JP2014153549A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the display performance of a transflective liquid crystal display device.SOLUTION: A liquid crystal display device 50 includes: substrates 21, 22 arranged facing one another; a liquid crystal layer 23 sandwiched by the substrates 21, 22, and vertically aligned (VA) when no electric field is applied; pixels 12 each having a reflection region and a transmission region and having the same cell gap between the reflection region and the transmission region; a pixel electrode 26 provided on a pixel region of the substrate 21; a common electrode 31 provided on the substrate 22; and a reflection film 25 provided on the substrate 21 and the reflection region. The area of the reflection film 25 is equal to or smaller than 30% of that of the pixel electrode 26.

Description

  The present invention relates to a liquid crystal display device, and more particularly to a transflective liquid crystal display device that achieves both a transmissive display and a reflective display.

  As a liquid crystal display device that improves outdoor visibility, a transflective liquid crystal display device capable of displaying an image in both a transmission mode and a reflection mode is known. In this transflective liquid crystal display device, a reflective metal film provided inside a liquid crystal cell is patterned to form a reflective region and a transmissive region, and the reflective display by the reflective region and the transmissive display by the transmissive region are combined. Further, a transparent step film or the like is provided in the reflection region, and the cell gap between the reflection region and the transmission region is changed (multi-gap) to optimize the optical characteristics (for example, Patent Document 1 and Patent Document 2). reference).

  In a liquid crystal display device having such a multi-gap, a step is formed between the reflective region and the transmissive region. Therefore, alignment failure occurs at the boundary of the step and light leakage occurs. For this reason, there is a problem that the contrast is lowered. In addition, when the step portion is shielded from light in order to prevent a decrease in contrast, there is a problem that the effective aperture ratio decreases.

  Further, since a transparent step film is required in the reflective region in order to change the cell gap, the number of manufacturing steps is increased as compared with a transmissive liquid crystal display device or the like. Furthermore, in order to form a transparent step film in the reflection region, there is a restriction on the arrangement of the reflection region, for example, the reflection region is collected and arranged in a part.

JP 2003-262852 A JP 2003-270627 A JP 2012-141351 A

  The present invention provides a liquid crystal display device capable of improving display performance while achieving both reflective display and transmissive display.

  A liquid crystal display device according to one embodiment of the present invention includes a first substrate and a second substrate which are arranged to face each other, and a liquid crystal layer which is sandwiched between the first and second substrates and has vertical alignment (VA) when no electric field is applied. A pixel having a reflective region and a transmissive region, the cell region of the reflective region and the transmissive region being the same, a pixel electrode provided in a pixel region of the first substrate, and the second substrate A common electrode provided; and a reflective film provided on the first substrate and the reflective region. The area of the reflective film is 30% or less of the area of the pixel electrode.

  ADVANTAGE OF THE INVENTION According to this invention, the liquid crystal display device which can improve display performance can be provided, making a reflective display and a transmissive display compatible.

Sectional drawing of the liquid crystal display panel which concerns on this embodiment. The layout diagram of a liquid crystal display panel. FIG. 3 is a cross-sectional view of a liquid crystal display panel along the line AA ′ shown in FIG. 2. FIG. 3 is a cross-sectional view of the liquid crystal display panel along the line BB ′ shown in FIG. 2. 1 is a block diagram of a liquid crystal display device. The circuit diagram of a pixel array. FIG. 6 is a block diagram illustrating an example of a signal driver illustrated in FIG. 5. The graph which shows the VT curve of the reflective display of a liquid crystal display panel, and a transmissive display. Schematic explaining polarization of a liquid crystal display panel. The graph which shows the relationship between reflection area ratio and the chromaticity change of white. The figure which expanded the part which plotted the chromaticity change of white of FIG.

  Hereinafter, embodiments will be described with reference to the drawings. However, it should be noted that the drawings are schematic or conceptual, and the dimensions and ratios of the drawings are not necessarily the same as the actual ones. Further, even when the same portion is represented between the drawings, the dimensional relationship and ratio may be represented differently. In particular, the following embodiments exemplify an apparatus and a method for embodying the technical idea of the present invention, and the technical idea of the present invention depends on the shape, structure, arrangement, etc. of components. Is not specified. In the following description, elements having the same function and configuration are denoted by the same reference numerals, and redundant description will be given only when necessary.

[1. Configuration of liquid crystal display panel 10]
FIG. 1 is a cross-sectional view of a liquid crystal display panel 10 according to the present embodiment. The liquid crystal display panel 10 according to the present embodiment includes a reflection area that displays an image by selectively reflecting outside light and a transmission area that displays an image by selectively transmitting backlight light within one pixel. A transflective liquid crystal display panel.

  A surface light source (backlight) 11 is disposed opposite to the surface opposite to the display surface of the liquid crystal display panel 10. As the backlight 11, for example, a sidelight type (edge light type) backlight device is used. That is, the backlight 11 is configured such that a plurality of light emitting elements such as LEDs (light emitting diodes) are incident from the end surface of the light guide plate, and light is emitted from one plate surface of the light guide plate toward the pixel array. Is done. For example, the backlight 11 is configured by laminating a reflection sheet, a light guide plate, a diffusion sheet, and a prism sheet.

