JP2009229894A - Twisted vertical alignment type liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a vertical alignment type liquid crystal display device which has a good sharpness of a change in light transmittance when a voltage is applied and has less color shift. <P>SOLUTION: A twisted vertical alignment type liquid crystal display device includes a pair of substrates on the surface of which electrodes and vertical alignment films formed on the electrodes, are formed and which are disposed to be opposite to each other with a gap d therebetween and in which only one of vertical alignment films is alignment-treated; a vertical alignment liquid crystal layer which is disposed between the pair of substrates and has a chiral agent added thereto and satisfies 0<(d/p)≤0.5556 wherein a chiral pitch is defined as p; and a pair of polarizers in a crossed Nicol state, which are disposed on the outside of a liquid crystal cell. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a liquid crystal display device, and more particularly to a vertical alignment type liquid crystal display device.

  When a vertical alignment type liquid crystal display device in which liquid crystal molecules are aligned perpendicularly to a substrate in the absence of voltage application, when combined with a polarizing plate having a crossed Nicol arrangement, light transmittance at the time of off can be reduced and high contrast can be provided.

  FIG. 1 shows a typical configuration example of a vertical alignment type liquid crystal display device (cited from JP-A-2005-234254). The first substrate 1 and the second substrate 2 face each other, and the liquid crystal layer 3 is sandwiched therebetween. The first substrate 1 is obtained by forming a transparent electrode 14 on the opposite surface of the transparent substrate 13, applying a polymer vertical alignment film 15 thereon, and rubbing the surface in a direction indicated by 18; A viewing angle compensation plate 12 and a polarizing plate 11 are disposed on the outer surface. Similar to the first substrate 1, the second substrate 2 is formed with a transparent electrode 24 on the opposite surface of the transparent substrate 23, covered with a polymer vertical alignment film 25, and rubbed in the direction of the arrow 28. Is. A viewing angle compensation plate 22 and a polarizing plate 21 are disposed on the outer surface. The rubbing directions 18 and 28 are arranged in antiparallel so that the liquid crystal molecules pretilt in a certain direction. The liquid crystal layer 3 includes liquid crystal molecules having a property of being aligned perpendicular to the planes of the substrates 1 and 2, and pre-tilts 19 and 29 having a certain angle (less than 90 degrees) from the surface of the substrate by rubbing treatments 18 and 28. Have.

  As for the electro-optical characteristics of the vertical alignment ECB-LCD, the transmitted light is well blocked in a low applied voltage state, and a good black display can be obtained. However, the angular distribution of the transmitted light spectrum may not necessarily be flat for incident light in a direction in which the liquid crystal has refractive index anisotropy, and a phenomenon may occur in which coloring is observed in an observation from an oblique direction. In vertical alignment ECB-LCD, it is generally necessary to optimize the combination of driving signal conditions such as duty ratio and cell conditions such as liquid crystal materials and spacers, and different materials are combined for each driving condition. Defective products may occur due to mixing of materials used.

  It has been proposed to add a chiral agent to the vertical alignment type liquid crystal layer so that the pretilt angle is greatly separated from the normal direction of the substrate (Japanese Patent Application Laid-Open Nos. 2004-355032 and 2007-101808). The pretilt angle measured from the substrate surface is set in the range of 85 ° to 60 ° in Japanese Patent Laid-Open No. 2004-355032, and in the range of 80 ° to 86 ° in Japanese Patent Laid-Open No. 2007-101808. It is not easy to control the orientation to give these pretilt angles. Further, it is desired that the optimum cell configuration changes depending on the duty ratio, and that the cell configuration be changed when the duty ratio is changed.

JP 2005-234254 A JP 2004-355032 A JP 2007-101808 A

  An object of the present invention is to provide a vertical alignment type liquid crystal display device with little color shift.

According to one aspect of the present invention,
A pair of substrates formed on the surface with an electrode and a vertical alignment film formed on the electrode and arranged to face each other at a distance d, wherein only one of the vertical alignment films is subjected to an alignment treatment When,
A vertically aligned liquid crystal layer to which a chiral agent is added, disposed between the pair of substrates, wherein 0 <(d / p) ≦ 0.5556 when the chiral pitch is p;
A pair of polarizers arranged in a crossed Nicol arrangement disposed outside the liquid crystal cell;
A vertically twisted liquid crystal display device is provided.

  A vertically aligned liquid crystal display device with little color shift can be provided.

