JP2869450B2 - Liquid crystal display device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a liquid crystal display device, and more particularly, to a super twisted nematic (STN) type liquid crystal display device having a liquid crystal color compensating plate.

(Problems to be Solved by the Related Art and the Invention) A display mode of a liquid crystal display element which has been mainly used in the past is called a twisted nematic (TN) type, in which liquid crystal molecules are rotated by about 90 ° between a pair of upper and lower substrates. It has a twisted structure and utilizes the rotation of the polarization plane by the liquid crystal and the disappearance of the effect when a voltage is applied. This display method has an advantage that since it is a black-and-white display, it has an excellent shutter effect, and multicolor display can be performed relatively easily by providing a color filter for each pixel. However, there is a disadvantage that it is difficult to perform high-time-division driving because of poor performance. In a large-capacity display, there are problems such as a decrease in contrast and a narrow viewing angle.

In order to improve the sharpness of the voltage-transmittance characteristics, a method has been proposed in which the twist angle of the liquid crystal molecules is increased and the polarization axis of the polarizing plate is shifted from the alignment direction of the liquid crystal to utilize the birefringence effect of the liquid crystal. , SBE (super twisted birefringen
ce effect) or STN (super twisted nematic) mode. This method has an excellent threshold characteristic, so that the contrast is less reduced even in time-division driving, and has an excellent characteristic that the viewing angle is wide. Then, it was difficult to colorize.

Recently, in order to reduce the coloring phenomenon of the STN mode liquid crystal display element, two liquid crystal cells whose liquid crystal layers have opposite twist directions are stacked, one of which is used for driving and the other is used as a compensator. A two-layer STN-type liquid crystal display device has been developed which performs black and white display by compensating for coloring due to birefringence. However, this two-layer system is a black-and-white display when viewed from the front. However, when viewed from an oblique direction, coloring occurs, and since two liquid crystal cells are used, the element becomes thicker and heavier. There is a problem of bad.

These problems can be improved by replacing the compensation cell with a birefringent polymer film (phase plate type monochrome STN liquid crystal display device), but this phase plate method cannot provide sufficient contrast. In addition, there is a problem that the viewing angle becomes narrower.

On the other hand, there has been proposed a method of using a twisted-aligned liquid crystal molecule compensator in place of the compensation cell in the two-layer system. This method uses a coated and aligned liquid crystalline polymer as a main component of the compensator, and the liquid crystalline polymer can be fixed in an alignment state in a liquid crystal state by cooling below a glass transition point. . That is, after a liquid crystalline polymer having a glass transition point of room temperature or higher is twisted in a liquid crystal state, and then cooled, a compensating performance equivalent to that of a compensating liquid crystal cell can be exhibited. If the self-holding property in the solid phase is used, only one substrate is needed to hold the liquid crystalline polymer, the compensator can be made thinner, and the contrast is as excellent as the two-layer method. However, the method using such a liquid crystal polymer having a fixed orientation as a compensator has the following insufficient points in terms of temperature characteristics. In general, in a two-layer system or a system using a liquid crystal polymer as a compensator, the retardation of the liquid crystal layer to be operated and the retarder of the compensator are set to be substantially equal, and almost perfect polarization compensation is performed. However, the refractive index anisotropy of the liquid crystal greatly depends on the temperature, and the retardation decreases as the temperature increases. In the two-layer system, since the liquid crystal is used for the compensation layer, the retardation of the compensation plate is similarly reduced, and the compensation performance does not depend much on the temperature. However,
In a liquid crystalline polymer with fixed orientation, there is almost no temperature change of the refractive index anisotropy in the solid layer, so that the temperature change, especially at high temperatures, deviates from the perfect compensation condition, and the contrast tends to decrease. Was. This limits the use of the liquid crystal display device in a severe environment such as outdoors.

The present invention has been made in view of the above-described problems of the related art, and an object of the present invention is to provide a liquid crystal display device that can perform high-contrast display over a wide temperature range, is thin, and has a wide viewing angle. It is in.

