Organic Displays May Be Feasible

April 01, 1997

San Francisco -- In the future, cheaper, more durable and more easily manufactured liquid crystal computer displays may be manufactured from organic thin films, according to a team of Penn State researchers.

Pentacene, an organic compound, has the key materials characteristics necessary to fabricate useable thin film electronic devices, the researchers told attendees at the annual meeting of the Materials Research Society today (April 1) in San Francisco.

Liquid crystal flat screen displays are composed of millions of cells that must be turned on and off to provide an image. The cells are activated by a charge storage capacitor that is controlled by a transistor. For the screen to work, the cell has to be turned on very rapidly, remain on long enough to create the image and turn off when the image no longer requires activation in that cell.

Pentacene has a field effect mobility and on/off current ratio comparable to amorphous silicon and, with a bit of engineering, also can have an acceptable subthreshold slope -- a measure of the amount of voltage needed before an electronic device can be turned from on to off.

"Organic semiconductors were investigated in the 1940s and 1950s, but had properties that were considered unsuitable for practical use," says Dr. Thomas Jackson, professor of electrical engineering. "About 10 years ago, people started looking at small molecule organic compounds like sexithienyl. While the compound's semiconductor properties were still questionable, they approached a level that might be useful."

While others were looking at sexithienyl, Jackson and electrical engineering graduate students Yen Li Lin and David J. Gundlach, began looking at pentacene in research sponsored by the Defense Advanced Research Project Agency and NSF.

Pentacene is a blue solid at room temperature. It can be deposited as a thin film at low temperatures and, therefore, can be deposited on light-weight, durable plastics, perhaps using continuous web manufacturing techniques.

The market for thin film applications, including smart cards, liquid crystal displays and emissive displays is enormous. The active matrix liquid crystal display industry is already a $12 billion business. Most active matrix liquid crystal displays currently use hydrogenated amorphous silicon thin film transistors, but high-operating temperatures require glass substrates.

The first characteristic the group investigated was mobility. Initially, pentacene showed very low mobility, but refinement of the deposition process gave a mobility acceptable for semiconductor devices.

"We began looking at mobility, because if a compound doesn't have high enough mobility, there is little point in looking at other characteristics," says Jackson. "Once we knew the mobility was acceptable, we turned to on/off ratio, an important characteristic for flat screen displays.

"With purified pentacene, we found we could get a very acceptable on/off ratio that is in the range of amorphous silicon semiconductors," he adds.

Once mobility and on/off ratio proved acceptable, the researchers turned to subthreshold slope. The Penn State researchers found the subthreshold voltage for pentacene 10 times higher than for amorphous silicon devices, making it unsuitable for most electronic applications.

"However, the subthreshold slope is greatly reduced if the active pentacene layer is deposited on a dielectric treated with a self-assembly material," says Jackson.

The researchers deposited pentacene on oxidized silicon treated with octadecyltricholorsilane, a self-assembly material, and achieved a subthreshold slope similar to amorphous silicon devices, showing that large threshold slope is not an intrinsic characteristic of the material.

"Now that we have acceptable mobility, on/off ratio and subthreshold voltage, we can begin to consider whether pentacene thin film devices can be manufactured," says Jackson. "Now we need circuit demonstrations."

***aem***

EDITORS: Dr. Jackson can be contacted at tnj1@psuvm.psu.edu on the Internet.

Penn State

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