JPH05165430A - Active matrix type liquid crystal display device - Google Patents

Abstract

PURPOSE:To provide an active matrix type liquid crystal display device capable of suppressing a display defect such as the reduction of a contrast ratio or display nonuniformity, etc., and the deterioration of display quality and keeping high display quality extending over a long period, and with high reliability. CONSTITUTION:A liquid crystal composition 6 can be prevented from being deteriorated by eliminating approximately potential difference between the signal line 23 of a liquid crystal display element 1 and a counter electrode 25 by lowering the reference potential VB of a video signal voltage Vx applied from a liquid crystal driving circuit to the signal line 23 and making it approach potential equivalent to the potential Vc of the counter electrode 25. To make the reference potential VB of the video signal voltage Vx approach the voltage Vc by reducing, the potential Vxv in the vertical blanking period of the video signal voltage Vx is set at potential less than the voltage Vc applied to the counter electrode 25.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a thin film transistor (TF).
The present invention relates to an active matrix type liquid crystal display device using a switching element made of T).

[0002]

2. Description of the Related Art Liquid crystal display devices are widely used as display elements for televisions and graphic displays.

Among them, the active matrix type liquid crystal display device is particularly excellent in high-speed response and suitable for increasing the number of pixels, and is intended to realize high image quality, large size and color screen of the display screen. Some have been expected and have been researched and developed, and some have already been put to practical use.

In the display element portion of this active matrix type liquid crystal display device, an active element substrate on which an active element such as a thin film transistor and a pixel electrode are arranged, and a counter electrode which is arranged so as to face the active element substrate are arranged. The main part is composed of a counter substrate, a liquid crystal composition sandwiched between these two substrates, and a polarizing plate attached to each of the two substrates.

The active element substrate is provided on a transparent insulating substrate such as a glass plate or a plastic film so that the scanning lines and the signal lines are orthogonal to each other, and a switching element and a pixel electrode are provided at each intersection of the scanning lines and the signal lines. Are respectively arranged.

Since the switching elements control the driving of each pixel in a distributed manner and the variation in the voltage applied to the liquid crystal is significantly reduced, the pixels can be driven at high speed, and the number of pixels can be increased and the area can be increased. It will be possible.

As a switching element, a thin film transistor (Thin Film Transistor) having a steep on / off characteristic which is suitable for the purpose of use is usually used.
r, hereinafter abbreviated as TFT) is used. TFT is thin film I
It is a type of insulated gate field effect transistor formed using the C technique.

FIG. 4 is an equivalent circuit diagram of a display element portion of a conventional active matrix type liquid crystal display device. This shows a case where an n-type TFT switching element 404 in which the gate (G) is connected to the scanning line 401, the drain (D) is connected to the signal line 402, and the source (S) is connected to the pixel electrode 403 is used. ..

Further, each pixel electrode 403 forms a pixel composed of an individual liquid crystal cell 406 with the counter electrode 405 via a liquid crystal. The liquid crystal cell 406 can be represented by an electrostatic capacitance C _LC as an equivalent circuit.

An auxiliary capacitance C _S 409 for holding charges is connected in parallel with the liquid crystal cell 406. That is, one end 407 of this auxiliary capacitance C _S 409 is connected to the pixel electrode 403 of the liquid crystal cell 406, and the other end 408 is connected to the same potential as the counter electrode 405 of the liquid crystal cell 406. This auxiliary capacitance C
_After once applying the output voltage pulse of the TFT switching element 404, the _S 409 continues to excite the liquid crystal by applying a constant voltage to the connected liquid crystal cell 406 until the next frame scanning.

FIG. 5 is a waveform diagram showing a video signal voltage V _X applied to a conventional liquid crystal display device. Video signal voltage V _X
Is applied to the drain, the scan pulse V _Y
When applied to the gate of the FT switching element 404,
This TFT becomes conductive, and a current flows between the drain and the source, so that T is applied to the pixel electrode 403 connected to the source.
The FT output voltage V _S is applied. At this time, the auxiliary capacitance C _S
Similarly, this T is also applied to one end 407 of the pixel electrode 409 on the pixel electrode side.
The FT output voltage V _S is applied and the charge is held. This charge is supplementarily applied to the liquid crystal so as to supplement the potential of the capacitance C _LC of the liquid crystal cell 406 and excite the liquid crystal for a certain period of time.

