JP2004279930A - Reflective liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reflective liquid crystal display device which is constituted of a reflective liquid crystal display panel of a single polarizing plate type composed of a liquid crystal layer formed with a reflection electrode within a liquid crystal cell and subjected to homogeneous alignment treatment, a phase compensation plate and a polarization plate and which makes contrast higher by realizing an ideal black display state at the time of black gradation, is free of coloring, light, and wide in an angle of visibility and is high speed on response. <P>SOLUTION: Positive uniaxial or biaxial phase compensation plates are successively laminated between the liquid crystal cell and a polarizing plate and the phase differences and slow axis azimuths of the phase compensation plates are so regulated that external light passes the liquid crystal cell and thereafter the reflected light thereof is modulated to linearly polarized light after successively passing the respective phase compensation plates. Also, the constitution to arrange the axes of polarization of the polarizing plate at the azimuth of -45° or +45° in relation to the optical axis azimuth of the liquid crystal layer is employed and the physical property parameters of the liquid crystal are specified to a prescribed range. Further, a cell gap is regulated to 3.0 to 3.5 μm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a reflection type liquid crystal display device using a thin film transistor.
[0002]
[Prior art]
Liquid crystal display devices are characterized by being thin and light, and capable of reducing power consumption. They are practical and high-performance display devices compared to other display devices. It is applied to portable information terminals, network PCs, and the like. However, since the liquid crystal display device is originally a non-light emitting display device, a high-luminance backlight must be disposed on the back of the panel, and at present, the liquid crystal display device does not always make full use of its essential features.
[0003]
In order to solve this problem, there is a reflection type liquid crystal display device. Since a reflection type liquid crystal display device displays an image using reflection by external light, it can be made thinner and lighter than a transmission type liquid crystal display device, and is particularly excellent in low power consumption. Another feature is that excellent image visibility under daylight can be obtained. Further, there is an advantage that the number of parts can be reduced as compared with the transmission type, there is no replacement part, and the influence on the environment is small. (Patent Document 1)
[0004]
Conventional reflection type liquid crystal display devices are mainly used for portable terminals of monochrome or multicolor (4 to 16 colors). Among them, colorization can be roughly classified into two types, and there are an additive color mixture type in which color display is performed using RGB three dots using a micro color filter, and a light interference type in which an interference color due to birefringence of liquid crystal is used.
[0005]
The respective configurations and features are as follows. In the additive color mixing type in which color display is performed using a micro color filter, as shown in FIG. 12, a conventional transmissive TN cell or STN cell is used as a liquid crystal panel, polarizing plates 14 are provided on both sides thereof, and diffusion is performed on the back surface of the cell. A configuration in which the reflection plate 17 is disposed, a reflection electrode inside the liquid crystal cell or a reflection plate outside the cell, a dichroic dye (guest) mixed with the liquid crystal (host), and transmission and absorption of light by voltage control. There is a configuration for displaying. This type of liquid crystal display device can realize excellent gradation display and high contrast. On the other hand, the latter optical interference type does not require a polarizing plate, so that a bright display with a wider viewing angle can be realized. In the light interference type, generally, an STN cell of an external reflector type is used, and a multi-color display of one dot and one pixel has high light efficiency and can realize a bright display.
[0006]
[Patent Document 1]
JP-A-9-218403
[Problems to be solved by the invention]
In recent years, the display capacity of video and information displays has dramatically increased due to the development of advanced information communication technology and infrastructure development, and advanced information network systems capable of receiving and transmitting information not only in offices but also outdoors have been constructed. . Assuming a future multimedia terminal, a high-resolution, full-color reflective color liquid crystal display device is demanded as a display device for a portable terminal.
[0008]
However, the configuration of the conventional reflective liquid crystal display device has many display performance issues. For example, the micro color filter method has a problem of loss of transmitted light of the original color filter. When a transmission type TN or STN cell is used, the brightness is reduced due to the use of two polarizing plates as shown in FIG. 12, and character blur occurs due to parallax between the liquid crystal layer and the reflecting plate. However, at higher resolutions, visibility is further impaired. Further, in the guest-host system, since hysteresis occurs in electro-optical characteristics, gradation display is difficult, and there is a problem of high voltage driving. On the other hand, the optical interference type has the same problems as the STN cell, such as a problem of brightness loss due to the polarizing plate, a limitation of the number of display colors due to interference colors, and a narrower operating temperature range. For the above reasons, the conventional display method cannot realize a high-resolution, full-color reflective liquid crystal display device.
