Organic light-emitting diode

Prototype OLED lighting panels
Type	LED

An organic light-emitting diode (OLED), also known as organic electroluminescent (organic EL) diode,^[1][2] is a type of light-emitting diode (LED) in which the emissive electroluminescent layer is an organic compound film that emits light in response to an electric current. This organic layer is situated between two electrodes; typically, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, and portable systems such as smartphones and handheld game consoles. A major area of research is the development of white OLED devices for use in solid-state lighting applications.^[3][4][5]

There are two main families of OLED: those based on small molecules and those employing polymers. Adding mobile ions to an OLED creates a light-emitting electrochemical cell (LEC) which has a slightly different mode of operation. An OLED display can be driven with a passive-matrix (PMOLED) or active-matrix (AMOLED) control scheme. In the PMOLED scheme, each row and line in the display is controlled sequentially, one by one,^[6] whereas AMOLED control uses a thin-film transistor (TFT) backplane to directly access and switch each individual pixel on or off, allowing for higher resolution and larger display sizes.

OLED is fundamentally different from LED, which is based on a p-n diode structure. In LEDs, doping is used to create p- and n-regions by changing the conductivity of the host semiconductor. OLEDs do not employ a p-n structure. Doping of OLEDs is used to increase radiative efficiency by direct modification of the quantum-mechanical optical recombination rate. Doping is additionally used to determine the wavelength of photon emission.^[7]

An OLED display works without a backlight because it emits its own visible light. Thus, it can display deep black levels and can be thinner and lighter than a liquid crystal display (LCD). In low ambient light conditions (such as a dark room), an OLED screen can achieve a higher contrast ratio than an LCD, regardless of whether the LCD uses cold cathode fluorescent lamps or an LED backlight. OLED displays are made in the same way as LCDs, including manufacturing of several displays on a mother substrate that is later thinned and cut into several displays and comes in the same sizes as those used for manufacturing LCDs, but after TFT (for active matrix displays), addressable grid (for passive matrix displays) or indium-tin oxide (ITO) segment (for segment displays) formation, the display is coated with hole injection, transport and blocking layers, as well with electroluminescent material after the first 2 layers, after which ITO or metal may be applied again as a cathode and later the entire stack of materials is encapsulated. The TFT layer, addressable grid or ITO segments serve as or are connected to the anode, which may be made of ITO or metal.^[8][9] OLEDs can be made flexible and transparent, with transparent displays being used in smartphones with optical fingerprint scanners and flexible displays being used in foldable smartphones.

History

André Bernanose and co-workers at the Nancy-Université in France made the first observations of electroluminescence in organic materials in the early 1950s. They applied high alternating voltages in air to materials such as acridine orange dye, either deposited on or dissolved in cellulose or cellophane thin films. The proposed mechanism was either direct excitation of the dye molecules or excitation of electrons.^{[10][11][12][13]}

In 1960, Martin Pope and some of his co-workers at New York University in the United States developed ohmic dark-injecting electrode contacts to organic crystals.^[14][15][16] They further described the necessary energetic requirements (work functions) for hole and electron injecting electrode contacts. These contacts are the basis of charge injection in all modern OLED devices. Pope's group also first observed direct current (DC) electroluminescence under vacuum on a single pure crystal of anthracene and on anthracene crystals doped with tetracene in 1963^[17] using a small area silver electrode at 400 volts. The proposed mechanism was field-accelerated electron excitation of molecular fluorescence.

Pope's group reported in 1965^[18] that in the absence of an external electric field, the electroluminescence in anthracene crystals is caused by the recombination of a thermalized electron and hole, and that the conducting level of anthracene is higher in energy than the exciton energy level. Also in 1965, Wolfgang Helfrich and W. G. Schneider of the National Research Council in Canada produced double injection recombination electroluminescence for the first time in an anthracene single crystal using hole and electron injecting electrodes,^[19] the forerunner of modern double-injection devices. In the same year, Dow Chemical researchers patented a method of preparing electroluminescent cells using high-voltage (500–1500 V) AC-driven (100–3000 Hz) electrically insulated one millimetre thin layers of a melted phosphor consisting of ground anthracene powder, tetracene, and graphite powder.^[20] Their proposed mechanism involved electronic excitation at the contacts between the graphite particles and the anthracene molecules.

The first Polymer LED (PLED) to be created was by Roger Partridge at the National Physical Laboratory in the United Kingdom. It used a film of poly(N-vinylcarbazole) up to 2.2 micrometers thick located between two charge-injecting electrodes. The light generated was readily visible in normal lighting conditions though the polymer used had 2 limitations; low conductivity and the difficulty of injecting electrons.^[21] Later development of conjugated polymers would allow others to largely eliminate these problems. His contribution has often been overlooked due to the secrecy NPL imposed on the project. When it was patented in 1974^[22] it was given a deliberately obscure "catch all" name while the government's Department for Industry tried and failed to find industrial collaborators to fund further development.^[23] As a result publication was delayed until 1983.^{[24][25][26][27]}

Working principle

Schematic of a bilayer OLED: 1. Cathode (−), 2. Emissive Layer, 3. Emission of radiation, 4. Conductive layer, 5. Anode (+)

