(Solar) Cell a Million?


Cell a Million 20131102_stp501

SOLAR cells were once a bespoke product, reserved for satellites and military use. In 1977 a watt of solar generating capacity cost $77. That has now come down to about 80 cents, and solar power is beginning to compete with the more expensive sort of conventionally generated electricity. If the price came down further, though, solar might really hit the big time—and that is the hope of Henry Snaith, of Oxford University, and his colleagues. As he described recently in Science, Dr Snaith plans to replace silicon, the material used to make most solar cells, with a substance called a perovskite. This, he believes, could cut the cost of a watt of solar generating capacity by three-quarters.

When light falls on a solar cell, it knocks electrons away from the cell’s material and leaves behind empty spaces called holes. Electrons and holes then flow in opposite directions and the result is an electric current.

 

The more electrons and holes there are, and the faster they flow, the bigger the current will be. Electrons, however, often get captured by holes while still inside the cell, and cannot therefore contribute to the current. The average distance an electron travels in a material before it gets captured is known as that material’s diffusion length. The larger the diffusion length, the more efficient the cell.

The silicon used in commercial solar cells has a diffusion length of ten nanometres (billionth of a metre), which is not much. Partly for this reason a silicon cell’s efficiency at converting incident light into electricity is less than 10%. Dr Snaith’s perovskite does better. It has a diffusion length of 1,000 nanometres, giving it an efficiency of 15%. And this, Dr Snaith says, has been achieved without much tweaking of the material. The implication is that it could be made more efficient still.

Perovskites are substances composed of what are known as cubo-octahedral crystals—in other words, cubes with the corners cut off. They thus have six octagonal faces and eight triangular ones. Perovskite itself is a natually occuring mineral, calcium titanium oxide, but lots of other elemental combinations adopt the same shape, and tinkering with the mix changes the frequency of the light the crystal absorbs best.

Dr Snaith’s perovskite is a particularly sophisticated one. It has an organic part, made of carbon, hydrogen and nitrogen, and an inorganic part, made of lead, iodine and chlorine. The organic part acts as a dye, absorbing lots of sunlight. The inorganic part helps conduct the electrons thus released.

It is also cheap to make. Purifying silicon requires high (and therefore costly) temperatures. Dr Snaith’s perovskite can be blended at room temperature. Laboratory versions of cells made from it cost about 40 cents per watt (ie, about half the cost of commercial silicon-based solar cells). At an industrial scale, Dr Snaith expects, that will halve again.

There are caveats, of course. The new perovskite is such a recent invention that its durability has not been properly tested. Many otherwise-promising materials fail to survive constant exposure to the sun, a sine qua non of being a solar cell. And the process of converting a laboratory-made cell into a mass-manufactured one is not always straight forward.

If it leaps these hurdles, though, Dr Snaith’s material will be a strong challenger for silicon. As solar power-generation becomes a mainstream technology over the next few years, the once-strange word “perovskite” may enter everyday language.

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New findings open the way for research on spin properties and device applications of graphene


