GB2080561A

GB2080561A - Liquid crystal display device

Info

Publication number: GB2080561A
Application number: GB8120490A
Authority: GB
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1980-07-04
Filing date: 1981-07-02
Publication date: 1982-02-03
Also published as: DE3126362A1; GB2080561B; JPS5716087A

Abstract

A liquid crystal display device contains a nematic liquid crystal material satisfying the following conditions (a) to (c) i.e. (a) a threshold voltage of 2 V or less, (b) DELTA epsilon / epsilon 1</=2, and (c) having substantially no absorption in the visible light region. The device has excellent wider viewing angle properties, low voltage driving and a colorless display surface.

Description

SPECIFICATION Liquid crystal display device This invention relates to a liquid crystal display device having excellent display performance, particularly to a twisted nematic (TN) type liquid crystal display device.

In a TN type display method, there are used nematic liquid crystals having positive dielectric anistoropy (the dielectric constant e" at a direction of long axis of liquid crystal molecules being larger than the dielectric constant E1 at a direction of short axis of liquid crystal molecules). But when a driving voltage for liquid crystal display is lowered (for example, 3 V or less), there arises a defect in that an area wherein a driving state of liquid crystals is difficult to be seen or not seen at all takes place depending on an angle (a viewing angle) to the device surface, also in relation to a rising angle (a tilt angle) of the liquid crystals and the TN display construction.Such a defect is not so important in the case of wrist watches, small electronic computers wherein the viewing angle is almost perpendicular to display surfaces, while such a defect is a serious problem in the case of clocks, liquid crystal display televisions, etc., wherein the viewing angle is not always perpendicular to display surfaces, or in the case of devices having larger display areas, especially matrix driving displays with high duty ratio: that is, the viewing angle as large as possible is required.

As to viewing angle properties of nematic liquid crystals having influence on display quality as mentioned above, there have been known that optical anisostropy of liquid crystal materials An (An = nO - ne, that is, a difference between optical anisotropy of normal light nO and that of abnormal light ne in double refraction) and the thickness of liquid crystal layers (d) have influences on the viewing angle properties. But there are limitations to change the values of An and d. That is, as to the gap (d) control of a display device, it is difficult to make the gap less than about 10 Rm according to the mass production technique now employed, while if the gap is larger than 10 yam, there arises a problem of making the response speed slower.Further, in order to solve a problem of uneveness of display due to double refraction, it is necessary to make the value An.dB1.5. That is, when d = 10 Fm, then it is required to be An z 0.15. There have not been found liquid crystal materials having such a quick response speed. Therefore, at present, no desirable result was obtained by changing An and d.

The present inventors have studied and found that dielectric constants of liquid crystals have great influence on the viewing angle properties as physical factors in the form of AE/1 = Ess = E - E1, wherein E and E1 are as defined above).

On the other hand, liquid crystal display devices having a colorless display surface have recently been demanded strongly. Such a colorless display surface depends greatly on chemical structure of liquid crystal compositions and is an important factor for improving the display quality.

It is an object of this invention to provide a liquid crystal display device improved in viewing angle properties with a low driving voltage compared with known liquid crystal display devices based on the above-mentioned findings. It is another object of this invention to provide a liquid crystal display device having excellent responsiveness as well as the above-mentioned properties. Other objects and advantages of this invention will be clear from the following descriptions.

Features of this invention are to provide a liquid crystal display device comprising a pair of substrates which are placed in parallel and at least one of which is transparent, the peripheries of said substrates being sealed by a sealant to form a cell of closed structure, said cell being filled with one or more nematic liquid crystals, the inner surfaces of said substrates facing each other having electrode thereon and having a twisted nematic structure when no voltage is applied to said liquid crystals, characterized in that said nematic liquid crystals satisfy the following conditions (a) to (c):: (a) a threshold voltage of 2 V or less, an preferably a response time of 100 msec or less at normal temperatures when applied a voltage of 2 times as large as the threshold voltage, (b) (E"- E1)/Er being 2 or less, and (c) having substantially no absorption in the visible light region.

