JP2003241223A - Thin film transistor type liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin film transistor type liquid crystal display device which can realize both of the reduction of a flicker and quick response compatibly when being used as a liquid crystal TV or a liquid crystal monitor. <P>SOLUTION: A liquid crystal composition having ≤1.7 steepness γ of T-V characteristics of a liquid crystal panel and having 130 mPa×s viscosity at 25°C is used to improve the display quality by reduction of a flicker phenomenon. Furthermore, a capacitive coupling driving system is combined with the above composition, and the difference (ΔV) between a maximum voltage and a minimum voltage applied to a liquid crystal layer of the liquid crystal panel is set to ≥4 V to reduce the flicker without sacrificing the response speed. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor type liquid crystal display device using a twisted nematic liquid crystal material, which is a flicker phenomenon for a liquid crystal display device such as a liquid crystal TV or a liquid crystal monitor which requires moving image display performance. The present invention relates to a liquid crystal display device that is effective in reducing the noise and obtaining excellent moving image display characteristics.

[0002]

2. Description of the Related Art A conventional thin film transistor (hereinafter referred to as "TFT") type liquid crystal display device using a capacitive coupling drive system is provided with an active matrix substrate and a counter electrode substrate, and a spacer of several microns is provided between them. Gaps are formed, and the liquid crystal composition is filled therein.
To each pixel electrode provided on the active matrix substrate, an AC signal whose polarity is inverted every vertical period is applied between the pixel electrode and the common electrode provided on the counter electrode substrate.

FIG. 6 shows a TF using a capacitive coupling drive system.
An electrically equivalent circuit of a pixel portion in a T-type liquid crystal display device is shown. Each display element has a TFT 3 at the intersection of the scanning electrode 1 and the image signal wiring 2. The TFT 3 has a gate-drain capacitance Cgd6 as a parasitic capacitance, and a liquid crystal capacitance Clc7 and a storage capacitance Cs as capacitances intentionally formed.
There is t8. Here, the Cst 8 is formed between the pre-stage scan electrode 4 and the pixel electrode (drain) 5.

In actual driving, first the TFT is turned on.
The signal voltage Vs is supplied to the pixel electrode during the N period. After that, the voltage change of the preceding scan electrode signal (compensation voltage Vge
(+) Or Vge (-) capacitive coupling voltage Vcc
Are supplied to the pixel electrode via the storage capacitor Cst. The capacitive coupling voltage Vcc is finally changed to the pixel voltage Vp by changing the liquid crystal capacitance Clc due to the dielectric anisotropy of the liquid crystal.
It has the characteristic of overshooting.

That is, in comparison with the normal driving method, the capacitive coupling driving method automatically applies an overdrive voltage in the direction of amplifying the change at the moment the display image changes, and the dynamic behavior of the capacitive coupling voltage. Has the effect of significantly improving the response characteristics of the TFT liquid crystal display device.

[0006]

However, the TFT
In a liquid crystal display device, an AC signal whose polarity is inverted every vertical cycle is applied between each pixel electrode provided on an active matrix substrate and a common electrode provided on a counter electrode substrate. Due to the DC anisotropy generated in the liquid crystal panel due to the dielectric anisotropy of the composition, the wiring capacitance, etc., the absolute value of the effective voltage applied to the liquid crystal differs between the positive polarity and the negative polarity. Therefore, the brightness of the display screen changes depending on whether the polarity is positive or negative, and flicker (flicker) occurs, which causes deterioration of image quality. Flicker is particularly noticeable in halftone display.

In the capacitive coupling drive type liquid crystal display device, since the parasitic capacitance Cst is formed, the wiring capacitance of the scanning electrode increases,
Within the gate-on time of the TFT, a charging error occurs in which the image signal cannot be transmitted to the pixel electrode through the TFT. Alternatively, there is a time delay until the scanning voltage reaches the gate-off level, the charged charges leak, the pixel potential changes, and flicker becomes noticeable.

Furthermore, if the response speed is increased by using a low-viscosity liquid crystal composition for the purpose of improving moving image display characteristics and capacitive coupling drive, the change in luminance due to the change in pixel potential will become faster. More and more flicker becomes noticeable.

Therefore, in order to solve such a problem, the present invention combines a low-viscosity twisted nematic (TN) liquid crystal composition and a capacitive coupling drive system to
We have realized a liquid crystal display device with a response characteristic of one frame or less. That is, it is an object of the present invention to provide a TFT type liquid crystal display device using a capacitive coupling drive system, which can achieve both reduction of flicker and high-speed response.