A typical OLED is composed of a layer of organic materials situated between two electrodes, the anode and cathode, all deposited on a substrate. The organic molecules are electrically conductive as a result of delocalization of pi electrons caused by conjugation over part or all of the molecule. These materials have conductivity levels ranging from insulators to conductors, and are therefore considered organic semiconductors. The highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of organic semiconductors are analogous to the valence and conduction bands of inorganic semiconductors.^[41]

Originally, the most basic polymer OLEDs consisted of a single organic layer. One example was the first light-emitting device synthesised by J. H. Burroughes et al., which involved a single layer of poly(p-phenylene vinylene). However multilayer OLEDs can be fabricated with two or more layers in order to improve device efficiency. As well as conductive properties, different materials may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile,^[42] or block a charge from reaching the opposite electrode and being wasted.^[43] Many modern OLEDs incorporate a simple bilayer structure, consisting of a conductive layer and an emissive layer. Developments in OLED architecture in 2011 improved quantum efficiency (up to 19%) by using a graded heterojunction.^[44] In the graded heterojunction architecture, the composition of hole and electron-transport materials varies continuously within the emissive layer with a dopant emitter. The graded heterojunction architecture combines the benefits of both conventional architectures by improving charge injection while simultaneously balancing charge transport within the emissive region.^[45]

During operation, a voltage is applied across the OLED such that the anode is positive with respect to the cathode. Anodes are picked based upon the quality of their optical transparency, electrical conductivity, and chemical stability.^[46] A current of electrons flows through the device from cathode to anode, as electrons are injected into the LUMO of the organic layer at the cathode and withdrawn from the HOMO at the anode. This latter process may also be described as the injection of electron holes into the HOMO. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole. This happens closer to the electron-transport layer part of the emissive layer, because in organic semiconductors holes are generally more mobile than electrons. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of this radiation depends on the band gap of the material, in this case the difference in energy between the HOMO and LUMO.

As electrons and holes are fermions with half integer spin, an exciton may either be in a singlet state or a triplet state depending on how the spins of the electron and hole have been combined. Statistically three triplet excitons will be formed for each singlet exciton. Decay from triplet states (phosphorescence) is spin forbidden, increasing the timescale of the transition and limiting the internal efficiency of fluorescent devices. Phosphorescent organic light-emitting diodes make use of spin–orbit interactions to facilitate intersystem crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and improving the internal efficiency.

Indium tin oxide (ITO) 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 HOMO level of the organic layer. A second conductive (injection) layer is typically added, which may consist of PEDOT:PSS,^[47] as the HOMO level of this material generally lies between the work function of ITO and the HOMO of other commonly used polymers, reducing the energy barriers for hole injection. Metals such as barium and calcium are often used for the cathode as they have low work functions which promote injection of electrons into the LUMO of the organic layer.^[48] Such metals are reactive, so they require a capping layer of aluminium to avoid degradation. Two secondary benefits of the aluminum capping layer include robustness to electrical contacts and the back reflection of emitted light out to the transparent ITO layer.

Experimental research has proven that the properties of the anode, specifically the anode/hole transport layer (HTL) interface topography plays a major role in the efficiency, performance, and lifetime of organic light-emitting diodes. Imperfections in the surface of the anode decrease anode-organic film interface adhesion, increase electrical resistance, and allow for more frequent formation of non-emissive dark spots in the OLED material adversely affecting lifetime. Mechanisms to decrease anode roughness for ITO/glass substrates include the use of thin films and self-assembled monolayers. Also, alternative substrates and anode materials are being considered to increase OLED performance and lifetime. Possible examples include single crystal sapphire substrates treated with gold (Au) film anodes yielding lower work functions, operating voltages, electrical resistance values, and increasing lifetime of OLEDs.^[49]

Single carrier devices are typically used to study the kinetics and charge transport mechanisms of an organic material and can be useful when trying to study energy transfer processes. As current through the device is composed of only one type of charge carrier, either electrons or holes, recombination does not occur and no light is emitted. For example, electron only devices can be obtained by replacing ITO with a lower work function metal which increases the energy barrier of hole injection. Similarly, hole only devices can be made by using a cathode made solely of aluminium, resulting in an energy barrier too large for efficient electron injection.^[50][51][52]

Carrier balance

Balanced charge injection and transfer are required to get high internal efficiency, pure emission of luminance layer without contaminated emission from charge transporting layers, and high stability. A common way to balance charge is optimizing the thickness of the charge transporting layers but is hard to control. Another way is using the exciplex. Exciplex formed between hole-transporting (p-type) and electron-transporting (n-type) side chains to localize electron-hole pairs. Energy is then transferred to luminophore and provide high efficiency. An example of using exciplex is grafting Oxadiazole and carbazole side units in red diketopyrrolopyrrole-doped Copolymer main chain shows improved external quantum efficiency and color purity in no optimized OLED.^[53]

