JP3305990B2 - Liquid crystal display device and driving method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device and a driving method thereof, and more particularly, to an active matrix type liquid crystal display device using a liquid crystal material having a spontaneous polarization induced by an intrinsic or electric field. It relates to the driving method.

[0002]

2. Description of the Related Art Liquid crystal display devices have characteristics such as low power consumption and light weight, and are widely used as displays for word processors, personal computers, car navigation systems and the like. At present, such a liquid crystal display device includes a thin film transistor (T
FT) and other active elements as switching elements, TFT-TN method using nematic liquid crystal, and STN method using twisted angle using nematic liquid crystal as well. Already full-color display of about 10 inches has been achieved. It is used for information terminal displays.

[0003] These liquid crystal display devices have almost satisfactory characteristics for limited applications such as word processors and spreadsheets. However, the STN method is still insufficient in response speed for these applications. Further, the viewing angle is extremely narrow, and an improvement for enlarging the viewing angle using a retardation film or the like is currently being studied, but a sufficient viewing angle has not yet been obtained.

On the other hand, a TN type liquid crystal display device using a TFT or the like as a switching element is almost satisfactory in response speed, but it is difficult to manufacture a larger device in terms of response speed. is expected. Furthermore, the viewing angle is more advantageous than the STN method,
In particular, in the case of full-color display, the display becomes extremely narrow, which is expected to limit the use of this display method.

As a display method for solving such a problem of the liquid crystal display device, a ferroelectric liquid crystal FLC (Ferroe) has recently been used.
lectric Liquid Crystal), anti-ferroelectric liquid crystal AFLC
(AntiFerroelectric Liquid Crystal), DHF (Dis
torted Helical Ferroelectric Liquid Crystal, Twisted Ferroelectric Liquid Cry
stal), TLAF (Thresholdless Antiferroelectric)
A display method using a liquid crystal material having a spontaneous polarization induced by applying an electric field, such as a liquid crystal, has attracted attention.

[0006] As a display system using such a liquid crystal material, a surface stabilized ferroelectric liquid crystal (Surface Stabilized F liquid crystal) disclosed by Clark and Lagabal in 1980 is known.
erroelectric Liquid Crystal: SSFLC, NA Clark and
ST Lagerewall Appl. Phys. Lett., 36, 899 (1980)).

According to this method, the response speed is increased by two to three orders of magnitude, and the viewing angle is increased like a cathode ray tube. This method uses chiral smectic C of smectic liquid crystal.
Solving the helical structure of the phase by the interaction between the alignment film and the liquid crystal,
The switching is performed by the torque generated by the interaction between the spontaneous polarization generated at that time and the electric field.

In this method, only two states in which spontaneous polarization is directed in two directions perpendicular to the interface of the alignment film are stabilized. Therefore, the method has a memory property. At first, TFT, TFD (thin film diode), MIM, etc. It has been greatly expected as a display system that does not require a switching element composed of a nonlinear active element.

However, in this method, since only two states are used, it is impossible to display a halftone. However, considering the display in the future, the display of the halftone is indispensable, and several examples of studies for obtaining the display of the halftone are currently known.

As one of them, there are several attempts to perform halftone display using the above-mentioned surface stabilized ferroelectric liquid crystal (for example, WJAMHarmann, Ferroelectrics, 199).
1, 122, p1). However, since the surface-stabilized ferroelectric liquid crystal exhibits discontinuous switching whose response is called domain inversion, it can be said that halftone display cannot be performed without using an active element.

On the other hand, an antiferroelectric liquid crystal is used, and a display is performed by using the antiferroelectric liquid crystal phase (SmCa phase) (A.
DLChandani, T.Hagiwara, T.Suzuki, Y.Ouchi, H.Tak
ezoeand A. Fukuda, Jpn. J. Appl. Phys., 27L729 (198
8)) is known. In this system, the ferroelectric liquid crystal 2
In addition to the two stable states, it adopts an antiferroelectric liquid crystal structure when no voltage is applied.In recent years, it has been announced that this method enables halftone display without using switching elements consisting of active elements. (N.Koshoubu, K.Mo
ri, K. Nakamura, Y. Yamada, Ferroelectrics, 1993, 14
9, p295).

In recent years, a display device using a chiral smectic C phase in combination with a switching element composed of an active element has been proposed. Specifically, a method using DHF (J. Funfschilling and M. Sc)
hadt, J. Appl. Phys. 66 (8), 15), a method using TFLC (JSPate, Appl. Phys. Lett. 60 (3), p280), or TLAF (Thresholdless Antiferroelectric Liqu
id Cryatal) has been proposed.

A display device using these methods is inferior to the above methods in terms of cost because a switching element composed of an active element is also used. However, this method is superior to the above methods in the following points.

First, the reliability of halftone display is excellent. In other words, these methods do not cause a problem that the change in transmittance with respect to the applied voltage is relatively gentle, and halftone display becomes difficult as in the case of the surface stabilized ferroelectric liquid crystal. Second, the liquid crystal material of this system can be driven at a low voltage (0 to 5 V), and can be a liquid crystal display device with low power consumption. Third, the display device of this type is resistant to mechanical shock, and does not cause alignment breakdown due to mechanical shock unlike the surface stabilized ferroelectric liquid crystal.

Here, the orientation and electric field of a thresholdless antiferroelectric liquid crystal, which is an example of a liquid crystal material having spontaneous polarization induced by application of an intrinsic or electric field (hereinafter, abbreviated as liquid crystal having spontaneous polarization). 17 is shown in FIG. In the state A when no voltage is applied, the molecules 1 of the antiferroelectric liquid crystal are arranged alternately to cancel spontaneous polarization. In this case, the optical axis 2 of the average molecule 1 is in the vertical direction. Therefore, when two polarizing plates are arranged so as to be in the same direction and the orthogonal direction to the optical axis 2 as shown by arrows 3 and 4, a dark state (normally black) is obtained.

However, when a positive voltage or a non-voltage is applied, B
In the state or the C state, the molecules 1 of the antiferroelectric liquid crystal are arranged in one direction in accordance with the direction of the electric field 5, the optical axis 2 is shifted from the polarization direction of the polarizing plate, and a bright state is obtained. That is, the antiferroelectric liquid crystal is different from the nematic liquid crystal in that the arrangement of liquid crystal molecules is different between the application of a positive voltage and the application of a negative voltage.

Further, the non-threshold antiferroelectric liquid crystal has no voltage applied state (A state), positive voltage applied state (B state), and negative voltage applied state (C state) depending on the intensity of the voltage applied between the electrodes. In addition to the three orientations (state), any orientation intermediate between these states is possible. Therefore, although the memory property is poor or not, it is applied to an active matrix type display device in which a switching element including an active element such as a TFT is formed in a plurality of pixels, and a voltage that takes the arbitrary alignment state even during a non-selection period. Is maintained, gradation display is possible.

[0018]

FIG. 18 shows a liquid crystal display device in which a nematic liquid crystal and a liquid crystal having spontaneous polarization are sandwiched between a pixel electrode and a counter electrode, which are arranged in a matrix. In this case, the voltage and light transmittance applied to an arbitrary pixel are shown. In this case, the polarizing plate is arranged to be normally black.

In a nematic liquid crystal, it is assumed that a gate signal 7 is periodically input from a gate line as shown in FIG. In this case, the cycle of the gate signal 7 is the frame frequency fF. On the other hand, as shown in FIG.
A voltage 8 whose polarity is inverted at a period equal to the frame frequency is applied to the signal line (the potential of the counter electrode is displayed as 0 V).

When the gate signal is input and applied to the gate as described above, the switching element is turned on at t1 during that time as shown in FIG. 4C, and the voltage of the signal line is applied to the pixel electrode as a write voltage 9. Supplied, and as shown in (d), the holding voltage 10a of the nematic liquid crystal cell is:
Since the liquid crystal cell and the auxiliary capacitance line function as a capacitor by the voltage supplied to the pixel electrode shown in (c), the holding ratio hardly decreases and is kept almost constant.

That is, when impurities are mixed in the liquid crystal, the holding voltage is lowered, but when a fluorine-based liquid crystal containing almost no ionic impurities is used, it is kept almost constant as in this example. Dripping. The light transmittance of the liquid crystal cell in this case is shown in FIG.

The nematic liquid crystal has a slow response speed, so that the light transmittance 11a rises slowly. However, regardless of whether the voltage held by the pixel electrode is positive or negative, it does not affect the orientation of the liquid crystal, and hence the nematic liquid crystal does not affect the alignment. The light transmittance 11a gradually increases, and becomes substantially constant after several to several tens of frames.

On the other hand, in the liquid crystal having spontaneous polarization, the gate signal 7 from the gate line shown in FIG.
And the voltage 8 applied to the signal line shown in (b) supplies the writing voltage 9 shown in (c) to the pixel. In this case, as shown in (f), the holding voltage 10b of the liquid crystal cell decreases after writing and exhibits extremely poor holding characteristics. In this case, the light transmittance 11b of the liquid crystal cell becomes as shown by the solid line in FIG.

When the liquid crystal cell is statically driven by supplying the write voltage 12 shown in (h), the light transmittance 11c shown by the broken line in (g) is obtained. When the active matrix driving (holding driving) is performed, the light transmittance at the time of ON is significantly reduced as compared with the static driving. As a result, the liquid crystal display device using the liquid crystal having spontaneous polarization has a problem that the contrast is lowered and the display quality is deteriorated.

