JP2012198351A - Liquid crystal display element and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that in a liquid crystal element using a cholesteric liquid crystal, a portion out of a pixel where a phase state cannot be controlled by application of a voltage remains bright.SOLUTION: A matrix driving type liquid crystal display element is provided, including: a cholesteric liquid crystal held between a pair of substrates having transparent electrodes; a first alignment layer positioned in a pixel part on an interface between the transparent electrode and the cholesteric liquid crystal, for forming a predetermined pretilt angle in the cholesteric liquid crystal; and a second alignment layer positioned in a non-pixel part out of the pixel part, for forming a pretilt angle in the cholesteric liquid crystal larger than the pretilt angle formed by the first alignment layer.

Description

  The present invention relates to a liquid crystal display element and a manufacturing method thereof.

  In a display element using cholesteric liquid crystal, display is controlled according to the alignment state of the liquid crystal molecules. Hereinafter, the principle of a reflective liquid crystal display element using cholesteric liquid crystal will be described with reference to FIG.

  Cholesteric liquid crystal has two states, a planar (PL) state that reflects incident light and a focal conic (FC) state that transmits light. These two states are controlled by applying a voltage to the cholesteric liquid crystal 100 between the transparent electrodes 101/102. Cholesteric liquid crystal is characterized by maintaining a stable alignment state even in the absence of an electric field after the voltage is turned off.

  In the PL state, light having a wavelength corresponding to the helical pitch (one period of the helical structure) of the liquid crystal molecules A is selectively reflected (referred to as selective reflection).

Now, assuming that the average refractive index of the liquid crystal is n and the helical pitch is p, the wavelength λ at which reflection is maximum is
λ = n * p (1)
It is represented by

  The selectively reflected wavelength band Δλ increases with the refractive index anisotropy Δn of the liquid crystal, and a predetermined color can be expressed by adjusting the refractive index anisotropy Δn. For example, when a liquid crystal with λ of 545 nm is used, a display element that exhibits green in the PL state can be formed.

  In the PL state, the reflected light is either left or right circularly polarized light according to the spiral pitch, and the circularly polarized light that is not reflected is transmitted through the cholesteric liquid crystal 100. When natural light 10 is incident on the cholesteric liquid crystal 100 in the PL state, since the natural light 10 is in a state where left and right circularly polarized light is mixed, 50% of the incident light is reflected light 20 and 50% is transmitted light 30. It is done.

  Conventionally, in the reflective liquid crystal system, the transmitted light 30 that has passed through the cholesteric liquid crystal 100 is absorbed by the light absorption layer 105 provided on the side opposite to the observation side of the display screen, whereby the PL state is changed to the “bright state”, The FC state is displayed as “dark state”.

  In addition to the PL state and the FC state described above, the alignment state of the cholesteric liquid crystal includes a preplanar state (hereinafter referred to as a PPL state) as a state before the voltage is applied after the liquid crystal is injected into the cell and isotropically processed. Called). This PPL state is affected by the materials of the alignment films 104 and 105 disposed at the interfaces between the transparent electrodes 101 and 102 and the cholesteric liquid crystal 100, and the pretilt angle of the liquid crystal molecules A (the tilt angle of the liquid crystal molecules A near the interface). Changes.

  Conventionally, a low pretilt material having a low tilt angle has been applied as an alignment film material in order to improve the reflectance of a liquid crystal display element.

Japanese Patent Laid-Open No. 04-258924

  However, when a low pretilt material is applied to the alignment film material in the matrix drive type liquid crystal display element, the PPL state portion corresponding to the outside of the pixel remains shining, and the contrast ratio of the entire liquid crystal display element is increased. I have a problem that it falls.

  Therefore, the present invention provides a liquid crystal display element that improves the contrast ratio by disposing a high pretilt material as an alignment film on the outside of the pixel.

  One aspect of the invention is a matrix-driven liquid crystal display element, in which a cholesteric liquid crystal sandwiched between a pair of substrates having a transparent electrode and a pixel portion at the interface between the transparent electrode and the cholesteric liquid crystal. And a first alignment film that forms a predetermined pretilt angle on the cholesteric liquid crystal and a non-pixel portion other than the pixel portion, and has a pretilt angle larger than that of the first alignment film on the cholesteric liquid crystal. And a second alignment film to be formed.