According to the present invention, a pair of substrates having electrodes and a liquid crystal layer which is sandwiched between the substrates and has a positive dielectric anisotropy and is substantially horizontally aligned when no voltage is applied are provided. A liquid crystal cell, a polarizing plate disposed outside the substrate, and a compensator having a liquid crystal polymer layer, which is provided between the liquid crystal layer and the polarizing plate and is substantially horizontally aligned and fixed in alignment, as a main component. A liquid crystal display device characterized in that the ratio R _C / R _L between the retardation R _C of the compensator and the retardation _RL of the liquid crystal layer is 0.7 or more and 0.95 or less.

 Hereinafter, the configuration of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a cross-sectional view showing one configuration example of a liquid crystal display device according to the present invention. In this liquid crystal display device, a first light-transmitting substrate 11 and a second light-transmitting substrate 21 are separated from each other and face each other. Liquid crystal is sealed in a space formed by the substrates 11, 21 and the outer peripheral seal 14 to form a liquid crystal layer 15, and a liquid crystal cell 16 is formed. Liquid crystal layer on the inner surface of substrates 11 and 21
Transparent electrodes 12 and 22 for applying a voltage to 15 and alignment films 13 and 23 for aligning the liquid crystal in a certain direction are formed. 17, 27 are polarizing plates. Between the liquid crystal cell 16 and the polarizing plate 27, a compensator 30 having a liquid crystalline polymer layer 35 as a main functional component
Is arranged. Reference numeral 31 denotes a substrate on which the liquid crystalline polymer layer 35 is formed, and a highly transparent material such as glass or plastic is used.

In the liquid crystal layer 15, the liquid crystal is composed of a nematic or cholesteric liquid crystal having a positive dielectric anisotropy.
In the state where no voltage is applied due to 13,23, the alignment is substantially parallel to the substrate surface. The liquid crystal preferably has a twisted orientation between the upper and lower substrates 11, 21 with the helical axis directed perpendicular to the substrate surface, and the twist angle is preferably 160 ° to 360 °. When the torsion angle is small, the sharpness of the voltage-transmittance characteristic decreases, and the time-division driving characteristic decreases. As shown in FIG. 2, the twist angle ω _L of the liquid crystal in the liquid crystal cell 16 depends on the alignment processing direction (R ₁ ) of the alignment film 13 of the lower substrate 11, the alignment processing direction (R ₂ ) of the alignment film 23 of the upper substrate 21, and The control can be easily performed by controlling the pitch of the liquid crystal and the thickness of the liquid crystal layer 15. Retardation R _L of the liquid crystal layer 15 is defined by the product delta _n of thickness d _L of the refractive index anisotropy [Delta] n _L and the liquid crystal layer 15 of liquid crystal.
d _L is 0.4-1.5 μm to obtain good contrast
, And particularly preferably in the range of 0.6 to 1.0 μm.

FIG. 3a shows the temperature change of the refractive index anisotropy of a general liquid crystal. The refractive index anisotropy decreases almost linearly as the temperature rises, and sharply decreases to zero near the transition temperature of the nematic-isotropic liquid. The product is defined by the product Δn _L · d _L of the thickness d _L of the liquid crystal layer 15. Since d _L can be considered to hardly change with temperature, the retardation R _L of the liquid crystal layer 15 also shows the same change as in FIG. 3A. Although it depends on the liquid crystal used, compared to the retardation at 20 ° C
At 40 ° C it is about 10% smaller.

