JPH06160812A - Liquid crystal electrooptical device - Google Patents

Abstract

PURPOSE:To make digital gradation display possible by using a liquid crystal material having spontaneous polarization of an adequate value. CONSTITUTION:A liquid crystal material exhibiting a ferroelectric or antiferroelectric property or a material 17 formed by dispersing this material into a high-polymer compd. is clamped in the spacing of a liquid crystal cell formed by arranging a pair of substrates 11 and 12 which have electrodes 13, 14, have liquid missivity on at least on thereof, have thin-film transistors 15 on at least either one substrate and further, have films 16 subjected to a uniaxial orientation treatment, such as rubbing treatment, on at least one thereof in such a manner that the surfaces subjected to the orientation treatment face each other. The liquid crystal material having the small value of the spontaneous polarization is so used that the liquid crystal molecules can be inverted even by small charges, by which the response of the liquid crystal molecules even at the pulse width shorter than the response time of the liquid crystal material exhibiting the ferroelectric or antiferroelectric property or the material formed by dispersing such material into the high-polymer compd. is possible.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal electro-optical device using a thin film transistor (hereinafter referred to as a TFT) as a switching element for driving, and the liquid crystal material used is a ferroelectric liquid crystal or an antiferroelectric material excellent in high speed response. Liquid crystal or a liquid crystal electro-optical device characterized by being limited to so-called polymer liquid crystal (also referred to as dispersion type liquid crystal) in which they are dispersed in a high molecular compound. In particular, in order to perform gradation display for obtaining an intermediate color tone or shade expression, gradation display is performed without externally applying any analog signal to the active element, so-called completely digital gradation display is performed. The present invention relates to a liquid crystal electro-optical device.

[0002]

2. Description of the Related Art A liquid crystal composition is one whose light transmission amount and refractive index are changed by an external electric field, and by using this property, an electric signal can be converted into an optical signal for display. . Liquid crystal materials include TN (twisted nematic) liquid crystal, STN (super twisted nematic) liquid crystal, ferroelectric or antiferroelectric liquid crystal, and recently, nematic liquid crystal and ferroelectric or antiferroelectric liquid crystal. A material called polymer liquid crystal (also referred to as dispersion type liquid crystal) in which is dispersed in a polymer material is known. It is known that liquid crystal does not respond to an external voltage in an infinitely short time, but it takes a certain time to respond. The value is unique to each liquid crystal material, and in the case of TN liquid crystal, it is several tens of ms.
ec, several 100 msec in the case of STN liquid crystal, several 10 μsec in the case of ferroelectric liquid crystal, several 10 msec in the case of dispersion type or polymer liquid crystal using nematic liquid crystal.
msec.

Among the display devices using liquid crystals, the ones which can obtain the most excellent image quality are those using the active matrix system. In a conventional active matrix type liquid crystal electro-optical device, a thin film transistor (TFT) is used as an active element, an amorphous or polycrystalline semiconductor is used for the TFT, and one pixel is either P type or N type. It used a type of TFT. That is, in general, N-channel TFT (N
(TFT) is connected to the pixel in series. And
By turning on / off the liquid crystal pixels by applying a signal voltage to the signal lines of the matrix and applying signals from both sides to the TFTs provided at the orthogonal positions of the respective signal lines, the TFTs are turned on. It was controlled individually.
By controlling the pixels by such a method, a liquid crystal electro-optical device having a large contrast can be realized.

[0004]

However, in the conventional active matrix method, there are differences in brightness and color tone.
It was extremely difficult to perform gradation display. Conventionally, a method of utilizing the fact that the light transmittance of the liquid crystal changes depending on the magnitude of the applied voltage has been studied for gradation display. For example, by applying an appropriate voltage between the source and drain of the TFT in the matrix from the peripheral circuit and applying a signal voltage to the gate electrode in that state, a voltage of that magnitude is applied to the liquid crystal pixel. It was something to try.

However, in such a method, the voltage applied to the liquid crystal pixel actually varies from pixel to pixel by at least several percent due to, for example, the non-uniformity of the TFT and the non-uniformity of the matrix wiring. Oops. On the other hand, the voltage dependence of the light transmittance of the liquid crystal is extremely non-linear, and the light transmittance changes abruptly at a specific voltage. It could be significantly different. Therefore, the conventional analog gradation display system has a limit to achieve 16 gradations. For example, in a TN liquid crystal material, the so-called transition region where the light transmittance changes is only 1.2V wide and 1
In order to achieve 6 gradations, it is necessary to control a voltage as small as 75 mV, and therefore the manufacturing yield is remarkably low.

The difficulty of gradation display is extremely disadvantageous in that the liquid crystal display device competes with the CRT (cathode ray tube) which is a conventional general display device. On the other hand, the present inventors have found that the gradation can be visually obtained by controlling the time during which a voltage is applied to the liquid crystal. The details are shown in Japanese Patent Application No. 3-169306.

