JPH0737687A - Resistance type ac thin film el element - Google Patents

Abstract

PURPOSE:To provide a resistance type AC thin film EL element having high luminous efficiency and a good gradation display and capable of being driven at low voltage by sandwiching a luminescence layer with dielectric films having a prescribed range of thickness and a prescribed range of electric resistivity. CONSTITUTION:A transparent electrode 3, a dielectric layer 4, a luminescence layer 5, a dielectric layer, a current limiting layer 6 fixed with conducting fine powder by an organic resin, and a back electrode 7 are laminated in sequence on a transparent, electrically insulating substrate 2. The electric resistivity of the dielectric layers is set to 1.0X10<8>-1.0X10<10>OMEGAcm, and the thickness is set to 10-100nm. When the AC electric field is applied to the transparent electrode 3 and the back electrode 7, electrons pass through the dielectric layers by the tunneling effect, and the luminescence center in the luminescence layer 7 is excited.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electroluminescence (EL) element used for displaying characters and graphics, and more particularly to a resistance type AC thin film EL element.

[0002]

2. Description of the Related Art An EL display using EL elements is
It is one of the promising flat displays that is rapidly spreading in recent years to portable computer terminals and workstation terminals as a display that can display characters and graphics with high display quality.
Is one. The EL element used in the currently commercially available EL display is an EL element having a structure in which the light emitting layer 4 is sandwiched between two electric insulating layers 8 as shown in FIG. 5 (hereinafter referred to as a capacitive AC thin film EL element). is there. In this device, when an alternating electric field is applied between the transparent electrode 3 and the back electrode 7, electrons at the interface between the electric insulating layer 8 and the light emitting layer 4 are emitted into the light emitting layer 4 by the electric field and are accelerated, and the emission center is changed. Excitation causes emission of light. The emitted light is a transparent electrically insulating substrate 2
And is released to the outside.

Further, as shown in FIG. 2, a transparent electrode 3, a light emitting layer 4, an intermediate layer 8, a current limiting layer 6 in which conductive fine powder is fixed with an organic resin, and a back electrode 7 are provided for the EL element. A powder / thin film mixed molding EL element (hereinafter referred to as a DCHEL element) capable of being driven by a DC voltage sequentially laminated is known. In the above-mentioned DCHEL element, electrons are injected into the light emitting layer from the outside of the EL element. By modulating the pulse width of the voltage waveform applied to the EL element, the amount of current flowing in the light emitting layer can be adjusted, and good gradation display can be easily obtained. It has an advantage that it can be driven at a lower voltage than a thin film EL element.

[0004]

In the above capacitive AC thin film EL element, the electrons contributing to light emission are the electrons charged at the interface between the light emitting layer and the insulating layer, and these electrons are accelerated by a high AC electric field. Then, the emission center is excited to emit light. Therefore, the device has an advantage that the luminous efficiency of the device is high, but the insulating layer needs to be formed of a thick dielectric in order to prevent dielectric breakdown, resulting in a drawback that the driving voltage becomes high. Due to the thin film laminated structure, this EL element emits from the interface inside the light emitting layer only at t ₁ and t ₂ when the applied voltage pulse rises as shown in FIGS. 3 (a) and 3 (b). Since a conduction current due to the generated electrons flows, most of the electric charges that move are largely independent of the length of the voltage application time and change only according to the magnitude of the applied voltage. Therefore, the brightness modulation has a characteristic of being applied voltage modulation. However, since the change width of the luminance is large with respect to the change width of the voltage, it is difficult to obtain a subtle gradation display. In addition, it is necessary to increase the thickness of the electric insulating layer in order to prevent dielectric breakdown of the element, which results in 200 V
It has the drawback that it must be driven at a high voltage of the order of magnitude.

On the other hand, since the DCHEL element injects electrons from the outside of the EL element into the light emitting layer, it is a capacitive AC thin film EL element.
The clamping electric field in the light emitting layer (insulating property when a voltage of a certain voltage or lower is applied, but the voltage value when the semiconductor has characteristics that a current flows when a voltage of a certain voltage or higher is applied) is lower than that of the device, The EL element has a drawback that the luminous efficiency is low. SUMMARY OF THE INVENTION It is an object of the present invention to solve the problems of the above-mentioned conventional techniques, and to provide an EL element which has high luminous efficiency, can perform good gradation display, and can be driven at a low voltage.

