EP0420129A2

EP0420129A2 - Liquid crystal electro-optical device and method for driving the same

Info

Publication number: EP0420129A2
Application number: EP90118351A
Authority: EP
Inventors: Takashi Fukui; Toshio Watanabe
Original assignee: Semiconductor Energy Laboratory Co Ltd
Current assignee: Semiconductor Energy Laboratory Co Ltd
Priority date: 1989-09-25
Filing date: 1990-09-24
Publication date: 1991-04-03
Also published as: JP2873616B2; EP0420129A3; JPH03174116A

Abstract

A liquid crystal electro-optical device and a method for driving the same are disclosed. The liquid crystal electro-optical device comprises a pair of substrates and nematic liquid crystals disposed between said pair of substrates in a helical structure. In the present invention the liquid crystal electro-optical device having a duty number of 200 or more is driven at a frame frequency of 200 to 280Hz, and thereby high contrast can be obtained.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method of driving a liquid crystal electro-optical device with high contrast and the liquid crystal electro-optical device having high contrast.

Description of the Prior Art

Nematic liquid crystals having positive dielectric anisotropy including optically active substances therein are disposed between a pair of substrates and are arranged in a helical structure wherein the uppermost and lowermost nematic liquid crystals between said pair of substrates are oriented in different directions by an angle of 90°. Liquid crystal panels of twisted nematic (TN) type having such a helical structure above mentioned have been used for watches, electric calculators and so on for years.
However, in a liquid crystal electro-optical device of TN type the threshold value of applied voltage against switching of liquid crystals is not definite and when said device is used as a comparatively large sized display and so on, contrast becomes extremely small. So that it was practically impossible to use said device as a large sized display.
On the other hand, liquid crystal electro-optical devices of super twisted nematic (STN) type have been popular as comparatively large sized displays such as displays of word processors. The difference of the structures of the liquid crystal electro-optical device of STN type from that or the liquid crystal electro-optical device or TN type is that, particularly, the angle at which the uppermost and lowermost nematic liquid crystals between a pair of substrates in said device of STN type are crossed is not 90° but 180° to 270°. Thereby, in the liquid crystal electro-optical device of STN type, the threshold value of voltage against switching or liquid crystals becomes definite compared with the case of the liquid crystal electro-optical device of TN type. And the application for comparatively large displays can be actualized though the conventional liquid crystal electro-optical devices of TN type could not be used as such large displays.
Conventionally, the comparative large display has a large duty number, for example 200 to 240, in the case of the liquid crystal electro-optical device of STN type.
And a frame frequency during driving the conventional liquid crystal electro-optical device of STN type is in the range of 120 to 160Hz.
Fig. 2(A) shows a waveform of pulsed voltage applied to liquid crystals at one picture element of the liquid crystal electro-optical device of STN type having a duty number of 200 to 240. In Fig. 2(A) the pulsed voltage is applied at a frame frequency of 120 to 160Hz. When the pulsed voltage is applied in accordance with the waveform shown in Fig. 2(A), a quantity of light transmitted through said one picture element varies as shown in Fig. 2(B). Fig. 2(B) shows a waveform appearing on an oscilloscope display by inputting in the oscilloscope a voltage into which the quantity of light is converted by photomultiplier.
In Fig. 2(A) every pulsed voltage acts as ON voltage. Pulsed voltage 5 is applied to the picture element of the conventional liquid crystal electro-optical device of STN type in order to obtain a black display and a low voltage 6 is applied to the same picture element in order to obtain a white display. However in the conventional liquid crystal electro-optical device or STN type having a small frame frequency, the quantity of transmitted light becomes too large when the low voltage is applied. For this reason, an application of pulsed voltage 5 after the application of the low voltage 6 results in almost white display rather than black display. Since users recognize white or black display according to a value obtained by integrating the quantity of transmitted light of a picture element for a regular term, such an almost white display is caused and tires users' eyes a lot and also causes a decline of their eyesight. Practically the contrast of the display was measured, and the contrast was lowered. This is because the quantity of transmitted light are not sufficiently small during black display.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of driving a liquid crystal electro-optical device with high contrast and to provide a liquid crystal electro-optical device having high contrast.
In order to attain this object, a liquid crystal electro-optical device of STN type, in which the uppermost and lowermost nematic liquid crystals are oriented in different directions by an angle of 180° to 270° and which has a duty number of 200 or more, is driven at a frame frequency of 200 to 280Hz.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig.1(A) shows a waveform of applied voltage used in the present invention.
Fig.1(B) shows a waveform of voltage into which a quantity of transmitted light is converted in the present invention.
Fig.2(A) shows a waveform of applied voltage used in a conventional liquid crystal electro-optical device.
Fig.2(B) shows a waveform of voltage into which a quantity of transmitted light is converted in a conventional liquid crystal electro-optical device.
Fig.3 is a cross sectional view of a liquid crystal electro-optical device in accordance with the present invention.
Fig.4 is a cross sectional view of a liquid crystal electro-optical device in accordance with the present invention.
Fig.5 shows orientation directions and absorption axes in the present invention.
Fig.6 is a cross sectional view of a liquid crystal electro-optical device in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention has an object to provide a liquid crystal electro-optical device having high contrast. The device in accordance with the present invention has nematic liquid crystals having positive dielectric anisotropy including optically active substances which are disposed between a pair of substrates and the nematic liquid crystals are arranged in a helical structure wherein the uppermost and lowermost nematic liquid crystals are oriented in different directions at an angle of 180° to 270°. In the present invention the device has a duty number of 200 or more. In order to obtain high contrast, such a device is driven at a frame frequency of 200 to 280Hz.
In the present invention a frame frequency is a reciprocal of the time from the moment that a common electrode is selected till the moment that the common electrode is again selected.
The present invention is described hereinafter using Fig. 1(A) and (B).
In Fig. 1(A) is illustrated a waveform of voltage applied to one picture element of display and in Fig. 1(B) is illustrated a waveform of the moment that a quantity of transmitted light on said picture element is converted into voltage and the voltage is inputted in an oscilloscope by the use of a photomultiplier.
At first in a selection term 2 a pulsed voltage 1 is applied to liquid crystals and the display is made black. Then in a non-selection term 3 the quantity of transmitted light becomes large. However, since in the present invention the frame frequency is larger than that of a conventional device, a variation 4 of the quantity of transmitted light is less than that of a conventional device. Therefore, when the quantity of transmitted light is integrated within one term, the integral value becomes smaller than that of a conventional case, and black display can be obtained. Accordingly, contrast of display improves largely compared with the contrast of a conventional display.

