NL8303545A - DISPLAY DEVICE. - Google Patents

Description

* .. ♦ VO 5094

Display device.

Bistable nematic liquid crystal displays generally require large AC alternating voltages to initiate interstate switching between bistable ones. An important reason for such large electrical switching AC voltages is that sufficient electrical energy must be supplied to each display cell to release and move drifts from "pinning" sites.

An embodiment of a nematic liquid crystal display device exhibits a configurable bistability between two states. See U.S. Patent 4,333,708. The two states, which are separately present in the absence of a retention potential, are topologically inequivalent and derive their stability from declination pinning. Switching between states occurs by loosening and moving drifts from a "pinning" site in response to an applied switching AC voltage exceeding a large, sharp switching threshold value.

A reduction of the switching threshold level for this type of liquid crystal display is achieved by pre-biasing selected cells in the display with a small alternating voltage before applying the larger switching voltage.

With regard to the above-described displays, it is pointed out that switching alternating voltages are used to effect switching between states. The signals which generate these switching AC voltages generally belong to the host of constant envelope signals and more particularly are gated AC pulse signals with substantially constant envelope. Constant-envelope AC signals are preferred over DC or constant-amplitude signals because the latter give rise to space charge polarization effects, which reduce the amplitude of the applied electric field.

For the above-mentioned displays, the problems of relatively large switching AC voltages and a switching due to the declination movement still occur.

8303545 j * i -2-

According to the invention, a small DC electric voltage is applied to the nematic liquid crystal display cell to initiate a bistable interstate switching between two topologically equivalent horizontal orientational alignment configurations.

5 The polarity of the DC voltage determines the configuration to which an intermediate state switching period is initiated. For example, after switching begins, a small alternating electrical voltage of less than 10 volts is supplied to the cell to complete the switching period and maintain the orientational alignment configuration in either of the two horizontal states. The bistable nematic liquid crystal display cell comprises upper and lower substrates, liquid liquid crystal material located between the two substrates, and a combination of elements which are integrally connected to the substrates and are capable of directing elements of the liquid Preferably orienting crystal material in an asymmetric horizontal state with an inversion layer located substantially at and parallel to a predetermined substrate in the presence of a symmetrically breaking DC electric field, followed by a certain applied alternating electric voltage.

In one embodiment of the invention, the liquid crystal display cell comprises upper and lower parallel substrates with electrically conductive strips and topographically textured tilt-centering surfaces disposed thereon, a nematic liquid crystal material sandwiched between opposed textured surfaces, and a variable voltage source connected to the conductive strips to generate electrical switching fields throughout the liquid crystal material. A cell is split into an active area and an isolation area surrounding the active area. In the active region of the cell, the opposed textured tilt center faces exhibit an equal opposite tilt boundary state and a twist or angular difference between azimuth orientations of the opposed textured tilt center faces for optical differentiation of states.

With each textured tilt center face, the isolation area is characterized by a parallel boundary condition. An intermediate state switching is effected by applying a first DC voltage to the liquid crystal material to initiate the centering of the orientational orienters in a first asymmetric horizontal state. A small AC holding voltage greater than a critical voltage is applied perpendicular to the substrates to complete the switchover to the first state. Transitions to the second state are made by applying a second DC voltage to the liquid crystal material to initiate proper centering of the orientational alignment elements in a second asymmetric horizontal state. Again, a small holding-AC voltage is applied to complete a switch to the second state.

In another embodiment of the invention, the liquid crystal display cell similarly comprises upper and lower parallel substrates with electrically conductive strips and topographically textured tilt center faces disposed thereon, the nematic liquid crystal material disposed between opposed textured surfaces and the variable voltage source connected to the conductive strips to generate electric switching fields throughout the liquid crystal material. This embodiment differs from the previously described embodiment in that topographically textured tilt center faces on opposed substrates exhibit an inverted tilt limit state in their entirety. Active regions of the latter embodiment are not defined by the texture of the tilt center planes, but instead the active regions are defined by the overlapping region between conductive strips on opposed substrates.

