Brown liquor and solar cells to provide sustainable electricity

 A breakthrough for inexpensive electricity from solar cells, and a massive investment in wind power, will mean a need to store energy in an intelligent way. According to research at Linköping University, published in Science, batteries of biological waste products from pulp mills could provide the solution.

Organic solar cells based on conductive plastic is a low cost alternative that has achieved high enough performance to be upscaled and, in turn, become competitive. However, solar electricity must be able to be stored from day to night, as well as electricity from wind turbines from windy to calm days.

In conventional batteries metal oxides conduct the charge. Materials, such as cobalt, are expensive and a limited resource, therefore, low cost solutions are sought preferably with renewable materials.

"Nature solved the problem long ago," says Olle Inganäs, professor of biomolecular and organic electronics at Linköping University (LiU) and lead author of the article in a recent edition of Science.

He drew inspiration from the process of photosynthesis, where electrons charged by solar energy are transported by quinones; electrochemically active molecules based on benzene rings composed of six carbon atoms. Inganäs chose the raw material brown liquor that is a by-product from the manufacture of paper pulp. The brown liquor is largely composed of lignin, a biological polymer in the plant cell walls.

To utilise the quinones as charge carriers in batteries, Inganäs and his Polish colleague Grzegorz Milczarek devised a thin film from a mixture of pyrrole and lignin derivatives from the brown liquor. The film, 0.5 microns in thickness, is used as a cathode in the battery.

The goal is to offer ways to store renewable electricity where it is produced, without constructing up large grids. In several countries, major wind power investments are planned. Meanwhile, the performance of cheap organic solar cells has now reached a critical level. A research team at the University of California, Los Angeles, has recently reported efficiency of more than 10 percent of the energy of the captured sunlight.

According to Inganäs who for many years conducted research on organic solar cells, the efficiency is sufficient to initiate an industrial scale up of the technology.

"Now we need more research into new energy storage based on cheap and renewable raw materials. Lignin constitutes 20-30 percent of the biomass of a tree, so it's a source that never ends."

Story Source:

The above story is reprinted from materials provided by Linköping University.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Grzegorz Milczarek and Olle Inganäs. Renewable Cathode Materials from Biopolymer/Conjugated Polymer Interpenetrating Networks. Science, 2012 DOI: 10.1126/science.1215159]

Towards artificial photosynthesis for solar hydrogen generation

 Water splitting in photo-electrochemical cells to yield hydrogen is a promising way to sustainable fuels. A team of Swiss and US scientists now made major progress in developing highly efficient electrodes – made of an algal protein, thus mimicking a central step in natural photosynthesis.

Photosynthesis is considered the «Holy Grail» in the field of sustainable energy generation because it directly converts solar energy into storable fuel using nothing but water and carbon dioxide (CO2). Scientists have long tried to mimic the underlying natural processes and to optimize them for energy device applications such as photo-electrochemical cells (PEC), which use sunlight to electrochemically split water – and thus directly generate hydrogen, cutting short the more conventional approach using photovoltaic cells for the electrolysis of water.

Traditionally, PEC electrodes are made of semiconducting materials such as metal oxides, some of which are also known for their photocatalytic properties. For quite some time, researchers at Empa’s Laboratory for High Performance Ceramics (LHPC) have been investigating nanoparticles of these materials, for instance titanium dioxide (TiO2), for the neutralization of organic pollutants in air and water. Collaborating with colleagues at the University of Basel and at Argonne National Laboratory in the US, they now succeeded in making a nano-bio PEC electrode, consisting of iron oxide conjugated with a protein from blue-green algae (also known as cyanobacteria), which is twice as efficient in water splitting as iron oxide alone.

Iron oxide, in particular hematite (alpha-Fe2O3), is a promising electrode material for PEC because it is susceptible to visible wavelengths and thus uses sunlight more efficiently than photocatalysts like TiO2, which can only use the UV part of solar radiation. What’s more, hematite is a low-cost and abundant material.

The second ingredient in the novel electrode «recipe» is phycocyanin, a protein from blue-green algae. «I was inspired by the natural photosynthetic machinery of cyanobacteria where phycocyanin acts as a major light-harvesting component. I wanted to make artificial photosynthesis using ceramics and proteins», recalls Debajeet K. Bora who designed the new electrode during his PhD thesis at Empa. «The concept of hematite surface functionalization with proteins was completely novel in PEC research.»

After Bora covalently cross-coupled phycocyanin to hematite nanoparticles that had been immobilized as a thin film, the conjugated hematite absorbed many more photons than without the algal protein. In fact, the induced photocurrent of the hybrid electrode was doubled compared to a «normal» iron oxide electrode.

Somewhat surprisingly, the light harvesting protein complex does not get destroyed while in contact with a photocatalyst in an alkaline environment under strong illumination. Chemists would have predicted the complete denaturation of biomolecules under such corrosive and aggressive conditions. «Photocatalysts are designed to destroy organic pollutants, which are a burden to the environment. But here we have a different situation», says Artur Braun, group leader at Empa’s LHPC and principal investigator of the study. «There seems to be a delicate balance where organic molecules not only survive harsh photocatalytic conditions, but even convey an additional benefit to ceramic photocatalysts: They double the photocurrent. This is a big step forward».

The project was fully funded by the Swiss Federal Office of Energy (SFOE). Bora who will soon have completed his PhD thesis says he will continue what he started at Empa during a postdoc at the University of California, Berkeley, which he will assume early next year.

Solar alchemy: Photocatalysts to clean water and recover chemicals

 Polluted water can be easily cleaned and treated to extract valuable chemicals, e.g., used in drug manufacturing. No factories or plants are needed, the sun and a "magic" powder are enough. The nearly alchemic transformation is accomplished due to photocatalysts studied by researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw.

In many places of the world water is highly polluted by organic chemicals from industrial wastes. The experiments carried out at the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw prove that the biomass can be successfully transformed into useful chemicals and fuel. Due to appropriately selected photocatalysts, the transformation of polluted water into clean one and chemicals does not require specialized plants and takes place under conditions that are commonly met in nature.

Catalyst is a substance that participates in the chemical reaction, speeds its course and fully recovers after the reaction is completed. In typical catalytic processes the catalysts are activated at high temperatures, typically of several hundreds degrees centigrade, often at a significantly increased pressure.

"Photocatalysts studied by us differ in many respects from traditional catalysts. They are activated by light, and the temperature has no significant effect here," says Dr Juan Carlos Colmenares from the IPC PAS. The reactions with participation of photocatalysts occur at good exposure to sun rays, at temperature about 30 degrees centigrade and at normal atmospheric pressure -- so at conditions occurring naturally all year round in many equatorial countries.

The photocatalysts studied at the IPC PAS are solids based on titanium dioxide, TiO2. The catalysed reaction occurs in a liquid containing organic pollutants. After the reaction is completed, the catalyst can be isolated almost without losses and used again.

"My work resembles somewhat alchemy," jokes Colmenares. "I take a 'magic' powder, pour it into polluted water, stir and expose to the sun. After several hours, I get clean water plus chemicals that can be used to make useful things, for instance drugs."

The research on photochemical degradation of pollutants has been carried out in the world already in the late 1960's. By intensive UV irradiation chemical compounds with simple structures have been obtained at that time.

The research pursued at the IPC PAS aims at such a selection of photocatalysts and reaction conditions that the reaction can occur without using specialized equipment, and the degradation of biomass stops at a precisely defined stage. With titania-based photocatalysis the researchers managed to produce carboxylic acids used, e.g., in pharmaceutical and food industries. It is also possible to prepare a photocatalyst so as to have the reaction completed and yielding substances with the simplest structure, such as hydrogen or carbon dioxide. The latter compound is undesirable and would require disposal, hydrogen, however, has excellent prospects as the fuel of the future.

"In laboratory conditions, the reactions of the biomass with participation of photocatalysts are promising already now. In this year we are going to attempt the first tests in the pilot biochemical photoreactors at the University of Cordoba, Spain. The reactions will occur there in liquids with volumes measured in tens of litres," says Colmenares while making clear that still many tests and studies are to be carried out before the new technology gets disseminated.

