JPH1172765A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a circuit scale without deteriorating picture quality and to shorten the adjusting time of common electrode voltage in an active matrix liquid crystal display device having a laminated structure. SOLUTION: This active matrix liquid crystal display device having laminated structure is constituted so that the same common electrode voltage is supplied from the same common electrode drive circuit 22 to respective common electrodes 3, 6, 9, and the common electrode voltage is adjusted so that the voltage polarity difference in a first layer 4 and a second layer 7 large in reflectance change becomes small, and preferably, the voltage polarity difference in the first layer 4 becomes the smallest, and thus, the occurrence of a flicker in the liquid crystal display device is reduced while reducing the circuit scale.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an active matrix type liquid crystal display device having a laminated structure.

[0002]

2. Description of the Related Art Various portable information devices, such as personal computers, word processors, EWS, calculators, electronic books, electronic organizers,
Since a PDA, a portable television, a portable telephone, a portable FAX and the like are required to be driven by a battery, the display devices mounted on these various information devices must have low power consumption. Conventionally, a liquid crystal display (LCD), a plasma display, a flat CRT, and the like are known as thin displays. Among them, LCDs are the best in terms of low power consumption and are widely used.
An LCD in which the display surface of the display is directly viewed is referred to as a direct viewing type. Direct-view LCDs include a transmissive type that incorporates a light source such as a fluorescent lamp on the back and a reflective type that uses ambient light. The former requires a backlight and is not suitable for reducing power consumption. This means that the backlight requires power consumption of 1 W or more, and the usable time by battery driving is generally only about 2 to 3 hours.
From this point, the latter reflective type is the most widely used display for portable information devices.

[0003] In a reflection type LCD, a reflection plate and a polarizing plate made of aluminum foil having a matte surface are laminated and adhered on a rear glass substrate. Such a reflective LCD
Has no power consumption because it does not emit light. However, the conventional reflective LCD cannot display bright paper white display colors, and it is naturally difficult to realize a vivid color display. This is a major technical issue in developing a reflective LCD that is comparable to the image quality of a transmissive TFT-LCD.

In a reflection type LCD, an ECB (Electr
Controlled Birefrigennce) mode, GH (Guest
Host) mode, TN (Twisted Nematic) mode, and the like are used.

[0005] In the case of the ECB mode or the TN mode, a scratch plate is required. Since the polarizing plate has a light transmittance of about 40%, the light use efficiency is deteriorated. In the case of a reflective display device, the brightness of the display device is evaluated by a reflectance. This represents what percentage of the light incident on the display is reflected. This measurement is usually performed by integrating the diffuse reflected light with an integrating sphere. For example, newspaper paper has a reflectance of about 60%, high-quality paper has a reflectance of about 80%, and powder such as magnesium oxide and barium sulfate has a reflectance of 99% or more.

On the other hand, when a polarizing plate is provided in an LCD, a reflectance of 40% or more cannot be desired, which causes a problem in display performance. A paper white display requires a reflectance of 60% or more.

Therefore, the GH system which does not require a polarizing plate is most promising from the viewpoint of light use efficiency. When color display is performed by this method, it is necessary to stack three GH cells each containing cyan, magenta, and yellow dyes.
Generally, in order to realize a color display with a wide color reproduction range with a reflector, a method of laminating GH cells is most preferable. On the other hand, in the RGB parallel arrangement in which three GH cells are arranged side by side or in the cyan, magenta, and yellow parallel arrangement, the same color cannot be displayed on the entire surface, so that the color reproduction range is inevitably narrowed.

When a dot matrix display is performed in these three-layer GH cells, it is necessary to transmit image information in units of one pixel. As a method of matrix driving, there are simple matrix driving and active matrix driving. The former requires steepness in the VT (voltage-transmittance) characteristics, so that a G is present in which the ratio of liquid crystal is reduced due to the inclusion of a dye.
H liquid crystals are not very suitable. In the latter, the active element is a diode MIM method and a transistor TFT
There is a method. In the MIM method, as the number of pixels is increased, the effective voltage applied to one pixel is reduced and may be less than 5V. Therefore, the VT of the current GH liquid crystal is
Considering the characteristics, the MIM method is not suitable for driving the GH liquid crystal. On the other hand, in the TFT method, the voltage of one pixel can be set arbitrarily, and it can be said that it is suitable for GH liquid crystal.

From the above background, a high-quality reflective display device having a laminated structure (Japanese Patent Application Laid-Open No. 8-57531) has been proposed (hereinafter referred to as "three-layer GH liquid crystal"). Driving methods related to the three-layer GH liquid crystal are disclosed in Japanese Patent Application Nos. 7-235357 and 7-235357.