The invention will be explained in more detail below with reference to the drawing. In the drawing: Fig. 1 shows a three-dimensional view of a liquid crystal display cell; FIG. 2 is a conception of the top topographically textured tilt center plane 20 viewed from line II-II in FIG. 1; FIG. 3 is a conception of the bottom topographically structured tilt-center plane 21 as viewed from line III-III in FIG. 1; and FIGS. 4 to 7 are different horizontal orientational target element centers within the active region in the display cell of FIG. 1 according to the invention.

A new bistability effect is provided for nematic liquid crystals, in which the intermediate state switching 5 is initiated between two topologically equivalent states by applying a symmetrical breaking DC voltage. A state is maintained in its proper configuration by subsequently applying a small holding AC voltage. Each state has a boundary inversion layer, which contains substantially horizontally centered orientational orientations at a corresponding boundary. Switching from one state to another does not require any divergence movement due to the topological equivalence of the states.

Figure 1 shows a liquid crystal display cell. An exemplary embodiment according to the invention is shown in this display cell. The cell in Figure 1 is only one of a number of such cells, which are contained in an entire liquid crystal display. As shown in Fig. 1, the liquid crystal display cell comprises an upper substrate 10, a lower substrate 11, an upper topographically textured tilt center face 20, a lower topographically textured tilt center face 21, nematic liquid crystal material 30, an upper conductor. 40 and a lower conductor 41. Switching and holding voltages are supplied to the cell from a variable voltage source 50, which is connected to the upper conductor 40 and the lower conductor 41.

In the figures, a set of reference base vectors (x, y, z) is indicated to support an orientation of figure 1 with respect to figures 4 to 7.

The substrates 10 and 11 support the respective conductors 40 and 41 and also provide a means of accommodating the liquid crystal material 30. Each substrate consists essentially of a transparent dielectric material, such as silicon dioxide or glass or the like.

Conductors 40 and 41 are located on an inner opposing face of each respective substrate to allow an alternating AC or DC electric field to be applied substantially perpendicular to each substrate. Both interdigital and continuous uniform strip electrodes are means suitable for use with conductors 40 and 41.

As shown in Figure 1 for illustrative purposes only, conductors 40 and 41 consist of electrodes in the form of a continuous uniform strip, which electrodes are arranged perpendicular to each other. The conductor 40 is formed on an inner surface of the upper substrate 10, while the conductor 41 is similarly formed on an inner surface of the lower substrate 11 in a direction perpendicular to the direction of the conductor 40. Each conductor is deposited as a thin film on the inner surface of the respective substrate or etched therein by a conventional photolithographic method. For example, transparent films of indium-tin oxide are used as conductors on both substrates of display cells according to the transmission modes, while opaque films consisting of, for example, aluminum are used for conductors on one substrate in display cells according to the reflection modes.

Topographically textured tilt centering surfaces 20 and 21 are used to communicate a known tilt centering to the liquid crystal molecules at each surface. These planes are also called tilt-centering planes. The surfaces 20 and 21 consist of transparent non-conductive layers on the free inner surfaces of the substrates and conductors for defining a surface centering of the orientational alignment elements of the liquid crystal material 30. The surfaces 20 and 21 are integral with each respective substrate connected by an oblique electron beam deposit or a thermal evaporation of a material, such as titanium oxide or silicon oxide, both of which act as insulators. This results in a uniformly tilted column topography for each tilt center plane. The topography at each of the faces 20 and 21 determines a surface tilt angle 9g measured for each substrate normal (inner face) in the range of 0 ° to 90 °. Surface tilt angles greater than 45 ° are preferred to ensure dominance of the horizontal orientational straightening element configuration. The tilt centering surfaces 20 and 21 will be described more fully hereinafter with reference to Figures 2 and 3.

3303545 * -6-%

The liquid crystal material 30 is a liquid crystal substance in the nematic mesophase with a positive dielectric anisotropy at least in a certain frequency range. In an exemplary display cell, the material 30 consists of 5 cyanobiphenyl samples of E7 from Merck Chemical Company. The liquid crystal material 30 is located between opposed parallel substrates in which the distance between surface and surface of the substrates is less than 20 µm and more particularly about 10 µm.

10 Each display cell is split into an active area and an active area. The active region includes a volume of liquid crystal material 30 capable of intermediate switching in response to suitably applied electric fields.

Generally, for the cell type depleted in FIG. 1, the active region is determined as that region located between the overlapping of conductors 40 and 41. In FIG. 1, a bottom boundary of the active region is indicated as the crosshatched area at the surface 21.

