could be utilized, solar PV panels could come much closer to achieving their theoretical maximum efficiency of 66 percent.
A team of research collaborators from the University of Texas as Austin and the University of Minnesota, have now proven that this “hot” energy, which is essentially high-energy photons that exceed the conduction band energy level, can, in fact, be captured and utilized, resulting in cell efficiencies in excess of 60%.
In conventional collectors, these hot electrons are ejected from the conduction band and cool down extremely rapidly, before the energy can be recovered. But Xiaoyang Zhu, and his team at UT Austin, found that using colloidal lead-selenide nanocrystals, or quantum dots, coupled with an electron-conducting titanium dioxide layer, this energy could be recovered very rapidly (as in 50 quadrillionths of a second).
According to an article about this work in last month’s Scientific American (subscription required),
“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.”
Previous work had shown indirect evidence of this hot transfer, but “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.”
An article in the UT News a year ago, described Zhu’s work at the time when he had completed the first two of the three step process required to capture this energy, namely:
- Slow down the cooling of the hot electron, and
- Transfer the electron to a conducting layer.
The third and final step, which they recently achieved, was to move the electron onto a wire where it can be utilized.
Zhu cautions that the work is not yet ready for prime time. Much of the energy they are now successfully capturing ends up as heat, rather than electricity in the wires exiting the cell. It will require yet another materials engineering breakthrough to come up with an appropriate set of materials with the right set of characteristics to overcome this final hurdle.
[Image Credit/Marsha Miller/U of Texas News]
RP Siegel is the co-author of the eco-thriller Vapor Trails, the first in a series covering the human side of various sustainability issues including energy, food, and water. Like airplanes, we all leave behind a vapor trail. And though we can easily see others’, we rarely see our own.
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