JP2010060725A - Liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal display device which enables transmittance to be improved without increasing the cost nor power consumption. <P>SOLUTION: The liquid crystal display device includes a liquid crystal display panel LPN having such a configuration that a liquid crystal layer LQ is held between an array substrate AR and an opposite substrate CT, and the array substrate AR includes a first electrode E1, a second electrode E2 which is disposed so as to face the first electrode with an insulating layer IL therebetween and has a slit SL formed so as to face the first electrode, and a first alignment film AL1 which covers the second electrode and is in contact with the liquid crystal layer, and the opposite substrate CT includes a second alignment film AL2 which has a film thickness thicker than the first alignment film and is in contact with the liquid crystal layer. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having a structure in which a pair of electrodes is provided on one substrate constituting the liquid crystal display device.

  In recent years, flat display devices have been actively developed, and among them, liquid crystal display devices have been applied to various fields by taking advantage of features such as light weight, thinness, and low power consumption. Such a liquid crystal display device has a configuration in which a liquid crystal layer is held between a pair of substrates, and displays an image by controlling a modulation rate for light passing through the liquid crystal layer by an electric field between a pixel electrode and a common electrode. Is.

  In such a liquid crystal display device, a structure using a lateral electric field (including a fringe electric field) is particularly attracting attention from the viewpoint of wide viewing angle.

2. Description of the Related Art A horizontal electric field mode liquid crystal display device such as an In-Plane Switching (IPS) mode or a Ringe Field Switching (FFS) mode includes a pixel electrode and a common electrode formed on an array substrate. The liquid crystal molecules are switched by a substantially parallel lateral electric field (for example, see Patent Document 1).
JP 2001-290168 A

  An object of the present invention is to provide a liquid crystal display device capable of improving the transmittance without causing an increase in cost and power consumption.

A liquid crystal display device according to an aspect of the present invention includes:
A liquid crystal display panel having a configuration in which a liquid crystal layer is held between a first substrate and a second substrate;
The first substrate is
A first electrode;
A second electrode that is disposed to face the first electrode with an insulating layer interposed therebetween, and has a slit formed to face the first electrode;
A first alignment film that covers the second electrode and is in contact with the liquid crystal layer,
The second substrate includes a second alignment film having a thickness larger than that of the first alignment film and in contact with the liquid crystal layer.

  According to the present invention, it is possible to provide a liquid crystal display device capable of improving the transmittance without causing an increase in cost and power consumption.

  A liquid crystal display device according to an embodiment of the present invention will be described below with reference to the drawings. Here, a liquid crystal mode in which a first electrode and a second electrode are provided on one substrate, and liquid crystal molecules are switched mainly using a lateral electric field (an electric field substantially parallel to the main surface of the substrate) formed therebetween. As an example, an FFS mode liquid crystal display device will be described.

  As shown in FIGS. 1 to 3, the liquid crystal display device is an active matrix type liquid crystal display device and includes a transmissive liquid crystal display panel LPN. The liquid crystal display panel LPN includes an array substrate (first substrate) AR, a counter substrate (second substrate) CT arranged to face the array substrate AR, and between the array substrate AR and the counter substrate CT. And a held liquid crystal layer LQ. Such a liquid crystal display panel LPN includes a display area DSP for displaying an image. This display area DSP is composed of a plurality of pixels PX arranged in an m × n matrix (where m and n are positive integers).

  The array substrate AR is formed using an insulating substrate 20 having optical transparency such as a glass plate or a quartz plate. In the display area DSP, the array substrate AR extends in the n gate lines Y (Y1 to Yn) extending along the row direction H of each pixel PX and along the column direction V of each pixel PX. The m source lines X (X1 to Xm) and m × n switching elements W arranged in a region including the intersection of the gate line Y and the source line X in each pixel PX are provided. Such gate lines Y and source lines X are formed of a conductive material such as molybdenum, aluminum, or tungsten.