In the attached drawings, Figure 1 is a cross-sectional view of one example of a reflection type liquid crystal display device according to this invention, Figure2 is a graph showing the relationship between AE (= Ex,- - E1) and AE/Ea, Figures 3 and 4 are graphs explaining the definition of response time, Figure 5 is a graph showing the relationship between the viewing angle properties and physical properties of liquid crystals and device parameters, Figure 6 is a graph showing the relationship between the dielectric properties and the device parameters, and Figures 7 and 8 are graphs showing the relationship between liquid crystal compositions and physical and other properties of the liquid crystals.

Referring to Figure 1, a typical example of a liquid crystal display device according to this invention is shown, but this invention is not limited to it and can take various modifications. In Figure 1, numeral 1 denotes twisted nematic liquid crystals, numeral 2 denotes an orientation controlling film, numeral 3 denotes an electrode, numeral 4 denotes a substrate, numeral 5 denotes a polarizing plate, numeral 6 denotes a reflector, and numeral 7 denotes a sealant. In this invention, the nematic liquid crystals should satisfy the special conditions in order to improve the viewing angle properties without lowering the response rate and without raising the driving voltage.

The present inventors have found that there is a relationship between the viewing angle properties ss and physical parameters of liquid crystals represented by #n-"1/##.#αe/#V as shown in Figure 6.

The viewing angle properties: 40 10 ss = Vth Vth ......................................................... (1) wherein Vth10 is a voltage applied at a time of changing 10% of transmitted light amount at a position moved by an angle of 10 from the perpendicular line to the display surface of a liquid crystal display device, and is a voltage applied at a time of changing 10% of transmitted light amount at a position oved by an angle of 40 from the perpendicular line to the display surface of the liquid crystal display device. When ss = 1, that is, an ideal state, there is no viewing angle differences in the range of angles of 10 to 40 .

As shown in Figure 6, there is a linear relationship between the product of #n, #1/##(=#ff - #1), and #αe/#V (or simply #α/#V) and ss, that is ss # #n.#1/##.#α/#V ........................................ (2) wherein #α/#V is a device parameter and represented by the equation (3):: #α/#V = (αvo - αvth)/(Vo - Vth) ..................................... (3) in other words, #α/V is a factor showing the voltage dependency of an average tilt angle and means "easiness of moving" of the liquid crystals to the electric field direction at a time of applying the electric field.

In the above equation (3), V0 mrans a voltage applied at which the change of transmittance is maximum, Vth is a voltage applied at which transmittance changes 10%, that is, a so-called threshold voltage (also see Figure 3), αvo is an average tilt angle at Vo and αvm is an average tilt angle at Vth.The average tilt angle a can be obtained by measuring properties of electric capacity (C) of liquid crystal at bias voltage (V) according to the Schadt et al method (IEEE Transaction on Electron Devices, ED-25, 1125 (1979)) and calculating the following equation (4):

wherein C(V) : capacitance in homogeneously aligned liquid crystals at bias voltage (V), C1 : capacitance in homogeneously aligned liquid crystals at zero bias voltage, and C : capacitance in homeotropic aligned liquid crystals at zero bias voltage.

By using this method, the values of ESX and E1 can also be obtained.

According to the study of the present inventors, the value of AciAV has a good relationship to the value of ##/#1 as shown in Figure 6. Further, from Figures 5 and 6, it is clear that greatly depends on particularly the value of ##/#1.

On the other hand, phenylcyclohexane series liquid crystals are known to have good responsiveness but have the values of An . 0.1 and p < 0.8, that is, the viewing angle properties are not so good. In order to enlarge the viewing angle of such phenylcyclohexane liquid crystals having good responsiveness and small An, it is necessary to enlarge the value of E1/hE in the equation (2), in other words, it is necessary to make the value of A'/,1 smaller. But many known liquid crystals have a relationship between A and AE/E1 as shown in Figure 2, that is, when the value of AE/E1 becomes smaller, the value of AE also becomes smaller, which results in making the driving voltage undesirably higher.But in Figure 2, the liquid crystal marked with X belongs to dioxane series liquid crystals as shown in Example 1, Run No. 17, and has the larger value of ## when the ##/#1 is small compared with the other liquid crystals. Such a liquid crystal material having ## # 10 and #1 # 6 is very useful in this invention. As mentioned above, ss = 1 is the ideal value and hardly be attained practically, so that in order to obtain wider viewing angle, the value of ss # 0.8 is sufficient considering practical uses. From Figure 5, #n.#1/##.#α/#V # 1.6 when ss # 0.8, and in order to use liquid crystals having excellent responsiveness of #n # 0.1, the value of A'/,1 should be: AE/E1 S 2 considering the results of Figure 6. Among liquid crystals now available, those having An of 0.15 or less are usable considering responsiveness and those having An > 0.15 are not preferable because of bad responsiveness.