Material technologies

Small molecules

Alq₃,^[28] commonly used in small molecule OLEDs

Organic small-molecule electroluminescent materials have the advantages of a wide variety, easy to purify, and strong chemical modifications. In order to make the luminescent materials to emit light as required, some chromophores or unsaturated groups such as alkene bonds and benzene rings will usually be introduced in the molecular structure design to change the size of the conjugation range of the material, so that the photophysical properties of the material changes. In general, the larger the range of π-electron conjugation system, the longer the wavelength of light emitted by the material. For instance, with the increase of the number of benzene rings, the fluorescence emission peak of benzene, naphthalene, anthracene,^[54] and butyl gradually red-shifted from 283 nm to 480 nm. Common organic small molecule electroluminescent materials include aluminum complexes, anthracenes, biphenyl acetylene aryl derivatives, coumarin derivatives,^[55] and various fluorochromes. Efficient OLEDs using small molecules were first developed by Ching W. Tang et al.^[56] at Eastman Kodak. The term OLED traditionally refers specifically to this type of device, though the term SM-OLED is also in use.^[41]

Molecules commonly used in OLEDs include organometallic chelates (for example Alq₃, used in the organic light-emitting device reported by Tang et al.), fluorescent and phosphorescent dyes and conjugated dendrimers. A number of materials are used for their charge transport properties, for example triphenylamine and derivatives are commonly used as materials for hole transport layers.^[57] Fluorescent dyes can be chosen to obtain light emission at different wavelengths, and compounds such as perylene, rubrene and quinacridone derivatives are often used.^[58] Alq₃ has been used as a green emitter, electron transport material and as a host for yellow and red emitting dyes.

Because of the structural flexibility of small-molecule electroluminescent materials, thin films can be prepared by vacuum vapor deposition, which is more expensive and of limited use for large-area devices. The vacuum coating system, however, can make the entire process from film growth to OLED device preparation in a controlled and complete operating environment, helping to obtain uniform and stable films, thus ensuring the final fabrication of high-performance OLED devices.However, small molecule organic dyes are prone to fluorescence quenching^[59] in the solid state, resulting in lower luminescence efficiency. The doped OLED devices are also prone to crystallization, which reduces the luminescence and efficiency of the devices. Therefore, the development of devices based on small-molecule electroluminescent materials is limited by high manufacturing costs, poor stability, short life, and other shortcomings. Coherent emission from a laser dye-doped tandem SM-OLED device, excited in the pulsed regime, has been demonstrated.^[60] The emission is nearly diffraction limited with a spectral width similar to that of broadband dye lasers.^[61]

Researchers report luminescence from a single polymer molecule, representing the smallest possible organic light-emitting diode (OLED) device.^[62] Scientists will be able to optimize substances to produce more powerful light emissions. Finally, this work is a first step towards making molecule-sized components that combine electronic and optical properties. Similar components could form the basis of a molecular computer.^[63]

Polymer light-emitting diodes

poly(p-phenylene vinylene), used in the first PLED^[29]

Polymer light-emitting diodes (PLED, P-OLED), also light-emitting polymers (LEP), involve an electroluminescent conductive polymer that emits light when connected to an external voltage. They are used as a thin film for full-spectrum colour displays. Polymer OLEDs are quite efficient and require a relatively small amount of power for the amount of light produced.

Vacuum deposition is not a suitable method for forming thin films of polymers. If the polymeric OLED films are made by vacuum vapor deposition, the chain elements will be cut off and the original photophysical properties will be compromised. However, polymers can be processed in solution, and spin coating is a common method of depositing thin polymer films. This method is more suited to forming large-area films than thermal evaporation. No vacuum is required, and the emissive materials can also be applied on the substrate by a technique derived from commercial inkjet printing.^[64][65] However, as the application of subsequent layers tends to dissolve those already present, formation of multilayer structures is difficult with these methods. The metal cathode may still need to be deposited by thermal evaporation in vacuum. An alternative method to vacuum deposition is to deposit a Langmuir-Blodgett film.

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 colour of emitted light^[66] or the stability and solubility of the polymer for performance and ease of processing.^[67] While unsubstituted poly(p-phenylene vinylene) (PPV) is typically insoluble, a number of PPVs and related poly(naphthalene vinylene)s (PNVs) that are soluble in organic solvents or water have been prepared via ring opening metathesis polymerization.^[68][69][70] These water-soluble polymers or conjugated poly electrolytes (CPEs) also can be used as hole injection layers alone or in combination with nanoparticles like graphene.^[71]

Phosphorescent materials

Ir(mppy)₃, a phosphorescent dopant which emits green light^[72]

Main article: Phosphorescent organic light-emitting diode

Phosphorescent organic light-emitting diodes use the principle of electrophosphorescence to convert electrical energy in an OLED into light in a highly efficient manner,^[73][74] with the internal quantum efficiencies of such devices approaching 100%.^[75]

Typically, a polymer such as poly(N-vinylcarbazole) is used as a host material to which an organometallic complex is added as a dopant. Iridium complexes^[74] such as Ir(mppy)₃^[72] as of 2004 were a focus of research, although complexes based on other heavy metals such as platinum^[73] have also been used.

The heavy metal atom at the centre of these complexes exhibits strong spin-orbit coupling, facilitating intersystem crossing between singlet and triplet states. By using these phosphorescent materials, both singlet and triplet excitons will be able to decay radiatively, hence improving the internal quantum efficiency of the device compared to a standard OLED where only the singlet states will contribute to emission of light.

