OLED Seminar Report

Seminar Report 1

SEMINAR REPORT

ON

ORGANIC LIGHT EMITTING DIODE DISPLAY

Done By

VISHNU S S

DIPLOMA IN ELECTRONICS AND COMMUNICATION

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

GOVERNMENT POLYTECHNIC COLLEGE

NEYYATTINKARA

2017

Government Polytechnic College, Neyyattinkara Electronics and Communication

Seminar Report 2

SEMINAR REPORT

ON

ORGANIC LIGHT EMITTING DIODE DISPLAY

Done By

VISHNU S S

DIPLOMA IN ELECTRONICS AND COMMUNICATION



NEYYATTINKARA

2017


Seminar Report 3



NEYYATTINKARA

2017

Certificate

This is to certify that this seminar report is a bonafide record of the work done by

VISHNU S S under our guidance towards the partial fulfillment of the requirement

for the award of Diploma in Electronics and Communication Engineering of the

Department of Technical Education, Kerala during the year 2017

Guided By, Sri. Sulficar A

Sri. Divya.C HOD

Lecturer Electronics and communication


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ACKNOWLEDGEMENT

I take this opportunity to express our sincere gratitude and profound obligation to Sri. Sulficar A,

Head Of Department of Electronics and Communication Engineering, Government polytechnic College

Neyyattinkara.

I also wish to express my gratitude to Sri. Aravind Sekhar R, Sri. Pavitrakumar G, Smt. Reeya

George, Smt. Divya C for their help and encouragement done throughout this work.

Last but not the least, I am extremely grateful to all our friends without whose timely aid, could

not have completed the work successfully.

VISHNU S S

(Reg.no: 14200181)


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ABSTRACT

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. This layer of organic semiconductor is situated between two electrodes;

typically, at least one of these electrodes is transparent. OLEDs are used to create digital displays

in devices such as television screens, computer monitors, 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.

There are two main families of OLED: those based on small molecules and those

employing polymers. Adding mobile ions to an OLED creates a light-emitting electrochemical

cell (LEC) which has a slightly different mode of operation. OLED displays can use either

passive-matrix (PMOLED) or active-matrix (AMOLED) addressing schemes. Passive matrix

OLEDs (PMOLED) uses a simple control scheme in which you control each row (or line) in the

display sequentially whereas 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.


https://en.wikipedia.org/wiki/LED-backlit_LCD_display

https://en.wikipedia.org/wiki/Cold_cathode

https://en.wikipedia.org/wiki/Contrast_ratio

https://en.wikipedia.org/wiki/Liquid_crystal_display

https://en.wikipedia.org/wiki/Black_level

https://en.wikipedia.org/wiki/Backlight

https://en.wikipedia.org/wiki/Thin-film_transistor

https://en.wikipedia.org/wiki/Active-matrix_OLED

https://en.wikipedia.org/wiki/Active_matrix_addressing

https://en.wikipedia.org/wiki/Passive_matrix_addressing

https://en.wikipedia.org/wiki/Light-emitting_electrochemical_cell

https://en.wikipedia.org/wiki/Light-emitting_electrochemical_cell

https://en.wikipedia.org/wiki/Ion

https://en.wikipedia.org/wiki/Polymer

https://en.wikipedia.org/wiki/Solid-state_lighting

https://en.wikipedia.org/wiki/Personal_digital_assistant

https://en.wikipedia.org/wiki/Handheld_game_console

https://en.wikipedia.org/wiki/Mobile_phones

https://en.wikipedia.org/wiki/Mobile_phones

https://en.wikipedia.org/wiki/Computer_monitor

https://en.wikipedia.org/wiki/Television_set

https://en.wikipedia.org/wiki/Digital_display

https://en.wikipedia.org/wiki/Organic_semiconductor

https://en.wikipedia.org/wiki/Organic_compound

https://en.wikipedia.org/wiki/Electroluminescence

https://en.wikipedia.org/wiki/Emission_(electromagnetic_radiation)

