PARIS (February 16, 2000 2:00 p.m. EST http://www.nandotimes.com) - Scientists believe they have made strides toward boosting the efficiency of organic semiconductors, the materials that offer the prospect of flat and even flexible computer and television screens. The semiconductors comprise minute light-emitting diodes, or LEDs, made up of aluminium or silicon compounds, sandwiched between conductive materials such as magnesium. The LEDs have to generate the primary optical colours of red, green and blue, the basis for all other tints. When electricity passes through the sandwich, it generates an excited electrical state called an exciton. Part of this energy is emitted as fluorescent light. However, the process is not very efficient. Only 25 percent of the exciton's energy is converted into light -- meaning, in short, that a portable computer that used organic semiconductors would exhaust its batteries very quickly. Researchers at Princeton University and the University of Southern California at Los Angeles, writing in Thursday's issue of the British weekly journal Nature, say they have found a way to harness the wasted 75 percent. Their approach is to add phosphorescent compounds to one of the layers in the LED. This efficiently harvests the energy imparted by the voltage, and hands it on in turn to excite the diode's fluorescent material. The phosphorescent intermediary has been used successfully on red LEDs, yielding efficiency "as high as 100 percent," the authors write. The next hurdle will be green and blue LEDs. These colours have a lower frequency and would thus require the energy transfer to be braked in some way -- a problem that may require alternative materials. Separately, the British weekly New Scientist reports Thursday that Italian chemists have devised a tiny light switch made from a single molecule, which could be a step on the road to building a chemical computer. The switch comprises a nickel molecule which fluoresces blue in an acidic environment, but switches off in an alkaline environment. The principle could form, down the road, a simple "logic gate" for a computing circuit built out of chemical liquids, says the lead researcher, Luigi Fabbrizzi at

-- benoit (foo@bar.com), February 16, 2000

Answers

Another contender for future generations of display will be Light Emitting Polymers, being developed at Cambridge University, England. These are devices which emit a radioactive particle when excited by an electric current, and the (presumably alpha) particle in turn causes a plastic material to fluoresce, giving a short burst of visible light. Since the particle is short-lived, and contained within the material, there is very little biological danger from the radioactivity. It would almost certainly be less hazardous than the x-rays from a conventional CRT.

This material will be tough and flexible too, but research seems further on; materials have already been developed to give red, green and blue emission, at reasonable efficiency levels.

-- Pete Andrews (p.l.andrews@bham.ac.uk), February 17, 2000.