Record power conversion efficiency from a new graphene solar cell

GAINESVILLE, Fla. — Doping may be a no-no for athletes, but researchers in the University of Florida’s physics department say it was key in getting unprecedented power conversion efficiency from a new graphene solar cell created in their lab.

Graphene solar cells are one of industry’s great hopes for cheaper, durable solar power cells in the future. But previous attempts to use graphene, a single-atom-thick honeycomb lattice of carbon atoms, in solar cells have only managed power conversion efficiencies ranging up to 2.9 percent. The UF team was able to achieve a record breaking 8.6 percent efficiency with their device by chemically treating, or doping, the graphene with trifluoromethanesulfonyl-amide, or TFSA. Their results are published in the current online edition of Nano Letters.

“The dopant makes the graphene film more conductive and increases the electric field potential inside the cell,” said Xiaochang Miao, a graduate student in the physics department. That makes it more efficient at converting sunlight into electricity. And unlike other dopants that have been tried in the past, TFSA is stable — its effects are long lasting.

The solar cell that Miao and her co-workers created in the lab looks like a 5-mm-square window framed in gold. The window, a wafer of silicon coated with a monolayer of graphene, is where the magic happens. Graphene and silicon, when they come together, form what is called a Schottky junction — a one-way street for electrons that when illuminated with light, acts as the power conversion zone for an entire class of solar cells. Schottky junctions are commonly formed by layering a metal on top of a semiconductor. But researchers at the UF Nanoscience Institute for Medical and Engineering Technologies discovered in 2011 that graphene, a semi-metal, made a suitable substitute for metal in creating the junction.

“Graphene, unlike conventional metals, is transparent and flexible, so it has great potential to be an important component in the kind of solar cells we hope to see incorporated into building exteriors and other materials in the future,” said Arthur Hebard, distinguished professor of physics at UF and co-author on the paper. “Showing that its power-converting capabilities can be enhanced by such a simple, inexpensive treatment bodes well for its future.”

The researchers said that if graphene solar cells reach 10 percent power conversion efficiency they could be a contender in the market place, if production costs are kept low enough.

The prototype solar cell created in the UF lab was built on a rigid base of silicon, which is not considered an economical material for mass production. But Hebard said that he sees real possibilities for combining the use of doped graphene with less expensive, more flexible substrates like the polymer sheets currently under development in research laboratories around the world. -30-

Credits Writer Donna Hesterman, donna.hesterman@ufl.edu, 352-846-2573 Source Arthur Hebard, afh@ufl.edu, 352-222-6212 Source Xiaochang Miao, xxmiaophy@gmail.com, 352-871-4116

Topological Transitions in nanostructured Metamaterials

WEST LAFAYETTE, Ind. – Researchers are edging toward the creation of new optical technologies using "nanostructured metamaterials" capable of ultra-efficient transmission of light, with potential applications including advanced solar cells and quantum computing.

The metamaterial - layers of silver and titanium oxide and tiny components called quantum dots - dramatically changes the properties of light. The light becomes "hyperbolic," which increases the output of light from the quantum dots.

Such materials could find applications in solar cells, light emitting diodes and quantum information processing far more powerful than today's computers.

"Altering the topology of the surface by using metamaterials provides a fundamentally new route to manipulating light," said Evgenii Narimanov, a Purdue University associate professor of electrical and computer engineering.

This graphic depicts a new "nanostructured metamaterial" - layers of silver and titanium oxide and tiny components called quantum dots - to dramatically change the properties of light. Researchers are working to perfect the metamaterials, which might be capable of ultra-efficient transmission of light, with potential applications including advanced solar cells and quantum computing. Findings and this image appeared in the journal Science in April. (Image courtesy of CUNY)

Such metamaterials could make it possible to use single photons – the tiny particles that make up light – for switching and routing in future computers. While using photons would dramatically speed up computers and telecommunications, conventional photonic devices cannot be miniaturized because the wavelength of light is too large to fit in tiny components needed for integrated circuits. "For example, the wavelength used for telecommunications is 1.55 microns, which is about 1,000 times too large for today's microelectronics," Narimanov said. Nanostructured metamaterials, however, could make it possible to reduce the size of photons and the wavelength of light, allowing the creation of new types of nanophotonic devices, he said.

