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Research Alert: March 11, 2014

Tuesday, March 11th, 2014

New research could help make “roll up” screens a reality

A study, published today in Nature’s Scientific Reports identifies a new technology which could see flexible electronics such as roll-up tablet computers, widely available in the near future. So far, this area of electronic design has been hampered by unreliability and complexity of production.

Researchers from the University of Surrey worked together with scientists from Philips to further develop the ‘Source-Gated-Transistor’ (SGT) – a simple circuit component invented jointly by the teams.

Previously, they found that the component could be applied to many electronic designs of an analog nature, such as display screens. Through this current study, researchers have now shown that SGTs can also be applied to next-generation digital circuits.

SGTs control the electric current as it enters a semiconductor, which decreases the odds of circuit malfunction, improves energy efficiency and keeps fabrication costs to a minimum. These properties make SGTs ideal for next-generation electronic devices, and could enable digital technologies to be incorporated into those built using flexible plastics or clothing textiles.

Such technologies may include ultra-lightweight and flexible gadgets which can be rolled up to save space when not in use, smart plasters, thinner than a human hair, that can wirelessly monitor the health of the wearer, low-cost electronic shopping tags for instant checkout, and disaster prediction sensors, used on buildings in regions that are at high risk of natural disasters.

“These technologies involve thin plastic sheets of , similar to sheets of paper, but embedded with smart technologies. Until now, such technologies could only be produced reliably in small quantities, and that confined them to the research lab. However, with SGTs we have shown we can achieve characteristics needed to make these technologies viable, without increasing the complexity or cost of the design,” said lead researcher Dr. Radu Sporea, Advanced Technology Institute (ATI), University of Surrey.

Professor Ravi Silva, Director of the ATI and a co-author of the work, said, “This work is a classic example of academia working closely with industry for over two decades to perfect a concept which has wide-reaching applications across a variety of technologies. Whilst SGTs can be applied to mainstream materials such as silicon, used widely in the production of current consumer devices, it is the potential to apply them to new materials such graphene that makes this research so crucial.”

“By making these incredible devices less complex and implicitly very affordable, we could see the next generation of gadgets become mainstream much quicker than we thought,” Dr Sporea concluded.

Squeezing light into metals

Using an inexpensive inkjet printer, University of Utah electrical engineers produced microscopic structures that use light in metals to carry information. This new technique, which controls electrical conductivity within such microstructures, could be used to rapidly fabricate superfast components in electronic devices, make wireless technology faster or print magnetic materials.

A recently discovered technology called plasmonics marries the best aspects of optical and electronic data transfer. By crowding light into metal structures with dimensions far smaller than its wavelength, data can be transmitted at much higher frequencies such as terahertz frequencies, which lie between microwaves and infrared light on the spectrum of electromagnetic radiation that also includes everything from X-rays to visible light to gamma rays. Metals such as silver and gold are particularly promising plasmonic materials because they enhance this crowding effect.

“Very little well-developed technology exists to create terahertz plasmonic devices, which have the potential to make wireless devices such as Bluetooth – which operates at 2.4 gigahertz frequency – 1,000 times faster than they are today,” says Ajay Nahata, a University of Utah professor of electrical and computer engineering and senior author of the new study.

Using a commercially available inkjet printer and two different color cartridges filled with silver and carbon ink, Nahata and his colleagues printed 10 different plasmonic structures with a periodic array of 2,500 holes with different sizes and spacing on a 2.5-inch-by-2.5 inch plastic sheet.

The four arrays tested had holes 450 microns in diameter – about four times the width of a human hair – and spaced one-25th of an inch apart. Depending on the relative amounts of silver and carbon ink used, the researchers could control the plasmonic array’s electrical conductivity, or how efficient it was in carrying an electrical current.

“Using a $60 inkjet printer, we have developed a low-cost, widely applicable way to make plasmonic materials,” Nahata says. “Because we can draw and print these structures exactly as we want them, our technique lets you make rapid changes to the plasmonic properties of the metal, without the million-dollar instrumentation typically used to fabricate these structures.”

Plasmonic arrays are currently made using microfabrication techniques that require expensive equipment and manufacture only one array at a time. Until now, controlling conductivity in these arrays has proven extremely difficult for researchers.

Nahata and his co-workers at the University of Utah’s College of Engineering used terahertz imaging to measure the effect of printed plasmonic arrays on a beam of light. When light with terahertz frequency is directed at a periodic array of holes in a metal layer, it can result in resonance, a fundamental property best illustrated by a champagne flute shattering when it encounters a musical tone of the right pitch.

Terahertz imaging is useful for nondestructive testing, such as detection of anthrax bacterial weapons in packaging or examination of insulation in spacecraft. By studying how terahertz light transmits through their printed array, the Utah team showed that simply changing the amount of carbon and silver ink used to print the array could be used to vary transmission through this structure.