[0006]

According to the present invention, a transparent electrode, a first dielectric layer, a light emitting layer, a second dielectric layer and a conductive fine powder are fixed on a transparent electrically insulating substrate with an organic resin. In the electroluminescence device, in which the current limiting layer and the back electrode are sequentially laminated, the electric resistivity of the first and second dielectric layers is 1.0 × 10 ^{8 to} 1.0 × 10 ¹⁰ Ωcm,
And by making the thickness 10 to 100 nm,
When an alternating electric field is applied to the transparent electrode and the back electrode, electrons pass through the dielectric layer by a tunneling effect to excite the emission center in the light emitting layer. is there.

As the transparent electrically insulating substrate of the present invention, a glass plate or a plastic plate can be used, and in particular, glass of soda lime composition or borosilicate glass is preferably used. As the transparent electrode of the present invention, a transparent conductive film of tin-doped indium oxide or tin oxide can be used. These transparent conductive films are finely patterned into predetermined dimensions as necessary according to the characters and figures to be displayed.

In the first dielectric layer and the second dielectric layer of the present invention, the thickness range and the electric resistivity range are defined,
Therefore, the range of the resistance value in the thickness direction of the dielectric layer is defined. The electric resistivities of the first dielectric layer and the second dielectric layer are 1.0 × 10 ^{8 to} 1.0 × 10 ¹⁰ Ωcm, respectively.
And the thickness is 10 to 10 respectively.
The range is 0 nm. Therefore, the resistance value in the thickness direction of the first dielectric layer and the second dielectric layer is 1 per square cm.
It is in the range of 0 ² to 10 ⁵ Ω. When the resistance value is larger than 10 ⁵ Ω, it becomes difficult to excite the emission center of the light emitting layer to emit light by electrons passing through the dielectric layer due to the tunneling effect. When the resistance value is smaller than 10 ² Ω, the energy of the electrons flowing in the light emitting layer is small, the ratio of the electrons contributing to the light emission is reduced, and the light emission efficiency is reduced.
The more preferable range of the resistance value of the dielectric is 1 cm <2> from a practical viewpoint as an EL display that can be driven at a low voltage without lowering the luminous efficiency.
It is 10 ² to 10 ³ Ω. A rare earth oxide is preferably used as the dielectric having the above electrical resistivity, and specifically, europium oxide can be exemplified. In the case of europium oxide EuOx (0 <x ≦ 2), Eu, which is a constituent element, compensates for sulfur defects in the zinc sulfide light-emitting layer, so that the shallow level at the interface is reduced in terms of energy and the clamp electric field of the EL element is improved. Can be made. Therefore,
The luminous efficiency of the present invention is characterized by being higher than that of the prior art DCHEL device. In addition to the first dielectric layer and the second dielectric layer provided so as to sandwich the light emitting layer, the dielectric layer of the present invention is provided with a third dielectric layer so as to separate the light emitting layer into two layers. You can In this case, it is preferable that the divided light emitting layers have a thickness of 200 to 400 nm.
The thickness of each of the third dielectric layers is preferably 10 to 50 nm. Further, the light emitting layer may be divided into three layers by the third and fourth dielectric layers, or the light emitting layer may be divided into four or more layers.

Zinc sulfide ZnS can be used as the substance of the light emitting layer of the present invention, and known substances such as Mn and TbF ₃ can be used as the emission center depending on the emission color. When ZnS is used for the light emitting layer, its thickness is 40
The thickness is preferably 0 to 800 nm. If the thickness is less than 400 nm, sufficient emission brightness cannot be obtained, which is not preferable.
If it is thicker than 00 nm, the driving voltage of the EL element becomes high, which is not preferable.

For the current limiting layer of the present invention, a conductive fine powder fixed with an organic resin is used. As the conductive fine powder, one having a resistivity of 3 × 10 ³ to 1 × 10 ⁶ Ωcm is used, and zinc coated with copper, manganese dioxide, lead sulfide, cuprous oxide, terbium oxide (Tb ₄ O). ₇ ), europium oxide (Eu ₂ O ₃ ), praseodymium oxide (Pr)
O ₂ ), carbon, barium titanate and the like can be exemplified. To increase the display contrast, it is preferable to use a black or dark substance. Then, the particle diameter of these fine powders is set to 1 μm or less, preferably 0.2 μm or less, and the organic resin is used as a binder to be fixed in layers. Examples of the resin used for the binder include vinyl resins, polyester resins, polyamide resins, cellulose resins, polyurethane resins, epoxy resins, melamine resins, silicone resins, urea resins, but especially hydroxyl groups, Polymer materials having polar groups such as a carboxyl group, a sulfonyl group and a nitro group, and a reactive group such as an epoxy group, an isocyanuric group and a silanol group are preferably used.
Further, the volume mixing ratio of the fine powder and the organic resin is as follows:
The organic resin is preferably within the range of 2: 8 to 6: 4. As a result, the electrical resistivity of the current limiting layer after being dried and fixed is almost the resistance value of the conductive fine powder itself.

