JPS62289818A - Driving method for optical modulation element - Google Patents

Abstract

PURPOSE:To execute a gradation display having a good picture quality by providing a potential gradient in at lest one conductive film surface for constituting a picture element, and inputting a signal which has been modulated by a voltage, width of a pulse of the number of pulses. CONSTITUTION:A display conductive film 2, and parallel electric transmission electrodes of an equal internal are superposed on a substrate 1, an opposed electrode film 4 is placed so as to be orthogonal to the electrode 3 on an opposed substrate, a picture element A is formed in a crossing part, and a ferroelectric liquid crystal is inserted and held between the film 2 and 4. When electrodes 3a, 3c are set to a reference potential VE=0, and a signal voltage Va is applied to an electrode 3b, a potential gradient of Va is generated in length directions L1, L2 in the film 2 surface. When an inverted threshold value Vth=Va of a liquid crystal is set and -Vb is applied to the opposed electrode 4, Va+Vb is applied to a liquid crystal of length directions m1, m2 in the film 2 surface, and converted to dark. Accordingly, the gradient can be controlled by modulating -Vb in accordance with a gradation at every picture element, or modulating width of a pulse or the number of pulses in accordance with gradation information, and the gradation display can be executed with a good picture quality.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Industrial Field of Application] The present invention relates to a method for driving an optical modulation element for a display panel, and more particularly relates to a method for driving an optical modulation element for a display panel. This invention relates to a method for driving a display panel using dielectric liquid crystal, particularly a liquid crystal optical element suitable for gradation display.

[Prior Art] In a liquid crystal television panel using a conventional active matrix driving method, thin film transistors (TPTs) are arranged in a matrix for each pixel, and a gate-on pulse is applied to the TPTs to bring the source and drain into a conductive state. At this time, a video image signal is applied from the source and stored in the capacitor, and a liquid crystal (for example, twisted nematic, TN-liquid crystal) is driven in response to this stored image signal.
At the same time, gradation display is performed by modulating the voltage of the video signal.

[Problems to be solved by invention C] However, in active matrix drive type television panels using such TN liquid crystals, the TPT used is
Because TPT has a complicated structure, the number of structural steps is large, and high manufacturing costs are a bottleneck.In addition, the thin film semiconductors (e.g., polysilicon, amorphous silicon) that make up TPT cannot be spread over a large area. There are problems such as difficulty in forming a film over the entire area.

On the other hand, a display panel using a passive matrix drive method using 7N liquid crystal is known as a device that can be manufactured at a low manufacturing cost, but in this display panel, as the number of scanning lines (N) increases, During scanning, the time during which an effective electric field is applied to one selected point (duty ratio) decreases at a rate of 1/N, resulting in cross-resistance and high-contrast images. In addition, when the duty ratio becomes low, it becomes difficult to control the gradation of each pixel by voltage modulation, so it is not suitable for display panels with high density wiring, especially LCD television panels. Not yet.

In view of these problems, an object of the present invention is to provide a method for driving a display panel having high-density pixels over a wide area, and in particular, a method for driving an optical modulation element suitable for gradation display. be.

[Means for Solving the Problems] The present invention provides a first substrate with a conductive film and two or more transmission electrodes connected to the conductive film, and a second substrate with a conductive film and two or more transmission electrodes that intersect and face each other. comprising a plurality of restripe-shaped electrodes or conductive films arranged and two or more transmission electrodes connected to the conductive films, and an optical modulation material disposed between the first substrate and the second substrate. , Applying a sequential scanning gradation signal to the transmission electrodes, and connecting the transmission electrodes to which the sequential scanning gradation signal is not applied to a reference potential point, and synchronizing with the sequential scanning gradation signal to the striped electrode or the transmission electrode. This is a method of driving an optical modulation element in which a scanning signal having gradation information is applied to a reference potential point, and the method is characterized in that a fixed bias voltage or a bias voltage proportional to a gradation signal is applied to a reference potential point.

