Csk- Organic Display

ABSTRACT With the imaging appliance revolution underway, the need for more advanced handheld devices that will combine the attributes of a computer, PDA, and cell phone is increasing and the flat-panel mobile display industry is searching for a display technology that will revolutionize the industry. The need for new lightweight, low-power, wide viewing angled displays has pushed the industry to revisit the current flat-panel digital display technology used for mobile applications. Struggling to meet the needs of demanding applications such as e-books, smart networked household appliances, identity management cards, and display-centric handheld mobile imaging devices, the flat panel industry is now looking at new and revolutionary form of displays known as Organic Light Emitting Diodes (OLED). OLEDs offer higher efficiency and lower weight than many other types of displays, and are present in myriad forms that lend themselves to various applications. Many exciting virtual imaging applications will become a reality as new advanced OLED – on – silicon micro displays enter the market place over the next few years.

CONTENTS

1. INTRODUCTION

2

2. EVOLUTION OF DISPLAY TECHNOLOGIES

4

3. COMPARING TECHNOLOGIES ™ LIQUID CRYSTAL DISPLAYS

7

™ CATHODE RAY TUBES

8

4. PROS AND CONS

9

5. WHAT ARE OLEDs?

10

6. DESCRIPTION ™ HOW IT WORKS

11

™ THE LAYERS

12

7. POLEDs

16

8. IMPROVING POLED EFFICIENCY

18

9. TYPES OF OLED DISPLAYS

21

10. PASSIVE DISPLAYS

22

11. ACTIVE DISPLAYS

24

12. PIXEL UNIFORMITY

26

13. PRODUCTION OF OLEDs

30

14. A SOLID STATE SOLUTION

31

15. TOLEDs ™ TOLEDs CREATE NEW DISPLAY OPPORTUNITIES 16. FOLEDs ™ FOLEDs OFFER REVOLUTIONARY FEATURES

35 36 38 40

17. SOLEDs

42

18. ADVANTAGES OF OLEDs

43

19. DISADVANTAGES

45

20. FUTURE OUTLOOK AND APPLICATIONS

46

21. SUMMARY

48

22. REFERENCES

49

PRESENTED BY CHANDRA SEKHAR KARTHA S7 ECE 01 - 615

COORODINATOR: SMT. MUNEERA C R

INTRODUCTION

The field of semi conducting polymers has its root in the 1977 discovery of the semi conducting properties of polyacetylene. This breakthrough earned Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa the 2000 Nobel Prize in Chemistry for ‘the discovery and development of conductive polymers’. The physical and chemical understanding of these novel materials has led to new device applications as active and passive electronic and optoelectronic devices ranging from diodes and transistors to polymer LEDs, photodiodes, lasers, and solar cells. Much interest in plastic devices derives from the opportunities to use clever control of polymer structure combined with relatively economical polymer synthesis and processing techniques to obtain simultaneous control over electronic, optical, chemical, and mechanical features. With the imaging appliance revolution underway, the need for more advanced handheld devices that will combine the attributes of a computer, PDA, and cell phone is increasing and the flat-panel mobile display industry is searching for a display technology that will revolutionize the industry. The need for new lightweight, low-power, wide viewing angled, handheld portable communication devices have pushed the display industry to revisit the current flat-panel digital display technology used for mobile applications. Struggling to meet the needs of demanding applications such as e-books, smart networked household appliances, identity management cards, and display-centric handheld mobile imaging devices, the flat panel industry is now looking at new displays known as Organic Light Emitting Diodes (OLED).

For the preparation of the latest materials to prepare against this onslaught of demand for lighter and less power hungry display technologies, electrical engineers have enlisted the help of the humble jellyfish in their efforts to develop better light-emitting diodes (LEDs), according to a report published in the December 1 issue of the journal Advanced Materials. The Pacific Ocean jellyfish Aequorea victoria, it appears, produces just the sort of light that researchers try to coax from crystalline semiconductors such as gallium arsenide or indium phosphide. Moreover, the jellyfish accomplishes this with great efficiency: its light comes from a substance dubbed green fluorescent protein (GFP), which collects the energy produced in a certain cellular chemical reaction and emits it as green light from a molecular package known as a chromophore. An OLED is an electronic device made by placing a series of organic thin films between two conductors. When electrical current is applied, a bright light is emitted. This process is called electro phosphorescence. Even with the layered system, these systems are very thin, usually less than 500 nm (0.5 thousandths of a millimeter).

EVOLUTION OF DISPLAY TECHNOLOGIES

The rise in importance of electronic displays over the last forty years has been a direct consequence of the explosive proliferation of computers of all sizes, from the large mainframes of the 1960s and 1970s to the small handheld systems of the late 1990s. Initially, displays based on neon discharges were used to display binary and decimal digits, but these quickly gave way to displays which exploited the cathode ray tubes (CRTs) developed for television. Because of the economies of scale afforded by the huge television market, the CRT still represents nearly half of the information display market in dollars, and more than half in terms of units. In spite of the fact that the CRT is still the most economical technology for displaying 0.3–3.0 million picture elements (pixels), it has never been able to shed its most serious drawbacks of weight and volume. For this reason, even in the early days of television, engineers dreamed of making thin, light “flat-panel” displays that would capture the function of the CRT in a more attractive package, perhaps even one that would be easily portable. Unfortunately, the first major commercial flatpanel technology, based again on neon discharges, was not, when introduced, economically competitive with the CRTs it was intended to replace. As desktop displays, plasma displays (as they were then called) were too expensive and lacked the ability to render full color. For portable applications, they were too heavy and too inefficient. Since the advent of the personal computer in the early 1980s prompted everyone to look for a way to make a portable version, display attention shifted to liquid crystal displays (LCDs), which already had gained a reputation, in watches, for low power demands and low weight.