https://en.wikipedia.org/wiki/Light-emitting_diode

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CONTENT

INTRODUCTION………….……………………….……………...…......8

MODULE – 1

HISTORY & COMPONENTS OF OLED………………………..….….9

1.1 HISTORY…………………………………………………...…...9

1.2 COMPONENTS OF OLED……………………………………..14

MODULE – II

FABRICATION TECHNOLOGY OF OLED…………………………16

2.1 STEPS IN FABRICATION…………………………………....16

2.2 METHODS OF FABRICATION…………………………..….18

MODULE - III

WORKING & TYPES OF OLED………………………………………..……….22

3.1 WORKING PRINCIPLE…………………………...………….22

3.2 WORKING………………………………………………….....23

3.3 TYPES OF OLED…………………………………...…………29

3.4 COMPARISON OF OLED AND LCD……………………..…36


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MODULE – IV

ADVANTAGES & DISADVANTAGES…………………………..37

4.1 ADVANTAGES……………………………………………….37

4.2 DISADVANTAGES……………………………..…………….38

4.3 APPLICATIONS……………………………………………….39

4.4 EFFICIENCY OF OLED……………………………………….42

4.5 THE ORGANIC FUTURE………………………………..……43

CONCLUSIONS………………………………………….……….44

REFERENCE………………………………………………...……45


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INTRODUCTION

Scientific research in the area of semiconducting organic materials as the active substance

in light emitting diodes (LEDs) has increased immensely during the last four decades. Organic

semiconductors was first reported in the 60:s and then the materials were only considered to be

merely a scientific curiosity. (They are named organic because they consist primarily of carbon,

hydrogen and oxygen.). However when it was recognized in the eighties that many of them are

photoconductive under visible light, industrial interests were attracted. Many major electronic

companies, such as Philips and Pioneer, are today investing a considerable amount of money in

the science of organic electronic and optoelectronic devices. The major reason for the big

attention to these devices is that they possibly could be much more efficient than today’s

components when it comes to power consumption and produced light. Common light emitters

today, Light Emitting Diodes (LEDs) and ordinary light bulbs consume more power than organic

diodes do. And the strive to decrease power consumption is always something of matter. Other

reasons for the industrial attention are i.e. that eventually organic full color displays will replace

today’s liquid crystal displays (LCDs) used in laptop computers and may even one day replace

our ordinary CRT-screens.

Organic light-emitting devices (OLEDs) operate on the principle of converting electrical

energy into light, a phenomenon known as electroluminescence. They exploit the properties of

certain organic materials which emit light when an electric current passes through them. In its

simplest form, an OLED consists of a layer of this luminescent material sandwiched between

two electrodes. When an electric current is passed between the electrodes, through the organic

layer, light is emitted with a colour that depends on the particular material used. In order to

observe the light emitted by an OLED, at least one of the electrodes must be transparent.

When OLEDs are used as pixels in flat panel displays they have some advantages over

backlit active-matrix LCD displays - greater viewing angle, lighter weight, andquicker response.

Since only the part of the display that is actually lit up consumespower, the most efficient

OLEDs available today use less power.


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Figure.1Demonstration of a flexible OLED device

Based on these advantages, 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.


http://en.wikipedia.org/wiki/File:OLED_EarlyProduct.JPG

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MODULE - I

HISTORY & COMPONENTS OF OLED

1.1 HISTORY

Conductive materials are substances that can transmit electrical charges. Traditionally,

most known conductive materials have been inorganic. Metals such as copper and aluminum are

the most familiar conductive materials, and have high electrical conductivity due to their

abundance of delocalized electrons that move freely throughout the inter-atomic spaces. Some

metallic conductors are alloys of two or more metal elements, common examples of such alloys

include steel, brass, bronze, and pewter.

In the eighteenth and early nineteenth centuries, people began to study the electrical

conduction in metals. In his experiments with lightning, Benjamin Franklin proved that an

electrical charge travels along a metallic rod. Later, Georg Simon Ohm discovered that the

current passing through a substance is directly proportional to the potential difference, known as

Ohm's law. This relationship between potential difference and current became a widely used

measure of the ability of various materials to conduct electricity. Since the discovery of

conductivity, studies have focused primarily on inorganic conductive materials with only a few

exceptions.

Henry Letheby discovered the earliest known organic conductive material in 1862. Using

anodic oxidation of aniline in sulfuric acid, he produced a partly conductive material that was

later identified as polyaniline. In the 1950s, the phenomenon that polycyclic aromatic

compounds formed semi-conducting charge-transfer complex salts with halogens was

discovered, showing that some organic compounds could be conductive as well.

More recent work has expanded the range of known organic conductive materials. A high

conductivity of 1 S/cm (S = Siemens) was reported in 1963 for a derivative of tetraiodopyrrole.

In 1972, researchers found metallic conductivity (conductivity comparable to a metal) in the

charge-transfer complex TTF-TCNQ.


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In 1977, it was discovered that polyacetylene can be oxidized with halogens to produce

conducting materials from either insulating or semiconducting materials. In recent decades,

research on conductive polymers has prospered, and the 2000 Nobel Prize in Chemistry was

awarded to Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa jointly for their work on

conductive polymers.