The optical modulating substance used in the present invention has a first optically stable state (for example, a bright state) and a second optically stable state (for example, a dark state) depending on the applied electric field. ), that is to say has at least two stable states with respect to an electric field, in particular liquid crystals having such properties are used.

As the liquid crystal having bistability that can be used in the driving method of the present invention, chiral smectic liquid crystal having ferroelectricity is most preferable, and among these, chiral smectic liquid crystal
Phase (SmC・), H phase (SmH, 1 phase (Sm!”)
, F-phase (SmF) or G-phase (SmG) liquid crystals are suitable.This ferroelectric liquid crystal is described in "Le JOURNARD DE Physique L'Etre" (LE JOURN).
36th
Volume (L-f79) 1975 “Ferroelect
LIQUID LIQUID
``Ristas'' (rFerroe)
electric Liquid Crystals')
; “Applied Physics Letters” (
Applied Physics Letters'' Volume 36, No. 11, 1980
(rsubmicro 5econdBistable
Electrooptic Switching i
ferroelectric liquid crystals disclosed in these publications can be used in the present invention.

More specifically, an example of a ferroelectric liquid crystal compound used in the method of the present invention is decyloxybenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMB-
C), hexyloxybenzylidene-P'-amino-
2-Chloropropyl cinnamate (HOBACPC) and 4-o-(2-methyl)-butylresolsiliten-
Examples include 4'-octylaniline (MBRA8).

When constructing an element using these materials, the liquid crystal compound may be SmC", SmH umbrella, SmI", SmF",
In order to maintain the temperature state such that SmG'', the element can be supported by a copper block or the like in which a heater is embedded, if necessary.

Note that FIG. 13 schematically depicts an example of a ferroelectric liquid crystal cell. 101 and 101' are In203, 5n0
It is a substrate (glass plate) coated with a transparent electrode such as 2 or ITO (indium tin oxide), between which a liquid crystal molecular layer 102 is oriented perpendicular to the glass surface. A lCI phase liquid crystal is sealed. A thick line 103 represents a liquid crystal molecule, and this liquid crystal molecule 10
3 is a dipole moment in the direction perpendicular to the molecule) C
P, ) 104. When a voltage higher than a certain threshold is applied between the electrodes on the substrates 101 and 101', the helical structure of the liquid crystal molecules 103 is unraveled, and the liquid crystal molecules 103 are twisted so that all dipole moments (P) 104 are oriented in the direction of the electric field.
The orientation direction can be changed. The liquid crystal molecules 103 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, if polarizers are placed above and below the glass surface in a crossed nicol positional relationship, It is easily understood that this is a liquid crystal optical modulation element whose optical characteristics change depending on the polarity of applied voltage. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1 µl), the helical structure of the liquid crystal molecules unravels (non-helical structure) even when no electric field is applied, as shown in Figure 14, and the bipolar structure The child moment P or P' is directed upward (114) or downward (11
4'). As shown in FIG. 14, when an electric field E with different polarity above a certain threshold value is applied to such a cell, the dipole moment electric field E or E
114 upward or 1 downward corresponding to the electric field vector of '
14', and accordingly the liquid crystal molecules are in the first stable state 113 (bright state) or in the second stable state 113'.
(dark state).

There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. First, the response speed is extremely fast.
Second, the alignment of liquid crystal molecules has bistability.

To explain the second point with reference to FIG. 14, for example, the electric field E
When the electric field is applied, the liquid crystal molecules are aligned in the first stable state 113, but this state remains unchanged even when the electric field is cut off.
13 is maintained and an electric field E' in the opposite direction is applied, the liquid crystal molecules align to the second stable state 113' and change their orientation, but they remain in this state even after the electric field is cut off, and each It has a memory function in a stable state. In order to effectively realize such fast response speed and bistability, it is preferable for the cell to be as thin as possible, and generally 0.5 to 20 l, particularly Ig to 5 l is suitable. ing. A liquid crystal-electro-optical device having a matrix electrode structure using ferroelectric liquid crystals of this kind has been proposed by Clark and Ragabal in US Pat. No. 4,387,924, for example.

