LAKE WALES, Fla.—This week EE Times will feature updates on some of the carbon-reducing technologies being developed or already in use that could help companies, countries and citizens reduce their carbon footprint. These are some examples to show the infinite variety and to give credit to engineers and companies working on them.
There is little doubt that the climate is heating up (see NASA GISS chart below) and that it is due to the production of greens-house gases, which have been steadily increasing since the maturation of the industrial revolution circa 1880, according to The Intergovernmental Panel on Climate Change (IPCC 2015) meeting this week in Paris at the Conference of the Parties (COP21, Nov. 30–Dec. 11).
Last year the IPCC declared that scientists were 95 percent certain that global warming is being caused (mostly) by increasing concentrations of man-made greenhouse gases—carbon dioxide, methane and nitrous oxide—most of which is being produced by electrical power plants and internal combustion engines. The National Oceanic and Atmospheric Administration (NOAA) paints an even bleaker picture. Of course, there are other causes—from deforestation to livestock flatulence—but perhaps the easiest to address with technology are carbon-free electrical power generation and zero-emission vehicles.
Carbon-free sustainable electrical power generation has been accomplished with power-generating river dams since the invention of the electrical generator, but in many places the dams are being disassembled because of the negative impact they have had on fish runs. No matter. We now have even cleaner methods of electrical power generation.
The most promising zero-carbon electrical power generators are solar cells, which already come in all sorts of formulations, sizes and capacities. Several governments, for instance, have produced net-zero demonstration houses that collect enough electrical energy with solar panels to run the household all day, plus sell their excess to the grid. Then at night and on cloudy days, the house draws from the grid. With enough solar cells this can work out to a net-zero or even net-positive proposition, where the utility company sends you a check at the end of the month instead of a bill.
According to the U.S. Department of Energy, every hour, enough energy from the sun reaches Earth to meet the world’s energy usage for an entire year. Of course its impossible to cover the lighted half of the 198 million square miles of the Earth's surface in solar cells. Even collecting all 365 days of the year with widely distributed solar cell arrays illuminated half the day (12/7) at 20 percent efficiency would take over 225 thousand square miles to satisfy the entire world's need for energy—a seemingly unachievable goal.
However, that is not stopping the world's scientists from trying. One of the latest attempts comes from multi-band solar cells which attempt to fulfill at least one of the key requirements for obsoleting coal-fired power plants—solar cells that capture the entire spectrum of light from the sun instead of just a narrow band as is the case today.
For instance, Lawrence Livermore Berkeley Laboratory (Berkeley Labs) claims to be on the way to multi-band solar cells that demonstrate a key requirement for a wider use of the solar spectrum. And Berkeley Labs is not the only one, others are following suit with tuned nanoscale antennas that capture more and more of the light coming from the sun—rain or shine.
Berkeley Lab's trick is creating a defect-free atomically thin film of molybdenum disulfide (MoS2) to create ultra-high-efficiency solar cells (and bright yet transparent displays for that matter). Usually monolayers have too many defects to achieve high-efficiency, but Berkeley Labs and the University of California Berkeley (UC Berkeley) have invented a way to repair defects chemically with what they call an organic super-acid.
After treating a defective monolayer with the superacid, its efficiency is improved up to 100 percent. And since the MoS2 films are barely 7 angstrom thick (0.7 nanometer) the material is “optoelectronically perfect,” according to principle investigator Ali Javey, at UC Berkeley, who performed the work with doctoral candidate, Matin Amani.
“Solar cells are able to provide the highest possibly voltage when the photoluminescence quantum yield (a parameter that is extremely sensitive to defects) is perfect. In our recent work we were able to obtain this in monolayer MoS2 by passivating defects through treatment with an organic super acid,” Amani told EE Times. “One of the interesting results from our treated MoS2 samples is that the luminescence yield does not drop off when the pump is very weak. This type of recombination (called Shockley-Read-Hall recombination) is observed in almost all other semiconductors and from a solar point of view means that if you run your cell on a cloudy day you will get even less power than you expect from the reduced light. Since this does not happen in MoS2 it could be a very good solar cell for deep space applications or energy harvesting in areas where there is very little sunlight.”
The superacid “bistriflimide” (TFSI) works by giving up protons to vacancy defects in the MoS2 while simultaneously dissolving any contaminants on the surface of the monolayer, thus resulting in the world's thinnest perfect light absorbing monolayer (see figure below). The MoS2 monolayers are not fragile either, because they are held together with powerful van der Waals forces—keeping each layer aligned with atomic accuracy. The material can be tuned to absorb a wide range of frequencies, as well as emit different frequencies depending on the voltage applied, allowing a multi-layer version to absorb almost the entire solar spectrum.
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