  In the vertical alignment type liquid crystal display device, in order to reduce the light transmittance at the time of OFF as much as possible, it is preferable that the liquid crystal molecules at the time of OFF are aligned as perpendicular to the substrate surface as possible. In order for the liquid crystal molecules to tilt in a certain direction when turned on, it is desirable to apply a pretilt in a certain direction. There is a trade-off relationship between the off-light transmittance and the pretilt. The present inventor considered performing an alignment treatment such as rubbing on only one substrate in order to bring the pretilt angle of the entire liquid crystal layer or the pretilt angle of the liquid crystal molecules at the center of the liquid crystal layer in the thickness direction closer to the normal direction of the substrate. It was.

  In addition, in order to widen the viewing angle range and suppress color shift, it was considered to introduce a twisted alignment by adding a chiral agent to the liquid crystal layer.

  2A and 2B are cross-sectional views schematically illustrating a configuration of a vertically aligned liquid crystal display device according to an embodiment when a voltage is applied and when no voltage is applied. In FIG. 2A showing a state in which no voltage is applied, the first substrate 1 and the second substrate 2 face each other, and a vertically aligned liquid crystal layer 3 including liquid crystal molecules having negative dielectric anisotropy is sandwiched therebetween. The first substrate 1 is formed by forming a transparent electrode 14 on the opposite surface of the transparent substrate 13, applying a polymer vertical alignment film 17 thereon, and without rubbing, and compensating the viewing angle on the outer surface. A plate 12 and a polarizing plate 11 are arranged. The second substrate 2 is formed by forming a transparent electrode 24 on the opposite surface of the transparent substrate 23, covering the surface with a polymer vertical alignment film 25, and rubbing the vertical alignment film 25 in the direction of an arrow 28. is there. A viewing angle compensation plate 22 and a polarizing plate 21 are disposed on the outer surface. The viewing angle compensation plates 12 and 22 compensate for the refractive index anisotropy of the liquid crystal layer. The polarizing plates 11 and 21 are arranged in crossed Nicols.

  Only one substrate is subjected to orientation treatment by rubbing. The rubbing is performed, for example, by the method disclosed in paragraphs 0022-0035, 0039-0072 and FIGS. 2B-9B of JP-A-2005-234254, and the pretilt angle with respect to the substrate surface is 87 ° to 89.7 °, more preferably 88. Set to 4 ° to 89.7 °.

  The liquid crystal layer 3 includes liquid crystal molecules having a negative dielectric anisotropy having a property of being aligned perpendicular to the surfaces of the substrates 1 and 2, and the liquid crystal molecules in contact with the lower substrate by the rubbing treatment 28 are changed to the lower substrate. The pretilt 29 has a constant angle of less than 90 ° from the surface of the surface. In the figure, since the upper substrate is not rubbed, the liquid crystal molecules in contact with the upper substrate are aligned substantially in the normal direction of the substrate. As approaching the lower substrate, the influence of the alignment treatment of the lower substrate appears. Although the figure shows a case where only the vertical alignment film 25 of the lower substrate is rubbed, instead, only the vertical alignment film of the upper substrate may be subjected to alignment treatment.

  A chiral agent is added to the liquid crystal layer 3. By adding the chiral agent, the liquid crystal layer has a property that the tilt direction rotates along the thickness direction. The turning angle depends on the amount of chiral agent added. Hereinafter, the chiral pitch of the liquid crystal material is denoted by p, and the thickness of the liquid crystal layer is denoted by d.

  FIG. 2B schematically shows the alignment of liquid crystal molecules when a voltage is applied. By adding the chiral agent, the tilt direction of the liquid crystal molecules rotates along the thickness direction of the liquid crystal layer. The direction of collapse is affected by the orientation processing direction, chiral angle, applied voltage, and the like. Since the liquid crystal molecules in contact with the substrate have an anchoring force and are not easily tilted, the liquid crystal molecules are most strongly tilted at the central portion in the thickness direction of the liquid crystal layer by voltage application. When only one substrate is subjected to orientation treatment, the chiral angle obtained when a voltage is applied is not necessarily the same as when no voltage is applied, and is generally different.

  The liquid crystal display device shown in FIGS. 2A and 2B can be produced as follows, for example. First, a pair of transparent substrates on which transparent electrodes are formed are prepared. The transparent electrode is etched into a predetermined shape by photoetching. A vertical alignment film is applied by flexographic printing so as to cover the patterned transparent electrode and baked. Alignment treatment by rubbing is performed on the vertical alignment film on one of the substrates. A main sealant with a gap control material added to the opposite surface of one substrate is printed by, for example, screen printing. A gap control agent is sprayed on the opposite surface of the other substrate. A pair of substrates are stacked with the opposing surfaces facing each other, and baked to create an empty cell. A liquid crystal material is injected into the empty cell using vacuum injection, capillary action, or the like. The end sealing material is applied and cured while applying a predetermined pressure so as to obtain a predetermined liquid crystal layer thickness, and the liquid crystal cell is sealed. The liquid crystal cell is washed, and a viewing angle compensation plate and a polarizing plate set in a predetermined direction are attached to the outer surface.

