JP2005062901A - Liquid crystal display element and optically anisotropic element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a liquid crystal display element whose contrast ratio and dependency of display colors on the visual angles are improved, and an optically anisotropic element used in the same. <P>SOLUTION: The liquid crystal display element is equipped with two polarizing plates 1 and 4 and liquid crystal cells for driving which are arranged between those two polarizing plates and characterized in that between the polarizing plates and liquid crystal cells for driving, the optically anisotropic element 2 are arranged which has a 1st direction wherein optical rotary power oblique to normals of the substrates 3a and 3b is larger than optical rotary power in the directions of the normals of the substrates and optical rotary power inclined symmetrically based upon the normals of the substrates is small and a 2nd direction wherein the optical rotary power is large and the optically anisotropic element compensates asymmetry of the optical rotary power of the liquid crystal cells for driving. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a liquid crystal display element and an optical anisotropic element used therefor.

Since the liquid crystal display element has the great advantage of being thin, lightweight and low power consumption, it is not only used as a display for watches, calculators, Japanese word processors, personal computers, etc., but also a new concept that actively utilizes the advantages of liquid crystal display elements. It is also used in other products. In particular, liquid crystal display elements used in personal computers and the like have a large area and large capacity display, and a display surface of 10 inches diagonal and 640 × 480 pixels has become mainstream. The display methods used in this class of liquid crystal display elements can be roughly classified into two types. One is a simple matrix system and the other is an active matrix system.

The simple matrix system has a simple structure in which liquid crystal is sandwiched between two glass substrates with comb-shaped transparent electrodes. Therefore, high performance is required for the liquid crystal in the simple matrix system. Before describing this performance, the display principle of the liquid crystal display element will be described. In the display of the liquid crystal display element, the voltage applied to the liquid crystal is changed to change the direction of the liquid crystal molecules. In general, a large voltage difference is required to obtain a large contrast ratio. However, in order to realize a display of 640 × 480 pixels, the voltage difference between dark and light is as small as about 1V, and a large change in the state of liquid crystal molecules is required only by 1V difference. A lot of research has been done to realize this, but in 1985, the research group of Schaefer et al. Made the change in alignment sensitive to voltage by increasing the twist angle of the liquid crystal molecules. It was found that liquid crystal molecules must have a certain degree of inclination in order to change and to obtain a stable alignment with a large twist angle. Since this research report, orientation technology to realize this has been actively performed and successfully put into practical use.

In order to realize a display having an array of 640 × 480 pixels, generally, the twist angle is required to be 180 ° or more. Since the twist angle is large as described above, this liquid crystal is called a super twist nematic (STN). However, the initial STN display had a yellow background and a green character display, and was not a monochrome display. This is because the twist angle is large, and as a means for eliminating such display coloring, a second liquid crystal cell in which the alignment of the liquid crystal layer is twisted in the opposite direction is disposed between the polarizing plate and the liquid crystal cell. For example, Patent Document 1 reports that black and white display can be realized.

The principle of black-and-white conversion is that light transmitted through a first liquid crystal cell in which liquid crystal molecules are arranged in a twisted manner and causing optical rotation dispersion is transmitted through the first liquid crystal cell and the second liquid crystal cell of the target structure. The optical rotation dispersion has been eliminated. As a result, coloring due to optical rotatory dispersion of light is eliminated, and monochrome display can be realized. In order to perform such conversion accurately, the second liquid crystal cell, which is an optical compensator, has substantially the same retardation value as that of the first liquid crystal cell and the twist direction is opposite to each other. It is necessary to configure the arrangement of the liquid crystal molecules so that the orientation directions of the liquid crystal molecules closest to each other are orthogonal to each other.

  As other means, various methods using an optically anisotropic film instead of the above-described second liquid crystal cell have been proposed. This is a method of providing almost the same function as the second liquid crystal cell by laminating an optically anisotropic film on the liquid crystal cell.

  With the optical compensation described above, monochrome display is possible even on STN displays, and color display with higher added value can also be realized by combining with color filters. However, since the simple multiplex method is based on time-division driving based on the voltage averaging method, if the number of scanning lines is increased to increase the display capacity, the voltage value when light is blocked and the light is transmitted. There is an essential problem in that the difference from the voltage value at the time of the reduction is remarkably reduced, and as a result, the contrast ratio becomes small and the response speed of the liquid crystal becomes slow. In addition, such a conventional technique is observed as a phenomenon in which the display image appears to be reversed depending on the azimuth and angle when viewing the liquid crystal display element, or the display image is not visible at all, or the display is colored. When a liquid crystal display element having a high level is realized, it becomes a big problem.

  On the other hand, since the active matrix system includes a switching element formed of a thin film transistor or a diode for each display pixel, an arbitrary voltage ratio can be set in the liquid crystal layer of each pixel regardless of the number of scanning lines. Therefore, special performance as in the case of the simple matrix system is not required for the liquid crystal. There is no need to increase the twist angle as in STN, and it is 90 °.

  A liquid crystal cell (TN) having a twist angle of 90 ° has a small twist angle, and the light rotates following the twist faithfully. Therefore, the optical rotation dispersion is small, and an achromatic and high-contrast display can be obtained. Also, the response to voltage is faster than STN. By combining the active matrix method and TN, a liquid crystal display element having a large display capacity, a high contrast ratio, and a high response speed can be realized. Further, since there is a switching element for each pixel, an intermediate voltage can be applied, thereby enabling halftone display. Further, by combining with a color filter, full color display can be easily realized.

  However, even in the case of the active matrix method, it is not so much in the case of binary display, but when displaying a halftone, the display image may be reversed depending on the viewing direction, or the display image may not be visible at all, Alternatively, it is observed as a phenomenon that the display is colored, and it becomes a big problem when realizing a liquid crystal display element with higher display quality.

