JPS631593B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for electrically rewriting a displayed image in a storage type liquid crystal display device using so-called p-type cholesteric liquid crystal by utilizing the characteristics of the liquid crystal. Liquid crystal display devices have been widely put into practical use in recent years as light-receiving display devices that have the advantages of low voltage and low power consumption, and research and development for further improvements and applications are actively underway. In particular, cholesteric liquid crystals have a memory effect, ie, the ability to maintain a display for a long time even after the electric field is removed. Therefore, if this type of liquid crystal is used, it is expected that the display power will be further reduced, and there will be no need to refresh the screen to prevent image flickering when used in matrix type liquid crystal display devices. , and has the feature that restrictions on the number of scanning lines that can be displayed simultaneously are dramatically relaxed.
Among them, the so-called p-type cholesteric liquid crystal, which is a mixture of a p-type nematic liquid crystal, which has a positive dielectric anisotropy, and an optically active substance such as a cholesteric liquid crystal or a chiral nematic substance, The so-called n
Compared to cholesteric liquid crystals, it has features such as a higher contrast ratio and shorter writing time, so it is widely expected to be used mainly in matrix-type liquid crystal display cells. The present invention relates to a storage XY matrix type liquid crystal display device using this p-type cholesteric liquid crystal. ,
(referred to as X-electrode array and Y-electrode array for convenience)
The present invention relates to a method of selectively electrically rewriting a part of a plurality of display sections formed at the intersection portion of the present invention. A suitable method for partially rewriting such an XY matrix type liquid crystal display device has not been known in the past.
When a specific display section consisting of a pair of strip-shaped X-electrodes and a Y-electrode intersects is rewritten, that is, erased or written, a potential change is applied to the X-electrode and the Y-electrode, and a voltage is applied to the specific display section. is applied, but this potential change inevitably causes a voltage change not only to the specific display section but also to other display sections that are in contact with the X-electrode and Y-electrode. This is because it was thought that the specific display section could not be rewritten without a change in the display.
However, according to the research of the present inventor, the memory XY using such p-type cholesteric liquid crystal
It has been found that in a matrix type liquid crystal display device, partial rewriting of such a matrix type liquid crystal display device is possible by appropriately utilizing differences in response characteristics to applied voltages of liquid crystals in different states. More specifically, the partial rewriting method for a memory type liquid crystal display device of the present invention uses a pair of vertically aligned electrode substrates each having an X-electrode array and a Y-electrode array on their surfaces, each consisting of a plurality of spaced apart strip-shaped electrodes. ,X-
The electrode array and the Y-electrode array are arranged so as to intersect with each other, and a mixed liquid crystal containing a nematic liquid crystal having positive dielectric anisotropy and an optically active substance is sandwiched between the pair of electrode substrates, and the X- A storage XY matrix type liquid crystal display device in which the electrode intersections of the Y-electrode rows are used as unit display parts is first constructed by combining a transparent display part made of liquid crystal in a ground state and a light scattering part made of liquid crystal in a focal conic state. It is characterized by rewriting the display corresponding to a part of the band-shaped electrodes of the band-shaped electrode group included in the X-electrode array by placing the display in the low-voltage display state that is formed, and then performing operations including the following steps. be. (b) Applying a voltage X sufficient to shift the focal conic state liquid crystal to a homeotropic state.
