Screening-Engineered Field-Effect Photovoltaics Could Accelerate the Usage of Solar Energy

July 27, 2012

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.

Utilizing the electric field effect, a new technology called “screening-engineered field-effect photovoltaics,” or SFPV, reduces the cost and complexity of fabricating solar cells and could provide a cost-effective and environmentally friendly alternative to accelerate the usage of solar energy.

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 opens the door to the use of plentiful, relatively inexpensive semiconductors, such as the promising metal oxides, sulfides and phosphides, that have been considered unsuitable for solar cells because it is so difficult to tailor their properties by chemical means.

“It’s time we put bad materials to good use,” says physicist Alex Zettl, who led this 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 Lab’s 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.

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

Solar cells convert sunlight into electricity using semiconductor materials that exhibit the photovoltaic effect – meaning they absorb photons and release electrons that can be channeled 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.”

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.

Feng Wang

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.

Source: Lynn Yarris, Lawrence Berkeley National Laboratory

Image: Lawrence Berkeley National Laboratory; Roy Kaltschmidt

Sandia Engineers are Reevaluating Vertical Axis Wind Turbines

July 30, 2012

Technology

Basing their work on decades of wind energy research and experience, Sandia engineers are creating several concept designs, running those designs through modern modeling software and narrowing those design options down to a single, most-workable design for a VAWT turbine-blade. Results aren’t in, but the early favorite for further testing is the Darrieus design. Illustration by Josh Paquette and Matt Barone

Working to advanced rotor technologies for U.S. offshore wind power generation, Sandia scientists are searching for the most-workable design for a vertical axis wind turbine blade by creating several concept designs and running those designs through modern modeling software.

Albuquerque, New Mexico — Sandia National Laboratories’ wind energy researchers are re-evaluating vertical axis wind turbines (VAWTs) to help solve some of the problems of generating energy from offshore breezes.

Though VAWTs have been around since the earliest days of wind energy research at Sandia and elsewhere, VAWT architecture could transform offshore wind technology.

The economics of offshore wind power are different from land-based turbines, due to installation and operational challenges. VAWTs offer three big advantages that could reduce the cost of wind energy: a lower turbine center of gravity; reduced machine complexity; and better scalability to very large sizes.

A lower center of gravity means improved stability afloat and lower gravitational fatigue loads.

Additionally, the drivetrain on a VAWT is at or near the surface, potentially making maintenance easier and less time-consuming. Fewer parts, lower fatigue loads and simpler maintenance all lead to reduced maintenance costs.

Elegant in their simplicity

Sandia is conducting the research under a 2011 Department of Energy (DOE) solicitation for advanced rotor technologies for U.S. offshore windpower generation. The five-year, $4.1 million project began in January of this year.

Wind Energy Technologies manager Dave Minster said Sandia’s wind energy program is aimed at addressing the national energy challenge of increasing the use of low-carbon power generation.

“VAWTs are elegant in terms of their mechanical simplicity,” said Josh Paquette, one of Sandia’s two principal investigators on the project. “They have fewer parts because they don’t need a control system to point them toward the blowing wind to generate power.”

These characteristics fit the design constraints for offshore wind: the high cost of support structures; the need for simple, reliable designs; and economic scales that demand larger machines than current land-based designs.

Large offshore VAWT blades in excess of 300 meters will cost more to produce than blades for onshore wind turbines. But as the machines and their foundations get bigger — closer to the 10–20 megawatt (MW) scale — turbines and rotors become a much smaller percentage of the overall system cost for offshore turbines, so other benefits of the VAWT architecture could more than offset the increased rotor cost.

Challenges remain

However, challenges remain before VAWTs can be used for large-scale offshore power generation.

Curved VAWT blades are complex, making manufacture difficult. Producing very long VAWT blades demands innovative engineering solutions. Matt Barone, the project’s other principal investigator, said partners Iowa State University and TPI Composites will explore new techniques to enable manufacture of geometrically complex VAWT blade shapes at an unprecedented scale, but at acceptable cost.

VAWT blades must also overcome problems with cyclic loading on the drivetrain. Unlike horizontal axis wind turbines (HAWTs), which maintain a steady torque if the wind remains steady, VAWTs have two “pulses” of torque and power for each blade, based on whether the blade is in the upwind or downwind position. This “torque ripple” results in unsteady loading, which can lead to drivetrain fatigue. The project will evaluate new rotor designs that smooth out the amplitude of these torque oscillations without significantly increasing rotor cost.