The above three-layer GH liquid crystal is not used as a reflection type display device, but also as a transmission type liquid crystal display device using a backlight instead of a reflection plate. Powerful display devices can be realized.

[0011]

Since an active matrix type liquid crystal display device has a TFT which is an active element, a so-called pixel potential level shift occurs when the gate pulse of the TFT is changed from an ON voltage to an OFF voltage. (Reference: The Journal of the Institute of Television Engineers of Japan, Vol. 42, No. 1 (1988)
“Drive system”, ΔVGD in Nikkei BP “Flat Panel Display 1991” p85, K. Suzuki: “Compensat
ive Addressing for Switching Distortionina-Si TFT-
LCD "dV in Proc.of Eurodisplay 87p.107
s ^* ), image quality degradation due to pixel potential level shift occurs. As a general method of preventing image quality deterioration due to a pixel potential level shift, a common electrode voltage is shifted by a pixel potential level shift, and a potential difference between a pixel electrode and a common electrode in a positive polarity and a pixel electrode-common electrode in a negative polarity are used. This is a method of making the potential difference between them effectively equal.

In an active matrix type liquid crystal display device having a laminated structure (for example, a three-layer GH liquid crystal), a pixel potential level shift occurs in each layer and causes flicker, but the pixel potential level shift is injected into the TFT and each layer. (Ref .:
k.Suzuki; "Compensative Addressing for SwitchingDi
stortion in a-Si TFT-LCD "Proc.of Eurodisplay 87
p.107) Even when different liquid crystal materials are injected into each layer, even when the same liquid crystal material is injected, the cell gap varies between layers and the TFT characteristics vary. The potential level shift is different. Therefore, it is only necessary to adjust the common electrode voltage for each layer. However, in order to adjust the voltage for each layer, a different common electrode drive circuit is required for each layer, which leads to an increase in circuit size and power consumption. The adjustment time is simply required to be three times that of the conventional case.

An object of the present invention is to provide an active matrix type liquid crystal display device having a laminated structure with a small circuit size, low power consumption and no image quality deterioration such as flicker. .

[0014]

In order to achieve the above object, a liquid crystal display device according to the present invention comprises an active matrix type liquid crystal display device, comprising: a plurality of liquid crystal layers having different display colors; A plurality of common electrodes provided corresponding to the plurality of electrodes, and a common electrode for simultaneously supplying a common electrode voltage to the plurality of common electrodes and adjusting the common electrode voltage so as to minimize a voltage polarity difference of a specific liquid crystal layer. And an electrode driving means.

In particular, the present invention has a common electrode driving means for adjusting the common electrode voltage so as to minimize the voltage polarity difference between one or more liquid crystal layers including the liquid crystal layer having the largest change in reflectance.

According to the present invention, a common electrode voltage can be simultaneously supplied to a plurality of common electrodes by the same common electrode driving means, and a voltage polarity difference in a liquid crystal layer having a large change in reflectance in a plurality of liquid crystal layers is minimized. Since the common electrode voltage is adjusted to be as small as possible, it is possible to realize an active matrix type liquid crystal display device having a laminated structure with a small circuit size, low power consumption, and without image quality deterioration such as flicker.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

1 and 2 show examples of the configuration of a display device to which the present invention is applied.

The display device shown in FIG. 1 is disclosed in Japanese Patent Application No. Hei 7-2265.
FIG. 6 of Japanese Patent Publication No. 04 and SID97DIGEST “A Reflective Tri-La
yer Guest-Host Color TFT-LCD ''
The display device in FIG. 2 has the same configuration as that in FIG. 12 of Japanese Patent Application No. 7-226504.

In FIG. 1, 1 is a TFT, 2 is a reflective electrode, and 3 is a common electrode corresponding to the first layer 4 (“counter electrode”).
Also say. 4 is a first layer, 5 is a pixel electrode corresponding to the first layer 4, 6 is a common electrode corresponding to the second layer 7, and 7 is a second electrode.
8 is a pixel electrode corresponding to the second layer 7, 9 is a third layer 10
Are the third layer, 2 is a pixel electrode corresponding to the third layer 10, and 11 is a glass substrate. In FIG. 2, 1
1 is a glass substrate, 12 is an insulating film, 13 is a pixel electrode, 14
Is a pixel electrode, 15 is a pixel electrode, 16 is a copper-plated pillar, 17
Denotes a first layer, 18 denotes a second layer, and 19 denotes a third layer.

A liquid crystal layer is injected into each of the layers 4, 7, 10, 17, 18, and 19, and display is performed by utilizing the characteristics of the injected liquid crystal layer (reference: SID97DIGEST).
`` A Reflective Tri-Layer Guest-Host Color TFT-LCD
)).