As a result of a detailed investigation of the problem, the inventors have found that the problem is caused by the following. That is, in the case of the active matrix driving (holding driving), as shown in FIG. 18C, the supply of the voltage for writing in one frame is performed only partially. Usually, the liquid crystal has a slow response time (80 μs or more) as compared with the writing time (typically 64 μm or less).
The alignment change of the liquid crystal molecules does not end within the writing time t1.

Therefore, the remaining time t2 after the end of writing
Also in this case, the change in the arrangement of the liquid crystal molecules continues due to the electric charge held in the auxiliary capacitance, and the holding voltage decreases as shown in FIG. At this time, the liquid crystal molecules cannot change to the alignment obtained by static driving, and therefore, the transmittance is lower than that in static driving. Then, in the next frame, a voltage of the opposite polarity is written.

In a nematic liquid crystal having no spontaneous polarization, liquid crystal molecules respond to the absolute value of the applied voltage.
That is, the arrangement is the same when +5 V is applied and when -5 V is applied. For this reason, 1
Even if the liquid crystal alignment change is insufficient in the second frame,
The alignment of the liquid crystal molecules gradually changes with the frame,
After several tens of frames, the same arrangement is reached as when the same voltage is applied by static driving. That is, after several to several tens of frames, the transmittance is the same as that of the static drive. On the other hand, liquid crystals having spontaneous polarization have different arrangements of liquid crystal molecules depending on the polarity of an applied voltage. That is, the arrangement differs between when +5 V is applied and when -5 V is applied. Therefore, the liquid crystal molecules are arranged in a positive polarity in the first frame from the off state to the on state (because the response speed is low, the arrangement does not reach the arrangement when the same voltage is applied by static driving).

In the second frame, since the polarity is inverted, the liquid crystal molecules pass through the alignment when no voltage is applied from the positive polarity alignment in the first frame, and thus the static driving is performed in the same manner as in the first frame which is turned on from off. Does not reach the sequence obtained in Since the polarity of the subsequent frames is inverted for each frame, the arrangement does not reach the arrangement when the same voltage is applied by static driving. As a result, the transmittance is greatly reduced as compared with the static driving, and a display with low contrast is obtained.

The present invention has been made in view of the above-mentioned problems, and has been made between a pixel electrode and a counter electrode in which liquid crystals having spontaneous polarization induced by application of an electric field or an intrinsic electric field are arranged in a matrix. In a liquid crystal display device and a method of driving the liquid crystal display device, a high-contrast image with good image quality is displayed.

[0030]

SUMMARY OF THE INVENTION A method of driving a liquid crystal display device in which a liquid crystal having a spontaneous polarization induced by applying an intrinsic or electric field is sandwiched between a pixel electrode and a counter electrode arranged in a matrix. In the above, the frame time is defined as TF, and when the polarity inversion of the voltage applied between the electrodes in at least a partial area of the screen is performed every TS time, the following relationship is satisfied: TS / TF ≧ 2.

Further, in a method of driving a liquid crystal display device in which a liquid crystal having spontaneous polarization induced by application of an intrinsic or electric field is sandwiched between a pixel electrode and a counter electrode arranged in a matrix, TF, the response time of the liquid crystal is τ, and the writing time is TK. When the polarity of the voltage applied between the electrodes in at least a part of the screen is inverted every TS time, TS / TF ≧ τ / TK ≧ 2 was satisfied.

Further, when performing the polarity inversion, the writing time for the pixel for which the polarity inversion is performed is set longer than that for the pixel for which the polarity inversion is not performed. Further, the signal amplitude at the time of performing the polarity inversion is made larger than that at the time of not performing the polarity inversion. The polarity inversion performed so as to satisfy the relational expression TS / TF ≧ 2 is performed in one area of the screen, and in the other area, the polarity inversion is performed every frame. The polarity inversion performed so as to satisfy the relational expression TS / TF ≧ 2 is performed intermittently with the elapse of time, and the polarity inversion is performed every frame at other times.

In a driving method of a liquid crystal display device in which a liquid crystal having spontaneous polarization induced by application of an intrinsic or electric field is sandwiched between a pixel electrode and a counter electrode arranged in a matrix. The polarity of the voltage applied to the pixel is inverted only for a part of the pixels of the screen, and the pixels whose polarity is inverted are sequentially changed when rewriting the screen so that the polarity inversion of the entire screen is completed.

The polarity inversion in one screen is performed for at least one pixel. In addition, the polarity inversion in one screen is performed for all pixels located on at least one scanning line. Further, in an arbitrary area of 3 mm × 3 mm in the screen, the polarity inversion is performed so that the ratio of the positive polarity pixel to the negative polarity pixel is 0.5 or more and 2 or less.

Further, a liquid crystal having spontaneous polarization induced by application of an intrinsic or electric field is interposed between a pixel electrode arranged in a matrix and a counter electrode via an alignment film provided on these electrodes. In the method of driving the liquid crystal display device, the surface of the alignment film is made conductive, and is driven DC without inverting the polarity of the voltage applied between the electrodes.

In addition, a liquid crystal having spontaneous polarization induced by application of an electric field or an intrinsic electric field is sandwiched between a pixel electrode arranged in a matrix and an opposing electrode via an alignment film provided on these electrodes. In the driving method of the liquid crystal display device thus obtained, the surface of the alignment film is made conductive, and the polarity of the voltage applied between the electrodes is inverted every frame time that satisfies TS / TF ≧ 2.

The liquid crystal display device according to the present invention comprises: a first substrate;
A plurality of pixel electrodes arranged in a matrix on the first substrate; a second substrate provided to face a surface of the first substrate on which the plurality of pixel electrodes are formed; And a liquid crystal having a spontaneous polarization held between the first substrate and the second substrate on a second substrate, the counter electrode being formed to face the plurality of pixel electrodes. An operation of periodically inverting the polarity of at least a part of a voltage applied between the plurality of pixel electrodes and the counter electrode, and applying the voltage to the pixel electrodes;
A write operation for causing the pixel electrode to hold a display voltage based on the voltage, wherein the polarity inversion operation satisfies TS / TF ≧ 2 when a frame time is TF and a cycle of performing the polarity inversion is TS. Is performed.

It is desirable that the polarity inversion operation is performed so as to satisfy the following condition, where τ is the response time of the liquid crystal and TK is the writing time: TS / TF ≧ τ / TK ≧ 2.

The writing operation includes an operation of making the writing time for the pixel electrode of which polarity is inverted among the plurality of pixel electrodes longer than that of the pixel electrode of which the polarity is not inverted.

[0040] The writing operation may include an operation of increasing a writing voltage for a pixel electrode of which polarity is inverted among the plurality of pixel electrodes as compared with a pixel electrode of which polarity is not inverted.

The operation of inverting the polarity includes an operation of inverting the polarity of the voltage applied between the pixel electrode and the common electrode only for one pixel electrode of one screen, and inverting the polarity. The operation may include a plurality of operations for sequentially changing the part of the pixel electrodes to complete the polarity inversion of the entire screen.

The operation of inverting the polarity may include an operation of inverting the polarity of one screen for at least one pixel. The operation for inverting the polarity is as follows.
The operation of performing the polarity inversion on the screen for all the pixels located on at least one scanning line may be included.

In the operation of performing the polarity inversion, the polarity is inverted so that the ratio of the positive polarity pixel to the negative polarity pixel is 0.5 or more and 2 or less in an arbitrary area of 3 mm × 3 mm in one screen. It is desirable to include actions.

The above-mentioned response time of the liquid crystal is defined as a change in light intensity corresponding to a change in light intensity of 9 when the state is changed from the state where no voltage is applied to the state where voltage is applied or from the state where a voltage is applied to the state where no voltage is applied.
Time to complete 0%.

A specific example of a method for measuring the response time τ of the liquid crystal will be described below. When a backlight is attached to the liquid crystal display element to be measured, it is turned on. If there is no backlight, place it on a suitable light source. In order to facilitate the measurement, a small and simple liquid crystal cell without a TFT or the like may be manufactured and measured on a transmission polarization microscope. At this time, the capacitance between the pixel electrode and the counter electrode (the liquid crystal capacitance and the capacitance of the alignment film and the insulating film) needs to be equal between the simple liquid crystal cell and the liquid crystal display device actually manufactured.

The amount of light transmitted through the liquid crystal display device is measured with a photodiode, a photomultiplier, or a luminance meter. A voltage as shown in FIG. 19A is applied between the pixel electrode and the counter electrode. If the liquid crystal display element has a switching element such as a TFT, it is necessary to turn on the switching element by applying DC 20 V or more to the gate line.

The change in light intensity (transmittance) at this time is shown in FIG.
9 (b). The light intensity at t = 0 is T
1, the light intensity at t = 16.7 ms is defined as T2. The light intensity changes by 90%, that is, the light intensity is 0.9 × (T
If the time required to reach 2−T1) + T1 is obtained, this is the response time τ. If the absolute value of the applied voltage is V and the response times are different from 0 → + V, + V → 0, 0 → −V, −V → 0, the largest value among these is the response time.

[0048]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a structure of a liquid crystal display device according to one embodiment. This liquid crystal display device has a configuration in which a polarity inversion control circuit 20 is added to the configuration of a conventional active matrix type liquid crystal display device using nematic liquid crystal.

