Electroluminesecnce

PONDICHERRY UNIVERSITY“Center for Nanoscience and Technology”

NAST-621: Elements of Materials Science and Physical Properties of Nanostructured

Materials

By:

Sudama ChaurasiyaM. Tech IInd Sem. (NAST)

Submitted to:

Dr. A. Subramania

ELECTROLUMINESCENCE

ELECTROLUMINESCENCE

Electroluminescence (EL)is an optical phenomenon andelectrical phenomenon inwhich a material emits light inresponse to the passage of anelectric current or to a strongelectric field.

Electroluminescence (EL) isa non-thermal generation oflight.

MECHANISM INVOLVE IN ELECTROLUMINESCENCE

There are two distinct mechanisms that can produce electroluminescence in crystals:

1. Pure or intrinsic

2. Charge injection

1. PURE OR INTRINSIC : In the Pure or intrinsic , no net current passes through the phosphor

(electroluminescent material). The electric field liberate atomic electrons (from donorlevels) into the conduction band.

Light is emitted in the normal way as soon as an electron recombines with anionized atom of the centre.

2. CHARGE INJECTION When an electrode contacts a crystal to provide a flow of electrons or holes

(electron extraction) or a voltage is applied to a p–n junction causing a current to flow;i.e., electrons flow from the n-type material into the p-type material

In both cases, the electrons lose energy when recombining with centers or

positive holes accompanied by the emission of light.

MECHANISM INVOLVE IN ELECTROLUMINESCENCE

Electroluminescence is the result ofradiative recombination of electronsand holes in a material, usually asemiconductor.

The excited electrons release theirenergy as photons (light).

Prior to recombination, electronsand holes may be separated eitherby doping the material to form a p-njunction (in semiconductorelectroluminescent devices such aslight-emitting diodes) or throughexcitation by impact of high-energyelectrons accelerated by a strongelectric field (as with the phosphorsin electroluminescent displays).

Spectrum of a blue/green electroluminescent light source for a clock radio Peak wavelength is at 492 nm and the FWHM spectral bandwidth is quite wide at about 85 nm.

ELECTROLUMINESCENT MATERIALS

• There may be 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:

Powdered zinc sulfide 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 III and Group V elements, such as indium phosphide (InP), gallium arsenide (GaAs), and gallium nitride (GaN). Certain organic semiconductors, such as [Ru(bpy)3]2+(PF6

-)2, where bpy is 2,2'-bipyridine.

SCIENTIFIC ORIGIN

Electroluminescence was first observed in silicon carbide (SiC) by Captain Henry Joseph Round in 1907. The second reported observation of electroluminescence occur in 1923, when O.V. Lossev reported electroluminescence again in silicon carbide crystals. The first thin-film EL structures were fabricated in the late 1950s by Vlasenko and Popkov. Soxman and Ketchpel conducted research between1964 and 1970 that demonstrated the possibility of matrix addressing a TFEL display with high luminance. In 1968, Aron Vecht first demonstrated a direct current (DC) powered EL panel using powdered phosphors.

PRACTICAL IMPLEMENTATION

The most common electroluminescent (EL) devices are composed of either,• Powder (primarily used in lighting applications)• Thin films (for information displays.)

POWDER PHOSPHOR-BASED ELECTROLUMINESCENT PANELS• Frequently used as backlights to liquid crystal displays.• Consuming relatively little electric power.• Convenient for battery-operated devices such as pagers, wristwatches.• Gentle green-cyan glow is a common sight in the technological world.• They do, however, require relatively high voltage (between 60 and 600 volts). this voltage must be generated by a converter circuit within the device

THIN FILM PHOSPHOR ELECTROLUMINESCENT DEVICE• Manufactured for medical and vehicle applications.• TFEL allows for a very rugged and high-resolution display fabricated even on silicon substrates.

