PLED DISPLAYS
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PRESENTED BY:
SUNAINA.N

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INTRODUCTION
 PLEDs using LEP are the next generation display types that promise excellent results as well as cost and power efficiency
 LEPs can emit light when a voltage is applied across it.
 A thin semiconducting polymer sandwiched between two electrodes.
 They are usually made by Ink jetting process.
 film thickness uniformity is obtained by multi passing heads with drive per nozzle technology.
 Displays using LEPs have small size , no viewing angle restriction low power requirement etc.
 Future applications include flexible displays, wearable displays etc
Basic Concepts
The origin comes from the discovery that conducting polymers exhibit electroluminescence and LEP was born .
LIGHT EMITTING POLYMER
• It has a thin-film of semiconducting polymer sandwiched between two electrodes (anode and cathode)
• Electrons and holes are injected from the electrodes, recombination of charge carriers takes place, it leads to emission of light that escapes through glass substrate.
• Band gap determines the wavelength of emitted light.
CONSTRUCTION
 These devices consists of active/emitting layer sandwiched between cathode and anode.
 Indium-tin oxide used for anode
 Aluminum or calcium for cathode
 For manufacturing polymer spin coating machine used
INK JET PRINTING
 The line process starts with cleaning of the substrate or the glass plate onto which the PLEDs are printed. The glass plate has certain surface structure with ‘pits‘ that serve as pixels.
 The displays have 96 x 48 pixels; each pixel consists of three sub pixels. One subpixel has a size of 79 x 234 micrometers.
 Once the plate has been cleaned, it is taken to the inkjet printer. This printer fill each subpixel with a mixture of solvent and polymer with high precision. The glass plate is then baked in an oven.
 In next step, the emerging PLEDs are taken to a further inkjet printer that applies a mixture of solvents and colour components in three subsets.
 It is followed by another oven, where the solvent is removed from the product.
 The plate is then taken to a vacuum vessel, where a barium-aluminium cathode is vapor-deposited under a strong vacuum.
 The cathode is surrounded with several layers of silicon nitride and organic compounds : Thin film encapsulation.
 After the treatment in the vacuum vessel, the whole PLED is furnished with a further coating for protection against scratching and impact.
 In the last the PLEDs are cut from the glass plate.
 Inkjets should be at reliably high frequency.
Active and Passive Matrix
 displays consist of a matrix of pixels, formed at the intersection of rows and columns deposited on a substrate. Each pixel is a PLED.
 To control pixels, and so form the image required, either 'passive' or 'active' matrix driver methods are used
 Passive matrix electrical components that do not supply their own energy to turn‘on’ or ‘off’ desired pixels.
 Each row and each column of the display has its own driver, and to create an image, the matrix is rapidly scanned to enable every pixel to be switched on or off as required
 The active displays have a transistor built into each pixel. This thin film transistor(TFT) acts like a switch precisely controlling the voltage each pixel receives. .
 Active matrix displays solve the problem of efficiently addressing each pixel by incorporating a transistor (TFT) in series with each pixel which provides control over the current and hence the brightness of individual pixels.
Principles and Technology
• Electroluminescence is the principle used for light emission.
• It is the process in which EM radiation is emitted from a material by passing an electrical current through it.
• The frequency of the EM radiation is directly related to the energy of separation between electrons in the CB and in the VB.
• The device that accomplishes this electron-hole interaction is that of a diode, which consists of an n-type material interfaced with p-type material . When the diode is forward biased the electrons cross a neutralized zone at the interface to fill holes and thus emit energy.
 Band theory is used to explain semi conductance of PPV
 The VB is filled with ∏ electrons in the chain and is completely filled, while the CB is being made up of empty ∏* orbital's
 PPV sandwiched between an electron injector (or cathode), and an anode.
 Electron injector inject electrons of sufficient energy to exceed the band gap, anode operates by removing electrons from polymer , leaving regions of positive charge called holes.
 The anode is referred to as the hole injector.
 When captured, an electron and a hole form neutral-bound excited states (termed excitons) that quickly decay and produce a photon up to 25% of the time, 75% of the time, decay produces only heat, this is due to the possible multiplicities of the exciton.
 The frequency of the photon is tied to the band-gap of the polymer.
 Capture is essential for a current to be sustained
 Electronic polymers possesses a conjugated p-electron system along its backbone, giving it the ability to support positive and negative charge carriers with high mobility's along the chain.
 The application of semiconducting conjugated polymers, that are based on polythiophene, PPV, and polyfluorene (PFO), among others in emerging display technologies of organic electroluminescence has generated immense
 The popularity of polyfluorenes (PFOs) as LEPs is due to their efficient electroluminescence coupled with their high chemical stability.The related poly(fluorenylene ethynylenes) (PFEs) have an intense solid-state fluoresce interest.
 PPV has been one of the most studied series of light-emitting polymers (LEPs) due to its excellent luminescent and mechanical properties. the goal is to achieve electroluminescence that spans the visible and near-infrared regions.
