Conventionally, color liquid crystal display devices have been popularly used as color display devices having such features as flatness, lightweight, and the like. In recent years, a color liquid crystal display device having properties of a high contrast and a broad viewing angle has been developed thanks to advancement in a liquid crystal technique. The color liquid crystal display device is widely put into practice as the mainstream of a large-scale display.
Examples of currently widely used display modes of the color liquid crystal display device include: a twisted nematic mode (hereafter referred to as “TN mode”) for performing display by controlling optical rotation of a liquid crystal layer by an electric field; a birefringence mode (hereafter referred to as “ECB mode”) for performing display by controlling birefringence of the liquid crystal layer by the electric field; and the like.
However, in the color liquid display devices to which any of these modes is applied, a response speed is still slow, a phenomenon of a lasting image occurs, and a contour is blurred Therefore, there is a problem that such a color liquid display device is not suitable to display a moving image.
Accordingly, a large number of attempts to increase the response speed of the color liquid crystal display device have conventionally been made. Currently, examples of a liquid crystal mode capable of rapidly responding so as to be suitable to display the moving image include: a ferroelectric liquid crystal mode; an anti-ferroelectric liquid crystal mode; an OCB (Optically self-Compensated Birefringence) mode; and the like.
Out of such modes, it is known that the ferroelectric liquid crystal mode and the anti-ferroelectric liquid crystal mode is quite susceptible to mechanical impact because of their layered structure and thus there are a lot of problems in putting the modes into practice.
On the other hand, the OCB mode in which a normal nematic liquid crystal is used has a high shock resistance and is also workable in a wide temperature range. The OCB mode has a broad viewing angle and rapid response. Therefore, the OCB mode is recognized as the most suitable liquid crystal mode to display the moving image.
FIG. 7 is a schematic cross-sectional view of an essential part of a liquid crystal display device to which the OCB mode is applied. As illustrated in FIG. 7, in a liquid crystal display device 100 to which the OCB mode is applied, a liquid crystal layer 103 is sandwiched by transparent glass substrates 101 and 102. A pixel electrode 104 and a counter electrode 105 are transparent electrodes. The pixel electrode 104 and the counter electrode 105 as well as alignment films 106 and 107 which film the pixel electrode 104 and the counter electrode 105, respectively are provided on the glass substrates 101 and 102 so as to face the liquid crystal layer 103. The alignment films 106 and 107 have been subjected to aligning treatment by rubbing.
In the liquid crystal display device 100, in order to perform color display, a color filter (not illustrated) is produced on one side of the glass substrate 102. Furthermore, in order to drive the liquid crystal layer 103 in an active matrix mode, there are produced a gate bus line and a source bus line on one side of the glass substrate 101 as well as a TFT at a part where the gate bus line and the source bus line intersect (The gate bus line, the source bus line and the TFT are not illustrated). After both the glass substrates 101 and 102 are individually produced, the substrates are combined to each other with a globular spacer or a pillar spacer (not illustrated) therebetween to appropriately provide a gap. The liquid crystal layer 103 is formed by vacuum filling between both the glass substrates 101 and 102 combined to each other or by one drop filling when the glass substrates 101 and 102 are combined to each other. To one side or both sides of a liquid crystal cell thus obtained, a wave plate (not illustrated) is combined so as to improve the viewing angle property in display, and a polarizer (not illustrated) is combined to an outer side of the wave plate.
As illustrated in FIG. 8, liquid crystal molecules 103a are often oriented substantially horizontally in the liquid crystal layer 103 just after liquid crystal is filled thereinto. Such a state is called an initial orientation (spray orientation). When a desired voltage is applied to the pixel electrode 104 and the counter electrode 105 which are below and above the liquid crystal layer 103, respectively, the orientation of the liquid crystal molecules 103a in the liquid crystal layer 103 is sequentially changed from the spray orientation illustrated in FIG. 8 to bend orientation illustrated in FIG. 7. Once the liquid crystal molecules 103a have the bend orientation illustrated in FIG. 7, the orientation of the liquid crystal molecules 103a is rapidly changed during white display (refer to FIG. 9) and black display (refer to FIG. 10). This allows the liquid crystal molecules 103a to rapidly respond. For this reason, use of the OCB mode enables the most rapid display of the modes to which the nematic liquid crystal is applied. Moreover, a display state having the property of the broader viewing angle is realized by combining the OCB mode with the wave plate.
