Researchers @Imperial College of London uncover secret of nanomaterial that makes harvesting sunlight easier

Gold nanoparticles chemically guided inside the hot-spot of a larger gold bow-tie nanoantenna. Credit: Imperial College London

Using sunlight to drive chemical reactions, such as artificial photosynthesis, could soon become much more efficient thanks to nanomaterials.

 

This is the conclusion of a study published today led by researchers in the Department of Physics at Imperial College London, which could ultimately help improve solar energy technologies and be used for new applications, such as using sunlight to break down harmful chemicals.

Sunlight is used to drive many chemical processes that would not otherwise occur. For example, carbon dioxide and water do not ordinarily react, but in the process of photosynthesis, plants take these two chemicals and, using sunlight, produce oxygen and sugar.

The efficiency of this reaction is very high, meaning much of the energy from sunlight is transferred to the chemical reaction, but so far scientists have been unable to mimic this process in manmade artificial devices.

One reason is that many molecules that can undergo chemical reactions with light do not efficiently absorb the light themselves. They rely on photocatalysts – materials that absorb light efficiently and then pass the energy on to the molecules to drive reactions.

In the new study, researchers have investigated an artificial photocatalyst material using nanoparticles and found out how to make it more efficient.

This could lead to better solar panels, as the energy from the Sun could be more efficiently harvested. The photocatalyst could also be used to destroy liquid or gas pollutants, such as pesticides in water, by harnessing sunlight to drive reactions that break down the chemicals into less harmful forms.

Lead author Dr Emiliano Cortés from the Department of Physics at Imperial, said: “This finding opens new opportunities for increasing the efficiency of using and storing sunlight in various technologies.

“By using these materials we can revolutionize our current capabilities for storing and using sunlight with important implications in energy conversion, as well as new uses such as destroying pollutant molecules or gases and water cleaning, among others.”

The material that the team investigated is made of metal nanoparticles – particles only billionths of a metre in diameter. Their results are published today in the Journal Nature Communications.

The team, which included researchers from the Chemistry Department at University of Duisburg-Essen in Germany led by Professor Sebastian Schlücker and theoreticians from the Rensselaer Polytechnic Institute and Harvard University at the US, showed that light-induced chemical reactions occur in certain regions over the surface of these nanomaterials.

They identified which areas of the nanomaterial would be most suitable for transferring energy to chemical reactions, by tracking the locations of very small gold nanoparticles (used as a markers) on the surface of the silver nanocatalytic material.

Now that they know which regions are responsible for the process of harvesting light and transferring it to chemical reactions, the team hope to be able to engineer the nanomaterial to increase these areas and make it more efficient.

Lead researcher Professor Stefan Maier said: “This is a powerful demonstration of how metallic nanostructures, which we have investigated in my group at Imperial for the last 10 years, continue to surprise us in their abilities to control light on the nanoscale.

“The new finding uncovered by Dr Cortés and his collaborators in Germany and the US opens up new possibilities for this field in the areas photocatalysis and nanochemistry.”

Explore further: Artificial leaf as mini-factory for drugs

More information: Emiliano Cortés et al. Plasmonic hot electron transport drives nano-localized chemistry, Nature Communications (2017). DOI: 10.1038/NCOMMS14880

Journal reference: Nature Communications

Provided by: Imperial College London

MIT: “Re-Purposing” Coal into the Renewable and Electronics Economies

Jeffrey Grossman  MIT: Jeffrey Grossman Profile) thinks we’ve been looking at coal all wrong. Instead of just setting it afire, thus ignoring the molecular complexity of this highly varied material, he says, we should be harnessing the real value of that diversity and complex chemistry. Coal could become the basis for solar panels, batteries, or electronic devices, he and his research team say.

As a first demonstration of what they see as a broad range of potential high-tech uses for this traditionally low-tech material, Grossman, doctoral student Brent Keller, and research scientist Nicola Ferralis have succeeded in making a simple electrical heating device that could be used for defrosting car windows or airplane wings, or as part of a biomedical implant. In developing this initial application, they have also for the first time characterized in detail the chemical, electrical, and optical properties of thin films of four different kinds of coal: anthracite, lignite, and two bituminous types. Their findings have just been reported in the journal Nano Letters (“Rethinking Coal: Thin Films of Solution Processed Natural Carbon Nanoparticles for Electronic Devices”).

In this photo, a sample of pulverized coal (right) is shown with several test devices made from coal by the MIT researchers. (Photo courtesy of the researchers)

“When you look at coal as a material, and not just as something to burn, the chemistry is extremely rich,” says Grossman, the Morton and Claire Goulder and Family Professor in Environmental Systems in the Department of Materials Science and Engineering (DMSE). The question he wanted to ask is, “Could we leverage the wealth of chemistry in things like coal to make devices that have useful functionality?” The answer, he says, is a resounding yes.

