JP2001281623A - Liquid crystal device and method for driving liquid crystal element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve moving image quality such as color breakup and to make image quality uniform in a display screen. SOLUTION: When an active matrix type liquid crystal display element is driven field sequentially, a signal (for example, a black display signal) which hardly contributes to the signal for displaying the color and its display is transferred in each color field within a period shorter than a half of each field period, and also the light source of the color is kept to be turned on almost for the field period.

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 device including a display device using the same.

[0002]

2. Description of the Related Art Conventionally, a TFT (thin film transistor; Thin Fi
As a typical liquid crystal mode of a nematic liquid crystal display element widely used as a display element using an active element such as an lm transistor, for example, M. Schat (M.
dt) and Appl by W. Helfrich
Twisted Nemati (TN (Twisted Nemati) shown on pages 127 to 128 of ied Physics Letters Vol. 18, No. 4 (published February 15, 1971).
c)) Modes are widely used. On the other hand, recently, in-plane switching (IPS
(In-Plain Switching)) Mode and vertical alignment (VA (Ve
A liquid crystal display using the (rtical alignment)) mode has been announced, and the viewing angle characteristic, which was 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. Slow, further improvement in response speed is required.

In order to improve the response speed of such a conventional nematic liquid crystal device, recently, several liquid crystal modes using a liquid crystal exhibiting a chiral smectic phase have been proposed. For example, "short-pitch type ferroelectric liquid crystal", "polymer stable ferroelectric liquid crystal", "thresholdless antiferroelectric liquid crystal" and the like have been proposed, but although not yet commercialized, It is also reported that high-speed response of sub-millisecond or less can be realized.

[0004] On the other hand, we have disclosed in Japanese Patent Application No. 10-177145.
(Hereinafter referred to as "prior application 1"). In the device of the prior application 1, for example, the isotropic liquid phase (ISO.)-Cholesteric phase (C
h) Focusing on a phase-series material showing a chiral smectic C phase (SmC ^* ) or an isotropic liquid phase (ISO.)-chiral smectic C phase (SmC ^* ), a position inside the edge of the virtual cone is focused on. To make it monostable. The example, when the Ch-SmC ^* phase transition, or to apply either positive or negative DC voltage between isotropic phase-SmC ^* phase transition pair of substrates in such, to equalize the layer in one direction, such as by Accordingly, a high-speed response and gradation control can be performed, and a high-brightness liquid crystal element excellent in moving image quality can be realized with high mass productivity. The element of the prior application 1 can be reduced in spontaneous polarization value as compared with the above-described various smectic liquid crystal modes, so that it is an element having good matching with an active element such as a TFT.

[0005] Similarly, we have disclosed in JP-A-2000-01007.
6 (hereinafter referred to as “prior application 2”). In the device of the prior application 2, for example, the isotropic liquid phase (ISO.)-Cholesteric phase (C
h) Focusing on a phase-series material showing a chiral smectic C phase (SmC ^* ) or an isotropic liquid phase (ISO.)-chiral smectic C phase (SmC ^* ), monostable at the position of a virtual cone edge I am trying to make it. And, for example, at the time of Ch-SmC ^* phase transition or isotropic phase-
SmC ^{* During} phase transition, either positive or negative D
The layer direction is made uniform in one direction by applying a C voltage, etc., whereby high-speed response and gradation control are possible, and a high-brightness liquid crystal element with excellent moving image quality can be realized with high mass productivity. . Also, the device of the prior application 2 can realize stable halftone display with small hysteresis and can reduce the spontaneous polarization value as compared with the other various smectic liquid crystal modes described above. Is a good element.

Another important technology as an environment surrounding a liquid crystal display element is a trend toward higher resolution. In a conventional liquid crystal display device, when a color image is displayed, a color filter (for example, RGB) is generally disposed for each pixel, a white light source is constantly turned on, and spatial color synthesis is generally performed. However, according to this method, it is necessary to divide one picture element into three, so that a high definition of the pixel pitch is required. Further, in principle, the color filter layer can use only 1/3 of the light amount of the white light source (substantially 1/3).
4), the transmittance does not increase,
Also, the cost for forming the color filter layer is high. In order to solve this problem, in recent years, an attempt has been made to switch between the three color light sources of RGB at a high speed and turn on the light without using a color filter to perform white color display (this is called a field sequential (FSC) driving method). ). In this method, a white color can be displayed with one pixel without using a color filter. Therefore, as compared with the color filter method, the number of dots is one-third to obtain the same resolution.
The transmittance of a color filter is, in principle, one-third or less, whereas in this method, the transmittance can be set to one. However, in the FSC driving method, since it is necessary to display three different color signals of RGB during one frame, the liquid crystal needs to respond at high speed.

FIG. 8 shows a conventional FSC drive timing. Here, it is assumed that a matrix of M rows and N columns is driven. As described above, in the conventional FSC drive,
One frame is divided into three color fields. In each color field, first, a transfer period of a video signal to each pixel is provided, and then a timing is set to turn on the light source of each color. Between the video signal transfer period and the light source lighting period,
It is common to set a certain period of time. This is related to the response speed of the liquid crystal. This will be described with reference to FIG.

