NIST Measurements May Help Optimize Organic Solar Cells

Engineerblogger
March 8, 2012

Light that strikes this organic solar cell causes electrons to flow between its layers, creating an electric current. Measurements made by the NIST/NRL research team determined the best thickness for the layers, a finding that could help optimize the cells performance.  Credit: NIST

Organic solar cells may be a step closer to market because of measurements taken at the National Institute of Standards and Technology (NIST) and the U.S. Naval Research Laboratory (NRL), where a team of scientists has developed a better fundamental understanding of how to optimize the cells’ performance.

Prototype solar cells made of organic materials currently lag far behind conventional silicon-based photovoltaic cells in terms of electricity output. But if even reasonably efficient organic cells can be developed, they would have distinct advantages of their own: They would cost far less to produce than conventional cells, could cover larger areas, and conceivably could be recycled far more easily.

The cells the team studied are made by stacking up hundreds of thin layers that alternate between two different organic materials—zinc pthalocyanine and C60, the soccer-ball shaped carbon molecules sometimes called buckminsterfullerenes, or “buckyballs.” Light that strikes this multilayered film excites all its layers from top to bottom, causing them to give up electrons that flow between the buckyball and pthalocyanine layers, creating an electric current.

Each layer is only a few nanometers thick, and varying their thickness has a dramatic effect on how much electrical current the overall cell puts out. According to NIST chemist Ted Heilweil, determining the ideal thickness of the layers is crucial to making the best-performing cells.

“In essence, if the layers are too thin, they don’t generate enough electrons for a substantial current to flow, but if they are too thick, many of the electrons get trapped in the individual layers,” says Heilweil. “We wanted to find the sweet spot.”

Finding that “sweet spot” involved exploring the relationship between layer thickness and two different aspects of the material. When light strikes the film, the layers generate an initial “spike” in current that then decays fairly quickly; the ideal cell would generate electrons as steadily as possible. Changing the layer thickness affects the initial decay rate, but it also affects the overall capacity of the material to carry electrons, so the team wanted to find the optimum combination of these two factors.

Paul Lane of NRL grew a number of films that had layers of different thickness, and the team made measurements at both labs that took the two factors into account, finding that layers of roughly two nanometers thick give the best performance. Heilweil says the results encourage him to think prototype cells based on this geometry can be optimized, though one engineering hurdle remains: finding the best way to get the electricity out.

“It’s still unclear how to best incorporate such thin nanolayers in devices,” he says. “We hope to challenge engineers who can help us with that part.”

Source: NIST

Additional Information:

• P.A. Lane, P.D. Cunningham, J.S. Melinger, G.P. Kushto, O. Esenturk and E.J. Heilweil. Photoexcitation dynamics in films of C60 and Zn-Phthalocyanine with a layered nanostructure. Physical Review Letters, DOI: 10.1103/PhysRevLett.108.077402. Published 15 Feb 2012.

Stretching Exercises: Using Digital Images to Understand Bridge Failures

Engineerblogger
Jan 11, 2012

Photogrammetry at work: New video shows how NIST and the FHWA used random patterns of speckles painted on metal to better understand how stress and strain could lead to catastrophic failure.

With a random-looking spatter of paint specks, a pair of cameras and a whole lot of computer processing, engineer Mark Iadicola of the National Institute of Standards and Technology (NIST) has been helping the Federal Highway Administration (FHWA), in cooperation with the American Association of State Highway and Transportation Officials (AASHTO), to assure the safety of hundreds of truss bridges across the United States. Iadicola has been testing the use of a thoroughly modern version of an old technique—photographic measurement or “photogrammetry”—to watch the failure of a key bridge component in exquisite detail.

The impetus for the FHWA project was the disastrous collapse of the Interstate 35-W bridge in Minneapolis, Minnesota. On Aug. 1, 2007, in the middle of the evening rush hour, a thousand feet of the bridge’s main deck truss collapsed, part of it falling 108 feet into the Mississippi River. Thirteen people died. One hundred and forty five were injured.

According to FHWA engineer Justin Ocel, an investigation by the National Transportation Safety Board (NTSB), assisted by FHWA, determined that the immediate culprit was a failed gusset plate, a flat heavy piece of steel bolted in pairs to join the ends of the steel members that make up the bridge truss. As a result of a design error decades before, the gusset plates in the bridge were about half as thick as they should have been.

Although that design flaw was clearly a major factor in the disaster, Ocel says, the collapse highlighted the fact that gusset plates were not generally considered by engineers during periodic reviews of bridge capacity, a process called load rating. It was assumed that gusset plates were properly sized to be stronger than the members they connect. “One of the recommendations from the NTSB was that we include gusset plates in load ratings, and until that point it hadn't been done,” Ocel explains. “To assist the states with this process we developed a guidance document on how to load rate gusset plates.”

