Liquid crystal display device

Abstract: A liquid crystal display device which comprises first and second electrode substrates spaced a predetermined distance from each other, a nematic liquid crystal layer added with an optically active material and having a posi­ tive dielectric anisotropy, which layer is sandwiched between the first and second electrode substrates such that the liquid crystal molecules exhibit a helical structure in which they are twisted in a predetermined twist angle in a direction across the thickness of the liquid crystal layer, a light detecting element positioned on one side of the first electrode substrate remote from the liquid crystal layer, a polarizing element positioned on one side of the second electrode substrate remote from the liquid crystal layer, and a film-like sheet having a birefringence and disposed between the light detecting element and the first electrode substrate, and/or between the polarizing element and the second electrode substrate.

Description: The present invention generally relates to a liquid crystal display device and, more particularly, to a twisted nematic (TN) liquid crystal display device. Liquid crystal display devices are utilized in numerous applications, for example, clocks, wrist watches, electronic calculators, computer terminal devices, word-processor display devices, compact television receiver sets, and so on. As a display mode of the liquid crystal display device, a supertwisted birefringence effect (SBE) display mode in which liquid crystal molecules are twisted to about 180° to about 270° for the purpose of improving the contrast and which is capable of exhibiting an improved multiplex driving characteristic is currently widely used. An example of the liquid crystal display cell of SBE display mode is schematically illustrated in Fig. 5 of the accompanying drawings, reference to which will now be made. In Fig. 5, in order for the nematic liquid crystal having a positive dielectric anisotropy between a pair of electrode-formed substrates 17 and 18 to exhibit a helical structure having a twist angle , orientation has to be carried out by the use of, for example, a slant vapour-deposition method or a rubbing method in which a surface of each of the electrode substrates which is in contact with the liquid crystal is rubbed in one direction with a cloth, followed by the addition of optically active material to the nematic liquid crystal so that the ratio of the thickness d of the liquid crystal cell relative to the resultant helical pitch P, that is, d/P, can fall within the range expressed by the following equation. (/2π - 1/4) < d/P < (/2π + 1/4) The typical voltage-transmission characteristic curve of the SBE mode liquid crystal cell is illustrated in Fig. 6. The direction in which observation is made is a direction parallel to the normal to a liquid crystal display surface. In Fig. 6, as a parameter descriptive of a steep characteristic of a threshold characteristic of the voltage-transmission, a value α is defined which stands for the ratio of the voltage V 10% , at which the transmission attains 10%, relative to the voltage V 90% at which the transmission attains 90%. As the value α approaches 1, the threshold characteristic becomes steep, and the multiplex driving characteristic improves. In view of this, when the value α exhibited by the SBE mode liquid crystal cell wherein phenylcyclohexanes are employed as the liquid crystal material and the value α exhibited by the SBE mode liquid crystal cell wherein the 90° twisted nematic liquid crystal is employed are measured, it has been found that the value α exhibited by the SBE mode liquid crystal cell wherein phenylcyclohexanes are employed is 1.08 whereas the value α exhibited by the SBE mode liquid crystal cell wherein the conventional 90° twisted nematic liquid crystal is employed is 1.40. In this way, it will readily be understood that the SBE mode liquid crystal cell has a very steep threshold characteristic. Fig. 7 illustrates the dependency of the transmission on the wavelength when the SBE mode liquid crystal display cell is in an ON state and in an OFF state, and Fig. 8 illustrates the dependency of the transmission on the wavelength when the conventional 90° twisted nematic liquid crystal cell is in an ON state and in an OFF state. As can be understood from the respective graphs shown in Figs. 7 and 8, the dependency of the transmission of the SBE mode liquid display cell on the wavelength is more considerable than that of the twisted nematic liquid crystal cell. This is because the SBE display mode makes use of birefringent effects of the liquid crystal. Accordingly, in the SBE mode liquid crystal cell, it is not possible to accomplish a uniform switching over all the visible region of wavelength and an undesirable colouring occurs in the display, thereby constituting a cause of reduction in display quality. In