Caltech Researchers Create Highly Absorbing, Flexible Solar Cells with Silicon Wire Arrays
PASADENA, Calif. (2/14/10) Using arrays of long, thin silicon wires embedded in a polymer substrate, a team of scientists from the California Institute of Technology (Caltech) has created a new type of flexible solar cell that enhances the absorption of sunlight and efficiently converts its photons into electrons. The solar cell does all this using only a fraction of the expensive semiconductor materials required by conventional solar cells.
“These solar cells have, for the first time, surpassed the conventional light-trapping limit for absorbing materials,” says Harry Atwater, Howard Hughes Professor, professor of applied physics and materials science, and director of Caltech’s Resnick Institute, which focuses on sustainability research.
The light-trapping limit of a material refers to how much sunlight it is able to absorb. The silicon-wire arrays absorb up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight. “We’ve surpassed previous optical microstructures developed to trap light,” he says.
Atwater and his colleagues—including Nathan Lewis, the George L. Argyros Professor and professor of chemistry at Caltech, and graduate student Michael Kelzenberg—assessed the performance of these arrays in a paper appearing in the February 14 advance online edition of the journal Nature Materials.
Atwater notes that the solar cells’ enhanced absorption is “useful absorption.”
“Many materials can absorb light quite well but not generate electricity—like, for instance, black paint,” he explains. “What’s most important in a solar cell is whether that absorption leads to the creation of charge carriers.”
The silicon wire arrays created by Atwater and his colleagues are able to convert between 90 and 100 percent of the photons they absorb into electrons—in technical terms, the wires have a near-perfect internal quantum efficiency. “High absorption plus good conversion makes for a high-quality solar cell,” says Atwater. “It’s an important advance.”
The key to the success of these solar cells is their silicon wires, each of which, says Atwater, “is independently a high-efficiency, high-quality solar cell.” When brought together in an array, however, they’re even more effective, because they interact to increase the cell’s ability to absorb light.
“Light comes into each wire, and a portion is absorbed and another portion scatters. The collective scattering interactions between the wires makes the array very absorbing,” he says.
This effect occurs despite the sparseness of the wires in the array—they cover only between 2 and 10 percent of the cell’s surface area.
“When we first considered silicon wire-array solar cells, we assumed that sunlight would be wasted on the space between wires,” explains Kelzenberg. “So our initial plan was to grow the wires as close together as possible. But when we started quantifying their absorption, we realized that more light could be absorbed than predicted by the wire-packing fraction alone. By developing light-trapping techniques for relatively sparse wire arrays, not only did we achieve suitable absorption, we also demonstrated effective optical concentration—an exciting prospect for further enhancing the efficiency of silicon-wire-array solar cells.”
Each wire measures between 30 and 100 microns in length and only 1 micron in diameter. “The entire thickness of the array is the length of the wire,” notes Atwater. “But in terms of area or volume, just 2 percent of it is silicon, and 98 percent is polymer.”
In other words, while these arrays have the thickness of a conventional crystalline solar cell, their volume is equivalent to that of a two-micron-thick film.
Since the silicon material is an expensive component of a conventional solar cell, a cell that requires just one-fiftieth of the amount of this semiconductor will be much cheaper to produce.
The composite nature of these solar cells, Atwater adds, means that they are also flexible. “Having these be complete flexible sheets of material ends up being important,” he says, “because flexible thin films can be manufactured in a roll-to-roll process, an inherently lower-cost process than one that involves brittle wafers, like those used to make conventional solar cells.”
Atwater, Lewis, and their colleagues had earlier demonstrated that it was possible to create these innovative solar cells. “They were visually striking,” says Atwater. “But it wasn’t until now that we could show that they are both highly efficient at carrier collection and highly absorbing.”
The next steps, Atwater says, are to increase the operating voltage and the overall size of the solar cell. “The structures we’ve made are square centimeters in size,” he explains. “We’re now scaling up to make cells that will be hundreds of square centimeters—the size of a normal cell.”
Atwater says that the team is already “on its way” to showing that large-area cells work just as well as these smaller versions.
In addition to Atwater, Lewis, and Kelzenberg, the all-Caltech coauthors on the Nature Materials paper, “Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications,” are postdoctoral scholars Shannon Boettcher and Joshua Spurgeon; undergraduate student Jan Petykiewicz; and graduate students Daniel Turner-Evans, Morgan Putnam, Emily Warren, and Ryan Briggs.
Their research was supported by BP and the Energy Frontier Research Center program of the Department of Energy, and made use of facilities supported by the Center for Science and Engineering of Materials, a National Science Foundation Materials Research Science and Engineering Center at Caltech. In addition, Boettcher received fellowship support from the Kavli Neuroscience Institute at Caltech.
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‘Silicon ink’ for solar cells glides toward production
By Candace Lombardi, news.cnet.com
JA Solar, one of the big players in the solar industry, is working with Innovalight to commercialize the latter’s method for making silicon-ink-based, high-efficiency solar cells, the companies said this week.