  The n gate lines Y are drawn out of the display area and connected to the gate driver YD. The m source lines X are drawn out of the display area and connected to the source driver XD. The gate driver YD sequentially supplies scanning signals (drive signals) to the n gate lines Y based on control by the controller CNT. Further, the source driver XD supplies a video signal (drive signal) to the m source lines X at a timing when the switching elements W in each row are turned on by the scanning signal based on the control by the controller CNT.

  The array substrate AR further includes a pair of electrodes, that is, a first electrode E1 and a second electrode E2 arranged so as to face each other with the insulating layer IL interposed therebetween. The second electrode E2 is disposed closer to the liquid crystal layer LQ than the first electrode E1. The second electrode E2 is formed with a slit SL facing the first electrode E1.

  In the example shown here, the first electrode E1 corresponds to a common electrode having a common potential common to the plurality of pixels PX, and the second electrode E2 is m × n pixels arranged in each pixel PX. It corresponds to an electrode. In such a configuration example, the second electrode E2 of each row is set to a pixel potential corresponding to the video signal supplied via the corresponding switching element W with respect to the potential of the first electrode E1. Needless to say, the present invention can be applied to a configuration example in which the first electrode E1 corresponds to a pixel electrode and the second electrode E2 corresponds to a common electrode.

  The structure of the array substrate AR will be described in more detail. Each switching element W is configured by, for example, a thin film transistor (TFT). The semiconductor layer of the switching element W can be formed of, for example, polysilicon or amorphous silicon. The gate electrode WG of the switching element W is connected to the gate line Y (or formed integrally with the gate line Y). The source electrode WS of the switching element W is connected to the source line X (or formed integrally with the source line X) and is in contact with the source region of the semiconductor layer. The drain electrode WD of the switching element W is connected to the second electrode E2 (or formed integrally with the second electrode E2) and is in contact with the drain region of the semiconductor layer.

  For example, the first electrode E1 is arranged in an island shape in each pixel PX, and is electrically connected to a common wiring COM having a common potential. The first electrode E1 is made of a light-transmitting conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO). Such a first electrode E1 is covered with an insulating layer IL. The insulating layer IL is formed of an inorganic material such as silicon nitride (SiN).

  The second electrode E2 is formed in, for example, an island shape corresponding to the pixel shape in each pixel PX, for example, a substantially square shape, and is disposed on the insulating layer IL so as to face the first electrode E1. The second electrode E2 is electrically connected to the switching element W of each pixel PX. The second electrode E2 is provided with a plurality of slits SL facing the first electrode E1. The slit SL is formed in, for example, a rectangular shape or an oval shape. The slit SL is formed so that its long axis L is parallel to the column direction V, for example. Such a plurality of slits SL are arranged in the row direction H. Similar to the first electrode E1, the second electrode E2 is formed of a light-transmitting conductive material such as ITO or IZO.

  Such a second electrode E2 is covered with the first alignment film AL1. In other words, the surface in contact with the liquid crystal layer LQ of the array substrate AR is covered with the first alignment film AL1. The first alignment film AL1 is made of polyimide, for example.

  On the other hand, the counter substrate CT is formed using an insulating substrate 30 having optical transparency such as a glass plate or a quartz plate. In particular, in the color display type liquid crystal display device, as shown in FIG. 3, the counter substrate CT is a black matrix that partitions each pixel PX on the inner surface of the insulating substrate 30 (that is, the side facing the liquid crystal layer LQ). 32, a color filter layer 34 disposed in each pixel PX surrounded by the black matrix 32, and the like. Further, the counter substrate CT may further include an overcoat layer arranged with a relatively thick film thickness so as to flatten the unevenness of the surface of the color filter layer 34.

  The black matrix 32 is arranged on the insulating substrate 30 so as to face the gate lines Y and the source lines X provided on the array substrate AR, and further the wiring portions such as the switching elements W. The black matrix 32 is made of, for example, a colored resin colored black.

  The color filter layer 34 is disposed on the insulating substrate 30 and is formed of colored resins that are respectively colored in a plurality of different colors, for example, three primary colors such as red, blue, and green. The red colored resin, the blue colored resin, and the green colored resin are disposed corresponding to the red pixel, the blue pixel, and the green pixel, respectively.