The co-authors of the paper published in the Bioresource Technology journal, describing application of photocatalysts to glucose degradation and to production of valuable chemicals are Agnieszka Magdziarz and Dr Anna Bielejewska, who passed away late last year. The research has been financed from an international Marie Skłodowska-Curie reintegration grant under the 7th Framework Programme of the European Union.

Story Source:

The above story is reprinted from materials provided by Institute of Physical Chemistry of the Polish Academy of Sciences, via AlphaGalileo.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Juan C. Colmenares, Agnieszka Magdziarz, Anna Bielejewska. High-value chemicals obtained from selective photo-oxidation of glucose in the presence of nanostructured titanium photocatalysts. Bioresource Technology, 2011; 102 (24): 11254 DOI: 10.1016/j.biortech.2011.09.101

Atomic Layer Deposition (ALD) to enable novel, high efficiency silicon nanorod solar cells

 Picosun Oy reports successful final results of the European Union 7th Framework Programme funded research project ROD-SOL. The goal of this multinational, inter-European, three years (2009-2011) project combining the efforts of both scientific and industrial partners has been to dramatically increase the efficiency of solar cells and reduce the costs of their manufacturing. This has been achieved with novel, innovative, silicon nanorod based concept. The amount of active photovoltaic material (Si) can be significantly reduced by growing the light-trapping nanorod “forests” (thickness from < 1µm to a few µm at most) on cheaper substrates such as glass or flexible foils. This has led to already promising over 9% energy conversion efficiencies with very good long-term stabilities of cells. Due to their effectively 3D geometry, the nanorod forests have high active surface area which enables efficient light absorption – much more efficient than in convenient 2D thin film solar cells. Also, the location of the p-n junction much closer to the surface than in normal solar cells radically improves the minority carrier charge transport and thus the amount of electricity that can be extracted from the cell.

Due to the micrometer/sub-micrometer dimensions of the nanorod forests (dense packing, rod diameters typically few hundreds of nm and lengths < 1 µm) ALD has proven to be ideal technique for manufacturing some of the most crucial cell components. To prevent recombination losses in the active photovoltaic layer and thus cell efficiency decrease, a recombination barrier i.e. passivation layer needs to be coated on the rods’ surface. An ultrathin ALD-deposited Al2O3 film serves ideally this purpose, and the gas-phase, surface-controlled and self-limiting nature of the ALD process ensures that even the deepest and narrowest between-the-rods nooks and crannies will be reliably covered with 100 % uniform, conformal and pinhole- and defect-free passivation film. Another central cell component where ALD has shown its indispensability is the transparent conductive oxide (TCO) layer that works as the current collector on the top of the cell. Different TCO deposition methods were investigated in the course of the project, and ALD turned out to be the ideal method regarding both the TCO film quality and the scalability of the technique, due to Picosun’s fast, efficient and easy-to-use HVM (High Volume Manufacturing) batch ALD system, which was developed specifically during the project ROD-SOL.

Paint-on solar cells developed

ScienceDaily (Dec. 21, 2011) — Imagine if the next coat of paint you put on the outside of your home generates electricity from light -- electricity that can be used to power the appliances and equipment on the inside.

A team of researchers at the University of Notre Dame has made a major advance toward this vision by creating an inexpensive "solar paint" that uses semiconducting nanoparticles to produce energy.

"We want to do something transformative, to move beyond current silicon-based solar technology," says Prashant Kamat, John A. Zahm Professor of Science in Chemistry and Biochemistry and an investigator in Notre Dame's Center for Nano Science and Technology (NDnano), who leads the research.

"By incorporating power-producing nanoparticles, called quantum dots, into a spreadable compound, we've made a one-coat solar paint that can be applied to any conductive surface without special equipment."

The team's search for the new material, described in the journal ACS Nano, centered on nano-sized particles of titanium dioxide, which were coated with either cadmium sulfide or cadmium selenide. The particles were then suspended in a water-alcohol mixture to create a paste.

When the paste was brushed onto a transparent conducting material and exposed to light, it created electricity.

"The best light-to-energy conversion efficiency we've reached so far is 1 percent, which is well behind the usual 10 to 15 percent efficiency of commercial silicon solar cells," explains Kamat.

"But this paint can be made cheaply and in large quantities. If we can improve the efficiency somewhat, we may be able to make a real difference in meeting energy needs in the future."

"That's why we've christened the new paint, Sun-Believable," he adds.

Kamat and his team also plan to study ways to improve the stability of the new material.

NDnano is one of the leading nanotechnology centers in the world. Its mission is to study and manipulate the properties of materials and devices, as well as their interfaces with living systems, at the nano-scale.

This research was funded by the Department of Energy's Office of Basic Energy Sciences.

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Story Source:

The above story is reprinted from materials provided by University of Notre Dame. The original article was written by Arnie Phifer.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Matthew P. Genovese, Ian V. Lightcap, Prashant V. Kamat. Sun-BelievableSolar Paint. A Transformative One-Step Approach for Designing Nanocrystalline Solar Cells. ACS Nano, 2011; 111214102150001 DOI: 10.1021/nn204381g

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

Discovery of a 'dark state' could mean a brighter future for solar energy

The efficiency of conventional solar cells could be significantly increased, according to new research on the mechanisms of solar energy conversion led by chemist Xiaoyang Zhu at The University of Texas at Austin.

Zhu and his team have discovered that it's possible to double the number of electrons harvested from one photon of sunlight using an organic plastic semiconductor material.

"Plastic semiconductor solar cell production has great advantages, one of which is low cost," said Zhu, a professor of chemistry. "Combined with the vast capabilities for molecular design and synthesis, our discovery opens the door to an exciting new approach for solar energy conversion, leading to much higher efficiencies."

Zhu and his team published their groundbreaking discovery Dec. 16 in Science.

The maximum theoretical efficiency of the silicon solar cell in use today is approximately 31 percent, because much of the sun's energy hitting the cell is too high to be turned into usable electricity. That energy, in the form of "hot electrons," is instead lost as heat. Capturing hot electrons could potentially increase the efficiency of solar-to-electric power conversion to as high as 66 percent.

Zhu and his team previously demonstrated that those hot electrons could be captured using semiconductor nanocrystals. They published that research in Science in 2010, but Zhu says the actual implementation of a viable technology based on that research is very challenging.

"For one thing," said Zhu, "that 66 percent efficiency can only be achieved when highly focused sunlight is used, not just the raw sunlight that typically hits a solar panel. This creates problems when considering engineering a new material or device."

To circumvent that problem, Zhu and his team have found an alternative. They discovered that a photon produces a dark quantum "shadow state" from which two electrons can then be efficiently captured to generate more energy in the semiconductor pentacene.

Zhu said that exploiting that mechanism could increase solar cell efficiency to 44 percent without the need for focusing a solar beam, which would encourage more widespread use of solar technology.

The research team was spearheaded by Wai-lun Chan, a postdoctoral fellow in Zhu's group, with the help of postdoctoral fellows Manuel Ligges, Askat Jailaubekov, Loren Kaake and Luis Miaja-Avila. The research was supported by the National Science Foundation and the Department of Energy.

Science Behind the Discovery:

Absorption of a photon in a pentacene semiconductor creates an excited electron-hole pair called an exciton.The exciton is coupled quantum mechanically to a dark "shadow state" called a multiexciton.This dark shadow state can be the most efficient source of two electrons via transfer to an electron acceptor material, such as fullerene, which was used in the study.Exploiting the dark shadow state to produce double the electrons could increase solar cell efficiency to 44 percent.

Story Source:

The above story is reprinted from materials provided by University of Texas at Austin.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

W.-L. Chan, M. Ligges, A. Jailaubekov, L. Kaake, L. Miaja-Avila, X.- Y. Zhu. Observing the Multiexciton State in Singlet Fission and Ensuing Ultrafast Multielectron Transfer. Science, 2011; 334 (6062): 1541 DOI: 10.1126/science.1213986

'Fool's gold' aids discovery of new options for cheap, benign solar energy

 Pyrite, better known as "fool's gold," was familiar to the ancient Romans and has fooled prospectors for centuries -- but has now helped researchers at Oregon State University discover related compounds that offer new, cheap and promising options for solar energy.