After the video signal is transferred to the pixel at the address (1, 1) in the pixel matrix, the video signal is sequentially transferred to each pixel, and finally the signal is transferred to the pixel at the address (M, N). It takes several msec during this time. When the video signal of the pixel is transferred, the liquid crystal responds according to the signal level, and changes to a desired luminance level. However, at the address (1, 1) pixel (response curve 904) and the address (M, N) pixel (response curve 905), a time lag 901 occurs in the response start time of the liquid crystal, and the last pixel reaches a desired luminance level. And then turn on the light source. The conventionally used liquid crystal is a liquid crystal based on the above-described TN liquid crystal, and it is known that a response speed of 2 msec or more at room temperature is required even if the response speed is high. Therefore, the time 903 during which the light source is actually turned on and contributes to the display is displayed.
Is even shorter than one field period and less than or equal to sub-millisecond.

In the drawings, both the first address bits (pixels) and the last address bits (pixels) are written as if they reached the desired luminance level. This is because the response speed of the liquid crystal is sufficiently fast. Case, and in fact,
The brightness level may be slightly different between the first bit and the last bit. For example, when the environmental temperature changes to a lower temperature side, the response speed of the liquid crystal becomes slower, the reached luminance level differs between the first bit and the last bit, and color unevenness and luminance unevenness occur on the screen.

When a moving image is displayed on the liquid crystal display element driven at the timing as described above, color bleeding (color break) occurs before and after the moving direction of the outline of the image, and the image quality is significantly deteriorated. . This phenomenon is also caused by moving the eyes while displaying a still image.

This is because, as also shown in the timing chart of FIG. 8, the lighting timing (display timing of each color) of each color of RGB is discrete in time, and the lighting (display) period is one frame (display time). Field) due to the extremely small number of periods. That is, for example, when a white image moves on a black background (the eyes move in a still image), all the RGB colors must be colored (displayed) at all times. In terms of time alone, one of RGB (or 2
This occurs because a situation can occur in which only one light source is lit (colored). This phenomenon becomes remarkable at the edge in the traveling direction or the trailing edge of the image, and as a result, color breakup occurs.

[0012]

As described above,
In a conventional liquid crystal element, it is difficult to guarantee image quality, particularly moving image quality, in an FSC driving method for realizing high resolution.

The present invention has been made in view of the above-mentioned problems in the conventional example, and has solved the problem of color breakup in a liquid crystal device using a field sequential driving method.
An object is to obtain uniform image quality.

[0014]

The above object of the present invention is attained by the following invention. That is, in the liquid crystal device of the present invention, one frame for displaying an image is composed of a plurality of color fields, each of which is shorter than 1/2 of the color field period. And a transfer period of a signal (for example, a black display signal) that hardly contributes to display, and a light source of a color corresponding to the color field is turned on during most of the color field. The white color display is performed by sequentially performing the color fields.

Furthermore, the liquid crystal element preferably used in the present invention comprises a chiral smectic liquid crystal, an electrode for applying a voltage to the liquid crystal, and a uniaxial liquid crystal for sandwiching and opposing the liquid crystal and aligning the liquid crystal. A liquid crystal device comprising a pair of substrates subjected to an alignment treatment and a polarizing plate on at least one of the substrates, wherein a phase transition series of the chiral smectic liquid crystal is an isotropic liquid phase (ISO. )
-Cholesteric phase (Ch) -Chiral smectic C
Phase, or isotropic liquid phase (ISO.)-Chiral smectic C phase, wherein at least one of the substrates used for the liquid crystal element has an active element connected to an electrode corresponding to each pixel. Is a chiral smectic liquid crystal element.

In addition, when the liquid crystal element does not apply a voltage,
The liquid crystal shows the first state in which the average molecular axis is monostable,
When a voltage of the first polarity is applied, the average molecular axis of the liquid crystal tilts to one side from the mono-stabilized position at an angle corresponding to the magnitude of the applied voltage, and has a polarity opposite to the first polarity. When a voltage of the second polarity is applied, the average molecular axis of the liquid crystal is a liquid crystal element that tilts from the monostable position to the opposite side to that when a voltage of the first polarity is applied. The tilt angles of the maximum tilt state with respect to the monostable position in the first state of the average molecular axis of the liquid crystal when the voltage of the polarity is applied and the voltage of the second polarity are applied are β1 and β, respectively.
It is preferable that the liquid crystal element satisfy β1> β2 when 2. It is more preferable that the relationship between β1 and β2 is β1 ≧ 5 × β2, and it is further preferable that β2 is substantially zero.

[0017]

According to a preferred embodiment of the present invention, an active matrix type liquid crystal display device using a chiral smectic liquid crystal is field-sequentially driven, in which case, for each color field, in a period shorter than 1/2 of the field period. The signal source and the light source of the corresponding color
By illuminating substantially in the field period, uniform image quality without color breakup can be obtained as compared with the related art.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION As a preferred embodiment of the liquid crystal element according to the present invention, a liquid crystal, an electrode for applying a voltage to the liquid crystal, and the liquid crystal sandwiched and opposed to at least one of the opposing surfaces. Equipped with a pair of substrates that have been subjected to a uniaxial alignment treatment for alignment, and a polarizing plate on at least one of the substrates, images in a plurality of frames are displayed in one second, and the alignment state of the liquid crystal in each frame changes with time. The present invention provides a liquid crystal element which changes dynamically.

Further, according to the present invention, there is provided a liquid crystal device using a chiral smectic liquid crystal such that the liquid crystal exhibits a monostable state when no voltage is applied as described above, as a device manufactured by the above-described device manufacturing method. You.

