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Researchers find way to make solar cells lightweight, more efficient, bendable and easy to mass produce

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 IMAGE: Debashis Chanda helped create large sheets of nanotextured, silicon micro-cell arrays that hold the promise of making solar cells lightweight, more efficient, bendable and easy to mass produce.
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Converting sunshine into electricity is not difficult, but doing so efficiently and on a large scale is one of the reasons why people still rely on the electric grid and not a national solar cell network.

But a team of researchers from the University of Illinois at Urbana-Champaign and the University of Central Florida in Orlando may be one step closer to tapping into the full potential of solar cells. The team found a way to create large sheets of nanotextured, silicon micro-cell arrays that hold the promise of making solar cells lightweight, more efficient, bendable and easy to mass produce.

The team used a light-trapping scheme based on a nanoimprinting technique where a polymeric stamp mechanically emboss the nano-scale pattern on to the solar cell without involving further complex lithographic steps. This approach has led to the flexibility researchers have been searching for, making the design ideal for mass manufacturing, said UCF assistant professor Debashis Chanda, lead researcher of the study.

 IMAGE: This image shows a printed cell.
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The study’s findings are the subject of the November cover story of the journal Advanced Energy Materials.

Previously, scientists had suggested designs that showed greater absorption rates of sunlight, but how efficiently that sunlight was converted into electrical energy was unclear, Debashis said. This study demonstrates that the light-trapping scheme offers higher electrical efficiency in a lightweight, flexible module.

The team believes this technology could someday lead to solar-powered homes fueled by cells that are reliable and provide stored energy for hours without interruption.


Debashis Chanda joined UCF in Fall 2012 from University of Illinois at Urbana-Champaign with joint appointment in the Nanoscience Technology Center and the College of Optics and Photonics (CREOL). He has published multiple articles on light-matter interactions and metamaterials and is a reviewer for multiple journals in his field. For some of his pioneering works Debashis was awarded a Department of Energy solar innovation award and a Natural Sciences and Engineering Research Council award among others. He also earned a National Science Foundation Summer Institute Fellowship this year.

Other researchers on the project include Ki Jun Yu, Li Gao, Jae Suk Park, Yi Ri Lee, Christopher J. Cocoran, Ralph G. Nuzzo and John A. Rogers from the University of Illinois at Urbana-Champaign.

January 20, 2014 |

Cellulose nanocrystals could be a wonder material for strengthening construction materials

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 IMAGE: This transmission electron microscope image shows cellulose nanocrystals, tiny structures that give trees and plants their high strength, light weight and resilience. The nanocrystals might be used to create a…
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WEST LAFAYETTE, Ind. – The same tiny cellulose crystals that give trees and plants their high strength, light weight and resilience, have now been shown to have the stiffness of steel.

The nanocrystals might be used to create a new class of biomaterials with wide-ranging applications, such as strengthening construction materials and automotive components.

Calculations using precise models based on the atomic structure of cellulose show the crystals have a stiffness of 206 gigapascals, which is comparable to steel, said Pablo D. Zavattieri, a Purdue University assistant professor of civil engineering.

“This is a material that is showing really amazing properties,” he said. “It is abundant, renewable and produced as waste in the paper industry.”

Findings are detailed in a research paper featured on the cover of the December issue of the journal Cellulose.

“It is very difficult to measure the properties of these crystals experimentally because they are really tiny,” Zavattieri said. “For the first time, we predicted their properties using quantum mechanics.”

The nanocrystals are about 3 nanometres wide by 500 nanometres long—or about 1/1,000th the width of a grain of sand—making them too small to study with light microscopes and difficult to measure with laboratory instruments.

The paper was authored by Purdue doctoral student Fernando L. Dri; Louis G. Hector Jr., a researcher from the Chemical Sciences and Materials Systems Laboratory at General Motors Research and Development Center; Robert J. Moon, a researcher from the U.S. Forest Service’s Forest Products Laboratory; and Zavattieri.

