DE3140898A1 - BISTABLE LIQUID CRYSTAL TWIST CELL - Google Patents

Description

description

Bistable liquid crystal twist cell

The invention relates to liquid crystal twist cells which are characterized by at least two stable states and which can be switched between the stable states.

Liquid crystals have anisotropic physical and optical properties. As a result of the anisotropy, the order of the liquid crystal can be changed by applying suitable electric fields. Such a change in order can be associated with a change in the transmission properties of the liquid crystal versus the anisotropic optical properties of the liquid crystal. (While the term "transmission properties ^11" refers to the transmission of any electromagnetic radiation, the main area of application will be optical transmission, ie transmission of visible electromagnetic radiation, as it is commonly referred to as lying in the wavelength range from approximately 450 to approximately 800 nm) .

Controlled change in the liquid crystal orientation can change the transmission properties of the cells. Such components were the first liquid crystal cells with significant potential for use as display elements.

Initial devices were based on a phenomenon called dynamic scattering. In such Liquid crystal cells provided for components are distinguished by at least two different states. In one of these states the liquid crystal is ordered. In the other state, through the presence of an electric Field is induced, the liquid crystal is disordered. Since the transmission properties of the cell in the ordered state are different from those in the disordered state, the cell can be used as a display or playback device. In such a device, the disordered For example, zones look bright while the ordered zone then looks dark. Alphanumeric characters can be formed by appropriately selected supply of an electric field.

Other liquid crystal cells made earlier did not rely on the existence of disordered states. These components are from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971). They are now generally referred to as Schadt-Helfrich cells and have at least two associated states, both of which are ordered but have different molecular orientations. The transmission properties of these cells depend on the specific state of the liquid crystal. Thus, a display device can be constructed by taking advantage of the different transmission characteristics of the cell in the two states.

A special embodiment of the Schadt-Helfrich cell is a liquid crystal cell in which the liquid crystal is in at least one of the states in a helical configuration is oriented, the helix axis being approximately perpendicular to the boundary surfaces and the helix extending from extending one face of the cell to the other. Such cells are commonly referred to as liquid crystal "twist" cells designated. The way twist cells work depends on the effect the liquid crystal has on polarization the electromagnetic radiation transmitted through the cell. The use of suitable polisers and Analyzers make it possible to take advantage of this difference in polarization between the two different states To maintain transmission properties, draw. In such components, the liquid crystal cell of switched from one transmitting state to another, non-transmitting state by creating suitable fields the influence of the latter on the liquid crystal leading to a change in the orientation of the Liquid crystal and an accompanying change in the optical transmission properties.

In some cases, the optical anisotropy of the liquid crystal is sufficient not enough to obtain an optical discrimination between the two states. In these cases so-called Guest materials are added, the orientation of which follows that of the liquid crystal and their optical properties

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Properties are sufficiently anisotropic to allow the necessary discrimination between the two states to reach. One can also use guest materials in other situations to avoid the use of polarizers and / or analyzers. Such devices are generally known as guest / host cells (see G. H. Heilmeier, J. A. Castellano and L. A. Zanoni, Mol. Cryst. Liq. Cryst., 8, 293 (1969) and G. H. Heilmeier and L. A. Zanoni, Appl. Phys. Lett., 13, 91 (1968)).

In the case of the cells described above, the respective states are only stable if there is a suitable field (including zero field) on the cell. This field must depend on the state take on a different value. However, disadvantages result from the requirement of a large number Contacts to continuously address the cells with the correct fields to keep each cell in its correct state to keep. Such problems, although not going to be explored further here, fall within the general scope Multiplexing area and have limited the application of liquid crystal twist cells to relatively simple display devices (see A. R. Kmetz and F. K. von Willisen, editors "Matrix Addressing of Non-Emissive Displays", in Non-Emissive Electrooptic Displays, Plenum, New York, 1976).

