benefit is that the device structures themselves are simple. They consist of a transparent conductor on which lies some number of organic layers, usually four. The total thickness of the organic layer is only about 2,000 angstroms, and that layer is capped with a thin metal cathode.

The Advantage of Flexibility

Another benefit is that all the organic layers are amorphous and therefore bendable. This differentiates them from semiconductors, which are formed by the crystal-growing process of epitaxy2 and are therefore rigid. OLEDs are so flexible that they can be bent around any reasonable radius and grown as a passive matrix display on virtually any medium, such as plastic sheets or rolls. They can be applied easily to ceilings, walls, or other large surfaces or even embedded in fabrics or other soft elements. They can also be located on or in firm surfaces such as glass, metal, or silicon.

Straightforward Processing

Because organic LEDs are amorphous, the methods for preparing them are relatively straightforward, inexpensive, and can be scaled to large areas. All of the deposition tools for large-scale processing are commercially available. Finally, the lights can be readily tuned in terms of color and electronic properties using chemical means.

There are two types of materials used for making OLEDs: small molecules and polymeric thin films. Electronically the two materials are very similar, although there are some differences in preparation techniques. For small molecules, vacuum deposition is used for both organic thin films and the metal electrodes; the organic compounds are deposited from a heated source directly onto the substrate in a vacuum. Another technique is organic vapor phase deposition, which gives good control over film thickness and composition. For polymers, solution processing (spin coating) is the most common technique. Another technique that Dr. Thompson’s laboratory has been developing involves not a vacuum but vapor deposition. This type of process is much more convenient than standard deposition, although the quality of the polymer layers is typically low.

The Advantage of Transparency

Another benefit of these devices is that they are virtually transparent to their own radiation, because the organic layers themselves are very thin. If the cathode

2  

Epitaxy is the growth of the crystals of one mineral on the crystal face of another mineral, so that the crystalline substrates of both minerals have the same structural orientation. Semiconductors are grown from such solid crystalline substances as silicon and germanium.



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