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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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