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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 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.
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 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 Photoluminescence
luminescence excited by an electric field. Such luminescence is observed in gases or crystal phosphors whose atoms or molecules undergo a transition to an excited state when some type of electric discharge occurs.
The electroluminescence of gases—that is, the luminescence of electric discharges in gases—has been studied since the middle of the 19th century and is used in gas-discharge light sources. The electroluminescence of solids was discovered in 1923 by the Soviet scientist O. V. Losev in SiC and in 1936 by the French scientist G. Destriau in single crystals of ZnS doped with Cu or Cl. Of the various types of electroluminescence of solids, the most important are injection electroluminescence and high-field electroluminescence.
Injection electroluminescence is characteristic of an SiC or GaP p-n junction that is biased in the forward direction and that is connected to a source of direct voltage. In this case, either excess holes are injected into the n-region and excess electrons are injected into the p-region or both electrons and holes are injected into the thin high-resistivity layer between the n- and p-regions. Luminescence is produced when the electrons and holes recombine in the high-resistivity layer.
High-field electroluminescence is observed in, for example, ZnS powder that is doped with particles of a metal, such as Cu or Al, and that is embedded in a dielectric between the plates of a capacitor to which an alternating voltage is supplied. In each half-cycle, a region containing an intense electric field is formed on the sides of a ZnS crystal that face the cathode. Electrons emitted into the high-field region from the surface of the crystal are accelerated by the field and ionize atoms of the crystal lattice. The holes produced are trapped by luminescent centers. In each alternate half-cycle, the field is oriented in the opposite direction and causes the electrons to return to the luminescent centers, where they recombine with the holes to produce luminescence.
The electroluminescence of solids is used for display units based on electroluminescent panels (see Figure 1) or on light-emitting diodes. Such units include symbolic displays with luminescent numerals, letters, and other characters that may be changed by switching contacts. They also include matrix displays for the production of complex luminescent images (see), graphic panels, and image converters.
REFERENCESPrikladnaia elektroliuminestsentsiia. Moscow, 1974.
Vereshchagin, I. K. Elektroliuminestsentsiia kristallov. Moscow, 1974.
M. V. FOK