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Recently a liquid crystal twist cell was specified, which is characterized by stable states, which the supply of a field only during the switchover requires, but not the application of a field to maintain the cell in a certain state (see EUROPEAN PATENT APPLICATION No..80301168.3 and D. W. Berreman and ¥. R. Heffner, Appl. Phys. Lett., 37, 109 (1980)). Such cells, commonly referred to as bistable liquid crystal twist cells, have the Application of liquid crystal cells to large display or reproducing devices made possible in practice.

The invention now resides in a bistable liquid crystal twist cell, in which at least two states are maintained by applying the same given holding voltage. The cell differs from known cells because the latter requires the supply of different voltages to maintain different states (Schadt-Helfrich cell) require or not need a holding voltage at all. Switching between the at least two Stable states are brought about by applying an interference field during the short period of time it takes to convert of the liquid crystal from one state to the other is required. After switching, the field reverts to the withstand voltage value and the cell remains in its new configuration stable. The use of a holding voltage allows a bistable cell to be operated through a wider range of cell parameters. Characteristic for

this new class of cells is the fact that the switching is "adiabatic" without affecting any other dynamic Properties of the liquid crystal can occur. The addressed zone in the cell according to the invention is (in contrast to the non-addressed zones and boundaries) essentially free of disclinations and walls in the stable State, and switching occurs without passing a disclination through the active zone of the liquid crystal twist cell on.

The invention is described below with reference to the drawing; show it:

FIG. 1 shows an exemplary energy / voltage diagram to illustrate the behavior of the present invention Liquid crystal twist cell and

FIG. 2 shows a schematic representation of the liquid crystals in the individual states that are assigned to the current cell.

The present bistable liquid crystal twist cell is characterized by the use of a single holding voltage to keep the cell in the stable states. Switching between two states is made possible by a short Change in holding voltage causes.

Figure 1 shows the energy properties of a cell in The type in question. In Fig. 1, the voltage applied to the cell is plotted on the abscissa, and that of the cell associated energy on the ordinate. The two states described by the figure are the "down" state, which extends from point a to point d, and the "up" state, which extends from point f to point j. (The part of the curve that extends from point d to point f represents represents an unstable condition).

The behavior of the states when the voltage rises from zero explains how the cell works. For voltages less than V_, «+, only downward states exist. When the voltage is increased, a zone is reached in which either the up-state or the down-state can exist (voltage range from V _down to V _up ). However, the cell in the down state seeks to remain in this down state because the up state requires greater energy. If the tension is increased to ^ cut, then a point is reached where the two states are energetically degenerate. However, the cell continues to exist in the downward state again because it is in that state. For values greater than ^ cut * ^la ' ^t ' ^ ^he upward state the smaller energy. However, in the down state, the cell is also searching in this zone

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because a change in the molecular configuration from the downward state to the upward state would require overcoming an energy threshold between the two states. (The line df represents the energy value of the threshold which must be exceeded before a switchover occurs). When the voltage is above V ~ ». is increased, the cell can no longer exist in the down state and therefore switches to the up state. If the cell has switched to the upward state, then the voltage can be reduced to cut and the cell will remain in the upward state. Lowering the voltage below ^v _a ^, _W ^{ard will again} result in the cell being switched, this time from the up state to the down state. The return of the voltage to ^v _{s imitt} will then leave the cell in the down state. In practical operation of the cell, a holding voltage is applied to all active zones of the cells, and the voltage is only changed when switching is desired.

