Getting more electricity out of solar cells

New MIT model can guide design of solar cells that produce less waste heat, more useful current.

When sunlight shines on today’s solar cells, much of the incoming energy is given off as waste heat rather than electrical current. In a few materials, however, extra energy produces extra electrons — behavior that could significantly increase solar-cell efficiency.

An MIT team has now identified the mechanism by which that phenomenon happens, yielding new design guidelines for using those special materials to make high-efficiency solar cells. The results are reported in the journal Nature Chemistry by MIT alumni Shane R. Yost and Jiye Lee, and a dozen other co-authors, all led by MIT’s Troy Van Voorhis, professor of chemistry, and Marc Baldo, professor of electrical engineering.

In most photovoltaic (PV) materials, a photon (a packet of sunlight) delivers energy that excites a molecule, causing it to release one electron. But when high-energy photons provide more than enough energy, the molecule still releases just one electron — plus waste heat.

A few organic molecules don’t follow that rule. Instead, they generate more than one electron per high-energy photon. That phenomenon — known as singlet exciton fission — was first identified in the 1960s. However, achieving it in a functioning solar cell has proved difficult, and the exact mechanism involved has become the subject of intense controversy in the field.

For the past four years, Van Voorhis and Baldo have been pooling their theoretical and experimental expertise to investigate this problem. In 2013, they reported making the first solar cell that gives off extra electrons from high-energy visible light, which makes up almost half the sun’s electromagnetic radiation at the Earth’s surface. According to their estimates, applying their technology as an inexpensive coating on silicon solar cells could increase efficiency by as much as 25 percent.

While that’s encouraging, understanding the mechanism at work would enable them and others to do even better. Exciton fission has now been observed in a variety of materials, all discovered — like the original ones — by chance. “We can’t rationally design materials and devices that take advantage of exciton fission until we understand the fundamental mechanism at work — until we know what the electrons are actually doing,” Van Voorhis says.

To support his theoretical study of electron behavior within PVs, Van Voorhis used experimental data gathered in samples specially synthesized by Baldo and Timothy Swager, MIT’s John D. MacArthur Professor of Chemistry. The samples were made of four types of exciton fission molecules decorated with various sorts of “spinach” — bulky side groups of atoms that change the molecular spacing without altering the physics or chemistry. To detect fission rates — which are measured in femtoseconds (10^-15 seconds) — the MIT team turned to experts including Moungi Bawendi, the Lester Wolfe Professor of Chemistry, and special equipment at Brookhaven National Laboratory and the Cavendish Laboratory at Cambridge University, under the direction of Richard Friend.

Van Voorhis’ new first-principles formula successfully predicts the fission rate in materials with vastly different structures. In addition, it confirms once and for all that the mechanism is the “classic” one proposed in 1960s: When excess energy is available in these materials, an electron in an excited molecule swaps places with an electron in an unexcited molecule nearby. The excited electron brings some energy along and leaves some behind, so that both molecules give off electrons. The result: one photon in, two electrons out. “The simple theory proposed decades ago turns out to explain the behavior,” Van Voorhis says. “The controversial, or ‘exotic,’ mechanisms proposed more recently aren’t required to explain what’s being observed here.”

The results also provide practical guidelines for designing solar cells with these materials. They show that molecular packing is important in defining the rate of fission — but only to a point. When the molecules are very close together, the electrons move so quickly that the molecules giving and receiving them don’t have time to adjust. Indeed, a far more important factor is choosing a material that has the right inherent energy levels.

The researchers are pleased with the agreement between their experimental and theoretical data — especially given the systems being modeled. Each molecule has about 50 atoms, and each atom has six to 10 electrons. “These are complicated systems to calculate,” Van Voorhis says. “That’s the reason that 50 years ago they couldn’t compute these things — but now we can.”

David Reichman, a professor of chemistry at Columbia University who was not involved in this research, considers the new findings “a very important contribution to the singlet fission literature. Via a synergistic combination of modeling, crystal engineering, and experiment, the authors have provided the first systematic study of parameters influencing fission rates,” he says. Their findings “should strongly influence design criteria of fission materials away from goals involving molecular packing and toward a focus on the electronic energy levels of selected materials.”

This work was performed in the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy. Experimental measurements were supported by the British Engineering and Physical Sciences Research Council, and work at the Center for Functional Nanomaterials at Brookhaven National Laboratory was supported by the U.S. Department of Energy.

CdTe ink makes high-efficiency solar cell

Cadmium telluride nanocrystal colloids could be used as the photovoltaic “ink” in solar cells, according to new experiments by researchers at the National Renewable Energy Laboratory and the University of Chicago. Devices made using CdTe layers as thin as just 330 nm have a sunlight-to-power conversion of efficiency of 10% while those made with 550 nm thick layers have an efficiency of more than 11%. They also boast an impressive blue light response of nearly 80% external quantum efficiency – something that allows for improved photocurrent from these cells.

Thin-film photovoltaic materials could be alternatives to traditional silicon-based solar-cell materials because they absorb sunlight more efficiently – thanks to the fact that they have direct rather than indirect bandgaps. This means that less material, weight for weight, is needed to absorb the same amount of solar radiation. What is more, thin-film photovoltaics, such as cadmium telluride, can be easily and cheaply deposited onto a wide range of flexible and rigid substrates in solution.

