1366 Bets on Silicon Wafer Innovation with New Solar Plant

 

New method for making silicon wafers enables 1366 Technologies to survive and expand By Richard Martin on October 14, 2015   Why It Matters Declines in component prices are making solar power nearly as cheap as coal-fired plants. 1366 CEO Frank van Mierlo presents New York governor Andrew Cuomo with a custom silicon wafer at an event announcing the company’s plans to build a commercial-scale factory in Genesee County. Solar wafer maker 1366 Technologies has survived the carnage in the U.S. solar manufacturing industry over the last five years. Now the company, which uses a novel technology for making the silicon wafers used in most solar cells, is embarking on its next phase, building a large manufacturing plant in upstate New York (see “Solar Survivor”). Founded by MIT professor Ely Sachs in 2008, the startup said last week that it will build its first commercial-scale factory in Genesee County, near Rochester. The plant will cost around $100 million and will initially produce about 50 million wafers annually, equivalent to 250 megawatts of power-generation capacity. Eventually, says CEO Frank van Mierlo, 1366 (which is named for the “solar constant,” the amount of solar energy that reaches Earth, in watts per square meter) hopes to reach three gigawatts of annual production at the Genesee plant, supplying silicon wafers to a solar industry that’s growing rapidly in North America, Europe, and the developing world. The Massachusetts-based company says it was lured to New York, as opposed to building in China, through a combination of state incentives, access to cheap hydro power, and the desire to make its products in the United States. The booming market for residential solar and the rising demand from developing countries including India, which has committed to building 100 gigawatts of solar capacity in the next seven years, is driving a comeback for solar manufacturing in the U.S. (see “India’s Energy Crisis”). SolarCity’s planned factory, also being built in upstate New York, will have a gigawatt of production capacity when fully operational at the beginning of 2017 (see “Paying for Solar Power”). Last month, the U.S. unit of China-based Seraphim Solar Manufacturing said it will open a new solar factory in Jackson, Mississippi, that will reach one gigawatt of capacity by 2018. In Hillsboro, Oregon, SolarWorld Americas is spending $10 million to expand its factory, currently the largest solar PV production facility in the Western Hemisphere. A 1366 technician works in a lab at the company’s Bedford, Massachusetts, demonstration facility. Conventional factories produce silicon wafers via a multistep process of sawing, polishing, and slicing that wastes close to half of the silicon. Led by Sachs, who stepped aside as the company’s chief technology officer earlier this year, 1366’s team of several dozen engineers devised a way to make wafers directly from molten silicon, producing wafers for half the cost of traditional methods and dramatically reducing the amount of wasted silicon. The 1366 process also uses one-third the energy to produce each wafer. While the concept of producing wafers from molten silicon is not original to 1366, the company is the first to develop a production technology that can be expanded to commercial scale. In addition to a loan guarantee of $150 million from the U.S. Department of Energy that was made in 2011, the company will receive about $97 million in grants and tax incentives from the state of New York. Van Mierlo says 1366 has binding agreements for 60 percent of the plant’s production; he won’t name customers but says they are concentrated mostly China and Taiwan. However, 1366 still has to raise additional funds to build the Genesee facility. Van Mierlo says that “90 percent of that is secured at this point,” and that discussions are underway for the remaining $10 million. The state investment is part of New York governor Andrew Cuomo’s strategy of turning upstate New York into a major manufacturing hub for the solar industry. The state is investing $750 million to build the SolarCity plant, which it will lease back to the Silicon Valley company, essentially for free. Cuomo, who attended the 1366 launch event on November 7, called the 1366 announcement “a game changer” for New York’s tech sector and the U.S. solar industry. The track record of solar manufacturing companies built largely on public funding is not encouraging. But 1366 has survived so far in part because of its frugality. “The first line in a Dutch cookbook,” says van Mierlo, a U.S. citizen who was born in the Netherlands, “is ‘borrow an egg.’”   http://www.technologyreview.com/news/542416/1366-bets-on-silicon-wafer-innovation-with-new-solar-plant/

Efficiency from larger perovskite solar cells improved

 

