JPS6134523A - Liquid crystal device - Google Patents

Abstract

PURPOSE:To carry out display on large sized screen with many number of picture element as well as high speed image display by impressing two electric fields of different direction using active matrix to ferromagnetic liquid crystal which has rapid voltage response and state storability driving to two states, bright and dark. CONSTITUTION:Picture elements PN,N-PN+2,N+2 formed by opposed electrodes and ferroelectric liquid crystal placed between them are arranged in matrix by multiple rows and columns, and each picture element is connected in the unit of row or column commonly each other. Selected picture elements on rows are turned to display a state (drak) based on the first orienting state, and written in each row sequentially then selected picture elements (exclusive of shaded portion) are turned to a display state (bright) based on the second orienting state, and written in each row sequentially, to bright (second orienting state). Namely, depending on whether the display electrodes GN-GN+2 are selected or not on scanning electrode line, the picture elements are turned to bright (second orientating state) when selected, and when not selected, voltage impressed to the elements is kept lower than threshold voltage and maintain the orientation of signal state of the previous scanning.

Description

Detailed Description of the Invention "Field of Industrial Application" The present invention relates to liquid crystal devices such as optical shutter arrays and image display devices using liquid crystals, and more specifically to bistable liquid crystals, particularly ferroelectric liquid crystals. The present invention relates to a liquid crystal device that drives liquid crystal using an active matrix configuration.

[Prior Art] Conventionally, liquid crystal display elements have been used to display images or information by configuring a scanning electrode group and a signal electrode group in a matrix, filling a liquid crystal compound between the electrodes, and forming a large number of pixels. ,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 fatal drawbacks as described below.

That is, it is difficult to increase the pixel density or enlarge the screen. Among conventional liquid crystals, most of them have relatively high response speed and low power consumption, so most of them are in practical use as display elements.
(M, 5chadt) and Tableu, Helfrich (W, Helfrich) - Applied Physics Letters
! s Letters”) Volume 18, No. 4 (1971
(Published on February 15, 2015), pp. 8127-128゛°
Voltage Dependent Optical Activity - of a Twisted Nematic Liquid Crystal''
ntOptical Activity of a T
twisted Nematic LiquidCrys
Twisted nematic (tw
This type of liquid crystal has a structure (helical structure) in which the molecules of the nematic liquid crystal, which have positive dielectric anisotropy in the absence of an electric field, are twisted in the thickness direction of the liquid crystal layer. The liquid crystal molecules form a structure in which they are arranged parallel to each other on both electrode surfaces. On the other hand, when an electric field is applied, nematic liquid crystals with positive dielectric anisotropy are aligned in the direction of the electric field, resulting in light modulation. When a display element is constructed using this type of liquid crystal with a matrix electrode structure, the area where both the scanning electrode and the signal electrode are selected (selection point) has a threshold voltage required to align the liquid crystal molecules perpendicular to the electrode surface. The area where the following voltage is applied and neither the scanning electrode nor the signal electrode is selected (
No voltage is applied to the non-selected point, so the liquid crystal molecules maintain a stable alignment parallel to the electrode surface. By arranging linear polarizers that are in a cross-Nicol relationship with each other under -E of such a liquid crystal cell, light does not pass through selected points, but light passes through non-selected points, which is designated as image element f. It becomes ri (Noh).

[Problems to be solved by the invention] However, when a matrix electrode structure is constructed,
A finite electric field is also applied to a region where a scanning electrode is selected and a signal electrode is not selected, or a region where a scanning electrode is not selected and a signal electrode is selected (the so-called "select point"). 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 the liquid crystal molecules perpendicular to the electric field is set to an intermediate voltage value, then the display element f is It works normally. However, in this method, when the number of scanning lines (N) is increased, the time during which the effective electric field is distorted at one selected point (duty ratio) while scanning the entire screen (1 frame) is increased. )
decreases at the rate of I/H. For this reason, when repeated scanning is performed, the effective voltage difference between selected points and non-selected points becomes smaller as the number of scanning lines increases, resulting in a decrease in image contrast and crosstalk. This is a drawback that is difficult to avoid. Such a phenomenon is
Temporal accumulation of 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 plane, and they are aligned vertically only while an electric field is effectively applied). This is essentially a problem that occurs when driving using the effect (ie, repeatedly scanning). In order to improve this point, voltage averaging methods, dual frequency driving methods, multiple matrix methods, etc. have already been proposed, but all of these methods are insufficient, and it is difficult to increase the screen size and density of display elements. ,
Currently, the number of scanning lines has reached a plateau because the number of scanning lines cannot be increased sufficiently.

