JPH0682763A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To suppress the generation of contrast unevenness between a picture element part and a non-picture element part, enhance the visually confirming property of a display screen, and realize a liquid crystal display element improved in display quality. CONSTITUTION:An interelectrode material 3 is formed between respective electrodes of transparent electrodes 1, 2 so that the layer thickness d2 of a liquid cell corresponding to a non-picture element part to which at least one of the display electrodes of two bases is not opposed is smaller than the layer thickness d1 of a liquid crystal composition corresponding to a picture element part in which the transparent electrodes 1, 2 as the display electrodes on bases 6, 7 are mutually opposed. Therefore, the contrast unevenness between the picture element part and the non-picture element part which is caused from a difference in thickness of liquid crystal cell in the picture element part and the non-picture element part can be solved.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a liquid crystal display device.

[0002]

2. Description of the Related Art A liquid crystal display element is used as an OA device such as a word processor or a personal computer, an image display device such as a portable television or an information processing device, by taking advantage of its features such as thinness, light weight and low power consumption. . Furthermore, it is drawing attention as a next-generation image display device that replaces cathode ray tubes in televisions and various information devices, and has a large number of pixels and a high definition screen to realize a high-quality image display over a larger area. Research and development on a driving method in that case are underway.

In response to such a large display screen,
The size of a simple matrix type liquid crystal display device is being increased. Among them, STN (Super Twisted Nematic) type liquid crystal is used as a liquid crystal suitable for a simple matrix type liquid crystal display element because it has sufficient twist characteristics even at a low voltage.

In such a simple matrix type liquid crystal display device, the scanning electrodes and the signal electrodes arranged in an XY matrix are transparent electrodes made of ITO or the like having a high light transmittance. The transparent electrode has higher electric resistance than metal wiring, and as is well known, the brightness is continuously changed from the power supply end of the scanning electrode or the signal electrode toward the end thereof, that is, so-called tilt brightness unevenness display. The unevenness occurs.

It is considered that the cause is that the liquid crystal drive voltage applied to the transparent electrode is continuously attenuated from the feeding end to the end due to the electric resistance of the transparent electrode.

Such tilted luminance unevenness is also found in a conventional general matrix type liquid crystal display device having 640 × 400 pixels and about 10 inches.

By the way, in order to increase the size and definition of the display screen, it is necessary to make the transparent electrodes on the scanning side and the signal side which form the pixels long and thin, but with this, the electric resistance of the transparent electrodes is increased. Becomes even larger, and the above-mentioned tilted luminance unevenness becomes more prominent, which is a major obstacle to the increase in size and definition of the display screen of the simple matrix type liquid crystal display device.

Therefore, in order to eliminate such an uneven brightness, it is necessary to make the electric resistance of the transparent electrode smaller and to compensate for the thinner transparent electrode by increasing its thickness. .

[0009]

However, it is difficult to reduce the electric resistance of the transparent electrode made of a material such as ITO at present because of material limitations.

When the thickness of the transparent electrode is increased, the transparent electrode in the liquid crystal cell is divided into a pixel portion in which the transparent electrodes face each other and a non-pixel portion (pixel gap portion) in which the transparent electrodes do not face each other. As a result, a step is generated, and the layer thickness (so-called cell gap) d of the liquid crystal layer is different by the thickness of the transparent electrode.

Particularly in the case of a liquid crystal display device using STN type liquid crystal, the birefringence effect is due to the retardation Δn.
Since it is largely influenced by d, even a small difference in the cell gap d of the liquid crystal layer appears as a large difference in contrast in the display.

As a result, the contrast between the pixel portion and the non-pixel portion becomes uneven, and the quality of the display screen is extremely deteriorated.

The present invention has been made to solve such a problem, and an object thereof is to suppress the occurrence of contrast unevenness between the pixel portion and the non-pixel portion and to improve the visibility of the display screen. Another object of the present invention is to provide a liquid crystal display device having improved display quality.

[0014]

A liquid crystal display device of the present invention comprises two substrates, each having display electrodes facing each other to form a pixel array in an XY matrix, and the two substrates facing each other. In the liquid crystal display device having a liquid crystal composition sandwiched between the two substrates, the liquid crystal layer thickness of the two substrates is larger than the liquid crystal layer thickness corresponding to the pixel portion where the display electrodes of the two substrates face each other. It is characterized in that the liquid crystal layer thickness corresponding to the non-pixel portion where the respective display electrodes do not face each other and the pixel is not formed is smaller.

