DE2941199A1 - LIQUID CRYSTAL DISPLAY CELL - Google Patents

Description

General Electric Company, 1 River Road, Schenectady,

New York 12305 (USA)

Liquid crystal display cell

The invention is based on a liquid crystal display cell with a first and a second opposite Surface having layer of dichroic liquid crystal material.

It is known that liquid crystal display cells can be advantageously used to produce displays with low power consumption to create. The liquid crystal display cell is a passive device that is usually either in the reflective mode of operation, in which the ambient light optionally from parts of the display is reflected to characters, symbols or other markings to generate, or that is driven in the transmissive mode of operation, in the case of a light source behind the Outgoing light passes through selected areas of the display to produce the visible characters. The dichroic liquid crystal display cell that uses a dichroic dye in one as a host serving liquid crystal material is dissolved, has additional advantages; external elements, such as polarizers or the like. are generally not required. In both modes of operation of the dichroic liquid crystal display cell however, the brightness of the display is generally influenced by a number of factors

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and finally the absorption of light on the two opposing surfaces of the liquid crystal layer limited. The reduction of this absorption in the boundary layers of the liquid crystal layer is then particularly desirable when the display cell utilizes a dichroic liquid crystal layer is made because the dichroic liquid crystal material is capable of an enlarged To generate contrast ratio in the display cell.

It is therefore the object of the invention to provide a dichroic liquid crystal display cell whose contrast ratio and brightness is increased.

To solve this problem, the liquid crystal display cell is characterized by the features of the main claim.

The liquid crystal display cell according to the invention having a layer of liquid crystal material rather than the host serves for a dichroic dye, has on the two opposite surfaces of the liquid crystal layer such interfacial conditions that the molecules of the liquid crystal and the dichroic The dye next to a first surface is homogeneous, i.e. aligned with its longitudinal axes parallel to this surface while the liquid crystal molecules and the dichroic dye molecules are on the other surface oriented perpendicular to this, i.e. oriented homeotropically. The remaining liquid crystal and dichroic dye molecules within the layer between the surfaces have a state of orientation that depends on the presence or

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the lack of an electric field of suitable magnitude to align these molecules either essentially parallel to the planes of the surfaces (in the unexcited or field-free state) or substantially is controlled perpendicular to the planes of the surfaces (in the excited or field-affected state), so that the light in the bulk of the liquid crystal layer is only in the non-excited (first) state is absorbed. At the interface for parallel alignment, light absorption occurs while on the interface for the perpendicular alignment in both the excited and the non-excited state Light is absorbed.

In a preferred embodiment, the limit surface states of the dichroic liquid crystal layer by appropriate treatment of each of the two, essentially transparent and electrically conductive electrodes, next to an associated surface of the dichroic Liquid crystal layer are arranged, generated. The surface at the interface for the vertical Alignment is treated with a suitable surfactant, while the surface at the interface for parallel alignment the liquid crystal layer, on which the molecules have a relatively small angle of inclination have, by means of an oblique vapor deposition of SiIiziamoxidschichten is produced.

In the drawing is a known liquid crystal display cell and an embodiment of the subject matter of Invention shown. Show it:

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1a shows a known reflective, dichroic ^and liquid crystal display cell to illustrate the mode of operation and the light absorption at the interfaces of the liquid crystal layer in a perspective exploded view,

2 shows the definition of the angle of inclination and the angle of rotation between the axes of the liquid crystal or dichroic dye molecules relative to a molecular coating electrode interface in a perspective view and

Fig. 3a shows a reflective dichroic liquid crystal display cell according to the invention, ^un is a perspective exploded view.

