neetu report

8/4/2019 Neetu 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, i.e. 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

TOPIC

1) INTRODUCTION2) SUBJECT DETAILING

2.1) CONSTRUCTION OF LEP

2.1.1) INKJET PRINTING

2.1.2) ACTIVE & PASSIVE MATRIX

2.2) BASIC PRINCIPLE & TECHNOLOGY

2.3) LIGHT EMISSION

2.4) COMPARISON TABLE

3) ADVANTAGES & DISADVANTAGES

3.1) ADVANTAGES

3.2) DISADVANTAGES

4) APPLICATION & FUTUREDEVELOPMENTS

4.1) APPLICATION

4.2) FUTURE DEVELOPMENTS

5) CONCLUSION

6) REFERENCES

7) APPENDICES


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SUBJECT DETAILING

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 band gap, i.e. energy difference

between valence band and conduction band of the semiconducting

polymer determines the wavelength (color) of the emitted light.

CONSTRUCTION

Light-emitting devices consist of active/emitting layers sandwiched between a

cathode and an anode. Indium-tin oxides typically used for the anode and

aluminum or calcium for the cathode. Fig.2.1(a) shows the structure of a

simple single layer device with electrodes and an active layer.


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Single-layer devices typically work only under a forward DC bias. Fig.2.1(b)

shows a symmetrically configured alternating current light-emitting (SCALE)

device that works under AC as well as forward and reverse DC bias.

In order to manufacture the polymer, a spin-coating machine is used that has aplate spinning at the speed of a few thousand 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 baked in

an oven to evaporate any remnant liquid. The same technology is used to coat the

CDs.

2.1.1 INK JET PRINTING

Although inkjet printing is well established in printing graphic images, only

now are applications emerging in printing electronics materials.

Approximately a dozen companies have demonstrated the use of inkjet

printing for PLED displays and this technique is now at the forefront of

developments in digital electronic materials deposition. However, turning

inkjet printing into a manufacturing process for PLED displays has required

significant developments of the inkjet print head, the inks and the substrates

(see Fig.2.1.1).Creating a full colour, inkjet printed display requires the

precise metering of volumes in the order of pico liters. Red, green and blue

polymer solutions are jetted into well defined areas with an angle of flight

deviation of less than 5. To ensure the displays have uniform emission, the

film thickness has to be very uniform.


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Fig. 2.1.1 Schematic of the ink jet printing for PLED materials

For some materials and display applications the film thickness uniformity may

have to be better than 2 per cent. A conventional inkjet head may have volume

variations of up to 20 per cent from the hundred or so nozzles that comprise the

head and, in the worst case, a nozzle may be blocked. For graphic art this

variation can be averaged out by multi-passing with the quality to the print

dependent on the number of passes. Although multi-passing could be used for

PLEDs the process would be unacceptably slow. Recently, Spectra, the worlds

largest supplier of industrial inkjet heads, has started to manufacture heads

where the drive conditions for each nozzle can be adjusted individually so called

drive-per-nozzle (DPN). Litrex in the USA, a subsidiary of CDT, has developed

software to allow DPN to be used in its printers. Volume variations across the

head of 2 per cent can be achieved using DPN In addition to very good volume

control, the head has been designed to give drops of ink with a very small angle-

of-flight variation. A 200 dots per inch (dpi) display has colour pixels only 40

microns wide; the latest print heads have a deviation of less than 5 micronswhen placed 0.5 mm from the substrate. . In addition to the precision of the print

head, the formulation of the ink is key to making effective and attractive display

devices.


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The formulation of a dry polymer material into an ink suitable for PLED displays

requires that the inkjets reliably at high frequency and that on reaching the

surface of the substrate forms a wet film in the correct location and dries to a

uniformly flat film. . The film then has to perform as a useful electro-optical

material. Recent progress in ink formulation and printer technology has allowed

400 mm panels to be colour printed in under a minute.

2.1.2.ACTIVE AND PASSIVE MATRIX

Many displays consist of a matrix of pixels, formed at the intersection of rows

and columns deposited on a substrate. Each pixel is a light emitting diode such

as a PLED, capable of emitting light by being turned on or off, or any state in

between. Colored displays are formed by positioning matrices of red, green

and blue pixels very close together.

To control the pixels, and so form the image required, either 'passive' or

'active' matrix drive Pixel displays can either by active or passive matrix. Fig.

2.1.2 shows the differences between the two matrixes types, active displays

have transistors so that when a particular pixel is turned on it remains on until

it is turned off methods are used time. Active displays are preferred, however

it is technically challenging to incorporate so many transistors into such small a

compact area. The matrix pixels are accessed sequentially. As a result passive

passive displays are prone to flickering since each pixel only emits light for

such a small length of time. Active displays are preferred, however it is

technically challenging to incorporate so many transistors into such small a

compact area.


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Fig: 2.1.2Active & Passive Matrices

In passive matrix systems, each row and each column of the display has its

own driver, and to create an image, the matrix is rapidly scanned to enable

every pixel to be switched on or off as required. As the current required to

brighten a pixel increases (for higher brightness displays), and as the display

gets larger, this process becomes more difficult since higher currents have to

flow down the control lines. Also, the controlling current has to be present

whenever the pixel is required to light up. As a result, passive matrix displays

tend to be used mainly where cheap, simple displays are required. Active

matrix displays solve the problem of efficiently addressing each pixel by

incorporating a transistor (TFT) in series with each pixel which provides control

over the current and hence the brightness of individual pixels. Lower currents

can now flow down the control wires since these have only to program the TFT

driver, and the wires can be finer as a result. Also, the transistor is able to hold

the current setting, keeping the pixel at the required brightness, until it

receives another control signal.


