Emissive layer

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, small, portable system screens such as cell phones and PDAs, 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. Because there is no need for a backlight, an OLED display can be much thinner than an LCD panel. OLED-based display devices also can be more effectively manufactured than LCDs and plasma displays. However, degradation of OLED materials has limited their use.

History

A. Bernanose and co-workers at Université de Nancy, France, 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. In 1960, researchers at Dow Chemical developed AC-driven electroluminescent cells using doped anthracene.

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. They achieved a conductivity of 1 S/cm. Unfortunately, this discovery was "lost" , as was a 1974 report 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, Hideki Shirakawa et al. reported high conductivity in similarly oxidized and iodine-doped polyacetylene. Alan J. Heeger, Alan G. MacDiarmid & Hideki 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.

The first diode device was invented at Eastman Kodak by Dr. Ching Tang and Steven Van Slyke in the 1980s. This diode, giving rise to the term "OLED" used a novel two-layer structure with separate hole transporting and electron transporting layers such that recombination and light emission occurred in the middle of the organic layer. This resulted in a reduction in operating voltage and improvements in efficiency, and started the current era of OLED research and device production.

Later, this concept was adapted for use with polymers culminated in the Burroughs et al. 1990 paper in the journal Nature reporting a very-high-efficiency green-light-emitting polymer.

Working principle

A typical OLED is composed of an emissive layer, a conductive layer, a substrate, and anode and cathode terminals. The layers are made of special organic molecules that conduct electricity. Their levels of conductivity range from those of insulators to those of conductors, and so they are called organic semiconductors.

The first, most basic OLEDs consisted of a single organic layer, for example the first light-emitting polymer device synthesised by Burroughs et al. involved a single layer of poly(p-phenylene vinylene). Multilayer OLEDs can have more than two layers to improve device efficiency. As well as conductive properties, layers may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile, or block a charge from reaching the opposite electrode and being wasted.

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 they 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.

Just like passive-matrix LCD versus active-matrix LCD, OLEDs can be categorized into passive-matrix and active-matrix displays. Active-matrix OLEDs (AMOLED) require a thin film transistor backplane to switch the individual pixel on or off, and can make higher resolution and larger size displays possible.

Material technologies

Small molecules

OLED technology was first developed at Eastman Kodak Company by Dr. Ching W. Tang using small molecules. The production of small-molecule displays often involves 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) 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

Polymer light-emitting diodes (PLED), also light-emitting polymers (LEP), involve an electroluminescent conductive polymer that emits light when connected to an external voltage source. 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. The substrate used can be flexible, such as PET. Thus flexible PLED displays, also called Flexible OLED (FOLED), may be produced inexpensively.

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

Phosphorescent OLED

Phosphorescent OLED (PHOLED) uses the principle of electrophosphorescence to convert electrical energy in an OLED into light in a highly efficient manner.

Patterning technologies

Patternable OLED

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.

Inkjet

See "PLED" section.

Laser patterning

Color patterning by means of laser, such as Radiation-Induced Sublimation Transfer (RIST).

Backplane technologies

For a high resolution display like a TV, a TFT backplane is necessary to drive the pixels correctly. Currently LTPS-TFT (low temperature poly silicon) is used for commercial AMOLED displays. LTPS-TFT has variation of the performance in a display, so various compensation circuits have been reported. Due to the size limitation of the excimer laser used for LTPS, the AMOLED size was limited. To cope with the hurdle related to the panel size, amorphous-silicon/microcrystalline-silicon backplanes have been reported with large display prototype demonstrations.

OLED Structures

Bottom emission/Top emission

Bottom emission uses a transparent or semi-transparent bottom electrode to get the light through a transparent substrate. Top emission uses a transparent or semi-transparent top electrode to get the light through the counter substrate.

Transparent OLED

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. This technology is used in Head-up displays.

Stacked OLED

Stacked OLED (SOLED) uses a pixel architecture that stacks 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.

Inverted OLED

In contrast to a conventional OLED, in which the anode is placed on the substrate, an Inverted OLED (IOLED) uses a bottom cathode that can be connected to the drain end of an n-channel TFT especially for the low cost amorphous silicon TFT backplane useful in the manufacturing of AMOLED displays.

  • - AM OLED = Active Matrix OLED device
  • - FOLED = Flexible Organic Light Emitting Diode (UDC)
  • - OLED = Organic Light Emitting Diode/Device/Display
  • - PhOLED = Phosphorescent Organic Light Emitting Diode (UDC)
  • - PLED = Polymer Light Emitting Diode (CDT)
  • - PM OLED = Passive Matrix OLED device
  • - POLED = Polymer Organic Light Emitting Diode (CDT)
  • - RCOLED = Resonant Color Organic Light Emitting Diode
  • - SmOLED = Small Molecule Organic Light Emitting Diode (Kodak)
  • - SOLED = Stacked Organic Light Emitting Diode (UDC)
  • - TOLED = Transparent Organic Light Emitting Diode (UDC)

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 an inkjet printer or even screen printing technologies, 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 fabrics or 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 4-8 millisecond response time, an OLED can have less than 0.01ms response time.

