JP2014041239A - Liquid crystal element and liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a time period required for a state transition of a liquid crystal layer in a liquid crystal element using a transition between two alignment states.SOLUTION: The liquid crystal element includes a first substrate, a second substrate, a liquid crystal layer disposed between the first substrate and the second substrate, an electrode to impart to the liquid crystal layer a first electric field in a direction substantially perpendicular to the substrate surface and a second electric field substantially parallel to the substrate surface, and a drive circuit to supply a drive voltage to the electrode. Directions of an alignment process applied to the first substrate and the second substrate are set to induce a first alignment state where liquid crystal molecules in the liquid crystal layer are twisted in a first direction; and the liquid crystal layer contains a chiral material that induces a second alignment state where the liquid crystal molecules are twisted in a second direction opposite to the first direction. A drive voltage supplied from the drive circuit first imparts the first electric field to the liquid crystal layer and then imparts the second electric field. The liquid crystal layer transmits from the first alignment state to the second alignment state by application of the first electric field and the second electric field.

Description

  The present invention relates to a technique for improving electro-optical characteristics in a liquid crystal element and a liquid crystal display device.

  Japanese Patent Laying-Open No. 2011-203547 (Patent Document 1) discloses a novel liquid crystal display element (reverse TN liquid crystal element) that utilizes a transition between two alignment states. The liquid crystal display element according to the preceding example has a first substrate and a second substrate subjected to alignment treatment, and a liquid crystal layer disposed between them and twist-aligned, and the liquid crystal layer includes a chiral material. In the liquid crystal layer, when the swirl direction in which the liquid crystal molecules are twisted when the chiral material is not included is defined as the first swirl direction, the chiral material moves the liquid crystal molecules in the second swirl direction opposite to the first swirl direction. Gives turning ability. The first substrate and the second substrate are each subjected to orientation treatment so that a pretilt angle of 20 ° or more and 45 ° or less is developed. The first substrate and the second substrate are provided with electrodes capable of generating an electric field in each of the thickness direction of the liquid crystal layer and the direction orthogonal thereto. According to this configuration, it is possible to obtain a liquid crystal display element having a memory property capable of maintaining a display state and excellent display quality capable of obtaining a high contrast ratio.

  However, the above-described liquid crystal display element of the above-described example has a long time required to change the alignment state of the liquid crystal molecules in the liquid crystal layer from the state twisted in the second turning direction to the state twisted in the first turning direction. There was still room for improvement. In particular, improvement is desired in many applications (for example, a display unit of a watch, an information display panel for a vehicle, etc.) that need to write and erase the display in a relatively short time.

JP 2011-203547 A

  A specific aspect of the present invention provides a technique capable of reducing the time required for the state transition of a liquid crystal layer in a liquid crystal element using a transition between two alignment states and a liquid crystal display device using the same. One of the purposes.

  In one embodiment of the liquid crystal element according to the present invention, (a) each surface is subjected to an alignment treatment, and a first substrate and a second substrate which are disposed to face each other, and (b) one surface of the first substrate and a second substrate. A liquid crystal layer provided between one surface of the substrate and (c) a first electric field in a direction substantially perpendicular to each surface of the first substrate and the second substrate and substantially parallel to the one surface with respect to the liquid crystal layer An electrode for applying a second electric field in the direction, and (d) a drive circuit for supplying a drive voltage to the electrode. (E) The first substrate and the second substrate have the liquid crystal molecules of the liquid crystal layer being the first. The direction of alignment treatment is set so as to produce a first alignment state twisted in the direction. (F) The liquid crystal layer has a second alignment state in which liquid crystal molecules are twisted in a second direction opposite to the first direction. (G) the drive voltage supplied from the drive circuit is at least via the electrodes. The second electric field is applied after the first electric field is applied to the crystal layer. (H) The liquid crystal layer is changed from the first alignment state to the second alignment state by applying the first electric field and the second electric field. It is a liquid crystal element characterized by making a transition.

  According to the above configuration, the time required for the transition from the first alignment state to the second alignment state can be reduced by combining the two types of electric fields and applying them to the liquid crystal layer.

  In the liquid crystal element, the second electric field may be applied after the first electric field is applied.

  Thereby, the second electric field can be applied before the transition of the alignment state caused by the first electric field starts to return.

  In the above liquid crystal element, it is also preferable that the period for applying the first electric field and the period for applying the second electric field overlap at least partially. At this time, the overlapping period can be, for example, 3 seconds or more.

  Thereby, in the overlap period, the first electric field and the second electric field are simultaneously applied, and the time required for the transition of the alignment state can be further reduced.

  A liquid crystal display device according to one embodiment of the present invention is a liquid crystal display device including a plurality of pixel portions, and each of the plurality of pixel portions is configured using the liquid crystal element according to the present invention described above.

  According to the above configuration, by utilizing the bistability (memory property) of the two alignment states of the liquid crystal element, basically no power is required except during display rewriting, and the transition of the alignment state of the liquid crystal layer Thus, it is possible to obtain a liquid crystal display device in which the time required for the reduction is reduced (that is, the display switching time is reduced).