Conductive plastics have recently undergone development for applications in industry. In

1987, the first organic diode device of was produced at Eastman Kodak by Ching W. Tang and

Steven Van Slyke. Spawning the field of organic light-emitting diodes (OLED) research and

device production. For his work, Ching W. Tang is widely considered as the father of organic

electronics. Technology for plastic electronics constructed on thin and flexible plastic substrates

was developed in the 1990s. In 2000, the company Plastic Logic was founded as a spin-off of

Cavendish Laboratory to develop a broad range of products using the plastic electronics

technology

Attractive properties of polymer conductors include a wide range of electrical

conductivity that can be tuned by varying the concentrations of chemical dopants, mechanical

flexibility, and high thermal stability. Organic conductive materials can be grouped into two

main classes: conductive polymers and conductive small molecules.

ORGANIC ELECTRONICS

Organic electronics is a field of materials science concerning the design, synthesis,

characterization, and application of organic small molecules or polymers that show desirable

electronic properties such as conductivity. Unlike conventional inorganic conductors and

semiconductors, organic electronic materials are constructed from organic (carbon-based) small

molecules or polymers using synthetic strategies developed in the context of organic and

polymer chemistry. One of the benefits of organic electronics is their low cost compared to

traditional inorganic electronics.


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CONDUCTIVE MATERIALS

Conductive small molecules are usually used in the construction of organic

semiconductors, which exhibit degrees of electrical conductivity between those of insulators and

metals. Semiconducting small molecules include polycyclic aromatic compounds such as

pentacene, anthracene and rubrene.

Conductive polymers are typically intrinsically conductive. Their conductivity can be

comparable to metals or semiconductors. Most conductive polymers are not thermoformable,

during production. However they can provide very high electrical conductivity without showing

similar mechanical properties to other commercially available polymers. Both organic synthesis

and advanced dispersion techniques can be used to tune the electrical properties of conductive

polymers, unlike typical inorganic conductors. The well-studied class of conductive polymers is

the so-called linear-backbone “polymer blacks” including polyacetylene, polypyrrole,

polyaniline, and their copolymers.

Poly (p-phenylene vinylene) and its derivatives are used for electroluminescent

semiconducting polymers. Poly (3-alkythiophenes) are also a typical material for use in solar

cells and transistors.

APPLICATION OF ORGANIC ELECTRONICS

There are four major application areas: displays; lighting; photovoltaics and integrated

smart systems. While OLAE technology is currently used in many manufacturing processes, new

applications are entering the marketplace rapidly.

While organic light- emitting diodes (OLEDs) are already used commercially in displays

of mobile devices and significant progress has been made in applying organic photovoltaic cells

to light-weight flexible fabrics to generate low-cost solar energy, a brand new range of

applications is possible such as biomedical implants and disposable biodegradable RFID

packaging tags.


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In addition, low cost organic solar cells have the potential to drive down the cost of

photovoltaics to levels, which are not achievable with mono or poly-crystalline solar cells.

Similarly, organic light emitting diodes will revolutionize current lighting applications,

significantly reducing CO2 impact. Also, smart devices incorporating organic and printed

circuits, sensors and energy sources will enable new approaches in logistics and consumer

packaging, and new flexible displays with exceptionally low energy consumption will be used

anywhere and anytime.

WHAT ARE THE POSSIBILITIES?

The possibilities are limitless as the technology is evolving at such a rapid pace.

Industrial designers across all sectors and markets should be aware of the technology and looking

at ways of harnessing its power and benefits into new product design.

Possible applications could include:

Memory or logic devices

Detectors, lasers and light emitters

Information displays – advertising billboards and other media

Micro lenses

Batteries

Power or light sources

Subsystem packaging

Image patterning

Electrical or optical fibers

Transistors

Photoconductors


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ORGANIC LED

Why so much excitement about Organic LED?

Easy to process

Processing is low cost

Less temperature required to fabricate

They can possess to low –cost substrates (i.e., plastic, paper even cloth)

Directly integrated to packages as it is light weight.

1.2 COMPONENTS OF AN OLED

The components in an OLED differ according to the number of layers of the organic

material. There is a basic single layer OLED, two layer and also three layer OLED’s. As the

number of layers increase the efficiency of the device also increases. The increase in layers also

helps in injecting charges at the electrodes and thus helps in blocking a charge from being

dumped after reaching the opposite electrode. Any type of OLED consists of the following

components.

1. An emissive layer

2. A conducting layer

3. A substrate

4. Anode and cathode terminals.


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SUBSTRATE- The substrate supports the OLED.

