This seminar topic deals with ecological and economic aspects of flexible display along with some technical aspects of and applications. The basic functioning of the displays is discussed, as well as a few of its major implementations and concurrent developments. The core of the topic is an analysis of the economic and ecological impact of the aforementioned applications.
??????????????????? We all want a laptop display that looks as if it was printed on paper, where texts and images would not wash out in bright light and can be view from any angle. Better yet we want displays that can be conjured from our pockets and unrolled to access information anywhere while wireless internet keeps our roll-up display continuously updated. Some day food packages will display health messages and clothing will have displays sewn into the fabrics. You might think of these exciting prospects and doubt their validity. The truth is flexible displays are not commercialized yet, but they are on the brink of becoming a reality as enabling technologies pave the way. Currently there are four types of flexible display close to commercialization, flexible liquid crystal display, flexible organic light emitting diode, gyricon displays, and electrophoretic displays.
A durable and flexible display with low-power consumption, high-contrast ratio, has been a technical challenge for years. They have to be lightweight, rugged, and in some cases, conformal, wearable, rollable and unbreakable. The recent successful integration of flexible display technologies and the traditional web-based processing and/or inkjet technologies has opened up the possibility of low cost and high throughput roll-toroll manufacturing and has shown the potential to replace the paper used today.
A flexible display cannot rely on a normal layer of glass as used in displays common at the time since glass does not fulfill the criteria of flexibility. Instead of glass it is possible to build displays on metal foil and a variety of plastics, each of which pose many difficult issues waiting to be resolved. For example, a plastic substrate replacingglass would need to over some properties of glass, i.e. clarity, dimensional stability, thermal stability, barrier, solvent resistance and a low coefficient of thermal expansion coupled with a smooth surface. No plastic isomers all these properties, yet, so any plastic-based substrate will almost certainly be a multilayer composite structure.
1. ENABLING TECHNOLOGIES
Before introducing the different types of flex displays, an overview of the enabling
technologies is necessary. These technologies include many components that must be compatible and convergent to enable a truly flexible display. The necessary technologies include robust flexible substrates, conducting transparent conducting oxides and /or conducting polymers, electro-optic materials, inorganic and organic electronics, and packaging technologies.2 In addition to these technologies, many processes must also be developed and optimized in conjunction with the materials development, such as roll-toroll manufacturing, and printing.
1.1 Flexible Substrates
??????????????? The primary flexible substrate candidates are plastics and thin glass. Plastic substrates are inexpensive, roll-to roll processable and can be laminated to multi-layers,but they also impose limitations with respect to thermal processing and barrier performance. Companies are developing coatings for these substrates as well as new plastic substrates to compensate for these constraints. Thin glass substrates exhibit better thermal stability and have higher visual transparency than plastics, but cannot fully bend and are not compatible with roll to roll processing.The use of thin metal substrates is a complementary approach to the glass and plastic displays. Flexible metallic substrates provide excellent barrier properties, thermal and dimensional stability over a broad temperature range. In addition, they offer potential integration with backplane technology for active-matrix displays.
??????????????? Since flexible displays utilize polymer materials, a barrier layer is essential inprotecting and enclosing the functional materials and layers from oxygen and degraded water. Since organic materials tend to oxidize and hydrolyze, oxygen and water permeation through a flexible substrate is of particular importance flexible electronics. Although single-layer barrier layers do provide the packaged materials with some protection, it appears that multiple layers are necessary for organic light emitting diode applications for long-term stability.
1.3 Organic and Inorganic Conducting Layers
Indium tin oxide is the typical conducting layer used in display technology because ofits excellent sheet resistance and optical clarity. However, the process temperature required for ITO on glass is incompatible with plastic substrates. Therefore lower temperature processes have to be developed for ITO in order for it to be considered for flexible display applications. When ITO is deposited on a polymeric substrate, it can crack under tensile strain and cause catastrophic failure. Conducting polymers are also being considered for flexible display applications. Although their sheet resistance and optical properties are not as attractive as ITO, they donhave exceptional mechanical properties and low process temperatures. As ITO and conducting polymer technology compete for the conducting substrate solution, there is a new conducting substrate technology based on nanotechnology. Flexible and transparent electrodes have been formed from carbon nanotube dispersions in the combination with wet coating processes and printing techniques.
1.4 Electro-optic Materials
The various types of electro-optic materials for flexible display fall into three categories ? emissive, reflective, and transmissive. For emissive applications, small molecules and polymers are being used for OLED applications. In order to have a truly low power display, a reflection mode of operation will have to be implemented on flexible substrates. Polymer-dispersed liquid crystals, encapsulated electrophoretics, gyricon, and bichromic ball composites all operate in the reflective mode. For electronic book and paper applications, an efficient reflective mode display is crucial to eliminate the power consuming backlight.
