JPH0428197A - End-face radiating type electroluminescent element and its driving method - Google Patents

Abstract

PURPOSE:To provide an electroluminescent element of large luminous area and high luminous efficiency by varying the luminous brightness of each of luminous layers on both obverse and reverse faces of a substrate so as to control the luminous color of the layer. CONSTITUTION:When a power supply is connected between each of transparent electrodes 2 and each of cathodes 6 to impress voltage upon an end-face radiating type electroluminescent element, positive holes are injected respectively into positive hole transport layers 3a, 3b by the electrodes 2 while electrons are injected respectively into electron transport layers 5a, 5b by also the cathodes 6 when each of the electrodes 2 has been set to a voltage higher than that of each of the cathodes 6, and subsequently the positive holes and the electrons are transported respectively into luminous layers 4a, 4b. The positive holes and the electrons, all transported respectively into the luminous layers 4a, 4b, are recombined with one another so as to lead each of molecules present in the luminous layers 4a, 4b into its excited condition. Part of energy being discharged when each of the molecules in the excited condition returns again to the basic condition is discharged as EL light. The luminous color tone of each of the luminous layers 4a, 4b can be also controlled at the light of a wavelength range from blue to orange by varying each of a drive voltage, a frequency, a duty ratio and the like for the end-face radiating type electroluminescent element so as to vary the luminous brightness of each of the luminous layers 4a, 4b.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an electroluminescent device that directly converts electrical energy into optical energy by applying an electric field, and more specifically to an edge-emitting electroluminescent device that emits light from an end surface. Regarding elements.

[Prior Art] A conventional electroluminescent device (hereinafter abbreviated as EL device) has a small number of electrodes (one of which is a transparent electrode 8,
EL light emitted from the light emitting layer 10 was extracted to the outside through the glass substrate 7. However, since the EL light emitted from the light emitting layer 10 is emitted isotropically in all directions, the insulating layer 9
Due to the difference in refractive index between the transparent electrode 8 and the glass substrate 7, a portion of the light is totally reflected at their respective interfaces, and the amount of light extracted to the outside is at most 10% of the total amount of light emitted, which is poor efficiency.

In order to improve this, an edge-emitting type E as shown in Fig.
An L element has been proposed, in which the picture electrodes 14 and 18 that sandwich the light emitting layer 16 are metal electrodes with high reflectance, and the EL light emitted from the light emitting layer 16 is repeatedly totally reflected between both electrodes, and finally It is taken out to the outside of the EL element from the end face of the light emitting layer. In this case, most of the total amount of emitted light is taken out to the outside of the EL element, excluding the amount absorbed inside the EL element, resulting in very high efficiency.

[Problem to be solved by the invention] The thickness of the light-emitting layer of an EL device is 1,000 to 10,000 layers, but recently, EL devices have attracted attention because they can accommodate various emission wavelengths and can be easily made into a large area. In the case of an organic thin film type EL device that uses an organic thin film as a light emitting layer, the thickness of the light emitting layer is as thin as 50 to 1,000 people. The area becomes significantly smaller, which may be inconvenient depending on the application.

For example, when used as a light source for an image reading device, the reading speed is restricted due to the narrow irradiation area of the document. Similar problems also occur when an inorganic thin film type EL element is an edge-emitting type.

The present invention aims to solve the above-mentioned problems, and its purpose is to provide an electroluminescent device that has a large light emitting area and high luminous efficiency, and further provides an electroluminescent device that can control the color tone of light emission. It's about doing.

