JPH027072B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention uses EL (Electro
The present invention relates to the structure of a thin film light emitting device (thin film EL device) that emits light (luminescence), and particularly relates to a thin film light emitting device that obtains mixed color light emission by laminating a plurality of light emitting layers. Conventionally, for AC-operated thin film EL devices, a high electric field (10 ⁶ V/cm) is regularly applied to the light emitting layer.
In order to improve dielectric strength, luminous efficiency, and operation stability, 0.1 to 2.0 wt% of Mn (or Cu, Al,
A three-layer structure ZnS:Mn (or ZnSe:Mn) EL device was developed, in which a semiconductor light-emitting layer such as ZnS _or ZnSe doped with Br, etc. was sandwiched with a dielectric thin film such as Y ₂ O ₃ or TiO 2 , and the light emitting properties were improved. It has been confirmed that the characteristics have improved. This thin film EL element emits high-intensity light when an alternating current electric field of several KHz is applied, and has a long lifespan. Furthermore, regarding the light emission of this thin film EL element, it has a hysteresis characteristic in which the light emission brightness differs for the same applied voltage in the process of increasing the applied voltage and in the process of decreasing the voltage from the high voltage side. was discovered, and in the process of increasing the applied voltage to a thin film EL element with this hysteresis characteristic, when light, electric field, heat, etc. are applied, the thin film EL element is excited to a state of luminance corresponding to the intensity. The so-called memory phenomenon, in which the luminance remains high even after light, electric field, heat, etc. are removed and the display returns to its original state, has opened up a new field of application for display technology. As an example of a thin film EL device, the basic structure of a ZnS:Mn thin film EL device is shown in FIG. The structure of the thin film EL device will be explained in detail based on the attached drawings. A transparent electrode 2 made of ln ₂ O ₃ , SnO ₂ , etc. is placed on a glass substrate 1, and Y ₂ O ₃ , etc. is laminated thereon.
A first dielectric layer 3 made of TiO ₂ , Al ₂ O ₃ , Si ₃ N ₄ , SiO ₂ or the like is formed in an overlapping manner by sputtering or electron beam evaporation. A ZnS light emitting layer 4 is formed on the first dielectric layer 3 by electron beam evaporation of ZnS:Mn sintered pellets. At this time, the ZnS:Mn sintered pellets used for deposition are pellets in which the concentration of Mn, which is an active substance, is set to suit the purpose. A second dielectric layer 5 made of the same material as the dielectric layer 3 is laminated on the ZnS light emitting layer 4, and a back electrode 6 made of Al or the like is further deposited thereon. The transparent electrode 2 and the back electrode 6 are connected to an AC power source 7, and the thin film EL element is driven. When an AC voltage is applied between the electrodes 2 and 6, the above AC voltage is induced between the dielectric layers 3 and 5 on both sides of the ZnS luminescent layer 4, and therefore the electric field generated within the ZnS luminescent layer 4 Therefore, electrons that are excited and accelerated into the conduction band and have obtained sufficient energy can directly
The Mn luminescence center is excited, and when the excited Mn luminescence center returns to the ground state, it emits yellow-orange light. In other words, electrons accelerated by a high electric field enter the Zn site, which is the luminescent center in the ZnS luminescent layer 4.
When an atom's electrons are excited and fall to the ground state, approximately
It emits strong light in a wide wavelength range with a peak of 5850 Å. When a rare earth fluoride other than Mn is used as an active substance, green and other luminescent colors characteristic of this rare earth element can be obtained. The thin film EL element having the structure described above can be used as a flat thin display device that takes advantage of the space factor to display characters, symbols, still images, moving images, etc. on computer output display terminal equipment and various other display devices that contain characters and figures. It can be used as a means of displaying images, etc. Thin-film EL panels as flat flat display devices have a lower operating voltage than conventional cathode ray tubes (CRTs), and are superior in terms of weight and strength compared to plasma display panels (PDPs), which are the same flat display devices. It has many advantages over liquid crystals (LCDs), such as a wider operating temperature range and faster response speed. Furthermore, since it can be used as a pure solid matrix type panel, it has a long operating life, and its address accuracy makes it very effective as an input/output display means for computers and the like. Regarding the emission color of thin film light emitting devices, ZnS,
It has already been confirmed that when Mn is added as a luminescent center in a luminescent host material such as ZnSe, yellow-orange EL emission is obtained as mentioned above, when TbF ₃ is added, green EL emission, and when TmF ₃ is added, blue EL emission is obtained. It is being Incidentally, in recent years, from a practical standpoint, there has been a demand for something visually close to white. white light emitting thin film
The development of EL elements has made it possible to realize flat thin displays with black and white displays of good display quality (for example, displays for various office automation equipment and measuring instruments, black and white wall-mounted TVs, etc.). An object of the present invention is to provide a thin-film light-emitting element that emits white light at various color temperatures by stacking light-emitting layers having different emission colors. FIG. 2 is an element configuration diagram of a thin film light emitting element showing one embodiment of the present invention. In the figure, the same reference numerals as in FIG. 1 indicate the same contents, and the explanation will be omitted. The light emitting layer is a first layer 4a of ZnS doped with Mn,
Second layer 4b of ZnS doped with TbF ₃ , ZnS
It is composed of a three-layer stacked structure including a third layer 4c doped with TmF ₃ . FIG. 3 is a CIE chromaticity diagram illustrating the embodiment of FIG. 2, in which color temperature is plotted. In the figure, A is the ZnS:Mn layer that constitutes the first layer 4a, and B is the second layer 4.
b is _{the three} ZnS:TbF layers forming the third layer 4c, and C is the emission chromaticity position of the three ZnS:TmF layers forming _{the third} layer 4c. Here, in the thin film light emitting element whose chromaticity position is a in FIG. 3, the luminance ratio expressed by the first layer 4a:second layer 4b:third layer 4c is 20:10:1. Regarding this white light emitting thin film EL element, specific adjustment methods will be explained in detail for each film thickness ratio and impurity concentration, but the present invention is not limited thereto. Adjustment by film thickness ratio Figures 4a to 4c show the relationship between the film thickness of the luminescent layer and the luminance when the luminescent center concentration is optimized.
Figure c shows the relationship between ZnS:TbF ₃ and ZnS:TmF ₃ . As is clear from these figures, there is a nearly proportional relationship between film thickness and luminance, and when the film thickness is 8000 Å, the luminance of each layer is: ZnS:Mn approximately 2700cd/m ² ZnS:TbF ₃ approximately 600cd/m ² ZnS: TmF ₃ is approximately 6 cd/m ² . Therefore, in order to realize white light emission at chromaticity position a in FIG. 3, that is, the luminance ratio of the first layer 4a:second layer 4b:third layer 4c is 20:10:1. The film thickness design is as follows.