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Future demands on displays will in part require larger area displays so the active

matrix market segment will grow faster. PLED devices are especially suitable for

incorporating into active matrix displays, as they are processable in solution and

can be manufactured using ink jet printing over larger areas.


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BASIC PRINCIPLE & TECHNOLOGY

Polymer properties are dominated by the covalent nature of carbon bonds

making up the organic molecules backbone. The immobility of electrons that

form the covalent bonds explain why plastics were classified almost exclusively

insulators until the 1970s.

A single carbon-carbon bond is composed of two electrons being shared in

overlapping wave functions. For each carbon, the four electrons in the valence

bond form tetrahedral oriented hybridized sp3

orbitals from the s & p orbitals

described quantum mechanically as geometrical wave functions. The

properties of the spherical s orbital and bimodal p orbitals combine into four

equal, unsymmetrical, tetrahedral oriented hybridized sp3

orbitals. The bond

formed by the overlap of these hybridized orbitals from two carbon atoms is

referred to as a sigma bond. A conjugated pi bond refers to a carbon chain

or ring whose bonds alternate between single and double (or triple) bonds.

The bonding system tends to form stronger bonds than might be first indicated

by a structure with single bonds. The single bond formed between two double

bonds inherits the characteristics of the double bonds since the single bond isformed by two sp

2

hybrid orbitals. The p orbitals of the single bonded carbons

form an effective pi bond ultimately leading to the significant consequence of

pi electron de-localization. Unlike the sigma bond electrons, which are

trapped between the carbons, the pi bond electrons have relative mobility.

All that is required to provide an effective conducting band is the oxidation or

reduction of carbons in the backbone. Then the electrons have mobility, as do

the holes generated by the absence of electrons through oxidation with a

dopant like iodine.

2.2.1. LIGHT EMISSION

The production of photons from the energy gap of a material is very similar for

organic and ceramic semiconductors. Hence a brief description of the process

of electroluminescence is in order. Electroluminescence is the process in which


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electromagnetic (EM) radiation is emitted from a material by passing an

electrical current through it. The frequency of the EM radiation is directly

related to the energy of separation between electrons in the conduction band

and electrons in the valence band. These bands form the periodic arrangement

of atoms in the crystal structure of the semiconductor. In a ceramic

semiconductor like GaAs or ZnS, the energy is released when an electron from

the conduction band falls into a hole in the valence band. The electronic device

that accomplishes this electron-hole interaction is that of a diode, which

consists of an n-type material (electron rich) interfaced with p-type material

(hole rich). When the diode is forward biased (electrons across interface from

n to p by an applied voltage) the electrons cross a neutralized zone at the

interface to fill holes and thus emit energy. The situation is very similar for

organic semiconductors with two notable exceptions. The first exception

stems from the nature of the conduction band in an organic system while the

second exception is the recognition of how conduction occurs between two

organic molecules.

With non-organic semiconductors there is a band gap associated with Brillouin

zones that discrete electron energies based on the periodic order of the

crystalline lattice. The free electrons mobility from lattice site to lattice site is

clearly sensitive to the long-term order of the material. This is not so for theorganic semiconductor. The energy gap of the polymer is more a function of

the individual backbone, and the mobility of electrons and holes are limited to

the linear or branched directions of the molecule they statistically inhabit. The

efficiency of electron/hole transport between polymer molecules is also

unique to polymers. Electron and hole mobility occurs as a hopping

mechanism which is significant to the practical development of organic


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Fig2.2.1 a demonstration of full conjugation of electrons in PPV

Emitting devices. PPV has a fully conjugated backbone (figure 2.2.1), as a

consequence the HOMO (exp link remember 6th form!) of the macromolecule

stretches across the entire chain, this kind of situation is ideal for the transport of

charge; in simple terms, electrons can simply "hop" from one orbital to the next

since they are all linked. The delocalized electron clouds are colored yellow.

PPV is a semiconductor. Semiconductors are so called because they have

conductivity that is midway between that of a conductor and an insulator. While

conductors such as copper conduct electricity with little to no energy (in this case

potential difference or voltage) required to "kick-start" a current, insulators such

as glass require huge amounts of energy to conduct a current. Semi-conductors

require modest amounts of energy in order to carry a current, and are used in

technologies such as transistors, microchips and LEDs.

Band theory is used to explain the semi-conductance of PPV, see figure 5. In a

diatomic molecule, a molecular orbital (MO) diagram can be drawn showing a

single HOMO and LUMO, corresponding to a low energy orbital and a high

energy * orbital. This is simple enough, however, every time an atom is added to

the molecule a further MO is added to the MO diagram. Thus for a PPV chain

which consists of ~1300 atoms involved in conjugation, the LUMOs and HOMOs

will be so numerous as to be effectively continuous, this results in two bands, a

valence band (HOMOs, orbitals) and a conduction band (LUMOs, * orbitals).

They are separated by a band gap which is typically 0-10eV (check) and depends

on the type of material. PPV has a band gap of 2.2eV (exp eV). The valence band is

filled with all the electrons in the chain, and thus is entirely filled, while the


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conduction band, being made up of empty * orbitals (the LUMOs) is entirely

empty).

In order for PPV to carry a charge, the charge carriers (e.g. electrons) must be

given enough energy to "jump" this barrier - to proceed from the valence band to

the conduction band where they are free to ride the PPV chains empty

LUMOs.(Fig. 2.2.2)

Figure 2.2.2 A series of orbital diagrams

y A diatomic molecule has a bonding and an anti-bonding orbital, two

atomic orbital gives two molecular orbitals. The electrons arrange

themselves following, Auf Bau and the Pauli Principle.

y

A single atom has one atomic orbital.y A tri atomic molecule has three molecular orbitals, as before one

bonding, one anti-bonding, and in addition one non-bonding orbital.