Early screen images produced on liquid crystal flat panels were grossly inferior to CRT images, even if the computers incorporating them were highly portable. In 1983, however, workers at Seiko–Epson produced a small, backlit liquid crystal display with color filters, and a thin- film transistor at each pixel site, that yielded for the first time a flat-panel display with the image characteristics of a color CRT [1]. Within five years, this technology had been developed to a point at which displays suitable for portable computers became feasible. Although this active-matrix LCD technology was not, and is not, competitive in cost with CRT technology, the new function of portability was so highly valued that a major industry was enabled by its existence. The market for portable computers of various sizes is now in the range of $50 billion per year. Looking back, it is clear that a critical milestone for triggering the growth of the market was the ability to provide an image competitive with that of the CRT, after which cost became secondary. In spite of their great success, even directly viewed flatpanel displays have their limitations as portable devices. For instance, if one wants to provide a large amount of information, i.e., a large number of pixels, it is necessary to use a large flat panel because of the limitations of the human visual system, especially in adults. But a large display is too cumbersome for use while walking. Also, with the convergence of information system technology and entertainment, exemplified by the DVD (digital versatile disk), creating a large-viewing-angle experience similar to that encountered in a cinema requires a very large flat-panel display. Now, the emphasis is on developing display technologies that will meet the requirements of the emerging communication and multimedia applications. This is where most experts believe that Organic LEDs with their many advantages will be able to provide a comprehensive solution.

The beginning of worldwide interest in OLEDs first started in the early 1980’s. Then in 1987 the breakthrough of Chin Tang and Steve Van Slyke, from KODAK, catapulted the OLED industry forward with the publication of their “Organic Electroluminescent Diodes” paper. They proposed the use of two organic layers instead of one to be deposited between the conducting metal layers. This proposed design change was the first of its kind, and that design model is still used today in many OLED device applications. Since Tang and Slyke’s 1987 discoveries our knowledge has increased to the point where we are beginning to see electronic devices using OLED displays instead of liquid crystal displays (LCD). But between conceptualization and implementation lies a long path which many budding technologies have taken long to traverse. Here are a few examples:

Comparing Technologies

Liquid Crystal Displays (LCDs)

For

comparison, LCDs, which

are

widely

used

today, are

nonorganic, non emissive light devices, which means they do not produce any form of light. Instead they block/pass light reflected from an external light source or provided by a back lighting system. The back lighting system accounts for about half of the power requirements for LCDs, which is the reason for their increased power consumption (over OLED technologies). LCD production involves the same sort of layering technique used in OLED displays, with some modification. First there is the formation of electrodes on two glass substrates. Then the substrates are joined together and the liquid crystals are sealed within them. Backlights are used to spread light out by a thin light diffuser. Finally the system is placed into a metal frame.

CATHODE RAY TUBES

Displays made from CRTs are produced using electron tubes in which electrons are accelerated by high-voltage anodes, formed into a beam by focusing electrodes, and projected toward a phosphorescent screen that forms one face of the tube. The electrons beam leaves a bright spot wherever it strikes the phosphor screen.

Pros and Cons

CRTs

•

Cost less and produce a display capable of more colors than LCD displays.

•

CRTs also use emissive technology, meaning that they can provide their own light - this means you can view images from any angle.

LCDs

•

LCDs have gained popularity due to their smaller, lighter form factor and their lower power consumption.

•

Many users report lower eyestrain and fatigue due to the fact that LCD displays have no flicker.

•

LCDs

emit

fewer

emissions than CRTs.

low-frequency

electromagnetic

What Are Organic Light Emitting Diodes (OLED)?

Organic Light Emitting Diode technology pioneered and patented by Kodak/Sanyo, enables full color, full-motion flat panel displays with a level of brightness and sharpness not possible with other technologies. Unlike traditional LCD’s, OLEDs are self-luminous and do not require backlighting, diffusers, polarizers, or any of the other baggage that goes with liquid crystal displays. Essentially, the OLED consists of two charged electrodes sandwiched on top of some organic light emitting material. This eliminates the need for bulky and environmentally undesirable mercury lamps and yields a thinner, more versatile and more compact display. Their low power consumption provides for maximum efficiency and helps minimize heat and electric interference in electronic devices. Armed with this combination of features, OLED displays communicate more information in a more engaging way while adding less weight and taking up less space. Invented in 1963 and envisioned as a slimmed-down replacement for bulky cathode ray tubes or as screens for wall mounted televisions – a use never realized due to problems scaling up to large surfaces – liquid crystal displays have instead become the standard for everything from watches to laptop computers.

