JPH08248384A - Liquid crystal display device - Google Patents

Abstract

PURPOSE: To prevent residual images or the drop of the contrast ratio and to improve display quality by setting the Δn.d value of liquid crystal at a value larger than one obtained from a specified first minimum condition and thereby by making the lower limit of the driving voltage range higher than the threshold value which allows the liquid crystal to be driven. CONSTITUTION: The product, An-d, where Δn and d are double refraction index and the thickness of liquid crystal layer respectively is set at a value larger than the one obtained from the first minimum condition that is represented by the formula shown where u=πdΔn/θλ, θ: the tortional angle of liquid crystal, λ: the wavelength of light. Consequently a humpy section giving a peak of the transmission curve appears on the right-hand side of the threshold value voltage Vth. Therefore, the initial voltage Vor as the lower limit of the driving voltage region VR is set at the voltage value which maximizes transmittance T. And the upper limit of the driving voltage region VR is set at 7V. By this, the driving voltage region is reduced without lowering the contrast ratio and thereby the delay in the reaction of liquid crystal is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display capable of providing an image having no afterimage by preventing the delay of the response speed of the liquid crystal by optimally setting the driving voltage width. Regarding display device.

[0002]

2. Description of the Related Art Liquid crystal display devices have advantages such as small size, thin shape, and low power consumption, and are being put to practical use in fields such as OA equipment and AV equipment. In particular, a matrix type in which transparent electrodes for driving liquid crystal are crossed and voltage is applied while selecting display points in a matrix, and further, a plurality of pixel electrodes for driving liquid crystal are arranged to face a common electrode, By connecting switch elements to the electrodes, the active matrix type, which retains the signal voltage while selecting the rewriting pixels line-sequentially, enables high-definition, high-contrast ratio moving image display, and displays on a personal computer. It has been put to practical use in television, etc.

A liquid crystal display device is constructed by adhering an electrode substrate having a transparent electrode for driving the liquid crystal with a narrow gap and sealing the liquid crystal inside. FIG. 3 shows the structure of a liquid crystal display cell. Glass substrate (10) (20)
Transparent electrodes (11) and (21) made of ITO are formed on the upper surface of the liquid crystal layer (30), and the facing portions of the electrodes (11 and 21) are sealed between the substrates (10 and 20). ) Is partitioned to form the pixel capacitance. Each pixel capacitance is configured so that a desired voltage is applied. The liquid crystal has anisotropy in permittivity and refractive index, and its alignment state changes according to the electric field (31) formed in each pixel capacitance to modulate transmitted light. Since the transmittance is finely adjusted depending on the electric field strength, gradation control is performed by controlling the applied voltage for each pixel capacitance, and a desired display screen is created.

FIG. 4 shows the voltage-transmittance characteristics in the normally white (NW) mode (a) and the normally black (NB) mode (b). Each,
When no voltage is applied (V = 0), the transmittance is maximum (T = M) or minimum (T = m), the applied voltage (V) rises, and the threshold voltage (Vth) that drives the liquid crystal. When it exceeds, the transmittance (T) starts to change. The drive voltage range (VR) is set corresponding to the region where the transmittance (T) changes, and gradation display is performed.

[0005]

Normally, TN (Twisted)
In the nematic method, the liquid crystal has a slight tilt angle (pretilt angle) by controlling the alignment in the initial state by interacting with the alignment film made of a polymer film formed on the surfaces of both electrode substrates. And they are aligned in a certain direction. When a voltage is applied to the pixel capacitance, the liquid crystal undergoes an electric field effect to change its orientation, but at the edge of the pixel capacitance electrode, a reaction different from that of other regions occurs due to an oblique electric field. That is, although the change destination of the orientation is determined in advance by the pretilt, the action of the oblique electric field is predominant at the edge of the pixel capacitor electrode, so that the side opposite to the pretilt rises to form a so-called reverse tilt domain. hand,
It takes time to return to the normal alignment, and the response speed of the liquid crystal apparently decreases. This is remarkable even when observed with the naked eye, and particularly when the white in the NW mode or the black in the NB mode changes to another display color, the reaction delay of 1 to several seconds in the reverse tilt domain. Was seen, and it was an afterimage.

FIG. 5 shows the results of examining changes in the transmittance (T) with respect to time (t) in the conventional liquid crystal display device. The experiment was performed in NW mode, and the maximum transmission (transmittance of 9
0%) voltage value (V90) is changed to a voltage value (V0) where the transmittance is 0%, and when the voltage is again V90, the transmittance (T) changes with time, and the intermediate value is the same. The voltage value (V80) in the gradation state (transmittance of 80%) is changed to the voltage value (V0) of which the transmittance is 0%, and the transmittance (T) when the voltage is set to V80 again. With time change,
Each graph (III, IV) was created.

