ABSTRACT
In the Modem era where technology is at high state, need
for new machineries and instruments are a prerequisite. Demand for high
efficient measuring system and interactive displays make user-friendly
capabilities. In the entertainment section high precision imaging is needed for
efficient operation.
With advent of OLEDs, conventional LEDs and LCDs are
becoming history. High imaging techniques of OLEDs make the critical fields
such as defense and research more efficient in operation.
With this stage, the purpose of this seminar is to throw
light in to the capabilities of OLEDs and brief study of their technology.
INTRODUCTION
As thin display technology improves, TV sets will be
available in wider shapes and sizes.
Fold-up TVs and wearable TVs, similar to wearable PCs will be launched soon.
A plethora of display technologies catering to different
requirements of different applications are available. Electroluminescent(EL)
displays, vacuum fluorescent
displays (VFDs), and light emitting
diodes (LEDs) have a wide operating temperature range and luminescent
characteristics
OLEDs are lightweight, durable, power efficient and ideal
for portable applications. OLEDs have fewer process steps and also use both
fewer and lower- cost materials than LCD displays. Universal Display believes
that OLEDs can replace the current technology in many applications due to the following
performance advantages over LCD.
·
Greater brightness
·
Faster response time for full motion video
·
Fuller viewing angles
·
Lighter weight
·
Greater environmental durability
·
More power efficiency
·
Broader operating temperature ranges
·
Greater cost-effectiveness
CONSTRUCTION
A fundamental difference between small molecule and
polymeric device is the manner in which they are constructed.
Small molecule OLEDs are grown on a glass or plastic
substrate to form a multi-layer structure about 100 nm thick. The substrate is
first coated with a conducting transparent electrode such as indium tin oxide
(ITO) which serves as the anode. This is followed by a thin hole-transporting
organic layer HTL. Typically made from chemicals called diamines. An organic
light-emitting layer of comparable thickness is then deposited on the ETL surface.
A low work function is necessary to ensure efficient low-resistance injection
of electrons from the cathode on to the ETL.
Changing composition of the layers tunes the OLED
emission colors across the visible spectrum. Green emission can be achieved by doping
an electron-conducting organic matrix called Alq3, with either a
small amount of an iridium phosphor or fluorescent dyes. The pigment perylene
when doped in to an ETL known as CBP emits blue light. Lanthanide complexes and
porphyrin pigments have been used to efficiently emit red light when doped in
to Alq3 or CBP. As in small-molecule devices, changing the chemistry
of the polymer can tune the color of an OLED.
WORKING
Both polymeric & small molecule OLEDs operate by
accepting charge carriers of opposite polarities, electrons and holes, from the
cathode and anode contacts respectively. An externally applied voltage drives
these carriers into the recombination region where they form a neutral bound
state, or exciton. There are two types of excitons formed, called singlets and
triplets. On average one singlet and three triplets are formed for each four
electron hole pairs injected into the exciton formation region of the OLED.
Recombination of the singlet occurs within a few
nanoseconds of formation. This leads to a photon emission and is called
fluorescence. Recombination of the triplet exciton is slow (taking about 1 ms
to 1 second) and when it does occur usually results in heat rather than light.
But if a heavy — metal atom such as iridium or platinum is placed in an organic
molecule, the characteristics of singlet and triplet excitons mix speeding the
emission of light to within l00ns-l00ms. The kind of emission is
called phosphorescence.
Currently, efficiencies of the best doped polymer and
molecular OLEDs exceed that of incandescent light bulbs. Efficencies of 20
lumens per watt have been reported for yellow green—emitting polymer devices,
and 40lm/W attained for phosphorescent moleculer OLEDs, compared to less than
20 lm/W for a typical incandescent light bulb. Soon efficiencies of 80lm W a
value comparable to that of fluorescent room lighting will be achieved using
phosphorescent OLEDs.
