JP3742142B2 - Liquid crystal display element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an IPS (in-plane switching) type liquid crystal display element (LCD), and more particularly to improvement of display characteristics (improvement of white luminance and reduction of afterimage) of an active matrix LCD.
[0002]
[Prior art]
In recent years, LCDs have been used in various information equipment terminals, video equipments, etc., taking advantage of characteristics such as light weight, thinness, and low power consumption. Most of these LCDs are TN (twisted nematic) and STN (super twisted nematic) LCDs. However, although this conventional LCD has been put into practical use, there is a problem that the viewing angle is relatively narrow.
[0003]
In view of this point, proposals have been made for in-plane switching (IPS) LCDs (for example, Japanese Display '92 -547 to 550 R. Kiefer et al., “P2-30 In -Plane Switching of Nematic Liquid Crystals "). This IPS LCD has a structure in which liquid crystal is sealed between a substrate on which comb-like electrodes are formed facing each other and a substrate on which no electrode is formed.
[0004]
For example, as shown in FIG. 3, the periphery of transparent substrates 11 and 12 such as glass is sealed and fixed with a sealing material 13, and n-type liquid crystal (dielectric material) is formed in the space formed by these substrates 11 and 12 and the sealing material 13. Liquid crystal with negative rate anisotropy) 14 is enclosed. A pair of comb-like electrodes 15 and 16 as shown in FIG. 4A are formed on the inner surface of one substrate 12 so as to mesh with each other, and no electrode is formed on the other substrate 11. Alignment films 17 and 18 are formed on the inner surfaces of the substrates 11 and 12, respectively. The alignment films 17 and 18 are subjected to an alignment process in a direction perpendicular to the longitudinal direction of the teeth of the electrodes 15 and 16, respectively. The orientation directions are indicated by arrows 19 and 20, respectively. Therefore, the major axis of the liquid crystal molecules 14a of the liquid crystal 14 is aligned in a direction perpendicular to the longitudinal direction of the teeth of the electrodes 15 and 16 and parallel to the substrates 11 and 12, respectively. Polarizers 21 and 22 are formed on the outer surfaces of the substrates 11 and 12, respectively. The polarization direction 23 of one polarizer 21 is the same as the orientation direction 19, but the polarization direction 24 of the other polarizer 22 is the orientation direction. 20, the direction orthogonal to the polarization direction 23. Alternatively, p-type liquid crystal (liquid crystal having positive dielectric anisotropy) oriented in a direction parallel to the longitudinal direction of each tooth of the electrodes 15 and 16 and parallel to the substrates 11 and 12 may be used.
[0005]
In a state where no voltage is applied between the electrodes 15 and 16 shown in FIG. 3, the light incident on the LCD is linearly polarized by a polarizing plate, for example, 21 on the incident side, the polarization direction and the long axis of the liquid crystal molecules 14a. Since the direction coincides with the polarization direction, the polarization direction of the light is transmitted through the liquid crystal 14 without being changed. Therefore, the polarization direction of the light reaching the polarizing plate 22 on the emission side is orthogonal to the polarization direction of the polarizing plate 22 and is blocked. The
[0006]
However, when a voltage is applied between the electrodes 15 and 16, the electric field between the comb teeth of the electrodes 15 and 16 causes the major axis direction of the liquid crystal molecules 14a to be parallel to the longitudinal direction of the electrode teeth as shown in FIG. 4B. Bend. Therefore, light incident from the substrate 11 side and linearly deflected by the polarizing plate 21 is changed to elliptically polarized light by the birefringence of the liquid crystal 14 while being transmitted through the liquid crystal 14, and is transmitted through the polarizing plate 22.
[0007]
In order to display an image using such an LCD, for example, a pair of electrodes 15 and 16 shown in FIG. 4A are provided for each pixel, one of the electrodes is used as a scanning electrode, and the other is used as a signal electrode. There is a display method similar to a matrix (XY matrix) LCD. As another method, as in a conventional TFT (thin film transistor) active matrix LCD (referred to as AM-LCD), as shown in FIG. 5, as a switching element together with electrodes 15 and 16 for each pixel on the inner surface of the transparent substrate 12. There is a method of selectively displaying each pixel by forming TFTs 33, using source buses 35 formed in columns as signal electrodes, and gate buses 36 formed in rows as scanning electrodes. In the latter case, the electrode 15 is a pixel electrode and the electrode 16 is a common electrode. In any method, a region between the teeth of the electrodes 15 and 16 becomes a display region. In the AM-LCD, a two-terminal switching element such as a diode or a varistor may be provided in addition to the three-terminal switching element such as a TFT.
