Liquid crystal display device

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-285342, filed Sep. 19, 2001, the entire contents of which are incorporated herein by reference.

1. Field of the Invention

The present invention relates generally to a liquid crystal display device wherein a liquid crystal layer is held between a pair of electrode substrates, and more particularly to a liquid crystal display device whose liquid crystal layer is divided into a plurality of domains among which liquid crystal molecules are set in different tilt directions.

2. Description of the Related Art

Liquid crystal display (LCD) panels are applied to various technical fields of OA equipment, information terminals, watches, televisions, etc., because of their characteristics of lightness, thinness, and low power consumption. In particular, an active matrix type LCD panel has an excellent response characteristic obtained by thin film transistors for switching pixels. Thus, the active matrix type LCD panel is used as a monitor display for a portable TV or a computer, which is required to quickly display a large quantity of image information.

In these years, with an increase in quantity of information, there is a demand for enhancement of the resolution and display speed of the liquid crystal display panel. As a technique of enhancing the resolution, the number of pixels is increased by miniaturization of the TFT array structure. In this case, an LCD mode, which can provide a response speed of liquid crystal molecules twice to several tens of times higher than the current level, is required to change the alignment of liquid crystal molecules in a period shortened according to the increase in the number of pixels. Such an LCD mode can be selected from Optically Compensated Birefringence (OCB), Vertically Aligned Nematic (VAN), Hybrid Aligned Nematic (HAN), and π-alignment modes using a nematic liquid crystal, and Surface-Stabilized Ferroelectric Liquid Crystal (SSFLC) and Anti-Ferroelectric Liquid Crystal (AFLC) modes using a smectic liquid crystal, for example.

Particular attention has recently been paid to the VAN mode, which has a higher response speed than the conventional twisted nematic (TN) mode and adopts a vertical alignment treatment, thus dispensing with a conventional rubbing process that may cause defects such as electrostatic destruction. Moreover, a design for viewing angle compensation becomes easier when the VAN mode is employed. The viewing angle can be widened by a multi-domain structure that each pixel region of the liquid crystal layer is divided into a plurality of domains between which liquid crystal molecules are set in different tilt directions.

In the above-mentioned multi-domain structure, the plurality of domains are created, for example, by causing an electric field applied from each electrode into the liquid crystal layer to be non-uniform at an inside part or periphery of the electrode, so that the tilt directions can be determined according to the dielectric constant anisotropy of liquid crystal material and the non-uniformity of electric field. Conventionally, sufficient multi-directional components are not obtained in the non-uniform electric field, with a result that the viewing angle characteristic is made uneven. In addition, it is difficult to control the threshold characteristic defining a point from which a change in the transmittance is initiated.

An object of the present invention is to provide a liquid crystal display device which can control the threshold characteristic while remedying the unevenness in the viewing angle characteristic owing to the shape of each electrode.

According to an aspect of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal layer containing a nematic liquid crystal material; first and second light-transmissive substrates which hold the nematic liquid crystal material therebetween such that liquid crystal molecules are aligned in a predetermined directions; and first and second circular electrodes which are formed on the first and second light-transmissive substrates and apply to the liquid crystal layer an electric field for controlling the orientation of the liquid crystal molecules; wherein the first and second circular electrodes include radial light-shielding electrode lines and concentric light-shielding electrode lines, respectively, and the radial electrode lines and the concentric electrode lines are opposed to each other to divide the liquid crystal layer into a plurality of domains between which liquid crystal molecules are set in different tilt directions under the electric field.

In this liquid crystal display device, an electric field is applied to the liquid crystal layer from the radial light-shielding electrode lines and concentric light-shielding electrode lines. Under the electric field, the liquid crystal layer is divided into a plurality of domains between which liquid crystal molecules are set in different tilt directions. With this structure, the plural domains are made symmetric with respect to the center of each circular electrode, and thus the viewing angle characteristic can be prevented from being made uneven. In addition, the domain size gradually increases in the radial direction outward from the center of the circular electrode, and thus the threshold characteristic, i.e. the point from which a change in the transmittance is initiated can be controlled. Moreover, when an image is displayed using a region corresponding to the circular electrodes as a pixel, curves in the displayed image can be prevented from being degraded.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and together with the general description given above and the detailed description of the embodiment given below, serve to explain the principles of the invention.

FIG. 1 is a diagram showing a planar structure of a liquid crystal display (LCD) panel according to an embodiment of the present invention;

FIG. 2 a diagram showing a cross-sectional structure of pixels of the LCD panel shown in FIG. 1 ;

FIG. 3 is a diagram showing an arrangement of pixel electrodes and counter electrodes, which are opposed in the LCD panel shown in FIG. 2 ;

FIG. 4 is a diagram showing a planar structure of the pixel electrodes shown in FIG. 3 ;

FIG. 5 is a diagram showing a planar structure of the counter electrodes shown in FIG. 3 ;

FIG. 6 is a diagram showing a plurality of domains obtained by a combination of the pixel electrode shown in FIG. 4 and the counter electrode shown in FIG. 5 ;

FIG. 7 is a graph showing the voltage-to-transmittance characteristic of an LCD panel actually manufactured with a multi-domain structure shown in FIG. 6 ; and

FIG. 8 is a graph showing the viewing angle characteristic of the LCD panel actually manufactured with the multi-domain structure shown in FIG. 6 .

