JP2940287B2 - Antiferroelectric liquid crystal display device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an antiferroelectric liquid crystal display device.

[0002]

2. Description of the Related Art An anti-ferroelectric liquid crystal display device utilizes the stability of the molecular alignment state of an anti-ferroelectric liquid crystal. Are known. FIGS. 5 and 6 show a conventional antiferroelectric liquid crystal display device.
Is a sectional view of a part of the liquid crystal display element, and FIG. 6 is a plan view of one of the substrates.

The structure of this antiferroelectric liquid crystal display device will be described. In FIGS. 5 and 6, reference numerals 1 and 2 denote a pair of transparent substrates (a glass plate or the like) which are arranged to face each other with a liquid crystal layer interposed therebetween. A plurality of scanning electrodes (transparent electrodes) 3 are formed on one of the substrates 1 and 2 in parallel with each other.
On the other substrate 2, a plurality of signal electrodes (transparent electrodes) 4 orthogonal to the length direction of the scanning electrodes 3 are formed in parallel with each other.

Further, alignment films 5 and 6 are provided on the electrode forming surfaces of the substrates 1 and 2 respectively. Each of these alignment films 5 and 6 is a horizontal alignment film made of an organic polymer compound such as polyimide. Each of these alignment films 5 and 6 is an alignment treatment film whose surface is rubbed in a direction parallel to each other. I have.

The pair of substrates 1 and 2 are bonded at their outer peripheral edges via a frame-shaped sealing material (not shown), and are surrounded by the sealing material between the two substrates 1 and 2. An antiferroelectric liquid crystal 7 is sealed in the region.
The antiferroelectric liquid crystal 7 has a smectic layer structure, has a stable molecular alignment state, and changes the alignment direction of liquid crystal molecules according to an electric field.

That is, the antiferroelectric liquid crystal 7 has a certain tilt angle (22.5 to 30 °) with respect to the normal of the smectic layer structure.
Liquid crystal molecules are arranged in the direction inclined by
There are three stable states in the molecular arrangement state.

The first stable state is a state where there is no electric field or when a weak electric field is applied. In this state, the liquid crystal molecules are arranged alternately in each layer (in the normal direction of the smectic layer structure). They are alternately arranged in the opposite direction at the same tilt angle). Therefore, the average arrangement direction of the liquid crystal molecules in the entire liquid crystal layer in a state where no electric field or a weak electric field is applied is in a normal direction of the smectic layer structure.

The second stable state is a state in which an electric field having a strong unidirectional polarity is applied to the liquid crystal layer. In this case, the spontaneous polarization of the liquid crystal molecules acts on the electric field, and all the liquid crystal molecules are turned on. Are uniformly arranged in one direction at the tilt angle (22.5 to 30 °) with respect to the normal of the smectic layer structure.

The third stable state is a state in which a strong electric field having a reverse polarity is applied to the liquid crystal layer. In this case, the spontaneous polarization of the liquid crystal molecules acts on the reverse electric field to cause the liquid crystal molecules to move. The liquid crystal molecules are inverted, and all the liquid crystal molecules are uniformly arranged at a tilt angle (22.5 to 30 °) in a direction opposite to the second stable state with respect to a normal line of the smectic layer structure.

In addition, since the alignment direction of the liquid crystal molecules of the antiferroelectric liquid crystal 7 is regulated by the alignment films 5 and 6 of the two substrates 1 and 2, in the first stable state, the liquid crystal molecules are averaged. In the second and third stable states, all of the liquid crystal molecules are oriented such that the major axis direction is the same as the alignment treatment direction (rubbing direction) of the alignment films 5 and 6. They are arranged so as to face a direction inclined by the tilt angle with respect to the direction.

