Unsure what to make of this research report from Boston College about a means to double the power output of solar cells. “Researchers at Boston College,” states Technology Review’s Kevin Bullis, “have built solar cells that get a power boost from high-energy photons. This boost, the researchers say, is the result of extracting hot electrons.”
The results are a step toward solar cells that break conventional efficiency limits. Because of the way ordinary solar cells work, they can, in theory, convert at most about 35 percent of the energy in sunlight into electricity, wasting the rest as heat. Making use of hot electrons could result in efficiencies as high as 67 percent, says Matthew Beard, a senior scientist at the National Renewable Energy Laboratory in Golden, CO, who was not involved in the current work. Doubling the efficiency of solar cells could cut the cost of solar power in half.
The nanowires grow upward from the substrate, creating a surface that’s able to absorb more sunlight than a flat surface is. When photons hit their tips, they are thin enough to allow excited electrons to escape. The technology makes collection of electrons easier.
As noted some time ago, quantum dots exhibit outstanding photoelectric properties; “they exhibit a broad absorption spectrum, a narrow emission band and large absorption cross sections.” Even though many more solar panel manufacturers might be willing to embed quantum dots between layers so as to absorb energy that otherwise is wasted due to overheating, the fabrication of such semiconductor nanocrystals unfortunately is more complex and expensive when compared to other thin film development, e.g., organic dyes. Thus, some researchers convinced of the possibilities of quantum dots have focused upon the development of hybrid materials that would allow for low cost fabrication of TVPV (Thin Film Photo Voltaic) cells that demonstrate greater efficiency.
It would see that the Boston College researchers have been successful with the fabrication of nanowires. As previously noted, nanowires are better at light conversion because their length allows more absorption of energy.
When photons hit their tips, nanowires are thin enough to allow excited electrons to escape. The nanotechnology makes collection of electrons possible. Science Daily relays more explanation of the physics in the Boston College announcement.
When light is captured in solar cells, it generates free electrons in a range of energy states. But in order to snare these charges, the electrons must reach the bottom of the conduction band. The problem has been that these highly energized “hot” electrons lose much of their energy to heat along the way.
Hot electrons have been observed in other devices, such as semiconductors. But their high kinetic energy can cause these electrons, also known as “hot carriers,” to degrade a device. Researchers have long theorized about the benefits of harnessing hot electrons for solar power through so-called “3rd generation” devices.
By using ultrathin solar cells — a film fewer than 30 nanometers thick — the team developed a mechanism able to extract hot electrons in the moments before they cool — effectively opening a new “escape hatch” through which they typically don’t travel, said co-author Michael J. Naughton, the Evelyn J. and Robert A. Ferris Professor of Physics at Boston College.
The team’s success centered on minimizing the environment within which the electrons are able to escape, said Professor of Physics Krzysztof Kempa, lead author of the paper.
Kempa compared the challenge to trying to heat a swimming pool with a pot of boiling water. Drop the pot into the center of the pool and there would be no change in temperature at the edge because the heat would dissipate en route. But drop the pot into a sink filled with cold water and the heat would likely raise the temperature in the smaller area.
“We have shrunk the size of the solar cell by making it thin,” Kempa said. “In doing so, we are bringing these hot electrons closer to the surface, so they can be collected more readily. These electrons have to be captured in less than a picosecond, which is less than one trillionth of a second.”
The ultrathin cells demonstrated overall power conversion efficiency of approximately 3 percent using absorbers one fiftieth as thick as conventional cells. The team attributed the gains to the capture of hot electrons and an accompanying reduction in voltage-sapping heat. The researchers acknowledged the film’s efficiency is limited by the negligible light collection of ultra-thin junctions. However, combining the film with better light-trapping technology — such as nanowire structures — could significantly increase efficiency in an ultra-thin hot electron solar cell technology.
In addition to Naughton and Kempa, the research team included Professor of Physics Zhifeng Ren, Research Associate Professor and Laboratory Director Andrzej A. Herczynski, Research Scientist Yantao Gao, doctoral student Timothy Kirkpatrick, and Jakub Rybczynski of Solasta Corp., of Newton MA, which supported the research. Naughton, Kempa and Ren are principals in the clean energy firm as well.
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- Hot Electrons (peakenergy.blogspot.com)