20-01-2010, 01:48 PM
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ABSTRACT
This paper deals with the study of OLED, its structural
details. OLEDs are light weight durable power efficient, and ideal for
portable applications. OLEDs have fewer process steps and also use both
fewer and low cost materials than LCD display. A feed back circuit for
OLED based display is proposed and demonstrated. A demonstration system
is built proving the feasibility of a flat panel display using direct
optical feed back. The demonstration system consists of a 5 x 5 array
of OLEDs, pixels circuitry and feed back loop. The system shares a
single feedback loop among a number of pixels saving power and real
estate.
1. INTRODUCTION
Organic light emitting diodes (OLEDs) are optoelectronic
devices based on small molecules or polymers that emit light when an
electric current flows through them. simple OLED consists of a
fluorescent organic layer sandwiched between two metal electrodes.Under
application of an electric field, electrons and holes are injected from
the two electrodes into the organic layer, where they meet and
recombine to produce light. They have been developed for applications
in flat panel displays that provide visual imagery that is easy to
read, vibrant in colors and less consuming of power.
OLEDs are light weight, durable, power efficient and ideal for
portable applications. OLEDs have fewer process steps and also use both
fewer and low-cost materials than LCD displays. OLEDs can replace the
current technology in many applications due to following performance
advantages over LCDs.
¢ 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-effectivenes
LIMITATIONS OF LCD- EVOLUTION OF OLED
Most of the limitations of LCD technology come from the fact that LCD
is a non-emissive Display device. This means that they do not emit
light on their own. Thus, an LCD Operates on the basis of either
passing or blocking light that is produced by an external light Source
(usually from a backside lighting system or reflecting ambient light).
Applying an electric field across an LCD cell controls its transparency
or reflectivity. A cell blocking (absorbing) light will thus be seen as
black and a cell passing (reflecting) light will be seen as white. For
a color displays, there are color filters added in front of each of the
cells and a single pixel is represented by three cells, each
responsible for the basic colors: red, green and blue.
The basic physical structure of a LCD cell is shown in
Figure.The liquid crystal (LC) material is sandwiched between two
polarizers and two glass plates (or between one glass plate and one
Thin Film Transistor (TFT) layers). The polarizers are integral to the
working of the cell. Note that the LC material is inherently a
transparent material, but it has a property where its optical axis can
be rotated by applying an electric field across the material. When the
LC material optical axis is made to align with the two polarizersâ„¢
axis, light will pass through the second polarizer. On the other hand,
if the optical axis is rotated 90 degrees, light will be polarized by
the first polarizer, rotated by the LC material and blocked by the
second polarizer.
Note that the polarizers and the LC material absorb light. On a
typical monochrome LCD display the polarizers alone absorb 50% of the
incident light. On an active matrix display TFT layer, the light
throughput may be as low as 5% of the incident light. Such low light
output efficiency requires with a LC based displays to have a powerful
backside or ambient light illumination to achieve sufficient
brightness. This causes LCDâ„¢s to be bulky and power hungry.The LC cells
are in fact relatively thin and their operation relatively power
efficient. It is the backside light that takes up most space as well as
power. In fact with the advent of low power microprocessors, the LCD
module is the primary cause of short battery life in notebook
computers.
Moreover, the optical properties of the LC material and the
polarizer also causes what is known as the viewing angle effect. The
effect is such that when a user is not directly in front of the
display, the image can disappear or sometime seem to invert (dark
images become light and light images become dark).
With these disadvantages of a LC based display in mind, there
has been a lot of research to find an alternative. In recent years, a
large effort has been concentrated on Organic Light Emitting Device
(OLED) based displays. OLED-based displays have the potential of being
lighter, thinner, brighter and much more power-efficient than LC based
displays. Moreover, OLED-based displays do not suffer from the viewing
angle effect. Organic Optoelectronics has been an active field of
research for nearly two decades. In this time device structures and
materials have been optimized, yielding a robust technology. In fact,
OLEDs have already been incorporated into several consumer electronic
products. However, there are basic properties of organic molecules,
especially their instability in air, that hamper the commercialization
of the technology for high quality displays.
ORGANIC LED AND LIQUID CRYSTAL DISPLAY COMPARISON
An organic LED panel Liquid crystal Panel
A luminous form Self emission of light Back light or outside light is
necessary
Consumption of Electric power It is lowered to about mW though it is
a little higher than the reflection type liquid crystal panel It is
abundant when back light is used
Colour Indication form The flourscent material of RGB is arranged in
order and or a colour filter is used. A colour filter is used.
