JP4619707B2 - Liquid crystal display device - Google Patents

Description

  The present invention relates to a liquid crystal display device, and more particularly to a so-called transflective liquid crystal display device capable of performing both transmissive display using a backlight and reflective display using reflected light.

  For example, a liquid crystal display device mounted on a device that is used outdoors, such as a mobile phone, needs to be capable of reflective display using external light. Accordingly, in response to such demands, transflective liquid crystal displays capable of performing transmissive display mainly using a backlight and reflected light display outdoors where the illuminance of external light is strong have been developed.

  On the other hand, in recent years, liquid crystal display devices capable of color high-definition display have been demanded for camera-equipped mobile phones and the like. In a liquid crystal display device of a camera-equipped mobile phone, it is important that a viewing angle characteristic is good as a gray scale gradation characteristic when displaying an image captured by a camera in color. Therefore, in the transflective liquid crystal display device, in order to achieve both display performance and power consumption reduction, it is essential that the reflection mode and the transmission mode are both bright, have a high viewing angle, and have a high contrast.

Here, as a technique for improving the brightness and the viewing angle of the reflective display, for example, a technique described in Patent Document 1 is known. In the technique described in Patent Document 1, a reflector that reflects a large amount of external light in a direction different from regular reflection is used. When the observer views from a direction close to the normal direction of the display panel, the light is bright and has high contrast. A display with less reflection is realized. The reflector selectively scatters only incident light within a predetermined angle range and transmits incident light at other angles, reflects light of a predetermined linearly polarized component, and otherwise It is composed of a reflective polarizing plate that transmits light and a light absorbing plate disposed behind the reflective polarizing plate. The light control plate is described in detail in Patent Document 2, Patent Document 3, and the like.
JP-A-11-84115 JP-A-7-58361 JP 7-64069 A

  However, the technique described in Patent Document 1 basically assumes the reflection mode, and hardly considers the transmission mode. Furthermore, for the transflective liquid crystal display device, the reflection mode and the transmission mode are considered. No study has been made on optimizing the optical configuration to achieve both.

  The technique described in Patent Document 1 assumes a case where an observer views from a direction close to the normal direction of the display panel, and does not apply when observing from a regular reflection direction of external light. For example, in a liquid crystal display device incorporated in a mobile phone, the observation direction is substantially fixed in the regular reflection direction, and therefore the technique described in Patent Document 1 cannot be applied as it is.

  Furthermore, in the technique described in Patent Document 1, the reflector is configured by a combination of a light control plate, a reflective polarizing plate, and a light absorbing plate, and there is a problem that the structure is complicated. The liquid crystal display device for mobile phones is required to be further thinned and reduced in price, and specializes in observation in the regular reflection direction, so that the transmission mode and the reflection mode can be made with a simple configuration as much as possible. It is desired to realize a bright and high-contrast color high-definition display.

  The present invention has been proposed in view of these requirements. That is, the present invention provides an optical configuration that is optimized to ensure both the reflection mode and the transmission mode, while having an extremely simple structure, for example, corresponding to observation from the regular reflection direction, It is an object of the present invention to provide a transflective liquid crystal display device capable of bright and high-contrast color high-definition display in both the mode and the reflection mode.

In order to solve the above-described problem, the liquid crystal display device according to the present invention has, for each pixel, a transmissive display area that mainly displays light and a reflective display area that mainly displays light. a semi-transmissive type, a liquid crystal display device light control plate having an angle dependent on the degree of light scattering was arranged on the display surface side, the main scattering direction of the light control plate, is the front direction of the display surface And
The peak angle direction of the light scattering degree on the light control plate coincides with the angle direction in which gradation inversion occurs in the display by the light transmission, and the luminance inversion by gradation inversion is within 4 gradations out of 64 gradations. And
The light scattering degree peak angle direction in the light control plate is coincident with the external light incident angle direction .

  In the liquid crystal display device of the present invention, a light control plate having an angle dependency on the light scattering degree is arranged on the display surface side, and the main scattering direction on the light control plate is a direction in which gradation inversion occurs in the display by light transmission. Since the mounting direction is regulated so as to be approximately the same, the grayscale gradation characteristics in the transmission mode are improved, and the range of the viewing angle where the gradation black blur occurs is greatly reduced.

