LED efficiency puzzle solved by UC Santa Barbara theorists

(Santa Barbara, Calif., April 19, 2011) -- Researchers at the University of California, Santa Barbara, say they've figured out the cause of a problem that's made light-emitting diodes (LEDs) impractical for general lighting purposes. Their work will help engineers develop a new generation of high-performance, energy-efficient lighting that could replace incandescent and fluorescent bulbs.

"Identifying the root cause of the problem is an indispensable first step toward devising solutions," says Chris Van de Walle, a professor in the Materials Department at UC Santa Barbara who heads the research group that carried out the work.

Van de Walle and his colleagues are working to improve the performance of nitride-based LEDs, which are efficient, non-toxic and long-lasting light sources. They investigated a phenomenon referred to as "droop"?the drop in efficiency that occurs in these LEDs when they're operating at the high powers required to illuminate a room. The cause of this decline has been the subject of considerable debate, but the UC Santa Barbara researchers say they've figured out the mechanism responsible for the effect by performing quantum-mechanical calculations.

LED droop, they conclude, can be attributed to Auger recombination, a process that occurs in semiconductors, in which three charge-carriers interact without giving off light. The researchers also discovered that indirect Auger effects, which involve a scattering mechanism, are significant?a finding that accounts for the discrepancy between the observed degree of droop and that predicted by other theoretical studies, which only accounted for direct Auger processes.

In nitride LEDs, "These indirect processes form the dominant contribution to the Auger recombination rate," says Emmanouil Kioupakis, a postdoctoral researcher at UC Santa Barbara and lead author of a paper published online April 19 in Applied Physics Letters. The other authors are Van de Walle, Patrick Rinke, now with the Fritz Haber Institute in Germany, and Kris Delaney, a project scientist at UC Santa Barbara.
LED droop can't be eliminated because Auger effects are intrinsic, but it could be minimized, the researchers say, by using thicker quantum wells in LEDs or growing devices along non-polar or semi-polar growth directions in order to keep carrier density low.

"With Auger recombination now established as the culprit, we can focus on creative approaches to suppress or circumvent this loss mechanism," Van de Walle says.

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The work was supported by the Center for Energy Efficient Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, and by UC Santa Barbara's Solid State Lighting and Energy Center.
Computational resources were provided by the U.S. Department of Energy's National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory, the California NanoSystems Institute's Computing Facility at UC Santa Barbara, and the National Science Foundation-funded TeraGrid.

From EurekAlert!

Effect of cloud-scattered sunlight on earth's energy balance depends on wavelength of light

RICHLAND, Wash. -- Atmospheric scientists trying to pin down how clouds curb the amount of sunlight available to warm the earth have found that it depends on the wavelength of sunlight being measured. This unexpected result will help researchers improve how they portray clouds in climate models.
Additionally, the researchers found that sunlight scattered by clouds — the reason why beachgoers can get sunburned on overcast days — is an important component of cloud contributions to the earth's energy balance. Capturing such contributions will increase the accuracy of climate models, the team from the Department of Energy's Pacific Northwest National Laboratory reported in Geophysical Research Letters earlier this month.

Click image to enlarge

These cumulus clouds above Oklahoma both shade the earth and make shadows brighter. The larger ones have a distinct cauliflower shape, providing even more opportunities for light to bounce off of them.

"The amount of the sun's energy that reaches the earth's surface is the main driver of the earth's temperature. Clouds are one of the least understood aspects of climate change. They can block the sun, but light can also bounce off one cloud into another cloud's shadow and increase the solar energy hitting earth," said PNNL atmospheric scientist Evgueni Kassianov. 