High brightness 100 cd/m2 6 cd/m2
The dimension of the panel Several-inches type in the future to
about 10-inch type.Goal It is produced to 28-inch type in the future to
30-inch type.Goal
Contrast 100:14 6:1
The thickness of the panel It is thin with a little over 1mm
When back light is used it is thick with 5mm.
The mass of panel It becomes light weight more than 1gm more than
the liquid crystal panel in the case of one for portable telephone
With the one for the portable telephone.10 gm weak degree.
Answer time Several us Several ns
A wide use of temperature range 86 *C ~ -40 *C ~ -10 *C
The corner of the view Horizontal 180 * Horizontal 120* ~ 170*
ORGANIC LED STRUCTURE AND OPERATION
An Organic LED is a light emitting device whose p-n
junction is made from an organic compound such as: Alq3 (Aluminum tris
(8-hydroxyquinoline)) and diamine (TPD). A typical structure of an OLED
cell and the molecular structure of some typical organic materials used
are shown in Figure
Fig. 2 Typical structure of an Organic LED and the Molecular Structure
of Alq3 & TPd
For an Organic LED, the organic layer corresponding to the
p-type material is called the hole-transport layer (HTL) and similarly
the layer corresponding to the n-type material is called the electron-
transport layer (ETL). In Figure 2, Alq3 is the ETL and TPD is the HTL.
Similar to doped silicon, when ETL and HTL materials are placed to
create a junction, the energy bands equilibrates to maintain continuity
across the structure. When a potential difference is applied across the
structure, a drift current flows through the structure. The injected
carries recombination at the junction consists of both thermal and
optical recombination, which emits photons.
Figure 3 shows the optical recombination from the energy
band perspective. Note that LUMO is a short form for Lowest Unoccupied
Molecular Orbital, which corresponds to the conduction band in the
energy diagram of doped silicon, and HOMO is a short form for Highest
Occupied Molecular Orbital, which corresponds to the valence band in
the energy diagram of doped silicon.
Since an OLED emits light through a recombination
process, it does not suffer from the viewing angle limitation like an
LC based device. Note that for any device to become a viable candidate
for use in flat panel displays it has to be able to demonstrate high
brightness, good power efficiency, good color saturation and sufficient
lifetime. Reasonable lower limits specifications for any candidate
device should include the following: brightness of ~ 100cd/m 2 ,
operating voltage of 5-15V and a continuous lifetime of at least
10,000h.
OLEDs with brightness of up to 140,000 cd/m 2 , power
efficiencies of up to
40 lm/W , and low operating voltages from 3-10V have been reported.
Saturated-color OLEDs have been demonstrated, spanning almost the
entire visible spectrum. Moreover, the thickness of an OLED structure,
which typically is less than a micrometer, allows for mechanical
flexibility, leading to the development of bendable displays
indicating the potential development of rolled or foldable displays.
Furthermore, the recent development of vapor phase deposition
techniques for the OLED manufacturing process may well result in low-
cost large-scale production of OLED based flat panel displays as
opposed to LC based displays that require extra processes such as layer
alignment and tilt angle adjustment.
OLED lifetime exceeding 50,000 h [7] has been
reported. Note however, this lifetime number applies to any singular
OLED structure. The number does not capture the fact that each. OLED
pixelâ„¢s electrical characteristics in a display consisting of array of
pixels may vary differently than the characteristic of its neighboring
pixel. Although all the pixel in the array may have upto 6yrs lifetime
display consisting of pixels with differing characteristics will lose
its brightness and pixel to pixel accuracy if no adjustments are made
to compensate for this variation. OLED-based displays are not so
popular among consumer mobile computing device as LC based displays.
There are challenges in OLED based flat panel display design which are
not found in LC based design. OLED pixel in an array may not have
uniform electrical characteristic since OLED are organic devices whose
electrical properties are easily effected by the environment and its
pattern of usage. In OLED power efficiency degrades with time and use.
All pixel have different identical pattern of usage. This causes each
pixel to have different colors.
I-V characteristic variation
The I-V characteristics of OLED is also varying with time.
