WO2024184995A1 - Electroluminescent device, and manufacturing method and display device of same - Google Patents

Abstract

According to the present invention, an electroluminescent device (13) comprises: a first color electroluminescent layer to an n-th color electroluminescent layer (n≧2), which emit different colors, in a direction intersecting the stacking direction; and a first electroluminescent layer (34) and a second electroluminescent layer (53) positioned on the positive electrode-side and the negative electrode-side, respectively, of a charge generation layer (40) in the stacking direction. The ratio of the thickness of the second electroluminescent layer (53) to the thickness of the first electroluminescent layer (34) adjacent thereto in the stacking direction via the charge generation layer (40) is greater than 4/15 and less than 2/3.

Description

Electroluminescent device, its manufacturing method and display device

This disclosure relates to an electroluminescent device, a manufacturing method thereof, and a display device.

Mass production of displays incorporating organic light-emitting diodes (also known as "Organic Light Emitting Diodes" or "OLEDs") has begun in earnest with thin displays for high-end smartphones and TVs. Displays using organic light-emitting diodes have become so widespread that they can be said to have become a central replacement for LCD displays as thin displays.

Thin-film laminate-type organic EL elements using an aluminum quinolinol complex ( _Alq3 ) in the electron transport layer and electroluminescent layer are known. However, in order to further improve the luminous efficiency of organic EL elements, development is being carried out in the following order of (1) to (3).
(1) A host-guest material in which a guest compound as a dopant is added to a host compound is used as a material for the electroluminescent layer, carrier (electron, hole) transport layer, or carrier (electron, hole) injection layer of an organic EL element.
(2) A fluorescent material, or a fluorescent material and a phosphorescent material, are used as a guest compound (dopant) for the electroluminescent layer.
(3) Improved guest compounds such as thermally activated delayed fluorescent (TADF) materials or hyperfluorescent materials are used as dopants in the electroluminescent layer.

From the viewpoint of improving the current efficiency and life span of an organic EL element, the following (4) and (5) have been further investigated.
(4) The carrier balance is improved by modifying the carrier transport layer or the carrier injection layer.
(5) A tandem structure is adopted for the element structure of the light-emitting element (see, for example, Patent Documents 1 to 3).

Japanese Patent Application Publication No. 2015-32582 Japanese Patent Application Publication No. 2007-329054 International Publication No. 2010/113493

However, when the RGB light-emitting layers are painted separately to enable full-color display and a tandem structure is used, the advantages of using a tandem structure may not be fully realized.

One aspect of the present disclosure aims to provide a technology that can fully realize the advantages of adopting a tandem structure in an electroluminescent device capable of full-color display.

An electroluminescent device according to one embodiment of the present disclosure includes an anode layer, a cathode layer facing the anode layer in a stacking direction, a plurality of electroluminescent layers disposed between the anode layer and the cathode layer in the stacking direction and in a direction intersecting the stacking direction, and a charge generation layer disposed between two adjacent electroluminescent layers in the stacking direction, and the electroluminescent layers have a plurality of electroluminescent layers emitting the same color disposed in the stacking direction, and electroluminescent layers of a first color to an n-th color (n is an integer greater than or equal to 2) emitting different colors disposed in a direction intersecting the stacking direction, and in at least one pair of the two electroluminescent layers adjacent to each other in the stacking direction via the charge generation layer, the ratio of the thickness of the electroluminescent layer on one side of the stacking direction to the thickness of the electroluminescent layer on the other side of the stacking direction is greater than 4/15 and less than 2/3.

In addition, a display device according to one aspect of the present disclosure includes the electroluminescent device described above.

In addition, a method for manufacturing an electroluminescent device according to one embodiment of the present disclosure is a method for manufacturing the above-mentioned electroluminescent device, and includes a step of forming the electroluminescent layer and the charge generation layer alternately in the stacking direction on the anode layer or the cathode layer, and forming the electroluminescent layer by a vapor deposition method.

According to one aspect of the present disclosure, it is possible to fully realize the advantages of adopting a tandem structure in an electroluminescent device capable of full-color display.

1 is a plan view illustrating a schematic configuration of a display device according to a first embodiment of the present disclosure. FIG. 2 is a diagram illustrating a layer structure of the display device of FIG. 1. FIG. 3 is a diagram showing a schematic layer structure of the electroluminescent element having the layer structure shown in FIG. 2 . 4 is a flowchart showing an example of a method for manufacturing the electroluminescence device shown in FIG. 3 . 3 is a diagram for explaining the light emitting mechanism of an electroluminescent element of the electroluminescent device shown in FIG. 2 . 6 is a diagram for explaining the light emission mechanism of an electroluminescent element of an electroluminescent device according to a second embodiment of the present disclosure. FIG. FIG. 11 is a diagram illustrating a layer configuration of a display device according to a third embodiment of the present disclosure. 8 is a diagram for explaining the light emitting mechanism of an electroluminescent element of the electroluminescent device shown in FIG. 7 .

[Electroluminescent Device]
The electroluminescent element according to the embodiment of the present disclosure has an anode layer, a cathode layer, an electroluminescent layer, and a charge generation layer. The electroluminescent element according to the present disclosure has a plurality of electroluminescent layers overlapping in the stacking direction. In addition, in at least one pair of the two electroluminescent layers adjacent to each other via the charge generation layer, the thickness of one electroluminescent layer has a specific ratio to the thickness of the other electroluminescent layer. The electroluminescent element according to the present disclosure may adopt a known element configuration of a light-emitting element within the range in which the layers satisfy these conditions are provided. Hereinafter, the layer configuration of the electroluminescent element according to the present disclosure will be described mainly using an OLED as an example. In addition, in the present disclosure, the term "electroluminescent element" refers to a group of electroluminescent layers and various carrier functional layers arranged in the stacking direction. In addition, in the present disclosure, the term "electroluminescent device" refers to a group of a plurality of electroluminescent elements in a direction intersecting the stacking direction.

[Anode layer]
The anode layer is one of a pair of electrode layers, an anode and a cathode, and is an electrode layer for supplying holes to each layer constituting the electroluminescent device in the present disclosure. The anode layer has electrical conductivity. Furthermore, the anode layer has optical properties, for example, of reflecting a part of visible light and transmitting the rest. Typically, the anode layer includes both an electrode material that reflects visible light and an electrode material that transmits visible light.

In order to enhance hole injection properties, it is preferable to use a material with a relatively large work function (e.g., a material with a work function of 4.5 eV or more) as the material for the anode layer. Examples of electrode materials with large work functions include Pt (5.65 eV), Ir (5.25 eV), Ni (5.2 eV), Au and Pd (5.15 eV), as well as indium tin oxide (In-Sn-O).

Examples of electrode materials that reflect visible light include metal materials such as Al, Mg, Li, Ag, Pd, and Cu, as well as alloys of these metal materials (e.g., APC (Ag-Pd-Cu) alloy, etc.).

Examples of electrode materials that transmit visible light include thin films of transparent metal oxides (e.g., indium tin oxide (In-Sn-O), indium zinc oxide (In-Zn-O), and indium gallium zinc oxide (In-Ga-Zn-O)), thin films made of metallic materials such as Al, Mg, and Ag, and nanowires (NW) made of such metallic materials.

Among transparent metal oxides, In-Sn-O has a relatively high work function of 4.6 to 5.0 eV, making it suitable for use as a material for the anode layer. Furthermore, for the anode layer, a laminate (e.g., In-Sn-O/Ag) in which In-Sn-O is formed on the surface of a metal material can be used to improve the electrical conductivity of the electrode layer or to add the function of reflecting visible light.

[Cathode layer]
The cathode layer is the other of a pair of electrode layers, an anode and a cathode, and is an electrode layer for supplying electrons to each layer constituting the electroluminescent device in the present disclosure. The cathode layer is disposed opposite the anode layer in the stacking direction. The cathode layer has, for example, electrical conductivity and visible light transparency.

As the material for the cathode layer, a material with a relatively small work function is preferably used, for example, from the viewpoint of enhancing electron injection properties. Examples of electrode materials constituting the cathode layer include metal materials such as alkali metals, alkaline earth metals, and Al, alloys containing these, and nanowires (e.g., Ag nanowires, etc.). Examples of alloys include alloys of Mg and Ag, and Al doped with a small amount of Li.

[Electroluminescent Layer]
The electroluminescent layer is a layer that emits light of a predetermined color when an electric field is applied. The electroluminescent layer is usually composed of a light-emitting material, but may be a laminated structure of a plurality of functional layers that exhibit the function of electroluminescence as a whole, in which two or more functional layers corresponding to two or more functions for electroluminescence in a known electroluminescent layer are overlapped, such as a layer that emits immediate light and a layer that emits delayed light. In the present disclosure, the electroluminescent layer is arranged in a plurality of layers between the anode layer and the cathode layer in the stacking direction and in the direction crossing the stacking direction.

The electroluminescent layer has multiple electroluminescent layers emitting the same color arranged in the stacking direction. The number of electroluminescent layers emitting the same color overlapping in the stacking direction is not limited, but from the viewpoints of increasing the luminous efficiency and extending the element life, it is preferably 2 or more, and more preferably 3 or more. On the other hand, the number of electroluminescent layers emitting the same color overlapping in the stacking direction is preferably 5 or less, and more preferably 4 or less, from the viewpoints of suppressing an increase in the driving voltage, achieving a voltage resistance of the driving driver according to the driving voltage, and maintaining the flexibility of the electroluminescent element (suppressing an increase in the total thickness of the electroluminescent element).

Here, the same color refers to light colors in which, when there are two or more emission peak wavelengths, all emission peak wavelengths are within a range of ±5 nm from each other, and the maximum of the full width at half maximum of all emission peak wavelengths is 1.25 times or less the minimum of the full width at half maximum of the remaining emission peak wavelengths. Basically, when both the host material and guest compound constituting the electroluminescent layer are the same material or similar materials having the same skeleton, they emit the same color. However, in the top emission structure described below, the emission peak wavelength of each color may vary due to differences in the microcavity structure of each color. Therefore, in the top emission structure, the same color refers to light colors in which all emission peak wavelengths are within a range of ±10 nm, and the full width at half maximum satisfies the above range.

In addition, the electroluminescent layer is arranged in a direction intersecting the stacking direction, with electroluminescent layers of a first color to an n-th color (n is an integer greater than or equal to 2) emitting different colors. Here, different colors refer to colors of light that are not included in the same color as described above. A typical known example is a three-color luminescent layer of a red luminescent layer, a green luminescent layer, and a blue luminescent layer, as a multi-color electroluminescent layer for full-color display. In this way, arranging electroluminescent layers of multiple colors in a direction intersecting the stacking direction is sometimes called "color separation." Note that the number of electroluminescent layers of each color in the stacking direction is preferably the same for all luminescent colors. Below, the electroluminescent layer in this disclosure will be described mainly using the above three-color luminescent layers as an example, but the present disclosure is not limited to this. Note that the electroluminescent layers of different colors in the intersecting direction may be at the same position (height) in the stacking direction or at different positions (heights) (shifted).

In other words, in the present disclosure, when an electroluminescent device includes three electroluminescent layers of red, green, and blue, electroluminescent elements of each color are independently arranged in a direction intersecting the stacking direction, for example, a direction perpendicular to the stacking direction. In addition, in the stacking direction, red electroluminescent layers are overlapped with red electroluminescent layers, green electroluminescent layers are overlapped with green electroluminescent layers, and blue electroluminescent layers are overlapped with blue electroluminescent layers. Although electroluminescent layers of different colors may be further stacked in the stacking direction, from the viewpoint of improving color purity, it is preferable that only electroluminescent layers that emit the same color are arranged in the stacking direction. Stacking only electroluminescent layers of the same color in the stacking direction is advantageous from the viewpoint of more easily determining the thickness of the electroluminescent layer described later.