view of the foregoing, the inventor of the present invention has conducted a series of experiments to determine conditions required to accomplish a black-and-white display of high quality with no undesirable colouring taking place. In one aspect, the invention provides a liquid crystal display device comprising: first and second electrode substrates spaced a predetermined distance from each other; a nematic liquid crystal layer including an added optically active material and having a positive dielectric anisotropy, said nematic liquid crystal layer being sandwiched between the first and second electrode substrates such that the liquid crystal molecules exhibit a helical configuration in which they are twisted in a predetermined twist angle greater than 90° in a direction across the thickness of the liquid crystal layer; a light detecting element positioned on the side of the first electrode substrate remote from the liquid crystal layer; a polarizing element positioned on the side of the second electrode substrate remote from the liquid crystal layer; and one only film-like birefringent sheet disposed between the light detecting element and the first electrode substrate, said film-like birefringent sheet having an optically uniaxial property in a direction parallel to a surface thereof; wherein the following conditions are satisfied: 50° ≦ Θ ≦ 30° 60° ≦ β ≦ 10° 100° ≦ γ ≦ - 20° where Θ is the angle measured from the liquid crystal orientation axis at said first electrode substrate towards the optical axis of said birefringent sheet, β is the angle measured from the liquid crystal orientation axis at said first electrode substrate towards a polarizing axis of said light detecting element, and γ is the angle measured from the liquid crystal orientation axis at said second electrode substrate towards a polarizing axis of said polarizing element, a positive direction of each said angles being defined as the direction of twist of said liquid crystal molecules when viewed in a direction from said second electrode substrate towards said first electrode substrate. In another aspect, the invention provides a liquid crystal display device comprising: first and second electrode substrates spaced a predetermined distance from each other; a nematic liquid crystal layer including an added optically active material and having a positive dielectric anisotropy, said nematic liquid crystal layer being sandwiched between the first and second electrode substrates such that the liquid crystal molecules exhibit a helical configuration in which they are twisted in a predetermined twist angle greater than 90° in a direction across the thickness of the liquid crystal layer; a light detecting element positioned on the side of the first electrode substrate remote from the liquid crystal layer; a polarizing element positioned on the side of the second electrode substrate remote from the liquid crystal layer; and one only film-like birefringent sheet disposed between the polarizing element and the second electrode substrate, said birefringent sheet having an optically uniaxial property in a direction parallel to a surface thereof; wherein the following conditions are satisfied: 50° ≦ Θ ≦ 30° 60° ≦ β ≦ 10° 100° ≦ γ ≦ - 20° where Θ is the angle measured from the liquid crystal orientation axis at said second electrode substrate towards the optical axis of said birefringent sheet, β is the angle measured from the liquid crystal orientation axis at said first electrode substrate towards a polarizing axis of said light detecting element, and γ is the angle measured from the liquid crystal orientation axis at said second electrode substrate towards a polarizing axis of said polarizing element, a positive direction of each said angles being defined as the direction of twist of said liquid crystal molecules when viewed in a direction from said second electrode substrate towards said first electrode substrate. Hereinafter, the film-like sheet is referred to as a compensator plate. According to the present invention, because of the intervention of the compensator plate, the transmission of the SBE mode liquid crystal cell relative to the wavelength can exhibit a flat characteristic and, therefore, not only can a favourable black-and-white display be obtained, but also a steep threshold characteristic can be exhibited in the voltage-transmission characteristic thereof. EP-A2-0 246 842 forms part of the state of the art under Article 54(3) EPC. EP-A2-0 246 842 discloses a liquid crystal display device comprising a liquid crystal cell disposed between two polarizing plates. A respective optically anisotropic film is provided between the cell and each polarizing plate. Certain embodiments omit one of the two films. None of these embodiments meets the above-specified conditions for angles Θ, β and γ. The present invention will become clear from the following description of a