JP3507358B2 - Liquid crystal element and liquid crystal device having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a liquid crystal element which attains a state with high transmissivity, even in the case a liquid crystal which exhibits a chiral smetic phase is used and a liquid crystal device provided with it. SOLUTION: This liquid crystal element holds a liquid crystal 49, which exhibits a chiral smetic phase and moreover exhibits the first state with its monostabilized average molecular axis when no voltage is applied, of which the average molecular axis gets tilted, when a voltage with a first polarity or a reversed second polarity is applied, from the monostabilized position to the one side or to the reverse side with an angle corresponding to the applied voltage and of which, at the same time, the respective tilt angles of the average molecular axis under the voltage application at the maximum tilted state are identical based on the position of the first state as a reference, between a pair of substrates 81a, 81b. Also at least one of the anisotropy in refractive index of the liquid crystal 85 or the thickness of the liquid crystal layer is set so as to make the retardation under no voltage application larger than the optimum retardation of the liquid crystal 85 set corresponding to the arrangement of polarizing plates 87a, 87b and the wavelength of light transmitted by the liquid crystal layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal device used for a light valve used in a flat panel display, a projection display, a printer and the like, and a liquid crystal display device including the liquid crystal device, and particularly a chiral smectic. The present invention relates to one using a liquid crystal showing a phase.

[0002]

2. Description of the Related Art Conventionally, a TFT (Thin Film T
A typical liquid crystal mode of a nematic liquid crystal display element that is widely used as a display element using an active element such as a transistor is described in, for example, M. schadt and W. Helfrich ". Applied Phy
sics Letters Volume 18, Issue 4 (1971)
(Published February 15, 2002) "Twisted Nematic shown on pages 127-128.
The electronic mode is widely used.

On the other hand, recently, in-plane switching (In-Plane Switchc) utilizing a lateral electric field is used.
hing) mode and vertical alignment (Vertical A
A liquid crystal display using a liquid crystal mode has been announced, and the viewing angle characteristic, which has been a drawback of the conventional liquid crystal display, has been improved.

As described above, there are several liquid crystal modes for use in a TFT display element using such a nematic liquid crystal. In any of these modes, the response speed of the liquid crystal is several tens of milliseconds or more. And it is slow, and further improvement in response speed is required.

Therefore, in order to improve the response speed of such a conventional nematic liquid crystal element, some liquid crystal modes using a liquid crystal exhibiting a chiral smectic phase have been proposed in recent years. For example, "short pitch type ferroelectric liquid crystal", "polymer stable ferroelectric liquid crystal",
Although "threshold-free antiferroelectric liquid crystal" has been proposed and has not been put to practical use yet, it is reported that any of them can realize high-speed response of sub-millisecond or less.

On the other hand, we have invented and proposed an element described in Japanese Patent Application No. 11-39954 (hereinafter referred to as prior application element). Here, in this prior application device, the smectic layer of the chiral smectic C liquid crystal has a chevron structure, and the tilt angle δ of the smectic layer forming the chevron structure with respect to the substrate normal is at least the tilt of the chiral smectic liquid crystal at the operating temperature. The angle Θ is substantially equal to or more than that, and when no electric field is applied, the average molecular axis of the molecules of the chiral smectic liquid crystal substantially coincides with the average biaxial alignment treatment axis, and It has an alignment state that has only one darkest optical axis when sandwiched between two, and when an electric field is applied, the apparent tilt angle and transmitted light intensity of the chiral smectic liquid crystal are continuously changed according to the change in the applied electric field strength. Is characterized by changing.

The prior application device having such characteristics has a single alignment state having only one darkest optical axis when sandwiched between crossed Nicols polarizers when no electric field is applied. In addition, when the electric field is applied, the transmitted light intensity changes according to the electric field applied to the liquid crystal, and gradation display can be realized by utilizing this characteristic.

Both the ferroelectric liquid crystal and the antiferroelectric liquid crystal that perform such inversion switching by spontaneous polarization are liquid crystals exhibiting a chiral smectic liquid crystal phase. That is, in the sense that the problem regarding the response speed that the conventional nematic liquid crystal has had can be solved, realization of a liquid crystal element using a chiral smectic liquid crystal is expected.

[0009]

However, a liquid crystal element using a smectic liquid crystal having a gradation display capability in next-generation displays such as high-speed response performance is expected, but the optimum design of the liquid crystal element is desired. The current situation is that there are not enough guidelines for cell design, especially cell thickness design.

With respect to such cell thickness design, for example, in the case of SSFLC which takes only a binary state of brightness and darkness, when the liquid crystal molecules are aligned parallel to the substrate, the birefringence anisotropy value (Δn = N _// −n⊥) and the cell thickness d (Δnd) can be used to estimate the amount of transmitted light in the transmissive state (white display state).

For example, the transmitted light amount T1 under crossed Nicols
Is represented by T1 = Sin ² (πΔnd / λ) ... [1]. The liquid crystal element can be optimized by determining the cell thickness so as to obtain a desired wavelength-transmittance characteristic using this formula.

When the SSFLC has a large pretilt angle, the effective birefringence anisotropy value (Δn _effect ) in the liquid crystal element is taken into consideration by considering the rising angle of the liquid crystal molecules from the horizontal plane of the substrate. Can be obtained by calculation and can be designed by the same method by using it in the formula [1]. Therefore, the optimum cell thickness for the liquid crystal element can be easily obtained.

However, as a result of a detailed study by the inventors, when a device is manufactured according to the design guideline described above in the prior application device, the transmittance when actually driven is increased despite the same ferroelectric liquid crystal. The result is different from the predicted value. In other words, SSF with bistability
It was discovered by experiments that the LC and the monostable FLC of the prior application device behave differently.

For example, in the case of the prior application element, when an element having a zero pretilt is manufactured and the cell thickness is set to optimize Δnd as described above, the amount of transmitted light is lower than the desired transmitted light amount calculated by the above formula [1]. Only the intensity of transmitted light was obtained, and the white color temperature was higher than the predicted value, that is, the peak wavelength was shifted to the single wavelength side.

