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February 17, 2000

Scientists Develop New Material for Efficient Flat-Panel Displays

Discovery Could Accelerate Development of Better Laptops and Other Compact Displays

PRINCETON, N.J. -- Princeton scientists have created a variety of light-emitting materials that could greatly accelerate the development of flat-panel computer screens and other compact video displays. The discovery, a feat of engineering materials at the level of quantum mechanics, also may yield insights into the basic properties of light-emitting substances.

The researchers, led by professor of electrical engineering Stephen Forrest, found that they could combine the usually distinct phenomena of fluorescence and phosphorescence in a way that allows extremely efficient production of light. The result could allow developers of display devices to choose from a much wider range of materials than previously available, adding flexibility to their products and reducing costs of production.

The results will appear in the February 17 issue of Nature. Forrest and a graduate student in his lab, Marc Baldo, collaborated on the work with University of Southern California chemistry professor Mark Thompson.

The subject of the research is a type of device called an organic light-emitting diode (OLED). These are thin films of molecules that can be induced to emit light. They have advantages over liquid crystal displays, which are used in laptop computer screens for example, because they are brighter, use less electricity, offer potentially truer colors and allow smaller pixel size.

OLEDs can be made from two types of molecules, fluorescent and phosphorescent. Until now, the choice between the two has been a tradeoff. Fluorescence offers variety because scientists have identified many more fluorescent than phosphorescent molecules with suitable properties, such as good color quality and operational lifetime. Phosphorescence, thanks to a discovery published by the same team in 1998, is much more efficient in terms of energy consumption.

The new finding gives developers of OLED devices the best of both materials. The researchers found that adding small quantities of energy-efficient phosphorescent molecules to fluorescent materials resulted in final products that emitted fluorescent light in a highly efficient manner.

Forrest said electronics manufacturers could use the new technique within six months in certain applications such as car stereo displays. Eventually the technique could lead to the ubiquitous use of OLEDs in products such as palm pilots, cell phones and laptop computers.

"It offers manufacturers exactly what they want," said Forrest. "You want a laptop that doesn't run down the battery in three hours; you want the battery to last 10 hours."

The efficiency of light-emitting devices depends on a detail of quantum mechanics: How well do molecules take advantage of two "excited" states that they enter when they receive an electric charge? The two states are called singlets and triplets; they always occur with three triplets for every singlet. The material emits light when the singlets or triplets release their energy and return to a "ground state." Fluorescent materials are inefficient because only singlets produce light and the three triplets are wasted.

In their 1998 paper (also published in Nature), Forrest's group showed that they could engineer materials to use the singlets and the triplets and produce light through phosphorescence. Because they use all four excited states, these phosphorescent materials are four times more efficient than fluorescent materials.

The researchers then sought to bring the same efficiency to fluorescent materials, which exist in great variety. They found they could use one of their high-efficiency phosphors to "collect" all the triplet states, convert them into usable singlets and transfer them into a fluorescent material.

"The fluoresor that would miss all those triplets now gets them from the phosphor. The phosphor essentially sensitizes it," said Forrest. The process takes advantage of the fact that phosphorescence is a slow phenomenon compared to fluorescence; the fluorescent material grabs the converted triplets and turns them into light before the phosphorescence has a chance to occur.

Princeton University has applied for a patent on Forrest's work and has licensed rights to the discovery to Universal Display Corporation, which is dedicated to developing the display technologies from Forrest's lab and which partially funded the research. Additional funding came from the Department of Defense, the Air Force and the National Science Foundation.