The findings represent a milestone in understanding the fundamental mechanical behaviour of the cellulose nanocrystals.

“It is also the first step towards a multi-scale modelling approach to understand and predict the behaviour of individual crystals, the interaction between them, and their interaction with other materials,” Zavattieri said. “This is important for the design of novel cellulose-based materials as other research groups are considering them for a huge variety of applications, ranging from electronics and medical devices to structural components for the automotive, civil and aerospace industries.”

The cellulose nanocrystals represent a potential green alternative to carbon nanotubes for reinforcing materials such as polymers and concrete. Applications for biomaterials made from the cellulose nanocrystals might include biodegradable plastic bags, textiles and wound dressings; flexible batteries made from electrically conductive paper; new drug-delivery technologies; transparent flexible displays for electronic devices; special filters for water purification; new types of sensors; and computer memory.

Cellulose could come from a variety of biological sources including trees, plants, algae, ocean-dwelling organisms called tunicates and bacteria that create a protective web of cellulose.

 IMAGE: This illustration depicts structural details of cellulose nanocrystals.
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“With this in mind, cellulose nanomaterials are inherently renewable, sustainable, biodegradable and carbon-neutral like the sources from which they were extracted,” Moon said. “They have the potential to be processed at industrial-scale quantities and at low cost compared to other materials.”

Biomaterials manufacturing could be a natural extension of the paper and biofuels industries, using technology that is already well-established for cellulose-based materials.

“Some of the byproducts of the paper industry now go to making biofuels, so we could just add another process to use the leftover cellulose to make a composite material,” Moon said. “The cellulose crystals are more difficult to break down into sugars to make liquid fuel. So let’s make a product out of it, building on the existing infrastructure of the pulp and paper industry.”

Their surface can be chemically modified to achieve different surface properties.

“For example, you might want to modify the surface so that it binds strongly with a reinforcing polymer to make a new type of tough composite material, or you might want to change the chemical characteristics so that it behaves differently with its environment,” Moon said.

Zavattieri plans to extend his research to study the properties of alpha-chitin, a material from the shells of organisms including lobsters, crabs, mollusks and insects. Alpha-chitin appears to have similar mechanical properties as cellulose.

“This material is also abundant, renewable and waste of the food industry,” he said.

 


By Emil Venere, 765-494-4709, venere@purdue.edu. Sources: Pablo D. Zavattieri, 765-496-9644, zavattie@purdue.edu; Robert Moon, robertmoon@fs.fed.us.

December 30, 2013 |

A look at Omega’s living sewage treatment plant

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In a large greenhouse two hours north of New York City, banana trees, flowers, and other tropical plants grow amid frogs, fish and burbling waters. Outside, bullrushes and cattails wave in the Hudson Valley breeze. Each year, more than 20,000 people travel to the Omega Institute for Holistic Studies for yoga retreats, wellness workshops, and conferences on how to live simpler, cleaner, better.

And each year, they stop in their tracks, stunned to learn that those lush gardens are actually sewage-treatment plants at work. At Omega, plants, animals, and microorganisms annually transform 5 million gallons of human feces, urine and other waste into water rated pure enough to drink (though Omega uses it for irrigation).

“It looks like it came out of your kitchen faucet—there’s no odour, it’s perfectly clear,” says Omega’s CEO Skip Backus. “It’s really sort of a life-changer for people.”

Since 2009, when Omega’s leaders discovered that their 50-year-old septic system was falling apart, the forward-thinking institute has been at the forefront of a slowly growing international movement built on the idea that conventional sewage treatment—dependent on water- and energy-wasting technology—is unsustainable. To be blunt, there are better ways to handle our shit.

At Omega, the result is a 4,500-square-foot solar-powered greenhouse fed by a system of tanks, lagoons, and sand filters that process up to 52,000 gallons of human waste a day. When it enters the Eco-MachineTM the stuff is as rank and nasty as you’d imagine.