As described above, the orientation in the OCB mode is the spray orientation when no voltage is applied. In the case of actual performance of display, display is performed in a state where the bend orientation occurs. That is, in the liquid crystal display device 100 to which the OCB mode is applied, the bend orientation is maintained by constantly applying a voltage to the liquid crystal layer 103 when display is performed. For example, as illustrated in FIG. 9, the white display is performed when a voltage VL is applied. On the other hand, as illustrated in FIG. 10, the black display is performed when a voltage VH is applied. Furthermore, provided that a halfway state is displayed when a voltage between the voltage VL and the voltage VH is applied, the liquid crystal layer 103 has the bend orientation in a range of the voltages VL to VH.
In the OCB mode, the liquid crystal layer 103 in the display state keeps the bend orientation by a voltage being constantly applied thereto. On the other hand, in a state where a power supply of the liquid crystal display device 100 is off, the liquid crystal layer 103, to which no voltage is applied, has the spray orientation. For this reason, when the power supply of the liquid crystal display device 100 is turned on, the orientation in the liquid crystal layer 103 is changed from the spray orientation to the bend orientation (hereafter referred to as “transition from spray to bend”).
However, as disclosed in Patent Literatures 1 and 2, for example, it is known that the transition from spray to bend requires a high voltage or much time. The time for the transition from spray to bend to be carried out in the entire screen depends on a voltage to be applied to the liquid crystal layer 103.
Here, FIG. 11 shows a relationship between a voltage applied to the liquid crystal layer 103 at a room temperature (+25° C.) and a transition time required for the transition from spray to bend. In FIG. 11, an area of the electrode and a thickness of the cell are set as 1 cm2 and 5 μm, respectively. FIG. 11 shows that the transition time for the transition from spray to bend becomes shorter as the voltage to be applied to the liquid crystal layer 103 grows higher.
On the other hand, when a state of the transition from spray to bend is observed, it can be seen that the transition occurs from a specific place where several spacers are aggregated. Such a place is called transition nucleus. There are some cases where only several pieces of the transition nucleus are formed within 1 cm2, which elongates the time for the transition from spray to bend to be spread to the entire screen. A speed for the transition from spray to bend to be spread depends on viscosity of the liquid crystal. For example, at a low temperature of −30° C., the viscosity is greatly increased and thus the speed for the transition from spray to bend to be spread is 100 times or so lower than that at the room temperature.
In order to prevent such a problem, as illustrated in FIG. 12, Patent Literature 3 discloses a configuration such that a protrusion 201 composed of an electroconductive material or a recess (not illustrated) is formed at a predetermined position in the screen. With such a configuration, intensity of an electric field which is applied to the liquid crystal layer 203 on the protrusion 201 or the recess (not illustrated) is greater than intensity of an electric field of a circumference thereof. This promotes formation of the transition nucleus. Forming such a transition nucleus in each of the pixels facilitates the transition from spray to bend in all the pixels.
Moreover, as illustrated in FIG. 13, Patent Literature 4 discloses drive means for causing a potential difference between an auxiliary capacitor electrode 301 and a pixel electrode 303 which is provided so as to overlap the auxiliary capacitor electrode 301 via an insulator 302 and includes a cutaway portion 303a. With such a configuration, intensity of an electric field to be applied between the auxiliary capacitor electrode 301 and the pixel electrode 303 grows higher than intensity of an electric field of other areas and thereby the liquid crystal molecules provided on a periphery of the cutaway portion 303a become the transition nucleus. This facilitates the transition from spray to bend in all the pixels.
In this way, Patent Literatures 3 and 4 disclose that making a structure to form the transition nucleus for all the pixels allows the transition from spray to bend to be carried out in all the pixels, i.e. in the entire screen even if there is an isolated space where no voltage is applied to the liquid crystal layer.
Furthermore, in order to complete the transition from spray to bend in the entire screen, it is necessary to individually form the transition nucleus in each of the pixels. Patent Literature 2 shows that it is effective to apply a transverse electric field to the liquid crystal so as to form the transition nucleus.