Despite this, however, remains high

production and commercial expenses that have never come down enough to successfully mass market these displays, leaving the technology vulnerable to new innovations.

DESCRIPTION: How It Works: The basic OLED cell structure consists of a stack of thin organic layers sandwiched between a transparent anode and a metallic cathode. The organic layers comprise a hole-injection layer, a holetransport layer, an emissive layer, and an electron-transport layer. When an appropriate voltage (typically between 2 and 10 volts) is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light (electro luminescence). The structure of the organic layers and the choice of anode and cathode are designed to maximize the recombination process in the emissive layer, thus maximizing the light output from the OLED device. Both the electroluminescent efficiency and control of colour output can be significantly enhanced by "doping" the emissive layer with a small amount of highly fluorescent molecules.

THE LAYERS:

Today’s basic OLED structures may be comprised of four active layers of material to be deposited on top of a substrate. In choosing a good substrate candidate, one needs to find a material that has very smooth surface. Some possible choices for substrates are silicon, glass and plastic. The first layer to be deposited on top of the substrate is a transparent conducting anode layer. A current and widely used material for this part of the device is a 300nm layer of Indium Tin Oxide (ITO). The next thin layer is called the hole transport layer (HTL). An example of a compound that could be used here is N, N’-bis (3-methylphenyl)-[1,1’biphenyl]-4,4’-diamine (TPD, ~60nm). On top of the HTL layer is the thin electron transport layer (ETL). A proven ETL layer commonly used today is a 60nm thick layer of 8-tris-hydroxyquinoline aluminum (Alq3). This layer is where the recombination of holes and electrons occurs and the result is emitting light. Finally, a thin conductive cathode material with a low work function is applied over the ETL layer. A good candidate for this layer is 300nm of a combination of magnesium and silver (Mg: Ag) with a ratio of ten to one respectively. Since this layer is so thick, the light is emitted back through a transparent substrate. Another possible device structure consists of only three layers of material. This device is made up of a substrate coated with ITO, a single polymer to replace both the HTL and ETL layers, and a metal cathode layer. Polymers are beginning to be utilized more and more in OLED manufacturing due to a few appealing characteristics. First and foremost of these qualities is the ability of polymers to be incorporated into mass production processes with less difficulty than many organic compounds.

Rather than using a physical vapor deposition system for organic compounds, large ink presses or spin coaters can be used to deposit polymers. One polymer that is beginning to be incorporated into OLED production

is

Poly3,4-ethylenedioxythiophene/polystyrenesulfonate

(PEDOT/PSS).

This

polymeric

compound

is

both

transparent

and

conductive. Some experimentalists are using this compound as an interface in between the ITO layer and the HTL layer as a hole injection enhancer.

The key material is that used in the organic emitter layer. This semiconducting organic layer must contain a material with conjugated πbonds, but can be either a small molecule in a crystalline phase (small molecule OLEDs or SMOLEDs) or a polymer (polymer OLEDs or POLEDs). These molecules contain chemical groups called chromophores, which absorb incident energy and emit visible light. The color of the emitted light depends upon the precise composition of the material. Red, green, and

blue emissive materials can be used together to produce the full color spectrum. SMOLEDs use organic emissive materials with molecular weights up to several hundred daltons and do not contain repeating units as polymers do. Anthracene (three benzene rings fused in a chain) was the original material studied by Pope in his pioneering work on SMOLEDs. Fused benzene ring compounds (arenes) are now commonly used and recently another promising class of arenes, pentacenes (chain-like structures of five aromatic rings), has been found. Pioneered by Kodak, the technology has been licensed to IBM, UDX, and Ritek, who are now pursuing this approach. Researchers are working to overcome the common limitations of SMOLED organic emitters, which include limited efficiency in converting electrical energy into light (electroluminescence efficiency), rapid degradation upon contact with oxygen or water (which makes production difficult), and poor solubility (which leads to aggregation). For example, many SMOLED organic emitters incorporate transition metal-atom-containing species that convert a high fraction of the input electric charge to emitted light. Examples include adducts of the mercury trifunctional Lewis acid trimer and the arene compounds pyrene, naphthalene, and biphenyl. These adducts exhibit bright red, green, and blue phosphorescent emissions in the solid state at ambient temperatures. These trimer-arene adducts overcome three limitations of SMOLED emitters:

•

Typical

arenes

only

fluoresce,

limiting

electroluminescence

efficiency to 25%, while OLEDs containing phosphorescent trimerarene adducts have an upper limit of 100%;

•

Arene-arene intermolecular interactions in the solid state reduce device efficiency through self-quenching and excimer formation. These interactions are not present in trimer-arene adducts; and

•

The trimer-arene adduct compounds provide better control of color than arenes alone.

Encapsulation technology is also being developed to reduce the degradation that limits SMOLED operating life, while reducing the undesirable effects of aggregation in the organic emitter. For example, poly(benzylaryl ether) dendrimers have been used to encapsulate quinacridone. Because dendrimers are soluble in organic solvents, spincoating processes can be used. However, the limited solubility of SMOLED organic emitter materials means devices cannot usually be fabricated using solution processing but require more expensive vacuum vapor deposition.

