Researchers have been trying for years to develop flexible electronic displays, a breakthrough that would bring us tablet computers that fold and roll up and clothing with stretchable video screens embedded in the fabric. The biggest problem has always been finding a malleable, ultrathin base layer, or substrate, on which to build the display. Polymer LEDs (PLEDs)—a form of the organic light-emitting-diode displays now going into extravagantly expensive, vanishingly thin televisions—are mere microns thick and pliable. The problem is that they have always needed to be manufactured on a plastic or glass substrate, which is usually somewhere between 1,000 and 10,000 times thicker than the PLED itself and not particularly flexible.
By changing the method of manufacture, however, it is possible to get rid of those thick, inflexible substrates. Using a “peel-and-stick” technique, an international team of scientists made PLEDs just two microns thick—several times thinner than plastic kitchen wrap.
First, the researchers apply a 1.4-micron-thick sheet of Mylar film, which will serve as the substrate for the PLED, to a rigid glass support. The Mylar sheet is similar to the screen protector that today comes on most new, out-of-the-box smartphones and tablet computers, says researcher Matthew White, a materials scientist at Johannes Kepler University in Linz, Austria. “We just realized that we could apply the [peel-and-stick] method to device fabrication and push the limits of how thin you can make the substrate.”
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Once the Mylar film is in place, White and his colleagues deposit the PLED on top. The PLED has three layers: a 100-nanometer metal electrode, a 200-nanometer transparent electrode, and a light-emitting layer 225 to 330 nanometers thick sandwiched in between. When those elements are on the Mylar, the researchers peel the entire thing off the glass. The resulting display is as light, flexible and crushable as one would expect kitchen wrap to be.
Depositing the PLED elements on a thin, rubbery base layer makes a screen that is stretchable. The stretchiness is, theoretically, only limited by the stretchiness of the base layer.
This past summer the scientists fabricated two pliable, eight-pixel PLED displays, one that glows red and one that glows orange. Each pixel measures three by six millimeters, much larger than the tiny pixels in today's high-resolution screens. The PLEDs are nearly display-quality in brightness, however, and the researchers are confident they can shrink the pixel size readily.
Plenty of hurdles remain before these displays are ready for the market. The biggest: the metal electrodes are unstable in air; in about an hour the pixels gradually go dark. Different materials will be needed before the technology is ready for prime time. The displays are not particularly energy-efficient, either, although White says his team sees a clear path toward making them as efficient as conventional lighting.
