Graphene electrodes for organic solar cells

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A promising approach for making solar cells that are inexpensive, lightweight and flexible is to use organic (that is, carbon-containing) compounds instead of expensive, highly purified silicon. But one stubborn problem has slowed the development of such cells: Researchers have had a hard time coming up with appropriate materials for the electrodes to carry the current to and from the cells. Specifically, it has been hard to make electrodes using materials that can match the organic cells’ flexibility, transparency and low cost.

The standard material used so far for these electrodes is indium-tin-oxide, or ITO. But indium is expensive and relatively rare, so the search has been on for a suitable replacement. Now, a team of MIT researchers has come up with a practical way of using a possible substitute made from inexpensive and ubiquitous carbon. The proposed material is graphene, a form of carbon in which the atoms form a flat sheet just one atom thick, arranged in a chicken-wire-like formation.

An analysis of how to use graphene as an electrode for such solar cellswas published on Dec. 17in the journalNanotechnology, in a paper by MIT professors Jing Kong and Vladimir Bulović along with two of their students and a postdoctoral researcher.

Graphene is transparent, so that electrodes made from it can be applied to the transparentorganic solar cellswithout blocking any of the incoming light. In addition, it is flexible, like the organic solar cells themselves, so it could be part of installations that require the panel to follow the contours of a structure, such as a patterned roof. ITO, by contrast, is stiff and brittle.

The biggest problem with getting graphene to work as an electrode for organic solar cells has been getting the material to adhere to the panel. Graphene repels water, so typical procedures for producing an electrode on the surface by depositing the material from a solution won’t work.

The team tried a variety of approaches to alter the surface properties of the cell or to use solutions other than water to deposit the carbon on the surface, but none of these performed well, Kong says. But then they found that“doping” the surface— that is, introducing a set of impurities into the surface— changed the way it behaved, and allowed the graphene to bond tightly. As a bonus, it turned out the doping also improved the material’s electrical conductivity.

While the specific characteristics of the graphene electrode differ from those of the ITO it would replace, its overall performance in a solar cell is very similar, Kong says. And the flexibility and light weight of organic solar cells with graphene electrodes could open up a variety of different applications that would not be possible with today’s conventional silicon-based solar panels, she says. For example, because of their transparency they could be applied directly to windows without blocking the view, and they could be applied to irregular wall or rooftop surfaces. In addition, they could be stacked on top of other solar panels, increasing the amount of power generated from a given area. And they could even be folded or rolled up for easy transportation.

While this research looked at how to adapt graphene to replace one of the two electrodes on a solar panel, Kong and her co-workers are now trying to adapt it to the other electrode as well. In addition, widespread use of this technology will require new techniques for large-scale manufacturing of graphene— an area of very active research. The ongoing work has been funded by the Eni-MIT Alliance Solar Frontiers Center and an NSF research fellowship.

Peter Peumans, an assistant professor of electrical engineering at Stanford University, who was not involved in this study, says organicsolar cellswill probably become practical only with the development of transparent electrode technology that is both cheaper and more robust than conventional metal oxides. Other materials are being studied as possible substitutes, he says, but this work represents“very important progress” toward making graphene a credible replacement transparent electrode.

“Other groups had already shown that graphene exhibits good combinations of transparency and sheet resistance, but no one was able to achieve a performance with graphene electrodes that matches that of devices on conventional metal oxide (ITO)electrodes,” Peumans says.“This work is a substantial push toward making graphene a leading candidate.”

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This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

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New solar cell self-repairs like natural plant systems

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(PhysOrg.com) -- Researchers are creating a new type of solar cell designed to self-repair like natural photosynthetic systems in plants by using carbon nanotubes and DNA, an approach aimed at increasing service life and reducing cost.

"We've created artificial photosystems using opticalnanomaterialsto harvestsolar energythat is converted to electrical power,"said Jong Hyun Choi, an assistant professor of mechanical engineering at Purdue University.

