Record Efficiency of 18.7 Percent for Flexible Solar Cells on Plastics, Swiss Researchers Report

The measurements have been independently certified by the Fraunhofer Institute for Solar Energy Systems in Freiburg, Germany.

It's all about money. To make solar electricity affordable on a large scale, scientists and engineers worldwide have long been trying to develop a low-cost solar cell, which is highly efficient, easy to manufacture and has high throughput. Now a team at Empa's Laboratory for Thin Film and Photovoltaics, led by Ayodhya N. Tiwari, has made a major step forward."The new record value for flexible CIGS solar cells of 18.7% nearly closes the"efficiency gap" to solar cells based on polycrystalline silicon (Si) wafers or CIGS thin film cells on glass," says Tiwari. He is convinced that"flexible and lightweight CIGS solar cells with efficiencies comparable to the"best-in-class" will have excellent potential to bring about a paradigm shift and to enable low-cost solar electricity in the near future."

One major advantage of flexible high-performance CIGS solar cells is the potential to lower manufacturing costs through roll-to-roll processing while at the same time offering a much higher efficiency than the ones currently on the market. What's more, such lightweight and flexible solar modules offer additional cost benefits in terms of transportation, installation, structural frames for the modules etc., i.e. they significantly reduce the so-called"balance of system" costs. Taken together, the new CIGS polymer cells exhibit numerous advantages for applications such as facades, solar farms and portable electronics. With high-performance devices now within reach, the new results suggest that monolithically-interconnected flexible CIGS solar modules with efficiencies above 16% should be achievable with the recently developed processes and concepts.

At the forefront of efficiency improvements

In recent years, thin film photovoltaic technology based on glass substrates has gained sufficient maturity towards industrial production; flexible CIGS technology is, however, still an emerging field. The recent improvements in efficiency in research labs and pilot plants -- among others by Tiwari's group, first at ETH Zurich and since a couple of years now at Empa -- are contributing to performance improvements and to overcoming manufacturability barriers.

Working closely with scientists at FLISOM, a start-up company who is scaling up and commercializing the technology, the Empa team made significant progress in low-temperature growth of CIGS layers yielding flexible CIGS cells that are ever more efficient, up from a record value of 14.1% in 2005 to the new"high score" of 18.7% for any type of flexible solar cell grown on polymer or metal foil. The latest improvements in cell efficiency were made possible through a reduction in recombination losses by improving the structural properties of the CIGS layer and the proprietary low-temperature deposition process for growing the layers as well as in situ doping with Na during the final stage. With these results, polymer films have for the first time proven to be superior to metal foils as a carrier substrate for achieving highest efficiency.

Record efficiencies of up to 17.5% on steel foils covered with impurity diffusion barriers were so far achieved with CIGS growth processes at temperatures exceeding 550°C. However, when applied to steel foil without any diffusion barrier, the proprietary low temperature CIGS deposition process developed by Empa and FLISOM for polymer films easily matched the performance achieved with high-temperature procedure, resulting in an efficiency of 17.7%. The results suggest that commonly used barrier coatings for detrimental impurities on metal foils would not be required."Our results clearly show the advantages of the low-temperature CIGS deposition process for achieving highest efficiency flexible solar cells on polymer as well as metal foils," says Tiwari.

The projects were supported by the Swiss National Science Foundation (SNSF), the Commission for Technology and Innovation (CTI), the Swiss Federal Office of Energy (SFOE), EU Framework Programmes as well as by Swiss companies W.Blösch AG and FLISOM.

Scaling up production of flexible CIGS solar cells

The continuous improvement in energy conversion efficiencies of flexible CIGS solar cells is no small feat, says Empa Director Gian-Luca Bona."What we see here is the result of an in-depth understanding of the material properties of layers and interfaces combined with an innovative process development in a systematic manner. Next, we need to transfer these innovations to industry for large scale production of low-cost solar modules to take off." Empa scientists are currently working together with FLISOM to further develop manufacturing processes and to scale up production.

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Original Versus Copy: Researchers Develop Forgery-Proof Prototypes for Product Authentication

For this, all the data has to be electronically checked. In the framework of the"Crypta" project supported by the Federal Ministry for Transport, Innovation and Technology (BMVIT), scientists from Graz University of Technology have now developed a prototype which safeguards objects according to new standards.