These new compounds, unlike some solar cell materials made from rare, expensive or toxic elements, would be benign and could be processed from some of the most abundant elements on Earth. Findings on them have been published in Advanced Energy Materials, a professional journal.

Iron pyrite itself has little value as a future solar energy compound, the scientists say, just as the brassy, yellow-toned mineral holds no value compared to the precious metal it resembles. But for more than 25 years it was known to have some desirable qualities that made it of interest for solar energy, and that spurred the recent research.

The results have been anything but foolish.

"We've known for a long time that pyrite was interesting for its solar properties, but that it didn't actually work," said Douglas Keszler, a distinguished professor of chemistry at OSU. "We didn't really know why, so we decided to take another look at it. In this process we've discovered some different materials that are similar to pyrite, with most of the advantages but none of the problems.

"There's still work to do in integrating these materials into actual solar cells," Keszler said. "But fundamentally, it's very promising. This is a completely new insight we got from studying fool's gold."

Pyrite was of interest early in the solar energy era because it had an enormous capacity to absorb solar energy, was abundant, and could be used in layers 2,000 times thinner than some of its competitors, such as silicon. However, it didn't effectively convert the solar energy into electricity.

In the new study, the researchers found out why. In the process of creating solar cells, which takes a substantial amount of heat, pyrite starts to decompose and forms products that prevent the creation of electricity.

Based on their new understanding of exactly what the problem was, the research team then sought and found compounds that had the same capabilities of pyrite but didn't decompose. One of them was iron silicon sulfide.

"Iron is about the cheapest element in the world to extract from nature, silicon is second, and sulfur is virtually free," Keszler said. "These compounds would be stable, safe, and would not decompose. There's nothing here that looks like a show-stopper in the creation of a new class of solar energy materials."

Work to continue the development of the materials and find even better ones in the same class will continue at the National Renewable Energy Laboratory in Colorado, which collaborated on this research.

The work was done at the Center for Inverse Design, a collaborative initiative of the College of Science and College of Engineering at OSU, formed two years ago with a $3 million grant from the U.S. Department of Energy. It was one of the new Energy Frontier Research Centers set up through a national, $777 million federal program to identify energy solutions for the future.

The OSU program is different from traditional science, in which the process often is to discover something and then look for a possible application. In this center, researchers start with an idea of what they want and then try to find the kind of materials, atomic structure or even construction methods it would take to achieve it.

Finding cheap, environmentally benign and more efficient materials for solar energy is necessary for the future growth of the industry, researchers said.

"The beauty of a material such as this is that it is abundant, would not cost much and might be able to produce high-efficiency solar cells," Keszler said. "That's just what we need for more broad use of solar energy."

Story Source:

The above story is reprinted from materials provided by Oregon State University.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Liping Yu, Stephan Lany, Robert Kykyneshi, Vorranutch Jieratum, Ram Ravichandran, Brian Pelatt, Emmeline Altschul, Heather A. S. Platt, John F. Wager, Douglas A. Keszler, Alex Zunger. Iron Chalcogenide Photovoltaic Absorbers. Advanced Energy Materials, 2011; 1 (5): 748 DOI: 10.1002/aenm.201100351

'Fool's gold' aids discovery of new options for cheap, benign solar energy

These new compounds, unlike some solar cell materials made from rare, expensive or toxic elements, would be benign and could be processed from some of the most abundant elements on Earth. Findings on them have been published in Advanced Energy Materials, a professional journal.

Iron pyrite itself has little value as a future solar energy compound, the scientists say, just as the brassy, yellow-toned mineral holds no value compared to the precious metal it resembles. But for more than 25 years it was known to have some desirable qualities that made it of interest for solar energy, and that spurred the recent research.

The results have been anything but foolish.

"We've known for a long time that pyrite was interesting for its solar properties, but that it didn't actually work," said Douglas Keszler, a distinguished professor of chemistry at OSU. "We didn't really know why, so we decided to take another look at it. In this process we've discovered some different materials that are similar to pyrite, with most of the advantages but none of the problems.

"There's still work to do in integrating these materials into actual solar cells," Keszler said. "But fundamentally, it's very promising. This is a completely new insight we got from studying fool's gold."

Pyrite was of interest early in the solar energy era because it had an enormous capacity to absorb solar energy, was abundant, and could be used in layers 2,000 times thinner than some of its competitors, such as silicon. However, it didn't effectively convert the solar energy into electricity.

In the new study, the researchers found out why. In the process of creating solar cells, which takes a substantial amount of heat, pyrite starts to decompose and forms products that prevent the creation of electricity.

Based on their new understanding of exactly what the problem was, the research team then sought and found compounds that had the same capabilities of pyrite but didn't decompose. One of them was iron silicon sulfide.

"Iron is about the cheapest element in the world to extract from nature, silicon is second, and sulfur is virtually free," Keszler said. "These compounds would be stable, safe, and would not decompose. There's nothing here that looks like a show-stopper in the creation of a new class of solar energy materials."

Work to continue the development of the materials and find even better ones in the same class will continue at the National Renewable Energy Laboratory in Colorado, which collaborated on this research.

The work was done at the Center for Inverse Design, a collaborative initiative of the College of Science and College of Engineering at OSU, formed two years ago with a $3 million grant from the U.S. Department of Energy. It was one of the new Energy Frontier Research Centers set up through a national, $777 million federal program to identify energy solutions for the future.

The OSU program is different from traditional science, in which the process often is to discover something and then look for a possible application. In this center, researchers start with an idea of what they want and then try to find the kind of materials, atomic structure or even construction methods it would take to achieve it.

Finding cheap, environmentally benign and more efficient materials for solar energy is necessary for the future growth of the industry, researchers said.

"The beauty of a material such as this is that it is abundant, would not cost much and might be able to produce high-efficiency solar cells," Keszler said. "That's just what we need for more broad use of solar energy."

Provided by Oregon State University (news : web)

USC team develops promising polymer for solar cells

One way to do this, researchers believe, is to create a polymer based material that could be used instead of silicon. Such material would cost less to produce and have sufficient bendiness that it could be printed onto bendable surfaces in much the same way newspapers are mass printed, i.e. via giant rollers. Up to now though, figuring out how to create such a polymer that is as efficient at converting sunlight into energy as silicon-based cells, hasn’t really worked out.

Now though, a team working out of USC, headed by Alan Heeger, who along with Guillermo Bazan won the Nobel Prize in Physics back in 2000 for groundbreaking work they did on polymer cells, believe they have made another breakthrough. In their paper, published in Nature Materials, they say they’ve figured out a way to use an organic material with a low molecular weight (small molecule) to produce a solar cell that is every bit as efficient as current silicon technology.

The small molecule technology came about as the result of work done by Bazan, who used theory and lots of trial and error to produce just the right material; one that could, unlike many others that had been tried, be formed into a layer that could be applied to other materials. Heeger then took the lead in applying the new material in a solar cell. The end result the team says, is a solar cell capable of matching the 6.7% energy efficiency of silicon cells. And not only that, they believe with some tweaking, they can get it to 9%.

Unfortunately, there is a dark cloud looming ahead, and that is because the team isn’t sure just yet if the new material will work as designed once it’s ramped up to commercial size. In the past, when polymers have been sized up, their efficiencies went down.

More information: Solution-processed small-molecule solar cells with 6.7% efficiency, Nature Materials (2011) doi:10.1038/nmat3160

Abstract
Organic photovoltaic devices that can be fabricated by simple processing techniques are under intense investigation in academic and industrial laboratories because of their potential to enable mass production of flexible and cost-effective devices1, 2. Most of the attention has been focused on solution-processed polymer bulk-heterojunction (BHJ) solar cells3, 4, 5, 6, 7. A combination of polymer design, morphology control, structural insight and device engineering has led to power conversion efficiencies (PCEs) reaching the 6–8% range for conjugated polymer/fullerene blends8, 9. Solution-processed small-molecule BHJ (SM BHJ) solar cells have received less attention, and their efficiencies have remained below those of their polymeric counterparts10. Here, we report efficient solution-processed SM BHJ solar cells based on a new molecular donor, DTS(PTTh2)2. A record PCE of 6.7% under AM 1.5?G irradiation (100?mW?cm-2) is achieved for small-molecule BHJ devices from DTS(PTTh2)2:PC70BM (donor to acceptor ratio of 7:3). This high efficiency was obtained by using remarkably small percentages of solvent additive (0.25%?v/v of 1,8-diiodooctane, DIO) during the film-forming process, which leads to reduced domain sizes in the BHJ layer. These results provide important progress for solution-processed organic photovoltaics and demonstrate that solar cells fabricated from small donor molecules can compete with their polymeric counterparts.