The thickness of the current limiting layer is preferably 1 to 200 μm, more preferably 1 to 30 μm. When the thickness is less than 1 μm, the role of protecting the element is deteriorated when a large current flows due to minute breakage in the EL element, which is not preferable, and when the thickness exceeds 200 μm, E
Since the resistance component of the L element increases, an afterimage remains (the time constant increases) when the applied voltage is removed, and the driving voltage increases, which is not preferable.

As the back electrode film, a metal film of aluminum, nickel, chromium or the like can be used. The transparent conductive film, the dielectric layer, and the light emitting layer can be provided by a physical method such as a vapor deposition method or a sputtering method, and the current limiting layer can be provided by a coating method.

[0013]

According to the EL element of the present invention, by sandwiching the light emitting layer between the dielectric films having a predetermined thickness range and a predetermined electric resistivity range, electrons supplied from the outside tunnel through the dielectric layer. Electrons that flow in the light-emitting layer and the dielectric layer are allowed to pass through by the effect to excite the emission center in the light-emitting layer, and by providing a current-limiting layer on one side of the light-emitting layer, At, the electrons are prevented from locally concentrating and flowing. Thus, the EL of the present invention
The device can emit light at a lower drive voltage than that of the capacitive AC thin film EL device of the prior art, and current is not concentrated and does not flow in the device, so that the occurrence of voltage breakdown is prevented. Further, as shown in FIG. 4, the amount of current flowing through the EL element is controlled by adjusting the pulse width of the applied voltage waveform, and the integrated value of the light emission intensity of the element changes with the amount of current, and this integrated value is the luminance. Since it is proportional to, it is possible to obtain a better gradation display than the conventional capacitive AC thin film EL element.

Further, in AC driving, an internal electric field due to electrons charged up at the interface between the dielectric layer and the light emitting layer can be superposed when a voltage of opposite polarity is applied, and the electric field in the light emitting layer is It is higher than the electric field due to the driving voltage alone. For this reason, the driving voltage required to obtain the same brightness is reduced, and the luminous efficiency of the EL element is improved.

[0015]

EXAMPLES The present invention will be described below with reference to examples. FIG. 1 shows a partial cross-sectional structure of one embodiment of the EL device of the present invention.

EXAMPLE A transparent electrically insulating substrate 2 (a glass substrate having a thickness of 1 mm was used) was coated with an ITO transparent conductive film to a thickness of about 500 nm, and was formed in a direction parallel to the paper surface by a photolithography method. Transparent electrode 3 by patterning into multiple stripes
And Then, the Eu metal and an evaporation source by electron beam evaporation method, the oxygen partial pressure in the reduced pressure atmosphere with 5 × 10 ^{over 2} Pa dielectric film in which europium oxide EuOx (0 <x ≦
2) A thin film is formed to a thickness of about 20 nm to form the first dielectric layer 3,
Then, oxygen was stopped, and ZnS (hereinafter referred to as ZnS: Mn) doped with 0.3% by weight of Mn in a high vacuum was added to about 300.
nm to form a light emitting layer 4. After that, this operation is repeated once again to cover the third dielectric film 4 and the light emitting layer 5, and then the EuOx thin film is coated to a thickness of about 20 nm under the same conditions.
Of the dielectric film. And about 2 at 550 ° C in vacuum
Time annealing was performed. Subsequently, a paint prepared by dispersing MnO ₂ fine powder in a mixed liquid of an organic resin and a thinner is applied by a spray method and dried to obtain a resistivity of 1 × 10 ⁵ Ωcm.
To form a current limiting layer having a thickness of 12 μm. further,
Aluminum (Al) with a thickness of 1 μm was formed by an electron beam evaporation method, and then a back electrode and a current limiting layer were simultaneously scratched by a metal rod in a direction perpendicular to the paper surface to form a stripe pattern, and finally the entire device was formed. Dot-matrix EL that is covered with cover glass to prevent humidity
Element 1 was prepared.

