Liquid crystal display

This is a continuation of U.S. application Ser. No. 11/591,510, filed Nov. 2, 2006, which is a continuation of U.S. application Ser. No. 11/239,198, filed Sep. 30, 2005, which is a continuation of U.S. application Ser. No. 10/219,391, filed Aug. 16, 2002, now U.S. Pat. No. 6,975,374, which is a continuation of U.S. application Ser. No. 09/803,980, filed Mar. 13, 2001, now U.S. Pat. No. 6,462,799, which is a continuation of U.S. application Ser. No. 09/331,266, filed Jun. 18, 1999, now U.S. Pat. No. 6,532,053, and copending with U.S. application Ser. No. 09/804,190, filed Mar. 13, 2001, now U.S. Pat. No. 6,831,724, the subject matter of which is incorporated by reference herein.

The present invention relates generally to active-matrix liquid crystal display devices and, more particularly, to liquid crystal display devices of the lateral electric field type having wide view-angle characteristics suitable for improvement of the aperture ratio.

Liquid crystal display devices of the active matrix type, which employ active elements typically, including thin-film transistors (TFTS), are becoming more important in the manufacture of display terminals for use with OA equipment in view of the fact that these devices offer enhanced displayability with superior image quality in comparison to cathode ray tubes, not to mention the flatness and light-weight features thereof. Such liquid crystal display devices are generally categorized into two types.

In one type, a liquid crystal material is sandwiched between two substrates, with a plurality of transparent electrodes being arranged thereon, so that application of a voltage to such transparent electrode causes an electric field to be generated transverse to the substrate, thereby modulating rays of light failing onto the liquid crystal after passing through the transparent electrodes, to thereby generate a display—all of the currently available products are designed to employ this scheme.

The other type of device was a scheme for causing the liquid crystal to be modulated by an electric field that is generated substantially in parallel to a substrate surface between two electrodes arranged on the same substrate, thereby modulating light incident on the liquid crystal from a space between the two electrodes, to thereby generate a display in which the viewing angle is extremely wide. This technology, which shows great promise for improvements in active-matrix liquid crystal display devices, is called a “lateral electric field” type or, alternatively, an “in-plane switching” type device.

Some features of the latter type of device have been disclosed in Domestically Published Japanese PCT Application No. 5-505247 Published Japanese Patent Application No. 63-21907 (JP-A-63-21907), and JP-A-6-160878.

However, in the in-plane switching type device, since an opaque metal electrode is arranged into a comb-like shape on one substrate, the resulting ratio of the opening region permitting light to pass therethrough (aperture ratio) is significantly low, which results in the problem that active-matrix liquid crystal display devices of the in-plane switching type have a display screen which is dark, or, alternatively, a bright backlight with great power dissipation must be used in order to brighten the display screen, resulting in an increase in the power dissipation of the devices.

Another problem associated with the in-plane switching type device is that the use of a metal electrode leads to an increase the reflectivity at the electrode, which in turn creates a problem in that an image or the like appears like a ghost image on the screen due to reflection at the electrode, reducing the recognizability of the display.

The present invention is designed to solve the problems mentioned above, and an objective of the present invention is to provide an active-matrix liquid crystal display device employing the in-plane switching scheme, which device is capable of realizing a viewing angle equivalent to that of cathode ray tubes, and wherein the active-matrix liquid crystal display device is bright due to a high aperture ratio and yet is low in power dissipation and in reflection for achieving increased displayability.

To attain the foregoing object, the present invention offers, as its first aspect, an arrangement in which at least one of a pixel electrode and a counter electrode is provided as a transparent electrode; the normally-black mode is established for providing dark display in the absence of an electric field as applied thereto; the initial alignment state of the twistable liquid crystal layer is the homogeneous alignment state upon application of no electric fields; liquid crystal molecules between said electrodes and those on the electrodes upon application of an electric field rotate controllably in a direction substantially parallel to the substrate surface; the maximum value of the optical transmissivity of a liquid crystal display panel is 4.0% or greater; and, the view-angle range of the contrast ratio of 10 to 1 or greater is within the range of all-directional coverage as tilted by 40 degrees or more from the vertical direction relative to the display plane.

As a second aspect of the invention, at least one of the pixel electrode and counter electrode is provided as a transparent electrode, the normally-black mode is set for providing dark display upon application of no electric fields, the initial alignment state of the twistable liquid crystal layer is the homogeneous state upon application of no electric fields, and the twist elastic modulus is not greater than 10×10⁻¹² N (Newton).

As a third aspect of the invention, at least one of the pixel electrode and counter electrode is provided as a transparent electrode, the normally-black mode is set for providing dark display upon application of no electric fields, the initial alignment state of the twistable liquid crystal layer is the homogeneous state upon application of no electric fields, the initial pretilt angle of those liquid crystal molecules at the upper and lower interfaces of the liquid crystal layer is not more than 10 degrees, and the initial tilt state of liquid crystal molecules within the liquid crystal layer are in the splay state.

As a fourth aspect of the invention, at least one of the pixel electrode and counter electrode is provided as a transparent electrode, the normally black mode is set for providing dark display upon application of no electric fields, the initial alignment state of the twistable liquid crystal layer is the homogeneous state upon application of no electric fields, and the average tilt angle of liquid crystal molecules of the liquid crystal layer on the transparent electrode is less than 45 degrees even when applying an electric field thereto.

As a fifth aspect of the invention, in any one of the arrangements described above, a double structure of a transparent electrode and an opaque electrode is employed for at least either the pixel electrode or the counter electrode.

As a sixth aspect of the invention, in any one of the arrangements described above, a structure is used in which neighboring ones of contra-voltage signal lines are connected by a counter electrode within a pixel via more than one through-hole.

As a seventh aspect of the invention, in any one of the arrangements described above, a protective film is provided for use in covering or coating active matrix elements, and at least one of said pixel electrode or said counter electrode is formed overlying said protective film while permitting electrical connection via more than one through-hole as formed in said protective film to either active matrix elements or contra-voltage signal lines.

As an eighth aspect of the present invention, in any one of the arrangements described above, the counter electrode is made of a transparent electrode, and further use is made of a structure having an optical shield pattern between a counter electrode and an image signal line.

As a ninth aspect of the invention, in any one of the arrangements described above, the contra-voltage signal line for electrical connection between counter electrodes is made of a metal.