The liquid crystal element preferably applicable to the present invention is disclosed in Japanese Patent Application No. Hei 10-177145 or Japanese Patent Application Laid-Open No. 2000-2000.
01076, wherein the liquid crystal material has a phase transition series of isotropic liquid phase (ISO.)-Cholesteric phase (Ch) -chiral smectic C phase (SmC ^* ) or isotropic liquid phase (ISO. )-Indicates a chiral smectic C phase (SmC ^* ), and a positive or negative DC voltage is applied between a pair of substrates at the time of transition to the SmC ^* phase, whereby one of two layer directions is applied. In other words, the deviation direction between the average uniaxial alignment processing axis and the normal direction of the smectic layer is kept constant, and it is stabilized on or inside the virtual cone edge of the liquid crystal molecule with no voltage applied, and its memory performance Is obtained, and the orientation state of the SmC ^* phase is obtained.

Further, the liquid crystal used in the present invention is a chiral smectic liquid crystal, and the phase transition series is from an isotropic liquid phase (ISO.) To a cholesteric phase (C
h) -Chiral smectic C phase or isotropic liquid phase (ISO.)-chiral smectic C phase is preferred. Hereinafter, specific examples of preferred compounds constituting the liquid crystal composition used in the present invention are shown in (Chemical Formulas 1 to 4).

[0022]

Embedded image R1, R2: a linear or branched alkyl group having 1 to 20 carbon atoms and optionally having a substituent X1, X2: a single bond, O, COO or OOC Y1, Y2, Y3, Y4: H or F n: 0 or 1

[0023]

Embedded image R1, R2: a linear or branched alkyl group having 1 to 20 carbon atoms and optionally having a substituent X1, X2: a single bond, O, COO or OOC Y1, Y2, Y3, Y4: H or F

[0024]

Embedded image R1, R2: a linear or branched alkyl group having 1 to 20 carbon atoms and optionally having a substituent X1, X2: a single bond, O, COO or OOC Y1, Y2, Y3, Y4: H or F

[0025]

Embedded image R1, R2: a linear or branched alkyl group having 1 to 20 carbon atoms and optionally having a substituent X1, X2: a single bond, O, COO or OOC Y1, Y2, Y3, Y4: H or F

Hereinafter, a specific embodiment of the liquid crystal device of the present invention will be described with reference to FIG. In a liquid crystal element 80 shown in the figure, cells having a liquid crystal 85, preferably a liquid crystal exhibiting a chiral smectic phase, sandwiched between a pair of substrates 81a and 81b made of a highly transparent material such as glass and plastic have mutually orthogonal polarization axes. It has a structure sandwiched between a pair of polarizing plates 87a and 87b.

The substrates 81a and 81b are provided with electrodes 82a and 82b made of a material such as In ₂ O ₃ or ITO for applying a voltage to the liquid crystal 85, respectively. Dot-shaped transparent electrodes are arranged in a matrix, and a TFT or MIM (Metal-
Switching elements such as Insulator-Metal) are connected, and an opposing electrode having a predetermined pattern is formed on one surface of the other substrate to form an active matrix structure.

The electrodes 82a, On 82b, SiO ₂ having a function such as to prevent these short as necessary,
An insulating film 83a made of a material such as TiO ₂ , Ta ₂ O ₅ ,
83b are provided respectively.

Further, on the insulating films 83a and 83b, alignment control films 84a and 84b which are in contact with the liquid crystal 85 and function to control the alignment state are provided. At least one of the alignment control films 84a and 84b is subjected to a uniaxial alignment treatment. As such a film, for example, a film obtained by applying a rubbing treatment to the surface of a film obtained by applying a solution of an organic material such as polyimide, polyimide amide, polyamide, or polyvinyl alcohol, or an oxide such as SiO or a nitride obliquely to the substrate is used. An oblique deposition film of an inorganic material deposited at a predetermined angle from the direction can be used.

The orientation control films 84a and 84b may have different pretilt angles of the molecules of the liquid crystal 85 (the film surfaces near the liquid crystal molecule alignment control film interface) depending on the selection of the material and the conditions of the treatment (uniaxial orientation treatment and the like). Is adjusted.

When each of the alignment control films 84a and 84b is a film that has been subjected to a uniaxial alignment treatment, the direction of the uniaxial alignment treatment (particularly, the rubbing direction) of each film is antiparallel or parallel depending on the liquid crystal material used. , Or cross rubbing, or rubbing on one side only.

The substrates 81a and 81b are
Are opposed to each other. The spacer 86 is provided on the substrate 8.
The distance (cell gap) between 1a and 81b is determined, and silica beads or the like are used. Regarding the cell gap determined here, the optimum range and the upper limit are different depending on the difference of the liquid crystal material, but the uniform uniaxial orientation,
In order to develop an alignment state in which the average molecular axis of the liquid crystal molecules is substantially the same as the average direction axis of the alignment processing axis when no voltage is applied, it is preferable to set the average molecular axis in the range of 0.3 to 10 μm.

In addition to the spacers 86, adhesive particles made of a resin material such as an epoxy resin are dispersed to improve the adhesiveness between the substrates 81a and 81b and improve the impact resistance of the liquid crystal exhibiting a chiral smectic phase. (Not shown).

