PORTLAND, Ore.— Switching to purely organic phosphorescent emitters could drastically cut the cost of solid-state lighting panels, according to a team of researchers at the University of Michigan. The team claims to have found the first example of a purely organic—and thus ultra-cheap—compound that can compete with an metal-doped emitters in terms of brightness and color tuning capability.
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.
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.
Solid-state lighting (SSL) today is harnessing arrays of LEDs, some of which fit in the standard light bulb socket. However, some believe that in the future, consumers will abandon the "light bulb" form factor in favor of SSL lighting panels in whatever shape is convenient. Light panels using these organic phosphors could accelerate the day when OLED panels are as bright, but cheaper and more power efficient than LEDs.
Why would one have to wait to have the organic phosphors OLED pannels ? Today you can get high lumens based lighting panels using the same LED light guides technology used in LCD backlighting panels ! This can go up to several hundreds of lumens using only a limited numbers of LEDs.
How long do these organic materials continue to glow after the ongoing excitation? While some materials do glow for hours (dimly), many materials only continue to glow for a fraction of a second. This isn't all bad; people expect a light to go dark after they turn it off. However, a continuing glow for a few seconds might be an interesting feature, it would allow time to leave the room before it went completely dark. (Like the car headlights with a timer that turn them off after a delay.)
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