JP2001215471A - Method for driving liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a driving method by which a hysteresis of a threshod-free liquid crystal is reduced and a response speed can be improved depending on a liquid crystal. SOLUTION: A hysteresis of a threshold-free liquid crystal is prevented from occurring by arranging a '0 V' reset period before or after a gray scale display period. In a liquid crystal having small spontaneous polarization and taking time for switching a half tone period, the response speed is effectively improved by switching via '0 V'.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION In this specification, a liquid crystal display device is a display device in which a voltage is applied to a liquid crystal layer to change the polarization, scattering or wavelength characteristics of light incident on the liquid crystal layer, thereby displaying light and dark. Is shown.

[0002] In this specification, a liquid crystal having no threshold liquid crystal, V-shaped liquid crystal or a chiral smectic C _R phase, since the phase transition precursor phenomenon occurs remarkably, showing a liquid crystal having no clear threshold. Further, these liquid crystals have a characteristic that, when a positive polarity voltage and a negative polarity voltage having the same absolute value are applied, the tilt angles of the liquid crystal at the voltages are equal and the transmittance is uniquely determined.

[0003] In this specification, an active element refers to an element that controls power from an external power supply in accordance with an input signal. Examples of the active element include a thin film transistor, a field effect transistor, and a diode.

[0004] In this specification, a thin film transistor refers to a semiconductor element having a semiconductor layer, a gate electrode, a source electrode, and a drain electrode.

According to the present invention, among the liquid crystals having spontaneous polarization, it is possible to reduce the hysteresis peculiar to the material of the thresholdless liquid crystal which does not have a definite threshold value and can perform halftone display, and can perform a good gradation display. A driving method that can achieve high-speed response at the same time will be described.

[0006]

2. Description of the Related Art Liquid crystal panels have the advantages of being thin, lightweight, and low in power consumption. Recently, it is also used for display devices that require a high-speed response at a moving image level, such as portable TVs and wall-mounted TVs. In addition, since a large screen can be displayed, it can be used as a projector panel such as a 50-inch rear projector.

As a liquid crystal alignment mode, a TN mode having a cell gap of about 4 to 5 μm is widely used because of easy alignment control. However, in the TN mode, since the alignment regulating force of the alignment film is strong, the response speed of the halftone display close to the white level is slow, and when a high-speed moving image is displayed, the response of the liquid crystal cannot be applied, and the image appears as flicker.

[0008] As a high-speed response technology using a nematic liquid crystal instead of the TN mode, there are HAN mode, OCB mode, TN mode in which a cell gap is narrowed to 2 to 3 μm to increase electric field strength and achieve high-speed response, alignment of alignment film. There is a vertical alignment mode and the like in which a high-speed response is expected compared to TN because of a small regulating force. However, it is difficult to respond at the microsecond level with a nematic liquid crystal.

There is a liquid crystal having a spontaneous polarization as a material replacing the nematic liquid crystal. Switching is performed by the interaction between the spontaneous polarization and an electric field, so that a high-speed response on a microsecond level can be achieved. For example, a simple matrix panel uses tristable antiferroelectric liquid crystal and bistable ferroelectric liquid crystal. Fig. 7 (A) shows the relationship between the applied voltage and the transmittance of a tristable antiferroelectric liquid crystal.
FIG. 7B shows the relationship between the applied voltage and the transmittance of the bistable ferroelectric liquid crystal. Thus, a tristable antiferroelectric liquid crystal,
The bistable ferroelectric liquid crystal is stable in two values of a white level and a black level. In order to perform halftone display, there are methods such as area gray scale for displaying gray scale by adjusting the number of white and black pixels, and time gray scale for adjusting display time. When used as a panel, the response of the liquid crystal is usually between a white level and a black level.

The simple matrix panel (FIG. 8) has scanning lines (X1, X2... Xn) 801 and signal lines (Y1, Y2.
n) 802 are arranged in a matrix. In order to use a tristable antiferroelectric liquid crystal or a bistable ferroelectric liquid crystal in a simple matrix panel, a larger hysteresis width is better. Hysteresis refers to the difference in transmittance between voltage paths. The hysteresis width (701 in FIG. 7) is defined by a difference between voltage values showing the same transmittance. For example, to drive a tristable antiferroelectric liquid crystal with a simple matrix panel, it is appropriate to perform bias driving. When displaying the black level, the bias voltage is always applied to the liquid crystal panel. Therefore, in order to obtain a good black level, there is an area where the transmittance does not change even when the bias voltage is applied. It is desirable to use a liquid crystal having a value characteristic.

Instead of the bistable ferroelectric liquid crystal and the tristable antiferroelectric liquid crystal, a thresholdless liquid crystal or a liquid crystal called a V-shaped liquid crystal has attracted attention as a spontaneously polarized liquid crystal capable of analog gradation. (Casio Computer JP 9-500
No. 50, JP-A-10-301091). The thresholdless liquid crystal is chiral smectic C in the bulk state.
_Although it shows the _A phase, when it is sealed between the substrates, it is oriented with a tilt with respect to the main surface of the substrate, unlike usual, and this tilt changes continuously according to the applied electric field. Since the liquid crystal is in a mixed phase over its entire thickness, the director moves smoothly, there is no clear threshold, and hysteresis is hard to occur. For this reason, gradation display, which has been difficult with a tristable antiferroelectric liquid crystal or a bistable ferroelectric liquid crystal, can be easily performed. In addition, since there is spontaneous polarization, high-speed switching is performed by interaction with an electric field. Tristable antiferroelectric liquid crystal and bistable ferroelectric liquid crystal do not have domain-less switching,
Also in the halftone display, there is a feature that a display with a small number of domains and high contrast can be obtained.

[0012] thresholdless liquid crystal when no electric field is applied, the tilt of the liquid crystal molecules are also said to have SmC _R * phase does not have a correlation (chiral smectic C _R phase) between the layers. When no voltage is applied, the director of the liquid crystal is oriented at an arbitrary tilt angle in the cone, and each layer has a different tilt angle, so that the spontaneous polarization of the liquid crystal is offset to zero as a whole. The average optical axis of the liquid crystal is equal to the central axis of the cone (Japanese Patent Laid-Open No. 10-082985).

The hysteresis of the thresholdless liquid crystal is smaller than that of the tristable antiferroelectric liquid crystal and the bistable ferroelectric liquid crystal shown in FIG. Since the thresholdless liquid crystal does not have a clear threshold characteristic and the brightness increases monotonously according to the applied voltage, it is generally used for an active matrix panel. In a simple matrix panel, the liquid crystal is driven by a bias drive in which a bias voltage is always applied to the liquid crystal.However, when the thresholdless liquid crystal is driven by such a bias drive, the liquid crystal is switched by the bias voltage, resulting in good quality. Black level cannot be achieved.

[0014]

The present invention in a first [0008] reduces the hysteresis seen slightly liquid crystal having no threshold liquid crystal, V-shaped liquid crystal or a chiral smectic C _R phase by improving the driving method, a good A gradation display is performed.

[0015] The present invention can be used equally in a liquid crystal having no threshold liquid crystal, V-shaped liquid crystal or a chiral smectic C _R phase. However, in order to simplify the description, these liquid crystals will be described by using the term thresholdless liquid crystal as a representative.

FIGS. 9A and 9B show the relationship between the cell voltage and the transmittance, which will be described later. The thresholdless liquid crystal has some hysteresis depending on the conditions of the liquid crystal cell as shown in FIG. FIGS. 9A and 9B show the characteristics of the hysteresis of the thresholdless liquid crystal. FIG.
(1) shows a change in hysteresis depending on the thickness of the alignment film. FIG. 9A shows a case where the alignment film is thin, and FIG. 9B shows a case where the alignment film is thick. FIG.
(2) shows a change in hysteresis due to spontaneous polarization. FIG. 9A shows a case where the spontaneous polarization is small, and FIG. 9B shows a case where the spontaneous polarization is large. In particular, when the orientation film thickness is small (Liquid Crystal
s, 1998, Vol. 25 LCT100975 F
ukuda) When the spontaneous polarization of the liquid crystal is small, hysteresis tends to occur. The present invention is effective when the spontaneous polarization of the liquid crystal is small or when the thickness of the alignment film is small.

Second, the present invention aims to further improve the response speed of the thresholdless liquid crystal by improving the driving method. In particular, spontaneous polarization is 40 nC / cm
² , as a thresholdless liquid crystal, it is effective for liquid crystals having relatively small spontaneous polarization.

Third, the driving method of the present invention is to secure a sufficient white level and prevent hysteresis at the same time when there is a limit to an external voltage described later due to the withstand voltage of the active element in active matrix driving. As an issue. In this specification, having a withstand voltage means that a voltage value that the active element can apply to the liquid crystal has an upper limit. The present invention has the effect of reducing the hysteresis that occurs when the thickness of the alignment film is reduced to obtain a good white level in order to prevent voltage loss due to the alignment film when the external voltage is limited. There is something. Further, the present invention has an effect of reducing the hysteresis that occurs when the spontaneous polarization of the liquid crystal is reduced so as to obtain a good white level in order to prevent the potential from being lowered due to the inversion of the spontaneous polarization. .

As the active element, there are a thin film transistor, a diode (two-terminal element), and the like. In this specification, the thin film transistor will be described as an example. As shown in FIG. 10, there is a thin film transistor 1003 as a switching element for driving liquid crystal at an intersection where a scanning line 1002 and a signal line 1001 are arranged in a matrix. In accordance with a signal voltage given by the signal line, electric charges are supplied to an auxiliary capacitor using the liquid crystal layer 1004 as a dielectric and an auxiliary capacitor 1005 connected in parallel to the liquid crystal layer. In the present specification, an external voltage corresponds to a signal voltage in a thin film transistor. A voltage applied to the liquid crystal layer 1004 and the auxiliary capacitor 1005 during the selection period of the scanning line 1001. ○ Cell voltage: a voltage applied to the liquid crystal layer 1004, that is, a voltage applied to the alignment film and the liquid crystal. ○ a liquid crystal effective voltage: a voltage applied only to the liquid crystal. The cell voltage is lower than the external voltage due to the inversion of the spontaneous polarization of the thresholdless liquid crystal, and the liquid crystal effective voltage is lower than the cell voltage due to the voltage loss of the alignment film. The liquid crystal effective voltage is smaller than the voltage.

