Looking for Mr. GoodPhosphor

Researchers at the University of California, Santa Barbara have identified a key to optimizing phosphors for SSL.

A group from the university's Solid State Lighting & Energy Center (SSLEC) has published research that promises to make it easier to find new candidate phosphors for converting blue LED light into white light with desired spectral characteristics. They looked for ways to identify Ce3+ phosphor hosts with the most promise for high quantum efficiency.

In looking for phosphors with a high photoluminescence quantum yield (Φ), the group observed that Φ correlates well with a characteristic of the host crystal structure, the Debye temperature, ΘD. This is easily calculated, the researchers say. High values of ΘD are associated with crystal structures that display higher rigidity.

The other characteristic an efficient phosphor needs is a sufficiently large band gap, Eg (also easily calculated). The researchers plotted candidate phosphor materials on a graph with ΘD on one axis and Eg on the other. Good candidates should be found in the high-ΘD, large-Eg region of the resulting graph. Thus, potential phosphor materials can be screened before they are formulated and tested.

The researchers do note, however, that "large Eg with high ΘD are often contraindicated, suggesting a challenge in the search for new host structures."

The researchers believe that materials selected by their screening process will also tend to maintain their efficiency at elevated temperatures better than phosphors hosted on less rigid crystal structures.

While they worked with the class of phosphors that use Ce3+ as the activator ion, the researchers claim their method will be transferrable to phosphors based on Eu2+ as well.

How efficient is high-efficiency?

The researchers are trying to get to a phosphor efficiency of 90 percent, Steve DenBaars, co-director of the SSLEC, said in the UCSB press release. A phosphor-converted white LED with a phosphor efficiency in that range could achieve a luminous efficacy of 300 lm/W. DenBaars claimed that the team has achieved 60 percent efficiency in the laboratory.

Below, one of the authors of the paper, Ram Seshadri, explains the work in laymen's terms.

— Keith Dawson , Editor-in-Chief, All LED Lighting