“We give people tours—we want them to see the stinky brown start of the process, and then the constructed wetlands, the reeds, brush, cattails,” says Backus. “At the end, there’s this sense of awe and wonder. They say, ‘Really, this is my waste from last night?’ And we tell them, ‘yeah.’”

In communities from upstate New York to Ohio, Minnesota to Hawaii, a network of scientists, architects, and engineers has been rethinking the way we process our waste because the need is extreme. Jason McLennan, an architect, chief executive of the Cascadia Green Building Council, and one of YES! Magazine’s “Breakthrough 15,”points out that instead of taking our cues from nature—which processes waste on the spot and recycles the nutrients—every flush of a standard toilet takes a several-ounce problem and turns it into a several-gallon problem by adding clean water to feces and transporting the whole mess to treatment facilities via a Byzantine network of aging sewers.

“This system is nothing less than insane,” McLennan writes in “Flushing Outdated Thinking,” his manifesto for change.

Meanwhile, untreated effluent, pharmaceutical drugs, and whatever else we flush leaches from our 600,000-mile maze of pipes into rivers, farm fields, and, yes, tap water. An Associated Press study from 2008 found traces of prescription drugs—including antibiotics, anti-convulsants, mood stabilizers, and sex hormones—in the drinking water of 41 million Americans. The Environmental Protection Agency estimates that, within the next eight years, America’s deteriorating sewage system could cost $82.6 billion to fix.

Finally, our treated waste is dumped, rendering useless such elements within it as nitrogen and phosphorus, which could be valuable as fertilizers.

Perhaps the most intriguing characteristic of an answer like Omega’s is its rudimentary technology—if you can even call it that. In living machines, bugs, plants, and bacteria break down excrement similar to the way nature handles animal droppings in the wild. Composting toilets, too, mimic natural processes by covering the feces with sawdust and letting microbes digest the mixture, producing enough heat to neutralize pathogens.

But combine the word “feces” with composting and even the most progressive thinkers are likely to blanch. Describe the kind of eco-machine in use at Omega, and risk being tarred as a stinky, starry-eyed dreamer.

“This is a cultural thing. People have been taught that their feces are something evil and dreadful,” sighs Gene Logsdon, author of Holy Shit, a book on rethinking our handling of waste, both human and animal. “For years, shit has been seen as something so repugnant that the word itself was scrubbed from polite conversation.”

Logsdon, a plainspoken Ohioan who spent most of his life as a farmer, insists, however, that there’s gold in feces, and he’s not one to mince words. “Cities and villages have been putting human waste—not even all that well treated—on farmland for centuries,” he said. “You don’t hear anyone talk about it or suburbanites would lose their minds. But it’s commonly done. They take it out in trucks—it’s a liquid—and spread it on farm soil that’s going to be planted with corn or wheat.”

In California, Logsdon writes, half of the state’s annual haul of dried sludge—some 375,000 tons—is applied to the land. For him, practicality trumps all, and the soaring cost of chemical fertilizer makes its own argument for composting.

To get a sense of scale, consider that the average household flushes about 160 gallons of water to handle three pounds of poop and a gallon of urine each day. An equation that imbalanced strikes even the most conventional thinkers as hopelessly inefficient.

“It’s absolutely insane the amount of water we use to flush toilets in the U.S.,” says former Naval officer Todd Foret, who is working with the U.S. Department of Agriculture to reclaim wastewater for irrigation. “It’s sad. We commingle super-contaminated stuff with graywater from your dishwasher and shower that’s easy to treat. It’s just crazy.”

Mark Buehrer, a civil engineer based in Bellingham, Wash., is so deep into the field that he doesn’t even like to use the word “waste.” “There is no such thing,” he says. “I look at human waste as, really, a resource. We can’t just keep flushing our nutrients into the oceans and rivers.”

True believers can talk even the most squeamish past their misgivings. The real battleground is politics, where a tangle of regulations designed to keep us healthy thwart even the most ingenious inventors.

