JPS619624A - Driving method of liquid crystal element - Google Patents

Abstract

PURPOSE:To execute display of a large image screen at a high speed by impressing a scanning signal to the gates of the FETs corresponding to respective picture elements and prescribed display signals to selected counter electrodes to execute writing and impressing the prescribed display signals to another selected counter electrodes to execute writing. CONSTITUTION:A ferroelectric liquid crystal having a bistable state is held in place between a substrate provided with picture element electrodes corresponding to the FETs and a substrate provided with the counter electrodes, by which a liquid crystal element is constituted. The counter electrodes of the FETs constituting the active matrix are connected to display electrodes 7 and the gates are connected to scanning electrodes 6. The sources or drains are grounded. The prescribed scanning signal is impressed to the electrodes 6 and the prescribed display signals are impressed to the selected electrodes 7 to write the display state based on the 1st orientation state of the liquid crystal and the prescribed display signals are impressed to another selected electrodes 7 to write the display state based on the 2nd orientation state, by which the time dividing driving is executed. The display of the large screen image with a large number of the picture elements and at the high speed is thus executed.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to a method for driving optical shutter arrays, image display devices, etc. using liquid crystals, and more specifically relates to bistable liquid crystals, particularly ferroelectric liquid crystals. The present invention relates to a method of driving a liquid crystal using an active matrix configuration.

[Prior Art] Conventionally, a liquid crystal display device has a scanning electrode group and a signal electrode group configured in a matrix, and a liquid crystal compound is filled between the electrodes to form a large number of pixels, thereby displaying a C image or information. is well known. The driving method for this display element is as follows:
Time-division driving is used in which address signals are selectively applied to the scanning electrode group in a sequential and periodic manner, and predetermined information signals are selectively applied in parallel to the signal electrode group in synchronization with the address signal. Display elements and their driving methods have had major and potentially fatal drawbacks as described below.

In other words, it is important to increase the pixel density or make the screen larger.Among conventional liquid crystals, the response speed is relatively high, and the electricity consumption is low, making it difficult to use as a display device. Most of them are, for example, M,
5chadt and W. He1frich, “Appl.
ied Physics' Letters”, Vo
l, 18. No, 4 (1!171.2.15),
P, 127-1128 °'Voltage-
Dependent 0ptical Activit
y of a Twisted Nematic Liq
TN (twist
ed nematic) type liquid crystal (7)? l
- Yes, in this type of liquid crystal, the molecules of nematic liquid crystal, which has positive dielectric anisotropy in the absence of an electric field, form a twisted structure (helical structure) in the liquid crystal layer thickness direction, and both electrode surfaces The molecules of this liquid crystal form a structure in which they are arranged in parallel to each other.

On the other hand, in the state where an electric field is applied, Nemati Nku! has positive 1-shuden anisotropy! The latter are aligned in the direction of the electric field, resulting in optical modulation.

When a display element is constructed with a matrix electrode structure using this type of liquid crystal, liquid crystal molecules are arranged perpendicularly to the ':1j:polar plane in the area (selection point) where both the scanning electrode and the signal electrode are selected. A voltage higher than the threshold required to
No voltage is applied to a region where neither the scanning electrode nor the beam electrode is selected (non-selected point), and therefore the liquid crystal molecules maintain a stable alignment parallel to the electrode plane. By placing linear polarizers above and below such a liquid crystal cell in a cross-Nicol relationship with each other, light is transmitted at selected points and at non-selected points, making it possible to use it as an image element. becomes. However, when a matrix electrode structure is configured, there are areas where scan electrodes are selected and signal electrodes are not selected, or areas where scan electrodes are not selected and signal electrodes are selected (so-called "l'-selection point"). ′) is also subject to a finite electric field. If the difference between the voltage applied to the selected point and the voltage applied to the half-selected point is sufficiently large, and the voltage threshold required to align liquid crystal molecules perpendicular to the electric field is set to a voltage value in between, the display element will function normally. This is why it works. However, in this method, the scanning line Ml (N
), the time during which an effective electric field is applied to one selected point while scanning the entire screen (one frame) (duty ratio) decreases by one command at a rate of 1/N. For this reason, the effective voltage difference between the selected point and the non-blocked point when repeated scanning is performed becomes smaller as the number of scanning lines increases, resulting in a decrease in image contrast and Crosstalk is an unavoidable drawback.