More preferably, the liquid crystal layer thickness d ₁ corresponding to the non-pixel portion and the liquid crystal layer thickness d ₂ corresponding to the pixel portion have a tilt angle θ of the liquid crystal corresponding to the non-pixel portion.
_1. Let θ _{2 be} the tilt angle of the liquid crystal corresponding to the pixel portion when no voltage is applied, and pay attention to the retardation value of both, and satisfy Δn · d ₁ cos ² θ ₁ = Δn · d ₂ cos ² θ ₂ . It is desirable to set it to such a value. That is, it is desirable to set d ₁ and d ₂ to values that satisfy d ₁ = (cos θ ₂ / cos θ ₁ ) ² · d ₂ .

The present invention is effective for all liquid crystal display devices using the polarization effect due to the change in the molecular alignment of the liquid crystal, but is a particularly suitable technique for simple matrix liquid crystal display devices using STN type liquid crystals.

[0017]

The retardation value R of the liquid crystal display element is R = Δ
It is given by n · dcos ² θ. Where θ is the tilt angle. The transmittance and color of the liquid crystal display element depend on the retardation value of the liquid crystal cell, and the above-mentioned difference in transmittance and color is due to the difference in retardation value.

As is clear from the above-mentioned theoretical formula of retardation value, the retardation value R decreases as the tilt angle θ increases as a voltage is applied to the liquid crystal layer. That is, the retardation value R ₂ when the non-selection voltage is applied to the pixel portion is smaller than the retardation value R ₁ of the non-pixel portion.

Therefore, in the liquid crystal display element of the present invention, the cell thickness d ₁ of the non-pixel portion is adjusted to be smaller than the cell thickness d ₂ of the pixel portion, so that the non-selection voltage is applied to the pixel portion in the conventional liquid crystal display element. Retardation value R ₂ when applying
The retardation value R ₁ of the non-pixel portion, which is larger than the above, is reduced so as to approach the small retardation value R ₂ of the pixel portion when the non-selection voltage is applied so as to be substantially equal.

As described above, by decreasing the retardation value R ₁ of the non-pixel portion to approach the retardation value R ₂ of the pixel portion to which the non-selection voltage is applied, the same contrast as that of the pixel portion is obtained in the non-pixel portion. Black display is performed to eliminate display unevenness during black display between the pixel portion and the non-pixel portion,
It is possible to realize high-quality image display.

It is more preferable that the retardation value R ₁ of the non-pixel portion and the retardation value R ₂ of the pixel portion to which the non-selection voltage is applied are made equal.

Therefore, from the above equation, Δn · d ₁ cos ²
θ ₁ = d ₁ to a value that satisfies the Δn · d ^₂ cos ₂ θ ₂
And d ₂ are set, it is possible to realize the optimum black display in the non-pixel portion as in the pixel portion.

Particularly in the conventional liquid crystal display device using the STN type liquid crystal, the retardation value R _{1 of the} non-pixel portion is
And the retardation value R _{2 of} the pixel portion to which a non-selection voltage is applied
The display unevenness caused by the difference between Therefore, it can be said that the present invention is a suitable technique particularly for a liquid crystal display element using STN type liquid crystal.

[0024]

Embodiments of the liquid crystal display device of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) FIG. 1 is a diagram showing the structure of a liquid crystal display device according to a first embodiment of the present invention.

The liquid crystal display device of the first embodiment is arranged so that the gap between the transparent electrodes 1 and 2 having a sheet resistance of 7Ω / □ and the transparent electrodes 1 and 2 arranged in series as display electrodes is filled. Substrates 6 and 7 each having an inter-electrode material 3 and alignment films 4 and 5 made of polyimide disposed so as to cover the inter-electrode material 3 are arranged substantially parallel to each other with a spacer 8 interposed therebetween, and The liquid crystal composition 10 is sealed and fixed by a sealing agent 9 made of an epoxy adhesive, and is sandwiched between the substrates 6 and 7.

The transparent electrodes 1 and 2 form a pixel at each intersection of the transparent electrodes 1 and 2 when the substrates 6 and 7 are opposed to each other, and the pixels form a so-called XY matrix array. It is arranged to make up. The number of pixels is 640 × 400
And

As the liquid crystal composition 10, ZLI-2293 (E.
S-811 as a counterclockwise chiral agent
(E. Merck) 0.75% by weight was added.

On the liquid crystal composition 10 as described above, the alignment film 4,
By performing rubbing orientation treatment of 5
= 240 degree twist orientation is applied. The orientation directions were −30 ° and + 30 ° with respect to the reference direction parallel to one side of the substrate.

