Materials

=__** Substrates / Materials **__ =  __**ITO**__ Indium tin oxide is one of the most widely used transparent conducting oxides because of its two chief properties: 1. electrical conductivity 2. optical transparency Like other transparent conducting oxides, a compromise has, however, to be reached during its film deposition, as high concentration of charge carriers will increase the material's conductivity, but decrease its transparency. Thin films of indium tin oxide are most commonly deposited on surfaces by electron beam evaporation, physical vapor deposition, or a range of sputter deposition techniques.

"For smaller areas a transparent polyester foil with ITO coating is commonly used. Applications: LCD displays, membrane switches, etc. The foil has a resistance of 5 Ohm/sq. The coating can be applied on glass, polycarbonat or foil. Please specify the size in mm." [|www.faradaycages.com]
 * ITO foil**


 * Electroluminescent Ink**

There are two methods of producing light using electricity, incandescence and luminescence.

=Incandescence= Electric current is passed through a conductor (filament) whose resistance to the current, caused by collision of electrons, causes it to heat up and emit light. Greater the heat of the filament, higher is the amount of light it emits.

=Luminescence= It is defined as the emission of visible radiation which does not derive energy from the temperature of the body. Fluorescence, phosphorescence and bio-luminescence fall in this category. Energy for emitting visible light may be derived from chemical reaction (chemoluminescence), reaction of biologically produced enzymes with oxygen in deep sea marine life and glow worms and fireflies(bioluminescence), absorption and re-emission of photons (photoluminescence) or by passage of electric current or exposure to electric field (electroluminescence).

=Electroluminescence=

Electroluminescence is defined as the emission of light under the action of an electric field which is distinct from black body light emission which results from heat (incandescence)
 * øDefinition**


 * øMechanism of EL**

Electroluminescence is the result of radiative recombination of electrons and holes in a material, usually a semiconductor. The excited electron release their energy as light. The electrons and holes need to separated by doping the material to form a p-n junction (as in LEDs) or through excitation by impact of high-energy electrons accelerated by a strong electric field

There are two general types of electroluminescence: 1.) Electroluminescence that occurs under the action of passing electric current (Lossev effect). This type of electroluminescence is commonly used in electroluminescent devices. The mechanism of this electroluminescence is different for inorganic and organic semiconductors. Inorganic semiconductors usually emit light due to a process of recombination of electrons and holes that meet together in an emitting layer or on a border between n- and p-type semiconductors (p-n-junction). The mechanism of electroluminescence in organic semiconductors involves formation of excited molecules (excitons) followed by their recombination. This recombination may give rise to either light or heat emission (vibrational or thermal dissipation).

2.) Electroluminescence that occurs under the action of an electric field on a substance is known as "Destriau effect". This eletroluminescence occurs when a material is placed in the electric field without of directly attached electrodes. Electroluminescence may result in either fluorescence or phosphorescence, or involve both mechanisms.

Source: Hart, Jeffrey A. et al. "A History of Electroluminiescent Displays" Sept 1999, Nov 1 2008.


 * Band model to illustrate Electroluminescence (EL)**

Source: pmTUC; Prof. Arved Hübler: Lecture Display technologies.

The Green EL phosphor used in the printing the display of the prototype was manufactured by Gwent Electronic Materials Ltd.

=EL foil=

Another intended approach was to assemble pieces of a ready EL foil obtained from Schreiner Printonics.

With a thickness below 1 mm, exceptionally flexible, water-resistant and also resistent to extreme temperatures electroluminescence films are ideally suited for a wide range of applications. The shape of electroluminescence films can be unusual due to their material properties. So it is possible to reach shapes by cutting, die-cutting, deforming and bending. There are practically no limits to individual requirements. Depending on the material electroluminescence films can be combined with various fabrics, transparent materials and special-effects films to create attractive backlighting. When the system is switched off the electroluminescence film is completely invisible.
 * Ø Features and Design: **

Schreiner PrinTronics makes individually and flexibly printed electroluminescence films with colors, patterns, decors and graphic elements across the whole surface and in transparent versions.

With a look at the design you find the advantages in the homogeneous light, that means that the uniformly distributed luminescent pigments across the whole surface create a harmonious and flicker-free light. The film thickness below 1 mm and the connectors require little space and offer more room for new ideas. By comparing electroluminescence film with standard lighting solutions such as LEDs, el sheets require significantly less cables or connections, which results in space and cost savings. The advantages in the production are lying in the contacts which are simply connected. So there is no necessary for additional wiring of the luminescent elements as the conducting paths are printed on the el film. El films consume very less power as against lasers and don´t generate any heat. Therefore they enable flexible combinations with other materials. Also all materials are recyclable.
 * Ø Advantages: **

However the cutting of the foil and assembling it into a finished display would have made it extremely complicated and hence the idea to use the foil was dropped.


 * __Other possible methods /materials for realising a display__**


 * oLED Display**

An organic light emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes. Generally, at least one of these electrodes is transparent.

oLED is an upcoming and very versatile display technology, with prospective super-low production costs and higher degree of flexibility among other advantages.

The oLED materials we inquired about were not available for application by screen printing, eg. the back electrode needs to be applied by vacuum evaporation process which is a critical and expensive process. Hence we gave up the idea of printing with oLED. Another problem is the lesser reliability of oLED materials due to susceptibility to water (moisture) damage which needs advanced sealing methods and UV sensitivity and short lifetime all of which mean that it is not usable as a display material for this product at the moment.


 * Electrochromic inks**

Electrochromism is the reversible change in colour of electrochromic materials on application of a small DC charge. This effect could also be used as an approach for preparing the display. ( Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. Electrochromism: Fundamentals and Applications; VCH: Weinheim, Germany, 1995.)

The possible advantage was that no front mask would be needed and the ink could be overprinted with plain non-conductive ink to achieve an uniform look when no charge is applied. The disadvantage is that a front (or possibly back) light would be necessary to illuminate the display in the dark which will consume more power and result in the display being rendered less flexible.

Hence the idea was dropped in favour of the readily available and usable EL inks to have a model as a proof of principle.

Further research into the use of EC or even oLED displays can be done according to economics and feasibility of production.