Understanding Electroluminescence: A Bright Light Source

Electroluminescence Electroluminescence (EL) is an optical and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field. Electroluminescent display (ELD) Light-emitting diode display (LED) Both types of displays are based on electroluminescence

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 light-emitting diodes) or through excitation by impact of high-energy electrons accelerated by a strong electric field (as with the phosphors in electroluminescent displays)

The application of electroluminescence to display and image formation received great impetus from work on thin-film EL (TFEL), giving rise to devices that are different in structure and mechanism from EL panels using phosphor powder. The phosphor in these devices is not a powder but a thin (about 500 nanometers) continuous film prepared by sputtering or vacuum evaporation. The phosphor film is sandwiched between two thin (about 200 nm) transparent insulating films also prepared by evaporative means.

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 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. The low power consumption makes EL technology one of the most energy efficient light sources available.

Electroluminescent panels are capacitors where the dielectric between the outside plates is a phosphor that gives off photons when the capacitor is charged. http://www.ioa-advertising.com/static/images/en/el_drawing.jpg By making one of the contacts transparent, the large area exposed emits light.

http://www.ioa-advertising.com/static/images/en/el_drawing.jpghttp://www.ioa-advertising.com/static/images/en/el_drawing.jpg The structure of an electroluminescent cell is shown in the illustration; the light is observed through the transparent indium tin oxide electrode. http://www.answers.com/topic/electroluminescence#ixzz2XEhjgvV5

Electroluminescent Panels EL Panels, or EL Sheets are a great form of advertising medium. The thin and flexible nature of EL Panels allow them to be mounted in numerous locations, even curved surfaces. They grab customers' attention and allow your product to be seen during the day or at night, all while saving money. https://www.youtube.com/watch?v=HTWCrSgyFfo

Electroluminescent Panels EL Panels (also called Light Boxes, Light Panels, Lite Panels and Luminescent Panels) are a paper thin laminated panel made from the element Phosphor that has a great glow when an electrical current passes. These light-up panels can be made to nearly any shape, color, or size and provide a nice ambient glow when used at night. Typical EL Panels have a light output of around 100- 150cd/m2.

Electroluminescent Panels When comparing to other LED mediums, like televisions or monitors, which typically range from 200-350cd/m2, they do not appear as bright, however they use far less power, are flexible, and can even be made into various shapes, or even logos. These features make Electroluminescent glow panels a perfect medium for advertising in low lit environments, like subways, nightclubs, restaurants, and perfect for wayfinding for events and festivals. Applications: Bus Stops Wall mounted Displays Billboards Window Displays POS Display stands Reception Desks FSDU s Kiosks CDU's (Counter Display Units) Vending machines Floor Graphics Gaming machines Shelf-edge Displays Vehicle wraps Back Bar Displays

Electroluminescent (EL) Displays Probably the simplest display, at least conceptually, is the electroluminescent or EL panel. Phosphor materials, in some cases identical to those used in the more common CRT, will glow not only when struck with an electron beam but also when subjected to a sufficiently strong electric field. Therefore, placing these materials between electrodes in the now-common row and column arrangement can produce an emissive display with an attractively wide viewing angle.

Thin Film Electroluminescent Displays Thin Film Electroluminescent (TFEL) displays comprise a solid-state glass panel, an electronic control circuit and a power supply. The TFEL glass panel, the heart of the assembly, consists of a luminescent phosphorous layer sandwiched between transparent dielectric layers and a matrix of row and column electrodes (see schematic). The circuit board, which contains the drive and control electronics, is connected directly to the back of the glass panel. A pixel on the display is lit by applying voltage to the row and column electrodes, thus causing the area of intersection to emit light.

File:Rogers Shine-01.jpg Photo of the worlds first electroluminescent billboard campaign, manufactured for Canada by Top Right Optoelectronics Ltd - Winter 2005. Photo by Nick Perry / www.nickperry.ca.

Typical application of EL device is the panel lighting like automotive dash board panel. images It is also used in Audio equipments and other electronic gadgets having displays.

In some makes of Laptops, Powder Phosphor panel is used as the back-light. The lighting of EL device is superior than that of LCD. It is also used in Keypad illumination, Watch dials, Calculators, Mobile phones etc. EL-LIGHT-PANEL The power consumption of EL display is very low so that it is an ideal solution to save power in battery operated devices. The color of EL display may be Blue, Green, White etc.

The basis of the Electroluminescence technology is that small phosphor molecules convert the electric current into light. This process occurs without any heat production. Lamp3 Created with this technology, the electroluminescense lighting panel's characteristics are: the low energy consumption, the long lifetime, the extra thinness and ductility. Therefore it can be used excellently to unique advertising purposes. The light emission is completely homogeneous and broadcasts on one frequency so it can be seen in great distance too.

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: 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

Glass coated with indium 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. Products based on Electroluminescent Lamps Due to the manufacturing process, the EL products represent a higher price range compared to LED lighting. Therefore we specifically recommend this to support medium and long term advertising campaigns or for strengthening the brand image.

