JPH02176625A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To easily make a gradational display by enclosing a liquid crystal which has negative dielectric constant anisotropy between substrates where orienting films which perform vertical orienting operation are formed at least on electrodes. CONSTITUTION:This display device is provided with the transparent substrates 1 and 2, the electrodes 3 and 4 for displaying, the orienting films 5 and 6, a switching element 8, and the liquid crystal 7 which has the negative dielectric constant anisotropy. Then when a voltage V for displaying which is applied between the display electrodes 3 and 4 is 0V, the orienting films 5 and 6 orient the liquid crystal 7 vertically. Further, when V1 is applied as the voltage V for displaying, a part of the liquid crystal 7 rotates in a horizontal direction and when the voltage V for displaying is raised to V2 (>V1), the liquid crystal between the electrodes 3 and 4 rotates almost in the horizontal direction. Namely, the liquid crystal orientation state changes continuously corresponding to the voltage for displaying. Consequently, the gradational display is facilitated.

Description

[Detailed Description of the Invention] [Summary] Regarding an active matrix type liquid crystal display device in which switching elements such as thin film transistors and transistors are provided in correspondence with pixels, the present invention relates to an active matrix type liquid crystal display device in which switching elements such as thin film transistors and transistors are provided corresponding to pixels. Applying a voltage for display between a display electrode formed on the transparent substrate, an alignment film formed at least on the display electrode, a liquid crystal whose molecular orientation is controlled by the alignment film, and the display electrode. In a liquid crystal display device having a switching element for
has negative dielectric anisotropy, and the refractive index anisotropy Δ of the liquid crystal
The product of n and the gap length d for injecting the liquid crystal is Δn・
d-(m+V2) X (0.5~0.6) (
However, m is an integer), and the alignment film has a configuration in which the molecular alignment of the liquid crystal is vertically aligned at least on the display electrode when no voltage is applied, and is changed to a horizontal alignment state when a voltage is applied. And so.

[Industrial application field]

The present invention relates to an active matrix liquid crystal display device in which switching elements such as thin film transistors are provided corresponding to pixels.

Active matrix liquid crystal display devices in which switching elements such as thin film transistors (TPTs) are provided for each pixel have the advantage of providing high display quality because any pixel can be selectively driven with any applied voltage. Further, by providing a color filter, full color can be easily achieved, and this is applied to small color televisions. However, since the viewing angle characteristics are not sufficient, there is a demand for improvement.

[Conventional technology]

In conventional liquid crystal display devices, TN (twisted nematic) liquid crystal is relatively often used. In such a liquid crystal display device, for example, as shown in FIG. 9, when the alignment film of the lower substrate of the opposing substrates is laminated in the direction of the solid arrow, the upper The alignment film on the substrate is laminated in the direction of the dotted arrow to twist the molecular orientation of the liquid crystal 90 degrees, and the polarizing axes of the polarizing plates placed on both sides are arranged perpendicularly as shown by the double solid arrow and double dotted arrow. It is something to do. In the case of such orthogonal Nicols, the image is normally white. Furthermore, in the case of parallel Nicols, in which the polarization axes are arranged in parallel, the light becomes normally black.

The transmittance varies depending on the product Δn·d of the refractive index anisotropy Δn of the liquid crystal in the case of parallel Nicols and the gap length d that encloses the liquid crystal, and has, for example, the characteristics shown in FIG. 1O. In other words, the transmittance is minimum under the condition of Δn-d≠0.51.0.1.5, but as the value of Δn-d increases, the viewing angle characteristics and response characteristics deteriorate. . Therefore, normally, the gap length d is selected so that Δn-d:Q,5. The viewing angle characteristics in that case are shown in Part 11. In other words, the contrast in the horizontal direction decreases to about 70% or less at positions ±20 degrees from the center, as shown by the dotted line curve, and the contrast in the vertical direction decreases to about 70% or less at positions ±20 degrees from the center, as shown by the solid line curve. This is even lower than the viewing angle characteristics.

In this case, if Δn-d is set to 0.5 or less, the viewing angle characteristics will be improved compared to the case shown in FIG. 11, but as can be seen from FIG. 10, the transmittance characteristics will decrease and the desired contrast will not be achieved. In addition, in the case of black and white display, the display may become colored.