FIG. 1 is a schematic diagram schematically showing the operation of the reverse TN liquid crystal element. FIG. 2 is a conceptual diagram for explaining the relationship between the alignment state of the liquid crystal layer and the electric field direction when transitioning from the reverse twist state to the spray twist state. FIG. 3 is a cross-sectional view illustrating a configuration example of a reverse TN liquid crystal element. FIG. 4A is a schematic diagram showing the arrangement of the first to fourth electrodes in plan view. FIG. 4B to FIG. 4D are schematic views showing the arrangement of the first to fourth electrodes in cross section. 5A to 5C are diagrams showing microscopic observation images when a longitudinal electric field is applied after the liquid crystal layer of the liquid crystal element is in a reverse twist state. FIG. 5D is a diagram showing the rubbing direction of the liquid crystal element and the direction of the transmission axis of the polarizing plate used for this observation. FIG. 6 is a diagram showing the relationship between the magnitude of the longitudinal electric field that causes a temporary transition of the alignment state and the transmittance. FIG. 7A and FIG. 7B are waveform diagrams for explaining the drive voltage supplied from the drive circuit. FIG. 8 is a diagram illustrating a change with time in transmittance when the liquid crystal element of the example is driven using a driving voltage. FIGS. 9A to 9C are diagrams showing microscopic observation images of the liquid crystal element of the example, and FIG. 9D is a diagram showing a transmission axis and a rubbing direction of the polarizing plate. 10 (A) to 10 (D) are diagrams showing a microscopic observation image of the liquid crystal element of the comparative example, and FIG. 10 (E) is a diagram showing a transmission axis and a rubbing direction of the polarizing plate. FIG. 11 is a diagram illustrating the relationship between the frequency of the drive voltage and the transition time. FIG. 12 is a diagram schematically illustrating a configuration example of a liquid crystal display device.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic diagram schematically showing the operation of the reverse TN liquid crystal element. The reverse TN type liquid crystal element includes an upper substrate 1 and a lower substrate 2 which are arranged to face each other and a liquid crystal layer 3 provided therebetween as a basic configuration. Each surface of the upper substrate 1 and the lower substrate 2 is subjected to an alignment process such as a rubbing process. The upper substrate 1 and the lower substrate 2 are relatively arranged so that the directions of these alignment treatments (indicated by arrows in the drawing) intersect each other at an angle of about 90 °. The liquid crystal layer 3 is formed by injecting a nematic liquid crystal material between the upper substrate 1 and the lower substrate 2. The liquid crystal layer 3 is made of a liquid crystal material to which a chiral material that causes the liquid crystal molecules to twist in a specific direction in the azimuth angle direction (right-turning direction in the example of FIG. 1) is added. Such a reverse TN liquid crystal element is in a splay twist state in which the liquid crystal layer 3 is twisted while being splay aligned in the initial state due to the action of the chiral material. When a voltage exceeding the saturation voltage is applied to the liquid crystal layer 3 in the splay twist state in the layer thickness direction, the liquid crystal molecules transit to a reverse twist state (uniform twist state) in which the liquid crystal molecules are twisted in the left-turning direction. In the liquid crystal layer 3 in such a reverse twist state, since the liquid crystal molecules in the bulk are inclined, an effect of reducing the driving voltage of the liquid crystal element appears.

  FIG. 2 is a conceptual diagram for explaining the relationship between the alignment state of the liquid crystal layer and the electric field direction when transitioning from the reverse twist state to the spray twist state. As shown in FIG. 2A, with respect to an electric field in a direction parallel to the substrate surface, a liquid crystal molecule at the center in the thickness direction of the liquid crystal layer in the reverse twist state (the pattern in the figure is The application direction of the electric field is set so that the major axis direction of the attached liquid crystal molecules) is not parallel as much as possible, but is orthogonal or close thereto. As a result, the liquid crystal molecules at the center in the thickness direction of the liquid crystal layer are realigned along the electric field direction, so that the alignment state of the liquid crystal layer transitions from the reverse twist state to the spray twist state as shown in FIG. To do. When an electric field is applied to the liquid crystal layer in the reverse twist state so as to be in a state parallel to or close to the major axis direction of the liquid crystal molecules at the center in the layer thickness direction, the liquid crystal layer in the reverse twist state is sprayed from the reverse twist state. Transition to the twist state is unlikely to occur. This is because realignment of liquid crystal molecules due to an electric field hardly occurs at the approximate center in the layer thickness direction of the liquid crystal layer. From the above, in order to freely transition between two alignment states in a reverse TN liquid crystal element, basically, an electric field (longitudinal electric field) with respect to the layer thickness direction of the liquid crystal layer and an electric field in a direction perpendicular to the electric field. (Transverse electric field) must be generated, and the transverse electric field should be in a direction substantially perpendicular to or close to the major axis direction of the liquid crystal molecules in the center of the liquid crystal layer in the reverse twist state. Is preferred. A reverse TN liquid crystal element having an electrode structure for freely applying these vertical and horizontal electric fields will be described below with specific examples.

