JP2006507513A - Improved liquid crystal display device - Google Patents

Abstract

(A) A pair of transparent substrates are held and are opposed to each other with a general gap in this type of apparatus, and (b) each substrate is transparent on one of its surfaces and functions as an electrode. Having a coating of conductive material, (c) there is a further layer of polymer coating on the resulting substrate, and (d) a dual frequency addressable nematic liquid crystal is filled in the gap between the coated surfaces of the substrate. A liquid crystal device in which a cell is formed and (e) the cell is incorporated between a pair of crossed polarizers. The device has the following advantages over conventional TN liquid crystal displays: (i) wider symmetric viewing angle, (ii) faster response speed, (iii) higher multiplexing capability.

Description

  The present invention relates to an improved liquid crystal display (LCD) device. More particularly, the present invention relates to a dual frequency address rubbing free twisted nematic liquid crystal display device. The LCD industry is a multi-billion dollar industry whose products range from simple watch displays to flat panel color TV screens. Currently, nematic liquid crystal devices (such as twisted nematic (TN) devices and their super twisted nematic (STN) devices) are dominant in the global LCD market. The device according to the present invention has several distinct advantages over conventional TN devices. The most important of these are (i) a significantly faster response speed (about one to two orders of magnitude faster) and (ii) a wider and more symmetric viewing angle than a normal TN device. .

Nearly all commercially available LCDs use rod-like nematic liquid crystals. (The present invention is not related to discotic molecular liquid crystals or discotic liquid crystals). The TN device consists of two parallel glass substrates, which are separated from each other by spacers. The inner surface of the glass substrate is covered with a thin layer made of a transparent conductive material such as indium tin oxide, and further a thin layer of polyimide is applied. The macroscopic alignment of liquid crystal directors (alignment of the preferential axis of rod-like molecules) is performed by a polyimide one-way rubbing process using a cotton cloth or the like. The rubbing direction is orthogonal between the two substrates. The cell is then filled with nematic liquid crystal. Due to the boundary conditions, the director is oriented along the rubbing direction on each glass substrate, so that the director continuously undergoes a 90 degree twist from one substrate to the other. A polarizing sheet is attached to the outer surface of the glass substrate. The vibration axis (polarization axis) of each polarizing sheet is parallel to the director axis of the substrate to which it is attached. Non-polarized light is converted into linearly polarized light by a polarizer fixed on the entrance side of the cell, and the polarization axis rotates 90 degrees and appears on the exit side. The emerging light is transmitted by the second polarizer. In such a configuration, the so-called “normally white mode”, the display is bright in the off state. The addition of electric field perpendicular nematic film, the liquid crystal molecules as director along the direction of the applied electric field (positive dielectric anisotropy, epsilon _|| -ε _⊥ = Δε> 0) is oriented. In this on state, the polarization state of incident light is not rotated by the liquid crystal medium, and the display becomes dark. If one polarizer is kept parallel to the rubbing direction and the other polarizer is orthogonal, the light is dark in the off state and bright in the on state. This is the so-called “normally black mode”.

  In recent years, a non-contact alignment method has been developed by exposing a polyimide coating to polarized ultraviolet light. This photo-alignment method can be used instead of the rubbing method.

  The drawback of the TN device is that when viewed obliquely, the contrast between the on and off states is significantly reduced, and even contrast inversion occurs at high vertical angles. FIG. 1 of the drawings accompanying this specification shows a typical polar diagram of the contrast ratio in a conventional TN device. Here, θ and φ are called a polar angle and an azimuth angle, respectively. The polarizer and analyzer axes are orthogonal to each other. The contrast ratio is fairly asymmetric on the vertical axis.