Whether for checking the origin of foodstuffs or as proof of authenticity of drugs, the future will bring an increased use of electronic assistants to make sure that the quality is right. RFID (Radio Frequency Identification) technology enables objects to be identified wirelessly."You need a reading device and an RFID tag which communicate with each other," explains project leader Jörn-Marc Schmidt from the Institute of Applied Information Processing and Communications at Graz University of Technology. There is a difference between active and passive tags. The former are connected to a power source whereas the latter draw the required power directly from the field of the reading unit, which makes them particularly suitable, for instance, for applications in supermarkets.

Private Key

For a long time the same electronic keys were used for these energy-efficient passive tags and their readers -- using what experts call symmetrical methods."In asymmetrical methods, the transmitter and receiver possess different keys. Secure digital signatures are thus made possible," adds Jörn-Marc Schmidt. Together with the semiconductor manufacturer austriamicrosystems and RF-iT Solutions GmbH, an RFID software and services provider from Graz, the researchers have now developed a prototype which uses a standard method for passive tags for the first time.

"For every tag there is a public key and a private key which remains secret," explains Jörn-Marc Schmidt. This is a development which could be made use of everywhere where proof of authenticity is important. The research results are the gratifying outcome of the Crypta research project of the FIT-IT funding line of the BMVIT, which supports application-oriented research in information technology in particular.

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Forecast Calls for Nanoflowers to Help Return Eyesight: Physicist Leads Effort to Design Fractal Devices to Put in Eyes

These flowers are not roses, tulips or columbines. They will be nanoflowers seeded from nano-sized particles of metals that grow, or self assemble, in a natural process -- diffusion limited aggregation. They will be fractals that mimic and communicate efficiently with neurons.

Fractals are"a trademark building block of nature," Taylor says. Fractals are objects with irregular curves or shapes, of which any one component seen under magnification is also the same shape. In math, that property is self-similarity. Trees, clouds, rivers, galaxies, lungs and neurons are fractals, Taylor says. Today's commercial electronic chips are not fractals, he adds.

Eye surgeons would implant these fractal devices within the eyes of blind patients, providing interface circuitry that would collect light captured by the retina and guide it with almost 100 percent efficiency to neurons for relay to the optic nerve to process vision.

In an article titled"Vision of beauty" forPhysics World, Taylor, a physicist and director of the UO Materials Science Institute, describes his envisioned approach and how it might overcome the problems occurring with current efforts to insert photodiodes behind the eyes. Current chip technology is limited, because it doesn't allow sufficient connections with neurons.

"The wiring -- the neurons -- in the retina is fractal, but the chips are not fractal," Taylor says."They are just little squares of electrodes that provide too little overlap with the neurons."

Beginning this summer, Taylor's doctoral student Rick Montgomery will begin a yearlong collaboration with Simon Brown at the University of Canterbury in New Zealand to experiment with various metals to grow the fractal flowers on implantable chips.

The idea for the project emerged as Taylor was working under a Cottrell Scholar Award he received in 2003 from the Research Corporation for Science Advancement. His vision is now beginning to blossom under grants from the Office of Naval Research (ONR), the U.S. Air Force and the National Science Foundation.

Taylor's theoretical concept for fractal-based photodiodes also is the focus of a U.S. patent application filed by the UO's Office of Technology Transfer under Taylor's and Brown's names, the UO and University of Canterbury.

The project, he writes in thePhysics Worldarticle, is based on"the striking similarities between the eye and the digital camera." (Physics Worldarticle is available at:http://physicsworld.com/cws/article/indepth/45840)

"The front end of both systems," he writes,"consists of an adjustable aperture within a compound lens, and advances bring these similarities closer each year." Digital cameras, he adds, are approaching the capacity to capture the 127 megapixels of the human eye, but current chip-based implants, because of their interface, are only providing about 50 pixels of resolution.

Among the challenges, Taylor says, is determining which metals can best go into body without toxicity problems."We're right at the start of this amazing voyage," Taylor says."The ultimate thrill for me will be to go to a blind person and say, we're developing a chip that one day will help you see again. For me, that is very different from my previous research, where I've been looking at electronics to go into computers, to actually help somebody… if I can pull that off that will be a tremendous thrill for me."

