Drishti Seminar

SEMINAR ON (OLED TECHNOLOGY) Submitted in partial fulfillment of the requirement for the award of the degree of Bachelor of Technology in Electronics and Communication Engineering Department

Submitted by DRISHTI MALIK 1715306 Batch: 2015-2019

Department of Electronics & Communication Engineering Seth Jai Parkash Mukand Lal Institute of Engineering & Technology, Radaur – 135133 (Yamuna Nagar) (Affiliated to Kurukshetra University, Kurukshetra, Haryana, India)

ACKNOWLEDGEMENT I

would

like

to

thank

my

Department

i.e.-

Department

of

Electronics & Communication Engineering of my College especially my H.O.D. “Mr. Vishal Chaudhary” and all my teachers who gave me the opportunity to submit my seminar on innovation in electronics. I am highly obliged that I have received all your support in such a good way. The information submitted is true and genuine to the best of my knowledge. This kind of opportunity not only allows the student to be in touch with the latest inventions, but also motivates to take part in their field of electronics to invent new devices and machines to make the future better and easy to live in. Enhancing the capabilities of learning new skills and technologies in various fields of engineering. I would sincerely thank to all my teachers and mentors to provide the opportunity to present the report on the latest inventions in field of electronics.

Yours faithfully Drishti Malik

LIST OF FIGURES Figure No. 1.1

Figure Demonstration of flexible OLED device

2.1

Injet Mechanism

3.1

Triplet State

3.2

Two different ways of Decay

3.3

Single Organic Layer

3.4

Two Organic Layer

3.5

Multilayer Organic LED

3.6

Recombination Region

3.7

Passive Matrix

3.8

Active Matrix

3.9

OLED structure

3.10

OLED Top Emitting Structure

3.11

Foldable OLED

3.12

White OLED

4.1

Samsung Galaxy

4.2

SONY XEL-1

4.3

KODAK LS633

4.4 4.5

Turn light flap Wallpaper Lightining

4.6

Scroll Laptop

Page No.

CONTENTS 

CHPTER –1: INTRODUCTION TO OLED TECHNOLOGY

1.1 INTRODUCTION ................................................................................. 1.2 HISTORY & COMPONENTS OF OLED………………………..….….9 1.2 (i)HISTORY…………………………………………………...…...9 1.2 (ii)COMPONENTS OF OLED……………………………………..14 

CHAPTER -2: FABRICATION TECHNOLOGY OF OLED………16 2.1 STEPS IN FABRICATION…………………………………....16 2.2 METHODS OF FABRICATION…………………………..….18 

CHAPTER -3: 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……………...........



CHAPTER-:4 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



REFERENCES.................…............................................................…45

CHAPTER-1: INTRODUCTION 1.1 INTRODUCTION TO OLED TECHNOLOGY 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 CRTscreens. 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.

Figure.1.1Demonstration 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.

1.2 HISTORY & COMPONENTS OF OLED 1.2(i) 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.

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 lightemitting 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.

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.

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

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(ii) 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.



SUBSTRATEThe 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

CHAPTER-: 2 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.(i) 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.(ii) 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. 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.(iii) 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.(iv) 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 as it melts at low temperature.

2.1.(v) 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(vi)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.(i) 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

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(ii)SCREEN PRINTING Screen printing is a commonly used technique for fast, inexpensive deposition of dye. 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 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(4,4bis(2,2-diphenylvinyl)-1,1biphenyl, 99.95% purity, Gracel), -NPD (N,N-diphenylN,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.(iii)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

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.

Figure 2.1 Inkjet Mechanism 2.2.(iv) 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.(v) 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

CHAPTER -:3 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

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.3.1Triplet State On the average, one singlet and three triplets are formed for every four electronhole 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 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.3.2 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.

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.3.3 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.

Figure.3.4 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.3.5 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

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.

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.3.6 Recombination Region

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.

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.

Figure.3.7 OLED 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.

Figure.3.8 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.

Figure.3.9 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

Figure.3.10 OLED 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.3.11 Foldable OLED

 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 truecolor 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.3.12 White OLED

3.4 COMPARISON OF OLED AND LCD Organic LED panel A luminous form

Self emission of light

Liquid Crystal Panel Back light or outside light is necessary

Consumption of

Electric It is lowered to about mW

Power

It is abundant when back light

though it is a little higher is used than the reflection

type

liquid crystal panel Colour Indication form

The fluorescent

material A colour filter is used.

of RGB is arranged in order and or a colour filter is used. High brightness

100 cd/m2

6 cd/m2

The dimension of the panel

Several-inches type in the It is produced to 28-inch type in future to about 10-inch

the future to 30-inch type.Goal

type.Goal Contrast

100:14

6:1

The thickness of the panel

It is thin with a little over

When back light is used it is

1mm

thick with 5mm.

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more than 1gm more than telephone.10 gm weak degree. the liquid crystal panel in the

case

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for

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Several ns ~ -10 *C

Range The corner of the view

Horizontal 180 *

Horizontal 120* ~ 170*

CHAPTER -: 4 ADVANTAGES & 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.

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.

 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.4.1 Samsung Galaxy Round , Blackberry Q30  OLED TVs- OLED TVs had begun shipping in 2013 but their prices are still very high.

Figure.4.2 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.4.3 Kodak LS633  OLED Lamps- OLED lamps are currently very expensive, but already several companies are offering these in the premium lighting category.

Figure. 4.4 Turn 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.4.5 Wallpaper Lighting

 Scroll Laptop

Figure. 4.6 Scroll Laptop  Rollable OLED television

Figure. 4.7 Toshiba 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 batterypowered 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-greenemitting 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(2phenylpyridine) iridium[Ir(PPY)3] ,a green electro phosphorescent material. Thus phosphorescent emission originates from a long-lived triplet state. 4.5 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, 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

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 1) http://impnerd.com/the-history-and- future-of-oled 2) http://jalopnik.com/5154953/samsung-transparent-oled-display-pitched-asautomotive-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.pocketlint.com/news/news.phtml/23150/24174/samsung-say-oled-notready.phtml 7) http://www.technologyreview.com/energy/21116/page1/ 8) http://www.voidspace.org.uk/technology/top_ten_phone_techs.shtml#k eep-your-eye-on-flexible-displays-coming-soon