In an electroluminescent element in which three or more electroluminescent layers are arranged in the stacking direction, when a charge generation layer is interposed between each of the electroluminescent layers, optimizing the supply of carriers (electrons and/or holes) in the electroluminescent layer on one side of the stacking direction relative to the charge generation layer may cause the supply of electrons and/or holes to be rate-limiting, i.e., insufficient, in the electroluminescent layer on the other side. As a result, an imbalance in the carrier balance may occur between the multiple electroluminescent layers. In the present disclosure, the thickness of the electroluminescent layer in which the supply of carriers (electrons and/or holes) is rate-limiting is appropriately changed relative to the thickness of the adjacent electroluminescent layer in the stacking direction, thereby optimizing the carrier balance of all electroluminescent layers in the stacking direction.

In other words, in the electroluminescent device of the present disclosure, in at least one pair of two electroluminescent layers adjacent to each other in the stacking direction via a charge generation layer, the ratio of the thickness of the electroluminescent layer on one side in the stacking direction to the thickness of the electroluminescent layer on the other side in the stacking direction is greater than 4/15 and smaller than 2/3. In other words, in the present disclosure, the thickness of a specific electroluminescent layer is thinner than the thickness of the electroluminescent layer adjacent to it on the anode layer side or the electroluminescent layer adjacent to it on the cathode layer side by the above ratio. Note that "adjacent" in electroluminescent layers means that the electroluminescent layers included in the electroluminescent element are adjacent to each other in the stacking direction in terms of their positional relationship. In other words, another layer may be interposed between the electroluminescent layers adjacent to each other in the stacking direction.

The electroluminescent device of the present disclosure may also be configured to include an electroluminescent layer in which the thickness of the electroluminescent layer gradually decreases at a specific ratio from the anode layer side to the cathode layer side in the stacking direction, or the thickness of the electroluminescent layer gradually decreases at a specific ratio from the cathode layer side to the anode layer side.

Which side (cathode layer side or anode layer side) of adjacent electroluminescent layers in the stacking direction should have its thickness reduced by the above ratio can be determined according to the type of carrier that is rate-limiting and the location of the electroluminescent layer where the carrier is rate-limiting. For example, when either or both of the electrons supplied (injected and/or moved) from the cathode layer and the holes supplied from the charge generation layer are rate-limiting, the thickness of the electroluminescent layer on the cathode layer side in the stacking direction can be made thinner. Also, when either or both of the holes supplied from the anode layer and the electrons supplied from the charge generation layer are rate-limiting, the thickness of the electroluminescent layer on the anode layer side in the stacking direction can be made thinner. Furthermore, for example, when three or more electroluminescent layers are arranged in the stacking direction, the thickness of the electroluminescent layer with the larger carrier balance imbalance can be made thinner depending on the magnitude of the carrier balance imbalance between the electroluminescent layers, that is, the thickness of the electroluminescent layer with the larger carrier balance imbalance can be made thinner.

By adopting such a configuration, in the electroluminescent device of the present disclosure, the carrier balance in each electroluminescent layer in the stacking direction is optimized. The thickness of each electroluminescent layer in the stacking direction can be appropriately determined depending on various factors, such as the combination of materials of each layer in the electroluminescent element and the purpose of changing the thickness of the electroluminescent layer, as long as it is in accordance with the technical concept of the present disclosure.

If the ratio of the thickness of the electroluminescent layer on one side of the stacking direction to the thickness of the electroluminescent layer on the other side of the stacking direction is too small, the effect of correcting the carrier balance is small, and excess carriers are generated. Therefore, a current that does not contribute to light emission is generated, and power consumption may increase. On the other hand, if the ratio is too large, one of the carriers (electrons or holes) that generate excitons (excitons) in the electroluminescent layer described later may be clearly insufficient, and the expected light-emitting characteristics (exciton generation ability) of the electroluminescent layer may not be fully expressed. The above ratio is preferably greater than 4/15 from the viewpoint of suppressing the generation of excess carriers, and is preferably smaller than 2/3 from the viewpoint of fully expressing the light-emitting characteristics of the electroluminescent layer. In addition to the above viewpoint, the above ratio is more preferably 1/3 or more, and more preferably 1/2 or less, from the viewpoint of making it easier to change the thickness of each electroluminescent layer in the stacking direction.

By arranging a set of two electroluminescent layers with the above-mentioned specific ratio among the multiple electroluminescent layers arranged in the stacking direction, it is possible to reduce the unnecessary thickness that does not contribute to light emission in each electroluminescent layer. This makes it possible to reduce the driving voltage and the amount of material used. Furthermore, in the electroluminescent device of the present disclosure, the current injected into the electroluminescent element having multiple electroluminescent layers in the stacking direction can be made to contribute to light emission with less waste. Therefore, the electroluminescent device of the present disclosure can further improve the light emission efficiency compared to a conventional electroluminescent device with a tandem structure having multiple electroluminescent layers of the same thickness.

In addition, the thickness of each electroluminescent layer may be changed by the above ratio for only one color of an electroluminescent layer in an electroluminescent device having electroluminescent layers of multiple colors in a direction intersecting the stacking direction, or the thickness of any two or more colors may be changed uniformly or independently by the above ratio, within the range where the effects of the present disclosure can be obtained. Furthermore, the thickness ratio of each electroluminescent layer in the stacking direction may be constant for each luminescent color, or may be a different ratio within the above range.

Furthermore, the direction in which the thickness of the electroluminescent layer included in the electroluminescent element is changed in the stacking direction may be determined for each luminescent color within the range in which the effects of the present disclosure can be obtained. For example, the thickness of the electroluminescent layer of some luminescent colors may be set to decrease from one side to the other side, and the thickness of the electroluminescent layer of some other luminescent colors may be set to decrease from the other side to one side.

The direction in which the thickness of the electroluminescent layer in the stacking direction is changed and the individual thicknesses are determined, for example, according to the combination of materials of each layer in the electroluminescent device. The thickness of each electroluminescent layer in the stacking direction can be determined as a thickness according to the purpose by evaluating the carrier injection property and/or carrier transport property between each layer in the electroluminescent device by simulation and conducting a demonstration experiment based on the evaluation. In this case, by setting the multiple electroluminescent layers stacked in the stacking direction to be only the same color, the parameters of the carrier injection property and carrier transport property between different electroluminescent layers in the stacking direction can be made the same. This makes the simulation easier and makes it possible to determine the thickness of each electroluminescent layer more simply.

A preferred electroluminescent layer in the present disclosure is a host-guest electroluminescent layer that contains a host compound and a guest compound. A host-guest electroluminescent layer contains a small amount (e.g., about 0.1 to several mol%) of a fluorescent dopant or the like as a guest compound in a solid medium that is a host compound. In an electroluminescent layer doped with a guest compound in this way, the fluorescence of the host compound is completely lost, and instead, strong light emission that matches the fluorescence spectrum of the doped guest compound is obtained. This is because the excitation energy of the host compound is transferred to the guest compound. Due to this transfer of excitation energy, in a host-guest electroluminescent layer, light emission from a guest compound with higher quantum efficiency is obtained.

Furthermore, by doping the electroluminescent layer with a fluorescent dopant as a guest compound, the device life of the electroluminescent element is also improved dramatically. This is because the dopant, which is a guest compound, functions as a trap for carriers (electrons or holes) in the solid medium of the host compound, becoming a carrier recombination center and directly creating excitons in the solid medium. The process in which the created excitons relax to the ground state is called the deactivation process. The deactivation process includes a non-radiative process (thermal deactivation) and a radiative process (luminescence), and electroluminescence is the phenomenon in which luminescence occurs through the radiative process. By the guest compound functioning as a carrier trap, not only is the quantum efficiency of the electroluminescent layer improved, but the device life is also improved due to the increased probability of carrier recombination. As a result, the luminous efficiency of the electroluminescent layer and the device life of the electroluminescent element are improved.

In this way, the host-guest electroluminescent layer can effectively utilize carriers, and therefore the electroluminescent device disclosed herein can achieve high luminous efficiency of the electroluminescent layer and improved device life of the electroluminescent element.

The excitons generated during carrier recombination are classified into singlet excitons and triplet excitons. When a fluorescent dopant is used as the guest compound, singlet excitons contribute to light emission. When a phosphorescent dopant is used instead of a fluorescent dopant, triplet excitons contribute to light emission. According to the law of spin statistics, the generation ratio of singlet excitons and triplet excitons is 25% for singlet excitons and 75% for triplet excitons. Therefore, when a fluorescent dopant is used as the guest compound, that is, in the process of light emission of "fluorescence" emitted only from singlet excitons, the probability of generating excitons that can contribute to light emission is at most 25%.

In contrast, when a phosphorescent dopant is used as the guest compound, light can be extracted from triplet excitons, increasing the quantum efficiency three times that of a fluorescent dopant. Furthermore, by utilizing intersystem crossing, which is the inversion of the spin from singlet excitons to triplet excitons, theoretically all generated excitons can emit "phosphorescence" from triplet excitons. Therefore, when a phosphorescent dopant is used as the guest compound, quantum efficiency can be increased up to four times compared to when a fluorescent dopant is used.

　Known materials can be used as the light-emitting material for the electroluminescent layer. For example, examples of light-emitting materials constituting the blue electroluminescent layer include pyrene-based compounds and anthracene-based compounds, which are fluorescent dopants. Furthermore, examples of light-emitting materials constituting the red electroluminescent layer and the green electroluminescent layer include iridium complexes and palladium complexes, which are phosphorescent dopants.

In addition, complexes containing platinum group elements such as iridium or palladium that are used as phosphorescent dopants are very expensive even in small amounts, due to the fact that platinum group elements are produced in small quantities and are unevenly distributed, and it can be difficult to obtain a stable supply. Therefore, reducing the use of phosphorescent dopants that use these platinum group complexes is extremely important from the perspective of cost reduction and economic security.

In this way, a host-guest electroluminescent layer can be formed by doping a host compound with a guest compound. A host-guest electroluminescent layer can be formed by a co-evaporation method using multiple evaporation sources.

Various known examples of host-guest electroluminescent layer materials can be used. Examples of host compounds include known luminescent layer materials of each color. Examples of guest compounds include the above-mentioned fluorescent dopants and phosphorescent dopants, as well as TADF and hyperfluorescent materials. Further examples of fluorescent dopants include perylene, DPT, Coumarin 6, PMDFB, quinacridone, rubrene, BTX, ABTX, DCM, and DCJT. Further examples of phosphorescent dopants include Ir(ppy) ₃ , Ir(thpy) ₃ , Ir(t5m-thpy) ₃ , Ir(t- _5CF3 -py) ₃ , Ir(t-5t-py) ₃ , Ir(mt-5mt-py) ₃ , Ir(btpy) ₃ , Ir(tflpy) ₃ , Ir(piq) ₃ , Ir(tiq) ₃ , Ir(fliq) ₃ , FIrpic, FIr6, ppy, tpy, bzq, thp, op, bo, bt, bon, α-bsn, btp, ppo, C6, pq, β-bsn, and ppz.

In addition, in the currently mainstream electroluminescent elements, a fluorescent dopant is used as the guest compound in the electroluminescent layer that emits blue light, and a phosphorescent dopant is used as the guest compound in the electroluminescent layers that emit red and green light. In electroluminescent devices that include these electroluminescent elements, the quantum efficiency of the fluorescent dopant is lower than that of the phosphorescent dopant, both theoretically and in practice. Therefore, in the above electroluminescent devices, the amount of light emitted may be balanced by making the amount of current passed through the blue electroluminescent layer greater than the amount of current passed through the red and green electroluminescent layers.

