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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 theelectrodes, 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 (color) 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,
OLEDs 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. Unlike other flat panel displays OLED has a
wide viewing angle (upto 160 degrees), even in bright light. Their low power
consumption (only 2 to 10 volts) provides for maximum efficiency and helps minimize
heat and electric interference in electronic devices. Because of this combination of this
features, OLED displays communicate more information in a more engaging way while
adding less weight and taking up less space. Their application in numerous devices is not
only a future possibility but a current reality.
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LIST OF FIGURES
Figure No. Title Page No.
2.1 OLED schematic 6
2.2 OLED working principle 7
3.1 Alq3 9
3.2 poly (p-phenylene vinylene) 10
3.3 Ir (mppy)3 11
3.4 OLED structure 12
3.5 Schematic of the ink jet printing 13
3.6 Active and passive matrices 15
3.7 Conjugation of 18
3.8 Series of orbital diagrams 19
4.1 Active matrix OLED Structure 22
4.2 Passive matrix OLED structure 23
4.3 Transparent OLED structure 24
4.4 Top-emitting OLED structure 25
4.5 Foldable OLED 26
4.6 White OLED 27
6.1 A Sony PSP having foldable OLED display 32
6.2 Toshiba Laptop having Transparent OLED 33
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INTRODUCTION
History
Eastman Kodak Company & Universal laboratories, USA has started the research
towards the OLED technology but the cup of victory gone to the Kodak researchers havemade a number of major breakthroughs which led to patents on OLED material, device
structure, dopping techniques to drastically improve efficiency and colour control, thin
film deposition method, patterning methods as well as design & fabrication methods for
both active & passive matrix OLED panels.
The OLED technology initially grew from research on organic electronic devices used in
solar cells & electrophotography. At this time Kodak is the worlds only company who
has patent on this OLED technology. The intrinsic quality of this technology is superb
because of its high brightness & efficiency, low drive voltage fast response. Low cost
manufacturing methods are already in use for passive matrix OLED display. The advanceof the complementary low temperature polySi technology has enabled the fabrication of
high resolution, full colour, active matrix OLED display. An organic light emitting diode
(OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is
a film of organic compounds which emit light in response to an electric current. This
layer of organic semiconductor material is situated between two electrodes. Generally, at
least one of these electrodes is transparent. OLEDs are used in television screens,
computer monitors, small, portable system screens such as mobile phones and PDAs,
watches, advertising, information and indication. OLEDs are also used in light sources
for space illumination and in large-area light-emitting elements. Due to their early stage
of development, they typically emit less light per unit area than inorganic solid-state
based LED point-light sources.
An OLED display functions without a backlight. Thus, it can display deep black levels
and can be thinner and lighter than liquid crystal displays. In low ambient light conditions
such as dark rooms, an OLED screen can achieve a higher contrast ratio than an LCD
using either cold cathode fluorescent lamps or the more recently developed LED
backlight.
Evolution
With the imaging appliance revolution underway, the need for more advanced handheld
devices that will combine the attributes of a computer, PDA, and cell phone is increasing
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and the flat-panel mobile display industry is searching for a display technology that will
revolutionize the industry. The need for new lightweight, low-power, wide viewing
angled, handheld portable communication devices have pushed the display industry to
revisit the current flat-panel digital display technology used for mobile applications.
Struggling to meet the needs of demanding applications such as e-books, smart
networked household appliances, identity management cards, and display-centric
handheld mobile imaging devices, the flat panel industry is now looking at new displays
known as Organic Light Emitting Diodes (OLED).
Over the time there are many changes came into the field of output/display devices. In
this field first came the small led displays which can show only the numeric contains.
Then came the heavy jumbo CRTs (Cathode Ray Tubes) which are used till now. But the
main problem with CRT is they are very heavy & we couldnt carry them from one place
to another the result of this CRT is very nice & clear but they are very heavy & bulky &
also required quiet large area then anything else.
