Electroluminescence (EL) is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field. This is distinct from black body light emission resulting from heat (incandescence), from a chemical reaction (chemiluminescence), sound (sonoluminescence), or other mechanical action (mechanoluminescence).
Electroluminescence is the result of radiative recombination of electrons & holes in a material, usually a semiconductor. The excited electrons release their energy as photons - light. Prior to recombination, electrons and holes may be separated either by doping the material to form a p-n junction (in semiconductor electroluminescent devices such as light-emitting diodes) or through excitation by impact of high-energy electrons accelerated by a strong electric field (as with the phosphors in electroluminescent displays).
It has been recently shown that as a solar cell improves its light-to-electricity efficiency (improved open-circuit voltage), it will also improve its electricity-to-light (EL) efficiency.
Electroluminescent devices are fabricated using either organic or inorganic electroluminescent materials. The active materials are generally semiconductors of wide enough bandwidth to allow exit of the light.
The most typical inorganic thin-film EL (TFEL) is ZnS:Mn with yellow-orange emission. Examples of the range of EL material include:
The most common electroluminescent (EL) devices are composed of either powder (primarily used in lighting applications) or thin films (for information displays.)
Light-emitting capacitor, or LEC, is a term used since at least 1961  to describe electroluminescent panels. General Electric has patents dating to 1938 on flat electroluminescent panels that are still made as night lights and backlights for instrument panel displays. Electroluminescent panels are a capacitor where the dielectric between the outside plates is a phosphor that gives off photons when the capacitor is charged. By making one of the contacts transparent, the large area exposed emits light.
Electroluminescent automotive instrument panel backlighting, with each gauge pointer also an individual light source, entered production on 1960 Chrysler and Imperial passenger cars, and was continued successfully on several Chrysler vehicles through 1967.
Sylvania Lighting Division in Salem and Danvers, MA, produced and marketed an EL night lamp (right), under the trade name Panelescent at roughly the same time that the Chrysler instrument panels entered production. These lamps have proven extremely reliable, with some samples known to be still functional after nearly 50 years of continuous operation. Later in the 1960s, Sylvania's Electronic Systems Division in Needham, MA developed and manufactured several instruments for the Apollo Lunar Lander and Command Module using electroluminescent display panels manufactured by the Electronic Tube Division of Sylvania at Emporium, PA. Raytheon, Sudbury, MA, manufactured the Apollo guidance computer, which used a Sylvania electroluminescent display panel as part of its display-keyboard interface (DSKY).
Powder phosphor-based electroluminescent panels are frequently used as backlights for liquid crystal displays. They readily provide gentle, even illumination for the entire display while consuming relatively little electric power. This makes them convenient for battery-operated devices such as pagers, wristwatches, and computer-controlled thermostats, and their gentle green-cyan glow is common in the technological world. They require relatively high voltage (between 60 and 600 volts). For battery-operated devices, this voltage must be generated by a converter circuit within the device. This converter often makes an audible whine or siren sound while the backlight is activated. For line-voltage-operated devices, it may be supplied directly from the power line. Electroluminescent nightlights operate in this fashion. Brightness per unit area increases with increased voltage and frequency.
Thin film phosphor electroluminescence was first commercialized during the 1980s by Sharp Corporation in Japan, Finlux (Oy Lohja Ab) in Finland, and Planar Systems in the US. Here, bright, long-life light emission is achieved in thin film yellow-emitting manganese-doped zinc sulfide material. Displays using this technology were manufactured for medical and vehicle applications where ruggedness and wide viewing angles were crucial, and liquid crystal displays were not well developed. In 1992, Timex introduced its Indiglo EL display on some watches.
Recently, blue-, red-, and green-emitting thin film electroluminescent materials that offer the potential for long life and full color electroluminescent displays have been developed.