In the liquid crystal element 80 having the above structure, when a liquid crystal exhibiting a chiral smectic phase is used as the liquid crystal 85, the composition of the material is adjusted, and furthermore, the processing of the liquid crystal material and the element configuration, for example, the alignment control films 84a and 84b are performed.
By appropriately setting the material, processing conditions, and the like, when no voltage is applied, the liquid crystal exhibits an alignment state in which the average molecular axis (liquid crystal molecules) of the liquid crystal is monostable. Of the average molecular axis, the tilt angle changes continuously according to the magnitude of the applied voltage, and the voltage of the other polarity (second polarity) is applied. Sometimes, the average molecular axis of the liquid crystal exhibits characteristics such that it tilts at an angle corresponding to the magnitude of the applied voltage.
At this time, the characteristic may be such that the maximum tilt angle by applying the voltage of the first polarity is larger than the maximum tilt angle by applying the voltage of the second polarity. Alternatively, the characteristics may be such that the average molecular axis does not tilt regardless of the magnitude of the applied voltage when the voltage of the second polarity is applied.

The liquid crystal material 85 exhibiting a chiral smectic phase has the above-described properties (properties relating to the physical property cone angle 固有 inherent to the liquid crystal material, the layer spacing d of the smectic layer, and the inclination angle δ). A composition prepared by appropriately selecting a hydrocarbon-based liquid crystal material, a naphthalene-based liquid crystal material, or a polyfluorine-based liquid crystal material having a biphenyl skeleton, a phenylcyclohexane ester skeleton, a phenylpyrimidine skeleton, or the like is used.

In the liquid crystal device, the substrates 81a and 81
A color liquid crystal element may be provided by providing at least one of R, G, and B color filters on one of b. In addition, a method of performing full-color display using time-division color mixture by sequentially switching the R, G, and B light sources as the light source can also be used.

Note that the liquid crystal element is connected to the substrates 81a and 81
1b, a transmission type liquid crystal element in which a pair of polarizing plates is provided on both substrates, ie, both substrates 81a and 81b are translucent substrates, and incident light from one substrate side (for example, light from an external light source) Or a reflection type liquid crystal element in which a polarizing plate is provided on at least one substrate, that is, a reflection plate is provided on one of the substrates 81a and 81b, or one substrate is provided. The present invention can be applied to any type of element that modulates incident light and reflected light by using a reflective material as a member provided on itself or a substrate and emits light to the same side as the incident side.

In the present embodiment, a drive circuit for supplying a gradation signal to the above-mentioned liquid crystal element is provided, and a continuous tilt angle from a monostable position of the average molecular axis of the liquid crystal by applying the above-mentioned voltage is provided. , And a characteristic in which the amount of light emitted from the element changes continuously, a liquid crystal display element that performs gradation display can be configured. For example, by using an active matrix substrate provided with the above-described TFT or the like as one substrate of a liquid crystal element and performing active matrix driving by amplitude modulation with a driving circuit, analog gray scale display can be performed.

With reference to FIGS. 2 to 4, an example in which such an active matrix substrate is used in the liquid crystal element of the present embodiment will be described. FIG. 2 schematically shows the element with a driving unit, mainly focusing on the configuration of one substrate (active matrix substrate).

In the configuration shown in FIG. 2, in a panel section 90 corresponding to a liquid crystal element, horizontal gate lines G1, G2,... In the drawing corresponding to scanning lines connected to a scanning signal driver 91 as driving means, The source lines S1, S2,... In the vertical direction in the drawing corresponding to the information signal lines connected to the information signal driver 92 as the driving means are provided so as to be orthogonal to each other while being insulated from each other. Corresponding to the switching element (T
FT) 94 and a pixel electrode 95 (only a 5 × 5 pixel area is shown in the figure for simplicity). still,
As the switching element, an MIM element can be used in addition to the TFT. The gate lines G1, G2,...
4 is connected to the gate electrode (not shown)
Are connected to a source electrode (not shown) of the TFT 94, and the pixel electrode 95 is connected to a drain electrode (not shown) of the TFT 94. In such a configuration,
The gate lines G1, G2,... Are, for example, line-sequentially selected for scanning by the scanning signal driver 91, and a gate voltage is supplied.
The information signal driver 9 is synchronized with the gate line scanning selection.
2 are supplied to the source lines S1, S2,... According to the information to be written to each pixel, and are applied to each pixel electrode via the TFT 94.

FIG. 3 shows an example of a sectional structure of each pixel portion (for one bit) in the panel configuration shown in FIG. In the structure shown in FIG.
Matrix substrate 20 provided with the common electrode 4 and the common electrode 4
A liquid crystal layer 49 having spontaneous polarization is sandwiched between the liquid crystal layer 49 and a counter substrate 40 provided with a liquid crystal capacitor 2, thereby forming a liquid crystal capacitor (C _LC ) 31.

As for the active matrix substrate 20, an example is shown in which a bottom gate type TFT using amorphous Si (a-Si) is used as the TFT 94. In addition, a TFT having a similar structure using poly-Si or a top gate type TFT can be used. The TFT 94 is formed on a substrate 21 made of glass or the like, and an insulating film (gate insulating film) made of a material such as silicon nitride (SiN _x ) is formed on the gate electrode 22 connected to the gate lines G1, G2,. 23 through the a-Si layer 2
4 is provided, on the a-Si layer 24, respectively n ⁺
The source electrode 27 and the drain electrode 28 are provided separately from each other via the a-Si layers 25 and 26. The source electrode 27 is connected to the source lines S1, S2,.
The drain electrode 28 is connected to a pixel electrode 95 made of a transparent conductive film such as an ITO film. The channel protection film 29 covers the a-Si layer 24 of the TFT 94. The TFT 94 is turned on by applying a gate pulse to the gate electrode 22 during a period in which the corresponding gate line is selected for scanning.