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In FIGS. 1a and 1b, details that are identical to one another are provided with the same reference numerals; a known dichroic liquid crystal display 10 includes a liquid crystal display cell 11 having two at a distance from one another, flat, transparent substrates 12a and 12b, which advantageously made of materials such as optical glass or the like. can be made and substantially parallel to each other are arranged. A substantially transparent electrode 14a and 14b, respectively, on each inwardly facing Surface of one of the corresponding substrates 12a and 12b is applied, can at certain points etched to include alphanumeric symbols or other markings made by the liquid crystal display cell 10 should be shown. Advantageously, the electrodes 14a and 14b can be made of materials such as tin oxide (SnO-), indium oxide (In-O-) or the like. be educated. One (not for the sake of simplicity shown) layer of silicon oxide (SiO) is on the inwardly facing surfaces of the Electrodes 14a and 14b are grown obliquely to act as an alignment film, as shown in FIG U.S. Patent 3,834,792.

The space between the spaced electrodes 14a and 14b is filled with a layer, contains the molecules 15 of a liquid crystal material, which see as host for molecules 16 of a dichioi ti Serve dye, which is characterized by a parameter with a sufficiently high order of magnitude to a to give a high contrast ratio. The Molecules 13 as illustrated, belong to a liquid crystal composition having a positive dielectric

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Anisotropy, i.e. the dielectric constant of the material, is parallel to the longitudinal axis of the molecule or the director greater than the dielectric constant in the direction perpendicular to the molecule director. The directors of such a nematic liquid crystal composition tend to be at rest so that all molecules 15 align both parallel to one another and in a single direction, which is illustrated here as the Z-axis, the alignment being through the aforementioned silicon oxide alignment film is supported. The molecules 16 of the dichroic dye are in the nematic Liquid crystal composition dissolved in an amount such that sufficient light absorption described below is attainable. The dye molecules 16 are interspersed between the nematic liquid crystal molecules 15 and with their long axes or directors aligned parallel to the directors of the nematic liquid crystal molecules 15. A suitable liquid crisis II dye composition uses a liquid crystal mixture of ester-type compounds with a positive dielectric anisotropy, the mesophase also exists at room temperature and in the mixture a dichroic dye material, for example Sudan-Schwarz-B or the like. is dissolved in an amount of about 0.5 percent by weight of the liquid crystal mixture. The liquid crystal display cell 11 can either be transmissive or, as illustrated, reflective State operated. In the reflective mode of operation, a plate 17 is made of one optically birefringent material arranged so that its optical axis 17a at an angle of 45 °, based on the directors of the liquid crystal molecules and the dichroic dye molecules 16, proceeds.

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In this way, the optical axis 17a runs in the direction of the vector (X + Z) , the direction of the vector X being orthogonal to the direction of the vector Z and the vectors X and Z spanning a plane parallel to the planes of the substrates 12a and 12b . The plate 17 has two exactly parallel, spaced apart outer surfaces 17b and 17c, which are each so far apart that the distance is sufficient for the plate 17 to be a Ψ plate, ie that one of the doubly refracted optical waves is essentially is delayed by 90 ° (a quarter wavelength »4), based on the other of the optical waves . The plate 17 can advantageously be made of a material such as quartz, calcite, mica or the like.

A reflector 19, which is a polished mirror or the like. can be, has a planar shape and lies essentially completely against the outer surface 17c of the v- plate 17, the opposite outer surface 17b of which in turn lies almost completely against the outer surface of the rear substrate 12b.

The use of the alignment film on both electrodes 14a and 14b in operation causes the liquid crystal molecules 15 and dichroic dye molecules 16 to be aligned substantially parallel (homogeneously) to the plane of substrates 12a and 12b when there is no electric field between electrodes 14a and 14b (Fig. 1a) is applied. A light beam 20 of the ambient light, which, as indicated by the plurality of polarization vectors 21, is unpolarized, runs along the Y-axis in order to impinge on the transparent, front substrate 12a and through this and the

