JP2006163127A - Light scattering type display element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make a contrast ratio higher than before by increasing only a reflection factor of white display nearly without increasing a reflection factor of black display by optimizing refractive indexes or thicknesses of respective layers. <P>SOLUTION: A back-side substrate 10 has its refractive index made lower than the refractive index no of liquid crystal molecules 33. A display-side substrate 20, on the other hand, has its refractive index made equal to or higher than the refractive index no of liquid crystal molecules 33. More preferably, the refractive index of the display-side substrate 20 is equal to or higher than a mean refractive index n<SB>LC</SB>of the whole liquid crystal layer 30 in a scatter state (white display state). Thus, the refractive indexes are optimized to increase only the reflection factor of white display nearly without increasing the reflection factor of black display, thereby making the contrast ratio higher than before. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a light-scattering display element that is applied to a reflective display and is suitable for, for example, electronic paper.

  In recent years, with the development of mobile devices such as portable information terminals, there is an increasing demand for display elements with low power consumption and high image quality. As a first candidate, there is a reflective liquid crystal display element that does not require a backlight and consumes less power. However, since all of the liquid crystal display elements in the twisted nematic (TN) mode or the super-twisted nematic (STN) mode that are generally used at present must use polarizing plates, There is a problem that utilization efficiency is low and the display is very dark when used as a reflective display.

  As a liquid crystal display method that does not use a polarizing plate, there is a light scattering display mode in which an action with respect to incident light is switched between a scattering state (bright display) and a transmission state (dark display) by an electric field. FIG. 19 shows a general cross-sectional structure of a liquid crystal display element in a light scattering type display mode (hereinafter referred to as a light scattering type liquid crystal display element). This light scattering type liquid crystal display element includes a liquid crystal layer 130 made of polymer dispersed liquid crystal (PDLC), which is one of light scattering type display modes, for example, and a back side substrate 110 made of glass, for example. And the display-side substrate 120 are disposed so as to face each other. A light absorption layer 140 is disposed on the entire back surface of the back substrate 110 (the side opposite to the liquid crystal layer 130). Transparent electrodes 111 and 121 made of, for example, ITO (Indium Tin Oxide) are provided on the back substrate 110 and the display substrate 120 on the side in contact with the liquid crystal layer 130, and the liquid crystal layer is formed on a pixel basis. An electric field can be applied to 130.

In this light scattering type liquid crystal display element, when an electric field is applied to the liquid crystal layer 130 by the transparent electrodes 111 and 121, the light incident from the display side substrate 120 side is transmitted as it is without being scattered in the liquid crystal layer 130. Further, the light passes through the back substrate 110 and is absorbed by the light absorption layer 140. Thereby, black display is obtained. On the other hand, when an electric field is not applied to the liquid crystal layer 130, the incident light is scattered in the liquid crystal layer 130 and reflected to the display side substrate 120 side to display white. In this way, display can be performed without using a polarizing plate. Patent Documents 1 to 5 describe techniques related to such a light-scattering liquid crystal display element.
JP-A-5-313140 Japanese Patent Laid-Open No. 7-13139 Japanese Patent Laid-Open No. 11-174436 JP-A-7-253570 JP 7-56152 A

  However, even in the light scattering type liquid crystal display element, the light intensity (reflectance) particularly in the scattering state cannot be obtained sufficiently, so that the brightness in white display is not yet sufficient, so the contrast becomes insufficient and there is room for improvement. is there. The above Patent Documents 1 to 5 also disclose techniques for improving the reflectance and contrast, but there is room for further improvement. For example, in Patent Document 5, an attempt is made to solve the above problem by sandwiching a layer having a refractive index lower than that of the liquid crystal layer between the liquid crystal layer and the electrode (zinc oxide layer) so as to be in contact with the liquid crystal layer. It is said. However, in this case, since it is in contact with the liquid crystal layer, there is a problem that it is practically impossible to increase the thickness of the low refractive index layer. This is because if the low refractive index layer is made thicker, the distance between the electrodes above and below the liquid crystal layer becomes longer by that amount, so that even if the same voltage is applied, the electric field drops and the operation of the liquid crystal becomes insufficient. Therefore, the increase in reflectivity due to the provision of the low refractive index layer in Patent Document 5 is an effect of increasing the reflectivity due to the multilayered dielectrics having different refractive indexes having a thickness of about 1/4 of the wavelength. . However, in this case, the reflectance increases similarly for light incident in a direction perpendicular to the substrate surface, and the reflectance for white display increases, while the reflectance for black display also increases at the same time. As a result, the contrast ratio eventually decreases. In this case, in order to increase the contrast ratio, the reflectance in white display (corresponding to the luminance in white display) is increased without increasing the reflectance in black display (corresponding to the luminance in black display). Only need to be high.

  The present invention has been made in view of such problems, and its purpose is to optimize the refractive index or thickness of each layer so that the reflectance in white display is hardly increased without substantially increasing the reflectance in black display. It is an object of the present invention to provide a light-scattering display element that can increase the contrast ratio as compared with the prior art.

  A light-scattering display element according to a first aspect of the present invention is a light-scattering display element that displays an image by reflecting incident light, and can switch an action on incident light between a scattering state and a transmission state. A transparent display-side substrate and a back-side substrate disposed opposite each other through the transmission / scattering layer, and whether the refractive index of the display-side substrate is the same as the refractive index of the transmission / scattering layer Or something that ’s getting higher.

In the light-scattering display element according to the first aspect of the present invention, during black display, the transmissive / scattering layer is in a transmissive state, and incident light is transmitted through the display-side substrate and the back-side substrate. On the other hand, at the time of white display, the transmission / scattering layer is in a scattering state, and most of the incident light is reflected to the display side substrate and emitted to the outside of the display side substrate. In this white display, when the light reflected by the transmission / scattering layer is incident on the surface of the transmission side / scattering layer of the display side substrate obliquely with respect to the substrate surface, the incident angle with respect to the normal of the substrate surface is large. As a result, internal reflection occurs on the display-side substrate, and it returns to the transmission / scattering layer side without being emitted to the outside, which may reduce the reflectance in white display.
In the light scattering display element according to the first aspect, since the refractive index of the display side substrate is the same as or higher than the refractive index of the transmission / scattering layer, the light transmission type display element moves toward the transmission / scattering layer in the white display state. As a result, the ratio of the return light decreases, the ratio of the light emitted to the outside increases, the reflectance of the entire element in the white display state increases, and the white luminance increases. Thereby, only the reflectance in white display is increased without increasing the reflectance in black display, and the contrast ratio is increased.

