JP2949770B2 - Liquid crystal electro-optical element - Google Patents

Description

The present invention relates to an active matrix type liquid crystal electro-optical element.

[Prior Art] In a conventional active matrix type liquid crystal electro-optical element, a wholly aromatic polyimide having a structure as shown in Formula F is used as an alignment material.

In addition, among a plurality of liquid crystal compounds constituting a liquid crystal, a liquid crystal compound having a large Δε generally uses a liquid crystal compound having a terminal group such as G or H in addition to a liquid crystal compound having a terminal group represented by C, D, or E. Met.

[Problems to be Solved by the Invention] However, in a conventional active matrix type liquid crystal electro-optical element, when a still image is displayed for a long period of time, the displayed image remains even when it is not turned on. The phenomenon called was easy to occur. Further, if the liquid crystal display cell is kept turned on for a longer time, there is a problem that display unevenness occurs. These are phenomena that occur when a certain DC voltage is applied to the liquid crystal, impurity ions in the liquid crystal are adsorbed on the alignment layer, and polarization occurs in the cell. These phenomena have reduced the display quality and reliability of the liquid crystal electro-optical element. The voltage for driving the liquid crystal should be an AC signal, but in practice
The voltage applied to the liquid crystal does not become a perfect alternating current due to TFT push-down, DC voltage wrapping between the source line and the alignment substrate, and the asymmetry of the cell due to the presence of a color filter. Is applied to the liquid crystal.

The magnitude of this DC voltage depends on the type and structure of the switching element, but is caused by the DC voltage.
Conventionally, it has been difficult to completely eliminate the occurrence of display burn-in and display unevenness.

Thus, in the present invention, by using a combination of an alignment film that is less likely to adsorb impurity ions and a liquid crystal that captures less impurity ions, the display burn-in of the liquid crystal display element and the occurrence of display unevenness due to long-time continuous driving are suppressed, and reliability is reduced. It is intended to improve.

[Means for Solving the Problems] In a liquid crystal electro-optical element according to the present invention, a liquid crystal composition is sandwiched between a pair of opposed substrates, and an alignment film is formed on at least one of the substrates. An electro-optical element, wherein the alignment film has a structure represented by the following formula B, wherein the liquid crystal composition includes a plurality of liquid crystal compounds, and among the liquid crystal compounds, all the liquid crystal compounds having Δε of +3 or more are: And a terminal group selected from the following formulas C, D and E.

Further, the liquid crystal electro-optical element of the present invention is preferably used for a liquid crystal electro-optical element of a type having a switching element arranged in a matrix on one of the substrates and an electrode arranged on the other substrate. As a specific example of the switching element, a poly-si TFT, an a-si TFT, or a MIM is considered.

[Operation] The signal applied to the liquid crystal in the liquid crystal cell is usually designed to be basically an alternating current.

However, the voltage actually applied to the liquid crystal due to the characteristics of the switching element, the structural problem, the asymmetry of the cell, and the like is not a perfect alternating current but a slight DC voltage. When a DC voltage is applied to the liquid crystal in the cell, impurity ions in the liquid crystal are adsorbed on the alignment film to form an electric double layer.
The formation of the electric double layer proceeds until the DC voltage applied to the liquid crystal layer is completely cancelled. When the voltage applied to the cell turns off and the DC voltage decreases at the same time,
This time, the DC voltage of the electric double layer is applied to the liquid crystal layer. The state in which a voltage is applied to the liquid crystal by the electric double layer is image burn-in.

This burn-in of the display disappears when the impurity ions adsorbed on the alignment layer leave the interface of the alignment layer due to thermal energy or the like and are dispersed in the bulk of the liquid crystal layer, and the potential of the electric double layer becomes sufficiently small.

Further, when the liquid crystal is driven for a long time, adsorption and desorption of impurity ions at the interface of the alignment layer are repeated. If the time required from the time when the impurity ions are adsorbed at the interface of the alignment layer to the time when they are desorbed is set as the relaxation time, if the adsorption and desorption are repeated for a long time, the adsorption of the impurity ions having the long relaxation time will inevitably increase.

The adsorption of the impurity ions having a long relaxation time causes polarization in the cell and hinders application of a desired signal to the liquid crystal. The adsorption of the impurity ions having a long relaxation time causes display unevenness and is observed.

The adsorption of impurity ions in the liquid crystal onto the alignment layer is based on the Japanese Journal of Applied Physics 27
(1987) As reported by MADA and others in L1092, it is possible to evaluate by measuring the hysteresis width of the CV characteristic of the liquid crystal cell.

Table 1-1 and Table 1-2 show the measurement results of the hysteresis width of the liquid crystal.