It turns out, for example, that naturally occurring coal varieties, without the purifying or refining that is needed to make electronic devices out of silicon, have a range of electrical conductivities that spans seven orders of magnitude (ten million times). That means that a given variety of coal could inherently provide the electrical properties needed for a particular component.

Designing a process

Part of the challenge was figuring out how to process the material, Grossman says. For that, Keller developed a series of steps to crush the material to a powder, put it in solution, then deposit it in thin uniform films on a substrate — a necessary step in fabricating many electronic devices, from transistors to photovoltaics.

Even though coal has been one of the most widely used substances by human beings for centuries, its bulk electronic and optical properties had never really been studied for the purpose of advanced devices.

“The material has never been approached this way before,” says Keller, who carried out much of the work as part of his doctoral thesis in DMSE, “to find out what the properties are, what unique features there might be.” To do so, he developed a method for making thin films, which could then be tested in detail and used for device fabrication.

A simple heating device made by the researchers from unrefined pulverized coal, shown at left under visible light and at right in infrared light, showing the heat produced by the device. (Photo courtesy of the researchers)

Even this new, detailed characterization they carried out is just the tip of a large iceberg, the team says. The four varieties selected are just a few of the hundreds that exist, all with likely significant differences. And preparing and testing the samples was, from the outset, an unusual process for materials scientists. “We usually want to make materials from scratch, carefully combining pure materials in precise ratios,” says Ferralis, also in DMSE. In this case, though, the process involves “selecting from among this huge library of materials,” all with their own different variations.

Using nature’s complexity

While coal and other fossil fuels have long been used as feedstocks for the chemical industry, making everything from plastics to dyes and solvents, traditionally the material has been treated like other kinds of raw ore: something to be refined into its basic constituents, atoms, or simple molecules, which are then recombined to make the desired material. Using the chemistries that nature has provided, just as they are, is an unusual new approach. And the researchers found that by simply adjusting the temperature at which the coal is processed, they could tune many of the material’s optical and electrical properties to exactly the desired values.

The simple heating device the team made as a proof of principle provides an end-to-end demonstration of how to use the material, from grinding the coal, to depositing it as a thin film and making it into a functional electronic device. Now, they say, the doors are opened for a wide variety of potential applications through further research.

The big potential advantage of the new material, Grossman says, is its low cost stemming from the inherently cheap base material, combined with simple solution processing that enables low fabrication costs. Much of the expense associated with chip-grade silicon or graphene, for example, is in the purification of the materials. Silica, the raw material for silicon chips, is cheap and abundant, but the highly refined form needed for electronics (typically 99.999 percent pure or more) is not. Using powdered coal could provide a significant advantage for many kinds of applications, thanks to the tunability of its properties, its high conductivity, and its robustness and thermal stability.

Source: By David Chandler, MIT

 

Nanocones from “Down Under” ~ Boost Solar Cell Efficiency by 15 percent

A team of scientists at Royal Melbourne Institute of Technology in Australia has announced the development of a nanostructure material made of what they are calling nanocones—it is a type of nanomaterial that can be added to boost the efficiency of photovoltaics by increasing their light absorbing abilities. In their paper published in the journal Science Advances, the team describes the new material, how it works, and their hopes for its use in a wide variety of photovoltaic applications.

The new cone structured material’s positive attributes come about due to an ultrahigh refractive index—each cone is made of a type of material that acts inside as an insulator and outside as a conductor—under a microscope the material looks like a mass of bullets stood up on end atop a flat base. It, like other topological insulators, exploits oscillations that occur as a result of changes in the concentration of electrons that come about when the material is struck by photons. Each cone has a metal shell coating and a core that is based on a dielectric—a material made with them would be able to provide superior light absorption properties, making it ideal for not just solar cells, but a wide variety of photovoltaic applications ranging from optical fibers to waveguides and even lenses. The researchers suggest that if such a material were to be used as part of a traditional thin-film solar cell, it could increase light absorption by up to 15 percent in both the visible and ultraviolet range.

In interviews with the press, the researchers pointed out that theirs is the first time that such a nanocone structure has been created and perhaps just as importantly, noted that creating them would not require any new fabrication techniques. Also, they suggested that because of the better light absorption properties of the new material, “both the short circuit current and photoelectric conversion efficiency could be enhanced.”

The researchers also note that unlike other nanostructures the oscillations generated by the nanocones are polarization insensitive, which means they do not have to be directionally perpendicular to nanoslits making them more useful in a wider array of applications because they can be directly integrated into current hardware. They add that they next plan to shift their efforts towards focusing on plasmonics that occur in other sorts of structures with different types of shapes.