However, if the amount of current flowing through the blue electroluminescent layer, which is a fluorescent electroluminescent layer using a fluorescent dopant as a guest compound, is increased in this way, the life of the blue electroluminescent layer will inevitably be shortened (the rate of deterioration will be increased). To address this, the total number of stacked blue electroluminescent layers in the stacking direction may be made greater than the total number of stacked red and green electroluminescent layers, which are phosphorescent electroluminescent layers in which the guest compound is a phosphorescent dopant. In other words, the total number of stacked fluorescent electroluminescent layers may be made greater than the total number of stacked phosphorescent electroluminescent layers. This configuration is advantageous for balancing the life of the electroluminescent layer and the amount of light emitted between the fluorescent electroluminescent layer and the phosphorescent electroluminescent layer in the electroluminescent element. The total number of stacked fluorescent electroluminescent layers is preferably one or two more than the total number of stacked phosphorescent electroluminescent layers. If the number of fluorescent electroluminescent layers is three or more more than the number of phosphorescent electroluminescent layers, it may be difficult to balance the life and the amount of light emitted between the phosphorescent electroluminescent layers of different colors. Or, the balance of the element structure of the electroluminescent device may be poor, resulting in a low manufacturing yield.

[Charge Generation Layer]
The charge generation layer is disposed between two electroluminescent layers adjacent to each other in the stacking direction. The charge generation layer is a layer that generates one or both of electrons and holes. Charges (charges or carriers) generated in the charge generation layer are supplied to the electroluminescent layers located on the anode layer side and the cathode layer side in the stacking direction. The charge generation layer may be made of a known charge generation material that exhibits the above-mentioned functions.

The charge generation layer may be composed of an electron generation layer that generates electrons and a hole generation layer that generates holes. An example of an electron generation layer is an n-type charge generation layer, and an example of a hole generation layer is a p-type charge generation layer. When holes are supplied from the anode layer and electrons are supplied from the cathode layer, the n-type charge generation layer generates electrons and the p-type charge generation layer generates holes.

For the p-type charge generation layer, a material containing an organic hole transport material and an organic electron accepting material (hole supply material) added in the range of 1 to 10% can be used. For the organic hole transport material, a known triarylamine organic compound can be used. An example of an organic electron accepting material is tetracyanoquinodimethane tetrafluoride (TCNQ-4F). For the material of the p-type charge generation layer, a material with sufficient hole generation ability is realized using all organic materials, such as the hole transport material and electron accepting material described above.

For the n-type charge generation layer, for example, a material containing an organic electron transport material and an inorganic metal material, Yb (ytterbium) or Li (lithium), which is added in the range of 5 to 20% and acts as an electron supply material, can be used. Examples of organic electron transport materials include oxadiazole compounds. Development is also underway to use organic materials for both the electron transport layer and the electron supply material in the n-type charge generation layer. For example, development is underway for organic electron supply materials such as BUPH1, BPen, p-MeO-Phen, p-NMe ₂ -Phen, and p-Pyrrd-Phen.

Currently, no organic electron supply material with sufficient electron supply properties has been found that can be used in an n-type charge generation layer, and therefore no n-type charge generation layer has been found that is formed entirely from organic materials and has sufficient characteristics. According to the present disclosure, by setting the thickness of each electroluminescent layer in the stacking direction as described above, it is possible to optimize the balance (carrier balance) between the electrons and/or holes supplied from the charge generation layer and the holes and/or electrons supplied from each electrode layer. Therefore, the present disclosure realizes an electroluminescent device with a tandem structure having multiple electroluminescent layers in the stacking direction that suppress the generation of excess carriers and have excellent power consumption.

[Other configurations]
The electroluminescent device of the present disclosure may further include other layers as long as the effects of the present disclosure can be obtained. Examples of other configurations include carrier functional layers such as a hole injection layer, an electron injection layer, a hole transport layer, an electron blocking layer, a hole blocking layer, and an electron transport layer.

The hole injection layer is disposed, for example, adjacent to the anode layer. The hole injection layer may be composed of a hole transport material and an electron accepting material. These materials are the same organic materials as those described for the p-type charge generation layer, and the specific material of the hole injection layer in the electroluminescent device may be the same as that of the p-type charge generation layer, or may be different.

The electron injection layer is disposed adjacent to the cathode layer, for example. The electron injection layer may be made of an electron transport material. Examples of electron transport materials constituting the electron injection layer include lithium fluoride (LiF), which is an inorganic material. Similarly to the n-type charge generation layer, the electron injection layer may be configured by forming an electron transport material from an organic material such as an oxadiazole compound, and doping it with a metal material (e.g., Li or Yb).

The LiF used in the electron injection layer exhibits excellent electron injection properties. On the other hand, the deposition of the carrier function layer, which contains inorganic materials such as Yb, not limited to LiF, is generally carried out at a higher temperature than the deposition of the electroluminescent layer and carrier function layer, which are made of organic materials, because the melting point of the inorganic material is high. This raises the risk of thermal damage to the organic material deposited earlier. Therefore, it is preferable to form the electroluminescent element using only organic materials, whenever possible.

Therefore, organic electron injection materials such as BUPH1, BPen, p-MeO-Phen, p-NMe ₂ -Phen and p-Pyrrd-Phen have been developed as they can have sufficient characteristics when combined with a cathode layer made of aluminum (Al) or the like, which can form a vapor deposition layer at a relatively low temperature. However, the electron injection ability of an electron injection layer made of an organic material is still inferior to that of an electron injection layer made of an inorganic material such as LiF. Therefore, when an electron injection layer made of an organic material is disposed adjacent to a cathode layer, the amount of electrons supplied to the electroluminescent layer formed on the cathode layer side may decrease. In such a case, the thickness of the electroluminescent layer on the cathode layer side is made thinner than the theoretical value in order to reduce the effect of the reduction in the amount of electrons supplied, thereby allowing the advantages of adopting a tandem structure to be fully realized.

The hole transport layer can be composed of an organic hole transport material, for example, a triarylamine organic compound.

The electron blocking layer, like the hole transport layer, can be made of an organic hole transport material. The material of the electron blocking layer can be the same as or different from that of the hole transport layer.

The hole blocking layer may be composed of an organic electron transport material, such as an oxadiazole compound. The material of the hole blocking layer may contain lithium quinoline (Liq) in addition to the electron transport material.

　Like the hole blocking layer, the electron transport layer can be composed of the organic electron transport material described above. The material of the electron transport layer can be the same as or different from that of the hole blocking layer.

The hole injection layer and electron injection layer may be arranged corresponding to the electrode layers, and are usually arranged adjacent to each electrode layer in the stacking direction. The hole transport layer, electron blocking layer, the electroluminescent layer, the hole blocking layer, the electron transport layer, and the charge generation layer may be arranged in a repeated manner in the stacking direction of the electroluminescent element.

The electroluminescent device of the present disclosure is suitable for a top-emission type electroluminescent device capable of full-color display. In a top-emission type electroluminescent device, the light extraction efficiency may be improved by utilizing the microcavity effect by adjusting the distance between the electrode layers according to the wavelength of light from the light-emitting layers of each color. For this purpose, one of the layers of the carrier functional layer may be made thicker. The electroluminescent device of the present disclosure has a so-called tandem structure that includes multiple electroluminescent layers and carrier functional layers corresponding to each electroluminescent layer in the stacking direction. Therefore, the thickness of the carrier functional layer for adjusting the distance between the electrode layers is suppressed, making it possible to further suppress the consumption of functionally unnecessary materials.

[stack]
The stack is a set of layers including one electroluminescent layer disposed between the anode layer and the charge generation layer, between the charge generation layers, and between the charge generation layer and the cathode layer in the stacking direction. The stack may include a carrier functional layer in addition to the one electroluminescent layer. A plurality of stacks may be disposed between the anode layer and the cathode layer in the stacking direction. In the present disclosure, the anode layer, the cathode layer, and the charge generation layer are not included in the stack.

The thickness of the stack is determined so that the amount of light emitted in the electroluminescent layer is a theoretical value or a value close to it. In the present disclosure, the thickness of each electroluminescent layer in the stacking direction can be set within the range of the ratio described above, so from the viewpoint of increasing the light-emitting efficiency of the electroluminescent device, it is preferable to set the thickness of the stack according to the thickness of the electroluminescent layer set in this way. From this viewpoint, the ratio of the thickness of the electroluminescent layer in the stack to the thickness of the stack in the stacking direction is preferably 0.05 or more and preferably 0.35 or less. Note that the thickness of the stack is determined by the sum of the thicknesses of the electroluminescent layer and the carrier functional layer in the stack, but among the electroluminescent layer and the carrier functional layer, layers with a very small thickness (for example, layers that may have a thickness of less than 1 nm) may be ignored when calculating the thickness of the stack.

[Method of manufacturing electroluminescent device]
In the manufacture of the electroluminescent device of the present disclosure, the thickness of each electroluminescent layer is controlled so that the ratio of the thickness of the electroluminescent layer on one side of the stacking direction to the thickness of the electroluminescent layer on the other side of the stacking direction, which are adjacent to each other in the stacking direction via a charge generation layer, is greater than 4/15 and smaller than 2/3. Otherwise, the electroluminescent device of the present disclosure can be manufactured by a known manufacturing method capable of manufacturing an electroluminescent device having a plurality of electroluminescent layers (tandem structure). The manufacturing method of the electroluminescent device may be a method including a step of forming an electroluminescent layer and a charge generation layer alternately in the stacking direction on an anode layer or a cathode layer. In addition, from the viewpoint of precisely controlling the thickness of each electroluminescent layer in the electroluminescent device of the present disclosure, it is preferable that at least the electroluminescent layer is formed by a vapor deposition method in the manufacturing method. The electroluminescent layer formed by the vapor deposition method is generally preferable from the viewpoint of high brightness and low voltage driving, and is also preferable from the viewpoint of realizing a high-definition display device because an electroluminescent device consisting of fine pixels can be formed with high precision. Furthermore, if a co-evaporation method using a plurality of evaporation sources is used, it is more preferable because it is possible to form a host-guest electroluminescent layer.

The electroluminescent device of the present disclosure has a specific layer structure repeated, and therefore can be manufactured by repeating the formation of a specific layer multiple times. In the present disclosure, it is preferable to form the electroluminescent layers having different thicknesses in the stacking direction by changing only the deposition time. With such a manufacturing method, it is possible to keep constant the conditions other than the deposition time in forming the electroluminescent layer (for example, the deposition rate controlled by the crucible temperature (deposition temperature) or deposition temperature, the ratio of the host compound to the guest compound (doping concentration of the guest compound), and the deposition mask that defines the pixel shape, etc.). Therefore, the variation in characteristics due to changes in the conditions of each electroluminescent layer in the stacking direction is suppressed, and the effect due to the difference in thickness of the electroluminescent layer in the stacking direction can be more significantly expressed.

[Display Device]
The display device according to the present disclosure includes the electroluminescent device described above. The display device according to the present disclosure can be configured in the same manner as a known display device having a known light-emitting device, except for having the electroluminescent device described above. Examples of the display device include a television set and a smartphone.

[Description of Specific Embodiments]
Hereinafter, the electroluminescent device, its manufacturing method, and display device of the present disclosure will be described in more detail with reference to the drawings, taking an electroluminescent device equipped with an organic light emitting diode (OLED) as an example. In this specification, for configurations related to different colors among similar basic configurations, a code indicating the color is further added to the code of the basic configuration. For example, the code R is further added to the configuration related to red, the code G is further added to the configuration related to green, and the code B is further added to the configuration related to blue.

[Embodiment 1]
<Configuration>
Fig. 1 is a plan view showing a schematic configuration of a display device 100 according to a first embodiment of the present disclosure. Fig. 1 shows a smartphone as an example of a display device. As shown in Fig. 1, the display device 100 includes a frame area NDA and a display area DA. The display area DA of the display device 100 includes a plurality of pixels PIX, each of which includes a red sub-pixel RSP, a green sub-pixel GSP, and a blue sub-pixel BSP.

Note that the pixel configuration of the display device of the present disclosure is not limited to the above configuration. In the display device of the present disclosure, for example, one pixel PIX may include subpixels of other colors in addition to the red subpixel RSP, green subpixel GSP, and blue subpixel BSP.

FIG. 2 is a diagram showing a schematic layer structure of the display device 100 in FIG. 1. As shown in FIG. 2, the display device 100 has a substrate 11, a buffer layer 12, a TFT (thin film transistor) layer 20 including pixel circuits, an electroluminescent device 13 and an edge cover film 16, a sealing layer 14, and an external functional layer 15 laminated in this order.

