Editing Electroluminescence

{{Short description|Optical and electrical phenomenon}}
{{more citations needed|date=April 2014}}
[[File: Backlit LCD display.jpg|thumbnail|Views of a [[liquid crystal display]], both with electroluminescent backlight switched on (top) and switched off (bottom)]]
'''Electroluminescence''' ('''EL''') is an [[optical phenomenon|optical]] 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]]), chemical reactions ([[chemiluminescence]]), reactions in a liquid ([[electrochemiluminescence]]), sound ([[sonoluminescence]]), or other mechanical action ([[mechanoluminescence]]), or 
organic electroluminescence.

==Mechanism==
[[File:Electroluminescent panel spectrum.png|thumb|right|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&nbsp;nm and the [[full width at half maximum|FWHM]] spectral bandwidth is quite wide at about 85 nm.]]
Electroluminescence is the result of [[radiative recombination]] of [[electron]]s and [[electron hole|hole]]s in a material, usually a [[semiconductor]]. The excited electrons release their energy as [[photon]]s – light. Prior to recombination, electrons and holes may be separated either by [[doping (semiconductors)|doping]] the material to form a [[p-n junction]] (in semiconductor electroluminescent devices such as [[light-emitting diode]]s) or through excitation by impact of high-energy electrons accelerated by a strong electric field (as with the [[phosphor]]s in [[electroluminescent display]]s).

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.<ref>{{cite journal|last1=Raguse|first1=John|title=Correlation of Electroluminescence with Open-CIrcuit Voltage from Thin-Film CdTe Solar Cells|journal=Journal of Photovoltaics|volume=5|issue=4|date=April 15, 2015|pages=1175–1178|doi=10.1109/JPHOTOV.2015.2417761|doi-access=free}}</ref>

==Characteristics==
[[File:66ChargerDash2.jpg|right|thumb|1966 [[Dodge Charger]] instrument panel with "Panelescent Lighting". [[Chrysler]] first introduced cars with EL panel lighting in its 1960 model year.]]
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 can 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 reflectance|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. 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 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 400{{nbsp}}nm depending on the drive frequency; this is similar to the color-changing effect seen with aqua EL sheet but on a larger scale.

==Examples of electroluminescent materials==
Electroluminescent devices are fabricated using either organic or inorganic electroluminescent materials. The active materials are generally semiconductors of wide enough bandwidth to allow the 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:
* Powdered [[zinc sulfide]] [[dopant|doped]] with [[copper]] (producing greenish light) or [[silver]] (producing bright blue light)
* Thin-film zinc sulfide doped with [[manganese]] (producing orange-red color)
* Naturally blue [[diamond]], which includes a trace of [[boron]] that acts as a dopant.
* Semiconductors containing [[Group (periodic table)|Group]] III and Group V elements, such as [[Indium phosphide|indium phosphide (InP)]], [[Gallium arsenide|gallium arsenide (GaAs)]], and [[Gallium nitride|gallium nitride (GaN)]] ([[Light-emitting diode]]s).
* Certain [[organic semiconductor]]s, such as [Ru(bpy)<sub>3</sub>]<sup>2+</sup>(PF<sub>6</sub><sup>−</sup>)<sub>2</sub>, where bpy is 2,2'-bipyridine
* [[Terbium(III) oxide|Terbium oxide]] (yellow-green light)<ref>{{cite web |url=https://www.stanfordmaterials.com/blog/terbium-oxide-used-as-electroluminescent-materials.html |last=Loewen |first=Eric |title=Terbium Oxide Used as Electroluminescent Materials |website=Stanford Materials |access-date=Sep 3, 2024}}</ref>

==Practical implementations==
The most common electroluminescent (EL) devices are composed of either powder (primarily used in lighting applications) or [[thin film]]s (for information displays.)

=== Light-emitting capacitor (LEC) ===
[[File:NightLight.jpg|right|thumbnail|An electroluminescent [[nightlight]] in operation (uses 0.08&nbsp;W at 230&nbsp;V, and dates from 1960; lit diameter is 59&nbsp;mm)]]
''Light-emitting capacitor'', or '''LEC''', is a term used since at least 1961<ref>''Proceedings of the National Electronics Conference, Volume 17'', National Engineering Conference, Inc., 1961;  page 328</ref> to describe electroluminescent panels. [[General Electric]] has patents dating to 1938 on flat electroluminescent panels that are still made as [[night light]]s and backlights for [[Control panel (engineering)|instrument panel]] displays. Electroluminescent panels are a [[capacitor]] where the [[dielectric]] between the outside plates is a [[phosphor]] that gives off [[photon]]s when the capacitor is charged.  By making one of the contacts transparent, the large area exposed emits light.<ref>Raymond Kane, Heinz Sell, ''Revolution in lamps: a chronicle of 50 years of progress, 2nd ed.'', The Fairmont Press, Inc., 2001 {{ISBN|0881733784}},  pages 122–124</ref>

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 and marketed as "Panelescent Lighting".

=== Night lights ===
The Sylvania Lighting Division in Salem and [[Danvers, Massachusetts]], produced and marketed an EL night light, 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.{{when|date=March 2023}}

Later in the 1960s, Sylvania's Electronic Systems Division in [[Needham, Massachusetts]] developed and manufactured several instruments for the [[Apollo Lunar Module]] and [[Apollo Command Module|Command Module]] using [[electroluminescent display]] panels manufactured by the Electronic Tube Division of Sylvania at [[Emporium, Pennsylvania]]. [[Raytheon]] in [[Sudbury, Massachusetts]]  manufactured the [[Apollo Guidance Computer]], which used a Sylvania electroluminescent display panel as part of its display-keyboard interface ([[DSKY]]).

