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Driving LED efficiency using indium nitride nanostructures

09 Apr 2014  | Paul Buckley

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The U.S. Department of Energy's National Energy Research Scientific Computing Center (NERSC) has conducted simulations showing that nanostructures half the breadth of a DNA strand could enhance the efficiency of LEDs. According to them, the efficiency improvements were notable in the 'green gap', a portion of the spectrum where efficiency in traditional LED is known to fall.

Using NERSCs Cray XC30 supercomputer 'Edison', University of Michigan researchers Dylan Bayerl and Emmanouil Kioupakis found that the semiconductor indium nitride (InN), which typically emits infrared light, will emit green light if reduced to 1nm-wide wires. By varying their sizes, the nanostructures could be tailored to emit different colours of light, which could lead to more natural-looking white lighting while avoiding some of the efficiency loss current LEDs experience at high power.

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

Presently, LEDs are created as multilayered microchips. The outer layers are doped with elements that create an abundance of electrons on one layer and too few on the other. The missing electrons are called holes. When the chip is energized, the electrons and holes are pushed together, confined to the intermediate quantum-well layer where they are attracted to combine, shedding their excess energy (ideally) by emitting a photon of light.

At low power, nitride-based LEDs (most commonly used in white lighting) are efficient, converting most of their energy into light. But turn the power up to levels that could light up a room and efficiency plummets, meaning a smaller fraction of electricity gets converted to light. This effect is especially pronounced in green LEDs, giving rise to the term 'green gap'. Nanomaterials offer the tantalizing prospect of LEDs that can be 'grown' in arrays of nanowires, dots or crystals. The resulting LEDs could not only be thin, flexible and high-resolution, but efficient, as well.

If you reduce the dimensions of a material to be about as wide as the atoms that make it up, then you get quantum confinement. The electrons are squeezed into a small region of space, increasing the bandgap energy, Kioupakis said. That means the photons emitted when electrons and holes combine are more energetic, producing shorter wavelengths of light.

The energy difference between LED electrons and holes, called the bandgap, determines the wavelength of the emitted light. The wider the bandgap, the shorter the wavelength of light. The bandgap for bulk InN is quite narrow, only 0.6 electron volts (eV), so it produces infrared light. In Bayerl and Kioupakis simulated InN nanostructures, the calculated bandgap increased, leading to the prediction that green light would be produced with an energy of 2.3eV.

If we can get green light by squeezing the electrons in this wire down to 1nm, then we can get other colours by tailoring the width of the wire, said Kioupakis. A wider wire should yield yellow, orange or red. A narrower wire, indigo or violet.

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