For example, in the case of using TN (Twisted Nematic) liquid crystal which is a typical liquid crystal material,
Brightness can be changed by applying various pulse waveforms to the liquid crystal pixels as shown in FIG. That is, the brightness can be gradually increased in the order of “1”, “2”, ... “15” in FIG. 1, and in the example of FIG.
6-gradation display is possible. For example, in FIG.
At "1", a pulse having a length of 1 unit is applied. Further, at "2", a pulse having a length of 2 units is applied.
At "3", a pulse of 1 unit and a pulse of 2 units are applied, and as a result, a pulse of a length of 3 units is applied.
At "4", a pulse with a length of 4 units is applied.
At "5", 1 unit pulse and 4 unit pulse are applied, and at "6", 2 unit pulse and 4 unit pulse are applied. Furthermore, by providing a pulse length of 8 units, a pulse length of 15 units can be obtained as a result.

That is, by properly combining four types of pulses of 1 unit, 2 units, 4 units, and 8 units, it is possible to display 2 ⁴ = 16 gradations. Furthermore, 1
By preparing many pulses, such as 6 units, 32 units, 64 units, and 128 units, respectively,
High gradation display of 32 gradations, 64 gradations, 128 gradations and 256 gradations is possible. For example, in order to obtain 256 gradation display, eight kinds of pulses may be prepared.

Further, in the example of FIG. 1A, the duration of the voltage applied to the pixel is first T ₁ , second 2T ₁ , and then 4T ₁ , so that the durations increase in a geometric progression. In the example shown in FIG. 1B, for example, T ₁ may be _first , 8T ₁ may be the second, 2T ₁ may be the second, and 4T ₁ may be the last. By arranging in this way, the load on the device for transmitting data to the display device can be reduced.

However, when the TN liquid crystal is used, as a result, the applied voltage is required to have the same accuracy as in the conventional analog gradation display system. That is, when 5V is applied to the pixel as the ON voltage and "10" in FIG. 1 is displayed, the ON voltage is 5.
By applying a voltage of 1 V, it looks darker by about 2% than the case where the same "10" is displayed. That is, in such a digital gradation display system, it is required that there is no variation in TFT as in the conventional analog gradation display system.

A circuit diagram of a typical TFT active matrix is shown in FIG. The scanning signal (V
FIG. 3B shows changes in the potential V ₁ of the liquid crystal pixel when _G ) and the data signal (V _D ) are applied.

There are several factors that cause variations in V ₁ , but the major ones are the potential drop (ΔV) that occurs when the scanning signal is cut off due to the parasitic capacitance between the gate electrode of the TFT and the wiring on the pixel electrode side, and the leakage of the TFT. This is a voltage drop due to a current or a leak current of the liquid crystal, and when the driving capability of the TFT is not sufficient (the mobility is small), sufficient charging cannot be performed during the time t ₁ during which the scanning signal continues. It is the variation of the ultimate voltage.

Since these fluctuations are greatly affected by the characteristics of the TFT, if the variation of the TFT is large, the brightness of the pixel will be greatly different. For example, if the parasitic capacitances of the gate electrode and the wiring on the pixel electrode side are different, ΔV is different, and if the magnitude of the leak current of the TFT is different, the drop rate of the pixel voltage is also different. TF with low mobility such as amorphous silicon TFT (a-Si TFT)
At T, variation in charging is also a problem. For the above reason, even if the same signal is applied, the pixel potential V ₁ can be obtained as shown by the solid line in FIG. 3B or as shown by the dotted line. Of course, such variations are not desirable.

[0014]

As pointed out above, the precise control of the pixel potential is required because the TN liquid crystal changes the light transmissivity according to the effective voltage.
The same was true for STN liquid crystals or dispersion type liquid crystals using nematic liquid crystals, which are the basic materials of these.

The present invention solves such a drawback when a nematic liquid crystal is used. Specifically, the liquid crystal material used was a liquid crystal material exhibiting ferroelectricity or antiferroelectricity, or a so-called polymer liquid crystal (also referred to as dispersion type liquid crystal) in which they were dispersed in a polymer compound (polymer). .

The liquid crystal electro-optical device of the present invention basically has the following configuration. As shown in FIG. 4, the basic structure of the present invention has electrodes 13 and 14, at least one of which has a light-transmitting property, a thin film transistor 15 on one of the substrates, and a rubbing treatment on at least one of the substrates. Liquid crystal exhibiting ferroelectricity or antiferroelectricity in a gap of a liquid crystal cell in which a pair of substrates 11 and 12 having a film 16 subjected to uniaxial orientation treatment are arranged so that their surfaces subjected to orientation treatment face each other. It has a structure in which a material or a material 17 obtained by dispersing them in a polymer compound is sandwiched.

In the case of the display device of the present invention, it is possible to invert liquid crystal molecules with a small amount of electric charge,
By using a liquid crystal material having a small spontaneous polarization value,
Liquid crystal molecules can respond even with a pulse width shorter than the response time of liquid crystal materials exhibiting ferroelectricity or antiferroelectricity, or materials in which they are dispersed in a polymer compound, and good optical characteristics can be obtained. It was

Further, in the present invention, an auxiliary capacitance is added to the switching element in order to compensate for the insufficient amount of charge compared to the spontaneous polarization of the liquid crystal material due to the short pulse width and to compensate for the insufficient charge due to discharge. It was solved by installing it together. In such a case, the auxiliary capacitor is provided on the substrate having the thin film transistor.

