1

Introduction

1.1 Flat panel displays A display is an interface containing information which stimulates human vision. Information may be pictures, animation, movies and articles. One can say that the functions of a display are to produce or reproduce colors and images. Using ink to write, draw or print on paper is a traditional display, like a painting or a book. However, the content of such a traditional display is motionless and typically inerasable. In addition, a light source, synthetic or natural, is needed for reading a book or seeing a picture. There are lots of electronic displays that use an electronic signal to create images on a panel and stimulate the human eye. Typically, they can be classified as emissive and nonemissive. Emissive displays emit light from each pixel which constitutes an image on the panel. In contrast, nonemissive displays modulate light, by means of absorption, reflection, refraction and scattering, to display colors and images. For a nonemissive display, a light source is needed. Hence, these can be classified into transmissive and reflective displays. One of the most successful display technologies for home entertainment is the cathode ray tube (CRT), which is in widespread use in televisions (TVs). CRT is already a mature technology which has the advantages of self-emission, wide viewing angle, fast response, good color saturation, long lifetime and good image quality. However, a major disadvantage is its bulky size. The depth of a CRT is roughly equal to the length and width of the panel. For example, a monitor’s depth is about 40 cm for a 19-inch (38.6 cm × 30.0 cm) CRT with an aspect ratio of 4:3. Hence, it is not very portable. The bulky size and heavy weight limit its applications. In this book, we introduce various types of flat panel displays (FPDs). As the name implies, these displays have a relatively thin profile, i.e. several centimeters or less. For instance, the liquid crys- tal display (LCD) is presently the dominant FPD technology with diagonal sizes ranging from less than 1 inch (microdisplay) to over 100 inches. Such a display is usually driven by thin-film transistors (TFTs). A (LC) is a light modulator because it does not emit light. Hence, a backlight module is required for a transmissive LCD. In most LCDs, two crossed polarizers are employed in order to obtain a high contrast ratio. The use of two polarizers limits the maximum transmittance to about 35–40 %, unless a polarization conversion scheme is implemented. Moreover, the optical axes of two crossed polarizers areCOPYRIGHTED no longer perpendicular to each other MATERIAL when viewed at oblique angles. A LC is a birefringent medium which means its electro-optic effects are dependent on the incident light direction. Therefore, the viewing angle of a LCD is an important issue. Most wide-view LCDs require multiple optical phase compensation films; one for compensating the crossed polarizer and another for the birefrin- gent LC. Film-compensated transmissive LCDs exhibit a high contrast ratio, high resolution, crisp image, good color saturation and wide viewing angle. However, the displayed images can be washed out under