[0010]

In order to achieve the above object, a TFT type liquid crystal display device according to claim 1 of the present invention comprises:
An active matrix substrate having a switching element composed of a thin film transistor and a pixel electrode connected to the switching element, a counter electrode substrate having at least a common electrode facing the switching element, and a space between the active matrix substrate and the counter electrode substrate. A liquid crystal display device comprising a liquid crystal panel formed of a liquid crystal layer formed of the above liquid crystal compound, wherein the steepness γ of the transmittance vs. drive voltage characteristic curve of the liquid crystal panel is 1.7 or less, and The viscosity of the liquid crystal composition at 25 ° C. is 130 mPa · s
It is characterized by the following.

A TFT type liquid crystal display device according to a second aspect of the present invention is the TFT type liquid crystal display device according to the first aspect, wherein the panel has a plurality of scanning electrodes and image signal wiring on the active matrix substrate. The thin film transistor element is provided at each intersection, and the scan electrode and the pixel electrode connected to the thin film transistor element are capacitive coupling drive type liquid crystal panels which are coupled by a parasitic capacitance (Cst). To do.

A TFT type liquid crystal display device according to claim 3 of the present invention is the TFT type liquid crystal display device according to claim 1 or 2, wherein the liquid crystal layer filled between the active matrix substrate and the counter electrode substrate is formed. Thickness (d) is 3-6
and the difference (ΔV) between the maximum voltage and the minimum voltage applied to the liquid crystal layer is 4 V or more.

With this configuration, when used in a liquid crystal display device used as a liquid crystal television or a liquid crystal monitor, both flicker reduction and high-speed response performance can be achieved.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION A TFT type liquid crystal display device of the present invention has an active matrix substrate having a switching element composed of a thin film transistor and a pixel electrode connected to the switching element, and at least a common electrode facing the active matrix substrate. What is claimed is: 1. A liquid crystal display device, comprising: a counter electrode substrate; and a liquid crystal panel comprising a liquid crystal layer formed from a liquid crystal composition filled between the active matrix substrate and the counter electrode substrate, wherein the liquid crystal panel has a transmissivity to drive. The steepness γ of the voltage characteristic curve is 1.7 or less, and the viscosity of the liquid crystal composition filled in the liquid crystal layer at 25 ° C. is 130 mPa · s or less.

According to this TFT type liquid crystal display device, the transmissivity vs. applied voltage characteristic curve of the liquid crystal panel is made steep and the voltage difference between the gradations is made small, thereby slowing down the response speed in the halftone. Fluctuation can be reduced even in a liquid crystal display device that uses a capacitive coupling drive method by suppressing a change in luminance associated with a change in pixel potential.

Further, a high speed response can be maintained by setting the difference ΔV between the maximum voltage and the minimum voltage applied to the liquid crystal layer to 4 V or more.

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

(Embodiment) FIG. 1 shows an embodiment of the present invention.
FIG. 7 is a correlation diagram of the transmittance vs. drive voltage characteristic (hereinafter, referred to as TV characteristic) of the liquid crystal panel γ and the flicker level in the halftone display of the liquid crystal display device. Where T-
The steepness γ of the V characteristic has a transmittance of 90% as shown in FIG.
% Voltage is V90, and the transmittance is 10% is V
Ratio of V10 and V90 when defined as 10, V10 / V
It was set to 90.

There are two types of T-V characteristics of the liquid crystal panel, a normally white mode and a normally black mode. In the embodiment of the present invention, the normally white mode is used. However, the same effect as the normally white mode can be obtained by using the normally black mode liquid crystal panel.

Further, the steepness γ of the TV characteristic of the liquid crystal panel
In order to clarify the correlation between the flicker level and the flicker level, the correlation of the flicker level of the liquid crystal panel in which the cell gap was fixed and the threshold value of the liquid crystal composition was changed was confirmed.

As shown in FIG. 1, the steepness γ of the TV characteristic is shown.
There is a good correlation between the flicker level and the flicker level, and the flicker is reduced by reducing the steepness γ of the TV characteristic, that is, by making the gradient of the TV characteristic curve steep.

