Electroluminescence

    Electroluminescence (EL) is an optical phenomenon and electrical phenomenon where 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) or from the action of chemicals (chemoluminescence).

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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

• 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.
• Organic semiconductors - such as [Ru(bpy)₃]²⁺(PF₆^-)₂, where bpy is 2,2'-bipyridine

Practical implementations

Electroluminescent automotive IP 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. This system was reintroduced by Ford approximately 40 years later as a "new development".^{[citation needed]}

Sylvania 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.

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 Inc. in Japan and Planar Inc. in USA. Here, bright, long life light emission is achieved in thin film yellow-emitting manganese-doped zinc sulphide 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 or is coated with reflective metal. Additionally, other transparent conducting materials, such as carbon nanotubes coatings or PEDOT can be used as the front electrode.

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.

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.

EL devices have low power consumption when compared with neon signs, and have a wide range of applications such as their use on advertising boards and safety signs. Because an EL layer can be very thin (around 1 mm thick), it can be used as decoration added to everyday items, including clothing and accessories such as bags and earphone cords.

See also

• INDIGLO
• Illuminator
• Light emitting capacitor
• List of light sources