Nematic twisted electro optic - cell

Note In the off-state the electro-optical cell contains a thin film of a nematic liquid-crystal with mutually perpendicular directors at the upper and lower glass plates hence to reach the on-state the director performs a 90° twist over the thickness of the liquid crystal film. 

The times for a twisted nematic electro-optic cell to switch on or rise (tr) or to switch off or decay (t[Pg.313]

In an extensive work on polysiloxane polymer liquid crystals, Finkelmann et al. examined their performance in a twisted nematic electro-optical cell. In parallel with the polymer measurements, analogous studies were carried out on low molar mass nematic compounds of equivalent dielectric anisotropy (Ae). Both the low molar mass nematic materials and the polymer side-group moieties were mixtures of methoxy- (or alkyloxy-) and chlorine-terminated benzoate esters. Cells were prepared using rubbed polyimides and in the polymer case the samples were annealed for several hours to obtain defect-free aligned textures. Measurement of the threshold voltages, for both the low molar mass compounds and the polymer, showed ... 

Time required for the light intensity viewed through crossed polarizers to increase to 90% of the final value from the off-state to the on-state of an electro-optical twisted-nematic cell. 

Time required by an electro-optical twisted-nematic cell for a light-intensity change from 90% to 10 % of the maximum intensity on going from the on-state to the off-state. 

Conventional AMLCDs use nematic liquid crystals in a twisted configuration between two sheets of glass or plastic, one of which is the backplane containing the TFTs. The space between the two sheets is called the cell gap. Display operation relies on some subtle features of the electro-optic behavior of nematic liquid crystals... 

FIGURE 6.4.12 Electro-optic curve for a typical twisted-nematic liquid-crystal cell. 

Ferroelectric liquid crystals (FLC) are of great interest due to their fast electro-optical response which is about 1,000 times faster than conventional twisted nematic cells [131]. The geometry used is called a surface stabilized FLC cell which utilizes a very thin gap (=2 pm) to unwind the FLC supramolecular pitch (=1-2 pm) since the bulk FLC materials do not show macroscopic polarization. This very thin gap, however, leads to difficulties in manufacturing large panels and very poor shock resistance. Researchers have proposed the concept of microphase stabilized FLC [79,109, 130] using FLC-coil diblock copolymers for electro-optical applications as shown in Fig. 15. This concept takes advantage of ferroelectric liquid crystallinity and block copolymer microphase separation since the block... 

Electro-Optical Effects in Twisted Nematic Cells. 224... 

Room temperature nematic liquid crystals have been developed for electro-optical applications [13-15]. In particular, twisted nematic (TN) liquid crystal displays have been widely used for practical display devices [ 13-15,38). In the TN cells, nematic liquid crystals form twisted alignment due to the influence of rubbed aligmnent polymer layers coated on the substrates (Fig. 7a). The TN cells are placed between two crossed polarizers. Without electric fields, the twisted LC aligmnent induces optical rotation of incident polarized... 

At the present time, as shown by the pocket calculator, materials are available with adequate electro-optic performance where the duty cycle is 1 in 2, 3 or 4. Where temperature compensation is used, 1 in 7 duty cycle is possible O with mixtures of cyanobiphenyls and benzoate esters. For a duty cycle of 1 in 10, temperature compensation is a must and the electro-optic performance is determined principally by the threshold sharpness. By way of example. Fig. 36 shows a viewing cone plot for ZLl 1253 (dotted line curves) compared with a sharper threshold material (1.65) (solid line curves) in an 8y, low tilt cell. By single frequency addressing, a 1 in 10 duty cycle is approaching the maximum for an acceptable performance in a twisted nematic display. 

The majority of commercial low molar mass liquid crystal displays are based on the twisted nematic electro-optic effect. Not surprisingly, therefore, one of the first attempted applications of nematic polymer materials was in a similar device.In such a cell. Fig. 6, two... 

Fig. 21. Electro-optic response times for a twisted nematic device as a function of polymer concentration. The polymer was a polysiloxane backbone system (as in Figs 19 and 20) dissolved in a nematic cyanobiphenyl compound. Ihe applied voltage was 5Vn, , the cell thickness was 12jum and data are...

There are also electro-optic effects using either a different geometry of surface stabilization or a completely different mechanism In the twisted ferroelectric smectic-C cell [54] the moleeules form in the zero field state a quarter helix which is removed when a dc field of either polarity is applied the optical effect is achieved in the same way as in a twisted nematic cell. Compounds with a short chiral smectic-C pitch in a thick cell are used for the distorted helix ferroelectric (DHF) device [55] this effect uses the optical difference between the zero-field state eharacterized by a fully developed short-pitch helix, and structures with a distorted or almost unwound helix in the presence of an applied field optically addressed spatial light modulators can take advantage of the DHF effect [56]. Further applications of ferroelectric liquid crystals are switchable diffraction gratings [57]. 

In display applications, the twist cell is placed between two polarizers. They can either be oriented parallel or perpendicular to each other, the switching modes of these two configurations being complementary. The electro-optic characteristics of nematic twist cells have been studied by several authors [56—59]. Applications will be treated in Part III of this book. 

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 ... 

In the 1980s, the research and development of AM LCDs were very active, but it had not yet opened great application. On the other hand, STN-LCD (super twisted nematic LCDs) for PCs and Japanese word processors were realized in the 1980s [7], and also colored STN-LCDs for PCs were developed. The electro-optical response increased steeply with increasing applied voltage for those cells and enough contrast was provided even if the number of scanning lines increased, and... 

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. 

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