Several factors contribute to the I-V characteristic variation. The
first and foremost is temperature. As shown in Figure , the I-V
characteristic depends quite strongly on the operating temperature. The
I-V characteristic variation pose a challenge to the control of OLED
based displays as the I-V operating points have to be shifted depending
on the operating temperature. Besides temperature, the I-V
characteristic also depends strongly on the type of anode/cathode used
in the device as well as the thickness of the organic active Electro
Luminescence (EL) layer. In particular Figure shows the I-V
characteristic variation with the thickness of the organic layer.
Direct Optical Feedback
The electrical feedback signal, which will
represent the light output intensity level, is then used to control the
driving signal so that the output optical power consistently represents
the input reference signal. Figure shows the block diagram for this
idea. The idea has the potential to succeed since the sensor can be
designed to have a much more reliable and consistent characteristic
compared to the OLED.
The goal of the thesis is to create a working 5x5
pixels OLED display, which maintains uniform grayscale reliability
despite the varying characteristics of the individual pixels. The final
demonstration system includes the 5x5 pixels OLED based displays
together with the addressing, the feedback and the driving circuitry
implemented using discrete components.
A Feedback Loop Shared by a Column of Pixels
There are several considerations to be made for the
feedback loop implementation. Since the demonstration system is geared
to building a model for the later integrated implementation, there are
many more aspects to be considered. Ideally the discrete demonstration
implementation should: use the simplest circuits possible, use as small
number of devices as possible, be low power so that the power
efficiency potential of OLED-based displays can be achieved and
scalable to a much larger number of pixels.
The simplest implementation of the feedback loop of the display system
will be to have a loop for every single pixel. However, this is
expensive in term of the number of components, which translates to
space and complexity if the design is used in an integrated version.
Moreover, a continuous running feedback loop around every pixel will
also tend to be expensive in terms of Power since the feedback
circuitry is also consuming power.
On the other hand, a display design based on a single feedback
loop per pixel can be expanded easily to large number of pixels, as
every pixel and its control loop is then simply an exact copy of
another. Moreover, in the integrated implementation, the light sensor
as shown in Figure will be implemented using a simple silicon p-n
junction. The close spatial proximity of the sensor to the feedback
loop will make the sensing more accurate. As a result each pixel will
have less error and more consistent output.
Another possibility is to have a small number of feedback
loops, each reusable by a group of pixels using some addressing
mechanism. This alternative has the potential of being lower in Power
consumption and in the number of devices.
However, with this scheme there are extra requirements on the
feedback loop since each of the pixels only has access to the feedback
loop for a limited of time within each cycle. In other word, the
feedback loop must have a faster step response (larger bandwidth).
Furthermore, the Pixel design also has to include a relatively accurate
sample and hold circuit so that it can reliably store a driving signal
set by the shared feedback loop and maintain it through out a full
cycle of refresh time. The basic schematic for this shared feedback
loop is shown in Figure.
In this thesis project, a single feedback loop shared by a
single column of pixels is chosen as the method to drive the display
because a single feedback loop per pixel turns out to be prohibitively
expensive in terms of real estate and pixel complexity. Moreover, the
driving circuitry in the feedback loop can use the conventional display
driver circuitry since a loop per-column topology means that the
display is refreshed in a row by row fashion similar to the active
matrix topology in the commercially available LC based display. This
also means that the same buffering and data format used in any active
matrix display can be used to drive the proposed OLED based display. In
the demonstration system, a single feedback loop for each column of 5
Pixel is built, together with the sample and hold as well as the
addressing circuitry.
5x5 Pixels Demonstration System
Figure shows the overall system block diagram for the
demonstration system. The system can be generally divided into two
large parts: analog and digital. The analog part is responsible mainly
for the pixel circuitry, which includes the sample and hold (S/H), as
well as the feedback loop and its compensation network. The digital
part is responsible for the sensing (the CMOS camera in this case) and
the control circuitry (implemented using Complex Programmable Logic
Devices - CPLD).
MODERN TECHNOLOGIES IN OLEDs
OLEDS(Organic Light Emitting Device ) technology is focused on
a number of key areas, including:
¢ High Efficiency Materials
¢ Transparent OLED (TOLED)
¢ Flexible OLED (FOLED)
¢ Passive and Active Matrices
¢ Vertically Stacked, High Resolution OLED (SOLED)
¢ Organic Vapor Phase Deposition (OVPD)
¢ Stamping
¢ Organic Lasers
HIGH EFFICIENCY MATERIALS
These materials emit light through the process of
electrophosphorescence. In traditional OLEDs, the light emission is
based on fluorescence, a transition from a singlet excited state of a
material. According to theoretical and experimental estimation, the
upper limit of efficiency of an OLED doped with fluorescent material,
is approximately 25%.