First, looking at graph (IV), t = 130
At [msec], the voltage is changed from V = V80 to V = V0, and the transmittance is also T = 80 [%].
To T = 0 [%], the response is very steep and shows a high response characteristic. On the other hand, in graph (III), t =
At 130 [msec], the voltage is changed from V = V90 to V =
The transmittance is changed to V0, and the transmittance shows a steep change from T = 90 [%] to T = 10 [%], but then drops very slowly toward T = 0. . As a result, in the case of white, that is, when the maximum transmission state (T = 90) is changed to another display color, it takes time to reach a predetermined transmittance due to the delay of the reaction in the reverse tilt domain. I understand.

Such a delay in the reaction causes a reduction in the contrast ratio or a residual image in the display of a moving image, thus deteriorating the display quality. If the drive voltage voltage range (VR) is reduced to eliminate the voltage range in which the reverse tilt domain is generated in order to prevent such a delay in the reaction, sufficient white (or black) brightness cannot be obtained, and the contrast ratio is reduced. Leading to a decrease in

[0009]

The present invention has been made to solve this problem, in which a liquid crystal is sealed between electrode substrates, and a pixel capacitor for driving a liquid crystal is formed for each display pixel.
In a liquid crystal display device in which a voltage is applied to the pixel capacitance to change the transmission characteristics of the corresponding liquid crystal, the product Δn · d value of the birefringence Δn of the liquid crystal and the thickness d of the liquid crystal layer is expressed as

[0010]

[Equation 2]

(In the above equation, u = πdΔn / θλ, θ is the twist angle of the liquid crystal, and λ is the wavelength of light.) The value is set to be larger than the value obtained from the first minimum condition in the equation. At the same time, the lower limit of the voltage range applied to the pixel capacitance is set to a value that maximizes or minimizes the transmittance.

[0012]

With the configuration of the present invention, the Δn · d value of the liquid crystal is set to be larger than the first minimum value, and the lower limit of the applied voltage range is set to the maximum transmittance (in the NW mode) or minimum transmittance (in the NB mode). If the value is set to (case), the delay of the liquid crystal reaction can be prevented without simply reducing the drive voltage range and lowering the contrast ratio. That is, since the maximum transmission state (white in the NW mode) or the minimum transmission state (black in the NB mode) is realized in the voltage applied state, the liquid crystal is constantly driven.
The transition from a certain driving state to a driving state showing another display color becomes quicker, and the delay of the reaction is eliminated.

[0013]

EXAMPLES Next, examples of the present invention will be described in detail.
First, the transmittance T when no voltage is applied depends on the product Δn · d value of the birefringence Δn and the cell gap d according to the equation of the Goochand Tarry derived in the NB mode, It is represented as follows.

Normally, the Δn · d value is set to 0.3 to 0.5 μm according to the first minimum condition of the above equation, but in the present invention, Δn · d = 0.50 μm or more is set, and this Δn is set. The voltage value that minimizes the transmittance is set based on the correlation with the d value, and this is set as the lower limit of the drive voltage range.
In the NW mode, the transmittance becomes maximum at this time. FIG.
A graph of voltage-transmittance characteristics when the cell gap d is set to 4.3 μm in the NW mode is shown in FIG. In the graph (A), when Δn is 0.135, the graph (B) is Δn
Is 0.150, Δn is 0.200 in the graph (C).
Is the case. At this time, each Δn · d value is 0.
It becomes 58 μm, 0.65 μm, and 0.86 μm. As can be seen from the figure, the applied voltage (V) is 2 in any case.
The transmittance (T) (arbitrary unit) begins to change in the vicinity of [v], and this value is the threshold voltage (Vth) for driving the liquid crystal. The voltage that maximizes the transmittance (T) is V = 2.2 [v] in the graph (A) and V = in the graph (B).
2.3 [v], and V = 2.6 [v] in the graph (C). That is, when the Δn · d value is set to be larger than the first minimum value, a bump-shaped portion at the peak of the transmittance curve appears on the right side of the threshold voltage (Vth), and the voltage that maximizes the transmittance (T) is , Both are threshold voltage (Vth)
It is a larger value. In this embodiment, the voltage value that maximizes the transmittance (T) is set as the initial voltage (Vor) as the lower limit of the drive voltage range (VR). Further, from the figure, the upper limit of the drive voltage range (VR) is set to 7 [v].

With this setting, the liquid crystal is always driven, and the initial state is realized by voltage application. Therefore, the reverse state that occurs when the voltage is not applied is changed to another state. The reaction delay in the tilt domain is eliminated. In order to eliminate the reaction delay, it is effective that the initial voltage (Vor) is higher than the threshold voltage (Vth). That is, by increasing the Δn · d value and increasing the initial voltage (Vor) for obtaining the maximum transmission state, the transition to the state exhibiting another transmittance becomes rapid. However, in the NW mode, the contrast ratio is generally Δn
・ Because it begins to decrease when the d value increases from 0.4 μm, taking both the reaction delay and the contrast ratio into consideration,
The Δn · d value must be set appropriately.