In the case of the thresholdless liquid crystal, when the thickness of the oriented film is large as shown in FIG. 9B, the hysteresis tends to be small. However, it is not the case that the thickness of the alignment film should be blindly increased to suppress the hysteresis. Since the alignment film and the spontaneously polarized liquid crystal having a high dielectric constant are connected in series, if the alignment film is thick and the voltage loss is large, the liquid crystal effective voltage applied to the liquid crystal becomes low, and the liquid crystal cannot be sufficiently inverted and the white level is sufficient. Can not be removes. External voltage ± 10 ± 2
There is a method of increasing the liquid crystal effective voltage applied to the liquid crystal by increasing the voltage to 0 V. However, there is a limitation in increasing the external voltage in a thin film transistor whose external voltage is as low as ± 5 V to ± 7 V. For this reason, in order to improve the white level by active matrix driving in which the external voltage is limited, it is desirable to make the alignment film thin and increase the effective voltage applied to the liquid crystal. However, when the thickness of the alignment film is optimized so that the white level can be sufficiently obtained by the active matrix driving, the optimum alignment film thickness becomes thin depending on the magnitude of the spontaneous polarization of the liquid crystal, and hysteresis is easily generated.

That is, when the thresholdless liquid crystal is driven in an active matrix, if the thickness of the alignment film is increased,
Hysteresis is suppressed, but if the thickness of the alignment film is increased, a good white level cannot be obtained. However, by using the present invention, it is possible to reduce the hysteresis caused by reducing the thickness of the alignment film.

In the thresholdless liquid crystal, when the spontaneous polarization is large as shown in FIG. 9B, the hysteresis tends to be small. However, it is not the case if the spontaneous polarization should be increased blindly. The thin-film transistor is driven by a constant charge. If the spontaneous polarization is too large, it is impossible to supply a sufficient charge to invert the spontaneous polarization of the thresholdless liquid crystal, and the white level is deteriorated.

When the liquid crystal is driven by the thin film transistor, even if an external voltage is applied to the liquid crystal cell and the auxiliary capacitor connected in parallel during a selected scanning line, the spontaneous polarization is reversed and the liquid crystal cell and the auxiliary capacitor are applied to the liquid crystal cell and the auxiliary capacitor. The accumulated charge decreases, and the cell voltage after the spontaneous polarization has been inverted is lower than the external voltage applied during the period when the scanning line is selected. When the cell voltage decreases, the effective liquid crystal voltage decreases, and when the effective liquid crystal voltage decreases, the white level decreases. If the external voltage can be increased, a sufficient white level can be secured after the spontaneous polarization is reversed. However, in the active matrix driving, the height of the external voltage is limited due to the withstand voltage of the active element.

As described above, when the active element is driven by the thin film transistor having the upper limit of the external voltage, the thickness of the alignment film is reduced, the voltage loss due to the alignment film is prevented, the white level is improved, and the potential of the auxiliary capacitor is reduced. In order to prevent the drop, the spontaneous polarization of the liquid crystal may be reduced. However, when the spontaneous polarization is small or the thickness of the alignment film is small, hysteresis tends to occur easily.

In other words, when the thresholdless liquid crystal is driven in an active matrix, there is a relationship that the white level is deteriorated when the spontaneous polarization is increased, and the hysteresis is increased when the spontaneous polarization is reduced. However, the driving method of the present invention can reduce the hysteresis generated under the condition using the thresholdless liquid crystal having a small spontaneous polarization.

Fourth, in the conventional driving method disclosed to reduce the hysteresis, the black level is deteriorated.
According to the present invention, good black display can be performed. As conventional driving method, set pulse V _H, gray scale display pulse for a gradation display to reset pulse V _L for preventing seizure in the scan line selecting period 101 as shown in FIG. 1, the liquid crystal is oriented in a predetermined position there is to be applied to V _D (Casio JP-10-073803, JP-10-0
No. 82895). After a certain alignment state of the liquid crystal in setting pulse V _H, it is a driving method for reducing the hysteresis by causing switching. However, in this case, even when the user wants to perform the black display 102, the user recognizes the white level 104 displayed by the set pulse V _H2 103,
Color mixture of the black level and the white level occurs, and the quality of the black level deteriorates.

Fifthly, it is an object of the present invention to provide good gradation display when an auxiliary capacitor is connected in series to an active element such as a thin film transistor to drive a thresholdless liquid crystal. Assume that a voltage of +5 V or +1 V is applied to the liquid crystal layer in the first frame. It is assumed that the polarity of the voltage applied to the liquid crystal layer is changed between the first frame and the second frame by the AC driving, and a voltage of −3 V is applied to the liquid crystal layer in the second frame. At this time, the electric charge required for reversing the spontaneous polarization of the thresholdless liquid crystal changes according to the voltage of the first frame. Therefore, the spontaneous polarization of the liquid crystal was reversed in the second frame by the voltage applied to the liquid crystal layer in the first frame. Later brightness is different.
That is, the gradation of the second frame is darker when the first frame is at +5 V than when the first frame is at +1 V. When driving the thresholdless liquid crystal, the gradation changes depending on the voltage of the previous frame.

[0028]

Means for solving the problem will be described below.

In this specification, a period during which a voltage of 0 V is applied to the liquid crystal layer is referred to as a first period, a "0 V" reset period, or a reset period. In the first period, the liquid crystal switches to a reference position specified by a voltage of 0V. A voltage is applied to the liquid crystal during the first period, and
It is said that the liquid crystal is in the reset position when the liquid crystal is aligned according to the voltage of the liquid crystal. Further, a period in which a voltage corresponding to a display gradation is applied to the liquid crystal layer is referred to as a second period.

According to the present invention, when driving the thresholdless liquid crystal, a first period for applying a voltage of 0 V to the liquid crystal layer is provided before the second period, and the thresholdless liquid crystal is moved from a predetermined position. By driving and performing gradation display, hysteresis of the liquid crystal is prevented. By applying the present invention, good gradation display can be performed even when the alignment film is thin and hysteresis easily occurs, and when spontaneous polarization is small and hysteresis is easily generated.

The first period may be provided before or after the second period for performing gradation display. This is because, by alternately providing the first period and the second period, the thresholdless liquid crystal is returned to the reference position in the first period, and then gradation display is performed.

Further, according to the present invention, when the thresholdless liquid crystal is driven by connecting an auxiliary capacitor in series to an active element, for example, a thin film transistor, the first period is before or after the second period.
Is provided. As a result, the electric charge stored in the capacitance using the liquid crystal layer as a dielectric and the auxiliary capacitance connected in series to the active element is extinguished in the first period. Next, in the second period, charge is supplied to the capacitance and the auxiliary capacitance using the liquid crystal layer as a dielectric to switch the thresholdless liquid crystal. Thereby, regardless of the amount of charge stored in the auxiliary capacitor and the capacitor having the liquid crystal layer as a dielectric in the previous frame, the thresholdless liquid crystal is moved to a unique position corresponding to the supplied charge in the second period. Switch to

According to the present invention, by providing the first period for applying a voltage of 0 V to the liquid crystal layer, even when the time required for switching between halftones of different polarities of the thresholdless liquid crystal is slow, By switching the liquid crystal through the first period, the response time of the liquid crystal can be reduced. That is, the present invention has an effect of improving the response time when the switching between the halftones of the liquid crystal is significantly slower than when switching between the halftones is performed via “0 V”. In particular, when the spontaneous polarization of the liquid crystal is 40 nC / cm ² or less, the response time between halftones tends to be long.
The present invention is particularly useful for driving a liquid crystal display using a thresholdless liquid crystal having a spontaneous polarization of 40 nC / cm ² or less.

Further, according to the present invention, a black level is displayed by the liquid crystal orientation in the first period, so that even when black is displayed in gradation display, color mixing due to the first period does not occur. In addition, in the liquid crystal alignment in the first period, the spontaneous polarization is canceled, and therefore, the seizure of the liquid crystal due to the first period does not occur.

The present invention includes a pair of substrates, the alignment layer having a crystal orientation function is formed on the pair of substrates, the liquid crystal has a chiral smectic C _R phase, continuously switched in accordance with an applied electric field A period in which one image is displayed is defined as one frame period, the one frame period has a plurality of subframe periods, at least one subframe period is a first period (reset period), and Sets a voltage value applied to the liquid crystal to 0 V, and sets a voltage value corresponding to a predetermined gradation display in which a second period (a period for performing gradation display) is provided before or after the first period. A method for driving a liquid crystal display device, characterized in that a pulse having the same is applied to a liquid crystal layer.

The present invention is particularly effective when at least one of the pair of substrates has an active element for applying a voltage to the liquid crystal layer, and the withstand voltage of the active element limits the external voltage. In other words, in order to display a sufficient white level in a state in which the external voltage is limited, the alignment film is made thinner to suppress the voltage loss due to the alignment film, or the potential of the auxiliary capacitance due to the reversal of spontaneous polarization is reduced. It is necessary to reduce the spontaneous polarization of the thresholdless liquid crystal in order to suppress the drop of the threshold voltage. As shown in FIG. 9, when the thickness of the alignment film is small or when the spontaneous polarization of the thresholdless liquid crystal is small, the hysteresis of the thresholdless liquid crystal is likely to occur. Thus, the hysteresis of the thresholdless liquid crystal can be reduced. That is, the present invention is particularly useful when the external voltage has an upper limit. In the case of a thin film transistor, the upper limit of the external voltage is about ± 7V. In addition, the spontaneous polarization of the liquid crystal is 4
0 to 150 nC / cm ² , the thickness of the alignment film after post-baking is 15 to 75 nm, or the spontaneous polarization of the liquid crystal is 20 to 40 nC / cm ² , and the film of the alignment film after post-baking When the thickness is 30 to 150 nm, spontaneous polarization is small, and the alignment film is thin, so that hysteresis tends to occur. The present invention is particularly useful for a liquid crystal display device in which such hysteresis easily occurs.

[0037]

[Embodiment 1] By applying the present invention to a thresholdless liquid crystal having a relatively large spontaneous polarization, the hysteresis is reduced and a good gradation display is achieved. Regarding the response time of the liquid crystal, since the spontaneous polarization was as large as 100 nC / cm ² , the response time was short, and no significant difference was observed between the presence and absence of the “0 V” reset period.