Explore further: Nanocones could be key to making inexpensive solar cells

More information: Z. Yue et al. Intrinsically core-shell plasmonic dielectric nanostructures with ultrahigh refractive index, Science Advances (2016). DOI: 10.1126/sciadv.1501536

Abstract
Topological insulators are a new class of quantum materials with metallic (edge) surface states and insulating bulk states. They demonstrate a variety of novel electronic and optical properties, which make them highly promising electronic, spintronic, and optoelectronic materials. We report on a novel conic plasmonic nanostructure that is made of bulk-insulating topological insulators and has an intrinsic core-shell formation. The insulating (dielectric) core of the nanocone displays an ultrahigh refractive index of up to 5.5 in the near-infrared frequency range. On the metallic shell, plasmonic response and strong backward light scattering were observed in the visible frequency range. Through integrating the nanocone arrays into a-Si thin film solar cells, up to 15% enhancement of light absorption was predicted in the ultraviolet and visible ranges. With these unique features, the intrinsically core-shell plasmonic nanostructure paves a new way for designing low-loss and high-performance visible to infrared optical devices.

Journal reference: Science Advances

 

ORNL (Oak Ridge National Labortory): Researchers Find ‘Greener’ way to Assemble Materials for Solar Applications: “Self-assembly of polymers using surfactants provides huge potential in fabricating nanostructures with molecular-level controllability.”

IMAGE: A surfactant template guides the self-assembly of functional polymer structures in an aqueous solution. view more

Credit: Image credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; image by Youngkyu Han and Renee Manning.

OAK RIDGE, Tenn., Oct. 5, 2015–The efficiency of solar cells depends on precise engineering of polymers that assemble into films 1,000 times thinner than a human hair.

Today, formation of that polymer assembly requires solvents that can harm the environment, but scientists at the Department of Energy’s Oak Ridge National Laboratory have found a “greener” way to control the assembly of photovoltaic polymers in water using a surfactant– a detergent-like molecule–as a template. Their findings are reported in Nanoscale, a journal of the Royal Society of Chemistry.

“Self-assembly of polymers using surfactants provides huge potential in fabricating nanostructures with molecular-level controllability,” said senior author Changwoo Do, a researcher at ORNL’s Spallation Neutron Source (SNS).

The researchers used three DOE Office of Science User Facilities–the Center for Nanophase Materials Sciences (CNMS) and SNS at ORNL and the Advanced Photon Source (APS) at Argonne National Laboratory–to synthesize and characterize the polymers.

“Scattering of neutrons and X-rays is a perfect method to investigate these structures,” said Do.

The study demonstrates the value of tracking molecular dynamics with both neutrons and optical probes.

“We would like to create very specific polymer stacking in solution and translate that into thin films where flawless, defect-free polymer assemblies would enable fast transport of electric charges for photovoltaic applications,” said Ilia Ivanov, a researcher at CNMS and a corresponding author with Do. “We demonstrated that this can be accomplished through understanding of kinetic and thermodynamic mechanisms controlling the polymer aggregation.”

The accomplishment creates molecular building blocks for the design of optoelectronic and sensory materials. It entailed design of a semiconducting polymer with a hydrophobic (“water-fearing”) backbone and hydrophilic (“water-loving”) side chains. The water-soluble side-chains could allow “green” processing if the effort produced a polymer that could self-assemble into an organic photovoltaic material. The researchers added the polymer to an aqueous solution containing a surfactant molecule that also has hydrophobic and hydrophilic ends. Depending on temperature and concentration, the surfactant self-assembles into different templates that guide the polymer to pack into different nanoscale shapes–hexagons, spherical micelles and sheets.

In the semiconducting polymer, atoms are organized to share electrons easily. The work provides insight into the different structural phases of the polymer system and the growth of assemblies of repeating shapes to form functional crystals. These crystals form the basis of the photovoltaic thin films that provide power in environments as demanding as deserts and outer space.

“Rationally encoding molecular interactions to rule the molecular geometry and inter-molecular packing order in a solution of conjugated polymers is long desired in optoelectronics and nanotechnology,” said the paper’s first author, postdoctoral fellow Jiahua Zhu. “The development is essentially hindered by the difficulty of in situ characterization.”

In situ, or “on site,” measurements are taken while a phenomenon (such as a change in molecular morphology) is occurring. They contrast with measurements taken after isolating the material from the system where the phenomenon was seen or changing the test conditions under which the phenomenon was first observed. The team developed a test chamber that allows them to use optical probes while changes occur.

Neutrons can probe structures in solutions

Expertise and equipment at SNS, which provides the most intense pulsed neutron beams in the world, made it possible to discover that a functional photovoltaic polymer could self-assemble in an environmentally benign solvent. The efficacy of the neutron scattering was enhanced, in turn, by a technique called selective deuteration, in which specific hydrogen atoms in the polymers are replaced by heavier atoms of deuterium–which has the effect of heightening contrasts in the structure. CNMS has a specialty in the latter technique.

“We needed to be able to see what’s happening to these molecules as they evolve in time from some solution state to some solid state,” author Bobby Sumpter of CNMS said. “This is very difficult to do, but for molecules like polymers and biomolecules, neutrons are some of the best probes you can imagine.” The information they provide guides design of advanced materials.

