Published on February 13th, 2018 | By: April Gocha0
Materials science advances could light up new LED technologiesPublished on February 13th, 2018 | By: April Gocha
[Image above] Credit: Vladimir Agafonkin; Flickr CC BY-NC 2.0
There’s a reason why LEDs reign in the lighting world today.
Residential LEDs (light emitting diodes) use at least 75% less energy and last 25 times longer than incandescent bulbs, according to the U.S. Department of Energy.
Those are such drastic savings that the DOE says that widespread use of LED lighting by itself has the biggest potential for energy savings in America. “By 2027, widespread use of LEDs could save about 348 TWh (compared to no LED use) of electricity: This is the equivalent annual electrical output of 44 large electric power plants (1,000 megawatts each), and a total savings of more than $30 billion at today’s electricity prices.”
But what makes LEDs more efficient than incandescent or even CFL bulbs?
The biggest reason why LEDs use so much less energy is that they don’t emit much heat. So compared to incandescent bulbs, which lose about 90% of their energy as heat, and CFLs, which lose about 80%, LEDs are simply less wasteful with their energy.
LEDs’ energy efficiency alone doesn’t make them perfect, however—there are still R&D challenges to address before LEDs completely become the light of our lives.
So building on abundant research over the past several years to improve LED materials, new research continues to push the technology further forward.
Increasing indium content in indium gallium nitride
For instance, researchers at the Leibniz-Institut für Kristallzüchtung (Berlin, Germany) recently figured out why the indium content in blue LED material indium gallium nitride (InGaN) is limited to just 30%. Scientists have wanted to increase the indium content because that would shift LED light emission towards green and red portions of the spectrum—and manufacturers use a combination of blue, red, and green LEDs to create sought-after white LED light.
No matter how the Leibniz scientists tried to increase the amount of indium by growing single atomic layers of indium nitride on gallium nitride, however, their efforts failed—they couldn’t exceed 25%–30% indium content in the material.
So they took a closer look at what was going on. Atomic resolution transmission electron microscopy and in-situ reflection high-energy electron diffraction revealed that InGaN actually undergoes an atomic rearrangement at 25% indium content.
Instead of a predicted structure that bonds indium atoms with three neighboring atoms, InGaN instead favors a structure that bonds indium atoms with four nearby atoms. While this increased number of bonds creates a stronger structure, it also physically limits the amount of indium that can incorporate into the LED material.
Top view of the surface reconstruction of InGaN’s structure. Credit: IKZ
“Apparently, a technological bottleneck hampers all the attempts to shift the emission from the green towards the yellow and the red regions of the spectra. Therefore, new original pathways are urgently required to overcome these fundamental limitations,” Tobias Schulz, scientist at the Leibniz-Institut für Kristallzüchtung, says in a news release. “For example, growth of InGaN films on high quality InGaN pseudo-substrates that would reduce the strain in the growing layer.”
Although the team’s research indicates that there’s no solution to indium’s incorporation limit, it does provide potential for overcoming InGaN’s limited charge carrier localization due to variations of the compound’s chemical composition, according to the release.
The research, published in Physical Review Materials, is “Elastically frustrated rehybridization: Origin of chemical order and compositional limits in InGaN quantum wells” (DOI: 10.1103/PhysRevMaterials.2.011601).
Halide perovskite nanoantennas enhance light emission
Researchers from ITMO University (Saint Petersburg, Russia) are focusing on a different material to push forward the potential of LED lights—halide perovskites.
The researchers recently demonstrated that subwavelength nanoparticles of the materials can act as light emitters and nanoantennas to enhance light emission—halide pervoskite nanoparticles generate, enhance, and route emission through excited resonant modes coupled with excitons, according to an ITMO news release.
Excitons are bound pairs of electrons and holes that emit light when they recombine.
“The unique features of perovskite enabled us to make nanoantennas from this material,” first author Ekaterina Tiguntseva says in the release. “We basically synthesized perovskite films, and then transferred material particles from the film surface to another substrate by means of pulsed laser ablation technique. Compared to alternatives, our method is relatively simple and cost-effective.”
ITMO scientists celebrate their findings with LED light-colored cupcakes. Credit: ITMO
In addition, the emitted light color is easily tunable by simply varying anions in the material. “The structure of the material remains the same, we simply use another component in the synthesis of perovskite films,” Tiguntseva explains in the release. “Therefore, it is not necessary to adjust the method each time. It remains the same, yet the emission color of our nanoparticles changes.”
That research, published in Nano Letters, is “Light-emitting halide perovskite nanoantennas” (DOI: 10.1021/acs.nanolett.7b04727).
Halide perovskites push LED efficiency
Another team of scientists at the U.S. Naval Research Laboratory (Washington, D.C.) is also focusing on the power of halide perovskites to develop more efficient LEDs.
The scientists work shows that cesium lead halide perovskite nanocrystals (CsPbX3) emit light much faster than conventional lighting materials, suggesting that the materials could enable efficiency upgrades in solid-state lasers and LEDs.
“An optically active bright exciton in this material emits light much faster than in conventional light emitting materials and enables larger power, lower energy use, and faster switching for communication and sensors,” NRL research physicist Alexander Efros says in an NRL news release.
NRL researchers (clockwise from top left) Alex Efros, Noam Bernstein, John Lyons, and John Michopoulos. Credit: U.S. Naval Research Laboratory; James Marshall
By generating nanocrystals from lead halide perovskites containing either chlorine, bromine, or iodine, the researchers found that the materials rapidly emit light through the range of visible wavelengths.
“The increased rate of light emission of these materials holds great promise for various technological applications that rely on LEDs and lasers,” Efros adds in the release. “In principle, the 20 times shorter lifetime could therefore lead to 20 times more intense LEDs and lasers.”
The research, published in Nature, is “Bright triplet excitons in caesium lead halide perovskites” (DOI: 10.1038/nature25147).
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