Spheres, faceted nanocrystals and tetrapods

There is a problem, however, in that the power-conversion efficiencies of thin-film materials that have been processed from solution are typically lower than those produced by traditional vapour deposition techniques.

Now, a team led by Dmitri Talapin of Chicago and Joseph Luther at NREL has succeeded in synthesizing CdTe inks from solutions of nanocrystals that have controllable shapes, ranging from spheres to tetrapods, and controllable crystallographic phases: wurtzite and zincblende. The researchers found that the best performing solar-cell devices are those containing tetrapodal-shaped nanocrystals in the wurtzite phase. Following a relatively low-temperature short anneal, these crystals undergo a critical phase change from wurtzite to zincblende that coincides with the small grain soluble nanocrystals growing into large grain, photovoltaic quality, CdTe.

Layer-by-layer approach

“Rather than depositing the whole CdTe layer at once, we use a layer-by-layer approach to build up a very thin layer of the CdTe and control the grain growth,” explains team member Ryan Crisp, graduate student at the Colorado School of Mines. “We then deposit more nanocrystals and repeat the process until we reach the desired layer thickness.”

As the nanocrystals change phase and sinter (or grow) together, they form polycrystalline films, he adds. These films are unique in that they are exceptionally smooth and uniform (compared with films that are produced by traditional sublimation methods). “This means that further layers have a ‘nice’ surface on which we can deposit without fear of encountering short-circuits caused by irregularities and defects,” he tells nanotechweb.org.

“The crystal grains in our material extend from the top to the bottom in a finished device, allowing us to efficiently extract charge carriers (in this case photoexcited electrons) from it. We are able to do this since the electrons do not encounter many grain boundaries – something that minimizes their chance of being ‘lost’ to defect traps as they travel through the structure.”

Higher-efficiency, lower-cost devices

Solar cells made from the CdTe ink boast a sunlight-to-power conversion efficiency of 10–12%. This value might be further improved by placing the ink on the right type of substrate. “By employing this inexpensive solution-processed ink (instead of the more expensive, and slower throughput thin-film photovoltaic materials produced by sublimation), we can make potentially higher-efficiency, lower-cost devices,” says Crisp. “We explored several device structures and found that the ink-based films perform better in a simple ITO/CdTe/ZnO/Al structure rather than the traditional structure with CdS and ZnTe contacts.”

The main limiting factor to improving device efficiency is increasing the open circuit voltage. “We now plan on improving the quality of the ITO/CdTe interface (used in our highest efficiency device) to do this – and in particular by better controlling the energy levels (that is the band alignment) of the materials at this interface,” adds Crisp.

The new photovoltaic ink is described in ACS Nano

10 Emerging Technologies That Will Change/ Have Changed (?) Your World

Note to Readers: It is interesting (To Us at GNT anyway) that the BOLD predictions for technology, should always be IOHO “re-visited”. What follows is the “Top 10 List” from 2004. 10 Years … How have the technology “fortune-tellers” done?!

 

 

10 Emerging Technologies That Will Change Your World

Technology Review unveils its annual selection of hot new technologies about to affect our lives in revolutionary ways-and profiles the innovators behind them.

Full Article Link Here: http://www2.technologyreview.com/featured-story/402435/10-emerging-technologies-that-will-change-your/

Technology Review: February 2004

With new technologies constantly being invented in universities and companies across the globe, guessing which ones will transform computing, medicine, communication, and our energy infrastructure is always a challenge. Nonetheless, Technology Review’s editors are willing to bet that the 10 emerging technologies highlighted in this special package will affect our lives and work in revolutionary ways-whether next year or next decade. For each, we’ve identified a researcher whose ideas and efforts both epitomize and reinvent his or her field. The following snapshots of the innovators and their work provide a glimpse of the future these evolving technologies may provide.

10 Emerging Technologies That Will Change Your World
Universal Translation
Synthetic Biology
Nanowires
T-Rays
Distributed Storage
RNAi Interference
Power Grid Control
Microfluidic Optical Fibers
Bayesian Machine Learning
Personal Genomics

Excerpt: Nanowires:

(Page 4 of 11)

PEIDONG YANG

Nanowires

Few emerging technologies have offered as much promise as nanotechnology, touted as the means of keeping the decades-long electronics shrinkfest in full sprint and transfiguring disciplines from power production to medical diagnostics. Companies from Samsung Electronics to Wilson Sporting Goods have invested in nanotech, and nearly every major university boasts a nanotechnology initiative. Red hot, even within this R&D frenzy, are the researchers learning to make the nanoscale wires that could be key elements in many working nanodevices.

“This effort is critical for the success of the whole [enterprise of] nanoscale science and technology,” says nanowire pioneer Peidong Yang of the University of California, Berkeley. Yang has made exceptional progress in fine-tuning the properties of nanowires. Compared to other nanostructures, “nanowires will be much more versatile, because we can achieve so many different properties just by varying the composition,” says Charles Lieber, a Harvard University chemist who has also been propelling nanowire development.

The World Of Tomorrow: Nanotechnology: Interview with PhD and Attorney D.M. Vernon

The Editor interviews Deborah M. Vernon, PhD, Partner in McCarter & English, LLP’s Boston office.

