JPS62285397A - Dc driven thin film el 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 3. Detailed Description of the Invention Field of Industrial Application The present invention relates to a DC-driven thin film EL device, and more particularly to a DC-driven thin film EL device that is characterized by good contrast and stability and is easy to drive.

BACKGROUND OF THE INVENTION AC-driven thin film EL devices are being actively researched as flat panel displays used in computer terminals and the like. Conventionally, such thin EL elements have a transparent electrode made of indium and tin mixed crystal oxide (hereinafter referred to as ITO) on a glass substrate, yttrium oxide, titanium oxide, strontium titanate, tantalum oxide, silicon oxide, and aluminum oxide. and a first dielectric layer made of silicon nitride, etc.; a phosphor layer made of zinc sulfide doped with manganese, terbium, samarium, etc.; a second dielectric layer made of the same material as the first dielectric layer; It has a structure in which back electrodes consisting of the following are sequentially laminated.

To form a dot matrix, the ITO electrodes and aluminum electrodes are processed into stripes and are perpendicular to each other. By applying an AC pulse voltage between the two electrodes, the intersection between the two electrodes is made to emit light, thereby displaying characters and figures. AC-driven thin film EL devices have the major characteristics of high brightness and stable operation over long periods of time, but because they have a high operating voltage and are driven by forward pulses, the drive circuit is complicated and expensive. There were certain drawbacks. Another drawback was that it was difficult to modulate the brightness.

On the other hand, DC-driven thin film EL devices have a structure in which the EL light emitting layer is directly sandwiched between electrodes without using a dielectric layer, or a structure in which a resistor layer or a semiconductor layer is used instead of the dielectric layer. has been prototyped (Japanese Unexamined Patent Publication No. 181485/1985).

When a DC-driven thin film EL element is configured into a matrix type display device, it has the following characteristics: 1. A low-cost display device with a simple drive circuit is possible, and brightness modulation is easy.

Problems to be Solved by the Invention Although the DC-driven thin film EL device has the above-mentioned advantages,
The disadvantage is that it lacks stability. In other words, when a voltage is applied to cause the device to emit light, dielectric breakdown is likely to occur, and when the device is operated for a long time, there are problems in that pixel elements are missing or wire breaks occur. Further, although thin film EL elements generally have poor contrast in both AC and DC driven types, the present invention also improves the contrast of DC driven EL elements.

Means for Solving the Problems An element is provided in which a transparent electrode, an EL light emitter layer, a current control layer made of a mixed oxide thin film of Pr and Co, and a back electrode are arranged in sequence.

Function The mixed oxide thin film of Pr and Co has a resistivity of 4X10~
It has a value of 4×1Q Ω・a, and has a large dielectric breakdown field strength, so it operates as a stable resistor for a long period of time. As a result, it is thought that by using it as a current control layer of a DC-driven EL element, a direct iEL element that emits light stably over a long period of time will become possible. In addition, this material is completely black and has a large light absorption coefficient of 1o(7) or more, so it can effectively absorb external light, reduce the external light reflectance of the panel, and form a high-contrast element. be able to.

Example FIG. 1 shows a cross-sectional view of the thin film EL device of the present invention.

On the glass substrate 1 made of Corning 7059-glass,
A tin-doped indium oxide thin film (hereinafter abbreviated as IT◯ thin film) with a thickness of 2000 mm was formed by a sputtering method, and processed into a stripe shape by a photolithography technique to form a transparent electrode 2. Thereon, an EL phosphor layer 3 consisting of a manganese-doped zinc sulfide thin film was formed at a substrate temperature of 200° C. to a thickness of 4 o○ by co-evaporation of manganese and zinc sulfide. Thereafter, heat treatment was performed in vacuum at 450° C. for 1 hour to perform EI and activation of the light emitting layer 3.

A current control layer 4 made of a mixed oxide thin film of Pr and :Co was formed thereon. The preparation method and characteristics thereof will be explained in detail below.

P so that the atomic ratio of Pr and Co is 50 atoms each.
r e○11 and Coo oxide were weighed out and mixed well. Then, it was heat-treated in air at 1200° C. for 2 hours to effect atomic diffusion, and then pulverized to obtain a sputtering target. Using RF sputtering equipment, the substrate temperature is 20
A mixed oxide thin film of Pr and Co having a thickness of 4000 nm was formed by sputtering in Ar gas of 3 x 10-27' orr at 0°C. This mixed oxide thin film current control layer of Pr and Co is a completely black resistor, and a separate experiment showed that the specific resistance was 1.2 x 10 Ω@mouth.Therefore, when the thickness was 4 o○〇 people, the sheet resistance was 3. ×1090/mouth. The specific resistance varies greatly depending on the composition ratio, and decreases rapidly when Co exceeds 60 atomic %. 4XI with a composition of 60 atomic % or less
Specific resistance of Q3Ω・(7) or more, 10 with a film thickness of 4000 people
A sheet resistance value of 89/mouth or more can be obtained with good reproducibility under the above sputtering conditions. In order to absorb external light as a black resistor and lower the external light reflectance of the element, a light absorption coefficient of 105 Gff-' or more is desired at a wavelength of around 500 mm. For this purpose, it has been found that more than 10 atoms of co are required to be poisoned.

