Abstract :
Light emitting
polymers (LEPs) or polymer-based light-emitting diodes discovered by Friend et al in 1990
have been found better than other displays like liquid crystal displays (LCDs),
vacuum fluorescence displays, and electroluminescence displays. Though not commercialized
yet, these have proved to be a milestone in the field of Flat Panel Displays (FPDs).
Research on LEP is underway in Cambridge Display Technology Ltd, CDT, Cambridge,
UK. The Cathode Ray Tube (CRT), invented by German physicist Karl Ferdinand Braun
in 1897, remained the ubiquitous
display in the
last half of the 20th century. But the CRT’s long heritage in an environment where product life
cycles are measured in months rather than years doesn’t mean that it is an ideal display solution.
It is bulky, power hungry and expensive to manufacture.
It is a polymer
that emits light when a voltage is applied to it. The structure comprises a thin-film
semiconducting polymer sandwiched between two electrodes (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 escapes through
glass substrate. The bandgap i.e. the energy
difference between valence band and conduction band, of the semiconducting polymer
determines the wavelength (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 electroluminescence (EL).Efforts are on to improve
the efficiency of polymer devices by modifying their configuration.
Working
Like the CRT, LEP emits light as a function of its electrical operation. An LEP display solely consists of the polymer material manufactured on a substrate of glass or plastic and doesn’t require additional elements like the backlights, filters, and polarisers that are typical of LCDs. Fig. shows the structure of an LEP device. 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 ‘excitons’. These excitons are in excited state and go back to their initial state by emitting energy. When this energy drop occurs, light comes out from the device.
Light-emitting
devices consist of active/emitting layers sandwiched between a cathode and an anode.
Indium-tin oxide is typically used for the anode and aluminium or calcium for the
cathode. Fig.2 shows the structure of a simple single layer device with electrodes
and an active layer. Single-layer devices typically work only under a forward DC
bias. Also it shows a symmetrically configured alternating current light-emitting
(SCALE) device that works under AC as well as forward and reverse DC bias.
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