POLED

The alternative approach to small molecules is to use polymers as the organic emitter layer. Cambridge Display Technologies developed the POLED approach, which is also now being pursued by Philips, who have a number of products (Fig. 3), DuPont and its subsidiary Uniax, and Dow Chemical. POLEDs consist of a thin (0.1 µm) two-layer polymer film sandwiched between the two electrodes. The bilayer film consists of an emitting polymer layer, such as polyparaphenylene or polyfluorene, atop a conducting polymer layer, such as a combination of polyaniline and polystyrenesulfonate or polyethylenedioxythiophene and polystyrene sulfonate.

The combination of the electrical properties of metals and semiconductors with the mechanical properties of polymers enables POLEDs to be deposited on flexible substrates. Such flexible displays could find applications in portable computers, electronic books, and billboard-

type displays. This is a unique advantage over both SMOLEDs and current display technologies. POLED displays also operate at a lower voltage and are more power-efficient than SMOLED displays. In addition to these advantages, manufacturing costs are lower because solution processing (spin coating) and ink-jet printing methods can be used instead of vacuum vapor deposition.

However, there are important areas in which POLED technology needs improvement, including increased electroluminescence efficiency and longer operating life, particularly for blue light emitters.

Improving POLED efficiency

Until recently, it was thought that the light-emitting polymers used in POLEDs were inherently limited in efficiency, able to convert no more than 25% of their energy into light because of spin statistics. The theoretical studies of Jean-Luc Brédas of Georgia Institute of Technology indicate that higher efficiencies are possible. Brédas proposes that the key to higher efficiency is the two-step charge recombination process that begins when initially separated charges combine to form a loosely bound charge-transfer state (Fig. 4a). When the opposite charges meet, they neutralize one another and produce a singlet or triplet excited state (exciton) (Fig. 4b). The decay of that excited state results in the emission of light. During the chargerecombination process, the spin directions of the electrons involved can orient themselves into four possible combinations, each with an equal statistical likelihood. The first pattern, a ‘singlet’, can have only one of the four possible spin combinations. The other, a ‘triplet’, can have three different combinations. Thus spin statistics predict that singlets will be formed in only 25% of charge recombinations, and only singlets produce light in π-conjugated polymers. Brédas has shown theoretically that systems built from long polymer chains should be able to boost the percentage of light-emitting singlets to as high as 50%. This is because, with increasing molecular weight, triplets take longer to convert to neutral excitons. During this time, the triplet state can convert to a singlet, while singlet conversion to excitons remains rapid. As a result, spin statistics become biased in favor of singlet

formation, which accounts for more than 25% of the four possible spin combinations. This results in an increase in POLED efficiency beyond the 25% limit. The π -conjugated polymer molecular weight required is still being defined, however. “These results are important in the sense that they lead to an understanding of why polymer LEDs can have an efficiency that goes beyond the 25% limit predicted on the basis of simple spin statistics,” says Brédas. Besides improving efficiency by using longer polymer backbones to increase molecular weight, Brédas is also investigating the use of chemical group substituents on the polymer backbone to improve efficiency. Eric Meulenkamp, principal scientist at Philips Research, has reported a different method of increasing POLED efficiency. By dispersing a phosphorescent ‘guest’ material into a light-emitting polymer ‘host’, it is possible to use all the excited states, both singlet and triplet, for light emission provided that the triplet energy gap of the host is higher than that of the guest. Scientists at Philips Research and TNO Industrial Technology have developed a proprietary copolymer suitable for hosting a green triplet emitter and providing a high luminous efficacy of 24 cd/A. Still higher efficiencies and efficient blue emission could be achieved by further optimizing the copolymer composition. Meulenkamp also reports increasing POLED efficiency using a proprietary anode layer. The novel anode significantly reduces losses that arise from imbalances in the hole and electron partial currents. With present anode layers, the hole current can far exceed the electron current. This results in significant energy wastage, since the excess holes

cannot combine with electrons to generate light. The new proprietary anode layer introduces a barrier to hole injection, thereby reducing the number of excess holes. With balance between holes and electrons at high voltage, efficiency is increased from 2-4% in conventional devices to around 12%. This translates into a luminous efficacy of 35 cd/A for a yellow light-emitting polymer and 20 cd/A for blue.

TYPES OF OLED DISPLAYS

OLED displays are typically of two major types: Passive-matrix displays and Active-matrix displays.

Passive Displays:

The passive-matrix OLED display has a simple structure and is well suited for low-cost and low-information content applications such as alphanumeric displays. It is formed by providing an array of OLED pixels connected by intersecting anode and cathode conductors. Organic materials and cathode metal are deposited into a “rib” structure (base and pillar), in which the rib structure automatically produces an OLED display panel with the desired electrical isolation for the cathode lines. A major advantage of this method is that all patterning steps are conventional, so the entire panel fabrication process can easily be adapted to large-area, high-throughput manufacturing. To get a passive-matrix OLED to work, electrical current is passed through selected pixels by applying a voltage to the corresponding rows and columns from drivers attached to each row and column. An external controller circuit provides the necessary input power, video data signal and multiplex switches. Data signal is generally supplied to the column lines and synchronized to the scanning of the row lines. When a particular row is selected, the column and row data lines determine which pixels are lit. A video output is thus displayed on the panel by scanning through all the rows successively in a frame time, which is typically 1/60 of a second. The first OLED displays, like the first LCD (Liquid Crystal Displays), are addressed as a passive matrix. This means that to illuminate any particular pixel, electrical signals are applied to the row line and column line (the