FIG. 6 shows the relationship between the luminance and the applied voltage of the obtained EL element 1 and the conventional capacitive AC thin film EL element 30. The light emission threshold of the applied voltage was reduced by about 50V.

FIGS. 7A and 7B show current waveforms with respect to applied voltage waveforms of the obtained EL element. It was found that the gradation display can be performed by pulse width modulation because the conduction current flows during the voltage application. Therefore,
It is possible to obtain good gradation display.

Comparative Example 1 In the same manner as in Example 1, 500 nm was formed on a glass substrate.
Was coated with an ITO transparent electrode having a thickness of 100 nm, a light emitting layer made of ZnS having a thickness of 600 nm doped with 0.3% by weight of Mn, and then an intermediate layer 8 made of europium oxide having a thickness of 30 nm was coated. Further, a current limiting layer 6 and a back electrode 7 were provided thereon in the same manner as in the example, to prepare a DCHEL element.

Comparative Example 2 In the same manner as in the example, an ITO transparent electrode having a thickness of 500 nm, an electric insulating layer made of oxynitride of silicon having a thickness of 200 nm, and an Mn having a thickness of 600 nm are 0.3 on a glass substrate. A capacitive thin film AC EL device was prepared by coating a light emitting layer made of ZnS doped by weight% and an electric insulating layer made of oxynitride of silicon having a thickness of 200 nm, and further providing a back electrode made of aluminum having a thickness of 1 μm. .

Table 1 shows the luminous efficiency of the above three types of devices.
The AC element of the present invention driven by AC is a capacitive AC thin film EL element.
Compared to the device, it can be driven at a lower voltage without significantly lowering the luminous efficiency of the DCHEL type device,
It can be seen that the dielectric breakdown resistance and gradation display characteristics are improved.

[0022]

[Table 1]

[0023]

According to the present invention, the driving voltage is lower than that of the capacitive AC thin film EL element, and the driving power source can be made smaller and lighter, and at the same time, finer gradation display can be performed. It is possible to obtain a resistance type AC drive thin film EL element having improved dielectric breakdown resistance, which was insufficient in the element.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view of an embodiment of the present invention.

2 is a partial cross-sectional view of a fine powder / thin film mixed molding EL element of Comparative Example 1. FIG.

FIG. 3 is a diagram showing a voltage waveform applied to a capacitive AC thin film EL element and a conduction current waveform at that time.

FIG. 4 is a graph showing a change in emission intensity with respect to a pulse width of an applied voltage waveform of the present invention.

5 is a partial cross-sectional view of a capacitive AC thin film EL element of Comparative Example 2. FIG.

FIG. 6 is a diagram for comparatively explaining the luminance and applied voltage characteristics of the present invention and a capacitive AC thin film EL element.

FIG. 7 is a diagram for explaining the relationship between the applied voltage waveform and the conduction current waveform of the present invention.

[Explanation of symbols]

1 ... EL element of the present invention, 2 ... on transparent electrically insulating substrate, 3 ... transparent electrode, 4 ... dielectric layer, 5 ...
..Light-emitting layer, 6 ... Current-limiting layer, 7 ... Back electrode,
8 ... Intermediate layer, electric insulation layer, 20 ... Fine powder / thin film mixed molding EL element, 30 ... Capacitive AC thin film EL element

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiaki Yasuzaki 3-5-11 Doshomachi, Chuo-ku, Osaka-shi, Osaka Inside Nippon Sheet Glass Co., Ltd.

Claims (4)

[Claims] 1. A transparent electrode on a transparent electrically insulating substrate,
The first and second dielectric layers include a first dielectric layer, a light emitting layer, a second dielectric layer, a current limiting layer in which conductive fine powder is fixed by an organic resin, and a back electrode, which are sequentially laminated. Electrical resistivity of 1.0 × 10 ⁸ ~
1.0 × 10 ¹⁰ Ωcm and its thickness is 10 to 1
With a thickness of 00 nm, when an AC electric field is applied to the transparent electrode and the back electrode, electrons pass through the dielectric layer due to the tunneling effect to excite the emission center in the emission layer. And an electroluminescent element. 2. The electric resistivity of the current limiting layer is 3 × 10 ³
˜1 × 10 ⁶ Ωcm, and its thickness is 1 to 200
The electroluminescence device according to claim 1, wherein the electroluminescence device has a thickness of μm. 3. The electroluminescent element according to claim 1, wherein the dielectric layer is a layer made of a rare earth oxide. 4. The electroluminescent device according to claim 3, wherein the rare earth oxide is europium oxide.