JP2001142071A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain both of fast response and high display quality of a liquid crystal display device. SOLUTION: The light from a light emitting element 100 is linearly polarized by a first linear polarizing plate 200. In a display liquid crystal cell 300, the light passing through the first linearly polarizing plate 200 is transmitted through a part where a specified voltage is applied, while in a part where a specified voltage is applied, the light is elliptically polarized with different directions of the major axes of the elliptically polarized light depending on the wavelength. A compensation liquid crystal cell 400 is formed into larger thickness than the display liquid crystal cell 300. In the compensation liquid crystal cell 400, the polarized light from the display liquid crystal cell 300 is elliptically polarized with substantially the same direction of the major axes of the elliptically polarized light without depending on the wavelength, while the elliptically polarized light from the display liquid crystal cell 300 is linearly polarized. A second linearly polarizing plate 500 has the polarizing axis in the same direction as the direction of the major axis of the elliptically polarized light in the light from the compensation liquid crystal cell 400 or as the polarizing direction of the linearly polarized light from the compensation liquid crystal cell 400.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device having two liquid crystal layers.

[0002]

2. Description of the Related Art A liquid crystal display device is used for a display of a television or a computer. Some liquid crystal display devices have an STN (Super-Twisted Nemati) in which the directions of liquid crystal molecules are twisted between the substrates by 180 ° to 360 °.
c) There is an STN type liquid crystal display device formed from liquid crystal.

As shown in FIG. 4, for example, an STN-type liquid crystal display device includes a light source 10, a first linear polarizing plate 20 provided on the light source 10, a pair of first linear polarizing plates 20 provided on the first linear polarizing plate 20. Display liquid crystal cell 3 with STN liquid crystal sealed between substrates
0 and a second linear polarizing plate 40 provided on the display liquid crystal cell 30.

[0004] The light source 10 emits light for displaying an image, and the light emitted from the light source 10 is applied to the first linear polarizer 2.
The light is linearly polarized by transmitting 0. And the first
The linearly polarized light transmitted through the linear polarizing plate 20 enters the display liquid crystal cell 30. The STN liquid crystal of the display liquid crystal cell 30 makes the linearly polarized light elliptically polarized when a predetermined voltage is not applied. On the other hand, when a predetermined voltage is applied, the STN liquid crystal transmits linearly polarized light as it is. Therefore, by controlling the voltage applied to the display liquid crystal cell 30, the polarization of the light transmitted through the display liquid crystal cell 30 is controlled.

[0005] The second linear polarizing plate 40 is connected to the display liquid crystal cell 3.
When a predetermined voltage is applied to 0, the polarization direction of the linearly polarized light transmitted through the display liquid crystal cell 30 and the polarization axis (transmission axis) of the second linear polarizer 40 are at a predetermined angle (transmission axis). (For example, 90 °). Thus, by controlling the voltage applied to the display liquid crystal cell 30, light can be emitted from the second linear polarizing plate 40 or the second linear polarizing plate 40 can absorb light. That is, display and non-display of an image are controlled by controlling the voltage application to the display liquid crystal cell 30.

However, the direction of the principal axis of the elliptically polarized light included in the light transmitted through the display liquid crystal cell 30 differs for each wavelength.
For this reason, the screen displayed by the above-described STN liquid crystal display device may be colored.

Specifically, for example, a predetermined voltage is applied to the display liquid crystal cell 30, and the polarization direction of the linearly polarized light transmitted through the display liquid crystal cell 30 is the direction of the polarization axis of the second linear polarizing plate 40. When the light is orthogonal, the light transmitted through the display liquid crystal cell 30 is absorbed by the second linear polarizing plate 40. That is,
The screen is displayed in black. On the other hand, when a predetermined voltage is not applied to the display liquid crystal cell 30, the light transmitted through the display liquid crystal cell 30 is elliptically polarized in a direction in which the main axis is different for each wavelength. For this reason, of the light transmitted through the display liquid crystal cell 30, light having a component (wavelength) whose principal axis direction is closer to the polarization axis direction of the second linear polarizing plate 40 is closer to the second linear polarizing plate 40.
Through. As a result, the screen is displayed in, for example, yellow instead of white.