intersection of which defines the pixel). The more current pumped through each pixel diode, the brighter the pixel looks to our eyes. OLEDs were recently demonstrated as the light emitting component in a passively addressed display product. These passive matrix displays demonstrate the feasibility of OLEDs in these applications, but encounter a fundamental barrier as the display size and pixel density increase. Since the luminous output of an OLED is proportional to the charge injected through the device, the current densities required to operate passively addressed displays rapidly rise as the time available to drive each pixel decreases with increasing display resolution. These high currents cause large voltage drops in the ITO lines of the passive array, pushes the OLED operation to higher voltages and creates display driver issues that are not easily resolved.

Active Displays:

In contrast to the passive-matrix OLED display, active-matrix OLED has an integrated electronic back plane as its substrate and lends itself to high-resolution, high-information content applications including videos and graphics. This form of display is made possible by the development of polysilicon technology, which, because of its high carrier mobility, provides thin-film-transistors (TFT) with high current carrying capability and high switching speed. In an active-matrix OLED display, each individual pixel can be addressed independently via the associated TFT’s and capacitors in the electronic back plane. That is, each pixel element can be selected to stay “on” during the entire frame time, or duration of the video. Since OLED is an emissive device, the display aperture factor is not critical, unlike LCD displays where light must pass through aperture. Therefore, there are no intrinsic limitations to the pixel count, resolution, or size of an active-matrix OLED display, leaving the possibilities for commercial use open to our imaginations. Also, because of the TFT’s in the active-matrix design, a defective pixel produces only a dark effect, which is considered to be much less objectionable than a bright point defect, like found in LCD’s. Moving to an active matrix drive scheme can overcome a number of the issues posed by passively driven display schemes. The goal of the active matrix OLED (AMOLED) display is to generate a constant current

source at each pixel using thin film transistors which, in this work, were made in a polysilicon technology. Each pixel is programmed to provide a constant current during the entire frame time, eliminating the high currents encountered in the passive matrix approach. However, polysilicon thin film transistors suffer from significant initial output characteristic non-uniformity due to the nature of the polysilicon crystal growth. This makes it difficult to create a uniform current source at each pixel. A new AMOLED pixel has been designed which addresses this issue and the results which were presented show the improved performance compared to a standard pixel.

Pixel Uniformity

The most critical issue in the design of an AMOLED pixel is the pixel to pixel luminance uniformity. Driving the OLED with a constant current provides the best pixel to pixel uniformity since the OLED threshold variations no longer impact the charge passed through the devices. This requires that the active components at each pixel of the AMOLED display provide a constant current to the OLED. OLEDs are presently fabricated with the anode connected to the ITO on the active matrix plate and the cathode connected to a metal alloy back plane such as Mg:Ag or Al:Li. The effect of OLED threshold variations can be eliminated by using a PMOS device on the active plate since the OLED will be connected to the drain of the PMOS transistor. In this configuration, the transistor will provide a constant current to the OLED as long as the transistor stays in saturation.

FIGURE 1. Two-Transistor AMOLED Pixel

If an NMOS device were used on the active matrix plate, the OLED would be connected to the source of the transistor and additional techniques would be needed to ensure that the OLED threshold variation did not affect the gate to source voltage of the transistor. PMOS polysilicon devices are available, so the pixel designs use a PMOS device to drive the OLED and the transistor is operated in saturation to overcome the OLED threshold variations. The simplest pixel, shown in the above figure, uses two transistors: one drives the current for the OLED, MP2, and another, MP1, acts as a switch to sample and hold a voltage on the gate of the drive transistor. The major cause of luminance non-uniformity in the two-transistor pixel is the variation in the drive transistor MP2. This transistor has a high level of output characteristic variation due to the nature of the polysilicon grain growth. This local non-uniformity can be improved by process refinements, but it is difficult to reduce the variation across the array to a level necessary for good gray scale control. For example, if the pixel configuration of Fig. 1 was designed to provide maximum brightness with 3 volts of overdrive on the drive transistor, MP2, then the voltage division for one gray level with 8 bits of gray scale is 3 volts/256 = 12 mV. The threshold variation across the 2.7 inch diagonal display was 300 mV with local variations on the order of 100 mV. These numbers indicate that, even with a high quality process, the transistor threshold variations are much too large to provide the pixel to pixel uniformity necessary for a high quality flat panel display. This conclusion is confirmed in Fig. 2 which shows the visible effects of transistor non-uniformity on the two transistor pixel design of Fig. 1.

An improved AMOLED pixel has been designed which eliminates the effects of the polysilicon transistor threshold voltage variation. The advantage of the four-transistor pixel is that it uses an autozero cycle to reference the data against the transistor threshold voltage, eliminating the effects of the transistor threshold voltage variation. The improvement can be seen by comparing Fig. 2 where there is an obvious, visible improvement in the pixel to pixel luminance uniformity in the four transistor test pixel array.

