JPH07168173A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To suppress the decrease in the light quantity of a back light source, to improve the way of looking of interference fringes and to provide a device which is bright and has an excellent display grade by forming transparent electrodes and insulating film to film thicknesses of low reflectivity to one specific bright line of the back light source and making the half-amplitude value of the bright line exclusive a specific bright line wide in the wavelength distribusion of the back light source. CONSTITUTION:The transparent electrodes 12, 22 and insulating films 13, 23 on transparent substrates 11, 21 constituting a liquid crystal panel are formed at the film thicknesses of the low reflectivity to the one specific bright line of the back light source. The back light source is widened in the half-amplitude value of the bright lines exclusive of the specific bright line in a wavelength distribution. As a result, the specific bright line has high transmittance, increases the luminance of the panel and weakens the interference fringes by reflected light which is of a problem particularly with the steep bright lines. In addition, the bright lines exclusive of the specific bright line have a possibility that the reflectivity at the surfaces of the transparent electrodes 12, 22 and the insulating films 13, 23 increase slightly but the half-amplitude value thereof is set wide and the broad bright lines are set and, therefore, the interference fringes by the bright line are weakened and are made hardly visible.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmitted light type liquid crystal display device having a back light source and a liquid crystal panel.

[0002]

2. Description of the Related Art Conventionally, various techniques have been disclosed for increasing the brightness of a liquid crystal panel in a transmitted light type liquid crystal display device.

For example, in USP 4,737,018 and USP 4,895,432,
By setting the thickness of the transparent conductive film or insulating film on the transparent substrate used for the liquid crystal panel to 5500Å × N / (2 × n),
Measures have been taken to reduce the reflection on these film surfaces and increase the transmittance.

In Japanese Patent Publication No. 4-33009, a three-wavelength phosphor is used as a back light source and a bright line absorption type filter is used.

[0005]

However, in the method of increasing the transmittance by adjusting the film thickness of the above-mentioned transparent conductive film or insulating film, the wavelength range which is derived in a fixed relationship with these film thicknesses is used. Is effective, but it is difficult to increase the transmittance uniformly over the entire wavelength range. For example, Green5
In order to increase the transmittance with respect to the wavelength of 500Å, when N = 1 and n = 2 in the above equation, the film thickness is 5
500 × 1 / (2 × 2) = 1375Å, and with this film thickness, the transmittance for Blue and Red deteriorates (the reflectance is high). FIG. 7 shows the transmittance for Blue and Red for each film thickness of the transparent conductive film (ITO film).

As described above, it is difficult to design an optical film in order to increase the transmittance for all wavelengths in the visible region, and it is very difficult to use it for a liquid crystal device in terms of cost and manufacturing conditions. Is.

Further, in the method shown in the second example, a steep filter characteristic is required as a filter characteristic, so that a dyeing filter is indispensable, and the cost is very high.

Although the light transmittance has been described above, for example, in a large liquid crystal panel, the liquid crystal gap varies in-plane, and a membrane structure having an element such as a TFT is formed in the display portion. In a so-called membrane type display, since the thickness of the membrane structure is about 2 to 3 μm, the thickness of the liquid crystal cell is apt to change, which causes interference fringes as shown in FIG. There was.

The principle of occurrence of the above interference fringes will be described below.

FIG. 4 shows a schematic structure of a conventional general transmitted light type liquid crystal display device, in which 11 is a front substrate.
21 is a rear substrate, 12 and 22 are transparent conductive films, respectively.
3 and 23 are insulating films, and 14 and 24 are alignment films. Further, 31 is a liquid crystal, 41 is a sealing material, 51
Indicates a back light source, and arrows indicate interference luminous fluxes a and b including reflection of light.

In the figure, a light beam a directly transmitted and a light beam b reflected by the front transparent conductive film 12 and further reflected by the rear transparent conductive film 22 interfere with each other, and an interference fringe is formed in a certain relationship with the thickness d of the liquid crystal layer. Occurs. When the entire cell is formed with the same thickness, the transmittance is reduced (dark screen) instead of interference fringes. In the case of the large screen display or the membrane type display described above, the thickness d of the liquid crystal layer is not uniform in the cell plane, and the gap in the central portion is larger or smaller than that in the peripheral portion. Fig. 6
Multiple ring-shaped interference fringes as shown in FIG.

This interference fringe is naturally unlikely to occur when the light from the back light source is broad (especially white light source), and becomes clearly visible in the case of the bright line type. However,
In the case of a white light source, light conversion efficiency is poor and it is dark, and it is difficult to match the color with a color filter. In particular, in the case of a color display device, a three-wavelength bright line type back light source is usually used. FIG. 5 shows an example of the wavelength distribution of the back light source used conventionally.

