JP2548979B2 - Liquid crystal display - Google Patents

Description

The present invention relates to an active matrix type liquid crystal display device in which switching elements such as thin film transistors are provided corresponding to pixels, and a transparent substrate and a transparent substrate which are opposed to each other for the purpose of improving viewing angle characteristics. A display electrode formed on the display electrode, an alignment film formed on at least the display electrode, a liquid crystal whose molecular orientation is controlled by the alignment film, and a display voltage for applying a display voltage between the display electrodes. In a liquid crystal display device having a switching element, the liquid crystal has a negative dielectric anisotropy, and a product of a refractive index anisotropy Δn of the liquid crystal and a gap length d for injecting the liquid crystal. Is Δ
n · d = (m + 1/2) × (0.5 to 0.6) (where m is an integer), and the alignment film aligns the molecular alignment of the liquid crystal vertically at least on the display electrode when no voltage is applied. In this state, when the voltage is applied, the orientation is changed to one horizontal direction.

[Industrial applications]

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

An active matrix type liquid crystal display device provided with a switching element such as a thin film transistor (TFT) corresponding to a pixel has an advantage that high display quality can be obtained because an arbitrary pixel can be selectively driven by an applied voltage. Further, by providing a color filter, full color can be easily realized, and it is applied to a small color television.
However, since the viewing angle characteristic is not sufficient, its improvement is desired.

[Conventional technology]

In the conventional liquid crystal display device, TN (twisted nematic) liquid crystal is used relatively often. In such a liquid crystal display device, for example, as shown in FIG. 9, when the alignment film of the lower substrate among the substrates arranged to face each other is rubbed in the direction of the solid line arrow, the alignment film is aligned so as to be orthogonal to it. The alignment film of the substrate is rubbed in the direction of the dotted arrow to twist the molecular orientation of the liquid crystal by 90 degrees, and the polarization axes of the polarizing plates placed on both sides are arranged orthogonally as indicated by the double solid arrow and double dotted arrow. To do. In the case of such a crossed Nicol, it is normally white. In the case of parallel Nicols in which the polarization axes are arranged in parallel, normally black is obtained.

The transmittance differs depending on the product of the refractive index anisotropy Δn of the liquid crystal in the case of parallel Nicols and the gap length d that encloses this liquid crystal, Δn · d, and has the characteristics shown in FIG. 10, for example. That is, the transmittance becomes minimum under the condition of Δn · d≈0.5, 1.0, 1.5, ... However, as the value of Δn · d increases, the viewing angle characteristic and the response characteristic deteriorate. Therefore, normally, the gap length d is set so that Δn · d≈0.5.
Will be selected. The viewing angle characteristics in that case are shown in FIG. That is, as shown by the dotted line curve, the horizontal viewing angle characteristic shows that the contrast decreases to about 70% or less at the position of ± 20 degrees from the center, and the vertical viewing angle characteristic shows the horizontal direction as shown by the solid line curve. It is even lower than the viewing angle characteristic.

In this case, if Δn · d is set to 0.5 or less, the viewing angle characteristics are improved as compared with the case shown in FIG. 11, but as can be seen from FIG. 10, the transmittance characteristics are lowered and the desired contrast is obtained. There may be no display, and in the case of black and white display, color display may occur.

Ferroelectric liquid crystal is known as a liquid crystal mode that enables monochrome display with a smaller Δn · d. For example,
In the case of the birefringence type shown in FIG. 12, the liquid crystal 68 is enclosed between the electrodes 63 and 64 formed on the substrates 61 and 62, and the polarizing plates 65 and 66 are arranged on both sides to display between the electrodes 63 and 64. A voltage 67 is applied.Ordinary light and extraordinary light interfere with each other due to the birefringence effect, and when the display voltage is applied between the electrodes 63 and 64 to change the orientation direction of the molecules, the interference condition changes, resulting in black and white. Display can be performed.

In addition, the molecular orientation of the liquid crystal 68 is set to the polarization axis direction (solid arrow) of the polarizing plate 65 on the outgoing side as shown on the right side, and the polarizing plate 66 on the incoming side is used.
When the display voltage 67 is not applied, a dark state is obtained when the polarization axis direction of (1) is indicated (dotted arrow), and when the display voltage 67 is applied to change the alignment direction of the liquid crystal molecules, a bright state is obtained.

