JP2012168553A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that a twisted nematic liquid crystal in a TN mode or the like has a slow response speed and causes blur in motion pictures when displaying a high-speed image.SOLUTION: By setting the relation between the twist pitch p and the gap d of a twist liquid crystal to p/d<8 and stabilizing polymers to achieve high-speed stabilization which increases the torque for recovering an alignment state when no voltage is applied, the response speed of the liquid crystal during falling is increased. By applying the above technique to a liquid crystal display device which is driven to reset, overdrive or perform common modulation, a fast response speed is obtained even when a TN liquid crystal is used. Consequently, the liquid crystal can cope with a field sequential display which requires a faster response speed.

Description

  The present invention relates to a liquid crystal panel, a liquid crystal display device using the liquid crystal panel, and an electronic apparatus equipped with the liquid crystal display device.

  With the progress of the multimedia era, liquid crystal display devices are rapidly spreading from small ones used in projector devices and mobile phones to large ones used in notebook PCs, monitors, televisions and the like. is made of. In addition, medium-sized liquid crystal display devices are indispensable for electronic devices such as viewers and PDAs, and also for game tools such as portable game machines and pachinko machines. In addition, liquid crystal display devices are used everywhere from home appliances such as refrigerators and microwave ovens.

  Currently, most liquid crystal display elements are of the twisted nematic (TN) type. This TN type liquid crystal display element uses a nematic liquid crystal composition, and the liquid crystal driving system is roughly divided into two. One of them is a simple matrix driving method. The other one is an active matrix system in which a switching element is provided in each pixel. For example, a TN-TFT system using a thin film transistor (TFT) is generally used for a TN type display system.

  An STN (Super Twisted Nematic) system is available for the TN display system. This STN method is improved in contrast and viewing angle dependency as compared with the conventional simple matrix method using the TN type, but is not suitable for moving image display because of its low response speed. In addition, there is a drawback that display quality is lower than that of an active matrix system using TFTs. As a result, at present, the TN-TFT method has become the mainstream of the market.

  On the other hand, due to the demand for higher image quality, methods with improved viewing angles have been researched and put to practical use. As a result, the current mainstream of high-performance liquid crystal displays is a method using a compensation film in TN mode, in-plane switching (IPS) mode, or multi-domain vertical alignment (MVA). Three types of TFT type active matrix liquid crystal display devices in the “Vertical Aligned” mode are mainstream.

  In these active matrix liquid crystal display devices, the image signal is normally rewritten at 60 Hz because it is written at positive and negative at 30 Hz, and the time for one field is about 16.7 ms (milliseconds) (the total time for both positive and negative fields is It is called 1 frame and is about 33.3 ms). On the other hand, the response speed of the current liquid crystal is about this frame time even in the fastest state.

  There are two major demands for liquid crystal display devices: higher definition of pixels and improved response speed for images.

  The response speed to images is due to the fact that the screens displayed on the liquid crystal display more and more opportunities to display moving images as well as conventional still images. In a moving image, a sport image or a computer image (CG) in a game in which a fast image change occurs particularly requires a response speed faster than the current frame time.

  On the other hand, the current mainstream of pixels is 100 ppi (pixel per inch), but there are two types of methods for high definition. One method is to increase the processing accuracy and reduce the size of the image element, and the other method is to switch the backlight, which is the illumination light of the display device, to red, green, and blue in terms of time. This is a method for performing color display. This method is called a field sequential (time division) method, and a color liquid crystal display device is also being studied. In this method, since it is not necessary to divide the pixel into three and arrange the color filter spatially, it is said that it is possible to increase the resolution three times that of the conventional method and at the same time increase the aperture ratio and increase the light utilization efficiency. Yes. On the other hand, in the field sequential liquid crystal display device, since it is necessary to display one color in 1/3 time of one field, the time available for display is about 5 ms. Therefore, the response speed of the liquid crystal must be faster than 5 ms.

  Various technologies have been studied from the necessity of such a liquid crystal display device capable of responding to high-speed images, and high-speed liquid crystal display mode technologies have been developed. These high-speed liquid crystal display mode technologies are roughly divided into two tides. One is a technique for increasing the response speed of the twisted nematic liquid crystal as described above, and the other is a technique using a liquid crystal capable of high-speed response.

  As a liquid crystal capable of high-speed response, there is a technique that uses a spontaneous polarization type smectic liquid crystal, but there is also a method of increasing the speed by using a nematic liquid crystal and changing the display mode to another mode. However, there is a problem that the required level of accuracy and uniformity in manufacturing increases and the yield decreases. The reason is that the required accuracy of the thickness of the liquid crystal layer is increased, and optical elements such as a compensation plate and a retardation plate are required, and the uniformity thereof is required.

  For example, in the case of an ECB (electrically controlled birefringence) mode using birefringence, the requirement for the thickness accuracy of the liquid crystal layer to achieve contrast 100 is about 3% of the thickness of the liquid crystal layer. In that case, if the thickness is 3 microns or less, the thickness variation needs to be 90 nanometers or less. When the required display conditions such as contrast are high, the required accuracy with respect to the thickness becomes more severe.

  Similarly, there is an OCB (Optically compensated birefringence) mode.

  The OCB mode is one method of the wide viewing angle mode. The cell structure is a structure in which an anti-parallel cell having a pretilt angle is also provided with a phase difference compensation plate (biaxial phase difference plate). A bias voltage is applied to the homogeneous alignment to form a bend alignment, and a voltage is applied to perform switching. The OCB method has a wide viewing angle and a fast response time. However, it is difficult to obtain a uniform and stable bend alignment, and further, the cell gap (the thickness of the liquid crystal) and the uniformity of the characteristics of the compensation plate are required, so that a great deal of stability in the manufacturing process is required. Has a problem.

  In either case, the requirement for the thickness accuracy of the liquid crystal layer to realize the contrast 100 is about 3% of the thickness of the liquid crystal layer. In that case, if the thickness is 3 microns or less, the thickness variation needs to be 90 nanometers or less. When the required display conditions such as contrast are high, the required accuracy with respect to the thickness becomes more severe.

  Furthermore, it is difficult to obtain a stable display over a wide range of temperatures. The reason is that the dependence of the response speed of the liquid crystal on the temperature is large. As a result, at a low temperature, the response speed becomes extremely slow as the viscosity decreases, and it becomes difficult to ensure contrast and display a clear gradation.

  The first trend is to increase the response speed of the nematic liquid crystal mainly by the following means.

(1) Thinning the cell gap and increasing the electric field strength at the same voltage.
(2) A high voltage is applied to increase the electric field strength and promote state change (overdrive method).
(3) Increase the dielectric anisotropy and make the response to electric field sensitive.
(4) Lower the viscosity.
(5) Speed up by optimizing liquid crystal material and cell design.

  When the response speed of such a nematic liquid crystal is increased, the following problems occur. When the response speed of the nematic liquid crystal is increased, the capacitance of the liquid crystal varies greatly depending on the orientation of the liquid crystal due to the anisotropy of the dielectric constant (in the case of the product name DLC-43002, ε parallel = 11.8, ε vertical = 3.7). For this reason, when the response speed is increased, the capacitance change becomes extremely large, and the holding voltage to be written and held in the liquid crystal layer is lowered. When the holding voltage is lowered, that is, when the effective applied voltage is lowered, insufficient writing occurs and the liquid crystal does not change to a desired position, so that the contrast is lowered. When the same signal is repeated as in a still image, the luminance continues to change until the holding voltage does not decrease, and several frames are required to obtain a stable luminance.

  In order to prevent such a response requiring several frames, it is necessary to have a one-to-one correspondence between the applied signal voltage and the obtained transmittance. In active matrix driving, the transmittance after the liquid crystal response is determined not by the applied signal voltage but by the amount of charge stored in the liquid crystal capacitance after the liquid crystal response. This is because the active drive is a constant charge drive in which the liquid crystal responds with the held charge.

