Emerging Display Technologies

1. Emerging Display Technologies Wesley Holland

2. Organic Light Emitting Diode (OLED) Similarities to conventional LED: Has anode and cathode Electrons flow out through anode and in through cathode Operates like diode; must be forward-biased Photons are emitted when electrons fill holes in the emissive layer Differences from conventional LED: Emissive and conductive layers are organic compounds or polymers Substrate can be plastic Color displays can be made of triads of RGB OLEDs OLEDs are similar in construction and function to conventional LEDs. Like conventional LEDs, they have a cathode and anode and must be forward-biased to operate. Also like conventional LEDs, photons are emitted when electrons fill holes; this happens in the emissive layer. The main differences in OLEDs and conventional LEDs are that the emissive and conductive layers are made of organic compounds. These compounds can be either a crystallized organic molecule, or an organic polymer (possibly plastic).OLEDs are similar in construction and function to conventional LEDs. Like conventional LEDs, they have a cathode and anode and must be forward-biased to operate. Also like conventional LEDs, photons are emitted when electrons fill holes; this happens in the emissive layer. The main differences in OLEDs and conventional LEDs are that the emissive and conductive layers are made of organic compounds. These compounds can be either a crystallized organic molecule, or an organic polymer (possibly plastic).

3. Organic Light Emitting Diode (OLED) Advantages Can be printed onto a wide variety of substrates [1] Can be made using plastic screens; LCDs require glass backing [3] Low power requirements No backlight required No polarization filters necessary; no wasted light “Off” pixels consume no power Faster response time than LCDs [2] Larger field of view than LCDs; up to 170 degrees [3] Disadvantages Short lifetime (<1000 hours for blue OLEDs [3]) Currently, manufacturing is more expensive than LCDs More susceptible to water damage than LCDs One of the primary advantages to OLEDs is the flexibility of using a plastic substrate. Using a plastic substrate means that OLED displays can be printed on flexible plastic sheets that can be rolled-up when not in use. It also means that OLED displays can be woven into clothing. OLED displays also have lower power requirements when compared to LCDs. The reason for this is threefold. First, OLEDs generate light and, as such, do not require a backlight. Second, LCDs work by blocking light; OLEDs do not require these polarization filters, so all light goes into the display. Third, an off pixel in an OLED display consumes no power, while an off pixel in an LCD consumes power. OLEDs also offer a faster response time and a larger viewing field than LCDs. The largest single disadvantage to OLEDs is their short lifetime. While red and green OLEDs can last up to 40,000 hours, current technology puts blue OLEDs lasting at only 1,000 hours. This compares very poorly to even the worst of LCDs, which offer a minimum of 40,000 hours. Because of the novelty of OLEDs and the disadvantage of their short lifetime, current fabrication facilities are not geared for mass OLED production. Consequently, manufacturing is currently more expensive than LCDs. A final disadvantage is that OLEDs are more susceptible to water damage than LCDs. This is due to the tendency of crystalline organic molecules to dissolve in water.One of the primary advantages to OLEDs is the flexibility of using a plastic substrate. Using a plastic substrate means that OLED displays can be printed on flexible plastic sheets that can be rolled-up when not in use. It also means that OLED displays can be woven into clothing. OLED displays also have lower power requirements when compared to LCDs. The reason for this is threefold. First, OLEDs generate light and, as such, do not require a backlight. Second, LCDs work by blocking light; OLEDs do not require these polarization filters, so all light goes into the display. Third, an off pixel in an OLED display consumes no power, while an off pixel in an LCD consumes power. OLEDs also offer a faster response time and a larger viewing field than LCDs. The largest single disadvantage to OLEDs is their short lifetime. While red and green OLEDs can last up to 40,000 hours, current technology puts blue OLEDs lasting at only 1,000 hours. This compares very poorly to even the worst of LCDs, which offer a minimum of 40,000 hours. Because of the novelty of OLEDs and the disadvantage of their short lifetime, current fabrication facilities are not geared for mass OLED production. Consequently, manufacturing is currently more expensive than LCDs. A final disadvantage is that OLEDs are more susceptible to water damage than LCDs. This is due to the tendency of crystalline organic molecules to dissolve in water.