FIG. 6A is a sectional view of the liquid crystal cell, and FIG. 6B is a top view of the liquid crystal cell. Explaining with reference to the cross-sectional view of FIG. 6A, an ITO 602 of 120 nm is formed on a quartz substrate 601.
Is patterned. A low pretilt alignment film 603 is formed on the ITO through printing, pre-baking, and post-baking steps of the alignment film, and the alignment film is subjected to a rubbing process. In the present embodiment, the thickness of the alignment film is reduced to 60 nm in order to suppress the voltage loss of the alignment film and improve the white level by driving using an active element having an upper limit on the voltage value that can be applied to the liquid crystal layer, for example, a thin film transistor. are doing. The thresholdless liquid crystal has a tendency that the alignment axis of the liquid crystal is displaced from the rubbing axis by about 0 to 15 °, so that the rubbing axes 606 and 607 are attached when the pair of substrates 601 are bonded.
Are crossed so that the optical axis of the liquid crystal is fixed to one axis. Spacers are not sprayed to prevent alignment defects.
A sealing material 604 is provided at an end of the liquid crystal cell to form a gap between the two substrates, and the cell gap of the liquid crystal cell is 2.0 μm. Polarizing plates 608 and 609 are attached to the substrate. When a voltage is not applied to the liquid crystal cell, the transmission axis 608 of the polarizing plate set in crossed Nicols and the optical axis of the liquid crystal can be approximated to be almost parallel at the wavelength of the visible light level. Become. Further, when no voltage is applied to the liquid crystal cell, the liquid crystal is oriented at random positions of the cone, so that the spontaneous polarization of the liquid crystal is not canceled as a whole.

An arbitrary waveform generator (function generator) manufactured by Wavetek and having a model number of "MOD
A driving waveform of a liquid crystal cell was created by an EL 275 ″ device. When a thresholdless liquid crystal is driven by a thin film transistor, the amount of supplied charges is a constant charge drive determined by an external voltage, a gate selection time, and the like. When driving the liquid crystal cell, the characteristics were examined by constant current driving, which constantly supplies electric charges, during a period in which a voltage was applied to the liquid crystal layer for the constant current driving, that is, according to the reversal of spontaneous polarization of the liquid crystal within a pulse period. 3A and 3B, a charge is supplied to the liquid crystal layer.
The driving waveform and the driving result when the first period for applying the V voltage is provided are shown. FIG. 3A shows a driving waveform used for driving the liquid crystal cell. In FIG. 3A, the horizontal axis represents time, and the vertical axis represents cell voltage. There is a first period (reset period of “0” V) 301 before the second period 302 for performing grayscale display. Experimentally, the first period 301 and the second period 302 were set to 8.3 msec. The voltage path (A) 303 is from 0V to + 1V, -1V, + 2V, -2V, + 3V, -3V,
It is a waveform of a path in which the absolute value of the cell voltage applied to the liquid crystal cell is increased every 1 V while sequentially changing the polarity of the voltage to +4 V, -4 V, +5 V, and -5 V. Voltage path (B) 3
04 is + 4V, -4V, + 3V, -3V, + 2V, -2
It is a waveform of a path for decreasing the absolute value of the cell voltage every 1 V while sequentially changing the polarity of the voltage in the order of V, +1 V, -1 V, and 0 V. A waveform having one cycle 305 by combining the voltage path (A) and the voltage path (B) was continuously input to the liquid crystal cell.

FIG. 3B shows the relationship between the voltage value of the waveform used for driving the liquid crystal cell and the amount of light of the liquid crystal cell.
It is. The transmittance at each cell voltage is plotted. The transmittance is the brightness finally shown when the optical response of the liquid crystal changes with time during the time when each cell voltage is applied to the liquid crystal cell. In the graph, the voltage applied to the liquid crystal and the alignment film is shown on the horizontal axis as the cell voltage.

The driving voltage of a normal thin film transistor (± 5
(± 7 V), it is necessary to reduce the spontaneous polarization of the thresholdless liquid crystal and to reduce the thickness of the alignment film in order to obtain a sufficient white level. Spontaneous polarization 40 to 150 nC / c
m ² thresholdless liquid crystal, and the alignment film after post-baking is 1
It was desirable to reduce the voltage loss by reducing the thickness to 5 to 75 nm and increase the liquid crystal effective voltage. Spontaneous polarization 20-40n
With a thresholdless liquid crystal of C / cm ² , the thickness of the alignment film is 75 to
150 nm is considered desirable. However, such a combination of the spontaneous polarization of the thresholdless liquid crystal and the value of the thickness of the alignment film is a condition under which hysteresis easily occurs because the alignment film is thin and the spontaneous polarization is small.

The thickness of the alignment film of this embodiment is optimized to be driven by an active element having an upper limit on the voltage that can be applied to the liquid crystal and the alignment film, for example, a thin film transistor.
m, and hysteresis easily occurs. However, the first period (“0V” reset period) is provided, and by switching the liquid crystal from the reset position, a favorable floor in which a difference in brightness between the voltage path (A) and the voltage path (B) is hardly observed. Key display was completed.

FIG. 17A shows an oscilloscope photograph when the same liquid crystal cell is driven with a different waveform from that of FIG. FIG. 17 (A)
This represents the optical response of the liquid crystal when a “0 V” reset period (first period) is provided. The applied waveform 1710 has 0 V, +1.6
V, -1.6 V, +3.2 V, -3.2 V, +4.8
V, -4.8 V, +3.2 V, -3.2 V, +1.6
V and -1.6 V in this order by changing the polarity of the voltage and the amplitude of the voltage, and applying a voltage having an absolute value larger than 0 to provide a second period for performing gradation display, and then setting the voltage to "0 V There is a reset period (first period). The relationship between the applied voltage and the optical response of the liquid crystal can be seen from the two waveforms shown on the oscilloscope. A photograph of the oscilloscope will be described with reference to FIG. The horizontal axis (referred to as the X axis) indicates time, and one division is 25 msec. A first axis (referred to as Y ₁ axis) indicating the optical response of the liquid crystal on the vertical axis (referred to as Y axis) indicates the brightness measured by a photomultiplier (photo multiplier), and one division is 10 mV. . Of vertical axis (referred to as Y ₂ axis) a second axis indicating a voltage applied to the liquid crystal cell, one scale shows the cell voltage is 4V.

The optical response of the liquid crystal changes in brightness in accordance with the absolute value of the voltage applied to the liquid crystal cell. In the normally black mode, when a voltage having a large absolute value is applied to the liquid crystal cell, the liquid crystal cell has a high white level, and when a voltage having a small absolute value is applied to the liquid crystal cell, the gradation becomes close to the black level. During the “0 V” reset period (first period), the liquid crystal switches to the black display reset position. In the oscilloscope display, the optical responses 1701 and 1702 of the liquid crystal when a voltage of ± 1.6 V is applied, and the optical responses 1703, 1704, and ± 4. Of the liquid crystal when a voltage of ± 3.2 V is applied.
The optical response 1705 of the liquid crystal when a voltage of 8 V is applied is shown. There is a black display period by the first period between the optical responses 1701 to 1705 of the liquid crystal. In the present embodiment, the optical response of the liquid crystal when the same absolute value voltage is applied is almost the same brightness. When a voltage of ± 1.6 V is applied, there is a difference in the light amount of the optical response, but the difference between the maximum value and the minimum value of the light amount is still pp (peak to peak).
k) is as small as 2.2 mV.

Since the thresholdless liquid crystal does not have a memory property, the orientation when no voltage is applied to the liquid crystal cell should always be the same, but in practice 0 V during driving and no voltage after driving are applied. The black level changes with time.
In particular, as the thickness of the alignment film is smaller (70 nm or less), a difference is observed between black alignment when the liquid crystal cell is driven and black alignment when the liquid crystal cell is not driven. (Liqu
id Crystals, 1998, Vol. 25 L
CT100975 Fukuda). This is a feature not found in liquid crystals having memory properties, such as tristable antiferroelectric liquid crystals and bistable ferroelectric liquid crystals. However, even if a voltage value at which an unstable phenomenon is observed is intentionally set as the voltage value during the reset period, there is no problem in the gradation displayed by the thresholdless liquid crystal, and a voltage-transmittance characteristic without hysteresis can be obtained. Was. That is, by providing a reset period of “0 V”, which is predicted to exhibit unstable alignment in the thresholdless liquid crystal, the effect of actually reducing the hysteresis was obtained.

[Comparative Example 1] "0V" reset period (first
The driving result when the period is not provided is shown below.

FIG. 6A is a cross-sectional view of the liquid crystal cell, and FIG. 6B is a top view thereof. ITO 602 on quartz substrate 601
Are patterned with a thickness of 120 nm. ITO
An alignment film 603 having a low pretilt is formed thereon by printing, pre-baking, and post-baking the alignment film, and the alignment film is rubbed. Since the thresholdless liquid crystal has a tendency that the alignment axis of the liquid crystal is displaced from the rubbing axis by about 0 to 15 °, the rubbing axes 606 and 607 intersect when the pair of substrates 601 are bonded to each other, and the optical axis of the liquid crystal is Is fixed to one axis. Spacers are not sprayed to prevent alignment defects. A sealant 604 is provided at an end of the liquid crystal cell to form a gap between the two substrates.
The cell gap of the liquid crystal cell is 2.0 μm. Polarizing plates 608 and 609 are provided on each of the pair of substrates, and the optical axis of the liquid crystal is substantially parallel to the transmission axis 608 of one of the polarizing plates at a wavelength of visible light level. For this reason, when no voltage is applied, the transmission axis of the first polarizing plate is provided along the optical axis of the liquid crystal, and the transmission axis 609 of the second polarizing plate is arranged orthogonal to the first polarizing plate. Then, the liquid crystal cell displays black. When no voltage is applied, the liquid crystal is oriented at random positions of the cone, so that the spontaneous polarization of the liquid crystal is canceled out as a whole. For comparison, the film thickness of the alignment film and the spontaneous polarization of the liquid crystal were the same as those used in the first embodiment, so that the liquid crystal material and the film thickness of the alignment were the same as those used when acquiring the data of FIG.

FIGS. 2A and 2B show driving waveforms and driving results when the “0 V” reset period is not provided. FIG. 2A shows a driving waveform of this comparative example. A pair of AC pulses 204 having opposite polarities and equal absolute values are applied to the liquid crystal cell while changing the voltage amplitude. Although the thin film transistor is driven by a constant charge, the characteristics were temporarily examined by a constant current drive. In the constant current driving, when a voltage is applied to the liquid crystal cell, a charge corresponding to the reversal of the spontaneous polarization of the liquid crystal is supplied. Wave
Teck's function generator "MODE"
L275 ″. A drive waveform was created. Voltage path (A) 201
Are from 0V to + 1V, -1V, + 2V, -2V, + 3V,
This is a waveform of a path for increasing the absolute value of the cell voltage applied to the liquid crystal cell every 1 V while sequentially changing the polarity of the voltage in the order of -3 V, +4 V, -4 V, +5 V, and -5 V. Voltage path (B) 202 has + 4V, -4V, + 3V, -3V, +2
It is a waveform of a path in which the absolute value of the cell voltage is decreased for each 1 V while sequentially changing the polarity of the voltage in the order of V, -2 V, +1 V, -1 V, and 0 V. The voltage path (A) and the voltage path (B) have one cycle 203, and a voltage is continuously applied to the liquid crystal cell. The width of the pulses 205 to 208 was 16.6 msec.