Therefore, the present invention has been made in view of the above circumstances, and a liquid crystal element capable of obtaining a high transmittance even when a liquid crystal exhibiting a chiral smectic phase is used, and a liquid crystal including the same. The purpose is to provide a device.

[0016]

According to the present invention, a liquid crystal exhibiting a chiral smectic phase is sandwiched, while an electrode for applying a voltage to the liquid crystal is provided, and the liquid crystal is aligned on the facing surface of at least one substrate. A liquid crystal element comprising a pair of substrates that have been subjected to a uniaxial alignment treatment, and a polarizing plate provided on at least one of the substrates, wherein the liquid crystal is
When no voltage is applied, the average molecular axis of the liquid crystal shows a first state in which it is mono-stabilized, and when a voltage of the first polarity is applied, the average molecular axis of the liquid crystal is at an angle corresponding to the magnitude of the applied voltage. The average molecular axis of the liquid crystal is tilted to one side from the mono-stabilized position, and when a voltage having a second polarity opposite to the first polarity is applied, the average molecular axis of the liquid crystal is In addition to tilting to the side opposite to that when a voltage of polarity is applied, the monostable state of the average molecular axis of the liquid crystal during the application of the voltage of the first polarity and the voltage of the second polarity in the first state The tilt angles of the maximum tilt state with respect to the changed position are equal, and the optimum retardation amount of the liquid crystal is set according to the arrangement of the polarizing plate and the wavelength of light transmitted through the liquid crystal layer. , The amount of retardation when no voltage is applied is larger So as it is characterized in that setting the at least one of the thickness values and the liquid crystal layer with a refractive index anisotropy of the liquid crystal.

Further, the present invention is characterized in that the substrate has an active element and a drive circuit for performing active matrix drive.

According to the present invention, the liquid crystal has an inclination angle δ with respect to a substrate normal line of the smectic layer having a chevron structure.
However, chiral smectic C at least at operating temperature
It is characterized in that it is substantially the same as or larger than the tilt angle Θ of the liquid crystal.

In the present invention, the liquid crystal is Iso-Ch-
It has a phase sequence of SmA-SmC ^*, and is characterized in that the electric field application process is performed in Ch, SmA, or SmC ^* .

In the present invention, the liquid crystal is Iso-Ch-
It has a phase sequence of SmA-SmC ^*, and is characterized in that the pressure treatment is performed with SmA or SmC ^* .

In the present invention, the liquid crystal is Iso-Ch-
SmC ^* has a phase sequence that, Ch or SmC ^*
The method is characterized in that the electric field application process is carried out at.

In the present invention, the tilt angle δ of the smectic layer of the liquid crystal is larger than at least the size calculated from the temperature change of the layer spacing in the bulk state of the chiral smectic C liquid crystal at a use temperature. It is characterized by that.

According to the present invention, the liquid crystal is formed by cooling the layer inclination angle δ of the chevron structure in the element, which is changed by the electric field application process and / or the pressure process, from the high temperature side showing the isotropic liquid phase. Inclination angle δ ₀ of the initial chevron structure
It is characterized by being in a larger state.

The present invention is also characterized in that the helical pitch of the liquid crystal in the bulk state is longer than twice the cell thickness.

Further, the present invention is a transmissive liquid crystal device having a liquid crystal element, a drive circuit for driving the liquid crystal element, and a backlight light source, wherein the liquid crystal element is one of the above inventions. It is characterized by that.

Further, the present invention is a reflection type liquid crystal device having a liquid crystal element, a drive circuit for driving the liquid crystal element, and a reflector, wherein the liquid crystal element is the invention described in any one of the above. It is characterized by being.

Further, as in the present invention, while exhibiting a chiral smectic phase, it exhibits the first state in which the average molecular axis of the liquid crystal is mono-stabilized when no voltage is applied, and it exhibits the first polarity or the first polarity. When a voltage having a second polarity opposite to that of the above is applied, the average molecular axis of the liquid crystal tilts from the mono-stabilized position to one side or the opposite side at an angle according to the magnitude of the applied voltage,
The tilt angles of the maximum tilt state with respect to the mono-stabilized position in the first state of the average molecular axes of the liquid crystal when the voltage of the first polarity is applied and the voltage of the second polarity are applied are equal. The liquid crystal is sandwiched between a pair of substrates. In addition, the refractive index anisotropy of the liquid crystal is set so that the retardation amount when no voltage is applied is larger than the optimum retardation amount of the liquid crystal set according to the arrangement of the polarizing plate and the wavelength of light transmitted through the liquid crystal layer. At least one of the value and the thickness of the liquid crystal layer is set.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

FIG. 1 is a sectional view showing the structure of a liquid crystal device equipped with a liquid crystal element according to an embodiment of the present invention. In FIG. 1, 70 is a liquid crystal device and 80 is a pair of polarization axes orthogonal to each other. The liquid crystal element is sandwiched between the polarizing plates 87a and 87b. The liquid crystal element 80 has a substrate 81 made of a highly transparent material such as a pair of glass and plastic.
A liquid crystal 85, preferably a liquid crystal exhibiting a chiral smectic phase, is sandwiched between a and 81b.

Here, the substrates 81a and 81b are provided with In ₂ O ₃ and ITO for applying a voltage to the liquid crystal 85, respectively.
Electrodes 82a and 82b made of the same material are provided in, for example, a stripe shape, and these intersect with each other to form a matrix electrode structure. As will be described later, dot-shaped transparent electrodes are arranged in a matrix on one substrate, and a TFT or MIM (Metal-I) is provided on each transparent electrode.
It is preferable to connect a switching element such as an insulator-metal) and to provide an active matrix structure by providing a counter electrode on one surface of the other substrate or in a predetermined pattern.

