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A Seminar Report on

Light Emitting Polymer

Prepared by : Krishna D. Patel

Roll No. : 51

Class : B.E. 4th I & C

Semester : 8th

Year : 2010-2011

Guided by : Prof. Brijesh B. Naik

Department

Of

Instrumentation & Control Engineering

Sarvajanik College of Engineering & Technology

Dr R.K. Desai Road,

Athwalines, Surat - 395001, India.

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Sarvajanik College of

Engineering & Technology

Dr R.K. Desai Road,

Athwalines, Surat - 395001,

India.

Department

Of

Instrumentation & Control Engineering

CERTIFICATE

This is to certify that the Seminar report entitled Light Emitting

Polymer is prepared & presented by Mr. Patel Krishna D. Class

Roll No 51 of B.E.IV Sem VIII Instrumentation & Control

Engineering during year 2010-2011. His work is satisfactory.

Signature of Guide Head of Department

Inst. & Control Engineering

Signature of Jury Members

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Acknowledgement

I take this opportunity to express my sincere gratitude to Prof. Brijesh Naik for his

valuable suggestion and guidance, without which the successful completion of this seminar

report would not have been possible.

I would further like to thanks our Head of Department and all the faculty members for

giving me this opportunity, and also thanks to all my friends & other who have directly or

indirectly helped me in preparing this report.

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ABSTRACT

The seminar is about polymers that can emit light when a voltage is applied to it. The

structure comprises of a thin film of semiconducting polymer sandwiched between two

electrodes (cathode and anode).When electrons and holes are injected from the electrodes, the

recombination of these charge carriers takes place, which leads to emission of light .The band

gap, ie. The energy difference between valence band and conduction band determines the

wavelength (colour) of the emitted light.

They are usually made by ink jet printing process. In this method red green and blue

polymer solutions are jetted into well defined areas on the substrate. This is because, PLEDs

are soluble in common organic solvents like toluene and xylene .The film thickness

uniformity is obtained by multi-passing (slow) is by heads with drive per nozzle

technology .The pixels are controlled by using active or passive matrix.

The advantages include low cost, small size, no viewing angle restrictions, low power

requirement, biodegradability etc. They are poised to replace LCDs used in laptops and CRTs

used in desktop computers today.

Their future applications include flexible displays which can be folded, wearable

displays with interactive features, camouflage etc.

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Index

1.

1. INTRODUCTION............................................................................................62. LIGHT EMITTING POLYMER......................................................................8

2.1 CHEMISTRY BEHIND LEP......................................................................82.2 BASIC STRUCTURE AND WORKING...................................................92.3 TYPES OF OLED DISPLAYS.................................................................102.4 PIXEL UNIFORMITY..............................................................................142.5 MANUFACTURING................................................................................162.6 PRODUCTION OF OLEDs......................................................................182.7 A SOLID STATE SOLUTION.................................................................182.8 OTHER TYPES OF OLED.......................................................................20

2.8.1 TOLED................................................................................................202.8.2 FOLED................................................................................................212.8.3 SOLED................................................................................................22

3. ADVANTAGES AND DISAVANTAGES....................................................243.1 ADVANTAGES:.......................................................................................243.2 DISAVANTAGES:...................................................................................24

4. APPLICATIONS AND FUTURE DEVELOPMENTS.................................264.1 APPLICATIONS.......................................................................................26

4.1.1 PHOTOVOLTAICS ...........................................................................264.1.2 POLY LED TV ..................................................................................264.1.3 MP3 PLAYER DISPLAY ..................................................................27

4.2 FUTURE DEVELOPMENTS ..................................................................275. SUMMARY....................................................................................................306. REFERENCE..................................................................................................31

1. INTRODUCTION

The field of semi conducting polymers has its root in the 1977 discovery of the semi

conducting properties of polyacetylene. This breakthrough earned Alan Heeger, Alan

MacDiarmid, and Hideki Shirakawa the 2000 Nobel Prize in Chemistry for ‘the discovery

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and development of conductive polymers’. The physical and chemical understanding of these

novel materials has led to new device applications as active and passive electronic and

optoelectronic devices ranging from diodes and transistors to polymer LEDs, photodiodes,

lasers, and solar cells. Much interest in plastic devices derives from the opportunities to use

clever control of polymer structure combined with relatively economical polymer synthesis

and processing techniques to obtain simultaneous control over electronic, optical, chemical,

and mechanical features.