The compensator 30 basically includes a liquid crystal polymer layer 35 having a fixed orientation, and a light transmitting substrate 31 for holding the liquid crystal polymer layer 35. By using a liquid crystalline polymer that exhibits a solid phase at room temperature and exhibits liquid crystallinity at a temperature above room temperature,
The orientation at room temperature is stabilized, and the counter substrate can be omitted. The surface of the compensator substrate 31 that is in contact with the liquid crystal polymer layer 35 has been subjected to an alignment treatment for arranging the liquid crystal polymer in parallel to the substrate 31 and in a specific direction,
On the substrate 31, the liquid crystalline polymer is oriented in the direction of the alignment treatment (R ₃ ). R ₃ must be substantially orthogonal to R _2, and if this condition is not satisfied, the compensation effect will be reduced, resulting in reduced contrast and coloring.
In terms of specific angles, it is necessary that the intersection angle between the two is in the range of 60 ° to 120 °, and 70 ° to 110 °
More preferably, it is within the range.

As the liquid crystalline polymer used in the present invention, those having a cholesteric phase or a nematic phase in the liquid crystal phase are particularly preferably used, and the alignment of the liquid crystalline polymer is controlled at a temperature at which these phases are exhibited. In order to obtain a more complete compensation effect, it is preferable that the molecules in the liquid crystalline polymer layer 35 are twisted in the opposite direction to the molecules of the liquid crystal layer 15 and at an angle substantially equal to the twist angle of the liquid crystal layer 15. The twist angle ω _C of the liquid crystalline polymer can be arbitrarily set by controlling the thickness of the liquid crystalline polymer layer 35 and the natural pitch.

FIG. 3 (b) shows the temperature dependence of the refractive index anisotropy of a liquid crystalline polymer having an orientation fixed by taking as an example an orientation-fixed film of a polyester-based main chain type liquid crystal polymer having a glass transition temperature of about 100 ° C. It shows the nature. Almost no temperature change of the refractive index anisotropy is observed in the temperature range up to 70 ° C. The retardation R _C of the compensator 30 is defined by the product Δn _C · d _C of the refractive index anisotropy Δn _C of the liquid crystalline polymer and the thickness d _C of the liquid crystalline polymer layer 35.
Since d _C hardly changes with temperature, the retardation R _C of the compensator 30 also shows the same change as in FIG. 3B.

The greatest feature of the present invention is that the ratio R _C / R _L of the retardation of the liquid crystal layer 15 to the liquid crystal polymer layer 35 (compensator 30) is set to a specific range in order to minimize the temperature change of the contrast. It is in. Hereinafter, this point will be described based on specific data.

FIG. 4B shows the twist angle ω ₁ of the liquid crystal layer = 220 °, the twist angle ω _Z of the liquid crystalline polymer layer = −220 ° (the left-handed spiral is defined as positive), R _C = 0.82, R _L = The graph shows the temperature dependence of contrast when operated by time-division driving with a duty of 1/200 with 0.845 ( _RC / _RL = 0.97). Where R ₃
The intersection angle δ of R ₂ and 90 °, the upper and lower polarizers have transmitting axes of and lower polarizing plates orthogonally is arranged at an angle of R ₁ and 45 °. In this arrangement, when complete compensation is performed, light is completely shut off when a non-selection voltage is applied, and black display is obtained, and white display is obtained when a voltage is applied. This liquid crystal cell has good contrast at room temperature,
If the temperature is higher than ° C., light leakage occurs when a non-selection voltage is applied, and the contrast is greatly reduced. On the other hand, c, d, and e in FIG. 4 are the cases where the retardation ratios R _C / R _L are 0.95, 0.9, and 0.8, respectively. In these cases, the contrast at 30 ° C. or higher is greatly improved. When R _C / R _L is such a small value, when the voltage is not applied, the compensation condition is deviated from the compensation condition. Therefore, the liquid crystal molecules are slightly moved by applying the non-selection voltage, and the effective retardation is reduced. If it is reduced, almost perfect compensation is possible. If R _C / R _L is too small, the non-pixel portion becomes colored and the operating voltage increases, which is not preferable. Therefore, _RC / _RL needs to be in the range of 0.7 to 0.95, and more preferably in the range of 0.8 to 0.92.