When the flicker level is -35 dB or less, it is difficult for the human eye to recognize and to stand out. From this, it is desirable that the steepness γ of the TV characteristic of the liquid crystal panel is 1.7 or less in order to improve the flicker from FIG. However, it is not realistic to make the steepness γ extremely small, because it becomes difficult to display between the gray scales when performing 64 to 256 gray scales, and on the contrary, the image display quality is deteriorated. 6 to 1.7 is desirable.

FIG. 2 is a correlation diagram between the viscosity of the liquid crystal composition and the flicker level. As shown in FIG. 2, there was a good correlation between the viscosity of the liquid crystal composition and the flicker level, and it was confirmed that the higher the viscosity, the better the flicker level and the less noticeable.

From FIG. 2, it is desirable that the viscosity of the liquid crystal composition is 100 mPa · s or more in order to improve the flicker. However, increasing the viscosity of the liquid crystal composition means that the response characteristics of the liquid crystal panel and the liquid crystal composition are applied to the liquid crystal cell. When vacuum-injecting, a problem occurs that the injection time significantly increases, so that 130 mPa · s seems to be a practical limit. Therefore, in the present invention, the viscosity of the liquid crystal composition is preferably 100 to 130 mPa · s at 25 ° C.

From FIGS. 1 and 2, as a means for reducing flicker, the steepness γ of the TV characteristic of the liquid crystal panel is made small, that is, the steepness of the TV characteristic curve is made steep, and the viscosity of the liquid crystal composition is increased. It was found that increasing the value is effective.

Generally, the response speed τr from off to on and the response speed τd from on to off in the TN liquid crystal composition.
Is expressed by the following equation. τr = η · d ² / {Δε · (V ² −Vth ² )} (1) τd = η · d ² / π ² K (2) where η is the viscosity of the liquid crystal and Δε Is the dielectric anisotropy of the liquid crystal composition, V is the applied voltage, Vth is the threshold voltage, and d is the thickness (cell gap) of the liquid crystal layer.

From the above equation, by reducing the steepness γ of the TV characteristic of the liquid crystal panel, the potential difference between the gradations is reduced, and the response speed τr from off to on is slowed. Increasing the viscosity of the liquid crystal composition has the effect of slowing the response speed τr from off to on and the response speed τd from on to off.

That is, referring to FIGS. 1 and 2, to make the gradient of the T-V characteristic curve steep and to increase the viscosity of the liquid crystal composition to slow the response speed is particularly important for the halftone display. In this case, it is possible to suppress the change in luminance with respect to the change in pixel potential and make the flicker phenomenon inconspicuous.

In general, it is effective to use a liquid crystal composition having a low threshold voltage in order to reduce the steepness γ of the TV characteristic of the liquid crystal panel and increase the viscosity. The liquid crystal composition used may be a mixture of various known liquid crystal materials in order to obtain desired physical property values.

However, slowing the response speed deteriorates the moving image display performance when it is used in a liquid crystal display device used as a liquid crystal television or a liquid crystal monitor, and image quality such as blurring or tailing at the time of displaying a moving image. Will lead to a decline. Therefore, there has been a demand for a solution that can achieve both reduction of flicker and high-speed response characteristics.

Then, as a result of examining the improvement measures by using the above equations (1) and (2), the minimum applied voltage applied to the liquid crystal layer between the pixel electrode and the common electrode of the liquid crystal panel is shown in FIG. It has been found that the response characteristic is improved by increasing the difference ΔV between the maximum applied voltage, that is, the difference between the white display voltage and the black display voltage in the normally white mode liquid crystal panel. From FIG. 3, in order to maintain the response characteristics,
The applied voltage difference ΔV is preferably 4 V or more, but the applied voltage is limited by a drive circuit for driving the liquid crystal panel, and applying a high voltage also increases the power consumption of the liquid crystal display device. Not real. The applied voltage difference ΔV of the liquid crystal panel in the present invention is preferably 4 to 5V.

Further, the response speed depends on the thickness (cell gap) of the liquid crystal layer, and by narrowing the cell gap, the response speed can be maintained even if a liquid crystal composition having a high viscosity is used. However, it has been clarified by the present inventor that a narrow cell gap structure in the capacitive coupling drive system causes a large ratio of the storage capacity and the capacity of the liquid crystal composition, resulting in a slow response speed. There is concern about a decrease in yield due to foreign matter clogging when the active matrix substrate and the counter electrode substrate are adhered to each other during the fabrication of, and a decrease in productivity due to an increase in injection time when injecting the liquid crystal composition. Also, a thick cell gap is not realistic because the response speed is insufficient. The cell gap of the liquid crystal panel in the present invention is preferably 3 to 6 μm.