An object of the present invention is to provide a liquid crystal device using a novel bistable liquid crystal, particularly a ferroelectric liquid crystal, which solves all the problems of conventional liquid crystal display elements as described above.

That is, the present invention can increase the number of pixels by applying electric fields in two directions using an active matrix to drive a ferroelectric liquid crystal with a high voltage response speed and state memory into two states, bright and dark. The object of the present invention is to provide a liquid crystal device using ferroelectric liquid crystal that can display images on a large screen and at high speed.

[F-stage for solving problems E] and [Function J] The liquid crystal device of the present invention has pixels formed by opposing electrodes and a ferroelectric liquid crystal disposed between the electrodes in a plurality of rows and In a liquid crystal device arranged in a matrix along columns and in which each pixel is commonly connected in rows and columns, a first group of pixels selected in a row is brought into a display state based on a first alignment state. A first step in which writing is performed sequentially for each row, and a second step in which writing is performed sequentially in each row to bring the selected second group of pixels in one row into a display state based on the second arrangement state. It is characterized by having a writing means for writing on the screen.

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

As the ferroelectric 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 having chiral smectic C phase (SmC and H phase) is most preferable. This ferroelectric liquid crystal is suitable for this ferroelectric liquid crystal.
RNAL DE PHYSIQUELETTER3”)
1975, No. (L-89), "Ferroelectric Liquid Crystals" (rFerroel
etricliquid CrystalS')
; “Applied Physics Letters” JA
ppliedPhysics Letters”)19
1980, 3B(II)Q, [Submicro Second/
Hastiple Electro-Optic Switching in Liquid °Crystals (rSubmicr
o 5econdBistableElectroo
ptic Switching in 1quidC
rystals') ; "Solid State Physics" 1981,
18(+41), "Liquid Crystal", etc., and the ferroelectric liquid crystal disclosed therein can be used in the present invention.

More specifically, examples of ferroelectric liquid crystal compounds used in the method of the present invention include tesyloxybensiriten-p'-amino-2-methylbutylcinnamate (DOBAMBC).
:), hexyloxyhensyritene-P'-amino-2
-Chloroprovir cinname-1-(HOBACPC) and 4-o-(2-methyl)-butylenelucilithene-4'-octylaniline (MBRA8).

When constructing an element using these materials, in order to maintain the temperature state such that the liquid crystal compound becomes the SmG◆ phase or the SmH'' phase, the element may be placed in a copper block with a heater embedded, etc., as necessary. can be supported.

FIG. 1 schematically depicts an example of a ferroelectric liquid crystal cell. ■ and 1' are In203, 5n02 or ITO
It is a substrate (glass plate) coated with a transparent electrode such as (Indium-Tin-Owide), and Smc'' phase liquid crystal in which the liquid crystal molecular layer 2 is oriented perpendicular to the glass surface is sealed between the substrates (glass plates). A thick line 3 represents a liquid crystal molecule, and this liquid crystal molecule 3 has a dipole moment )(P, )4 in the direction orthogonal to the molecule.The electrodes on the substrates 1 and 1' When a voltage higher than a certain threshold is applied between them, the helical structure of the liquid crystal molecules 3 is unraveled, and the alignment direction of the liquid crystal molecules 3 can be changed so that all dipole moments (P↓) 4 are oriented in the direction of the electric field. The liquid crystal molecules 3 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 easy to understand that this is a liquid crystal optical modulation element whose optical properties change depending on the polarity of applied voltage.Furthermore, if the thickness of the liquid crystal cell is made sufficiently thin (for example, lu), as shown in Fig. 2, Even when no electric field is applied to the liquid crystal molecules −
The helical structure of
The moment P or P' is in the 1-direction (4d) or in the F direction (
Either state 4b) is taken. A second cell like this
As shown in the figure, electric fields E or E' with different polarities below a certain threshold value are applied for a predetermined period of time! j-, the dipole moment changes its direction to upward 4a or downward 4b in accordance with the electric field vector of electric field E or E', and accordingly, the liquid crystal molecules are in the first orientation state 5 or in the first orientation state. Orientation to either state 5'.