 

 

 

Why It Matters –

” … I would say the two most interesting areas in the last year or two have been in 3-D printing and nanotechnology. 3-D printing is an additive technology in which one is able to make a three-dimensional product, such as a screw, by adding material rather than using a traditional reduction process, like a CNC (milling) process or a grinding-away process.

The other interesting area has been nanotechnology. Nanotechnology is the science of materials and structures that have a dimension in the nanometer range (1-1,000 nm) – that is, on the atomic or molecular scale. A fascinating aspect of nanomaterials is that they can have vastly different material properties (e.g., chemical, electrical, mechanical properties) than their larger-scale counterparts. As a result, these materials can be used in applications where their larger-scale counterparts have traditionally not been utilized.”

Editor: Deborah, please tell us about the specific practice areas of intellectual property in which you participate.

 

 

Vernon: My practice has been directed to helping clients assess, build, maintain and enforce their intellectual property, especially in the technology areas of material science, analytical chemistry and mechanical engineering. Prior to entering the practice of law, I studied mechanical engineering as an undergraduate and I obtained a PhD in material science engineering, where I focused on creating composite materials with improved mechanical properties.

Editor: Please describe some of the new areas of biological and chemical research into which your practice takes you, such as nanotechnology, three-dimensional printing technology, and other areas.

Vernon: I would say the two most interesting areas in the last year or two have been in 3-D printing and nanotechnology. 3-D printing is an additive technology in which one is able to make a three-dimensional product, such as a screw, by adding material rather than using a traditional reduction process, like a CNC (milling) process or a grinding-away process. The other interesting area has been nanotechnology. Nanotechnology is the science of materials and structures that have a dimension in the nanometer range (1-1,000 nm) – that is, on the atomic or molecular scale.

A fascinating aspect of nanomaterials is that they can have vastly different material properties (e.g., chemical, electrical, mechanical properties) than their larger-scale counterparts. As a result, these materials can be used in applications where their larger-scale counterparts have traditionally not been utilized.

I was fortunate to work in the nanotech field in graduate school. During this time, I investigated and developed methods for forming ceramic composites, which maintain a nanoscale grain size even after sintering. Sintering is the process used to form fully dense ceramic materials. The problem with sintering is that it adds energy to a system, resulting in grain growth of the ceramic materials. In order to maintain the advantageous properties of the nanosized grains, I worked on methods that pinned the ceramic grain boundaries to reduce growth during sintering.

The methods I developed not only involved handling of nanosized ceramic particles, but also the deposition of nanofilms into a porous ceramic material to create nanocomposites. I have been able to apply this experience in my IP practice to assist clients in obtaining and assessing IP in the areas of nanolaminates and coatings, nanosized particles and nanostructures, such as carbon nanotubes, nano fluidic devices, which are very small devices which transport fluids, and 3D structures formed from nanomaterials, such as woven nanofibers.

Editor: I understand that some of the components of the new Boeing 787 are examples of nanotechnology.

Vernon: The design objective behind the 787 is that lighter, better-performing materials will reduce the weight of the aircraft, resulting in longer possible flight times and decreased operating costs. Boeing reports that approximately 50 percent of the materials in the 787 are composite materials, and that nanotechnology will play an important role in achieving and exceeding the design objective. (See, http://www.nasc.com/nanometa/Plenary%20Talk%20Chong.pdf).

While it is believed that nanocomposite materials are used in the fuselage of the 787, Boeing is investigating applying nanotechnology to reduce costs and increase performance not only in fuselage and aircraft structures, but also within energy, sensor and system controls of the aircraft.

Editor: What products have incorporated nanotechnology? What products are anticipated to incorporate its processes in the future?

Vernon: The products that people are the most familiar with are cosmetic products, such as hair products for thinning hair that deliver nutrients deep into the scalp, and sunscreen, which includes nanosized titanium dioxide and zinc oxide to eliminate the white, pasty look of sunscreens. Sports products, such as fishing rods and tennis rackets, have incorporated a composite of carbon fiber and silica nanoparticles to add strength. Nano products are used in paints and coatings to prevent algae and corrosion on the hulls of boats and to help reduce mold and kill bacteria. We’re seeing nanotechnology used in filters to separate chemicals and in water filtration.

The textile industry has also started to use nano coatings to repel water and make fabrics flame resistant. The medical imaging industry is starting to use nanoparticles to tag certain areas of the body, allowing for enhanced MRI imaging. Developing areas include drug delivery, disease detection and therapeutics for oncology. Obviously, those are definitely in the future, but it is the direction of scientific thinking.

Editor: What liabilities can product manufacturers incur who are incorporating nanotechnology into their products? What kinds of health and safety risks are incurred in their manufacture or consumption?

Vernon: There are three different areas that we should think about: the manufacturing process, consumer use and environmental issues. In manufacturing there are potential safety issues with respect to the incorporation or delivery of nanomaterials. For example, inhalation of nanoparticles can cause serious respiratory issues, and contact of some nanoparticles with the skin or eyes may result in irritation. In terms of consumer use, nanomaterials may have different material properties from their larger counterparts.

