JPH0733433Y2 - Thin film EL device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a thin film EL element which is a light emitting device utilizing electroluminescence (hereinafter referred to as EL).

[Prior Art] EL is a phenomenon that emits light when an electric field is applied to a phosphor.
An EL display is one that emits light by applying an electric field to a phosphor coated on a glass plate or a transparent organic film.
EL displays include a dispersion type EL in which phosphor powder such as ZnS is dispersed in a dielectric such as cellulose, and a thin film EL in which a vapor-deposited thin film in which ZnS is doped with Mn is sandwiched with an insulating layer.

The structure of a typical thin film EL device is shown in FIG. In FIG. 7, A indicates the viewing side, but IT is on the back surface of the glass substrate 1.
The front transparent electrode layer 2 made of O or the like is formed, the light emitting layer 4 is sandwiched by the front insulating layer 3 and the back insulating layer 5, and is disposed on the back surface of the front transparent electrode layer 2. A back electrode layer 6 made of a vapor deposition film or the like is formed, and a power source 7 is connected between the front transparent electrode layer 2 and the back electrode layer 6.

As described above, a normal thin film EL element is of a reflection type, but in some cases, a transmission type light emitting element is required.
Therefore, when a transmissive light emitting element is required, both the front electrode layer and the back electrode layer are formed of transparent electrode films.

[Problems to be Solved by the Invention] When the thin film EL element is a reflection type, the light emitted from the light emitting layer to the back surface is reflected to the front surface by the back electrode layer composed of a metal deposition film or the like. Excellent display brightness.
However, in the case of a transmissive EL element, the light emitted from the light emitting layer to the back surface does not pass through the transparent electrode layer on the back surface as it is and is reflected, so that the display brightness becomes weak.

The present invention has been made in view of the above problems of the transmissive thin film EL element, and an object of the present invention is to provide a transmissive thin film EL element having excellent display brightness.

[Means for Solving the Problems] The thin film EL device of the present invention comprises a transparent substrate, a transparent front electrode layer formed on the back surface of the substrate, and a low refractive index layer and a high refractive index layer. A front insulating layer formed of an insulating layer having a different refractive index on the front electrode layer such that the high refractive index layer is on the front electrode side; and a light emitting layer formed on the front insulating layer. A low-refractive index layer and a high-refractive index layer, two layers of insulating layers having different refractive indices, and a back insulating layer formed on the light-emitting layer so that the low-refractive index layer is on the light-emitting layer side; The refractive index of the low-refractive-index insulating layer consisting of a back electrode layer formed on the back insulating layer, n _{0 in} the light emitting layer serving as a medium into which light is incident, and the front or back low insulating layer serving as a thin film through which light is transmitted. Is n, and the refractive index of the front or back high-refractive-index insulating layer, which is a thin film through which light is transmitted, is ns. Can n _0> n
<Ns, where the refractive index of the front and back low-refractive-index insulating layers that become thin films that transmit light is n ₀ , and the refractive index of the front- or back-side high-refractive-index insulating layers that become thin films that transmit light Where n is n ₀ <n> ns, where n is the refractive index of the front or back electrode layer serving as a substrate that supports a thin film that transmits light, and the front or back surface is a thin film that transmits light. Where d is the film thickness of the low-refractive index or high-refractive index insulating layer and λ is the emission peak wavelength of the light emitting layer, each insulating layer on the viewing side has the following expression n · d = (2N−1) λ / 4 The gist of each insulating layer on the side opposite to the viewing side is to satisfy the following equation n · d = (2N) λ / 4 (where N is a natural number).

The transparent substrate used in the thin film EL device of the present invention may be a glass substrate or a resin substrate. Further, this transparent substrate may be slightly colored as long as it has translucency.

As the material of the transparent electrode layer formed on the front surface or the back surface of the thin film EL element, various transparent conductive materials can be used, but ITO (indium-tin oxide), tin dioxide or the like is usually used. The method of forming the electrode layer is
Various vapor deposition methods or sputtering, spray-firing methods can be used.