  According to one aspect of the present invention, by arranging an alignment film having a high pretilt angle in a portion outside the pixel where the alignment state cannot be controlled by voltage application, and by arranging an alignment film having a low pretilt angle in the pixel portion, It is possible to suppress a decrease in display contrast ratio.

It is a figure which shows the structural example of the liquid crystal display element of a matrix drive type which becomes embodiment of this invention. It is a figure which shows the structural example of the alignment film material applied to the liquid crystal display element of this invention. It is a figure which shows the influence of alignment film material which has on the reflection spectrum of PL state of a cholesteric liquid crystal. It is a figure explaining the definition of the aperture ratio of the liquid crystal display element of a matrix structure. It is a figure which shows the relationship between the brightness and contrast ratio of a liquid crystal display element, and an aperture ratio. It is a figure which shows the method (The low pretilt material is arrange | positioned to the 1-pixel part whole) which controls the in-plane pretilt angle which becomes embodiment of this invention. It is a figure which shows the method (The low pretilt material is arrange | positioned to the part of 2-pixel part) which controls the in-plane pretilt angle which becomes embodiment of this invention. It is a figure which shows the structure of the alignment film in the liquid crystal display element which becomes embodiment of this invention, and the relationship between brightness. It is a figure which shows the relationship between the structure of the alignment film in the liquid crystal display element which becomes embodiment of this invention, and contrast ratio. It is a figure which shows the manufacturing method of the alignment film in the liquid crystal display element which becomes embodiment of this invention. It is a figure explaining the display principle by a cholesteric liquid crystal.

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

  FIG. 1 shows a configuration example of a matrix driving type liquid crystal display element of the present invention. 1A shows a cross-sectional view of a liquid crystal display element, and FIG. 1B shows a top view of a matrix electrode structure portion of the liquid crystal display element.

A pair of substrates 1 and 2 each having transparent electrodes 11 and 21 formed by vapor deposition of a material such as In ₂ O _3, and a liquid crystal (hereinafter referred to as cholesteric liquid crystal) 100 sandwiched between the substrates 1 and 2; The alignment film 3 is provided on each of the transparent electrodes 11 and 21. The alignment film 3 includes a first alignment film 3 a that forms a predetermined pretilt angle in the cholesteric liquid crystal 100 located in the pixel unit 50 at the same interface between the transparent electrodes 11 and 21 and the cholesteric liquid crystal 100. In addition, the second alignment film 3 b is formed which causes the cholesteric liquid crystal 100 of the non-pixel portion 51 to form a pretilt angle larger than that of the first alignment film.

  As described above, in the liquid crystal display element using the cholesteric liquid crystal, the alignment film 3 is formed so as to have a high pretilt angle only in the vicinity of the liquid crystal interface of the non-pixel portion 51 other than the pixel portion 50 where the phase state cannot be controlled by voltage application. By controlling, it is possible to reduce the reflection component of this portion, improve the brightness, and improve the decrease in contrast.

  Here, the operation of a liquid crystal display element using cholesteric liquid crystal will be supplemented. In FIG. 1, the liquid crystal display element is a substrate on the upper surface side when the substrate 1 is viewed from an observer. Although not shown, a light absorption layer for absorbing transmitted light is provided on the lower surface of the substrate 2. It has been.

  Two states of the cholesteric liquid crystal 100, a planar (PL) state and a focal conic (FC) state, are controlled by applying a voltage across the transparent electrodes 11/21. This state exists stably even when the voltage is turned off.

  The wavelength λ selectively reflected by the cholesteric liquid crystal depends on the average refractive index n inherent to the liquid crystal material and the helical pitch (one period of the helical structure) p as shown in the equation (1). Therefore, when in the PL state, light having a wavelength corresponding to the helical pitch and refractive index of the liquid crystal molecules is selectively reflected (selective reflection). For example, when a cholesteric liquid crystal having λ of 545 nm is used, a display element that expresses green can be formed. In this way, the selective reflection wavelength λ in the cholesteric liquid crystal 100 can be in the visible light region.

