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ABSTRACT
PHOLED (phosphorescent organic light-emitting device) is a proprietary display technology developed by the Universal Display Corporation (UDC) that uses soluble phosphorescent small molecule materials to create organic light-emitting devices (OLEDs). PHOLED technology works on the principle that certain organic molecules emit light when an electric current is applied.
Although PHOLED was originally developed as a display technology for use in mobile phones, the U.S. Department of Energy . PHOLED-based lighting sources, such as glowing walls, are expected to provide cost-effective and energy-efficient alternatives to traditional incandescent and fluorescent lighting.
INTRODUCTION
A PHOLED is used to signify a phosphorescent OLED. This type of technology is currently under development by a Ewing, New Jersey company called Universal Display Corporation.
Like all types of OLEDs, PHOLEDs function via the following method: an electric current is applied to organic molecules, which then emit bright light. However, PHOLEDs use "...the principle of electro phosphorescence to convert up to 100% of the electrical energy in an OLED into light." . In comparison, traditional fluorescent OLEDs only convert approximately 25-30% of electrical energy into light. LCD displays, the current favorite technology for use in flat-screen devices, are even less efficient, converting only 10% of electrical energy into light.
Due to their extremely high level of energy efficiency, even when compared to other OLEDs, PHOLEDs are being studied for potential use in large-screen displays such as television monitors or TV screens, as well as general lighting needs. One potential use of PHOLEDs as lighting devices is to cover walls with gigantic PHOLED displays. This would allow entire rooms to glow uniformly, rather than require the use of light bulbs which distribute light unequally throughout a room.
The United States Department of Energy has recognized the potential for massive energy savings via the use of this technology and therefore has awarded $200,000 in contracts to develop PHOLED products for general lighting applications.
DISPLAY TECHNOLOGIES
Today, there are many different types of display technologies in the marketplace, each with their own unique characteristics to address specific applications. Below is a sample list of popular approaches used in display systems:
¢ Cathode Ray Tubes (CRT)
The technology used in most televisions and computer displayscreens. A CRT works by moving an electron beam back and forth across the back of the screen.
¢ Field Emission Displays (FED)
FED is a flat cathode ray tube that uses a matrix-addressed cold- cathode to produce light from a cathodoluminescent phosphor screen.
¢ Organic Light Emitting Diodes (OLED)
Unlike LCDs, which require backlighting, OLED displays are emissive devices that give off light rather than modulating transmitted light or reflected light.
¢ Plasma Display Panels (PDP)
Technology that operates by controlling discharges from ionized gases.Plasma displays are becoming an alternative to LCD as they can be easily manufactured in a large format. Compared to conventional CRT displays, plasma displays are about one-tenth the thickness--around 4'', and one-sixth the weight.
¢ Light Emitting Diodes (LED)
An LED is an electronic device that lights up when electricity is passed through it. LEDs have been around for decades and are used in everything from car dashboards, to large indoor/outdoor display systems, to portable electronics as indicator lamps.
¢ Liquid Crystal Displays (LCD)
Function through the use of pixels composed of liquid crystals that have the ability to polarize light in the presence of an electronic field. The core advantage to LCD is the usefulness, relatively low power consumption, and high contrast.
WHAT IS OLED
OLED (organic light-emitting diodes) is a display technology, pioneered and patented by Kodak, based on the use of organic polymer material as the semiconductor material in light-emitting diodes (LEDs). A polymer can be a natural or synthetic substance and macro or micro in size. Examples of organic polymers include proteins and DNA. OLED displays are used in cellular phones, digital video cameras, digital versatile disc (DVD) players, personal digital assistants (PDAs), notebooks, car stereos, and televisions. OLED displays are thinner and weigh less because they do not require backlighting. OLED displays also have a wide viewing angle up to 160 degrees even in bright light, and they use only two to ten volts to operate.
New technologies that build on the OLED include FOLED (flexible organic light-emitting display), which promises to make highly portable, roll-up displays possible within the next few years.
PROBLEMS WITH OLED
OLED seem to be the perfect technology for all types of displays, but
they also have some problems:
¢ Lifetime - While red and green OLED films have long lifetimes (10,000 to 40,000 hours), blue organics currently have much shorter lifetimes (only about 1,000 hours).
¢ Manufacturing - Manufacturing processes are expensive right now.
¢ Water - Water can easily damage OLEDs.
BACKGROUND OF PHOLED
PHOLEDs are energy conversion devices (electricity-to-light) based on electroluminescence.PHOLEDs first attracted the attention of researchers in the 1970s because of their potentially high quantum efficiency of luminescence and the ability to generate a wide variety of colors. Unfortunately, their high operating voltages (>1000V) prohibited them from becoming practical devices. However, in 1987, after C. Tang and Van Slyke (5) from Eastman Kodak devised a heterostructure double layered device containing active small molecules that combined a low operating voltages (<10V) with good brightness (>1000 cd/m2) and respectable luminous efficiency (1.5 lm/W), research gained the momentum.
PHOLEDs, both small molecular and polymeric, have already achieved emission in all colors of the spectrum - including white. Fine-tuning to any desired shade of color can be achieved by selecting an appropriate emitter or a mixture of emitters with the right emission spectra. Literally hundreds of emitters are already known and have been tested. Some are more efficient than others, and many more will be synthesized and optimized. Since the selection of basic structures and properties-modifying substituents of all types and sizes is nearly unlimited, organic chemistry provides endless opportunities in designing the desired color.
FEATURES OF PHOLED
These are the some of the features of PHOLED
¢ Vibrant colors
¢ High contrast
¢ Excellent grayscale
¢ Full-motion video
¢ Wide viewing angles from all directions
¢ A wide range of pixel sizes
¢ Low power consumption
¢ Low operating voltages
¢ Wide operating temperature range
¢ Long operating lifetime
¢ A thin and lightweight form factor
¢ Cost-effective manufacturability
DEVICES
PHOLEDs are extremely thin, practically twodimensional multi-layer devices of large square area. The thickness of all the active layers combined is only of the order of one hundred nanometers. This feature will be a benefit for applications where space is a premium, such as in airplanes. The nearly two-dimensional nature of PHOLEDs makes them also suitable for manufacturing by roll-to-roll coating technologies, which are inherently low-
cost. The roll-to-roll coating technologies operating at speeds of about 20 ft/sec have already been used successfully in manufacturing of organic photoreceptors for laser printers, with extremely stringent thickness tolerances. There is no restriction on the size and shape of the PHOLED devices. Every conceivable shape and form can be envisioned, and only the human imagination is the limiting factor.
The devices can be in form of fibers, and woven to fabrics. They can be on bent or rolled films or constitute the surface of spheres. For lighting applications, thin flat sheets possibly using thin glass substrates will probably be the shape of choice. The intensity of light can be controlled by conventional types of dimmers. The devices are conceptually simple, but the details of their structure are complicated. Many changes will be made before the final design is established.