  The performance of liquid crystal cell samples in which the value of d / p was changed to various values was evaluated. As a comparative example, as shown in FIG. 1, a sample was also evaluated in which alignment treatment by rubbing was performed on both substrates and a liquid crystal material to which no chiral agent was added was injected.

  FIGS. 3A and 3B are tables summarizing characteristics of comparative examples and samples obtained by simulation. The in-plane angles of the liquid crystal display device are as follows: rightward and 3 o'clock as viewed from the observer, 0 degrees up, 90 o'clock in the 12 o'clock direction, left direction, 180 o'clock in the 9 o'clock direction, down and 6 o'clock The direction is 270 degrees (-90 degrees).

  In the table, the sample number is written in the leftmost column. The next three columns show d / p, the twist angle in the cell when no voltage is applied, the optimum rubbing direction, and the twist angle during driving. The twist angle [(d / p) × 360] when no voltage is applied is given by d / p. Since the twist angle during driving varies from the twist angle when no voltage is applied, the optimum rubbing direction should be set based on the state during driving. The right side shows the steepness and the maximum transmittance determined from the VT curve indicating the change of the light transmittance (T) with respect to the voltage (V), and the right side shows three characteristics and methods determined from the contrast viewing angle characteristic diagram. The line direction contrast, the viewing angle characteristic determined from the spread, the viewing angle target determined from the left and right targets, and the result of the comprehensive determination are shown in the rightmost column. The comparative example is currently manufactured, and this characteristic is set as a standard ◯, ◎ if the characteristic is improved, and △ if the characteristic is deteriorated. X indicates performance that cannot be used.

  The comparative example is a liquid crystal display device in which an alignment process by rubbing is performed on both a pair of substrates and a liquid crystal material to which no chiral agent is added is injected. Samples 0 to 18 are liquid crystal display devices in which alignment treatment by rubbing is performed on only one substrate and a liquid crystal material added with a chiral agent is injected, and the twist angle when no voltage is applied is changed from 1 ° to 275 °. Thus, d / p was set to -0.0028 to -0.7639.

  In the comparative example, the rubbing direction is 270 degrees. In the one-side rubbing sample, the twist angle at the time of driving with a voltage applied is changed from the twist angle at the time of no voltage application. For this reason, the conditions for the liquid crystal molecules at the center of the thickness of the liquid crystal layer also change. This change affects the optimal rubbing direction.

  The steepness of the VT curve does not change significantly in the comparative examples, samples 0 to 14, the deterioration appears in the sample 15, clearly deteriorates in the samples 16 and 17, and the sample 18 has an unusable performance. . The maximum light transmittance is the same as that of the comparative example in the samples 0 to 14, slightly deteriorated in the sample 15, clearly deteriorated in the samples 16 and 17, and further deteriorated in the sample 18.

  The normal direction contrast is higher in the samples 0 to 14 than in the comparative example except for the sample 7. Sample 7 can be said to be comparable to the comparative example. Sample 15 is slightly worse, samples 16 and 17 are clearly reduced, and sample 18 is significantly worse.

  The viewing angle characteristics seen from the spread tend to be improved in comparison with the comparative examples in samples 0 to 5, clearly improved in samples 6 to 12, sample 13 shows a tendency to improve, and sample 14 can be said to be equivalent to the comparative example. The center of the visual angle starts to shift, and the samples 15, 16, and 17 are widened, but problems also occur, and the problem is large in the sample 18.

  From the viewpoint of symmetry, samples 0 to 11 are equivalent to the comparative example, and deterioration is apparent in samples 13 and 14, sample 15 is clearly deteriorated, and samples 16, 17, and 18 are unusable. It becomes.

  As a result of the comprehensive judgment, the samples 16 to 18 including the unusable characteristics are unusable, and the remaining samples will be considered. Samples 0-7 show a trend of improvement, and samples 8-12 clearly improve. Samples 13 and 14 show an improvement trend, and sample 15 is equivalent to the comparative example.

  Samples 0 to 15 (0 <(absolute value of d / p) ≦ 0.5556) can be used because they can provide an overall characteristic equivalent to or better than that of the comparative example, and samples 0 to 0 in which the overall characteristic shows a tendency to improve. 14 (0 <(absolute value of d / p) ≦ 0.5000) is desirable, and samples 0 to 11 (0 <(absolute value of d / p) ≦ 0.4167) having no deterioration element are more desirable.