Further, in the active matrix substrate 20, the insulating film 23 (continuously formed 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 side of the electrode. A storage capacitor (Cs) 32 is provided in parallel with the liquid crystal layer 50 by a structure sandwiching the film. When the area of the storage capacitor electrode is large, the aperture ratio is reduced, and thus the storage capacitor electrode is formed of a transparent conductive film such as an ITO film.

The TFT 9 of the active matrix substrate 20
On the pixel electrode 95 and the pixel electrode 95, there is provided an alignment film 43a that has been subjected to a uniaxial alignment process such as a rubbing process for controlling the alignment state of the liquid crystal. On the other hand, in the counter substrate 40, the common electrode 4 is formed on the glass substrate 41 with the same thickness as the entire surface.
2. Alignment film 43 for controlling alignment state of liquid crystal
b is laminated. The cell structure is sandwiched between a pair of polarizing plates whose polarization axes are orthogonal to each other (not shown).

In the pixel portion of the panel having the above structure, a liquid crystal having spontaneous polarization, for example, a liquid crystal exhibiting a chiral smectic phase is used as the liquid crystal layer 49. In the panel configuration shown in FIGS. 2 and 3, polycrystalline Si (p-Si) TF was used as an active matrix substrate.
A substrate with T can be used.

FIG. 4 shows an equivalent circuit of a pixel portion of the panel shown in FIG. The active matrix driving utilizing the characteristics of the liquid crystal element having the above structure will be described with reference to FIGS. In the active matrix driving in the liquid crystal element of the present embodiment, for example, a period (one frame) for displaying certain information in one pixel is divided into a plurality of fields (for example, 1F and 2F shown in FIG. 5), and in these two fields, The amount of emitted light according to predetermined information is obtained on average. Hereinafter, an example in which the liquid crystal layer 49 is divided into two fields in a case where the transmission light intensity is sufficient when a voltage of one polarity is applied and the transmission light intensity is smaller when the polarity is reversed, will be described.

FIG. 5A shows a voltage applied to one gate line serving as a scanning line connected to the pixel when focusing on one pixel. In the liquid crystal element having the above structure, the gate lines G1, G2,...
A predetermined gate voltage Vg is applied to one gate line during the selection period Ton, and a voltage Vg is applied to the gate electrode 22 to cause Tg.
FT94 is turned on. In a non-selection period Toff corresponding to a period in which another gate line is selected, the gate electrode 2
2, no voltage is applied to the TFT 2, and the TFT 94 is in a high resistance state (off state), a predetermined same gate line is selected for each Toff, and a 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. As shown in FIG. 5A, the gate electrode 22 in the selection period Ton in each field.
When a gate voltage is applied to the source electrode 27, a predetermined source voltage (information signal voltage) V is applied from the source lines S1, S2,.
s (the reference potential is the potential Vc of the common electrode 42) is applied.

Here, in the first field (1F) constituting one frame, optical information to be obtained by the pixel based on information written in the pixel, for example, a voltage-transmittance characteristic corresponding to the liquid crystal used. Source voltage of positive polarity (information signal voltage) of level Vx according to state or display information (transmittance)
(The reference potential is the potential Vc of the common electrode 42). At this time, since the TFT 94 is in the ON state, the voltage Vx applied to the source electrode 27
8 to the pixel electrode (95), and the liquid crystal capacitance (C
_LC ) 31 and the storage capacity (Cs) 32 are charged,
The potential of the pixel electrode becomes the information signal voltage Vx. Subsequently, during the non-selection period Toff of the gate line to which the pixel belongs, T
Since the FT 94 has a high resistance (off state), during this non-selection period, the liquid crystal cell (liquid crystal capacitor C _LC ) 31 and the storage capacitor (Cs) 32 store the charge accumulated in the selection period Ton during the non-selection period. The voltage Vx is maintained. Then, the first field 1F is added to the liquid crystal layer 49 in the pixel.
The voltage Vx is applied throughout the period, and an optical state (amount of transmitted light) corresponding to this voltage value is obtained in the liquid crystal portion of the pixel. At this time, if the response speed of the liquid crystal is slower than the gate-on period, the charging of the liquid crystal cell (liquid crystal capacitor C _LC ) 31 and the storage capacitor (Cs) 32 is completed, and switching starts in the non-selection period in which the gate is turned off. In such a case, the charged charge is offset by the reversal of spontaneous polarization,
The voltage applied to the liquid crystal layer takes a value of Vx ', which is smaller by Vd than Vx, as shown in FIG.

Next, in the selection period Ton of the second field (2F), the source voltage (-Vx) having the polarity opposite to that of the first field 1F and having substantially the same voltage value Vx is applied to the source electrode 27. Is applied to At this time, the TFT 14 is in the ON state, the voltage −Vx is applied to the pixel electrode 95, and the liquid crystal capacitance (C _LC ) 31 and the storage capacitance (Cs) 32
And the potential of the pixel electrode is changed to the information signal voltage −Vx.
become. Subsequently, in the non-selection period Toff, the TFT 94
Becomes a high resistance (off state), and during this non-selection period, the liquid crystal cell (liquid crystal capacitor C _LC ) 31 and the storage capacitor (C
In s) 32, the state in which the charge charged during the selection period Ton is accumulated is maintained, and the voltage -Vx is held. Then, the voltage −Vx is applied to the liquid crystal layer 49 in the pixel throughout the second field 2F, and the pixel obtains an optical state (light emission amount) corresponding to the voltage value. At this time, similarly, when the response speed of the liquid crystal is lower than the gate-on period, the liquid crystal cell (liquid crystal capacitor C _LC ) 31 and the storage capacitor (C
s) The charging is completed in 32, and the switching is started in the non-selection period in which the gate is turned off. In such a case, the charges charged by the reversal of the spontaneous polarization are cancelled, and the voltage applied to the liquid crystal layer becomes V from -Vx as shown in FIG.
It takes a value of -Vx 'which is d smaller.