1.5 Thin Film Transistor
For many electro-optic materials, such as OLEDs, polymer-dispersed liquid crystals, electrophoretics and Gyricon materials, an active matrix backplane will be required for high resolution. The success of TFTs for plastic substrates to date has been an enabler for flexible flat panel displays and constitutes a very vital component. Currently, poly and amorphous silicon are the standards for TFTs for flexible displays. However, organic thin film transistors on polymeric substrates are also being considered as a candidate for flexible, light weight and inexpensive switching device.
A TFT backplane may be deformed by internally produced forces.? These include stresses built-in by film growth, by differential thermal expansion or contraction, and by the uptake or release of humidity.? A backplane also may be deformed by an external force that bends it, shapes it conformally, or elastically stretches and relaxes it.? We survey how mechanical stress may be applied to or develop in a TFT backplane.
1.6 Roll to Roll Processing
????????????????? Flexible displays are amendable to a roll-to-roll manufacturing process which would be a revolutionary change from current batch process manufacturing. Roll to roll processing is where materials are processed and rolled back up. If roll-to-roll manufacturing technology matures for display processing, it promises to reduce capital equipment costs, reduce display part costs, significantly increase throughput, and it may potentially eliminate component supply chain issues if all processes are performed with roll-to-roll techniques. Although batch processing can still be employed to manufacture flexible flat panel displays, many researchers and technologists believe that roll-to-roll manufacturing will ultimately be implemented.
2. FLEXIBLE LCD
Flat panel displays generally consist of four layers: a back substrate providing mechanical strength; a plane of switches on the substrate that addresses each pixel; a light-controlling layer; and a front panel that holds the top electrodes, encapsulates the light-control layer and offers support. In liquid crystal displays, the substrate is usually glass coated with amorphous silicon or organic conductor, in which the pixel-switches (TFTs) are patterned. Trapped between these electronics and the front glass is the liquid crystal material, which acts as a light-controlling layer. The spacing between the two of glass pieces must be carefully controlled to make light-control layer work. Making this sandwich of materials flexible requires finding a set of technologies that can combine to create a matrix of individually addressable pixels that will flex. Since the rigidity of a device increases with the cube of its thickness, reducing the thickness of the glass substrate is an obvious step to take. One method to accomplish this is to etching the glass after the display is complete. Some researchers also tried to replacing the substrate with a flexible plastic, but producing reliable amorphous silicon electronics on a flexible substrate is very difficult using conventional lithographic patterning techniques.In addition, as the display is flexed to different radiuses, maintaining a fixed electrode gap is extremely demanding.
One flexible LCD very close to commercialization is the cholesteric LCD fromKent Displays Inc. This display utilizes a liquid crystal material originally derived fromanimal cholesterol, hence the name cholesteric. This LCD will be a full-color screen, 160mm across the diagonal, which is slightly larger than Pocket PC screens. In addition, each color pixel in the display consists of a red, a blue, and a green cell stacked on top of each other, instead of side by side as in today?s full-color laptop LCDs. As a result, the cholesteric LCD?s resolution is far superior than that of current laptop displays.
2.1Passive Matrix LCD
Apply small bias to perpendicular lines of electrodes.Bias strong enough to darken bit at line intersection.
Advantage: Simple to implement
Disadvantage: Can cause distortion(?ghosting? or ?crosstalk?)
Each cell has its own thin-film transistor (TFT)Addressed independently from behind LCD
Direct addressing scheme
Advantages: Sharp display, better viewing angle, 40:1 contrast
Disadvantages: Need better backlight, complex hardware
3. OLED DISPLAY
Organic light emitting diodes (OLED) display is another promising technologyfor flexible flat panel displays. Flexible OLEDs are very lightweight and durable. Theiruse in devices such as cell phones and PDAs can reduce breakage. Potentially, OLEDs can be embedded in fabrics to create ?smart? clothing.
3.1 How OLEDs work
OLEDs, like regular LEDs, operate on the principle of electroluminescence,where injected charge carriers recombine and generate light. All OLEDs have four basic components: substrate, anode, organic layers, and cathode. Flexible substrate materials are usually plastic, thin glass or metal foils. The anode is a transparent layer of metal of low work function which serves to remove electrons when a current flows through the device. The cathode is a metal layer of high work function which injects electrons when a current flows through the device. In between the cathode and the anode are the organic layer(s) where transport and recombination of the electrons and holes occur. Dependingon the device, the OLED could have one, two or multiple organic layers. Figure 1 showsthe structure of a bilayer device.4 Finally a top cover glass is used to encapsulate thedevice.