-Apply between the rewriting voltage in the electrode row and the counter electrode in the Y-electrode row, apply approximately the same potential as the counter electrode to the non-writing electrode in the X-electrode row, and apply to the rewriting electrode. (b) determining at least a part of the rewrite electrodes in the X-electrode array as write electrodes, and at least one of the Y-electrode arrays; Defining the electrodes as scanning electrodes and maintaining each electrode in the following voltage relationship: a. Setting the non-writing electrode in the X-electrode row and the non-scanning electrode in the Y-electrode row to approximately the same potential; b. Scanning. The electrodes are set with a potential corresponding to a voltage V at which both the focalconic state liquid crystal and the homeotropic state liquid crystal can exist stably or metastablely. Almost the same potential is set for the non-writing electrode, and an opposite potential of the same magnitude as that of the scanning electrode is set for the non-writing electrode. The liquid crystal is maintained for a sufficient time to form a graminian state in the display section and for a time period for the liquid crystal to undergo a phase transition from the ground state to the focal conic state under the application of the voltage V. (c) Setting the operating electrode to zero potential, applying a voltage V to the liquid crystal of the display to be written, and maintaining the voltage for a time sufficient for the liquid crystal in the grunge state to transition to the focal conic state. The present invention will be explained in more detail below. Although the memory type liquid crystal device used in the present invention and its basic operating principle are known (e.g., Electronic Materials, November 1975 issue, pages 28-33), they will be briefly repeated for understanding of the method of the present invention. . FIG. 1 is a conceptual cross-sectional view of an example of a memory-type liquid crystal display device used in the present invention, in which at least one side is provided with a strip-shaped X-electrode array 1X and a Y-electrode array 1Y, respectively. After vertically aligning the electrode surfaces of a pair of transparent electrode substrates 1, a p-type cholesteric liquid crystal 2 is sandwiched between them, and both electrodes are connected to an AC or DC power source 3 to apply an electric field to the liquid crystal 2. This is possible. Note that detailed illustrations of the electrode arrangement and their connections are omitted. FIG. 2 shows the light transmittance-voltage characteristics of a p-type cholesteric liquid crystal. When a p-type cholesteric liquid crystal is initially sandwiched between a pair of vertically aligned electrode substrates, the liquid crystal is in the ground state shown by S in the figure. This S state is also called a vortex state based on the results of microscopic observation, and as a result of the vertical alignment process, near the surface of the electrode substrate, liquid crystal molecules are aligned perpendicularly to the substrate surface, while as they move away from the surface, the
It is considered to be a complex state in which the liquid crystal molecules are aligned almost parallel to the substrate surface, and the direction of the molecular axis gradually changes as the distance from the substrate surface increases. It is being As a result of this molecular orientation, liquid crystals are substantially transparent in the S state. When a voltage is applied to the liquid crystal in the S state and the voltage is increased, the liquid crystal moves in the direction of the arrow along the solid line in Figure 2, and exceeds a certain threshold voltage V _L , as shown in the figure. The state changes to a focal conic state indicated by F. In this F state, the liquid crystal scatters light and becomes cloudy. When the applied voltage is further increased, the liquid crystal passes the threshold voltage V _H and shifts to the homeotropic state indicated by H in the figure. In this H state, the liquid crystal molecules are forcibly aligned perpendicular to the electrode substrate by the applied electric field, and their transparency is maximized. However, with the naked eye, the transparency of the H state and that of the S state are almost indistinguishable. Next, when the applied voltage is gradually lowered to the liquid crystal in the H state, it exhibits hysteresis, and the homeotropic state continues even after passing the threshold voltage V _H (H' state in the figure), until a certain voltage value V _H ' and moves to F state. When the voltage is further lowered from the F state, it does not return to the S state even after passing V _L , but remains in the focal conic state and becomes metastable at voltage 0. This metastable focal conic liquid crystal, indicated by F' in the figure, gradually shifts to the S state if left for a very long time, but is considered stable for several days to several months. On the other hand, if the voltage applied to a liquid crystal in the H or H' state is suddenly lowered to near 0 volts and maintained for a certain relaxation time τ _HG or longer, it will not enter the F state this time and will not reach the grunge range unique to cholesteric liquid crystals. The state becomes the G' state (herein referred to as the grandien state), which has a helical structure similar to that of the G' state. If the liquid crystal in the G′ state is kept at an applied voltage of 0 for a relaxation time τ _GS , it will return to the base S state, but
Here, when a voltage V under the condition of V _H '<V<V _H is applied and the relaxation time τ _GF or longer is maintained, the liquid crystal returns to the focal conic state. It has been experimentally confirmed that the following relationship holds true for relaxation times associated with various state changes. τ _SF = 200 to 300 mS τ _HG = 1 to 20 mS τ _GF = 5 to 50 mS τ _HF = τ _HG + τ _GF ≒30 mS≪τ _SF The light transmittance-voltage characteristics of the p-type cholesteric liquid crystal explained using Fig. 2 are as follows. It is used to display the memory. In other words, once the entire liquid crystal is set to H or
After transitioning to the H' state, the voltage for the display area is gradually lowered, passing through the F state and reaching the F' state, which is a light scattering state near OV, and for the non-display area, the voltage is suddenly decreased from the H or H' state. When the voltage decreases to the transparent S state, stable display continues in both states for a relatively long period of time. Memory display can also be done in other ways.
This is because both the F state and the H' state can be stabilized between the voltages V _H ' and V _H , and the liquid crystal in the H' state is dropped to 0 V for a time sufficient for a certain phase transition, that is, the relaxation time τ _HG , and then V This takes advantage of the fact that when the voltage is returned to between _H ' and _VH , it stabilizes in the F state. That is, once all the liquid crystal elements are in the H' state, then only the elements in the display section are dropped to 0 V for more than τ _HG , and then returned to the voltage between V _H ' and V _H , thereby changing between the F state and the H' state. Can be displayed.
