Sun-Free Photovoltaics Powered by Heat

Sun-Free Photovoltaics Powered by Heat

  6:24:00 PM    Science Lover

---

A new photovoltaic energy-conversion system developed at MIT can be powered solely by heat, generating electricity with no sunlight at all. While the principle involved is not new, a novel way of engineering the surface of a material to convert heat into precisely tuned wavelengths of light -- selected to match the wavelengths that photovoltaic cells can best convert to electricity -- makes the new system much more efficient than previous versions.

A variety of silicon chip micro-reactors developed by the MIT team. Each of these contains photonic crystals on both flat faces, with external tubes for injecting fuel and air and ejecting waste products. Inside the chip, the fuel and air react to heat up the photonic crystals. In use, these reactors would have a photovoltaic cell mounted against each face, with a tiny gap between, to convert the emitted wavelengths of light to electricity. (Credit: Photo by Justin Knight)

The key to this fine-tuned light emission, described in the journal Physical Review A, lies in a material with billions of nanoscale pits etched on its surface. When the material absorbs heat -- whether from the sun, a hydrocarbon fuel, a decaying radioisotope or any other source -- the pitted surface radiates energy primarily at these carefully chosen wavelengths.

Based on that technology, MIT researchers have made a button-sized power generator fueled by butane that can run three times longer than a lithium-ion battery of the same weight; the device can then be recharged instantly, just by snapping in a tiny cartridge of fresh fuel. Another device, powered by a radioisotope that steadily produces heat from radioactive decay, could generate electricity for 30 years without refueling or servicing -- an ideal source of electricity for spacecraft headed on long missions away from the sun.

According to the U.S. Energy Information Administration, 92 percent of all the energy we use involves converting heat into mechanical energy, and then often into electricity -- such as using fuel to boil water to turn a turbine, which is attached to a generator. But today's mechanical systems have relatively low efficiency, and can't be scaled down to the small sizes needed for devices such as sensors, smartphones or medical monitors.

"Being able to convert heat from various sources into electricity without moving parts would bring huge benefits," says Ivan Celanovic ScD '06, research engineer in MIT's Institute for Soldier Nanotechnologies (ISN), "especially if we could do it efficiently, relatively inexpensively and on a small scale."

It has long been known that photovoltaic (PV) cells needn't always run on sunlight. Half a century ago, researchers developed thermophotovoltaics (TPV), which couple a PV cell with any source of heat: A burning hydrocarbon, for example, heats up a material called the thermal emitter, which radiates heat and light onto the PV diode, generating electricity. The thermal emitter's radiation includes far more infrared wavelengths than occur in the solar spectrum, and "low band-gap" PV materials invented less than a decade ago can absorb more of that infrared radiation than standard silicon PVs can. But much of the heat is still wasted, so efficiencies remain relatively low.

An ideal match

The solution, Celanovic says, is to design a thermal emitter that radiates only the wavelengths that the PV diode can absorb and convert into electricity, while suppressing other wavelengths. "But how do we find a material that has this magical property of emitting only at the wavelengths that we want?" asks Marin Soljačić, professor of physics and ISN researcher. The answer: Make a photonic crystal by taking a sample of material and create some nanoscale features on its surface -- say, a regularly repeating pattern of holes or ridges -- so light propagates through the sample in a dramatically different way.



"By choosing how we design the nanostructure, we can create materials that have novel optical properties," Soljačić says. "This gives us the ability to control and manipulate the behavior of light."

The team -- which also includes Peter Bermel, research scientist in the Research Laboratory for Electronics (RLE); Peter Fisher, professor of physics; and Michael Ghebrebrhan, a postdoc in RLE -- used a slab of tungsten, engineering billions of tiny pits on its surface. When the slab heats up, it generates bright light with an altered emission spectrum because each pit acts as a resonator, capable of giving off radiation at only certain wavelengths.

This powerful approach -- co-developed by John D. Joannopoulos, the Francis Wright Davis Professor of Physics and ISN director, and others -- has been widely used to improve lasers, light-emitting diodes and even optical fibers. The MIT team, supported in part by a seed grant from the MIT Energy Initiative, is now working with collaborators at MIT and elsewhere to use it to create several novel electricity-generating devices.

Mike Waits, an electronics engineer at the Army Research Laboratory in Adelphi, Md., who was not involved in this work, says this approach to producing miniature power supplies could lead to lighter portable electronics, which is "critical for the soldier to lighten his load. It not only reduces his burden, but also reduces the logistics chain" to deliver those devices to the field. "There are a lot of lives at stake," he says, "so if you can make the power sources more efficient, it could be a great benefit."

The button-like device that uses hydrocarbon fuels such as butane or propane as its heat source -- known as a micro-TPV power generator -- has at its heart a "micro-reactor" designed by Klavs Jensen, the Warren K. Lewis Professor of Chemical Engineering, and fabricated in the Microsystems Technology Laboratories. While the device achieves a fuel-to-electricity conversion efficiency three times greater than that of a lithium-ion battery of the same size and weight, Celanovic is confident that with further work his team can triple the current energy density. "At that point, our TPV generator could power your smartphone for a whole week without being recharged," he says.

Celanovic and Soljačić stress that building practical systems requires integrating many technologies and fields of expertise. "It's a really multidisciplinary effort," Celanovic says. "And it's a neat example of how fundamental research in materials can result in new performance that enables a whole spectrum of applications for efficient energy conversion."

Note: The full version of the MITEI story is available at: http://web.mit.edu/mitei/research/spotlights/making-electricity-with-photovoltaics.html

Read More

Artificial leaves make fuel from sunlight

Artificial leaves make fuel from sunlight

  10:06:00 AM    Science Lover

---

Two teams of researchers in the US have taken important steps towards the creation of commercially viable "artificial leaf" – a hypothetical device that can turn sunlight into electrical energy or fuel by mimicking some aspects of photosynthesis.