With the imaging appliance revolution underway, the need for more advanced

handheld devices that will combine the attributes of a computer, PDA, and cell phone is

increasing and the flat-panel mobile display industry is searching for a display technology

that will revolutionize the industry. The need for new lightweight, low-power, wide viewing

angled, handheld portable communication devices have pushed the display industry to revisit

the current flat-panel digital display technology used for mobile applications. Struggling to

meet the needs of demanding applications such as e-books, smart networked household

appliances, identity management cards, and display-centric handheld mobile imaging devices,

the flat panel industry is now looking at new displays known as Organic Light Emitting

Diodes (OLED).

For the preparation of the latest materials to prepare against this onslaught of demand

for lighter and less power hungry display technologies, electrical engineers have enlisted the

help of the humble jellyfish in their efforts to develop better light-emitting diodes (LEDs),

according to a report published in the December 1 issue of the journal Advanced Materials.

The Pacific Ocean jellyfish Aequorea victoria, it appears, produces just the sort of light that

researchers try to coax from crystalline semiconductors such as gallium arsenide or indium

phosphide. Moreover, the jellyfish accomplishes this with great efficiency: its light comes

from a substance dubbed green fluorescent protein (GFP), which collects the energy produced

in a certain cellular chemical reaction and emits it as green light from a molecular package

known as a chromophore.

An OLED is an electronic device made by placing a series of organic thin films

between two conductors. When electrical current is applied, a bright light is emitted. This

process is called electro phosphorescence. Even with the layered system, these systems are

very thin, usually less than 500 nm (0.5 thousandths of a millimeter).

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2. LIGHT EMITTING POLYMER

It is a polymer that emits light when a voltage is applied to it. The structure comprises

a thin-film of semiconducting polymer sandwiched between two electrodes (anode and

cathode) as shown in fig.1. When electrons and holes are injected from the electrodes, the

recombination of these charge carriers takes place, which leads to emission of light that

escapes through glass substrate. The bandgap, i.e. energy difference between valence

band and conduction band of the semiconducting polymer determines the wavelength

(colour) of the emitted light.

FIGURE 2.1 – Structure of PLED

The first polymer LEPs used poly phinylene vinylene (PPV) as the emitting layer. Since

1990, a number of polymers Light-Emitting Polymers have been shown to emit light under

the application of an electric field; the property is called the electro luminescence(EL) PPV

and its derivatives, including poly thiophenes, polypyridines, poly phenylenes and

copolymers are still the most commonly used materials. Efforts are on to improve stability,

lifetime and efficiency of polymer devices by modifying their configuration.

2.1 CHEMISTRY BEHIND LEP

LEPs are constructed from a special class of polymers called conjugated polymers. Plastic

materials with metallic and semiconductor characteristics are called conjugated polymers.

These polymers posses delocalised pi electrons along the backbone, whose mobility shows

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properties of semiconductors. Also this gives it the ability to support positive and negative

charge carriers with high mobility along the polymer chain.

The charge transport mechanism in conjugated polymers is different from traditional

inorganic semiconductors. The amorphous chain morphology results in inhomogeneous

broadening of the energies of the chain segments and leads to hopping type transport.

Conjugated polymers have already found application as conductor in battery

electrodes, transparent conductive coatings, capacitor electrolytes and through hole platting in

PCB’s. There are fast displaying traditional materials such as natural polymers etc owing to

better physical and mechanical properties and amenability to various processes.

2.2 BASIC STRUCTURE AND WORKING

An LEP display solely consists of the polymer material manufactured on a substrate of

glass or plastic and doesn’t require additional elements like polarizers that are typical

of LCDs. LEP emits light as a function of its electrical operation.

The basic LEP consists of a stack of thin organic polymer layers sandwiched

between a transport anode and a metallic cathode. Figure shows the basic structure.

The indium-tin-oxide (ITO)coated glass is coated with a polymer. On the top of it,

there is a metal electrode of Al, Li, Mg or Ag. When a bias voltage is applied, holes

and electrons move into the polymer. These moving holes and electrons combine

together to form hole electron pairs known as “excitons’. These excitons are in excited

state and go back to their initial state by emitting energy.

When this energy drop occurs light comes out from the device. This

phenomenon is called electroluminescence. The greater the difference in

energy/between the hole and the electron, the higher the frequency of the emitted

light.

The development of blue LEF material enthused the world about the possibility

full colour display.

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FIGURE 2.2 – Structure and Working

2.3 TYPES OF OLED DISPLAYS

OLED displays are typically of two major types: Passive-matrix displays and Active-matrix

displays.

