JP2013003463A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-quality liquid crystal display device that hardly causes burn-in of a display image.SOLUTION: A liquid crystal display device includes: a pair of substrates at least one of which is transparent; a liquid crystal layer 240 that is disposed between the pair of substrates; a pair of electrode groups that are formed on one substrate 232 of the pair of substrates and applies an electric field to the liquid crystal layer 240; a plurality of active elements that are connected to the pair of electrode groups; and an alignment layer 237 that is formed on the surface contacting with the liquid crystal layer 240. In a region of one pixel, one electrode 233 of the pair of electrode groups is formed in a flat plate shape and the other electrode 236 is formed in a comb teeth shape, an insulating film 270 is disposed between the pair of electrode groups, the alignment layer 237 is disposed on the comb teeth-shaped electrodes, and a specific resistance ρ(Ωcm) and a film thickness d(nm) of the insulating film 270 and a specific resistance ρ(Ωcm) and a film thickness d(nm) of the alignment layer 237 satisfy relational expression: (ρ×ρ)/(ρ×d+ρ×d)≥8×10(Ω).

Description

  The present invention relates to a liquid crystal display device having an alignment film for aligning a liquid crystal layer sandwiched between two substrates.

  Liquid crystal display devices are used in a wide range of applications due to their high display quality, thinness, light weight, low power consumption, and other features such as mobile phone monitors, digital still camera monitors, desktop PC monitors, and printing. It is used in various applications such as monitors for monitors, medical monitors, and LCD TVs. As these applications are expanded, liquid crystal display devices are required to have higher quality adapted to each application.

  Usually, the display of a liquid crystal display device is performed by changing the alignment direction of the liquid crystal molecules by applying an electric field to the liquid crystal molecules of the liquid crystal layer sandwiched between a pair of substrates, and thereby changing the optical characteristics of the liquid crystal layer. Is called. The alignment direction of liquid crystal molecules when no electric field is applied is defined by an alignment film (also referred to as an “alignment control film”) obtained by rubbing the surface of the polyimide thin film. Conventionally, an active drive type liquid crystal display device provided with a switching element such as a thin film transistor (TFT) for each pixel is provided with electrodes on each of a pair of substrates sandwiching the liquid crystal layer, and the direction of the electric field applied to the liquid crystal layer is determined by the substrate surface Is set to be a so-called vertical electric field that is substantially perpendicular to the liquid crystal display, and display is performed using the optical rotation of liquid crystal molecules constituting the liquid crystal layer. As a typical vertical electric field type liquid crystal display device, a twisted nematic (TN) method is known. In a TN liquid crystal display device, a narrow viewing angle is one of the major problems. Therefore, an IPS (In-Plane Switching) method and a VA (Vertically Alignment) method are known as display methods for achieving a wide viewing angle.

  Further, in such a liquid crystal display device, it is known that afterimages and image sticking occur due to charge accumulation during driving. In order to solve this problem, Patent Document 1 describes that the accumulated charges can be easily diffused and eliminated by reducing the alignment film resistance as much as possible.

JP 2007-241249 A

  However, in a small and medium-sized liquid crystal display device that repeatedly turns on and off in a short time compared to a large-sized liquid crystal display device such as a TV, the initial afterimage is reduced by shortening the time for diffusing accumulated charges and eliminating the afterimage. A lower strength is desirable. According to the research of the inventors of the present application, a liquid crystal display device in which liquid crystal is driven by applying an electric field between a planar electrode and a comb-like electrode formed via an interlayer insulating film (protective insulating film) However, it has been found that if the alignment film resistance is lowered, the amount of charge accumulated during driving increases, and the intensity of the afterimage generated initially increases. It was also found that the resistance of the protective insulating film was greatly related to the afterimage strength together with the alignment film resistance, and that the afterimage strength could not be suppressed if these combined resistances were low.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a high-quality liquid crystal display device with less display image burn-in.

The liquid crystal display device of the present invention is formed on a pair of substrates, at least one of which is transparent, a liquid crystal layer disposed between the pair of substrates, and one of the pair of substrates, and applies an electric field to the liquid crystal layer A liquid crystal display device having a pair of electrode groups, a plurality of active elements connected to the pair of electrode groups, and an alignment film formed on a surface in contact with the liquid crystal layer, In the region, one of the pair of electrode groups is formed in a flat plate shape and the other electrode is formed in a comb-tooth shape, and an insulating film is disposed between the pair of electrodes. An alignment film is disposed on the insulating film, and the specific resistance ρ _p (Ωcm) and film thickness d _p (nm) of the insulating film, and the specific resistance ρ _A (Ωcm) and film thickness d _A (nm) of the alignment film are ρ _p · ρ _A ) / (ρ _p · d _A + ρ _A · d _p ) ≧ 8 × 10 ¹⁷ A liquid crystal display device satisfying (Ω).