Applications of OLEDs in solid state lighting require the achievement of high brightness with good CIE coordinates (for white emission). The use of macromolecular species like polyhedral oligomeric silsesquioxanes (POSS) in conjunction with the use of phosphorescent species such as Ir for printed OLEDs have exhibited brightnesses as high as 10,000 cd/m².^[76]

Device architectures

Structure

Bottom emission

a) Bottom-emitting and b) top-emitting OLED structures; c,d) Schematic diagrams based on bottom-emitting and top-emitting OLEDs with low and high contrast ratio, respectively.

The bottom-emission organic light-emitting diode (BE-OLED) is the architecture that was used in the early-stage AMOLED displays. It had a transparent anode fabricated on a glass substrate, and a shiny reflective cathode. Light is emitted from the transparent anode direction. To reflect all the light towards the anode direction, a relatively thick metal cathode such as aluminum is used. For the anode, high-transparency indium tin oxide (ITO) was a typical choice to emit as much light as possible.^[77] Organic thin-films, including the emissive layer that actually generates the light, are then sandwiched between the ITO anode and the reflective metal cathode. The downside of bottom emission structure is that the light has to travel through the pixel drive circuits such as the thin film transistor (TFT) substrate, and the area from which light can be extracted is limited and the light emission efficiency is reduced.

Top emission

An alternative configuration is to switch the mode of emission. A reflective anode, and a transparent (or more often semi-transparent) cathode are used so that the light emits from the cathode side, and this configuration is called top-emission OLED (TE-OLED). Unlike BEOLEDs where the anode is made of transparent conductive ITO, this time the cathode needs to be transparent, and the ITO material is not an ideal choice for the cathode because of a damage issue due to the sputtering process.^[78] Thus, a thin metal film such as pure Ag and the Mg:Ag alloy are used for the semi-transparent cathode due to their high transmittance and high conductivity.^[79] In contrast to the bottom emission, light is extracted from the opposite side in top emission without the need of passing through multiple drive circuit layers. Thus, the light generated can be extracted more efficiently.

Improvements

Micro-cavity theory

Sony's Super Top Emission OLED technology enhances the color purity of emitted lights.

When light waves meet while traveling along the same medium, wave interference occurs. This interference can be constructive or destructive. It is sometimes desirable for several waves of the same frequency to sum up into a wave with higher amplitudes.

Since both electrodes are reflective in TEOLED, light reflections can happen within the diode, and they cause more complex interferences than those in BEOLEDs. In addition to the two-beam interference, there exists a multi-resonance interference between two electrodes. Because the structure of TEOLEDs is similar to that of the Fabry-Perot resonator or laser resonator, which contains two parallel mirrors comparable to the two reflective electrodes),^[80] this effect is especially strong in TEOLED. This two-beam interference and the Fabry-Perot interferences are the main factors in determining the output spectral intensity of OLED. This optical effect is called the "micro-cavity effect."

In the case of OLED, that means the cavity in a TEOLED could be especially designed to enhance the light output intensity and color purity with a narrow band of wavelengths, without consuming more power. In TEOLEDs, the microcavity effect commonly occurs, and when and how to restrain or make use of this effect is indispensable for device design. To match the conditions of constructive interference, different layer thicknesses are applied according to the resonance wavelength of that specific color. The thickness conditions are carefully designed and engineered according to the peak resonance emitting wavelengths of the blue (460 nm), green (530 nm), and red (610 nm) color LEDs. This technology greatly improves the light-emission efficiency of OLEDs, and are able to achieve a wider color gamut due to high color purity.

Color filters

In "white + color filter method", red, green, and blue emissions are obtained from the same white-light LEDs using different color filters.^[81] With this method, the OLED materials produce white light, which is then filtered to obtain the desired RGB colors. This method eliminated the need to deposit three different organic emissive materials, so only one kind of OLED material is used to produce white light. It also eliminated the uneven degradation rate of blue pixels vs. red and green pixels. Disadvantages of this method are low color purity and contrast. Also, the filters absorb most of the emitted light, requiring the background white light to be relatively strong to compensate for the drop in brightness, and thus the power consumption for such displays can be higher.

Color filters can also be implemented into bottom- and top-emission OLEDs. By adding the corresponding RGB color filters after the semi-transparent cathode, even purer wavelengths of light can be obtained. The use of a microcavity in top-emission OLEDs with color filters also contributes to an increase in the contrast ratio by reducing the reflection of incident ambient light.^[82] In a conventional panel, a circular polarizer was installed on the panel surface. While this was provided to prevent the reflection of ambient light, it also reduced the light output. By replacing this polarizing layer with color filters, the light intensity is not affected, and essentially all ambient reflected light can be cut, allowing a better contrast on the display panel. This potentially reduced the need for brighter pixels and can lower the power consumption.

Color patterning technologies

Thin-film transistor backplanes

For a high resolution display like a TV, a thin-film transistor (TFT) backplane is necessary to drive the pixels correctly. As of 2019, low-temperature polycrystalline silicon (LTPS) – TFT is widely used for commercial AMOLED displays. LTPS-TFT has variation of the performance in a display, so various compensation circuits have been reported.^[100] 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.^[101] An indium gallium zinc oxide (IGZO) backplane can also be used.