The definition of this hysteresis width is 100mV / sec scanrate
The CV characteristic of ± 10 V is measured by using .DELTA.C = 10% of the change ΔC = (CMAX−CMIN) between the minimum capacitance CMIN and the maximum capacitance CMAX of the CV characteristic obtained as shown in FIG. Using V1, V2, V3, and V4, the voltage at which the capacitance change of (V1 + V2-
(V3-V4) / 2 was defined as the hysteresis width.

The liquid crystal I is a mixed liquid crystal using only a liquid crystal having a terminal group of fluorine or difluorine in a liquid crystal compound having Δε of +3 or more.

The liquid crystal II is a mixed liquid crystal using a liquid crystal compound having Δε of +3 or more and a liquid crystal having a terminal group of only fluorine or difluorine, which is different from the liquid crystal I.

The liquid crystal III is a mixed liquid crystal using only a liquid crystal compound having a terminal group of fluorine or difluorine in a liquid crystal compound having Δε of +3 or more, which is different from the liquid crystals I and II.

The liquid crystal IV is a mixed liquid crystal using only a liquid crystal compound having a terminal group of fluorine, difluorine or trifluoromethoxy group in a liquid crystal compound having Δε of +3 or more.

The liquid crystal V is a mixed liquid crystal including a liquid crystal compound having a Δε of +3 or more and a liquid crystal compound having a cyano group in addition to a liquid crystal compound having a terminal group of fluorine or difluorine.

The liquid crystal VI is a liquid crystal compound different from the liquid crystal V, which is a mixed liquid crystal including a liquid crystal compound having a terminal group of fluorine or difluorine and a liquid crystal material having a cyano group in addition to a liquid crystal compound having Δε of +3 or more.

The liquid crystal VII is a mixed liquid crystal containing a liquid crystal compound having a Δε of +3 or more and a liquid crystal compound having a cyano group in addition to a liquid crystal compound having a terminal group of fluorine or difluorine and different from the liquid crystal V and the liquid crystal VI.

The liquid crystal VIII is a mixed liquid crystal using only a liquid crystal compound having a terminal cyano group in a liquid crystal compound having Δε of +3 or more.

The liquid crystal IX is a mixed liquid crystal using a liquid crystal compound having a terminal group with a cyano group and fluorine and a liquid crystal compound with fluorine added to a liquid crystal compound having Δε of +3 or more.

The liquid crystal compositions such as liquid crystal I, liquid crystal II, liquid crystal III, and liquid crystal IV have a hysteresis width of liquid crystal V, liquid crystal VI, liquid crystal VII, liquid crystal VII.
I, the value was much smaller than the hysteresis width of the liquid crystal IX.

From this result, it can be seen that a mixed liquid crystal using only a liquid crystal compound having a terminal group of fluorine, difluorine, or trifluoromethoxy group in a liquid crystal compound having Δε of +3 or more has a cyano group or an impurity ion forming an electric double layer. It was found that the amount was smaller than that of a mixed liquid crystal using a liquid crystal compound to which a cyano group and fluorine were added.

Next, a liquid crystal material having Δε of +3 or more is used as a liquid crystal material, and a mixed liquid crystal using only a liquid crystal compound having a terminal group of fluorine or difluorine is used. When the material of the alignment film is changed, an impurity ion is formed on the alignment film. The polarization voltage in the cell was measured by the electric double layer.

This measurement was performed by applying a DC voltage of 5 V to the liquid crystal cell for evaluation for 100 hours in a high-temperature bath at 50 ° C., and then applying the same C at 25 ° C.
-V characteristics were measured. In the CV characteristic, the axis of symmetry of the CV curve is greatly deviated from 0 V as shown in FIG. 2 because internal polarization occurs due to the electric double layer in the cell. This symmetry shift was defined as polarization voltage. The polarization voltage was defined as (V1 + V2 + V3 + V4) / 4 using V1, V2, V3, and V4. Table 2 shows the measurement results of the polarization voltage when the material of the alignment film was changed.

The alignment film I is an alicyclic / aromatic polyimide having the structure shown in A.

The alignment film II is an alicyclic / aromatic polyimide having the structure shown in B.

The alignment film III is a wholly aromatic polyimide having a structure shown by F.

The alignment film IV is a wholly aromatic polyimide having the structure shown in J.

The alignment film V is an alicyclic / aromatic polyimide having a structure represented by K.

The alignment film VI is an all-alicyclic polyimide having the structure shown by L.

From Table 2, the alignment films I and II are the alignment films III and I.
V, the polarization voltage is significantly lower than that of the alignment film V or VI. This is because the alignment films I and II are desorbed in a short time even if impurity ions are adsorbed.