? 2011 PhysOrg.com

Why solar wind is rhombic-shaped: Temperature and energy equipartition in cosmic plasmas explained

 Why the temperatures in the solar wind are almost the same in certain directions, and why different energy densities are practically identical, was until now not clear. With a new approach to calculating instability criteria for plasmas, Bochum researchers led by Prof. Dr. Reinhard Schlickeiser (Chair for Theoretical Physics IV) have solved both problems at once. They were the first to incorporate the effects of collisions of the solar wind particles in their model. This explains experimental data significantly better than previous calculations and can also be transferred to cosmic plasmas outside our solar system.

The scientists report on their findings in Physical Review Letters.

Temperatures and pressures in the cosmic plasma

The solar wind consists of charged particles and is permeated by a magnetic field. In the analysis of this plasma, researchers investigate two types of pressure: the magnetic pressure describes the tendency of the magnetic field lines to repel each other, the kinetic pressure results from the momentum of the particles. The ratio of kinetic to magnetic pressure is called plasma beta and is a measure of whether more energy per volume is stored in magnetic fields or in particle motion. In many cosmic sources, the plasma beta is around the value one, which is the same as energy equipartition. Moreover, in cosmic plasmas near temperature isotropy prevails, i.e. the temperature parallel and perpendicular to the magnetic field lines of the plasma is the same.

Explaining satellite data

For over a decade, the instruments of the near-earth WIND satellite have gathered various solar wind data. When the plasma beta measured is plotted against the temperature anisotropy (the ratio of the perpendicular to the parallel temperature), the data points form a rhombic area around the value one. "If the values move out of the rhombic configuration, the plasma is unstable and the temperature anisotropy and the plasma beta quickly return to the stable region within the rhombus" says Prof. Schlickeiser. However, a specific, detailed explanation of this rhombic shape has, until now, been lacking, especially for low plasma beta.

Collisions in the solar wind

In previous models it was assumed that, due to the low density, the solar wind particles do not directly collide, but only interact via electromagnetic fields. "Such assumptions are, however, no longer justified for small plasma beta, since the damping due to particle collisions needs to be taken into account" explains Dipl.-Phys. Michal Michno. Prof. Schlickeiser's group included this additional damping in their model, which led to new rhombic thresholds i.e. new stability conditions. The Bochum model explains the solar wind data measured significantly better than previous theories.

Universally valid solution

The new model can be applied to other dilute cosmic plasmas which have densities, temperatures and magnetic field strengths similar to the solar wind. Even if the diagram of temperature anisotropy and plasma beta does not have exactly the rhombic shape that the researchers found for the solar wind, the newly discovered mechanism predicts that the values are always close to one. In this way, the theory also makes an important contribution to the explanation of the energy equipartition in cosmic plasmas outside of our solar system.

Journal Reference:

R. Schlickeiser, M. Michno, D. Ibscher, M. Lazar, T. Skoda. Modified Temperature-Anisotropy Instability Thresholds in the Solar Wind. Physical Review Letters, 2011; 107 (20) DOI: 10.1103/PhysRevLett.107.201102

Taking the pulse of charge-separation processes: Self-organization gives rise to more efficient organic solar cells

 Organic solar cells have the potential to convert sunlight into electrical energy in an economical and environmentally friendly fashion. The challenge is that they still work less efficiently than inorganic semiconductors. Ultrafast measurements on hybrid cells now reveal a route to double their efficiency.

The use of organic photovoltaics for the production of electricity from sunlight offers an attractive and promising basis for an innovative and environmentally friendly means of energy supply. They can be manufactured quite economically and, because they are as bendable as plastic wrap, they can be processed flexibly. The problem is that they are yet markedly less efficient than conventional inorganic semiconductor cells.

The most crucial process in the conversion of light into electric current is the generation of free charge carriers. In the first step of photoconversion, upon absorption of light one component of the organic solar cell, usually a polymer, releases electrons that are taken up by the second component of the cell -- in this case silicon nanoparticles -- and can then be transported further.

"The mechanisms and the timescale of charge separation have been the subject of controversial scientific debate for many years," says LMU physics professor Eberhard Riedle. In cooperation with investigators at the Technical University in Munich and at Bayreuth University, Riedle and his group have now been able to dissect the process in detail. To do so, the researchers used a novel hybrid cell type containing both organic and inorganic constituents, in which silicon serves as the electron acceptor. Based on the insights obtained with this system, they developed a processing strategy to improve the structural order of the polymer -- and found that this enhances the efficiency of charge separation in organic semiconductors by up to twofold. Their findings provide a new way to optimize the performance of organic solar cells.

The key to this breakthrough lies in a unique, laser-based experimental setup, which combines extremely high temporal resolution of 40 femtoseconds (fs) with a very broadband detection. This allowed the team to follow the ultrafast processes induced by photon absorption in real time as they occur. Instead of the fullerenes used in typical organic cells, the researchers used silicon as the electron acceptor, a choice that has two major advantages.

"First, with these novel hybrid solar cells, we were able to probe the photophysical processes taking place in the polymer with greater precision than ever before, and secondly through the use of silicon, a much larger segment of the solar spectrum can be harnessed for electricity," says Riedle. It turns out that free charge carriers -- so called polarons -- are not generated immediately upon photoexcitation, but with a delay of about 140 fs. Primary photoexcitation of a polymer molecule first leads to the formation of an excited state, called an exciton. This then dissociates, releasing an electron, which is then transferred to the electron acceptor.

The loss of electrons leaves behind positively charged "holes" in the polymer and, as oppositely charged entities are attracted to one another by the Coulomb force, the two have a tendency to recombine. "In order to obtain free charge carriers, electron and hole must both be sufficiently mobile to overcome the Coulomb force," explains Daniel Herrmann, the first author of the new study. The team was able to show, for the first time, that this is much easier to achieve in polymers with an ordered, regular structure than with polymers that are chaotically arranged. In other words, a high degree of self-organization of the polymer significantly increases the efficiency of charge separation.

"The polymer that we used is one of the few known to have a tendency to self-organize. This tendency can be inhibited, but one can also increase the polymer's intrinsic propensity for self-organization by choosing appropriate processing parameters," Herrmann explains. By cleverly optimizing the processing of the polymer P3HT, the researchers succeeded in doubling the yield of free charge carriers -- and thereby significantly enhancing the efficiency of their experimental solar cells.

Story Source:

The above story is reprinted from materials provided by Ludwig-Maximilians-Universitaet Muenchen (LMU).

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Daniel Herrmann, Sabrina Niesar, Christina Scharsich, Anna Köhler, Martin Stutzmann, Eberhard Riedle. Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption. Journal of the American Chemical Society, 2011; 111021142308005 DOI: 10.1021/ja207887q

 

Scientists lay out plans for efficient harvesting of solar energy

 Solar power could be harvested more efficiently and transported over long distances using tiny molecular circuits, according to research inspired by new insights into natural photosynthesis.

Incorporating the latest research into how plants, algae and some bacteria use quantum mechanics to optimise energy production via photosynthesis, scientists have set out how to design molecular "circuitry" that is 10 times smaller than the thinnest electrical wire in computer processors. Published in Nature Chemistry, the report discusses how tiny molecular energy grids could capture, direct, regulate and amplify raw solar energy.

Professor Gregory Scholes, lead author from the University of Toronto said: "Solar fuel production often starts with the energy from light being absorbed by an assembly of molecules. The energy is stored fleetingly as vibrating electrons and then transferred to a suitable reactor.

"It is the same in biological systems. In photosynthesis, for example, antenna complexes composed of chlorophyll capture sunlight and direct the energy to special proteins that help make oxygen and sugars. It is like plugging those proteins (called reaction centres) into a solar power socket."

In natural systems energy from sunlight is captured by 'coloured' molecules called dyes or pigments, but is only stored for a billionth of a second. This leaves little time to route the energy from pigments to the molecular machinery that produces fuel or electricity.

