thin display full report

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Thin Displays Seminar Report 03

INTRODUCTION

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.

Greater brightness

Faster response time for full motion video

Fuller viewing angles

Lighter weight

Greater environmental durability

More power efficiency

Broader operating temperature ranges

Greater cost-effectiveness

Dept. of ECE MESCE Kuttippuram1


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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 fastthe

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.



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EVOLUTION

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 materials 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 40atoms 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.



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CONSTRUCTION

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.



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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.



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WORKING

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

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 greenemitting 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-



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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 thesolvents 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 color display are locally deposited using ink

jet printing. Here, control of the thickness and shape of the droplet. which

eventually sets into a high resolution pixel, remains an as yetunsolved

problem. hence polymeric OLEDs not Used commonly



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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 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



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Less efficiency is lost by using a single blue or ultra violet OLED to pump

organic fluorescent wave length downconverters, also known as colour

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.



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TECHNOLOGY

Universal Display Corporations 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 singletexcited 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.



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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 FOLEDtechnology, the prospect of roll-to-roll processing is created.

TRANSPARENT OLED(TOLED)

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, siliconwafers 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 information-

content displays using active matrix silicon backplanes.



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Transparency: TOLED displays can be nearly as clear as

the glass or substrate theyre 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 helmetmounted or head-up systems for virtual reality

applications.

Enhanced highambient contrast: TOLED technology

offers enhanced Contrast ratio by using lowreflectance 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.

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



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of structural and processing constraints. Flexible materials have significant

performance advantages over traditional glass substrates.

FOLEDs Offer Revolutionary Features for Displays:

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.

Ultralightweight, 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, large televisionson

thewall.

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



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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 cancontrol 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 Corporations 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.



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VERTICALLY STALKED, HIGH RESOLUTION OLED (SOLED)

The stacked OLED (SOLED) Universal Display Corporations 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 worldwideweb 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) subpixel 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 Corporations 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.



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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:

Full colour tunability: SOLEDs offer fullcolour

tunability for true colour quality at each pixelvaluable when colour

fidelity k important.

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.



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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.

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.



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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 precisionsemiconductor production equipment for LEDs. The equipment will be sold

exclusively by AIXTRON under royalty-bearing licenses from UDC.

ORGANIC LASERS

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 commercialpotential, 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:

Greater color variety



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Tunability

Further miniaturization

Easier processing

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.



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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 siliconas 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 ITT- 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 superiorperformance 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



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ADVANTAGES OF OLEDs

Very slim flat panel

Low power consumption

High brightness

Wide visibility

Quick response time

Wider viewing angle

Self luminous

No environmental draw backs

No power intake when turned off.



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DISADVANTAGES OF OLEDs

Vulnerable to shorts due to contamination of substrate

surface by dust particles.

Voltage drops

Mechanically fragile

Potential not yet realized

APPLICATION OF OLEDs

Car display

Cellular phone

Mobile computer

Audio visual device

Household machine



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CONCLUSION

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

liquidcrystal 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.



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REFERENCES

ELECTRONICS FOR YOU MAY-2003

WWW.UNJVERSAL DISPLAY.COM

WWW.IEEE.ORG

WWW.EMAGIM.COM

http://www.emagim.com/http://www.emagim.com/


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ABSTRACT

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 asdefense and research more efficient in operation.

With this stage, the purpose of this seminar is to throw light in to the

capabilities of OLEDs and brief study of their technology.



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CONTENTS

INTRODUCTION

EVOLUTION

CONSTRUCTION

WORKING

FULL COLOUR DISPLAY

TECHNOLOGY

A MAJOR BREAKTHROUGH IN OLEDS

ADVANTAGES OF OLEDs

DISADVANTAGES OF OLEDs

APPLICATION OF OLEDs

CONCLUSION

REFERENCES



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ACKNOWLEDGEMENT

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 preparationand presentation of this seminar.

I also extend my sincere thanks to all the faculty members of

Electronics and Communication Department for their support and

encouragement.

Soorya.S

D f ECE MESCE K i

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