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April 9, 2014

‘Edison’ Helps Reinvent the Light Bulb

Tiffany Trader
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Today’s consumers have an array of lighting options to select from, giving them the light they want while saving energy and money. In addition to the standard incandescent bulb that’s been around since Thomas Edison invented it about 130 years ago, there are energy-efficient halogen incandescents, compact florescent lamps (CFLs) and a new breed of softer, warmer light-emitting diodes (LEDs) to choose from.

The problem with traditional incandescents is that 90 percent of the electricity they draw generates heat instead of light. Considering the huge potential for energy savings – the equivalent of $10 billion per year in the US alone – it should come as no surprise that scientists and engineers are hard at work developing and perfecting smart lighting choices. Of these, LEDs have the most potential to offer a pleasing, natural light while using a lot less energy.

A type of solid-state lighting, LEDs are semiconductors that convert electricity into photons. Once used mainly for indicator and traffic lights, LEDs for general illumination applications are one of today’s most energy-efficient and rapidly-developing technologies. ENERGY STAR-qualified LEDs use only 20-25 percent of the energy of a common incandescent and last up to 25 times longer. Besides having a lower energy consumption, LEDs can also claim a longer lifetime, improved physical robustness, smaller size, and faster switching.

Even though LEDs are one of the most promising light technologies to come along in decades, there is still room for improvement. Most notably, scientists are addressing the so-called “green gap,” a portion of the spectrum where LED efficiency drops significantly. Increasing the efficiency of green LEDs is a high-priority research area for the US Department of Energy.

Simulations undertaken at the DOE’s National Energy Research Scientific Computing Center (NERSC) have shown that nanostructures half a DNA strand-wide could ameliorate this gap, delivering more energy-efficient LEDs.

At low power, nitride-based LEDs – the ones most commonly used in white general-use lighting – are very efficient, converting most of their energy into light. But when the power is increased to create sufficient light for mainstream environmental and task lighting needs, a much smaller fraction of electricity gets turned to light. The effect is especially prominent in green LEDs, which is how the term “green gap” came into being.

University of Michigan researchers Dylan Bayerl and Emmanouil Kioupakis are seeking to bridge this gap with the help of NERSC’s Cray XC30 supercomputer “Edison.” They discovered that the semiconductor indium nitride (InN), which typically emits infrared light, will emit green light when 1 nanometer-wide wires are employed. They further determined that by tailoring the width of the wire, these nanostructures emit different colors of light – with a wider wire generating yellow, orange or red and a narrower wire making indigo or violet. It is thought that by mixing these colors, LED engineers can create natural-looking lighting that does not suffer from a steep efficiency drop as power is increased.

This direct method of making LEDs is not yet practical because green LEDs are not as efficient as blue and red versions. Today, most general use LED lights are made from blue LED light that has been passed through a phosphor. The process, akin to that used in conventional florescent tubes, does not fully exploit LED’s energy efficecy potential. As the related article at NERSC explains, “direct LED lights would not only be more efficient, but the color of light they produce could be dynamically tuned to suit the time of day or the task at hand.”

“Our work suggests that indium nitride at the few-nanometer size range offers a promising approach to engineering efficient, visible light emission at tailored wavelengths,” said Kioupakis.

A paper based on this study, called “Visible-Wavelength Polarized Light Emission with Small-Diameter InN Nanowires,” was published online in February. The work will also be featured on the cover of the July issue of Nano Letters.