preferred embodiment thereof with reference to Figs. 1 to 4 of the accompanying drawings, in which: Fig. 1 is a schematic sectional view of a SBE mode liquid crystal display device embodying the present invention; Fig. 2 is a perspective view of a compensator sheet used in the display device embodying the present invention; Fig. 3 is a diagram showing various relationships between optical axes of components of the display device embodying the present invention; Fig. 4 is a graph showing the dependency of the transmission of the display device embodying the present invention on the wavelength; Fig. 5 is a schematic sectional view of the prior art SBE mode liquid crystal display device; Fig. 6 is a graph showing the applied voltage versus transmission characteristic of the prior art display device of Fig. 5; Fig. 7 is a graph showing the dependency of the transmission of the prior art SBE mode liquid crystal display device on the wavelength; and Fig. 8 is a graph showing the dependency of the transmission of the prior art twisted nematic liquid crystal display device on the wavelength. Referring to Fig. 1, a SBE mode liquid crystal display device embodying the present invention comprises an upper glass substrate 1, a lower glass substrate 2, a compensator plate 3, transparent electrodes 4 and 5, orientation layers 6 and 7, a liquid crystal layer 8, a light detecting element (an upper polarizer plate) 9, a polarizing element (a lower polarizer plate) 10 and a light source 11. The upper and lower glass substrates 1 and 2 constitute paired electrode substrates positioned one above the other and are formed, on respective lower and upper surfaces thereof, with the transparent electrodes 4 and 5 of predetermined pattern made of indium oxide. Surfaces of the transparent electrodes 4 and 5 opposite to the respective glass substrates 1 and 2 are formed with the orientation layers 6 and 7, respectively, which layers 6 and 7 are in the form of a coating of polyimidesilane polymer, the surface of which has been rubbed in a predetermined direction with the use of a cloth. The liquid crystal layer 8 is disposed between the orientation layers 6 and 7. More specifically, the liquid crystal layer 8 is a layer of nematic liquid crystal with added photo-active material and having a positive dielectric anisotropy and is sandwiched between the orientation layers 6 and 7 so that the liquid crystal molecules can exhibit a helical structure wherein they are twisted at an angle greater than 90° in a direction across the thickness thereof. For the liquid crystal layer 8, the use of phenylcyclohexanes such as are listed in Table 1 in a quantity ex pressed in terms of percentage by weight in Table 1 is preferred. However, other than the phenylcyclohexanes, biphenyls, pyrimidines and esters may be employed. Between the upper glass substrate 1 and the light detecting element 9 positioned thereabove, the compensator plate 3 having a birefringence is disposed. As shown in Fig. 2, this compensator plate 3 is formed from a film-like sheet having an optically uniaxial property in a direction parallel to the surface thereof. As material for the compensator plate 3, polyesters are preferred. However, a film-like sheet of polyvinyl alcohol, polypropylene, nylon or acetate can be employed provided that it can exhibit an optically uniaxial property in a direction parallel to the surface thereof. It is to be noted that the compensator plate 3 may be disposed between the lower glass substrate 2 and the polarizing element 10 positioned therebelow or between the upper glass substrate 1 and the light detecting element 9 positioned thereabove. Relationships among the rubbing angle ψ, the twisting angle  of the liquid crystal molecules, the set angle β of the light detecting element (upper polarizing plate) 9, the set angle y of the polarizing element (lower polarizing plate) 10 and the set angle ϑ of the compensator plate 3 are illustrated in Fig. 3. In Fig. 3, the rubbing angle ψ is the one formed between the rubbing direction 12 of the upper glass substrate 1, that is, the longitudinal direction of the liquid crystal molecules on the upper glass substrate 1, and the rubbing direction 13 of the lower glass substrate 2, that is, the longitudinal direction of the liquid crystal molecules on the lower glass substrate 2. The twisting angle  represents that of the liquid crystal molecules and is adjusted to satisfy the relationship of (ψ = 360° - ). The twisting angle  depends on the ratio d/P of the thickness d of the liquid crystal layer relative to the particular helical pitch P induced by the addition of the photo-active material to the nematic liquid crystal and, therefore, it is well known that, in order to render the twisting angle of the liquid crystal molecules to be ψ, the ratio d/P has to be so adjusted as to fall within the range of (/2π - 1/4) < d/P < (/2π + 1/4). Accordingly, the