KR101331809B1 - Liquid crystal display device - Google Patents

Abstract

The present invention relates to a liquid crystal display device having a low black luminance by using a high elastic liquid crystal and a high stretching alignment layer. The liquid crystal display device according to the present invention includes a first substrate, a second substrate facing the first substrate, and It comprises a polymer alignment film formed on the opposite surface of the first substrate and the second substrate and a liquid crystal layer formed between the two substrates, the alignment film for the light passing through the short axis of light passing through the long axis of the molecules forming the alignment film The retardation value is 0.20 to 0.36 nm, the liquid crystal layer is characterized in that the torsional modulus is 6.0 ~ 7.0pN.

High elongation, high elasticity, black brightness, side chain alignment layer, matching

Description

[0001] LIQUID CRYSTAL DISPLAY DEVICE [0002]

1 is a plan view showing unit pixels of a horizontal electric field type liquid crystal display device;

FIG. 2 is a cross-sectional view taken along line II-II 'of the horizontal field type liquid crystal display unit pixel of FIG. 1. FIG.

3 is a graph showing Comparative Examples 1 to 4 and shows a degree of improvement of luminance with respect to a delay value of an alignment layer in%;

4 is a graph illustrating comparison of black luminance values of Comparative Example 1 and Example 1. FIG.

<Explanation of symbols for the main parts of the drawings>

T: thin film transistor 103: first substrate

104: second substrate 109: gate line

110: gate electrode 111: common electrode

112: data line 117: source electrode

118: drain electrode 119: pixel electrode

112a and 112b: alignment layer 130: liquid crystal layer

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal and an alignment film of a liquid crystal display device, and more particularly, to a liquid crystal display device having low black luminance by using a high elastic liquid crystal and a high stretching alignment film.

Recently, with the development of the information society, as the demand for various display devices increases, research on flat panel display devices such as liquid crystal display (LCD) and plasma display panel (PDP) has been actively conducted. .

Among them, a liquid crystal display device is a display device for displaying an image by adjusting the light transmittance using optical anisotropy of the liquid crystal. Liquid crystal display devices are most popular for their high quality, ease of driving, and low power consumption.

Liquid crystal display devices are classified into liquid crystal display devices having various display modes according to the type of liquid crystal compound being driven. Generally, a twisted nematic (TN) method is widely used.

The twisted nematic method turns on / off an electric field perpendicular to the substrate, driving the liquid crystal compound so that the director of the liquid crystal compound has an angle between 0 ° and 90 ° with respect to the substrate. It is. The twisted nematic method has a disadvantage in that the viewing angle characteristic is not excellent because the liquid crystal compound is vertically driven.

Therefore, in order to overcome this disadvantage, a horizontal electric field type liquid crystal display device has been developed in which the liquid crystal compound is driven while the director of the liquid crystal compound is horizontal to the substrate. The horizontal electric field type liquid crystal display device has an advantage that a wide viewing angle is secured as compared to a conventional TN type liquid crystal display device by switching the liquid crystal molecules in a horizontal state with respect to the substrate.

In the process of manufacturing the liquid crystal display device as described above, an alignment film is used to initially align the liquid crystal molecules in a direction of a predetermined angle.

In general, a polyimide-based polymer organic material is mainly used for the alignment layer, and is aligned in a predetermined direction through rubbing. Rubbing is a process of rubbing the alignment layer in a certain direction using a rubbing cloth, which is suitable for mass production, and has an advantage of stable orientation and easy control of the pretilt angle.

The polyimide-based alignment layer is composed of a main chain type and a side chain type according to the structure of a polymer. In general, in an in-plane switching liquid crystal display device, the polyimide alignment layer is formed on a main chain type polyimide alignment layer. It is prepared by injecting a normal liquid crystal.

By the way, when aligning the liquid crystal with the main chain alignment film, the dispersion of the director when rubbing is large because the surface area is smaller and the interaction for aligning the liquid crystal is smaller than that of the side chain alignment film. Therefore, the alignment alignment of the liquid crystal is low, light leakage occurs when black is implemented. The light leakage increases the brightness when no voltage is applied in the normally black state, that is, the black brightness.

In fact, in the horizontal field type liquid crystal display device, the black luminance is about 50% higher than that of the non-rubbing vertical alignment (VA) type liquid crystal display device.

As a result, the increase in the black luminance lowers the contrast ratio (CR), which causes a problem of deteriorating display quality.