  The liquid crystal display panel 10 includes a TFT substrate 21 on which a TFT (Thin Film Transistor) as a switching element and a pixel electrode are formed, and a color filter substrate on which a color filter and a common electrode are formed and disposed opposite to the TFT substrate 21. (CF substrate) 22 and a liquid crystal layer 23 sandwiched between the TFT substrate 21 and the CF substrate 22. The TFT substrate 21 and the CF substrate 22 are each composed of a transparent substrate (for example, a glass substrate).

  The liquid crystal layer 23 is made of a liquid crystal material sealed with a sealing material (not shown) that bonds the TFT substrate 21 and the CF substrate 22 together. In the liquid crystal material, the alignment of liquid crystal molecules is manipulated in accordance with the electric field applied between the TFT substrate 21 and the CF substrate 22, and the optical characteristics change. The liquid crystal display panel 10 of the present embodiment uses a VA mode using a vertical alignment (VA) type liquid crystal. That is, negative (N-type) nematic liquid crystal is used as the liquid crystal layer 23, and the liquid crystal molecules are aligned substantially perpendicular to the substrate surface when there is no voltage (no electric field). In the VA mode liquid crystal molecule alignment, the long axis (director) of the liquid crystal molecule is vertically aligned when no voltage is applied, and the director of the liquid crystal molecule is inclined in the horizontal direction when the voltage is applied (electric field application). The liquid crystal mode may be a homogeneous mode, but the VA mode is more optimal from the viewpoint of improving contrast.

  A TFT 24, a reflective film 25, and a pixel electrode 26 are provided on the TFT substrate 21 for each pixel 12. In addition, an alignment film 27 is provided on the TFT substrate 21 so as to cover the TFT 24, the reflective film 25, and the pixel electrode 26. One display pixel (pixel) is composed of three sub-pixels having red (R), green (G), and blue (B) color filters. In the following description, the sub-pixel is referred to as a pixel unless it is particularly necessary to distinguish between the display pixel and the sub-pixel.

  The pixel 12 has a reflective region 12R and a transmissive region 12T. The reflective region 12R corresponds to a region of the pixel 12 where the reflective film 25 is provided, and the transmissive region 12T corresponds to a region of the pixel 12 where the reflective film 25 is not provided. The area of the pixel 12 corresponds to the area of the pixel electrode 26. In the present embodiment, the thickness (cell gap) of the liquid crystal layer 23 in the reflective region 12R and the transmissive region 12T is the same (flat gap). The cell gap can be kept uniform over the entire screen by arranging, for example, columnar spacers 28 in the liquid crystal layer 23.

The area ratio occupied by the reflection region in the pixel (display portion) is called a reflection area ratio.
Reflection area ratio = reflection area / pixel area (= reflection film area / pixel electrode area)
Pixel Area = Reflection Area Area + Transmission Area Area In this embodiment, the reflection area ratio is set to be greater than 0% and 30% or less. That is, the area of the reflective film 25 is set to be greater than 0% and 30% or less of the pixel area (area of the pixel electrode 26). Details of the reflection area ratio will be described later.

  The TFT 24 is a switching element that is formed on the liquid crystal layer 23 side of the TFT substrate 21 and switches the pixel 12 on and off. That is, the liquid crystal display panel 10 of the present embodiment uses an active matrix type driving method.

  The reflective film 25 has a function of reflecting external light incident from the display screen of the liquid crystal display panel 10. The reflective film 25 is disposed above the TFT 24 that cannot be used for transmissive display. Thereby, reflective display and transmissive display are possible without reducing the aperture ratio of the transmissive region 12T (area of the transmissive region 12T). The reflective film 25 is disposed, for example, in a layer where a storage capacitor (Cs) is formed or a layer where a pixel electrode is formed.

  The pixel electrode 26 is provided for each pixel 12 and is formed on the entire surface of the pixel 12. The pixel electrode 26 is for applying a voltage to the liquid crystal layer 23, and is electrically connected to one end of the current path of the TFT 24. The pixel electrode 26 is made of a transparent conductive film such as ITO (indium tin oxide). The alignment film 27 controls the alignment of liquid crystal molecules.

  The CF substrate 22 is provided with a color filter 30, a common electrode 31, an alignment film 32, and a protrusion 33. The color filter 30 is provided on the liquid crystal layer 23 side of the CF substrate 22 and includes three color filters 30-R, 30-G, and 30-B of red, green, and blue. The color filters 30-R, 30-G, and 30-B are arranged at positions facing the pixel electrode 26 and constitute sub-pixels. A light shielding film (black matrix) 30-BK for preventing color mixture is formed between the color filters 30-R, 30-G, and 30-B.

  The common electrode 31 is provided on the liquid crystal layer 23 side of the color filter 30 and has a size that covers all the pixel electrodes 26. The common electrode 31 is made of a transparent conductive film such as ITO. The alignment film 32 is paired with the alignment film 27 and controls the alignment of liquid crystal molecules. In the present embodiment, the alignment films 27 and 31 are in a state where there is almost no potential difference between the pixel electrode 26 and the common electrode 31, that is, in a state where no electric field is applied between the pixel electrode 26 and the common electrode 31. The liquid crystal molecules are aligned substantially perpendicular to the substrate surface.

  A plurality of protrusions 33 are provided at predetermined positions, and have, for example, a conical shape. The protrusion 33 controls the direction in which the liquid crystal molecules are tilted. Due to the plurality of protrusions 33, the liquid crystal layer 23 is composed of a plurality of regions having different directions in which liquid crystal molecules are inclined, and the liquid crystal display panel 10 is applied with a so-called multi-domain method. Viewing angle dependency can be improved by using the multi-domain method.

