Solar Cells Use Nanoparticles to Capture More Sunlight

Optical antennas could help solar cells produce more energy.

By Katherine Bourzac

Solar antenna: The square at the center is an array of test solar cells being used to evaluate a coating that contains metallic nanoantennas tuned to the solar spectrum.
Credit: Brongersma lab, Stanford

Inexpensive thin-film solar cells aren't as efficient as conventional solar cells, but a new coating that incorporates nanoscale metallic particles could help close the gap. Broadband Solar, a startup spun out of Stanford University late last year, is developing coatings that increase the amount of light these solar cells absorb.

Based on computer models and initial experiments, an amorphous silicon cell could jump from converting about 8 percent of the energy in light into electricity to converting around 12 percent. That would make such cells competitive with the leading thin-film solar cells produced today, such as those made by First Solar, headquartered in Tempe, AZ, says Cyrus Wadia, codirector of the Cleantech to Market Program in the Haas School of Business at the University of California, Berkeley. Amorphous silicon has the advantage of being much more abundant than the materials used by First Solar. The coatings could also be applied to other types of thin-film solar cells, including First Solar's, to increase their efficiency.

Broadband believes its coatings won't increase the cost of these solar cells because they perform the same function as the transparent conductors used on all thin-film cells and could be deposited using the same equipment.

Broadband's nanoscale metallic particles take incoming light and redirect it along the plane of the solar cell, says Mark Brongersma, professor of materials science and engineering at Stanford and scientific advisor to the company. As a result, each photon takes a longer path through the material, increasing its chances of dislodging an electron before it can reflect back out of the cell. The nanoparticles also increase light absorption by creating strong local electric fields.

The particles, which are essentially nanoscale antennas, are very similar to radio antennas, says Brongersma. They're much smaller because the wavelengths they interact with are much shorter than radio waves. Just as conventional antennas can convert incoming radio waves into an electrical signal and transmit electrical signals as radio waves, these nanoantennas rely on electrical interactions to receive and transmit light in the optical spectrum.

Their interaction with light is so strong because incoming photons actually couple to the surface of metal nanoparticles in the form of surface waves called plasmons. These so-called plasmonic effects occur in nanostructures made from highly conductive metals such as copper, silver, and gold. Researchers are taking advantage of plasmonic effects to miniaturize optical computers, and to create higher-resolution light microscopes and lithography. Broadband is one of the first companies working to commercialize plasmonic solar cells.

In his lab at Stanford, Brongersma has experimented with different sizes and shapes of metallic nanostructures, using electron-beam lithography to carve them out one at a time. Different sizes and shapes of metal particles interact strongly with different colors of light, and will direct them at varying angles. The ideal solar-cell coating would contain nanoantennas varying in size and shape over just the right range to take advantage of all the wavelengths in the solar spectrum and send them through the cell at wide angles. However, this carving process is too laborious to be commercialized.

Through his work with Broadband, Brongersma is developing a much simpler method for making the tiny antennas over large areas. This involves a technique called "sputter deposition" that's commonly used in industry to make thin metal films (including those that line some potato-chip bags). Sputtering works by bombarding a substrate with ionized metal. Under the right conditions, he says, "due to surface tension, the metal balls up into particles like water droplets on a waxed car." The resulting nanoparticles vary in shape and size, which means they'll interact with different wavelengths of light. "We rely on this randomness" to make the films responsive to the broad spectrum found in sunlight, he says.

Broadband is currently developing sputtering techniques for incorporating metal nanoantennas into transparent conductive oxide films over large areas. Being able to match the large scale of thin-film solar manufacturing will be key to commercializing these coatings.

The company has been using money from angel investors to test its plasmonic coatings on small prototype cells. So far, says Brongersma, enhanced current from the cells matches simulations. Broadband is currently seeking venture funding to scale up its processes, says CEO Anthony Defries.

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Scientists Turn Light Into Electrical Current Using a Golden Nanoscale System

ScienceDaily (Feb. 12, 2010) — Material scientists at the Nano/Bio Interface Center of the University of Pennsylvania have demonstrated the transduction of optical radiation to electrical current in a molecular circuit. The system, an array of nano-sized molecules of gold, respond to electromagnetic waves by creating surface plasmons that induce and project electrical current across molecules, similar to that of photovoltaic solar cells.