George Destriau observes that zinc sulfide(ZnS) glows if electric voltage is impress on it. He names this effect after Losev Lossew-Light 1936 First researches because of the development of transparent conductors(ITO) 1950s 1960 Focusing on thin-film LEC Improvements through the use of double layer insulators 1980s 1967 1980s Application to monochromatic displays Today Developing of televisions with LECs

EL Device Structures The four basic types of EL devices

ac Thin-Film EL Technology The basic structure of ac thin-film EL technology is the double- insulating structure shown below.

The central layer in the insulator sandwich is the phosphor layer which emits light when a large electric field is applied (100V/mm). When thin film insulating layers are used, a current limiting layer is placed on both sides of the phosphor layer making it a more reliable device. The insulating layers limit the maximum current of the capacitive charging and discharging displacement current level. The approximate layer thicknesses of the insulating layers range from 0.3 - 0.5 mm and the phosphor layer tends to be between 0.5 - 1.0 mm.

Color EL-Display The basic structure of an color EL display is shown below. TDEL: TDEL or thick film dielectric EL technology is known for providing a solution to the blue problem. It provides the only full color RGB display technology available at this time.

Color EL-Display TDEL: Thick film dielectric displays have proven to be effective: they have a good brightness (luminosity) and have a decent efficiency. iFire Group and TDK Corporation currently hold the patents for this technology. The phosphor in TDEL is 10K - 20K nanometers thick. Some TDEL like that used in displays uses two layers of phosphors. The bottom thick layer is resistant to dielectric breakdown, so it can transport a higher current and make a brighter light. Above the thick dielectric layer is colored phosphors of ZnMgS:Mn (green) and BaAl2S4:Eu (blue). With this system RGB can be created.

In order to increase the light output, the rear substrate can be made reflective, although this can reduce the display contrast as it also reflects incoming ambient light. Another common design is to place a black (light-absorbing) layer at the rear of the structure, and/or place a circular polarizing layer on the front surface, both done in order to increase the display contrast . The circular polarizer will pass light produced by the display, but ambient light entering the panel and reflecting off the rear surface will not exit the panel due to the reversal of polarization occurring upon reflection.

The energy band diagram for ac EL thin-film is shown in the next figure

ac Powder EL Technology This technology is predominantly used for backlight technology rather than as stand alone display. The device structure is shown below.

To understand the EL mechanism in powder EL, it is first worthwhile to describe the observation of the emission. When the electric field is comparable to the EL threshold, a pair of bright spots form. When the electric field is increased above threshold, the bright spots turn into comet-shaped emission regions. This phenomena is depicted above and can be explained in the following way. EL powders at high temperatures, usually have hexagonal structure, but subsequent cooling transforms them into a cubic structure. During this structural transformation, copper exceeds the solubility limit precipitates on defects in ZnS particles. The result is embedded CuxS conducting needles. Between the CuxS precipitates and ZnS powder heterojunctions are formed. This is depicted below.

Between the CuxS precipitates and ZnS powder heterojunctions are formed. This is depicted below

These CuxS conducting needles act to concentrate or focus an applied electric field at their tips. Therefore, an applied field of 1 - 10 V/mm can create local fields > 100 V/mm. The electric field is strong enough to induce tunneling of holes from one end and electrons from the other. The holes are trapped on copper recombination centers and upon reversal of fields, the emitted electrons recombine with the trapped holes to produce light. The EL emission therefore occurs along the CuxS precipitates. Larger particle sizes lend to longer needles and greater field enhancement. Since EL powder is typically used in backlight applications, lifetime is a key issue. Long lifetime and high luminous efficiency are of course the tradeoff. The half-life of current EL is approximately 2500 - 3000 hours at 200 V (400 Hz).

dc Thin Film EL Technology

The dc thin film EL technology has the simplest structure compared to other EL techniques. However, these devices are mostly experimental and have not yet seen the commercial market. The basic drawback with these dc devices is catastrophic dielectric breakdown, so current limiting layers must always be considered.

dc Powder EL Technology

Although this technology has been predominantly a research curiosity, there have been demonstrations of panels with 640 X 200 pixel resolution. The device structure was shown in the previous figure, consisting of a phosphor layer (30 - 50 mm). This layer is made from fine (0.5 - 1.0 mm) Mn-doped ZnS powder and a small amount of binder. The ZnS powder is deliberately prepared with a CuxS coating. This has no insulating layer in contrast to the ac powder EL technology. To obtain stability and emission uniformity, a forming process is necessary. When a voltage is first applied, a large current flows, the layer heats up and gradually a narrow region ~1 mm thick, adjacent to the transparent anode begins to luminesce.

The figure below depicts the microscopic model of dc powder EL. Electromigration drives all copper out of a region adjacent to the anode, creating a highly resistive copper-free formed region near the anode. This resutls in a very large electric field in the region (~100 V/mm). Under a large electric field electrons, which have tunneled out of the CuxS at the edge of the formed region, are accelerated and excite Mn2+ luminescent centers by impact. This is shown below.

Organic Thin-Film EL Technology One can construct a three layer structure made of a hole-transport layer, a luminescent layer or an electron-transport layer. By separating the luminescent function from the hole or electron transport layer, it becomes possible to select luminescent layer materials freely to obtain different emission colors.

https://www.youtube.com/watch?v=Nq-Ru8j1DGI EL-Applications https://www.youtube.com/watch?v=ZesXSJ0inBg How to make a EL plate https://www.youtube.com/watch?v=RJ2xGFfrND0 EL applications