Four atomic orbitals give four molecular orbitals. Many atoms results in so

many closely spaced orbitals that they are effectively continuous and non-

quantum. The orbital sets are called bands. In this case the bands are


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separated by a band gap, and thus the substance is either an insulator or a

semi-conductor. It is already apparent that conduction in polymers is not

similar to that of metals and inorganic conductors; however there is more to

this story! First we need to imagine a conventional diode system, i.e. PPV

sandwiched between an electron injector (or cathode), and an anode. It is

already apparent that conduction in polymers is not similar to that of metals

and inorganic conductors ; however there is more to this story.

First we need to imagine a conventional diode system, i.e.

PPV sandwiched between an electron injector (or cathode), and an anode. The

electron injector needs to inject electrons of sufficient energy to exceed the

band gap, the anode operates by removing electrons from the polymer andconsequently leaving regions of positive charge called holes. The anode is

consequently referred to as the hole injector.

In this model, holes and electrons are referred to as charge carriers, both are

free to traverse the PPV chains and as a result will come into contact. It is

logical for an electron to fill a hole when the opportunity is presented and they

are said to capture one another. The capture of oppositely charged carriers is

referred to as recombination. When captured, an electron and a hole form

neutral-bound excited states (termed excitons) that quickly decay and produce

a photon up to 25% of the time, 75% of the time, decay produces only heat,

this is due to the the possible multiplicities of the exciton. The frequency of the

photon is tied to the band-gap of the polymer; PPV has a band-gap of 2.2eV,

which corresponds to yellow-green light.

Not all conducting polymers fluoresce, poly acetylene, one of the first conducting-

polymers to be discovered was found to fluoresce at extremely low levels of

intensity. Excitons are still captured and still decay, however they mostly decay torelease heat. This is what you may have expected since electrical resistance in

most conductors causes the conductor to become hot. Capture is essential for a

current to be sustained. Without capture the charge densities of holes and

electrons would build up, quickly preventing any injection of charge carriers. In

effect no current would flow.


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LIGHT EMITTINGDIODES BASED ON CONJUGATED POLYMERS

Conjugated polymers are organic semiconductors, the semiconducting behavior

being associated with the molecular orbitals delocalized along the polymer chain.

Their main advantage over non-polymeric organic semiconductors is thepossibility of processing the polymer to form useful and robust structures. The

response of the system to electronic excitation is nonlinearthe injection of an

electron and a hole on the conjugated chain can lead to a self-localized excited

state which can then decay radiatively, suggesting the possibility of using these

materials in electroluminescent devices.

We demonstrate here that poly (p-phenylene vinylene), prepared by way of a

solution-processable precursor, can be used as the active element in a large-area

light-emitting diode. The combination of good structural properties of this

polymer, its ease of fabrication, and light emission in the green-yellow part of the

spectrum with reasonably high efficiency, suggest that the polymer can be used

for the development of large-area light-emitting displays. There has been long-standing interest in the development of solid-state light-emitting devices. Efficient

light generation is achieved in inorganic semiconductors with direct band gaps,

such as GaAs, but these are not easily or economically used in large-area displays.

For this, systems based on polycrystalline ZnS have been developed, although low

efficiencies and poor reliability have prevented large-scale production. Because of

the high photoluminescence quantum yields common in organic molecular

semiconductors, there has long been interest in the possibility of light emission by

these organic semiconductors through charge injection under a high applied field

(electroluminescence).

Light-emitting devices are fabricated by vacuum sublimation of the organic layers,

and although the efficiencies and selection of colour of the emission are very

good, there are in general problems associated with the long-term stability of the

sublimed organic film against recrystallization and other structural changes.

One way to improve the structural stability of these organic layers is to movefrom molecular to macromolecular materials, and conjugated polymers are a

good choice in that they can, in principle, provide both good charge transport and

also high quantum efficiency for the luminescence. Much of the interest in

conjugated polymers has been in their properties as conducting materials, usually

achieved at high levels of chemical doping, and there has been comparatively


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little interest in their luminescence. One reason for this is that poly acetylene, the

most widely studied of these materials, shows only very weak

photoluminescence. But conjugated polymers that have larger semiconductor

gaps, and that can be prepared in a sufficiently pure form to control non-radiative

decay of excited states at defect sites, can show high quantum yields forphotoluminescence. Among these, poly (p- phenylene vinylene) or PPV can be

conveniently made into high-quality films and shows strong photoluminescence in

a band centred near 2.2 eV, just below the threshold for to interband transitions.

We synthesized PPV (I) using a solution-processable precursor polymer (II), as

shown in Figure. This precursor polymer is conveniently prepared from dichloride-

p-xylene (III), through polymerization of the sulphonium salt intermediate (IV).

We carried out the polymerization in a water/methanol mixture in the presence

of base and, after termination, dialyzed the reaction mixture against distilledwater. The solvent was removed and the precursor polymer redissolved in

methanol. We find that this is a good solvent for spin-coating thin films of the

precursor polymer on suitable substrates. After thermal conversion (typically

250C, in vacuo, for 10h), the films of PPV (typical thickness 100 nm) arehomogeneous, dense and uniform Furthermore, they are robust and intractable,

stable in air at room temperature, and at temperatures >300C in a vacuum.

SYNTHETIC ROUTE TO PPV

Current-voltage characteristic for an electroluminescent device having a PPV film


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70 mm thick are of 2 mm2, a bottom contact of indium oxide, and a top contact of

aluminium. The forward-bias regime is shown (indium oxide positive with respect

to the aluminium electrode).

