JPH0990345A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a reflection type color liquid crystal display device capable of making full-color display by setting the areas of respective sub-pixels so as to vary. SOLUTION: Flattening films for TFTs are formed. These flattening films are silicon nitride films and are formed by a plasma CVD apparatus and the film thickness thereof is 1μm. Pixel electrodes 57 are formed on the flattening films and the material thereof is aluminum. The three sub-pixels are formed at each pixel electrode 57 so as to constitute one pixel. The area ratios of the respective sub-pixel are set at x1 :x2 :x3 =1:2:3. Through-holes 55 are formed in the flattening films and source electrodes 46 and the pixel electrodes 57b are connected. A driving device is combined with the display panel of the liquid crystal display device constituted in such a manner and the change in the display colors according to a change in the driving voltage (source voltage) when the gates are open is widened in the measurement range to a range including about 10 pixels. Seven gradations are obtd. in the measurement of the chromaticity of the respective gradations between the green display and the white display. The respective gradations are distributed at nearly equal intervals on chromaticity coordinates.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflective color liquid crystal display device characterized by low power consumption.

[0002]

2. Description of the Related Art Reflective liquid crystal display devices are thin and have low power consumption because they use external light as a light source for display. Further, the reflective liquid crystal display device can be mounted on a portable information terminal which is expected to spread rapidly in the future.

In a color liquid crystal display device, one pixel is composed of a plurality of independently controllable parts. For example, one pixel of a transmissive color liquid crystal display device has R, G, B3
It is composed of three parts corresponding to the color filters of the colors. Hereinafter, each part that constitutes one pixel will be referred to as a subpixel.

The transmissive color liquid crystal display device performs bright and dark display and color display separately. That is, a liquid crystal layer, a phase plate and a polarizing plate are used to display light and dark, and a color filter is used for color display. Then, the polarizing plate absorbs light to display light and dark, and the color filter absorbs light to perform color display. Since the two parts absorb light in this way, the transmittance was low.

On the other hand, the reflective color display device has one
Bright and dark display and color display are performed in one part. That is, the liquid crystal layer, the phase plate, and the polarizing plate are set so that the display color changes according to the applied voltage value. Without using color filters,
Bright and dark display and color display are performed by the birefringence interference color of the liquid crystal layer, the phase plate and the polarizing plate. Since there is only one portion that absorbs light, the transmittance is high, and reflective color display is possible.

In the transmissive color liquid crystal display device, one sub pixel always displays the same color. By controlling the liquid crystal orientation and changing the transmittance of each sub-pixel, the display color and gradation of one pixel are changed. The area of each sub-pixel is almost the same.

On the other hand, in the reflective color liquid crystal display device, one sub-pixel displays different colors depending on the applied voltage value. That is, color display of at least three colors including white display and black display is performed. As described above, in the reflective color liquid crystal display device, the operation of the sub-pixel is different from that of the transmissive color liquid crystal display device, and thus a new gradation display method is required.

In the conventional reflection type color liquid crystal display device, for example, in JP-A-6-95151, the area of each sub-pixel is almost equal.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to devise a gradation display method for a reflective color liquid crystal display device and to realize a reflective color liquid crystal display device capable of full color display.

In the conventional reflective color liquid crystal display device, the areas of the sub-pixels are equal, but the gradation display in this case will be described by taking the gradation between the green display and the white display as an example. In this case, each subpixel displays green or white, and the number of combinations of display colors of each subpixel is expressed by the following equation.

[0011]

(Equation 3)

In Expression 3, n is the number of subpixels in one pixel. When n = 3 as in the transmissive color liquid crystal display device, the number of display color combinations of each sub-pixel becomes eight.
An example of the pixel shape in this case is shown in FIG. Subpixels are numbered (1, 2, 3, ...) And the area of the i-th subpixel is represented as x _i . That is, in this case, x ₁ : x ₂ : x ₃ = 1: 1: 1. All combinations of display colors of each subpixel are shown in FIG. In FIG. 3, white subpixels indicate white display, and hatched subpixels indicate green display.