  FIG. 3 is a cross-sectional view illustrating a configuration example of a reverse TN liquid crystal element. The liquid crystal element shown in FIG. 3 has a basic configuration in which a liquid crystal layer 60 is interposed between a first substrate (upper substrate) 51 and a second substrate (lower substrate) 54, and a driving voltage is applied to the liquid crystal layer 60. It has the drive part 65 for supplying. Hereinafter, the structure of the liquid crystal element will be described in more detail. Note that illustration and description of members such as a sealing material for sealing the periphery of the liquid crystal layer 60 are omitted.

  The first substrate 51 and the second substrate 54 are transparent substrates such as a glass substrate and a plastic substrate, respectively. As illustrated, the first substrate 51 and the second substrate 54 are bonded to each other with a predetermined gap (for example, several μm) so that one surface of the first substrate 51 and the second substrate 54 face each other. Although not particularly shown, a switching element such as a thin film transistor may be formed on any substrate.

  The first electrode 52 is provided on one surface side of the first substrate 51. The second electrode 55 is provided on one surface side of the second substrate 54. The first electrode 52 and the second electrode 55 are each configured by appropriately patterning a transparent conductive film such as indium tin oxide (ITO), for example.

  The insulating film (insulating layer) 56 is provided on the second substrate 54 so as to cover the second electrode 55. The insulating film 56 is, for example, an inorganic insulating film such as a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a laminated film thereof, or an organic insulating film (for example, an acrylic organic insulating film).

  The third electrode 58 and the fourth electrode 59 are respectively provided on the above-described insulating film 56 on the second substrate 54. The third electrode 58 and the fourth electrode 59 in this embodiment are comb-like electrodes each having a plurality of electrode branches, and are arranged so that the electrode branches are alternately arranged (see FIG. 4 described later). ). The third electrode 58 and the fourth electrode 59 are each configured by appropriately patterning a transparent conductive film such as indium tin oxide (ITO), for example. The electrode branches of the third electrode 58 and the fourth electrode 59 have, for example, an electrode width of about 20 to 30 μm and an electrode interval of about 20 to 200 μm.

  The alignment film 53 is provided on one surface side of the first substrate 51 so as to cover the first electrode 52. The alignment film 57 is provided on one surface side of the second substrate 54 so as to cover the third electrode 58 and the fourth electrode 59. Each alignment film 53, 57 is subjected to a predetermined alignment process (for example, a rubbing process). The pretilt angle given to the liquid crystal molecules of the liquid crystal layer 60 at the interface between the alignment films 53 and 57 and the liquid crystal layer 60 is relatively high, for example, 20 ° or more.

  The liquid crystal layer 60 is provided between the first substrate 51 and the second substrate 54. The dielectric anisotropy Δε of the liquid crystal material constituting the liquid crystal layer 60 is positive (Δε> 0). A chiral material for twisting and aligning liquid crystal molecules is added to the liquid crystal material.

  The drive circuit 65 is connected to the first electrode 53, the second electrode 56, the third electrode 58, and the fourth electrode 59, and supplies a drive voltage to these electrodes.

  FIG. 4A is a schematic diagram showing the arrangement of the first to fourth electrodes in plan view. FIG. 4B to FIG. 4D are schematic views showing the arrangement of the first to fourth electrodes in cross section. The electric field applied to each liquid crystal layer using each electrode will be described with reference to these drawings.

  As shown in FIG. 4A, the first electrode 52 and the second electrode 55 are arranged to face each other, and the third electrode 58 and the fourth electrode 59 are arranged in a region where both overlap. Further, the plurality of electrode branches of the third electrode 58 and the plurality of electrode branches of the fourth electrode 59 are arranged so as to be alternately repeated one by one.

  As shown in FIG. 4B, an electric field can be generated between both electrodes by applying a voltage between the first electrode 52 and the second electrode 55 by the drive circuit 65. The electric field in this case is an electric field along the thickness direction (cell thickness direction) of the first substrate 51 and the second substrate 54 as shown in the drawing, that is, a “longitudinal electric field (first electric field)”.

  As shown in FIG. 4C, an electric field can be generated between the electrodes by applying a voltage between the third electrode 58 and the fourth electrode 59 by the drive circuit 65. The electric field in this case is an electric field in a direction substantially parallel to each surface of the first substrate 51 and the second substrate 54 as shown in the figure, that is, a “lateral electric field (second electric field)”. Hereinafter, a mode using such an electric field may be referred to as an “IPS mode”.

  Further, as shown in FIG. 4D, by applying a voltage between the second electrode 55 and the third electrode 58 and the fourth electrode 59 that are arranged to face each other with the insulating film 56 interposed therebetween by the drive circuit 65. An electric field can be generated between both electrodes. The electric field in this case is an electric field along a direction substantially parallel to one surface of each of the first substrate 51 and the second substrate 54 as shown in the drawing, that is, a “lateral electric field (second electric field)”. Hereinafter, a mode using such an electric field may be referred to as an “FFS mode”.