In the case of the guest host type shown in FIG. 13, the substrates 71, 72
Liquid crystal 7 in which dichroic dye is mixed between electrodes 73 and 74 formed on
7, the polarizing plate 75 is arranged outside the substrate 71, the electrodes 73,
A display voltage 76 is applied between 74, and the liquid crystal 77 in that case is composed of liquid crystal molecules 77a and dye molecules 77b, and the dye molecules 77b are parallel to the liquid crystal molecules 77a as shown on the right side. It has an orientation property, and when the orientation of the liquid crystal molecule 77a is changed by applying an electric field, the orientation direction of the dye molecule 77b also changes, and it may or may not be colored depending on the orientation direction of the dye molecule 77b. Display can be performed by applying a 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, a dark state occurs and the display voltage is
When 76 is applied to change the molecular orientation direction of the liquid crystal 77, a bright state is obtained.

[Problems to be Solved by the Invention]

As described above, in the liquid crystal display device using the TN liquid crystal of the conventional example, even if an attempt is made to reduce Δn · d to improve the viewing angle characteristic, the transmittance characteristic is deteriorated, so it is difficult to reduce the value to 0.5 or less. is there.

Further, in the conventional example using the ferroelectric liquid crystal shown in FIG. 12 and FIG. 13, although the viewing angle characteristic is excellent, since the display is performed by utilizing the bistable state, it is difficult to display gradation. However, it has a drawback that it is weak against mechanical shock and stable display is difficult.

The present invention aims to improve the viewing angle characteristics.

[Means for solving the problem]

The liquid crystal display device of the present invention is an active matrix liquid crystal display device provided with a switching element such as a TFT, and will be described with reference to FIG.

The transparent substrates 1 and 2 facing each other, the display electrodes 3 and 4 formed on the transparent substrates 1 and 2, the alignment films 5 and 6 formed on at least the display electrodes 3 and 4, and the alignment film 5 In a liquid crystal display device having a liquid crystal 7 whose molecular orientation is controlled by the electrodes 6 and 6, and a switching element 8 such as a TFT for applying a display voltage between the display electrodes 3 and 4, the liquid crystal 7 is It has a negative dielectric anisotropy, and the product of the refractive index anisotropy Δn of this liquid crystal 7 and the gap length d into which this liquid crystal 7 is injected is Δn · d = (m + 1 /
2) × (0.5 to 0.6), and the alignment films 5 and 6 make the molecular alignment of the liquid crystal 7 in a vertical alignment state at least on the display electrodes 3 and 4 when no voltage is applied, and when the voltage is applied, the alignment film is aligned in one horizontal direction. A structure in which transparent plates 1 and 2 are provided with a polarizing plate (not shown) having a configuration of changing to an alignment state and having a polarization axis of approximately 45 degrees with respect to the molecular alignment direction of the liquid crystal 7 when a voltage is applied. Is.

[Action]

The molecules of the liquid crystal in FIG. 1 are shown in an exaggerated form, and when the display voltage V applied between the display electrodes 3 and 4 is set to 0 (no voltage is applied), the alignment film 5, 6, liquid crystal 7
Is in a vertical alignment state as shown on the left side, and by setting the display voltage V to V ₁ , a part of the liquid crystal 7 is rotated in one horizontal direction as shown at the center, and Voltage V
By increasing the value to V ₂ (> V ₁ ), the liquid crystal between the electrodes 3 and 4 rotates almost in one horizontal direction as shown on the right side. That is, since the liquid crystal alignment state changes continuously according to the display voltage, gradation display is possible.

Further, the liquid crystal 7 is vertically aligned, and by applying the display voltage V, the alignment direction is changed in one direction in the horizontal plane, and the polarization axis of the polarizing plate is inclined by 45 degrees with respect to this one direction. Further, when the crossed Nicols are used, the transmitted light intensity I becomes I = I ₀ sin ² (π · Δn ′ · d / λ). Here, Δn ′ is the effective refractive index anisotropy that changes to the maximum Δn (the original refractive index anisotropy of the liquid crystal molecules) according to the degree of deformation of the liquid crystal molecules, and λ is the wavelength of light. . Therefore, when the liquid crystal molecules change completely in the horizontal direction, Δn ′ → Δn, and the gap length d is selected so that the transmittance at this time is maximized. That is, m is an arbitrary integer, λ As the wavelength of light, Δn · d = [(1/2) + m]
If λ is set, white is produced by applying the display voltage. If parallel Nicols are used, black is produced by applying a display voltage. Further, when m = 0, Δn · d = λ / 2, and therefore Δn · d = the minimum value when the conventional TN liquid crystal is used.
It can be halved compared to λ.