MATERIALS
PHOLED devices contain the substrate materials, electrodes and functional organic substances. All are environmentally safe. Two types of electrodes are used. An extremely thin layer of indium - tin oxide (extremely thin, because it has to be optically transparent), is used as anode. Low work-function metals such as Mg, Li, and their alloys with Ag, and in some cases
Al, are now used as cathodes. Several types of organic materials are used in the functional layers: Polymers or small molecules that transport injected charges to the recombination zone, fluorescent or recently introduced more efficient phosphorescent dopants that emit light, and charge-injection modifying compounds, such as conducting polymers near the anode or salts such as LiF or CsF etc near the cathode.
The charge transporting polymers are typically polyconjugated, such as derivatives of polyphenylene vinylene. The small molecules are substituted aromatic amines for hole transport and a variety of polynuclear aromatic complexes with high electron affinity for electron transport. In some cases, the charge transporting polymers or small molecules themselves assume the role of emitters. These materials can be deposited in many separate layers or mixed into one or several layers. Organic chemistry offers an endless variety of structures, and therefore, the choices of charge transporters, emitters and other dopants are virtually unlimited. Polymeric PHOLEDs have the advantage that the active layers can be deposited from solution, while in "small molecule" PHOLEDs, the active layers are typically deposited by vapor deposition techniques
WHITE LIGHT FROM PHOLEDs
PHOLEDs are uniquely suitable as sources of white light. The structure of light emitting fluorescence or phosphorescence additives can be tailored to emit any desired color. Mixing light from two or more sources (dopants or layers) gives light whose color is determined by the weighted average of the CIE coordinates of these sources. Given the enormous variety of known and yet-to-be synthesized dopants, both fluorescent and phosphorescent, with broad emission spectra of choice, practically any shade of white or any "temperature" of white light can be generated in PHOLEDs. Many devices have already been made in the laboratory scale and tested and some of them almost perfectly simulate the sunlight.
RED PHOLED EMITTERS IN COMBINATION WITH NSCCâ„¢s HOST MATERIALS
Tokyo, Japan and Ewing, New Jersey “ Nippon Steel Chemical Co., Ltd. (NSCC), the Company that™s a leading manufacturer of OLED materials and owns the equipment and the technology for manufacturing super-purified products with consistent quality in commercial scale in advance of any other manufacturers in the world, and Universal Display Corporation (NASDAQ: PANL), the Company that™s lighting the way in developing and commercializing OLED technology for flat panel displays, lighting and other opto-electronics with its proprietary PHOLED or phosphorescent OLED technology, today announced a collaboration to develop new markets for red phosphorescent OLED materials. As the background of this collaboration, there is the fact that a commercially-available red host material from NSCC is compatible with commercially-available red phosphorescent OLED emitters from Universal Display.
By using red phosphorescence, overall power consumption of the display can be reduced by more than 40% as compared to using fluorescent red OLEDs. The combination of Universal Displayâ„¢s red PHOLED materials and NSCCâ„¢s host material, called NS11, offers very efficient, highly desirable red colors with long operational lifetime. These products are currently available for evaluation and commercial use from the two companies.
Since the time when fluorescent materials were introduced into the OLED market, we started to supply the OLED materials as commercial base. Since then, we have been involved in providing our materials to most of the OLED manufacturers, which have already started to mass-produce OLED devices. However, we recognize that it should be very difficult for the fluorescent OLED system to compete against the LCD system because of the small distinction in power consumption between the both systems. In order to pursue the further enlargement of the OLED market, we consider that it must become a key factor to introduce phosphorescent materials into the OLED market. Based on this point of view, we have mainly focused our efforts on the development of phosphorescent materials.
Universal Display™s proprietary PHOLED technology offers up to four times higher efficiency than conventional OLED technology “ a feature that is very important for today™s battery-operated cell phones and other portable devices, as well as for tomorrow™s large-area TVs and solid-state lighting products.
Over the past few years, the Company has announced a series of record-breaking performance milestones for its red, green and blue PHOLED systems. Universal Displayâ„¢s vacuum-deposited PHOLED materials, manufactured by PPG Industries exclusively for Universal Display, are currently being evaluated and used in commercial production by a number of electronics manufacturers.
EWING, N.J. and DRESDEN, Germany - Phosphorescent organic light-emitting diode (PHOLED) developer Universal Display Cooperation and OLED research company Novaled AG have created a record-breaking red PHOLED device that achieves new milestones in power efficiency and operating voltage. The device has a luminous efficiency of 15 cd/A at an operating voltage of less than 4V, resulting in record power efficiency of 12 lumens per watt, according to a joint press release.
The device was achieved using a combination of UDC's proprietary PHOLED technology, which offers up to 100% internal quantum efficiency that is as much as 4 times higher than that of conventional OLED technology, and Novaled's proprietary doping technology and materials, which offer up to 3 times lower operating voltage than conventional OLEDs
.
RECORD POWER EFFICIENCY
The UDC and Novaled Team produced Record Breaking Red Phosphor-escent OLED Devices. This was achieved through the use of Universal Displayâ„¢s high-efficiency PHOLED phosphorescent OLED technology and materials in combination with Novaledâ„¢s low-voltage conductivity doped Novaled PIN OLED technology and transport material.
The red PHOLED device has a peak external quantum efficiency of 16% and a drive voltage of less than 4V at 1,000 cd/m², and demonstrates excellent projected operational lifetimes of over 150,000 hours at an initial luminance of 500 cd/m² with a luminous efficacy exceeding 15 cd/A.
DEVICE ARCHITECTURE
A typical structure of the PH OLED device.
Phosphorescent Organic Light Emitting Diodes (PHOLEDs) are thin-film multi-layer devices consisting of a substrate foil, film or plate (rigid or flexible), an electrode layer, and layers of active materials, a counter electrode layer, and a protective barrier layer.
At least one of the electrodes must be transparent to light. The PHOLEDs operate in the following manner: Voltage bias is applied on the electrodes. The voltages are low, from 2.5 to ~ 20 V, but the active layers are so thin (~10Ã… to 100nm) that the electric fields in the active layers are very high, of the order of 105 “ 107 V/cm. These
high, near-breakdown electric fields support injection of charges across the electrode / active layers interfaces. Holes are injected from the anode, which is typically transparent, and electrons are injected from the cathode. The injected charges migrate against each other in the opposite directions, and eventually meet and recombine. Recombination energy is released and the molecule or a polymer segment in which the recombination occurs, reaches an exited state. Excitons may migrate from molecule to molecule. Eventually, some molecules or a polymer segments release the energy as photons or heat. It is desirable that all the excess excitation energy is released as photons (light). The materials that are used to bring the charges to the recombination sites are usually, but not always, poor photon emitters (most of the excitation energy is released as heat). Therefore, suitable dopants are added, which first transfer the energy from the original excitons, and release the energy more efficiently as photons.