Polymer OLED structures can be simpler than small-molecule
structures. The first polymer layer (in contact with ITO) can serve solely as a
hole-injecting/conducting layer, in some cases a single layer is used for
electron and hole injection, conduction and light emission. Polymer OLEDs also
often operate at lower power than small-molecule devices. Due to their high
conductivity, polymer- based devices have operating voltages in the 2-5V range,
which is l-2V lower than small molecule OLEDs.
The challenge to making full-color polymer-based displays
is very different from that for making such displays using small-molecule
OLEDs. Solution chemistry makes it difficult to deposit and pattern a polymer
pixel of one color, and then repeat the process using a second color emitter
because the solvents employed may dissolve or attack the devices already on the
substrate, Several schemes have been suggested to dodge this problem. One
particularly promising method involves depositing a single blue-emitting
polymer, and then selectively diffusing green and red dyes into adjecent
regions. However, it has proved difficult to keep the diffusing dyes from
bleeding into regions nearby. Seiko Epson Corp. of Nagano , Japan Cambridge England
FULL COLOUR DISPLAY
One the of the principal reason that OLED technology has
attracted such intense interest is its potential for use use in-full colour
displays that might eventually replace active matrix LCDs. A display consists
of a matrix of contacts made to the bottom and top surfaces of each organic
light — emitting element or pixel. To generate a full-colour image, it is
necessary to vary the relative intensities of three closely spaced,
independently addressed pixels, each emitting one of the three primary colours
of red, green or blue.
Optical filtering of white OLEDs can produce acceptable
red, green and blue emission as in the diagram. But this method sacrifices
efficiency due to the large amount of light absorbed in the filters
Less efficiency is lost by using a single blue or ultra
violet OLED to pump organic fluorescent wave length down—converters, also known
as colour—changing media (CCM) as in diagram. Each CCM filter consists of a
material that efficiently absorbs blue
light and re-emit the energy as either green or red light, depending on the compound used.
Organic thin films may lead to the practical realization
of low-cost very high-resolution, full-colour displays.
TECHNOLOGY
Universal Display Corporation’s OLED (Organic Light
Emitting Device) technology is focused on a number of key areas, including:
a) High Efficiency Materials
b) Transparent OLED (TOLED)
c) Flexible OLED(FOLED)
d) Passive and Active Matrices
e) Vertically Stacked, High
Resolution OLED (SOLED)
f) Organic Vapor Phase Deposition
(OVPD)
g) Organic Lasers
h) Patterning by Stamping
TRANSPARENT OLED(TOLED)
The Transparent OLED (TOLED) uses a proprietaly
transparent contact to create displays that can be made to be top-only
emitting, bottom-only emitting, or both top and bottom emitting (transparent).
TOLEDs can greatly improve contrast, making it much easier to view displays in
bright sunlight.
Because TOLEDs are 70 % transparent when turned off, they
may be integrated into car windshields, architectural windows, and eyewear.
Their transparency enables TOLEDs to be used with metal
foils, silicon wafers and other opaque substrates for top-emitting devices.
Directed Top Emission: TOLEDs also provided an excellent way
to achieve better fill factor and characteristics in high resolution, high —
information-content displays using active matrix silicon backplanes.
·
Transparency: TOLED displays can be
nearly as clear as the glass or substrate they’re built on. This feature paves
the way for TOLED to be built into applications that rely on maintaining vision
area. Today, “smart” windows are penetrating the multi-billion dollar flat
glass architectural and automotive marketplaces. Before long, TOLEDs may be
fabricated on windows for home entertainment and teleconferencing purposes, on
windshields and cockpits for navigation and warming systems and into
helmet—mounted or “head-up” systems for virtual reality applications.
·
Enhanced high—ambient
contrast:
TOLED technology offers enhanced Contrast ratio by using low—reflectance
absorber (a black backing) behind either top or bottom TOLED surface, contrast
ratio can be significantly improved over that in most reflective LCDs and
OLEDs. This feature is particularly important in daylight readable
applications, such as on cell phones and in military fighter aircraft cockpits.