Further, in the case of the display element of the present invention, when a liquid crystal material having a large spontaneous polarization is used, the operating temperature is raised to reduce the value of the spontaneous polarization so that the liquid crystal molecules can be inverted even with a small charge. This enables liquid crystal molecules to respond even with a pulse width shorter than the response time of liquid crystal materials exhibiting ferroelectricity or antiferroelectricity, or materials in which they are dispersed in polymer compounds, and good optical characteristics can be obtained. I was able to.

The magnitude of spontaneous polarization of the liquid crystal material required in the present invention is 10 nC / cm ² or less, preferably 8
nC / cm ² or less. When the auxiliary capacitance is provided, the spontaneous polarization of the liquid crystal material is 20 nC / cm ² or less, preferably 18 nC / cm ² or less, and the ratio of the pixel capacitance to the auxiliary capacitance is 1: 10000 or less.

Of course, if the spontaneous polarization of the liquid crystal is small and the discharge of the pixel is sufficiently small, the auxiliary capacitance may be omitted. In particular, the presence of excessive storage capacity
The charging or discharging operation is time consuming and not desirable in the practice of the invention. In order to reduce the pixel discharge, for example, it is necessary to sufficiently increase the OFF resistance of the thin film transistor, reduce the leak current, and sufficiently increase the interelectrode resistance of the pixel itself such as liquid crystal. Particularly for the latter purpose, it is effective to coat the pixel electrode with an insulating material such as tantalum oxide or aluminum oxide such as silicon nitride or silicon oxide.
Further, increasing the capacitance of the pixel itself is also effective in reducing discharge, and it suffices to take measures such as increasing the dielectric constant of liquid crystal or narrowing the distance between substrates.

To implement the present invention, for example, a matrix circuit using thin film transistors as shown in FIG. 5 may be assembled. The circuit shown in FIG. 5 is the same as the circuit used in a conventional active matrix type display device using TFTs.

In such a circuit, the gate voltage of each thin film transistor and the voltage between the source and the drain are controlled to turn ON / OFF the voltage applied to the pixel.
It is possible to control OFF. In this example, the matrix is 640 × 480 dots, but for simplicity, only the vicinity of the nth row and the mth column is shown. If you expand the same thing up, down, left and right, you can get the perfect one. An operation example using this circuit is shown in FIG. Here, the operation is shown focusing on only one pixel.

The signal line X _n (scanning line) is connected to the gate electrode of each TFT. Then, as shown in FIG.
A rectangular pulse signal is applied. On the other hand, the signal line Y
_{The m} (data line) is connected to the source (or drain electrode) of each TFT, to which a pulse train showing either a positive or negative state is applied. 480
In the row matrix, this pulse train contains 480 pieces of information during one unit of time T ₁ . In the present invention, one frame is composed of a plurality of subframes (5 in the example of FIG. 2).
The feature is that it is composed of. Also, FIG.
In this example, each subframe has a different duration.

In the following, for simplicity of explanation, it is assumed that the potential of the counter substrate is 0 and constant. As shown, V _D was positive when V _G was first applied, so
The pixel potential V _LC becomes positive. At this time, the potential drops by ΔV as described with reference to FIG. 3, and then the potential V _LC of the pixel gradually approaches 0 due to spontaneous discharge. However, paying attention to the transmittance T _LC of the pixel, the transmittance T _LC is kept constant despite the pixel potential V _LC decreasing. If the liquid crystal material has a good memory property, the pixel potential V _LC
Descent is not so problematic. However, it is necessary to set the other parameters (V _G, V _D) to tolerate a reduction in pixel potential V _LC in the case of using the memory of the poor liquid crystal material.

A design point is to set V _G and V _D with reference to the TFT having the worst characteristic. For example, V _D is set so that the potential V _LC after the longest sub-frame duration 16T _{0 in the} case where the pixel has the worst charge retention characteristic is +9 V or more, preferably +11 V or more in the case shown in FIG. Then, V _G appropriate for driving V _D on the left is set.

In FIG. 2, the potential V is set in any subframe.
_It should be noted that although the _LC drops are written in the same manner, the longer the subframe duration is, the larger the drop in potential is.

Time T has passed since the first pulse V _G was applied.
After ₀ , the second pulse V _G is applied. Since the data signal V _{D at} this time was also positive, the pixel potential V _LC remains positive. However, new charges are injected and the potential rises again. The pixel transmittance T _LC does not change.

Then, after a time 16T ₀ , a third pulse V
_{When G} is applied, the data signal V _D is negative, so the pixel potential V _LC is inverted negative. Then, the transmittance also changes. However, this transition is relatively gradual, and if the applied voltage is 10 V or less, it takes about 50 μsec. On the other hand, the width of the pulse V _G is 3
Although it is 0 μsec or less, there is no problem because the optical response transition of the liquid crystal is performed not by the pulse V _G but by the pixel potential V _LC .

Thereafter, after the time 2T ₀ , the third pulse V _G is applied and the data signal V _{D at} that time is negative, so the state of the pixel does not change. Further, after the time 8T ₀ , the fourth pulse V _G is applied and the data signal V _{D at} that time is positive, so that the pixel potential V _LC becomes positive again and the transmittance T _{LC of the} pixel also changes. Finally, time 4T ₀
Later, the first pulse V _G of the next frame is applied to end one frame. It is possible to display 32 gradations by properly combining such 5 sub-frames. With the above operation, 1 + 16 +
A brightness of 4 = 21 [gray scale] was obtained.