White clouds
Clouds both cool down and warm up the earth's surface. They cool the earth by reflecting some sunlight up into outer space, and they warm it by bouncing some sunlight down to the surface. Overall, most clouds have a net cooling effect, but atmospheric scientists need to accurately measure when they cool and warm to produce better climate models that incorporate clouds faithfully.
But it's a hard number to get. Fair-weather clouds are big puffy white objects that bounce a lot of light around. They can make the sky around them look brighter when they're there, but they float about and reform constantly. Cloud droplets and aerosol particles in the sky — tiny bits of dirt and water in the air that cause haziness — scatter light in three dimensions, even into cloud shadows.
To determine the net cloud effect, researchers need two numbers. First they need to measure the total amount of sunlight in a cloudy sky. Then they need to determine how bright that sky would be without the clouds, imagining that same sky to be blue and cloudless, when aerosols are in charge of a sky's brightness. The difference between those numbers is the net cloud effect.

Rainbow energy
Researchers have traditionally estimated the net cloud effect by measuring a broad spectrum of sunlight that makes it to the earth's surface, from ultraviolet to infrared. But clouds are white — that's because the large water droplets within them scatter light of all colors almost equally in the visible spectrum, the part of the electromagnetic spectrum that includes the colors of the rainbow.
On the other hand, aerosols — both within clouds and in the open sky — bounce different-colored light unequally. Broadband measurements that fail to distinguish color differences might be covering up important details, the researchers thought.
Instead of taking one broadband measurement that covers everything from ultraviolet to infrared, Kassianov and crew wanted to determine how individual wavelengths contribute to the net cloud effect. To do so, the team used an instrument that can measure brightness at four different wavelengths of color — violet, green, orange, red — and two of infrared.
In addition, this instrument, a spectral radiometer at DOE's Atmospheric Radiation Measurement Climate Research Facility located on the southern Great Plains in Oklahoma, allowed the team to calculate what the brightness would be if the day sported a cloudless, blue sky. The spectral measurements taken by the radiometer can be converted into the amount and properties of aerosols. Then aerosol properties can be used to calculate clear blue sky brightness.

Clouds Gone Wild
Comparing measured values for cloudy sky to the calculated values for clear sky, the researchers found that, on average, puffy fair-weather clouds cool down the earth's surface by several percent on a summer day. Although clouds cool overall, two components that the researchers looked at — from direct and scattered sunlight — had opposite effects.
The direct component accounts for the shade provided by clouds and cools the earth. The second component accounts for the sunlight scattered between and under clouds, which makes the sky brighter, warming the earth.
"The sunlight scattered by clouds can heat the surface," said Kassianov. "We all know that we can still get sunburned on cloudy days. This explains why."
In the Oklahoma summer, the scattered-light effect measured by the researchers could be quite large. For example, if a cloud passed over the instrument, the measured cloudy sky brightness exceeded calculated clear sky value by up to 30 percent. Kassianov attributes that large difference to scattered sunlight being "caught on tape" by the radiometer.
"Sunlight scattered by three-dimensional, irregular clouds is responsible for the observed large difference. The one-dimensional cloud simulations currently used in large-scale climate models don't capture this diffuse light," said Kassianov.

Aerosols' Day in the Sky
The team also found that the effect changed depending on the measured visible-spectrum wavelength, and whether the light was direct or scattered.
With direct light, the cooling caused by clouds was weakest on the violet end of the spectrum and strongest at infrared. With scattered light, warming caused by clouds was also weakest at violet and the strongest at infrared. Overall, the least cooling and warming occurred at violet, and the most cooling and warming occurred at infrared.
Because large droplets in clouds scatter sunlight almost uniformly across the spectrum, the clouds themselves can't be the reason why different wavelengths contribute differently to the net cloud effect. Compared to cloud droplets, aerosols are more than 100 times smaller and scatter wavelengths differently. These results suggest that aerosols — which not only cause haziness but contribute to cloud formation as well — are responsible for the wavelength differences, something researchers need to be aware of as they study clouds in the sky.
"If you want to study how aerosols and clouds interact," said Kassianov, "you need to look in the region of the spectrum where aerosol effects are significant. If you want to fish, you go where the fish are biting."