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semi-permeable front electrode 14a to pass therethrough. Incident polarized light 20 is preferred absorbed by the parallel aligned dichroic dye molecules 16, so that a light beam propagating in the Y direction through the rear electrode 14b and the rear substrate 12b 22 is transmitted with linear polarization in the X direction (orthogonal to the longitudinal axis of the dye molecules 16, which are aligned in the Z ^ direction). The polarization of the exiting light beam 22 is indicated by a polarization vector 2 3. The light beam 22 linearly polarized in the X direction arrives through the j plate 17 and becomes circularly polarized Light beam 22 is converted with a left-handed (counterclockwise) polarization vector 25, as shown. The circularly polarized light beam 24 strikes the reflector 18 and is reflected there, whereby the direction of rotation is determined by the reflection of the circularly polarized light beam 26 is reversed into a right-turning (clockwise) Polarization vector 27. The reverse circularly polarized light beam 26 occurs along the X-axis of the light beam 23 through the 4 * - plate 17 in reverse Direction to the transmission direction of the light beam 22 passes through and consequently emerges from the outer surface 17b of the plate 17 as a light beam 28 which is linearly polarized in the Z-direction, like this is indicated by a polarization vector 29. The polarization vector 29 is parallel to the directors of the dichroic dye molecules 16 and it is, because the parallel alignment of the directors the is the strongly absorbing alignment of the dichroic dye molecules used, essentially the total optical radiation of the beam 28 from the MoIe-

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kulen 16 absorbed, so that the intensity of the through the optical beam 30 illustrated by the dashed line emitted by the liquid crystal display 10 is reflected is very small (essentially zero); the optical energy of the incident light beam 20 is partly due to scattering and absorption when passing through the various components of the Display 10 is lost, but the energy is mainly absorbed by the dye molecules 16.

Between the electrodes 14a and 14b, which are at a distance from one another, there is a voltage source 35 (FIG. 1b) switched to generate an electric field E in the Y direction, which causes the liquid crystal molecules 15 ', essentially aligning themselves with their directors so that they are mainly in the Y direction lie. It is important to note that a thin "layer" 38a and 38b of molecules 15 and 16, which are closest to substrates 14a and 14b, respectively, substantially in their home orientation remain and parallel to the planes of the liquid crystal layer and those of the electrodes 14a and 14b, even then when the field is switched on. In the central region 39 of the liquid crystal layer are the dichroic dye molecules 16 'dissolved in the liquid crystal material serving as the host aligned along the liquid crystal molecules so that their directors lie essentially in the Y direction. In this active or "ON" state, a light beam 20 'of incident, unpolarized light is only through the Alignment of the dichroic dye molecules 16 ' damped in the thin "layers" 38a and 38b, in which the molecular axes are still parallel to the layer surfaces are aligned, the preferred absorption still occurring in the thin "layers" 38a and 38b.

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The optical energy of the light beam 40 exiting the outer surface of the rear substrate 12b is substantially randomly polarized, as illustrated by polarization vectors 41, such that the energy is transmitted through the 4- plate 17 with substantially no effect to be optical Exit beam 42 with unpolarized light, the light beam 42 being reflected by the reflector 18 as light beam 43. The light beam 4 3 is again represented by ■ £ - transmitted -Platte 17 as light beam 44, which in turn almost completely passes through the aligned in the Y direction molecules of the display cell 11 'and an additional attenuation due to the absorption in the two thin "layers" 38a and 38b before it emerges as a reflected beam 46 with optical energy. The brightness of the reflected beam is slightly less than that of the incident light beam 20 because it passes through a total of four absorbing "layers" (ie layers 38a and 38b in the + Y direction and layers 38b and 38a in the - Y direction) '; the brightness of the "reflected" light beam 30 is minimal in the "OFF" or inactive state of the display cell 11 compared to the incident beam 20. The ratio of the reflected beam 42 in the "ON" state to the light beam 30 in the "OFF" state "State gives the contrast ratio specified for a display cell 10; this contrast ratio is directly dependent on the parameter order of magnitude of the dichroic dye used there, especially if scattering and transmission loss losses are minimized by choosing sufficient high-quality optical material for the substrates 12a, 12b, the plate 17 and the reflector 18

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can be. The higher the magnitude of the parameter, the greater the damping that results at rest as well as the contrast ratio. The overall brightness and, to a certain extent, that too However, the contrast ratio is determined by the amount of absorption in the "layers" 38a and 38b.