[0008]
As shown in FIG. 3, the IPS LCD has a symmetric structure with respect to a plane parallel to the substrates 11 and 12 in the middle between the substrates 11 and 12, and is therefore said to have a wide viewing angle. In the literature, it is shown that the result of the simulation by the electronic computer has a smaller viewing angle dependency than the conventional TN or STN type LCD.
[0009]
[Problems to be solved by the invention]
In a conventional IPS AM-LCD, a comb-like pixel electrode 15 and a common electrode 16 are formed in close proximity to each other on the inner surface of the transparent substrate 12, and the entire display area between these electrodes is displayed. Since the proportion of the screen, that is, the aperture ratio, is smaller than that of an AM-LCD that does not use the conventional IPS method, there is a problem that white luminance is lowered.
[0010]
In addition, the conventional IPS AM-LCD has a problem that an afterimage is very likely to occur.
An object of the present invention is to reduce the decrease in white luminance and the occurrence of afterimages in a conventional IPS AM-LCD.
[0011]
[Means for Solving the Problems]
(1) In the first aspect of the invention, the first and second substrates are disposed in close proximity to each other with the liquid crystal layer interposed therebetween, a black matrix is disposed on the inner surface of the first substrate, and the switching element is disposed on the inner surface of the second substrate. The present invention relates to an IPS (in-plane switching) type active matrix liquid crystal display element in which pixel region portions including pixel electrodes and common electrodes are formed in a matrix. In particular, the black matrix is made of a conductive material, and the black matrix and the common electrode are electrically short-circuited.
[0012]
(2) In the invention of claim 2, the black matrix is made of a conductive material, a conductive film is formed on the inner surface or the outer surface thereof, and the conductive film and the common electrode are electrically short-circuited.
(3) The invention of claim 3 is that in the above (1) or (2), conductivity is imparted to the adhesive of the polarizing plate to be attached to the first or second substrate.
[0013]
(4) According to the invention of claim 4, in (1) or (2), a conductive film is formed on an outer surface or an inner surface of a polarizing plate to be attached to the first or second substrate.
(5) The invention of claim 5 is the method according to (1) or (2), wherein a conductive film is formed on the outer surface of the first or second substrate.
[0014]
(6) In the invention of claim 6, the components of the polarizing plate to be attached to the first or second substrate have conductivity.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described with reference to the examples of FIGS. However, in these drawings, the same reference numerals are given to portions corresponding to those in FIGS.
In the AM-LCD of the present invention, a black matrix 41 is formed between the respective pixels on the inner surface of the transparent substrate 11, and in the case of color display, R, G, B color filters 42 are provided in the pixels of the black matrix. And correspondingly formed. The black matrix 41 and the color filter 42 are generally used conventionally. The black matrix 41 has a function of improving contrast and color purity.
[0016]
By the way, in order to solve the above-mentioned problem that the IPS system AM-LCD is liable to cause a decrease in white luminance and an afterimage, various experimental studies have been made. As a result, the black matrix 41 is formed of a conductive material. It was found effective to short-circuit the common electrode 16 with a conductive paste (generally a conductive material) 43. In this case, it has been experimentally confirmed that the white luminance is increased by about 5%.
[0017]
In this way, since the black matrix 41 has the same potential (common potential) as the common electrode 16, the direction from the teeth of the conventional pixel electrode 15 toward the teeth of the common electrode 16, that is, perpendicular to the alignment direction 24 of the polarizing plate 22. In addition to the horizontal electric field in the direction of, in the same manner as a normal AM-LCD (a common electrode is formed on the inner surface of the transparent substrate 11) which is not a conventional IPS system, the transparent electrode directly above the pixel electrode 15 is transparent. Since a vertical electric field is generated in a direction toward the substrate 11, that is, a direction perpendicular to the transparent substrates 11 and 12, it is considered that white luminance is increased.
[0018]
Further, when the display is performed for a long time, charges are accumulated in the alignment film 17 and an afterimage is generated. However, since this charge leaks to the black matrix 41 having a constant voltage (common voltage), it is considered that the afterimage is reduced. It is done.
When the black matrix 41 is formed of a nonconductive material, a conductive film 41a is formed on the inner surface (liquid crystal side) as shown in FIG. 1C, and the conductive film 41a and the common electrode 16 are short-circuited by the conductive material 43. However, the same effect can be obtained.
[0019]
In the example of FIG. 1, the black matrix 41 and the common electrode 16 are short-circuited outside the sealing material 13, that is, outside the liquid crystal cell, but may be short-circuited inside the cell depending on circumstances.