If the voltage is rapidly reduced, the F state and H' state will transition to the F' and S states, respectively. This method has the advantage that storage and display can be done more quickly than the above-mentioned method. Now, the partial rewriting method of a matrix type liquid crystal display device according to the present invention is performed by utilizing the difference in response relaxation time due to the various state changes described above, especially the relationship τ _HF = τ _HG + τ _GF ≪ τ _SF . . An example of this method will be described below using an example of an XY matrix type liquid crystal display device. An X-electrode array consisting of five strip-shaped electrodes X ₁ to _{X 5} and a Y-electrode array consisting of three strip-shaped electrodes Y ₁ to _{Y 3} are provided on the opposing substrate, and 15
It is assumed that a liquid crystal display device having a flat matrix arrangement as shown in FIG. 3 in which a display section is formed is used. In the following description, for example, electrodes X ₄ and electrodes
The display section formed at the intersection with _Y3 is indicated as _D43 . Now, the electrodes X ₃ to _{X 5} in the display device in the display state shown in Figure 4a (the shaded display area is in the light scattering state and the other display parts are in the transparent state) are used as rewriting electrodes. The display state shown in FIG. 4b is finally obtained by performing a rewriting operation. In the state shown in FIG. 4a, the potentials in all electrode rows are equal (typically 0 potential), and the voltage applied to the liquid crystal of each display section is 0. According to the present invention, each electrode is first given a potential as shown in FIG. 5a. The values shown in the margins here are the potentials applied to each electrode, and the values shown in each display section are the voltages (potential of the Y electrode - potential of the X electrode) applied to the liquid crystal of that display section. Here, the voltage V is the voltage at which both the focalconic state liquid crystal and the homeotropic state liquid crystal can exist stably or metastablely (V _H ′<V< _VH ), and 2V is the voltage at which the focalconic state liquid crystal and the homeotropic state liquid crystal can exist stably or metastablely. The voltage is sufficient to shift the liquid crystal to a homeotropic state (2V> _VH ). If this state is maintained for more than time τ _SH , Figure 5b
The display state of , that is, erasure of the display sections D ₃₁ to _{D 53} formed by the rewriting electrodes X ₃ to X ₅ is obtained, and the state of the liquid crystal of each display section at that time is as shown in FIG. 5c. Next, the electrode X ₃ is set as a write electrode and the electrode Y ₃ is set as a scan electrode, and the potential shown in FIG. 6a is applied to each electrode and held for a time t ₁ (τ _HG <t ₁ <<τ _SF ). As a result,
As shown in FIG. 6c, the G' state (grandien state) is formed in the display area _D33 , but the display area _D23 is in the S state.
The display state shown in Fig. 6b (display section D ₃₃
The dotted diagonal line corresponds to the G' state and indicates a state with somewhat reduced transparency). Note that the voltage -V applied to the display section _D32 and the like has exactly the same effect as the voltage V on the liquid crystal. Here, as shown in FIG. 7a, scan electrode _Y3 is set to 0.
When the potential is lowered and held for a time t ₁ '(>τ _GF ), the display portion D ₃₃ changes to the F state as shown in FIGS. 7b and 7c, and writing is performed. The series of operations described above is the most basic embodiment of the method of the present invention, and means that a partial rewriting has been performed from the display state of FIG. 4a to the display state of FIG. 7b.
Conventionally, in the process of the above operation, in particular,
It was thought that partial rewriting of such an XY matrix type liquid crystal display device was impossible because the voltage state of _FIG . By maintaining the voltage state of τ _HG < t ₁ < τ _SF for the time t ₁ satisfying the condition, the display section
It is now possible to selectively write to the display portion D _{33 without changing D 23 (or including this when the display portion D 13 is} originally in the S state). As already mentioned, the above series of operations completes the implementation of the method of the present invention. However, in order to rewrite to the display state shown in FIG. 4b, the following operations are additionally performed. That is, from the state shown in FIG. 6, without passing through the state shown in FIG. 7, electrodes X ₄ and X ₅ are defined as write electrodes and electrode Y ₂ is defined as a scan electrode, and each electrode is successively exposed to the state shown in FIG. 8. Apply the potential of a and time t ₂ (τ _HG < t ₂ ≪ τ _SF )
Hold. As a result, the display states shown in FIGS. 8b and 8c are obtained. That is, speaking only about the electrode _Y3 , 0 potential is applied, and the display section _D33 changes from the G' state to the F state, that is, a writing operation is performed, while the corresponding electrode _Y2 This means that the display sections _D42 and _D52 are being rewritten to the G' state, that is, a preliminary write operation is being performed. _{Furthermore , the electrode}
＜t ₃ ≪τ _SF ) held. As a result, the display states shown in FIGS. 9b and 9c are obtained. The changes here are substantially the same as those described in FIG. Next, as _{shown in FIG} .