  The amount of charge supplied from the active element is determined by accumulated charge before writing a predetermined signal and newly written write charge, if a minute leak or the like is ignored. In addition, the accumulated charge after the liquid crystal has responded also varies depending on pixel design values such as the physical constants and electrical parameters of the liquid crystal and the accumulation capacitance.

For this reason, to take a correspondence between the signal voltage and the transmittance,
(1) Correspondence between signal voltage and write charge (2) Accumulated charge before writing (3) Information for calculating accumulated charge after response and actual calculation based on the information are required. As a result, a frame memory for storing (2) over the entire screen and the calculation units (1) and (3) are required. This leads to an increase in the number of parts of the system, which is not preferable.

  As a method for solving these problems, a reset pulse method is often used in which a reset voltage is applied so as to align with a predetermined liquid crystal state before writing new data. As an example, the technology described on pages L-66 to L-69 of IDRC 1997 will be described.

  In this document, the above-described OCB (Optically compensated birefringence) mode in which the alignment of the nematic liquid crystal is a pi-type alignment and a compensation film is added is used. The response speed of this liquid crystal mode is about 2 to 5 milliseconds, which is much faster than the conventional TN-TFT method, so the response should be completed within one frame. However, as described above, several frames are required as in the conventional case until the holding voltage is significantly lowered due to the change in the dielectric constant due to the response of the liquid crystal and a stable transmittance is obtained. Therefore, a method of writing a black display after writing a white display within one frame is shown.

  This will be described with reference to FIG.

  The horizontal axis is time, and the vertical axis is luminance. A dotted line represents a luminance change in the case of normal driving, and has reached a stable luminance in the third frame. According to this reset pulse method, since a predetermined state is always obtained when new data is written, a one-to-one correspondence of constant transmittance is seen with respect to the written constant signal voltage. This one-to-one correspondence makes it very easy to generate driving signals, and at the same time eliminates the need for means such as a frame memory for storing previous writing information.

  Next, the pixel configuration of the active matrix liquid crystal display device will be described. FIG. 14 shows an example of a pixel circuit for one pixel of a conventional active matrix liquid crystal display device. As shown in the figure, an active matrix type liquid crystal display device includes an n-type MOS transistor (hereinafter referred to as an n-type transistor (Qn)) 904 having a gate electrode connected to a scanning line 901 and one electrode connected to a signal line. 902, the other electrode is connected to the pixel electrode 903, and is sandwiched between the storage capacitor 906 formed between the pixel electrode 903 and the storage capacitor electrode 905, and the pixel electrode 903 and the counter electrode Vcom 907. And liquid crystal 908.

  Currently, liquid crystal display devices for notebook PCs and mobile phones that form a large application market for liquid crystal display devices are usually amorphous silicon thin film transistors (hereinafter referred to as a-Si TFTs) or poly (transistors) as transistors (Qn) 904. Silicon thin film transistors (hereinafter referred to as p-Si TFTs) are used. As the liquid crystal material, twisted nematic liquid crystal (hereinafter referred to as TN liquid crystal) is used.

  FIG. 15 shows an equivalent circuit of a TN liquid crystal. As shown in the figure, the equivalent circuit of the TN liquid crystal is a circuit in which a liquid crystal capacitance component C3 (its electrostatic capacitance Cpix), a resistance R1 value Rr and a capacitance C1 (its electrostatic capacitance Cr) are connected in parallel. Can be represented. Here, the resistance value Rr and the capacitance Cr are components that determine the response time constant of the liquid crystal.

  FIG. 16 is a timing chart of the gate scanning voltage Vg, the data signal voltage Vd, and the voltage of the pixel electrode 903 (hereinafter referred to as pixel voltage) Vpix when such a TN liquid crystal is driven by the pixel circuit shown in FIG. Shown in As shown in FIG. 16, when the gate scanning voltage Vg becomes the high level VgH during the horizontal scanning period, the n-type MOS transistor (Qn) 904 is turned on, and the data signal voltage Vd input to the signal line 902 is obtained. Is transferred to the pixel electrode 903 via the n-type transistor (Qn) 904. A TN liquid crystal normally operates in a so-called normally white mode in which light is transmitted when no voltage is applied.

  Here, as the data signal voltage Vd, a voltage that increases the light transmittance through the TN liquid crystal is applied over several fields. When the horizontal scanning period ends and the gate scanning voltage Vg becomes low level, the n-type transistor (Qn) 904 is turned off, and the data signal voltage transferred to the pixel electrode 903 is generated by the storage capacitor 906 and the liquid crystal capacitor Cpix. Retained. At this time, the pixel voltage Vpix causes a voltage shift called a feedthrough voltage via the gate-source capacitance of the n-type transistor (Qn) 904 at the time when the n-type transistor (Qn) 904 is turned off. This voltage shift is indicated by Vf1, Vf2, and Vf3 in FIG. 16, and the amount of the voltage shifts Vf1 to Vf3 can be reduced by designing the value of the storage capacitor 906 to be large.

  The pixel voltage Vpix is held in the next field period until the gate scanning voltage Vg becomes high level again and the transistor (Qn) 904 is selected. The TN liquid crystal is switched according to the held pixel voltage Vpix, and the transmitted light of the liquid crystal transitions from a dark state to a bright state as indicated by the light transmittance T1. At this time, as shown in FIG. 16, in the holding period, the pixel voltage Vpix varies by ΔV1, ΔV2, and ΔV3 in each field, respectively. This is because the capacitance of the liquid crystal changes according to the response of the liquid crystal. Normally, the storage capacitor 906 is designed with a large value of 2 to 3 times or more the pixel capacitance Cpix so as to minimize this variation. As described above, the TN liquid crystal can be driven by the pixel circuit shown in FIG.

  Further, as a technique having an effect such as a mixture of the overdrive method and the reset method, a common voltage (common electrode (common electrode)) which is a voltage of a common electrode arranged to face a pixel electrode disclosed in JP-T-2001-506376 is disclosed. For example, there is a technique for modulating the voltage of the counter electrode). This technique will be described with reference to FIG.

  Conventionally, the common voltage is driven to be maintained at a constant value for one frame period (t0 to t2 (and t2 to t4) in FIG. 18 are defined as one frame period), and one frame period is divided into two. Common inversion drive that changes between two voltage values was performed.

  On the other hand, as shown in FIG. 18, the technique for modulating the common voltage modulates the common voltage, which is the voltage of the common electrode arranged to face the pixel electrode. The upper diagram of FIG. 18 shows the temporal change of the common voltage (VCG), and the lower diagram shows the temporal change of the change of the light transmittance (I) due to the liquid crystal response. That is, the voltage waveform 151 is a voltage waveform applied to the common electrode, the light intensity waveform 152 is a corresponding light intensity waveform at a time corresponding to the waveform 151, and 153 to 156 are pixel light intensity curves.

  One frame period is divided into two, and during the period from t1 to t2 (and t3 to t4), a voltage having substantially the same amplitude as that of the conventional common inversion drive is applied. On the other hand, during a period from t0 to t1 (and t2 to t3) in one frame period, a voltage higher than the amplitude of common inversion (for example, a voltage higher than the amplitude of common inversion by the voltage during black display) is applied. In this technique, the entire display region can be changed to black display at high speed due to the effect of increasing the voltage difference between the pixel electrode and the common electrode during a period from t0 to t1 when a high voltage is applied to the common electrode. That is, driving corresponding to reset driving is performed. Further, even if image data is written on the pixel electrode side during the period from t0 to t1, the potential difference from the common electrode is sufficiently large (for example, equal to or higher than the black display voltage), so that it is not observed on the display. After writing the image data in the entire display area, the voltage of the common electrode is returned to the common inversion amplitude at the timing t1. As a result, the liquid crystal layer starts to respond to the transmittance corresponding to each gradation level in accordance with the voltage stored in the pixel electrode. That is, at the start of response, the voltage difference always changes from the high voltage difference state to the gradation voltage value. In this respect, a kind of overdrive is performed during the period from t0 to t1.

