JP3308154B2 - Liquid crystal panel and its driving method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid crystal panel which is bright and is of a low cost and is capable of performing a full-color display in a color display liquid crystal panel utilizing a double refraction effect and its driving method. SOLUTION: One pair of comb-line electrodes 14, 15 arranged so as to be meshed with each other are formed on the inner surface of a lower substrate 1 in two sheets of substrates holding a liquid crystal layer there between and counter electrodes 21 are formed on the inner surface of an upper substrate 2. Then, luminance is modulated by changing orientational angles of liquid crystal molecules by the traversal voltage between comb-line electrodes 14, 15 and hue is modulated by changing inclinations of liquid crystal molecules by the potential difference between the voltage of the counter electrode 21 and the mean voltage of the comb-line electrodes 14, 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-cost, bright color liquid crystal display device, and more particularly to a reflective color liquid crystal display device.

[0002]

2. Description of the Related Art Liquid crystal display elements have been widely used as displays for portable information terminals by making use of their thin and light characteristics. The liquid crystal is a light-receiving element that does not emit light by itself, and can be driven with a low voltage of several volts, so that display can be performed with low power consumption. In particular, a reflection type liquid crystal display element in which a display is illuminated with external light by placing a reflection plate on the back surface to view a display consumes extremely low power.

However, a reflection type liquid crystal display device is usually a black and white display, and a color type display uses a transmission type liquid crystal display device with a backlight placed behind. The color display employs a method in which three pixels are covered with fine color filters of three primary colors of red, blue and green, and white incident light is absorbed by the color filters so that each transmitted light becomes three primary colors. . When all three pixels of red, blue, and green are in the transmissive state, white display is performed and the state becomes the brightest.
Therefore, the incident light is absorbed by the color filter, and the transmittance becomes 1/3 or less. In addition, since a liquid crystal display element usually uses a polarizing plate, one polarized light is absorbed, and the transmittance is reduced to half or less. For this reason, a liquid crystal display device using a color filter requires a backlight that consumes a large amount of power, which increases the thickness and weight, and impairs the characteristics of the liquid crystal display device, that is, low power consumption. Further, the production process of the color filter is complicated and the cost is high.

On the other hand, there has been proposed a liquid crystal display element which performs color display using color development by a birefringence effect without using a color filter (for example, Shoichi Matsumoto and Ryo Tsunoda, "Latest Technology of Liquid Crystal"). 130-132). In this method, for example, a nematic liquid crystal having a positive dielectric anisotropy is horizontally aligned, and a liquid crystal layer is sandwiched between a polarizing plate having a polarization axis having a direction of an angle θ in the alignment direction and a polarizing plate orthogonal thereto. And the transmitted light intensity I due to birefringence is expressed by the following equation with respect to the incident light intensity I ₀ .

I = I ₀ sin ² (2θ) sin ² (πΔnd / λ) (1) where λ is the wavelength of the incident light, d is the thickness of the liquid crystal layer, and Δn is the birefringence anisotropy of the liquid crystal. . When the angle θ is 45 degrees, the transmitted light intensity I becomes the highest (brighter).

As can be seen from the above equation, the transmitted light intensity has a wavelength dependence, and the birefringence (refractive index anisotropy Δn) of the liquid crystal layer is
And the thickness d). Since Δnd becomes smaller as the liquid crystal molecules rise by applying a voltage, the wavelength of high transmittance changes according to the applied voltage, and the color of display can be controlled. In the case of the reflection type, the incident light is emitted back and forth through the liquid crystal panel, and therefore has an intensity distribution obtained by squaring Expression (1). Therefore, the wavelength dependency becomes more remarkable, and the color purity increases.