·
Multi-stacked devices: TOLEDs are a fundamental
building block for many multi-structure (i.e. SOLEDs) and hybrid devices.
Bi-directional TOLEDs can provide two independent displays emitting form
opposite faces of the display. With portable products shrinking and desired
information content expanding, TOLEDs make it possible to get twice the display
area for the same display size.
FLEXIBLE OLED (FOLED)
FOLEDs are organic light emitting devices built on
flexible substrates. Flat panel displays have traditionally been fabricated on
glass substrates because of structural and processing constraints. Flexible materials
have significant performance advantages over traditional glass substrates.
FOLEDs Offer Revolutionary Features for Displays:
·
Flexibility: For the first time,
FOLEDs may be made on a wide variety of substrates that range form
optically-clear plastic films to reflective metal foils. These materials
provide the ability to conform, bend or roll a display into any shape. This
means that a FOLED display may be laminated onto a helmet face shield, a
military uniforms shirtsleeve, an aircraft cockpit instrument panel or an
automotive windshield.
·
Ultra—lightweight, thin
form: The
use of thin plastic substrates will also significantly reduce the weight of the
f1at panel displays in cell phones, portable computers and especially, large—
televisions—on—the—wall.
Durability: FOLEDs will also generally be less breakable, more impact resistant
and more durable compare to their glass-based counterpart.
PASSIVE AND ACTIVE MATRICES
How passive Matrix works?
Passive Matrix displays consist of an array of picture
elements, or pixels, deposited on a patterned substrate in a matrix of rows and
columns. In an OLED display, each pixel is an organic light emitting diode,
formed at the intersection of each column and row line. The first OLED displays
like the first LCD (Liquid Crystal Displays), are addressed as a passive
matrix. This means that to illuminate any particular pixel, electrical signals
are applied to the row line and column line (the intersection of which defines
the pixel). The more current pumped through each pixel diode, the brighter the
pixel looks to our eyes.
How Active Matrix works?
In an active matrix display, the array is still divided
into a series of row and column lines, with each pixel formed at intersection of
a row and column line. However, each pixel now consists of an organic light
emitting diode (OLED) in series with a thin film transistor (TFT). The TFT is a
switch that can control the amount of current flowing through the OLED.
In an active matrix OLED display (AMOLED). information is
sent to the transistor in each pixel, telling it how bright the pixel should
shine. The TFT then stores this information and continuously controls the
current flowing through the OLED. In this way the OLED is operating all the
time, avoiding the need for the very high current necessary iii a passive
matrix display.
Universal Display Corporation’s proprietary technologies
should enable extremely efficient active matrix OLEDs. Our new high efficiency
material systems are ideally suited for use in active matrix OLED displays, and
their high efficiencies should result in greatly reduced power consumption. The
TOLED architecture enables the organic diode, which is placed in each pixel to
emit its light upwards away from the substrate. This means that the diode can
be placed over the TFT back plane, resulting in a brighter display.
UDC is collaborating with other organizations to develop
TFT technology compatible with plastic substrates, in order to commercialize
highly efficient bright active matrix flexible OLEDs.
VERTICALLY STALKED, HIGH RESOLUTION
OLED (SOLED)
The stacked OLED (SOLED) Universal Display Corporation’s
award- winning, novel pixel architecture that is based on stacking the red,
green, and blue subpixels on top of one another instead of next to one another
as is commonly done in CRTs and LCDs. This improve display resolution up to
three-fold and enhances full-color quality. SOLEDs my provide the high
resolution needed for wireless worldwide—web applications.
What is a SOLED?
SOLED display consists of an array vertically-stacked
TOLED sub- pixels. To separately tune color and brightness, each of the red,
green and blue (R- GB) sub—pixel elements is individually controlled. By
adjusting the ratio of current in the three elements, color is tuned. By
varying the total current through the stack,brightness is varied. By modulating
the pulse width, gray scale is achieved. With this SOLED architecture, each
pixel can, in principle, provide full color. Universal Display Corporation’s
SOLED technology may be the first demonstration of an vertically integrated structure,
where intensity, colour and gray scale can be independently tuned to achieve
high-resolution full-colour.