Taylor also is working under a Research Corp. grant to pursue fractal-based solar cells.

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Electronic Life on the Edge: Scientists Discover the Edge States of Graphene Nanoribbons

A graphene nanoribbon is a strip of graphene that may be only a few nanometers wide (a nanometer is a billionth of a meter). Theorists have envisioned that nanoribbons, depending on their width and the angle at which they are cut, would have unique electronic, magnetic, and optical features, including band gaps like those in semiconductors, which sheet graphene doesn't have.

"Until now no one has been able to test theoretical predictions regarding nanoribbon edge-states, because no one could figure out how to see the atomic-scale structure at the edge of a well-ordered graphene nanoribbon and how, at the same time, to measure its electronic properties within nanometers of the edge," says Michael Crommie of Berkeley Lab's Materials Sciences Division (MSD) and UC Berkeley's Physics Division, who led the research."We were able to achieve this by studying specially made nanoribbons with a scanning tunneling microscope."

The team's research not only confirms theoretical predictions but opens the prospect of building quick-acting, energy-efficient nanoscale devices from graphene-nanoribbon switches, spin-valves, and detectors, based on either electron charge or electron spin. Farther down the road, graphene nanoribbon edge states open the possibility of devices with tunable giant magnetoresistance and other magnetic and optical effects.

Crommie and his colleagues have published their research inNature Physics, available May 8, 2011 in advanced online publication.

The well-tempered nanoribbon

"Making flakes and sheets of graphene has become commonplace," Crommie says,"but until now, nanoribbons produced by different techniques have exhibited, at best, a high degree of inhomogeneity" -- typically resulting in disordered ribbon structures with only short stretches of straight edges appearing at random. The essential first step in detecting nanoribbon edge states is access to uniform nanoribbons with straight edges, well-ordered on the atomic scale.

Hongjie Dai of Stanford University's Department of Chemistry and Laboratory for Advanced Materials, a member of the research team, solved this problem with a novel method of"unzipping" carbon nanotubes chemically. Graphene rolled into a cylinder makes a nanotube, and when nanotubes are unzipped in this way the slice runs straight down the length of the tube, leaving well-ordered, straight edges.

Graphene can be wrapped at almost any angle to make a nanotube. The way the nanotube is wrapped determines the pitch, or"chiral vector," of the nanoribbon edge when the tube is unzipped. A cut straight along the outer atoms of a row of hexagons produces a zigzag edge. A cut made at a 30-degree angle from a zigzag edge goes through the middle of the hexagons and yields scalloped edges, known as"armchair" edges. Between these two extremes are a variety of chiral vectors describing edges stepped on the nanoscale, in which, for example, after every few hexagons a zigzag segment is added at an angle.

These subtle differences in edge structure have been predicted to produce measurably different physical properties, which potentially could be exploited in new graphene applications. Steven Louie of UC Berkeley and Berkeley Lab's MSD was the research team's theorist; with the help of postdoc Oleg Yazyev, Louie calculated the expected outcomes, which were then tested against experiment.

Chenggang Tao of MSD and UCB led a team of graduate students in performing scanning tunneling microscopy (STM) of the nanoribbons on a gold substrate, which resolved the positions of individual atoms in the graphene nanoribbons. The team looked at more than 150 high-quality nanoribbons with different chiralities, all of which showed an unexpected feature, a regular raised border near their edges forming a hump or bevel. Once this was established as a real edge feature -- not the artifact of a folded ribbon or a flattened nanotube -- the chirality and electronic properties of well-ordered nanoribbon edges could be measured with confidence, and the edge regions theoretically modeled.

Electronics at the edge

"Two-dimensional graphene sheets are remarkable in how freely electrons move through them, including the fact that there's no band gap," Crommie says."Nanoribbons are different: electrons can become trapped in narrow channels along the nanoribbon edges. These edge-states are one-dimensional, but the electrons on one edge can still interact with the edge electrons on the other side, which causes an energy gap to open up."

