JP2002107773A - Display element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a display element which has improved in contrast and long term reliability, and modulates light by applying an electric field to a medium containing dipole particulates. SOLUTION: In the display element in which a medium containing dipole particulates is arranged at least between a pair of electrodes and light is modulated by applying the electric field to the medium across the electrodes, an alternating electric field is to be applied to the medium. Thus, the contrast and reliability are improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel display device that performs light modulation by applying a voltage.

[0002]

2. Description of the Related Art Japanese Patent Application Laid-Open No. 11-271815 discloses a display element in which dipole fine particles are dispersed in a liquid, a polymer, or at least one medium on a transparent electrode, and a voltage is applied thereto to perform display. ing. This element controls the orientation state of the dipole fine particles dispersed in the medium by applying a voltage and modulates the light, and has a feature that the element can be operated at an extremely low electric field intensity. For example, according to the embodiment of the publication, an optical switch is operated by applying a DC electric field of about 67 V / cm. Accurate comparison is difficult due to differences in the configuration such as the thickness of the display element, but the characteristics are clear when considering that the electric field strength of the liquid crystal display element is estimated to be several thousand to several tens of thousands V / cm. However,
There is a problem that the contrast of the display element of this publication is not high for practical use. As a means to solve this, a method of increasing the electric field strength can be considered, but about 300 V / cm
When the above-described DC electric field is applied, there is a problem that dipole fine particles electrophoreses and the light modulation operation becomes irreversible. Further, even when a DC electric field of about 300 V / cm or less is applied, if the device is operated for a long period of time, the contrast as an optical switch is reduced, that is, there is a problem of reliability in long-term use.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a display element which modulates light by applying an electric field to a medium containing dipole fine particles, in which the contrast and the reliability over a long period of time are improved. It is to provide an element.

[0004]

According to the present invention, as a result of studying to solve the above problems, a display element having the following structure has been found.

Invention 1. A display element in which a medium containing dipole particles is disposed between at least a pair of electrodes, and a display element that modulates light by applying an electric field to the medium between the electrodes is applied to the medium. A display element characterized in that an AC electric field is applied. Invention 2. The display element according to Invention 1, wherein an electric field of 300 V / cm or more is applied. Invention 3. The display element according to Invention 1 or 2, wherein the alternating current frequency is 1 Hz or more.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The display element of the present invention has been completed by finding that the electrophoresis of dipole fine particles between electrodes can be suppressed by setting an electric field applied to a medium containing dipole fine particles to an AC electric field. Things. This has improved long-term reliability. In addition, the upper limit of the electric field intensity that can be applied to the display element can be made higher than that of the conventional display element. Specifically, an electric field of 300 V / cm or more required for high contrast can be applied, and as a result, the contrast is reduced. improves. Further, with respect to the frequency of the AC electric field, it has been found that the lowest frequency at which the electrophoresis of the dipole fine particles can be suppressed and the contrast can be increased is 1 Hz, thereby improving the perfection as a display element.

[0007] The display principle of the display element of the present invention is the same as that of JP-A-11-271815. However, in the embodiment of the publication, the electric field applied to the medium is only a DC electric field, which is different from the present invention. Although the publication states that static driving or multiplex driving used for driving a liquid crystal display element can be performed as a driving method of an element, the influence of electrophoresis of dipole fine particles by a driving method and improvement of contrast are described. There is no description or suggestion, and there is no description about the frequency of the driving AC electric field.

The fine particles having a dipole which can be used in the display element of the present invention can be used without particular limitation as long as they have an electric dipole and are dispersed in a medium. Among such fine particles, when the dipoles of the molecules constituting the fine particles are oriented so as to be aligned in one direction, the dipole moment of the fine particles increases,
As a result, the driving voltage as a display element can be reduced, and thus such fine particles are preferable. From this viewpoint, as a form of the fine particles having a dipole, microcrystals having anisotropy in the molecular orientation direction are particularly preferable.