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 14,000 hours (5 years at 8 hours a day) 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. Toshiba and Panasonic have come up with a way to solve this problem with a new technology that can double the lifespan of OLED displays, pushing its expected life past that of LCD displays. A metal membrane helps deliver light from polymers in the substrate throughout the glass surface more efficiently than current OLEDs. The result is the same picture quality with half the brightness and a doubling of the screen's expected life.

In 2007, experimental PLEDs were created which can sustain 400 cd/m² of luminance for over 198,000 hours for green OLEDs and 62,000 hours for blue OLEDs.

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.

Technology demos

Sony applications

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 released a commercial version of this television in Japan in December, 2007.

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

Sony has begun selling an 11-inch OLED Digital TV (XEL-1) for $2499.99 CAN

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

On October 1 2007, Sony announced it will sell 11-Inch OLED TVs for 200,000 yen (1,962.51 USD as of 4/1/08) from December 2007, only in Japan and with an initial production of 2000 units per month.

On April 16 2008, at "Display 2008", Sony showed a 0.2 mm (0.0079 inch) thick 3.5 inch display with a resolution of 320x200 pixels and a 0.3 mm thick 11 inch display with 960x540 pixels resolution. That's one-tenth the thickness of the XEL-1 (which is also 11 inch and the same resolution).

On July 10 2008, the Japanese government announced that it was backing Sony, Toshiba, Sharp, Matsushita Electric and some other companies in joint research of OLED TV Panels.

An agency set up for encouraging research, The New Energy and Industrial Technology Development Organization, or NEDO, said it had chosen the companies with the aim of developing a 40-inch OLED display sometime after 2015.

Other companies

The Optimus Maximus keyboard developed by the Art. Lebedev Studio and released early 2008 uses 113 48×48-pixel OLEDs (10.1×10.1 mm) for its keys.

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.

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 a source of general illumination; an example is the EU OLLA project.

On March 11, 2008 GE Global Research demonstrated the first successful roll-to-roll manufactured OLED, marking a major milestone towards cost effective production of commercial OLED technology. The four year, $13 million research project was carried out by GE Global Research, Energy Conversion Devices, Inc and the U.S. Commerce Department’s National Institute of Standards and Technology (NIST).

Chi Mei EL Corp of Tainan, Taiwan, demonstrated a 25" Low-Temperature Polycrystalline silicon Active Matrix OLED at the Society of Information Displays (SID) conference in Los Angeles, CA, USA on May 20–22, 2008.

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. It is also found in the Creative Zen V/V Plus series of MP3 players. Nokia has also introduced recently some OLED products, including the 7900 Prism and Nokia 8800 Arte.

On August 26, 2008 Mobile phone maker Nokia has introduced a new member to its famous Nseries lineup, the N85. Nokia N85 will pack a 2.6” AMOLED display, which will be the largest of its kind among GSM phones.

On October 1, 2007, Sony became the first company to announce an OLED television.The XEL-1 11" OLED (Organic Light Emitting Diode) Digital Television sells for $2,499.99 in Canada (as of April 24, 2008). They are available in the United States at select Sony Style stores for US$2,499.99.

Newer OLED applications include signs and lighting.

Use of OLEDs may be subject to patents held by Eastman Kodak and others. Kodak has licensed its patents to other firms for commercialization.

Manufacture

Toshiba Matsushita

According to reports by Bloomberg.com, Toshiba Matsushita Display Technology Co. has announced that it intends to produce a million 2.5 inch OEL panels per month by Fall 2009. The screens will be produced for use in cellphones, GPS navigation systems, and other assorted mobile devices. The news broke after Japan's Nikkei newspaper reported that the Toshiba/Matsushita joint-venture have begun building OEL production lines in their Ishikawa Prefecture factories.

Although Nikkei did not list a source for the announcement, it seems fairly certain that OEL technology is growing ever closer to mainlined viability. As it stands, Toshiba Matsushita Display Technology Co. will be among the first Japanese companies to mass produce OEL panels. While their initial run will be limited only to mobile sized screens, the process will presumably garner a refined, more cost effective means of production, which may lead to mainstream availability of larger OEL screens.

See also

References

Further reading

  • Shinar, Joseph (Ed.), Organic Light-Emitting Devices: A Survey. NY: Springer-Verlag (2004). ISBN 0-387-95343-4.
  • Hari Singh Nalwa (Ed.), Handbook of Luminescence, Display Materials and Devices, Volume 1-3. American Scientific Publishers, Los Angeles (2003). ISBN 1-58883-010-1. Volume 1: Organic Light-Emitting Diodes
  • Hari Singh Nalwa (Ed.), Handbook of Organic Electronics and Photonics, Volume 1-3. American Scientific Publishers, Los Angeles (2008). ISBN 1-58883-095-0.
  • Yersin, Hartmut (Ed.), Highly Efficient OLEDs with Phosphorescent Materials. Wiley-VCH (2007). ISBN 3-527-40594-1
  • Müllen, Klaus (Ed.), Organic Light Emitting Devices: Synthesis, Properties and Applications. Wiley-VCH (2006). ISBN 3-527-31218-8

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