Example: clear plastic, glass, foil.

ANODE- The anode removes electrons when current flows through the device.

Example: indium tin oxide

ORGANIC LAYERS- These layers are made of organic molecules or polymers.

CONDUCTIVE LAYER- This layer is made of organic plastic

molecules that send electrons out from the anode.

Example: polyaniline, polystyrene

EMISSIVE LAYER- This layer is made of organic plastic

molecules (different ones from the conducting layer) that transport

electrons from the cathode; this is where light is made.

Example: polyfluorine, Alq3

CATHODE- The cathode injects electrons when a current flows through the device. (It

may or may not be transparent depending on the device)

Example: Mg, Al, Ba, Ca


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MODULE – II

FABRICATION TECHNOLOGY OF OLED

2.1 STEPS IN FABRICATION

In general OLEDs are fabricated in a class 1000 cleanroom to produce results with as

high a consistency as possible. However, OLEDs are relatively tolerant to dust, as it is insulating

and generally only stops the device working where the dust has landed on the surface.

In this section, a generalized fabrication process is dis-cussed. There are six basic steps in

the fabrication process from the substrate to devices ready for use. These are described below

2.1.1 SUBSTRATE CLEANING

Preparing the ITO surface for coating simply consists of sonicating the substrates in a

sodium hydroxide (NaOH) solution to remove the photoresist, followed by a rinse in de-ionized

(DI) water and blow dry. The first step is to load the substrates into the cleaning rack such that

they all have the same orientation. The loaded substrate rack is then placed in a beaker and

submerged in a 10% solution of NaOH in water. The substrates are then sonicated to remove the

photoresist. Depending upon the power and temperature of the sonicator the photoresist may

either dissolve or de-laminate as sheets. The time that it takes for this to occur will depend on the

ultrasonic bath used as well as the temperature. After sonication the substrates should be

thoroughly rinsed with water to wash away the photoresist. To ensure that they is no residual

layer of photoresist present they should be put back in the ultrasonic bath in a fresh NaOH

solution for about the same time again. Following this second sonication, the substrate should be

again rinsed thoroughly with water and keep immersed in water until ready to blow dry to avoid

contamination by dust.

2.1.2 APPLYING PEDOT: PSS

PEDOT: PSS is a common hole injection layer material The chemical name of it is

poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate). Getting a high quality PEDOT:PSS

film is critical for effective device performance and is often the most difficult part of device


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fabrication. PEDOT:PSS requires a pristine and hydrophilic surface in order to coat properly,

which should have been achieved with the cleaning routine above. It is also critical to ensure that

the active areas have not come into contact with any other surfaces as this will affect how well

the ITO will spin. For typical use in OLEDs, the PEDOT:PSS are spin coated at 5000 rpm for 30

seconds to produce a film thickness of around 40 nm. To minimise material use this can be done

by pipetting 20 to 30 L into the middle of a spinning substrate. After spinning has completed

visually inspect the PEDOT:PSS films for defects and for best performance discard any

substrates with imperfections near the active pixels. After spin coating, the PEDOT:PSS should

be wiped off the cathode with a cotton bud soaked in DI water. Then the substrates are placed

either in a box with the lid closed to avoid dust settling on devices, or if kept in air for more than

a few minutes place directly on a hotplate.

2.1.3 APPLYING ACTIVE LAYER

The active layer can be applied either in air or in a glovebox with little difference in

performance provided exposure time and light levels are minimised. Pipetting 20 L of the

solution onto a substrate spinning at 2000 rpm should provide a good even coverage with

approximately the desired thickness. The substrate needs to be spun until dry, which is typically

only a few seconds. Following spin coating, the samples can be solvent or thermally annealed if

desired. For the OLED reference solution thermal annealing is recommended to be done at 80 C

for 10 minutes. Before cathode deposition, the cathode strip needs to be wiped clean. Finally, the

substrates need to be placed face down in the evaporation shadow mask with the cathode strip at

the wide end of the apertures.

2.1.4 CATHODE EVAPORATION

Typically, aluminium of 100 nm is evaporated at a rate of around 1.5 A/s, but thinner

cathodes (50 nm) have also been used with no decrease in initial performance noted. Calcium

evaporation is relatively trivial as it melts at low temperatures, however it can only be used

effectively in conjunction with a glovebox otherwise degradation occurs.


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2.1.5 ANNEALING

After cathode deposition, thermal annealing can be per-formed if required. Annealing at a

temperature of approxi-mately 150 C for 15 minutes gives optimal performance.

