Display device

Abstract

To provide a display device capable of shortening a time required to change from a transparent state to a screen state.SOLUTION: A display device comprises: an image display body having a display function layer whose light scattering property is increased by ultraviolet light and whose light scattering property is reduced by visible light; an ultraviolet light projection unit that projects the ultraviolet light onto the image display body; and a controller. The image display body is formed by sandwiching the display function layer between two transparent substrates. The two transparent substrates each have an electrode on the display function layer side. The display function layer includes liquid crystal molecules, azobenzene, and an electrolyte, Since the controller applies an alternating electric field to the display function layer to swing the liquid crystal molecules before projecting the ultraviolet light onto the image display body, it is possible to improve responsiveness to ultraviolet light irradiation, to shorten a time required to change from a transparent state to a screen state.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a display device, and more particularly to a display device having an image display that switches between a transparent state and a screen state. [Background technology]

[0002] 2. Description of the Related Art Display devices are known that have an image display whose optical state changes between a light transmitting state and a light scattering state, and a projector that displays an image by projecting visible light onto the image display.

[0003] Patent Document 1 discloses a display device in which an image display is irradiated with ultraviolet light to increase the light scattering property of the image display, thereby changing the image display from a transparent state to a cloudy white screen state.

[0004] This image display contains liquid crystal and azobenzene, and the azobenzene molecules are changed from trans to cis by ultraviolet light, and the bent molecular structure of the cis azobenzene disrupts the alignment of the liquid crystal molecules, thereby increasing the light scattering properties. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2018-185511 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the image display described in Patent Document 1, a certain amount of time is required from the start of irradiation with ultraviolet light until the state changes to the screen state, so it is desirable to improve the response speed from the transparent state to the screen state.

[0007] The present invention has been made in consideration of the problems associated with the conventional technology, and an object of the present invention is to provide a display device capable of shortening the time required for changing from a transparent state to a screen state. [Means for solving the problem]

[0008] As a result of extensive research into achieving the above-mentioned object, the inventors discovered that the above-mentioned object can be achieved by vibrating the liquid crystal molecules before projecting ultraviolet light onto the image display, and thus completed the present invention.

[0009] That is, the display device of the present invention comprises an image display body having a display function layer whose light scattering properties are increased by ultraviolet light and decreased by visible light, an ultraviolet light projection unit that projects the ultraviolet light onto the image display body, and a control device. The image display is formed by sandwiching the display function layer between two transparent substrates, each of the two transparent substrates has an electrode on the display function layer side, the display function layer contains liquid crystal molecules, azobenzene, and an electrolyte, and the control device applies an alternating electric field to the display function layer before projecting the ultraviolet light onto the image display. and at the same time as irradiating the image display with the ultraviolet light, stopping the application of the alternating electric field increases the light scattering property. It is characterized by: Effect of the Invention

[0010] According to the present invention, an alternating electric field is applied to the display function layer to vibrate the liquid crystal molecules before projecting ultraviolet light to change the state from the transparent state to the screen state, thereby making it possible to provide a display device that shortens the time it takes to change from the transparent state to the screen state. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a cross-sectional view of an image display according to the present invention. [Diagram 2] FIG. 13 is a diagram illustrating a state in which the response speed of the image display device of the present invention is measured. [Diagram 3]10 is a graph showing the relationship between the time elapsed from the start of ultraviolet light irradiation and the visible light transmittance of the display function layer when the image display is irradiated with ultraviolet light without applying an alternating electric field. [Figure 4] 11 is a graph showing the relationship between the time elapsed from the start of ultraviolet light irradiation and the visible light transmittance of the display function layer when an alternating electric field is applied before the ultraviolet light irradiation to the image display member. [Diagram 5] 1 is a graph showing the relationship between screening response time and applied electric field. [Figure 6] 10 is a graph showing the relationship between the electrolyte content and the haze value of an image display in a transparent state. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The display device of the present invention will now be described in detail. The display device includes an image display, an ultraviolet light projector, and a control device.

[0013] <Image display body> As shown in Figure 1, the image display comprises a display function layer, whose light scattering properties increase with ultraviolet light and decrease with visible light, sandwiched between two transparent substrates, and each of the two transparent substrates has a transparent electrode on the display function layer side.

[0014] The display function layer has a structure in which liquid crystal molecules, azobenzene molecules, and an electrolyte are sandwiched between two vertical alignment films.

[0015] The mechanism by which the display function layer changes from a transparent state to a cloudy screen state due to light scattering will be described.

