JPH07104257A - High polymer dispersion type liquid crystal display element - Google Patents

Abstract

PURPOSE:To improve the responsiveness when a display turns to a dark state from a bright state so as to enable time-division driving at a high duty by and to enhance a light scattering effect in a non-electric field state so as to improve the contrast of the display by providing a liquid crystal/high polymer composite film dispersed with cholesteric liquid crystals between a pair of substrates formed with electrodes. CONSTITUTION:The front surface side substrate 11 and the rear surface side substrate 12 are joined via a frame-shaped sealing material in the outer peripheral part thereof. The region enclosed by the sealing material between these substrates 11 and 12 is provided with the liquid crystal/high polymer composite film 30. The liquid crystal/high polymer composite film 30 is formed by dispersing the cholesteric liquid crystals 32 into a high polymer layer 31. The composite film 30 has a structure formed by confining the respective cholesteric liquid crystals 32 into the respective gap parts of the polymerized high polymer layer 31 having a sponge-like section.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polymer dispersion type liquid crystal display device.

[0002]

2. Description of the Related Art Recently, polymer dispersion type liquid crystal display elements have been attracting attention as liquid crystal display elements. In this polymer dispersion type liquid crystal display element, a liquid crystal / polymer composite film in which a liquid crystal is dispersed in a polymer layer is provided between a pair of substrates on which electrodes are formed.

In the liquid crystal / polymer composite film of the polymer dispersion type liquid crystal display device, the liquid crystal is roughly confined in each void portion of the polymer layer polymerized to have a cross section like a sponge. There is a structure and a structure in which liquid crystal is microencapsulated and the liquid crystal capsules are dispersed in a polymer layer.A nematic liquid crystal with a positive dielectric anisotropy is used for both composite films. There is.

FIG. 4 is a sectional view of a part of a liquid crystal / polymer composite film in a conventional polymer dispersed liquid crystal display device.
(A) shows a non-electric field state, and (b) shows an electric field application state. The composite film shown in FIG. 4 has a structure in which the nematic liquid crystal 2 is confined in each void of the polymer layer 1 polymerized to have a cross-section like a sponge. The molecules of the nematic liquid crystal 2 are shown.

The above polymer dispersion type liquid crystal display device is
The display is made by utilizing the scattering and transmission of light in the polymer composite film, and in the state where no electric field is applied between the electrodes of both substrates, the nematic liquid crystal 2 in the composite film is As shown in FIG. 4 (a), it is in a random direction.

Therefore, in the non-electric field state, the polymer layer 1 and the nematic liquid crystal 2 have different refractive indexes, and the light incident on the liquid crystal display element is scattered by the composite film, and the scattered white turbid light causes darkness. The status is displayed.

When an electric field is applied between the electrodes of both substrates, the molecules 2a of the nematic liquid crystal 2 in the composite film are
As shown in FIG. 4B, the nematic liquid crystals 2 are uniformly arranged in a rising direction in a direction in which an electric field is applied, that is, in a direction substantially perpendicular to the substrate surface, and the refractive index of the nematic liquid crystal 2 becomes substantially equal to that of the polymer layer 1. The incident light passes through the composite film with little scattering, and the non-scattered transmitted light displays a bright state.

That is, the polymer dispersion type liquid crystal display device displays a dark state due to cloudy light and a bright state due to transmitted light by utilizing the scattering and transmission of light in the liquid crystal / polymer composite film. This polymer-dispersed liquid crystal display device can display light and dark without using a polarizing plate, and therefore, there is no loss of light amount due to light absorption in the polarizing plate such as a commonly used TN type liquid crystal display device. Therefore, it has a feature that the screen is significantly brighter than that of the TN type liquid crystal display element.

[0009]

However, the conventional polymer-dispersed liquid crystal display device has good responsiveness when the display changes from the dark state to the bright state, but it does not respond when the display changes from the bright state to the dark state. There is a problem that the responsiveness is poor and therefore it is difficult to perform time division driving with high duty.

