JP2605064B2 - Liquid crystal display device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a liquid crystal display device.

(Prior art)

The liquid crystal display mode that has been mainly used in the past is called a twisted nematic (TN) type, in which the liquid crystal molecules are twisted by about 90 ° between a pair of upper and lower substrates. , Utilizing the disappearance of its effect by voltage. This display mode was sufficient for low-time-division driving such as a clock or a calculator. However, when high-time-division driving was performed to increase the display capacity, there was a disadvantage that the contrast was reduced and the viewing angle was reduced. there were. This is because the ratio of the voltage applied to the selected point and the voltage applied to the non-selected point approaches 1 in the case of high time division driving. In order to obtain a display element with a high contrast and a wide viewing angle, the relative transmittance of the element is 10%. it is necessary to minimize the steepness γ represented by the ratio of the voltage V ₅₀ that changes the voltage V ₁₀ 50% varying (V ₅₀ / V _10).

In the case of the twisted nematic type, the γ value is about 1.13. In order to reduce the γ value, a method has been proposed in which the twist angle of the liquid crystal molecules is increased and the polarization axis is shifted from the alignment direction, which is called an SBE mode or an STN mode.
According to such a method, the γ value can be reduced to 1.1 or less, and high time division driving of about 1/400 duty can be performed. However, in such a method, since the coloring due to birefringence and the change due to the voltage are used, the demand for the liquid crystal layer thickness accuracy is intense. For example, the cell must be manufactured with an accuracy within ± 0.1 μm. As a result, with the SBE or STN type, product production is extremely difficult, the non-defective rate is reduced, and the cost is increased. In addition, since these devices use coloring due to birefringence and a change due to the voltage, they are displayed on a colored background, and it is difficult to perform monochrome display. In addition, there is light leakage depending on the wavelength, and it is difficult to use it as an optical shutter for color display.

〔Purpose〕

The present invention overcomes the disadvantages of the prior art, has a high time-division driving characteristic, can perform black display on a white background or white display on a black background, and can perform color display with less color unevenness. It is an object to provide a liquid crystal display element. It is another object of the present invention to provide a liquid crystal display element which is lightweight and does not have a double image or a feeling of floating characters.

[Structure and operation]

According to the present invention, a liquid crystal sandwiched between a pair of polarizers, oriented substantially horizontally with respect to a substrate, has a structure twisted between 120 ° and 360 ° between upper and lower substrates, and has a positive dielectric anisotropy. Layers and
A birefringent medium layer provided between the substrate and the polarizer, the substrate adjacent to the birefringent medium is a uniaxially stretched plastic film, and a liquid crystal on the substrate adjacent to the polarizer The angle between the alignment direction of the polarizer and the transmission axis or absorption axis of the polarizer, the angle between the optical axis of the substrate adjacent to the birefringent medium layer and the alignment direction of the liquid crystal on the substrate, the optical axis of the substrate And the angle between the optical axis of the birefringent medium layer adjacent to the substrate and the optical axis of the birefringent medium layer adjacent to the polarizer and the absorption axis or transmission axis of the polarizer adjacent to the birefringent medium layer The liquid crystal display element is characterized in that the angle formed by each of them is in the range of 20 ° to 70 °.

 Next, the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view showing a configuration example of a conventional STN type liquid crystal display element.

In the figure, a first substrate 11 'having a transparent electrode 13' on which an alignment film 15 'has been formed and having been subjected to an alignment treatment, and a similar second substrate 21' are disposed so as to be separated and opposed to each other. A liquid crystal 16 'is sealed between the two to form a liquid crystal cell. twenty three'
Indicates a transparent electrode, 25 'indicates an alignment film, and 10' indicates a sealant. This liquid crystal cell comprises a first polarizer 17 'and a second polarizer 27'.
To constitute a liquid crystal display element.

FIG. 2 is a sectional view showing a configuration example of the liquid crystal display element of the present invention. The liquid crystal layer 16 is formed between the first substrate 11 on which the transparent electrode 13 is formed and subjected to the alignment treatment, and the second substrate 21 subjected to the same treatment. A birefringent medium layer 26 is formed between the substrate 21 and the polarizer 27 on the side opposite to the liquid crystal layer. In the liquid crystal layer 16, the liquid crystal is aligned so as to be substantially horizontal with respect to the substrate, and the liquid crystal molecules are preferentially aligned along the alignment processing direction. in this case,
Substantially horizontal with respect to the orientation of liquid crystal molecules means that the tilt angle of the liquid crystal molecules with respect to the substrate is in the range of approximately 0 to 30 °. In the liquid crystal layer 16, the liquid crystal
It has a twisted structure of ゜ to 360 ゜.