On the other hand, polarizing plates 8 and 9 are laminated on the outer surfaces of the substrates 1 and 2, respectively. These polarizing plates 8, 9
Are arranged in crossed Nicols such that their transmission axes are substantially orthogonal to each other.
In the above, the direction of the transmission axis of the lower incident-side polarizing plate 8 is substantially parallel to the average arrangement direction of the liquid crystal molecules in the first stable state.

FIG. 7 is an equivalent circuit diagram of the above-mentioned liquid crystal display element. In FIG. 7, an anti-ferroelectric liquid crystal 7 and an intersecting portion between the scanning electrode 3 of one substrate 1 and the signal electrode 4 of the other substrate 2 are provided. Since the constituted pixel is equivalent to a capacitor, the equivalent circuit of the liquid crystal display element is represented by a circuit in which the scanning electrode 3 and the signal electrode 4 are connected by a capacitance (hereinafter referred to as a pixel capacitance) CLC of the pixel. .

The display operation of the liquid crystal display device will be described. In a state where no voltage is applied between the electrodes 3 and 4 of the substrates 1 and 2 or a state where the applied voltage is low, the liquid crystal molecules are in the first stable state. Array. In the first stable state, the light that has passed through the incident-side polarizing plate 8 and has become linearly polarized light has the same polarization direction as the average alignment direction of the liquid crystal molecules. The liquid crystal layer is emitted, and is blocked by the emission-side polarizing plate 9 arranged in crossed Nicols. Therefore, at this time, almost no light is emitted from the liquid crystal display element, and the display is turned off (dark).

On the other hand, when an ON voltage (voltage higher than the threshold voltage of the liquid crystal) in one direction is applied between the electrodes 3 and 4 of the substrates 1 and 2, the liquid crystal molecules are arranged in the second stable state. . In the second stable state, the major axis direction of the liquid crystal molecules is substantially equal to the tilt angle (22.5 to 30 °) with respect to the polarization direction of the light that has passed through the incident-side polarizing plate 8 and has become linearly polarized light. Since the angles are shifted by the same angle, the linearly polarized light incident on the liquid crystal layer becomes elliptically polarized light due to the birefringence effect of the liquid crystal layer, and the light of the component in the transmission axis direction of the emission side polarizing plate 9 becomes the emission light of the liquid crystal display element. , The display is turned on (bright).

This is the same when an ON voltage of the opposite polarity is applied between the electrodes 3 and 4 of the substrates 1 and 2. In this case, the liquid crystal molecules are arranged in the third stable state. Even in the third stable state, the linearly polarized light incident on the liquid crystal layer becomes elliptically polarized light due to the birefringence effect of the liquid crystal layer, and light of the component in the transmission axis direction of the output side polarizing plate 9 becomes light emitted from the liquid crystal display element. The display is turned on (bright).

FIG. 8 shows a typical relationship between the voltage applied between the electrodes 3 and 4 and the light transmittance in the antiferroelectric liquid crystal display element. When the applied voltage is around 0 V, the transmittance is low. Therefore, although the liquid crystal display element is in the OFF state, when the applied voltage is increased to the + side, the transmittance increases when the voltage value exceeds a certain threshold value, and the liquid crystal display element is turned off.
It becomes N state. Further, when the applied voltage is reduced from this state, the transmittance becomes low when the applied voltage becomes equal to or lower than a certain threshold voltage which is lower than the voltage value at which the liquid crystal display element is turned on. become. This is the same when the applied voltage is changed to the negative side. In this case, the liquid crystal display element is turned on when the absolute value of the applied voltage exceeds a certain threshold value. When the absolute value of the voltage is reduced, the liquid crystal display element is turned off again when the absolute value of the voltage becomes equal to or lower than a certain threshold value lower than the voltage value for turning on the liquid crystal display.

Accordingly, in FIG. 8, voltage values + VB and -VB near the middle between the voltage value when the liquid crystal display element is turned on and the voltage value when the liquid crystal display element is turned off again are used as the reference voltage, and this reference voltage + VB , -VB with a driving voltage superimposed on the image data signal applied between the electrodes 3 and 4,
The liquid crystal display device can display an image.