Then came the very compact LCDs (Liquefied Crystal Displays). They are very lighter in
weight as well as easy to carry from one place to the other. But the main problem with
the LCDs is we can get the perfect result in the some particular direction. If we see from
any other direction it will not display the perfect display. To overcome this problems of
CRTs & LCDs the scientist of Universal Laboratories, Florida, United States & Eastman
Kodak Company both started their research work in that direction & the overcome of
their efforts is the new generation of display technologies named OLED (Organic Light
Emitting Diode) Technology.
In the flat panel display zone unlike traditional Liquid-Crystal Displays OLEDs are self
luminous & do not required any kind of backlighting. This eliminates the need for bulky
& environmentally undesirable mercury lamps and yields a more thinner ,more compact
display. Unlike other flat panel displays OLED has a wide viewing angle (up to 160
degrees), even in bright light. Their low power consumption (only 2 to 10 volts) provides
for maximum efficiency and helps minimize heat and electric interference in electronic
devices. Because of this combination of this features, OLED displays communicate more
information in a more engaging way while adding less weight and taking up less space.
Their application in numerous devices is not only a future possibility but a current reality.
An organic light emitting diode (OLED) is a light-emitting diode (LED) in which theemissive electroluminescent layer is a film of organic compounds which emit light in
response to an electric current. This layer of organic semiconductor material is situated
between two electrodes. Generally, at least one of these electrodes is transparent. OLEDs
are used in television screens, computer monitors, small, portable system screens such as
mobile phones and PDAs, watches, advertising, information and indication. OLEDs are
also used in light sources for space illumination and in large-area light-emitting elements.
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Due to their early stage of development, they typically emit less light per unit area than
inorganic solid-state based LED point-light sources.
An OLED display functions without a backlight. Thus, it can display deep black levels
and can be thinner and lighter than liquid crystal displays. In low ambient light conditions
such as dark rooms, an OLED screen can achieve a higher contrast ratio than an LCDusing either cold cathode fluorescent lamps or the more recently developed LED
backlight.
There are two main families of OLEDs: those based upon small molecules and thoseemploying polymers. Adding mobile ions to an OLED creates a Light-emitting
Electrochemical Cell or LEC, which has a slightly different mode of operation. OLED
displays can use either passive-matrix (PMOLED) or active-matrix addressing schemes.Active-matrix OLEDs (AMOLED) require a thin-film transistor backplane to switch each
individual pixel on or off, and can make higher resolution and larger size displays
possible.
WORKING PRINCIPLE
A typical OLED is composed of an emissive layer, a conductive layer, a substrate, and
anode and cathode terminals. The layers are made of special organic molecules that
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conduct electricity. Their levels of conductivity range from those of insulators to those of
conductors, and so they are called organic semiconductors. The first, most basic OLEDs
consisted of a single organic layer, for example the first light-emitting polymer device
synthesized by Burroughs et al involved a single layer of poly(p-phenylene vinylene).
Multilayer OLEDs can have more than two layers to improve device efficiency. As well
as conductive properties, layers may be chosen to aid charge injection at electrodes by
providing a more gradual electronic profile, or block a charge from reaching the opposite
electrode and being wasted.
A voltage is applied across the OLED such that the anode is positive with respect to the
cathode. This causes a current of electrons to flow through the device from cathode to
anode.
1. The battery or power supply of the device containing the OLED applies a voltage
across the OLED.
2. An electrical current flows from the cathode to the anode through the organic layers
(an electrical current is a flow of electrons).
3. The cathode gives electrons to the emissive layer of organic molecules.
4. The anode removes electrons from the conductive layer of organic molecules. (This is
the equivalent to giving electron holes to the conductive layer.)
5. At the boundary between the emissive and the conductive layers, electrons find
electron holes.