The design exploits the unusualelectrical propertiesof structures called single-wall carbon nanotubes, using them as"molecular wires in light harvesting cells,"said Choi, whose research group is based at the Birck Nanotechnology and Bindley Bioscience centers at Purdue's Discovery Park.

"I think our approach offers promise for industrialization, but we're still in the basic research stage,"he said.

Photoelectrochemical cells convert sunlight into electricity and use anelectrolyte- a liquid that conducts electricity - to transport electrons and create the current. The cells contain light-absorbing dyes called chromophores, chlorophyll-like molecules that degrade due to exposure to sunlight.

"The critical disadvantage of conventionalphotoelectrochemical cellsis this degradation,"Choi said.

The new technology overcomes this problem just as nature does: by continuously replacing the photo-damaged dyes with new ones.

"This sort of self-regeneration is done in plants every hour,"Choi said.

The new concept could make possible an innovative type of photoelectrochemical cell that continues operating at full capacity indefinitely, as long as new chromophores are added.

Findings were detailed in a November presentation during the International Mechanical Engineering Congress and Exhibition in Vancouver. The concept also was unveiled in anonline articlefeatured on the Web site for SPIE, an international society for optics andphotonics.

The talk and article were written by Choi, doctoral students Benjamin A. Baker and Tae-Gon Cha, and undergraduate students M. Dane Sauffer and Yujun Wu.

The carbon nanotubes work as a platform to anchor strands of DNA. The DNA is engineered to have specific sequences of building blocks called nucleotides, enabling them to recognize and attach to the chromophores.

"The DNA recognizes the dye molecules, and then the system spontaneously self-assembles,"Choi said

When the chromophores are ready to be replaced, they might be removed by using chemical processes or by adding new DNA strands with different nucleotide sequences, kicking off the damaged dye molecules. New chromophores would then be added.

Two elements are critical for the technology to mimic nature's self-repair mechanism: molecular recognition and thermodynamic metastability, or the ability of the system to continuously be dissolved and reassembled.

The research is an extension of work that Choi collaborated on with researchers at the Massachusetts Institute of Technology and the University of Illinois. The earlier work used biological chromophores taken from bacteria, and findings were detailed in a researchpaper publishedin November in the journalNature Chemistry.

However, using natural chromophores is difficult, and they must be harvested and isolated from bacteria, a process that would be expensive to reproduce on an industrial scale, Choi said.

"So instead of using biological chromophores, we want to use synthetic ones made of dyes called porphyrins,"he said.

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Researchers print solar cells on toilet paper, other delicate materials (w/ Video)

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To demonstrate how a new fabrication technique can print solar cells on extremely thin, flexible materials, researchers from MIT have patterned solar cells onto ordinary toilet paper. While toilet paper may be an unlikely substrate for practical solar cell applications, it illustrates the versatility of the technique for low-cost printing on a wide variety of materials.

Karen Gleason, a chemical engineering professor at MIT, along with graduate student Miles Barr and others, showed that the technique could be used to printsolar cellson a variety of delicate materials. One example is rice paper, which is used to make spring rolls in restaurants and usually dissolves in wet processes. Since the researchers’ technique is a dry, solvent-free process, the rice paper remains intact. The researchers also demonstrated the technique on plastic Saran wrap, which repels water and would normally be difficult to coat.

The new method, called oxidative chemical vapor deposition (oCVD), involves spraying a vapor of a monomer and an oxidizing agent onto asubstrate. The monomer and oxidizing agent polymerize when they meet and form PEDOT plastic. The plastic itself is conductive, but the conductivity can be further increased up to 1,000 times by controlling the substrate temperature so that small nanopores form, which can be laced with highly conductive silver particles.

The printed solar cells can also withstand a great deal of bending and stretching with minimal effect on their properties. In tests, the researchers bent a printed plastic substrate to a radius of less than 5 mm more than 1,000 times, and found that its efficiency was still 99% of what is was before bending. The electrodes could also be bent and stretched, and still retained their conductivity. To further demonstrate the method’s robustness, Barr folded a piece of paper printed with solar cells into a paper airplane, and showed that the device still generated a current.