  Further, two or more liquid crystal display elements composed of such cholesteric liquid crystals can be stacked to form a stacked color liquid crystal display element having different selective reflection wavelengths in each layer.

  Furthermore, a stable and easy-to-see color liquid crystal display element is realized by arranging the boundaries of the portions having different pretilt angles in the same position in each layer of the multilayer liquid crystal display element as viewed from the observation surface (upper surface).

  Next, the alignment film in the liquid crystal display element of the present invention will be described with reference to FIGS.

  FIG. 2 shows a structural example of an alignment film material applied to the liquid crystal display element of the present invention.

  As the alignment film material disposed at the interface with the cholesteric liquid crystal, a polyimide-based material that is conventionally used for TN liquid crystal is applied. FIG. 2 shows a general structure of polyimide. For example, an alignment chain material having a different pretilt angle can be obtained by arranging an alkyl chain in the side chain portion of the R ′ portion and changing the length of the alkyl chain. In general, it is said that the longer the side chain, the higher the pretilt film.

  In a normal TN liquid crystal element or the like, a pretilt angle equal to or greater than a certain value is provided in order to stabilize the display quality by setting the liquid crystal molecules to rise when a voltage is applied. It is generally known that the higher the pretilt, the greater the effect of making the rising direction of the liquid crystal molecules constant, but the viewing angle characteristics are reduced accordingly.

  However, the influence of the pretilt angle on display characteristics of cholesteric liquid crystals is generally not well known. Therefore, the relationship between the pretilt angle and the display characteristics was experimentally obtained below.

  FIG. 3 shows the influence of the alignment film material on the display characteristics of the cholesteric liquid crystal. FIG. 3 shows the reflection characteristics of the liquid crystal display device having the configuration shown in FIG. 1 when the same liquid crystal material is used and only the alignment film disposed at the liquid crystal interface is changed to a low pretilt alignment film and a high pretilt alignment film. Was investigated. FIG. 3 shows the reflection characteristics in the PPL state before voltage is applied to the liquid crystal display element.

  For example, the high pretilt material includes SE5300 manufactured by Nissan Chemical, and the low pretilt material includes SE5291 manufactured by Nissan Chemical.

  As shown in FIG. 3, as the alignment film, it was found that the low pre-tilt material has a higher selective reflection reflectance in the PPL state than the high pre-tilt material. This is presumably because the lower the pretilt angle, the more the amount of light reflected by the liquid crystal increases because the PPL state takes an alignment state closer to the PL state.

  Here, the relationship between the brightness and contrast ratio and the aperture ratio will be described in the evaluation of the brightness and contrast ratio of the liquid crystal display element having the matrix structure hereinafter with reference to FIGS.

  FIG. 4 is a diagram illustrating the definition of the aperture ratio of a liquid crystal display element having a matrix structure. As shown in FIG. 4, in the matrix structure, the upper transparent electrode 11 and the lower transparent electrode 21 in the stripe shape cross each other to form a pixel portion 50 (region surrounded by a thick solid line) and a non-pixel portion 51. The

Now, assuming that the size of the pixel portion 50 is D and the pixel interval is d, the aperture ratio S is the ratio of the area E of the pixel portion 50 to the area F of the portion including the non-pixel portion (the region surrounded by the thick dotted line). S = E / F = D ² / D + d) ²

  For example, when D is 90 μm and d is 10 μm, the aperture ratio S is 81%.

  FIG. 5 shows the relationship between the brightness and contrast ratio of the liquid crystal display element and the aperture ratio.

  In a liquid crystal display element using cholesteric liquid crystal, the PL state is used as a bright state and the FC state is used as a dark state. First, a reflection spectrum in the PL state is measured, and a Y value is calculated from the obtained reflection spectrum in consideration of visibility. This Y value (Y (PL)) is defined as “brightness”.