[0016] The liquid crystal molecules are nematic liquid crystals having a combination of a rigid mesogen skeleton and a flexible long-chain alkyl group and having optical anisotropy and dielectric anisotropy.

[0017] In addition, the azobenzene molecule has the property of changing its structure from a trans form to a cis form when exposed to ultraviolet light. The trans form of the azobenzene molecule has a planar structure, whereas the cis form has a curved three-dimensional structure. In FIG. 1, a trans azobenzene molecule is shown as a straight line, and a cis azobenzene molecule is shown as an L-shape.

[0018] In the transparent state, the molecular structure of azobenzene is a planar trans form, and the liquid crystal molecules form a nematic phase with directional order, as shown on the left side of Figure 1. The aligned liquid crystal molecules in the nematic phase are aligned in a direction perpendicular to the vertical alignment film by the vertical alignment film, allowing light to pass through the display function layer.

[0019] When the display function layer is irradiated with ultraviolet light, azobenzene isomerizes to a cis form. As azobenzene changes from a planar structure to a curved structure, the alignment of the liquid crystal molecules becomes disordered, as shown on the right side of Figure 1, causing light scattering and making the display function layer opaque.

[0020] The photoisomerization of azobenzene is very fast, occurring in picoseconds (10 -12 This occurs on a time scale of seconds.

[0021] However, liquid crystal molecules are subject to association forces that orient them so that they have a directional order and line up in a fixed direction, as well as forces that align the aligned liquid crystal molecules toward the direction of the vertical alignment film.

[0022] These effects of aligning the liquid crystal molecules counteract the effect of azobenzene isomerized into a cis form, which disrupts the alignment of the liquid crystal molecules, and therefore the response speed from the transparent state to the screen state in response to ultraviolet light irradiation becomes slow.

[0023] In the present invention, an alternating electric field is applied to the display function layer before irradiation with ultraviolet light to vibrate the liquid crystal molecules, thereby weakening the interaction between the liquid crystal molecules and the vertical alignment film that orients the liquid crystal molecules in the thickness direction of the display function layer, and the association force that aligns the liquid crystal molecules with directional order.

[0024] Therefore, when ultraviolet light is irradiated and azobenzene isomerized to the cis form, the alignment of the liquid crystal molecules is immediately disturbed, resulting in a reduction in the amount of azobenzene required to isomerize to the cis form to achieve the screen state, thereby shortening the time it takes to change from the transparent state to the screen state.

[0025] The cis azobenzene molecules are converted back to the trans form by exposure to visible light or exposure to visible light, whereupon the liquid crystal molecules are converted back to their original alignment, allowing light to pass through the display function layer, making the display function layer transparent.

[0026] The liquid crystal molecules may be any known liquid crystal molecules used in display devices. The liquid crystal molecules may be composed of a single type of liquid crystal molecules having the same molecular structure, but are preferably a mixture containing multiple types of liquid crystal molecules having different molecular structures.

[0027] When a mixture contains multiple types of liquid crystal molecules with different molecular structures, the liquid crystal molecules differ in their sensitivity to changes in the electric field and the speed at which they follow the direction of the electric field, resulting in differences in their responsiveness to changes in the electric field.

[0028] Therefore, instead of all the liquid crystal molecules simultaneously orienting in the same direction in response to a change in the direction of the electric field, variation occurs in the orientation of each liquid crystal molecule, weakening the binding force between the liquid crystal molecules, further improving the responsiveness to ultraviolet light irradiation.

[0029] As the liquid crystal molecules, for example, E44 and MLC2172 (both manufactured by Merck) can be used as P-type nematic liquid crystal having dielectric anisotropy.

[0030] As the azobenzene molecule, a conventionally known azobenzene molecule used in display devices can be used. However, the azobenzene molecule represented by the following structural formula (1) has a strong power to twist liquid crystal, and therefore can shorten the time until the display functional layer becomes cloudy and reaches a screen state.

[0031] [ka] In the structural formula (1), R represents a linear alkyl group having 5 to 10 carbon atoms.

[0032] The electrolyte provides electrical conductivity to the display function layer. When an alternating electric field is applied to the display function layer containing the electrolyte, a charge distribution occurs, and the liquid crystal molecules can be swung.

[0033] As the electrolyte, a conventionally known electrolyte that does not separate from but mixes with the liquid crystal molecules can be used. Examples include TEMPO (2,2,6,6-Tetramethylpiperidine-1-Oxyl Free Radical), BMIN-BF4 ([1-butyl-3-methylimideazolium]BF4), CTAB (Cetrimonium bromide), TBAB (Tetrabutyl ammonium bromide), CTAI (Cetrimonium iodide), and CTAI3 (Cetrimonium triiodide).