A new fabrication method enabled researchers to make larger perovskite cells with few defects, helping to maintain efficiency at larger cell sizes. Credit: Brown University / NREL Using a newly developed fabrication method, a research team has attained better than a 15-percent energy conversion efficiency from perovskite solar cells larger than one square centimeter area. The researchers, from Brown University and the National Renewable Energy Lab (NREL), have reported their findings in the journal Advanced Materials. Perovskites, materials with a particular crystalline structure, have caused quite a buzz in the solar energy world. Perovskite solar cells are relatively cheap to make, and the efficiency with which they can convert sunlight into electricity has been increasing rapidly in recent years. Researchers have reported efficiency in perovskite cells of higher than 20 percent, which rivals traditional silicon cells. Those high efficiency ratings, however, have been achieved using cells only a tenth of a square centimeter -- fine for lab testing, but too small to be used in a solar panel. "The use of tiny cells for efficiency testing has prompted some to question comparison of perovskite solar cells with other established photovoltaic technologies," said Nitin Padture, professor of engineering at Brown, director of Brown's Institute for Molecular and Nanoscale Innovation, and one of the senior authors of the new research. "But here we have shown that it is feasible to obtain 15-percent efficiency on cells larger than a square centimeter through improved processing. This is real progress." Maintaining high efficiency on larger perovskite cells has proved to be a challenge, Padture says. "The problem with perovskite has been that when you try to make larger films using traditional methods, you get defects in the film that decrease efficiency." The fabrication process that the Brown and NREL researchers reported in this latest paper builds on a previously reported method developed by Yuanyuan Zhou, a graduate student in Padture's lab. Perovskite precursors are dissolved in a solvent and coated onto a substrate. Then the substrate is bathed in a second solvent (called anti-solvent) that selectively grabs the precursor-solvent and whisks it away. What's left is an ultra-smooth film of perovskite crystals. In this new study Zhou and Mengjin Yang, a postdoctoral researcher at NREL, developed a trick to grow the perovskite crystals to a larger size. The trick is to add excess organic precursor that initially "glues" the small perovskite crystals and helps them merge into larger ones during a heat-treatment, which then bakes away the excess precursor. "The full coverage and uniformity over a large area come from the solvent method," Padture said. "Once we have that coverage, then we increase the size of the crystals. That gives us a film with fewer defects and higher efficiency." The 15-percent efficiency reached in this latest work is a good start, Padture said, but there's still room to improve. Ultimately, he would like to reach 20 to 25 percent in large-area cells, and he thinks that mark could be within reach using this method or a similar one. Padture and colleagues at the University of Nebraska-Lincoln were recently awarded a $4-million grant by the National Science Foundation to expand their perovskite research. Story Source: The above post is reprinted from materials provided by Brown University. Note: Materials may be edited for content and length. Journal Reference: Mengjin Yang, Yuanyuan Zhou, Yining Zeng, Chun-Sheng Jiang, Nitin P. Padture, Kai Zhu. Square-Centimeter Solution-Processed Planar CH3NH3PbI3Perovskite Solar Cells with Efficiency Exceeding 15%.Advanced Materials, 2015; DOI: 10.1002/adma.201502586   http://www.sciencedaily.com/releases/2015/10/151005132851.htm

Microscopic rake doubles efficiency of low-cost solar cells

 