Further, the viewing angle characteristics are improved as Δn · d is decreased. As described above, the viewing angle characteristics can be improved by reducing Δn · d to 1/2 as compared with the conventional example.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a sectional view of an embodiment of the present invention, in which 11 and 12 are transparent substrates such as glass, 13 and 14 are transparent electrodes such as ITO, and 15 and 16 are shown.
Is an alignment film, 17 is a liquid crystal, 18 is a TFT (thin film transistor), 1
Reference numerals 9 and 20 denote polarizing plates, G denotes a gate, D denotes a drain, and S denotes a source.

The TFTs 18 that form the switching element are arranged orthogonally,
The intersection is arranged at the intersection of the data bus line and the scan bus line (not shown) which are insulated from each other, the drain D of the TFT 18 is connected to the data bus line, the gate G is connected to the scan bus line, and the source S is displayed. Electrode for
It is connected to the.

Further, the liquid crystal 17 is one having a negative dielectric constant anisotropy. In general, a liquid crystal has a Schiff group, an azo group, an azoxy group, an ester group, and the like as a central group, and an alkyl group, an alkoxy group, a cyano group, and the like are bonded via a benzene ring. Most liquid crystals that do not have negative dielectric anisotropy. Examples of the liquid crystal having such a negative dielectric anisotropy include, for example, an alkylcyclohexylcarbonitrile system (manufactured by Merck).
Liquid crystal is known.

Further, the alignment films 15 and 16 are used by adding, for example, a silane coupling agent having a vertical alignment action, and when the voltage is not applied between the electrodes 13 and 14 as shown by exaggerating the liquid crystal molecules. It is an oriented state. Electrode 1
When a voltage is applied between 3 and 14, a slight horizontal alignment action is given so that the alignment direction of the liquid crystal molecules changes in the same direction in the horizontal plane. This horizontal alignment action in the same direction can be obtained by a rubbing treatment, and the rubbing directions in the alignment films 15 and 16 on the transparent substrates 11 and 12 arranged opposite to each other are opposite to each other.

Further, polarizing plates 19 and 20 are arranged outside the transparent substrates 11 and 12, and the polarization axis is inclined by 45 degrees with respect to the rubbing direction in the alignment films 15 and 16 to form orthogonal Nicols or parallel Nicols. .

In the liquid crystal display device having the above-described structure, when the scan bus lines are sequentially selected and the scan pulse is applied, and at the same time, the display voltage according to the display information is applied to the data bus line, the scan pulse is applied to the gate G. The TFT 18 applied to the TFT is turned on, and the display voltage applied to the data bus line is applied to the electrode 14 via the drain D to the source S of the TFT 18 in the turned-on state, and the opposite electrode 13 is applied.
When, for example, is grounded, a display voltage is applied between the electrodes 13 and 14, and the alignment direction of the liquid crystal molecules changes from the vertical direction to the horizontal direction according to the voltage value, so that gray scale 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 orthogonal to each other, the molecules of the liquid crystal 17 are oriented perpendicular to the paper surface, and the transmitted light is almost zero. is there. Then, when a voltage is applied between the electrodes 13 and 14, as shown in (b), the molecules of the liquid crystal 17 rotate in the direction of 45 degrees with respect to the polarization axes of the polarizing plates 19 and 20 in the direction parallel to the paper surface. To do. Thereby, the transmitted light is maximized.

The alignment films 15 and 16 described above are spun on the entire surface including the electrodes 13 and 14 by adding 1 wt% of a silane coupling agent (octadecyldimethylammonium chloride) having a vertical alignment action to an aqueous solution of 3 wt% of polyvinyl alcohol. After coating and treatment at 160 ° C., a rubbing treatment was applied to form. This rubbing process is performed on the transparent substrate as described above.
The orientations of the orientation film 15 on the 11 side and the orientation film 16 on the transparent substrate 12 side are opposite to each other. When the pretilt angle in that case was optically measured, it was about 85 degrees.