The results may provide a technological approach for higher efficiency energy harvesting with a nano-sized circuit that can power itself, potentially through sunlight. Recently, surface plasmons have been engineered into a variety of light-activated devices such as biosensors.

It is also possible that the system could be used for computer data storage. While the traditional computer processor represents data in binary form, either on or off, a computer that used such photovoltaic circuits could store data corresponding to wavelengths of light.

Because molecular compounds exhibit a wide range of optical and electrical properties, the strategies for fabrication, testing and analysis elucidated in this study can form the basis of a new set of devices in which plasmon-controlled electrical properties of single molecules could be designed with wide implications to plasmonic circuits and optoelectronic and energy-harvesting devices.

Dawn Bonnell, a professor of materials science and the director of the Nano/Bio Interface Center at Penn, and colleagues fabricated an array of light sensitive, gold nanoparticles, linking them on a glass substrate. Minimizing the space between the nanoparticles to an optimal distance, researchers used optical radiation to excite conductive electrons, called plasmons, to ride the surface of the gold nanoparticles and focus light to the junction where the molecules are connected. The plasmon effect increases the efficiency of current production in the molecule by a factor of 400 to 2000 percent, which can then be transported through the network to the outside world.

In the case where the optical radiation excites a surface plasmon and the nanoparticles are optimally coupled, a large electromagnetic field is established between the particles and captured by gold nanoparticles. The particles then couple to one another, forming a percolative path across opposing electrodes. The size, shape and separation can be tailored to engineer the region of focused light. When the size, shape and separation of the particles are optimized to produce a "resonant" optical antennae, enhancement factors of thousands might result.

Furthermore, the team demonstrated that the magnitude of the photoconductivity of the plasmon-coupled nanoparticles can be tuned independently of the optical characteristics of the molecule, a result that has significant implications for future nanoscale optoelectronic devices.

"If the efficiency of the system could be scaled up without any additional, unforeseen limitations, we could conceivably manufacture a one-amp, one-volt sample the diameter of a human hair and an inch long," Bonnell said.

The study, published in the current issue of the journal ACS Nano, was conducted by Bonnell, David Conklin and Sanjini Nanayakkara of the Department of Materials Science and Engineering in the School of Engineering and Applied Science at Penn; Tae-Hong Park of the Department of Chemistry in the School of Arts and Sceicnes at Penn; Parag Banerjee of the Department of Materials Science and Engineering at the University of Maryland; and Michael J. Therien of the Department of Chemistry at Duke University.

This work was supported by the Nano/Bio Interface Center, National Science Foundation, the John and Maureen Hendricks Energy Fellowship and the U.S. Department of Energy.

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Plasmon-Induced Electrical Conduction in Molecular Devices

• Abstract
• Full Text HTML
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• Supporting Info

Parag Banerjee†, David Conklin‡, Sanjini Nanayakkara‡, Tae-Hong Park§, Michael J. Therien and Dawn A. Bonnell‡*

^† Department of Materials Science and Engineering, University of Maryland, College Park, Maryland, 20742

^‡ Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, 19104

^§ Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania, 19104

Department of Chemistry, Duke University, Durham, North Carolina, 27708

ACS Nano, Article ASAP

DOI: 10.1021/nn901148m

Publication Date (Web): January 22, 2010

Copyright © 2010 American Chemical Society

* Address correspondence to bonnell@lrsm.upenn.edu.

Abstract

Metal nanoparticles (NPs) respond to electromagnetic waves by creating surface plasmons (SPs), which are localized, collective oscillations of conduction electrons on the NP surface. When interparticle distances are small, SPs generated in neighboring NPs can couple to one another, creating intense fields. The coupled particles can then act as optical antennae capturing and refocusing light between them. Furthermore, a molecule linking such NPs can be affected by these interactions as well. Here, we show that by using an appropriate, highly conjugated multiporphyrin chromophoric wire to couple gold NP arrays, plasmons can be used to control electrical properties. In particular, we demonstrate that the magnitude of the observed photoconductivity of covalently interconnected plasmon-coupled NPs can be tuned independently of the optical characteristics of the molecule—a result that has significant implications for future nanoscale optoelectronic devices.