Alternative operating characteristics can be used in actual components. For example, in a Cell of approximately constant thickness an operating zone to which voltage is applied and a zone surrounding it, to which no voltage is applied. In such embodiments, is in the upward state Operating zone the use of a holding voltage, which by the intersection of upward and downward states is defined, to instability of the operating zone and to

their eventual falling back into the downward state. This is because, for the defined conditions, the energy of the upward and downward states are degenerate at the holding voltage, and the operating zone in the upward state is surrounded by a zone that is in the energetically degenerate downward state. Under these circumstances, the surrounding zone located in the Abwürts-Zuntand will eventually grow into the operating zone through a "wall". Thus, practical embodiments take advantage of the use of a hold voltage that is above Vg-.J _1n J ^. In general, the withstand voltage will be greater than Vg ^ j ^, and by an amount greater than 10% of the voltage difference between Vg j ^. ^ And ^v _au f _W g _r -J- _s ^is "t · Such a holding voltage prevents the upward state from being emptied into the downward state, since with this holding voltage the upward state has less energy than the downward state, which means that a transition from the upward state to the downward state requires additional energy, which is not available, would require.

Needless to say, however, the holding voltage should never reach the value of V- f «+, because for these values the down state switches to the up state. Alternatively, the surrounding zones can be used in the case of a supplied Voltage of different thickness.

The discussion of the preferred values for the withstand voltage is based on the use of a liquid crystal with positive dielectric anisotropy outlined. In the case of a liquid crystal with negative dielectric anisotropy, similar considerations apply except that the up and down labels in FIG. 1 are reversed.

The is characteristic of some of the present cells The fact that the switching phenomenon is not due to the dynamics associated with the movement of the liquid crystals needs. Instead, a switchover can also occur when the switching points f and d slowly approach will. This differs from a switching that only occurs when there is movement of the liquid crystal molecules assigned dynamics is considered. The class of liquid crystal cells whose switching is not based on liquid crystal dynamics deigned, are roughly characterized as being adiabatically switchable. However, despite the fact that such cells do not rely on the dynamics of the liquid crystal molecules when switching, an advantage of one such dynamics to obtain faster changeover properties to be pulled. For example, the switching can be effected by changing the holding voltage to a point outside the points d or f. Under these circumstances requires the precise mathematical description of the cell behavior taking into account the energy contributions as a result of the dynamic properties of the movement of the liquid crystal molecules assigned. Within the

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In the terminology taken, the liquid crystal twist cell would still be considered adiabatically switchable in so far The person skilled in the art using this cell does not inherit the flow dynamic properties of the liquid crystal needs to influence to switch. Generally applies to the present Cells that they also include those in which only one switching point can be switched adiabatically, during the other switching point can only be switched dynamically.

It has been assumed that the unswitched Zones are held at a given withstand voltage value, however, the use of multi-zone cells may involve a change in voltage, all of which addressable zones is supplied during switching, although such zones are not switched. Such circumstances would occur by changing the voltage supplied to the zones to be switched to sufficient for switching Sizes, and changing the voltage on non-switching zones in sizes smaller than that required for switching Size. The importance of such alternative embodiments lies in that obtained in a multiplex operation comparative simplification when a changing voltage is applied to all addressable zones, only the zones to be switched are exposed to a voltage change that is sufficient for switching.

FIG. 2 shows a schematic representation of the orientation of the liquid crystal molecules in one embodiment of the present cell. Note that the illustration of FIG. 2 with the energy curve after Fig. 1 is compatible. The upward state is called because in this state the liquid crystal molecules in the central cell area are largely vertical are oriented to the cell surface. In the down state and in the no-field state, which is also a down state is, the molecules in the central cell area are largely oriented parallel to the cell surface. A visual distinction between these two states can be achieved through the use of a polarizer, a polarization analyzer or a guest material. In the downward state, the polarization of the incident radiation rotated by a different amount than in the upward state, creating an optical discrimination between the both states is enabled.

Design considerations

The complexity of liquid crystal devices has recently led to the development of liquid crystal technologies where this development is limited solely to experimental analysis of numerous cell configurations was based. The variety of such configurations makes their study in the laboratory so time consuming that advances

cannot be expected with a reasonable probability can. Such an examination method is therefore impractical. As a result, the various cell configurations were started using computers to analyze, and significant advances have also been made. the Results of these analyzes were used to direct the experimental effort. The present invention came in this way from the knowledge that within the unlimited number of mathematical descriptions a class of liquid crystal twist cell configurations of cells exists, which can be described by energy curves, to which at least two states are assigned and these states are stable when a single suitable holding voltage is applied. Between these states can be adiabatic and without going through a disclination through the active zone of the liquid crystal twist cell be switched.