  Such a color filter layer 34 is covered with the second alignment film AL2. In other words, the surface in contact with the liquid crystal layer LQ of the counter substrate CT is covered with the second alignment film AL2. The second alignment film AL2 is made of polyimide, for example.

  The array substrate AR and the counter substrate CT as described above are arranged so that the first alignment film AL1 and the second alignment film AL2 face each other. At this time, a spacer (not shown) (for example, a columnar spacer formed integrally with one substrate by a resin material) is disposed between the array substrate AR and the counter substrate CT, thereby forming a predetermined gap. Is done. The array substrate AR and the counter substrate CT are bonded together with a sealing material in a state where a predetermined gap is formed.

  The liquid crystal layer LQ has a positive dielectric anisotropy including liquid crystal molecules LM sealed in a gap formed between the first alignment film AL1 of the array substrate AR and the second alignment film AL2 of the counter substrate CT. This liquid crystal composition is used.

  The first alignment film AL1 and the second alignment film AL2 are rubbed so as to regulate the alignment of the liquid crystal molecules LM contained in the liquid crystal layer LQ. The liquid crystal molecules LM are homogeneously aligned by the regulation force by the first alignment film AL1 and the second alignment film AL2. The rubbing direction S of the first alignment film AL1 and the second alignment film AL2 is a non-parallel and non-perpendicular direction with respect to the major axis L of the slit SL.

  Such a liquid crystal display device further includes an illumination unit, that is, a backlight unit BL disposed on the array substrate AR side with respect to the liquid crystal display panel LPN. The backlight unit BL illuminates the liquid crystal display panel LPN from the array substrate AR side. As such a backlight, various forms are applicable, and any of those using a light emitting diode (LED) or a cold cathode tube (CCFL) as a light source can be applied. A description of the detailed structure is omitted.

  The liquid crystal display device also includes an optical element OD1 provided on one outer surface of the liquid crystal display panel LPN (ie, the surface opposite to the surface in contact with the liquid crystal layer LQ of the array substrate AR), and the liquid crystal display panel LPN. The optical element OD2 is provided on the other outer surface (that is, the surface opposite to the surface in contact with the liquid crystal layer LQ of the counter substrate CT). Each of these optical elements OD1 and OD2 includes a polarizing plate, and for example, no potential difference is formed between the first electrode E1 and the second electrode E2 (that is, between the first electrode E1 and the second electrode E2). In the absence of an electric field), a normally black mode in which the transmittance of the liquid crystal display panel LPN is lowest (that is, a black screen is displayed) is realized.

  That is, in such a liquid crystal display device, when there is no electric field, the liquid crystal molecules LM are aligned such that the major axis D is oriented in a direction parallel to the rubbing direction S of the first alignment film AL1 and the second alignment film AL2. ing. In such a state, the backlight light from the backlight unit BL passes through the optical element OD1, passes through the liquid crystal display panel LPN, and is absorbed by the optical element OD2 (black screen display).

  When a potential difference is formed between the first electrode E1 and the second electrode E2 (that is, when a voltage having a potential different from that of the first electrode E1 is applied to the second electrode E2), the first A lateral electric field (fringe field) is formed between the electrode E1 and the second electrode E2. This transverse electric field is formed in the direction orthogonal to the major axis L through the slit SL.

  At this time, the alignment state of the liquid crystal molecules LM changes so that the major axis D is directed from the rubbing direction S in the direction parallel to the transverse electric field. As described above, when the orientation of the major axis D of the liquid crystal molecule LM changes from the rubbing direction S, the modulation factor for the light transmitted through the liquid crystal layer LQ changes. Therefore, part of the backlight light emitted from the backlight unit BL and transmitted through the liquid crystal display panel LPN is transmitted through the second optical element OD2 (white screen display). In this way, the backlight is selectively transmitted through the liquid crystal display panel LPN to display an image.