Advantages

The different manufacturing process of OLEDs has several advantages over flat panel displays made with LCD technology.

Lower cost in the future

OLEDs can be printed onto any suitable substrate by an inkjet printer or even by screen printing,^[102] theoretically making them cheaper to produce than LCD or plasma displays. However, fabrication of the OLED substrate as of 2018 is costlier than that for TFT LCDs.^[103] Roll-to-roll vapor-deposition methods for organic devices do allow mass production of thousands of devices per minute for minimal cost; however, this technique also induces problems: devices with multiple layers can be challenging to make because of registration — lining up the different printed layers to the required degree of accuracy.

Lightweight and flexible plastic substrates

OLED displays can be fabricated on flexible plastic substrates, leading to the possible fabrication of flexible organic light-emitting diodes for other new applications, such as roll-up displays embedded in fabrics or clothing. If a substrate like polyethylene terephthalate (PET)^[104] can be used, the displays may be produced inexpensively. Furthermore, plastic substrates are shatter-resistant, unlike the glass displays used in LCD devices.

Better picture quality

OLEDs enable a greater contrast ratio and wider viewing angle compared to LCDs, because OLED pixels emit light directly. This also provides a deeper black level, since a black OLED display emits no light. Furthermore, OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90° from the normal.

Better power efficiency

LCDs filter the light emitted from a backlight, allowing a small fraction of light through. Thus, they cannot show true black. However, an inactive OLED element does not produce light or consume power, allowing true blacks.^[105] Removing the backlight also makes OLEDs lighter because some substrates are not needed.

Response time

OLEDs also have a much faster response time than an LCD. Using response time compensation technologies, the fastest modern LCDs can reach response times as low as 1 ms for their fastest color transition, and are capable of refresh frequencies as high as 240 Hz. According to LG, OLED response times are up to 1,000 times faster than LCD,^[106] putting conservative estimates at under 10 μs (0.01 ms), which could theoretically accommodate refresh frequencies approaching 100 kHz (100,000 Hz). Due to their extremely fast response time, OLED displays can also be easily designed to be strobed, creating an effect similar to CRT flicker in order to avoid the sample-and-hold behavior seen on both LCDs and some OLED displays, which creates the perception of motion blur.^[107]

Disadvantages

Light-emitting polymer (LEP) display showing partial failure

An old OLED display showing wear

Manufacturers and commercial uses

Magnified image of the AMOLED screen on the Google Nexus One smartphone using the RGBG system of the PenTile Matrix Family

A 3.8 cm (1.5 in) OLED display from a Creative ZEN V media player

OLED lighting in a shopping mall in Aachen, Germany

Almost all OLED manufacturers rely on material deposition equipment that is only made by a handful of companies,^[139] the most notable one being Canon Tokki, a unit of Canon Inc. Canon Tokki is reported to have a near-monopoly of the giant OLED-manufacturing vacuum machines, notable for their 100-metre (330 ft) size.^[140] Apple has relied solely on Canon Tokki in its bid to introduce its own OLED displays for the iPhones released in 2017.^[141] The electroluminescent materials needed for OLEDs are also made by a handful of companies, some of them being Merck, Universal Display Corporation and LG Chem.^[142] The machines that apply these materials can operate continuously for 5–6 days, and can process a mother substrate in 5 minutes.^[143]

OLED technology is used in commercial applications such as displays for mobile phones and portable digital media players, car radios and digital cameras among others, as well as lighting.^[144] Such portable display 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 organic displays is less of an issue. Prototypes have been made of flexible and rollable displays which use OLEDs' unique characteristics. Applications in flexible signs and lighting are also being developed.^[145] OLED lighting offers several advantages over LED lighting, such as higher quality illumination, more diffuse light source, and panel shapes.^[144] Philips Lighting has made OLED lighting samples under the brand name "Lumiblade" available online^[146] and Novaled AG based in Dresden, Germany, introduced a line of OLED desk lamps called "Victory" in September, 2011.^[147]

Nokia introduced OLED mobile phones including the N85 and the N86 8MP, both of which feature an AMOLED display. OLEDs have also been used in most Motorola and Samsung color cell phones, as well as some HTC, LG and Sony Ericsson models.^[148] OLED technology can also be found in digital media players such as the Creative ZEN V, the iriver clix, the Zune HD and the Sony Walkman X Series.

The Google and HTC Nexus One smartphone includes an AMOLED screen, as does HTC's own Desire and Legend phones. However, due to supply shortages of the Samsung-produced displays, certain HTC models will use Sony's SLCD displays in the future,^[149] while the Google and Samsung Nexus S smartphone will use "Super Clear LCD" instead in some countries.^[150]

OLED displays were used in watches made by Fossil (JR-9465) and Diesel (DZ-7086). Other manufacturers of OLED panels include Anwell Technologies Limited (Hong Kong),^[151] AU Optronics (Taiwan),^[152] Chimei Innolux Corporation (Taiwan),^[153] LG (Korea),^[154] and others.^[155]

DuPont stated in a press release in May 2010, that they can produce a 50-inch OLED TV in two minutes with a new printing technology. If this can be scaled up in terms of manufacturing, then the total cost of OLED TVs would be greatly reduced. DuPont also states that OLED TVs made with this less expensive technology can last up to 15 years if left on for a normal eight-hour day.^[156][157]

