April 2001

A Bright Future for Displays

Continued from page 1

By Bob Johnstone

A Strange Blue Glow

If ever a technology has begged to be disrupted, it is liquid crystal displays. Invented in 1963 and originally envisioned as a slimmed-down replacement for bulky cathode-ray tubes or as screens for wall-mounted televisions-a use never realized due to problems scaling up to large surfaces-liquid crystal displays have instead become the standard for everything from watches to laptop computers.

In spite of its spread, however, this ubiquitous technology has an Achilles' heel: the screens are hard to make and therefore expensive-especially when it comes to high-end versions used in color displays. Indeed, they account for as much as a third of the cost of a laptop, and the failure to bring down the price of portables as dramatically as the price of personal computers has been due largely to the inability to simplify display-screen production.

Perhaps the remarkable thing about liquid crystal displays is not that they are so expensive but that, given their technological complexity, they are affordable at all. The core of the screen is a sandwich of two flat sheets of glass a few microns apart, with the liquid crystals that form the display medium poured in between. Since the crystals themselves cannot produce light, it's necessary to provide a source-a backlight. A diffuser is then needed to distribute the light evenly across the crystals, as well as front-and-back polarizers to orient the light. And that's just for monochromatic screens. Full-color displays also require expensive red/green/blue filters made of dichromated gelatin-fish glue. To make things fast and bright for, say, a laptop requires another pricey addition: an active-matrix backplane that puts a thin-film transistor behind every pixel. Finally, to manufacture this Byzantine monster takes a superclean factory that won't leave much change out of a billion-dollar bill.

The result is that for the dozen or so firms, mostly Japanese, that have overcome these obstacles to produce active-matrix liquid crystal displays-only to see them become a commodity item, with wafer-thin margins to match-victory has often been Pyrrhic.

Into this chasm of opportunity, with the potential for cheaper but higher-margin and more versatile displays made of common materials like the dyes used in photocopying and photographic paper, come organic light-emitting diodes. Their story begins in 1979, with a scene straight out of a low-budget sci-fi movie. That's when Ching Tang, a Hong Kong-born chemist at Kodak's research laboratories in Rochester, NY, noticed that one of the organic solar cells he was working on was giving off...well, a strange, blue glow. Curiosity aroused, the Kodak scientist launched a long investigation into this phenomenon, known as "organic electroluminescence." His seminal work, reported in Applied Physics Letters in 1987, showed that organic materials were efficient converters of electricity into light that could be switched on and off quickly-especially crucial for showing video, where images are updated 50 or 60 times a second and get blurred if the screen can't keep up. Furthermore, Tang noted, these effects could be obtained with low voltage. In short, organic light-emitting diodes had all the makings of a sensational display technology.

Another breakthrough occurred a year later, when Jeremy Burroughes, a doctoral student at the University of Cambridge's famous Cavendish Laboratory, showed that electroluminescence was characteristic not just of the small molecules studied by Tang but also of far larger polymer molecules (see "Displaying a Winning Glow," TR January/February 1999). This was important because it's much easier in principle to make displays out of polymers than out of small molecules. The smaller materials must be vaporized in a vacuum, then patterned through a perforated metal foil or shadow mask-a costly and delicate process. At least for monochrome displays, however, polymers can be deposited by inexpensive "spin-coating," in which they are simply squirted at a rotating target to achieve a uniform surface.