Figure 2.1 OLED schematic
6. When an electron finds an electron hole, the electron fills the hole (it falls into an
energy level of the atom that's missing an electron). When this happens, the electron
gives up energy in the form of a photon of light (see How Light Works).
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Figure 2.2 OLED working principle
7. The OLED emits light.
8. The color of the light depends on the type of organic molecule in the emissive layer.Manufacturers place several types of organic films on the same OLED to make color
displays.
9. The intensity or brightness of the light depends on the amount of electrical current
applied: the more current, the brighter the light.
CONSTRUCTION
Material technologies
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A) Small molecules:
Figure 3.1 Alq3, commonly used in small molecule OLEDs.
Efficient OLEDs using small molecules were first developed by Dr. Ching W. Tanget al.
at Eastman Kodak. The term OLED traditionally refers specifically to this type of device,
though the term SM-OLED is also in use.
Molecules commonly used in OLEDs include organometallic chelates (for exampleAlq3,
used in the organic light-emitting device reported by Tang et al.), fluorescent and
phosphorescent dyes and conjugated dendrimers. A number of materials are used for their
charge transport properties, for example triphenylamine and derivatives are commonly
used as materials for hole transport layers. Fluorescent dyes can be chosen to obtain light
emission at different wavelengths, and compounds such as perylene, rubrene and
quinacridone derivatives are often used. Alq3 has been used as a green emitter, electron
transport material and as a host for yellow and red emitting dyes.
The production of small molecule devices and displays usually involves thermal
evaporation in a vacuum. This makes the production process more expensive and of
limited use for large-area devices than other processing techniques. However, contrary to
polymer-based devices, the vacuum deposition process enables the formation of well
controlled, homogeneous films, and the construction of very complex multi-layer
structures. This high flexibility in layer design, enabling distinct charge transport and
charge blocking layers to be formed, is the main reason for the high efficiencies of the
small molecule OLEDs.
Coherent emission from a laser dye-doped tandem SM-OLED device, excited in the
pulsed regime, has been demonstrated. The emission is nearly diffraction limited with a
spectral width similar to that of broadband dye lasers.
B) Polymer light-emitting diodes:
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http://en.wikipedia.org/wiki/Tris(8-hydroxyquinolinato)aluminiumhttp://en.wikipedia.org/wiki/Tris(8-hydroxyquinolinato)aluminiumhttp://en.wikipedia.org/wiki/Ching_W._Tanghttp://en.wikipedia.org/wiki/Eastman_Kodakhttp://en.wikipedia.org/wiki/Eastman_Kodakhttp://en.wikipedia.org/wiki/Chelationhttp://en.wikipedia.org/wiki/Tris(8-hydroxyquinolinato)aluminiumhttp://en.wikipedia.org/wiki/Tris(8-hydroxyquinolinato)aluminiumhttp://en.wikipedia.org/wiki/Tris(8-hydroxyquinolinato)aluminiumhttp://en.wikipedia.org/wiki/Dendrimerhttp://en.wikipedia.org/wiki/Triphenylaminehttp://en.wikipedia.org/wiki/Perylenehttp://en.wikipedia.org/wiki/Rubrenehttp://en.wikipedia.org/wiki/Quinacridonehttp://en.wikipedia.org/wiki/Evaporation_(deposition)http://en.wikipedia.org/wiki/Evaporation_(deposition)http://en.wikipedia.org/wiki/File:AlumQ3.pnghttp://en.wikipedia.org/wiki/Tris(8-hydroxyquinolinato)aluminiumhttp://en.wikipedia.org/wiki/Ching_W._Tanghttp://en.wikipedia.org/wiki/Eastman_Kodakhttp://en.wikipedia.org/wiki/Chelationhttp://en.wikipedia.org/wiki/Tris(8-hydroxyquinolinato)aluminiumhttp://en.wikipedia.org/wiki/Dendrimerhttp://en.wikipedia.org/wiki/Triphenylaminehttp://en.wikipedia.org/wiki/Perylenehttp://en.wikipedia.org/wiki/Rubrenehttp://en.wikipedia.org/wiki/Quinacridonehttp://en.wikipedia.org/wiki/Evaporation_(deposition)http://en.wikipedia.org/wiki/Evaporation_(deposition) -
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Figure 3.2poly (p-phenylene vinylene), used in the first PLED.