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Glitter-sized solar photovoltaics produce competitive results

December 22 2009

Enlarge

These are representative thin crystalline-silicon photovoltaic cells -- these are from 14 to 20 micrometers thick and 0.25 to 1 millimeter across. Credit: Murat Okandan

Sandia National Laboratories scientists have developed tiny glitter-sized photovoltaic cells that could revolutionize the way solar energy is collected and used.

The tiny cells could turn a person into a walking solar battery charger if they were fastened to flexible substrates molded around unusual shapes, such as clothing.

The solar particles, fabricated of crystalline silicon, hold the potential for a variety of new applications. They are expected eventually to be less expensive and have greater efficiencies than current photovoltaic collectors that are pieced together with 6-inch- square solar wafers.

The cells are fabricated using microelectronic and microelectromechanical systems (MEMS) techniques common to today's electronic foundries.

Sandia lead investigator Greg Nielson said the research team has identified more than 20 benefits of scale for its microphotovoltaic cells. These include new applications, improved performance, potential for reduced costs and higher efficiencies.

"Eventually units could be mass-produced and wrapped around unusual shapes for building-integrated solar, tents and maybe even clothing," he said. This would make it possible for hunters, hikers or military personnel in the field to recharge batteries for phones, cameras and other electronic devices as they walk or rest.

Even better, such microengineered panels could have circuits imprinted that would help perform other functions customarily left to large-scale construction with its attendant need for field construction design and permits.

Said Sandia field engineer Vipin Gupta, "Photovoltaic modules made from these microsized cells for the rooftops of homes and warehouses could have intelligent controls, inverters and even storage built in at the chip level. Such an integrated module could greatly simplify the cumbersome design, bid, permit and grid integration process that our solar technical assistance teams see in the field all the time."

For large-scale power generation, said Sandia researcher Murat Okandan, "One of the biggest scale benefits is a significant reduction in manufacturing and installation costs compared with current PV techniques."

Part of the potential cost reduction comes about because microcells require relatively little material to form well-controlled and highly efficient devices.

From 14 to 20 micrometers thick (a human hair is approximately 70 micrometers thick), they are 10 times thinner than conventional 6-inch-by-6-inch brick-sized cells, yet perform at about the same efficiency.

Enlarge

Sandia project lead Greg Nielson holds a solar cell test prototype with a microscale lens array fastened above it. Together, the cell and lens help create a concentrated photovoltaic unit. Credit: Randy Montoya

100 times less silicon generates same amount of electricity

"So they use 100 times less silicon to generate the same amount of electricity," said Okandan. "Since they are much smaller and have fewer mechanical deformations for a given environment than the conventional cells, they may also be more reliable over the long term.

"Another manufacturing convenience is that the cells, because they are only hundreds of micrometers in diameter, can be fabricated from commercial wafers of any size, including today's 300-millimeter (12-inch) diameter wafers and future 450-millimeter (18-inch) wafers. Further, if one cell proves defective in manufacture, the rest still can be harvested, while if a brick-sized unit goes bad, the entire wafer may be unusable. Also, brick-sized units fabricated larger than the conventional 6-inch-by-6-inch cross section to take advantage of larger wafer size would require thicker power lines to harvest the increased power, creating more cost and possibly shading the wafer. That problem does not exist with the small-cell approach and its individualized wiring.

Other unique features are available because the cells are so small. "The shade tolerance of our units to overhead obstructions is better than conventional PV panels," said Nielson, "because portions of our units not in shade will keep sending out electricity where a partially shaded conventional panel may turn off entirely.

"Because flexible substrates can be easily fabricated, high-efficiency PV for ubiquitous solar power becomes more feasible, said Okandan.

A commercial move to microscale PV cells would be a dramatic change from conventional silicon PV modules composed of arrays of 6-inch-by-6-inch wafers. However, by bringing in techniques normally used in MEMS, electronics and the light-emitting diode (LED) industries (for additional work involving gallium arsenide instead of silicon), the change to small cells should be relatively straightforward, Gupta said.

Each cell is formed on silicon wafers, etched and then released inexpensively in hexagonal shapes, with electrical contacts prefabricated on each piece, by borrowing techniques from integrated circuits and MEMS.

Offering a run for their money to conventional large wafers of crystalline silicon, electricity presently can be harvested from the Sandia-created cells with 14.9 percent efficiency. Off-the-shelf commercial modules range from 13 to 20 percent efficient.