[0009]
An object of the present invention is to solve the above-mentioned problems and to provide a reflective color liquid crystal display panel having high contrast, a wide viewing angle, high-speed response, and high reflectance.
[0010]
[Means for Solving the Problems]
The present invention provides a thin film transistor array substrate having a thin film transistor and a reflective electrode formed for each pixel, a counter electrode substrate formed with a counter electrode, and a homogeneous alignment treatment sandwiched between the thin film transistor array substrate and the counter substrate. Liquid crystal cell, a positive uniaxial or biaxial phase compensator laminated on the counter electrode substrate side, and one polarizing plate laminated on the phase compensator The phase difference and the slow axis azimuth of the phase compensator are adjusted so that the reflected light passing through the liquid crystal cell sequentially passes through the phase compensator and is then modulated into linearly polarized light. The polarizing plate is arranged such that its polarization axis is at an angle of -45 ° or + 45 ° with respect to the optical axis direction of the liquid crystal layer, and the liquid crystal of the liquid crystal layer has a dielectric anisotropy Δε. Is 6.5 ≦ △ ε 8.0, the refractive index anisotropy Δn is in the range of 0.07 ≦ Δn ≦ 0.09, and the elastic constant ratio K33 / K11 is in the range of 1.0 ≦ K33 / K11 ≦ 2.0. The cell gap d is in the range of 3.0 μm ≦ d ≦ 3.5 μm, and a voltage higher than a threshold voltage is applied to the liquid crystal layer to perform display driving.
[0011]
Further, in the reflection type liquid crystal display device of the present invention, it is preferable that at least three or more positive uniaxial or biaxial phase compensators laminated on the counter electrode substrate side are provided.
[0012]
When three phase compensators are configured, the slow axis of the first phase compensator is arranged perpendicular to the optical axis direction of the liquid crystal, and the light reflected on the reflective electrode is reflected by the liquid crystal cell and the first phase compensator. , The phase difference of the first phase compensator is adjusted so that the light is modulated into circularly polarized light. Further, the phase difference and the slow axis azimuth of the second and third phase compensators are adjusted so that the light is modulated into linearly polarized light after passing through the second and third phase compensators. Further, the polarization axis of the polarizing plate is disposed at -45 or +45 with respect to the optical axis direction of the liquid crystal.
[0013]
According to the reflective liquid crystal display device of the present invention, a low reflectance characteristic independent of wavelength in a black display can be obtained in a wide viewing angle range, so that a high contrast image and a wide viewing angle can be realized. Further, it is possible to realize a wide viewing angle without grayscale inversion between grayscale displays. On the other hand, even in white display, high reflectance characteristics can be obtained in a wide viewing angle range, and a bright image display can be realized. In addition, since the liquid crystal is formed in a homogeneous alignment mode and is driven and displayed at a threshold voltage or higher, a display image that responds at a high speed can be realized regardless of a gradation level. Further, since the change in the reflectance characteristic of the black display with respect to the operating temperature is very small, there is no image deterioration due to the temperature change, and a high-quality image display can be realized.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a configuration of a homogeneously oriented reflective liquid crystal panel using three reflective biaxial phase compensating plates with a reflective electrode as a mirror body, and the polarization state of light during monochrome operation. FIG. 1A shows a case where the display state is black display, and FIG. 1B shows a case where the display state is white display. FIG. 2 is a cross-sectional view showing a panel configuration of a reflective liquid crystal panel used in the reflective liquid crystal display device of the present embodiment.