The key to transferring and storing energy very quickly is to harness the collective quantum properties of antennae, which are made up of just a few tens of pigments.

Dr Alexadra Olaya-Castro, co-author of the paper from UCL's department of Physics and Astronomy said: "On a bright sunny day, more than 100 million billion red and blue "coloured" photons strike a leaf each second.

"Under these conditions plants need to be able to both use the energy that is required for growth but also to get rid of excess energy that can be harmful. Transferring energy quickly and in a regulated manner are the two key features of natural light-harvesting systems.

"By assuring that all relevant energy scales involved in the process of energy transfer are more or less similar, natural antennae manage to combine quantum and classical phenomena to guarantee efficient and regulated capture, distribution and storage of the sun's energy."

Summary of lessons from nature about concentrating and distributing solar power with nanoscopic antennae:

The basic components of the antenna are efficient light absorbing molecules. These photo-energy absorbers should be appropriately distributed to guarantee that there is an even probability of converting sun energy into vibrating electrons across the whole antennae.Take advantage of the collective properties of light-absorbing molecules by grouping them close together. This will make them exploit quantum mechanical principles so that the antenna can: i) absorb different colours ii) create energy gradients to favour unidirectional transfer and iii) possibly exploit quantum coherence for energy distribution -several energy transfer pathways can be exploited at once.Make sure that the relevant energy scales involved in the energy transfer process are more or less resonant. This will guarantee that both classical and quantum transfer mechanisms are combined to create regulated and efficient distribution of energy across short and long-range distances when many antennae are connected.An antenna should transfer energy not as fast as possible but as fast as necessary. This means that regulatory mechanisms need to be integrated in the antenna. For instance, if necessary, combine light-absorbing molecules with a few local "sinks" that dissipate excess of damaging energy.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University College London.

Journal Reference:

Gregory D. Scholes, Graham R. Fleming, Alexandra Olaya-Castro, Rienk van Grondelle. Lessons from nature about solar light harvesting. Nature Chemistry, 2011; 3 (10): 763 DOI: 10.1038/nchem.1145

Note: If no author is given, the source is cited instead.

Researchers use carbon nanotubes to make solar cells affordable, flexible

Researchers from Northwestern University have developed a carbon-based material that could revolutionize the way solar power is harvested. The new solar cell material -- a transparent conductor made of carbon nanotubes -- provides an alternative to current technology, which is mechanically brittle and reliant on a relatively rare mineral.

Due to Earth's abundance of carbon, carbon nanotubes have the potential to boost the long-term viability of solar power by providing a cost-efficient option as demand for the technology increases. In addition, the material's mechanical flexibility could allow solar cells to be integrated into fabrics and clothing, enabling portable energy supplies that could impact everything from personal electronics to military operations.

The research, headed by Mark C. Hersam, professor of materials science and engineering and professor of chemistry, and Tobin J. Marks, Vladimir N. Ipatieff Professor of Catalytic Chemistry and professor of materials science and engineering, is featured on the cover of the October 2011 issue of Advanced Energy Materials, a new journal that specializes in science about materials used in energy applications.

Solar cells are composed of several layers, including a transparent conductor layer that allows light to pass into the cell and electricity to pass out; for both these actions to occur, the conductor must be both electrically conductive and also optically transparent. Few materials concurrently possess both of these properties.

Currently, indium tin oxide is the dominant material used in transparent conductor applications, but the material has two potential limitations. Indium tin oxide is mechanically brittle, which precludes its use in applications that require mechanical flexibility. In addition, Indium tin oxide relies on the relatively rare element indium, so the projected increased demand for solar cells could push the price of indium to problematically high levels.

"If solar technology really becomes widespread, as everyone hopes it will, we will likely have a crisis in the supply of indium," Hersam said. "There's a great desire to identify materials -- especially earth-abundant elements like carbon -- that can take indium's place in solar technology."

Hersam and Marks' team has created an alternative to indium tin oxide using single-walled carbon nanotubes, tiny, hollow cylinders of carbon just one nanometer in diameter.

The researchers have gone further to determine the type of nanotube that is most effective in transparent conductors. Nanotubes' properties vary depending on their diameter and their chiral angle, the angle that describes the arrangement of carbon atoms along the length of the nanotube. These properties determine two types of nanotubes: metallic and semiconducting.

Metallic nanotubes, the researchers found, are 50 times more effective than semiconducting ones when used as transparent conductors in organic solar cells.

"We have now identified precisely the type of carbon nanotube that should be used in this application," Hersam said.

Because carbon nanotubes are flexible, as opposed to the brittle indium tin oxide, the researchers' findings could pave the way for many new applications in solar cells. For example, the military could incorporate the flexible solar cells into tent material to provide solar power directly to soldiers in the field, or the cells could be integrated into clothing, backpacks, or purses for wearable electronics.

"With this mechanically flexible technology, it's much easier to imagine integrating solar technology into everyday life, rather than carrying around a large, inflexible solar cell," Hersam said.

Researchers are now examining other layers of the solar cell to explore also replacing these with carbon-based nanomaterials.

Besides Hersam and Marks, other authors include Timothy P. Tyler, Ryan E. Brock, and Hunter J. Karmel. This work was supported by the Argonne Northwestern-Northwestern Solar Energy Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Northwestern University, via EurekAlert!, a service of AAAS.

Journal Reference:

Timothy P. Tyler, Ryan E. Brock, Hunter J. Karmel, Tobin J. Marks, Mark C. Hersam. Organic Solar Cell Characterization: Electronically Monodisperse Single-Walled Carbon Nanotube Thin Films as Transparent Conducting Anodes in Organic Photovoltaic Devices (Adv. Energy Mater. 5/2011). Advanced Energy Materials, 2011; 1 (5): 701 DOI: 10.1002/aenm.201190021

Nature offers key lessons on harvesting solar power, say chemists

Clean solutions to human energy demands are essential to our future. While sunlight is the most abundant source of energy at our disposal, we have yet to learn how to capture, transfer and store solar energy efficiently. According to University of Toronto chemistry professor Greg Scholes, the answers can be found in the complex systems at work in nature.

"Solar fuel production often starts with the energy from light being absorbed by an assembly of molecules," said Scholes, the D.J. LeRoy Distinguished Professor at U of T. "The energy is stored fleetingly as vibrating electrons and then transferred to a suitable reactor. It is the same in biological systems. In photosynthesis, for example, antenna complexes composed of chlorophyll capture sunlight and direct the energy to special proteins called reaction centres that help make oxygen and sugars. It is like plugging those proteins into a solar power socket."

In an article in Nature Chemistry to be published Sept. 23, Scholes and colleagues from several other universities examine the latest research in various natural antenna complexes. Using lessons learned from these natural phenomena, they provide a framework for how to design light harvesting systems that will route the flow of energy in sophisticated ways and over long distances, providing a microscopic "energy grid" to regulate solar energy conversion.

A key challenge is that the energy from sunlight is captured by coloured molecules called dyes or pigments, but is stored for only a billionth of a second. This leaves little time to route the energy from pigments to molecular machinery that produces fuel or electricity. How can we harvest sunlight and utilize its energy before it is lost?

"This is why natural photosynthesis is so inspiring," said Scholes. "More than 10 million billion photons of light strike a leaf each second. Of these, almost every red-coloured photon is captured by chlorophyll pigments which feed plant growth." Learning the workings of these natural light-harvesting systems fostered a vision, proposed by Scholes and his co-authors, to design and demonstrate molecular "circuitry" that is 10 times smaller than the thinnest electrical wire in computer processors. These energy circuits could control, regulate, direct and amplify raw solar energy which has been captured by human-made pigments, thus preventing the loss of precious energy before it is utilized.

Last year, Scholes led a team that showed that marine algae, a normally functioning biological system, uses quantum mechanics in order to optimize photosynthesis, a process essential to its survival. These and other insights from the natural world promise to revolutionize our ability to harness the power of the sun.

"Lessons from nature about solar light harvesting" was written by Scholes, Graham Fleming of the University of California, Berkeley, Alexandra Olaya-Castro of University College, London UK and Rienk van Grondelle of VU University in Amsterdam, The Netherlands.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by University of Toronto.

