JPS5958420A - Bistable liquid crystal display and switching of state ther-eof - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD This invention relates to electrically addressed bistable liquid crystal devices.

BACKGROUND OF THE INVENTION Matrix addressing of stratiform displays using bistable nematic liquid crystals has conventionally used continuous linear electrodes, combined with thermal erasing and field-effect threshold switching for writing. It was carried out. The memory function of this display is based on a bistable director structure, which allows memory to be retained even when no retention potential is applied.

In this type of display, the speed at which each display cell is addressed, and therefore the speed at which the array is switched, is directly proportional to the magnitude of the applied potential, or in other words, the strength of the applied electric field. Therefore, the speed of addressing can be increased if the threshold value at which the director switches is set to a high potential and the applied potential exceeds this threshold. The need for higher applied potentials for faster addressing is one obstacle in using liquid crystal displays for high speed matrix addressing applications.

SUMMARY OF THE INVENTION In bistable nematic liquid crystal displays, applying a low priming potential before switching cell states allows for faster addressing at lower applied potentials. The cell state is switched by applying a write potential higher than the preliminarily applied threshold switching potential.

In one embodiment of the invention, the cells of a bistable liquid crystal display are addressed by continuous uniform linear electrodes disposed orthogonally on two opposing substrates. However, if a small vertical electric field is applied to this, it will cause disturbance to the orderly array of horizontally oriented directors near one of the substrates. A larger vertical electric field is then applied to the cell, switching the director to vertical alignment. In this example,
Potentials well below the threshold switching potential without priming pulses allowed for fast addressing on the order of milliseconds.

DESCRIPTION OF THE EMBODIMENTS A cell of a liquid crystal display, as shown in the cross-sectional view of FIG. It may also be configured. Furthermore, the area of the KM crystal material 13 above the display cell is shown depending on the state of the director. The states in Figure 1 are horizontal (H) and neutral (
N).

The substrates 10 and 14 hold the conductors], I and 15 and also serve as containers for the liquid crystal material 13. Each substrate is primarily composed of a transparent insulator such as silicon dioxide or glass.

The conductors Jl and 15 are placed inside each substrate so that the voltage rise is sufficiently perpendicular to each substrate. Conductor 1
1 and 15 may be interdigital (@teeth) type electrodes or continuous and uniform linear electrodes.

Since FIG. 1 is shown merely by way of example, the conductors 11 and 1
5 shows the case of a continuous and uniform linear electrode 1-. conductor J
1 is formed inside the substrate 10, and a conductor 15 is similarly formed inside the substrate 14, but this is similar to the conductor 11.
It faces in a direction perpendicular to the direction of. Each conductor is formed as a thin film on the inner surface of its respective substrate by vapor deposition or etching using conventional optical phosphorography techniques. In a transmissive display cell, a transparent F3A film such as indium soot oxide is used as the conductor, and in a reflective display cell, an opaque film of aluminum, for example, is used as the conductor.

As mentioned above, conductors 11 and 15 are connected to the vertical electric field, i.e.
A virtually perpendicular electric field will be created in the liquid crystal material 13 for each substrate.

This electric field causes the liquid crystal material 13 to have its director vertically aligned when the electric field strength exceeds a certain threshold. (See Figure 5) In fact, it is also necessary to switch the soil director from a vertical orientation to a horizontal orientation. Depending on the type of conductor employed, thermal or electric field effects are suitable for this switching. Applying a horizontal electric field to the liquid crystal material is possible by means of interdigitated electrodes. See, for example, US Pat. No. 4,333,708, issued June 2, 1982. In the case of a conductor for a uniform linear electrode as shown in Fig. 1, switching from vertical alignment to horizontal alignment is achieved by electrothermal action using resistance heating, for example, by applying current to the electrode 15 and once isotropically dissolving the liquid crystal material. This is achieved by cooling the liquid crystal material so that it becomes horizontally aligned.

The inclined array surfaces 12, 16 and 17 are the first of each board]]
It is a transparent oxide film obliquely deposited on the surface, and is a conductor that determines the alignment of the boundary surface of the liquid crystal material. The inclination angle θ measured from the substrate normal by the structures 12 and 16. is determined in the range of 22.5° to 67.5°.

The inclined arrangement surface 17 is formed on the conductor 15, and the inclined state is opposite to that of the conductor 12 and 16.

In other words, the structure of surface 17 is -0 with respect to the perpendicular to the substrate. The inclination angle is 7a, and since the inclination is reversed in this way, a discontinuity point in the inclination direction occurs at the location where the horizontal edge of the surface 17 intersects with the surface 16. Discontinuities are indicated by small circular black dots in Figures 1, 4 and 5.

