ORGANIC LEDS: New method of making sol-gel micropixels

Makers of high-definition displays, such as computer screens and head-mounted displays, are looking at organic light-emitting devices (OLEDs) as a potential component of the systems they would like to build.

Nov 1st, 1999
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Makers of high-definition displays, such as computer screens and head-mounted displays, are looking at organic light-emitting devices (OLEDs) as a potential component of the systems they would like to build. Such OLEDs work efficiently with low power, have tunable red-green-blue output, and are bright and lightweight. So far, however, there are no products on the market in large volume (see Laser Focus World, July 1999, p. 16).

Nasser Peyghambarian, a professor in the Optical Sciences Center at the University of Arizona (Tucson, AZ), who recently founded NP Photonic Technologies (Tucson), and his team have a method of making OLEDs that he says has good commercial potential. Peyghambarian builds the diodes using sol-gel technology, a hybrid of organic glass that is produced chemically at a low temperature.

In one attempt, the researchers started with a substrate of indium tin oxide and deposited a sol-gel film approximately 1.5 µm thick on top of it. They then placed a photomask with a pattern of 45 x 45-µm squares over the sol-gel and cured it with an ultraviolet lamp. They washed away the unexposed sol-gel, leaving an array of openings to act as pixels, and fabricated OLEDs on this structure, creating an array of 45 x 45 µm2 pixels each surrounded by a 1.5-µm-high sol-gel step (see figure).

Peyghambarian says that he has made circular pixels as small as 2 µm in diameter. Light output of the devices based on micropixels was appoximately five times as great as that of conventional LEDs at a given voltage, so Peyghambarian's device reached light-output levels suitable for display applications—approximately 100 to 200 cd/m2 at 6 to 7 V.

Click here to enlarge image

Peyghambarian says the advantage of his method comes from the fact that sol—gel is photoimageable—a pattern can be inscribed directly on the sol-gel by exposing it to light. "There are several steps in the manufacturing that you don't have to do when you use this process," he said.

FIGURE. Thin film of sol-gel is deposited on an indium tin oxide (ITO) glass substrate (top) and exposed through a photomask (middle), creating an array of pixels (bottom) for organic light-emitting diodes.

Another refinement that Peyghambarian and his colleagues are trying to achieve is to make the LED itself in the sol-gel layer, eliminating even more steps. The process is very close to being ready to be applied to the manufacture of OLEDs, Peyghambarian said.

Neil Savage

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