This phenomenon is caused by liquid crystals that do not have a bistable state (the stable state is when the liquid crystal molecules are aligned horizontally with respect to the electrode surface, and they are aligned vertically only while an electric field is effectively applied). ) is an unavoidable problem that arises when driving (that is, repeatedly scanning) using the temporal accumulation effect. In order to improve this point, the voltage averaging method, 2
Frequency driving methods and multiple matrix methods have already been proposed, but none of these methods are sufficient, and the ability to increase the screen size and density of display devices has reached a plateau due to the inability to increase the number of scanning lines sufficiently. This is the current situation.

[Problems to be Solved by the Invention] An object of the present invention is to provide a new method for driving a ferroelectric liquid crystal device, especially a ferroelectric liquid crystal device, which solves all the problems in conventional liquid crystal display devices as described above. It's about providing people.

That is, in the present invention, a ferroelectric liquid crystal having a fast voltage response speed and a memory property of 2 is formed by an active matrix.
By applying a directional electric field, there are two states: bright and dark! The object of the present invention is to provide a method for driving a ferroelectric liquid crystal that can display a large screen with a large number of pixels and display images at high speed by driving the ferroelectric liquid crystal in a large number of pixels.

[Means for Solving the Problems] and [Operations] The method for driving a liquid crystal element according to the invention is such that a pixel electrode connected to a first terminal, which is a terminal other than the gate of an FET (field effect transistor), corresponds to the FET. a ferroelectric liquid crystal having a bistable state with respect to an electric field between the pixel electrode and the counter electrode; A method for driving a liquid crystal element having a sandwiched structure, in which an electric field is formed between a first terminal and a second terminal, which are terminals other than the gate of the FET, in synchronization with the application of a signal that turns the gate of the FET into a gate-on state. The first phase controls the alignment of the ferroelectric liquid crystal in the first alignment state, and the electric field of opposite polarity to the electric field formed between the first terminal and the second terminal is applied to the first terminal and the second terminal. A second phase is formed between the terminals to control the alignment of the ferroelectric liquid crystal in a second alignment state, and when a display signal is applied to the counter electrode (a plurality of striped counter electrode groups), This is a time division drive in which the source or drain of the FET terminals corresponding to each pixel is connected to a common terminal and a scanning signal is applied to the gate, and a predetermined scanning signal is applied to the scanning signal line (gate). , apply a predetermined display signal ′ to the selected display signal line (striped counter electrode group) to write a display state based on the first orientation state, and then write a predetermined display signal to another selected display signal line. The display state is written based on the second orientation state by applying a display signal.

The ferroelectric liquid crystal used in the driving method of the present invention takes either the first optically stable state IU' or the second optically stable state depending on the applied electric field, that is, it is bistable with respect to the electric field. A substance having a state, particularly a liquid crystal having such properties, is used.

As the ferroelectric liquid crystal with bistability that can be used in the driving method of the present invention, chiral smectic liquid crystals with ferroelectricity are most preferable, among which chiral smectic C phase (SmC*) or H phase (SmH* ) is suitable. Regarding this ferroelectric liquid crystal, please refer to “LE
JOURNAL DE PHYSIOUELE T
T E, RS″313 (L-Ei9) 1975.
r FerroelectricLiquid Cr
ystals J; Applied physics
Let-ters''313 (11) 1980, r
Submicro 5econd B1-5table
eElectrooptic Switching
In Liquid Crystals J; "Solid State Physics" 1B (141) 1981 r Liquid Crystals' (), and the ferroelectric liquid crystals disclosed therein can be used in the present invention.