Cathode ray tube
 It is an emissive display
 A vacuum tube using a hot filament to generate thermo electrons.
 Electrostatic and/or magnetic fields to focus the electrons into a beam attracted to the high voltage anode which is the phosphor coated screen
 Electrons colliding with the phosphor emit luminous radiation
 Although wide viewing angle, wide operating temperature range, moderate life is there it requires high voltage, difficult to recycle and is heavy
Liquid crystal display
 it is a reflective display
 it uses the property of liquid crystal in an electric field to guide light from oppositely polarized front and back display plates
 when the driver presents the correct electric field it guides the light through 90° from one plate
 Although they are of low cost backlight adds cost and often limits the useful life and is having viewing angle restrictions
 LEPs achieve high brightness at low drive voltage and current, resulting in low power consumption.
 Figure shows the cross section of 3 addressable sub pixels in a PLED.
 Each pixel consists of glass substrate, indium tin oxide (ITO) anode, a hole conduction layer, the light-emitting polymer layer, and the top cathode.
 In an active-matrix display, the array is divided into a series of row and column lines,with each pixel at the intersection of row and column line.
 Each pixel has an organic light-emitting diode (OLED) in series with a thin-film transistor (TFT).The TFT is a switch which controls the amount of current flowing through the OLED
 In an active-matrix OLED display information is sent to the transistor in each pixel, dictating the brightness of the pixel.
 The TFT then stores this information and continuously controls the current flowing through the OLED. In this manner the OLED is operating continuously, avoiding the need for the very high currents necessary in a passive-matrix display.
 The technology used in an active-matrix screen tends to be a more expensive but better quality than a passive-matrix display.
 Fabrication method depend on materials used.
 In molecular system, based on photo luminescent dye, vacuum deposition can be used but some times aggregation of vacuum deposited molecules may cause destruction of layout structure and degradation of device.
 But polymeric material has stronger mechanical strength and are less crystalline than low molecular weight material.
 Thin polymer films can be obtained by spin coating, ink jet printing etc…
In spin-coating, liquefied organic material is applied to a substrate which is then spun, at rates of 1200-1500 revolutions per minute, to uniformly spread the organic material and it may then be patterned as required.
With ink-jet printing techniques, the substrates can be made more flexible while keeping the production costs low. This means that PLEDs can be used for larger displays such as monitors or television sets.
 fe
 Feature that make LEP prime candidate for next generation flat panel displays
Here's just a few ideas which build on the versatility of light emitting materials.
 High efficiency displays running on low power and economical to manufacture will find many uses in the consumer electronics field. Bright, clear screens filled with information and entertainment data of all sorts may make our lives easier, happier and safer.
 Demands for information on the move could drive the development of 'wearable' displays, with interactive features.
 It can be fabricated on flexible substrates opens up fascinating possibilities for formable or even fully flexible displays
 Because the plastics can be made in the form of thin films or sheets, they offer a huge range of applications..
 Clothes made of the polymer and powered by a small battery pack could provide their own cinema show.
 Camouflage, generating an image of its surroundings picked up by a camera would allow its wearer to blend perfectly into the background
 A fully integrated analytical chip that contains an integrated light source and detector could provide powerful point-of-care technology.
 Ultra-light, ultra-thin displays, with low power consumption and excellent readability features thus brightens product incorporating this technology
 Polymer light-emitting diodes (PLED) can easily be processed into large-area thin films using simple and inexpensive technology.
 Philips will demonstrate its first 13-inch PolyLED TV prototype based on polymer OLED.It uses high-accuracy multi-nozzle, multi-head inkjet printers.
 The excellent and sparkling image quality of Philips' PolyLED TV prototype illustrates the great potential of this new display technology for TV applications.
ADVANTAGES
 Require only 3.3 volts and have lifetime of more than 30,000 hours.
 Low power consumption.
 Self luminous.
 No viewing angle dependence.
 Display fast moving images with optimum clarity.
 Cost much less to manufacture and to run than CRTs because the active material is plastic.
 Can be scaled to any dimension.
 Fast switching speeds that are typical of LEDs.
 No environmental draw backs.
 No power in take when switched off.
 All colours of the visible spectrum are possible by appropriate choose of polymers.
 Simple to use technology than conventional solid state LEDs and lasers.
 Very slim flat panel.
DISADVANTAGES
 Vulnerable to shorts due to contamination of substrate surface by dust.
 Voltage drops.
 Mechanically fragile.
 Philips and pioneer and smaller companies such as Cambridge Display Technology are betting that the future holds tremendous opportunity for low cost and surprisingly high performance offered by organic electronic and optoelectronic devices.
 organic full colour displays may eventually replace LCDs in laptop and even desktop computers.
 many manufactures are working to introduce cell phoned and personal digital assistants with organic displays within the next few years.
 LCDs using passive or active matrices captured portable devices and are expanding into large screen applications
 Next generation displays will be lighter ,thinner, flexible, more adaptable,and power efficient
 Alorich Material science magazine
 Introduction to Optoelectronics by Wilson and Hawkes
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