It is an object of the present invention to provide a high quality liquid crystal display device by lowering the black luminance of the liquid crystal display device. In particular, it is an object of the present invention to provide a liquid crystal display device having high display quality by increasing the contrast ratio by lowering black brightness through proper matching between the liquid crystal constituting the liquid crystal layer and the alignment layer for determining the initial alignment angle of the liquid crystal.

(여기서, X와 Y는 탄소 수 10개 이하의 알킬기, 알콕시기, 페닐기)According to an aspect of the present invention, a liquid crystal display device includes: a first substrate; A second substrate bonded to the first substrate; A polymer alignment layer formed on opposite surfaces of the first substrate and the second substrate; And a liquid crystal layer formed between the first substrate and the second substrate, wherein the alignment layer has a phase difference delay value of 0.20 to 0.36 nm for light passing through a short axis of light passing through the long axis of the molecules forming the alignment layer. The liquid crystal layer is characterized in that the torsional elastic modulus of the liquid crystal mixture containing a substance of the formula (2) 6.0 ~ 7.0pN.
(2)

(Where X and Y are an alkyl group having 10 or less carbon atoms, an alkoxy group, or a phenyl group)

In this case, the alignment layer is characterized in that the phase difference delay value for the light passing through the short axis of the light passing through the long axis of the polymer forming the alignment layer is 0.20 ~ 0.36nm, the liquid crystal layer is characterized in that the torsional elastic modulus of 6.0 ~ 7.0pN It is done.

Here, the liquid crystal display device according to the present invention is characterized in that the liquid crystal display device of the horizontal electric field method.

Hereinafter, a liquid crystal display according to the present invention will be described with reference to the drawings. The present invention can be applied to various types of liquid crystal display devices such as a twisted nematic method or a vertical alignment method, but for the convenience of description, a liquid crystal display device having a horizontal electric field method will be described as an embodiment. Here, the present invention is not limited to the liquid crystal display device of the horizontal electric field system.

In the following description, a liquid crystal display device of a horizontal electric field method according to the present invention will be described first, and then matching between the liquid crystal and the alignment layer will be described later.

1 and 2 are a plan view and a sectional view taken along the line II-II 'of the horizontal pixel type liquid crystal display unit pixel, respectively. The liquid crystal display device is composed of a plurality of pixels, but in this drawing, only one pixel is shown for convenience of description.

Referring to the drawings, a liquid crystal display according to an exemplary embodiment of the present invention includes a second substrate 104 facing the first substrate 103 and the first substrate 103, and the first substrate 103 and the second substrate. It comprises a liquid crystal layer 130 formed between the (104).

The liquid crystal molecules forming the liquid crystal layer 130 are materials having optical anisotropy, and since the alignment is different depending on the applied voltage, light transmittance may be adjusted. Accordingly, a still image or a moving image corresponding to the light transmittance of the liquid crystal layer 130 is displayed on the liquid crystal display device.

In the horizontal electric field type liquid crystal display, a liquid crystal molecule of a liquid crystal layer is driven on the same plane by arranging electrodes forming an electric field in parallel to form an electric field horizontally with the surface of the substrate.

In the horizontal field type liquid crystal display, a data line 112 and a gate line 109 are arranged on the first substrate 103 to define a pixel area, and the intersection point of the gate line 109 and the data line 112 is described. And a pixel electrode 119 and a common electrode 111 arranged substantially parallel to the data line 112 in the pixel.

The thin film transistor T is formed on the first substrate 103 and is connected to the gate line 109, and the silicon nitride (SiNx) or silicon oxide (SiOx) stacked on the gate electrode 110. A gate insulating film 113 made of the same material as that of the semiconductor layer, a semiconductor layer 115 formed on the gate insulating film 113, an ohmic contact layer 116 formed on the semiconductor layer 115, and the ohmic contact layer ( A source electrode 117 and a drain electrode 118 are formed on the 116 and connected to the data line 112 and the pixel electrode 119, respectively.

On the source electrode 117 and the drain electrode 118, a protective film 120 is stacked over the entire substrate. A pixel electrode 119 is formed on the passivation layer 120 to be connected to a drain electrode 118 of the thin film transistor T through a contact hole, and to be substantially parallel to the pixel electrode 119. 111 is formed and connected to the common line 105 formed on the first substrate through a contact hole (not shown).

At this time, although not shown, the common electrode 111 may be formed on the substrate simultaneously with the same material as the gate electrode 110.

The first alignment layer 112a is formed on the passivation layer 120 including the pixel electrode 119 and the common electrode 111, and thus a pretilt angle, which is an initial alignment angle of the liquid crystal layer, is determined.

