Solar Cells that See Red

Solar Cells that See Red

  10:25:00 AM    Science Lover

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Metamaterials that convert lower-energy photons to usable wavelengths could offer solar cells an efficiency boost.

Light switch: In a process that could make
solar cells more efficient, green laser light
is "upconverted" to blue light by a
solution of dyes and metal nanoparticles.
Credit: Jennifer Dionne

Researchers at Stanford University have demonstrated a set of materials that could enable solar cells to use a band of the solar spectrum that otherwise goes to waste. The materials layered on the back of solar cells would convert red and near-infrared light—unusable by today's solar cells—into shorter-wavelength light that the cells can turn into energy. The university researchers will collaborate with the Bosch Research and Technology Center in Palo Alto, California, to demonstrate a system in working solar cells in the next four years.

Even the best of today's silicon solar cells can't use about 30 percent of the light from the sun: that's because the active materials in solar cells can't interact with photons whose energy is too low. But though each of these individual photons is low energy, as a whole they represent a large amount of untapped solar energy that could make solar cells more cost-competitive.

The process, called "upconversion," relies on pairs of dyes that absorb photons of a given wavelength and re-emit them as fewer, shorter-wavelength photons. In this case, the Bosch and Stanford researchers will work on systems that convert near-infrared wavelengths (most of which are unusable by today's solar cells). The leader of the Stanford group, assistant professor Jennifer Dionne, believes the group can improve the sunlight-to-electricity conversion efficiency of amorphous-silicon solar cells from 11 percent to 15 percent.



The concept of upconversion isn't new, but it's never been demonstrated in a working solar cell, says Inna Kozinsky, a senior engineer at Bosch. Upconversion typically requires two types of molecules to absorb relatively high-wavelength photons, combine their energy, and re-emit it as higher-energy, lower-wavelength photons. However, the chances of the molecules encountering each other at the right time when they're in the right energetic states are low. Dionne is developing nanoparticles to add to these systems in order to increase those chances. To make better upconversion systems, Dionne is designing metal nanoparticles that act like tiny optical antennas, directing light in these dye systems in such a way that the dyes are exposed to more light at the right time, which creates more upconverted light, and then directing more of that upconverted light out of the system in the end.

The ultimate vision, says Dionne, is to create a solid. Sheets of such a material could be laid down on the bottom of the cell, separated from the cell itself by an electrically insulating layer. Low-wavelength photons that pass through the active layer would be absorbed by the upconverter layer, then re-emitted back into the active layer as usable, higher-wavelength light.

Kozinsky says Bosch's goal is to demonstrate upconversion of red light in working solar cells in three years, and upconversion of infrared light in four years. Factoring in the time needed to scale up to manufacturing, she says, the technology could be in Bosch's commercial solar cells in seven to 10 years.

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A novel enzymatic catalyst for biodiesel production

A novel enzymatic catalyst for biodiesel production

  4:47:00 PM    Science Lover

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Continuous production of biodiesel can now be envisaged thanks to a novel catalyst developed by a French team at CNRS's Centre de Recherches Paul Pascal (CRPP). The results, which have been patented, have just been published in the journal Energy & Environmental Science.

Diagram showing the enzyme biocatalytic reactor and its unidirectional continuous flow operation that uses enzymatic catalysis to turn triesters into biodiesel. © CNRS

Biofuel production provides an alternative to fossil fuels. Biodiesels, for instance, are processed products based on oils from oleaginous plants such as oilseed rape, palm, sunflower and soybeans. They result from a chemical reaction, catalyzed in either an acidic or preferably a basic medium, between a vegetable oil (90%) and an alcohol (10%). This reaction, known as transesterification, converts the mixture into a methyl ester (the main constituent of biodiesel) and glycerol. A saponification side reaction (methyl ester conversion into the corresponding acid salt) reduces methyl ester yield. To increase the yield, it was therefore necessary to develop alternative catalysts.

For this type of reaction, certain enzymatic catalysts such as those belonging to the family of lipases (triglyceride hydrolases) are particularly efficient and selective. However, their high cost and low conformational stability restrict their industrial use, unless they can be irreversibly confined in porous matrices, allowing good accessibility and enhanced mass transport. This has now been achieved by the team led by Professor Renal Backov.



In an initial study, they had already demonstrated the possibility of efficient catalysis, by developing modified silica-based cellular matrices that make it possible to confine lipases in order to obtain exceptional yields for hydrolysis, esterification and transesterification reactions. Their work had also shown that unpurified enzymes could be used in the matrices. The fact that they were unpurified was a first step to significantly reducing the cost of biocatalysts. However, the methodology did not allow continuous biodiesel production. This obstacle has now been overcome.