In conventional IPS AM-LCDs, the display quality deteriorates due to local influences on the inside and outside of the liquid crystal cell due to some influence from outside or inside, resulting in light leakage when black is displayed. However, in the LCD of the present invention, the black matrix 41 is held at a constant common voltage, so that it is less susceptible to external electromagnetic or electrostatic influences, and the charged charge is black matrix. Therefore, there is an effect of suppressing deterioration in display quality due to static electricity or EMI.
[0020]
In order to further enhance the effect of preventing the influence of the static electricity and EMI, as shown in FIG. 2A, the conductive material such as carbon, gold, silver or the like is mixed into the pressure-sensitive adhesive 21a or 22a of the polarizing plate 21 or 22, and polarized. It is also possible to impart conductivity to the plate itself so that it functions as a shield plate. In this way, EMI from the outside can be prevented, and even if the polarizing plate or the transparent substrate is locally charged due to external or internal static electricity, the charge is diffused over the entire surface, and the influence of charging is affected. Weakened.
[0021]
In FIG. 2B, in order to obtain the same effect as FIG. 2A, a transparent conductive film 21b or 22b such as ITO can be provided on the outer surface of the polarizing plate 21 or 22. These transparent conductive films 21b or 22b may be provided on the inner surface of the polarizing plate 21 or 22.
Further, as shown in FIG. 2C, the same effect as in FIGS. 2A and 2B can be obtained by providing a transparent conductive film 11a or 12a such as ITO on the outer surface of the transparent substrate 11 or 12.
[0022]
Further, in the structure in which the polarizing element is disposed between the two polarizing plates, the same effect can be obtained even if conductivity is imparted to one or both of the supporting members, that is, the components of the polarizing plate. .
[0023]
【The invention's effect】
As described above, in the present invention, the black matrix 41 is short-circuited to the common electrode 16 and is held at a constant common voltage. Therefore, in addition to the lateral electric field peculiar to the IPS method, the conventional AM-LCD which is not the IPS method is used. As described above, when the vertical electric field is generated, the white luminance can be improved accordingly.
[0024]
Further, when the image is displayed for a long time, the charge (which becomes an afterimage) charged in the alignment film 17 leaks to the black matrix having a constant potential, so that the afterimage can be reduced.
Furthermore, since the black matrix 41 is held at a constant potential, the shielding effect also has an effect of suppressing deterioration in display quality due to the influence of external EMI and external or internal static electricity. When a transparent conductive film (including the case of an adhesive) is provided between the outer surface of the polarizing plate or the polarizing plate and the transparent substrate, or when conductivity is imparted to the constituent elements of the polarizing plate, the above effects are even more remarkable. is there.
[Brief description of the drawings]
FIG. 1 is a view showing an embodiment of the invention of claim 1, wherein A is a cross-sectional view, B is a plan view of a black matrix 41 of A, and C is a cross-sectional view of an essential part for explaining the embodiment of claim 2. .
FIGS. 2A and 2B are cross-sectional views showing embodiments of claims 3, 4 and 5, respectively.
FIGS. 3A and 3B are diagrams for explaining a conventional IPS LCD, in which A is an exploded perspective view of a main part, and B is a sectional view.
4A is a plan view showing an example of electrodes 15 and 16 on the transparent substrate 12 in FIG. 3, and B is a cross-sectional view showing a state in which an electric field is applied between the electrodes 15 and 16 in FIG. 3B.
5A is a plan view of an essential part of a TFT array including electrodes 15 and 16 formed on the inner surface of the transparent substrate 12 when the LCD of FIG. 3 is a TFT active matrix LCD; FIG. a 'sectional drawing.

Claims (5)

  A first and second substrate are arranged in close proximity to each other with a liquid crystal layer interposed therebetween, a black matrix is arranged on the inner surface of the first substrate, and a pixel region including a switching element, a pixel electrode, and a common electrode on the inner surface of the second substrate In an IPS (in-plane switching) type active matrix type liquid crystal display element in which the portions are formed in a matrix, the black matrix is made of a non-conductive material, and a conductive film is formed on the inner surface or outer surface thereof. A liquid crystal display element, wherein the film and the common electrode are electrically short-circuited. The liquid crystal display device according to claim 1, characterized in that prior SL adhesive polarizing plate to be pasted to the first or second substrate is electrically conductive. The liquid crystal display device according to claim 1, characterized in that the conductive layer on the outer surface or inner surface of the polarizing plate to be pasted before Symbol first or second substrate is formed. The liquid crystal display device according to claim 1, characterized in that the conductive film on the outer surface of the front Symbol first or second substrate is formed. The liquid crystal display device according to claim 1, characterized in that the component of the polarizing plate to be pasted before Symbol first or second substrate is electrically conductive.

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