With this new printing technique, Nahata says, “we have an extra level of control over both the transmission of light and electrical conductivity in these devices – you can now design structures with as many different variations as the printer can produce.” Nahata says these faster plasmonic arrays eventually could prove useful for:

  • Wireless devices, because the arrays allow data to be transmitted much more quickly. Many research groups are actively working on this application now.
  • Printing magnetic materials for greater functionality (lower conductivity, more compact) in different devices. This technology is more than five years away, Nahata says.

Although the Utah team used two different kinds of ink, up to four different inks in a four-color inkjet printer could be used, depending on the application.

Promising news for solar fuels from Berkley Lab researchers at JCAP

There’s promising news from the front on efforts to produce fuels through artificial photosynthesis. A new study by Berkeley Lab researchers at the Joint Center for Artificial Photosynthesis (JCAP) shows that nearly 90-percent of the electrons generated by a hybrid material designed to store solar energy in hydrogen are being stored in the target hydrogen molecules.

Gary Moore, a chemist and principal investigator with Berkeley Lab’s Physical Biosciences Division, led an efficiency analysis study of a unique photocathode material he and his research group have developed for catalyzing the production of hydrogen fuel from sunlight. This material, a hybrid formed from interfacing the semiconductor gallium phosphide with a molecular hydrogen-producing cobaloxime catalyst, has the potential to address one of the major challenges in the use of artificial photosynthesis to make renewable solar fuels.

“Ultimately the renewable energy problem is really a storage problem,” Moore says. “Given the intermittent availability of sunlight, we need a way of using the sun all night long. Storing solar energy in the chemical bonds of a fuel also provides the large power densities that are essential to modern transport systems. We’ve shown that our approach of coupling the absorption of visible light with the production of hydrogen in a single material puts photoexcited electrons where we need them to be, stored in chemical bonds.”

Moore is the corresponding author of a paper describing this research in the journal Physical Chemistry Chemical Physics titled “Energetics and efficiency analysis of a cobaloxime-modified semiconductor under simulated air mass 1.5 illumination.” Co-authors are Alexandra Krawicz and Diana Cedeno.

Bionic leaves that produce energy-dense fuels from nothing more than sunlight, water and atmosphere-warming carbon dioxide, with no byproducts other than oxygen, represent an ideal sustainable energy alternative to fossil fuels. However, realizing this artificial photosynthesis ideal will require a number of technological breakthroughs including high performance photocathodes that can catalyze fuel production from sunlight alone.

Last year, Moore and his research group at JCAP took an important step towards the photocathode goal with their gallium phosphide/cobaloxime hybrid. Gallium phosphide is an absorber of visible light, which enables it to produce significantly higher photocurrents than semiconductors that only absorb ultraviolet light. The cobaloxime catalyst is also Earth-abundant, meaning it is a relatively inexpensive replacement for the highly expensive precious metal catalysts, such as platinum, currently used in many solar-fuel generator prototypes.

“The novelty of our approach is the use of molecular catalytic components interfaced with visible-light absorbing semiconductors,” Moore says. “This creates opportunities to use discrete three-dimensional environments for directly photoactivating the multi-electron and multi-proton chemistry associated with the production of hydrogen and other fuels.”

The efficiency analysis performed by Moore and his colleagues also confirmed that the light-absorber component of their photocathode is a major bottleneck to obtaining higher current densities. Their results showed that of the total number of solar photons striking the hybrid-semiconductor surface, measured over the entire wavelength range of the solar spectrum (from 200 to 4,000 nanometers) only 1.5-percent gave rise to a photocurrent.

“This tells us that the use of light absorbers with improved spectral coverage of the sun is a good start to achieving further performance gains, but it is likely we will also have to develop faster and more efficient catalysts as well as new attachment chemistries. Our modular assembly method provides a viable strategy to testing promising combinations of new materials,” Moore says.

Research News: Oct. 29, 2013

Tuesday, October 29th, 2013

ASU, Georgia Tech create breakthrough for solar cell efficiency

Crystals are at the heart of diodes. Not the kind you might find in quartz, formed naturally, but manufactured to form alloys, such as indium gallium nitride or InGaN. This alloy forms the light emitting region of LEDs, for illumination in the visible range, and of laser diodes (LDs) in the blue-UV range.

Research into making better crystals, with high crystalline quality, light emission efficiency and luminosity, is also at the heart of studies being done at Arizona State University by Research Scientist Alec Fischer and Doctoral Candidate Yong Wei in Professor Fernando Ponce’s group in the Department of Physics.

In an article recently published in the journal Applied Physics Letters, the ASU group, in collaboration with a scientific team led by Professor Alan Doolittle at the Georgia Institute of Technology, has just revealed the fundamental aspect of a new approach to growing InGaN crystals for diodes, which promises to move photovoltaic solar cell technology toward record-breaking efficiencies.