In PHOLEDs, approximately 25% of the excitons are in the singlet states and 75% in the triplet states (8). Emission of photons from the singlet states (fluorescence), in most cases facilitated by fluorescent dopants, was believed to be the only applicable form of energy release, thus limiting the internal quantum efficiency (IQE) of PHOLEDs to the maximum of 25%. Triplet states in organic materials were considered useless, since the energy of triplets was believed to dissipate non-radiatively, as heat. This low ratio of
singlet states to the triplet states and, consequently, low device efficiency, would make the application of PHOLEDs as sources of light extremely difficult, if not unlikely.
The utilization of the triplet states was virtually ignored until 1998 when researchers from University of Southern California, (USC) and Princeton University (PU) demonstrated that by using phosphorescent dopants, the energy from all the triplet states could be harnessed as light (phosphorescence). The energy is transferred from the triplet excitons to the dopant molecules. However, not only excitons in the triplet states are utilized; these dopants, typically containing heavy atoms such as Ir or Pt, facilitate the forbidden "intersystem crossing" from the singlets to the triplet states, thus allowing for up to 100% IQE. In the recent experiments, green- and red emitting phosphorescent OLEDs (PHOLEDS) show indeed nearly 100% IQE, and 19% external quantum efficiencies (EQE) (which, under the experimental conditions, translates to 40 lm/W). This represents a quantum leap over the fluorescent systems . The onset voltage, sometimes as low as 2.4 V is the voltage at which the current begins to flow and enough hole-electron pairs recombine to generate light visible by naked eye. The current and the corresponding light intensity increase with increasing the drive voltage. Two types of materials are needed to bring charges to the recombination sites: hole transport polymers or small molecules, and electron transport polymers or molecules. The energy mismatch between the electrode and the charge transport layer may require another layer to be sandwiched in between, to facilitate charge injection and thus to reduce the operating voltage. Some add a "buffer" layer, which may serve the same purpose. Injection of holes is in most cases energetically easier than injection of electrons.
This may result in the injection of excess of holes, which could drift to the cathode without meeting electrons.
The excessive current would be wasteful and would heat the device. Usually,the electron transport layer acts as a hole blocker, but in some cases a hole-blocking layer is added between the electron and hole transport layers to prevent the escape of holes to the cathode. This has an additional benefit: the excess holes accumulate near the blocking layer and the resulting strong electric field across the cathode-electron transporter interface enhances injection of electrons to the system. This automatically balances the injection rates of both charge carriers, and maximizes recombination. In some cases, exciton blocking layers are added to prevent excitons to reach the electrodes and decay non-radiatively.
In other cases, a separate emission layer is sandwiched between the electron transport and hole transport layer. In white-color emitting devices there may be three separate emission layers, each emitting a different color. So today's devices may have a total of 7 - 9 layers - including electrodes, deposited by different techniques (sputtering, vapor deposition, solvent coating, etc). In spite of the large number of layers the total thickness of the device is typically less than 100 - 200 nm. The deposition of all layers requires humidity- and oxygen-free conditions and all will require class 10 clean room. The cost consequence of such complexity is high. The deposition of each layer negatively impacts the manufacturing yield of the final device. The number of layers depends primarily on the type of materials used.
It is still not clear how many layers will be ultimately needed to achieve the best performance.
POLYMERIC PHOLED DEVICES
Polymeric PHOLED devices have usually fewer layers. The electroactive polymers may serve multiple functions: both electron and hole transport and light emission, even though dopant emitters can be used to tune the color. The electron transporting polymer and hole transporting polymer may be in one or two separate layers. In some cases, very thin layers of p-doped and n-doped semi-conducting polymers are sandwiched between the transport polymers and the cathode and anode, respectively, to facilitate charge injection. The active polymers and the injection layers are solution-coatable, but the electrodes are deposited by different techniques such as vapor deposition or ion sputtering, as in "small molecular" devices. To date, a large number of polymers have been synthesized and tested, and new structures are still emerging.
The polymers have an extended chain of conjugated double bonds or aromatic rings, and pendant groups,which determine the emission characteristics. The polymers are members of the polyphenylene vinylene family, polyfluorene homo-and copolymers and a new class of polyspiro emitters.
"SMALL MOLECULAR PHOLED DEVICES
As the name indicates, the active components are "small" molecules. These small molecules are deposited by vapor deposition. Most "small molecules" would crystallize
when deposited from solutions and crystallization would damage the device performance. Also, solution coating may result in uncontrollable mixing of layers.
Most of the hole-transport small molecules contain one or several aromatic amine groups (a key pre-requisite for hole transport) and a variety of pendant substituents.
Electron transport molecules are typically complexes of a metal such as aluminum (such as Alq3), boron, etc. with aromatic groups, bisbiphenyl anthracene, or, recently developed silacyclo-pentadienes. These molecules have a relatively high electron affinity and must form stable anion-radicals. Some silacyclopentadiene may be unstable but new structures are being synthesized. The detailed description of the structures of charge transport materials is beyond the scope of this overview. Also, there is a need to fabricate the devices with extremely uniform thicknesses of each layer. Nonuniformities may lead to localized surges of electric current, localized overheating, and gradual destruction of the device. The complexity makes the fabrication of PHOLEDs difficult and slows down testing of new materials
DEVICE EFFICIENCY
The efficiency of PHOLEDs can be characterized by its quantum efficiency, power efficiency (lm/W), luminous efficiency (cd/A), sometimes called luminous yield.
The device quantum efficiency çq has two parts: internal and external:
¢ Internal quantum efficiency
çint or IQE, is the number of photons generated inside thedevice per number of injected hole “ electron pairs. A large fraction of generated photons stays trapped and absorbed inside the device.
¢ External quantum efficiency
çext or EQE, is the number of photons released from the device per number of injected hole “ electron pairs.
¢ Luminous (Power) efficiency
çp is the ratio of the lumen output to the input electrical
watts (lm/W).
¢ Luminous efficacy
çàrepresents the ratio of the lumen output to the optical watts (radiative power). The luminous efficiency and luminous efficacy of a device account for a spectral sensitivity of a human eye. Therefore, two devices with similar quantum efficiencies can have different luminous performance, depending on the spectrum of the emitted light. In the process of converting electrical power into optical power, losses are incurred due to non-radiative processes (thermal relaxation of excitons, internal reflection and absorption of photons). The luminous efficiency and luminous efficacy are related as çp = çà(Pin / Ö) where Pin is input el. watts, and Ö is lumen output.