2.1.6 ENCAPSULATION

Encapsulating the devices protects them against degradation by oxygen and moisture

once removed from the glovebox. True encapsulation for lifetimes of thousands of hours requires

the use of glass welding technology and/or getter layers of calcium.

2.2 METHODS OF FABRICATION

Physical vapor deposition Screen

Screen Printing

Inkjet printing

In-line fabrication

Roll to roll process

2.2.1 PHYSICAL VAPOR DEPOSITION SCREEN

Physical Vapor Deposition (PVD) is a group of vacuum coating techniques used to deposit

thin films of various mate-rials on different surface.This technique is based on the for-mation of

vapor of the material to be deposited as a thin film. The material in solid form is either heated

until evaporation (thermal evaporation) or sputtered by ions (sputtering).It is also possible to

bombard the sample with an ion beam from an external ion source.Thermal vapor evaporation of

small molecules is carried out on glass surface.Multicolor displays are made by properly

matched shadow masks for depositing RGB emitting material.

PHYSICAL VAPOR DEPOSITION TECHNOLOGIES: There are two technologies

which are often used for physical vapor depo-sition (PVD). Physical vapor deposition is

done by thermal evaporator. Here, the material is heated to attain gaseous state. Besides,

Electron Beam Evaporator is also used. Another method is Sputtering which is carried


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out under high vacuum condition. Here plasma as the particle source is used to strike the

target.

THERMAL EVAPORATOR: Thermal evaporator uses an elec-tric resistance heater to

melt the material and raise its vapor pressure to a useful range. This is done in a high

vacuum environment.An electron beam evaporator fires a high energy beam from an

electron gun to boil a small spot of the material

SPUTTERING: Sputtering is a physical process whereby atoms in a solid target material

are ejected into the gas phase due to bombardment of the material by energetic ions.The

ions for the sputtering process are supplied by the plasma that is induced in the sputtering

equipment. Sputtering relies on a plasma (usually a noble gas, such as argon) to knock

material from a surface.

2.2.2 SCREEN PRINTING

Screen printing is a commonly used technique for fast, inexpensive deposition of dye

films over large areas. In addi-tion, screen printing allows patterning to easily define which areas

of the substrate receive deposition. It is mainly used industrially. The essential components of a

screen printing process consist of a cloth of interwoven threads. Cloth is stretched tightly in a

frame. A patterned mask is prepared on the stretched cloth in the frame. Ink is poured onto the

top surface of the cloth. A substrate onto which the ink is to be printed is placed underneath the

framed cloth so that it does not directly contact the bottom surface of the cloth. A squeegee

spreads the ink lightly over the patterned open cloth area without pressing down on the substrate.

This fills the openings in the cloth with ink. Then the squeegee presses the cloth from the top

against a substrate underneath and by sliding horizontally over the surface, squeezes out the ink

in the open cloth areas onto the substrate, leaving the printed pattern. This remains wet first. So

then the printed image is dried. This can be repeated many times by replacing the substrate and

printing a new.

A variety of cloth types is available. Polyester is common; nylon cloth and metal cloth

are also made. The specific limits we have found to our process apply to polyester cloth;

however, nylon and metal cloth will give essentially similar results. Mesh count is the number of

threads per inch in the cloth. The Theoretical Ink Volume is the volume of ink in all mesh


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openings per unit area of substrate. This volume is the thickness of the ink deposit as if the ink

were coating the substrate below the open cloth as a uniform, continuous layer. A high tension is

maintained on the cloth to keep it from sagging in the screen. A higher mesh count cloth gives

both higher print definition and lower theoretical ink volume, but the mesh opening and percent

open area decrease. In general, the printed layers of light-emitting polymer lamp construction

need to be as thin as possible which entails using higher mesh count screens with lower

theoretical ink volume values.

In a typical single layer white OLED fabrication by screen printing method ITO (Indium

Tin Oxide) glasses are ultra-sonically cleaned, followed by rinsing with deionized water,

trichloroethylene, acetone and methanol. The cleaned ITO glasses are patterned via a standard

micro lithographic process. HCl (37%, Aldrich) is used as the etchant for the ITO. For the

surface treatment of the ITO, the patterned ITO glasses were treated by oxygen plasma for some

minutes as RMS roughness is lower in plasma treated ITO than bare ITO glasses. The pinholes

are also reduced due to plasma treat-ment. For white OLED, DPVBi(4,4-bis(2,2-diphenylvinyl)-

1,1biphenyl, 99.95% purity, Gracel), -NPD (N,N-diphenyl-N,N-bis(1-naphthyl)-1,1 biphenyl-4,4

diamine, 99.95% purity, Gracel) and rebrene(99.96% purity, Gracel) are dissolved in a

previously prepared solution of polystyrene in chlorobenzene. The solution is then screen printed

using mask. Then LiF and Al layer is deposited to form OLED device.