  Next, the reflection spectrum in the FC state is measured, the Y value taking into account the visibility is calculated from the obtained reflection spectrum, and the Y value in the PL state is divided by the Y value in the FC state (Y (FC)). Is the contrast ratio.

  In an actual liquid crystal display element, the aperture ratio affects the brightness and contrast ratio. For example, in the case of an element using a cholesteric liquid crystal, when the element has a matrix structure with an aperture ratio of S, the actual brightness and contrast ratio can be calculated by the following equations as shown in FIG.

Brightness of light state I _L = Y (PL) × S + Y (PPL) × S _N (2)
Brightness in the dark state I _D = Y (FC) × S + Y (PPL) × S _N (3)
Contrast ratio C = I _L / I _D (4)
Here, PPL is a state before applying a voltage after injecting liquid crystal into a cell and applying an isotropic process by heat treatment. S represents an aperture ratio, and S _N is an area ratio of an uncontrollable region due to voltage, and is represented by S _N = 1−S.

  As described so far, when the pre-tilt angle material and the high pre-tilt angle material are used as the alignment film material arranged at the liquid crystal interface, the brightness of the PL state is different, and the lower the pre-tilt, the more PL It turned out that Y value which shows the brightness of a state becomes large.

  However, when an alignment film material having a low pretilt is used, the portion of the PPL state outside the pixel remains shining and the contrast ratio of the entire display element is lowered. Therefore, in the following, FIGS. 6 and 7 propose a structure of an alignment film for eliminating the light state in the PPL state portion for the outside of the pixel.

  FIG. 6 shows a method for controlling the in-plane pretilt angle according to the present invention (part 1—a low pretilt material is disposed over the entire pixel portion). FIG. 6 shows an example in which a low pre-tilt material is arranged over the entire pixel portion. 6A shows a matrix structure composed of an upper electrode and a lower electrode, FIG. 6B shows a top view of the vicinity of one pixel existing in the circular dotted line portion of FIG. 6A, and FIG. (C) has shown sectional drawing of the AA 'line of (b).

  The present liquid crystal display element includes a pair of substrates 1 and 2 each having transparent electrodes 11 and 21, a cholesteric liquid crystal 100 sandwiched between the substrates 1 and 2, and an orientation provided on each transparent electrode 11 and 21. It has a membrane 3. In addition, the alignment film 3 includes a first alignment film 3 a provided on the entire pixel unit 50 and a second alignment film provided on the non-pixel unit 51 at the same interface between the transparent electrodes 11 and 21 and the cholesteric liquid crystal 100. 3b. The first alignment film 3 a forms a predetermined pretilt angle in the cholesteric liquid crystal 100 located in the entire pixel unit 50 in contact with the first alignment film 3 a, and the second alignment film 3 b It has a role of forming a pretilt angle larger than 3a.

  In this embodiment, the alignment film 3 has a different pretilt state at the same interface in contact with the cholesteric liquid crystal 100 due to the configuration in which the entire pixel unit 50 is made of a low pretilt material and the non-pixel part 51 that cannot be controlled by voltage is made of a high pretilt material. The contrast ratio is improved by increasing the reflected light in the pixel unit 50 while suppressing the reflected light in the non-pixel unit 51 in the presence of (PPL state).

  FIG. 7 shows a method for controlling the in-plane pretilt angle according to the present invention (the low pretilt material is arranged in a part of the 2-pixel portion). FIG. 7 shows an example in which a low pretilt material is arranged in a part of the pixel portion. FIG. 7A shows a matrix structure composed of upper and lower electrodes, FIG. 7B shows a top view of the vicinity of one pixel existing in the circular dotted line portion of FIG. 7A, and FIG. (C) has shown sectional drawing of the AA 'line of (b).

  The present liquid crystal display element includes a pair of substrates 1 and 2 each having transparent electrodes 11 and 21, a cholesteric liquid crystal 100 sandwiched between the substrates 1 and 2, and an orientation provided on each transparent electrode 11 and 21. It has a membrane 3. In addition, the alignment film 3 includes the first alignment film 3 a provided in a part of the pixel unit 50 and the second film provided in the non-pixel unit 51 at the same interface between the transparent electrodes 11 and 21 and the cholesteric liquid crystal 100. It has an alignment film 3b.