WORKING OF PHOLED
An PHOLED is a monolithic, solid-state device that typically consists of a series of organic thin films sandwiched between two thin-film conductive electrodes. The choice
of organic materials and the layer structure determine the deviceâ„¢s performance features: emitted color, operating lifetime and power efficiency.
When electricity is applied to an PHOLED, charge carriers (holes and electrons) are injected from the electrodes into the organic thin films. They migrate through the device under the influence of an electrical field. The charge carriers then recombine, forming excitons. In the past, conventional wisdom suggested that only about 25% of these excitons could generate light, with the remaining 75% lost as heat. This was known as fluorescent emission.
100% of the excitons can be converted into light using a process known as electrophosphorescence, commonly referred to as phosphorescence. Thus, the efficiency of a phosphorescent OLED is up to four times higher than that of a conventional fluorescent OLED.
ADVANTAGES OF PHOLED
This are the some of the advantages of pholed
¢ Phosphorescent OLEDs (PHOLEDs) have up to 4 times the efficiency of traditional fluorescent OLEDs.
¢ PHOLEDs provide reduced power consumption for portable, batterypowered devices.
¢ PHOLEDs reduce display temperatures extending operational lifetimes.
¢ PHOLEDs provide reduced power losses and heat dissipation issues for large-area displays.
¢ PHOLEDs offer the potential to use existing low cost amorphous-
Silicon (a-Si) backplane infrastructure in addition to emerging poly -
Silicon (poly- Si) backplane technology.
OTHER TECHNOLOGIES
¢ WOLED White OLEDs
¢ TOLED Transparent and Top-emission OLEDs
¢ FOLED Flexible OLEDs
¢ P2OLED Printable Phosphorescent OLEDs
¢ SOLED Stacked OLEDs
In the United States, lighting consumes 765TWh of electricity each year. This represents 18% of total building energy consumption and 8.3% of total energy use. As one consequence, white organic light-emitting diode technology (WOLED) is under development due to its potential to achieve over 10 times the efficacy of incandescent lamps. However, for a WOLED to be as operationally long-lived as a fluorescent light and to emit optical power at levels comparable to a 60W incandescent bulb, its substrate area (0.10m2) tends to be larger than that of a typical light bulb.
Although a simple method to increase the flux of a WOLED would be to increase input power, efficacy thereby decreases and diode lifetime is inversely proportional to optical output power.
One approach to creating a powerful WOLED light source on a reasonably small (0.02m2) substrate would seek to improve total power efficacy by enhancing the
outcoupling efficiency through a reduction in operating voltage and elevation of the quantum yield. Given the current trend, which has seen an exponential increase in efficacy over the last 10 years, a 100lm/W device might be expected within 5 years.
Another solution currently in development that would make small-area WOLED lighting sources more optically powerful. This involves electrically connecting WOLEDs that are vertically stacked away from the substrate. This architecture enables maximum optical output power increases of 2 to 4 times that of a single WOLED.
Technology for the stacked organic light-emitting device (SOLED) was originally developed for full-color displays, and is currently being adapted for illumination applications. To fabricate a white SOLED, the first WOLED is grown on a substrate, with a second directly on top of the first. A third can be grown on top of the second, and so on. A key component is the electrical connection between stacked units. Our team has been developing a single metal connector that serves as both a cathode for one device and an anode for another.
To demonstrate this approach, a SOLED that uses a 70nm-thick aluminum floating electrode to serially connect two vertically stacked OLEDs was fabricated and characterized.
TOLED (transparent organic light-emitting device) is a display technology being developed by the Universal Display Corporation (UDC) that uses transparent electrodes and light emitting materials in an organic light-emitting device . TOLED embeds OLED technology in thin, transparent glass or plastic substrates that are capable of emitting light from the top or bottom, or both. Because TOLED displays that emit from both top and bottom are 70% transparent when not in use, they could be incorporated into eyeglasses, car windshields, or windows.
A green phosphorescent OLED (PHOLED) was first grown on the substrate; then a red PHOLED was grown directly on the Al cathode of the green PHOLED. The top electrode, which is the cathode of the top red OLED, consists of a thin transparent metal layer. As shown in Figure the color emissions do not mix due to the reflectivity of the connecting aluminum electrode.
Figure. In this picture, red and green stacked OLEDs emit from both sides of a glass substrate. The green OLED emissions are reflected in a mirror on the left side of the picture.
The bottom green device has characteristics that are identical to a single OLED, while the thin metal electrode limits optical output for the red device to 40% due to transmission loss. Such loss can be eliminated by employing a transparent top cathode such as indium tin oxide.
The next step in development is to replace the red and green OLEDs with WOLEDs to produce light sources that resemble the one shown in Figure .
Figure 2. A white organic light emitting diode is shown illuminating red, blue, and yellow flowers.
The most significant advantage of this SOLED structure is ease of fabrication. There are no doped transport layers, and the connecting electrode is identical to a standard OLED cathode. All organic layers similarly use materials commonly employed in the OLED industry. New deposition systems, or evaporation sources to accommodate materials needed solely for the purpose of electrically connecting stacked OLEDs, are not required. Moreover, operational stabilities of individual SOLED units are expected to be similar to equivalent OLEDs.
To meet future energy demands, solid-state lighting systems containing WOLEDs may help significantly to reduce energy consumption by replacing inefficient incandescent lighting. Our work indicates that the total Optical power emitted from these sources can be doubled, an increase in output power that would translate into a reduction of the active area of WOLEDs required to replace 60W bulbs. Our next step will be to develop stacks of three and four WOLEDs to further increase output power.
CONCLUSION
As PHOLED display technology matures, it will be better able to improve upon certain existing limitations of LCD and other flat panel display technologies including: high power consumption, costly manufacturing, limited viewing angles, and poor contrast ratios. While LCD penetration into major display applications is expected to be robust for the next decade, a small and increasing portion of flat panel display growth could come from PHOLEDs.
REFERENCE
¢ www.searchsmb.com
¢ www.ledsmagazine.com
¢ www.wekipedia.com
SEMINAR ON
PHOLED
(PHOSPHORESCENT ORGANIC LIGHT EMITTING DIODE)
PRESENTED BY
PREETHISHA.VP
03EC36
ROLL.NO.35
VJEC
CONTENTS
ABSTRACT
INTRODUCTION
DISPLAY TECHNOLOGIES
WHAT IS OLED
BACKGROUND OF PHOLED
FEATURES OF PHOLED
DEVICES
MATERIALS
WHITE LIGHT FROM PHOLED
RED PHOLED
DEVICE ARCHITECTURE
POLYMERIC PHOLED DEVICES
SMALL MOLECULAR PHOLED DEVICES
DEVICE EFFICIENCY
WORKING OF PHOLED
ADVANTAGES OF PHOLED
OTHER TECHNOLOGIES
CONCLUSION
REFERENCE
ABSTRACT
PHOLED (phosphorescent organic light-emitting device) is a proprietary display technology developed by the Universal Display Corporation (UDC) that uses soluble phosphorescent small molecule materials to create organic light-emitting devices (OLEDs). PHOLED technology works on the principle that certain organic molecules emit light when an electric current is applied.
