In the Modem era where technology is at high state, need for new machineries and instruments are a prerequisite. Demand for high efficient measuring system and interactive displays make user-friendly capabilities. In the entertainment section high precision imaging is needed for efficient operation.
With advent of OLEDs, conventional LEDs and LCDs are becoming history. High imaging techniques of OLEDs make the critical fields such as defense and research more efficient in operation.
With this stage, the purpose of this seminars is to throw light in to the capabilities of OLEDs and brief study of their technology.
As thin display technology improves, TV sets will be available in wider shapes and sizes. Fold-up TVs and wearable TVs, similar to wearable PCs will be launched soon.
A plethora of display technologies catering to different requirements of different applications are available. Electroluminescent(EL) displays, vacuum fluorescent displays (VFDs), and light emitting diodes (LEDs) have a wide operating temperature range and luminescent characteristics
OLEDs are lightweight, durable, power efficient and ideal for portable applications. OLEDs have fewer process steps and also use both fewer and lower- cost materials than LCD displays. Universal Display believes that OLEDs can replace the current technology in many applications due to the following performance advantages over LCD.
Â¢ Greater brightness
Â¢ Faster response time for full motion video
Â¢ Fuller viewing angles
Â¢ Lighter weight
Â¢ Greater environmental durability
Â¢ More power efficiency
Â¢ Broader operating temperature ranges
Â¢ Greater cost-effectiveness
Displays are an essential element of industrial and Consumer products. Once considered as simple indicator lights, these have become vital in the performance of computing, medical imaging, process control and entertainment systems. From video systems to kitchen appliances and portable computers, or even your digital watch, displays are inevitable.
Most devices provide users with useful information on a display. In a way, the display is an indicator of the quality of that product. If the information depicted is consistently wrong-for instance, a watch always running fastâ€the quality of the product is perceived as poor. The same is true with information display. The poor quality of the image displayed means that the product is also of poor quality.
A plethora of display technologies catering to different requirements of different applications are available. Electroluminescent (EL) displays, vacuum fluorescent displays (VFDs), and light-emitting diodes (LEDs) have a wide operating temperature range and luminance characteristics, but limited colour reproduction. EL displays are suited to instrumentation and military applications, while VFDs are used in automotive dashboards and traffic lights. Organic LEDs (OLEDs) are likely to replace liquid crystal displays (LCDs) in the near future.
Digital display technology (DDT) is a breakthrough in visual technology. It enables engineers to create thin wide-screen TVs, flat TVs, wall TVs, and even TV screens that you can fold up like newspapers. Professor Ifor Samuel at the University of St Andrews in Scotland, who is conducting research on this technology, predicts, In a few years, it will be possible to make TV screens which can he rolled up when not in use, information displays on roller blinds, and light-emitting clothing for fashion or safety applications. So days are not far off when you will be able to customise the shape of your TV as if it were an article of clothing.
Organic semiconductors have been the subjects of intense scientific investigation for the past 50 years. These materials primarily consist of carbon, hydrogen and oxygen. Organic materialsâ„¢ weak intermolecular bonds in the solid state give them properties of both semiconductors and insulators. Organic semiconductors attracted industrial interest when it was recognized that many of them are photoconductive under visible light. This discovery led to their use in electro photography and as light valves in LCDs.
Organic materials have often proved to be unstable. When exposed to air, water, or ultraviolet light, their electronic properties can degrade rapidly. Low carrier mobility characteristic of organic materials obviates their use in high- frequency (greater than 10 MHZ) applications. These shortcomings are compounded by the difficulty of both purifying and doping the materials.
But in1987 Ching Tang and Steven Van Slyke of Eastman Kodak Co., Rochester, N.Y., successfully addressed many of these problems when they produced the first efficient light emission from a two-layer organic structure resembling a pn junction. The Kodak group used a class of synthetic dyes to develop a device called a small-molecule OLED that produced light with about 1 percent efficiency. The materials used consist of often no more than 30 or 40 atoms covalently bonded into stable individual molecular units called monomers.
Unlike small molecule compounds, polymers are long chain molecules whose monomer segments are attached in a continuous covalently bonded high- molecular weight chain. Polymers tend to be environmentally rugged and flexible although, like small molecules, their electronic properties can rapidly degrade when exposed to oxygen or water.