As a result, we are not quite sure how these materials will affect the human body insofar as they might have a higher toxicity level than in their larger counterparts. With respect to an environmental impact, waste or recycled products may lead to the release of nanoparticles into bodies of water or impact wildlife. The National Institute for Occupational Safety and Health has established the Nanotechnology Research Center to develop a strategic direction with respect to occupational safety and nanotechnology. Guidance and publications can be found at http://www.cdc.gov/niosh/topics/nanotech.

Editor: The European Union requires the labeling of foods containing nanomaterials. What has been the position of the Food & Drug Administration and the EPA in the United States about food labeling?

Vernon: So far the FDA has taken the position that just because nanomaterials are smaller, they are not materially different from their larger counterparts, and therefore there have been no labeling requirements on food products. The FDA believes that their current standards for safety assessment are robust and flexible enough to handle a variety of different materials. That being said, the FDA has issued some guidelines for the food and cosmetic industries, but there has not been any requirement for food labeling as of now. The EPA has a nanotechnology division, which is also studying nanomaterials and their impact, but I haven’t seen anything that specifically requires a special registration process for nanomaterials.

Editor: What new regulations regarding nanotech products are expected? Should governmental regulations be adopted to prevent nanoparticles in foods and cosmetics from causing toxicity?

Vernon: The FDA has not telegraphed that any new regulations will be put into place. The agency is currently in the data collection stage to make sure that these materials are being safely delivered to people using current FDA standards – that materials are safe for human consumption or contact with humans. We won’t really understand whether or not regulations will be coming into place until we see data coming out that indicates that there are issues that are directly associated with nanomaterials. Rather than expecting regulations, I would suggest that we examine the data regarding nano products to optimize safe handling and use procedures.

Editor: Have there ever been any cases involving toxicity resulting from nano products?

Vernon: There are current investigations about the toxicity of carbon nano tubes, but the research is in its infancy. There is no evidence to show any potential harm from this technology. Unlike asbestos or silica exposure, the science is not there yet to demonstrate any toxicity link. The general understanding is that it may take decades for any potential harm to manifest. I believe my colleague, Patrick J. Comerford, head of McCarter’s product liability team in Boston, summarizes the situation well by noting that “if any supportable science was available, plaintiff’s bar would have already made this a high-profile target.”

Editor: While some biotech cases have failed the test of patentability before the courts, such as the case of Mayo v. Prometheus, what standard has been set forth for a biotech process to pass the test for patentability?

Vernon: There is no specified bright-line test for determining if a biotech process is patentable. But what the U.S. Patent and Trademark Office has done is to issue some new examination guidelines with respect to the Mayo decision that help examiners figure out whether a biotech process is patent eligible. Specifically, the guidelines look to see if the biotech process (i.e., a process incorporating a law of nature) also includes at least one additional element or step. That additional element needs to be significant and not just a mental or correlation step. If a biotech process patent claim includes this significant additional step, there still needs to be a determination if the process is novel and non-obvious over the prior art. So while this might not be a bright-line test to help us figure out whether a biotech process is patentable, it at least gives us some direction about what the examiners are looking for in the patent claims.

Editor: What effect do you think the new America Invents Act will have in encouraging biotech companies to file early in the first stages of product development? Might that not run the risk that the courts could deny patentability as in the Ariad case where functional results of a process were described rather than the specific invention?

Vernon: The AIA goes into effect next month. What companies, especially biotech companies, need to do is file early. Companies need to submit applications supported by their research to include both a written description and enablement of the invention. Companies will need to be more focused on making sure that they are not only inventing in a timely manner but are also involving their patent counsel in planned and well-thought-out experiments to make sure that the supporting information is available in a timely fashion for patenting.

Editor: Have there been any recent cases relating to biotechnology or nanotechnology that our readers should be informed about?

Vernon: The Supreme Court will hear oral arguments in April in the Myriad case. This case involves the BRCA gene, the breast cancer gene – and the issue is whether isolating a portion of a gene is patentable. While I am not a biotechnologist, I think this case will also impact nanotechnology as a whole. Applying for a patent on a portion of a gene is not too far distant from applying for a patent on a nanoparticle of a material that already exists but which has different properties from the original, larger-counterpart material. Would this nanosize material be patentable? This will be an important case to see what guidance the Supreme Court delivers this coming term.

Editor: Is there anything else you’d like to add?

Vernon: I think the next couple of years for nanotech will be very interesting. As I mentioned, I did my PhD thesis in the nanotechnology area a few years ago. My studies, like those of many other students, were funded in part with government grants. There is a great deal of government money being poured into nanotechnology. In the next ten years we will start seeing more and more of this research being commercialized and adopted into our lives. To keep current of developments, readers can visit www.nano.gov.

The Metropolitan Corporate Counsel
The Leading Resource For Corporate Counsel

As a leading publication in the corporate counsel community, MCC offers unique editorial content covering legal, regulatory, legislative and business developments, featuring original articles and interviews from experts at prestigious law firms, bar associations, accounting firms and legal service providers, as well as educators, business executives and high-level state, national and international officials.

 

“At the Speed of Light” – New ‘Nanowires’ Support Integrated Nanophotonic Circuits

A new combination of materials can efficiently guide electricity and light along the same tiny wire, a finding that could be a step towards building computer chips capable of transporting digital information at the speed of light.