MIT Professor Karen K. Gleason explains how graduate student Miles Barr folds a solar cell into a paper airplane. The research is part of the Eni-MIT Solar Frontiers Center. Video credit: MIT.

As the researchers noted, paper is not typically considered a good substrate for photovoltaics because it’s not transparent. However, the ability to print solar cells at low-cost on flexible, stretchable materials could be very useful for making solar cells more widespread. Since the technique can also be used to print other electronic devices besides solar cells, it could be used for novel applications such as printing electronics on fabric and other flexible displays.

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Polymer scientists make imprint on nanolithography

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(PhysOrg.com) -- Nanolithography, or surface patterning on a nanoscale, is critical for modern technology, but has been developed largely for patterning flat surfaces until recently. A team of University of Akron scientists discovered a new method for patterning curved surfaces. The technique creates patterns on curved or topographically uneven surfaces with stand-alone nanoparticles, opening new technology opportunities.

Findings by University of Akron graduate students Sarang P. Bhawalkar, Jun Qian (a visiting student from Tianjin University, China), Michael C. Heiber, and assistant professor of polymer science Dr. Li Jia are available in the Nov. 16, 2010 issue ofLangmuir, a publication of the American Chemical Society.

“Nanoparticles arranged in hexagonal patterns have been widely used for surfacepatterningbefore our work, but these particles touch and support each other,” explains Jia.“We were curious to learn if we could use stand-alone particles not supporting each other. There are several advantages to this. Among them is the possibility of patterning curved or uneven surfaces. Consider traditional photolithography, which is highly efficient in putting complex circuits on flat computer chips, but inapt at patterning surfaces that are not flat.”

The challenge, according to Jia, was to secure the pattern against the lateral capillary force. When this challenge was presented to Sarang, his solution was to dip-coat a layer of polymer adhesive.

“It worked like a charm,” Jia says.

According to Jia, the method is a breakthrough due to adaptation to topographic features ranging from macroscopic to microscopic scales. The team is currently working on fabrication of surfaces with a combination of several advanced properties such as self-cleaning, anti-reflection and anti-icing, says Jia, who notes the desirability of thesesurfaceproperties in skyscrapers, aircrafts, solar panels and residential windows.

The researchers are testing their lithography method on large surfaces and durability of the patterns when subjected to temperature fluctuations and abrasion. Jia adds that he and his colleagues’ next step, in collaboration with other experts, is to explore the applications of their lithography method in optical circuitry, imaging and sensing, and bioengineering.

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Japan nano-tech team creates palladium-like alloy: report

Japanese researchers have created an alloy with properties similar to palladium, a precious metal used in many high-tech goods, a news report said Thursday, dubbing the breakthrough"present-day alchemy".

Kyoto University professor Hiroshi Kitagawa and his team said they used nano-technology to combine rhodium and silver, elements which do not usually mix, to produce the new composite, the Yomiuri daily said.

The alloy has similar properties topalladium, which is used in cars' emission-reducing catalytic converters as well as in computers, mobile phones, flatscreen TVs and dentistry instruments.

Like other white metals, such as silver and platinum, palladium is expensive, with its deposits largely limited to South Africa and Russia.

Palladium also has applications in the production of fuel cells -- a clean andrenewable energy sourcethat produces electricity by combining hydrogen and oxygen, with water as the only byproduct.

To make the new alloy, the Kyoto team used nano-technology to"nebulise"the rhodium and silver and gradually mixed them with heated alcohol, with the two metals mixed stably at the atomic level, the report said.

Japan's industry ministry has listed 31 rare metals, including palladium and lithium, which are used in industrial products, such aselectronic devicesand batteries. Of these, 17 elements are called rare earth minerals.

Resource-poor Japan has tried to shift from its dependence on China, which controls the bulk of global rare earth production.

Kitagawa said he hopes to create morealloysusing nano-technology, without specifying which ones, the Yomiuri said.

