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Organic light-emitting diode
An organic light-emitting diode (OLED) is a thin-film light-emitting diode (LED) in which the emissive layer is an organic compound. These devices promise to be much cheaper to fabricate than inorganic LEDs. When the emissive layer is polymeric, varying amounts of OLEDs can be deposited in arrays on a screen using simple "printing" methods to create a graphical colour display, for use as television screens, computer displays, portable system screens, and in advertising and information board applications. OLED panels may also be used as lighting devices. OLEDs are available as distributed sources while the inorganic LEDs are point sources of light. Prior to standardization, OLED technology was also referred to as OEL or Organic Electro-Luminescence.
One of the great benefits of an OLED display over the traditional LCD displays found in computer displays is that OLED displays don't require a backlight to function. This means that they draw far less power and they can be used with small portable devices which have mostly been using monochrome low-resolution displays to conserve power. This will also mean that they will be able to last for long periods of time with the same amount of battery charge.
Two main directions
There are two main directions in OLED, Small Molecules and Polymers.
The first technology was developed by Eastman-Kodak and is usually referred to as "small-molecule" OLED. The production of Small-molecule displays requires vacuum deposition which makes the production process expensive and not so flexible. The term OLED traditionally referrs to this type of device, though some are using the term SM-OLED.
A second technology, developed by Cambridge Display Technologies or CDT, is called LEP or Light-Emitting Polymer, though these devices are better known as Polymer Light Emitting Devices (PLEDs). Although this technology lags the Small-Molecule development by several years (primarily in efficiency and lifetime), it is more promising because of an easier production technique. No vacuum is required, and the emissive materials can be applied on the substrate by a technique derived from commercial ink-jet printing. This means that PLED displays can be made in a very flexible and cheap way.
Recently a third hybrid light emitting layer has been developed that uses nonconductive polymers doped with light-emitting, conductive molecules. The polymer is used for its production and mechanical advantages without worrying about optical properties. The small molecules then emit the light and have the same longevity that they have in the Small-Molecule OLEDs.
How OLEDs work
OLEDs work on the principle of Electroluminescence. The key to the operation of an OLED is an organic lumophore . Excitons, which consist of an bound excited electron and hole pairs (empty state) are generated inside the emissive layer. When the electron and hole combine, a photon can be emitted. A major challenge in OLEDs is tuning the devices such that holes and electrons meet in the emissive layer in equal quantities. This is difficult because the mobilities of holes are typically much higher than that of electrons in organic compounds. Light emission can only occur when singlet excitons form in the emissive because the materials currently employed are typically fluorophors and cannot emit light from a triplet state. This is a problem because only one in four excitons is a singlet. By incorporating transition metals into small-molecule OLEDs, the triplet and singlet states can be mixed by spin-orbit coupling, which leads to emission from both states, but triplet emission is always red-shifted from the corresponding singlet emission, thus blue light is nearly impossible to achieve from a triplet excited state.
To create the excitons, a thin film of the lumophor is sandwiched between electrodes of differing work functions . Electrons are injected into one side from a metal cathode, while holes are injected in the other from an anode (think of the anode as sucking electrons out). These electrons and holes move into the emissive layer and can meet to form excitons.
Derivatives of PPV, poly(p-phenylene vinylene) and poly(fluorene), are commonly used as polymer lumophors in OLEDs. Indium tin oxide is a common transparent anode, while aluminum or calcium are common cathode materials. Other materials are added in between the cathode/anode and the emissive layer to enhance the efficiency by facilitating or hindering hole or electron injection. You may find more materials for this technology.
The radically different manufacturing process of OLEDs lends itself to many advantages over traditional flat panel displays. Since OLEDs can be printed onto a substrate using traditional ink-jet technology they can have a significantly lower cost than LCDs or plasma displays. A more scalable manufacturing process enables the possibility of much larger displays. Unlike LCDs which employ a back-light and are incapable of showing true black, an off OLED element produces no light allowing for infinite contrast ratios. The range of colors, brightness, and viewing angle possible with OLEDs is greater than that of LCDs or plasma displays.
The fact that OLEDs can be printed onto flexible substrates opens the door to new applications such as roll-up displays or displays embedded in clothing.
The biggest technical problem left to overcome now is lifetime. Red and green OLED elements already have life-times of well over 20,000 hours but blue OLED life-times lag significantly behind at 1,000 hours.
The lifetime problems are not so significant in small molecule OLEDs, particularly as a result of doping of OLEDs has led to much better device performance both electrically and optically. Universal Display for example have produced a blue OLED that has a lifetime of 10,000 hours. There are still a number of problems to overcome though, and one of these is intrusion of water into displays which damages and destroys the organics, as well as outcoupling, which can result in the loss of much of the light in waveguided modes within the substrates.
In October 2004 Cambridge Display Technology announced a blue OLED with a lifetime of 70,000 hours.
Commercial development of the technology is also hampered by intellectual property issues since even the basics of OLED technology is heavily patented by Kodak and other firms, requiring outside research teams to acquire a license.
- Howard, Webster E. (Feb. 2004). Better Displays with Organic Films. Scientific American, p. 76.
- Shinar, Joseph (Ed.) (2004). Organic Light-Emitting Devices: A Survey. NY: Springer-Verlag. ISBN 0-387-95343-4.
- OLED-Info - news, articles, forums and more.
- OLEDToday.com - Daily News, Gallery, and Patent Watch for OLEDs
- CDT achieves blue lifetime of 70,000 hours
- Samsung announces 21" OLED 01/05
- Samsung announces 'largest ever' organic LED display (ZDNet uk, 21 May 2004)
- On line Forum to Discuss about Organic device such as OLED
- Overview of OLED Display Technology
- Infos and News about the OLED/Pled Display-Technology
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