The inactive region surrounding each active region consists of a volume of liquid crystal material which maintains a fixed orientational alignment element configuration independent of the configurations in adjacent active regions. Each inactive region, also known as a neutral isolation region, separates, isolates and stabilizes the surrounding active region of a corresponding cell in the liquid crystal display. A theory of neutral, isolation areas is explained by J. Cheng in "Surface Pinning of Disciplines and the Stability of Bistable Nematic Storage Displays", J. Appl. Phys. 52_, p. 724-727 (1981).

Further information regarding physical aspects and the construction of the basic display cell enclosed in FIG. 1 is found in U.S. Patent 4,333,708.

The variable voltage source 50 generates a number of electrical signals which are supplied to the upper conductor 40 and the lower conductor 41 to pass through the entire liquid crystal material 30 and substantially perpendicularly to the substrates 10 and 11 different electrodes. . * · -7- to create AC and DC electrical fields. Depending on the characteristics of the electric field applied in the active region of the display cell, the orientational alignment element configuration of the liquid crystal material 30 is transformed via 5 a distorted horizontal configuration (Fig. 5) or an upper asymmetric horizontal state (Fig. 6) or a lower asymmetric horizontal state (Fig. 7) After the switch to an asymmetrical state is initiated, the source 50 generates a hold AC voltage signal to complete the switch period and an asymmetric horizontal in the display cell condition with a holding voltage.

The signals generated by the source 50 generally belong to the multitude of constant envelope and constant amplitude signals. More specifically, signals with constant envelope gated are AC pulse signals with substantially constant envelope, while signals with constant amplitude are gated DC pulse signals.

In order to effect the switching according to the invention, signals from the source 50 generate voltages which are related to a critical potential V, which will be described in more detail later. The signals are classified into large categories, namely, a DC recording signal, a DC lead-in or DC exchange signal, and an AC hold signal. A recording signal from the source 50 applies a DC voltage 25 of a value V across the display cell to initiate the switch π of the cell to a first (top or bottom) asymmetric horizontal state, while the potential V is either above or below the critical potential V is located to obtain the desired switching c characteristics. An exchange signal applies a potential 30 with a value V across the display cell to switch the

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cell to a second (bottom or top) asymmetric horizontal state, where V has substantially the same value as but is opposite to V in polarity. A holding AC voltage signal is generated by the source 50 to complete the switching period and to keep orientational orienters in the particular asymmetric horizontal state to which they have switched.

8303545 / ψ i -8-

The hold signal generates an AC voltage of a value V.

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the cell, where V is at least greater than the critical potential V. The value of the holding potential V_ can be increased to improve the C n optical contrast between the first and second asymmetric horizontal states. It is noted that the potentials V, V, E Ξ V and V depend on the dimensions and other properties w c of the liquid crystal display cell. By way of example, however, it is known that with a thin cell (a distance between the substrates of 10 ^ nn) containing E7, the preferred potentials are = 1.5 volts, 10 Vw and V are between 1.5 volts and 5.0 volts, and is less than 10.0 volts. More detailed information regarding the variable voltage source 50 and the bistable switching of the liquid crystal display cell can be found below with reference to Figures 5 to 7.

Fig. 2 shows a view of the top tilt center plane 15 from a position along the line II-II in FIG. 1. The tilt center plane 20 comprises the active area plane 201 (ellipses with a dark outline) and an isolation area plane 202 (ellipses with a light outline). perimeter). Ellipses have been drawn to represent tilted molecular columns in the tilted topography of the surface 20. A vector is drawn along the major axis of each of a number of ellipses at the active region plane 201 as an orthogonal projection of the major axis of each ellipse, that is, the molecular axis of the metallic column, on the tilt center plane. Since the vector indicates a direction in which the columns are tilted from the tilt center plane, it can be said that the vector indicates a direction of the surface tilt for the metallic oxide columns and, therefore, a direction of azimuthal bias for the tilt center plane.

The azimuthal bias for an active area plane is measured as an angular displacement from the reference line.

In the figures, line 213 is the reference line. Line 203 is parallel to the vectors on the surface 201 to indicate the direction of the azimuthal bias for the active area plane 201 at an angle α, where α is an acute angle between -90 and + 90 °. It should be noted that the isolation area plane 202 is centered parallel to the azimuthal bias direction of the active area plane 201.