The use of OLEDs may be subject to patents held by Universal Display Corporation, Eastman Kodak, DuPont, General Electric, Royal Philips Electronics, numerous universities and others.^[158] By 2008, thousands of patents associated with OLEDs, came from larger corporations and smaller technology companies.^[41]

Flexible OLED displays have been used by manufacturers to create curved displays such as the Galaxy S7 Edge but they were not in devices that can be flexed by the users.^[159] Samsung demonstrated a roll-out display in 2016.^[160]

Samsung foldable smartphones

On 31 October 2018, Royole, a Chinese electronics company, unveiled the world's first foldable screen phone featuring a flexible OLED display.^[161] On 20 February 2019, Samsung announced the Samsung Galaxy Fold with a foldable OLED display from Samsung Display, its majority-owned subsidiary.^[162] At MWC 2019 on 25 February 2019, Huawei announced the Huawei Mate X featuring a foldable OLED display from BOE.^[163][164]

The 2010s also saw the wide adoption of tracking gate-line in pixel (TGP), which moves the driving circuitry from the borders of the display to in between the display's pixels, allowing for narrow bezels.^[165]

In 2023 the German startup Inuru has announced to manufacture low-cost OLED with printing for packaging and fashion applications. ^[166]

Company-specific applications

Samsung

Samsung AMOLED displays

By 2004, Samsung Display, a subsidiary of South Korea's largest conglomerate and a former Samsung-NEC joint venture, was the world's largest OLED manufacturer, producing 40% of the OLED displays made in the world,^[172] and as of 2010, has a 98% share of the global AMOLED market.^[173] The company is leading the world of OLED industry, generating $100.2 million out of the total $475 million revenues in the global OLED market in 2006.^[174] As of 2006, it held more than 600 American patents and more than 2800 international patents, making it the largest owner of AMOLED technology patents.^[174]

Samsung SDI announced in 2005, the world's largest OLED TV at the time, at 21 inches (53 cm).^[175] This OLED featured the highest resolution at the time, of 6.22 million pixels. In addition, the company adopted active matrix-based technology for its low power consumption and high-resolution qualities. This was exceeded in January 2008, when Samsung showcased the world's largest and thinnest OLED TV at the time, at 31 inches (78 cm) and 4.3 mm.^[176]

In May 2008, Samsung unveiled an ultra-thin 12.1 inch (30 cm) laptop OLED display concept, with a 1,280×768 resolution with infinite contrast ratio.^[177] According to Woo Jong Lee, Vice President of the Mobile Display Marketing Team at Samsung SDI, the company expected OLED displays to be used in notebook PCs as soon as 2010.^[178]

In October 2008, Samsung showcased the world's thinnest OLED display, also the first to be "flappable" and bendable.^[179] It measures just 0.05 mm (thinner than paper), yet a Samsung staff member said that it is "technically possible to make the panel thinner".^[179] To achieve this thickness, Samsung etched an OLED panel that uses a normal glass substrate. The drive circuit was formed by low-temperature polysilicon TFTs. Also, low-molecular organic EL materials were employed. The pixel count of the display is 480 × 272. The contrast ratio is 100,000:1, and the luminance is 200 cd/m². The colour reproduction range is 100% of the NTSC standard.

As of 2020, the world's largest OLED television is an 88-inch with an 8K resolution, frame rate up to 120 fps and cost of 34,676 US Dollars.^[180]

At the Consumer Electronics Show (CES) in January 2010, Samsung demonstrated a laptop computer with a large, transparent OLED display featuring up to 40% transparency^[181] and an animated OLED display in a photo ID card.^[182]

Samsung's 2010 AMOLED smartphones used their Super AMOLED trademark, with the Samsung Wave S8500 and Samsung i9000 Galaxy S being launched in June 2010. In January 2011, Samsung announced their Super AMOLED Plus displays, which offer several advances over the older Super AMOLED displays: real stripe matrix (50% more sub pixels), thinner form factor, brighter image and an 18% reduction in energy consumption.^[183]

At CES 2012, Samsung introduced the first 55" TV screen that uses Super OLED technology.^[184]

On 8 January 2013, at CES Samsung unveiled a unique curved 4K Ultra S9 OLED television, which they state provides an "IMAX-like experience" for viewers.^[185]

On 13 August 2013, Samsung announced availability of a 55-inch curved OLED TV (model KN55S9C) in the US at a price point of $8999.99.^[186]

On 6 September 2013, Samsung launched its 55-inch curved OLED TV (model KE55S9C) in the United Kingdom with John Lewis.^[187]

Samsung introduced the Galaxy Round smartphone in the Korean market in October 2013. The device features a 1080p screen, measuring 5.7 inches (14 cm), that curves on the vertical axis in a rounded case. The corporation has promoted the following advantages: A new feature called "Round Interaction" that allows users to look at information by tilting the handset on a flat surface with the screen off, and the feel of one continuous transition when the user switches between home screens.^[188]

Samsung released a new line of OLED TVs in 2022, its first using the technology since 2013.^[189] They use panels sourced from Samsung Display; previously, LG was the sole manufacturer of OLED panels for TVs.^[190]