FIG. 3 shows the definition of an angle according to the present invention. The torsion angle ω is the orientation processing direction of the first substrate.
D ₁ is determined by the orientation processing direction D ₂ of the second substrate. This orientation treatment can be performed by rubbing with a cotton cloth or the like after forming a conventionally known oblique vapor deposition or inorganic or organic coating.

In the present invention, a liquid crystal obtained by adding a cholesteric liquid crystal or a chiral nematic liquid crystal to a nematic liquid crystal having a positive dielectric anisotropy is preferably used. In this case, if the torsion angle ω is small, the steepness is deteriorated, and the time-division driving characteristics are deteriorated. On the other hand, if ω is too large, a scattering structure is generated when an electric field is applied, and display quality is undesirably reduced. Therefore, it is preferable that ω is defined in the range of 120 ° to 360 °.

In FIG. 3, the twist direction is counterclockwise from the first substrate to the second substrate, but the clockwise direction is selected by the direction of the alignment treatment and the selection of the cholesteric liquid crystal and the chiral nematic liquid crystal. Can also.

In the present invention, a medium layer 26 having birefringence is formed between the substrate 21 and the polarizer 27.

Such a birefringent medium may be any medium having in-plane refractive index anisotropy, and specifically, triacetyl cellulose, polyester, polyvinyl alcohol,
Examples thereof include a stretched film of a polymer such as polyethylene and cellophane, and a thin slice of a crystal such as mica, calcite, and quartz cut out on a plane parallel to the optical axis.
As shown in FIG. 3, the optical axis 1 of the birefringent medium is arranged at an angle of δ with the orientation direction of the liquid crystal layer on the adjacent substrate.

In the present invention, a liquid crystal can be used as the birefringent medium. FIG. 4 shows the configuration in this case. 4th
In the drawing, a second substrate 21 is provided between a second substrate 21 and a third substrate 41.
Liquid crystal layer 46 is formed. 35 and 45 are alignment films.
For the alignment treatment, the same method as in the case of the first liquid crystal layer can be used. Examples of the liquid crystal used include a smectic liquid crystal, a nematic liquid crystal, and a liquid crystal obtained by adding a cholesteric liquid crystal to a nematic liquid crystal.
Similarly to the first liquid crystal layer 16, the second liquid crystal layer 46 can have a twisted structure between the upper and lower substrates. In this case, δ in FIG. 3 is the angle between the alignment direction of the first liquid crystal layer and the alignment direction of the second liquid crystal layer on the substrate 21.

Polarizers 17, 27 arranged so as to sandwich the cell,
As shown in FIG. 3, the polarization axes P ₁ and P _{2 correspond} to the liquid crystal alignment directions D _1.
Alternatively, they are arranged so as to form angles of β ₁ and β ₂ with the optical axis 1 of the birefringent medium.

In the liquid crystal display element of the present invention, the product of the refractive index anisotropy Δn of the first liquid crystal and the liquid crystal layer thickness d is Δnd, the refractive index anisotropy Δn ′ of the birefringent medium or the second liquid crystal layer, Assuming that the product of the thickness d 'is Δn'd', the color tone when no voltage is applied can be changed to white (transparent) by appropriately selecting the angle arrangement of β ₁ , β ₂ , δ and the values of Δnd, Δnd '. ) Or black, and black or white, respectively, when a voltage is applied.

A first feature of the present invention resides in that in a liquid crystal display device having such a configuration, at least one of the substrates is formed of a light-transmitting plastic. By doing so, the thickness of the substrate can be reduced from 1 to 0.5 mm to about 100 μm. By making all the substrates plastic, the cell thickness can be reduced from about 3 mm to 0.4 mm or less.

Although it is possible in principle to achieve the above object by using thin glass, it is practically impossible because the glass is liable to be broken, and there are serious problems in production and use.

Examples of the plastic film include polymer films such as polyvinyl alcohol, polyester, polyethersulfone, polysulfone, polyetheretherketone, and triacetylcellulose. On the side of these films in contact with the liquid crystal, an alignment film and, if necessary, a transparent electrode are formed. The second
It is not always necessary to form a transparent electrode on the liquid crystal layer, but the color of the cell can be adjusted by forming and operating the electrode.