With such a structure, it is possible to realize a TFT type liquid crystal display device which can reduce flicker in halftone and improve image display quality.

Further, by adopting the capacitive coupling drive system and arbitrarily setting the difference between the maximum applied voltage and the minimum applied voltage, it is possible to achieve both flicker reduction and high-speed response.
An FT type liquid crystal display device can be realized.

[0035]

EXAMPLES The present invention will be specifically described below with reference to examples. The present invention is not limited to the following examples.

(Embodiment 1) A switching element composed of a TFT, a pixel electrode connected to the switching element, an active matrix substrate having a pixel structure for capacitive coupling driving, and a portion other than the pixel electrode are shielded from light. Using two substrates consisting of a light-shielding layer and a counter electrode substrate having a common electrode and a colored layer facing the pixel electrode, a polyimide resin film is applied by any appropriate technique and baked,
The alignment treatment was performed by rubbing so that the twist angle of the liquid crystal molecules in the liquid crystal layer was 90 °. Cell gap is 5μ
A liquid crystal cell was produced by dispersing appropriate spacers so that the thickness would be m, and then pasting both substrates together through a sealing resin.

A TN liquid crystal material having a steepness γ of the TV characteristic of 1.7 and a viscosity of 100 mPa · s at 25 ° C. was injected into the prepared liquid crystal cell by a vacuum injection method,
A liquid crystal panel was produced, and polarizing plates were attached to both substrates so as to be in a normally white mode to complete a liquid crystal display device.

As a result of evaluating the response characteristics of the liquid crystal display device thus obtained, the response speed between white and black display is τr.
+ Τd = 23 msec, and the response speed during the halftone display was τr + τd = 50 msec. When halftones were displayed using this liquid crystal display device, flicker was significantly reduced and was hardly visible. Furthermore, there was no problem in gradation display.

Example 2 Using the same liquid crystal cell as in Example 1, a TN liquid crystal material having a steepness γ of the TV characteristic of 1.6 and a viscosity of 130 mPa · s at 25 ° C. is vacuum-injected. The liquid crystal cell was injected by the method to prepare a liquid crystal panel, and polarizing plates were attached to both substrates so as to obtain a normally white mode to complete a liquid crystal display device.

As a result of evaluating the response characteristics of the liquid crystal display device thus obtained, the response speed τr + between white and black display was obtained.
τd = 30 msec, the response speed τ between halftone displays
It was r + τd = 60 msec. When halftones were displayed using this liquid crystal display device, flicker was significantly reduced and was hardly visible. Furthermore, there was no problem in gradation display.

(Comparative Example 1) A liquid crystal cell similar to that of Example 1 and Example 2 was used, and the steepness of the TV characteristic γ = 1.9, 2
A high-speed response TN liquid crystal material having a viscosity of 5 ° C. of 80 mPa · s was injected into a liquid crystal cell by a vacuum injection method to prepare a liquid crystal panel, and a polarizing plate was attached to both substrates so as to obtain a normally white mode. The display device was completed.

As a result of evaluating the response characteristics of the liquid crystal display device thus obtained, the response speed τr + between the white-black display is obtained.
τd = 14 msec, and the response speed τ between halftone displays
It was r + τd = 30 msec. When a halftone was displayed by using this liquid crystal display device, a flicker phenomenon occurred severely, and the image was difficult to see.

FIG. 4 is a diagram showing the TV characteristics of the liquid crystal panels according to Examples 1 and 2 and Comparative Example 1. As shown in FIG. 4, the TV characteristic curve 9 of the liquid crystal panel of Example 1
In comparison with the T-V characteristic curve 11 of the liquid crystal panel of Comparative Example 1, it can be seen that the shift is toward the lower threshold side. Further, by comparing the steepness of the TV characteristics, it was confirmed that the steepness 12 of Example 1 is smaller than that of Comparative Example 1, that is, the steepness is steeper. Further, the TV characteristic curve 10 of the liquid crystal panel of Example 2 is further shifted to the lower threshold value side, and the steepness 13 of the TV characteristic is shown.
Also, as compared with the steepness of Example 1, it was confirmed that the steepness was further increased.