[Function] That is, the present invention includes a first substrate having a first conductive film, a second substrate having a second conductive film facing the first conductive film, and the first substrate. an optical modulating substance disposed between the first conductive film and the second conductive film;
The optical modulation element is characterized by a method of driving the optical modulation element in which a signal having gradation information is applied to the conductive film or the second conductive film. That is, the present invention applies an in-plane potential gradient to at least one of two opposing conductive films constituting one pixel, and applies a pulse signal or a pulse signal having a peak value corresponding to the gray scale to the other conductive film. The gradation is expressed by applying a signal with a pulse width or number of pulses corresponding to the gradation, and forming regions within the pixel where the inversion threshold voltage is exceeded and regions where the inversion threshold voltage is not exceeded.

In the present invention, this area is set by applying a bias voltage, and the inversion of the liquid crystal is controlled by making the bias voltage correspond to the gradation.

[Example] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a perspective view showing one embodiment of an optical modulation element used in the present invention. In FIG. 1, 1 is one substrate, and 2 is a display conductive film laminated on the substrate l. Reference numeral 3 denotes transmission electrodes made of a low-resistance metal film, which are laminated in parallel on the display conductive film 2 at equal intervals. Further, another substrate (not shown) is opposed to the substrate, and a counter conductive film (or counter electrode) 4 is arranged to intersect with the transmission electrode 3 on the other substrate. 1 display conductive film 2 and opposing conductive film II in which pixel region A is formed! 4, the optical modulating material is sandwiched between them.

In the liquid crystal optical element configured as described above, the transmission electrode 3
By applying a potential gradient within the surface of the display conductive film 20 by the signal voltage applied to the display conductive film 20, a potential difference gradient is generated in the electric field between the display conductive film 20 and the counter electrode 4. At this time, when the transmission electrodes 3a and 3c are connected to the reference potential point VE (for example, O port) and a predetermined signal voltage Va is applied to another transmission electrode 3b, the transmission electrode 3a and 3b
A potential gradient of Va can be applied in the in-plane length directions L1 and L2 of the conductive film 2 between 3b and 3C. At this time, the inversion threshold voltage Vth of the ferroelectric liquid crystal is set as Va,
When -Vb is applied to the counter electrode 4, as shown in FIG. 2(b), a potential difference v greater than the inversion threshold voltage Vth between the in-plane length direction ml of the conductive film 2 and the ferroelectric liquid crystal corresponding to ml,
+vb will be applied, and the resulting ml and ml
For example, the area corresponding to the area can be inverted from a bright state to a dark state.

Therefore, in the present invention, gradation can be expressed by applying Vb to each pixel at a value corresponding to the gradation. At this time, the voltage value of the voltage signal -Vb applied to the counter electrode 4 may be modulated according to the gradation information, or the pulse width may be modulated according to the gradation information, or the pulse width may be modulated according to the gradation information. The gradation can also be controlled by modulating the number.

Further, in the present invention, prior to applying the gradation signal,
After going through an erasing step that puts the pixel into either a bright state or a dark state, an inversion voltage that reverses that state is controlled according to the gradation and applied to the ferroelectric liquid crystal. It is necessary.

However, in reality, to prevent crosstalk, va
<Vth and Vb <vth. Therefore, in FIG. 2(b), ml<Ll, ml<L2, and in the vicinity of electrodes 3a and 3C, Va +Vb<
Vth, and the liquid crystal no longer performs inversion operation in this portion.

In order to prevent this, in the present invention, as shown in FIG. 3, every other transmission electrode, for example, 3a and 30 are combined into 3e, and this 3e is set to a fixed bias voltage VE (
(not shown). However, in order to prevent crosstalk, VE<Vth must be satisfied.

