Organic light-emitting diode

An organic light-emitting diode (OLED), also Light Emitting Polymer (LEP) and Organic Electro-Luminescence (OEL), is any light-emitting diode (LED) whose emissive electroluminescent layer is composed of 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 significant 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.

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History

Bernanose and co-workers first produced electroluminescence in organic materials in the early 1950s 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][7][8] They achieved a conductivity of 1 S/cm. Unfortunately, this discovery was "lost"^[clarify], as was a 1974 report^[9] 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.^[10] 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.^[11]

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.^[12] The OLED timeline since 1996 is well documented on oled-info.com site.^[13]

Related technologies

Small molecules

OLED technology was first developed at Eastman Kodak Company by Dr. Ching Tang using Small-molecules. 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^[14]) 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.

PLED

Polymer light-emitting diodes (PLED), also Light-Emitting Polymers (LEP) involve an electroluminescent conductive polymer that emits light when subjected to an electric current^[15]. 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.^[16][17] The substrate used can be flexible, such as PET.^[18] 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^[19] or the stability and solubility of the polymer for performance and ease of processing.^[20]

POLED

Patternable organic light-emitting device (POLED) uses a light or heat activated electroactive layer. A latent material (PEDOT-TMA) is included in this layer that, upon activation, becomes highly efficient as a hole injection layer. Using this process, light-emitting devices with arbitrary patterns can be prepared^[21].

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.

IOLED

Inverted OLED (IOLED) uses a bottom cathode that can be connected to the drain end of n-channel TFT especially for the low cost a-Si TFT backplane useful in manufacturing of AMOLED display.^[22] In contrast to a conventional OLED which anode is placed on the substrate.

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.

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.^[23]

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 technologies,^[24] they can theoretically have a significantly lower cost than LCDs or plasma displays. Printing 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 currently has an average of 8-12 millisecond response time, an OLED can have less than 0.01ms response time. ^[25]

Disadvantages

The biggest technical problem for OLEDs is the limited lifetime of the organic materials. In particular, blue OLEDs historically have had a lifetime of around 5,000 hours when used for flat-panel displays, which is lower than typical lifetime of LCD, LED or PDP technology – each currently rated for about 60,000 hours, depending on manufacturer and model. But in 2007, experimental PLEDs were created which can sustain 400cd/sq.m of output for over 198,000 hours.^[26]

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.^[27] In the past, many display technologies have become widespread only once the patents have expired; a classic example is the aperture grille CRT.^[28]

Technology demos

At the Las Vegas CES 2007, Sony showcased 11-inch (28 cm, resolution 1,024×600) and 27-inch (68.5 cm, full HD resolution at 1920×1080) models claiming million-to-one contrast ratio and total thickness (including bezels) of 5 mm. Sony plans on releasing a commercial version of this television in Japan in December, 2007.^[29]

The Optimus Maximus keyboard currently in development by the Art. Lebedev Studio is expected to use 113 48×48-pixel OLEDs (10.1×10.1 mm) for its keys.

Sony plans to begin manufacturing 1000 11-inch OLED TVs per month for market testing purposes.^[30]

On October 1 2007, Sony announced it will sell 11-Inch OLED TVs for 200,000 yen (1714 USD) from December 2007, only in Japan ^[31] and with an initial production of 2000 units per month.

On May 25 2007, Sony publicly unveiled a video of a 2.5-inch flexible OLED screen which is only 0.3 millimeters thick.^[32] The screen displayed images of a bicycle stunt and a picturesque lake while the screen was flexed.

OLEDs can be used in High-Resolution Holography (Volumetric Display). Professor Orbit showed on May 12 2007, EXPO Lisbon the potential application of these materials to reproduce three-dimensional video.^{[citation needed]}

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.^[33]

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 OLED's unique characteristics. OLEDs have been used in most Motorola and Samsung color cell phones, as well as some Sony Ericsson phones, notably the Z610i, and some models of the Sony Walkman^[34] it is also found in the Creative Zen V/V Plus series of MP3 players. Nokia has also introduced recently some OLED products: Nokia 7900 Prism and Nokia 8800 Arte.

On October 1st, 2007, Sony became the first company to announce an OLED television, which will be released in Japan in December 2007.^[35]

Newer OLED applications include signs and outdoor lighting, from companies such as CeeLite.^[36]

Samsung will unveil a 31-inch OLED TV at the January CES in Las Vegas and is promising much larger screens to come. “We have the technological ability to make 40-inch OLED,” said a spokesman, before adding that it won’t be until 2010 that the company will be in a position to mass produce such panels.

See also

References

Further reading

• Shinar, Joseph (Ed.), Organic Light-Emitting Devices: A Survey. NY: Springer-Verlag (2004). ISBN 0-387-95343-4.
• Yersin, Hartmut (Ed.), Highly Efficient OLEDs with Phosphorescent Materials. Wiley-VCH (2007). ISBN 3-527-40594-1