  In the liquid crystal element of this embodiment, the liquid crystal molecules of the liquid crystal layer 60 are aligned in a spray twist state in the initial state. In contrast, when a vertical electric field is generated using the first electrode 52 and the second electrode 55 as described above, the alignment state of the liquid crystal molecules in the liquid crystal layer 60 transitions to the reverse twist state. Thereafter, when a lateral electric field is generated using the third electrode 58 and the fourth electrode 59 (IPS mode), the alignment state of the liquid crystal layer 60 transitions to the spray twist state. Similarly, when a lateral electric field is generated using the second electrode 55, the third electrode 58, and the fourth electrode 59 (FFS mode), the alignment state of the liquid crystal layer 60 similarly changes from the reverse twist state to the spray twist state. . In comparison with the IPS mode, the FFS mode tends to shift the alignment state of the liquid crystal layer 60 more uniformly. This is considered to be because a lateral electric field is also applied to each of the third electrode 58 and the fourth electrode 59. Therefore, it can be said that the FFS mode is more suitable in terms of the aperture ratio (transmittance, contrast ratio).

  The reason why the alignment state of the liquid crystal layer 60 can be switched between the spray twist state and the reverse twist state is considered as follows. In the spray twist state, the liquid crystal molecules at the approximate center in the layer thickness direction of the liquid crystal layer 60 are aligned substantially horizontally. However, after the reverse twist state is caused by the vertical electric field, the liquid crystal molecules at the approximate center in the layer thickness direction are vertical. Oriented in a close state. Thereafter, a lateral electric field is applied to the liquid crystal molecules at the approximate center in the thickness direction of the liquid crystal layer 60 in the reverse twist state by a lateral electric field in the IPS mode or the FFS mode, and the liquid crystal molecules at the approximate center of the liquid crystal layer 60 in the spray twist state. Since it is in the direction of the director, there is a transition to the spray twist state which is the initial state again. As described above, it is considered that the spray twist state and the reverse twist state can be switched by utilizing the vertical electric field and the horizontal electric field.

  Next, examples of the liquid crystal element will be described in detail.

  By patterning the ITO film of the glass substrate with the ITO film, the first substrate 51 having the first electrode 52 is produced. Here, the ITO film can be patterned by a general photolithography technique. As the ITO etching method, wet etching (ferric chloride) is used. The shape pattern of the first electrode 52 here is such that the ITO film remains in the extraction electrode portion and the portion corresponding to the display pixel. Similarly, the 2nd board | substrate 54 which has the 2nd electrode 55 is produced by patterning the ITO film | membrane of the glass substrate with an ITO film | membrane.

Next, an insulating film 56 is formed on the second electrode 55 of the second substrate 54. At that time, it is necessary to devise so that the insulating film 56 is not formed on the extraction electrode portion. As the method, a resist is formed in advance on the extraction electrode portion and lifted off after the formation of the insulating film 56, or the insulating film 56 is formed by sputtering or the like with the extraction electrode portion hidden by a metal mask or the like. The method etc. are mentioned. Examples of the insulating film 56 include an organic insulating film, an inorganic insulating film such as a silicon oxide film or a silicon nitride film, and a combination thereof. Here, a laminated film of an acrylic organic insulating film and a silicon oxide film (SiO ₂ film) is used as the insulating film 56.

A heat-resistant film (polyimide tape) is attached to the extraction electrode portion (terminal portion), and the material liquid of the organic insulating film is spin-coated in that state. For example, a film thickness of 1 μm is obtained under the condition of spinning at 2000 rpm for 30 seconds. This is baked in a clean oven (eg, 220 ° C., 1 hour). A SiO ₂ film is formed by sputtering (alternating current discharge) while a heat resistant film is stuck. For example, the substrate is heated to 80 ° C. to form 1000 Å. Here, when the heat-resistant film is peeled off, both the organic insulating film and the SiO ₂ film can be peeled off cleanly. Thereafter, baking is performed in a clean oven (for example, 220 ° C., 1 hour). This is to increase the insulation and transparency of the SiO ₂ film. It is not always necessary to form the SiO ₂ film, but it is preferable to form the SiO ₂ film because the adhesion and patterning of the ITO film formed thereon are improved. Also, the insulation is improved. On the other hand, be a method of taking only the insulating SiO ₂ film without forming an organic insulating film can be considered, SiO ₂ film in which case the securing of about 4000Å~8000Å thickness for easily becomes porous desirable. Also, a laminated film with SiNx may be used. Although the sputtering method has been described as a method for forming the inorganic insulating film, a forming method such as a vacuum evaporation method, an ion beam method, or a CVD method (chemical vapor deposition method) may be used.