Daniel Nocera in his lab at the
Massachusetts Institute of Technology.
(Courtesy: Donna Coveney/MIT)

Earlier this year, the chemist Daniel Nocera at the Massachusetts Institute of Technology (MIT) announced artificial-leaf prototypes at the annual meeting of the American Chemical Society in California. Now, working with two different teams of researchers, he has published two papers on different devices that represent progress towards effective and commercially viable versions of the artificial leaf.

Here comes the Sun

Both teams made their devices from silicon wafers that are coated with catalytic metals and protective layers. The prototype solar cells are about the size of a credit card and can capture sunlight and then use the energy to split water into its constituent oxygen and hydrogen. This is different to conventional photovoltaic cells, which convert light directly into electricity. With these new devices, the ultimate plan is to recombine the two gases in an integrated fuel cell, thus converting the chemical energy to electrical energy. Producing fuel rather than electricity has the advantage that the fuel can be easily stored until it is needed.

Both artificial leaves use a silicon n–p junction: a bilayer of n-type and p-type silicon. An incident photon is absorbed to create an electron–hole pair in the semiconductor. The electrons migrate to the n-side and the holes to the p-side. The holes then drive the splitting of water in a process mediated by the outermost layer of the cell, which is a photocatalyst. Unlike some of the exotic photocatalysts used in earlier devices, the catalyst in these new devices are made of cobalt phosphate, which is an abundant and cheap material.

The main challenge in creating both devices was how to prevent the silicon from reacting with the water. The two teams took different approaches to the problem. One group led by electrical engineer Vladimir Bulovic used the catalyst itself as a protective layer, binding a thin film of pure cobalt firmly to the silicon before converting it to the phosphate form. The other team, led by mechanical engineer Tonio Buonassisi, used a thin film of conductive indium tin oxide in front of the p-type silicon as the protective layer.

Bubbles needed



Buonassisi and colleagues connected two of their cells in series and managed to split water with a solar-to-oxygen conversion efficiency of 0.25%. While this does not sound like much, the efficiency of photosynthesis is only a few per cent. However, the cells make hydrogen ions, and turning this into gas could add considerable cost to the device. "Platinum electrodes are good catalysts for reducing hydrogen ions to hydrogen gas", says Devens Gust of Arizona State University, who was not involved in the research. "However, the rarity of platinum limits its usefulness."

Gust describes the MIT work as "very important in that it demonstrates a workable, inexpensive water-oxidation catalyst". However, he says that the technology is entering a crowded market, pointing out that there is already a production technology for solar fuel that is "pretty much ready to go now". This system uses photovoltaic cells coupled to an electrolyzer that splits water into oxygen and hydrogen. "Electrolyzer efficiencies can be as high as 70–80%, and currently available photovoltaic efficiencies are as high as 15–20%", he points out. "None of the artificial photosynthetic systems can compete with this at the moment."

The MIT technology must also compete with other water-splitting systems based on silicon solar cells coated with photocatalysts. These have been in development since at least 1998 and some have reached solar-to-hydrogen conversion efficiencies of 7% or better.

"Challenges remain"

One of these cells was developed at California Institute of Technology by Nathan Lewis and Harry Atwater. Atwater told physicsworld.com that "Nocera's work is interesting, but many challenges remain." It is not clear, for example, whether the catalyst and devices remain stable beyond the few days of operation for which they have so far been tested. Atwater also thinks there is room for improvement in the materials themselves.

Gust agrees, pointing out that while cobalt and other catalysts based on common materials are promising, researchers have yet to develop an inexpensive catalyst that works near the thermodynamic potential for water oxidation/reduction. This property would help to optimize the performance of an artificial-leaf system. Nocera hopes to have a fully working device within about three years, and he has formed a company called SunCatalytix to develop it.

The work by Bulovic's group is published in Energy & Environmental Science, while the research by Buonassisi's group is outlined in Proceedings of the National Academy of Sciences USA.

Read More

Solar Cells that See Red

Solar Cells that See Red

  10:25:00 AM    Science Lover

---

Metamaterials that convert lower-energy photons to usable wavelengths could offer solar cells an efficiency boost.

Light switch: In a process that could make
solar cells more efficient, green laser light
is "upconverted" to blue light by a
solution of dyes and metal nanoparticles.
Credit: Jennifer Dionne

Researchers at Stanford University have demonstrated a set of materials that could enable solar cells to use a band of the solar spectrum that otherwise goes to waste. The materials layered on the back of solar cells would convert red and near-infrared light—unusable by today's solar cells—into shorter-wavelength light that the cells can turn into energy. The university researchers will collaborate with the Bosch Research and Technology Center in Palo Alto, California, to demonstrate a system in working solar cells in the next four years.

Even the best of today's silicon solar cells can't use about 30 percent of the light from the sun: that's because the active materials in solar cells can't interact with photons whose energy is too low. But though each of these individual photons is low energy, as a whole they represent a large amount of untapped solar energy that could make solar cells more cost-competitive.

The process, called "upconversion," relies on pairs of dyes that absorb photons of a given wavelength and re-emit them as fewer, shorter-wavelength photons. In this case, the Bosch and Stanford researchers will work on systems that convert near-infrared wavelengths (most of which are unusable by today's solar cells). The leader of the Stanford group, assistant professor Jennifer Dionne, believes the group can improve the sunlight-to-electricity conversion efficiency of amorphous-silicon solar cells from 11 percent to 15 percent.



