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."

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

Foam that lasts and lasts and lasts, and disappears when you want

There's nothing special about foaming soap solutions; however, a soap foam that lasts several months, even at 60°C, is unusual. Especially if the foam is made from a natural substance and can quickly be destroyed or restored only by changing ambient temperature. This sums up the work of the teams of INRA, CEA and CNRS, which makes possible new applications that will be of interest to manufacturers of cosmetics and detergents.

The results of this research have been published in the August 29th issue of Angewandte Chemie.

Due to their particular texture and the molecules that compose them, foams often have detergent properties. In physical chemistry, such molecules, which must be dispersed in water to create foam, are called "surface-active." They are located spontaneously in water and air, so that very thin films of water can stabilize around air bubbles of foam with a special architecture. Due to such properties, various foams have numerous applications in cleaning, decontamination, cosmetics, battling pollution and fire, agribusiness and mining.

In this case, the researchers of INRA, CEA and CNRS have studied a particular surface-active molecule, the 12-hydroxystearic fatty acid, produced from castor oil. In order to disperse the molecule which is initially insoluble in water, they added a salt. They then demonstrated the special advantages of the surfactant: even in small quantities, it produces abundant foam and, above all, remains stable for more than six months, in contrast with traditional surfactants that stabilize foams for only several hours. The researchers observed and explained this phenomenon using microscopy and neutron scattering, so as to monitor structural change on a nanometric scale in situ.

They thus demonstrated that within a range of average temperatures between 20 and 60°C, 12-hydroxystearic acid, mixed with the "right" salt, disperses in water in the form of tubes that are several microns in size. The tubes form a structure that is perfectly stable and rigid in very thin films of water located between air bubbles, which explains the foam's resistance.

Above 60°C, the tubes merge into spherical assemblies that are a thousand times smaller (several nanometers), which the researchers call "micelles." The previously stable foam then collapses because the rigid structure disappears. The researchers have demonstrated that this transition from an assembly of tubes to an assembly of micelles is "reversible." If the foam's temperature is increased, its volume will diminish when micelles start to form, and if the temperature is again reduced to between 20 and 60°C, the tubes will form again and the form will re-stabilize (to regain the initial volume of the foam, air must be re-injected).

This is the first time where such a stable foam has been created with such a simple, natural surface-active molecule. The transition temperature between the state where the foam contains tubes and the "micelle" state depends on the salt chosen to disperse the molecule in water, which increases its potential uses.

This "green" chemistry, since it is produced from an organic molecule, creates new possibilities because foams have many industrial uses. For example, it should be possible to produce detergents and shampoos in which the quantity of foam can be controlled simply by adjusting temperature, thus facilitating drainage. Some cosmetic products require numerous chemical ingredients to produce a stable foam; use of 12-hydroxystearic acid would limit the quantity of synthetic ingredients while retaining "foaming" properties over a longer period.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Commissariat a l'Energie Atomique (CEA).

Journal Reference:

Anne-Laure Fameau, Arnaud Saint-Jalmes, Fabrice Cousin, Bérénice Houinsou Houssou, Bruno Novales, Laurence Navailles, Frédéric Nallet, Cédric Gaillard, François Boué, Jean-Paul Douliez. Smart Foams: Switching Reversibly between Ultrastable and Unstable Foams. Angewandte Chemie, 2011; DOI: 10.1002/ange.201102115

Innovative nanoparticle purification system uses magnetic fields

A team of Penn State University scientists has invented a new system that uses magnetism to purify hybrid nanoparticles -- structures that are composed of two or more kinds of materials in an extremely small particle that is visible only with an electron microscope.

Team leaders Mary Beth Williams, an associate professor of chemistry, and Raymond Schaak, a professor of chemistry, explained that the never-before-tried method will not only help scientists to remove impurities from such particles, it also will help researchers to distinguish between hybrid nanoparticles that appear to be identical when viewed under an electron microscope, but that have different magnetism -- a great challenge in recent nanoparticle research. The system holds the promise of helping to improve drug-delivery systems, drug-targeting technologies, medical-imaging technologies, and electronic information-storage devices.

The paper will be published in the journal Agewandte Chemie and is available on the journal's early-online website.

Schaak explained that purifying hybrid nanoparticles presents an enormous challenge, especially when nanoparticles are designed for human use -- for example, for drug delivery or as a contrast-dye alternative for patients undergoing MRI studies. "The problem is that although molecules are synthesized and purified using well-known methods, there have not been analogous methods for purifying nanoparticles," Schaak said. "Hybrid particles are especially challenging because the methods that are used to make them often leave impurities that are not easily detected or removed. Impurities can change the properties of a sample, for example, by making them toxic, so it is a major challenge to find ways to remove such impurities."

The team combined forces to figure out a way to purify hybrid nanoparticles. "We had to find a way to separate impurities from the target nanoparticles, even when these particles are similar in size and shape, because of the potentially very big consequences of impurities on the ultimate use of nanoparticles," Schaak said. The team's new system does just that. The innovative technique uses the magnetic components of nanoparticles to tell them apart and to separate impurities from the target nanoparticle structures.

"Our method uses magnetic fields to slow the flow of particles through tiny glass tubes called capillaries," Williams explained. "We use a magnet to pull magnetic particles against the wall of the tube and, when the magnetic field is reduced, the particles flow out of the capillary. Magnetism increases as particle volume increases, so small and gradual changes in the magnetic field let us slowly separate and distinguish between nanoparticles based on even minute magnetic and structural differences."

