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  • Phosphors Optimized for Better, Brighter LEDs

Photonics.com
Nov 2013
SANTA BARBARA, Calif., Nov. 1, 2013 — New guidelines for optimizing phosphors — a key component in white LEDs — could lead to brighter and more efficiently produced solid-state lighting.

Researchers at the University of California, Santa Barbara’s Solid State Lighting & Energy Center (SSLEC) developed the guidelines based on the rigidity of phosphor crystalline structure. 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.


This illustration demonstrates how bright blue LED light, shone through its complementary yellow phosphor, yields white light. Images courtesy of UCSB. 


“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 Materials department, and in the Chemistry and Biochemistry department.

All of the recent advances in solid-state lighting have come from devices based on gallium nitride LEDs, Seshadri said, a technology that is largely credited to UCSB materials professor Shuji Nakamura, who invented the first high-brightness blue LED. In solid-state white-lighting technology, phosphors are applied to the LED chip in a way that allows the photons from the blue gallium nitride LED to pass through the phosphor, converting and mixing the blue light into the green-yellow-orange spectrum band. When combined evenly with the blue, the green-yellow-orange wavelength emits white 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, when they are used in vehicle front lights — they also tend to get warmer, which adversely affects the phosphor’s properties.

“Very few phosphor materials retain their efficiency at elevated temperatures,” said postdoc Jakoah Brgoch. “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.”


The researchers behind the breakthrough are (l-r) Steve DenBaars, Jakoah Brgoch and Ram Seshadri.


However, using calculations based on density functional theory, which was developed by UCSB professor and 1998 Nobel laureate Walter Kohn, the researchers determined that the rigidity of the crystalline host structure is a key factor in the efficiency of phosphors: the higher the rigidity, the better the phosphor. They also found that 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, they said.

“Our target is to get to 90 percent efficiency, or 300 lumens per watt,” said materials professor Steven DenBaars, who also is a professor of electrical and computer engineering and co-director of the SSLEC. Current incandescent light bulbs, by comparison, are at roughly 5 percent efficiency, and fluorescent lamps are a little more efficient at about 20 percent.

“We have already demonstrated up to 60 percent efficiency in lab demos,” DenBaars said.

The results appear in The Journal of Physical Chemistry C.

For more information, visit: www.ucsb.edu 


GLOSSARY
phosphor
A chemical substance that exhibits fluorescence when excited by ultraviolet radiation, x-rays or an electron beam. The amount of visible light is proportional to the amount of excitation energy. If the fluorescence decays slowly after the exciting source is removed, the substance is said to be phosphorescent.
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