Photon-to-electron conversion
Throughout decades of research on solar cells, one formula has been considered an absolute limit to the efficiency of such devices in converting sunlight into electricity: Called the Shockley-Queisser efficiency limit, it posits that the ultimate conversion efficiency can never exceed 34% for a single optimized semiconductor junction.  But now, researchers at MIT have shown there is a way to blow past that limit.

The principle behind the barrier-busting technique has been known theoretically since the 1960s, the researchers aid, but it was a somewhat obscure idea that nobody had succeeded in putting into practice. The MIT team was able, for the first time, to perform a successful “proof of principle” of the idea, which is known as singlet exciton fission. An exciton is the excited state of a molecule after absorbing energy from a photon.

In a standard photovoltaic (PV) cell, each photon knocks loose exactly one electron inside the PV material. That loose electron then can be harnessed through wires to provide an electrical current. But in the new technique, each photon can instead knock two electrons loose. This makes the process much more efficient: In a standard cell, any excess energy carried by a photon is wasted as heat, whereas in the new system the extra energy goes into producing two electrons instead of one.

While others have previously “split” a photon’s energy, they have done so using ultraviolet light, a relatively minor component of sunlight at Earth’s surface. The new work represents the first time this feat has been accomplished with visible light, laying a pathway for practical applications in solar PV panels. This was accomplished using an organic compound called pentacene in an organic solar cell. While that material’s ability to produce two excitons from one photon had been known, nobody had previously been able to incorporate it within a PV device that generated more than one electron per photon.

Wireless “smart skin” sensors
Major bridge failures in recent years have focused attention on the need to monitor America’s highway bridges and other infrastructure. As thousands of bridges, parking garages and other structures age, improved methods for detecting deterioration could save lives and prevent economic disruption.

Researchers at the Georgia Institute of Technology are developing a novel technology that would facilitate close monitoring of structures for strain, stress and early formation of cracks. Their approach uses wireless sensors that are low cost, require no power, can be implemented on tough yet flexible polymer substrates, and can identify structural problems at a very early stage. The only electronic component in the sensor is an inexpensive radio-frequency identification (RFID) chip.

These sensor designs can be inkjet-printed on various substrates, using methods that optimize them for operation at radio frequency. The result would be low-cost, weather-resistant devices that could be affixed by the thousands to various kinds of structures.

For many engineering structures, one of the most dangerous problems is the initiation of stress concentration and cracking, which is caused by overloading or inadequate design and can lead to collapse – as in the case of the I-35W bridge failure in Minneapolis in 2007, the researchers said.  Placing a ‘smart skin’ of sensors on structural members, especially on certain high-stress hot spots that have been pinpointed by structural analysis, could provide early notification of potential trouble.

The Georgia Tech research team is focusing on wireless sensor designs that are passive, which means they need no power source. Instead, these devices respond to radio-frequency signals sent from a central reader or hub. One such reader can interrogate multiple sensors, querying them on their status at frequent intervals. The researchers’ approach utilizes a small antenna mounted on a substrate and tuned to a specific radio frequency. This technique enables the antenna itself to function as a stress sensor.

As long as the structural member to which the antenna/sensor is affixed remains entirely stable, its frequency stays the same. But even a slight deformation in the structure also deforms the antenna and alters its frequency response. The reader can detect that change at once, initiating a warning months or years before an actual collapse.

Gentler robot hands
What use is a hand without nerves, that can’t tell what it’s holding? A hand that lifts a can of soda to your lips, but inadvertently tips or crushes it in the process? Researchers at the Harvard School of Engineering and Applied Sciences (SEAS) have developed a very inexpensive tactile sensor for robotic hands that is sensitive enough to turn a brute machine into a dextrous manipulator.

Designed by researchers in the Harvard Biorobotics Laboratory at SEAS, the sensor, called TakkTile, is intended to put what would normally be a high-end technology within the grasp of commercial inventors, teachers, and robotics enthusiasts.

Despite decades of research, tactile sensing hasn’t moved into general use because it’s been expensive and fragile, normally costing about $16,000, give or take, to put tactile sensing on a research robot hand, which is really limited where people can use it. The traditional technology also uses very specialized construction techniques, which can slow down your work. Now, Takktile changes that because it’s based on much simpler and cheaper fabrication methods, the researchers said.