A fundamental difference between small molecule and polymeric device is the manner in which they are constructed.
Small molecule OLEDs are grown on a glass or plastic substrate to form a multi-layer structure about 100 nm thick. The substrate is first coated with a conducting transparent electrode such as indium tin oxide (ITO) which serves as the anode. This is followed by a thin hole-transporting organic layer HTL. Typically made from chemicals called diamines. An organic light-emitting layer of comparable thickness is then deposited on the ETL surface. A low work function is necessary to ensure efficient low-resistance injection of electrons from the cathode on to the ETL.
Changing composition of the layers tunes the OLED emission colors across the visible spectrum. Green emission can be achieved by doping an electron-conducting organic matrix called Alq3, with either a small amount of an iridium phosphor or fluorescent dyes. The pigment perylene when doped in to an ETL known as CBP emits blue light. Lanthanide complexes and porphyrin pigments have been used to efficiently emit red light when doped in to Alq3 or CBP. As in small-molecule devices, changing the chemistry of the polymer can tune the color of an OLED.
Both polymeric & small molecule OLEDs operate by accepting charge carriers of opposite polarities, electrons and holes, from the cathode and anode contacts respectively. An externally applied voltage drives these carriers into the recombination region where they form a neutral bound state, or exciton. There are two types of excitons formed, called singlets and triplets. On average one singlet and three triplets are formed for each four electron hole pairs injected into the exciton formation region of the OLED.
Recombination of the singlet occurs within a few nanoseconds of formation. This leads to a photon emission and is called fluorescence. Recombination of the triplet exciton is slow (taking about 1 ms to 1 second) and when it does occur usually results in heat rather than light. But if a heavy â€ metal atom such as iridium or platinum is placed in an organic molecule, the characteristics of singlet and triplet excitons mix speeding the emission of light to within l00ns-l00s. The kind of emission is called phosphorescence.
Currently, efficiencies of the best doped polymer and molecular OLEDs exceed that of incandescent light bulbs. Efficencies of 20 lumens per watt have been reported for yellow greenâ€emitting polymer devices, and 40lm/W attained for phosphorescent moleculer OLEDs, compared to less than 20 lm/W for a typical incandescent light bulb. Soon efficiencies of 80lm W a value comparable to that of fluorescent room lighting will be achieved using phosphorescent OLEDs.
Polymer OLED structures can be simpler than small-molecule structures. The first polymer layer (in contact with ITO) can serve solely as a hole-injecting/conducting layer, in some cases a single layer is used for electron and hole injection, conduction and light emission. Polymer OLEDs also often operate at lower power than small-molecule devices. Due to their high conductivity, polymer- based devices have operating voltages in the 2-5V range, which is l-2V lower than small molecule OLEDs.
The challenge to making full-color polymer-based displays is very different from that for making such displays using small-molecule OLEDs. Solution chemistry makes it difficult to deposit and pattern a polymer pixel of one color, and then repeat the process using a second color emitter because the solvents employed may dissolve or attack the devices already on the substrate, Several schemes have been suggested to dodge this problem. One particularly promising method involves depositing a single blue-emitting polymer, and then selectively diffusing green and red dyes into adjecent regions. However, it has proved difficult to keep the diffusing dyes from bleeding into regions nearby. Seiko Epson Corp. of Nagano, Japan, and Cambridge Display Technology Ltd., Cambridge. England are pursuing a second approach in which the various polymer constituents of a full â€ color display are locally deposited using inkâ€jet printing. Here, control of the thickness and shape of the droplet. which eventually sets into a high resolution pixel, remains an as yetâ€unsolved problem. hence polymeric OLEDs not Used commonly
FULL COLOUR DISPLAY
One the of the principal reason that OLED technology has attracted such intense interest is its potential for use use in-full colour displays that might eventually replace active matrix LCDs. A display consists of a matrix of contacts made to the bottom and top surfaces of each organic light â€ emitting element or pixel. To generate a full-colour image, it is necessary to vary the relative intensities of three closely spaced, independently addressed pixels, each emitting one of the three primary colours of red, green or blue.
