JPH0344619A - Manufacture of active matrix type liquid crystal display element - Google Patents

Abstract

PURPOSE:To suppress the temperature dependency of a driving voltage which is applied to a liquid crystal layer by equalizing the temperature coefficient of the effective driving voltage to the temperature coefficient of a voltage holding rate. CONSTITUTION:The variation rate [V50(60 deg.C)/V50(20 deg.C)] of the driving voltage V50 between 20 and 60 deg.C required for the standardized transmitted light intensity in a liquid crystal material orienting film to reach 50% is nearly constant with out depending upon the setting temperature of the orienting film. The tempera ture variation rate of the voltage holding rate, on the other hand, depends greatly upon the setting temperature of the orienting film. The orienting film is so formed that the temperature variation rate of the voltage holding rate is nearly equal to that of the V50, and the active matrix type liquid crystal display element which uses an a-SiTFT as an active element is manufactured. The liquid crystal display element which is thus obtained does not very in voltage-transmissivity characteristic even if the temperature of the element varies, excellent picture quality and contrast are always obtained, therefore its practical value is high.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a liquid crystal display element used in an electro-optical liquid crystal display, and more particularly to a method for manufacturing an active matrix type liquid crystal display element. The elements are characterized by light weight and low power consumption, and although the demand for them is increasing year by year, there are three types of drive methods: 1) Static drive, 2) Simple matrix drive, and 3) Active matrix drive. Among them, active matrix drive is one in which each pixel is provided with a switching element, and although it is characterized by excellent image quality, it is generally used in the design of liquid crystal display elements (based on the required characteristics of the display). The liquid crystal material, panel gap, alignment film material, rubbing method, etc. are determined. Normally, the panel configuration and driving conditions are determined so that the contrast is maximized at room temperature (around 20°C or 25°C). Normally, the relationship between the light transmittance of the panel and the drive voltage is determined from the voltage-transmittance curve of the panel at room temperature, and gradation display is performed based on this.

Problems to be Solved by the Invention However, considering the way in which liquid crystal display elements are used, they are often used in various temperature environments, and the temperature of the panel is not necessarily constant (t Also, by using a backlight, The temperature of the panel increases by 5 to 15 degrees Celsius.The voltage-transmittance characteristics of the liquid crystal display element change as the temperature changes. In response to this, attempts have been made to reduce the temperature dependence of various physical properties of liquid crystal materials. At present, the potential of the opposing electrode is changed each time the drive voltage is varied due to the change in the driving voltage, so that sufficient contrast can be obtained.

The present invention aims to solve the problems of conventional liquid crystal display elements. In an active matrix liquid crystal display element, the factors that determine the driving voltage are broadly divided into the following: There are three factors that can be cited: 1), dielectric loss of the element (c), 2) elastic energy of the liquid crystal material, and anchoring energy of the alignment film. (\1) works to increase the driving voltage, and 2) and 3) work to lower the driving voltage. Therefore, if the panel is designed so that the temperature dependence of the effect 1) exactly cancels out the temperature dependence of the combined effect 2) and 3>, then by configuring it as above, The present invention also makes it possible to obtain a panel with no temperature dependence of driving voltage.The voltage-transmittance characteristics do not change even if the panel temperature changes. Active matrix type with extremely small temperature dependence of contrast. It provides liquid crystal display elements, and its practical value is extremely limited.
% EXAMPLES Before explaining specific examples, in the case of an active matrix type liquid crystal display element, a detailed explanation of the present invention is given below.
When the switching element is turned on, a signal voltage is applied to the pixel electrode through the switching element. The voltage waveform at this time is shown by the solid line in a of FIG. However, in reality, the potential between the pixel electrodes provided above and below the liquid crystal layer gradually decreases within the time of one field, as shown in b in Figure 1. Although it varies greatly depending on the combination of materials and alignment film materials, the ratio between the average value of the effective voltage applied to the liquid crystal layer and the magnitude of the signal pulse voltage (V in Figure 1) Defining the voltage holding ratio R, even though 1-Rv represents the magnitude of dielectric loss, Rv is normally 0.8 at 20°C.
<Rv<1. It becomes 0. Then, as the panel temperature increases, it gradually decreases. Now, if Rv does not depend on the magnitude of the applied voltage and has the temperature dependence shown in Figure 2, then at a temperature of 20°C it will have the voltage-transmittance characteristic shown by a in Figure 3. Due to the temperature dependence of dielectric loss on liquid crystal panels, the voltage-transmittance curves at temperatures of 40°C and 60°C are as shown in Figure 3b and Figure 3C, respectively. Although each effect can be quantified as the flexible threshold voltage and binding energy by measuring the capacitance-voltage characteristics of the element, in practical terms, each effect increases the temperature of the panel. These two effects can also be quantified in a combined form (1
Specifically, a rectangular wave (for example, 3
Measure the voltage-transmittance characteristics while applying a 0H9 rectangular wave), and use the vll to quantify the effects of 2) and 3) (where vn is the maximum transmitted light intensity due to voltage application of the liquid crystal display element). Let the minimum transmitted light intensity be l and O, respectively.
When normalized to α, it means the magnitude of the driving voltage necessary to obtain an arbitrary normalized transmitted light intensity n (%), and 0<
n<100).