QDOTS imagesCAKXSY1K 8(Nanowerk News) Using a spin-polarized metastable  helium beam, a group headed by Dr. Shiro Entani, who is a limited-term  researcher at the Advanced Science Research Center, Japan Atomic Energy Agency  and Dr. Yasushi Yamauchi, a Group Leader in the Nano Characterization Unit, National Institute  for Materials Science succeeded in detecting the electronic spin state of only  the graphene contacted to a magnetic metal in devices.
spin detector
Schematic diagram of the experimental method. When a low velocity  spin-polarized metastable He beam is irradiated on the specimen surface, the He  atoms rebound above the surface without penetrating to the interior. As a  result, it is possible to selectively detect surface information. During the  collision, the 1s-hole (black broken line arrow) is filled by a surface electron  and the He atom returns to its ground state by releasing the 2s electron. Since  the surface electron filing the 1s-hole should have the same spin to the hole,  the ejected electron carries the spin information of surface electrons. By  aligning the spin direction of 1s-hole a metastable He becomes a spin detector.  In the present study the spin state of topmost layer of graphene/Ni(111), namely  graphene, has been selectively detected with this method.
Graphene is considered a promising substrate material for  next-generation spintronics, as it possesses many properties that are suitable  for transmission of electronic spin information. In order to utilize graphene in  spin devices, techniques for controlling its spin state are indispensable, and  among these, the development of a spin injection technique using a magnetic  electrode is a key issue. In developing these techniques, first, it is necessary  to know the spin state of the graphene which is contacted to the magnetic metal  electrode. It was difficult, however, for conventional techniques to selectively  obtain the spin information of the graphene because the weak signal from the  graphene, which comprises a single atomic layer, is buried in the strong signal  from the magnetic substrate.
In this study, the JAEA-NIMS research group succeeded for the  first time in observing the electronic spin state of only the graphene by  measuring a junction of graphene and magnetic metal (nickel) with a  spin-polarized metastable helium (He) beam. The results revealed that, in  conduction electrons of graphene contacted to nickel, spin polarization occurs  with the same orientation as the spin of the nickel.
This research achievement is expected to greatly advance  research on the spin properties of various 2-dimensional materials including  graphene which are a focus of attention as new spintronics materials, as well as  device applications such as development of spin injection techniques, etc.
These results were published in the online edition of the  scientific journal Carbon (“Spin polarization of single-layer graphene  epitaxially grown on Ni(1 1 1) thin film”).
Source: NIMS

Read more: http://www.nanowerk.com/news2/newsid=31176.php#ixzz2YP9nqz47

Quantum Dots that Assemble Themselves


QDOTS imagesCAKXSY1K 8A paper on the new technology, “Self-assembled Quantum Dots in a Nanowire System for Quantum Photonics,” appears in the current issue of the scientific journal Nature Materials. Quantum dots are tiny crystals of semiconductor a few billionths of a meter in diameter. At that size they exhibit beneficial behaviors of quantum physics such as forming electron-hole pairs and harvesting excess energy.

The scientists demonstrated how quantum dots can self-assemble at the apex of the gallium arsenide/aluminum gallium arsenide core/shell nanowire interface. Crucially, the quantum dots, besides being highly stable, can be positioned precisely relative to the nanowire’s center. That precision, combined with the materials’ ability to provide quantum confinement for both the electrons and the holes, makes the approach a potential game-changer.

Electrons and holes typically locate in the lowest energy position within the confines of high-energy materials in the nanostructures. But in the new demonstration, the electron and hole, overlapping in a near-ideal way, are confined in the quantum dot itself at high energy rather than located at the lowest energy states. In this case, that’s the gallium-arsenide core. It’s like hitting the bulls-eye rather than the periphery.

The quantum dots, as a result, are very bright, spectrally narrow and highly anti-bunched, displaying excellent optical properties even when they are located just a few nanometers from the surface – a feature that even surprised the scientists. “Some Swiss scientists announced that they had achieved this, but scientists at the conference had a hard time believing it,” said NREL senior scientist Jun-Wei Luo, one of the co-authors of the study. Luo got to work constructing a quantum-dot-in-nanowire system using NREL’s supercomputer and was able to demonstrate that despite the fact that the overall band edges are formed by the gallium Arsenide core, the thin aluminum-rich barriers provide quantum confinement both for the electrons and the holes inside the aluminum-poor quantum dot. That explains the origin of the highly unusual optical transitions.

Several practical applications are possible. The fact that stable quantum dots can be placed very close to the surface of the nanowires raises a huge potential for their use in detecting local electric and magnetic fields. The quantum dots also could be used to charge converters for better light-harvesting, as in the case of photovoltaic cells.

The team of scientists working on the project came from universities and laboratories in Sweden, Switzerland, Spain, and the United States.