Optical filtering of white OLEDs can produce acceptable red, green and blue emission as in the diagram. But this method sacrifices efficiency due to the large amount of light absorbed in the filters
Less efficiency is lost by using a single blue or ultra violet OLED to pump organic fluorescent wave length downâ€converters, also known as colourâ€changing media (CCM) as in diagram. Each CCM filter consists of a material that efficiently absorbs blue light and re-emit the energy as either green or red light, depending on the compound used.
Organic thin films may lead to the practical realization of low-cost very high-resolution, full-colour displays.
Universal Display Corporationâ„¢s OLED (Organic Light Emitting Device) technology is focused on a number of key areas, including:
a) High Efficiency Materials
b) Transparent OLED (TOLED)
c) Flexible OLED(FOLED)
d) Passive and Active Matrices
e) Vertically Stacked, High Resolution OLED (SOLED)
f) Organic Vapor Phase Deposition (OVPD)
g) Organic Lasers
h) Patterning by Stamping
HIGH EFFICIENCY MATERIALS
UDC and its research partners have developed families of highly efficient OLED materials. These materials emit light through the process of electrophosphorescence.
In traditional OLEDs, the light emission is based on fluorescence, a transition from a singletâ€excited state of a material. According to theoretical and experimental estimation, the upper limit of efflciency of an OLED doped with fluorescent material, is approximately 25%.
With our electrophosphorescent materials used as a dopant, which exploits both singlet and triplet excited states, this upper limit is virtually eliminated. Equipped with the potential of 100% efficiency. UDC is working towards the commercialization of electrophosphorescent devices by optimizing the device efficiency, color purity and device storage and operation durabilities.
Such a process is facilitated by the development and modification of charge transport materials, charge blocking materials and luminescent materials and their incorporation into devices. In addition to the fabrication of high quality devices. UDC is also committed to a high standard of device testing. Our scientists and engineers have custom-developed sophisticated test hardware and software for this purpose.
Cost effective processing: OLEDs are projected to have full-production level cost advantage over most flat panel displays. With the advent of FOLED technology, the prospect of roll-to-roll processing is created.
The Transparent OLED (TOLED) uses a proprietaly transparent contact to create displays that can be made to be top-only emitting, bottom-only emitting, or both top and bottom emitting (transparent). TOLEDs can greatly improve contrast, making it much easier to view displays in bright sunlight.
Because TOLEDs are 70 % transparent when turned off, they may be integrated into car windshields, architectural windows, and eyewear.
Their transparency enables TOLEDs to be used with metal foils, silicon wafers and other opaque substrates for top-emitting devices.
Directed Top Emission: TOLEDs also provided an excellent way to achieve better fill factor and characteristics in high resolution, high â€ information-content displays using active matrix silicon backplanes.
Â¢ Transparency: TOLED displays can be nearly as clear as the glass or substrate theyâ„¢re built on. This feature paves the way for TOLED to be built into applications that rely on maintaining vision area. Today, smart windows are penetrating the multi-billion dollar flat glass architectural and automotive marketplaces. Before long, TOLEDs may be fabricated on windows for home entertainment and teleconferencing purposes, on windshields and cockpits for navigation and warming systems and into helmetâ€mounted or head-up systems for virtual reality applications.
Â¢ Enhanced highâ€ambient contrast: TOLED technology offers enhanced Contrast ratio by using lowâ€reflectance absorber (a black backing) behind either top or bottom TOLED surface, contrast ratio can be significantly improved over that in most reflective LCDs and OLEDs. This feature is particularly important in daylight readable applications, such as on cell phones and in military fighter aircraft cockpits.
Â¢ Multi-stacked devices: TOLEDs are a fundamental building block for many multi-structure (i.e. SOLEDs) and hybrid devices. Bi-directional TOLEDs can provide two independent displays emitting form opposite faces of the display. With portable products shrinking and desired information content expanding, TOLEDs make it possible to get twice the display area for the same display size.
FLEXIBLE OLED (FOLED)
FOLEDs are organic light emitting devices built on flexible substrates. Flat panel displays have traditionally been fabricated on glass substrates because of structural and processing constraints. Flexible materials have significant performance advantages over traditional glass substrates.