The video signal voltage V _X applied to each pixel is
As shown in FIG. 5, a set of pulses having the same amplitude in the positive direction and the negative direction with respect to the reference potential V _B is configured as one unit. Pulses of this one set is positive pulse for each frame, the polarity for negative pulses ... reference potential V _B, as is added as a pulse to reverse, the reference potential V _B on average the voltage for each even frame Will be. Therefore, the same result as that in which a DC voltage having the same voltage value as the reference potential V _B is applied to the liquid crystal for a long period of time is obtained. As is well known, when a DC voltage is applied for a long period of time, the deterioration of the liquid crystal is accelerated and the display performance is deteriorated. Therefore, from the DC voltage applying unit 412 to the counter electrode 405 and the auxiliary capacitance C _S 4
09 is applied to the other end 408 of the video signal voltage V _X , which is the same as the reference potential V _B of the video signal voltage V _X , to make the difference between the average potential of the pixel electrode 403 and the potential of the counter electrode 405 zero, thereby promoting deterioration of the liquid crystal. I try not to do anything.

It should be noted that although it takes some time when shifting from one frame to the next frame, this is called a vertical blanking period. The voltage V _XV applied to the pixel electrode 403 during this vertical blanking period has a value set to an appropriate value and is synchronized with the video signal voltage V _X , and the reference potential V _B is centered on the same polarity as the video signal voltage V _X. Is applied as a voltage whose polarity reverses.

By the way, the parasitic capacitance C _GS 410 actually exists between the gate and the source of the TFT switching element 404 as described above. Due to this, a level shift ΔV1 occurs in the output voltage of the TFT switching element 404 when the waveform of the scanning pulse V _Y sharply falls for each scanning period T _F.

Further, between the drain and source of the TFT switching elements 404 are present parasitic capacitance C _DS 411, due to this, the video signal voltage V _X voltage V _B
At the time of the polarity reversal with reference to, a level shift ΔV2 occurs in the output voltage V _S of the TFT switching element 404.

Since the level shift ΔV1 and the level shift ΔV2 are inevitably present due to the structure of the TFT element, the level shift ΔV1 and the level shift ΔV are present.
It is difficult to eliminate 2 itself.

Of these, the level shift ΔV 2 is a potential fluctuation of the same amplitude in which the polarity of the TFT output voltage V _S with respect to the reference potential V _B is reversed every frame, so that it is averaged over a plurality of (even) frames, which is the TFT output voltage V V. _S
Does not change the average potential V _C. However, the level shift ΔV1 is
Uniformly because potential fluctuations varies to the low potential side with respect to the output voltage V _S, the reference potential of the average potential V _C is a video signal voltage when averaged over a plurality of frames TFT output voltage V _S V
_It means that it fluctuated to the low potential side with respect to _B. Therefore, the DC voltage applied from the DC voltage application unit 412 to the counter electrode 405 and the other end 408 of the auxiliary capacitance C _S 409 is set to the average potential V _{C of} the TFT output voltage V _S that has fluctuated to a potential lower than the reference potential V _B. It is set to the same potential.

However, when the voltage applied to the counter electrode 405 is set to the average potential V _C of the TFT output voltage V _S , the video signal voltage V _X of the reference potential V _B before the level shift ΔV 2 occurs on the signal line 402. Is applied, a potential difference is generated between the signal line 402 and the counter electrode 405, and the liquid crystal corresponding to that portion has the same result as that when a DC voltage is applied over a long period of time on average. Therefore, there is a problem in that the deterioration of the liquid crystal is promoted to cause a display defect such as a decrease in contrast ratio and display unevenness and a deterioration in display quality.

