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Abstract and Figures

An organic light-emitting diode (OLED) is a light-emitting diode (LED), in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. OLED’s are used to create digital displays in devices such as television screens, computer, portable systems such as mobile phones, handheld game consoles and PDAs. A major area of research is the development of white OLED devices for use in solid-state lighting applications. OLED display devices use organic carbon-based films, sandwiched together between two charged electrodes. One is a metallic cathode and the other a transparent anode, which is usually glass. OLED displays can use either passive-matrix (PMOLED) or active-matrix (AMOLED) addressing schemes. Active-matrix OLEDs (AMOLED) require a thin-film transistor backplane to switch each individual pixel on or off, but allow for higher resolution and larger display sizes. An OLED display works without a backlight; thus, it can display deep black levels and can be thinner and lighter than a liquid crystal display (LCD). In low ambient light conditions (such as a dark room), an OLED screen can achieve a higher contrast ratio than an LCD, regardless of whether the LCD uses cold cathode fluorescent lamps or an LED backlight.
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1
Optical Test and Measurements (OME)
Prof. Dr.-Ing. G. Wenke
Report On
Organic Light Emitting Diode (OLED)
Due Date: 31/05/2016
Submitted By:
Name
Matriculation No
Signature
Rishabh Chikker
5006237
Navaneetha C M
5006274
Onur Can
5006235
Bartyr Barakov
5006234
2
Contents
1. Introduction …………………………………………………. 4
2. Construction …………………………………………………. 4
3. Emission Spectrum ………………………………………………….. 9
4. OLED Architectures ………………………………………………….10
5. Manufacturing OLED ………………………………………………… 11
6. Fabrication Methods ………………………………………………… 11
6.1 Physical Vapor Deposition ………………………………………………… 12
6.2 Spin Coating ………………………………………………… 13
6.3 Inkjet Printing ………………………………………………… 14
6.4 Roll to roll Printing ………………………………………………… 15
6.5 Vacuum Sputtering ………………………………………………… 15
7. Technical Characteristics …………………………………………………16
8. Types of OLED …………………………………………………17
9. Comparison Between LED and OLED …………………………………………...19
10. Advantages of OLED ………………………………………………….21
11. Disadvantages ………………………………………………….22
12. Application of OLED …………………………………………………23
13. Challenges ………………………………………………….24
14. Future Possibilities ………………………………………………….24
15. Conclusion ………………………………………………… 25
16. References ………………………………………………… 25
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List of Figures
Figure 1: Basic OLED Structure …………………………………………......... 4
Figure 2: Basic OLED Working ……………………………………………….. 6
Figure 3: Energy Level Diagram ……………………………………………….. 7
Figure 4: Typical emission spectra of organic materials ……………………………………... 9
Figure 5: Transparent OLED structure ………..…………………………………….. 10
Figure 6: Physical Vapor Deposition (PVD) in vacuum chamber …………………………… 12
Figure 7: Spin coating ……………………………………………....13
Figure 8: Ink jet printing to pattern polymers (Full Color Applications)…………………. 14
Figure 9: Roll to roll printing production of flexible OLED’s …………………................ 15
Figure 10: OLED Mass Production System ……………………………………………. 16
Figure 11: AMOLED ..………………………………………..... 17
Figure 12: Structure of Top-emitting OLED …………...…………………………….. 18
Figure 13: Foldable OLED ……………………………………….... 18
Figure 14: White OLED ……………………………………….... 19
Figure 15: Comparison of LED and OLED ………………………………………... 20
Figure 16: Comparison of LED and OLED Display …………………………………….21
Figure 17: Comparison of LCD, LED and OLED displays ……………………………... 22
Figure 18: Applications of OLED ………………………………………... 23
Figure 19: Future of OLED ………………………………………… 24
List of Tables
Table 1: Technical Characteristics of OLED’s …………………………………………… 16
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1 Introduction
An organic light-emitting diode (OLED) is a light-emitting diode (LED), in which
the emissive electroluminescent layer is a film of organic compound that emits light in
response to an electric current. OLEDs are used to create digital displays in devices such
as television screens, computer, portable systems such as mobile phones, handheld game
consoles and PDAs. A major area of research is the development of white OLED devices for
use in solid-state lighting applications.