As a means for reducing the viewing angle dependency of such display, for example, in Patent Document 2, a liquid crystal cell and a birefringent layer having a negative optical anisotropy in the thickness direction are arranged between two polarizing plates. Is disclosed. On the other hand, Patent Document 3 discloses disposing a birefringent layer made of a liquid crystal compound (or polymer liquid crystal) exhibiting a cholesteric liquid crystal phase having a product of a helical pitch length and a refractive index of 400 nm or less on a liquid crystal cell. . These two proposals have been devised only in the case of a liquid crystal cell with a vertical alignment (liquid crystal molecules aligned vertically with respect to an alignment substrate), and a liquid crystal cell with a twisted alignment such as the TN mode or STN mode. In case not considered. Japanese Patent Application Laid-Open No. 2004-228867 proposes controlling the viewing angle of a liquid crystal display element with an optical compensation element having a twist angle of 360 ° or more and having a tilt angle. However, in the case of gradation display, the effect of widening the viewing angle is proposed. Is not enough.
Japanese Patent Publication No. 63-53528 Japanese Patent Laid-Open No. Sho 62-21423 JP-A-3-67219 Japanese Patent Laid-Open No. 3-121578

  The basic display principle of the liquid crystal display element described above is that light is controlled by changing the direction of liquid crystal molecules by a voltage applied to the liquid crystal and causing an optical change in the liquid crystal cell.

    Therefore, tilting the liquid crystal display element has a viewing angle dependency that changes the orientation of the liquid crystal molecules and changes the appearance, especially when displaying subtle halftones, the tilt angle of the liquid crystal molecules is changed finely, so the viewing angle is increased. The dependency is remarkable.

    Due to the viewing angle dependence of the appearance of the liquid crystal molecules, it is observed as a phenomenon that the display image appears reversed or cannot be identified at all, especially when full color display is performed in combination with a color filter. As a result, the reproducibility of the display image is significantly reduced, which is a serious problem.

    The present invention solves the above inconveniences, and provides a liquid crystal display element having improved contrast ratio and viewing angle dependency of display color, and an optical anisotropic element used therefor.

  The present invention is the following liquid crystal display element and optical anisotropic element.

(1) In a liquid crystal display element comprising two polarizing plates and a driving liquid crystal cell disposed between the two polarizing plates,
Between the polarizing plate and the driving liquid crystal cell, the optical rotation in the direction inclined with respect to the normal of the substrate is larger than the optical rotation in the direction of the normal of the substrate, and the normal of the substrate is An optically anisotropic element having a first direction with a small optical rotation and a second direction with a large optical rotation that is symmetrically tilted with respect to a reference is arranged, and the optical asymmetry of the liquid crystal cell for driving is reduced. A liquid crystal display element that is compensated by the element.

(2) In an optical anisotropic element composed of an optically anisotropic material layer, the major axis directions of the optically anisotropic units constituting the optically anisotropic material layer are aligned on a single axis. An optically anisotropic element characterized in that the optical rotation in a direction inclined with respect to the normal of the element surface is greater than the optical rotation in the direction of the normal.

(3) The optical anisotropy is characterized in that the angle of the major axis of the optical anisotropic element changes continuously or stepwise in the layer thickness direction of the optical anisotropic element with respect to the surface of the optical anisotropic element. Element.

(4) The angle of the major axis of the optical anisotropic element is substantially parallel to the surface on one side of the optical anisotropic element, and is substantially along the normal line of the surface on the other side of the optical anisotropic element. An optical anisotropic element changing in the layer of the optical anisotropic element.

    Here, the optically anisotropic unit in the present invention refers to each of these layers when the optically anisotropic element having a thickness has a laminated structure of a plurality of layers as an example. Each layer is a unit having an optical axis directed in a specific direction, and when these layers are stacked, the optical axis gradually changes continuously or stepwise. In the present invention, the optical anisotropic element does not have a layer structure, but the configuration in which the optical axis changes in the thickness direction also changes the optical axis of the optical anisotropy unit continuously in the thickness direction. Define.

  ADVANTAGE OF THE INVENTION According to this invention, the contrast of a liquid crystal display element and the viewing angle characteristic of a display color can be improved, and the liquid crystal display element of a high quality display which is excellent in visibility can be provided.

    In the present invention, only the TN-LCD using the TFT has been described, but it goes without saying that an excellent effect can be obtained when applied to an active matrix using an MIM or the like, a simple matrix liquid crystal display element such as an STN, or the like. .

  The present invention solves the above-described problems, and simultaneously reduces the contrast ratio of the liquid crystal display element, the brightness at the time of gradation display, and the viewing angle dependency of the display color, or a specific contrast of the liquid crystal display element. The region in which the ratio is obtained is to be controlled to a specific orientation and viewing angle, and its operation will be described below.

    In liquid crystal display elements such as TN and STN, the polarization state of light propagating in the liquid crystal display element differs depending on whether light is incident on the display surface of the liquid crystal display element perpendicularly or obliquely. This difference directly reflects the inversion phenomenon and coloring phenomenon of the display image. Such a phenomenon is observed when the viewing angle of the liquid crystal display element is greatly tilted from the normal to the display surface (front), and in particular, a liquid crystal cell (hereinafter referred to as a driving cell) having means for applying a voltage to the liquid crystal layer. This is noticeable in pixels in which a voltage is applied to the liquid crystal layer.

    23 (a) and 23 (b) show the angle dependency of display luminance when tilted from 0 ° to 60 ° in the horizontal and vertical directions from the normal line of the display surface (substrate surface) of the conventional TN liquid crystal display element. FIG. The levels 1 to 8 are indicated by the gradation numbers of gradation display, and the voltages applied to the liquid crystal cells are sequentially different. Level 1 is 0V and level 8 is 5V applied to the liquid crystal cell. For example, in the case of the upward orientation, the brightness gradually increases as the angle (viewing angle) tilted from the normal line of the display surface of the display increases from 0 ° (front) to 60 °. In the actual display, the display color is observed to become whitish (clear white).

    On the other hand, as shown in FIG. 23 (b), when viewed from the lower position, when the viewing angle is tilted from the front (0 °) to 60 °, the luminance decreases contrary to the upper direction. This phenomenon is observed in the actual display image as a dark display color (blackout). In the front, the brightest display level 1 and the lower gradation level 2 are reversed in magnitude at an upward viewing angle of 35 °, and in an actual display image, it looks like a photographic film negative. Observed as an inverted display (inverted). Ideally, the transmittance does not change even if the viewing angle changes for any gradation level. However, as shown in FIG. 23, the actual viewing angle characteristics of TN are relatively good in the horizontal direction, but poor in the vertical direction.

    Such a phenomenon occurs because the viewing angle characteristics of the liquid crystal display element are caused by the polarization state being different depending on the incident angle of light incident on the liquid crystal display element as described above. This will be explained in detail.