A second feature of the present invention resides in that an optically isotropic film is used as a substrate in the STN-type liquid crystal display device having the above configuration. Here, “optically isotropic” means that the retardation is 0.05 μm or less. In this case, as a preferred example of the isotropic film, a polymer film such as polyethersulfone, polysulfone, diacetylcellulose, and triracetylcellulose can be exemplified. This optically isotropic film is
Any of 11, 21 and 41 may be used, and all of them may be constituted by this isotropic film. By using an isotropic film, the thickness of the element can be reduced while maintaining the same high display quality as that of a glass substrate, and the weight can be dramatically reduced. In particular, a substrate 21 for separating an isotropic film substrate from a first liquid crystal layer and a birefringent medium or a second liquid crystal layer.
It is particularly preferred to use If the substrate 21 is optically anisotropic, the background will not be white or black,
Contrast tends to be significantly reduced.

Another feature of the present invention relates to the use of a uniaxially stretched plastic film for a substrate of a two-layer STN liquid crystal display device. Here, the uniaxially stretched plastic film means that the retardation is 2 or more. Among the uniaxially stretched films, polyethylene terephthalate, in particular, has excellent characteristics as a liquid crystal display element substrate, such as excellent heat resistance, mechanical strength, gas barrier properties, and dimensional stability. Even if a two-layer type liquid crystal display element is formed, the contrast is significantly reduced, and black-and-white display cannot be performed. As a method of solving this, the inventor has found that these disadvantages can be solved by the following method, and that excellent display quality and substrate characteristics can be achieved at the same time.

(1) In the first method, when a uniaxially stretched film is used for a substrate adjacent to a polarizer, the optical axis of the film and the absorption axis or the transmission axis of the adjacent polarizer are substantially parallel. Here, "parallel" means that the angle between the two is within 5 degrees, preferably within 3 degrees. If this angle exceeds this range, the contrast will be significantly reduced.

In the above configuration, Δnd, Δn′d ′, β
It is preferable that the values such as ₁ , β ₂ , and δ be in the above-described suitable ranges.

(2) In the second method, when a uniaxially stretched film is used as a substrate separating two liquid crystal layers, the orientation direction of the liquid crystal on the substrate adjacent to the polarizer and the transmission axis or the absorption axis of the polarizer are different. The angle formed and the orientation direction of the liquid crystal on the uniaxially stretched film substrate that separates the liquid crystal layer from the other liquid crystal layer or the birefringent medium, and the angle formed by the optical axis of the substrate are both in the range of 20 to 70 °, Preferably, it is in the range of 30 to 60 °. If the angle is out of the angle range, the background color does not become white or black. For example, when the background color is white, the brightness decreases, and when the background color is black, the darkness decreases and the contrast decreases.

By this method, for the first time, a uniaxially stretched film having optical anisotropy can be used as a substrate for separating two liquid crystal layers. In addition, by combining with the first method, the whole substrate can be made of a uniaxially stretched film. In this case, the cell can be made extremely thin.

In the above description, the liquid crystal layer and the birefringent medium are separated by one substrate. However, as shown in FIG. 5, the liquid crystal layer and the birefringent medium may be formed by two substrates 21, 31 or more substrates. Good. When the film is a uniaxially stretched film, the optical axes of these films are preferably parallel.

As described above, in the liquid crystal display element of the present invention, by satisfying the above conditions, the background color of the cell can be white or black. Further, when a voltage is applied, black or white display is performed, and black and white display is possible.

The steepness of the liquid crystal display element of the present invention is the same as that of the conventional STN.
It is 1.05 or less as in the case of the liquid crystal display element, and has excellent time-division driving characteristics.

〔effect〕

In the liquid crystal display device of the present invention, color change due to a change in cell thickness is small, so that color unevenness is less likely to occur. In addition, since it has excellent steepness, it can be used as a liquid crystal display device having a large display capacity. In addition, since black-and-white display, which was impossible in the past, can be performed, and light leakage due to wavelength is small, it can be used as a light shutter for color display. Also, as a substrate,
Since a plastic film is used, the device can be made thinner, and since the substrate is thin, the image does not appear as a double image, and the display quality is extremely high.

〔Example〕

 Next, the present invention will be described in more detail with reference to examples.

Example 1 Polyimide was coated on a polyethersulfone film (100 μm thick) having a transparent electrode, dried, rubbed, and subjected to an alignment treatment (first substrate).

Next, a polyethersulfone film (second substrate) subjected to the same treatment was prepared, and this substrate was placed on the first substrate.
The substrate was bonded with an interval so that the twist angle was 200 °. In this case, the distance between the upper and lower substrates was set to 7 μm by the spacer. Δn between the upper and lower substrates
A nematic liquid crystal of 0.10 was filled with a liquid crystal composition to which a chiral nematic liquid crystal S811 for inducing a left-handed twist was added.