The concept of upconversion isn't new, but it's never been demonstrated in a working solar cell, says Inna Kozinsky, a senior engineer at Bosch. Upconversion typically requires two types of molecules to absorb relatively high-wavelength photons, combine their energy, and re-emit it as higher-energy, lower-wavelength photons. However, the chances of the molecules encountering each other at the right time when they're in the right energetic states are low. Dionne is developing nanoparticles to add to these systems in order to increase those chances. To make better upconversion systems, Dionne is designing metal nanoparticles that act like tiny optical antennas, directing light in these dye systems in such a way that the dyes are exposed to more light at the right time, which creates more upconverted light, and then directing more of that upconverted light out of the system in the end.

The ultimate vision, says Dionne, is to create a solid. Sheets of such a material could be laid down on the bottom of the cell, separated from the cell itself by an electrically insulating layer. Low-wavelength photons that pass through the active layer would be absorbed by the upconverter layer, then re-emitted back into the active layer as usable, higher-wavelength light.

Kozinsky says Bosch's goal is to demonstrate upconversion of red light in working solar cells in three years, and upconversion of infrared light in four years. Factoring in the time needed to scale up to manufacturing, she says, the technology could be in Bosch's commercial solar cells in seven to 10 years.

Read More

'Cling-film' solar cells could lead to advance in renewable energy

'Cling-film' solar cells could lead to advance in renewable energy

  9:16:00 PM    Science Lover

---

A scientific advance in renewable energy which promises a revolution in the ease and cost of using solar cells, has been announced today. A new study shows that even when using very simple and inexpensive manufacturing methods - where flexible layers of material are deposited over large areas like cling-film - efficient solar cell structures can be made.

A polymer solar cell ready for testing; the metal connections allow us to measure different areas of the film and measure the device efficiency amongst other parameters. Credit: Andrew Parnell

The study, published in the journal Advanced Energy Materials, paves the way for new solar cell manufacturing techniques and the promise of developments in renewable solar energy. Scientists from the Universities of Sheffield and Cambridge used the ISIS Neutron Source and Diamond Light Source at STFC Rutherford Appleton Laboratory in Oxfordshire to carry out the research.

Plastic (polymer) solar cells are much cheaper to produce than conventional silicon solar cells and have the potential to be produced in large quantities. The study showed that when complex mixtures of molecules in solution are spread onto a surface, like varnishing a table-top, the different molecules separate to the top and bottom of the layer in a way that maximises the efficiency of the resulting solar cell.

ISIS' Target Station 2 at STFC Rutherford Appleton Laboratory in Oxfordshire. Credit: STFC

Dr Andrew Parnell of the University of Sheffield said, "Our results give important insights into how ultra-cheap solar energy panels for domestic and industrial use can be manufactured on a large scale. Rather than using complex and expensive fabrication methods to create a specific semiconductor nanostructure, high volume printing could be used to produce nanoscale (60 nanometers) films of solar cells that are over a thousand times thinner than the width of a human hair. These films could then be used to make cost-effective, light and easily transportable plastic solar cell devices such as solar panels."

Dr Robert Dalgliesh, one of the ISIS scientists involved in the work, said, "This work clearly illustrates the importance of the combined use of neutron and X-ray scattering sources such as ISIS and Diamond in solving modern challenges for society. Using neutron beams at ISIS and Diamond's bright X-rays, we were able to probe the internal structure and properties of the solar cell materials non-destructively. By studying the layers in the materials which convert sunlight into electricity, we are learning how different processing steps change the overall efficiency and affect the overall polymer solar cell performance."

This image shows how eutrons are scattered from one of the solar cell layers. Modelling this information helps us understand the composition and structure within the layer. The intense horizontal line is the mirror-like reflection (specular reflectivity) from the solar cell. The data was taken on the instrument Offspec at ISIS' Target Station 2. Credit: STFC

"Over the next fifty years society is going to need to supply the growing energy demands of the world's population without using fossil fuels, and the only renewable energy source that can do this is the Sun", said Professor Richard Jones of the University of Sheffield. "In a couple of hours enough energy from sunlight falls on the Earth to satisfy the energy needs of the Earth for a whole year, but we need to be able to harness this on a much bigger scale than we can do now. Cheap and efficient polymer solar cells that can cover huge areas could help move us into a new age of renewable energy."

Solar cells

Photovoltaics are semiconductor devices that are used to generate low-cost renewable energy - most commonly as solar panels. When sunlight hits a photovoltaic cell, it is absorbed and its energy is converted into an electrical current. Most photovoltaic devices are made with silicon; however, devices can also be made from plastic (organic photovoltaic devices).



Plastic films can be deposited from solution by low-cost, roll to roll printing techniques resulting in significant overall savings in energy and cost. This is where the film is put on a roll and goes through a series of processes similar to the way newspapers are printed and taken off a roll at the end. There are currently products using this type of technology. To increase usage further, however, the technology needs to be more efficient. Polymer solar cells are currently 7-8% efficient.The next step is to develop cells which are 10% efficient or more for commercial viability.

The materials used in the research carried out by the collaboration are called PCDTBT (poly [N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di- 2-thienyl- 2′,1′,3′-benzothiadiazole): PCBM ([6,6]- phenyl-C61-butyric acid methylester), a material based on Nobel-prize-winning (Chemistry 1996) work of Professor Richard Smalley and Professor Harry Kroto (amongst others) on the C60 Buckminsterfullerene or buckyball form of carbon.Bright X-rays using instruments at Diamond Light Source were used to study the crystallinity of the material; neutrons at ISIS were used to examine the material's composition profile.

More information: The research is published in Advanced Energy Materials, volume 1, issue 4 July 2011. The paper is also available to view online.