Ferroelectric liquid crystal is known as a liquid crystal mode that enables black and white display with an even smaller Δn-d. For example, in the case of the birefringent type shown in FIG. 12, a liquid crystal 68 is sealed between electrodes 63 and 64 formed on a substrate 61 and 62, polarizing plates 65 and 66 are arranged on both sides, and A display voltage 67 is applied, and ordinary light and extraordinary light interfere due to the birefringence effect, and when the display voltage is applied between the electrodes 63 and 64 to change the orientation direction of molecules, the interference conditions change.
It is possible to perform black and white display.

If the molecular orientation of the liquid crystal 68 is set to the direction of the polarization axis of the polarizing plate 65 on the output side (solid line arrow) and the direction of the polarization axis of the polarizing plate 66 on the input side (dotted line arrow) as shown on the right side, then the display voltage 6
When voltage 7 is not applied, the display becomes dark, and when display voltage 67 is applied to change the alignment direction of the liquid crystal molecules, the display becomes bright.

In the case of the guest-host type shown in FIG. 13, the substrate 71.
A liquid crystal 77 mixed with a dichroic dye is sealed between electrodes 73 and 74 formed on the substrate 72, a polarizing plate 75 is placed outside the substrate 71, and a display voltage 76 is applied between the electrodes 73 and 74. In this case, the liquid crystal 77 is composed of liquid crystal molecules 77a and dye molecules 77b.As shown on the right side, the dye molecules 77b have a property of being aligned parallel to the liquid crystal molecules 77a, and when an electric field is applied, the liquid crystal molecules When the orientation of the dye molecules 77a is changed, the orientation direction of the dye molecules 77b also changes, and although coloring may or may not occur depending on the orientation direction of the dye molecules 77b, display can be performed by applying the display voltage 76. .

When the polarization axis of the polarizing plate 75 is set in the direction of the solid line arrow, if the display voltage 76 is not applied, the state will be dark, and if the display voltage 76 is applied to change the molecular orientation direction of the liquid crystal 77, the state will be the bright state. Become.

[Problem to be solved by the invention]

As mentioned above, in a liquid crystal display device using a conventional TN liquid crystal, even if an attempt is made to improve the viewing angle characteristics by reducing Δn-d, the transmittance characteristics will decrease, so it is impossible to reduce it to 0.5 or less. Have difficulty.

Furthermore, although the conventional examples using ferroelectric liquid crystals shown in Figs. 12 and 13 have excellent viewing angle characteristics, it is difficult to display gradation because the display is performed using a bistable state. can be,
Furthermore, it has the disadvantage that it is susceptible to mechanical shock, making stable display difficult.

The present invention aims to improve viewing angle characteristics.

[Means to solve the problem]

The liquid crystal display device of the present invention is an active matrix type liquid crystal display device provided with switching elements such as TPT, and will be explained with reference to FIG.

Transparent substrates 1 and 2 arranged opposite to each other, display electrodes 3.4 formed on the transparent substrates 1.2, alignment films 5 and 6 formed at least on the display electrodes 3 and 4, and the alignment film 5. ,6
A liquid crystal 7 whose molecular orientation is controlled by a display electrode 3.
In a liquid crystal display device having a switching element 8 such as TPT for applying a display voltage between 4 and 4, the liquid crystal 7 has negative dielectric constant anisotropy, and the refractive index anisotropy of this liquid crystal 7 The product of the orientation Δn and the gap length d into which this liquid crystal 7 is injected is
Δn−d −(m+V2) The structure is such that the alignment state is changed to one horizontal direction when a voltage is applied.

[Effect]

The molecules of the liquid crystal in FIG. 1 are exaggerated, and when the display voltage ■ applied between the display electrodes 3 and 4 is set to 0 (when no voltage is applied), the alignment film 5, 6, the liquid crystal 7 is vertically aligned as shown on the left, and by setting the display voltage V to 1, some of the liquid crystals 7 are rotated horizontally in one direction as shown in the center. By further increasing the display voltage (2) to V, (>v,), the liquid crystal between the electrodes 3 and 4 rotates almost horizontally in one direction, as shown on the right side. That is, since the liquid crystal alignment state changes continuously in response to the display voltage, gradation display is possible.