Journal Reference:

Gregory D. Scholes, Graham R. Fleming, Alexandra Olaya-Castro, Rienk van Grondelle. Lessons from nature about solar light harvesting. Nature Chemistry, 2011; 3 (10): 763 DOI: 10.1038/nchem.1145

Solar rays could replace petroleum fuels, research shows

Alternative fuel sources for cars may have a glowing future as a Kansas State University graduate student is working to replace petroleum fuels with ones made from sunlight.

Yen-Ting Kuo, a doctoral candidate in chemistry, Taiwan, has spent several years in K-State's chemistry program working to create new materials that better use sunlight in chemical reaction processes to generate energy.

"People tend to think of chemistry as test-tube experiments and not really creating practical things. That's just not true," Kuo said. "A big focus now is on 'green chemistry.' This means wanting to have the same quality of life that we have right now, but using chemistry to replace some things with materials that are more eco-friendly, such as biodegradable products or clean fuel."

As a way to advance the clean fuel research, Kuo is making and studying metal-oxide catalysts that react with light. These catalysts, called photocatalysts, cause a chemical reaction when triggered by sunlight, but are not destroyed during the reaction. Photocatalysts are crucial to producing new fuels, like solar gasoline, which use hydrogen.

To make solar gasoline, sunlight is channeled into a tank of water that contains photocatalysts. The sunlight triggers the photocatalysts to react with the water. This reaction causes the water to split into hydrogen and oxygen. When the hydrogen is combined with carbon monoxide it forms a synthetic gas -- called syngas -- that is the basic building block in fossil fuel and can be used to power cars.

In recent years solar gasoline has been getting more mileage as more international laboratories attempt to improve and perfect the process. But developing a photocatalyst that efficiently uses sunlight to create a chemical reaction and produce hydrogen is proving difficult for researchers. It also is needed for production to reach commercial levels. Kuo is working to solve that problem by creating and analyzing new photocatalysts in the lab.

To make a photocatalyst, Kuo mixes various elements in powdered form, and then cooks them at temperatures between 700 degrees Celsius and 850 degrees Celsius.

Once the material is made, its structure is studied with a transmission electron microscope and ultraviolet spectrums. Doing this allows Kuo to look at ways to structurally improve the photocatalyst and its performance.

In addition to improving the material's photocatalytic properties -- which will intensify reaction with the sunlight -- Kuo focuses on increasing the material's surface area. An increased surface area means bigger and better reactions, and a material with a high surface area and with high photocatalytic properties could mean a bright future for solar gasoline and other alternative fuels.

Engineering a photocatalyst that efficiently splits water into hydrogen and oxygen could also be a boon to fuel cell technology, Kuo said. Fuel cells operate by essentially reversing the chemical reaction that's used to split water. Hydrogen is converted into electrical power, and water is given off as a byproduct.

"Even though the mature technology of fuel cells is in the near future, the source of hydrogen is still a question since most of the hydrogen sources now are from petroleum," Kuo said. "Therefore, water splitting using photocatalysts is one of the solutions providing a new pathway to obtain hydrogen."

Kuo came to K-State after reading work published from his adviser, Ken Klabunde. Klabunde, a university distinguished professor of chemistry, is an expert in turning chemistry into new environmentally-friendly materials. He's created inorganic materials and nanotechnology that filter water and air; control odor, bacteria and viruses; and detoxify hazardous chemical spills.

Kuo will defend his dissertation, "Novel photocatalytic water splitting with the N-doped In2O3/TiO2 D10-D0 configuration composite oxide semiconductors," in mid-September. He then will begin a postdoctoral position at the University of Michigan, one of the few universities in the U.S. to study solar gasoline.

Provided by Kansas State University (news : web)

Millions of molecules screened in search for the ideal organic solar cell material

Currently, the cost of electricity from commercial silicon solar cells is about 10 times higher than the cost of utility-scale electricity. In order to make solar cells cost-competitive with currently available energy sources, some researchers are looking to organic materials. Not only are organic materials less expensive than inorganic materials like silicon, but they’re also non-hazardous, lightweight, easily processed, and can be made semi-transparent and molded into almost any shape. The problem is there are literally millions of organic materials to choose from, and identifying those few that have the best optical and electronic properties is extremely challenging.

To address this problem, a team of researchers from Harvard University, the National Autonomous University of Mexico, and Haverford College in Haverford, Pennsylvania, has developed an extremely large-scale automated computational screening method to study potential molecular structures for organic photovoltaic devices (OPVs). They introduced the initiative, called the Harvard Clean Energy Project (CEP), http://cleanenergy.harvard.edu/ in a recent issue of the Journal of Physical Chemistry Letters where they present some early results, with more studies to follow.

CEP’s overall goal is to identify an organic material that can increase the efficiency of OPVs from the current record of 9.2% to 10-15%, as well as expand the currently limited lifetimes to more than 10 years. A solar cell with these two features could push the power generation costs of organic solar cells below that of other currently available energy sources.

To achieve this goal, the project has taken a highly collaborative approach. It relies on input and feedback from experimentalists from Zhenan Bao’s group at Stanford and other research groups. To analyze the large number of molecules, the project combines conventional modeling strategies with strategies from modern drug discovery, along with ideas from machine learning, pattern recognition, and cheminformatics. Also, the project utilizes volunteer computing by IBM’s World Community Grid (WCG) to supply part of the large-scale computational power. Volunteers who would like to donate computer time can download a free and virus-free program from the IBM website that uses their computers for screening the materials while their computer is idle.

“Roughly, every 12 hours of donated free CPU time will result in a new molecule added to our database of candidate organic materials for solar cells,” Alán Aspuru-Guzik of Havard, who is one of the project’s leaders, told PhysOrg.com. “The database will aid scientists in accelerating the discovery of novel solar materials.”

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With traditional approaches to analyzing and characterizing materials, researchers typically rely on their past experiences with certain materials and their own empirical intuition. Due to the long time required for synthesis and characterization, only a few examples can be experimentally studied per year. In contrast, CEP can characterize thousands of molecules per day, and the CEP library already contains about 10 million molecular motifs of potential interest.

“The great variety in properties found in our candidate library is quite remarkable, as is the small parameter space that makes for promising OPVs and that has to be hit,” said Johannes Hachmann of Harvard, another of the project’s leaders. “The latter underlines the value of our high-throughput approach.”

Using a calculation hierarchy, the method rates each candidate motif at each step with respect to the desired properties, and expedites further characterization for the most promising candidates. The hierarchy technique is already proving valuable: a preliminary analysis has revealed that only about 0.3% (3,000–5,000) of the screened structures have the necessary energetic levels to realize organic solar cells with 10% or higher efficiency. While an unaided search would have a very small chance of identifying these molecules, CEP can move all of the promising candidates forward for additional analysis.

“So far, we have made a proof-of-principle study in collaboration with Zhenan Bao’s group at Stanford,” said Aspuru-Guzik. “We screened eight different variants of a parent compound for organic semiconductors, and this resulted in a compound with an astonishing large hole mobility. This gives us confidence that the type of approach followed in the WCG will yield useful information to the community.”

In addition to searching for molecules with specific structures, the project also gives researchers a better understanding of structure-property relationships of molecules in general. Knowing these design principles will allow scientists to not only improve screening, but also to actively engineer novel organic electronics at a future stage.

“On the one hand, the collection provides on-demand access to specific compounds with a wide range of desired properties and electronic structures for all sorts of applications, not only for OPVs,” Hachmann said. “On the other hand it forms a solid foundation to learn about structure-property relationships. Lastly, it will be a useful resource for theoreticians to assess the performance of different computational methods and can serve as a parameter repository in this chemical space.”

As more technical results arrive, the researchers are building a reference database that will be available to the public by 2012. The data should accelerate the search for optimal OPV materials, and provide valuable data for the development of organic electronics in general. The researchers hope that one day the search will lead to a clean source of electricity that can compete with conventional energy sources, although it’s difficult to predict exactly when that will be.

“In principle, we want the search to last as little as possible,” Aspuru-Guzik said. “Obviously, things are more complicated: It takes human time to catalog and understand the results, as well as to select molecules for further screening. We are in the process of selecting the top candidates of our initial screening and releasing them in a publication. We want to further screen the most promising candidates with more calculations to ascertain and verify their potential as organic solar cell materials.”