In either case, the EL material must be enclosed between two electrodes and at least one electrode must be transparent to allow escape of the produced light. Glass coated with indium tin oxide is commonly used as the front (transparent) electrode while the back electrode is coated with reflective metal. Additionally, other transparent conducting materials, such as carbon nanotube coatings or PEDOT can be used as the front electrode.
The display applications are primarily passive (i.e., voltages are driven from edge of the display cf. driven from a transistor on the display). Similar to LCD trends, there have also been Active Matrix EL (AMEL) displays demonstrated, where circuitry is added to prolong voltages at each pixel. The solid-state nature of TFEL allows for a very rugged and high-resolution display fabricated even on silicon substrates. AMEL displays of 1280x1024 at over 1000 lines per inch (lpi) have been demonstrated by a consortium including Planar Systems.
Electroluminescent technologies have low power consumption compared to competing lighting technologies, such as neon or fluorescent lamps. For example, EL panels achieve around 6 lumens per watt, so a nightlight might use about .02 watts. This, together with the thinness of the material, has made EL technology valuable to the advertising industry. Relevant advertising applications include electroluminescent billboards and signs. EL manufacturers are able to control precisely which areas of an electroluminescent sheet illuminate, and when. This has given advertisers the ability to create more dynamic advertising that is still compatible with traditional advertising spaces.
An EL film is a so-called Lambertian radiator: unlike with neon lamps, filament lamps, or LEDs, the brightness of the surface appears the same from all angles of view; electroluminescent light is not directional and therefore hard to compare with (thermal) light sources measured in lumens or lux. The light emitted from the surface is perfectly homogeneous and is well-perceived by the eye. EL film produces single-frequency (monochromatic) light that has a very narrow bandwidth, is absolutely uniform and visible from a great distance.
In principle, EL lamps can be made in any color. However, the commonly used greenish color closely matches the peak sensitivity of human vision, producing the greatest apparent light output for the least electrical power input. Unlike neon and fluorescent lamps, EL lamps are not negative resistance devices so no extra circuitry is needed to regulate the amount of current flowing through them. A new technology now being used is based on multispectral phosphors that emit light from 600 to 400nm depending on the drive frequency; this is similar to the colour changing effect seen with aqua EL sheet but on a larger scale.
Electroluminescent lighting is now used as an application for public safety identification involving alphanumeric characters on the roof of vehicles for clear visibility from an aerial perspective.
Electroluminescent lighting, especially electroluminescent wire (EL wire), has also made its way into clothing as many designers have brought this technology to the entertainment and night life industry.
Engineers have developed an electroluminescent "skin" that can stretch more than six times its original size while still emitting light. This hyper-elastic light-emitting capacitor (HLEC) can endure more than twice the strain of previously tested stretchable displays. It consists of layers of transparent hydrogel electrodes sandwiching an insulating elastomer sheet. The elastomer changes luminance and capacitance when stretched, rolled and otherwise deformed. In addition to its ability to emit light under a strain of greater than 480% its original size, the group's HLEC was shown to be capable of being integrated into a soft robotic system. Three six-layer HLEC panels were bound together to form a crawling soft robot, with the top four layers making up the light-up skin and the bottom two the pneumatic actuators. The discovery could lead to significant advances in health care, transportation, electronic communication and other areas.
André Bernanose (17 June 1912 – 18 March 2002) was a 20th-century French physicist, chemist and pharmacologist.
He studied chemiluminescence during the late 1940s - early 1950s, which led him to discover the electroluminescence. He is for this reason considered the father of the OLED.Conductive polymer
Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers is their processability, mainly by dispersion. Conductive polymers are generally not thermoplastics, i.e., they are not thermoformable. But, like insulating polymers, they are organic materials. They can offer high electrical conductivity but do not show similar mechanical properties to other commercially available polymers. The electrical properties can be fine-tuned using the methods of organic synthesis and by advanced dispersion techniques.Electrical phenomena
Electrical phenomena are commonplace and unusual events that can be observed and that illuminate the principles of the physics of electricity and are explained by them.