  Next, the third electrode 58 and the fourth electrode 59 are formed on the insulating film 56. Specifically, first, an ITO film is formed on the insulating film 56 by a sputtering method (AC discharge). This is heated to, for example, 100 ° C., and an ITO film of about 1200 mm is formed on the entire surface. The ITO film is patterned by a general photolithography technique. As the photomask at this time, a photomask having a light shielding portion corresponding to the comb-like electrode as shown in FIG. 4 is used. For example, the width of the electrode branch may be 20 μm to 30 μm, and the electrode interval may be 20 μm to 200 μm. If there is no pattern in the extraction electrode portion, the lower ITO film is also removed by etching. Therefore, a photomask having a pattern formed on the extraction electrode portion is used.

  The first substrate 51 and the second substrate 54 manufactured as described above are cleaned. Specifically, first, washing with water (brush washing or spray washing, pure water washing) is performed, followed by UV washing after draining, and finally IR drying.

  Next, alignment films 53 and 57 are formed on the first substrate 51 and the second substrate 54, respectively. As the alignment films 53 and 57, for example, a polyimide film in which a side chain density of a material normally used as a vertical alignment film is lowered is used. An alignment film material liquid (alignment material) is applied to one surface of each of the first substrate 51 and the second substrate 54, and these are baked in a clean oven (for example, 160 to 260 ° C., 1 hour). As a method for applying the material liquid for the alignment film, flexographic printing, inkjet printing, or spin coating is used. Here, spin coating is used, but the results are the same even if other methods are used. The film thickness of the alignment films 53 and 57 is, for example, 500 to 800 mm. Next, a rubbing process as an alignment process is performed on the alignment films 53 and 57. The pushing amount at the time of rubbing is set to 0.8 mm, for example. Thereby, each alignment film 53 and 57 can express a pretilt angle of about 20 ° to 60 ° with respect to the liquid crystal molecules. The rubbing direction is set so that the twist angle φ in the initial state (spray twist state) is, for example, 70 °.

  Next, the first substrate 51 and the second substrate 54 are bonded together. On the first substrate 51, a seal material mixed with approximately 2 wt% of a rod-shaped glass spacer having a particle size of about 4 μm is applied in a desired pattern with a dispenser. Further, a plastic spacer having a particle size of about 4 μm is sprayed on the second substrate 54 by a dry spraying method. After the two substrates are bonded together, the sealing material is cured by thermocompression bonding.

  Next, a liquid crystal layer 60 is formed by injecting a liquid crystal material between the first substrate 51 and the second substrate 54. For example, CB15 is added to the liquid crystal material as a chiral material. The amount of chiral material added is such that d / p is 0.20 to 0.53.

  Finally, polarizing plates are bonded to the outer sides of the first substrate 51 and the second substrate 54. The polarizing plate has a crossed Nicols arrangement. Here, the polarizing plates are arranged so that the transmission axis of each polarizing plate and the rubbing direction have an angle of 10 °.

  Thus, the liquid crystal element of the example is completed. In this liquid crystal element, the liquid crystal layer 60 is aligned in a spray twist state at the time of completion. At this time, the transmittance (or reflectance) is relatively high and the appearance is bright. When a vertical electric field is applied to the liquid crystal layer 60, the orientation of the liquid crystal layer 60 changes to a reverse twist state, and this state is maintained even after the electric field is turned off. At this time, the transmittance (or reflectance) is relatively low and the appearance is dark. Furthermore, when a lateral electric field is applied to the liquid crystal layer 60, the orientation of the liquid crystal layer 60 again transitions to the spray twist state, and this state is maintained even after the electric field is turned off.

  Next, the driving method of the liquid crystal element of this embodiment will be described in detail.

  5A to 5C are diagrams showing microscopic observation images when a longitudinal electric field is applied after the liquid crystal layer of the liquid crystal element is in a reverse twist state. FIG. 5D is a diagram showing the rubbing direction of the liquid crystal element and the direction of the transmission axis of the polarizing plate used for this observation. As shown in FIG. 5D, in this liquid crystal element, the transmission axis of each polarizing plate is set to 45 ° clockwise and 45 ° counterclockwise in the figure, and the rubbing direction of the first substrate 51 is set. RL is set to 35 ° clockwise, and the rubbing direction RU of the second substrate 54 is set to 35 ° counterclockwise. As a result of investigations by the inventors of the present application, when a vertical electric field is applied to a liquid crystal layer in a reverse twist state as shown in FIG. 5A, the electric field is turned off, as shown in FIG. 5B. It was found that the orientation state of the layer temporarily transited from the reverse twist state to the spray twist state, and after a few seconds, again returned to the reverse twist state as shown in FIG. 5C. Here, the vertical electric field is set to 0 V after a voltage of 2.5 V is applied between the first electrode 52 and the second electrode 55. As a result of detailed observation, it has been found that the re-transition from the temporary spray twist state to the reverse twist state starts to start from the spacer (gap material).

