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Flat Panel Displays

1.1 INTRODUCTION

Displays provide a man-machine interface through which information can be passed to the human visual system. The information may include pictures, animations, and movies, as well as text. One can say that the most basic functions of a display are to produce, or re-produce, colors and images. The use of ink to write, draw, or print on a paper as in a painting or a book might be regarded as the longest established display medium. However, the content of such a traditional medium is static and is typically difficult or impossible to modify or update. Also, a natural or artificial source of light, is needed for reading a book or viewing a picture. In contrast, there are now many electronic display technologies, which use an electronic signal to create images on a panel and stimulate the eyes. In this chapter, we first introduce flat panel display (FPD) classifications in terms of emissive and non-emissive displays, where non-emissive displays include both transmissive and reflective displays. Then, specifications of FPDs will be outlined. Finally, the FPD technologies described in the later chapters of this book will be briefly introduced.

Displays can be subdivided into emissive and non-emissive technologies. Emissive displays emit light from each pixel which forms a part of the image on the panel. On the other hand, non-emissive displays modulate light by means of absorption, reflection, refraction, and scattering, to display colors and images. For a non-emissive display, a light source is needed. Such non-emissive displays can then be further classified into transmissive and reflective types. In historical terms, one of the most successful technologies for home entertainment has been the cathode ray tube (CRT), which enabled the widespread adoption of television (TV). It exhibits the advantages of being self-emissive and offering wide viewing angle, fast response, good color saturation, long lifetime, and good image quality. However, one of its major disadvantages is its size and bulk. The depth of a CRT is roughly equal to the length or width of the panel. For example, for a 19 in. (38.6 cm × 30.0 cm) CRT with aspect ratio of 4 : 3 the depth of a monitor is about 40 cm. Hence, it is hardly portable; its bulky size and heavy weight limit its applications.

In this book, we introduce various types of FPDs. As the name implies, these displays have a relatively thin profile, several centimeters or less, which is largely independent of the screen diagonal. Specifying a display or the design and optimization of a display-based product require selection of an appropriate technology, and are strongly dependent on the application and intended conditions of use. These issues, together with the intense pace of FPD development, which has made available many options and variations of the different display types, have made a thorough understanding of displays essential for product engineers. The options can be illustrated by some typical examples. For instance, the liquid crystal display (LCD) is presently the dominant FPD technology and is available with diagonal sizes ranging from less than 1 in. (microdisplay) to over 100 in. Such a display is usually driven by thin-film-transistors (TFTs). The liquid crystal cell acts as a light modulator which does not itself emit light. Hence, a backlight module is usually used behind a transmissive LCD panel to form a complete display module. In most LCDs, two crossed polarizers are employed which can provide a high contrast ratio. However, the use of polarizers limits the maximum optical transmittance to about 35-40%, unless a polarization conversion scheme is implemented. Moreover, at oblique angles the optical performance of the assembly is degraded by two important effects. Firstly the projections of optic axes of two crossed polarizers onto the E vector of the light are no longer perpendicular to each other when light is incident at an oblique angle, so it is difficult to maintain a good dark state in the display over a wide viewing cone. Secondly, the liquid crystal (LC) is a birefringent medium, which means that electro-optic effects based on switching an LC are dependent on the relative directions of the incident light and the LC alignment in the cell. Hence, achieving a wide viewing angle and uniform color rendering in an LCD requires special care. To achieve wide-view, multi-domain architectures and phase compensation films (either uniaxial or biaxial) are commonly used; one for compensating the light leakage of crossed polarizer at large angles and another for compensating the birefringent LC layer. Using this phase compensation technique, transmissive multi-domain LCDs exhibit a high contrast ratio, high resolution, crisp image, vivid colors (when using quantum dots or narrow-band light emitting diodes), and a wide viewing angle. It is still possible for the displayed images to be washed out under direct sunlight. For example, if we use a smartphone or notebook computer in the high ambient light conditions found outdoors in clear weather, the images may not be readable. This is because the reflected sunlight from the LCD surface is much brighter than that transmitted from the backlight, so the ambient contrast ratio is greatly reduced. A broadband anti-reflection coating and adaptive brightness control help improve the sunlight readability.

Another approach to improve sunlight readability is to use reflective LCDs [1]. A reflective LCD uses ambient light to illuminate the displayed images. It does not need a backlight, so its weight, thickness, and power consumption are reduced. A wrist watch is such an example. Most reflective LCDs have inferior performance compared to transmissive ones in terms of contrast ratio, color saturation, and viewing angle. Moreover, in fully dark conditions a reflective LCD is not readable at all. As a result, its application is rather limited.

To overcome the sunlight readability issue while maintaining high image quality, a hybrid display termed a transflective liquid crystal display (TR-LCD) has been developed [2]. In a TR-LCD, each pixel is subdivided into two sub-pixels which provide, respectively, transmissive (T) and reflective (R) functions. The area ratio between T and R can be adjusted depending on the applications. For example, if the display is mostly used out of doors, then a design which has 80% reflective area and 20% transmissive area might be used. In contrast, if the display is mostly used indoors, then we can use 80% transmissive area and 20% reflective area. Within this TR-LCD family, there are various designs: double cell gap versus single cell gap, and double TFTs versus single TFT. These approaches attempt to solve the optical path-length disparity between the T and R sub-pixels. 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 can make the cell gap of the T sub-pixels twice as thick as that of the R sub-pixels. This is the dual cell gap approach. The single cell gap approach, however, 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 (providing a stronger field for the T region and a weaker field in the R region), and dual alignments. Although TR-LCDs can improve sunlight readability, the fabrication process is much more complicated and the performance inferior to transmissive devices. Therefore, TR-LCD has not been widely adopted in products.

Light-emitting diodes (LEDs) consist of a semiconductor p-n junction, fabricated on a crystalline substrate. Under a forward bias, electrons and holes are injected into the device where they recombine and emit light. The emission wavelength of the LED is determined by the bandgap of the semiconductor. For longer wavelength (such as red and yellow) emission, an AlGaInP-based semiconductor is needed. Three group III (Al, Ga, and In) and one group IV (P) atoms are needed to allow tuning of the emission wavelength and lattice-matching to the substrate (e.g. GaAs). However, for shorter wavelength (green and blue) emission, it was not easy to find a lattice-matched substrate. Besides, there were other technological difficulties in fabricating nitride-based LEDs such as p-type doping and InGaN growth. In recognition of their successful demonstration of the InGaN-based blue LED, Professor Isamu Akasaki, Professor Hiroshi Amano, and Professor Shuji Nakamura were awarded the Nobel Prize in Physics in 2014. By combining the blue LED with phosphors, white emission is possible from a single chip. LEDs have been used for many display and lighting applications, such as traffic lights, very large diagonal (over 100 in.) signage, backlights of LCD, and general lighting, due to their long lifetime and high efficiency. A detailed description of LEDs from the viewpoints of materials, devices, fabrication, and applications will be presented in Chapter 5.

In Chapter 6, organic light-emitting devices (OLEDs) will be introduced. The operating principle of OLEDs is quite similar to that of the LED. It is also an electroluminescence (EL) device, but fabricated from organic materials rather than a semiconductor. In contrast to LEDs, it is not necessary to fabricate OLEDs on a crystalline...