New Materials Improve Color Quality of LEDs

Under UV light, the phosphor emits either green-yellow or blue light depending on the chemical activator mixed in. (Source: D. Baillot, UCSD)

A team led by engineers at the Univer­sity of Cali­fornia San Diego has used data mining and compu­tational tools to discover a new phosphor material for white LEDs that is inex­pensive and easy to make. Researchers built proto­type white LED light bulbs using the new phosphor. The proto­types exhi­bited better color quality than many commer­cial LEDs cur­rently on the market.

Phosphors, which are substances that emit light, are one of the key ingre­dients to make white LEDs. They are crystal­line powders that absorb energy from blue or near-UV light and emit light in the visible spectrum. The combi­nation of the different colored light creates white light. The phosphors used in many commer­cial white LEDs have several disad­vantages, however. Many are made of rare-earth elements, which are expensive, and some are difficult to manu­facture. They also produce LEDs with poor color quality. Researchers at UC San Diego and Chonnam National Univer­sity in Korea disco­vered and developed a new phosphor that avoids these issues. It is made mostly of earth-abundant elements (strontium, lithium, aluminum and oxygen); it can be made using indus­trial methods; and it produces LEDs that render colors more vividly and accu­rately.

The new phosphor, SLAO, was disco­vered using a systematic, high-through­put compu­tational approach developed in the lab of Shyue Ping Ong, a nanoengi­neering professor at the UC San Diego Jacobs School of Engi­neering. Ong’s team used super­computers to predict SLAO, which is the first known material made of the elements strontium, lithium, aluminum and oxygen. Calcu­lations also predicted this material would be stable and perform well as an LED phosphor. For example, it was predicted to absorb light in the near-UV and blue region and have high photo­luminescence, which is the material’s ability to emit light when excited by a higher energy light source.

Researchers in the lab of Joanna McKittrick, a materials science professor at the Jacobs School of Engi­neering, then figured out the recipe needed to make the new phosphor. They also confirmed the phosphor’s predicted light absorption and emission proper­ties in the lab. A team led by materials science pro­fessor Won Bin Im at Chonnam National Uni­versity in Korea optimized the phosphor recipe for industrial manu­facturing and built white LED proto­types with the new phosphor. They evaluated the LEDs using the Color Rendering Index (CRI), a scale that rates from 0 to 100 how accurate colors appear under a light source. Many commercial LEDs have CRI values at around 80. LEDs made with the new phosphor yielded CRI values greater than 90.

Thanks to the compu­tational approach developed by Ong’s team, disco­very of the phosphor took just three months – a short time frame compared to the years of trial-and-error experi­ments it typically takes to discover a new material. “Calcu­lations are quick, scalable and cheap. Using computers, we can rapidly screen thousands of materials and predict candidates for new materials that have not yet been dis­covered,” Ong said. He uses a combi­nation of high-throughput calcu­lations and machine learning to discover next-gene­ration materials for energy appli­cations, including batteries, fuel cells and LEDs. The calcu­lations were performed using the National Science Foun­dation’s Extreme Science and Engi­neering Disco­very Environ­ment at the San Diego Super­computer Center.

Now, Ong’s team first compiled a list of the most frequently occurring elements in known phosphor materials. To the researchers’ surprise, they found that there are no known materials containing a combi­nation of strontium, lithium, aluminum and oxygen, which are four common phosphor elements. Using a data mining algorithm, they created new phosphor candi­dates containing these elements and performed a series of first-principles calcu­lations to predict which would perform well as a phosphor. Out of 918 candidates, SLAO emerged as the leading material. It was predicted to be stable and exhibit excellent photo­luminescence pro­perties.

The phosphor’s main limitation is its less than ideal quantum effi­ciency of about 32 percent. However, researchers note that it retains more than 88 percent of its emission at typical LED operating tempera­tures. In commer­cial LEDs, there’s usually a tradeoff with color quality, Ong noted. “But we want the best of both worlds. We have achieved excellent color quality. Now we are working on opti­mizing the material to improve quantum effi­ciency,” Ong said. (Source: UCSD)

Reference: Z. Wang et al.: Mining Unexplored Chemistries for Phosphors for High-Color-Quality White-Light-Emitting Diodes, Joule, online 19 February 2018; DOI: 10.1016/j.joule.2018.01.015

Link: Dept. of Nanoengineering, University of California, San Diego, La Jolla, USA

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