Abstract

The continually increasing demands for higher-speed and lower-operating-power devices have resulted in the continued impetus to shrink photonic components. We demonstrate a primitive nanophotonic integrated circuit element composed of a single silver nanowire and single-layer molybdenum disulfide (MoS2 ) flake.

Using scanning confocal fluorescence microscopy and spectroscopy, we find that nanowire plasmons can excite MoS2 photoluminescence and that MoS2 excitons can decay into nanowire plasmons. Finally, we show that the nanowire may serve the dual purpose of both exciting MoS2 photoluminescence via plasmons and recollecting the decaying exciton as nanowire plasmons. The potential for subwavelength light guiding and strong nanoscale light–matter interaction afforded by our device may facilitate compact and efficient on-chip optical processing.

© 2014 Optical Society of America

Funding By: Directorate for Mathematical and Physical Sciences (MPS)10.13039/100000086 (DMR-1309734); Office of Science, U.S. Department of Energy10.13039/100006132 (DE-FG02-05ER46207); NSF IGERT (DGE-0966089); Institute of Optics.

To read the Full Disclosure Paper Go Here:

http://www.opticsinfobase.org/optica/fulltext.cfm?uri=optica-1-3-149&id=300737

 

 

Genesis Nanotech News: Latest Updates

Genesis Nanotech: Latest News & Updates in Nanotechnology

U of Maryland Researchers Discover New synthesis Method: Could Impact the Futures of Nanostructures, Clean Energy

 

 

New Patent Issued to Samsung for Quantum Dot Organic Light Emitting Device (QDLED)

 

 

Simpler process to grow germanium nanowires could improve lithium ion batteries

 

 

Nanotechnology will leapfrog development.|Hiru News Official Web Site|Sri Lanka

Eco-Friendly ‘pre-fab nanoparticles’ Could Revolutionize Nano Manufacturing

 

Eco-friendly ‘pre-fab nanoparticles’ could revolutionize nano manufacturing: UMass Amherst team invents a way to create versatile, water-soluble nano-modules

Amherst. MA | Posted on August 13th, 2014

 

A team of materials chemists, polymer scientists, device physicists and others at the University of Massachusetts Amherst today report a breakthrough technique for controlling molecular assembly of nanoparticles over multiple length scales that should allow faster, cheaper, more ecologically friendly manufacture of organic photovoltaics and other electronic devices. Details are in the current issue of Nano Letters.

Lead investigator, chemist Dhandapani Venkataraman, points out that the new techniques successfully address two major goals for device manufacture: controlling molecular assembly and avoiding toxic solvents like chlorobenzene. “Now we have a rational way of controlling this assembly in a water-based system,” he says. “It’s a completely new way to look at problems. With this technique we can force it into the exact structure that you want.”

Materials chemist Paul Lahti, co-director with Thomas Russell of UMass Amherst’s Energy Frontiers Research Center (EFRC) supported by the U.S. Department of Energy, says, “One of the big implications of this work is that it goes well beyond organic photovoltaics or solar cells, where this advance is being applied right now. Looking at the bigger picture, this technique offers a very promising, flexible and ecologically friendly new approach to assembling materials to make device structures.”

 

Lahti likens the UMass Amherst team’s advance in materials science to the kind of benefits the construction industry saw with prefabricated building units. “This strategy is right along that general philosophical line,” he says. “Our group discovered a way to use sphere packing to get all sorts of materials to behave themselves in a water solution before they are sprayed onto surfaces in thin layers and assembled into a module. We are pre-assembling some basic building blocks with a few predictable characteristics, which are then available to build your complex device.”

“Somebody still has to hook it up and fit it out the way they want,” Lahti adds. “It’s not finished, but many parts are pre-assembled. And you can order characteristics that you need, for example, a certain electron flow direction or strength. All the modules can be tuned to have the ability to provide electron availability in a certain way. The availability can be adjusted, and we’ve shown that it works.”

The new method should reduce the time nano manufacturing firms spend in trial-and-error searches for materials to make electronic devices such as solar cells, organic transistors and organic light-emitting diodes. “The old way can take years,” Lahti says.

“Another of our main objectives is to make something that can be scaled up from nano- to mesoscale, and our method does that. It is also much more ecologically friendly because we use water instead of dangerous solvents in the process,” he adds.

For photovoltaics, Venkataraman points out, “The next thing is to make devices with other polymers coming along, to increase power conversion efficiency and to make them on flexible substrates. In this paper we worked on glass, but we want to translate to flexible materials and produce roll-to-roll manufactured materials with water. We expect to actually get much greater efficiency.” He suggests that reaching 5 percent power conversion efficiency would justify the investment for making small, flexible solar panels to power devices such as smart phones.

If the average smart phone uses 5 watts of power and all 307 million United States users switched from batteries to flexible solar, it could save more than 1500 megawatts per year. “That’s nearly the output of a nuclear power station,” Venkataraman says, “and it’s more dramatic when you consider that coal-fired power plants generate 1 megawatt and release 2,250 lbs. of carbon dioxide. So if a fraction of the 6.6 billion mobile phone users globally changed to solar, it would reduce our carbon footprint a lot.”

Doctoral student and first author Tim Gehan says that organic solar cells made in this way can be semi-transparent, as well, “so you could replace tinted windows in a skyscraper and have them all producing electricity during the day when it’s needed. And processing is much cheaper and cleaner with our cells than in traditional methods.”