Further, for example, a predetermined voltage is applied to the display liquid crystal cell 30, and the polarization direction of the linearly polarized light transmitted through the display liquid crystal cell 30 is the same as the direction of the polarization axis of the second linear polarizing plate 40. In some cases, almost all of the light transmitted through the display liquid crystal cell 30 is transmitted through the second linear polarizing plate 40.
That is, the screen is displayed by white light. On the other hand, when a predetermined voltage is not applied to the display liquid crystal cell 30, the light transmitted through the display liquid crystal cell 30 is elliptically polarized in different directions for each wavelength. For this reason, some light transmits through the second linear polarizing plate 40. As a result, the screen is not displayed in black, but is displayed in blue, for example.

In order to prevent the display screen from being colored as described above, a liquid crystal is provided between the first linear polarizing plate 20 and the second linear polarizing plate 40 on the light incident side or the light emitting side of the display liquid crystal cell 30. There is a two-layer STN type liquid crystal display device provided with a cell (compensation liquid crystal cell). The compensation liquid crystal cell is formed by sealing STN liquid crystal between a pair of substrates, similarly to the display liquid crystal cell 30.

In order to prevent the emitted light from being colored as described above, the product (retardation value; Δt) of the birefringence Δn of the STN liquid crystal and the thickness (cell gap) d of the liquid crystal is required.
nxd) must match between the display liquid crystal cell and the compensation liquid crystal cell. However, when driving a liquid crystal display device employing the multiplex driving method, crosstalk or the like may occur, and a voltage may be applied to liquid crystal cells in a non-selected portion. For this reason, there is a problem that light is emitted also from a non-selected portion.

In order to avoid the above problems, in a liquid crystal display device employing a multiplex drive system, the retardation value of the compensating liquid crystal cell is set smaller than that of the display liquid crystal cell. Specifically, in the two-layer STN liquid crystal display device, STN liquid crystals having substantially the same birefringence are used, and the cell gap of the compensation liquid crystal cell is set smaller than that of the display liquid crystal cell. This prevents light from being emitted from unselected portions.

[0012]

However, in recent years, a liquid crystal cell having a small cell gap has been manufactured in order to quickly switch between displaying and non-displaying an image by a liquid crystal display device, that is, to realize a high-speed response of the liquid crystal. It is supposed to be.

As described above, a display liquid crystal cell and a compensation liquid crystal cell having a small cell gap are also adopted in the above-mentioned two-layer STN type liquid crystal display device. In particular, display and non-display of an image are selected by controlling the application of voltage to the liquid crystal cell for display. Therefore, high-speed response can be realized by reducing the cell gap of the liquid crystal cell for display.

However, as described above, the cell gap of the compensation liquid crystal cell is set smaller than the cell gap of the display liquid crystal cell. Specifically, for example, when the cell gap of the display liquid crystal cell is about 4.0 μm, the cell gap of the compensation liquid crystal cell is set to about 3.0 μm. Therefore, if the display liquid crystal cell is formed sufficiently thin in order to realize a high-speed response, the compensation liquid crystal cell is too thin, and it is difficult to form the compensation liquid crystal cell to have a uniform thickness. As a result, there is a problem that the image is likely to be disturbed and the display quality is deteriorated.

On the other hand, if the cell gap of the compensating liquid crystal cell is set to be large so that the compensating liquid crystal cell can be formed with a uniform thickness, the display liquid crystal cell becomes too thick to realize a high-speed response. That is, if the cell gap of the compensating liquid crystal cell is set smaller than the cell gap of the display liquid crystal cell, it is impossible to simultaneously achieve both high-speed response and high display quality of the liquid crystal. Further, the yield of a manufactured liquid crystal display device is reduced by forming the liquid crystal cell thinner to realize a high-speed response.

