JP4385912B2 - Liquid crystal display - Google Patents

Description

  The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device capable of performing good gradation display with respect to changes over time.

In recent years, displays are becoming larger and thinner, and the use of liquid crystal displays is particularly widespread. Among them, as an active matrix liquid crystal display device, for example, one disclosed in Patent Document 1 or 2 is known.
Here, a conventional general liquid crystal display device will be described. FIG. 3 is a conceptual diagram showing a liquid crystal display device, and FIG. 4 is a conceptual diagram showing an optical system of the liquid crystal display device. In FIG. 3 and FIG. 4, “2” is a liquid crystal display panel, and a liquid crystal that modulates incident light is sealed therein as described later. “4” is a liquid crystal drive circuit, and a video signal inputted thereto is processed by the image processing unit 4A to form a data signal, and a drive control signal is formed by the drive control unit 4B, together with the data signal. The liquid crystal display panel 2 is supplied. As shown in FIG. 4, in the liquid crystal display device, the light emitted from the light source 6 is polarized and reflected by the polarization beam splitter 8, and is incident on the liquid crystal display panel 2 as incident light. In the liquid crystal display panel 2, the incident light is modulated (polarized light) based on the data signal and reflected, and the modulated reflected light passes through the polarization beam splitter 8 and is projected onto the screen 10. In the case of light measurement, the light is incident on a light receiver.

  Specifically, in the liquid crystal drive circuit 4, the video signal is subjected to image processing such as signal modulation, digitization, and image quality processing, and a data signal for optimally driving the liquid crystal display panel 2 is output together with the drive control signal. The Usually, the signal processing in the liquid crystal driving circuit 4 is performed by a digitized signal. Due to the characteristics of A / D conversion, D / A conversion, etc., the display of the liquid crystal display device depends on the design of the liquid crystal driving circuit 4. The number of gradations is determined. Thereafter, this data signal is transmitted to the liquid crystal display panel 2 together with the drive control signal.

Next, the configuration of the liquid crystal display panel 2 will be described. 5 is a schematic view showing a liquid crystal display panel, FIG. 6 is a three-dimensional perspective view showing a part of the liquid crystal display panel, and FIG. 7 is a cross-sectional view showing a pixel portion of the liquid crystal display panel.
As shown in FIG. 5, the liquid crystal display panel 2 includes a plurality of pixels PX arranged in a lattice shape (matrix shape) vertically and horizontally. In the vertical direction, a plurality of data lines 12 that transmit a voltage obtained by sampling the data signal are arranged in parallel, and a plurality of scanning lines 14 are arranged in a direction orthogonal to the data lines 12. The intersections of the plurality of data lines 12 and the plurality of scanning lines 14 are the pixels PX, and a switch transistor TR is disposed in each pixel PX. The plurality of data lines 12 extend from the horizontal address circuit 16, and the plurality of scanning lines 14 extend from the vertical address circuit 18. Regarding the switch transistor TR, the drain D is connected to the data line 12, the gate G is connected to the scanning line 14, and the source S is connected to the storage capacitor CS and the pixel electrode 20 in parallel. Here, the pixel electrodes 20 are arranged in a matrix corresponding to the pixels PX. As will be described later, a liquid crystal layer 24 is interposed between each pixel electrode 20 and a transparent common electrode 22. Each storage capacitor CS is connected to the storage capacitor common line 26.
The horizontal address circuit 16 receives a horizontal address circuit control signal and a data signal, and applies the data signal to each data line while sampling. A vertical address circuit control signal is input to the vertical address circuit 18, and each pixel PX is selected by sending a scanning signal to the scanning line 14 in synchronization with the data signal.

  Next, the cross-sectional structure of the liquid crystal display panel 2 will be described. FIG. 6 is a schematic perspective view of the liquid crystal display panel, and FIG. 7 is a cross-sectional view of the liquid crystal layer portion of the pixel portion. As shown in the figure, the switch transistor TR, the storage capacitor CS, and the like are formed on a first substrate 30 made of a semiconductor substrate of, for example, a silicon wafer. The pixel electrodes 20 are formed in a matrix on the first substrate 30, and an alignment film 32 is formed on the entire surface of the first substrate 30 including the pixel electrodes 20.