FIG. 9 shows the relationship between the waveform of the drive voltage applied between the electrodes 3 and 4 of one pixel of the liquid crystal display device and the light transmittance of the pixel. In the figure, TS is the pixel selection period, and this selection period TS is a period obtained by equally dividing one frame TF for displaying one screen by the number of rows of pixels (the number of scanning electrodes).

As shown in FIG. 9, if a voltage sufficiently higher than the reference voltage VB is applied between the electrodes 3 and 4 of the selected pixel during the first selection period T S, for example, The pixels are turned on (bright) to arrange them in the second stable state. When the selection period TS elapses and the non-selection period starts, the potential of the scanning electrode 3 of this pixel becomes the non-selection potential, but the data signal for driving the pixels of another row is applied to the signal electrode 4. Therefore, a voltage having a waveform corresponding to the data signal is applied to the pixel even during the non-selection period. However, during this non-selection period, since the potential of the scanning electrode 3 is at the non-selection potential, the voltage applied during the non-selection period is a voltage near + VB. Although there are fluctuations,
The arrangement state of the liquid crystal molecules does not change from the second stable state to another stable state.

If a voltage (-voltage) having a polarity opposite to that of the previous selection is applied between the electrodes 3 and 4 of the selected pixel during the next selection period Ts, the arrangement state of the liquid crystal molecules is changed to the second state. Changes from the stable state to another stable state. When the absolute value of the applied voltage is smaller than the reference voltage VB as shown in FIG. 9, the alignment state of the liquid crystal molecules is not inverted until the third stable state. The first stable state is set, and the pixel is turned off (dark). Further, during the non-selection period, as described above, a voltage having a waveform corresponding to a data signal for driving pixels in another row is applied. However, the voltage applied during the non-selection period is -VB. The voltage is close to the voltage, and therefore, although the transmittance varies due to slight movement of the liquid crystal molecules, the alignment state of the liquid crystal molecules does not change from the first stable state to another stable state.

Further, although not shown in FIG. 9, when a voltage having an absolute value sufficiently higher than the reference voltage VB is applied between the electrodes 3 and 4 of the selected pixel, the liquid crystal molecules are brought into the third stable state. When the pixels are arranged to be in an ON (bright) state, and when a + voltage whose absolute value is smaller than the reference voltage VB is applied, the liquid crystal molecules are arranged in a first stable state and the pixels are in an OFF (dark) state. become.

In the antiferroelectric liquid crystal display device, the liquid crystal molecules arranged in one of the first, second and third stable states are applied with an electric field for changing the arrangement state. Until the above, the above-mentioned stable state is maintained, so that a higher contrast display can be obtained as compared with a generally used TN mode or STN mode liquid crystal display element, and there is a possibility that time-division driving can be performed at a high duty. Therefore, realization of a high-definition and large-screen liquid crystal display device can be expected.

[0023]

However, all of the currently known antiferroelectric liquid crystals have a problem that the response time to an electric field is long. Since the conventional antiferroelectric liquid crystal display device is of the simple matrix type as described above, the frame frequency for writing one screen cannot be increased.

This is because, in the simple matrix liquid crystal display device, an electric field for changing the arrangement state of liquid crystal molecules is applied to the liquid crystal only during the selection period of each pixel.
This is because writing to the state or the OFF state needs to be completed within the selection period of the pixel.

That is, in the simple matrix liquid crystal display device, the electric field for changing the arrangement state of the liquid crystal molecules acts on the liquid crystal only during the selection period. Becomes writing failure, and the image cannot be displayed.

For this reason, in the conventional antiferroelectric liquid crystal display element, it is necessary to set the selection period of the pixels in each column to a period necessary for changing the arrangement state of the liquid crystal molecules. Since the response time to the electric field is long as described above, if the selection period is set in accordance with the response time of the antiferroelectric liquid crystal, the frame frequency becomes less than the practical value.