8303545 Μ * + -9-

Fig. 3 shows a view of the bottom tilt center plane 21 from a position along the line III-III in FIG. 1. The plane 21 includes the active area plane 211 (dark outline ellipses) and the isolation area plane 212 (light outline ellipses). The reference 5 line 213 again shows the direction of the azimuthal bias for the active region plane 211, such that the azimuthal bias for the surface 211 is 0 °. The azimuthal bias for the surface 212 is parallel to the direction of the bias for the surface 211.

In the active region of the display cell, surfaces 20 and 21 form an inverse tilt limit state. An inverse tilt occurs because the azimuthal bias α of the surface 201 is between -90 and + 90 ° and, when measured as an acute angle to each respective substrate normal (inner surface), the surface tilt angle for the surface 201 is a has opposite polarity to the surface tilt angle for the surface 211. As shown, for example, in Figs. 2 and 3, the surface tilt angle for the surface 201 in the left direction is measured from the inner surface normal of the substrate 10 as an acute angle, while the tilt angle for the surface 211 in the right sense from the inner surface is normally measured for the substrate 11. As noted above, the surface tilt angles for the surfaces 201 and 211 must have absolute values in the range of 0 ° to 90 ° relative to the respective substrate normals, and preferably greater than 45 °, to achieve a horizontal orientation target configuration. Furthermore, it is important for the invention that the inverse tilt is equal so that the absolute value tilt angle of the surface 201 is substantially equal to the absolute value tilt angle of the surface 211.

In the isolation region, surfaces 20 and 21 form a uniform parallel boundary state, which is centered parallel to the azimuthal bias of the corresponding active region surfaces. That is, the insulating area surfaces 202 and 212 have columns that exhibit surface tilt angles of approximately 90 ° to the substrate normal (see Figures 2 and 3).

8303545

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It has been found that, for simple fabrication, the parallel boundary state of the surfaces 201 and 202 is obtained by an oblique evaporation of SiO 2 with the incident plane in a direction perpendicular to the azimuthal preferred bias direction relative to an angle of approximately 65 ° relative to the substrate normal.

The top and bottom tilt centering surfaces, individually and in combination, are important for bistable switching of the liquid crystal display cell. The top and bottom tilt center planes are fabricated in such a way that a preference of one asymmetric horizontal state over the others is eliminated in the absence of a particular electrical switching field and optical differentiation of the asymmetric states is provided. More specifically, the difference between the azimuthal biases of the top and bottom active region planes provides for an optical differentiation between the bistable states. A symmetry of the surfaces in the display cell eliminates a preference for establishing an asymmetric horizontal state at a given surface in the absence of the symmetry breaking field. These features will become more apparent later in the description of FIGS. 4 to 7.

Fig. 4 shows a three-dimensional view of the volume of liquid crystal material in the active region of the display cell, with the orientational alignment elements shown in an undistorted horizontal configuration. This is the quiescent configuration of the cell 25 because the orientation elements of the liquid crystal material adopt this configuration in the absence of an electric field. The planar section 401 of a boundary layer contains alignment elements of the liquid crystal material, which are oriented substantially below the surface tilt angle of the surface 211, while the planar sections 403 of a boundary layer comprise alignment elements, which are below the surface tilt angle of the surface 201 are oriented. Planar section 402 of an inversion layer includes orientational orienters that are generally horizontal (substantially parallel to) relative to each substrate surface.

For the sake of simplicity, FIG. 4 shows only enough detail to make planar section 402 as a single section of coplanar orientations. §303545 9. » -11- see rational target elements in the inversion layer. Obviously, there are a number of identical planar sections parallel to the planar section 402, including the entire inversion layer. Similarly, there is a corresponding number of identical planar sections parallel to each of planar sections 401 and 403, which include boundary layers at respective surfaces 20 and 21. This simplification of details has also been applied to Figures 5, 6 and 7.

