JPH07294881A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To provide the active matrix type liquid crystal display device which has small visual angle dependency. CONSTITUTION:Plural voltages which are different b >=20% are applied in a one-frame period and set so that liquid crystal sufficiently responds to them. In concrete, there are a method which performs writing >=2 times in the one- frame period 26, a method which varies a common voltage >=1 time in the one-frame period 26, or a method which makes the time constant of the liquid crystal smaller than the one-frame period 26. In either case, the response speed of the liquid crystal is preferably faster than the one-frame period 26.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device,
In particular, it relates to an active matrix liquid crystal display device.

[0002]

2. Description of the Related Art TN (twisted nemeti)
c) Liquid crystal is applied to a TFT (thin film transistor)
tor) and MIM (Metal Insulator)
The active matrix type liquid crystal display device combined with an active element such as a diode is now the mainstream of the flat panel display because of its features such as high contrast, high speed response, large display capacity, full color display and low power consumption. .

The TN liquid crystal has a poor steepness in the voltage transmittance characteristic, and a high-contrast large-capacity display cannot be performed by the voltage averaging method. Therefore, the active element is used as a switch, and the necessary charges are injected (writing) by closing for a certain short period of time, and opened and held for the other periods. By doing so, it is possible to apply a required voltage to a required pixel even in a large-capacity display, and a high-contrast display is realized.

A driving method of a conventional TFT type liquid crystal display device and a voltage applied to the liquid crystal will be described with reference to FIG. Generally, an NTSC video signal is composed of two interlaced fields, and the first field and the second field are combined to form one picture. However, in the case of an active matrix type liquid crystal display device, a 30 Hz signal that is AC-inverted every 1 field period (1/60 second) is normally applied to the liquid crystal in a non-interlaced mode, so one image is generated in one field period. Make up the picture. In the following, the period that constitutes one picture will be referred to as one frame. Normally, in a liquid crystal display device, one frame is set to about 1/60 seconds. In FIG. 7, 21 and 22 are signals of scanning lines and (video) signal lines, 23 is a pixel electrode potential, 24 is a common electrode potential, and 2 is a common electrode potential.
The hatched area in 5 is the voltage applied to the liquid crystal,
26 is one frame period. T in the selection period 27
When the FT is turned on, the pixel electrode potential becomes equal to the signal line potential. At the moment when the TFT is turned off, the pixel electrode potential is voltage-shifted due to the parasitic capacitance between the gate and the drain, but during the non-selection period 28, the TFT is turned off and the signal voltage written in the liquid crystal capacitance is held. The above write-to-hold operation is the same for the MIM diode.

[0005]

However, the above-mentioned conventional liquid crystal display device includes a plasma display and an EL device.
There is a problem that the viewing angle dependence of the display is larger than that of the light emitting display. FIG. 8 shows typical viewing angle characteristics of a conventional TFT type TN type liquid crystal display device. Here, the center of the figure is the substrate normal direction, and the six concentric circles surrounding it are from the inside in order of inclination angles of 10 degrees and 20 degrees from the basic normal.
The directions of degrees, 30 degrees, 40 degrees, 50 degrees, and 60 degrees are shown. The contrast ratios of 31, 32, 33, 34, and 35 are 1: 1, 1: 3, 1:10, 1:30, 1: respectively.
100 is an isocontrast curve.

As described above, in the conventional TFT type TN type liquid crystal display device, the viewing angle characteristics are wide in the left-right direction but narrow in the up-down direction, and the problem of widening the viewing angle in the up-down direction has been a problem.

As a means for alleviating the viewing angle dependence of such display, Japanese Patent Laid-Open No. 54-5754 discloses a means for dividing a pixel so that the orientations of the pixels are different from each other.
In the publication, a means for dividing a pixel to make each driving voltage different is proposed. Such a pixel division method has a great effect, and some have started mass production. However, on the other hand,
There is also a drawback that the manufacturing process becomes very complicated.

