JPH079508B2 - Liquid crystal display device and driving method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a liquid crystal display device and a driving method thereof, and more particularly to a liquid crystal display device using a ferroelectric liquid crystal and a driving method thereof.

[Prior Art] Conventionally, a liquid crystal display element that forms a scanning electrode group and a signal electrode group in a matrix form, fills a liquid crystal compound between the electrodes, and forms a large number of pixels to display an image or information is known. well known. The driving method of this display element is a time-division drive in which an address signal is selectively applied to the scanning electrode group sequentially and cyclically, and a predetermined information signal is applied to the signal electrode group in parallel in synchronization with the address signal. However, this display element and its driving method had a serious drawback that can be said to be fatal as described below.

That is, it is difficult to increase the pixel density or enlarge the screen. Among the conventional liquid crystals, most of them are practically used as display devices because of their relatively high response speed and low power consumption. For example, "Applied Physics Letters"
ers ") December 15, 1971, 127-128, by M. Schadt and W. Helfrich," Voltage-Dependent Optical Activity of a Twisted. " Nematic
Liquid Crystal "(" Voltage Dependent Optica
l Activity of a Twisted Nematic Liquid Crystal ")
The twisted nematic liquid crystal shown in Fig. 2 is used, and this type of liquid crystal has a structure in which the molecules of the nematic liquid crystal having a positive dielectric anisotropy in the absence of an electric field are twisted in the thickness direction of the liquid crystal layer (helical Structure is formed, and the molecules of the liquid crystal are aligned in parallel with each other on both electrode surfaces. On the other hand, when an electric field is applied, nematic liquid crystals having positive dielectric anisotropy are aligned in the direction of the electric field, and as a result, light modulation can occur. When a display element is constructed with a matrix electrode structure using this type of liquid crystal, in a region (selection point) in which both the scanning electrode and the signal electrode are selected, the liquid crystal molecules are equal to or more than the threshold value required to be aligned vertically to the electrode surface. The voltage is applied, and no voltage is applied to a region (non-selection point) in which neither the scanning electrode nor the signal electrode is selected, so that the liquid crystal molecules maintain a stable alignment parallel to the electrode surface. By arranging linear polarizers having a crossed Nicols relationship above and below such a liquid crystal cell, light does not pass at selected points and light passes at non-selected points, so it can be used as an image device. And However, when the matrix electrode structure is formed, the scan electrode is selected and the signal electrode is not selected, or the scan electrode is not selected and the signal electrode is selected (so-called “half-selected point”). A finite electric field is applied. If the difference between the voltage applied to the selection point and the voltage applied to the semi-selection point is sufficiently large and the voltage threshold value required to align the liquid crystal molecules perpendicularly to the electric field is set to an intermediate voltage value, the display element will operate normally. It works.

[Problems to be Solved by the Invention] The problem to be solved by the present invention is that when the number of scanning lines is increased by the above-described method, one
Time for which two selectively effective electric fields are applied (duty ratio)
Will decrease at a rate of 1 / N.Therefore, the voltage difference as the effective value applied to the selected point and the non-selected point when repeating scanning becomes smaller as the number of scanning lines increases. As a result, a decrease in image contrast and crosstalk are inevitable drawbacks, and such a phenomenon is caused by a liquid crystal that does not have a bistable state (liquid crystal molecules are aligned horizontally with respect to the electrode surface). Is stable and vertically aligned only when the electric field is effectively applied), which is essentially unavoidable when driven (that is, repeatedly scanned) using the temporal accumulation effect. There is a problem, and in order to improve this point, the voltage averaging method, the two-frequency driving method, the multiple matrix method, etc. have already been proposed, but none of them is sufficient, and the display device has a large screen. For higher density, increase the number of scanning lines It has peaked by Ikoto.

Further, in the image display using the ferroelectric liquid crystal, if the state of the ferroelectric liquid crystal molecules is a state having a memory property,
Attempting to stabilize in one of the two bistable states does not at least ideally take an intermediate molecular position, so that conventional analog-like molecular behavior cannot be obtained and it is considered unsuitable for gradation expression. It was However, in gradation display by the dither method, the resolution is lowered,
It was not preferable because the demand for display quality could not be met, and especially when the electrodes were formed by etching, the size of one pixel was limited.