Further, the electrode 82a, On _{82b, SiO 2, TiO 2,} Ta 2 O consists of ₅ such material insulating film 83a having a function such as to prevent these short as necessary, 83 b is Insulating film 83
Alignment control films 84a and 84b which are in contact with the liquid crystal 85 and function to control the alignment state thereof are provided on the a and 83b.

Here, at least one of the orientation control films 84a and 84b is uniaxially oriented. The orientation control films 84a and 84b are, for example, those obtained by applying a rubbing treatment to the surface of a solution-coated film of an organic material such as polyimide, polyimide amide, polyamide, or polyvinyl alcohol, or an oxide such as SiO or nitride. It is composed of an oblique vapor deposition film of an inorganic material in which an object is vapor-deposited from a substrate at a predetermined angle.

Further, regarding the alignment control films 84a and 84b, the pretilt angle of the molecules of the liquid crystal 85 (the film near the interface of the alignment control film of the liquid crystal molecules) depends on the selection of the material, the condition of the treatment (uniaxial alignment treatment, etc.) and the like. The angle to the surface) is adjusted.

When the orientation control films 84a and 84b are both uniaxially oriented films, the uniaxial orientation direction of each film (particularly the rubbing direction) is parallel or antiparallel depending on the liquid crystal material used. Alternatively, it can be set to cross in the range of 45 ° or less.

The substrates 81a and 81b are opposed to each other with the spacer 86 interposed therebetween. Such spacer 86
Is for determining the distance (cell gap) between the substrates 81a and 81b, and silica beads or the like is used.

Here, regarding the cell gap determined by the spacer 86, the optimum range and the upper limit value differ depending on the liquid crystal material, but the uniform uniaxial orientation and the average molecular axis of the liquid crystal molecules when no voltage is applied. It is preferable to set it in the range of 0.3 to 10 μm in order to develop an alignment state that is substantially the same as the average axis of the alignment treatment axes. Further, it is preferable that the value of the cell thickness is appropriately adjusted and set so as to be a retardation amount when no voltage is applied, which will be described later.

In addition to the spacer 86, the substrate 81a
Adhesive particles made of a resin material such as an epoxy resin may be dispersedly arranged (not shown) in order to improve the adhesion between the liquid crystal layer 81 and 81b and to improve the impact resistance of the liquid crystal 85 exhibiting a chiral smectic phase.

On the other hand, in such a liquid crystal element 80, when a liquid crystal exhibiting a chiral smectic phase is used as the liquid crystal 85, the composition of the material is adjusted, and further the treatment of the liquid crystal material and the element constitution, for example, the alignment control film 84a. , 84b
By appropriately setting the material, the processing conditions, etc., the average molecular axis (liquid crystal molecule) of the liquid crystal exhibits a mono-stabilized orientation state when no voltage is applied, and one polarity (first When the voltage of (polarity) is applied, the tilt angle based on the mono-stabilized position of the average molecular axis continuously changes according to the magnitude of the applied voltage, and when the voltage of the other polarity (second polarity) is applied. The average molecular axis of the liquid crystal is made to exhibit a characteristic that it tilts at an angle according to the magnitude of the applied voltage. Further, in the present embodiment, the maximum tilt angle β1 due to the voltage application of the first polarity and the maximum tilt angle β2 due to the voltage application of the second polarity exhibit the same characteristic.

As the liquid crystal material exhibiting such a chiral smectic phase, the characteristics of the above-mentioned prior application device (characteristic value cone angle Θ peculiar to the liquid crystal material, layer spacing d of the smectic layer, inclination angle δ) are described. A hydrocarbon-based liquid crystal material having a biphenyl skeleton, a phenylcyclohexane ester skeleton, a phenylpyrimidine skeleton, or the like,
A composition prepared by appropriately selecting a naphthalene liquid crystal material or a polyfluorine liquid crystal material is used.

Further, in these liquid crystal materials, (a) whether the inclination angle δ of the smectic layer having the chevron structure with respect to the substrate normal is substantially the same as the tilt angle Θ of the chiral smectic C liquid crystal at the operating temperature or not. Greater than (B) The tilt angle δ of the smectic layer is larger than the size calculated from the temperature change of the layer spacing in the bulk state of the chiral smectic C liquid crystal at least at the use temperature. (C) The layer inclination angle δ of the chevron structure in the element changed by the electric field application treatment and / or the pressure treatment is the layer inclination angle of the initial chevron structure formed by cooling from the high temperature side showing the isotropic liquid phase. The state is larger than δ ₀ . (D) The spiral pitch in the bulk state is longer than twice the cell thickness.

It shows characteristics such as.

Further, (a) it has a phase sequence of Iso-Ch-SmA-SmC ^* , and the electric field application process is performed with Ch, SmA or SmC ^* . (B) It has a phase sequence of Iso-Ch-SmA-SmC ^* , and pressure treatment is performed with SmA or SmC ^* . (C) It has a phase sequence of Iso-Ch-SmC ^* , and the electric field application process is performed with Ch or SmC ^* .

Processing such as the above is performed.

Incidentally, the liquid crystal device 7 according to the present embodiment.
0, a drive circuit for supplying a gradation signal to the liquid crystal element 80 shown in FIG. 1 is provided, and by applying the voltage as described above, a continuous tilt angle from the monostable position of the average molecular axis of the liquid crystal is provided. And a characteristic that the amount of light emitted from the liquid crystal element 80 continuously changes are used to perform gradation display.

For example, as one substrate of the liquid crystal element 80, an active matrix substrate provided with the above-mentioned TFT or the like is used, and active matrix driving by amplitude modulation is performed by a driving circuit, thereby enabling analog gradation display. .

Next, a gradation display using such an active matrix substrate will be described with reference to FIGS.

FIG. 2 is a diagram schematically showing the structure of one substrate (active matrix substrate) of the liquid crystal element 80. As shown in FIG. 2, the panel section 9 corresponding to the liquid crystal element is provided.
0, the horizontal gate lines G1 and G2 corresponding to the scanning lines connected to the scanning signal driver 91 which is the driving unit.
, G5 and the source line S1 in the vertical direction corresponding to the information signal line connected to the information signal driver 92 as the driving means.
S2, ..., S5 are provided so as to be orthogonal to each other while being insulated from each other.

