A Traffic Light that uses Red, Amber and Green Light-emitting Diodes Rather than Incandescent Bulbs
But for flat-screen color displays, inorganic semiconductors now face stiff competition from organic materials: light-emitting polymers. These work on much the same principles: the emitter is a semiconductor, which luminesces when charge is injected (although the mechanisms of charge transport and recombination are quite different). The polymers acquire an electrical conductivity from the presence of delocalized electron orbitals along their chains. The first polymer LED, fashioned in 1990, was made from the hydrocarbon polymer poly(p-phenylene vinylene), which emits in the yellow part of the spectrum. Conducting polymers have since been devised that glow right across the visible range, so that full color displays are now possible in principle from polymer LEDs. The advantages are that these materials are very lightweight, flexible (a polymer LED can be rolled up like a sheet of paper), easy to process and fabricate into patterns (this can be done using a kind of printing process) and are amenable to fine-tuning of the emission wavelength by chemical modification of the polymer chain. But the drawbacks are that the emission efficiencies (power out relative to power in) can be very low, and that the polymers may be susceptible to chemical degradation after many hours of use. These shortcomings are being overcome, however, and a full-color polymer LED display is already commercially available.