Although PHOLED was originally developed as a display technology for use in mobile phones, the U.S. Department of Energy . PHOLED-based lighting sources, such as glowing walls, are expected to provide cost-effective and energy-efficient alternatives to traditional incandescent and fluorescent lighting.
INTRODUCTION
A PHOLED is used to signify a phosphorescent OLED. This type of technology is currently under development by a Ewing, New Jersey company called Universal Display Corporation.
Like all types of OLEDs, PHOLEDs function via the following method: an electric current is applied to organic molecules, which then emit bright light. However, PHOLEDs use "...the principle of electro phosphorescence to convert up to 100% of the electrical energy in an OLED into light." . In comparison, traditional fluorescent OLEDs only convert approximately 25-30% of electrical energy into light. LCD displays, the current favorite technology for use in flat-screen devices, are even less efficient, converting only 10% of electrical energy into light.
Due to their extremely high level of energy efficiency, even when compared to other OLEDs, PHOLEDs are being studied for potential use in large-screen displays such as television monitors or TV screens, as well as general lighting needs. One potential use of PHOLEDs as lighting devices is to cover walls with gigantic PHOLED displays. This would allow entire rooms to glow uniformly, rather than require the use of light bulbs which distribute light unequally throughout a room.
The United States Department of Energy has recognized the potential for massive energy savings via the use of this technology and therefore has awarded $200,000 in contracts to develop PHOLED products for general lighting applications.
DISPLAY TECHNOLOGIES
Today, there are many different types of display technologies in the marketplace, each with their own unique characteristics to address specific applications. Below is a sample list of popular approaches used in display systems:
¢ Cathode Ray Tubes (CRT)
The technology used in most televisions and computer displayscreens. A CRT works by moving an electron beam back and forth across the back of the screen.
¢ Field Emission Displays (FED)
FED is a flat cathode ray tube that uses a matrix-addressed cold- cathode to produce light from a cathodoluminescent phosphor screen.
¢ Organic Light Emitting Diodes (OLED)
Unlike LCDs, which require backlighting, OLED displays are emissive devices that give off light rather than modulating transmitted light or reflected light.
¢ Plasma Display Panels (PDP)
Technology that operates by controlling discharges from ionized gases.Plasma displays are becoming an alternative to LCD as they can be easily manufactured in a large format. Compared to conventional CRT displays, plasma displays are about one-tenth the thickness--around 4'', and one-sixth the weight.
¢ Light Emitting Diodes (LED)
An LED is an electronic device that lights up when electricity is passed through it. LEDs have been around for decades and are used in everything from car dashboards, to large indoor/outdoor display systems, to portable electronics as indicator lamps.
¢ Liquid Crystal Displays (LCD)
Function through the use of pixels composed of liquid crystals that have the ability to polarize light in the presence of an electronic field. The core advantage to LCD is the usefulness, relatively low power consumption, and high contrast.
WHAT IS OLED
OLED (organic light-emitting diodes) is a display technology, pioneered and patented by Kodak, based on the use of organic polymer material as the semiconductor material in light-emitting diodes (LEDs). A polymer can be a natural or synthetic substance and macro or micro in size. Examples of organic polymers include proteins and DNA. OLED displays are used in cellular phones, digital video cameras, digital versatile disc (DVD) players, personal digital assistants (PDAs), notebooks, car stereos, and televisions. OLED displays are thinner and weigh less because they do not require backlighting. OLED displays also have a wide viewing angle up to 160 degrees even in bright light, and they use only two to ten volts to operate.
New technologies that build on the OLED include FOLED (flexible organic light-emitting display), which promises to make highly portable, roll-up displays possible within the next few years.
PROBLEMS WITH OLED
OLED seem to be the perfect technology for all types of displays, but
they also have some problems:
¢ Lifetime - While red and green OLED films have long lifetimes (10,000 to 40,000 hours), blue organics currently have much shorter lifetimes (only about 1,000 hours).
¢ Manufacturing - Manufacturing processes are expensive right now.
¢ Water - Water can easily damage OLEDs.
BACKGROUND OF PHOLED
PHOLEDs are energy conversion devices (electricity-to-light) based on electroluminescence.PHOLEDs first attracted the attention of researchers in the 1970s because of their potentially high quantum efficiency of luminescence and the ability to generate a wide variety of colors. Unfortunately, their high operating voltages (>1000V) prohibited them from becoming practical devices. However, in 1987, after C. Tang and Van Slyke (5) from Eastman Kodak devised a heterostructure double layered device containing active small molecules that combined a low operating voltages (<10V) with good brightness (>1000 cd/m2) and respectable luminous efficiency (1.5 lm/W), research gained the momentum.
PHOLEDs, both small molecular and polymeric, have already achieved emission in all colors of the spectrum - including white. Fine-tuning to any desired shade of color can be achieved by selecting an appropriate emitter or a mixture of emitters with the right emission spectra. Literally hundreds of emitters are already known and have been tested. Some are more efficient than others, and many more will be synthesized and optimized. Since the selection of basic structures and properties-modifying substituents of all types and sizes is nearly unlimited, organic chemistry provides endless opportunities in designing the desired color.
FEATURES OF PHOLED
These are the some of the features of PHOLED
¢ Vibrant colors
¢ High contrast
¢ Excellent grayscale
¢ Full-motion video
¢ Wide viewing angles from all directions
¢ A wide range of pixel sizes
¢ Low power consumption
¢ Low operating voltages
¢ Wide operating temperature range
¢ Long operating lifetime
¢ A thin and lightweight form factor
¢ Cost-effective manufacturability
DEVICES
PHOLEDs are extremely thin, practically twodimensional multi-layer devices of large square area. The thickness of all the active layers combined is only of the order of one hundred nanometers. This feature will be a benefit for applications where space is a premium, such as in airplanes. The nearly two-dimensional nature of PHOLEDs makes them also suitable for manufacturing by roll-to-roll coating technologies, which are inherently low-
cost. The roll-to-roll coating technologies operating at speeds of about 20 ft/sec have already been used successfully in manufacturing of organic photoreceptors for laser printers, with extremely stringent thickness tolerances. There is no restriction on the size and shape of the PHOLED devices. Every conceivable shape and form can be envisioned, and only the human imagination is the limiting factor.
The devices can be in form of fibers, and woven to fabrics. They can be on bent or rolled films or constitute the surface of spheres. For lighting applications, thin flat sheets possibly using thin glass substrates will probably be the shape of choice. The intensity of light can be controlled by conventional types of dimmers. The devices are conceptually simple, but the details of their structure are complicated. Many changes will be made before the final design is established.