Passive Displays:

The passive-matrix OLED display has a simple structure and is well suited for low-cost and

low-information content applications such as alphanumeric displays. It is formed by

providing an array of OLED pixels connected by intersecting anode and cathode conductors.

Organic materials and cathode metal are deposited into a “rib” structure (base and

pillar), in which the rib structure automatically produces an OLED display panel with the

desired electrical isolation for the cathode lines. A major advantage of this method is that all

patterning steps are conventional, so the entire panel fabrication process can easily be adapted

to large-area, high-throughput manufacturing. To get a passive-matrix OLED to work,

electrical current is passed through selected pixels by applying a voltage to the corresponding

rows and columns from drivers attached to each row and column. An external controller

circuit provides the necessary input power, video data signal and multiplex switches. Data

signal is generally supplied to the column lines and synchronized to the scanning of the row

lines. When a particular row is selected, the column and row data lines determine which

pixels are lit. A video output is thus displayed on the panel by scanning through all the rows

successively in a frame time, which is typically 1/60 of a second. The first OLED displays,

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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. OLEDs were recently demonstrated

as the light emitting component in a passively addressed display product. These passive

matrix displays demonstrate the feasibility of OLEDs in these applications, but encounter a

fundamental barrier as the display size and pixel density increase. Since the luminous output

of an OLED is proportional to the charge injected through the device, the current densities

required to operate passively addressed displays rapidly rise as the time available to drive

each pixel decreases with increasing display resolution. These high currents cause large

voltage drops in the ITO lines of the passive array, pushes the OLED operation to higher

voltages and creates display driver issues that are not easily resolved.

Active Displays:

In contrast to the passive-matrix OLED display, active-matrix OLED has an integrated

electronic back plane as its substrate and lends itself to high-resolution, high-information

content applications including videos and graphics. This form of display is made possible by

the development of polysilicon technology, which, because of its high carrier mobility,

provides thin-film-transistors (TFT) with high current carrying capability and high switching

speed. In an active-matrix OLED display, each individual pixel can be addressed

independently via the associated TFT’s and capacitors in the electronic back plane. That is,

each pixel element can be selected to stay “on” during the entire frame time, or duration of

the video. Since OLED is an emissive device, the display aperture factor is not critical, unlike

LCD displays where light must pass through aperture. Therefore, there are no intrinsic

limitations to the pixel count, resolution, or size of an active-matrix OLED display, leaving

the possibilities for commercial use open to our imaginations. Also, because of the TFT’s in

the active-matrix design, a defective pixel produces only a dark effect, which is considered to

be much less objectionable than a bright point defect, like found in LCD’s. Moving to an

active matrix drive scheme can overcome a number of the issues posed by passively driven

display schemes. The goal of the active matrix OLED (AMOLED) display is to generate a

constant current source at each pixel using thin film transistors which, in this work, were

made in a polysilicon technology. Each pixel is programmed to provide a constant current

during the entire frame time, eliminating the high currents encountered in the passive matrix

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approach. However, polysilicon thin film transistors suffer from significant initial output

characteristic non-uniformity due to the nature of the polysilicon crystal growth. This makes

it difficult to create a uniform current source at each pixel. A new AMOLED pixel has been

designed which addresses this issue and the results which were presented show the improved

performance compared to a standard pixel.

FIGURE 2.3 – Active and Passive matrix

Introducing a major new development in driver technology…

Until now, there have been two ways to drive an OLED display: passive matrix (PM),

and active matrix (AM). PM displays are simpler and lower cost, but are limited to

small sizes as the power consumption becomes dominated by resistive and capacitive

losses at higher resolutions and/or larger panel sizes. On the other hand, AM, based on

thin film transistor driving technology, is scalable with respect to both resolution and

panel size, but is more complex and very capital intensive, and is therefore out of

reach for many manufacturers including those in developing economies with no LCD

manufacturing capacity.

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Now there is a third driving solution:

Total Matrix Addressing (TMA™)

� TMA is a new driving technology for PM displays, which has the potential to

reduce power consumption and to extend panel lifetime for a given pixel count

(resolution or information content).

� TMA extends PM display resolution into the hitherto AM-dominated display space,

whilst being competitive on power consumption.

� TMA should significantly increase the range of panel sizes which can be addressed

using PM driver schemes.

� TMA is applicable to polymer and small molecule OLED technologies, and both

fluorescent and phosphorescent materials.

How does total matrix addressing work?