This value Vn is the effective driving voltage of the liquid crystal material, and corresponds to VLl, which is obtained by applying a rectangular wave with the switching element always on in an active matrix type liquid crystal display element. Figure 4 shows such a voltage application. Although this figure shows the temperature dependence of the voltage-transmittance characteristics measured by the method, it can be seen from the figure that the driving voltage shifts to the lower voltage side as the panel temperature increases. The effect 1) shown in FIG. 3 and the effects 2) and 3) shown in FIG. 4 appear in combination. The change in drive voltage based on the temperature dependence of dielectric loss is opposite in sign (direction of increase/decrease in node plate) to the change in drive voltage based on the temperature dependence of the elastic energy of the liquid crystal material or the anchoring energy of the alignment film. By making the temperature change rate of Rv equal to the temperature change rate of VLl, it is possible to provide an active matrix liquid crystal display element whose voltage-transmittance characteristics change extremely little due to temperature. In the example, voltage holding ratio Rv and effective driving voltage v, it are measured using a liquid crystal cell without a switching element for convenience of experiment.
Voltage holding ratio Rvi & was determined by applying a pulse whose signal voltage V, (t) at time t (seconds) is expressed by the following formula to the liquid crystal cell (hereinafter, this drive will be abbreviated as pseudo-TPT drive).

Here, m is a positive integer. The voltage-transmittance characteristics of the cell were determined by applying a 30H2 rectangular wave to a liquid crystal cell.Example 1: #RN-707 was oriented on a glass substrate with an ITO electrode on the surface so that the film thickness after drying was 80OA. (Nissan Chemical polyimide paint) applied, 150t, 170t
, 200t, and 250℃.
Next, after the rubbing treatment, the respective substrates were bonded together via glass beads with a diameter of 5.5 μm to create TN type liquid crystal display devices A, B, C, and D.7. Thereafter, Chisso Petrochemical's liquid crystal composition LI
Is the injection port sealed with epoxy resin? , after that,
After attaching a polarizing plate to achieve a positive display mode, a rectangular wave was applied for 30 hours between the upper and lower electrodes, and the voltage-transmittance characteristics were measured at 20 tons and 60 degrees Celsius. Also pseudo TP
20t by T drive? l, Voltage holding rate R at 60℃
Table 1 shows the results of measuring v and their temperature change rates.
and shown in Table 2. V2O has a normalized transmitted light intensity of 50%
In Table 1, the temperature change rate is defined as the ratio of the value at 60℃ and the value at 20℃. 6V in the area
Rate of change of 50 (V2O (60℃)/V50 (20℃))
(Y) It is almost constant regardless of the curing temperature of the alignment film,
It shows a value of 0.83. On the other hand, the temperature change rate of the voltage holding ratio largely depends on the curing temperature of the alignment film L0.52 to 0.95
Here, liquid crystal display element B has a temperature change rate of voltage holding ratio that is approximately the same as the temperature change rate of V2O. Next, we created an active matrix liquid crystal display element (3 inches, 240x372 dots, no auxiliary capacitance) using a-8i TFT as an active element using the same combination of liquid crystal material and alignment film as for element B. As a spacer: 1.X using 5.5 μm glass beads, L lX0N-915 by vacuum method
The temperature dependence of the voltage-transmittance characteristics was determined in the positive display mode using the usual method. Measurements were made under the same conditions as TPT drive.
As is clear from Table 3, the voltage-transmittance characteristics of the liquid crystal display element of the present invention change even when the temperature of the element changes, and it is possible to always obtain good image quality and contrast, and its practical value is extremely high. Table 3 Example 2 An alignment film RN-725 (polyimide paint made by Sansan Kagaku Co., Ltd.) was coated on a glass substrate having an IT○ electrode on its surface so that the film thickness after drying was 800A.The film was coated at a temperature of 170°C for 2 hours. Let it harden? Q.Next, the substrates were bonded together through glass beads with a diameter of 5.5 μm using a rubbing machine to create TN type liquid crystal display devices E, F, G, and H. After that, each liquid crystal display device was coated with trans-4-pentyl ( 4-cyanophenyl)cyclohexane (abbreviated as PCl3)
Weight% and Merck liquid crystal ZLI-3376-000, (
PCl3 in each display element was injected under reduced pressure and the injection port was sealed with epoxy resin.
Table 4 shows the mixing ratio (X weight %).

After attaching a polarizing plate to achieve the true positive display mode, a 30Hz square wave is applied between the upper and lower electrodes for 2Q.
The voltage-transmittance characteristics were measured at each temperature of 60°C and 20t by pseudo-TPT driving.

The voltage holding ratio at 60° C. was also measured, and the t4 measurement results and their temperature change rates are shown in Tables 5 and 6.

Table 5 Table 6 In Table 5, V in the temperature range from 20°C to 60°C
The temperature change rate of 2O depends on the content ratio of i & PcH5.
．． It changes from 88 to 0.86.