Venkataraman credits organic materials chemist Gehan, with postdoctoral fellow and device physicist Monojit Bag, with making “crucial observations” and using “persistent detective work” to get past various roadblocks in the experiments. “These two were outstanding in helping this story move ahead,” he notes. For their part, Gehan and Bag say they got critical help from the Amherst Fire Department, which loaned them an infrared camera to pinpoint some problem hot spots on a device.

It was Bag who put similar sized and charged nanoparticles together to form a building block, then used an artist’s airbrush to spray layers of electrical circuits atop each other to create a solar-powered device. He says, “Here we pre-formed structures at nanoscale so they will form a known structure assembled at the meso scale, from which you can make a device. Before, you just hoped your two components in solution would form the right mesostructure, but with this technique we can direct it to that end.”

###

This work at the Polymer-Based Materials for Harvesting Solar Energy is part of an EFRC supported by the U.S. DOE’s Office of Basic Energy Science.

 

Copyright © University of Massachusetts at Amherst

 

Translating Science into Business: The Business of Organic Semiconductors

 

“There are many things which can go wrong when starting a company; but the worst thing that can go wrong is to not do it,” said Prof. Karl Leo, Director of KAUST’s Solar & Photovoltaics Engineering Research Center, when speaking at an Entrepreneurship Center speaker series event this past spring. Wearing the dual hats of scientist and entrepreneur, Prof. Leo is the author of 440 publications, holds more than 50 patents, and has co-created 8 companies which have generated over 300 jobs.

A physicist by training, Prof. Leo highlighted the point that he is primarily a scientist who stumbled onto business by chance. “For me it’s always started with and been about the science,” he says. All his spin-off companies came about as a result of basic research he and his group conducted on organic semiconductors. Speaking specifically to the young KAUST researchers hoping to emulate his success as academics and entrepreneurs, Prof. Leo said: “The message I want to pass along is if you really want to do things, just be curious. Don’t say I want to do research to make a company. Do very basic research and the spin-off ideas will come along.”

The Growing Influence of Organic Semiconductors

Prof. Karl Leo started doing research on organic semiconductors about 20 years ago. He has since been passionate about this field’s developments and future potential. Despite his early skepticism resulting from the ephemeral lifetime of organic semiconductors in the ’90s, the performance levels of LED devices for instance have gone from just a few minutes of useful life then to virtually not aging today. “In the long-term, as in 20 to 30 years from now, almost everything will be organics,” he believes. “Silicon has dominated electronics for a long time but organic is something new.” Organic products have evolved into a variety of applications such as: small OLED displays, OLED televisions, OLED lighting, OPV and organic electronics.

Organics, as opposed to traditional silicon-based semiconductors, are by nature essentially lousy semiconductors. Mobility, or the speed at which electrons move on these materials, is a really important property. However, when looking at the electronic properties of semiconductors, carbon offers interesting developments for the performance of organics. For instance, graphene, which is a carbon-based organic material, has even higher mobility than silicon.

One of the companies Prof. Karl Leo co-founded and began operating out of Dresden, Germany in 2003, Novaled, became a leader in in organic light-emitting diode (OLED) field. OLEDs are made up of multiple thin layers of organic materials, known as OLED stacks. They essentially emit light when electricity is applied to them. Novaled became a pioneer in developing highly efficient and long-lifetime OLED structures; and it currently holds the world record in power efficiency. They key to Novaled’s success, as Prof. Leo explains, is “the simple discovery that you can dope organics.” This was a major breakthrough achieved simply adding a very little amount of another molecule.

This organic conductivity doping technology, used to enhance the performance of OLED devices, was the main factor leading to the company being purchased by Samsung in 2013.

Organic Photovoltaics: Technology of the Future

Following the successful commercial penetration of OLED displays in the consumer electronics market, Prof. Karl Leo has since turned his focus on organic photovoltaics. “I think organic PV is something that can change the world,” said Leo. Among the many advantages of organic photovoltaics are that they are thin organic layers which can be applied on flexible plastic substrates. They consume little energy, can be made transparent, and are compatible with low-cost large-area production technologies. Because they are transparent, they can be made into windows for instance, and also be manufactured in virtually any color. All these characteristics make organic PV ideal for consumer products.

Again based on basic research conducted by his group, Prof. Leo also started a company, Heliatek, which is now a world-leader in the production of organic solar film. Heliatek has developed the current world record in the efficiency of transparent solar cells. The company also holds the record for efficiency of opaque cells at 12 percent. Leo believes that it’s possible to achieve up to 20 percent efficiency in the near future, which will be necessary to compete with silicon and become commercially viable.

Don’t Believe Business Plans

Prof. Leo explained that the experience he and his team gained from launching a successful company like Novaled helped them to both define the objectives and obtain funding from investors for his solar cell company, Heliatek. “Once you create a successful company, things get much easier,” he said. But Leo also cautioned the budding entrepreneurs in the audience to be willing to adapt as they present and implement their ideas.