It is an object of the present invention to solve all of these problems, and to provide a liquid crystal display device using a novel bistable liquid crystal, particularly a ferroelectric liquid crystal, and a driving method excellent in gradation operation thereof. And

[Means for Solving Problems] Means for solving the above problems in the present invention include a liquid crystal layer using a ferroelectric liquid crystal, an insulating layer, and a display pixel electrode. A liquid crystal display element having a shape, wherein an insulating layer is formed on a display pixel electrode and the layer thickness is continuously or stepwise changed and distributed. And, as a driving method thereof, driving of a liquid crystal display element including a liquid crystal layer using a ferroelectric liquid crystal, a display pixel electrode, and an insulating layer formed on the display pixel electrode and having a different layer thickness. A method for driving a liquid crystal display element is characterized in that a signal voltage, which is changed in frequency or voltage peak value in accordance with the layer thickness of an insulating layer, is applied to a display pixel electrode.

The ferroelectric liquid crystal used in the driving method of the present invention has one of a first optical stable state and a second optical stable state depending on an applied electric field, that is, has a bistable state with respect to an electric field. A substance, especially a liquid crystal having such a property is used.

As the ferroelectric liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic C phase (SmC *) or H phase (SmH *) liquid crystal having ferroelectricity is suitable. For this ferroelectric liquid crystal, refer to "LE JOURNAL D
E PHYSIQUE LETTERS ") 1975, No. 36 (L-69)," Ferroelectric Liquid Crystal "(" Ferroe
lectric Liquid Crystals ");" Applied Physics Letters "19
1980, No. 36 (11), "Sub-micro second bistable ectotroobtic switching in
Liquid Crystals "(" Submicro Second Bistab
le Electrooptic Switching in Liquid Crystals ”);
"Solid state physics", 1981, 16 (141), "Liquid crystal", etc., and the ferroelectric liquid crystal disclosed therein is also used in the present invention.

More specifically, examples of the ferroelectric liquid crystal compound used in the method of the present invention include desiloxybenzylidene-p'-amino-2-methylbutylcinnamate (DOBAMBC), hexyloxybenzylidene-p'-amino- 2-chloropropyl cinnamate (HOBACPC) and 4-o- (2-methyl) -butylresorcylidene-4'-octylaniline (MBRA8) and the like can be mentioned.

When using these materials to construct an element, the element is supported by a copper block with a heater embedded, if necessary, in order to maintain the temperature at which the liquid crystal compound becomes the SmC * phase or SmH * phase. can do.

In addition to the above-mentioned SmC * and SmH *, chiral smectic I phase (SmI *), J phase (SmJ *), G phase (SmG *), F phase (SmF *), K phase (SmK *), etc. A ferroelectric liquid crystal that appears can also be used.

[Operation] FIG. 3 schematically illustrates an example of a ferroelectric liquid crystal cell. 1 and 1'are In ₂ O ₃ , SnO ₂ and ITO (Indium-Tin O
xide) and the like coated with a transparent electrode (glass plate), in which SmC * phase liquid crystal oriented so that the liquid crystal molecular layer 2 is perpendicular to the glass surface is enclosed. A thick line 3 represents a liquid crystal molecule, and this liquid crystal molecule 3
_{Has a} dipole moment (P _⊥ ) 4 in a direction orthogonal to its molecule. When a voltage above a certain threshold is applied between the electrodes on the substrates 1 and 1 ', the helical structure of the liquid crystal molecules 3 is unraveled, and all the dipole moments (P _⊥ ) 4 are oriented in the direction of the electric field. You can change direction. The liquid crystal molecules 3 have an elongated shape, and exhibit refractive index anisotropy in the major axis direction and the minor axis direction thereof. Therefore, for example, if polarizers arranged in a crossed Nicols position above and below a glass surface are placed. It is easily understood that the liquid crystal optical modulation element has optical characteristics that change depending on the polarity of voltage application. Further, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1 μ), the helical structure of the liquid crystal molecules is unwound (non-helical structure) even if no electric field is applied as shown in FIG. The child moment P or P'takes either an upward (4a) or downward (4b) state. As shown in FIG. 4, when an electric field E or E'having a polarity different from a certain threshold value is applied to such a cell for a predetermined time, the dipole moment is directed upward 4a or corresponding to the electric field vector of the electric field E or E ', or The direction is changed to downward 4b, and the liquid crystal molecule is changed to the first alignment state 5 or the second alignment state 5'according to it.
Orient one of them.

There are two advantages of using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has a bistable state. Explaining the second point with reference to FIG. 2, for example, when an electric field E is applied, the liquid crystal molecules are aligned in the first alignment state 5, but this state is stable even when the electric field is cut off. When a reverse electric field E'is applied, the liquid crystal molecules are oriented in the second alignment state 5'to change the orientation of the molecules, but they remain in this state even when the electric field is cut off. Also, the applied electric field E
As long as the values do not exceed a certain threshold, they are still maintained in their respective orientations. In order to effectively realize such a high response speed and bistability, it is preferable that the cell is as thin as possible, and generally 0.5 μ to 20 μ, particularly 1 μ to 5 μ is suitable. A liquid crystal-electro-optical device having a matrix electrode structure using this type of ferroelectric liquid crystal,
For example, proposed by Clark and Gabal in US Pat. No. 4,367,924.