Introduction to Flat Panel Displays J.-H. Lee, D.N. Liu and S.-T. Wu c 2008 John Wiley & Sons, Ltd 2 Introduction to Flat Panel Displays direct sunlight. For example, if we use a notebook computer at outdoor ambient, the images may not be readable. This is because the reflected sunlight from the LCD surface is much brighter than that trans- mitted from the backlight so that the signal-to-noise ratio is low. A broadband antireflection coating will definitely help to improve the sunlight readability. Another way to improve sunlight readability is to use reflective LCDs.1 A reflective LCD uses ambient light to produce the displayed images. It does not carry a backlight; thus, its weight is reduced. A wrist- watch is such an example. Most reflective LCDs have inferior performances compared to the transmissive ones in contrast ratio, color saturation and viewing angle. Moreover, at dark ambient a reflective LCD is not readable. As a result, its application is rather limited. To overcome the sunlight readability issue while maintaining high image quality, a hybrid display called a transflective liquid crystal display (TR-LCD) has been developed.2 In a TR-LCD, each pixel is divided into two subpixels: transmissive (T) and reflective (R). The area ratio between T and R can be adjusted depending on the application. For example, if the display is mostly used outdoors, then one can design to have 80 % reflective area and 20 % transmissive area. In contrast, if the display is mostly used indoors, then one can have 80 % transmissive area and 20 % reflective area. Within this TR-LCD family, there are still some varieties: double cell gap versus single cell gap, and double TFTs versus single TFT. These approaches are trying to solve the optical path length disparity between the T and R subpixels. In the transmissive mode the light from the backlight unit passes through the LC layer once, but in the reflective mode the ambient light traverses the LC medium twice. To balance the optical path length, we could make the cell gap of the T subpixels twice as thick as that of the R subpixels. This is the so-called dual cell gap approach. The single cell gap approach has a uniform cell gap throughout the T and R regions. To balance the different optical path lengths, several approaches have been developed, e.g. dual TFTs, dual fields (stronger field for T region and weaker field for R region) and dual alignments. Presently, the majority of TR-LCDs adopt the double cell gap approach for two reasons: (1) both T and R modes can achieve maximum light efficiency, and (2) the gamma curve matching between the voltage-dependent transmittance (VT) and reflectance (VR) is almost perfect. However, the double cell gap approach has two shortcomings: first, the T region has a slower response time than the R region because its cell gap is about twice as thick as that of the R region; second, the viewing angle is relatively narrow, especially when homogeneous cells are employed. To widen the viewing angle, a special rod-like LC polymeric compensation film has to be used. Chapter 4 gives detailed descriptions of various types of LCDs. A panel (PDP) is an emissive display which can be thought of as very many miniature fluorescent lamps on a panel. As an emissive display it typically has a better display performance, such as good color saturation and wide viewing angle. Due to the limitation of fabrication, the pixel size of a PDP cannot be too small. For a finite pixel size, the video content is increased by enlarging the panel size. PDPs are suitable for large-screen applications. In 2008, Panasonic demonstrated a 150-inch PDP TV with 4096 × 2160 pixels. This resolution is four times higher than that of the present full high-definition television (HDTV). Light-emitting diodes (LEDs) and organic light-emitting devices () are electroluminescent devices with semiconductor and organic materials, respectively. Electrons and holes recombine within the emissive materials, where the bandgap of the materials determines the emission wavelength. A field emission display (FED) uses sharp emitters to generate electrons. These electrons bombard the phosphors that are present to emit red (R), green (G) and blue (B) light. A FED is like a ‘flat’ CRT. Due to the mature technologies developed in CRTs, FEDs exhibit all the advantages of CRTs plus the smaller panel thickness. Compared to conventional displays (such as books, magazines and newspapers), electronic displays (such as TVs, mobile phones and monitors) are rigid because they are typically fabricated on glass substrates. Flexible FPDs are emerging. Several approaches have been developed, such as electrophoretic displays and polymer-stabilized cholesteric displays. Flexible displays are thin, robust and lightweight. In the remainder of this chapter, we first introduce FPD classifications in terms of emissive and none- missive displays, where nonemissive displays include transmissive and reflective displays. Specifications Introduction 3 of FPDs are then outlined. Finally, the FPD technologies described in the later chapters of this book are briefly introduced.

1.2 Emissive and nonemissive displays Both emissive and nonemissive FPDs have been developed. For emissive displays, each pixel emits light with different intensity and color which stimulate the human eye directly. CRTs, PDPs, LEDs, OLEDs and FEDs are emissive displays. An emitter is called Lambertian when the luminances from different viewing directions are the same. Most emissive displays are Lambertian emitters which results in a wide viewing angle performance. Also, due to the self-emissive characteristics, they can be used even under very low ambient light. When such displays are turned off, they are completely dark (ignoring the ambient reflection). Hence, ratios (see also Section 1.3.3) are high. Displays that do not emit light themselves are called nonemissive displays. A LCD is a nonemissive display in which the LC molecules in each pixel work as an independent light switch. The external voltage reorients the LC directors which causes phase retardation. As a result, the incident light from the backlight unit or ambient is modulated. Most high-contrast LCDs use two crossed polarizers. The applied voltage controls the transmittance of the light through the polarizers. If the light source is behind the display panel, the display is called a transmissive display. It is also possible to use ambient light as the light source. This resembles the concept of a conventional display, such as reading a book, which is called a reflective display. Since no backlight is needed in a reflective display, its power consumption is relatively low. In a very bright environment, images of emissive displays and transmissive LCDs can be washed out. In contrast, reflective displays exhibit an even higher luminance as the ambient light increases. However, they cannot be used in a dim environment. Hence, transflective LCDs have been developed, which are described in Chapter 4.