Because first-generation VAWT development ended decades ago, updated designs must incorporate decades of research and development already built into current HAWT designs. Reinvigorating VAWT research means figuring out the models that will help speed up turbine design work.

“Underpinning this research effort will be a tool development effort that will synthesize and enhance existing aerodynamic and structural dynamic codes to create a publicly available aeroelastic design tool for VAWTs,” Barone said.

Needed: aerodynamic braking

Another challenge is brakes. Older VAWT designs didn’t have an aerodynamic braking system, and relied solely on a mechanical braking system that is more difficult to maintain and less reliable than the aerodynamic brakes used on HAWTs.

HAWTS use pitchable blades, which stop the turbine within one or two rotations without damage to the turbine and are based on multiple redundant, fail-safe designs. Barone said new VAWT designs will need robust aerodynamic brakes that are reliable and cost-effective, with a secondary mechanical brake much like on modern-day HAWTs. Unlike HAWT brakes, new VAWT brakes won’t have actively pitching blades, which have their own reliability and maintenance issues.

VAWT technology: A long history at Sandia

A Sandia team completes installation in the late 1980s of a vertical axis wind turbine test platform in Bushland, Texas. Photo by Randy Montoya

In the 1970s and 1980s, when wind energy research was in its infancy, VAWTs were actively developed as windpower generators. Although strange looking, they had a lot going for them: They were simpler than their horizontal-axis cousins so they tended to be more reliable. For a while, VAWTs held their own against HAWTs. But then wind turbines scaled up.

“HAWTs emerged as the predominant technology for land-based wind over the past 15 years primarily due to advantages in rotor costs at the 1 to 5 megawatt scale,” Paquette said.

In the 1980s, research focused more heavily on HAWT turbines, and many VAWT manufacturers left the business, consigning VAWTs to an “also ran” in the wind energy museum.

But the winds of change have blown VAWTs’ way once more.

Sandia is mining the richness of its wind energy history. Wind researchers who were among the original wind energy engineers are going through decades of Sandia research and compiling the lessons learned, as well as identifying some of the key unknowns described at the end of VAWT research at Sandia in the 1990s.

The first phase of the program will take place over two years and will involve creating several concept designs, running those designs through modern modeling software and narrowing those design options down to a single, most-workable design. During this phase, Paquette, Barone and their colleagues will look at all types of aeroelastic rotor designs, including HVAWTs and V-shaped VAWTs. But the early favorite rotor type is the Darrieus design.

In phase two researchers will build the chosen design over three years, eventually testing it against the extreme conditions that a turbine must endure in an offshore environment.

In addition to rotor designs, the project will consider different foundation designs: Early candidates are barge designs, tension-leg platforms and spar buoys.

The project partners will work on many elements.

Another partner, the University of Maine, will develop floating VAWT platform dynamics code and subscale prototype wind/wave basin testing. Iowa State University will develop manufacturing techniques for offshore VAWT blades and subscale wind tunnel testing. TPI Composites will design a proof-of-concept subscale blade and develop a commercialization plan. TU-Delft will work on aeroelastic design and optimization tool development and modeling. Texas A&M University will work on aeroelastic design tool development.

“Ultimately it’s all about the cost of energy. All these decisions need to lead to a design that’s efficient and economically viable,” said Paquette.

Source: Sandia National Laboratory

Images: Josh Paquette and Matt Barone; Randy Montoya

SLIPS Coating Prevents Bacteria from Forming on Solid Surfaces

July 31, 2012

Technology

The SLIPS technology for preventing biofilm formation as compared with a Teflon-coated surface. Image courtesy of Joanna Aizenberg and Tak-Sing Wong

By applying a technology called SLIPS (slippery-liquid-infused porous surfaces) to solid surfaces, a team of Harvard scientists are able to trick bacteria into thinking they have nowhere to attach to and deprive the bacteria of the static interface that they need to grip to and use to form biofilms.

Biofilms may no longer have any solid ground upon which to stand. A team of Harvard scientists has developed a slick way to prevent the troublesome bacterial communities from ever forming on a surface.

Biofilms stick to just about everything, from copper pipes to steel ship hulls to glass catheters. The slimy coatings are more than simply a nuisance, resulting in decreased energy efficiency, contamination of water and food supplies, and — especially in medical settings — persistent infections. Even cavities in teeth are the unwelcome result of bacterial colonies.

In a study published in the Proceedings of the National Academy of Sciences (PNAS), lead co-authors Joanna Aizenberg, Alexander Epstein, and Tak-Sing Wong coated solid surfaces with an immobilized liquid film to trick the bacteria into thinking they had nowhere to attach and grow.