Polymer light-emitting diodes (PLED), also light-emitting polymers (LEP), involve an
electroluminescent conductive polymer that emits light when connected to an external
voltage. They are used as a thin filmforfull-spectrum colour displays. Polymer OLEDs
are quite efficient and require a relatively small amount of power for the amount of light
produced.
Vacuum deposition is not a suitable method for forming thin films of polymers.
However, polymers can be processed in solution, and spin coatingis a common method
of depositing thin polymer films. This method is more suited to forming large-area films
than thermal evaporation. No vacuum is required, and the emissive materials can also be
applied on the substrate by a technique derived from commercial inkjet printing.
However, as the application of subsequent layers tends to dissolve those already present,
formation of multilayer structures is difficult with these methods. The metal cathode may
still need to be deposited by thermal evaporation in vacuum.
Typical polymers used in PLED displays include derivatives of poly(p-phenylene
vinylene)andpolyfluorene. Substitution of side chains onto the polymer backbone may
determine the colour of emitted light or the stability and solubility of the polymer for
performance and ease of processing.
While unsubstituted poly(p-phenylene vinylene) (PPV) is typically insoluble, a number
of PPVs and related poly(naphthalene vinylene)s (PNVs) that are soluble in organic
solvents or water have been prepared via ring opening metathesis polymerization.
C) Phosphorescent materials:
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http://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene)http://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene)http://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene)http://en.wikipedia.org/wiki/Electroluminescencehttp://en.wikipedia.org/wiki/Conductive_polymerhttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Thin_filmhttp://en.wikipedia.org/wiki/Thin_filmhttp://en.wikipedia.org/wiki/Full-spectrumhttp://en.wikipedia.org/wiki/Spin_coatinghttp://en.wikipedia.org/wiki/Spin_coatinghttp://en.wikipedia.org/wiki/Substrate_(printing)http://en.wikipedia.org/wiki/Substrate_(printing)http://en.wikipedia.org/wiki/Substrate_(printing)http://en.wikipedia.org/wiki/Inkjet_printerhttp://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene)http://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene)http://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene)http://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene)http://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene)http://en.wikipedia.org/wiki/Polyfluorenehttp://en.wikipedia.org/wiki/Substitution_reactionhttp://en.wikipedia.org/wiki/Ring_opening_metathesis_polymerizationhttp://en.wikipedia.org/wiki/Ring_opening_metathesis_polymerizationhttp://en.wikipedia.org/wiki/File:Polyphenylene_vinylene.pnghttp://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene)http://en.wikipedia.org/wiki/Electroluminescencehttp://en.wikipedia.org/wiki/Conductive_polymerhttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Thin_filmhttp://en.wikipedia.org/wiki/Full-spectrumhttp://en.wikipedia.org/wiki/Spin_coatinghttp://en.wikipedia.org/wiki/Substrate_(printing)http://en.wikipedia.org/wiki/Inkjet_printerhttp://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene)http://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene)http://en.wikipedia.org/wiki/Polyfluorenehttp://en.wikipedia.org/wiki/Substitution_reactionhttp://en.wikipedia.org/wiki/Ring_opening_metathesis_polymerization -
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these charge carriers takes place, which leads to emission of light that escapes through glass
substrate. The bandgap, i.e. energy difference between valence band and conduction band of the
semiconducting polymer determines the wavelength (colour) of the emitted light.
Figure 3.4 OLED structure
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.
In order to manufacture the polymer, a spin-coating machine is used that has a plate
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.
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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.