Figure 1. Bilayer OLED consists of two organic layers: emissive and conducting
3.2 Manufacturing OLED
The major part of manufacturing OLEDs is applying the organic layers to the substrate. This can be economically done in two ways, organic vapor phase depositionand inkjet printing. Organic vapor phase deposition involves a carrier gas and a lowpressure, hot-walled reactor chamber. The carrier gas transports evaporated organic molecules onto cooled substrates, where they condense into thin films. Using a carrier gas increases the efficiency and reduces the cost of making OLEDs.With inkjet technology, the organic layers are sprayed onto substrates just like inks are sprayed onto paper during printing. Inkjet printing greatly reduces the cost of OLED manufacturing by enabling roll to roll processing. In addition, it allows OLEDs to be printed onto very large films for large displays like electronic billboards.
3.3How do OLEDs Emit Light?
OLEDs emit light in a similar manner to LEDs, through a process called?? electro-phosphorescence.
The process is as follows:
The cathode gives electrons to the emissive layer of organic molecules.
The anode removes electrons from the conductive layer of organic molecules. (This is the equivalent to giving electron holes to the conductive layer.)
When an electron finds an electron hole, the electron fills the hole (it falls into an energy level of the atom that's missing an electron).
When this happens, the electron gives up energy in the form of a photon oflight (see How Light Works).
Each type has different uses. In the following sections, we'll discuss each type of OLED. Let's start with passive-matrix and active-matrix OLEDs.
3.4Passive-matrix OLED (PMOLED)
PMOLEDs have strips of cathode, organic layers and strips of anode. The anode strips are arranged perpendicular to the cathode strips. The intersections of the cathode and anode make up the pixels where light is emitted. External circuitry applies current to selected strips of anode and cathode, determining which pixels get turned on and which pixels remain off. Again, the brightness of each pixel is proportional to the amount of applied current.
PMOLEDs are easy to make, but they consume more power than other types of OLED, mainly due to the power needed for the external circuitry. PMOLEDs are most efficient for text and icons and are best suited for small screens (2- to 3-inch diagonal) such as those you find in cell phones, PDAs and MP3 players. Even with the external circuitry, passive-matrix OLEDs consume less battery power than the LCDs that currently power these devices.
3.5Active-matrix OLED (AMOLED)
AMOLEDs have full layers of cathode, organic molecules and anode, but the anode layer overlays a thin film transistor (TFT) array that forms a matrix. The TFT array itself is the circuitry that determines which pixels get turned on to form an image.
AMOLEDs consume less power than PMOLEDs because the TFT array requires less power than external circuitry, so they are efficient for large displays. AMOLEDs also have faster refresh rates suitable for video. The best uses for AMOLEDs are computer monitors, large-screen TVs and electronic signs or billboards.
Even when the technological challenges are met, there still is a piece missingfrom the flexible displays puzzle, the production equipment. 1 Currently there is noinfrastructure to produce plastic displays in any volume. It will be a few years beforethere are sufficient roll-to-roll lines to produce displays that will significantly increase the market share. 1 In addition, OLED?s commercialization is restrained by key patents held by Kodak and other firms. It is expected that OLED display technology become widespread once the patents had expired.
Many researchers have attempted to create displays using a light controlling material that require a cell. For example, Xerox Corp experimented with a material called Gyricon. Gyricon are spherical beads with one black and one white hemisphere. The spheres are only 100um in diameter and make a disply that is only 200um thick. In the display, the beads are dispersed in a transparent rubber sheet and suspended in oil, allowing it to rotate in response to an electric field. For a one polarity, the white hemisphere faces the viewing direction. Reversing the field polarity will cause the black sphere to be seen. The orientation of the beads stays the same even after the field is removed, allowing images to be stored. In addition, no backlight is needed to view an image on the rubber sheet. The display consumes energy only when forming an image and even this is at very low power. The Gyricon rubber sheet is thin, robust and highly flexible. It can be made in large sheets or cut by designers to fit the application. The optical properties of the Gyricon are similar to those of paper, making it attractive for future display applications such as book and newspaper readers.
In the 1970s a researcher at Xerox Palo Alto Research Centre (PARC), Nicholas K. Sheridon, had the idea of embedding microscopic beads into a flexible film.? Each bead is half black and half white.? An electric field applied beneath a bead will cause the bead to rotate in place in order to show either its white (paper) or black (ink) side, as shown in figure 1 below.? Sheridon named this technology ?Gyricon?, meaning ?rotating image? in Greek.? Some years later, Sheridon and Xerox PARC founded Gyricon Media  to market this idea.