More specifically, as an example of the ferroelectric liquid crystal compound used in the method of the present invention, decyloxybenzyritene-P'-amino-2-methylbutylcinnamate (DOBAMBC
), hexyloxybensyritene-P'-7 minnow 2
-Chloroprovir cinname-1. (HOBACPC) and 4-o-(2-methyl)-butylresorsilitene-4'-octylaniline (MBRA8).

When constructing an element using these materials, 7□', which becomes a yield crystal compound, SmC'i phase or SmH* phase,
In order to hold the element in a single ship shape, the element can be supported by a copper block or the like in which a heater is embedded, if necessary.

FIG. 1 schematically depicts an example of a ferroelectric overnight cell. 1 and 1' are Ir+403, '5n
02 and ITO (Inclium-Tin Oxide)
It is coated with transparent electrodes such as (glass plate), and SmC4 phase liquid crystal with liquid crystal molecular layer 2 oriented perpendicular to the glass surface is sealed between them. represents a liquid crystal molecule, and this liquid crystal molecule 3 has a dipole moment (P) in the direction perpendicular to the molecule.
4). 7i1. ; When a constant voltage of 1,'ζ1"- is applied between the electrodes of the plates 1 and 1'-L, the helical structure of the liquid crystal molecules 3 is unraveled, and the dipole moment I・(P
, ) 4 can change the orientation direction of the liquid crystal molecules 3 so that they all face in the direction of the electric field. The liquid crystal molecule 3 has an elongated shape and exhibits refraction anisotropy in its long axis direction and short axis direction. Place the arranged polarizer,
It is easy to understand that -C optical properties change depending on the polarity of voltage application (it becomes a high optical modulation element. Furthermore! f
If the thickness of the K-product cell is sufficiently reduced by 1 (for example, 1 cell), the state in which no electric field is applied is y, as shown in Figure 2.
Even in bees, the helical structure of liquid crystal molecules is broken (non-helical 4
^1 construction), its dipole moment P or P' is upward (
4a) or downward (4b). When an electric field E or E' with a different polarity than the I value is applied for a predetermined time to such a cell as shown in Fig. 2, y,
The polar moment changes its direction upward 4a or downward 4b in response to the electric field vector of the electric field E or E', and accordingly, the liquid crystal molecules change from the first orientation state 5 to the second orientation state jm'45'. Orient yourself towards something.

There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has a bistable state. To explain the second point with reference to FIG. 2, for example, when the electric field E is applied, the liquid crystal molecules are aligned in the first alignment state 5, but in this state y! 1. is stable even when the electric field is turned off. When an electric field E' in the opposite direction is applied, the liquid crystal molecules are oriented to a second orientation state 5' and the orientation of the molecules is changed, but they remain in this state even after the electric field is turned off. Further, as long as the applied electric field E does not exceed a certain threshold value, each orientation state is maintained. 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, the thickness is 0.5 to 2.
0 ru, especially 1 ru to 5 kawa are suitable. A liquid crystal-electro-optical device having a matrix electrode structure using ferroelectric liquid crystals of this type has been proposed by Clark and Ragaval in US Pat. No. 4,387,924, for example.

The present invention utilizes TPTs constituting an active matrix.
It is based on the fact that an element with an FET (field effect transistor) structure, such as a thin film transistor (thin film transistor), can be used as either the drain or the source by reversing the voltages applied to the drain and source.

FET is the element that constitutes the active matrix.
As for the structural element, either amorphous silicon TPT or polycrystalline silicon TFT':g can be used. Further, it is also possible to perform the same operation with a bipolar transistor other than the FET structure.

An N-type FET is in a conductive state when v > ■, where ■ is the drain voltage, Vc is the gate voltage, ■ is the source voltage, and ■ is the threshold voltage between the gate and source. Then, vGkuv,
+V, it becomes a non-conducting state.

P-type FET i: Oite is VD vs vs, vG
Continuity is 7g with v8+vP, vG>vs+v
P becomes non-conductive.

