Organic lightemitting 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 less costly to fabricate than traditional LEDs. When the emissive electroluminescent 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 may also be used in 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 is that OLEDs do not require a backlight to function. This means that they draw far less power, can last longer on the same battery charge, and be of use with small portable devices which have mostly used monochrome low-resolution displays to conserve power.
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 refers 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 diodes (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 inkjet printing. This means that PLED displays can be made in a very flexible and inexpensive 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 SM-OLEDs.
How OLED works
An OLED works on the principle of electroluminescence. The key to the operation of an OLED is an organic luminophore. An exciton, which consists of a bound, excited electron and hole pair, is generated inside the emissive layer. When the exciton's electron and hole combine, a photon can be emitted. A major challenge in OLED manufacture is tuning the device such that an equal number of holes and electrons meet in the emissive layer. This is difficult because, in an organic compound, the mobility of a hole is much lower than that of an electron.
An exciton can be in one of two states, singlet or triplet. Only one in four excitons is a singlet. The materials currently employed in the emissive layer are typically fluorophors, which can only emit light when a singlet exciton forms, which reduces the OLED's efficiency.
Luckily, by incorporating transition metals into a small-molecule OLED, the triplet and singlet states can be mixed by spin-orbit coupling, which leads to emission from the triplet state. However, this emission is always red-shifted, making blue light more difficult to achieve from a triplet excited state. It is pointed out that triplet emitters can have a four times higher OLED efficiency (see ref. 1).
To create the excitons, a thin film of the luminophore 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. The electron and hole move into the emissive layer and can meet to form an exciton. (Mechanisms and details of exciton formation are discussed in ref.s 1 and 2)
Derivatives of PPV, poly(p-phenylene vinylene) and poly(fluorene), are commonly used as polymer luminophores in OLEDs. Indium tin oxide is a common transparent anode, while aluminium or calcium are common cathode materials. Other materials are added between the emissive layer and the cathode or the anode to facilitate or hinder hole or electron injection, thereby enhancing the OLED efficiency. 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 inkjet 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 are 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 has been the limited lifetime of the devices. Red and green OLED elements already had lifetimes of well over 20,000 hours but blue OLED lifetimes had lagged significantly behind. However, in May 2005, Cambridge Display Technology announced a blue OLED with a lifetime of over 100,000 hours and 100 cd/m². Unfortunately, according to the CDT press release "Lifetimes for devices made using the new blue materials at 200cd/m², 300cd/m² and 400cd/m² are greater than 25,000 hours, 10,000 hours and 6,000 hours respectively". 400 cd/m² is the brightness of LCD panels currently on the market.
According to Kodak, which is developing small molecule OLED, lifetime problems are not so significant for that type of OLED, mainly as a result of doping the base material of the OLEDs, which, they claim, 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. One of these is intrusion of water into displays which damages and destroys the organics. Therefore, improved sealing processes are important for practical manufacturing. Also, efficient outcoupling of waveguided light within the substrates is an area of continued research.
Commercial development of the technology is also restrained by patents held by Kodak and other firms, requiring other companies to acquire a license. In the past, many display technologies have become widespread only once the patents had expired; aperture grille CRT is a classic example.
Many proponents and investors in the burgeoning field of OLED research and development are optimistic regarding the technology because it offers the potential to revolutionize the flat-panel display (FPD) industry, and therefore change how and where people can watch television or use computers.
OLED technology is already finding commercial applications as diverse as heads-up displays in aircraft, displays inside high-end sports cars, in head-mounted displays and even as a replacement for lightbulbs. More speculative uses include ideas as varied as clothing that incorporates flexible OLED screens in order to change its color at the click of a button, or high definition virtual reality rooms where OLED screens cover every surface.
According to data compiled by the Society for Information Display: in 2003, the world OLED market was only $251 million. As of 2004, the world-wide OLED market was approximately $408 million. By 2008, experts are unsure exactly how fast it will have grown - conservative estimates are as low as $3 billion while other industry analysts feel it could reach as high as $8 billion. If such a high number of sales are reached, it will have a substantial economic impact for developers and vendors of LCD displays and CRT displays.
- 1) Hartmut Yersin, Triplet emitters for OLEDs. Introduction to exciton formation, charge transfer states, and triplet harvesting.
- 2) H. Yersin, Triplet emitters for OLED applications. Mechanisms of exciton trapping and control of emission properties. Top. Curr. Chem. 241, page 1 (2004). ISSN: 0340-1022.
- 3) Howard, Webster E., Better Displays with Organic Films. Scientific American, 290(2), page 76 (2004).
- 4) Shinar, Joseph (Ed.), Organic Light-Emitting Devices: A Survey. NY: Springer-Verlag (2004). ISBN 0-387-95343-4.
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