FIG. 5C shows the voltage value Vpix actually held in the liquid crystal capacitance and the storage capacitor of the pixel as described above and applied to the liquid crystal layer 49, and FIG. FIG. 1 schematically shows the actual optical response (optical response in the case of a transmission type liquid crystal element). As shown in FIG.
The applied voltages through the fields (1F and 2F) are at the same level (absolute value) Vx ′ whose polarity is just inverted. On the other hand, as shown in FIG.
In F, a gradation display state (emission light amount) according to Vx 'is obtained, and in the second field 2F, a gradation display state according to -Vx' is obtained. Only a change in the amount of transmitted light is obtained, and the amount of transmitted light is smaller than Tx and becomes Ty close to the 0 level.

In the above-described active matrix driving, when liquid crystal exhibiting a chiral smectic phase is used, gradation display based on good high-speed response can be performed, and at the same time, gradation display at a certain level in one pixel can be performed. Since it is divided into a first field that obtains a high transmitted light amount and a second field that obtains a low transmitted light amount and is continuously performed, the temporal aperture ratio is 50% or less, and the high-speed moving image response characteristics perceived by human eyes are also improved. Further, in the second field, the transmitted light amount does not become completely zero due to a slight switching operation of the liquid crystal molecules, so that the luminance perceived by human eyes during the entire frame period is secured.

Further, since the voltage of the same level is inverted and applied to the liquid crystal layer 49 in the first and second fields, the voltage actually applied to the liquid crystal layer 49 is converted into an alternating current, thereby preventing the deterioration of the liquid crystal. Is done.

In the above-described active matrix driving, 2
In one entire frame composed of fields, a transmitted light amount obtained by averaging Tx and Ty is obtained. Therefore, as the information signal voltage Vs, a voltage value capable of obtaining a large amount of transmitted light by a predetermined level is selected according to image information (gradation information) to be actually obtained by the pixel in the frame. It is also preferable to display a gradation state with a higher level of transmitted light than the desired gradation state in the first field 1F.

[0055]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 6 shows the drive timing of the liquid crystal element according to the first embodiment of the present invention. A period for displaying one entire screen is defined as one frame, which is divided into three fields of an R field, a G field, and a B field. If one frame frequency is 60 Hz, each color field frequency is
180 Hz.

A description will be given using the timing in the R field. As described with reference to FIG. 5, the liquid crystal used in the embodiment of the present invention has either a positive or negative signal (V in FIG. 5).
(positive side across c) Output light quantity corresponding to Vpix is obtained,
In the case of a signal of the opposite polarity, a characteristic is obtained in which almost no emitted light amount is obtained (almost close to black display).

As shown in FIG. 6A, in one color field (for example, R field) period, a video signal transfer period 601 that contributes to video display is provided in the first half, and hardly contributes to video display in the second half. A signal transfer period 602 (almost close to black display) is provided. At this time, as shown by (2) in FIG.
3 is lit. However, the light-on timing is almost the same as the start of the video signal transfer period, and the light-off timing is when the optical response state of the liquid crystal of each pixel changes after the latter half of the signal transfer period 602 ends for all the pixels. It is carried out at substantially the same level and at a timing when the level does not contribute to the display.

FIG. 7 shows the optical response waveform of the liquid crystal at the first address (1,1) pixel and the last address (M, N) pixel. The signal transfer starts sequentially from the (1,1) pixel and continues until the last (M, N) pixel is transferred. The video signal contributing to the video display in the first half transfer period 701 is
Transferred sequentially. Also, the time required for the transfer is set shorter than half the time of one field. As described above, the chiral smectic liquid crystal used in the embodiment of the present invention has a very high optical response speed, and the transfer period 701
All the pixels can reach the desired luminance level. In the case of the active matrix drive as shown in FIG. 2, a so-called line sequential drive for sequentially selecting the gate lines G1, G2,... Is used. In this case, the pixels (1, 1) to (1, N) connected to the gate line G1 are simultaneously selected, and the pixels (M, 1) connected to the gate line GM are set to (M, N). Pixels up to the pixel are simultaneously selected.

In the transfer period 702 in the latter half, signals having the opposite polarity to the video signal transferred in the period 701 are also sequentially transferred. The transfer start is set after a lapse of a certain period of time from the end of the first half transfer period 701, in other words, after a lapse of exactly の of one field. The time required for the transfer is set to be the same as the transfer period 701 in the first half.

As a result of this signal transfer, the luminance levels of all the pixels are aligned to a level that hardly contributes to video display (almost black display). Also,
By providing the periods 606 and 706 between the first and second signal transfer periods, it is possible to always maintain a constant luminance level in response to a change in the optical response time of the liquid crystal due to a change in temperature or the like.

