JPH0762749B2 - Emissive display device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION INDUSTRIAL FIELD OF APPLICATION The present invention is particularly applicable to. The present invention relates to a low-cost light emitting display device having excellent contrast and viewing angle characteristics.

2. Description of the Related Art Currently, most display devices used in word processors, personal computers, televisions, CAD / CAM, etc. use a CRT (cathode ray tube), ranging from monochrome devices capable of displaying A4 full pages to high A variety of sizes ranging from 0.5 "to 40" are used for office or entertainment purposes, even those that can display full-color quality. However, since the CRT has a large volume, it is difficult to make it thin, and high voltage is required. Therefore, there is a strong demand for a flat plate type large capacity, low cost full color display device.

Conventionally, flat panel type color display devices include plasma displays, flat CRTs, fluorescent display tubes, etc., which are still under development, and liquid crystal pocket televisions, which have already been commercialized. The former three have problems that the light emission efficiency is low at present, the panel structure is complicated and expensive, and it is difficult to increase the size. On the other hand, liquid crystal full-color display devices are being actively developed toward full-color enlargement because of the ease of upsizing. As an example, it is described in Technical Report of the Institute of Television Engineers of Japan, Vol. 8, No. 4, pp. 1 to 6, 1984. To realize full-color display with liquid crystal,
Usually, the liquid crystal is simply used as a panchromatic light valve, and by providing red, green, and blue color filters in the form of strips or dots, a color image is displayed by additive color mixture on a two-dimensional surface.

FIG. 3 shows the configuration and operation of a conventional full-color panel using a twisted nematic type (hereinafter abbreviated as TN) liquid crystal display mode. In the TN type full color matrix panel, a pair of glass substrates 2 and 8 sandwich a transparent row electrode 3 and a transparent column electrode 7 made of indium oxide or the like, and a mannethetic liquid crystal 4 having a positive dielectric anisotropy. Therefore, a pair of polarizing plates 1 and 9 are provided outside the glass substrates 2 and 8, respectively.

Red R, green G, and blue B color filter layers 5 are regularly provided on the column electrodes (or row electrodes) to form a color panel. Although the panel is shown in a simplified manner, an alignment treatment layer for regulating the alignment of liquid crystal molecules is usually provided on the color filter layer side surface and the row electrode side surface. On the surface, they are arranged in a specific direction almost parallel to the substrate, and the arrangement direction of molecules is different by 90 ° between one substrate and the other substrate, and the molecules are arranged from one substrate to the other substrate. The directions are gradually twisted, and in the end, an orientation treatment is applied to the surfaces of both substrates so that a twist of about 90 ° is generated between both substrates.

In a liquid crystal color panel, a transmissive type is usually used to obtain a bright display. That is, the white back light source on the back of the panel
Ten are provided. When a linear light source such as a fluorescent lamp is used as the light source 10, a light diffusing plate (not shown) is provided between the light source 10 and the liquid crystal in order to obtain uniform brightness without unevenness on the two-dimensional display surface. It is provided between the panels. If the light source 10 is a planar light source such as electroluminescence, the light diffusion plate is not necessary. The above is one example of the conventional full-color liquid crystal panel, but the most difficult point in the conventional technology is: (1) Generally, in a simple XY matrix display panel, a panel having N scanning lines is used as a line sequential signal. When driven by, the ratio R of the effective value voltage applied between the electrodes sandwiching the pixel to be turned on and the pixel to be turned off is maximized by adopting a driving method called a so-called voltage averaging method. When optimized like this, R = {(N ^1/2 +1) / (N ^1/2 -1)} ^1/2 . That is, in a panel having a simple matrix structure, a crosstalk voltage is applied to off-pixels as well, resulting in a reduction in contrast. For example, if N = 100,
Since R = 1.1, only 10% of the effective value voltage applied between the electrodes corresponding to the OFF pixels is applied to the ON pixel, and the contrast of the display must be provided by the voltage difference of 10%. That is, the display medium used in the simple matrix panel cannot display a high contrast unless the brightness-voltage characteristic is sharp and the threshold value characteristic is clear. In the conventional TN cell, since the sharpness is insufficient, it is the actual situation that even if N = 64 (R = 1.134), the contrast is not comparable to that of the active matrix panel. On the other hand, in the TN cell, the light transmission characteristic of the cell generally depends on the wavelength as shown in the above-mentioned reference, so-called optical rotation dispersion occurs, and the luminance-voltage characteristic varies considerably depending on the wavelength. Also, when a dielectric layer called a color filter is provided on the transparent electrode as shown in the figure, since the filter layer is inserted in series with the liquid crystal, this on and off applied between the electrodes There was a drawback that the contrast of the color panel, in which the voltage ratio of 1 was further lowered in the liquid crystal layer, was considerably worse than that of the monochrome panel.