Total Matrix Addressing reduces the effective multiplex rate by driving lines with

common information components simultaneously. It allows the driving of multiple

rows and columns simultaneously, substantially reducing the peak luminance

requirements.

Implementation of TMA has been made possible by the development of new

driver and high speed image processing technologies. A high speed image processing

algorithm was required to make TMA cost effective and to enable implementation

with full speed video.

Measurements on small passive matrix displays show at least a fifty percent

reduction in power consumption, or double the display luminance at the same power

consumption. Simulation shows that the benefit improves significantly for higher

resolutions, and suggests that power consumption will be competitive with a

substantial range of AM-OLED displays.

The high speed algorithm has been demonstrated running full 25 frames-

persecond video in programmable silicon. This has allowed CDT to evaluate the

feasibility of implementing this technology in various types of OLED product. While

development continues, CDT is presently considering how to bring this technology to

market in the shortest possible time. If this technology can be implemented

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successfully in PM display driver chips, it will provide a major boost to the PM OLED

industry.

2.4 PIXEL UNIFORMITY

The most critical issue in the design of an AMOLED pixel is the pixel to pixel luminance

uniformity. Driving the OLED with a constant current provides the best pixel to pixel

uniformity since the OLED threshold variations no longer impact the charge passed through

the devices. This requires that the active components at each pixel of the AMOLED display

provide a constant current to the OLED. OLEDs are presently fabricated with the anode

connected to the ITO on the active matrix plate and the cathode connected to a metal alloy

back plane such as Mg:Ag or Al:Li. The effect of OLED threshold variations can be

eliminated by using a PMOS device on the active plate since the OLED will be connected to

the drain of the PMOS transistor. In this configuration, the transistor will provide a constant

current to the OLED as long as the transistor stays in saturation.

FIGURE 2.4 - Two-Transistor AMOLED Pixel

If an NMOS device were used on the active matrix plate, the OLED would be connected to

the source of the transistor and additional techniques would be needed to ensure that the

OLED threshold variation did not affect the gate to source voltage of the transistor. PMOS

polysilicon devices are available, so the pixel designs use a PMOS device to drive the OLED

and the transistor is operated in saturation to overcome the OLED threshold variations. The

simplest pixel, shown in the above figure, uses two transistors: one drives the current for the

OLED, MP2, and another, MP1, acts as a switch to sample and hold a voltage on the gate of

the drive transistor. The major cause of luminance non-uniformity in the two-transistor pixel

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is the variation in the drive transistor MP2. This transistor has a high level of output

characteristic variation due to the nature of the polysilicon grain growth. This local non-

uniformity can be improved by process refinements, but it is difficult to reduce the variation

across the array to a level necessary for good gray scale control. For example, if the pixel

configuration of Fig. 1 was designed to provide maximum brightness with 3 volts of

overdrive on the drive transistor, MP2, then the voltage division for one gray level with 8 bits

of gray scale is 3 volts/256 = 12 mV. The

threshold variation across the 2.7 inch diagonal display was 300 mV with local variations on

the order of 100 mV. These numbers indicate that, even with a high quality process, the

transistor threshold variations are much too large to provide the pixel to pixel uniformity

necessary for a high quality flat panel display. This conclusion is confirmed in Fig. 2 which

shows the visible effects of transistor non-uniformity on the two transistor pixel design of

Fig. 1. An improved AMOLED pixel has been designed which eliminates the effects of the

polysilicon transistor threshold voltage variation. The advantage of the four-transistor pixel is

that it uses an autozero cycle to reference the data against the transistor threshold voltage,

eliminating the effects of the transistor threshold voltage variation. The improvement can be

seen by comparing Fig. 2 where there is an obvious, visible improvement in the pixel to pixel

luminance uniformity in the four transistor test pixel array.

FIGURE 2.5 - Four-Transistor AMOLED Pixel

Power dissipation in an OLED display is a critical issue. Measurements and modeling show

that the temperature increase due to power dissipation in the active plate and in the OLEDs

can be severe because the heat transfer from the display to the ambient is relatively

inefficient. It is essential that the active matrix electronics consume a minimum of power

since the OLEDs themselves dissipate power while generating light. It is important to note

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that while the OLEDs dissipate considerable power, the overall efficiency can be over 3

lm/W with all the drive electronics while AMLCDs are typically 1 to 2lm/W. The design

trade off to address in the active matrix electronics design is that, while some power must be

consumed in order to provide current control, this power must be kept to a minimum.