(Embodiment No.1)

First of all, on a first substrate made from soda glass is formed an ITO thin film as transparent electrodes by means of DC sputtering method, and subsequently the ITO (indium tin oxide) thin film is patterned into 640 of segment electrodes by means of well-known photolithography method.
On a second substrate made from soda glass are formed 240 of common electrodes made from an ITO thin film by means of the same methods as the preceding methods.
Then on the first and second substrates is applied polyamic acid (polyamide carboxylic acid) by the use of an offset press, and the substrates are heated at a temperature of 300 degrees Centigrade for 3 hours to form polyimide thin films thereon.
The polyimide thin films formed on the both substrates are rubbed by the use of cotton cloth. The rubbing treatment is done to cross the rubbing directions of the first and the second substrates at an angle of 240°.
On one substrate are scattered ball-like fine particles made of SiO₂ having the diameter of 7.5µm (which are called spacers) and on the other substrate is screen-printed a sealing material consisting mainly of epoxy resin, and the substrates are mated.
After that, liquid crystals are injected by means of well-known vacuum injection method to produce a liquid crystal panel. But the center of the panel swells by the injection, so that pressing is again carried out. Namely, in order to take the excessive liquid crystals out from the interior of the panel, pressure is added toward the inside. Then an injection port of liquid crystals is sealed with ultraviolet ray curable agent, and the liquid crystal panel is completed.
A pair of polarizing plates is disposed sandwiching the liquid crystal panel.
Then the liquid crystal panel and a driving circuit are connected by the use of FPC (flexible print circuit).
Such a liquid crystal electro-optical device completed in this way was driven in the conditions of duty number of 240 and frame frequency of 280Hz. Then the liquid crystal panel was radiated with white light and the light transmitted through the liquid crystal panel was converted into voltage by the use of photomultiplier and the voltage was inputted in an oscilloscope to measure the contrast. As the result high contrast of 35 could be obtained. Further, contrast was measured in the conditions of a fixed duty number of 240 and varied frame frequency. The result is shown in Table 1. Table 1