That is, in this liquid crystal display device, a signal line driver 25 for driving a liquid crystal display element 24 and a scanning line driver 26 are connected to a display timing controller 23 to which a display signal 21 and a synchronization signal 22 are inputted. A polarity inversion control circuit 20 for appropriately inverting the polarity of the display signal 21 is connected to the timing controller 23.

As shown in FIG. 2, the liquid crystal display element 24 has a matrix-like TFT on the inner surface of one of the pair of glass substrates 28 and 29 opposed to each other.
And a switching element 30 composed of
pixel electrode 31 made of a transparent conductive film such as um Tin Oxide)
And an alignment film 32a made of a polyimide resin or the like is provided on the switching element 30 and the pixel electrode 31.

A color filter 33 is provided on the inner surface of the other second substrate 29, and the color filter 33 is provided on the color filter 33.
A counter electrode 34 made of a transparent conductive film such as TO is provided, and an alignment film 32b made of a polyimide resin or the like is provided on the counter electrode 34.

The ferroelectric liquid crystal FLC and the anti-ferroelectric liquid crystal are placed between the switching element 30 and the pixel electrode 31 provided on the first substrate 28 and the counter electrode 34 provided on the second substrate 29. Liquid crystal AFLC, DHF, TFL
Liquid crystals having spontaneous polarization induced by application of an electric field, such as C and TALF, are sandwiched. Further, the first and second substrates 28 and 29 are formed in a structure in which polarizing plates 35a and 35b are adhered to the outer surfaces. Note that 36 shown in FIG. 2C is a signal line, and 37 is a gate line. In FIG. 2, the Cs (auxiliary capacitance) line is omitted.

In this liquid crystal display device, a display signal and a scanning signal are supplied from the display timing controller 23 to the signal line driver 25 and the scanning line driver 26, respectively, according to the synchronization signal 22 inputted to the display timing controller 23. At this time, the polarity of the display signal is appropriately inverted by the polarity inversion control circuit 20 and the display signal is driven.

The polarity inversion of the display signal is performed by the following (A).
(E).

(A) When the frame time is TF, the polarity of the voltage applied between the electrodes is inverted every TS time that satisfies TS / TF ≧ 2. This polarity inversion causes the response time of the liquid crystal to be τ,
Assuming that the writing time is TK, it is preferable that the writing is performed so as to satisfy TS / TF ≧ τ / TK ≧ 2.

According to this method, the following effects can be obtained. If the frame immediately after the polarity inversion is the first frame and the subsequent frames are the second and third frames, and the response time τ of the liquid crystal is longer than the writing time TK, the response of the liquid crystal molecules is not completed by writing the first frame. In this method, a voltage having the same polarity is applied to the second and subsequent frames, so that the response of the liquid crystal molecules progresses, and the light intensity of the second and subsequent frames is increased more than that of the first frame. As a result, the contrast is improved as compared with AC driving (changing the polarity of each frame).

(B) The writing time for the pixel on which the polarity inversion is performed is made longer than that of the pixel on which the polarity is not inverted.

In the case (a), the light intensity of the first frame is necessarily low because the light intensity is reduced due to the polarity inversion. If the area where the polarity is reversed is large, it means that the area of the region where the light intensity is reduced is large, and the display quality may be degraded due to visual recognition. In the method (b), since the writing time of the pixel whose polarity is inverted is increased, the decrease in light intensity can be reduced. For example, τ = 150μ
When a liquid crystal having a spontaneous polarization of s is driven at Tk = 42 μs, the writing time of only the pixel whose polarity is inverted is 200 μm.
If s, the light intensity does not decrease at the time of polarity inversion.

(C) The amplitude of the signal when the polarity is inverted is made larger than when the polarity is not inverted.

In this method, since the absolute value (signal amplitude) of the voltage applied to the pixel whose polarity is inverted is increased, it is possible to reduce a decrease in light intensity at the time of polarity inversion. For example, a liquid crystal having a spontaneous polarization of τ = 150 μs is expressed by Tk =
Consider the case of driving at 42 μs. When the polarity is inverted from -5V to + 5V, a voltage of (5 + α) V is applied only to the frame (first frame) whose polarity has been inverted. During the writing time, the liquid crystal molecules change to be aligned at (5 + α) V during static driving, but the change in alignment stops halfway because the response is longer than the writing time.

If the arrangement state at this time is an arrangement of +5 V at the time of static driving, the light intensity does not decrease at the time of polarity inversion. If the value of α is determined in advance for all gradations and the signal amplitude is increased by α at the time of polarity inversion, a decrease in light intensity at the time of polarity inversion can be prevented.

When α exceeds the limit of the voltage that can be output by the driver IC, it is desirable to reduce the light intensity at the time of polarity inversion by using the methods (b) and (c) together.

(D) The polarity of the voltage applied between the electrodes is inverted for one pixel of one screen, and the polarity inversion of all pixels is completed by rewriting the screen a plurality of times.

This polarity inversion method includes performing at least one pixel on one screen and performing all the pixels located on at least one scanning line. In these methods, it is preferable that the polarity inversion method be performed on one screen. The operation is performed so that the negative pixels are present at almost the same ratio.

When the polarity of the entire screen is reversed at a time, a decrease in the light intensity at the time of the polarity reversal is easily recognized. By reducing the region where the polarity is reversed as in the method (d), a decrease in light intensity is less likely to be visually recognized. If the polarity is inverted one pixel at a time in one frame, the decrease in light intensity is most difficult to be visually recognized.

When the polarity inversion is partially performed, driving is easy if all the pixels connected to one scanning line (gate line) are used as a unit for performing the polarity inversion. That is, the polarity of the pixel connected to the n-th gate line is inverted in a certain frame, and the polarity of the pixel connected to the m-th gate line is inverted in the next frame. In this case, when the writing at the time of the polarity inversion is lengthened as in (c), it is only necessary to lengthen the ON time of the gate line for which the polarity is inverted.

The operation of inverting the polarity in two or more frames is described as
Ordinary polarity reversal for each frame may be performed in one part of the screen and in another part. Further, the polarity inversion in two or more frames may be performed at a certain time point on the time axis, and the other polarity may be inverted in each frame.

Also, since the liquid crystal having spontaneous polarization has optically uniaxial anisotropy, when the liquid crystal display element is viewed from an oblique direction, the applied voltage is different between when the positive polarity is applied and when the negative polarity is applied. Even if the absolute values are equal, the light intensity and color will change.
In order to reduce this change, it is preferable to drive by signal line inversion or dot inversion, and it is preferable that the ratio of pixels to which positive polarity is applied and the ratio of pixels to which negative polarity is applied are substantially equal. Specifically, it is preferable that the number of pixels of positive polarity / the number of pixels of negative polarity is 0.5 or more and 2 or less in an arbitrary area of 3 mm × 3 mm in one screen.

When the liquid crystal display element is viewed obliquely, the intensity and chromaticity of the transmitted light change depending on the polarity of the voltage applied to the liquid crystal molecules. When signal line inversion or dot inversion is performed, adjacent signal lines or pixels have a wide viewing angle to compensate for a change in intensity or a change in chromaticity of transmitted light when viewed obliquely.

When the polarity of one portion of the screen is reversed, the balance between the positive polarity pixel and the negative polarity pixel is disturbed, and the above-described compensation cannot be performed. In order to prevent this, the polarity inversion is performed so that the ratio of the number of pixels of the positive polarity to the number of pixels of the negative polarity is 0.5 or more and 2 or less in an arbitrary 3 mm × 3 mm area in one screen. It is preferable (unevenness in this range does not pose a practical problem).

More preferably, an arbitrary 2 m in one screen
Polarity inversion is performed such that the ratio of the number of positive polarity pixels to the number of negative polarity pixels is 0.75 or more and 1.33 or less in an area of mx 2 mm. As a result, even if the liquid crystal display element is viewed from any angle, no unevenness is seen.

(E) The surface of the alignment film of the liquid crystal display element is made conductive, and the polarity of the voltage applied between the electrodes is inverted every predetermined frame time. According to this method, when there is a large amount of impurities in the liquid crystal material and burn-in occurs, this can be reduced.

Further, when the period of the polarity inversion is increased as described above, the number of times of charging and discharging the electric charge to and from the storage capacitor and the capacitor component such as the liquid crystal cell is reduced, so that the power consumption can be reduced. Hereinafter, embodiments of the present invention described above will be described.

(First Embodiment) A liquid crystal having spontaneous polarization induced by applying an intrinsic or electric field shown in FIG. 2 is sandwiched between a pixel electrode and a counter electrode arranged in a matrix. When the liquid crystal cell 24 is driven by the circuit configuration shown in FIG. 1, the frame time is TF, the response time of the liquid crystal is τ, the writing time is TK, and the polarity of the voltage applied between the electrodes is inverted every TS time. At this time, the operation is performed so as to satisfy TS / TF ≧ τ / TK ≧ 2.

In this case, TF = 1 / fF with respect to the frame frequency fF, and τ / TK in the above equation indicates how much the response time is longer than the writing time.

For example, when the writing time TK is 60 μs, if a liquid crystal having a response time τ of 120 μs is used, τ / TK = 2, which indicates that the response time is twice as long as the writing time.