MATERIALS
PHOLED devices contain the substrate materials, electrodes and functional organic substances. All are environmentally safe. Two types of electrodes are used. An extremely thin layer of indium - tin oxide (extremely thin, because it has to be optically transparent), is used as anode. Low work-function metals such as Mg, Li, and their alloys with Ag, and in some cases
Al, are now used as cathodes. Several types of organic materials are used in the functional layers: Polymers or small molecules that transport injected charges to the recombination zone, fluorescent or recently introduced more efficient phosphorescent dopants that emit light, and charge-injection modifying compounds, such as conducting polymers near the anode or salts such as LiF or CsF etc near the cathode.
The charge transporting polymers are typically polyconjugated, such as derivatives of polyphenylene vinylene. The small molecules are substituted aromatic amines for hole transport and a variety of polynuclear aromatic complexes with high electron affinity for electron transport. In some cases, the charge transporting polymers or small molecules themselves assume the role of emitters. These materials can be deposited in many separate layers or mixed into one or several layers. Organic chemistry offers an endless variety of structures, and therefore, the choices of charge transporters, emitters and other dopants are virtually unlimited. Polymeric PHOLEDs have the advantage that the active layers can be deposited from solution, while in "small molecule" PHOLEDs, the active layers are typically deposited by vapor deposition techniques
WHITE LIGHT FROM PHOLEDs
PHOLEDs are uniquely suitable as sources of white light. The structure of light emitting fluorescence or phosphorescence additives can be tailored to emit any desired color. Mixing light from two or more sources (dopants or layers) gives light whose color is determined by the weighted average of the CIE coordinates of these sources. Given the enormous variety of known and yet-to-be synthesized dopants, both fluorescent and phosphorescent, with broad emission spectra of choice, practically any shade of white or any "temperature" of white light can be generated in PHOLEDs. Many devices have already been made in the laboratory scale and tested and some of them almost perfectly simulate the sunlight.
RED PHOLED EMITTERS IN COMBINATION WITH NSCCâ„¢s HOST MATERIALS
Tokyo, Japan and Ewing, New Jersey “ Nippon Steel Chemical Co., Ltd. (NSCC), the Company that™s a leading manufacturer of OLED materials and owns the equipment and the technology for manufacturing super-purified products with consistent quality in commercial scale in advance of any other manufacturers in the world, and Universal Display Corporation (NASDAQ: PANL), the Company that™s lighting the way in developing and commercializing OLED technology for flat panel displays, lighting and other opto-electronics with its proprietary PHOLED or phosphorescent OLED technology, today announced a collaboration to develop new markets for red phosphorescent OLED materials. As the background of this collaboration, there is the fact that a commercially-available red host material from NSCC is compatible with commercially-available red phosphorescent OLED emitters from Universal Display.
By using red phosphorescence, overall power consumption of the display can be reduced by more than 40% as compared to using fluorescent red OLEDs. The combination of Universal Displayâ„¢s red PHOLED materials and NSCCâ„¢s host material, called NS11, offers very efficient, highly desirable red colors with long operational lifetime. These products are currently available for evaluation and commercial use from the two companies.
Since the time when fluorescent materials were introduced into the OLED market, we started to supply the OLED materials as commercial base. Since then, we have been involved in providing our materials to most of the OLED manufacturers, which have already started to mass-produce OLED devices. However, we recognize that it should be very difficult for the fluorescent OLED system to compete against the LCD system because of the small distinction in power consumption between the both systems. In order to pursue the further enlargement of the OLED market, we consider that it must become a key factor to introduce phosphorescent materials into the OLED market. Based on this point of view, we have mainly focused our efforts on the development of phosphorescent materials.
Universal Display™s proprietary PHOLED technology offers up to four times higher efficiency than conventional OLED technology “ a feature that is very important for today™s battery-operated cell phones and other portable devices, as well as for tomorrow™s large-area TVs and solid-state lighting products.
Over the past few years, the Company has announced a series of record-breaking performance milestones for its red, green and blue PHOLED systems. Universal Displayâ„¢s vacuum-deposited PHOLED materials, manufactured by PPG Industries exclusively for Universal Display, are currently being evaluated and used in commercial production by a number of electronics manufacturers.
EWING, N.J. and DRESDEN, Germany - Phosphorescent organic light-emitting diode (PHOLED) developer Universal Display Cooperation and OLED research company Novaled AG have created a record-breaking red PHOLED device that achieves new milestones in power efficiency and operating voltage. The device has a luminous efficiency of 15 cd/A at an operating voltage of less than 4V, resulting in record power efficiency of 12 lumens per watt, according to a joint press release.
The device was achieved using a combination of UDC's proprietary PHOLED technology, which offers up to 100% internal quantum efficiency that is as much as 4 times higher than that of conventional OLED technology, and Novaled's proprietary doping technology and materials, which offer up to 3 times lower operating voltage than conventional OLEDs
.
RECORD POWER EFFICIENCY
The UDC and Novaled Team produced Record Breaking Red Phosphor-escent OLED Devices. This was achieved through the use of Universal Displayâ„¢s high-efficiency PHOLED phosphorescent OLED technology and materials in combination with Novaledâ„¢s low-voltage conductivity doped Novaled PIN OLED technology and transport material.
The red PHOLED device has a peak external quantum efficiency of 16% and a drive voltage of less than 4V at 1,000 cd/m², and demonstrates excellent projected operational lifetimes of over 150,000 hours at an initial luminance of 500 cd/m² with a luminous efficacy exceeding 15 cd/A.
DEVICE ARCHITECTURE
A typical structure of the PH OLED device.
Phosphorescent Organic Light Emitting Diodes (PHOLEDs) are thin-film multi-layer devices consisting of a substrate foil, film or plate (rigid or flexible), an electrode layer, and layers of active materials, a counter electrode layer, and a protective barrier layer.
At least one of the electrodes must be transparent to light. The PHOLEDs operate in the following manner: Voltage bias is applied on the electrodes. The voltages are low, from 2.5 to ~ 20 V, but the active layers are so thin (~10Ã… to 100nm) that the electric fields in the active layers are very high, of the order of 105 “ 107 V/cm. These
high, near-breakdown electric fields support injection of charges across the electrode / active layers interfaces. Holes are injected from the anode, which is typically transparent, and electrons are injected from the cathode. The injected charges migrate against each other in the opposite directions, and eventually meet and recombine. Recombination energy is released and the molecule or a polymer segment in which the recombination occurs, reaches an exited state. Excitons may migrate from molecule to molecule. Eventually, some molecules or a polymer segments release the energy as photons or heat. It is desirable that all the excess excitation energy is released as photons (light). The materials that are used to bring the charges to the recombination sites are usually, but not always, poor photon emitters (most of the excitation energy is released as heat). Therefore, suitable dopants are added, which first transfer the energy from the original excitons, and release the energy more efficiently as photons.