1.3 Display specifications In this section, we introduce some specifications which are generally used to describe and judge FPDs from the viewpoints of mechanical, electrical and optical characteristics. FPDs can be smaller than 1 inch for projection displays, 2–4 inches for mobile phones and personal digital assistants, 7–9 inches for car navigation systems, 8–18 inches for notebook computers, 10–25 inches for desktop computers and more than 100 inches for direct-view TVs. For different FPDs, their requirements for pixel resolutions also differ. Luminance and color are two important characteristics which directly affect the display performances. Dependences of these two parameters to viewing angles, uniformity, lifetime and response time should be addressed when describing the performances of an FPD. Contrast ratio is another important parameter, which changes with different ambient environments.

1.3.1 Physical parameters The basic physical parameters of an FPD include display size, aspect ratio, resolution and pixel format. The size of a display is typically described by diagonal length, in units of inches. For example, a 15-inch display means the diagonal of the viewable area of this display is 38.1 cm. There are three kinds of display format: landscape, equal and portrait, corresponding to the display width being larger than, equal to and smaller than its length. Most monitors and TVs use landscape format with a width-to-length ratio, which is called the ‘aspect ratio’, of 4:3, 16:9 or 16:10, typically. An FPD typically consists of a ‘dot matrix’ which can display images and characters. To increase resolution, one may use more dots in a display. Table 1.1 lists some standard resolutions of FPDs. For 4 Introduction to Flat Panel Displays

Table 1.1 Resolution of FPDs.

Abbreviation Full name Resolution

VGA Video graphics array 640 × 480 SVGA Super video graphics array 800 × 600 XGA Extended graphics array 1024 × 768 SXGA Super extended graphics array 1280 × 1024 UXGA Ultra extended graphics array 1600 × 1200 WXGA Wide extended graphics array 1366 × 768 WSXGA Wide super extended graphics array 1680 × 1050 WUXGA Wide ultra extended graphics array 1920 × 1200 example, VGA means the display is 640 dots in width and 480 dots in length. Higher resolution typically (but not necessarily) means better image quality. There are some resolutions listed in Table 1.1 starting with the letter ‘W’, which means wide screen with an aspect ratio larger than 4:3. Once the resolution, display size and aspect ratio are known, one may obtain the pitch of the pixels. For example, a 19-inch display with aspect ratio of 4:3 and resolution of UXGA has a pitch of 190.5 m. Note that not all of the pixel area contributes to the display. One can define the ‘fill factor’ or ‘aperture ratio’ as the ratio of the display area in a pixel over the whole pixel size, with its maximum value of 100 %. Besides, for a full-color display, at least three primary colors are needed to compose a color pixel. Hence, each color pixel is divided into three subpixels (RGB) sharing the area. For example, let us assume a color pixel has size of 240 m × 240 m; then the dimension of each subpixel is 80 m × 240 m. If the fill factor is 81 % which actually contributes to light emission or transmission, then the usable pixel area is reduced to 72 × 216 m2. There are different layouts for RGB subpixels, as shown in Figure 1.1. For the stripe configuration, it is straightforward and easy for fabrication and driving circuit design. However, it has a poor color mixing performance for the same display area and resolution. For mosaic and delta configurations, their fabrication and/or driving circuit are more complicated but their image quality is better because of better color mixing capability. Also, displays with mosaic and delta configurations exhibit faster response times since the moving distance between the pixels is shorter. Actually, as the resolution gets high enough the subpixel arrangement becomes less critical. For medium and large displays, the stripe configuration is typically used. In contrast, for a small-size display which requires high resolution, e.g. video cameras, one may use the mosaic or delta configuration.