[0007] An example in which about four colors are displayed by a simple matrix drive using a super-twisted nematic (STN) liquid crystal with the same birefringence effect has been reported (for example, see Japanese Patent Application Laid-Open No. Hei 6 (1994) -106).
-301006). A liquid crystal display device that drives this STN liquid crystal cell using TFTs has also been proposed (see Japanese Patent Application Laid-Open No. 7-159752). When the TFT is driven, a voltage value of a color to be displayed can be easily and accurately applied. In the driving method and the driving circuit described in Japanese Patent Application Laid-Open No. 7-159752, when displaying several birefringent colors such as red, green, and blue that can be displayed by one pixel, the displayed birefringent colors are displayed. Are prepared, and the applied driving voltage is switched according to the color signal.

In a normal liquid crystal panel, an electric field is applied in the thickness direction of a liquid crystal layer with a liquid crystal layer interposed between electrodes provided on opposing substrates, and display is performed by standing liquid crystal molecules. Therefore, when viewed obliquely, the birefringence is small from the direction in which the liquid crystal molecules rise, and the birefringence is large in the opposite direction. In order to improve this, a lateral electric field method in which an electric field is applied in a direction substantially parallel to the substrate using a comb-shaped electrode has been proposed (for example, Japanese Patent Application Laid-Open No. 6-160878 or Japanese Patent Publication No. Sho 63-16378). No. 21907). In this method, an achromatic display can be obtained by using the birefringence effect of Δnd of the liquid crystal layer.
By setting about 3 and rotating the liquid crystal molecules in a plane parallel to the substrate by a lateral electric field, the brightness is changed by changing θ in Expression (1).

[0009]

The cost is high in the color display using the color filter, and only a very dark display can be obtained in the reflection type. On the other hand, in the method in which the color is changed using the birefringence effect without using a color filter, the color that changes according to the voltage is determined by the configuration of the liquid crystal panel, and only a specific color on the chromaticity diagram can be displayed. . Although the color changes depending on the voltage, gradation display, that is, luminance adjustment cannot be performed. Due to these restrictions, the method based on the birefringence effect has an extremely small number of display colors as compared with a transmissive liquid crystal panel using a color filter, and can be used only for limited applications.

It is an object of the present invention to provide a liquid crystal panel which can solve the above-mentioned problems in the prior art and which can perform a low-cost, bright, full-color display using a method based on the birefringence effect and a driving method thereof. Aim.

[0011]

In order to achieve this object, a liquid crystal panel according to the present invention has a liquid crystal layer sandwiched between two substrates, and has a light incident side and a light exit side on the liquid crystal layer. A polarizing plate is provided, a pair of comb-shaped electrodes arranged so as to be engaged with each other is formed on the inner surface of one substrate, and a counter electrode is formed on the inner surface of the other substrate, and a voltage applied between the pair of comb-shaped electrodes is provided. The azimuth of the liquid crystal molecules (in a plane parallel to the substrate) is changed by controlling, and by controlling the potential difference between the average potential of the pair of comb electrodes and the counter electrode,
The tilt of the liquid crystal molecules (relative to the substrate surface) changes ,
The color of the emitted light changes due to the tilt of the liquid crystal molecules,
It characterized in that it is configured so that the luminance of the emitted light is changed by the azimuth angle of the child.

Further, in the method of driving a liquid crystal panel according to the present invention, a voltage applied between a pair of comb-shaped electrodes arranged so as to engage with each other on the inner surface of one substrate is controlled in accordance with the luminance of the pixel, and the other is controlled. The potential difference between the potential of the counter electrode formed on the inner surface of the substrate and the average potential of the pair of comb electrodes is controlled according to the hue of the pixel.

According to the liquid crystal panel and its driving method as described above, a color change is produced by a birefringence effect without a color filter, and the brightness can be arbitrarily set. It becomes possible.

