ABSTRACT
Liquid crystals (LC) are an important class of materials with applications ranging from display devices, optoelectronic devices, sensors, and biological and structural materials. The focus of this chapter will be on LCs for display applications. In general, most substances have a single melting point where a solid possessing a positional and orientational order changes upon melting to an isotropic liquid that has neither positional nor orientational order. However, some materials when melted from the solid state change into a cloudy liquid with orientational order at one temperature, and upon further heating change into an isotropic liquid that has no order, as shown in Figure 32.1. Thus, an LC is a mesophase existing between the melting temperature
T
, of a crystalline phase, and clearing point
T
, of the liquid phase; i.e., below
T
, the material has a crystalline phase, above
T
, it has a liquid (isotropic) phase, and between
T
and
T
, it has a liquid crystal phase. This type of LC in which the mesophase is defined by the temperature (between
T
and
T
) is called a
thermotropic
LC. When the mesophase is defined by a solvent concentration, it is called a
lyotropic
LC. Thermotropic LCs are used for display applications. The orientational order in LC materials results in important physical properties, such as birefringence, that make these materials useful for display devices. Because LCs have the attributes of low drive voltage, low power consumption, thin form factor (flat panel displays), light weight, full-color, gray scale with a wide dynamic range, full motion video, superior image quality, and high reliability, LC displays (LCDs) are the preferred approach for battery-powered (portable) applications ranging from wristwatch displays and handheld TVs to laptop computer displays. They are also replacing cathrode ray tubes (CRTs) in select applications such as avionic displays because of their high brightness and readability in sunlight. LCs are also being used in projection display devices for head-mounted display (HMD) and for largescreen display applications. The following will discuss the various types of LC materials, their properties,
LCD materials and fabrication processes, various LCD modes, and display addressing methods. There are many good general references, for example, References 1 through 7, on LCs and LCDs. At the time of this writing, LCD technology is advancing very rapidly with respect to technology development for LCD products with improved viewing angles, improved image quality, lower power consumption, and larger display sizes. The purpose of this chapter, however, is to present the basic LCD principles and technologies, as opposed to reviewing the current state of the art.
