INK-JET PRINTING OF LIGHT-EMITTING POLYMERS onto a thin film has been demonstrated by a Princeton group (James Sturm, 609-258-5610), bringing about a new way to fabricate a light-emitting diode (LED) made of polymers. An LED is typically built by surrounding a semiconducting material with two electrodes. When an electron from one electrode and a hole from the other meet in the semiconductor, they can annihilate each other and release the energy as light. LEDs in which the semiconductor materials are polymers instead of inorganic materials such as gallium phosphide would be cheaper and easier to manufacture. To make polymer LEDs, the Princeton researchers replaced the ink cartridges of a conventional ink-jet printer with a polymer solution containing the semiconducting polymer polyvinylcarbazol (PVK) and a light-emitting dye dissolved in a chloroform solvent. The researchers printed this solution onto a thin polyester film coated with indium tin oxide (ITO), which served as one of the electrodes. Over the polymer layer they deposited a metal film, which served as the other electrode. With this technique, they produced LEDs emitting green light. In separate experiments, they used the ink-jet printer to make dot patterns of PVK mixed with either red, green, or blue dyes on the ITO-coated polyester film, although they have not yet used these patterned films to make LEDs. (T.R. Hebner et al., Applied Physics Letters, 2 February 1998.)