Provided by Science and Technology Facilities Council

Read More

Inkjet Printing Could Change the Face of Solar Energy Industry

Inkjet Printing Could Change the Face of Solar Energy Industry

  9:33:00 AM    Science Lover

---

Inkjet printers, a low-cost technology that in recent decades has revolutionized home and small office printing, may soon offer similar benefits for the future of solar energy.

Solar cell. This scanning electron microscope, 
cross-sectional image shows the various compounds 
of a new chalcopyrite solar cell only a few microns
thick, which can be created much less expensively with
inkjet printing. (Credit: Oregon State University)

Engineers at Oregon State University have discovered a way for the first time to create successful "CIGS" solar devices with inkjet printing, in work that reduces raw material waste by 90 percent and will significantly lower the cost of producing solar energy cells with some very promising compounds.

High performing, rapidly produced, ultra-low cost, thin film solar electronics should be possible, scientists said.

The findings have been published in Solar Energy Materials and Solar Cells, a professional journal, and a patent applied for on the discovery. Further research is needed to increase the efficiency of the cell, but the work could lead to a whole new generation of solar energy technology, researchers say.

"This is very promising and could be an important new technology to add to the solar energy field," said Chih-hung Chang, an OSU professor in the School of Chemical, Biological and Environmental Engineering. "Until now no one had been able to create working CIGS solar devices with inkjet technology."

Part of the advantage of this approach, Chang said, is a dramatic reduction in wasted material. Instead of depositing chemical compounds on a substrate with a more expensive vapor phase deposition -- wasting most of the material in the process -- inkjet technology could be used to create precise patterning with very low waste.

"Some of the materials we want to work with for the most advanced solar cells, such as indium, are relatively expensive," Chang said. "If that's what you're using you can't really afford to waste it, and the inkjet approach almost eliminates the waste."



One of the most promising compounds and the focus of the current study is called chalcopyrite, or "CIGS" for the copper, indium, gallium and selenium elements of which it's composed. CIGS has extraordinary solar efficiency -- a layer of chalcopyrite one or two microns thick has the ability to capture the energy from photons about as efficiently as a 50-micron-thick layer made with silicon.

In the new findings, researchers were able to create an ink that could print chalcopyrite onto substrates with an inkjet approach, with a power conversion efficiency of about 5 percent. The OSU researchers say that with continued research they should be able to achieve an efficiency of about 12 percent, which would make a commercially viable solar cell.

In related work, being done in collaboration with Greg Herman, an OSU associate professor of chemical engineering, the engineers are studying other compounds that might also be used with inkjet technology, and cost even less.

Some approaches to producing solar cells are time consuming, or require expensive vacuum systems or toxic chemicals. OSU experts are working to eliminate some of those roadblocks and create much less costly solar technology that is also more environmentally friendly. New jobs and industries in the Pacific Northwest could evolve from such initiatives, they say.

If costs can be reduced enough and other hurdles breached, it might even be possible to create solar cells that could be built directly into roofing materials, scientists say, opening a huge new potential for solar energy.

"In summary, a simple, fast, and direct-write, solution-based deposition process is developed for the fabrication of high quality CIGS solar cells," the researchers wrote in their conclusion. "Safe, cheap, and air-stable inks can be prepared easily by controlling the composition of low-cost metal salt precursors at a molecular level."

This work was supported by the Daegu Gyeongbuk Institute of Science and Technology, the U.S. Department of Energy and OSU's University Venture Development Fund, which helps donors receive tax benefits while sponsoring projects that will bring new technology, jobs and economic growth to Oregon.

Read More

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

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

  1:09:00 PM    Science Lover

---

Scientists at Empa, the Swiss Federal Laboratories for Materials Science and Technology, have further boosted the energy conversion efficiency of flexible solar cells made of copper indium gallium (di)selenide (also known as CIGS) to a new world record of 18.7 percent -- a significant improvement over the previous record of 17.6 percent achieved by the same team in June 2010.

Flexible thin film CIGS solar cell on polymer substrate 
developed at Empa. (Credit: Copyright: Empa)

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

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

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

At the forefront of efficiency improvements

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

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

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

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

Scaling up production of flexible CIGS solar cells

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

Read More

Improving Photosynthesis? Solar Cells Beat Plants at Harvesting Sun's Energy, for Now

Improving Photosynthesis? Solar Cells Beat Plants at Harvesting Sun's Energy, for Now

  6:50:00 PM    Science Lover

---

In a head-to-head battle of harvesting the sun's energy, solar cells beat plants, according to a new paper in Science. But scientists think they can even up the playing field, says researcher David Kramer at Michigan State University.

RuBisCO crucial photosynthetic enzyme sometimes reacts 
with oxygen rather than carbon, then goes through a 
long, complicated and energy-expensive process called 
photorespiration just to recover the carbon and get it back 
to the starting line. Scientists are trying to fix the problem by 
stealing ideas from C4 plants. C4 plants (right) prevent 
RuBisCO from binding oxygen by concentrating carbon 
dioxide in special photosynthetic cells called bundle-sheath 
cells. The high concentrations of carbon dioxide in these 
cells suppress oxygen binding, allowing RuBisCO to work 
more efficiently. (Credit: Viten, a service of the 
Norwegian Centre for Science Education)

Plants are less efficient at capturing the energy in sunlight than solar cells mostly because they have too much evolutionary baggage. Plants have to power a living thing, whereas solar cells only have to send electricity down a wire. This is a big difference because if photosynthesis makes a mistake, it makes toxic byproducts that kill the organism. Photosynthesis has to be conservative to avoid killing the organisms it powers.

"This is critical since it's the process that powers all of life in our ecosystem," said Kramer, a Hannah Distinguished Professor of Photosynthesis and Bioenergetics. "The efficiency of photosynthesis, and our ability to improve it, is critical to whether the entire biofuels industry is viable."