As the liquid crystal cell or the liquid crystal polymer substrate, light-transmitting glass, plastic, or the like can be used. A liquid crystal polymer substrate can also serve as a liquid crystal cell or a polarizing plate. As the plastic substrate, an optically isotropic substrate such as polysulfone, polycarbonate, or polyarylate is particularly preferably used. In addition, polyethylene terephthalate, polyether ether ketone and the like can be used.

The alignment treatment on each substrate of the liquid crystal display device of the present invention includes:
The liquid crystal molecules are aligned substantially horizontally when no voltage is applied, and the liquid crystal molecules are aligned along the alignment processing direction. In this case, “substantially horizontal” with respect to the alignment of the liquid crystal molecules means that the tilt angle of the liquid crystal molecules with respect to the substrate is in a range of approximately 0 ° to 30 °. The orientation can be controlled by oblique vapor deposition or rubbing with a cotton cloth after forming an inorganic or organic film on the substrate. Specifically, those obtained by rubbing a polymer coating such as polyamide or polyimide, or those obtained by obliquely depositing SiO, MgO, MgF ₂ or the like can be suitably used.

The liquid crystalline polymer can be oriented at a temperature exhibiting a high temperature liquid crystal phase and then rapidly cooled to around room temperature to obtain a film oriented in a solid phase. By utilizing this, the liquid crystalline polymer layer can be formed on the substrate in such a manner that only one side is in contact with the substrate as in the present configuration example, and two layers can be formed as in the two-layer system.
There is no need to form a cell using a single substrate. Therefore, the compensator can be made thin and lightweight. For such a reason, the temperature at which the liquid crystalline polymer exhibits a liquid crystal phase is particularly preferably higher than room temperature, and particularly preferably 60 ° C. or higher. The structure of the liquid crystalline polymer used in the present invention is not particularly limited. Examples of the liquid crystalline polymer used in the present invention include a generally known acrylic or polysiloxane-based side chain type liquid crystalline polymer in which a mesogen is introduced into a side chain, and biphenyl or phenyl benzoate in a main chain. A main chain type liquid crystalline polymer containing a mesogen such as an ester is typical. Examples of the main structure are shown below. A main chain type liquid crystalline polymer having the following structure and having a liquid crystal residue in the main chain of polyester, polyesteramide, polycarbonate, polyether, or the like: M ¹ -X ¹ A ^1- X ² X ¹ , X ² : -COO-, -CONH-, -OCO-, -O-, etc. (However, Ph is a phenylene group, Or And * represents a carbon atom and n represents an integer of 0 to 18. ) Alternatively, a side chain type liquid crystalline polymer having a liquid crystalline residue in a side chain such as a vinyl polymer or polysiloxane having the following structure: ^{M 2: -Ph-Ph-R} 3, -O-Ph-Ph-R 3, -Ph-COO-Ph
^{-R 3, -O-Ph-COO} -Ph-R 3, -Ph-COO-Ph-R 3, -
O-Ph-OCO-Ph- R 3, -Ph-Ph-COO-Ph-R 3, -O-
Ph-Ph-COO-Ph- R 3, -Ph-COO-Ph-Ph-R 3, -O-
Ph-COO-Ph-Ph- R 3, -Ph-Ph-OCO-Ph-R 3, -O-
Ph-OCO-Ph-Ph- R 3, -Ph-OCO-Ph-Ph-R 3, -O-
Ph-OCO-Ph-Ph- R 3 , etc. (wherein, R ³ is an alkyl group, an alkoxy group, a halogen atom, a nitro group or a cyano group, n represents. An integer of 0 to 18), and the like.

In order to orient the liquid crystalline polymer on the substrate, a method of applying shear stress to the liquid crystalline polymer in a liquid crystal state, applying an aligning agent such as polyimide to the substrate, forming a liquid crystalline polymer film, And a heat treatment method. The latter method is preferably employed in terms of uniformity. The liquid crystalline polymer preferably has a nematic phase or a cholesteric phase in its phase series.