In Fig. 2 there is now a liquid crystal or a dye molecule 50 illustrated with a longitudinal axis 50a, the one with its longitudinal axis 50a at an angle of inclination Θ, based on the surface of a plane 52 and at an angle of rotation 0, based on a straight line 52a this surface can be arranged. According to this definition are known in the case of the above-described Liquid crystal display the molecules of liquid crystal and dichroic dye in thin ones "Layers" 38a and 38b (near the surfaces of the liquid crystal layers and the flat faces of the adjacent electrodes) arranged at an angle of inclination essentially of 0, since the longitudinal axes of the molecules 50a in these areas essentially parallel to the interfaces between the liquid crystal layer and the neighboring ones Electrodes. It should also be noted that the angle of rotation 0 is very small and at dichroic liquid crystal compositions having a high-order parameter in these layers can even become zero. Similarly, it is found that the excited liquid crystal layer at 1b has liquid crystal and dye molecules in the central region 39 of the layer, the Angle of inclination increases progressively from about 0 ° to a maximum of about 90 ° and then at the opposite Interface decreases to a critical inclination angle of about 0.

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3a and 3b, a preferred embodiment of a dichroic liquid crystal display 60 according to the invention is now shown, which is used in the reflective mode and in which the same elements with reference to FIGS. 1a and 1b are identified by the same reference numerals; a liquid crystal display cell 61 is successively supplemented by a -f- plate 17 and a reflector 18. The liquid crystal display cell 61 includes front and rear substrates 12a and 12b each having a substantially transparent, electrically conductive electrode 14a and 14b, respectively, on the inwardly facing surfaces. In the space between the plane parallel electrodes 14a and 14b, a layer 63 of a dichroic liquid crystal material is enclosed. One of the electrodes 14a and 14b is pretreated, for example, by oblique vapor deposition of a layer of silicon oxide (SiO) so that liquid crystal molecules 65 and dichroic molecules dissolved therein align parallel to the electrode surface next to the interface defined by this electrode. In the illustrated embodiment, the state with the parallel alignment (homogeneous) is generated at the interface defined by the inner surface of the electrode 14a. The angle of inclination of the molecules next to the surface 14a is accordingly relatively small and is generally less than 30 °, the angles of inclination ideally approaching 0 °.

The inner surface of the remaining electrode 14b is coated with a surfactant or a silane coupling agent treated to create a homeotropic interfacial condition in which the areas in the vicinity of this boundary

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surface-located liquid crystal molecules 65a and dichroic dye molecules 66a with their longitudinal axes Align essentially perpendicular to the electrode plane. That way is the majority the liquid crystal and dichroic dye molecules between the spaced apart ones Electrodes 14a, 14b oriented homogeneously (parallel), with only a relatively small proportion of the molecules next to the electrode 14b has angles of inclination approaching 90.

In operation, the use of different surface treatments on electrodes 14a and 14b that the liquid crystal and dichroic dye molecules that are next to the electrode with the obliquely vapor-deposited silicon oxide layer are always in the homogeneous state, while the liquid crystal and the dichroic dye molecules next to the other electrode with a surfactant or a Sihn coupling tangent is always essentially dealt with in the interfacial homeotropic state linger. A light beam 70 (FIG. 3a) coming along the X-axis strikes the essentially transparent one front substrate 12a and is passed therethrough with essentially no attenuation. The unpolarized Light beam 20 penetrates the front electrode 14a and strikes those aligned in parallel dichroic dye molecules 66 adjacent to the front electrode 14a and in most of the thickness of the dichroic liquid crystal layer 63, so that preferably an absorption of polarized in the Z "direction Light occurs. The light is thus in front of the passage through the homeotropic layer region 63a in which because of the orthogonal alignment of the directors and

