JPH07122708B2 - Optical modulator - Google Patents

Description

The present invention relates to a method for driving an optical modulator for a display panel, and more particularly to a display using a liquid crystal substance having bistability, particularly a ferroelectric liquid crystal. The present invention relates to a driving method of a liquid crystal optical element suitable for a panel, particularly a gradation display.

[Conventional technology]

In a liquid crystal television panel using the conventional active matrix driving method, thin film transistors (TFTs) are arranged in a matrix for each pixel, and a gate-on pulse is applied to the TFTs to establish a conduction state between the source and the drain. Is applied from a source and is accumulated in the capacitor. A liquid crystal (for example, twisted nematic; TN-liquid crystal) is driven in response to the accumulated image signal, and at the same time, the voltage of the image signal is modulated to display a gradation. Has been done.

[Problems to be solved by the invention]

However, in such an active matrix drive type television panel using TN liquid crystal, since the TFT used has a complicated structure, the number of structural steps is large and the high manufacturing cost is a problem. In addition, there is a problem that it is difficult to form a thin film semiconductor (for example, polysilicon or amorphous silicon) forming a TFT over a wide area.

On the other hand, a display panel of a passive matrix drive system using a TN liquid crystal is known as one that can be manufactured at a low manufacturing cost. In this display panel, one screen (one frame ) While scanning
The time (duty ratio) during which an effective electric field is applied to one selected point is reduced by a ratio of 1 / N, which causes crosstalk and has the drawback of not producing a high-contrast image. Moreover, when the duty ratio becomes low, it becomes difficult to control the gradation of each pixel by voltage modulation, and therefore it is not suitable for a display panel with a high-density wiring number, particularly a liquid crystal television panel.

[Means for Solving Problems] and [Operation] An object of the present invention is to solve the above-mentioned drawbacks, and more specifically, a driving method of a display panel having high-density pixels over a large area, particularly gray scale. An object is to provide a driving method of an optical modulation element suitable for display.

That is, according to the present invention, a plurality of pixels in which an optical modulation material is arranged between a pair of conductive films facing each other are arranged in a matrix, and a plurality of pixels are commonly connected to each row, and a plurality of pixels are connected to each column. A group of information electrodes in which the pixels are connected in common, and the scanning electrodes each have the conductive film and a transmission line having a resistance lower than that of the conductive film, and apply different voltages to two adjacent transmission lines. An optical modulation element comprising a driving means for driving the optical modulation element so that a potential difference gradient is generated between the pair of conductive films forming the pixel, wherein the driving means is one of the optical modulation elements. An optical modulator, comprising: a scan electrode driving circuit provided at an end portion; and a canceling voltage superimposing circuit that superimposes a voltage for canceling a voltage drop of a transmission line on the information electrode group. Is. According to the present invention, it is possible to cancel fluctuations in the electric potential along the transmission line and accurately perform gradation display based on the potential difference gradient method.

〔Example〕

The present invention will be described below with reference to the drawings. As the optical modulator used in the driving method of the present invention, a first optically stable state (for example, a bright state is formed) and a second optically stable state (for example, a dark state) depending on an applied electric field are used. A substance having a stable state against an electric field, that is, a liquid crystal having such a property is used.

As a liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable, and a chiral smectic C among them is most preferable.
Phase (SmC *), H phase (SmH *), I phase (SmI *), F phase (SmF *)
Liquid crystals of G phase (SmG *) are suitable. For this ferroelectric liquid crystal, please refer to "Le Journal de Fiji
36th "LE JOURNAL DE PHYSIQUE LETTRE"
Volume (L-69) 1975 "Ferroelectric Liquid Crystals"
s ");" Applied Fuzzy Letters "
("Applied Physics Letters") Volume 36, Issue 11, 1980
"Submicro Second Bistable Electroop, Switching In Liquid Crystals"("Submicro Second Bistable Electroop"
tic Switching in Liquid Crystels ”);“ Solid physics 16
(141) 1981 “Liquid crystal” and the like, and the ferroelectric liquid crystal disclosed therein can be used in the present invention.