The orientational alignment element centering is not changed from the deformed horizontal configuration until a field breaking the symmetry is applied to the cell. Furthermore, this change is sustained when a holding voltage equal to or greater than the critical potential is subsequently applied to the display cell. The critical potential V is determined as the potential

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15 above which the liquid crystal material 30 behaves in a bistable manner relative to horizontal configurations. The critical potential is described as follows. It is believed that the boundary and inversion layers are completely separated and exhibit a uniform "splay bend" distortion energy Uq per unit of volume, 20 where TT _ k Γ π 2 üo “2 2ξ 'and r - I "4Trk1 T 4" E Δε where ξ is the electrical coherence length, determined as the characteristic distance in which liquid crystal molecules with an average 25 splay bend modulus k and dielectric anisotropy Δε from the perpendicular state to rotate the parallel state relative to an applied electric field E. The unit energy density of area for each boundary layer is proportional to the thickness of the particular layer, as shown in the following table: 8303545 -12-

Layer Type Thickness Energy Density (Reference Numbers) per unit area

Limit 5 (501, 503) ξ / 2 ϋοξ / 2

Inversion (502) 52 2α0ξ

Inversion limitation 3_ 3 „_ ö-ξ tü 5 10 (504, 505)

The above table shows that the distorted horizontal configuration shown in Figure 5 has a total energy per unit area of 3µ ^ ξ, while the asymmetric horizontal states of Figures 6 and 7 each have a total energy per unit area of 15 of 2üqC. However, the argument does not apply to an applied field, where the boundary and inversion layers merge over the total thickness, d, of the display cell. Therefore, the cell thickness d is at least equal to 3ξ and the critical potential is given by the relationship: 20 V = dE = 3ξΕ.

c c c

For a sample of cyanobiphenyl E7 and surface tilt values with an absolute value of approximately 53 °, the critical potential Vc is approximately 1.3-1.7 volts.

When a potential greater than the critical potential Vq, 25 is applied to the display cell, the orientational orienters of the active region are transformed for a short time into a distorted horizontal configuration, as shown in Fig. 5. The distorted horizontal configuration includes planar sections 501 of the lower boundary layer, planar section 502 of the inversion layer and planar section 503 of the upper boundary layer, hereinafter referred to as the lower boundary layer 501, the inversion layer 502 and the upper boundary layer 503, respectively. This state is unstable because the total elastic and dielectric energy of the orienting straightener configuration can be decreased 3303545 -13- when the inversion layer 502 fuses with either the top boundary layer 503 (Fig. 6) to form the top asymmetric horizontal state, or with the bottom boundary layer 501 (Fig. 7) for forming the bottom shaft symmetrical horizontal state. The two resulting asymmetric horizontal permissions have the same energy, are topologically equivalent, and are separated by an energy barrier represented by the distorted horizontal configuration.

If the DC voltage applied to the cell in the distorted horizontal configuration 10 is V corresponding to the recording w signal from the source 50, an orientational directional transform is introduced from the distorted horizontal configuration (Fig. 5) to the one shown in Fig. 6. upper asymmetric horizontal position. The transformation occurs by a direct vertical movement 15 of the inversion layer 502 to the boundary layer 503. This results in the formation of the boundary inversion layer 504 at the active region plane 201 of the surface 20. Then, to the cell via the hold signal from the source 50 the holding AC voltage V is applied, the switching period is completed, and the orientational alignment elements are kept in the upper asymmetric horizontal state. The orientational alignment elements in the boundary inversion layer 504 are in the plane containing both the substrate normal and the azimuthal bias line for the active region plane 201, i.e. the line 203.

On the other hand, if the DC voltage applied to the cell in distorted horizontal configuration is equal to V, corresponding to £ the erasure signal from the source 50, an orientational target transform from the distorted horizontal configuration to the bottom asymmetric horizontal state shown in FIG. 7 is initiated. . The transformation occurs by a downward vertical movement from the inversion layer 502 to the lower boundary layer 501. When thereafter the holding AC voltage V is applied to the cell via the holding signal from the source 50, the switching period is completed and the orienting target configuration in the lower asymmetric horizontal state. The orientational alignment elements in the boundary inversion layer 505 are located in a plane containing both the substrate normal and the azimuthal bias line for the active 3303545 t -14 region plane 211, i.e., the reference line 213.

The intermediate state switching between the asymmetric horizontal states, for example from the top to the bottom or from the bottom to the top, is effected by no longer supplying the holding alternating current signal to the cell and allowing the liquid crystal material 30 to momentarily transitions to the distorted horizontal configuration (Fig. 5) or the undistorted horizontal configuration (Fig. 4). After a short relaxation period, a. registration DC signal or an erase DC signal applied to the 10 cell to initiate the switchover properly.