Yellow-orange emission ZnS: Mn (emission peak wavelength λ = 58
5 nm), green emission ZnS: TbF ₃ (emission peak wavelength λ = 550 n
m), CaS: Eu emitting red light (emission peak wavelength λ = 610 nm), and the like can be used.

As the low refractive index material used for the insulating layer, MgF ₂ (refractive index n
= 1.38), SiO ₂ (n = 1.45), Al ₂ O ₃ (n = 1.62), etc., and high refractive index materials include Y ₂ O ₃ (n = 1.89) and Zr.
O ₂ (n = 2.1), TiO ₂ (n = 2.3), etc., but thin film EL
It is necessary to select an appropriate combination for the insulating layer of the device, paying attention to the characteristics such as high dielectric constant and high breakdown voltage.

The combination of the low refractive index insulating layer and the high refractive index insulating layer is n ₀ <n> ns or n _{0 in} relation to the refractive index of the light emitting layer.
It is determined to satisfy the relationship of> n <ns. Further, the relationship between the medium into which light is incident, the thin film through which light is transmitted, and the substrate supporting the thin film is determined for each low-refractive index insulating layer or high-refractive index insulating layer, and the film thickness is set.

[Operation] When a voltage is applied between the front electrode layer and the back electrode layer, light having a peak wavelength λ is emitted from the light emitting layer.

Therefore, in FIG. 1, when the viewpoint is on the left side and the viewing side is the front surface, the refractive index of the light-emitting layer or the low-refractive-index insulating layer on the front surface, which is the medium into which light is incident, is n ₀ , and the thin film through which light is transmitted. N is the refractive index of the low or high refractive index insulating layer on the front side, ns is the refractive index of the high refractive index insulating layer or the front electrode layer on the front side that is the substrate that supports the thin film that transmits light, and transmits the light. Where d is the thickness of the low-refractive index or high-refractive index insulating layer on the front surface, which is a thin film, and λ is the emission peak wavelength of the light-emitting layer, the combination of n ₀ , n, and ns in the reference numeral of FIG. (4, 3
2, 31) and (31, 32, 2), but there is a relationship of n ₀ <n> ns or n ₀ > n <ns, and a low refractive index or a high refractive index of the front surface that becomes a thin film that transmits light. The film thickness d of the insulating layer is set so as to satisfy the following equation: n · d = (2N−1) λ / 4 (1) (where N is a natural number). The reflectance of the light emitted to the front surface by the emission of the layer is minimal.

Further, the refractive index of the light-emitting layer serving as a medium into which light enters or the low-refractive-index insulating layer on the back surface is n ₀ , and the low-refractive index or the high-refractive-index insulating layer on the back surface serving as a thin film that transmits light is n, Ns is the refractive index of the high refractive index insulating layer or the back electrode layer on the back surface that serves as a substrate that supports the thin film that transmits light, and ns is the thickness of the low refractive index or high refractive index insulating layer on the back surface that is the thin film that transmits light. d,
If the emission peak wavelength of the emission layer was changed to lambda, in the code of Figure 1, n _0, n, the combination of the ns is a (4,51,52) and (5152,6), n ₀ <n > Ns or n ₀ > n <ns
And the film thickness d of the low-refractive-index or high-refractive-index insulating layer on the back surface, which is a thin film that transmits light, is expressed by the following equation: n · d = (2N) λ / 4 (2) ( However, since N is set so as to satisfy the relation of (N is a natural number), the reflectance of the light emitted to the back surface by the light emission of the light emitting layer is maximized.