Researchers have developed a new method that generates the cellular hybrid biocatalyst in situ inside a chromotography column. This novel approach makes it possible to carry out continuous, unidirectional flow synthesis over long periods, since catalytic activity and ethyl ester production are maintained at high, practically steady levels during a two-month period of time. These results are amongst the best ever obtained in this field.

Research is continuing into solvent-free conversion of triesters, aimed at minimizing waste production and curbing the use of solvents and metals in chemical transformation processes. This work, which meets current energy and environmental requirements, shows how much chemists are working in the public interest, and confirms the importance of integrative chemistry.

More information: References:

-- “Enzyme-Based Biohybrid Foams Designed for Continuous Flow Heterogeneous Catalysis and Biodiesel Production”, N.Brun, A.Babeau-Garcia, M.-F.Achard, C.Sanchez, F.Durand, L.Guillaume, M.Birot, H.Deleuze and R.Backov - Energy & Environmental Science, 2011 DOI:10.1039/C1EE01295A

-- Catalyseur enzymatique hétérogène, procédé de préparation et utilisation pour la catalyse enzymatique en flux continu. N.Brun, H.Deleuze, C.Sanchez and R.Backov. French patent 2010. File number FR10-56099. Provided by CNRS

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Solar Cells Thinner Than Wavelengths of Light Hold Huge Power Potential

Solar Cells Thinner Than Wavelengths of Light Hold Huge Power Potential

  10:21:00 AM    Science Lover

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Ultra-thin solar cells can absorb sunlight more efficiently than the thicker, more expensive-to-make silicon cells used today, because light behaves differently at scales around a nanometer (a billionth of a meter), say Stanford engineers. They calculate that by properly configuring the thicknesses of several thin layers of films, an organic polymer thin film could absorb as much as 10 times more energy from sunlight than was thought possible.

This schematic diagram of a thin film organic solar 
cell shows the top layer, a patterned, roughened
scattering layer, in green. The organic thin film layer, 
shown in red, is where light is trapped and electrical
current is generated. The film is sandwiched between 
two layers that help keep light contained within 
the thin film. (Credit: Reproduced with permission
from Proceedings of the National Academy 
of Sciences USA)

In the smooth, white, bunny-suited clean-room world of silicon wafers and solar cells, it turns out that a little roughness may go a long way, perhaps all the way to making solar power an affordable energy source, say Stanford engineers.

Their research shows that light ricocheting around inside the polymer film of a solar cell behaves differently when the film is ultra thin. A film that's nanoscale-thin and has been roughed up a bit can absorb more than 10 times the energy predicted by conventional theory.

The key to overcoming the theoretical limit lies in keeping sunlight in the grip of the solar cell long enough to squeeze the maximum amount of energy from it, using a technique called "light trapping." It's the same as if you were using hamsters running on little wheels to generate your electricity -- you'd want each hamster to log as many miles as possible before it jumped off and ran away.

"The longer a photon of light is in the solar cell, the better chance the photon can get absorbed," said Shanhui Fan, associate professor of electrical engineering. The efficiency with which a given material absorbs sunlight is critically important in determining the overall efficiency of solar energy conversion. Fan is senior author of a paper describing the work published online by Proceedings of the National Academy of Sciences.

Light trapping has been used for several decades with silicon solar cells and is done by roughening the surface of the silicon to cause incoming light to bounce around inside the cell for a while after it penetrates, rather than reflecting right back out as it does off a mirror. But over the years, no matter how much researchers tinkered with the technique, they couldn't boost the efficiency of typical "macroscale" silicon cells beyond a certain amount.

Eventually the scientists realized that there was a physical limit related to the speed at which light travels within a given material.

But light has a dual nature, sometimes behaving as a solid particle (a photon) and other times as a wave of energy, and Fan and postdoctoral researcher Zongfu Yu decided to explore whether the conventional limit on light trapping held true in a nanoscale setting. Yu is the lead author of the PNAS paper.

"We all used to think of light as going in a straight line," Fan said. "For example, a ray of light hits a mirror, it bounces and you see another light ray. That is the typical way we think about light in the macroscopic world.

"But if you go down to the nanoscales that we are interested in, hundreds of millionths of a millimeter in scale, it turns out the wave characteristic really becomes important."