This is because the nematic liquid crystal 2 used in the liquid crystal / polymer composite film has liquid crystal molecules 2a aligned in the direction of the electric field in a relatively short time when an electric field is applied, depending on the strength of the applied electric field. However, when the electric field applied state is changed to the non-electric field state, the direction of the liquid crystal molecules 2a cannot be controlled by the electric field, and therefore the liquid crystal molecules 2a naturally have to return to a randomly oriented state. It takes time to change from a bright state to a dark state.

Moreover, in the conventional polymer-dispersed liquid crystal display device, the molecules 2a of the nematic liquid crystal 2 in the liquid crystal / polymer composite film tend to be aligned along the surface of the polymer layer 1 in the non-electric field state. Even when the liquid crystal molecules 2a are oriented in random directions, when viewed partially, the orientation of the liquid crystal molecules 2a is almost the same as shown in FIG. 4A near the interface with the polymer layer 1. I have them all.

Therefore, in the conventional polymer dispersion type liquid crystal display element, the difference between the refractive index of the nematic liquid crystal 2 and the refractive index of the polymer layer 1 in the non-electric field state is small, and therefore the light scattering effect is low. However, there is a problem that the contrast of the display is poor because the darkness of the dark display due to the cloudy light is not sufficient.

According to the present invention, not only the responsiveness when the display changes from the dark state to the bright state, but also the responsiveness when the display changes from the bright state to the dark state, the time division driving at high duty is performed. It is an object of the present invention to provide a polymer-dispersed liquid crystal display device that is capable of enhancing the light scattering effect in a non-electric field state and improving the display contrast.

[0014]

In a polymer dispersed liquid crystal display device of the present invention, a liquid crystal / polymer composite film in which a cholesteric liquid crystal is dispersed in a polymer layer is provided between a pair of substrates on which electrodes are formed. It is characterized by that. In the present invention, it is desirable to add a black dichroic dye to the cholesteric liquid crystal.

[0015]

In other words, the polymer dispersed liquid crystal display device of the present invention uses a cholesteric liquid crystal in which liquid crystal molecules are arranged in a twisted state at a helical pitch peculiar to the liquid crystal in a liquid crystal / polymer composite film liquid crystal state in the absence of an electric field. In this cholesteric liquid crystal, when an electric field is applied, liquid crystal molecules are aligned in the electric field direction in a relatively short time according to the strength of the applied electric field, and when the application of the electric field is cut off, The alignment state of the liquid crystal molecules returns to the original twist alignment state.

Therefore, the polymer dispersed liquid crystal display device has good responsiveness not only when the display changes from the dark state to the bright state, but also when the display changes from the bright state to the dark state. High-duty time-division driving is possible.

Moreover, in this polymer dispersion type liquid crystal display element, the molecules of the liquid crystal of the liquid crystal / polymer composite film are arranged in a twisted state at a spiral pitch peculiar to the cholesteric liquid crystal in the non-electric field state, so that there is no electric field state. The difference between the refractive index of the liquid crystal and the refractive index of the polymer layer is large, and therefore, the light scattering effect in the non-electric field state can be enhanced and the display contrast can be improved.

In this polymer-dispersed liquid crystal display device, if a black dichroic dye is added to the cholesteric liquid crystal, scattered light in the non-electric field state is absorbed by the dichroic dye. Therefore, it is possible to make the dark display closer to black and further increase the contrast.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. The polymer-dispersed liquid crystal display element of this example is an active matrix type and is a reflective element that reflects incident light from the front surface side by a reflective film provided on the back side substrate to display. is there.

First, the structure of the polymer dispersion type liquid crystal display element will be described. FIG. 2 is a sectional view of a part of the liquid crystal display element. In the figure, the upper substrate 11 is the front side substrate and the lower substrate 12 is This is the back side substrate.