Therefore, the luminance of light emission is superior to that of the conventional transmissive thin film EL element. Further, since the applied voltage may be lower than that of the conventional element in order to obtain the same brightness, the life characteristics represented by the emission time at which the initial brightness is reduced by half are remarkably improved. In the above description, the case where the viewpoint is on the left side in FIG. 1 has been described. However, when the viewpoint is on the left side in FIG. 1 and the viewing side is the back side, the back side should satisfy the first expression. The front side may satisfy the second formula.

[Example] A preferred example of the present invention will be described together with a comparative example.
The effect of the present invention will be clarified.

(Example 1) FIG. 1 shows a partial cross-sectional view of a thin film EL device of the present invention. First, a transparent front electrode layer 2 made of ITO is formed on the back surface of a glass substrate 1 with a film thickness of 200 nm by an ion plating method.
Formed by.

Then, the front insulating layer 3 including the high refractive index layer 31 and the low refractive index layer 32 was formed on the front electrode layer 2 by the electron beam evaporation method. The high refractive index layer 31 is made of a high refractive index material Y ₂ O ₃ (refractive index ns = 1.89), and the low refractive index layer 32 is made of a low refractive index material SiO ₂ (refractive index n = 1.45). And n ₀ > n <
I tried to satisfy the relationship of ns. Further, based on the equation (1), the reflectance of the light from the light emitting layer is reduced.
The front high-refractive-index insulating layer 31 was formed with a thickness of 200 nm, and the front low-refractive-index insulating layer 32 was formed with a thickness of 100 nm.

Then, on the formed front low-refractive-index insulating layer 32, yellow-orange light emission (emission peak wavelength λ = 58
The light emitting layer 4 made of ZnS: 0.5 wt% Mn (5 nm) was formed to a film thickness of 600 nm. Further, a back insulating layer 5 composed of a low refractive index layer 51 and a high refractive index layer 52 was laminated on the surface of the formed light emitting layer 4. Like the front insulating layer 3, the high refractive index layer 52 is made of a high refractive index material Y ₂ O ₃ (refractive index ns = 1.89), and the low refractive index layer 51 is made of a low refractive index material SiO 2. ₂ (Refractive index n = 1.4
5) is used to satisfy the relationship of n ₀ > n <ns. In addition, the thickness of the back low-refractive-index insulating layer 51 is set to 20 according to the formula (2) so as to increase the reflectance of light from the light emitting layer.
The thickness of the back high-refractive-index insulating layer 52 was 200 nm.

Finally, a transparent back electrode layer 6 made of ITO is formed on the back high-refractive-index insulating layer 52 by the ion plating method to a film thickness of 20.
A 0 nm-thick film was formed to produce a thin film EL device.

A voltage was applied to the manufactured thin film EL element of this example to cause it to emit light, and the relationship between the wavelength of light from the light emitting layers in the front insulating layer 3 and the back insulating layer 5 and the reflectance was measured. ) And the results shown in FIG. 2 (b) were obtained. FIG. 2 (a) shows the wavelength and reflectance of light from the light emitting layer incident on the front insulating layer, and it was confirmed that the reflectance was minimum at the peak wavelength λ of 585 nm. Also, the second
FIG. 6B shows the wavelength and reflectance of the light from the light emitting layer incident on the back insulating layer, and it was confirmed that the reflectance was maximum at the peak wavelength λ of 585 nm.

Subsequently, the change in luminance according to the driving voltage of the light emitting device of this example was measured. For comparison, the front insulating layer 3 is set to Y ₂ O ₃
(Thickness 300 nm) One layer, and the back insulating layer 5 is Y ₂ O ₃ (thickness 400
For a comparative example in which the thickness is one layer and other configurations are exactly the same as in Example 1, the change in luminance due to the driving voltage was measured in the same manner. The obtained results are shown in FIG.

As is clear from FIG. 5, in order to obtain the same luminance of 80 cd / m ² , the driving voltage of this example is lower than that of the comparative example by about 30 V, and the luminance of this example is also lower than that of the comparative example. It was confirmed that 20 to 30% higher value could be obtained.