Reduced power dissipation in the supporting electronics leads to a smaller control voltage

range with a subsequent requirement of more accurate electronics. The electronics must

provide the necessary gray scale control while dissipating as little power as possible. Finally,

the pixel should use a minimum of control lines, storage capacitors and transistors. For

example, the two-transistor pixel of Fig. 1 requires two transistors, select and data control

lines, one power line and a storage capacitor. The four-transistor pixel requires more

components than the two-transistor pixel. This may be the greatest weakness of the four

transistor pixel but theextra devices are essential to provide the necessary pixel to pixel

luminance uniformity.

2.5 MANUFACTURING

In order to manufacture the polymer two techniques are used.

Spin coating process

This technique involves spinning a disk, that is glass substrate at a fixed angular velocity and

letting a small amount of polymer solution to drop on the top of the disk. It is shown in the

figure. Spin coating machine used has a few thousands rotations per minute.

The robot pours the plastic over the rotating plate, which in turn, evenly spreads the

polymer on the plate. This results in an extremely fine layer of the polymer having a

thickness of 100 nanometers. Once the polymer is evenly spread, it is a\baked in an oven to

evaporate any remnant liquid.

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FIGURE 2.6 – Spin Coating

Printer based technique

LEPs can be patterned using a wide variety of printing techniques. The most advanced is ink-

jet printing (figure). Resolution as high as 360 dpi have been demonstrated, and the approach

are scalble to large-screen displays. Printing promises much lower manufacturing cost.

FIGURE 2.7 – Ink jet printing

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2.6 PRODUCTION OF OLEDs

As OLED materials are extremely thin -- and some are chemically reactive and oxidize

immediately on exposure to water or oxygen, creating black spots that ruin the display -- they

can be 10,000 times more sensitive to moisture and oxygen than LCDs. To protect them,

display

makers currently use glass as the display substrate (the same as LCDs) and glue a glass lid oil

top, with a desiccant powder inside the display to absorb moisture that comes through the

glue line. This design works but is awkward and costly.

2.7 A SOLID STATE SOLUTION

Currently, a number of FPD (Flat Panel Display) makers are evaluating a thin-film solution

that offers moisture and oxygen permeability approximately equal to a sheet of glass. It

comprises alternating layers of polymer and ceramic films applied in vacuum. The total

thickness of the coating is only ~3[micro]m, and it can be applied directly on top of an OLED

display, eliminating mechanical packaging components. A liquid precursor is flash-

evaporated to a gas, which then flows into a vacuum chamber where it condenses back to a

liquid and onto a substrate. It is not a traditional vacuum process such as evaporation,

sputtering, or chemical vapor deposition. All these are gas-to-solid deposition processes in

which atoms or molecules hit a substrate in a line of sight path and are converted back to the

solid state. By their very nature, these deposition processes create conformal layers that have

the same topography and surface roughness as the underlying substrate. In contrast, the

polymer layer formed in this new vacuum process is actually condensation of gas to liquid.

The precursor gas molecules travel to the substrate and condense on all its surfaces, thereby

encapsulating and planarizing the entire structure. The coating covers all the imperfections

and provides a flat surface. In addition, because it is a liquid, the flat surface of the monomer

is atomically smooth. The substrate next moves to an ultraviolet light source, which

polymerizes the liquid to create a solid polymer film, still with an atomically smooth top

surface. This provides an ideal surface on which to deposit a barrier film. Next, a ceramic

film, ~500 [Angstrom] thick, is deposited on top of the polymer layer. Because the surface is

so smooth, the ceramic film has very few defects and is therefore an almost perfect moisture

barrier. An OLED display, however, requires an even better barrier, so the process is

repeated, creating a stack of multiple polymer and ceramic layers in which each ceramic film

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is a near-perfect moisture barrier. This combination of ceramic and polymer layers, with a

total thickness of ~3microm, creates a moisture barrier with a water permeability in the range

of [10.sup.-6]gm of water/[m.sup.2]/day. This is the water impermeability required by an

OLED display. The application of this multilayer polymer/ceramic encapsulating thin film is

challenging, and the following factors have to be considered:

• The organic emissive layers in the OLED display are extremely thin, on the order of

nanometers, and have little mechanical strength. Subjecting the OLED layers to shear stresses

when the monomer is polymerized and solidifies, which involves about 2% shrinkage, is a

concern.

• OLED materials are sensitive to the UV light used to initiate polymerization. Consequently,

the formulation of the monomer as well as the UV intensity and duration must be carefully

controlled. This is especially critical with top-emission displays, since they have transparent

cathodes and the OLED layers would be directly exposed to the UV light.