  A light-scattering display element according to a second aspect of the present invention is a light-scattering display element that displays an image by reflecting incident light, and can switch an action on incident light between a scattering state and a transmission state. A transparent display-side substrate and a back-side substrate that are arranged opposite to each other through the transmission / scattering layer, and the refractive index of the back-side substrate is lower than the refractive index of the transmission / scattering layer It is what has become.

In the light-scattering display element according to the second aspect of the present invention, during black display, the transmissive / scattering layer is in a transmissive state, and incident light is transmitted through the display-side substrate and the back-side substrate. On the other hand, at the time of white display, the transmission / scattering layer is in a scattering state, and most of the incident light is reflected to the display side substrate and emitted to the outside of the display side substrate. In the case of this white display, a part of the light may pass through the transmission / scattering layer and enter the back side substrate side, which may reduce the reflectance in the white display.
In the light-scattering display element according to the second aspect, the refractive index of the back side substrate is lower than the refractive index of the transmission / scattering layer. When the incident light is incident on the back side substrate obliquely with respect to the substrate surface, internal reflection tends to occur on the back side substrate as the incident angle with respect to the normal of the substrate surface increases, and the light enters the transmission / scattering layer again. It returns and it becomes easy to reflect toward the display side board | substrate side. That is, the proportion of light incident on the back side substrate in the white display state is reduced, and as a result, the proportion of light emitted to the outside from the display side substrate side is increased, and the reflectance of the entire element in the white display state is increased. , White brightness increases. Thereby, only the reflectance in white display is increased without increasing the reflectance in black display, and the contrast ratio is increased.

  A light-scattering display element according to a third aspect of the present invention is a light-scattering display element that displays an image by reflecting incident light, and can switch an action on incident light between a scattering state and a transmission state. The transparent / scattering layer, the transparent display-side substrate and the back-side substrate disposed opposite to each other via the transparent / scattering layer, and the display-side transparent electrode layer disposed between the transparent / scattering layer and the display-side substrate A backside transparent electrode layer disposed between the transmission / scattering layer and the backside substrate, a light absorption layer disposed on the surface of the backside substrate opposite to the transmission / scattering layer, and a backside transparent It is disposed between the electrode layer and the light absorption layer, and includes a low refractive index layer having a refractive index lower than that of the transmission / scattering layer.

In the light-scattering display element according to the third aspect of the present invention, during black display, the transmissive / scattering layer is in a transmissive state, and incident light is transmitted through the display-side substrate and the back-side substrate. On the other hand, at the time of white display, the transmission / scattering layer is in a scattering state, and most of the incident light is reflected to the display side substrate and emitted to the outside of the display side substrate. In the case of this white display, a part of the light passes through the transmission / scattering layer, enters the back side substrate, is absorbed by the light absorption layer, and the reflectance in the white display may be lowered.
In the light-scattering display element according to the third aspect, the low refractive index layer is disposed between the back-side transparent electrode layer and the light absorption layer, so that the light is transmitted through the transmission / scattering layer in the white display state. When some light is incident on the low refractive index layer obliquely with respect to the laminated surface, internal reflection tends to occur in the low refractive index layer as the angle of incidence with respect to the normal of the laminated surface increases, and transmission and scattering again. It returns to the layer, and it becomes easy to reflect to the display side substrate side. That is, in the white display state, the ratio of the light that enters the back side substrate and is absorbed by the light absorption layer decreases, and as a result, the ratio of the light that is emitted from the display side substrate side to the outside increases. The reflectance of the entire element is increased, and white brightness is increased. Thereby, only the reflectance in white display is increased without increasing the reflectance in black display, and the contrast ratio is increased.

A light-scattering display element according to a fourth aspect of the present invention is a light-scattering display element that displays an image by reflecting incident light, and can switch an action on incident light between a scattering state and a transmission state. A transparent display-side substrate and a back-side substrate, which are arranged to face each other via the transmission / scattering layer, and the outermost surface of the display-side substrate opposite to the transmission / scattering layer are laminated to display And a low refractive index layer having a refractive index n _AR lower than that of the side substrate, and satisfying the following conditional expression (1) with respect to the thickness d _AR (nm) of the low refractive index layer: is there.
0.5 × 540 / (4 × n _AR ) ≦ d _AR ≦ 1.5 × 540 / (4 × n _AR ) (1)

In the light-scattering display element according to the fourth aspect of the present invention, during black display, the transmissive / scattering layer is in a transmissive state, and incident light is transmitted through the display-side substrate and the back-side substrate. On the other hand, at the time of white display, the transmission / scattering layer is in a scattering state, and most of the incident light is reflected to the display side substrate and emitted to the outside of the display side substrate. In the case of this white display, when the light reflected by the transmission / scattering layer is incident on the outermost surface of the display side substrate obliquely with respect to the substrate surface, the display side substrate increases as the incident angle with respect to the normal of the substrate surface increases. This causes internal reflection at the outermost surface, and returns to the transmission / scattering layer side without being emitted to the outside, which may reduce the reflectance in white display. Further, when external reflection occurs on the outermost surface of the display-side substrate, light incident on the element itself is reduced, which may hinder appropriate display.
In the light-scattering display element according to the fourth aspect, the low refractive index layer is laminated on the outermost surface of the display side substrate, and the thickness d _AR is set to an appropriate value. Functions as an antireflection film, reduces internal reflection at the outermost surface of the display side substrate, reduces the ratio of light returning to the transmission / scattering layer side in the white display state, and as a result, the light emitted to the outside This increases the reflectance of the entire element in the white display state, and the white luminance increases. In addition, since the ratio of external reflection at the outermost surface of the display-side substrate is reduced, the amount of light incident on the element itself increases, resulting in an increase in white luminance. Thereby, only the reflectance in white display is increased without increasing the reflectance in black display, and the contrast ratio is increased.