An object of the present invention is to provide a liquid crystal display device in which the viewing angle dependence of the display is small without complicating the manufacturing process by dividing the pixels spatially and not temporally. is there.

[0009]

According to the present invention, a pixel is formed from a pair of substrates provided with electrodes on opposing inner surfaces, a liquid crystal sandwiched between the substrates, and an active element provided on the one substrate. The liquid crystal display device thus constructed is provided with means for applying different voltages to the liquid crystal of each pixel during one frame period.

Further, in the invention, it is preferable that the different voltages have a difference of 20% or more.

More preferably, the different voltage is 50%
It is characterized by the above difference.

More preferably, the voltage applying means has means for varying the voltage applied to the active element.

More preferably, the voltage applying means has means for varying the common electrode voltage of the liquid crystal.

More preferably, the voltage applying means is characterized in that the time constant defined by the product of the resistance of the liquid crystal and the capacitance of the liquid crystal is shorter than the one frame period.

[0015]

The principle that the viewing angle dependence is alleviated by the structure of the present invention is basically the same as in Japanese Patent Laid-Open No. 2-12. In this publication, pixels are spatially divided and different voltages are applied,
Create areas with different tilt angles of the liquid crystal. Since the human eye sees this as an average, it was the principle that the viewing angle dependence was alleviated. In the present invention, a period in which the tilt angle of the liquid crystal is different is created in one frame, and the viewing angle dependency is alleviated by averaging the periods.

[0016]

【Example】

(Example 1) A liquid crystal display device in Example 1 of the present invention includes an upper polarizing plate 1, an upper substrate 2 of a liquid crystal cell, a lower substrate 3 and a lower polarizing plate 4, as schematically shown in FIG. A backlight or a reflector is arranged under the lower polarizing plate as needed. Between the upper and lower substrates of the liquid crystal cell, a color filter 5, a common electrode 6, a pixel electrode 7,
A liquid crystal 8, a polysilicon TFT 9, a scanning line 10 and a (video) signal line 11 were provided. In FIG. 1, only 9 pixels are shown for simplicity, but actually 320 (horizontal) × 240
(Vertical) × 3 (R, G, B primary colors) pixels were provided. In FIG. 1, the orientation film, the insulating film, the storage capacitor and the like are omitted in order to avoid making the drawing complicated, but these are not particularly different from the liquid crystal display device which is generally made in the past.

As the liquid crystal 8, a liquid crystal mixture was used in which a liquid crystal having fluorine as a terminal group was mainly used as a p-type liquid crystal and a liquid crystal having a tolan skeleton was blended in order to increase the birefringence Δn. The Δn is 0.176 and the rotational viscosity η is 2
It was 7.7 cps. Δn × d of liquid crystal cell (d: gap of liquid crystal cell) 0.48 μm of first minimum
The cell thickness was set to 2.7 μm in order to meet the above requirement. Further, after adding a small amount of chiral agent, the rubbing directions of the upper and lower substrates were made orthogonal to each other, and the liquid crystal was twisted by 90 degrees. The response speed of the liquid crystal was ton = 7 ms and toff = 14 ms.
In addition, the response speed of the liquid crystal is faster than one frame period. If it is slower than this, it becomes necessary to greatly vary the voltages applied to a plurality of elements applied during one frame period as described later. The response speed of the liquid crystal referred to here is
"Measurement method of liquid crystal display panel (JISC7072-19
88) ”, and is defined by the average value of ton and toff defined in Section 7.2. In the liquid crystal display mode, the screen is black when no voltage is applied.
It is a TN mode of rally black).

