Art to science
Until recently, the preparation of phosphor materials was more an art than a science, 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?"
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."
However, using calculations based on density functional theory, which was developed by UCSB professor and 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. Lower-efficiency incandescent and fluorescent bulbs which use relatively more energy to produce light could become antiquated fixtures of the past.
"Our target is to get to 90 percent efficiency, or 300 lumens per watt," said DenBaars, who also is a professor of electrica
|Contact: Sonia Fernandez|
University of California - Santa Barbara