29-08-2014, 02:18 PM
Light emitting polymers
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Introduction
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.
The fact is that researchers haven’t come up with a better solution. Liquid Crystal Display (LCD) was pitched as the savior of the display industry. Its creators claimed that a slim profile would quickly make it the display of choice. But today, LCDs are far more pervasive. These offer a little bit benefit over their predecessor, the CRT. The cost of a LCD as well as a CRT monitor one-third of the total price of a computer. Says David Mentley, Vice President and display industry analyst at Stanford Resources, California, USA, “Although LCD is a highly successful technical achievement, the manufacturing archetype must change if flat panel displays are to compete directly across all applications.” In the last decade, several other 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 the broad demands of the computer industry.
What if a new type of display could combine the characteristics of a CRT with the performance of an LCD and the added design benefits of formability and low power? Cambridge Display Technology Ltd (CDT) 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 is plastic.
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 possess delocalized 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 applications in battery electrodes, transparent conductive coatings, capacitor electrolytes and through hole platting in PCBs. 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
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.
SPIN COATING PROCESS
A spin-coating machine is used that has a plate spinning at the speed of a few thousand rotations per minute. The robot pours the plastic over the rotating plate, which, in turn, evenly spreads the polymer on the plate. This results in an extremely fine layer of the polymer having a thickness of 100 nanometres. Once the polymer is evenly spread, it is baked in an oven to evaporate any remnant liquid. The same technology is used to coat the CDs.
CONCLUSION
LEPs are promising, low-cost solutions for today’s flat-panel displays. Although not commercialised yet, these may replace bulky and heavy CRT displays in the near future. At the Wall Street Journal CEO Forum that took place in London, the UK, a panel of industry leaders predicted that
LEP technology would storm the market in the next few years.