  Here, the insulating film includes a gate insulating film, a protective insulating film, and the like, and means a PAS film in Examples described later.

In the liquid crystal display device of the present invention, the insulating film has a specific resistance ρ _p (Ωcm) and a film thickness d _p (nm), and a specific resistance ρ _A (Ωcm) and a film thickness d _A (nm) of the alignment film. The relational expression (ρ _p · ρ _A ) / (ρ _p · d _A + ρ _A · d _p ) ≧ 2 × 10 ¹⁸ (Ω) may be satisfied.

In the liquid crystal display device of the present invention, the specific resistance ρ _p of the insulating film may be 1 × 10 ¹⁴ (Ωcm) or more and less than 1 × 10 ¹⁷ (Ωcm), and the specific resistance of the insulating film. the ^{_{ρ p, 5 × 10 15 (}} Ωcm) or more and may be less than 1 × ^{10 17} (Ωcm).

In the liquid crystal display device of the present invention, the specific resistance ρ _A of the alignment film may be 1 × 10 ¹⁴ (Ωcm) or more and less than 1 × 10 ¹⁷ (Ωcm), and further the specific resistance of the alignment film. ρ _A may be 5 × 10 ¹⁵ (Ωcm) or more and less than 1 × 10 ¹⁷ (Ωcm).

In the liquid crystal display device of the present invention, the thickness d _{p of the} insulating film may be 200 (nm) or more and 1000 (nm) or less.

In the liquid crystal display device of the present invention, the thickness d _{A of the} alignment film may be 20 (nm) or more and 200 (nm) or less.

In the liquid crystal display device of the present invention, the specific resistance of the liquid crystal layer may be 1 × 10 ¹⁴ (Ωcm) or more.

  In the liquid crystal display device of the present invention, the alignment film may be provided with liquid crystal alignment ability by irradiating polarized ultraviolet rays.

  In the liquid crystal display device of the present invention, the alignment film may include polyimide having a polyamic acid ester as a precursor.

  According to the present invention, it is possible to provide a high-quality liquid crystal display device with less display image burn-in. Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

It is a figure which shows schematically about the liquid crystal display device which concerns on the Example of this invention. It is a figure which shows roughly the cross section about a certain pixel of the liquid crystal module of FIG. It is a schematic plan view of 1 pixel vicinity of a TFT substrate. It is a table | surface which shows the relationship between the film thickness and specific resistance of a PAS film | membrane and each alignment film, the specific resistance of a liquid crystal, and afterimage intensity. It is a table | surface for comparing about the difference in the specific resistance of a PAS film | membrane. It is a table | surface for comparing about the difference in the specific resistance of an alignment film. It is a table | surface for comparing about the difference in the specific resistance of a liquid crystal.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description shows specific examples of the contents of the present invention, and the present invention is not limited to these descriptions. Various modifications by those skilled in the art are within the scope of the technical idea disclosed in this specification. Changes and modifications are possible. In all the drawings for explaining the embodiments, the same reference numerals are given to those having the same function, and the repeated explanation thereof is omitted.

  FIG. 1 schematically shows a liquid crystal display device 100 according to an embodiment of the present invention. As shown in this figure, the liquid crystal display device 100 includes a liquid crystal module 200 fixed so as to be sandwiched between an upper frame 110 and a lower frame 120.

  FIG. 2 is a diagram schematically showing a cross section of a certain pixel of the liquid crystal module 200 of FIG. 1, and shows a cross section taken along line II-II of FIG. 3 to be described later. As shown in FIG. 2, the liquid crystal module 200 includes a backlight 260 composed of a light source (not shown) and the like, and a thin film transistor substrate (hereinafter referred to as “TFT substrate”) that generates an electric field based on the gradation value of each pixel. 230), a color filter substrate 220 having RGB color filters, and a liquid crystal layer 240 made of a liquid crystal composition sealed between the TFT substrate 230 and the color filter substrate 220.

  Further, the TFT substrate 230 is disposed on the glass substrate 232, the backlight 260 side of the glass substrate 232, the polarizing plate 231 that polarizes the light emitted from the backlight 260, and the flat surface on the liquid crystal layer 240 side of the glass substrate 232. Formed on the common electrode 233, the gate insulating film 234 that is an insulating film formed on the common electrode 233, the protective insulating film 235 formed on the gate insulating film 234, and the protective insulating film 235. And an alignment film 237 that is formed on the pixel electrode 236 and aligns the liquid crystal composition when no voltage is applied.