Visible light has wavelengths around 400 to 700 nanometers (billionths of a meter), but even at that small scale, Fan said, many of the structures that Yu analyzed had a theoretical limit comparable to the conventional limit proven by experiment.

"One of the surprises with this work was discovering just how robust the conventional limit is," Fan said.

It was only when Yu began investigating the behavior of light inside a material of deep subwavelength-scale -- substantially smaller than the wavelength of the light -- that it became evident to him that light could be confined for a longer time, increasing energy absorption beyond the conventional limit at the macroscale.

"The amount of benefit of nanoscale confinement we have shown here really is surprising," said Yu. "Overcoming the conventional limit opens a new door to designing highly efficient solar cells."

Yu determined through numerical simulations that the most effective structure for capitalizing on the benefits of nanoscale confinement was a combination of several different types of layers around an organic thin film.

He sandwiched the organic thin film between two layers of material -- called "cladding" layers -- that acted as confining layers once the light passed through the upper one into the thin film. Atop the upper cladding layer, he placed a patterned rough-surfaced layer designed to send the incoming light off in different directions as it entered the thin film.

By varying the parameters of the different layers, he was able to achieve a 12-fold increase in the absorption of light within the thin film, compared to the macroscale limit.

Nanoscale solar cells offer savings in material costs, as the organic polymer thin films and other materials used are less expensive than silicon and, being nanoscale, the quantities required for the cells are much smaller.

The organic materials also have the advantage of being manufactured in chemical reactions in solution, rather than needing high-temperature or vacuum processing, as is required for silicon manufacture.

"Most of the research these days is looking into many different kinds of materials for solar cells," Fan said. "Where this will have a larger impact is in some of the emerging technologies; for example, in organic cells."

"If you do it right, there is enormous potential associated with it," Fan said.

Aaswath Raman, a graduate student in applied physics, also worked on the research and is a coauthor of the paper.

The project was supported by funding from the King Abdullah University of Science and Technology, which supports the Center for Advanced Molecular Photovoltaics at Stanford, and by the U.S. Department of Energy.

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Mimicking Nature, Water-Based Artificial Leaf Produces Electricity

Mimicking Nature, Water-Based Artificial Leaf Produces Electricity

  1:11:00 PM    Science Lover

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A team led by a North Carolina State University researcher has shown that water-gel-based solar devices -- "artificial leaves" -- can act like solar cells to produce electricity. The findings prove the concept for making solar cells that more closely mimic nature. They also have the potential to be less expensive and more environmentally friendly than the current standard-bearer: silicon-based solar cells.

Just as real leaves do, water-gel-based solar devices can act like solar cells to produce electricity, new research shows. (Credit: iStockphoto/Chaikovskiy Igor)

The bendable devices are composed of water-based gel infused with light-sensitive molecules -- the researchers used plant chlorophyll in one of the experiments -- coupled with electrodes coated by carbon materials, such as carbon nanotubes or graphite. The light-sensitive molecules get "excited" by the sun's rays to produce electricity, similar to plant molecules that get excited to synthesize sugars in order to grow, says NC State's Dr. Orlin Velev, Invista Professor of Chemical and Biomolecular Engineering and the lead author of a paper published online in the Journal of Materials Chemistry describing this new generation of solar cells.

Velev says that the research team hopes to "learn how to mimic the materials by which nature harnesses solar energy." Although synthetic light-sensitive molecules can be used, Velev says naturally derived products -- like chlorophyll -- are also easily integrated in these devices because of their water-gel matrix.

Now that they've proven the concept, Velev says the researchers will work to fine-tune the water-based photovoltaic devices, making them even more like real leaves.

"The next step is to mimic the self-regenerating mechanisms found in plants," Velev says. "The other challenge is to change the water-based gel and light-sensitive molecules to improve the efficiency of the solar cells."

Velev even imagines a future where roofs could be covered with soft sheets of similar electricity-generating artificial-leaf solar cells.

"We do not want to overpromise at this stage, as the devices are still of relatively low efficiency and there is a long way to go before this can become a practical technology," Velev says. "However, we believe that the concept of biologically inspired 'soft' devices for generating electricity may in the future provide an alternative for the present-day solid-state technologies."

Researchers from the Air Force Research Laboratory and Chung-Ang University in Korea co-authored the study. The study was funded by the Air Force Research Laboratory and the U.S. Department of Energy. The work is part of NC State's universitywide nanotechnology program, Nano@NC State.

NC State's Department of Chemical and Biomolecular Engineering is part of the university's College of Engineering.