An active matrix liquid crystal display (LCD) panel according to an embodiment of the present invention will now be described with reference to the accompanying drawings.

FIG. 1 shows a planar structure of the LCD panel, and FIG. 2 shows a cross-sectional structure of pixels of the LCD panel. The LCD panel operates in a multi-domain VAN mode. The LCD panel comprises light-transmissive electrode substrates 11 and 12 , and a liquid crystal layer 13 containing a nematic liquid crystal material held between the electrode substrates 11 and 12 .

The electrode substrate 11 comprises a plurality of pixel electrodes EL 1 arrayed substantially in a matrix form; a plurality of scanning lines Y (Y 1 to Ym) disposed along rows of the pixel electrodes EL 1 ; a plurality of signal lines X (X 1 to Xn) disposed along columns of the pixel electrodes EL 1 ; a plurality of switching elements SW disposed near intersections of the scanning lines Y and signal lines X; a scanning line driving circuit YD for driving the scanning lines Y; and a signal line driving circuit XD for driving the signal lines X. The pixel electrodes EL 1 are formed on an insulating substrate G 1 shown in FIG. 2 , and covered with an alignment film 14 which aligns liquid crystal molecules of the liquid crystal layer 13 substantially vertical to the electrode substrate 11 plane in a voltage non-application state. The switching elements SW are formed of amorphous-silicon or polysilicon thin-film transistors. The scanning line driving circuit YD and signal line driving circuit XD are formed of, e.g. polysilicon thin-film transistors or Si peripheral circuit. Each switching element SW is driven via a corresponding scanning line Y to electrically connect a corresponding signal line X to a corresponding pixel electrode EL 1 . Although not shown in FIG. 2 , the scanning lines Y, signal lines X, switching elements SW, scanning line driving circuit YD and signal line driving circuit YD are disposed on the insulating substrate G 1 .

The electrode substrate 12 includes a color filter 15 formed on an insulating substrate G 2 ; a plurality of counter electrodes EL 2 formed on the color filter 15 and opposed to the pixel electrodes EL 1 ; and an alignment film 16 which covers the counter electrodes EL 2 and aligns liquid crystal molecules of the liquid crystal layer 13 substantially vertical to the electrode substrate 12 plane in a voltage non-application state. The color filter 15 includes three color layers of red (R), green (G) and blue (B) formed in pixel regions defined by the electrodes EL 1 and EL 2 , and shield layers formed around the pixel regions defined by the electrodes EL 1 and EL 2 .

It is preferable that the insulating substrate G 1 , G 2 be formed of such a transparent material as glass, polycarbonate, or a cycloolefin resin, and has low hygroscopicity, thermal resistance, low birefringence, and high dimensional stability. The electrode substrates G 1 and G 2 and liquid crystal layer 13 are integrated by means of a peripheral sealing member surrounding the liquid crystal layer 13 between the electrode substrates 11 and 12 . The cell thickness of the liquid crystal layer 13 is kept substantially constant by columnar spacers SP which are formed in contact with the electrode substrates 11 and 12 at a central area of the pixel region and an outside area of the pixel region. It is preferable that a pre-tilt angle of liquid crystal molecules, which is provided by the alignment film 14 , 16 , be 90°. The LCD panel further comprises polarization plates PL 1 and PL 2 covering those surfaces of the electrode substrates 11 and 12 , which are located opposite to the liquid crystal layer 13 . The polarization plates PL 1 and PL 2 are affixed to the electrode substrates G 1 and G 2 in such orientations that their polarization axes are set in a cross-Nicol.

The pixel electrodes EL 1 and counter electrodes EL 2 are, as shown in FIG. 3 , circular electrodes for applying an electric field to the liquid crystal layer 13 to control orientation of liquid crystal molecules serving as a pixel of a circular form defined by these circular electrodes. The pixel electrodes EL 1 and counter electrodes EL 2 are arrayed substantially in a matrix form. In fact, however, these electrodes are slightly displaced to eliminate useless regions among the pixels, as shown in FIG. 3 . Each of the pixel electrodes EL 1 is formed of light-shielding electrode lines CM 1 which are arranged radially in the pixel region, as shown in FIG. 4 . Each of the counter electrodes EL 2 is formed of light-shielding electrode lines CM 2 which are arranged concentrically about the center of the pixel region, as shown in FIG. 5 . The light-shielding electrode lines CM 1 are connected to a bridge fine line BR disposed, e.g. along periphery of the pixel region. The light-shielding electrode lines CM 2 are connected to a cruciform bridge fine lines BR disposed, e.g. to extend vertically and horizontally and intersect at the center of the pixel region. The light-shielding electrode lines CM 1 and CM 2 are opposed to each other as shown in FIG. 6 , so as to apply an electric field to a pixel region of the liquid crystal layer 13 and divide the pixel region into a plurality of domains DM between which liquid crystal molecules are set in different tilt directions. In the above-described structure, it is considered that there is no substantial movement of liquid crystal molecules adjacent to the electrode lines CM 1 and CM 2 because of the symmetry of the electrode lines CM 1 and CM 2 . It is necessary, however, to prevent light leak from disclination lines created according to distribution of the electric field. The electrode lines CM 1 and CM 2 are formed of the light-shielding material in order to prevent the light leak from the disclination lines. The radius of each of the electrodes EL 1 and EL 2 is set at, e.g. 60 μm, and the line widths of the electrode lines CM 1 and CM 2 are set at 2 μm and 3 μm, respectively.