Scalable to large pixel size: In large screen displays,
individual pixels are frequently large enough to be seen by the eye at short
range. With the S x S format, the eye may perceive the individual red, green
and blue instead of the intended colour mixture. With a SOLED, each pixel emits
the desire colour and, thus, is perceived correctly no matter what size it is
and from where it is viewed.
In addition to being a transparent light emitter, the top
indium-tin-oxide surface of the TOLED can serve as the hole-injecting electrode
for a second TOLED built on top of the first device as shown
Each device in the stack is then independently
addressable and can be tailored to emit its own colour through the adjacent
transparent organic layers, the transparent contacts, and the glass substrate.
This allows the entire area of the vertically stacked pixel to emit any mixture
of the three primary colours.
SOLED architecture is a significant departure from the
traditional side by side (SxS) approach used in CRTs and LCDs today compared to
SxS configuration, SOLEDs offer compelling performance enhancements:
·
Full colour tunability: SOLEDs offer full—colour
tunability for ‘‘true’’ colour quality at each pixel—valuable when colour
fidelity k important.
·
High resolution: SOLEDs also offer 3x
higher resolution than the comparable SxS display. While it takes 3 SxS pixels
(an R,G,B)lo generate full colour display, it takes only one SOLED pixel or
one-third the area to achieve the same. This is especially advantageous when
maximizing pixel density is important.
·
Nearly 100% fill factor: SOLEDs also maximize fill
factor. For example. when a full colour calls for green, the red &blue
pixels are turned off in the SxS structure. By comparison all the pixels turn
on green in a SOLED under the same conditions. This means that SOLED color
definition and picture quality are superior.
·
Scalable to large pixel
size: In
large screen displays, individual pixels are frequently large enough to be seen
by the eye at short range. With the S x S format, the eye may perceive the
individual red, green and blue instead of the intended colour mixture. With a
SOLED, each pixel emits the desire colour and, thus, is perceived correctly no
matter what size it is and from where it is viewed.
ORGANIC VAPOR PHASE DEPOSITION
(OVPD)
Universal Display Corporation and its research partners
at Princeton University have developed a transformational technology which can
reduce the cost and increase the efficiency of the OLED production process.
The technology, Organic Vapor Phase Deposition (OVPD),
can enable a low cost, precise, high throughput process for fabricating OLEDs.
The OVPD production process utilizes a carrier gas stream
in a hot walled reactor at very low pressure to precisely deposit the thin
layers of organic materials used in OLED displays. Conventional OLED
fabrication equipment evaporates the organic molecules at high temperature and
pressure. OVPD offers the ability to precisely control the multi-source
deposition required for full-color OLED displays. The OVPD design should also
be adaptable to the rapid, uniform deposition of organics on large-area
substrates and for roll-to-roll processing.
Universal Display is developing the production equipment
in partnership with AIXTRON AG of Aachen ,
Germany
ORGANIC LASERS
Organic lasers, based on UDC's pioneering work with
Princeton University, have the potential to revolutionize yet another industry.
An organic laser is a solid-state device based on organic
materials and structures similar to those used in UDC's display technologies.
An optically-pumped organic laser demonstrates five key laser characteristics:
spatial coherence, a clear threshold, strongly polarized light emission,
spectral line narrowing, and the existence of laser cavity modes. To realize
commercial potential, the key technical challenge today is to demonstrate a
mechanism for the electrical pumping of these lasers.
The use of small-molecule organic materials opens the
door to an entirely new class of light emitters for diode lasers. These organic
lasers may offer:
·
Greater color variety
·
Tunability
·
Further miniaturization
·
Easier processing
·
Lower cost in a host of end uses
Potential applications include optical memories (e.g.,
compact discs and digital versatile discs (DVDs), CD-ROMs, optical scanners,
sensors, and laser printers.