With our electro phosphorescent materials used as a dopant,
which exploits both singlet and triplet excited states, this upper
limit is virtually eliminated. Equipped with the potential of 100%
efficiency, we are working towards the commercialization of electro
phosphorescent devices by optimizing the device efficiency, color
purity and device storage and operation durabilities.
Such a process is facilitated by the development and
modification of charge transport materials, charge blocking materials
and luminescent materials, and their incorporation into devices. In
addition to the fabrication of high quality devices, UDC is also
committed to a high standard of device testing. Our scientists and
engineers have custom developed sophisticated test hardware and
software for this purpose.
TOLED
The Transparent OLED (TOLED) uses a proprietary transparent
contact to create displays that can be made to be top-only emitting,
bottom-only emitting, or both top and bottom emitting (transparent).
TOLEDs can greatly improve contrast, making it much easier to view
displays in bright sunlight. Because TOLEDs are 70% transparent when
turned off, they may be integrated into car windshields, architectural
windows, and eyewear. Their transparency enables TOLEDs to be used with
metal, foils, silicon wafers and other opaque substrates for top-
emitting devices.
TOLED Creates New Display Opportunities:
¢ Directed top emission: Because TOLEDs have a transparent
structure, they may be built on opaque surfaces to effect top emission.
Simple TOLED displays have the potential to be directly integrated with
future dynamic credit cards. TOLED displays may also be built on metal,
e.g., automotive components. Top emitting TOLEDs also provide an
excellent way to achieve better fill factor and characteristics in high
resolution, high-information-content displays using active matrix
silicon backplanes.
¢ Transparency: TOLED displays can be nearly as clear as the
glass or substrate they're built on. This feature paves the way for
TOLEDs to be built into applications that rely on maintaining vision
area. Today, "smart" windows are penetrating the multi-billion dollar
flat glass architectural and automotive marketplaces. Before long,
TOLEDs may be fabricated on windows for home entertainment and
teleconferencing purposes; on windshields and cockpits for navigation
and warning systems; and into helmet-mounted or "head-up" systems for
virtual reality applications.
¢ Enhanced high-ambient contrast: TOLED technology offers
enhanced contrast ratio. By using a low-reflectance absorber (a black
backing) behind either top or bottom TOLED surface, contrast ratio can
be significantly improved over that in most reflective LCDs and OLEDs.
This feature is particularly important in daylight readable
applications, such as on cell phones and in military fighter aircraft
cockpits.
¢ Multi-stacked devices: TOLEDs are a fundamental building block
for many multi-structure (i.e. Solids) and hybrid devices. Bi-
directional TOLEDs can provide two independent displays emitting from
opposite faces of the display. With portable products shrinking and
desired information content expanding, TOLEDs make it possible to get
twice the display area for the same display size.
FOLED
FOLEDs are organic light emitting devices built on flexible
substrates. Flat panel displays have traditionally been fabricated on
glass substrates because of structural and/or processing constraints.
Flexible materials have significant performance advantages over
traditional glass substrates.
FOLEDs Offer Revolutionary Features for Displays:
¢ Flexibility: For the first time, FOLEDs may be made on a wide
variety of substrates that range from optically-clear plastic films to
reflective metal foils. These materials provide the ability to conform,
bend or roll a display into any shape. This means that a FOLED display
may be laminated onto a helmet face shield, a military uniform
shirtsleeve, an aircraft cockpit instrument panel or an automotive
windshield.
¢ Ultra-lightweight, thin form: The use of thin plastic
substrates will also significantly reduce the weight of flat panel
displays in cell phones, portable computers and, especially, large-area
televisions-on-the-wall. For example, the weight of a display in a
laptop may be significantly reduced by using FOLED technology.
¢ Durability: FOLEDs will also generally be less breakable, more
impact resistant and more durable compared to their glass-based
counterpart.
¢ Cost-effective processing: OLEDs are projected to have full-
production level cost advantage over most flat panel displays. With the
advent of FOLED technology, the prospect of roll-to-roll processing is
created. To this end, our research partners have demonstrated a
continuous organic vapor phase deposition (OVPD) process for large-area
roll-to-roll OLED processing. While continuous web FOLED processing
requires further development, this process may provide the basis for
very low-cost, mass production
PASSIVE AND ACTIVE MATRIX
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 the
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 currents necessary in
a passive matrix display.