  Here, attention is paid to the fact that the response time of liquid crystal is generally given by the following two formulas (refer to pages 24 to 25, edited by Japan Society for the Promotion of Science, Organic Materials for Information Science, 142nd Committee, Liquid Crystal Division, Japan Society for the Promotion of Science) . That is, in the rising response that turns on by applying a voltage higher than the threshold voltage,

  On the other hand, in the falling response that suddenly sets the applied voltage above the threshold to 0,

  Where d is the thickness of the liquid crystal layer, η is the rotational viscosity, Δ is the dielectric anisotropy, V is the applied voltage, Vc is the threshold voltage, and K is a constant according to the Frank's elastic constant.

Given in.

Here, K ₁₁ is an elastic constant of spread, K ₂₂ is an elastic constant of torsion, and K ₃₃ is an elastic constant of bending. As can be seen from Equation (1), in the rising response, the response time of the liquid crystal depends on the reciprocal of the square of the magnitude of the applied voltage. That is, it depends on the reciprocal of the square according to the voltage value that differs for each gradation level. Therefore, the response time varies greatly depending on the gradation level, and when there is a voltage difference of 10 times, a difference in response time of 100 times occurs. On the other hand, there is a difference in response time depending on the gradation level even in the falling response, but it falls within the range of about twice.

  From the above contents, the speed is increased by the overdrive effect in which a very high voltage is applied during the rising response. Also, since all responses used for actual image display are falling responses, the dependence on the gradation level is extremely small. As a result, almost the same response time can be obtained over all gradations.

JP-T-2001-506376 Japanese Patent Laid-Open No. 04-186227

IDR 1997, pages L-66-69 Japan Society for the Promotion of Science, Organic Materials for Information Science, 142nd Committee, Section A (Liquid Crystal Materials), 91st Study Group, 2003, pp. 28-30

  The response speed of the TN display device has been improved by the prior art. However, as can be seen from FIG. 17, only the display screen in a state where the original luminance is not obtained is obtained. As a result, there still remains a problem that the light utilization efficiency is low and sufficient light transmittance cannot be obtained.

  The reason is that the response speed of the normal nematic liquid crystal is slow, and the response speed of the most widely used TN liquid crystal is particularly slow. Since the reaction speed is slow, the required light transmittance cannot be obtained when the image display mode is raised from the black display at the time of reset because the image cannot be sufficiently raised within the required period. That is, for example, when switching from black display to white display, the display is not completely white but gray. On the other hand, when switching from white display to black display, the display is not completely black but is also gray. Furthermore, as described above, the response speed is further slower in halftone display, and the expected gradation cannot be displayed in moving image display or the like.

  Further, the reset method has a problem due to the slow response speed.

  For example, the liquid crystal may not respond completely until the black display mode is reset. In this case, the same transmittance may not be obtained even if the same data is written a plurality of times. The cause is that when the reset is insufficient, the predetermined orientation state is not completely achieved at the time of reset. Therefore, the response after the reset shows the transmittance according to the history of the previous frame. As a result, there is no one-to-one correspondence between applied voltage and transmittance.

  Furthermore, there is a problem that the optical response of the liquid crystal after reset is delayed or an abnormal optical response is observed before the normal optical response starts. The reason is that at the time of shifting from a predetermined orientation state realized by reset to a normal response, the direction of operation at the time of response is not clear and the response is uneven and unstable.

  It is said that the response time of the TN liquid crystal is generally the sum of the rise response time and the fall response time is about several tens of milliseconds, and the response between halftones is further delayed to be over 100 milliseconds.

  Usually, since the response time at the time of rising is proportional to the square of the thickness as shown in Equation 1, the transmission type has a response time four times that of the reflection type. As a result, the light use efficiency is extremely deteriorated in the transmissive display device.

In order to improve the slow response speed, as can be seen from Equation 1 and Equation 2, (1) the thickness d of the liquid crystal layer is reduced, (2) the viscosity η is reduced, and (3) the dielectric anisotropy Δε. (4) Increase applied voltage (only rising response), (5) Increase K11 and K33 among elastic constants, and decrease K22 (only falling response), etc. Is effective. However, the thickness of the liquid crystal layer (1) can be changed only within a certain range of the refractive index anisotropy Δn in order to obtain a sufficient optical effect. Furthermore, since the viscosity, dielectric anisotropy, and elastic constant of (2), (3), and (5) are all physical property values, it greatly depends on the material, and it is difficult to make it above a certain condition. In addition, it is very difficult to change only each physical property value alone, and it is difficult to realize the speed-up effect assumed from the mathematical formula. For example, K ₁₁ , K ₂₂ and K ₃₃ are independent elastic constants, but according to actual material measurement results (oral presentation by Merck)
K ₁₁ : K ₂₂ : K ₃₃ = 10: 5: 14 (5)
The relationship is almost established and cannot always be treated as an independent constant.

  On the other hand, the method of increasing the applied voltage value in (4) is also greatly restricted from the viewpoint of power consumption and the high cost driving circuit. At the same time, when an active element such as a thin film transistor is provided in the display device for driving, the display device is restricted by the withstand voltage of the element. As described above, there is a great limit to increasing the response speed by the conventional device.

  Although there is a technique using a spontaneous polarization type smectic liquid crystal capable of a high-speed response, the spontaneous polarization type smectic liquid crystal has a high level of accuracy and uniformity in manufacturing, and a yield is lowered. The reason for this is that when a nematic liquid crystal mode other than the TN mode is used for speeding up the response, the required accuracy of the thickness of the liquid crystal layer is increased, or an optical element such as a compensation plate or a phase difference plate is required. This is because uniformity is required. Further, the spontaneous polarization type smectic liquid crystal has a remarkably higher level of flatness with respect to the substrate surface than the nematic liquid crystal such as the TN type, and minute irregularities on the surface of the ITO electrode which is a transparent electrode become a problem. On the other hand, in the ECB (Electrically Controlled Birefringence) mode using birefringence, the requirement for the thickness accuracy of the liquid crystal layer to achieve contrast 100 is about 3% of the thickness of the liquid crystal layer. . In that case, if the thickness is 3 microns or less, the thickness variation needs to be 90 nanometers or less. When the required display conditions such as contrast are high, the required accuracy with respect to the thickness becomes more severe.

  Furthermore, it is difficult for liquid crystals to obtain a stable display over a wide range of temperatures. The reason is that the liquid crystal is solidified at a low temperature, and thus the response speed is highly dependent on the temperature. As a result, at low temperatures, the response speed becomes extremely slow as the viscosity increases, and it is difficult to ensure contrast and display a clear gradation. At high temperatures, the response speed increases and a sufficient response is possible. On the other hand, since the response speed is not sufficiently high at room temperature and a sufficient response cannot be obtained for halftone display, the transmittance at a predetermined gradation greatly varies.

  In the case of the TN display method, if a liquid crystal having a short chiral pitch is used in order to increase the response speed, the response at the time of voltage application, that is, the response to the ON state is delayed. This is because the liquid crystal having a short chiral pitch stabilizes the twisted liquid crystal alignment in a state where no voltage is applied. As a result of this stabilization, the threshold voltage, which is a voltage necessary for shifting to the ON state, increases, and the response time inversely proportional to the square of the difference between the applied voltage and the threshold voltage becomes longer. Further, when this problem is solved by applying a high voltage to the pixel electrode, power consumption is increased.

  Furthermore, in the liquid crystal display device in which the polymer is stabilized in the state where no voltage is applied, the response to voltage application, that is, the response to the ON state is delayed. This problem also arises because of the same cause as the problem related to the chiral pitch, that is, to stabilize the twisted liquid crystal alignment in the state where no voltage is applied. As a result, power consumption also increases. Thus, stabilizing the orientation when no voltage is applied causes the problem of slow response and the problem of increased power consumption.