The substrate 11 is a glass substrate or a flexible substrate whose main component is a resin such as polyimide. For example, the substrate 11 can be made of two polyimide films and an inorganic film sandwiched between them. The buffer layer 12 can be made of an inorganic insulating layer that prevents the intrusion of foreign matter such as water and oxygen. The TFT layer 20 includes pixel circuits that control the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B.

The electroluminescent device 13 includes a red electroluminescent element 10R, a green electroluminescent element 10G, and a blue electroluminescent element 10B. The red electroluminescent element 10R includes an anode layer 21R and a cathode layer 22R, and includes a first electroluminescent layer 34R and a second electroluminescent layer 53R between these electrode layers. Similarly, the green electroluminescent element 10G includes an anode layer 21G, a first electroluminescent layer 34G, a second electroluminescent layer 53G, and a cathode layer 22G, and the blue electroluminescent element 10B includes an anode layer 21B, a first electroluminescent layer 34B, a second electroluminescent layer 53B, and a cathode layer 22B.

Here, FIG. 2 shows a configuration in which the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B are provided with the cathode layer 22R, the cathode layer 22G, and the cathode layer 22B, respectively. The electroluminescent device 13 of the present disclosure is not limited to this configuration. For example, the cathode layer 22R, the cathode layer 22G, and the cathode layer 22B may be a common electrode layer provided across the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B.

The sealing layer 14 that covers the electroluminescent device 13 is a layer that prevents foreign substances such as water and oxygen from penetrating into the electroluminescent device 13, and can be composed of, for example, two layers of inorganic sealing films and an organic film formed between them. The external functional layer 15 is a layer that adds various functions to the display device 100, such as optical control, a touch sensor, and surface protection.

The edge cover film 16 is insulating and covers the edges of each anode layer 21R, 21G, 21B. The edge cover film 16 is formed by applying an organic material such as polyimide or acrylic resin, and then patterning it by photolithography. The red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B are, for example, organic light emitting diodes (OLEDs).

The element configuration of the electroluminescent element 10 shown in FIG. 2 will be described with reference to FIG. 3. FIG. 3 is a schematic diagram showing the layer configuration of the electroluminescent element having the layer configuration shown in FIG. 2.

As shown in FIG. 3, the substrate 11, buffer layer 12, and TFT layer 20 are stacked in this order, and the electroluminescent element 10 is configured by stacking an anode layer 21, a set of layers 30, a first charge generation layer 40, a set of layers 50, and a cathode layer 22 on the TFT layer 20 in this order. The electroluminescent element 10 according to the present disclosure is of a top emission type (a structure in which light is extracted from the upper side, i.e., the cathode layer 22 side). In the electroluminescent element 10, for example, the anode layer 21 functions as the anode, and the cathode layer 22 functions as the cathode.

The layer set 30 is composed of a hole injection layer 31, a first hole transport layer 32, a first electron blocking layer 33, a first electroluminescent layer 34, a first hole blocking layer 35, and a first electron transport layer 36. The layer set 50 is composed of a second hole transport layer 51, a second electron blocking layer 52, a second electroluminescent layer 53, a second hole blocking layer 54, a second electron transport layer 55, and an electron injection layer 56. Between the layer set 30 and the layer set 50, a first charge generation layer 40 including an n-type first charge generation layer (electron generation layer) 41 and a p-type first charge generation layer (hole generation layer) 42 is disposed. In the electroluminescent element 10 of FIG. 3, the layer set 30 is also called a "first stack", and the layer set 50 is also called a "second stack". The layer set 30, the first charge generation layer 40, and the layer set 50 constitute an organic laminate 60.

The hole injection layer 31, the first hole transport layer 32, the first electron blocking layer 33, the first hole blocking layer 35, the first electron transport layer 36, the first n-type charge generation layer 41, the first p-type charge generation layer 42, the second hole transport layer 51, the second electron blocking layer 52, the second hole blocking layer 54, the second electron transport layer 55, and the electron injection layer 56 are carrier functional layers that contribute to at least one of the injection, movement, and generation of carriers (electrons or holes).

The electroluminescent element 10 is an electroluminescent element of a so-called tandem structure in which two electroluminescent layers, a first electroluminescent layer 34 and a second electroluminescent layer 53, are disposed between the anode layer 21 and the cathode layer 22 in the stacking direction. The first electroluminescent layer 34 and the second electroluminescent layer 53, which are stacked in the stacking direction, are both electroluminescent layers that emit the same color.

The thickness of the first electroluminescent layer 34 varies depending on the luminescent color. For example, the thickness of the first electroluminescent layer 34 of the red electroluminescent element and the green electroluminescent element is 25 nm to 50 nm, and the thickness of the first electroluminescent layer 34 of the blue electroluminescent element is 10 to 25 nm. The ratio (T2/T1) of the thickness (T2) of the second electroluminescent layer 53 to the thickness (T1) of the first electroluminescent layer 34 is greater than 4/15 and less than 2/3. More preferably, the ratio of the thickness of the second electroluminescent layer 53 to the thickness of the first electroluminescent layer 34 is greater than 1/3 and less than 1/2. Both the first electroluminescent layer 34 and the second electroluminescent layer 53 are host-guest electroluminescent layers.

<Method of manufacturing electroluminescent device>
Next, an example of a method for manufacturing the electroluminescent element 10 will be described with reference to Fig. 4. Fig. 4 is a flow chart showing an example of a method for manufacturing the electroluminescent element shown in Fig. 3.

As shown in FIG. 4, in step S1, an anode layer 21 is formed on the TFT layer 20. Specifically, an Ag layer and an In-Sn-O layer are formed in sequence using a sputtering method.

In step S2, a hole injection layer 31 is formed on the anode layer 21. Specifically, the hole transport material and the electron acceptor material are co-evaporated at a predetermined deposition rate by adjusting the deposition temperature and deposition time of each material so that they are laminated at a predetermined film thickness and ratio. Here, a deposition film is formed uniformly over the entire surface of the workpiece without using a fine metal mask.

In the specific description of the manufacturing method based on FIG. 4, a common layer is formed between electroluminescent elements of all colors for some of the carrier functional layers, but the manufacturing method of the present disclosure is not limited to this. The thickness of each carrier functional layer may be different for each color, for example, depending on the electroluminescent layer of each color. A carrier functional layer having a partially different thickness in this way can be formed by vapor deposition through a mask.

In step S3, a first hole transport layer 32 is formed on the hole injection layer 31. Specifically, the hole transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layer is deposited to a predetermined thickness. Here, the evaporated film is formed without using a fine metal mask.

In step S4, a first electron blocking layer 33 is formed on the first hole transport layer 32. Specifically, the hole transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the hole transport material is deposited to a predetermined thickness. Here, a fine metal mask is used to evaporate the material to a first thickness corresponding to each color. The first thickness may be the same for the electroluminescent element of each color, or it may be different.

In step S5, the first electroluminescent layer 34 is formed on the first electron blocking layer 33. Specifically, the host compound and the guest compound (dopant) are co-evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layers are laminated with a predetermined film thickness and guest compound concentration (dopant concentration). Here, a fine metal mask is used to evaporate materials according to each color while precisely controlling the thickness and guest compound concentration. The first electroluminescent layer 34 is evaporated so that the ratio of the thickness of the first electroluminescent layer 34 to the thickness of the layer set 30 in the stacking direction is 0.05 to 0.35.

In step S6, the first hole blocking layer 35 is formed on the first electroluminescent layer 34. Specifically, the deposition temperature and deposition time are adjusted to deposit an electron transport material at a predetermined deposition rate so that the material is deposited to a predetermined thickness. Here, the deposition film is formed without using a fine metal mask.

In step S7, the first electron transport layer 36 is formed on the first hole blocking layer 35. Specifically, the deposition temperature and deposition time are adjusted to deposit the electron transport material at a predetermined deposition rate so that the layer is formed to a predetermined thickness. The deposition may be co-deposition of the electron transport material and lithium quinoline. Here, the deposition film is formed without using a fine metal mask.

In step S8, the n-type first charge generation layer 41 is formed on the first electron transport layer 36. Specifically, co-evaporation of an organic electron transport material and an inorganic metal material, Yb or Li, which is an electron supply material, is performed at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layers are laminated to a predetermined film thickness and ratio. Here, the evaporated film is formed without using a fine metal mask.

In step S9, a p-type first charge generation layer 42 is formed on the n-type first charge generation layer 41. Specifically, the co-evaporation of an organic hole transport material and an organic electron acceptor material is performed at a predetermined deposition rate by adjusting the deposition temperature and deposition time of each material so that the material is laminated at a predetermined film thickness and ratio. Here, the deposition film is formed without using a fine metal mask.

In step S10, the second hole transport layer 51 is formed on the p-type first charge generation layer 42. Specifically, the hole transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the hole transport material is deposited to a predetermined thickness. Here, the evaporated film is formed without using a fine metal mask.

In step S11, a second electron blocking layer 52 is formed on the second hole transport layer 51. Specifically, the hole transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the hole transport material is deposited to a predetermined thickness. Here, a fine metal mask is used to evaporate the material to a second thickness corresponding to each color. As with the first thickness, the second thickness may be the same or different for the electroluminescent elements of each color.

In step S12, the second electroluminescent layer 53 is formed on the second electron blocking layer 52. Specifically, the host compound and the guest compound (dopant) are co-evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layers are laminated to a predetermined film thickness and guest compound concentration (dopant concentration). Here, a fine metal mask is used to evaporate materials according to each color while precisely controlling the thickness and guest compound concentration of each.

Here, as a result of the selection of the material of the carrier functional layer, it is assumed that the electron injection from the cathode layer 22 is lower than the hole injection from the anode layer 21, and/or the hole injection from the p-type first charge generation layer 42 is lower than the electron injection from the n-type first charge generation layer 41. More specifically, it is assumed that the amount of electron injection from the cathode layer 22 is lower than the amount of hole injection from the anode layer 21, and/or the amount of hole injection from the p-type first charge generation layer 42 is lower than the amount of electron injection from the n-type first charge generation layer 41. Alternatively, it is assumed that the difference between the electron injection from the cathode layer 22 and the hole injection from the p-type first charge generation layer 42 is greater than the difference between the hole injection from the anode layer 21 and the electron injection from the n-type first charge generation layer 41, i.e., the difference between the electron injection amount and the hole injection amount in the second electroluminescent layer 53 is greater than the difference between the electron injection amount and the hole injection amount in the first electroluminescent layer 34. At this time, the second electroluminescent layer 53 is deposited so that the ratio of the thickness of the second electroluminescent layer 53 to the thickness of the first electroluminescent layer 34 in the stacking direction is greater than 4/15 and less than 2/3. More preferably, the ratio of the thickness of the second electroluminescent layer 53 to the thickness of the first electroluminescent layer 34 is greater than or equal to 1/3 and less than or equal to 1/2. In addition, the second electroluminescent layer 53 is deposited so that the ratio of the thickness of the second electroluminescent layer 53 to the thickness of the set of layers 50 in the stacking direction is greater than or equal to 0.05 and less than or equal to 0.35.

Alternatively, as a result of the selection of the material of the carrier functional layer, the electron injection from the cathode layer 22 is higher than the hole injection from the anode layer 21, and/or the hole injection from the p-type first charge generation layer 42 is higher than the electron injection from the n-type first charge generation layer 41. More specifically, the amount of electron injection from the cathode layer 22 is higher than the amount of hole injection from the anode layer 21, and/or the amount of hole injection from the p-type first charge generation layer 42 is higher than the amount of electron injection from the n-type first charge generation layer 41. Alternatively, the difference between the hole injection from the anode layer 21 and the electron injection from the n-type first charge generation layer 41 is larger than the difference between the electron injection from the cathode layer 22 and the hole injection from the p-type first charge generation layer 42, i.e., the difference between the electron injection amount and the hole injection amount in the first electroluminescent layer 34 is larger than the difference between the electron injection amount and the hole injection amount in the second electroluminescent layer 53. At this time, the second electroluminescent layer 53 is deposited so that the ratio of the thickness of the first electroluminescent layer 34 to the second electroluminescent layer 53 in the stacking direction is greater than 4/15 and less than 2/3, more preferably greater than 1/3 and less than 1/2. In this case, too, the second electroluminescent layer 53 is deposited so that the ratio of the thickness of the second electroluminescent layer 53 to the thickness of the set of layers 50 in the stacking direction is greater than 0.05 and less than 0.35.

