JPH0326368B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a liquid crystal twist cell characterized by at least two stable states and capable of switching alternately between the two stable states.

As a result of the anisotropy, the alignment of the liquid crystal can be changed by exposing it to a suitable electric field. Such a change in arrangement may also be accompanied by a change in the transmission characteristics of the liquid crystal due to the optical anisotropy of the liquid crystal (here, "transmission characteristics" refers to the transmission of any electromagnetic radiation, but the main application is Field of optical transmission, i.e. usually about 4500~
(It consists in the transmission of visible electromagnetic radiation in the wavelength range of approximately 8000 Å).

By controlling the liquid crystal alignment and polarizing the light, the transmission characteristics of the cell can be changed. These devices were the first liquid crystal cells to have considerable potential use as display elements.

Early devices operated based on a phenomenon known as dynamic scattering. Liquid crystals used in such devices are characterized by at least two distinct states. In one of these states, the liquid crystals are regularly arranged. In other states induced by the presence of an electric field, the liquid crystals assume a disordered arrangement. Since the cell's transmission properties are different in the ordered and irregularly arranged states, the cells can be used as display devices. In such a device, irregularly arranged areas, for example, appear bright, whereas regularly arranged areas appear dark. Alphanumeric characters can be formed by application of suitably selective electric fields.

Other early liquid crystal cells did not utilize the presence of irregularly aligned states. Such devices are characterized by the work of Schadt and Helfrich [M. Schadt and W. Helfrich, Appl.
See Phys. Lett., 18, 127 (1971)]. Such a shutter-helfrichsel, as it is usually called, is
All of them had a regular arrangement, but had at least two states with different molecular orientations. The transmission properties of such cells depend on the particular state of the liquid crystal. As a result, display devices can be made by exploiting the difference in transmission properties from one state of the cell to another. In a special embodiment of a shutter-helfrich cell, the liquid crystal in at least one state is oriented in a helical configuration, the helical axis being approximately perpendicular to the interface, and the helix extending from one surface of the cell to the other. It includes the existing liquid crystal cell. Such cells are commonly referred to as liquid crystal "twist" cells. The operation of twisted cells is based on the effect that liquid crystals have on the polarization of electromagnetic radiation passing through the cell. By using appropriate polarizers and analyzers, this difference in polarization can be exploited to obtain different transmission characteristics between the two states. In such devices, the liquid crystal cell can be switched from one transmissive state to another non-transmissive state by applying a suitable electric field, and the effect of such electric field on the liquid crystal results in an alignment of the liquid crystal. changes, and its optical transmission properties change.

Under certain circumstances the optical anisotropy of the liquid crystal may not be sufficient to obtain an optical difference between the two states. In such cases, a "guest" material whose orientation follows that of the liquid crystal and whose optical properties are sufficiently anisotropic to provide the required difference between the two states is added. can do. Guest materials can also be used in other cases to avoid the use of polarizers and/or analyzers. Such devices are usually referred to collectively as guest host cells [GH
Heilmeier, JACastellano and LAZanoni,
Mol.Cryst.Lig.Cryst., 8, 293 (1969), and G.
H. Heilmeier and LAZanovi, Appl. Phys.
See Lett. 13, 91 (1968)] In the above cell, the special state is stable only when an appropriate electric field (including zero electric field) is applied to the cell. This electrolysis must be different for each state. A disadvantage arises from the need to make numerous contacts in order to apply a suitable electric field to the cells in succession to keep each cell in its proper state. Although such problems will not be explained in detail here, they belong to a problem collectively called "multiplexing," and for this reason, the application of liquid crystal twist cells has been limited to relatively simple display devices. (Edited by ARKmetz and FKvon WillisenNon
−“Matrix Addressing of Non Emissive” in Emissive Electrooptic Displays
Plenum, New York, 1976).

Recently, liquid crystal twist cells have been disclosed which are characterized by a stable state that requires the application of an electric field only during switching and does not require the application of an electric field to maintain the cell in a particular state. For example, Japanese Patent Application No. 55-48805 and DWBerreman and WRHeffner, Appl.Phys.Lett.37109 (1980)
reference〕. Such cells, commonly referred to as bistable liquid crystal twist cells, are the first to make liquid crystal cells practical for large-scale display applications.