The front-side substrate 11 is a transparent insulating substrate made of a glass plate or the like, and on the front-side substrate 11, a plurality of transparent pixel electrodes 13 arranged in rows and columns are formed. , A plurality of active elements 14 corresponding to the respective pixel electrodes 13 are provided.

The active element 14 is, for example, a TFT (thin film transistor), and the TFT 14 includes a gate electrode 15 formed on the substrate 1, a gate insulating film 16 covering the gate electrode 15, and the gate insulating film 16. A semiconductor film 17 made of a-Si (amorphous silicon) or the like formed above the gate electrode 15 and a source electrode 18 and a drain electrode 19 formed on both sides of the semiconductor film 17. It is configured.

Although not shown, a gate line (address line) for supplying a gate signal to the TFT 14 and a data line for supplying a data signal corresponding to image data to the TFT 14 are provided on the front substrate 11. , The gate electrode 15 of the TFT 14 is formed integrally with the gate line, and the drain electrode 19 is connected to the data line.

The gate insulating film 16 of the TFT 14 is also used.
Is a transparent insulating film made of Si N (silicon nitride) or the like, and the pixel electrode 13 is formed on the gate insulating film 16 and is connected to the source electrode 8 of the corresponding TFT 14 at one end thereof.

On the other hand, the back side substrate 12 is an insulating substrate made of a glass plate or the like (however, it need not be transparent).
The rear surface side substrate 12 is provided with the counter electrode 20 facing substantially all the pixel electrodes 13 of the front surface side substrate 11 over substantially the entire surface thereof. This counter electrode 20
Is made of a metal film having a high light reflectance such as Al (aluminum) and also serves as a reflection film for reflecting light incident from the surface side of the liquid crystal display element.

On the counter electrode 20, there is provided a phosphor film 21 which emits fluorescence when exposed to light. The phosphor film 21 is a transparent resin base material in which granular phosphor materials are mixed in a scattered state. In this embodiment, as the phosphor film 21, a plurality of phosphor films emitting fluorescence of different colors, for example, a phosphor film emitting red fluorescence, a phosphor film emitting green fluorescence, and a blue fluorescence film. And a phosphor film that emits light, and these phosphor films are alternately arranged so as to face each pixel electrode 13 of the front substrate 11.

The front-side substrate 11 and the back-side substrate 12 are joined to each other at their outer peripheral edges via a frame-shaped sealing material (not shown), and are surrounded by the sealing material between the substrates 11 and 12. The liquid crystal / polymer composite film 30 is provided in the isolated region.

The liquid crystal / polymer composite film 30 is a polymer layer 31 in which a cholesteric liquid crystal 32 is dispersed, and the composite film 30 is a polymer polymerized so as to have a cross section like a sponge. The layer 31 has a structure in which the cholesteric liquid crystal 32 is confined in each void.

FIG. 1 is an enlarged cross-sectional view of a part of the liquid crystal / polymer composite film 30 in the non-electric field state and the electric field applied state. The cholesteric liquid crystal 32 is shown in FIG. ), The liquid crystal molecules 32a are arranged in a twisted state with a helical pitch peculiar to the liquid crystal, and this twisted arrangement is present randomly. As shown in FIG.

The liquid crystal display element is formed, for example, by joining a pair of substrates 11 and 12 via a sealing material, and then removing a part of the sealing material between the substrates 11 and 12. A mixed solution of a polymer material that undergoes a polymerization reaction by light and a cholesteric liquid crystal is injected and filled from the above injection port by a vacuum injection method, and the filling solution is irradiated with ultraviolet rays from the side of the front surface side substrate 11 which is a transparent substrate. It can be produced by a method of photopolymerizing a polymer material. The injection port is sealed after filling the mixed solution or after photopolymerization of a polymer.

As described above, when the mixed solution filled between the substrates 11 and 12 is irradiated with ultraviolet rays, the polymer material in a monomer or oligomer state is radicalized by thawing the double bond, and adjacent molecules are adsorbed. Radicals are bound to each other to form a polymer by a radical polymerization reaction, and the cholesteric liquid crystal is phase-separated by polymerizing the polymer material.