The InGaN crystals are grown as layers in a sandwich-like arrangement on sapphire substrates. Typically, researchers have found that the atomic separation of the layers varies; a condition that can lead to high levels of strain, breakdowns in growth, and fluctuations in the alloy’s chemical composition.

“Being able to ease the strain and increase the uniformity in the composition of InGaN is very desirable,” says Ponce, “but difficult to achieve. Growth of these layers is similar to trying to smoothly fit together two honeycombs with different cell sizes, where size difference disrupts a periodic arrangement of the cells.”

As outlined in their publication, the authors developed an approach where pulses of molecules were introduced to achieve the desired alloy composition. The method, developed by Doolittle, is called metal-modulated epitaxy. “This technique allows an atomic layer-by-layer growth of the material,” says Ponce.

Analysis of the atomic arrangement and the luminosity at the nanoscale level was performed by Fischer, the lead author of the study, and Wei. Their results showed that the films grown with the epitaxy technique had almost ideal characteristics and revealed that the unexpected results came from the strain relaxation at the first atomic layer of crystal growth.

New device stores electricity on silicon chips

Solar cells that produce electricity 24/7, not just when the sun is shining. Mobile phones with built-in power cells that recharge in seconds and work for weeks between charges.

These are just two of the possibilities raised by a novel supercapacitor design invented by material scientists at Vanderbilt University that is described in a paper published in the Oct. 22 issue of the journal Scientific Reports.

It is the first supercapacitor that is made out of silicon so it can be built into a silicon chip along with the microelectronic circuitry that it powers. In fact, it should be possible to construct these power cells out of the excess silicon that exists in the current generation of solar cells, sensors, mobile phones and a variety of other electromechanical devices, providing a considerable cost savings.

Instead of storing energy in chemical reactions the way batteries do, “supercaps” store electricity by assembling ions on the surface of a porous material. As a result, they tend to charge and discharge in minutes, instead of hours, and operate for a few million cycles, instead of a few thousand cycles like batteries.

These properties have allowed commercial supercapacitors, which are made out of activated carbon, to capture a few niche markets, such as storing energy captured by regenerative braking systems on buses and electric vehicles and to provide the bursts of power required to adjust of the blades of giant wind turbines to changing wind conditions. Supercapacitors still lag behind the electrical energy storage capability of lithium-ion batteries, so they are too bulky to power most consumer devices. However, they have been catching up rapidly.

Researchers advance scheme to design seamless integrated circuits etched on graphene

Researchers in electrical and computer engineering at UC Santa Barbara have introduced and modeled an integrated circuit design scheme in which transistors and interconnects are monolithically patterned seamlessly on a sheet of graphene, a 2-dimensional plane of carbon atoms. The demonstration offers possibilities for ultra energy-efficient, flexible, and transparent electronics.

Bulk materials commonly used to make CMOS transitors and interconnects pose fundamental challenges in continuous shrinking of their feature-sizes and suffer from increasing “contact resistance” between them, both of which lead to degrading performance and rising energy consumption. Graphene-based transistors and interconnects are a promising nanoscale technology that could potentially address issues of traditional silicon-based transistors and metal interconnects.

“In addition to its atomically thin and pristine surfaces, graphene has a tunable band gap, which can be adjusted by lithographic sketching of patterns – narrow graphene ribbons can be made semiconducting while wider ribbons are metallic. Hence, contiguous graphene ribbons can be envisioned from the same starting material to design both active and passive devices in a seamless fashion and lower interface/contact resistances,” explained Kaustav Banerjee, professor of electrical and computer engineering and director of the Nanoelectronics Research Lab at UCSB. Banerjee’s research team also includes UCSB researchers Jiahao Kang, Deblina Sarkar and Yasin Khatami.

“Accurate evaluation of electrical transport through the various graphene nanoribbon based devices and interconnects and across their interfaces was key to our successful circuit design and optimization,” explained Jiahao Kang, a PhD student in Banerjee’s group and a co-author of the study. Banerjee’s group pioneered a methodology using the Non-Equilibrium Green’s Function (NEGF) technique to evaluate the performance of such complex circuit schemes involving many heterojunctions. This methodology was used in designing an “all-graphene” logic circuit reported in this study.

“This work has demonstrated a solution for the serious contact resistance problem encounterd in conventional semiconductor technology by providing an innovative idea of using an all-graphene device-interconnect scheme. This will significantly simplify the IC fabrication process of graphene based nanoelectronic devices.” commented Philip Kim, professor of physics at Columbia University, and a renowned scientist in the graphene world.

As reported in their study, the proposed all-graphene circuits have achieved 1.7X higher noise margins and 1-2 decades lower static power consumption over current CMOS technology. According to Banerjee, with the ongoing worldwide efforts in patterning and doping of graphene, such circuits can be realized in the near future.

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