The team's paper shows how magnetic fields can be used to separate and distinguish between hybrid nanoparticles in a mixture of slightly different structures and shapes. In one example, the researchers separated "nano-flowers," so named because of their petal-like arrangement around a solid core, from spherically shaped particles. Williams explained that the magnetism of the particles depends on their shape, so particles of a different shape adhere to the capillary wall when different magnetic fields are applied, thus allowing the researchers to distinguish between the different particles.

In another example in the paper, the researchers showed how the magnetic-field method can be used with a class of nanoparticle dubbed the "nano-olive," which is a spherical particle composed of two different materials joined in a shape reminiscent of an olive. The nano-olives, which are composed of iron, platinum, and oxygen, may look alike, but they may have slightly different internal compositions that are impossible to detect under a microscope. "For example, some may have more iron content," Schaak explained. "This is a property we can use for purification with our method because these nanoparticles are a bit more magnetic. They stick along the walls of the capillary tubes more easily, while more magnetically weak particles flow out."

The new purification and separation method has many applications, especially within the fields of medicine and diagnostics. For example, nanoparticles could be used in lieu of contrast dye when patients undergo MRI imaging studies. Such particles could be used to track where a drug is traveling in the human body. "Some patients are allergic to traditional contrast dyes, so nanoparticles offer a promising alternative," Williams said.

Williams also explained that one of the very futuristic dreams of nanoparticle research is that it one day may be used to improve cancer-fighting drugs. "Unfortunately, chemotherapy drugs don't discriminate: They attack healthy tissue, as well as cancerous tissue," Williams said. "If we could use nanoparticle technology to manipulate exactly where the drugs are going, which tissue they attack, and which they leave alone, we could greatly reduce some of the bad side effects of chemotherapy, such as hair loss and nausea. But to do this we need to be able to separate out nanoparticle impurities to make them safe for medical use. That's where this new technology comes in."

In addition to Williams and Schaak, other members of the research team include Jacob S. Beveridge, Matthew R. Buck, and James F. Bondi of the Department of Chemistry at Penn State; and Rajiv Misra and Peter Schiffer of the Department of Physics and the Materials Research Institute at Penn State.

The above story is reprinted (with editorial adaptations) from materials provided by Penn State.

Journal Reference:

Jacob S. Beveridge, Matthew R. Buck, James F. Bondi, Rajiv Misra, Peter Schiffer, Raymond E. Schaak, Mary Elizabeth Williams. Purification and Magnetic Interrogation of Hybrid Au-Fe3O4 and FePt-Fe3O4 Nanoparticles. Angewandte Chemie International Edition, 2011; DOI: 10.1002/anie.201104829

A 'nano,' environmentally friendly, and low toxicity flame retardant protects fabric

The technology in "fire paint" used to protect steel beams in buildings and other structures has found a new life as a first-of-its-kind flame retardant for children's cotton sleepwear, terrycloth bathrobes and other apparel, according to a report presented in Denver at the 242nd National Meeting & Exposition of the American Chemical Society (ACS).

"People are concerned about the potential toxicity of flame retardants that are currently used on a variety of products, especially children's pajamas and the foam in children's car seats," said Jaime C. Grunlan, Ph.D., who led the research. "The water-based ingredients in this new coating are much less toxic to humans than the so-called 'halogenated' or 'brominated' flame retardants used in the past, and they are more environmentally friendly."

Grunlan explained that flame retardants are used on cotton, the most popular fabric in the world, because it can catch fire easily and burns rapidly with a hot flame. Flame retardants make it more difficult for fabrics to ignite, make them burn slower and make fabrics self-extinguish when the flame is removed. That margin of safety is especially important for clothing fires, which can cause severe and disfiguring injuries. Flame retardants allow time to remove the clothing or put out the flames.

In responding to the need for more environmentally friendly flame retardants, Grunlan's team turned to a technology termed "intumescence," long used to fireproof exposed interior steel beams in buildings. At the first lick of a flame, an intumescent coating swells up and expands like beer foam, forming tiny bubbles in a protective barrier that insulates and shields the material below. The researchers are at Texas A&M University in College Station.

"This work is the first demonstration of a polymer-based 'nano intumescent'," said Grunlan. "We believe it has great potential for use as flame retardants on clothing and other materials in order to avoid some of the disadvantages of existing products."

The material is "nano," or ultra-small, built up from layers of alternating positively and negatively charged polymers so thin that almost 50,000 would fit across the width of a human hair. Size has an advantage, Grunlan explained. Because these layers are so thin, the polymer liquid seeps deep into cotton fabric and onto each individual cotton fiber. Existing flame retardants, in contrast, simply settle on fiber bundles like armor and slow the spread of flames, but the fabric still burns and turns black. When the new nano coating is exposed to a flame, it expands slightly and stops the fire from igniting and burning the fabric -- which remains white, except for the small area where the cotton directly touches the flame.

Grunlan noted that the new flame retardant addresses public concerns about the potential toxicity of flame retardants now used on a variety of products, especially children's pajamas and the foam inside children's car seats and toys, and pointed out that the water-based polymers used in the nanocoating are much less toxic to humans than other flame retardants used today.

Would clothing coated with the nano intumescent be stiff and dull? "The look and texture of the fabric would depend on the thickness of the coating and also on the specific polymer we use," Grunlan pointed out. The nanocoating is deposited in alternate layers of positively and negatively charged polymers; swapping one of those polymers out for a different one in the recipe could offer similar anti-flammable protection while making the fabric softer.

Grunlan's team is in the process of optimizing the flame retardant so that it remains on cotton fabrics despite frequent laundering. "We haven't done anything yet to protect the coating, but we believe that with further research, we could make this an almost permanent addition to the fabric," he explained.

They also plan to test the coating on other materials, such as polyester and foam, possibly with commercial partners.

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