Writing is performed on the display section _D51 as explained using the figures, and the display states shown in FIGS. 10b and 10c are obtained.
In addition, in FIGS. 8 and 9, if the holding times t _{2 and} t ₃ are not greater than τ _GF , the display portion D ₃₃ (FIG. 8 b, c) and the display portion D ₄₂ , D ₅₂ (FIG. 9 b, c) may remain in the G' state and the illustrated state change may not have occurred. However, in any case, since the voltage V is applied to these display sections in the subsequent operation, the change to the F state is ensured, although there may be a difference in time. This completes the steps from Figure 4a to Figure 4b, that is, Figure 10.
This means that a series of rewriting operations to the display state shown in FIG. b have been performed. In order to fix the rewritten image obtained in this way as a memory image, it is necessary to
The electrodes involved in the display of the electrode array may be set at the same potential (typically 0 potential) as shown in FIG. 11a. As a result, stored images in display states as shown in FIGS. 11b and 11c are obtained. In the above, parallel straight strips X-, Y-
An orthogonal matrix type liquid crystal display device consisting of a combination of electrodes has been described. However, the shape of the strip electrode is arbitrary, and it can be made of not only an oblique matrix but also a combination of curved strip X-, Y-electrodes.
It is easy to understand that the present invention can also be applied to segment digital display matrices, etc. Further, as an application, by sandwiching the display device used in the present invention between a pair of polarizing plates, color contrast can be performed using different birefringence states in the scattering portion and the transmission portion. Note that the voltage applied between the electrodes may be an alternating current voltage or a direct current voltage. However, threshold voltages such as V _H , V _H ′, etc. and relaxation times may differ somewhat between alternating current and direct current. Generally speaking, in reality, with direct current, the response relaxation speed tends to be slow due to the influence of the capacitance component. The above-mentioned values are AC values of about 30Hz to 10KHz. As mentioned above, the p-type cholesteric liquid crystal used in the present invention is a mixture of a p-type nematic liquid crystal and an optically active substance, and the optically active substance is a mixture of a p-type nematic liquid crystal and an optically active substance, in addition to the cholesteric liquid crystal. It contains a chiral nematic substance having carbon, and other optically active substances having the effect of inducing a helix in a nematic liquid crystal can also be used. The mixed liquid crystal used in the present invention generally contains 5 to 35% by weight of an optically active substance, the remainder being substantially p-type nematic liquid crystal, and has a helical pitch of about 1 to about 5 μm.
Those are preferably used. If the helix pitch exceeds the upper limit, the helix pitch will be too large.
A display device with an electrode gap of about 10 μm cannot provide sufficient contrast and has a short storage time, making it impractical. Display devices with an electrode gap of more than 15μ can use a display device with a pitch of 5μ or more, but the response speed decreases and the voltage increases significantly, making it impractical.
Further, below the lower limit, the contrast is high and the storage time is long, but the transition from the H or H' state to the S state takes too much time and is impractical. Display devices with an electrode gap of less than 3 μm have low contrast and are difficult to control evenly in terms of manufacturing technology, making them impractical. An example of a suitable mixed liquid crystal is one having the following composition (weight ratio).