   At the same time, the mounting direction of the light control plate is regulated so that the main scattering direction in the light control plate substantially coincides with the main external light incident direction in light reflection display. Reflectivity and contrast ratio are greatly improved.

  According to the liquid crystal display device of the present invention, it is possible to optimize the optical configuration in both the reflection mode and the transmission mode simply by controlling the mounting direction of the light control plate. In addition, it is possible to provide a transflective liquid crystal display device capable of bright and high-contrast color high-definition display in both the reflection mode and the reflection mode.

  Hereinafter, a liquid crystal display device to which the present invention is applied will be described in detail with reference to the drawings.

  First, a cross-sectional structure of the transflective liquid crystal display panel of this embodiment is shown in FIG. The transflective liquid crystal display panel 1 has basically the same structure as a normal liquid crystal display panel, and a liquid crystal cell is formed by a pair of glass substrates, and a liquid crystal material is sealed in the gap to form a liquid crystal layer. Has been. The transflective liquid crystal display panel 1 of this embodiment is no exception, and a liquid crystal layer 4 is sealed between the counter substrate 2 and the TFT substrate 3. For the liquid crystal layer 4, any liquid crystal material used for a normal liquid crystal display panel can be used.

  Although not shown, a color filter layer corresponding to each pixel is formed on the surface of the counter substrate 2 on the liquid crystal layer 4 side, and a transparent electrode made of a transparent conductive material such as ITO covering the surface. Is formed on the entire surface. The color filter layer is a resin layer colored in colors by pigments or dyes, and is configured by combining filter layers of R, G, B colors, for example. A so-called black matrix layer is formed at the boundary between the color filter layers for the purpose of improving contrast and the like.

  On the other hand, the TFT substrate 3 corresponds to an array substrate, and although not shown in the figure, thin film transistors (TFTs) that are pixel electrodes and switching elements corresponding to each pixel are arranged in a matrix. A signal line for transmitting an electric signal to the pixel electrode and a gate line for supplying a signal to the thin film transistor serving as a switching element are wired orthogonally to each other.

  In the transflective liquid crystal display panel 1 of the present embodiment, the area is divided into a reflective opening A in which a reflective electrode 5 is formed as each pixel electrode and a transparent opening B in which a transparent electrode 6 is formed. . In such a transflective liquid crystal display panel, external light is used in the reflective opening A, and image display is performed in the transmissive opening B using the backlight 7 arranged on the back side as a light source.

  An alignment film for aligning the liquid crystal molecules constituting the liquid crystal layer 4 in a predetermined direction is formed on the surface of the counter substrate 2 and the TFT substrate 3 in contact with the liquid crystal layer 4. A normal alignment film used for a liquid crystal display element can be used, and the description thereof is omitted here. In addition, circularly polarizing plates 8 and 9 are arranged on the surfaces of the counter substrate 2 and the TFT substrate 3 opposite to the surfaces in contact with the liquid crystal layer 4, respectively. The circularly polarizing plates 8 and 9 have a structure in which, for example, a polarizing plate and a retardation plate are combined.

  The basic structure of the transflective liquid crystal display panel 1 has been described above. In this embodiment, light control is performed on the circularly polarizing plate 8 provided on the display surface side of the transflective liquid crystal display panel 1, that is, on the counter substrate 2 side. By providing the plate 10 and appropriately setting the main scattering direction, the optical configuration is optimal for a transflective liquid crystal display device. Therefore, in the following, the mounting orientation of the light control plate 10 will be described.

  First, the light control plate 10 has a property of selectively scattering only incident light within a predetermined angle range and transmitting incident light at other angles. The structure has a structure in which a transparent layer having a high refractive index and a transparent layer having a low refractive index are laminated obliquely.

  Regarding the configuration of the light control plate 10, for example, IDW'03 Development of a Directional Diffuser (LMMurillo-Mora, H. Honma, Y. Maekawa, M. Takano, K. Hirose, A. Sato and F. Iwata) FMC10- 2 Details on p701-704. Any commercially available light control plate 10 can be used.

  FIG. 2 schematically shows the main scattering direction of the light control plate 10. As described above, the light control plate 10 has an angle dependency on the light scattering degree, and the light scattering degree is high in a specific direction. For example, in FIG. 2, when the angle θ of the incident light is changed with respect to the normal line Z of the light control plate 10, the light scattering degree (haze value) changes accordingly as shown in FIG. At this time, the angle θ1 at which the degree of scattering is greatest is the peak angle, and the direction indicating the peak angle is the peak angle direction A1, which is projected to represent the direction around the normal Z as the main scattering direction. A2. On the contrary, on the −θ side, the degree of scattering is small, so this direction is the transmission angle region.