[Means for Solving the Problems] The present invention provides an end face characterized in that a transparent electrode, at least one light-emitting layer, and another electrode having light reflecting ability are laminated in this order on both the front and back surfaces of a transparent substrate. When the emission spectra of the light-emitting layers of the light-emitting electroluminescent element and the edge-emitting electroluminescent element are different on both the front and back sides of the transparent substrate, the color of the emitted light is controlled by changing the luminance of the light-emitting layers on both the front and back sides of the substrate. The present invention relates to a method for driving an edge-emitting electroluminescent device, characterized in that:

[Example] FIG. 1 is a sectional view of a main part of an edge-emitting type organic thin film type EL element which is an example of the present invention. In FIG. 1, a substrate 1 transmits EL light emitted from light emitting layers 4a and 4b, and also serves as a light guide path for the EL light.
It is sufficiently transparent in the wavelength range of EL light, and its material is generally a transparent resin sheet such as glass, polyethylene terephthalate, or polyimide.

The transparent electrode 2 is an anode, and the hole transport layers 3 a, 3 b
E is injected into the hole and released in the light-emitting layers 4a and 4b.
It allows L light to pass through the substrate 1, and is made of nickel, gold, platinum, palladium, alloys of these, or tin oxide (SiOx).
), indium tin oxide (TO), iodide steel, and other metals with a large work function, their alloys, and compounds are used, and the thickness is 100 to 5000, preferably 2.
00-1500 people.

The hole transport layers 3a and 3b are layers that transport holes injected from the transparent electrode 2 to the light emitting layers 4a and 4b. Examples include high-molecular compounds with excellent hole-transporting ability and low-molecular-weight compounds with excellent hole-transporting ability. Examples of low molecular weight compounds include triphenylamines, stilbene derivatives, oxadiazoles, etc. Specific examples thereof include the following.

(H-1) (H-31 (H-4) The thickness thereof is 200 to 5000 layers, preferably 500 to 1000 layers. Also, the hole transport layers 3a and 3b
The compounds used may be the same or different.

The light emitting layers 4a and 4b are layers that emit light by being excited by the recombination of holes transported from the hole transport layers 3a and 3b and electrons transported from the electron transport layers 5a and 5b, and are made of fluorescent organic compounds. Specific examples thereof include the compounds shown below.

(E-1) (E-2) Also, its thickness is 50 to 5000 people, preferably 1
00 to 1000 people. In addition, the light emitting layers 4 a, 4 b
The compounds used in the light-emitting layer 4a may be the same or different, but when different compounds are used, for example, (E
When the compound (E-9) is used in the light-emitting layer 4b and the compound (E-1) is used in the light-emitting layer 4b, as shown in FIG. This emits blue light (emission peak is about 460 nm), and when both are emitted at the same time, a mixture of these colors can be obtained.

The electron transport layers 5a and 5b are layers that transport electrons injected from the cathode 6 to the light emitting layers 4a and 4b, and various conventionally known electron injection and transport materials have been used, such as those shown below. oxadiazole derivatives can be used.

(T-3) (T-4) The thickness is 200 to 5000 people, preferably 500 to 1000 people. In addition, the charge transport layer 5a,
The compounds used for 5b may be the same or different.

The cathode 6 not only injects electrons into the electron transport layers 5a and 5b, but also serves as a reflector that completely reflects the EL light emitted from the light emitting layers 4a and 4b and confines it within the EL element. Small metals such as silver, tin, magnesium, manganese, aluminum or alloys thereof are used. Also, its thickness is relatively thick, usually 8.
The thickness is 00 Å or more, preferably 1500 Å or more.

All of the above layers are normally formed one after another by a vacuum evaporation method, but the transparent electrode 2 and cathode 6 may be formed by a sputtering method or the like.

Depending on the compound used as the light-emitting layer, holes or electrons may be injected well without going through the hole transport layer or electron transport layer, and in this case, either the hole transport layer or the electron transport layer, Or it is also possible to omit both. For example, when compound (E-7) is used in the light emitting layer, the electron transport layer is omitted, and when compound (E-8) is used in the light emitting layer, even if the hole transport layer is omitted, the brightness of the device is almost negligible. It does not change. Further, a protective layer may be provided on the cathode 6 if necessary.

