The topic is about polymers that can emit light when a voltage is applied to it. The structure comprises of a thin film of semiconducting polymer sandwiched between two electrodes (cathode and anode).When electrons and holes are injected from the electrodes, the recombination of these charge carriers takes place, which leads to emission of light .The band gap, ie. The energy difference between valence band and conduction band determines the wavelength (colour) of the emitted light.
They are usually made by ink jet printing process. In this method red green and blue polymer solutions are jetted into well defined areas on the substrate. This is because, PLEDs are soluble in common organic solvents like toluene and xylene .The film thickness uniformity is obtained by multi-passing (slow) is by heads with drive per nozzle technology .The pixels are controlled by using active or passive matrix. The advantages include low cost, small size, no viewing angle restrictions, low power requirement, biodegradability etc. They are poised to replace LCDs used in laptops and CRTs used in desktop computers today.Their future applications include flexible displays which can be folded, wearable displays with interactive features, camouflage etc.
Introduction-Imagine these scenarios
- After watching the breakfast news on TV, you roll up the set like a large handkerchief, and stuff it into your briefcase. On the bus or train journey to your office, you can pull it out and catch up with the latest stock market quotes on CNBC.
- Somewhere in the Kargil sector, a platoon commander of the Indian Army readies for the regular satellite updates that will give him the latest terrain pictures of the border in his sector. He unrolls a plastic-like map and hooks it to the unit's satellite telephone. In seconds, the map is refreshed with the latest high resolution camera images grabbed by an Indian satellite which passed over the region just minutes ago.
Don’t imagine these scenarios at least not for too long.The current 40 billion-dollar display market, dominated by LCDs (standard in laptops) and cathode ray tubes (CRTs, standard in televisions), is seeing the introduction of full-color LEP-driven displays that are more efficient, brighter, and easier to manufacture. It is possible that organic light-emitting materials will replace older display technologies much like compact discs have relegated cassette tapes to storage bins.
The origins of polymer OLE D technology go back to the discovery of conducting polymers in 1977,which earned the co-discoverers- Alan J. Heeger , Alan G. MacDiarmid and Hideki Shirakawa - the 2000 Nobel prize in chemistry. Following this discovery , researchers at Cambridge University UK discovered in 1990 that conducting polymers also exhibit electroluminescence and the light emitting polymer(LEP) was born!.
LIGHT EMITTING POLYMER: It is a polymer that emits light when a voltage is applied to it. The structure comprises a thin-film of semiconducting polymer sandwiched between two electrodes(anode and cathode) as shown in fig.1. 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. energy difference between valence band and conduction band of the semiconducting polymer determines the wavelength (colour) of the emitted light.
CONSTRUCTION: Light-emitting devices consist of active/emitting layers sandwiched between a cathode and an anode. Indium-tin oxides typically used for the anode and aluminum or calcium for the cathode. Fig.2.1(a) shows the structure of a simple single layer device with electrodes and an active layer.
ADVANTAGES
• Require only 3.3 volts and have lifetime of more than 30,000 hours.
• Low power consumption.
• Self luminous.
• No viewing angle dependence.
• Display fast moving images with optimum clarity.
• Cost much less to manufacture and to run than CRTs because the active material is plastic.
• Can be scaled to any dimension.
• Fast switching speeds that are typical of LEDs.
• No environmental draw backs.
• No power in take when switched off.
• All colours of the visible spectrum are possible by appropriate choose of polymers.
• Simple to use technology than conventional solid state LEDs and lasers.
• Very slim flat panel.
DISADVANTAGES
• Vulnerable to shorts due to contamination of substrate surface by dust.
• Voltage drops.
• Mechanically fragile.
• Potential not yet realized.
CONCLUSION: Organic materials are poised as never before to trans form the world of display technology. Major electronic firms such as Philips and pioneer and smaller companies such as Cambridge Display Technology are betting that the future holds tremendous opportunity for low cost and surprisingly high performance offered by organic electronic and opto electronic devices. Using organic light emitting diodes, organic full colour displays may eventually replace LCDs in laptop and even desktop computers. Such displays can be deposited on flexible plastic coils, eliminating fragile and heavy glass substrate used in LCDs and can emit light without the directionality inherent in LCD viewing with efficiencies higher than that can be obtained with incandescent light bulbs.
Organic electronics are already entering commercial world. Multicolor automobile stereo displays are now available from Pioneer Corp., of Tokyo And Royal Philips Electronics, Amserdam is gearing up to produce PLED backlights to be used in LCDs and organic ICs.
The first products using organic displays are already in the market. And while it is always difficult to predict when and what future products will be introduced, many manufactures are working to introduce cell phoned and personal digital assistants with organic displays within the next few years. The ultimate goal of using high efficiency, phosphorescent
flexible organic displays in laptop computers and even for home video applications may be no more than a few years in to the future. The portable and light weight organic displays will soon cover our walls replacing the bulky and power hungry cathode ray tubes.