  In addition, the color filter substrate 220 partitions each color of each pixel on the glass substrate 222, the polarizing plate 221 disposed on the opposite side of the glass substrate 232 from the liquid crystal layer 240 side, and the liquid crystal layer 240 side of the glass substrate 222. A black matrix 226 that prevents light entering the adjacent pixels from entering, and a color that is formed in a region surrounded by the black matrix 226 and transmits a wavelength corresponding to one of RGB colors A filter 223, an overcoat layer 224 formed so as to cover the black matrix 226 and the color filter 223, and an alignment film 225 formed on the overcoat layer 224 and aligning the liquid crystal composition when no voltage is applied. And have.

  The pixel electrode 236 and the common electrode 233 are made of ITO (Indium Tin Oxide), and the common electrode 233 is a flat electrode that covers almost the entire pixel. With this configuration, the electrode can be used as a transmission portion, and the aperture ratio can be improved. In addition, the distance between the electrodes can be shortened, and an electric field can be efficiently applied to the liquid crystal. It is also possible to drive the liquid crystal by inverting the voltage applied to the pixel electrode 236 and the common electrode 233.

  In the manufacture of the TFT substrate 230, a glass substrate 232 having a thickness of 0.7 mm and a polished surface is used. On the glass substrate 232, the common electrode 233, the pixel electrode 236, the signal wiring 238, and the scanning are used. A gate insulating film 234 for preventing a short circuit of the wiring 239 (see FIG. 3) and a protective insulating film 235 for protecting the thin film transistor 280 (see FIG. 3), the pixel electrode 236, and the signal wiring 238 are formed.

  3 is a schematic plan view of the vicinity of one pixel of the TFT substrate 230, and FIG. 2 is a cross-sectional view taken along the line II-II in FIG. The thin film transistor 280 includes a pixel electrode (source electrode) 236, a signal wiring (drain electrode) 238, a scanning wiring (gate electrode) 239, and amorphous silicon 281. The scanning wiring (gate electrode) 239 is formed by patterning an aluminum film, the signal wiring (drain electrode) 238 is formed by patterning a chromium film, and the common electrode 233 and the pixel electrode 236 are formed by patterning ITO.

The gate insulating film 234 and the protective insulating film 235 are made of silicon nitride. The capacitor element is formed to have a structure in which the gate insulating film 234 and the protective insulating film 235 are sandwiched between the pixel electrode 236 and the common electrode 233, and these two insulating films are combined to form a PAS film 270. For silicon nitride, the amount of nitrogen in the film was changed by changing the flow rate of N ₂ gas or NH ₄ gas during film formation, and silicon nitride films having different specific resistances were formed. The PAS film may be an organic insulating film such as an acrylic resin, and the specific resistance can be changed by changing the resin material.

  The pixel electrode 236 is disposed so as to overlap the upper layer of the solid-shaped common electrode 233. The number of pixels is 1024 × 3 × 768 including 1024 × 3 (corresponding to R, G, and B) signal wirings 238 and 768 scanning wirings 239. On the glass substrate 222 of the color filter substrate 220, a color filter 223 with a black matrix 226 is formed.

In this embodiment, the alignment film 237 is made of polyimide and imparted with a liquid crystal alignment ability by irradiation with polarized ultraviolet rays. Since the specific resistance of the alignment film is increased by irradiating ultraviolet rays, the present invention can be effectively operated. Polyimide was formed by applying a precursor polyamic acid or polyamic acid ester to a substrate, followed by heating and baking. In this example, three types of precursors having different specific resistances were used: polyamic acid 1 (1 × 10 ¹⁴ Ωcm), polyamic acid 2 (1 × 10 ¹⁵ Ωcm), and polyamic acid methyl ester (5 × 10 ¹⁵ Ωcm). An alignment film was used. Since the polyamic acid ester has a higher specific resistance than the polyamic acid, the present invention can be effectively operated. The liquid crystal alignment ability can also be imparted by a rubbing alignment method.

  The alignment directions of the alignment film 237 of the TFT substrate 230 and the alignment film 225 of the color filter substrate 220 are substantially parallel to each other. Polymer beads having an average particle diameter of 4 μm are dispersed as spacers between these substrates, and liquid crystal molecules are sandwiched between the TFT substrate 230 and the color filter substrate 220 to form a liquid crystal layer 240.