On the other hand, the light source turns on 703 for most of one field period. However, the lighting start time is turned on in accordance with the start timing of the video signal transfer periods 601 and 701 in the first half, and the turn-off timing is performed after a certain period of time has elapsed from the signal transfer end timing in the latter half.

By setting such timings,
A period contributing to image display (between the transfer periods 601 and 701 and the transfer periods 602 and 702 when viewed from each pixel)
However, it is possible to take a sufficiently long time as compared with the conventional art (as will be described later, in practice, a time of about 3 msec was able to be secured), and the lighting of the light sources which are discrete in time is considerably improved, and the image moves on the screen. No color breakup phenomenon was observed at that time,
Good moving image quality video was obtained.

By setting such timing, it is possible to almost eliminate the difference in luminance of the entire screen. That is, by setting both the first half of the video signal transfer period and the second half of the signal transfer period of a signal that hardly contributes to video display to a period shorter than １ ／ of one field period, the display luminance level and display Both of the luminance levels that do not contribute to the liquid crystal display provide a sufficient time for the liquid crystal to respond sufficiently even if the response speed of the liquid crystal changes due to a change in temperature or the like. Also, by setting the start time of the latter half of the signal transfer period after a lapse of exactly １ ／ of one field, and setting the lighting period of the light source as described above, the same duty ratio is always applied to the lighting period. Can be displayed.

As described with reference to FIG. 5, both signals are bipolar signals, and are set so that the DC component of both signals becomes substantially zero. By doing so, it is possible to drive the liquid crystal without applying and remaining a DC component, and the reliability of the element is improved.

As described above, the chiral smectic liquid crystal, which is a liquid crystal material used in the embodiment of the present invention, has an extremely high optical response speed as compared with a conventionally used TN liquid crystal, and is sufficiently desired within one signal transfer period. Brightness levels can be reached. When the operating temperature range is set to 0 ° C to 60 ° C, the time to respond from the black state to the white state is about 1 hour.
The response time from the white state to the black state is about 0.5 msec. Therefore, in this embodiment, the field frequency is set to 1 during each signal transfer period.
Assuming 80 Hz, each is set to 2 msec.

In this embodiment, a polysilicon TFT having a high driving capability is used to sequentially transfer signals to all the pixels in 2 msec. With the number of pixels of VGA (640 × 480), the dot frequency is about 150 MHz. Therefore, by dividing the screen and transferring signals in parallel, it is possible to transfer signals to all pixels even in a short transfer period of 2 msec. It is now possible.

(Preparation of Liquid Crystal Composition) A liquid crystal composition LC-1 was prepared by mixing the following liquid crystal compounds. The numerical values described in the structural formula are weight ratios at the time of mixing.

[0068]

Embedded image The physical parameters of the liquid crystal composition LC-1 are shown below.

[0069]

(Equation 1)

(Preparation of Liquid Crystal Cell) An active matrix substrate having an a-Si TFT having the above-described pixel structure as one substrate and a silicon nitride film as a gate insulating film, and a substrate having a transparent electrode as an opposing substrate A pair of substrates having a thickness of 1.1 mm was prepared. The screen size was 10.4 inches and the number of pixels was 800 × 600.
A commercially available alignment film SE79 for TFT is provided on the transparent electrode of the substrate.
9 (manufactured by Nissan Chemical Industries, Ltd.) was applied by spin coating, followed by pre-drying at 80 ° C. for 5 minutes, followed by heating and baking at 200 ° C. for 1 hour to obtain a polyimide film having a thickness of 500 °.

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

Subsequently, as a spacer on one of the substrates,
Silica beads having an average particle size of 1.5 μm were scattered, and the rubbing directions of the substrates were opposed to each other so as to be antiparallel to each other, thereby obtaining cells having a uniform cell gap (active matrix panel A).

The liquid crystal composition LC-1 was injected into the active matrix panel A manufactured by the above process at the temperature of the Ch phase, and the liquid crystal was cooled to a temperature showing a chiral smectic liquid crystal phase. ^* Before and after the phase transition, a cooling process was performed by applying an offset voltage (DC voltage) of +2 V or -2 V to produce a liquid crystal panel. The sample was evaluated for image quality, especially for moving image quality.

In the above-described embodiment, a polysilicon TFT is used as an active element. However, if the driving capability and the disclosed driving timing can be realized, for example, MI
An active matrix substrate using M elements may be used, or a simple matrix substrate may be used.

[0075]

As described in detail above, according to the present invention,
It is possible to provide a liquid crystal element in which moving image quality such as color breakup, which has been a problem in the related art, is improved and uniform image quality is obtained in a display surface.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a liquid crystal element according to one embodiment of the present invention.

FIG. 2 is a schematic diagram of an active matrix substrate used for the device of FIG.

FIG. 3 is a sectional structural view of the active matrix element in FIG. 2;

FIG. 4 is an equivalent circuit diagram of a pixel portion in FIG.

FIG. 5 is a diagram illustrating a basic driving method of the liquid crystal element shown in FIGS.

FIG. 6 is a diagram illustrating a method for driving a liquid crystal element according to one embodiment of the present invention.

FIG. 7 is a diagram illustrating a method for driving a liquid crystal element according to an embodiment of the present invention.

FIG. 8 is a diagram illustrating a problem of a conventional driving method.

FIG. 9 is a diagram illustrating a problem of a conventional driving method.