Therefore, the polymerized polymer layer 31 has a cross section like a sponge, and the cholesteric liquid crystals 32 are confined in the gaps of the polymer layer 31, respectively, and the liquid crystal / polymer composite having the above-described structure is formed. The film 30 is formed. The method of forming the composite film 30 is a method called a photopolymerization phase separation method.

That is, the polymer dispersion type liquid crystal display device uses the cholesteric liquid crystal 32 in which liquid crystal molecules are arranged in a twisted state at a helical pitch peculiar to the liquid crystal in the liquid crystal of the liquid crystal / polymer composite film 30 in a non-electric field state. When the electric field is applied, the cholesteric liquid crystal 32 is aligned in the electric field direction in a relatively short time according to the strength of the applied electric field, and when the application of the electric field is stopped, the liquid crystal molecules are aligned. The state quickly returns to the original twist arrangement state.

Therefore, when no electric field is applied between the pixel electrode 13 and the counter electrode 20 of both substrates 11 and 12, the cholesteric liquid crystal 32 of the composite film 30 becomes as shown in FIG. , The liquid crystal molecules 32a are arranged in a twisted state with a helical pitch peculiar to the liquid crystal, and this twisted arrangement is present randomly, and in this state, the refractive index of the polymer layer 31 and the cholesteric liquid crystal 32 are different. Therefore, the light incident on the liquid crystal display element from the surface side is scattered by the composite film 30, and the display becomes a cloudy dark state.

Of the light scattered in the composite film 30, a certain amount of light is the phosphor film 21 on the rear substrate 12.
And reaches the counter electrode 20 which also serves as a reflection film thereunder, a part of the light strikes the fluorescent substance in the fluorescent substance film 21 to generate fluorescence, and the fluorescent substance film 21 is prevented from hitting the fluorescent substance film 21 without hitting the fluorescent substance. The transmitted light is reflected by the counter electrode 20, but in the non-electric field state, the amount of light reaching the phosphor film 21 and the counter electrode 20 thereunder is small, and therefore the amount of fluorescence emitted by the phosphor film 21 and Since the amount of light reflected from the counter electrode 20 is small, and these lights are scattered as described above when passing through the composite film 30 again, there is almost no light emitted to the surface side, and therefore the dark display dark. That is enough.

When an electric field is applied between the pixel electrode 13 and the counter electrode 20, the cholesteric liquid crystal 32 of the composite film 30 changes the direction of the electric field, as shown in FIG.
In other words, the liquid crystal molecules 32 are uniformly stood up and aligned in a direction substantially perpendicular to the substrate surfaces 11 and 12, and the refractive index of the cholesteric liquid crystal 32 becomes substantially equal to the refractive index of the polymer layer 31. Light is transmitted through the composite film 30 with almost no scattering, and the non-scattered transmitted light is reflected by the counter electrode 20 serving also as the reflective film on the rear substrate 12 to display a bright state.

In this case, in the liquid crystal display element of this embodiment, since the phosphor film 21 which emits fluorescence when light strikes is provided on the counter electrode 20, of the light reflected by the counter electrode 20. Since a certain amount of light strikes the fluorescent substance in the phosphor film 21 to generate fluorescence, the light reflected by the counter electrode 20 and the fluorescence emitted by the phosphor film 21 are transmitted through the composite film 30 again to the surface side. The light is emitted to obtain a bright display with high brightness and a fluorescent color. The bright display color corresponds to the color of the fluorescence emitted by the phosphor film 21, and the chromaticity increases as the amount of the fluorescent substance in the phosphor film 21 increases.

Since the light reflected by the counter electrode 20 again passes through the phosphor film 21 and enters the composite film 30, some of the reflected light hits the phosphor substance in the phosphor film 21. However, the light that hits the fluorescent substance becomes energy that causes the fluorescent substance to generate fluorescence. Further, since the fluorescent substance emits fluorescence not only in visible light but also in light having a wavelength outside the visible light band, the fluorescence emitted from the phosphor film 21 has high brightness.