Further, the liquid crystal 7 is brought into a vertical alignment state, and the alignment direction is changed in one direction in the horizontal plane by applying a display voltage (2), and the polarization axis of the polarizing plate is tilted at 45 degrees with respect to this one direction. In addition, in the case of crossed Nicols, the transmitted light intensity I becomes 1=Iosin2(g-ΔH'-d/λ). Here, Δn' is the maximum Δn depending on the degree of deformation of liquid crystal molecules.
(the refractive index anisotropy inherent to liquid crystal molecules), and λ is the wavelength of light. Therefore, when the liquid crystal molecules change completely horizontally, Δn゛−Δn
If the gear knob length d is selected so as to maximize the transmittance at this time, Δn-d= ((1/2)+m ) It is sufficient to set it to λ, and it becomes white when a display voltage is applied. If parallel Nicols are used, the color becomes black when a display voltage is applied. Also, if m = o, then Δn-d-λ/2, so
Minimum value Δn-d-λ when using conventional TN liquid crystal
It can be reduced to 1/2 compared to .

The viewing angle characteristics are improved as Δn-d is reduced, and as described above, the viewing angle characteristics can be improved by reducing Δn-d to 1/2 compared to the conventional example.

〔Example〕

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

FIG. 2 is a cross-sectional view of an embodiment of the present invention, in which 1112 is a transparent substrate such as glass, 13.14 is a transparent electrode such as ITO, 15.16 is an alignment film, 17 is a liquid crystal, and 18 is a TFT (
thin film transistor), 1920 is a polarizing plate, G is a gate,
D indicates a drain, and S indicates a source.

The TFTs 18 constituting the switching element are arranged orthogonally, and the intersection points are connected to data bus lines and scan lines which are insulated from each other.
TPT]
The drain D of 8 is connected to the data bus line, the gate 1-G is connected to the scan bus line, and the source S is connected to the display electrode 14.

or? The night crystal 17 used has negative dielectric anisotropy. In general, liquid crystals have a Schiff group, azo group, azoxy group, ester group, etc. as a central group, and an alkyl group, an alkoxy group, etc. are bonded through a benzene ring. Most liquid crystals without groups have negative dielectric anisotropy. As a liquid crystal having such negative dielectric constant anisotropy, for example, an alkyl cyclohexyl carbonitrile type liquid crystal (manufactured by Merck & Co., Ltd.) is known.

In addition, the alignment film 1.5.16 is used by adding, for example, a silane coupling agent having a vertical alignment effect,
As shown in an exaggerated manner, the liquid crystal molecules are vertically aligned when no voltage is applied between the electrodes 13 and 14.

Further, when a voltage is applied between the electrodes 13 and 14, the liquid crystal molecules have a slight horizontal alignment effect so that the alignment direction of the liquid crystal molecules changes in the same direction in the horizontal plane. This horizontal alignment effect in the same direction can be obtained by a rubbing process, and the rubbing directions of the alignment films 15 and 16 on the transparent substrates 11 and 12 arranged opposite to each other are opposite directions.

In addition, a polarizing plate 1920 is arranged outside the transparent substrate 11.12, and the polarization axis is inclined at 45 degrees with respect to the rubbing direction in the alignment films 15 and 1.6, so that the polarization axis is crossed Nicols or parallel Nicols. .

In a liquid crystal display device having the above-mentioned configuration, if the scan path lines are sequentially selected and a scan pulse is applied, and at the same time a display voltage according to the display information is applied to the data bus line, the scan pulse becomes The TPT 18 applied to -G is turned on, and the display voltage applied to the data bus line is applied from the drain D of the turned-on TFT 18 to the electrode 14 via the source S, and the opposing electrode 13 is grounded, for example. Then, a display voltage is applied between the electrodes 13 and 14, and the alignment direction of the liquid crystal molecules changes from vertical to horizontal in accordance with the voltage value, so that gradation display is also possible.

For example, as shown in FIG. 3(a), in the case of crossed Nicols, the polarization axes of the polarizing plates 19 and 20 are perpendicular to each other, the molecules of the liquid crystal 17 are oriented perpendicular to the plane of the paper, and the transmitted light is almost zero. be. Then, when a voltage is applied between electrodes 13 and 14, fb
), 45° relative to the polarization axis of the polarizing plate 1920.
The molecules of the liquid crystal 17 rotate in a direction parallel to the plane of the paper. The transmitted light is thereby maximized.

The above-mentioned alignment films 15 and 16 are prepared, for example, by adding 1 wt % of a silane coupling agent (octadecyldimethylammonium chloride) having a vertical alignment effect to an aqueous solution of 3 wt % of polyvinyl alcohol to form the electrodes 13 and 14. The entire surface was spin-coded, treated at 160° C., and then rubbed. As described above, this rubbing treatment is performed in opposite directions for the alignment film 15 on the transparent substrate 11 side and the alignment film 16 on the transparent substrate 12 side. When the pretilt angle in that case was optically measured, it was about 85 degrees.