  Many methods have been proposed so far to improve the viewing angle. For example, an external retardation film [H. Mori, Jpn. J. et al. Appl. Phys. 36, 1068-1072 (1997); Mori, Yoji Itoh, Yosuke Nishiura, Taka Nakamura, Yukio Shinaganea, Jpn. J. et al. Appl. Phys. 36, 143-147 (1997); Ong, Mol. Cryst. Liq. Cryst. 320, 59-67 (1998)], compensation LC layer [E. Wiener-Avnea, “Twisted nematic liquid crystal light valve with birefringence compensation”, US Pat. No. 4,408,839 (October 11, 1983); Wiener-Avnea and J.A. Grimberg, US Pat. No. 4,466,702 (1984); Kobayashi, U.K. Mitshiro, U.S.A. Ishihara, S .; Yokoyama, K .; Adachi, K .; Fujimoto, H.M. Taneka, Y. et al. Miyatake and S.M. Hota, SID 1989, Digest of Technical Papers, P-114 (1989)], and a new driving method such as a multi-domain method including vertical alignment [K. H. Yang, Jpn. J. et al. Appl. Phys. 31, L 1603-1605 (1992); Chen, P.A. J. et al. Bos, D.C. R. Bryant, D.W. L. Johnson, S.M. H. Jamal, J. et al. P. Kelly, SID95, Digest, 865-868 (1995)], bend orientation technique [P. J. et al. Bos and K.K. R. Kochler / Beran, Mol. Cryst. Liq. Cryst. 113, 329-339 (1984)], horizontal electric field driving method [G. Baur, R.A. Kiefer, H.C. Klausmann and F.K. Windseheid, Liquid Crystal Today, 5, 13-14 (1995); M-Oh-e, M .; Yoneya and K.K. Kondo, J .; Appl. Phys. 82, 528-535 (1997); H. Lu, H. et al. Y. Kim, I.D. C. Park, B.M. G. Rho, J. et al. S. Park, H.C. S. Park and C.I. H. Lu, Appl. Phys. Lett. 71, 2851-2853 (1997)], and amorphous TN-LCD technique [Y. Toko, T .; Sugiyama, K .; Katoh, Y. et al. Iimura and S.M. Kobayashi, SID 93 Digest, 622-625 (1993); Appl. Phys. 74, 2071-75 (1993); Katoh and S.K. Kobayashi, Display Devices, 26-28 (1993)].

  Among these, the so-called amorphous TN-LCD is notable because it requires only a relatively simple production process and has a wide viewing angle and symmetry. The liquid crystal material used is nematic doped with chiral molecules, and the director concentration in the cell is twisted by 90 degrees by adjusting the dopant concentration. The polymer film is applied on the transparent conductive substrate but is not rubbed. The polymer film without rubbing has optical isotropy. Thus, in the off state, the nematic director is parallel to the surface of each substrate, but is randomly oriented in the substrate plane. In the on state, the director is perpendicular to the substrate. With this device, the viewing angle characteristics are improved and contrast inversion does not occur.

  Furthermore, while conventional TN-LCDs have been made by mechanically rubbing a polyimide layer to align LC molecules, it is pointed out that this technique produces dust and charging. This may destroy a thin film transistor (TFT) for an active matrix drive type TN-LCD, which is a very serious drawback. Therefore, in order to improve the product yield and make the production process simple and cost-effective, it is essential to adopt rubbing-free technology.

Another major drawback of conventional TN devices is that their electro-optic response is rather slow. In general, when the cell gap is about 8 μm, the switch-off time τ _OFF (that is, the time required to reach 90% transmittance from the dark state) is about 30 ms. This is a significant disadvantage, especially when trying to use the device for multiplex displays with fast addresses.

  Furthermore, today's color TFT-LCD operates in the TN mode. One of the characteristics that TFT-LCD should have in order to cut into the huge CRT market is the speed of response time suitable for moving video.

  Therefore, it is necessary to develop a new simple matrix type LCD having a very fast response time.

  The main object of the present invention is to provide a rubbing-free device with a much faster response and at the same time a wide viewing angle and symmetry.