[0034] The content of the electrolyte in the display function layer is preferably 0.2% by mass or less. Since the electrolyte and the liquid crystal molecules do not have the same refractive index, light is easily scattered at the interface between the electrolytes. By setting the electrolyte content to 0.2 mass % or less, the transparency of the display function layer can be improved.

[0035] The lower limit of the electrolyte content is not particularly limited as long as it can generate charge distribution in the display function layer, but is substantially about 0.05% by mass.

[0036] The transparent substrate may be made of glass, resin, or the like.

[0037] <Ultraviolet light projection unit> The ultraviolet light projecting unit is a light source that emits ultraviolet light that changes the light scattering properties of the image display. This ultraviolet light may have a peak wavelength in the absorption band of the azobenzene, and is preferably ultraviolet light having a peak within ±5 nm of the maximum absorption peak of azobenzene.

[0038] As the light source, in addition to light sources that emit light of a single wavelength, such as LEDs and semiconductor lasers, high-pressure mercury lamps that emit continuous light can be used.

[0039] <Control device> The control device controls the alternating electric field applied to the display function layer and the ultraviolet light irradiated to the display function layer. Specifically, the control device applies an alternating electric field to the display function layer and stops the application of the alternating electric field simultaneously with or after the start of irradiation of the ultraviolet light to the display function layer.

[0040] The alternating electric field applied to the display function layer is preferably greater than Ec (V / m) represented by the following formula (1).

[0041]

number

[0042] When the alternating electric field is greater than Ec, the force that shakes the liquid crystal molecules exceeds the force that aligns the liquid crystals, shortening the time it takes for the liquid crystal to change from a transparent state to a cloudy state.

[0043] The frequency of the alternating electric field applied to the display function layer is preferably 40 to 70 Hz, depending on the liquid crystal molecules in the display function layer, and the time for applying the alternating electric field before irradiating the display function layer with ultraviolet light is preferably about 10 ms.

[0044] The ultraviolet light to be irradiated to the display function layer may be irradiated to the entire display function layer or to a part of the display function layer. By irradiating a part of the display function layer with the ultraviolet light to form a transparent part and a cloudy part, an image can be displayed on the display function layer.

[0045] The display device may have, as necessary, a first visible light projector that projects a first visible light that makes a cloudy image display body transparent, and a second visible light projector that projects a second visible light that displays a colored image on the cloudy image display body. The first visible light projector and the second visible light projector may be an integrated visible light projector, and the control device also controls them. EXAMPLES

[0046] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples.

[0047] A transparent substrate was obtained by forming a transparent electrode (indium tin oxide: ITO film) on the entire surface of one side of a transparent glass, and a vertical alignment film (polyimide) was formed on the entire surface of the transparent electrode.

[0048] A mixed solution was prepared containing 83.65 mass% of a nematic liquid crystal (E44 manufactured by Merck) containing multiple types of liquid crystal molecules, 5.1 mass% of azobenzene molecules represented by the following structural formula (2), 2.9 mass% of a non-photoresponsive chiral molecule represented by the following structural formula (3), 0.1 mass% of an electrolyte (Cetrimonium bromide; CTAB), 7.5 mass% of a polymerizable monomer represented by the following structural formula (4), and 0.75 mass% of a polymerization initiator (IRGAUCERE819 manufactured by IGM Resins BV).

[0049] [ka]

[0050] [ka]

[0051] [ka]

[0052] Two transparent substrates were placed with the vertical alignment film facing inward, and the above mixture was poured between the two transparent substrates while heating. One of the transparent substrates was irradiated with light having a wavelength of 420 to 450 nm to polymerize the above monomer, thereby obtaining an image display having partitions in the thickness direction of the display function layer.

[0053] The specifications of this image display are as follows: Liquid crystal dielectric constant (εa): 1.5849E-10 (F / m) Liquid crystal splay elastic constant (K 11 ) :6.37E-12(N) Liquid crystal bend elastic constant (K 33 ) :8.6E-12(N) Convection wavelength (λx): 0.000002 (m) Display functional layer thickness (d): 0.00001 (m)

[0054] This image display was irradiated with ultraviolet light as shown in FIG. 2, and the response time from the start of irradiation with ultraviolet light until the image became cloudy and turned into a screen state was measured.