Structures for electroluminescence studies were fabricated with the PPV film

formed on a bottom electrode deposited on a suitable substrate (such as glass),

and with the top electrode formed onto the fully converted PPV film. For the

negative, electron-injecting contact we use materials with a low work function,

and for the positive, hole-injecting contact, we use materials with a high work

function. At least one of these layers must be semi-transparent for light emission

normal to the plane of the device, and for this we have used both indium oxide,

deposited by ion-beam sputtering and thin aluminium (typically 7.15 nm). We

found that aluminium exposed to air to allow formation of a thin oxide coating,

gold and indium oxide can all be used as the positive electrode material, and that

aluminium, magnesium silver alloy and amorphous silicon hydrogen alloys

prepared by radio frequency sputtering are suitable as the negative electrode

materials. The high stability of the PPV film allows easy deposition of the top

contact layer, and we were able to form this contact using thermal evaporation

for metals and ion-beam sputtering for indium oxide.

Figure show typical characteristics for devices having indium oxide as the bottom

contact and aluminium as the top contact. The threshold for substantial charge

injection is just below 14 V, at a field of 2 106

V cm-1

, and the integrated light

output is approximately linear with current. Figure shows the spectrally resolvedoutput for a device at various temperatures. The spectrum is very similar to that

measured in photoluminescence, with a peak near 2.2 eV and well resolved

phonon structure. These devices therefore emit in the green-yellow part of the

spectrum, and can be easily seen under normal laboratory lighting. The quantum

efficiency (photons emitted per electron injected) is moderate, but not as high as


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reported for some of the structures made with molecular materials. The quantum

efficiencies for our PPV devices were up to 0.05%. We found that the failure mode

of these devices are usually associated with the failure of polymer/thin metal

interface and is probably due to local joule heating there.

Integrated light output plotted against current for the electroluminescent device giving the

current-voltage characteristic in Fig

Spectrally resolved output for an electroluminescent device at various temperatures.

The observation and characterization of electroluminescence in this conjugated

polymer is of interest in the study of the fundamental excitations of this class ofsemiconductor. Here, the concept of self-localized charged or neutral excited

states in the nonlinear response of the electronic system has been a useful one.

For polymers with the symmetry of PPV, these excitations are polarons, either

uncharged (as the polaron exciton) or charged (singly charged as the polaron, and

doubly charged as the bipolaron). We have previously assigned the

photoluminescence in this polymer to radiative recombination of the singlet

polaron exciton formed by intra chain excitation and, in view of the identical

spectral emission here, we assign the electroluminescence to the radiative decay

of the same excited state. The electroluminescence is generated byrecombination of the electrons and holes injected from opposite sides of the

structure, however, and we must consider what the charge carriers are. We have

previously noted that bipolarons, the more stable of the charged excitations in

photo excitation and chemical doping studies, are very strongly self-localized,

with movement of the associated pair of energy levels deep into the


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semiconductor gap, to within 1 eV of each other. In contrast, the movement of

these levels into the gap for the neutral polaron exciton, which one-electron

models predict to be the same as for the bipolaron, is measured directly from the

photoluminescence emission to be much smaller, with the levels remaining more

than 2.2 eV apart. For electroluminescence then, bipolarons are very unlikely tobe the charge carriers responsible for formation of polaron excitons, because

their creation requires coalescence of two charge carriers, their mobilities are low

and the strong self-localization of the bipolaron evident in the positions of the gap

states probably does not leave sufficient energy for radiative decay at the photon

energies measured here. Therefore, the charge carriers involved are probably

polarons. The evidence that they can combine to form polaron excitons requires

that the polaron gap states move no further into the gap than those of the

polaron exciton and may account for the failure to observe the optical transitions

associated with the polaron.

The photoluminescence quantum yield of PPV has been estimated to be ~8%. It

has been shown that the non-radiative processes that limit the efficiency of

radiative decay as measured in photoluminescence are due to migration of the

excited states to defect sites which act as non-radiative recombination centres,

and also, at high intensities, to collisions between pairs of excited states. These

are processes that can, in principle, be controlled through design of the polymer,

and therefore there are excellent possibilities for the development of this class of

materials in a range of electroluminescence applications.

EFFICIENT LIGHT EMITTINGDIODES BASED ON POLYMERS WITHHIGHELECTRON

AFFINITIES

CONJUGATED polymers have been incorporated as active materials into several

kinds of electronic device, such as diodes, transistors and light-emitting diodes.

The first polymer light-emitting diodes were based on poly (p-phenylene vinylene)

(PPV), which is robust and has a readily processible precursor polymer.

Electroluminescence in this material is achieved by injection of electrons into the

conduction band and holes into the valence band, which capture one another

with emission of visible radiation. Efficient injection of electrons has previously

required the use of metal electrodes with low work functions, primarily calcium;

but this reactive metal presents problems for device stability. Here we report the

fabrication of electroluminescent devices using a new family of processible poly

(cyanoterephthalylidene) s. As the lowest unoccupied orbital of these polymers


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(from which the conduction band is formed) lie at lower energies than those of

PPV, electrodes made from stable metals such as aluminium can be used for

electron injection. For hole injection, we use indium tin oxide coated with a PPV

layer; this helps to localize charge at the interface between the PPV and the new

polymer, increasing the efficiency of recombination. In this way, we are able toachieve high internal efficiencies (photons emitted per electrons injected) of up to

4% in these devices.

2.3 COMPARISON TABLE

This table

compares the

main

electronic

displays

technologies.

Each display

type is

described

briefly, and the

relative

advantages

and

disadvantages

are reviewed

display type

Acronym Emissive or

Reflective

Technology Advantages Disadvantages

Cathode Ray

Tube

CRT Emissive The CRT is a

vacuum tube

using a hot

filament to

generate

thermo-

electrons,

electrostatic

and/or

magnetic fields

to focus the

Very bright

Wide viewing

angle

No mask, so no

pixel size

limitation for

mono

Minimum pixel

size 0.2mm

(color)

High (5kV to

20kV+) drive

voltages

Not a flat panel

(rare

exceptions)

Can be fragile,

particularly

neck-end

Heavy


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electrons into a

beam attracted

to the high

voltage anode

which is the

phosphor

coated screen.