FOLEDs Offer Revolutionary Features for Displays:
Â¢ Flexibility: For the first time, FOLEDs may be made on a wide variety of substrates that range form optically-clear plastic films to reflective metal foils. These materials provide the ability to conform, bend or roll a display into any shape. This means that a FOLED display may be laminated onto a helmet face shield, a military uniforms shirtsleeve, an aircraft cockpit instrument panel or an automotive windshield.
Â¢ Ultraâ€lightweight, thin form: The use of thin plastic substrates will also significantly reduce the weight of the f1at panel displays in cell phones, portable computers and especially, largeâ€ televisionsâ€onâ€theâ€wall.
For eg.. the weight of a display in a laptop may be significantly reduced by using FOLED technology.
Durability: FOLEDs will also generally be less breakable, more impact resistant and more durable compare to their glass-based counterpart.
PASSIVE AND ACTIVE MATRICES
How passive Matrix works
Passive Matrix displays consist of an array of picture elements, or pixels, deposited on a patterned substrate in a matrix of rows and columns. In an OLED display, each pixel is an organic light emitting diode, formed at the intersection of each column and row line. The first OLED displays like the first LCD (Liquid Crystal Displays), are addressed as a passive matrix. This means that to illuminate any particular pixel, electrical signals are applied to the row line and column line (the intersection of which defines the pixel). The more current pumped through each pixel diode, the brighter the pixel looks to our eyes.
How Active Matrix works
In an active matrix display, the array is still divided into a series of row and column lines, with each pixel formed at intersection of a row and column line. However, each pixel now consists of an organic light emitting diode (OLED) in series with a thin film transistor (TFT). The TFT is a switch that can control the amount of current flowing through the OLED.
In an active matrix OLED display (AMOLED). information is sent to the transistor in each pixel, telling it how bright the pixel should shine. The TFT then stores this information and continuously controls the current flowing through the OLED. In this way the OLED is operating all the time, avoiding the need for the very high current necessary iii a passive matrix display.
Universal Display Corporationâ„¢s proprietary technologies should enable extremely efficient active matrix OLEDs. Our new high efficiency material systems are ideally suited for use in active matrix OLED displays, and their high efficiencies should result in greatly reduced power consumption. The TOLED architecture enables the organic diode, which is placed in each pixel to emit its light upwards away from the substrate. This means that the diode can be placed over the TFT back plane, resulting in a brighter display.
UDC is collaborating with other organizations to develop TFT technology compatible with plastic substrates, in order to commercialize highly efficient bright active matrix flexible OLEDs.
VERTICALLY STALKED, HIGH RESOLUTION OLED (SOLED)
The stacked OLED (SOLED) Universal Display Corporationâ„¢s award- winning, novel pixel architecture that is based on stacking the red, green, and blue subpixels on top of one another instead of next to one another as is commonly done in CRTs and LCDs. This improve display resolution up to three-fold and enhances full-color quality. SOLEDs my provide the high resolution needed for wireless worldwideâ€web applications.
What is a SOLED
SOLED display consists of an array vertically-stacked TOLED sub- pixels. To separately tune color and brightness, each of the red, green and blue (R- GB) subâ€pixel elements is individually controlled. By adjusting the ratio of current in the three elements, color is tuned. By varying the total current through the stack,brightness is varied. By modulating the pulse width, gray scale is achieved. With this SOLED architecture, each pixel can, in principle, provide full color. Universal Display Corporationâ„¢s SOLED technology may be the first demonstration of an vertically integrated structure, where intensity, colour and gray scale can be independently tuned to achieve high-resolution full-colour.
Scalable to large pixel size: In large screen displays, individual pixels are frequently large enough to be seen by the eye at short range. With the S x S format, the eye may perceive the individual red, green and blue instead of the intended colour mixture. With a SOLED, each pixel emits the desire colour and, thus, is perceived correctly no matter what size it is and from where it is viewed.
In addition to being a transparent light emitter, the top indium-tin-oxide surface of the TOLED can serve as the hole-injecting electrode for a second TOLED built on top of the first device as shown
Each device in the stack is then independently addressable and can be tailored to emit its own colour through the adjacent transparent organic layers, the transparent contacts, and the glass substrate. This allows the entire area of the vertically stacked pixel to emit any mixture of the three primary colours.
