JPH0377297A - Gradation indication method for el display device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a gradation display method of an EL display device using an EL (electroluminescence) element that emits light in response to the application of an electric field.

<Conventional technology> There are roughly two types of gradation display methods.

The first method is to divide pixels. For example, 4
By configuring one pixel with two small pixels, all four small pixels are in a light emitting state, only three small pixels are in a light emitting state, only two small pixels are in a light emitting state, only one small pixel is in a light emitting state, and all small pixels are in a light emitting state. Five emission brightness levels are obtained, each consisting of a non-emission state. That is, in this case, five gradation levels can be displayed.

The second method is to control the luminance by changing the voltage applied to the pixel. There are various methods for this; for example, there are methods that take advantage of the fact that luminance is approximately proportional to the frequency of the applied pulse voltage, and methods that take advantage of the fact that the higher the applied voltage, the higher the brightness.

<Problems to be Solved by the Invention> Currently, there is an increasing need for higher definition in X----Y matrix type displays, and the first method is to configure pixels from a plurality of small pixels. Therefore, the electrode width should be narrowed.
1. This has the disadvantage of significantly lowering productivity due to poor yield, increasing the number of peripheral circuit components, and increasing costs.

The second method does not have the drawbacks of the first method, but new problems arise. In the case of a panel having 640×400 pixels, the maximum frame frequency is limited to about 60 Hz, and this frame frequency is also the lower limit in order to prevent flickering. Therefore, it is not practical to change the frequency of the applied voltage. The most practical method is to control the luminance by changing the value of the applied voltage, but this also has the following problems. First, EL
The structure of the device and the characteristics between luminance and applied voltage (LV) are shown in FIG. 5.6. In FIG. 5, l is a glass substrate, 2
3 is a transparent electrode, 3 is a lower insulating layer, 4 is a light emitting layer, 5 is an upper insulating layer, and 6 is a back electrode. Then, as shown in FIG. 6, when the applied voltage (V) is increased, the brightness (L) rapidly increases and then becomes saturated. This LV characteristic may change slightly over operating time. 1 (1)
When displaying 0 (zero), a non-emission region and a saturated region are used, so a slight change in the L-V characteristic will result in a wide change in brightness, as shown in Figure 6. do not have.

However, in the case of gradation display, since a steep region of the LV characteristic is used, when the LV characteristic changes from curve a to curve a', the luminance changes greatly from Lm to Lm'. That is, in order to obtain gradations by controlling the applied voltage, L-
Remarkable stability of the V characteristics is essential. For EL panels that use a ZnS:Mn film as a light-emitting layer, the panel manufacturing technology for obtaining highly stable L-V characteristics has been established, and it is possible to use other light-emitting layers to see if this is not a major problem. At present, it is difficult to display high-quality gradations in EL panels.

SUMMARY OF THE INVENTION An object of the present invention is to provide a gradation display method that can display high quality gradations even if the L-V characteristics are not stable.

Means for Solving the Problems> This invention basically utilizes the fact that the LV characteristic changes greatly depending on the waveform of the applied voltage. FIG. 1 shows how the LV characteristics change depending on how the applied pulse voltage rises. For the sake of simplicity, we will change the way the voltage applied to the EL element rises by changing the electrical resistance R provided between the AC power source and the EL element, and
-V characteristics were measured. As can be seen from FIG. 1, when the resistance value R increases, the LV characteristic shows a gradual rise, and the brightness in the saturated region decreases. Therefore, in this example, the operating voltage is fixed at Vs, and the resistance value R is changed from 0Ω to 12Ω.
This means that when the value is changed to Ω, the brightness changes from LM to LO.

The phenomenon in which brightness changes depending on how the applied voltage rises is due to the following mechanism.

One of the basic operating mechanisms of a thin film EL device with a double insulation structure is the action of polarization voltage (electric field). Conduction electrons flowing through the light-emitting layer during application of an external voltage of a certain polarity are trapped near the interface between the insulating layer and the light-emitting layer.