FIG. 9 shows a driving device of the liquid crystal display device shown in FIG. The video signal (NTSC signal) from the video signal source 50 is supplied to the video signal conversion circuit 51 and the sync separation circuit 52. The sync separation circuit 52 separates the vertical and horizontal sync signals from the input video signal and outputs it. Reference numeral 53 is a timing controller, which produces and outputs a timing clock supplied to each component described later in synchronization with the synchronization signal from the synchronization separation circuit 52. The video signal conversion circuit 51 receives the interlaced scanned NTS.
An image signal of the C system is alternately stored in two frame memories for each 1/30 second (that is, each of 1/60 second is composed of consecutive odd and even fields of interlaced scanning). (One frame image is stored) and one frame image video signal is taken out alternately from the two frame memories every 1/30 seconds, and is output to the first and second video signal generation circuits 54 and 55. Commonly supplied. These circuits 54 and 55 output video signals whose levels have been adjusted so that the respective levels have a relationship as shown in FIG. 4, which will be described later, and output them to the first and second memories (frame memories) 56 and 57. accumulate. From the first and second memories 56 and 57, one frame image video signal is read alternately every 1/120 second at a horizontal scanning frequency four times that of the NTSC system and supplied to the video signal selection circuit 58. To do. The video signal selection circuit 58 alternately selects the video signals from the first and second memories 56 and 57 every 1/120 second so as to obtain a video signal of a level as shown in FIG. It is supplied to the line drive circuit 59. The signal line drive circuit 59 sequentially supplies the video signal from the video signal selection circuit 58 to each signal line 11 of the liquid crystal display device having the structure shown in FIG. 1 selected in response to the clock from the timing controller 53 (that is, (Horizontal scanning), as shown in FIG. 2, voltage 23 applied to TFT 9 every 1/60 seconds
Reverse the polarity of. The scanning line drive circuit 60 is 1/120.
The same scan line 10 is driven every second (that is, the scan line 1 concerned).
The scanning lines 10 are sequentially driven in response to the clock from the timing controller 53 so that the gates of all the TFTs on 0 are turned on. FIG. 2 shows the relationship between the driving method of the liquid crystal device by the driving device shown in FIG. 9 and the voltage applied to each liquid crystal. In the figure, 21 and 22 are signals of scanning lines and signal lines, 23 is a pixel electrode potential, 24 is a common electrode potential, 25 is a voltage applied to the liquid crystal in a hatched region, and 26 is one frame period (1 / 60 seconds).
In the first embodiment, with respect to each liquid crystal, four consecutive selection periods 27a to 27d and a non-selection period 28 are performed during two consecutive frame periods, that is, during the period in which the same frame image is displayed.
, And two different signal voltages are alternately written to the pixels in each selection period. That is, the signal voltage from the first memory 56 in the first selection period 27a, the signal voltage from the second memory 57 in the next period 27b, and the first voltage in the next 27c.
The memory 56, and the next 27d write the respective signal voltages in the second memory 57. Two different signal voltages are determined as follows. FIG. 4 shows the voltage transmittance characteristics of the liquid crystal device according to the first embodiment of the present invention. Generally, the transmittance T ₀
To obtain (41), the voltage V ₀ (42) is applied to the liquid crystal. However, the voltages V ₁ (43) and V ₂ (4
It is also possible to obtain the transmittance T ₀ by applying 4) every half period, and this reduces the viewing angle dependence of the display. V ₁
When V ₂ and V ₂ are set so that the average transmittance thereof is T ₀ , and V ₂ / V ₁ is not set to 1.2 or more, the viewing angle expansion effect is small. Also, V ₂ / V ₁ is 1.2
Even if it is more than the above, the voltage applied momentarily has no effect, and it is necessary to continuously apply the voltage for a period of at least about 1/10 frame.

FIG. 3 shows the viewing angle characteristics of the liquid crystal device according to the first embodiment of the present invention. Here, the center of the figure is the substrate normal direction,
The six concentric circles surrounding it are arranged from the inside in order of inclination angles of 10 degrees, 20 degrees, 30 degrees, 40 degrees, 50 degrees,
The direction of 60 degrees is shown. Also 32, 33, 34, 3
5 is the contrast ratio 1: 3, 1:10, 1: 3, respectively
It is an isocontrast curve of 0, 1: 100. Compared with FIG. 8, there was a great effect that the viewing angle in the vertical direction, which was narrower in the past, was widened, and the so-called inversion phenomenon in which the contrast ratio was 1: 1 or less was eliminated.