A transparent substrate 11, .
During 1.2, a liquid crystal containing ester and ethane as main components was injected. The dielectric anisotropy Δε of this mixed liquid crystal is −2
, the refractive index anisotropy Δn was 0.05. And Δn
-d = 0.275, gear 2 blade length d is 5.
It was set to 5 μm.

The viewing angle characteristics of this liquid crystal display device were as shown in FIG. That is, the viewing angle characteristics in the left-right direction and the viewing angle characteristics in the up-down direction are similar, and as is clear from the comparison with the viewing angle characteristics of the conventional example shown in FIG. 11, the viewing angle characteristics are improved.

Further, the transmittance characteristics are as shown by the dotted line curve in FIG. 5, and the slope is gentler than that of the conventional example shown by the solid line curve B, and it can be seen that gradation display is easy. That is, in the conventional example, the bright state changes to the dark state with a voltage of ΔV,
In the embodiment of the present invention, the bright state changes to the dark state at Δ■゛.

Therefore, since Δ■ minus Δ■1, it becomes easy to set the voltage for gradation display.

Table 1 shows comparative characteristics between the case where a conventional TN liquid crystal is used and the case where a liquid crystal with negative dielectric anisotropy according to an embodiment of the present invention is used.

Table 1 It is desirable that the specific resistance of the liquid crystal be large in order to efficiently apply display voltage to the liquid crystal, and in the embodiment of the present invention, the specific resistance is one order of magnitude larger than that of the conventional example. It became resistance. Further, the writing efficiency is the ratio of the effective voltage applied to the liquid crystal to the display voltage, and in the present invention, the entire display voltage is the effective voltage. Furthermore, it is desirable that the static electricity during rubbing be small, and in the embodiment of the present invention, since the horizontal dispensing effect is slightly generated, weak rubbing is sufficient. Since the values are low, this is advantageous in terms of preventing electrostatic damage to the TFT 18.

Image retention is the performance of whether or not the pattern remains as an afterimage when the screen is switched when a certain display pattern is displayed continuously, and it depends on the △ε/ε of the liquid crystal. In the example, no afterimage occurred even after one hour (IH) or more had elapsed.

From the above points, the refractive index anisotropy Δn of the liquid crystal 17,
The product Δn d of the gap length d for injecting the liquid crystal 17 is
The range of 0.25<Δn−d<0.3 was suitable.

Also, where m is an arbitrary integer and λ is the wavelength of light, Δn・d#(
m10%) Xo, it was suitable to set it to 55. Therefore, if m=Q, Δn-d=0.27
5, which is the condition for the above-mentioned embodiment. Therefore,
In the present invention, the refractive index anisotropy Δn, gear, and length d
The product of Δn-d-(m++A)X (0,5~0.
6).

FIG. 6 (b) is an explanatory diagram of the method for manufacturing an alignment film according to the first embodiment of the present invention, in which, as shown in (a), display electrodes 32 are formed on a transparent substrate 31. . This electrode 32 corresponds to electrodes 13, 14 in FIG. An alignment film 33 having a vertical alignment effect is formed on the entire surface including the electrode 32, and rubbed in one direction with a rubbing brush 34 or the like as shown in FIG. 3B. As a result, the liquid crystal molecules are vertically aligned, and the direction in which they rotate in one horizontal direction according to the rubbing direction when a voltage is applied is determined, making it possible to display uniformly and with high contrast.

FIG. 7 (al to tc+ are explanatory diagrams of the method for manufacturing an alignment film according to the second embodiment of the present invention; as shown in FIG. 7(a), a display electrode 42 is formed on a transparent substrate 41, This electrode 42
An alignment film 43 made of polyimide or the like having a horizontal dispensing effect is formed on the entire surface including the wafer. Then, as shown in (bl), rubbing is performed in one direction with a rubbing brush 44, etc., and then (C
As shown in FIG. 1, an alignment film 45 made of the above-mentioned silane coupling agent or the like having a vertical alignment effect is formed on the alignment film 43. In this case, an alignment film 45 having a vertical alignment effect is used.
As a result, the liquid crystal molecules become vertically aligned, and the horizontal alignment direction of the liquid crystal molecules when voltage is applied is regulated by the horizontal alignment effect of the alignment film 43.

FIG. 8 is an explanatory diagram of the arrangement of alignment films according to the third embodiment of the present invention, in which 51 is a scan line, 52 is a data bus line, and the orthogonally arranged intersections are insulated by an insulating layer (not shown). has been done.