TakkTile takes an existing device—a tiny barometer, which senses air pressure—and adds a layer of vacuum-sealed rubber to it, protecting it from as much as 25 pounds of direct pressure. The creators said that the chips can even survive a strike from a hammer or a baseball bat. At the same time, Takktile is sensitive enough to detect a very slight touch.

An 8x5 tactile array provides gram-level sensitivity in hardware created from MEMS barometers and standard manufacturing processes. (Source: Harvard)

The result, when added to a mechanical hand, is a robot that knows what it’s touching. It can pick up a balloon without popping it. It can pick up a key and use it to unlock a door.

~Ann Steffora Mutschler

Hybrid carbon/silicon solar cells
Hoping to help pave the way for the next generation of solar cells, researchers at Yale University have developed a renewable energy technology that directly converts solar energy into electricity.

They reported  a novel and cost-effective way to improve the efficiency of crystalline silicon solar cells through the application of thin, smooth carbon nanotube films. The films could be used to produce hybrid carbon/silicon solar cells with far greater power-conversion efficiency than reported in this system to date, the researchers said.

New research by Yale University scientists helps pave the way for the next generation of solar cells, a renewable energy technology that directly converts solar energy into electricity. (Source: Yale University)

The researchers said the approach bridges the cost-effectiveness and electrical and optical properties of novel nanomaterials with well-established, high efficiency silicon solar cell technologies.

While silicon is an ideal material for solar cells because its optical properties make it an intrinsically efficient energy converter, the high cost of processing single-crystalline silicon at necessarily high temperatures has hindered widespread commercialization.

Organic solar cells are an existing alternative to high-cost crystalline silicon solar cells and allow for simpler, room-temperature processing and lower costs, but have low power-conversion efficiency, the Yale researchers said.

So instead of using only organic substitutes, the team applied thin, smooth carbon nanotube films with superior conductance and optical properties to the surface of single crystalline silicon to create a hybrid solar cell architecture through a method called superacid sliding.

This allowed for the desirable photovoltaic properties of single-crystalline silicon to be leveraged through a simpler, low-temperature, lower-cost process, and allows for both high light absorption and high electrical conductivity.

This work suggests that the photovoltaic properties of single-crystalline silicon can be realized by a simple, low-temperature process the researchers concluded.

Micro-supercapacitors
While the demand for ever-smaller electronic devices has spurred the miniaturization of a variety of technologies, one area has lagged behind in this downsizing revolution: energy-storage units, such as batteries and capacitors.

UCLA researchers believe they may have changed the game through the development of a groundbreaking technique that uses a DVD burner to fabricate micro-scale graphene-based supercapacitors. These are devices that can charge and discharge a hundred to a thousand times faster than standard batteries.

Called micro-supercapacitors, the devices are made from a one-atom–thick layer of graphitic carbon, which can be easily manufactured and readily integrated into small devices such as next-generation pacemakers.

The cost-effective fabrication method holds promise for the mass production of these supercapacitors, which have the potential to transform electronics and other fields, the researchers said.

The integration of energy-storage units with electronic circuits is challenging and often limits the miniaturization of the entire system because the necessary energy-storage components scale down poorly in size and are not well suited to the planar geometries of most integrated fabrication processes.

The micro-supercapacitors by UCLA researchers (Source: UCLA)

Traditional methods for fabricating micro-supercapacitors involve labor-intensive lithographic techniques that have proven difficult for building cost-effective devices, which has limited their commercial application, so researchers used a consumer-grade LightScribe DVD burner to produce graphene micro-supercapacitors over large areas at a fraction of the cost of traditional devices. Using this technique, they have been able to produce more than 100 micro-supercapacitors on a single disc in less than 30 minutes, using inexpensive materials.

The micro-supercapacitors are also highly bendable and twistable, making them potentially useful as energy-storage devices in flexible electronics like roll-up displays and TVs, e-paper, and even wearable electronics.

~Ann Steffora Mutschler

Solar Cell Spray Painting
With the potential to possibly reduce the cost of solar cells, researchers in the University of Sheffield’s Department of Physics and Astronomy and the University of Cambridge have created a method of spray-coating a photovoltaic active layer by an air based process – similar to spraying regular paint from a can – to develop a cheaper technique which can be mass produced.

Professor David Lidzey from the University of Sheffield said spray coating can be used to make solar cells using specially designed plastic semiconductors and that maybe in the future surfaces on buildings and even car roofs will routinely generate electricity with these materials.