The color perceived by humans is determined by the area ratio of each color within one pixel. In (b), (c), and (d) in FIG. 3, the area ratio of green display and white display is 2: 1, and humans perceive these three to be the same color. Also, (e), (f), (g)
In both cases, the area ratio of green display and white display is 1: 2, and humans perceive these three as the same color. Therefore, the eight combinations shown in FIG. 3 can display only four gradations.

[0014]

In order to solve the above problems, in the present invention, the area of each subpixel is set to be different.

As in the previous section, consider the case where n = 3. First, x ₁ : x ₂ : x ₃ = 1: 2: 3.
An example of the pixel shape in this case is shown in FIG. Regarding the gradation between the green display and the white display, all combinations of display colors of each subpixel are shown in FIG. Since x ₁ + x ₂ = x ₃ , the colors (d) and (e) in FIG. 5 have the same color, but other combinations do not have the same color. The eight combinations shown in FIG. 5 can display seven gradations.

Next, consider the case where x ₁ : x ₂ : x ₃ = 1: 2: 5. An example of the pixel shape in this case is shown in FIG.
Shown in As shown in FIG. 7, all combinations do not form the same color, and display of 8 gradations, which is the maximum number of gradations, can be performed by three sub-pixels. Even when n = 3,
If the relationship of Mathematical 1 is established for x _i , not all combinations have the same color, and the maximum number of gradations can be obtained.

In FIG. 7, the combinations of display colors are arranged in descending order of white display area. Area ratio of green display is 0/8, 1 /
8, 2/8, 3/8, 5/8, 6/8, 7/8, 8/8
And change. The area ratio of red display is 0/8 to 3/8, 5 /
It increases by 1/8 between 8 and 8/8, but it is 3/8 and 5
It increases by 2/8 between / 8 and the way of changing the gradation is not constant. Therefore, next, x ₁ : x ₂ : x ₃ = 1: 2: 4. FIG. 8 shows an example of the pixel shape in this case, and FIG. 9 shows all combinations of display colors of the sub-pixels. Area ratio of green display is 0/7, 1/7, 2/7, 3/7, 4/7, 5
/ 7, 6/7, 7/7, and the area ratio of green display increases by 1/7 in any gradation. Even if n = 3 is not satisfied, the maximum number of gray levels can be obtained and the way of changing the gray level becomes constant if the relationship of Equation 2 holds for x _i .

If the color purity of each display color is high, the range of display colors is widened, and the number of gradations that can be visually recognized by human eyes is also increased.
The liquid crystal display device of the present invention uses an active element such as a TFT in order to apply a voltage that maximizes the color purity of each color to the liquid crystal layer. In order to prevent a decrease in aperture ratio due to the active element, a display electrode for directly applying a voltage to the liquid crystal layer is placed in a layer above the active element. The display electrode is made of a high-reflectance conductor such as aluminum, and also serves as a reflector.
The display electrode is connected to the source electrode of the active element through a through hole.

When using the active element, the voltage of the liquid crystal layer must be kept constant during the non-selection time. If the voltage of the liquid crystal layer changes during the non-selection time, the display color also changes, and the color purity recognized by the human eye decreases. A storage capacitor is used to prevent this. The storage capacitor is formed between the common electrode and the source electrode of the next row by extending the source electrode to above the common electrode of the next row.

Alternatively, a reference electrode is newly provided.
The reference electrode is grounded and electrically neutral. The source electrode is extended above the reference electrode, and a storage capacitor is formed between the reference electrode and the source electrode. At this time, the i-th subpixel, the source electrode connected to the i-th subpixel, and the storage capacitor are configured so as not to stereoscopically intersect with other subpixels. This can prevent the occurrence of parasitic capacitance between the i-th subpixel and other subpixels.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The contents and effects of the present invention will be described below with reference to specific examples.

(Embodiment 1) FIG. 1 is a plan view showing an example of the configuration of one pixel and its periphery of a liquid crystal display device of the present invention.
FIG. 10 and FIG. 11 are sectional views taken along line 1-1 ′ and 2- of FIG.
It is sectional drawing in a 2'cut line. The configuration and manufacturing method of the liquid crystal display device of the present invention will be described in order from the bottom of FIGS.