The calculation of liquid crystal configurations falling under the present description is carried out according to methods easily accessible to those skilled in the art. The calculation concerns the consideration of energy contributions as a result of the elastic expansion and the applied fields. The elastic expansion field is created by the expansion of the liquid crystal as this results from the boundary conditions and the applied electric field. the elastic strain energy was first applied to the description of liquid crystal twist cells by

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Oseen and Frank (see C. W. Osecn, Trans. Discuss. Faraday Soc, 29, 883 (1933) and F. C. Frank, Discuss. Faraday Soc, 25, 17 (1958)). The electrostatic field contribution has been described by Gruler, Scheffer and Meir (see H. Gruler, T. J. Scheffer and G. Meier, Zeit. Naturforsch., 27A, 966-976 (1972), "Elastic Constants of Nematic Liquid Crystals I. Theory of the Normal Deformation). The properties of the liquid crystal formulation happens with the help of suitable elastic and dielectric constants. The equations are based on the well-known Euler-LaGrange method solved to finally describe certain cell configurations using an energy / voltage curve to obtain. The solution to the Euler-LaGrange equations provides certain cell configurations for which the energy can be calculated. Obviously, innumerable Curves can be obtained depending on the choice of the boundary conditions and the constants. Within this large number of Solutions, according to the present invention, are the solutions afflicted with the properties of the cells described here found before.

On the basis of the calculations outlined above, stress / dislocation field diagrams are obtained under the assumption different output parameters. Numerical methods are then used to determine the behavior of the fed Stress as a function of the dislocation field in equilibrium to determine how the latter results from the assumed

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System parameters results. In practice, the equations are solved by assuming a given dislocation field and computing the resulting ones Tension. The solutions are then selected that are trivalent in the dislocation as a function of the voltage are. Such solutions then generally become the general Gibbs free energy / stress behavior shown in FIG. 1 have and will therefore be bistable at a given withstand voltage as described.

Preferred output parameters for the numerical calculation include liquid crystals with dielectric anisotropies greater than 5 and cell thicknesses of about one-half a twist length. The present cells generally comprise liquid crystals that are twisted in at least one of the stable states (twisted) and forced into another of the states can be either to be untwisted or to have a twist other than the casual value. Usually the total twist in the two states will either be the same or an integer number of turns differentiate. In at least one of the stable states, the liquid crystal can have a zero torque about a perpendicular be subject to the axis oriented to the surface. However, in at least one other of the stable states the liquid crystal will have a torque about an axis oriented perpendicular to the surface. In contrast to the pitch that the liquid crystal directors

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is associated with the tilt of the liquid crystal molecules assigned directors always have a different value than that assigned by the directors would be taken in the absence of limitations, d. i.e., the inclination will always have a value other than 90 ° opposite of the vertical at one or both of the boundaries.

While the numerical method of building the present cell is believed to be the most effective and less than one half an hour of computing time on simple computers can be used to set up the present cell experimentally Methods are applied. Such methods include, for example, the manufacture of a wedge in which the twisted liquid crystal to be examined is arranged. The wedge surface would provide a means of aligning adjacent ones Have liquid crystal molecules. When there is an increasing voltage difference between the two surfaces of the wedge-shaped When the cavity is created, it is found that a "wall" traversing the wedge occurs. Bistability becomes then by the disappearance of the wall in one place and its reappearance in another place, or by a faster wall movement, indicated for a given voltage. The properties of the wedge cell in that Zone where the wall appeared and disappeared are then used to produce a cell with the present properties used.