  A phase difference plate 34, a polarizing plate 35, a diffusion adhesive material (diffusion sheet) 36, and a reflective polarizing plate (brightness enhancement film) 37 are provided on the backlight 11 side of the TFT substrate 21. On the display surface side of the CF substrate 22, a retardation plate 38, a diffusion adhesive material (diffusion sheet) 39, and a polarizing plate 40 are provided. The diffusion adhesive materials 36 and 39 have a function of making transmitted light and reflected light uniform by diffusing (scattering) light, and a function of reducing interference (rainbow color) of reflected light. The diffusion adhesive 39 may be disposed between the CF substrate 22 and the phase difference plate 38.

  The polarizing plates 35 and 40 extract light having a vibrating surface in one direction parallel to the transmission axis, that is, light having a polarization state of linearly polarized light, from light having a vibrating surface in a random direction. The polarizing plates 35 and 40 are arranged so that the absorption axis and the transmission axis are orthogonal to each other in the plane.

  The phase difference plates 34 and 38 have refractive index anisotropy, and are arranged so that the slow axis and the fast axis are orthogonal to each other in the plane. The phase difference plates 34 and 38 provide a predetermined retardation (a phase difference of λ / 4 when the wavelength of light transmitted through λ is used) between light of a predetermined wavelength transmitted through the slow axis and the fast axis. Has the function to give. That is, the phase difference plates 34 and 38 are composed of λ / 4 plates. The slow axes of the phase plates 34 and 38 are set to 45 ° with respect to the transmission axes of the polarizing plates 35 and 40, respectively.

  The reflective polarizing plate 37 is a film (for example, DBEF manufactured by Sumitomo 3M Co., Ltd.) having a function of improving the light utilization efficiency by the recycling effect and improving the luminance of the light passing through the reflective polarizing plate 37. The reflective polarizing plate 37 is disposed so that the reflection axis and the transmission axis are orthogonal to each other in the plane, and the transmission axis is set to be parallel to the transmission axis of the polarizing plate 35.

[2. Specific example of liquid crystal display panel 10]
Next, a more specific configuration example of the liquid crystal display panel 10 will be described. FIG. 2 is a layout diagram of the liquid crystal display panel 10. The layout diagram of FIG. 2 mainly shows the configuration of the TFT substrate 21. FIG. 3 is a cross-sectional view of the liquid crystal display panel 10 taken along the line AA ′ shown in FIG. FIG. 4 is a cross-sectional view of the liquid crystal display panel 10 taken along the line BB ′ shown in FIG.

  A scanning line GL extending in the X direction is provided on the TFT substrate 21. The scanning line GL functions as a gate electrode of the TFT 24. An insulating film 41 is provided on the TFT substrate 21 so as to cover the gate electrode GL. The insulating film 41 on the gate electrode GL functions as a gate insulating film of the TFT 24.

  A semiconductor layer 42 is provided above the gate electrode GL and on the insulating film 41. The semiconductor layer 42 is made of, for example, amorphous silicon or polysilicon. A source electrode 43 and a drain electrode 44 are provided on both sides of the gate electrode GL and on the insulating film 41. The source electrode 43 and the drain electrode 44 are in partial contact with the semiconductor layer 42. The TFT 24 includes a gate electrode GL, a gate insulating film 41, a source electrode 43 and a drain electrode 44.

  A signal line SL extending in the Y direction is provided on the insulating film 41. The signal line SL is electrically connected to the source electrode 43. The signal line SL is disposed below the black matrix 30-BK.

  An insulating film 45 is provided on the TFT 24 and the signal line SL. A reflective film 25 is provided on the insulating film 45 so as to cover the TFT 24. The reflective film 25 forms a reflective region 12 </ b> R in the pixel 12. A storage capacitor line 48 extending in the X direction is provided on the insulating film 45. The storage capacitor line 48 constitutes a storage capacitor Cs provided for each pixel 12.

  An insulating film 46 is provided on the reflective film 25 and the storage capacitor line 48. A pixel electrode 26 is provided on the insulating film 46. The pixel electrode 26 is electrically connected to the drain electrode 44 by a contact plug 47. In the configuration example of FIG. 2, the drain electrode 44 includes a first electrode that partially contacts the semiconductor layer 42 and a second electrode that extends from the first electrode to below the contact plug 47. Composed.

  The layout shown in FIG. 2 employs a two-divided pixel structure in which one pixel 12 is divided into two domains. The area of one pixel 12 corresponds to the area of one pixel electrode 26 extending in the Y direction. The pixel electrode 26 has a slit at the center, and two domains constituting one pixel 12 are arranged on both sides with the slit as a boundary. The protrusion 33 that controls the alignment of the liquid crystal molecules is provided for each domain. Reflective films 25-1 and 25-2 are provided at both ends of the pixel 12 in the Y direction. The region where the pixel electrode 26 and the reflective film 25-1 overlap is the first reflective region 12R-1, and the region where the pixel electrode 26 and the reflective film 25-2 overlap is the second reflective region 12R-2. A region including the reflection regions 12R-1 and 12R-2 is a reflection region 12R of one pixel 12. A region of the pixel electrode 26 that does not overlap with the reflective films 25-1 and 25-2 is a transmissive region 12T. Therefore, in this embodiment, the total area of the reflection regions 12R-1 and 12R-2 is set to 30% or less of the area of the pixel 12 (area of the pixel electrode 26).