[0055] (Response speed measurement method) Ultraviolet light emitted from the projector, wavelength 365 nm, 20 mW / cm 2 The image display was irradiated with ultraviolet light of 100 nm and an alternating voltage was applied to the image display, and the change in visible light transmittance of the image display during irradiation with ultraviolet light was measured.

[0056] The change in visible light transmittance was measured by projecting light with a wavelength of 650 nm from a visible light projecting unit (laser diode) onto the image display, receiving the light that had passed through the image display with a photodiode, and using an oscilloscope. The distance between the visible light projector and the image display was approximately 8 cm, and the distance between the image display and the photodiode was approximately 20 cm.

[0057] (Voltage application method) A control voltage with a given frequency and waveform was output from a waveform generator, boosted by a voltage amplifier, and then applied to the image display.

[0058] FIG. 3 shows the relationship between the time elapsed from the start of ultraviolet light irradiation and the visible light transmittance of the display function layer when ultraviolet light was irradiated without applying an alternating electric field.

[0059] As shown in the graph in Figure 3, there is almost no decrease in visible light transmittance for approximately 2 seconds after the start of UV light irradiation, which indicates that even if the azobenzene changes to a cis form, the liquid crystal molecules maintain their oriented and aligned state for a while, and that the alignment is not immediately disturbed even after the azobenzene isomerizes.

[0060] In Fig. 4, a square wave alternating electric field with an electric field frequency of 50 Hz is applied to the display function layer. 6 4 shows the relationship between the time elapsed from the start of ultraviolet irradiation and the visible light transmittance of the display function layer when a voltage of 1 V (V / m) is applied.

[0061] The graph in Figure 4 shows that by shaking the liquid crystal molecules, the visible light transmittance begins to decrease almost simultaneously with the irradiation of UV light, and that the liquid crystal molecules immediately respond to the isomerization of azobenzene, releasing their orientation and disrupting their alignment.

[0062] The alternating electric field applied to this image display was changed, and the time until the image display became cloudy and entered the screen state was measured. Figure 5 shows the relationship between the screen response time and the applied electric field. The screen state was determined when the haze value of the image display was 50% or more and the visible light transmittance was 1 / e of that when it was transparent.

[0063] It can be seen that by applying an alternating electric field Ec of more than 2.75 (V / m), the force that shakes the liquid crystal molecules exceeds the force that aligns the liquid crystals, thereby shortening the time it takes for the liquid crystal to change from the transparent state to the screen state.

[0064] In addition, displays with different electrolyte contents were produced and the haze value of the transparent display was measured. Figure 6 shows the relationship between the electrolyte content and the haze value of the display.

[0065] From FIG. 6, it can be seen that when the electrolyte content is 0.2 mass % or less, the haze value is 20% or less and the film is transparent. [Explanation of symbols]

[0066] 1 Image display 2 Display function layer 21 Liquid crystal molecules 22 Azobenzene molecule 23 Electrolytes 24 Vertical alignment film 3 Transparent substrate 31 Transparent electrode 4. Ultraviolet light projection section 5 Visible light projection unit 6. Control device 61 Waveform Generator 62 Voltage Amplifier 7 Photodiode 8 Oscilloscope

Claims

1. an image display having a display function layer whose light scattering property increases with ultraviolet light and decreases with visible light; an ultraviolet light projection unit that projects the ultraviolet light onto the image display; A display device comprising: The image display body is configured by sandwiching the display function layer between two transparent substrates, the two transparent substrates each have an electrode on the display function layer side, the display function layer includes liquid crystal molecules, azobenzene molecules, and an electrolyte; The display device is characterized in that the control device applies an alternating electric field to the display function layer before projecting the ultraviolet light onto the image display body, and stops applying the alternating electric field at the same time as irradiating the image display body with the ultraviolet light, thereby increasing light scattering.

2. The display device as described in claim 1, characterized in that the frequency of the alternating electric field is 40 to 70 Hz.

3. The display device as described in Claim 2, characterized in that the electrolyte content of the display function layer is 0.2 mass % or less.

4. A display device as described in claim 2 or 3, characterized in that the alternating electric field applied to the display function layer is greater than Ec (V / m) expressed by the following formula (1). [0010] In formula (1), ε a represents the liquid crystal dielectric constant (F / m), K 11 represents the liquid crystal splay elastic constant (N), K 33 represents the liquid crystal bend elastic constant (N), λx represents the convection wavelength (m), and d represents the thickness (m) of the display function layer.

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