* R G B

(a) (b) (c)

Figure 1.1 Subpixel layout of an FPD: (a) stripe, (b) mosaic and (c) delta configurations. Introduction 5

1.3.2 Brightness and color Luminance and color are two important optical characteristics of an FPD. A display with high luminance looks dazzling in a dark room. On the other hand, a display with insufficient brightness appears washed out under high ambient. Typically, the luminance of an FPD should be as bright as (or slightly brighter than) the real object. Under an indoor lighting environment, a monitor has a luminance of 200–300 cd m−2 (Section 2.3.6). For a large-screen TV, a higher luminance (500–1000 cd m−2) may be needed. An FPD is used to produce or reproduce colors; hence, how many colors of an FPD and how real the color is (color fidelity) between an FPD and a real object are two important characteristics of an FPD. Since the color of an FPD is mixed by (at least) three primary colors, i.e. RGB, more ‘pure’ (saturated) primaries results in a broader range of the possibly displayed colors, which is called ‘color gamut’ (Section 2.3.4). One can equally divide the stimuli to the eyes from dark to bright with 2, 4, 8 or more spacings, which is called ‘gray level’ or ‘gray scale’ (Section 2.3.3). For example, an FPD can display 16 million colors (28 × 28 × 28 ≈ 16.8 million) when each RGB subpixel is divided into 8 gray scales.

1.3.3 Contrast ratio The device contrast ratio (CR) of an FPD is defined as L CR = w , (1.1) Lb where Lw and Lb are the luminance at white and black states, respectively. Higher CR means higher on/off ratio and hence better image quality and higher color saturation. When CR is equal to or less than 1, the human eye cannot distinguish the on and off colors so that the information content of an FPD is lost or distorted. For most emissive displays, the off-state luminance is zero. Hence, the contrast ratio is infinity in a perfectly dark room. However, due to the surface reflection from the ambient, Equation (1.1) should be modified to L + L A-CR = w ar , (1.2) Lb + Lar where A-CR is the ambient contrast ratio and Lar is the luminance from ambient reflection. A-CR is used to specify the ambient contrast ratio, to distinguish from the intrinsic ‘device’ contrast ratio as described in Equation (1.1). From Equation (1.2), as the ambient reflection increases, A-CR decreases sharply. To keep a good ambient contrast, one can: (1) increase the on-state luminance, and (2) reduce the reflectivity of the display surface. However, for a very strong ambient, e.g. in sunshine outdoors, luminance from the direct sun is four orders of magnitude higher than that of an FPD, which severely washes out the information content of the FPD. Sunlight readability is an important issue especially for mobile displays. In contrast, an adequate ambient light is required for conventional displays, such as books or newspapers. A similar situation applies to reflective displays, such as reflective LCDs.

1.3.4 Spatial and temporal characteristics Uniformity of an FPD means the luminance and color change over a display area. Human eyes are sensitive to luminance and color differences. For example, a 5 % luminance difference is noticeable between two adjacent pixels. For a gradual change, human eyes can tolerate up to 20 % luminance change over the whole display. Optical characteristics (luminance and colors) may also change at different viewing angles. For Lambertian emitters, such as CRTs, PDPs and FEDs, viewing angle performances are quite good. The emission profile of LEDs and OLEDs can be engineered by packaging and layer structure. However, the viewing angle of LCDs is one of the major issues because LC material is birefringent and crossed 6 Introduction to Flat Panel Displays polarizers are no longer crossed when viewed at oblique angles. There are several ways to define the view- ing angle of an FPD. For example, to find the viewing cone with: (1) a luminance threshold; (2) minimum contrast ratio, say 10:1; or (3) maximum value of color shift. For some cases that contrast ratio is smaller than 1; this is called ‘gray level inversion’. Response time is another important metric. If an FPD has a slow response time, one may see blurred images for fast moving objects. By switching the pixel from ‘off’ to ‘on’ and from ‘on’ to ‘off’, and calculating the time required from 10 to 90 % and 90 to 10 % luminance levels, one can obtain rise and fall time, respectively. One may also define the response time from one gray level to another, which is called the ‘gray-to-gray’ (GTG) response time. Most display scenes contain rich grayscales. Therefore, GTG response time is more meaningful. For LCDs, this GTG response time can be much longer than the black-to-white rise and fall time.3 A TFT is a holding type of active matrix. It is different from the CRT’s impulse type. Therefore, a motion picture response time4 is commonly used to define the response time of a TFT LCD. After long-term operation, the luminance of an FPD (especially an emissive display) decays. In an emissive display, if a fixed pattern is lit on for a long period of time before all the pixels are turned on for the full white screen, one can see nonuniformity of the fixed pattern with a lower brightness, which is called the ‘residual image’. As mentioned before, the human eye can detect less than 5 % nonuniformity between two adjacent pixels. Hence the lifetime of an FPD is crucial for static images. An alternative solution is to use moving pictures, rather than static images, for information display. Then the luminances of all pixels decay uniformly, since the average on time for all pixels is the same.