Since such a bright line type back light source is used, the problem of the above-mentioned interference fringes is conspicuously manifested in a membrane type display having various film structures that cause light reflection.

In view of the above-mentioned problems of the prior art, an object of the present invention is to suppress the reduction of the light amount of the back light source in a transmitted light type liquid crystal display device, improve the appearance of interference fringes, and have a bright and excellent display quality. It is to realize a display device at low cost.

[0015]

Means and Actions for Solving the Problems The present invention made to achieve the above object is

In a transmitted light type liquid crystal display device including a rear light source and a liquid crystal panel, the transparent electrode and the insulating film on the front transparent substrate and / or the rear transparent substrate which compose the liquid crystal panel are the back light sources 1 and 2. The liquid crystal display device is characterized in that the back light source is formed with a film thickness having a low reflectance with respect to one specific bright line, and the half-width of the bright line other than the specific bright line is wide in the wavelength distribution.

According to the present invention, since the reflection of the specific bright line on the transparent electrode and the surface of the insulating film is suppressed to a low level, the specific bright line has a high transmittance and enhances the brightness of the panel, and there is a problem with a particularly sharp bright line. The interference fringes due to the reflected light are weakened. Further, although the bright lines other than the specific bright line may have a slightly higher reflectance on the transparent electrode and the insulating film surface,
Since the full width at half maximum is set wide and the line is broad, the interference fringes due to this line are weakened and are difficult to visually recognize. Furthermore, even if the reflectance is slightly higher at the center wavelength of this bright line, since it has a broad component, the reflectance of this single bright line as a whole does not increase so much, so that the amount of light decreases significantly. Will not be invited.

The present invention is particularly applicable to a color display device.
This is effective when using a three-wavelength bright line type back light source such as a three-wavelength fluorescent lamp. In this case, since there is currently no blue bright line type with high brightness, the coherence is weak. Therefore, the specific bright line is defined as the bright line in the red light region, and the bright lines having a wide half-value width are defined as the green and blue light regions. Alternatively, the specific bright line can be a bright line in the green light region, and the bright lines having a wide half-value width can be a red light region and a blue light region.

In the present invention, the transparent substrate is not limited to a substrate whose entire surface is made of glass or the like, but includes a substrate in which the membrane structure as described above is formed and which is transparent in the display section.

[0020]

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

Embodiment 1 FIG. 1 is a diagram showing a first embodiment of the present invention. In the figure,
A front substrate 11 is a so-called membrane type TFT substrate. The membrane type TFT substrate is disclosed in Japanese Patent Application Laid-Open No. 3-19411.
Using the technique of wafer bonding shown in Japanese Patent Publication No. 5
An OI (Silicon On Insulator) substrate is prepared, and then a semiconductor wafer process is used.
This is a so-called Active Matrix substrate. Since the SOI substrate is opaque as it is, the Si wafer is etched using an organic alkaline solution to make a portion corresponding to the display portion into a membrane to be transparent. The membrane type TFT substrate 11 has a complicated film structure. However, in order to simplify the description here, the description will be made on the simplified film structure shown in FIG.

In FIG. 1, 21 is a rear substrate, and 12 and 2.
Reference numeral 2 is a transparent electrode made of an ITO film, 13 and 23 are insulating films, and 14 and 24 are alignment films made of polyimide.

In this figure, in particular, the detailed film portion and the flattening film of the front substrate (membrane type TFT substrate) 11, the color filter layer and the passivation layer of the rear substrate (color filter substrate) 21 are omitted. Reference numeral 15 indicates a TFT layer on the substrate 11.

As shown in FIG. 1, the light flux a emitted from the back light source 51 is totally transmitted through the liquid crystal panel, and the light flux b is reflected by the ITO film 13 of the membrane type TFT substrate 11 and further ITO of the color filter substrate 21. By being reflected by the film 23, it interferes with the light flux a under an interference condition having an optical path difference of about 2d.

Therefore, in this embodiment, the back light source 51 is used.
Of the light source wavelengths of Blue and Green, a relatively wide half width (Blue is 430 to 470 nm, Gr is
een uses a phosphor having 515 to 555 nm),
For example, manufactured by Nichia Corporation and used as a blue material, NP
-105-01, NP-105-02, and NP-200 and NP-230 are used as the material of Green, and the ITO film thickness design combined with them improves the transparency to Red (around 6200Å) (low reflectance). ) 6200 ÷ 2 ÷
By setting 2≈1550Å, even if the bright line type NP-340 (manufactured by Nichia Corporation) was used for the red phosphor, the interference fringes could not be seen. The wavelength distribution of the back light source of this embodiment is shown in FIG. In this example using the back light source designed in this way, the brightness of the panel could be increased as compared with the example in which the half widths of all three bright lines were widened.