In the liquid crystal display device, the liquid crystal (cholesteric liquid crystal) 3 of the liquid crystal / polymer composite film 30 is used.
2 shows that when an electric field is applied, the liquid crystal molecules are aligned in the direction of the electric field in a relatively short time according to the strength of the applied electric field, and when the application of the electric field is stopped, the alignment state of the liquid crystal molecules is promptly increased. Since the liquid crystal display element returns to the original twisted alignment state, this liquid crystal display element has good responsiveness not only when the display changes from the dark state to the bright state, but also when the display changes from the bright state to the dark state, and therefore, the high responsiveness. Time-divisional drive with duty is possible.

In addition, in this liquid crystal display device, the liquid crystal molecules 32a of the composite film 30 are arranged in a twisted state at a helical pitch peculiar to the cholesteric liquid crystal in the non-electric field state, so that the liquid crystal 32 in the non-electric field state. Since the difference between the refractive index and the refractive index of the polymer layer 31 is large, it is possible to enhance the light scattering effect in the non-electric field state, darken the dark display, and improve the display contrast.

The spiral pitch p of the cholesteric liquid crystal 32 is determined by the liquid crystal portion of the composite film 30, that is, the polymer layer 3.
1 is preferably shorter than the average diameter d of the cavity in which the liquid crystal 32 is confined. If the spiral pitch p of the cholesteric liquid crystal 32 and the average diameter d of the liquid crystal portion are p <d, it is more effective. Light can be diffused.

In addition, in the liquid crystal display element of the above-mentioned embodiment, the counter electrode 20 provided on the back side substrate 12 is used as an electrode also serving as a reflection film, and a phosphor that emits fluorescence when light strikes the counter electrode 20. Since the film 21 is provided, the light reflected by the counter electrode 20 passes through the phosphor film 21 and causes the phosphor film 21 to generate fluorescence, which brightens the bright display, and thus dark. In combination with the display becoming darker as described above, a high contrast display can be obtained.

In the above embodiment, the liquid crystal / polymer composite film 30 has a structure in which only the cholesteric liquid crystal 32 is dispersed in the polymer layer 31, but the cholesteric liquid crystal 32 has a black dichroic dye. If added, the scattered light in the non-electric field state is absorbed by the dichroic dye,
It is possible to make the dark display closer to black and further increase the contrast.

That is, FIG. 3 is a cross-sectional view of a liquid crystal / polymer composite film 30 showing another embodiment of the present invention in an enlarged view of a part of the liquid crystal / polymer composite film 30 in a non-electric field state and an electric field applied state. In the example, the composite film 30 has a structure in which a cholesteric liquid crystal 32 to which a black dichroic dye is added is dispersed in a polymer layer 31.

If a black dichroic dye is added to the cholesteric liquid crystal 32 of the liquid crystal / polymer composite film 30 as in this embodiment, in the non-electric field state shown in FIG. Since the molecules 33 of the dichroic dye are randomly oriented due to the twisted arrangement of the molecules 32a of the cholesteric liquid crystal 32, when the incident light passes through the composite film 30, the refractive index of the polymer layer 31 and the liquid crystal 32 is changed. As well as being scattered by the difference, most of this scattered light is absorbed by the dichroic dye, thus making the dark display almost black.

When an electric field is applied, the molecules 32a of the cholesteric liquid crystal 32 as shown in FIG. 3 (b).
Since the molecules 33a of the dichroic dye rise almost vertically as the ridges rise, the composite film 30 is
Light passing through is hardly absorbed by the dichroic dye.

Further, in the above-mentioned embodiment, the phosphor film 21 is provided on the back side substrate 12 in order to increase the brightness of bright display, but this phosphor film 21 is provided on the front side substrate 11 for each pixel electrode 13. May be provided corresponding to each. However, the phosphor film 21 may be omitted.