The optical response when a pulse was applied to the liquid crystal cell was examined with a photomultiplier tube. Since the optical response of the liquid crystal and the waveform input to the liquid crystal cell are displayed synchronously on an oscilloscope, the transmittance of the liquid crystal with respect to each cell voltage can be determined.

FIG. 2B is a graph showing the relationship between the cell voltage and the amount of light at this time. FIG. 2B plots the transmittance at each cell voltage. The transmittance indicates the final brightness when the optical response of the liquid crystal changes with time while the cell voltage is applied. Cell voltage 2V-3V
In the halftone display area, a shift (hysteresis) of the voltage-transmittance characteristic of 0.2 V at the maximum occurs. The AC drive is performed to average the transmittance 211 of the positive pulse and the transmittance 212 of the negative pulse in the path A, and the transmittance 209 of the positive pulse and the transmittance 210 of the negative pulse of the same voltage in the path (B). In the averaged one, the brightness is clearly different by ％ 5% even at the same voltage of 2V.

FIG. 2A shows an applied waveform and FIG. 2 shows an optical response of the liquid crystal.
According to (B), when driving the thresholdless liquid crystal, the liquid crystal indicates whether the absolute value of the previous voltage is higher or lower than the voltage for displaying the current gradation. It can be seen that the brightness changes. That is, the transmittance 20 when responding from the (−3V) pulse 205 having a high absolute voltage value
9 and the transmittance 211 when responding from the pulse (206) having a low absolute voltage value, the transmittances 209 and 211 are different even at the same voltage of 2V.

FIG. 13 shows an oscilloscope photograph when the same liquid crystal cell is driven with a different voltage amplitude from that of FIG. 2A. 13 (A) and 13 (B),
5 shows an optical response of the thresholdless liquid crystal when no “0V” reset period is provided. The applied waveforms are 0V, + 1.6V, -1.
6V, + 3.2V, -3.2V, + 4.8V, -4.8
V, + 3.2V, -3.2V, + 1.6V, -1.6V
The voltage polarity and the voltage amplitude are changed in this order. From the waveform displayed on the oscilloscope, the relationship between the voltage applied to the liquid crystal cell and the optical response of the liquid crystal can be understood. Horizontal axis (referred to as X axis)
Indicates the passage of time. One division of the oscilloscope waveform is 25 msec. A first axis (referred to as Y ₁ axis) indicating the optical response of the liquid crystal on the vertical axis (referred to as Y axis) indicates the brightness of the liquid crystal cell measured by a photomultiplier tube, and one scale is 1 scale.
0 mV. Of vertical axis (referred to as Y ₂ axis) a second axis indicating a voltage applied to the liquid crystal cell shows the cell voltage 1
The scale is 4V.

The brightness of the liquid crystal cell changes with time according to the applied voltage. The liquid crystal cell is in a normally black mode. When a voltage having a large absolute value is applied, the liquid crystal switches in a direction to increase the white level, and when a voltage having a small absolute value is applied, the liquid crystal switches in a direction to increase the black level. The waveforms of the oscilloscope display are liquid crystal optical responses 1301 and 1302 when a voltage of ± 1.6 V is applied, and liquid crystal optical responses 1303, 1304 and ± when a voltage of ± 3.2 V is applied.
Liquid crystal optical response 1305 when a voltage of 4.8 V is applied
There is. The absolute value of the voltage applied to the liquid crystal cell increases from 0 V every 1.6 V, and after applying the voltage of ± 4.8 V, the absolute value of the voltage applied to the liquid crystal cell decreases every 1.6 V. As a result, the brightness of the liquid crystal cell gradually increases from the black level 1306 of the liquid crystal optical response to ± 4.8.
When a voltage of V is applied, the white level of the liquid crystal optical response 1305 becomes maximum. Next, as the absolute value of the voltage applied to the liquid crystal cell decreases, the white level changes with the liquid crystal optical response 130.
4, 1302. Even if the same voltage is applied to the liquid crystal cell, the light quantity of the liquid crystal cell slightly changes. The light amount difference of the optical response of the liquid crystal is large when a voltage of ± 1.6 V is applied, and pp (peak) indicating the difference between the maximum value and the minimum value of the light amount.
There is 5.8 mV in the to peak). In particular, the light amount difference between the liquid crystal optical responses 1301 and 1302 when a voltage having an absolute value of 1.6 V is applied is larger than the optical response of the liquid crystal in which the “0 V” reset period shown in FIG. 17 is introduced.

When the comparative example 1 is compared with the first embodiment, “0
The usefulness of providing the “V” reset period is understood. "0
Hysteresis is suppressed by the "V" reset period.

[0055]

[Embodiment 1] In the case of active matrix driving in which an external voltage is limited, a thresholdless liquid crystal having a small spontaneous polarization is used in order to secure a good white level even if the auxiliary capacitance is small. Therefore, a first embodiment shows an example in which the present invention is applied to a liquid crystal having a small spontaneous polarization. If the spontaneous polarization is small, hysteresis tends to occur, but good gradation display can be achieved by applying the present invention. At the same time, the response speed of the liquid crystal can be improved to achieve a high-speed response.

FIG. 6 shows a sectional view and a top view of the liquid crystal cell. ITO 602 is patterned on a quartz substrate 601 to a thickness of 120 nm. A low pretilt alignment film 603 is printed, pre-baked, and post-baked on the ITO, and the surface of the alignment film is rubbed. In the thresholdless liquid crystal, the orientation axis of the liquid crystal is approximately 0 with respect to the rubbing axis.
Since there is a tendency to shift by about 15 °, a pair of substrates 60
The rubbing axes 606 and 607 intersect at 1 so that the liquid crystal optical axis is substantially parallel in the cell thickness direction. Spacers are not sprayed to prevent alignment defects. Seal material 6
The configuration is such that a gap is formed only in the cell 04, and the completed cell gap is 2.0 μm. Polarizing plates 608 and 609 are provided on each of the pair of substrates. At the wavelength of the visible light level, the optical axis of the liquid crystal has one polarizer optical axis 608.
Are approximately parallel to For this reason, when no voltage is applied, if the polarizers are arranged in crossed Nicols, the liquid crystal cell will display black. When no voltage is applied,
Since the liquid crystal is oriented at random positions of the cone, the spontaneous polarization of the liquid crystal is canceled out as a whole. In this embodiment, the thickness of the alignment film is reduced, the spontaneous polarization of the liquid crystal is reduced, and the white level is optimized by active matrix driving. The alignment film after post-baking has a thickness of 60 nm. Spontaneous polarization of liquid crystal 605 40 nC / cm
²

Driving waveforms were created with a function generator “MODEL 275” manufactured by Wavetek. Although the thin film transistor is driven by a constant charge, the characteristics are temporarily examined by a constant current drive. Charges corresponding to the reversal of the spontaneous polarization of the liquid crystal are supplied during the pulse period for constant current driving. FIGS. 11A and 11B show that the spontaneous polarization of the liquid crystal is 40.
The driving waveform and the driving result when the “0 V” reset period is provided at nc / cm ² are shown. FIG. 11A shows a driving waveform of the liquid crystal cell. There is a first period 1101 which is a reset period of “0 V” before a second period in which a gradation is displayed by the gradation display pulse 1102. In the experiment, the first period 1101 and the second period 1102 were set to 8.3 msec. The voltage path (A) 1103 is from 0V to + 1V, -1
V, + 2V, -2V, + 3V, -3V, + 4V, -4
V, + 5V, -5V, while sequentially changing the polarity of the voltage,
This is a waveform in which the absolute value of the cell voltage applied to the liquid crystal cell is increased every 1V. The voltage path (B) 1104 is + 4V,
-4V, + 3V, -3V, + 2V, -2V, + 1V,-
This is a waveform in which the absolute value of the cell voltage is decreased every 1 V while sequentially changing the polarity of the voltage to 1 V and 0 V. The voltage path (A) and the voltage path (B) have one cycle 1105, and a voltage is continuously applied to the liquid crystal cell with one cycle of the waveform as one unit.

The optical response when a voltage was applied to the liquid crystal cell was examined with a photomultiplier tube. Since the optical response of the liquid crystal and the pulse waveform input to the liquid crystal cell are displayed in synchronization on an oscilloscope, the transmittance of the liquid crystal with respect to the cell voltage can be determined.

FIG. 11B is a graph showing the relationship between the voltage applied to the liquid crystal cell and the amount of light. FIG.
In (B), the transmittance at each cell voltage is plotted. The transmittance indicates the final brightness when the optical response of the liquid crystal changes with time while the voltage is applied.
In FIG. 11B, the voltage applied to the liquid crystal and the alignment film when the liquid crystal cell is driven is defined as the cell voltage.

By providing the “0 V” reset period, it is possible to perform a good gradation display with almost no difference in brightness between the voltage path (A) and the voltage path (B).

As a first effect, spontaneous polarization of 40 nC / c
The thickness of the alignment film optimized so that it can be driven by an active matrix panel having a limited driving voltage for a thresholdless liquid crystal of m ² is as thin as 60 nm, which is a condition where hysteresis is relatively easy to occur. However, by providing the “0 V” reset period, display with almost no hysteresis can be performed.

Next, as a second effect of this embodiment, when a “0 V” reset period is provided for the thresholdless liquid crystal, the response time can be improved simultaneously with the improvement of the hysteresis.

FIG. 12 shows the measurement results of the response time of the present liquid crystal. FIG. 12A shows a response time between positive voltages, and FIG.
FIG. 12B shows the response time between the negative polarity voltages, and FIG. 12C shows the response time between the different polarity voltages. 12 (A) to 12
Each figure in (C) shows the relationship between the initial voltage, the end voltage, and the response time. The configuration of the liquid crystal cell is such that the spontaneous polarization of the liquid crystal is 40n.
C / cm ² , and the thickness of the alignment film after post-baking is 6
0 nm, which is the same as that of the liquid crystal cell for which the data in FIG. When the response time when switching from the positive voltage to the negative voltage is examined, especially from -5V to +1
It was found that the response at V took a long time (32 msec). 14. Switching from -3V to + 1V
It takes a long time of 6 msec (FIG. 12C). The thresholdless liquid crystal takes a long time to switch between white level, halftone and halftone in switching between different polarity voltages. When compared with the spontaneous polarization of the liquid crystal, such a tendency was conspicuous in the liquid crystal having a small spontaneous polarization of 40 nC / cm ² as in this example. Since the response time tends to be longer when the spontaneous polarization is small, the spontaneous polarization is 40 nC / c.
It is considered that such a tendency is more conspicuous at m ² or less.