  According to the light scattering display element of the first aspect of the present invention, the refractive index of the display side substrate is the same as or higher than the refractive index of the transmission / scattering layer. Even if the light reflected by the scattering layer is incident on the transmission side / scattering layer side surface of the display side substrate obliquely with respect to the substrate surface, the ratio of causing internal reflection on the display side substrate is reduced, and the white display state The ratio of the return light to the transmission / scattering layer side can be reduced. As a result, the proportion of light emitted to the outside increases, the reflectance of the entire element in the white display state increases, and white luminance can be increased. As a result, the contrast ratio can be increased by increasing only the reflectance in white display without increasing the reflectance in black display.

  According to the light scattering display element of the second aspect of the present invention, the refractive index of the back side substrate is made lower than the refractive index of the transmission / scattering layer. When some of the light that has passed through is incident on the back side substrate obliquely with respect to the substrate surface, internal reflection tends to occur on the back side substrate as the incident angle with respect to the normal of the substrate surface increases. The light returns to the scattering layer and is easily reflected toward the display side substrate. That is, the ratio of incident light to the back side substrate in the white display state can be reduced. As a result, the proportion of light emitted from the display side substrate side to the outside increases, the reflectance of the entire element in the white display state increases, and white luminance can be increased. As a result, the contrast ratio can be increased by increasing only the reflectance in white display without increasing the reflectance in black display.

  According to the light scattering display element of the third aspect of the present invention, since the low refractive index layer is disposed between the back side transparent electrode layer and the light absorption layer, transmission / scattering in the white display state. When some light transmitted through the layer is incident on the low refractive index layer obliquely with respect to the laminated surface, internal reflection is likely to occur in the low refractive index layer as the incident angle with respect to the normal of the laminated surface increases. It returns to the transmission / scattering layer again and is easily reflected toward the display side substrate. That is, the ratio of light that is incident on the back substrate and absorbed by the light absorption layer in the white display state can be reduced. As a result, the proportion of light emitted from the display side substrate side to the outside increases, the reflectance of the entire element in the white display state increases, and white luminance can be increased. As a result, the contrast ratio can be increased by increasing only the reflectance in white display without increasing the reflectance in black display.

According to the light-scattering display element according to the fourth aspect of the present invention, the low refractive index layer is laminated on the outermost surface of the display side substrate, and the thickness d _AR is set to an appropriate value. The low refractive index layer functions as an antireflection film, and internal reflection at the outermost surface of the display side substrate is reduced, so that the ratio of return light to the transmission / scattering layer side in the white display state can be reduced. As a result, the proportion of light emitted to the outside increases, the reflectance of the entire element in the white display state increases, and white luminance can be increased. In addition, since the ratio of external reflection at the outermost surface of the display-side substrate is reduced, the amount of light incident on the element itself increases, and as a result, white luminance can be increased. As a result, the contrast ratio can be increased by increasing only the reflectance in white display without increasing the reflectance in black display.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[First Embodiment]
1 and 2 show a configuration example of a light scattering display element according to the first embodiment of the present invention. 1 and 2 show a configuration example in which the light scattering display element according to the present embodiment is applied to a PNLCD (Polymer Network Liquid Crystal Display). 1 also shows the state of incident light when no electric field E is applied (white display), and FIG. 2 also shows the state of incident light when an electric field E is applied (black display). .

  The light-scattering display element 1 includes a liquid crystal layer 30 and transparent back-side substrate 10 and display-side substrate 20 made of, for example, glass and arranged to face each other with the liquid crystal layer 30 interposed therebetween. The light scattering display element 1 also includes a back side transparent electrode layer 11 disposed between the liquid crystal layer 30 and the back side substrate 10, and a display side disposed between the liquid crystal layer 30 and the display side substrate 20. A transparent electrode layer 21 is provided. The light scattering display element 1 further includes a light absorption layer 12 formed over the entire surface on the back surface (surface opposite to the liquid crystal layer 30) of the back substrate 10.

  Each of the transparent electrode layers 11 and 21 is for applying an electric field E to the liquid crystal layer 30 in units of pixels. The display-side transparent electrode layer 21 is made of, for example, an ITO film, and is formed on the entire surface of the display-side substrate 20 on the side in contact with the liquid crystal layer 30. On the other hand, although not shown in FIGS. 1 and 2, a plurality of transparent pixel electrodes made of ITO and a voltage applied to each pixel electrode are controlled on the back side transparent electrode layer 11. A TFT (Thin Film Transistor) and a vertical wiring and a horizontal wiring for applying a control signal to the TFT are formed.

  The light absorption layer 12 is provided to absorb incident light 40 transmitted through the liquid crystal layer 30 and the back side substrate 10 in the black display state, and is made of, for example, a material containing carbon.

  The liquid crystal layer 30 can switch the action on the incident light 40 between a scattering state and a transmission state in accordance with the application of the electric field E, and is composed of a polymer network type liquid crystal. The liquid crystal layer 30 is configured by randomly dispersing spheroidal liquid crystal molecules 33 in a polymer 32 as spherical liquid crystal droplets 31. FIG. 3 shows the structure of the liquid crystal molecules 33. The liquid crystal molecules 33 have birefringence, and the refractive index ne in the longitudinal direction (z direction) has a relationship of ne> no with respect to the refractive index no in the vertical direction (x, y direction). In the liquid crystal molecules 33, the refractive index of the light incident from the z direction is no because the vibration direction of the light is in the xy direction perpendicular to the z direction. Conversely, for light incident from the xy direction, the refractive index is ne because the vibration direction of the light is in the z direction perpendicular to the xy direction. The refractive index of the polymer 32 is set to the same value as the refractive index no of the liquid crystal molecules 33 on the low refractive index side.

The rear substrate 10 has a refractive index n _sub 1 lower than the refractive index of the liquid crystal layer 30. Desirably, the refractive index n _sub 1 is lower than the refractive index no of the liquid crystal molecules 33. On the other hand, the refractive index n _sub 2 of the display side substrate 20 is the same as or higher than the refractive index of the liquid crystal layer 30. Desirably, the refractive index n _sub 2 is the same as or higher than the refractive index no of the liquid crystal molecules 33. More desirably, the refractive index n _sub 2 is equal to or higher than the average refractive index n _{LC of the} entire liquid crystal layer 30 in the scattering state (white display state). More specifically, the refractive index n _sub 2 is preferably higher than the average refractive index n _{LC of the} entire liquid crystal layer 30 within the following range of Δn _sub . The basis for setting such a refractive index will be described later.
0.01 ≦ Δn _sub ≦ 0.03

Thus, the point that the refractive indexes n _sub 1 and n _sub 2 of the back side substrate 10 and the display side substrate 20 are optimized is greatly different from the conventional PNLCD. In the present embodiment, the liquid crystal layer 30 corresponds to a specific example of a “transmission / scattering layer” in the present invention.