The performance of the spray coated solar cells was found to be the same as cells made with more traditional research methods, but is impossible to scale in manufacturing. The researchers’ goal is to reduce the amount of energy and money required to make a solar cell, which means solar cell materials are needed that have low embodied energy. Also, manufacturing processes are needed that are efficient, reliable and consume less energy.

Most solar cells are manufactured using special energy intensive tools and using materials like silicon that themselves contain large amounts of embodied energy. Plastic, by comparison, requires much less energy to make. By spray-coating a plastic layer in air the researchers hope the overall energy used to make a solar cell can be significantly reduced.

Under Pressure
Like turning coal to diamond, adding pressure to an electrical material enhances its properties and researchers from the University of Illinois at Urbana-Champaign have devised a method of making ferroelectric thin films with twice the strain, resulting in exceptional performance.

Ferroelectric materials, metal oxides with special polarization properties, are used in a number of advanced electronics applications. When electricity is applied, they can switch their polarization, or the direction of their internal electric field, which makes them useful in devices such as computer memories and actuators. Ferroelectric materials are especially useful in aerospace applications because they are less susceptible to radiation than traditional semiconductors.

Strain in these materials can alter their properties and improve their performance. A lot of research in ferroelectric materials has focused on making strained thin films with alternating layers only a few nanometers thick of materials with slightly different crystal structures.

The films are made of lead zirconate titanate (commonly called PZT). The relative amounts of zirconium (Zr) and titanium (Ti) in the films determine the shape of the crystals. Traditionally, films of PZT have been made up of a single composition, grown on a substrate with a slightly different crystal structure to cause strain in the PZT. However, too much strain causes the PZT to revert to its original crystal structure. This limits researchers’ ability to change the properties of these materials for better device performance.

The Illinois researchers overcame this limitation by gradually shifting the concentrations of Zr and Ti as they grew the thin films, incrementally changing the crystal structure. From layer to layer, the structures are very similar, yet the composition of the PZT at the top and bottom of the film is very different, transitioning from a PZT composition with 80 percent Zr to 80 percent Ti. This gradual change, instead of the usual layered approach, results in little localized strain but large overall strain.

Thanks to the large strain, the compositionally graded PZT films not only have improved properties, but also entirely new properties. Most notably, the films have a built-in electric field, called an intrinsic potential. This means that it can perform some functions without needing an external current or field applied to it. In addition, it means that the material has a preferred polarity, which opens the door for new applications.

~Ann Steffora Mutschler

Thinnest p-n junction
In a technological advance that may support the creation of more efficient photo detectors, solar cells and night vision systems, researchers at Peking University said they have created a scalable technique for the synthesis of high quality graphene p-n junctions.

The project, which began in mid-2011, adds to a growing body of research on the characteristics of graphene, an extremely thin carbon framework that exhibits extraordinary properties in electronics and optics and shows promise as the core material of next generation transistors, the researchers said.

The process enhances researchers’ efforts to create a batch of mosaic graphene, named for the distinct doping levels of its parts, in a more efficient and stable way.

The p-n junctions, which serve to make connections within mosaic graphene, are believed to be the thinnest in the world, increasing the speed and reducing the energy cost of devices that may one day use them. P-n junctions are the building blocks of most semiconductor electronic devices, including diodes, solar cells and LEDs.

Based on the improved doping, mosaic graphene should benefit not only graphene based photocurrent generation but also fuel cells, lithium batteries and super capacitors, the researchers said. But before such advances can take place, graphene itself would have to become commercially available.

–Ann Steffora Mutschler

Solar Cell Powered by Spinach
A team of five engineering seniors from Vanderbilt University has designed a truly green biohybrid solar panel: it substitutes a protein from spinach for expensive silicon wafers that are energy intensive to produce, and is capable of producing electricity.

An individual bio-solar cell. (Source: Vanderbilt University)

The large-scale biohybrid solar panel for power production is made up of many individual photoelectrochemical cells that employ photosynthetic proteins as the active elements for light-harvesting and energy conversion. A miniature bio-cell can produce minute electricity from Photosystem I (PSI), the protein in plant chloroplasts that converts light to electrochemical energy. The team extracted PSI from spinach and used it as the working medium in the bio-photovoltaic cells. The two scaled-up panels consist of 24 cells connected in series with each cell measuring 75 x 38 mm.

The researchers standardized a production and assembly scheme that can serve as a footprint for future scale up of next generation cells, which has not been attempted previously.