  The insulating films 41, 45, and 46 are made of a transparent insulating material, and for example, a silicon nitride film is used. The contact plug 47 and the storage capacitor line 48 are made of a transparent conductive material, and for example, ITO is used. As the reflective film 25, for example, aluminum (Al) is used. The source electrode 43, the drain electrode 44, the scanning line GL, and the signal line SL are made of, for example, aluminum (Al), molybdenum (Mo), chromium (Cr), tungsten (W), or one or more of these. Including alloys are used.

[3. Circuit configuration of liquid crystal display device]
Next, a configuration example of the liquid crystal display device 50 including the liquid crystal display panel 10 configured as described above will be described. In the present embodiment, an active matrix liquid crystal display device 50 will be described as an example.

  FIG. 5 is a block diagram of the liquid crystal display device 50. The liquid crystal display device 50 includes a liquid crystal display panel 10, a scanning driver (scanning line driving circuit) 51, a signal driver (signal line driving circuit) 52, a common voltage supply circuit 53, a voltage generation circuit 54, and a control circuit 55. .

  The liquid crystal display panel 10 is provided with a plurality of scanning lines GL each extending in the row direction (X direction) and a plurality of signal lines SL each extending in the column direction (Y direction). A pixel 12 is disposed in each of the intersection regions of the plurality of scanning lines GL and the plurality of signal lines SL. The plurality of pixels 12 are arranged in a matrix.

  FIG. 6 is a circuit diagram of the pixel array. In FIG. 6, four pixels are extracted and shown. The pixel 12 includes a TFT 24, a liquid crystal capacitor Clc, and a storage capacitor Cs.

  The source of the TFT 24 is electrically connected to the signal line SL. The gate of the TFT 24 is electrically connected to the scanning line GL. The drain of the TFT 24 is electrically connected to the pixel electrode 26. The pixel electrode 26 constitutes a liquid crystal capacitor Clc together with the common electrode 31 disposed opposite thereto and the liquid crystal layer 23 filled between the pixel electrode 26 and the common electrode 31.

  The storage capacitor Cs is connected in parallel to the liquid crystal capacitor Clc. The storage capacitor Cs suppresses the potential fluctuation generated in the pixel electrode 26 and holds the pixel voltage applied to the pixel electrode 26 until the next pixel voltage is applied again. The storage capacitor Cs includes a storage electrode 48 disposed opposite to the pixel electrode 26 and an insulating film 46 formed between the pixel electrode 26 and the storage electrode 48. A common voltage Vcom is applied to the common electrode 31 and the storage electrode 48 by the common voltage supply circuit 53.

  In the pixel 12 configured as described above, when the TFT 24 connected to the pixel electrode 26 is turned on, a drive voltage is applied to the pixel electrode via the signal line SL, and a voltage difference between the drive voltage and the common voltage Vcom is obtained. Accordingly, the alignment state of the liquid crystal changes. Thereby, the transmission state of the liquid crystal with respect to the incident light and the reflected light is changed to perform image display.

  The scanning driver 51 is connected to the plurality of scanning lines GL, and sequentially drives the plurality of scanning lines GL based on the vertical control signal Vs from the control circuit 55. The vertical control signal Vs from the control circuit 55 is applied every frame period. A “frame” is a period during which one image is displayed by supplying a display signal to all the pixels of the liquid crystal display panel 10. Each time the scanning driver 51 receives the horizontal control signal Hs from the control circuit 55, the scanning driver 51 outputs a scanning signal for turning on the TFTs 24 for one row (one scanning line) from the gate off level Vgl to the gate on level. Switch to Vgh. The horizontal control signal Hs is applied every horizontal period which is a period for supplying a driving voltage to the pixels 12 for one row. In synchronization with the horizontal control signal Hs, the scanning driver 51 sequentially applies the gate-on level Vgh to the scanning lines GL. At this time, the drive voltage applied to the signal line SL is applied to the pixel electrode 26 through the TFT 24 that is turned on.

  The signal driver 52 is connected to the plurality of signal lines SL, and takes in the gradation data D2 for one horizontal period based on the horizontal control signal Hs from the control circuit 55. Then, the signal driver 52 generates a drive voltage corresponding to the gradation data D2, and applies this drive voltage to the signal line SL.

  Generally, in a liquid crystal display device, inversion driving (AC driving) is performed to invert the polarity of an electric field between a pixel electrode filled with liquid crystal and a common electrode at a predetermined period. In the liquid crystal display panel, as described above, the alignment of the liquid crystal is determined according to the electric field between the pixel electrode and the common electrode. However, if an electric field having the same polarity is continuously applied between the pixel electrode and the common electrode, burn-in occurs. Or cause deterioration or destruction of the liquid crystal. For this reason, the polarity of the electric field between the pixel electrode and the common electrode is periodically reversed to suppress deterioration of the liquid crystal. As the inversion driving method, line inversion driving and frame inversion driving are generally used. Line inversion driving is a method of inverting the polarity of an electric field for each scanning line and also for each frame period. The frame inversion driving is a method for inverting the polarity of the electric field for each pixel every frame period.

