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PLEDS


Published on Dec 17, 2015

Abstract

Polymers are chemical substances that consist of large molecules that are, themselves, made from many smaller and simpler molecules: proteins and DNA are examples of naturally occurring polymers; many others, such as nylon, are artificially created. Because of their flexibility and strength, polymers are used for products such as car bumpers and bulletproof vests.

One of the major display technology developed is Organic Light Emitting Diodes (OLED), a thin-film LED whose benefits include high flexibility and lack of backlight requirements. But now perhaps the most exciting development in the display industry in the last fifteen years has been the discovery and development of polymer light emitting diodes (PLEDs).

Polymer, chemical compound with high molecular weight consisting of a number of structural units linked together by covalent bonds. Meaning "many parts," it is a material constructed of smaller molecules of the same substance that form larger molecules. For example, plastic is a synthetic polymer, while protein is a natural polymer. The simple molecules that may become structural units are themselves called monomers; two monomers combine to form a dimer, and three monomers, a trimer. A structural unit is a group having two or more bonding sites.

In 1989, it was found in the Cavendish Laboratory of Cambridge University that 'organic' LEDs could be made using conjugated polymers.In particular, polyphenylene vinylene (PPV) was found to emit yellow-green light when sandwiched between a pair of electrodes. The initial device efficiencies were very low, but the researchers quickly realised the commercial potential of this discovery, especially for the manufacture of displays which emit their own light. These would offer significant advantages over the main display technology we still use today (liquid crystal display or LCD), in which a separate light source has to be filtered in several stages to produce the image we see.

Among the marvels of the Polymer Light Emitting Diode is that the polymers it is made of can be laid down much like the way ink-jet printers lay ink.PLED displays are made by applying a thin film of light-emitting polymer onto a glass or plastic substrate coated with a transparent electrode. A metal electrode is sputtered or evaporated on top of the polymer.

Application of an electric field between these two electrodes results in emission of light from the polymer. When a current is applied, electrons from the cathode migrate through the cell and meet positive 'holes' migrating from the anode. When they meet, they form so-called excitons, and as the electrons drop into the holes, energy is released as light.














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