In developing the guidance, Ocel says, FHWA used the best available data on the failure modes of gusset plates in major bridges—but there wasn’t much. So at the FHWA's Turner-Fairbank Highway Research Center in Virginia they began building full-scale models of bridge gusset plate joints and pulling them apart with a huge hydraulic test machine.

NIST’s Iadicola is there to watch what happens as the plate stretches and fails. He covers the plate with an irregular pattern of paint speckles and then trains a pair of carefully calibrated, high-definition digital cameras on it. The cameras repeatedly image the plate, send the pictures to a computer that uses custom software to compare each image to the previous one, and calculate which of the paint spots have moved, in what direction and by how much. Using two cameras allows the computer to “see” the plate in three dimensions, so it can tell if points on the surface move in or out as well as up, down or sideways.

“The NIST digital image correlation method is a good complement to the FHWA measurement methods,” Iadicola explains. “Their techniques—strain gages and photoelasticity—are very good for the normal range of stress in which the plate will stretch and spring right back to its original shape. Our method can tell you a little about that, but it really shines in showing you what happens past that point, when the plate starts permanently deforming and finally rips apart. The failure modes.”

After more than a year of experiments, Ocel says, the FHWA has learned a lot about how to predict what loads will cause a gusset plate to fail. Currently, FHWA is working with AASHTO to translate those findings into language that can be adopted into the AASHTO Bridge Design Specification and Manual for Bridge Evaluation, two documents used throughout the country for designing and load rating bridges.

The FHWA project is just one of a range of applications for digital image correlation being studied at NIST, Iadicola says. “We’ve been using it in looking at sheet metal forming—you have very high strains during the forming process—and we’ve used it at very small scales, looking at targets with an optical microscope.”

Source: The National Institute of Standards and Technology (NIST)

Slippery When Stacked:Theorists Quantify the Friction of Graphene

Engineerblogger
Jan 11, 2012

NIST software simulates the tip of an atomic force microscope moving left across a stack of four sheets of graphene. Research using this software indicates that graphene's friction is reduced as more layers are added to the stack. Credit: A Smolyanitsky/NIST

Similar to the way pavement, softened by a hot sun, will slow down a car, graphene—a one-atom-thick sheet of carbon with wondrous properties—slows down an object sliding across its surface. But stack the sheets and graphene gets more slippery, say theorists at the National Institute of Standards and Technology (NIST), who developed new software to quantify the material's friction.

"I don't think anyone expects graphene to behave like a surface of a three-dimensional material, but our simulation for the first time explains the differences at an atomic scale," says NIST postdoctoral researcher Alex Smolyanitsky, who wrote the modeling program and co-authored a new paper* about the study. "If people want to use graphene as a solid-state lubricant or even as a part of flexible electrodes, this is important work."

With the capacity to be folded, rolled or stacked, graphene is super-strong and has unusual electronic and optical properties. The material might be used in applications ranging from electronic circuits to solar cells to "greasing" moving parts in nanoscale devices.

Friction is the force that resists the sliding of two surfaces against each other. Studying friction at the atomic scale is a challenge, surmountable in only the past few years. The NIST software simulates atomic force microscopy (AFM) using a molecular dynamics technique. The program was used to measure what happens when a simulated AFM tip moves across a stack of one to four graphene sheets (see image) at different scanning rates.

The researchers found that graphene deflects under and around the AFM tip. The localized, temporary warping creates rolling friction or resistance, the force that exerts drag on a circular object rolling along a surface. Smolyanitsky compares the effect to the sun melting and softening pavement in the state where he got his doctoral degree, Arizona, causing car tires to sink in slightly and slow down. The NIST results are consistent with those of recent graphene experiments by other research groups but provide new quantitative data.

Most significantly, the NIST study shows why friction falls with each sheet of graphene added to the stack (fast scanning also has an effect on the friction). With fewer layers, the top layer deflects more, and the friction per unit of AFM contact force rises. The top surface of the stack becomes less yielding and more slippery as graphene layers are added. By contrast, the friction of three-dimensional graphite-like material is virtually unaffected by deformation and rolling friction, and is due instead to heat created by the moving tip.


Source: The National Institute of Standards and Technology (NIST) 


Additional Information:

• A. Smolyanitsky, J.P. Killgore and V.K. Tewary. "Effect of elastic deformation on frictional properties of few-layer graphene". Physical Review B. Posted online Jan. 9, 2012.

New Magnetic-Field-Sensitive Alloy Could Find Use in Novel Micromechanical Devices

Engineerblogger
Nov 24, 2011

TEM (transmission electron microscope) image taken at NIST of an annealed cobalt iron alloy. The high magnetostriction seen in this alloy is due to the two-phase iron-rich (shaded blue) and cobalt-rich (shaded red) structure and the nanoscale segregation. Credit: Bendersky/NIST

Led by a group at the University of Maryland (UMd), a multi-institution team of researchers has combined modern materials research and an age-old metallurgy technique to produce an alloy that could be the basis for a new class of sensors and micromechanical devices controlled by magnetism.* The alloy, a combination of cobalt and iron, is notable, among other things, for not using rare-earth elements to achieve its properties. Materials scientists at the National Institute of Standards and Technology (NIST) contributed precision measurements of the alloy's structure and mechanical properties to the project.