Whether the FET is P-type or N-type, which terminal of the FET acts as the drain and which acts as the source is determined by the direction of voltage application. That is, for N type, the lower voltage side acts as a source, and for P type, the higher voltage side acts as a source.

In ferroelectric liquid crystals, either positive or negative voltage applied to the liquid crystal cell is in the "bright" state, and any deviation is "υ".
The 1"7" state can be freely set depending on the orientation of the polarized optical axes of a pair of polarizers in a crossed nicol state arranged in the upper part of the cell and the long sleeve of the liquid crystal molecules.

Since the present invention performs display by controlling the electric field applied to the liquid crystal cell by controlling the voltage between the terminals of each element of the active matrix, the voltage level of each signal is taken as shown in the following example. This can be carried out without any problems as long as the potential difference between each signal is maintained relatively.

[Example] Next, a specific example of a method for driving a ferroelectric liquid crystal using an active matrix of the present invention will be described based on FIGS. 3 to 71/1.

FIG. 3 is a circuit diagram of an active matrix, FIG. 4 is an explanatory diagram showing addresses of corresponding pixels, and FIG. 5 is an explanatory diagram showing an example of display of corresponding pixels.

6 is a scanning electrode group, and 7 is a display electrode group.

FIG. 6(a) is a diagram showing a scanning signal, in which the phase tl
+t2 + . . . shows electrical signals applied to each selected scan electrode and electrical signals applied to other scan electrodes (unselected scan electrodes). FIG. 6(b) is a diagram showing the display signal, and the phase tl
+j2 + . . . shows electrical signals applied to selected display electrodes and unselected display electrodes, respectively.

In FIG. 6, the horizontal axis represents time and the vertical axis represents voltage. For example, when displaying a moving image, the scanning electrode groups 6 are sequentially and periodically selected. As shown in FIG. 6(a), the electrical signal applied to the selected scanning electrode is +0 during "bright" writing, and is 0 during "dark" writing.

Further, the electrical signals applied to the other unselected scanning electrodes are <Vc as shown in FIG. 6(a). On the other hand, the electrical signal applied to the selected display electrode is +V during "bright" writing, as shown in FIG. 6(b). -V when writing "dark". It is. Furthermore, all electrical signals given to unselected display electrodes are 0. In the above, each voltage value is set to a desired value that satisfies the following relationship.

When writing "bright" to the scan electrode m=q line using the signal line of display electrode n=11, and then writing "dark" to the scan electrode m=q line using the display electrode n=Q2, v cIllv p > V L C'+ V S
, (m=q, n 2 sentences 1) Vs +vLckuvcn (n 2 sentences 1) vGm=
o (m=q, n=sentence,)
vS −vLC””Cn (n =
up ) v cmv p < vcn,
(m≠q) However, each symbol represents the following car terms.

■, Gate electrode (scanning signal) voltage m V: Counter electrode (display signal) voltage Cn ■,: Source or drain (common terminal) voltage vLC' Absolute value of threshold voltage of ferroelectric liquid crystal v,: Between gate and source Writing is performed repeatedly for q=l−N operations exceeding the threshold value. At this time, the counter electrode can be formed into a stripe shape as shown in FIG.

Among the pixels when such an electric signal is applied, the write operation of the pixel in FIG. 4, for example, represents the seventh state.