SOLED architecture is a significant departure from the traditional side by side (SxS) approach used in CRTs and LCDs today compared to SxS configuration, SOLEDs offer compelling performance enhancements:
Â¢ Full colour tunability: SOLEDs offer fullâ€colour tunability for ËœËœtrueâ„¢â„¢ colour quality at each pixelâ€valuable when colour fidelity k important.
Â¢ High resolution: SOLEDs also offer 3x higher resolution than the comparable SxS display. While it takes 3 SxS pixels (an R,G,B)lo generate full colour display, it takes only one SOLED pixel or one-third the area to achieve the same. This is especially advantageous when maximizing pixel density is important.
Â¢ Nearly 100% fill factor: SOLEDs also maximize fill factor. For example. when a full colour calls for green, the red &blue pixels are turned off in the SxS structure. By comparison all the pixels turn on green in a SOLED under the same conditions. This means that SOLED color definition and picture quality are superior.
Â¢ Scalable to large pixel size: In large screen displays, individual pixels are frequently large enough to be seen by the eye at short range. With the S x S format, the eye may perceive the individual red, green and blue instead of the intended colour mixture. With a SOLED, each pixel emits the desire colour and, thus, is perceived correctly no matter what size it is and from where it is viewed.
ORGANIC VAPOR PHASE DEPOSITION (OVPD)
Universal Display Corporation and its research partners at Princeton University have developed a transformational technology which can reduce the cost and increase the efficiency of the OLED production process.
The technology, Organic Vapor Phase Deposition (OVPD), can enable a low cost, precise, high throughput process for fabricating OLEDs.
The OVPD production process utilizes a carrier gas stream in a hot walled reactor at very low pressure to precisely deposit the thin layers of organic materials used in OLED displays. Conventional OLED fabrication equipment evaporates the organic molecules at high temperature and pressure. OVPD offers the ability to precisely control the multi-source deposition required for full-color OLED displays. The OVPD design should also be adaptable to the rapid, uniform deposition of organics on large-area substrates and for roll-to-roll processing.
Universal Display is developing the production equipment in partnership with AIXTRON AG of Aachen, Germany, the leading manufacturer of precision semiconductor production equipment for LEDs. The equipment will be sold exclusively by AIXTRON under royalty-bearing licenses from UDC.
Organic lasers, based on UDC's pioneering work with Princeton University, have the potential to revolutionize yet another industry.
An organic laser is a solid-state device based on organic materials and structures similar to those used in UDC's display technologies. An optically-pumped organic laser demonstrates five key laser characteristics: spatial coherence, a clear threshold, strongly polarized light emission, spectral line narrowing, and the existence of laser cavity modes. To realize commercial potential, the key technical challenge today is to demonstrate a mechanism for the electrical pumping of these lasers.
The use of small-molecule organic materials opens the door to an entirely new class of light emitters for diode lasers. These organic lasers may offer:
Â¢ Greater color variety
Â¢ Further miniaturization
Â¢ Easier processing
Â¢ Lower cost in a host of end uses
Potential applications include optical memories (e.g., compact discs and digital versatile discs (DVDs), CD-ROMs, optical scanners, sensors, and laser printers.
PATTERNING BY STAMPING
Universal Display Corporation and its research partners, Princeton University and University of Southern California, have developed a novel process for patterning electrodes in OLEDs which shows promise of making them more efficient and less expensive to manufacture.
This invention is based on a cold welding process to pattern electrodes to sizes as small as 12 microns. UDC has been granted the exclusive worldwide license for these and other associated technologies.
This technology is potentially cost-effective, offers high throughput and is well-suited for large-area and roll-to-roll fabrication. It is a valuable addition to UDC's portfolio of innovative OLED technologies for this emerging industry.
A MAJOR BREAKTHROUGH IN OLEDS
Chi Mei Optoelectronics Corporation (CMO), Taiwan, and IBM Japan, have jointly developed OLEDs based on advanced amorphous silicon. The full-colour 50.8 cm (20-inch) OLED displays consumes less power than competitive flat-panel technologies and allows full-size computer displays and flat-panel TV screens. The costs involved in fabrication are less.