2.2.3 INKJET PRINTING

Ink jet printing is another way to deposit the organic layers, especially organic polymers.

In this method we can use simply an inkjet printer. Organic layers are sprayed onto substrates

like ink sprayed on paper during printing. For example, there may be three ink cartridges and

three nozzles enabling the printer to print three different colours simultaneously. As the printer

head scans the page and the piezoelectric materials are pulsed, ink is squirted from the nozzles

onto the page. The only modification to the ink-jet printer for printing OLEDs was to replace the

ink cartridges with polymer solutions. Different colors are achieved with different layer

materials. For example, if green is desired it is common to use the combination Mq3, where M is

a Group III metal and q3 is 8-hydroxyquinolate. Blue is achieved by using Alq2OPh and red is

done with perylene derivatives. Organic solutions used here are a solution of hole transport layer


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and emissive layer organic materials. When using polymers, ink-jet technology is commonly

used. We can use an electron transport material layer for better device efficiency.

Inkjet Mechanism

2.2.4 IN-LINE FABRICATION

In-line fabrication is a mass process technique. Vertical in-line tool operates with

continuous substrate flow. Linear sources of depositing organic and metallic materials are used

in this process. In this process in-line sources are used where material is deposited from a linear

tube (as opposed to the point sources that are more commonly used in OLED manu-facture), It

improves material usage by a factor of 10.

Cheaper mass production technique and excellent thickness homogeneity can be achieved

by this process. Deposition stability is excellent in this method. Complicated stack struc-tures

can be implemented using in-line fabrication. Deposition rate and throughput are high. This

process can handle large substrate.

2.2.5 ROLL TO ROLL PROCESS

Roll to Roll processing could revolutionize the fabrication of OLED flexible flat panel

displays. The prerequisite of this method is flexible substrate, so that the substrate can be rolled.

We can divide this process into three parts.

Deposition

Patterning

Packaging


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MODULE - III

WORKING & TYPES OF OLED

3.1 WORKING PRINCIPLE

As previously mentioned, OLEDs are an emissive technology, which means they emits

light instead of diffusing or reflecting a secondary source, as LCDs and LEDs currently do.

Below is a graphic explanation of how the technology works


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3.2 WORKING

The organic light emitting diode (OLED) is a p-n diode, in which charge-carriers (e-h

pairs) recombine to emit photons in an organic layer. The thickness of this layer is approximately

100 nm (experiments have shown that 70 nm is an optimal thickness). When an electron and a

hole recombine, an excited state called an exciton is formed. Depending on the spin of the e-h

pair, the excitation is either a singlet or a triplet. An electron can have two different spins, spin

up and spin down. When the spin of two particles is the same, they are said to be in a spin-

paired, or a triplet state, and when the spin is opposite they are in a spin-paired singlet state.

Figure.3Triplet State

On the average, one singlet and three triplets are formed for every four electron-hole pairs, and

this is a big inefficiency in the operation of the diodes. A singlet state decays very quickly,

within a few nanoseconds, and thereby emits a photon in a process called fluorescence. A triplet

state, however, is much more long-lived (1 ms - 1 s), and generally just produce heat. One


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method of improving the performance is to add a phosphorescent material to one of the layers in

the OLED. This is done by adding a heavy metal such as iridium or platinum. The excitation can

then transfer its energy to a phosphorescent molecule which in turn emits a photon. It is however

a problem that few phosphorescent materials are efficient emitters at room temperature.

Figure.4 Two different ways of decay

There have been devices manufactured which transforms both singlet and tripletstates in a host to

a singlet state in the fluorescent dye. This is done by using a phosphorescent compound which

both the singlets and triplets transfer their energy to, after which the compound transfer its

energy to a fluorescent material which then emits light.

Using one organic layer has some problems associated with it. The electrodes energy

levels have to be matched very closely, otherwise the electron and hole currents will not be

properly balanced. This leads to a waste in energy since charges can then pass the entire structure


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without recombining, and this lowers the efficiency of the device. With two organic layers, the

situation improves dramatically. Now the different layers can be optimized for the electrons and

holes respectively. The charges are blocked at the interface of the materials, and “waits” there for

a “partner”.

Figure.5 Single Organic layer

Considerably better balance can be achieved by using two organic layers one ofwhich is matched

to the anode and transports holes with the other optimized for electron injection and transport.

Each sign of charge is blocked at the interface between the two organic layers and tend to "wait"

there until a partner is found.