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Reference: Kassianov E., Barnard J., Berg L.K., Long C.N., and C. Flynn, Shortwave Spectral Radiative Forcing of Cumulus Clouds from Surface Observations, Geophys Res Lett, April 2, 2011, DOI 10.1029/2010GL046282 (http://www.agu.org/pubs/crossref/2011/2010GL046282.shtml).
This work was supported by the U.S. Department of Energy Office of Science.

From EurekAlert!

Mathematical Model Simulating Rat Whiskers Provides Insight Into Sense of Touch

ScienceDaily (Apr. 7, 2011) — Researchers at Northwestern University have developed a mathematical model that will allow them to simulate how rats use their whiskers to sense objects around them. The model enables further research that may provide insight into the human sense of touch.

Hundreds of papers are published each year that use the rat whisker system as a model to understand brain development and neural processing. Rats move their whiskers rhythmically against objects to explore the environment by touch. Using only tactile information from its whiskers, a rat can determine all of an object's spatial properties, including size, shape, orientation and texture.
But there is a big missing piece that prevents a full understanding of the neural signals recorded in these studies: no one knows how to represent the "touch" of a whisker in terms of mechanical variables.
"We don't understand touch nearly as well as other senses," says Mitra Hartmann, associate professor of biomedical engineering and mechanical engineering at the McCormick School of Engineering and Applied Science. "We know that visual and auditory stimuli can be quantified by the intensity and frequency of light and sound, but we don't fully understand the mechanics that generate our sense of touch."

Click on image to enlarge

To create a model that starts to quantify these mechanics, Hartmann's team first studied the structure of the rat whisker array -- the 30 whiskers arranged in a regular pattern on each side of a rat's face. By analyzing them in both two- and three-dimensional scans, they defined the relationship between the size and shape of each whisker and its placement on the face of the rat.
Using this information, the team created a model that quantifies the full shape and structure of the rat head and whisker array. The model now allows the team to simulate the rat "whisking" against different objects and to predict the full pattern of inputs into the whisker system as a rat encounters an object. The simulations can then be compared against real behavior.
The research is published online in the journal PLoS Computational Biology.
Understanding the mechanics of the rat whisker system may provide a step toward understanding the human sense of touch.
"The big question our laboratory is interested in is how do animals, including humans, actively move their sensors through the environment and somehow turn that sensory data into a stable perception of the world," Hartmann says.
To determine how a rat can sense the shape of an object, Hartmann's team previously developed a light sheet to monitor the precise locations of the whiskers as they came in contact with the object. Using high-speed video, the team can also analyze how the rat moves its head to explore different shapes. These behavioral observations can then be paired with the output from the model.
These advances will provide insight into the sense of touch but may also enable new technologies that could make use of the whisker system. For example, Hartmann's lab created arrays of robotic whiskers that can, in several respects, mimic the capabilities of mammalian whiskers. The researchers demonstrated that these arrays can sense information about both object shape and fluid flow.
"We show that the bending moment, or torque, at the whisker base can be used to generate three-dimensional spatial representations of the environment," Hartmann says. "We used this principle to make arrays of robotic whiskers that can replicate much of the basic mechanics of rat whiskers." The technology, she said, could be used to extract the three-dimensional features of almost any solid object.
Hartmann envisions that a better understanding of the whisker system may be useful for engineering applications in which the use of cameras is limited. But most importantly, a better understanding of the rat whisker system could translate into a better understanding of ourselves.
"Although whiskers and hands are very different, the basic neural pathways that process tactile information are in many respects similar across mammals," Hartmann says. "A better understanding of neural processing in the whisker system may provide insights into how our own brains process information."
In addition to Hartmann, other authors of the paper are Blythe Towal, Brian Quist and Joseph Solomon, all of Northwestern, and Venkatesh Gopal of Elmhurst College.

Source: Science Daily