By combining expertise in topics including neutron scattering, high-throughput data analysis, theory, modeling and simulation, the scientists developed a test chamber for monitoring phase transitions as they happened. It tracks molecules under conditions of changing temperature, pressure, humidity, light, solvent composition and the like, allowing researchers to assess how working materials change over time and aiding efforts to improve their performance.

Scientists place a sample in the chamber and transport it to different instruments for measurements. The chamber has a transparent face to allow entry of laser beams to probe materials. Probing modes–including photons, electrical charge, magnetic spin and calculations aided by high-performance computing–can operate simultaneously to characterize matter under a broad range of conditions. The chamber is designed to make it possible, in the future, to use neutrons and X-rays as additional and complementary probes.

“Incorporation of in situ techniques brings information on kinetic and thermodynamic aspects of materials transformations in solutions and thin films in which structure is measured simultaneously with their changing optoelectronic functionality,” Ivanov said. “It also opens an opportunity to study fully assembled photovoltaic cells as well as metastable structures, which may lead to unique features of future functional materials.”

Whereas the current study examined phase transitions (i.e., metastable states and chemical reactions) at increasing temperatures, the next in situ diagnostics will characterize them at high pressure. Moreover, the researchers will implement neural networks to analyze complex nonlinear processes with multiple feedbacks.

The title of the Nanoscale paper is “Controlling molecular ordering in solution-state conjugated polymers.”

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Zhu, Do and Ivanov led the study. Zhu, Ivanov and Youngkyu Han conducted synchrotron X-ray scattering and optical measurements. Sumpter, Rajeev Kumar and Sean Smith performed theory calculations. Youjun He and Kunlun Hong synthesized the water-soluble polymer. Peter Bonnesen conducted thermal nuclear magnetic resonance analysis on the water-soluble polymer. Do, Han and Greg Smith performed neutron measurement and analysis of the scattering results. This research was conducted at CNMS and SNS, which are DOE Office of Science User Facilities at ORNL. Moreover, the Advanced Photon Source, a DOE Office of Science User Facility at Argonne National Laboratory, was used to perform synchrotron X-ray scattering on the polymer solution. Laboratory Directed Research and Development funds partially supported the work.

UT-Battelle manages ORNL for DOE’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit http://science.energy.gov/.

Graphene – Silicon-Perovskite Tandem Solar Cells = Greater Conversion Efficiency

Silicon absorbers primarily convert the red portion of the solar spectrum very effectively into electrical energy, whereas the blue portions are partially lost as heat. To reduce this loss, the silicon cell can be combined with an additional solar cell that primarily converts the blue portions. Teams at Helmholtz Zentrum Berlin (HZB) have already acquired extensive experience with these kinds of tandem cells. A particularly effective complement to conventional silicon is the hybrid material called perovskite. It has a band gap of 1.6 electron volts with organic as well as inorganic components. However, it is very difficult to provide the perovskite layer with a transparent front contact. While sputter deposition of indium tin oxide (ITO) is common practice for inorganic silicon solar cells, this technique destroys the organic components of a perovskite cell.

Graphene as transparent front contact

Now a group headed by Prof. Norbert Nickel has introduced a new solution. Dr. Marc Gluba and PhD student Felix Lang have developed a process to cover the perovskite layer evenly with graphene (“Perovskite Solar Cells with Large-Area CVD-Graphene for Tandem Solar Cells”). Graphene consists of carbon atoms that have arranged themselves into a two-dimensional honeycomb lattice forming an extremely thin film that is highly conductive and highly transparent.

The perovskite film (black, 200-300 nm) is covered by Spiro.OMeTAD, Graphene with gold contact at one edge, a glass substrate and an amorphous/crystalline silicon solar cell. (Image: F. Lang / HZB)

Fishing for graphene

As a first step, the scientists promote growth of the graphene onto copper foil from a methane atmosphere at about 1000 degrees Celsius. For the subsequent steps, they stabilise the fragile layer with a polymer that protects the graphene from cracking. In the following step, Felix Lang etches away the copper foil. This enables him to transfer the protected graphene film onto the perovskite.

“This is normally carried out in water. The graphene film floats on the surface and is fished out by the solar cell, so to speak. However, in this case this technique does not work, because the performance of the perovskite degrades with moisture. Therefore we had to find another liquid that does not attack perovskite, yet is as similar to water as possible”, explains Gluba.

Ideal front contact

Subsequent measurements showed that the graphene layer is an ideal front contact in several respects. Thanks to its high transparency, none of the sunlight’s energy is lost in this layer. But the main advantage is that there are no open-circuit voltage losses, that are commonly observed for sputtered ITO layers. This increases the overall conversion efficiency.

“This solution is comparatively simple and inexpensive to implement”, says Nickel. “For the first time, we have succeeded in implementing graphene in a perovskite solar cell. This enabled us to build a high-efficiency tandem device.”

Source: Helmholtz Zentrum Berlin

Case Western University: Using Solar Cells (Energy) to Charge a lithium-ion Batteries for Electric Vehicles

Consumers aren’t embracing electric cars and trucks, partly due to the dearth of charging stations required to keep them moving. Even the conservation-minded are hesitant to go electric in some states because, studies show, if fossil fuels generate the electricity, the car is no greener than one powered with an efficient gasoline.