Therefore, in order to lengthen the period of the polarity inversion of the display signal by this time, TS / TF ≧ τ / TK = 2, and the polarity inversion has to be performed every two frames or more.

FIG. 3 shows an example in which the polarity is inverted every two frames. As shown in FIG. 3 (a), a gate signal 38 is periodically input from the gate line, while as shown in FIG. 3 (b), the signal line is twice as long as the period of the gate signal, that is, every two frames. , A display signal voltage 39 whose polarity is inverted is applied (the potential of the counter electrode is displayed as OV).

At this time, as shown in (c), when the pixel is turned on in the first frame and a positive voltage 40a is applied to the pixel electrode, as shown in (f), the optical axis 4 of the liquid crystal molecule is changed.
1 deviates from the polarization directions 42 and 43, resulting in a bright state.

Also in the second frame, a positive voltage of 4 is applied to the pixel electrode.
0a is applied, resulting in a similar bright state. FIG. 11D shows the change of the holding voltage 44 in this case. In this case, since the time required for the alignment change of the liquid crystal molecules is longer than the writing time, the alignment change is not completed within the writing time of the first frame, and the transmittance is the same as the transmittance of the first frame (e). 45
a, the transmittance of the second frame is higher than that of the first frame as indicated by the transmittance of the second frame of 45b. Next, in the third frame, the display signal is inverted, and the negative voltage 40b is applied to the pixel electrode. The holding voltage 44 changes as shown in FIG.

In this case, the liquid crystal molecules change from the arrangement of the positive electric field to the arrangement of the negative electric field through the arrangement of no electric field (FIG. 1).
7). Since the time required for the array change is longer than the write time, the array change is not completed within the write time.
Therefore, as shown in (e), the transmittance 45a decreases. Since a negative voltage is applied also in the next fourth frame, the writing in this fourth frame almost completes the change in the arrangement of liquid crystal molecules that was not completed in the third frame, and has a high transmittance 45b equivalent to that in the static drive. Is obtained.

As a result, a high-contrast and clear gradation display can be performed. Moreover, according to this driving method, only the period of the polarity inversion of the display signal 39 is lengthened, so that the driving circuit can be configured without adding a driver IC or drastically changing the circuit design. be able to. Further, there is an effect that power consumption can be reduced by reducing the number of polarity inversions.

In the above description, the voltage of the display signal 39 is the same in one frame and the second frame, and the voltage of the display signal 39 is the same in the third frame and the fourth frame. However, actually, it is necessary to change the voltage depending on the image displayed in each frame. is there. For example, when performing a display that gradually becomes white, the first frame may be changed to +2 V, the second frame to +3 V, the third frame to -5 V, and the fourth frame to -6 V.

In the first embodiment, the polarity of the display signal applied to the pixel electrode is inverted every two frames or more. However, the optimum upper limit of the period of the polarity inversion of the voltage applied to the pixel electrode is set. The value is determined by the amount of ionic impurities in the liquid crystal and the ease of charging the alignment film.

That is, when a voltage of the same polarity is continuously applied to the pixel electrode, the ionic substance contained in the liquid crystal moves to the interface between the alignment film and the liquid crystal near the pixel electrode and charges the alignment film. As a result, when the effective electric field applied to the liquid crystal decreases and the display changes, a sticking phenomenon occurs in which the previous display remains thin. In order to prevent this image sticking phenomenon, for example, when the alignment film is formed of a polyimide resin, it is desired to invert the polarity of the voltage of the display signal applied to the pixel electrode within 60 minutes, preferably within 5 minutes.

Further, it is preferable to provide a changeover switch in the liquid crystal display device so that the polarity of the display signal can be arbitrarily changed. For example, when displaying the same screen for a relatively long time, such as a personal computer screen, increase the polarity inversion cycle. When displaying fast-moving video such as TV or video display, set the polarity inversion cycle to a frame. It is preferable that the polarity inversion cycle can be optimally selected according to the purpose of use of the liquid crystal display device, for example, by shortening the cycle so as to approach the cycle.

In the first embodiment, when the polarity is inverted every n frames, a frequency component of 1 / 2n of the frame frequency is generated. For example, if the frame frequency is 60 Hz
If the polarity of the display signal is inverted every two frames, a 15 Hz component is generated in the transmittance response, and in some cases, the 15 Hz component appears as flicker.

Therefore, in such a case, FIG.
To (f) show the first to fourth frames F1 to F1, respectively.
As shown for the adjacent four pixel electrodes 31 in F4, by driving the pixel electrodes 31 while appropriately changing the polarity, the flicker can be made less visible.
In particular, when pixel electrodes on a plurality of scanning lines (gate lines) are simultaneously driven as in a dual scan or the like, the polarity inversion shown in (c) is preferable.

Next, some specific examples of the first embodiment will be described.

(Specific Example 1) First, a first substrate on which TFT elements and pixel electrodes are formed in a matrix, and a second substrate on which a color filter and a black matrix are formed are prepared. The configuration of the TFT element of this specific example will be described below with reference to FIG.

The gate line 37 formed on the first substrate
Is covered with a gate insulating film having a laminated structure of a gate oxide film and a silicon oxide film, and a semiconductor thin film made of an amorphous silicon thin film is formed on the gate insulating film.

On the semiconductor thin film, a channel protective film made of a silicon nitride film for protecting the semiconductor thin film when forming a channel is formed. On the semiconductor thin film and the channel protective film, a source electrode electrically connected to the semiconductor thin film via an ohmic contact layer and a drain electrode integrated with the signal line are arranged. Further, the source electrode is electrically connected to the pixel electrode.

The switching element (TFT) 3 having the above configuration
0, the signal line 36, the gate line 37, and the pixel electrode are covered with a protective film of silicon oxide or silicon nitride. In this manner, by covering the signal line 36, the pixel electrode, and the like with the protective film, occurrence of a defect due to a short circuit with the common electrode on the second substrate can be reduced.

Next, the second substrate will be described. Second
A color filter and a black matrix are formed inside the substrate. A resin layer (acrylic, benzocyclobutene polymer,
(Formed of polyimide or the like).

On this, a counter electrode made of a transparent conductive film such as ITO is provided. This counter electrode is not formed on the entire surface of the substrate. That is, when the first substrate and the second substrate are opposed to each other, a portion of the first substrate facing the signal line and the TFT element is removed by the PEP process. This prevents a short circuit between the signal line and the TFT element and the common electrode due to dust or protrusions of the color filter.

When the opposing electrode is formed on the entire surface of the second substrate, since the opposing electrode and the signal line sandwich a dielectric (a liquid crystal material and an alignment film), a voltage is applied to the signal line. Although the waveform of the signal to be output is rounded, in the configuration of this specific example, since the counter electrode is not provided on the signal line, the waveform is not rounded.

The above-mentioned short-circuit and rounding of the waveform become more serious as the cell gap (the distance between the first substrate and the second substrate) becomes narrower. In this specific example, the cell gap is 2 μm. However, when the cell gap is small,
Partial removal of the counter electrode is extremely effective.

A first substrate on which a TFT element is formed and a second substrate on which a color filter and a black matrix are formed
On a substrate of the above, a thin film of soluble polyimide (AL-1051 manufactured by Nippon Synthetic Rubber Co., Ltd.) is offset-printed as an alignment film,
Bake at 90 ° C. for 3 minutes using a hot plate and further at 200 ° C. for 30 minutes in a nitrogen atmosphere.

The rubbing treatment is performed on the polyimide alignment film (thickness: 40 nm) thus formed. At this time, the first substrate and the second substrate are rubbed while being heated to 100 ° C. This makes it possible to sufficiently perform a rubbing alignment treatment on a portion having a step due to a TFT or the like without peeling off the polyimide. The rubbing direction is a cross rubbing angle of 5 degrees so that the first substrate and the second substrate are antiparallel to each other.

Next, spacer particles (2 μm in diameter) are scattered on the first substrate. This spacer is made of silica (S
The core made of iO ₂ ) is coated with an organic resin.
Further, an ultraviolet-curable sealing material is printed on a peripheral portion of the second substrate. In addition, it is preferable to provide two or more injection ports to shorten the injection time. The first substrate and the second substrate are combined to face each other, and ultraviolet light is applied in a pressurized state to cure the sealing material. Thereafter, the sealing material is completely cured by heating at 160 ° C. for 1 hour, and a liquid crystal cell is completed.

This liquid crystal cell is placed in a vacuum chamber,
Thresholdless antiferroelectric liquid crystal composition (solid phase → -3
(Having a phase sequence that changes from 0 ° C. → smectic C phase → 80 ° C. → smectic A phase → 85 ° C. → isotropic and has a response time of 80 μs), and vacuum-injected while heating at 120 ° C. Thereafter, the inlet is sealed with an epoxy-based adhesive.
The cell gap of the cell thus formed is 2 μm
Met.

On the outside of the first substrate, the polarizing plate is set so that the transmission axis is substantially perpendicular to the rubbing direction (about 92.5 degrees).
Attach a polarizing plate. Further, a sheet-like heater is attached to the outside of the second substrate, on which a polarizing plate has a transmission axis substantially parallel to the rubbing direction (about 2.5 degrees).
A polarizing plate is attached to the substrate.

The sheet-shaped heater is formed by forming a transparent conductive film such as ITO on a glass or plastic substrate, and is mounted for the purpose of heating the liquid crystal so that a good display quality can be maintained even in a use environment of 0 ° C. or less. Have been.