    FIG. 3 shows the operation principle of a TN-LCD (TN type liquid crystal element). FIG. 3A shows the alignment state of the liquid crystal molecules LM in the TN cell when no voltage is applied to the electrodes 3c and 3d. When the voltage V is not applied, the liquid crystal molecules are arranged in a continuously twisted manner with the liquid crystal molecules parallel to the thickness direction of the liquid crystal layer (in the figure, the Z-axis direction) substantially parallel to the substrate. ing. Liquid crystal molecules have an optical axis in the molecular long axis direction, and a parallel arrangement of liquid crystal molecules forms one optical axis plane. When light Li polarized by the polarizer Pi out of the incident light LA is incident on this arrangement, the plane of polarization rotates in accordance with the twisted arrangement of the liquid crystal molecules LM, and the plane of polarization before the liquid crystal layer enters the liquid crystal layer. Is rotated by the twist angle of the liquid crystal layer with respect to the plane of polarization. When the transmission axis Pot of the analyzer Po is aligned with the rotated direction, the transmitted light Lo is obtained.

    FIG. 3B shows the alignment state of the liquid crystal molecules in the TN cell when a voltage is applied. By applying the voltage V, the liquid crystal molecules LM rise, and the liquid crystal molecules LMc near the center of the cell are tilted more than the liquid crystal molecules LMs near the electrode. The reason why the inclination of the liquid crystal molecules LMs in the vicinity of the electrodes 3c and 3d is small is that there is an alignment regulating force (necessary for aligning the liquid crystal) at the electrode-liquid crystal layer interface. The inclination of the liquid crystal molecules increases according to the magnitude of the voltage V, and at the same time, the twisted arrangement is distorted. When the voltage is further increased, the twist is finally released. When polarized light Li is incident in such a state, the polarization plane Lp does not rotate and travels through the liquid crystal layer because it is not twisted, that is, the optical axis plane is on a single axis, and exits the liquid crystal layer. The polarization plane is not different from that before entering the liquid crystal layer. Therefore, since the transmission axis Pot of the analyzer Po is orthogonal to the polarization plane Lp, polarized light cannot be transmitted. In order to display halftones, the voltage applied to the liquid crystal layer is set smaller than this, the twisted arrangement of the arrangement is left slightly, and the polarization plane that exits the liquid crystal layer is rotated slightly to obtain intermediate transmitted light. Get.

    Based on the above principle, the TN device controls the transmitted light by using the distortion of the twisted arrangement. Next, what kind of phenomenon occurs with respect to light in an oblique direction will be described.

    FIG. 4 is a diagram illustrating a state in which light is incident obliquely on the molecular arrangement state when displaying a halftone. FIG. 4A is a perspective view showing the relationship between the molecular arrangement state LMint at the time of halftone display and the directions L and U of the two incident lights. In order to make this easier to understand, FIG. 4A is a view seen from the Y-axis direction. 4 (b) and 4 (c). Here, the normal direction of the substrate of the driving liquid crystal cell is represented by the Z axis, and the substrate surface is represented by the XY axis. The liquid crystal molecules LMs near the electrodes 3c and 3d on the upper and lower substrates are arranged with a slight inclination with respect to the substrate surface. This tilt is called a pretilt. Generally, the pretilt indicates the tilt of liquid crystal molecules at the substrate-liquid crystal interface, and the tilt angle is referred to as a pretilt angle α0. When no voltage is applied, the upper and lower substrates 3a and 3b are inclined at the same angle. If there is a predetermined tilt (pretilt) over the region to which the voltage V is applied, the tilt direction when the voltage is applied is aligned with the pretilt direction, so that uniform display can be achieved. If there is no pretilt, the direction in which the liquid crystal molecules incline varies when a voltage is applied, and a defect line is generated at the boundary between regions having different inclination directions, which causes a significant deterioration in display quality. Therefore, pretilt is indispensable for obtaining a uniform display, and the angle is generally 1 ° to 6 °. All the liquid crystal display elements are given a pretilt.

    Therefore, as shown in FIGS. 4B and 4C, the alignment state of the liquid crystal molecules is asymmetric with respect to the Z axis, particularly when a halftone is displayed. With respect to the polarized light L obliquely incident in the direction from the + X axis to the + Z axis in FIG. 4B, as shown in LM-L in FIG. 5, the alignment is in a state in which the liquid crystal molecules LM are not inclined (as if no voltage was applied). The polarization plane can be greatly rotated. As a result, the transmitted light becomes larger than the intensity of the outgoing light with respect to the incident light from the front (light parallel to the Z axis). On the other hand, as shown in FIG. 4C, for the polarized light U incident from a symmetric direction (obliquely in the direction of −Z axis to + Z axis) with reference to the opposite substrate normal, FIG. As shown in LM-U, the alignment is in a state where the liquid crystal molecules LM are greatly inclined (as if an even larger voltage was applied), and the polarization plane cannot be rotated. As a result, the transmitted light becomes smaller than the intensity of the outgoing light with respect to the incident light from the front (light parallel to the Z axis). 23 corresponds to the upper direction of FIG. 23B, and the direction of U of FIG. 4 corresponds to the lower direction of FIG. 23B.

    As described above, the orientation dependency of transmitted light in a halftone is due to the asymmetry of the alignment of liquid crystal molecules. This asymmetry of the arrangement causes the rotation (optical rotation) angle of the polarization plane to be different depending on the direction in which light is incident, resulting in a change in transmittance. In TN-LCD, it can be said that optical rotation occurs in the upper direction and optical rotation tends to decrease in the lower direction. Therefore, in order to improve this, the viewing angle dependency of the liquid crystal display element can be improved by adding an optical anisotropic element in which the optical rotation is reduced in the upper direction and the optical rotation is generated in the lower direction. The gist of the present invention is how to provide an optically anisotropic element having such characteristics.

    Next, the optical anisotropic element of the present invention will be specifically described.