In the actual manufacturing, the precision of a stepper for forming electrodes is about 1.5 μm in the state of the art. The precision in alignment for attaching the electrode substrates 11 and 12 is about 2 μm in the state of the art. If the precision of the stepper and the precision in alignment with these values are available, the domains DM can be obtained, as shown in FIG. 6 , by precisely dividing the liquid crystal layer 13 at the intervals of about 10 μm for each pixel.

A description will now be given of a liquid crystal display (LCD) panel manufactured as a sample. In this LCD panel, the radius of each electrode EL 1 , EL 2 was set at 60 μm, as mentioned above. The radial electrode lines CM 1 were patterned with a line width of 2 μm so as to equally divide the electrode EL 1 into 16 portions in its circumferential direction. The concentric electrode lines CM 2 were patterned with a line width of 3 μm so as to divide the electrode EL 2 at radial points of 10 μm, 20 μm, 30 μm, 40 μm, 50 μm and 60 μm from the center of the electrode EL 2 . As a result, the number of domains was set at 80. The liquid crystal layer 13 was formed of a liquid crystal material with anisotropic dielectric constant Δε=17.3 and birefringence Δn=0.286. Each of the alignment films 14 and 16 was formed by vertical alignment treatment of a full-coated silane coupling material.

FIG. 7 shows the voltage-to-transmittance characteristic of the actually manufactured LCD panel. The transmittance gradually rises with a threshold point of about 2V, and the contrast ratio is 210:1. A voltage was applied between the pixel electrode EL 1 and counter electrode EL 2 , and the liquid crystal alignment was observed by a polarization microscope. It was confirmed that there was a multi-domain structure wherein the liquid crystal alignment differs between the domains of the pixel region partitioned by the radial electrode lines CM 1 and concentric electrode lines CM 2 .

FIG. 8 shows the viewing angle characteristic of the LCD panel. The viewing angle characteristic is measurement results obtained by applying voltages for attaining transmittances of 100%, 80%, 60%, 40%, 20% and 0% in a front view. Although the pixel was divided in units of 22.5° in the circumferential direction, the measurement results show that the variation in transmittance was substantially symmetric in all viewing angles.

In the LCD panel of the present embodiment, an electric field is applied to the liquid crystal layer 13 from the radial light-shielding electrode lines CM 1 and concentric light-shielding electrode lines CM 2 . Under the electric field, the liquid crystal layer 13 is divided into domains DM between which liquid crystal molecules are set in different tilt directions. With this structure, the plural domains DM are made symmetric with respect to the center of the electrodes EL 1 and EL 2 , and thus the unevenness in the viewing angle characteristic can be remedied. In addition, the domain size is gradually varied in the radial direction outward from the center of the electrodes EL 1 and EL 2 , and thus the threshold characteristic, i.e. the point from which a change in the transmittance is initiated can be controlled. Moreover, when an image is displayed using a region corresponding to the circular electrodes EL 1 and EL 2 as a pixel, curves in the displayed image can be prevented from being degraded.

The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the invention.

In the above embodiment, the alignment films 14 and 16 are subjected to alignment treatment such that the liquid crystal molecules of the liquid crystal layer 13 are aligned substantially vertical to the planes of the electrode substrates 11 and 12 in a voltage non-application state. Alternatively, the alignment films 14 and 16 may be subjected to alignment treatment such that the liquid crystal molecules of the liquid crystal layer 13 are aligned substantially horizontal with respect to the planes of the electrode substrates 11 and 12 in a voltage non-application state. In this case, it is preferable that the pre-tilt angle of liquid crystal molecules provided by the alignment films 14 and 16 be 0°.

In the above embodiment, the pixel electrodes EL 1 , as well as the scanning line driving circuit YD, signal line driving circuit XD and switching elements SW, are provided on the electrode substrate 11 , and the counter electrodes EL 2 are provided on the electrode substrate 12 so as to face the pixel electrodes EL 1 . Alternatively, the structural relationship between the pixel electrodes EL 1 and the counter electrodes EL 2 may be reversed since the transmittance of the liquid crystal layer 13 is controlled by a potential difference between the pixel electrodes EL 1 and the counter electrodes EL 2 . For example, each of the pixel electrodes EL 1 is formed of concentric light-shielding electrode lines CM 2 , and each of the counter electrodes EL 2 is formed of radial light-shielding electrode lines CM 1 .

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.