It is noted that the cell will transition to a substantially undistorted horizontal configuration in the presence of a potential less than or even slightly above the critical potential v. Therefore, the intermediate state switch can also be obtained by lowering the potential on the cell from the holding potential level to a level slightly above or below the critical potential.

It is advantageous for the operation of the display cell in any asymmetrical state that the orientational alignment elements are prevented from switching to a vertical configuration. Vertical configuration switching can be prevented by operating the variable voltage source 50 operating below the threshold level at which dislocation is released. This threshold level generally appears to be of the order of 60 volts.

In a second embodiment of the display cell, the essential elements and operations are the same as described above for the display cell shown in FIGS. 1 to 7. In addition, the isolation regions 202 and 212 exhibit a tilted column topography corresponding to the respective active region surrounding each isolation region.

Although not indicated in the figures, a suitable combination of linear polarizers and perhaps a fixed retardation plate can be used to increase the optical contrast between the asymmetric states.

An application for this type of nematic liquid crystal display cell is found in fast-acting matrix-addressable storage displays. Although the addressing speeds are currently 9303545

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It is clear that some modifications of the display cell characteristics may improve the operation of the display cell. More specifically, these modifications include reducing the distance between the substrates and reducing the viscosity of the nematic liquid crystal material.

8303545

Claims (5)

1. Liquid crystal display cell, which can be switched to either a first state or a second state, comprising first and second substrates arranged in parallel to each other, and a nematic liquid crystal material with orienting elements, which material is located between the both substrates, characterized in that each substrate has a topographically textured surface, which exhibits uniform tilt at a sharp surface tilt angle relative to a respective surface normal, and means are provided for generating a DC voltage of first polarity across the initiating liquid crystal material makes a first change in the orientational alignment member configuration by breaking the symmetry of the liquid crystal display cell to favor an orientational alignment member configuration in which an inversion layer with orientational aligners substantially at and parallel to the first sub street, and means are provided which are connected to each substrate to generate an electrical potential throughout the liquid crystal material in order to complete and maintain the first change in the orientational alignment member configuration such that the orientational alignment elements the first state. 2. Display cell according to claim 1, characterized in that the surface tilt angle for the topographically textured inner surface of the first substrate is substantially equal to and has an opposite polarity to the surface tilt angle for the topographically textured inner surface of the first substrate. second substrate, so that the two topographically textured inner surfaces form an equal opposite tilt-limiting state. Display cell according to claim 1 or 2, characterized in that each topographically textured inner surface has an azimuthal bias with respect to a predetermined reference line, the topographically textured inner surface of the first substrate having an azimuthal bias, which is only in the range of - 90 ° to + 90 °, and the topographically textured inner surface of the second substrate has an azimuthal bias of 0® 35. 8303545 -17- Display cell according to claim 1, 2 or 3, characterized in that the means for initiating the first change are provided with means for generating a DC voltage of second polarity over the liquid crystal material to initiate a second change in the orientational alignment element configuration by breaking the symmetry of the liquid crystal display cell to favor the orientational alignment element configuration, in which an inversion layer of orientational alignment elements is located substantially at and parallel to the second substrate, and the means 10 for generating an electrical potential further includes means connected to each substrate to generate an electrical potential throughout the liquid crystal material in order to complete and maintain the second change in the orienting alignment configuration such that the orientational 15 tional alignment elements in the second t state. 5. A method of switching a liquid crystal display cell with a nematic crystal material having orientational alignment elements, which material is disposed between parallel disposed first and second substrates, each substrate having a topographically textured inner surface which has a uniform tilt at a sharp surface tilt angle relative to a respective surface normal, a DC voltage with a first polarity is generated across the liquid crystal material to initiate a first change in the orientational target configuration by breaking the symmetry of the liquid crystal display cell to produce a favor orientational alignment element configuration, wherein an inversion layer of orientational alignment elements is substantially adjacent to and parallel to the first substrate, and an electric potential is generated throughout the liquid crystal material to change the first complete and maintain the orientational alignment element configuration such that the orientational alignment elements are configured in the first state. 8303545