  The control circuit 55 generates various control signals such as a horizontal control signal Hs, a vertical control signal Vs, an inverted signal Pol, and a clock CLK, and sends these control signals to the scanning driver 51, the signal driver 52, and the common voltage supply circuit 53. send. The control circuit 55 receives image data D1 from the outside, and generates gradation data D2 based on the image data D1. Then, the gradation data D2 is sent to the signal driver 52.

  Based on the inverted signal Pol from the control circuit 55, the common voltage supply circuit 53 generates a common voltage Vcom whose polarity is inverted every predetermined period (one frame period) and applies it to the display pixels.

  The voltage generation circuit 54 generates signal voltages Vgl and Vgh necessary for generating a scanning signal and supplies them to the scanning driver 51. In addition, the voltage generation circuit 54 generates a signal voltage Vsh necessary for generating a gradation voltage and supplies it to the signal driver 52. In addition, the voltage generation circuit 54 generates a logic power supply and supplies it to the scan driver 51 and the signal driver 52.

  FIG. 7 is a block diagram showing an example of the signal driver 52 shown in FIG. The signal driver 52 includes a shift register 60, a data latch circuit 61, a level shifter 62, a D / A converter (DAC) 63, an output buffer 64, and a gradation voltage generation circuit 65. Note that the number of signal lines SL is m.

  The shift register 60 is configured by connecting m flip-flops in series, and sequentially shifts the horizontal control signal Hs in synchronization with the clock CLK. Then, the shift register 60 sequentially outputs sampling pulses Smp from the m flip-flops.

  The data latch circuit 61 includes m latch circuits corresponding to the m flip-flops of the shift register 60. The data latch circuit 61 holds the gradation data D2 serially input from the control circuit 55 in synchronization with the sampling pulse Smp. Thus, the gradation data D2 corresponding to m pixels corresponding to one horizontal period is taken into the data latch circuit 61.

  The level shifter 62 receives the gradation data D2 fetched by the data latch circuit 61, and shifts the voltage level of the gradation data D2 so that it becomes an appropriate input signal in the subsequent DAC 63.

  The gradation voltage generation circuit 65 generates a plurality of types of gradation voltages corresponding to a plurality of gradation levels using the power supply voltage Vsh supplied from the voltage generation circuit 54, and supplies these gradation voltages to the DAC 63. .

  The DAC 63 includes m DACs 63-1 to 63-m corresponding to the m latch circuits of the data latch circuit 61. The DAC 63 selects a gradation voltage corresponding to the gradation data D2 sent from the level shifter 62 from among a plurality of kinds of gradation voltages generated by the gradation voltage generation circuit 65. In this embodiment, since the inversion driving method is used, the gradation voltage generation circuit 65 generates a positive gradation voltage and a negative gradation voltage. The DAC 63 selects either a positive gradation voltage or a negative gradation voltage according to the polarity signal Pol. The positive polarity and the negative polarity of the gradation voltage are based on the common voltage Vcom. The gradation voltage selected by the DAC 63 is applied to the signal lines SL1 to SLm via the output buffers 64-1 to 64-m.

[4. Operation]
In flat-gap transflective panels, when viewing an image display in an environment with high illuminance where the reflective display is relatively stronger than the transmissive display, the optimal voltage for the transmissive display is maintained without changing the drive voltage. In addition, the yellowishness of the reflective display and black-and-white reversal (white display is displayed darkly) become strong, and the visibility is greatly reduced. As a countermeasure, it is conceivable to reduce the yellowness and black-and-white reversal of the reflective display by lowering the drive voltage. However, the brightness of the transmissive display is lowered by reducing the drive voltage. Further, in order to automatically change the drive voltage, a sensor for monitoring the illuminance of the surrounding environment is required.

  Therefore, in this embodiment, as described above, the cell gap between the reflective region and the transmissive region is made the same, and the reflective area ratio, which is the area ratio occupied by the reflective region in the pixel (display portion), is 30% or less. Set. Further, the drive voltage is set to the optimum voltage for transmissive display. In this way, the reflective display and the transmissive display are optimized without changing the drive voltage according to the surrounding environment.

  First, a method for setting a gradation voltage applied to the liquid crystal display panel 10 will be described. FIG. 8 is a graph showing V (voltage) -T (relative luminance) curves for the reflective display and the transmissive display of the liquid crystal display panel 10. The horizontal axis in FIG. 8 represents voltage (V), and the vertical axis represents relative luminance with luminance corresponding to the maximum gradation (all gradations) being 100%.

  The relative luminance corresponding to each gradation level is expressed by the following equation (1).

Relative luminance = {gradation level / (all gradations-1)} ^γ (1)
In FIG. 8, for simplification, all gradations are set to 8 (from 0 gradation to 7 gradations). In addition, the gamma value γ = 2.2. However, only the 0th gradation is appropriately set in order to reduce the voltage sweep width.

  The VT curve of the reflective display shown in FIG. 8 is an optimum voltage considering only the reflective display. The VT curve of the transmissive display shown in FIG. 8 is an optimum voltage considering only the transmissive display. The optimum reflective display (or transmissive display) means that smooth gradation display and accurate color expression can be realized, and there is no luminance unevenness caused by gradation skip (gradation reduction) or the like. The reflective display refers to displaying an image using light reflected by the reflective region 12 </ b> R where the reflective film 25 is formed in the pixel 12. The transmissive display means that an image is displayed by transmitting light from the backlight 11 through a transmissive region 12T in the pixel 12 where the reflective film 25 is not formed.

