IV.26 - Organic LEDs?

This section was contributed by Aneek Roy, aroy@ccf.nrl.navy.mil:

A recent advance in light emitting device technology is the development of LEDs whose active layers consist entirely of organic materials. These organic materials possess the electronic and optical properties of semiconductors as well as the processability of organic compounds. Spin coating of polymers and vacuum deposition of low molecular weight organics have both been applied to fabricate these LEDs.

Light emission in organic LEDs is accomplished through the recombination of electrons and holes injected into the device from electrodes at opposite ends, which results in the production of singlet and triplet excitons at the emitter layer. The decay of singlet excitons to the ground state produces light. Emitter layer materials include polyphenylene vinylene (PPV) in polymeric devices and aluminum III 8-hydroxyquinoline (Alq3) in low weight molecular devices. Triphenyldiamine derivative and polysilane have been used extensively for hole transport in organic LEDs, as they have relatively high hole drift mobilities, and indium tin oxide (ITO) is commonly used as a transparent anode to fabricate organic LEDs on glass or plastic substrates.

Current research in this field of organic LEDs include the study of mechanisms microscopic surface and interface properties, tuning of metal-organic Schottky barrier heights, increasing quantum efficiency and improving packaging to prevent degradation from water vapor and oxygen in the atmosphere.

Organic LEDs have been able to achieve high luminance levels (as high as 100,000 cd/m^2 in pulsed operation) at voltages of <15V. (For information on driving diodes with such high on-voltages, click here.) Possible applications include color flat panel displays (as organic LEDs can be produced with emission colors spanning the visible spectrum), low-level backlights, and portable segmented/dot matrix displays.

Companies:

• Planar America, Inc.
  1400 NW Compton Dr., Beaverton, Oregon 97006
  Tel: +1 503 690 6967
  Fax: +1 503 690 1493
  http://www.planar.com
• Eastman Kodak Corp., Rochester, New York
• Xerox Corp., Webster Research and Technology Center
  114-39D, Webster, New York 14680
• Uniax Corp., Santa Barbara, California