  The first alignment film 3 a forms a predetermined pretilt angle in the cholesteric liquid crystal 100 located in a part of the pixel unit 50 in contact with the first alignment film 3 a, and the second alignment film 3 b forms the first cholesteric liquid crystal 100 in the non-pixel unit 51 on the first cholesteric liquid crystal 100. It has a role of forming a pretilt angle larger than that of the alignment film 3a.

  In this embodiment, the alignment film 3 is in contact with the cholesteric liquid crystal 100 in the same manner as in FIG. 6 by using a configuration in which a part of the pixel portion 50 is made of a low pretilt material and a non-pixel portion 51 that cannot be controlled by voltage is made of a high pretilt material. Different pre-tilt states (PPL states) can be present at the same interface, the reflected light in the non-pixel portion 51 can be suppressed, and the reflected light in the pixel portion 50 can be increased. As in this embodiment, the low pretilt portion is arranged in the pixel, and the manufacturing tolerance is increased as compared with the case where the boundary of the low pretilt portion is extended to the pixel boundary as shown in FIG. is there.

  Hereinafter, the arrangement configuration of the alignment film including the low pretilt part and the high pretilt part and the effect thereof will be described.

  Here, in the configuration of the present invention, as the cholesteric liquid crystal, for example, each Y value (actually measured value by chromaticity measurement) in the PL state and the PPL state in FIG. , Y (PPL) is 0.098, low pretilt, Y (PL) is 30.8, Y (PPL) is 4.05, and each Y value (actually measured value) in FC state is high pretilt A liquid crystal material having Y (PL) of 1.2 and low pretilt Y (FC) of 1.4 is used.

  The results of calculating the brightness and contrast with respect to the area of the low tilt portion of the alignment film according to FIG. 5 using these values are shown in FIGS. 8 and 9 below.

  FIG. 8 shows the relationship between the configuration of the alignment film and the brightness in the liquid crystal display element of the present invention. FIG. 8 shows the result of evaluating the brightness with respect to the area ratio of the low pretilt part. Note that the area ratio of the low pretilt portion being 0% corresponds to the case where the high pretilt material is disposed in all regions.

  When the display element has a matrix structure with an aperture ratio of 81%, the actual brightness and contrast ratio can be calculated using the calculation formula of FIG. At this time, if the boundary between the high pre-tilt part and the low pre-tilt part is arranged outside the pixel unit 50, the contrast ratio is remarkably affected (described later in FIG. 9), so that the boundary is within the pixel unit 50. It is good to arrange.

  In FIG. 8, Y (PL) is 24.6 and Y (PPL) is 0.098 in the case of high pretilt, and Y (PL) is 30.8 and Y (PPL) is 4.046 in the case of low pretilt. In this case, the cholesteric liquid crystal material is used and the aperture ratio is 81%.

  According to FIG. 8, if the area ratio of the low pretilt portion in the pixel unit 50 can be secured at 50% or more, the brightness is improved by 10% or more as compared with the case where all the regions are set to the high pretilt. I understand.

  Next, FIG. 9 shows the relationship between the configuration of the alignment film and the contrast ratio in the liquid crystal display element of the present invention. FIG. 9 shows the result of evaluating the contrast ratio with respect to the area ratio of the low pretilt part.

  The Y values (actually measured values) in the FC state of the cholesteric liquid crystal used are 1.4 for low pretilt Y (FC) and 1.2 for high pretilt Y (PL). Further, the Y value and the aperture ratio in the PL state and the PPL state are the same as those in FIG.

  According to FIG. 9, it can be seen that when the area ratio of the low pretilt part exceeds 80%, which is the aperture ratio, the contrast ratio is significantly reduced.

  As described above, when FIGS. 8 and 9 are combined, when the area ratio of the low pretilt portion is increased to about 50%, the brightness is maintained as high pretilt while maintaining the contrast ratio without decreasing. Compared to the case, it can be improved 1.1 times or more.