  On the other hand, the active matrix type liquid crystal element uses a TFT element as a switching element. For this purpose, a semiconductor layer, an insulating film, a wiring layer, and a pixel electrode are provided in the liquid crystal element. As a result, irregularities are formed on the substrate surface.

  If the substrate surface is uneven, the orientation of the liquid crystal is disturbed. When the alignment of the liquid crystal is disturbed, a reverse tilt that tilts in the reverse direction of the pretilt angle of the liquid crystal occurs, and further, a reverse twist that tilts the liquid crystal in the reverse direction of the pretilt occurs.

  When such alignment failure occurs, a boundary (disclination) is formed between the normal alignment portion and the abnormal alignment portion of the liquid crystal layer.

  As a technique for preventing this phenomenon and obtaining a stable alignment in a TN-TFT type liquid crystal display device, Japanese Patent Application Laid-Open No. 4-186227 discloses a chiral pitch (twist pitch. Liquid crystal molecules are twisted 360 ° without external influence. In addition, a liquid crystal having a distance of about 70 to 150 μm is used, and the pretilt angle of the alignment film is set to about 1 °. Thereby, the occurrence of alignment defects such as reverse twist and reverse tilt is prevented, and no alignment defects occur in the OFF state where a low voltage is applied to the display pixels. On the other hand, the following technique is used in this document in order to prevent alignment failure that occurs in the ON state where a high voltage is applied to the display pixel. That is, a liquid crystal having a chiral pitch of 86 μm or less and an alignment film that is aligned with a pretilt angle of 2 to 10 ° with respect to the glass substrate.

  In addition, as a technology for stabilizing the liquid crystal alignment, there are technologies shown on pages 28 to 30 of the "Japan Society for the Promotion of Science Information Science Organic Materials No. 142 Committee A Section (Liquid Crystal Materials) 91st Study Group Materials". . In this technique, a polymer is stabilized by adding a photocurable monomer to twisted nematic liquid crystal and irradiating it with light. In this state, the twisted nematic liquid crystal maintains the twisted structure before the polymer is stabilized.

In particular, by using a liquid crystalline monomer having a liquid crystal skeleton as the photocurable monomer, the liquid crystallinity and orientation of the liquid crystal material for display are not lost. As an example of the effect of this document, after adding a photocurable monomer by diacrylate, which is denoted as Monomer A in the document, to a twisted nematic liquid crystal host at a concentration of several percent, it is irradiated with light with no voltage applied. An example of the produced polymer stable liquid crystal element is shown. FIG. 2 of this document is referred to as FIG. This figure shows the voltage and dielectric constant characteristics of the polymer-stable liquid crystal element. The polymer stabilization element uses DLC-43002, which has a parallel dielectric constant of 11.8 and a vertical dielectric constant of 3.7 as a liquid crystal host. It is manufactured by injecting into a glass cell with a 6 μm gap subjected to rubbing treatment and irradiating with 1 mW / cm ² of ultraviolet rays for 600 seconds. That is, this is an element capable of obtaining a homogeneous alignment liquid crystal alignment substantially parallel to the substrate surface when no voltage is applied and rising from the substrate surface when a voltage is applied. The dotted line in FIG. 19 shows the voltage-dielectric constant characteristics of the element to which no liquid crystalline monomer is applied, and the voltage-dielectric when the liquid crystalline monomer is changed by 2%, 3%, 4%, and 5%. Rate characteristics are shown. As can be clearly seen from the figure, as the addition rate of the liquid crystalline monomer increases, the response of the dielectric constant to the voltage is limited, and the steepness tends to deteriorate.

  The present invention relates to a transistor array substrate on which at least a pixel electrode and a thin film transistor for driving the pixel electrode are formed, a counter substrate on which a common electrode for driving liquid crystal is opposed to the pixel electrode, and a pixel electrode on the transistor array substrate. A common electrode on a counter substrate is arranged to face each other with a gap, and has a twisted nematic liquid crystal sandwiched in the gap between the transistor substrate and the counter substrate, and the twist pitch p of the twisted nematic liquid crystal and the twisted nematic liquid crystal The liquid crystal panel is characterized by having a relationship of p / d <20 with the gap d which is the thickness of the liquid crystal layer.

  It is more preferable to have a relationship of p / d <8 between the twist pitch p (micron) of the twisted nematic liquid crystal and the gap d (micron) as the thickness of the twisted nematic liquid crystal layer.

  The twisted nematic liquid crystal layer is preferably polymer-stabilized, and polymer stabilization can be achieved by adding a photocurable monomer to the twisted nematic liquid crystal and irradiating it with light.

  Here, the photocurable monomer is preferably a liquid crystalline monomer having a liquid crystal skeleton, and diacrylate can be used as the liquid crystalline monomer.

  The liquid crystalline monomer is preferably a monoacrylate in which a polymerizable functional group and a liquid crystal skeleton are bonded without a methylene spacer.

  Further, the liquid crystal panel may include a drive circuit that drives the twisted nematic liquid crystal, and the drive circuit may be formed on the transistor array substrate.

  At this time, the driving circuit is preferably overdrive driving, and a high voltage can be applied to the liquid crystal without increasing the power supply voltage by modulating the common voltage.

  The direction of projection of the maximum electric field other than the electric field between the pixel electrode and the common electrode onto the substrate surface is almost parallel to the direction of projection of the liquid crystal alignment substrate surface at the center of the liquid crystal layer when no voltage is applied to the twisted nematic liquid crystal. It is preferable that

  When the pretilt angle with the substrate surface of the twisted nematic liquid crystal is equal to or smaller than the angle that stabilizes the reverse twist alignment, a torque that returns to the normal twist alignment at the fall of the liquid crystal works. In this case, the pretilt angle is preferably 16 degrees or less.

  The pretilt angle with the substrate surface of the twisted nematic liquid crystal preferably has a large difference between the energy when the electric field of the twisted nematic liquid crystal is not applied and the energy when the electric field is applied, and the pretilt angle at this time is 5 degrees or less. It is preferable.

The anchoring strength in the pretilt angle direction of the twisted nematic liquid crystal is preferably an anchoring strength that does not destabilize the normal twist alignment, and the anchoring strength at this time is 10 ⁻⁵ [J / m ² ] or more. preferable.

  A liquid crystal panel can be mounted on a liquid crystal display device, and the liquid crystal display device has a back surface light source that emits a plurality of colors on the back surface, and a plurality of color images corresponding to a plurality of colors of the back surface light source. Dividing means for dividing data, synchronizing means for synchronizing the color image data of a plurality of colors corresponding to the plurality of colors of the back-emitting light source that emits a plurality of colors, and the multiple color image data are sequentially displayed in time. A field sequential drive type liquid crystal display device including sequential display means can be obtained.

  Further, these liquid crystal display devices can be mounted on electronic devices.

  In the twisted nematic liquid crystal, the present invention establishes a relationship of p / d <20 between the twist pitch p and the thickness (gap between the substrates) d of the twisted nematic liquid crystal layer. Is to increase the response speed.

  By increasing the response speed at the time of falling of the liquid crystal, an unstable alignment state such as bounce does not occur, so that the image can be stabilized within one frame, and the deterioration of the image due to the influence of the history (gradation variation, A display image without flicker can be obtained. Furthermore, it is possible to obtain a display image in which no moving image blur occurs, and at the same time, even if the environmental temperature changes, it is possible to display a good image at a low temperature which has been a problem in the past.

  Furthermore, since the response speed of the fall of the liquid crystal is increased to reach a stable transmittance quickly, the light utilization efficiency is increased, so that a liquid crystal display device with low power consumption can be obtained.

