SIEMENS

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 (see article “Let There Be Light”, Pictures of the Future 1/2007). 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 nanometers 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 lightemitting 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 made 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 had their largest public showing to date at the Light & Building trade show in April 2010 in Frankfurt, Germany. There, Osram made the topic of OLEDs a special focus of its presentation and pulled out all the stops by showing a variety of lighting installations and illumination techniques in order to give architects and light designers food for thought.

OLEDs are manufactured in a high vacuum. A glass substrate less than a millimeter 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.