In the above operation, it is important to determine the optimum minimum time unit T ₀ . As already mentioned,
The optical response time of a ferroelectric (or antiferroelectric) liquid crystal depends on the applied voltage. If the voltage is about 15 V as described above, the response time is 50 μsec. Generally, the optical response time is inversely proportional to the applied voltage. By the way, 1 frame is 1 for the purpose of preventing display characteristics and flicker of moving images.
The frame is 100 msec or less, preferably 30 mse
It must be c or less. For example, 1 frame is 30 ms
Assuming ec, the maximum gradation frequency is 30 msec to 50
The limit is 600 gradations divided by μsec, but in reality, several times the optical response time is required for complete optical response. Become. Although such a constraint is improved by increasing the magnitude of the voltage (or electric field) applied to the liquid crystal and shortening the optical response time, the breakdown voltage of the TFT needs to be improved accordingly.

[0032]

According to the structure of the present invention, when the digital gradation display as described above is carried out, the TF having a slight variation.
It was revealed that even in T, uniform gradation display is possible. This is because the ferroelectric liquid crystal or the anti-ferroelectric liquid crystal does not substantially respond to the effective value voltage but responds to the change in the polarity of the electric field. That is, in the case of the ferroelectric liquid crystal or the anti-ferroelectric liquid crystal, ON
This is because when the voltage is continuously applied for 1 msec or more, the same light transmissivity is exhibited at 5V and 5.1V.

As an example showing the above effect, a ferroelectric liquid crystal (phenylpyrimidine type) is used, and the contrast is controlled by changing the duration of this pulse, and a gradation display is shown in FIG. A similar effect was observed in a material in which a ferroelectric liquid crystal or an antiferroelectric liquid crystal was dispersed in a polymer. However, when the TN liquid crystal is used, such linear gradation display cannot be obtained even when gradation display is performed by the same method of changing the pulse width.

However, the ferroelectric or antiferroelectric liquid crystal which can be used in the liquid crystal electro-optical device which performs gradation display by controlling the time when the electric field is applied to the liquid crystal as in the present invention is limited. . The reason will be described in detail below.

The driving principle of the ferroelectric liquid crystal is to switch the liquid crystal molecules by the torque generated by the interaction between the spontaneous polarization of the liquid crystal material and the external electric field. Therefore, in order to drive the ferroelectric liquid crystal or the anti-ferroelectric liquid crystal, an electric field for reversing the spontaneous polarization of the liquid crystal material is formed between the electrodes, and it is necessary to charge a sufficient electric charge between the electrodes. is there. When a continuous electric field is applied to the liquid crystal material from the outside, electric charges are always supplied from the outside to the electrodes, so that the liquid crystal molecules react to the electric field and the spontaneous polarization is reversed, so that a current flows between the electrodes. Even if the charge is consumed, the voltage between the electrodes is kept at a voltage equal to the voltage of the external power supply.

However, when the TFT is driven as in the present invention, the charges are externally supplied to the electrodes only while the pulse is being applied to the gate of the device. Therefore, after the TFT is turned off, the electrodes are in an open state, so that power is consumed only between the electrodes when the spontaneous polarization is inverted.

The ferroelectric liquid crystal or the anti-ferroelectric liquid crystal has a fast response speed of several μsec. In order to perform finer gradation, for example, if the pulse width must be 1 μs, the inversion of liquid crystal molecules is performed when the TFT is in the OFF state.

Therefore, when the response time of the liquid crystal is longer than the pulse width, when most of the liquid crystal molecules are inverted when the TFT is OFF, the spontaneous polarization is inverted during the time when the TFT is in the ON state. Sufficient charge must be charged to the electrodes.

In the case of a ferroelectric liquid crystal or an antiferroelectric liquid crystal having a large spontaneous polarization, the electric charges that must be charged between the electrodes are naturally large.

When the electric charge supplied to the electrodes is small due to a short pulse width, there is a problem that the optical response becomes insufficient because the spontaneous polarization is not completely inverted.

If the voltage between the electrodes is not kept at a certain level or more by the time the next pulse is applied and the voltage between the electrodes is open, the liquid crystal molecules once switched back to the original state. This will cause deterioration of optical characteristics.

In practice, when the display method of the present invention is applied to a liquid crystal electro-optical device using a ferroelectric or anti-ferroelectric liquid crystal, the optical characteristics of the liquid crystal electro-optical device greatly depend on the physical properties of the liquid crystal material. It is necessary to select the spontaneous polarization of the liquid crystal material and the size of the auxiliary capacitance so as to be suitable for this method.

Here, in order to enable the liquid crystal electro-optical device of the present invention to perform digital gradation, the magnitude of spontaneous polarization of the liquid crystal material that the liquid crystal must have and the auxiliary capacitance are required. If so, calculate the auxiliary capacity.

In the ferroelectric or antiferroelectric liquid crystal display device, when the optical response is obtained, the applied voltage, the pixel capacitance, the spontaneous polarization of the liquid crystal material, and the voltage immediately before the change from the most subframe to the next subframe. It is considered that the relation of the following expression (1) must be established between and.