=== Display backlighting ===
[[File: Casio W-86 digital watch electroluminescent backlight (ii).jpg|thumb|A Casio digital LCD watch with an electroluminescent backlight]]
Powder phosphor-based electroluminescent panels are frequently used as backlights for [[liquid crystal display]]s. 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.

EL backlights require relatively high voltage (between 60 and 600 volts).<ref name=Handbook>Donald G. Fink and H. Wayne Beaty, ''Standard Handbook for Electrical Engineers, Eleventh Edition'', McGraw-Hill, New York, 1978, {{ISBN|0-07-020974-X}}  pp 22-28</ref> For battery-operated devices, this voltage must be generated by a [[boost converter]] circuit within the device. This converter often makes a faintly audible whine or siren sound while the backlight is activated. Line-voltage-operated devices may be activated directly from the power line; some electroluminescent nightlights operate in this fashion. Brightness per unit area increases with increased voltage and frequency.<ref name=Handbook/>

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.  In these devices, 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 Group USA|Timex]] introduced its [[Indiglo]] EL display on some watches.

Recently,{{when|date=March 2023}} blue-, red-, and green-emitting thin film electroluminescent materials that offer the potential for long life and full-color electroluminescent displays have been developed.

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 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 the 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 the 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 1280×1024 at over 1000 lines per inch (LPI) have been demonstrated by a consortium including Planar Systems.<ref>Ron Khormaei, et al., "High-Resolution Active Matrix Electroluminescent Display", Society for Information Display Digest, p. 137, 1994.</ref><ref>{{Cite web|url=http://www.planar.com/support/design-resources/docs/overview.pdf |title=Active Matrix Electroluminescence (AMEL) |url-status=dead |archive-url=https://web.archive.org/web/20120722221137/http://www.planar.com/support/design-resources/docs/overview.pdf |archive-date=2012-07-22 }}</ref>

==Thick-film dielectric electroluminescent technology==
'''Thick-film dielectric electroluminescent''' '''technology''' ('''TDEL''') is a [[phosphor]]-based [[flat panel]] display [[technology]] developed by [[Canada|Canadian]] company iFire Technology Corp. TDEL is based on inorganic electroluminescent (IEL) technology that combines both thick-and thin-film processes.<ref>{{cite news |last1=Akkad |first1=Omar El |title=The next big (flat) thing |url=https://www.theglobeandmail.com/report-on-business/the-next-big-flat-thing/article1096424/ |work=The Globe and Mail |date=17 March 2006 |language=en-CA}}</ref> The TDEL structure is made with glass or other substrates, consisting of a thick-film dielectric layer and a thin-film phosphor layer sandwiched between two sets of electrodes to create a matrix of pixels. Inorganic phosphors within this matrix emit light in the presence of an alternating electric field.

===Color By Blue===
Color By Blue (CBB) was developed in 2003.<ref>{{cite web |title=iFire: Sparking a New Revolution in Flat Panel Technology |url=http://www.ifire.com/ |website=www.ifire.com}}</ref> The Color By Blue process achieves higher [[luminance]] and better performance than the previous triple pattern process, with increased contrast, grayscale rendition, and color uniformity across the panel. Color By Blue is based on the physics of [[photoluminescence]]. High luminance inorganic blue phosphor is used in combination with specialized color conversion materials, which absorb the blue light and re-emit red or green light, to generate the other colors.

==New applications==
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.<ref>{{cite web |url=http://www.Air-EL.com/ |title=air-el |publisher=Federal Signal |access-date=July 23, 2016}}</ref>

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 nightlife industry.<ref>Diana Eng. [https://books.google.com/books?id=Wn9cPkCtzcsC "Fashion Geek: Clothes Accessories Tech"]. 2009.</ref> From 2006, t-shirts with an electroluminescent panel stylized as an audio [[Equalization (audio)|equalizer]], the T-Qualizer, saw a brief period of popularity.<ref>{{Cite web |last=Jain |first=Bupesh |title=T-Qualizer: The beat goes on |url=https://www.cnet.com/tech/computing/t-qualizer-the-beat-goes-on/ |access-date=2022-12-08 |website=CNET |language=en}}</ref>

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% of its original size, the group's HLEC was shown to be capable of being integrated into a [[soft robotics|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.<ref>{{Cite news|url=https://www.sciencedaily.com/releases/2016/03/160303150224.htm|title=Super elastic electroluminescent 'skin' will soon create mood robots|last=Cornell University|date=March 3, 2016|work=Science Daily|access-date=March 4, 2016}}</ref>

==See also==
*[[List of light sources]]
*[[OLED]]
*[[Photoelectric effect]]

==References==
{{Reflist}}

==External links==
* [http://www.indiana.edu/~hightech/fpd/papers/ELDs.html Overview of electroluminescent display technology, and thediscovery of electroluminescence] {{Webarchive|url=https://web.archive.org/web/20120430024805/http://www.indiana.edu/~hightech/fpd/papers/ELDs.html |date=2012-04-30 }}
* [http://www.imperialclub.com/Yr/1960/Panelescent/index.htm Chrysler Corporation press release introducing Panelescent (EL) Lighting on] {{Webarchive|url=https://web.archive.org/web/20061112163831/http://www.imperialclub.com/Yr/1960/Panelescent/index.htm |date=2006-11-12 }}
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* [http://www.imperialclub.com/Yr/1960/Panelescent/index.htm 8 September, 1959.] {{Webarchive|url=https://web.archive.org/web/20061112163831/http://www.imperialclub.com/Yr/1960/Panelescent/index.htm |date=2006-11-12 }}

{{Artificial light sources}}
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[[Category:Condensed matter physics]]
[[Category:Electrical phenomena]]
[[Category:Light sources]]
[[Category:Lighting]]
[[Category:Luminescence]]