INK JET PRINTING PROCESS
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 .Creating a full color, 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.
Fig. 3.5 Schematic of the ink jet printing
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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 microns when placed 0.5 mm from the
substrate. In addition to the precision of the print head, the formulation of the ink is keyto making effective and attractive display devices. 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. 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 Creating a full color,
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 offlight deviation of less than 5 are used. In order to manufacture the polymer, a spin-
coating machine is used that has a plate spinning at the speed of a few thousand rotations
per minute.
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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. Coloured
displays are formed by positioning matrices of red, green and blue pixels very closetogether. To control the pixels, and so form the image required, either 'passive' or 'active'
matrix driver methods are used. Pixel displays can either by active or passive matrix. Fig.
2.1.2 shows the differences between the two matrix types, active displays have transistors
so that when a particular pixel is turned on it remains on until it is turned off.
The matrix pixels are accessed sequentially. As a result 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.
Fig 3.6 Active and 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.
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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. 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.
BASIC PRINCIPLE AND TECHNOLOGY
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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 tetrahedraloriented 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 tend 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 sp2 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.
LIGHT EMISSION
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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 electromagnetic(EM) radiation is emitted
from a material by passing an electrical current through it. The frequency of the EMradiation 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 the organic 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 statisticallyinhabit. 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 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.
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Figure 3.7 A demonstration of the full conjugation of
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 amountsof 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 effectivelycontinuous, 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 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.
Figure 3.8 a series of orbital diagrams.
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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, polyacetylene, 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 to release 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. A diatomic molecule has a bonding and an anti-bonding
orbital, two atomic orbitals gives two molecular orbitals. The electrons arrange
themselves following, Auf Bau and the Pauli Principle. A single atom has one atomic
obital. A triatomic 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 separated by a band gap, and thus the substance is either an insulator or asemi-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. The electron injector needs to inject electrons of sufficient
energy to exceed the band gap, the anode operates by removing electrons from the
polymer and consequently leaving regions of positive charge called holes. The anode is
consequently referred to as the hole injector.
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TYPES OF OLED
Active-matrix OLED - AMOLED
AMOLEDs have full layers of cathode, organic molecules and anode, but the anode layer
overlays a thin film transistor (TFT) array that forms a matrix. The TFT array itself is the
circuitry that determines which pixels get turned on to form an image. AMOLEDs
consume less power than PMOLEDs because the TFT array requires less power than
external circuitry, so they are efficient for large displays. AMOLEDs also have faster
refresh rates suitable for video. The best uses for AMOLEDs are computer monitors,
large screen TVs and electronic signs or billboards.
Active-matrix OLED displays provide the same beautiful video-rate performance as their passive-matrix OLED counterparts, but they consume significantly less power. This
advantage makes active-matrix OLEDs especially well suited for portable electronics
where battery power consumption is critical and for displays that are larger than 2 to 3
in diagonal
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Figure 4.1 Active matrix OLED Structure
An active-matrix OLED (AMOLED) display consists of OLED pixels that have been
deposited or integrated onto a thin film transistor (TFT) array to form a matrix of pixels
that illuminate light upon electrical activation. In contrast to a PMOLED display, where
electricity is distributed row by row, the active-matrix TFT backplane acts as an array of
switches that control the amount of current flowing through each OLED pixel.
Passive-matrix OLED - PMOLED
PMOLEDs have strips of cathode, organic layers and strips of anode. The anode strips
are arranged perpendicular to the cathode strips. The intersections of the cathode and
anode make up the pixels where light is emitted. External circuitry applies current to
selected strips of anode and cathode, determining which pixels get turned on and which
pixels remain off. Again, the brightness of each pixel is proportional to the amount of
applied current.