  The present invention describes a dual frequency addressable amorphous TN-LCD. It has been clearly confirmed that the response time of a dual frequency address TN display is much faster than a conventional (single frequency) TN-LCD [M. Schadt, Mol. Cryst. Liq. Cryst. 89, 77-92 (1982); C. Koo and S.K. T.A. Wu, Singapore, World Scientific Publisher, “Optics and Nonlinear Optics of Liquid Crystals”; Haase, New York, Chapman & Hall, “Side chain liquid crystal polymers” Ch. 11, C.I. B. Edited by McArdle; Kitzerow, Mol. Cryst. Liq. Cryst. 321, 457-472 (1998)]. Furthermore, it has been found that in a dual frequency address TN-LCD, the multiplexing capability is increased by 30 times or more compared to the conventional addressing. [M. Schadt, Mol. Cryst. Liq. Cryst. 89, 77-92 (1982)]. In the present invention, dual frequency addressable nematic LC is used in an amorphous TN-LCD configuration. It can be seen that a very fast response time and a wide viewing angle can be obtained.

The dual frequency addressable nematic LC material used in the present invention has the following characteristics. In the nematic phase, the sign of dielectric anisotropy (Δε) varies with frequency. At low frequencies, Δε is positive and the molecules are oriented parallel to the direction of the applied field of sufficient strength. In the case of high frequency, Δε becomes negative, and the molecules are oriented so as to be orthogonal to the direction of the applied electric field. That is, there is a crossover frequency (F _c ) where the sign of Δε changes. Thus it is possible to orient the molecules to be parallel or perpendicular to the electric field depending on whether they are respectively _{f <f c, f> f} c. Any of these materials can be used in the device, for example, homologous series 4-alkyloxybenzoyloxy-4'-cyanoazobenzene fourth, sixth, eighth. (N = 4, 6, 8 nOBCAB) and other pure compounds and commercially available dielectric switch materials [ZLI-2461, M1 mixture (Merck), RO-TN-2851 (Roche), EK11650 (Eastman Kodak) ]].

Compound 6OBCAB transitions in the following order in the cooling mode: isotropic → ^{275 ° C.} → nematic → ^{127 ° C.} → smectic A ₁ → ^{90 ° C.} → crystal. A crossover frequency _{F c} is 200kHz at 130 ° C., to evaluate the performance of device at this temperature. As another example, we used a commercial dielectric switch material (2F-3333, Rolic) exhibiting a nematic phase at room temperature in this experiment to demonstrate the operating principle of the device. This material is a multi-component mixture, and an ester and pyridazine having four benzene rings are used as main components.

The specifications for this material are as follows:
Clearing temperature: 68 ℃
Melting temperature: <10 ° C
ε _：: 9.4
Dielectric anisotropy, Δε (low frequency): +4.1
Δε (high frequency): −4.7
Crossover frequency F _c : 3.2 kHz
Normal refractive index, n ₀ . : ~ 1.5
Optical anisotropy, Δn: ˜0.10
Viscosity (+ 22 ° C.): 71 mP

The liquid crystal device according to the present invention comprises:
(A) a pair of transparent substrates is held and has a gap common in this type of device;
(B) The substrate has a coating film of a transparent conductive material functioning as an electrode on one of its surfaces,
(C) there is a further layer of polymer coating on the resulting substrate;
(D) a dual frequency addressable nematic liquid crystal is filled in the gap between the coated surfaces of the substrate to form a cell;
(E) A cell is incorporated between a pair of crossed polarizers.

  In one embodiment of the invention, a transparent material such as glass, plastic, or other similar material can be used as the substrate.

  In another embodiment of the invention, the resulting substrate is coated with silica and used as a barrier layer to prevent elution of ions from the glass into the liquid crystal material.

  In yet another embodiment of the present invention, the resulting substrate incorporates a fixed pattern of red, green and blue color filters corresponding to the pixel pattern of the color matrix TN display.

  In yet another embodiment of the present invention, a conductive material selected from indium tin oxide and tin oxide is used.

  In yet another embodiment of the invention, the resulting substrate is coated with a further layer of polymer selected from polyimide, polyamide, etc. used as alignment layer.

  In yet another embodiment of the present invention, the substrates are separated by using spacers such as polyethylene terephthalate film, polyimide film, glass spherules and the like.

  In one embodiment of the present invention, for example, the fourth, sixth, eighth, etc. pure compounds of homologous 4-alkyloxybenzoyloxy-4′-cyanoazobenzene, or commercially available dielectric switch materials [ZLI-2461, Dual frequency addressable nematic materials such as M1 mixture (Merck), RO-TN-2851 (Roche), EK11650 (Eastman Kodak), mixture 2F-3333 (Rolic)) are used.