Charging cars by solar cell would appear to be the answer. But most cells fail to meet the power requirements needed to directly charge lithium-ion batteries used in today’s all-electric and plug-in hybrid electric vehicles.

Researchers at Case Western Reserve University, however, have wired four perovskite solar cells in series to enhance the voltage and directly photo-charged lithium batteries with 7.8 percent efficiency–the most efficient reported to date, the researchers believe.

The research, published in the Aug. 27 issue of Nature Communications, holds promise for cleaner transportation, home power sources and more.

“We found the right match between the solar cell and battery,” said Liming Dai, the Kent Hale Smith Professor of macromolecular science and engineering and leader of the research. “Others have used polymer solar cells to charge lithium batteries, but not with this efficiency.”

In fact, the researchers say their overall photoelectric conversion and storage outperformed all other reported couplings of a photo-charging component with lithium-ion batteries, flow batteries or super-capacitors.

Perovskite solar cells have active materials with a crystalline structure identical to the mineral perovskite and are considered a promising new design for capturing solar energy. Compared to silicon-based cells, they convert a broader spectrum of sunlight into electricity.

In short order, they have matched the energy conversion of silicon cells, and researchers around the world are pursuing further advances.

Dai’s lab made multilayer solar cells, which increases their energy density, performance and stability. Testing showed that, as desired, the three layers convert into a single perovskite film.

By wiring four lab-sized cells, about 0.1 centimeter square each, in series, the researchers further increased the open circuit voltage. The solar-to-electric power conversion efficiency was 12.65 percent.

To charge button-sized lithium-ion batteries, they used a lithium-ion-phosphate cathode and a lithium-titanium-oxide anode. The photoelectric conversion and storage efficiency was 7.8 percent. Through 10 photo-charge/galvanostatic (steady current) discharge cycles lasting nearly 18 hours, the technology maintained almost identical discharge/charge curves over all cycles, showing high cycling stability and compatibility of the components.

“We envision, in the not too distant future, this is a system that you could have at home to refuel your car and, eventually, because perovskite solar cells can be made as a flexible film, they would be on the car itself,” said Jiantie Xu, who, with Yonghua Chen, is an equally contributing first author of the study. Both are macromolecular science and engineering research associates in Case School of Engineering.

The researchers are developing small-scale prototypes and working to further improve the perovskite cell’s stability and optimize the system.

 

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Story Source:

The above post is reprinted from materials provided by Case Western Reserve University. Note: Materials may be edited for content and length.

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Journal Reference:

1. Jiantie Xu, Yonghua Chen, Liming Dai. Efficiently photo-charging lithium-ion battery by perovskite solar cell. Nature Communications, 2015; 6: 8103 DOI: 10.1038/ncomms9103

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MIT: Analysis sees many Promising Pathways for Solar Photovoltaic Power

New study identifies the promise and challenges facing large-scale deployment of solar photovoltaics.

In a broad new assessment of the status and prospects of solar photovoltaic technology, MIT researchers say that it is “one of the few renewable, low-carbon resources with both the scalability and the technological maturity to meet ever-growing global demand for electricity.”

Use of solar photovoltaics has been growing at a phenomenal rate: Worldwide installed capacity has seen sustained growth averaging 43 percent per year since 2000. To evaluate the prospects for sustaining such growth, the MIT researchers look at possible constraints on materials availability, and propose a system for evaluating the many competing approaches to improved solar-cell performance.

The analysis is presented in the journal Energy & Environmental Science; a broader analysis of solar technology, economics, and policy will be incorporated in a forthcoming assessment of the future of solar energy by the MIT Energy Initiative.

The team comprised MIT professors Vladimir Bulović, Tonio Buonassisi, and Robert Jaffe, and graduate students Joel Jean and Patrick Brown. One useful factor in making meaningful comparisons among new photovoltaic technologies, they conclude, is the complexity of the light-absorbing material.

The report divides the many technologies under development into three broad classes: wafer-based cells, which include traditional crystalline silicon, as well as alternatives such as gallium arsenide; commercial thin-film cells, including cadmium telluride and amorphous silicon; and emerging thin-film technologies, which include perovskites, organic materials, dye-sensitized solar cells, and quantum dots.

With the recent evolution of solar technology, says Jean, the paper’s lead author, it’s important to have a uniform framework for assessment. It may be time, he says, to re-examine the traditional classification of these technologies, generally into three areas: silicon wafer-based cells, thin-film cells, and “exotic” technologies with high theoretical efficiencies.

“We’d like to build on the conventional framework,” says Jean, a doctoral student in MIT’s Department of Electrical Engineering and Computer Science. “We’re seeking a more consistent way to think about the wide range of current photovoltaic technologies and to evaluate them for potential applications. In this study, we chose to evaluate all relevant technologies based on their material complexity.”