[0015]
The reflection type liquid crystal panel includes a third phase compensator 13, a diffusion film 15, a first phase compensator 11, a second phase compensator 12, and a polarizer 14 on an upper surface of a liquid crystal cell 16 including the reflective electrode 1. Are sequentially laminated. As shown in FIG. 2, the liquid crystal cell 16 includes a TFT glass substrate 3 and a thin film transistor array substrate having the reflective electrode 1 formed on the array chip 2 thereon, a CF glass substrate 4 and a color filter layer 9 It comprises a counter electrode substrate on which a counter electrode 8 is formed, and a liquid crystal layer 7 which is homogeneously aligned between the two electrodes.
[0016]
Here, a polycarbonate polymer film is used as the first and second phase compensators 11 and 12, and the polarizing plate 14 is made of a material that has been subjected to a low reflection surface treatment for preventing external light reflection. use.
[0017]
Here, the polarization state of light passing through the polarizing plate 14, the second, first and third phase compensating plates 12, 11, 13 and the liquid crystal layer 7 of the liquid crystal cell 16 will be described with reference to FIG. In the case of black display, as shown in FIG. 1A, the color changes from linearly polarized light to elliptically polarized light → circularly polarized light → circularly polarized light → elliptically polarized light → circularly polarized light. On the contrary, the return path starts from circularly polarized light, and becomes elliptically polarized light → circularly polarized light → circularly polarized light → elliptically polarized light → linearly polarized light. However, since the linearly polarized light after passing through the polarizing plate 14 on the path and the linearly polarized light after passing through the second phase compensator on the return path are orthogonal to each other, black display is performed.
[0018]
On the other hand, in the case of white display, as shown in FIG. 1B, the polarization state of the path is linearly polarized light → elliptical polarized light → circular polarized light → circular polarized light → elliptical polarized light → elliptical polarized light. On the return path, starting from elliptically polarized light on the reflective electrode, the order is elliptically polarized light → elliptically polarized light → elliptically polarized light → elliptically polarized light → elliptically polarized light.
[0019]
Further, the refractive indices of the liquid crystal in the optical axis direction and the vertical direction thereof are respectively n _p and n _v , the cell thickness of the liquid crystal cell is d, the tilt angle of the liquid crystal molecules is α, and the third phase compensating plate is provided. each 13 of the slow axis, fast axis and the refractive index in the thickness _{direction, n x (3), n} y (3), n z (3), when the films film thickness and _{D, n} x (3 ), _Ny (3), _nz (3) and D are designed as follows.
_{n x (3) = [(} n v × cosα) 2 + (n p × sinα) 2] / n p
n _y (3) = [(n _v × cos α) ² + (n _p × sin α) ² ] ^1/2
_nz (3) = n _v
D = n _p × n _v / [(n _v × cos α) ² + (n _p × sin α) ² ] × d
The slow axis direction is arranged in a direction perpendicular to the liquid crystal optical axis.
[0020]
The optical design of the second phase compensator 12 is such that its slow axis is arranged at + 30 ° or −30 ° with respect to the optical axis direction of the liquid crystal, and the slow axis, the fast axis and the refractive index in the thickness direction. Let _nx (4), _ny (4), _nz (4) be
_{[N x (4) -n z} (4)] / [n x (4) -n y (4)] = 0.5
Is created to satisfy
[0021]
The optical design of the first phase compensator 11 is such that its slow axis is -30 ° with respect to the optical axis direction of the liquid crystal (in this case, the slow axis direction of the second phase compensator 12 is + 30 °). ) Or + 30 ° (the slow axis direction of the second phase compensator 12 in this case is -30 °). The slow axis, _n x ₍₅₎ a refractive index of the fast axis and the thickness _direction, n y (5), When _n z (5),
_{[N x (5) -n z} (5)] / [n x (5) -n y (5)] = 0.5
Is created to satisfy
[0022]
On the other hand, the polarization axis of the polarizing plate 14 is arranged at + 45 ° or −45 ° with respect to the optical axis direction of the liquid crystal.