Further, in the above-described embodiment, the substrate 12 provided with the counter electrode 20 is used as the back side substrate and the counter electrode 20 also serves as a reflection film, but conversely, the pixel electrode 13 is provided. The substrate 11 may be used as the back side substrate, and the pixel electrode 13 may be used as an electrode that also serves as a reflective film. In that case, the substrate provided with the counter electrode 20 serves as the front surface side substrate, and thus the substrate may be a transparent substrate and the counter electrode 20 may be a transparent electrode.

Further, the present invention can be applied to a reflection type liquid crystal display element having a reflection film provided on the outer surface of the back side substrate and a transmission type liquid crystal display element having no reflection film, in which case Both substrates 11 and 12 may be transparent substrates, and both the pixel electrode 13 and the counter electrode 20 may be transparent electrodes.

Further, the liquid crystal / polymer composite film 30 in the above embodiment has a structure in which the cholesteric liquid crystal 32 is confined in each void portion of the polymer 31 layer polymerized so as to have a cross section like a sponge. In Although,
The liquid crystal / polymer composite film may have a structure in which cholesteric liquid crystal is microencapsulated and the liquid crystal capsules are dispersed in the polymer layer.

Further, the polymer dispersion type liquid crystal display element of the above embodiment is an active matrix type which uses the TFT 14 as an active element, but the present invention is an active matrix type polymer which uses MIM or the like as an active element. A segment display type polymer dispersion liquid crystal display in which a segment electrode having a shape corresponding to a display pattern is also provided on one substrate of a dispersion type liquid crystal display element, and a common electrode facing the segment electrode is provided on the other substrate. It can also be applied to devices.

[0052]

The polymer dispersed liquid crystal display device of the present invention is
Since a liquid crystal / polymer composite film in which a cholesteric liquid crystal is dispersed in a polymer layer is provided between a pair of substrates on which electrodes are formed, it is only necessary to respond when the display changes from a dark state to a bright state. In addition, the responsiveness when the display changes from the bright state to the dark state is improved, time-division driving with high duty is possible, and the light scattering effect in the no electric field state is enhanced to improve the display contrast. Can be made.

In this polymer-dispersed liquid crystal display device, if a black dichroic dye is added to the cholesteric liquid crystal, scattered light in the non-electric field state is absorbed by the dichroic dye. Therefore, it is possible to make the dark display closer to black and further increase the contrast.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a liquid crystal / polymer composite film in an enlarged view of a part of a liquid crystal / polymer composite film according to an embodiment of the present invention in a non-electric field state and an electric field applied state.

FIG. 2 is a partial cross-sectional view of a polymer dispersed liquid crystal display device according to an exemplary embodiment of the present invention.

FIG. 3 is a cross-sectional view showing a liquid crystal / polymer composite film in an enlarged view of a part of a liquid crystal / polymer composite film according to another embodiment of the present invention in a non-electric field state and an electric field applied state.

FIG. 4 is a cross-sectional view showing a state where no electric field is applied and a state where an electric field is applied, which is an enlarged view of a part of a liquid crystal / polymer composite film in a conventional polymer dispersed liquid crystal display device.

[Explanation of symbols]

 11, 12 ... Substrate 13 ... Pixel electrode 14 ... Active element (TFT) 20 ... Counter electrode 21 ... Fluorescent substance film 30 ... Liquid crystal / polymer composite film 31 ... Polymer layer 32 ... Cholesteric liquid crystal 32a ... Liquid crystal molecule 33 ... Two Chromatic dye molecule

Claims (2)

[Claims] 1. A polymer dispersion type liquid crystal display device comprising a liquid crystal / polymer composite film in which a cholesteric liquid crystal is dispersed in a polymer layer, provided between a pair of substrates having electrodes formed thereon. 2. A polymer dispersed liquid crystal display device according to claim 1, wherein a black dichroic dye is added to the cholesteric liquid crystal.