  The peak angle direction A1 of the light control plate 10 is preferably in the direction of 10 ° to 50 °, although it depends on the gradation inversion angle direction and the external light incident direction described later. The light scattering degree (haze value) of the light control plate 10 is preferably in the range of 50% to 100% in the peak angle direction, and is preferably 50% or less in the transmission angle region.

  On the other hand, in the transflective liquid crystal display panel 1, a TN (twisted nematic) mode is most commonly employed as a display mode because it has an advantage of high definition and high transmittance. In the TN mode, when a halftone is displayed in light transmission display, if the screen is observed with the viewing angle being changed, the gradation is reversed, and the display quality is deteriorated.

  In the TN mode, liquid crystal molecules are rotated in one direction to selectively transmit backlight light, and each gradation is reproduced by applying an electric field to each pixel to control the standing of the liquid crystal molecules. The For this reason, when the angle is changed in the major axis direction of the liquid crystal molecules, a phenomenon occurs in which the halftone luminance is reversed at a certain tilt angle. In other words, each tilt angle-luminance curve at each gradation has a minimum value, but since the state of liquid crystal molecules at each gradation is different, each minimum value is shifted little by little for each gradation. For this reason, as shown in FIG. 4, at a certain tilt angle, the luminance of the gradation close to black may be higher than the luminance of the gradation close to white. In FIG. 4, the area surrounded by a broken line is a halftone gradation reversal point. When such gradation inversion occurs, the screen becomes difficult to see, leading to deterioration in display quality.

  FIG. 5 illustrates the gradation inversion direction (direction in which gradation inversion occurs) and the gradation inversion angle direction (angle direction in which gradation inversion occurs) in the transflective liquid crystal display panel 1. As described above, when the screen is observed with the viewing angle α, the gradation is reversed in the predetermined angle region α1. The angle direction in which this gradation inversion occurs is the gradation inversion angle direction B1, and the direction in which the direction is projected with the normal Z as a center is the gradation inversion direction B2.

  In the case of the transflective liquid crystal display panel 1, it is necessary to consider not only the light transmission display but also the light reflection display. In the display by light reflection, the incident direction of external light is important, and as shown in FIG. 6, most of the light that is viewed from a direction symmetric to the viewing direction in the transflective display panel 1 is obtained. Incident. The incident direction of the light with respect to the viewing direction is the external light incident angle direction C1, and the external light incident direction C2 is a projection of the incident direction and representing the direction around the normal line Z.

  In the present invention, the main scattering direction A2 in the light control plate 10, the gradation inversion direction B2 in the transmissive display of the transflective liquid crystal display panel 1, and the external light incident direction C2 in the reflective display are substantially matched. In both the transmissive display and the reflective display, a bright, wide viewing angle and high contrast display is realized. FIG. 7 schematically shows an arrangement state of the light control plate 10 and the transflective liquid crystal display panel 1. As shown in FIG. 7, the light control plate 10 is placed on the surface of the transflective liquid crystal display panel 1 so that the main scattering direction A2 of the light control plate 10 substantially coincides with the gradation inversion direction B2 and the external light incident direction C2. Paste to. In this case, in the transflective liquid crystal display panel 1, it is premised that the gradation inversion direction B2 and the external light incident direction C2 are substantially coincident with each other. It is necessary to keep this in mind.

  In addition, the external light incident direction C2, for example, in the case of a liquid crystal display device for a mobile phone, it is normal to use reflected light from the front direction (approximately 12 o'clock direction of the display surface). The main scattering direction A2 in the light control plate 10 is preferably directed in the direction of about 12:00 on the display surface. Here, the expression “what time direction” indicates the azimuth of the display surface by using a number representing the time carved on the dial plate, assuming that the dial plate of the watch is superimposed on the liquid crystal display device. Therefore, for example, the 12 o'clock direction corresponds to the upward direction of the display surface.