[0014]

Embodiments of the present invention will be described below with reference to examples and drawings. Embodiment 1 As shown in the sectional view of FIG. 1, TFT elements 10 and 11 and comb-shaped electrodes 14 and 15 are provided on a lower substrate 1 side constituting a liquid crystal panel, and a stripe-shaped common electrode is provided on an upper substrate 2 side. An electrode 21 (a transparent electrode made of indium tin oxide) is provided. The opposing common electrode 21 is a comb-shaped electrode 14 on the lower substrate,
It is arranged at a position corresponding to the middle of the fifteen comb teeth. The upper and lower substrates 1 and 2 were rubbed after the application of the polyimide alignment film 3 and bonded together with a spherical spacer having a substrate spacing of 4 microns. Then, a chlor-based positive nematic liquid crystal having a birefringence of 0.24 was injected between the substrates to make a homogeneous alignment.

This panel is sandwiched between polarizing plates 4 and 5, a reflecting plate 6 is adhered to the back surface, and a polycarbonate retardation film 7 (retardation 150 nm) is set with the slow axis set in a direction perpendicular to the rubbing direction. It was inserted between the polarizing plate 4 and the upper substrate 2.

FIG. 2 shows a plan view of the lower substrate 1 provided with the TFT elements. FIG. 1 is a cross section taken along line AA of FIG. After patterning the chromium film 200 nm and forming one of the comb electrodes 14 at the same time as the gate lines 12 and 13, an insulating film 16 (500 nm) of silicon dioxide is formed.
A contact hole 17 was formed in the insulating film 16. then,
After forming the amorphous silicon portion 18, the aluminum film 200 is formed.
A source line 19 of nm and the other comb-shaped electrode 15 were formed. At the same time, a relay electrode 20 for conducting the drain of the TFT element 10 and the comb-shaped electrode 14 through the contact hole 17 was formed, and connected to the drain of the TFT element 11.

Although the comb electrodes 14 and 15 can be formed at the same time, in order to reduce short-circuit defects, they are formed as separate layers with the insulating film 16 interposed therebetween. The pitch of the comb teeth of each comb electrode is 40 microns and the width is 5 microns.
The distance between adjacent comb teeth of 4 and 15 is 20 microns.

The initial alignment of the liquid crystal molecules 21 is indicated by an arrow 24 in FIG.
As shown by, rubbing was performed in a direction forming an angle of 20 degrees with the comb teeth. Then, a stripe-shaped counter electrode 2 indicated by a broken line
After bonding the upper and lower substrates so that 1 was located at the center of the comb teeth, liquid crystal was injected. The polarizing axes of the polarizing plates 4 and 5 were orthogonal as indicated by arrows 22 and 23, and one of the polarizing axes was parallel to the initial alignment direction 24 of the liquid crystal molecules, and black display was performed when no voltage was applied to the liquid crystal. This is because black display in the initial orientation can reduce the sinking of black display and increase contrast, as compared with setting white display.

FIGS. 3A to 3D show TFT elements 10 and
FIG. 2 is a cross-sectional view of the liquid crystal panel when a voltage is applied to the comb-shaped electrodes 14 and 15 via 11. The outline of the electric field applied to the liquid crystal layer is drawn by a broken equipotential line, and the inclination of the liquid crystal molecules 25 is shown by an ellipse. The common electrode 21 of the opposing substrate has a potential of 1.5 volts.

FIGS. 3A and 3B show the comb electrodes 14 and 1
5 is made equal to the potential of the common electrode 21 located at the center of these comb teeth at 1.5 volts, and the voltage between the comb electrodes 14 and 15 is 3 volts in FIG. In b), 2 volts is used. In these cases, the potential of the opposing common electrode is equal to the potential of the intermediate portion of the comb teeth of the lower substrate immediately below the common electrode, so that the equipotential lines are perpendicular to the substrate, and a uniform horizontal electric field is applied to the liquid crystal layer. Liquid crystal molecules rotate horizontally. In FIG. 3A where the horizontal electric field intensity is strong, the liquid crystal molecules rotate about 45 degrees and become the brightest. The color at this time is green. In FIG. 3B, since the horizontal electric field is weakened and the rotation angle is reduced, the luminance is reduced, but the Δnd of the liquid crystal layer is (a)
The hue does not change, so it becomes dark green.