When a thresholdless liquid crystal is driven by an active element such as a thin film transistor, an AC drive is generally used in order to prevent a DC component from being accumulated in the liquid crystal and causing burn-in. In other words, a positive voltage is applied to the liquid crystal panel,
Since the negative voltage is applied alternately, if the liquid crystal response time is long between the positive and negative voltages (that is, between the different polarities), it becomes difficult to produce a natural picture when displaying a moving image.

However, when the liquid crystal is switched through the “0 V” reset period as in the present invention, the response time can be improved. That is, comparing the response time of the liquid crystal shown in FIG. 12C, it takes 32 msec to directly switch from −5 V to +1 V,
Switching from -5 V to 0 V is 0.4 msec, and switching from 0 V to +1 V is 0.3 msec.
Therefore, when switching from -5 V to 0 V and then switching from 0 V to +1 V, the response time can be 0.7 msec.

When the response time of the liquid crystal is compared, -3 is obtained.
Switching from V to +1 V takes 15.6 msec, but switching from -3 V to 0 V is 2.6 msec, and switching from 0 V to +1 V is 0.3 msec. When switching from 0 V to +1 V is newly performed after switching, the response time can be set to 2.9 msec.

As described above, when the “0V” reset period is provided, the response time is five times or more shorter than when the “0V” reset period is not provided. That is, assuming that a response time between a first voltage having a voltage value of -3 V and a second voltage having a voltage value of +1 V having a polarity different from that of the first voltage is a first response time, Although the response time is 15.6 msec, assuming that the response time of the liquid crystal when 0 V passes between the first voltage and the second voltage is the second response time, the second response time is 2.9 msec. The difference between the first response time and the second response time was five times or more.

Of course, the “0 V” reset period may be set to a length that allows the liquid crystal to return to the reset position. For example, in the case of the liquid crystal having the response characteristics shown in FIG. 12, the maximum value of the response time of the liquid crystal from a voltage of 5 V or less to 0 V is 2.6 msec.
For c, the “0 V” reset period may be set to 2.6 msec, and the gradation display period may be set to 14 msec. 1. “0 V” reset period as a subframe within one frame
When 6 msec and the gradation display period of 14 msec are provided,
The liquid crystal can be switched to a predetermined gradation in one frame, and it is not necessary to make the liquid crystal make a cumulative response over a plurality of frames in order to display the predetermined gradation.

As described above, as a second effect when the “0 V” reset period is provided, when a liquid crystal having a long response time between a white level and a halftone or a halftone is AC-driven,
Providing a “0 V” reset period has the effect of improving the response time. Of course, even in the case where the liquid crystal is driven by DC, if the switching between the halftones of the liquid crystal is significantly slower than the switching between the halftones via 0 V, the “0 V” reset period of the present invention is provided. The effect of improving the response time can be estimated.

[Embodiment 2] To make the effect of the "0 V" reset period effective in a panel having hysteresis, the liquid crystal needs to be at a predetermined position or near a predetermined position within the reset period. Response time when relaxation liquid crystal returns to "0V" in this example (T ₃₎ is, using short (T ₃ <T ₄₎ material than the response time (T ₄₎ at which the liquid crystal responds to the electric field Thus, an attempt was made to shorten the period of black display by the reset period (T ₁ ) with respect to the gradation display period (T ₂ ) (T ₁ <T ₂ ), and to make the panel brighter.

A drive waveform was created by a function generator “MODEL 275” manufactured by Wavetek. The driving is a constant current driving, and a charge corresponding to the reversal of the spontaneous polarization of the liquid crystal is supplied within a pulse period. 4A and 4B show a reset period of 2 msec and a gradation display period of 1.
The driving waveform and the driving result when the “0 V” reset period is shortened to 4.6 msec are shown. Fig. 4 shows the driving waveform of this experiment.
It is shown in (A). Before the second period 402 in which the gradation is displayed by the gradation display pulse, the first period which is the “0 V” reset period is performed.
Period 401. One frame period 403 is 16.6
msec, and the reset period 401 is 2 msec and 1
The feature is that the time is about 1/8 of the frame period. The voltage path (A) 404 is from 0V to + 1V, -1V,
+ 2V, -2V, + 3V, -3V, + 4V, -4V, +
This is a waveform in which the absolute value of the cell voltage applied to the liquid crystal cell is increased every 1 V while sequentially changing the polarity of the voltage to 5 V and −5 V. The voltage path (B) 405 is + 4V, -4V, +
3V, -3V, + 2V, -2V, + 1V, -1V, 0V
And waveforms in which the absolute value of the cell voltage is decreased every 1 V while sequentially changing the polarity of the voltage. The voltage path (A) and the voltage path (B) were combined to form one cycle 406, and a voltage was continuously applied to the liquid crystal cell using one cycle of the waveform as one unit.
The spontaneous polarization of the liquid crystal is 100 nC / cm ² , and the thickness of the alignment film is 60 nm, which is the same as in the first embodiment.

FIG. 4B is a graph showing the transmittance of each cell voltage at each gradation display pulse. The transmittance indicates the final brightness when the optical response of the liquid crystal changes with time during the voltage pulse application time. In particular, the difference in brightness between the voltage path (A) and the voltage path (B) at a voltage of 2 V is improved compared to when there is no “0V” reset period in FIG. Although the measured black level was floating by 2% due to the noise of the photomultiplier tube, the actual black was almost the same as FIG. Even if the reset period is shorter than the gradation display period, gradation display can be performed without any problem. Since the black display time is shortened by the reset period, the brightness of the panel is hardly impaired even if the reset period is provided.

In this embodiment, by using a liquid crystal having a large spontaneous polarization, the response time for resetting to "0 V" is shortened, and the reset period for displaying black is shortened. In order to shorten the response time, it is possible not only to increase the spontaneous polarization but also to raise the panel temperature and decrease the viscosity of the liquid crystal as in a projector panel.

[Embodiment 3] This embodiment is characterized in that it is a driving method for preventing burn-in of liquid crystal and suppressing hysteresis.

The configuration of the experimental cell conforms to FIG. Liquid crystal 60
Reference numeral 5 uses the same liquid crystal as the data obtained in the graphs of FIGS. W to apply arbitrary waveform
A drive waveform is created by a function generator “MODEL275” manufactured by avetek. The driving is a constant current driving, and a charge corresponding to the reversal of the spontaneous polarization of the liquid crystal is supplied within a pulse period. FIG. 14 shows the applied pulse in this experiment. One frame period 1404 includes four subframe periods 1401 to 1403. Gradation display pulse 140
A first period 1402, which is a “0 V” reset period, is provided before a second period for performing gray scale display by 1, 1403, and returns the liquid crystal to the “0 V” reset position. In one frame period (16.6 msec) 1404, voltage pulses 1401 and 14 having the same absolute value and opposite polarities to each other
03 is characterized in that AC driving is performed.

Even when a frame whose 1 frame period is 16.6 msec is divided into four subframes, the response time of the liquid crystal is short, so that the period during the reset period is 4.15 msec.
Then, the display returns to the predetermined reset position, and a good gradation display can be performed. In addition, since the DC component is canceled within one frame period, good gradation display in which image sticking is unlikely to occur even in long-term display can be performed. A reset period may be provided after the gradation display pulse, and the liquid crystal may be returned to the “0 V” reset position after writing the image data with the gradation display pulse.

[Comparative Example 2] In this comparative example, the optical response when the reset voltage is set to 1 V is examined and compared with the case where a reset period of "0 V" is provided.

The configuration of the experimental cell conforms to FIG. For the purpose of comparison, the same cells as those used for data in FIGS. The driving waveform is shown in FIG. Reset voltage is set to 1V. The active matrix substrate is driven by a constant charge, but we are temporarily experimenting with a constant current drive. Waveform that can be applied is Wave
Teck's function generator "MOD
EL275 ". The pulse widths 501 and 502 are set to 8.
3 msec. The voltage path (A) 503 is from + 1V to + 2V, 0V, + 3V, -1V, + 4V, -2V, +5
With V, −3 V, +6 V, −4 V, and +1 V as reset voltages, the applied voltage increases by 1 V for the positive polarity voltage and decreases by 1 V for the negative polarity voltage. This is a direction in which the difference between the voltage values applied to the reset voltage increases.
Conversely, the voltage path (B) 504 has + 4V, -3V, + 3V,
With the reset voltage of −2V, + 2V, −1V, + 2V and + 1V, the applied voltage of the positive polarity voltage decreases by + 1V,
As for the voltage of the negative polarity, the applied voltage increases by -1V. This is a direction in which the difference between the voltage values applied to the reset voltage becomes smaller. The voltage path (A) and the voltage path (B) are combined to form one cycle 505, and a waveform is continuously applied to the liquid crystal cell with one cycle as one unit.

FIG. 5B is a graph showing the relationship between the cell voltage when applied to the liquid crystal cell and the transmittance of the liquid crystal cell. Although the hysteresis due to the voltage path is small, the voltage-transmittance characteristic shifts from 0V to the positive voltage side. When the thresholdless liquid crystal is driven with such a voltage-transmittance characteristic, the direct current component of the positive voltage is integrated and causes burning.

In driving the thresholdless liquid crystal, when a voltage pulse having a single polarity is used as a reset pulse, the voltage-transmittance characteristic shifts. This shows that providing a “0 V” reset period having no polarity is also effective in preventing a shift in the voltage-transmittance characteristic.

[Embodiment 4] The present invention is applied to an active matrix panel and its effect is examined. Panel specification is V
GA (640 × 480 pixels). Pixel pitch is 42
μm × 126 μm. Spontaneous polarization 4 of the data in FIG.
A liquid crystal of 0 nC / cm ² was used, and the thickness of the alignment film was 60 nm.
And

In driving the thresholdless liquid crystal, when the liquid crystal has not completely reversed its spontaneous polarization within the scanning line selection period,
The spontaneous polarization is inverted by the charge stored in the auxiliary capacitance of the thin film transistor. If the auxiliary capacitance is too small, the accumulation of charges in the capacitance using the liquid crystal layer as a dielectric becomes insufficient, and the spontaneous polarization of the liquid crystal cannot be sufficiently reversed, and the white level decreases. For this reason, it is desirable that the auxiliary capacity be large. In this embodiment, an auxiliary capacitor having a relatively large capacity of 0.48 fF / μm ² per pixel unit area is used.

The structure of the thin film transistor substrate of this embodiment will be described as follows. The configuration of the thin film transistor is not limited to the following. Although a thin film transistor of a top gate type is shown in this embodiment, a thin film transistor of a bottom gate type may be used.

In FIG. 15, a glass substrate such as barium borosilicate glass or alumino borosilicate glass typified by Corning # 7059 glass or # 1737 glass can be used as the substrate 1501. In order to avoid the influence of mobile ions such as sodium ions on the glass substrate, a base film (not shown) can be formed on the glass substrate.