That is, as is clear from FIG. 7, phase 1. The pixel P on the scanning line selected in , has a threshold value V
V L C< V sN+1. N
A voltage of LCVc is applied. Therefore, in FIG. 4, pixel "N+1.N" changes orientation and becomes "dark".
transfer (sinch) to. Next, at phase t2, pixels P, PN+2, . N to N
, N is the threshold 1 (i-V voltage V L C> V
S V C is applied. Therefore, pixel P
, PN, N N+2. N transfers (switches) to "light". The operation in phases t3 to t6 after phase t2 is similar to t1 to t2 above, in which "dark j" is first written to the pixels on the selected scanning line and display line, and then the previous pixel on the same scanning line is written. "Bright" is written to the pixels that were not selected. As can be seen from each operation described above, regardless of whether a display electrode is selected on the selected scanning electrode line, when the display electrode is selected, the liquid crystal molecules are in the first alignment state or the second alignment state. Align the orientation to the state,
The pixel becomes ON (bright) or OFF (dark),
On unselected scan lines, none of the voltages applied to all pixels exceeds the threshold voltage. Therefore, as shown in FIG. 7, the liquid crystal molecules in each pixel other than the selected scanning line "2" maintain the orientation corresponding to the signal state (QN-1) when scanned last time without changing the orientation state. ,
It is kept as is. That is, when a scanning electrode is selected, one tin's worth of signals is written, and the signal state yu4 can be held until the next selection after one frame is completed. Therefore, even if the number of scanning electrodes increases, the actual duty ratio does not change and the contrast does not deteriorate at all.

In FIG. 5, scanning electrodes G'G, GN+2.・
...N' N formed at the intersection of N' N+1 and display electrode CCC...
+1'N+2' Of the pixels, the pixels in the shaded area are 'IJIf J-shaped 7U:
It is assumed that the pixels shown in white correspond to the "bright" state. Now, note the display above the display electrode SN in FIG. ; is. The display pattern shown in FIG. 5 is completed by each operation of the phases t1 to t6.

In FIG. 6, the drive waveform is a voltage signal with three levels for both the scanning signal and the display signal, but the potential of the counter electrode used as a common electrode is set to GND when writing the first display state. The second display state 1;; includes +
(2) By setting it to 8, both the scanning signal 1j and the display signal can be driven with a voltage signal of 2 levels.

An example of a driving waveform using two levels of voltage is shown in the eighth [/1].

In the method for driving a ferroelectric liquid crystal of the present invention, the arrangement of the scanning electrode and the (iM electrode) is arbitrary, for example, as shown in FIG.
, As shown in (b), it is also possible to arrange the pixels at 1j, and by arranging them in this way, it can be used as a shack-area, r, etc.

Next, in the embodiment described above, the forcible strength l(
Preferred specific values for driving the mouth OBAMBG as a k product are, for example, human power frequency f, = lXlO4~lXl06H210<
I v61 <80V (peak value) 0.3 < l
VSl < IOV (wave height value).

Figure 1O shows F in the TFT used in the present invention.
11 is a cross-sectional view of a ferroelectric liquid crystal cell using TPT, FIG. 12 is a perspective view of the TPT substrate, FIG. 13 is a plan view of the TPT substrate, and FIG. 14 is a cross-sectional view showing the configuration of ET. 13
FIG. 15 is a partial cross-sectional view taken along line A-A' in the figure, and FIG. 15 is a partial cross-sectional view taken along line B-B' in FIG. It shows Mr. Le.

FIG. 11 shows one specific example of a liquid crystal device that can be used in the method of the present invention. Semiconductor film 16 (amorphous silicon with 1-hinged hydrogen atoms) formed on a substrate 20 of glass or plastic film via a gate electrode 24 and an insulating film 22 (silicon nitride film doped with hydrogen atoms, etc.) A TPT composed of two terminals 8 and Ll that are in contact with this semiconductor film, and a pixel electrode 12 (ITO; Indnium Tin) connected to the terminal 11 of the TPT.
O! 1de) is formed.

Furthermore, an insulating layer 13 (polyimide I, polyamide, polyhinyl alcohol, polyparaxylylene, Si
A light shielding film 9 made of O, SiO2), aluminum, or chromium is provided. Substrate 20 serving as a counter substrate
'' is a counter electrode 2+ (ITO; Indniu
nTin O++1de) and an insulating film 22 are formed.

Between the substrates 20 and 20', the above-mentioned ferroelectric liquid 1,
xjJ 23 is being held. Also, this board 2o and 2
A sealing material 25 for sealing the enhanced cardiac liquid crystal 23 is provided around 0'.