As mentioned above, when liquid crystals having the value of ~E/E1 S 2 are used, there can be obtained liquid crystal display elements having a wider viewing angle. Such liquid crystals can be obtained by mixing a small amount of liquid crystal materials having negative dielectric anisotropy or near to zero to liquid crystal materials having positive dielectric anisotropy, A E = (Eti - E1) 3 O. But in general, AE may be lowered remarkably so as to make Vth undesirably higher. This must be avoided.

In order to obtain the value of ~ E/EA S 2 without lowering the value of A, it is preferable to use materials having high EAZ and high E1 values. Examples of liquid crystals having high values both in 'ii and E1 are dioxane series and ester series liquid crystals. Thus, at least one kind of these liquid crystals is preferably added to one or more liquid crystals having dielectric anisotropy of near zero.

Examples of usable liquid crystals in this invention are listed in Table 1.

TABLE 1 No. Kind Example 1 Cyclohexyl cyclohexane series 2 Phenylcyclo hexane series 3 Biphenyl series 4 Ester series 5 Dioxane series 6 Thioester series 7 Pyrimidine series

8 Polycyclic series

9 Diester series

10 Biphenyl ester series

Note) R' = CnH2n+1, CnH2n+1-O, etc.

X = NO2, CN, CnH2n+1, CnH2n+1, CnH2n+1-O, etc.

Needless to say, the resulting mixed liquid crystals should have the value of ##/#1 # 2. More concretely, preferable combinations of liquid crystals usable in this invention are as follows:

In the above combinations (a) to (g), R is an alkyl or alkoxy group and Xis an alkoxy or cyano group. It is preferable to use one of the above-mentioned combinations (a) to (g) in an amount of 50% by weight or more, more preferably 60% by weight or more. Further, in order to broaden a mesomorphic range, one or more compounds of No. 8 to No. 10 in Table 1 can be added to one of the above-mentioned combinations (a) to (g) in an amount of not more than 30% by weight. These liquid crystal compositions mentioned above also satisfy the condition (a), the threshold voltage of 2 V or less.

In the case of liquid crystal display devices in which fast response time is required such as watches or clocks which require second display, the viscosity of the liquid crystal material used therein is preferably 40 centipoises or less at the temperature of driving state (usually at room temperatureof 20 C). By using these liquid crystal materials, the response time of about 100 msec or less at the static driving of 2 Vth can be obtained. If necessary, liquid crystal compositions having better responsiveness can be obtained by lowering their viscosities.

Response time is defined as follows. A rise time (TON) iS a time required for making relative transmittance of light from 0 to 90% when a voltage of 2 times as large as the threshold voltage (Vth) is applied (V0 = 2 Vth) and a decay time (TOFF) iS a time required for making relative transmittance of light from 100 to 10% when a supply of voltage is cut off, as shown in Figure 4, when incident light is measured at a perpendicular direction to the device surface (i.e. angle of 0 ). Response time means either TON or TOFF.

The viscosity was measured by using a rotational viscometer (E type manufactured by Tokyo Keiki K.K.). A viscosity of 40 mPas (= centipoises) or less at 200C corresponds to a response time of 100 msec or less at 20"C.

Liquid crystal compositions having a response time of 100 msec or less are those containing phenylcyclohexane series liquid crystals, phenylcyclohexylphenyl series liquid crystals, cyclohexylcyclohexylcarboxylate series liquid crystals or those as mentioned above plus biphenyl series liquid crystals or pyrimidine series liquid crystals.