The present invention is to devise a cell structure and a driving method when the above-mentioned ferroelectric liquid crystal is used in a bistable state, in which insulating layers having different layer thicknesses are formed on the electrodes of the ferroelectric liquid crystal element. The provision of the film enables gradation display.

In principle, when a liquid crystal display element is constructed by using a substrate with an insulating layer formed on electrodes on one side or both sides of the liquid crystal layer, for example, the capacitance of the insulating layer is C ₁ and the capacitance of the liquid crystal layer is C _2. , Pulse of positive direction is applied, and the voltage peak value is
Assuming V ₀ , the liquid crystal layer has V ₀ · C ₁ / (C
₁ + C ₂ ) initial voltage is applied, and thereafter, if the insulation layer resistance value R ₁ is sufficiently large, it decays with a time constant τ = R ₂ (C ₁ + C ₂ ) proportional to the resistance value R ₂ of the liquid crystal layer. To go. When a pulse is applied for a time almost equal to the leakage of electric charge in the liquid crystal layer, an electric field of the opposite polarity to the applied voltage is applied to the liquid crystal layer when the pulse waveform falls, so in this case, the resistance R _{2 of the} liquid crystal layer,
When the capacitance C ₂ is constant, the initial voltage value and the time constant vary depending on the capacitance C ₁ of the insulating layer. Here, if the capacitance distribution is realized by forming the thickness distribution of the insulating layer on the pixel electrode, the liquid crystal is changed in a state corresponding to the capacitance distribution by changing the frequency of the applied voltage and the voltage peak value. The display can be controlled.

[Examples] Hereinafter, the present invention will be described in detail with reference to the drawings and examples.

FIG. 1 (a) is a sectional view showing an example of the structure of a liquid crystal display element embodying the present invention. In FIG. 1, a liquid crystal display element is configured by sandwiching a transparent electrode 1, a liquid crystal layer 2, and an insulating layer 5 between glass substrates 6 in a laminated shape. The transparent electrode 1 is a known ITO electrode, the liquid crystal layer 2 is made of the above-mentioned ferroelectric liquid crystal DOBAMBC, and the insulating layer 5 is a polyimide resin, which is formed on the lower ITO electrode 1 in the figure. The liquid crystal display device is divided into three pixel regions (1), (2), and (3) in the drawing, and the insulating layer 5 is patterned to have different layer thicknesses in each region, and accordingly, the region (1) is formed. ) Is C ₁ ′, the capacitance in region (2) is C ₁ ″, and the capacitance in region (3) is C _1. Similarly, the capacitance and resistance of the liquid crystal layer are C ₂ ′, C ₂ ″, C ₂ and R ₂ ′, R ₂ ″, R ₂
Then, the capacitance distribution of this liquid crystal display element becomes as shown in the circuit diagram of FIG. Here, assuming that the areas of the three regions are equal, assuming that the thickness of the liquid crystal layer is sufficiently larger than the thickness of the insulating layer, it is approximately C ₂ ′.
It can be said that ≈C ₂ ″ ≈C ₂ and R ₂ ′ ≈R ₂ ″ ≈R ₂ , and in this case, the voltage waveform applied to the liquid crystal layer is a function of the capacitance in the insulating layer.

FIGS. 2A to 2F are waveform diagrams showing examples of the voltage waveform applied to one pixel and the voltage waveform between the liquid crystal layers, and FIG. 2A is a pulse width T and a peak value V. By setting the pulse width T and the voltage V ₀ of the applied voltage pulse of ₀ appropriately, C ₁ > C ₁ ″> C ₁ is established, and the region (3),
The voltage waveforms applied to (2) and (1) are (b),
(C) and (d) can be used. That is, in the region (3), the liquid crystal molecules are in the first stable state, and in the region (2), the liquid crystal molecules are immediately inverted to the second stable state, and then the region (first). ) Keeps the state Qn _-1 before voltage application. At this time, the pulse (V ₁ ≤V ₀ , T ₀ <for the region (2) as shown in FIG.
When T) is applied, a voltage as shown in FIG. 6F is applied to the liquid crystal, and the region (2) is written in the first stable state. The table below lists the above operations.