Still, there may be room for new thinking. Washington state Representative Kevin Ranker (D-Orcas Island) asked Buehrer to help draft legislation that would loosen composting regulations. A housing development in Vancouver, B.C., uses sewage to generate heat for 26,000 homes. A Norwegian firm has invented a composting toilet that captures methane for cooking.

2-Omega.jpg

Photo Essay: Inside the Omega Institute

All well and good as an experiment, perhaps, or out in the country. But how practical is composting for city dwellers?

That question led Seattle’s Bertschi School to install a vacuum-powered composting toilet in its new science wing. Similar to an airplane commode, the Envirolet whisks waste into a heated chamber where bacteria attack the feces.

“Some parents were intrigued,” said Stan Richardson, director of technology and campus planning at the private school. “Others thought we were crazy.”

Results after the first year have been mixed. The toilet demands regular maintenance, is prone to clogging, and requires 1,100 watts of energy to run. But when greener toilets do shrink costs, conserve water, and safely replenish soil, sooner or later other arguments fall away.

“Mainstream developers come into our office, and I know they’re not here to save the environment,” says Buehrer in his soft-spoken way. “That’s fine. Now that we’ve shown that we can do these designs better, at lower cost, they can raise their green flag for that.”


Claudia Rowe wrote this article for What Would Nature Do?, the Winter 2013 issue of YES! Magazine. Claudia has been an award-winning social issues journalist for more than 20 years. Her work has appeared in Mother Jones, The New York Times, The Seattle Times, and The Seattle Post-Intelligencer. Republished under a Creative Commons BY-NC-SA license.

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  • Vandana Shiva: Everything I Need to Know I Learned in the Forest
    Today, at a time of multiple crises, we need to move away from thinking of nature as dead matter to valuing her biodiversity, clean water, and seeds. For this, nature herself is the best teacher.
August 4, 2013 |

Passive solar radiator that cools buildings in full sunlight

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Homes and buildings chilled without air conditioners. Car interiors that don’t heat up in the summer sun. Tapping the frigid expanses of outer space to cool the planet. Science fiction, you say? Well, maybe not any more.

A team of researchers at Stanford has designed an entirely new form of cooling structure that cools even when the sun is shining. Such a structure could vastly improve the daylight cooling of buildings, cars and other structures by reflecting sunlight back into the chilly vacuum of space. Their paper describing the device was published March 5 in Nano Letters.

“People usually see space as a source of heat from the sun, but away from the sun outer space is really a cold, cold place,” explained Shanhui Fan, professor of electrical engineering and the paper’s senior author. “We’ve developed a new type of structure that reflects the vast majority of sunlight, while at the same time it sends heat into that coldness, which cools manmade structures even in the daytime.”

The trick, from an engineering standpoint, is twofold. First, the reflector has to reflect as much of the sunlight as possible. Poor reflectors absorb too much sunlight, heating up in the process and defeating the purpose of cooling.

The second challenge is that the structure must efficiently radiate heat back into space. Thus, the structure must emit thermal radiation very efficiently within a specific wavelength range in which the atmosphere is nearly transparent. Outside this range, Earth’s atmosphere simply reflects the light back down. Most people are familiar with this phenomenon. It’s better known as the greenhouse effect—the cause of global climate change.

Two goals in one

The new structure accomplishes both goals. It’s an effective broadband mirror for solar light that reflects most of the sunlight and it also emits thermal radiation very efficiently within the crucial wavelength range needed to escape Earth’s atmosphere.

Radiative cooling at nighttime has been studied extensively as a mitigation strategy for climate change, yet peak demand for cooling occurs in the daytime.

“No one had yet been able to surmount the challenges of daytime radiative cooling—of cooling when the sun is shining,” said Eden Rephaeli, a doctoral candidate in Fan’s lab and a co-first-author of the paper. “It’s a big hurdle.”

The Stanford team has succeeded where others have come up short by turning to nanostructured photonic materials. These materials can be engineered to enhance or suppress light reflection in certain wavelengths.