[Explanation of symbols]

701: transfer period of a signal for color display, 702: transfer period of a signal that hardly contributes to display, 703: lighting period of a light source, 704: response curve of a pixel to which a color display signal is transferred first, 705: last Response curve 706 of the pixel to which the color display signal is transferred, the difference between 差 of the color field and the transfer period of the color display signal.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G09G 3/20 642 G09G 3/20 642J 642C 3/36 3/36 F-term (Reference) 2H090 HB08Y HB13Y KA14 LA02 LA04 LA16 MA11 MB01 2H093 NA16 NA43 NC16 NC34 NC42 ND01 ND09 ND34 NE06 NF17 5C006 AA22 AC02 AC24 AC25 AF44 AF50 AF71 BA11 BB16 BC03 BC06 BC07 BC13 BF34 BF37 EA03 FA22 FA56 5C080 AA10 BB05 JJ02 EJ02 FF02

Claims (11)

[Claims] An active matrix type liquid crystal element;
A plurality of light sources, and one frame for displaying an image on the liquid crystal element is composed of a plurality of color fields, and the color field is less than half of the color field period. Transfer period and 1 of the color field period
The light source of the corresponding color is turned on almost simultaneously with the start of the transfer period of the signal for color display, and the transfer of the signal hardly contributes to the display is included. A liquid crystal device which performs white color display by turning off a light source of a corresponding color after a lapse of a predetermined time after the end of the period and sequentially performing the light sources in the plurality of color fields. 2. The transfer period of the color display signal starts at the same time as the start of the color field period, and the transfer period of the signal that hardly contributes to the display starts when half of the color field period elapses. The display device according to claim 1, wherein: 3. The display device according to claim 1, wherein the signal that hardly contributes to the display is a signal obtained by inverting the polarity of the color display signal. 4. A liquid crystal device comprising: a chiral smectic liquid crystal; an electrode for applying a voltage to the liquid crystal; and a pair having a uniaxial orientation treatment for sandwiching and opposing the liquid crystal and for aligning the liquid crystal. And a polarizing plate provided on at least one of the substrates, wherein the phase transition series of the chiral smectic liquid crystal is isotropic liquid phase (ISO.)-Cholesteric phase from the high temperature side. (C
The display device according to any one of claims 1 to 3, wherein h) is a chiral smectic C phase or an isotropic liquid phase (ISO.)-chiral smectic C phase. 5. The liquid crystal device according to claim 4, wherein the active matrix element of the liquid crystal element is constituted by a thin film transistor. 6. The liquid crystal element exhibits a first state in which the average molecular axis of the liquid crystal is monostable when no voltage is applied, and the average molecular axis of the liquid crystal is applied when a voltage of the first polarity is applied. Is tilted to one side from the mono-stabilized position at an angle corresponding to the magnitude of the applied voltage, and when a voltage having a second polarity opposite to the first polarity is applied, the average molecule of the liquid crystal is tilted. The axis tilts from the mono-stabilized position to the opposite side when a voltage of the first polarity is applied, and the liquid crystal of the liquid crystal when the voltage of the first polarity is applied and when the voltage of the second polarity is applied. The liquid crystal element satisfies β1> β2 when the tilt angles in the maximum tilt state with respect to the mono-stabilized position in the first state of the average molecular axis are β1 and β2, respectively. The liquid crystal device according to claim 4. 7. The liquid crystal device, wherein a monostabilized position in the first state of the average molecular axis of the liquid crystal when the voltage of the first polarity is applied and when the voltage of the second polarity is applied. The tilt angle of the maximum tilt state
7. The liquid crystal device according to claim 6, wherein, when 1, and β2, β1 ≧ 5 × β2. 8. The liquid crystal element shows a first state in which the average molecular axis of the liquid crystal is monostable when no voltage is applied, and the average molecular axis of the liquid crystal is applied when a voltage of the first polarity is applied. Is tilted to one side from the mono-stabilized position at an angle corresponding to the magnitude of the applied voltage, and when a voltage having a second polarity opposite to the first polarity is applied, the average molecule of the liquid crystal is tilted. The liquid crystal device according to claim 7, wherein the axis is a liquid crystal element that does not substantially change from the mono-stabilized position. 9. The liquid crystal device according to claim 6, wherein a helical pitch of the chiral smectic liquid crystal in a bulk state is longer than twice a cell thickness of the liquid crystal device. Liquid crystal devices. 10. A chiral smectic liquid crystal, an electrode for applying a voltage to the liquid crystal, and a pair of substrates that have been subjected to a uniaxial alignment treatment for sandwiching the liquid crystal and for aligning the liquid crystal. A method for driving an active matrix liquid crystal element including a polarizing plate on one substrate, wherein the phase transition series of the chiral smectic liquid crystal is
From the high temperature side, it is an isotropic liquid phase (ISO.)-Cholesteric phase (Ch) -chiral smectic C phase or an isotropic liquid phase (ISO.)-Chiral smectic C phase, and one frame for displaying an image. Are composed of a plurality of color fields, and the color fields substantially contribute to a transfer period of the signal for color display of less than half of the color field period and a display of less than half of the color field period. And a light source of a corresponding color is turned on almost simultaneously with the start of the transfer period of the signal for color display, and after a lapse of a fixed time after the end of the transfer period of the signal that hardly contributes to the display. A method for driving a liquid crystal element, wherein a white color display is performed by extinguishing a light source of a corresponding color and sequentially performing these in the plurality of color fields. 11. The method according to claim 10, wherein the liquid crystal element is driven by line-sequential driving.