  FIG. 6 is a diagram showing the relationship between the magnitude of the longitudinal electric field that causes a temporary transition of the alignment state and the transmittance. In FIG. 6, the horizontal axis corresponds to the elapsed time, the left vertical axis corresponds to the transmittance measured from the front direction of the liquid crystal element, the right vertical axis corresponds to the magnitude of the vertical electric field (applied voltage), the solid line indicates the transmittance, A bar in the form of a bar graph indicates the applied voltage. During the elapsed time from 0 to 15 seconds, the liquid crystal layer of the liquid crystal element is in the initial state (spray twist state), and the transmittance at this time is relatively high. During the elapsed time of 15 to 30 seconds, the liquid crystal layer of the liquid crystal element transitions to a reverse twist state by applying a vertical electric field, and the transmittance at this time is relatively low. During the elapsed time of 30 to 50 seconds, no voltage is applied to the liquid crystal layer of the liquid crystal element, and the transmittance is low because the reverse twist state is maintained. After the elapsed time of 50 seconds, AC voltages of 10V (volt), 9V, 8V, 7V, 6V, 5V, 4V, 3.5V, 3V, 2.5V, 2V, 1V are applied to the liquid crystal layer at 10 second intervals. It was. Note that the frequency of the applied AC voltage is 20 Hz, and the application time is approximately 1 second. As shown in the figure, in this experimental example, when the applied voltage is 2.5 V to 3.5 V, the transmittance instantaneously increases to the same level as the initial state after the electric field is turned off. It was found to be maintained for 2 seconds. However, under the condition of the applied voltage of 2V and the condition of 3.5V or more, the phenomenon that the transmittance rapidly increased after the electric field was turned off did not appear. That is, by applying a relatively low vertical electric field to the liquid crystal layer in the reverse twist state, it is possible to instantaneously transit to the spray twist state, and the time required for the temporary transition is relatively short. was gotten.

  Based on this knowledge, the inventors of the present application have studied that a longitudinal electric field is applied to the liquid crystal layer in the reverse twist state to temporarily transit to the spray twist state, and then a lateral electric field is further applied. Therefore, it is possible to stabilize the state transitioned to the spray twist state, and to the spray twist state as compared with the case where the horizontal electric field is applied to the liquid crystal layer in the reverse twist state without applying the vertical electric field. The idea was that the time required for transition could be shortened. Next, a driving method for realizing this idea will be described in detail.

  FIG. 7A and FIG. 7B are waveform diagrams for explaining the drive voltage supplied from the drive circuit. The drive circuit 65 of this embodiment has voltage outputs of V1, V2, and V3, all of which are rectangular waves. When the alignment state of the liquid crystal layer 60 of the liquid crystal element is changed from the spray twist state to the reverse twist state, the drive circuit 65 applies a voltage V1 between the first electrode 52 and the second electrode 55 (vertical electric field). On the other hand, when the orientation state of the liquid crystal layer 60 of the liquid crystal element is changed from the reverse twist state to the spray twist state, the drive circuit 65 first applies a voltage V2 between the first electrode 52 and the second electrode 55 (vertical electric field). Then, a voltage V3 is applied between the third electrode 58 and the fourth electrode 59 (lateral electric field).

  Specifically, first, as shown in FIG. 7A, the voltage V2 is applied between the first electrode 52 and the second electrode 55 for a time t1. Next, as shown in FIG. 7B, the voltage V3 is applied between the third electrode 58 and the fourth electrode 59 for a time t3 after the time t2 has elapsed since the application of the voltage V2. At this time, when the time t2 is set smaller than the time t1, the application period of the voltage V2 and the application period of the voltage V3 are temporarily overlapped as in the illustrated example. That is, the vertical electric field and the horizontal electric field are temporarily applied to the liquid crystal layer 60 at the same time. Note that the frequency and the magnitude of the voltage can be appropriately adjusted for any of the voltages V1 to V3. When the voltage V3 is larger than the voltages V1 and V2, high impedance driving is preferable to ground driving in order to prevent the voltage V3 from entering the first electrode 52 and the second electrode 55 as noise.

  Examination of detailed driving conditions in the case of using such a driving voltage revealed that it is preferable to satisfy the condition of t1> t2 in order to further shorten the transition time from the reverse twist state to the spray twist state. It was. That is, it has been found that it is preferable to have a period in which the vertical electric field and the horizontal electric field are applied simultaneously. Note that t1 = t2 may be set, in which case the voltage V3 is applied after the application of the voltage V2. Thereby, after the vertical electric field is applied to the liquid crystal layer 60, the horizontal electric field is continuously applied. Alternatively, t1 <t2, and in this case, the voltage V2 is applied after a voltage non-application period after the voltage V1 is applied. Accordingly, a vertical electric field is applied to the liquid crystal layer 60, and then a horizontal electric field is applied across the period of the electric field 0. In this case, it is necessary to satisfy the condition of t1 + 3 (seconds) ≧ t2 in order to express the effect of improving the response by the combination of the vertical electric field and the horizontal electric field. As for “3 seconds” to be added to the time t1, as shown in FIG. 6 described above, after the electric field is turned off, the time during which the transmittance instantaneously increases to the same level as the initial state is approximately 3 times. The ground is for the second.