1.3.5 Efficiency and power consumption Power consumption is a key parameter, especially for mobile displays, as it affects battery life. For displays with wall-plug electrical input, lower power consumption implies lower heat generation, which means heat dissipation is less serious. Typically, one uses the unit lm W−1 to describe power efficiency of an FPD (Section 2.3.6). Lumen (lm) and watt (W) are units for describing light output and electrical input. A portable display with lower power consumption leads to a longer battery life. For notebooks and TVs, high optical efficiency also translates into less heat dissipation and a lower electricity bill. Thermal management in a small-chassis notebook is an important issue.

1.3.6 Flexible displays An FPD is usually fabricated on thin glass plates. Glass is a kind of rigid substrate. In contrast, conventional displays are printed on paper, which is flexible. An interesting research topic is to fabricate FPDs on flexible substrates, as a ‘paper-like’display.5 Compared to the glass-based FPDs, flexible displays are thin and lightweight.Also, flexible displays can be fabricated by the roll-to-roll process, which is potentially of low cost. Substrate selection of flexible FPDs includes ultrathin glass, plastic and stainless steel. Bendable ultrathin glass substrate is achievable, but the cost is high. Plastic substrate is suitable for flexible displays, but the highest durable temperature is typically lower than 200 ◦C. Stainless steel substrate is bendable, and durable for high temperature; however, it is opaque hence not suitable for transmissive displays. There are many technical bottlenecks for flexible FPDs, such as material selection, fabrication processes, device configurations, display package and measurement.

1.4 Applications of flat panel displays The following subsections briefly outline the applications of each technology. Detailed mechanisms are described in the related chapters. Introduction 7

1.4.1 Liquid crystal displays Although LC materials were discovered more than a century ago,6,7 their useful electro-optic effects and stability were developed only in the late 1960s and 1970s. In the early stage, passive matrix LCDs were found useful in electronic calculators and wristwatches.8 With the advance of TFTs,9 color filters10 and low-voltage LC effects,11 active matrix LCDs have gradually penetrated into the market of notebook computers, desktop monitors and TVs. Today, LCDs have found widespread uses in everyday life, including (1) mobile applications, such as mobile phones, personal digital assistants, navigation systems, notebook personal computers; (2) office applications, such as desktop computers and video projectors; and (3) home applications, such as large-screen TVs.12 To satisfy these wide-spectrum applications, three types of LCDs have been developed: transmissive, reflective and transflective. Transmissive LCDs can be further separated into projection and direct-view. In a small-size, high-resolution LCD, the pixel size is around 40 m × 40 m. Here, the aperture ratio becomes particularly important because it affects the light throughput.13 To enlarge the aperture ratio, poly-silicon (p-Si) TFTs are commonly used because their electron mobility is about two orders of magnitude higher than that of amorphous silicon (a-Si). High mobility allows a smaller TFT to be used which, in turn, enlarges the aperture ratio. For the detailed structure of a TFT LCD, see Figure 4.1. For direct-view transmissive TFT LCDs, the pixel size (∼300 m × 300 m) is much larger than that of a microdisplay. Thus, a-Si is adequate although its electron mobility is relatively low. Amorphous silicon is easy to fabricate and has good uniformity. Thus, a-Si TFTs dominate the large-screen (>10 inches) LCD panel market. Similarly, reflective LCDs can also be divided into projection and direct-view displays. In projection displays using liquid-crystal-on-silicon (LCoS),14 the pixel size can be as small as ∼10 m × 10 m because of the high electron mobility of crystalline silicon (c-Si). In a LCoS device, the electronic driving circuits are hidden beneath the metallic reflector. Therefore, the aperture ratio can reach 90 % and the displayed picture is film-like. In contrast, most reflective direct-view LCDs use a-Si TFTs and a circular polarizer. Their sunlight readability is excellent, but they are not readable in dark ambient. Therefore, the application of reflective direct-view LCDs is rather limited. To maintain high-quality transmissive display and good sunlight readability, a hybrid TR-LCD has been developed. In a TR-LCD, each pixel is divided into two subpixels: one for transmissive and another for reflective display.15 In dark to normal ambient, the backlight is on and the TR-LCD works as a transmissive display. Under direct sunlight, the TR-LCD works in reflective mode. Therefore, its dynamic range is wide and its functionality does not depend on the ambient lighting conditions. TR-LCDs have been widely adopted in portable devices, such as mobile phones. For a detailed discussion of TR-LCDs, see Chapter 4.