twisting angle  is within the range of 0° to 300°. The set angle β is the angle formed between the direction 14 of a polarizing axis of the light detecting element 9 and the longitudinal direction 12 of the liquid crystal molecules on the electrode substrate 1 adjacent the light detecting element 9, and the set angle γ is the angle formed between the direction 15 of a polarizing axis of the polarizing plate 10 and the longitudinal direction 13 of the liquid crystal molecules on the electrode substrate 2 adjacent the polarizing element 10. The set angle ϑ is the angle formed between the direction 16 of the optical axis of the compensator plate 3, made of the film-like sheet having the uniaxial property, and the longitudinal direction 12 of the liquid crystal molecules on the electrode substrate 1. Fig. 4 illustrates the wavelength-dependence characteristic of the transmission of the SBE mode liquid crystal cell of the above described construction wherein the compensator plate is employed. The liquid crystal material is chosen from the phenylcyclohexanes and exhibits 0.13 in anisotropy Δn in refractive index and 5 µm in thickness d and, therefore, the value Δn·d reads 0.65 µm. Also, since the compensator plate 3 is made of polyester and exhibits 0.04 in anisotropy Δn′ in refractive index in a direction parallel to the surface thereof and 6 µm in film thickness d′, the value Δn′·d′ reads 0.24 µm. From Fig. 4, it can be readily understood that the transmission of the SBE mode liquid crystal display cell provided with the compensator plate exhibits a sufficiently flat characteristic relative to the wavelength. Also, since the value α in this case is 1.04 while the value a in the case where no compensator plate is utilized as hereinbefore discussed is 1.08, it can also be understood that the multiplex driving characteristic is improved. In the liquid crystal display element of the construction described above, in order for it to exhibit a sufficiently flat transmission acceptable in practice, it is preferred to make the value Δn·d equal to or greater than 0.3 µm, but equal to or smaller than 0.9 µm. It is also preferred to make the value Δn′·d′ of the compensator plate 3 equal to or greater than 0.1 µm, but equal to or smaller than 0.9 µm. With respect to the set angles β and γ of the polarizing element 10 and the light detecting element 9, respectively, it is required that, assuming that the direction in which the liquid crystal molecules are twisted from the electrode substrate 2 adjacent the polarizing element 10 towards the electrode substrate 1 adjacent the light detecting element 9 is positive, the angles β, γ and ϑ are within the respective ranges of (-60° ≦ β ≦ 10°), (-100 ≦ γ ≦ -20°) and (-50° ≦ ϑ ≦ 30°). By so doing, the wavelength-dependent characteristic of the transmission can be minimized with the consequence that a favourable black-and-white display can be obtained. It is to be noted that, even when the nematic liquid crystal having the positive dielectric anisotropy other than the phenylcyclohexanes is employed, and also even when material other than polyesters is used for the film-like sheet having the optically uniaxial property, effects similar to those described above could be exhibited. Moreover, even when the unaxial film-like sheet is interposed between the polarizing element 10 and the electrode substrate 2, effects similar to those described above could be obtained. By way of example, a liquid crystal material of a composition shown in Table 2 was employed. The twist angle  of the liquid crystal molecules was 270° and the thickness d of the liquid crystal layer was 5.0 µm. In this liquid crystal, 1.17 wt% of S-811, which is an optically active material manufactured and sold by Merck, had been added so that the twist angle  could become 270°. In this example, the value d/P is about 0.65. Also, since the refractive index anisotropy Δn of the nematic liquid crystal tabulated in Table 2 is 0.12, the value Δn·d is 0.6 µm. From the foregoing description, it has now become clear that, since according to the present invention the optically uniaxial film-like sheet is interposed between the light detecting element and the electrode substrate or between the polarizing element and the electrode substrate, in the SBE mode liquid crystal display device, the transmission characteristic can exhibit a flat characteristic relative to the wavelength and, therefore, the black-and-white display of high contrast and, therefore, of high quality can be obtained. Moreover, in the applied voltage versus transmission characteristic, the steep threshold characteristic can be obtained. In view of the foregoing, the present invention can be equally applicable to a graphic display, a character display and a liquid crystal television display. Although the present invention has fully been described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom.

