JPH0210320A - Active matrix type liquid crystal display element - Google Patents

Abstract

PURPOSE:To suppress the change of an electric double layer formed in the border surface between the surface of an orienting film and a liquid crystal material due to an external magnetic field by setting the polarity item component of the surface free energy of the orienting film to <=10 dyne/cm. CONSTITUTION:The orienting film which is used for the active matrix type liquid crystal display element is so constituted that the polarity item component of the surface free energy is <=10 dyne/cm. When there are atoms of oxygen or sulfur having large polarity or atoms with a small polarization rate, an extremely small amount of polar impurities or liquid crystal molecules with strong polarity in liquid crystal are attracted in the surface of the orienting film to form the electric double layer. In this case, the rate of the presence of the atoms of oxygen or sulfur with large polarity or atoms with a large polarization rate in the film surface is small and the leak current of the liquid crystal display device is reducible.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a liquid crystal display element used in an electro-optical liquid crystal display, and more particularly to a liquid crystal display element for an active matrix TN-FEM incorporating a switching element. .

Conventional technology Active matrix type TN-FE incorporating switching elements such as C-MOS or TPT has been used in the past.
M liquid crystal display elements have been widely used in television receivers because of their excellent display quality. The display modes at this time include normally white (NW) mode, in which the display changes from white to black as voltage is gradually applied from a state where no voltage is applied, and normally black (NW) mode, in which the display changes from black to white. NB) mode.

In general, NW mode has excellent features such as very small color shift due to changes in applied voltage and the ability to increase contrast, but in principle, in order to produce a sufficient black level, There is a problem in that a sufficiently large voltage must be applied relative to the saturation voltage. To deal with this, a method has been taken in which a material with a large dielectric anisotropy Δε is used as the liquid crystal material and its threshold voltage and saturation voltage are lowered. However, such a liquid crystal material with a large dielectric anisotropy Δε causes the problem of increasing leakage current of a liquid crystal display element in which it is encapsulated.

) In α-crystal display elements, it is important to reduce leakage current in the liquid crystal panel; an increase in leakage current causes problems such as an increase in driving voltage, a decrease in contrast, an increase in power consumption, and a decrease in reliability. In particular, in the case of an active matrix type liquid crystal display element in which a switching element is provided for each pixel, as the leakage current increases, the voltage loss increases, and the liquid crystal cannot be kept in an excited state during the frame cycle, making it difficult for the screen to A problem arises in that the effective voltage applied to the liquid crystal layer is different between the upper and lower parts, resulting in a difference in contrast. One way to solve this problem is to increase the capacity of the auxiliary capacitor, but this increases the cost of manufacturing the device and is not practical.

Therefore, in order to reduce the leakage current of a liquid crystal display element, a high resistance liquid crystal material is used which has been purified to a high degree of purity to remove as much ionic impurities as possible.

Problems to be Solved by the Invention However, even if a high-resistance liquid crystal material is used, when it is sealed in a liquid crystal display element, an increase in leakage current generally occurs. Moreover, its size varies greatly depending on the type of alignment film material used in the liquid crystal display element. That is, in order to reduce leakage current, it is important to develop an alignment film that comes into contact with the liquid crystal.

The present invention aims to solve the problems of the prior art described above.

Means for Solving the Problems The present invention as claimed in claim 1 provides an active matrix liquid crystal display element having at least an alignment film provided on a transparent conductive film and a switching element, in which the surface free energy of the alignment film is reduced. The polar term component is 10 dylle/c
The invention of claim 2 is characterized in that the above object is achieved by setting the alignment film to methylene iodide or less.
The present invention according to claim 3, characterized in that the above object is achieved by exhibiting a contact angle of 5 degrees or more, is the active matrix type liquid crystal display element, wherein the tilt angle of the encapsulated liquid crystal material is 4 degrees or more and less than 10 degrees. By doing so, the above objectives will be achieved.

The cause of leakage current in liquid crystal display elements is not clear, but its magnitude may be several times larger depending on the material of the alignment film.
■When different types of alignment film materials are stacked, the magnitude of the leakage current is determined by the alignment film material on the side that is in contact with the liquid crystal material.The molecular structure of the alignment film surface that is in contact with the liquid crystal material has a large influence. seems to be doing so. In other words, if highly polar atoms such as oxygen or sulfur or atoms with a large polarizability exist on the film surface, trace amounts of polar impurities in the liquid crystal or highly polar liquid crystal molecules will be adsorbed to the alignment film surface, causing electrical discharge. It is thought that a superimposed layer is formed and its displacement due to the external electric field contributes to an increase in leakage current.

It is known that molecular chains on the surface of a polymer tend to change to a lower energy alignment state depending on the external environment, and the reason why the magnitude of leakage current differs depending on the film formation conditions even with the same alignment film material is because This is thought to be due to the difference in the arrangement of functional groups on the surface of the alignment film.

