OLED - Organic Light Emitting Diode

Organic light-emitting diode

From Wikipedia, the free encyclopedia (Thanks very much Wikpedia)

I have reproduced this excellent article for reference from my previous post Plasma Screens Are Bad For The Environment.

An organic light-emitting diode (OLED) is any light-emitting diode (LED) whose emissive electroluminescent layer comprises a film of organic compounds. The layer usually contains a polymer substance that allows suitable organic compounds to be deposited. They are deposited in rows and columns onto a flat carrier by a simple "printing" process. The resulting matrix of pixels can emit light of different colors.

Such systems can be used in television screens, computer displays, portable system screens, advertising, information and indication. OLEDs can also be used in light sources for general space illumination, and large area light-emitting elements. OLEDs typically emit less light per area than inorganic solid-state based LEDs which are usually designed for use as point light sources.

A great benefit of OLED displays over traditional liquid crystal displays (LCDs) is that OLEDs do not require a backlight to function. Thus they draw far less power and, when powered from a battery, can operate longer on the same charge. OLED-based display devices also can be more effectively manufactured than LCDs and plasma displays. But degradation of OLED materials has limited the use of these materials. See Drawbacks.

OLED technology was also called Organic Electro-Luminescence (OEL), before the term "OLED" became standard.


History

Bernanose and coworkers first produced electroluminescence in organic materials by applying a high-voltage alternating current (AC) field to crystalline thin films of acridine orange and quinacrine.[1][2][3][4] In 1960, researchers at Dow Chemical developed AC-driven electroluminescent cells using doped anthracene. [5]

The low electrical conductivity of such materials limited light output until more conductive organic materials became available, especially the polyacetylene, polypyrrole, and polyaniline "Blacks". In a 1963 series of papers, Weiss et al. first reported high conductivity in iodine-"doped" oxidized polypyrrole.[6] They achieved a conductivity of 1 S/cm. Unfortunately, this discovery was "lost", as was a 1974 report[7] of a melanin-based bistable switch with a high conductivity "ON" state. This material emitted a flash of light when it switched.

In a subsequent 1977 paper, Shirakawa et al. reported high conductivity in similarly oxidized and iodine-doped polyacetylene. [8] Heeger, MacDiarmid & Shirakawa received the 2000 Nobel Prize in Chemistry for "The discovery and development of conductive organic polymers". The Nobel citation made no reference to the earlier discoveries.[citation needed]

Modern work with electroluminescence in such polymers culminated with Burroughs et al. 1990 paper in the journal Nature reporting a very high efficiency green-light-emitting polymer. [9] The OLED timeline since 1996 is well documented on oled-info.com site.[10]

Related technologies

Small molecules

Small-molecule OLED technology was developed by Eastman Kodak Company. 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.

Molecules commonly used in OLEDs include organo-metallic chelates (for example Alq3, used in the first organic light emitting device[11]) and conjugated dendrimers.

Recently a 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.


Polymer light-emitting diodes (PLED) involve an electroluminescent conductive polymer that emits light when subjected to an electric current. Developed by Cambridge Display Technology, they are also known as Light-Emitting Polymers (LEP). They are used as a thin film for full-spectrum color displays and require a relatively small amount of power for the light produced. No vacuum is required, and the emissive materials can be applied on the substrate by a technique derived from commercial inkjet printing.[12][13] The substrate used can be flexible, such as PET.[14] Thus, flexible PLED Displays may be produced inexpensively.

Typical polymers used in PLED displays include derivatives of poly(p-phenylene vinylene) and poly(fluorene). Substitution of side chains onto the polymer backbone may determine the color of emitted light[15] or the stability and solubility of the polymer for performance and ease of processing.[16]

TOLED

Transparent organic light-emitting device (TOLED) uses a proprietary transparent contact to create displays that can be made to be top-only emitting, bottom-only emitting, or both top and bottom emitting (transparent). TOLEDs can greatly improve contrast, making it much easier to view displays in bright sunlight.

SOLED

Stacked OLED (SOLED) uses a novel pixel architecture that is based on stacking the red, green, and blue subpixels on top of one another instead of next to one another as is commonly done in CRTs and LCDs. This improves display resolution up to threefold and enhances full-color quality.