The lower substrate 10 is made of non-alkali borosilicate glass 11 and has silicon oxide layers 12 on the upper and lower sides.

The gate electrode 20 is formed so as to vertically project from the scanning lines 27 and 27 'and extend beyond the active area of the TFT. An aluminum film 21 was used for the gate electrode and was formed by a sputtering method. Further, an anodized aluminum film 22 was formed on this. A silicon nitride film was used as the insulating film 25 and was formed by a plasma CVD method to a film thickness of 2000 Å.

The TFT operates so that the channel resistance between the source and the train decreases when a positive bias is applied to the gate electrode, and the channel resistance increases when the bias is zero. The TFT has a gate electrode 20, an insulating film 25,
i-type semiconductor layer 30 made of i-type (intrinsic, intrinsic, conductivity-type determining impurity-free) amorphous silicon
Having. The i-type semiconductor layer was also formed at the intersection of the scanning line and the signal line. Here, the i-type semiconductor layer is distributed between the scanning line and the signal line to reduce a short circuit between them.

Amorphous layer silicon 31 was used for the i-type semiconductor layer, and the layer thickness was 2000 Å. Above the portion of the i-type semiconductor layer that overlaps with the source electrode and the drain electrode, N is formed.
A (+) type amorphous silicon semiconductor layer 32 was formed. The N (+) type amorphous silicon semiconductor layer is formed by doping amorphous silicon with phosphorus for ohmic contact.

The source electrode 40 and the drain electrode 45 are composed of two layers. Of these layers, the upper layers 41 and 46 are made of aluminum and are formed by the sputtering method, and the layer thickness is 4000 Å. The lower layers 42 and 47 are made of chromium and are similarly formed by the sputtering method, and the layer thickness is 600Å. The source electrode was extended above the adjacent scanning electrode 27 ', and the storage capacitor 38 was formed between the two.

A flattening film 50 was formed on the TFT.
The flattening film is a silicon nitride film, formed by a plasma CVD apparatus, and has a film thickness of 1 μm. A pixel electrode 57 was formed on the flattening film, and the material was aluminum. The pixel electrode was formed so that three sub-pixels form one pixel. The area ratio of each sub-pixel is x ₁ : x ₂ : x ₃ = 1:
It was set to 2: 3. Forming a through hole 55 in the flattening film,
The source electrode and the pixel electrode were connected. A polyimide organic polymer (RN718) manufactured by Nissan Chemical Industries, Ltd. was used for the organic alignment film 60. HA-5073XX made of nitrogen was used for the liquid crystal layer 70. HA-
5073XX contains no chiral agent and exhibits a nematic layer at room temperature. The maximum thickness of the liquid crystal layer in the pixel electrode is 6.2μ
m, and the retardation of the liquid crystal layer when no voltage was applied was 0.86 μm.

The organic alignment film 61 on the upper substrate 80 is the same as that on the lower substrate. The organic alignment films on the upper and lower substrates have alignment treatment directions antiparallel to each other and a pretilt angle of 5 °. The common transparent pixel electrode 75 on the upper substrate is made of an Indium-Tin-Oxide (ITO) film formed by sputtering and has a film thickness of 1400Å. The upper substrate 80 is made of non-alkali borosilicate glass 81 like the lower substrate,
A silicon oxide layer 82 is provided on the top and bottom.

A polycarbonate phase plate manufactured by Nitto Denko was used as the phase plate 90. Retardation of the phase plate is 0.22
μm 2 and the slow axis was oriented perpendicular to the liquid crystal alignment direction of the liquid crystal layer. The polarizing plate 95 is G1225DUAG25 manufactured by Nitto Denko.
The orientation of the absorption axis was set to 45 ° with respect to the liquid crystal orientation direction.