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. The second point will be explained 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, and this state remains stable even when the electric field is turned off. Furthermore, when an opposite electric field E' is applied, the liquid crystal molecules are oriented to the second alignment 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 darkness 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 in general, the cell should be as thin as possible.
．． 5 well to 20 l, especially 111°5μ is suitable. A liquid crystal-electro-optical device having a matrix electrode structure using this type of ferroelectric liquid crystal has been proposed, for example, by Clark and Lagerle in US Pat. No. 4,387,924.

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 train or the source by reversing the applied voltages to the train and source.

FET is the element that constitutes the active matrix.
Any of amorphous silicon TPT, polycrystalline silicon TPT, etc. can be used as long as the element has a structure.

Furthermore, it is also possible to apply the same method to bipolar transistors other than FET structures.

In an N-type FET, where v is the train voltage, ■6 is the gate voltage, VS is the source voltage, and v+ is the threshold voltage between the gate and source, VD>Vs, and VG>Vs +Vp (
7) It becomes conductive when VG < VS + Vl. It becomes non-conductive.

P-type FET C has V o < V S ,
Continuity state is established at VG > v5 + vP, VG > vs +
It becomes non-conductive at vl-.

Whether the FET is P-type or N-type, which end f of the FET acts as the 1ζ rain and which acts as the source is determined by the direction of voltage application. That is, N
In the type, the one with the lower voltage acts as the source, and in the P-type, the one with the higher voltage acts as the source.

In ferroelectric liquid crystals, the cross nicols placed at F of the cell determine which is in the "bright" state and which is in the "dark" state in response to positive and negative voltages applied to the liquid crystal cell. It can be freely set depending on the direction of the polarization axes of the pair of polarizers and the long axis of the liquid crystal molecules.

According to the invention, the electric field applied to the liquid crystal cell is controlled by controlling the voltage between the terminals of each element of the active matrix to perform display. Therefore, the voltage level of each signal is determined by the following example. It can be implemented by maintaining the relative potential difference of each signal without being limited to the above.

[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 7.

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.

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. The electric signal applied to the selected scanning electrode has a phase (time) of 1 to 1 at t3, as shown in FIG. 6(a). VS.
Phase (+v5 from time t4 to t6).

On the other hand, the electrical signals returned to the other unselected scanning electrodes become O in the phases t1 to t6, as shown in FIG. 6(a). Further, the electric signal given to the selected display electrode has a phase of t1 to t3 as shown in FIG. 6(b).
0, and +v6 during phases t4 to t6. Furthermore, the electrical signal applied to the unselected display electrodes has a phase t.
From 1 to t3, it is -, VG, and from t4 to t6, it is 0. Hereinafter, each voltage value is set to a desired value that satisfies the following relationship.

On the line of scanning electrode 1 = 1-N, display electrode I+ = 1
Write "bright" sequentially on the entire screen using the + signal line (first step), then write the display electrode on the same -=l-N line.-2
When writing "dark" sequentially to the entire screen (second stage) using the signal line 2.

Vc −Vt, c>Vsm (ol
=1~N, n=j? +)Ven=O(n=I!+
)(n/p2)VGn-vp >Vlc+Vc
(m=] ~N, nJ2) V C+ V l,
c< V Sm (n=b) V Gn
-V p < V Cm (n〆p1) However, each symbol represents the following items.

VGn: Gate electrode (display signal) voltage vc: Opposite electrode (common terminal) voltage V+c: Absolute value of threshold voltage of ferroelectric liquid crystal vP: Operation below the threshold between gate and source from q-1 to N Write repeatedly.

Of each pixel when such electrical signals are received,
For example, FIG. 7 shows the write operation of the pixel in FIG. 4. In FIG. 7, the horizontal axis represents time, and the vertical axis represents each display state, with ON (dark) on the upper side and OFF (light) on the lower side.