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the polarization vectors are relatively low Absorption occurs, linearly polarized. The linear -X-polarized light beam 22 penetrates the ^ Plate 17, where it is circular in a light beam 24 polarized light is converted, which the illustrated left-rotating polarization vector 25 having. The circularly polarized light beam 24 is reflected by the reflector 18 to the opposite circularly polarized light beam 26 passing in the -Y direction through the 4- plate is to be transmitted through and from this as a light beam 28 with linear polarization in the Z direction, as illustrated by the polarization vector 29 emerges. The light beam 28 essentially arrives undeveloped again by the rear substrate 12b and the rear electrode 14b and hits first on the homeotropic region 63a of the liquid crystal layer, in which a relatively low absorption occurs. The light beam then penetrates the remaining part of the liquid crystal layer 63 ', which is homogeneously oriented where the directors of the dichroic dye molecules 66 are parallel to the polarization vector 29 of the Light beam 28 are aligned so that the remaining light energy is absorbed and a "beam" 30, the light energy of which is substantially zero emerges from the liquid crystal display 60. The liquid crystal display 60 is thus visible in the "OFF" or non-excited state as a relatively dark display, in which all of the incoming light is absorbed, albeit in a liquid crystal interface state of the is homeotropic type.

In Fig. 3b, a voltage source 25 is connected between the spaced electrodes 14a and 14b to

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to generate an electric field E in the Y direction, so that the liquid crystal molecules 65 'and the dichroic dye molecules 66' in the mean bulk of the Liquid crystal layer 63 'adopt the homeotropic orientation which is parallel to molecules 65a and 66a of the layer region 63a. A thin "layer" 63b of the dichroic liquid crystal material next to the front electrode 14a remains in the homogeneous state because of the effect of the homogeneous interfacial surface condition at this point the molecule directors are aligned parallel to the electrode surface. The molecules next to the surface of the front electrode 14b thus have relatively small angles of inclination, which ideally approach 0 °, with the angle of inclination from behind towards the rear Electrode 14b subsequently arranged molecules one after the other and very quickly to an angle of inclination of 90 in the bulk of the liquid crystal layer 63 and the layer region 63a adjacent to the rear electrode 14b grows.

The penetrating, randomly polarized light beam 25 passes essentially unattenuated through the front substrate 12a and the front electrode 14a and is then transmitted through the homogeneous region 13b of the liquid crystal layer 63 ', in which there is little absorption. The partially absorbed light beam 20 then passes through the remainder of the liquid crystal layer 63 'with essentially no additional absorption, since the remaining molecules are in the homeotropic state. Thus, an essentially randomly polarized light beam 40 emerges from the liquid crystal cell 61 ' and passes through the 4- plate 17, which has essentially no influence on the polarization and

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emerges as beam 42 with unpolarized light from the I plate 17; the light beam 42 is called a light beam 43 with unpolarized light reflected by the reflector 18 and again by the 4- plate 17 transmitted in order to emerge from this as an essentially unpolarized light beam 44, which is directed to the rear Substrate 12b of the liquid crystal cell 61 * hits. The light beam 44 successively penetrates the layer regions 63a and 63a, essentially without being attenuated the main mass of layer 63 ″ because of the homeotropic orientation of the dichroic dye molecules present there 66a and 66 '. The passage of light through the layer region 63b brings about a certain additional effect Attenuation because at this point the dichroic dye molecules are aligned homogeneously, i.e. orthogonal to the direction of preparation of the light and parallel to the front flat electrode surface. After passing through the front electrode 14a and the front The beam 46 emerges from substrate 12a with an essentially random polarization and has a brightness which because of the two passages through the homogeneous layer region 63b is slightly below the brightness of the entering light beam 20 'lies. It can be seen that this weakening is only half the attenuation of the known liquid crystal cell according to FIG. 1b, since in this two passages through the two "layers" 38a and 38b occur in which a homogeneous alignment at both crystal layer interfaces is used. When using only one homogeneous interface state (the other interface state is a homeotropic interface) consequently a brighter display is visible while the visible one Contrast appears increased.

