April 4, 2014
Building A High Power LED That Remains Efficient: Study
Brett Smith for redOrbit.com - Your Universe Online
At low power, commonly used LEDs are very efficient. However, when these same LEDs receive enough power to light a room, they lose a significant amount of efficiency, a term referred to as the “green gap” because the efficiency loss is most pronounced with green LEDs.
According to a new online report published in Nano Letters, two engineers from the University of Michigan have modeled nanostructures with less width than a DNA strand capable of improving the efficiency of LEDs.
Using computer models, the Michigan duo discovered that the semiconductor indium nitride (InN), which generally gives off infrared light, will emit green light if decreased to 1 nanometer-wide wires. Furthermore, by simply varying their sizes, these nanostructures might be customized to give off distinct colors of light, which may lead to more natural-looking white lighting while steering clear of some of the performance loss today's LEDs encounter at high power.
"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 study author Emmanouil Kioupakis, a physics professor at the university.
LEDs, short for light emitting diodes, are semiconductor devices that produce light when an electrical current is applied to them. Modern LEDs are made as multilayered microchips. The outer layers have components that create a large amount of electrons on a single layer and an insufficient amount on the other, with the lacking electrons called holes. If the chip is energized, the electrons and holes are forced into each other causing them to combine and shed their excess energy by emitting a photon of light.
The researchers said their novel nanostructures could be "grown" in arrays of nanowires, dots or crystals, resulting in thin, flexible and high-resolution, and very efficient, LEDs.
The energy contrast between an LED's electrons and holes, known as the bandgap, decides the wavelength of the released light – with the broader the bandgap, the smaller the wavelength of light. The bandgap for common InN is fairly narrow, only 0.6 electron volts (eV), resulting in infrared light. With the new nanostructures, the modeled bandgap was increased, resulting in the prediction that green light would be generated with an energy generation of 2.3eV.
"If we can get green light by squeezing the electrons in this wire down to a nanometer, then we can get other colors by tailoring the width of the wire," Kioupakis explained.
The new structures also use pure InN, instead of layers of alloy nitride materials, eliminating one factor that adds to the inefficiency of green LEDs: miniscule composition fluctuations in the alloys. The purer materials also eliminate the "lattice mismatch" problem of layered devices.
"When the two materials don't have the same spacing between their atoms and you grow one over the other, it strains the structure, which moves the holes and electrons further apart, making them less likely to recombine and emit light," Kioupakis said. "In a nanowire made of a single material, you don't have this mismatch and so you can get better efficiency.”
The researchers noted that such small nanowires are currently difficult to synthesize. However, they said their study results could be generalized to other types of nanostructures which have already been successfully created.