4. Electronic Paper Display (EPD) (a.k.a E-Ink) E-ink consists of capsules containing negatively charged white plastic suspended in blue or black oil E-ink is sandwiched between two electrode layers When field orients one way, the white particles rise to the top. When field orients the other way, the oil color is visible Color displays can be made by adding color filters on top of transparent electrodes and organizing pixels in CMY triads There are two competing types of electronic paper. One was developed by Xerox in the 1970s, the other was developed by E-Ink in the 1990s. Since both technologies are similar and operate on the same principle, we will concentrate on the more advanced of the two, that developed by E-Ink. E-Ink is a substance which consists of many capsules. These capsules contain black or blue oil, in which is suspended a number of negatively charged pieces of white plastic. When exposed to an electric field, all of the pieces of white plastic are attracted to one side of the capsule, leaving the oil on the other side. This is the mechanism by which E-Ink is controlled. In devices, it is sandwiched between two electrode layers. These electrodes are actually made of flexible plastic transistors. The electrodes are used to control individual pixels on an E-Ink page.There are two competing types of electronic paper. One was developed by Xerox in the 1970s, the other was developed by E-Ink in the 1990s. Since both technologies are similar and operate on the same principle, we will concentrate on the more advanced of the two, that developed by E-Ink. E-Ink is a substance which consists of many capsules. These capsules contain black or blue oil, in which is suspended a number of negatively charged pieces of white plastic. When exposed to an electric field, all of the pieces of white plastic are attracted to one side of the capsule, leaving the oil on the other side. This is the mechanism by which E-Ink is controlled. In devices, it is sandwiched between two electrode layers. These electrodes are actually made of flexible plastic transistors. The electrodes are used to control individual pixels on an E-Ink page.

5. Electronic Paper Display (EPD) (a.k.a E-Ink) Advantages Can be printed onto flexible plastic [4] Extremely large viewing angle (light is reflected, not generated) Extremely low power requirements Power is only consumed while image is switching Disadvantages Currently, manufacturing is more expensive than LCDs Slower response time than LCDs [5] Like OLED displays, E-Ink can be printed onto flexible plastic sheets. This lends itself to a number of portable applications. The primary advantage of E-Ink is its extremely low power requirements. Due to the properties of the capsules from which E-Ink is composed, power is only used while the image is changing. That is, the capsules are constructed such that, in the absence of an electric field, they stay oriented in the direction in which they were last oriented. The primary disadvantage of E-Ink is its slow response time. It is meant to be used in applications where switching time is relatively unimportant, and an image will stay the same for a relatively long period of time.Like OLED displays, E-Ink can be printed onto flexible plastic sheets. This lends itself to a number of portable applications. The primary advantage of E-Ink is its extremely low power requirements. Due to the properties of the capsules from which E-Ink is composed, power is only used while the image is changing. That is, the capsules are constructed such that, in the absence of an electric field, they stay oriented in the direction in which they were last oriented. The primary disadvantage of E-Ink is its slow response time. It is meant to be used in applications where switching time is relatively unimportant, and an image will stay the same for a relatively long period of time.

6. Surface-Conduction Electron-Emitter Display (SED) SEDs work on the same principle as CRTs One side of a glass is covered in phosphor that lights up when hit by electrons Electrons are fired at the phosphor to create the picture However, where CRTs use one electron gun for the whole screen, SEDs use an electron emitter for each pixel A SED is an array of Surface-Conduction Electron-Emitters (SCEs), each of which has a separate emitter for RGB SEDs are based on the same principle as CRTs – namely, that electrons are fired at a phosphor-covered glass to generate light. However, where CRTs use one electron gun for the whole screen, SEDs use an electron emitter for each pixel. This has numerous advantages. The first advantage is that the display can be made much thinner, as it is not necessary that it be long enough for a single electron gun to focus on all parts of the screen. The second advantage is that it is not necessary that each of the many electron emitters have deflection plates to aim the electron beam, as in the case of SEDs the electron emitters are very close to the phosphor-coated glass. Each pixel of a SED is made of three emitters, each of which is opposite a phosphor which emits a certain color light when struck. SEDs are based on the same principle as CRTs – namely, that electrons are fired at a phosphor-covered glass to generate light. However, where CRTs use one electron gun for the whole screen, SEDs use an electron emitter for each pixel. This has numerous advantages. The first advantage is that the display can be made much thinner, as it is not necessary that it be long enough for a single electron gun to focus on all parts of the screen. The second advantage is that it is not necessary that each of the many electron emitters have deflection plates to aim the electron beam, as in the case of SEDs the electron emitters are very close to the phosphor-coated glass. Each pixel of a SED is made of three emitters, each of which is opposite a phosphor which emits a certain color light when struck.