FIGS. 1 and 2 show a so-called reflection type liquid crystal display device. The present invention is basically an active matrix type and is effective for a liquid crystal display device having a laminated structure. For example, the present invention is also effective for a transmission type liquid crystal display device having the structure shown in FIG. 3 in which the display device of FIG. In FIG. 3, a transparent pixel electrode 20 is arranged on a glass substrate 11 instead of the reflective electrode 2, and light from a backlight 21 is irradiated from outside the glass substrate 11.

Since each of the display devices shown in FIGS. 1, 2 and 3 has a TFT 1 as an active element,
A pixel potential level shift occurs when the gate pulse of T1 is changed from the on-voltage to the off-voltage (Reference: The Journal of the Institute of Television Engineers of Japan, Vol. 42, No. 1 (1988) "Drive system", Nikkei BP, "Flat" Panel Display 1991, p85, ΔVGD, K. Suzuki: "Compensative Addressing for
Switching Distortionin a-Si TFT.LCD "Proc.of Eurod
dVs ^* ) in isplay 87 p.107, image quality degradation occurs due to pixel potential level shift.

FIG. 4 shows the pixel potential of one pixel in the case of active matrix driving (reference: Nikkei BP).
FIG. 7 of the company “Flat Panel Display 1991” P84). ΔVGD in FIG. 4 is a pixel potential level shift. As a general method for preventing image quality deterioration due to ΔVGD, the common electrode voltage in FIG. 4 is shifted by ΔVGD, and the potential difference between the pixel electrode and the common electrode in the positive polarity and the potential difference between the pixel electrode and the common electrode in the negative polarity. Is a method of effectively making However, ΔVGD
Depends on the signal line voltage, and as shown in FIG. 5, it is common to shift the common voltage by ΔVGD corresponding to the steepest voltage-transmittance characteristic (VT characteristic) and the signal line voltage at which flicker is likely to occur. It is.

Also in the liquid crystal display device having the laminated structure shown in FIGS. 1 and 3, ΔVGD occurs in each layer and causes flicker, but ΔVGD depends on the characteristics of the TFT and the liquid crystal material injected into each layer. For (references; K. Suzuki;
"Compensative Addressing for Switching Distortion
in a-Si TFT-LCD "Proc.of Eurodisplay 87P.107),
Even if the same liquid crystal material is injected into each layer as well as when the same liquid crystal material is injected, ΔVGD differs between layers due to variations in cell gaps and TFT characteristics. Therefore, it is sufficient to adjust ΔVGD for each layer. However, in order to adjust for each layer, a different common electrode drive circuit is required for each layer, which causes an increase in circuit scale and power consumption (see Japanese Patent Application No. Hei. 1842
No. 59), the adjustment time is simply required to be three times that of the conventional case.

A material mixed with a magenta dye is injected into the first layer 4 of FIG. 1, a material mixed with a cyan dye is injected into the second layer 7, and a material mixed with a yellow dye is injected into the third layer 10. When the voltage-reflectance characteristics (VR characteristics) of each layer were examined, the results shown in FIG. 6 were obtained.
The layers are basically kept transparent to light). FIG.
, The reflectance of BaSO ₄ is set to 100% (reference: S1D 97DIGEST83 “A Relfective Tri-Layer”).
Guest-Host Color TFT-LCD ", AM-LCD'96 / IDW'96,329
"A Single-Polarizer Relfective TFT-LCD"). The optimum common electrode voltage of each layer (the common electrode voltage that minimizes flicker and provides the best image quality) was 7.53 V for the common electrode 3, 7.5 V for the common electrode 6, and 7.48 V for the common electrode 9. .

In the present invention, as shown in FIG. 7, the same common electrode voltage is supplied from the same common electrode drive circuit 22 to each of the common electrodes 3, 6 and 9, and the first common electrode has a large change in reflectance.
The common electrode voltage is adjusted so that the voltage polarity difference between the layer 4 and the second layer 7 is reduced, and more preferably, the voltage polarity difference is adjusted so as to minimize the voltage polarity difference in the first layer 4.

The principle of the present invention will be described with reference to FIGS. 8 and 9 show VR characteristics of a certain active matrix type liquid crystal display device. The vertical axis indicates the reflectance [%], and the horizontal axis indicates the voltage applied to the liquid crystal.
The reflectance is 100% for BaSO ₄ . The reflectivity corresponds to the luminance of the display device, and the change in reflectivity means a change in luminance. FIG. 8 shows VR characteristics of an active matrix type liquid crystal display device using a liquid crystal having a large change in reflectance (the change in reflectance at this time is about 75%).
Shows the VR characteristics of an active matrix type liquid crystal display device using a liquid crystal having a small change in reflectance (the change in reflectance at this time is about 25%). Here, the pixel potential level shift in FIG. 8 and FIG.
It is assumed that the voltage is 5V.