• A plasma is typically used in depositing the ceramic barrier layers, which can also damage

the OLED layers and must be carefully controlled.

• Temperature excursions during UV curing and sputtering must be avoided, as many OLED

materials would be damaged by temperatures 100°C.

• As with most manufacturing processes for semiconductors or flatpanel displays, applying

thin-film encapsulation to an OLED display demands tight control of particles. The thin-film

encapsulation tool is connected directly to the OLED vacuum tool, where particulate is

already tightly controlled. It is essential to ensure that the organic and inorganic deposition

processes used in building the multilayer barrier stack do not create any particulates.

• Finally, to qualify the thin-film encapsulation process, encapsulated OLED displays must be

subjected to high temperature and humidity-- typically 60[degrees]C/90%RH for 500 hours--

as well as thermal shock testing to ensure that the displays will satisfy the requirements of

mobile electronic-device manufacturers (i.e., for cell phones and PDAs). Besides being

extremely thin yet impermeable to water, the Barix coating is also transparent to visible light.

This means OLED display makers could conceivably avoid having to make a bottom-

emission display in which the light path is partially blocked by the TFT silicon transistors on

the substrate, thereby reducing the display efficiency and placing a limit on resolution. If the

mechanical packaging--metal cans, glass lids, and desiccant--were replaced by transparent

thin-film encapsulation, then the display could be designed so that all the light exits the top of

the display,significantly boosting efficiency and enabling much higher resolution. This

efficiency increase means more than just saving electrical power. The other limiting factor

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with OLED displays (aside from protection from moisture) is the lifetime of the emissive

materials, especially blue emitters.Unlike LCDs, which are voltage-driven, OLED displays

are current-driven. Moreover, the amount of current that flows through the emissive materials

has a major effect on lifetime. More efficient top-emitter displays require much less current

for a given brightness because they avoid the inefficiencies of bottom-emitters where light is

partially blocked, and thus have longer lifetimes. A thin-film moisture barrier that meets

OLED display requirements is therefore an enabling technology, for OLED TVs.

2.8 OTHER TYPES OF OLED

2.8.1 TOLED

The Transparent OLED (TOLED) uses a proprietary 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.

TOLED Creates New Display Opportunities:

• Directed top emission: Because TOLEDs have a transparent structure, they may be built on

opaque surfaces to effect top emission. Simple TOLED displays have the potential to be

directly integrated with future dynamic credit cards. TOLED displays may also be built on

metal, e.g., automotive components. Top emitting TOLEDs also provide an excellent way to

achieve better fill factor and characteristics in high resolution, high-information-content

displays using active matrix silicon backplanes.

• Transparency: TOLED displays can be nearly as clear as the glass or substrate they're built

on. This feature paves the way for TOLEDs 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 warning systems; and into helmet-mounted or "head-up" systems for

virtual reality applications.

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• Enhanced high-ambient contrast: TOLED technology offers enhanced contrast ratio. By

using a low-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.

• Multi-stacked devices: TOLEDs are a fundamental building block for many multi-structure

and hybrid devices. Bi-directional TOLEDs can provide two independent displays emitting

from 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.

2.8.2 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/or processing

constraints. Flexible materials have significant performance advantages over traditional glass

substrates. In display technology, FOLED (flexible organic light emitting device) is an

organic light emitting device (OLED) built on a flexible base material, such as clear plastic

film or reflective metal foil, instead of the usual glass base. FOLED displays can be rolled up,

folded, or worn as part of a wearable computer. The devices are said to be lighter, more

durable, and less expensive to produce than the traditional glass-based alternatives. FOLED's

light-weight base materials significantly decrease the overall weight of a screen. This

capacity makes FOLED displays especially useful for portable devices, such as laptop

computers and other displays where weight is a consideration, such as large wall-mounted

screens. Furthermore, a FOLED display is less prone to breakage than a glass-based display

and compared to the silicon-based LCD displays used for small displays and flat-screen

monitors, are much less expensive to produce. Time Magazine named Universal Display

Corporation (UDC)'s rollable FOLED wireless monitor prototype one of the best 10

environmentally -- friendly technologies for 2002. UDC is working on such FOLED-based

products as rollable, refreshable electronic newspapers and video screens embedded in car

windshields, walls, windows, and office partitions. According to UDC, such products could

be on the market within five years.