In PHOLEDs, approximately 25% of the excitons are in the singlet states and 75% in the triplet states (8). Emission of photons from the singlet states (fluorescence), in most cases facilitated by fluorescent dopants, was believed to be the only applicable form of energy release, thus limiting the internal quantum efficiency (IQE) of PHOLEDs to the maximum of 25%. Triplet states in organic materials were considered useless, since the energy of triplets was believed to dissipate non-radiatively, as heat. This low ratio of
singlet states to the triplet states and, consequently, low device efficiency, would make the application of PHOLEDs as sources of light extremely difficult, if not unlikely.
The utilization of the triplet states was virtually ignored until 1998 when researchers from University of Southern California, (USC) and Princeton University (PU) demonstrated that by using phosphorescent dopants, the energy from all the triplet states could be harnessed as light (phosphorescence). The energy is transferred from the triplet excitons to the dopant molecules. However, not only excitons in the triplet states are utilized; these dopants, typically containing heavy atoms such as Ir or Pt, facilitate the forbidden "intersystem crossing" from the singlets to the triplet states, thus allowing for up to 100% IQE. In the recent experiments, green- and red emitting phosphorescent OLEDs (PHOLEDS) show indeed nearly 100% IQE, and 19% external quantum efficiencies (EQE) (which, under the experimental conditions, translates to 40 lm/W). This represents a quantum leap over the fluorescent systems . The onset voltage, sometimes as low as 2.4 V is the voltage at which the current begins to flow and enough hole-electron pairs recombine to generate light visible by naked eye. The current and the corresponding light intensity increase with increasing the drive voltage. Two types of materials are needed to bring charges to the recombination sites: hole transport polymers or small molecules, and electron transport polymers or molecules. The energy mismatch between the electrode and the charge transport layer may require another layer to be sandwiched in between, to facilitate charge injection and thus to reduce the operating voltage. Some add a "buffer" layer, which may serve the same purpose. Injection of holes is in most cases energetically easier than injection of electrons.
This may result in the injection of excess of holes, which could drift to the cathode without meeting electrons.
The excessive current would be wasteful and would heat the device. Usually,the electron transport layer acts as a hole blocker, but in some cases a hole-blocking layer is added between the electron and hole transport layers to prevent the escape of holes to the cathode. This has an additional benefit: the excess holes accumulate near the blocking layer and the resulting strong electric field across the cathode-electron transporter interface enhances injection of electrons to the system. This automatically balances the injection rates of both charge carriers, and maximizes recombination. In some cases, exciton blocking layers are added to prevent excitons to reach the electrodes and decay non-radiatively.
In other cases, a separate emission layer is sandwiched between the electron transport and hole transport layer. In white-color emitting devices there may be three separate emission layers, each emitting a different color. So today's devices may have a total of 7 - 9 layers - including electrodes, deposited by different techniques (sputtering, vapor deposition, solvent coating, etc). In spite of the large number of layers the total thickness of the device is typically less than 100 - 200 nm. The deposition of all layers requires humidity- and oxygen-free conditions and all will require class 10 clean room. The cost consequence of such complexity is high. The deposition of each layer negatively impacts the manufacturing yield of the final device. The number of layers depends primarily on the type of materials used.
It is still not clear how many layers will be ultimately needed to achieve the best performance.
POLYMERIC PHOLED DEVICES
Polymeric PHOLED devices have usually fewer layers. The electroactive polymers may serve multiple functions: both electron and hole transport and light emission, even though dopant emitters can be used to tune the color. The electron transporting polymer and hole transporting polymer may be in one or two separate layers. In some cases, very thin layers of p-doped and n-doped semi-conducting polymers are sandwiched between the transport polymers and the cathode and anode, respectively, to facilitate charge injection. The active polymers and the injection layers are solution-coatable, but the electrodes are deposited by different techniques such as vapor deposition or ion sputtering, as in "small molecular" devices. To date, a large number of polymers have been synthesized and tested, and new structures are still emerging.
The polymers have an extended chain of conjugated double bonds or aromatic rings, and pendant groups,which determine the emission characteristics. The polymers are members of the polyphenylene vinylene family, polyfluorene homo-and copolymers and a new class of polyspiro emitters.
"SMALL MOLECULAR PHOLED DEVICES
As the name indicates, the active components are "small" molecules. These small molecules are deposited by vapor deposition. Most "small molecules" would crystallize
when deposited from solutions and crystallization would damage the device performance. Also, solution coating may result in uncontrollable mixing of layers.
Most of the hole-transport small molecules contain one or several aromatic amine groups (a key pre-requisite for hole transport) and a variety of pendant substituents.
Electron transport molecules are typically complexes of a metal such as aluminum (such as Alq3), boron, etc. with aromatic groups, bisbiphenyl anthracene, or, recently developed silacyclo-pentadienes. These molecules have a relatively high electron affinity and must form stable anion-radicals. Some silacyclopentadiene may be unstable but new structures are being synthesized. The detailed description of the structures of charge transport materials is beyond the scope of this overview. Also, there is a need to fabricate the devices with extremely uniform thicknesses of each layer. Nonuniformities may lead to localized surges of electric current, localized overheating, and gradual destruction of the device. The complexity makes the fabrication of PHOLEDs difficult and slows down testing of new materials
DEVICE EFFICIENCY
The efficiency of PHOLEDs can be characterized by its quantum efficiency, power efficiency (lm/W), luminous efficiency (cd/A), sometimes called luminous yield.
The device quantum efficiency çq has two parts: internal and external:
¢ Internal quantum efficiency
çint or IQE, is the number of photons generated inside thedevice per number of injected hole “ electron pairs. A large fraction of generated photons stays trapped and absorbed inside the device.
¢ External quantum efficiency
çext or EQE, is the number of photons released from the device per number of injected hole “ electron pairs.
¢ Luminous (Power) efficiency
çp is the ratio of the lumen output to the input electrical
watts (lm/W).
¢ Luminous efficacy
çàrepresents the ratio of the lumen output to the optical watts (radiative power). The luminous efficiency and luminous efficacy of a device account for a spectral sensitivity of a human eye. Therefore, two devices with similar quantum efficiencies can have different luminous performance, depending on the spectrum of the emitted light. In the process of converting electrical power into optical power, losses are incurred due to non-radiative processes (thermal relaxation of excitons, internal reflection and absorption of photons). The luminous efficiency and luminous efficacy are related as çp = çà(Pin / Ö) where Pin is input el. watts, and Ö is lumen output.
