Electroluminescence

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 nm and the FWHM spectral bandwidth is quite wide at about 85 nm.

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).

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 may be separated either by doping 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 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:

• 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 0.08 W at 230 V, and dates from 1960; lit diameter 59 mm)

The most common electroluminescent (EL) devices are composed of either powder (primarily used in lighting applications) or thin films (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 instrument panels 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 (between 60 and 600 volts).^[1] 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.^[2]

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. In 1992 Timex introduced its Indiglo EL display on some watches.

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 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.^[3][4]

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.

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.

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

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.

See also

• Indiglo
• Illuminator
• Light emitting capacitor
• List of light sources
• Electroluminescent wires
• Electroluminescent display
• Organic light-emitting diode
• Photoelectric effect

References

1. ^ Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition,McGraw-Hill, New York, 1978, ISBN 0-07-020974-X pg22-28
2. ^ Fink 1978 p. 22-28
3. ^ Ron Khormaei, et al., "High Resolution Active Matrix Electroluminescent Display", Society for Information Display Digest, p. 137, 1994.
4. ^ http://www.planar.com/advantages/whitepapers/docs/overview.pdf

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.
• Technology and Colours