FIGS. 3C and 3D show the counter common electrode 21.
And the average potential of the comb-shaped electrodes 14 and 15 does not match. At this time, the equipotential lines are obliquely inclined as shown in the figure. 3 (c) and 3 (d), the average potential of the comb teeth is 3 volts, and there is a potential difference of 1.5 volts between the comb electrodes. The voltage between the comb teeth is 3 volts in FIG.
In (d), it is 2 volts. In FIG. 3 (c), the liquid crystal molecules are rotated by approximately 45 degrees in the horizontal direction and become brightest, and the display color is blue. In FIG. 3D, a blue display having the same hue and a luminance of about 60% as in FIG. 3C was obtained. Judging from the luminance, the rotation angle in the horizontal direction is about 20 degrees.
The inclination angle should be substantially the same as that of FIG.

The absolute value | Vc−Vm | = | Vs | of the difference between the potential Vc of the opposing common electrode and the average potential Vm of the comb electrode is given by
The electric field strength in the thickness direction changes, the rising angle of the liquid crystal molecules changes, and the Δnd of the liquid crystal layer changes, so that the hue changes. FIG. 4 illustrates the change in hue according to | Vs |
FIG. 4 shows a y chromaticity diagram. Although the voltage between the comb teeth is set to be 3 volts, almost the same chromaticity diagram is obtained with other voltages. ｜ V
The locus of the color change due to the change of s | is shown by a curve 40. Green at 0 volts (point 41), Blue at 1.8 volts (point 4)
2) Red at 3.0 volts (point 43) and white at 3.8 volts (point 44).

FIG. 5 is a characteristic diagram showing the relationship between the transmittance (luminance) and the voltage between the comb teeth. Although | Vs | is set to 3.0 volts, almost the same characteristics are obtained with other voltages. In FIG. 5, the horizontal axis represents voltage, and the vertical axis represents relative luminance. As shown by curve 50, the luminance starts increasing around 1.1 volts, reaches a maximum luminance around 3 volts, and starts decreasing when the voltage is further increased. As can be seen from the above equation (1), the rotation is 45 degrees at 3 volts, and when the voltage is further increased, the rotation is 45 degrees or more.

As described above, according to the present invention, it is possible to control both the hue and brightness (luminance) of a pixel in a birefringent liquid crystal panel that performs color display without a color filter.

It is to be noted that the main object of the present invention is to increase the .DELTA.nd of the liquid crystal layer to cause a color change by applying a voltage, and to perform bright color display without a color filter.
If d is about 0.3 microns, a black and white display is obtained. At this time, the viewing angle can be controlled by controlling the luminance by the azimuth angle of the liquid crystal molecules and changing the tilt angle. By the viewing angle control, for example, an application in which a parallax image is shown to the left and right eyes to perform pseudo three-dimensional display is possible.

In the above-described embodiment, the liquid crystal layer has a horizontal alignment and a homogeneous alignment having no twist. However, a nematic liquid crystal (Nn liquid crystal) having a negative dielectric anisotropy may be vertically aligned. In this case, as the polyimide alignment film 3 in FIG. 1, a polyimide vertical alignment film SE1211 manufactured by Nissan Chemical Co., Ltd. is applied with a thickness of about 60 nm, and rubbed in the same direction as in the case of homogeneous. When an empty panel is assembled by dispersing spacers so that the substrate interval becomes 6.8 μm, and Nn liquid crystal having Δn of 0.14 is injected, vertical alignment inclined several degrees in the rubbing direction is obtained. When the voltage between the comb electrodes 14 and 15 is set to 0 volt and the potential difference with the counter electrode is increased, the liquid crystal molecules fall in the rubbing direction and become colored.
It is very dark because the falling direction is parallel to the polarization axis. When the voltage between the comb electrodes is increased and a voltage is applied between the comb electrodes, the direction in which the liquid crystal molecules fall is rotated. The direction in which the inter-electrode voltage is inclined at about 1.7 volts rotates 45 degrees and becomes the brightest. The control method of luminance and hue and the order of color change are the same as in the case of homogeneous, but the voltage | Vs |
Reaches white at 3.5 volts. As an advantage of the vertical alignment type, it is possible to lower the voltage between the comb electrodes.
As a disadvantage, since there is no liquid crystal material having a large Δn, the thickness of the liquid crystal layer increases and the response speed decreases. In any case,
As in the case of the homogeneous type, an effect that a bright color display can be performed without a color filter can be obtained.