Innovalight first got noticed in 2007 for perfecting a process in which it could essentially ink-jet-manufacture solar cells using a proprietary silicon ink it had developed. The solar cells are created by pouring an ink solution incorporated with silicon nanoparticles and then decanting the excess liquid to leave behind a crystalline silicon structure.
At the time of the 2007 announcement, Sunnyvale, Calif.-based Innovalight claimed its method not only resulted in solar cells that were cheaper to produce by as much as half, but that the crystalline structure resulting from the process made its cells more efficient at converting electricity.
Those claims now appear to be validated.
On Tuesday, Innovalight announced that an independent study of its method by the U.S. Department of Energy’s National Renewable Energy Laboratory and the Fraunhofer Institute for Solar Energy Systems in Germany confirmed that its silicon ink-based cells “demonstrated a record 18 percent conversion of efficiency.”
Shanghai, China-based JA Solar said the process will lower its production cost for this type of solar cell.
“Innovalight’s silicon ink in conjunction with JA Solar’s leadership in high-volume solar cell manufacturing with demonstrated yield, conversion efficiency, and low production costs, provides a very promising solution to enhance the conversion efficiency of solar cells utilizing our existing solar cell manufacturing lines,” Qingtang Jiang, JA Solar’s chief technology officer, said in a statement Tuesday.
JA Solar plans to further develop the process at its research and development plant in Yangzhou, a city on China’s coast about 630 miles south of Beijing.
Dye-sensitized Solar Cells To Power Air Force Unmanned Aerial Vehicles
From ScienceDaily.com
Dye-sensitized solar cells (DSSCs) are expected to power Air Force unmanned aerial vehicles (UAVs) in the future because they are an optimum energy harvesting source that may lead to longer flight times without refueling.
The University of Washington’s Multidisciplinary University Research Initiative (MURI) project team, with lead researcher Dr. Minoru Taya is working on airborne solar cells by using a flexible film and a thin glass coating with transparent conductive electrodes. He has found that DSSCs made from organic materials, which use (dyes) and moth-eye film, are able to catch photons and convert them into synthesized electrons that can harvest high photon energy.
A few years ago the team mounted dye-sensitized solar cells on the wings of a toy airplane. The propeller was effectively powered, but the plane was not able to become airborne because the glass based solar cells they were using were too heavy. Upon experimentation, they decided to use film battery technology, which worked and in fact, enabled the plane to fly.
“These kinds of solar cells have more specific power convergence efficiency (PCE), very clean energy and easy scalability to a larger skin area of the craft, as well as, low-temperature processing, which leads to lower costs overall,” said Taya.
The team is currently working on DSSCs with higher PCEs using bioinspired dyes, which are installed in the wings of the UAV (airborne energy harvesters).
“Any airborne energy harvester must satisfy additional requirements, like weight and durability in airborne environments. If those are met, then there may even be longer UAV flight times,” said Taya.
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New Jersey is now a true solar power
By Joe Tyrrell, newjerseynewsroom.com
As Congress wrestles with national energy policies and gubernatorial candidates tout their plans here, New Jersey officials say the state deserves credit as a leader in promoting solar power.
In just a few years of coordinated efforts, New Jersey has gone from a non-factor to number two among the states in solar installations connected to the power grid. While far behind California, New Jersey currently generates about twice as many solar kilowatt hours as number three Colorado.
While applauding the gains, many in the industry also say the state, like the nation, has fallen well short of performance goals. New Jersey rose to the top of solar charts in a period when there was little competition from other states.
Now, as the federal government begins to pay attention to renewable energy, New Jersey is in the midst of a challenging transition away from an easy to understand program, which gave rebates to install solar power cells.
The new program shifts the focus away from consumers to utility companies and investors by creating a marketplace for renewable energy credits. The concept has its supporters, though many are more hopeful than confident.
Still, at a time when solar businesses believe the technology is on the verge of a belated boom in the United States, recent New Jersey statistics wowed some attendees at a recent industry conference in Philadelphia.
“Making this even more remarkable is that in 2001 New Jersey had only six” solar cell installations connected to the power grid, compared to more than 4,000 today, wrote Bob Haavind of Photovoltaics World.
His report can be viewed here.
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Dye-sensitized Solar Cells To Power Air Force Unmanned Aerial Vehicles
From ScienceDaily.com
Dye-sensitized solar cells (DSSCs) are expected to power Air Force unmanned aerial vehicles (UAVs) in the future because they are an optimum energy harvesting source that may lead to longer flight times without refueling.
The University of Washington’s Multidisciplinary University Research Initiative (MURI) project team, with lead researcher Dr. Minoru Taya is working on airborne solar cells by using a flexible film and a thin glass coating with transparent conductive electrodes. He has found that DSSCs made from organic materials, which use (dyes) and moth-eye film, are able to catch photons and convert them into synthesized electrons that can harvest high photon energy.
A few years ago the team mounted dye-sensitized solar cells on the wings of a toy airplane. The propeller was effectively powered, but the plane was not able to become airborne because the glass based solar cells they were using were too heavy. Upon experimentation, they decided to use film battery technology, which worked and in fact, enabled the plane to fly.
Click link above for complete article.