On a glass substrate, there are active layers 1502 to 1504 each having a thickness of 25 to 80 nm (preferably 30 to 60 nm) and made of a silicon semiconductor film of a thin film transistor. For the active layer of the thin film transistor, either a-Si (amorphous silicon) or poly-Si (polycrystalline silicon) can be used. In this embodiment, the resistance of polycrystalline silicon is smaller than that of amorphous silicon. Polycrystalline silicon is used because the write current can be increased. The write current is 1 μA at 5V. The active layer doped with impurities is used as a capacitance electrode of the auxiliary capacitance 1525. The active layer is provided with a mask such as a resist as necessary, and is doped with an impurity to form an impurity region and a channel region. In FIG. 15, an intrinsic semiconductor layer region 150 is formed in the active layer.
2, n-type impurity region 1503, p-type impurity region 1504
Is provided.

[0086] The gate insulating film 1505 is formed as an insulating film containing silicon by a plasma CVD method or a sputtering method. The gate insulating film is simultaneously formed with the auxiliary capacitance 152.
It is desirable to use a silicon nitride film having a dielectric constant as high as about 7 because it becomes an insulating film when forming 5. The thickness of the gate insulating film is desirably small in order to increase the auxiliary capacitance, but cannot be too small in order to prevent short circuit and dielectric breakdown. In this embodiment, the thickness of the gate insulating film is 30 nm. In this embodiment, the area of the auxiliary capacitance is 784 μm ² . When calculated from the film thickness and dielectric constant of the insulating film material, the auxiliary capacitance of this embodiment is 376 fF per pixel.
It is. With this capacity, the VGA-level panel can drive the spontaneously polarized liquid crystal of this embodiment.

The heat-resistant conductive layer 1 is formed on the gate insulating film 1505.
506, forming a scanning electrode and a capacitance electrode of an auxiliary capacitance. The heat-resistant conductive layer may be formed as a single layer, or may be formed as a multilayer structure including a plurality of layers such as two layers or three layers as necessary. The conductive layer is formed of an element selected from tantalum (Ta), titanium (Ti), molybdenum (Mo), and tungsten (W), an alloy containing the above-described element as a main component, or an alloy film combining the above-described elements (typically, May be formed of a Mo—W alloy film or a Mo—Ta alloy film). In this embodiment, the heat-resistant conductive layer 1506 is made of 350 tantalum.
nm. In this embodiment, the heat-resistant conductive layer 1506 is used.
And the active layer intersects a plurality of times, and has a plurality of intrinsic semiconductor regions.

[0088] A protective insulating film 1507 is provided over the heat-resistant conductive layer 1506 and the gate insulating film 1504. The protective insulating film may be formed using a silicon oxide film, a silicon oxynitride film, a silicon nitride film, or a stacked film including a combination thereof. In any case, the protective insulating film 1507 is formed from an inorganic insulating material. The thickness of the protective insulating film 1507 is 100 to
It is set to 200 nm.

On the protective insulating film 1507, there is an interlayer insulating film 1508 made of an organic insulating material. Since organic resin materials generally have a low dielectric constant, parasitic capacitance can be reduced. However, since it is hygroscopic and not suitable as a protective film, it is necessary to use it in combination with a silicon oxide film, a silicon oxynitride film, a silicon nitride film, or the like formed as the protective insulating film 1507 as in this embodiment.

The orientation of the thresholdless liquid crystal is greatly affected by surface irregularities. Since an alignment defect is induced by a difference in pretilt due to surface irregularities, it is desirable that the alignment surface of the liquid crystal is as flat as possible. Therefore, it is desirable to form an interlayer insulating film made of an organic insulating material having a planarizing effect on the protective insulating film. Interlayer insulating film 150
The thickness of 8 is desirably 1000 to 6000 nm. As the thickness of the interlayer insulating film 1508 is larger, the flattening effect of the liquid crystal alignment surface is higher. Therefore, it is desirable that the thickness of the organic resin film be as large as possible within a range where there is no major problem in the process.

A contact hole is made in the interlayer insulating film 1508, and a transparent pixel electrode 1510 made of ITO or the like is patterned to have a voltage control function for liquid crystal alignment. A signal electrode 1509 is patterned on the same insulating film. It is desirable to use a metal having a small resistance value as the signal electrode in order to prevent the signal waveform from being rounded. From the above, the first p-channel TFT 1520 and the second n-channel TFT 1
521, a second p-channel TFT 1522, a sampling circuit 1527 having a second n-channel TFT 1523, a pixel TFT 152
4. A storage capacitor 1525 is formed. Logic circuit part 1
526 and the sampling circuit section 1527 are formed in the driving circuit section 1528 of the active matrix substrate, and the pixel T
The FT 1524 and the storage capacitor 1525 are formed in the pixel portion 1529.

Counter substrate 1 of active matrix panel
The 511 may be provided with a black matrix (not shown) in order to prevent light leakage at a black level due to an alignment defect of the liquid crystal. In particular, since thresholdless liquid crystal tends to have large alignment defects on a surface having unevenness, it is preferable to provide a black matrix at a place where the surface unevenness of the thin film transistor is large. As a material of the black matrix, there are chromium, chromium oxide, and resin BM. When the resin BM is employed in the direct-view panel, reflection due to the reflection of light by the black matrix can be avoided, and a panel without glare can be obtained. In a projector panel, adopting a metal black matrix having a high reflectance can reduce light leakage to the active layer by reflecting unnecessary light. In any case, in the case of the thresholdless liquid crystal, it is necessary to prevent the unevenness due to the thickness of the black matrix from affecting the liquid crystal alignment.

Further, a transparent conductive film 151 having a function of controlling the alignment by applying an electric field to the liquid crystal is provided on the opposite substrate 1511.
There are two. As the transparent conductive film, there is ITO or the like. In any case, it is desirable that the surface unevenness is small for the alignment of the thresholdless liquid crystal. ITO with a film thickness of about 2
In the case of, when the wavelength is set to 100 nm to 120 nm, the transmittance in the visible light region can be increased depending on the interference condition.

A column spacer (not shown) is formed on the opposing substrate to maintain the cell gap of the panel.
When the diameter of the spacer material increases, the alignment defects of the liquid crystal induced by the spacer tend to increase. Therefore, it is desirable that the diameter of the columnar spacer be as small as possible. By arranging the columnar spacer on the contact hole of the pixel electrode of the thin film transistor substrate, it is possible to cover the unevenness on the surface of the thin film transistor.

As the cell gap maintaining means, not only the columnar spacer but also a wall spacer (not shown) can be employed for the purpose of preventing the alignment defect of the liquid crystal. By designing the wall spacer so as to be located at an uneven position such as a boundary between pixel electrodes on the thin film transistor substrate, the unevenness on the thin film transistor surface can be covered, and the liquid crystal alignment surface can be made as flat as possible. In this embodiment, the spacer material is patterned and provided on the opposing substrate. However, the spacer material may be formed on the opposing substrate or on an active matrix substrate.

When color display is performed by an active matrix panel, it is necessary to appropriately provide a color filter (not shown) for a direct-view panel and a single-panel projector panel. However, in this case, there is a possibility that the alignment defect of the liquid crystal is induced by the unevenness of the portion where the color filter resins of different colors overlap, so it is necessary to provide an overcoat agent on the color filter and sufficiently planarize the liquid crystal alignment surface. is there.

An alignment film 1513 is formed by an alignment film printing method. The thickness of the alignment film is made as thin as 60 nm, with the aim of suppressing voltage loss due to the alignment film.

Rubbing is performed after pre-baking and post-baking of the alignment film. In the thresholdless liquid crystal, an alignment defect is generated due to dripping of the tip of the rubbing cloth, and the black level is reduced. For this reason, it is necessary to determine the material of the rubbing cloth and the number of times of aging to find a condition that can obtain a good orientation. According to the experimental results of the applicant, the rubbing cloth tends to have a uniform rubbing state and a good black level because of the uniform hair tips of the rubbing cloth than the cotton. The rubbing cloth which has been aged to some extent tends to have less frayed hair tips and fewer orientation defects.

A seal pattern 1514 is formed between a pair of substrates. It is preferable to use a sealant that can be crushed to 1.5 to 2.0 μm by a cell assembly hot pressing process.

The thresholdless liquid crystal 1515 is injected into the liquid crystal cell. The thresholdless liquid crystal has a high viscosity, and injection at room temperature takes time. It is preferable to perform the liquid crystal injection at a temperature higher than the isotropic phase. However, in the vacuum heating injection, the low viscosity component of the liquid crystal tends to volatilize due to the conditions of high vacuum and high temperature, and the composition itself of the liquid crystal changes,
It may affect the orientation of the liquid crystal. It is necessary to determine the conditions of the vacuum injection program and injection method, take care not to expose the liquid crystal to high vacuum and high temperature as much as possible, and to suppress the volatilization of low viscosity components. In this embodiment, the liquid crystal is injected in an isotropic phase or higher, and after the injection, the liquid crystal is gradually cooled to perform a realignment treatment. After the realignment, the liquid crystal injection port is sealed with a UV curing resin (not shown).

Next, a flexible printed wiring board (FP)
C: Flexible Printed Circuit
t) (not shown) is attached, and an external signal is input from the FPC to drive the liquid crystal panel. An electric field is applied to the liquid crystal / alignment film by the pixel electrode 1510 connected to the thin film transistor and the pixel electrode 1511 provided on the substrate facing the thin film transistor.

In the case of driving the thresholdless liquid crystal in driving the active matrix panel, line-sequential driving is performed.
Image data is written for each scanning line. In the line-sequential driving, the writing time of image data is longer than that in the point-sequential driving, and a writing current required for reversing the spontaneous polarization can be input for a long time.

When displaying a moving image on the liquid crystal panel, one image is written in one frame period of 16.6 msec. FIG. 16 shows a driving waveform in one frame period. In one frame period 1601 for displaying one image, the sub-frame 160
2, 1603 are provided. Each of the two subframes is 8.3 msec. Calculating from the number of VGA pixels, one scanning line selection period 1604 in each subframe is 17.2 μsec. The first subframe 1602 is a “0 V” reset period, that is, a first period, and the signal voltage 1606 becomes 0 V. The second sub-frame 1603 is a gradation display period, that is, a second period,
During a scanning line selection period 1605, a gradation display pulse 1607 of a signal voltage corresponding to image data is input to each pixel from a signal electrode. In an actual moving image display, a gradation display pulse 1607 corresponding to image data changes a voltage value in each frame,
Form a continuous image.