A drive device was combined with the display panel of the liquid crystal display device having this structure, and the dependency of the reflectance on the drive voltage (source voltage) when the gate was opened was measured. The measurement range was within 1 sub-pixel. The result is shown in FIG. The drive voltage with which each display color is obtained is indicated by an arrow in FIG. In addition to white and black, red, green, cyan, purple, yellow, and blue colors were displayed in the driving voltage range of 0V to 3V. In addition, FIG. 12 shows a change in display color due to a change in driving voltage by a UCS (Unifom Chrom
(aticity System) is a diagram plotted on chromaticity coordinates (u, v) of a display system. Next, the measurement range was expanded to a range including about 10 pixels, and the chromaticity of each gradation between green display and white display was measured. Seven gradations were obtained as shown in FIG. 17, and each gradation was distributed on the chromaticity coordinates at substantially equal intervals.

As described above, one pixel is composed of three sub-pixels, and the area ratio of each sub-pixel is x ₁ : x ₂ : x ₃ =
By setting the ratio to 1: 2: 3, display with 7 gradations was obtained.

(Embodiment 2) In the liquid crystal display device of Embodiment 1, the area ratio of each sub-pixel is x ₁ according to the formula ₁ :
x ₂ : x ₃ = 1: 2: 5. When the chromaticity of each gradation between the green display and the white display was measured, as shown in FIG. 18, eight gradations, which is the maximum number of gradations when one pixel was composed of three sub-pixels, were obtained. .

(Embodiment 3) In the liquid crystal display device of Embodiment 1, the area ratio of each sub-pixel is x _{1 in} accordance with equation 2:
x ₂ : x ₃ = 1: 2: 4. When the chromaticity of each gradation between the green display and the white display was measured, as shown in FIG. 19, eight gradations, which is the maximum number of gradations when one pixel was composed of three subpixels, were obtained. . Further, each gradation is distributed on the chromaticity coordinates at substantially equal intervals, and a gradation display that continuously changes from green display to white display was obtained.

(Example 4) In the liquid crystal display device of Example 1, the pixel electrode was formed so that four sub-pixels form one pixel as shown in FIG. The area ratio of each sub-pixel is x ₁ : x ₂ : x ₃ : x ₄ = 1: 2: 4:
It was set to 8. When the chromaticity of each gradation between green display and white display was measured, 16 gradations, which is the maximum number of gradations, was obtained when one pixel was composed of four sub-pixels. Further, each gradation is distributed on the chromaticity coordinates at substantially equal intervals, and a gradation display that continuously changes from green display to white display was obtained.

(Embodiment 5) In the liquid crystal display device of Embodiment 4, a reference electrode 35 is newly installed as shown in FIG. The reference electrode is parallel to the scanning line and is formed in the same layer as the gate electrode as shown in the sectional views of FIGS. In addition, the aluminum film 3 is used as the reference electrode, similarly to the gate electrode.
6 was used, and the film was formed by the sputtering method. Further, an anodized aluminum film 37 was formed on this. The source electrode was extended above the reference electrode, and a storage capacitor 38 was formed between the two. As shown in FIG. 14, the storage capacitor 38 connected to the i-th subpixel was formed with a reference electrode so as not to three-dimensionally intersect with other subpixels. The reference electrode was electrically neutralized by extending to the outside of the display area of the liquid crystal display device and grounding.

When the chromaticity of each gradation between green display and white display was measured, 16 gradations, which was the maximum number of gradations when one pixel was composed of four sub-pixels, were obtained. Further, each gradation is distributed on the chromaticity coordinates at substantially equal intervals, and a gradation display that continuously changes from green display to white display was obtained.

(Comparative Example 1) In the liquid crystal display device of Example 1, the area ratio of each sub-pixel is x ₁ : x ₂ : x ₃ = 1:
It was set to 1: 1.

When the chromaticity of each gradation between green display and white display was measured, four gradations were obtained as shown in FIG. When one pixel is composed of three sub-pixels, a maximum of 8 gradations can be obtained, but since the area ratio of each sub-pixel is made equal, only half of that, 4 gradations can be obtained.

[0040]

According to the present invention, a reflective liquid crystal display device capable of multi-gradation display can be obtained.

[Brief description of drawings]

FIG. 1 is a front view showing an example of the configuration of a liquid crystal display device of the present invention.

FIG. 2 is an explanatory diagram showing a pixel configuration of a conventional liquid crystal display device.

FIG. 3 is an explanatory diagram showing gradation display in a pixel configuration of a conventional liquid crystal display device.