    First, when the characteristics required for the optical anisotropic element are summarized, the required characteristics are “the rotation direction of the optical rotation is opposite between the upper direction and the lower direction”. FIG. 6 is a diagram showing an arrangement state of optical axes of the optical anisotropic element of the present invention, and FIG. 6A is a cross-sectional view of the optical anisotropic element of the embodiment of the present invention, which is shown by an ellipse. Indicates an optically anisotropic body LD constituting an optically anisotropic element, and the major axis of the ellipse corresponds to the optical axis OL. The inclination of the major axis continuously changes from the lower substrate 3d to the upper substrate 3c, and is substantially parallel to the substrate surface in the vicinity of the lower substrate 3d and substantially vertical in the vicinity of the upper substrate 3c (hybrid orientation). ). An example of this arrangement as seen from above is shown in FIG. The arrow in the ellipse in the figure indicates the direction of the optical axis. The directions of the optical axes in the layer are on the same plane, that is, they are aligned on a single axis. FIG. 6C is an arrangement diagram when observed obliquely from the Z axis. The tilt direction is indicated by XYZ axes in the figure. A view seen from the opposite oblique direction is shown in FIG. As can be seen from FIGS. 6C and 6D, when the array of FIG. 6A is observed obliquely from the Z-axis, in FIG. ) Is arranged with the reverse right twist. By the optically anisotropic elements arranged obliquely in this way, the above-described property that “the rotation direction of the optical rotation is opposite between the upper direction and the lower direction” can be realized.

    Next, how such an optical anisotropic element is combined with a driving liquid crystal cell to obtain a good compensation effect will be described next.

    FIG. 7A is a diagram showing the driving liquid crystal cell shown in FIGS. 3, 4 and 5 with an arrow added as in FIG. 6, where the symbol Lip is the polarization axis of the incident light and the symbol Lop is the outgoing light. Represents the polarization axis. FIG. 7A shows an optical anisotropic element, and FIG. 7B shows a driving liquid crystal cell (TN) to which a voltage corresponding to a halftone is applied, as viewed from the Z axis. (C) is the figure which showed the arrangement | sequence of the optical axis of an optical anisotropic element when it sees from the Z-axis to the + X side, and showed the optical rotation state when linearly polarized light injects into the figure. In this direction, the optical anisotropic element has the property of rotating the polarization plane of incident light in the left direction (left rotation ability). (D) shows the arrangement state of the driving cells when viewed from the same direction as (c). The liquid crystal molecules are tilted obliquely because a voltage equivalent to halftone (a voltage slightly higher than the critical voltage (threshold voltage) at which the liquid crystal operates) is applied, and when viewed from this direction, the long axis of the liquid crystal molecules An orientation portion is produced in which the length in the direction and the length in the minor axis direction are substantially the same. Therefore, the incident polarized light is transmitted without rotating much, and the direction of the polarization axis Lop of the outgoing light is almost the same as the polarization axis Lip of the incident light. This is the cause of the display abnormality called “blackout” that darkens the display. In this case, if the polarized light is rotated counterclockwise (increases the optical rotation ability), this is improved. For this purpose, the optically anisotropic element shown in FIG. The optical anisotropic element shown in FIG. 3C has a left optical rotation ability, and the driving liquid crystal cell compensates for the insufficient optical rotation.

    On the other hand, the opposite direction will be described with reference to FIGS. (E) and (f) show the arrangement of the optical axes when the optical anisotropic element in FIG. (A) is observed from the -X axis from the Z-axis direction. It has the property of turning to the right (right rotation ability). Fig. (F) shows a state in which a halftone voltage is applied as in Fig. (D). From this direction, although the liquid crystal molecules are actually tilted, it does not appear to tilt. For this reason, a large optical rotation is achieved. This causes a display anomaly called “white spot” that makes the display brighter than necessary, and if the right rotation that suppresses counterclockwise rotation is applied, the extra rotation can be eliminated and “white spot” is improved. . The optical anisotropic element shown in FIG. 3E has a right optical rotation ability, and characteristics can be improved by combining this with a drive cell.

    As described above, the principle of the viewing angle expansion is exemplified by the optically anisotropic element having a hybrid orientation in which the optical rotation in the direction inclined with respect to the normal to the surface of the optical anisotropic element is larger than the optical rotation in the direction of the normal. Although explained, an optical anisotropic element that is twist-aligned by hybrid alignment and an optical anisotropic element that is tilt-aligned uniformly between the upper and lower substrates can obtain characteristics similar to those of the hybrid alignment optical anisotropic element. Can be selected according to the design specifications.

    Further, as shown in FIG. 8 (a), when an optical anisotropic element having a splay arrangement of the optical axis OL is used in the layer between the front and back surfaces 2d and 2e, as shown by the arrow TW in FIGS. The twist angle of the array observed from the direction becomes larger and a good compensation effect can be obtained. The same applies to the bend arrangement.

    In addition, although TN has been described as an example, since the same principle can be applied to STN, it can be used as means for improving the viewing angle of STN.

    The above-described optical anisotropic element has a great improvement effect mainly on display abnormality such as “blackout” and “whiteout”. The inventors have found that when a further optical anisotropic element having an optical axis in the thickness direction of the optical anisotropic element and a negative optical anisotropy is further added, a better improvement effect is obtained. Next, the principle of improving the viewing angle characteristics when using an optical anisotropic element having a negative optical anisotropy will be described.

    A state in which a voltage higher than the threshold voltage is applied to the driving liquid crystal cell is represented by a three-dimensional refractive index ellipsoid as shown in FIG. The Z axis is the thickness direction of the liquid crystal cell, and the XY plane corresponds to the substrate surface of the liquid crystal cell. The birefringence phenomenon is a normal line on the central point of the refractive index ellipsoid RA, which is a line connecting the observation point when the central point of the refractive index ellipsoid RA is viewed from a certain direction and the central point of the refractive index ellipsoid RA. The surface is indicated by the shape of an elliptical cut surface formed when the refractive index ellipsoid RA is cut (herein referred to as a refractive index body in a two-dimensional plane). If the difference between the major axis and minor axis length of the refractive index body in the two-dimensional plane corresponds to the phase difference between ordinary light and extraordinary light, and the transmission axes of the polarizing plates sandwiching the liquid crystal cell are orthogonal to each other, When the phase difference is zero, the transmitted light of the liquid crystal cell is blocked, and when the phase difference is not zero, transmitted light corresponding to the phase difference and the wavelength of the incident light is generated.

    When light is incident perpendicularly to the substrate surface of the liquid crystal cell (that is, when the liquid crystal cell is viewed from the front), the refractive index body RA4 in the two-dimensional plane is a circle, and the phase difference between ordinary light and extraordinary light is zero. However, when light is incident from the direction RA1 tilted from the substrate surface of the liquid crystal cell, the refractive index RA5 becomes an ellipse, causing a phase difference between ordinary light and extraordinary light, and light transmitted through the liquid crystal cell in the front direction and the oblique direction. The polarization states of are different.