As a countermeasure against this problem, there is known a method of covering the signal line with an insulating film to suppress the voltage applied to the liquid crystal from the signal line, but there is a pattern shift or a pinhole defect due to a hillock of the signal line. However, it is actually difficult to form the insulating film on the fine pattern of the signal line without any defects.

[0020]

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and its object is to solve the problem of deterioration of liquid crystal due to a potential difference between a signal line and a counter electrode. An object of the present invention is to provide a highly reliable active matrix type liquid crystal display device that can eliminate display defects such as a decrease in contrast ratio and display unevenness and display quality, and can maintain high display quality for a long period of time. ..

[0021]

An active matrix type liquid crystal display device of the present invention comprises a plurality of scanning lines to which a scanning pulse is applied and a plurality of signal lines to which a video signal voltage having a vertical blanking period is applied. A matrix wiring, a pixel electrode provided at each intersection of the matrix wiring, a transistor switching element interposed between the pixel electrode and the matrix wiring and connected to the pixel electrode and the matrix wiring, In an active matrix liquid crystal display device having a counter electrode facing a pixel electrode and a liquid crystal composition sandwiched by the pixel electrode and the counter electrode, the video signal voltage during the vertical blanking period is set to the counter electrode. Applied to the signal line as a voltage having a potential lower than the average potential of the voltage applied to the It is characterized by comprising a video signal voltage applying means.

[0022]

The voltage applied to the counter electrode is set to the same potential and the same polarity as the average potential V _C of the TFT output voltage V _S. This potential V _C is the TFT output voltage of the TFT switching element. When the voltage drop of the level shift ΔV1 of V _S is subtracted and averaged, the potential is lower than the reference voltage V _B of the video signal voltage V _X applied to the signal line. Therefore, a potential difference occurs between the potential V _{C of the} voltage applied to the counter electrode and the reference potential V _B of the video signal voltage V _X applied to the signal line. As a result, the liquid crystal in the portion sandwiched between the counter electrode and the signal line has the same result as a case where a DC voltage is applied over a long period of time, which promotes deterioration of the liquid crystal.

Therefore, in order to eliminate the difference in average potential between the counter electrode and the signal line, in the apparatus of the present invention, the potential of the video signal voltage V _X during the vertical blanking period is applied to the counter electrode. Set to a potential lower than the applied voltage.
As a result, the average value (average potential) of the potential of the video signal voltage V _X including the voltage during the vertical blanking period is lowered to the same level as the potential of the DC voltage applied to the counter electrode, and the potential of the counter electrode is reduced. The potential difference between the potential and the average potential of the signal line is eliminated. In this way, the problem of liquid crystal deterioration due to the potential difference between the signal line and the counter electrode is solved, and display defects such as a decrease in contrast ratio and display unevenness and display quality are suppressed, and high display quality is maintained for a long period of time. An active matrix liquid crystal display device with high reliability that can be maintained is realized.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below.

FIG. 1 is a diagram showing the structure of the liquid crystal display device of the present invention, FIG. 2 is a side view of the liquid crystal display device of the present invention showing a structure of a display element portion, and FIG. It is a wave form diagram which shows the video signal voltage of an apparatus.

The liquid crystal display device of the present invention comprises a liquid crystal display element 1
And a liquid crystal drive circuit 2 for driving the liquid crystal display element 1.

The liquid crystal display element 1 includes a TFT substrate 20, a counter substrate 30, a liquid crystal composition 6 sandwiched between these substrates, and a gap agent (not shown) for keeping a constant gap between these substrates. The main part is composed of.

The TFT substrate 20 includes a pixel electrode 21, a TFT switching element 22 using amorphous silicon,
It has a signal line 23 and a scanning line 24, and these constitute a total of 10000 pixels of 100 × 100. in addition,
The polyimide thin film whose surface is rubbed is an alignment film 26.
Is formed as.

TFT used in the liquid crystal display element 1 of this embodiment
The switching element 22 is an n-type field effect transistor. The gate (G) is connected to the scanning line 24, the drain (D) is connected to the signal line 23, and the source (S) is connected to the pixel electrode 21.