Claims: A liquid crystal display device comprising: first and second electrode substrates (1,2) spaced a predetermined distance from each other; a nematic liquid crystal layer (8) including an added optically active material and having a positive dielectric anisotropy, said nematic liquid crystal layer being sandwiched between the first and second electrode substrates such that the liquid crystal molecules exhibit a helical configuration in which they are twisted in a predetermined twist angle greater than 90° in a direction across the thickness of the liquid crystal layer; a light detecting element (9) positioned on the side of the first electrode substrate (1) remote from the liquid crystal layer (8); a polarizing element (10) positioned on the side of the second electrode substrate (2) remote from the liquid crystal layer (8); and one only film-like birefringent sheet (3) disposed between the light detecting element (9) and the first electrode substrate (1), said film-like birefringent sheet (3) having an optically uniaxial property in a direction parallel to a surface thereof; wherein the following conditions are satisfied: 50° ≦ Θ ≦ 30° 60° ≦ β ≦ 10° 100° ≦ γ ≦ - 20° where Θ is the angle measured from the liquid crystal orientation axis (12) at said first electrode substrate (1) towards the optical axis (16) of said birefringent sheet (3), β is the angle measured from the liquid crystal orientation axis (12) at said first electrode substrate (1) towards a polarizing axis (14) of said light detecting element (9), and γ is the angle measured from the liquid crystal orientation axis (13) at said second electrode substrate (2) towards a polarizing axis (15) of said polarizing element (10), a positive direction of each said angles being defined as the direction of twist of said liquid crystal molecules when viewed in a direction from said second electrode substrate (2) towards said first electrode substrate (1). A liquid crystal display device comprising: first and second electrode substrates (1,2) spaced a predetermined distance from each other; a nematic liquid crystal layer (8) including an added optically active material and having a positive dielectric anisotropy, said nematic liquid crystal layer being sandwiched between the first and second electrode substrates such that the liquid crystal molecules exhibit a helical configuration in which they are twisted in a predetermined twist angle greater than 90° in a direction across the thickness of the liquid crystal layer; a light detecting element (9) positioned on the side of the first electrode substrate (1) remote from the liquid crystal layer (8); a polarizing element (10) positioned on the side of the second electrode substrate (2) remote from the liquid crystal layer; and one only film-like birefringent sheet (3) disposed between the polarizing element (10) and the second electrode substrate (2), said birefringent sheet (3) having an optically uniaxial property in a direction parallel to a surface thereof; wherein the following conditions are satisfied: 50° ≦ Θ ≦ 30° 60° ≦ β ≦ 10° 100° ≦ γ ≦ - 20° where Θ is the angle measured from the liquid crystal orientation axis (13) at said second electrode substrate (2) towards the optical axis (16) of said birefringent sheet (3), β is the angle measured from the liquid crystal orientation axis (12) at said first electrode substrate (1) towards a polarizing axis (14) of said light detecting element (9), and γ is the angle measured from the liquid crystal orientation axis (13) at said second electrode substrate (2) towards a polarizing axis (15) of said polarizing element (10), a positive direction of each said angles being defined as the direction of twist of said liquid crystal molecules when viewed in a direction from said second electrode substrate (2) towards said first electrode substrate (1). A liquid crystal display device according to claim 1 or claim 2, wherein said liquid crystal layer (8) exhibits a retardation value Δn.d in the range 0.3 µm ≦ Δn.d ≦ 0.9 µm. A liquid crystal display device according to any one of claims 1 to 3, wherein said film-like birefringent sheet (3) exhibits a retardation value Δn′.d′ in the range 0.1 µm ≦ Δn′.d′ ≦ 0.9 µm. A liquid crystal display device according to claim 4, as appendent to claim 3 wherein the value Δn.d is 0.65 µm and the value Δn′.d′ is 0.24 µm. A liquid crystal display device according to claim 1 or claim 2, wherein said birefringent sheet (3) comprises acetate. A liquid crystal display device according to claim 1 or claim 2, wherein said birefringent sheet (3) comprises nylon.

Inventors: KOZAKI SHYUICHI, KOZAKI, SHYUICHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G02F1
IPC codes: G09F9, G02F1, C09K19