Electrons

colliding with

the phosphor

emit luminous

radiation. Color

CRTs typically

use 3 electron

sources (guns)

to target red,

green, and blue

patterns on

phosphor

the screen.

Low cost

standard sizes

Low cost high-

res color

Wide operatingtemperature

range

Moderate

(20khrs+) life

Source of X-rays

unless screened

Affected by

magnetic fields

Difficult torecycle or

dispose of

Liquid Crystal

Display

LCD Reflective An LCD uses

the properties

of liquid crystals

in an electricfield to guide

light from

oppositely

polarized front

and back display

plates. The

liquid crystal

works as a

helical director

(when thedriver presents

the correct

electric field) to

guide the light

through 90

Small, static,

mono panels

can be very low

cost

Both mono and

color panels

widely available

Backlight adds

cost, and often

limits the useful

life

Requires AC

drive waveform

Fragile unless


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from one plate

Polymeric Luminescent Material Development

The design of luminescent materials for use in LED devices is as critical to device

performance as the process of constructing the device itself. Processability,

purity, thermal and oxidative stability , color of emission, luminance efficiency,

balance of charge carrier mobility, and others are among many important

materials properties required for a system to be viable in commercial LED device

applications. Great strides have been made toward the development of newpolymeric materials for luminescent applications .Although electroluminescent

materials have been known for decades, and some were available for purchase in

the early 1960s, they were limited until recently to inorganic semiconductors and

small-molecule organic dyes. To be used in diode display devices, these materials

require deposition as very thin films by vapor deposition or sublimation

processes. Such processes are expensive, equipment intensive, and do not adapt

well for coating large-area substrates .To overcome the device processing

drawbacks of these emitting systems, a great deal of research has been focussed

on the development of luminescent polymeric equivalents that offer the ability tocoat substrates efficiently by any of a variety of common solution processes. A

series of fluorescent polymers with extended conjugated backbones has emerged

from this work. The year 1990 marked the discovery of the first

electroluminescent polymer, PPV. [13] Since that time a variety of PPV derivatives

with improved performance and processability have been developed. Other

conjugated polymer systems such as polyfluorenes, polyphenylenes, poly

(phenylene ethynylenes), and polyalkylthiophenes are also interesting emissive

materials investigated over the past decade. Additional approaches to solution-

processable polymeric emitting materials that have been explored includeblending of inorganic semiconductors or organic dyes with polymers (inert and/or

electroactive) as well as synthesizing polymers that bear discrete chromophores

bound either pendant from or within the polymer backbone. Out of the host of

materials investigated, two classes of polymers have emerged as leading

candidates for PLED applications, the PPVs and the polyfluorenes. Through fine

tuning of their chemical structures via polymerization variations, both of these


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polymer families have achieved ease of processability, high purity, and tenability

of emission color. Many high-efficiency, high-brightness, and long-lived devices

have been constructed from these materials. An excellent review of the state-of-

the-art in electroluminescent conjugated polymer development was published by

Holmes and co-workers in 1998.[58] The reader is referred to this review for anextensive overview of the classes of polymeric materials that exist, and as a

reference to many other previous reviews. A summary of the synthetic processes

that yield the emissive polymers, materials properties reported, as well as device

structures and performances derived from them are well covered, especially for

the PPV family. Polyphenylenes, poly (phenylene ethynylenes),

polyalkylthiophenes, and other polymer systems containing isolated

chromophores either in the polymer chain or pendant to it are also briefly

described. Polyfluorene homopolymers are lightly mentioned primarily due to the

early stage of their development in 1998. In the past few years, much progresshas been made within this class of material. The major groups of polymeric

materials developed in the past decade will be summarized here. The discussion is

by no means exhaustive (given the recent explosion of research in the field) and is

limited to the groups of materials that have shown the greatest overall potential

to date to be adopted as emissive materials in PLED applications: the poly

(phenylene vinylene) s, the poly (fluorene) s, and the poly (phenylene) s, as

illustrated in Figure 3. Polyfluorene homo- and co-polymers are intentionally

highlighted since they have not been well reviewed in the past, much progress

has been made very recently, and this class of material has quickly emerged as anextremely viable LED polymeric material of great commercial interest.

Polyfluorene Alternating Copolymers

Tertiary aromatic amines have been known as excellent hole-transport materials

and have found utility in photoconductors and in LEDs. With the Pd-catalyzed

polymerization process, it is possible to prepare high-molecular-weight,

alternating copolymers consisting of 9,9 -dialkylfluorene and various aromatic

amines. These alternating polymers are all blue emitters, excellent film formers,

and soluble in conventional organic solvents. Films of these polymers show

distinct and reversible oxidation potentials by cyclic voltammetry and can be

cycled without any appreciable change. Hole mobilities of these amine

copolymers are quite high (3 104 to 1 103 cm2/V s) [6466] equaling hole

mobility values for small molecules such as tetraphenyl diamine (TPD) doped into

polycarbonates. The high hole mobility of these materials suggests that they are


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potentially useful as photoconductors as well as hole transporters in LED devices.