In the thresholdless liquid crystal of this embodiment, the maximum transmittance of the brightness obtained when the white level of the liquid crystal is saturated at the cell voltage of 3.5 V in the experimental cell and the voltage is increased by the voltage-transmittance characteristic. However, in the active matrix driving, the cell voltage (the voltage applied to the alignment film and the liquid crystal) and the potential of the auxiliary capacitor decrease due to the reversal of spontaneous polarization, and the cell voltage becomes lower than the signal voltage of the thin film transistor. I will. To make the cell voltage 3.5V, the signal voltage is 6V.
Need to be raised. For this reason, the relationship between the signal voltage and the transmittance has a more gradual threshold characteristic than the relationship between the liquid crystal cell voltage and the transmittance, and gradation can be easily obtained.

The liquid crystal was driven by using a constant charge driving active matrix panel and providing a reset period of “0 V” with the driving waveform of FIG. 16, but the effect of improving the hysteresis tested in the constant current driving was the same in the active matrix driving. Can be confirmed. In addition, a good gradation can be obtained even with an alignment film thickness as small as 60 nm, which is optimized to prevent a voltage loss of the alignment film and to obtain a white level in active matrix driving with a limited driving voltage, and in which hysteresis is likely to occur.

Although the liquid crystal was driven by providing a “0 V” reset period in the active matrix panel driven by the constant charge, it was confirmed that the effect of improving the response speed tested by the constant current drive was also achieved by the active matrix drive. As compared with the driving method without the reset period, the provision of the “0 V” reset period enables smooth switching of the halftone display when displaying a moving image, and the response time is improved.

The embodiment of the present invention is not limited to the liquid crystal material, spontaneous polarization and alignment film thickness of the thresholdless liquid crystal of this embodiment.

When the liquid crystal is driven by providing an auxiliary capacitor in the thin film transistor, the voltage is accumulated in the capacitor using the liquid crystal layer as a dielectric and the auxiliary capacitor connected in series to the active element according to the voltage value when the liquid crystal is driven. The charges are different. However, by inserting the “0 V” reset period, the charge accumulated in the capacitor having the liquid crystal layer as a dielectric and the auxiliary capacitor connected in series to the active element can be eliminated, and the gradation in the next frame is applied. It can be displayed according to the voltage.

In recent years, among smectic liquid crystals, when a voltage of the first polarity is applied, a constant black level is shown, and when a voltage of the other polarity, that is, a voltage of the second polarity, is applied, the liquid crystal is changed according to the voltage value. A liquid crystal in which the transmittance of a cell continuously changes is known. Such a characteristic is conveniently called a Half-V characteristic. R2402 (LZ-972) and R24 manufactured by Clariant as liquid crystals exhibiting Half-V characteristics.
01 (LZ-972). In the liquid crystal exhibiting the Half-V characteristic, the inversion of the spontaneous polarization does not occur when the voltage of the first polarity is applied, and the inversion of the spontaneous polarization occurs only when the voltage of the second polarity is applied. As long as a liquid crystal having a Half-V characteristic is used, a constant charge is injected into a capacitor having a liquid crystal layer as a dielectric regardless of the value of the first polarity voltage when AC driving is performed. The same gradation can be displayed.
That is, in the liquid crystal exhibiting the Half-V characteristic, a “0 V” reset period, that is, a first period is provided during driving.
It is not necessary to extinguish the charges stored in the auxiliary capacitor and the capacitor using the liquid crystal layer as a dielectric.

In the present invention, when an auxiliary capacitor is connected in series to an active element, for example, a thin film transistor, and a thresholdless liquid crystal is driven, a “0 V” reset period, that is, a first period is provided to allow the auxiliary capacitor and the liquid crystal layer to be connected. The electric charge stored in the capacitor serving as the dielectric is eliminated. From the description in the preceding paragraph, such a configuration is particularly effective for smectic liquid crystals, in which the transmittance is uniquely determined when a voltage having the same absolute value and different polarity is applied to the liquid crystal layer, for example, a thresholdless liquid crystal. It can be seen that it is.

The active matrix substrate and the liquid crystal display device manufactured according to the present invention can be used for various electro-optical devices. The present invention can be applied to all electronic devices incorporating such an electro-optical device as a display medium. Examples of the electronic device include a personal computer, a digital camera, a video camera, a portable information terminal (such as a mobile computer, a mobile phone, and an electronic book), and a navigation system.

[Embodiment 5] In this embodiment, the thickness of the alignment film and the manner of occurrence of hysteresis due to spontaneous polarization of the liquid crystal are shown by experimental data. In this embodiment, the thickness of the alignment film is 30
nm, 75 nm, 110 nm, and 220 nm. As the alignment film, RN1286 manufactured by Nissan Chemical Industries, Ltd. is used.

The rubbing conditions for the alignment film are as follows.
mm, the number of roll rotations is 100 rpm, the diameter of the rubbing roll is 130 mmφ, the stage speed is 10 mm / sec,
The number of times of rubbing is one.

The liquid crystal is MX having a spontaneous polarization of 40 nC / cm ² .
-Z19 and MX-Y having a spontaneous polarization of 100 nC / cm ²
102 is used. All liquid crystals are manufactured by Mitsubishi Gas Chemical Company.

Since both liquid crystals MX-Y102 and MX-Z19 have a phase transition temperature of 96 ° C. from the SmA (smectic A) phase to the isotropic phase, these liquid crystals were injected in an isotropic phase of 100 ° C. Place the liquid crystal cell on the hot plate,
After heating and injecting the liquid crystal, 1 ° C./min to 2 ° C./m
The mixture was gradually cooled to room temperature in.

The value of the cell gap was in the range of 1.5 μm ± 0.5 μm in all the liquid crystal cells.

The liquid crystal cell was driven using a function generator “MODEL 275” manufactured by Wavetek. The appearance of the hysteresis changes depending on the level of the frequency.However, since it was generally said that the hysteresis in the triangular wave driving at a frequency of 0.1 Hz can be approximately approximated to the magnitude of the hysteresis generated when driving in an actual liquid crystal display device, The frequency for driving the cell was 0.1 Hz.

A liquid crystal cell was provided between polarizing plates having transmission axes arranged in crossed Nicols, and electro-optical characteristics when a voltage was applied to the liquid crystal cell were measured using a photomultiplier tube. The data displayed on the oscilloscope screen will be described with reference to FIG. The X axis indicates the value of the cell voltage applied to the liquid crystal cell. When the X coordinate is a positive value, it indicates that a positive voltage is applied to the liquid crystal cell. A negative value of the X coordinate indicates that a negative voltage is applied to the liquid crystal cell. The Y-axis shows the amount of photoelectrons incident on the photomultiplier tube converted into a voltage. That is, the Y-axis 2002 is the brightness measured using the photomultiplier tube as a light receiver, and is conveniently referred to as the transmittance of the liquid crystal cell. When the Y coordinate is 0 mV, it indicates that the liquid crystal cell is displaying black, and that no photoelectrons are incident on the photomultiplier.

In the thresholdless liquid crystal, the transmittance gradually increases due to the birefringence effect when the absolute value of the voltage applied to the liquid crystal cell is gradually increased, and the transmittance of the liquid crystal cell becomes constant when the applied voltage is further increased. become. In the experiment, the liquid crystal cell was driven within a range that does not exceed a voltage value at which the transmittance of the liquid crystal cell becomes constant. This is because, if the absolute value of the voltage applied to the liquid crystal is too large, the orientation of the liquid crystal becomes non-uniform due to electric energy, and hysteresis tends to occur.

FIG. 18 shows M having a spontaneous polarization of 40 nC / cm ² .
13 shows electro-optical characteristics of a liquid crystal cell using XZ19. 18A shows a film thickness of the alignment film of 30 nm, FIG. 18B shows a film thickness of 75 nm, FIG. 18C shows a film thickness of 110 nm, and FIG. When the presence or absence of hysteresis was confirmed by changing the thickness of the alignment film, it was found that the alignment film had a thickness of 30 nm to 110 nm.
It was confirmed that there was hysteresis in the range of nm.

FIG. 19 shows the electro-optical characteristics of a liquid crystal cell using MX-Y102 having a spontaneous polarization of 100 nC / cm ² . FIG. 19A shows that the thickness of the alignment film is 30 nm and FIG.
(B) is 75 nm, FIG. 19 (C) is 110 nm, FIG.
(D) is 220 nm. Alignment film is 30nm ~ 75n
It was confirmed that there was hysteresis in the range of m.

In FIGS. 18 and 19, the difference between the maximum values of the transmittances of the respective liquid crystal cells is mainly due to the difference in the retardation of the liquid crystal due to the difference in the cell gap.

That is, the experiment showed that the spontaneous polarization of the liquid crystal was 40
In ^{nC / cm 2 ~100nC / cm 2} , the alignment film is 30nm
It was confirmed that hysteresis appeared in the range of 〜75 nm. Considering that hysteresis is more likely to occur as the thickness of the alignment film is more likely to appear, and that hysteresis is more likely to occur as the spontaneous polarization of the liquid crystal is smaller, the thickness of the alignment film is 75 n.
m and the spontaneous polarization of the liquid crystal is 100 nC / cm ²
It was confirmed that hysteresis easily occurred in the following cases.

Of course, as in the present invention, before or after the second period in which the voltage corresponding to the gradation is applied to the liquid crystal,
By providing the first period for applying the voltage of V, the effect of reducing the hysteresis even in a liquid crystal cell in which the thickness of the alignment film is 75 nm or less and the spontaneous polarization of the liquid crystal is 100 nC / cm ² or less. There is.

[0125]

According to the present invention, the thresholdless liquid crystal is used as a reference position,
That is, by switching from the voltage value of 0V,
Hysteresis of thresholdless liquid crystal is reduced.

First, by introducing a reset period of "0 V", hysteresis is hardly generated even with a spontaneously polarized material that easily undergoes hysteresis and an oriented film thickness, and good gradation display can be performed. Since the thresholdless liquid crystal tends to exhibit hysteresis as the spontaneous polarization is smaller, the present invention is more effective with a material having a smaller spontaneous polarization (40 nC / cm ² or less). The thresholdless liquid crystal is more effective for a panel having a thin alignment film because the hysteresis easily occurs when the alignment film thickness is small.

Second, the effect of improving the response speed can be seen by providing the “0 V” reset period. It has a spontaneous polarization of 4
It is particularly effective for a liquid crystal as small as 0 nC / cm ² or less. It is particularly effective when switching between halftones of the liquid crystal is slower than when switching between halftones is performed via “0 V”.