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A New Way to Use the Sun's Energy

A New Way to Use the Sun's Energy

  1:47:00 AM    Science Lover

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Researchers have demonstrated a new mechanism for converting both sunlight and heat into electricity.

A new type of device that uses both heat and light from the sun should be more efficient than conventional solar cells, which convert only the light into electricity.

Bright heat: Nicholas Melosh has
developed a device for simultaneously
converting the sun’s light and heat into
electricity. Melosh makes and tests the
device in this vacuum chamber in his
lab at Stanford University.
Credit: Technology Review

The device relies on a physical principle discovered and demonstrated by researchers at Stanford University. In their prototype, the energy in sunlight excites electrons in an electrode, and heat from the sun coaxes the excited electrons to jump across a vacuum into another electrode, generating an electrical current. The device could be designed to send waste heat to a steam engine and convert 50 percent of the energy in sunlight into electricity--a huge improvement over conventional solar cells.

The most common silicon solar cells convert about 15 percent of the energy in sunlight into electricity. More than half of the incoming solar energy is lost as heat. That's because the active materials in solar cells can interact with only a particular band of the solar spectrum; photons below a certain energy level simply heat up the cell.

One way to overcome this is to stack active materials on top of one another in a multijunction cell that can use a broader spectrum of light, turning more of it into electrical current instead of heat, for efficiencies up to about 40 percent. But such cells are complex and expensive to make.

Looking for a better way to take advantage of the sun's heat, Stanford's Nicholas Melosh was inspired by highly efficient cogeneration systems that use the expansion of burning gas to drive a turbine and the heat from the combustion to power a steam engine. But thermal energy converters don't pair well with conventional solar devices. The hotter it is, the more efficient thermal energy conversion becomes. Solar cells, by contrast, get less efficient as they heat up. At about 100 °C, a silicon cell won't work well; above 200 °C, it won't work at all.

The breakthrough came when the Stanford researchers realized that the light in solar radiation could enhance energy conversion in a different type of device, called a thermionic energy converter, that's conventionally driven solely by heat. Thermionic converters consist of two electrodes separated by a small space. When the positive electrode, or cathode, is heated, electrons in the cathode get excited and jump across to the negative electrode, or anode, driving a current through an external circuit. These devices have been used to power Russian satellites but haven't found any applications on the ground because they must get very hot, about 1,500 °C, to operate efficiently. The cathode in these devices is typically made of metals such as cesium.

Melosh's group replaced the cesium cathode with a wafer of semiconducting material that can make use of not only heat but also light. When light strikes the cathode, it transmits its energy to electrons in the material in a way that's similar to what happens in a solar cell. This type of energy transfer doesn't happen in the metals used to make these cathodes in the past, but it's typical of semiconductor materials. It doesn't take quite as much heat for these "preëxcited" electrons to jump to the anode, so this new device can operate at lower temperatures than conventional thermionic converters, but at higher temperatures than a solar cell.

The Stanford researchers call this new mechanism PETE, for photon-enhanced thermionic emission. "The light helps lift the energy level of the electrons so that they will flow," says Gang Chen, professor of power engineering at MIT. "It's a long way to a practical device, but this work shows that it's possible," he says.

The Stanford group's prototype, described this month in the journal Nature Materials, uses gallium nitride as the semiconductor. It converts just about 25 percent of the energy in light into electricity at 200 °C, and the efficiency rises with the temperature. Stuart Licht, professor of chemistry at George Washington University, says the process would have an "advantage over solar cells" because it makes use of heat in addition to light. But he cautions: "Additional work will be needed to translate this into a practical, more efficient device."

The Stanford group is now working to do just that. The researchers are testing devices made from materials that are better suited to solar energy conversion, including silicon and gallium arsenide. They're also developing ways of treating these materials so that the device will work more efficiently in a temperature range of 400 °C to 600 °C; solar concentrators would be used to generate such high temperatures from sunlight.

Even at high temperatures, the photon-enhanced thermionic converter will generate more heat than it can use; Melosh says this heat could be coupled to a steam engine for a solar-energy-to-electricity conversion efficiency exceeding 50 percent. These systems are likely to be too complex and expensive for small-scale rooftop installations. But they could be economical for large solar-farm installations, says Melosh, a professor of materials science and engineering. He hopes to have a device ready for commercial development in three years.