Next, the operation of this embodiment will be explained. When a power source is connected between the transparent electrode 2 and the cathode 6 and a voltage is applied, when the voltage of the transparent electrode 2 becomes higher than that of the cathode 6, holes are transferred from the transparent electrode 2 to the hole transport layers 3a and 3b. Electrons are injected from the cathode 6 into the electron transport layers 5a and 5b, respectively, and then transported to the light emitting layers 4a and 4b. Luminescence N4a, 4
The holes and electrons transported into the light emitting layer 4 are recombined.
a, 4b to an excited state (When the molecules in the excited state return to the ground state, part of the energy released is emitted as EL light. At this time, the EL light is emitted equally in all directions. Although the light is emitted in the direction, it undergoes repeated total reflection at the cathode 6, and finally its traveling direction aligns with the end face direction.

At this time, the substrate 1 includes the transparent electrode 2 and the hole transport layer 3a.

3b, light emitting layers 4a, 4b and electron transport layer 5a,
5b (and its cross-sectional area is also considerably large), the substrate 1 serves as a light guide path for the EL light, and most of the EL light is emitted from the end surface of the substrate 1.

Further, when different compounds are used in the light-emitting layers 4a and 4b, for example, when the above-mentioned compounds (E-1) and (E-9) are used, they emit red and blue light respectively when used alone, but
When both are emitted simultaneously, spectral components exist over the entire visible light range, resulting in white light.

In addition, by changing the driving voltage, frequency, duty ratio, etc., and changing the luminance of each of the light emitting layers 4a and 4b, the color tone of the light emitted by the EL element can be controlled to light in any wavelength range from blue to Enoki. I can do it.

As described above, by making the organic thin film type EL element an edge-emitting type, the light emitting area is large (and the luminous efficiency is high.
It became possible to use it as an L element. Furthermore, by using materials with different emission spectra for each light-emitting layer and changing the luminance of each light-emitting layer, it has become possible to control the color tone of the light emitted by the EL element.

In this embodiment, an example in which an organic thin film type EL element is an edge-emitting type has been described, but it goes without saying that the present invention can also be applied to an inorganic thin film type EL element.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, it is possible to provide an edge-emitting thin film electroluminescent device that has a large light-emitting area, good light-emitting efficiency, and can control the color tone of light emission.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of essential parts of an edge-emitting type organic thinned electroluminescent device according to an embodiment of the present invention, and FIG.
, 4b is a graph showing the relationship between the wavelength and emission spectrum intensity of each light-emitting layer when fluorescent organic compounds E-1 and E-9 are used, and FIG. 3 is a cross-sectional view of the main part of a conventional electroluminescent device. , FIG. 4 is a sectional view of a main part of a conventional edge-emitting type electroluminescent device. 1 Substrate, 2... Transparent electrode (anode), 3 Hole transport layer, 4... Light emitting layer, 5... Electron transport layer, 6 Cathode, 7... Glass substrate, 8... Transparent electrode , 9... Insulating layer, lO... Light emitting layer, 11... Insulating layer, 12...
Metal electrode, 13...Glass substrate, 14 Metal electrode, 15.
Insulating layer, 16. Light emitting layer, 17. Insulating layer, 18. Metal electrode.

Claims (3)

[Claims] (1) An edge-emitting electroluminescent device characterized in that a transparent electrode, at least one light-emitting layer, and another electrode having light reflecting ability are laminated in this order on both the front and back surfaces of a transparent substrate. (2) The edge-emitting electroluminescent device according to claim 1, wherein the emission spectrum of the light-emitting layer is different on both the front and back surfaces of the substrate. (3) Edge emission in which a transparent electrode, at least one light-emitting layer, and another electrode having light reflecting ability are laminated in this order on both the front and back sides of a transparent substrate, and the emission spectrum of the light-emitting layer is different on both the front and back sides of the transparent substrate. In the type electroluminescent device,
A method for driving an edge-emitting electroluminescent device, the method comprising controlling the color of emitted light by changing the luminance of light-emitting layers on both the front and back surfaces of a substrate.