Phosphorescent emitters cut solid-state lighting costs

Researchers at the University of Michigan have proposed the use of purely organic phosphorescent emitters, claiming it would considerably reduce the cost of solid-state lighting panels. According to the group, it has discovered the first example of wholly organic and extremely cheap compounds that can equal the brightness and color tuning capacity of metal-doped emitters.

Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs, but rather continues to glow for up to several hours after the original excitation—a property that has previously been seen only in compounds doped with metals, called organometallics. However, these new metal-free organic compounds developed at the University of Michigan, which appear white in visible light, radiate blue, green, yellow and orange after absorbing ultraviolet light.

"This is the first example of an organic that can compete with an organometallic in terms of brightness and color tuning capability," said University of Michigan professor Jinsang Kim.

The novel compounds, called aromatic carbonyls, form strong bonds with halogens in the crystal, allowing molecules to be tightly packed, thus suppressing vibration and heat losses during recombination, leading to strong yet tunable phosphorescence. Color is determined by the precise chemical formula, and quantum yields were about 55 percent.

Research associate Kangwon Lee discovered the unique properties of aromatic carbonyls, which was developed further by fellow researcher Onas Bolton. Funding was provided by the National Science Foundation and the National Research Foundation of Korea.

Figure caption: Organic phosphors developed at the University of Michigan could one day lead to cheaper organic light-emitting diodes. Here, they glow in blue and orange when triggered by ultraviolet light. Credit: Marcin Szczepanski, U-M College of Engineering.

R. Colin Johnson
EE Times