(Embodiment 2) The liquid crystal display device and the driving device in Embodiment 2 of the present invention also use FIGS. 1 and 9.
It is operated by the driving method shown in FIG. However, since the liquid crystal used in Example 2 is a mixture in which a liquid crystal having a tolan skeleton is not blended, Δn is as small as 0.094 and the cell thickness is as thick as 5.0 μm. A thick cell has the advantage of facilitating manufacturing, but the response speed is ton = 23 ms,
It was delayed to toff = 46 ms. Therefore, even if two signal voltages are applied during one frame period as in the first embodiment, the liquid crystal does not respond sufficiently and the effect of expanding the viewing angle is reduced. Therefore, in the second embodiment, the ratio V _{2 of the two} signal voltages is
/ V ₁ is set to be larger than 1.5 and the difference is set to be larger than 1V so that the liquid crystal can easily respond. As a result, the same viewing angle expanding effect as in Example 1 appeared. The second embodiment can be applied to the third and fourth embodiments.

(Embodiment 3) A liquid crystal display device according to a third embodiment of the present invention has the same structure as that of the liquid crystal display device according to the first embodiment shown in FIG. 1, and its driving device is shown in FIG. The video signal conversion circuit 51A alternately stores one frame image video signal of each NTSC system in two frame memories every 1/30 seconds, and every 1/30 seconds at twice the scanning frequency of the NTSC system. Alternately from 2 frame memories to 2
The same frame image signal is read out each time and supplied to the signal line drive circuit 59A. The signal line drive circuit 59A sequentially applies the input video signal voltage to each of the selected signal lines 11 in response to the clock having a frequency twice that of the NTSC system from the timing controller 53A, and as shown in FIG. The polarity of the voltage 23 applied to the TFT 9 is reversed every 60 seconds. The scanning line drive circuit 60A drives the same scanning line every 1/60 seconds, so as to drive the same scanning line.
In response to the flocs from A, the scanning lines 10 are sequentially driven. The common electrode drive source 61 responds to the clock from the timing controller 53A for the liquid crystal for each horizontal line and synchronizes with the drive timing (selection period 27) of the scanning line 10 as shown in FIG. Each time, the voltage of the common electrode 6 is changed so as to satisfy the relationship shown in FIG. FIG. 5 shows the relationship between the driving method of the liquid crystal display device and the voltage applied to the liquid crystal in the third embodiment. In the figure, 21 and 22 are signals of scanning lines and signal lines, 23 is a pixel electrode potential, 24 is a common electrode potential, 25 is a voltage applied to the liquid crystal in a hatched region, and 26 is one frame period (1
/ 60 seconds), 27 is a selection period, and 28 is a non-selection period. The third embodiment is characterized in that the common voltage (common electrode voltage) is changed once or more during one frame period and two different voltages V ₁ and V ₂ are applied to the liquid crystal. Variation of the common voltage, V ₂ / V ₁ was set to be 1.2 or more. As a result, the same viewing angle expanding effect as in Example 1 appeared.

(Embodiment 4) The liquid crystal display device according to the fourth embodiment of the present invention has the same structure as that of the liquid crystal display device according to the first embodiment shown in FIG. 1, and the driving device shown in FIG. It is the same as FIG. 10 except that the common electrode 6 is grounded without using the power supply 61. Further, FIG. 6 shows the relationship between the driving method of the liquid crystal display device and the voltage applied to the liquid crystal in Example 4. In the figure, 21 and 22 are signals of scanning lines and signal lines, 23 is a pixel electrode potential, 24 is a common electrode potential, 25 is a voltage applied to the liquid crystal in a hatched region, and 26 is one frame period (1 / 60
Seconds), 27 is a selection period, and 28 is a non-selection period. This driving method is the same as the conventional driving method shown in FIG. 7, but the voltage applied to the liquid crystal changes greatly during one frame period. This is due to the following two ideas.
The storage capacity provided for each pixel has been completely abolished.The other is to reduce the resistance by mixing a small amount of organic electrolyte, such as tetrabutylammonium chloride, with the liquid crystal, and the product of the resistance and capacity of the liquid crystal. The value of the time constant defined by is adjusted to 12 milliseconds, which is shorter than one frame period. Therefore, the voltage written to the pixel is 1
It cannot be held for the frame period. The liquid crystal responds to this voltage change, and the viewing angle characteristics are greatly improved.

