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Light Emitting Polymers . . . the new flexible flat-panel display technology By JULIAN EDGAR Imagine a flat panel display made out of plastic. Or how about a panel that’s flexible and can made to conform to any shape such as a car dashboard? That’s the promise offered by new display technology based on semiconducting polymers. 42  Silicon Chip O VER THE LAST 30 YEARS, there has been increasing interest in the use of plastic polymers as conductors or semiconductors. Polymers that have semi­ conduct­ing characteristics are called “conjugated” polymers and they behave as semiconductors for reasons that are different to those of inorganic devices. Despite this, semiconductor polymers are engineered using many of the lessons learned with traditional semiconductors. As a result, progress in the use of semiconducting polymers has been quite rapid. This prototype light emitting polymer display screen has been developed by Cambridge Display Technology. How it started Polymer semiconductor technology was invented in 1989 at the Cavendish Laboratory at Cambridge University in the UK. It began when physicist Richard Friend and chemist Andrew Holmes were experimenting with organic polymers. Quoted in the Cambridge “Alumni Magazine”, Holmes says: “we started with a plastic material called PPV, made by a process that allows it to be coated in thin films over large surface areas. If the material is chemically ‘doped’, it can conduct electricity nearly as well as a good metallic conductor”. However, the two scientists were interested in seeing how good the ‘undoped’ material was as an insulator. “We sandwiched a thin film between metal electrodes and subjected it to a high voltage. What happened next was pure serendipity. Someone switched out the lights by mistake and the plastic was seen to emit a yellow-green light. We had discovered the plastic version of a light-emitting diode.” In 1992, a company called Cambridge Display Technology was formed to develop commercial applications for light-emitting polymers (LEP), sometimes also called organic light emitting diodes (OLED). Joint ventures have since been signed with Seiko-Epson, Philips, DuPont, Hoechst and UNIAX. Conductors Conjugated polymers have found their first uses as conductors. In fact, doped conjugated polymers have achieved conductivities close to that of copper! The potential commercial applications include battery electrodes, conductive coatings for electrostatic speakers, capacitor electrolytes, transparent conductive coatings, through-hole plating of PC boards and electrostatic discharge coatings. Japanese company Matsushita is currently using polypyrrole in the manufacture of polymer capacitors, for example. Another major goal is to use conducting polymers to replace the copper tracks on PC boards. However, it will be necessary to improve the stability of the highest conductivity plastics before this can occur. Another promising use for conducting polymers is in electromagnetic shielding. That’s because of their relatively high conductivity and dielectric constant. It’s also easy to control these properties through chemical processing. Polyaniline is an especially good candidate for electromagnetic shielding. How they work Light emitting polymers sandwich a thin-film semiconducting polymer between two electrodes. Electrons and holes are injected from the electrodes and the recombination of these charge carriers leads to luminescence. The bandgap – ie, the energy difference between the valance band and conduction band of the semiconducting polymer – determines the wavelength of the light that is emitted. Fig.1 shows the basic layout. To make a device, a very thin (50300nm) uniform coating of polymer is spin-cast or extruded onto a glass or plastic film substrate that has been precoated with a transparent electrode material. The substrates can be chosen freely, with flexible and even 3-dimensional substrates suitable for use. The electrodes are either conducting oxides (indium tin oxide is often used) or conducting polymers, with one electrode transparent to allow the light to escape. In order to define the final configuration, the transparent electrode is patterned before the polymer layer is added. The other electrode is deposited by vacuum metallisation FEBRUARY 2000  43 TOP ELECTRODE ORGANIC LAYER BOTTOM ELECTRODE GLASS SUBSTRATE Fig.1: light emitting polymers consist of a polymer layer which is sandwiched between two electrodes, one of which is transparent. and patterned. The device is then encapsulated in a hermetically-sealed package. In practice, multiple devices can be fabricated on a single large substrate which is then scribed and broken before the leads are attached. By manipulating the structure of the polymer, light in the full colour spectrum of 450-740nm can be obtained. Display technology One hot topic of interest is the use of conjugated polymers in display technology. Five years ago, light output efficiencies of only 0.01 lumens/W were being reported but recent developments have seen efficiencies 10,000 times higher. Indeed, the polymer mat­ erials now being used have efficiencies close to that of inorganic LEDs. The display lifetimes that are now being quoted are also impressive. For example, Philips recently measured a display lifetime of more than 30,000 hours using light-emitting polymers. The display has high brightness and contrast and operates from only The UNIAX company has recently completed a prototype manufacturing line to produce this flexible alpha­ numeric display. 3.3V. Another company, UNIAX, has recently completed a clean room and prototype manufacturing line for its first light emitting polymer product – a flexible alphanumeric display. One major advantage of polymer displays is that the light emitting device can be patterned by simple pixellation of the metal. Large area pixellated displays made from one Table 1: Benefits Of Light Em itting Polym ers Feature LEP Processing Benefit Fast Swi tching Speed Fl exibl e substrates possible; large area coatings. No backlights required; no colour fil ters; no aperture loss; 180° viewing angle. Simple to define complex light emission patterns; very high resolution possible wrequired; any pi xel size and shape possible. Battery dri ven devices; DC dri ve. Innovati ve designs for end products; di spl ays shaped to products; easy manufacturing i ntegration wi th product; continuous coating for manufacture. Video displ ay capabili ty. Light Emi tting Pattern Formation Low Vol tage Operation Formabl e Substrates Light Weight Portabili ty. Solid State Devices Ruggedness. Thin Films Allows use of pol ari sers to gi ve high contrast. 44  Silicon Chip sheet are possible. Dot-matrix alphanumeric displays can also be made. The commercial collaboration between Cambridge Display Technology and Seiko-Epson is aimed at using ink-jet technology to print the pixels of the display directly on top of the pixel switching elements in the active matrix. It is hoped that this will lead to the development of a fast-switching, robust solid-sate device with a wide viewing angle, that can be used as a flat-screen display. When developed, it should combine both thinness and light weight with the look and feel of a traditional colour CRT. Thus far, this technology has only been showcased in a small (50mm square) b&w TV display that’s just 2mm thick! However, it’s being suggested that when combined with poly­silicon TFT technology and inkjet printing, light-emitting polymers will deliver superior performance to existing display technologies such as LCDs. Cambridge Display Technology suggest that the advantages of the light emitting polymer displays are varied and many. Table 1 shows some of the advantages cited by the company. Recently, researchers at Princeton in the US replaced the ink cartridges of a conventional inkjet printer with a polymer solution containing the semiconducting polymer polyvinyl­ carbazol (PVK) and a light-emitting dye dissolved in a chloroform solvent. This solution was then “printed” onto a thin polyester film coated with indium tin oxide, which served as one of the electrodes. Finally, they deposited a metal film over the polymer layer to form the other electrode. This technique produced a light-emitting polymer that emitted green light. They then used the inkjet printer to make dot patterns of PVK mixed with either red, green or blue dyes on the coated polyester film. While this latter process has not yet been used to develop light-emitting polymers, it’s possible that this technology may lead to the development of a large, flat screen with mixed red, green and blue dot patterns. This in turn could lead to full-colour plastic TV screens, or even car indicator and dashboard lights that blend seamlessly into the bodywork and become visible only when they are on. It could even lead to the development of flexible TV and PC SC display screens.