The present invention provides that at least two stable states are maintained by application of the same given sustaining voltage,
Thus, the bistable liquid crystal twist cell is different from prior art cells that require the application of different sustaining voltages to maintain different states (shut-helfrich cells) or cells that require no sustaining voltage at all. . Switching between at least two stable states is accomplished by applying a perturbing electric field for a period of time necessary to convert from one liquid crystal state to the other. Following the switch, the electric field is returned to the holding voltage value and the cell stabilizes in its new configuration. The use of a holding voltage allows operation of bistable cells over a wide range of cell parameters. A feature of this new type of cell is that the switch occurs ``adiabatic'' without resorting to any dynamic properties of the liquid crystal. In the cell of the present invention, the addressed region (as opposed to the non-addressed region and the boundary) is substantially free of disclination and walls in the steady state, and the switch does not pass through the disclination, but the liquid crystal twist cell occurs through the active region of

The bistable liquid crystal twist cell of the present invention is characterized by the use of a single sustain voltage to maintain the cell in a stable state. FIG. 1 displays the energy characteristics of a particular cell of the invention. In this figure, the voltage applied to the cell is shown on the horizontal axis, and the energy associated with the cell is shown on the vertical axis. The two states illustrated by the figure are the "down" state extending from point a to point d and the "up" state extending from point f to point j (extending from point d to point f). (The part of the curve shown below represents the lower stable state).

The behavior as the voltage is increased from zero is indicative of the operation of the cell. For V down voltages only down states exist. As the voltage increases, a region is reached where either an up state or a down state can exist (ie, a voltage region ranging from V down to V up). However, since the up state often requires more energy, the cell tends to remain in the down state if it is in the down state. As the voltage is increased to the V-intersection, a point is reached where the two states become energetically degenerate. However, since the cell is also in the down state here, it continues to exist in the down state. For values greater than the V-intersection, the up state has less energy. however,
Cells tend to remain down in this region as well. This is because the change in molecular configuration from the down state to the up state requires overcoming an energy barrier between the two states (the line d-f must be crossed before the switch occurs). ).
When the voltage is increased above V UP, the cell can no longer survive in the DOWN state and is switched to the UP state. Once the cell has been switched to the UP state, if the voltage is returned to the V crossing, the cell will remain in the UP state. Switching of the cell occurs again by lowering the voltage to V down. In this case, a switch is made from the up state to the down state. The cell continues to remain down by returning the voltage to the V intersection. Therefore, the actual operation of the cell is to apply a sustain voltage to the entire active area of the cell, and changes in voltage are made only when a switch is required.

Also, other operating characteristics can be used in practical devices. For example, when operating a cell having a substantially constant thickness, it is possible to provide an operating area to which a voltage is applied and a surrounding area to which no voltage is applied. In such embodiments,
When the operating range is in the UP state, using a sustaining voltage defined by the intersection of the UP and DOWN states will lead to instability of the operating range, which will eventually collapse to the DOWN state. This is because under such conditions, the operating region which is in an up state due to energy degeneration is surrounded by an energy degenerate region which is in a down state. In such a situation, the surrounding area that is down will eventually grow through the wall into the operating area. Advantages can therefore be obtained in practice by using a sustain voltage above the V-crossing point. Typically, the sustain voltage value is large, on the order of more than 10% of the voltage difference between V-cross and V-up. At such a sustaining voltage, the up state has less energy than the down state, i.e., the conversion from the up state to the down state requires extra energy that is not available. Collapse into a down state is prevented. Of course, the sustain voltage should never approach the value of V-up. This is because, at these values, a switch from the down state to the up state always occurs. Also, the surrounding regions may have different thicknesses for a given applied voltage.

The above description of preferred values of the sustaining voltage is based on the use of liquid crystals with positive dielectric anisotropy. In the case of a liquid crystal having negative dielectric anisotropy, however, similar considerations may be made with the up and down directions in FIG. 1 reversed.

A feature of some of the cells of the invention is that the switching phenomenon need not depend on the dynamics associated with the movement of the liquid crystal. Rather, switch points f and d
A switch can also occur if the wind slowly approaches. This is distinguished from a switch that occurs only when dynamic characteristics associated with the movement of liquid crystal molecules are taken into account. Types of liquid crystal cells that do not need to rely on the dynamic properties of the liquid crystal for switching can be characterized, somewhat less precisely, as "adiabaticly switchable." However, despite the fact that such cells do not have to rely on the dynamic properties of the liquid crystal molecules for switching, faster switching properties can be obtained by exploiting such dynamics. It is therefore also possible to do this, for example, by changing the holding voltage to a point above points d or f. Under these circumstances, a proper numerical description of cell behavior requires consideration of the energy contribution due to the dynamic properties associated with the movement of liquid crystal molecules. Within the terminology employed herein, this liquid crystal twist cell is "adiabaticly switchable" insofar as the person using the cell does not inherently have to rely on liquid dynamic properties for switching. Can be done. Within the broad definition of the invention, cells are also included in which only one switch point is switchable adiabatically and the other switch points are only dynamically switchable.