The annually averaged efficiency of photovoltaic electrolysis based on silicon semiconductors to produce fuel in the form of hydrogen is about 10 percent, while a plant's annually averaged efficiency using photosynthesis to form biomass for fuel is about 1 or 2 percent.

Plants, following the path of evolution, are primarily interested in reproducing and repairing themselves. The efficiency at which they produce stored solar energy in biomass is secondary.

Still, things can change.

Just as early Native Americans manipulated skinny, non-nutritious Teosinte into fat, juicy kernel corn, today's plants can be manipulated to become much better sources of energy.

Researcher Arthur J.Nozik, a NREL senior research fellow, and Senior Scientist Mark Hanna working at DOE's National Renewable Energy Laboratory (NREL), recently demonstrated how a multi-junction, tandem solar cell for water splitting to produce hydrogen can provide higher efficiency -- more than 40 percent -- by using multiple semiconductors and/or special photoactive organic molecules with different band gaps arranged in a tandem structure.

The coupling of different materials with different gaps means photons can be absorbed and converted to energy over a wider range of the solar spectrum.

"In photovoltaics, we know that to increase power conversion efficiency you have to have different band gaps (i.e., colors) in a tandem arrangement so they can more efficiently use different regions of the solar spectrum," Nozik said. "If you had the same gap, they would compete with each other and both would absorb the same photon energies and not enhance the solar conversion efficiency."

Photosynthesis does use two gaps based on chlorophyll molecules to provide enough energy to drive the photosynthesis reaction. But the two gaps have the same energy value, which means they don't help each other to produce energy over a wider stretch of the spectrum of solar light and enhance conversion efficiency.

Furthermore, most plants do use the full intensity of sunlight but divert some of it to protect the plant from damage. Whereas photovoltaics use the second material to gain that photoconversion edge, plants do not, Nozik noted.

One of NREL's roles at the DOE workshop was to help make it clear how the efficiency of photosynthesis could be improved by re-engineering the structure of plants through modern synthetic biology and genetic manipulation based on the principles of high efficiency photovoltaic cells, Nozik said. In synthetic biology plants can be built from scratch, starting with amino acid building blocks, allowing the formation of optimum biological band gaps.

The newly engineered plants would be darker, incorporating some biological pigments in certain of nature's flora that would be able to absorb photons in the red and infrared regions of the solar spectrum.

As plants store more solar energy efficiently, they potentially could play a greater role as alternative renewable fuel sources. The food that plants provide also would get a boost. And that would mean less land would be required to grow an equivalent amount of food.

The new information in the Science manuscript will help direct the development of new plants that have a better propensity for reducing carbon dioxide to biomass. This could spur exploration of blue algae, which not only comprise about one quarter of all plant life, but are ideal candidates for being genetically engineered into feedstock, because they absorb light from an entirely different part of the spectrum compared to most other plants.

"It would be the biological equivalent of a tandem photovoltaic cell," said Robert Blankenship, one of the lead authors in the Science paper who studies photosynthesis at Washington University in St. Louis. "And those can have very high efficiencies."

Read More

New Solar Cell Technology Greatly Boosts Efficiency

New Solar Cell Technology Greatly Boosts Efficiency

  7:17:00 PM    Science Lover

---

With the creation of a 3-D nanocone-based solar cell platform, a team led by Oak Ridge National Laboratory's Jun Xu has boosted the light-to-power conversion efficiency of photovoltaics by nearly 80 percent.

Nanocone-based solar cell consisting of n-type 
nanocones, p-type matrix, transparent conductive 
oxide (TCO) and glass substrate. (Credit: Image 
courtesy of DOE/Oak Ridge National Laboratory)

The technology substantially overcomes the problem of poor transport of charges generated by solar photons. These charges -- negative electrons and positive holes -- typically become trapped by defects in bulk materials and their interfaces and degrade performance.

"To solve the entrapment problems that reduce solar cell efficiency, we created a nanocone-based solar cell, invented methods to synthesize these cells and demonstrated improved charge collection efficiency," said Xu, a member of ORNL's Chemical Sciences Division.

The new solar structure consists of n-type nanocones surrounded by a p-type semiconductor. The n-type nanoncones are made of zinc oxide and serve as the junction framework and the electron conductor. The p-type matrix is made of polycrystalline cadmium telluride and serves as the primary photon absorber medium and hole conductor.

With this approach at the laboratory scale, Xu and colleagues were able to obtain a light-to-power conversion efficiency of 3.2 percent compared to 1.8 percent efficiency of conventional planar structure of the same materials.

"We designed the three-dimensional structure to provide an intrinsic electric field distribution that promotes efficient charge transport and high efficiency in converting energy from sunlight into electricity," Xu said.

Key features of the solar material include its unique electric field distribution that achieves efficient charge transport; the synthesis of nanocones using inexpensive proprietary methods; and the minimization of defects and voids in semiconductors. The latter provides enhanced electric and optical properties for conversion of solar photons to electricity.

Because of efficient charge transport, the new solar cell can tolerate defective materials and reduce cost in fabricating next-generation solar cells.

"The important concept behind our invention is that the nanocone shape generates a high electric field in the vicinity of the tip junction, effectively separating, injecting and collecting minority carriers, resulting in a higher efficiency than that of a conventional planar cell made with the same materials," Xu said.

Research that forms the foundation of this technology was accepted by this year's Institute of Electrical and Electronics Engineers photovoltaic specialist conference and will be published in the IEEE Proceedings. The papers are titled "Efficient Charge Transport in Nanocone Tip-Film Solar Cells" and "Nanojunction solar cells based on polycrystalline CdTe films grown on ZnO nanocones."