Between the transparent substrates 11 and 12 having such alignment films 15 and 16,
A liquid crystal containing an ester type and an ethane type as main components was injected. The dielectric anisotropy Δε of this mixed liquid crystal was −2, and the refractive index anisotropy Δn was 0.05. The gap length d was set to 5.5 μm so that Δn · d = 0.275.

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

Further, the transmittance characteristic is as shown by the dotted curve A in FIG. 5, and the inclination is gentler than that of the characteristic of the conventional example shown by the solid curve B, which shows that gradation display is easy. That is, in the conventional example, the voltage changes from the bright state to the dark state with a voltage of ΔV, and in the embodiment of the present invention, the voltage changes from the bright state to the dark state with ΔV ′. Therefore, since ΔV <ΔV ′, it becomes easy to set the voltage for gradation display.

Table 1 shows comparative characteristics between the case of using the TN liquid crystal of the conventional example and the case of using the liquid crystal of the negative dielectric anisotropy of the embodiment of the present invention.

It is desirable that the specific resistance of the liquid crystal is large in order to efficiently apply the display voltage to the liquid crystal, and in the embodiment of the present invention, the specific resistance is one digit higher than that of the conventional example. . The writing efficiency is the ratio of the effective voltage applied to the liquid crystal to the display voltage, and in the present invention, all the display voltages are effective voltages. Further, it is desirable that the static electricity during rubbing is small, and in the embodiment of the present invention, since a slight horizontal alignment action is caused, weak rubbing will suffice. Since it is a value, it is advantageous from the viewpoint of preventing electrostatic breakdown of the TFT 18.

Image sticking is the performance of whether or not the pattern remains as an afterimage when the screen is switched when a certain display pattern is continuously displayed, and depends on the liquid crystal Δε / ε.
In the examples of the present invention, an afterimage did not occur even after 1 hour (1H) or more.

From the above points, the refractive index anisotropy Δn of the liquid crystal 17 and
The product Δn · d of the gap length d for injecting the liquid crystal 17 is 0.
It was preferable that the range was 25 <Δn · d <0.3.

In addition, m is an arbitrary integer and λ is a wavelength of light, and Δn · d≈
It was preferable to set it to be (m + 1/2) × 0.55. Therefore, when m = 0, Δn · d = 0.275, which is the condition for the above-described embodiment. Therefore, in the present invention, the product of the refractive index anisotropy Δn and the gap length d is within the range of Δn · d = (m + 1/2) × (0.5 to 0.6).

FIGS. 6 (a) and 6 (b) are explanatory views of the method for producing an alignment film of the first embodiment of the present invention. As shown in FIG. 6 (a), a display electrode 32 is formed on a transparent substrate 31. Form. This electrode 32
Corresponds to the electrodes 13 and 14 in FIG.
An alignment film 33 having a vertical alignment action on the entire surface including the electrode 32.
Are formed and are rubbed in one direction by a rubbing brush 34 or the like as shown in (b). Thereby, the liquid crystal molecules become
Along with the vertical orientation, the direction of rotation in one horizontal direction according to the rubbing direction is determined when a voltage is applied, and uniform and high-contrast display is possible.

FIGS. 7 (a) to 7 (c) are explanatory views of a 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. An alignment film 43 having a horizontal alignment action such as polyimide is formed on the entire surface including the electrode 42.
To form. Then, as shown in (b), it is rubbed in one direction by a rubbing brush 44 or the like, and then, as shown in (c), it comprises the above-mentioned silane coupling agent or the like having a vertical alignment action on the alignment film 43. An alignment film 45 is formed. In this case, the liquid crystal molecules are vertically aligned by the alignment film 45 having the vertical alignment action, and the horizontal alignment direction of the liquid crystal molecules when voltage is applied is regulated by the horizontal alignment action by the alignment film 43.

FIG. 8 is a layout explanatory view of an alignment film of a third embodiment of the present invention, 51 is a scan bus line, 52 is a data bus line, and the intersecting points arranged orthogonally are insulated by an insulating layer (not shown). Has been done. Further, 53 is a TFT including a gate G, a drain D and a source S, 54 is a display electrode, and 55 is an alignment film having a vertical alignment action. The alignment film 55 having the vertical alignment function can be formed only on the electrode 54, and the other portions can be an alignment film having the normal horizontal alignment function. In order to selectively form such an alignment film 55, screen printing or etching technology may be used.