FREQUENCY CONTRAST

120 9

140 12

160 14

180 16

200 21

220 25

240 30

260 33

280 35
As apparent in Table 1, when a frame frequency is 200Hz or more, high contrast of 20 or more can be obtained.
On the other hand, when a frame frequency is 120 to 180Hz, contrast is low.

(Embodiment No.2)

First of all, on a first substrate made from soda glass is formed an ITO thin film as transparent electrodes by means of DC sputtering method, and subsequently the ITO thin film is patterned into 640 of segment electrodes by means of well-known photolithography method.
On a second substrate made from soda glass are formed 240 of common electrodes made from an ITO thin film by means of the same methods as the preceding methods.
Then on the first and second substrates is applied polyamic acid by the use of an offset press, and the substrates are heated at a temperature of 300 degrees Centigrade for 3 hours to form polyimide thin films thereon.
The polyimide thin films formed on the both substrates are rubbed by the use of cotton cloth. The rubbing treatment is done to cross the rubbing directions of the first and the second substrates at an angle of 240°.
On one substrate are scattered ball-like fine particles made of SiO₂ having the diameter of 7.5µm (which are called spacers) and on the other substrate is screen-printed a sealing material consisting mainly of epoxy resin, and the substrates are mated.
After that, liquid crystals are injected by means of well-known vacuum injection method to produce a liquid crystal panel. But the center of the panel swells by the injection, so that pressing is again carried out. Namely, in order to take the excessive liquid crystals out from the interior of the panel, pressure is added toward the inside. Then an injection port of liquid crystals is sealed with ultraviolet ray curable agent, and the liquid crystal panel is completed.
A pair of polarizing plates is disposed sandwiching the liquid crystal panel.
Then the liquid crystal panel and a driving circuit are connected by the use of FPC.
Contrast of such a liquid crystal electro-optical device completed in this way was measured in the respective conditions of duty number of 240 and 60 and frame frequency of 280Hz and 140Hz. The result is shown in Table 2. In Table 2, 1/60 means a duty number of 60 and 1/240 means a duty number of 240. Table 2

140Hz 280Hz

1/60 37 39

1/240 12 35
As apparent in Table 2, driving the liquid crystal electro-optical device at a frame frequency of 200Hz to 280Hz mentioned in the present invention is effective when the duty number is large. When the duty number is small such as 60, high contrast is obtained independently of the frequency.

(Embodiment No.3)