For their work, the researchers won a Phase II $90,000 grant at the 8th Annual National Sustainable Design Expo held at the National Mall in Washington, D.C., co-sponsored by the U.S. Environmental Protection Agency. The students – Eric Dilbone, Phil Ingram, Trevan Locke, Paul McDonald and Jason Ogg – had won a Phase I $15,000 grant in November from the EPA for their bio-inspired solar panel. Dilbone and Ingram are senior mechanical engineering majors; Locke, McDonald and Ogg are senior chemical engineering majors.

The funding will allow the team to work on the research aspects of achieving the higher energy conversion goals for the individual cells.

Viruses Convert Mechanical Energy to Electricity
The futuristic concept of charging your cell phone as you walk is a bit closer to reality with a paper-thin generator embedded in the sole of your shoe developed by scientists from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory. The Berkeley Lab scientists developed a way to generate power using harmless viruses that convert mechanical energy into electricity.

To test their approach, the scientists created a generator that produces enough current to operate a small LCD that works by tapping a finger on a postage stamp-sized electrode coated with specially engineered viruses, which convert the force of the tap into an electric charge.

This generator is the first to produce electricity by harnessing the piezoelectric properties of a biological material, the scientists said. Piezoelectricity is the accumulation of a charge in a solid in response to mechanical stress.

The bottom 3-D atomic force microscopy image shows how the viruses align themselves side-by-side in a film. The top image maps the film's structure-dependent piezoelectric properties, with higher voltages a lighter color. (Source: Berkeley Lab)

This discovery could lead to tiny devices that harvest electrical energy from the vibrations of everyday tasks such as shutting a door or climbing stairs and point to a simpler way to make microelectronic devices since the viruses arrange themselves into an orderly film that enables the generator to work.

—Ann Steffora Mutschler

Decorating Nanowires

To increase the electrical and catalytic performance of nanowires, engineers at Stanford University have discovered a new method of decorating nanowires with chains of tiny particles, which they said is simpler, faster and provides greater control than earlier methods. This could someday lead to better lithium-ion batteries, more efficient thin-film solar cells and improved catalysts that yield new synthetic fuels, the researchers said.

Xiaolin Zheng, an assistant professor of mechanical engineering and senior author of the study explained, “You can think of it like a tree. The nanowires are the trunk, very good at transporting electrons, like sap, but limited in surface area. The added nanoparticle decorations, as we call them, are like the branches and leaves, which fan out and greatly increase the surface area.”

Since nanoscale surface area matters a great deal in engineering applications like solar cells, batteries and especially catalysts—where the catalytic activity is dependent on the availability of active sites at the surface of the material—greater surface area means greater opportunity for reactions and therefore better catalytic capabilities in, for example, water-splitting systems that produce clean-burning hydrogen fuel from sunlight. Other applications such as sensing small concentrations of chemicals (e.g., toxins or explosives) might also benefit from the greater likelihood of detection made possible by increased surface area.

Prof. Xiaolin Zheng has discovered a new way to 'decorate' nanowires with coatings of metal nanoparticles that greatly improve surface area. The decorated nanowires look like tiny pipe cleaners and are formed of sinuous chains of metal oxide or noble metal nanoparticles. (Source: Stanford)

The sol-flame method, named for the combination of solvent and flame that yields the nanoparticle structures, appears general enough to work with many nanowire and nanoparticle materials and, perhaps more important, provides an unprecedented degree of engineering control in creating the nanoparticle decorations.

“Though more research is needed, such precision is crucial and could bolster the wider adoption of the process,” said Zheng.

Liquid Solar Cells

In other nano-related news, scientists at USC have developed a potential pathway to cheap, stable solar cells made from nanocrystals so small they can exist as a liquid ink and be painted or printed onto clear surfaces.

The solar nanocrystals are about four nanometers in size and can float in a liquid solution, so as Richard L. Brutchey, assistant professor of chemistry at the USC Dornsife College of Letters, Arts and Sciences explained, “like you print a newspaper, you can print solar cells.”

Brutchey and USC postdoctoral researcher David H. Webber developed a new surface coating for the nanocrystals, which are made of the semiconductor cadmium selenide. While liquid nanocrystal solar cells are cheaper to fabricate than available single-crystal silicon wafer solar cells, they are not nearly as efficient at converting sunlight to electricity. The scientists solved one of the key problems of liquid solar cells: how to create a stable liquid that also conducts electricity.