To the thus manufactured liquid crystal display device having a diagonal size of 15 inches, a direct current of 25 V is applied to the gate line to keep the gate on, and a ± 10 V rectangle (10 Hz) is applied to the signal line. Then, while applying 0 V to the common electrode, a voltage application alignment process of gradually cooling the liquid crystal display element from 90 ° C. to room temperature over 30 minutes is performed. Thereby, the liquid crystal alignment becomes uniform.

A drive circuit is mounted on this liquid crystal display element.
Further, a backlight is provided outside the first substrate, and the liquid crystal display device is completed by placing the backlight in a housing. Note that the sheet-shaped heater also plays a role of a protective plate (shock absorbing plate) for preventing the alignment breakage of the liquid crystal. The alignment breakage means that the alignment of liquid crystal molecules is disturbed by strongly pressing the liquid crystal display element with a finger or the like.

This liquid crystal display element was set to a frame frequency of 60
Hz, frame time 16.67 ms, writing time 64μ
s, and the polarity inversion is 33.33 m in order to invert the polarity of the display signal applied to the pixel electrode every two frames.
Performed every s.

As a result, one frame (16.67 ms)
When the polarity of the display signal was inverted every time, the contrast was 10: 1, which could be greatly improved to 80: 1. In addition, the adjacent pixel electrode corresponds to FIG.
As a result of reversing the polarity in the relationship shown in (e), a flicker was not observed at all, a wide viewing angle was obtained, and a liquid crystal display device having extremely good display characteristics with no afterimage or image sticking was obtained.

(Specific Example 2) Next, a cell was formed in the same manner as in the above specific example, except that the rubbing treatment was performed while heating the alignment film at 140 ° C., and a DHF liquid crystal (response time 150 μs) was added to the cell. By injecting, a liquid crystal display element having a diagonal diameter of 10 inches was manufactured.

Then, the liquid crystal display device was driven by dual scan at a frame frequency of 60 Hz and a frame time of 16.67 ms. In this case, the write time is 1
28 μs, but 64 in the first half of writing after the gate is turned on.
During μs, the same voltage as that of the pixel electrode whose gate is turned on earlier for one scanning time (t1) when the same signal line is connected is applied. Further, in order to invert the polarity of the display signal applied to the pixel electrode every two frames, the polarity was inverted every 33.33 ms.

As a result, one frame (16.67 ms)
When the polarity of the display signal was inverted every time, the contrast was 9: 1, which could be greatly improved to 80: 1. In addition, the polarity of the adjacent pixel is inverted in the relationship shown in FIG. 4C (the inversion method is V (vertical)).
Dual scan drive is possible because of line inversion.)
As a result, it was possible to obtain a liquid crystal display device having no flicker at all, a wide viewing angle, and excellent display characteristics with no afterimage or image sticking.

(Specific Example 3) A cell was formed in the same manner as in the above specific example, except that the rubbing treatment was performed while heating the alignment film at 50 ° C., and an antiferroelectric liquid crystal (response speed of 65 μs) was formed in the cell. ) Was injected to produce a liquid crystal display element having a diagonal diameter of 10 inches.

The liquid crystal display element has a frame frequency of 60 Hz, a frame time of 16.67 ms, a writing time of 32 μs, and inverts the polarity of the display signal applied to the pixel electrode every 5 minutes. Performed every minute.

As a result, one frame (16.67 ms)
When the polarity of the display signal was inverted every time, the contrast was 10: 1, which could be greatly improved to 100: 1. Further, by using a liquid crystal composed of a fluorine element-containing organic compound containing almost no ionic impurities, the resistivity was as high as 10 <15> [Omega] .cm, and no baking occurred even if polarity inversion was not performed for 5 minutes.

(Specific Example 4) As a fourth specific example, the following liquid crystal display element was manufactured. On a first substrate on which TFTs and pixel electrodes are formed in a matrix, and on a second substrate on which a color filter and a black matrix are formed, a soluble polyimide (low pretilt angle) as an alignment film
Offset printing of thin film of 9
Bake at 0 ° C. for 30 minutes. A rubbing treatment is performed on the thus formed polyimide alignment film (65 nm in thickness).

Next, spacer particles are dispersed on the first substrate. An ultraviolet-curable sealing material is printed on the peripheral portion of the second substrate. The first substrate and the second substrate are combined to face each other, ultraviolet rays are applied in a pressurized state to cure the sealing material, and then heated at 160 ° C. for 1 hour to form a cell. This cell is placed in a vacuum chamber, and after a thresholdless antiferroelectric liquid crystal composition (response speed τ = 80 μs) is injected from the injection port, the sealing port is sealed with an epoxy-based adhesive. Further, by attaching polarizing plates to both sides of the cell, a 10-inch diagonal liquid crystal display device is completed.

The liquid crystal display device manufactured as described above is
It was driven under the following conditions. Since the screen resolution was XGA (768 scanning lines), the frame frequency was 60 Hz, the frame time was 16.67 ms, and the screen was driven in two upper and lower parts, the writing time was 42 μs.

When a fast-moving image such as a baseball broadcast is displayed on the entire screen, the polarity of the voltage applied between the pixel electrodes is inverted every two frames. As a result, one frame (1
When the polarity was inverted every 6.67 ms), the contrast was 20: 1, but the contrast was 20: 1.
When the polarity is inverted every s), the contrast is 50:
It was remarkably improved to 1.

Also, a slow-moving image such as teletext is displayed on one portion of the screen (display portion 1), and a fast-moving image such as a baseball broadcast is displayed on another portion (display portion 2). In this case, the polarity of the display unit 1 was inverted every two frames, and the polarity of the display unit 2 was inverted every frame. As a result, a high contrast was obtained in the display unit 1 and a fast moving image was seen in the display unit 2 without trailing, though the contrast was low.

That is, when the response time τ of the liquid crystal is longer than the writing time TK and the polarity is changed every two frames, the response of the liquid crystal is not completed within the frame time in which the polarity is inverted, and the same polarity is applied. Next frame,
The liquid crystal molecules respond. That is, the response time of the liquid crystal is 33.3.
This is as long as about 3 ms (the length of two frames).

When the polarity is inverted for each frame, the response time does not exceed 16.7 ms (the length of one frame). As a result, when the same polarity is applied over a plurality of frames, the ball may appear to trail in the case of a fast-moving display such as a video in which a baseball ball flies.

Which part of the screen is to be inverted for each frame and which part is inverted for every two or more frames is as follows.
The user of the liquid crystal display device may determine it according to the image desired by the user, or may detect the speed of movement of the image from the magnitude of the change in the input signal 21 and automatically display the display unit 1. And the display unit 2 may be set.

When the screen is divided into n areas and the area of the i-th area is Ai and inverted every m frames,
Ts is defined by the following equation by taking the average of the entire screen.

[0124]

(Equation 1)

(Second Embodiment) In the first embodiment,
In order to avoid a decrease in contrast that occurs when the polarity of the display signal voltage is inverted, TS / TF ≧ 2, preferably, a display signal applied to the pixel electrode at every TS time that satisfies TS / TF ≧ τ / TK ≧ 2. Has been described.

In the second embodiment, as shown in FIG. 5, the polarity of the display signal 39 applied to the pixel electrode is inverted by two.
The supply time of the voltages 40a and 40b to be applied to the pixel electrodes when the polarity is inverted is performed for each frame or more.
Make the supply time longer than when the polarity is not reversed.

In this way, for example, if the pre-driving of the next line is performed during the driving time of the previous line, and then the main driving is performed, an image having a vertical correlation can be doubled at the maximum. The driving time can be reduced. Since this is performed only at the time of polarity inversion, the portion where the contrast is instantaneously reduced can be reduced. As a result, it is possible to reduce flicker and to prevent deterioration in contrast that occur when the polarity of the display signal is inverted, and to obtain a display with good image quality.

The same effect as in the case where the writing time is lengthened when the polarity is inverted is obtained by increasing the absolute value of the writing voltage when the polarity is inverted as shown in FIG. can get. That is, the voltage held in the pixel electrode becomes substantially constant as shown in FIG.

In FIGS. 5 and 6, (a) is the gate (scanning line) voltage, (b) is the display signal voltage, (c) is the writing voltage to the pixel electrode, and (d) is the holding of the pixel electrode. The voltages are drawn on the same time scale.

(Third Embodiment) FIG. 7 shows the configuration of the polarity inversion control circuit 20 of the liquid crystal display device shown in FIG. 1 as the configuration of the liquid crystal display device according to the third embodiment, and FIG.
FIG. 2 shows a timing chart for explaining the operation of the polarity inversion control circuit 20.

The polarity inversion control circuit 20 inverts the polarities of all the plurality of pixel electrodes located on one scanning line (line) when rewriting the screen of one frame. A line counter 49 for counting the number of scanning lines, a frame counter 50 for counting the number of times of rewriting of the screen, and an inversion determination circuit 52 including a comparator 51
And the display timing controller 2 shown in FIG.
The display signal is controlled to invert the polarity by the timing signal 22 supplied from 3.

The line counter 49 is cleared (reset) by the negative synchronizing signal 53 shown in FIG. 7C every time the screen is rewritten, and as shown in FIGS. The number of lines 54 is counted.