The molecules constituting the fine particles having dipoles can be used without any particular limitation as long as they are composed of two or more molecules and have a dipole in an aggregated state. From the viewpoint of reducing driving voltage and improving contrast as a display element, it is preferable to use a molecule having a large dipole moment. For this reason, it is preferable that the molecule has an elongated shape and the whole molecule is conjugated, and it is more preferable that the molecule has a bias in the molecular long axis direction. In particular, it is particularly preferable that an electron donating group and an electron withdrawing group are bonded to both ends of the elongated molecule long axis. In addition, since it is important to have a large dipole moment in the assembled state from the viewpoint of reducing the driving voltage, select a molecule that expresses a large dipole moment when the molecules are aggregated into a microcrystalline state. Is preferred. Therefore, selection of a molecule and a crystal form having a large second-order nonlinear effect is an important guideline for selecting a particulate material. Examples of useful materials for obtaining fine particles having a large second-order nonlinear optical effect include dimethylaminostilbazolium salt, 2-cyclooctylamino-5-nitropyridine, and 4-nitro-1,2-phenylene Diamine, 4-bromo-4'-methoxychalcone, 3-methyl
4-methoxy-4'-nitrostilbene, 2- (N-prolinol) -5-nitropyridine, methylnitroaniline, 4-dimethylamino-1-ethylpyridinium salt. Among these, compounds such as formula (1)

Embedded image Is preferred. Compounds of formula (2)

Embedded image Is preferable because the second-order nonlinear optical effect is large. Further, compounds having no absorption in the visible light region, such as the formula (3), are also preferable.

Embedded image Even if the compound does not absorb light in the visible light region, it can be applied to the display element of the present invention, and is particularly preferable when a change in the refractive index due to application of an electric field is used for display.

The external shape and size of the fine particles having dipoles are not particularly limited as long as the fine particles have dipoles as a whole and are well dispersed in a medium. The shape may be spherical, cubic, square flaky, or acicular, but non-isotropic shapes such as flaky or acicular are preferred. In this case, it is preferable that the angle between the dipole direction and the major axis direction or the minor axis direction of the fine particles be as small as possible. Although the size depends on the shape of the fine particles, it is preferably 0.001 to 100 μm in the short side direction, and 0.005 to 100 μm.
-10 μm is more preferable, and 0.01-1 μm is particularly preferable. In the long side direction, 0.005 to 500 μm is preferable, and 0.025 to
-50 μm is more preferable, and 0.05-5 μm is particularly preferable. The size of the fine particles having dipoles is preferably as uniform as possible (preferably closer to monodispersion). In the display element of the present invention, when it is desired to suppress light scattering due to isotropic thermal fluctuation of fine particles having a dipole in a state where an AC electric field is not applied to the medium, the size of the fine particles having a dipole is 0.5. It is preferably at most μm, more preferably at most 0.4 μm, particularly preferably at most 0.3 μm.

The concentration of the dipole-containing fine particles in the medium is not particularly limited as long as the dipole-containing fine particles are well dispersed, and is set so that both the dispersibility and the contrast as a display element can be satisfied. Is preferred, and 0.00
It is preferably 001 to 5% by mass, more preferably 0.0001 to 0.5% by mass, and particularly preferably 0.001 to 0.05% by mass. As a dispersion medium for dispersing the dipole-containing fine particles, a low-molecular-weight dispersion medium, a high-molecular-weight dispersion medium, or the like can be used without any particular limitation as long as it can disperse the dipole-containing fine particles satisfactorily. Among these, aprotic dispersion media,
A dispersion medium having a low polarity is preferred. Specific examples of such a dispersion medium include alicyclic solvents such as cyclohexane and decalin.

In the display element of the present invention, it is preferable to add an appropriate dispersant to the medium in order to disperse the dipole-containing fine particles well. Dispersants include amphiphilic molecules, especially surfactants. A specific example of such a compound is dodecyltrimethylammonium chloride.

