1. Field of the Invention
The invention relates to a liquid crystal composition suitable for use in an active matrix (AM) device, and an AM device containing the composition. In particular, it relates to a liquid crystal composition having a negative dielectric anisotropy, and a device of an IPS (in-plane switching) mode or a VA (vertical alignment) mode containing the composition.
2. Related Art
On a liquid crystal display device, classification based on an operating mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), and so forth. Classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and so forth, and AM is classified into a thin film transistor (TFT), a metal insular metal (MIM) and so forth. TFT is further classified into amorphous silicon, polycrystal silicon and continuous grain silicon. The polycrystal silicon is classified into a high temperature type and a low temperature type according to a production process. Classification based on a light source includes a reflection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both the natural light and the backlight.
These devices contain a liquid crystal composition having suitable characteristics. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes a relationship between the general characteristics of the two. The general characteristics of the composition will be explained further based on a commercially available AM device. A temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or more and a desirable minimum temperature is approximately −20° C. or less. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying a moving image by means of the device. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General characteristics of a liquid crystal composition and an AM deviceGeneralGeneral characteristics of an AMNocharacteristics of a compositionDevice1Temperature range of a nematicUsable temperature range is widephase is wide2Viscosity is small 1)Response time is short3Optical anisotropy is suitableContrast ratio is large4Dielectric anisotropy isDriving voltage is low and electricpositively or negatively largepower consumption is small5Specific resistance is largeVoltage holding ratio is large and acontrast ratio is large1) A liquid crystal composition can be injected into a liquid crystal cell in a short time.
The optical anisotropy of the composition relates to the contrast ratio of the device. The devices having a VA mode, an IPS mode and so forth utilize electrically controlled birefringence. A product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed to be a constant value to maximize the contrast ratio of the device having a VA mode, an IPS mode and so forth. An example of the value is approximately 0.30 to approximately 0.35 micrometers (VA mode) or approximately 0.20 to approximately 0.30 micrometers (IPS mode). Since the cell gap (d) is generally approximately 3 micrometers to approximately 6 micrometers, the optical anisotropy of the composition is always in a range of approximately 0.05 to approximately 0.11. A large dielectric anisotropy of the composition contributes to a small driving voltage of the device. Accordingly, a large dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at room temperature and also at a high temperature in the initial stage. A composition having a large specific resistance is desirable at room temperature and also at a high temperature after it has been used for a long time. Since an ultraviolet ray is used on production of the device, a composition having a large specific resistance is desirable at room temperature and also at a high temperature after it is irradiated with an ultraviolet ray.
A composition having a positive dielectric anisotropy is used in an AM device having a TN mode. On the other hand, a composition having a negative dielectric anisotropy is used in an AM device having a VA mode. A composition having a positive or negative dielectric anisotropy is used in an AM device having an IPS mode. A liquid crystal composition having a negative dielectric anisotropy is disclosed in the following patent documents. JPH10-176167 A/1998 (U.S. Pat. No. 5,965,060), JP 2000-96055 A/2000 (U.S. Pat. No. 6,376,030 B1), JP 2001-354967 A/2001 (U.S. Pat. No. 6,764,722 B2) and JP 2003-13065 A/2003 (U.S. Pat. No. 6,764,723 B2).