Electrical phenomena are a somewhat arbitrary division of
Some examples are
Biefeld–Brown effect — Thought by the person who coined the name, Thomas Townsend Brown, to be an anti-gravity effect, it is generally attributed to electrohydrodynamics (EHD) or sometimes electro-fluid-dynamics, a counterpart to the well-known magneto-hydrodynamics.
Bioelectrogenesis — The generation of electricity by living organisms.
Contact electrification — The phenomenon of electrification by contact. When two objects were touched together, sometimes the objects became spontaneously charged (οne negative charge, one positive charge).
Direct Current — (old: Galvanic Current) or "continuous current"; The continuous flow of electricity through a conductor such as a wire from high to low potential.
Electroluminescence — The phenomenon wherein a material emits light in response to an electric current passed through it, or to a strong electric field.
Electrical conduction — The movement of electrically charged particles through transmission medium.
Electric shock — Physiological reaction of a biological organism to the passage of electric current through its body.
Ferroelectric effect — The phenomenon whereby certain ionic crystals may exhibit a spontaneous dipole moment.
Inductance — The phenomenon whereby the property of a circuit by which energy is stored in the form of an electromagnetic field.
Lightning — powerful natural electrostatic discharge produced during a thunderstorm. Lightning's abrupt electric discharge is accompanied by the emission of light.
Photoconductivity — The phenomenon in which a material becomes more conductive due to the absorption of electro-magnetic radiation such as visible light, ultraviolet light, or gamma radiation.
Photoelectric effect — Emission of electrons from a surface (usually metallic) upon exposure to, and absorption of, electromagnetic radiation (such as visible light and ultraviolet radiation).
Piezoelectric effect — Ability of certain crystals to generate a voltage in response to applied mechanical stress.
Plasma — Plasma occur when gas is heated to very high temperatures and it disassociates into positive and negative charges.
Pyroelectric effect — The potential created in certain materials when they are heated.
Redox — (short for reduction-oxidation reaction) A chemical reaction in which the oxidation states of atoms are changed.
Static electricity — Class of phenomena involving the imbalanced charge present on an object, typically referring to charge with voltages of sufficient magnitude to produce visible attraction (e.g., static cling), repulsion, and sparks.
Sparks — Electrical breakdown of a medium that produces an ongoing plasma discharge, similar to the instant spark, resulting from a current flowing through normally nonconductive media such as air.
Telluric currents — Extremely low frequency electric current that occurs naturally over large underground areas at or near the surface of the Earth.
Thermionic emission — the emission of electrons from a heated electrode, usually the cathode, the principle underlying most vacuum tubes.
Thermoelectric effect — the Seebeck effect, the Peltier effect, and the Thomson effect
Thunderstorm — also electrical storm, form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder.
Triboelectric effect — Type of contact electrification in which objects become electrically charged after coming into contact and are then separated.