As described above, the present invention has been described based on the configuration example shown in FIG. 1, but the present invention is not limited to this configuration example, and various modifications and changes are possible. For example, the liquid crystal polymer layer may be two or more layers, the substrate for the liquid crystal polymer layer may be omitted, or the compensation substrate may be adjacent to the polarizing plate and the liquid crystal polymer layer may be adjacent to the liquid crystal cell. Is also possible. The same effect can be obtained even if the vertical relationship in FIG. 1 is exchanged. Further, it is also possible to provide a reflection plate and use it as a reflection type liquid crystal display element.

(Examples) Next, the present invention will be described in more detail with reference to Examples.
The present invention is not limited to only these examples.

Example 1 A polyimide varnish PIQ manufactured by Hitachi Chemical Co., Ltd. was applied to a thickness of about 1000 mm on a glass substrate by spin coating, and then applied to a glass substrate.
It was baked at ℃ to form a polyimide film. Next, the polyimide film was rubbed in one direction with a Tetron flocking cloth to perform a rubbing treatment.

Next, a nematic liquid crystal polysiloxane liquid crystal polymer having a glass transition point of 70 ° C. represented by the following formula (A) and a polysiloxane liquid crystal polymer having an optically active group represented by the following formula (B) are used. It was dissolved in a phenol / tetrachloroethane mixed solvent (weight ratio: 50:50) to a concentration of 25% by weight. The ratio between the polymers (A) and (B) was 3: 1 (weight ratio).

This solution is applied on the alignment film by spin coating, then dried at 70 ° C., heat-treated at 170 ° C. for 30 minutes at which the polymer (A) exhibits a nematic phase, then rapidly cooled to room temperature and twisted. A compensator having an angle of 220 ° C. (right twist) and a retardation _RC of 0.82 μm was prepared.

On the other hand, a polyimide varnish LQ1800 made by Hitachi Chemical Co., Ltd.
And then fired at 270 ° C. to form a polyimide film. Then, a rubbing treatment was performed by rubbing the polyimide film in one direction with a Tetron flocking cloth. The rubbing direction of this substrate with the substrate subjected to the same treatment is
The substrates were bonded to each other via a spacer having a diameter of 6.8 μm so as to form an angle of 220 °, and a mixed liquid crystal obtained by adding S811 as an optically active substance to a nematic liquid crystal ZLI2293 made of Merck was injected into a gap between the substrates. The obtained liquid crystal cell has a twist angle of 220 °.
(Left twist), and the retardation _RL was 0.91 μm.

A liquid crystal display device was manufactured by stacking the thus obtained compensator and a liquid crystal cell, and sandwiching the upper and lower portions thereof with polarizing plates. The retardation ratio R _C / R _L between the compensator of this liquid crystal display element and the liquid crystal cell was 0.9. Note that the angle arrangement is δ = 90 °,
α = β = 45 °.

When this liquid crystal display element was driven by time-division driving with a duty of 1/200, excellent black-and-white display was performed as shown in FIG. Also, when this liquid crystal display element was heated in a thermostat, a contrast of 15: 1 or more was obtained even at 40 ° C.
It was confirmed that excellent display performance was obtained even at high temperatures.

Example 2 A liquid crystal cell having a retardation R _L = 1.0 μm was prepared in the same manner as in Example 1 except that a spacer having a diameter of 7.3 μm was used. And Example 1
And the liquid crystal cell were overlapped with each other, and the upper and lower portions were sandwiched between polarizing plates to produce a liquid crystal display element. The retardation ratio R _C / R _L of this liquid crystal display device was 0.8. The angle arrangement was δ = 90 ° and α = β = 45 °.

When this liquid crystal display element was driven by time-division driving with a duty of 1/200, excellent black-and-white display could be performed as in Example 1. Further, when this liquid crystal display element was heated in a constant temperature bath, a contrast of 20: 1 or more was obtained even at 40 ° C., and it was confirmed that the display performance was excellent even at a high temperature.

