Comet Aids Discovery of New, Inexpensive Material to Make LEDs with Excellent Color Quality

Published February 20, 2018

Written by Liezel Labios, UC San Diego Jacobs School of Engineering

A team led by engineers at the University of California San Diego has used data mining and the Comet supercomputer at the San Diego Supercomputer Center, based at UC San Diego, to discover a new phosphor material for white LEDs that is inexpensive and easy to make.

As reported in the Feb. 19 issue of the journal Joule, the researchers built prototype white LED light bulbs using the new phosphor that 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.

Under UV light, the phosphor emits either green-yellow or blue light depending on the chemical activator that is mixed in. Photo by David Baillot, UC San Diego Jacobs School of Engineering.

The new phosphor, Sr₂LiAlO₄ or simply 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 and lead principal investigator of the study.

Ong’s team used the Comet supercomputer at SDSC 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.

"Comet resources, which are designed specifically for computations of modest scale, were especially well-suited for our project,” said Ong. “By leveraging on our open-source high-throughput software infrastructure and Comet resources, we were able to compute the stability and photoluminescent properties of thousands of candidate materials within a matter of weeks, something which would have taken years to obtain experimentally. This large dataset was then carefully mined to identify the optimal target LED phosphor for further experimental development.” Allocations to use Comet were provided by the National Science Foundation’s Extreme Science and Engineering Discovery Environment (XSEDE).

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.

The Computational Quest for a New Material

Ong, who leads the Materials Virtual Lab and is a faculty member in the Sustainable Power and Energy Center at UC San Diego, uses a combination of high-throughput calculations and machine learning to discover next-generation materials for energy applications, including batteries, fuel cells and LEDs.

In this study, 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.

“It’s not only remarkable that we were able to predict a new phosphor compound, but one that’s stable and can actually be synthesized in the lab,” said Zhenbin Wang, a nanoengineering Ph.D. candidate in Ong’s research group and co-first author of the study.

The phosphor’s main limitation is its less than ideal quantum efficiency—how efficiently it converts incoming light to light of a different color—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.

Paper title: “Mining Unexplored Chemistries for Phosphors for High-Color-Quality White-Light-Emitting Diodes.” Authors of the study are Zhenbin Wang*, Jungmin Ha*, Joanna McKittrick and Shyue Ping Ong at UC San Diego; and Yoon Hwa Kim* and Won Bin Im at Chonnam National University, Republic of Korea.

*These authors contributed equally to this work

This work was supported by the National Science Foundation, Ceramics Program (grant 1411192). Computational resources were provided by the Triton Shared Computing Cluster at UC San Diego, the National Energy Research Scientific Computing Centre, and the Extreme Science and Engineering Discovery Environment supported by the National Science Foundation (grant ACI-1053575). This work was also supported by the Basic Science Research Program through the National Research Foundation of Korea, which is funded by the Ministry of Science and ICT (NRF-2017R1A2B3011967).

About SDSC

As an Organized Research Unit of UC San Diego, SDSC is considered a leader in data-intensive computing and cyberinfrastructure, providing resources, services, and expertise to the national research community, including industry and academia. Cyberinfrastructure refers to an accessible, integrated network of computer-based resources and expertise, focused on accelerating scientific inquiry and discovery. SDSC supports hundreds of multidisciplinary programs spanning a wide variety of domains, from earth sciences and biology to astrophysics, bioinformatics, and health IT. SDSC’s petascale Comet supercomputer is a key resource within the National Science Foundation’s XSEDE (Extreme Science and Engineering Discovery Environment) program.