That is, as is clear from FIGS. 6 and 7, the phase t
The pixels PN, N, and l at the intersection of the scanning line and display line selected in 1 are
I C> -V 3 V (, is applied. Therefore, in FIG. 4, "bright" ib is added to the pixels PN, N, + (first pixel group). Thereafter, the phase t/
and tJ, pixels PNI1 . N, PsI+N+2.
P.N., 7. NIl, PN, :Nl2 (first pixel group) are filled with "bright" one after another. Phase t+”
t3 (In the first stage, the first pixel group of the entire screen is set to "bright".
After the interpolation is performed, "dark" interpolation is performed on the second pixel group of the entire screen during the phase t4 to ta (second stage). That is, a voltage of Vl (+<Vs - Vl) exceeding the threshold value V is applied to the pixels PN, ++, P++N+;+ on the scanning line selected in phase t4. Therefore, in FIG. N
, P.N., N. / (second pixel group) r tk't
Writing of J is performed. At phases t5 and t6, the pixel pH of the selected scanning line is 1.1.
1.1. P)+・7. "Dark" to N (second pixel group)
Only writing is performed.

As can be seen from the above operations, depending on whether or not a display electrode is selected on the selected scanning electrode line, if 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 is ON (dark) or OFF
(bright), and when 1 is not selected, the voltages applied to all pixels do not exceed the threshold voltage. Therefore, the liquid crystal molecules in each pixel other than the selected scanning line "2" maintain the orientation corresponding to the signal S (QN-1) when scanned last time without changing the orientation state. That is, when a scanning electrode is selected, a signal for one line is written, and the signal state can be maintained until the next selection after one frame ends. 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 SN, SN, l.

S.N. 2,... and display electrodes GN, GNII
, G)l,? , . . . Among the pixels formed at the intersections of , . Now, if we pay attention to the display of the display electrode GN"2 in FIG. 5, the pixels corresponding to the scanning electrodes Ss, SN, 2 are in the "Sho" state, and the other pixels are in the "bright" state yE. . The display pattern shown in FIG. 5 is completed by each of the operations in phases 11 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, and When one display state is written, by setting it to one second, both the scanning signal and the display signal can be driven with two-level voltage signals.

FIG. 8 shows an example of drive waveforms using two levels of voltage.

In the method for driving a ferroelectric liquid crystal of the present invention, the arrangement of the scanning electrode and the signal electrode can be arbitrary. For example, as shown in FIG. 9(a),
It is also possible to arrange pixels in columns as shown in (b), and when arranged in this way, it can be used as a shunter array or the like.

Next, in the embodiment described above, specific numerical values preferable for driving DOBAMBG as a ferroelectric liquid crystal are shown, for example, input frequency fo = IX104 to IX106Hz.
10< l VG l <80V (peak value) 0.3
< l Vs I < IOV (wave height value).

Figure 1θ shows F in TPT used in the present invention.
FIG. 11 is a cross-sectional view of a ferroelectric liquid crystal cell using TPT, FIG. 12 is a perspective view of a TPT substrate, FIG. 13 is a plan view of a TFT substrate, and FIG. 14 is a cross-sectional view showing the structure of an ET. 13
A partial cross-sectional view taken along the line A-A' in the figure, Figure 15 is the first
FIG. 3 is a partial cross-sectional view taken along line BB' in FIG. 3, and each of the above figures shows one embodiment of the present invention.

FIG. 11 shows one specific example of a liquid crystal element that can be used in the method of the invention. A semiconductor film 1B (amorphous silicon doped with q hydrogen atoms) is formed on a substrate 20 made of glass, plastic, etc. via a gate electrode 24 and an insulating film 22 (such as a silicon nitride film doped with hydrogen atoms), and this semiconductor A TFT composed of two ends f8 and 11 in contact with the film 16,
A pixel electrode 12 (ITO; connected to the end f11 of the TFT)
Indium-Tin-Owide) is formed. Furthermore, this second insulating layer 13 (polyimide, polyamide, polyvinyl alcohol, polyparaxylylene, S
A light shielding film 9 made of aluminum, chromium, or the like is provided. Substrate 20 serving as a counter substrate
' -L is a counter electrode 21 (ITO; Indium
-Tin-Oxide) and insulation 1122 are formed.

