JPH0289020A - Ferroelectric liquid crystal - Google Patents

Abstract

PURPOSE:To decrease the influence of ionic impurities and to improve performance by using a ferroelectric liquid crystal having spontaneous polarization of a specific value or below. CONSTITUTION:The ferroelectric liquid crystal having the spontaneous polarization of <=15nC/cm<2> is used. An electric field in the liquid crystal layer by this polarization charge even in the state of not impressing an external electric field to the liquid crystal is, therefore, not formed and the unequal presence of the ionic impurities in the liquid crystal layer is obviated by the presence of the spontaneous polarization. As a result, the ionic impurities do not restrain the spontaneous polarization of the liquid crystal molecules on the contrary and, therefore, the ferroelectric liquid crystal which exhibits bistability in a normal state do not have monostability and the degradation in the contrast ratio is prevented. With respect to the response speed, a satisfactory solution is obtainable by a method for lowering the viscosity of the ferroelectric liquid crystal or for increasing the intensity of the external field or the like. The reliability of the ferroelectric liquid crystal element is improved while the dominance of the ferroelectric liquid crystal over a TN type liquid crystal is maintained.

Description

[Detailed Description of the Invention] [Prior Art] Ferroelectric liquid crystals, which are currently being researched worldwide, have been conventionally applied to watches, calculators, etc.
It is superior to electrified type liquid crystals in terms of faster response speed and wider viewing angle. The fast response speed of ferroelectric liquid crystals is due to the spontaneous polarization of the liquid crystal itself. In general, the response speed of ferroelectric liquid crystals is inversely proportional to spontaneous polarization and external electric field, and proportional to viscosity. Conventionally, ferroelectric liquid crystals with small spontaneous polarization were considered unusable due to their slow response speeds.
The development of ferroelectric liquid crystals with large spontaneous polarization has been progressing. However, this spontaneous polarization has a significant effect on the switching characteristics of ferroelectric liquid crystals. In other words, due to the presence of spontaneous polarization, this polarized charge forms an electric field within the liquid crystal layer even when no external electric field is applied, causing uneven distribution of ionic impurities within the liquid crystal layer. In order to restrain the spontaneous polarization of ferroelectric liquid crystal molecules, which exhibit bistability under normal conditions, they are forced to become monostable, resulting in a decrease in contrast ratio.

This problem will be explained with reference to FIGS. 1 and 2, which show the I-characteristics of a ferroelectric liquid crystal element using a liquid crystal having a spontaneous polarization of 32 nC/cm2. FIG. 1 shows the measurement immediately after the liquid crystal was injected, and FIG. 2 shows the measurement 60 days after the measurement in FIG. 1. As shown in FIG. 1, immediately after the liquid crystal was injected, the graph was almost symmetrical, indicating that the liquid crystal was well bistable. However, after 60 days, the peak of the current shifted considerably to the right, and under the same driving conditions as immediately after liquid crystal injection, it was predicted that the contrast ratio would drop significantly, and in fact, in this cell, the contrast ratio was 23.8 immediately after liquid crystal injection. The contrast ratio decreased to 5.6 after 60 days.

However, the contrast ratio is 640 x 400 dots.
A matrix cell was prepared, the cell was placed between crossed Nicol polarizing plates as shown in FIG. 3, and the amount of transmitted light was measured using a 2no+φ slit. Then, measurements were taken at five arbitrary locations on the cell, and the average thereof is shown in FIG.

Further, FIG. 4 shows how the contrast ratio of the same cell changes over time until 60 days have elapsed.

From the figure, since the contrast ratio is almost stable after about 40 days, it seems that even after 60 days or more, the contrast ratio is almost the same as the contrast ratio after 60 days.

By the way, since displays generally require a contrast ratio of at least 10, this cell cannot be applied to displays and the like.

[Structure of the invention]

To solve this problem, the present invention reduces the influence of ionic impurities by using a ferroelectric liquid crystal having a spontaneous polarization of 15 nc/cm or less, thereby significantly improving the performance of the ferroelectric liquid crystal element. Table 1 shows the contrast ratio immediately after liquid crystal injection and the contrast ratio 60 days later for 10 types of ferroelectric liquid crystals having different spontaneous polarizations.

Table 1 When applied to displays etc., the contrast ratio is 1.
Considering that 0 or more is required, the ferroelectric liquid crystals that can be used in Table 1 are A-L, and therefore the spontaneous polarization of the ferroelectric liquid crystal must be 15 nC/cm" or less. I can say that.

〔effect〕

From the above, the liquid crystal element of the present invention using a ferroelectric liquid crystal having a spontaneous polarization of 15 nC/cm or less solves the conventional problems in switching characteristics, and also reduces the viscosity of the ferroelectric liquid crystal in terms of response speed. Alternatively, this problem can be sufficiently solved by methods such as increasing the external electric field strength. Therefore, the present invention significantly improves the reliability of ferroelectric liquid crystal elements while maintaining the superiority of ferroelectric liquid crystal elements over TN type liquid crystals. It is something that

[Brief explanation of the drawing]

Figures 1 and 2 show the results of a liquid crystal element using a ferroelectric liquid crystal with a spontaneous polarization of 32 nC/cm'', immediately after liquid crystal injection, and 600 nm.
The IV characteristics are shown after a day has passed. FIG. 3 shows a contrast ratio measurement system. FIG. 4 shows the change in contrast ratio of a ferroelectric liquid crystal device having a spontaneous polarization of 32 nC/cm 2 from immediately after liquid crystal injection until 60 days have elapsed. Light source slit polarizer LCD cell detector oscilloscope power supply

Claims (1)

[Claims] 1. Ferroelectric liquid crystal element having ferroelectric liquid crystal with spontaneous polarization of 15 nC/cm^2 or less