More specifically, examples of the ferroelectric liquid crystal compound used in the method of the present invention include desiloxybenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMBC) and hexyloxybenzylidene-P'-amino. 2-chloropropyl cinnamate (HOBACPC) and 4-o- (2
-Methyl) -butyl resorcylidene-4'-octylaniline (MBRA8) and the like.

When an element is constructed using these materials, the element is heated by a heater as necessary in order to keep the liquid crystal compound in a temperature state where it becomes SmC *, SmH *, SmI *, SmF *, SmG *. It can be supported by embedded copper blocks or the like.

FIG. 1 is a schematic drawing of an example of a ferroelectric liquid crystal cell. 11 and 11 ′ are substrates (glass plates) coated with transparent electrodes such as In ₂ O ₃ , SnO ₂ and ITO (Indium-Thein-Oxide), between which the liquid crystal molecular layer 12 is perpendicular to the glass surface. A liquid crystal of SmC * phase oriented so that it is enclosed. A thick line 13 represents a liquid crystal molecule, and the liquid crystal molecule 13 has a dipole moment (⊥) 14 in a direction orthogonal to the molecule. When a voltage above a certain threshold is applied between the electrodes on the substrates 11 and 11 ', the helical structure of the liquid crystal molecules 13 is unraveled, and all dipole moments (⊥) 14 are oriented in the electric field direction. Can be changed. The liquid crystal molecules 13 have an elongated shape, and exhibit refractive index anisotropy in the major axis direction and the minor axis direction thereof, and therefore, for example, if polarizers arranged in a crossed Nicols positional relationship are placed above and below a glass surface. It is easily understood that the liquid crystal optical modulation element has optical characteristics that change depending on the polarity of voltage application. Further, when the liquid crystal cell is made sufficiently thin (for example, 1 μ), the helical structure of the liquid crystal molecules is unraveled (non-helical structure) even when no electric field is applied, as shown in FIG. The moment P or P'takes either an upward (24) or downward (24 ') orientation state. When an electric field E having a polarity equal to or larger than a certain threshold is applied to such a cell as shown in FIG. 2, the dipole moment electric field E is directed upwards corresponding to the electric field vector of E '.
Or, the liquid crystal molecules are turned downward 24 ', and the liquid crystal molecules are corresponding to the first stable state 23 (bright state) or the second stable state 23'.
Oriented to either one of (dark state).

There are two advantages of using such a ferroelectric liquid crystal as an optical modulation element. First, the response speed is extremely fast,
Secondly, the alignment of liquid crystal molecules has bistability.
The second point will be explained, for example, with reference to FIG. 2. When the electric field E is applied, the liquid crystal molecules are aligned in the first stable state 23.
In this state, the first stable state 23 is maintained even when the electric field is cut off, and when an opposite electric field E'is applied, the liquid crystal molecules are oriented to the second stable state 23 'and the orientation of the molecule is changed. Although it changes, it keeps this state even when the electric field is cut off, and has a memory function in each stable state. In order to effectively realize such a high response speed and bistability, it is preferable that the cell be as thin as possible, and generally 0.5% or less.
μ to 20 μ, particularly 1 μ to 5 μ are suitable. A liquid crystal-electro-optical device having a matrix electrode structure using a ferroelectric liquid crystal of this kind has been proposed by Clarke and Lagabal in US Pat. No. 4,367,924.

Next, details of the liquid crystal optical element used in the present invention will be described with reference to FIG.

Reference numeral 31 in FIG. 3 is one of the substrates. Reference numeral 32 denotes a display conductive film, which is laminated on the substrate of 31. Reference numeral 33 denotes a transfer electrode made of a low-resistance metal film, which is laminated on the display conductive film 32 in parallel at equal intervals. Further, the other substrate (not shown) is opposed to the substrate 31, and a counter conductive film (counter electrode) 34 is arranged in a region corresponding to the region of the pixel A in the figure on the other substrate. There is. The above-mentioned optical modulator is sandwiched between the display conductive film 32 and the counter electrode 34.