The work was a collaboration of researchers from Queens and City Colleges of City University of New York (CUNY), Purdue University, and University of Alberta. The experimental study was led by the CUNY team, while the theoretical work was carried out at Purdue and Alberta.

The Science paper is authored by CUNY researchers Harish N.S. Krishnamoorthy, Vinod M. Menon and Ilona Kretzschmar; University of Alberta researcher Zubin Jacob; and Narimanov. Zubin is a former Purdue doctoral student who worked with Narimanov.

The approach could help researchers develop "quantum information systems" far more powerful than today's computers. Such quantum computers would take advantage of a phenomenon described by quantum theory called "entanglement." Instead of only the states of one and zero, there are many possible "entangled quantum states" in between.

The research has been funded by the National Science Foundation and the U.S. Army Research Office.

Purdue University Writer: Emil Venere, 765-494-4709, venere@purdue.edu Source: Evgenii Narimanov, 765-494-1622, enarimanov@gmail.com

Cloak of invisiblity a device that can see without being seen, an invisible machine that detects light and controls it's flow at the nanoscale

“plasmonic cloaking” a device that can see without being seen, an invisible machine that detects light and controls the flow of light at the nanoscale

A team of engineers at Stanford and the University of Pennsylvania has for the first time used “plasmonic cloaking” to create a device that can see without being seen – an invisible machine that detects light. It is the first example of what the researchers describe as a new class of devices that controls the flow of light at the nanoscale to produce both optical and electronic functions.

It may not be intuitive, but a coating of reflective metal can actually make something less visible, engineers at Stanford and UPenn have shown. They have created an invisible, light-detecting device that can “see without being seen.”

At the heart of the device are silicon nanowires covered by a thin cap of gold. By adjusting the ratio of metal to silicon – a technique the engineers refer to as tuning the geometries – they capitalize on favorable nanoscale physics in which the reflected light from the two materials cancel each other to make the device invisible.

Pengyu Fan is the lead author of a paper demonstrating the new device published online May 20th in the journal Nature Photonics. He is a doctoral candidate in materials science and engineering at Stanford University working in Professor Mark Brongersma’s group. Brongersma is senior author of the study.

Cloak of invisiblity

An image showing light scattering from a silicon nanowire running diagonally from bottom left to top right. The brighter areas are bare silicon while the dimmer sections are coated with gold demonstrating how plasmonic cloaking reduces light scattering in the gold-coated sections. Photo: Stanford Nanocharacterization Lab.

Light detection is well known and relatively simple. Silicon generates electrical current when illuminated and is common in solar panels and light sensors today. The Stanford device, however, is a departure in that for the first time it uses a relatively new concept known as plasmonic cloaking to render the device invisible. The field of plasmonics studies how light interacts with metal nanostructures and induces tiny oscillating electrical currents along the surfaces of the metal and the semiconductor. These currents, in turn, produce scattered light waves. By carefully designing their device – by tuning the geometries – the engineers have created a plasmonic cloak in which the scattered light from the metal and semiconductor cancel each other perfectly through a phenomenon known as destructive interference.

The rippling light waves in the metal and semiconductor create a separation of positive and negative charges in the materials – a dipole moment, in technical terms. The key is to create a dipole in the gold that is equal in strength but opposite in sign to the dipole in the silicon. When equally strong positive and negative dipoles meet, they cancel each other and the system becomes invisible.

“We found that a carefully engineered gold shell dramatically alters the optical response of the silicon nanowire,” said Fan. “Light absorption in the wire drops slightly – by a factor of just four – but the scattering of light drops by 100 times due to the cloaking effect, becoming invisible.”

“It seems counterintuitive,” said Brongersma, “but you can cover a semiconductor with metal – even one as reflective as gold – and still have the light get through to the silicon. As we show, the metal not only allows the light to reach the silicon where we can detect the current generated, but it makes the wire invisible, too.”

Broadly effective

The engineers have shown that plasmonic cloaking is effective across much of the visible spectrum of light and that the effect works regardless of the angle of incoming light or the shape and placement of the metal-covered nanowires in the device. They likewise demonstrate that other metals commonly used in computer chips, like aluminum and copper, work just as well as gold.

To produce invisibility, what matters above all is the tuning of metal and semiconductor.