PATTERNING BY STAMPING
Universal Display Corporation and its research partners,
Princeton University and University of Southern California, have developed a
novel process for patterning electrodes in OLEDs which shows promise of making
them more efficient and less expensive to manufacture.
This invention is based on a cold welding process to
pattern electrodes to sizes as small as 12 microns. UDC has been granted the
exclusive worldwide license for these and other associated technologies.
This technology is potentially cost-effective, offers
high throughput and is well-suited for large-area and roll-to-roll fabrication.
It is a valuable addition to UDC's portfolio of innovative OLED technologies
for this emerging industry.
A MAJOR BREAKTHROUGH IN OLEDS
Chi Mei Optoelectronics Corporation (CMO), Taiwan
OLEDs have long been heralded as the display technology
of the future, but have so far failed to compete with conventional technologies
beyond displays of small size and law information content, such as car radio
and cell phone displays that are available in the market. A major limitation
has been the expensive polycrystalline silicon which drives light emission in
the organic layers.
The revolutionary prototype by CMO and IBM offers
amorphous silicon as a perfect alternative. Amorphous silicon is an unordered
material structure, which can, unlike polycrystalline silicon, be implemented
easily and cost-effectively over large areas. The use of existing ‘ITT- LCD
manufacturing prpcess and facility for commercial production of OLEDs is
another milestone achieved with the tech nology.
The advanced amorphous silicon cir cuitry, together with
superior performance characteristics of the light-emitting layers themselves as
well as the overall device architecture, results in a display that is
comparable to a high-end LCD display of the same size and resolution, with half
the power consumption at typical desktop-dis play brightness, better colour
saturation, and larger viewing angle. The display has WXGA resolution (1280 x
768) and bright ness of 300 cd/rn and consumes 25W power. The full video
capability extends its usab to large flat-panel TVs. 0
ADVANTAGES OF OLEDs
·
Very slim flat panel
·
Low power consumption
·
High brightness
·
Wide visibility
·
Quick response time
·
Wider viewing angle
·
Self luminous
·
No environmental draw backs
·
No power intake when turned off.
DISADVANTAGES OF OLEDs
·
Vulnerable to shorts due to contamination of substrate
surface by dust particles.
·
Voltage drops
·
Mechanically fragile
·
Potential not yet realized
APPLICATION OF OLEDs
·
Car display
·
Cellular phone
·
Mobile computer
·
Audio visual device
·
Household machine
CONCLUSION
Organic materials are poised as never before to transform
the world of circuit and display technology. Major electronics firms such as
Philips and Pioneer, and smaller companies such as Cambridge Display
Technology, Universal Display and Uniax, are betting that the future holds
tremendous opportunity for the low cost and some times surprisingly high
performance offered by organic electronic and opto electronic devices. Using
organic light-emitting devices (OLEDs), organic full colour displays may
eventually replace liquid—crystal displays (LCDs) for use with laptop and eve
desktop computers. Such displays c deposited on flexible plastic coils, eliminating
the fragile and heavy glass substrates used in LCDs, and can emit bright light
without the pronounced directionality inherent in LCD viewing, all with
efficiencies higher than can be obtained with in incandescent light bulbs.
Organic electronics are already entering commercial
world. Multi colour automobile stereo displays are now available from Pioneer
Corp., of Tokyo and Royal Philips Electronics ,Amsterdam is gearing up to
produce born OLED back lights to be used in LCDs and organic integrated
circuits.
The first products using organic displays are already
being introduced into the market place. And while it is always difficult to
predict when and what future products will be introduced, many manufactures are
now working to introduce cell phones and 1 digital assistants with OLED
displays with in the next one or two years. The ultimate goal of using high
efficiency, phosphorescent flexible OLED displace in lap top computers and even
for home video applications may be no more than a few years in to the future.
The portable and light weight OLED displays will soon
cover our walls replacing the bulky and power hungry cathode ray tube.
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