“We’ve taken a very different approach compared to previous efforts in this field,” said Aaswath Raman, a doctoral candidate in Fan’s lab and a co-first-author of the paper. “We combine the thermal emitter and solar reflector into one device, making it both higher performance and much more robust and practically relevant. In particular, we’re very excited because this design makes viable both industrial-scale and off-grid applications.”

Using engineered nanophotonic materials the team was able to strongly suppress how much heat-inducing sunlight the panel absorbs while it radiates heat very efficiently in the key frequency range necessary to escape Earth’s atmosphere. The material is made of quartz and silicon carbide, both very weak absorbers of sunlight.

Net cooling power

The new device is capable of achieving a net cooling power in excess of 100 watts per square meter. By comparison, today’s standard 10-per cent-efficient solar panels generate the about the same amount of power. That means Fan’s radiative cooling panels could theoretically be substituted on rooftops where existing solar panels feed electricity to air conditioning systems needed to cool the building.

To put it a different way, a typical one-storey, single-family house with just 10 per cent of its roof covered by radiative cooling panels could offset 35 per cent of its entire air conditioning needs during the hottest hours of the summer.

Radiative cooling has another profound advantage over all other cooling strategies such as the air conditioner. It’s a passive technology. It requires no energy. It has no moving parts. It’s easy to maintain. You put it on the roof or the sides of buildings and it starts working immediately.

A changing vision of cooling

Beyond the commercial implications, Fan and his collaborators foresee a broad potential social impact. Much of the human population on Earth lives in sun-drenched regions huddled around the equator. Electrical demand to drive air conditioners is skyrocketing in these places, presenting an economic and an environmental challenge. These areas tend to be poor and the power necessary to drive cooling usually means fossil-fuel power plants that compound the greenhouse gas problem.

“In addition to these regions, we can foresee applications for radiative cooling in off-the-grid areas of the developing world where air conditioning is not even possible at this time. There are large numbers of people who could benefit from such systems,” Fan said.

 


This article was written by Andrew Myers, associate director of communications for the Stanford University School of Engineering.

Source: Eurekalert.org

April 26, 2013 |

PRESSURE RETARDED OSMOSIS: Making energy from freshwater flowing into saltwater

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red beachThe latest episode in the American Chemical Society’s (ACS’) award-winning Global Challenges/Chemistry Solutions podcast series describes a process that could pave the way for a new genre of electric power-generating stations. These stations could supply electricity for more than a half billion people by tapping just one-tenth of the global potential of a little-known energy source that exists where rivers flow into the ocean.

Based on a report by Menachem Elimelech, Ph.D., and Ngai Yin Yip in the ACS journal Environmental Science & Technology, the new podcast is available without charge at iTunes and from www.acs.org/globalchallenges.

In the report, Elimelech and Yip explain that the little-known process, called pressure-retarded osmosis (PRO), exploits the difference in saltiness between freshwater and seawater. PRO requires no fuel, is sustainable and releases no carbon dioxide (the main greenhouse gas).

In PRO, freshwater flows naturally through a special membrane to dilute seawater on the other side. The pressure from the flow spins a turbine generator and produces electricity. The world’s first PRO prototype power plant was inaugurated in Norway in 2009. With PRO appearing to have great potential, the scientists set out to make better calculations on how much it actually could contribute to future energy needs under real-world conditions.

Elimelech and Yip concluded that PRO power-generating stations using just one-tenth of the global river water flow into the oceans could generate enough power to meet the electricity needs of 520 million people, without emitting carbon dioxide. The same amount of electricity, if produced by a coal-fired power plant, would release more than 1 billion metric tons of greenhouse gases every year.