“People have tried all sorts of things to deter biofilm buildup — textured surfaces, chemical coatings, and antibiotics, for example,” says Aizenberg, Amy Smith Berylson Professor of Materials Science at the Harvard School of Engineering and Applied Sciences (SEAS) and a core faculty member at the Wyss Institute for Biologically Inspired Engineering at Harvard. “In all those cases, the solutions are short-lived at best. The surface treatments wear off, become covered with dirt, or the bacteria even deposit their own coatings on top of the coating intended to prevent them. In the end, bacteria manage to settle and grow on just about any solid surface we can come up with.”

Taking a completely different approach, the researchers used their recently developed technology, dubbed SLIPS (slippery-liquid-infused porous surfaces) to effectively create a hybrid surface that is smooth and slippery due to the liquid layer that is immobilized on it. First described in the Sept. 22, 2011, issue of the journal Nature, the super-slippery surfaces have been shown to repel both water- and oil-based liquids and even to prevent ice or frost from forming.

The word “SLIPS” is coated with the SLIPS technology to show its ability to repel liquids and solids and even prevent ice or frost from forming. The slippery discovery has now been shown to prevent more than 99 percent of harmful bacterial slime from forming on surfaces. Image courtesy of Joanna Aizenberg, Rebecca Belisle, and Tak-Sing Wong

“By creating a liquid-infused structured surface, we deprive bacteria of the static interface they need to get a grip and grow together into biofilms,” says Epstein, a recent Ph.D. graduate who worked in Aizenberg’s lab at the time of the study.

“In essence, we turned a once bacteria-friendly solid surface into a liquid one. As a result, biofilms cannot cling to the material, and even if they do form, they easily ‘slip’ off under mild flow conditions,” adds Wong, a researcher at SEAS and a Croucher Foundation Postdoctoral Fellow at the Wyss Institute.

Aizenberg and her collaborators reported that SLIPS reduced by 96 to 99 percent the formation of three of the most notorious, disease-causing biofilms — Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus — over a seven-day period.

The technology works in both a static environment and under flow, or natural conditions, making it ideally suited for coating implanted medical devices that interact with bodily fluids. The coated surfaces can also combat bacterial growth in environments with extreme pH levels, intense ultraviolet light, and high salinity.

SLIPS is also nontoxic, readily scalable, and most importantly, self-cleaning, needing nothing more than gravity or a gentle flow of liquid to stay unsoiled. As previously demonstrated with a wide variety of liquids and solids, including blood, oil, and ice, everything seems to slip off surfaces treated with the technology.

To date, this may be the first successful test of a nontoxic synthetic surface that can almost completely prevent the formation of biofilms over an extended period of time. The approach may find applications in medical, industrial, and consumer products and settings.

In future studies, the researchers aim to better understand the mechanisms involved in preventing biofilms. In particular, they are interested in whether any bacteria transiently attach to the interface and then slip off, if they just float above the surface, or if any individuals can remain loosely attached.

“Biofilms have been amazing at outsmarting us. And even when we can attack them, we often make the situation worse with toxins or chemicals. With some very cool, nature-inspired design tricks, we are excited about the possibility that biofilms may have finally met their match,” concludes Aizenberg.

Aizenberg and Epstein’s co-authors included Rebecca A. Belisle, research fellow at SEAS, and Emily Marie Boggs ’13, an undergraduate biomedical engineering concentrator at Harvard College. The authors acknowledge support from the Department of Defense Office of Naval Research; the Croucher Foundation; and the Wyss Institute for Biologically Inspired Engineering at Harvard University.

Source: Michael Patrick Rutter, SEAS Communications; Harvard Gazette

Images: Joanna Aizenberg and Tak-Sing Wong; Joanna Aizenberg, Rebecca Belisle, and Tak-Sing Wong

iCVD Process Produces Precisely Controlled Patterns in Wrinkled Surfaces

August 1, 2012

Technology

Images taken with a 3-D microscope show wrinkled surfaces produced using a method developed by the MIT team. The size, spacing and angles of the wrinkles vary depending on how much the original underlying surface was stretched, and how the stretching was released. Images by Jose Luis Yague and Felice Frankel

Using an initiated chemical vapor deposition (iCVD) process, a team of scientists at MIT has discovered a way to create wrinkled surfaces while having the ability to control the exact size, periodic spacing and angles in both directions for the first time.

The wrinkles on a raisin result from a simple effect: As the pulp inside dries, the skin grows stiff and buckles to accommodate its shrinking size. Now, a team of researchers at MIT has discovered a way to harness that same principle in a controlled and orderly way, creating wrinkled surfaces with precise sizes and patterns.