Δn ′ is placed on the upper part of the liquid crystal cell thus manufactured.
d '= 0.7 μm cellulosic film (thickness 20 μm)
m) are bonded together so that the optical axis (the direction with the highest refractive index) forms an angle of 90 ° with the liquid crystal alignment direction on the upper substrate (δ = 90 °), and the whole is a pair of neutral gray systems. And a liquid crystal display device. Here, the direction of the polarizing plate was adjusted so that β ₁ = 45 ° and β ₂ = 45 °.

This element was white (colorless) when no voltage was applied, and a black display was possible by applying a voltage. The steepness is
The time-sharing drive characteristic was extremely excellent at 1.04. Also,
When one polarizer was rotated by 90 °, the white display was inverted on a black background. The thickness of the element is a polarizing plate (120 μm thick)
And about 0.5 mm. Form a pattern on the electrode,
When a character was displayed, no double image was seen at all, and even when the cell thickness varied by 0.5 μm, almost no color unevenness of the background was observed.

Example 2 In the same manner as in Example 1, except that a liquid crystal cell layer using polyethersulfone was used as a substrate instead of the cellulose-based film layer (second liquid crystal cell),
An element was manufactured. The second liquid crystal cell has a twist angle of 180
゜, Δn′d ′ were prepared to be 0.7 μm. δ,
β ₁ and β ₂ were the same as in Example 1. This element also exhibited excellent characteristics as in Example 1.

Example 3 A polyester film having a transparent electrode (100 μm thick)
m, Δnd = 10), a polyimide was applied, dried, rubbed, and arranged. The rubbing direction was set to 45 ° with respect to the polyester stretching direction. A polyester film and the substrate subjected to the same treatment,
They were bonded at intervals so that the twist angle was 200 °. The distance between the upper and lower substrates is 7 μm. A liquid crystal composition satisfying Δn = 0.13 was filled between the substrates to form a liquid crystal cell. Δn′d ′ = 0.9 μm similarly prepared on the upper part of the liquid crystal cell,
The liquid crystal cell (second cell) having a twist angle of 200 ° was bonded so that the liquid crystal alignment on the overlapping substrates was parallel.
With this configuration, δ = 0 ° and both cells are partitioned. 2
In one substrate, the optical axis (stretching axis) of the substrate and the liquid crystal alignment direction form an angle of 45 °. Further, the whole was sandwiched between a pair of neutral gray polarizing plates to form a liquid crystal display device.
The transmission axis of the polarizing plate was set to be parallel to the optical axes of the adjacent substrates, that is, the uppermost and lowermost substrates. This element was almost white when no voltage was applied, showed a sufficient contrast, showed no double image or the like, and showed excellent characteristics in steepness.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a configuration example of a conventional liquid crystal display element, FIG. 2 is a cross-sectional view showing a configuration example of a liquid crystal display element of the present invention, and FIG. FIG. FIG. 4 and FIG. 5 are cross-sectional views showing another example of the configuration of the present invention. 11 ′, 21 ′, 11,21,31,41 …… Substrate, 15 ′, 25 ′, 15,25,35,
45 …… Orientation film, 13 ′, 23 ′, 13,23,33,43 …… Electrode, 17 ′,
27 ', 17,27 …… Polarizer.

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Takamichi Enomoto 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Co., Ltd. (56) References JP-A-62-159116 (JP, A) JP-A Sho 61-73924 (JP, A) JP-A-64-46728 (JP, A) Nikkei Micro Device (1987-10) 84-88

Claims (3)

(57) [Claims] 1. A liquid crystal layer sandwiched between a pair of polarizers, oriented substantially horizontally with respect to a substrate, has a structure twisted between 120 ° and 360 ° between upper and lower substrates, and has a positive dielectric anisotropy. And a birefringent medium layer provided between the substrate and the polarizer, wherein the substrate adjacent to the birefringent medium is a uniaxially stretched plastic film, and on the substrate adjacent to the polarizer. Angle between the liquid crystal orientation direction and the transmission axis or absorption axis of the polarizer, the angle between the optical axis of the substrate adjacent to the birefringent medium layer and the orientation direction of the liquid crystal on the substrate, The angle between the optical axis and the optical axis of the birefringent medium layer adjacent to the substrate, and the absorption axis or transmission of the optical axis of the birefringent medium layer adjacent to the polarizer and the polarizer adjacent to the birefringent medium layer The angle between the axis and
A liquid crystal display element having a range of 20 to 70 mm. 2. The liquid crystal display device according to claim 1, wherein the birefringent medium layer is a liquid crystal layer formed between substrates. 3. The liquid crystal display device according to claim 2, wherein the film sandwiching the liquid crystal layer used as the birefringent medium layer is a plastic film.