ELECTROLUMINESCENT DISPLAY (ELDs)

An electroluminescent (EL) device is similar to a laser in that photons are produced by the return of an excited substance to its ground state. But unlike lasers EL devices require much less energy to operate and do not produce coherent lightThere are four steps necessary to produce electroluminescence in ELDs:1. Electrons tunnel from electronic states at the insulator/phosphor interface.2. Electrons are accelerated to ballistic energies by high fields in the phosphor.3. The energetic electrons impact-ionize the luminescent center or create electron-hole pairs that lead to the activation of the luminescent center, and4. The luminescent center relaxes toward the ground state and emits a photon.

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.

All ELDs have the same basic structure:•There are at least six layers to the device. •The first layer is a baseplate (usually a rigid insulator like glass), •The second is a conductor,•The third is an insulator,•The fourth is a layer of phosphors•The fifth is an insulator, and •The sixth is another conductor

STRUCTURE OF AN ELECTROLUMINESCENT DISPLAY

• Can be produced to any size or shape.• Paper Thin - Typically between 0.25mm - 0.5mm.• Lightweight.• Flexibility - Can be applied to a flat or curved surface.• Strength - Almost unbreakable.• Low power consumption - Consumes between 75-90% less electricity than any other light source.• Efficiency - 80% of energy is converted to light.• Brightness - Brightness can be varied to suit customer requirements.• Visibility - Can be seen from far distances in darkness, smoke and fog.• Low operating temperature - EL is a cold lighting source• Does not generate heat due to its electronic luminous emission.• No Glare - EL produces a soft even light over the entire surface.• Waterproof.• Landfill friendly - EL does not use any hazardous materials.• Long Life- Over 30,000 hours depending upon brightness.

CHARACTERISTICS OF THE ELECTROLUMINESCENT PANELS

ADVANTAGES:-Low wattage-Long life-No external circuitry required (no ballast needed to limit current, it can be plugged directly into AC power and will self-regulate power through it's own resistivity)-Can be manufactured into flat flexible panels, narrow strings, and other small shapes-Can be made into waterproof computer monitors which are more durable and light weight than LCDs or Plasma screens.-Not directional like LCDs when used as a computer monitor, looks good at all angles-EL displays can handle an impressive -60 C to 95 C temperature range, which LCD monitors cannot do

DISADVANTAGES:-Not practical for general lighting of large areas due to low lumen output of phosphors (so far)-Poor lumens per watt rating, however typically the lamp is not used for high lumen output anyway-Reduced lumen output over time, although newer technologies are better than older phosphors on this point-Flexible flat EL sheets wear out as they get flexed, durability is being worked on-The lamps can use significant amount of electricity: 60-600 volts-Typical EL Needs a converter when used with DC sources such as on watches (to create higher frequency AC power, this is audible)

CONCLUSION

Electroluminescent displays (ELDs) have a venerable history starting with the experiments of Captain Henry J. Round in 1907, O.V. Lossev in the Soviet Union, and Georges Destriau in France. Electroluminescence was mostly a scientific curiosity until the invention of thin film deposition techniques and the discovery that a sandwich of conductors, insulators and phosphors could result in a very efficient and long-lasting form of emissive display. Electroluminescent display technology is unique and relevant for today’s embedded display solutions. The performance and visual characteristics of high-performance electroluminscent displays make it an ideal solution for the most challenging and demanding applications.

REFERENCES

1. Alt, Paul M., "Thin Film Electroluminescent Devices: Device Characteristics and Performance," Proceedings of the Society for Information Display, (1984)

2. “A History of Electroluminescent Displays” By Jeffrey A. Hart, Ann Lenway and Thomas Murtha.

3. Electronic Industries Association of Japan (EIAJ), Research Report on the Visions of the Electronic Display Industry in the Year 2000, transl. By InterLingua (Tokyo: EIAJ, 1993)

4. http://en.wikipedia.org/wiki/electroluminescence5. http://www.indiana.edu/hightech/fpd/papers/ELDs.html