    As the angle of viewing the refractive index ellipsoid RA in FIG. 9, ie, the viewing angle RA3 is increased, the refractive index body RA5 in the two-dimensional plane of the visual axis RA1 increases in the length direction of nRA1, and from the direction of the visual axis RA1. Transmitted light larger than when viewed is observed. Ideally, it is desirable that the shape of the refractive index body in the two-dimensional plane does not change when the viewing angle is changed in any orientation.

    In such optical compensation, a disc-shaped refractive index ellipsoid RB as shown in FIG. 10 is arranged on the Z axis of the refractive index ellipsoid RA in FIG. 9 (that is, adjacent to the upper or lower portion of the liquid crystal cell). Can be realized. In this way, when the viewing angle RA3 is increased, the refractive index body RA5 in the two-dimensional plane of the refractive index ellipsoid RA increases in the length direction of nRA1, whereas the nRA2 of the refractive index ellipsoid RB The refractive index in the lengthwise direction increases, and as a result, the combined refractive index body in the two-dimensional plane becomes a circle, and the refractive index ellipsoid RA can be optically compensated, and the viewing angle characteristics are improved.

    In an actual liquid crystal display element, the refractive index ellipsoid of the driving liquid crystal cell has a long axis of the ellipse that is not perpendicular to the display surface but slightly tilted as shown in FIG. Accordingly, it is preferable that the refractive index ellipsoid RB of the optical anisotropic element shown in FIG.

    Actually, the refractive index ellipsoid as shown in FIG. 10 has an optical anisotropic element composed of an optically anisotropic material layer in which the optical axis is continuously twisted, and a refractive index in the in-plane direction rather than the thickness direction. This can be realized by using a smaller material.

    Hereinafter, a description will be given of how to realize an optically anisotropic element having a negative optical anisotropy using an optically anisotropic element made of an optically anisotropic material layer having an optical axis continuously twisted.

    In general, the liquid crystal cell for driving is displayed by actively changing the polarization direction of light in a visible wavelength region (generally, a region from 380 nm to 750 nm) by a voltage applied to the liquid crystal cell.

    On the other hand, in the case of the optically anisotropic element for optical compensation of the present invention, the optical axis of the optically anisotropic material layer is continuously twisted, so that optical rotation may occur depending on the optical conditions of the optically anisotropic element. . Here, the optical rotation indicates the property that the vibration direction of the light turns to the left or right around the traveling direction as the light travels in the medium. When the retardation value of an optically anisotropic element whose optical axis is continuously twisted is constant, if the twist pitch of the optical axis is long, the light rotates its polarization plane according to the twist of the optical axis, but the twist of the optical axis When the pitch is short, the light cannot follow the twist of the optical axis, and the optical rotation phenomenon does not occur. If the optical rotatory power of the optical anisotropic element is large, the polarization plane of the light transmitted through the element is changed.As a result, the contrast ratio is decreased, and in some cases, the polarization plane changes variously depending on the wavelength of the light, There arises a problem that light transmitted through the optical anisotropic element is colored.

    Therefore, at least the optical rotatory power with respect to visible light of the optically anisotropic element needs to be smaller than the optical rotatory power with respect to visible light of the driving liquid crystal cell. The optical rotation depends largely on the wavelength of light transmitted through the medium and the medium through which the light is transmitted. The magnitude of the optical rotation is represented by the degree of change in the retardation value of the medium with respect to the change in the optical axis.

Therefore, the optical rotation of the liquid crystal cell for driving is determined by the difference between the refractive index no for the ordinary light and the refractive index ne for the extraordinary light of the liquid crystal of the driving liquid crystal cell by Δn1 (= ne−no: refractive index anisotropy), If the thickness of the liquid crystal layer is d1, and the twisted angle (twist angle) of the liquid crystal layer is T1,
Δn 1 · d 1 / T 1 = R 1 / T 1 [1.1]
However, R1 = Δn1 · d1 (retardation value)
It can be expressed as

Similarly, the optical rotatory power of the compensating optical anisotropic element is determined by the refractive index anisotropy of the optical anisotropic material layer of the compensating optical anisotropic element being Δn2, and the thickness of the laminated optical anisotropic material layer. Is d2, and the total twist angle of the optical axis of the optically anisotropic material layer is T2.
Δn 2 · d 2 / T 2 = R 2 / T 2 [1.2]
However, it can be expressed by R2 = Δn2 · d2.

Therefore, the magnitude relationship between the optical rotatory power of the compensating optical anisotropic element and the optical rotatory power of the driving liquid crystal cell is expressed by the equations [1.1] and [1.2]:
(R1 / T1)> (R2 / T2) [1.3]
It becomes.

The propagation of light in an optical anisotropic element in which the optical axis of the optically anisotropic material layer is continuously twisted can be expressed by a parameter represented by the following equation (C.Z.Van Doorn, Physics Letters 42A, 7 (1973)).

f = λ / (p × Δn) [1.4]
Where λ is the wavelength of light in the vacuum (visible wavelength range)
p is the twist pitch length of the optical axis (p = d / T).

In the case of f << 1, the polarization plane of the light in the optical anisotropic element changes according to the twist angle of the optical axis and has optical rotation. As described above, it is desirable that the optical anisotropic element has a small optical rotation, and the optical anisotropic element needs to satisfy the condition of f >> 1. Therefore, the optical anisotropic element is expressed by the equation [1.4]
p × △ n <λ [1.5]
It is necessary to hold.

By the way, a liquid crystal having a very large twist angle, that is, a liquid crystal having a short helical pitch is generally referred to as a cholesteric liquid crystal. The product of the length p of the helical pitch of the liquid crystal and the average refractive index n of the cholesteric liquid crystal is n × p. When the value is in the visible wavelength range (depending on conditions, the short wavelength end is 360 nm to 400 nm and the long wavelength end is 760 nm to 830 nm), selective scattering occurs (JLFergason; Molecular Crystals. 1. 293 (1966 )). Such a phenomenon is not only observed in the cholesteric liquid crystal cell, but can also occur in an optical anisotropic element in which the optical axis of the optical anisotropic body is continuously twisted. When selective scattering occurs, a coloring phenomenon of the optical anisotropic element occurs and the display color changes. Accordingly, if the product n × p of the average refractive index n of the optically anisotropic material layer forming the optically anisotropic element and the twist pitch p of the optical axis is excluded from the visible wavelength range, the coloring phenomenon can be prevented.