According to the present invention, while using an antiferroelectric liquid crystal, the frame period is increased by shortening the selection period of the pixels in each column, and the arrangement state of the liquid crystal molecules is surely brought to its stable state. It is an object of the present invention to provide an antiferroelectric liquid crystal display device capable of performing high contrast display by changing the display.

[0028]

According to the present invention, there is provided an antiferroelectric liquid crystal display device comprising a plurality of pixels arranged vertically and horizontally on one of a pair of transparent substrates opposed to each other with an antiferroelectric liquid crystal layer interposed therebetween. An electrode, a semiconductor active element connected to each of the pixel electrodes, a signal line for supplying a drive signal to the active element, an alignment film directly covering the pixel electrode and the active element, and having a planarized surface. And a counter electrode facing the pixel electrode is provided on the other substrate.

[0029]

[Action] The present invention is for an antiferroelectric liquid crystal display element is an active matrix of active elements
A planarized alignment film is formed on the surface of the substrate provided with
Therefore, the orientation of the liquid crystal becomes uniform. In the active matrix liquid crystal display element, the active element is turned on during the selection period, and the pixel is charged with the electric charge corresponding to the drive signal supplied from the signal line. Is electrically cut off, so that the discharge from the pixel during the non-selection period is performed only gradually, so that the pixel can hold sufficient electric charge even during the non-selection period.

Since the molecules of the antiferroelectric liquid crystal are arranged in any stable state by an electric field corresponding to the charge held in the pixel, if sufficient charge is held in the pixel even during the non-selection period, Even if the switching of the liquid crystal molecule alignment state is not completed within the selection period, the liquid crystal molecule alignment state can be switched during the non-selection period.

Therefore, according to the antiferroelectric liquid crystal display device of the present invention, even if the response time of the antiferroelectric liquid crystal is long,
It is not necessary to lengthen the selection period accordingly, so that the selection period of the pixels in each column can be shortened to increase the frame frequency, and the arrangement state of the liquid crystal molecules can be reliably changed to its stable state. Thus, a high-contrast display can be performed.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is a cross-sectional view of a part of an antiferroelectric liquid crystal display element, and FIG. 2 is a plan view of a part of a substrate on which pixel electrodes and active elements are formed.

The antiferroelectric liquid crystal display device of this embodiment is
This is an active matrix liquid crystal display element having a thin film transistor (hereinafter referred to as TFT) as an active element. Among a pair of transparent substrates 11 and 12 opposed to each other with a liquid crystal layer interposed therebetween,
In FIG. 1, a lower substrate (hereinafter referred to as a TFT substrate) 11
, A transparent pixel electrode 13 and a TFT 14 as an active element connected to the pixel electrode 13 are vertically and horizontally arranged.

The TFT 14 has, for example, an inverted stagger structure, and includes a gate electrode G formed on the substrate 11,
A gate insulating film 15 made of SiN (silicon nitride) or the like covering the gate electrode G; a semiconductor layer 16 made of a-Si (amorphous silicon) formed on the gate insulating film 15; And a source electrode S and a drain electrode D formed thereon.

The TFT substrate 11 is provided with a gate line GL corresponding to a row between the pixel electrodes 13 and a data line DL corresponding to a column between the pixel electrodes 13. The gate electrode G of the TFT 14 is connected to the gate line GL, and the drain electrode D is connected to the data line DL. The gate line GL is covered with a gate insulating film 15 of the TFT 14 except for a terminal portion (not shown), and the data line DL is formed on the gate insulating film 15. The pixel electrode 13 is formed on the gate insulating film (transparent film) 15 and has one end connected to the source electrode S of the TFT 14.

On the other hand, a transparent counter electrode 17 facing each pixel electrode 13 of the TFT substrate 11 is formed on an upper substrate (hereinafter referred to as a counter substrate) 12 in FIG. The counter electrode 17 is a single electrode having an area covering the entire display area.