Whistlers — Very low frequency radio wave generated by lightningElectroluminescent display
Electroluminescent Displays (ELDs) are a type of Flat panel display created by sandwiching a layer of electroluminescent material such as GaAs between two layers of conductors. When current flows, the layer of material emits radiation in the form of visible light. Electroluminescence (EL) is an optical and electrical phenomenon where a material emits light in response to an electric current passed through it, or to a strong electric field.Electroluminescent wire
Electroluminescent wire (often abbreviated as EL wire) is a thin copper wire coated in a phosphor that produces light through electroluminescence when an alternating current is applied to it. It can be used in a wide variety of applications—vehicle and structure decoration, safety and emergency lighting, toys, clothing etc.—much as rope light or Christmas lights are often used. Unlike these types of strand lights, EL wire is not a series of points, but produces a 360 degree unbroken line of visible light. Its thin diameter makes it flexible and ideal for use in a variety of applications such as clothing or costumes.Field-induced polymer electroluminescent technology
Field-induced polymer electroluminescent (FIPEL) technology is a low power electroluminescent light source. Three layers of moldable light-emitting polymer blended with a small amount of carbon nanotubes glow when an alternating current is passed through them. The technology can produce white light similar to that of the Sun, or other tints if desired. It is also more efficient than compact fluorescent lamps in terms of the energy required to produce light. As cited from the Carroll Research Group at Wake Forest University, "To date our brightest device - without output couplers - exceeds 18,000 cd/m2." This confirms that FIPEL technology is a viable solution for area lighting.FIPEL lights are different from LED lighting, in that there is no junction. Instead, the light emitting component is a layer of polymer containing an iridium compound which is doped with multi-wall carbon nanotubes. This planar light emitting structure is energized by an AC field from insulated electrodes. The lights can be shaped into many different forms, from mimicking conventional light bulbs to unusual forms such as 2-foot-by-4-foot flat sheets and straight or bent tubes. The technology was developed by a team headed by Dr. David Carroll of Wake Forest University in Winston-Salem, North Carolina.Georges Destriau
Georges Destriau (1 August 1903 - 20 January 1960) was a French Physicist and early observer of electroluminescence.H. J. Round
Captain Henry Joseph Round (2 June 1881 – 17 August 1966) was an English engineer and one of the early pioneers of radio. He was the first to report observation of electroluminescence from a solid state diode, leading to the discovery of the light-emitting diode. He was a personal assistant to Guglielmo Marconi.
Round was the eldest child of Joseph and Gertrude Round. He spent his early years in the small town of Kingswinford in Staffordshire and received his early education at Cheltenham Grammar School. He later attended the Royal College of Science, a constituent college of Imperial College London where he gained a first class honours degree.
Round joined the Marconi Company in 1902; not long after Marconi had made his transatlantic wireless transmission. He was sent to the USA where he experimented with a variety of different aspects of radio technology, focusing on technologies such as powdered iron cored tuning inductors. He also performed some experiments with transmission paths over land and sea at different times of the day and investigated direction finding, for which he used a frame antenna.Jenny Rosenthal Bramley
Jenny Rosenthal Bramley (July 31, 1909 – May 26, 1997) was a Russian-born American physicist. She was the first woman to earn a doctorate in physics from an American institution, and she was the second woman elected as a fellow of the IEEE. She holds numerous patents on Electroluminescence and Electro-optics and is cited by the IEEE as being "well known for her innovative work in lasers.”Large Underground Xenon experiment
The Large Underground Xenon experiment (LUX) aims to directly detect weakly interacting massive particle (WIMP) dark matter interactions with ordinary matter on Earth. Despite the wealth of (gravitational) evidence supporting the existence of non-baryonic dark matter in the Universe, dark matter particles in our galaxy have never been directly detected in an experiment. LUX utilizes a 370 kg liquid xenon detection mass in a time-projection chamber (TPC) to identify individual particle interactions, searching for faint dark matter interactions with unprecedented sensitivity.The LUX experiment, which cost approximately $10 million to build, is located 1,510 m (4,950 ft) underground at the Sanford Underground Laboratory (SURF, formerly the Deep Underground Science and Engineering Laboratory, or DUSEL) in the Homestake Mine (South Dakota) in Lead, South Dakota. The detector is located in the Davis campus, former site of the Nobel Prize-winning Homestake neutrino experiment led by Raymond Davis. It is operated underground to reduce the background noise signal caused by high-energy cosmic rays at the Earth's surface.Light-emitting diode
A light-emitting diode (LED) is a semiconductor light source that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. This effect is called electroluminescence. The color of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the band gap of the semiconductor. White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device.Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared light. Infrared LEDs are used in remote-control circuits, such as those used with a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red. Modern LEDs are available across the visible, ultraviolet, and infrared wavelengths, with high light output.
Early LEDs were often used as indicator lamps, replacing small incandescent bulbs, and in seven-segment displays. Recent developments have produced white-light LEDs suitable for room lighting. LEDs have led to new displays and sensors, while their high switching rates are useful in advanced communications technology.