Thirdly, in active matrix driving in which the driving voltage is limited, when the alignment film thickness is reduced in order to prevent a voltage loss due to the alignment film, or when the thresholdless liquid crystal is used in order to prevent a voltage drop due to a decrease in the potential of the auxiliary capacitor. When the spontaneous polarization is reduced, hysteresis is likely to occur. However, by introducing a reset period of “0 V”, it is possible to suppress hysteresis and perform favorable gradation display. The cell conditions used in the active matrix driving will be described taking a thin film transistor as an example. If the spontaneous polarization is 40 to 150 nC / cm ² and the orientation film is 15 to 75 n
This is the case where the orientation film is as thin as 75 to 150 nm at m or spontaneous polarization of 20 to 40 nC / cm ² , but under such conditions, the use of the present invention can suppress the hysteresis.

Fourth, since the reset period voltage is "0 V" as compared with the conventional drive which aims to reduce the hysteresis with the conventional thresholdless liquid crystal, the spontaneous polarization of the liquid crystal is not canceled out on average, and burn-in caused by the reset period is eliminated. Can be suppressed. In addition, since the display during the reset period is at the black level, even when black is displayed in gradation, color mixture due to the display during the reset period does not occur, and stable black can be obtained.

Fifth, the present invention is characterized in that a "0 V" reset period is provided to prevent a shift in voltage-transmittance characteristics. That is, when the reset voltage is set to +1 V as shown in Comparative Example 1, the voltage-transmittance characteristic tends to shift to the positive voltage side. If the voltage-transmittance characteristic shifts to the positive voltage side, a bias of the positive polarity voltage occurs, which leads to burn-in. Reset voltage to "0"
V ", and it is desirable to prevent the shift of the voltage-transmittance characteristic as much as possible.

Sixth, in the present invention, if the liquid crystal returns to the approximate reset position within the “0 V” reset period, the effect of reducing the hysteresis can be exerted. Therefore, it is assumed that the liquid crystal returns to the approximate reset position within the “0 V” reset period. It is also possible to change the driving method as described below. The “0 V” reset period can be freely set according to the response speed of the liquid crystal. That the liquid crystal is "0V" response time is reset to (T _3), may be short relative to the reset period _{(T 1) (T 3 <} T 1) for a reset period (T ₁₎ a gradation display period (T ₂ ) can be shortened (T ₁ <T ₂ ), and the bright display period can be lengthened. In the present invention, the effect is exhibited when the liquid crystal returns to the approximate reset position within the “0 V” reset period. Therefore, it is desirable that the response speed of the liquid crystal is relatively high in the driving voltage range.
The response speed of the liquid crystal returning to the “0 V” reset position can be increased by lowering the viscosity of the liquid crystal on a panel having a high environmental temperature such as a projector panel. In order to prevent burn-in, it is also possible to provide voltage pulses of opposite polarity in one frame period.

Seventh, in the present invention, by providing the “0 V” reset period, when driving the liquid crystal by providing the auxiliary capacitance in the thin film transistor, the data is stored in the liquid crystal layer and the auxiliary capacitance determined by the gradation of the previous frame. The charge can be canceled by the “0 V” reset period. For this reason, good gradation display can be performed regardless of the voltage value of the previous frame.

[Brief description of the drawings]

FIG. 1 shows an example of a conventional driving method.

FIG. 2 shows a driving waveform and a driving result when a “0 V” reset period is not provided in Comparative Example 1.

FIG. 3 shows a driving waveform and a driving result when a “0 V” reset period of the present invention is provided.

FIG. 4 shows a driving waveform and a driving result when a “0 V” reset period of Example 2 is shortened.

FIG. 5 shows a driving waveform and a driving result when a “1V” reset period of Comparative Example 2 is provided.

FIG. 6 shows a liquid crystal cell used in an example.

FIG. 7 shows a hysteresis curve of a tristable antiferroelectric liquid crystal and a bistable ferroelectric liquid crystal.

FIG. 8 shows a configuration of a simple matrix panel.

FIG. 9 shows characteristics of hysteresis of a thresholdless liquid crystal.

FIG. 10 illustrates a circuit of an active matrix panel using thin film transistors as an example of a panel for driving thresholdless liquid crystal.

FIG. 11 shows a driving waveform and a driving result when a “0 V” reset period is provided for driving a thresholdless liquid crystal having a small spontaneous polarization in Example 1.

FIG. 12 shows a response time of the thresholdless liquid crystal having a small spontaneous polarization in Example 1.

FIG. 13 shows actual optical response data of the thresholdless liquid crystal when there is no “0V” reset period in Comparative Example 1.

FIG. 14 shows a driving example of a third embodiment of the present invention for preventing image sticking.

FIG. 15 shows an active matrix panel of Example 4.

FIG. 16 illustrates an active matrix panel according to a fourth embodiment.
4 shows a driving waveform during a frame period.

FIG. 17 shows data of the optical response of the thresholdless liquid crystal when the “0 V” reset period of the present invention is provided.

FIG. 18 shows the dependence of the electro-optical properties of the thresholdless liquid crystal MX-Z19 of Example 5 on the thickness of the alignment film.

FIG. 19 shows the dependency of the electro-optical characteristics of the thresholdless liquid crystal MX-Y102 of Example 5 on the thickness of the alignment film.

FIG. 20 shows the relationship between the cell voltage and the transmittance of the liquid crystal cell shown in FIGS. 18 and 19 in Example 5.

[Explanation of symbols]

101 Scan line selection period 102 Black display by gradation display pulse 103 Setting pulse 104 White level due to setting pulse 201, 303, 404, 503, 1103 Voltage path A
(Direction of increasing absolute value of applied voltage) Voltage path B 202, 304, 405, 504, 1104
(Direction of increasing the absolute value of applied voltage) 203, 305, 406, 505, 1105 One cycle of experimental waveform 204 AC pulse 205-208 Voltage pulse 209-212 Transmittance of voltage pulse 301, 401, 501, 1101, 1402 "0
V "reset period 302, 402, 502, 1102, 1401, 140
3 Gradation display pulse 403 One frame of image display 601 Quartz substrate 602 ITO 603 Low pretilt alignment film 604 Sealant 606, 607 Rubbing direction of thresholdless liquid crystal 608, 609 Polarizer 701 Definition of hysteresis width 1001 Signal of active matrix panel Line 1002 scanning line of active matrix panel 1003 thin film transistor of active matrix panel 1004 liquid crystal layer 1005 auxiliary capacitance 1301-1306, 1701-1705 optical response of liquid crystal 1501 glass substrate 1502 intrinsic semiconductor layer 1503 n-channel region 1504 p-channel region 1505 gate insulating film 1506 Heat resistant conductive layer 1507 Protective insulating film 1508 Organic resin interlayer insulating film 1509 Signal electrode 1510 Transparent pixel electrode 1511 Opposite substrate 1 512 Transparent conductive film 1513 Alignment film 1514 Seal pattern 1515 Thresholdless liquid crystal 1601 One frame period 1602 First sub-frame 1603 Second sub-frame 1604, 1605 Scan line selection period 1606 Signal voltage of "0 V" reset period 1607 Gradation Display pulse signal voltage

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G09G 3/20 621 G09G 3/20 642A 642 3/36 3/36 G02F 1/137 510

Claims (16)

[Claims] [1 claim] and an alignment film on the substrate, and a liquid crystal on the alignment layer, the liquid crystal is continuously switched in response to an applied electric field having a chiral smectic C _R phase, the liquid crystal is black level And a second period in which gradation is displayed by applying a voltage to the liquid crystal, and the second period comes before or after the first period. For driving a liquid crystal display device. And alignment film 2. A substrate, and a liquid crystal on the alignment layer, the liquid crystal is continuously switched in response to an applied electric field having a chiral smectic C _R phase, spontaneous polarization of the liquid crystal And a second period in which a voltage is applied to the liquid crystal to perform a gray scale display, wherein the second period comes before or after the first period. Driving method for a liquid crystal display device. 3. A alignment film on the substrate, and a liquid crystal on the alignment layer, the liquid crystal is continuously switched in response to an applied electric field having a chiral smectic C _R phase, 0V of the liquid crystal A first period in which a voltage is applied, and a second period in which a voltage is applied to the liquid crystal to perform a gray scale display.
Wherein the period is before or after the first period. 4. The method according to claim 1, wherein
A method for driving a liquid crystal display device, comprising a plurality of active elements on the substrate. 5. The method according to claim 4, wherein the active element applies a voltage to the liquid crystal, and the value of the voltage has an upper limit. 6. The method according to claim 5, wherein the upper limit value of the voltage is an absolute value of 7 V or less. 7. The method according to claim 1, wherein
The method of driving a liquid crystal display device, wherein the liquid crystal has a spontaneous polarization of 40 nC / cm ^{2 to} 150 nC / cm ² , and the alignment film has a thickness of 15 nm to 75 nm. 8. The method according to claim 1, wherein
The method of driving a liquid crystal display device, wherein the liquid crystal has a spontaneous polarization of ²⁰ nC / cm ^{2 to} 40 nC / cm ² , and the alignment film has a thickness of 30 nm to 150 nm. 9. The method according to claim 1, wherein
A method for driving a liquid crystal display device, wherein the liquid crystal has a spontaneous polarization of 40 nC / cm ² or less. 10. The liquid crystal display device according to claim 1, wherein a response time of the liquid crystal between a first voltage and a second voltage having a different polarity from the first voltage is a first response time. The response time of the liquid crystal when a voltage of 0 V is applied between the first voltage and the second voltage is defined as a second response time, and the first response time is smaller than the second response time. 5
A method for driving a liquid crystal display device, wherein the driving method is at least twice as long. 11. A driving method for a liquid crystal display device according to claim 4, further comprising an auxiliary capacitor connected in series to said active element. 12. A liquid crystal display device comprising: a thin film transistor provided on a substrate; an auxiliary capacitor connected in series to the thin film transistor; an alignment film above the thin film transistor; and a liquid crystal on the alignment film. Has a spontaneous polarization, switches continuously according to an applied electric field, and applies a voltage of 0 V to the liquid crystal;
A second period during which gradation is displayed by applying a voltage to the liquid crystal, wherein the second period comes before or after the first period. 13. A driving method for a liquid crystal display device according to claim 12, wherein a transmittance is uniquely determined when voltages of the same absolute value and different polarities are applied to said liquid crystal. 14. A driving method for a liquid crystal display device according to claim 12, wherein when the liquid crystals are applied with voltages of the same absolute value and different polarities, the voltages have the same tilt angle. 15. The method for driving a liquid crystal display device wherein the liquid crystal according to claim 12, characterized by having a chiral smectic C _R phase. 16. The liquid crystal display according to claim 1, wherein the liquid crystal has a spontaneous polarization of 100 nC / cm ² or less, and the alignment film has a thickness of 75 nm or less. How to drive the device.