Phosphors: Turning Blue Light White

Friday, December 15, 2017

An LED is the sum of many parts, and one key component for generating all-important white light from an LED is the phosphor. The phosphor — which has nothing to do with the element phosphorus — is a material that is applied over the surface of the blue LED. When the blue light source hits the phosphor, some of it is transformed to appropriate complementary colors that enable the totality of the emission to appear white.

It is in the area of phosphors that Ram Seshadri, professor in materials, chemistry, and biochemistry and director of the UCSB Materials Research Laborator, has made his contributions to SSLEEC research. When he came to UCSB in 2002, he had a colleague who was working on phosphors to complement LED research at what was then the Solid State Lighting and Energy Center (SSLEC). When the colleague left UCSB for Cambridge in 2007, SSLEEC co-director Steve DenBaars asked Seshadri if he would be willing to continue working on phosphors. 

Seshadri agreed, and since 2008, he and his colleagues have worked to ensure that the phosphor is not a source of inefficiency, since the whole reason to use an LED is that it is more efficient than other bulbs. “We want every blue photon that comes in and hits the phosphor to be converted to a yellow photon,” he says. “We don’t want there to be losses of blue photons.”

He worked on that for several years, while also trying to develop a detailed understanding of the mechanisms behind efficiency loss, and codifying rules for identifying good phosphor materials, to avoid having to try out multiple materials that would likely not work. 

More recently, Seshadri has been using a blue laser, rather than a blue LED, to excite the phosphors. “The goal is stadium lighting,” he says. “You can’t do that currently with LEDs. It can be bright if you make a panel of them, but the panel has to be too large. It’s hard to achieve focused white light with an LED, but you can use a blue laser and phosphor.” 

By this point, he says, “The existing materials for LED phosphors are so good that LEDs actually have to catch up with the phosphors. The goal was to match the way colors are rendered under sunlight. We’ve accomplished that. Our goal now is to do it with lasers instead of with blue LEDs.” And, in fact, in February 2017, he and Steve DenBaars registered the first patent for a “laser-driven white-lighting system for high-brightness applications."

A phosphor is a coating applied to a blue LED to turn blue light white

A phosphor is a coating applied to a blue LED to turn blue light white