“If the dipoles do not align properly, the cloaking effect is lessened, or even lost,” said Fan. “Having the right amount of materials at the nanoscale, therefore, is key to producing the greatest degree of cloaking.”

In the future, the engineers foresee application for such tunable, metal-semiconductor devices in many relevant areas, including solar cells, sensors, solid-state lighting, chip-scale lasers, and more.

In digital cameras and advanced imaging systems, for instance, plasmonically cloaked pixels might reduce the disruptive cross-talk between neighboring pixels that produces blur. It could therefore lead to sharper, more accurate photos and medical images.

“We can even imagine reengineering existing opto-electronic devices to incorporate valuable new functions and to achieve sensor densities not possible today,” concluded Brongersma. “There are many emerging opportunities for these photonic building blocks.”

Brongersma lab alumnus Professor Linyou Cao and doctoral candidate Farzaneh Afshinmanesh contributed to this research. This work is a collaboration with Professor Nader Engheta and post-doctoral researcher Uday Chettiar from University of Pennsylvania.

Andrew Myers is associate director of communications for the Stanford University School of Engineering.

Last modified Sun, 20 May, 2012 at 20:09 Media Contacts Andrew Myers Associate Director of Communications 650.736.2245 admyers@stanford.edu Jamie Beckett Director of Communications and Alumni Relations 650.736.2241 jbeckett@stanford.edu Stanford University. School of Engineering, 475 Via Ortega, Stanford, California 94305-4121. 650.725.1575

Producing graphene quantum dots of controlled shape and size at large densities

Kansas State University researchers have come closer to solving an old challenge of producing graphene quantum dots of controlled shape and size at large densities, which could revolutionize electronics and optoelectronics.

Vikas Berry, William H. Honstead professor of chemical engineering, has developed a novel process that uses a diamond knife to cleave graphite into graphite nanoblocks, which are precursors for graphene quantum dots. These nanoblocks are then exfoliated to produce ultrasmall sheets of carbon atoms of controlled shape and size.

By controlling the size and shape, the researchers can control graphene’s properties over a wide range for varied applications, such as solar cells, electronics, optical dyes, biomarkers, composites and particulate systems. Their work has been published in Nature Communications and supports the university's vision to become a top 50 public research university by 2025. The article is available online.

"The process produces large quantities of graphene quantum dots of controlled shape and size and we have conducted studies on their structural and electrical properties," Berry said.

Graphene is a single atom thick sheet of sp2 hybridized carbon atoms arranged in a honeycomb lattice. With its dense cloud of charge carriers confined in atomic thickness and its large chemically modifiable surface area, graphene is a promising material for electronic sensing systems, electro-switches, biotechnology, and defense applications.

While other researchers have been able to make quantum dots, Berry's research team can make quantum dots with a controlled structure in large quantities, which may allow these optically active quantum dots to be used in solar cell and other optoelectronic applications. "There will be a wide range of applications of these quantum dots," Berry said. "We expect that the field of graphene quantum dots will evolve as a result of this work since this new material has a great potential in several nanotechnologies." It has been know that because of the edge states and quantum confinement, the shape and size of graphene quantum dots dictate their electrical, optical, magnetic and chemical properties. This work also shows proof of the opening of a band-gap in graphene nanoribbon films with a reduction in width. Further, Berry’s team shows through high-resolution transmission electron micrographs and simulations that the edges of the produces structures are straight and relatively smooth.

Other collaborators on this work include Zhiping Xu from Tsinghua University in China and David Moore from the University of Kansas. Xu conducted the molecular dynamics simulations. The co-authors from Kansas State University include Nihar Mohanty, 2011 doctoral graduate; T. S. Sreeprasad, postdoctoral fellow; Alfredo A. Rodriguez, 2012 graduate; and Ashvin Nagaraja, 2009 graduate.

The project was funded by the National Science Foundation and the office of naval research.

Berry earned his bachelor's degree in chemical engineering from the Indian Institute of Technology in Delhi, India, in 1999. He received his master's degree in chemical and petroleum engineering from the University of Kansas in 2003, followed by his doctorate in chemical engineering from Virginia Polytechnic Institute and State University in 2006.