  In the present invention, as described above, it is necessary to make the main scattering direction A2 substantially coincide with the gradation inversion direction B2 and the external light incident direction C2, but in particular, the peak angle of the light control plate 10 Ideally, the direction A1 is substantially coincident with the gradation inversion angle direction B1 and the external light incident angle direction C1 of the transflective liquid crystal display panel 1. For example, with regard to the peak angle direction A1 and the gradation inversion angle direction B1, it is preferable to match these angles so that the inversion of the luminance due to the gradation inversion is within 4 gradations out of 64 gradations.

  Note that since the gradation inversion angle direction B1 has a certain width, the peak angle direction A1 may be made to coincide with the angle that is substantially the center thereof. In addition, the external light incident angle direction C1 varies greatly depending on the usage pattern of the user. For example, in the case of a liquid crystal display device for a mobile phone, the viewing direction is almost determined. The peak angle direction A1 may be matched with the light incident angle direction C1.

  Thus, the effect can be reliably acquired by making each said angle direction correspond mutually. FIG. 8 schematically shows the light transmission characteristics in the transmission mode. In the transmission mode, the transmitted light from the backlight 7 is observed. At this time, the transmitted light is almost scattered by the light control plate 10 from the normal direction to the good gradation direction of the transflective display panel 1. Not observed, light scattering is small. In this region, gradation inversion does not occur in the transflective display panel 1. On the other hand, the transmitted light in the gradation inversion angle direction is transmitted through the main scattering azimuth A2 of the light control plate 10 and further in the peak angle direction A1, so that light scattering is large. As a result, the display with gradation inversion is diffused and cannot be clearly observed, so that the gradation viewing angle on the gray scale in this direction is greatly improved.

  On the other hand, FIG. 9 schematically shows light reflection characteristics in the reflection mode. In the reflection mode, the display is performed mainly by the reflected light from the regular reflection direction. At this time, the reflection due to the regular reflection becomes a factor of reducing the contrast and the like. In the present embodiment, the reflected light incident from the regular reflection direction is transmitted through the main scattering azimuth A2 of the light control plate 10 and further the peak angle direction A1, so that it is greatly scattered. As a result, the reflection is performed. Is eliminated and the contrast is greatly improved. Further, the light from the direction other than the regular reflection direction is also used for the reflective display due to the scattering effect of the light control plate 10, so that the reflectance in the front direction in the reflective display is substantially improved.

  As mentioned above, although embodiment of the liquid crystal display device to which this invention was applied was described, it cannot be overemphasized that this invention is not limited to this embodiment. For example, in the above-described embodiment, the light control plate 10 is disposed on the outer side (upper side) of the front-side circularly polarizing plate 8, but other locations, for example, between the counter substrate 2 and the front-side circularly polarizing plate 8. You may arrange | position between. In addition, the same effect can be obtained even if the light reflection plate is disposed outside the liquid crystal cell. Further, the light reflection plate itself may have a diffusion function.

  Next, a liquid crystal display device was actually assembled and the effect of the present invention was confirmed. Hereinafter, this specific embodiment will be described.

  The basic configuration of the liquid crystal display device of this embodiment is the same as that shown in FIG. 1, and also in this embodiment, a light control plate that selectively scatters light is viewed from the front side circularly polarized light as viewed from the liquid crystal cell. Arranged outside the plate. The pixel of the liquid crystal element is a so-called transflective liquid crystal display device having a structure in which a region where display is performed mainly by light transmission and a region where display is performed mainly by light reflection are combined for each pixel. The reflective electrode in the reflective region has a flat structure having specular characteristics. The liquid crystal cell has a homogeneous alignment, and the cell gaps for transmission and reflection are different from each other. Further, the pixel driving method is an active matrix method, and the display mode is normally white. The circularly polarizing plate has a structure in which a polarizing plate and a retardation plate are combined.

  In the liquid crystal display device of the present embodiment, the viewing angle characteristics in the transmissive display are gray scale gradation characteristics on the upper side and black blurring / reversal orientation. The main external light incident direction in the reflective display is the upper direction (12 o'clock direction) of the liquid crystal cell.

  FIG. 10 shows the angle dependence of the light scattering characteristics of the light control plate used in this example. The light control plate used had a peak angle of 30 °, and the light scattering degree (haze value) at that time was 90%.