Further, these gate lines G1, G2 ..., G
5 and the source lines S1, S2 ..., S5, corresponding to the pixels at the respective intersections, a thin film transistor (T
FT) 94 and a pixel electrode 95 are provided (only 5 × 5 pixel region is shown in the figure for simplification). As the switching element, an MIM element can be used instead of the TFT.

Further, the gate lines G1, G2, ..., G5 are TF.
, S5 are connected to the source electrode (not shown) of the TFT 94, and the pixel electrode 15 is connected to the drain electrode (not shown) of the TFT 94. Has been done.

Then, in the substrate having such a structure,
The scanning signal driver 91 causes the gate lines G1, G2, ..., G
5 is, for example, line-sequentially scanned and selected to supply a gate voltage, and in synchronization with the scanning selection of the gate lines G1, G2, ..., G5, the information signal driver 92 outputs information according to information to be written in each pixel. Signal voltage is source line S1, S2
..., each pixel electrode 1 supplied to S5 and via the TFT 94.
5 is applied.

On the other hand, FIG. 3 is a diagram showing an example of a sectional structure of each pixel portion (for one bit) in the panel structure as shown in FIG. 2. In the structure shown in FIG. 3, the TFT 94 and the pixel electrode 95 are formed. Active matrix substrate 20 provided
And a counter substrate 40 having a common electrode 32, a liquid crystal layer 49 having spontaneous polarization is sandwiched between the liquid crystal capacitor (C1) and
c) 31 is configured. In this active matrix substrate 20, the amorphous S is used as the TFT 94.
iTFT is used.

Here, the TFT 94 is formed on the substrate 21 made of glass or the like, and the gate line G1 shown in FIG.
An a-Si layer 24 is provided on the gate electrode 22 connected to G2, ..., G5 via an insulating film (gate insulating film) 23 made of a material such as silicon nitride (SiNx), and the a-Si layer 24. A source electrode 27 and a drain electrode 28 are provided on the upper side of the n ⁺ a-Si layers 25 and 26, respectively, separated from each other.

The source electrode 27 is connected to the source lines S1, S2 ..., S5 shown in FIG. 2, and the drain electrode 28 is I.
It is connected to the pixel electrode 95 made of a transparent conductive film such as a TO film. In addition, the channel protection film 29 covers the a-Si layer 24 of the TFT 94. Furthermore, this TFT
In the period 94, a gate pulse is applied to the gate electrode 22 during a period in which the corresponding gate line is scan-selected, and is turned on.

On the other hand, in the active matrix substrate 20, the insulating film 23 (a film continuously provided with the insulating film on the gate electrode 22) is formed by the pixel electrode 95 and the storage capacitor electrode 30 provided on the glass substrate 21 side. The holding capacitor (CS) 32 is provided in parallel with the liquid crystal layer 49 due to the structure of sandwiching (1). When the area of the storage capacitor electrode 30 is large, the aperture ratio is lowered, so that the storage capacitor electrode 30 is formed of a transparent conductive film such as an ITO film.

Further, T of the active matrix substrate 20
On the FT 94 and the pixel electrode 95, there is provided an alignment film 43a for controlling the alignment state of the liquid crystal 49, for example, which has been subjected to a uniaxial alignment process such as a rubbing process.
On the glass substrate 41 of 0, the common electrode 4 having the same thickness as the entire surface is formed.
2 and the alignment film 43b are stacked. In the panel structure as shown in FIGS. 2 and 3, a polycrystalline Si (p-Si) TFT is used as the active matrix substrate 20.
A substrate provided with can be used.

Next, with reference to FIGS. 4 and 5, active matrix driving utilizing the characteristics of the liquid crystal element having the above structure will be described. 4 shows an equivalent circuit of the pixel portion of the panel shown in FIG. Further, FIG. 5 is a diagram showing an applied voltage for driving a liquid crystal element and an optical response.

Here, FIG. 5A shows a voltage applied to one gate line which is a scanning line connected to the pixel when one pixel is focused. In the liquid crystal element having the above structure, as described above, the gate lines G1 and G are provided for each field.
, G5 are selected, for example, line-sequentially, a predetermined gate voltage Vg is applied to one gate line in the selection period Ton, the voltage Vg is applied to the gate electrode 22, and the TFT 94 is connected.
Turns on.

In the non-selection period Toff corresponding to the period in which another gate line is selected, the voltage is not applied to the gate electrode 22 and the TFT 12 is in a high resistance state (off state), whereby a predetermined Toff is performed. The same gate line is selected and the gate voltage Vg is applied to the gate electrode 22.

FIG. 5B shows the voltage Vs applied to the information signal line (source line) of the pixel. And FIG.
As shown in (a), when the gate voltage is applied to the gate electrode 22 in the selection period Ton in each field, the source lines S1 and S that become the information lines connected to the pixel in synchronization with this are applied.
, S5 applies a predetermined source voltage (information signal voltage) Vs (the reference potential is the potential Vc of the common electrode 42) to the source electrode 27.

Here, in the first field (1F) constituting one frame, the optics to be obtained in the pixel based on the information written in the pixel, for example, the voltage-transmittance characteristic according to the liquid crystal used. A positive source voltage (information signal voltage) of level Vx according to the state or display information (transmittance) is applied.

At this time, since the TFT 94 is in the ON state, the voltage Vx applied to the source electrode 27 is applied to the pixel electrode 95 via the drain electrode 28, and the liquid crystal capacitance (C1c) 31 and the storage capacitance (Cs). 32 is charged, and the potential of the pixel electrode becomes the information signal voltage Vx.