The liquid crystal molecules 102 are aligned in the rubbing direction of the alignment layer 112a between the common electrode 111 and the pixel electrode 119 when no voltage is applied.

In addition, the first substrate 103 and the second substrate 104 corresponding to the light shielding layer 106, red (R, green, G) and blue ( The color filter layer 107 and the overcoat layer 108 which consist of the color filter elements of blue and B) are laminated | stacked one by one. The second alignment layer 112b is formed on the overcoat layer 108, and the initial alignment angle of the liquid crystal layer 130 is determined by the second alignment layer 112b.

Although not shown, the first substrate 103 and the second substrate 104 are bonded by a sealing material, and a liquid crystal layer 130 is formed therebetween to form the first substrate 103. The liquid crystal molecules are driven by the transistor to control the amount of light passing through the liquid crystal layer 130 to display information.

In general, the pretilt angle and the anchoring energy on the surface of the polymer forming the alignment layer depend not only on the rubbing strength but also on the type of the material and the liquid crystal forming the alignment layer. In particular, since the pretilt angle changes according to the type and characteristics of the alignment film and the liquid crystal, it is important to match the alignment film and the liquid crystal to have a predetermined physical quantity, and the types and characteristics of the alignment film and the liquid crystal to provide the best display quality. Optimizing is called matching.

The orientation of the liquid crystal molecules on the rubbed polymer surface is mainly due to the interfacial intermolecular interaction and the surface topological interaction between the alignment layer and the liquid crystal. For example, van der Waals interactions acting between the liquid crystal molecules. By rubbing, the side chains or main chains of the polymer are arranged in the rubbing direction, and the orientation of the liquid crystal is determined by acting between the side chains or main chains of the rubbed polymer and the liquid crystal molecules. For this reason, there is a need for matching between the liquid crystal and the polymer that can optimize the interaction between the interfacial molecules between the alignment layer and the liquid crystal and the surface shape effect.

In the matching of the polymer and the liquid crystal, the main chain polymer material is used to form a small pretilt angle and the side chain polymer material is used to align the liquid crystal to form a relatively large pretilt angle. In a conventional liquid crystal display device having a horizontal electric field type requiring a small pretilt angle around 1 °, an alignment film is formed using a main chain polymer material.

However, when the liquid crystal is oriented using the main chain polymer material, there is a problem in that the alignment energy is lower than that of the side chain polymer material due to a lower fixed energy. As a result, the scattering of the director of the liquid crystal becomes larger at the time of rubbing, thereby increasing the black luminance.

According to the present invention, in the conventional horizontal field type liquid crystal display device, the alignment layer is formed of the main chain type polymer material, the polymer material forming the alignment layer is a high-stretched polymer material in which side chains are elongated by rubbing in one direction. It is characterized by.

The higher the stretched molecule is, the larger the phase difference delay value is. In the present invention, the phase difference retardation value due to the stretching of the side chain is characterized in that 0.20 to 0.36 nm.

In this case, the retardation delay value refers to a retardation delay value of light passing through a long axis and a short axis in the polymer forming the alignment layer. In the case of a general alignment layer, the delay value is about 0.16 nm.

Here, in the case of a general alignment film having a low degree of stretching and a delay value smaller than 0.20 nm, since liquid crystals are arranged along the alignment film, light leakage occurs even when no voltage is applied in a normally black state. Therefore, the present invention is characterized by having a phase difference delay value larger than 0.20 nm.

On the contrary, however, when the degree of stretching is large and the phase difference retardation value is larger than 0.36 nm, there is a problem in that the hardness of the alignment film is too low. In the alignment film, the stretching and the hardness have a relationship that the hardness decreases as the degree of stretching increases and the hardness increases as the degree of stretching decreases.

In fact, when the general polyimide is used as the alignment layer, the hardness is about 4H, but when the highly stretchable polyimide is used, the hardness is about 2B, and the softness is deepened. If the hardness is too low, it becomes vulnerable to rubbing which aligns the side chains of the alignment layer in a certain direction by applying a physical force, thereby causing a problem of easy deformation. Therefore, the alignment layer according to the embodiment of the present invention is characterized by having a phase difference delay value smaller than 0.36 nm.

Here, as the material for forming the alignment film, a side chain type polymer material is preferable, and polyimide or polyamic acid is more preferable. However, the material for forming the alignment layer is not limited thereto, and the alignment layer may be formed in addition to the polyimide or the polyamic acid in the side chain type polymer material.