Using an STM in spectroscopy mode (STS), the team measured electronic density changes as an STM tip was moved from a nanoribbon edge inward toward its interior. Nanoribbons of different widths were examined in this way. The researchers discovered that electrons are confined to the edge of the nanoribbons, and that these nanoribbon-edge electrons exhibit a pronounced splitting in their energy levels.

"In the quantum world, electrons can be described as waves in addition to being particles," Crommie notes. He says one way to picture how different edge states arise is to imagine an electron wave that fills the length of the ribbon and diffracts off the atoms near its edge. The diffraction patterns resemble water waves coming through slits in a barrier.

For nanoribbons with an armchair edge, the diffraction pattern spans the full width of the nanoribbon; the resulting electron states are quantized in energy and extend spatially throughout the entire nanoribbon. For nanoribbons with a zigzag edge, however, the situation is different. Here diffraction from edge atoms leads to destructive interference, causing the electron states to localize near the nanoribbon edges. Their amplitude is greatly reduced in the interior.

The energy of the electron, the width of the nanoribbon, and the chirality of its edges all naturally affect the nature and strength of these nanoribbon electronic states, an indication of the many ways the electronic properties of nanoribbons can be tuned and modified.

Says Crommie,"The optimist says, 'Wow, look at all the ways we can control these states -- this might allow a whole new technology!' The pessimist says, 'Uh-oh, look at all the things that can disturb a nanoribbon's behavior -- how are we ever going to achieve reproducibility on the atomic scale?'"

Crommie himself declares that"meeting this challenge is a big reason for why we do research. Nanoribbons have the potential to form exciting new electronic, magnetic, and optical devices at the nanoscale. We might imagine photovoltaic applications, where absorbed light leads to useful charge separation at nanoribbon edges. We might also imagine spintronics applications, where using a side-gate geometry would allow control of the spin polarization of electrons at a nanoribbon's edge."

Although getting there won't be simple --"The edges have to be controlled," Crommie emphasizes --"what we've shown is that it's possible to make nanoribbons with good edges and that they do, indeed, have characteristic edge states similar to what theorists had expected. This opens a whole new area of future research involving the control and characterization of graphene edges in different nanoscale geometries."

"Spatially resolving edge states of chiral graphene nanoribbons," by Chenggang Tao, Liying Jiao, Oleg V. Yazyev, Yen-Chia Chen, Juanjuan Feng, Xiaowei Zhang, Rodrigo B. Capaz, James M. Tour, Alex Zettle, Steven G. Louie, Hongjie Dai, and Michael F. Crommie, appears inNature Physicsand is available online athttp://www.nature.com/nphys/index.html. Funded at Berkeley Lab by DOE's Helios Energy Research Center, this collaborative project was supported jointly by the Office of Naval Research, the National Science Foundation, and the U.S. Department of Energy's Office of Science.

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High Temperature Milestone Achieved in Silicon Spintronics

The electron possesses an internal angular momentum called the spin. The International Technology Roadmap for Semiconductors has identified the electron's spin as a new state variable that should be explored as an alternative to the electron's charge for use beyond Moore's Law, a projection named after Intel co-founder Gordon E. Moore. Moore predicted in 1965 that the number of transistors per unit area in an integrated circuit would double approximately every two years as advances in fabrication technology enabled the devices to be made smaller. Although this approach has been remarkably successful, critical device dimensions now approach atomic length scales, so that further size scaling becomes untenable."Researchers have been forced to look beyond the simple reduction of size to develop future generations of electronic devices," states NRL senior scientist Dr. Berry Jonker."Electrical generation, manipulation and detection of significant spin polarization in silicon at temperatures that meet commercial and military requirements are essential to validate spin as an alternative to charge for a device technology beyond Moore's Law."

Using ferromagnetic metal / silicon dioxide contacts on silicon, NRL scientists Connie Li, Olaf van 't Erve and Jonker electrically generate and detect spin accumulation and precession in the silicon transport channel at temperatures up to 225°C, and conclude that the spin information can be transported in the silicon over distances readily compatible with existing fabrication technology. They thus overcome a major obstacle in achieving control of the spin variable at temperatures required for practical applications in the most widely utilized semiconductor.

