REDUCING WASTEFUL CHARGE-CARRIER INTERACTIONS THAT COMPETE WITH LIGHT PRODUCTION

FROM:  LOS ALAMOS NATIONAL LABORATORY 
Nanoscale Engineering Boosts Performance of Quantum Dot Light Emitting Diodes

Making the light at the end of the tunnel more efficient

LOS ALAMOS, N.M., October 25, 2013—Dramatic advances in the field of quantum dot light emitting diodes (QD-LEDs) could come from recent work by the Nanotechnology and Advanced Spectroscopy team at Los Alamos National Laboratory.

Quantum dots are nano-sized semiconductor particles whose emission color can be tuned by simply changing their dimensions. They feature near-unity emission quantum yields and narrow emission bands, which result in excellent color purity. The new research aims to improve QD-LEDs by using a new generation of engineered quantum dots tailored specifically to have reduced wasteful charge-carrier interactions that compete with the production of light.

“QD-LEDs can potentially provide many advantages over standard lighting technologies, such as incandescent bulbs, especially in the areas of efficiency, operating lifetime and the color quality of the emitted light,” said Victor Klimov of Los Alamos.

Incandescent bulbs, known for converting only 10 percent of electrical energy into light and losing 90 percent of it to heat, are rapidly being replaced worldwide by less wasteful fluorescent light sources. However, the most efficient approach to lighting is direct conversion of electricity into light using electroluminescent devices such as LEDs.

Due to spectrally narrow, tunable emission, and ease of processing, colloidal QDs are attractive materials for LED technologies. In the last decade, vigorous research in QD-LEDs has led to dramatic improvements in their performance, to the point where it nearly meets the requirements for commercial products. One outstanding challenge in the field is the so-called efficiency roll-off (known also as “droop”), that is, the drop in efficiency at high currents.

“This ‘droop’ problem complicates achieving practical levels of brightness required especially for lighting applications,” said Wan Ki Bae, a postdoctoral researcher on the nanotech team.

By conducting spectroscopic studies on operational QD-LEDs, the Los Alamos researchers have established that the main factor responsible for the reduction in efficiency is an effect called Auger recombination. In this process, instead of being emitted as a photon, the energy from recombination of an excited electron and hole is transferred to the excess charge and subsequently dissipated as heat.

A paper, “Controlling the influence of Auger recombination on the performance of quantum-dot light-emitting diodes” is being published Oct. 25 in Nature Communications. In addition, an overview article on the field of quantum-dot light-emitting diodes and specifically the role of Auger effects appeared in the September Materials Research Society Bulletin, Volume 38, Issue 09, also authored by researchers of the Los Alamos nanotech team.

Not only has this work identified the mechanism for efficiency losses in QD-LEDs, Klimov said, but it has also demonstrated two different nano-engineering strategies for circumventing the problem in QD-LEDs based on bright quantum dots made of cadmium selenide cores overcoated with cadmium sulfide shells.

The first approach is to reduce the efficiency of Auger recombination itself, which can be done by incorporating a thin layer of cadmium selenide sulfide alloy at the core/shell interface of each quantum dot.

The other approach attacks the problem of charge imbalance by better controlling the flow of extra electrons into the dots themselves. This can be accomplished by coating each dot in a thin layer of zinc cadmium sulfide, which selectively impedes electron injection. According to Jeffrey Pietryga, a chemist in the nanotech team, “This fine tuning of electron and hole injection currents helps maintain the dots in a charge-neutral state and thus prevents activation of Auger recombination.”

NATURE AND INNOVATIVE MATERIALS

FROM:  NATIONAL SCIENCE FOUNDATION 
Inspired by nature: textured materials to aid industry and military
Innovation Corps team developed metals and plastic that repel water, capture sunlight and prevent ice build-up

The lotus leaf has a unique microscopic texture and wax-like coating that enables it to easily repel water. Taking his inspiration from nature, a University of Virginia professor has figured out a way to make metals and plastics that can do virtually the same thing.

Mool Gupta, Langley Distinguished Professor in the university's department of electrical and computer engineering, and director of the National Science Foundation's (NSF) Industry/University Cooperative Research Center for Lasers and Plasmas, has developed a method using high-powered lasers and nanotechnology to create a similar texture that repels water, captures sunlight and prevents the buildup of ice.

These textured materials can be used over large areas and potentially could have important applications in products where ice poses a danger, for example, in aviation, the automobile industry, the military, in protecting communication towers, blades that generate wind energy, bridges, roofs, ships, satellite dishes, and even snowboards.

In commercial and military aviation, for example, these materials could improve airline safety by making current de-icing procedures, which include scraping and applying chemicals, such as glycol, to the wings, unnecessary.

For residents in the frigid northeast, many of whom rely on satellite systems, "it could mean they won't lose their signal, and they won't have to go outside with a hammer and chisel and break off the ice," Gupta says.

The materials' ability to trap sunlight also could enhance the performance of solar cells.

Gupta and his research team first made a piece of textured metal that serves as a mold to mass-produce many pieces of plastic with the same micro-texture. The replication process is similar to the one used in manufacturing compact discs. The difference, of course, is that the CD master mold contains specific information, like a voice, whereas, "in our case we are not writing any information, we are creating a micro-texture," Gupta says.

"You create one piece of metal that has the texture," Gupta adds. "For multiple pieces of plastic with the texture, you use the one master made of metal to stamp out multiple pieces. Thus, whatever features are in your master are replicated in the special plastic. Once we create that texture, if you put a drop of water on the texture, the water rolls down and doesn't stick to it, just like a lotus leaf. We have created a human-made structure that repels water, just like the lotus leaf."

The process of making the metal with the special texture works like this: the scientists take high-powered lasers, with energy beams 20 million times higher than that of a laser pointer, for example, and focus the beams on a metal surface. The metal absorbs the laser light and heats to a melting temperature of about 1200 degrees Centigrade, or higher, a process that rearranges the surface material to form a microtexture.

"All of this happens in less than 0.1 millionth of a second," Gupta says. "The microtexture is self-organized. By scanning the focused laser beam, we achieve a large area of microtexture. The produced microtexture is used as a stamper to replicate microtexture in polymers. The stamper can be used many, many times, allowing a low cost manufacturing process. The generated microtextured polymer surface shows very high water repellency."

In the fall of 2011, Gupta was among the first group of scientists to receive a $50,000 NSF Innovation Corps (I-Corps) award, which supports a set of activities and programs that prepare scientists and engineers to extend their focus beyond the laboratory into the commercial world.

Such results may be translated through I-Corps into technologies with near-term benefits for the economy and society. It is a public-private partnership program that teaches grantees to identify valuable product opportunities that can emerge from academic research, and offers entrepreneurship training to faculty and student participants.

The other project members are Paul Caffrey, a doctoral candidate under Gupta's supervision, and Martin Skelly of Charleston, S.C., a veteran of banking in the former Soviet Union who serves as business mentor and is involved in new business investments.

The team participated in a three-day entrepreneurship workshop at Stanford University run by entrepreneurs from Silicon Valley. "We are still pursuing the commercial potential," Gupta says. "The idea is to look at what market can use this technology, how big the market is, and how long it will take to get into it."

-- Marlene Cimons, National Science Foundation