26-08-2012, 11:09 PM
22-12-2012, 04:34 PM
Light emitting polymers
1Light emitting.pdf (Size: 944.44 KB / Downloads: 53) INTRODUCTION Light emitting polymers or polymer based light emitting diodes discovered by Friend et al in 1990 has been found superior than other displays like, liquid crystal displays (LCDs) vacuum fluorescence displays and electro luminescence displays. Though not commercialised yet, these have proved to be a mile stone in the filed of flat panel displays. Research in LEP is underway in Cambridge Display Technology Ltd (CDT), the UK. In the last decade, several other display contenders such as plasma and field emission displays were hailed as the solution to the pervasive display. Like LCD they suited certain niche applications, but failed to meet broad demands of the computer industry. WHAT IS LEP ? LEP is a polymer that emits light when a voltage is applied to it. The structure comprises a thin film semi conducting polymer sandwiched between two electrodes namely anode and cathode. When electrons and holes are injected from the electrodes, the recombination of these charge carriers takes place, which leads to emission of light that escape through glass substrate. The ban gap, that is energy difference between valence band and conduction band of the semi conducting polymer determines the wave length, that is colour of the emitted light. The first polymer LEPs used poly phinylene vinylene (PPV) as the emitting layer. CHEMISTRY BEHIND LEP LEPs are constructed from a special class of polymers called conjugated polymers. Plastic materials with metallic and semiconductor characteristics are called conjugated polymers. These polymers posses delocalised pi electrons along the backbone, whose mobility shows properties of semiconductors. Also this gives it the ability to support positive and negative charge carriers with high mobility along the polymer chain. BASIC STRUCTURE AND WORKING An LEP display solely consists of the polymer material manufactured on a substrate of glass or plastic and doesn’t require additional elements like polarizers that are typical of LCDs. LEP emits light as a function of its electrical operation. The basic LEP consists of a stack of thin organic polymer layers sandwiched between a transport anode and a metallic cathode. Figure shows the basic structure. The indium-tin-oxide (ITO)coated glass is coated with a polymer. On the top of it, there is a metal electrode of Al, Li, Mg or Ag. When a bias voltage is applied, holes and electrons move into the polymer. Printer based technique LEPs can be patterned using a wide variety of printing techniques. The most advanced is ink-jet printing (figure). Resolution as high as 360 dpi have been demonstrated, and the approach are scalble to large-screen displays. Printing promises much lower manufacturing cost. CONCLUSION LEPs are promising, low cost solutions for today’s flat panel displays. Although not commercialised yet, these replace bulky and heavy CRT displays in the near future. However research is underway to improve the efficiency and lifetime of the polymer displays. A panel of industry leaders predicted that LEP technology would storm the market in the near few years and we will find LEP in every sphere of life about ten years from now. LEP technology is now set to change the products we use to view the world. |
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