Formula 1 shows a polymer material according to a preferred embodiment of the present invention. Preferably, the polymer having Formula 1 may be formed as an alignment layer, but the alignment layer material is not limited thereto, and main chains or side chains may be variously applied to other functional groups as necessary.

In the embodiment of the present invention, the torsional modulus of the liquid crystal layer matched with the alignment layer is 6.0 to 7.0 pN. The elastic modulus of a general liquid crystal has a value smaller than 6.0 pN. In the present invention, the liquid crystal layer is formed of a high elastic liquid crystal having an elastic modulus of 6.0 pN or more.

Here, the elastic modulus of the liquid crystal compound is not only a torsional elastic modulus but also a concept in consideration of three factors such as a spray modulus and a bend modulus, but in a horizontal electric field type liquid crystal display, the orientation of the liquid crystal compound Since the values of the spray modulus and the bending modulus are negligibly small compared to the torsion modulus, the elastic modulus in the present invention means the torsion modulus.

In general, when a polymer alignment film having a large retardation value is used for a general liquid crystal, black luminance is reduced. However, according to the present invention, a synergy effect between the alignment film and the liquid crystal is achieved by matching a highly stretched alignment film with a high elastic liquid crystal. There is an effect of further increasing the degree of alignment of the liquid crystal with respect to rubbing, and the black luminance can be greatly reduced.

Elastic force refers to the force to return to the original state when the liquid crystal is deformed under a certain direction, the elastic modulus is a constant proportional to the elastic force. Therefore, when the elastic modulus is smaller than a predetermined level, the property of returning to the original position when the electric field is removed after applying the electric field is reduced, which causes light leakage.

In particular, when the elastic modulus is less than 6.0pN, there is a problem in that the black luminance value becomes large due to insufficient elastic force. Therefore, the elastic modulus of the liquid crystal in the embodiment according to the present invention is characterized by having a large value of 6.0 pN or more.

However, when the elastic modulus of the liquid crystal has a value larger than a predetermined value, especially when the value of the liquid crystal has a value greater than 7.0 pN, the rotational viscosity according to the rotation of the liquid crystal becomes large. Therefore, since the voltage to be applied to rotate the liquid crystal becomes large, and the time until the liquid crystal rotates to show a specific state increases, an increase in the applied voltage results in a decrease in the response speed.

Therefore, the present invention is characterized in that the torsional modulus has a value of 7.0 pN or less.

Formula 2 shows a liquid crystal according to a preferred embodiment of the present invention.

Formula (2) is a main single liquid crystal to exhibit high elastic liquid crystal properties, wherein X and Y may be composed of an alkyl group having 10 or less carbon atoms, an alkoxy group, or the like. By mixing several other single liquid crystals with this single liquid crystal, it becomes a highly elastic commercial liquid crystal which can be used.

According to an embodiment of the present invention, when the highly stretched alignment film and the high elastic liquid crystal are matched, the degree of improvement in black luminance is remarkably improved than in the respective cases in which the general alignment film and the general liquid crystal, the high stretching alignment film and the general liquid crystal, and the general liquid crystal and the high elastic liquid crystal are matched. high.

That is, black luminance is improved even when only an alignment layer is matched with a general liquid crystal with a high-stretch alignment layer that satisfies the above value, or when only a liquid crystal is matched with a general alignment layer with a high-elastic liquid crystal that satisfies the above value. It shows a significantly smaller improvement than the case of matching.

The following table shows an example in which a general liquid crystal, a high elastic liquid crystal, a general polymer alignment layer and a high elastic polymer alignment layer are matched, and the degree of improvement of black luminance in each case is expressed in%.

In Comparative Example 1, the general alignment film was matched with the general liquid crystal, and Comparative Examples 2 to 4 matched the high-stretch alignment film with the general liquid crystal. In Comparative Example 5, the general alignment film and the high elastic liquid crystal were matched, and Example 1 was the high stretching alignment film and the high elastic liquid crystal.

In this embodiment, polyimide was used as the polymer alignment layer.

Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Example 1 Alignment film Kinds Normal High stretch High stretch High stretch Normal High stretch Phase difference delay value (nm) 0.16 0.24 0.29 0.36 0.16 0.36 Liquid crystal Kinds Normal Normal Normal Normal High elasticity High elasticity Modulus of elasticity (pN) 5.8 5.8 5.8 5.8 6.5 6.5 Black luminance improvement degree 0% 7.7% 8.7% 9.7% 7.3% 30.3%

FIG. 3 is a graph showing Comparative Examples 1 to 4 to show the degree of improvement in luminance with respect to the retardation delay value of the alignment layer in%. At this time, in Comparative Example 1, the general alignment film was matched with the general liquid crystal, and was set as a reference value of black luminance.