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example

In one embodiment of the present cell, sheets of glass used with tin oxide were used to define the liquid crystal. The wafers were also coated with silicon monoxide, which was vapor-deposited at an angle of 5 ° to the surface parallel. The tin oxide was formed into an electrode pattern and used to apply suitable voltages, for example to obtain an alphanumeric arrangement, while the evaporated silicon monoxide was used to align the directors near the cell walls (see J. L, Janning, Appl. Phys Lett., 21, 173 (1972)). The liquid crystal used was E7 from the British Drug House and was doped with about 1.342% cholesteryl nonanoate. After the introduction of the liquid crystal into the cell, the cell surfaces were glued with epoxy glue along the circumference of the cell with a cell thickness of 13.95 μm. The total twist was 360 ° in the cells. The angle of inclination of the liquid crystal in the vicinity of the surfaces was determined to be 55 to the normal using the method of Crossland et al. (Journ. Phys. D., Appl. Phys., 9, 100 (1976)). The holding voltage in this case is 1.7 volts. The switch from the down to the up state first occurs at around 1.8 volts, while the switch from the up to the down state first occurs at around 1.5 volts. For quick switching

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the dynamic properties of the cell were also used, and a dynamic switch from the up state the downward state was achieved by a 0.075 sec. ' causes a long voltage drop to zero. Dynamic Switching from the down state to the up state was achieved by increasing the voltage for 0.055 seconds Get 3 volts. Dynamic switching of this nature depends on both the magnitude of the voltage change and on the time during which the voltage change is applied will. Thus, it has been found that similar voltages are supplied for a shorter time or that the supply is of the same length Smaller voltage changes for dynamic switching ineffective. Therefore, an embodiment with matrix addressing, which requires that all elements receive some voltage, even when used in dynamic switching mode will. A polarizer and an analyzer were used for optical discrimination between the two states. The analyzer and the polarizer were arranged parallel to the direction of vapor deposition of the silicon monoxide coating oriented perpendicular to this direction. The cell was oriented in an optical bench and in different Directions inclined to investigate the contrast dependence of the incident radiation. Calculations indicate that a cell with a total twist of 270 ° can have useful optical properties if comparable angles of inclination can be obtained in the two states.

Claims (3)

BLUMBACH · WESER * · "g'ERGEN · KRÄMER ZWIRNER · HOFFMANN PATENTANWÄLTE IN MUNICH AND WIESBADEN Patentconsull Radeckestrasse 43 8000 Munich 60 Telephone (089) 883603/883604 Telex 05-212313 Telegrams Patentconsull Patentconsult Sonnenberger Straße 562943/06 4312 6200 Wiesbaden 561998 Telex 04-186237 Telegrams Patentconsult Western Electric Company BERREMAN 14 Incorporated New York NY Claims 1. Liquid crystal twist cell with - a liquid crystal which assumes a helical configuration in an unstretched state, - Boundary surfaces, at least one of which is electromagnetic Transmits radiation at a wavelength of interest, ■ - - a device for aligning the liquid crystal molecules that adjoin the boundary surfaces, a device for applying a voltage difference to at least part of the cell and - A device for optical discrimination between at least two different orientations of the Liquid crystal, Munich: R. Kramer Dipl.-Ing. · W. Weser Dipl.-Phys. Or. Rer. nat. · E. Hoffmann Dipl.-Ing. Wiesbaden: P. G. Blumbach Dipl.-Ing. · P. Bergen Prof. Dr. jur. Dipl.-Ing., Pat.-Ass., Pat.-Anw. until 1979 G. Zwirner Dipl.-Ing. Dipl.-W -.- lng. _ 2 - characterized in that - the liquid crystal in the presence of a single ,, of Zero different holding voltage has at least two stable states and - the stable states are switched from one to the other without going through a disclination through the cell can be. 2. Cell according to claim 1,
characterized in that the stable states can be switched from one to the other by changing the size of the applied voltage. 3. Cell according to claim 1 or 2,
characterized in that the liquid crystal can be switched adiabatically.