Engineers Double Efficiency of Solar Film Cells


nanomanufacturing-6Engineers and materials scientists at University of California in Los Angeles improved the design of solar cells built in a thin semi-transparent film that nearly doubles their ability to generate power. A team from the lab of engineering professor Yang Yang described its findings online in Friday’s issue of the journal Energy and Environmental Science (free registration required).

Yang’s lab developed an earlier form of the solar cell with a near-infrared light-sensitive polymer. The cell produces energy by absorbing mainly infrared light, not visible light. The cell developed in that first round was 70 percent transparent, and achieved a power-generating efficiency of 4 percent.

The new version of the solar cell from Yang’s lab is a tandem device with two thin light-activated polymer solar cells that absorb more light than the single-cell version. The new device also combines transparent and semi-transparent polymer cells, and a layer between the two cells to reduce energy loss.

Tests conducted by Yang’s team show the tandem device achieves a conversion rate — percentage of energy from the sun converted to electric power — of 7.3 percent, compared to 4 percent in the earlier version. The new device captures up to 80 percent of infrared light, with a small amount of light from the visible spectrum, compared to about 40 percent of infrared light absorbed in the earlier single-cell version.

The process to generate the solar cells, say the researchers, uses low temperatures, which makes production of the cells more feasible. The cells can also be produced to appear in various shades of light gray, green, or brown to blend in with building exteriors, windows, or electronic surfaces.

“We anticipate this device,” says Yang, “will offer new directions for solar cells, including the creation of solar windows on homes and office buildings.”

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Integration Of Photonic And Electronic Components


QDOTS imagesCAKXSY1K 8Better integration of photonic and electronic components in nanoscale devices may now become possible, thanks to work by Khuong Phuong Ong and Hong-Son Chu from the A*A*STAR Institute of High Performance Computing and their co-workers in Singapore and the US. From computer simulations, they have identified that the compound BiFeO3 has the potential to be used to efficiently couple light to electrical charges through light-induced electron oscillations known as plasmons. The researchers propose that this coupling could be activated, controlled and switched off, on demand, by applying an electrical field to an active plasmonic device based on this material. If such a device were realized on a very small footprint it would give scientists a versatile tool for connecting components that manipulate light or electric currents.

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Thin poles standing in water barely affect waves rolling past them. Similarly, nanostructured devices typically do not interact with light waves

Many devices used in everyday life — whether they be televisions, mobile phones or barcode scanners — are based on the manipulation of electric currents and light. At the micro- and nano-scales, however, it is typically challenging to integrate electronic components with photonic components. At these small dimensions, the wavelengths of light become long relative to the size of the device. Consequently, the light waves are barely detectable by the device, just as passing waves simply roll past thin poles in a water body (see image).

“The fact that, in theory, the properties of BiFeO3 [could] be [so readily controlled] by applying an electric field makes it a promising material for high-performance plasmonic devices,” explains Ong. He says that they expected such favorable properties after they had calculated the behavior of the material. But when they studied the behavior of the proposed BiFeO3-based device, they found that it could outperform devices based on BaTiO3, which is one of the best materials currently used for such applications.

Like BaTiO3, BiFeO3 can be fabricated relatively easily and cheaply. The new material is therefore a particularly promising candidate for device applications. Ong, Chu and their collaborators will now explore that potential. “We will design BiFeO3 nanostructures optimized for applications such as optical devices for data communication, sensing and solar-energy conversion,” says Ong.

According to Ong and Chu, an important step on the path to producing practical devices will be assessing the compatibility of BiFeO3-based structures with standard technologies, which typically use materials known as metal-oxide semiconductors. This future work will involve collaborations with experimental groups at the A*STAR Institute of Materials Research and Engineering and at the National University of Singapore.

 

The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance

 

Super Effective Camera Chips And Solar Cells Possible With Graphene


QDOTS imagesCAKXSY1K 8Theoretically expected, it was now experimentally proven by scientists for the first time that the fascinating new material graphene is also highly efficient at converting light into electricity–which makes it an ideal candidate to boost the sensitivity of imaging sensors and also to increase the maximum conversion efficiency of photovoltaic cells.

piezoelectric-graphene

 

 

 

 

 

 

 

 

 

(Photo : Mitchell Ong, Stanford School )
This illustration shows lithium atoms (in red) adsorbed to a layer of graphene to create electricity when the graphene is bent, squeezed or twisted.