When the next external voltage of opposite polarity is applied, the electric field (polarization electric field) due to the positive space charge formed by the electrons trapped near the interface and their source is added to the electric field due to the external voltage, causing light emission. A very high electric field is induced in the layer. EL, which emits light by collision-exciting a luminescent center with hot electrons, exhibits high-intensity light emission due to the effect of this polarization voltage. Incidentally, the kinetic energy of electrons for exciting the luminescence center depends on the luminescence center material, and for example, 2.3 eV is required for M-" and 3.3 eV for Tb". On the other hand, the stronger the electric field strength in the light emitting layer, the higher the kinetic energy the electrons gain from the electric field. That is, in this example, the electric field strength to excite Tb'' is higher than the electric field strength required to excite Mn2+.

By further researching the operating mechanism, we discovered the following.

The above-mentioned polarization electric field can be used more effectively as the externally applied voltage rises more steeply. If the rise is slow, the voltage does not rise sufficiently, and when the voltage is still low, the electrons that were accumulated near the interface between the insulating layer and the light emitting layer during the previous polarity will move, causing the external voltage to reach its peak value. By this time, the polarization voltage has decreased 1 and the electric field in the emissive layer has become lower. For this reason, the steeper the external voltage, the higher the electric field can be induced in the light emitting layer.

Due to the above mechanism, for example, ZnS:T
In an EL device having a B film as a light emitting layer, the LV characteristic exhibits a significant dependence on the applied voltage waveform as shown in FIG. still,"
As is clear from the mechanism described above, in ZnS:MnEL devices where the energy required for excitation of the luminescence center is low, the change in L-v characteristics due to the way the applied voltage rises is as follows.
It is not as pronounced as in ZnS:TbEL devices.

We propose a gradation display method for an EL display device that is capable of highly reliable gradation display by actively utilizing the phenomena described above. That is, in the gradation display method of the EL display device of the present invention, the pulse voltage applied to the thin film EL element is set in the substantially saturated region of the L-V characteristic, and the rise h (rise is made steep or gradual). It is characterized by controlling the light emission brightness depending on the conditions, and performing gradation display.

The sheet resistance of the ITO (tin-doped indium oxide) film used as the transparent electrode is 10-2007. Electrode width 25
In an XY7 trix panel with a 0μ shoulder and an electrode length of 10cm, the resistance between both ends of the electrode is 5 to lOKΩ. That is,
Compared to pixels closer to the electrode terminal, pixels farther from the electrode terminal are 5
It will be connected to a pulse power source via a resistance of ~IOKΩ. Therefore, in order to apply the present invention to a large display device, it is necessary to lower the electrode resistance as much as possible so that the voltage waveform actually applied to the pixel does not depend on the distance from the pixel electrode terminal. be. However, the sheet resistance of transparent electrodes currently available is at most 10Ω/mouth. Therefore, by providing a metal wiring in contact with the transparent electrode, the electrode resistance can be lowered and the applications of the present invention can be expanded.

<Operation> In the case of gradation display using a steep region of L-V characteristic, L
Display quality deteriorates significantly even with subtle changes in the −V characteristics, but according to the present invention, a pulse voltage in the approximately saturated region of the L characteristics is applied while changing the way it rises, resulting in gradation display. Ru. Therefore, high display quality can be maintained even if the LV characteristics change somewhat.

<Example 1> In this embodiment, the EL panel shown in FIG. 5 is used.

This EL panel is made of a transparent ITO film processed into stripes on a glass substrate. Polar 2.5iOt film and 5
Lower insulating layer 3 consisting of a multilayer N film with an iaNa film, Zy+S
:Light emitting layer consisting of Tb, F film 4.5LN4 film and Al2O
It is fabricated by sequentially stacking an upper insulating layer 5 cut out from a stacked film of three films and a back electrode 6 made of an AR film processed into a stripe shape. The above A & back electrode 6 are on the scanning side, and the IT
The O transparent electrode 2 is on the data side.