WORKING OF PHOLED
An PHOLED is a monolithic, solid-state device that typically consists of a series of organic thin films sandwiched between two thin-film conductive electrodes. The choice
of organic materials and the layer structure determine the deviceâ„¢s performance features: emitted color, operating lifetime and power efficiency.
When electricity is applied to an PHOLED, charge carriers (holes and electrons) are injected from the electrodes into the organic thin films. They migrate through the device under the influence of an electrical field. The charge carriers then recombine, forming excitons. In the past, conventional wisdom suggested that only about 25% of these excitons could generate light, with the remaining 75% lost as heat. This was known as fluorescent emission.
100% of the excitons can be converted into light using a process known as electrophosphorescence, commonly referred to as phosphorescence. Thus, the efficiency of a phosphorescent OLED is up to four times higher than that of a conventional fluorescent OLED.
ADVANTAGES OF PHOLED
This are the some of the advantages of pholed
¢ Phosphorescent OLEDs (PHOLEDs) have up to 4 times the efficiency of traditional fluorescent OLEDs.
¢ PHOLEDs provide reduced power consumption for portable, batterypowered devices.
¢ PHOLEDs reduce display temperatures extending operational lifetimes.
¢ PHOLEDs provide reduced power losses and heat dissipation issues for large-area displays.
¢ PHOLEDs offer the potential to use existing low cost amorphous-
Silicon (a-Si) backplane infrastructure in addition to emerging poly -
Silicon (poly- Si) backplane technology.
OTHER TECHNOLOGIES
¢ WOLED White OLEDs
¢ TOLED Transparent and Top-emission OLEDs
¢ FOLED Flexible OLEDs
¢ P2OLED Printable Phosphorescent OLEDs
¢ SOLED Stacked OLEDs
In the United States, lighting consumes 765TWh of electricity each year. This represents 18% of total building energy consumption and 8.3% of total energy use. As one consequence, white organic light-emitting diode technology (WOLED) is under development due to its potential to achieve over 10 times the efficacy of incandescent lamps. However, for a WOLED to be as operationally long-lived as a fluorescent light and to emit optical power at levels comparable to a 60W incandescent bulb, its substrate area (0.10m2) tends to be larger than that of a typical light bulb.
Although a simple method to increase the flux of a WOLED would be to increase input power, efficacy thereby decreases and diode lifetime is inversely proportional to optical output power.
One approach to creating a powerful WOLED light source on a reasonably small (0.02m2) substrate would seek to improve total power efficacy by enhancing the
outcoupling efficiency through a reduction in operating voltage and elevation of the quantum yield. Given the current trend, which has seen an exponential increase in efficacy over the last 10 years, a 100lm/W device might be expected within 5 years.
Another solution currently in development that would make small-area WOLED lighting sources more optically powerful. This involves electrically connecting WOLEDs that are vertically stacked away from the substrate. This architecture enables maximum optical output power increases of 2 to 4 times that of a single WOLED.
Technology for the stacked organic light-emitting device (SOLED) was originally developed for full-color displays, and is currently being adapted for illumination applications. To fabricate a white SOLED, the first WOLED is grown on a substrate, with a second directly on top of the first. A third can be grown on top of the second, and so on. A key component is the electrical connection between stacked units. Our team has been developing a single metal connector that serves as both a cathode for one device and an anode for another.
To demonstrate this approach, a SOLED that uses a 70nm-thick aluminum floating electrode to serially connect two vertically stacked OLEDs was fabricated and characterized.
TOLED (transparent organic light-emitting device) is a display technology being developed by the Universal Display Corporation (UDC) that uses transparent electrodes and light emitting materials in an organic light-emitting device . TOLED embeds OLED technology in thin, transparent glass or plastic substrates that are capable of emitting light from the top or bottom, or both. Because TOLED displays that emit from both top and bottom are 70% transparent when not in use, they could be incorporated into eyeglasses, car windshields, or windows.
A green phosphorescent OLED (PHOLED) was first grown on the substrate; then a red PHOLED was grown directly on the Al cathode of the green PHOLED. The top electrode, which is the cathode of the top red OLED, consists of a thin transparent metal layer. As shown in Figure the color emissions do not mix due to the reflectivity of the connecting aluminum electrode.
Figure. In this picture, red and green stacked OLEDs emit from both sides of a glass substrate. The green OLED emissions are reflected in a mirror on the left side of the picture.
The bottom green device has characteristics that are identical to a single OLED, while the thin metal electrode limits optical output for the red device to 40% due to transmission loss. Such loss can be eliminated by employing a transparent top cathode such as indium tin oxide.
The next step in development is to replace the red and green OLEDs with WOLEDs to produce light sources that resemble the one shown in Figure .
Figure 2. A white organic light emitting diode is shown illuminating red, blue, and yellow flowers.
The most significant advantage of this SOLED structure is ease of fabrication. There are no doped transport layers, and the connecting electrode is identical to a standard OLED cathode. All organic layers similarly use materials commonly employed in the OLED industry. New deposition systems, or evaporation sources to accommodate materials needed solely for the purpose of electrically connecting stacked OLEDs, are not required. Moreover, operational stabilities of individual SOLED units are expected to be similar to equivalent OLEDs.
To meet future energy demands, solid-state lighting systems containing WOLEDs may help significantly to reduce energy consumption by replacing inefficient incandescent lighting. Our work indicates that the total Optical power emitted from these sources can be doubled, an increase in output power that would translate into a reduction of the active area of WOLEDs required to replace 60W bulbs. Our next step will be to develop stacks of three and four WOLEDs to further increase output power.
CONCLUSION
As PHOLED display technology matures, it will be better able to improve upon certain existing limitations of LCD and other flat panel display technologies including: high power consumption, costly manufacturing, limited viewing angles, and poor contrast ratios. While LCD penetration into major display applications is expected to be robust for the next decade, a small and increasing portion of flat panel display growth could come from PHOLEDs.
REFERENCE
¢ www.searchsmb.com
¢ www.ledsmagazine.com
¢ www.wekipedia.com
SEMINAR ON
PHOLED
(PHOSPHORESCENT ORGANIC LIGHT EMITTING DIODE)
PRESENTED BY
PREETHISHA.VP
03EC36
ROLL.NO.35
VJEC
CONTENTS
ABSTRACT
INTRODUCTION
DISPLAY TECHNOLOGIES
WHAT IS OLED
BACKGROUND OF PHOLED
FEATURES OF PHOLED
DEVICES
MATERIALS
WHITE LIGHT FROM PHOLED
RED PHOLED
DEVICE ARCHITECTURE
POLYMERIC PHOLED DEVICES
SMALL MOLECULAR PHOLED DEVICES
DEVICE EFFICIENCY
WORKING OF PHOLED
ADVANTAGES OF PHOLED
OTHER TECHNOLOGIES
CONCLUSION
REFERENCE