To make a semiconductor spintronic device, one needs contacts that can both generate a current of spin-polarized electrons (called a spin injector), and detect the spin polarization of the electrons (spin detector) in the semiconductor. Because the magnetic contact interface is likely to introduce additional scattering and spin relaxation mechanisms not present in the silicon bulk, the region of the semiconductor directly beneath the contact is expected to be a critical factor in the development of any future spin technology. The NRL scientists probe the spin environment directly under the magnetic metal / silicon dioxide contact using the three terminal geometry illustrated in the accompanying

figure

. Demonstration of spin precession and dephasing in a magnetic field transverse to the injected spin orientation, known as the Hanle effect, is conclusive evidence of spin accumulation, and enables a direct measure of the spin lifetime, a critical parameter for device operation. The NRL researchers observed Hanle precession of the electron spin accumulation in the silicon channel under the contact for biases corresponding to both spin injection and extraction, and determine the corresponding spin lifetimes.

Electronic states can form at the contact interface and introduce deleterious effects for both charge and spin transport. These undesirable states can serve as traps which prevent propagation of either charge or spin in the silicon channel. In bulk silicon, the spin lifetime is known to depend upon the carrier density, and generally decreases as the electron density increases.."In this study we show that the spin lifetime determined from our measurements changes systematically as one changes carrier concentration of the particular silicon sample used," adds Jonker."Our results were obtained for a number of different carrier densities and show this trend, thus making it very clear that we obtain spin injection and accumulation in the silicon itself rather than in interface defect states." The result of this research rules out spin accumulation in interface states and demonstrates spin injection, accumulation and precession in the silicon channel.

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Transistors Reinvented Using New 3-D Structure

The three-dimensional Tri-Gate transistors represent a fundamental departure from the two-dimensional planar transistor structure that has powered not only all computers, mobile phones and consumer electronics to-date, but also the electronic controls within cars, spacecraft, household appliances, medical devices and virtually thousands of other everyday devices for decades.

"Intel's scientists and engineers have once again reinvented the transistor, this time utilizing the third dimension," said Intel President and CEO Paul Otellini."Amazing, world-shaping devices will be created from this capability as we advance Moore's Law into new realms."

Scientists have long recognized the benefits of a 3-D structure for sustaining the pace of Moore's Law as device dimensions become so small that physical laws become barriers to advancement. The key to this latest breakthrough is Intel's ability to deploy its novel 3-D Tri-Gate transistor design into high-volume manufacturing, ushering in the next era of Moore's Law and opening the door to a new generation of innovations across a broad spectrum of devices.

Moore's Law is a forecast for the pace of silicon technology development that states that roughly every 2 years transistor density will double, while increasing functionality and performance and decreasing costs. It has become the basic business model for the semiconductor industry for more than 40 years.

Unprecedented Power Savings and Performance Gains

Intel's 3-D Tri-Gate transistors enable chips to operate at lower voltage with lower leakage, providing an unprecedented combination of improved performance and energy efficiency compared to previous state-of-the-art transistors. The capabilities give chip designers the flexibility to choose transistors targeted for low power or high performance, depending on the application.

The 22nm 3-D Tri-Gate transistors provide up to 37 percent performance increase at low voltage versus Intel's 32nm planar transistors. This incredible gain means that they are ideal for use in small handheld devices, which operate using less energy to"switch" back and forth. Alternatively, the new transistors consume less than half the power when at the same performance as 2-D planar transistors on 32nm chips.

"The performance gains and power savings of Intel's unique 3-D Tri-Gate transistors are like nothing we've seen before," said Mark Bohr, Intel Senior Fellow."This milestone is going further than simply keeping up with Moore's Law. The low-voltage and low-power benefits far exceed what we typically see from one process generation to the next. It will give product designers the flexibility to make current devices smarter and wholly new ones possible. We believe this breakthrough will extend Intel's lead even further over the rest of the semiconductor industry."

Continuing the Pace of Innovation -- Moore's Law

Transistors continue to get smaller, cheaper and more energy efficient in accordance with Moore's Law -- named for Intel co-founder Gordon Moore. Because of this, Intel has been able to innovate and integrate, adding more features and computing cores to each chip, increasing performance, and decreasing manufacturing cost per transistor.