A scanning electron microscope image shows the rigid pillar-like bristles of the FLUENCE rake, which is used to apply light-harvesting polymers to a solar cell. The distance between the pillars is 1 micrometer, about one-hundredth the diameter of a human hair. Credit: Z. Bao et al, Nature Communications Researchers from the Department of Energy's SLAC National Accelerator Laboratory and Stanford University have developed a manufacturing technique that could double the electricity output of inexpensive solar cells by using a microscopic rake when applying light-harvesting polymers. When commercialized, this advance could help make polymer solar cells an economically attractive alternative to those made with much more expensive silicon-crystal wafers. In experiments, solar cells made with the tiny rake double the efficiency of cells made without it and are 18 percent better than cells made using a microscopic straightedge blade. The research was led by Zhenen Bao, a chemical engineering professor at Stanford and a member of the Stanford Institute for Materials and Energy Sciences (SIMES), which is run jointly by SLAC and Stanford. The team reported its results August 12 in Nature Communications. "The fundamental scientific insights that come out of this work will give manufacturers a rational approach to improving their processes, rather than relying simply on trial and error," Bao said. "We also expect this simple, effective and versatile concept will be broadly applicable to making other polymer devices where properly aligning the molecules is important." The Problem With Polymers Although prices for silicon-based solar cells are dropping, it still takes five to 15 years before they produce enough electricity to offset their purchase and installation. Silicon solar cells also require a large amount of energy to manufacture, which partly offsets their value as renewable energy sources. Polymer-based photovoltaic cells are much cheaper because they're made of inexpensive materials that can be simply painted or printed in place. They are also flexible and require little energy to manufacture. While small, lab-scale samples can convert more than 10 percent of sunlight into electricity, the large-area coated cells have very low efficiency -- typically converting less than 5 percent, compared with 20-25 percent for commercial silicon-based cells. Polymer cells typically combine two types of polymers: A donor, which converts sunlight into electrons, and an acceptor, which stores the electrons until they can be removed from the cell as usable electricity. But when this mixture is deposited on a cell's conducting surface during manufacturing, the two types tend to separate as they dry into an irregular assortment of large clumps, making it more difficult for the cell to produce and harvest electrons. The SLAC/Stanford researchers' solution is a manufacturing technique called "fluid-enhanced crystal engineering," or FLUENCE, which was originally developed to improve the electrical conduction of organic semiconductors. In the current work, as the polymers are painted onto a conducting surface, they are forced through a slightly angled rake containing several rows of stiff microscopic pillars. The rake is scraped along the surface at the relatively slow speed of 25-100 micrometers per second, which translates to 3.5-14.2 inches per hour. The large polymer molecules untangle and mix with each other as they bounce off and flow past the pillars, ultimately drying into tiny nanometer-sized crystals of uniform size with enhanced electrical properties. Simulations and X-rays The researchers used computer simulations and X-ray analyses at two DOE Office of Science User Facilities -- SLAC's Stanford Synchrotron Radiation Lightsource (SSRL) and Lawrence Berkeley National Laboratory's Advanced Light Source (ALS) -- to customize the FLUENCE rake for making solar cells. "At SSRL, the team used X-ray diffraction to measure the degree to which the polymers formed crystals and X-ray scattering to determine how clearly the two polymers segregated themselves," said Mike Toney, SSRL Materials Sciences group leader and a co-author on the paper. "These are bread-and-butter techniques for which we've developed some novel approaches at SSRL in recent years." To achieve the polymer patterns they wanted for the solar cells, the researchers made the pillars in the rake much shorter and more densely packed than those used earlier for organic semiconductors. They were 1.5 micrometers high and 1.2 micrometers apart; for comparison, a human hair is about 100 micrometers in diameter. Close, But Not Too Close "Ideally, the two types of photovoltaic polymers should be close enough to each other for electrons to move quickly from donor to acceptor, but not so close that the acceptor gives back its electrons before they can be harvested to electricity," said Yan Zhou, a Stanford researcher on Bao's team. "Our new FLUENCE rake achieves this happy medium. Because we understand what's happening, we can tune the rake design and processing speed to alter the final polymer structures." Future research will be aimed at applying the FLUENCE technique to other polymer blends and adapting it to rapid industrial-scale roll-to-roll printing processes -- which can reach speeds of 50 miles per hour -- that promise the lowest solar-cell manufacturing costs. Story Source: The above post is reprinted from materials provided by SLAC National Accelerator Laboratory. Note: Materials may be edited for content and length. Journal Reference: Ying Diao, Yan Zhou, Tadanori Kurosawa, Leo Shaw, Cheng Wang, Steve Park, Yikun Guo, Julia A. Reinspach, Kevin Gu, Xiaodan Gu, Benjamin C. K. Tee, Changhyun Pang, Hongping Yan, Dahui Zhao, Michael F. Toney, Stefan C. B. Mannsfeld, Zhenan Bao. Flow-enhanced solution printing of all-polymer solar cells. Nature Communications, 2015; 6: 7955 DOI:10.1038/ncomms8955

Study claims perovskite solar cells can recoup their energy cost within three months

 

Perovskite solar cells are reportedly able to recoup their energy cost much faster than normal silicon cells, although their short lifetime remains an issue (Credit: Northwestern University)

Scientists at Northwestern University and the U.S. Department of Energy have found that perovskite cells, one of the most promising solar technologies of recent years, can repay their energy cost over 10 times faster than traditional silicon-based solar cells. The finding confirms that, once issues related to cell longevity are ironed out, perovskite cells could soon bring us solar energy on the cheap, and do so with less impact on the environment over their lifetime.

Solar panel installations are doubtlessly having a positive impact on the environment, but quantifying their carbon footprint with some degree of precision – which is useful for comparing them to other means of energy production, including other renewables – is not a straightforward process. To get a more complete picture, it's important to consider not only the carbon emissions saved during the panel's operating life, but also the amount of energy that goes into materials processing, manufacture, repair, maintenance and, once it is no longer useful, disposal of the panel.