(2) Another drawback of the conventional TN type full color display panel is that the luminous flux utilization rate from the light source 10 is low. That is, for the purpose of full-color display, the light spectrum from the light source 10 must include red, blue, and green color light components. Since light from an ordinary light source is natural light, when passing through the polarizing plate 1, about 50% of the luminous flux is absorbed by the polarizing plate and is lost. Whether the display medium is a liquid crystal or an electro-optic crystal plate, a display system using a polarizing plate,
This 50% light loss is unavoidable for both monochrome and color panels. A further disadvantage of the color panel as compared with the monochrome panel is that the color filter layer 5 is inserted. That is, the polarizing plates 1 and 9 shown in FIG. 3 are assumed to be neutral polarizing plates, and the white light flux from the back light source 10 becomes white linearly polarized light by the polarizing plate 1. This white linearly polarized light, whether it enters the liquid crystal layer 4 through the color filter layer or exits the liquid crystal layer 4 and enters the filter layer 5 as shown in the figure, absorbs light of a specific wavelength by each color filter layer.

That is, when passing through the red filter layer, the green and blue components are
The red and green components are absorbed when passing through the blue filter layer, and the red and blue components are absorbed when passing through the green filter layer, so the original white light energy is reduced to about one-third. If the light passing through the liquid crystal layer 4 or the color filter layer, which is to be lost, is linearly polarized light and the polarization axis thereof coincides with the polarization axis of the polarizing plate 9, the light is basically transmitted when passing through the polarizing plate 9. Has no light loss. As described above, the polarizing plate 1,9
And even if the color filter 5 is ideal, the light energy passing through the color panel is almost 50% x 33% = 1
It will be about 6.5%. The liquid crystal panel itself usually features low voltage, constant current and low power, but when it comes to liquid crystal color panels, as mentioned above, it requires a back light source and uses only part of the light flux of the back light source. The fact is that the liquid crystal feature of constant power is greatly impaired because it is not possible.

Problems to be Solved by the Invention Problems to be solved by the present invention include (1) Poor threshold characteristics due to polychromatic light in a conventional simple matrix panel (2) Many problems in a conventional simple matrix panel Coloring based on colored light (3) Poor threshold characteristics due to color filter in conventional simple matrix color panel (4) Reduction in brightness due to color filter in conventional simple matrix color panel (5) Conventional simple It is difficult to use a plastic substrate in a matrix panel.

Means for Solving the Problems In order to solve the above problems, in the light emitting display device of the present invention, the electrode surface sides of a pair of transparent substrates each having a transparent electrode face each other, and the transparent electrodes are mutually It is composed of a strip-shaped transparent electrode group, and the pair of transparent electrode groups are arranged so as to be orthogonal to each other to form an XY matrix type electrode, and a positive dielectric anisotropy is provided between both electrodes. The nematic liquid crystal having is aligned substantially horizontally on the substrate and is initially aligned so that it is twisted by about 90 degrees from one substrate to the other, and a polarizing plate is provided outside each transparent substrate. , A phosphor layer on the polarizing plate side for observing the display, a monochromatic light source capable of exciting and emitting the phosphor on the other polarizing plate side, and means for applying a voltage to the transparent electrode. It is a thing.