Polarizers 18 and 18' in a crossed Nicol state are arranged on both sides of a liquid crystal element with such a cell structure, and the observation point A is Mitsushige Okawa. A reflector plate 18 (diffuse reflective aluminum sheet or plate) is placed 1□9 behind the polarizer 18' so that the four people on the display can see the reflected light. .

Also, in each of the above figures, the source electrode, i・rain′tuni! The term "i" is used only when current flows from the drain to the source. It is also possible for the FET to function as either a source or a drain.

[Effects of the Invention] A consisting of 4i1'J ]'h in -1-. Solved at high speed.

It is possible to display both clear images.

[Brief explanation of drawings]

1, and 2 are perspective views schematically showing the ferroelectric liquid crystal used in the method of the present invention, FIG. 3 is a circuit diagram of a matrix electrode used in the method of the present invention, and FIG. 4 is a corresponding pixel. FIG. 5 is an explanatory diagram showing an example of a table of corresponding pixels. FIGS. 6(a) and (b) are explanatory diagrams showing electrical signals applied to the scanning electrode and display electrode. The figure is an explanatory diagram showing the write operation to each pixel, and FIG. 8 is based on two levels of voltage! 5 (Explanatory diagram of drive waveform, Fig. 9 (a) and (b)
) is a wiring diagram showing an example of an active matrix circuit and pixel arrangement (Fig. 10 is a cross-sectional view showing the configuration of FET in a TFT, Fig. 11 is a cross-sectional view of an accentuated liquid crystal cell using TPT, 12 is a perspective view of the TPT substrate, FIG. 13 is a plan view of the TPT substrate, FIG. 14 is a partial sectional view taken along line A-A', and FIG. 15 is a partial sectional view taken along line B-B'. 1.1 ', the transparent electrode is coated] (plate 2, liquid crystal molecule layer 3, liquid crystal molecule 4; dipole moment (P) 4a; upward dipole moment 4b, downward dipole moment 5: first orientation State 5′; Second orientation yl 9; Light shielding film 10; N+ layer 11: Drain′ITr, pole (source electrode) 12;
Pixel electrode 13; Insulating layer 14; Substrate 15; Light shielding film 16 directly under the semiconductor: Semiconductor 17; Transparent electrode 18 in gate wiring portion: Reflector
19. l! 1', polarizing plate 20, 20'; transparent substrate 21 made of glass, plastic, etc.; counter electrode 22; insulating film 23; ferroelectric liquid crystal layer 24; gate electrode 25; sealing material 26. Thin film semiconductor 27: gate wiring 28; panel, (plate 28; gate portions 1' to M' having a light blocking effect, scanning electrodes 1-N, display electrode, common electrode LC;

Claims (1)

[Claims] (1) It has a first substrate provided with a plurality of pixel electrodes corresponding to the FETs connected to a first terminal which is a terminal other than the gate of the FET, and a second substrate provided with a counter electrode facing the pixel electrodes. , a method for driving a liquid crystal element having a structure in which a ferroelectric liquid crystal having a bistable state with respect to an electric field is sandwiched between the pixel electrode and the counter electrode, the method comprising: synchronizing with application of a signal to turn the gate of the FET into a gate-on state; a first phase that controls the alignment of the ferroelectric liquid crystal to a first alignment state by forming an electric field between a first terminal and a second terminal that are terminals other than the gate of the FET; A second phase that controls the alignment of the ferroelectric liquid crystal to a second alignment state by forming an electric field between the first terminal and the second terminal with the opposite polarity to the electric field formed between the terminal and the second terminal. It is time-division driving in which a display signal is applied to the counter electrode, and the source or drain of the FET terminals corresponding to each pixel is connected to a common terminal, and a scanning signal is applied to the gate. A predetermined scanning signal is applied to the scanning signal line and a predetermined display signal is applied to the selected display signal line to write a display state based on the first orientation state, and then another selected display signal is applied. 1. A method for driving a liquid crystal element, characterized in that a predetermined display signal is applied to a line to write a display state based on a second alignment state. Driving method of the liquid crystal element described.