[0045]

[Equation 1]

Here, C _LC is the capacitance between the pixel electrodes, V is the voltage of the signal applied to the pixel, Ps is the spontaneous polarization of the ferroelectric or antiferroelectric liquid crystal material, and S is the electrode area of the pixel electrode. , V _rem indicates the voltage between the electrodes after the gate pulse is applied and immediately before the next pulse is applied. Further, in equation (1), the balance of charges when the liquid crystal material is inverted is considered. Regarding C _LC of the formula (1), it can be expressed as the formula (2) as follows.

[0047]

[Equation 2]

Here, ε is the dielectric constant of the ferroelectric or antiferroelectric liquid crystal material, S is the area of the pixel electrode, and d is the distance between the pixel electrodes of two substrates facing each other. In particular, in the case of a ferroelectric or antiferroelectric liquid crystal, it is necessary to consider the contribution of spontaneous polarization to the dielectric constant ε of the formula (2). Therefore, if the dielectric constant of the ferroelectric or antiferroelectric liquid crystal material is divided into a component related to spontaneous polarization and a component other than that, it can be expressed as the following formula (3).

[0049]

[Equation 3]

Where ε ₀ is the dielectric constant of a vacuum (8.854 × 10
^-12 [F / m]), a is a constant, P is a value related to spontaneous polarization, ε _r
Is the value of the part not related to spontaneous polarization. Generally, in a ferroelectric or antiferroelectric liquid crystal, when the optical response is perfectly taken, the value in the parentheses of the value of the formula (3) is 10 to 15 due to the effect of spontaneous polarization. Therefore, the dielectric constant of the liquid crystal material is 106 pF / m. However, it is assumed that the value in the parentheses in the formula (3) is equal to 12.

When the area of the pixel electrode is 2.5 × 10 ^-5 m ² and the distance between the pixel electrodes on the two opposing substrates is 2.5 × 10 ^-6 m, the capacitance of the pixel is 1.06 nF. is there.

The present inventors have investigated the optical characteristics of a liquid crystal display device by various driving methods according to the present invention with respect to various ferroelectric or antiferroelectric liquid crystals. As a result, the voltage between electrodes when the optical characteristics can be completely obtained. Was confirmed to be 9-10V.

Therefore, if V _rem is 9V,
The spontaneous polarization of the liquid crystal material is 10.6 nC / cm from the formula (1).
It turns out to be ² .

Furthermore, when the discharge of the pixel electrode is large and an auxiliary capacitance is required, the required spontaneous polarization and the required auxiliary capacitance are obtained by replacing C _LC with C _LC + C _AD in the equation (1). To be That is, the formula (1) is
Expression (4) can be expressed as the following expression.

[0055]

[Equation 4]

Assuming that the pulse width is short, the liquid crystal does not respond completely, and there is almost no fluctuation in the liquid crystal, it is hardly affected by spontaneous polarization, and the values in the parentheses in the equation (3) are 2-3. Therefore, the dielectric constant of the liquid crystal is 0.0221 pF / m, assuming that the value in the parentheses in the formula (3) is equal to 2.5. Therefore, the pixel capacitance is 0.221 pF. As mentioned above, when the optical characteristics are perfectly taken, V _rem
Is 9 V, the spontaneous polarization is 20 nC / cm ²
, The auxiliary capacitance added to the pixel is
It turns out that it becomes 2000 pF. As above
The values of the spontaneous polarization and the auxiliary capacitance of the liquid crystal material necessary for carrying out the present invention can be obtained.

[0057]

【Example】

Example 1 An example of a liquid crystal electro-optical device utilizing the present invention will be shown below, and the present invention will be described in accordance with the example. The structure of the liquid crystal electro-optical device manufactured in this example is shown in FIG. As one substrate 11 of the cell, an active matrix 15 using a crystalline silicon TFT formed on a non-alkali glass substrate was prepared. In the circuit configuration of the active matrix substrate, as shown in FIG. 4, no auxiliary capacitance was provided. A single-gate PMOS is used as the TFT because the leak current is small and ON / OFF can be large. Typically, leakage current is 1 pA or less (gate voltage +15 V, drain voltage -10 V) or less, ON /
OFF ratio 7.5 digits or more (gate voltage -15V / + 15
V, drain voltage -10V).

As the other substrate 12, there was used a substrate having an ITO film 14 formed on the entire surface and a silicon oxide film 21 for preventing short circuit formed thereon.

The size of the pixel 13 was 20 μm × 60 μm, and the scale of the matrix was 1920 × 480. When the charge retention characteristics of each pixel were examined, the worst one when -10 V was applied as a data signal was 3 msec.
The latter voltage was about -9V.

Therefore, in this embodiment, the width of the scanning signal pulse applied to the matrix is 1 μsec, the pulse height is −15 V, and the data signal is ± 15 V.

Next, a polymer resin 16 in which a solvent was dissolved was applied onto the substrate by spin coating. The polymer resin used here was a polyimide resin (manufactured by Toray Industries, Inc.), and n-methyl-2-pyrrolidone was used as the solvent. The dilution concentration of the polymer resin is 8 times. The substrate coated with the polymer resin was heated at 280 ° C. for 2.5 hours to dry the solvent and imidize the resin. Next, the resin on this substrate is 1000 rpm with a roller wrapped with cloth such as velvet.
Rubbed in one direction at the number of revolutions. Next, the substrates are separated by a distance of 1 to 7
An inorganic spacer having a thickness of μm was sandwiched between them to apply pressure. The liquid crystal material 17 was injected between these two substrates.