Effect The present invention has the above-described structure, in which the TN liquid crystal is used as a light valve, and the rotatory dispersion is prevented by using a monochromatic excitation light source, and a fluorescent substance layer having a different emission color is used without using a conventional color filter. Since the energy loss of light transmitted through the liquid crystal cell is prevented by applying different colors, a full-color image that is bright and has excellent contrast in each color can be displayed.

EXAMPLE As shown in FIG. 1, a full-color light emitting type liquid crystal display device of the present invention is basically a monochromatic excitation light source 13, a first polarizing plate 1,
It is composed of a TN liquid crystal cell 14, a second polarizing plate 9, and a phosphor layer 11 which emits R, G, B lights. As the light source 13, a light source that emits substantially single wavelength light in the blue or ultraviolet range is used. As the R, G, B phosphor layer 11, one that emits light into R, G, B by excitation light from the light source 13 is used. Although they are provided in a stripe shape in the figure, they may be provided in a mosaic shape. The phosphor layer 11 may be provided on the polarizing plate 9, or may be provided on a substrate 12 transparent to visible light as shown in FIG. The TN liquid crystal cell 14 includes a pair of glasses provided with a transparent conductive film (hereinafter abbreviated as ITO) such as indium oxide, tin oxide, and a metal thin film, or between the electrode surfaces of the transparent substrates 2 and 8 made of a plastic film, A nematic liquid crystal 4 having a positive Δε is horizontally aligned. Here, means such as an alignment film for aligning liquid crystal molecules in a specific direction and having an appropriate tilt angle (to prevent disclination defects) with respect to the electrode surface and a power source for applying a voltage to the electrodes Is not shown. For the molecular orientation treatment, an organic thin film such as polyimide is applied to the electrode surface, dried, and then rubbed in one direction with a cloth, oblique deposition of SiO, etc. on the electrode surface, or molecular orientation agent is applied to the substrate by dipping or the like. It is performed by adsorption.

In the present invention, the material forming the TN cell 14, that is, the transparent substrate, the transparent electrode, the alignment film, the liquid crystal, the polarizing plate, etc., has a high light transmissivity from the light source 13. In the present invention, “transparent” means having high transparency to the excitation light from the light source 13.

Biphenyl-based and ester-based liquid crystals have a small absorption around 250 nm and above 300 nm, and phenylcyclohexane-based liquid crystals have a small absorption above 250 nm.

For the polarizing plate, a uniaxially stretched oriented film such as polyvinyl alcohol (PVA) (1) a multi-halogen polarizing film in which iodine or the like is arranged, (2) a dichroic dye is arranged and adsorbed 2
Color dye system, (3) In addition to arraying metal such as Au, Ag, Hg, Fe, PVA and polyvinylene polarization system in which polyvinyl chloride double bonds, PVA is potassium iodide and sodium thiosulfate. A near-ultraviolet polarizing film treated with a boric acid solution containing The light transmittance of the TN cell sandwiched between a pair of polarizing plates is
Since it also depends on the liquid crystal layer thickness d, it is important to provide a spacer (not shown) between the electrodes to keep d as constant as possible in order to obtain a uniform display. In the XY matrix type panel for displaying characters, images, etc., the pair of IT
As shown in the drawing, each O is formed into a strip-shaped panel, and the strip-shaped ITOs are arranged so as to be orthogonal to each other, and the intersections of both electrodes form one pixel.

When the matrix panel of the present invention shown in FIG. 1 is line-sequentially driven by a normal voltage averaging method, when the number of scanning lines is N, a voltage R times as large as that of an off cell is applied to an on cell and optical rotation of both pixels. Since the functions are different, the intensity of the excitation light source passing through the second polarizing plate is different. Therefore, the emission intensity from the phosphor layer is different, and contrast is provided. What is important here is that in the present invention, the light rays passing through the liquid crystal layer are monochromatic, so that optical rotation dispersion does not occur as pointed out in the cited example, and since there is no color filter layer on the electrode, the net The voltage R is applied to both pixels, so that a much better contrast than before is obtained.