If the display is performed by using a pair of interdigitated comb-shaped electrodes as one sub-pixel and two or three sub-pixels as a unit pixel, the saturation can be controlled.
Full color display is possible. In the case of two sub-pixels, the saturation can be changed by displaying a color with one pixel and displaying an achromatic color with the other pixel. In the case of three sub-pixels, any color can be reproduced by independently displaying the three primary colors of red, blue, and green, as in a normal color filter, but any two of the three colors can be displayed. A method of selecting a hue, displaying an achromatic color with the remaining one pixel, and adjusting the saturation can also be adopted.

FIG. 6 is a block diagram of a driving circuit that embodies a method of driving a liquid crystal panel according to the present invention. Red-green,
Image signals (Yr, Yg,
When Yb) is input to the component separation unit 60, the achromatic component is removed and the color component is separated. Specifically, since the minimum value of the three components is the achromatic component, for example, when the luminance Yr of the red component is the minimum, the image signal after removing the achromatic component is (0, Yg-Yr, Yb-Yr). ) = (0, Yg ′,
Yb ′). Then, the ratio of the remaining two green and blue components (Yg ′ / Yb ′) is used as a hue signal as a component
Is output to the voltage generator 61. At the same time, Yg ′ + Yb ′ is also output as a luminance signal.

The voltage generator 61 calculates the voltages V10 = Vm + Va / 2 and V11 = Vm-Va / applied to the TFT elements 10 and 11 from the voltage Vm corresponding to the hue signal and the voltage Va corresponding to the luminance signal. 2 is calculated. These voltages are applied to the gate lines 12, 1 from the strobe signal generator 63.
3 and transferred to the source driver LSI 64 via the sample and hold circuit 62 in synchronization with the strobe signal to
Source driver LSI 64 and gate driver LSI 65
Thus, the TFT element and the liquid crystal panel are driven.
When there are two sub-pixels, one sub-pixel may display a color as described above, and a voltage at which the luminance and hue of the achromatic component become white may be applied to the other sub-pixels in the same manner as described above.

[0030]

As described above, according to the liquid crystal panel and the method of driving the same according to the present invention, the inclination of the liquid crystal molecules with respect to the substrate surface and the azimuth angle in the plane parallel to the substrate can be controlled by the voltage. The color and brightness of a pixel can be controlled without a color filter. As a result, a bright full-color display of the reflection type can be realized. Further, even in the transmission type, the power consumption of the backlight can be reduced by improving the light use efficiency, and the cost can be reduced by eliminating the color filter.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a liquid crystal panel according to an embodiment of the present invention.

FIG. 2 is a plan view of a lower substrate of the liquid crystal panel of FIG. 1;

FIG. 3 is a sectional view showing an electric field distribution and tilt of liquid crystal molecules in the liquid crystal panel of FIG.

