Matrix liquid crystal display

Details Matrix liquid crystal display Matrix liquid crystal display

1999-12-21

2003-05-13

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299010, C252S299630, C252S299660

Reexamination Certificate

active

06562419

ABSTRACT:

The invention relates to a matrix liquid crystal display containing
two plane parallel support plates which together with a frame form a cell,
integrated non-linear elements for switching individual picture elements on the support plates and
a nematic liquid crystal mixture which is present in the cell and has a positive dielectric anisotropy and high resistivity, the liquid crystal mixture being based on the following components:
a) at least 10% by weight of a liquid-crystalline component B comprising one or more compounds having a dielectric anisotropy of more than +1.5,
b) up to 90% by weight of a liquid-crystalline component A comprising one or more compounds having a dielectric anisotropy of −1.5 to +1.5 of the general formula I
 in which
R
1
and R
2
are each, independently of one another, n-alkyl, &ohgr;-fluoroalkyl or n-alkenyl having up to 9 carbon-atoms,
 the rings A
1
, A
2
and A
3
are each, independently of one another, 1,4-phenylene, 2- or 3-fluoro-1,4-phenylene, trans-1,4-cyclohexylene or 1,4-cyclohexenylene,
Z
1
and Z
2
are each, independently of one another, —CH
2
CH
2
— or a single bond, and
m is 0, 1 or 2, and
c) 0 to 20% by weight of a liquid-crystalline component C comprising one or more compounds having a dielectric anisotropy of less than −1.5, and the nematic liquid crystal mixture having a nematic phase range of at least 60° C., a maximum viscosity at 20° C. of 30 mpa·s and a mean dielectricity constant ∈≦8.
Matrix liquid crystal displays (MLC displays) according to the preamble are known. For example, active elements (i.e. transistors) can be used as non-linear elements for the individual switching of the individual picture elements. This is referred to as an “active matrix”, in which two types can be distinguished:
1. MOS (metal oxide semiconductor) transistors on a silicon wafer as the substrate.
2. Thin film transistors (TFT) on a glass plate as the substrate.
In the case of type 1, dynamic scattering or the guest/host effect is usually used as the electrooptical effect. The use of single-crystal silicon as the substrate material limits the size of the display, since, even if different partial displays are put together in the form of modules, difficulties arise at the joints.
In the case of the more promising, type 2, which is preferred, the TN effect is usually used as the electrooptical effect. Two technologies are distinguished: TFTs consisting of compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. The latter technology is the subject of intense development work worldwide.
The TFT matrix is disposed on the inside surface of one of the glass plates of the display, while the other glass plate carries the transparent counter electrode on its inside surface. Compared with the size of the picture element electrode, the TFT is very small and essentially does not interfere with the picture. This technology can also be extended to picture displays in fully satisfactory colours by arranging a mosaic of red, green and blue filters in such a manner that each filter element is opposite to a switchable picture element.
The TFT displays usually operate as TN cells which contain crossed polarizers in transmission and are illuminated from behind.
The term MLC displays in this context comprises each matrix display which has integrated non-linear elements, i.e. apart from the active matrix also displays which contain passive elements such as varistors or diodes (MIM=metal/insulator/metal).
MLC displays of this type are in particular suitable for TV applications (e.g. portable TVs) or for highly informative displays in automobile and aircraft construction. In addition to problems regarding the angle dependency of the contrast and the switching times, difficulties in MLC displays arise from the insufficient resistivity of the liquid crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. With decreasing resistivity, the contrast of an MLC display deteriorates. Since the resistivity of the liquid crystal mixture usually decreases by interaction with the inside surfaces of the displays over the lifetime of an MLC display, a high (initial) resistance is very important for achieving acceptable service lives.
Therefore, there is still a high demand for MLC displays which have very high resistivity in combination with a large range of operating temperatures, short switching times and low threshold voltage.
The object of the invention is to provide MLC displays which do not or only to a small extent have the above disadvantages and, at the same time, have very high resistivities.
It has now been found that this object can be achieved by using nematic liquid crystal mixtures in these display elements, which mixtures are based on the abovementioned components A, B and C, B and C, or B.
Accordingly, the invention relates to an MLC display containing
two plane parallel support plates which together with a frame form a cell,
integrated non-linear elements for switching individual picture elements on the support plates and
a nematic liquid crystal mixture which is present in the cell and has a positive dielectric anisotropy and high resistivity,
 the liquid crystal mixture being based on the following components:
a) at least 10% by weight of a liquid-crystalline component B comprising one or more compounds having a dielectric anisotropy of more than +1.5,
b) up to 90% by weight of a liquid-crystalline component A comprising one or more compounds having a dielectric anisotropy of −1.5 to +1.5 of the general formula I
 in which
R
1
and R
2
are each, independently of one another, n-alkyl, &ohgr;-fluoroalkyl or n-alkenyl having up to 9 carbon atoms,
 the rings A
1
, A
2
and A
3
are each, independently of one another, 1,4-phenylene, 2- or 3-fluoro-1,4-phenylene, trans-1,4-cyclohexylene or 1,4-cyclohexenylene,
Z
1
and Z
2
are each, independently of one another, —CH
2
CH
2
— or a single bond, and
m is 0, 1 or 2, and
c) 0 to 20% by weight of a liquid-crystalline component C comprising one or more compounds having a dielectric anisotropy of less than −1.5,
and the nematic liquid crystal mixture having a nematic phase range of at least 60° C., a maximum viscosity at 20° C. of 30 mPa·s and a mean dielectricity constant ∈≦8.
The invention also relates to the corresponding liquid crystal mixtures, in particular for use in MLC displays. However, the mixtures are also suitable for many other applications, such as, for example, TN, STN or OMI.
Nematic liquid crystal mixtures which instead of the compounds of the formula I contain analogous compounds in which one of the radicals R
1
and R
2
is n-alkyl and the other is n-alkoxy are known and commercially utilized in various designs. However, these liquid crystal mixtures are distinguished by values for the resistivity which are too low and are often between 5×10
9
and 1.1×10
11
&OHgr;cm or less at 20°. The corresponding MLC displays have values for the resistivity which are too low for some commercial applications.
The resistivity of liquid crystal mixtures is in general high, if the dielectric anisotropy is small, since the polar components which are present in mixtures which have a high &Dgr;∈ have a stabilizing effect on ions and thus lead to high conductivity or low resistance. Surprisingly, it has now been found that the resistivity is particularly high, if the mean dielectricity constant ∈[=
3
1
(2∈
⊥
+∈
11
)] is small and, at the same time, the dielectrically neutral (&Dgr;∈ from −1.5 to +1.5) components of the liquid crystal mixture do not contain a

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