FIG. 2 shows the time (t) for the liquid crystal display device in which the drive voltage range is set based on the graph (C) of FIG.
The change in transmittance (T) was investigated. The voltage is V = Vor
To a voltage value (V0) that changes the transmittance to 0%, and then changes the transmittance (T) with time when the voltage Vor is set again, and a voltage value (80%) for comparison for comparison. V80)
Was changed to a voltage value (V0) at which the transmittance was 0%, and the time change of the transmittance (T) when the voltage was again set to V80 was measured to prepare respective graphs (I, II). . In the graph (I), V = V at t = 130 [msec]
The transmittance is also T = 90 with the change from or to V = V0.
There is a sharp change from [%] to T = 0 [%]. Also in the graph (II), at t = 130 [msec], T = 80 with the change from V = V80 to V = V0.
There is a sharp change from [%] to T = 0 [%]. From this, it can be seen that the transition from white (V = Vor) to another display color is performed as quickly as the transition from a display color other than white to another display color. Needless to say, the above is the same in the NB mode.

Although the TN mode has been described above mainly for the purpose of preventing the delay of the operation of the liquid crystal in the reverse tilt domain, the present invention is not limited to this, and the present applicant Has already filed Japanese Patent Application No. 5-84696,
Japanese Patent Application No. 5-153671, Japanese Patent Application No. 5-157120, Japanese Patent Application No. 5-169087, Japanese Patent Application No. 5-169088, Japanese Patent Application No. 5-216441, Japanese Patent Application No. 5-295731, Japanese Patent Application No. 6-21152, Japanese Patent Application No. Japanese Patent Application No. 6-92283, Japanese Patent Application No. 6-1
04044, Japanese Patent Application No. 6-207589, Japanese Patent Application No. 6-23
It can also be applied to those already filed in 7482. That is, in addition to the pixel capacitance electrode, an alignment control electrode for controlling the directivity of the alignment of the liquid crystal, an alignment control window formed as an electrode absent region in the pixel capacitance electrode to fix the boundary between regions having different alignment directions of the liquid crystal, Alternatively, by providing an alignment control layer or the like that partially bulges or dents the contact interface with the liquid crystal layer to give directivity of the alignment of the liquid crystal, and controls the alignment of the liquid crystal to be divided into multiple directions, Even in the structure with enlarged corners, the speed at which the liquid crystal operates in response to an electric field can be increased by setting the Δn · d value and the applied voltage range based on the present application.

[0018]

According to the present invention, the Δn · d value of the liquid crystal can be calculated by
By setting the value higher than the value obtained from the first minimum condition in the expression of h and Tarry and setting the lower limit of the driving voltage range higher than the threshold value for driving the liquid crystal,
In black or white display, no voltage is applied, and the liquid crystal is always driven. Therefore, the liquid crystal is not applied with no voltage, and when the voltage is changed to another voltage applied state, the difference in the response speed due to the difference in the rising direction of the liquid crystal is eliminated, and the afterimage or the decrease in the contrast ratio is prevented. The display quality is improved. Further, since the maximum or minimum transmission state is realized when a voltage is applied, the contrast ratio is not reduced by simply reducing the driving voltage range.

[Brief description of drawings]

FIG. 1 is a voltage of a liquid crystal display device showing the effect of the present invention.
It is a characteristic view of transmittance.

FIG. 2 is a time chart of a liquid crystal display device showing the function and effect of the present invention.
It is a characteristic view of transmittance.

FIG. 3 is a cross-sectional view of a liquid crystal display device.

FIG. 4 is a voltage-transmittance characteristic diagram of a conventional liquid crystal display device.

FIG. 5 is a characteristic diagram of time-transmittance of a conventional liquid crystal display device.

[Explanation of symbols]

 T Transmittance VR Drive voltage range Vor Initial voltage Vth Threshold voltage for driving liquid crystal V0 Voltage with 0% transmittance V80 Voltage with 80% transmittance V90 Voltage with 90% transmittance 10,20 Substrate 11, 21 transparent electrode 30 liquid crystal layer 31 electric field

Claims (1)

[Claims] 1. A liquid crystal display device in which liquid crystal is sealed between electrode substrates, a liquid crystal driving pixel capacitance is formed for each display pixel, and a transmission characteristic of the corresponding liquid crystal is changed by applying a voltage to the pixel capacitance. , The product Δn of the birefringence Δn of the liquid crystal and the thickness d of the liquid crystal layer
Let the d value be (In the above equation, u = πdΔn / θλ, θ is the twist angle of the liquid crystal, and λ is the wavelength of the light.) In addition to the value obtained from the first minimum condition in the equation, According to the above, the lower limit of the voltage range applied to the pixel capacitance is set to a value that maximizes or minimizes the transmittance.