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.
Today the trend is towards the non_crt flat panel displays. As LEDs are inexpensive devices these can be extremely handy in constructing flat panel displays. The idea was to combine the characteristics of a CRT with the performance of an LCD and added design benefits of formability and low power. Cambridge Display Technology Ltd is developing a display medium with exactly these characteristics.
The technology uses a light-emitting polymer (LEP) that costs much less to manufacture and run than CRTs because the active material used is plastic.
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. Since 1990, a number of polymers have been shown to emit light under the application of an electric field; the property is called the electro luminescence(EL)
PPV and its derivatives, including poly thiophenes, poly pyridines, poly phenylenes and copolymers are still the most commonly used materials.
Efforts are on to improve stability, lifetime and efficiency of polymer d3evices by modifying their configuration.
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.
The charge transport mechanism in conjugated polymers is different from traditional inorganic semiconductors. The amorphous chain morphology results in inhomogeneous broadening of the energies of the chain segments and leads to hopping type transport.
Conjugated polymers have already found application as conductor in battery electrodes, transparent conductive coatings, capacitor electrolytes and through hole platting in PCBâ„¢s. There are fast displaying traditional materials such as natural polymers etc owing to better physical and mechanical properties and amenability to various processes.
BASIC STRUCTURE AND WORKING
An LEP display solely consists of the polymer material manufactured on a substrate of glass or plastic and doesnâ„¢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. These moving holes and electrons combine together to form hole-electron pairs known as excitonsâ„¢. These excitons are in excited state and go back to their initial state by emitting energy.
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