According to this metric, called the cradle-to-grave life cycle assessment, a typical solar panel takes a fairly long time, between two and three years, to offset the energy costs that went into producing it. This is because silicon-based solar panels must be manufactured inside a clean room using high-purity crystalline silicon wafers that can only form inside specialized high-temperature furnaces.

Scientists at Northwestern University have now calculated that, by contrast,perovskite-based solar cells have an energy payback time (EPBT) of only two to three months. According to the researchers, this is not only much faster than a silicon-based cell, but also significantly better than any other type of commonly available solar cell.

Energy payback time, or EPBT, for some of the best-known types of solar cells

Perovskite cells are the fastest-growing technology in the solar arena. Although they aren't quite as efficient at converting sunlight into electricity as silicon-based cells, they are catching up very quickly. More importantly, they are much cheaper to produce than normal panels, meaning that their commercialization could lead to a drastic drop in the cost of clean electricity.

Unlike traditional silicon-based cells, perovskites can be manufactured at a very low energy cost, without the need for sophisticated equipment, and in very few steps. A solution containing the electrode materials is coated onto a substrate and, once it evaporates, this solution produces dense layers of crystallized perovskite at a fraction of the cost and energy expenditure of other common solar panels.

According to the study, which analyzed the detailed energy expenditure for two different types of perovskite cells, raw materials contribute about 80 percent of the primary energy consumption for making the panels, suggesting that a better choice of materials could reduce the energy costs even further.

There are indeed plenty of issues with the current choice of materials for perovskite cells, which often make use of potentially toxic lead to absorb sunlight and improve conversion efficiency. The researchers also found that the use of gold, another common raw material, was even more problematic, since the process of mining this precious metal is extremely damaging to the environment.

But perhaps the biggest issue that perovskite cells are currently facing is that they are unable to brave the environment, since they are partly made from organic molecules that degrade quickly when exposed to the elements. Most perovskite cell designs currently lack a protective layer that could lengthen their lifetime, as this would reduce conversion efficiency.

Because of their very short lifetime, the researchers found that the overall CO2 impact of perovskite cells is still significantly higher than that of traditional silicon-based cells, which are much more durable with a reported average lifetime of approximately 20 years.

But if these issues are solved (which the researchers say could happen in as little as two years), perovskite cells could indeed rise to take the lion's share of the solar landscape in the near future, providing clean energy while having an even lower impact on the environment than the solar cells of today.

A paper describing the study appears in the latest issue of the journal Energy & Environmental Science.

Source: Northwestern University

Sticky tape the key to ultrathin solar cells

 

Tue, 07/21/2015 - 11:06am Australian National University Jiajie Pei with crystals of black phosphorus. Courtesy of Stuart Hay, ANU Scientists studying thin layers of phosphorus have found surprising properties that could open the door to ultrathin and ultralight solar cells and LEDs. The team used sticky tape to create single-atom thick layers, termed phosphorene, in the same simple way as the Nobel-prize winning discovery of graphene. Unlike graphene, phosphorene is a semiconductor, like silicon, which is the basis of current electronics technology. "Because phosphorene is so thin and light, it creates possibilities for making lots of interesting devices, such as LEDs or solar cells," said lead researcher Dr. Yuerui (Larry) Lu, from The Australian National University (ANU). "It shows very promising light emission properties." The team created phosphorene by repeatedly using sticky tape to peel thinner and thinner layers of crystals from the black crystalline form of phosphorus. As well as creating much thinner and lighter semiconductors than silicon, phosphorene has light emission properties that vary widely with the thickness of the layers, which enables much more flexibility for manufacturing. "This property has never been reported before in any other material," said Dr. Lu, from ANU College of Engineering and Computer Science, whose study is published in the Nature serial journal Light: Science and Applications. "By changing the number of layers we can tightly control the band gap, which determines the material's properties, such as the color of LED it would make. "You can see quite clearly under the microscope the different colors of the sample, which tells you how many layers are there," said Dr. Lu. Dr. Lu's team found the optical gap for monolayer phosphorene was 1.75 electron volts, corresponding to red light of a wavelength of 700 nanometers. As more layers were added, the optical gap decreased. For instance, for five layers, the optical gap value was 0.8 electron volts, an infrared wavelength of 1550 nanometers. For very thick layers, the value was around 0.3 electron volts, a mid-infrared wavelength of around 3.5 microns. The behavior of phosphorene in thin layers is superior to silicon, said Dr. Lu. "Phosphorene's surface states are minimized, unlike silicon, whose surface states are serious and prevent it being used in such a thin state." SOURCE: Australian National University