Next, the liquid crystal material will be described. The liquid crystal material used in this embodiment is CS-1014, a ferroelectric liquid crystal manufactured by Chisso Corporation. This liquid crystal has a phase sequence of I
So-N ^* -SmA-SmC ^* -Cry, which has a transition temperature of Iso-N ^* of 81 ° C and N ^* -Sm.
A is 69 ° C, SmA-SmC ^* is 54 ° C, SmC ^* -C
ry was -21 degreeC. The thickness of the liquid crystal cell is 1.6 μm
And The spontaneous polarization of the liquid crystal was 5 nC / cm 2.

It was confirmed that the domain structure was generated in the liquid crystal when the voltage applied to the liquid crystal was 5 V or less. Since such a domain structure deteriorates the characteristics when performing digital gradation display, it is desired to increase the applied voltage so that the domain does not occur.

FIG. 9 shows the dependence of the contrast ratio of the liquid crystal electro-optical device of this embodiment on the data signal application time. As shown in FIG. 9, it can be seen that a complete response is obtained even when the pulse width is 1 μsec.

Digital gradation display was performed by such a liquid crystal electro-optical device. That is, as shown in FIG. 2, one frame is composed of five subframes, and
Two-gradation digital gradation display was performed. The duration of each subframe is 179 μsec for the first subframe, 2.87 msec for the second subframe, and 3 for the third subframe.
58 μsec, the fourth sub-frame is 1.43 msec,
The fifth sub-frame was 717 μsec, and one frame was 5.5 msec, that is, 180 Hz.

As a result, with the above liquid crystal electro-optical device, it was possible to obtain a display with a maximum contrast ratio of 180 and 32 gradations.

Comparative Example As a comparative example, except that a ferroelectric liquid crystal having a spontaneous polarization value of 17 nC / cm ² was used,
FIG. 10 shows the results of measuring the transmitted light intensity of the liquid crystal electro-optical device having the same structure as in Example 1 by changing the time of the scanning signal pulse applied to the matrix. As can be seen from FIG. 10, the response becomes incomplete when the pulse width is shorter than 40 μs. From this, it can be seen that selecting an appropriate material so that the value of the spontaneous polarization of the liquid crystal material satisfies the formula (1) is effective in exerting the characteristics of the device.

[Embodiment 2] FIG. 11 shows the structure of a liquid crystal electro-optical device manufactured according to this embodiment. As one substrate 11 of the cell, an active matrix 15 using a crystalline silicon TFT formed on a non-alkali glass substrate was prepared. In this example, since a liquid crystal material having a large spontaneous polarization was used as shown below, an auxiliary capacitor was provided in the circuit configuration of the active matrix substrate as shown in FIG. The auxiliary capacitance was provided on the substrate 11 as shown in FIG. The size of the auxiliary capacitance 22 is 0.05 pF. T
A single-gate PMOS is used for the FT because the leak current is small and ON / OFF can be large. Typically, leakage current is 1 pA or less (gate voltage +15 V, drain voltage -10 V) or less, ON / OFF
The ratio was 7.5 digits or more (gate voltage -15V / + 15V, drain voltage -10V).

As the other substrate 12, a substrate having an ITO film 14 formed on the entire surface and a silicon oxide film 21 for preventing short circuit formed thereon was used.

The size of the pixel 13 was 20 μm × 60 μm, and the scale of the matrix was 1920 × 480. When the charge retention characteristics of each pixel were examined, the worst one when -10 V was applied as a data signal was 3 msec.
The latter voltage was about -9V.

The width of the scanning signal pulse applied to the matrix was 1 μsec, the pulse height was −15 V, and the data signal was ± 15 V.

Next, a polymer resin 16 in which a solvent was dissolved was applied onto the above substrate by spin coating. The polymer resin used here was a polyimide resin (manufactured by Toray Industries, Inc.), and n-methyl-2-pyrrolidone was used as the solvent. The dilution concentration of the polymer resin is 8 times. The substrate coated with the polymer resin was heated at 280 ° C. for 2.5 hours to dry the solvent and imidize the resin. Next, the resin on this substrate is 1000 rpm with a roller wrapped with cloth such as velvet.
Rubbed in one direction at the number of revolutions. Next, the substrates are separated by a distance of 1 to 7
An inorganic spacer having a thickness of μm was sandwiched between them to apply pressure. The liquid crystal material 17 was injected between these two substrates.

Next, the liquid crystal material will be described. The liquid crystal used in this example is a phenylpyrimidine ferroelectric liquid crystal, and its phase sequence is Iso-SmA-SmC ^* -Cr.
The transition temperature is 71.7 ° C. for Iso-SmA, 46.3 ° C. for SmA-SmC ^* , and SmC ^*.
-Cry was -9.7 ° C. The spontaneous polarization of liquid crystal is 18
It was nC / cm2. The thickness of the liquid crystal cell was 2.5 μm.

It was confirmed that a domain structure was generated in the liquid crystal when the voltage applied to the liquid crystal was 5 V or less. Since such a domain structure deteriorates the characteristics when performing digital gradation display, it is desired to increase the applied voltage so that the domain does not occur.

