JPH0473880B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a thin film EL device, and more particularly to a matrix type thin film EL display panel that performs gradation display by changing pixel occupancy.

Conventional technology Conventionally, for AC-operated thin-film EL elements, a high electric field (about 10 ⁶ V/cm) is regularly applied to the light emitting layer, and in order to improve dielectric strength, luminous efficiency, stability of operation, etc. 1.0WT% Mn (or Cu,
_A three-layer _{structure ZnS} : _Mn (or ZnSe:
Mn) EL devices have been developed, and improvements in various light-emitting properties have been confirmed. 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.

FIG. 1 shows the basic structure of a Zn S:Mn thin film EL device as an example of a thin film EL device.

The structure of the thin film EL element will be explained in detail based on FIG. 1. A transparent electrode 2 made of In ₂ O ₃ , SnO ₂ , etc. is placed on a glass substrate 1, and Y ₂ O ₃ , etc.
A first dielectric layer 3 made of Ta ₂ O ₅ , 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. First dielectric layer 3
On top is formed a ZnS light emitting layer 4 obtained by electron beam evaporation of ZnS:Mn sintered pellets. At this time, the ZnS:Mn sintered pellets used for vapor 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 first dielectric layer 3 is laminated on the ZnS light-emitting layer 4, and further on top of this is a second dielectric layer 5 made of the same material as the first dielectric layer 3.
A back electrode 6 made of Al or the like is formed by vapor deposition.
The transparent electrode 2 and the back electrode 6 are formed into a band shape as shown in FIG. 2, and a matrix electrode structure is adopted in which a plurality of electrodes are arranged perpendicularly to each other. The intersecting position (hatched area) corresponds to one pixel 7 on the panel. The transparent electrode 2 and the back electrode 6 are connected to an AC power source 10 via switches 8 and 9, respectively.

Close switches 8 and 9 and connect the AC between electrodes 2 and 6.
When a voltage is applied, the above AC voltage will be induced between the dielectric layers 3 and 5 on both sides of the ZnS light emitting layer 4, and therefore the electric field generated within the ZnS light emitting layer 4 will cause the conductor to The excited and accelerated electrons, which have obtained sufficient energy, directly excite the Mn luminescent center, and when the excited Mn luminescent center returns to its ground state, it emits orange-yellow light. In other words, the electrons accelerated by a high electric field excite the electrons of the Mn atoms that enter the Zn site, which is the luminescent center in the ZnS luminescent layer 4, and when they fall to the ground state, the electrons are strong in a wide wavelength range with a peak of approximately 5850 Å. Exhibits luminescence.

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. It can be used as a means of displaying images, etc., and is very effective.

However, in the thin film EL element described above, each pixel 7 changes only in two stages, emitting light or not emitting light, and cannot display intermediate gradations. This is because the thin film EL element has a sharp applied voltage-luminance characteristic as shown in FIG. 3, and therefore it is difficult to display halftones by changing the applied voltage.

Therefore, several proposals have been made for thin film EL elements capable of displaying gradations, and the following are the main ones.

A method in which a plurality of EL elements are layered on a substrate and the number of layers of the EL elements that emit light is varied (JP-A-56-139126, JP-A-58-113078, etc.).

The thickness of the dielectric layer is partially varied within one pixel to form multiple regions with light emission threshold voltages corresponding to the layer thickness, and the light emitting area within one pixel is gradually increased depending on the applied voltage. Method of changing (Japanese Unexamined Patent Publication No. 57-100468, etc.).

However, in method 2, each pixel can be made smaller, so it is advantageous for increasing resolution, but it requires a large number of electrodes and the drive circuit becomes complicated. light emission is upward
Absorbed by the EL element. In the method (2), the number of electrodes is small and the drive circuit is simple, but since the withstand voltage of each pixel is determined by the dielectric layer with the smallest thickness, it is not possible to have a large variation range of the applied voltage. There was a problem that there was a limit to the range of gradation changes.

OBJECTS OF THE INVENTION Accordingly, it is an object of the present invention to provide a thin film EL element with a small number of electrodes, a simple drive circuit, a high withstand voltage, and a wide range of gradation changes.

Structure of the Invention The present invention is characterized in that the thickness of the light emitting layer in one pixel is made partially different, and the thickness of the dielectric layer is made the same.

In other words, according to the above configuration, the thinner the layer thickness of the light emitting layer, the smaller the light emission threshold voltage.
When the applied voltage is increased stepwise, light is emitted stepwise from a portion where the layer thickness is small, and the light emitting area increases sequentially, resulting in an intermediate gradation. In addition, since the dielectric layers can be made to have the same layer thickness, it is easy to set the withstand voltage of the part with the lowest light emission threshold voltage to the same level as the withstand voltage of the part with the highest light emission threshold voltage, Furthermore, since the thickness of the light-emitting layer is changed, the saturation luminance of each part is different, so the range of gradation change can be widened.

Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

Fig. 4 is a partial plan view of the thin film EL element, Fig. 5 is an enlarged plan view of the main part of Fig. 4 by one pixel, Fig. 6 is a sectional view taken along the - line in Fig. 5, Fig. 7 5 is a sectional view taken along the - line in FIG. 5. FIG. The figure is the same as FIGS. 1 and 2 except for the following points, so the same reference numerals are given and the explanation thereof will be omitted. In this embodiment, the light emitting layer 4 in one pixel 7 is
The layer thicknesses of the light-emitting layers are varied in four stages from a to b, thereby dividing the light-emitting layer into four parts 4a to 4b. The first dielectric layer 3 and the second dielectric layer 5 have the same layer thickness.

According to the above configuration, since the light emitting layer 4 in each pixel 7 is formed by the light emitting layers 4a to 4d having the layer thicknesses a to b, the pixel regions 7a to 7b corresponding to each of the light emitting layers 4a to 4d The light emission threshold voltage and brightness level differ depending on the layer thickness of the light emitting layers 4a to 4d. That is, the pixel region 7a including the light emitting layer 4a with the thinnest layer thickness a has a light emission threshold voltage of Va as shown in FIG. 8 and a luminance of Ba. The pixel region 7b including the light emitting layer 4b having the second thinnest layer thickness b does not emit light when the voltage V _a is applied, and has a light emission threshold voltage V _b shown in FIG _. It has a brightness of Pixel region 7c including the light emitting layer 4c with the third thinnest layer thickness c
does not emit light when a voltage of V _b is applied, has a light emission threshold voltage of V _c in FIG. 8, and has a luminance of B _c . The pixel region 7d including the light emitting layer 4d with the thickest layer thickness d does not emit light when the voltage of V _c is applied, has a light emission threshold voltage of V _d in FIG. 8, and has a luminance B _d . .

By partially varying the layer thickness of the light emitting layer 4 in each pixel 7 in this way, the pixel regions 7a to 7
The emission threshold voltage of d can be changed from V _a to V _d . Therefore, when a voltage V _a is applied between the transparent electrode 2 and the back electrode 6, only the pixel region 7a emits light with a brightness B _a . Next, when the applied voltage is set to V _b ,
Pixel regions 7a and 7b emit light with a brightness of B _a +B _b .
Moreover, when the applied voltage is increased to V _c , the pixel area 7a
~7c emits light with a brightness of B _a +B _b +B _c . Furthermore,
When the applied voltage is set to _Vd , the pixel regions 7a to 7d,
That is, the pixel 7 emits light with a brightness of B _a +B _b +B _c +B _d . In this way, by changing the voltage applied between the transparent electrode 2 and the back electrode 6, the light emitting area of each pixel 7 can be changed, making it possible to display five levels of gradation including the time when no light is emitted.

Note that the layer thickness a of the light emitting layers 4a to 4d in each pixel 7 is
By setting d, the brightness difference between each stage can be set arbitrarily.

Further, in the above embodiment, a case has been described in which each pixel 7 is divided into four, but the invention is not limited to this, and it is also possible to divide into other numbers.

Effects of the Invention As described above, in this invention, the thickness of the light emitting layer in each pixel is partially different, and the thickness of the dielectric layer is made the same, so that the light emission of each pixel can be adjusted depending on the applied voltage. It becomes possible to change the area and therefore the brightness, making it possible to display gradations of pixels, etc. Compared to the conventional method of multi-layering EL elements mentioned above, the number of electrodes is smaller, the drive circuit is simpler, and gradation display can be achieved by simply changing the drive voltage while using the conventional drive method. Even if the number of gradations is increased, the light emitted from the lower layer will not be absorbed by the EL element above. Also, compared to the method of partially varying the thickness of the dielectric layer, since the thickness of the dielectric layer is constant, the withstand voltage does not decrease locally, making it more reliable. A panel with high brightness can be obtained, and even with the same number of gradations, the range of change in luminance gradation can be widened.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a conventional thin film EL element, FIG. 2 is a plan view of the counter electrode shown in FIG. 1, and FIG. 3 is a voltage-luminance characteristic diagram. FIG. 4 is a plan view of a thin film EL device according to an embodiment of the present invention, FIG. 5 is an enlarged plan view of a main part of one pixel element in FIG. 4, and FIG. 6 is a cross-sectional view taken along the - line in FIG. 5. , FIG. 7 is a sectional view taken along the - line in FIG. 5, and FIG. 8 is a voltage-luminance characteristic diagram. 1... Glass substrate, 2... Transparent electrode, 3, 5...
...Dielectric layer, 4, 4a to 4d...Light emitting layer, 7...
Pixels, 7a to 7d...pixel area.

Claims (1)

[Scope of Claims] 1 A plurality of striped electrodes, at least one of which is transparent, are arranged facing each other in a matrix, and a dielectric layer and a light emitting layer are sandwiched between the electrodes, and the intersections of the electrodes are In a matrix-type thin film EL element in which a pixel is formed with a plurality of regions having different emission threshold voltages, the thickness of the light emitting layer in the plurality of regions of the pixel is made different, and the layer thickness of the dielectric layer in the plurality of regions is made the same. A thin film EL element characterized by the following.