【table】

[Table] The electrode substrate used in the present invention must have liquid crystal molecules in contact with the electrode surface vertically aligned at least in the vicinity of the surface. In order to impart such properties to the electrode surface, amines having a straight-chain alkyl group, silanes having a straight-chain alkyl group, polydimethylsiloxane or some of its methyl groups are substituted with phenyl groups, etc. An example of this is application of a silicone-based vertical alignment treatment agent, but the present invention is not limited to this, and any vertical alignment treatment such as an inorganic coating such as silicon oxide is also possible. In addition, as an application, by sandwiching the liquid crystal display device formed in this way between a pair of polarizing plates, or using a retardation plate between them, or using a polarizing plate on the front side and a retardation plate and a reflection plate on the back side, Color contrast can also be performed by utilizing different birefringence states in the scattering part and the transmitting part. As described above, according to the present invention, a storage XY matrix type liquid crystal display is produced by utilizing the difference in response characteristics to applied voltage of various liquid crystal states used for display, especially the difference in relaxation time accompanying state changes. A preferred method for performing a partial rewrite of a device is provided. The inventor of the present invention also developed another method for partially rewriting a storage XY matrix type liquid crystal display device by utilizing the difference in relaxation time accompanying various state changes of p-type cholesteric liquid crystal. A separate patent application has been filed (patent application (1) on the same day). However, in this method, when a working electrode or a scanning electrode is selected among the rewriting electrodes, a voltage V is continuously applied to the non-rewriting electrode during a series of rewriting operations. In the area matrix display device, it is necessary to interrupt the rewriting operation in the middle. In contrast, the method of the present invention has the advantage that rewriting operations can be continued permanently on any large-area display device.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view conceptually showing an example of a memory matrix type liquid crystal display device to which the method of the present invention is applied, FIG. 2 is a light transmittance-voltage curve of p-type cholesteric liquid crystal, and FIGS. 3 to 11 are 3 is a conceptual plan view of an example of a display matrix of a display device, FIG. 3 is a layout diagram of electrodes and unit display parts, and FIGS. 4a and 4b are diagrams of display states before and after partial rewriting according to the method of the present invention, respectively. . 5a to 11a are diagrams showing the potential distribution to each electrode and the voltage distribution to each display section, and FIGS. 5b to 11b are diagrams showing the corresponding display state, FIG. 5c to FIG. 11c are diagrams showing the presence state of liquid crystal in each corresponding display section. 1... Electrode substrate, 2... P-type cholesteric liquid crystal,
1X, 1Y...Transparent electrodes forming X-electrode array and Y-electrode array, respectively, 3...Power supply, S...Ground state, F...Focalconic state, H...Homeotropic state, H'...Quasi homeotropic state,
F'...quasi-focalconic state, G'...grangene state, V _L ...threshold voltage of S→F state change,
V _H ′…H′→F Threshold voltage for state change, V _H …F
→Threshold voltage for H state change, V...Voltage at which both the homeotropic state and the focal conic state can exist stably or metastablely, or a potential equivalent thereto.

Claims (1)

[Claims] 1. A pair of vertically aligned electrode substrates each having an X-electrode array and a Y-electrode array on their surfaces, each consisting of a plurality of spaced apart strip-shaped electrodes. a mixed liquid crystal containing a nematic liquid crystal having positive dielectric anisotropy and an optically active substance is sandwiched between the pair of electrode substrates so that the electrodes intersect,
A storage XY matrix type liquid crystal display device in which the electrode intersections of the X- and Y-electrode arrays are used as unit display parts is first constructed using a transparent display part made of liquid crystal in a ground state and a light scattering part made of liquid crystal in a focal conic state. and a low voltage display state formed by the X-electrode array, and then performs operations including the following steps to rewrite the display corresponding to a part of the strip electrodes of the strip electrode group included in the X-electrode array. A method for partially rewriting a memory type liquid crystal display device. (b) Applying a voltage X sufficient to shift the focal conic state liquid crystal to a homeotropic state.
- Applied between the rewriting electrode in the electrode row and the counter electrode in the Y-electrode row, and approximately the same potential as the counter electrode is applied to the non-rewriting electrode in the X-electrode row so that it is in contact with the rewriting electrode. (b) determining at least a part of the rewriting electrodes in the X-electrode array as write electrodes, and at least one electrode in the Y-electrode array; are defined as scanning electrodes, and each electrode is held in the following voltage relationship: a. The non-writing electrode in the X-electrode row and the non-scanning electrode in the Y-electrode row are held at approximately the same potential; b. The scanning electrode In this case, a potential corresponding to a voltage V at which both the focal conic state liquid crystal and the homeotropic state liquid crystal can exist stably or metastablely is set. The same potential is set for the non-writing electrode, and an opposite potential of the same magnitude as that of the scanning electrode is set for the non-writing electrode. The liquid crystal is held for a sufficient period of time to form a graminian state in the liquid crystal, and for a period of time required for the liquid crystal to undergo a phase transition from the ground state to the focal conic state by application of the voltage V. (c) Setting the scanning electrode to zero potential, applying a voltage V to the liquid crystal of the written display area, and maintaining the voltage for a time sufficient for the liquid crystal to transition to a focal conic state. 2. Determine a new write electrode that is the same as or different from the write electrode from the rewrite electrodes in the X-electrode row, and
An electrode different from the scanning electrode in the electrode array is set as a new scanning electrode, and the step (b) is repeated, and the Y electrode array is scanned in this way to contact the rewriting electrode in the X-electrode array. 2. A partial rewriting method for a storage type liquid crystal display device according to claim 1, wherein a writing operation is sequentially performed on a memory type liquid crystal display device. 3. The memory according to claim 1 or 2, which includes a step of setting the electrodes of the X-electrode array and the Y-electrode array at the same potential after the step (c) to create a stored image. A method for partially rewriting a type liquid crystal display device.