Current materials used for these applications include silicon and gallium arsenide, but they just generate a single electron for each photon absorbed. Since a photon contains more energy than one electron can carry, much of the energy contained in the incoming light is lost as heat. Graphene on the other hand can generate multiple electrons from absorbing one photon, according to theoretical research that was now confirmed in the lab as described this week in Nature Physics.

Previous work had inspired hope that graphene had this property, says Frank Koppens, a group leader at the Institute of Photonic Sciences in Spain, who led the research. To conduct the experiment, the researchers used two ultrafast light pulses. The first sent a known amount of energy into a single layer of graphene. The second served as a probe that counted the electrons the first one generated.

Koppens said he is “reasonably confident” that the group can enhance the performance of light sensors like those used in cameras, night vision goggles, and certain medical sensors quite soon–after all, his lab is already working on a prototype device to demonstrate the new found capability of graphene.

A second but more difficult application would be solar cells. The material could help to increase the theoretical efficiency limit to about 60%, about twice as much as the 30% limit possible with today’s silicon cells, which currently reach about 20% in the field and 25% in the lab. But Koppens cautions that key engineering challenges stand in the way of that, which includes figuring out how to extract power from a system at all.

The new paper illustrates a “very important concept,” since future devices will depend on an understanding of the physical processes that occur when graphene absorbs light, says says he and colleagues have a still-unpublished paper that describes a similar result. Demonstrating this property in graphene opens a promising new field of research, he says.

Graphene was already exciting as a photovoltaic material because of its unique optical properties, says Andrea Ferrari, a professor of nanotechnology at the University of  Cambridge in the U.K. who was not involved in this research. The material “can work with every possible wavelength you can think of,” he says. “There is no other material in the world with this behavior.” It is also flexible, robust, relatively cheap, and easily integrated with other materials. The new research “adds a third layer of interest to graphene for optics,” he says.

Among Koppens’s collaborators were MIT physics professor Leonid Levitov  and Justin Chien Wen Song, a graduate student in Levitov’s lab, who  helped Koppens interpret the data through theoretical modeling.

Self-assembling Solar-harvesting Films Reveals New Low-Cost Tool for 3D Circuit Printing


4 March 2013 (created 4 March 2013)

QDOTS imagesCAKXSY1K 8Scientists from Imperial College London, working at the Institut Laue-Langevin, have presented a new way of positioning nanoparticles in plastics, with important applications in the production of coatings and photovoltaic material that harvest energy from the sun.  The study used neutrons to understand the role that light – even ambient light – plays in the stabilisation of these notoriously unstable thin films. As a proof of concept the team have shown how the combination of heat and low intensity visible and UV light could in future be used as a precise, low-cost tool for 3D printing of self-assembling, thin-film circuits on these films.
Thin films made up of long organic molecule chains called polymers and fullerenes (large football-shaped molecules composed entirely of carbon) are used mainly in polymer solar cells where they emit electrons when exposed to visible or ultraviolet sun rays. These so-called photovoltaic materials can generate electrical power by converting solar radiation into direct electrical current.
Polymer solar cells are of significant interest for low-power electronics, such as autonomous wireless sensor networks used to monitor everything from ocean temperature to stress inside a car engine. These fullerene-polymer mixtures are particularly appealing because they are lightweight, inexpensive to make, flexible, customisable on the molecular level, and relatively environmentally-friendly.
However current polymer solar cells only offer about one third of the efficiency of other energy harvesting materials, and are very unstable.
In order to improve science’s understanding of the dynamics of these systems and therefore their operational performance, the team carried out neutron reflectometry experiments at the ILL, the world’s flagship centre for neutron science, on a simple model film made up of pure fullerenes with a flexible polymer. Neutron reflectometry is a non-destructive technique that allows you to ‘shave’ layers off these thin films to look at what happens to the fullerenes and the polymers separately, at atomic scale resolution, throughout their depth.
Whilst previous theories suggested that thin film stabilisation was linked to the formation of an expelled fullerene nanoparticle layer at the substrate interface, neutron reflectometry experiments showed that the carbon “footballs” remain evenly distributed throughout the layer. Instead, the team revealed that the stabilisation of the films was caused by a form of photo-crosslinking of the fullerenes. The process imparts greater structural integrity to films, which means that ultrathin films, (down to 10000 times smaller than a human hair) readily become stable with trace amounts of fullerene.
The implications of this finding are significant, particularly in the potential to create much thinner plastic devices which remain stable, with increased efficiency and lifetime (whilst the smaller amount of material required minimises their environmental impact).