The research was supported by the Laboratory Directed Research and Development program and the Department of Energy's Office of Nonproliferation Research and Engineering.

Other contributors to this technology are Sang Hyun Lee, X-G Zhang, Chad Parish, Barton Smith, Yongning He, Chad Duty and Ho Nyung Lee.

Read More

Solar Power Goes Viral: Researchers Use Virus to Improve Solar-Cell Efficiency

Solar Power Goes Viral: Researchers Use Virus to Improve Solar-Cell Efficiency

  10:02:00 AM    Science Lover

---

Researchers at MIT have found a way to make significant improvements to the power-conversion efficiency of solar cells by enlisting the services of tiny viruses to perform detailed assembly work at the microscopic level.

In this diagram, the M13 virus consists of a strand 
of DNA (the figure-8 coil on the right) attached to a 
bundle of proteins called peptides -- the virus coat 
proteins (the corkscrew shapes in the center) which 
attach to the carbon nanotubes (gray cylinders) 
and hold them in place. A coating of titanium 
dioxide (yellow spheres) attached to dye molecules 
(pink spheres) surrounds the bundle. More of the 
viruses with their coatings are scattered across the 
background. (Credit: Matt Klug, Biomolecular 
Materials Group)

In a solar cell, sunlight hits a light-harvesting material, causing it to release electrons that can be harnessed to produce an electric current. The new MIT research, published online in the journal Nature Nanotechnology, is based on findings that carbon nanotubes -- microscopic, hollow cylinders of pure carbon -- can enhance the efficiency of electron collection from a solar cell's surface.

Previous attempts to use the nanotubes, however, had been thwarted by two problems. First, the making of carbon nanotubes generally produces a mix of two types, some of which act as semiconductors (sometimes allowing an electric current to flow, sometimes not) or metals (which act like wires, allowing current to flow easily). The new research, for the first time, showed that the effects of these two types tend to be different, because the semiconducting nanotubes can enhance the performance of solar cells, but the metallic ones have the opposite effect. Second, nanotubes tend to clump together, which reduces their effectiveness.

And that's where viruses come to the rescue. Graduate students Xiangnan Dang and Hyunjung Yi -- working with Angela Belcher, the W. M. Keck Professor of Energy, and several other researchers -- found that a genetically engineered version of a virus called M13, which normally infects bacteria, can be used to control the arrangement of the nanotubes on a surface, keeping the tubes separate so they can't short out the circuits, and keeping the tubes apart so they don't clump.

The system the researchers tested used a type of solar cell known as dye-sensitized solar cells, a lightweight and inexpensive type where the active layer is composed of titanium dioxide, rather than the silicon used in conventional solar cells. But the same technique could be applied to other types as well, including quantum-dot and organic solar cells, the researchers say. In their tests, adding the virus-built structures enhanced the power conversion efficiency to 10.6 percent from 8 percent -- almost a one-third improvement.

This dramatic improvement takes place even though the viruses and the nanotubes make up only 0.1 percent by weight of the finished cell. "A little biology goes a long way," Belcher says. With further work, the researchers think they can ramp up the efficiency even further.

The viruses are used to help improve one particular step in the process of converting sunlight to electricity. In a solar cell, the first step is for the energy of the light to knock electrons loose from the solar-cell material (usually silicon); then, those electrons need to be funneled toward a collector, from which they can form a current that flows to charge a battery or power a device. After that, they return to the original material, where the cycle can start again. The new system is intended to enhance the efficiency of the second step, helping the electrons find their way: Adding the carbon nanotubes to the cell "provides a more direct path to the current collector," Belcher says.

The viruses actually perform two different functions in this process. First, they possess short proteins called peptides that can bind tightly to the carbon nanotubes, holding them in place and keeping them separated from each other. Each virus can hold five to 10 nanotubes, each of which is held firmly in place by about 300 of the virus's peptide molecules. In addition, the virus was engineered to produce a coating of titanium dioxide (TiO2), a key ingredient for dye-sensitized solar cells, over each of the nanotubes, putting the titanium dioxide in close proximity to the wire-like nanotubes that carry the electrons.

The two functions are carried out in succession by the same virus, whose activity is "switched" from one function to the next by changing the acidity of its environment. This switching feature is an important new capability that has been demonstrated for the first time in this research, Belcher says.

In addition, the viruses make the nanotubes soluble in water, which makes it possible to incorporate the nanotubes into the solar cell using a water-based process that works at room temperature.

Prashant Kamat, a professor of chemistry and biochemistry at Notre Dame University who has done extensive work on dye-sensitized solar cells, says that while others have attempted to use carbon nanotubes to improve solar cell efficiency, "the improvements observed in earlier studies were marginal," while the improvements by the MIT team using the virus assembly method are "impressive."

"It is likely that the virus template assembly has enabled the researchers to establish a better contact between the TiO2 nanoparticles and carbon nanotubes. Such close contact with TiO2 nanoparticles is essential to drive away the photo-generated electrons quickly and transport it efficiently to the collecting electrode surface."

Kamat thinks the process could well lead to a viable commercial product: "Dye-sensitized solar cells have already been commercialized in Japan, Korea and Taiwan," he says. If the addition of carbon nanotubes via the virus process can improve their efficiency, "the industry is likely to adopt such processes."

Belcher and her colleagues have previously used differently engineered versions of the same virus to enhance the performance of batteries and other devices, but the method used to enhance solar cell performance is quite different, she says.

Because the process would just add one simple step to a standard solar-cell manufacturing process, it should be quite easy to adapt existing production facilities and thus should be possible to implement relatively rapidly, Belcher says.