1.4.2 Light-emitting diodes A LED is an electroluminescent device based on crystalline semiconductors.16 To convert electrical to optical power, one has to inject carriers into the LED through electrodes, and then they recombine to give light. The emission wavelength is mainly determined by the semiconductor materials, and can be fine tuned by device design. Since it is difficult to grow large-size single crystals, the wafer diameter of LEDs is limited to about 8 inches. After device processing, LEDs are diced from the wafer followed by the package process. The dimension of a single LED is typically several millimeters, which means the ‘pixel size’ of the LED is large. Hence, it is difficult to use a LED as a small display or it will have a very low resolution. An exception is to dice LED arrays from a wafer and use as a microdisplay with a size less than 1 to 2 inches. Due to their self-emissive characteristic, LEDs are commonly used for large displays, such as outdoor signages (single color, multicolor and full color), traffic signals and general lighting to replace light bulbs. Compared to conventional displays enabled by light bulbs, LED displays exhibit the advantages of lower 8 Introduction to Flat Panel Displays power consumption, greater robustness, longer lifetime and lower driving voltage (so safer). There are also lots of outdoor screens with diagonals of over 100 inches which consist of millions of LED pixels. Rather than a display itself, a LED can also be used as the light source, such as the backlight module for a LCD, and general lighting. Compared to a conventional cold cathode fluorescent lamp (CCFL), which resembles a thin fluorescent tube, a LED exhibits a better color performance, longer lifetime and faster response. Another important driving force to the use of LEDs as LCD backlights is that the mercury in CCFLs is harmful to the environment. When using LEDs for general lighting applications, a broad spectrum is preferred to simulate natural light, such as sunlight, for obtaining a high color rendering of reflective objects (Section 2.3.5). This is quite different from the requirements for LED displays and LCD backlights, which usually need a narrow spectrum.

1.4.3 Plasma display panels The typical structure and operation principle for PDPs are similar to those of a fluorescent lamp. In the structure of a fluorescent lamp, two filament electrodes are formed in two ends of an inner glass tube. The wall of the inner glass tube is coated with phosphor. The cavity of the glass tube is filled with a gas mixture of argon and mercury. When a certain voltage is applied to the electrodes, plasma is generated from a gas discharge. Due to the energy level system of the plasma, ultraviolet (UV) radiation is generated with peak wavelength at λ = 254 nm. The phosphor of the fluorescent lamp is excited by the UV radiation which, in turn, emits light. PDPs use a similar operation mechanism to fluorescent lamps but the gases commonly used in PDPs are neon and xenon instead of the argon and mercury used in fluorescent lamps. Neon and xenon gases generate peak wavelengths at 147 and 173 nm which belong to the vacuum ultraviolet (VUV) region. VUV radiation can only propagate in a vacuum because it is strongly absorbed by air. Although the PDP structure is similar to a fluorescent lamp which is composed of two electrodes, phosphor and gases, an additional barrier rib structure is needed in PDPs to sustain the space between upper plate and lower plate.17 Because of the structure of the barrier rib, the unit cell size of PDPs cannot be made too small. In addition, PDP operation voltage is high because a typical plasma generation is needed. The high operation voltage demands a high voltage driver integrated circuit (IC) and results in a high cost of the electronics. However, PDPs exhibit a wider view angle, faster response time and wider temperature range than LCDs. In other words, PDPs remain good candidates for large-panel displays spanning from static pictures to motion pictures, from cold ambient to hot ambient and from personal use to public use. In addition to these performance advantages, PDPs can be fabricated with a low-cost and simple manufacturing process. For these reasons, many different PDP structures intended for a wide spectrum of applications have been developed.18−20