The light sensitivity also suggests a unique and simple tool for imparting patterns and designs onto these notoriously unstable films. To prove the concept the team used a photomask to spatially control the distribution of light and added heat. The combination causes the fullerenes to self-assemble into well-defined connected and disconnected patterns, on demand, simply by heating the film until it starts to soften. This results in spontaneous topography and may form the basis of a low-cost tool for 3D printing of thin film circuits.

Other potential applications could include patterning of sensors or biomedical scaffolds.
In the future, the team is looking to apply its findings to conjugated polymers and fullerene derivatives, more common in commercial films, and industrial thin film coatings.

Source: From A neutron investigation into self-assembling solar-harvesting films reveals new low-cost tool for 3D circuit printing. This work is detailed in the paper “Patterning Polymer–Fullerene Nanocomposite Thin Films with Light” by Him Cheng Wong, Anthony M. Higgins, Andrew R. Wildes, Jack F. Douglas, João T. Cabral.

Nano-rod solar cell generates hydrogen


QDOTS imagesCAKXSY1K 8A new type of solar collector that uses gold nano-rods could convert sunlight into energy without many of the problems associated with traditional photovoltaic solar cells.

24 February 2013 Will Parker

 

The developers of the new technique, from the University of California – Santa Barbara, say it is “the first radically new and potentially workable alternative to semiconductor-based photovoltaic devices to be developed in the past 70 years.” They provide details of the new solar hydrogen generator in the journal Nature Nanotechnology.

In conventional photovoltaic cells, sunlight hits the surface of semiconductor material, one side of which is electron-rich, while the other side is not. The photon excites the electrons, causing them to leave their positions, and create positively-charged “holes.” The result is a current of charged particles – electricity.

In the new technique, it is not semiconductor materials that provide the electrons and venue for the conversion of solar energy, but a “forest” of gold nano-rods operating in water. Specifically, gold nano-rods capped with a layer of crystalline titanium dioxide and platinum, and a cobalt-based oxidation catalyst deposited on the lower portion of the array.

“When nanostructures, such as nano-rods, of certain metals are exposed to visible light, the conduction electrons of the metal can be caused to oscillate collectively, absorbing a great deal of the light,” explained Martin Moskovits (pictured front center), a professor of chemistry at UCSB. “This excitation is called a surface plasmon.”

As the “hot” electrons in these plasmonic waves are excited by light particles, some travel up the nano-rod, through a filter layer of crystalline titanium dioxide, and are captured by platinum particles. This causes the reaction that splits hydrogen ions from the bond that forms water. Meanwhile, the holes left behind by the excited electrons head toward the cobalt-based catalyst on the lower part of the rod to form oxygen.

The researchers say that hydrogen production was clearly observable after about two hours. Importantly, the nano-rods were not subject to the photo-corrosion that often causes traditional semiconductor materials to fail and Moskovits says the device operated with no hint of failure for “many weeks.”

Though still in its infancy, the research promises a more robust method of converting sunlight into energy. “Despite the recentness of the discovery, we have already attained ‘respectable’ efficiencies. More importantly, we can imagine achievable strategies for improving the efficiencies radically,” Moskovits said.

moskovits_m

Related: Discuss this article in our forum Stanford announces peel-and-stick solar panels Solar steam generator outshines photovoltaic solar cells Solar power’s dirty secret: skyrocketing lead pollution Much simpler catalyst could fast-track hydrogen economy

See Summary of article here: http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2013.18.html

Source: University of California – Santa Barbara

Engineers create device that can focus light into a nanoscale point (w/video)