“If you have a good idea and you are convinced you have a good idea, never give up,” he said. But being able to adapt to market needs is also crucial. For instance, Leo’s original business plan for Novaled focused on manufacturing displays. But the realities of the market, and the prohibitive cost of manufacturing displays, convinced his team that the smarter way to go was to supply materials. At the end of the day, what really succeeded in getting a venture capital firm’s attention, after haven been told no 49 times, was his team’s ability to demonstrate the value of the technology.

“Business plans are useful but they must not be overestimated,” said Prof. Leo. Business plans are a good indicator of how entrepreneurs are able to structure their thoughts, identify markets and create a roadmap, but “nobody is able to predict the future in a business plan; it’s not possible.”

 

Definition of Organic Semi-Conductors: Background

An organic semiconductor is an organic material with semiconductor properties, that is, with an electrical conductivity between that of insulators and that of metals. Single molecules, oligomers, and organic polymers can be semiconductive. Semiconducting small molecules (aromatic hydrocarbons) include the polycyclic aromatic compounds pentacene, anthracene, and rubrene. Polymeric organic semiconductors include poly(3-hexylthiophene), poly(p-phenylene vinylene), as well as polyacetylene and its derivatives.

There are two major overlapping classes of organic semiconductors. These are organic charge-transfer complexes and various linear-backbone conductive polymers derived from polyacetylene. Linear backbone organic semiconductors include polyacetylene itself and its derivatives polypyrrole, and polyaniline.

At least locally, charge-transfer complexes often exhibit similar conduction mechanisms to inorganic semiconductors. Such mechanisms arise from the presence of hole and electron conduction layers separated by a band gap.

Although such classic mechanisms are important locally, as with inorganic amorphous semiconductors, tunnelling, localized states, mobility gaps, and phonon-assisted hopping also significantly contribute to conduction, particularly in polyacetylenes. Like inorganic semiconductors, organic semiconductors can be doped. Organic semiconductors susceptible to doping such as polyaniline (Ormecon) and PEDOT:PSS are also known as organic metals

 

Further Information

• Novaled AG
• Heliatek GmbH

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SUBCOMMITTE EXAMINES BREAKTHROUGH NANOTECHNOLOGY OPPORTUNITIES FOR AMERICA

Chairman Terry: “Nanotech is a true science race between the nations, and we should be encouraging the transition from research breakthroughs to commercial development.”

WASHINGTON, DC – The Subcommittee on Commerce, Manufacturing, and Trade, chaired by Rep. Lee Terry (R-NE), today held a hearing on:

“Nanotechnology: Understanding How Small Solutions Drive Big Innovation.”

 

 

“Great Things from Small Things!” … We Couldn’t Agree More!

 

Nanoscale Desalination of Seawater Through Nanoporous Graphene

By Silvia Román

Perhaps the most repeated words in the last few years when talking about graphene — since scientists Geim and Novoselov were awarded the Nobel Prize in Physics in 2010 for their groundbreaking experiments — are “the material of the future”. There are some risks regarding so many expectations about everything related to materials science, since similar breakthroughs have ended up confined to the limits of the lab bench. Nevertheless, the outstanding properties of this one-atom thick 2D lattice of carbon atoms promise equally outstanding developments in the future.

In the meantime, new and exciting potential uses appear from time to time that keep us in suspense. One of the prospective uses currently attracting more attention is that of the nanoporous graphene materials. An interesting review recently published in Materials Today examines the last discoveries both in the production and application of this graphene-based nanocellular structure. Nanoporous graphene consists of a two-dimensional graphene sheet in which a pattern of nano-sized porous distribution has been achieved by means of different techniques, among them, electron-beam irradiation or chemical vapor deposition.

Perhaps the most repeated words in the last few years when talking about graphene – since scientists Geim and Novoselov were awarded the Nobel Prize in Physics in 2010 for their groundbreaking experiments – are “the material of the future”. There are some risks regarding so many expectations about everything related to materials science, since similar breakthroughs have ended up confined to the limits of the lab bench. Nevertheless, the outstanding properties of this one-atom thick 2D lattice of carbon atoms promise equally outstanding developments in the future. In the meantime, new and exciting potential uses appear from time to time that keep us in suspense.

One of the prospective uses currently attracting more attention is that of the nanoporous graphene materials. An interesting review¹ recently published in Materials Today examines the last discoveries both in the production and application of this graphene-based nanocellular structure. Nanoporous graphene consists of a two-dimensional graphene sheet in which a pattern of nano-sized porous distribution has been achieved by means of different techniques, among them, electron-beam irradiation or chemical vapor deposition.

 

Figure 1. Graphene has a unique one-atom thick structure made of carbon atoms arranged in a honeycomb lattice (left). Nanoporous graphene consists of creating nano-sized porous along this hexagonal graphene lattice (right). Credit: Wikimedia Commons (left); National Energy Research Scientific Computer Center (NERSC) (right)

Creating a porous structure in any kind of material always leads to new interesting properties. We have learned a lot from nature, as porous structures are constantly appearing in naturally occurring materials such as bones or wood. In the last few years materials scientists are paying much more attention to the properties arising from materials with nano-sized porosity, also known as nanofoams. In the case of graphene, this nanoporosity seems to considerably widen its range of uses. To name a few, it opens the band gap of the graphene sheets, allowing its use in field effect transistors (FETs), very limited in pristine graphene due to its zero band gap; moreover, the presence of porous in its structure increases the surface area per volume which in turn increases the content of edges acting as adsorbing sites suitable for molecular sensing applications.