From these results, all of the liquid crystal compounds constituting the liquid crystal and having Δε of +3 or more are the terminal groups represented by CE. And an organic film having a structure shown in A or B. Is used in combination with an alignment film, even if a DC voltage is applied to the liquid crystal layer, the formation of an electric double layer due to the adsorption of impurity ions at the interface of the alignment layer is further reduced. It can be seen that a highly reliable liquid crystal display cell in which display burn-in and display unevenness due to continuous driving for a long time are less likely to occur as compared with the display cell is obtained.

 Hereinafter, the present invention will be described in detail with reference to examples.

Example Reference Example 1 As a first reference example of the present invention, po
A 1-inch diagonal liquid crystal electro-optical device for a viewfinder using a ly-Si TFT was fabricated.

The alignment film is formed by applying an alicyclic / aromatic polyimide having a structure shown in A dissolved in γ-butyllactone to a substrate constituting a liquid crystal electro-optical element by spin coating at 150 ° C.
It was formed by firing for an hour.

As the liquid crystal, a mixed liquid crystal composed of a plurality of liquid crystal compounds was used. Among the liquid crystal compounds constituting the liquid crystal, Δε
Are all liquid crystal compounds having either the terminal group shown by C or the terminal group shown by D. Δε of this liquid crystal was +3.9, and Δn was 0.080.

The cell thickness was set to 5.0 μm so that Δnd was 0.40 μm.

As a result of a reliability test of this liquid crystal electro-optical element and the conventional liquid crystal electro-optical element, the conventional liquid crystal electro-optical element showed image sticking when the window pattern was turned on for 1 minute and then turned off. The display of the liquid crystal electro-optical element did not show any burn-in even after being turned on for 24 hours. In addition, as a result of conducting current aging in a high-temperature chamber at 50 ° C., the conventional liquid crystal electro-optical element had a display failure of 50% or more within 500 hours, whereas the liquid crystal electro-optical element of the present invention had a display defect of 1000%. Even after time aging, the occurrence rate of display defects was 10% or less. The liquid crystal electro-optical device of the present invention has extremely high reliability.

(Reference Example 2) As a second reference example of the present invention, po
A 2 inch diagonal liquid crystal electro-optical element for a light valve using a ly-Si TFT was prepared.

The alignment film was formed in the same manner as in Reference Example 1. The liquid crystal is
A mixed liquid crystal composed of a plurality of liquid crystal compounds was used. Among the liquid crystal compounds constituting the liquid crystal, all of the liquid crystal compounds having Δε of +3 or more were liquid crystal compounds having any one of a terminal group represented by C, a terminal group represented by D, and a terminal group represented by E. . Δε of this liquid crystal is +6.8 and Δn is 0.088.
Met.

The cell thickness was set to 4.5 μm so that Δnd was 0.40 μm.

As a result of a reliability test of this liquid crystal electro-optical element and the conventional liquid crystal electro-optical element, the conventional liquid crystal electro-optical element showed image sticking when the window pattern was turned on for 1 minute and then turned off. The display of the liquid crystal electro-optical element did not show any burn-in even after being turned on for 24 hours. In addition, as a result of conducting current aging in a high-temperature chamber at 50 ° C., the conventional liquid crystal electro-optical element had a display failure of 50% or more within 500 hours, whereas the liquid crystal electro-optical element of the present invention had a display defect of 1000%. Even after time aging, the occurrence rate of display defects was 10% or less. The liquid crystal electro-optical device of the present invention has extremely high reliability.

Reference Example 3 As a third embodiment of the present invention, a liquid crystal electro-optical element for a display of a 10-inch diagonal laptop computer using MIM and having 640 × 400 × 3 pixels was prepared.

The alignment film was formed in the same manner as in Reference Examples 1 and 2. As the liquid crystal, a mixed liquid crystal composed of a plurality of liquid crystal compounds was used. Among the liquid crystal compounds constituting the liquid crystal, Δ
All the liquid crystal compounds having ε of +3 or more were liquid crystal compounds having a terminal group shown in D. This liquid crystal did not contain any ester-based liquid crystal compound. Δε of this liquid crystal is +
3.2, Δn was 0.088.

The cell thickness was set to 4.5 μm so that Δnd was 0.40 μm.

As a result of a reliability test of this liquid crystal electro-optical element and the conventional liquid crystal electro-optical element, the conventional liquid crystal electro-optical element showed image sticking when the window pattern was turned on for 1 minute and then turned off. No burn-in of the display was observed even when the liquid crystal electro-optical element was lit for 48 hours. In addition, as a result of conducting current aging in a high-temperature chamber at 50 ° C., the conventional liquid crystal electro-optical element had a display failure of 40% or more within 1000 hours, whereas the liquid crystal electro-optical element of the present invention had a display defect of 1000%. Even after time aging, the rate of occurrence of display defects was 5% or less. The liquid crystal electro-optical device of the present invention has extremely high reliability.