  By the way, the display mode mainly using the transverse electric field as in the present embodiment can widen the viewing angle. However, the display mode such as the twisted nematic (TN) mode and the electric field controlled birefringence (ECB) mode can be used. The transmittance is low compared. For this reason, in order to obtain the transmittance (or luminance) equivalent to the TN mode in the present situation, it is necessary to devise on the backlight unit side, for example, increase the number of light sources such as LEDs or increase the voltage to the light source. For example, measures such as applying However, such a measure causes an increase in cost and an increase in power consumption.

  Therefore, in this embodiment, in order to improve the transmittance of the liquid crystal display panel LPN, the film thickness of the alignment film is optimized, and the film thickness of the second alignment film AL2 on the counter substrate CT side is set to the array substrate AR side. The first alignment film AL1 is thicker than the first alignment film AL1.

  That is, when a potential difference is formed between the first electrode E1 and the second electrode E2, the voltage is distributed according to the respective capacitances C of the liquid crystal layer LQ that is a dielectric and the first alignment film AL1. For this reason, the proportion of the voltage distributed to the first alignment film AL1 increases as the thickness of the first alignment film AL1 increases. In other words, the ratio of the voltage applied to the liquid crystal layer LQ decreases.

  For this reason, when a predetermined voltage is applied to the liquid crystal layer LQ, the higher the film thickness of the first alignment film AL1, the higher the driving voltage is required. In other words, in the case of a predetermined drive voltage, the voltage applied to the liquid crystal layer LQ decreases as the film thickness of the first alignment film AL1 increases, so that a desired alignment distribution cannot be obtained.

  As in the example shown in FIG. 4, in the normally black mode, the drive voltage (peak voltage) for obtaining the maximum transmittance of the liquid crystal display panel LPN tends to increase as the thickness of the first alignment film AL1 increases. It is in.

  Further, when the first alignment film AL1 is thick, an electric field spreads in the first alignment film AL1, and the electric field component contributing to the modulation is wasted mainly in the display mode using the horizontal electric field. End up. For this reason, even if the drive voltage is increased, the maximum transmittance of the liquid crystal display panel LPN is lowered.

  As in the example shown in FIG. 5, the maximum transmittance of the liquid crystal display panel LPN tends to decrease as the thickness of the first alignment film AL1 increases. As a result, the luminance is reduced.

  Based on such a verification result, when driving the liquid crystal display panel LPN, the thickness of the first alignment film AL1 that is affected by the electric field between the pair of electrodes provided on the array substrate AR side is normally applied. It is desirable to make it thinner than the thickness (for example, about 70 nm). Thereby, while being able to obtain a high transmittance, the peak voltage for obtaining the maximum transmittance can be reduced.

  The film thickness of the first alignment film AL1 is desirably as thin as possible. However, when the film is excessively thin, the leveling property of the surface of the array substrate AR in contact with the liquid crystal layer LQ is deteriorated. That is, since various wirings and electrodes are arranged on the array substrate AR, a step is easily formed according to their thickness. A large step is likely to cause distortion of the horizontal electric field and orientation failure. Moreover, in the case of a thin alignment film, the alignment regulating force of liquid crystal molecules may be reduced.

  For this reason, the first alignment film AL1 needs to have a film thickness that can sufficiently reduce the step on the surface of the array substrate AR while ensuring the alignment regulating force. According to the inventors' investigation, it was confirmed that the thickness of the first alignment film AL1 is preferably in the range of 30 nm to 50 nm. Thereby, the transmittance can be improved while maintaining the characteristics as the alignment film.

  On the other hand, in the liquid crystal display panel LPN, since the alignment film is sandwiched between layers having different refractive indexes, reflection occurs at the interface. In particular, on the counter substrate CT side, the second alignment film AL2 is located between the liquid crystal layer LQ and the color filter layer. When reflection occurs at these interfaces, interference occurs, but the interference conditions differ depending on the thickness of the second alignment film AL2. At this time, by setting the film thickness of the second alignment film AL2 so as to satisfy an interference condition in which transmitted light transmitted from the liquid crystal display panel LPN is strengthened, it is possible to contribute to improvement of the transmittance.