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After the brightness of the liquid crystal cell is increased in the "ON" or excited state, the visible contrast by increasing the absorption of the liquid crystal cell in the "OFF" or unexcited Condition due to the addition of a small amount of a chiral composition to the liquid crystal layer 63 can be increased. The addition of a small amount of a chiral addition tends to decrease of the mean inclination angle θ of the molecules within the bulk of the liquid crystal layer 63 in the "OFF" state because the chiral neumatic liquid crystal mixture has a preferred state with a helical structure, with an angle of inclination zero occurs in a plane parallel to the plane of the electrodes. The amount of chiral addition is chosen to lie between two boundary conditions: The slope of the resulting chiral neumatic dichroic liquid crystal material should be at the wavelength of the light in the liquid crystal material be relatively long so that the light is as close as possible to the linear polarization spreads; and the slope should be sufficiently long, i.e. that the slope is greater than or approximately the same of the layer thickness, so that a uniform single crystal layer can be formed. The result when using of chiral, neumatic, dichroic liquid crystal material in a liquid crystal display cell with a homogeneous and a homeotropic interface Generating a high "OFF" absorption or a reduced "ON" absorption can result in an amount equal to the square of the Contrast ratio, as it was previously achieved with a liquid crystal display cell according to FIGS. 1a and 1b, lead to improved contrast ratio. It is understandable that while here, for reasons of illustration, a

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reflective dichroic liquid crystal display is used, also a transmissively operating dichroic Liquid crystal display can be created using the principle of opposite interfacial conditions used for the liquid crystal layer to obtain the same advantages. A transmissive Liquid crystal display can be made by replacing the • j = - plate 17 with a linear polarizer whose polarization vector 17 '(Fig. 3b) is orthogonal to the directors of the dichroic dye molecules in the homogeneous "layer" 6 3b runs next to one of the electrodes; the reflector 18 becomes then replaced by a light source arranged behind it.

Claims (10)

Claims 1. Liquid crystal display cell with a first and a second opposing surface layer of dichroic liquid crystal material, characterized in that due to the interface conditions on the two surfaces, the molecules (65, 66) of the liquid crystal material next to the first surface are homogeneously oriented and the molecules (65, 66) of the liquid crystal material next to the second surface are oriented homeotropically ^and that the molecules (65, 66) in the remainder of the liquid crystal layer can optionally be brought into the homogeneous or the homeotropic orientation. 2. Liquid crystal display cell according to claim 1, characterized in that on the first and on the second surface of the liquid crystal layer (63) in each case a first and a second essentially transparent electrode (141, 14b) is applied. 3. Liquid crystal display cell according to claim 2, characterized in that on the surface of the first electrode (14a) in contact with the first surface of the liquid crystal layer (63) a film is applied obliquely, which defines the interface condition on the first surface. 030017/0824 ORIGINAL INSPECTED 4. Liquid crystal display cell according to claim 3, characterized characterized in that the film consists of silicon oxide. 5. A liquid crystal display cell according to claim 2, characterized in that a surfactant or a silane coupling agent is applied to the surface of the second electrode (14b) which is in contact with the second surface of the liquid crystal layer (63) which satisfies the interface condition at the second surface specifies. 6. Liquid crystal display cell according to claim 2, characterized in that an optional between the first and the second electrode (14a, 14b) switchable electrical voltage source is provided which on the dichroic liquid crystal layer (63) to change the orientation of the molecules in the The remaining dichroic liquid crystal layer (63) generates an electric field of sufficient size. 7. Liquid crystal display cell according to claim 2, characterized in that it has a first or a second substantially transparent substrate (12a, 12b), the respective associated first and second electrodes (14a, 14b), respectively. 8. Liquid crystal display cell according to claim 7, characterized in that it is either reflective or can be operated transmissively. 030017/0824 9. Liquid crystal display cell according to claim 1, characterized characterized in that the mean inclination angle (Θ) of the dichroic liquid crystal molecules is reduced in the homogeneous state. 10. Liquid crystal display cell according to claim 9, characterized in that the angle of inclination (Θ) is reduced by a chiral addition contained in the dichroic liquid crystal material is resolved. I) 30017/0824