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. OLED technology is intended primarily as picture elements in practical display devices. These devices promise to be much less costly to fabricate than traditional LCD displays. When the emissive electroluminescent layer is polymeric, varying amounts of OLEDs can be deposited in rows and columns 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 and, when powered from a battery, can operate longer on the same charge.

Small Molecules and Polymers

The largest OLED display prototype as of May 2005, at 40 inches.

Small-molecule OLED technology was developed by Eastman-Kodak. The production of small-molecule displays requires vacuum deposition which makes the production process more expensive than other processing techniques (see below). Since this is typically carried out on glass substrates, these displays are also not flexible, though this limitation is not inherent to small molecule organic materials. 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). 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.

Functionality

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 an electron is much lower than that of a hole.

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 redshifted, making blue light more difficult to achieve from a triplet excited state. It is pointed out that triplet emitters can be four times more efficient than OLED technology Hartmut Yersin, Triplet emitters for OLEDs. Introduction to exciton formation, charge transfer states, and triplet harvesting.

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. 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 and H. Yersin, Triplet emitters for OLED applications. Mechanisms of exciton trapping and control of emission properties. Top. Curr. Chem. 241,.

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 materialsOD Software Incorporated - Material Knowledge Base 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.

Advantages

The radically different manufacturing process of OLEDs lends itself to many advantages over flat panel displays made with LCD technology. Since OLEDs can be printed onto any suitable substrate using inkjet printer technology, they can theoretically have a significantly lower cost than 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 even displays embedded in clothing.

The range of colors, brightness, and viewing angle possible with OLEDs are greater than that of LCDs because OLED pixels directly emit light. LCDs employ a backlight and are incapable of showing true black, while an "off" OLED element produces no light and consumes no power. In LCDs, energy is also wasted because the liquid crystal acts as a Polarizer which filters out about half of the light emitted by the backlight.

Drawbacks

The biggest technical problem left to overcome has been the limited lifetime of the organic materials. Particularly, blue OLEDs typically have lifetimes of around 1,000 hours when used for flat panel displays, which is lower than typical lifetimes of LCD or Plasma technology. However, recent experimentation has shown that it's possible to swap the chemical component for a phosphorescent one, if the subtle differences in energy transitions are accounted for, resulting in lifetimes of up to 15,000 hours for blue PHOLEDs.

Also, the intrusion of water into displays can damage or destroy the organic materials. Therefore, improved sealing processes are important for practical manufacturing and may limit the longevity of more flexible displays.

Commercial development of the technology is also restrained by patents held by Eastman 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.

Commercial Uses

OLED technology is being used in commercial applications such as small screens for mobile phones and portable digital music players (mp3 players), car radios and digital cameras and also in high resolution microdisplays for head-mounted displays. Also, prototypes have been made of flexible and rollable displays which take advantage of OLEDs unique characteristics.

OLEDs could also be used as solid state light sources. As by now the OLED efficacies and lifetime already go beyond those of tungsten bulbs, white OLEDs are under worldwide investigation as source for general illumination (e.g. the EU OLLA project[1]).

References

* Shinar, Joseph (Ed.), Organic Light-Emitting Devices: A Survey. NY: Springer-Verlag (2004). ISBN 0387953434.

External links

News & Information Sources

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News Stories, Information Articles, Announcements (not chronologically ordered)

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*Scientific American Magazine (February 2004 Issue) Better Displays with Organic Films

Organic light-emitting diode: Encyclopedia BETA

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