The 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 backplane, resulting in a brighter display.
VERTICALLY STACKED HIGH RESOLUTION OLED (SOLED)
The Stacked OLED (SOLED) uses 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 improves display
resolution up to three-fold and enhances full-color quality. SOLEDs may
provide the high resolution needed for wireless worldwide-web
applications.
SOLED DISPLAY
A SOLED display consists of an array of vertically-stacked
TOLED sub-pixels. To separately tune color and brightness, each of the
red, green and blue (R-G-B) sub-pixel elements is individually
controlled. By adjusting the ratio of currents 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, color and gray scale can be independently tuned to
achieve high-resolution full-color.
Performance Enhancements
The SOLED architecture is a significant departure from the
traditional side-by-side (SxS) approach used in CRTs and LCDs today.
Compared to SxS configurations, SOLEDs offer compelling performance
enhancements
¢ Full-color tunability: SOLEDs offer dynamic full-color
tunability for "true" color quality at each pixel -- valuable when
color fidelity is important.
¢ High resolution: SOLEDs also offer 3X higher resolution than
the comparable SxS display. While it takes three SxS pixels (an R, G
and B) to generate full-color, 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-color display calls for green, the red and 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 SxS format, the eye may perceive the individual
red, green and blue instead of the intended color mixture. With a
SOLED, each pixel emits the desired color and, thus, is perceived
correctly no matter what size it is and from where it is viewed.
ORGANIC VAPOUR PHASE DEPOSITION (OVPD)
OVPD Research System Schematic
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.
The technology, Organic Vapour Phase Deposition can enable low cost,
precise, high throughput process for fabricating OLEDs.
ORGANIC LASERS
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.
PRODUCT CONCEPTS
Universal Display Corporation has only begun to imagine what
our OLED technology can create in the way of products for our world and
our future. The technology has the potential to not only improve
existing products, but also to create exciting, new product
possibilities, for example:
¢ Low-power, bright, colorful cell phones
¢ Full color, high-resolution, personal communicators
¢ Wrist-mounted, featherweight, rugged PDAs
¢ Wearable, form-fitting, electronic displays
¢ Full-color, high resolution, portable Internet devices and palm
size computers
¢ High-contrast automotive instrument and windshield displays
¢ Heads-up instrumentation for aircraft and automobiles
¢ Automobile light systems without bulbs
¢ Flexible, lightweight, thin, durable, and highly efficient
laptop screens
¢ Roll-up, electronic, daily-refreshable newspaper
¢ Ultra-lightweight, wall-size television monitor
¢ Office windows, walls and partitions that double as computer
screens
¢ Color-changing lighting panels and light walls for home and
office
¢ Low-cost organic lasers
¢ Computer-controlled, electronic shelf pricing for supermarkets
and retail stores
¢ Smart goggles/helmets for scuba divers, motorcycle riders
¢ Medical test equipment
¢ Wide area, full-motion video camcorders
¢ Global positioning systems (GPS)
¢ Integrated computer displaying eyewear
¢ Rugged military portable communication devices
ADVANTAGES, APPLICATIONS AND DRAWBACKS OF OLED
ADVANTAGES:
The important advantages are
¢ Very thin panel.
¢ Low power consumption.
¢ High brightness/ High contrast.
¢ Wide visibility.
¢ Quick response time.
¢ Viewer order wide angle.
¢ Self luminous.
¢ Thinner than LCD.
¢ No environmental drawbacks.
¢ No power intake when turned off.
APPLICATIONS:
¢ Car display
¢ Car navigation
¢ Display panel
¢ Cellular phone.
¢ Mobile computer.
¢ Audio visual device
o Digital camera
o Digital VTR
¢ House hold machine.
o Game machine
o Portable
Light sources made from organic materials are of immense
potential value for a range of applications. Large area, flat light
sources with surface brightness have potential applications such as
space lighting, back lighting or advertising displays. Organic light
emitting devices(OLEDs) offer the potential for such a source. OLEDs
promise a cheap, light weight source which potentially can be made any
size and on to a range of substrates(including flexible plastic).
DRAWBACKS:
Despite outstanding properties of organic materials regarding
usage in display technologies, their potential is by far not realized
yet. Still present disadvantages of state of the art organic LED make
competition with established principles difficult. Low driving voltages
below 5v are needed to be compatible with typical integrated
electronics used for passive addressed matrix displays.