Subsequently, in the non-selection period Toff of the gate line to which the pixel belongs, the TFT 94 has a high resistance (off state).
Therefore, the liquid crystal cell (liquid crystal capacitance C
1c) 31 and the storage capacitor (Cs) 32, the selection period To
The state in which the charge charged by n is accumulated is maintained and the voltage V
x is retained. As a result, the voltage Vx is applied to the liquid crystal layer 49 of the pixel through the period of the first field 1F, and the liquid crystal portion of the pixel obtains an optical state (amount of transmitted light) corresponding to the voltage value.

On the other hand, if the response speed of the liquid crystal is slower than the gate-on period at this time, the liquid crystal cell (liquid crystal capacitance C1c) 31
The charging of the storage capacitor (Cs) 32 is completed, and switching is started in the non-selection period in which the gate is turned off. Then, in such a case, the charged charges are canceled by the inversion of the spontaneous polarization, and the voltage value Vpix applied to the liquid crystal layer 49 takes a value Vx 'smaller than Vx as shown in FIG. 5C.

Next, in the selection period Ton of the second field (2F), the source voltage (-Vx) having the polarity value Vx which is opposite in polarity to the first field 1F and has substantially the same voltage value Vx.
Is applied to the source electrode 27. At this time, the TFT 94 is in the ON state, the voltage −Vx is applied to the pixel electrode 95, and the liquid crystal capacitance (C1c) 31 and the storage capacitance (Cs) 32 are provided.
Is charged, and the potential of the pixel electrode is changed to the information signal voltage -Vx.
become.

Then, in the non-selection period Toff, TF
Since T94 has a high resistance (off state), during this non-selection period, the liquid crystal cell (liquid crystal capacitance Clc) 31 and the storage capacitor (Cs) 32 maintain the state in which the charge charged during the selection period Ton is accumulated. Then, the voltage −Vx is held.

Then, the voltage -Vx is applied to the liquid crystal layer 49 of the pixel through the second field 2F period, and the optical state (amount of emitted light) corresponding to the voltage value is obtained in the pixel. At this time as well, when the response speed of the liquid crystal is slower than the gate-on period, the liquid crystal cell (the liquid crystal capacitance C1
c) The charging of 31 and the storage capacitor (Cs) 32 is completed, and switching is started in the non-selected period when the gate is turned off.

In such a case, the charged charges are canceled by the reversal of the spontaneous polarization, and the voltage value Vpix applied to the liquid crystal layer 49 is smaller than -Vx, as shown in FIG. 5C.
Takes the value VX '.

On the other hand, the actual optical response of the liquid crystal in the pixel (optical response in the case of using a transmissive liquid crystal element) is, as shown in (d), gradation display according to Vx 'in the first field 1F. The state (amount of emitted light) is obtained, and the second field 2F
In, a gradation display state corresponding to -Vx 'is obtained, and these have the same gradation level.

As described above, in the above-mentioned active matrix driving, it is possible to perform gradation display based on excellent high-speed response when using a liquid crystal exhibiting a chiral smectic phase. Further, in the first and second fields 1F and 2F, the voltages of the same level are inverted in polarity and applied to the liquid crystal layer 49, so that the voltage actually applied to the liquid crystal layer 49 is converted into an alternating current, which causes deterioration of the liquid crystal. To be prevented.

The gradation display by the active matrix driving is performed by the transparent substrate 81a, as shown in FIG.
81b and a pair of polarizing plates, not only a transmissive liquid crystal element of a type that modulates light incident from one substrate side by a backlight (not shown) and emits the light to the other side, but at least one substrate A reflection type liquid crystal element having a polarizing plate provided on the substrate, that is, a reflection plate is provided on one side of the substrates 81a and 81b, or a reflective material is used as one of the substrates itself or a member provided on the substrate. It can also be applied to an element of a type that modulates reflected light and emits light to the same side as the incident side. Further, it can be applied to a color liquid crystal element in which at least one of R, G, and B color filters is provided on one of the substrates 81a and 81b.

By the way, regarding the phenomenon that the SSFLC having the bistability and the monostable FLC of the prior application element behave differently, which is described in the problem to be solved by the above-mentioned invention, the bright state of the SSFLC or The most different point between the alignment state in the halftone display state and the alignment state in driving the ferroelectric liquid crystal having gradation display capability of the prior application is that the latter liquid crystal element is composed of a plurality of frames per second. It is a liquid crystal device that displays an image, and it is considered that the molecular orientation state is constantly changed in each frame when AC driving is performed such that voltages having different polarities are applied for each frame.

For example, since SSFLC has a memory property, a voltage for maintaining gradation information is basically unnecessary, and therefore AC driving is rarely performed as a driving method.

On the other hand, a monostable ferroelectric liquid crystal used in a liquid crystal element having a gradation display capability such as a prior application element is premised on TFT driving, and forms an alignment state according to an applied voltage value. . In this TFT driving, if a direct current is applied to the liquid crystal layer, it causes a burn-in phenomenon due to uneven distribution of impurity ions. Therefore, the above-mentioned alternating current driving is generally used.

However, unlike the nematic liquid crystal that responds to the effective value, in these liquid crystals having spontaneous polarization, the alignment state changes according to the polarity of the applied AC voltage. In other words, each frame always includes a transient state in which the orientation state changes, and such a state is observed. In view of the fact that the inversion of bulk molecules is generally faster than the inversion of molecules near the interface, it is conceivable that the liquid crystal molecules always undergo a twisted state in the inversion of molecules in such a transient state.

Here, regarding this twisted state, the orientation state can be approximated as follows. That is, in the monostable mode, it is considered that the vicinity of the interface maintains the alignment state in the state where no voltage is applied even if a voltage is applied. , Ξ = ξ (V), and this value is a function of the applied voltage.

Note that due to the symmetry of the element, ξ (0) = d / 2 when the voltage is zero, but the value of ξ decreases as the applied voltage increases. From this, the thickness d _sw of the liquid crystal layer which is inverted in the bulk is expressed as d _sw = d−2 × ξ (V) ... [3].

Here, in the case of an element in which the polarization axis direction of one of the crossed Nicols and the average molecular orientation direction in the state where no voltage is applied are matched, the liquid crystal molecules existing within the distance of ξ above are incident polarized light. Does not contribute to the rotation of polarized light.