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Figure 4.2 Passive matrix OLED structure
PMOLEDs are easy to make, but they consume more power than other types of OLED,
mainly due to the power needed for the external circuitry. PMOLEDs are most efficient
for text and icons and are best suited for small screens (2- to 3-inch diagonal) such as
those you find in cell phones, PDAs and MP3 players. Even with the external circuitry,
passive-matrix OLEDs consume less battery power than the LCDs that are currently used
in these devices.
Transparent OLED
Transparent OLEDs have only transparent components (substrate, cathode and anode)
and, when turned off, are up to 85 percent as transparent as their substrate. When a
transparent OLED display is turned on, it allows light to pass in both directions. A
transparent OLED display can be either active- or passive-matrix. This technology can be
used for heads-up displays.
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Figure 4.3 Transparent OLED structure
Top emission: Using the same transparent structure, TOLED technology can also be used
for top-emitting structures for active-matrix displays and with opaque substrates.
Especially desirable for high-resolution, active-matrix OLED applications, a top-
emittingstructure can improve the effective active area and the power consumption of the
display by directing the emitted light away from the thin film transistor (TFT) backplane
rather than through it (see schematic below). Top-emitting OLEDs can also be built on
opaque surfaces such as metallic foil and silicon wafers. To illustrate this point, the video(to the right) shows an icon-format TOLED demonstrator that Universal Display
Corporation built on metallic foil with Palo Alto Research Center (PARC), a subsidiary
of Xerox Corporation, and Vitex Systems, Inc. Potential TOLED applications include
smart cards or displays on furniture, automotive parts and other opaque surfaces, to
suggest a few.
Top-emitting OLED
Top-emitting OLEDs have a substrate that is either opaque or reflective. They are best
suited to active-matrix design. Manufacturers may use top-emitting OLED displays insmart cards. TOLED transparent and top-emitting OLED technology uses a proprietary
transparent contact structure to create displays that can be transparent, that is, top- and
bottom-emitting or, selectively, top-emitting only.
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Figure 4.4 Top-emitting OLED structure
TOLEDs can significantly enhance display performance and open up many new product
applications. Transparency: TOLEDs can be 70% to 85% transparent when switched off,
nearly as clear as the glass or plastic substrate on which they are built. To better picture
this, please refer to the video (to the right) where a simple transparent OLED pixel is
shown turning on and off. This feature paves the way for TOLEDs to be built into vision-
area applications, such as architectural windows for home entertainment and
teleconferencing purposes, and automotive windshields for navigation and warning
systems. TOLEDs may also enable the development of novel helmet-mounted or "heads-
up" systems for virtual reality, industrial and medical applications.
Foldable OLED
Foldable OLEDs have substrates made of very flexible metallic foils or plastics. Foldable
OLEDs are very lightweight and durable. Their use in devices such as cell phones and
PDAs can reduce breakage, a major cause for return or repair. Potentially, foldable
OLED displays can be sewn into fabrics for "smart" clothing, such as outdoor survival
clothing with an integrated computer chip, cell phone, GPS receiver and OLED display
sewn into it.
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Figure 4.5 Foldable OLED
FOLED flexible OLEDs are organic light emitting devices that are built on flexible
substrates such as plastic or metallic foil. FOLED displays can offer significant
performance advantages over LCD displays that are typically built on rigid glass
substrates and contain a bulky backlight.Today, the primary substrate candidates are thin
plastics, such as PET and PEN polyester films. While these materials offer many
attractive features, they also currently impose limitations with respect to thermal
processing and barrier performance. Companies are developing coatings for these
substrates as well as new plastic substrates to compensate for these constraints. UniversalDisplay Corporation is actively working with a number of these companies.
White OLED
A white organic LED (OLED) incorporating a blue phosphorescent dye and a down-
conversion phosphor has achieved a luminous efficacy of 25 lm/W. This high-efficacy
device was enabled by lowering the device operating voltage, increasing the outcoupling
efficiency, and incorporating highly efficient phosphorescent emitters.