On the other hand, the frame counter 50 counts the number of times the screen has been rewritten 55 as shown in FIG.
When the count for (L in FIG. 7 (b)) is completed, 1 again
Repeat counting from.

The comparator 51 is supplied with two kinds of numerical values, a numerical value updated every time the screen is rewritten from the frame counter 49 and a numerical value updated every scanning line from the line counter 50. 4
When the numerical value from the line counter 9 and the numerical value from the line counter 50 coincide with each other with a certain numerical value n, the coincidence output 57 is output to the exclusive OR circuit 59 as shown in FIG. In addition,
Reference numeral 38 shown in FIG. 7A is a vertical synchronization signal among the synchronization signals supplied to the display timing controller 23 shown in FIG.

The exclusive OR circuit 59 has a memory 60 for holding a polarity inversion signal in addition to the coincidence output 57.
The output from the memory 60 is inverted only when the output from the memory 60 is input and the coincidence output 57 is present. That is, when the output of the frame counter 50 is n, the polarity is inverted only for n lines.

The output of the inverted, that is, updated exclusive OR circuit 59 is supplied to switching circuit 61 and latch circuit 6.
2 to the display timing controller 23 shown in FIG.

The output of the exclusive OR circuit 59 is fed back to the memory 60 via the switching circuit 61 and held until the next update. The address of the memory 60 is controlled by a memory address counter 63. The address of the memory address counter 63 is created at the same address as the line counter 49.

The polarity inversion signal generated by the polarity inversion control circuit 20 is output to the display timing controller 23 shown in FIG. 1, and the display timing controller 23 outputs the display signal based on the polarity inversion signal. Performs polarity reversal and display control.

When the liquid crystal display device is driven by such a method, when the output of the frame counter is n as shown in FIG. 9 for the P frame and the P + 1 frame, only the n lines 65 are rewritten when the screen of one frame is rewritten. The polarity is inverted, and when the output of the frame counter becomes the next n + 1, the polarity is inverted only for the (n + 1) th line 66. When the number of scanning lines on one frame screen is L, rewriting of the L frame screen ends the polarity reversal of the entire screen.

Therefore, when the liquid crystal display element is driven by the above driving circuit, a decrease in contrast caused when the polarity of the display signal is inverted, that is, a change in transmittance caused by passing through a state of zero electric field, is caused in a part of the screen. Limited to
It can prevent the deterioration of the contrast of the entire screen,
A high-contrast image with good image quality can be obtained.

The liquid crystal display device of this embodiment is a 15-inch XG
In A, the pixel size is 300 μm in length × 100 μm in width, and horizontal line inversion driving (polarity is inverted between pixels connected to adjacent gate lines). 3mm x 3mm of screen
Focusing on the region, there is one scanning line (gate line) there.
There are 0 and 300 pixels.

When the polarity of one scanning line is inverted in a certain frame, the number of pixels to which a positive voltage is applied is not equal to the number of pixels to which a negative voltage is applied. In the case where the polarity is inverted as shown in FIG. 9, in the area of 3 mm × 3 mm, there are 120 positive polarity pixels and 180 negative polarity pixels. Dividing the number of positive polarity pixels by the number of negative polarity pixels gives 0.667.
With such a difference in the number of pixels of the positive polarity and the number of pixels of the negative polarity, the liquid crystal display element was not visually recognized when viewed from within an oblique angle of 70 degrees.

The order of the polarity inversion is determined by the line counter 49 and the frame counter 50 of the polarity inversion control circuit 20.
And the configuration of the comparator 51. For example, as shown in FIG. 10, the output of line counter 49 and the output of frame counter 50 are both the most significant bit MSB.
(Most Signifcant Bit) to the least significant bit LSB (Le
If the same sequence up to the ast signifcant bit), the comparator 5
The coincidence output from 1 is output in order from the first line to the Lth line, and polarity inversion is performed in order. Therefore, the position where the polarity inversion is performed on one screen is indicated by a line 68 in FIG. 11, and the adverse effect of the polarity inversion due to the movement of the polarity inversion position from top to bottom may be visually recognized. There is.

Therefore, in order to make the polarity inversion position difficult to see, rather than changing the polarity inversion position in an orderly manner, the polarity inversion is performed randomly in an adjacent frame, preferably in a line, and so that the regularity is not recognized. It is good to do. FIG. 12 shows, as an example, a case where the wiring operation is performed so that the output of the line counter 49 and the output of the frame counter 50 are exchanged between the most significant bit MSB and the least significant bit LSB. In this case, FIG.
As indicated by a point 69, the polarity can be apparently inverted at random.

When the polarity inversion is performed by the above-described method, when the screen of one frame is composed of L lines, the polarity inversion of all the lines is completed in the time of the L frame. However, in this case, since the polarity is reversed line by line, a positive or negative display signal is applied to all the pixel electrodes on the screen at a certain time.

When a display signal of a positive polarity or a negative polarity is applied to all the pixel electrodes in this manner, there is a possibility that image quality deterioration such as flicker may occur due to a difference between the positive polarity and the negative polarity. Therefore, in such a case, it is preferable that the display region of the positive polarity and the display region of the negative polarity exist in the screen of one frame at almost the same ratio before and after performing the polarity inversion.

For this purpose, an initial pattern generation circuit 71 is provided in the polarity inversion control circuit 20 shown in FIG. 7, and a polarity pattern in which the positive polarity and the negative polarity exist at the same ratio is input to the initial pattern generation circuit 71. In addition, a polarity pattern in which the positive polarity and the negative polarity are present at the same ratio when the power is turned on or when a reset signal is given may be input to the memory 60.

For example, assuming that a pattern in which even lines are positive and odd lines are negative is set in the initial pattern generating circuit 71 as initial values, a positive display signal and a negative display signal are displayed on a screen of one frame. Are supplied at the same rate. When the display signal has such a polarity, adjacent two
By inverting the polarity of the lines collectively, the ratio of the positive polarity and the negative polarity can be prevented from changing before and after the polarity inversion. That is, FIG.
As shown in the case where the polarities of the 5 and n + 1 lines 66 are inverted, the ratio of the positive polarity and the negative polarity existing in the P frame before the polarity inversion is changed when the next P + 1 frame is reached. However, the existence ratio does not change.

As described above, in order to invert the polarity of two lines at the time of rewriting a screen of one frame, it is necessary to reduce the number of bits of the line counter 41 and the frame counter 42 by one bit as compared with the case where the polarity is inverted one line at a time. By doing so, it can be easily realized.

(Fourth Embodiment) In the third embodiment,
Although the case where the polarity inversion is performed for each line has been described, the case where the polarity is inverted for each pixel electrode can be realized by a similar configuration except for the polarity inversion control circuit.

FIG. 15 shows a polarity inversion control circuit when the polarity is inverted for each pixel electrode. The polarity inversion control circuit 20 basically replaces the line counter of the polarity inversion control circuit shown in FIG. 7 with a pixel counter 73, and stores the frame counter 50, the comparator 51, and the memory 60 in a screen of one frame. The polarity inversion can be performed by the same operation as in the case where the polarity is inverted for each line, except that the polarity is inverted for each pixel except that the number of bits is expanded to the number of bits corresponding to the number of existing pixels.

Therefore, in this case, a polarity pattern in which the positive polarity and the negative polarity are present in the same ratio, for example, a polarity pattern in which even pixels are positive and odd pixels are negative is input to the initial pattern generation circuit 71. By inverting the polarity of the even-numbered pixel electrodes at the same time, the polarity inversion operation can be performed while display signals having different polarities are present at the same ratio for each pixel electrode in the screen of one frame.

The order of the polarity inversion for each pixel, that is, the position of the pixel to be inverted in the adjacent frame is input to the comparator 51 in the same manner as in the case of performing the polarity inversion for each line. The polarity inversion can be performed in an arbitrary order by exchanging the output bits of the pixel counter 73 and the frame counter 50.

By performing the polarity inversion for each pixel as described above, the polarity inversion is performed in a very small area as compared with the case where the polarity inversion is performed in the line. Can be prevented, and a high-contrast image with good image quality can be obtained.

(Fifth Embodiment) A liquid crystal display device in which a liquid crystal having spontaneous polarization or spontaneous polarization induced by applying an electric field is sandwiched between a pixel electrode and a counter electrode, has a period of polarity inversion of a display signal. , The image sticking phenomenon that the previous image remains is unlikely to occur. However, when a still image is displayed for a long time, a burn-in phenomenon in which the image remains remains.

The image sticking of the liquid crystal display device in which the liquid crystal having the spontaneous polarization is sandwiched between the pixel electrode and the counter electrode is caused by the alignment films 32a and 3 of the liquid crystal display element 24 shown in FIG.
By imparting conductivity to the surface of 2b, charges accumulated on the surface of the alignment film can be dispersed and eliminated, thereby preventing image sticking.

As a result, without inverting the polarity of the display signal, no seizure occurs even when driven in a DC manner.
The direct current drive can completely prevent a decrease in contrast that occurs at the time of polarity inversion.

Note that the liquid crystal having spontaneous polarization is sandwiched between the pixel electrode and the counter electrode, and the alignment films 32a, 32
In the liquid crystal display device having conductivity on the surface of 2b,
It is optional to invert the polarity of the display signal every predetermined frame time.