On the other hand, the liquid crystal layer d _sw inverted in the bulk
The liquid crystal in the range of is a smectic liquid crystal, the order of liquid crystal molecules is higher than that of nematic liquid crystal, the cell thickness of the smectic liquid crystal element is very thin, generally about 1 to 2 μm, and d _sw is a sufficiently small value. Therefore, it can be approximated to switching while maintaining a uniform arrangement.

That is, it can be approximated by taking an inversion process in which the length of ξ (V) becomes shorter as the applied voltage becomes larger, and at the same time, the switching angle becomes larger while being uniformly inverted in d _sw .

These are not only a function of voltage but also a function of time because they are transient response phenomena. That is, when the switching angle at d _sw is ψ, the instantaneous transmittance T3 at time t is expressed as T3 = sin ² (2ψ) × sin ² (R3 / λ) ... [4]. Here, R3 is a transient retardation amount at time t, and is an effective refractive index anisotropy (Δn
_effect ) and d _sw . Further, the Δn _effect matches the Δn peculiar to the liquid crystal molecules when the liquid crystal molecules are inverted only in the plane parallel to the substrate, but as the inversion based on the Goldstone mode, when switching on the cone of the SmC ^* liquid crystal. It is considered that the angle raised from the substrate surface is smaller than Δn peculiar to liquid crystal molecules.

In any case, the transient retardation R3
And the switching angle ψ at d _sw is a function of the applied voltage and time, so R3 = R3 (V, t), and ψ = ψ
(V, t). Therefore, the average value of the transmitted light intensity Tr when the voltage V is applied within one frame is

[0082]

[Equation 1] Becomes

Here, T is 1 when the backlight is always lit and used in a transmissive element, for example.
It represents the frame period. In addition, Japanese Patent Application No. 10-177
Similarly, in the case where one frame is composed of a plurality of fields such as the element described in 145, the entire period of one frame is represented. On the other hand, when the backlight is displayed by blinking, it represents the lighting period of the backlight.

From the above, by considering the value of ξ depending on the liquid crystal material used for the liquid crystal element and its alignment state, and the value of R3 obtained thereby, it is desirable to consider the above formula [5]. The optimum cell thickness can be set so that the amount of transmitted light can be obtained.

By the way, the retardation amount during driving is a function of time as described above, but when the average retardation amount within the period of T is taken into consideration, the transmitted light is affected by the transient retardation amount R3. The amount of retardation to be applied should be a weighted average considering the switching angle ψ which is also a function of time. Therefore, at least in the present invention, when the average retardation amount within the period of T is,

[0086]

[Equation 2] Is defined as the value of.

The value of the average retardation amount within the period T obtained by the equation [6] may be simply referred to as "retardation at the time of voltage application" in this specification.

(Method of Measuring Transient Retardation Amount) As described above, the switching angle ψ and the bulk retardation R3 are functions of time, and it is necessary in the present invention to measure these values in real time. However, for example, the retardation amount has conventionally been generally used to be obtained by the Senarmont method or Bereck method, but these methods are difficult to measure in real time. Therefore, the inventors newly devised a method for measuring a transitional switching angle and a retardation amount, and measured using the method. The method is shown below.

As described above, the orientation of the liquid crystal molecules when a voltage is applied to the liquid crystal element is such that the liquid crystal layer near the interface that does not contribute to the polarization rotation of the incident polarized light and the thickness d _sw that contributes to the polarization rotation. It is divided into a bulk layer. The purpose of this measurement is to obtain the transient switching angle and the amount of retardation in this bulk layer in real time.

The inventors have considered that it is possible to measure by using a phase compensator to obtain these values. That is, a phase compensating plate having the same value as the retardation amount of the bulk layer is prepared, and its angle is adjusted so that the optical axis direction thereof is orthogonal to the liquid crystal molecule alignment direction of the bulk layer and laminated.
That is, when these are completely orthogonal, the retardation of the bulk layer is compensated, the polarization rotation does not occur as a result of the retardation amount becomes zero, it is considered that the transmitted light intensity in the state of no voltage application, that is, the darkest state. .

Then, the switching angle of the bulk layer can be estimated by measuring the angle at which the phase compensation plate is arranged at this time. Further, the transitional switching angle can be obtained in real time by performing this measurement using an oscilloscope and obtaining the angle that produces the darkest state at any time.

Further, as the above-mentioned phase compensating plate, an ECB mode cell of nematic liquid crystal can be substituted.
As a result, the amount of retardation can be changed simply by changing the voltage applied to the nematic liquid crystal cell. Therefore, while performing the above-mentioned operation of adjusting the stacking angle and the voltage applied to the nematic liquid crystal cell at the same time, the darkest state Both the retardation amount and the switching angle can be obtained at the same time by finding the condition that takes

The relationship between the voltage applied to the nematic liquid crystal and the amount of retardation can be measured with a Senarmont or Bereck conventor. Also regarding this measurement, the transitional switching angle and the retardation amount can be simultaneously obtained in real time by performing the measurement using an oscilloscope and obtaining the condition of the darkest state at an arbitrary time.

Then, by designing the device so that the optimum cell thickness obtained by the above equation [5] is obtained, it becomes possible to obtain a desired transmission characteristic.

By the way, in the liquid crystal element 80 according to the present embodiment having the above-described structure, the product of the cell thickness is the optimum retardation, which is the product of the refractive index anisotropy value (Δn) of the liquid crystal material. It is desirable to set the cell thickness to a value different from the value of the cell thickness designed to have the above value.

That is, in the liquid crystal element 80, the effective Δn value in the voltage applied state is smaller than the Δn value in the no voltage applied state. It is desirable to determine the cell thickness so that the product of the value (Δn) and the cell thickness is larger than the value of the cell thickness designed to have the optimum retardation value.