  Here, in particular, when the dependence of the transmittance of light with a green wavelength (550 nm) having high visibility was measured with respect to the film thickness of the alignment film, the result shown in FIG. 6 was obtained. From this result, it was confirmed that the thicker the alignment film, the more satisfying the interference condition that transmitted light strengthens, and the higher the transmittance.

  Therefore, it is desirable that the film thickness of the second alignment film AL2 provided on the counter substrate CT side is larger than the normally applied film thickness (for example, about 70 nm). Thereby, a high transmittance can be obtained.

  The film thickness of the second alignment film AL2 is desirably as thick as possible. However, if it is excessively thick, the influence of absorption by the alignment film material cannot be ignored, and the transmittance may be reduced. In addition, it is important to determine the thickness of the second alignment film AL2 in consideration of not only the interference condition of light of green wavelength but also the interference condition of light of blue wavelength or red wavelength. According to the inventor's investigation, it was confirmed that the thickness of the second alignment film AL2 is preferably in the range of 100 nm to 130 nm. Thereby, it is possible to improve the transmittance of light of each wavelength while maintaining a good color balance.

  As described above, the transmittance is increased by reducing the thickness of the alignment film for driving, and the transmittance is increased by increasing the thickness of the alignment film for interference conditions. In order to satisfy the contradictory conditions, the second alignment film AL2 on the counter substrate CT side that hardly affects driving is thickened, and the first alignment film AL1 on the array substrate AR side is thinned.

  As described above, in particular, in the horizontal electric field mode liquid crystal display device in which the pair of electrodes necessary for driving are provided on the array substrate AR, the thicknesses of the alignment films of the array substrate AR and the counter substrate CT are independently optimized. As a result, the transmittance can be improved.

  Therefore, according to this embodiment, it is possible to provide a liquid crystal display device capable of improving the transmittance and realizing a good display quality without causing an increase in cost and power consumption.

  In addition, this invention is not limited to the said embodiment itself, In the stage of implementation, it can change and implement a component within the range which does not deviate from the summary. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

FIG. 1 is a diagram schematically showing a configuration of a liquid crystal mode liquid crystal display device using a lateral electric field according to an embodiment of the present invention. FIG. 2 is a plan view schematically showing a configuration of a pixel applied to the liquid crystal display device shown in FIG. FIG. 3 is a cross-sectional view schematically showing the structure of the liquid crystal display device shown in FIG. FIG. 4 is a diagram illustrating an example of the relationship of the peak voltage (relative value) to the thickness of the alignment film. FIG. 5 is a diagram illustrating an example of the relationship of the maximum transmittance with respect to the film thickness of the alignment film. FIG. 6 is a diagram illustrating an example of the relationship of the transmittance with respect to the thickness of the alignment film.

Explanation of symbols

LPN ... Liquid crystal display panel AR ... Array substrate CT ... Counter substrate LQ ... Liquid crystal layer DSP ... Display area PX ... Pixel Y ... Gate line X ... Source line W ... Switching element E1 ... First electrode E2 ... Second electrode SL ... Slit AL1 ... first alignment film AL2 ... second alignment film

Claims (5)

A liquid crystal display panel having a configuration in which a liquid crystal layer is held between a first substrate and a second substrate;
The first substrate is
A first electrode;
A second electrode that is disposed to face the first electrode with an insulating layer interposed therebetween, and has a slit formed to face the first electrode;
A first alignment film that covers the second electrode and is in contact with the liquid crystal layer,
The liquid crystal display device, wherein the second substrate includes a second alignment film having a thickness greater than that of the first alignment film and in contact with the liquid crystal layer.   The liquid crystal display device according to claim 1, wherein the first alignment film has a thickness of 30 to 50 nm.   The liquid crystal display device according to claim 1, wherein the second alignment film has a thickness of 100 to 130 nm.   The liquid crystal display device according to claim 1, wherein the first electrode and the second electrode are formed of a light-transmitting conductive material.   The liquid crystal display device according to claim 1, further comprising an illumination unit that illuminates the liquid crystal display panel from the first substrate side.

Images