Power

dissipation

in

an

OLED

display

is

a

critical

issue.

Measurements and modeling show that the temperature increase due to power dissipation in the active plate and in the OLEDs can be severe because the heat transfer from the display to the ambient is relatively inefficient. It is essential that the active matrix electronics consume a minimum of power since the OLEDs themselves dissipate power while generating light.

It is important to note that while the OLEDs dissipate considerable power, the overall efficiency can be over 3 lm/W with all the drive electronics while AMLCDs are typically 1 to 2lm/W. The design trade off to address in the active matrix electronics design is that, while some power must be consumed in order to provide current control, this power must be kept to a minimum. Reduced power dissipation in the supporting electronics leads to a smaller control voltage range with a subsequent requirement of more accurate electronics. The electronics must provide the necessary gray scale control while dissipating as little power as possible. Finally, the pixel should use a minimum of control lines, storage capacitors and transistors. For example, the two-transistor pixel of Fig. 1 requires two transistors, select and data control lines, one power line and a storage capacitor. The four-transistor pixel requires more components than the two-transistor pixel. This may be the greatest weakness of the four transistor pixel but the extra devices are essential to provide the necessary pixel to pixel luminance uniformity.

PRODUCTION OF OLEDs

As OLED materials are extremely thin -- and some are chemically reactive and oxidize immediately on exposure to water or oxygen, creating black spots that ruin the display -- they can be 10,000 times more sensitive to moisture and oxygen than LCDs. To protect them, display makers currently use glass as the display substrate (the same as LCDs) and glue a glass lid oil top, with a desiccant powder inside the display to absorb moisture that comes through the glue line. This design works but is awkward and costly.

A SOLID STATE SOLUTION

Currently, a number of FPD (Flat Panel Display) makers are evaluating

a

thin-film

solution

that

offers

moisture

and

oxygen

permeability approximately equal to a sheet of glass. It comprises alternating layers of polymer and ceramic films applied in vacuum. The total thickness of the coating is only ~3[micro]m, and it can be applied directly on top of an OLED display, eliminating mechanical packaging components. A liquid precursor is flash-evaporated to a gas, which then flows into a vacuum chamber where it condenses back to a liquid and onto a substrate. It is not a traditional vacuum process such as evaporation, sputtering, or chemical vapor deposition. All these are gas-to-solid deposition processes in which atoms or molecules hit a substrate in a lineof-sight path and are converted back to the solid state. By their very nature, these deposition processes create conformal layers that have the same topography and surface roughness as the underlying substrate. In contrast, the polymer layer formed in this new vacuum process is actually condensation of gas to liquid. The precursor gas molecules travel to the substrate and condense on all its surfaces, thereby encapsulating and planarizing the entire structure. The coating covers all the imperfections and provides a flat surface. In addition, because it is a liquid, the flat surface of the monomer is atomically smooth. The substrate next moves to an ultraviolet light source,

which polymerizes the liquid to create a solid polymer film, still with an atomically smooth top surface. This provides an ideal surface on which to deposit a barrier film. Next, a ceramic film, ~500 [Angstrom] thick, is deposited on top of the polymer layer. Because the surface is so smooth, the ceramic film has very few defects and is therefore an almost perfect moisture barrier. An OLED display, however, requires an even better barrier, so the process is repeated, creating a stack of multiple polymer and ceramic layers in which each ceramic film is a near-perfect moisture barrier. This combination of ceramic and polymer layers, with a total thickness of ~3microm, creates a moisture barrier with a water permeability in the range of [10.sup.-6]gm of water/[m.sup.2]/day. This is the water impermeability required by an OLED display. The application of this multilayer polymer/ceramic encapsulating thin film is challenging, and the following factors have to be considered: •

The organic emissive layers in the OLED display are extremely thin, on the order of nanometers, and have little mechanical strength. Subjecting the OLED layers to shear stresses when the monomer is polymerized and solidifies, which involves about 2% shrinkage, is a concern.

•

OLED materials are sensitive to the UV light used to initiate polymerization. Consequently, the formulation of the monomer as well as the UV intensity and duration must be carefully controlled. This is especially critical with top-emission displays, since they have transparent cathodes and the OLED layers would be directly exposed to the UV light.

•

A plasma is typically used in depositing the ceramic barrier layers, which can also damage the OLED layers and must be carefully controlled.

•

Temperature excursions during UV curing and sputtering must be avoided, as many OLED materials would be damaged by temperatures 100˚C.

•

As with most manufacturing processes for semiconductors or flat-

panel displays, applying thin-film encapsulation to an OLED display demands tight control of particles. The thin-film encapsulation tool is connected directly to the OLED vacuum tool, where particulate is already tightly controlled. It is essential to ensure that the organic and inorganic deposition processes used in building the multilayer barrier stack do not create any particulates.