7. Surface-Conduction Electron-Emitter Display (SED) Advantages Thin when compared to CRTs; approximately the same width as LCDs Higher contrast ratio and better viewing angle than LCDs [6] Faster response time than LCDs Comparable power consumption to LCDs Disadvantages Phosphor screens are subject to “burn-in” Due to ongoing patent litigation, not currently mass-produced SEDs have the picture-quality advantages of CRTs while continuing to be competitive with LCDs in the areas of width and power consumption. Aside from this, it is difficult to numerically quantify the advantages and disadvantages of SEDs, due to the fact that none are in wide-spread use or mass production.SEDs have the picture-quality advantages of CRTs while continuing to be competitive with LCDs in the areas of width and power consumption. Aside from this, it is difficult to numerically quantify the advantages and disadvantages of SEDs, due to the fact that none are in wide-spread use or mass production.

8. Plasma Display Each pixel is composed of three gas-filled capsules (RGB) that operate on the same principle fluorescent lights. When a voltage is applied across the gas, free electrons bombard gas atoms, causing some electrons to jump to higher energy levels. When they fall back to stable energy level, a photon is emitted. The Xeon and Neon atoms in plasma screens release ultra-violet photons. These ultra-violet photons collide with a phosphor coating and create visible light. An electrode grid is used to turn individual pixels on and off. A plasma display is essentially composed of many tiny fluorescent lights. Each little cell contains a mixture of Xeon and Neon. When a voltage is applied across this gas, free electrons excite the gas atoms and cause them to emit photons. Unlike fluorescent lights, however, the cells of a plasma screen emit ultraviolet photons. Visible light is actually created when these ultraviolet photons hit the phosphor coating on the inside of each cell. By arranging these cells in a grid of alternating blue, green, and red rows, an entire color display can be formed.A plasma display is essentially composed of many tiny fluorescent lights. Each little cell contains a mixture of Xeon and Neon. When a voltage is applied across this gas, free electrons excite the gas atoms and cause them to emit photons. Unlike fluorescent lights, however, the cells of a plasma screen emit ultraviolet photons. Visible light is actually created when these ultraviolet photons hit the phosphor coating on the inside of each cell. By arranging these cells in a grid of alternating blue, green, and red rows, an entire color display can be formed.

9. Plasma Display Advantages Thin when compared to CRTs; approximately the same width as LCDs Higher contrast ratio and better viewing angle than LCDs [7] Easy to produce in large sizes Disadvantages Phosphor cells are subject to “burn-in” High production cost The main advantages to plasma displays are picture quality and easy production of large sizes. Large size production is difficult for LCDs. The main disadvantages are that plasma screens, like all phosphor-based screens, are subject to burn-in. Also, plasma screens are currently more expensive than LCDs for small to medium sizes.The main advantages to plasma displays are picture quality and easy production of large sizes. Large size production is difficult for LCDs. The main disadvantages are that plasma screens, like all phosphor-based screens, are subject to burn-in. Also, plasma screens are currently more expensive than LCDs for small to medium sizes.