The above-mentioned Tables 1 and 2 are tables showing state changes in each region in the pixel, wherein Table 1 is based on voltage modulation and Table 2 is based on frequency modulation. In the table, "0" and "1" respectively indicate one of the bistable states, and an electric field is applied in the direction of "1". Qn _-1 shows the state before voltage application, and if this state is taken into consideration when driving, it becomes possible to make the state of a partial region on one pixel different from that of another region, and gradation display is possible. To be achieved.

Example 1 A known DOBAMBC was used as a liquid crystal, a liquid crystal cell gap was set to about 2.0 μm, and an insulating layer having a cross section as shown in FIG. I went. Here, the area of all the regions is made uniform to 1 cm ^2, and the layer thickness of the insulating layer is set to the region (1),
In the case of 400 Å, 800 Å, 2000 Å for each of (2) and (3), assuming that the initial voltage of the applied voltage pulse is V ₀ , the regions (1), (2) and (3) are approximately 62% and 89%, respectively. , 94
% Initial voltage is applied, and each time constant is about 0.4,
0.8 and 1.6 msec. Therefore, the voltage values corresponding to Table 1 are: i) 1V> V ₁ ii) 1V <V ₂ <5V iii) 5V <V ₃ <30V iv) 30V <V ₄ <70V v) 70V <V ₅ I was able to achieve it. As frequencies corresponding to those in Table 2, i) 625HZ> f ₁ ii) 625HZ <f ₂ <1250HZ iii) 1250HZ <f ₃ could be achieved.

5 (a), (b) and (c) are a plan view, a side view and a front view showing another pixel configuration example of the liquid crystal display element embodying the present invention. In each drawing of FIG. 5, the ITO electrode 1 is formed on the glass substrate 6, and the polyimide insulating layer 5 is laminated on the ITO electrode 1 in six layers. Since each layer of the insulating layer has a different area, as a result, a plan view (a)
Areas A to F shown in Fig. 3 are shown in the side view (b) d ₁
It becomes the same as have a layer thickness of _{~d 6, d 1 <d 2} <d 3 < than _{d 4 <d 5 <d 6} , C A <C B <C C <C D <C E <C F Therefore, assuming that the pixel area is S, the respective capacitances are as follows.

C _A = S (ε ₀ · εr) / d ₆ C _B = S (ε ₀ · εr) / d ₅ C _C = S (ε ₀ · εr) / d ₄ C _D = S (ε ₀ · εr) / d ₃ C _E = S (ε ₀ · εr) / d ₂ C _F = S (ε ₀ · εr) / d ₁ where ε _{0 is} the dielectric constant of vacuum, ε r is the relative dielectric constant of the medium. ) And (B) are a plan view and a front view showing still another pixel configuration example of the liquid crystal display element embodying the present invention, and as shown in the drawing, each region of the pixel is not uniformly divided. Alternatively, in FIG. 6, assuming that the area ratio of the insulating layer 5 in each region is A: B: C = 3: 2: 1, d ₁ <d ₂ <
d ₃

In addition, in the present embodiment, the example in which the layer thickness of the insulating layer is changed stepwise for each region has been described, but even if the layer thickness is continuously changed and distributed, the present invention can be implemented. It doesn't matter at all.

As described above, according to the present invention, the bistable liquid crystal,
In particular, it is possible to provide a liquid crystal display element which uses a ferroelectric liquid crystal and realizes gradation display by controlling pixels by operating a voltage or a frequency, and a driving method thereof.

[Brief description of drawings]

1 (a) and 1 (b) are sectional views and equivalent circuit diagrams of one embodiment of the present invention, FIGS. 2 (a) to 2 (f) are voltage waveform diagrams thereof, and FIGS. FIG. 5 and FIG. 6 of the dielectric liquid crystal are explanatory views of another embodiment of the present invention. 1 ...... electrode, 2 ...... liquid crystal layer, 5 ...... insulating layer, 6 ...... glass substrate, V ₀ ...... voltage peak value, f ...... frequency.

Claims (2)

[Claims] 1. A liquid crystal display device comprising a liquid crystal layer using a ferroelectric liquid crystal, an insulating layer, and a display pixel electrode in a laminated form, the insulating layer being formed on the display pixel electrode. A liquid crystal display device, characterized in that the thickness is distributed continuously or stepwise. 2. A method of driving a liquid crystal display device comprising a liquid crystal layer using a ferroelectric liquid crystal, a display pixel electrode, and an insulating layer formed on the display pixel electrode and having a different layer thickness. A method for driving a liquid crystal display element, characterized in that a signal voltage whose frequency or voltage peak value is changed corresponding to the layer thickness of an insulating layer is applied to a display pixel electrode.