It is sectional drawing which shows the cross-section of the planar type | mold polysilicon TFT switch used in the 1st Embodiment of this invention. It is a figure which shows the relationship between the twist pitch / thickness and the inclination in the transmittance | permeability 50% in the response at the time of the fall of the 1st Embodiment of this invention. It is a figure which shows the structure of the liquid crystalline monomer by the diacrylate of the 2nd Embodiment of this invention. It is a figure which shows the structure of the liquid crystalline monomer of the monoacrylate which the polymeric functional group and liquid crystal frame | skeleton of the 2nd Embodiment of this invention couple | bonded without interposing a methylene spacer. 1 is a schematic view of an entire field sequential display system according to the present invention. It is a time change of the transmittance | permeability in the liquid crystal panel of this invention. It is the figure which calculated | required the time until the transmittance | permeability became 90% and 50% in the liquid crystal panel of this invention, and plotted the horizontal axis p / d and the vertical axis | shaft. In the liquid crystal display panel of this invention, it is the figure which calculated | required the elapsed time until the rise of an optical response begins to occur after reset. It is the figure which calculated | required the average transmittance | permeability of the bounce which has generate | occur | produced in the liquid crystal panel of this invention until the rising period of an optical response. In the liquid crystal display panel of this invention, it is a figure which shows the transmittance | permeability obtained when driving on different temperature conditions. It is a figure which shows the result of having calculated | required the energy difference of reverse twist orientation and normal twist orientation by simulation when changing the pretilt angle of one board | substrate. It is sectional drawing for demonstrating the main processes of preparation of the display panel board | substrate used by this invention. It is sectional drawing for demonstrating the main processes of preparation of the display panel board | substrate used by this invention. It is a figure which shows the example of the pixel circuit which comprises the conventional liquid crystal display device. It is a figure which shows the equivalent circuit of TN liquid crystal. It is a timing chart in the case of driving a TN liquid crystal with a conventional liquid crystal display device. It is a figure which shows the effect of the conventional reset drive, A dotted line is a figure which shows the normal drive and a solid line is a figure which shows the light intensity change of the drive by reset drive. It is a figure explaining the drive which modulates the conventional common voltage, the upper figure shows the voltage waveform applied to a common electrode, and the lower figure is a figure which shows light intensity. It is a figure which shows the effect of the polymer stabilization by the conventional liquid crystalline monomer, and is the figure which plotted the relationship between an applied voltage and a dielectric constant change according to the addition amount of a monomer.

  Before an embodiment of the present invention is described in detail with reference to the drawings, a TFT array used in the present invention will be described.

  A unit structure of a polysilicon TFT array for modifying amorphous silicon into polysilicon will be described with reference to FIG. 1 which is a schematic diagram.

  In the polysilicon TFT of FIG. 1, after forming a silicon oxide film 28 on a glass substrate 29, amorphous silicon was grown. Next, annealing was performed using an excimer laser to convert amorphous silicon into polysilicon 27, and a 10 nm silicon oxide film 28 was further grown. After the patterning, the source region (electrode) 26 and the drain region (electrode) 25 were formed by patterning the photoresist slightly larger than the gate shape (to form the LDD regions 23 and 24 later) and doping with phosphorus ions. Thereafter, a silicon oxide film 28 to be a gate oxide film is grown, then amorphous silicon to be a gate electrode and tungsten silicide (WSi) are grown, and then a photoresist is patterned, and the amorphous silicon and tungsten are masked using the photoresist as a mask. Silicide (WSi) was patterned into a gate electrode shape. Further, LDD regions 23 and 24 were formed by doping phosphorus ions only in necessary regions using the patterned photoresist as a mask. Thereafter, after the silicon oxide film 28 and the silicon nitride film 21 were continuously grown, a contact hole was formed, and aluminum and titanium were formed by sputtering and patterned to form a source electrode 26 and a drain electrode 25. Thereafter, a silicon nitride film 21 was formed on the entire surface, a contact hole was formed, an ITO film was formed on the entire surface, and patterning was performed to form a transparent pixel electrode 22. In this way, a planar TFT pixel switch as shown in FIG. 1 was prepared and a TFT array was formed, so that a pixel array and a scanning circuit using TFT switches were provided on a glass substrate.

  In FIG. 1, a TFT in which amorphous silicon is made into polysilicon is formed. However, after growing the polysilicon, the TFT may be formed by a method of improving the grain size of polysilicon by laser irradiation. In addition to the excimer laser, a continuous wave (CW) laser may be used as the laser.

  Furthermore, an amorphous silicon TFT array can be formed by omitting the step of converting amorphous silicon into polysilicon by laser irradiation.

  12 to 13 are process sectional views showing a method of manufacturing a polysilicon TFT (planar structure) array. The manufacturing method of the polysilicon TFT array will be described in detail with reference to FIGS.

  After forming the silicon oxide film 11 on the glass substrate 10, the amorphous silicon 12 was grown. Next, annealing was performed using an excimer laser to convert amorphous silicon into polysilicon (FIG. 12A).

  Further, after a 10 nm-thickness silicon oxide film 13 is grown and patterned (FIG. 12B), a photoresist 14 is applied and patterned (masking the p-channel region) and doped with phosphorus (P) ions. As a result, n-channel source and drain regions were formed (FIG. 12C).

  Further, after a 90 nm-thick silicon oxide film 15 to be a gate insulating film is grown, amorphous silicon 16 and tungsten silicide (WSi) 17 for forming a gate electrode are grown and patterned into a gate shape (FIG. 12 (d)).

  Photoresist 18 was applied and patterned (masking the n-channel region), and boron (B) was doped to form n-channel source and drain regions (FIG. 13E).

  After the silicon oxide film and the silicon nitride film 19 were continuously grown, a contact hole was formed (FIG. 13F), and aluminum and titanium 20 were formed by sputtering and patterned (FIG. 13G). ). By this patterning, CMOS source / drain electrodes of the peripheral circuit, data line wiring connected to the drain of the pixel switch TFT, and contacts to the pixel electrode are formed.

  Subsequently, a silicon nitride film 21 as an insulating film was formed, holes for contact were made, ITO (indium tin oxide) 22 as a transparent electrode was formed for the pixel electrode, and patterned (FIG. 13 (h)).

  Thus, a TFT pixel switch having a planar structure was produced, and a TFT array was formed.

  A liquid crystal panel is formed by sandwiching the liquid crystal between the TFT array substrate thus manufactured and the counter substrate on which the counter electrode is formed.

  The counter electrode is formed by forming an ITO film on the entire surface of a glass substrate serving as a counter substrate and patterning it, and then forming a light-shielding chromium patterning layer. The light shielding chromium patterning layer may be formed before the ITO film is formed on the entire surface.

  Further, a 2 μm patterned pillar was produced on the counter substrate side. This pillar was used as a spacer for maintaining the cell gap and at the same time had impact resistance.

  This column is for maintaining the cell gap, and the height of the column can be appropriately changed depending on the design of the liquid crystal panel.

  An alignment film was printed on the surfaces of the TFT array substrate and the counter substrate facing each other and rubbed so that an alignment direction having an angle of 90 degrees was obtained after assembly. Thereafter, an ultraviolet curing sealing material was applied to the outside of the pixel region of the counter substrate. After the TFT array substrate and the counter substrate are bonded to face each other, liquid crystal is injected to form a liquid crystal panel.

  Although the patterning layer made of chromium serving as a light shielding film is provided on the counter substrate side, it can also be provided on the TFT array substrate side. Needless to say, the light shielding film can be used as long as it is a material that can shield light even if it is other than chromium. For example, WSi (tungsten silicide), aluminum, or the like can be used.

  When forming a light-shielding chromium patterning layer on the TFT array substrate, there are two types of structures. In the first structure, a light-shielding chromium patterning layer is formed on a glass substrate. After forming the light shielding patterning layer, the first embodiment can be manufactured in the same manner. In the second structure, after the TFT array substrate is manufactured as in the first embodiment, a light-shielding chromium patterning layer is finally provided.