[Table] Adjustment by impurity concentration Figures 5a to 5c show the relationship between the concentration of impurities that serve as luminescence centers and luminescence _brightness . , Figure c shows the relationship between ZnS:TmF ₃ . At this time,
ZnS:TmF ₃ has the lowest luminance, so it is necessary to set the film thickness thicker than the other two layers in advance.
Here, ZnS:TmF ₃ was set to 8000 Å, and ZnS:Mn and ZnS:TbF ₃ were each set to 1000 Å. In this case, it is preferable to select the impurity concentration of ZnS:TmF ₃ , which has the lowest luminance, as the optimum concentration.
From FIG. 5c, the optimal TmF ₃ concentration is about 2 mol %, at which time blue light emission with a brightness of 6 cd/m ² can be obtained. Therefore, in order to realize white light emission at chromaticity position a in FIG. 3, that is, the luminance ratio of the first layer 4a:second layer 4b:third layer 4c is 20:10:1. , ZnS:Mn, and ZnS:TbF ₃ need to have luminances of 120 cd/m ² and 60 cd/m ² , and the impurity concentration design is as follows.

[Table] By changing the film thickness ratio, concentration, etc. of the three types of light emitting layers 4a, 4b, 4c, the luminance brightness ratio of each light emitting layer can be adjusted.
Emission colors of arbitrary chromaticity can be obtained within the range surrounded by the triangle connecting the three points B and C. The brightness ratio of each layer and the chromaticity of the resulting luminescent color are shown in the table below.

[Table] In the thin film light emitting device according to the present invention, even if the luminance of the ZnS:TmF ₃ layer, which has the lowest luminous efficiency, becomes low, it is easy to adjust the luminance of the other two layers to the luminance of the ZnS:TmF ₃ layer. Emission colors in the white range can be obtained, and the color temperature can be freely controlled. In addition, ZnS:Mn layer, ZnS:TbF ₃ layer, and
By stacking ZnS:TmF _three layers in order of luminance efficiency, the higher the layer, the better the crystallinity, making even materials with low luminance efficiency brighter.
This makes it possible to take advantage of the characteristics of ZnS films.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the basic structure of a conventional thin film light emitting device. FIG. 2 is a configuration diagram of a thin film light emitting device showing one embodiment of the present invention. FIG. 3 is a CIE chromaticity diagram illustrating the embodiment of FIG. 2. Figure 4a
~c is a diagram showing the relationship between the film thickness of the light emitting layer and the luminance,
FIGS. 5a to 5c are diagrams showing the relationship between the impurity concentration of the light emitting layer and the luminance. 4a...first light emitting layer, 4b...second light emitting layer, 4
c...Third light emitting layer.

Claims (1)

[Claims] 1 A thin film light-emitting device using ZnS as a light-emitting layer,
A first light-emitting layer doped with an impurity that produces yellow-orange EL light, a second light-emitting layer doped with an impurity that produces green EL light, and a third light-emitting layer doped with an impurity that produces blue EL light. A three-layer structure with a light-emitting layer of
A thin film light emitting device characterized in that the mixed color of EL light is located in a substantially white region on a chromaticity diagram.