Sustaining the progress of Moore's Law becomes even more complex with the 22nm generation. Anticipating this, Intel research scientists in 2002 invented what they called a Tri-Gate transistor, named for the three sides of the gate. This announcement follows further years of development in Intel's highly coordinated research-development-manufacturing pipeline, and marks the implementation of this work for high-volume manufacturing.

The 3-D Tri-Gate transistors are a reinvention of the transistor. The traditional"flat" two-dimensional planar gate is replaced with an incredibly thin three-dimensional silicon fin that rises up vertically from the silicon substrate. Control of current is accomplished by implementing a gate on each of the three sides of the fin -- two on each side and one across the top -- rather than just one on top, as is the case with the 2-D planar transistor. The additional control enables as much transistor current flowing as possible when the transistor is in the"on" state (for performance), and as close to zero as possible when it is in the"off" state (to minimize power), and enables the transistor to switch very quickly between the two states (again, for performance).

Just as skyscrapers let urban planners optimize available space by building upward, Intel's 3-D Tri-Gate transistor structure provides a way to manage density. Since these fins are vertical in nature, transistors can be packed closer together, a critical component to the technological and economic benefits of Moore's Law. For future generations, designers also have the ability to continue growing the height of the fins to get even more performance and energy-efficiency gains.

"For years we have seen limits to how small transistors can get," said Moore."This change in the basic structure is a truly revolutionary approach, and one that should allow Moore's Law, and the historic pace of innovation, to continue."

World's First Demonstration of 22nm 3-D Tri-Gate Transistors

The 3-D Tri-Gate transistor will be implemented in the company's upcoming manufacturing process, called the 22nm node, in reference to the size of individual transistor features. More than 6 million 22nm Tri-Gate transistors could fit in the period at the end of this sentence.

Intel has demonstrated the world's first 22nm microprocessor, codenamed"Ivy Bridge," working in a laptop, server and desktop computer. Ivy Bridge-based Intel® Core™ family processors will be the first high-volume chips to use 3-D Tri-Gate transistors. Ivy Bridge is slated for high-volume production readiness by the end of this year.

This silicon technology breakthrough will also aid in the delivery of more highly integrated Intel® Atom™ processor-based products that scale the performance, functionality and software compatibility of Intel® architecture while meeting the overall power, cost and size requirements for a range of market segment needs.

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'Swiss Cheese' Design Enables Thin Film Silicon Solar Cells With Potential for Higher Efficiencies

One long-term option for low-cost, high-yield industrial production of solar panels from abundant raw materials can be found in amorphous silicon solar cells and microcrystalline silicon tandem cells (a.k.a. Micromorph) -- providing an energy payback within a year.

A drawback to these cells, however, is that the stable panel efficiency is less than the efficiency of presently dominate crystalline wafer-based silicon, explains Milan Vanecek, who heads the photovoltaic group at the Institute of Physics in Prague.

"To make amorphous and microcrystalline silicon cells more stable they're required to be very thin because of tight spacing between electrical contacts, and the resulting optical absorption isn't sufficient," he notes."They're basically planar devices. Amorphous silicon has a thickness of 200 to 300 nanometers, while microcrystalline silicon is thicker than 1 micrometer."

The team's new design focuses on optically thick cells that are strongly absorbing, while the distance between the electrodes remains very tight. They describe their design in the American Institute of Physics' journalApplied Physics Letters.

"Our new 3D design of solar cells relies on the mature, robust absorber deposition technology of plasma-enhanced chemical vapor deposition, which is a technology already used for amorphous silicon-based electronics produced for liquid crystal displays. We just added a new nanostructured substrate for the deposition of the solar cell," Vanecek says.

This nanostructured substrate consists of an array of zinc oxide (ZnO) nanocolumns or, alternatively, from a"Swiss cheese" honeycomb array of micro-holes or nano-holes etched into the transparent conductive oxide layer (ZnO).

"This latter approach proved successful for solar cell deposition," Vanecek elaborates."The potential of these efficiencies is estimated within the range of present multicrystalline wafer solar cells, which dominate solar cell industrial production. And the significantly lower cost of Micromorph panels, with the same panel efficiency as multicrystalline silicon panels (12 to 16 percent), could boost its industrial-scale production."

The next step is a further optimization to continue improving efficiency.

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