Accordingly, an object of the present invention is to provide a liquid crystal display device capable of simultaneously realizing high-speed response of liquid crystal and high display quality. Another object of the present invention is to provide a liquid crystal display device having a high yield.

[0017]

In order to achieve the above object, a liquid crystal display device according to the present invention comprises a first linear polarizing plate and a second linear polarizing plate having a layer thickness d ₁ and a birefringence. Rate Δn ₁
And a compensating liquid crystal cell having a second liquid crystal layer having a birefringence Δn ₂ with a layer thickness of d ₂ , wherein a retardation value Δn ₂ · d ₂ is a retardation value. Δn ₁ · d ₁ and the liquid crystal layer thickness d
₁ is characterized in that the smaller than the liquid crystal layer thickness d _2.

According to the present invention, the yield of the compensation liquid crystal cell is high because the influence of the thickness unevenness is small. Further, the response at a low temperature is good, and the contrast ratio at a high temperature can be increased. For this reason, disturbance of an image is less likely to occur, and high display quality can be realized.

The birefringence Δn ₂ of the second liquid crystal layer may be smaller than the birefringence Δn ₁ of the first liquid crystal layer. The nematic phase-isotropic liquid phase transition temperature of the second liquid crystal layer may be higher than the nematic phase-isotropic liquid phase transition temperature of the first liquid crystal layer. The first liquid crystal layer and the second liquid crystal layer may be formed from a super twisted nematic (STN) liquid crystal.

[0020]

Next, a liquid crystal display device according to an embodiment of the present invention will be described with reference to the drawings. The liquid crystal display device according to the embodiment of the present invention has one liquid crystal molecule.
STN (Su) which is oriented to twist from 80 ° to 360 °
per-Twisted Nematic) An STN type liquid crystal display using a liquid crystal.

FIG. 1 is a sectional view showing the structure of the liquid crystal display device. As shown in FIG. 1, the liquid crystal display device includes a light emitting element 100, a first linear polarizing plate 200, a display liquid crystal cell 300, a compensating liquid crystal cell 400, a second linear polarizing plate 500, and a driving circuit 600. And is composed of

The light emitting element 100 is a light source for displaying an image, and is connected to the drive circuit 600. The light emitting element 100 is, for example, an organic EL (electroluminescence) element that emits light when a predetermined voltage is applied. The first linear polarizing plate 200 is a light emitting device 100.
Linearly polarized light emitted from. That is, the first linearly polarizing plate 200 includes, among the light emitted from the light emitting element 100,
Light of a component (wavelength) having a vibration surface in a predetermined direction is transmitted.

The display liquid crystal cell 300 is provided on the first linear polarizing plate 200, and for example, as shown in FIG.
A first transparent substrate 301, a second transparent substrate 302, a first electrode 303, a second electrode 304, a first alignment film 306 provided on the first electrode 303, and provided on the second electrode 304. A second alignment film 307 and an STN liquid crystal layer 305 interposed between the two alignment films 306 and 307. The first transparent substrate 301 and the second transparent substrate 302 are formed of an optically transparent material, for example, glass or the like, and are provided to face each other. In addition, both alignment films 30
Rubbing treatments 6 and 307 are performed so that the liquid crystal molecules of the STN liquid crystal layer 305 are arranged in a predetermined direction.

The first electrode 303 is optically transparent,
A plurality of first transparent substrates 301 are provided at predetermined intervals so as to be parallel to each other. Also, the first electrode 303
It is connected to the drive circuit 600. The second electrode 304 is
It is optically transparent and is provided on the second transparent substrate 302 at a plurality of predetermined intervals so as to be substantially orthogonal to the first electrode 303. The second electrode 304 is connected to the driving circuit 600
It is connected to the.

