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UV-Induced Cross-Linking Yields Organic LEDs

Photonics Spectra
Apr 2003
Kevin Robinson

Scientists from Universität München and Technische Universität München in Munich, and Covion Organic Semiconductors GmbH in Frankfurt, all in Germany, have devised a technique for the production of organic polymer LEDs that promises to be faster and more versatile than existing processes. Although still in development, the method may be a boon to the industry by providing it with an inexpensive and reliable means of assembling organic LED displays on a large scale.

UV-Induced Cross-Linking Yields Organic LEDs
A new manufacturing technique promises to allow multicolor organic LEDs to be made more efficiently and on a larger scale. Courtesy of Klaus Meerholz. ©Nature.

There are several approaches to making organic LEDs. The most common, vacuum deposition, is costly and cannot lay down material over a large area. Another uses ink-jet printing technology to lay down the polymers, but it requires additional steps to ensure that the fluorescent polymers stick properly to the substrate. A third, spin coating, only allows the complete coverage of a substrate and cannot be used to create display-quality pixels.

The researchers employ polymers common to these other approaches, but their first step is to take these common molecules and engineer them into polymers that will react to ultraviolet radiation by cross-linking. Cross-linking modifies the chemical bonds in a polymer to make it more rugged: The individual molecules "join hands" to form a much larger molecular network.

To make a display with red, green and blue pixels, they begin by depositing a polymer that will emit one of the colors onto the surface of a treated glass substrate. Then they use a UV laser to expose a pixel pattern on the surface of the chip, inducing cross-linking. Next, as in photolithography, they apply a solvent to remove the noncross-linked polymer. They repeat these steps for the remaining two colors, filling in the gaps in the pattern.

Markus Rojahn, who worked with Oskar Nuyken's group at Technische Universität München and with Heinrich Becker at Covion to synthesize the materials, said that the size of the pixels in a 17-in. computer monitor is 0.2 to 0.4 mm, which is in reach of their method.

For smaller displays, colleague and principal investigator Klaus Meer-holz, currently at the Universität zu Köln in Cologne, Germany, said that 0.05- to 0.1-mm pixels would be useful. The team has demonstrated higher resolutions, but both researchers cautioned that they remain far from presenting a finished display.

"The method is exceptional concerning ease of preparation and the patterning resolution," Rojahn said. "Of course, we only showed the basic principle. Real TV or PC displays need a lot more electronics, so this will be a future project for our industrial partners."

He and Meerholz said that the researchers will continue to work to improve the materials and the process and that they will investigate the long-term stability of the devices.


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