The research team also included Paula Hammond, the Bayer Professor of Chemical Engineering; Michael Strano, the Charles (1951) and Hilda Roddey Career Development Associate Professor of Chemical Engineering; and four other graduate students and postdoctoral researchers. The work was funded by the Italian company Eni, through the MIT Energy Initiative's Solar Futures Program.

Read More

Solar Power Without Solar Cells: A Hidden Magnetic Effect of Light Could Make It Possible

Solar Power Without Solar Cells: A Hidden Magnetic Effect of Light Could Make It Possible

  9:05:00 AM    Science Lover

---

A dramatic and surprising magnetic effect of light discovered by University of Michigan researchers could lead to solar power without traditional semiconductor-based solar cells.

Researchers have discovered a dramatic and surprising magnetic effect of light that could lead to solar power without traditional semiconductor-based solar cells. (Credit: © Sean Gladwell / Fotolia)

The researchers found a way to make an "optical battery," said Stephen Rand, a professor in the departments of Electrical Engineering and Computer Science, Physics and Applied Physics.

In the process, they overturned a century-old tenet of physics.

"You could stare at the equations of motion all day and you will not see this possibility. We've all been taught that this doesn't happen," said Rand, an author of a paper on the work published in the Journal of Applied Physics. "It's a very odd interaction. That's why it's been overlooked for more than 100 years."

Light has electric and magnetic components. Until now, scientists thought the effects of the magnetic field were so weak that they could be ignored. What Rand and his colleagues found is that at the right intensity, when light is traveling through a material that does not conduct electricity, the light field can generate magnetic effects that are 100 million times stronger than previously expected. Under these circumstances, the magnetic effects develop strength equivalent to a strong electric effect.

"This could lead to a new kind of solar cell without semiconductors and without absorption to produce charge separation," Rand said. "In solar cells, the light goes into a material, gets absorbed and creates heat. Here, we expect to have a very low heat load. Instead of the light being absorbed, energy is stored in the magnetic moment. Intense magnetization can be induced by intense light and then it is ultimately capable of providing a capacitive power source."

What makes this possible is a previously undetected brand of "optical rectification," says William Fisher, a doctoral student in applied physics. In traditional optical rectification, light's electric field causes a charge separation, or a pulling apart of the positive and negative charges in a material. This sets up a voltage, similar to that in a battery. This electric effect had previously been detected only in crystalline materials that possessed a certain symmetry.

Rand and Fisher found that under the right circumstances and in other types of materials, the light's magnetic field can also create optical rectification.

"It turns out that the magnetic field starts curving the electrons into a C-shape and they move forward a little each time," Fisher said. "That C-shape of charge motion generates both an electric dipole and a magnetic dipole. If we can set up many of these in a row in a long fiber, we can make a huge voltage and by extracting that voltage, we can use it as a power source."

The light must be shone through a material that does not conduct electricity, such as glass. And it must be focused to an intensity of 10 million watts per square centimeter. Sunlight isn't this intense on its own, but new materials are being sought that would work at lower intensities, Fisher said.

"In our most recent paper, we show that incoherent light like sunlight is theoretically almost as effective in producing charge separation as laser light is," Fisher said.

This new technique could make solar power cheaper, the researchers say. They predict that with improved materials they could achieve 10 percent efficiency in converting solar power to useable energy. That's equivalent to today's commercial-grade solar cells.

"To manufacture modern solar cells, you have to do extensive semiconductor processing," Fisher said. "All we would need are lenses to focus the light and a fiber to guide it. Glass works for both. It's already made in bulk, and it doesn't require as much processing. Transparent ceramics might be even better."

In experiments this summer, the researchers will work on harnessing this power with laser light, and then with sunlight.

The paper is titled "Optically-induced charge separation and terahertz emission in unbiased dielectrics." The university is pursuing patent protection for the intellectual property.

Read More

A Greener 'Artificial Leaf' New device offers distinct advantages over previous attempts to improve upon natural photosynthesis.

A Greener 'Artificial Leaf' New device offers distinct advantages over previous attempts to improve upon natural photosynthesis.

  12:47:00 AM    Science Lover

---

Photosynthesis, nature's way of converting sunlight to fuel, happens all around us, from leaves on a tree to the smallest blade of grass. But finding a way to mimic the ability cheaply and efficiently has confounded engineers for decades.

Leaves that are not green: Silicon coated with inexpensive catalysts splits water into hydrogen and oxygen inside an illuminated container of water.
Credit: Daniel Nocera, MIT

Now researchers have taken a step toward this elusive feat, with a device that is even more efficient than natural photosynthesis and relies on low-cost, abundant materials.

Conventional solar cells produce electricity when a photovoltaic material is exposed to light. The new device goes a step further, using the resulting electricity to split water into hydrogen and oxygen, which can be stored and used to generate electricity via a fuel cell.

The new device is still in early laboratory development, and significant challenges remain before it can be commercialized.

Daniel Nocera, a professor at MIT, revealed preliminary details of the device, which he calls the first practical "artificial leaf," at the national meeting of the American Chemical Society in California on March 27. The device combines a commercially available solar cell with a pair of inexpensive catalysts made of cobalt and nickel that split water into oxygen and hydrogen. Using this approach, a solar panel roughly one square meter bathed in water could produce enough hydrogen to supply a house in a developing country with electricity for both day and night, Nocera says.



Using a thin-film silicon solar cell that converts the energy in light with 7 percent efficiency, Nocera says his group achieved 5 percent efficiency for the conversion of sunlight to hydrogen. Natural photosynthesis is less than 1 percent efficient at converting sunlight to energy.

The device is not the first to attempt to improve upon natural photosynthesis. It does, however, offer distinct advantages over previous devices, which either used costly precious-metal catalysts to split water into hydrogen and oxygen, or performed the water splitting indirectly with a separate device, which is less efficient and more expensive.