Usually, in the TN mode, the polarization axis crossing angle of both polarizing plates is set to 90 degrees, and one polarization axis is used in a so-called optical waveguide mode in which the polarization axis is aligned with or orthogonal to the molecular axis of the liquid crystal. It is known that if the polarization axis is not parallel or perpendicular to the molecular axis, a birefringence mode is produced, and the optical rotatory dispersion becomes remarkable and coloring occurs, which is not preferable, but the brightness-voltage characteristics are sharp. Since monochromatic light is used in the present invention, the crossing angles of the polarizing plates and the positional relationship between the polarization axis and the molecular axis can be widely selected. That is, with respect to the wavelength of the light source used and the refractive index anisotropy (Δn) of the liquid crystal, the cell gap (d), the polarization axis crossing angle, and the polarization axis position have the highest ratio of the transmittance of the excitation light from the light source. It may be set to be large.

With this, the contrast can be greatly improved as compared with the conventional cell configuration.

FIG. 2 shows the transmittance-voltage characteristics of the TN cell. Both are parallel Nicols and the solid line (a) is the characteristic of the conventional cell configuration in which the polarization axis of one polarizing plate is aligned with one molecular axis, and the dotted line (b) is the polarization axis 45 degrees with respect to the molecular axis. It is a characteristic when arranged at an angle of. As is clear from the figure, the dotted line shows the luminance −
The voltage characteristics are extremely sharp, and when used in the A region or B region, the brightness is dark-bright or bright-depending on the magnitude of the voltage.
It can be modulated implicitly.

In the present invention, the monochromatic light source, as well as the spectral line,
An excitation light source having a full width at half maximum that does not significantly reduce the contrast.

The fluorescent material used in the present invention may be one widely used for so-called photoluminescence such as a fluorescent lamp.
That is, Eu ³⁺ activated phosphor is typical for red emission, and Y ₂ O ₃ : Eu ³⁺ , Y (P, V) O ₄ : Eu ³⁺ , (Y, Gd) BO ₃ : Eu ³⁺ , Y ₂ O ₂ S: Eu ³⁺ Other 3.5MgO, 0.5MgF ₂ , GeO ₂ : Mn Tb ³⁺ activated phosphor is typical for green emission, and Y ₂ O ₂ S: Tb ³⁺ Ga ₂ O ₂ S: Tb ³⁺ Y ₂ SiO ₅ : Ce ³⁺ , Tb ³⁺ (Ce, Tb) MgAl ₁₁ O ₁₉ , LaPO ₄ : Ce ³⁺ , Tb ³⁺ Other ZnO: Zn, ZnSiO ₄ : MnZn ₂ SiO ₄ : Mn LaPO ₄ : Ce ³⁺ , Tb ³⁺ Eu ²⁺ activated phosphor is typical for blue emission, and BaMg ₂ Al ₁₆ O ₂₇ : Eu ²⁺ , (Sr , Ca) ₅ (PO ₄ ) ₃ Cl: Eu ²⁺ , Sr ₂ P ₂ O ₇ : Eu ²⁺ , Ca ₅ (PO ₄ ) ₃ (F, Cl): Sb ³⁺ (Ba, Ca, Mg) ₅ (PO ₄ ) ₃ Cl: Eu ²⁺ Sr ₅ (PO ₄ ) ₃ Cl: Eu ²⁺ , other MgWO ₄ , CaWO ₄ , Sr ₂ P ₂ O ₇ : Sn Ca ₅ (PO ₄ ) ₃ (F, Cl) : Sb ³⁺ Y ₂ O ₂ S: Tm ³⁺ etc. can be used.

