A display device applying a liquid crystal material is used extensively as a light-weight and thin display device. The liquid crystal display device interposes a liquid crystal material between two pieces of flat plates such as glass substrates. By applying an electric field by at least one electrode arranged on a flat surface where the liquid crystal of a flat plate is arranged, a liquid crystal molecule is switched by the intensity and existence or nonexistence of the electric field. Since optical property of the liquid crystal is changed in conjunction with it, displaying of an image or the like is performed using this change of optical property.
A nematic liquid crystal is generally used as a liquid crystal material used for a liquid crystal element. A nematic liquid crystal means one of the categories distinguished by arranging status of liquid crystal molecules constituting a liquid crystal material. In addition, for example, there is a material such as a smectic liquid crystal. However, a nematic liquid crystal has a liquid property and flowability within an operating temperature range and is used extensively as a display in easiness of initial orientation of liquid crystal molecules and stability of orientation state, and breadth of operatable temperature range.
A hight of a contrast is given as one of the factorsto determine display performance of a display. A contrast is represented by a ratio of luminance of a light state and luminance of a dark state, and a high-grade image can be provided as the ratio becomes higher.
However, conventional TN (Twisted Nematic) mode and the like were difficult to get an ideal dark state since it is affected by a slight phase difference that a liquid crystal represented. This is because in the case of a TN mode, though a molecular major axis is shifted from a parallel state to a perpendicular state in an orientation state of liquid crystal molecules by means of applying an electric field, it is not easy particularly that a molecular in a place near to a substrate surface is shifted perpendicular to the substrate so that birefringence of liquid crystal in that portion remains.
Thus, a liquid crystal display device using a perpendicular orientation mode is suggested as a method to make contrast ratio better. (for example, see Non-Patent Document 1).
In a perpendicular orientation mode, a liquid crystal molecule is oriented in perpendicular direction with respect to a substrate surface of a liquid crystal display device beforehand, therefore, when an electric field is not applied to an electrode, transmitted light is interrupted by a polarizing plate. Meanwhile when an electric field is applied to an electrode (at a time of driving), a liquid crystal molecule exists with the state that declined from the perpendicular direction. It is a display method using that light-transmitting property is obtained by birefringence of these liquid crystal molecules.
In addition, in the perpendicular orientation mode, the state in which an electric field is not applied is a perpendicular orientation state and a black display is obtained. In the perpendicular orientation state, since a birefringence of liquid crystal does not appear, it is a driving mode in which light leak is small and a black display is theoretically easy to obtain.
Furthermore, in the case of a liquid crystal display device using nematic liquid crystal, a challenge is reducing time required for an operation of a liquid crystal molecule (hereinafter, response time) when a display signal is switched from one displaying state to the next displaying state. Note that, the response time which is needed when display is switched from state of OFF to a state of ON is referred to as “rising response time” and when display is switched from state of ON to the state of OFF is referred to as “falling response time”. A method in which “rising response time” is shortened by driving wave form has been suggested. (for example, see Non-Patent Document 2).
In the case where display is made a state of ON from a state of OFF (in the case of rising), if electric field is made strongly, response speed can be made quickly since an orientation direction of liquid crystal molecules is controlled by electric field. However, in the case of display is made to a state of OFF from a state of ON (in the case of falling), control to improve response speed is difficult as compared to the case of rising since the OFF state can be obtained by returning liquid crystal molecules released from the control by the electric field to the orientation state before electric field has been applied. Thus, more improvement has been needed.
[Non-Patent Document 1]
K. Ohmuro, S. Kataoka, T. Sasaki, and Y. Koike, SID' 97 Digest of Tech. Papers, (1997) 845,
[Non-Patent Document 2]
S. Sang, SID' 04 Digest of Tech. Papers, (2004) 760