 


Global Challenges/Chemistry Solutions is a series of podcasts describing some of the 21st century’s most daunting problems, and how cutting-edge research in chemistry matters in the quest for solutions. Global Challenges is the centerpiece in an alliance on sustainability between ACS and the Royal Society of Chemistry. Global Challenges is a sweeping panorama of global challenges that includes dilemmas such as providing a hungry and thirsty world with ample supplies of safe food and clean water, developing alternatives to petroleum to fuel society, preserving the environment and ensuring a sustainable future for our children and improving human health.

For more entertaining, informative science videos and podcasts from the ACS Office of Public Affairs, view Prized Science, Spellbound, Science Elements and Global Challenges/Chemistry Solutions.

The American Chemical Society is a nonprofit organization chartered by the U.S. Congress. With more than 164,000 members, ACS is the world’s largest scientific society and a global leader in providing access to chemistry-related research through its multiple databases, peer-reviewed journals and scientific conferences. Its main offices are in Washington, D.C., and Columbus, Ohio.

Source: EurekAlert!

August 1, 2012 |

SOLAR CELL BREAKTHROUGH: U of T-led research team develops next-generation CQD solar cells

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CQD solar cells

CQD solar cellsResearchers from the University of Toronto and King Abdullah University of Science & Technology have made a breakthrough in the development of colloidal quantum dot (CQD) films, leading to the most efficient CQD solar cell ever. Their work is featured in a letter published in Nature Nanotechnology.

The researchers, led by U of T Engineering Professor Ted Sargent, created a solar cell out of inexpensive materials that was certified at a world-record 7.0% efficiency.

“Previously, quantum dot solar cells have been limited by the large internal surface areas of the nanoparticles in the film, which made extracting electricity difficult,” said Dr. Susanna Thon, a lead co-author of the paper. “Our breakthrough was to use a combination of organic and inorganic chemistry to completely cover all of the exposed surfaces.”

Quantum dots are semiconductors only a few nanometres in size and can be used to harvest electricity from the entire solar spectrum – including both visible and invisible wavelengths. Unlike current slow and expensive semiconductor growth techniques, CQD films can be created quickly and at low cost, similar to paint or ink. This research paves the way for solar cells that can be fabricated on flexible substrates in the same way newspapers are rapidly printed in mass quantities.

The U of T cell represents a 37% increase in efficiency over the previous certified record. In order to improve efficiency, the researchers needed a way to both reduce the number of “traps” for electrons associated with poor surface quality while simultaneously ensuring their films were very dense to absorb as much light as possible. The solution was a so-called “hybrid passivation” scheme.

“By introducing small chlorine atoms immediately after synthesizing the dots, we’re able to patch the previously unreachable nooks and crannies that lead to electron traps,” explained doctoral student and lead co-author Alex Ip. “We follow that by using short organic linkers to bind quantum dots in the film closer together.”

Work led by Professor Aram Amassian of KAUST showed that the organic ligand exchange was necessary to achieve the densest film.

“The KAUST group used state-of-the-art synchrotron methods with sub-nanometer resolution to discern the structure of the films and prove that the hybrid passivation method led to the densest films with the closest-packed nanoparticles,” stated Professor Amassian.

The advance opens up many avenues for further research and improvement of device efficiencies, which could contribute to a bright future with reliable, low cost solar energy.

According to Professor Sargent, “Our world urgently needs innovative, cost-effective ways to convert the sun’s abundant energy into usable electricity. This work shows that the abundant materials interfaces inside colloidal quantum dots can be mastered in a robust manner, proving that low cost and steadily-improving efficiencies can be combined.”

SOURCE: EurekAlert!

July 31, 2012 |

WIDESPREAD SOLAR: Berkeley lab develops technology to make photovoltaics out of any semiconductor

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Berkeley Lab develops photovoltaics from any semiconductor

Alex Zettl (left) and Will Regan can make low-cost, high efficiency solar cells from virtually any semiconductor material.

A technology that would enable low-cost, high efficiency solar cells to be made from virtually any semiconductor material has been developed by researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley. This technology allows for plentiful, relatively inexpensive semiconductors, such as metal oxides, sulfides and phosphides that had previously been considered unsuitable for solar cells because of the difficulty in tailoring their properties by chemical means.