The liquid crystal on the alignment film 55 has a vertical alignment, and the liquid crystal around the alignment film has a horizontal alignment in a direction according to the rubbing direction of the alignment films other than the alignment film 55, and when a voltage is applied to the electrode 54, the liquid crystal is vertically aligned. Since the oriented liquid crystal changes according to the orientation direction of the horizontally oriented liquid crystal adjacent thereto, the orientation film 55 does not need to be rubbed for the horizontal orientation.

The present invention is not limited to the above-described embodiments, and it goes without saying that the alignment film can be formed using a material having a vertical alignment action which is already known, and a color filter is provided. It is also possible to perform full-color display. Further, in the electrode arrangement shown in FIG. 2 or FIG. 8, a liquid crystal display having a structure in which the data bus line of the scan bus line and the data bus line on the same transparent substrate is provided on the other transparent substrate. The present invention can also be applied to a device.

〔The invention's effect〕

As described above, according to the present invention, the liquid crystal 7 having the negative dielectric anisotropy is formed between the substrates 1 and 2 on which the alignment films 5 and 6 having the vertical alignment action 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 a TFT. It is possible to easily perform gradation display by selection.

Further, the product of the refractive index anisotropy Δn and the gap length d is set to be in the range of Δn · d = (m + 1/2) × (0.5 to 0.6), where m is an integer. When m = 0, Δn · d of the conventional TN liquid crystal is used.
It is possible to select 1/2, which has the advantage that the viewing angle characteristics can be improved.

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

[Brief description of drawings]

1 is an explanatory view of the principle of the present invention, FIG. 2 is a sectional view of an embodiment of the present invention, FIGS. 3 (a) and 3 (b) are explanatory views in the case of an orthogonal Nicol, and FIG. 4 is an illustration of the present invention. FIG. 5 is a view angle characteristic curve diagram of the embodiment, FIG. 5 is a transmittance characteristic curve diagram, and FIGS. 6 (a), (b) and FIGS. 7 (a) to 7 (c) are manufacture of the alignment film of the embodiment of the present invention. FIG. 8 is an explanatory view of a method, FIG. 8 is an explanatory view of an alignment film arrangement of an embodiment of the present invention, FIG. 9 is an explanatory view of an alignment treatment direction and a polarization axis of a conventional TN liquid crystal, and FIG. 10 is a case of parallel nicols. A relational curve diagram between Δn · d and transmittance, FIG. 11 is a view angle characteristic curve diagram of a conventional example,
FIG. 12 is an explanatory view of a birefringence type of a conventional example, and FIG. 13 is an explanatory view of a guest host type of a conventional example. 1, 2 are transparent substrates, 3 and 4 are electrodes, 5 and 6 are alignment films, 7 is liquid crystal,
8 is a switching element.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kaichi Toyama, Kaichi Toyama 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Within Fujitsu Limited (72) Inventor Susuke Kobayashi, Nishiozumi, Nerima-ku, Tokyo 3-13-40 (56) ) Reference JP 59-81621 (JP, A) JP 62-180326 (JP, A) JP 2-151830 (JP, A)

Claims (1)

(57) [Claims] 1. A transparent substrate (1, 2) facing each other, a display electrode (3, 4) formed on the transparent substrate (1, 2), and at least on the display electrode (3, 4). Alignment film (5,6)
A liquid crystal (7) whose molecular orientation is controlled by the orientation films (5, 6) and a switching element (8) for applying a display voltage between the display electrodes (3, 4). In the liquid crystal display device having the liquid crystal (7), the liquid crystal (7) has a negative dielectric anisotropy, and a refractive index anisotropy Δn of the liquid crystal (7) and a gap for injecting the liquid crystal (7). The product of length d and Δn · d = (m + 1/2) ×
(0.5 to 0.6) (where m is an integer), the alignment film (5, 6) defines the molecular alignment of the liquid crystal (7),
A molecular orientation of the liquid crystal (7) when the voltage is applied is such that the liquid crystal (7) is vertically aligned at least on the display electrodes (3, 4) when no voltage is applied, and is changed to a horizontal unidirectional orientation when voltage is applied. A polarizing plate having a polarization axis of about 45 degrees to the transparent substrate (1,
A liquid crystal display device characterized in that it is provided in 2).