  An optically anisotropic element is a laminate of retardation films in which optical anisotropy is produced by stretching a polymer film, a twisted alignment liquid crystal cell, and a polymer liquid crystal. This can be realized by an arrayed thin film. In this case, for example, the polymer layer is obtained by applying the polymer layer to at least one of the substrates of the driving liquid crystal cell, which facilitates manufacturing and provides a more desirable liquid crystal display element. For example, a polymer copolymer having a polysiloxane main chain and biphenylbenzoate and cholesteryl groups in an appropriate ratio in the side chain can be used. Alternatively, the optical anisotropic element of the present invention can also be realized by using a liquid crystal that is cured by ultraviolet rays to which a polymerizable functional group using an acryloyloxy group or the like is added.

  Hereinafter, embodiments of the liquid crystal display element of the present invention will be described in detail.

(Embodiment 1)
1 and 2 are cross-sectional views of the liquid crystal display element according to this embodiment. The liquid crystal display element includes two polarizing plates 1 and 4 (LLC2-92-18: manufactured by SANRITZ), and a liquid crystal cell 2 and a driving liquid crystal cell 3 which are optical anisotropic elements for viewing angle compensation between them. It has a sandwiching structure. The polarizing plate 1 is formed by sandwiching the polarizing film 1a inside the transparent substrate 1b, and the polarizing plate 4 is similarly formed by attaching the polarizing film 4a to the transparent substrate 4b.

    A viewing angle compensating liquid crystal cell 2 as an optically anisotropic element is disposed between the polarizing plates 1 and 4 and has a liquid crystal cell structure in which a liquid crystal 2c is interposed between the transparent substrates 2a and 2b. Substrates 2a and 2b have SiO2 deposited obliquely on the surface, and an optically anisotropic material layer which is a liquid crystal layer in which a chiral agent S811 (manufactured by E. Merck Co., Ltd.) is mixed in a twisted nematic liquid crystal has a twist angle. Introduced at 270 °, the liquid crystal molecules are twisted counterclockwise from the lower substrate 2b to the upper substrate 2a while maintaining a pretilt angle of 50 ° (left twist). The Δn of the liquid crystal material as the optically anisotropic material layer used in the viewing angle compensation liquid crystal cell 2 is 0.189, the helical pitch is 1. The thickness is 33 μm and the layer thickness is 1 μm.

    The driving liquid crystal cell 3 is disposed between the viewing angle compensation liquid crystal cell 2 and the polarizing plate 4. The two upper substrates 3a and the lower substrate 3b form transparent electrodes 3c and 3d, respectively, and are connected to the drive power supply 3f. Twisted nematic liquid crystal (ZLI-4287, manufactured by E. Merck Co. Ltd.) having a positive dielectric anisotropy between the substrates 3a and 3b and a chiral agent S811 (trade name, manufactured by E. Merck Co. Ltd.) Is introduced at a twist angle of 90 °, and the state changes in accordance with the applied voltage from the drive power supply 3f. The torsional arrangement is maintained when no voltage is applied.

    The Δn of the liquid crystal used in the driving liquid crystal cell 3 is 0.093, and the thickness of the liquid crystal layer is 5.5 μm. The liquid crystal molecules of the driving liquid crystal cell 3 are twisted counterclockwise from the lower substrate 3b to the upper substrate 3b (left twist). The cell 3 operates as a TN cell with a 90 ° twist angle, and performs light control by an optical rotation action.

    FIG. 2A is an exploded perspective view showing the configuration of the liquid crystal display element in the present embodiment. (1.1) and (4.1) are the transmission axes of the two polarizing plates 1 and 4, which are orthogonal to each other. (1.1) is counterclockwise when viewed from the + Z direction, which is the normal direction of the substrate with respect to the Y axis. Arranged at 135 °. (3.1) and (3.2) are the rubbing axes of the upper substrate 3a and the lower substrate 3b of the driving liquid crystal cell 3, that is, the alignment treatment direction, which are orthogonal to each other and formed by the rubbing axis (3.1) with respect to the Y axis. Are arranged at 45 ° counterclockwise as viewed from the + Z direction.

    (2.1) and (2.2) of the viewing angle compensation liquid crystal cell 2 which is an optically anisotropic element are the rubbing axes of the upper and lower substrates 2a and 2b, respectively, which are orthogonal to each other, and the viewing angle compensation liquid crystal cell 2 is rubbed. The axis (2.2) is arranged so as to be parallel to the rubbing axis (3.1) of the driving liquid crystal cell 3. In other words, the optical axis OL (FIG. 6) of the liquid crystal molecules LM is arranged along these rubbing axes, and becomes the optical axis of the liquid crystal layer on the side where the liquid crystal layer is in contact with the rubbed surface of the substrate.

    The polarizing plate 1 was arranged so that the transmission axis (1.1) was orthogonal to the rubbing axis (2.1) of the viewing angle compensation liquid crystal cell 2.

    The liquid crystal display element of this configuration was measured for electro-optical characteristics in the coordinate system of FIG. The voltage value at the time of measurement (voltage applied between the driving power supply 3f and the electrodes 3c-3d of the driving liquid crystal cell 3) was changed from 1V to 5V. The results are shown in FIG. FIG. 11 shows applied voltage-transmittance characteristics in four directions, top, bottom, left and right, respectively, and shows the transmittance when the viewing angle is changed by 30 ° from the front to 60 °. Ideally, any viewing angle should be identical to the transmittance curve of the front (viewing angle θ = 0 °). In the front direction, when a certain voltage is exceeded, the transmittance decreases as the voltage increases.

    FIG. 12 is a graph showing applied voltage-transmittance characteristics of a TN-LCD of a comparative example of the prior art. In the lower characteristics, the transmittance decreases as the viewing angle increases. This corresponds to the occurrence of “blackout” when the gradation is actually displayed. Further, the re-increase of the transmittance near 3 V at a viewing angle of 60 ° corresponds to “inversion” in the actual display. Looking at the upper direction, the transmittance increases as the viewing angle increases from 0 ° to 60 ° at a voltage of 3V. This corresponds to “whitening” in actual display.