Further, alignment films 18 and 19 are provided on the electrode forming surfaces of the TFT substrate 11 and the counter substrate 12, respectively. Each of these alignment films 18 and 19 is a horizontal alignment film made of an organic polymer compound such as polyimide, and its film surface is subjected to an alignment treatment by rubbing. Incidentally, in a liquid crystal display device using an antiferroelectric liquid crystal,
If the alignment film of at least one of the pair of substrates 11 and 12 facing each other with the liquid crystal layer interposed therebetween has a function of regulating the alignment direction of the liquid crystal molecules, the alignment direction of the liquid crystal molecules can be sufficiently regulated. The alignment film of any of the substrates may be a non-rubbing film having only horizontal alignment.

The TFT substrate 11 and the counter substrate 12 are
The outer peripheral edge is bonded via a frame-shaped sealing material (not shown), and an antiferroelectric liquid crystal 20 is sealed in a region between the substrates 11 and 12 surrounded by the sealing material. . The antiferroelectric liquid crystal 20 is the same as that used in the conventional antiferroelectric liquid crystal display device, and the arrangement state of the liquid crystal molecules according to the electric field is changed to the first, second, and second liquid crystal molecules. 3 changes to a stable state.

On the other hand, polarizing plates 21 and 22 are laminated on the outer surfaces of the substrates 11 and 12, respectively. Like the conventional antiferroelectric liquid crystal display device, the two polarizing plates 21 and 22 are arranged in crossed Nicols so that their transmission axes are substantially orthogonal to each other.
In the figure, the direction of the transmission axis of the lower incident-side polarizing plate 21 is substantially parallel to the alignment direction of liquid crystal molecules in the first stable state (average alignment direction in the entire liquid crystal layer).

FIG. 3 is an equivalent circuit diagram of the antiferroelectric liquid crystal display element. A pixel formed by the pixel electrode 13, the counter electrode 17, and the liquid crystal 20 between them is equivalent to a capacitor. Capacity (hereinafter referred to as pixel capacity) C
Expressed in LC. CS is the stray capacitance CS of the TFT 14, and this stray capacitance CS is the pixel capacitance Cs.
Connected in parallel to LC.

The anti-ferroelectric liquid crystal display device is driven for display by sequentially applying a gate signal to each gate line GL and applying an image data signal to each data line DL in synchronization with the gate signal.

FIG. 4 shows the waveforms of the gate signal and the data signal applied to the TFT 14 of one pixel from the gate line GL and the data line DL, the waveform of the voltage between the electrodes of the pixel, and the change of the light transmittance of the pixel. Is shown. In the figure, TS is a selection period of the pixel, and this selection period TS
Is a period in which one frame TF for displaying one screen is equally divided by the number of rows of pixels (the number of scanning electrodes).

The display operation of the antiferroelectric liquid crystal display device will be described. The gate electrode G of the TFT 14 during the selection period TS is described.
When the gate signal -VG is applied to the TFT, the TFT 14 is turned ON.
Then, a charge corresponding to the data signal VD supplied from the data line DL to the pixel via the TFT 14 is charged in the pixel (pixel capacitance CLC and floating capacitance CS). in this case,
The pixel is rapidly charged, and the voltage between the electrodes of the pixel (the voltage between the pixel electrode 13 and the counter electrode 17) is reduced by the gate signal-.
It becomes a voltage corresponding to the potential difference between VG and the data signal VD.

When the selection period Ts has elapsed and the non-selection period has come, the application of the gate signal to the gate electrode G of the TFT 14 is cut off and the TFT 14 is turned off. In the non-selection period, the pixels (pixel capacitance CLC and floating capacitance CS)
)), The TFT 14 is turned off during this non-selection period, and the pixel and the data line D
Since the connection to L is electrically cut off, the discharge from the pixel during the non-selection period is performed only gradually, so that the pixel can hold a sufficient charge even during the non-selection period.