FIG. 4 is an explanatory diagram showing an example of a pixel configuration of a liquid crystal display device of the present invention.

FIG. 5 is an explanatory diagram showing gradation display in the pixel configuration of the liquid crystal display device of the present invention.

FIG. 6 is an explanatory diagram showing an example of a pixel configuration of a liquid crystal display device of the present invention.

FIG. 7 is an explanatory diagram showing gradation display in the pixel configuration of the liquid crystal display device of the present invention.

FIG. 8 is an explanatory diagram showing an example of a pixel configuration of a liquid crystal display device of the present invention.

FIG. 9 is an explanatory diagram showing gradation display in the pixel configuration of the liquid crystal display device of the present invention.

FIG. 10 is a cross-sectional view showing an example of a configuration of a TFT portion of the liquid crystal display device of the present invention.

FIG. 11 is a cross-sectional view showing an example of the configuration of a through hole portion of the liquid crystal display device of the present invention.

FIG. 12 is a characteristic diagram showing a voltage change in chromaticity of a display color.

FIG. 13 is a perspective view showing voltage changes in display color and reflectance.

FIG. 14 is a front view showing an example of the configuration of a liquid crystal display device of the present invention.

FIG. 15 is a cross-sectional view showing an example of the configuration of a TFT section of a liquid crystal display device of the present invention.

FIG. 16 is a cross-sectional view showing an example of the configuration of a through hole portion of the liquid crystal display device of the present invention.

FIG. 17 is a characteristic diagram showing chromaticity of each gradation between green display and white display of the liquid crystal display device of the present invention.

FIG. 18 is a characteristic diagram showing chromaticity of each gradation between green display and white display of the liquid crystal display device of the present invention.

FIG. 19 is a characteristic diagram showing chromaticity of each gradation between green display and white display of the liquid crystal display device of the present invention.

FIG. 20 is a characteristic diagram showing the chromaticity of each gradation between the green display and the white display of the conventional liquid crystal display device.

[Explanation of symbols]

20 ... Gate electrode, 27, 27 '... Scan line, 30 ... i-type semiconductor layer, 38 ... Storage capacitor, 40 ... Source electrode, 45 ...
Drain electrode, 50 ... Flattening film, 57 ... Pixel electrode, 55
… Through holes.

Claims (6)

[Claims] 1. A liquid crystal layer, a driving device, a polarizing plate, a phase plate, and two substrates. The two substrates are provided with display electrodes and alignment films, and the display electrodes on the lower substrate are reflective. It also serves as a plate, the display electrode of the lower substrate is connected to the active element, the two transparent substrates are arranged to face each other with the liquid crystal layer sandwiched therebetween, and the polarizing plate is arranged above the upper substrate. In the reflective liquid crystal display device, the phase plate is disposed between the polarizing plate and the upper substrate, and the absorption axis of the polarizing plate and the alignment direction in which the liquid crystal layer is close to each other are not parallel to each other. If each unit has a plurality of possible units and each unit is a sub-pixel, each sub-pixel can display white and black and at least one color, and the areas of the sub-pixels are different from each other. Characteristic liquid crystal display device. 2. A liquid crystal display device according to claim 1, wherein when the sub-pixels within one pixel are numbered in ascending order of area, the area x _i of the i-th sub-pixel is expressed by the following equation. [Equation 1] 3. A liquid crystal display device according to claim 2, wherein when the sub-pixels within one pixel are numbered in ascending order of area, the area x _i of the i-th sub-pixel is expressed by the following equation. [Equation 2] 4. The display electrode on the lower substrate of each subpixel is connected to a source electrode of an active element through a through hole, and the source electrode is provided between an adjacent common electrode. A liquid crystal display device that forms a storage capacitor. 5. The display electrode on the lower substrate of each subpixel is connected to a source electrode of an active element through a through hole, and the source electrode is between an electrically neutral reference electrode. A liquid crystal display device that forms a storage capacitor in a liquid crystal display device. 6. The liquid crystal display device according to claim 5, wherein the source electrode connected to the i-th subpixel and the storage capacitor do not stereoscopically intersect with other subpixels.