In FIG. 8, after 1.5 V (point A) is applied to the liquid crystal, the actual voltage applied to the liquid crystal shifts due to the pixel potential level shift. It is 2 V (point C) in the characteristics. At this time, the reflectance difference between the positive polarity and the negative polarity is about 53%. Note that such a difference between the voltage applied to the liquid crystal of the positive polarity and that of the negative polarity is referred to as a “voltage polarity difference”.

On the other hand, in FIG. 9, after 1.5 V (point D) is applied to the liquid crystal, the actual liquid crystal applied voltage shifts due to the pixel potential level shift. Therefore, in the case of the positive polarity, the liquid crystal applied voltage is 1 V (point E). , 2V for negative polarity (point F)
Becomes At this time, the difference in reflectance between the positive polarity and the negative polarity is about 20.
%. In general, the greater the difference in reflectance between positive and negative polarities, the greater the flicker. Therefore, as is clear from FIGS. 8 and 9, it can be said that, in general, the greater the change in reflectance, the greater the difference in reflectance (ΔR) between positive polarity and negative polarity.

As described above, in the present invention, as shown in FIG. 7, the same common electrode voltage is supplied from the same common electrode drive circuit 22 to each of the common electrodes 3, 6, and 9, and the change in reflectance is large. Since the common electrode voltage is adjusted so that the voltage polarity difference between the first layer 4 and the second layer 7 is reduced, and more optimally, the voltage polarity difference between the first layer 4 is minimized. It is possible to reduce flicker of the liquid crystal display device while reducing the scale.

The present invention is particularly effective when the laminated structure is such that each layer is a magenta layer, a cyan layer, and a yellow layer (reference: SID97DIGEST83 “A Relfective”).
Tri-Layer Guest-Host Color TFT-LCD "). This is because, as shown in FIG. 6, the change in reflectivity of the yellow layer is extremely smaller than that of the magenta layer and the cyan layer, so that even if the common electrode voltage is greatly deviated from the optimum common electrode voltage in the yellow layer, the optimum common electrode This is because flicker can be suppressed smaller than when the voltage is largely shifted. In the case of FIG. 6, the common electrode voltage is set to 7.5 V to 7.53 V.
, And more preferably 7.515 V to 7.5
It is desirable to set to 3V.

[0033]

As described above, according to the present invention, the common electrode voltage can be supplied to a plurality of common electrodes at the same time by the same common electrode driving means, and the reflectance change in the plurality of liquid crystal layers is large. Since the common electrode voltage is adjusted to minimize the voltage polarity difference in the liquid crystal layer, it is possible to realize an active matrix type liquid crystal display device having a laminated structure with a small circuit size, low power consumption and no image quality deterioration such as flicker. it can.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a display device to which the present invention is applied.

FIG. 2 is a diagram showing a configuration of another display device to which the present invention is applied.

FIG. 3 is a diagram showing a configuration of still another display device to which the present invention is applied;

FIG. 4 is a diagram showing a pixel potential in active matrix driving.

FIG. 5 is a diagram showing a voltage-transmittance characteristic in active matrix driving.

FIG. 6 is a diagram showing VR characteristics in each layer of the display device of FIG.

FIG. 7 illustrates a configuration of a display device according to an embodiment of the present invention.

FIG. 8 is a diagram showing VR characteristics when a difference in reflectance is large in an active matrix liquid crystal display device.

FIG. 9 is a diagram showing VR characteristics when the difference in reflectance is small in an active matrix liquid crystal display device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... TFT2 ... Reflection substrate 3,6,9 ... Common electrode 5,8 ... Pixel electrode 4 ... First layer 7 ... Second layer 10 ... Third layer 11 ... Glass substrate 22 ... … Common electrode drive circuit

Claims (2)

[Claims] 1. An active matrix type liquid crystal display device, comprising: a plurality of liquid crystal layers having different display colors; a plurality of common electrodes provided corresponding to the individual liquid crystal layers; and a common electrode provided to the plurality of common electrodes. A liquid crystal display device comprising: common electrode driving means for simultaneously supplying a voltage and adjusting the common electrode voltage so as to minimize the voltage polarity difference of a specific liquid crystal layer. 2. The liquid crystal display device according to claim 1, wherein the specific liquid crystal layer includes at least one liquid crystal layer having a largest change in reflectance.