On the other hand, the light source used in the present invention must have a radiation spectrum suitable for exciting and emitting the fluorescent substance. Further, it is desirable that the conversion efficiency from electric energy to radiant energy is as high as possible. As an example, a fluorescent lamp that utilizes the ultraviolet radiation emitted by the arc discharge in mercury vapor is suitable. A fluorescent lamp has a glass tube filled with a small amount of mercury and a few Torr of an odd gas such as argon, and a fluorescent substance is applied to the inner wall of the tube. A pair of electrodes is enclosed at both ends of the tube. The surface of the electrode is coated with an electron emitting substance. In a low-pressure mercury discharge lamp, about 60% of the electric energy input to the lamp is converted into ultraviolet energy having a wavelength of 254 nm. It is not impossible to use 254 nm directly, but as is clear from FIG. 1, it must pass through the polarizing plate, transparent substrate, transparent electrode, alignment film, liquid crystal layer, etc. before exciting the phosphor layer. Don't you? Therefore, the preferable light source spectrum used in the present invention is that the energy attenuation in these layers is small, the damage to these layers is small, and the luminous efficiency to the phosphor layer used is high. If the mercury vapor pressure is increased, longer wavelength components such as 313 nm and 365 nm are increased. Alternatively, the wavelength generated by the discharge
It is also possible to use a fluorescent lamp provided with a fluorescent material for converting ultraviolet light of 254 nm into longer wavelength light of up to about 450 nm provided on the inner wall of the lamp tube.

450 nm is usually required as a blue light component for full color display.

If the excitation light has a wavelength in the vicinity of 450 nm, the B phosphor layer is unnecessary, and a light diffusion plate may be used, and only the G and R phosphor layers that efficiently emit the excitation light of 450 nm may be used.

Effect of the Invention In the conventional full-color liquid crystal panel, TN mode liquid crystal light valves, stripe-shaped or mosaic-shaped R, G, B
The basic combination was a color filter and a white back light source. Since the liquid crystal cell is in TN mode and R increases as N increases, and because the so-called optical rotatory dispersion does not have the same threshold characteristics for all R, G, B color lights, the contrast and color limit range are limited in a simple matrix panel. The problem was the poor display quality such as the narrowness of the screen and the large viewing angle dependency. In order to overcome this limitation, a method called an active matrix for improving R by providing a switching element such as TFT in each pixel has been developed and put into practical use, and it has been proved that the display quality is improved even if the same TN mode is used. However, there is a drawback that the cost of the panel is increased. In the present invention, attention is paid to the fact that even in a TN cell, monochromatic light has a sharper threshold characteristic than polychromatic light, and the color filter layer on the electrode, which has conventionally caused deterioration of the threshold characteristic, is removed, and the number of scanning lines is increased. High contrast and wide color reproducibility have been realized with a simple matrix panel with large N. Even if the brightness-voltage characteristic is steep and N is large, it is easy to obtain the contrast, and the excitation light transmittance of the liquid crystal panel is changed by changing the effective value voltage applied to the cell by pulse width modulation based on the normal voltage averaging method. Therefore, the intensity of the radiated light from the phosphor layer changes and gradation display is easy,
Low cost small to large sized full color TV
It can be said that it is suitable for display.

The second feature of the present invention is the brightness of the display. That is, instead of using a conventional color filter, since the R, G, B phosphor layers each having a different emission spectrum are used, most of the light energy of the light source is set to the excitation energy of each R, G, B phosphor. Can be used. Therefore, if the material selection for the light source, substrate, polarizing plate, transparent electrode, and fluorescent material is optimized to minimize the absorption loss until reaching the fluorescent material, the efficiency is three times higher than the conventional one. It can be a display panel. The third feature of the present invention is that it is easy to make a lightweight and thin plastic panel. Conventionally, TN-mode plastic liquid crystal panels have been put to practical use in thin calculators, etc., but the plastic substrate inside the pair of polarizing plates has no birefringence (optical cube) or is optically uniaxial. Was used and the optical axis direction had to be aligned with the polarization axis of the polarizing plate. Therefore, the plastics that can be used are greatly restricted, and only polyether sulfone resin, phenoxy resin, uniaxial PET (polyethylene terephthalate) resin, etc. can be used. However, in the present invention, since the light passing through the liquid crystal layer is monochromatic, the Δn, d (Δn: refractive index anisotropy, d: cell gap) of the substrate is set to the polarization axis crossing angle of both polarizing plates, the polarization In relation to the angle of the axis with respect to the molecular axis and the twist angle of the Δn · d molecule of the cell, both the excitation light entering the polarizing plate 9 in the ON pixel and the OFF pixel become substantially linearly polarized light, and both polarization axes are The contrast is maximized by optimizing it to form an angle of about 90 degrees, and even if a plastic film having ordinary birefringence is used for the substrate, the contrast characteristic is not deteriorated and the coloring phenomenon does not occur. Thin on substrate (several tens / several hundreds)
The use of micron) plastic films is particularly advocated in the present invention. Because in the panel structure of the present invention, the phosphor layer is provided on the outside of the polarizing plate, so if the substrate thickness on the front side is thicker than the size of the pixel, the color shift will occur unless the excitation light from the back light source is a parallel light beam. , The display quality is deteriorated due to color mixing.