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Tiny channels carry big information

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They say it's the little things that count, and that certainly holds true for the channels in transmembrane proteins, which are small enough to allow ions or molecules of a certain size to pass through, while keeping out larger objects. Artificial fluidic nanochannels that mimic the capabilities of transmembrane proteins are highly prized for a number of advanced technologies. However, it has been difficult to make individual artificial channels of this size– until now.

Researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory have been able to fabricate nanochannels that are only two nanometers (2-nm) in size, using standard semiconductor manufacturing processes. Already they've used these nanochannels to discover that fluid mechanics for passages this small are significantly different not only from bulk-sized channels, but even from channels that are merely 10 nanometers in size.

"We were able to study ion transport in our 2-nm nanochannels by measuring the time and concentration dependence of the ionic conductance,"says Arun Majumdar, Director of DOE's Advanced Research Projects Agency– Energy (ARPA-E), who led this research while still a scientist at Berkeley Lab."We observed a much higher rate of proton and ionic mobility in our confined hydrated channels– up to a fourfold increase over that in larger nanochannels (10-to-100 nm). This enhanced proton transport could explain the high throughput of protons in transmembrane channels."

Majumdar is the co-author with Chuanhua Duan, a member of Majumdar's research group at the University of California (UC) Berkeley, of a paper on this work, which was published in the journalNature Nanotechnlogy. The paper is titled"Anomalous ion transport in 2-nm hydrophilic nanochannels."

In their paper, Majumdar and Duan describe a technique in which high-precision ion etching is combined with anodic bonding to fabricate channels of a specific size and geometry on a silicon-on-glass die. To prevent the channel from collapsing under the strong electrostatic forces of the anodic bonding process, a thick (500 nm) oxide layer was deposited onto the glass substrate.

"This deposition step and the following bonding step guaranteed successful channel sealing without collapsing,"says Duan."We also had to choose the right temperature, voltage and time period to ensure perfect bonding. I compare the process to cooking a steak, you need to choose the right seasoning as well as the right time and temperature. The deposition of the oxide layer was the right seasoning for us."

The nanometer-sized channels in transmembrane proteins are critical to controlling the flow ofionsandmoleculesacross the external and internal walls of a biological cell, which, in turn, are critical to many of the biological processes that sustain the cell. Like their biological counterparts, fluidic nanochannels could play critical roles in the future of fuel cells and batteries.

"Enhanced ion transport improves the power density and practical energy density of fuel cells and batteries,"Duan says."Although the theoretical energy density in fuel cells and batteries is determined by the active electrochemical materials, the practical energy density is always much lower because of internal energy loss and the usage of inactive components. Enhanced ion transport could reduce internal resistance in fuel cells and batteries, which would reduce the internal energy loss and increase the practical energy density."

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Artificial fluidic nanochannels, like these 30-nm channels shown under fluorescence, mimic the capabilities of transmembrane proteins and are highly prized for advanced technology applications. Credit: Image courtesy of Majumdar group, UC Berkeley

The findings by Duan and Majumdar indicate that ion transport could be significantly enhanced in 2-nm hydrophilic nanostructures because of their geometrical confinements and high surface-charge densities. As an example, Duan cites the separator, the component placed between the between the cathode and the anode in batteries and fuel cells to prevent physical contact of the electrodes while enabling free ionic transport.

"Current separators are mostly microporous layers consisting of either a polymeric membrane or non-woven fabric mat,"Duan says."An inorganic membrane embedded with an array of 2-nm hydrophilic nanochannels could be used to replace current separators and improve practical power and energy density."

The 2-nm nanochannels also hold promise for biological applications because they have the potential to be used to directly control and manipulate physiological solutions. Current nanofluidic devices utilize channels that are 10-to-100 nm in size to separate and manipulate biomolecules. Because of problems with electrostatic interactions, these larger channels can function with artificial solutions but not with natural physiological solutions.

"For physiological solutions with typical ionic concentrations of approximately 100 millimolars, the Debye screening length is 1 nm,"says Duan."Since electrical double layers from two-channel surfaces overlap in our 2-nm nanochannels, all current biological applications found in larger nanochannels can be transferred to 2-nm nanochannels for real physiological media."