In the liquid crystal optical element configured as described above, a potential gradient is generated in the electric field between the counter electrode 34 and the potential gradient in the plane of the display conductive film 32 by the signal voltage applied to the transmission electrode 33. . At this time, the transfer electrode 33b is connected to the reference voltage point V _E (for example, 0 volt), and the transfer electrodes 33a and 33a are connected.
When a predetermined signal voltage Va is applied to the transmission electrode adjacent to the transmission electrode connected to the reference potential point as in 3c, the conductive film 32 between the transmission electrodes 33a and 33b or 33b and 33c is applied as shown in FIG. 11 (a). A potential gradient of Va can be applied in the in-plane length directions l ₁ and l ₂ . At this time, when the inversion threshold voltage Vth of the ferroelectric liquid crystal is set to Va, if -Vb is applied to the counter electrode 34, FIG.
Will be inversion threshold voltage Vth or higher potential difference Va + Vb to the ferroelectric liquid crystal corresponding to the length direction m ₁ and m ₂ in the plane of the conductive film 32 as shown in is applied, in such m ₁ and m ₂ The corresponding area can be inverted, for example from a bright state to a dark state. Therefore, in the present invention, the gradation can be expressed by applying Vb with a value according to the gradation for each pixel. On this occasion,
The voltage value of the voltage signal −Vb applied to the counter electrode 34 may be modulated according to the gradation information, or its pulse width may be modulated according to the gradation information, or the number of its pulses may be modulated. Therefore, the gradation can be controlled.

Further, in the present invention, prior to applying the above-mentioned gradation signal,
After the erasing step for setting the pixel to either the bright state or the dark state, the inversion voltage for inverting the state is controlled according to the gradation and applied to the ferroelectric liquid crystal. It is necessary.

Furthermore, preferred specific examples of the present invention will be described.

In FIG. 3, a SnO ₂ film, which is a transparent conductive film having a thickness of about 20 Å, was formed on the glass substrate 31 by a sputtering method to form a display conductive film 32. The sheet resistance of this SnO ₂ film is 105Ω /
It was □. Then, Al having a thickness of 1000 Å was vacuum-deposited on the above-mentioned SnO ₂ film and patterned again to form a plurality of transfer electrodes 33 as shown in FIG. In this example, the transfer electrode 33
The sheet resistance of this transmission electrode 33 was about 0.4Ω / □, and its width was about 20μ. On the other hand, an ITO film covering the area A was provided as the counter electrode 34 on the counter substrate. The sheet resistance of the ITO film serving as the counter electrode 34 was about 20Ω / □.

A polyvinyl alcohol layer of about 500 liters was formed as a liquid crystal alignment film on each surface of the two substrates thus manufactured, and subjected to rubbing treatment.

Next, the two substrates are opposed to each other and the gap is adjusted to be about 1 μ, and the ferroelectric liquid crystal (pn-octyloxybenzoic acid-P '-(2-methylbutyloxy) phenyl ester and p-n-octyloxybenzoic acid-p- A liquid crystal composition containing η-nonyloxybenzoic acid-P ′-(2-methylbutyloxy) phenyl ester as a main component was injected. The shape of the partial pixel A in which the display conductive film 32 and the counter electrode 34 overlap was 230 μ × 230 μ, and the electrostatic capacity after liquid crystal injection was about 3 PF. However, the width of pixel A is And

Polarizing plates were arranged in crossed Nicols on both sides of the liquid crystal cell thus formed, and optical characteristics were observed.

FIG. 4 schematically shows a method of applying an electric signal, and FIGS. 5 and 6 show electric signals. FIG. 5 shows the waveform of the signal (a) generated by the drive circuit 43 of FIG.
FIGS. 6 (i) to 6 (v) show the waveforms of the signal (b) generated in the drive circuit 44 of FIG.

An erasing step for providing a −12 V 200 μsec pulse as the signal (a) and an 8 V 200 μsec pulse as the signal (b) in advance in synchronization (referred to as an erase pulse) is provided. Then, the liquid crystal is switched to the first stable state, and the entire pixel A is in the bright state (the cross polarizing plate is arranged in this way). From this state, when various pulses as shown in FIGS. 6 (i) to 6 (v) are given to the counter electrode 34 in synchronization with the pulse of FIG. 5 applied to the transmission electrode 33 as the signal (b). FIG. 7 shows the optical state of the pixel A of FIG.