Referring to Table 1 and FIG. 3, the improvement in black luminance is gradually increased to 7.7%, 8.7%, and 9.7% as the general liquid crystal having an elastic modulus of 5.8 and the phase difference delay values are increased to 0.24 nm, 0.29 nm, and 0.36 nm. Therefore, it can be seen that as the degree of stretching increases, the black luminance decreases. Here, the alignment films whose phase difference delay values are 0.24 nm, 0.29 nm and 0.36 nm are HB, B and 2B, respectively. This means that the higher the draw, the weaker the hardness. If the hardness is less than 2B, rubbing defects (for example, the alignment film tearing phenomenon) appears.

As shown in Comparative Examples 1 to 4 of Table 1, when the high-stretched alignment film is matched to a general liquid crystal, luminance is improved according to the degree of stretching. It is because a high oriented oriented film increases the alignment degree of a liquid crystal as mentioned above.

In addition, looking at Comparative Example 5, it can be seen that even when using a general polyimide alignment layer, the black luminance is improved by 7.3% when the high elastic liquid crystal is matched.

However, as compared with the comparative example described above, when the highly elastic polyimide alignment layer having a phase difference delay value of 0.36 nm is matched with a high elastic liquid crystal having an elastic modulus of 6.5 pN as in Example 1, the black luminance is rapidly reduced to improve 30.3%. You can see that there is.

4 is a graph illustrating comparison of black luminance values of Comparative Example 1 and Example 1. FIG. Referring to the drawings, the black luminance of Comparative Example 1 is 1.61 cd / m 2 and the black luminance of Example 1 is 1.12 cd / m 2. This is due to the synergistic effect of matching the alignment film and the liquid crystal, and is due to the effect that the high elastic liquid crystal is further aligned by the high stretching alignment film.

Therefore, compared with the case where a high elasticity liquid crystal is matched with a general alignment film or a general liquid crystal is matched with a high stretched alignment film, the black brightness is much improved.

Therefore, the liquid crystal display according to the exemplary embodiment of the present invention can improve the black luminance by matching the high stretchable alignment layer with the high elastic liquid crystal. When the black luminance is lowered, the contrast ratio is increased, so that the quality of the image of the liquid crystal display device is increased.

The detailed description of the invention should be construed as illustrative of preferred embodiments rather than as limiting the scope of the invention. For example, the description of the present specification describes a horizontal electric field type liquid crystal display device as a preferred embodiment, but is not limited thereto and may be applied to another type of liquid crystal display device as necessary.

Therefore, the invention should not be defined by the described embodiments, but should be defined by the claims and their equivalents.

As described above, the present invention provides a high quality liquid crystal display device by lowering the black luminance of the liquid crystal display device. In particular, the present invention provides a liquid crystal display device having high display quality by increasing the contrast ratio by lowering black brightness through proper matching between the liquid crystal constituting the liquid crystal layer and the alignment layer for determining the initial alignment angle of the liquid crystal.

Claims (7)

A first substrate; A second substrate bonded to the first substrate; A polymer alignment layer formed on opposite surfaces of the first substrate and the second substrate; And It comprises a liquid crystal layer formed between the first substrate and the second substrate, The alignment layer has a phase difference delay value of 0.20 to 0.36 nm for light passing through a short axis of light passing through the long axis of the molecules forming the alignment layer. The liquid crystal layer is a liquid crystal display comprising a liquid crystal mixture containing a substance of the formula (2) to a torsional modulus of 6.0 ~ 7.0pN. (2)

(Where X and Y are an alkyl group having 10 or less carbon atoms, an alkoxy group, or a phenyl group) The method of claim 1, Gate lines and data lines intersecting the first substrate to define a unit pixel area; A thin film transistor formed at an intersection of the gate line and the data line; And And at least one common electrode and a pixel electrode arranged parallel to the first substrate to generate a horizontal electric field with respect to the surface of the first substrate. 3. The method of claim 2, A color filter formed on the second substrate; And And a light blocking layer formed on the second substrate. The method of claim 1, And the alignment layer is polyimide or polyamic acid. 5. The method of claim 4, The alignment layer is a liquid crystal display, characterized in that the side chain type polyimide. The method of claim 5, The alignment layer is a liquid crystal display, characterized in that it comprises a material of the formula (1). [Formula 1]

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