LEDs have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Light-emitting diodes are used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, lighted wallpaper and medical devices.Unlike a laser, the color of light emitted from an LED is neither coherent nor monochromatic, but the spectrum is narrow with respect to human vision, and functionally monochromatic.Light-emitting electrochemical cell
A light-emitting electrochemical cell (LEC or LEEC) is a solid-state device that generates light from an electric current (electroluminescence). LECs are usually composed of two metal electrodes connected by (e.g. sandwiching) an organic semiconductor containing mobile ions. Aside from the mobile ions, their structure is very similar to that of an organic light-emitting diode (OLED).
LECs have most of the advantages of OLEDs, as well as additional ones:
The device is less dependent on the difference in work function of the electrodes. Consequently, the electrodes can be made of the same material (e.g. gold). Similarly, the device can still be operated at low voltages.
Recently developed materials such as graphene or a blend of carbon nanotubes and polymers have been used as electrodes, eliminating the need for using indium tin oxide for a transparent electrode.
The thickness of the active electroluminescent layer is not critical for the device to operate. This means that:
LECs can be printed with relatively inexpensive printing processes (where control over film thicknesses can be difficult).
In a planar device configuration, internal device operation can be observed directly.There are two distinct types of LECs, those based on inorganic transition metal complexes (iTMC) or light emitting polymers. iTMC devices are often more efficient than their LEP based counterparts due to the emission mechanism being phosphorescent rather than fluorescent.While electroluminescence had been seen previously in similar devices, the invention of the polymer LEC is attributed to Pei et al. Since then, numerous research groups and a few companies have worked on improving and commercializing the devices.
In 2012 the first inherently stretchable LEC using an elastomeric emissive material (at room temperature) was reported. Dispersing an ionic transition metal complex into an elastomeric matrix enables the fabrication of intrinsically stretchable light-emitting devices that possess large emission areas (∼175 mm2) and tolerate linear strains up to 27% and repetitive cycles of 15% strain. This work demonstrates the suitability of this approach to new applications in conformable lighting that require uniform, diffuse light emission over large areas.In 2012 fabrication of organic light-emitting electrochemical cells (LECs) using a roll-to-roll compatible process under ambient conditions was reported.In 2017, a new design approach developed by a team of Swedish researchers promised to deliver substantially higher efficiency: 99.2 cd A−1 at a bright luminance of 1910 cd m−2.List of light sources
This is a list of sources of light, including both natural and artificial processes that emit light. This article focuses on sources that produce wavelengths from about 390 to 700 nanometers, called visible light.OLED
An organic light-emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound 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, portable systems such as smartphones, handheld game consoles and PDAs. A major area of research is the development of white OLED devices for use in solid-state lighting applications.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, whereas AMOLED control uses a thin-film transistor backplane to directly access and switch each individual pixel on or off, allowing for higher resolution and larger display sizes.
An OLED display works without a backlight because it emits 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.Optical properties of carbon nanotubes
Within materials science, the optical properties of carbon nanotubes refer specifically to the absorption, photoluminescence (fluorescence), and Raman spectroscopy of carbon nanotubes. Spectroscopic methods offer the possibility of quick and non-destructive characterization of relatively large amounts of carbon nanotubes. There is a strong demand for such characterization from the industrial point of view: numerous parameters of the nanotube synthesis can be changed, intentionally or unintentionally, to alter the nanotube quality. As shown below, optical absorption, photoluminescence and Raman spectroscopies allow quick and reliable characterization of this "nanotube quality" in terms of non-tubular carbon content, structure (chirality) of the produced nanotubes, and structural defects. Those features determine nearly any other property, such as optical, mechanical, and electrical.