In step S13, the second hole blocking layer 54 is formed on the second electroluminescent layer 53. Specifically, the deposition temperature and deposition time are adjusted to deposit an electron transport material at a predetermined deposition rate so that the material is deposited to a predetermined thickness. Here, the deposition film is formed without using a fine metal mask.

In step S14, the second electron transport layer 55 is formed on the second hole blocking layer 54. Specifically, the deposition temperature and deposition time are adjusted to deposit the electron transport material at a predetermined deposition rate so that the layer is formed to a predetermined thickness. Co-deposition of the electron transport material and lithium quinoline may also be performed. Here, the deposition film is formed without using a fine metal mask.

In step S15, an electron injection layer 56 is formed on the second electron transport layer 55. Specifically, lithium fluoride is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that a layer with a predetermined thickness is formed. Here, the evaporated film is formed without using a fine metal mask.

In step S16, the cathode layer 22 is formed on the electron injection layer 56. Specifically, for example, a magnesium-silver alloy thin film is formed using a deposition method.

<Light Emitting Mechanism>
The light emitting mechanism of the electroluminescent element 10 of the electroluminescent device 13 capable of full color display will be described with reference to Fig. 5. Fig. 5 is a diagram for explaining the light emitting mechanism of the electroluminescent element 10 of the electroluminescent device 13 shown in Fig. 2. In Fig. 5, only the essential parts of the configuration of the electroluminescent element 10 are shown.

The electroluminescent element 10 shown in FIG. 5 is an organic EL element having a tandem structure in which two electroluminescent layers, a first electroluminescent layer 34 and a second electroluminescent layer 53, are formed between an anode layer 21 and a cathode layer 22. A first charge generation layer 40 is disposed between the first electroluminescent layer 34 and the second electroluminescent layer 53. In the example shown in FIG. 5, the first electroluminescent layer 34 and the second electroluminescent layer 53 that emit light of the same color are laminated for each of the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B.

More specifically, as shown in FIG. 5, in the red electroluminescent element 10R, the anode layer 21R, the first electroluminescent layer 34R, the first charge generation layer 40R, the second electroluminescent layer 53R, and the cathode layer 22R are arranged in this order. In the red electroluminescent element 10R, the set of layers 30R including the first electroluminescent layer 34R, which is arranged between the anode layer 21R and the first charge generation layer 40R in the stacking direction, is the first stack. And the set of layers 50R including the second electroluminescent layer 53R, which is arranged between the first charge generation layer 40R and the cathode layer 22R, is the second stack.

Similarly, in the green electroluminescent element 10G, the anode layer 21G, the first electroluminescent layer 34G, the first charge generation layer 40G, the second electroluminescent layer 53G, and the cathode layer 22G are arranged in this order. The first stack in the green electroluminescent element 10G is a set of layers 30G including the first electroluminescent layer 34G, which is arranged between the anode layer 21G and the first charge generation layer 40G in the stacking direction. The second stack in the green electroluminescent element 10G is a set of layers 50G including the second electroluminescent layer 53G, which is arranged between the first charge generation layer 40G and the cathode layer 22G.

In addition, in blue electroluminescent element 10B, anode layer 21B, first electroluminescent layer 34B, first charge generation layer 40B, second electroluminescent layer 53B, and cathode layer 22B are stacked in this order. The first stack in blue electroluminescent element 10B is a set of layers 30B including first electroluminescent layer 34B, which is disposed between anode layer 21B and first charge generation layer 40B in the stacking direction. The second stack in blue electroluminescent element 10B is a set of layers 50B including second electroluminescent layer 53B, which is disposed between first charge generation layer 40B and cathode layer 22B.

Here, cathode layers 22R, 22G, and 22B are provided for each of the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B. The cathode layers 22R, 22G, and 22B may be a common electrode layer provided across the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B.

Furthermore, in FIG. 5, a sealing layer 14 is shown provided at the top of each of the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B to prevent the intrusion of oxygen and moisture. The sealing layer 14 may also be a common layer provided across the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B.

In addition, the layer assemblies 30R, 30G and 30B, 50R, 50G and 50B included in the electroluminescent elements 10R, 10G and 10B of the respective colors have carrier functional layers (electron injection layer, electron transport layer, hole transport layer, hole injection layer, etc.) arranged therein.

The light emission mechanism of the electroluminescent element 10 will be further explained below. In the following explanation, the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B all have the same light emission mechanism, so the symbols R, G, and B that indicate the colors added to the symbols indicating each basic configuration will be omitted.

When a current flows through the electroluminescent element 10, holes are supplied from the anode layer 21 to the first electroluminescent layer 34, and electrons are supplied from the cathode layer 22 to the second electroluminescent layer 53. In addition, when a current flows through the electroluminescent element 10, electrons generated by the n-type first charge generation layer 41 are supplied to the first electroluminescent layer 34, and holes generated by the p-type first charge generation layer 42 are supplied to the second electroluminescent layer 53.

As a result, electrons and holes recombine in the first electroluminescent layer 34 to generate electron-hole pairs (also called excitons), which transition to the ground state to emit light in a predetermined wavelength range. Similarly, electrons and holes recombine in the second electroluminescent layer 53 to generate electron-hole pairs, which transition to the ground state to emit light in a predetermined wavelength range (light of the same color as the first electroluminescent layer 34). For example, the first electroluminescent layer 34R and the second electroluminescent layer 53R of the red electroluminescent element 10R each emit red light. Similarly, the first electroluminescent layer 34G and the second electroluminescent layer 53G of the green electroluminescent element 10G each emit green light, and the first electroluminescent layer 34B and the second electroluminescent layer 53B of the blue electroluminescent element 10B each emit blue light.

At this time, the first electroluminescent layer 34 and the second electroluminescent layer 53 can both emit light with a luminous efficiency substantially equal to the theoretical value. Therefore, each of the multiple light-emitting layers of the same color arranged in the stacking direction emits light substantially equal to the theoretical value, contributing to the formation of a high-brightness, high-definition full-color image.

However, the amount of light emitted by a conventional electroluminescent element that does not have the electroluminescent layer configuration described in this disclosure, i.e., a conventional tandem-structure electroluminescent element in which two host-guest electroluminescent layers that emit light of the same color are stacked to the same thickness, may not reach twice the amount of light emitted by an electroluminescent element having a single electroluminescent layer (i.e., the theoretical value).

Here, the results of the verification by the present inventors are introduced. First, an electroluminescent device (single prototype) having one layer of blue electroluminescent layer (also called single layer; single structure) and an electroluminescent device (comparative tandem prototype) having two layers of blue electroluminescent layer in a tandem structure were fabricated. The single prototype was manufactured under the following conditions. The comparative tandem prototype was manufactured under three conditions, i.e., condition i to condition iii, below. The single prototype has the same layer structure as the first stack under condition i between the cathode layer and the anode layer. The comparative tandem prototype is an electroluminescent device having the same thickness of each electroluminescent layer stacked in the stacking direction, that is, a conventional tandem structure. Then, the element life and driving voltage were measured for each prototype, and the current efficiency was calculated using the Blue Index (unit: cd/A/y, y is one of the chromaticity coordinates in CIE1931).
(Single prototype)
Layer configuration: cathode layer/first stack (electron transport layer/hole blocking layer/first blue electroluminescent layer (thickness: 15 nm)/electron blocking layer/hole transport layer/hole injection layer)/anode layer. Thickness ratio: the ratio of the first electroluminescent layer thickness (15 nm) to the first stack thickness (165 nm) is 0.091.
(Condition i)
Layer configuration: cathode layer/second stack (electron injection layer/electron transport layer/hole blocking layer/second blue electroluminescent layer (thickness: 15 nm)/hole transport layer/hole injection layer)/charge generation layer/first stack (electron transport layer/hole blocking layer/first blue electroluminescent layer (thickness: 15 nm)/electron blocking layer/hole transport layer/hole injection layer)/anode layer. Thickness ratio: the ratio of the second electroluminescent layer thickness (15 nm) to the second stack thickness (70 nm) is 0.21, and the ratio of the first electroluminescent layer thickness (15 nm) to the first stack thickness (165 nm) is 0.091.
(Condition ii)
Layer structure: same as condition i.
Thickness ratio: Same as condition i, except that the thickness of the carrier functional layer in the second stack layer was adjusted to a total thickness of 80 nm. The ratio of the thickness of the second electroluminescent layer (15 nm) to the thickness of the second stack layer (80 nm) was 0.188.
(Condition iii)
Layer structure: same as condition i.
Thickness ratio: Same as condition i, except that the thickness of the carrier functional layer in the second stack layer was adjusted to a total thickness of 90 nm. The ratio of the thickness of the second electroluminescent layer (15 nm) to the thickness of the second stack layer (90 nm) was 0.167.

As a result, the current efficiency of the single prototype was 206 cd/A/y. In contrast, the current efficiency of the comparative tandem prototype was (condition i) 305 cd/A/y (current efficiency 1.48 times that of the single prototype), (condition ii) 320 cd/A/y (same 1.55 times), and (condition iii) 219 cd/A/y (same 1.1 times). In addition, when the element life was measured, the element life of the comparative tandem prototype was 2.01 times that of the single prototype (condition i), 2.12 times that of the single prototype (condition ii), and 1.92 times that of the single prototype (condition iii). In addition, when the driving voltage was measured, the driving voltage of the comparative tandem prototype was 1.9 times that of the single prototype (condition i), 1.9 times that of the single prototype (condition ii), and 2.0 times that of the single prototype (condition iii).

From the above results, it can be seen that the life span of electroluminescent devices with a tandem structure having two conventional electroluminescent layers in which the thickness of the electroluminescent layers is the same and the conditions of the carrier functional layers other than the electroluminescent layer are changed is improved by about twice as much as that of electroluminescent devices with a single structure having a single electroluminescent layer. On the other hand, electroluminescent devices with a tandem structure having two conventional electroluminescent layers of the same thickness have a smaller increase in current efficiency compared to the increase in driving voltage compared to electroluminescent devices with a single structure, and even if the conditions of the carrier functional layer are changed, it can be seen that the luminous efficiency does not reach twice that of electroluminescent devices with a single structure by a large margin, even though the driving voltage is about twice as high.

Based on these results, one possible measure to double the luminous efficiency of an electroluminescent device having a conventional two-layer electroluminescent layer (tandem structure) compared to an electroluminescent device having a single structure (i.e., an electroluminescent device having a single electroluminescent layer) would be to apply a higher driving voltage and supply more current, but it is clear that this would result in increased power consumption and a shortened device life. In other words, it can be seen that there is still room for improvement in order to fully realize the advantages of adopting a tandem structure in the conventional structure. According to the inventors' investigations into this matter, the following reasons are considered.

In organic materials, the mobility of holes is generally higher than that of electrons, but depending on the material selection of the electron transport layer and the hole transport layer, the mobility of electrons may be higher than that of holes. In addition, the mobility of electrons may be higher than that of holes in terms of the injection of carriers (electrons and holes) from the electrode layer, depending on the combination of the cathode layer material and the electron injection layer material, and the combination of the anode layer material and the hole injection layer material. In other words, the generation of excitons that contribute to light emission is due to the recombination of electrons and holes supplied to the electroluminescent layer, but the carrier that is the rate-limiting factor for the generation of excitons changes whether it is electrons or holes depending on the material selection. In conventional electroluminescent elements having a single electroluminescent layer, the amount of carriers supplied to the electroluminescent layer is matched, that is, carrier balance is achieved by optimizing the material selection of these carrier functional layers other than the electroluminescent layer or the thickness of the carrier functional layer.