5. ELECTROPHORETIC DISPLAY
In 1995, independent of Sheridon, Joseph Jacobson thought of transparent microcapsules containing both a dark liquid dye and particles of white titanium dioxide.? An electric charge applied beneath a microcapsule would either draw the titanium oxide to the bottom, revealing the dark dye (ink) or move the titanium oxide to the top, revealing white (paper), as shown in figure 2 below.? This movement of charged particles in liquid due to electric fields, called electrophoresis, resulted in Jacobson naming his technology ?electrophoretic ink?, or e-ink.? Following further research at Massachusetts Institute of Technology (MIT) Media Laboratory, Jacobson founded the E In.
Even thinner than the Gyricon are electrophoretic displays created by E-ink of Massachusettes. The electrophoretic material consists of a gel suspension of tiny capsules, each containing positively-charged white particles and negatively-charged black particles as shown in figure 4. 1 A monolayer of the material is sandwiched betweena substrate and a top glass electrode layer. When an electric field is applied between the top and bottom electrodes, the particles move within the capsules to reflect or absorb incident light. Varying the field strength or the addressing time on each pixel can also provide some control of grey scale.
Figure 4. E-ink?s electrophoretic display. White particles are positively charged, black ones negative.
E-Ink's technology has many advantages for flexible flat panel displays. First, the location of individual pixels is defined by the addressing electrodes, and the electrode gap is not critical (unlike in LCD). In addition, electrophoretic display is ideally suited for flexible display applications due to their thin form factor and inherent flexibility. It uses ultra-low power and is easily read under any lighting from all viewing angles. While E Ink's display materials already enable fully flexible displays, flexible backplanetechnology for high-resolution, active matrix displays is in the development stage.
6. E-PAPER PRODUCTION
Both Gyricon Media and E Ink Corp. have been continuing their research toward the common goal of producing a thin, flexible ?sheet? of high-resolution display material.? The first commercial application for each company?s technology has been store signage; producing signs that can be changed electronically and that consume no power between changes.? While Gyricon has continued to work in this field, E Ink has concentrated research on developing their technology for use in portable devices such as eBooks.
E Ink has generated much high profile interest, which bodes well on the future of the eBook.? Lucent Technologies licensed their plastic transistor technology to E Ink, enabling flexible displays to be produced, as demonstrated in November 2000 ; in April 2001, using IBM?s active matrix technology, a display was produced with a size and resolution comparable to laptop displays of the time; and working with the TOPPAN Printing Company of Japan, May 2001 saw E Ink unveil a 3-bit colour display capable of eight different colours.
With all the technology and support behind them, E Ink look set to achieve their ultimate goal in as little as two years  ? ?radio paper?.? Radio paper will be a flexible electronic paper capable of producing at least 12-bit colour (over 4,000 different hues) at a resolution more than comfortable for close reading.? The displayed content of radio paper will be updated via a wireless data network, keeping it entirely portable.
This technology sets the scene for not only electronic books, but also electronic newspapers.? Picture a double-page spread from a broadsheet newspaper that you can hold, fold and roll up like any newspaper, where, instead of turning a page,
3. Transparent Car Navigation System on Windshield
By the developments in flexible diplay we are able tomake scroll laptops.This makes portability reliable.requires only less space.
While prototypes of flexible displays have been available for several years, thefirst commercial flexible flat panel display product has just become available in 2004 in the form of a display in digital cameras by Kodak.7 Currently, most interest in flexible display is from the military, where ?smart? clothing for outdoor survival is highly desired. The public still thinks of flexible flat panel display as ?cool? but fictional. While flexible displays could capture revenues in the growing handheld device market, much will depend on whether low-cost manufacturing can be achieved. As for large area displays like computer monitor and electronic billboards, much time and processing improvementwill be needed before flexible display can take over. Despite the obstacles, flexible display market is estimated to grow 5-7% over the next 2-5 years and the current market projections range anywhere form $100-500 million by 2010.
As the components and manufacturing processes of flexible electronics mature, the concept of flexible flat panel display will eventually become a reality. Flexible displays offer tremendous advantages over conventional flat panel displays, like light weight, durability, low power consumption, portability etc. In particular, OLED displays offer bright sharp images at wide viewing angles and bright light, but are difficult to encapsulate. Gyricon and electrophoretic displays have thin form factors and can beviewed at a great range of angels, but their high resolution displays still require development. LCD displays are already mature, but making the sandwich-structured device flexible is still challenging. Once technical difficulties are overcome and roll to roll processing becomes feasible, flexible flat panel displayswill widely commercialize and enter all of our lives.
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