The alignment film used in the active matrix liquid crystal display element of the present invention has a polar term component of surface free energy of l.
It is characterized by being small at 0 dyne/cm or less, and has a small proportion of highly polar atoms such as oxygen and sulfur, or atoms with high polarizability on the membrane surface, and the formation of the electric double layer is This reduces the leakage current of the liquid crystal display element.

In addition, an alignment film that exhibits a contact angle of 35 degrees or more with methylene iodide, which has a large interaction with atoms with high polarity or atoms with high polarizability, has a small proportion of polar sites on its surface, and the electric double layer described above The generation of is suppressed, and the leakage current of the liquid crystal display element is similarly reduced.

On the other hand, alignment film materials that exhibit a relatively high tilt angle of 4 degrees or more and less than 10 degrees of the encapsulated liquid crystal material are conventionally used alignment film materials (tilt angles of less than 2 degrees).
The alignment regulating force for the liquid crystal material is weaker than that of the liquid crystal material, and the displacement of the generated electric double layer due to an external electric field is relatively small, resulting in a small leakage current.

EXAMPLES The present invention will be explained below based on drawings showing examples thereof. FIG. 1 is a cross-sectional view of an evaluation cell used for measuring leakage current.

Measurement of surface free energy by Fowkes
Measured based on the formula (F, M, Fowkes; 1.E
,C., ,Vol.

56, p40 (1962), and Kitazaki and Hata, Japan Adhesive Association Journal, Vol. 8, No. 3, p. 131 (1972)
reference).

Now, the surface free energy of the alignment film is γ1, and the surface free energy of the medium in contact with the alignment film is γ2, and each consists of a component based on dispersion force (represented by chronological d) and a component based on polarization (expressed by chronological p). If so, they can be written as the following formula:

γ1=γ1+γ1...#(1)γ
2: γ2d + γ2° ... (2)
In addition, the adhesion work between the alignment film l and the medium 2 is expressed as "A' +
2, this is expressed by the following equation.

W12: γ1 + γ2 - γ12 ・-(
3) Here, γI2 is the interfacial free energy. On the other hand, assuming that each free energy that decreases as a result of the interaction at the interface between two substances can be expressed as the sum of the geometric mean of the corresponding surface free energy components, the interfacial free energy can be expressed as Ru.

γ12=γ1+γ2-2(γ, dγ2')"-2(γ1
3γ2p) I' 2...(4) Substituting equation (4) into equation (3), W12=2(γ1dγ2d)172+2(γ1pγ29
)! /2...(5) In this case, since one is solid, the adhesion work WI2 can also be determined from the measurement of the contact angle θ. That is, W, = 72 (1 + cO8 θ) --- (6) Here, methylene iodide (72' = 46.8 dyne/cm, γ2) is used as a standard medium with known surface free energy.
o: 4.0dyne/cm) and water (r 2”29.1d
W+2 was determined by measuring the contact angle with the alignment film, and (5
), the component 1d due to the dispersion force of the surface free energy of the alignment film and the component 1ρ due to polarity were determined by simultaneous equations.

On the other hand, the tilt angle of the liquid crystal material in the liquid crystal display element sealed with the liquid crystal material was measured by the crystal rotation method. At this time, the configuration of the liquid crystal display element used in the measurement was a homogeneous alignment cell (anti-parallel rubbing, liquid crystal layer thickness 15 to 20 μm), and the known liquid crystal material with refractive index anisotropy was 9150 (made by Chisso Petrochemical, ne=1.588, n.

= 1.490 (25° C., measurement wavelength 589 nm)). (For detailed measurement methods, please refer to G, Hatsuy(B
aur), v. Wittwer
and D.W., Berreman:
Phys, Lett, 56A, p142. (1976)
and F, Nakano, M, Isoriki”i (
Iso3ai) and M, 9) (Sato)
;,1-j-A,P,, Vol, 19. No, 10°p
2013. (1980). ) The present invention provides an active matrix liquid crystal display element with low leakage current, but in the following examples, in order to exclude leakage current due to switching elements, the leakage current is reduced using an evaluation cell that does not have a switching element. was evaluated.

As mentioned above, FIG. 1 is a block diagram of an evaluation cell for measuring leakage current. As the spacer 4, 6 μm glass beads were used, and the shape of the electrodes 2.7 was circular with an area of 2 cyr+”, and they were opposed to each other. The leakage 'rL current was measured at ±10 V.
, 30 Hz square wave was applied to the panel, and after the electric field was reversed, 7
After m5ec, the value of the current flowing through the panel was measured at 25°C.