That is, in the case of a diffusion type back light source, it is desirable to make the thickness of the front side substrate sufficiently smaller than the width of the fluorescent body, but since the monochromatic light source is used, the present invention has an advantage that the selection of the resin substrate is extremely easy. .

The fourth feature of the present invention is the expansion of the viewing angle. A drawback of liquid crystal displays is that they generally have a large viewing angle dependency, but this is because the equivalents Δn and d of the liquid crystal layer differ depending on the viewing direction. In the present invention, since the light emitted from the fluorescent substance stimulated by the excitation light is diffuse, it has an advantage that the viewing angle is much wider than the conventional one.

In the present invention, an example in which full-color display is performed has been mainly described, but it goes without saying that a monochromatic light-emitting phosphor layer may be used instead of the R, G, B phosphor layer in FIG. Good. In this case, there are advantages such as improvement of contrast and expansion of viewing angle.

[Brief description of drawings]

FIG. 1 is a perspective view of a full-color display device of the present invention, FIG. 2 is a transmittance-voltage characteristic diagram of a TN liquid crystal cell, and FIG. 3 is a perspective view of a conventional full-color liquid crystal panel. 1 ... First polarizing plate, 2 ... First substrate, 3 ... Transparent row electrode, 4 ... Liquid crystal layer, 7 ... Transparent column electrode, 8 ... Second substrate, 9 ... Second polarizing plate, 12 ... Third substrate, 11 ... Fluorescent material layer, 6 ... Black matrix layer, 13 ... Monochromatic excitation light source, 14 ... TN cell liquid crystal cell.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hiroyuki Onishi 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Shuko Oba 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. 56) References JP 57-204078 (JP, A) JP 50-98345 (JP, A)

Claims (6)

[Claims] 1. A pair of transparent substrates each having a transparent electrode are opposed to each other on their electrode surface sides, said transparent electrodes are constituted by strip-shaped transparent electrode groups, and said pair of transparent electrode groups are orthogonal to each other. Are arranged as described above to form an XY matrix type electrode, and nematic liquid crystal having a positive dielectric anisotropy is oriented substantially horizontally between the two electrodes, and the one substrate to the other substrate. Initially oriented so as to twist about 90 degrees toward each other, a polarizing plate is provided on the outside of each transparent substrate, a fluorescent substance layer is provided on the polarizing plate side for observing the display, and a polarizing plate is provided on the other polarizing plate side. A light-emitting display device comprising a monochromatic light source capable of exciting and emitting a fluorescent substance, and means for applying a voltage to the transparent electrode. 2. A light-emitting display device according to claim 1, wherein the light source uses a gas discharge of low-pressure mercury vapor. 3. The light emitting display device according to claim 1, wherein the light source emits light having a wavelength shorter than at least 450 nm. 4. The polarizing plate is any of a multi-halogen type, a dichroic dye type, a metal polarizing film type, a polyvinylene polarizing system, and a near-ultraviolet polarizing film obtained by treating PVA with a boric acid solution containing potassium iodide and sodium thiosulfate. The light-emitting display device according to claim (1), which is selected from the above. 5. The light emitting display device according to claim 1, wherein the phosphor layers are separately coated so as to emit red, green and blue lights, respectively. 6. The light emitting display device according to claim 1, wherein the substrate on the side of observing the display of the display device is made of at least a plastic film.