Furthermore, the liquid crystal materials used in this invention should also satisfy the condition (a), that is, having Vth S 2 V in order to achieve lower driving voltage on static and dynamic drive displays. Examples for, on matrix display with high duty ratio (1/32), the liquid crystal materials have to satisfy driving voltage < 12andVth%2V.

The liquid crystal material used in this invention should satisfy the condition (c), that is, having substantially no absorption in the visible light region in order to satisfy the requirement of having a colorless display surface and a good stability. Schiff's base and azoxy series liquid crystals having absorption in the visible light region are poor in stability for ultra violet light and humidity.

This invention is illustrated by way of the following Examples, in which all percents are by weight unless otherwise specified.

Example I A mixture of dioxane series liquid crystals represented by the formula:

wherein n is 4,5 and 6, proportions of individual dioxane series liquid crystals being equimolar amounts, was prepared.

The resulting mixture had AE = 13 and E1 = 9.

Typical conventional liquid crystals had the values of A, EA and AElE1 as shown in Table 2, wherein individual components are listed in Table 3.

TABLE 2 No. Kind AE E1 A,/1 Note Composition 11 Biphenylseries 12 5.3 2.3 is listed in Table 3 12 Phenylcyclohexane series 10.6 4.7 2.3 13 Biphenyl-pyrimi dine series 14.7 5.3 2.8 14 Biphenyl-ester series 8.3 5.3 1.3 15 Ester series 15 7.2 2.2 16 Phenylcyclohexane series 6.6 3.9 1.7 17 Dioxaneseries 13 9 1.4 Example 1 As shown in Table 2, conventional liquid crystals do not satisfy the both conditions of ## # 10, the value giving a practical driving voltage, and ##/#1 # 2, the value giving a wider viewing angle properties. On the other hand, the dioxane series liquid crystals of Example 1 satisfy the both conditions of ## # 10 and ##/#1 # 2.

Table

No. Composition c37oMc 15 c511ffc 50 11 C7H15-(0) 15 C8H170 My C 15 gHll-Q /7 CN 5 5 C3H7 35 12 C5H11MCN 35 C7H15 HCN 30 - Cont ' . - Table 3 (Cont'd)

C5H11Mff Cii 45 5 Cii 25 13 C5ESS < CN 5 C7H15 (0 cii 10 C1IH9 + Cii 10 C5H11 Nii#mo Cii 5 C3H7ThffcN 35 C5H11M CN I 10 C6H13 --(0) 10 14 C5H11 < CN 10 C5Hll {H} COO-OCgHll 10 C5H11 VCOO~OC2H5 5 C3H7 {+COO40 > 0C5S11 10 C4H9 < cocCco JN 10 - Cont'c - Table 3 (ContVd)

CLiH9 X C Z < F C & 25 15 C6E13&commat; COO X CN 30 C:'17&commat; COOXC ; 25 C4,H9 CO w COO e 0) CN CN 20 C3H1 ff C2H5 20 C3H7 S CN 30 16 C3H7 -OCllHg 10 C5BI11 HCN -(O--CN 25 C 11HC2E5 C2H5 11 c3:-:7 JE f 5'-' C J. Li CSsg CN 33 17 C5Ell CN 33 C 1 I % 13 XCZ 3 i The dioxane series liquid crystals (No. 17) have a slower response time but can be improved in response time by mixing other liquid crystals as mentioned below.

Example 2 A mixture of liquid crystals was prepared by mixing the phenylcyclohexane series liquid crystals as shown No. 16 in Table 2 with the dioxane series liquid crystals as shown No. 17 in Table 2 in the weight percents of 0 - 100%. Vth and hE/eq of individual liquid crystal compositions were measured and shown in Figure 7, curves 1a and 1b.

As shown in Figure 7, in order to satisfy the condition of Vth # 2 V, it is necessary to add 20% or more of No.

17 liquid crystals. In all the cases, ##/#1 is less than 2.0.