  Next, functions and effects of the light scattering display element 1 will be described.

  In the light scattering display element 1, by applying an electric field E to the liquid crystal layer 30 as shown in FIG. 2, the liquid crystal molecules 33 are aligned in one direction in the vertical direction. In this case, the refractive index of the liquid crystal droplet 31 is no with respect to the light 40 incident from above, and is equal to the refractive index of the polymer 32 around the liquid crystal droplet 31 (the difference in refractive index Δn between the liquid crystal droplet 31 and the polymer 32). Becomes 0). Accordingly, the liquid crystal layer 30 is in a transmissive state, and the incident light 40 is transmitted as it is without being scattered in the liquid crystal layer 30, further passes through the back side substrate 10 and is absorbed by the light absorption layer 12. Thereby, black display is obtained.

On the other hand, when the electric field E is not applied, since the liquid crystal molecules 33 are randomly oriented as shown in FIG. 1, the refractive index of the liquid crystal droplet 31 is higher than the refractive index of the polymer 32. Usually, the average refractive index of the liquid crystal droplet 31 at this time is (ne + 2 × no) / 3 in consideration of the components in the triaxial directions of the x, y, and z directions.
Accordingly, the refractive index difference Δn between the polymer 32 and the liquid crystal droplet 31 is generated with respect to the light 40 incident from the upper side, so that the light is scattered in the liquid crystal droplet 31 and reflected to the display side substrate 20 side. Is displayed in white.

  Here, when the light reflected by the liquid crystal layer 30 in the white display state (scattering state) is incident obliquely on the surface of the display side substrate 20 on the liquid crystal layer 30 side, as shown in FIG. Consider reflection. First, consider a multilayer film composed of the liquid crystal layer 30, the display-side transparent electrode layer 21, and the display-side substrate 20, and reflectivity R in the case where light is incident from the liquid crystal layer 30 side to the display-side substrate 20 side during white display. The incident angle dependency is calculated by the effective Fresnel coefficient method.

Here, the calculation conditions are as follows. First, the refractive index of the liquid crystal molecules 33 was set to no = 1.52, ne = 1.80, and the refractive index of the polymer 32 was set to n _poly = 1.52. And the volume ratio of the liquid crystal droplet 31 and the polymer 32 was set to 8: 2, and the average refractive index of the whole was calculated | required as follows. The value = 1.59 obtained thereby was taken as the total refractive index n _LC of the liquid crystal layer 30 in the white display state.
((Ne + 2 × no) / 3) × 0.8 + n _poly × 0.2
Further, as shown in FIG. 4, the thickness d _ITO and the refractive index n _ITO of the display side transparent electrode layer 21 are d _ITO = 0.03 μm (30 nm) and n _ITO = 1.78, respectively. The calculation was performed by changing the rate to n _sub 2 = 1.32, 1.52, 1.59, 1.65. Further, the reflectance R was calculated for each of the S wave and the P wave. The S wave is light polarized in a direction perpendicular to the paper surface of FIG. 4, and the P wave is light polarized in a direction parallel to the paper surface of FIG. .

  FIG. 5 shows the calculation result. The vertical axis represents the reflectance R, and the horizontal axis represents the angle θ (deg). The angle θ is an incident angle with respect to the normal of the incident surface as shown in FIG. The wavelength of light was λ = 540 nm. The reason for setting λ = 540 nm is that the wavelength has the highest visibility. In the case of red light or blue light, the result is almost the same as the calculation result of FIG.

From the result of FIG. 5, reflection is suppressed by setting the refractive index n _sub 2 of the display side substrate 20 to be the same as or higher than the refractive index n _LC (= 1.59) of the liquid crystal layer 30. the reflectance by lower than the refractive index n _LC is increased at high angles side seen. In particular, when the incident angle θ exceeds the critical angle θr, the total reflection condition is satisfied and the reflectance becomes 1. By the way, considering the case where light isotropically diffuses by repeating scattering in the liquid crystal layer 30, it is considered that the same light intensity is obtained when the angle component of the light is 0 ° to 90 °. In this case, the total reflectance Rtotal of light is a value obtained by integrating the reflectance R at each incident angle from 0 ° to 90 ° with all the reflectances from 0 ° to 90 ° being 1. It is obtained by dividing by.

FIG. 6 shows the result of the total reflectance Rtotal estimated in this way. Moreover the refractive index n _sub 2 of the display-side substrate 20 is varied from 1.30 to 1.80 and shows the results of calculating the total reflectance Rtotal in FIG. From this result, the vicinity where the refractive index n _sub 2 of the display-side substrate 20 is shifted to the plus side as much as Δn _{sub in} the range of 0.01 ≦ Δn _sub ≦ 0.03 from the refractive index n _LC of the liquid crystal layer 30. It can be seen that the total reflectance Rtotal is minimized and becomes the minimum value (around refractive index 1.61).

In view of the above, it is possible to increase the reflectance of the entire element in white display by adopting the following structure. First, the display side substrate 20 on the light extraction side is desirable to transmit more light from the liquid crystal layer 30 side to the display side substrate 20 side in the light incident state shown in FIG. If the light is not transmitted, internal reflection occurs on the display-side substrate 20 side, and the light returns to the liquid crystal layer 30 side without being emitted to the outside. As a result, the reflectance of white display is reduced as a whole as a whole. Therefore, if the refractive index n _sub 2 of the display side substrate 20 is equal to or higher than the refractive index no of the liquid crystal molecules 33, the reflectance R of light from the liquid crystal layer 30 side to the display side substrate 20 side can be kept low. The ratio of return light to the liquid crystal layer 30 side decreases, and as a result, the ratio of light transmitted through the display side substrate 20 and emitted to the outside may increase. Thereby, the reflectance of the whole element in the white display state is increased, and the white luminance is increased. More preferably, the refractive index n _sub 2 of the display-side substrate 20 may be equal to or higher than the refractive index n _LC of the liquid crystal layer 30. More specifically, it is even better if the refractive index n _sub 2 of the display-side substrate 20 is made higher than the refractive index n _LC of the liquid crystal layer 30 by Δn _sub = 0.02 ± 0.01.
That is, the refractive index n _sub 2 is smaller than the refractive index n _LC .
0.01 ≦ Δn _sub ≦ 0.03
It is preferable that the refractive index be as high as possible.