Since the molecules of the antiferroelectric liquid crystal are arranged in one of the stable states by an electric field corresponding to the charge held in the pixel, if sufficient charge is held in the pixel even during the non-selection period, Even if the switching of the liquid crystal molecule alignment state is not completed within the selection period, the liquid crystal molecule alignment state can be switched during the non-selection period.

Therefore, according to the antiferroelectric liquid crystal display element, even if the response time of the antiferroelectric liquid crystal is long, it is not necessary to take a long selection period in accordance with the response time. Can be increased to increase the frame frequency, and the arrangement state of the liquid crystal molecules can be reliably changed to its stable state to perform a high-contrast display. In the above embodiment, the TFT
Although the (thin film transistor) is an active element, the active element may be, for example, a thin film diode.

[0047]

According to the present invention, a plurality of pixel electrodes arranged vertically and horizontally are connected to one of a pair of transparent substrates opposed to each other with a layer of antiferroelectric liquid crystal therebetween, and these pixel electrodes are respectively connected to the transparent electrodes. A semiconductor active element, a signal line for supplying a drive signal to the active element, and a direct connection between the pixel electrode and the active element.
An anti-ferroelectric liquid crystal display element is formed into an active matrix by providing an orientation film that covers and has a planarized surface, and a counter electrode facing the pixel electrode on the other substrate. The layer thickness of the dielectric liquid crystal becomes uniform, and the occurrence of alignment failure can be prevented. Further, according to the present invention, while using an antiferroelectric liquid crystal, the frame period is increased by shortening the selection period of the pixels in each column, and the alignment state of the liquid crystal molecules is ensured to its stable state. And a high-contrast display can be performed.

[Brief description of the drawings]

FIG. 1 is a sectional view of a part of an antiferroelectric liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a plan view of a part of the substrate on which pixel electrodes and active elements are formed.

FIG. 3 is an equivalent circuit diagram of the antiferroelectric liquid crystal display device.

FIG. 4 is a diagram showing waveforms of a gate signal and a data signal applied to an active element (TFT) of one pixel, a waveform of a voltage between electrodes of the pixel, and a change in light transmittance of the pixel.

FIG. 5 is a sectional view of a part of a conventional antiferroelectric liquid crystal display device.

FIG. 6 is a plan view of a part of one of the substrates.

FIG. 7 is an equivalent circuit diagram of a conventional antiferroelectric liquid crystal display device.

FIG. 8 is a diagram showing a typical relationship between a voltage applied between electrodes and light transmittance in an antiferroelectric liquid crystal display device.

FIG. 9 is a view showing a relationship between a waveform of a driving voltage applied between electrodes of one pixel of a conventional antiferroelectric liquid crystal display element and light transmittance of the pixel.

[Explanation of symbols]

11, 12: substrate, 13: pixel electrode, 14: TFT (active element), GL: gate line, G: gate electrode, 15
... gate insulating film, 16 ... semiconductor layer, S ... source electrode, D
... Drain electrode, DL ... Data line, 17 ... Counter electrode, 18, 19 ... Alignment film, 20 ... Anti-ferroelectric liquid crystal, 2
1,22 ... Polarizing plate.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02F 1/141 G02F 1/1337 G02F 1/136 G02F 1/133

Claims (1)

(57) [Claims] In a liquid crystal display device using an antiferroelectric liquid crystal, a plurality of pixel electrodes arranged vertically and horizontally are formed on one of a pair of transparent substrates facing each other with the antiferroelectric liquid crystal layer interposed therebetween. A semiconductor active element connected to each of the pixel electrodes, a signal line for supplying a drive signal to the active element,
An antiferroelectric liquid crystal display, comprising: an alignment film that directly covers the pixel electrode and the active element and has a planarized surface; and a counter electrode facing the pixel electrode is provided on the other substrate. element.