OLED display devices use organic carbon-based films, sandwiched together between two
charged electrodes. One is a metallic cathode and the other a transparent anode, which is
usually glass. OLED displays can use either passive-matrix (PMOLED) or active-
matrix (AMOLED) addressing schemes. Active-matrix OLEDs (AMOLED) require a thin-
film transistor backplane to switch each individual pixel on or off, but allow for higher
resolution and larger display sizes.
An OLED display works without a backlight; thus, it can display deep black levels and can
be thinner and lighter than a liquid crystal display (LCD). In low ambient light conditions
(such as a dark room), an OLED screen can achieve a higher contrast ratio than an LCD,
regardless of whether the LCD uses cold cathode fluorescent lamps or an LED backlight.
2 Construction
A typical OLED is composed of a layer of organic materials situated between two electrodes,
the anode and cathode, all deposited on a substrate. The organic molecules are electrically
conductive as a result of delocalization of pi electrons caused by conjugation over part or the
entire molecule. These materials have conductivity levels ranging from insulators to
conductors, and are therefore considered organic semiconductors. The highest occupied and
lowest unoccupied molecular orbitals (HOMO and LUMO) of organic semiconductors are
analogous to the valence and conduction bands of inorganic semiconductors.
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Figure 1: Basic OLED structure
Source: http://electronics.howstuffworks.com/oled1.htm
Originally, the most basic polymer OLEDs consisted of a single organic layer. One example
was the first light-emitting device synthesized by J. H. Burroughes et al., which involved a
single layer of poly(p-phenylene vinylene). However multilayer OLEDs can be fabricated
with two or more layers in order to improve device efficiency.
As well as conductive properties, different materials 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. Many modern OLEDs incorporate a simple
bilayer structure, consisting of a conductive layer and an emissive layer.
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Figure 2: Basic OLED working
Source: http://electronics.howstuffworks.com/oled1.htm
During operation, a voltage is applied across the OLED such that the anode is positive with
respect to the cathode. Anodes are picked based upon the quality of their optical
transparency, electrical conductivity, and chemical stability. A current of electrons flows
through the device from cathode to anode, as electrons are injected into the LUMO of the
organic layer at the cathode and withdrawn from the HOMO at the anode. This latter process
may also be described as the injection of electron holes into the HOMO. Electrostatic forces
bring the electrons and the holes towards each other and they recombine forming an exciton,
a bound state of the electron and hole. This happens closer to the emissive layer, because in
organic semiconductors holes are generally more mobile than electrons. The decay of this
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excited state results in a relaxation of the energy levels of the electron, accompanied by
emission of radiation whose frequency is in the visible region. The frequency of this radiation
depends on the band gap of the material, in this case the difference in energy between the
HOMO and LUMO.
When a DC bias is applied to the electrodes, the injected electrons and holes can recombine
in the organic layers and emit light of a certain color depending on the properties of the
organic material. Since charge carrier transport in organic semiconductors relies on individual
hopping processes between more or less isolated molecules or along polymer chains, the
conductivity of organic semiconductors is several orders of magnitude lower than that of their
inorganic counterparts. Before actually decaying radiatively, an electron-hole pair will form
an exciton in an intermediate step, which will eventually emit light when it decays.
Depending on its chemical structure, a dye molecule can be either a fluorescent or a
phosphorescent emitter. Only in the latter, all excitons singlets and triplets are allowed to
decay radiatively. In the former, however, three quarters of all excitons the triplet excitons
do not emit any light. Fluorescent emitters therefore have a maximum intrinsic efficiency of
only 25 % and their application is avoided if possible. However, up to now, the lifetimes of
phosphorescent emitters, especially at a short wavelength (blue), are inferior to those of
fluorescent ones.