However, in an electroluminescent element having multiple electroluminescent layers (tandem structure), there is usually a charge generation layer and multiple adjacent electroluminescent layers interposed therebetween in the stacking direction. In this case, the balance of electrons and holes generated by the charge generation layer may not be 1:1. Therefore, the carriers that become rate-limiting and the degree of rate-limiting may differ for carrier injection from the charge generation layer depending on the combination of an electron injection layer and an electron transport layer for an n-type charge generation layer, or the combination of a hole injection layer and a hole transport layer for a p-type charge generation layer.

　Conventionally, the problem of the carrier balance of each electroluminescent layer in an electroluminescent device having multiple electroluminescent layers (tandem structure) has been addressed by the same measures as in electroluminescent devices having a single electroluminescent layer, that is, by optimizing the carrier functional layer around the electroluminescent layer other than the electroluminescent layer as described above. However, as shown by the above-mentioned verification results, the effect is still not sufficient in some cases. Furthermore, there is still room for consideration in terms of measures to address the new problem that the amount of light emitted by a conventional electroluminescent device having a tandem structure with two electroluminescent layers having the same thickness may not reach twice the amount of light emitted by an electroluminescent device having a single electroluminescent layer.

In response to this problem, the present inventors have found that in an electroluminescent device having a tandem structure, in addition to matching the supply amounts of electrons and holes in each of the multiple electroluminescent layers, i.e., the carrier balance, it is necessary to consider the carrier balance and carrier supply amount between each electroluminescent layer, which did not need to be considered in an electroluminescent device having a single electroluminescent layer, and that in a conventional electroluminescent device having multiple electroluminescent layers (tandem structure), excess carriers (electrons or holes) generated in one of the electroluminescent layers can be a cause of the current efficiency of the electroluminescent device not being doubled. And, in order to make the amount of light emitted by an electroluminescent device having a tandem structure substantially twice that of an electroluminescent device having a single electroluminescent layer, it has been found that it can be an effective measure to not only balance the supply of electrons and holes to one electroluminescent layer, but also to balance the carriers in each of the multiple electroluminescent layers and between them.

The inventors have introduced a new design concept to an electroluminescent device having a tandem structure with multiple electroluminescent layers, in which the configuration (thickness) of each electroluminescent layer itself is changed as in the present disclosure, thereby achieving carrier balance between the multiple electroluminescent layers and solving the problem of imbalance in carrier supply. As a result, the present disclosure makes it possible to generate an appropriate amount and balance of excitons in each electroluminescent layer in the stacking direction without generating excess carriers as in conventional electroluminescent devices with a tandem structure. As a result, the present disclosure realizes an electroluminescent device with a tandem structure that can reduce current consumption and driving voltage and improve luminous efficiency.

Specifically, when the electron injection from the cathode layer side is inferior to the hole injection from the anode layer side, and/or when the hole injection from the p-type charge generation layer is inferior to the electron injection from the n-type charge generation layer, the amount of excitons generated in the electroluminescent layer formed on the cathode side layer is small. More specifically, examples of such cases include cases where the amount of electron injection from the cathode layer is smaller than the amount of hole injection from the anode layer, and/or cases where the amount of hole injection from the p-type charge generation layer is smaller than the amount of electron injection from the n-type charge generation layer. Alternatively, examples of the above cases include cases where the difference between the electron injection from the cathode layer and the hole injection from the p-type charge generation layer is larger than the difference between the hole injection from the anode layer and the electron injection from the n-type charge generation layer, that is, cases where the difference between the electron injection amount and the hole injection amount in the electroluminescent layer formed on the cathode side is larger than the difference between the electron injection amount and the hole injection amount in the electroluminescent layer formed on the anode side. Therefore, the thickness of the electroluminescent layer formed on the cathode layer side is made thinner. In this case, the electroluminescent layer on the cathode layer side is formed so that the ratio of the thickness of the electroluminescent layer on the cathode layer side to the thickness of the electroluminescent layer on the anode layer side in the stacking direction is greater than 4/15 and less than 2/3. More preferably, the ratio of the thickness of the electroluminescent layer on the cathode layer side to the thickness of the electroluminescent layer on the anode layer side is greater than 1/3 and less than 1/2.

Conversely, if the hole injection from the anode layer side is inferior to the electron injection from the cathode layer side, and/or if the electron injection from the n-type charge generation layer is inferior to the hole injection from the p-type charge generation layer, the amount of excitons generated in the electroluminescent layer formed on the anode layer side is small. More specifically, examples of such cases include cases where the amount of electron injection from the cathode layer is greater than the amount of hole injection from the anode layer, and/or cases where the amount of hole injection from the p-type charge generation layer is greater than the amount of electron injection from the n-type charge generation layer. Alternatively, examples of the above cases include cases where the difference between the hole injection from the anode layer and the electron injection from the n-type charge generation layer is greater than the difference between the electron injection from the cathode layer and the hole injection from the p-type charge generation layer, that is, cases where the difference between the electron injection amount and the hole injection amount in the electroluminescent layer formed on the anode side is greater than the difference between the electron injection amount and the hole injection amount in the electroluminescent layer formed on the cathode side. Therefore, the thickness of the electroluminescent layer formed on the anode layer side is made thinner. In this case, the electroluminescent layer on the anode layer side is formed so that the ratio of the thickness of the electroluminescent layer on the anode layer side to the thickness of the electroluminescent layer on the cathode layer side in the stacking direction is greater than 4/15 and less than 2/3. More preferably, the ratio of the thickness of the electroluminescent layer on the anode layer side to the thickness of the electroluminescent layer on the cathode layer side is greater than 1/3 and less than 1/2.

In particular, when the electroluminescent element has a top-emission configuration in which light is extracted from the cathode layer side, even the slightest defect in the sealing layer formed on the cathode layer can cause the cathode layer to deteriorate due to the intrusion of oxygen or moisture, reducing the electron injection properties, and as a result, the amount of electrons supplied to the electroluminescent layer directly below the cathode layer can be reduced. However, by anticipating such a case and forming the thickness of the electroluminescent layer on the cathode layer side thinner than the theoretical value from the initial design stage, it is possible to prevent the generation of excess holes in the electroluminescent layer on the cathode layer side. In this case, the thickness of the electroluminescent layer on the anode layer side may be formed to be a specific ratio to the thickness of the electroluminescent layer on the cathode layer side thinner than the theoretical thickness.

In this disclosure, attention is paid to the mobility of carriers in the stacking direction and the injection property of carriers, and further to the supply amount of carriers and the generation amount of excess carriers in each electroluminescent layer in the stacking direction. In the electroluminescent device of embodiment 1, the thickness of the electroluminescent layer formed on the cathode layer side is made thinner than that of the electroluminescent layer formed on the anode layer side. As a result, carriers (electrons and holes) corresponding to the thickness of the electroluminescent layer are supplied to the electroluminescent layer on the cathode layer side. Therefore, excitons are generated without the generation of excess carriers, and the amount of excitons generated in the electroluminescent layer on the anode side is substantially the same as the theoretical value. Therefore, each electroluminescent layer overlapping in the stacking direction emits light with a brightness according to its thickness. Furthermore, since the generation of excess carriers can be prevented in each electroluminescent layer overlapping in the stacking direction, a reduction in current consumption is realized, and therefore a reduction in power consumption can also be realized.

Here, we will explain the evaluation by simulation of the examples and comparative examples of the present disclosure for a full-color electroluminescent device having two electroluminescent layers (tandem structure) in the stacking direction. The conditions for the electroluminescent device are that the thickness of the electroluminescent layer on the cathode layer side is 17 nm for the red and green electroluminescent elements and 7 nm for the blue electroluminescent element, and that the thickness of the electroluminescent layer on the anode layer side is 35 nm for the red and green electroluminescent elements and 15 nm for the blue electroluminescent element.

The electroluminescent device of the example has the layer structure of this embodiment. The ratio of the thickness of the cathode-side electroluminescent layer to the thickness of the anode-side electroluminescent layer is 0.49 for the red and green electroluminescent layers, and 0.47 for the blue electroluminescent layer. The thickness of each stack of each color is 67 nm to 255 nm, in the range of 60 nm to 260 nm, and the ratio of the thickness of the electroluminescent layer to the thickness of the stack in each stack is in the range of 0.10 to 0.13. Each layer of the electroluminescent device of the comparative example has the same thickness as each layer of the stack on the anode layer side described above, and is a full-color electroluminescent device in which each color has a single electroluminescent layer (not a tandem structure).

According to the results of a simulation under the above conditions, the current efficiency (also called "luminous efficiency") of the full-color electroluminescent device of the above example was 1.6 times that of the full-color electroluminescent device of the above comparative example, the element life was 2.6 times, and the driving voltage was 1.6 times. From these results, it can be seen that the electroluminescent element of the above example achieves an improvement in luminous efficiency commensurate with the increase in driving voltage. Therefore, it can be seen that the electroluminescent device having the tandem structure disclosed herein can suppress an increase in driving voltage and extend the element life, as well as achieve an improvement in luminous efficiency commensurate with the increase in driving voltage, compared to a full-color electroluminescent device having a conventional tandem structure.

Furthermore, when the driving voltage was further increased to further improve the current efficiency (luminous efficiency) under the conditions in the simulation of the above example, the current efficiency also doubled when the driving voltage was doubled, and furthermore, the element life under those conditions became 2.5 times longer. In other words, it can be seen that the full-color electroluminescent device having two electroluminescent layers (tandem structure) in the stacking direction according to the present disclosure can achieve characteristics more than twice those of a single-layer full-color electroluminescent device.

In addition, when comparing the blue electroluminescent device in the above Example with the Comparative Example, the current efficiency of the electroluminescent device in the above Example is increased by 12.0% compared to that of the electroluminescent device in the Comparative Example, the device life is increased by 13.5%, and the driving voltage is decreased by 12.1%. As is clear from these results, the electroluminescent device of this embodiment can achieve a significant improvement in characteristics. In addition, the electroluminescent device of this embodiment also has the effect of reducing the amount of guest compound (dopant) used compared to conventional electroluminescent devices having a tandem structure with multiple electroluminescent layers of the same thickness.

[Embodiment 2]
The second embodiment of the present disclosure will be described below with reference to Fig. 6. Fig. 6 is a diagram for explaining the light emission mechanism of the electroluminescent element 10A of the electroluminescent device 13A according to the second embodiment of the present disclosure. The electroluminescent device 13A according to the second embodiment is different from the electroluminescent device 13 described above in that it includes the electroluminescent element 10A having three electroluminescent layers in the stacking direction.

In FIG. 6, only the essential configuration of the electroluminescent element 10A included in the electroluminescent device 13A is shown. In the following description of the embodiment, for the sake of convenience, the same explanation as in the above-described embodiment will not be repeated, and the same reference numerals will be used for components having the same functions as those described in the above-described embodiment, and the explanation will not be repeated.

As shown in FIG. 6, in the red electroluminescent element 10AR, a third electroluminescent layer 71R and a second charge generation layer 80R are further disposed between the cathode layer 22R and the second electroluminescent layer 53R. In the red electroluminescent element 10AR, the set of layers 50R including the second electroluminescent layer 53R, which is disposed between the first charge generation layer 40R and the second charge generation layer 80R in the stacking direction, is the second stack. And the set of layers 70R including the third electroluminescent layer 71R, which is disposed between the second charge generation layer 80R and the cathode layer 22R, is the third stack.

Similarly, in the green electroluminescent element 10AG, a third electroluminescent layer 71G and a second charge generation layer 80G are further disposed between the cathode layer 22G and the second electroluminescent layer 53G. In the green electroluminescent element 10AG as well, the set of layers 50G including the second electroluminescent layer 53G, which is disposed between the first charge generation layer 40G and the second charge generation layer 80G in the stacking direction, is the second stack. And the set of layers 70G including the third electroluminescent layer 71G, which is disposed between the second charge generation layer 80G and the cathode layer 22G, is the third stack.