“It’s time we put bad materials to good use,” says physicist Alex Zettl, who led the research along with colleague Feng Wang. “Our technology allows us to sidestep the difficulty in chemically tailoring many earth abundant, non-toxic semiconductors and instead tailor these materials simply by applying an electric field.”

Zettl, who holds joint appointments with Berkeley Labs’ Materials Sciences Division and UC Berkeley’s Physics Department where he directs the Center of Integrated Nanomechanical Systems (COINS), is the corresponding author of a paper describing this work in the journal Nano Letters. The paper is titled “Screening-Engineered Field-Effect Solar Cells.” Co-authoring it were William Regan, Steven Byrnes, Will Gannett, Onur Ergen, Oscar Vazquez-Mena and Feng Wang.

Solar cells convert sunlight into electricity using semiconductor materials that exhibit the photovoltaic effect–meaning they absorb photons and release electrons that can be channelled into an electrical current. Photovoltaics are the ultimate source of clean, green and renewable energy but today’s technologies utilize relatively scarce and expensive semiconductors, such as large crystals of silicon, or thin films of cadmium telluride or copper indium gallium selenide, that are tricky or expensive to fabricate into devices.

“Solar technologies today face a cost-to-efficiency trade-off that has slowed widespread implementation,” Zettl says. “Our technology reduces the cost and complexity of fabricating solar cells and thereby provides what could be an important cost-effective and environmentally friendly alternative that would accelerate the usage of solar energy.”

This new technology is called “screening-engineered field-effect photovoltaics,” or SFPV, because it utilizes the electric field effect, a well understood phenomenon by which the concentration of charge-carriers in a semiconductor is altered by the application of an electric field. With the SFPV technology, a carefully designed partially screening top electrode lets the gate electric field sufficiently penetrate the electrode and more uniformly modulate the semiconductor carrier concentration and type to induce a p-n junction. This enables the creation of high quality p-n junctions in semiconductors that are difficult if not impossible to dope by conventional chemical methods.

“Our technology requires only electrode and gate deposition, without the need for high-temperature chemical doping, ion implantation, or other expensive or damaging processes,” says lead author William Regan. “The key to our success is the minimal screening of the gate field which is achieved through geometric structuring of the top electrode. This makes it possible for electrical contact to and carrier modulation of the semiconductor to be performed simultaneously.”

SFPV technology

The SFPV technology was tested for two top electrode architectures: (A) the top electrode is shaped into narrow fingers; (B) top electrode is uniformly ultrathin.

Under the SFPV system, the architecture of the top electrode is structured so that at least one of the electrode’s dimensions is confined. In one configuration, working with copper oxide, the Berkeley researchers shaped the electrode contact into narrow fingers; in another configuration, working with silicon, they made the top contact ultra-thin (single layer graphene) across the surface. With sufficiently narrow fingers, the gate field creates a low electrical resistance inversion layer between the fingers and a potential barrier beneath them. A uniformly thin top contact allows gate fields to penetrate and deplete/invert the underlying semiconductor. The results in both configurations are high quality p-n junctions.

Says co-author Feng Wang, “Our demonstrations show that a stable, electrically contacted p-n junction can be achieved with nearly any semiconductor and any electrode material through the application of a gate field provided that the electrode is appropriately geometrically structured.”

The researchers also demonstrated the SFPV effect in a self-gating configuration, in which the gate was powered internally by the electrical activity of the cell itself.

“The self-gating configuration eliminates the need for an external gate power source, which will simplify the practical implementation of SFPV devices,” Regan says. “Additionally, the gate can serve a dual role as an antireflection coating, a feature already common and necessary for high efficiency photovoltaics.”


This research was supported in part by the DOE Office of Science and in part by the National Science Foundation.

Lawrence Berkeley National Laboratory (Berkeley Lab) addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

SOURCE: EurekAlert!

July 29, 2012 |
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