A widely used commercial tool called a pick-and-place machine — the current standard for the mass assembly of electronics — can place up to 130,000 pieces of glitter per hour at electrical contact points preestablished on the substrate; the placement takes place at cooler temperatures. The cost is approximately one-tenth of a cent per piece with the number of cells per module determined by the level of optical concentration and the size of the die, likely to be in the 10,000 to 50,000 cell per square meter range. An alternate technology, still at the lab-bench stage, involves self-assembly of the parts at even lower costs.

Solar concentrators — low-cost, prefabricated, optically efficient microlens arrays — can be placed directly over each glitter-sized cell to increase the number of photons arriving to be converted via the photovoltaic effect into electrons. The small cell size means that cheaper and more efficient short focal length microlens arrays can be fabricated for this purpose.

High-voltage output is possible directly from the modules because of the large number of cells in the array. This should reduce costs associated with wiring, due to reduced resistive losses at higher voltages.

Other possible applications for the technology include satellites and remote sensing.

Provided by Sandia National Laboratories

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Hot Electrons Could Double Solar Power

A novel approach could turn more sunlight into electricity.

By Kevin Bullis FRIDAY, DECEMBER 18, 2009

For decades researchers have investigated a theoretical means to double the power output of solar cells--by making use of so-called "hot electrons." Now researchers at Boston College have provided new experimental evidence that the theory will work. They built solar cells that get a power boost from high-energy photons. This boost, the researchers say, is the result of extracting hot electrons.

Hot solar: This solar cell is made of thin layers of amorphous silicon with aluminum dots serving as back electrical contacts. It provides evidence that it may be possible to double the output of solar cells. Credit: Michael Naughton

The results are a step toward solar cells that break conventional efficiency limits. Because of the way ordinary solar cells work, they can, in theory, convert at most about 35 percent of the energy in sunlight into electricity, wasting the rest as heat. Making use of hot electrons could result in efficiencies as high as 67 percent, says Matthew Beard, a senior scientist at the National Renewable Energy Laboratory in Golden, CO, who was not involved in the current work. Doubling the efficiency of solar cells could cut the cost of solar power in half.

Conventional solar cells can only efficiently convert the energy of certain wavelengths of light into electricity. For example, when a solar cell optimized for red wavelengths of light absorbs photons of red light, it produces electrons with energy levels similar to those of the incoming photons. When the cell absorbs a higher-energy blue photon, it first produces a similarly high-energy electron--a hot electron. But this loses much of its energy very quickly as heat before it can escape the cell to produce electricity. (Conversely, cells optimized for blue light don't convert red light into electricity, so they sacrifice the energy in this part of the spectrum.)

The Boston College researchers made ultra-thin solar cells just 15 nanometers thick. Because the cells were so thin, the hot electrons could be pulled out of the cell quickly, before they cooled. The researchers found that the voltage output of the cells increased when they illuminated them with blue light rather than red. "Now we're getting the electrons from the blue light out before they lose all of their excess energy," says Michael Naughton, a professor of physics at Boston College.

The problem is that because they're so thin, the solar cells let most of the incoming light pass through them. As a result, they convert only 3 percent of the energy in incoming light into electricity. "I think it's promising," Beard says. But he adds that so far they're only showing "a pretty small effect."

Naughton says that his team plans to address this problem using nanowires. The basic idea,put forward by many different researchers now, is to make forests of nanowires that will absorb light along their lengths. And because each nanowire is thin, the electrons won't have far to travel to escape to a conductive layer on its surface. This could make it possible to replicate the hot electron effect seen in the thin solar cells. Naughton and colleagues are commercializing such nanowires via a startup called Solasta, based in Newton, MA, which is being funded by the respected venture capital firm Kleiner Perkins Caufield & Byers.

The researchers also hope to increase the number of hot electrons they collect from the absorbed light. To do this, they are turning to an approach taken by Martin Green, a professor at the University of New South Wales in Australia and a leader in using hot electrons in solar cells. This method involves incorporating a layer of quantum dots, which act as a sort-of filter, selectively extracting higher-than-normal-voltage electrons, Beard says. Naughton says that Solasta has already demonstrated that it's possible to incorporate such quantum dots into the company's nanowires.