On the other hand, the refractive index n _sub 1 of the back side substrate 10 is opposite to that of the display side substrate 20 so that the light does not enter or pass through the back side substrate 10 in the case of white display. It is better to be lower than. In this way, when a part of the light transmitted through the liquid crystal layer 30 in the white display state is incident on the rear substrate 10 obliquely with respect to the substrate surface, the incident angle θ with respect to the normal of the substrate surface is large. As it becomes, internal reflection easily occurs on the back side substrate 10, and returns to the liquid crystal layer 30 again and is easily reflected on the display side substrate 20. That is, the ratio of incident and transmitted light to the back side substrate 10 in the white display state is decreased, and as a result, the ratio of light transmitted through the display side substrate 20 and emitted to the outside may be increased. Thereby, the reflectance of the whole element in the white display state is increased, and the white luminance is increased.

  However, even if the reflectance R at the time of white display on the back side substrate 10 increases as described above, if the reflection at the time of black display similarly increases (+ α), the contrast (= white luminance / black luminance). ) Is reduced, leading to degradation of image quality. Therefore, the reflectance at the time of black display will be considered.

In the case of black display, since the refractive index of the liquid crystal droplet 31 and the polymer 32 of the liquid crystal layer 30 is the same as the refractive index no of the liquid crystal molecules 33, the light incident from the surface is transmitted without being scattered by the liquid crystal layer 30. To the rear substrate 10 on the side. At this time, the distribution of transmitted light with respect to the incident angle θ of light in each layer does not exist beyond a certain angle in order to satisfy Snell's law (however, all interfaces and surfaces are flat). That is, even if it is incident at any angle from the air side with refractive index n = 1.0, if it is incident on the device side having a higher refractive index than air, the angle is smaller than the critical angle θr = arcsin (1 / n _D ) at that layer. Will be distributed. Here, n _D is representative of the refractive index of each layer of the device. In the case of the liquid crystal layer 30, n _D = n _LC . Now, assuming that the refractive index of the liquid crystal layer 30 is 1.52, the critical angle is θr = 41.1 °. In this case, from the angle dependence of the reflectance R shown in FIG. 5, even if a layer having a refractive index of 1.32 is present under the liquid crystal layer 30, the angle component up to θ = 41.1 ° may be used. In this case, light is hardly reflected. Accordingly, in the case of black display, the reflection of light does not increase, and thus no increase in luminance is observed. On the other hand, in the case of white display, light is present at all angles due to light scattering in the liquid crystal layer 30. As a result, the reflection of light is increased and the luminance is increased. Accordingly, the contrast is increased.

As described above, according to the present embodiment, the refractive index n _sub 2 is optimized so as to suppress internal reflection on the surface of the display-side substrate 20 on the liquid crystal layer 30 side. The ratio of return light to the liquid crystal layer 30 side in the white display state can be reduced. Further, since the refractive index n _sub 1 is optimized so that internal reflection is likely to occur when oblique incidence is made on the liquid crystal layer 30 side surface of the back side substrate 10, the back side in the white display state. The ratio of incident and transmitted light to the substrate 10 can be reduced. As a result, when viewed as the whole element, the contrast ratio can be increased by increasing only the reflectance in white display without increasing the reflectance in black display.

Thereby, for example, when applied to electronic paper, a more paper-like white display is possible. In addition, since a high contrast can be achieved without increasing the reflectance of light in black display, a display that is easier to see can be realized. Furthermore, by applying to PNLCD, the response speed is faster than other light scattering displays, and high-quality moving image display is possible.
[Second Embodiment]

Next, a second embodiment of the present invention will be described. Note that substantially the same parts as those of the light-scattering display element 1 (FIGS. 1 and 2) according to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted as appropriate. FIG. 8 shows a configuration example of the light scattering display element according to the present embodiment. In the first embodiment, the advantage of setting the refractive index n _sub 1 of the back substrate 10 to be lower than the refractive index no of the liquid crystal molecules 33 has been described. Currently, a substrate having a low refractive index is used. In many cases, the substrate is expensive and versatility is poor. Therefore, in the present embodiment, a low refractive index layer is separately provided between the back side transparent electrode layer 11 and the light absorption layer 12 without lowering the refractive index n _sub 1 of the back side substrate 10 itself. Thus, an effect similar to that of the first embodiment is obtained. In the light scattering display element 2 shown in FIG. 8, a transparent low refractive index layer 13 is provided between the back substrate 10 and the back transparent electrode layer 11.

Hereinafter, conditions for obtaining the same effect as the case where the refractive index n _sub 1 of the back substrate 10 is lowered by providing the low refractive index layer 13 will be described. Here, as shown in FIG. 9, a multilayer film composed of the liquid crystal layer 30, the back-side transparent electrode layer 11, the low refractive index layer 13, and the back-side substrate 10 is considered. The incidence angle dependence of the reflectance R when light enters the index layer 13 side will be calculated by the effective Fresnel coefficient method.

Here, the calculation conditions are as follows. As shown in FIG. 9, in a structure in which a film having a refractive index n3 = 1.40 is inserted as a low refractive index layer 13 on a substrate having a refractive index n _sub 1 = 1.52 as the back side substrate 10. The reflectance R was calculated by changing the thickness d3 of the layer. The refractive index of the liquid crystal layer 30 was set to the total refractive index n _LC = 1.59 in the white display state, similarly to the case where the calculation of FIG. 5 was performed. As for the back side transparent electrode layer 11, as in the case of the calculation of FIG. 5, the thickness d _ITO and the refractive index n _ITO are d _ITO = 0.03 μm (30 nm) and n _ITO = 1.78, respectively. did. Further, the reflectance R was calculated for each of the S wave and the P wave. The S wave is light polarized in a direction perpendicular to the paper surface of FIG. 9, and the P wave is light polarized in a direction parallel to the paper surface of FIG. .