[0023]
Next, the thickness d of the liquid crystal cell, the black voltage and the viewing angle characteristics will be described. FIG. 3A shows a change in the viewing angle at which the reflectance R becomes 50% with respect to the black voltage (V) with respect to the front, with respect to the black voltage (V), and graphs for the film thickness d of 3.0 μm, 4.0 μm, and 5.0 μm. Is shown. FIG. 3B shows an angle where the contrast CR with respect to the black voltage is 10 or more. Setting a black voltage is important to obtain good viewing angle characteristics of reflectance and contrast. It is desirable that the black voltage is reduced from the viewpoint of reducing the panel power consumption, but it is necessary to increase the black voltage beyond the threshold voltage of the liquid crystal. The thickness d of the liquid crystal cell is set to 3.0 to 3.5 μm so that the viewing angle characteristic of the reflectance is 50 ° or more and the viewing angle characteristic of the contrast is 60 ° or more. The physical constants of the liquid crystal used here are Δε = 3.5, Δn = 0.090, and K33 / K11 = 1.5. In addition, K33 indicates a bend elastic constant of liquid crystal physical properties, and K11 indicates a splay elastic constant.
[0024]
FIGS. 4A and 4B show the viewing angle characteristics of the reflectance and the viewing angle characteristics of the contrast when the refractive index anisotropy Δn is changed from 0.07 to 0.15. Also in this case, Δn is set to 0.07 to 0.09 in order to set the viewing angle characteristic of the reflectance to 50 ° or more and the viewing angle characteristic of the contrast to 60 ° or more.
[0025]
FIGS. 5A and 5B show the viewing angle characteristics of the reflectance and the viewing angle characteristics of the contrast when the dielectric anisotropy Δε is changed to 3.5 to 8.0. Also in this case, the dielectric anisotropy Δε is set to 6.5 to 8.0 so that the viewing angle characteristic of the reflectance is 50 ° or more and the viewing angle characteristic of the contrast is 60 ° or more.
[0026]
6 (a) and 6 (b) show the viewing angle characteristics of the reflectance, and FIG. 6 (b) shows the viewing angle characteristics of the contrast for the elastic constant ratio K33 / K11. In these figures, the elastic constant ratio K33 / K11 is in the range of 1.0 to 2.0 in order to make the viewing angle characteristic of the reflectance 50 ° or more and the viewing angle characteristic of the contrast 60 ° or more.
[0027]
According to the reflective liquid crystal panel of the present embodiment, the reflectance in the front direction of the panel when the black display voltage is applied can be made as small as possible, and a characteristic that is flat with respect to the wavelength can be obtained. Can be provided. Further, even if the viewing angle is tilted with respect to the optical axis direction of the liquid crystal, the phase difference of the entire liquid crystal panel hardly changes, so that inversion between each gradation does not occur, and an image display with a wide viewing angle can be realized. In addition, since the driving voltage of the liquid crystal panel is used at a voltage higher than the threshold voltage of the liquid crystal, a high-speed response characteristic can be obtained between each gradation including the halftone. On the other hand, in order to minimize the change in retardation of the liquid crystal layer even at high temperatures, the appropriate liquid crystal physical properties are selected, and the display quality such as contrast, viewing angle, and reflectance is optimized by optimizing the black display voltage. Thus, it is possible to realize an image which is less affected by deterioration due to a temperature change.
[0028]
【Example】
Here, an embodiment of the present invention will be described. As the liquid crystal layer 7, a dielectric anisotropy Δε = 7.8, a refractive index anisotropy Δn = 0.0074, an elastic constant ratio K33 / K11 = 2.0, and a wavelength dispersion ratio Δnd (α) / △. A liquid crystal material having a physical property value of nd (550 nm) of 1.15 ≦ △ nd (α) / △ nd (550 nm) ≦ 0.95 was used, and the cell thickness d of the liquid crystal cell was formed to d = 3 μm. Further, the phase difference of the third phase compensator 13 is △ nd (3), the slow axis is θ (3), the phase difference of the second phase compensator 12 is △ nd (4), and the slow axis is θ (4), the phase difference of the first phase compensator 11 is Δnd (5), the slow axis is θ (5), the polarization axis of the polarizer is θp (2), and these are respectively described below. I did it.