  When the light shielding chromium patterning layer is formed on the TFT array substrate side, it is not necessary to form the light shielding chromium patterning layer on the counter substrate. The counter substrate can be formed by patterning after forming the ITO film on the entire surface.

  Next, details of the first embodiment of the present invention will be described.

  Liquid crystals with different twist pitches were prepared as liquid crystal materials in the gap between the TFT substrate and the counter substrate manufactured by the above method, and a liquid crystal panel was produced for each. Further, a pair of polarizing plates is arranged outside the panel so that a normally white display can be obtained.

  A liquid crystal having a substrate gap (liquid crystal layer thickness) of 2 μm and a twist pitch of 6 μm, 20 μm, or 60 μm was used.

  The thickness of the liquid crystal layer depends on the square of the response speed. For example, if the thickness of the liquid crystal layer is 6 μm (three times the thickness), the response speed becomes 1/9. For this reason, the thickness of the liquid crystal layer is preferably 4 μm or less, and more preferably 3 μm or less. Although there is no limitation on the thickness, the thickness is preferably 0.5 μm or more, more preferably 1 μm or more in consideration of the limitation of the twist pitch of the liquid crystal and the difficulty in manufacturing the gap between the substrates.

  Further, an overdrive driving circuit is provided outside, and overdrive driving is performed during rising driving that requires a high voltage, and overdrive driving is not performed during falling driving when the applied voltage is low.

  In this state, the time-transmittance characteristics of the liquid crystal at the time of falling (optical response at the time of falling of the liquid crystal (that is, response from a dark state to a bright state in the normally white arrangement)) were observed. From the black display state to the completely transmissive white display state, the slope of the transmittance change when the transmittance is around 50% was determined from the observed time-transmittance characteristics. The reason why the vicinity of the transmittance of 50% is selected is that the transmittance change in this vicinity is the largest. FIG. 2 shows a plot in which the obtained slope (% / ms) is plotted on the vertical axis and (p (twist pitch) / d (liquid crystal layer thickness)) is plotted on the horizontal axis. Needless to say, the thickness of the liquid crystal layer is equivalent to the distance between the substrates.

  From FIG. 2, it can be seen that as (twist pitch / liquid crystal layer thickness) decreases, the tilt increases and the response at the fall of the liquid crystal increases. In particular, in the first embodiment, a sharp increase in inclination is observed from (twist pitch / thickness) of about 15, and when (twist pitch / thickness) is about 3, the inclination is 50 (% / ms). Over. That is, ideally, a response within 2 milliseconds is possible. In this figure, when the twist pitch / thickness is 30 and the case of 3, the slope of 3 is almost doubled by 30, and the optical response time at the fall of the liquid crystal may be halved. I understand that. Further, the speed can be increased by 15% or more even under 10 conditions with respect to 30.

  In short, this effect is achieved by a large torque that returns to an initial alignment state in which no voltage or the like is applied (that is, an alignment state that is twisted almost uniformly between the substrates). This effect of the present invention is not directly derived from the response time equations, Equation 1 and Equation 2. However, it can be understood by considering energy. An expression of a free energy density due to the physical effect of the liquid crystal material itself (that is, not including an external effect such as an electric field) is shown as Expression 4.

  Here, the first term, the second term, and the third term are a term related to spread deformation, a term related to torsional deformation, and a term related to bending deformation, respectively. As can be seen from this energy equation, the twist pitch p has the effect of changing the value of the term relating to the elastic constant K22. Specifically, as the twist pitch becomes shorter, the term related to K22 increases. Applying to the response time equation 2 at the time of falling corresponds to an increase in K, and the response time is shortened.

  Due to these effects, in the present invention, it is possible to increase the response speed of the fall of the liquid crystal which is limited by the physical property value of the material and reaches the limit.

  Although the overdrive drive is used in the first embodiment, it goes without saying that the response time of the fall of the liquid crystal can be increased without the overdrive drive.

  Next, the TFT array substrate constituting the liquid crystal panel has irregularities on the surface because the TFT (thin film transistor) is formed on the substrate as compared with the substrate used for simple matrix driving. Since the alignment of the liquid crystal is disturbed if there are irregularities on the surface of the substrate, what stabilizes the alignment of the liquid crystal with respect to the irregularities of the substrate will be described as a second embodiment.

  In the second embodiment of the present invention, alignment stabilization is performed on the liquid crystal to prevent disorder of the alignment of the liquid crystal due to unevenness on the substrate surface. In the present embodiment, the alignment of the liquid crystal is stabilized even when the TFT array substrate has an uneven structure.

  As in the first embodiment, the liquid crystal is sandwiched between the TFT substrate and the counter substrate.

  Also in the second embodiment, twisted nematic liquid crystal of the same material having the same pitch as that used in the first embodiment is used.

As the liquid crystal, twisted nematic liquid crystal added with 2% of a photocurable diacrylate liquid crystalline monomer having the structural formula shown in FIG. 3 is injected, and light irradiation {ultraviolet (1 mW / cm ² × 600 sec. )} And polymerized to obtain a normally white display TN type display device.

  In addition, a twisted nematic liquid crystal in which 2% of a photocurable monoacrylate liquid crystalline monomer in which a polymerizable functional group having the structural formula shown in FIG. 4 and a liquid crystal skeleton are bonded via a methylene spacer is added to this configuration is provided. The same results as in the case of the diacrylate liquid crystal monomer were obtained also when the polymerization was carried out by injecting and irradiating with light without applying voltage.

  This is because the use of a monomer not via a methylene spacer is less likely to be limited in response to the voltage of the liquid crystal with respect to the addition of the monomer. It goes without saying that other liquid crystalline monomers can be used by adjusting the amount of monomer added.

  In order to stabilize the orientation of the liquid crystal with respect to the unevenness of the substrate, the amount of the monomer added may be 0.5% or more with respect to the liquid crystal, but more preferably 1% or more. If it is 5% or less, the responsiveness of the liquid crystal is not impaired, but 3% or less is more preferable.

  In the case of the second embodiment, the light shielding film is preferably formed on the TFT substrate side.

  When polymerizing by light irradiation, light is irradiated from the counter substrate side. Since TFTs, pixel electrodes, wirings, and the like are formed on the TFT array substrate side, the loss of light is large and it is difficult to uniformly irradiate the monomer with light. Therefore, light irradiation from the TFT array substrate side is not performed.

  If a light shielding film is formed on the counter substrate side during light irradiation, unreacted monomers remain in the shaded area of the light shielding film. If unreacted monomer remains, problems such as alignment of the liquid crystal will not occur because the alignment state of the liquid crystal is unknown when the unreacted monomer is polymerized by external light. This is because problems occur.

  When the light-curing chrome patterning layer for shielding light is not formed on the counter substrate side, when the liquid crystalline monomer is photocured, the irradiated light is irradiated to the polymerizable monomer without being shielded, so that it is unreacted. No monomer remains.

  Also in the present embodiment, a high-speed response is realized as in the first embodiment.

  Polymer stabilization adds the free energy density fstab due to polymer stabilization, and fstab produces the same effect as increasing the term related to K22 in Equation 4 by stabilizing in a twisted orientation. In particular, when a photocurable monomer is added with a liquid crystal monomer having a liquid crystal skeleton, it can be stabilized without impairing the liquid crystallinity and orientation of the liquid crystal.

  By using diacrylate as the liquid crystalline monomer, the density of the three-dimensional crosslinked structure can be increased, and a good polymer stabilizing effect can be obtained. In addition, by using monoacrylate in which a polymerizable functional group and a liquid crystal skeleton are bonded without a methylene spacer as a liquid crystal monomer, the liquid crystal skeleton is directly bonded to the polymer main chain after being polymerized. As a result, the liquid crystal skeleton contributes to alignment stabilization. By these actions, it is considered that the torque to return to the initial alignment state where no voltage or the like is applied is increased, and a high-speed response can be obtained at the time of the falling response.