Since the thickness of the membrane type TFT substrate is only about 2 to 3 μm as described above, in the liquid crystal display device using this, the liquid crystal cell gap d is likely to change, and therefore the interference fringes are easily visible. However, the apparatus of the present embodiment described above was able to completely eliminate the interference fringes, and at the same time, a bright display was possible.

Embodiment 2 In this embodiment, in the apparatus configuration as shown in FIG.
As the back light source 51, a bright line type is used for Green (for example, NP-220 manufactured by Nichia Corporation), B
A relatively wide half-value width for blue and red (Blue is 430
˜470 nm, Red is 640 to 665 nm) (for example, Blue is the one shown in Example 1, and Red is NP-32 manufactured by Nichia Corporation).
0, NP-300). The wavelength distribution of the back light source of this example is shown in FIG.

In this embodiment, since the back light source is a green bright line type, the thickness of the ITO film takes into consideration the thickness of the inorganic insulating film formed thereon, and in particular Green.
5500 ÷ 2 ÷ with improved transparency to (around 5500 Å)
The film thicknesses of the ITO and the inorganic insulating film were adjusted so that n≈1375Å (where n is the refractive index of the ITO and the inorganic insulating layer). In this example, the LP-SiN film was used as the inorganic insulating film.

Also in this embodiment, no interference fringe was visible. However, the bright line of the liquid crystal panel was slightly reduced as compared with Example 1. This is a phosphor with a relatively wide half width,
This is because there is currently no red light with higher brightness than green light.

[0030]

As described above, according to the present invention, it is possible to improve the appearance of interference fringes and at the same time suppress the reduction of the light amount of the back light source, thereby realizing a bright liquid crystal display device having a high light transmittance.

Further, the color design of the three-color phosphor which is preferably used for the back light source is very easy, and it is not necessary to completely re-examine the balance of each color phosphor, and the matching property with the current color filter is satisfied. Is also good enough. Further, in terms of production as well, only one kind of phosphors currently mixed is changed, and there are few problems in cost increase and production control. In addition, since the wavelength of the specific bright line that is the non-reflection (low reflection) condition is clear and there is only one type, optical design such as film design is easy.

As described above, in addition to the original purpose of the present invention to simply reduce the interference fringes and become a bright panel device, the product development period is shortened, cost is prevented from increasing, smoothness of production start-up, and color design are easy. A secondary effect such as the following is also obtained, and it can be said that the industrial utility value of the present invention is great.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a wavelength distribution of a back light source used in the present invention.

FIG. 3 is a diagram showing an example of a wavelength distribution of a back light source used in the present invention.

FIG. 4 is a schematic configuration diagram of a liquid crystal display device used for explaining a conventional example.

FIG. 5 is a diagram showing a wavelength distribution of a back light source of a conventional example.

FIG. 6 is a diagram of interference fringes generated on a liquid crystal panel display unit.

FIG. 7 is a diagram showing a relationship between a film thickness of a transparent conductive film and light transmittance.

[Explanation of symbols]

 11 Front Substrate 12 Transparent Electrode 13 Insulation Film 14 Alignment Film 15 TFT Layer 21 Rear Substrate 22 Transparent Electrode 23 Insulation Film 24 Alignment Film 31 Liquid Crystal 41 Sealing Material 51 Rear Light Source

Claims (3)

[Claims] 1. In a transmitted light type liquid crystal display device comprising a rear light source and a liquid crystal panel, the transparent electrode and the insulating film on the front transparent substrate and / or the rear transparent substrate constituting the liquid crystal panel are the rear light source. 2. A liquid crystal display device, wherein the back light source is formed to have a film thickness having a low reflectance with respect to one specific bright line, and the half width of a bright line other than the specific bright line is wide in the wavelength distribution. 2. The liquid crystal according to claim 1, wherein the specific bright line is a bright line in a red light region, and the bright line having a widened half-value width is a bright line in a green light region and a blue light region. Display device. 3. The liquid crystal according to claim 1, wherein the specific bright line is a bright line in a green light region, and the bright line having a widened half-value width is a bright line in a red light region and a blue light region. Display device.