FIG. 12 shows the dependency of the contrast ratio on the auxiliary capacitance value when the data signal application time is fixed to 1 μs in the liquid crystal electro-optical device of this embodiment. Figure 1
As shown in FIG. 2, by providing the auxiliary capacitance of 0.5 pF, it can be seen that even a liquid crystal material having a large spontaneous polarization responds completely.

Digital gradation display was performed by such a liquid crystal electro-optical device. That is, as shown in FIG. 2, one frame is composed of five subframes, and
Two-gradation digital gradation display was performed. The duration of each subframe is 179 μsec for the first subframe, 2.87 msec for the second subframe, and 3 for the third subframe.
58 μsec, the fourth sub-frame is 1.43 msec,
The fifth sub-frame was 717 μsec, and one frame was 5.5 msec, that is, 180 Hz.

With the above liquid crystal electro-optical device, a display with a maximum contrast ratio of 180 and 32 gradations could be obtained.

Example 3 The structure of the liquid crystal electro-optical device manufactured by this example is the same as that of Example 1 as shown in FIG. In this embodiment, one substrate 11 of the cell is crystalline silicon TF formed on a non-alkali glass substrate.
An active matrix 15 using T was prepared. In the circuit configuration of the active matrix substrate, as shown in FIG. 4, no auxiliary capacitance was provided. Although a single-gate PMOS was used for the TFT, this has a small leak current and
This is because a large N / OFF can be taken. Typically, the leakage current was 1 pA or less (gate voltage +15 V, drain voltage -10 V) or less, and the ON / OFF ratio was 7.5 digits or more (gate voltage -15 V / + 15 V, drain voltage -10 V).

As the other substrate 12, a substrate having an ITO film 14 formed on the entire surface and a silicon oxide film 21 for preventing short circuit formed thereon was used.

The size of the pixel 13 was 20 μm × 60 μm, and the scale of the matrix was 1920 × 480. When the charge retention characteristics of each pixel were examined, the worst one when -10 V was applied as a data signal was 3 msec.
The latter voltage was about -9V.

Therefore, in this embodiment, the width of the scanning signal pulse applied to the matrix is 1 μsec, the pulse height is −15 V, and the data signal is ± 15 V.

Next, a polymer resin 16 in which a solvent was dissolved was applied onto the above substrate by spin coating. The polymer resin used here was a polyimide resin (manufactured by Toray Industries, Inc.), and n-methyl-2-pyrrolidone was used as the solvent. The dilution concentration of the polymer resin is 8 times. The substrate coated with the polymer resin was heated at 280 ° C. for 2.5 hours to dry the solvent and imidize the resin. Next, the resin on this substrate is 1000 rpm with a roller wrapped with cloth such as velvet.
Rubbed in one direction at the number of revolutions. Next, the substrates are separated by a distance of 1 to 7
An inorganic spacer having a thickness of μm was sandwiched between them to apply pressure. The liquid crystal material 17 was injected between these two substrates.

Next, the liquid crystal material will be described. The liquid crystal used in this example is a phenylpyrimidine ferroelectric liquid crystal, and its phase sequence is Iso-SmA-SmC ^* -Cr.
The transition temperature is 71.7 ° C. for Iso-SmA, 46.3 ° C. for SmA-SmC ^* , and SmC ^*.
-Cry was -9.7 ° C. The spontaneous polarization of liquid crystal is 18
It was nC / cm2. The thickness of the liquid crystal cell was 2.5 μm.

It was confirmed that the domain structure was generated in the liquid crystal when the voltage applied to the liquid crystal was 5 V or less. Since such a domain structure deteriorates the characteristics when performing digital gradation display, it is desired to increase the applied voltage so that the domain does not occur.

FIG. 13 shows the operating temperature dependence of the contrast ratio when the data signal application time is fixed to 1 μs in the liquid crystal electro-optical device of this embodiment. Also,
FIG. 14 shows the temperature dependence of the spontaneous polarization of the liquid crystal material used in this example. The liquid crystal electro-optical device of this embodiment is
At room temperature, as shown in FIG. 13, the contrast ratio is low and good optical characteristics are not obtained. It is considered that this is because the spontaneous polarization has a large value as shown in FIG. 14 and the liquid crystal molecules do not completely respond in the driving method in this example. However, it can be seen that increasing the temperature of the liquid crystal electro-optical device increases the contrast ratio, and the liquid crystal material responds completely when the operating temperature is 40 ° C. This is
It is considered that this is because the temperature of the liquid crystal material increased and the spontaneous polarization became small enough to perform the driving method in the present example, as shown in FIG. Therefore, in this embodiment, when the liquid crystal electro-optical device was driven, the temperature was adjusted so that the temperature was always 40 ° C. or higher. Each characteristic value during driving in this example is measured at a place such as a temperature-controlled room where the temperature can be kept constant.

Digital gradation display was performed by such a liquid crystal electro-optical device. That is, as shown in FIG. 2, one frame is composed of five subframes, and
Two-gradation digital gradation display was performed. The duration of each subframe is 179 μsec for the first subframe, 2.87 msec for the second subframe, and 3 for the third subframe.
58 μsec, the fourth sub-frame is 1.43 msec,
The fifth sub-frame was 717 μsec, and one frame was 5.5 msec, that is, 180 Hz.