•

Finally, to qualify the thin-film encapsulation process, encapsulated

OLED displays must be subjected to high temperature and humidity-typically 60[degrees]C/90%RH for 500 hours--as well as thermal shock testing to ensure that the displays will satisfy the requirements of mobile electronic-device manufacturers (i.e., for cell phones and PDAs). Besides being extremely thin yet impermeable to water, the Barix coating is also transparent to visible light. This means OLED display makers could conceivably avoid having to make a bottom-emission display in which the light path is partially blocked by the TFT silicon transistors on the

substrate, thereby reducing the display efficiency and placing a limit on resolution. If the mechanical packaging--metal cans, glass lids, and desiccant--were replaced by transparent thin-film encapsulation, then the display could be designed so that all the light exits the top of the display, significantly boosting efficiency and enabling much higher resolution. This efficiency increase means more than just saving electrical power. The other limiting factor with OLED displays (aside from protection from moisture) is the lifetime of the emissive materials, especially blue emitters. Unlike LCDs, which are voltage-driven, OLED displays are current-driven. Moreover, the amount of current that flows through the emissive materials has a major effect on lifetime. More efficient top-emitter displays require much less current for a given brightness because they avoid the inefficiencies of bottom-emitters where light is partially blocked, and thus have longer lifetimes. A thin-film moisture barrier that meets OLED display requirements is therefore an enabling technology, for OLED TVs.

TOLED

The Transparent OLED (TOLED) uses a proprietary transparent contact to create displays that can be made to be top-only emitting, bottom-only emitting, or both top and bottom emitting (transparent). TOLEDs can greatly improve contrast, making it much easier to view displays in bright sunlight. Because TOLEDs are 70% transparent when turned off, they may be integrated into car windshields, architectural windows, and eyewear. Their transparency enables TOLEDs to be used with metal, foils, silicon wafers and other opaque substrates for top-emitting devices.

TOLED Creates New Display Opportunities:

•

Directed top emission: Because TOLEDs have a transparent structure, they may be built on opaque surfaces to effect top emission. Simple TOLED displays have the potential to be directly integrated with future dynamic credit cards. TOLED displays may also be built on metal, e.g., automotive components. Top emitting TOLEDs also provide an excellent way to achieve better fill factor and characteristics in high resolution, high-information-content displays using active matrix silicon backplanes.

•

Transparency: TOLED displays can be nearly as clear as the glass or substrate they're built on. This feature paves the way for TOLEDs to be built into applications that rely on maintaining vision area. Today, "smart" windows are penetrating the multi-billion dollar flat glass architectural and automotive marketplaces. Before long, TOLEDs may be fabricated on windows for home entertainment and teleconferencing purposes; on windshields and cockpits for navigation and warning systems; and into helmet-mounted or "head-up" systems for virtual reality applications.

•

Enhanced

high-ambient

contrast:

TOLED

technology

offers

enhanced contrast ratio. By using a low-reflectance absorber (a black backing) behind either top or bottom TOLED surface, contrast ratio can be significantly improved over that in most reflective LCDs and OLEDs. This feature is particularly important in daylight readable applications, such as on cell phones and in military fighter aircraft cockpits.

•

Multi-stacked devices: TOLEDs are a fundamental building block for many multi-structure and hybrid devices. Bi-directional TOLEDs can provide two independent displays emitting from opposite faces of the

display.

With

portable

products

shrinking

and

desired

information content expanding, TOLEDs make it possible to get twice the display area for the same display size.

FOLEDs

FOLEDs are organic light emitting devices built on flexible substrates. Flat panel displays have traditionally been fabricated on glass substrates because of structural and/or processing constraints. Flexible materials have

significant

performance

advantages

over

traditional

glass

substrates. In display technology, FOLED (flexible organic light emitting device) is an organic light emitting device (OLED) built on a flexible base material, such as clear plastic film or reflective metal foil, instead of the usual glass base. FOLED displays can be rolled up, folded, or worn as part of a wearable computer. The devices are said to be lighter, more durable, and less expensive to produce than the traditional glass-based alternatives. FOLED's light-weight base materials significantly decrease the overall weight of a screen. This capacity makes FOLED displays especially useful for portable devices, such as laptop computers and other displays where weight is a consideration, such as large wall-mounted screens. Furthermore, a FOLED display is less prone to breakage than a glass-based display and compared to the silicon-based LCD displays used for small displays and flat-screen monitors, are much less expensive to produce.

Time Magazine named Universal Display Corporation (UDC)'s rollable

FOLED

wireless

monitor

prototype

one

of

the

best

10

environmentally -- friendly technologies for 2002. UDC is working on such FOLED-based products as rollable, refreshable electronic newspapers and video screens embedded in car windshields, walls, windows, and office partitions. According to UDC, such products could be on the market within five years.

FOLEDs Offer Revolutionary Features for Displays:

•

Flexibility: For the first time, FOLEDs may be made on a wide variety of substrates that range from optically-clear plastic films to reflective metal foils. These materials provide the ability to conform, bend or roll a display into any shape. This means that a FOLED display may be laminated onto a helmet face shield, a military uniform shirtsleeve, an aircraft cockpit instrument panel or an automotive windshield.

•

Ultra-lightweight, thin form: The use of thin plastic substrates will also significantly reduce the weight of flat panel displays in cell phones, portable computers and, especially, large-area televisions-on-thewall. For example, the weight of a display in a laptop may be significantly reduced by using FOLED technology.

