Density functional theory indicates that the rigidity of the crystalline host structure is a key factor in the efficiency of phosphors used to create white LEDs. Currently, the best phosphors possess a highly rigid structure

By determining simple guidelines, researchers at UC Santa Barbara's Solid State Lighting & Energy Centre (SSLEC) have made it possible to optimise phosphors – a key component in white LED lighting – allowing for brighter, more efficient lights.

"These guidelines should permit the discovery of new and improved phosphors in a rational rather than trial-and-error manner," says Ram Seshadri, a professor in the university's Department of Materials and Department of Chemistry and Biochemistry.

The results of this research, performed jointly with materials professor Steven DenBaars and postdoctoral associate researcher Jakoah Brgoch, appear in The Journal of Physical Chemistry.



The researchers behind the breakthrough, from left to right : Steve DenBaars, Jakoah Brgoch and Ram Seshadri

LED lighting has been a major topic of research due to the many benefits it offers over traditional incandescent or fluorescent lighting. LEDs use less energy, emit less heat, last longer and are less hazardous to the environment than traditional lighting. Already utilised in devices such as street lighting and televisions, LED technology is becoming more popular as it becomes more versatile and brighter.

Most, if not all of the recent advances in solid-state lighting (SSL) have come from devices based on GaN LEDs.

In solid-state white lighting technology, phosphors are applied to the LED chip in such a way that the photons from the blue GaN LED pass through the phosphor, which converts and mixes the blue light into the green-yellow-orange range of light. When combined evenly with the blue, the green-yellow-orange light yields white light.

Seshadri explains that for a good phosphor "we mean something that is efficient, something that takes the blue photons that go in and push out yellow or orange photons and the same number of them rather than wasting some of the blue photons. The challenge is two-fold - not just having a material that is efficient at room temperature - but also a material that retains its efficiency at elevated temperatures. And our studies suggest a solution to both."

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.



This illustration demonstrates how bright blue LED light, shone through its complementary yellow phosphor, yields white light

"So far, there has been no complete understanding of what make some phosphors efficient and others not," Seshadri notes. "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 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 says. "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.

Sheshadri says, "We have found through a combination of computational studies as well as experimental studies that the best phosphors have rather rigid structures."

The images below show examples of a rigid phosphor which is highly connected  and a phosohor where the units  are not connected and the structure is what Sheshadri describes as "somewhat floppy."



Rigid connected phosphor      (Credit UCSB)



"Floppy" unconnected phosphor  (Credit UCSB)

This new breakthrough will also allow the determination of structural rigidity which 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, SSL 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.

"We can now start looking for cheaper materials from which we can construct the same rigid hosts and that should decrease the cost of phosphors and by looking at increasingly rigid hosts we can also start finding materials for niche applications where very high brightness is key, things like the front lighting of cars and even perhaps stadium lighting," concludes Sheshadri.

"Our target is to get to 90 percent efficiency, or 300 lumens per watt," says 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 concludes.

Further details of this work has been published in the paper, " Article Proxies from Ab Initio Calculations for Screening Efficient Ce3+ Phosphor Hosts," by Jakoah Brgoch et al in the Journal of Physical Chemistry, 2013, 117 (35), pp 17955–17959. DOI: 10.1021/jp405858e