The optimum retardation value referred to here is a value at which Δnd / λ becomes 1/2 under crossed Nicols when used as a transmissive liquid crystal element, but this value depends on the wavelength. Therefore, it is necessary to determine the wavelength to be optimized in advance. Here, from our experience, when used as a color display element, as the center wavelength to be optimized from the viewpoint of color temperature during white display,
It has been found that 400 nm to 480 nm is suitable.
Therefore, the optimum retardation value is 200 nm to 240
nm.

As described above, in the present embodiment, the product of the refractive index anisotropy value (Δn) of the liquid crystal material and the cell thickness is 20.
The cell thickness is determined so as to be larger than the range of 0 nm to 240 nm.

That is, in the liquid crystal element using the chiral smectic liquid crystal in which the liquid crystal exhibits a monostable state when no voltage is applied as described above, the retardation amount when the voltage is applied is smaller than the retardation amount when the voltage is not applied. When designing such an element, for example, in the case of a liquid crystal element that displays black when no voltage is applied, the retardation amount when no voltage is applied is designed to be larger than the optimum retardation amount.

By thus designing the retardation amount when no voltage is applied to the optimum retardation amount, that is, the product of the refractive index anisotropy value Δn of the liquid crystal material and the cell thickness d is increased, A desired amount of transmitted light can be obtained.

Examples and comparative examples of the present embodiment will be described below.

(Example) (Production of Liquid Crystal Cell) In this example, a pair of glass substrates having a thickness of 1.1 mm on which a 700 L IT0 film was formed as a transparent electrode were prepared. A polyimide precursor having the following repeating unit PI-a was applied on the transparent electrode of the substrate by a spin coating method, followed by predrying at 80 ° C. for 5 minutes and then heating at 200 ° C. for 1 hour. Firing was performed to obtain a polyimide coating having a film thickness of 50 Å.

Subsequently, the polyimide film on the substrate was subjected to rubbing treatment with a nylon cloth as uniaxial orientation treatment. The condition of the rubbing treatment is a rubbing roll in which nylon (NF-77 / manufactured by Teijin) is bonded to a roll having a diameter of 10 cm, and the pushing amount is 0.3 mm and the feeding speed is 10
cm / sec, the number of revolutions was 1000 rpm, and the number of feeds was 4 times.

[0104]

[Chemical 1] Then, as a spacer on one of the substrates, an average particle size of 1.
Disperse 7 μm silica beads and face each other so that the rubbing directions of the substrates are parallel to each other.
A cell having a uniform cell gap (empty cell) was obtained. And
The pretilt angle of this cell was 2.0 ° as measured by the crystal rotation method.

(Preparation of Liquid Crystal Composition) The following liquid crystalline compounds were mixed in the following formulation to prepare a liquid crystal composition LC-1.

[0106]

[Chemical 2] The parameters of such a liquid crystal composition are shown below.

Phase transition temperature (° C.) Spontaneous polarization (30 ° C.): Ps = 0.57 nC / cm ² Tilt angle (30 ° C.): Θ = 22.2 ° Δn (30 ° C.): 0.157 SmC ^* Helical pitch in phase (30 ° C.): 20 μm Liquid crystal composition LC-1 was added to the empty cell prepared by the above process.
Was injected at a temperature of an isotropic phase, and the liquid crystal was gradually cooled to a temperature showing a chiral smectic liquid crystal phase to prepare a liquid crystal element sample A.

As a result of observing the initial alignment state of the liquid crystal of this sample A with a polarizing microscope, it was found that it was at room temperature (30 ° C.).
Was in a bistable state in which two domains were mixed.

(Electric field application treatment) In the sample A, a rectangular wave of ± 50 V and 10 Hz was applied to the liquid crystal at room temperature (30 ° C.) to apply an electric field, and then the alignment state was observed under a polarizing microscope. However, it was observed that the bistable state in which two domains were mixed, which was observed before the electric field application process, was changed to a uniform alignment state having the darkest axis in the rubbing direction.

Sample A after the electric field application treatment was placed in a polarization microscope with a photomultiplier so that the polarization axis was aligned with the rubbing direction to form a dark field, and the voltage value was 0.
Observing the optical response (transmittance) when a rectangular wave of ± 15 V and 0.1 Hz is applied, switching is performed without a domain, and there is no V-shaped hysteresis around 0 V as shown in FIG. Curve voltage-transmittance curve V-T
I found out to draw.

The saturation voltage of the voltage transmittance characteristic of this cell is
Both the positive side and the negative side were about 10.0V. Further, when the application of the electric field to the sample A was stopped, the darkest axis returned to the same uniform state as the rubbing direction, and it was found that the most stable position of the molecule in this liquid crystal element was in the same direction as the rubbing direction.

Since the sample A exhibits a continuous optical response to the applied voltage as shown in FIG. 6, analog gray scale display can be performed by amplitude modulation by driving the TFT active matrix.

(Measurement of Retardation) The retardation of Sample A was measured. The measurement was performed on two values, that is, a value when no voltage was applied and a value when a rectangular wave alternating current of 30 Hz and ± 10 V was being applied. In addition, the measurement method uses a Berek compensator for the value when no voltage is applied,
For the value during voltage application, the method for measuring the average retardation during driving described in this specification was used.

As a result, the value of retardation when no voltage was applied was 265 [nm], whereas the value of retardation when voltage was applied was 230 [nm].

Comparative Example Sample A except that silica beads having an average particle size of 1.4 μm were used as the spacer.
Sample B was manufactured by the same process as above, and the same electric field application process was performed.

(Measurement of Retardation) The retardation of Sample B was measured. In addition, the measurement was performed on two values, a value when no voltage was applied and a value when a rectangular wave alternating current of 30 Hz and ± 10 V was being applied. In addition, the measurement method used was a Berek compensator for the value when no voltage was applied, and the value during voltage application was the method for measuring the average retardation amount during driving described in this specification.

As a result, the retardation value when no voltage was applied was 220 [nm], whereas the retardation value when voltage was applied was 195 [nm].