Under this scheme, traditional silicon — a single-element crystalline material — is the simplest material. While crystalline silicon is a mature technology with advantages including high efficiency, proven reliability, and no material scarcity constraints, it also has inherent limitations: Silicon is not especially efficient at absorbing light, and solar panels based on silicon cells tend to be rigid and heavy. At the other end of the spectrum are perovskites, organics, and colloidal quantum dots, which are “highly complex materials, but can be much simpler to process,” Jean says.

Illustration shows the MIT team’s proposed scheme for comparing different photovoltaic materials, based on the complexity of their basic molecular structure. The complexity increases from the simplest material, pure silicon (single atom, lower left), to the most complex material currently being studied for potential solar cells, quantum dots (molecular structure at top right). Materials shown in between include gallium aresenide, perovskite and dye-sensitized solar cells.

Courtesy of the researchers

The authors make clear that their definition of material complexity as a key parameter for comparison does not imply any equivalency with complexity of manufacturing. On the contrary, while silicon is the simplest solar-cell material, silicon wafer and cell production is complex and expensive, requiring extraordinary purity and high temperatures.

By contrast, while some complex nanomaterials involve intricate molecular structures, such materials can be deposited quickly and at low temperatures onto flexible substrates. Nanomaterial-based cells could even be transparent to visible light, which could open up new applications and enable seamless integration into windows and other surfaces. The authors caution, however, that the conversion efficiency and long-term stability of these complex emerging technologies is still relatively low. As they write in the paper: “The road to broad acceptance of these new technologies in conventional solar markets is inevitably long, although the unique qualities of these evolving solar technologies — lightweight, paper-thin, transparent — could open entirely new markets, accelerating their adoption.”

The study does caution that the large-scale deployment of some of today’s thin-film technologies, such as cadmium telluride and copper indium gallium diselenide, may be severely constrained by the amount of rare materials that they require. The study highlights the need for novel thin-film technologies that are based on Earth-abundant materials.

The study identifies three themes for future research and development. The first is increasing the power-conversion efficiency of emerging photovoltaic technologies and commercial modules.

A second research theme is reducing the amount of material needed per cell. Thinner, more flexible films and substrates could reduce cell weight and cost, potentially opening the door to new approaches to photovoltaic module design.

A third important research theme is reducing the complexity and cost of manufacturing. Here the researchers emphasize the importance of eliminating expensive, high-temperature processing, and encouraging the adoption of roll-to-roll coating processes for rapid, large-scale manufacturing of emerging thin-film technologies.

“We’ve looked at a number of key metrics for different applications,” Jean says. “We don’t want to rule out any of the technologies,” he says — but by providing a unified framework for comparison, he says, the researchers hope to make it easier for people to make decisions about the best technologies for a given application.

Martin Green, a professor at the Australian Centre for Advanced Photovoltaics at the University of New South Wales who was not involved in this work, says the MIT team has produced “some interesting new insights and observations.” He says the paper’s main significance “lies in the attempt to take a unifying look at the issues involved in choosing between PV technologies.”

“The issues involved are complex,” Green adds, “and the authors abstain from betting on any particular PV technology.”

Can Nanotechnology Solve the Energy Crisis?

This post is part of a series examining the connections between nanotechnology and the top 10 trends facing the world, as described in the Outlook on the Global Agenda 2015. All authors are members of the Global Agenda Council on Nanotechnology.

Special to the World Economic Forum: By Tim Harper

The late Richard Smalley, often considered to be one of the fathers of nanotechnology following his Nobel Prize-winning work on fullerenes, had a keen interest in energy. In many presentations he would ask the audience to call out what they considered to be the most pressing issues facing humanity. The answers were often similar to those identified in the World Economic Forum’s Global Risks Report, including persistent worries such as disease, clean water, poverty, inequality and access to resources. Smalley would then rearrange the list to put energy at the top and proceed to explain how a happy, healthy world of 9 billion could be achieved if we could only fix the problem of providing cheap and abundant clean energy.

Back in the early 2000s, most of the imagined solutions to the energy challenge involved novel materials such as carbon nanotubes for lossless electricity transmission, or hydrogen storage to enable fuel-cell vehicles. While novel materials like nanotubes never quite lived up to their promise, 15 years later many nanotechnologies, including the latest carbon-based material graphene, are now promising to deliver huge leaps in the way that we generate, store and use energy.

But these advances are not enabled by nanotechnologies in isolation. Many of the technologies identified in the Forum’s top 10 emerging technologies list for the past three years, from gene editing to additive manufacturing, also play a role, supporting our ability to understand the nanoscale processes in nature, generating new insights into how to move beyond conventional solar cells and copy some of nature’s tricks, such as photosynthesis.

Solar solutions

The problem is that conventional silicon-based solar cells, while effective, have many drawbacks. They are brittle, which means that they need to be fixed to a rigid support, and they only harvest a small amount of the spectrum of light generated by the sun. For instance, silicon is transparent to infrared light, which means a lot of potential energy available is not harvested.