In addition, in the blue electroluminescent element 10AB, a third electroluminescent layer 71B and a second charge generation layer 80B are further disposed between the cathode layer 22B and the second electroluminescent layer 53B. In the blue electroluminescent element 10AB as well, the set of layers 50B including the second electroluminescent layer 53B, which is disposed between the first charge generation layer 40B and the second charge generation layer 80B in the stacking direction, is the second stack. And the set of layers 70B including the third electroluminescent layer 71B, which is disposed between the second charge generation layer 80B and the cathode layer 22B, is the third stack.

In addition, the layers 30R, 30G and 30B, 50R, 50G and 50B, and 70R, 70G and 70B included in the electroluminescent elements 10R, 10G and 10B of each color have carrier function layers (electron injection layer, electron transport layer, hole transport layer, hole injection layer, etc.) arranged therein. In the following description, the third electroluminescent layers 71R, 71G, and 71B included in the electroluminescent elements 10R, 10G and 10 of each color are collectively referred to as the "third electroluminescent layer 71."

In the stacking direction, when the thickness of the first electroluminescent layer 34 is 1, for example, the thickness of the second electroluminescent layer 53 is 0.5, and the thickness of the third electroluminescent layer 71 is 0.3. Thus, the ratio of the thickness of the electroluminescent layer on the cathode side to the electroluminescent layer on the anode side is greater than 4/15 and less than 2/3. Furthermore, the thickness of each stack of each color is in the range of 60 nm to 260 nm, and the ratio of the thickness of the electroluminescent layer to the thickness of the stack in each stack is in the range of 0.05 to 0.35. Within the above range, the thicknesses of the electroluminescent layers 34, 53, and 71 included in the electroluminescent element 10A are thicker on the anode layer 21 side and thinner on the cathode layer 22 side.

6, the cathode layer 22 may be a common electrode layer provided across the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B. Similarly, the sealing layer 14 may be a common layer provided across the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B.

Even when there are three electroluminescent layers in the stacking direction as shown in FIG. 6, the supply of excess carriers (electrons or holes) is prevented in the electroluminescent layers closer to the cathode layer, as in the display device 100 shown in FIG. 5, and each electroluminescent layer emits light with a brightness and luminous efficiency that is substantially the same as the theoretical values.

[Embodiment 3]
A display device according to a third embodiment of the present disclosure will be described with reference to Figs. 7 and 8. Fig. 7 is a diagram showing a layer structure of a display device 200 according to a third embodiment of the present disclosure. Fig. 8 is a diagram for explaining the light emission mechanism of the electroluminescent element of the electroluminescent device 13B shown in Fig. 7. The electroluminescent device 13B included in the display device 200 according to this embodiment is different from the display device 100 described above in that it includes an electroluminescent device 13B including an electroluminescent element 210 in which the thickness of the second electroluminescent layer 253 on the cathode layer 22 side is made thicker and the first electroluminescent layer 234 on the anode layer 21 side is made thinner. Note that Figs. 7 and 8 show only the main configuration of the electroluminescent element 210 included in the electroluminescent device 13B.

As shown in FIG. 7, the electroluminescent device 13B laminated on the display device 200 includes an electroluminescent element 210. The electroluminescent element 210 is an organic EL element having a tandem structure in which two electroluminescent layers, a first electroluminescent layer 234 and a second electroluminescent layer 253, are formed between the anode layer 21 and the cathode layer 22. The electroluminescent element 210 includes a red electroluminescent element 210R, a green electroluminescent element 210G, and a blue electroluminescent element 210B. The first electroluminescent layer 234 includes a first electroluminescent layer 234R, a first electroluminescent layer 234G, and a first electroluminescent layer 234B. The second electroluminescent layer 253 includes a second electroluminescent layer 253R, a second electroluminescent layer 253G, and a second electroluminescent layer 253B.

As shown in FIG. 8, a first charge generation layer 40 is disposed between the first electroluminescent layer 234 and the second electroluminescent layer 253. This first charge generation layer 40 has a two-layer structure in which an n-type first charge generation layer and a p-type first charge generation layer are laminated. The n-type first charge generation layer is configured by doping an oxadiazole compound, which is an organic electron transport material, with BUPH1, which is an organic electron supply material. In the example shown in FIG. 7 and FIG. 8, a first electroluminescent layer 234 and a second electroluminescent layer 253 that emit light of the same color are laminated for each of the red electroluminescent element 210R, the green electroluminescent element 210G, and the blue electroluminescent element 210B.

More specifically, as shown in FIG. 8, in the red electroluminescent element 210R, the anode layer 21R, the first electroluminescent layer 234R, the first charge generation layer 40R, the second electroluminescent layer 253R, and the cathode layer 22R are arranged in this order. In the red electroluminescent element 210R, the set of layers 230R including the first electroluminescent layer 234R, which is arranged between the anode layer 21R and the first charge generation layer 40R in the stacking direction, is the first stack, and the set of layers 250R including the second electroluminescent layer 253R, which is arranged between the first charge generation layer 40R and the cathode layer 22R, is the second stack.

Similarly, in the green electroluminescent element 210G, the anode layer 21G, the first electroluminescent layer 234G, the first charge generation layer 40G, the second electroluminescent layer 253G, and the cathode layer 22G are arranged in this order. The first stack in the green electroluminescent element 210G is a set of layers 230G including the first electroluminescent layer 234G, which is arranged between the anode layer 21G and the first charge generation layer 40G in the stacking direction. The second stack in the green electroluminescent element 210G is a set of layers 250G including the second electroluminescent layer 253G, which is arranged between the first charge generation layer 40G and the cathode layer 22G.

In addition, in the blue electroluminescent element 210B, the anode layer 21B, the first electroluminescent layer 234B, the first charge generation layer 40B, the second electroluminescent layer 253B, and the cathode layer 22B are stacked in this order. The first stack in the blue electroluminescent element 210B is a set of layers 230B including the first electroluminescent layer 234B, which is disposed between the anode layer 21B and the first charge generation layer 40B in the stacking direction. The second stack in the blue electroluminescent element 210B is a set of layers 250B including the second electroluminescent layer 253B, which is disposed between the first charge generation layer 40B and the cathode layer 22B.

In addition, the layer assemblies 230R, 230G, 230B, 250R, 250G, and 250B included in the electroluminescent elements 210R, 210G, and 210B of the respective colors have carrier functional layers (electron injection layer, electron transport layer, hole transport layer, hole injection layer, etc.) arranged therein.

In the present embodiment shown in Figures 7 and 8, the ratio of the thickness of the first electroluminescent layer 234 to the thickness of the second electroluminescent layer 253 is greater than 4/15 and less than 2/3. The thickness of each stack of each color is in the range of 50 nm to 350 nm, and the ratio of the thickness of the electroluminescent layer to the thickness of the stack in each stack is in the range of 0.05 to 0.35.

The results of simulation evaluation of the examples and comparative examples based on the present embodiment shown in Figures 7 and 8 are shown below. In the examples of this embodiment, the thickness of the electroluminescent layer on the cathode layer side was 35 nm for the red and green electroluminescent elements and 15 nm for the blue electroluminescent element, and the thickness of the electroluminescent layer on the anode layer side was 18 nm for the red and green electroluminescent elements and 8 nm for the blue electroluminescent element. The ratio of the thickness of the anode side electroluminescent layer to the thickness of the cathode side electroluminescent layer was 0.34 for the red and green electroluminescent layers, and 0.33 for the blue electroluminescent layer. The thickness of each stack of each color was in the range of 100 nm to 240 nm, and the ratio of the thickness of the electroluminescent layer to the thickness of the stack in each stack was in the range of 0.05 to 0.11. The comparative example in this embodiment was an electroluminescent device having a single electroluminescent layer of each color that satisfied the above-mentioned conditions for the thickness on the anode layer side.

As a result, the current efficiency (luminous efficiency) in the above example of this embodiment was 1.6 times that of the comparative example of this embodiment, the luminous lifetime was 2.6 times, and the driving voltage was 1.6 times.

As mentioned above, in many organic materials, the mobility of holes is higher than that of electrons. For example, in the case of _Alq3 , the mobility of electrons is about two orders of magnitude higher than that of holes, so when this is used in the electron transport layer or electroluminescent layer of the carrier functional layer, the movement of electrons may not be the rate-limiting factor. As a result, the movement of holes may become the rate-limiting factor, and in the generation of excitons in the electroluminescent layer on the anode layer side, holes may be insufficient and electrons may be insufficient and electrons may be insufficient. Furthermore, as mentioned above, when an n-type charge generation layer made of an all-organic material is used, the carrier injection from the charge generation layer is generally inferior to the hole injection, so the supply amount of carriers (here, electrons) supplied to the electroluminescent layer on the anode layer side may be reduced. As a result, the generation amount of excitons in the electroluminescent layer on the anode layer side may be reduced.

However, as described above, when the balance of carrier supply is lost in the electroluminescent layer on the anode layer side, if the thickness of the electroluminescent layer formed on the anode layer side is appropriately made thinner than that of the electroluminescent layer formed on the cathode layer side as in embodiment 3, carriers (electrons and holes) corresponding to the thickness of the electroluminescent layer are supplied to the electroluminescent layer on the anode layer side. Therefore, excitons are generated without the generation of surplus holes or electrons that cannot be combined with electrons or holes, and the amount of excitons generated in the electroluminescent layer on the cathode layer side is substantially the same as the theoretical value. Therefore, each electroluminescent layer overlapping in the stacking direction emits light with a brightness according to its thickness. Furthermore, since the generation of surplus carriers (electrons or holes) can be prevented, a reduction in current consumption is realized, and therefore a reduction in power consumption can also be realized.

[Embodiment 4]
As in the second embodiment corresponding to the first embodiment, the second electroluminescent layer 253 on the cathode layer 22 side is thicker and the first electroluminescent layer 234 on the anode layer 21 side is thinner, so that the electroluminescent layer may have three or more layers in the stacking direction. Such an embodiment can be illustrated by a diagram in which the electroluminescent layer in FIG. 6 is modified to have a thickness that gradually decreases from the anode layer side to the cathode layer side in the stacking direction. This embodiment has the same effect as the third embodiment, just as the second embodiment has the same effect as the first embodiment.

[Embodiment 5]
This embodiment is the same as the above-described embodiment 2, except that only the thicknesses of two electroluminescent layers adjacent to each other in the stacking direction among the three electroluminescent layers in the above-described embodiment 2 have the above-described specific ratio. Such an embodiment can be illustrated, for example, by a diagram in which the thicknesses of electroluminescent layer 34 and electroluminescent layer 53 in FIG. 6 are the same, and the thickness of electroluminescent layer 71 is changed to be thinner than that of electroluminescent layer 53.

As described above, this embodiment is suitable for adjusting the carrier balance in the remaining stack when a good carrier balance cannot be obtained in the remaining stack as a result of adjusting the carrier balance in two adjacent stacks among the three stacks by a method other than the control of the thickness of the electroluminescent layer in this disclosure. Examples of the configuration of such an electroluminescent device include a configuration in which the thicknesses of corresponding carrier functional layers in the first stack and the second stack are different, a configuration in which the materials of corresponding carrier functional layers in the first stack and the second stack are different, and a configuration in which the material of the first electroluminescent layer contained in the first stack and the material of the second electroluminescent layer contained in the second stack are different, and in which the ratio of the thickness of the electroluminescent layer between the second stack and the third stack is the ratio of the thickness of the electroluminescent layer described in this disclosure.

In this embodiment, the same effect as in embodiment 1 is achieved in the two electroluminescent layers adjacent to each other in the stacking direction in embodiment 2.

[Embodiment 6]
This embodiment is similar to the above-described embodiment 4, except that only the thicknesses of two electroluminescent layers adjacent to each other in the stacking direction among the three electroluminescent layers in the above-described embodiment 4 have the above-described specific ratio. Such an embodiment can be illustrated, for example, by a diagram modified so that the thicknesses of electroluminescent layer 34 and electroluminescent layer 53 in FIG. 6 are the same, and the thickness of electroluminescent layer 71 is made thicker than that of electroluminescent layer 53.

