Supertwisted nematic cells

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... 

H. Seiberle and M. Schadt, LC-conductivity and cell parameters their influence on twisted nematic and supertwisted nematic liquid crystal displays. Mol. Cryst. Liq. Cryst. 239(1), 229-244, (1994). 

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°. 

The geometry of a SBE display is typical for the supertwisted displays (Fig. 4.20). A 270 supertwisted nematic layer is oriented with a 28 director tilt at the boundaries to prevent the appearance of light-scattering domain structures. In an SBE cell the ratio d/P = 0.75, i.e., three-quarters of the helix pitch, is fitted within the layer thickness d. When the field is switched on the director reorients to nearly homeotropic configuration (dielectric anisotropy Ae > 0). Two polars used in the SBE display are located at angles (3 and 7 with respect to the director projection on the input (Li) and output substrates. 

Besides these two standard cell alignments, there are many other variations such as hybrid, twisted, supertwisted, fingerprint, multidomain vertically aligned, etc. Industrial processing of these nematic cells, as well as the transparent conductive coating of the cell windows for electro-optical device applications, is understandably more elaborate. 

High contrast and uniformity of transmission characteristics at oblique incidence are also beneficial features of the SBE mode which considerable improve the legibility of supertwist displays. Better viewing angles than that for the 90° twist structure seem to be a peculiar feature of highly twisted chiral nematics. Figure 4.23 demonstrates this for a 200° supertwist cell in comparison with the usual 90° twist cell [127]. 

TABLE 4.8. Effect of various chiral nematic parameters and cell geometry on the electrooptical characteristics of supertwist cells. 

The problems of supertwisted displays are response times. Current nematic mixtures have response times of 200 ms, which is much slower than the 80 ms required in mouse and cursor applications in personal computers, and far slower than the 15 ms required for video applications. This problem is being solved by using higher birefringence nematic mixtures and decreasing the cell gap down to 4-5 microns. For instance, Seiko-Epson have developed a 50 ms response time display [6]. 

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