From FIG. 8, the gradation voltages for the reflective display and the transmissive display are determined. The gradation voltage of the reflective display is expressed as Vr ^{(0) to} Vr ^{(7) in} the order of 0 to 7 gradations. The gradation voltage of transmissive display is expressed as Vt ^{(0) to} Vt ^{(7) in} the order of 0 to 7 gradations. The liquid crystal display panel 10 of the present embodiment is, for example, a normally black type. Therefore, Vr ⁽⁰⁾ and Vt ⁽⁰⁾ are the darkest black display, and Vr ⁽⁷⁾ and Vt ⁽⁷⁾ are the most dark. Bright white display.

When the gradation voltage is changed according to the surrounding environment, the gradation voltage V ⁽ⁿ⁾ is expressed by a linear sum of the gradation voltage Vr ^{(n) for} reflective display and the gradation voltage Vt ^{(n) for} transmissive display. The The gradation voltage V ⁽ⁿ⁾ is a voltage applied to the pixel electrode 26 during transflective display. n is an integer of 0-7. The gradation voltage V ⁽ⁿ⁾ is expressed by the following equation (2).
V ⁽ⁿ⁾ = [alpha] .Vr ⁽ⁿ⁾ + (1- [alpha]). Vt ⁽ⁿ⁾
(0 ≦ α ≦ 1) (2)
In the present embodiment, the liquid crystal display panel 10 is operated using a driving voltage optimum for transmissive display regardless of the illuminance of the surrounding environment. That is, “α = 0” is set, and “V ⁽ⁿ⁾ = Vt ⁽ⁿ⁾ ”. As described above, by using the optimum driving voltage for transmissive display, it is not necessary to monitor the illuminance of the surrounding environment, and the level control of the driving voltage is facilitated.

Next, the driving operation of the liquid crystal display device 50 using the driving voltage calculated by the above method will be described. The control circuit 55 stores the gradation voltages V ^{(0) to} V ⁽⁷⁾ (= Vt ^{(0) to} Vt ⁽⁷⁾ ) in a storage circuit (not shown). The control circuit 55 receives image data D1 from a predetermined external video source (CPU or the like).

Subsequently, the control circuit 55 calculates the gradation level of the image data D1. Subsequently, the control circuit 55 calculates a gradation voltage V ⁽ⁿ⁾ corresponding to the gradation level n of the image data D1. Then, the control circuit 55 generates the gradation voltage V ⁽ⁿ⁾ as a digital signal (gradation data) D2, and sends the gradation data D2 to the signal driver 52.

Subsequently, the signal driver 52 takes in the gradation data D2 and generates a gradation voltage V ⁽ⁿ⁾ corresponding to the gradation data D2. Similarly, a gradation voltage for one horizontal line is generated by the control circuit 55 and the signal driver 52. Then, the signal driver 52 applies the gradation voltage for one horizontal line to the signal line SL. Thereby, the gradation voltage is supplied to the pixel electrode 26 from the signal line SL via the TFT 24. The liquid crystal display panel 10 can obtain optimum luminance by such pixel driving operation.

  FIG. 9 is a schematic diagram for explaining the polarization of the liquid crystal display panel 10. The arrows of the polarizing plates 35 and 40 represent the transmission axis, and the lines of the retardation plates (λ / 4 plates) 34 and 38 represent the slow axis.

  Since the liquid crystal layer 23 is in the VA mode, in the off state (the potential difference between the common electrode 31 and the pixel electrode 26 is almost zero), the major axis of the liquid crystal molecules is oriented in the vertical direction, while the on state (the common electrode 31). Since the liquid crystal layer 23 is a multi-domain type, the major axis of the liquid crystal molecules tilts in any direction by an angle θ (0 <θ <90 ° with respect to the horizontal direction). . When the phase difference of the liquid crystal layer 23 in the ON state is Re_L, “λ / 4 <Re_L <λ / 2”.

  In the reflection region 12 </ b> R in the ON state, external light (linearly polarized light) transmitted through the polarizing plate 40 becomes circularly polarized light by the λ / 4 plate 38 and becomes elliptically polarized light by the liquid crystal layer 23. Subsequently, the elliptically polarized light reflected by the reflective film 25 is given a phase difference Re_L by the liquid crystal layer 23 and a phase difference λ / 4 by the λ / 4 plate 38, and then passes through the polarizing plate 40 to display white. . Since the light incident on the polarizing plate 40 from the λ / 4 plate 38 is not completely linearly polarized light, the light intensity transmitted through the polarizing plate 40 is somewhat reduced.

  Even in the transmissive region 12T, there is reflected display light in which external light passes through the panel and is reflected on the backlight. In the transmission region 12T in the ON state, the external light (linearly polarized light) transmitted through the polarizing plate 40 becomes circularly polarized light by the λ / 4 plate 38 and becomes elliptically polarized light by the liquid crystal layer 23. Subsequently, the elliptically polarized light transmitted through the liquid crystal layer 23 is transmitted through the polarizing plate 35 after being given a phase difference λ / 4 by the λ / 4 plate 34. Subsequently, the linearly polarized light reflected by the backlight 11 passes through the polarizing plate 35, becomes circularly polarized light by the λ / 4 plate 34, and becomes elliptically polarized light by the liquid crystal layer 23. Subsequently, the elliptically polarized light that has passed through the liquid crystal layer 23 is given a phase difference λ / 4 by the λ / 4 plate 38 and then passes through the polarizing plate 40 to display white. Since the light incident on the polarizing plate 40 from the λ / 4 plate 38 is not completely linearly polarized light, the light intensity transmitted through the polarizing plate 40 is somewhat reduced.