  The two polarizing plates 231 and 221 sandwiching the TFT substrate 230 and the color filter substrate 220 are arranged in a crossed Nicol manner. A normally closed characteristic is adopted in which a dark state is obtained at a low voltage and a bright state is obtained at a high voltage.

FIG. 4 is a table showing the relationship between the film thickness and specific resistance of the PAS film 270 and the alignment film 237, the specific resistance of the liquid crystal, and the measured afterimage intensity in the liquid crystal display device 100 manufactured in this example. It is shown. In the table of FIG. 1 to No. 13 types of measurement results up to 13 are shown. In this example, in order to quantitatively measure the image sticking and afterimage of the liquid crystal display device 100, evaluation was performed using a luminance meter. First, the window pattern is displayed on the screen at the maximum brightness for 2 hours, and then the halftone display in which the afterimage is most noticeable. Here, the entire surface is switched so that the brightness is 50% of the maximum brightness, and the brightness of the window pattern display section is displayed. B ₁ and the luminance B ₀ of the non-display area were measured using a luminance meter, and the afterimage intensity was determined by the formula (B ₁ -B ₀ ) / B ₀ × 100. Here, the allowable afterimage intensity is less than 1.0%, and if it is less than 1.0%, the afterimage is hardly recognized. The combined resistance in the table is the specific resistance ρ _p (Ωcm) and film thickness d _p (nm) of the insulating film, and the specific resistance ρ _A (Ωcm) and film thickness d _A (nm) of the alignment film. (Ρ _p · ρ _A ) / (ρ _p · d _A + ρ _A · d _p )), and indicates the combined resistance in the in-plane direction of the PAS film and the alignment film.

As shown in the table of FIG. 4, the afterimage strengths were high values of 4.5% and 4.4% with the combined resistance of 7 × 10 ¹⁷ (Ω) and the combined resistance of 2 × 10 ¹⁷ (Ω), respectively. When the combined resistance was 8 × 10 ¹⁷ (Ω) or more, a good value with an afterimage intensity of 0.9% or less was obtained. In particular, when the combined resistance is 2 × 10 ¹⁸ (Ω) or more, the afterimage intensity is 0.7% or less, which is more preferable.

In the table of FIG. 4, the PAS film (insulating film) has a thickness of 400 (nm), the alignment film has a thickness of 100 (nm), a specific resistance of 1 × 10 ¹⁴ (Ωcm) or more, A table for comparing the difference in specific resistance of the PAS film extracted for the measurement in which the specific resistance of the liquid crystal is 1 × 10 ¹⁴ (Ωcm) is shown. As shown in the table of FIG. 5, when the specific resistance of the PAS film is 1 × 10 ¹³ (Ωcm), the afterimage intensity is a high value of 4.4%, but the specific resistance of the PAS film is 1 At x10 ¹⁴ (Ωcm) or more, a good value with an afterimage intensity of 0.5% or less is obtained. Further, when the specific resistance of the PAS film is 5 × 10 ¹⁵ (Ωcm), the afterimage strength is 0.1%, and the specific resistance of the PAS film is more preferably 5 × 10 ¹⁵ (Ωcm) or more. I understand that.

In the table of FIG. 4, the PAS film thickness is 400 (nm), the specific resistance is 1 × 10 ¹⁴ (Ωcm), the alignment film thickness is 100 (nm), and the specific resistance of the liquid crystal is shown in FIG. The table | surface for comparing about the difference of the specific resistance of the alignment film extracted about the measurement which is 1 * 10 < ¹⁴ > ((omega | ohm) cm) is shown. As shown in the table of FIG. 6, when the specific resistance of the alignment film is 1 × 10 ¹³ (Ωcm), the afterimage intensity is a high value of 4.5%, but the specific resistance of the alignment film is 1 At x10 ¹⁴ (Ωcm) or more, a good value with an afterimage intensity of 0.5% or less is obtained. Further, when the specific resistance of the alignment film is 5 × 10 ¹⁵ (Ωcm), the afterimage strength is 0.3%, and the specific resistance of the alignment film is more preferably 5 × 10 ¹⁵ (Ωcm) or more. I understand that.

FIG. 7 shows an extracted measurement of the combined resistance of 8 × 10 ¹⁷ (Ω) or more in the table of FIG. 4, and the combined resistance of 8 × 10 ¹⁷ (Ω) or more. No. having a specific resistance of 1 × 10 ¹³ (Ωcm). Fourteen measurement results are added. As shown in the table of FIG. 7, even if the combined resistance is 8 × 10 ¹⁷ (Ω) or more, when the specific resistance of the liquid crystal is 1 × 10 ¹³ (Ωcm), the afterimage intensity is 3. It shows a high value of 8%, which indicates that the specific resistance of the liquid crystal is preferably 1 × 10 ¹⁴ (Ωcm) or more.