  The results are shown in FIG. 10 for the S wave and FIG. 11 for the P wave. The vertical axis represents the reflectance R, and the horizontal axis represents the angle θ (deg). The angle θ is an incident angle with respect to the normal line of the incident surface as shown in FIG. Note that, for the same reason as the case of the calculation in FIG. 5, the wavelength of light is set to λ = 540 nm, which has the highest visibility.

  From these results, both the S wave and the P wave have an effect of increasing the reflection if the thickness d3 of the low refractive index layer 13 is 0.25 μm or more as the thickness d, and more desirably 0.5 μm or more. It can be seen that the effect is obtained. In order to expect such a reflection effect, it is necessary that the thickness d3 of the low refractive index layer 13 is equal to or larger than the light penetration distance (evanescent field). This is because here, a mechanism is used in which light is reflected by substantially satisfying the total reflection condition. Therefore, such a low refractive index layer 13 does not necessarily need to be directly under the back side transparent electrode layer 11 such as ITO, but is below the liquid crystal layer 30 and the back side transparent electrode layer 11 and above the light absorption layer 12. If so, the same effect can be expected.

  By providing such a low refractive index layer 13, when a part of light transmitted through the liquid crystal layer 30 in the white display state is incident on the low refractive index layer 13 obliquely with respect to the stacked surface, the stacked layer As the incident angle θ with respect to the normal of the surface increases, internal reflection easily occurs at the low refractive index layer 13, and returns to the liquid crystal layer 30 side again, and easily reflects to the display-side substrate 20. That is, in the white display state, the proportion of light that enters the back side substrate 10 and is absorbed by the light absorption layer 12 decreases, and as a result, the proportion of light that passes through the display side substrate 20 and is emitted to the outside increases. It will be good. Thereby, the reflectance of the whole element in the white display state is increased, and the white luminance is increased.

  Note that even if there is a low refractive index layer below the liquid crystal layer 30, the luminance in black display does not increase, but only the luminance in white display increases, resulting in an increase in contrast. This is the same as in the first embodiment.

As described above, according to the second embodiment, the low refractive index layer 13 is laminated at an arbitrary position between the back side transparent electrode layer 11 and the light absorption layer 12 below the liquid crystal layer 30. In the white display state, the ratio of light that is incident on the back-side substrate 10 and is absorbed by the light absorption layer 12 is set so that internal reflection is likely to occur when it is incident obliquely on the laminated surface on the liquid crystal layer 30 side. Less. As a result, when viewed as the entire element, it is possible to increase only the reflectance in white display without increasing the reflectance in black display, thereby increasing the contrast ratio.
[Third Embodiment]

Next, a third embodiment of the present invention will be described. Note that substantially the same parts as those of the light-scattering display element 1 (FIGS. 1 and 2) according to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted as appropriate. FIG. 12 shows a configuration example of the light scattering display element according to this embodiment. The light scattering display element 3 has an antireflection film 22 laminated on the outermost surface of the display side substrate 20 opposite to the liquid crystal layer 30 with respect to the light scattering display element 1 shown in FIGS. It is a thing. The antireflection film 22 has a refractive index n _AR that is lower than the refractive index _sub 2 of the display-side substrate 20.

  Here, as shown in FIGS. 13A and 13B, the reflection at the interface between the outermost surface of the element and air is considered. In the case of white display, as shown in FIG. 13A, when the light reflected by the liquid crystal layer 30 is incident on the outermost surface of the display-side substrate 20 obliquely with respect to the substrate surface, the normal to the substrate surface is obtained. As the incident angle θ increases, internal reflection occurs at the outermost surface of the display-side substrate 20, returning to the liquid crystal layer 30 side without being emitted to the outside, and there is a concern that the reflectance in white display may be reduced. As shown in FIG. 13B, when external reflection occurs on the outermost surface of the display-side substrate 20, light incident on the element itself is reduced, which may hinder appropriate display. Therefore, by providing the antireflection film 22, internal reflection at the interface between the outermost surface of the display-side substrate 20 and the air can be reduced, the amount of emitted light increases toward the air side, and the white luminance increases. Since the amount of incident light from the air side to the display-side substrate 20 is also increased, white luminance is increased as a result.

FIG. 14 shows the calculation result of the incident angle dependency of the reflectance R of the light emitted from the inside of the display side substrate 20 to the outside. Conversely, FIG. 15 shows the result of calculating the incident angle dependence of the reflectance R of light incident from the outside to the inside of the display-side substrate 20. The refractive index of the display-side substrate 20 was n _sub 2 = 1.65, and the antireflection film 22 had a refractive index n _AR = 1.32 and a thickness d _AR = 100 nm. The refractive index n _air of air (outside the substrate) was 1.0. As a comparative example, the case of no antireflection film 22 (only the display-side substrate 20) is calculated and shown. Moreover, the reflectance R was calculated for each of the S wave and the P wave. Note that the S wave is light polarized in the direction perpendicular to the paper surface of FIGS. 13A and 13B, and the P wave is relative to the paper surface of FIGS. 13A and 13B. The light is polarized in a parallel direction.

From the result of FIG. 14, it can be seen that by applying the antireflection film 22, internal reflection is suppressed at angles up to the critical angle θr = 37.3 °. FIG. 15 also shows that external reflection is suppressed over almost all angles. In order to obtain such an antireflection effect, the refractive index n _AR of the antireflection film 22 is lower than the refractive index n _sub 2 of the display side substrate 20 (n _AR <n _sub 2), and the thickness d _AR ( nm) is λ / 4n _AR , ideal reflection conditions can be obtained. λ is the wavelength of light, for example, it may be adjusted to the wavelength of 540 nm with the highest visibility. Considering that there is an error in practice and that the wavelength range used ranges from red light to blue light,
0.5 × 540 / (4 × n _AR ) ≦ d _AR ≦ 1.5 × 540 / (4 × n _AR ) (1)
If it satisfies, it is effective. Preferably
0.8 × 540 / (4 × n _AR ) ≦ d _AR ≦ 1.2 × 540 / (4 × n _AR ) (2)
If this condition is satisfied, it will be closer to the ideal condition, which is more effective.