Recombination therefore occurs with the excitation forming in the organic material with the

lower energy gap. The fact that it forms near the interface is also beneficial in preventing

quenching of the luminescence that can occur when the excitation is near one of the electrodes.


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Figure.6 Two Organic layers

Another improvement is to introduce a third material specifically chosen for its luminescent

efficiency. Now the three organic materials can be separately optimized for electron transport,

for hole transport and for luminescence.

Figure.7 Multilayer organic light emitting diode

The principle of operation of organic light emitting diodes (OLEDs) is similar to that of

inorganic light emitting diodes (LEDs). Holes and electrons are injected from opposite contacts

into the organic layer sequence and transported to the emitter layer. Recombination leads to the

formation of singlet excitons that decay radiatively. In more detail, electroluminescence of


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organic thin film devices can be divided into five processes that are important for device

operation:

(a) Injection: Electrons are injected from a low work function metal con-tact, e. g. Ca or Mg.

The latter is usually chosen for reasons of stability. A wide-gap transparent indium-tin-oxide

(ITO) or polyaniline thin film is used for hole injection. In addition, the efficiency of carrier

injection can be improved by choosing organic hole and electron injection layers with a low

HOMO (high occupied molecular orbital) or high LUMO (lowest unoccupied molecular orbital)

level, respectively.

(b) Transport: In contrast to inorganic semiconductors, high p- or n-conducting organic thin

films can only rarely be obtained by doping. Therefore, preferentially hole or electron

transporting organic compounds with sufficient mobility have to be used to transport the charge

carriers to the re-combination site. Since carriers of opposite polarity also migrate to some

extent, a minimum thickness is necessary to prevent non-radiative recombination at the opposite

contact. Thin electron or hole blocking layers can be inserted to improve the selective carrier

transport.

(c) Recombination: The efficiency of electron-hole recombination leading to the creation of

singlet excitons is mainly influenced by the overlap of electron and hole densities that originate

from carrier injection into the emitter layer. Recombination of filled traps and free carriers may

also attribute to the formation of excited states. Energy barriers for electrons and holes to both

sides of the emitter layer allow to spatially confine and improve the recombination process.

(d) Migration and (e) decay:Singlet excitons will migrate with an average diffusion length of

about 20 nm followed by a radiative or non-radiative decay. Embedding the emitter layer into

transport layers with higher singlet excitation energies leads to a confinement of the singlet

excitons and avoids non-radiative decay paths. Doping of the emitter layer with organic dye

molecules allows to transfer energy from the host to the guest molecule in order to tune the

emission wavelength or to increase the luminous efficiency.


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When biased, charge is injected into the highest occupied molecular orbital (HOMO) at the

anode (positive), and the lowest unoccupied molecular orbital (LUMO) at the cathode (negative),

and these injected charges (referred to as “holes” and “electrons,” respectively) migrate in the

applied field until two charges of opposite polarity encounter each other, at which point they

annihilate and produce a radiative state emitting photons with energy hf =Eg . The energy gap is

the difference between the HOMO and LUMO level of the emitting layer, and it is largely

responsible for the observed color of the light.

Figure.8Recombination Region


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Figure.9 Layer sequences and energy level diagrams for OLEDs with (a) single layer,

(b) single hetero structure, (c) double hetero structure, and (d)multiplayer structure

with separate hole and electron injection and transport layers.


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3.3 TYPES OF OLED

There are several types of OLEDs:

Passive-matrix OLED

Active-matrix OLED

Transparent OLED

Top-emitting OLED

Foldable OLED

White OLED

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.

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,PDA’s and MP3 Players. Even with the external circuitry, passive-matrix OLEDs

consume less battery power than the LCDs that currently power these devices.


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Figure.10OLED Passive Matrix

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.


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Figure.11 OLED Active Matrix

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.12 OLED Transparent Structure

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 in SMART CARDS


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Figure.13OLED Top-Emitting Structure

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

attached to fabrics to create "smart" clothing, such as outdoor survival clothing with an

integrated computer chip, cell phone, GPS receiver and OLED display sewn into it.

Figure.14 Foldable OLED


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WHITE OLED

White OLEDs emit white light that is brighter, more uniform and more energy efficient than that

emitted by fluorescent lights. White OLEDs also have the true-color qualities of incandescent

lighting. Because OLEDs can be made in large sheets, they can replace fluorescent lights that are

currently used in homes and buildings. Their use could potentially reduce energy costs for

lighting.