Figure 3: Energy Level diagram
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As electrons and holes are fermions with half integer spin, an exciton may either be in
a singlet state or a triplet state depending on how the spins of the electron and hole have been
combined. Statistically three triplet excitons will be formed for each singlet exciton. Decay
from triplet states (phosphorescence) is spin forbidden, increasing the timescale of the
transition and limiting the internal efficiency of fluorescent devices. Phosphorescent organic
light-emitting diodes make use of spinorbit interactions to facilitate intersystem
crossing between singlet and triplet states, thus obtaining emission from both singlet and
triplet states and improving the internal efficiency.
Indium tin oxide (ITO) is commonly used as the anode material. It is transparent to visible
light and has a high work function which promotes injection of holes into the HOMO level of
the organic layer. A typical conductive layer may consist of PEDOT:PSS as the HOMO level
of this material generally lies between the work function of ITO and the HOMO of other
commonly used polymers, reducing the energy barriers for hole injection. Metals such
as barium and calcium are often used for the cathode as they have low work functions which
promote injection of electrons into the LUMO of the organic layer. Such metals are reactive,
so they require a capping layer of aluminum to avoid degradation.
Experimental research has proven that the properties of the anode, specifically the anode/hole
transport layer (HTL) interface topography plays a major role in the efficiency, performance,
and lifetime of organic light emitting diodes. Imperfections in the surface of the anode
decrease anode-organic film interface adhesion, increase electrical resistance, and allow for
more frequent formation of non-emissive dark spots in the OLED material adversely
affecting lifetime. Mechanisms to decrease anode roughness for ITO/glass substrates include
the use of thin films and self-assembled monolayers. Also, alternative substrates and anode
materials are being considered to increase OLED performance and lifetime. Possible
examples include single crystal sapphire substrates treated with gold (Au) film anodes
yielding lower work functions, operating voltages, electrical resistance values, and increasing
lifetime of OLEDs.
Single carrier devices are typically used to study the kinetics and charge transport
mechanisms of an organic material and can be useful when trying to study energy transfer
processes. As current through the device is composed of only one type of charge carrier,
either electrons or holes, recombination does not occur and no light is emitted. For example,
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electron only devices can be obtained by replacing ITO with a lower work function metal
which increases the energy barrier of hole injection. Similarly, hole only devices can be made
by using a cathode made solely of aluminum, resulting in an energy barrier too large for
efficient electron injection.
3 Emission spectrum
Typical emission spectra of organic molecules are broad (as shown in figure X). As stated before, the
emission color is a material property. Thus, the total emission can be tuned to virtually any color,
including white at any color temperature, by stacking several different emitting layers in a single
device. This is possible since the organic layers are almost transparent in the visible spectral range.
Most white OLEDs contain a red, a green and a blue emission layer to create high-quality white light.
Figure 4: Typical emission spectra of organic materials.
The diagram shows spectra of red, green and blue emitters and their superposition which yields white
emission at a high color rendering index.
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4 OLED Architectures
Bottom or top emission
Bottom or top distinction refers not to orientation of the OLED display, but to the
direction that emitted light exits the device. OLED devices are classified as bottom
emission devices if light emitted passes through the transparent or semi-transparent
bottom electrode and substrate on which the panel was manufactured. Top emission
devices are classified based on whether or not the light emitted from the OLED device
exits through the lid that is added following fabrication of the device. Top-emitting
OLEDs are better suited for active-matrix applications as they can be more easily
integrated with a non-transparent transistor backplane. The TFT array attached to the
bottom substrate on which AMOLEDs are manufactured are typically non-
transparent, resulting in considerable blockage of transmitted light if the device
followed a bottom emitting scheme.[53]
Transparent OLEDs
Transparent OLEDs use transparent or semi-transparent contacts on both sides of the
device to create displays that can be made to be both top and bottom emitting
(transparent). TOLEDs can greatly improve contrast, making it much easier to view
displays in bright sunlight. This technology can be used in Head-up displays, smart
windows or augmented reality applications.