In this way, a potential gradient as shown in FIG. 4(a) is generated between the transmission electrodes 3a and 3b, and between 3b and 3C. When -Vb is applied to the opposing electrode 4 in FIG. A potential gradient as shown in FIG. 4(b) is applied to the regions L1 and L2 in FIG. Here, Vb <Vt
h, VE <Vth, but vb and VE can be selected so that vb+■E>vth, so L
Va + vfi + VE over the entire region of l, L2 >
The condition of Vth is achieved, the entire area becomes an effective pixel area, and the effective aperture ratio can be increased.

In the above method, the reference point 3e in FIG.
If V death,
There is a method of interposing a fixed resistor R[. However, fixed resistance R
As a constraint on E, in FIG. 4, in order to prevent crosstalk, V, +VE <Vth. Also, since V is a scanning signal or a gradation signal, the maximum value of Va, Vaffiax, is actually VallaX
≦Vth.

Therefore, it is desirable for vE to satisfy the following conditions.

0<VE≦Vth/2 Furthermore, in FIG. 3, when the electrical resistance between the electrodes 3b and 3e is Ra, the relationship between ■[ and va is as follows.

VE = RE / (Ra + RE) ・V.

That is, when va is a grayscale signal, the bias value vE is proportional to the grayscale signal ■a. Also, considering the preferable constraint conditions of V[ mentioned above, since V, < Vth, VE = RE / (Ra + RE)
- Since Va ≦Vth/2, RE < Ra, and more preferably, for example, VE = Vth/10 t 6 (!l”, or VE + V6 < Vth
In other words, the fixed resistance Rε that provides the bias voltage Vε is desirably smaller than 1/8 of the combined resistance between the transmission electrode and the transmission electrodes on both sides thereof.

As described above, in the present invention, by providing the bias voltage vE, it is possible to increase the effective aperture ratio.

Further, a description will be given of a yet-to-be-discovered preferred specific example.

A transparent conductive film of 5n02 with a thickness of about 20 OA was formed on the glass substrate l shown in FIG. 1 by sputtering to form a display conductive film 2. The sheet resistance of this 5n02 film was 106Ω/mouth. Next, Aβ was vacuum-deposited to a thickness of 100 Å on the 5n02 film and patterned again to form a plurality of transmission electrodes 3 as shown in FIG. did. The sheet resistance of this electrical transmission electrode 3 was about 0.4Ω/hole, and its width was about 201L. On the other hand, an ITO film covering area A was provided as a counter electrode 4 on the counter substrate. The sheet resistance of the ITO film serving as the counter electrode 4 was about 20Ω/hole.

A polyvinyl alcohol layer of about 50 OA was formed as a liquid crystal alignment film on the surface of each of the two substrates thus created, and a rubbing treatment was performed.

Next, the two substrates were placed facing each other, the gap was adjusted to about 1 mm, and the ferroelectric liquid crystal (p-η-octyloxybenzoic acid-p'-(2-methylbutyloxy) phenyl ester) and p -η-7nyloxybenzoic m-p'-(2-methylbutyloxy)phenyl ester-based liquid crystal composition) was injected. The shape of the partial pixel A where the display conductive film 2 and the counter electrode 4 overlapped was 230 g x 230 g, and the capacitance after the liquid crystal was injected was about 3PF. However, the width of pixel A was set to I, +/2 + L2/2. Then, polarizing plates were arranged in a crossed nicol configuration on both sides of the liquid crystal cell formed in this way, and the optical characteristics were observed.

FIG. 6 is a schematic diagram showing a method of applying an electric signal, in which a display conductive film 2 is formed on a substrate 1, a power transmission electrode 3 is further arranged on it, and a counter substrate 61 is provided with an electrically conductive film 2. A conductive layer 82 is formed, and a ferroelectric liquid crystal 83 is sandwiched between the two substrates. The counter conductive film 62 is connected to a first drive circuit 64 , and the display conductive film 2 or the power transmission electrode 3 is connected to a second drive circuit 65 . The resistor 68 is a fixed resistor RE that provides the bias voltage vE shown in FIG. 5, and is 80Ω. 7 shows the waveform of the signal (a) generated in the drive circuit 64 of FIG. 6, and FIG. 8 shows the waveform of the signal (b) generated in the drive circuit 65 of FIG.