The STN liquid crystal layer 305 is formed on the first transparent substrate 30.
It is sealed between the first and second transparent substrates 302. STN liquid crystal layer 305 in a portion where a predetermined voltage is not applied
Turns elliptically polarized light of linearly polarized light transmitted through the first linearly polarizing plate 200. The direction of the main axis of the elliptically polarized light included in each component of the elliptically polarized light differs depending on the wavelength. on the other hand,
STN liquid crystal layer 305 in a portion where a predetermined voltage is applied
Causes the linearly polarized light transmitted through the first linear polarizing plate 200 to be emitted as it is. That is, the polarization direction of the linearly polarized light is preserved.

The compensating liquid crystal cell 400 is a liquid crystal cell for compensating the optical phase difference of the light transmitted through the display liquid crystal cell 300. That is, the compensation liquid crystal cell 400 is provided on the display liquid crystal cell 300 in order to prevent coloring of light emitted from the liquid crystal display device and to prevent light from being emitted from a non-selected portion due to crosstalk or the like. Have been.

The compensation liquid crystal cell 400 is, for example, shown in FIG.
It is configured as shown in FIG. Specifically, the compensation liquid crystal cell 400 includes a first transparent substrate 401, a second transparent substrate 402, a first alignment film 403, and a second alignment film 404.
And an STN liquid crystal layer 405. The compensation liquid crystal cell 400 (or the STN liquid crystal layer 405)
Of the liquid crystal cell 300 for display (cell gap d ₂ )
(Or STN liquid crystal layer 305) is thicker than cell gap _{d 1} of.

First transparent substrate 401 and second transparent substrate 40
Numerals 2 are made of an optically transparent material, for example, glass, and provided so as to face each other. The alignment films 403 and 404 are rubbed so that the liquid crystal molecules of the STN liquid crystal layer 405 are arranged in a predetermined direction.

The STN liquid crystal layer 405 is formed on the first transparent substrate 40.
It is sealed between the first and second transparent substrates 402. Ma
The molecules of the STN liquid crystal layer 405 are
Are oriented so as to be twisted in the opposite direction to the molecule. In addition,
The birefringence of the STN liquid crystal changes according to the applied voltage.
Therefore, the birefringence Δn of the STN liquid crystal layer 405₂Is displayed
Birefringence of STN Liquid Crystal Layer 305 of Liquid Crystal Cell 300
Rate Δn₁It is set smaller than. In addition, compensation liquid
Value of crystal cell 400 (birefringence x liquid crystal cell)
Is the retardation value of the display liquid crystal cell 300.
Smaller than Δn₁× d₁> Δn₂× d₂Becomes
It is set as follows. Thus, the compensation liquid crystal cell 40
Δn of 0_Two・ D_TwoOf the liquid crystal cell for display₁・ D₁
Of the display liquid crystal cell 300
In the initial state where no voltage is applied between the opposing electrodes,
Through both cells 300 and 400 to second linear polarizer 500
Although there is a small amount of incident light, the display liquid crystal cell 300
When a voltage is applied between the electrodes, the electric field between the electrodes causes
Δn of the display liquid crystal cell 300₁・ D₁Becomes smaller,
Δn of the display liquid crystal cell 300₁・ D ₁Is the compensation LCD
400n_Two・ D_TwoWhen it is almost equal to
Light incident on the two linear polarizers 500 is almost eliminated.

That is, the STN liquid crystal layer 405 has a predetermined elliptically polarized light generated by the display liquid crystal cell 300 when the voltage applied between the electrodes 303 and 304 of the display liquid crystal cell 300 does not exceed the threshold value. Is linearly polarized. The polarization axis of the second linear polarizing plate 500 is set to be orthogonal to the polarization axis of the linearly polarized light. And
When the voltage applied between the electrodes 303 and 304 of the display liquid crystal cell 300 exceeds the threshold value, light polarized according to the elliptically polarizing action of the display liquid crystal cell 300 and the compensation liquid crystal cell 400 is generated. Transmitted through the linear polarizing plate 500,
Is emitted.

