Polymer light emitting diodes (PLEDs) are optoelectronic devices based on small molecule or polymers that emit light when an electric current flow through them. A simple PLED consist of a fluorescent polymer layer sandwiched between two metal electrodes. Under application of an electric field electrons and holes are injected from the two electrodes in to the polymer layer, where they meet and recombine to produce light. They have been developed for application in flat panel displays. Envision light weight, portable and flexible flat panel displays that provide visual imagery that is easy to read, vibrant in colour and less consuming of power.
PLEDs are light weight, durable power efficient and ideal for portable application. PLEDs fewer process steps and also use both fewer low cost materials than LCD displays. PLEDs can replace the current technology in many applications due to following performance advantages over LCD s
*Faster response time for full motion video
*Full viewing angle
*Greater environmental durability
*More power efficiency
*Broader operating temperature range
*Greater cost effectiveness
2. EVELUTION OF PLED
Light Emitting Polymers (LEPs) are semi-conducting devices that exhibit electroluminescent characteristics. The phenomenon known as electroluminescence (EL) is caused by the emission of light generated from the recombination of electrons and holes electrically injected into a semi-conductor. So far conventional and marketable electroluminescent devices (ED s) have always been inorganic semi-conductors but recent developments in the late 1990s are opening the doors for organic materials. Obtaining organic materials with electroluminescent properties was first achieved in the 1960s on anthracene crystals by Pope et al at New York University.
However these early devices had high operating voltages and very low quantum efficiency thus they did not attract much attention for two decades. In 1987 a breakthrough was made by Tang and VanSlyke at Eastman Kodak who, by using multi layers of sublimated organic molecules, succeeded in reducing the operating voltage dramatically and increasing the quantum efficiency significantly. Another important discovery was made in 1990 by Burroughs Et al at Cambridge University who detected electroluminescence from diodes based on luminescent conjugated polymers.
Polymer means many parts. Polymers are very large organic molecules formed by thousands of repeating units linked together. One can view these heavy organic molecules (molecular weight between 5,000 and 150,000) as very long chains mostly made of C-C, C-O and C-N bonds. Polymer technology has existed for decades.
Use of natural rubber and common plastics such as polythene dates back to as early as 1820s. However finding polymers with intrinsic properties of semi-conductors is very recent. In 1967, electrically conducting polymers from pyrrole, thiopene and furan were characterized and the electrical conductivity of poly(anilines) noted. The report in 1977about doping polyacetylene to achieve conductivity opened up important vistas for chemistry and physics.
Polymer based light-emitting diodes (LEDs) were discovered in 1990 by Friend et al. The first polymer LED used 'poly phenylene vinylene' (PPV) as the emitting layer. Since 1990 many different polymers have been shown to emit light under the application of an electric field (EL). PPV and its derivatives are still the most commonly used materials, but polythiophenes polyphenylenes, and polypyridines are now being tested for higher efficiency, longer lifetime and lower power requirements.