Researchers at the University of California, Riverside, are helping to solve this by working with hybrid material combining inorganic semiconductor nanoparticles with organic compounds. These first capture two infrared photons that would normally pass right through a solar cell without being converted to electricity, then add their energies together to make one higher energy photon.

An alternative approach is the use of quantum dots. These are nanoscale particles where the response to different wavelengths can be tuned by altering their sizes. Because of their unique optical properties, they are finding increasing uses in lighting and televisions, but these properties are also useful in solar cells. While the efficiency of quantum-dot solar cells reported in recent studies is increasing to as high as 9%, the real breakthrough is that the new devices can be produced at room temperature and in an atmosphere, rather than an expensive and hard-to-maintain vacuum. Perhaps the most exciting aspect of quantum-dot solar cells, though, is that the quantum dots can be dispersed in other materials, leading to “spray on” low-cost and large-area solar cells that can be applied to buildings or vehicles.

A leaf out of nature’s book

But the big prize in advanced photovoltaics will come with achieving artificial photosynthesis. The aim is to enable the production of useful chemicals and fuels directly from sunlight and carbon dioxide, just as plants do. By combining nanotechnology and biology, researchers are mimicking the processes that occur in the leaf of a plant to produce fuels such as butanol and biodegradable plastics. Once combined with synthetic biology to precisely engineer the bacteria, the possibilities are endless.

Generating energy is only half the solution, though. It also has to be stored for later use. This is an addressable issue for energy utilities, who balance peaks and troughs in demand by using techniques such as pumping water uphill into hydro-electric dams. But such large-scale engineering solutions are not an option for off-grid communities in much of the developing world. Local energy use requires a cheap and efficient way of storing energy, as do electric vehicles and smartphones.

Nanomaterials, and graphene in particular, have been attracting significant interest as potential game-changers for energy storage. One driver for this is the high surface area of many nanomaterials, which increases the ability to store charge within a given volume. Graphene – which is formed from layers of carbon a single atom thick – has a tremendous surface area for a given amount of material, and has created a lot of excitement about graphene-based supercapacitors and anodes for lithium ion batteries.

One of the biggest problems with the lithium ion batteries is the amount of charge that can be stored in the conventional graphite-based anodes they use. Lithium is added to the graphite when the battery is charging and removed as it discharges, but the low capacity of graphite means that the anode is limited in the amount of energy it can store. Researchers have been looking at silicon anodes that promise 10 times better capacity for the best part of decade, but the constant stresses on the material results in a short lifetime. One way of addressing this issue has been to place the silicon in cage of fullerenes, nanotubes or nanowires. Companies such as XG Sciences and California Lithium Battery are developing graphene-coated silicon, or “silicon-graphene nano-composite anode material”.

Fast-charging batteries

Taking a more bio-inspired approach, the Israeli company StoreDot is combining nanotechnology and biology to create nanoscale peptide crystals to produce a battery that will charge in less than a minute, while researchers in Singapore have recently developed a nanotube-based battery that could last more than 10 times as long as normal ion batteries and can also charge in minutes.

In the meantime, while we wait for current nanotechnology research to bear fruit, the biggest contribution that nanotechnology can make today is simply to reduce the amount of energy required to perform common tasks, such as heating and cooling.

The UK company Xefro, for instance, is making use of graphene to create a smart home-heating system which promises savings of up to 70%. The heaters make use of the high surface area of what is effectively a two-dimensional material to create an efficient heating material which is then applied as an ink. The ink can be printed on a variety of materials and in just about any shape, including water heaters. In a two-dimensional material, energy isn’t wasted in heating up the heater, so the heat can be turned on and off quickly. This both reduces energy use and makes the system ideal for use with smart thermostats.

Cool fractals

Meanwhile, another UK start-up called Inclusive Designs is addressing the problem of keeping things cool by combining nanomaterials and fractals with 3D printing. The company prints 3D fractal structures designed to absorb infrared (heat) and then removes the heat by making use of the high thermal conductivity of graphene, creating a cooling system with no liquids or moving parts.

Since Richard Smalley’s untimely death in 2005, the energy situation has improved, with an increasing number of countries now generating the majority of their power from renewable sources; electric vehicles are now a common sight. But cheap, efficient renewable-energy production – together with its storage and transmission – remains a challenge. The combination of nanotechnology, with a wide range of other emerging and transformative technologies, promises to make Smalley’s dream of a world of abundant, cheap, clean energy a reality over the coming decade.