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Carbon nanotubes could make efficient solar cells

September 10th, 2009 By Anne Ju

Enlarge

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. Image: Nathan Gabor

(PhysOrg.com) -- Cornell researchers fabricated, tested and measured a simple solar cell called a photodiode, formed from an individual carbon nanotube.

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.

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.

Source

Nanoparticle ink could print solar cells like newspaper

28 August 2009

Country: United States

Nanoparticle inks could be used to print solar cells like newspaper, or painted onto the side of buildings or rooftops to produce electricity.

Manufacturing solar cells using gas-phase deposition in a vacuum chamber requiring high temperatures is relatively expensive. However, according to Professor Brian Korgel, University of Texas, Austin, and co-founder of Innovalight, it may be possible to reduce these costs by an amazing 90% - making the price of solar energy more competitive with other technologies.

Chemical engineering Professor Brian Korgel tests one of his printed solar cells. Source: University of Texas.

His team has been working on a solution of nanomaterials using copper indium gallium selenide (CIGS) which is not only cheaper than silicon but you also use less of it when manufacturing solar cells. The light-absorbing nanomaterials are 10,000 times thinner than a strand of hair, because their microscopic size allows for new physical properties that can help enable higher-efficiency devices.

Researchers apply the nanoparticle "inks" as a spray on the solar cells. Source: University of Texas.

His team has only developed a solar cell prototype at 1% efficiency, but in order to make the product commercially viable, they need to achieve 10% efficiencies, which Korgel believes could be possible within the next 3-5 years.

Not only could the inks, which are semi-transparent be printed on a roll-to-roll printing process on a plastic substrate or stainless steel, they could also be used on windows that double up as solar cells. Korgel says the prospect of being able to paint the "inks" onto a rooftop or building is not far-fetched - "you would have to paint the light-absorbing material and a few other layers as well," he concludes.

They recently demonstrated proof-of-concept in a recent issue of "Journal of the American Chemical Society". The work has attracted the interest of industrial partners.

Innovalight recently installed a system to inkjet print silicon-ink, which they claim can halve the number of costly manufacturing processes required to produce highly efficient solar cells.

Whilst researchers at the UCLA Henry Samueli School of Engineering and Applied Science have developed a low-cost solution processing method for CIGS-based solar cells . They used hydrazine as the solvent to dissolve copper sulfide and indium selenide in order to form the constituents for the copper-indium-diselenide material. Their material, which is simply dissolved into a liquid, can be easily painted or coated evenly onto a surface and baked. The solar cells achieved over 9% efficiency in the lab.

Top image: At the core of Korgel's research are the nanoparticle "inks" (as shown here) which are the sunlight-absorbing materials of his solar cells. Source: University of Texas.

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Solar Power Game-changer: 'Near Perfect' Absorption Of Sunlight, From All Angles

ScienceDaily (Nov. 4, 2008) — Researchers at Rensselaer Polytechnic Institute have discovered and demonstrated a new method for overcoming two major hurdles facing solar energy. By developing a new antireflective coating that boosts the amount of sunlight captured by solar panels and allows those panels to absorb the entire solar spectrum from nearly any angle, the research team has moved academia and industry closer to realizing high-efficiency, cost-effective solar power.

“To get maximum efficiency when converting solar power into electricity, you want a solar panel that can absorb nearly every single photon of light, regardless of the sun’s position in the sky,” said Shawn-Yu Lin, professor of physics at Rensselaer and a member of the university’s Future Chips Constellation, who led the research project. “Our new antireflective coating makes this possible.”

An untreated silicon solar cell only absorbs 67.4 percent of sunlight shone upon it — meaning that nearly one-third of that sunlight is reflected away and thus unharvestable. From an economic and efficiency perspective, this unharvested light is wasted potential and a major barrier hampering the proliferation and widespread adoption of solar power.

After a silicon surface was treated with Lin’s new nanoengineered reflective coating, however, the material absorbed 96.21 percent of sunlight shone upon it — meaning that only 3.79 percent of the sunlight was reflected and unharvested. This huge gain in absorption was consistent across the entire spectrum of sunlight, from UV to visible light and infrared, and moves solar power a significant step forward toward economic viability.

Lin’s new coating also successfully tackles the tricky challenge of angles.