Sony

Sony XEL-1, the world's first OLED TV^[36] (front)

The Sony CLIÉ PEG-VZ90 was released in 2004, being the first PDA to feature an OLED screen.^[191] Other Sony products to feature OLED screens include the MZ-RH1 portable minidisc recorder, released in 2006^[192] and the Walkman X Series.^[193]

At the 2007, Las Vegas Consumer Electronics Show (CES), Sony showcased a 11-inch (28 cm), (resolution 960×540) and 27-inch (69 cm), full HD resolution at 1920 × 1080 OLED TV models.^[194] Both claimed 1,000,000:1 contrast ratios and total thicknesses (including bezels) of 5 mm. In April 2007, Sony announced it would manufacture 1000 11-inch (28 cm) OLED TVs per month for market testing purposes.^[195] On 1 October 2007, Sony announced that the 11-inch (28 cm) model XEL-1, was the first commercial OLED TV^[36] and it was released in Japan in December 2007.^[196]

In May 2007, Sony publicly unveiled a video of a 2.5-inch (6.4 cm) flexible OLED screen which is only 0.3 millimeters thick.^[197] At the Display 2008 exhibition, Sony demonstrated a 0.2 mm thick 3.5 inches (8.9 cm) display with a resolution of 320×200 pixels and a 0.3 mm thick 11-inch (28 cm) display with 960×540 pixels resolution, one-tenth the thickness of the XEL-1.^[198][199]

In July 2008, a Japanese government body said it would fund a joint project of leading firms, which is to develop a key technology to produce large, energy-saving organic displays. The project involves one laboratory and 10 companies including Sony Corp. NEDO said the project was aimed at developing a core technology to mass-produce 40 inch or larger OLED displays in the late 2010s.^[200]

In October 2008, Sony published results of research it carried out with the Max Planck Institute over the possibility of mass-market bending displays, which could replace rigid LCDs and plasma screens. Eventually, bendable, see-through displays could be stacked to produce 3D images with much greater contrast ratios and viewing angles than existing products.^[201]

Sony exhibited a 24.5" (62 cm) prototype OLED 3D television during the Consumer Electronics Show in January 2010.^[202]

In January 2011, Sony announced the PlayStation Vita handheld game console (the successor to the PSP) will feature a 5-inch OLED screen.^[203]

On 17 February 2011, Sony announced its 25" (63.5 cm) OLED Professional Reference Monitor aimed at the Cinema and high end Drama Post Production market.^[204]

On 25 June 2012, Sony and Panasonic announced a joint venture for creating low cost mass production OLED televisions by 2013.^[205] Sony unveiled its first OLED TV since 2008 at CES 2017 called A1E. It revealed two other models in 2018 one at CES 2018 called A8F and other a Master Series TV called A9F. At CES 2019 they unveiled another two models one the A8G and the other another Bravia Series TV called A9G. Then, at CES 2020, they revealed the A8H, which was effectively an A9G in terms of picture quality but with some compromises due to its lower cost. At the same event, they also revealed a 48-inch version of the A9G, making this its smallest OLED TV since the XEL-1.^{[206][207][208][209]}

LG

On 9 April 2009, LG acquired Kodak's OLED business and started to utilize white OLED technology.^[210][211] As of 2010, LG Electronics produced one model of OLED television, the 15-inch (38 cm) 15EL9500^[212] and had announced a 31-inch (79 cm) OLED 3D television for March 2011.^[213] On 26 December 2011, LG officially announced the "world's largest 55-inch (140 cm) OLED panel" and featured it at CES 2012.^[214] In late 2012, LG announces the launch of the 55EM9600 OLED television in Australia.^[215]

In January 2015, LG Display signed a long-term agreement with Universal Display Corporation for the supply of OLED materials and the right to use their patented OLED emitters.^[216]

Mitsubishi

Lumiotec is the first company in the world developing and selling, since January 2011, mass-produced OLED lighting panels with such brightness and long lifetime. Lumiotec is a joint venture of Mitsubishi Heavy Industries, ROHM, Toppan Printing, and Mitsui & Co. On 1 June 2011, Mitsubishi Electric installed a 6-meter OLED 'sphere' in Tokyo's Science Museum.^[217]

Recom Group

On 6 January 2011, Los Angeles-based technology company Recom Group introduced the first small screen consumer application of the OLED at the Consumer Electronics Show in Las Vegas. This was a 2.8" (7 cm) OLED display being used as a wearable video name tag.^[218] At the Consumer Electronics Show in 2012, Recom Group introduced the world's first video mic flag incorporating three 2.8" (7 cm) OLED displays on a standard broadcaster's mic flag. The video mic flag allowed video content and advertising to be shown on a broadcasters standard mic flag.^[219]

Dell

On 6 January 2016, Dell announced the Ultrasharp UP3017Q OLED monitor at the Consumer Electronics Show in Las Vegas.^[220] The monitor was announced to feature a 30-inch (76 cm) 4K UHD OLED panel with a 120 Hz refresh rate, 0.1 millisecond response time, and a contrast ratio of 400,000:1. The monitor was set to sell at a price of $4,999 and release in March, 2016, just a few months later. As the end of March rolled around, the monitor was not released to the market and Dell did not speak on reasons for the delay. Reports suggested that Dell canceled the monitor as the company was unhappy with the image quality of the OLED panel, especially the amount of color drift that it displayed when you viewed the monitor from the sides.^[221] On 13 April 2017, Dell finally released the UP3017Q OLED monitor to the market at a price of $3,499 ($1,500 less than its original spoken price of $4,999 at CES 2016). In addition to the price drop, the monitor featured a 60 Hz refresh rate and a contrast ratio of 1,000,000:1. As of June, 2017, the monitor is no longer available to purchase from Dell's website.