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Trapping Sunlight With Silicon Nanowires

Trapping Sunlight With Silicon Nanowires

  4:38:00 PM    Science Lover

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Solar cells made from silicon are projected to be a prominent factor in future renewable green energy equations, but so far the promise has far exceeded the reality. While there are now silicon photovoltaics that can convert sunlight into electricity at impressive 20 percent efficiencies, the cost of this solar power is prohibitive for large-scale use. Researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab), however, are developing a new approach that could substantially reduce these costs. The key to their success is a better way of trapping sunlight.

This photovoltaic cell is comprised of 36 individual arrays of silicon nanowires featuring radial p-n junctions. The color dispersion demonstrates the excellent periodicity over the entire substrate. (Credit: Photo from Peidong Yang)

"Through the fabrication of thin films from ordered arrays of vertical silicon nanowires we've been able to increase the light-trapping in our solar cells by a factor of 73," says chemist Peidong Yang, who led this research. "Since the fabrication technique behind this extraordinary light-trapping enhancement is a relatively simple and scalable aqueous chemistry process, we believe our approach represents an economically viable path toward high-efficiency, low-cost thin-film solar cells."

Yang holds joint appointments with Berkeley Lab's Materials Sciences Division, and the University of California Berkeley's Chemistry Department. He is a leading authority on semiconductor nanowires -- one-dimensional strips of materials whose width measures only one-thousandth that of a human hair but whose length may stretch several microns.

"Typical solar cells are made from very expensive ultrapure single crystal silicon wafers that require about 100 micrometers of thickness to absorb most of the solar light, whereas our radial geometry enables us to effectively trap light with nanowire arrays fabricated from silicon films that are only about eight micrometers thick," he says. "Furthermore, our approach should in principle allow us to use metallurgical grade or "dirty" silicon rather than the ultrapure silicon crystals now required, which should cut costs even further."

Yang has described this research in a paper published in the journal NANO Letters, which he co-authored with Erik Garnett, a chemist who was then a member of Yang's research group.

Generating Electricity from Sunlight

At the heart of all solar cells are two separate layers of material, one with an abundance of electrons that functions as a negative pole, and one with an abundance of electron holes (positively-charged energy spaces) that functions as a positive pole. When photons from the sun are absorbed, their energy is used to create electron-hole pairs, which are then separated at the interface between the two layers and collected as electricity.

Because of its superior photo-electronic properties, silicon remains the photovoltaic semiconductor of choice but rising demand has inflated the price of the raw material. Furthermore, because of the high-level of crystal purification required, even the fabrication of the simplest silicon-based solar cell is a complex, energy-intensive and costly process.

Yang and his group are able to reduce both the quantity and the quality requirements for silicon by using vertical arrays of nanostructured radial p-n junctions rather than conventional planar p-n junctions. In a radial p-n junction, a layer of n-type silicon forms a shell around a p-type silicon nanowire core. As a result, photo-excited electrons and holes travel much shorter distances to electrodes, eliminating a charge-carrier bottleneck that often arises in a typical silicon solar cell. The radial geometry array also, as photocurrent and optical transmission measurements by Yang and Garrett revealed, greatly improves light trapping.

"Since each individual nanowire in the array has a p-n junction, each acts as an individual solar cell," Yang says. "By adjusting the length of the nanowires in our arrays, we can increase their light-trapping path length."

While the conversion efficiency of these solar nanowires was only about five to six percent, Yang says this efficiency was achieved with little effort put into surface passivation, antireflection, and other efficiency-increasing modifications.

"With further improvements, most importantly in surface passivation, we think it is possible to push the efficiency to above 10 percent," Yang says.

Combining a 10 percent or better conversion efficiency with the greatly reduced quantities of starting silicon material and the ability to use metallurgical grade silicon, should make the use of silicon nanowires an attractive candidate for large-scale development.

As an added plus Yang says, "Our technique can be used in existing solar panel manufacturing processes."

This research was funded by the National Science Foundation's Center of Integrated Nanomechanical Systems.

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Photosynthesis: A New Source of Electrical Energy? Biofuel Cell Works in Cactus

Photosynthesis: A New Source of Electrical Energy? Biofuel Cell Works in Cactus

  1:08:00 AM    Science Lover

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Scientists in France have transformed the chemical energy generated by photosynthesis into electrical energy by developing a novel biofuel cell. The advance offers a new strategy to convert solar energy into electrical energy in an environmentally-friendly and renewable manner. In addition, the biofuel cell could have important medical applications.

Biofuel cell inserted in a cactus and graph showing the course of electrical current as a function of illumination of the cactus (black: glucose, red: O2)

These findings have just been published in the journal Analytical Chemistry.