In the medium containing the dipole-containing fine particles as described above, for example, a material that becomes the dipole-containing fine particles is dissolved in a good solvent, and the solution is dropped into a poor solvent of the material. It can be manufactured by the following.
It is possible to control the external shape and size of the fine particles by the concentration when dissolved in a good solvent, the temperature when dropping into a poor solvent, the dropping speed and stirring efficiency, the presence or absence of a dispersant and its concentration, etc. is there. As another method, a material containing fine particles having dipoles can be prepared by directly adding a material to be fine particles having dipoles to a poor solvent and stirring vigorously, or by exposing to ultrasonic waves. It is possible. The medium produced in this manner may be used as it is, or may be used after dispersing and evaporating the dispersion medium and dispersing it in another dispersion medium. Also, to make the size of the fine particles uniform,
It is also preferable to use separation means such as chromatography such as centrifugation, filter separation, gel filtration separation alone or in combination of two or more.

In the display element of the present invention, at least one pair of electrodes is indispensable as a means for applying an AC electric field to a medium containing fine particles having dipoles. There is no particular limitation on the shape of the electrodes, their spacing and arrangement method as long as an AC electric field can be applied to the medium. For example, an arrangement in which the traveling direction of the light to be modulated and the direction of the electric field may be the same, or an arrangement in which the traveling direction of the light to be modulated and the direction of the electric field are orthogonal to each other is preferable. In the case of an arrangement in which the traveling direction of the modulated light is orthogonal to the direction of the electric field, a comb-shaped electrode can be used. The distance between the electrodes is preferably 1 μm to 2 cm, more preferably 2 μm to 1.5 cm, and particularly preferably 5 μm to 1 cm. The optical path length, which is the length of the light to be modulated passing through the medium, is preferably 2 μm to 3 cm, and 5 μm
To 2 cm is more preferable, and 10 μm to 1 cm is particularly preferable.

In order to obtain the maximum contrast, the strength of the AC electric field applied is preferably 300 V / cm or more, and 500 V / cm or more.
cm or more, more preferably 1,000 V / cm or more. The frequency of the AC electric field is preferably 1 Hz to 10 kHz, and 15 Hz
-5 kHz is more preferable, and 50 Hz-1 kHz is particularly preferable.
As a method for applying an AC electric field, a driving method established in the technical field such as a liquid crystal display device, a plasma display device, and an electroluminescence display device can be applied.

This display element may be used as a light transmission type having a light source or a light reflection type provided with a light reflection plate. When a change in the refractive index due to the application of an electric field is used for display, a polarizing plate may be used in combination. Further, for the purpose of suppressing deterioration of the fine particles, an ultraviolet cut filter or an infrared cut filter may be used in combination as necessary. Further, in order to enable color display, a color filter may be used in combination.

[0017]

EXAMPLES The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Example 1 The concentration of the compound of the formula (1) is 5.0 m
M,

Embedded image Dodecyl trimethyl ammonium chloride concentration of 5.0m
A 100 μl ethanol solution of M was dropped into 10 ml of decalin with vigorous stirring to prepare a dispersion in which microcrystals having dipoles were dispersed (the concentration of dipole microcrystals was about 0.02% by mass). The size of the microcrystal was estimated to be 0.65 μm using a dynamic light scattering meter DLS-7000 manufactured by Otsuka Electronics.

As shown in FIG. 1, Teflon (registered trademark)
Dispersion prepared in a quartz glass cell with a 1 cm optical path length that has a pair of ITO transparent electrodes whose spacing is adjusted to 1 cm by a spacer and that allows light that modulates from a direction perpendicular to the direction in which the electric field is applied to enter. The liquid was poured. Thus, an element having a structure in which a medium in which microcrystals having a dipole were dispersed was arranged between a pair of electrodes was manufactured.

An AC voltage having a frequency of 50 Hz was applied between the pair of electrodes, and the rate of change in absorbance was measured at a measurement wavelength of 0.55 μm. The absorbance change rate (%) is a value defined by a change amount of absorbance / absorbance when no electric field is applied × 100. When an AC voltage was applied so as to have an electric field strength of 300 V / cm, the rate of change in absorbance was 6%. Also, 1,300V / c
When an AC voltage was applied so as to have an electric field strength of m, the absorbance change rate was 9%.