The next step for the researchers will be to study the transport of ions and molecules in hydrophilic nanotubes that are even smaller than 2-nm. Ion transport is expected to be even further enhanced by the smaller geometry and stronger hydration force.

"I am developing an inorganic membrane with embedded sub-2 nm hydrophilic nanotube array that will be used to study ion transport in both aqueous and organic electrolytes,' Duan says."It will also be developed as a new type of separator for lithium-ion batteries."

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New method for making tiny catalysts holds promise for air quality

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Fortified with iron: It's not just for breakfast cereal anymore. University of Illinois researchers have demonstrated a simpler method of adding iron to tiny carbon spheres to create catalytic materials that have the potential to remove contaminants from gas or liquid.

Civil and environmental engineering professor Mark Rood, graduate student John Atkinson and their team described their technique in the journalCarbon.

Carbon structures can be a support base for catalysts, such as iron and other metals. Iron is a readily available, low-costcatalystwith possible catalytic applications for fuel cells and environmental applications for adsorbing harmful chemicals, such as arsenic or carbon monoxide. Researchers produce a carbon matrix that has manyporesor tunnels, like a sponge. The large surface area created by the pores provides sites to disperse tiny iron particles throughout the matrix.

A common source of carbon is coal. Typically, scientists modify coal-based materials into highly porous activated carbon and then add a catalyst. The multi-step process takes time and enormous amounts of energy. In addition, materials made with coal are plagued by ash, which can contain traces of other metals that interfere with the reactivity of the carbon-based catalyst.

The Illinois team's ash-free, inexpensive process takes its carbon from sugar rather than coal.

In one continuous process, it produces tiny, micrometer-sized spheres of porous, spongy carbon embedded with ironnanoparticles– all in the span of a few seconds.

"That's what really sets this apart from other techniques. Some people have carbonized and impregnated with iron, but they have no surface area. Other people have surface area but weren't able to load it with iron,"Atkinson said."Our technique provides both the carbon surface and the iron nanoparticles."

The researchers built upon a technique called ultrasonic spray pyrolysis (USP), developed in U. of I. chemistry professor Kenneth Suslick's lab in 2005. Suslick used a household humidifier to make fine mist from a carbon-rich solution, then directed the mist through an extremely hot furnace, which evaporated the water from each droplet and left tiny, highly porous carbon spheres.

Atkinson used USP to make his carbon spheres, but added an iron-containing salt to a carbon-rich sugar solution. When the mist is piped into the furnace, the heat stimulates achemicalreaction between the solution ingredients that creates carbon spheres with iron particles dispersed throughout.

"We were able to take advantage of Dr. Suslick's USP technique, and we are building upon it by simultaneously impregnating the porous carbons with metal nanoparticles,"Atkinson said."It's simple because it's continuous. We can isolate the carbon, add pores, and impregnate iron into the carbon spheres in a single step."

Another advantage of the USP technique is the ability to create materials to address particular needs. By fabricating the material from scratch, rather than trying to modify off-the-shelf products, scientists and engineers can develop materials for specific problem-solving scenarios.

"Right now, you take coal out of the ground and modify it. It's difficult to tailor it to solve a particular air quality problem,"Rood said."We can readily change this new material by how it's activated to tailor its surface area and the amount of impregnated iron. This method is simple, flexible and tailorable."

Next, the researchers will explore applications for the material. Rood and Atkinson have received two grants from the National Science Foundation to develop the carbon-iron spheres to remove nitric oxide, mercury, and dioxin from gas streams– bioaccumulating pollutants that have caused concern as emissions from combustion sources.

Currently, the three pollutants can be dealt with separately by carbon-based adsorbents and catalysts, but the Illinois team and collaborators in Taiwan hope to harness carbon's adsorption properties and iron's reactivity to remove all three pollutants from gas streams simultaneously.

"We're looking at taking advantage of their porosity and, ideally, their catalytic applications as well,"Atkinson said."Carbon is a very versatile material. What's in my mind is a multi-pollutant control where you can use the porosity and the catalyst to tackle two problems at once."

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