Between the substrates 20 and 20', the ferroelectric liquid crystal 2
3 is being held. Further, a sealing material 25 for sealing the ferroelectric liquid crystal 23 is provided around the substrates 20 and 20'.
is provided.

Polarized lights -F1a and 18' in a crossed Nicol state are arranged on both sides of a liquid crystal element with such a cell structure, and the viewer A can see the display states □, 6o, 7 by the reflected light 11 from the incident light IO.
, -c 3' 6 m k= 4jA X f 1゜'
o'**t;: A radiation plate 1B (a minium sheet or plate of the radiation plate 1B) is provided.

In each of the figures of the above embodiments, the terms "source electrode" and "train electrode" are used only when current flows from the train to the source. In FET operation, it is also possible for the source to function as a train.

Further, in the above embodiment, the first orientation state is set to "bright".
In the above, the case where the second orientation state corresponds to "dark" was explained, but it is also possible to correspond to either the first or second orientation state as "bright".
or "dark" can be arbitrarily selected.

Further, in the above embodiment, the entire screen is written in the first orientation state, and then the entire screen is written in the second orientation state, but the range of pixels on the screen to which writing is performed is not necessarily the entire screen. Writing is not limited to the screen, and writing may be performed only on a specific portion of the screen.

Note that another write step may be inserted between the first step and the second step as the write procedure.

[Effects of the Invention L] A liquid crystal device using the ferroelectric liquid crystal of the present invention having the structure shown in L can display a large screen with a large number of pixels in the active matrix and display clear images at high speed. .

[Brief explanation of the drawing]

Figures 1 and 2 are perspective views schematically showing the ferroelectric liquid crystal used in the present invention, Figure 3 is a circuit diagram of the matrix electrode used in the present invention, and Figure 4 is an explanation showing the addresses of corresponding pixels. Figure 5 is an explanatory diagram showing an example of display of corresponding pixels, and Figure 6 (
a) and (b) are explanatory diagrams showing electrical signals applied to scanning electrodes and display electrodes, FIG. 7 is an explanatory diagram showing writing operation to each pixel, and FIG. 8 is an explanation of drive waveforms using two levels of voltage. 9(a) and (b) are wiring diagrams showing an example of an active matrix circuit and pixel arrangement, and FIG.
A cross-sectional view showing the configuration of FET in FT, FIG.
A cross-sectional view of a ferroelectric liquid crystal cell using FT, Figure 12 is T.
FIG. 13 is a perspective view of the PT board, FIG. 13 is a plan view of the TPT board, FIG. 14 is a partial sectional view taken along line A-A', and FIG. 15 is B-B.
'It is a partial cross-sectional view. 1, l', substrate coated with transparent electrode 2; liquid crystal molecule layer 3; liquid crystal molecule 4; dipole moment (P↓) 4a; upward dipole moment 4b; downward dipole moment 5; first alignment state 5 '; Second orientation state 8; Source electrode (train electrode) 9; Light shielding film 10; N' layer 11; Train electrode (source electrode) 12; Pixel electrode 13; Insulating layer 14; Substrate 15; Shielding film, 16-semiconductor 17; Transparent electrode 18 in gate wiring portion; Reflector + S, IS'; 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 substrate 29; Gate portions 1' to M' having light blocking effect; Scanning Electrodes 1 to N; Display electrode L; Common electrode LC; Liquid crystal

Claims (1)

[Claims] (1) Pixels formed by opposing electrodes and ferroelectric liquid crystal placed between the electrodes are arranged in a matrix along a plurality of rows and columns, and each pixel is divided into rows and columns. In a commonly connected liquid crystal device, a first stage in which writing is performed sequentially for each row to set the selected first pixel group on the row to a display state based on the first alignment state; 1. A liquid crystal device characterized by having a writing means for performing screen writing in a second step of sequentially performing writing for each row to set a second pixel group to a display state based on a second arrangement state.