electroluminescence

1. Direct conversion of electric energy to light by a solid phosphor subjected to an alternating electric field.
2. Emission of light caused by electric discharge in a gas.

A general term for the luminescence excited by the application of an electric field to a system, usually in the solid state. Solid-state electroluminescent systems can be made quite thin, leading to applications in thin-panel area light sources and flat screens to replace cathode-ray tubes for electronic display and image formation. See also Luminescence.

Modern interest in electroluminescence dates from the discovery by G. Destriau in France in 1936 that when a zinc sulfide (ZnS) phosphor powder is suspended in an insulator (oil, plastic, or glass ceramic) and an intense alternating electric field is applied with capacitorlike electrodes, visible light is emitted. The phosphor, prepared from zinc sulfide by addition of a small amount of copper impurity, was later shown to contain particles of a copper sulfide (Cu₂S) phase in addition to copper in its normal role as a luminescence activator in the zinc sulfide lattice. The intensification of the applied electric field by the sharp conductive or semiconductive copper sulfide inhomogeneities is believed to underlie the mechanism of Destriau-type electroluminescence. Minority carriers are ejected from these high-field spots into the low- or moderate-field regions of the phosphor, where they recombine to excite the activator centers. The structure of a Destriau-type electroluminescent cell is shown in the illustration; the light is observed through the transparent indium–tin oxide electrode. See also Light panel.

Structure of powdered-phosphor (Destriau) electroluminescent cell, edge view (not to scale).

The application of electroluminescence to display and image formation received great impetus from work in the late 1960s and mid-1970s on thin-film electroluminescence (TFEL), giving rise to devices that are different in structure and mechanism from the Destriau conditions. The phosphor in these devices is not a powder but a thin (about 500 nanometers) continuous film prepared by sputtering or vacuum evaporation. The luminescence activators are manganese or rare-earth ions, atomic species with internal electronic transitions that lead to characteristic luminescence. The phosphor film does not contain copper sulfide or any other separate phase, and is sandwiched between two thin (about 200 nm) transparent insulating films also prepared by evaporative means. Conducting electrodes are applied to the outside of each insulating film; one of the electrodes is again a transparent coating of indium–tin oxide on glass, which serves as supporting substrate. If an imaging matrix is desired, both electrodes consist of grids of parallel lines, with the direction of the grid on one insulator (row) orthogonal to the other grid (column). By approximate circuitry the entire matrix can be scanned, applying voltage where desired to a phosphor element that is located between the intersection of a row and column electrode. A thin-film electroluminescent device acts like a pure capacitor at low applied voltage; no light is emitted until the voltage reaches a threshold value determined by the dielectric properties of the insulator and phosphor films. Above this threshold a dissipative current flows, and light emission occurs. The brightness increases very steeply with the applied voltage but is finally saturated. The light output, or average brightness, is roughly proportional to the frequency up to at least 5 kHz, and also depends on the waveform of the applied voltage.

The best thin-film electroluminescent phosphor is manganese-activated zinc sulfide, which emits yellow light peaking at 585 nm. Activation of zinc sulfide and certain alkaline earth sulfides with different rare earths has yielded many other promising electroluminescent phosphors emitting blue, green, red, and white, and making full-color matrix-addressed thin-film electroluminescent displays possible. The light output of thin-film electroluminescent displays has been very reliable, with typically only 10% loss after tens of thousands of hours of operation.

Injection electroluminescence results when a semiconductor pn junction or a point contact is biased in the forward direction. This type of emission, first observed from silicon carbide (SiC) in 1907, is the result of radiative recombination of injected minority carriers, with majority carriers being a material. Such emission has been observed in a large number of semiconductors. The wavelength of the emission corresponds to an energy equal, at most, to the forbidden band gap of the material, and hence in most of these materials the wavelength is in the infrared region of the spectrum. If a pn junction is biased in the reverse direction, so as to produce high internal electric fields, other types of emission can occur, but with very low efficiency. See also Junction diode; Semiconductor; Semiconductor diode.

Light emission may also occur when electrodes of certain metals, such as Al or Ta, are immersed in suitable electrolytes and current is passed between them. In many cases this galvanoluminescence is electroluminescence generated in a thin oxide layer formed on the electrode by electrolytic action. In addition to electroluminescence proper, other interesting effects (usually termed electrophotoluminescence) occur when electric fields are applied to a phosphor which is concurrently, or has been previously, excited by other means. These effects include a decrease or increase in steady-state photoluminescence brightness when the field is applied, or a burst of afterglow emission if the field is applied after the primary photoexcitation is removed. See also Photoluminescence.

The generation of light by applying electricity to a material such as a semiconductor or phosphor. LEDs, OLEDs, laser diodes and electroluminescent displays are examples. See LED, OLED, laser diode and EL display.

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The emission of light from a phosphor excited by electromagnetic energy.

Conversion of electrical energy into light energy.

Animation of LCD, both unlit and with electroluminescent backlight switched on.