-X The rate of temperature change in voltage holding ratio depends largely on the content of PCl3, and can take values from 0.92 to 0.73.
Here, the liquid crystal display element whose rate of change in voltage holding characteristics is approximately the same as the rate of temperature change of V2O is liquid crystal display element F. A-8iTFT as an active element
An active matrix type liquid crystal display element (3 inches, 240 x 372 dots, no auxiliary capacitance) was fabricated using 5.5 μm glass beads as spacers.
The temperature dependence of the voltage-transmittance characteristics was obtained in the positive display mode according to the conventional method, and the results shown in Table 7 were obtained. At this time, the measurement was carried out under the same conditions as the pseudo-TPT drive.As is clear from Table 7, the liquid crystal display element of the present invention has voltage-transmittance characteristics even when the temperature of the element changes. It is possible to always obtain good image quality and contrast, and its practical value is very large.Table 7 Example 3 Voltage-transmittance characteristics and voltage of liquid crystal display element A prepared in Example 1 The temperature dependence of the retention rate was measured with polypropylene capacitors of various capacities placed in parallel with element A. Table 8 shows the measurement system numbers and the capacities of the polypropylene capacitors used.

Table 8 In each measurement system, a 30Hz rectangular wave was applied between the upper and lower electrodes of liquid crystal display element A for 20t.

Voltage-transmittance characteristics were measured at each temperature of 60°C.

Table 9 and Table 10 show the measurement results of the voltage holding rate at 20 t and 60° C. also measured by pseudo TPT driving.

Table 10 In Table 9, the rate of change in V2O in the temperature range from 0°C to 0°C is almost constant regardless of the size of the propylene capacitor, and shows a value of 0.83.-6 Changes in voltage holding characteristics The rate depends largely on the size of the propylene capacitor capacitance and takes a value from 0.59 to 0.90.Here, the rate of change in the voltage holding characteristic is V2O
Measurement system number 2 is a liquid crystal display element that takes a value approximately equal to the temperature change rate of . Next, using the same combination of liquid crystal material and alignment film as element A, a-3i was used as an active element.
Active matrix liquid crystal display element using TFT (
3 inches, 240 x 372 dots, with auxiliary capacitor) The size of the auxiliary capacitor is the specific force between the capacitance of element A and the capacitor capacitance in the measurement of measurement system number 2.
The spacer t is adjusted to be 5.5 so that it also holds true in the active matrix liquid crystal display element of the present invention.
uX using μm glass beads, L I X0 by vacuum method
The temperature dependence of the voltage-transmittance characteristics was obtained in t and % positive display modes according to the usual method with N-9150CB sealed. The results shown in Table 11 were obtained and the timing of applying the signal voltage waveform at this time (The measurements were carried out under the same conditions as the pseudo-TPT drive, and as is clear from Table 11, the liquid crystal display element of the present invention does not change the voltage-transmittance characteristics even if the temperature of the element changes, and always has good image quality. Contrast can be obtained and its practical value is very large.

Table 11 Some active matrix liquid crystal display elements have a method of adding an auxiliary capacitance in parallel with the liquid crystal layer as in this embodiment to suppress voltage loss due to dielectric loss. In this case, k Active matrix of the present invention It is clear that the method for manufacturing a type liquid crystal display element is effective. In this example, the liquid crystal material was Alignment film material Rikimon with optimization of auxiliary capacitance, etc. Depending on the usage of the liquid crystal display element, the temperature dependence of the voltage holding rate may be 30 Hz, which is almost the same as the temperature dependence of VIOL or V2O when applying a square wave. It goes without saying that the optical element may be optimized.

According to the present invention, it is possible to obtain an active matrix type liquid crystal display element in which the temperature dependence of the driving voltage is extremely small.From the viewpoint of power consumption, the voltage holding ratio Rv is preferably 70% or more in the operating temperature range.

In this example, a driving voltage waveform height of 5V was adopted when measuring the voltage holding ratio. This is not intended to limit the present invention in any way. Even if changes, the voltage −
It provides an active matrix type liquid crystal display element with almost no change in transmittance characteristics, and its practical value is extremely high.

[Brief explanation of drawings]

Figure 1 is a graph for explaining the relationship between the signal voltage applied to the pixel electrode of an active matrix liquid crystal display element and the effective voltage applied to the liquid crystal layer, and Figure 2 is a graph of common liquid crystal materials - orientation A graph showing the temperature dependence of voltage holding ratio in a film system; FIG. 3 is a graph explaining the temperature change of the voltage-transmittance curve based on the temperature dependence of dielectric loss;
FIG. 4 is a graph for explaining the temperature change of the voltage-transmittance curve based on the elastic energy of the liquid crystal material and the anchoring energy of the alignment film.

Claims (1)

[Claims] 1. A method of manufacturing an active matrix liquid crystal display element, characterized in that the temperature coefficient of an effective driving voltage applied to a liquid crystal layer is designed to be equal to the temperature coefficient of a voltage holding rate.