10. Digital Light Processing (DLP) The heart of a DLP system is a DLP chip Contains many microscopic mirrors (one for each pixel) Each mirror can be in one of two positions (on or off) A light source is shined on the DLP chip When a mirror is “on”, light is reflected onto the screen, lighting that pixel To create color images, light is filtered through a color wheel. When the light source is red, red mirrors turn on. Likewise for green and blue The basic elements of a DLP system are a DLP chip, a light source, and a screen. A DLP chip contains hundreds of thousands to millions of tiny mirrors, each one corresponding to a pixel on the screen. These mirrors can be in one of two positions. In one position they reflect light from the light source; in the other they don’t. In this way, pixels can be turned on and off. This, however, allows only grayscale images. Color can be added by placing a color filter in front of the light source. When the light source is filtered to one color, only pixels that should be that color reflect; other mirrors turn off. In this way, the three colors can be displayed on each pixel.The basic elements of a DLP system are a DLP chip, a light source, and a screen. A DLP chip contains hundreds of thousands to millions of tiny mirrors, each one corresponding to a pixel on the screen. These mirrors can be in one of two positions. In one position they reflect light from the light source; in the other they don’t. In this way, pixels can be turned on and off. This, however, allows only grayscale images. Color can be added by placing a color filter in front of the light source. When the light source is filtered to one color, only pixels that should be that color reflect; other mirrors turn off. In this way, the three colors can be displayed on each pixel.

11. Digital Light Processing (DLP) Advantages Thin when compared to CRTs, though not as thin as LCDs Higher contrast ratio than LCDs[8] Low production costs Extremely long lifetime No possibility of “burn-in” Disadvantages Only one color is displayed at a time; if colors are not rotated fast enough, this becomes noticeable and is called the “rainbow effect” The manner in which light is reflected toward the screen results in poor viewing angles when compare to other technologies[8] DLP displays are thin and low-cost and also have high contrast ratios and a extremely long lifetime. Additionally, since they do not use phosphor, they are not subject to “burn-in”. They do, however, have a number of disadvantages specific to DLP. First is the rainbow effect. This problem stems from the fact that only one color is displayed at a time (i.e. first the red pixels, then the green, then the blue, etc.). If these colors are not switched fast enough, the switching becomes noticeable and can cause headaches and dizziness. Also, because the micro-mirrors are so small, they have a limited focal width. This means that most of the light is reflected straight forward. Consequently, DLP displays have a poor viewing angle.DLP displays are thin and low-cost and also have high contrast ratios and a extremely long lifetime. Additionally, since they do not use phosphor, they are not subject to “burn-in”. They do, however, have a number of disadvantages specific to DLP. First is the rainbow effect. This problem stems from the fact that only one color is displayed at a time (i.e. first the red pixels, then the green, then the blue, etc.). If these colors are not switched fast enough, the switching becomes noticeable and can cause headaches and dizziness. Also, because the micro-mirrors are so small, they have a limited focal width. This means that most of the light is reflected straight forward. Consequently, DLP displays have a poor viewing angle.

12. Comparison Summary

13. Future Market Share OLED Will capture the low-power electronics market (cell phones and PDAs) Will capture emerging market of wearable displays and find limited use in electronic billboards due to flexibility E-Ink Will capture E-book market due to extremely low power requirements Will dominate electronic billboard market due to flexibility SED Will enter production and compete with LCDs in all areas Plasma Will continue to compete with LCDs in the medium-sized television market Will increase dominance of large-sized television market DLP Will continue to compete in all-sized television markets

14. References [1] D. A. Pardo, G. E. Jabbour, N. Peyghambarian, Application of Screen Printing in the Fabrication of Organic Light-Emitting Devices, Adv. Mater. 2000, 12, No. 17, 1249 [2] Samsung SDI, OLED - Passive Matrix (PM), http://www.samsungsdi.com/contents/en/product/oled/type01.html , retrieved on July 28 2007 [3] http://electronics.howstuffworks.com/oled1.htm [4] Comiskey, B.; Albert, J. D.; Yoshizawa, H.; Jacobson, J. "An electrophoretic ink for all-printed reflective electronic displays" Nature 1998, 394, (6690), 253-255. [5] http://www.eink.com/products/matrix/High_Res.html [6] http://entertainment.howstuffworks.com/sed-tv2.htm [7] http://electronics.howstuffworks.com/plasma-display1.htm [8] http://electronics.howstuffworks.com/dlp1.htm [9] http://www.dlp.com/