In light of the success in creating polymers with unique properties, the alternating

copolymer approach has been extended to other conjugated monomers, as Figure

4 exemplifies (e.g., thiophene, bithiophene, triarylamine, etc.). All copolymers are

of high molecular weight, they are highly photo luminescent, and their emissivecolors can be qualitatively correlated to the extent of delocalization in the co

monomers. For example, the thiophene copolymer emits bluish green light, the

cyanostilbene copolymer emits green light, and the bithiophene copolymer emits

yellow light. Thus, the choice of comonomer in the fluorene-based polymer family

has served as an excellent synthetic tool for designing polymers with well-

balanced hole- and electron-transport properties and fine color control. A

portfolio of fluorene copolymers that emit colors spanning the entire visible range

has been prepared using the improved Suzuki route. [63] Rich red-, green-, and

blue-emitting polymers are of highest interest. No other polymer class offers thefull range of color with high efficiency, low operating voltage, and high lifetime

when applied in a device configuration. Thus the polyfluorene based molecules

are the most viable LEPs for commercialization; and since making these materials

available, we have positive feedback from a variety of customers who have

validated this concept to us.

Poly (p-phenylene vinylene) and Derivatives

Since the discovery about a decade ago of the ability of PPV to emit light (yellow-

green) under electrical stimulation, numerous research groups have continued towork towards optimization of this promising class of material. Improvements

have developed primarily in ease of synthesis and processability, control of charge

carrier balance, overall device power efficiency and lifetime, and to some extent

emission wavelength tunability. A variety of solvent-processable precursor

polymers (converted thermally to PPV following the coating process), solution-

processable versions of PPV, and PPV copolymers (to facilitate electron injection

and modulate color) have resulted from these efforts. The first convenient

synthetic route to PPVs was first described by Wessling et al. in 1968 via bis

sulfonium salt monomers. [14, 15] Since these original accounts, many variationson the theme have evolved. Routes have primarily afforded the ability to

reproducibly generate high-molecular-weight, high-purity polymers, vary

functional groups bound to the parent backbone (particularly solubilizing groups

and groups to enhance electron affinity), and produce PPVs with improved

efficiency in LED devices. The chemistry has remained somewhat restricted in


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enabling development of rich, saturated red and blue emitters, and the chemical

structure of the PPV backbone is inherently photo-oxidatively unstable due to the

vinylene linkages. While these issues are likely to continue to challenge the

introduction of PPVs into commercial display devices (particularly multi- and full-

color applications) in the future, significant progress has been made toward theoptimization and tuning of these materials, making them very good candidates for

PLED applications.

POLYPHENYLENE

Poly (1, 4-phenylene) (PPP) represents another class of conjugated polymer of

interest to the PLED field. Wide band gaps are typical for PPPs and allow emission

of blue light. Since the design of efficient long-lived blue emitters remains a

significant challenge to the field, these polymers, like the poly fluorenes, areattractive candidates for consideration. As with the PPVs, most PPPs are insoluble

and intractable. Thus, research activities in the PPP field have emphasized routes

to PPP films via soluble thermally converted precursor polymers, as well as the

development of soluble PPPs. A wide variety of processable PPP precursor routes

have been described. [76] Similar to the PPV precursor chemistries, the PPP

precursor routes typically involve the thermal elimination of a leaving group(s) to

generate the fully conjugated system. For example, elimination of two equivalents

of acetic acid (per monomer unit) from poly-1, 4-(5, 6-diaceto-2, 3-cyclohexene)

enables aromatization of the polymer chain rings and formation of the PPPstructure. Relatively high temperatures are required to effect the conversion, and

side reactions that can lead to reduction of molecular weight (and reduce

polymer film integrity) are problematic. Therefore, progress in the precursor

approach to PPP emissive devices has been slow and has met with very limited

success. A great deal of attention has been focused on the preparation and

characterization of PPPs derived from monomers functionalized with a variety of

solubility-enhancing groups, including alkyl, alkoxy, and aryl units, analogous to

the development of soluble PPV systems. Since themonomers and resulting

polymers are soluble if appropriately long or bulky substituents are employed, thepolymerization reaction can occur in solution using well-known aryl or Suzuki

coupling approaches. Molecular weights high enough to cast films with good

integrity have been achieved, and blue-emitting diodes have been constructed

and demonstrated reasonable efficiencies.


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Operational Characteristics: -The primary attributes of PLED device performance

are its emissive ability or perceived brightness, color matching within targeted

specifications, and control and longevity in operation. The term brightness is a

physiological description describes the yellow-green portion of the spectrum to

which of light intensity with respect to the wavelength sensitivity of the humaneye. The average observer possesses daylight sensitivity to wavelength described

by the photopic efficacy curve depicted in Figure. The maximum of this response

curve to which we are most sensitive.The response is almost Gaussian, falling off

rapidly towards either the red or blue. This photopic response has significant

implications for the materials designer of light-emitting polymers. For a green

emitter, a given number of photons corresponding to a proportional number of

electronic transitions will be perceived as attaining certain brightness. For a red

emitter, however, the material must support a significantly greater number of

electronic transitions in the same time interval in order to be perceived at thesame brightness level. For materials with similar quantum efficiencies, more

emitted photons come at the expense of increased current, which is typically

achieved at higher operating voltages. Matching similar operating characteristics

and brightness levels require dissimilar material EL efficiencies. Color tuning is an

important parameter when addressing the scope of LEPs. Due to the disorder of

the polymer matrix, emission peaks will be broad, with a full width at half

maximum (FWHM) approaching 60 to 70 nm for monochromatic sources. Color

is calculated using the integral of spectral emission as a function of wavelength

against the chromatic sensitivity of the eye. This results in a set of coordinates ona chromaticity graph defined by international standards maintained by the

Commission International de l'Eclairage (CIE). Figure 6 illustrates typical PLED

device data from two different color emitters, one red and the other green.

Identical device structures were employed to duplicate conditions that are

present on a multicolor display. Both devices utilize 40 nm of PEDOT as the HTL

and 80 nm of the respective EML. Both devices also use calcium as the cathode

contact. The green device has a maximum EL peak at 534 nm and the red device

has its maximum emission at 634 nm. The color coordinates according to the CIE

1931 chromaticity diagram for the green emitter are (0.385, 0.579), and for thered emitter the CIE coordinates are (0.683, 0.313). A desirable feature with LEPs is

that they can be fabricated into a device possessing a relatively constant

efficiency over a wide luminance range. An example of the near-linearity of the

material efficiency of the same green emitter described above is shown in Figure.