The thickness of the liquid crystal layer (liquid crystal cell), so-called cell gap, is determined by the transparent electrodes 1, 2 on the main surfaces of both substrates.
Has a cell gap d ₂ of 6.5 μm in the pixel portion facing each other through the liquid crystal composition 10 and faces the gap between the adjacent electrodes of the transparent electrode 1 and the transparent electrode 2, or the transparent electrode 2 and the transparent electrode. The cell gap d ₁ of the non-pixel portion which faces the gap between the adjacent electrodes is 6.4 μm. The cell gap d ₃ in the non-pixel portion where the gap between the adjacent electrodes of the transparent electrode 1 and the gap between the adjacent electrodes of the transparent electrode 2 face each other is 6.3 μm.

Since the cell gaps d ₁ and d ₃ in the non-pixel portion are smaller than the cell gap d _{2 in} the pixel portion as described above, the value of the retardation R ₂ of the liquid crystal layer in the pixel portion and the non-pixel portion. And the value of the retardation R ₁ of the liquid crystal layer approaches and becomes substantially equal, and the contrast when the pixel portion is not selected, that is, the black display, and the contrast on the display screen of the non-pixel portion become substantially equal, and the image It is possible to eliminate display unevenness during black display.

A negative photoresist OMR-83 (manufactured by Tokyo Ohka Kogyo Co., Ltd.) was used as the interelectrode material 3 for adjusting the cell gaps d ₁ and d ₃ in the non-pixel portion as described above. By filling the gaps between adjacent electrodes of the transparent electrodes 1 and 2 with this interelectrode material 3 and making the cell gaps d ₁ and d ₃ in the non-pixel portion smaller than the cell gap d ₂ in the pixel portion, The unevenness of the contrast between the pixel portion and the non-pixel portion, which is caused by the difference between the non-display retardation R ₂ of the liquid crystal cell and the retardation R _{1 of the} non-pixel portion of the liquid crystal cell, is eliminated. .

An organic film made of polycarbonate and having a retardation value of 0.370 μm was used as the optical delay plate 11, and was arranged and stuck so that the optical axis was 55 ° with respect to the reference direction.

A polarizing plate 12 is provided on the outer side of the substrates 6 and 7.
13 are arranged and pasted at polarization angles of 70 degrees and -20 degrees, respectively,
The display mode was normally black mode. LLC-2-81-18 (manufactured by Sanritsu Electric Co., Ltd.) as the polarizing plates 12 and 13
Was used.

Next, a method of manufacturing the above liquid crystal display element will be described with reference to FIG.

First, as shown in FIG. 2A, the substrate 6,
An ITO film 14 having a thickness of 2500 angstrom is formed on the substrate. On top of that, OFP is used as a positive photoresist 15.
R800 (manufactured by Tokyo Ohka Co., Ltd.) is applied (b), the desired electrode pattern is exposed using the positive type photomask 16 on which the desired electrode pattern is drawn (c), and then the ITO film 14 is patterned by etching. The transparent electrodes 1 and 2 as display electrodes are disposed on the substrates 6 and 7, respectively, by performing a process such as resist stripping (d). Then, OMR-83 (manufactured by Tokyo Ohka Co., Ltd.) is applied as a negative photoresist 17 on the substrate, and ultraviolet rays are irradiated from the back surface of the main surface of the substrate (the back surface of the surface on which the ITO film or the like is arranged) to irradiate the negative film. The mold photoresist 17 is exposed in a self-aligned manner (e), and the resist in the unexposed portion, which is an unnecessary portion, is peeled off to form the interelectrode material 3 (f).

That is, since the transparent electrodes 1 and 2 made of the ITO film have a low ultraviolet transmittance, only the negative photoresist corresponding to the portion where the transparent electrodes 1 and 2 are not polymerized. Then, the negative photoresist on the portion where the transparent electrodes 1 and 2 are provided is peeled off. In this way, the negative photoresist remains only in the gaps between the transparent electrodes 1 and 2, and the inter-electrode material 3 fills the gaps between the transparent electrodes 1 and 2. At this time, the film thickness of the negative photoresist 17 is controlled during the film formation so that the gap between the interelectrode material 3 and the transparent electrode 1 or the transparent electrode 2 on the opposite substrate side facing the interelectrode material 3 is 6.4 μm. To do. Thus, the liquid crystal display device of this example was manufactured.

