Bistable Nematic Displays

There are several types of bistable nematic LCDs that have good performance characteristics. They are twisted-untwisted bistable nematic LCD [9-13], zenithal bistable nematic LCD [14,15], surface-induced bistable nematic LCD [16-19], mechanically bistable nematic LCD and bistable STN-LCD [20-24], A bistable nematic material has two bistable states with 

The degree of stability of supertwisted bistable twisted nematic (SBTN) displays is shown to improve considerably in nematic twist configurations generated by photoaligned and domain-stabilized bias tilt angle patterns on one or both display substrates. Incorporation of uniaxial optical retarders into SBTN-LCD was shown to result in black and white display operation. 

Photoalignment of Liquid Crystalline Materials Physics and Applications V. Chigrinov, V. Kozenkov and H.-S. Kwok 2008 John Wiley Sons, Ltd 

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The third electro-optical effect using calamitic nematic liquid crystals makes use of a flexoelectric effect manifested by a curved asymmetrical nematic medium. This corresponds to piezoelectricity in crystals. The existence of flexoelectricity in a nematic phase under certain boundary conditions was predicted in the late 1960s and then confirmed experimentally several years later. However, LCDs using this effect, such as bistable nematic displays are only in the development stage and as such they will not be discussed in this monograph. 

Joubert, C., Angele, J., Boissier, A., Pecout, B., Forget, S.L., Dozov, I., Stoenescu, D., Lallemand, S., Lagarde, P.M. Reflective bistable nematic displays (BiNem ) fabricated by standard manufacturing equipment. J. SID 11, 17-24 (2003)... 

R. Barberi, M. Giocondo, J. Li and R. Bartolino, Fast bistable nematic display with gray scale, AppL Phys. Lett., 71, 3495 (1997). 

I. Dozov, M. Nobili, and G. Durand, Fast bistable nematic display using monostable surface switching,... 

Early Attempts at Bistable Nematic Devices 7.3.1. Nematic displays... 

J. Cheng and G.D. Boyd, Threshold and switching characteristics of a bistable nematic liquid-crystal storage display, Appl. Phys. Lett. 37(12), 1072-1074, (1980). doi 10.1063/1.91881... 

I. Dozov, A. Boissier and T. Laboureau, Nemoptic s bistable nematic liquid-crystal technology - Cholesterics or ferroelectrics are not necessary to make a bistable LCD, and using conventional nematics offers some substantial advantages, Information Display 18(1), 10 13, (2002). 

The spontaneous molecular polarization of ferroelectric liquid crystals, arising from their structure when constrained in small cell gaps, results in unique features that can be exploited in display devices. A low electric field of only a few volts can switch the ferroelectric liquid crystal between two equally stable states with opposing polarization directions. This is commonly referred to as bistability. In contrast, nematic displays generally require the electric field to maintain the ON state. The power required to run ferroelectric liquid crystal displays is consequently much less than that required for a nematic display. Since active switching is used in both directions, ferroelectric liquid crystals can switch hundreds of times faster than a... 

When a bistable TN display is optimized, the transmittance of one of the stable states should be 0 and the transmittance of the other stable state should be 1. The parameters of the display are the twist angles (f). In + (p) of the stable states, the angle a,- of the entrance polarizer, the angle of the exit polarizer, and the retardation F of the liquid crystal. As discussed in Chapter 3, the transmittance of a uniformly twisted nematic display in the geometry shown in Figure 3.3 is... 

In LCDs, liquid crystals are usually sandwiched between two substrates. A certain alignment of the liquid crystal at the surface of the substrates is usually necessary in order for a display to operate properly. Bistable nematic liquid crystals can be created by using surface alignment layers. They are divided into two categories zenithal bistable TN and azimuthal TN. 

G. P. Bryan-Brown, M. J. Towler, M. S. Bancroft, and D. G. McDonnell, Bistable nematic alignment using bigratings, Proc. Intnl. Display Research Conf. 94, 209 (1994). 

In addition to the TN and IPS modes in nematic displays, himdreds of other displays methods were invented and tested over the last 30 years. Out of them the bistable nematic development based on surface flexoelectric interactions, the bistable cholesterics displays based on switching between planar and focal conic textures and the polymer dispersed liquid crystal displays found some applications. None of them, however, offer better than a few milliseconds switching time. 

Today s ferroelectric liquid crystals are made on small sizes on silicon chips (liquid crystals on silicon) and presently are serving only a relatively small market. This is partially because of the problems with field-induced mechanical stresses related to their piezoelectricity. However, we believe that sooner or later the ferroelectric displays should be good enough to replace the nematic displays. They are inherently bistable, i.e., only those pixels should be readdressed that are showing change, and they offer switching with 1-100 microsecond ranges. Their main drawback is that they have two-dimensional... 

Fig. I. Schematic diagrams of the contrast versus the applied field for (a) a twisted nematic, (b) a cholesteric guest-host bistable display (Wysocki ei al., 1972 Ohtsuka and Sukamoto, 1973), and (c) a bistable LCD (Boyd et al., 1982).

Another bistable device is the Binem display, which does not involve flexoelectric effects but is sufficiently similar for a brief mention. This device uses a chirally doped nematic in a cell with parallel alignment directions... 

The concept of defects came about from crystallography. Defects are dismptions of ideal crystal lattice such as vacancies (point defects) or dislocations (linear defects). In numerous liquid crystalline phases, there is variety of defects and many of them are not observed in the solid crystals. A study of defects in liquid crystals is very important from both the academic and practical points of view [7,8]. Defects in liquid crystals are very useful for (i) identification of different phases by microscopic observation of the characteristic defects (ii) study of the elastic properties by observation of defect interactions (iii) understanding of the three-dimensional periodic structures (e.g., the blue phase in cholesterics) using a new concept of lattices of defects (iv) modelling of fundamental physical phenomena such as magnetic monopoles, interaction of quarks, etc. In the optical technology, defects usually play the detrimental role examples are defect walls in the twist nematic cells, shock instability in ferroelectric smectics, Grandjean disclinations in cholesteric cells used in dye microlasers, etc. However, more recently, defect structures find their applications in three-dimensional photonic crystals (e.g. blue phases), the bistable displays and smart memory cards. 

The rotation around the (small) cone is another reason for the fast switching speed. For Ps = 10 nC/cm = 1 V/pm, y = 0.1 poise and = 22.5, r 10 ps The fast switching speed is a merit of ferroelectric liquid crystal devices. Regarding the bistability, on the one hand, it is good because it enables multiplexed displays of the ferroelectric liquid crystal on passive matrices on the other hand, the bistability is a problem because it makes it difficult to produce gray scales. Another issue with SSFLC is that it is more challenging to achieve uniform orientation in SSFLC than in nematic liquid crystals. 

T. Tanaka, Y. Sato, A. Inoue, et al., A bistable twisted nematic (BTN) LCD driven by a passive-matrix addressing, Proc. Asia Display 95, 259 (1995). 

Z. L. Xie and H. S. Kwok, New bistable twisted nematic liquid crystal displays, J. Appl. Phys. Lett, 84, 77 (1998). 

H. Cheng and H. Gao, Optical properties of reflective bistable twisted nematic liquid crystal display,... 

F. Zhou and D.-K. Yang, Analytical solution of film compensated bistable twisted nematic liquid crystal displays, J. Display Tech., 1, 217 (2005). 

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