Since it is difficult to manufacture a PAS film (insulating film) and an alignment film having a specific resistance value of 1 × 10 ¹⁷ (Ωcm) or more, the specific resistance values of the PAS film (insulating film) and the alignment film are It is less than 1 × 10 ¹⁷ (Ωcm).

  On the other hand, if the PAS film is thinner than 200 nm, dielectric breakdown tends to occur, and forming it thicker than 1000 nm is not preferable from the viewpoint of cost and productivity.

  Also, if the alignment film is thinner than 20 nm, it is difficult to maintain the coating uniformity, and unevenness in the alignment direction of the liquid crystal is generated. This is not preferable.

  Further, when the display quality of the liquid crystal display device according to this example was evaluated, a high-quality display with a contrast ratio of 500 to 1 was confirmed, and a wide viewing angle during halftone display was confirmed.

  100 liquid crystal display device, 110 upper frame, 120 lower frame, 200 liquid crystal module, 220 color filter substrate, 221 polarizing plate, 222 glass substrate, 223 color filter, 224 overcoat layer, 225 alignment film, 226 black matrix, 230 TFT substrate 231 polarizing plate, 232 glass substrate, 233 common electrode, 234 gate insulating film, 235 protective insulating film, 236 pixel electrode, 237 alignment film, 238 signal wiring, 239 scanning wiring, 240 liquid crystal layer, 260 backlight, 270 PAS film 280 Thin film transistor, 281 Amorphous silicon.

Claims (11)

A pair of substrates at least one of which is transparent;
A liquid crystal layer disposed between the pair of substrates;
A pair of electrodes formed on one of the pair of substrates for applying an electric field to the liquid crystal layer;
A plurality of active elements connected to the pair of electrodes;
A liquid crystal display device having an alignment film formed on a surface in contact with the liquid crystal layer,
In one pixel region, either one of the pair of electrode groups is formed in a flat plate shape, and the other is formed in a comb shape.
An insulating film is disposed between the pair of electrodes,
An alignment film is disposed on the comb-like electrode,
The specific resistance ρ _p (Ωcm) and film thickness d _p (nm) of the insulating film, and the specific resistance ρ _A (Ωcm) and film thickness d _A (nm) of the alignment film are expressed by the relational expression (ρ _p · ρ _A ) / (Ρ _p · d _A + ρ _A · d _p ) ≧ 8 × 10 ¹⁷ (Ω)
A liquid crystal display device characterized by satisfying The specific resistance ρ _p (Ωcm) and film thickness d _p (nm) of the insulating film, and the specific resistance ρ _A (Ωcm) and film thickness d _A (nm) of the alignment film are expressed by the relational expression (ρ _p · ρ _A ) / (Ρ _p · d _A + ρ _A · d _p ) ≧ 2 × 10 ¹⁸ (Ω)
The liquid crystal display device according to claim 1, wherein: 3. The liquid crystal display device according to claim 1, wherein a specific resistance ρ _p of the insulating film is 1 × 10 ¹⁴ (Ωcm) or more and less than 1 × 10 ¹⁷ (Ωcm). 4. The liquid crystal display device according to claim 3, wherein a specific resistance ρ _p of the insulating film is 5 × 10 ¹⁵ (Ωcm) or more and less than 1 × 10 ¹⁷ (Ωcm). 5. The liquid crystal display according to claim 1, wherein a specific resistance ρ _A of the alignment film is 1 × 10 ¹⁴ (Ωcm) or more and less than 1 × 10 ¹⁷ (Ωcm). apparatus. 6. The liquid crystal display device according to claim 5, wherein a specific resistance ρ _A of the alignment film is 5 × 10 ¹⁵ (Ωcm) or more and less than 1 × 10 ¹⁷ (Ωcm). The liquid crystal display device according to claim 1, wherein a thickness d _{p of the} insulating film is 200 (nm) or more and 1000 (nm) or less. 8. The liquid crystal display device according to claim 1, wherein a thickness d _{A of the} alignment film is 20 (nm) or more and 200 (nm) or less. The liquid crystal display device according to claim 1, wherein a specific resistance of the liquid crystal layer is 1 × 10 ¹⁴ (Ωcm) or more.   The liquid crystal display device according to claim 1, wherein the alignment film is provided with a liquid crystal alignment ability by irradiating polarized ultraviolet rays.   The liquid crystal display device according to claim 1, wherein the alignment film includes polyimide having a polyamic acid ester as a precursor.

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