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 liquid crystal (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 plasma display 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 (OLEDs) 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, display contrast 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.