(Embodiment 1) As a first embodiment of the present invention, if the number of pixels is 320 × 220,
A 1-inch diagonal liquid crystal electro-optical device for a viewfinder using a ly-Si TFT was fabricated.

The alignment film was formed by applying an alicyclic / aromatic polyimide having a structure shown in B in which N-methylpyrrolidone was dissolved to a substrate constituting a liquid crystal electro-optical element by spin coating at 150 ° C. for 1 hour. .

As the liquid crystal, a mixed liquid crystal composed of a plurality of liquid crystal compounds was used. Among the liquid crystal compounds constituting the liquid crystal, Δε
Are all liquid crystal compounds having either the terminal group shown by C or the terminal group shown by D. Δε of this liquid crystal was +3.9, and Δn was 0.080.

The cell thickness was set to 5.0 μm so that Δnd was 0.40 μm.

As a result of a reliability test of this liquid crystal electro-optical element and the conventional liquid crystal electro-optical element, the conventional liquid crystal electro-optical element showed image sticking when the window pattern was turned on for 1 minute and then turned off. The display of the liquid crystal electro-optical element did not show any burn-in even after being turned on for 24 hours. In addition, as a result of conducting current aging in a high-temperature chamber at 50 ° C., the conventional liquid crystal electro-optical element had a display failure of 50% or more within 500 hours, whereas the liquid crystal electro-optical element of the present invention had a display defect of 1000%. Even after time aging, the occurrence rate of display defects was 10% or less. The liquid crystal electro-optical device of the present invention has extremely high reliability.

Embodiment 2 As a second embodiment of the present invention, when the number of pixels is 480 × 440 and po
A 2 inch diagonal liquid crystal electro-optical element for a light valve using a ly-Si TFT was prepared.

The alignment film was formed in the same manner as in Example 1. The liquid crystal is
A mixed liquid crystal composed of a plurality of liquid crystal compounds was used. Among the liquid crystal compounds constituting the liquid crystal, all of the liquid crystal compounds having Δε of +3 or more were liquid crystal compounds having any one of a terminal group represented by C, a terminal group represented by D, and a terminal group represented by E. . Δε of this liquid crystal is +6.8 and Δn is 0.088.
Met.

The cell thickness was set to 4.5 μm so that Δnd was 0.40 μm.

As a result of a reliability test of this liquid crystal electro-optical element and the conventional liquid crystal electro-optical element, the conventional liquid crystal electro-optical element showed image sticking when the window pattern was turned on for 1 minute and then turned off. The display of the liquid crystal electro-optical element did not show any burn-in even after being turned on for 24 hours. In addition, as a result of conducting current aging in a high-temperature chamber at 50 ° C., the conventional liquid crystal electro-optical element had a display failure of 50% or more within 500 hours, whereas the liquid crystal electro-optical element of the present invention had a display defect of 1000%. Even after time aging, the occurrence rate of display defects was 10% or less. The liquid crystal electro-optical device of the present invention has extremely high reliability.

Although the embodiments have been described above, the present invention is not limited to the TN (twisted nematic) mode liquid crystal display element as in the above embodiments, but also to the STN (super twisted nematic) mode.
It can be applied to a liquid crystal electro-optical element of another liquid crystal display mode such as a mode.

[Effects of the Invention] As described above, according to the invention of the present application, an impurity film is adsorbed on an alignment film by combining an alignment film that is hardly adsorbed with an impurity ion and a liquid crystal composition that has a small amount of impurity ions taken in. Since the amount of impurity ions can be suppressed, and as a result, an electric double layer is hardly formed, an effect of realizing a liquid crystal electro-optical element free from display burn-in or display unevenness is achieved.

[Brief description of the drawings]

FIG. 1 is a diagram showing hysteresis appearing in the measurement of CV characteristics of a liquid crystal cell. FIG. 2 is a diagram showing deviation of a symmetric axis of a CV curve due to internal polarization in a liquid crystal cell, which appears in measurement of a CV characteristic of the liquid crystal cell.

Claims (1)

(57) [Claims] 1. A liquid crystal electro-optical element having a liquid crystal composition sandwiched between a pair of substrates disposed opposite to each other, and an alignment film formed on at least one of the substrates. Wherein the liquid crystal composition includes a plurality of liquid crystal compounds, and among the liquid crystal compounds, all the liquid crystal compounds having Δε of +3 or more are selected from the following formulas C, D, and E A liquid crystal electro-optical element comprising a terminal group.