  Also in the second embodiment, the optical response at the time of falling of the liquid crystal, as in the first embodiment, shows that the twist pitch / thickness increases sharply from about 15 to the twist pitch / thickness. When the value of 3 was about 3, the inclination exceeded 50.

  As a modification of the second embodiment, the case where the light shielding film is changed from chromium to aluminum and a liquid crystal material having a different twist pitch is injected as in the first embodiment, and the second embodiment is modified. The response speed of the liquid crystal was examined using a polymer-stabilized liquid crystal panel. As a result, it was found that good contrast can be obtained in addition to the improvement in the responsiveness of the liquid crystal obtained in the first embodiment and the second embodiment. The reason is that the light shielding film made of aluminum has a higher reflectance than chromium, and can greatly reduce the light leakage current.

  In the first embodiment and the second embodiment, the optical response speed of the liquid crystal, in particular, the optical response speed when the liquid crystal falls can be improved. When reset driving is performed in the first and second embodiments, the problem that the same transmittance may not be obtained even if the same data is written a plurality of times, which is a problem in the prior art, is eliminated.

  This is because the response speed at the time of falling of the liquid crystal is improved, and the alignment state of the liquid crystal becomes a predetermined state every time it is reset. Thereby, the response time is defined by the time from the reset state to the desired gradation state. In particular, when the black display state is selected as the reset state, it is possible to improve moving image characteristics that cause tailing by a hold-type display element unique to liquid crystals. That is, the black reset shows a shutter effect, and the hold-type response can be brought close to the impulse-type response. When this black reset is performed, the response at the time of gradation display is defined by the falling response of the liquid crystal. At this time, speeding up the falling response of the liquid crystal according to the present invention is very effective.

  It can be seen that in the second embodiment, the luminance unevenness on the entire surface of the liquid crystal panel is reduced and it is less affected by the unevenness of the substrate than in the first embodiment.

  The present invention is particularly effective because it is not necessary to set the power supply voltage high when a drive circuit that modulates a common voltage for driving such as overdrive driving and reset driving is used.

  A third embodiment of the present invention will be described.

  In the present embodiment, the same liquid crystal panel as in the first embodiment is used.

  A field sequential display system was configured with the configuration shown in FIG. Specifically, normal image data 110 is processed by a controller IC (3) including a controller 105, a pulse generator 104, and a high-speed frame memory 106, thereby converting the image data into image data for each color of red, blue, and green. . This image data is input to a liquid crystal panel (LCD) 100 via a digital analog converter (DAC: Digital Analog Converter) 102. The scanning circuit in the LCD is controlled by drive pulses from a pulse generator of the controller IC. Further, three-color LEDs 101 are used as light sources. The LED 101 is controlled by an LED control signal 108 from the controller IC.

  In the present embodiment, driving for modulating the common voltage in FIG. 18 was performed. At this time, one subframe (that is, the period from t0 to t2 in FIG. 18) was set to 5.56 ms. The length of the period from t0 to t1 during which the common voltage corresponding to reset and overdrive is changed to a high voltage was 0.8 ms. FIG. 6 shows the results of measuring the change in transmittance over time in this state. Here, the same p / d as in the first embodiment is shown as a result of three conditions of 30, 10, and 3. Under the conditions where p / d is 30 and 10, the transmittance once rises after resetting. When this increase in transmittance (hereinafter referred to as bounce) occurs, the response time becomes longer. Under the condition of p / d = 3, almost no bounce is observed. Furthermore, the slope with which the transmittance changes is not steep at p / d = 30, and is the steepest at p / d = 3. At the end of the subframe, the transmittance does not reach 90% at p / d = 30, is slightly over 90% at p / d = 10, and is almost 100% at p / d = 3.

  FIG. 7 shows a graph in which the time until the transmittance reaches 90% and 50% is obtained from the results of FIG. 6, and the horizontal axis is p / d and the vertical axis is time. As can be seen from FIG. 7, when p / d is less than 20, a response with a transmittance of 90% can be reached in a subframe of 5.56 ms. When p / d is less than 20, a response with a transmittance of 50% can be reached in 2.78 ms, which is half the time of a subframe. This shows that sufficient transmittance can be achieved under the condition of p / d <20.

  Further, the elapsed time until the start of the rise of the optical response after resetting was obtained from FIG. That is, each graph after resetting was extrapolated to a transmittance of 0% along the slope at the point of the transmittance of 10%. Then, the difference between the time of the intersection between the extrapolated graph and the transmittance of 0% and the end of reset (here, 0.8 ms) was obtained. The result is shown in FIG. From FIG. 8, under the condition that p / d is less than 8, the time until the response starts after resetting is less than 1 ms. By cutting 1 ms, the time in the subframe can be used effectively. Further, the average transmittance of the bounce generated after the reset in FIG. 6 until the rising period of the optical response obtained in FIG. 8 was obtained. That is, the integral of the transmittance at the time of bounce was divided by the time until the response in FIG. The result is shown in FIG. As can be seen from this figure, the average transmittance during the bounce tends to bend at a point where p / d = 8, and the average transmittance suddenly decreases. Under the condition of p / d <8, a sudden drop in bounce is obtained. The fact that this bounce is reduced means that there is no relationship with the data of the previous subframe and that a one-to-one correspondence can be obtained, and that an unstable alignment state does not occur and a good display can be obtained over a long period of time. Show.

  The driving for modulating the common voltage is already known, but driving corresponding to normal common inversion driving can be applied. As a result, for example, when the conventional data signal is ± 4.5 V with respect to the common voltage, driving with an amplitude width of 9 V is necessary, but the amplitude width is reduced to 4.5 V by half by common inversion driving. Is possible. Further, in the configuration for modulating the common voltage, the display is always reset to the black display state, and the voltage necessary for the actual gradation display is only the difference from the black display voltage. That is, when the threshold value of the liquid crystal is 1.5V, 3V which is a difference from the voltage 4.5V at the time of black display is an actually required voltage. Therefore, by devising the method of modulating the common voltage, the required data amplitude becomes 3V, which is 1/3 of the conventional one. As a result, unlike the conventional overdrive method and reset method, a high-speed response can be obtained by driving with very low power consumption. In particular, according to the present invention, the response speed is accelerated by nearly 50%, and a field sequential display with low power consumption and high luminance can be obtained even when a TN liquid crystal is used. In this field sequential driving, in addition to the very good color reproducibility, the moving image display performance is also very good because the reset is always performed.

  By combining the overdrive drive, the reset pulse drive, and the common modulation drive with the present invention, the response speed can be increased with low power consumption.

  Thus, since field sequential driving can be satisfactorily realized with a TN liquid crystal, it is not necessary to pay much attention to cell gap uniformity and compensation plate uniformity as in the OCB method. Further, since a general material can be used without using a special material such as a ferroelectric liquid crystal, the long-term reliability is high.

  Furthermore, the result of having measured by changing temperature conditions is shown. Here, two liquid crystals with p / d = 30 and p / d = 3 were used. As temperature conditions, normal temperature (25 ° C.) and low temperature (3 ° C.) were selected. Three conditions of white display, black display, and gray display were selected under normal temperature conditions. The results are shown in FIG. In the figure, the horizontal axis represents temperature, and the vertical axis represents transmittance. Under the condition of p / d = 30, almost no white display transmittance can be obtained when the temperature reaches 3 ° C. On the other hand, under the condition of p / d = 3, a transmittance of 60% or more is obtained even at a temperature of 3 ° C. From this, it can be seen that the present invention can also improve the temperature dependence of the liquid crystal response.

  This improvement in temperature dependence is thought to be due to the following two effects. The first point is that the response of the liquid crystal is accelerated. Thereby, a sufficient response speed can be obtained even at a low temperature. The second point is that no bounce occurs. As a result, an unstable orientation state is prevented and the influence of the previous frame is not affected. Therefore, it is considered that gradation display is performed well even if the environmental temperature is changed.