Nocera's device is the first to use inexpensive and abundant catalyst materials that are incorporated into the solar cell. "You just have a piece of silicon coated with catalysts that you can put in a glass of water, and it starts splitting the water into hydrogen and oxygen," he says.

The device is made possible by several recent advances. Nocera first developed a cobalt catalyst capable of splitting oxygen from water in 2008, but the catalyst couldn't be applied directly to silicon because it would block incoming sunlight. For his new device, Nocera applied a thin film of cobalt to the silicon that blocks only 2 to 3 percent of incoming light. Prior to applying the catalyst, he coated the silicon with a thin membrane that protects it from oxidization but allows electrical current to pass through.

A novel nickel-based catalyst also developed recently by Nocera is applied to the other side of the silicon to split hydrogen from water. The nickel catalysts already used in other water splitting devices known as electrolyzers would quickly be rendered useless by phosphate and borate present in the water. The results of initial tests of the device have been submitted for publication. They show that it can operate for at least six days without a drop in efficiency, Nocera says.

John Turner, a research fellow at the National Renewable Energy Laboratory in Golden, Colorado, says the ability to use a virtually transparent cobalt catalyst is a key advance, and the reported efficiency is promising. "He is getting most of the efficiency out of the cell," Turner says. "If he [starts with] an 11 or 12 percent cell, which is commercially available, he should be able to do much better. But we would need to see what he can do once he gets a better cell."

However, Turner says, Nocera will have to demonstrate significantly longer run times—tens of thousands of hours. "They may have the durability, but they need to continue to show it," he says.

Sun Catalytix, a company founded by Nocera, will now work with Indian industrial giant Tata to commercialize the technology for residential use in developing countries. The companies are already working together to develop another artificial photosynthesis device developed previously by Nocera. This initial device will be based on a 100-watt solar panel and will require a separate electrolyzer connected by wires to the panel. It should sell for around $100.

The device would also have to be paired with a fuel cell to convert stored hydrogen to electricity. Nocera says he hopes to deliver this initial solar-powered hydrogen-producing device to Tata by the end of 2011. The new artificial leaf should be less expensive, but it could be another two and a half years before a commercial prototype would be ready, Nocera says.

James Stevens, a research fellow at Dow Chemical, says the technology still has a long way to go. "There is a lot that has to be done before this could be practical," he says. "The efficiency is low and the capital costs of these things are very high."

Other practical issues, such as safely storing hydrogen gas and preventing the system from freezing in subzero temperatures, are also significant challenges, Stevens says. "We're not really interested in the state of the art as it now stands," he says.

Read More

Smaller Particles Could Make Solar Panels More Efficient

Smaller Particles Could Make Solar Panels More Efficient

  1:24:00 AM    Science Lover

---

Studies done by Mark Lusk and colleagues at the Colorado School of Mines could significantly improve the efficiency of solar cells. Their latest work describes how the size of light-absorbing particles--quantum dots--affects the particles' ability to transfer energy to electrons to generate electricity.

Illustration of multiple-exciton generation (MEG), a 
theory that suggests it is possible for an electron that has 
absorbed light energy, called an exciton, to transfer that 
energy to more than one electron, resulting in more electricity 
from the same amount of absorbed light. The left side shows an 
electron promoted to a high energy state (blue) plus the "hole" 
vacated by the electron (red). The right side shows the original 
exciton (now dark green/red) and a new exciton (light green
 /orange) after MEG. The top image shows a conceptualized 
version of the idea, while the bottom shows an actual exciton 
and bi-exciton using the same color scheme. (Credit: Mark 
T. Lusk, Department of Physics, Colorado School of Mines)

The results are published in the April issue of the journal ACS Nano.
 
The advance provides evidence to support a controversial idea, called multiple-exciton generation (MEG), which theorizes that it is possible for an electron that has absorbed light energy, called an exciton, to transfer that energy to more than one electron, resulting in more electricity from the same amount of absorbed light.

Quantum dots are human-made atoms that confine electrons to a small space. They have atomic-like behavior that results in unusual electronic properties on a nanoscale. These unique properties may be particularly valuable in tailoring the way light interacts with matter.

Experimental verification of the link between MEG and quantum dot size is a hot topic due to a large degree of variation in previously published studies. The ability to generate an electrical current following MEG is now receiving a great deal of attention because this will be a necessary component of any commercial realization of MEG.

For this study, Lusk and collaborators used a National Science Foundation (NSF)-supported high performance computer cluster to quantify the relationship between the rate of MEG and quantum dot size.

They found that each dot has a slice of the solar spectrum for which it is best suited to perform MEG and that smaller dots carry out MEG for their slice more efficiently than larger dots. This implies that solar cells made of quantum dots specifically tuned to the solar spectrum would be much more efficient than solar cells made of material that is not fabricated with quantum dots.


According to Lusk, "We can now design nanostructured materials that generate more than one exciton from a single photon of light, putting to good use a large portion of the energy that would otherwise just heat up a solar cell."

The research team, which includes participation from the National Renewable Energy Laboratory, is part of the NSF-funded Renewable Energy Materials Research Science and Engineering Center at the Colorado School of Mines in Golden, Colo. The center focuses on materials and innovations that will significantly impact renewable energy technologies. Harnessing the unique properties of nanostructured materials to enhance the performance of solar panels is an area of particular interest to the center.

"These results are exciting because they go far towards resolving a long-standing debate within the field," said Mary Galvin, a program director for the Division of Materials Research at NSF. "Equally important, they will contribute to establishment of new design techniques that can be used to make more efficient solar cells."

Read More