 

Figure 2. Transmission electron microscopy (TEM) images of nanoporous in a graphene sheet. The scale bars correspond to 2nm (a) and 10 nm (b). | Credit: Yuan et al (2014)

But without any doubt, one of the most remarkable uses of nanoporous graphene might well be that of selective molecular sieving. There are several properties that make nanoporous graphene the ultimate selective membrane: its exceptional mechanical strength, its atomic thickness and the possibility to physically or chemically modify the graphene pores in order to create different barriers for different molecules. As soon as this potential use of nanoporous graphene as a filter for certain molecules was envisaged, the possibility of applying this new material to desalination of seawater became a focal point for materials scientists. It is well known that the shortage of water resources for human activities is one of the most urgent problems worldwide, while oceans and seas contain around 97% of the planet’s water.

That’s why desalination could become an ultimate solution where depletion and deterioration of water resources are already unavoidable. However, as easy as this solution could appear at first sight, desalination technologies still require high technical investments and large energy consumption. The most efficient and cost-effective technology at the moment is that of reverse osmosis (RO), but the transport of water across membranes using RO is still quite slow.

It seems that the use of nanoporous membranes as filters would considerably speed up this water flow through the nano-sized channels. Furthermore, taking into account that the flux across a membrane scales inversely with the membrane’s thickness, the one-atom thick graphene structure would become the ideal material for this purpose. Like most of the ongoing studies related to nanoporous graphene, its use for desalination of sea water has been analyzed computationally, mostly using classical molecular dynamics simulations.

That is the case of a very well known study from Professor Jeffrey C. Grossman², from the Massachusetts Institute of Technology (MIT), in which a complete study of the nanoporous graphene behaviour has been carried out mainly considering three critical parameters: pore size, chemical functionalization of pore’s edges and applied pressure over the membrane.

Figure 3. Nanoporous graphene acts as a filter allowing water flow while rejecting salt ions. | Credit: Grossman and Cohen-Tanugi (2012)

Chemical functionalization of pore’s edges has been proved to have an important impact in the flux across nanoporous membranes. In this study, they altered the pore chemistry using both hydrogen groups (H-) and hydroxyl groups (OH-). The hydrogenated pores were obtained by passivating the graphene’s carbon atoms at the pore edge with hydrogen atoms, while hydroxylated pores were obtained by bonding hydrogen groups and hydroxyl groups alternatively to the unsaturated carbon atoms along the pore edge.

The result can be seen in figure 4 (a and b). Of course, pore size has to be large enough to allow water molecules flow, but narrow enough to hinder the passage of salt ions. As the pore size increases, the water permeability is higher, and then the water flow speeds up. However, larger pore areas also lead to less effective salt rejection, so that a compromise between water permeability and salt rejection must be achieved.

It seems that hydroxylated pores behave better than hydrogenated pores regarding water permeability. The authors attribute this result to an entropic effect. The hydrophobic nature of H-pores restricts the number of configurations in which water molecules can cross the membrane, while the hydrophilic nature of OH-pores allows different conformations for water molecules inside the pore, then accelerating the water flow. On the contrary, regarding salt rejection, the authors found that H-pores behave better than OH-pores. The latter are more likely to bond with salt ions and, as a result, the free energy barrier to ionic passage is reduced.

Figure 4. (a) Hydrogen groups (H-) and (b) hydroxyl groups (OH-) attached to the carbon atoms along the pore edges alter the pore chemistry and thus the water permeability of the graphene membrane. (c) Under an external applied pressure the water flows through the nano-channels while salt ions’ passage is restricted. | Credit: Grossman and Cohen-Tanugi (2012)

All in all, using these chemically modified nanoporous graphene membranes results in an increase of several orders of magnitude in the water permeability than that of the reverse osmosis (RO) membranes.

Nevertheless, there are also some critical aspects that will have to be properly improved: first of all, mechanical stability under applied pressure, although inherent in this material, could be improved by adding a support layer to the graphene membrane; on the other hand, a narrower pore size distribution would considerably improve the salt rejection performance of the membrane, allowing lower applied pressures and energy requirements. The authors suggest that the use of improved bottom-up methods in the production of nanoporous graphene will result in a remarkable progress of this kind of structures.

 

Figure 5. Functionalized porous graphene exhibits higher water permeability than other existing desalination methods without reducing its salt rejection performance. | Credit: Grossman and Cohen-Tanugi (2012)

While everybody is waiting for the graphene revolution to translate into real-world applications, the experts claim that graphene market should start to take off after 2015, and it will take some years for all these new technologies to live up to its full potential. Graphene will have to attract technological markets enough for them to make large investments in its mass production and finally allow these high expectations turn into large-scale industrial applications.

References

1. Yuan W. & Gaoquan Shi (2014). Nanoporous graphene materials, Materials Today, 17 (2) 77-85. DOI: http://dx.doi.org/10.1016/j.mattod.2014.01.021 ↩
2. Cohen-Tanugi D., Grossman J. C., (2012) Water Desalination across Nanoporous Graphene, Nano Letters, 12, p. 3602-3608. dx.doi.org/10.1021/nl3012853 ↩