On the other hand, the second substrate 34 facing the first substrate 30 is made of, for example, a transparent glass plate, and an antireflection film 36 is formed on the upper surface thereof. A transparent common electrode 22 made of, for example, ITO is formed on the entire lower surface side of the second substrate 34, and an alignment film 38 is laminated on the entire lower surface side. The first and second substrates 30 and 34 are bonded to each other with the alignment films 32 and 38 facing each other with a predetermined gap therebetween, and the liquid crystal layer 24 is sealed between them. Has been. A voltage is applied between the pixel electrode 20 and the common electrode 22, and incident light enters from the second substrate 34 side.
Here, as the material of the alignment films 32 and 38, for example, there are two kinds of materials such as an organic material such as polyimide and an inorganic material such as silicon dioxide. In general, organic materials are more mass-productive, but there is a problem that they are discolored when light continues to strike. In the case of liquid crystal display devices that are exposed to strong light, inorganic materials are superior in terms of characteristics.

JP-A-11-183912 JP 2001-249351 A

By the way, the voltage applied to the liquid crystal of the liquid crystal layer 24 is ideally a potential difference between the pixel electrode 20 and the common electrode 22, but actually there are alignment films 32 and 38 having electric resistance between them. As a result, the voltage applied to the liquid crystal is reduced accordingly. For example, in general, the liquid crystal layer 24 needs to have a thickness of 2 μm or more in order to obtain sufficient light modulation, and the alignment films 32 and 38 have a thickness of about 0.05 μm in order to regularly align the liquid crystals. Is needed. Since the resistivity of a general liquid crystal is about 10 ¹⁴ Ωcm and the alignment films 32 and 38 are insulators, if the resistivity is about 10 ¹⁵ Ωcm, the resistance Rlc = 2 × 10 ¹⁰ per unit area of the liquid crystal layer 24. The resistance per unit area of the alignment films 32 and 38 is Ral = 5 × 10 ⁹ Ω, and about 80% of the voltage applied between the pixel electrode 20 and the common electrode 22 is about 24% due to the series resistance. Will join.

The problem here is the change over time in the resistance values of the alignment films 32 and 38. In general, the electrical resistance of the alignment film easily changes due to the influence of impurities, the use environment, and the like. In particular, when an inorganic material is used as the material of the alignment film, it reacts sensitively to substances in the atmosphere and the electric resistance tends to decrease. Here, when the electrical resistance is lowered due to a change with time, the voltage applied to the liquid crystal is changed, so that the light intensity of the liquid crystal is also changed, and there is a problem that the display quality is remarkably deteriorated. For example, when an acceleration test is performed in a specific environment, the electrical resistance decreases from Ral = 5 × 10 ⁹ Ω to Ral ′ = 5 × 10 ⁸ Ω per initial unit area of the alignment film. As a result, the voltage applied to the liquid crystal layer becomes relatively high, and the display quality deteriorates.

Specifically, FIG. 8 is a graph showing a voltage-relative light intensity characteristic curve of the liquid crystal when the alignment film is in the initial stage (before change over time), and FIG. 9 is a liquid crystal when the alignment film changes over time. It is a graph which shows the characteristic curve of voltage-relative light intensity.
As shown in FIG. 8, when a voltage is applied to the liquid crystal layer, the relative light intensity increases accordingly, and shows a characteristic of slightly decreasing after passing the peak. However, as shown in FIG. 9, when the alignment film changes with time, the characteristic curve changes from a solid line to a broken line. That is, the characteristic curve changes as described above before and after the environmental test. This means that when the maximum light intensity is 100%, the relative light intensity of the liquid crystal is 50% brighter than the conventional light, but the brightness is 90% or more at the same voltage, so the display gradation is significantly impaired. It means to end.
The present invention has been developed in order to effectively solve the above-mentioned problems, and an object of the present invention is to provide a good gradation display even when the alignment film changes with time. An object of the present invention is to provide a liquid crystal display device that can be maintained and can maintain high display quality.

  According to a first aspect of the present invention, there is provided a first substrate in which a plurality of pixel electrodes are arranged in a matrix and an alignment film is formed on a surface of the pixel electrode, and a transparent common electrode having an alignment film formed on the surface. A second substrate having a plurality of display gradation levels applied between the pixel electrode and the common electrode, with the alignment films facing each other with a liquid crystal layer interposed therebetween. In the liquid crystal display device in which incident light is modulated by the above, the relationship between the resistance Rlc per unit area of the liquid crystal layer and the resistance Ral per unit area of the alignment film with respect to the application of the voltage is represented by a display floor. The liquid crystal display device is configured to have a relationship of “Rlc / Ral> n” with respect to the characteristic number n.