  In yet another embodiment of the present invention, a light reflector may be provided at the bottom of the device for use in a reflective type.

  The device is created as described below. The surface of the glass substrate was covered with a transparent conductive material such as indium tin oxide (ITO). A further layer of polyimide was then coated on the ITO coated substrate. No rubbing was performed. The coated surfaces of the two substrates are held facing each other with a gap of about 8 μm between them, this gap being defined by a non-conductive spacer. The gap is then filled with the commercial dual frequency mixture 2F-3333 or pure compound 6OBCAB. In either case, the dielectric switch material is doped with a chiral component. The concentration of the chiral component is adjusted to be d / p = 1/4, where d and p represent the cell thickness and the chiral pitch. This results in a 90 degree twist of the director within the cell. The created cell is positioned between the crossed polarizers.

  The invention is described in detail in the following examples, which are merely illustrative of the invention and therefore should not be construed as limiting the scope of the invention.

Example 1
The surface of the glass substrate was coated with indium tin oxide (ITO), which is a transparent conductive material. A further layer of polyimide (Liquicoat® PI ZLI-2650, Merck) is then applied onto the ITO coated substrate. No rubbing was performed. The coated surfaces of the two substrates are held as opposed with an 8 μm gap in between, this gap being defined by a non-conductive spacer. The gap is then filled with a commercially available dual frequency nematic mixture (2F-3333, Rolic) doped with a chiral component (CM-9209F, Rolic). The concentration of the chiral component is adjusted to 0.2% by weight, and the director is twisted 90 degrees in the cell. The mixture is filled into the cell in an isotropic phase. The created cell is positioned between the crossed polarizers.

Example 2
The surface of the glass substrate was coated with indium tin oxide (ITO), which is a transparent conductive material. A further layer of polyimide (Liquicoat® PI ZLI-2650, Merck) is then applied onto the ITO coated substrate. No rubbing was performed. The coated surfaces of the two substrates are held as opposed with an 8 μm gap in between, this gap being defined by a non-conductive spacer. The gap is then filled with a dual-frequency addressable compound 6OBCAB doped with the chiral component 4- [4- (S-Methylheptyloxy) benzoyloxy] -4-cyanoazobenzene, which in the following order: Transition: isotropic → ^{183 ° C.} → cholesteric → ^{178 ° C.} → smectic A → ^{77 ° C.} → crystal. The concentration of the chiral component is adjusted to 1% by weight, and the director is twisted 90 degrees in the cell. The mixture is filled into the cell with a nematic phase. The created cell is positioned between the crossed polarizers.

  The two devices described in the first and second embodiments are used as test devices. The results of an investigation conducted using the first device created in Example 1 called device 1 and the second device created in Example 2 called device 2 are shown below.

Device 1
To investigate the electro-optic response time, a sinusoidal or square AC voltage with constant amplitude is applied to the sample and the frequency is switched between f _low = 1 kHz (Δε> 0) and f _high = 20 kHz (Δε <0). It was. A typical electro-optic response curve obtained from the apparatus is shown in FIG. The regions where “f _low ” and “f _high ” are shown represent the time when the frequency of the applied voltage (60 V _rms ) was 1 kHz (f _low ) and 20 kHz (f _high ), respectively. Changing the operating frequency from f _high to f _low causes the device to switch from a bright state to a dark state. An enlarged view of the electro-optic response during switching is shown in FIG. The time τ _on required for this switching (that is, the time required to reach 10% transmittance from the bright state) is 1.2 ms. Switching the frequency from f _low to f _high leads to switching from the dark state to the bright state, and the corresponding electro-optic response curve is shown in FIG. The time τ _off required for this switching is 550 μs. As shown in FIG. 5, as the voltage increases, both τ _on and τ _off become shorter.