Δnd (3) = 180 nm
θ (3) = + 90 ° or −90 ° nd (4) = 137.5 nm
θ (4) = + 30 ° or −30 ° nd (5) = 265 nm
θ (5) = + 30 ° (when the slow axis θ (4) = − 30 °) or = −30 ° (when the slow axis θ (4) = + 30 °)
θp (2) = + 45 ° or −45 °
FIG. 7 shows a voltage-reflectance characteristic of the reflection type liquid crystal display panel according to this embodiment. Regarding the measurement of the voltage-reflection characteristics, measurement was performed in the front direction using MgO as a reference using a measurement system shown in FIG. However, after the applied voltage Vc of the black gradation is fixed to Vc = 1.5 (V) in the RGB pixel so that the voltage-reflectance characteristics of each pixel match, the voltage-reflectance characteristics of the other gradations are obtained. Were corrected for each of the RGB pixels. As is clear from the characteristics of FIG. 7, by performing the voltage correction, a high-quality image display without coloring at all gradation levels can be realized.
[0030]
FIG. 8 shows the voltage-retardation ratio characteristics of the reflective liquid crystal panel at a high temperature of 60 ° C. Here, the retardation ratio is represented by a retardation ratio of 25 ° C. and a retardation ratio of 60 ° C. Curve A is a homogeneous liquid crystal having improved temperature characteristics according to the present embodiment, and B is a homogeneous liquid crystal of a conventional example. As shown in FIG. 8, as the voltage is increased, the retardation ratio temporarily changes abruptly at the boundary of the threshold voltage, but increases as the voltage shifts to the higher voltage side. Therefore, as the selection of the black gradation voltage of the present reflection type liquid crystal panel, (1) that it is larger than the threshold voltage, (2) that a black and white modulation rate of 100% can be ensured (that is, the retardation of the liquid crystal cell △ nd ≧ 175 nm), 3) Vc = 1.5 (V) on condition that the influence of the retardation ratio is small.
[0031]
FIG. 9 is a graph showing the viewing angle-contrast characteristics. FIG. 9A is a graph of a liquid crystal having improved temperature characteristics according to the present embodiment, and FIG. 9B is a graph of a conventional liquid crystal at 25 ° C. and 60 ° C., respectively. The measurement system shown in FIG. 13 was used. FIG. 9 shows that under the condition of 60 ° C., not only in the front direction but also in the entire viewing angle range, the liquid crystal of the present embodiment can obtain higher contrast characteristics than the conventional example.
[0032]
FIG. 10 is a diagram illustrating the viewing angle-gradation characteristics at 25 ° C. of the reflective liquid crystal panel including the liquid crystal of the present embodiment. The measurement system shown in FIG. 13 was used. R4 (1.6 V), R26 (2.1 V), R54 (2.5 V), R75 (2.9 V), and R96 (4.2 V) in FIG. The voltage value in parentheses indicates the voltage value applied at the time of halftone display. FIG. 10 shows that the reflection type liquid crystal panel according to the present embodiment can obtain high reflectance characteristics even in a viewing angle range other than the specular reflection azimuth, and can realize a wide viewing angle characteristic in which the reflectance characteristics are not inverted between gradations. Understand.
[0033]
FIG. 11 is a diagram illustrating the response characteristics at 25 ° C. of the reflective liquid crystal panel including the liquid crystal of the present example. From FIG. 11, it can be seen that the reflective liquid crystal panel of the present embodiment can provide a display image that responds at high speed between each gradation including the halftone.
[0034]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to the reflective liquid crystal display device of this invention, the low reflectance characteristic which does not depend on a wavelength in black display can be obtained in a wide viewing angle range, and high contrast and wide viewing angle of an image can be realized. Further, it is possible to realize a wide viewing angle without grayscale inversion between grayscale displays. On the other hand, even in white display, high reflectance characteristics can be obtained in a wide viewing angle range, and a bright image display can be realized. In addition, the liquid crystal physical property parameters of the liquid crystal layer are adjusted in a range of 6.5 ≦ Δε ≦ 8.0, 0.07 ≦ Δn ≦ 0.09, 1.0 ≦ K33 / K11 ≦ 2.0, and The liquid crystal cell has a cell gap in the range of 3.0 μm ≦ d ≦ 3.5 μm. In addition, since display driving is performed with the voltage applied to the liquid crystal layer being equal to or higher than the threshold voltage, it is not limited to black and white gradations, It is possible to realize a display image that responds at a high speed even during a key interval. Furthermore, since the change in the reflectance characteristic of the black display with respect to the operating temperature can be designed to be minimized, image deterioration due to the temperature change is small, and high-quality image display can be realized.