  FIG. 8 is a diagram showing a change with time in transmittance when the liquid crystal element of the example is driven using the above driving voltage. Here, the drive voltage is set to voltage V1 = 10V, voltage V2 = 2.5V, voltage V3 = 10V, time t1 = 1.0 second, time t2 = 0.5 second, time t3 = 1.0 second, The change over time in the transmittance at the time of transition from the reverse twist state to the spray twist state was measured. As a comparative example, the temporal change in transmittance when only a lateral electric field was used was also measured. The voltage V3 at this time was 10V. As shown in the figure, in the liquid crystal element of the comparative example in which only the transverse electric field is applied, it takes about 25 seconds until the alignment state of the liquid crystal layer transitions from the reverse twist state to the spray twist state and the transmittance changes accordingly. In contrast to this, in the liquid crystal element of the example, this time was significantly shortened to about 6 seconds.

  FIGS. 9A to 9C are diagrams showing microscopic observation images of the liquid crystal element of the example, and FIG. 9D is a diagram showing a transmission axis and a rubbing direction of the polarizing plate. In the liquid crystal element of the embodiment, as shown in FIG. 9A, when the driving voltage combining the vertical electric field and the horizontal electric field is supplied to the place where the liquid crystal layer is in the reverse twist state, the spray is instantaneously generated by the vertical electric field. A transition to the twist state (FIG. 9B) was observed, and then a state in which the transition to the spray twist state was stabilized by the lateral electric field was observed (FIG. 9C).

  10A to 10D are diagrams showing microscopic images of the liquid crystal element of the comparative example, and FIG. 10E is a diagram showing the transmission axis and the rubbing direction of the polarizing plate. In the liquid crystal element of the comparative example, as shown in FIG. 10A, when the drive voltage of the lateral electric field is supplied to the place where the liquid crystal layer is in the reverse twist state, the electrode branches of the comb-like third electrode and the fourth electrode The region that has transitioned to the spray twist state occurs randomly (FIG. 10B), and then the region that has transitioned to the spray twist state gradually expands (FIG. 10C: after 5 seconds), and further spray It was observed that the region transitioning to the twisted state was extended to the whole (FIG. 10D: after 25 seconds had elapsed).

  Compared to the liquid crystal element of the above-described embodiment, the comparative example is different in the state of transition from the reverse twist state to the spray twist state. That is, in the embodiment, a region that has transitioned to the spray twist state is generated in a wide area by the first application of the vertical electric field, and is maintained by the subsequent application of the horizontal electric field. For this reason, it looks good in appearance. On the other hand, in the comparative example, after the region that has transitioned to the spray twist state occurs locally between the electrode branches, the region expands. For this reason, it does not look good in appearance.

  FIG. 11 is a diagram illustrating the relationship between the frequency of the drive voltage and the transition time. The transition time from the reverse twist state to the spray twist state when the driving voltage is supplied with the frequency f2 of the voltage V2 and the frequency f3 of the voltage V3 being variably set between 20 Hz and 100 Hz respectively for the liquid crystal element of the embodiment. Measured. The transition time was the time until the change in transmittance was saturated. As for conditions other than the frequency, the voltage V2 is set to two types of 2.5V and 3.0V, the voltage V3 is set to 10V in any case, the time t1 is set to 1.0 second, and the time t2 is set to 0. The time t3 was set to 1.0 second for 5 seconds. As shown in the figure, when the frequency is relatively high, specifically, when the frequency is 50 Hz to 100 Hz, the dependence of the transition time T on the frequency is hardly seen, but when the frequency is relatively low, In the case of 20 to 40 Hz, the transition time T tended to be shortened as the frequency decreased.

  Next, a configuration example of a liquid crystal display device capable of low power consumption driving using the memory property of the liquid crystal element will be described.

  FIG. 12 is a diagram schematically illustrating a configuration example of a liquid crystal display device. The liquid crystal display device illustrated in FIG. 12 is a simple matrix type liquid crystal display device configured by arranging a plurality of pixel portions 74 in a matrix, and the liquid crystal element described above is used as each pixel portion 74. Specifically, the liquid crystal display device includes m control lines B1 to Bm extending in the X direction, a driver 71 that gives control signals to the control lines B1 to Bm, and control lines B1 to Bm, respectively. The n control lines A1 to An that cross and extend in the Y direction, the driver 72 that gives control signals to the control lines A1 to An, and the control lines B1 to Bm that cross each other and extend in the Y direction. Each of n control lines C1 to Cn and D1 to Dn, a driver 73 for giving a control signal to these control lines C1 to Cn and D1 to Dn, each of the control lines B1 to Bm and the control lines A1 to An And a pixel portion 74 provided at the intersection.