Apple

Apple began using OLED panels in its watches in 2015 and in its laptops in 2016 with the introduction of an OLED touchbar to the MacBook Pro.^[222] In 2017, Apple announced the introduction of their tenth anniversary iPhone X with their own optimized OLED display licensed from Universal Display Corporation.^[223] With the exception of the iPhone SE line, iPhone XR and iPhone 11 all iPhones released since then have also featured OLED displays.

Nintendo

A third model of Nintendo's Switch, a hybrid gaming system, features an OLED panel in place of the original model's LCD panel. Announced in the summer of 2021, it was released on 8 October 2021.^[224]

Research

In 2014, Mitsubishi Chemical Corporation (MCC), a subsidiary of Mitsubishi Chemical Holdings, developed an OLED panel with a 30,000-hour life, twice that of conventional OLED panels.^[225]

The search for efficient OLED materials has been extensively supported by simulation methods; it is possible to calculate important properties computationally, independent of experimental input,^[226][227] making materials development cheaper.

On 18 October 2018, Samsung showed of their research roadmap at their 2018 Samsung OLED Forum. This included Fingerprint on Display (FoD), Under Panel Sensor (UPS), Haptic on Display (HoD) and Sound on Display (SoD).^[228]

Various venders are also researching cameras under OLEDs (Under Display Cameras). According to IHS Markit Huawei has partnered with BOE, Oppo with China Star Optoelectronics Technology (CSOT), Xiaomi with Visionox.^[229]

In 2020, researchers at the Queensland University of Technology (QUT) proposed using human hair which is a source of carbon and nitrogen to create OLED displays.^[230]

See also

• Comparison of display technology
• Field-emission display – type of displayPages displaying wikidata descriptions as a fallback
• Flat-panel display – Electronic display technology
• Flexible electronics – Mounting of electronic devices on flexible plastic substrates
• Flexible organic light-emitting diode
• List of emerging technologies
• List of flat panel display manufacturers
• Molecular electronics – Branch of chemistry and electronics
• Organic light-emitting transistor – form of transistor that emits lightPages displaying wikidata descriptions as a fallback
• Printed electronics – Electronic devices created by various printing methods
• Quantum dot display – Type of display device
• Roll-to-roll processing – countinous printing method that prints directly onto a roll of fabric or other materialsPages displaying wikidata descriptions as a fallback
• Light-on-dark color scheme – Type of color scheme (Dark Mode)
• Surface-conduction electron-emitter display – CRT screen
• LED display
• microLED
• Mini LED

Notes

References

Further reading

• T. Tsujimura, OLED Display Fundamentals and Applications, Wiley-SID Series in Display Technology, New York (2017). ISBN 978-1-119-18731-8.
• P. Chamorro-Posada, J. Martín-Gil, P. Martín-Ramos, L.M. Navas-Gracia, Fundamentos de la Tecnología OLED (Fundamentals of OLED Technology). University of Valladolid, Spain (2008). ISBN 978-84-936644-0-4. Available online, with permission from the authors, at the webpage: Fundamentos de la Tecnología OLED
• Kordt, Pascal; et al. (2015). "Modeling of Organic Light Emitting Diodes: From Molecular to Device Properties". Advanced Functional Materials. 25 (13): 1955–1971. doi:10.1002/adfm.201403004. hdl:21.11116/0000-0001-6CD1-A. S2CID 18575622.
• 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.
• Müllen, Klaus (Ed.), Organic Light Emitting Devices: Synthesis, Properties and Applications. Wiley-VCH (2006). ISBN 3-527-31218-8
• Yersin, Hartmut (Ed.), Highly Efficient OLEDs with Phosphorescent Materials. Wiley-VCH (2007). ISBN 3-527-40594-1
• Kho, Mu-Jeong, Javed, T., Mark, R., Maier, E., and David, C. (2008) 'Final Report: OLED Solid State Lighting – Kodak European Research' MOTI (Management of Technology and Innovation) Project, Judge Business School of the University of Cambridge and Kodak European Research, Final Report presented on 4 March 2008 at Kodak European Research at Cambridge Science Park, Cambridge, UK., pages 1–12.
• Display, New Vision (12 February 2018). "Between cut-throat competition, Which is better OLED, LCD or PMOLED".

External links

Wikimedia Commons has media related to OLED.

• OLED, LCD & TFT - Construction and Difference, advantages and disadvantages 08. Juli 2020
• Structure and working principle of OLEDs and electroluminescent displays
• MIT introduction to OLED technology (video)
• Historical list of OLED products from 1996 to present