To participate and download the client software developed by IBM and Harvard, go to http://cleanenergy.harvard.edu and click “Download.” The website contains video tutorials for the installation.

More information: Johannes Hachmann, et al. “The Harvard Clean Energy Project: Large-Scale Computational Screening and Design of Organic Photovoltaics on the World Community Grid.” The Journal of Physical Chemistry Letters, 2011, DOI:10.1021/jz200866s

 

Colloidal quantum dots: Performance boost next-generation solar cell technology

Researchers from the University of Toronto (U of T), the King Abdullah University of Science & Technology (KAUST) and Pennsylvania State University (Penn State) have created the most efficient solar cell ever made based on colloidal quantum dots (CQD).

The discovery is reported in the latest issue of Nature Materials.

Quantum dots are nanoscale semiconductors that capture light and convert it into an energy source. Because of their small scale, the dots can be sprayed on to flexible surfaces, including plastics. This enables the production of solar cells that are less expensive to produce and more durable than the more widely-known silicon-based version. In the work highlighted by the Nature Materials paper, the researchers demonstrate how the wrappers that encapsulate the quantum dots can be shrunk to a mere layer of atoms.

"We figured out how to shrink the passivating materials to the smallest imaginable size," states Professor Ted Sargent, corresponding author on the work and holder of the Canada Research Chair in Nanotechnology at U of T.

A crucial challenge for the field has been striking a balance between convenience and performance. The ideal design is one that tightly packs the quantum dots together. The greater the distance between quantum dots, the lower the efficiency.

However the quantum dots are usually capped with organic molecules that add a nanometer or two. When working on a nanoscale, that is bulky. Yet the organic molecules have been an important ingredient in creating a colloid, which is a substance that is dispersed in another substance. This allows the quantum dots to be painted on to other surfaces.

To solve the problem, the researchers have turned to inorganic ligands, which bind the quantum dots together while using less space. The result is the same colloid characteristics but without the bulky organic molecules.

"We wrapped a single layer of atoms around each particle. As a result, they packed the quantum dots into a very dense solid," explains Dr. Jiang Tang, the first author of the paper who conducted the research while a post-doctoral fellow in The Edward S. Rogers Department of Electrical & Computer Engineering at U of T.

The team showed the highest electrical currents, and the highest overall power conversion efficiency, ever seen in CQD solar cells. The performance results were certified by an external laboratory, Newport, that is accredited by the US National Renewable Energy Laboratory.

"The team proved that we were able to remove charge traps -- locations where electrons get stuck -- while still packing the quantum dots closely together," says Professor John Asbury of Penn State, a co-author of the work.

The combination of close packing and charge trap elimination enabled electrons to move rapidly and smoothly through the solar cells, thus providing record efficiency.

"This finding proves the power of inorganic ligands in building practical devices," states Professor Dmitri Talapin of The University of Chicago, who is a research leader in the field. "This new surface chemistry provides the path toward both efficient and stable quantum dot solar cells. It should also impact other electronic and optoelectronic devices that utilize colloidal nanocrystals. Advantages of the all-inorganic approach include vastly improved electronic transport and a path to long-term stability."

"At KAUST we were able to visualize, with incredible resolution on the sub-nanometer length scale, the structure and composition of this remarkable new class of materials," states Professor Aram Amassian of KAUST, a co-author on the work.

"We proved that the inorganic passivants were tightly correlated with the location of the quantum dots; and that it was this new approach to chemical passivation, rather than nanocrystal ordering, that led to this record-breaking colloidal quantum dot solar cell performance," he adds.

As a result of the potential of this research discovery, a technology licensing agreement has been signed by U of T and KAUST, brokered by MaRS Innovations (MI), which will will enable the global commercialization of this new technology.

"The world -- and the marketplace -- need solar innovations that break the existing compromise between performance and cost. Through U of T's, MI's, and KAUST's partnership, we are poised to translate exciting research into tangible innovations that can be commercialized," said Sargent.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University of Toronto, via EurekAlert!, a service of AAAS.

Journal Reference:

Jiang Tang, Kyle W. Kemp, Sjoerd Hoogland, Kwang S. Jeong, Huan Liu, Larissa Levina, Melissa Furukawa, Xihua Wang, Ratan Debnath, Dongkyu Cha, Kang Wei Chou, Armin Fischer, Aram Amassian, John B. Asbury, Edward H. Sargent. Colloidal-quantum-dot photovoltaics using atomic-ligand passivation. Nature Materials, 2011; DOI: 10.1038/nmat3118

SNS, HFIR experiments help refine thin-film solar cells

 Solar cells that convert sunlight into electricity could be a widely used renewable energy source. Getting to that point, though, requires breakthroughs in their cost and their efficiency at turning sunbeams into electric current. Neutron scattering experiments conducted at Oak Ridge National Laboratory are helping solar cell makers obtain the hard data they need to refine their materials and manufacturing processes.

One of the most promising options for lowering costs is to make solar cells from thin films made up of combinations of plastics called polymers. These devices are easy to produce in large numbers because they use conventional industrial processing methods, which are relatively cheap and energy-efficient compared to the processes used to make the silicon solar cells that are most widely used now. Also, panels made from organic solar cells are lighter and less expensive to install than the bulky solar panels made from silicon cells.

The drawback to these easily fabricated thin-film devices is their power conversion efficiency, or how well they convert solar radiation to electricity. They're much less efficient than silicon cells (which are almost 30 percent efficient). To be inexpensive enough to compete with silicon cells, thin-film solar cells must be more than 10 percent efficient, but so far, the best ones are only about 8.3 percent efficient. To make solar cells efficient enough, scientists need to understand the molecular structure of the thin films they're made of, how the structure relates to the efficiency of the solar cell, and how to tailor the structure for the greatest efficiency.

Recent studies of polymer-based solar cells at ORNL's Spallation Neutron Source and High Flux Isotope Reactor revealed important details about their molecular structure and showed that annealing (heat treating) the devices improves their power conversion efficiency. The experiments showed that annealing solar cells appropriately as they are fabricated improves their efficiency by more than 20 percent compared to films that aren't annealed.

"We are trying to use mixtures of photoactive polymers to absorb light over a broad wavelength range to improve efficiency," said principal investigator Thomas Russell of University of Massachusetts-Amherst. Haiyun Lu of U-Mass and Bulent Akgun of the NIST Center for Neutron Research and the University of Maryland are co-investigators. Studies such as this one are key to improving the performance of polymer-based solar cells so that they can compete in the marketplace.

The device studied consisted of two semiconductor materials deposited in a thin film on an underlying plate. The films were examined in their original state after being deposited and then after annealing. The MAGICS magnetism reflectometer at the SNS investigated the vertical arrangement of the layers in the film, and the General Purpose Small-Angle Neutron Scattering instrument at HFIR showed how well the two semiconductors blended.

"Structural characterization of thin films has always represented a challenge for small-angle neutron scattering," said Yuri Melnichenko, lead scientist at GP SANS. A powerful neutron beam is needed to monitor the subtle structural changes that occur during the formation of the film, and HFIR provides one of the strongest neutron beams for SANS in the world. The experiments at HFIR were completed within approximately 24 hours, while similar measurements at less intense neutron sources would require five to seven days, Melnichenko said.

How well the semiconductor materials in the thin film blend is important to their performance. The measurements on MAGICS showed that the blending of the two semiconductors increased steadily as the sample was annealed for up to one minute, said Valeria Lauter, lead scientist for MAGICS. As heating continued beyond one minute, there was little further change in the blending.

The experiments determined that annealing the solar cell at 150 degrees Celsius for one minute at a particular point in the process improved its efficiency by slightly over 20 percent compared to the original film. Annealing for shorter times improved the efficiency by lesser amounts. Annealing for more than a minute caused it to decline, as did annealing it at a different point in the process.