Most surfaces and coatings are designed to absorb light — i.e., be antireflective — and transmit light — i.e., allow the light to pass through it — from a specific range of angles. Eyeglass lenses, for example, will absorb and transmit quite a bit of light from a light source directly in front of them, but those same lenses would absorb and transmit considerably less light if the light source were off to the side or on the wearer’s periphery.

This same is true of conventional solar panels, which is why some industrial solar arrays are mechanized to slowly move throughout the day so their panels are perfectly aligned with the sun’s position in the sky. Without this automated movement, the panels would not be optimally positioned and would therefore absorb less sunlight. The tradeoff for this increased efficiency, however, is the energy needed to power the automation system, the cost of upkeeping this system, and the possibility of errors or misalignment.

Lin’s discovery could antiquate these automated solar arrays, as his antireflective coating absorbs sunlight evenly and equally from all angles. This means that a stationary solar panel treated with the coating would absorb 96.21 percent of sunlight no matter the position of the sun in the sky. So along with significantly better absorption of sunlight, Lin’s discovery could also enable a new generation of stationary, more cost-efficient solar arrays.

“At the beginning of the project, we asked ‘would it be possible to create a single antireflective structure that can work from all angles?’ Then we attacked the problem from a fundamental perspective, tested and fine-tuned our theory, and created a working device,” Lin said. Rensselaer physics graduate student Mei-Ling Kuo played a key role in the investigations.

Typical antireflective coatings are engineered to transmit light of one particular wavelength. Lin’s new coating stacks seven of these layers, one on top of the other, in such a way that each layer enhances the antireflective properties of the layer below it. These additional layers also help to “bend” the flow of sunlight to an angle that augments the coating’s antireflective properties. This means that each layer not only transmits sunlight, it also helps to capture any light that may have otherwise been reflected off of the layers below it.

The seven layers, each with a height of 50 nanometers to 100 nanometers, are made up of silicon dioxide and titanium dioxide nanorods positioned at an oblique angle — each layer looks and functions similar to a dense forest where sunlight is “captured” between the trees. The nanorods were attached to a silicon substrate via chemical vapor disposition, and Lin said the new coating can be affixed to nearly any photovoltaic materials for use in solar cells, including III-V multi-junction and cadmium telluride.

Along with Lin and Kuo, co-authors of the paper include E. Fred Schubert, Wellfleet Senior Constellation Professor of Future Chips at Rensselaer; Research Assistant Professor Jong Kyu Kim; physics graduate student David Poxson; and electrical engineering graduate student Frank Mont.

Funding for the project was provided by the U.S. Department of Energy’s Office of Basic Energy Sciences, as well as the U.S. Air Force Office of Scientific Research.

---

Journal reference:

1. Kuo et al. Realization of a near-perfect antireflection coating for silicon solar energy utilization. Optics Letters, 2008; 33 (21): 2527 DOI: 10.1364/OL.33.002527

Adapted from materials provided by Rensselaer Polytechnic Institute.

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MIT opens new 'window' on solar energy

Cost effective devices expected on market soon

Elizabeth A. Thomson, News Office
July 10, 2008

Imagine windows that not only provide a clear view and illuminate rooms, but also use sunlight to efficiently help power the building they are part of. MIT engineers report a new approach to harnessing the sun's energy that could allow just that.

The work, to be reported in the July 11 issue of Science, involves the creation of a novel "solar concentrator." "Light is collected over a large area [like a window] and gathered, or concentrated, at the edges," explains Marc A. Baldo, leader of the work and the Esther and Harold E. Edgerton Career Development Associate Professor of Electrical Engineering.

As a result, rather than covering a roof with expensive solar cells (the semiconductor devices that transform sunlight into electricity), the cells only need to be around the edges of a flat glass panel. In addition, the focused light increases the electrical power obtained from each solar cell "by a factor of over 40," Baldo says.

Because the system is simple to manufacture, the team believes that it could be implemented within three years--even added onto existing solar-panel systems to increase their efficiency by 50 percent for minimal additional cost. That, in turn, would substantially reduce the cost of solar electricity.

• Fact sheet: MIT's solar concentrators

In addition to Baldo, the researchers involved are Michael Currie, Jon Mapel, and Timothy Heidel, all graduate students in the Department of Electrical Engineering and Computer Science, and Shalom Goffri, a postdoctoral associate in MIT's Research Laboratory of Electronics.