The alignment films 32a, 3 of the liquid crystal display element 24
The conductivity of the surface 2b can be obtained by dissolving a conductive material on the surfaces of the alignment films 32a and 32b made of, for example, a polyimide resin.

The conductive substance is not particularly limited, but an organic charge transfer complex obtained by reacting an electron donor (donor) with an electron acceptor (acceptor) at a molar ratio of 1: 1 can be used. preferable. As the electron donor, bis (ethylenedithio) tetrathiafulvalene bis (methylenedithio) tetrathiafulvalene bis (trimethylenedithio) tetrathiafulvalene 4,4′-dimethyltetrathiafulvalene tetrakis (octadecylthio) tetrathiafulvalene tetrakis (N-bentylthio) tetrathiafulvalene tetrakis (alkylthio) tetrathiafulvalene tetrathiafulvalene tris (tetrathiafulvalene) bis (tetrafluoroborate) and the like. As the electron acceptor, bis (tetra-n-butylammonium) tetracyanodiphenoquinomethanide 2,5-dimethyl-7,7,8,8-tetracyanoquinodimethane 11,11,12,12 -Tetracyanonaphth-2, 6
-Quinodimethane 7,7,8,8-tetracyanoquinodimethane tetracyanoquinodimethane and the like.

The organic charge transfer complex comprising the electron donor and the electron acceptor is used in an amount of 0.1 to 0.2% with respect to the polyimide resin.
It is obtained by dissolving 001 to 20% by weight, preferably 0.1 to 2% by weight. If the amount is less than 0.001% by weight or the effect of preventing the seizure phenomenon is insufficient. If it exceeds 20% by weight, it remains undissolved and cannot be used as an alignment film.

Hereinafter, a specific example of the fifth embodiment will be described.

In the liquid crystal display element 24 having the structure shown in FIG. 2, a first substrate 28 on which TFTs and pixel electrodes 31 are formed in a matrix, and a second substrate on which a color filter 32 and a counter electrode 34 are formed 29, each has a soluble polyimide resin, for example, A
On the surface of a thin film made of L1031, tetrakis (n-bentylthio) tetrathiafulvalene is used as an electron donor.
1% by weight of an organic charge transfer complex comprising 7,7,8,8-tetracyanoquinodimethane as an electron acceptor is dissolved to form alignment films 32a and 32b.

An MCL manufactured by Mitsui Petrochemical Co., Ltd. is interposed between the pixel electrode 31 and the counter electrode 34 via the alignment films 32a and 32b.
-0049 is sealed with an antiferroelectric liquid crystal,
A liquid crystal display device having a width of 107 mm, a width of 107 mm and a cell gap of 2 μm was manufactured.

The polarity of the voltage applied between the pixel electrode and the counter electrode was inverted every 150 minutes. As a result of displaying an NTSC television image, a high contrast of 100: 1 was obtained. After the still image was displayed for 120 minutes, the image was switched to a moving image, but the still image did not overlap with the subsequent moving image. That is, the image sticking phenomenon did not occur.

On the other hand, in the liquid crystal display device having the same structure manufactured without dissolving the organic charge transfer complex on the surface of the alignment film, the polarity of the voltage applied between the pixel electrode and the counter electrode is inverted every 150 minutes. went. As a result of displaying an NTSC television image, a high contrast of 100: 1 was obtained. However, in this liquid crystal display device, after displaying a still image for 120 minutes, the image was switched to a moving image, and the still image appeared to overlap with the subsequent moving image. That is, the image sticking phenomenon occurred, and the display quality deteriorated.

Although the embodiment is characterized in that the surface of the alignment film is conductive, the resistivity of the surface of the alignment film is preferably set to 10 ⁷ to 10 ⁹ Ω · cm. (Sixth Embodiment) In the present embodiment, the polarity inversion cycle is set to 6.
First embodiment in which 4 seconds are set, signal line inversion driving is performed, polarity inversion is performed using a plurality of pixels connected to one scanning (gate) line as one unit, and the configuration of a liquid crystal display element and the like. The other points are the same as in the first embodiment.

First, a first substrate on which TFT elements and pixel electrodes are formed in a matrix and a second substrate on which a color filter and a black matrix are formed are prepared. Referring to FIGS. 2B and 2C, the configuration of the TFT element will be described below. The gate line 37 formed on the first substrate is covered with a gate insulating film having a stacked structure of a gate oxide film and a silicon oxide film, and a semiconductor thin film made of a polysilicon thin film is formed on the gate insulating film. Have been.

On the semiconductor thin film, a channel protective film made of a silicon nitride film for protecting the semiconductor thin film when forming a channel is formed. On the semiconductor thin film and the channel protection film, a source electrode electrically connected to the semiconductor thin film via an ohmic contact layer and a drain electrode integrated with the signal line are arranged.

A flattening film made of a photosensitive resin of a benzocyclobutene polymer and having a thickness of 4 μm is formed thereon. The pixel electrode is formed on this flattening film,
Because a through hole is formed in this flattening film,
The source electrode and the pixel electrode are connected via this.
In this manner, by covering the signal line 36 with the flattening film, occurrence of a defect due to a short circuit with the common electrode on the second substrate can be reduced.

Next, the structure of the second substrate and a method for forming the second substrate will be described. A black matrix made of chromium is formed on the inner surface of the second substrate. Then, a photosensitive resin in which a red pigment is dispersed is formed by a PEP process to form a red color filter. Similarly, green and blue color filters are formed.

At this time, a red pigment-dispersed photosensitive resin, a green pigment-dispersed photosensitive resin, and a blue pigment-dispersed photosensitive resin are overlapped and arranged on a part of the black matrix facing the gate line, and have a length of 7 μm. , A columnar projection having a width of 4 μm and a height of 2 μm is formed. The projection serves as a spacer for keeping the distance between the first substrate and the second substrate constant when a cell is formed.

A counter electrode made of a transparent conductive film of ITO is formed thereon. Further, an insulating film made of silicon oxide is formed with a thickness of 100 nm over the entire pixel region on the transparent conductive film. This prevents a short circuit between the pixel, the signal line, the TFT element, and the common electrode on the first substrate due to dust or the like.

A first substrate on which a TFT element is formed and a second substrate on which a color filter and a black matrix are formed
A thin film of thermosetting polyimide (SE-150, manufactured by Nissan Chemical Industries, Ltd.) is offset-printed as an alignment film on the substrate, and cured using a hot plate at 90 ° C. for 30 minutes and further in a nitrogen oven at 220 ° C. for 60 minutes. I do.

The rubbing treatment is performed on the polyimide alignment film (thickness: 60 nm) thus formed. The rubbing direction is parallel to each other on the first substrate and the second substrate, and the cross rubbing angle is 5 degrees.

Next, a sealing material made of a thermosetting epoxy resin is printed on the peripheral portion of the second substrate so as to form one on each side facing the injection port. The first substrate and the second substrate are combined to face each other, and 160
The sealing material is cured by heating at 3 ° C. for 3 hours to form cells.

A tube was connected to one inlet of this cell, and the pressure was reduced. From the other inlet, a thresholdless antiferroelectric liquid crystal composition (MLC series manufactured by Mitsui Petrochemical Co., Ltd., phase series: solid phase → −30 ° C.) → Smectic C phase → 75 ° C → Smectic A phase → 80 ° C → Isotropic phase; response time = 300μs)
Is suction-injected while heating to 100 ° C. Thereafter, the inlet is sealed with an epoxy resin. The cell gap is 2.0 μm.

By setting the transmission axis of the polarizing plate outside the second substrate so as to be substantially parallel to the rubbing direction (about 2.5 degrees),
A polarizing plate is attached to the first substrate. A sheet heater is attached on the polarizing plate. The sheet-shaped heater has a configuration in which a transparent conductive film of ITO is formed on a polycarbonate substrate, and is mounted for the purpose of heating liquid crystal so that good display quality can be maintained even in an operating environment of 0 ° C. or less.

When a force such as pressing a liquid crystal display element with a finger is applied, the sheet-shaped heater also functions as a buffer for reducing the force. Thus, a 15-inch diagonal liquid crystal display device is completed.

Thereafter, while applying 25 V DC to the gate line to keep the gate on, applying a triangular wave of ± 20 V (1 Hz) to the signal line and 0 V to the common electrode, the liquid crystal display device was cooled from 90 ° C. to room temperature. Cool slowly over 30 minutes (voltage application orientation treatment). Thereby, the liquid crystal alignment becomes uniform. A drive circuit is mounted on this liquid crystal display element. Further, a backlight is placed outside the first substrate, and the backlight is placed in a housing to complete a liquid crystal display device.

The driving method will be described below. FIG. 16 (a),
(B) shows a gate signal for a pixel connected to one signal line and a voltage (writing voltage) applied between the pixel electrode and the counter electrode. Since the definition of the liquid crystal display device of this embodiment is at the XGA level, the number of gate lines is 7
There are 68. However, since the upper half and the lower half of the pixel of the liquid crystal display device are driven by being divided into two, one frame (1
6.67 ms), the number of gate lines to be scanned is 384.
It becomes a book.

In this embodiment, the gate on time is set to 42 μm.
16.67−42 μs × 384 = 0.5
4 ms is the blanking time. Using this blanking time, the gate on time (writing time) is extended to 504 μs at the time of polarity inversion.