As described above, this embodiment is suitable for adjusting the carrier balance in one remaining stack when, as a result of adjusting the carrier balance in two adjacent stacks out of three stacks by a method other than controlling the thickness of the electroluminescent layer in this disclosure, a good carrier balance cannot be obtained in the remaining stack. An example of the configuration of such an electroluminescent device is the same as in embodiment 5. This embodiment achieves the same effect as embodiment 3 in the two electroluminescent layers adjacent in the stacking direction in embodiment 4.

[Major effects]
In the electroluminescent device of the present disclosure, a plurality of electroluminescent layers are formed by overlapping in the stacking direction for each luminescent color, and in at least one pair of two electroluminescent layers adjacent in the stacking direction, the ratio of the thickness of the electroluminescent layer on one side in the stacking direction to the thickness of the electroluminescent layer on the other side in the stacking direction is greater than 4/15 and smaller than 2/3. For example, when the electron injection from the cathode layer side is inferior to the hole injection from the anode layer side, and/or when the hole injection from the p-type charge generation layer is inferior to the electron injection from the n-type charge generation layer, the thickness of the electroluminescent layer is made thicker on the anode layer side and thinner on the cathode layer side. With this configuration, the generation of excess carriers is prevented in each electroluminescent layer in the stacking direction, and light emission with approximately theoretical brightness and efficiency can be realized.

On the other hand, if the hole injection from the anode layer side is inferior to the electron injection from the cathode layer side, and/or if the electron injection from the n-type charge generation layer is inferior to the hole injection from the p-type charge generation layer, the thickness of the electroluminescent layer is made thicker on the cathode layer side and thinner on the anode layer side. With this configuration, the generation of excess carriers is prevented in each electroluminescent layer in the stacking direction, and light emission with approximately theoretical brightness and efficiency can be achieved.

In addition, in the electroluminescent device of the present disclosure, the thickness of the electroluminescent layer on one side is thinner than the thickness of the electroluminescent layer on the other side, so the total thickness of the electroluminescent layer from the anode layer to the cathode layer is reduced compared to when the thickness of each electroluminescent layer in the stacking direction is constant. Therefore, when the electric field (unit: V/m) applied to each electroluminescent layer in the stacking direction is the same, a reduction in the driving voltage (unit: V) can be achieved. In addition, since the generation of excess carriers in each stacked electroluminescent layer is suppressed, the current consumption is reduced. Therefore, a reduction in power consumption (unit: W = V x A) is also achieved. Note that by gradually reducing the thickness of the electroluminescent layer from one side to the other side in the stacking direction, the effects of improving the brightness (light emitting efficiency) and reducing power consumption described above can be achieved throughout the stacking direction of the electroluminescent element.

As a result, the electroluminescent device of the present disclosure achieves higher brightness emission, lower voltage operation, and lower power consumption compared to electroluminescent devices having a conventional tandem structure in which the thickness of each electroluminescent layer is the same in the stacking direction. Therefore, according to the present disclosure, in an electroluminescent device capable of full-color display without using color filters or the like, it is possible to achieve high brightness and/or low power emission, which are advantages of a tandem structure.

Furthermore, by using a host-guest electroluminescent layer as the electroluminescent layer, high luminous efficiency and improved device life can be achieved, and according to the present disclosure, the carrier balance (balance between electrons and holes) in each electroluminescent layer can be further improved. This is advantageous from the viewpoint of being able to fully utilize the performance of each host-guest electroluminescent layer, which has high luminous efficiency and excellent life. It is also advantageous from the viewpoint of being able to reduce the amount of expensive and rare phosphorescent dopant used as the guest compound.

In addition, when the electroluminescent layer includes a fluorescent electroluminescent layer containing a fluorescent dopant and a phosphorescent electroluminescent layer containing a phosphorescent dopant, the number of fluorescent electroluminescent layers in the stacking direction may be greater than the number of phosphorescent electroluminescent layers, for example, by 1 or 2. This configuration is even more effective in terms of optimizing both the balance of the lifetime and the balance of the amount of light emitted between the fluorescent electroluminescent layer and the phosphorescent electroluminescent layer.

In addition, by making the multiple electroluminescent layers stacked in the stacking direction only the same color, it is advantageous in terms of improving color purity in an electroluminescent device that enables full-color display by painting each RGB color separately. Furthermore, when the same material that emits the same color is stacked in each electroluminescent layer, it becomes easier to evaluate the carrier injection property and carrier transport property between each electroluminescent layer by simulation. In this way, the above configuration is even more effective in terms of more easily realizing an electroluminescent device capable of full-color display including electroluminescent elements having desired characteristics.

The number of electroluminescent layers in the stacking direction may be 2 or more and 5 or less. This configuration is even more effective in terms of improving the luminous efficiency, extending the life span, and reducing the driving voltage.

In addition, by setting the ratio of the thickness of the electroluminescent layer in the stack to the thickness of the stack to be 0.05 or more and 0.35 or less, the thickness of the carrier functional layer is optimized, and the risk of a shortage of carriers supplied to the electroluminescent layer can be further reduced. This is therefore even more effective in terms of achieving light emission with theoretical brightness.

The electroluminescent device of the present disclosure may also be a top-emission type. This configuration is even more effective in terms of optimizing the thickness of the carrier functional layer.

The display device of the present disclosure also includes the electroluminescent device described above. Thus, it is possible to make the ratio of the thickness of the electroluminescent layer on one side of the stacking direction to the thickness of the electroluminescent layer on the other side of the stacking direction greater than 4/15 and smaller than 2/3. This is advantageous in that the generation of excess carriers is prevented in each electroluminescent layer in the stacking direction, allowing the injected current to contribute to light emission more efficiently. Thus, according to the present disclosure, high-brightness, low-power light emission is achieved in the display device, and the advantages of adopting a tandem structure in the electroluminescent device can be fully realized.

Furthermore, in the electroluminescent device of the present disclosure, forming at least the electroluminescent layer by a vapor deposition method is advantageous in realizing an electroluminescent device capable of full color display, since it is possible to form a host-guest electroluminescent layer and to paint a highly precise luminescent layer, compared to forming an electroluminescent layer by other manufacturing methods. In addition, it is also advantageous in realizing high-precision control of the thickness of each electroluminescent layer. In particular, when forming an electroluminescent layer by a coating method using an inkjet device or the like, it is difficult to form an electroluminescent layer while doping a guest compound into a host compound, and to control the thickness of the electroluminescent layer to the order of several nm. Therefore, it is considered difficult to form the electroluminescent device of the present disclosure capable of full color display by a coating method.

Furthermore, in the manufacture of the electroluminescent device of the present disclosure, electroluminescent layers of different thicknesses in the stacking direction may be formed by changing only the deposition time. With this configuration, electroluminescent layers of different thicknesses can be formed in the same manufacturing equipment by applying the same conditions except for the deposition time, which is even more effective in terms of reducing manufacturing costs and improving productivity.

In addition, in electroluminescent devices including electroluminescent elements having multiple electroluminescent layers (tandem structure), the thickness of the electroluminescent layers that emit light of the same color in the stacking direction is usually constant. On the other hand, there are also technologies that have a thicker electroluminescent layer on the cathode layer side and a thinner electroluminescent layer on the anode layer side (JP Patent Publication No. 2007-329054, WO 2010/113493, etc.), and technologies that have a thicker electroluminescent layer on the anode layer side and a thinner electroluminescent layer on the cathode layer side (JP Patent Publication No. 2015-153587, JP Patent Publication No. 2015-32582, etc.). However, in these conventional technologies, the reason for making the thicknesses of the electroluminescent layers different in the stacking direction is not made clear, and the above publications do not disclose the realization of full-color display by painting each of the RGB colors separately.

According to the present disclosure, in an electroluminescent display device, it is possible to achieve even higher luminous efficiency, higher brightness, and/or power savings by controlling the thickness of each electroluminescent layer in the stacking direction. Therefore, the electroluminescent device and display device of the present disclosure are expected to contribute to the achievement of, for example, Goal 9.4 of the Sustainable Development Goals (SDGs) proposed by the United Nations, which states, "Improve sustainability by improving infrastructure and industry through increased resource efficiency and the expanded introduction of clean technologies and environmentally friendly technologies and industrial processes."

This disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in the different embodiments are also included in the technical scope of this disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in the respective embodiments.

10, 10A, 210 Electroluminescent element 11 Substrate 12 Buffer layer 13 Electroluminescent device 14 Sealing layer 15 External functional layer 16 Edge cover film 20 TFT layer 21 Anode layer 22 Cathode layer 30, 50, 70, 230, 250 Assembly of layers 31 Hole injection layer 32 First hole transport layer 33 First electron blocking layer 34, 234 First electroluminescent layer 35 First hole blocking layer 36 First electron transport layer 40 First charge generation layer 41 First n-type charge generation layer 42 First p-type charge generation layer 51 Second hole transport layer 52 Second electron blocking layer 53, 253 Second electroluminescent layer 54 Second hole blocking layer 55 Second electron transport layer 56 Electron injection layer 60 Organic laminate 71 Third electroluminescent layer 80 Second charge generation layer 100, 100A, 200 Display device DA Display area NDA Frame area PIX Pixel

Claims (11)

an anode layer;
a cathode layer facing the anode layer in a stacking direction;
a plurality of electroluminescent layers disposed between the anode layer and the cathode layer in the stacking direction and in a direction intersecting the stacking direction;
a charge generation layer disposed between two of the electroluminescent layers adjacent to each other in the stacking direction;
The electroluminescent layer is
A plurality of electroluminescent layers that emit light of the same color are arranged in the stacking direction,
In a direction intersecting the lamination direction, electroluminescent layers of a first color to an n-th color (n is an integer of 2 or more) that emit light of different colors are arranged, and
in at least one pair of two electroluminescent layers adjacent to each other in the stacking direction with the charge generation layer interposed therebetween, a ratio of a thickness of the electroluminescent layer on one side in the stacking direction to a thickness of the electroluminescent layer on the other side in the stacking direction is greater than 4/15 and smaller than 2/3;
Electroluminescent device. The electroluminescent device according to claim 1, wherein each of the electroluminescent layers is a host-guest electroluminescent layer containing a host compound and a guest compound. the electroluminescent layer includes a fluorescent electroluminescent layer in which the guest compound is a fluorescent dopant and a phosphorescent electroluminescent layer in which the guest compound is a phosphorescent dopant according to the emission color;
3. The electroluminescent device according to claim 2, wherein the number of said fluorescent electroluminescent layers in said stacking direction is greater than the number of said phosphorescent electroluminescent layers in said stacking direction. The electroluminescent device according to claim 3, wherein the number of the fluorescent electroluminescent layers in the stacking direction is one or two more than the number of the phosphorescent electroluminescent layers in the stacking direction. The electroluminescent device according to any one of claims 1 to 4, in which only a plurality of electroluminescent layers that emit light of the same color are arranged in the stacking direction. The electroluminescent device according to any one of claims 1 to 5, wherein the number of electroluminescent layers in the stacking direction is 2 or more and 5 or less. When a set of layers including one electroluminescent layer disposed between the anode layer and the charge generation layer, between the charge generation layers, and between the charge generation layer and the cathode layer in the stacking direction is defined as one stack,
7. The electroluminescent device according to claim 1, wherein a ratio of a thickness of the electroluminescent layer in the stack to a thickness of the stack in the stacking direction is 0.05 to 0.35. The electroluminescent device according to any one of claims 1 to 7, which is a top emission type. A display device comprising the electroluminescent device according to any one of claims 1 to 8. 9. A method for manufacturing the electroluminescent device according to claim 1, comprising the step of alternately forming the electroluminescent layer and the charge generation layer on the anode layer or the cathode layer in the stacking direction, the method comprising the step of forming the electroluminescent layer by a vapor deposition method. The method for manufacturing an electroluminescent device according to claim 10, in which the electroluminescent layers having different thicknesses in the stacking direction are formed by changing only the deposition time.

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