Liquid crystal material] 3 is a nematic liquid crystal material mixed with a pleochroic dye in order to cause a change in the arrangement of the director to change the optical characteristics.

For typical displays, the liquid crystal material 13 is cyanohyphenylated (Merk Chemical).
E7) Added 0.5-20% pleochroic dye (DS from Merck & Co., Ltd.) and adjusted the substrate spacing to 10-5.
0 μm, usually 20 μm, between which the material is sandwiched.

The liquid crystal material 13 exhibits positive anisotropy. Substance 13 is also
It is also possible to use a material with positive anisotropy without additives and detect changes in optical properties using a polarizer and analyzer. In order to properly bias the overall ordered structure with a torsional chirality, some cholesteric mesophase liquid crystal material is added to the nematic material - this creates a mixture of torsional domains and torsional walls, which leads to optical It is known that it is desirable to prevent deterioration of characteristics.

In FIG. 1, liquid crystal material 13 is shown in a horizontal alignment of directors in the area where conductors 11 and 15 overlap. A neutral separation area surrounds this horizontal arrangement area. The neutral separation region maintains a constant director structure when adjacent bistable cells are in one of two stable states, either designated state 2, i.e., the director is horizontally or vertically oriented. This refers to the part of the liquid crystal material. These regions surround the individual cells to separate, isolate, and stabilize each cell of the display device.

The theory of the neutral separation region was described by J. Cheng in the Journal of Applied Physics, Vol. 52, pp. 724-727 (1 ,98
1).

Additional information regarding the operation, physical properties and construction of the display cell of FIG. 1 is found in U.S. Pat.
Published in.

Data entry into a display cell, also referred to as writing the cell, is accomplished by providing write pulse signals to conductors 11 and 15. 1st
For the illustrated device, the input pulse signal is typically an alternating current signal that creates an electric field in the liquid crystal material 13 perpendicular to the substrate surface. If this applied electric field exceeds a certain switching threshold for at least a certain amount of time, then
This electric field causes the director to be switched from horizontal to vertical orientation.

The AC switching characteristics from horizontal to vertical orientation in the display cell of FIG. 1 are shown in FIG.
The duration tw of the applied electric field is from 2m5ec to ■ (6 seconds)
has been changed. The relationship between the electric field strength between both groove bodies based on this applied potential, that is, the write pulse signal applied to conductors 1 and 15, and the ratio of written SE/L is 3.
The case of two write pulse signals is illustrated.

It should be noted that the frequency of each pulse signal is ],, OKHz. In addition, each cell is further subcelled with a neutral isolation region of 40 μm square and each 10 μm wide]
, 6 X l , thereby localizing cell defects to one or several subcells and preventing them from interfering with the switching operation of the entire cell.

As shown in FIG. 2, a write pulse signal of substantially infinite duration (tw-■, however, in this case,
■ means the duration is 6 seconds or more.

), the threshold value of the electric field is at its minimum, and the threshold value is determined by the strength of binding the disclination to the periphery of each cell. A write pulse signal of finite duration (
For tw < ”), the field strength threshold moves to higher levels as the duration - becomes shorter. The dependence of the threshold on the pulse duration makes addressing faster and faster. In order to achieve fast switching of liquid crystal display cells addressed by write pulse signals such as those described in connection with Figures 1 and 2, this means that high electric fields are required. do.

In accordance with the principles of the present invention, a sufficiently low field strength threshold and the potential created by applying switching signals R8W and C8W of FIG. 3 to conductors 11 and 15 of the display cell (of FIG. Address specification can be performed. The switching signals R8W and CSW are synthesized by a priming pulse signal and a write pulse signal, respectively.

Each priming pulse has an amplitude of substantially V.

/2 Volt and lasts for t2 seconds, and each write pulse has a substantially constant envelope and a duration of t, -(t3t2) seconds.

A typical example of a switching signal is a cutout of an alternating current signal with a frequency of 1/t, Hz, as shown in FIG. Other alternating current signals that provide a constant envelope are preferable to constant amplitude signals because constant amplitude switching signals cause space charge deflection effects, which weaken the applied electric field. In the following, the fourth
Let us consider signals C8W and R8W while referring to FIG. Switching signals R3W and C
A signal source that can make SW mature can be easily prepared by anyone involved, so I will not discuss this in detail.