The alloy exhibits a phenomenon called "giant magnetostriction," an amplified change in dimensions when placed in a sufficiently strong magnetic field. The effect is analogous to the more familiar piezoelectric effect that causes certain materials, like quartz, to compress under an electric field. They can be used in a variety of ways, including as sensitive magnetic field detectors and tiny actuators for micromechanical devices. The latter is particularly interesting to engineers because, unlike piezoelectrics, magnetostrictive elements require no wires and can be controlled by an external magnetic field source.

To find the best mixture of metals and processing, the team used a combinatorial screening technique, fabricating hundreds of tiny test cantilevers—tiny, 10-millimeter-long, silicon beams looking like diving boards— and coating them with a thin film of alloy, gradually varying the ratio of cobalt to iron across the array of cantilevers. They also used two different heat treatments, including, critically, one in which the alloy was heated to an annealing temperature and then suddenly quenched in water.

Quenching is a classic metallurgy technique to freeze a material's microstructure in a state that it normally only has when heated. In this case, measurements at NIST and the Stanford Synchrotron Radiation Lightsource (SSRL) showed that the best-performing alloy had a delicate hetereogenous, nanoscale structure in which cobalt-rich crystals were embedded throughout a different, iron-rich crystal structure. Magnetostriction was determined by measuring the amount by which the alloy bent the tiny silicon cantilever in a magnetic field, combined with delicate measurements at NIST to determine the stiffness of the cantilever.

The best annealed alloy showed a sizeable magnetostriction effect in magnetic fields as low as about 0.01 Tesla. (The earth's magnetic field generally ranges around roughly 0.000 045 T, and a typical ferrite refrigerator magnet might be about 0.7 T.)

The results, says team leader Ichiro Takeuchi of UMd, are lower than, but comparable to, the values for the best known magnetostrictive material, a rare-earth alloy called Tb-Dy-Fe**—but with the advantage that the new alloy doesn't use the sometimes difficult to acquire rare earths. "Freezing in the heterogeneity by quenching is an old method in metallurgy, but our approach may be unique in thin films," he observes. "That's the beauty—a nice, simple technique but you can get these large effects."

The quenched alloy might offer both size and processing advantages over more common piezoelectric microdevices, says NIST materials scientist Will Osborn. "Magnetorestriction devices are less developed than piezoelectrics, but they're becoming more interesting because the scale at which you can operate is smaller," he says. "Piezoelectrics are usually oxides, brittle and often lead-based, all of which is hard on manufacturing processes. These alloys are metal and much more compatible with the current generation of integrated device manufacturing. They're a good next-generation material for microelectromechanical machines."

Source: The National Institute of Standards and Technology (NIST) 

Additional Information:

• In Study "Giant magnetostriction in annealed Co1−xFex thin-films" by D. Hunter, W. Osborn, K. Wang, N. Kazantseva, J. Hattrick-Simpers, R. Suchoski, R. Takahashi, M.L. Young, A. Mehta, L.A. Bendersky, S.E. Lofland, M. Wuttig and I. Takeuchi. in Nature Communications, published 01 November 2011

Nanowire-based Sensors Offer Improved Detection of Volatile Organic Compounds

The National Institute of Standards and Technology (NIST)
June 21, 2011

A team of researchers from the National Institute of Standards and Technology (NIST), George Mason University and the University of Maryland has made nano-sized sensors that detect volatile organic compounds—harmful pollutants released from paints, cleaners, pesticides and other products—that offer several advantages over today's commercial gas sensors, including low-power room-temperature operation and the ability to detect one or several compounds over a wide range of concentrations.

The recently published work is proof of concept for a gas sensor made of a single nanowire and metal oxide nanoclusters chosen to react to a specific organic compound. This work is the most recent of several efforts at NIST that take advantage of the unique properties of nanowires and metal oxide elements for sensing dangerous substances.

Modern commercial gas sensors are made of thin, conductive films of metal oxides. When a volatile organic compound like benzene interacts with titanium dioxide, for example, a reaction alters the current running through the film, triggering an alarm. While thin-film sensors are effective, many must operate at temperatures of 200° C (392° F) or higher. Frequent heating can degrade the materials that make up the films and contacts, causing reliability problems. In addition, most thin-film sensors work within a narrow range: one might catch a small amount of toluene in the air, but fail to sniff out a massive release of the gas. The range of the new nanowire sensors runs from just 50 parts per billion up to 1 part per 100, or 1 percent of the air in a room.
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