There is no change from the bright state 71 (corresponding to FIG. 7 (a)) at the pulse applied voltage −2V (corresponding to FIG. 6 (a)) and −5V (corresponding to FIG. 6 (b)). , Pulse applied voltage −8V
In (corresponding to FIG. 6 (c)), the liquid crystal in the vicinity of the transmission electrode 33 switches to the dark state 72 (corresponding to FIG. 7 (b)).
Further, when the applied voltage is increased to −14V (corresponding to FIG. 6 (d)), the area of the dark state 72 becomes wide as shown (corresponding to FIG. 7 (c)), and the applied voltage 20V (corresponding to FIG. 7 (c)). Figure 6 (e)
The entire pixel A is switched to the dark state 72 (corresponding to FIG. 7 (d)). In this way, an image with gradation can be formed.

Further, various signals (b) having different pulse widths as shown in FIGS. 9 (a) to (e) and a signal (a) having a triangular wave as shown in FIG. 8 were given in synchronization. At any time, the optical state change illustrated in FIG. 7 can be shown above. At this time, the pulse shown in FIG. 8 is applied to the transmission electrode, and the pulses shown in FIGS. 9 (a) to 9 (e) are applied to the counter electrode 34 according to the gradation in synchronization with this pulse. It can express tonality.

In FIG. 4, reference numeral 41 is a ferroelectric liquid crystal, preferably a chiral smectic liquid crystal in a bistable state, and 42 is a counter substrate.

Further, in the present invention, the aluminum (Al) used in the above-mentioned example
In addition to the transmission electrode 33, a metal such as silver, copper, gold, or chromium can be used as the transmission electrode 33, and the sheet resistance thereof can be preferably 102Ω / □ or less. Moreover, as the conductive film 32 to which the potential gradient is applied, 10 KΩ / □ to 1 MΩ
A transparent conductive film having a sheet resistance of / □ can be used. Such sheet resistance can be designed to an appropriate value by adjusting the film thickness of the transparent conductive film.

FIG. 10 shows a specific example when the gradation expression method according to the present invention is applied to matrix driving.

The display panel shown in FIG. 10 has a plurality of stripe-shaped conductive films 101 (101a, 101b, 101c) arranged on a glass substrate 31,
Further, the low-resistance transmission electrodes 102 (102a, 102b, 102) are provided on both ends of each stripe-shaped conductive film 101 in the longitudinal direction.
c) and 103 (103a, 103b, 103c) are wired. Board 31
A counter electrode 104 (104a, 104b) made of a stripe-shaped conductive film provided on a counter substrate (not shown) facing the above is disposed, and the ferroelectric film is provided between the stripe-shaped conductive film 101 and the counter electrode 104 described above. A liquid crystal is arranged.

FIG. 12 shows an electrical equivalent circuit of the display panel shown in FIG. 10, 121 is the resistance of the stripe-shaped conductive film 111,
122 and 123 are resistances of the transmission electrodes that are electrically connected and wired to both ends of the stripe-shaped conductive film 111 in the longitudinal direction. Incidentally, the respective resistances are not concentrated but are uniformly distributed. Now, the sheet resistance of the striped conductive film 111 is 10 ⁶ Ω / □, and the transmission electrode lines 112 and 1
The sheet resistance of 13 is 0.4Ω / □. In addition, the width of the transmission electrode lines 112 and 113 is 10 μm and the length is 200 mm.
And 113 are set to 250 μm, terminal 124 is grounded, and terminal 1
1V was applied to 25 and terminals 126 and 127 were opened.

FIG. 13 shows the potential distribution at that time. A curve 131 in FIG. 13 (a) is a potential curve showing the potential distribution in the longitudinal direction (between the terminals 124 and 126) of the transmission electrode line 112. Further, the curve 132 in FIG. 13 (b) is the transmission electrode line 1
13 is a potential curve showing a potential distribution in the longitudinal direction of 13 (between terminals 125 and 127). A curve 133 in FIG. 13C shows a stripe-shaped conductive film 11 narrowed by the transmission electrode lines 112 and 113.
6 is a potential difference curve showing a state in which the potential difference in the lateral direction of 1 is distributed in the longitudinal direction of the stripe-shaped conductive film 111. As can be seen from FIG. 13 (c), the potential difference in the lateral direction of the striped conductive film 111 on the terminals 124 and 125 side is 1 V, but the potential difference decreases as the distance from the terminals 124 and 125 increases. On the side of the terminals 126 and 127, the potential difference is almost 0V.