Photosynthesis is the process by which plants convert solar energy into chemical energy. In the presence of visible light, carbon dioxide (CO₂) and water (H₂0) are transformed into glucose and O₂ during a complex series of chemical reactions. Researchers at the Centre de Recherche Paul Pascal (CNRS) developed a biofuel cell that functions using the products of photosynthesis (glucose and O₂) and is made up of two enzyme-modified electrodes.

The cell was then inserted in a living plant, in this case a cactus. Once the electrodes, highly sensitive to O₂ and glucose, had been implanted in the cactus leaf, the scientists succeeded in monitoring the real-time course of photosynthesis in vivo. They were able to observe an increase in electrical current when a desk lamp was switched on, and a reduction when it was switched off. During these experiments, the scientists were also able to make the first ever observation of the real-time course of glucose levels during photosynthesis. This method could offer a new means of better understanding the mechanisms of photosynthesis.

Furthermore, the researchers showed that a biofuel cell inserted in a cactus leaf could generate power of 9 μW per cm². Because this yield was proportional to light intensity, stronger illumination accelerated the production of glucose and O₂ (photosynthesis), so more fuel was available to operate the cell. In the future, this system could ultimately form the basis for a new strategy for the environmentally-friendly and renewable transformation of solar energy into electrical energy.

Alongside these results, the initial objective of this work was to develop a biofuel cell for medical applications. This could then function autonomously under the skin (in vivo), drawing chemical energy from the oxygen-glucose couple that is naturally present in physiological fluids. It could thus provide power for implanted medical devices such as, for example, autonomous subcutaneous sensors to measure glucose levels in diabetic patients.

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Carbon Nanotubes Could Make Efficient Solar Cells

Carbon Nanotubes Could Make Efficient Solar Cells

  1:37:00 PM    Science Lover

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Using a carbon nanotube instead of traditional silicon, Cornell researchers have created the basic elements of a solar cell that hopefully will lead to much more efficient ways of converting light to electricity than now used in calculators and on rooftops.

In a carbon nanotube-based photodiode, electrons (blue) and holes (red) - the positively charged areas where electrons used to be before becoming excited - release their excess energy to efficiently create more electron-hole pairs when light is shined on the device. (Credit: Nathan Gabor)

The researchers fabricated, tested and measured a simple solar cell called a photodiode, formed from an individual carbon nanotube. Reported online Sept. 11 in the journal Science, the researchers -- led by Paul McEuen, the Goldwin Smith Professor of Physics, and Jiwoong Park, assistant professor of chemistry and chemical biology -- describe how their device converts light to electricity in an extremely efficient process that multiplies the amount of electrical current that flows. This process could prove important for next-generation high efficiency solar cells, the researchers say.

"We are not only looking at a new material, but we actually put it into an application -- a true solar cell device," said first author Nathan Gabor, a graduate student in McEuen's lab.

The researchers used a single-walled carbon nanotube, which is essentially a rolled-up sheet of graphene, to create their solar cell. About the size of a DNA molecule, the nanotube was wired between two electrical contacts and close to two electrical gates, one negatively and one positively charged. Their work was inspired in part by previous research in which scientists created a diode, which is a simple transistor that allows current to flow in only one direction, using a single-walled nanotube. The Cornell team wanted to see what would happen if they built something similar, but this time shined light on it.

Shining lasers of different colors onto different areas of the nanotube, they found that higher levels of photon energy had a multiplying effect on how much electrical current was produced.

Further study revealed that the narrow, cylindrical structure of the carbon nanotube caused the electrons to be neatly squeezed through one by one. The electrons moving through the nanotube became excited and created new electrons that continued to flow. The nanotube, they discovered, may be a nearly ideal photovoltaic cell because it allowed electrons to create more electrons by utilizing the spare energy from the light.

This is unlike today's solar cells, in which extra energy is lost in the form of heat, and the cells require constant external cooling.

Though they have made a device, scaling it up to be inexpensive and reliable would be a serious challenge for engineers, Gabor said.

"What we've observed is that the physics is there," he said.

The research was supported by Cornell's Center for Nanoscale Systems and the Cornell NanoScale Science and Technology Facility, both National Science Foundation facilities, as well as the Microelectronics Advanced Research Corporation Focused Research Center on Materials, Structures and Devices. Research collaborators also included Zhaohui Zhong, of the University of Michigan, and Ken Bosnick, of the National Institute for Nanotechnology at University of Alberta.

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