  FIG. 4 is a graph and diagram showing the results of a more detailed examination of the characteristics of the sample 10 and the comparative example, which are approximately the center of the d / p range where improved characteristics can be exhibited. C in the fractional code indicates a comparative example, and E indicates a sample example.

  4C1 and 4E1 show VT curves. In the comparative example, as the applied voltage increases, the light transmittance increases and then immediately begins to decrease. In the sample example, the decrease in light transmittance is relatively very slow and does not decrease to near zero. Obviously, the sample example is more stable and easier to use.

  The viewing angle characteristics based on the isocontrast curves in FIGS. 4C2 and 4E2 clearly show that the viewing angle characteristics of the sample example can be improved in all directions compared to the comparative example.

  4C3 and 4E3 show a color shift with respect to a change in viewing angle in the vertical direction, and FIGS. 4C4 and 4E4 show a color shift with respect to a change in viewing angle in the horizontal direction. The color shift of the sample example is clearly reduced compared to the comparative example.

  It can be seen that the viewing angle range can be widened and the color shift can be reduced by rubbing only one substrate and adding the chiral agent, as compared with the comparative example in which both substrates are rubbed and no chiral agent is added.

  Note that the matching between the driving condition and the cell condition also changes.

  In the comparative example, in the case of ¼ duty driving, the cell thickness is 4.0 μm, the negative dielectric anisotropy liquid crystal material has a refractive index anisotropy of 0.091, and a dielectric anisotropy of −5.1. It is preferable to use a liquid crystal material. In the case of ½ duty driving, a liquid crystal material having a cell thickness of 4.0 μm, a negative dielectric anisotropy liquid crystal material having a refractive index anisotropy of 0.092, and a dielectric constant anisotropy of −2.7 is used. Is preferred.

  In the embodiment, in the case of ¼ duty drive, it is preferable to use the same liquid crystal material as in the ¼ duty drive comparison example, to give a leftward twist with a chiral agent, and to set the rubbing direction to −45 degrees. In the case of ½ duty driving, it is preferable to use the same liquid crystal material as in the ¼ duty driving comparative example, to impart a leftward twist with a chiral agent, and to set the rubbing direction to −25 degrees. There is no need to change the material or configuration of the liquid crystal material or the spacer for each driving condition. The possibility of material contamination can be reduced, and an effect can be expected for a reduction in the defect occurrence rate.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, substitutions, improvements, combinations, and the like can be made.

FIG. 1 is a cross-sectional view showing a typical configuration example of a vertical alignment type liquid crystal display device. 2A and 2B are cross-sectional views schematically illustrating a configuration when no voltage is applied and when a voltage is applied, according to an embodiment of the present invention. / FIGS. 3A and 3B are tables summarizing characteristics of comparative examples and samples obtained by simulation. FIG. 4 is a graph and diagram showing the results of examining the characteristics of the sample 10 and the comparative example in more detail.

Explanation of symbols

1, 2 Substrate 3 Liquid crystal layer 11, 21 Polarizing plate 12, 22 Visual compensation plate 13, 23 Transparent substrate 14, 24 Transparent electrode 15, 25 Polymer vertical alignment film 18, 28 Rubbing direction 19, 29 Pretilt angle 17 (rubbing treatment Polymer vertical alignment film

Claims (5)

A pair of substrates formed on the surface with an electrode and a vertical alignment film formed on the electrode and arranged to face each other at a distance d, wherein only one of the vertical alignment films is subjected to an alignment treatment When,
A vertically aligned liquid crystal layer to which a chiral agent is added, disposed between the pair of substrates, wherein 0 <(d / p) ≦ 0.5556 when the chiral pitch is p;
A pair of polarizers arranged in a crossed Nicol arrangement disposed outside the liquid crystal cell;
A vertically twisted alignment liquid crystal display device.   The vertical twist-alignment liquid crystal display device according to claim 1, wherein the d / p satisfies 0 <(d / p) ≦ 0.5000.   The vertical twist alignment liquid crystal display device according to claim 2, wherein the d / p satisfies 0 <(d / p) ≦ 0.4167.   The vertical twist alignment liquid crystal display device according to claim 1, wherein a pretilt angle by the alignment treatment is in a range of 87.0 ° to 89.7 ° with respect to the substrate surface.   The vertical twist alignment liquid crystal display device according to claim 4, wherein a pretilt angle by the alignment treatment is in a range of 88.4 ° to 89.7 ° with respect to the substrate surface.

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