[Brief description of the drawings]
FIG. 1 is a diagram showing a panel configuration and a polarization state of light of a reflective liquid crystal display panel of the present embodiment. FIG. 2 is a diagram showing a panel cross section of the reflective liquid crystal display panel of the present embodiment. (A) is a view showing a viewing angle characteristic when the film thickness d of the liquid crystal cell is changed, and (b) is a view showing a viewing angle (contrast) characteristic. FIG. 5 (a) is a diagram showing the viewing angle characteristics of contrast, and FIG. 5 (a) is a diagram showing the viewing angle characteristics of contrast when the dielectric anisotropy Δε is changed. 6 (a) is a diagram showing reflectance when the elastic constant ratio K33 / K11 is changed, and FIG. 6 (b) is a diagram showing viewing angle characteristics of contrast. FIG. 7 is a diagram showing a voltage of a reflection type liquid crystal display panel of the present embodiment. FIG. 8 is a graph showing reflectance characteristics. FIG. 8 is a graph showing the relationship between the voltage and the retardation of the reflective liquid crystal display panel of the present embodiment. FIG. 9 is a diagram showing a viewing angle-contrast characteristic of the reflective liquid crystal display panel of the present embodiment. FIG. 10 is a diagram showing a viewing angle-gradation characteristic of the reflective liquid crystal display panel of the present embodiment. FIG. 11 is a diagram showing the response characteristics of the reflective liquid crystal display panel of the present embodiment. FIG. 12 is a panel configuration diagram of a conventional reflective liquid crystal display panel. FIG. 13 is a diagram of the reflective liquid crystal panel of the present embodiment. Diagram showing a measurement system for measuring optical characteristics [Description of symbols]
DESCRIPTION OF SYMBOLS 1 Reflective electrode 2 Array chip 3 TFT glass substrate 4 CF glass substrate 5 Organic insulating film 6 Alignment film 7 Liquid crystal layer 8 Counter electrode 9 Color filter layer 10 Light shielding layer 11 First phase compensator 12 Second phase compensator 13 3 phase compensation plate 14 polarizing plate 15 diffusion film 16 liquid crystal cell 17 diffusion reflection plate 18 light source 19 detector 20 liquid crystal panel

Claims (2)

A thin film transistor array substrate having a thin film transistor and a reflective electrode formed for each pixel; a counter electrode substrate having a counter electrode formed thereon; and a liquid crystal layer sandwiched between the thin film transistor array substrate and the counter substrate and homogeneously aligned. And a liquid crystal cell having:
A positive uniaxial or biaxial phase compensator laminated on the counter electrode substrate side;
One polarizing plate laminated on the phase compensator,
The phase difference and the slow axis azimuth of the phase compensator are adjusted so that the reflected light passing through the liquid crystal cell sequentially passes through the phase compensator, and is then modulated into linearly polarized light.
The polarizing plate is disposed such that its polarization axis has an orientation of -45 ° or + 45 ° with respect to the optical axis orientation of the liquid crystal layer,
The liquid crystal of the liquid crystal layer has a dielectric anisotropy Δ △ of 6.5 ≦ Δε ≦ 8.0, a refractive index anisotropy Δn of 0.07 ≦ Δn ≦ 0.09, and an elastic constant ratio K33. / K11 is in the range of 1.0 ≦ K33 / K11 ≦ 2.0,
The reflection type liquid crystal display device, wherein a cell gap d of the liquid crystal cell is in a range of 3.0 μm ≦ d ≦ 3.5 μm, and a voltage higher than a threshold voltage is applied to the liquid crystal layer to perform display driving. 2. The reflection type liquid crystal display device according to claim 1, comprising at least three or more positive uniaxial or biaxial phase compensators laminated on the counter electrode substrate side.

Images