    In the case of this embodiment, when FIG. 11 is seen, the fall of the transmittance | permeability of a lower rank and the reincrease of the transmittance | permeability of 3V vicinity are improved. That is, the blackening and inversion of the lower order are improved. Further, it can be seen from the comparison of transmittance at viewing angles of 30 and 60 degrees in the 3 to 5 V region that the white spots in the upper direction are slightly improved.

    When a TFT-LCD with a screen size diagonal of 10 inches with a color filter in the liquid crystal cell was actually created with this configuration, a good full-color display was obtained that could distinguish the display contents even when the orientation and viewing angle were changed. It was.

(Comparative Example 1)
In the first embodiment, voltage-transmittance characteristics were measured when the viewing angle compensation liquid crystal cell 2 was not provided. The measurement results are shown in FIG. In this comparative example, depending on the angle, a phenomenon that the display turned white in the upper direction and the display turned black or the gradation was reversed in the lower direction was observed.

(Comparative Example 2)
In the first embodiment, the alignment film of the viewing angle compensating liquid crystal cell 2 was prepared using polyimide having a tilt angle of 1 °. The other conditions are exactly the same as in the first embodiment. FIG. 14 shows the measurement results of the voltage-transmittance characteristics. By reducing the pretilt angle of the viewing angle compensation liquid crystal cell 2, the transmittance cannot be completely lowered in the front direction, and the contrast ratio is lowered. Further, as can be seen from the characteristics in the oblique direction as compared with the characteristic diagram 12 of the conventional example, the characteristics of the azimuth other than the upper azimuth deteriorated.

(Embodiment 2)
FIG. 13 is an exploded perspective view showing the configuration of the liquid crystal display element in the present embodiment. In the first embodiment, the viewing angle compensation liquid crystal cell 2 which is an optically anisotropic element is coated with polyimide AL-1051 (manufactured by Nippon Synthetic Rubber) on the side of the lower substrate 2b in contact with the liquid crystal, and the surface is subjected to rubbing treatment. Has been. The pretilt angle is 1 °. On the other hand, a vertical alignment process is performed on the side of the upper substrate 2a in contact with the liquid crystal. The Δn of the liquid crystal material used is 0.039, and the thickness of the liquid crystal layer is 4.4 μm. The optical axis of the liquid crystal molecules, that is, the optical axis of the optical anisotropic element is parallel to the cell on the driving liquid crystal cell 3 side, and continuously changes in the layer thickness direction and is normal to the cell substrate on the side away from the liquid crystal cell 3. It is almost along the direction. The twist angle is 0 °.

      (1.1) and (4.1) are the transmission axes of the polarizing plates 1 and 4, which are orthogonal to each other, and (1.1) is arranged at 135 ° counterclockwise when viewed from the + Z direction with respect to the Y axis. (3.1) and (3.2) are the rubbing axes of the upper and lower substrates 3a and 3b of the driving liquid crystal cell 3, which are orthogonal to each other, and the angle formed by the rubbing axis (3.1) with respect to the Y axis is the + Z direction. It is arranged at 45 ° counterclockwise when viewed from the side.

      The optical axis (2.2) of the viewing angle compensation liquid crystal cell 2 is the rubbing axis of the lower substrate 2b, orthogonal to the rubbing axis (3.1) of the upper substrate of the driving liquid crystal cell 3, and the rubbing axis (3.2) of the lower substrate. Are arranged in parallel with each other.

      The transmission axis (1.1) of the polarizing plate 1 is arranged so as to be parallel to the rubbing axis (3.1) of the upper substrate of the driving liquid crystal cell 3.

      The liquid crystal display element of this configuration was measured for electro-optical characteristics in the coordinate system of FIG. The voltage value at the time of measurement (voltage applied between the driving power supply 3f and the electrodes 3c-3d of the driving liquid crystal cell 3) was changed from 1V to 5V. The results are shown in FIG. Compared with the voltage-transmittance characteristics of Comparative Example 1 (FIG. 12), the darkness and the inversion of gradation decreased and the viewing angle expanded in the vertical direction, particularly in the lower direction. A TFT-LCD with a 10-inch diagonal screen size with a color filter in the liquid crystal cell in this configuration was actually manufactured, and a good full-color display that can distinguish the display contents even when the orientation or viewing angle was changed was obtained. It was.

(Embodiment 3)
FIG. 16 shows a configuration diagram of this embodiment. In the first embodiment, the optical anisotropy between the viewing angle compensation liquid crystal cell 2 and the driving liquid crystal cell 3 which is the first optical anisotropic element becomes negative when the thickness direction is the optical axis. An optical anisotropic element 5 which is an optical anisotropic element of FIG. The optical anisotropic element 5 has an optical axis (shown as nz in FIG. 16) inclined by 60 ° (δ) in the XZ plane with respect to the Z axis in the thickness direction, and the refractive index of this optical axis normal plane is the optical axis direction. Greater than the refractive index of. The retardation value is -140 nm. FIG. 17 shows the result of measuring the electro-optical characteristics of the liquid crystal display element of this configuration according to the orientation defined by the coordinate axes in FIG. As can be seen from comparison with FIG. 12, which is the characteristic diagram of Comparative Example 1, the upper azimuth characteristics of Embodiment 1 are further improved.

(Embodiment 4)
The configuration of this embodiment is shown in FIG. An optical anisotropy having negative optical anisotropy as a second optical anisotropic element between the viewing angle compensating liquid crystal cell 2 and the driving liquid crystal cell 3 which is the first optical anisotropic element in the second embodiment. Element 5 was arranged. The configuration is shown in FIG. The optical anisotropic element 5 has an optical axis (shown as nZ in FIG. 18) parallel to the Z axis, and the refractive index of this optical axis normal surface is larger than the refractive index in the optical axis direction. The retardation value is -140 nm. FIG. 19 shows the result of measuring the electro-optical characteristics of the liquid crystal display element of this configuration in the orientation defined by the coordinate system of FIG. As can be seen from comparison with the characteristic diagram 12 of Comparative Example 1, the upper direction characteristic of the second embodiment was further improved.