Inkjet printing process for kesterite solar cells

 

Wed, 05/06/2015 - 11:33am Antonia Rotger, Helmholtz-Zentrum Berlin   This is an illustration of the working principle of inkjet printing. Image: HZBThe drop-on-demand inkjet printing is a promising approach allowing patterning of materials with negligible materials waste; hence, significant reduction of raw materials cost can be achieved. Furthermore, inkjet printing can be easily adapted to a roll-to-roll process, which is suitable for large scale production. From the industrial application perspective, both of these two features of the inkjet printing technology are of great interest. A critical requirement for using inkjet printing is to develop a suitable ink in terms of viscosity and stability which leads to compact and homogeneous films. Tuning the molecular ink Dr. Xianzhong Lin from the Institute for Heterogeneous Material Systems of HZB used a molecular ink which was originally developed for spin coating technologies. The ink is produced by dissolving Cu, Zn, Sn metal salt and thiourea in dimethyl sulfoxide solvent. Lin tested its suitability for inkjet printing. He found that the viscosity of the ink can be tuned by adjusting the ink concentration and the ink composition can also be easily controlled by adding or reducing the amount of each chemical added. The CZTSSe absorbers were formed by annealing the inkjet-printed Cu-Zn-Sn-S precursor film under an atmosphere containing Selenium. Economical process Initial optimization of the processing conditions such as ink composition and printing parameters have already yielded solar cells with efficiencies up to 6.4 %. The huge advantage of inkjet printing versus spin coating to obtain thin film absorbers is the lesser amount of waste: Whereas with spin coating, a large quantity of the ink material is wasted, the inkjet printing is very economical: For example, less than 20 microliter ink is needed to build up a micrometer CZTSSe thin film absorber on an inch by inch substrate in this study. Low toxicity and low waste "Although the solar cell performance is still far below the record efficiency of 12.7 % for CZTSSe based solar cells, the great advantage of our approach is the low toxic and low material wastage process," Prof. Martha Lux-Steiner explains. The team is now working on the optimization of processing conditions for the kesterite absorbers to further improve the solar cell performance and on the deposition of buffer and TCO layers by inkjet printing. The goal is to print a complete device with high efficiency without relying on expensive vacuum technology. This work opens up a promising route for the fabrication of kesterite thin film solar cells. Source: Helmholtz-Zentrum Berlin

Perovskites provide big boost in silicon solar cells

 