As described above, according to the third embodiment, the antireflective film 22 having a low refractive index is laminated on the outermost surface of the display-side substrate 20, and the thickness d _AR is set to an appropriate value. As a result, the internal reflection at the outermost surface of the display-side substrate 20 is reduced, the ratio of the return light to the liquid crystal layer 30 side in the white display state is reduced, and as a result, the ratio of the light emitted to the outside is increased. The reflectance of the entire element in the display state is increased, and white luminance is increased. In addition, since the ratio of external reflection at the outermost surface of the display-side substrate 20 is reduced, the amount of light incident on the element itself increases, and as a result, white luminance can be increased. As a result, the contrast ratio can be increased by increasing only the reflectance in white display without increasing the reflectance in black display.

  Of course, a combination of the second and third embodiments is also possible.

  Next, a specific configuration example of the light scattering display element is shown as an example. Here, as an example, an example corresponding to a configuration obtained by combining the configurations of the second and third embodiments will be described.

FIG. 16 shows the main structure of the light scattering display element according to this example. As the back side substrate 10, a silica glass substrate having a refractive index _nsub1 = 1.52 and a thickness of 0.7 mm was used. An MgF ₂ film having a refractive index n3 = 1.32 and a thickness of 500 nm was deposited as a low refractive index layer 13 on one surface of the back substrate 10 by a vacuum heating method. Then, a transparent electrode film made of ITO having a thickness of 20 nm was deposited on the low refractive index layer 13 as a back side transparent electrode layer 11 by sputtering. FIG. 17 shows a more specific structure of the back side transparent electrode layer 11. More specifically, the back side transparent electrode layer 11 includes a plurality of transparent pixel electrodes 61 made of an ITO transparent electrode film, and a TFT 62 connected to each pixel electrode 61 and controlling a voltage applied to each pixel electrode 62. A gate line 63 for applying a control signal to the gate electrode of the TFT 62 and a source line 64 for applying a control signal to the source electrode of the TFT 62 were formed. The pixel electrode 61 made of ITO was patterned in a substantially square shape with a width of 90 μm by photolithography using a metal mask as shown in the figure. The TFTs 62 are originally formed on the back substrate 10 with a pixel pitch of 100 μm in the vertical and horizontal directions before depositing the ITO film. Correspondingly, there are a plurality of gate lines 63 extending in the horizontal direction and a plurality of source lines 64 extending in the vertical direction. The gate line 63 and the source line 64 were formed by metal wiring (Al).

As the display-side substrate 20, a silica-based glass substrate having a refractive index n _sub2 = 1.65 and a thickness of 0.7 mm was used. A film of MgF ₂ having a refractive index n _AR = 1.32 and a thickness of 100 nm was deposited on one surface of the display side substrate 20 by a vacuum heating method as the antireflection film 22. Further, as the display-side transparent electrode layer 21, an ITO film having a thickness of 20 nm was deposited by sputtering on the side where the MgF ₂ film was not deposited, as in the case of the back-side transparent electrode layer 11. Here, the ITO film to be the display-side transparent electrode layer 21 is deposited over the entire surface of the display-side substrate 20.

  On the substrate surface (ITO side) of the back side substrate 10 and the display side substrate 20 on which these ITO films and the like were laminated, 50 μm diameter plastic balls were evenly dispersed as spacers. Further, the two substrates 10 and 20 were fixed in close contact with the spacers sandwiched between the ITO films facing each other. In this state, the gap between the substrates is 50 μm. Next, the monomer-containing PN liquid crystal is thermally melted, put into the gap between the substrates using capillary action, and then irradiated with ultraviolet rays to cure and network the monomer to obtain a light scattering state. A liquid crystal layer 30 was obtained. Next, after returning to room temperature, the light absorption layer 12 was attached to the back side of the back side substrate 10. As the light absorption layer 12, a carbon solidified with a resin was used.

  In this light scattering display element, the pixel corresponding to the place where the voltage is applied to both the source line 63 and the gate line 64 (FIG. 17) is displayed in black. Using this light scattering display element, the change in the reflectance of light as a whole when a voltage was applied was measured. The results are shown in FIG. From this result, it can be seen that the reflectivity when no voltage is applied (white display) is 45%, whereas the reflectivity when a voltage of 20 V is applied (black display) is only 4%. As a result, the contrast ratio becomes 11. For comparison, the result of the same measurement for an element when the structure of this example is not used (the structure in which the refractive index of the substrate is not optimized and the low refractive index layer 13 and the antireflection film 22 are not provided). Then, the reflectance in white display was 38% and the contrast ratio was 10. For this reason, in the present embodiment, the reflectance of black display hardly increases despite the high reflectance of white display.

  In addition, this invention is not limited to each embodiment and Example which were demonstrated above, Furthermore, various deformation | transformation implementation is possible. For example, all the effects described above are not necessarily limited to PNLCD, but are also effective in a general light-scattering reflective liquid crystal display such as a PDLCD. In addition, the present invention is not limited to a liquid crystal display, and has the same effect in a light scattering display such as electronic paper in general.