Working principle

An OLED is composed of an emissive layer, a conductive layer, a substrate, and anode and cathode terminals. The layers are made of special organic polymer molecules that conduct electricity. Their levels of conductivity range from those of insulators to those of conductors, and so they are called organic semiconductors.
OLED schematic - 1. Cathode (-), 2. Emissive Layer, 3. Emission of radiation, 4 . Conductive Layer, 5. Anode (+)
OLED schematic - 1. Cathode (-), 2. Emissive Layer, 3. Emission of radiation, 4 . Conductive Layer, 5. Anode (+)

A voltage is applied across the OLED such that the anode is positive with respect to the cathode. This causes a current of electrons to flow through the device from cathode to anode. Thus, the cathode gives electrons to the emissive layer and the anode withdraws electrons from the conductive layer; in other words, the anode gives electron holes to the conductive layer.

Soon, the emissive layer becomes negatively charged, while the conductive layer becomes rich in positively charged holes. Electrostatic forces bring the electrons and the holes towards each other and recombine. This happens closer to the emissive layer, because in organic semiconductors holes are more mobile than electrons, (unlike in inorganic semiconductors). The recombination causes a drop in the energy levels of electrons, accompanied by an emission of radiation whose frequency is in the visible region. That is why this layer is called emissive.

The device does not work when the anode is put at a negative potential with respect to the cathode. In this condition, holes move to the anode and electrons to the cathode, so they are moving away from each other and do not recombine.

Indium tin oxide is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the polymer layer. Metals such as aluminium and calcium are often used for the cathode as they have low work functions which promote injection of electrons into the polymer layer.[17]

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 or even screen printing[18] technologies, they can theoretically have a significantly lower cost than LCDs or plasma displays. Printed OLEDs onto flexible substrates opens the door to new applications such as roll-up displays and displays embedded in clothing.

OLEDs enable a greater range of colors, brightness, and viewing angle than LCDs, because OLED pixels directly emit light. OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90 degrees from normal. LCDs use a backlight and cannot show true black, while an "off" OLED element produces no light and consumes no power. Energy is also wasted in LCDs because they require polarizers which filter out about half of the light emitted by the backlight. Additionally, color filters in color LCDs filter out two-thirds of the light.

OLEDs also have a faster response time than standard LCD screens. Whereas a standard LCD has around 10ms response time, an OLED can have less than 0.01ms response time. [19]

Drawbacks

The biggest technical problem for OLEDs is the limited lifetime of the organic materials. In particular, blue OLEDs typically have lifetimes of around 5,000 hours when used for flat panel displays, which is lower than typical lifetimes of LCD or Plasma technology. But recent experiments have shown that it is 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 20,000 hours for blue PHOLEDs. [20]

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.[citation needed] In the past, many display technologies have become widespread only once the patents had expired; a classic example is aperture grille Cathode ray tube. [21]

Technology demos

At the Las Vegas CES 2007 Summit Sony showcased 11 inch (28 cm, resolution 1,024 x 600) and 27 inch (68.5 cm, full HD resolution at 1920 x 1080) models claiming million-to-one contrast ratio and total thickness (including bezels) of 5 mm. According to news reports, Sony plans to begin releasing TVs this year.[22][23]

The upcoming Optimus Maximus keyboard,developed by Art Lebedev Studios, will use 113 48x48 pixel OLEDs (10.1×10.1 mm) for its keys. The keys will allow for full keyboard customization.

Sony plans to begin manufacturing just 1000 11 inch OLED TVs per month, and then see how the business develops from there.[24]

On May 25th, 2007, Sony publicly unveiled a video of a 2.5 inch flexible OLED screen which is only 0.3 millimeters thick.[25] The screen displayed images of a bicyclist stuntman and a picturesque lake while being bent.[26]

Commercial uses

OLED technology is used in commercial applications such as small screens for mobile phones and portable digital audio players (MP3 players), car radios, digital cameras and high-resolution microdisplays for head-mounted displays. Such portable applications favor the high light output of OLEDs for readability in sunlight, and their low power drain. Portable displays are also used intermittently, so the lower lifespan of OLEDs is less important here. Prototypes have been made of flexible and rollable displays which use unique OLEDs characteristics. OLEDs have been found in models of the Sony Walkman and some Sony Ericsson phones, notably the Z610i, as well as most Motorola color cell phones.

OLEDs could also be used as solid-state light sources. OLED efficacies and lifetime already exceed those of Incandescent light bulbs, and OLEDs are investigated worldwide as source for general illumination; an example is the EU OLLA project[27]).

eMagin Corporation is the only manufacturer of active matrix OLED-on-silicon displays. These are currently being developed for the US military, the medical field and the future of entertainment where an individual can immerse themselves in a movie or a video game. Laviva Online Store

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