Previously, organic ligand molecules were attached to the nanocrystals to keep them stable and to prevent them from sticking together. These molecules also insulated the crystals, making the whole thing terrible at conducting electricity. However, Brutchey and Webber discovered a synthetic ligand that not only works well at stabilizing nanocrystals but builds tiny bridges connecting the nanocrystals to help transmit current.

With a relatively low-temperature process, the technique allows for the possibility that solar cells can be printed onto plastic instead of glass without any issues with melting, resulting in a flexible solar panel that can be shaped to fit anywhere.  Continuing research will examine nanocrystals built from materials other than cadmium, which is restricted in commercial applications due to toxicity.

—Ann Steffora Mutschler

Solar Cells Like LEDs
Researchers from the University of California at Berkeley have suggested and demonstrated a solar cell concept more like that of LEDs to allow solar cells to be able to emit light as well as absorb it—a counterintuitive concept because currently to produce the maximum amount of energy, solar cells are designed to absorb as much light from the Sun as possible.

Principal researcher and UC Berkeley professor of electrical engineering Eli Yablonovitch said, “What we demonstrated is that the better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce.”

Yablonovitch and his colleagues have been trying to understand why there has been such a large gap between the theoretical limit of 33.5% and the limit that researchers have been able to achieve of 26%. As they worked, a “coherent picture emerged,” explained Owen Miller, a graduate student at UC Berkeley and a member of Yablonovitch’s group. They came across a relatively simple, if perhaps counterintuitive, solution based on a mathematical connection between absorption and emission of light. “Fundamentally, it’s because there’s a thermodynamic link between absorption and emission.”

Eli Yablonovitch and Owen Miller, who worked out the theory for the new solar cell efficiency. The monitor in the picture illustrates the new physics concept where increased light emission yields higher efficiency. Source: The Optical Society (OSA)

Designing solar cells to emit light—so that photons do not become “lost” within a cell—has the natural effect of increasing the voltage produced by the solar cell. If there is a solar cell that is a good emitter of light, it also makes it produce a higher voltage, which in turn increases the amount of electrical energy that can be harvested from the cell for each unit of sunlight.

The Berkeley research team will present its findings at the Conference on Lasers and Electro Optics (CLEO: 2012), to be held May 6-11 in San Jose, Calif.

Harvesting Energy with Nanotechnology
With 58% of the energy generated in the United States wasted as heat, researchers at Purdue University are working on a technique that uses nanotechnology to harvest energy from hot pipes or engine components to potentially recover energy wasted in factories, power plants and cars.  “If we could get just 10% back, that would allow us to reduce energy consumption and power plant emissions considerably,” said Yue Wu, a Purdue University assistant professor of chemical engineering.

The technique utilizes glass fibers coated with a thermoelectric material containing nanocrystals of lead telluride they developed, then exposed to heat in an annealing process to fuse the crystals together.  When the materials are heated on one side, electrons flow to the cooler side, generating an electrical current. These coated fibers could be used to create a solid-state cooling technology that does not require compressors and chemical refrigerants or even be woven into a fabric to make cooling garments. The fibers could be wrapped around industrial pipes in factories and power plants, as well as on car engines and automotive exhaust systems, to recapture much of the wasted energy. The ‘energy harvesting’ technology might dramatically reduce how much heat is lost, Wu said.

This image shows glass fibers coated with a thermoelectric material that generates electrical current when exposed to heat. The technology might be used to harvest energy from hot pipes or engine components, possibly representing a way to recover energy wasted in factories, power plants and cars. Source: Purdue University

These findings were detailed in a research paper appearing last month in the journal Nano Letters. The paper was written by Daxin Liang, a former Purdue exchange student from Jilin University in China; Purdue graduate students Scott Finefrock and Haoran Yang; and Wu.

Today’s high-performance thermoelectric materials are brittle. The devices are formed from large discs or blocks, which requires using a lot of material. But the new flexible devices would conform to the irregular shapes of engines and exhaust pipes while using a small fraction of the material required for conventional thermoelectric devices, researchers said.

“This approach yields the same level of performance as conventional thermoelectric materials but it requires the use of much less material, which leads to lower cost and is practical for mass production,” Wu said. Further, this approach promises a method that can be scaled up to industrial processes, making mass production feasible.

—Ann Steffora Mutschler