The green emitter is more efficient, with higher luminance levels at lower voltage


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levels than can be achieved with the red emitter in an identical device

configuration. If we approximate the relative EL spectral curves to follow the

same shape, then according to the photopic efficacy model of the eye, the red

emitter must produce four times more photons than the green emitter in the

same period of time to stimulate an equal sensation of brightness. For deep-blueemitters, the relationship is less favorable. Lifetime reliability is also an important

characteristic of this technology. Although different end-users have their own

unique lifetime tolerances, we shall adhere to the current definition that lifetime

refers to that duration of operating time corresponding to the decay of device

luminance to one-half of the initial value. We shall also describe lifetime

parameters that are obtained under DC (or continuously ON) conditions. When

describing device lifetime, it is also important to specify the corresponding initial

luminance level from which it is measured. For example, an early candidate for a

polyfluorene- based red EL was measured for device lifetime, of which the resultsof one device are shown in Figure 8. The data were obtained using DC operation

under constant current driving conditions. The initial luminance value was 100

cd/m2 and was directly proportional to the current applied to the device. Of

interest are both the voltage rise with time. As well as the luminance decay.

Varying the molecular design of EL polymers will permit long lifetimes, but with

varying degrees of voltage change during the operational lifetime of a device. The

lifetime of this specific device was later found to be approximately 5500 h. Such

data were used to compile the engineering diagram shown in Figure 9. For the

same red material, data were obtained from many DC lifetime evaluationsstarting with various initial luminances, all at room-temperature and -pressure

conditions. None of the data were extrapolated; all data represent empirical

observations of device performance in the laboratory. The relationship between

initial luminance and lifetime follows an inverse power law very well; with L0

describing the initial luminance and T1/2 was representing the half-life. Note that

lifetimes of 1 000 000 h are possible with this early material candidate with this

specific device configuration, but only with an initial luminance value of 5 to 7

cd/m2. Such graphical relationships are essential to the design engineer

considering certain performance levels over a desired operating duration.Electroluminescent material applied to a transparent surface and excited to

emission creates a Lambertian source of light, in that the surface can be

adequately described by the surface integral of a two-dimensional array of diffuse

point emitters, each generating light equally. As such, potential applications that

are replacements of filament-based sources of light are not straight forward,


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especially if product specifications are defined in terms of the luminous intensity

of localized point emitters. In these circumstances, different means of describing

product performance are required. Descriptions of filament bulb performance

describe luminance intensity in units of candelas (cd), whereby surface emitters

are described in terms of luminance with units in cd/m2. The two systems ofphotometric nomenclature can be reconciled by considering requirements need

to be redefined. For example, the luminance intensity for some safety indicators

is specified on the central optical axis, which is directly in-line with the filament

source. It is implicitly assumed that there will be some intensity drop-off at other

spatial locations on the front lens. The same intensity available at all points over

an extended emitter would be perceived as uncomfortably bright, even though

that is what would be required to meet the defining product specifications.

Therefore, as this technology is considered for incumbent technology

replacement, existing product specifications will need to be broadened to includeLambertian sources. Specific applications intended for the marketplace will

require their own unique engineering and tooling. But given the commonality of

the emissive medium, there are engineering aspects that are common to most

application embodiments. Most applications of either monochrome or multicolor

formats for information display will require pixilation of the emissive area. There

are two primary methodologies associated with achieving this, and both require

etching rows of ITO strips on the transparent substrate, and later depositing

columns of cathode electrodes across the display. Where the cathode and anode

overlap defines a pixel. Simple excitation of a given row and column definespassive-matrix operation. Passive-matrix driving schemes rely on pulsing each row

of pixels in sequence where the entire display format is scanned with a period of

less than 1/30 s. This corresponds to the limit of visual temporal resolution,

otherwise known as the physiological persistence of the latent image. For a

display built of N rows, each row will be pulsed (1/N)(1/30) s each. In that way,

the observer will not notice image flicker and will perceive a continuous image

display. The addition of individual thin-film transistors on the substrate for each

pixel permits individual pixels to be independently turned on and off, and

alleviates the need to raster each row sequentially within 1/30 s. This operationdefines active-matrix driving. Fine display resolution is achieved with small,

closely spaced pixels. For very fine resolution, neighboring pixels could be so

closely spaced that interpixel communication could distort the intended

operation with what is referred to as cross-talk. Cross-talk will result in a

blooming effect whereby nearest neighbor pixels will display some unintended


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gray-scale activation state. The use of highly conducting polymers as the HTL can

aggravate this problem by providing a low resistive path to nearest neighbor sites.

This can be overcome by modifications to the conductivity of the HTL, or to the

driver electronics, or both. In the passive matrix display, a pixel that will be in the

active state for 1/N of the scan period and which is designed to be perceived at aluminance of L will have to be pulsed to a luminance of NL for (1/N) (1/30) s. In

addition, improved image contrast from the display is achieved by using a

polarizer front filter, which also reduces the perceived luminance by

approximately 50 %. Therefore, a 64 128 pixel display running with a maximum

perceived luminance of 100 cd/m2 will be designed to operate with a maximum

luminance of 200 cd/ m2 (filter accommodation), and a given pixel will be

required to emit with 12 800 cd/m2 for 0.5 ms every 33 ms. Such pixels may

require 150 mA/cm2 given common materials characteristics. For portable

displays with small pixel sizes operating under short pulse duration, this is notexpected to introduce any undue Joule-heating effects. In fact, the pulsing

operation will permit higher device efficiencies and luminance levels than would

be achievable with DC (continuously ON) operation. [79] However, for large

display formats where the number of rows is large, the pulsed requirements for a

pixel can be substantial. For displays beyond approximately 6 inch (1 inch 2.5

cm) diagonal, active-matrix operation will be required. Then the minimum time

for each pixel to be operated will be 1/30 s, and the maximum luminance will be

the value required for observation (200 cd/m2 if used with a polarizer filter). It is

expected that the product lifetime based on an active matrix operation willexceed that achievable with a passive-matrix drive scheme. Also common to

