Green Organic LED Shows High Efficiency

Dr. James P. Smith

An organic light-emitting diode (LED) that relies on electrophosphorescence may prove a breakthrough in organic LED efficiency, with quantum and power efficiencies reaching 8 percent and 31 lm/W, respectively. The green device, described in the July 5 issue of Applied Physics Letters, is based on fac tris(2-phenylpyridine) iridium [Ir(ppy)₃], a green electrophosphorescent material.

University of Southern California chemists teamed with engineers at the Center for Photonics and Optoelectronic Materials at Princeton University in New Jersey to pursue the development of organic LED displays. Last year, they announced the development of a red electrophosphorescent material based on a platinum/porphyrin compound. Although this material's efficiency is competitive with red fluorescent dyes, the platinum-based phosphor is limited by a long decay time and becomes saturated at higher currents. The iridium device has a very short decay time compared with most organic molecules, and does not saturate as easily.

"One of the things that kept people from thinking about using phosphorescence before was that, historically, people thought that phosphorescence was incredibly long-lived," explained Mark E. Thompson, a chemistry professor at the University of Southern California.

Phosphorescent emission originates from a long-lived triplet state. This slow triplet relaxation can form a bottleneck in electrophosphorescence; the advantage of Ir(ppy)₃ is that its short triplet lifetime (~500 ns) substantially widens the bottleneck. To maximize the performance of an electrophosphorescent device, the phosphor is dispersed into a conductive host material.

Thompson reported that he has found other platinum and iridium compounds with short lifetimes that can be used to make yellow, orange and red/orange high-efficiency organic LEDs. "We are working in the direction of a blue," he said. "I don't see any reason to believe we won't eventually get to a suitable blue phosphorescent material."

Potential advantages of organic-based displays are compelling. Lower materials costs and simpler manufacturing processes should make them less expensive than cathode-ray tube or semiconductor displays. Organic LEDs are lightweight; and they can be made thin, flexible and transparent.

Just a few phosphorescent compounds have been investigated, but the purely organic materials have not shown strong phosphorescence at room temperature. The transition-metal complexes used in this study show promise and are the most likely to have reasonable photoluminescent efficiencies paired with lifetimes in the 1-µs range.