Thu, 01/22/2015 - 1:04pm Mark Shwartz, Stanford Univ. Stacking perovskites, a crystalline material, onto a conventional silicon solar cell dramatically improves the overall efficiency of the cell, according to a new study led by Stanford Univ. scientists. The researchers describe their novel perovskite-silicon solar cell in Energy & Environmental Science. "We've been looking for ways to make solar panels that are more efficient and lower cost," said study co-author Michael McGehee, a professor of materials science and engineering at Stanford. "Right now, silicon solar cells dominate the world market, but the power conversion efficiency of silicon photovoltaics has been stuck at 25% for 15 years." One cost-effective way to improve efficiency is to build a tandem device made of silicon and another inexpensive photovoltaic material, he said. "Making low-cost tandems is very desirable," McGehee said. "You simply put one solar cell on top of the other, and you get more efficiency than either could do by itself. From a commercial standpoint, it makes a lot of sense to use silicon for the bottom cell. Until recently, we didn't have a good material for the top cell, then perovskites came along." Perovskite is a crystalline material that is inexpensive and easy to produce in the lab. In 2009, scientists showed that perovskites made of lead, iodide and methylammonium could convert sunlight into electricity with an efficiency of 3.8%. Since then, researchers have achieved perovskite efficiencies above 20%, rivaling commercially available silicon solar cells and spawning widespread interest among silicon manufacturers. "Our goal is to leverage the silicon factories that already exist around the world," said Stanford graduate student Colin Bailie, co-lead author of the study. "With tandem solar cells, you don't need a billion-dollar capital expenditure to build a new factory. Instead, you can start with a silicon module and add a layer of perovskite at relatively low cost." Sunlight to electricitySolar cells work by converting photons of sunlight into an electric current that moves between two electrodes. Silicon solar cells generate electricity by absorbing photons of visible and infrared light, while perovskite cells harvest only the visible part of the solar spectrum where the photons have more energy. "Absorbing the high-energy part of the spectrum allows perovskite solar cells to generate more power per photon of visible light than silicon cells," Bailie said. A key roadblock to building an efficient perovskite-silicon tandem has been a lack of transparency. "Colin had to figure out how to put a transparent electrode on the top so that some photons could penetrate the perovskite layer and be absorbed by the silicon at the bottom," McGehee said. "No one had ever made a perovskite solar cell with two transparent electrodes." Perovskites are easily damaged by heat and readily dissolve in water. This inherent instability ruled out virtually all of the conventional techniques for applying electrodes onto the perovskite solar cell, so Bailie did it manually. "We used a sheet of plastic with silver nanowires on it," he said. "Then we built a tool that uses pressure to transfer the nanowires onto the perovskite cell, kind of like a temporary tattoo. You just need to rub it to transfer the film." Remarkable efficiencyFor the experiment, the Stanford team stacked a perovskite solar cell with an efficiency of 12.7% on top of a low-quality silicon cell with an efficiency of just 11.4%. "By combining two cells with approximately the same efficiency, you can get a very large efficiency boost," Bailie said. The results were impressive. "We improved the 11.4% silicon cell to 17% as a tandem, a remarkable relative efficiency increase of nearly 50%," McGehee said. "Such a drastic improvement in efficiency has the potential to redefine the commercial viability of low-quality silicon." In another experiment, the research team replaced the silicon solar cell with a cell made of copper indium gallium diselenide (CIGS). The researchers stacked a 12.7% efficiency perovskite cell onto a CIGS cell with a 17% efficiency. The resulting tandem achieved an overall conversion efficiency of 18.6%. "Since most, if not all, of the layers in a perovskite cell can be deposited from solution, it might be possible to upgrade conventional solar cells into higher-performing tandems with little increase in cost," the authors wrote. A big unanswered question is the long-term stability of perovskites, McGehee added. "Silicon is a rock," he said. "You can heat it to about 600 degrees Fahrenheit, shine light on it for 25 years, and nothing will happen. But if you expose perovskite to water or light, it likely will degrade. We have a ways to go to show that perovskite solar cells are stable enough to last 25 years. My vision is that some day we'll be able to get low-cost tandems that are 25% efficient. That's what companies are excited about. In five to 10 years, we could even reach 30% efficiency." Source: Stanford Univ.

Chip-Making Tools Produce Ultra-Efficient Solar Cells

 

Equipment for making microchips has led to solar cells that are twice as efficient as conventional ones. By Kevin Bullis on December 16, 2014 A wafer bearing 500 tiny solar cells, made by Soitec, has produced a new world record. Soitec, a French manufacturing company, says it has used techniques designed for making microprocessors to produce solar cells with a record-setting efficiency of 46 percent, converting more than twice as much sunlight into electricity as conventional cells. Although the cells are more complicated to produce, using established manufacturing techniques promises to keep production costs down. Ordinary solar cells use one semiconductor to convert sunlight into electricity. The cells made by Soitec have four semiconductors, each designed to target a different part of the solar spectrum. Soitec produced its first four-semiconductor cell about a year ago. Since then, it’s been improving efficiencies rapidly, and it looks on track to be the first company to hit the long-awaited milestone of 50 percent efficiency. Over the last several years, the costs of solar power have come down by over 80 percent, mostly because companies have found cheaper ways to manufacture conventional silicon solar cells. But solar power is still more expensive than fossil fuels in most places. Soitec is one of several companies attempting to lower costs by making solar cells more efficient, so fewer are needed to generate the same amount of power. That cuts installation costs, which can account for more than half the cost of solar power (see “Solar Panels That Configure Themselves”). The challenge is achieving high efficiencies without significantly increasing the cost of making the cells. Combining multiple semiconductors in a solar cell is an old idea that’s hard to execute in practice. It is possible to grow the semiconductor materials separately and then bond them together, but that requires multiple crystalline templates, which is expensive, and it can result in imperfect bonds. To make its four-semiconductor solar cells, Soitec starts by growing two atomically compatible semiconductor materials on one template and two different compatible semiconductors on another. One of the templates is then removed so it can be reused (the structure of the final solar cell makes it difficult to remove the other one). Finally, the two pairs of semiconductors are stacked together. Soitec has already used the process of reusing the template and bonding the semiconductors for years to make components for microprocessors and other electronics. The company plans to begin high-volume manufacturing of its four-semiconductor cells in 2016. Some questions remain about how cheap its process will be, though. The company isn’t providing specific estimates for the cost per kilowatt of solar power using its technology, saying the numbers depend on location. Other companies are vying to be the first to reach 50 percent efficiency. This year the startup Semprius demonstrated four-semiconductor cells that were 44.1 percent efficient, and the company says it’s on track to break the world record next year. source : MIT Technology Review

Blu-ray disc can be used to improve solar cell performance

 

 

An interdisciplinary research team has discovered that the pattern of information written on a Blu-ray disc -- and it doesn't matter if it's Jackie Chan's "Supercop" or the cartoon "Family Guy" -- works very well for improving light absorption across the solar spectrum. And better yet, the researchers know why.