(Comparative Example 1) The rate of change in absorbance at 0.55 μm was measured for the device having the structure manufactured in Example 1 while changing the DC electric field intensity. As a result, when a DC electric field was applied so as to have an electric field strength of 300 V / cm, the rate of change in absorbance was 6%. However, when a DC electric field was applied so as to have an electric field strength of 300 V / cm or more, the particles having dipoles electrophoresed, and light could not be modulated by the electric field.

From Example 1 and Comparative Example 1, when compared at the same electric field strength, the rate of change in absorbance is the same, but the device of the present invention to which an AC voltage is applied is compared with the conventional device to which a DC voltage is applied. Thus, it can be seen that electrophoresis does not occur even when the electric field intensity is increased, and the absorbance change rate can be increased, that is, the contrast can be improved.

Example 2 Using the same procedure and materials as in Example 1, a dispersion liquid in which microcrystals having a dipole were dispersed was prepared. Otsuka Electronics' Dynamic Light Scatterometer DLS-7000
The size of the microcrystal was estimated to be 500 nm. This dispersion was poured into the same quartz glass cell as used in Example 1, the prepared dispersion was poured, and an element having a configuration in which a medium in which microcrystals having dipoles were dispersed was disposed between a pair of electrodes was prepared. Produced. An AC voltage was applied to the device so that the electric field strength became 600 V / cm, and the frequency dependence of the absorbance change rate was measured. As a result, the absorbance change rate is 1.2% at 1 Hz, 6.3% at 5 Hz, 7.8% at 10 Hz, and 15% at 15 Hz.
It was 8.1%, 7.9% at 20Hz, and 8.2% at 30Hz. From this, to obtain the absorbance change rate efficiently, 5Hz
It is understood that the above frequency is preferable, the frequency is more preferably 10 Hz or more, and particularly preferably the frequency is 15 Hz or more.

(Example 3) An AC voltage having a frequency of 50 Hz was applied to the device having the structure manufactured in Example 1 so as to have an electric field strength of 1,300 V / cm, and the time change of the rate of change in absorbance was measured. The result is shown in figure 2. Even if this measurement was repeated, the display characteristics did not deteriorate.

(Comparative Example 2) A DC voltage was applied to the device having the structure manufactured in Example 1 so as to have an electric field strength of 300 V / cm, and the time change of the rate of change in absorbance was measured. The results are shown in Figure 3. When this measurement was repeated, the display characteristics deteriorated. This is considered to be due to the effect of electrophoresis of microcrystals having dipoles.

From Example 3 and Comparative Example 2, an AC of 1,300 V / cm
When the electric field strength of is applied, the rate of change in absorbance is larger than when the electric field strength of 300 V / cm is applied at DC,
That is, it can be seen that the contrast has been improved and the response speed at the rise has also become faster. From the above results, it can be seen that the display device of the present invention has improved contrast and reliability as compared with the conventional device.

[0027]

The display element of the present invention is characterized by being driven by an AC electric field. Thereby, the contrast and the reliability are improved as compared with the conventional display element, which is extremely useful.

[Brief description of the drawings]

FIG. 1 is a quartz glass cell used for the element of the present invention.

FIG. 2 is a graph showing a time change of an absorbance change rate of the device of the present invention.

FIG. 3 is a graph showing a time change of a rate of change in absorbance of a conventional device.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Hidetoshi Oikawa Room 503, 5-13-13 Koriyama, Taishiro-ku, Sendai, Miyagi Prefecture (72) Inventor Hitoshi Kasai 1-4-9, Kongosawa, Taishiro-ku, Sendai, Miyagi Prefecture Room 202 (72) Inventor Hiroshi Hasebe 4-12-8-107 Midorioka, Ageo-shi, Saitama

Claims (3)

[Claims] 1. A display element in which a medium containing dipole particles is disposed between at least a pair of electrodes, and a display element that modulates light by applying an electric field to the medium between the electrodes is applied to the medium. A display element characterized by an alternating electric field. 2. The display device according to claim 1, wherein an electric field of 300 V / cm or more is applied. 3. The display element according to claim 1, wherein an alternating current frequency is 1 Hz or more.