Figure.15 White OLED


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3.4 COMPARISON OF OLED AND LCD

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 fluorescent 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*

MODULE – IV


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ADVANRTAGES & DISADVANTAGES

4.1 ADVANTAGES

The different manufacturing process of OLEDs lends itself to several advantages over flat-panel

displays made with LCD technology.

Lower cost in the future: OLEDs can be printed onto any suitable substrate by an inkjet

printer or even by screen printing, theoretically making them cheaper to produce than

LCD or plasma display. However, fabrication of the OLED substrate is more costly than

that of a TFT LCD, until mass production methods lower cost through scalability.

Light weight & flexible plastic substrates: OLED displays can be fabricated on flexible

plastic substrates leading to the possibility of flexible organic light-emitting diodes being

fabricated or other new applications such as roll-up displays embedded in fabrics or

clothing.

Wider viewing angles & improved brightness: OLEDs can enable a greater artificial

contrast ratio (both dynamic range and static, measured in purely dark conditions) and

viewing angle compared to LCDs because OLED pixels directly emit light.

Better power efficiency: LCDs filter the light emitted from a back light

Response time: OLEDs can also have a faster response time than standard LCD screens.


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4.2 DISADVANTAGES

OLED seem to be the perfect technology for all types of displays;however, they do have some

problems, including:

Outdoor performance: As an emissive display technology, OLEDs rely completely

upon converting electricity to light, unlike most LCDs which are to some extent reflective

Power consumption: While an OLED will consume around 40% of the power of an

LCD displaying an image

Screen burn-in: Unlike displays with a common light source, the brightness of each

OLED pixel fades depending on the content displayed. The varied lifespan of the organic

dyes can cause a discrepancy between red, green, and blue intensity. This leads to image

persistence, also known as burn in

UV sensitivity: OLED displays can be damaged by prolonged exposure to UV light. The

most pronounced example of this can be seen with a near UV laser (such as a Bluray

pointer) and can damage the display almost instantly with more than 20mW leading to

dim or dead spots where the beam is focused.

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

Manufacturing - Manufacturing processes are expensive right now.


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Color balance issues: Additionally, as the OLED material used to produce blue light

degrades significantly more rapidly than the materials that produce other colors, blue

light output will decrease relative to the other colors of light. This differential color

output change will change the color balance of the display and is much more noticeable

than a decrease in overall luminance.

Water damage: Water can damage the organic materials of the displays. Therefore,

improved sealing processes are important for practical manufacturing. Water damage

may especially limit the longevity of more flexible displays.

4.3APPLICATIONS

Currently, OLEDs are used in small screen devices like cell phones, digital cameras etc.

Some examples of OLED applications are as follows:

Mobile Phones- Mobile phones were the first to adopt AMOLED displays and is the

largest market for OLEDs today.

Figure.16Samsung Galaxy RoundFigure.17Blackberry Q30

OLED TVs- OLED TVs had begun shipping in 2013 but their prices are still very high.


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Figure.18 Sony XEL-1, world’s 1st OLED TV

Digital Cameras- Several compact and high-end cameras use AMOLED displays that

offer rich colors and high contrast and brightness. Kodak was the first to release a digital

camera with an OLED display in March 2003, the EasyShare LS633.

Figure.19 Kodak LS633

OLED Lamps- OLED lamps are currently very expensive, but already several

companies are offering these in the premium lighting category.


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Figure.20Turn lights flaps

Other devices-OLEDs are also used in wrist watches, headsets, car audio systems,

remote controllers, digital photo frames and many other kinds of devices.

Future uses of OLED-

Wallpaper lighting defining new ways to light a space

Figure.21 Wallpaper Lighting

Scroll Laptop


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Figure.22 Scroll Laptop

Rollable OLED television

Figure.23Toshiba ultra thin flexible OLED

4.4 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.


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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.

4.5 THE ORGANIC FUTUREThe 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, phosphorescent, 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 worldwide 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


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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 going 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.

REFERENCES


Seminar Report 45

1) http://impnerd.com/the-history-and-future-of-oled

2) http://jalopnik.com/5154953/samsung-transparent-oled-display-pitched-as-automotive-hud

3) http://optics.org/cws/article/industry/37032

4) http://www.cepro.com/article/study_future_bright_for_oled_lighting_market/

5) http://www.oled-research.com/oleds/oleds-history.html

6) http://www.pocket-lint.com/news/news.phtml/23150/24174/samsung-say-oled-not-

ready.phtml

7) http://www.technologyreview.com/energy/21116/page1/

8) http://www.voidspace.org.uk/technology/top_ten_phone_techs.shtml#keep-your-eye-on-

flexible-displays-coming-soon