The line-sequential driving method of this EL panel is carried out in the same manner as the known conventional method. However, here, only the voltage waveform on the data side applied to the transparent electrode 2 is new; for example, the waveforms A o , A I, A as shown in FIG.
t , . . . , A7 are selectively used. This waveform A o
, A + , ... The highest level of A is in the saturation region of the L - V characteristic, and compared to the light emission intensity that responded to the waveform with a gradual rise, strong light emission intensity was obtained with the waveform A + with a steep rise. It will be done. Depending on how this voltage waveform rises, the luminance of light emission can be controlled, that is, gradation can be displayed, as shown in FIG. The selection of the voltage waveform is performed, for example, by selecting one of a plurality of resistors provided between the pulsed power source and the transparent electrode 2 using a switch. Alternatively, the pulse voltage waveform may be generated by operating one of a plurality of waveform generation circuits including capacitance and resistance.

In this way, gradation display is performed by changing the rise of the pulse voltage waveform in the substantially saturated region of the LV characteristic, so even if the LV characteristic is not very stable, high display quality can be maintained.

<Example 2> Here, an example for lowering the electrical resistance on the transparent electrode side will be shown. On the glass substrate 1, a 2° pattern of metal wiring is formed as shown in FIG. 3(a). The gold scrap material is selected from a group of materials that have low electrical resistance, are unlikely to react with the ITO film, and are thermally stable. Here, an AQ film or a W film manufactured by a sputtering method was used. The AC film has the advantage of low electrical resistance, which is convenient for the present invention, but it has a high reflectance of visible light, so it does not appear on the display screen.
This A12 wiring stands out. Therefore, it is necessary to reduce the reflection of external light using a filter.

Although the W film is inferior to AQ@ in terms of resistivity, it has a low reflectance of visible light, so when patterned as shown in Figure 4 (a), the periphery of the pixel becomes bordered in black, and in this respect it is Improvements in display quality can be expected.

Figure 3(b) above the patterned metal wiring 2°.

As shown in FIG. 4(b), an ITO film 2 was formed to form a data side electrode, and the metal wiring 2' made the electrical resistance smaller than that of the ITO film 2 alone.

Since the transparent electrode 2 and the metal wiring 2' are connected in this way, the electrical resistance is reduced, and the voltage waveform actually applied to the pixel does not depend on the distance from the pixel electrode terminal, resulting in high quality. A gradation display can be obtained.

<Effects of the Invention> As is clear from the above, according to the present invention, a gradation display is performed by applying a pulse voltage that is approximately in the saturation region of the L-V characteristic to the thin film EL element while changing the way it rises. , L
Highly reliable gradation display is possible without requiring significant stability of -V characteristics. Further, according to the present invention, l,
- Not only due to changes in V characteristics over time, but also due to the location of the display screen caused by, for example, the film thickness distribution of element constituent films.
The gradation display is not significantly affected by differences in V characteristics.

Furthermore, the present invention provides a thin film EL element having a light emitting layer containing a rare earth element, such as a ZnS:Tb light emitting layer, in which the L-V characteristic is significantly dependent on the applied voltage waveform. Since the pulse voltages forming the area are applied with different rising directions, highly reliable and high quality gradation display can be performed.

Further, according to the present invention, since a thin film EL element having metal wiring connected to a transparent electrode is used, the voltage waveform actually applied to the pixel does not depend on the distance from the pixel's electrode terminal, resulting in high quality. A gradation display can be obtained.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the dependence of the LV characteristic on the rise of the applied pulse, Fig. 2 is a diagram showing the applied voltage pulse waveform, and Fig. 3.4
The figures each show the pattern shape of the metal wiring, FIG. 5 shows the structure of the thin film EL element, and FIG. 6 shows the LV characteristics. 1...Glass substrate, 2...Transparent electrode, 3...
lower insulating layer, 4... light emitting layer, 5... upper insulating layer,
6... Back electrode, 2°... Metal wiring. Figure 1

Claims (3)

[Claims] (1) An EL display device characterized by performing gradation display by controlling the luminance by applying a pulse voltage whose rise is approximately in the saturation region of the luminance-voltage characteristic to the thin film EL element with different rising directions. gradation display method. (2) Claim 1 characterized in that a thin film EL element having a light emitting layer containing a rare earth element is used as a light emitting center.
A gradation display method for an EL display device according to . (3) Thin film EL with metal wiring connected to transparent electrodes
2. The gradation display method for an EL display device according to claim 1, characterized in that the EL display device uses an element.