Unwanted voltage drops are partially due to the low
conductivity of organic materials and interface barriers typically
encountered in organic devices. Surprisingly enough, the doping
concepts fundamental for the triumph of classical semiconductors have
not been employed for organic devices.
EFFICIENCY OF OLED
Recent advantages in boosting the efficiency of OLED light
emission have led to the possibility that OLEDs will find early uses in
many battery-powered electronic appliances such as cell phones, game
boys and personal digital assistants. Typical external quantum
efficiencies of OLEDs made using a single fluorescent material that
both conducts electrons and radiates photons are greater than 1
percent. But by using guest-host organic material systems where the
radiative guest fluorescent or phosphorescent dye molecule is doped at
low concentration into a conducting molecular host thin film, the
efficiency can be substantially increased to 10 percent or higher for
phosphorescence or up to approximately 3 percent for fluorescence.
Currently, efficiencies of the best doped OLEDs exceed that of
incandescent light bulbs. Efficiencies of 20 lumens per watt have been
reported for yellow-green-emitting polymer devices and 40 lm/W for a
typical incandescent light bulb. It is reasonable to that of
fluorescent room lighting will be achieved by using phosphorescent
OLEDs.
The green device which shows highest efficiency is based on
factris(2-phenylpyridine) iridium[Ir(PPY)3],a green electro
phosphorescent material. Thus phosphorescent emission originates from a
long-lived triplet state.
THE ORGANIC FUTURE
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
manufacturers are now working to introduce cell phones and personal
digital assistants with OLED displays within the next one or two years.
The ultimate goal of using high-efficiency, phosphorescenct, flexible
OLED displays in lap top computers and even for home video applications
may be no more than a few years into future.
However, there remains much to be done if organics are to
establish a foothold in the display market. Achieving higher
efficiencies, lower operating voltages, and lower device life times are
all challenges still to be met. But, given the aggressive world wide
efforts in this area, emissive organic thin films have an excellent
chance of becoming the technology of choice for the next generation of
high-resolution, high-efficiency flat panel displays.
In addition to displays, there are many other opportunities for
application of organic thin-film semiconductors, but to date these have
remained largely untapped. Recent results in organic electronic
technology that may soon find commercial outlets in display black
planes and other low-cost electronics.
CONCLUSION
Performance of organic LEDs depend upon many parameters such as
electron and hole mobility, magnitude of applied field, nature of hole
and electron transport layers and excited life-times. Organic materials
are poised as never before to transform the world IF circuit and
display technology. Major electronics firms are betting that the future
holds tremendous opportunity for the low cost and sometimes
surprisingly high performance offered by organic electronic and
optoelectronic devices.
Organic Light Emitting Diodes are evolving as the next
generation of light sources. Presently researchers have been gong on to
develop a 1.5 emitting device. This wavelength is of special interest
for telecommunications as it is the low-loss wavelength for optical
fibre communications. Organic full-colour displays may eventually
replace liquid crystal displays for use with lap top and even desktop
computers. Researches are going on this subject and it is sure that
OLED will emerge as future solid state light source.
REFERENCE
1. Electronics for You ; Volume 35, No: 5, May 2003
2. universaldisplay.com
3. edtn.com
4. emagin.com
5. pearsonptg.com
CONTENTS
1. INTRODUCTION 1
2. LIMITATIONS OF LCD- EVOLUTION OF OLED 2
3. ORGANIC LED AND LCD COMPARISON 5
4. ORGANIC LED STRUCTURE AND OPERATION 6
5. MODERN TECHNOLOGIES IN OLEDS 14
6. ADVANTAGES, APPLICATIONS AND
DRAWBACKS OF OLED 25
7. THE ORGANIC FUTURE 28
8. CONCLUSION
29
9. REFERENCE
30
ACKNOWLEDGEMENT
I express my sincere gratitude to Dr.Nambissan, Prof. & Head,
Department of Electrical and Electronics Engineering, MES College of
Engineering, Kuttippuram, for his cooperation and encouragement.
I would also like to thank my seminar guide Asst. Prof. Mrs.
Sobha.M. (Department of EEE), Asst. Prof. Gylson Thomas. (Staff in-
charge, Department of EEE) for their invaluable advice and wholehearted
cooperation without which this seminar would not have seen the light of
day.
Gracious gratitude to all the faculty of the department of EEE
& friends for their valuable advice and encouragement.