Next, the transmission type liquid crystal device using Sample A and Sample B was subjected to relative evaluation with respect to brightness and tint. The results are shown in the table below.

[0119]

[Table 1] From this table, the optimum retardation value (200 nm to 24
It is understood that a desired amount of transmitted light can be obtained by setting the cell thickness d such that the retardation value when no voltage is applied is larger than that of 0 nm).

In the above description, a transmissive liquid crystal device using a transmissive liquid crystal element has been described, but the present invention is not limited to this, and a reflective liquid crystal device having the above-described reflector is provided. It goes without saying that it can also be applied to.
In the case of such a reflection type liquid crystal device, the cell thickness d is set so that the value of the retardation amount when no voltage is applied is 120 [nm] or more.

Furthermore, in the above description, the case where the cell thickness d is set so that the retardation value when no voltage is applied is larger than the optimum retardation value (200 nm to 240 nm) has been described. The invention is not limited to this, and the retardation value may be changed by preparing a liquid crystal material.

[0122]

As described above, according to the present invention, the value of the refractive index anisotropy of the liquid crystal and the liquid crystal layer are controlled so that the retardation amount when no voltage is applied is larger than the optimum retardation amount of the liquid crystal. By setting at least one of the thicknesses, a high transmittance state can be obtained even when a liquid crystal showing a chiral smectic phase is used.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the structure of a liquid crystal device including a liquid crystal element according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing the configuration of one substrate of the liquid crystal element.

FIG. 3 is a diagram showing a cross-sectional structure of a pixel portion of the one substrate.

FIG. 4 is an equivalent circuit diagram of the pixel portion.

FIG. 5 is a diagram showing drive signals and optical characteristics applied to the pixel portion.

FIG. 6 is a diagram showing a voltage-transmittance curve of the liquid crystal element.

[Explanation of symbols]

20 Active matrix substrate 40 Counter substrate 43a, 43b Alignment film 49 Liquid crystal layer 80 Liquid crystal element 81a, 81b substrate 85 LCD 87a, 87b Polarizing plate 82a, 82b electrodes 84a, 84b Orientation control film 94 TFT 95 pixel electrode

─────────────────────────────────────────────────── --- Continuation of the front page (56) Reference JP-A-3-242624 (JP, A) JP-A-10-307292 (JP, A) JP-A-5-107541 (JP, A) JP-A-11- 311812 (JP, A) JP 5-203919 (JP, A) JP 4-212126 (JP, A) JP 7-334130 (JP, A) JP 8-95091 (JP, A) Patent 3403114 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/13-1/141

Claims (11)

(57) [Claims] 1. A liquid crystal exhibiting a chiral smectic phase is sandwiched, electrodes having a voltage applied to the liquid crystal, and a uniaxial alignment treatment for aligning the liquid crystal on at least one of the opposing surfaces of the substrates. A liquid crystal element comprising a pair of substrates and a polarizing plate provided on at least one of the substrates, wherein the liquid crystal is a first-stabilized average molecular axis of the liquid crystal when no voltage is applied. When a voltage of the first polarity is applied, the average molecular axis of the liquid crystal tilts from the mono-stabilized position to one side at an angle according to the magnitude of the applied voltage, When a voltage having a second polarity opposite to that of the first liquid crystal is applied, the average molecular axis of the liquid crystal tilts from the mono-stabilized position to the opposite side to that when a voltage of the first polarity is applied, and When a voltage of the second polarity is applied and the voltage of the second polarity is applied The tilt angles of the maximum tilt state with respect to the mono-stabilized position in the first state of the average molecular axis of the liquid crystal at the same time are equal to each other. At least the value of the refractive index anisotropy of the liquid crystal and the thickness of the liquid crystal layer so that the retardation amount when no voltage is applied is larger than the optimum retardation amount of the liquid crystal that is set according to the wavelength of light to be made. A liquid crystal device characterized by setting one of them. 2. The liquid crystal element according to claim 1, wherein the substrate has an active element and is provided with a drive circuit for performing active matrix drive. 3. The liquid crystal is characterized in that a tilt angle δ of a smectic layer having a chevron structure with respect to a substrate normal is substantially equal to or larger than a tilt angle Θ of a chiral smectic C liquid crystal at a working temperature. The liquid crystal element according to claim 1. 4. The liquid crystal is Iso-Ch-SmA-Sm.
It has a phase sequence of C ^* and has Ch or SmA or S
The liquid crystal element according to claim 1 or 3, wherein the electric field application process is performed with mC ^* . 5. The liquid crystal is Iso-Ch-SmA-Sm.
The liquid crystal element according to claim 1 or 3, which has a phase sequence of C ^{* and} is subjected to a pressure treatment with SmA or SmC ^* . 6. The liquid crystal device according to claim 1, wherein the liquid crystal has a phase sequence of Iso-Ch-SmC ^* , and the electric field application process is performed with Ch or SmC ^* . 7. The liquid crystal has a tilt angle δ of the smectic layer at least at a use temperature larger than a size calculated from a temperature change of the layer spacing in the bulk state of the chiral smectic C liquid crystal. The liquid crystal element according to claim 1 or 3. 8. The liquid crystal is initially formed when the layer inclination angle δ of the chevron structure in the element, which is changed by an electric field application process and / or a pressure process, is cooled from the high temperature side showing an isotropic liquid phase. The liquid crystal device according to claim 1, wherein the liquid crystal device has a chevron structure having a layer inclination angle larger than δ ₀ . 9. The liquid crystal device according to claim 1, wherein the helical pitch of the liquid crystal in the bulk state is longer than twice the cell thickness. 10. A transmissive liquid crystal device having a liquid crystal element, a drive circuit for driving the liquid crystal element, and a backlight light source, wherein the liquid crystal element is according to any one of claims 1 to 9. A liquid crystal device characterized by being a thing. 11. A reflection type liquid crystal device having a liquid crystal element, a drive circuit for driving the liquid crystal element, and a reflector, wherein the liquid crystal element is according to any one of claims 1 to 9. Is a liquid crystal device.