"We had a hunch that Blu-ray discs might work for improving solar cells, and, to our delight, we found the existing patterns are already very good," said Jiaxing Huang, a materials chemist and an associate professor of materials science and engineering in the McCormick School of Engineering and Applied Science. "It's as if electrical engineers and computer scientists developing the Blu-ray technology have been subconsciously doing our jobs, too."

Blu-ray discs contain a higher density of data than DVDs or CDs, and it is this quasi-random pattern, perfected by engineers over decades for data storage, that, when transferred to the surface of solar cells, provides the right texture to improve the cells' light absorption and performance.

Working with Cheng Sun, an associate professor of mechanical engineering at McCormick, Huang and his team tested a wide range of movies and television shows stored on Blu-ray discs, including action movies, dramas, documentaries, cartoons and black-and-white content, and found the video content did not matter. All worked equally well for enhancing light absorption in solar cells.

The findings will be published Nov. 25 in the journal Nature Communications.

In the field of solar cells, it is known that if texture is placed on the surface of a solar cell, light is scattered more effectively, increasing a cell's efficiency. Scientists have long been searching for the most effective texture with a reasonable manufacturing cost.

The Northwestern researchers have demonstrated that a Blu-ray disc's strings of binary code 0s and 1s, embedded as islands and pits to store video information, give solar cells the near-optimal surface texture to improve their absorption over the broad spectrum of sunlight.

In their study, the researchers first selected the Jackie Chan movie "Supercop." They replicated the pattern on the active layer of a polymer solar cell and found the cell was more efficient than a control solar cell with a random pattern on its surface.

"We found a random pattern or texture does work better than no pattern, but a Blu-ray disc pattern is best of all," Huang said. "Then I wondered, why did it work? If you don't understand why, it's not good science."

Huang puzzled over the question of why for some time. One day, his wife, Shaorong Liu, a database engineer at IBM, suggested it likely had something to do with data compression. That was the insight Huang needed.

Huang and Sun then turned to McCormick colleague Dongning Guo, an expert in information theory, to investigate this idea. Guo is an associate professor of electrical engineering and computer science.

The researchers looked closely at the data processing algorithms in the Blu-ray standard and noted the algorithms serve two major purposes:

• Achieving as high a degree of compression as possible by converting the video signals into a seemingly random sequence of 0s and 1s; and
• Increasing error tolerance by adding controlled redundancy into the data sequence, which also limits the number of consecutive 0s and 1s.

These two purposes, the researchers said, have resulted in a quasi-random array of islands and pits (0s and 1s) with feature sizes between 150 and 525 nanometers. And this range, it turns out, works quite well for light-trapping applications over the entire solar spectrum.

The overall broadband absorption enhancement of a Blu-ray patterned solar cell was measured to be 21.8 percent, the researchers report.

"In addition to improving polymer solar cells, our simulation suggests the Blu-ray patterns could be broadly applied for light trapping in other kinds of solar cells," Sun said.

"It has been quite unexpected and truly thrilling to see new science coming out of the intersection of information theory, nanophotonics and materials science," Huang said.

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Story Source:

The above story is based on materials provided by Northwestern University. Note: Materials may be edited for content and length.

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Journal Reference:

1. Alexander J. Smith, Chen Wang, Dongning Guo, Cheng Sun, Jiaxing Huang. Repurposing Blu-ray movie discs as quasi-random nanoimprinting templates for photon management. Nature Communications, 2014; 5: 5517 DOI: 10.1038/ncomms6517

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Northwestern University. "Blu-ray disc can be used to improve solar cell performance." ScienceDaily. ScienceDaily, 25 November 2014. <www.sciencedaily.com/releases/2014/11/141125111816.htm>.