Now, as signal (a) -12V, 200IL
sec pulse, 8V as signal (b), 200μs
Synchronize the ec pulse in advance (this is called an erase pulse)
With the erase step provided, the liquid crystal is switched to the first stable state, and the entire pixel A is in the bright state (so arranged the cross polarizers). From this state, the 8th
The optical characteristics of pixel A when various pulses as shown in Figures (a) to (e) are applied to the counter electrode 4 in synchronization with the pulse shown in Figure 7 which is applied to the transmission electrode 3 as a signal (b). The state is shown in FIG.

When the pulse applied voltage is -2V as shown in FIG. 8(a) and -5V as shown in FIG. 8(b), the change from the bright state 31 is as shown in FIG. 9(a). Although this does not occur at all, when the pulse applied voltage is 18'V as shown in FIG. 8(C), the liquid crystal near the transmission electrode 3 switches to the dark state 92 as shown in No. 91N(b).

Furthermore, when the applied voltage is increased to -14V as shown in FIG. 8(d), the area of the dark state 92 becomes wider as shown in FIG. 9(C), and the applied voltage is increased to 14 V as shown in FIG. 8(e). At 20V shown in FIG. 9(c), the entire screen A is in a dark state 9.
Switched to 2. In this way, an image with gradation can be formed over the entire area between the electrodes.

Also, a triangular wave signal (a) as shown in Figure 1θ
Even when signals (b) with various pulse widths as shown in FIGS. 11(a) to (e) are applied synchronously, the optical state change shown in FIG. 9 can be shown. Can be done. At this time, by applying the pulse shown in FIG. 10 to the transmission electrode, and in synchronization with this pulse, applying the pulse shown in FIGS. 11(a) to (e) to the counter electrode 4 according to the gradation, It is possible to express gradation.

The liquid crystal 83 of this embodiment shown in FIG. 6 is a ferroelectric liquid crystal, more preferably a chiral smectic liquid crystal in a bistable state.

Further, as the transmission electrode 3 and the counter electrode 4 of the present invention, metals such as gold, silver, copper, chromium, etc. can be used in addition to aluminum (Ai'), and preferably the sheet resistance thereof is 102Ω/portion. The following shall apply. Furthermore, as the conductive film 2 to which a potential gradient is applied, a transparent conductive film having a sheet resistance of 10 KΩ/hole to IMΩ/hole can be used. sheet resistance.

can be designed to an appropriate value by adjusting the thickness of the transparent conductive film.

FIG. 12 is a perspective view showing a specific example in which the gradation expression according to the present invention is applied to matrix driving.

The display panel shown in FIG. 12 has a plurality of striped conductive films 11a, llb, llc, . Transmission electrodes 12a, 12
b.

12c,... and 13a, 13b, 13c,...
- is wired.

Opposing electrodes 14a and 14b made of a striped conductive film are arranged on a counter substrate (not shown) facing the substrate 1, and a ferroelectric liquid crystal is arranged between the striped conductive film 11 and the counter electrode 14. ing.

According to the driving method of the present invention, prior to writing, all or a predetermined part of the pixel formed at the intersection of the striped conductive film 11 and the striped counter electrode 14 is placed in either a bright state or a dark state. or, for each writing line, all or a predetermined part of the pixels on the line are placed in either the bright state or the dark state at one time prior to writing, and then one of the states is set. Transmission electrode 12a.

12b, 12c, . . . , the pulses shown in FIG. 2 volts), the electrical transmission electrodes 12 and 1 are sequentially connected to the striped conductive film 11.
A potential gradient between 3 and 3 can be applied. At this time, it is preferable that the scan selection signal be a pulse having a voltage equal to the inversion threshold voltage of the ferroelectric liquid crystal.