More information: http://www.nature.com/nmat/journal/vaop/ncurrent/fig_tab/nmat3557_F4.htmlJournal reference: Nature Materials Provided by National Renewable Energy Laboratory

Read more at: http://phys.org/news/2013-04-team-quantum-dots.html#jCp

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.

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

Light-Based Hydrogen Production


Nanocrystals and Nickel Catalyst Substantially Improve

Light-Based Hydrogen Production

November 8, 2012

Hydrogen is an attractive fuel source because it can easily be converted into electric energy and gives off no greenhouse emissions. A group of chemists at the University of Rochester is adding to its appeal by increasing the output and lowering the cost of current light-driven hydrogen-production systems.

The work was done by graduate students Zhiji Han and Fen Qiu, as part of a collaboration between chemistry professors Richard Eisenberg, Todd Krauss, and Patrick Holland, which is funded by the U.S. Department of Energy. Their paper will be published later this month (Nov. 23) in the journal Science.

The chemists say their work advances what is sometimes considered the “holy grail” of energy science—efficiently using sunlight to provide clean, carbon-free energy for vehicles and anything that requires electricity.

One disadvantage of current methods of hydrogen production has been the lack of durability in the light-absorbing material, but the Rochester scientists were able to overcome that problem by incorporating nanocrystals. “Organic molecules are typically used to capture light in photocatalytic systems,” said Krauss, who has been working in the field of nanocrystals for over 20 years. “The problem is they only last hours, or, if you’re lucky, a day. These nanocrystals performed without any sign of deterioration for at least two weeks.”

Richard Eisenberg, the Tracy H. Harris Professor of Chemistry, has spent two decades working on solar energy systems. During that time, his systems have typically generated 10,000 instances—called turnovers—of hydrogen atoms being formed without having to replace any components. With the nanocrystals, Eisenberg and his colleagues witnessed turnovers in excess of 600,000.

The researchers managed to overcome other disadvantages of traditional photocatalytic systems. “People have typically used catalysts made from platinum and other expensive metals,” Holland said. “It would be much more sustainable if we used metals that were more easily found on the Earth, more affordable, and lower in toxicity. That would include metals, such as nickel.”

Holland said their work is still in the “basic research stage,” making it impossible to provide cost comparisons with other energy production systems. But he points out that nickel currently sells for about $8 per pound, while the cost of platinum is $24,000 per pound.

While all three researchers say the commercial implementation of their work is years off, Holland points out that an efficient, low-cost system would have uses beyond energy. “Any industry that requires large amounts of hydrogen would benefit, including pharmaceuticals and fertilizers,” said Holland.

The process developed by Holland, Eisenberg, and Krauss is similar to other photocatalytic systems; they needed a chromophore (the light-absorbing material), a catalyst to combine protons and electrons, and a solution, which in this case is water. Krauss, an expert in nanocrystals, provided cadmium selenide (CdSe) quantum dots (nanocrystals) as the chromophore. Holland, whose expertise lies in catalysis and nickel research, supplied a nickel catalyst (nickel nitrate). The nanocrystals were capped with DHLA (dihydrolipoic acid) to make them soluble, and ascorbic acid was added to the water as an electron donor.

Photons from a light source excite electrons in the nanocrystals and transfer them to the nickel catalyst. When two electrons are available, they combine on the catalyst with protons from water, to form a hydrogen molecule (H2).

This system was so robust that it kept producing hydrogen until the source of electrons was removed after two weeks. “Presumably, it could continue even longer, but we ran out of patience!” said Holland.

One of the next steps will be to look at the nature of the nanocrystal. “Some nanocrystals are like M&Ms – they have a core with a shell around it,” said Eisenberg. “Ours is just like the core. So we need to consider if they would they work better if they were enclosed in shells.”

Method and apparatus for nondestructive corrosion detection using quantum dots – The Boeing Company


Boeing’s patent US7925452 deals with method and apparatus for nondestructive corrosion detection using quantum dots – the boeing company .