PLED-based products will be the requirement for airtight device packaging. The

PLED is a semiconducting optoelectronic device operating at fields near half a

million volts per centimeter. With current flowing through the matrix, trapped

chemisorbed water molecules can undergo electrolysis, liberating radical oxygen-

bearing ions, which are detrimental to organic molecules. The preferred methods

of device encapsulation will have to stay below the glass transition temperature

of the polymers, because going above this precipitates device shorts and failure.

[80] Therefore, device encapsulation schemes are not likely to greatly exceed 100_C. The engineering of flexible displays offers greater challenges, but comes with

greater rewards. Some of the issues that will need to be addressed include

providing a substrate that seals against moisture and oxygen, maintaining

flexibility while maintaining the adhesion and integrity of a conducting anode, and

accommodating an encapsulation scheme with package flexibility. Dow is


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uniquely positioned with access to a broad portfolio of materials expertise across

the company to address these and related challenges that face the final stage of

product definition.

3.1 ADVANTAGES

y Requires only 3.3 volts and have lifetime of more than 30,000 hours.

y Low power consumption.

y Self luminous.

y No viewing angle dependence.

y Display fast moving images with optimum clarity.

y

Cost much less to manufacture and to run than CRTs because the activematerial is plastic.

y Can be scaled to any dimension.

y Fast switching speeds that are typical of LEDs.

y No environmental draw backs.

y No power in take when switched off.

y All colours of the visible spectrum are possible by appropriate choose of

polymers.

y Simple to use technology than conventional solid state LEDs and lasers.

y

Very slim flat panel.

3.2 DISADVANTAGES

y Vulnerable to shorts due to contamination of substrate surface by dust.

y Voltage drops.

y Mechanically fragile.

y Potential not yet realized.


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APPLICATIONS & FUTUREDEVELOPMENTS

4. 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 PHOTO-VOLTAICS

CDTs 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

4.2 POLYLED TV

Philips will demonstrate its first 13-inch Poly LED 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,


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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' Poly LED TV prototype illustrates

the great potential of this new display technology for TV applications. Accordingto current predictions, a polymer OLED-based TV could be a reality in the next five

years.

4.3 BABY MOBILE

BABY MOBILE

This award winning baby mobile uses light weight organic light emitting diodes to

realize images and sounds in response to gestures and speech of the infant.


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4.4 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 Go

Dot Technology. The unit employs a polymeric light-emitting diode (PLED) display

supplied by Delta Optoelectronics, Taiwan, which is made with green Lumationlight-emitting polymers furnished by Dow Chemical Co., Midland, Mich.


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5. FUTUREDEVELOPMENTS

Here are 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 filledwith information and entertainment data of all sorts may make our lives easier,

happier and safer.

Demands for information on the move could drive the development of 'wearable'

displays, with interactive features.

Eywith changing information cole would give many brand ownerve edge


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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 MOREDEVELOPMENTS

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 computerscreens 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.

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 wouldgreatly 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


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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 in roads 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 ofchoice 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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CONCLUSION

Light-emitting polymers have been studied intensively as materials for light

emitting diodes (LEDs). Here research efforts toward developing these materialsfor commercial applications are reviewed. The Figure shows the preferred two-

layer device structure for commercial polymer LEDs as well as polyfluorene, one

of the polymers discussed. Light-emitting polymers (LEPs) to the unique class of

aromatic organic molecules that exhibit semiconducting behavior and give off

light when electrically stimulated.

They are, consequently, electroluminescent as well as photo luminescent when

stimulated by long-wave ultraviolet (UV) irradiation. Electro active devices based

on LEPs typically operate at or below 5 V direct current (DC), have demonstrated

high efficiencies in excess of 20 lm/W, and are approaching average operatinglifetimes of 10 000 h at 200 cd/m2 intensity. As polymers, these materials exhibit

mechanical, electrical, and luminescent properties that hold great potential for

display application. The mechanical properties of polymers enable them to be

cast in a variety of shapes and sizes, and their simplified processing offers fewer

steps to manufacture, thereby providing economic advantages over many other

more elaborate device fabrication techniques.

Light is produced in the polymer by the fast decay of excited molecular states, the

color of which depends on the energy difference between those excited states

and the molecular ground level. A light-emitting device comprises a thin filmstructure of one or two layers typically no more than 0.1 lm thick, sandwiched

between two electrodes.


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APPENDIX

This report covers topics on Light Emitting Polymers. Polymeric

electroluminescence (EL) is a phenomenon based on amorphous, disordered

organic semiconducting materials that has stimulated technological activity across

a wide interdisciplinary reach within academic, industrial, and governmentalsectors. Efforts aimed at developing associated technology have created a rare

opportunity to develop high value products and have contributed fundamental

scientific insight in this area, which is poised for commercial success. Recent

materials advances have endowed the concept of plastic light with the optical,

electrical, and mechanical characteristics that truly make it disruptive technology

within the display and lighting industries in that it is compatible with conventional

device replacement and it offers new opportunities for exploitation. We review

the materials science and engineering of this field within the framework of

anticipated impact on product definition, as well as highlight contributions to the

field originating from within The Dow Chemical Company.


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REFERENCES

1. www.google.co.in

2.

Researches made by The Dow Chemical Company


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