Therefore, the composition of the mixed oxide current control layer of Pr and Co suitable for the present invention is one containing 10 to 60 atoms of Co. Although the above description is about a thin film formed by sputtering in Ar gas, it is not preferable to mix oxygen into Ar gas.

This is because although the light absorption coefficient of the resistor thin film does not change, the resistance decreases somewhat due to its P-type properties, and the underlying phosphor layer is adversely affected by oxygen plasma. In addition, the sputtering rate is low, resulting in poor production efficiency. The substrate temperature and Ar gas pressure of the sputtering conditions are the commonly used 200°C and 3 x 10-2 Torr as described above.
You can do it before and after.

Although the resistivity changes to some extent depending on these conditions, it is very small compared to the change due to the composition, so it may be selected within the commonly used range mentioned above. A thin film prepared in a composition range containing 10 to 60 atoms of Co shows a halo pattern when examined by X-ray diffraction, and is amorphous from an X-ray perspective, which is convenient from the viewpoint of uniformity of film quality. often,
It is thought that there are fewer defects within the film plane.

After forming the current limiting layer as described above, finally, aluminum is vacuum-deposited to a thickness of 2,000 yen, and a striped back electrode 5 is formed in a direction perpendicular to the ITO transparent electrode using photolithography technology. A thin film EL device was completed.

For comparison, an EL device was also fabricated with the same structure, except that the current control layer was replaced with a well-known T a 205 thin film with a thickness of 2000 mm.

The Ta 205 thin film was prepared by Ar using a Ta metal target.
It is formed by an active sputtering method in a +02 atmosphere, and the manufacturing conditions are optimized for a DC EL device.

The EL element of the present invention has a duty 11520 as shown in FIG. 2, with the back electrode 5 being exposed. When line-sequential driving was performed using pulses with a frequency of 60 and a pulse width of 32 μs, brightness voltage characteristics as shown in FIG. 3 were obtained. The light-emitting portion of the EL element corresponds to 64% of the total area, and the figure shows the average brightness including the light-emitting and non-light-emitting portions. A practical 44 cd/m 2 was obtained at 65 V rising from 45 V. The brightness change characteristic after driving at esV for 1000 hours is shown by a broken line. As shown in the figure, the change was small, and the brightness was only 9.1 inches lower at 66 inches, and the device operated stably for 1,000 hours without exhibiting any electrode breakage due to dielectric breakdown. Furthermore, in the EL element of the present invention, the current control layer is provided between the phosphor and the aluminum back electrode, and since it is black, reflection of external light by the aluminum electrode is effectively prevented, and the external light reflectance is 6%.
It was low. Therefore, a high contrast EL element can be obtained.

On the other hand, in the conventional EL element using a T a 20 s thin film, the light emission rises from ssV, and unlike the case of the black-brown current control layer of the present invention, it is transparent and the light emission is reflected by the aluminum back electrode. 80 cd/rpf with ssV
However, when similarly driven at 85V for 1000 hours, the brightness decreased to 60% and some wire breakage occurred. Also, the external light reflectance is as high as θγ. If 200
cd/yp? If an external light source of 2 is specularly reflected on the surface of the device, the device of the present invention can obtain a contrast of 3.7:1 at a luminance of 44 cd/772'. On the other hand, the conventional EL element has a luminance of 0.6:1 even in BOcdlyd.

As explained above, the DC thin film EL element of the present invention has more stable brightness voltage characteristics, driving stability, and deeper contrast characteristics than the conventional example.

Furthermore, as an EL emitting layer, other than the manganese-doped zinc sulfide mentioned above, zinc sulfide doped with rare earth elements such as terbium and samarium, and calcium sulfide and strontium sulfide also doped with rare earth elements, can also be used. It was possible to create a DC-driven thin film EL device.

Effects of the Invention According to the present invention, it is possible to form a DC-driven thin film EL element that has high contrast and can be driven stably over a long period of time. Since it is a DC drive type, brightness gradation control is easy, the drive circuit can be configured at low cost, and the element configuration is simple, so it can be put to practical use as an inexpensive thin display with excellent visibility.

[Brief explanation of drawings]

FIG. 1 shows a DC 1-driven thin film EL in one embodiment of the present invention.
FIG. 2 is a sectional view of the device, FIG. 2 is a waveform diagram showing DC pulses used to drive the device, and FIG. 3 is a graph showing the brightness voltage characteristics of the device of the present invention. 1...Glass substrate, 2...Transparent electrode,
3... EL light emitter layer, 4... Current control layer, 5... Back electrode. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2

Claims (3)

[Claims] (1) A direct current driven thin film EL device characterized in that it is constructed by sequentially arranging a transparent electrode, an EL light emitter layer, a current control layer made of a mixed oxide thin film of Pr and Co, and a back electrode. (2) In a mixed oxide thin film of Pr and Co, Co
2. The direct current driven thin film EL device according to claim 1, wherein the content of is 10 to 60 atomic %. (3) The mixed oxide thin film of Pr and Co is composed of Pr oxide and C
The DC-driven thin film according to claim 1, which is produced by heat-treating a mixture of O oxides in an air atmosphere, and then using the mixture as a target by sputtering in an Ar atmosphere. EL element.