In order to evaluate the improvement of the dual frequency driving method over the conventional single frequency method, the performance of the single frequency driving TN-LCD device was examined. FIGS. 6 and 7 show electro-optic responses obtained in the on / off state by driving the apparatus using a 1 kHz sine wave pulse having a voltage of 60 V _rms and a repetition rate of 2 Hz and a duration of 0.5 seconds, respectively. The values of τ _on and τ _off extracted from these figures are 1.2 ms and 110 ms, respectively. Thus, dual frequency addressing provides a switch-off time that is 200 times faster than conventional single frequency addressing schemes.

  FIG. 8 shows a polar coordinate diagram of the contrast ratio between the on-state luminance and the off-state luminance in the apparatus of the present invention. The advantage of the device is clear because the curve showing the contrast ratio is symmetrical in the vertical and horizontal directions at a fixed value θ. The decrease in contrast ratio at oblique viewing angles in the middle of the polarizer axis is not due to the LC material optics. This is due to incomplete light shielding properties at the oblique angles of the crossed polarizers (eg, M. Oh-E, M. Yoneya, M. Ohta and K. Kondo, Liquid Crystals, 22, 391-400 (1997). Year); see H. Bock, Appl.Phys.Lett., 73, 2905-2907 (1998)).

Device 2
To investigate the electro-optic response time, a sinusoidal or square AC voltage with constant amplitude is applied to the sample and the frequency is switched between f _low = 10 kHz (Δε> 0) and f _high = 600 kHz (Δε <0). It was. A typical electro-optic response curve obtained from the apparatus is shown in FIG. The regions where “f _low ” and “f _high ” are shown represent the time during which the frequency of the applied voltage (30 V _rms ) was 10 kHz (f _low ) and 600 kHz (f _high ), respectively. Changing the operating frequency from f _high to f _low causes the device to switch from a bright state to a dark state. An enlarged view of the electro-optic response during switching is shown in FIG. The time τ _on required for this switching (that is, the time required to reach 10% transmittance from the bright state) is 50 μs. Switching the frequency from f _low to f _high leads to switching from the dark state to the bright state, and the corresponding electro-optic response curve is shown in FIG. The time τ _off required for this switching is 300 μs. As shown in FIG. 12, as the voltage increases, both τ _on and τ _off become shorter.

In order to evaluate the improvement of the dual frequency driving method over the conventional single frequency method, the performance of the single frequency driving TN-LCD device was examined. Electricity obtained in the on / off state at the same temperature (130 ° C.) by driving the apparatus with a voltage of 10 kHz with an amplitude of 30 V _rms , a repetition rate of 0.5 s ⁻¹ and a pulse duration of 0.5 seconds and a frequency of 10 kHz. The optical responses are shown in FIGS. 13 and 14, respectively. The values of τ _on and τ _off extracted from these figures are 50 μs and 60 ms, respectively. Thus, dual frequency addressing provides a switch-off time that is 200 times faster than conventional single frequency addressing schemes.

  FIG. 15 shows a polar coordinate diagram of the contrast ratio between on-state luminance and off-state luminance in the apparatus of the present invention. The advantage of the device is clear because the curve showing the contrast ratio is symmetrical in the vertical and horizontal directions at a fixed value θ. The decrease in contrast ratio at oblique viewing angles in the middle of the polarizer axis is not caused by the LC material optics, as mentioned above, which is due to incomplete light blocking characteristics at the oblique angles of the crossed polarizers. It is.

Comparison and Inference of Results From an investigation with two test devices, it can be seen that dual frequency addressing results in a 200 times faster switch-off time compared to a single frequency addressing scheme. However, because the physical properties of the two types of nematic LC mixtures used in the device cell are different, there are differences in operating frequency and voltage. In addition, since the device 2 operates at a very high temperature (that is, 130 ° C.), the response time is faster than that of the device 1 that operates at room temperature.

  Comparing FIGS. 8 and 15 to FIG. 1, it is clear that the technique employed in the present invention widens the viewing pyramid and looks more symmetric. However, the contrast ratio of the device 2 is much smaller than that of the device 1, while the contrast ratio of the device 1 is almost the same as that of the conventional TN-LCD. The reason for this is that the clearing temperature is very high (275 ° C.), so the device 2 is not filled with an isotropic LC mixture in the isotropic phase, whereas the clearing temperature is quite low (68 ° C. This is because the device cell 1 is filled with an isotropic phase nematic LC mixture. It is known that when LC of a nematic phase is filled in a cell of a polyimide film without rubbing, LC molecules become non-uniformly aligned mainly by flow alignment [Y. Toko, T .; Sugiyama, K .; Katoh, Y. et al. Iimura and S.M. Kobayashi, J. et al. Appl. Phys. 74, 2071-75 (1993)].