"Professor Baldo's project utilizes innovative design to achieve superior solar conversion without optical tracking," says Dr. Aravinda Kini, program manager in the Office of Basic Energy Sciences in the U.S. Department of Energy's Office of Science, a sponsor of the work. "This accomplishment demonstrates the critical importance of innovative basic research in bringing about revolutionary advances in solar energy utilization in a cost-effective manner."

Solar concentrators in use today "track the sun to generate high optical intensities, often by using large mobile mirrors that are expensive to deploy and maintain," Baldo and colleagues write in Science. Further, "solar cells at the focal point of the mirrors must be cooled, and the entire assembly wastes space around the perimeter to avoid shadowing neighboring concentrators."

The MIT solar concentrator involves a mixture of two or more dyes that is essentially painted onto a pane of glass or plastic. The dyes work together to absorb light across a range of wavelengths, which is then re-emitted at a different wavelength and transported across the pane to waiting solar cells at the edges.

In the 1970s, similar solar concentrators were developed by impregnating dyes in plastic. But the idea was abandoned because, among other things, not enough of the collected light could reach the edges of the concentrator. Much of it was lost en route.

The MIT engineers, experts in optical techniques developed for lasers and organic light-emitting diodes, realized that perhaps those same advances could be applied to solar concentrators. The result? A mixture of dyes in specific ratios, applied only to the surface of the glass, that allows some level of control over light absorption and emission. "We made it so the light can travel a much longer distance," Mapel says. "We were able to substantially reduce light transport losses, resulting in a tenfold increase in the amount of power converted by the solar cells."

This work was also supported by the National Science Foundation. Baldo is also affiliated with MIT's Research Laboratory of Electronics, Microsystems Technology Laboratories, and Institute for Soldier Nanotechnologies.

Mapel, Currie and Goffri are starting a company, Covalent Solar, to develop and commercialize the new technology. Earlier this year Covalent Solar won two prizes in the MIT $100K Entrepreneurship Competition. The company placed first in the Energy category ($20,000) and won the Audience Judging Award ($10,000), voted on by all who attended the awards.

Video

Click To Play. Marc Baldo discusses MIT's solar concentrator

Images

Image courtesy / Nicolle Rager Fuller, NSF

An artist's representation shows how a cost effective solar concentrator could help make existing solar panels more efficient. Enlarge image

Photo / Donna Coveney

Organic solar concentrators collect and focus different colors of sunlight. Solar cells can be attached to the edges of the plates. By collecting light over their full surface and concentrating it at their edges, these devices reduce the required area of solar cells and consequently, the cost of solar power. Stacking multiple concentrators allows the optimization of solar cells at each wavelength, increasing the overall power output. Enlarge image

Photo / Donna Coveney

Marc Baldo, associate professor of electrical engineering and computer science (left) and Shalom Goffri, postdoc in MIT's Research Laboratory of Electronics (right) hold examples of organic solar concentrators. Enlarge image

Teresa Herbert
MIT News Office
Phone: 617-258-5403
E-mail: therbert@mit.edu

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Nanosolar Printing Video

Nanosolar's proprietary nanoparticle ink.

Applied Nanotech, Inc. - SBIR & STTR Grants

DEPARTMENT OF ENERGY

SMALL BUSINESS INNOVATION RESEARCH PROGRAM

AND

SMALL BUSINESS TECHNOLOGY TRANSFER PROGRAM

PHASE I GRANT APPLICATIONS SELECTED FOR FY 2008 AWARDS BY TOPIC

TOPIC: ADVANCED COAL RESEARCH

STTR Project

Applied Nanotech, Inc. Advanced Coal Research, New CO2 Monitoring Devices

3006 Longhorn Boulevard

Suite 107

Austin, TX 78758-7631

Scientists have proposed reducing greenhouse gas by injecting carbon dioxide (CO₂) into geologic subsurface structures. Sophisticated monitoring tools are essential to track CO₂ in the geologic structures and assure safe retention. This project will develop and field test an improved CO₂ sensor.

TOPIC: SOLAR ENERGY

Applied Nanotech, Inc. Solar Energy, Materials Solutions for Cells and Modules

3006 Longhorn Boulevard

Suite 107

Austin, TX 78758-7631

This project will develop metal nanoparticle inks and processes for printing photovoltaic cells, thus lowering the costs of fabrication while maintaining or improving cell performance over conventional techniques.

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