  As described above, the range of the configuration of the low pre-tilt part and the high pre-tilt part arranged in the pixel is determined in the alignment film of the liquid crystal display element based on the results of the brightness and contrast ratio of the liquid crystal display element in FIGS.

  A manufacturing method for realizing an alignment film which is a main component of the liquid crystal display element described above will be described below.

  FIG. 10 shows a method for producing an alignment film in the liquid crystal display element of the present invention. In this example, a method for producing an alignment film by a light exposure method will be described.

First, in step S11, a transparent electrode was formed to a thickness of 0.13 μm by vapor-depositing In ₂ O ₃ on each transparent substrate.

  Next, in step S12, a photo-alignment film material whose alignment was aligned by light irradiation was uniformly applied as an alignment film on the transparent electrode to a thickness of about 50 nm with a spinner at a rotation speed of 2000 rpm. Thus, the same photo-alignment film material was uniformly formed on the transparent electrode, and post-baked.

  In step S13, using a high-pressure mercury lamp, ultraviolet light of 365 nm was shielded with a photomask and irradiated with 30 millijoules.

  Through the above manufacturing process, for example, a photo-alignment film having an azobenzene portion in the side chain portion can be modified in surface state toward the low pretilt angle side by irradiation with ultraviolet light.

  As described above, a low pretilt portion and a high pretilt portion having different pretilt angles can be easily formed on the same plane by light irradiation using a material that can be surface-modified.

  In addition to the above light irradiation, as a method for forming alignment films having different pretilt angles, as a high pretilt material, for example, a high pretilt material such as Nissan Chemical SE5300 and a low pretilt material such as Nissan Chemical SE5291 are used. It may be created with a spin coater after masking with a resist.

  The present invention relates to a structure of a reflective liquid crystal element in which a cholesteric liquid crystal (chiral nematic liquid crystal) is sandwiched between upper and lower substrates.

DESCRIPTION OF SYMBOLS 1, 2 Substrate 3 Alignment film 3a First alignment film 3b Second alignment film 11, 21 Transparent electrode 50 Pixel part 51 Non-pixel part 100 Cholesteric liquid crystal

Claims (6)

A matrix-driven liquid crystal display element,
A cholesteric liquid crystal sandwiched between a pair of substrates with transparent electrodes;
A first alignment film disposed at a position of a pixel portion at an interface between the transparent electrode and the cholesteric liquid crystal, and forming a predetermined pretilt angle in the cholesteric liquid crystal;
A second alignment film disposed at a position of a non-pixel part other than the pixel part, and forming a pretilt angle larger than the first alignment film in the cholesteric liquid crystal;
A liquid crystal display element comprising:   The first alignment film is disposed inside the pixel portion, and a ratio of the area of the first alignment film to the area of the pixel portion is 50% or more, and a pixel portion with respect to an area of a region including a non-pixel portion The liquid crystal display element according to claim 1, wherein the liquid crystal display element is less than an aperture ratio expressed as an area ratio.   The liquid crystal display element according to claim 1, wherein a selective reflection wavelength in the cholesteric liquid crystal is in a visible light region.   A multi-layer liquid crystal display element that performs color display by laminating two or more liquid crystal display elements having the structure according to claim 1.   5. The liquid crystal display element according to claim 4, wherein, when viewed from the observation surface, in each layer of the multilayer liquid crystal display element, boundaries of portions having different pretilt angles are arranged at the same position. A cholesteric liquid crystal sandwiched between a pair of substrates having a transparent electrode, and a first pre-tilt angle formed on the cholesteric liquid crystal that is disposed at the pixel portion at the same interface between the transparent electrode and the cholesteric liquid crystal. A matrix-driven liquid crystal display element having a first alignment film and a second alignment film that is disposed at a position of a non-pixel portion other than the pixel portion and that causes the cholesteric liquid crystal to form a pretilt angle larger than that of the first alignment film. A manufacturing method of
Creating a transparent electrode on the substrate;
Forming a photo-alignment film on the transparent electrode;
After forming the same photo-alignment film material, light-shielding with a photomask and irradiating with ultraviolet light;
A method for producing a liquid crystal display element, comprising: