PARIS Researchers at Philips, collaborating with the University of Amsterdam, have developed the first electroluminescent material that can generate either red or green light. Unlike today's electroluminescent materials which can emit light of one color only, the new material can switch the emitted light between two colors depending on the direction of the applied current flow.
The material is "unique and revolutionary," said Hans Hofstraat, who heads up the polymers and organic chemistry department at Philips Research and works part time as a professor at the University of Amsterdam. Although still in the basic research phase, this new material could lead to a new generation of "much brighter, easier to prepare and longer lasting" polymer-based flat-panel light-emitting displays, according to Hans Hofstraat.
The researchers "stumbled" on this discovery while working on the development of a brighter electroluminescent device.
Their basic research results are published in the Jan. 2, 2003 issue of Nature.
The new material makes possible full-color displays by using pixels of only two instead of three materials, making the device "much easier to manufacture," Hofstraat said, in an interview with EE Times. Moreover, "a bigger advantage is that you can increase the brightness by more than 50 percent, since a larger part of their surfaces can be used for emitting light," he added. "That will also result in a longer lifetime."
The newly developed material is a homogeneous mix of a semiconducting polymer and a metal complex, each with different energy in its excited state. For direction dependence, a non-symmetric device is needed, so a single layer of the material is sandwiched between electrodes of different materials, one of gold and one of indium tin oxide (ITO).
When the ITO electrode is forward biased, only the light-emission process of the metal complex is triggered, showing up as red. Reversing the bias voltage on ITO reverses the direction of the current. The light-emission process in the metal complex ceases, but then the polymer gets excited. The device now emits green light, corresponding to the bandgap of the polymer. Both colors are well saturated, according to Philips, as they are not mixed with other colors.
Hofstraat said that the invention of the new electroluminescent material was "a coincidence," since the primary objective of Philips' research project was to explore technologies to improve the brightness of polymer-based LED displays.
With the newly developed electroluminescent material Hofstraat believes the industry can devise a polymer-based full color LED display whose brightness matches that of organic electroluminescent displays, while its polymer-based LED display can leverage a conventional printing technology for depositing materials.
Philips does not expect to see the first application of the newly developed electroluminescent material complex until 2006 or 2007. That device will be a small polymer-based, full-color LED display "a few-inch screen" in a small appliance, perhaps something like a shaver, according to Hofstraat.