The Twisted Nematic Display

FIGURE 2.32 The geometry of a twisted nematic liquid crystal device. Alignment layers are designed such that the molecules adjacent to one glass plate are twisted 90° to those at the other glass plate. 

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Unlike the twisted nematic displays which require an external reflector because of the external polarizers, the reflector for a guest-host display can be located inside the cell in direct contact with the liquid crystal layer. The internal reflector completely eliminates the parallax and shadow effects that are encountered in displays with external reflectors, and allows full use to be made of the intrinsically wider field of view available to the guest-host effect. An internal reflector can also improve the uniformity of the layer thickness because thicker cell plates can be employed without introducing parallax effects. 

The twisted nematic display is a prime example. Where direct drive for low information content displays is concerned, the primary requirements are stable materials with low thresholds, wide temperature ranges, good contrast over a wide viewing cone and fast response times. 

Chiral materials, not necessarily liquid crystalline, must be added to nematic mixtures for some display devices (see Chapter 13). For example, in the twisted nematic display a quarter-helix (90°) twist is caused by the perpendicular molecular alignment at the top and bottom plates and there are two possible twist directions. In order to ensure the same twist direction throughout the device, a very small quantity of a chiral material (e.g, compound 3) is added to the nematic mixture. Significantly too, a chiral material is required in the so-called supertwisted nematic (STN) displays where a twist in the nematic director of more than 90° is employed (usually between 180° and 240°). In this case, a chiral material with an appropriate helical pitch length (P) is chosen in conjunction with a particular cell spacing (d) for example, a d/P ratio of 0.75 induces a twist angle of 270°. 

As the first experiment of this new series, the twisted nematic display is the most appropriate because, on the one hand, the explanation of its operational principle is based on properties of the nematie phase which are usually... 

Such a 90° twist cell is of particular importance for electro-optic displays as it forms the basis of the twisted nematic display [23], which is the dominant technology both for panels of a low to medium information content and for use over an active backplane in complex displays. The important features of the rotation of light by the twisted layer may be summarized as follows ... 

Since the LCD was first developed, there have been many variations on the simple display mode shown. Different combinations of molecular orientations, surface treatments, and electric fields have yielded new, faster displays with higher contrast ratios. The liquid crystal material itself has also been highly optimized by the synthesis of a host of different liquid crystal molecules and the preparation of finely tuned liquid crystal mixtures. In this book, we do not discuss the many iterations of LCD improvement. Instead, we discuss one additional mode for the LCD, the twisted nematic display, as this is one of the most commercially successful and long-lasting modes of operation. 

We first discuss the classical Freedericksz transitions and critical thresholds for a nematic. The understanding of these phenomena is crucial to the basic traditional idea of switching liquid crystal cells by fields having magnitudes above the critical threshold. The commercial exploitation of these results in liquid crystal display devices, especially the twisted nematic display to be discussed in Section 3.7 below, has greatly increased the general interest in theoretical and experimental aspects of Freedericksz transitions, and vice-versa. 

The first stable commercial liquid crystal display (LCD) device was the twisted nematic (TN) [110], still widely... 

A slight variant of the classic twisted nematic display has been iatroduced due to its wider viewiag angle and improved switching characteristics. 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

Among the various LCDs, the three most widely used are (i) dynamic scattering display, (ii) twisted nematic display and (iii) guest-host display (Shanks, 1982). 

Afterwards there appeared what has become the main application liquid crystal displays (LCDs) based on the twisted nematic (TN) mode. These are commonly used for flat panel displays (e.g., desk calculators). Thin film transistor (TFT) LCDs enabled a large number of segments (e.g., 640 x 1024) to be used and they had advantages like... 

Liquid crystals can display different degrees of long-range order, dependent on temperature, chemical composition, and the presence or absence of electric fields. In the nematic phase, the molecular axes point in a common direction, denoted by the director n but the molecular centers are otherwise arranged randomly. Because of the low degree of long-range order, nematic LCs have viscosities typical of ordinary liquids, and displays based on nematic LCs can operate at television frame rates. The most popular nematic-based display, the twisted nematic (TN), will be discussed in more detail below. 

A twisted nematic display consists of a top electrode and bottom electrode, typically spaced 10 pim apart, whose surface alignments are perpendicular. The director, which is parallel to the surface, therefore rotates by 90° from top to bottom. Light enters the device at the top through a... 

The twisted nematic liquid crystal display (TN-LCD) was reported by Schadt and Helfrich of F. Hoffman-La Roche in Basle, Switzerland in 1970. This was part of a tripartite collaboration between F. Hoffman-La Roche in Basle, Brown Boveri of Baden and Ebauche in Neuchatel, all in Switzerland. The intention was to design and develop flat panel displays, e.g. for digital watches. The first LCD factory was constructed in Lenzburg, Switzerland in the mid-1970s by Videlec, a subsidiary of Brown Boveri. Since then the TN-LCD has... 

The twisted nematic (TN) and supertwisted nematic (STN) liquid crystals are widely used in liquid crystal displays. The former is used in wrist watches and calculators while the latter is used in notebook computers. 

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