Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-07-22
2002-08-06
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S141000
Reexamination Certificate
active
06429914
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a polymer dispersion type liquid crystal display panel used for a portable information terminal and a manufacturing method thereof, and particularly a polymer dispersion type liquid crystal display panel appropriately applicable to a display panel in a lateral electric field mode (PS ‘In-plane Switching’ mode) and a manufacturing method thereof.
BACKGROUND ART
[1] In recent years, a polymer dispersion type liquid crystal display panel wherein a polymer and a liquid crystal composition are dispersed in an incompatible state is noticeable. Since the polymer dispersion type panel has such a characteristic that a polarizer is unnecessary and a display with a high luminance can be obtained easily because of using a scattering mode for displaying. (refer to S. Shikama et al,
Society for Information Display '
95, pp. 231 to 234)
A polymer dispersion type panel is characterized by displaying by utilizing a refractive index matching of a liquid crystal and a polymer. A display mode of a polymer dispersion type panel is divided roughly into two kinds by a method of the refractive index matching. The one is a normal mode wherein a scattering display is executed under no application of voltage and a transparent display is executed under an application of voltage (for instance, the specification U.S. Pat. No. 3,600,060. The other is a reverse mode wherein a transparent display is executed under no application of voltage and a scattering display is executed under an application of voltage (for instance, Japanese Unexamined Patent Applications No. 2-309316 and No. 3-13268). Both of these two display modes are characterized by holding a polymer dispersion type liquid crystal between a pair of substrates with a transparent electrode and executing a switching of light by aligning a liquid crystal molecule in a vertical direction to the substrates under an application of voltage between the substrates.
Meanwhile, in the case of a liquid crystal panel wherein a display is executed by using a birefingence and a polarizer, a liquid crystal panel in a lateral electric field mode has been developed wherein an electric field is applied in parallel with a substrate for the purpose of a wide viewing angle (for instance, Japanese Unexamined Patent Publication No. 63-21907).
A liquid crystal panel is disclosed wherein an incident light is reflected into a gap between substrates by devising an electrode structure for intending a higher luminance with a lateral electric field mode (Japanese Unexamined Patent Publication No. 8-286211).
However, the above-mentioned conventional examples have various problems.
(1) A problem in a conventional polymer dispersion type liquid crystal panel
(a) In the case of using as a transmission type panel such as a projection type display, the panel has a defect of low contrast due to no disappearance of a black display resulting from insufficient scattering performance. Meanwhile, in order to solve such a defect, it is preferred to improve the scattering performance by increasing a panel gap and an optical length. However, the increase of a panel gap results in a high driving voltage, and the panel can not be utilized practically.
(b) In the case of using as a reflection type panel with a color filter such as a portable type display, the panel has a defect of low white luminance due to insufficient scattering of outside light in a white display resulting from insufficient scattering performance. Also in this case, the increase of a panel gap for improving the scattering performance results in a high driving voltage similarly to the above, and the panel can not be utilized practically.
(c) When a polymer dispersion type panel in a reverse mode as composed above is driven with a lateral electric field mode, a scattering intensity in applying voltage is increased, and a higher scattering and a higher contrast are obtained. Meanwhile, the problem is that a pixel aperture ratio is reduced and it is difficult to obtain a higher luminance since a driving electrode is arrayed in parallel at a pixel.
(2) A problem in a conventional liquid crystal display panel in a lateral electric field mode
(a) A luminance is low due to the use of a polarizer.
(b) An opaque electrode such as metal is usually used as an electrode used for applying an electric field in order to obtain a uniform display with a high contrast (refer to, for instance, M. Ohta et al,
Proceedings of Asia Display '
95, pp. 707 to 710). Accordingly, a pixel aperture ratio is low, approximately 30%, and the reduction of luminance is caused.
(c) When a gap between adjacent electrodes (equivalent to a gap in a direction of electric field between a first driving electrode and a second driving electrode in the present invention) is L (&mgr;m) and a panel gap is d (&mgr;), the L and d usually satisfy L>d in view of the reduction of driving voltage, the improvement of panel luminance and the optimization of voltage-transmittance characteristic (refer to, for instance, M. Ohta et al,
Proceedings of Asia Display '
95, pp. 577-580). Accordingly, a conventional panel in a lateral electric field mode using a nematic liquid crystal and a polarizer has the restriction in increasing the panel gap.
(d) When a conventional liquid crystal panel using a polarizer is driven with a lateral electric field mode, a wide viewing angle is actualized, while the problem is that a pixel aperture ratio is reduced and it is difficult to obtain a higher luminance due to a similar reason. As described in Japanese Unexamined Patent Publication No. 8-286211, in the case of composing so that a cross sectional form of a driving electrode is made a triangular form and incident light is reflected in a pixel aperture in order to intend a higher luminance, if an electrode exists in a liquid crystal layer, a black display does not disappear since an alignment direction of the liquid crystal is irregular on the side of the electrode, and a contrast gets lower. In the case of making an electrode a triangular form, a ratio h/W of its height h to its width W requires over 0.5. That is, in the case of electrode width W of 3 &mgr;m, electrode height h requires over 1.5 &mgr;m. Generally, a panel gap in a lateral electric field mode is 2 to 5 &mgr;m. It is necessary to uniformize a panel gap with high precision. However, it is difficult to keep the panel gap below 5 &mgr;m uniformly in a state of the electrode height of 1.5 &mgr;m. Consequently, in the case of making an electrode form a triangular form with a conventional lateral electric field mode, it is necessary to form a flattened layer on an electrode completely. Then, the flattened layer is, for instance, 1.6 &mgr;m in thickness for covering the electrode. However, when the flattened layer is thick, the problem is that display irregularity occurs since an electric field is not applied uniformly to a liquid crystal layer.
[2] A conventional liquid crystal display panel has the following problems.
That is, various scattering type display panels have been discussed since before. PDLC Polymer Dispersed Liquid Crystal) type liquid crystal display panel wherein a liquid crystal is dispersed into a polymer in a droplet (U.S. Pat. No. 4,688,900 and PNLC (Polymer Network Liquid Crystal) type liquid crystal display panel wherein a liquid crystal is dispersed into a polymer in a range of networks (No. 2724596 of Japanese Patent) are referred to as a typical example. In these conventional polymer dispersion type liquid crystal panels (hereinafter referred to as ‘the first conventional example’), under an application of voltage, a liquid crystal molecule is aligned in a direction vertical to a substrate, and a refractive index of a liquid crystal molecule and a refractive index of a polymer are equalized, and thereby a transparent state is obtained. However, since a refractive index of a liquid crystal molecule and a refractive index of a polymer are different against a diagonal incident light, a sufficient transparent state is not obtained
Kubota Hirofumi
Nakao Kenji
Nishiyama Seiji
Uemura Tsuyoshi
Parker Kenneth
Parkhurst & Wendel LLP
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