By the way, in preparing the above-mentioned liquid crystal display element, an experimental liquid crystal cell in which the transparent electrodes 1 and 2 were formed in advance according to the above specifications and the interelectrode material 3 was not provided was prepared. And the retardation value Δn · d ₂ cos ² θ ₁ when no voltage is applied to the pixel portion,
The retardation value Δn · d ₂ cos ² θ ₂ when a voltage was applied to the pixel portion when it was not selected was obtained. Where Δn · d ₂
Is a value that is determined by the liquid crystal composition 10 and the cell gap of the pixel portion and is known, so that (cos θ ₂ / cos
The value of θ ₁ ) ² is obtained. Then, using this value, the optimum value of the cell gap d _{1 in} the non-pixel portion is calculated by the relational expression; d ₁ =
It can be obtained by (cos θ ₂ / cos θ ₁ ) ² · d ₂ .

In this example, d ₂ = 6.5 μm, and from the above experiment, (cos θ ₂ / cos θ ₁ ) ²
= 0.98. Therefore, the optimum value of d ₁ is 6.5 × 0.98
Was calculated to be about 6.4 μm. Therefore, a liquid crystal display device was prepared so that the cell gap d ₁ of the non-pixel portion had such a value.

The liquid crystal display device of this embodiment as described above is
When multiplex drive with 1/240 duty,
It has been confirmed that the color and contrast of the pixel portion and the non-pixel portion are substantially uniform, and an image display with excellent visibility and high display quality can be realized.

Further, when the electro-optical characteristics at 25 ° C. were measured, it was confirmed that the contrast ratio was 20: 1 and that good display could be realized.

Comparative Example 1 A liquid crystal display element having the same structure as the conventional liquid crystal display element without the interelectrode material 3 in the first example was prepared. When this liquid crystal display device was driven in a multiplex manner with a duty ratio of 1/240, the colors of the pixel part and the ratio pixel part were different, and good display was not obtained.

Further, when the electro-optical characteristics at 25 ° C. were measured, the contrast ratio was 12: 1, and it was confirmed that the contrast was clearly lowered as compared with the above-mentioned first embodiment.

In the above embodiment, the case where the technique of the present invention is applied to the STN type liquid crystal display element in which the retardation value largely changes depending on the value of the cell gap has been described. The present invention is not limited to STN type liquid crystal display elements. Besides this, for example, TN
It goes without saying that the present invention can be applied to general liquid crystal display devices using the polarization effect due to a change in the alignment of liquid crystal molecules such as (Twisted Nematic) type liquid crystal.

Further, in the above embodiment, the cell gap d ₁ of the non-pixel portion is the optimum value d ₁ = 6.4 μ in this case.
However, it is needless to say that the display quality can be further improved by setting the cell gap d _{3 to} d ₃ = 6.4 μm as an optimum value.

However, in reality, the portion having the cell gap d ₃ in the non-pixel portion is not so wide in the entire screen as compared with the portion having the cell gap d ₁ , and the cell gap d ₃ has an optimum value. As described above, in order to dispose the interelectrode material 3, a step is provided so as to be orthogonal to the longitudinal direction of the interelectrode material 3, and the manufacturing process thereof is very complicated, which is not practical.

Therefore, if the interelectrode material 3 is arranged so as to set the cell gap d ₁ in the non-pixel portion to an optimum value as in the first embodiment, the cell gap d ₃ will be reduced. Becomes a value smaller than the cell gap d ₂ of the pixel portion, and it is possible to realize image display with good contrast without any practical problems.

[0048]

As described above in detail, according to the present invention, it is possible to suppress the occurrence of the contrast unevenness between the pixel portion and the non-pixel portion to improve the visibility of the display screen and to improve the display quality. It is possible to provide a simple matrix type liquid crystal display device having improved characteristics. The present invention is a particularly effective and suitable technique for a simple matrix type liquid crystal display device using STN type liquid crystal.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure of a liquid crystal display element according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a method of manufacturing the liquid crystal display element according to the first embodiment of the present invention.

[Explanation of symbols]

1, 2 ... Transparent electrodes, 3 ... Interelectrode material, 4, 5 ... Alignment film,
6, 7 ... Substrate, 8 ... Spacer, 9 ... Sealing agent, 10 ... Liquid crystal composition, 11 ... Optical delay plate, 12, 13 ... Polarizing plate

Claims (1)

[Claims] 1. A liquid crystal composition sandwiched between two substrates, each having a display electrode facing each other to form a pixel array in an XY matrix, and a liquid crystal composition sandwiched between the two substrates facing each other. In the liquid crystal display element, the display electrodes of the two substrates do not oppose each other and the pixels are formed more than the liquid crystal layer thickness corresponding to the pixel portion where the display electrodes of the two substrates oppose each other. A liquid crystal display element characterized in that the thickness of the liquid crystal layer corresponding to the non-pixel portion is smaller.