Further, Example 1 showing the TV characteristic of FIG.
FIG. 5 shows the change in the transmittance of each of the liquid crystal panels of Example 2 and Comparative Example 1 at each transmittance. 5A is a liquid crystal panel of Comparative Example 1, FIG. 5B is a liquid crystal panel of Example 1, and FIG.
Of the liquid crystal panel of Example 2 is 100% and 80%, respectively.
The optical response waveforms at the respective transmittances of 60%, 60%, and 40% are shown. As shown in FIG. 5, a liquid crystal panel having a high response speed and a steep gradient of the TV characteristic curve as in Comparative Example 1 has a large amount of change in the panel transmittance for each constant period, but the liquid crystal of the present embodiment. In the panel, the amount of change in transmittance is reduced. Further, it was confirmed that the steeper the gradient of the TV characteristic curve, the smaller the amount of change in transmittance, and the more the flicker phenomenon was reduced.

[0045]

As described above, according to the thin film transistor type liquid crystal display device of the present invention, the steepness γ of the TV characteristic of the liquid crystal panel is 1.7 or less and the viscosity at 25 ° C. is 13 or less.
By using the liquid crystal composition having 0 mPa · s, the flicker phenomenon can be reduced and the display quality can be improved. Further, by combining with the capacitive coupling drive method and setting the difference ΔV between the maximum voltage and the minimum voltage applied to the liquid crystal layer of the liquid crystal panel to 4 V or more, flicker can be reduced without sacrificing the response speed. .

Further, in combination with the capacitive coupling drive, high-speed response characteristics can be obtained without narrowing the cell gap, so that the yield is reduced due to foreign matter clogging, and the productivity is reduced due to an increase in liquid crystal injection time into the liquid crystal cell. Can be prevented. Therefore, its industrial value is great.

[Brief description of drawings]

FIG. 1 is a correlation diagram of a transmittance versus steepness γ of a driving voltage characteristic of a liquid crystal panel of the present invention and a flicker level.

FIG. 2 is a correlation diagram between the viscosity of a liquid crystal composition and a flicker level.

FIG. 3 is a correlation diagram between a voltage difference ΔV applied to a liquid crystal layer and a response speed.

FIG. 4 is a diagram showing the transmittance versus applied voltage characteristics of the liquid crystal panels of the present example and the comparative example.

FIG. 5 is a diagram showing changes in optical response waveforms in transmittance of liquid crystal panels of this example and a comparative example ((a) Comparative example 1, (b) Example 1, (c) Example 2).

FIG. 6 is a diagram showing an electrical equivalent circuit of a pixel portion of a capacitive coupling drive system.

[Explanation of symbols]

1 Scanning Electrode 2 Image Signal Wiring 3 TFT 4 Previous Scanning Electrode 5 Pixel Electrode 6 Gate-Drain Capacitance 7 Liquid Crystal Capacitance 8 Storage Capacitance 9 TV Characteristic 10 of Liquid Crystal Panel in Example 1 T- of Liquid Crystal Panel in Example 2 V characteristic 11 TV characteristic of liquid crystal panel in Comparative Example 1 12 Sharpness of TV characteristic of liquid crystal panel in Example 13 13 Sharpness of TV characteristic of liquid crystal panel in Example 2 14 Liquid crystal in Comparative Example 1 The steepness of the TV characteristics of the panel

Claims (3)

[Claims] 1. An active matrix substrate having a switching element formed of a thin film transistor and a pixel electrode connected to the switching element, a counter electrode substrate having at least a common electrode facing the active matrix substrate, the active matrix substrate and the counter electrode. A liquid crystal display device comprising a liquid crystal panel comprising a liquid crystal layer formed of a liquid crystal compound filled between electrode substrates, wherein a steepness γ of a transmittance vs. drive voltage characteristic curve of the liquid crystal panel.
Is 1.7 or less, and the viscosity of the liquid crystal composition at 25 ° C. is 130 mPa · s or less. 2. The liquid crystal panel, wherein a plurality of scanning electrodes and image signal wirings are provided on the active matrix substrate, the thin film transistor elements are arranged at respective intersections, and the scanning electrodes and the thin film transistor elements are connected. The liquid crystal display device according to claim 1, wherein the pixel electrode is a capacitive coupling drive type liquid crystal panel coupled with a parasitic capacitance (Cst). 3. The thickness (d) of the liquid crystal layer filled between the active matrix substrate and the counter electrode substrate is 3 to 6 μm, and the difference (ΔV) between the maximum voltage and the minimum voltage applied to the liquid crystal layer. ) Is 4V or more, The liquid crystal display device according to claim 1 or 2, wherein.