On the other hand, a counter electrode 25, which is a common transparent electrode made of an ITO film, is formed on the counter substrate 30, and a polyimide thin film is rubbed on the surface of the counter electrode 25 to form an alignment film 27.
Is formed as.

These TFT substrate 20 and counter substrate 3
0 are opposed to each other, and the alignment films are combined so that the angle formed by the rubbing directions is 90 degrees, and the liquid crystal composition 6 and the gap agent 5 are sandwiched between the substrates, and the opposing surfaces of both substrates 20 and 30. An adhesive and sealing material (not shown) is attached so as to surround the periphery of the. The liquid crystal injection port formed on the side surface of the adhesive / sealing material is sealed with an ultraviolet curable resin in a usual manner.

As the liquid crystal composition 6, E. Merck Z
LI-1565 to E. A product obtained by adding 0.1 wt% of S811 manufactured by Merck is used.

As the interstitial agent, Micropearl manufactured by Sekisui Fine Chemical Co., Ltd. having a particle size of 6 μm is used.

The outward resistance of each of the TFT substrate 20 and the counter substrate 30 is 200 Ω / cm ² on the surface.
A polarizing plate 7 formed by sputtering ITO is attached.

The liquid crystal drive circuit 2 includes a scanning line driver IC 3
The signal line driver IC 4 and the drive voltage generating circuit 5 are the main components of the circuit. The scanning line 23 of the liquid crystal display element 1 is supplied with the scanning pulse V _Y from the scanning line driver IC 3 and the signal line 24. The video signal voltage V _X is applied from the signal line driver IC 4 to the.

FIG. 4 shows the waveform of the video signal voltage V _X applied from the liquid crystal drive circuit 2 to the liquid crystal display element 1 in the liquid crystal display device having such a configuration.

The video signal voltage V _X input to each pixel from the liquid crystal drive circuit 2 having the signal line driver IC 4 as the video signal voltage applying means is the reference potential V _X as shown in FIG.
_{A set} of video signal pulses having the same amplitude in the positive direction and the negative direction with respect to _B is configured as one unit. This set of video signal pulses is input as a pulse whose polarity with respect to the reference potential V _B is reversed, such as a positive pulse and a negative pulse for each frame. The reference potential V _B is averaged for each.

However, the parasitic capacitance C _GS exists between the gate and the source of the TFT switching element 22, and due to this, the level shift occurs when the waveform of the scan pulse sharply falls for each scanning cycle. ΔV1 occurs, and T
The average potential V _C of the TFT output voltage V _S output from the source of the FT switching element 22 and applied to the liquid crystal is a voltage lower than the reference potential V _B of the input video signal voltage V _X. Correspondingly, the voltage applied to the counter electrode 25 is set to the average potential V _C of the TFT output voltage V _S. Thus, the voltage applied to the counter electrode 25 is V _C
And the video signal voltage V _X input to the signal line 23
Since the potential is lower than the reference potential V _{B of the} above, a potential difference occurs between them.

Therefore, in the present invention, the reference potential V _B of the video signal voltage V _X is lowered to bring it closer to a potential equal to the voltage applied to the counter electrode 25 having a value of V _C , so that the signal line 23 The potential difference between the counter electrode 25 and the counter electrode 25 is eliminated so that deterioration of the liquid crystal is not promoted. The reference potential V _B of this video signal voltage V _X is lowered to lower the potential V _C.
In order to bring the image signal voltage V _X closer to, the potential V _XV of the video signal voltage V _X during the vertical blanking period is set to be lower than the potential V _C of the counter electrode 25.

In this embodiment, the polarity is always negative with respect to the reference potential V _B regardless of the video signal voltage V _X for each frame, and the potential is always the minimum of the video signal voltage V _X as shown in FIG. The potential is set lower than the potential. In this way, the potential V during the vertical blanking period
The average potential V _Z of the video signal voltage V _X including _XV is lowered to a potential lower than the reference potential V _B so as to eliminate the potential difference from the potential V _{C of the} counter electrode. Since no image is displayed on the liquid crystal display element 1 during the vertical blanking period, setting the video signal voltage V _X during this period has substantially no effect on the display image.

