Electroluminescence

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Liquid crystal display with and without EL foil backlight

Electroluminescence (EL), also known as the Destriau effect , is a form of luminescence in which a solid is excited by applying an electric field or voltage to emit electromagnetic radiation , e.g. B. in the form of light to emit .

history

Dashboard of a 1966 Dodge Charger with electroluminescent lighting

The effect was discovered by the scientist Georges Destriau , who in 1936 experimented on a metal alloy in the laboratory of nuclear physicist Marie Curie to test its conductivity . The zinc sulfide he used was accidentally contaminated with copper and showed light emission when he applied strong electric fields to the alloy.

Technical use was not yet possible because of the very short service life of the test set-ups at the time. This mostly resulted from insufficient sealing of the light-emitting layers from oxygen and water. Destriau further developed the material into a product ready for series production, the so-called electroluminescent film , which was initially used by the military in cockpits and for wing lighting . Today, due to advances in materials science and better encapsulation (in the sense of the above "isolation"), a use z. B. possible as background lighting for LC displays .

In 1962, the light-emitting diode (LED) was invented, a semiconductor component that emits light when an electrical voltage is applied. The underlying phenomenon is also referred to as electroluminescence in this case, although the physical mechanism differs from the effect discovered by Destriau (see “ Mechanisms and components ”). Since then, light-emitting diodes have been widely used as display and lighting elements. With the development of organic light-emitting diodes (OLED) since 1987, which are also counted among the electroluminescent components, the field of application of LEDs has expanded again.

Mechanisms

Schematic representation of electroluminescent components

Of the conceivable physical mechanisms of stimulating a solid body to emit light by applying an electric field, the following two are of technical importance.

Alternating field excitation (Destriau effect)

The radiator, for example an electroluminescent film , is constructed like a capacitor . The electroluminescent material, usually zinc sulfide doped with metals (e.g. ZnS: Mn), is insulated between the electrodes. By applying a strong alternating electric field, electrons are accelerated in the electroluminescent layer. Impacts at the impurities generated by the doping move electrons into excited states , which fall back into the ground state with the emission of light. The field strength required for excitation is 10 kV / cm. A few 100 V with a frequency of 200 to 4000 Hz are sufficient for thin layers.

DC voltage excitation (pn junction)

Doped semiconductors generate light in the pn interface of a light-emitting diode at low DC voltage. Various semiconductor materials are used, e.g. B. AlGaAs , GaAsP , GaP and GaN , whose band gap determines in particular the color of the emitted light.

Organic light emitting diodes operate on a similar principle, but they are as Schottky diodes built up, so have two electrodes with different in contrast to inorganic LEDs work functions on. As emitting materials, organic semiconductors such. B. Alq3 is used.

Other suggestions

If the definition of electroluminescence is not limited to solids, gas discharge lamps are also included in the strict sense , but the term is not common in this context.

Components

See also

literature

  • RH Mauch: Electroluminescence in thin films . In: Applied Surface Science . tape 92 , 1996, pp. 589-597 , doi : 10.1016 / 0169-4332 (95) 00301-0 .
  • Leni Akcelrud: Electroluminescent polymers . In: Progress in Polymer Science . tape 28 , no. 6 , 2003, p. 875-962 , doi : 10.1016 / S0079-6700 (02) 00140-5 .
  • Ullrich Mitschke, Peter Bauerle: The electroluminescence of organic materials . In: Journal of Materials Chemistry . tape 10 , no. 7 , 2000, pp. 1471-1507 , doi : 10.1039 / a908713c .

Web links

Individual evidence

  1. ↑ George Destriau: AC electroluminesence in ZnS . In: J. Chimie Phys . tape 33 , 1936, pp. 587 .
  2. ↑ Nick Holonyak, Jr., SF Bevacqua: Coherent (visible) light emission from Ga (As 1-x P x ) junctions . In: Applied Physics Letters . tape 1 , 1962, pp. 82 .
  3. ↑ CW Tang, SA VanSlyke: Organic electroluminescent diodes . In: Applied Physics Letters . tape 51 , 1987, pp. 913 .
  4. ^ RH Mauch: Electroluminescence in thin films . In: Applied Surface Science . tape 92 , 1996, pp. 589 .