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FOLEDs Offer Revolutionary Features for Displays:

• Flexibility: For the first time, FOLEDs may be made on a wide variety of substrates that

range from 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 uniform shirtsleeve, an aircraft cockpit

instrument panel or an automotive windshield.

• Ultra-lightweight, thin form: The use of thin plastic substrates will also significantly

reduce the weight of flat panel displays in cell phones, portable computers and, especially,

large-area televisions-on-thewall. For example, 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 compared to their glass-based counterpart.

• Cost-effective processing: OLEDs are projected to have fullproduction level cost advantage

over most flat panel displays. With the advent of FOLED technology, the prospect of roll-to-

roll processing is created. To this end, a continuous organic vapor phase deposition (OVPD)

process for large-area roll-to-roll OLED processing has been demonstrated. While continuous

web FOLED processing requires further development, this process may provide the basis for

very low-cost, mass production.

2.8.3 SOLED

SOLED (Stacked Organic Light - Emitting Diode device) is a display technology from the

Universal Display Corporation (UDC) that uses a stack of transparent organic light-emitting

devices (TOLEDs) to improve resolution and enhance full-color quality. SOLEDs use a pixel

architecture developed at UDC that stacks sub pixels (the red, blue and green elements in

each pixel) vertically rather than arranging them side by side, as is usually done in CRT and

LCD displays. Within a SOLED display, each sub-pixel element can be controlled

independently. Pixel color can be adjusted by varying the currents through the three color

elements and gray scale can be adjusted by pulse-width modulation. Brightness is controlled

by manipulating current through the stack. According to UDC, their SOLED technology

enables a three-fold improvement in resolution and better color quality over CRT and LCD

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displays. The company expects that SOLEDs may in the future enable high resolution Web-

enabled devices.

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3. ADVANTAGES AND DISAVANTAGES

3.1 ADVANTAGES:

• Robust Design - OLED’s are tough enough to use in portable devices such as cellular

phones, digital video cameras, DVD players, car audio equipment and PDA’s.

• Viewing Angles – Can be viewed up to 160 degrees, OLED screens provide a clear and

distinct image, even in bright light.

• High Resolution – High information applications including videos and graphics, active-

matrix OLED provides the solution. Each pixel can be turned on or off independently to

create multiple colors in a fluid and smooth edged display.

• “Electronic Paper” – OLED’s are paper-thin. Due to the exclusion of certain hardware

goods that normal LCD’s require, OLED’s are as thin as a dime.

• Production Advantages – Up to 20% to 50% cheaper than LCD processes. Plastics will

make the OLED tougher and more rugged. The future quite possibly could consist of these

OLED’s being produced like newspapers, rather than computer “chips”.

• Video Capabilities – They hold the ability to handle streamlined video, which could

revolutionize the PDA and cellular phone market.

• Hardware Content – Lighter and faster than LCD’s. Can be produced out of plastic and is

bendable. Also, OLED’s do not need lamps, polarizers, or diffusers.

• Power Usage – Takes less power to run (2 to 10 volts).

3.2 DISAVANTAGES:

• Engineering Hurdles – OLED’s are still in the development phases of production. Although

they have been introduced commercially for alphanumeric devices like cellular phones and

car audio equipment, production still faces many obstacles before production.

• Overcoming LCD’s – LCD’s have predominately been the preferred form of display for the

last few decades. Tapping into the multi-billion dollar industry will require a great product

and continually innovative research and development. Furthermore, LCD manufacturers will

not likely fold up and roll over to LCD’s. They will also continue to improve displays and

search for new ways to reduce production costs.

• Aging of LEP – One of the major barriers to the commercial development of LEP is its

useful lifetime. Even under ideal conditions, the light intensity gradually decreases and some

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discrete regions become totally dark. This phenomenon is the ‘aging of LEP’. One method to

reduce or stop aging is that the final soldering of the displays is to be done in an airtight

environment because as soon as the LEP molecules come in contact with oxygen, these

would disintegrate. The solution was to do the final soldering in a glass jar filled nitrogen.

The enclosure protects the device from impurities and provides a higher degree of efficiency

by giving the screen an estimated life span of 30,000 working hours.

• Space charge effect – The effect of space charge on the voltage-current characteristics and

current-voltage characteristics becomes more pronounced when the difference in the electron

hole mobilities is increased. Consequences of space charge include lowering of the electric

fields near the contacts and therefore suppression of the injected tunnel currents and strongly

asymmetric recombination profiles for unequal mobility thereby decreasing the luminescence

and hence decreases the efficiency. Research is underway to overcome this barrier Even

though this limitations are there LEPs found to be superior to other flat panel displays like

LCD, FED (field emission display) and etc.