  Furthermore, it is known that the temperature dependence is related to the structure of the twisted nematic liquid crystal material. It is obvious for those skilled in the art what kind of material should be used. In particular, the composition of the composition is such that the temperature dependence of the twist pitch of the composition is small and a pitch almost equal to that at room temperature can be obtained in a wide temperature range, or the pitch becomes short at low temperatures. It is effective to do.

  The reason why no bounce occurs is as follows.

  In the conventional method, the liquid crystal alignment sufficiently rises when a high voltage is applied. When the voltage is turned off from this state, the liquid crystal tends to move to a lying orientation, but in which state the liquid crystal rises sufficiently, it is not defined in which direction it falls. As a result, the orientation becomes unstable and bounce occurs. In addition, if it does not stand up sufficiently, it is stored as a history and an unstable display occurs. On the other hand, in the present invention, when a high voltage is applied, the effect of stabilizing when no voltage is applied according to the present invention and the rising effect due to the high voltage are balanced in a certain balance. As a result, the liquid crystal does not rise too much, so that it always falls in a certain direction when the voltage is turned off. Further, since this balance relationship is made between large torques, the balance is always stable in substantially the same state. For this reason, a good reset effect is always obtained without storing the history.

  As described above, in the twisted orientation state, there are a normal twist orientation that twists along the twist pitch and a reverse twist orientation in which the twist direction of the twist pitch differs from the actual twist direction. By setting (twist pitch / liquid crystal layer thickness) <8, the torque to return to the normal twist alignment can be increased, so that defects due to the reverse twist alignment can be prevented and the speed of the present invention can be increased. It is done.

  In addition to the electric field between the pixel electrode and the common electrode, an electric field is generated between the wiring (for example, a scanning line) disposed on the outer edge of the pixel. The direction of projection of the maximum electric field other than the electric field generated between the pixel electrode and the common electrode onto the substrate surface is the direction of projection of the substrate surface of the liquid crystal orientation at the center of the liquid crystal layer when no voltage is applied to the twisted nematic liquid crystal , The torque returning to the liquid crystal alignment of the normal twist alignment (realized when no voltage is applied) can be increased. Conversely, if these projections are orthogonal to each other, reverse twist orientation is likely to occur.

  Furthermore, the normal twist alignment can be stabilized under the condition that the liquid crystal alignment angle (rise angle, pretilt angle) on the substrate surface is constant. FIG. 11 is a diagram illustrating a result of obtaining the energy difference between the reverse twist orientation and the normal twist orientation by simulation when the pretilt angle is changed. Up to a pretilt angle of about 16 degrees, the energy difference increases as the pretilt angle increases. That is, within that range, the higher the pretilt angle, the more stable the normal twist alignment. However, when the angle exceeds 16 degrees, the energy difference decreases and the reverse twist orientation becomes stable. Further, the difference in torque of the falling response is obtained by the energy difference between the liquid crystal alignment state when the voltage is applied and the liquid crystal alignment state when no voltage is applied. According to this simulation result, the energy difference increases as the pretilt angle decreases. In particular, a significant energy difference was obtained when the pretilt angle was 5 degrees or less.

On the other hand, the anchoring strength indicating the magnitude of the interaction between the substrate and the liquid crystal on the substrate surface also causes an energy difference due to the difference in the pretilt angle of the liquid crystal, resulting in a difference in torque. From the simulation, it was found that the normal twist orientation suddenly becomes unstable when the anchoring strength becomes smaller than 10 ⁻⁵ [J / m ² ].

  By field-sequentially driving the liquid crystal display device of the present invention in synchronism with the light source, a field-sequential display device that has been considered difficult with the TN-TFT type can be realized.

  In the present invention, the twist angle (twist angle) does not necessarily have to be around 90 degrees, as long as the structure has a twist, and the angle is not specified.

  The present invention is effective when mounted on an electronic device for television or a game in which the responsiveness of a transmissive display device can be improved and the image has a lot of movement. In the present invention, the twist angle (twist angle) does not necessarily have to be around 90 degrees, as long as the structure has a twist, and the angle is not specified.

DESCRIPTION OF SYMBOLS 10 Glass substrate 11 Silicon oxide film 12 Amorphous silicon 13 Silicon oxide film 14 Photoresist 15 Silicon oxide film 16 Amorphous silicon 17 Tungsten silicide (WSi)
18 Photoresist 19 Silicon oxide film / silicon nitride film 20 Metal (aluminum and titanium)
21 Silicon nitride film 22 Pixel electrode (ITO)
23, 24 LDD region 25 Drain electrode 26 Source electrode 27 Polysilicon 28 Silicon oxide film 29 Glass substrate 51 Signal electrode line 52 Thin film transistor 53 Scan electrode line 54 Pixel electrode 100 LCD
101 LED
102 DAC
103 Controller IC
104 Pulse generator 105 Controller 106 High-speed frame memory 107 Synchronization signal 108 LED control signal 109 Drive pulse 110 Image data 151 Voltage waveform 152 applied to the common electrode 152 Corresponding light intensity waveforms 153, 154, 155 at the time corresponding to the waveform 151 156 Pixel light intensity curve 901 Scan line 902 Signal line 903 Pixel electrode 904 MOS transistor 905 Storage capacitor electrode 906 Storage capacitor 907 Counter electrode 908 Liquid crystal

Claims (8)

An active element substrate having at least a pixel electrode and a plurality of active elements for driving the pixel electrode;
A counter substrate having a common electrode facing the active element substrate and driving a liquid crystal together with at least the pixel electrode;
A twisted nematic liquid crystal sandwiched in a gap between the active element substrate and the counter substrate;
Have
The gap is in the range of 0.5 to 3 μm, and between the twist pitch p of the twisted nematic liquid crystal sandwiched in the gap and the gap d,
p / d <20
In the liquid crystal display device having the relationship
Of the electric field between the common electrode and the wiring arranged outside the pixel other than the electric field between the pixel electrode and the common electrode, the direction of projection of the maximum electric field onto the substrate surface is the voltage of the twisted nematic liquid crystal. A liquid crystal display device characterized by not being orthogonal to the direction of projection of the substrate surface of the liquid crystal alignment at the center of the liquid crystal layer when no voltage is applied. An active element substrate having at least a pixel electrode and a plurality of active elements for driving the pixel electrode;
A counter substrate having a common electrode facing the active element substrate and driving a liquid crystal together with at least the pixel electrode;
A twisted nematic liquid crystal sandwiched in a gap between the active element substrate and the counter substrate;
Have
The gap is in the range of 0.5 to 3 μm, and between the twist pitch p of the twisted nematic liquid crystal sandwiched in the gap and the gap d,
p / d <20
In the liquid crystal display device having the relationship
The active element substrate has at least irregularities corresponding to the pixel electrodes on the surface thereof, and a twisted nematic liquid crystal sandwiched in the gap is polymer-stabilized.   A driving circuit for driving the twisted nematic liquid crystal, wherein the driving circuit performs reset pulse driving for applying a reset voltage for aligning a predetermined liquid crystal state within one frame before writing new data. Item 3. A liquid crystal display device according to item 1 or 2.   The liquid crystal display device according to claim 3, further comprising a drive unit that modulates a common voltage that is a voltage of the common electrode.   The liquid crystal display device according to claim 4, further comprising a drive unit that performs overdrive driving when the liquid crystal is driven to rise. Between the twist pitch p of the twisted nematic liquid crystal and the gap d,
p / d <8
The liquid crystal display device according to claim 5, wherein:   The liquid crystal display device according to claim 6, wherein a pretilt angle with the substrate surface of the twisted nematic liquid crystal is 5 degrees or less. 8. The liquid crystal display device according to claim 7, wherein the anchoring strength in the pretilt angle direction on the substrate surface of the twisted nematic liquid crystal is 10 ⁻⁵ [J / m ² ] or more.