Spectrum of a blue/green electroluminescent light source for a clock radio (similar to the one seen in the above image). Peak wavelength is at 492 nanometers and the FWHM spectral bandwidth is quite wide at about 85nm.

Electroluminescence (EL) is an optical phenomenon and electrical phenomenon in which a material emits light in response to an electric current passed through it, or to a strong electric field. This is distinct from light emission resulting from heat (incandescence), chemical reaction (chemiluminescence), sound (sonoluminescence), or other mechanical action (mechanoluminescence).

Contents 1 Mechanism 2 Examples of electroluminescent materials 3 Practical implementations 4 See also 5 External links

Mechanism

Electroluminescence is the result of radiative recombination of electrons and holes in a material (usually a semiconductor). The excited electrons release their energy as photons - light. Prior to recombination, electrons and holes are separated either as a result of doping of the material to form a p-n junction (in semiconductor electroluminescent devices such as LEDs), or through excitation by impact of high-energy electrons accelerated by a strong electric field (as with the phosphors in electroluminescent displays).

Examples of electroluminescent materials

Electroluminescent devices can be fabricated using thin films of either organic (see Organic Electro-Luminescence) or inorganic materials. The thin film layers contain a bulk semiconductor (or host material for organic EL) and a dopant which defines the visible color emitted. The semiconductor needs to have wide enough bandwidth to allow exit of the light.

The most typical inorganic Thin Film EL (TFEL), for example, is ZnS:Mn with its yellow-orange emission. Examples of the range of EL material include:

• Powder zinc sulfide doped with Copper or Silver
• Thin film zinc sulfide doped with Manganese
• Natural blue diamond (diamond with boron as a dopant).
• III-V semiconductors - such as InP,GaAs,and GaN.
• Inorganic semiconductors - such as [Ru(bpy)₃]²⁺(PF₆^-)₂, where bpy is 2,2'-bipyridine

Practical implementations

An electroluminescent nightlight in operation (uses 0.08W at 230V, and dates from 1960; lit diameter 59 mm)

The most common EL devices are either powder (primarily used in lighting applications) or thin film (for information displays.)

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 IP's entered production. These lamps have proven incredibly 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 to liquid crystal displays. They readily provide a gentle, even illumination to 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 a common sight in the technological world. They do, however, require relatively high voltage. 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.

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 USA. 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.

Recently, blue, red, and green emitting thin film electroluminescent materials have been developed that offer the potential for long life and full color electroluminescent displays.

In either case, the EL material must be enclosed between two electrodes and at least one electrode must be transparent to allow the escape of the produced light. Glass coated with indium oxide or 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.) 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. (A couple of references for AMEL include Ron Khormaei, et al., " High Resolution Active Matrix Electroluminescent Display," Society for Information Display Digest, p. 137, 1994. And at Planar's http://www.planar.com/advantages/whitepapers/docs/overview.pdf)

The world's first electroluminescent billboard campaign, Canada, Winter 2005

Electroluminescent technologies have low power consumption compared to competing lighting technologies, such as neon or fluorescent lamps. 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 which is still compatible with traditional advertising spaces.

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.

1966 Dodge Charger instrument panel with electroluminescent lighting. Chrysler began building cars with EL panel lighting for the 1960 model year.

See also

• Indiglo
• Illuminator
• Light emitting capacitor
• List of light sources
• Electroluminescent wires
• Electroluminescent display
• Organic LED

External links

• Overview of electroluminescent display technology, and the discovery of electroluminescence
• Chrysler Corporation press release introducing Panelescent (EL) Lighting on 8 September, 1959.
• First large format EL advertising campaign in Vienna made by elontech.eu.
• Electroluminescent Commercial Application

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Dansk (Danish)
adj. - elektroluminiscent

idioms:

• electroluminescent display    elektroluniniscent display

Français (French)
adj. - électroluminescent

idioms:

• electroluminescent display    affichage électroluminescent

Deutsch (German)
adj. - elektrolumineszent

idioms:

• electroluminescent display    elektroluminiszierendes Bildschirm

Ελληνική (Greek)
adj. - ηλεκτροφθορισμός, ηλεκτροφωταύγεια

idioms:

• electroluminescent display    επίπεδη οθόνη φορητού υπολογιστή

Español (Spanish)
adj. - electroluminiscente

idioms:

• electroluminescent display    representación visual electroluminiscente

Svenska (Swedish)
adj. - elektriskt självlysande

中文（简体）(Chinese (Simplified))
场致发光的, 电致发光的

idioms:

• electroluminescent display    场致发光显示

中文（繁體）(Chinese (Traditional))
adj. - 場致發光的, 電致發光的

idioms:

• electroluminescent display    場致發光顯示

한국어 (Korean)
adj. - 두 전극 사이에 형광 물질로 된 물체를 끼우고 교류 전압을 가하면 발광하는 현상의

עברית (Hebrew)
adj. - ‮של תאורה חשמלית או קשור בה‬

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