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Posted on June 8, 2010 by  & 

Walls of light

Organic LEDs (OLEDs) are extremely thin and lightweight surface-emitting lights that will radically change the way we provide illumination. Although mostly confined to labs, OLED technology is moving toward commercialization. In 2009 Osram became the first manufacturer to put an OLED tile on the market.
"Would you like to have a look?" says Dr. Christoph Gärditz, who works in business development for LED and OLED lights at Osram Opto Semiconductors, a Siemens subsidiary. Gärditz is referring to "Orbeos," the world's first commercially available OLED light tile. In his hand is a thin, palm-sized sheet of non-reflective glass that glows a pleasant white. It weighs little more than an envelope. "This is a pioneering product on the road to making OLEDs fit for general-purpose lighting," says Gärditz, who points out that it is a good example of why organic light-emitting diodes (OLEDs) will completely change our idea of lighting. Most lamps in use today, whether in the form of an incandescent bulb, a halogen spotlight, or a light-emitting diode (LED), are point light sources. OLEDs, on the other hand, are flat and emit colored or white light uniformly across their entire surface.
At its core, an OLED consists of several layers of specially designed materials that together are only 500 nm thick—a hundredth of a human hair. These layers are sandwiched between two electrically conductive contact surfaces and a cover and base made of glass. Each layer of plastic consists of chains of small organic molecules. When an electrical current is applied, charge carriers, in this case electrons and electron "holes," move along these chains. The holes are places that are available to electrons. Starting from a higher energy level, the electrons can fall into these empty places and in the process emit their excess energy in the form of light. As a result, the layer glows, and the type of molecule that is involved determines the color of the light. The color of the light is not restricted as much as that of an LED but instead spans a fairly wide range. This is important for white OLEDs, which consist of red, green, and blue light-emitting layers stacked on top of one another—because the more continuous the spectrum of a lamp is, the more true-to-life colors will appear in its light.
Because they are so thin and light, OLEDs can be mounted almost anywhere, and they can therefore convert walls into light sources. With their diffuse light and their good color rendering, large white OLED ceiling lights will make us feel as though we are sitting under the open sky. In laboratories, developers are also working on transparent OLEDs that could be commercially available in two to three years. Among other things, this requires replacing one of the two metallic contact layers with a different material. The plastic layers themselves are already transparent. Glass coated with transparent OLEDs could one day be used in doors, display windows or room dividers either to provide transparent visibility or to produce light itself.
Researchers are also working on making OLEDs more stable with respect to ultraviolet light. This would make it possible to produce windows that would let sun in during the day and give off light themselves at night. In principle, OLEDs would also be flexible if it weren't for their glass and brittle contact layers. In the lab, researchers are experimenting with plastic foils, thin-film techniques, and other contact materials to make flexible OLED lamps. In a few years we could encounter these as luminous roof linings in cars or as lighting columns. Further into the future, OLEDs will be flexible, and will be able to provide illumination in unprecedented ways as light films.
OLEDs are manufactured in a high vacuum. A glass substrate less than 1 mm in thickness is supplied with a transparent, electrically conductive contact layer, and then the individual substances are vapor-deposited on this layer one after another, followed by another metallic layer. At the end, a desiccant and a glass cover are added in order to protect the plastic layers from oxygen and moisture. Finally, the finished substrate is divided into individual light tiles that are checked in a quality control inspection. OLEDs emit light through the glass substrate, while the metal contact at the back of the plastic layer reflects the light like a mirror.
The Orbeos delivers 25 lm/W and thus already surpasses modern halogen lamps. In the lab, researchers can already get 60 lm/W from OLEDs. And in the next few years, they want to increase the efficiency to 100 lm/W—which corresponds to the level of LEDs in use today. To achieve this, Osram developers have to use special films to prevent the light leaving the OLED from being reflected at the boundary where the glass meets the air, which causes it to remain unused inside the lamp. When it comes to generating more light inside an OLED, the structure of the layers is crucial, says Dr. Karsten Heuser, who manages the OLED department at Osram. "Without good component architecture—the intelligent combination of molecules and right layer thicknesses—you can't achieve good results even with the best materials," he says.
The material itself is also important. Electrons don't always release their energy as light when they connect with a hole. But the probability of producing light can be increased by integrating metals like iridium into the layers. In addition, an OLED's service life—the time it takes for its brightness to diminish by half—depends on the stability of the molecules. "As a rule, an OLED ages faster when it's operated at a higher brightness," says Heuser.
At the moment, OLEDs reach about 5,000 hours, which is five times longer than an incandescent bulb. In a few years they will be able to last for 10,000 to 20,000 hours—new robust substances are expected to increase the longevity of the molecules that emit blue light in particular. But OLEDs can also age in storage if moisture and oxygen seep into their plastic layers. Good encapsulation is therefore a key issue for developers. OLEDs now last for about eight years in storage.
An OSRAM employee checks the coated OLED panels for regularity. The focus here is on purity as well as the displacement of different coatings.
Today, OLEDs are still expensive, because they are made in small batches in labs. In its present form the Orbeos tile costs about 250 €. But high-volume production lines will lower the costs considerably—and this also applies to organic materials, which are still being produced in very small amounts. Instead of glass substrates from the LCD industry, developers want to someday coat window glass or even plastic films; the latter is a possible solution for flexible OLEDs.
Researchers would also like to replace the glass cover with a special thin-film encapsulation. This technique offers such good protection that no desiccant is needed. That would reduce costs and increase transparency. Still needed, however, is a substitute for the transparent contact layer that now consists of brittle indium tin oxide—and new production strategies for flexible OLEDs.
Fresh from OSRAM's lab - the ORBEOS OLED.
It will thus be at least five years, Heuser believes, before the first flexible product is ready. And light-emitting wallpaper is still a relatively long way away. "It's one thing to bend the OLED once into a certain shape, but being able to roll it up and unroll it repeatedly is something else. That poses a much more complex challenge, especially when it comes to encapsulation," says Heuser. Nonetheless, one day we'll wonder how we ever did without the lightweight panel lights. In three to four years, estimates Gärditz, glass-based OLEDs will be so bright, have such a long service life, and be so cheap to make that they'll start popping up in living rooms and bedrooms.
Reference: Osram and Siemens
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