On the other hand, the plurality of striped counter electrodes 14 are arranged in synchronization with the scanning selection signal applied to the transmission electrode 12 for each electrode, as shown in FIGS. 8(a) to (e) or FIGS. e)
By applying a voltage signal corresponding to gradation information as shown in , it is possible to write to pixels on a scanning line according to the gradation. Therefore, by performing the above-mentioned writing in line-sequential writing, it is possible to form one screen having gradation.

Further, in the present invention, the pulses shown in FIG. 7 or 10 are sequentially applied to each of the striped electrodes 14 as a scanning signal, and in synchronization with this scanning signal, the pulses shown in FIG. ) to (e) or a voltage signal corresponding to gradation information as shown in FIGS. 11(a) to (e) is applied, and the other power transmission electrode 13a.

A screen with gradation can also be formed by connecting 13b, 14c, . . . to a reference potential point (for example, 2 volts).

Furthermore, in the present invention, as shown in FIG. 12, since it is possible to write on the entire surface of pixel A by using only one side 12 of the pair of transmission electrodes as a signal electrode, both sides 12 and 13 of the pair of transmission electrodes can be used as signal electrodes.
(For example, when driving the reference potential point at 0 port, as explained above, it is not possible to invert the liquid crystal over the entire pixel surface by driving only one side of the transmission electrode, and the pixel In order to invert the entire surface, both sides of the pair of transmission electrodes must be driven alternately)
Compared to this, the number of signal driving elements can be reduced to 1/2.

Regarding the optical modulation element, the most preferable example is a ferroelectric liquid crystal, particularly a ferroelectric liquid crystal having at least two stable states, but the present invention is not limited thereto, and may be applied to other optical modulation elements. It can also be applied to twisted nematic liquid crystals, guest host liquid crystals, etc. as modulation elements.

[Effects of the Invention] As explained above, according to the present invention, a potential gradient is formed in the plane of at least one conductive film constituting a pixel, and is modulated by the voltage value, pulse width, number of pulses, etc. as an input signal. By applying the gradation signal, it is possible to perform gradation display, and it is possible to provide a method for driving an optical modulation element that significantly improves the image quality in gradation display.

[Brief explanation of the drawing]

1 and 12 are perspective views of the basic structure of an optical modulation element implementing the driving method of the present invention, FIGS. 2 and 4 are potential gradient diagrams thereof, and FIGS. Figure 6 is a wiring diagram, Figures 7, 8, 10, and 11 are voltage waveform diagrams, and Figure 9 is a wiring diagram.
The figure is a state diagram of a pixel, and FIGS. 13 and 14 are schematic orientation diagrams of ferroelectric liquid crystal. 2, 12.82: conductive film, 3, 13.83: power transmission electrode, 4, 14.64: counter electrode.

Claims (2)

[Claims] (1) a first substrate having a conductive film and two or more transmission electrodes connected to the conductive film; a second substrate having a plurality of striped electrodes arranged opposite to each other and intersecting with the transmission electrodes; an optical modulation material disposed between a first substrate and a second substrate, a transmission electrode to which a sequential scanning grayscale signal is applied to the transmission electrode and to which the sequential scanning grayscale signal is not applied; is connected to a reference potential point, and a scanning signal having gradation information is sequentially applied to the striped electrode in synchronization with the scanning gradation signal. A method for driving an optical modulation element, characterized by applying a bias voltage proportional to a signal. (2) A first substrate having a conductive film and two or more power transmission electrodes connected to the conductive film, a conductive film disposed to face and intersect with the power transmission electrode, and two or more power transmission electrodes connected to the conductive film. a second substrate having an electrode; and an optical modulation material disposed between the first substrate and the second substrate, and sequentially applying scanning grayscale signals to the transmission electrodes of the first substrate. Optical modulation in which the transmission electrode to which the sequential scanning gradation signal is not applied is connected to a reference potential point, and a scanning signal having gradation information is applied to the electrode of the second substrate in synchronization with the sequential scanning gradation signal. A method for driving an optical modulation element, the method comprising applying a fixed bias voltage or a bias voltage proportional to a gradation signal to a reference potential point.