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Figure 4.6 White OLED
Solid-state white lighting using PHOLED, TOLED and FOLED technologies represents a
true breakthrough for next-generation lighting. Among the exciting advances in white
OLED lighting technology are the PHOLED technology and materials present the
potential to combine the power efficiencies of fluorescent tubes with the pleasing color
quality associated with incandescent bulbs in a thoroughly new flat form factor. In
collaboration with Toyota Industries Corporation, at the 2004 Society for Information
Display Symposium and Exhibition, we reported record-breaking white PHOLED
performance exceeding 18 lm/W at an operating voltage of < 6.5 V, brightness of 1000
cd/m2 and CIE color coordinates of (0.38, 0.38).
ADVANTAGES & DRAWBACKS
OLEDs offer many advantages over both LCDs and LEDs:
Require only 3.3 volts and have lifetime of more than 30,000 hours.
Low power consumption.
Self luminous.
No viewing angle dependence.
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Display fast moving images with optimum clarity.
Cost much less to manufacture and to run than CRTs because the active material
is plastic.
Can be scaled to any dimension.
Fast switching speeds that are typical of LEDs.
No environmental draw backs.
No power in take when switched off.
All colours of the visible spectrum are possible by appropriate choose of
polymers.
Simple to use technology than conventional solid state LEDs and lasers.
Very slim flat panel. The plastic, organic layers of an OLED are thinner, lighter
and more flexible than the crystalline layers in an LED or LCD.
Because the light-emitting layers of an OLED are lighter, the substrate of an
OLED can be flexible instead of rigid. OLED substrates can be plastic rather than the
glass used for LEDs and LCDs. OLEDs are brighter than LEDs. Because the organic layers of an OLED are much
thinner than the corresponding inorganic crystal layers of an LED, the conductive and
emissive layers of an OLED can be multi-layered. Also, LEDs and LCDs require glass
for support, and glass absorbs some light. OLEDs do not require glass.
OLEDs do not require backlighting like LCDs. LCDs work by selectively
blocking areas of the backlight to make the images that you see, while OLEDs generate
light themselves. Because OLEDs do not require backlighting, they consume much less
power than LCDs (most of the LCD power goes to the backlighting). This is especially
important for battery-operated devices such as cell phones.
OLEDs are easier to produce and can be made to larger sizes. Because OLEDs areessentially plastics, they can be made into large, thin sheets. It is much more difficult to
grow and lay down so many liquid crystals.
OLEDs have large fields of view, about 170 degrees. Because LCDs work by
blocking light, they have an inherent viewing obstacle from certain angles. OLEDs
produce their own light, so they have a much wider viewing range.
OLED seems to be the perfect technology for all types of displays, but it also has
some problems:
Lifetime - While red and green OLED films have longer lifetimes (46,000 to 230,000
hours), blue organics currently have much shorter lifetimes (up to around 14,000 hours.
The major drawback is the limited lifetime of organic materials. This problem still needs
to be solved to push OLED technology to be more successful in the future. Blue OLEDs
have only a lifetime of around 5,000 hours, when used in flat panel displays, which is
much lower than the typical lifetimes of LCDs or plasma displays. But there are various
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experimentations to increase the lifetime, some are reporting that they already reached a
lifetime up to 10,000 hours and above.
Water - Water can easily damage OLEDs. Organic materials can easily be damaged by
water intrusion into the displays. Therefore an improved sealing process is necessary for
OLED displays.
Vulnerable to shorts due to contamination of substrate surface by dust.
Voltage drops.
Mechanically fragile.
Potential not yet realized.
The development of the technology is restrained by patents held by Kodak and other
companies. For commercial development of OLED technology it is often necessary toacquire a license.
APPLICATIONS & FUTURE SCOPE
Applications of OLEDs
OLEDs have been proposed for a wide range of display applications including magnified
micro displays, wearable, head-mounted computers, digital cameras, personal digital
assistants, smart pagers, virtual reality games, and mobile phones as well as medical,
automotive, and other industrial applications. This OLEDs with its full color displays will
replace todays liquid crystal displays (LCDs) used in laptop computers and may even
one day replace ordinary CRT-screens. OLED technology is already used in some
devices. On this page we will name some products that are powered by OLED displays.