  According to the liquid crystal display device of the present invention, it is possible to maintain a good gradation display even when the alignment film undergoes a change over time, thereby maintaining a high display quality.

Hereinafter, an embodiment of a liquid crystal display device according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a sectional view showing a pixel portion of a liquid crystal display panel of a liquid crystal display device according to the present invention, and FIG. 2 is a graph showing a voltage-relative light intensity characteristic curve of liquid crystal for explaining the effect of the present invention.
The configuration of the liquid crystal display device of the present invention is exactly the same as the configuration described with reference to FIGS. 3 to 7 except for the configuration of the alignment film. Therefore, the description is omitted here and the alignment is performed with reference to FIG. The description will focus on the structure of the film. 1, the same components as those shown in FIG. 7 are denoted by the same reference numerals.
As shown in the figure, the switch transistor TR, the storage capacitor CS, and the like are formed on a first substrate 30 made of, for example, a silicon wafer semiconductor substrate. The pixel electrodes 20 are formed in a matrix on the first substrate 30, and the alignment film 40, which is a feature of the present invention, is formed on the entire surface of the first substrate 30 including the pixel electrodes 20. Yes.

  On the other hand, the second substrate 34 facing the first substrate 30 is made of, for example, a transparent glass plate, and an antireflection film 36 is formed on the upper surface thereof. A transparent common electrode 22 made of, for example, ITO is formed on the entire lower surface of the second substrate 34, and an alignment film 42, which is a feature of the present invention, is laminated on the entire lower surface. . The first and second substrates 30 and 34 are bonded to each other with the alignment films 40 and 42 opposed to each other with a predetermined gap therebetween, and the liquid crystal layer 24 is sealed between them. Has been. A voltage is applied between the pixel electrode 20 and the common electrode 22, and incident light enters from the second substrate 34 side.

Here, in the present invention, the relationship between the resistance Rlc per unit area of the liquid crystal layer 24 and the resistance Ral per unit area of the alignment films 40 and 42 with respect to the voltage application direction is the number of display gradations n. Is configured to have a relation of “Rlc / Ral> n”.
Here, the display gradation number of the data signal is designed to be output with, for example, 256 gradations, and therefore the display gradation number is “256”.

  The reason why the resistance per unit area of the alignment films 40 and 42 is set as described above is as follows. That is, when a certain voltage is applied to the liquid crystal display panel, the ratio of the voltage applied to the liquid crystal layer and the alignment film is the ratio of the resistance Rlc of the liquid crystal per unit area to the resistance Ral of the alignment film per unit area. It is determined as described above. Therefore, if Rlc is sufficiently higher than Ral, even if Ral changes, the influence on the voltage applied to the liquid crystal layer is small. Therefore, paying attention to the number of display gradations, it is possible to guarantee the prevention of gradation changes by making the voltage change due to deterioration over time less than the reciprocal of the display gradation number, that is, less than the minimum gradation voltage range. Become. Therefore, the voltage ratio, that is, the ratio of Rlc and Ral as described above is related to the number of display gradations. In other words, by satisfying the condition of the above formula, the voltage applied to the liquid crystal layer 24 causes only a voltage fluctuation of 1 / n or less of the display gradation number n, and has almost no adverse effect on the gradation.

Here, when an inorganic material is used as the alignment films 40 and 42, the resistivity can be controlled relatively easily. Therefore, in the present invention, a silicon oxide (SiOx: 1.0 ≦ x ≦ 2.0) film is formed as the alignment films 40 and 42 by the vapor deposition apparatus. At that time, 0.1 μm is deposited on the first substrate 30 and the second substrate 34 as the film thickness necessary for aligning the liquid crystal. At this time, the resistivity of the liquid crystal to be used is 1.2 × 10 ¹⁵ Ωcm in terms of specifications, but generally the resistance of the liquid crystal is 1.2 × 10 ¹⁴ Ωcm because the resistance drops by about an order of magnitude in actual use. When the thickness of the liquid crystal layer 24 is calculated at 3.2 μm, the resistance Rlc per unit area of the liquid crystal layer 24 is 3.84 × 10 ¹⁰ Ω. From the relational expression of Rlc / Ral> n according to the present invention, since the display gradation number n = 256 of the output of the liquid crystal driving circuit 4 (see FIG. 3), the resistance Ral of the alignment films 40 and 42 is 1.5 × 10 ^8. Must be:

Therefore, since each of the alignment films 40 and 42 has a thickness of 0.1 μm, the resistivity is 7.5 × 10 ¹² Ωcm or less [= Ral / (0.1 μm × 2)]. As described above, the alignment films 40 and 42 having a predetermined film thickness are formed under appropriate film forming conditions such as adjusting the deposition rate and the ratio of x in SiOx. After the alignment films 40 and 42 are formed, the alignment film surfaces of the first substrate 30 and the second substrate 34 are opposed to each other so that the thickness of the liquid crystal layer 24 is 3.2 μm. Laminate with a gap. Liquid crystal is injected to fill this gap, and then the hole used for injection is sealed. Then, wiring is performed on the first substrate 30 side so that a signal from the liquid crystal driving circuit 4 can be input, whereby a liquid crystal display panel is manufactured.

This liquid crystal display panel was connected to the liquid crystal driving circuit 4 and actually operated. FIG. 2 shows the direct modulation degree of the liquid crystal with respect to the voltage without performing gamma correction or the like. The initial voltage-relative light intensity curve is shown by a solid line, and after performing an acceleration test in a specific environment (after aging of the alignment film), the conventional voltage-relative light intensity curve is shown by a one-dot chain line. The voltage-relative light intensity curve of the present invention is indicated by a dotted line. The intensity of liquid crystal modulation is most sensitive to changes in the voltage applied to the liquid crystal, and the light intensity of the conventional device increased by about 16% after the acceleration test with respect to the voltage when the initial light intensity is 50%. According to the device of the present invention, it was within 1%, and it was confirmed that good display characteristics could be maintained.
Although the present invention has been described by using a reflective liquid crystal on a semiconductor substrate as a specific example, the present invention is not limited to this example. For example, the present invention is not limited to a substrate in which a thin film transistor is formed on an insulating substrate or a transmissive liquid crystal. Even if it exists, it is effective.

It is sectional drawing which shows the pixel part of the liquid crystal display panel of the liquid crystal display device which concerns on this invention. It is a graph which shows the characteristic curve of the voltage-relative light intensity of the liquid crystal for demonstrating the effect of this invention. It is a conceptual diagram which shows a liquid crystal display device. It is a conceptual diagram which shows the optical system of a liquid crystal display device. It is a schematic diagram which shows a liquid crystal display panel. It is a three-dimensional perspective view which shows a part of liquid crystal display panel. It is sectional drawing which shows the pixel part of a liquid crystal display panel. It is a graph which shows the characteristic curve of the voltage-relative light intensity of the liquid crystal at the time of the alignment film of an initial stage (before time-dependent change). It is a graph which shows the characteristic curve of the voltage-relative light intensity of a liquid crystal when an alignment film changes with time.

Explanation of symbols

DESCRIPTION OF SYMBOLS 12 ... Data line, 14 ... Scan line, 16 ... Horizontal address circuit, 18 ... Vertical address circuit, 20 ... Pixel electrode, 22 ... Common electrode, 24 ... Liquid crystal layer, 30 ... 1st board | substrate (semiconductor substrate), 34 ... Second substrate (glass plate), 40, 42, alignment film, PX, pixel, TR, switching transistor.

Claims (1)

A first substrate having a plurality of pixel electrodes arranged in a matrix and having an alignment film formed on the surface of the pixel electrode; and a second substrate having a transparent common electrode having an alignment film formed on the surface. The alignment films are opposed to each other with a liquid crystal layer interposed therebetween, and a plurality of display gradation level voltages are applied between the pixel electrode and the common electrode so as to modulate incident light. In the liquid crystal display device
When the voltage is applied, the relationship between the resistance Rlc per unit area of the liquid crystal layer and the resistance Ral per unit area of the alignment film is “Rlc / Ral> n” with respect to the display gradation number n. A liquid crystal display device characterized by having the following relationship.

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