Next, with respect to the light emitting elements of this example and the comparative example, changes in luminance with respect to the light emitting time of the light emitting element were measured, and the obtained results are shown in FIG. As is clear from FIG. 6, the emission time at which the initial luminance is reduced by half is found to be about 2 to 5 times longer in this example than in the comparative example, and the life characteristics of this example are excellent. It was confirmed.

(Example 2) The light emitting layer 4 emits green light (emission peak wavelength λ = 550 nm).
ZnS: 3 wt% TbF ₃ (thickness: 600 nm) was used, and a transmissive thin-film EL element having the same structure as in Example 1 was manufactured. In addition,
The front surface insulating layer 3 has a thickness of the front low refractive index insulating layer 32 of 95 nm and a front high refractive index insulating layer 31 based on the equation (1) so as to reflect and reduce the emission peak wavelength of 550 nm of the light emitting layer 4. Is 200 nm, and the back surface insulating layer 5 has a back surface low refractive index insulating layer 51 based on the formula (2) so as to increase reflection.
The thickness of the back high-refractive-index insulating layer 52 was 190 nm.

A voltage was applied to the produced thin film EL element of this example to cause it to emit light, and the relationship between the wavelength of light from the light emitting layers in the front insulating layer 3 and the back insulating layer 5 and the reflectance was measured. ) And the results shown in FIG. 3 (b) were obtained. FIG. 3 (a) shows the wavelength and reflectance of light from the light emitting layer incident on the front insulating layer, and it was confirmed that the reflectance was minimum at the peak wavelength λ of 550 nm. Also, the third
FIG. 6B shows the wavelength and the reflectance of the light from the light emitting layer incident on the back insulating layer, and it was confirmed that the reflectance was maximum at the peak wavelength λ of 550 nm.

Next, when the change in the luminance due to the driving voltage was measured in the same manner as in Example 1, it was confirmed that, as in Example 1, the present example had better luminance and a lower driving voltage than the comparative example. . Furthermore, when the life characteristics were measured by the same method as in Example 1, the emission time at which the initial luminance was reduced to half was found to be in Example 1.
Similarly, it was found that this example is about 2 to 5 times longer than the comparative example, and it was confirmed that the life characteristics of this example are excellent.

Example 3 The light emitting layer 4 emits red light (emission peak wavelength λ = 610 nm).
Using CaS: 0.1 wt% Eu (film thickness 600 nm), a transmissive thin film EL device having the same structure as in Example 1 was manufactured. In addition,
The front surface insulating layer 3 has a film thickness of the front low refractive index insulating layer 32 of 105 nm and a front high refractive index insulating layer 31 based on the equation (1) so as to reflect and reduce the emission peak wavelength of 610 nm of the light emitting layer 4.
Of the rear low-refractive-index insulating layer 51 based on the formula (2) so that the back-side insulating layer 5 increases reflection.
Was 210 nm, and the film thickness of the back high-refractive-index insulating layer 52 was 210 nm.

A voltage was applied to the fabricated thin film EL device of this example to cause it to emit light, and the relationship between the wavelength of light from the light emitting layers in the front insulating layer 3 and the back insulating layer 5 and the reflectance was measured. ) And the results shown in FIG. 4 (b) were obtained. FIG. 4 (a) shows the wavelength and reflectance of the light from the light emitting layer incident on the front insulating layer, and it was confirmed that the reflectance was minimum at the peak wavelength λ of 610 nm. Also, the fourth
FIG. 6B shows the wavelength and the reflectance of the light from the light emitting layer incident on the back insulating layer, and it was confirmed that the reflectance was maximum at the peak wavelength λ of 610 nm.

Next, when the change in the luminance due to the driving voltage was measured in the same manner as in Example 1, it was confirmed that, as in Example 1, the present example had better luminance and a lower driving voltage than the comparative example. . Furthermore, when the life characteristics were measured by the same method as in Example 1, the emission time at which the initial luminance was reduced to half was found to be in Example 1.
Similarly, it was found that this example is about 2 to 5 times longer than the comparative example, and it was confirmed that the life characteristics of this example are excellent.