OLEDs have long been heralded as the display technology of the future, but have so far failed to compete with conventional technologies beyond displays of small size and law information content, such as car radio and cell phone displays that are available in the market. A major limitation has been the expensive polycrystalline silicon which drives light emission in the organic layers.
The revolutionary prototype by CMO and IBM offers amorphous silicon as a perfect alternative. Amorphous silicon is an unordered material structure, which can, unlike polycrystalline silicon, be implemented easily and cost-effectively over large areas. The use of existing ËœITT- LCD manufacturing prpcess and facility for commercial production of OLEDs is another milestone achieved with the tech nology.
The advanced amorphous silicon cir cuitry, together with superior performance characteristics of the light-emitting layers themselves as well as the overall device architecture, results in a display that is comparable to a high-end LCD display of the same size and resolution, with half the power consumption at typical desktop-dis play brightness, better colour saturation, and larger viewing angle. The display has WXGA resolution (1280 x 768) and bright ness of 300 cd/rn and consumes 25W power. The full video capability extends its usab to large flat-panel TVs. 0
ADVANTAGES OF OLEDs
Â¢ Very slim flat panel
Â¢ Low power consumption
Â¢ High brightness
Â¢ Wide visibility
Â¢ Quick response time
Â¢ Wider viewing angle
Â¢ Self luminous
Â¢ No environmental draw backs
Â¢ No power intake when turned off.
DISADVANTAGES OF OLEDs
Â¢ Vulnerable to shorts due to contamination of substrate surface by dust particles.
Â¢ Voltage drops
Â¢ Mechanically fragile
Â¢ Potential not yet realized
APPLICATION OF OLEDs
Â¢ Car display
Â¢ Cellular phone
Â¢ Mobile computer
Â¢ Audio visual device
Â¢ Household machine
Organic materials are poised as never before to transform the world of circuit and display technology. Major electronics firms such as Philips and Pioneer, and smaller companies such as Cambridge Display Technology, Universal Display and Uniax, are betting that the future holds tremendous opportunity for the low cost and some times surprisingly high performance offered by organic electronic and opto electronic devices. Using organic light-emitting devices (OLEDs), organic full colour displays may eventually replace liquidâ€crystal displays (LCDs) for use with laptop and eve desktop computers. Such displays c deposited on flexible plastic coils, eliminating the fragile and heavy glass substrates used in LCDs, and can emit bright light without the pronounced directionality inherent in LCD viewing, all with efficiencies higher than can be obtained with in incandescent light bulbs.
Organic electronics are already entering commercial world. Multi colour automobile stereo displays are now available from Pioneer Corp., of Tokyo and Royal Philips Electronics ,Amsterdam is gearing up to produce born OLED back lights to be used in LCDs and organic integrated circuits.
The first products using organic displays are already being introduced into the market place. And while it is always difficult to predict when and what future products will be introduced, many manufactures are now working to introduce cell phones and 1 digital assistants with OLED displays with in the next one or two years. The ultimate goal of using high efficiency, phosphorescent flexible OLED displace in lap top computers and even for home video applications may be no more than a few years in to the future.
The portable and light weight OLED displays will soon cover our walls replacing the bulky and power hungry cathode ray tube.
Â¢ ELECTRONICS FOR YOU MAY-2003
Â¢ WWW://UNJVERSAL DISPLAY.COM
Â¢ FULL COLOUR DISPLAY
Â¢ A MAJOR BREAKTHROUGH IN OLEDS
Â¢ ADVANTAGES OF OLEDs
Â¢ DISADVANTAGES OF OLEDs
Â¢ APPLICATION OF OLEDs
I extend my sincere thanks to Prof. P.V.Abdul Hameed, Head of the Department, Electronics and Communication Engineering, for providing me his invaluable guidance for the Seminar.
I express my sincere gratitude to my Seminar Coordinator and Staff in Charge Mr. Manoj K, for his cooperation and guidance in the preparation and presentation of this seminars.
I also extend my sincere thanks to all the faculty members of Electronics and Communication Department for their support and encouragement.