It is sectional drawing which shows the structure with respect to the light-scattering type liquid crystal display element which concerns on the 1st Embodiment of this invention, and the effect | action with respect to incident light in the state (white display) which has not applied the electric field. It is sectional drawing which shows the effect | action with respect to incident light in the state (black display) to which the structure of the light-scattering type liquid crystal display element which concerns on the 1st Embodiment of this invention is applied, and an electric field. It is a figure which shows the structure of a liquid crystal molecule. It is a figure explaining the state of reflection of the light which injects into a display side board | substrate from the liquid crystal layer side. FIG. 6 is a characteristic diagram showing the incident angle dependence of the reflectance R of light incident on the display side substrate from the liquid crystal layer side. It is the figure which showed the value of the total reflectance Rtotal of the light which injects into a display side board | substrate from the liquid crystal layer side in the case of changing the refractive index of a display side board | substrate. It is a characteristic view which shows the refractive index dependence of the total reflectance Rtotal of the light which injects into a display side board | substrate from the liquid crystal layer side. It is a figure which shows the structure of the light-scattering type liquid crystal display element which concerns on the 2nd Embodiment of this invention. It is a figure explaining the state of reflection of the light which injects into a low refractive index layer from the liquid crystal layer side in the light-scattering type liquid crystal display element which concerns on the 2nd Embodiment of this invention. It is a characteristic view showing the relationship between the reflectance R for the incident light of the S wave incident on the low refractive index layer from the liquid crystal layer side and the thickness of the low refractive index layer. It is a characteristic view showing the relationship between the reflectance R for the incident light of the P wave incident on the low refractive index layer from the liquid crystal layer side and the thickness of the low refractive index layer. It is a figure which shows the structure of the light-scattering type liquid crystal display element which concerns on the 3rd Embodiment of this invention. It is the figure explaining the state of the reflection of the light in the outermost surface of the display side board | substrate in the light-scattering type liquid crystal display element which concerns on the 3rd Embodiment of this invention. FIG. 11 is a characteristic diagram showing the incident angle dependence of the reflectance of light emitted from the inside of the display side substrate to the outside. It is a characteristic view which shows the incident angle dependence of the reflectance of the light which injects into the inside from the display side board | substrate. It is a figure which shows the principal part structure of the light-scattering type display element which concerns on one Example of this invention. It is a top view which shows the structure of the back side transparent electrode layer in the light-scattering type display element which concerns on an Example. It is a characteristic view which shows the relationship between the applied voltage in the light-scattering type display element which concerns on an Example, and the reflectance of the whole element. It is sectional drawing which shows one structural example of the conventional light-scattering type liquid crystal display element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Back side substrate, 11 ... Back side transparent electrode layer, 12 ... Light absorption layer, 13 ... Low refractive index layer, 20 ... Display side substrate, 21 ... Display side transparent electrode layer, 22 ... Antireflection film (low refractive index) Layer), 30 ... liquid crystal layer, 31 ... liquid crystal droplet, 32 ... polymer, 40 ... incident light.

Claims (16)

A light-scattering display element that displays an image by reflecting incident light,
A transmission / scattering layer capable of switching the action on incident light between a scattering state and a transmission state;
A transparent display-side substrate and a back-side substrate, which are arranged to face each other via the transmission / scattering layer,
A light-scattering display element, wherein a refractive index of the display-side substrate is the same as or higher than a refractive index of the transmission / scattering layer. The transmission / scattering layer includes liquid crystal molecules having birefringence, and is configured such that a refractive index of the liquid crystal molecules with respect to light incident perpendicularly in a transmission state is no.
The light scattering display element according to claim 1, wherein a refractive index of the display-side substrate is the same as or higher than a refractive index no of the liquid crystal molecules. The transmission / scattering layer includes liquid crystal molecules having birefringence,
2. The light scattering display according to claim 1, wherein a refractive index of the display side substrate is equal to or higher than an average refractive index n _{LC of the} entire transmission / scattering layer in a scattering state. element. The refractive index of the display side substrate is higher than the average refractive index n _{LC of the} whole transmission / scattering layer in the scattering state within the following Δn _sub range: 0.01 ≦ Δn _sub ≦ 0 .03
The light-scattering display element according to claim 3. The light-scattering display element according to claim 1, wherein the light-scattering display element is used as a PNLCD (Polymer Network Liquid Crystal Display). A light-scattering display element that displays an image by reflecting incident light,
A transmission / scattering layer capable of switching the action on incident light between a scattering state and a transmission state;
A transparent display-side substrate and a back-side substrate, which are arranged to face each other via the transmission / scattering layer,
The light scattering display element, wherein a refractive index of the back side substrate is lower than a refractive index of the transmission / scattering layer. The transmission / scattering layer includes liquid crystal molecules having birefringence, and is configured such that a refractive index of the liquid crystal molecules with respect to light incident perpendicularly in a transmission state is no.
The light-scattering display element according to claim 6, wherein a refractive index of the back side substrate is lower than a refractive index no of the liquid crystal molecules. The light scattering display element according to claim 6, which is used as a PNLCD (Polymer Network Liquid Crystal Display). A light-scattering display element that displays an image by reflecting incident light,
A transmission / scattering layer capable of switching the action on incident light between a scattering state and a transmission state;
Transparent display-side substrate and back-side substrate, which are arranged to face each other through the transmission / scattering layer,
A display-side transparent electrode layer disposed between the transmission / scattering layer and the display-side substrate;
A backside transparent electrode layer disposed between the transmission / scattering layer and the backside substrate;
A light absorption layer disposed on a surface of the back side substrate opposite to the transmission / scattering layer;
A light-scattering type display comprising: a low-refractive index layer disposed between the back-side transparent electrode layer and the light-absorbing layer and having a refractive index lower than that of the transmission / scattering layer element. The light scattering display element according to claim 9, wherein the low refractive index layer has a thickness of 0.25 μm or more. The light scattering display element according to claim 9, wherein the low refractive index layer has a thickness of 0.5 μm or more. The transmission / scattering layer includes liquid crystal molecules having birefringence, and is configured such that a refractive index of the liquid crystal molecules with respect to light incident perpendicularly in a transmission state is no.
The light scattering display element according to claim 9, wherein a refractive index of the low refractive index layer is lower than a refractive index no of the liquid crystal molecules. The light-scattering display element according to claim 9, which is used as a PNLCD (Polymer Network Liquid Crystal Display). A light-scattering display element that displays an image by reflecting incident light,
A transmission / scattering layer capable of switching the action on incident light between a scattering state and a transmission state;
Transparent display-side substrate and back-side substrate, which are arranged to face each other through the transmission / scattering layer,
A low refractive index layer laminated on the outermost surface of the display side substrate opposite to the transmission / scattering layer, and having a refractive index n _AR lower than the refractive index of the display side substrate,
With respect to the thickness d _AR (nm) of the low refractive index layer, the following conditional expression (1) is satisfied: 0.5 × 540 / (4 × n _AR ) ≦ d _AR ≦ 1.5 × 540 / (4 × n _AR ) ...... (1)
A light-scattering display element. Regarding the thickness d _AR (nm) of the low refractive index layer, the following conditional expression (2) is further satisfied: 0.8 × 540 / (4 × n _AR ) ≦ d _AR ≦ 1.2 × 540 / (4 × n _AR ) (2)
The light scattering display element according to claim 14. The light scattering display element according to claim 14, which is used as a PNLCD (Polymer Network Liquid Crystal Display).

Images