  On the other hand, in the off state, the phase difference of the liquid crystal layer 23 is substantially zero. Therefore, in the reflective region 12R, the light reflected by the reflective film 25 is absorbed by the polarizing plate 40 and becomes black. In the transmissive region 12T, the light reflected by the backlight 11 is absorbed by the polarizing plate 40 and becomes black.

Next, the validity of setting the reflection area ratio to 30% or less will be described. As shown in FIG. 9, the reflectance of light reflected by the reflective film 25 (the reflectance of light reflected by the reflective region 12R) is R ₁ , and the reflectance of light reflected by the backlight 11 (transmits through the transmissive region 12T). the reflectance of light) is expressed as _{R 2.} Further, the reflectance of the entire pixel 12 including the reflective region 12R and the transmissive region 12T is represented by I. The reflectance is the ratio of the light intensity reflected by the reflective film 25 (or the backlight 11) and emitted from the polarizing plate 40 when the light intensity incident on the polarizing plate 40 is 1. R ₁ , R ₂ , and I used in the simulation are represented by the following formulas (3) to (5), respectively.

R ₁ (λ) = cos ² (πRe / λ) cos ² (2πΔn ^* d / λ) + sin ² (2πΔn ^* d / λ) {1- (1/2) cos ² (2πRe / λ)} (3)
R ₂ (λ) = sin ⁴ (πΔn ^* d / λ) {1-cos ² (2πRe / λ) + (3/8) cos ⁴ (2πRe / λ)} (4)
I (λ) = γR ₁ (λ) + (1−γ) R ₂ (λ) (5)
γ: reflection area ratio = reflection area / (reflection area + transmission area) area ratio Re: retardation value of λ / 4 plate (= 135 nm)
Δn ^* : Apparent refractive index (birefringence) of VA liquid crystal
d: thickness of liquid crystal layer λ: wavelength Apparent refractive index (birefringence) Δn ^* is a refraction considering the angle dependence when the major axis of liquid crystal molecules is inclined by an angle θ with respect to the horizontal direction. In this embodiment, this corresponds to the refractive index viewed from the traveling direction of light. The apparent refractive index Δn ^* is expressed by the following formula (6).

n _o : refractive index in the minor axis direction of liquid crystal molecules n _e : refractive index in the major axis direction of liquid crystal molecules FIG. 10 is a graph showing the relationship between the reflection area ratio and the change in chromaticity of white. FIG. 11 is an enlarged view of a portion where the change in white chromaticity in FIG. 10 is plotted. FIGS. 10 and 11 show on the xy chromaticity diagram the results of simulating the change in chromaticity of white when the reflection area ratio is changed from 0% to 100% in increments of 5% in the on state (white display). ing. The crosses in FIG. 10 represent the white coordinates (0.310, 0.316) of the C light source. In the simulation, the color filter is not taken into consideration.

  In FIG. 11, a white display closer to the white coordinate of the C light source is a good white display. In FIG. 11, the allowable range (allowable range 1 to 3) of the reflection area ratio for realizing good white display is indicated by a circle centered on the white coordinate of the C light source. As the reflection area ratio increases, yellow and green that the user dislikes are visually recognized.

  Therefore, when a range of chromaticity coordinates that is close to the white coordinate of the C light source and does not go in the yellow direction is selected from the locus on the chromaticity diagram using the reflection area ratio as a parameter, the reflection area ratio is less than 0%. Largely set to 30% or less. By setting the reflection area ratio to be larger than 0% and not more than 30%, it is possible to display good white without yellow and green.

  Further, it is desirable that the reflective film 25 completely covers the TFT 24. In this case, the lower limit of the reflection area ratio corresponds to the proportion of the area of the TFT 24 in the pixel area. The pixel area and the area of the TFT 24 change according to the size of the liquid crystal display panel 10 and the size of the TFT 24. For this reason, the lower limit of the reflection area ratio is optimally designed according to the size of the liquid crystal display panel 10 and the size of the TFT 24.

[5. effect]
As described above in detail, in this embodiment, in the transflective liquid crystal display panel 10, the reflection display and the transmission display are optimized without changing the cell gap between the reflection area and the transmission area of the pixel (flat gap). ing. As a result, the alignment defect at the boundary between the reflective region and the transmissive region, which has occurred in the conventional multi-gap transflective panel, does not occur, and light shielding is not required to prevent light leakage due to this, and the aperture ratio is reduced. Can be suppressed. In addition, since it is not necessary to form a transparent step film or the like for changing the cell gap between the reflective region and the transmissive region as in the prior art, the manufacturing process can be reduced, and the manufacturing cost can be reduced.

  Conventionally, since the transparent step film is formed in the reflection region, there is a restriction that the reflection region is collected and arranged in a part. However, in the present embodiment, since the flat gap structure is used, there is no restriction on the position and shape of the reflection region in the pixel. As a result, the manufacturing process can be simplified and the manufacturing cost can be reduced.