The second linear polarizing plate 500 is provided on the compensating liquid crystal cell 400, and sandwiches the display liquid crystal cell 300 and the compensating liquid crystal cell 400 together with the first linear polarizing plate 100. The driving circuit 600 includes the light emitting element 100 and the display liquid crystal cell 30.
The liquid crystal display device is driven by applying a predetermined voltage to the light emitting element 100 and the display liquid crystal cell 300 by being connected to the liquid crystal cell 400 for compensation and 0, and displays an image.

As described above, the compensation liquid crystal cell 40
0 is formed thicker than the display liquid crystal cell 300.
Therefore, even if the display liquid crystal cell 300 is formed as thin as possible, the compensation liquid crystal cell 400 is
It can be more easily manufactured. That is, the compensation liquid crystal cell 400 having a uniform thickness can be formed. In addition, even if small foreign matter (dust or the like) is mixed in the STN liquid crystal layer 405, the disorder of the alignment generated in the liquid crystal molecules is small. As described above, by forming the display liquid crystal cell 300 thin, a high-speed response of the liquid crystal display device can be realized. By manufacturing the compensating liquid crystal cell 400 having a uniform thickness and little disturbance of liquid crystal molecules, a high display quality of the liquid crystal display device can be realized.

As described above, black-and-white display can be performed by the liquid crystal display device having the two-layer STN liquid crystal. Further, since the cell gap of the compensating liquid crystal cell 400 is larger than the cell gap of the display liquid crystal cell 300, the display liquid crystal cell 300 can be formed as thin as possible. Thereby, a high-speed response of the liquid crystal display device can be realized. Further, since the cell gap of the compensation liquid crystal cell 400 is larger than the cell gap of the display liquid crystal cell 300, even if the cell gap is uneven, the influence on the display is small, and the liquid crystal display device can be manufactured with a high yield. .

In general, the liquid crystal becomes viscous at a low temperature, and crystallizes at a temperature lower than a certain temperature (transition temperature) T _{C -S.} Tend to be worse.
On the other hand, when the temperature increases, the alignment direction of the liquid crystal molecules becomes non-uniform, and when the temperature exceeds a certain temperature (transition temperature) T _NI , the alignment direction of the liquid crystal molecules becomes isotropic. However, as described above, the compensation liquid crystal cell 400 is formed thicker than the display liquid crystal cell 300, and the birefringence of the STN liquid crystal layer 405 is set to STN.
By setting the birefringence of the liquid crystal layer 305 to be smaller than that of the liquid crystal layer 305, the change of the birefringence of the STN liquid crystal layer 405 due to a change in ambient temperature becomes smaller than that of the STN liquid crystal layer 305. That is, as shown in FIG. 3, the retardation value (Δn × d)
The change due to the temperature becomes gentler in the compensation liquid crystal cell 400 than in the display liquid crystal cell 300. Therefore, the temperature is in between the _{T C-S} and _{T N-I,} good response of the display even at a low temperature, the retardation value of the STN liquid crystal layer 405 and the STN liquid crystal layer 305 at a high temperature is more approximate, S
Even if the voltage applied to the TN liquid crystal layer 305 is low, a sufficiently dark black (dark) display can be performed, and the contrast ratio of the image increases,
Regardless of the temperature change, high-speed response and high display quality can be realized at the same time.

The fact that the temperature dependence of the compensation liquid crystal cell 400 (STN liquid crystal layer 405) is low means that T _{C -S}
And T _N-I are wide, and the liquid crystal display device can operate in a wide temperature range. That is, the material of the STN liquid crystal layer 405 can be freely selected as much as the temperature dependency is reduced. Actually, the torsion angle is 220 °
The liquid crystal cell 300 for display and the liquid crystal cell 400 for compensation as shown in Table 1 were manufactured when the ambient temperature was 25 ° C. using the STN liquid crystal layer 305 and the STN liquid crystal layer 405. Also in this case, as described above, the compensation liquid crystal cell 40
Thus, a liquid crystal display device having a high-speed response and high display quality regardless of a change in temperature with substantially no thickness variation of 0 was manufactured.