First of all, on a first substrate made from soda glass is formed an ITO thin film as transparent electrodes by means of DC sputtering method, and subsequently the ITO thin film is patterned into 640 of segment electrodes by means of well-known photolithography method.
On a second substrate made from soda glass are formed 240 of common electrodes made from an ITO thin film by means of the same methods as the preceding methods.
Then on the first and second substrates is applied polyamic acid by the use of an offset press, and the substrates are heated at a temperature of 300 degrees Centigrade for 3 hours to form polyimide thin films thereon.
The polyimide thin films formed on the both substrates are rubbed by the use of cotton cloth. The rubbing treatment is done to cross the rubbing directions of the first and the second substrates at an angle of 240°.
On one substrate are scattered ball-like fine particles made of SiO₂ having the diameter of 5.6µm (which are called spacers) and on the other substrate is screen-printed a sealing material consisting mainly of epoxy resin, and the substrates are mated.
After that, liquid crystals are injected by means of well-known vacuum injection method to produce a liquid crystal panel. But the center of the panel swells by the injection, so that pressing is again carried out. Namely, in order to take the excessive liquid crystals out from the interior of the panel, pressure is added toward the inside. Then an injection port of liquid crystals is sealed with ultraviolet ray curable agent, and the liquid crystal panel is completed.
Then, as shown in Fig.3 film 30 having anisotropy of refractive index whose retardation dn_o-dn_e is 305nm and film 31 having anisotropy of refractive index whose retardation dn_o-dn_e is 390nm (dn_o is an optical distance of ordinary ray and dn_e is an optical distance of extraordinary ray) are stuck on one surface of the liquid crystal panel 33 in order to remove the coloring which is characteristic of a conventional liquid crystal electro-optical device of STN type. Further a pair of polarizing plates 32 and 34 is disposed sandwiching the liquid crystal panel.
Then the liquid crystal panel and a driving circuit are connected by the use of FPC.
Such a liquid crystal electro-optical device completed in this way was driven in the conditions of duty number of 240 and frame frequency of 280Hz. Then the liquid crystal panel was radiated with white light and the light transmitted through the liquid crystal panel was converted into voltage by the use of photomultiplier and the voltage was inputted in an oscilloscope to measure the contrast. As the result, high contrast of 64 could be obtained. Further, contrast was measured in the conditions of a fixed duty number of 240 and varied frame frequency. The result is shown in Table 3. Table 3

FREQUENCY CONTRAST

120 18

140 25

160 30

180 45

200 51

220 55

240 57

260 60

280 64
As apparent in Table 3, when a frame frequency is 200Hz or more, high contrast of 50 or more can be obtained.
On the other hand, when a frame frequency is 120 to 180Hz, contrast is low.

(Embodiment No.4)

In the same way as Embodiment No.3, a liquid crystal panel is produced followed by sticking both film 30 having anisotropy of refractive index whose retardation dn_o-dn_e is 305nm and film 31 having anisotropy of refractive index whose retardation dn_o-dn_e is 390nm on one surface of the liquid crystal panel 33. Further a pair of polarizing plates 32 and 34 is disposed sandwiching the liquid crystal panel. Then the liquid crystal panel and a driving circuit are connected by the use of FPC.
Contrast of such a liquid crystal electro-optical device completed in this way was measured in the respective conditions of duty number of 240 and 60 and frame frequency of 280Hz and 140Hz. The result is shown in Table 4. In Table 4, 1/60 means a duty number of 60 and 1/240 means a duty number of 240. Table 4

140Hz 280Hz

1/60 37 39

1/240 12 35
As apparent in Table 4, driving the liquid crystal electro-optical device at a frame frequency of 200Hz to 280Hz mentioned in the present invention is effective when the duty number is large. When the duty number is small such as 60, high contrast is obtained independently of the frequency.

(Embodiment No.5)

First of all, on a first substrate made from soda glass is formed an ITO thin film as transparent electrodes by means of DC sputtering method, and subsequently the ITO thin film is patterned into 640 of segment electrodes by means of well-known photolithography method.
On a second substrate made from soda glass are formed 240 of common electrodes made from an ITO thin film by means of the same methods as the preceding methods.
Then on the first and second substrates is applied polyamic acid by the use of an offset press, and the substrates are heated at a temperature of 300 degrees Centigrade for 3 hours to form polyimide thin films thereon.
The polyimide thin films formed on the both substrates are rubbed by the use of cotton cloth. The rubbing treatment is done to cross the rubbing directions of the first and the second substrates at an angle of 240°.
On one substrate are scattered ball-like fine particles made of SiO₂ having the diameter of 5.5µm (which are called spacers) and on the other substrate is screen-printed a sealing material consisting mainly of epoxy resin, and the substrates are mated.
After that, liquid crystals are injected by means of well-known vacuum injection method to produce a liquid crystal panel. But the center of the panel swells by the injection, so that pressing is again carried out. Namely, in order to take the excessive liquid crystals out from the interior of the panel, pressure is added toward the inside. Then an injection port of liquid crystals is sealed with ultraviolet ray curable agent, and the liquid crystal panel is completed.
Then films 38 and 39 having anisotropy of refractive index whose retardation dn_o-dn_e is 350nm are stuck on the both surfaces of the liquid crystal panel 35 in order to remove the coloring which is characteristic of a conventional liquid crystal electro-optical device of STN type. Further a pair of polarizing plates 36 and 37 is disposed sandwiching the liquid crystal panel.
Then the liquid crystal panel and a driving circuit are connected by the use of FPC.
Such a liquid crystal electro-optical device completed in this way was driven in the conditions of duty number of 240 and frame frequency of 280Hz. Then the liquid crystal panel was radiated with white light and the light transmitted through the liquid crystal panel was converted into voltage by the use of photomultiplier and the voltage was inputted in an oscilloscope to measure the contrast. As the result, high contrast of 62 could be obtained. Further, contrast was measured in the conditions of a fixed duty number of 240 and varied frame frequency. The result is shown in Table 5. Table 5