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4. APPLICATIONS AND FUTURE DEVELOPMENTS

4.1 APPLICATIONS

Polymer light-emitting diodes (PLED) can easily be processed into large-area thin films using

simple and inexpensive technology. They also promise to challenge LCD's as the premiere

display technology for wireless phones, pagers, and PDA's with brighter, thinner, lighter, and

faster features than the current display.

4.1.1 PHOTOVOLTAICS

CDT’s PLED technology can be used in reverse, to convert light into electricity.

Devices which convert light into electricity are called photovoltaic (PV) devices, and are at

the heart of solar cells and light detectors. CDT has an active program to develop efficient

solar cells and light detectors using its polymer semiconductor know-how and experience,

and has filed several patents in the area.

Digital clocks powered by CDT's polymer solar cells.

4.1.2 POLY LED TV

Philips will demonstrate its first 13-inch PolyLED TV prototype based on polymer OLED

(organic light-emitting diode) technology Taking as its reference application the wide-screen

30-inch diagonal display with WXGA (1365x768) resolution, Philips has produced a

prototype 13-inch carve-out of this display (resolution 576x324) to demonstrate the feasibility

of manufacturing large-screen polymer OLED displays using 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.

According to current predictions, a polymer OLED-based TV could be a reality in the next

five years.

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4.1.3 MP3 PLAYER DISPLAY

Another product on the market taking advantage of a thin form-factor, light-emitting polymer

display is the new, compact, MP3 audio player, marketed by GoDot Technology. The unit

employs a polymeric light-emitting diode (pLED) display supplied by Delta Optoelectronics,

Taiwan, which is made with green Lumation light-emitting polymers furnished by Dow

Chemical Co., Midland, Mich.

4.2 FUTURE DEVELOPMENTS

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.

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Demands for information on the move could drive the

development of 'wearable' displays, with interactive features.

Eywith changing information cole woul give many brand

ownerve edge

The ability of PLEDs to be fabricated on flexible substrates

opens up fascinating possibilities for formable or even fully flexible displays e catching

packaging intent at the point of sa d s a valuable competition.

FEW MORE DEVELOPMENTS

• Because the plastics can be made in the form of thin films or sheets, they offer a huge

range of applications. These include television or computer screens that can be rolled

up and tossed in a briefcase, and cheap videophones.

• Clothes made of the polymer and powered by a small battery pack could provide their

own cinema show.

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• 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. This would greatly extend the tools

available to a doctor and would allow on-the-spot quantitative analysis, eliminating

the need for patients to make repeat visits. This would bring forward the start of

treatment, lower treatment costs and free up clinician time.

The future is bright for products incorporating PLED displays. Ultra-light, ultra-thin

displays, with low power consumption and excellent readability allow product

designers a much freer rein. The environmentally conscious will warm to the absence

of toxic substances and lower overall material requirements of PLEDs, and it would

not be an exaggeration to say that all current display applications could benefit from

the introduction of PLED technology. CDT sees PLED technology as being first

applied to mobile communications, small and low information content

instrumentation, and appliance displays. With the emergence of 3G

telecommunications, high quality displays will be critical for handheld devices.

PLEDs are ideal for the small display market as they offer vibrant, full-colour displays

in a compact, lightweight and flexible form. Within the next few years, PLEDs are

expected to make significant inroads into markets currently dominated by the cathode

ray tube and LCD display technologies, such as televisions and computer monitors.

PLEDs are anticipated as the technology of choice for new products including virtual

reality headsets; a wide range of thin, technologies, such as televisions and computer

monitors. PLEDs are anticipated as the technology of choice for new products

including virtual reality headsets; a wide range of thin, lightweight, full colour

portable computing; communications and information management products; and

conformable or flexible displays.

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5. SUMMARY

The Organic Light Emitting Diode forms of display still have many obstacles to overcome

before it’s popularity and even more importantly, its reliability are up to par with standards

expected by consumers. Although the technology presents itself as a major player in the field

of displays, overcoming these obstacles will prove to be a difficult task. However, the

OLED’s advantages over LCD’s and future outlook have many in the industry goggle-eyed at

the realm of possibilities. For all we know and can hope for OLED’s could change the ways

in which we see things.

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6. REFERENCE

• www.cdtltd.co.uk

• www.research.philips.com

• www.covion.com

• www.techalone.com

• www.scribd.com

• www.lep-light.com