It was assumed that the area where switching is not performed is maintained at a given maintenance voltage value, but when using an actual multi-area cell, all addressable areas, including the area where switching is not performed, are This includes cases in which the voltage applied to the switch is changed at the time of switching. This situation is
This occurs, for example, when changing the polarization of the voltage applied to a region that is switched to a magnitude that is required for the switch, and the voltage in a region that is not switched to a magnitude less than that required for the switch. The significance of such an alternative implementation is that the relative voltages obtained in multiplexing where the various voltages are applied to all addressable areas and provide sufficient voltage polarization to switch only the areas that are switched are It's about simplification.

FIG. 2 is a schematic diagram of the orientation of liquid crystal molecules in a particular embodiment of the cell of the invention. Note that the representation in FIG. 2 corresponds to the energy curve in FIG. The up state originates from the fact that in this state, the liquid crystal molecules near the cell center are oriented at right angles to the cell surface.
In the down state and in the no-field state, which is the down state, molecules near the center of the cell are oriented parallel to the cell surface. Distinction between these two states can be achieved by using a polarizer or a guest material. In the down state, the polarization of the incident light is rotated by a different amount than in the up state, allowing optical differentiation of the two states.

In recent years, the development of liquid crystal technology was carried out exclusively through experimental analysis of the three-dimensional configuration of various cells.
The complexity of liquid crystal devices has limited the development of such liquid crystal technology. The diversity of their configurations makes their laboratory studies extremely time-consuming, so progress cannot be expected with any degree of certainty, and such research methods are thereby rendered impractical. becomes. As a result, recent researchers in the field have begun to analyze the configuration of various types of cells using computer techniques, and have thus made considerable progress. The results of these analyzes are used to direct experimental efforts. The present invention has thus been realized, in which a mathematical description of an infinite number of liquid crystals involves at least two states that are stable when a single suitable sustaining voltage is applied. It turns out that there are groups of cells described by energy curves. These states can be switched adiabatically to each other and can be switched adiabatically and through the active region of the liquid crystal twist cell without passing through the disclination.

Calculations of liquid crystal configurations within the scope of the present description employ techniques readily within the skill of those skilled in the art. This calculation includes consideration of energy contributions due to elastic strain and the applied electric field. An elastic strain electric field is a strain in a liquid crystal caused by boundary conditions and an applied electric field. Such elastic strain energy was applied by Oseen and Frank in the description of liquid crystal twist cells [C.
W.Oseen, Trans.Discass.Faraday.Soc., 29,
883 (1933) and FCFrank, Discuss.Faraday
See Soc., 25 (1958)]. Regarding the contribution of electrostatic fields, see Gruler, Scheffer and Meier [H.Gruler, TJScheffer and G.Meier, Zett.
Naturforsch, 27A.966–976 (1972), “Elastic
Constants of Nematic Liquid Crystals I.
See “Theory of the Normal Deformation”]
Characterization of the liquid crystal formulation is expressed by appropriate elastic and dielectric constants. These equations can be solved by the well-known Euler-Lagrange technique to finally obtain a description of the particular cell configuration in terms of energy versus voltage curves. A particular cell configuration can be obtained whose energy can be calculated by solving the Euler-Lagrange equation. Obviously, depending on the boundary conditions and the constants chosen, different curves can be obtained. Among these many solutions, the present applicant discovered a solution having the characteristics of the cell of the present invention.

Based on the above calculations and assuming various starting parameters, a plot of voltage versus displacement electric field is obtained. Using the given system parameters, computational techniques are then used to determine the behavior of the applied voltage as a function of the displacement electric field in equilibrium. In practice, the equation is solved by assuming a constant displacement electric field and calculating the resulting voltage. Then choose the solution whose displacement value is 3 times the function of the voltage. Such solutions usually involve the general Gibbs-free energy versus voltage behavior shown in FIG. It is therefore bistable for a given sustaining voltage as described above.

A preferred starting parameter for the calculation is 5
Liquid crystals with greater dielectric anisotropy and cell thicknesses of approximately one twist cell length are included. Cells of the present invention typically include liquid crystals that are in a twisted state in at least one stable state and are constrained to have an untwisted state or a twist other than its unconstrained value in the other state. Usually the net twist in the two conditions is the same or differs by an integer number of revolutions. In at least one stable state, the liquid crystal is placed in a state of zero torque about an axis perpendicular to the surface. However, in at least one other stable state, the liquid crystal has a torque about the normal axis of the surface. Unlike the pitch associated with the liquid crystal director, the slope of the direction vector associated with the liquid crystal molecules always has a value different from the value taken by the direction vector in the absence of boundaries. That is, the slope angle always has a value other than 90 degrees from the normal on one or both of the boundaries.