FIG. 4 is a chromaticity diagram showing an example of a color change due to a voltage change in the liquid crystal panel of FIG. 1;

FIG. 5 is a characteristic diagram showing a relationship between a voltage between comb teeth and luminance in the liquid crystal panel of FIG. 1;

FIG. 6 is a block diagram showing a method for driving a liquid crystal panel according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Lower substrate 2 Upper substrate 3 Alignment film 4,5 Polarizer 6 Reflector 7 Phase difference film 10,11 TFT element 12,13 Gate line 14,15 Comb electrode 16 Insulating film 17 Contact hole 18 Amorphous silicon part 19 Source line 20 Relay electrode 21 Counter electrode 22, 23 Deflection axis direction 25 Liquid crystal molecule 60 Component separation unit 61 Voltage generation unit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-273803 (JP, A) JP-A-7-318959 (JP, A) JP-A-6-301006 (JP, A) JP-A-6-301006 160878 (JP, A) JP-A-6-130394 (JP, A) JP-A-59-9630 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/1343 G02F 1 / 137 G02F 1/1362 G09F 9/35

Claims (7)

(57) [Claims] 1. A liquid crystal layer is sandwiched between two substrates, and a polarizing plate is provided on a light incident side and a light exit side of the liquid crystal layer, and is disposed so as to engage with an inner surface of one of the substrates. A pair of comb electrodes are formed, a counter electrode is formed on the inner surface of the other substrate, and the azimuth of the liquid crystal molecules is changed by controlling a voltage applied between the pair of comb electrodes. the inclination of the liquid crystal molecules is changed, the color of the emitted light is changed by the inclination of the liquid crystal molecules by controlling a potential difference between the average potential and the counter electrode, said liquid
A liquid crystal panel configured so that the luminance of emitted light changes depending on the azimuth angle of crystal molecules . Wherein said counter electrode is positioned approximately midway between adjacent comb teeth of the pair of comb-shaped electrodes, and liquid crystal panel according to claim 1, wherein the parallel stripe electrodes on the comb. 3. When no voltage is applied to the liquid crystal,
The liquid crystal molecules are oriented in a direction parallel to the polarization axis or absorption axis of the polarizer, and the liquid crystal molecules are oriented in a direction crossing the polarization axis when a voltage is applied to a pair of comb-shaped electrodes. LCD panel. 4. When no voltage is applied to the liquid crystal,
When the liquid crystal molecules are oriented perpendicular to the substrate and the potential difference between the counter electrode and the average potential of the pair of comb electrodes exceeds a predetermined threshold voltage, the liquid crystal molecules tilt from a direction perpendicular to the substrate, and the voltage between the pair of comb electrodes 2. The liquid crystal display panel according to claim 1, wherein the azimuthal angle of the liquid crystal molecules increases from the direction of the polarization axis or the absorption axis of the polarizing plate to 45 degrees with respect to the polarization axis as the number of liquid crystal molecules increases. 5. The liquid crystal display device according to claim 1, wherein the pair of comb-shaped electrodes are used as sub-pixels, and two or three sub-pixels are used as one unit pixel to perform color display. 6. A liquid crystal layer is sandwiched between two substrates, and a polarizing plate is provided on a light incident side and a light exit side of the liquid crystal layer, and is disposed so as to engage with an inner surface of one of the substrates. A method for driving a liquid crystal panel in which a pair of comb-shaped electrodes is formed and a counter electrode is formed on the inner surface of the other substrate, wherein a voltage applied between the pair of comb-shaped electrodes is controlled in accordance with pixel luminance,
A method for driving a liquid crystal panel, comprising: controlling a potential difference between an average potential of the pair of comb-shaped electrodes and a counter electrode in accordance with a hue of a pixel. 7. A liquid crystal layer is sandwiched between two substrates, and a polarizing plate is provided on a light incident side and a light exit side of the liquid crystal layer, and is arranged on an inner surface of one of the substrates so as to be engaged with each other. A liquid crystal panel having a pair of comb-shaped electrodes formed thereon and a counter electrode formed on the inner surface of the other substrate; and controlling a voltage applied between the pair of comb-shaped electrodes in accordance with a luminance signal of a pixel. A liquid crystal display device comprising: a driving circuit that controls a potential difference between an average potential of an electrode and a counter electrode in accordance with a hue signal of a pixel.