QDOTS imagesCAKXSY1K 8(Nanowerk News) As technology advances, it tends to  shrink. From cell phones to laptops—powered by increasingly faster and tinier  processors—everything is getting thinner and sleeker. And now light beams are  getting smaller, too.
Engineers at the California Institute of Technology (Caltech)  have created a device that can focus light into a point just a few nanometers  (billionths of a meter) across—an achievement they say may lead to  next-generation applications in computing, communications, and imaging.
Because light can carry greater amounts of data more efficiently  than electrical signals traveling through copper wires, today’s technology is  increasingly based on optics. The world is already connected by thousands of  miles of optical-fiber cables that deliver email, images, and the latest video  gone viral to your laptop.
As we all produce and consume more data, computers and  communication networks must be able to handle the deluge of information.  Focusing light into tinier spaces can squeeze more data through optical fibers  and increase bandwidth. Moreover, by being able to control light at such small  scales, optical devices can also be made more compact, requiring less energy to  power them.
But focusing light to such minute scales is inherently  difficult. Once you reach sizes smaller than the wavelength of light—a few  hundred nanometers in the case of visible light—you reach what’s called the  diffraction limit, and it’s physically impossible to focus the light any  further.
But now the Caltech researchers, co-led by assistant professor  of electrical engineering Hyuck Choo, have built a new kind of waveguide—a  tunnellike device that channels light—that gets around this natural limit. The  waveguide, which is described in a recent issue of the journal Nature  Photonics (“Nanofocusing in a metal-insulator-metal gap plasmon  waveguide with a three-dimensional linear taper”), is made of amorphous  silicon dioxide—which is similar to common glass—and is covered in a thin layer  of gold. Just under two microns long, the device is a rectangular box that  tapers to a point at one end.
As light is sent through the waveguide, the photons interact  with electrons at the interface between the gold and the silicon dioxide. Those  electrons oscillate, and the oscillations propagate along the device as  waves—similarly to how vibrations of air molecules travel as sound waves.  Because the electron oscillations are directly coupled with the light, they  carry the same information and properties—and they therefore serve as a proxy  for the light.
Instead of focusing the light alone—which is impossible due to  the diffraction limit—the new device focuses these coupled electron  oscillations, called surface plasmon polaritons (SPPs). The SPPs travel through  the waveguide and are focused as they go through the pointy end.
Because the new device is built on a semiconductor chip with  standard nanofabrication techniques, says Choo, the co-lead and the  co-corresponding author of the paper, it is easy integrate with today’s  technology
Previous on-chip nanofocusing devices were only able to focus  light into a narrow line. They also were inefficient, typically focusing only a  few percent of the incident photons, with the majority absorbed and scattered as  they traveled through the devices.
With the new device, light can ultimately be focused in three  dimensions, producing a point a few nanometers across, and using half of the  light that’s sent through, Choo says. (Focusing the light into a slightly bigger  spot, 14 by 80 nanometers in size, boosts the efficiency to 70 percent). The key  feature behind the device’s focusing ability and efficiency, he says, is its  unique design and shape.
“Our new device is based on fundamental research, but we hope  it’s a good building block for many potentially revolutionary engineering  applications,” says Myung-Ki Kim, a postdoctoral scholar and the other lead  author of the paper.
This video shows the final step of the fabrication process:
For example, one application is to turn this nanofocusing device  into an efficient, high-resolution biological-imaging instrument, Kim says. A  biologist can dye specific molecules in a cell with fluorescent proteins that  glow when struck by light. Using the new device, a scientist can focus light  into the cell, causing the fluorescent proteins to shine. Because the device  concentrates light into such a small point, it can create a high-resolution map  of those dyed molecules. Light can also travel in the reverse direction through  the nanofocuser: by collecting light through the narrow point, the device turns  into a high-resolution microscope.
The device can also lead to computer hard drives that hold more  memory via heat-assisted magnetic recording. Normal hard drives consist of rows  of tiny magnets whose north and south poles lay end to end. Data is recorded by  applying a magnetic field to switch the polarity of the magnets.
Smaller magnets would allow more memory to be squeezed into a  disc of a given size. But the polarities of smaller magnets made of current  materials are unstable at room temperature, causing the magnetic poles to  spontaneously flip—and for data to be lost. Instead, more stable materials can  be used—but those require heat to record data. The heat makes the magnets more  susceptible to polarity reversals. Therefore, to write data, a laser is needed  to heat the individual magnets, allowing a surrounding magnetic field to flip  their polarities.
Today’s technology, however, can’t focus a laser into a beam  that is narrow enough to individually heat such tiny magnets. Indeed, current  lasers can only concentrate a beam to an area 300 nanometers wide, which would  heat the target magnet as well as adjacent ones—possibly spoiling other recorded  data.
Because the new device can focus light down to such small  scales, it can heat smaller magnets individually, making it possible for hard  drives to pack more magnets and therefore more memory. With current technology,  discs can’t hold more than 1 terabyte (1,000 gigabytes) per square inch. A  nanofocusing device, Choo says, can bump that to 50 terabytes per square inch.
Then there’s the myriad of data-transfer and communication  applications, the researchers say. As computing becomes increasingly reliant on  optics, devices that concentrate and control data-carrying light at the  nanoscale will be essential—and ubiquitous, says Choo, who is a member of the  Kavli Nanoscience Institute at Caltech. “Don’t be surprised if you see a similar  kind of device inside a computer you may someday buy.”
The next step is to optimize the design and to begin building  imaging instruments and sensors, Choo says. The device is versatile enough that  relatively simple modifications could allow it to be used for imaging,  computing, or communication.
Source: California Institute of Technology

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