Further, 53 is a TF consisting of a gate G, a drain D, and a source S.
T and 54 are electrodes for display, and 55 is an alignment film having a vertical alignment function. The alignment film 55 having the vertical alignment effect can be formed only on the electrode 54, and the other portions can be made into an alignment film having a normal horizontal alignment effect. In order to selectively form such an alignment film 55, screen printing or etching technology may be used.

The liquid crystal on this alignment film 55 becomes vertically aligned, and the liquid crystal around it becomes horizontally aligned in the direction following the rubbing direction of the alignment film other than the alignment film 55, and when a voltage is applied to the electrode 54, the liquid crystal around it becomes vertically aligned. Since the aligned liquid crystal changes in accordance with the alignment direction of the water-oriented liquid crystal adjacent thereto, the alignment film 55 does not need to be rubbed for horizontal alignment.

The present invention is not limited only to the above-mentioned embodiments (
For example, the alignment film can of course be constructed using a known material having a vertical alignment effect, and it is also possible to provide a full color display by providing a color filter.

In addition, in the electrode arrangement shown in FIG. 2 or FIG. 8, a liquid crystal display having a structure in which the scan bus line and the data bus line are provided on the same transparent substrate, and the data bus line is provided on the other transparent substrate. The present invention can also be applied to devices.

〔Effect of the invention〕

As explained above, in the present invention, the liquid crystal 7 having negative dielectric constant anisotropy is placed between the substrates 1 and 2 on which alignment films 5 and 6 having a vertical alignment effect are formed at least on the electrodes 3 and 4. The liquid crystal molecules continuously change their alignment direction in one horizontal direction in response to the voltage applied between the electrodes 3 and 4 via the switching element 8 such as TPT.
Gradation display can be easily performed by selecting the applied voltage.

In addition, the product of the refractive index anisotropy Δn and the gap length d, where m is an integer, is Δn−d−(m+%) X (0.5 to 0.6
), so that m
= Q, Δ when using the conventional TN liquid crystal
It becomes possible to select 1/2 of n-d, which has the advantage of improving viewing angle characteristics.

Furthermore, when an intermediate level voltage is applied as a display voltage, a difference in transmittance occurs depending on the wavelength, so it is possible to perform color display without using a color filter. Therefore, it can also be used as a projection display device.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of the principle of the present invention, Fig. 2 is a sectional view of an embodiment of the present invention, Fig. 3 is an explanatory diagram in the case of crossed Nicols (a+, (b+), and Fig. 4 is an embodiment of the present invention). visual temporal curve diagram,
FIG. 5 is a transmittance characteristic curve diagram, FIG. 6 (al, (b) and FIG. 7 fal to (C) are explanatory diagrams of the method for manufacturing an alignment film according to an embodiment of the present invention, and FIG. 8 is a diagram of the present invention. FIG. 9 is an explanatory diagram of the orientation processing direction and polarization axis of a conventional TN liquid crystal, and FIG. 10 is an illustration of Δn-d in the case of parallel Nicols.
FIG. 11 is a viewing angle characteristic curve diagram of a conventional example, FIG. 12 is an explanatory diagram of a birefringence type of a conventional example, and FIG.
The figure is an explanatory diagram of a conventional guest-host type system. 1.2 is a transparent substrate, 3 and 4 are electrodes, 5.6 is an alignment film, and 7
8 is a liquid crystal, and 8 is a switching element.

Claims (1)

[Claims] Transparent substrates (1, 2) arranged opposite to each other;
2) Display electrodes (3, 4) formed on the display electrodes (3, 4) and alignment films (5, 6) formed at least on the display electrodes (3, 4).
, a liquid crystal (7) whose molecular orientation is controlled by the alignment film (5, 6), and a switching element (8) for applying a display voltage between the display electrodes (3, 4). In the liquid crystal display device, the liquid crystal (7) has negative dielectric anisotropy, and the refractive index anisotropy Δn of the liquid crystal (7) and the gap into which the liquid crystal (7) is injected The product with the length d is Δn・d=(m+1/2)×
(0.5 to 0.6) (where m is an integer), and the alignment film (5, 6) adjusts the molecular alignment of the liquid crystal (7) at least to the display electrodes (3, 4) when no voltage is applied. ) A liquid crystal display device characterized in that the liquid crystal display device is configured to have a vertically aligned state at the top thereof, and to change to a horizontally unidirectionally aligned state when a voltage is applied.