  Also, good white display is realized by controlling the reflection area ratio to 30% or less. Therefore, it is not necessary to optimize the driving voltage depending on the illuminance of the surrounding environment, and the liquid crystal display panel 10 can be driven with the driving voltage optimum for transmissive display. Furthermore, even when the optimum driving voltage is used for transmissive display, yellowness and black-and-white reversal of reflective display can be reduced and the luminance of transmissive display can be prevented from decreasing. At the same time, since a sensor or the like for monitoring the illuminance of the surrounding environment is not required, the configuration of the liquid crystal display panel 10 and the control of the drive voltage can be simplified.

  Further, by disposing the reflective region on the TFT 24 that cannot be used as a display, it is possible to perform reflective display and transmissive display without reducing the aperture ratio of the transmissive region, and the reflective film 25 also serves as a light shield for the TFT 24. Therefore, malfunctions of the TFT 24 due to light being applied to the TFT 24 can be reduced. Furthermore, a decrease in the aperture ratio can be suppressed as compared with the case where the TFT 24 is shielded from light on the color filter side.

  In the present embodiment, the VA mode and normally black liquid crystal display panel 10 is realized. Thereby, the contrast in the display image of the liquid crystal display panel 10 can be improved.

  The front polarizing plate 40 is provided with a diffusion adhesive material 39 in order to add a light scattering function. Thereby, when viewing the display in an environment with high illuminance such as outdoors, interference of reflection on the back surface (color filter 30 side) of the CF substrate 22 can be suppressed. The back polarizing plate 35 is provided with a diffusion adhesive material 36 and a reflective polarizing plate 37 on the back surface (backlight 11 side) of the polarizing plate 35. As a result, the luminance of the transmissive display increases and the intensity (reflectance) of the reflected light from the backlight 11 increases.

  The liquid crystal display panel 10 of the present embodiment can be applied to various electronic devices having an image display function, such as a mobile phone, a portable information terminal, a digital video camera, a digital still camera, and a scanner. Applicable to.

  The present invention is not limited to the above embodiment, and can be embodied by modifying the constituent elements without departing from the scope of the invention. Further, the above embodiments include inventions at various stages, and are obtained by appropriately combining a plurality of constituent elements disclosed in one embodiment or by appropriately combining constituent elements disclosed in different embodiments. Various inventions can be configured. For example, even if some constituent elements are deleted from all the constituent elements disclosed in the embodiments, the problems to be solved by the invention can be solved and the effects of the invention can be obtained. Embodiments made can be extracted as inventions.

  GL ... scanning line, SL ... signal line, 10 ... liquid crystal display panel, 11 ... back light, 12 ... pixel, 12R ... reflection region, 12T ... transmission region, 21 ... TFT substrate, 22 ... color filter substrate, 23 ... liquid crystal layer 24 ... TFT, 25 ... reflective film, 26 ... pixel electrode, 27, 32 ... alignment film, 28 ... spacer, 30 ... color filter, 31 ... common electrode, 33 ... projection, 34, 38 ... retardation plate, 35, DESCRIPTION OF SYMBOLS 40 ... Polarizing plate, 36, 39 ... Diffusion adhesive material, 37 ... Reflective polarizing plate, 41, 45, 46 ... Insulating film, 42 ... Semiconductor layer, 43 ... Source electrode, 44 ... Drain electrode, 47 ... Contact plug, 48 ... Storage capacitor line 50 ... Liquid crystal display device 51 ... Scan driver 52 ... Signal driver 53 ... Common voltage supply circuit 54 ... Voltage generation circuit 55 ... Control circuit 60 ... Shift register 61 ... Data Latch circuit, 62 ... a level shifter, 63 ... D / A converter, 64 ... output buffer, 65 ... gradation voltage generating circuit.

Claims (6)

First and second substrates disposed opposite to each other;
A liquid crystal layer sandwiched between the first and second substrates and having vertical alignment (VA) when no electric field is applied;
A pixel having a reflective region and a transmissive region, the cell gap of the reflective region and the transmissive region being the same;
A pixel electrode provided in a pixel region of the first substrate;
A common electrode provided on the second substrate;
A reflective film provided on the first substrate and the reflective region;
Comprising
The area of the reflective film is 30% or less of the area of the pixel electrode. A drive circuit for applying a drive voltage to the pixel electrode;
The liquid crystal display device according to claim 1, wherein the driving voltage is set in accordance with gradation display of the transmissive region. A transistor provided on the first substrate and transferring a driving voltage to the pixel electrode;
The liquid crystal display device according to claim 1, wherein the reflective film is disposed so as to cover the transistor. First and second polarizing plates provided so as to sandwich the opposed first and second substrates;
A diffusion layer provided between the second substrate and the second polarizing plate and diffusing light;
The liquid crystal display device according to claim 1, further comprising: First and second polarizing plates provided so as to sandwich the opposed first and second substrates;
A reflective polarizing plate provided on the opposite side of the first polarizing plate from the first substrate;
A diffusion layer provided between the first polarizing plate and the reflective polarizing plate for diffusing light;
The liquid crystal display device according to claim 1, further comprising: A first retardation plate provided between the first substrate and the first polarizing plate;
A second retardation plate provided between the second substrate and the second polarizing plate;
Further comprising
6. The liquid crystal display device according to claim 4, wherein a phase difference between the first and second retardation plates is λ / 4.

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