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Most of them are cellular phones or portable music players, but also other products use
this new technology. Cellular/mobile phones There are many mobile phones that use
OLED displays. Samsung has several models like the SGH-E700, E715 or E730. All
these models use an external OLED screen with different resolutions (64 x 96, 96 x 96
pixels) and different color depths (either 256 colours or 65k colours). The Samsung SGH-
X120 uses a main OLED screen with 128 x 128 pixels. The S88 phone from BenQ-
Siemens uses a two inch active-matrix OLED display with about 262k colors and 176 x
220 pixels. LG Electronic offers several mobile phones with an OLED technology. LG
LP4100 has an external display powered with the new technology.
Figure 6.1 A Sony PSP having foldable OLED display
LG's model VX8300 has an organic light-emitting diode display with 262,000 colors and
a resolution of 176 x 220 pixels.
Other mobile phone manufacturers like Motorola, Nokia, Panasonic or Sony Ericsson are
also using organic light emitting diodes for their external displays. MP3 playersMobiBLU ships an mp3 player that features an OLED display, the DAH-1500i model.
The popular Creative Zen Micro has also an organic LED display with 262k colors. The
Sony NW-A3000 and NW-A1000 both have an OLED display. The Zen Sleek music
player from Creative has a new 1.7 inch organic LED display. The Giga beat audio player
from Toshiba features also an OLED screen. The Kodak Easy Share LS633 is the world's
first digital camera with an organic LED display. The Sanyo Xacti HD1 is a high
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definition camera that features an OLED display. Other digital cameras with an OLED
screen are from Hasselblad (H2D-39 and 503CWD for example).
Figure 6.2 Toshiba Laptop having Transparent OLED
Future scope of OLEDs
In OLEDs as crystalline order is not required, organic materials, both molecular and
polymeric, can be deposited far more cheaply than the inorganic semiconductors ofconventional LEDs. Patterning is also easier, and may even be accomplished by
techniques borrowed from the printing industry. Displays can be prepared on flexible,
transparent substrates such as plastic. These characteristics form the basis for a displaytechnology that can eventually replace even paper, providing the same resolution and
reading comfort in a long-lived, fully reusable (and eventually recyclable) digital
medium.
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OLED is emerging as the new technology for thin panel displays. It can be used for mp3
players, cell phones, digital cameras or hand-held gaming devices. The field of
applications for OLED displays has a wide scale. According to a report of Maxim Group
revenues will rise from 600 million dollars in 2005 to more than five billion dollars by
2009. Other reports have shown that the total number of sold OLED units grew up to
over fifty percent in the past year. It is expected that this number will rise up to 80 or 90
percent in the following year. One of the future visions is to roll out OLEDs or to stick
them up like post-it notes. Another vision is the transparent windows which would
function like a regular window by day. At night it could be switched on and become a
light source. This could be possible because OLED allows transparent displays and light
sources. It will take considerably longer, of course, for OLED to keep its promise of
cheap manufacturing costs. The challenge is to compete against the industrial powers that
overwhelmingly support LCD and therefore achieve massive price advantages. Currently,
the wallpaper screen is nothing more than a vision, a clever one though. For future,
further improvement of Lifetime will be necessary while improving power efficiency. If adevice of longer Lifetime is realized, the foot of the application spreads out greatly. We
hope that the development discussed in this paper opens up a course to practical use of
OLED as lighting sources for illumination use, backlights and others.
REFERENCE
1. www.Whatis.com
2. www.Infopedia.com
3. www.Wikipedia.com4. www.Answers.com
5. www.Webopedia.com
6. www.About.com
7. www.Engiguide.com