The work is detailed in the paper "Morphological characterization of low-bandgap crystalline polymer: PCBM bulk heterojunction solar cells," in Advanced Energy Materials, available online at http://neutrons.or … _russell.pdf

Provided by Oak Ridge National Laboratory (news : web)

Down to the wire: Inexpensive technique for making high quality nanowire solar cells

Solar or photovoltaic cells represent one of the best possible technologies for providing an absolutely clean and virtually inexhaustible source of energy to power our civilization. However, for this dream to be realized, solar cells need to be made from inexpensive elements using low-cost, less energy-intensive processing chemistry, and they need to efficiently and cost-competitively convert sunlight into electricity.

A team of researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) has now demonstrated two out of three of these requirements with a promising start on the third.

Peidong Yang, a chemist with Berkeley Lab's Materials Sciences Division, led the development of a solution-based technique for fabricating core/shell nanowire solar cells using the semiconductors cadmium sulfide for the core and copper sulfide for the shell. These inexpensive and easy-to-make nanowire solar cells boasted open-circuit voltage and fill factor values superior to conventional planar solar cells. Together, the open-circuit voltage and fill factor determine the maximum energy that a solar cell can produce. In addition, the new nanowires also demonstrated an energy conversion efficiency of 5.4-percent, which is comparable to planar solar cells.

"This is the first time a solution based cation-exchange chemistry technique has been used for the production of high quality single-crystalline cadmium sulfide/copper sulfide core/shell nanowires," Yang says. "Our achievement, together with the increased light absorption we have previously demonstrated in nanowire arrays through light trapping, indicates that core/shell nanowires are truly promising for future solar cell technology."

Yang, who holds a joint appointment with the University of California (UC) Berkeley, is the corresponding author of a paper reporting this research that appears in the journal Nature Nanotechnology. The paper is titled "Solution-processed core-shell nanowires for efficient photovoltaic cells." Co-authoring this paper with Yang were Jinyao Tang, Ziyang Huo, Sarah Brittman and Hanwei Gao.

Typical solar cells today are made from ultra-pure single crystal silicon wafers that require about 100 micrometers in thickness of this very expensive material to absorb enough solar light. Furthermore, the high-level of crystal purification required makes the fabrication of even the simplest silicon-based planar solar cell a complex, energy-intensive and costly process.

A highly promising alternative would be semiconductor nanowires -- one-dimensional strips of materials whose width measures only one-thousandth that of a human hair but whose length may stretch up to the millimeter scale. Solar cells made from nanowires offer a number of advantages over conventional planar solar cells, including better charge separation and collection capabilities, plus they can be made from Earth abundant materials rather than highly processed silicon. To date, however, the lower efficiencies of nanowire-based solar cells have outweighed their benefits.

"Nanowire solar cells in the past have demonstrated fill factors and open-circuit voltages far inferior to those of their planar counterparts," Yang says. "Possible reasons for this poor performance include surface recombination and poor control over the quality of the p-n junctions when high-temperature doping processes are used."

At the heart of all solar cells are two separate layers of material, one with an abundance of electrons that function as a negative pole, and one with an abundance of electron holes (positively-charged energy spaces) that function as a positive pole. When photons from the sun are absorbed, their energy is used to create electron-hole pairs, which are then separated at the p-n junction -- the interface between the two layers -- and collected as electricity.

About a year ago, working with silicon, Yang and members of his research group developed a relatively inexpensive way to replace the planar p-n junctions of conventional solar cells with a radial p-n junction, in which a layer of n-type silicon formed a shell around a p-type silicon nanowire core. This geometry effectively turned each individual nanowire into a photovoltaic cell and greatly improved the light-trapping capabilities of silicon-based photovoltaic thin films.

Now they have applied this strategy to the fabrication of core/shell nanowires using cadmium sulfide and copper sulfide, but this time using solution chemistry. These core/shell nanowires were prepared using a solution-based cation (negative ion) exchange reaction that was originally developed by chemist Paul Alivisatos and his research group to make quantum dots and nanorods. Alivisatos is now the director of Berkeley Lab, and UC Berkeley's Larry and Diane Bock Professor of Nanotechnology.

"The initial cadmium sulfide nanowires were synthesized by physical vapor transport using a vapor-liquid-solid (VLS) mechanism rather than wet chemistry, which gave us better quality material and greater physical length, but certainly they can also be made using solution process" Yang says. "The as-grown single-crystalline cadmium sulfide nanowires have diameters of between 100 and 400 nanometers and lengths up to 50 millimeters."

The cadmium sulfide nanowires were then dipped into a solution of copper chloride at a temperature of 50 degrees Celsius and kept there for 5 to 10 seconds. The cation exchange reaction converted the surface layer of the cadmium sulfide into a copper sulfide shell.

"The solution-based cation exchange reaction provides us with an easy, low-cost method to prepare high-quality hetero-epitaxial nanomaterials," Yang says. "Furthermore, it circumvents the difficulties of high-temperature doping and deposition for typical vapor phase production methods, which suggests much lower fabrication costs and better reproducibility. All we really need are beakers and flasks for this solution-based process. There's none of the high fabrication costs associated with gas-phase epitaxial chemical vapor deposition and molecular beam epitaxy, the techniques most used today to fabricate semiconductor nanowires."

Yang and his colleagues believe they can improve the energy conversion efficiency of their solar cell nanowires by increasing the amount of copper sulfide shell material. For their technology to be commercially viable, they need to reach an energy conversion efficiency of at least ten-percent.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by DOE/Lawrence Berkeley National Laboratory.

Journal Reference:

Jinyao Tang, Ziyang Huo, Sarah Brittman, Hanwei Gao, Peidong Yang. Solution-processed core–shell nanowires for efficient photovoltaic cells. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.139

Innovative organic solar cell architecture sets new performance level, Belgian researchers demonstrate

 The Belgian research centre imec, together with Plextronics and Solvay, present this week at the European Photovoltaic Solar Energy Conference and Exhibition (PVSEC) in Hamburg an organic polymer-based single junction solar cell with 6.9% performance in an innovative inverted device stack. Combining imec's scalable inverted device architecture and Plextronics' polymers, new levels of cell efficiency were achieved. The polymer was also integrated into a module resulting in excellent module level efficiencies of 5% for an aperture area of 25cm2.

Organic solar cells are regarded as an emerging technology to become one of the low-cost thin-film alternatives to the current dominating silicon photovoltaic technology, due to their intrinsic potential for low-cost processing (high-speed and at low temperature). Inverted architectures are developed to extend the lifetime of organic solar cells, an investigation which is currently ongoing for this new architecture. By combining architecture improvements with optimizations to the active layer using different types of polymers, imec aims at making the organic photovoltaic technology ready for market introduction.

The dedicated inverted bulk heterojunction architecture developed by imec improved the device performance by at least 0.5% over standard architectures used for organic solar cells. In the active layer, a new buffer layer was introduced to optimize the light management in the device. Imec's innovative device architecture, combined with Plextronics' low band-gap p-type polymer with a fullerene derivate, resulted in a stabilized certified conversion efficiency of 6.9%, which is the highest performance obtained for this polymer material and, to our knowledge, the highest efficiency reported for inverted architectures. In this new inverted device architecture, similar performance boosts have also been achieved for other polymer materials. The module level efficiencies confirm the suitability towards upscaling.

Tom Aernouts, R&D Team Leader Organic Photovoltaics at imec: "We are delighted to present these excellent results, achieved by combining imec's expertise and knowhow in organic photovoltaics R&D with Plextronics' innovative material. With further optimizations to the material as well as to the architecture, for example by introducing a multi-junction featuring different layers of different polymers each capturing another part of the light spectrum, we envision organic solar cell lifetimes of over 10 years and conversion efficiencies of 10% in two to three years, ultimately aiming at industry-relevant solutions."

Andy Hannah, president and CEO of Plextronics adds, "Partnering with industry leaders like imec and Solvay allows us the opportunity to explore new approaches to accelerate the performance of OPV technology incorporating our proprietary polymers.

Patrick Francoisse, Sustainable Energy Platform Manager, Innovation Center, Solvay: "We are delighted to work with imec and develop new OPV architectures which will demonstrate the performance of materials being developed at Plextronics. We believe organic photovoltaics will play a bigger role in the future, when we can boost efficiency and lifetime, at a reduced cost price. Our collaboration with imec contributes to build this confidence and offer new products to this emerging market."

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Interuniversity Microelectronics Centre (IMEC).