Example 3 A liquid crystal cell having a retardation R _L = 0.96 μm was prepared in the same manner as in Example 1 except that a 7.7 μm diameter spacer was used. And Example 1
And the liquid crystal cell were overlapped with each other, and the upper and lower portions were sandwiched between polarizing plates to produce a liquid crystal display element. The retardation ratio _RC / _RL of this liquid crystal display device was 0.85. The angle arrangement was δ = 90 ° and α = β = 45 °.

When this liquid crystal display element was driven by time-division driving with a duty of 1/200, excellent black-and-white display was performed as in the above-described embodiments. Also, when this liquid crystal display element was heated in a constant temperature bath, a contrast of 20: 1 or more was obtained even at 40 ° C.,
It was confirmed that excellent display performance was obtained even at high temperatures.

Comparative Example A liquid crystal cell having a retardation R _L = 0.85 μm was prepared in the same manner as in Example 1 except that a spacer having a diameter of 6.4 μm was used. And Example 1
And the liquid crystal cell were overlapped with each other, and the upper and lower portions were sandwiched between polarizing plates to produce a liquid crystal display element. The retardation ratio R _C / R _L of this liquid crystal display device was 0.97. The angle arrangement was δ = 90 ° and α = β = 45 °.

When this liquid crystal display element was driven by time-division driving with a duty of 1/200, excellent black-and-white display was performed.However, when this liquid crystal display element was heated in a constant temperature bath, the contrast was 3: 1 at 40 ° C. It has dropped.

(Effect of the Invention) In the liquid crystal display device of the present invention, the liquid crystal molecular layer is used for the compensator, and the retardation ratio between the liquid crystal layer and the compensator is in a specific range. The present invention provides a liquid crystal display element which is suitable for use in a severe environment such as an outdoor environment without deterioration. Further, since the compensator is thin, it can be made thinner and lighter at the same time. By utilizing this feature, it can be preferably applied as a liquid crystal display element for portable information equipment. Further, since the liquid crystal polymer layer having a twisted orientation is used as a compensating plate, there is an ideal compensating effect, and therefore, excellent display performance such as a wide viewing angle and high contrast can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing an example of a configuration of a liquid crystal display element by economization, FIG. 2 is a view showing an angular relationship between elements constituting a liquid crystal element of the present invention, and FIG. FIG. 4 is a diagram showing a temperature change of contrast, and FIG. 5 is a diagram showing a display characteristic of a liquid crystal display device according to an embodiment of the present invention. 11,21 Substrate 12,22 Transparent electrode 13,33 Alignment film 14 Peripheral seal 15 Liquid crystal layer 16 Liquid crystal cell 17,27 Polarizer 30 Compensator 31 Substrate 35: Liquid crystalline polymer layer

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-149624 (JP, A) JP-A-64-519 (JP, A) JP-A-2-28618 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) G02F 1/133 500 G02F 1/1335 510

Claims (3)

(57) [Claims] 1. A liquid crystal cell comprising a pair of substrates having electrodes, a liquid crystal layer sandwiched between the substrates and having a positive dielectric anisotropy and being substantially horizontally aligned when no voltage is applied, and a polarized light disposed outside the substrates. In a liquid crystal display element comprising a liquid crystal polymer layer provided between a liquid crystal layer and a polarizing plate, and a liquid crystal polymer layer that is substantially horizontally aligned and fixed in alignment, the retardation R of the compensator _C and liquid crystal layer retardation
The liquid crystal display element characterized by the ratio R _C / R _L and R _L was 0.7 to 0.95 at room temperature. 2. The liquid crystal display device according to claim 1, wherein the liquid crystal layer has a structure twisted between 160 ° C. and 360 ° C. in the thickness direction between the upper and lower substrates when no voltage is applied. 3. The liquid crystal display device according to claim 2, wherein the liquid crystalline polymer layer has a structure in which the liquid crystal layer is twisted by the same twist angle in the opposite direction to the liquid crystal layer.