[Advantages of the Invention] As described in detail above, the present invention provides the front insulating layer and the back insulating layer sandwiching the light emitting layer with two or more optical multilayer films each including a low refractive index layer and a high refractive index layer. By limiting the thickness of the insulating layer according to the emission peak wavelength of the light emitting layer and the refractive index of the insulating layer, the emission peak wavelength of the light emitting layer is selectively reflected and reduced in the front insulating layer, and the back insulating layer is reduced. This is to selectively increase the emission peak wavelength of the light emitting layer by reflection, and compared with the conventional transmissive thin film EL element,
The brightness can be significantly improved. Although the present invention is particularly effective when applied to a transmissive thin film EL element, it is an ordinary reflective thin film using an Al film as a back electrode layer.
It is also effective when applied to EL devices. The thin film of the present invention
In order to obtain the same brightness, the EL device requires a lower applied voltage than the conventional device, so that the burden on the device is reduced and the life characteristics represented by the emission time at which the initial brightness is reduced by half are significantly improved.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view of a thin film EL device of the present invention, and FIGS. 2 (a) and 2 (b) are wavelengths and reflectances of light from the light emitting layers incident on the front insulating layer and the back insulating layer in Example 1. 3 (a) and 3 (b) are diagrams showing the relationship between the wavelength of light from the light emitting layer incident on the front insulating layer and the back insulating layer and the reflectance in Example 2. 4 (a) and 4 (b) are diagrams showing the relationship between the wavelength and the reflectance of the light from the light emitting layer incident on the front insulating layer and the back insulating layer in Example 3, and FIG. FIG. 6 is a diagram showing changes, FIG. 6 is a diagram showing changes in luminance with respect to light emission time, and FIG. 7 is a sectional view of a conventional thin film EL element. 1 ... Substrate, 2 ... Front electrode layer, 3 ... Front insulating layer, 31
...... Front high refractive index insulating layer, 32 …… Front low refractive index insulating layer,
4 ... Emitting layer, 5 ... Back insulating layer, 51 ... Back low refractive index insulating layer, 52 ... Back high refractive index insulating layer, 6 ... Back electrode layer

Claims (1)

[Scope of utility model registration request] 1. A high refractive index comprising a transparent substrate, a transparent front electrode layer formed on the back surface of the substrate, and two insulating layers having different refractive indices, a low refractive index layer and a high refractive index layer. A front insulating layer formed on the front electrode layer so that the layer is on the front electrode side; a light emitting layer formed on the front insulating layer; and a low refractive index layer and a high refractive index layer. A back insulating layer formed on the light emitting layer so that the low refractive index layer is on the light emitting layer side, and a back electrode layer formed on the back insulating layer. consists of a, the front of the light emitting layer in which light as a medium which enters n _0, becomes the refractive index of the light becomes a thin film that transmits the front or back of the low refractive index insulating layer n, a thin film through which light is transmitted or ns the refractive index of the back of the high refractive index insulating layer, and the time a n _0> n <ns, thin to transmit light N ₀ the refractive index of the front and rear of the low refractive index insulating layer becomes, the refractive index of the light becomes a thin film that transmits the front or back of the high refractive index insulating layer n, light support the thin film that transmits When the refractive index of the front or back electrode layer serving as a substrate is ns, n ₀ <n> ns, and the refractive index of the low or high refractive index insulating layer of the front or back is a thin film through which light is transmitted. When the film thickness is d and the emission peak wavelength of the light emitting layer is λ, each insulating layer on the viewing side has the following formula: n · d = (2N-1) λ / 4 A thin film EL device characterized by satisfying the expression n · d = (2N) λ / 4 (N is a natural number).