Leading the way to better LEDs

Adjusting the levels of rare earth elements can tune light-emitting diodes for wider applications

Light-emitting diodes (LEDs) are steadily replacing more conventional lighting sources. They are low-energy and long-lasting, but despite this there are still ways they could be improved. In the journal Materials Today Sustainability a research team based in China report a method for making LEDs that are more efficient and easier to control in terms of the spectrum of light they emit.

“We are developing a more efficient tunable material with potential for making white light-emitting LEDs,” says Biao Kong at Fudan University in Shanghai.

He highlights that in addition to efficiency and lifetime gains, LEDs are more environmentally friendly and far more flexible lighting options than traditional bulbs. So there should be considerable interest in and a good market for further innovations.

Until now, getting the often-preferred white light from LEDs has generally required a combination of different materials, called phosphors, that emit light at different wavelengths. Kong explains that some of the most generally applied systems rely on combining a phosphor that emits yellow light with one that emits blue light.

Using mixtures of phosphors causes problems, however, due to one material reabsorbing some of the emissions from the other, and also a lack of uniformity in the light-emitting properties of each type.

“These problems make it essential to explore full-colour phosphors emitting red, green and blue light,” says Kong, explaining the motivation driving the team’s current research, and also their plans for the future.

In this article, they report the development of a phosphor whose emissions can be tuned between blue and green light by varying the ratio of cerium (Ce³⁺), terbium (Tb³⁺) and lithium (Li⁺) ions. Demonstrating such tunability of emissions is a key step on the way to gaining full control.

Other researchers have noted the suitability of such ions for activating LED emissions, but Kong and his colleagues have taken it a step further by exploring the potential of using them in different quantities. “As far as we know, nobody else has looked into the effect of varying the doping of levels of these ions in this material,” says Kong.

The tuned phosphors can be made using standard chemical reactions for the main component, composed of sodium, calcium, silicon and oxygen (Na4CaSi3O9). This allowed the researchers to readily investigate the effects of changing the ratio of the added doping ions.

The results, especially with varying cerium and terbium ion doping, clearly confirm the potential for making more efficient white-light LEDs with the modified single phosphor material. The researchers were also able to figure out some technical details governing the mechanisms that allow the absorption and emission of light by the doped materials to be adjusted.

Cerium and terbium belong to the group of elements known as rare earth elements. Kong believes that rare earth doped phosphors have a bright future, both literally and metaphorically, and he expects them to play an important role in developing new white LEDs for a wide range of applications.

Article details:

Kong, B. et al.: “Tunable blue–green–emitting Na4CaSi3O9:Ce³⁺,Li⁺,Tb³⁺ phosphor with energy transfer for near-UV white LEDs,” Materials Today Sustainability (2020)