In the liquid crystal display device of the present invention, the video signal voltage V
_By setting the potential during the vertical blanking period of _{X to} the low potential V _XV as described above, the average potential V _Z of the video signal voltage V _X is approximately the same as the voltage applied from the reference potential V _B to the counter electrode 25. The potential of the counter electrode 25 and the average potential of the signal line are made sufficiently small or approximately eliminated. Actually, the video signal voltage V _X
The potential of the video representation is changing every moment, the average potential V _Z between the potential V _XV in potential and the vertical blanking period of the video representation of the video signal voltage V _X is changing from moment to moment for this Therefore, the potential is not exactly the same as the potential V _{C of the} counter electrode, but the potential is sufficiently close to the potential V _C on average over a long period of time.

In this way, the problem of deterioration of the liquid crystal due to the potential difference between the signal line and the counter electrode 25 is solved to suppress display defects such as display unevenness and a decrease in contrast ratio and deterioration of display quality, and for a long period of time. A highly reliable active matrix liquid crystal display device capable of maintaining high display quality is realized.

The liquid crystal display device of the present invention has a temperature of 60 degrees Celsius.
When a high-temperature energizing test was performed for 10 hours, no display unevenness or reduction in contrast ratio occurred even after 1000 hours had passed. From this experimental result, it was confirmed that the liquid crystal display device of the present invention has excellent durability and high reliability.

(Comparative Example) The liquid crystal drive circuit 2 of the liquid crystal display device used in the above-mentioned embodiment was changed to a liquid crystal drive circuit according to the prior art, and an insulating film was arranged on the signal line 23 to obtain a video signal voltage. The voltage during the vertical blanking period of V _{X is} the conventional waveform as shown in FIG. 5, that is, the reference potential V _B for each frame.
Is applied to the signal line 23 as a voltage having a waveform in which the polarity is inverted with respect to, and the other conditions are the same as those of the embodiment, a high-temperature energization test of 60 degrees Celsius is performed, and the indication is displayed when 1000 hours have passed. When the quality was visually inspected, it was confirmed that display unevenness occurred in the display image of that portion due to deterioration of the liquid crystal corresponding to the position of the signal line 23, and the display quality was significantly deteriorated.

[0045]

As is apparent from the above detailed description, the active matrix type liquid crystal display device of the present invention eliminates the problem of liquid crystal deterioration due to the potential difference between the signal line and the counter electrode and reduces the contrast. It is a highly reliable active matrix type liquid crystal display device that realizes a high display quality for a long period of time by suppressing display defects such as a reduction in ratio and display unevenness and a reduction in display quality.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a liquid crystal display device of the present invention.

FIG. 2 is a partially omitted side sectional view showing a structure of a display element portion of a liquid crystal display device of the present invention.

FIG. 3 is a waveform diagram showing a video signal voltage V _X of the liquid crystal display device of the present invention.

FIG. 4 is an equivalent circuit diagram of a display element portion of a conventional active matrix type liquid crystal display device.

FIG. 5 is a waveform diagram showing a video signal voltage V _X applied to a conventional liquid crystal display device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Liquid crystal display element 2 ... Liquid crystal drive circuit 3 ... Scan line driver IC 4 ... Signal line driver IC 5 ... Drive voltage generation circuit 6 ... Liquid crystal composition

Claims (1)

[Claims] 1. A matrix wiring composed of a plurality of scanning lines to which a scanning pulse is applied and a plurality of signal lines to which a video signal voltage having a vertical blanking period is applied, and a matrix wiring provided at each intersection of the matrix wirings. A pixel electrode,
A transistor switching element inserted between the pixel electrode and the matrix wiring and connected to the pixel electrode and the matrix wiring, a counter electrode facing the pixel electrode, and sandwiched by the pixel electrode and the counter electrode. In the active matrix liquid crystal display device including the liquid crystal composition, the video signal voltage during the vertical blanking period is applied to the signal line as a voltage having a potential lower than the average potential of the voltage applied to the counter electrode. An active matrix type liquid crystal display device comprising: a video signal voltage applying means for