FIG. 4 shows the behavior of the liquid crystal material 13 based on the electric field E (the direction of the electric field is indicated by the arrow) caused by the priming pulse portion of the signals CSW and R8W. Several new regions are shown in the figure: NA/N is the boundary between the neutral separation region and the asymmetrically strained horizontal alignment region (HA), and W is the boundary between the symmetric and asymmetrically strained horizontal The domain wall between sequence regions Hs and HA is shown.

In the bistable region of the display cell, conductors 15 and 11
When an electric field E is applied that occurs behind the priming pulse portion of signals C8W and R3W applied to , respectively, the alignment of liquid crystal material 13 will be distorted in the boundary layer near 12 and 17. . As the applied electric field increases, the liquid crystal material 13 is oriented entirely parallel to the electric field, whereas
This distortion is successively confined within a number of boundary layers whose thickness is the electrical coherence distance ξ. Spray
Due to the bent strain, the direction of local molecular alignment φ measured from the perpendicular to each substrate is now 12°]6 and 17 to 87
φ within 2. 2φ<π/2. Furthermore, the line (φ-π/2) along which the director is horizontally self-oriented is located in the middle of 11 display cells in area H8, and has a thickness curve ξ
, while in the area HA the horizontally oriented line exists in the vicinity of the inclined alignment plane, i.e. 17, to which the disk 1 nation is constrained.

To warp a horizontal array into a horizontal array with asymmetric distortion,
The priming potential V applied to suppress the electric field E must be higher than the critical potential VC. In the display Q cell described below, the dielectric anisotropy Δε is approximately]
Voltage ■ for liquid crystal material J3 consisting of a cyanobiphenyl sample of 3. is 10-2. It is within the range of OVolt.

Note that the critical voltage V. It should be noted that 5 Δε varies as a function of Δε.

For more information on critical voltage vc, see J. Cheng.
) et al.'s Journal of Applied Physics (JAppLphys, )
Volume 52 (4) I) I), 2756-2765 (19
8]-).

When a priming potential higher than V 2 is applied to the display cell, for a short period of time, due to molecular motion within the liquid crystal material 13, the horizontal alignment first becomes a symmetrically distorted horizontal alignment H8. For a low potential V, e.g. in the range of 2.0 to 5.0 Volt for a cyanobiphenyl sample, the liquid crystal material]3 is generally affected by the applied voltage and By moving the orientation line in the vertical direction, it is switched from H8 to a horizontally aligned shaft with asymmetrical distortion. ■ Higher voltage Vp, e.g.
5. At Vp higher than QVolt, the domain wall W is formed around the active region of the cell and propagates from the periphery to the inside of the cell, and along with this, the domain wall W is
Then it switches.

The direction of propagation of the domain wall W is as indicated by the arrow 18.

By arranging the liquid crystal material 13 near the constraint point (the location marked with a black circle) either in the C arrangement as described above or in a mixture of Kite and H8 arrangements, the display cell can be constructed as shown in FIG. This reduces the potential threshold required to switch to vertical alignment. This is HllZN
This is because there is a difference in stress between Alx and (Alx). The accumulated strain energy of the surface, part torsion or Bloch wall of the former is lower than that of the latter, that is, the spray vent wall. Therefore, After the HA/N plane is sufficiently primed, a lower threshold electric field is sufficient than the unprimed H/N plane.

In particular, the Bloch (twisted) wall W serves to relieve the distortion of H3/N, which would otherwise leave a spray vent wall on this surface.

If the Bloch wall remains on the HsZN surface, it lowers the energy of this surface and stabilizes it. When an electric field is applied to the liquid crystal material 13, as Hs replaces HA, the wall surface moves toward the center of the cell due to the difference in Halke energy. This movement increases energy, and as the H8/N plane replaces the HA/N plane, the stress on the new plane increases, which causes the plane to become unstable and lower the threshold electric field. These changes begin as soon as the domain wall W begins to move, and are completed when the domain wall leaves the interaction zone of the constraint point.

Since the stress is released by the presence of the domain wall W, its presence stabilizes the binding point and the domain wall W
Dislocations from the binding point to the disclination are suppressed until the binding point is far enough away from the binding point. Its interactions with the acid domain walls and binding points are not effective in releasing stress. The distance satisfies this condition and is inferred from the dimension of the domain wall and the geometric meaning of the twisted coherence distance ξ2. In the case of the cyanobiphenyl sample, the domain wall W moves to the center of the display cell at a speed of several microns per second due to the electric field based on the priming potential, so the domain wall W moves within 100 Sec over a distance of 7<2 μm. It will be possible.