EFFECT OF THE INVENTION This device has a much faster response time (about 200 times faster) than a conventional TN-LCD device, and can therefore be used to produce a simple matrix type TN-LCD with a fast response time.
The device has a wider and more uniform viewing angle than conventional TN-LCDs. Therefore, it is not necessary to add a retardation plate, a compensation plate, or a complicated electrode pattern for enlarging the viewing angle of the conventional TN-LCD, and this apparatus is economical.
・ This device is not only easy to produce, but also improves the yield by avoiding the mechanical rubbing treatment of the polymer.
The device has high multiplexing capabilities due to the dual frequency addressing technology.

Claims (16)

A dual frequency dual amplitude address twisted nematic liquid crystal device with a wide viewing angle,
(A) a pair of transparent substrates held in an opposing state via a spacer;
(B) an insulating layer of silica covering the surface of the substrate;
(C) a film of a conductive transparent material that functions as an electrode on one surface of each substrate;
(D) filling the gap between the coated surfaces of the substrate with a further layer of the polymer coating without rubbing treatment on the resulting substrate with a nematic phase dual frequency addressable nematic liquid crystal doped with a chiral compound. Used in conjunction with the resulting cell,
(E) A device comprising a pair of polarizers having a cell between them.   The apparatus according to claim 1, wherein the transparent substrate is selected from a glass plate and a plastic material.   The apparatus according to claim 1, wherein the conductive transparent material is selected from indium tin oxide, tin oxide, and other conductive polymers.   2. An apparatus according to claim 1, wherein each substrate has a film made of a material selected from silica functioning as a barrier layer on one of its surfaces to prevent elution of ions from the substrate to the liquid crystal.   The apparatus according to claim 1, wherein the surface of the obtained substrate is coated with a polymer selected from polyimide and polyamide to form an alignment layer.   2. An apparatus according to claim 1, wherein the coated surfaces of the substrates are held as opposed with a gap of about 2 to 10 [mu] m, preferably 8 [mu] m.   The apparatus according to claim 1, wherein the substrates are separated by using a spacer selected from a polyethylene terephthalate film, a polyimide film, a glass sphere, and a rod-shaped body.   The nematic phase dual-frequency addressable liquid crystal material is one in which the sign of dielectric anisotropy (Δε) is positive at low frequencies, and the molecules are aligned parallel to the direction of the applied electric field with sufficient strength. In this case, Δε is negative, and the molecules are oriented so as to be orthogonal to the direction of the applied electric field.   Dual frequency addressable nematic material is pure 4th, 6th, 8th, commercially available ZLI 2461, M1 mixture (Merck) of homologous 4-alkoxybenzoyloxy-4'-cyanoazobenzene, The apparatus of claim 1 selected from RO-TN2851 (Roche), EK11650 (Eastman Kodak), mixture 2F-3333 (Rolic).   The device of claim 1, wherein the dual frequency addressable nematic material is doped with a chiral component selected from CM-9209F (Rolic), CB-15 (Merck) and cholesterol esters.   2. An apparatus according to claim 1, wherein the dopant concentration is adjusted to give a director twist in the cell of 90 degrees.   2. An apparatus according to claim 1, wherein a light reflector is optionally provided at the bottom of the apparatus for use in a reflective type.   2. The apparatus according to claim 1, wherein the magnitude of the applied voltage between the low frequency and the high frequency is variable in the two amplitude method.   The apparatus of claim 1, wherein the high frequency is applied in a pulsed manner with a short duration in the range of 1-10 ms.   2. An apparatus according to claim 1, wherein the apparatus has a wide and symmetric viewing angle of polar angle ± 40 [deg.] Or more. 2. The device according to claim 1, wherein the electro-optic response time [tau] _off of the device is in the range of 10 [mu] s to 20 ms.

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