  In this embodiment, the optical angle is set by (1) the main scattering direction of the light control plate, (2) the external light direction of reflection (upper side of the panel), and (3) the black blur direction of transmission (tone inversion direction). ) Match each other. Therefore, (4) the transmission azimuth of the light control plate, (5) the observer azimuth (front or lower side of the panel), and (6) the good gradation azimuth of transmission are in agreement.

  The produced liquid crystal display was measured for optical characteristics (transmission display characteristics and reflection display characteristics). The measured items are the front transmittance, the front contrast ratio, and the viewing angle (up / down / left / right) for the transmissive display characteristics, and the reflectance and contrast ratio for the reflective display characteristics. The viewing angle of the transmissive display characteristic is an angle at which black blurring or gradation inversion does not occur in L * 8 gradation display. The reflection characteristics were measured under the condition that light from the light source was incident at 30 ° from above the liquid crystal cell and received at the front. Table 1 shows the optical characteristics of the liquid crystal display device of this example. Table 1 also shows optical characteristics of a conventional transflective liquid crystal cell (comparative example) that does not use a light control plate.

  As is clear from Table 1, the grayscale viewing angle in the upward gray scale in the transmissive display is greatly improved from 20 ° to 60 ° in the comparative example. Further, with respect to the reflective display, the reflectance and contrast ratio in the front direction were 2% and 5 in the comparative example, respectively, but were significantly improved to 4% and 20 in the example, respectively.

It is a schematic sectional drawing which shows the structural example of a transflective liquid crystal display panel. It is a schematic diagram explaining the main scattering direction and peak angle direction of a light control board. It is a characteristic view which shows the angle dependence of the light-scattering characteristic in a light control board. It is a characteristic view which shows the mode of gradation inversion. It is a schematic diagram explaining the gradation inversion direction and the gradation inversion angle direction in the liquid crystal display panel. It is a schematic diagram explaining the external light incident direction and external light incident angle direction in reflective display. It is a schematic perspective view which shows the attachment direction to the liquid crystal display panel of a light control board. It is a schematic sectional drawing which shows the light transmission characteristic in the transmission mode in the liquid crystal display panel of embodiment. It is a schematic sectional drawing which shows the light reflection characteristic in the reflection mode in the liquid crystal display panel of embodiment. It is a characteristic view which shows the angle dependence of the light-scattering characteristic in the light control board used in the Example.

Explanation of symbols

1 transflective liquid crystal display panel, 2 counter substrate, 3 TFT substrate, 4 liquid crystal layer, 5 reflective electrode, 6 transparent electrode, 7 backlight, 8, 9 circularly polarizing plate, 10 light control plate

Claims (6)

For each one pixel, mainly the transmissive display region for performing display by light transmission, mainly in semi-transmission type and a reflective display region for performing display by light reflection, the light control plate having an angle dependent on the degree of light scattering is A liquid crystal display device arranged on the display surface side, the main scattering direction in the light control plate is the front direction of the display surface,
The peak angle direction of the light scattering degree on the light control plate coincides with the angle direction in which gradation inversion occurs in the display by the light transmission, and the luminance inversion by gradation inversion is within 4 gradations out of 64 gradations. And
The liquid crystal display device, wherein a peak angle direction of a light scattering degree in the light control plate is coincident with an external light incident angle direction . Peak angle light scattering degree is maximum in the light control plate, a liquid crystal display device according to claim 1, characterized in that the 10 ° to 50 ° with respect to the normal direction of the display surface. 3. The liquid crystal display device according to claim 2, wherein a light scattering degree in a peak angle direction indicating a peak angle of the light control plate is a haze value of 50% to 100%. 3. The liquid crystal display device according to claim 2 , wherein the light scattering degree in the transmission angle region of the light control plate is less than 50% in terms of haze value. The light control plate, the refractive index of different layers the liquid crystal display device according to any one of claims 1, characterized in that it has a laminated structure in a tilted state with respect to the plate surface 4. An array substrate having an independent electrode for each pixel, a counter substrate disposed opposite to the array substrate, a liquid crystal layer sandwiched between the array substrate and the counter substrate, the array substrate, and the counter substrate And a polarizing plate disposed on the outer surface of
The said light control board is arrange | positioned between the said opposing board | substrate and the polarizing plate arrange | positioned on the outer surface, or the outer side of the polarizing plate arrange | positioned on the outer surface of the said opposing substrate, The any one of Claim 1 to 5 characterized by the above-mentioned. A liquid crystal display device according to claim 1.

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