Also Read: Genesis Nanotech Home Page

“Great Things from Small Things”

Also Read Our Online “Nano-News and Updates” at: GNT OnLine: “Great Things from Small Things”

More reading:
What does nanotech mean for geopolitics?
How new nanomaterials can boost renewables
Why energy poverty is the real energy crisis

Author: Tim Harper, CEO G2O Water International and co-founder of Xefro

Image: Solar panels are seen in the Palm Springs area, California April 13, 2015. REUTERS/Lucy Nicholson

University of Toronto: Two Great Photovoltaic Materials Brought Together Make Better LEDs

Ted Sargent at the University of Toronto has built a reputation over the years as being a prominent advocate for the use of quantum dots in photovoltaics. Sargent has even penned a piece for IEEE Spectrum covering the topic, and this blog has covered his record breaking efforts at boosting the conversion efficiency of quantum dot-based photovoltaics a few times.

Earlier this year, however, Sargent started to take an interest in the hot material that has the photovoltaics community buzzing: perovskite. Now, he and his research team at the University of Toronto have combined perovskite and quantum dots  into a hybrid that they believe could transform LED technology.

Read the Full Article Here: Two Great Photovoltaic Materials Brought Together Make Better LEDs.

New Graphene Compound Could “Revolutionise” Clean Tech: What IS GraphExeter – And Why Should We Take Note?

January 8th, 2015

If you’re thinking that the screamingly fast drop in oil prices will beat clean tech into the ground for the foreseeable future, guess again. This is not your father’s oil price cycle, and the next generation of transformative energy technology is already on the horizon. Case in point: graphene, the “nanomaterial of the new millennium.”

In the latest development, researchers at the University of Exeter have developed a graphene compound that could help resolve some problems with a material commonly used in solar cells and other clean tech electronic devices.

Meanwhile, over here in the US, the Energy Department’s cutting edge funding agency ARPA-E has kicked off 2015 with an open call for $125 million in transformative energy technology, and the new LITECAR challenge for transformative auto design curated by Local Motors. We’re guessing graphene could play a role in some of those projects as well.

Meet Graphene’s Cousin GraphExeter

For those of you new the topic, graphene is a sheet of carbon only one atom thick, which you can DIY by taking a piece of sticky tape to a chunk of graphite (think pencils and now you know what graphite is used for, among other things).

The discoverers of graphene did exactly that back in 2004, and ever since then the materials research field has been head over heels in love with the stuff, generating thousands of papers detailing its unique and powerful conductive properties.

However, at one atom thick graphene presents some enormous problems in terms of commercial manufacturing, which is why some teams have taken to figuring out ways to combine graphene with other, more compliant materials.

The problem is how to find materials that will preserve graphene’s superior characteristics.

At the University of Exeter, the solution was developed in 2012. It’s a graphene sandwich, with two thick layers of graphene for the bread, and a nice thick layer of ferric chloride molecules for the meat (or veggies for you vegans out there).

The beauty of GraphExeter is the combination of the new and exotic — graphene — with a widely used, commercially available material. Also called iron chloride, ferric chloride is a common industrial material used for copper etching, sewage treatment, and water purification among other things.

The Next Step For Clean Tech, Via Graphene

So, here’s where things get interesting. It’s been two years since the development of GraphExeter was announced, and the folks over at Exeter haven’t been cooling their heels since then.

Apparently the team was not initially aware that GraphExeter was particularly durable, partly because ferric chloride has a tendency to melt at room temperature. Also it dissolves easily in water, which is a problem.

In other words, you can’t use ferric chloride all by itself, because it falls apart when exposed to air and weather.

When the team started putting GraphExeter through some stress tests, they found that graphene provides the stability that ferric chloride lacks. The results showed that their new graphene compound could even beat out  indium tin oxide (ITO), which is commonly used as a conductive material in solar cells, LEDs, and other clean tech applications.

Specifically, they found that GraphExeter could hold up under high humidity, to the tune of 100 percent at room temperature, for 25 days.

They also found that it could withstand temperatures of up to 150 degrees Celsius (that’s 302 degrees Fahrenheit for those of you in the US).

In a vacuum, GraphExeter showed even better results, performing at up to 620 degrees Celsius (1,148 degrees Fahrenheit).

The figure below shows the results of subjecting a GraphExeter sample to heat at room temperature and up. The white scale bar corresponds to five nanometers (a nanometer is one billionth of a meter):

Here’s lead researcher Dr. Monica Craciun enthusing over the results:

By demonstrating its stability to being exposed to both high temperatures and humidity, we have shown that it is a practical and realistic alternative to ITO. This is particularly exciting for the solar panel industry, where the ability to withstand all weathers is crucial.

New Uses For Graphene-Enhanced Materials

Did we mention that GraphExeter is transparent? We didn’t? We must have skipped that part in the press materials, but we looked up the study online and we finally put two and two together.

ITO (indium tin oxide) is a transparent material, which makes it ideal for solar cells, LEDs, “smart” windows, and display electronics, but it has a couple of limitations, one major one being its brittleness.

If you can find something to sub in for ITO that’s flexible as well as transparent, and can at least equal ITO in efficiency and cost, then you’re talking transformation.

If you’re interested, the results of the study have just been published at Nature, in the journal Scientific Reports, under the title “Unforeseen high temperature and humidity stability of FeCl3 intercalated few layer graphene.”