Example 1 Evaluation cells A to E for measuring leakage current having the configuration shown in FIG. 1 were created. A polyimide resin was used for the alignment film 3.6, and the polyimide varnish shown in Table 1 manufactured by Yasan Kagaku Kogyo was applied onto a glass substrate 1.8 having an ITO electrode 2.7 so that the film thickness after curing would be 100 OA. It was applied using the spin code method. Thereafter, the applied polyimide resin was cured at a temperature of 250° C. for 6 hours in a nitrogen stream. Next, the prepared glass substrates 1 and 8 were rubbed and bonded together with 6 μm glass beads as spacers 4 in accordance with a conventional method, and a liquid crystal 5 was prepared.
0-000 was sealed under reduced pressure, and the injection port was sealed with epoxy resin 9 to form evaluation cells A-E.

Table 1 shows the measurement results of leakage current of evaluation cells A to E.

Table 1 The surface free energy of each of the inner membranes was measured by the method described above. The results are shown in Table 2. put it here,
θ1 is the contact angle to methylene iodide, θ2 is the contact angle to water, γp is the polar term of the surface free energy, and γ6 is the dispersion term of the surface free energy.

Table 2 Also, as is clear from Tables 1 and 2, which were created under the same conditions as evaluation cells A-E, the leakage current is small when the polar term component of the surface free energy of the alignment film is 10 dyne/cm or less. , suitable for active matrix type liquid crystal display elements. In place of the polar term component of the surface free energy of the alignment film, a certain solvent (e.g. methylene iodide)
It goes without saying that the degree of influence of the alignment film on the leakage current may be evaluated based on the contact angle.

From the relationship between the contact angle θ1 of the alignment film with respect to methylene iodide and the polar component p of the surface free energy, θ1 is 35 for an alignment film for active matrix type liquid crystal display elements.
It can be seen that an alignment film with a temperature higher than 100% is suitable. In addition to methylene iodide, liquids for contact angle measurement include tetrabromoethane, α-bromnaphthalene, tricresyl phosphate, tetrachloroethane, hexachlorobutadiene,
Suitable materials include polydimethylsiloxane.

Example 2 As the alignment film material, a mixed polyimide of polyimide 5E-610 (manufactured by Yasan Kagaku Kogyo) and polyimide 5P-710 (manufactured by Toshi) was used, and as the liquid crystal material to be sealed, liquid crystal ZLI-3097-000 manufactured by BDH was used. A liquid crystal cell F- for leakage current evaluation was prepared in almost the same manner as in Example 1 except for using
Created J. At this time, polyimide 5E-610 and 5P
By appropriately controlling the mixing ratio of -710, the curing temperature and time, and the magnitude of the rubbing load, the tilt angle of the liquid crystal material was controlled over a range of 1.5 degrees to 25 degrees. Table 3 shows the leakage current and tilt angle values measured by the crystal rotation method for evaluation cells F to J.

Table 3 As is clear from Table 3, liquid crystal cells in which the tilt angle of liquid crystal molecules is 4 degrees or more have a small leakage current, and their practical value is extremely large. However, if the tilt angle exceeds 10 degrees, it is generally difficult to maintain a high contrast of the liquid crystal cell, so it is practically preferable to use an alignment film material with a tilt angle of 4 degrees or more and less than 10 degrees.

In Examples 1 and 2, ZLI-3097-000 and ZLI-3097-000, which have large dielectric anisotropy, were used as liquid crystal materials.
3700-000, the leakage current value is relatively large, but even if other liquid crystal materials are used, Table 1
And the trends in Table 3 remain unchanged.

Effects of the Invention The active matrix liquid crystal display element of the present invention has the effect of suppressing the displacement of the electric double layer formed at the interface between the alignment film surface and the liquid crystal material due to an external electric field, and the leakage current of the display element is reduced. can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS The figure is a sectional view of an evaluation cell used for measuring leakage current in an embodiment of the present invention. 1, 8...φ glass substrate, 2.7・φ-ITO electrode,
3.6... Alignment film material, 4... Spacer, 5...
Liquid crystal material, 9...Name of sealing resin agent, patent attorney Toshio Nakao, 1 person], 8- is the base plate 2.7-ITO electric thickness 3.6-alignment film material 4-space 51 Liquid crystal material 9-Sealing resin

Claims (3)

[Claims] (1) In an active matrix liquid crystal display element having at least an alignment film provided on a transparent conductive film and a switching element, the polar term component of the surface free energy of the alignment film is 10 dyne/cm or less. Features an active matrix type liquid crystal display element. (2) At least in an active matrix liquid crystal display element having an alignment film provided on a transparent conductive film and a switching element, it is required that the alignment film exhibits a contact angle of 35 degrees or more with respect to methylene iodide. Features an active matrix type liquid crystal display element. (3) An active matrix liquid crystal display element having at least an alignment film provided on a transparent conductive film and a switching element, characterized in that the tilt angle of the encapsulated liquid crystal material is 4 degrees or more and less than 10 degrees. Active matrix type liquid crystal display element.