Seiko Epson Corporation

If seeing is believing, then journalists attending Epson's press conference in Tokyo this May announcing the company's prototype 40-inch organic light-emitting diode (OLED) display came away as true believers. Next-generation OLED flat-panel technology has a number of advantages over today's liquid crystal displays (LCDs) and plasma display panels, and these differences were on view in the new full-color prototype, which is the industry's largest OLED by far to date. Just as impressive is how Epson used an innovative manufacturing process to fabricate the display: a process based on its expertise in inkjet printing.

OLED displays use direct light-emitting materials. This means that there is no need for power-hungry backlighting in order to produce a bright image, and the image itself is superior in contrast and resolution when matched against displays currently on the market. Moreover, OLEDs boast high color purity to produce rich reds, greens, and blues, while the picture-viewing angle is a wide 165 degrees.

Epson's new prototype OLED display uses a substrate made up of four low-temperature TFT polysilicon panels joined together to form a single 40-inch display. Of the two types of organic substances used by OLED researchers-small molecules and polymer materials-Epson has chosen to work with the latter.

"We use polymer materials because they can be transformed into a liquid with the help of solvents, which we then pattern onto the substrate using our inkjet printer technology," explains Shoichi Iino, general administrative manager of Epson's OLED Technology Development Division. "This gives us the flexibility to fabricate panels of any size, from very small to very large."

To achieve the accuracy required in fabricating an OLED, Epson first uses photolithography to create an ultrafine pattern consisting of round cavities on the substrate. Then solutions of alternating red, green, and blue polymers are fired (or printed) into the cavities, creating a series of RGB subpixels. The display's electronics combines three RGB subpixels to form each full-color pixel.

To enhance accuracy at the patterning stage, Epson has introduced a further innovation to ensure the highest possible precision. "We coat the surface of the cavities with a hydrophilic material and the surrounding area with a hydrophobic material," explains Iino. "These chemical coatings then naturally attract or repulse the polymer solution, giving us the precision we need."

The end result is that the polymer material can be printed "on demand" with pinpoint accuracy, thereby minimizing waste. Iino believes that this inkjet printing method has the potential to eventually fabricate much larger displays. Currently, the largest printing area possible using commercialized inkjet technology is 1,200 mm × 820 mm with a resolution of 2,880 dots per inch.

As Iino points out, OLED displays of 10 to 40 inches will fill the gap between the company's small displays and projector offerings. This middle segment, he believes, will cover a wide range of products, from mobile TVs through to personal TVs and Internet viewers and all the way up to home theater systems.