As a result, with the above liquid crystal electro-optical device, a display with a maximum contrast ratio of 180 and 32 gradations could be obtained.

[0088]

INDUSTRIAL APPLICABILITY In the present invention, in a liquid crystal electro-optical device using a ferroelectric or antiferroelectric liquid crystal material or a polymer liquid crystal material thereof, a spontaneous polarization having an appropriate value so as to enable digital gradation display. It is characterized by using a liquid crystal material having Moreover, in order to use a liquid crystal material having a large spontaneous polarization, in this case, in particular, an auxiliary capacitance is provided on the substrate. As a result, digital gradation display can be performed, and extremely fine gradation display is possible.

For example, 256,0 of 640 × 400 dots
When analog gray scale display is performed using normal nematic liquid crystal on a liquid crystal electro-optical device in which 00 TFTs are formed in 100 mm square, 16 gray scale display is limited due to the influence of variations in TFT characteristics. there were. However,
When digital gradation display according to the present invention is performed, TF
Since it is less susceptible to the characteristic variation of the T element, 64
It becomes possible to display more than gradation, and what is 1 in color display.
A wide variety of 6,777,216 colors and subtle colors can be displayed.

[Brief description of drawings]

FIG. 1 shows an example of drive waveforms according to the present invention.

FIG. 2 shows an example of drive waveforms according to the present invention.

FIG. 3 shows an example of a conventional active matrix circuit and its drive waveform.

FIG. 4 shows a schematic diagram of a liquid crystal electro-optical device of the present invention.

FIG. 5 shows an example of a matrix configuration according to the first embodiment of the present invention.

FIG. 6 shows an example of a matrix configuration according to a second embodiment of the present invention.

FIG. 7 shows an example of gradation display according to the present invention. (Vertical axis: pulse duration, horizontal axis: contrast ratio)

FIG. 8 shows a schematic diagram of a liquid crystal electro-optical device according to a first embodiment of the present invention.

FIG. 9 shows the data signal application time dependency of the contrast ratio of the liquid crystal electro-optical device according to Example 1 of the invention.

FIG. 10 shows the dependence of the contrast ratio of the data signal application time on the liquid crystal electro-optical device of the comparative example of Example 1 of the present invention.

FIG. 11 is a schematic view of a liquid crystal electro-optical device according to a second embodiment of the invention.

FIG. 12 shows the auxiliary capacitance dependence of the contrast ratio of the liquid crystal electro-optical device according to Example 1 of the invention.

FIG. 13 shows the operating temperature dependence of the contrast ratio of the liquid crystal electro-optical device according to Example 3 of the invention.

FIG. 14 shows temperature dependence of spontaneous polarization of a liquid crystal material used in a liquid crystal electro-optical device according to Example 3 of the invention.

[Explanation of symbols]

11 ... Substrate 12 ... Substrate 13 ... Pixel electrode 14 ... Electrode 15 ... Thin film transistor 16 ... Aligned film 17 ... Ferroelectric or antiferroelectric Liquid crystal material or polymer resin in which the liquid crystal material is dispersed 21 ... Silicon oxide film 22 ... Storage capacitor

Claims (4)

[Claims] 1. A liquid crystal material having ferroelectricity or antiferroelectricity between a pair of substrates each having an electrode, at least one of which has a light-transmitting property, and one of which has a thin film transistor, or A liquid crystal electro-optical device in which a material in which they are dispersed in a polymer compound is sandwiched, and a data signal application time to the thin film transistor is shorter than a response time of a liquid crystal material or a material in which they are dispersed in a polymer compound. . 2. A liquid crystal material having ferroelectricity or antiferroelectricity between a pair of substrates having electrodes, at least one of which has a light-transmitting property, and which has a thin film transistor on one of the substrates, or A material in which these are dispersed in a polymer compound is sandwiched, and twice the product of the spontaneous polarization value of the liquid crystal material and the pixel electrode area is the applied data signal voltage,
A liquid crystal electro-optical device having a size equal to or larger than a product of a difference between a voltage held between electrodes until the next pulse is applied and a capacitance between pixel electrodes. 3. A liquid crystal material having ferroelectricity or antiferroelectricity between a pair of substrates having electrodes, at least one of which has translucency and which has a thin film transistor on either one of the substrates, or A material in which these are dispersed in a polymer compound is sandwiched, and twice the product of the spontaneous polarization value of the liquid crystal material and the pixel electrode area is the applied data signal voltage,
The difference between the voltage held between the electrodes until the next pulse is applied is multiplied by the sum of the capacitance between the pixel electrodes and the auxiliary capacitance provided for each pixel. Liquid crystal electro-optical device. 4. A liquid crystal material having ferroelectricity or antiferroelectricity between a pair of substrates each having an electrode, at least one of which has a light-transmitting property, and one of which has a thin film transistor, or A material in which these are dispersed in a polymer compound is sandwiched, and twice the product of the spontaneous polarization value of the liquid crystal material and the pixel electrode area is the applied data signal voltage,
A liquid crystal electro-optical device characterized by being used in a temperature range such that the difference between the voltage held between the electrodes until the next pulse is applied is multiplied by the capacitance between the pixel electrodes and larger. .