New type of silicon could find use in solar cells and LEDs

 

A view through the channels of the new zeolite-type allotrope of silicon (Image: Timothy Strobel)

You probably wouldn't be reading this if it weren't for silicon. It's the second most-abundant element in the Earth's crust as well as the key to modern technology – used in the integrated circuits that power such electronics as computers, mobile phones, and even some toasters and refrigerators. It's also used in compound form in building, ceramics, breast implants, and many other areas. And now the ubiquitous element may have a plethora of new applications, thanks to a team of Carnegie scientists who synthesized an allotrope (new/different physical form) with the chemical formula Si24.

The diamond-structured form of silicon normally used in technology applications has a semiconducting property called an indirect band gap, which differs from a direct band gap in that it requires an extra step to excite bound electrons into a free state so that they can participate in electrical conduction. Direct band gap semiconductors need only two entities to intersect; a photon imparts momentum on an electron. But indirect band gap semiconductors require a third entity – a lattice vibration called a phonon – because the minimum energy state of the conduction band and the maximum energy state of the valence band occur at different values of momentum.

This new form of silicon is a quasi-direct band gap material, which means not only that it can conduct electricity more efficiently than diamond-structured silicon but also that it can absorb and emit light – a property never before achieved. (I say quasi-direct because it is technically a very small and almost flat indirect band gap.) These properties make it ripe for use in next-generation solar cells, LEDs, and other semiconductor technologies.

To create Si24, the researchers first formed a polycrystalline compound of silicon and sodium (Na4Si24) with help from a tantalum capsule, very high temperature, and a 1,500 ton multi-anvil press that gradually reached a pressure of 10 gigapascals (1,450,377 pounds per square inch). This compound was then "degassed" in a vacuum at 400 Kelvin (260 F) for eight days, after which they had pure Si24 in an open framework called a zeolite-type structure.

The structure is comprised of five-, six-, and eight-membered silicon rings through which small atoms and molecules could spread, with potential applications in electrical energy storage and molecular-scale filtering, among other things.

Si24 could be just the tip of the iceberg for desirable new materials formed at high pressure, the researchers suggest. Lead researcher Timothy Strobel has gone so far as to call high-pressure precursor synthesis "an entirely new frontier in novel energy materials" that goes above and beyond silicon. And the stability of the new structures at atmospheric pressure means that low-pressure methods such as chemical vapor deposition could potentially allow large-scale production.

A paper describing the research was published in the journal Nature Materials.

Source: Carnegie Institution for Science

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Sharp Demonstrates Ultra-Efficient Solar Cells

 

New technology could be twice as efficient at converting sunlight to electricity.

• By Kevin Bullis on June 20, 2014

Why It Matters

Solar cells are still relatively inefficient at converting light to electricity, one of the biggest reasons solar can’t compete with fossil fuels.

The best solar cells convert less than one-third of the energy in sunlight into electricity, although for decades researchers have calculated that exotic physics could allow them to convert far more. Now researchers at Sharp have built a prototype that demonstrates one of these ideas. If it can be commercialized, it would double the amount of power a solar cell can generate, offering a way to make solar power far more economical.

The researchers figured out a way around a bothersome phenomenon: when sunlight strikes a solar cell, it produces some very high-energy electrons, but within a few trillionths of a second, those electrons shed most of their energy as waste heat.

The Sharp team found a way to extract these electrons before they give up that energy, thereby increasing the voltage output of their prototype solar cell. It’s far from a practical device—it’s too thin to absorb much sunlight, and for now it works only with a single wavelength of light—but it’s the first time that anyone has been able to generate electrical current using these high-energy electrons. In theory, solar cells that exploit this technique could reach efficiencies over 60 percent.

The approach is one of several that could someday break open the solar industry and make fossil fuels expensive in comparison. High-efficiency solar cells would lower the cost of installation, which today is often more expensive than the cells themselves.

Exploiting exotic physics requires both understanding the behavior of certain materials and figuring out how to make them with high precision (see “Capturing More Light with a Single Solar Cell” and “Nanocharging Solar”). The Sharp device relies on the ability to make high-quality, nanometers-thick layers of semiconducting materials (such as gallium arsenide), which create a shortcut for high-energy electrons to move out of the solar cell.

Another way to achieve ultra-high efficiencies now is by stacking up different kinds of solar cells (see “Exotic, Highly Efficient Solar Cells May Soon Get Cheaper”), but doing so is very expensive. Meanwhile, MIT researchers are studying the transient behavior of electrons in organic materials to find inexpensive ways to make ultra-efficient solar cells.

Each of the alternative approaches is at an early stage. James Dimmock, the senior researchers who developed the new device at Sharp, says he expects that his technique will initially be used to help boost the efficiency of conventional devices, not to create new ones.

MIT Technology Review