Contact: Vikas Berry vberry@k-state.edu 785-532-5519 Kansas State University

New Plasmonic Component for Near-Infrared Metamaterials

WEST LAFAYETTE, Ind. - Researchers have taken a step toward overcoming a key obstacle in commercializing "hyperbolic metamaterials," structures that could bring optical advances including ultrapowerful microscopes, computers and solar cells.

The researchers have shown how to create the metamaterials without the traditional silver or gold previously required, said Alexandra Boltasseva, a Purdue University assistant professor of electrical and computer engineering.

Using the metals is impractical for industry because of high cost and incompatibility with semiconductor manufacturing processes. The metals also do not transmit light efficiently, causing much of it to be lost. The Purdue researchers replaced the metals with an "aluminum-doped zinc oxide," or AZO.

"This means we can have a completely new material platform for creating optical metamaterials, which offers important advantages," Boltasseva said.

Alexandra Boltasseva

Doctoral student Gururaj V. Naik provided major contributions to the research, working with a team to develop a new metamaterial consisting of 16 layers alternating between AZO and zinc oxide. Light passing from the zinc oxide to the AZO layers encounters an "extreme anisotropy," causing its dispersion to become "hyperbolic," which dramatically changes the light's behavior. "The doped oxide brings not only enhanced performance but also is compatible with semiconductors," Boltasseva said. Research findings are detailed in a paper appearing Monday (May 14) in the Proceedings of the National Academy of Sciences.

The list of possible applications for metamaterials includes a "planar hyperlens" that could make optical microscopes 10 times more powerful and able to see objects as small as DNA; advanced sensors; more efficient solar collectors; quantum computing; and cloaking devices.

The AZO also makes it possible to "tune" the optical properties of metamaterials, an advance that could hasten their commercialization, Boltasseva said.

"It's possible to adjust the optical properties in two ways," she said. "You can vary the concentration of aluminum in the AZO during its formulation. You can also alter the optical properties in AZO by applying an electrical field to the fabricated metamaterial."

This switching ability might usher in a new class of metamaterials that could be turned hyperbolic and non-hyperbolic at the flip of a switch.

"This could actually lead to a whole new family of devices that can be tuned or switched," Boltasseva said. "AZO can go from dielectric to metallic. So at one specific wavelength, at one applied voltage, it can be metal and at another voltage it can be dielectric. This would lead to tremendous changes in functionality."

The researchers "doped" zinc oxide with aluminum, meaning the zinc oxide is impregnated with aluminum atoms to alter the material's optical properties. Doping the zinc oxide causes it to behave like a metal at certain wavelengths and like a dielectric at other wavelengths.

The material has been shown to work in the near-infrared range of the spectrum, which is essential for optical communications, and could allow researchers to harness "optical black holes" to create a new generation of light-harvesting devices for solar energy applications.

The PNAS paper was authored by Naik, Boltasseva, doctoral student Jingjing Liu, senior research scientist Alexander V. Kildishev, and Vladimir M. Shalaev, scientific director of nanophotonics at Purdue's Birck Nanotechnology Center, a distinguished professor of electrical and computer engineering and a scientific adviser for the Russian Quantum Center.

Current optical technologies are limited because, for the efficient control of light, components cannot be smaller than the size of the wavelengths of light. Metamaterials are able to guide and control light on all scales, including the scale of nanometers, or billionths of a meter.

Unlike natural materials, metamaterials are able to reduce the "index of refraction" to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. It causes the bent-stick-in-water effect, which occurs when a stick placed in a glass of water appears bent when viewed from the outside. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material

Natural materials typically have refractive indices greater than one. Metamaterials, however, can make the index of refraction vary from zero to one, which possibly will enable applications including the hyperlens.

The layered metamaterial is a so-called plasmonic structure because it conducts clouds of electrons called "plasmons."

"Alternative plasmonic materials such as AZO overcome the bottleneck created by conventional metals in the design of optical metamaterials and enable more efficient devices," Boltasseva said. "We anticipate that the development of these new plasmonic materials and nanostructured material composites will lead to tremendous progress in the technology of optical metamaterials, enabling the full-scale development of this technology and uncovering many new physical phenomena."

This work has been funded in part by the U.S. Office of Naval Research, National Science Foundation and Air Force Office of Scientific Research

Writer: Emil Venere, 765-494-4709, venere@purdue.edu Source: Alexandra Boltasseva, 765-494-0301, aeb@purdue.edu