In the above-mentioned embodiments, the polysilicon TFT is used as the active element, but the same effect can be obtained even if the amorphous silicon TFT or the MIM diode is used.

[0024]

As described above, according to the present invention, it is possible to provide a liquid crystal display device having a small viewing angle dependency by applying a plurality of different voltages to the liquid crystal during one frame period.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a structure of a liquid crystal device in an example of the present invention.

FIG. 2 is a diagram showing a driving method of a liquid crystal device and a voltage applied to a liquid crystal in Embodiments 1 and 2 of the present invention.

FIG. 3 is a diagram showing viewing angle characteristics of the liquid crystal device according to the first embodiment of the present invention.

FIG. 4 is a diagram showing a voltage transmittance characteristic of the liquid crystal device in Example 1 of the present invention.

FIG. 5 is a diagram showing a driving method of a liquid crystal device and a voltage applied to a liquid crystal according to a third embodiment of the present invention.

FIG. 6 is a diagram showing a driving method of a liquid crystal device and a voltage applied to a liquid crystal according to a fourth embodiment of the present invention.

FIG. 7 is a diagram showing a driving method of a conventional liquid crystal device and a voltage applied to a liquid crystal.

FIG. 8 is a diagram showing viewing angle characteristics of a conventional liquid crystal device.

FIG. 9 is a block diagram of a drive device according to the first embodiment.

FIG. 10 is a block diagram of a drive device according to a third embodiment.

FIG. 11 is a block diagram of a drive device according to a fourth embodiment.

[Explanation of symbols] 1 upper polarizing plate 2 upper substrate of liquid crystal cell 3 lower substrate of liquid crystal cell 4 lower polarizing plate 5 color filter 6 common electrode 7 pixel electrode 8 liquid crystal 9 TFT 10 scanning line 11 signal line 21 of scanning line Signal 22 Signal of signal line 23 Pixel electrode potential 24 Common electrode potential 25 Voltage applied to liquid crystal 26 1 frame period 27 Selection period 28 Non-selection period 31 Isocontrast curve with contrast ratio 1: 1 32 Contrast ratio 1: 3 etc. Contrast curve 33 Equal contrast curve with a contrast ratio of 1:10 34 Equal contrast curve with a contrast ratio of 1:30 35 Equal contrast curve with a contrast ratio of 1: 100 41 Transmittance T ₀ 42 Applied voltage V ₀ 43 Applied voltage V ₁ 44 Applied voltage V ₂

Claims (6)

[Claims] 1. A liquid crystal display device in which a pixel is composed of a pair of substrates provided with electrodes on opposing inner surfaces, a liquid crystal sandwiched between the substrates, and an active element provided on the one substrate. A liquid crystal display device comprising means for applying different voltages to the liquid crystal of each pixel during one frame period. 2. The voltage according to claim 1, wherein the different voltage is 2
A liquid crystal display device having a difference of 0% or more. 3. The voltage according to claim 1, wherein the different voltage is 5
A liquid crystal display device having a difference of 0% or more. 4. The liquid crystal display device according to claim 1, wherein the voltage applying unit has a unit that changes a voltage applied to the active element. 5. The liquid crystal display device according to claim 1, wherein the voltage applying unit has a unit that changes a common electrode voltage of the liquid crystal. 6. The liquid crystal according to claim 1, wherein the voltage applying unit has a time constant defined by the product of the resistance of the liquid crystal and the capacitance of the liquid crystal shorter than the one frame period. Display device.