FREQUENCY CONTRAST

120 28

140 35

160 42

180 48

200 50

220 52

240 54

260 58

280 62
As apparent in Table 5, when a frame frequency is 200Hz or more, high contrast of 50 or more can be obtained.
On the other hand, when a frame frequency is 120 to 180Hz, contrast is low.

(Embodiment No.6)

In the same way as Embodiment No.5, a liquid crystal panel is produced followed by sticking films 38 and 39 having anisotropy of refractive index whose retardation dn_o-dn_e is 350nm on both surfaces of the liquid crystal panel 35. Further a pair of polarizing plates 36 and 37 is disposed sandwiching the liquid crystal panel. Then the liquid crystal panel and a driving circuit are connected by the use of FPC.
Contrast of such a liquid crystal electro-optical device completed in this way was measured in the respective conditions of duty number of 240 and 60 and frame frequency of 280Hz and 140Hz. The result is shown in Table 6. In Table 6, 1/60 means a duty number of 60 and 1/240 means a duty number of 240. Table 6

140Hz 280Hz

1/60 72 84

1/240 35 62
As apparent in Table 6, driving the liquid crystal electro-optical device at a frame frequency of 200Hz to 280Hz mentioned in the present invention is effective when the duty number is large. When the duty number is small such as 60, high contrast is obtained independently of the frequency.

(Embodiment No.7)

First of all, on a first substrate made from soda glass is formed an ITO thin film as transparent electrodes by means of DC sputtering method, and subsequently the ITO thin film is patterned into segment electrodes by means of well-known photolithography method.
On a second substrate made from soda glass are formed common electrodes made from an ITO thin film by means of the same methods as the preceding methods.
Then on the first and second substrates is applied polyamic acid by the use of an offset press, and the substrates are heated at a temperature of 300 degrees Centigrade for 3 hours to form polyimide thin films thereon.
The polyimide thin films formed on the both substrates are rubbed by the use of cotton cloth. The rubbing treatment is done to cross the rubbing directions of the first and the second substrates at an angle of 240°.
On one substrate are scattered ball-like fine particles made of SiO₂ having the diameter of 6.2µm (which are called spacers) and on the other substrate is screen-printed a sealing material consisting mainly of epoxy resin, and the substrates are mated.
After that, liquid crystals are injected by means of well-known vacuum injection method to produce a panel for driving liquid crystals. But the center of the panel swells by the injection, so that pressing is again carried out. Namely, in order to take the excessive liquid crystals out from the interior of the panel, pressure is added toward the inside. Then an injection port of liquid crystals is sealed with ultraviolet ray curable agent, and the panel for driving liquid crystals is completed.
Next, a panel for optical compensation is produced.
On third and fourth substrates made from soda glass is applied polyamic acid by the use of an offset press, and subsequently the substrates are heated at a temperature of 300 degrees Centigrade for 3 hours to form polyimide thin films thereon.
The polyimide thin films formed on the both substrates are rubbed by the use of cotton cloth. The rubbing treatment is done to cross the rubbing directions of the third and the fourth substrates at an angle of 240° in the opposite direction to the case of the panel for driving liquid crystals. Further the rubbing direction of the third substrate and the rubbing direction of the second substrate are crossed at an angle of 80°.
On one substrate are scattered ball-like fine particles made of SiO₂ having the diameter of 7.5µm (which are called spacers) and on the other substrate is screen-printed a sealing material consisting mainly of epoxy resin. Then the substrates are mated.
After that, liquid crystals are injected by means of well-known vacuum injection method to produce a panel for optical compensation. But the center of the panel swells by the injection, so that pressing is again carried out. Namely, in order to take the excessive liquid crystals out from the interior of the panel, pressure is added toward the inside. Then an injection port of liquid crystals is sealed with ultraviolet ray curable agent, and the panel for optical compensation is completed.
Then polarizing plates 16 and 17, a light source 20, the panel for driving liquid crystals 11, and the panel for optical compensation 12 are disposed as shown in Fig.5 and Fig.6. As apparent in Fig.5 and Fig.6, the orientation direction of liquid crystals on a substrate 14 in the panel for driving liquid crystals 11 and the orientation direction of liquid crystals on a substrate 15 in the panel for optical compensation 15 are crossed at an angle of 80°. Thereby, coloring which is characteristic of the conventional liquid crystal electro-optical device of STN type is removed and display in black and white can be obtained.
Then the panel for driving liquid crystals and a driving circuit are connected by the use of FPC.
Such a liquid crystal electro-optical device completed in this way was driven in the conditions of duty number of 240 and frame frequency of 280Hz. Then the liquid crystal electro-optical device was radiated with white light and the light transmitted through the liquid crystal electro-optical device was converted into voltage by the use of photomultiplier and the voltage was inputted in an oscilloscope to measure the contrast. As the result high contrast of 84 could be obtained. Further, contrast was measured in the conditions of a fixed duty number of 240 and varied frame frequency. The result is shown in Table 7. Table 7