Since the No. 17 liquid crystals have an elastic constant of about 5.5 x 10-12 N, which is lower than the value of 7 - 10 x 10-12 N of usual liquid crystals, they have an advantage of lowering the driving voltage to a lower level than other liquid crystals having the same value of Ae.

The liquid crystal display device containing a mixture of No. 17 and No. 16 liquid crystals in weight ratio of No. 17 20% or more had the viewing angle properties ss of about 0.82 and had substantially no absorption in the visible light region.

Example 3 A mixture of liquid crystals was prepared by mixing the biphenyl-phenylcyclohexylcarboxylate series liquid crystals as shown No. 14 in Table 2 with the dioxane series liquid crystals as shown No. 17 in Table 2 in weight percents as shown in Figure 7. Vth and A1 of individual liquid crystal compositions were measured and shown in Figure 7, curves 2a and 2b.

As is clear from Figure 7, in order to satisfy the conditions of Vth S 2 V and AE/E1 S 2, it is necessary to add about 15% or more of No. 17 liquid crystals.

The mixture had the viewing angle properties p of about 0.84 and had good stability.

Example 4 An ester series liquid crystal mixture (No. 18 liquid crystals) containing equimolar amounts of the following compounds:

was mixed with the No. 16 liquid crystals (see Table 2) in weight percents as shown in Figure 8. Vth and AElE1 of individual liquid crystal compositions were measured and shown in Figure 8, curves 3a and 3b.

As is clear from Figure 8, in order to satisfy the conditions of Vth < 2 V and AElE1 S 2, it is necessary to add about 30 to 60% of No. 18 liquid crystals.

When a liquid crystal display device was made by using a mixture of No. 16 (70%) and No. 18 (30%) liquid crystals, the resulting device showed p of about 0.83 and substantially no absorption in the visible light region.

As mentioned above, the liquid crystal display devices according to this invention have good viewing angle properties (p > 0.8) at low voltage driving (2 V or less) and substantially no absorption in the visible light region, these excellent display properties being not expected from known liquid crystal display devices.

Example 5 A liquid crystal mixture containing 80% of No. 16 liquid crystals and 20% of No. 17 liquid crystals, each shown in Table 2, was placed in a liquid crystal display device as shown in Figure 1 having a cell gap of 10 m. Response properties of the resulting display device were measured. The rise time at a static driving applying 2 Vth voltage at room temperature (20 - 250C) (TON) was 90 msec and the decay time (TOFF) was 94 msec, these values showing remarkably good response properties.

Claims

1. A liquid crystal display device comprising a pair of substrates which are placed in parallel and at least one of which is transparent, the peripheries of said substrates being sealed by a sealant to form a cell of closed structure, said cell being filled with one or more nematic liquid crystals, the inner surfaces of said substrates facing each other having electrode thereon and having a twisted nematic structure when no voltage is applied to said liquid crystals, characterized in that said nematic liquid crystals statisfy the following conditions (a) to (c):: (a) a threshold voltage of 2 V or less, (b) (E,1 - E1/E1 being 2 or less, wherein E11 is a dielectric constant at a direction of long axis of liquid crystal molecules and E1 is a dielectric constant at a direction of short axis of liquid crystal molecules, and (c) having substantially no absorption in the visible light region.

2. A liquid crystal display device according to Claim 1, wherein the nematic liquid crystals contain one or more phenylcyclohexane series liquid crystals.

3. A liquid crystal display device according to Claim 1 or 2, wherein the nematic liquid crystals further contain one or more liquid crystal materials having the values of E1 6 and (E" - E1) 10.

4. A liquid crystal display device according to Claim 3, wherein the liquid crystal materials having the values of ea > 6 and (e" - E1) 10 are dioxane series liquid crystals.

5. A liquid crystal display device according to Claim 1 or 2, wherein the nematic liquid crystals have a response time of 100 msec or less at 20"C when applied a voltage of 2 times as large as the threshold voltage.

6. A liquid crystal display device according to Claim 1, wherein the nematic liquid crystals contains one of the following combinations of liquid crystals at least 50% by weight:

wherein R is an alkyl or alkoxy group and Xis an alkoxy or cyano group.

7. A liquid crystal display device substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.