•

Durability: FOLEDs will also generally be less breakable, more impact resistant and more durable compared to their glass-based counterpart.

•

Cost-effective processing: OLEDs are projected to have fullproduction level cost advantage over most flat panel displays. With the advent of FOLED technology, the prospect of roll-to-roll processing is created. To this end, a continuous organic vapor phase deposition (OVPD) process for large-area roll-to-roll OLED processing has been demonstrated. While continuous web FOLED processing requires further development, this process may provide the basis for very low-cost, mass production.

SOLED

SOLED (Stacked Organic Light - Emitting Diode device) is a display technology from the Universal Display Corporation (UDC) that uses a stack of transparent organic light-emitting devices (TOLEDs) to improve resolution and enhance full-color quality. SOLEDs use a pixel architecture developed at UDC that stacks sub pixels (the red, blue and green elements in each pixel) vertically rather than arranging them side by side, as is usually done in CRT and LCD displays. Within a SOLED display, each sub-pixel element can be controlled independently. Pixel color can be adjusted by varying the currents through the three color elements and gray scale can be adjusted by pulse-width modulation. Brightness is controlled by manipulating current through the stack. According to UDC, their SOLED technology enables a three-fold improvement in resolution and better color quality over CRT and LCD displays. The company expects that SOLEDs may in the future enable high resolution Web-enabled devices.

Advantages of OLED displays:

•

Robust Design - OLED’s are tough enough to use in portable devices such as cellular phones, digital video cameras, DVD players, car audio equipment and PDA’s.

•

Viewing Angles – Can be viewed up to 160 degrees, OLED screens provide a clear and distinct image, even in bright light.

•

High Resolution – High information applications including videos and graphics, active-matrix OLED provides the solution. Each pixel can be turned on or off independently to create multiple colors in a fluid and smooth edged display.

•

“Electronic Paper” – OLED’s are paper-thin. Due to the exclusion of certain hardware goods that normal LCD’s require, OLED’s are as thin as a dime.

•

Production Advantages – Up to 20% to 50% cheaper than LCD processes. Plastics will make the OLED tougher and more rugged. The future quite possibly could consist of these OLED’s being produced like newspapers, rather than computer “chips”.

•

Video Capabilities – They hold the ability to handle streamlined video, which could revolutionize the PDA and cellular phone market.

•

Hardware Content – Lighter and faster than LCD’s.

Can be

produced out of plastic and is bendable. Also, OLED’s do not need lamps, polarizers, or diffusers.

•

Power Usage – Takes less power to run (2 to 10 volts).

Disadvantages:

•

Engineering Hurdles – OLED’s are still in the development phases of

production.

Although

they

have

been

introduced

commercially for alphanumeric devices like cellular phones and car audio equipment, production still faces many obstacles before production.

•

Color – The reliability of the OLED is still not up to par. After a month of use, the screen becomes non uniform. Red and blues die first, leaving a very green display. 100,000 hours for red, 30,000for green and 1,000 for blue. Good enough for cell phones, but not laptop or desktop displays.

•

Overcoming LCD’s – LCD’s have predominately been the preferred form of display for the last few decades. Tapping into the multi-billion dollar industry will require a great product and continually innovative research and development. Furthermore, LCD manufacturers will not likely fold up and roll over to LCD’s. They will also continue to improve displays and search for new ways to reduce production costs.

Future Outlook:

The OLED technology faces a bright future in the display market, as the ever-changing market environment appears to be a global race to achieve new success. Eventually, the technology could be used to make screens large enough for laptop and desktop computers. Because production is more akin to chemical processing than semiconductor manufacturing, OLED materials could someday be applied to plastic and other materials to create wall-size video panels, roll-up screens for laptops, and even head wearable displays. Close to 100 manufacturers are at work developing applications for organic light emitters. Here are some examples:

Application: Small displays Uses: Personal electronic equipment Products (Display Makers): Digital camera (Kodak/Sanyo); cellular phones (Pioneer, RiTdisplay); car audio components (Pioneer, TDK); electric razor (Philips) Status: On the market

Application: Large displays Uses: TVs; computers; billboards; vehicle windshields Products (Display Makers): 15.5-inch OLED (Samsung SDI); 17-inch PLED (Toshiba); 20-inch OLED (ChiMei/IBM); 24-inch multipanel screen (Sony) Status: Prototype; two to four years from market

Application: Bendable displays Uses: Clothing; portable devices Products (Display Makers): Wearable computer (Pioneer); rollable display (Universal Display Corporation) Status: Prototype; several years from market

The OLED market appears to be expanding at a rapid pace. Sales of passive OLED displays rose from $2 million to $18 million this year. Projected sales by 2005 are expected to reach $717 million, with active matrix sales accounting for half of that.

Summary:

The Organic Light Emitting Diode forms of display still have many obstacles to overcome before it’s popularity and even more importantly, its reliability are up to par with standards expected by consumers. Although the technology presents itself as a major player in the field of displays, overcoming these obstacles will prove to be a difficult task. However, the OLED’s advantages over LCD’s and future outlook have many in the industry goggle-eyed at the realm of possibilities. For all we know and can hope for OLED’s could change the ways in which we see things.

BIBILIOGRAPHY

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