(Embodiment 5)
In the second embodiment, a second viewing angle compensation liquid crystal cell having a twist angle of 720 ° is arranged between the viewing angle compensation liquid crystal cell 2 and the driving liquid crystal cell 3. The liquid crystal material used had a Δn of 0.039 and a pitch length of 3.5 μm. The thickness of the liquid crystal layer is 7.0 μm, and the alignment is horizontally aligned by rubbing the polyimide film in the opposite direction between the upper and lower substrates. The second compensation cell was arranged so that the rubbing axis of the compensation cell was parallel to the rubbing axis of the lower substrate of the driving liquid crystal cell 3. FIG. 20 shows the result of measuring the electro-optical characteristics of the liquid crystal display element of this configuration in the orientation defined by the coordinate system of FIG. As can be seen from comparison with the characteristic diagram 12 of the comparative example 1 which is a conventional example, the upper azimuth characteristic of the second embodiment is improved.

(Embodiment 6)
As shown in FIG. 21, between the viewing angle compensation liquid crystal cell 2 and the driving liquid crystal cell 3, which is an optical anisotropic element composed of a liquid crystal cell in which the optically anisotropic material layer is formed of a liquid crystal layer in the second embodiment. The optically anisotropic element 6 made of a polymer copolymer having negative optical anisotropy was disposed. The optical anisotropic element 6 has an optical axis (shown as nz in the figure) parallel to the Z axis, and the refractive indexes nx and ny of the normal surface of the optical axis are larger than the refractive index nz in the optical axis direction. The retardation value is −100 nm. The optical anisotropic element 7 identical to the optical anisotropic element 6 is disposed between the driving liquid crystal cell 3 and the polarizing plate 4 having a thickness of 3.4 μm in the viewing angle compensation liquid crystal cell 2. The viewing angle compensation liquid crystal cell 5 is disposed between the optical anisotropic element 7 and the polarizing plate 4. The viewing angle compensation liquid crystal cell 5 is made of the same alignment film as the viewing angle compensation liquid crystal cell 2, and the upper substrate is rubbed in the direction of the arrow (5.1). A vertical alignment treatment is performed on the surface of the substrate on the side of the polarizing plate 4 that is in contact with the liquid crystal of the viewing angle compensating liquid crystal cell 5. The rubbing axis 5.1 is parallel to the rubbing axis 3.1 of the upper substrate of the driving liquid crystal cell 3.

      FIG. 22 shows the result of measuring the electro-optical characteristics of the liquid crystal display element of this configuration in the orientation defined by the coordinate system of FIG. As can be seen from comparison with FIG. 12 showing the characteristics of the conventional example, the viewing angle characteristics of the present embodiment are improved over those of the second embodiment with respect to all the vertical and horizontal directions.

(Embodiment 7)
In the second embodiment, the viewing angle compensating liquid crystal cells 2 and 5 were made of a polymer copolymer having a polysiloxane main chain and biphenylbenzoate and cholesteryl groups in an appropriate ratio in the side chain. Similar characteristics were obtained. Furthermore, a thinner liquid crystal display element is realized by producing an optically anisotropic element with a polymer copolymer.

Sectional drawing which shows the structure of Embodiment 1 of this invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating a first embodiment of the present invention, in which (a) is an exploded perspective view illustrating a configuration, and (b) is a diagram illustrating a coordinate system for measuring electro-optical characteristics. The figure explaining the operation principle of TN-LCD. The figure explaining the generation | occurrence | production principle of the viewing angle characteristic of TN-LCD. The figure explaining the generation | occurrence | production principle of the viewing angle characteristic of TN-LCD. The figure which shows the arrangement | sequence state of the optical anisotropic element of this invention. The figure explaining the optical compensation principle at the time of using the optically anisotropic element of this invention. The figure explaining the optical compensation principle at the time of using the optically anisotropic element of the splay arrangement | sequence in this invention. The figure which shows the refractive index ellipsoid of the drive cell at the time of a voltage application. The figure which showed the refractive index ellipsoid of the optically anisotropic element whose refractive index anisotropy is a negative sign in the thickness direction. 2 is an electro-optical characteristic of the liquid crystal display element of Embodiment 1. Electro-optical characteristics of conventional liquid crystal display elements. FIG. 9 illustrates a configuration of a second embodiment. Electro-optical characteristics of the comparative example. FIG. 6 is a diagram illustrating an effect of the second embodiment. FIG. 6 illustrates a configuration of a third embodiment. FIG. 10 is a diagram for explaining the effect of the third embodiment. FIG. 6 illustrates a configuration of a fourth embodiment. FIG. 10 is a diagram illustrating an effect of the fourth embodiment. FIG. 10 is a diagram illustrating an effect of the fifth embodiment. FIG. 10 illustrates a configuration of a sixth embodiment. FIG. 10 is a diagram for explaining the effect of the sixth embodiment. The curve figure which shows the viewing angle dependence of the brightness | luminance of the conventional TN-LCD.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 4 ... Polarizing plate 2 ... Optical anisotropic element 2c ... Optical anisotropic material layer 3 ... Liquid crystal cell for a drive

Claims (4)

In a liquid crystal display element comprising two polarizing plates and a driving liquid crystal cell disposed between the two polarizing plates,
Between the polarizing plate and the driving liquid crystal cell, the optical rotation in the direction inclined with respect to the normal of the substrate is larger than the optical rotation in the direction of the normal of the substrate, and the normal of the substrate is An optically anisotropic element having a first direction having a small optical rotation and a second direction having a large optical rotation that is symmetrically tilted with respect to a reference is disposed, and the optical rotation asymmetry of the driving liquid crystal cell is reduced. A liquid crystal display element that is compensated by the element. In an optical anisotropic element comprising an optically anisotropic material layer, the major axis directions of the optically anisotropic units constituting the optically anisotropic material layer are aligned on a single axis, and the element surface An optically anisotropic element characterized in that the optical rotatory power in the direction inclined with respect to the normal line is greater than the optical rotatory power in the normal line direction. The angle of the major axis of the optical anisotropic element changes continuously or stepwise in the layer thickness direction of the optical anisotropic element with respect to the surface of the optical anisotropic element. Optical anisotropic element. The optical anisotropic element so that the major axis angle of the optical anisotropic element is substantially parallel to the surface on one side of the optical anisotropic element and substantially along the normal line of the surface on the other side of the optical anisotropic element. The optical anisotropic element according to claim 2, wherein the optical anisotropic element changes in the layer of the element.

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