Although the computational techniques required to construct the cells of the present invention are likely to be simple and efficient, requiring less than half an hour of computer time, experimental techniques may also be utilized to construct the cells of the present invention. can do. Such techniques include, for example, techniques for creating wedges on which twisted liquid crystals are placed. The wedge surface includes means for aligning adjacent liquid crystal molecules. When an increasing voltage difference is applied between the two surfaces of the wedge-shaped cavity, the "wall" across the wedge is seen to move. Bistability is indicated by the disappearance of the wall at one location and its reappearance or more rapid movement everywhere at a given voltage. The features of the wedge cell in the areas where the walls are exposed and disappeared can then be utilized to produce cells having features of the present invention.

EXAMPLE In one embodiment of the cell of the present invention, a glass slide coated with tin oxide was used to seal the liquid crystal. The slide was further coated with silicon monoxide deposited on the surface at an angle of 5 degrees from the equilibrium direction. Tin oxide is formed in an electrode pattern,
Appropriate voltages are applied to obtain, for example, an alphanumeric configuration, and evaporated silicon monoxide is used to align the directional vectors close to the cell walls [JL Janning, Appl. Phys. Lett., 21 ,173
(1972)]. The liquid crystal used was a British drug doped with approximately 1.342% cholesteryl nonanoate.
E7 manufactured by British Drug House. E7 is 4-
Breitate Shudra for Eutectic Mixtures Consisting of Cyano-4'-n-alkylbiphenyl, 4-cyano-4'-n-alkoxybiphenyl and 4-cyano-4''-n-alkyl-P-terphenyl This is the symbol name for Tsughaus, for example, E7 is 4-cyano-4'-n-pentylbiphenyl, 4-cyano-
4'-n-peptylbiphenyl, 4-cyano-4'-
n-octoxybiphenyl and 4-cyano-
4'' represents a eutectic mixture of n-pentyl-P-terphenyl. After inserting a liquid crystal into the cell, the cell surface was bonded with epoxy around the periphery of the cell so that the cell thickness was 13.95 microns. Inside the cell The total twist angle was 360 degrees.The tilt angle of the liquid crystal near the surface was determined by the method of Crossland et al. [Journ.Phys.D., Appl.Phys., 9, 100 (1976)]
It was measured as 55 degrees from the normal. The sustaining voltage in this case was 1.7 volts. The switch from the down state to the up state starts at about 1.8 volts,
On the other hand, the switch from up state to down state is approximately
Start at 1.5 volts. The dynamic properties of the cell are also used to quickly switch. A dynamic switch from an up state to a down state changes the voltage
Obtained by dropping to zero for 0.075 seconds. or,
A dynamic switch from the down state to the up state was obtained by increasing the voltage to 3 volts for 0.055 seconds. Dynamic switches of this type depend on the magnitude of the voltage change and the time for which such voltage change is applied. Therefore, it has been found that applying a similar voltage for a shorter period of time, or applying fewer voltage changes for the same period of time, is not effective for dynamic switches. Therefore, aspects of matrix addressing that require all elements to receive some voltage can also be utilized in the dynamic switching method. A polarizer and analyzer were used to differentiate between the two states. The analyzer was placed in equilibrium with the direction of deposition of the silicon monoxide coating, and the polarizer was placed perpendicular to that direction. The cell was placed on an optical bench and tilted in various directions to examine the contrast dependence of incident light. Calculations have shown that a cell with a total twist angle of 270 DEG has useful optical properties if comparable tilt angles in the two states are obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the energy characteristics of the liquid crystal twist cell of the present invention, and FIG. 2 is a schematic diagram of the liquid crystal in various states of one embodiment of the cell of the present invention.

Claims (1)

[Scope of Claims] 1. A cholesteric liquid crystal that takes a spiral shape when no strain is applied to it; a boundary surface at least one of which transmits electromagnetic radiation of a predetermined wavelength; a means for arranging liquid crystal molecules close to the boundary surface; In a liquid crystal twist cell comprising means for applying a potential difference to a portion of the cell and means for optically discriminating between at least two different orientations of the liquid crystal, the liquid crystal is in the presence of a single non-zero sustaining voltage. 1. A liquid crystal twist cell characterized in that it has at least two stable states in which the two stable states can be switched from one to the other without going through disclination of the cell. 2. The liquid crystal twist cell according to claim 1, wherein the two stable states are switched from one to the other by changing the magnitude of the applied voltage. 3. A liquid crystal twist cell according to claim 1 or 2, characterized in that the liquid crystal can be switched adiabatically.