1.4.4 Organic light-emitting devices An OLED is also an electroluminescent device, like a LED, except its materials are organic thin films with amorphous structures.21 Amorphous organic material has a much lower mobility (typically five order of magnitude lower) than crystalline semiconductors, which results in a higher driving voltage of OLEDs. Also, the operation lifetime of OLEDs is one order of magnitude shorter than semiconductor LEDs. However, due to the amorphous characteristics, fabrication with large size (>40 inches) is possible. Since the conductivity of amorphous organic materials is very low, very thin organic films (100– 200 nm in total) are required to reduce the driving voltage to a reasonable value (i.e. <10 V). This is quite a challenge in thin-film formation, especially for large-size substrates. There are several fabrication technologies proposed, such as physical vapor deposition, spin coating, ink-jet printing and laser-assisted patterning. Prototypes of OLED panels of 40 inches have been demonstrated, and OLEDs of 11 inches Introduction 9

(or less) are also commercially available.22,23 Another challenge for large displays is the reliability. A TV must have a longer lifetime than a mobile phone. Recently, commercial OLED products for mobile displays and monitors have emerged. However, for TV applications, OLED panel lifetime still falls short. Two advantages of OLEDs are: (1) low process temperature, and (2) nonselective to the substrate material, which is suitable for flexible displays. One of the strategies for OLED development is to improve device performance (especially driving voltage and lifetime) so as to be as good as (or not too much worse than) LEDs. Also, due to the possibility of large-size fabrication, the potential manufacture cost of OLEDs is lower than that of LEDs. Because OLEDs have some advantages in performance and fabrication cost over LEDs, they have a chance to replace LEDs in some applications since they are both electroluminescent devices with similar operation principles. Besides, in comparison with LEDs, OLEDs have two unique advantages: larger panel size and higher resolution.

1.4.5 Field emission displays A FED is a display using electrons generated by field emission to excite phosphors and generate lumin- ance. There are several different approaches to generate electron emission, such as thermionic emission, photoemission and field emission.24,25 Thermionic emission electrons are thermally excited over the potential energy barrier while photoemission electrons are excited over the potential energy barrier by incoming photons. In field emission, the electrons tunnel though the surface potential energy barrier, which has been thinned and shaped by the influence of a strong electric field. The field emitter plays an important role in the electron emission of FEDs. The structure of the field emitter can be in the shape of a cone, a wedge, a cylinder or a tube.26 The emitting region is a tip for a conic-shape emitter while the emitting region is an edge for wedge, cylinder and tube shapes. There are also many types of emitters such as the Spindt emitter, carbon nanotube (CNT) emitter and surface conduction emitter (SCE).27 A Spindt emitter uses sharp conic material as an emitter while a CNT emitter uses a carbon tube of nanometric diameter as an emitter. A SCE uses a material named PdO as an emitter with a nano-gap structure to generate surface electrons. These types of emitters can be undesirably damaged by the ions generated from residual gas. This undesirable damage usually results in a short lifetime of operation. Therefore, less residual gas and lower operating voltage are strongly demanded in FEDs. In order to have a lower operating voltage, an emitter material of low work function with a sharp structure is desired. In addition, vacuum is also required so that residual gas can be eliminated. Both FEDs and CRTs use phosphors to generate visible light which demands a vacuum to ensure long life of electron emission. The structure of a FED consists of an emitter, electrode and phosphor which is similar to that of a CRT. The display performance is also similar to a CRT. However, a spacer structure is needed in a FED to maintain the space between phosphor plate and field emission plate. The emission uniformity which is caused by the process of emitter formation is the major challenge for FEDs. Above all, FED structure is simple because it does not require backlight, color filters, polarizers or other optical films which are needed in LCDs. Furthermore, FEDs have higher luminance efficiency, faster response time, a wider view angle and greater temperature range than LCDs.28 FEDs can be widely applied from static pictures to motion pictures, from cold ambient to hot ambient and from personal use to public use.29

