by Steven Schultz

Princeton researchers have created new 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 research, led by professor and chairman of electrical engineering Stephen Forrest, combines the usually distinct phenomena of fluorescence and phosphorescence in a way that allows extremely efficient production of light.

The results were published in the Feb. 17, 2000, issue of Nature (vol. 403, pp. 750-53).

Professor Forrest and a graduate student in his lab, Marc Baldo, collaborated with University of Southern California chemistry professor Mark Thompson.

Organic light-emitting diodes (OLED) are the subject of the research. These are thin films of molecules that can be induced to emit light. They have advantages over liquid crystal displays because they are brighter, use less electricity, offer potentially truer colors, and allow smaller pixels.

OLEDs can be made from two types of molecules, fluorescent and phosphorescent. Fluorescence offers variety because scientists have identified more fluorescent molecules with suitable properties. Phosphorescence is much more efficient in terms of energy consumption.

This new finding gives developers of OLED devices the best of both materials. By adding small quantities of energy-efficient phosphorescent molecules to fluorescent materials, the final products emitted fluorescent light in a highly efficient manner.

"It offers manufacturers exactly what they want," Professor Forrest said. "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 how well molecules take advantage of two "excited" states they enter when an electric charge is applied. 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), Professor 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. 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," Professor Forrest said. The process takes advantage of the fact that phosphor escence 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 Professor Forrest's work and has licensed rights to the discovery to Universal Display Corp., which partially funded the research. Additional funding came from the Department of Defense, the Air Force, and the National Science Foundation.

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.