Carbon nanotubes are unique "one-dimensional systems" which can be envisioned as rolled single sheets of graphite (or more precisely graphene). This rolling can be done at different angles and curvatures resulting in different nanotube properties. The diameter typically varies in the range 0.4–40 nm (i.e. "only" ~100 times), but the length can vary ~10,000 times, reaching 55.5 cm. The nanotube aspect ratio, or the length-to-diameter ratio, can be as high as 132,000,000:1, which is unequalled by any other material. Consequently, all the properties of the carbon nanotubes relative to those of typical semiconductors are extremely anisotropic (directionally dependent) and tunable.
Whereas mechanical, electrical and electrochemical (supercapacitor) properties of the carbon nanotubes are well established and have immediate applications, the practical use of optical properties is yet unclear. The aforementioned tunability of properties is potentially useful in optics and photonics. In particular, light-emitting diodes (LEDs) and photo-detectors based on a single nanotube have been produced in the lab. Their unique feature is not the efficiency, which is yet relatively low, but the narrow selectivity in the wavelength of emission and detection of light and the possibility of its fine tuning through the nanotube structure. In addition, bolometer and optoelectronic memory devices have been realised on ensembles of single-walled carbon nanotubes.Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness (> 1 ms), and fluorescent materials, where the emission decay takes place over tens of nanoseconds. Phosphorescent materials are known for their use in radar screens and glow-in-the-dark materials, whereas fluorescent materials are common in cathode ray tube (CRT) and plasma video display screens, fluorescent lights, sensors, and white LEDs.
Phosphors are often transition-metal compounds or rare-earth compounds of various types. The most common uses of phosphors are in CRT displays and fluorescent lights. CRT phosphors were standardized beginning around World War II and designated by the letter "P" followed by a number.
Phosphorus, the chemical element named for its light-emitting behavior, emits light due to chemiluminescence, not phosphorescence.Printed segmented electroluminescence
Printed Segmented Electroluminescence (pSEL) is a technology (developed by UK company MFlex, formerly known as Pelikon) that builds on the phenomenon of electroluminescence. It was designed for use in devices that require multiple displays within a fixed area, such as flexible displays and interfaces. pSEL Display panels use an encapsulated printed electroluminescent phosphor layer with various capacitive, insulation and conducting layers to create iconic and segmented lit areas.Examples of use include domestic appliances, consumer electronics and control panels.
pSEL hybrid offers flexible printed displays that are fully visible in daylight. This technology was designed for displays in portable products with displays requiring visible in full strength sunlight, such as mobile phones, MP3 players, and car dashboards.Solid-state lighting
Solid-state lighting (SSL) refers to a type of lighting that uses semiconductor light-emitting diodes (LEDs), organic light-emitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination rather than electrical filaments, plasma (used in arc lamps such as fluorescent lamps), or gas.
The term "solid state" refers commonly to light emitted by solid-state electroluminescence, as opposed to incandescent bulbs (which use thermal radiation) or fluorescent tubes. Compared to incandescent lighting, SSL creates visible light with reduced heat generation and less energy dissipation. Most common "white" LEDs convert blue light from a solid-state device to an (approximate) white light spectrum using photoluminescence, the same principle used in conventional fluorescent tubes.
The typically small mass of a solid-state electronic lighting device provides for greater resistance to shock and vibration compared to brittle glass tubes/bulbs and long, thin filament wires. They also eliminate filament evaporation, potentially increasing the life span of the illumination device.
Solid-state lighting is often used in traffic lights and is also used frequently in modern vehicle lights, street and parking lot lights, train marker lights, building exteriors, remote controls etc. Controlling the light emission of LEDs may be done most effectively by using the principles of nonimaging optics.Solid-state lighting has made significant advances in industry. In the entertainment lighting industry, standard incandescent tungsten-halogen lamps are being replaced by solid-state light lighting fixtures.Triphenylamine
Triphenylamine is an organic compound with formula (C6H5)3N. In contrast to most amines, triphenylamine is nonbasic. Its derivatives have useful properties in electrical conductivity and electroluminescence, and they are used in OLEDs as hole-transporters.Triphenylamine can be prepared by arylation of diphenylamine.