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Revealing organic LEDs’ true colors

U. MICHIGAN (US) — Pure organic compounds that glow in jewel tones have the potential to lead to cheaper, more efficient, and flexible display screens, among other applications.

Scientists have made metal-free organic crystals that are white in visible light and radiate blue, green, yellow, and orange when triggered by ultraviolet light. By changing the materials’ chemical composition, the researchers can make them emit different colors.

The new luminous materials, or phosphors, could improve upon current organic light-emitting diodes (OLEDs) and solid-state lighting. Bright, low-power OLEDs are used in some small screens on cell phones or cameras but are not practical for use in larger displays because of material costs and manufacturing issues.

The OLEDs of today aren’t 100 percent organic, or made of carbon compounds. The organic materials used in them must be spiked with metal to get them to glow.

“Purely organic materials haven’t been able to generate meaningful phosphorescence emissions. We believe this is the first example of an organic that can compete with an organometallic in terms of brightness and color tuning capability,” says Jinsang Kim, associate professor of materials science and engineering, chemical engineering, macromolecular science and engineering, and biomedical engineering at the University of Michigan.

The work appears online in Nature Chemistry.

The new phosphors exhibit “quantum yields” of 55 percent. Quantum yield, a measure of a material’s efficiency and brightness, refers to how much energy an electron dissipates as light instead of heat as it descends from an excited state to a ground state. Current pure organic compounds have a yield of essentially zero.

In Kim’s phosphors, the light comes from molecules of oxygen and carbon known as “aromatic carbonyls,” compounds that produce phosphorescence, but weakly and under special circumstances such as extremely low temperatures.

What’s unique about these new materials is that the aromatic carbonyls form strong halogen bonds with halogens in the crystal to pack the molecules tightly. The arrangement suppresses vibration and heat energy losses as the excited electrons fall back to the ground state, leading to strong phosphorescence.

“By combining aromatic carbonyls with tight halogen bonding, we achieve phosphorescence that is much brighter and in practical conditions,” says Onas Bolton, the paper’s co-author who recently received his Ph.D. in Materials Science and Engineering.

The new method offers an easier way to make high-energy blue organic phosphors, which are difficult to achieve with organometallics.

Organic light emitting diodes are lighter and cheaper to manufacture than their non-organic counterparts, which are made primarily of ceramics. Today’s OLEDs still contain small amounts of precious metals, though.

Organic phosphors glowing blue and orange could lead to cheaper organic light-emitting diodes. (Credit: Marcin Szczepanski)

The new compounds can bring the price down even further, because they don’t require precious metals. They’re made primarily of inexpensive carbon, oxygen, chlorine, and bromine.

“This is in the beginning stage, but we expect that it will not be long before our simple materials will be available commercially for device applications,” Kim says.

“And we expect they will bring a big change in the LED and solid-state lighting industries because our compounds are very cheap and easy to synthesize and tune the chemical structure to achieve different colors and properties.”

Former doctoral student Kangwon Lee discovered the unique properties of these materials while developing a biosensor—a compound that detects biological molecules and can be used in medical testing and environmental monitoring. The phosphors have applications in this area as well. After Lee’s discovery, Bolton developed the metal-free pure-organic phosphors.

The work was funded in part by the National Science Foundation and the National Research Foundation of Korea.

More news from University of Michigan: www.ns.umich.edu/

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