Figure 5: Transparent OLED structure
Source: http://electronics.howstuffworks.com/oled1.htm
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Graded Heterojunction
Graded hetero junction OLEDs gradually decrease the ratio of electron holes to
electron transporting chemicals. This results in almost double the quantum efficiency
of existing OLEDs.
Stacked OLEDs
Stacked OLEDs use a pixel architecture that stacks the red, green, and blue sub pixels
on top of one another instead of next to one another, leading to substantial increase in
gamut and color depth, and greatly reducing pixel gap. Currently, other display
technologies have the RGB (and RGBW) pixels mapped next to each other decreasing
potential resolution.
Inverted OLED
In contrast to a conventional OLED, in which the anode is placed on the substrate, an
Inverted OLED uses a bottom cathode that can be connected to the drain end of an n-
channel TFT especially for the low cost amorphous silicon TFT backplane useful in
the manufacturing of AMOLED displays.
5 Manufacturing OLED
The technological process of manufacturing OLEDs does not have fundamental differences.
In all cases, the process involves four basic steps: preparation of the substrate with the anode
layer, applying polymer layers, applying cathode layer and encapsulation, i.e. coating the
device with dense chemical resistant material layer, or gluing between glass plates to isolate
from the surrounding atmosphere. This method allows to greatly increasing the lifetime of the
device, which is critical for industrial designs. In the production of model devices intended
for research purposes, the last stage is often omitted, since the encapsulation does not affect
the basic operating characteristics of the OLED (except for the duration of the operation), but
considerably complicates the process. Significant differences from the mentioned schemes
have roll technology, which promising for making large luminous surfaces.
6 Fabrication Methods
There are two main methods of fabricating the OLED devices, which differ in the method of
applying nanolayers of polymer materials: a method of evaporation-condensation of material
in a vacuum, and the method of coating layers of solutions. In both cases, deposition of the
metallic cathode layer is nearly always carried out by evaporation in a vacuum.
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Mandatory and an important step in the fabrication of OLEDs, regardless of the method, is
the step of preparing the substrate surface. Insufficient clarity causes to the low efficiency, or
complete absence of luminescence even using efficient fluorescent materials. In most cases,
the substrate is a glass plate covered with a layer of ITO, i.e. the surface of this particular
layer is subjected to the treatment. Sufficiently clean surface provides a primary rinsing
sample in distilled water with containing detergents, mechanical cleaning, followed by
washing with deionized water and then with isopropyl alcohol in an ultrasonic bath. Good
results are obtained by subsequent irradiation with UV simultaneously treated the surface
with ozone. In this case, not only additional cleaning is achieved, but improved hole injection
properties of the ITO layer.
6.1 Physical vapor deposition
When evaporation-condensation method is used, the polymer layers of the device formed by
evaporating material with a thermal resistive evaporator at high vacuum (at least 5 • 10-4 Pa)
and its condensation on the substrate installed above the evaporator at a distance of 10-20 cm.
Figure 6: Physical Vapor Deposition (PVD) in vacuum chamber
Source: http://www.ewh.ieee.org/soc/cpmt/presentations/cpmt0401a.pdf
The method of vacuum evaporation-condensation has significant limitations. The main
limitation is substances, that have to be capable to sublime without decomposition, i.e. having
sufficient volatility and thermal stability. This dramatically reduces the number of potential
electro luminous.
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When using small molecule layers, evaporative techniques are commonly chosen. The small
molecules are evaporated in a vacuum chamber onto a substrate and form a thin layer.