In FIG. 16A, the polarity of the pixel connected to the i-th gate line is inverted from negative to positive in the first frame. Note that the pixel adjacent to the i-th pixel shown in FIG.
In the frame, the polarity is inverted from positive to negative. The polarity inversion is performed at a rate of one gate line per frame. Therefore, all the pixels have been inverted, in other words,
The period in which the polarity of a certain pixel is inverted is 16.67 ms × 38.
4 = 6.4 seconds.

In writing at the time of polarity inversion, Vh is applied in the first 168 μs, and voltage Va corresponding to the display signal is applied in the next 336 μs. In other words, the signal voltage of the signal line (not shown) is Vh in the first 168 μs.
However, the voltage Va corresponding to the display signal is supplied in the remaining time of the subsequent two or more frame times.

By setting the absolute value of Vh> the absolute value of Va, it is possible to prevent a decrease in light intensity at the time of polarity inversion. Vh
Is ± 7 V, the maximum and minimum values of Va are ± 5 V, and when the display signal does not change between the first frame and the second frame, the light intensity of the first frame and the light intensity of the second frame become equal, and the polarity is inverted. There was no change in light intensity at that time.

The order in which the polarity inversion is performed is n = 1, 13, 2
5,. . . , 373, 2, 14,. . . , 384. That is, when the polarity of the ith pixel is inverted in the first frame, the polarity of the (i + 12) th pixel is inverted in the second frame. As described above, as the distance between the gate line of which polarity is inverted between a certain frame and the next frame is increased, the light change at the time of polarity inversion is more difficult to be visually recognized.

[0187]

As described above, the driving of a liquid crystal display device in which a liquid crystal having spontaneous polarization induced by applying an intrinsic or electric field is sandwiched between a pixel electrode arranged in a matrix and a counter electrode. In the method, when the frame time is TF, when the polarity of the voltage applied between the electrodes is inverted every TS time that satisfies TS / TF ≧ 2, a decrease in contrast when the polarity is inverted is reduced, A high-contrast image with good image quality can be obtained.

Also, by giving conductivity to the surface of the alignment film,
Even if the driving is performed in a DC manner without performing the polarity inversion, or the polarity is inverted every predetermined frame time, a high-contrast image with good image quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a liquid crystal display device according to an embodiment of the present invention.

FIG. 2A is a plan view illustrating a configuration of a liquid crystal display device which is one configuration of the liquid crystal display device, and FIG.
FIG. 2A is a sectional view taken along line AA ′, and FIG.
It is an enlarged view of the C section of (a).

FIGS. 3 (a) to 3 (d) are voltage waveform diagrams of respective parts for describing a driving method of the liquid crystal display device according to the first embodiment of the present invention, and FIG. rate,
(F) shows the optical axis of the liquid crystal.

FIGS. 4A to 4F are diagrams for explaining polarity inversion of a pixel electrode in the first embodiment.

FIGS. 5A to 5D are diagrams illustrating a method of inverting the polarity of a pixel electrode according to the second embodiment.

FIGS. 6A to 6D are diagrams illustrating a method of inverting the polarity of a pixel electrode according to a modification of the second embodiment.

FIG. 7 is a diagram illustrating a configuration of a polarity inversion control circuit for controlling polarity inversion of a pixel electrode according to a third embodiment.

FIGS. 8A to 8F are views for explaining a polarity inversion method of a pixel electrode according to the third embodiment.

FIG. 9 is a diagram illustrating a polarity inversion of a pixel electrode according to the third embodiment.

FIG. 10 is a diagram illustrating a configuration of a polarity inversion determination circuit of a polarity inversion control circuit according to a third embodiment.

FIG. 11 is a diagram for explaining the order of polarity inversion by the polarity inversion determination circuit.

FIG. 12 is a diagram illustrating another configuration of the polarity inversion determination circuit of the polarity inversion control circuit according to the third embodiment.

FIG. 13 is a diagram illustrating the order of polarity inversion by the polarity inversion determination circuit.

FIG. 14 is a diagram illustrating a different polarity inversion of a pixel electrode according to the third embodiment.

FIG. 15 is a diagram illustrating a configuration of a polarity inversion control circuit that controls polarity inversion of a pixel electrode according to a fourth embodiment.

FIGS. 16A and 16B are diagrams for explaining the operation of the sixth embodiment, wherein FIG. 16A shows a gate voltage and FIG. 16B shows a write voltage.

FIG. 17 is a diagram for explaining an electric field response of a liquid crystal having spontaneous polarization.

18 (a) to 18 (h) are diagrams for explaining a conventional driving method of a liquid crystal display device.

FIG. 19 is a diagram for explaining the definition of the response time of the liquid crystal.

[Explanation of symbols]

 Reference Signs List 30 switching element 31 pixel electrode 32a, 32b alignment film 34 counter electrode 35a, 35b deflection plate 36 signal line 37 gate line 38 gate signal 39 display signal 44 holding voltage 45a, 45b transmittance

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Takaki Takato 33, Shinisogocho, Isogo-ku, Yokohama-shi, Kanagawa Prefecture Inside Toshiba Institute of Industrial Science (72) Inventor Hiroyuki Nagata Shinisogocho, Isogo-ku, Yokohama-shi, Kanagawa 33 Toshiba Production Technology Research Institute Co., Ltd. (72) Inventor Rieko Iida 33 Toshiba Production Technology Research Institute Co., Ltd. (72) Inventor Haruhiko Okumura Shinisogocho, Isogo-ku, Yokohama-shi, Kanagawa Prefecture No. 33 Toshiba Production Technology Laboratory Co., Ltd. (72) Inventor Hisao Fujiwara No. 33 Shinisogo-cho, Isogo-ku, Yokohama-shi, Kanagawa Prefecture Toshiba Production Technology Laboratory Co., Ltd. (56) References International Publication 96/35976 (WO, A1) ( 58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/133 550 G02F 1/133 560 G09G 3/36

Claims (10)

(57) [Claims] 1. A method for driving a liquid crystal display device in which a liquid crystal having a spontaneous polarization induced by applying an intrinsic or electric field is sandwiched between a pixel electrode and a counter electrode arranged in a matrix. TF, the response time of the liquid crystal is τ, and the writing time is TK. When the polarity of the voltage applied between the electrodes in at least a part of the screen is inverted every TS time, TS / TF ≧ τ / TK A method for driving a liquid crystal display device, characterized by satisfying ≧ 2. 2. The driving method of a liquid crystal display device according to claim 1, wherein, when performing the polarity inversion, a writing time for the pixel for which the polarity inversion is performed is longer than a writing time for the pixel for which the polarity inversion is not performed. 3. The method of driving a liquid crystal display device according to claim 1, wherein the signal amplitude when performing the polarity inversion is made larger than when not performing the polarity inversion. 4. The method according to claim 1, wherein the polarity inversion performed so as to satisfy a relational expression of TS / TF ≧ 2 is performed in a partial area of the screen, and the polarity inversion is performed every frame in another area. method for driving a liquid crystal display device according to any one of claims 1 to 3,. 5. The polarity reversal performed so as to satisfy the relational expression TS / TF ≧ 2 is performed intermittently with the passage of time, and the polarity reversal is performed every frame at other times. method for driving a liquid crystal display device according to any one of claims 1 to 3,. 6. A method for driving a liquid crystal display device in which a liquid crystal having spontaneous polarization induced by applying an intrinsic or electric field is sandwiched between a pixel electrode and a counter electrode arranged in a matrix. The liquid crystal display device according to claim 1, wherein the polarity of the voltage applied between the pixels is inverted only for a part of the pixels of the screen, and the pixels whose polarity is inverted are sequentially changed when the screen is rewritten to complete the polarity inversion of the entire screen. Drive method. 7. A first substrate, a plurality of pixel electrodes arranged in a matrix on the first substrate, and a surface of the first substrate on which the plurality of pixel electrodes are formed. A second substrate provided; a counter electrode formed on the second substrate so as to face the plurality of pixel electrodes; held between the first substrate and the second substrate Was
And a liquid crystal having a spontaneous polarization induced by applying an electric field or an electric field, wherein the liquid crystal display device periodically changes the polarity of at least a part of a voltage applied between the plurality of pixel electrodes and the counter electrode. An inversion operation, and a writing operation of applying the voltage to the pixel electrode to hold a display voltage based on the voltage in the pixel electrode. The period of polarity inversion is TS, the response time of the liquid crystal is τ, and
When the time is TK , the liquid crystal display device is operated so as to satisfy the following condition : TS / TF ≧ τ / TK ≧ 2. Wherein said write operation, among the plurality of pixel electrodes, according to claim 7, characterized in that it comprises an operation for longer than the pixel electrode is not performed polarity inversion writing time to the pixel electrode that performs polarity reversal 3. The liquid crystal display device according to 1. Wherein said write operation, among the plurality of pixel electrodes, according to claim 7, characterized in that it comprises an operation to be larger than the pixel electrode is not performed the write voltage polarity inversion to the pixel electrode that performs polarity reversal 3. The liquid crystal display device according to 1. 10. The operation of inverting the polarity includes an operation of inverting the polarity of a voltage applied between the pixel electrode and the common electrode only for one pixel electrode of a screen. 8. The liquid crystal display device according to claim 7 , comprising a plurality of operations for completing the polarity inversion of the entire screen by sequentially changing the part of the pixel electrodes to be inverted.