New research enables brighter LED lights

Researchers, including one of Indian-origin, have made it possible to optimise phosphors - a key component in white LED lighting - allowing for brighter, more efficient lights.
The discovery was enabled by determining simple guidelines, said researchers at University of California - Santa Barbara's Solid State Lighting & Energy Center (SSLEC).
"These guidelines should permit the discovery of new and improved phosphors in a rational rather than trial-and-error manner," said Ram Seshadri, a professor in the university's Department of Materials as well as in its Department of Chemistry and Biochemistry.
The research was performed jointly with materials professor Steven DenBaars and postdoctoral associate researcher Jakoah Brgoch.

Until recently, preparation of phosphor materials was based on finding crystal structures that act as hosts to activator ions, which convert the higher-energy blue light to lower-energy yellow/orange light.
"So far, there has been no complete understanding of what make some phosphors efficient and others not," Seshadri said.

"In the wrong hosts, some of the photons are wasted as heat, and an important question is: How do we select the right hosts?" he said.
As LEDs become brighter, for example a they are used in vehicle front lights, they also tend to get warmer, and, inevitably, this impacts phosphor properties adversely.

"Very few phosphor materials retain their efficiency at elevated temperatures," Brgoch said.
"There is little understanding of how to choose the host structure for a given activator ion such that the phosphor is efficient, and such that the phosphor efficiency is retained at elevated temperatures," Brgoch said.
However, using calculations based on density functional theory, which was developed by 1998 Nobel Laureate Walter Kohn, the researchers have determined that the rigidity of the crystalline host structure is a key factor in the efficiency of phosphors: The better phosphors possess a highly rigid structure.
Furthermore, indicators of structural rigidity can be computed using density functional theory, allowing materials to be screened before they are prepared and tested.

This breakthrough puts efforts for high-efficiency, high-brightness, solid-state lighting on a fast track.
"Our target is to get to 90 per cent efficiency, or 300 lumens per watt," said DenBaars.
Current incandescent light bulbs, by comparison, are at roughly 5 per cent efficiency, and fluorescent lamps are a little more efficient at about 20 per cent.
"We have already demonstrated up to 60 per cent efficiency in lab demos," DenBaars said.
The study was published in The Journal of Physical Chemistry.