Another method is called chemical vapor deposition (CVD). In CVD, a substrate is placed in
a vacuum and a chemical is introduced causing the film to condense onto the substrate. A
disadvantage of this method is that everything inside the vacuum will get coated, leading to
waste of material. [1]
6.2 Spin coating
In spin- coating method the organic materials are deposited in liquid form.To obtain uniform
layers in designing laboratory samples special centrifuges are used, which allow changing the
acceleration, speed, and duration of rotation. The substrate is mounted in the center of the
centrifuge and one or more drops of solution are dropped on it. The substrate is rotated at
high speed causing the liquid to spread out and dry. The liquid will uniform thin solid layer
of dissolved compounds.
The thickness of the layer is determined by the amount of rotating time and the drying rate of
the material. Films produced this way tend to have an inconsistent thickness as well as poor
surface smoothness.
[1]- OLED Fabrication for Use in Display Systems / Chris Summitt / 06.12.2006
Figure 7: In Spin coating, a drop of the material is deposited onto a substrate and rotated at high
speed until it spreads to the desired thickness.
Source: OLED Fabrication for Use in Display Systems / Chris Summitt / 06.12.2006
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6.3 Ink Jet Printing
Ink jet technology has many advantages in comparison with photolithography. Since, it is
used much a small volume of material, and the process is "dry", i.e. there is no air pollution
harmful liquids. In addition, the manufacturing process consists of fewer steps. This
technology is best suites for multi-layer structures since the interlayer connection can be
directly applied to the substrate. Thus, ink jet printing allows making multilayer OLED,
which has a low cost, and without any harm to the environment. Two kinds of ink used in the
process of manufacturing multilayer structures by ink jet printing - conductive and insulating.
In other methods of ink jet printing is used heated colorant with a very high temperature
before being discharged, which is not suitable for printing polymer materials.
To get the necessary precision in display manufacturing, at first micro-grooves are made on a
substrate by photolithography. Then they are filled (printed by technological jet printer) in
series red, blue, and green polymer, forming the structure of the RGB-subpixels. Electronics
of display combines every three sub-pixels in one full-color pixel. This method provides a
pixel pitch of 128 microns with the size of each sub-pixel 40 microns. To improve the clarity
of the print OLEDs, used another technological trick. Grooves on the substrate are covered
with a hydrophilic material, and the surface between them - hydrophobic. These substances
respectively attract or repel the polymer solution, providing the required printing accuracy.
All the microdroplets of liquid polymer falling into the grooves with minimum smearing the
polymer on barier layer. In the industrial production the solutions are applied to the substrate
in the form of dots by devices such ink jet printers.
Figure 8: Ink jet printing to pattern polymers (Full Color Applications)
Source: http://www.ewh.ieee.org/soc/cpmt/presentations/cpmt0401a.pdf
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6.4 Roll to roll Printing (R2R)
Method of ink jet printing and using a polymer base allows producing flexible, large area
luminous panel, but this technology has number of limitations related with protection of
polymer layers from the environment. Roll printing (lamination technology) allows to solve
these problems and improve OLED performance. The key point of this method is using two
components of panel - an anode and a cathode, which each of them is prepared individually
in the preliminary stage of the process. The cathode material is a polymer film or metal foil
with deposited cathode, electron injection and emission layers. The anode material is a
polymer film deposited with anode and hole transport layers. Then two components are
combined to form a multi-layered OLED flexible material
Figure 9: Roll to roll printing production of flexible OLEDs
Source: http://www.schott.com/newsfiles/com/foto_tesa_oleds_produktionsprozess.jpg
6.5 Vacuum sputtering
In the production of full-color OLED-panels an active matrix are used as the base, on which
polymer layers sputter. Each element (pixel) is an independent OLED-cell containing a
controling field tranzistor OFET. There are two types of facilities for industrial production -
with radial and linear arrangement of chambers for the preparation of the substrate, the
application of the polymer layers, cathode and encapsulation.
In a linear arrangement all cameras are disposed in series, which allows to assemble panels
in a continuous mode. Most industrial facilities, providing high quality products with high
performance, combined radial and linear sections.
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