References 1. Wu, S.T. and Yang, D.K. (2001) Reflective Liquid Crystal Displays, John Wiley & Sons, Ltd, Chichester. 2. Okamoto, M., Hiraki, H. and Mitsui, S. (2001) US Patent 6,281,952. 3. Wang, H., Wu, T.X., Zhu, X. and Wu, S.T. (2004) Correlations between liquid crystal director reorientation and optical response time of a homeotropic cell. J. Appl. Phys., 95, 5502. 4. Song, W., Li, X., Zhang, Y. et al. (2008) Motion-blur characterization on liquid-crystal displays. J. SID, 16, 587. 5. Crawford, G.P. (2005) Flexible Flat Panel Displays, John Wiley & Sons, Ltd, Chichester. 10 Introduction to Flat Panel Displays

6. Reinitzer, F. (1888) Monatsh. Chem., 9, 421. 7. Lehmann, O. (1889) Z. Phys. Chem., 4, 462. 8. Ishii, Y. (2007) The world of liquid crystal display TVs: past, present and future. J. Display Technol., 3, 351. 9. Lechner, B.J., Marlowe, F.J., Nester, E.O. and Tults, J. (1971) Liquid crystal matrix displays. Proc. IEEE, 59, 1566. 10. Fischer, A.G. et al. (1972) Design of a liquid crystal color TV panel. Proceedings of the IEEE Conference on Display Devices, p. 64. 11. Schadt, M. and Helfrich, W. (1971) Voltage-dependent optical activity of a twisted nematic liquid crystal. Appl. Phys. Lett., 18, 127. 12. Liu, C.T. (2007) Revolution of the TFT LCD technology. J. Display Technol., 3, 342. 13. Stupp, E.H. and Brennesholtz, M. (1998) Projection Displays, John Wiley & Sons, Inc., New York. 14. Armitage, D., Underwood, I. and Wu, S.T. (2006) Introduction to Microdisplays, John Wiley & Sons, Ltd, Chichester. 15. Zhu, X., Ge, Z., Wu, T.X. and Wu, S.T. (2005) J. Display Technol., 1, 15. 16. Round, H.J. (1907) A note on carborundum. Electrical World, 19, 309. 17. Fischer-Cripps, A.C., Collins, R.E., Turner, G.M. and Bezzel, E. (1995) Stress and fracture probability in evacuated glazing. Building Environ., 30, 41. 18. Oversluizen, G. and Dekker, T. (2006) High efficacy PDP design. SID Symp. Dig., 37, 1110. 19. Hirakawa, H., Shinohe, K., Tokai, A. et al. (2004) Dynamic driving characteristics of plasma tubes array. SID Symp. Dig., 35, 810. 20. Sano, Y., Nakamura, T., Numomura, K. et al. (1998) High-contrast 50-in color ac plasma display with 1365 × 768 pixels. SID Symp. Dig., 29, 275. 21. Tang, C.W. and Vanslyke, S.A. (1987) Organic electroluminescent diodes. Appl. Phys. Lett., 51, 913. 22. Iino, S. and Miyashita, S. (2006) Printable OLEDs promise for future TV market. SID Symp. Dig., 37, 1463. 23. Hirano, T., Matsuo, K., Kohinata, K. et al. (2007) Novel laser transfer technology for manufacturing large-sized OLED displays. SID Symp. Dig., 38, 1592. 24. Gomer, R. (1961) Theory of Field Emission: Field Emission and Field Ionization, Harvard University Press, Cambridge, MA. 25. Dyke, W.P. and Dolan, W.W. (1956) Field emission, in Advances in Electronics and Electron Physics, Vol. 8 (ed. L. Marton), Academic Press, New York. 26. Liu, D., Ravi, T.S., Gmitter, T. et al. (1991) Fabrication of wedge-shaped silicon field emitters with nm scale radii. Appl. Phys. Lett., 58, 1042. 27. Okuda, M., Matsutani, S., Asai, A. et al. (1998) Electron trajectory analysis of surface conduction electron emitter displays. SID Symp. Dig., 29, 185. 28. Utsumi, T. (1991) Keynote address vacuum microelectronics: what’s new and exciting. IEEE Trans. Electron Dev., ED-38, 2276. 29. Itoh, S. et al. (2007) Development of field-emission display. SID Symp. Dig., 38, 1297.