Further, as shown in FIG. 13 (b), the potential of the transmission electrode drops as it gets away from the input terminal. Therefore, a feature of the present invention is to solve this problem by superimposing a canceling voltage corresponding to such a voltage drop on the information signal (grayscale signal).

FIG. 14 is a block diagram for explaining the present invention, in which 141 is a display panel, 142 is an information line driving circuit, 143 is a scanning line driving circuit, and 14 is a scanning line driving circuit.
Reference numeral 4 is a canceling voltage superimposing circuit. The cancellation voltage superposition circuit 144 is
It shows the characteristics as shown in FIG. A solid line 151 in FIG. 15 shows a potential curve showing the potential distribution of the transmission electrode, and a broken line 151 shows the cancellation voltage of the cancellation voltage superposition circuit. By superimposing this offset voltage on each information line, the operating point of liquid crystal driving can be maintained at a constant value of Vs.

FIG. 16 shows a concrete example of the cancellation voltage superposition circuit 144. In FIG. 16, I ₁ , I ₂ …… _IN are gradation information lines, and A ₁ …… A _N
Is an output amplifier, 161 is an offset input terminal of each output amplifier, and 162 is a dummy wiring for offset voltage. Here, the offset voltage dummy wiring 162 is connected to the transmission electrode 33.
It can be formed by making the film of Al of the same material as the above, and by making the film thickness, width and length the same as those of the transmission electrode 33.

In this embodiment, the applied voltage is 1 V and the ground potential, but the two potentials may be any potential.

In the present invention, twisted nematic liquid crystals, guest-host liquid crystals and the like can be used in addition to the above-mentioned ferroelectric liquid crystals, but the most preferable are ferroelectric liquid crystals, especially ferroelectrics having at least two stable states. Liquid crystals are suitable.

〔The invention's effect〕

According to the present invention, stable display without display unevenness is possible, and gradation display is performed by applying a gradation value modulated by a voltage value, pulse width, pulse number, or the like as an input signal. be able to.

[Brief description of drawings]

1 and 2 are perspective views schematically showing a ferroelectric liquid crystal element used in the present invention. FIG. 3 is a perspective view showing one substrate used in the present invention. FIG. 4 is a sectional view of a liquid crystal optical element used in the present invention. 5 and 6 (a) to 6 (e) are explanatory views showing pulse waveforms used in the present invention. 7A to 7D are schematic diagrams showing the gradation of pixels. 8 and 9 (a) to (e) are explanatory views showing waveforms of other pulses used in the present invention. FIG. 10 is a perspective view showing another liquid crystal optical element used in the present invention. 11A and 11B are explanatory views schematically showing the potential gradient used in the present invention. FIG. 12 is an equivalent circuit diagram of an element other than the present invention, and FIGS. 13 (a) to 13 (c) are explanatory diagrams showing the potential distribution at that time. FIG. 14 is a block diagram used in the device of the present invention, and FIG. 15 is an explanatory diagram of the potential distribution used in the present invention. FIG. 16 is an equivalent circuit diagram of the offset voltage superimposing circuit used in the present invention.

Front page continued (72) Inventor Shuzo Kaneko 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Tsutomu Toyono 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP 61-201217 (JP, A) JP 52-20851 (JP, A) JP 52-122098 (JP, A)

Claims (2)

[Claims] 1. A scanning electrode group in which a plurality of pixels in which an optical modulation substance is arranged between a pair of conductive films facing each other are arranged in a matrix, and a plurality of pixels are commonly connected in each row, and a plurality of pixels in each column. An information electrode group in which pixels are connected in common, and the scan electrodes each have the conductive film and a transfer line having a resistance lower than that of the conductive film. By applying different voltages to two adjacent transfer lines. An optical modulation element comprising driving means for driving the optical modulation element so that a potential difference gradient is generated between the pair of conductive films forming the pixel, wherein the driving means is one end of the optical modulation element. And an offset voltage superimposing circuit that superimposes a voltage for offsetting a voltage drop of a transmission line on the information electrode group. 2. The optical modulation element according to claim 1, wherein the optical modulation substance is a ferroelectric liquid crystal.