[0036]

[Table 1]

Note that the liquid crystal display device described in the above embodiment is capable of multi-color display. In the case of performing full-color display using the liquid crystal display device, an R filter for separating each of red, green and blue colors, A color filter composed of a filter and a B filter, in which an R filter, a G filter, and a B filter are respectively arranged in stripes or dots corresponding to pixels, is formed on the second transparent substrate 402 of the liquid crystal cell 400 for compensation. Is also good.

Further, the STN liquid crystal layer 405 of the compensating liquid crystal cell 400 may be made of an STN liquid crystal having a low temperature dependency without applying a voltage. That is, the STN liquid crystal layer 305
Transition temperature T _C-S lower than the transition temperature T _{C-S of}
And the transition temperature T _{NI of} the STN liquid crystal layer 305.
An STN liquid crystal having a higher transition temperature T _NI may be used.

The positional relationship between the display liquid crystal cell 300 and the compensation liquid crystal cell 400 may be reversed.

[0040]

As is apparent from the above description, according to the present invention, disturbance of an image is hardly generated, and high display quality can be realized. As a result, a high yield of the manufactured liquid crystal display device can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a liquid crystal display device according to an embodiment of the present invention.

FIG. 2A is a cross-sectional view illustrating a configuration of the display liquid crystal cell illustrated in FIG. FIG. 2B is a cross-sectional view illustrating a configuration of the compensation liquid crystal cell illustrated in FIG. 1.

FIG. 3 is a diagram showing a relationship between a retardation value and a temperature of a display liquid crystal cell and a compensation liquid crystal cell.

FIG. 4 is a configuration diagram of a conventional liquid crystal display device.

[Explanation of symbols]

100: light emitting element, 200: first linear polarizing plate, 300
... Display liquid crystal cell, 301 ... First transparent substrate, 302 ...
2nd transparent substrate, 303 ... 1st electrode, 304 ... 2nd electrode, 305 ... STN liquid crystal layer, 306 ... 1st alignment film, 3
07: second alignment film, 400: liquid crystal cell for compensation, 401
... 1st transparent substrate, 402 ... 2nd transparent substrate, 403 ...
First alignment film, 404: second alignment film, 405: STN liquid crystal layer, 500: second linear polarizing plate, 600: driving circuit

Claims (4)

[Claims] 1. Between a first linear polarizing plate and a second linear polarizing plate
And the layer thickness is d₁And the birefringence Δn₁Having a first liquid crystal layer
Display liquid crystal cell and layer thickness d₂And the birefringence Δn₂No.
And a compensating liquid crystal cell having two liquid crystal layers.
Option value Δn_Two・ D_TwoIs the retardation value Δn₁・ D₁
Smaller and the liquid crystal layer thickness d₁Is the liquid crystal layer thickness d. ₂
A liquid crystal display device characterized by being smaller. 2. The liquid crystal display device according to claim 1, wherein a birefringence Δn ₂ of the second liquid crystal layer is smaller than a birefringence Δn ₁ of the first liquid crystal layer. 3. The liquid crystal layer according to claim 1, wherein the second liquid crystal layer has a nematic phase-isotropic liquid phase transition temperature higher than the first liquid crystal layer nematic phase-isotropic liquid phase transition temperature.
Or the liquid crystal display device according to 2. 4. The method according to claim 1, wherein the first liquid crystal layer and the second liquid crystal layer are
The liquid crystal display device according to any one of claims 1 to 3, wherein the liquid crystal display device is formed from a TN (super twisted nematic) liquid crystal.