1. Field

The disclosure is related generally to quantum dots and in particular to a method and apparatus for nondestructive inspection of structures. More particularly, the disclosure is directed to a method, apparatus, and computer usable program code for enhancing detection of corrosion on a surface of a structure by detecting quantum dots associated with the surface.

2. Background

During the manufacture, maintenance, and/or rework of many commodities, such as aircraft commodities, it can be extremely important to ensure that external and/or internal surfaces of the commodity do not have any corrosion.

Therefore, detection of corrosion may be very important. However, corrosion on a surface may be hidden or masked underneath layers of paint or other surface coatings. Destructive means of corrosion detection require the stripping or removal of paint and/or disassembly of parts and assemblies to identify corrosion. These processes are destructive, slow, inefficient, and may be cost prohibitive.

Currently available nondestructive corrosion inspection (NDI) is generally performed visually, using electromagnetic inspection, eddy current or ultrasonic inspection methods, which can measure metal thinning due to corrosion. However, these approaches require more inspections and disassemblies than would otherwise be required for a very early detection and monitoring capability. In addition, visual inspections require a technician or other maintenance personnel to visually inspect all surfaces for signs and evidence of corrosion, such as visible rust. This can be a time consuming, expensive, and unreliable process.

In addition, corrosion is often very difficult to detect under paint or other coatings, in remote areas, and/or in difficult to reach areas. For remote or limited access areas, visual inspection may be made possible through borescopes. Interpretation may frequently be difficult and sensitivity to corrosion may be limited with this approach because corrosion may appear similar to dirt, paint chips, or other foreign material.

Moreover, maintenance personnel and corrosion inspectors must wait until corrosion on a surface is substantial enough to be detected visually. By the time the corrosion is detected by these means, the corrosion may have resulted in greater damage to the commodity, and a correspondingly higher cost of rework, than if the corrosion had been detected at an earlier time.

Therefore, it would be advantageous to have an improved method, apparatus, and computer usable program code for non-destructive corrosion detection.

 

The Boeing patent solves the following problem:

A method, apparatus, and computer usable program code for non-destructive detection of corrosion using quantum dots. In one embodiment, a surface of an area on a commodity associated with a set of quantum dots is tested. A pattern of wavelengths emitted by the set of quantum dots associated with the surface of the commodity is detected to form a quantum dot pattern. The quantum dot pattern is analyzed to determine whether corrosion has occurred in the area on the surface of the commodity.
An embodiment of the disclosure provides a method, apparatus, and computer usable program code for non-destructive detection of corrosion using quantum dots. A surface of a commodity associated with a set of quantum dots is tested to form a test area. A pattern of wavelengths emitted by the set of quantum dots associated with the test area is detected to form a quantum dot pattern. The quantum dot pattern is analyzed to determine whether corrosion has occurred in the test area of the commodity.
Another advantageous embodiment provides a system for non-destructive detection of corrosion using quantum dots. The system includes a commodity; a set of quantum dots associated with an area of the commodity to form a test area; and a quantum dot detector, wherein the quantum dot detector detects a pattern of wavelengths emitted by the set of quantum dots associated with the test area of the commodity to form a quantum dot pattern; and analyzes the quantum dot pattern to determine whether corrosion has occurred in the test area of the commodity.
In another illustrative embodiment, a computer program product having a computer usable medium including computer usable program code for non-destructive detection of corrosion using quantum dots is provided. The computer program product comprises computer usable program code for detecting a pattern of wavelengths emitted by the set of quantum dots associated with the surface of the commodity to form a quantum dot pattern in response to a test of a surface of an area on a commodity associated with a set of quantum dots to form a test area; and computer usable program code for analyzing the quantum dot pattern to determine whether corrosion has occurred in the test area of the commodity.
The features, functions, and advantages can be achieved independently in various embodiments of the disclosure or may be combined in yet other embodiments in which further details can be seen with reference to the following description and drawings.