Therefore, the priming part of signals C8W and R8W is t2'-1,00 m in the cyanobiphenyl sample.
Must be 5ec or more.

FIG. 5 shows the display cells in a vertically aligned state after application and removal of the write portions of signals CSW and R8W, ie, after time t3. The transition from salt to vertical alignment or composite HA/H8 alignment (Figure 4) to vertical alignment is due to self-contraction and annihilation of freely floating disclination loops under the electric field based on the write pulse signal. It is something to do. The strength of the alternating electric field based on this write pulse signal, as shown in FIG. 6, is well below the field strength of a similar frequency and duration without the prior application of a priming electric field.

The results in FIG. 6 relate the display cell parameters to FIG. 2, where the frequency of the alternating electric field is I KHz (1/t+). For example, in a primed cell], OOVolt (3Vo)
, fully addressed and switched with a write pulse signal of 2.0 m5ec (tW), whereas 155 Vol for unprimed cells.
t (3Vo) is required. It is clear from FIG. 6 that if the molecular alignment of the liquid crystal material 13 can be disturbed by the priming signal, millisecond auto-addressing can be made possible at low applied potentials or electric field strengths.

Another advantage of employing a priming pulse signal in this type of display cell is evident in multiplex operation, such as a 2:1 or 3:] multiplex. The lower limit of the signal level that will not be selected for the primed display, i.e. 2
．． 55Vo with 0 msec (tw) addressing
The 34 Volt vs. lt serves to prevent spurious switching due to surface defects away from the tail of the threshold open line. Therefore, switching at low electric fields is possible without compromising addressing speed.

Various modifications to this embodiment are possible. For example, the priming potential V is V. In other words, 1/3 of the glue potential
, it would be possible to simplify the design of the generator of the signals CSW and R8W. When dealing with larger arrays of display cells, it becomes more practical to apply the initial priming potential to the entire array rather than to each cell individually and sequentially.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a bistable nematic liquid crystal display cell with the liquid crystal director oriented horizontally. FIG. 2 shows AC switching characteristic curves for write pulse signals of various durations. FIG. 6 is a diagram showing an example of a combination of the C double priming signal and the write pulse signal used for the conductor of the cell shown in FIG. 1. Figures 4 and 5 are diagrams showing the situation in which the cells in Figure 1 are rearranged based on the electric field generated by the signal in Figure 6 (Figure 6). This is a diagram comparing the AC switching characteristics of a display cell and a non-display cell C. [Explanation of symbols of main parts] Substrate 10.14 Liquid crystal material 1.

Claims (1)

[Claims] 1. A first and second substrate; a nematic intermediate phase liquid crystal material having a director oriented between the two substrates; processed so that the director is oriented in a predetermined pattern; a bistable liquid crystal display comprising on an inner surface of each of said substrates, applying a first priming electric field in said liquid crystal material to cause a director of a first selected portion of said liquid crystal material to be in a strained alignment in a first specified state; (signal between t.-t2 in FIG. 3);
writing means (t2
- a bistable liquid crystal display characterized by a signal between t3). 2. The display device according to claim 1, wherein the first electric field is generated by a first potential difference between the two substrates, the first potential difference is larger than a critical potential, and the first electric field is generated by a first potential difference between the two substrates.
A display device characterized in that the threshold switching potential of the display device is substantially lower than the threshold switching potential of the display device. 6. The display device according to claim 2, wherein the second electric field is generated by a second potential difference between the two substrates, and the second potential difference is a first threshold switching potential. , a second threshold switching potential. 4. first and second substrates, a nematic intermediate phase liquid crystal material having a director oriented between the two substrates, and each of the substrates processed so that the director is oriented in a predetermined pattern; a method for state switching of a bistable liquid crystal display of the type comprising: applying a first priming electric field to the director of the first selected portion of the liquid crystal material in a distorted alignment of a first specified state; generated in the liquid crystal material, the second selected portion is configured to switch the director arrangement of the first selected portion from the distorted arrangement of the Jth designated state to the second designated state.
1. A state switching method for a bistable liquid crystal display, characterized in that an electric field of 1 is generated in the liquid crystal material. 5. The method according to claim 4, wherein the first electric field is caused by a first potential difference between the two substrates, the first potential difference is greater than a critical potential, and the first threshold switching A method characterized in that the electrical potential is substantially lower than the electrical potential. 6. The method according to claim 5, wherein the second electric field is caused by a second potential difference between the two substrates, and the second potential difference is a first threshold switching potential, A method characterized in that the switching potential is lower than the second threshold switching potential.