FREQUENCY CONTRAST

120 46

140 51

160 63

180 64

200 71

220 74

240 76

260 81

280 84
As apparent in Table 7, when a frame frequency is 200Hz or more, high contrast of 70 or more can be obtained.
On the other hand, when a frame frequency is 120 to 180Hz, contrast is low.
Since other modification and changes (varied to fit particular operating requirements and environments) will be apparent to those skilled in the art, the invention is not considered limited to the examples chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.

Claims

1. A liquid crystal electro-optical device comprising:
a pair of substrates;
nematic liquid crystals having positive dielectric anisotropy including optically active substances therein which are disposed between said pair of substrates, said nematic liquid crystals being arranged in a helical structure so that the uppermost and lowermost nematic liquid crystals are oriented in different directions by an angle in the range of 180° to 270°;
and
means for driving said liquid crystal electro-optical device at a frame frequency in the range of 200 to 280Hz,
wherein a duty number of said liquid crystal electro-optical device is not less than 200.

2. The liquid crystal electro-optical device of claim 1 further comprising:
another pair of substrates; and
nematic liquid crystals having positive dielectric anisotropy including optically active substances therein which are disposed between said another pair of substrates, said nematic liquid crystals being arranged in a helical structure so that the uppermost and lowermost nematic liquid crystals are oriented in different directions by an angle in the range of -180° to -270°.

3. The liquid crystal electro-optical device of claim 1 further comprising a pair of films drawn in different directions to function as a filter having an anisotropy of refractive index,
wherein said pair of substrates and said pair of films are arranged so that the directions in which said films are drawn are different from each other by an angle in the range of 0° to 40°.

4. The liquid crystal electro-optical device of claim 3 wherein said films are arranged on one side of said pair of substrates.

5. The liquid crystal electro-optical device of claim 3 wherein said films are arranged on both sides of said pair of substrates.

6. A method for driving a liquid crystal electro-optical device having a duty number not less than 200 which device comprises a pair of substrates and nematic liquid crystals having positive dielectric anisotropy including optically active substances therein which crystals are arranged between said substrates in a helical structure so that the uppermost and lowermost nematic liquid crystals are oriented in different directions by an angle in the range of 180° to 270° wherein said device is driven at a frame frequency of 200 to 280Hz.