This basic method, they say, could be harnessed for a wide variety of useful structures: microfluidic systems for biological research, sensing and diagnostics; new photonic devices that can control light waves; controllable adhesive surfaces; antireflective coatings; and antifouling surfaces that prevent microbial buildup.

A paper describing this new process, co-authored by MIT postdocs Jie Yin and Jose Luis Yagüe, former student Damien Eggenspieler SM ’10, and professors Mary Boyce and Karen Gleason, is being published in the journal Advanced Materials.

The process uses two layers of material. The bottom layer, or substrate, is a silicon-based polymer that can be stretched, like canvas mounted on a stretcher frame. Then, a second layer of polymeric material is deposited through an initiated chemical vapor deposition (iCVD) process in which the material is heated in a vacuum so that it vaporizes, and then lands on the stretched surface and bonds tightly to it. Then — and this is the key to the new process — the stretching is released first in one direction, and then in the other, rather than all at once.

When the tension is released all at once, the result is a jumbled, chaotic pattern of wrinkles, like the surface of a raisin. But the controlled, stepwise release system developed by the MIT team creates a perfectly orderly herringbone pattern.

The size and spacing of the herringbone ribs, it turns out, is determined by exactly how much the underlying material was originally stretched in each direction, the coating’s thickness, and in which order the two directions are released. The MIT team has shown the ability to control the exact size, periodic spacing and angles in both directions for the first time.

The system is unusual in its ability to produce precisely controlled patterns without the need for masks or complex printing, molding or scanning processes, Gleason says.

Controlling the patterns

Fundamentally, “it’s the same process that gives you your fingerprints,” says Gleason, the Alexander and I. Michael Kasser Professor of Chemical Engineering. But in this case, precise control over the resulting patterns requires the iCVD process, which Gleason and her colleagues have been developing for years. This gives a high degree of control over the thickness of layers deposited, and also enables control of the surface chemistry of the coatings.

Additionally, the iCVD method provides the high degree of adhesion that is needed to form buckled patterns. Without sufficient adhesion, the surface layer would simply separate from the substrate.

“One distinguishing feature of what we’re showing is the ability to create deterministic two-dimensional patterns of wrinkles,” such as a zigzag herringbone pattern, says Boyce, the Ford Professor of Engineering and head of MIT’s Department of Mechanical Engineering. “The deterministic nature of these patterns is very powerful and yields principles for designing desired surface topologies.”

“One important application is the measurement of ultra-thin-film material properties without knowing the thin-film thickness,” Yagüe says. The film’s material stiffness and thickness could be measured by analyzing the pattern, he says.

Many potential uses

Another possible application, the researchers say, is microfluidic devices such as those used to test for molecules in a biological sample, where tiny channels of precise dimensions need to be produced on a surface. These could potentially be used as sensors for contaminants, or as medical diagnostic devices. Another possible use is in the control of reflections or the wettability of a surface — making it attract or repel water, properties that depend both on the surface shape and the chemical composition of the material.

Such patterns can also be used to make surfaces adhere to each other — and in this case, the degree of adhesion can also be controlled. “You can dynamically tune the patterning — direct stretching or other actuation can be used to tune the pattern and corresponding properties actively during use,” Boyce says, even letting surfaces return to perfectly flat. This could, for example, be used to provide secure bonding with quick-release capability or to actively alter reflectivity or wettability.

Many techniques have been used to create surfaces with such tiny patterns, whose dimensions can range from nanometers (billionths of a meter) to tens of micrometers (millionths of a meter). But most such methods require complex fabrication processes, or can only be used for very tiny areas.

The new method is both very simple (consisting of just two or three steps) and can be used to make patterned surfaces of larger sizes, the team says. “You don’t need an external template” to create the pattern, says Yin, the paper’s lead author.

The predictability of the resulting patterns was a big surprise, members of the team say. “One of the amazing things is to note how beautifully the experiments and the simulation match,” Gleason says.

John Hutchinson, a professor of engineering and of applied mechanics at Harvard University who was not involved in this research, says, “Wrinkling phenomena are highly nonlinear and answers to questions concerning pattern formation have been slow to emerge.” He says the MIT team’s work “is an important step forward in this active area of research that bridges the chemical and mechanical engineering communities. The advance rests on theoretical insights combined with experimental demonstration and numerical simulation — it covers all the bases.”

The work was funded by the King Fahd University of Petroleum and Minerals in Saudi Arabia.

Source: David L. Chandler, MIT News Office

Image: Jose Luis Yague and Felice Frankel