  Each control line B1-Bm, A1-An, C1-Cn, and D1-Dn consists of transparent conductive films, such as ITO, for example. The portions where the control lines B1 to Bm and A1 to An intersect function as the first electrode 52 and the second electrode 55 described above (see FIG. 4B). The control lines C1 to Cn are connected to a comb-like electrode branch (not shown in FIG. 12) provided in a region corresponding to each pixel portion 74 as the third electrode 58. Similarly, the control lines D1 to Dn are connected to a comb-like electrode branch (not shown in FIG. 12) provided as a fourth electrode 59 in a region corresponding to each pixel portion 74.

  Various methods are conceivable as driving methods of the liquid crystal display device having the configuration shown in FIG. For example, a control line B1, B2, B3... Bm and a method of performing display rewriting for each line (line sequential driving method) will be described. In this case, a vertical electric field drive voltage is applied to the pixel portion 74 that is desired to have a relatively bright display (reverse twist state), and a vertical electric field is applied to the pixel portion 74 that is desired to have a relatively dark display (spray twist state). And a driving voltage combining the horizontal electric field may be applied.

  .. Are sequentially selected and applied to the corresponding pixel portion 74, thereby enabling dot matrix display. The display state rewritten by such driving can be held semipermanently. In order to rewrite this display, the above control may be executed again from the control line B1. Note that although an example in which the present invention is applied to a so-called simple matrix liquid crystal display device is described here, the present invention can also be applied to an active matrix liquid crystal display device using a thin film transistor or the like. In the case of an active matrix liquid crystal display device, it is not necessary to rewrite each line such as the control line B1, so that the rewriting time can be shortened. Further, since it is possible to apply a voltage more than twice the threshold, rewriting can be performed at a higher speed. However, since there are electrodes for a horizontal electric field and a vertical electric field on one substrate, two thin film transistors or the like are required per pixel.

  As described above, according to this embodiment and each example, it is possible to reduce the time required for the state transition of the liquid crystal layer in the liquid crystal element using the transition between the two alignment states.

  In addition, the manufacturing process of the liquid crystal element is basically the same as the manufacturing process of a general liquid crystal element, and there are few factors that increase the cost. That is, it can be manufactured at low cost by the same manufacturing technique as that of a general liquid crystal element.

  In addition, since the liquid crystal element of this embodiment or the like does not require electric power except when rewriting the display, it can be driven with ultra-low power consumption, and is suitable for both a transmissive display and a reflective display. Can be realized. In particular, when it is applied to a reflective display, the merit is great.

  In addition, since it is possible to apply a driving method (line sequential rewriting method or the like) using the memory property of the alignment state, a large-capacity dot matrix display can be performed by a simple matrix display without using a switching element such as a thin film transistor. . Therefore, large capacity display is possible at low cost.

  In addition, this invention is not limited to the content of embodiment mentioned above, In the range of the summary of this invention, it can change and implement variously. For example, in the above-described embodiments and the like, the rubbing process is described as a specific example of the alignment process, but other alignment processes (for example, photo-alignment method, oblique vapor deposition method, etc.) can be used. Further, the numerical conditions and the like listed in the description are only suitable examples, and are not necessarily limited thereto.

1: Upper substrate 2: Lower substrate 3: Liquid crystal layer 51: First substrate 52: First electrode 53, 57: Alignment film 54: Second substrate 55: Second electrode 56: Insulating film 58: Third electrode 59: 4th electrode 60: Liquid crystal layer 65: Drive circuit 71, 72, 73: Driver 74: Pixel part A1-An, B1-Bm, C1-Cn, D1-Dn: Control line

Claims (5)

Each one surface has been subjected to an alignment treatment, and a first substrate and a second substrate disposed to face each other,
A liquid crystal layer provided between one surface of the first substrate and one surface of the second substrate;
An electrode for applying a first electric field in a direction substantially perpendicular to each surface of the first substrate and the second substrate and a second electric field in a direction substantially parallel to the one surface to the liquid crystal layer;
A drive circuit for supplying a drive voltage to the electrodes;
Including
The first substrate and the second substrate are set in a direction of the alignment treatment so as to generate a first alignment state in which liquid crystal molecules of the liquid crystal layer are twisted in a first direction;
The liquid crystal layer contains a chiral material having a property of causing a second alignment state in which the liquid crystal molecules are twisted in a second direction opposite to the first direction,
The driving voltage supplied from the driving circuit applies the second electric field after applying the first electric field to the liquid crystal layer through at least the electrode,
The liquid crystal layer transitions from the first alignment state to the second alignment state by applying the first electric field and the second electric field.
Liquid crystal element.   The liquid crystal element according to claim 1, wherein the second electric field is applied after the first electric field is applied.   The liquid crystal element according to claim 2, wherein the period for applying the first electric field and the period for applying the second electric field overlap at least partially.   The liquid crystal element according to claim 3, wherein the overlapping period is 3 seconds or more.   A liquid crystal display device comprising a plurality of pixel portions, wherein each of the plurality of pixel portions is configured using the liquid crystal element according to claim 1.