Light-emitting device, method for manufacturing same, and display device

Abstract

The present disclosure provides a technique capable of sufficiently expressing advantages of a tandem structure in a light-emitting element of a light-emitting device. A blue light-emitting element (120B) of three color light-emitting elements (120R, 120G, 120B) of a light-emitting device (110) includes a tandem light-emitting unit, and the thickness of a second blue light-emitting layer (53B) on a cathode layer (22B) side in the unit is less than the thickness of a first blue light-emitting layer (34B) on an anode layer (21B) side.

Description

Light-emitting device, manufacturing method thereof, and display device

[0001] The present disclosure relates to a light-emitting device, a manufacturing method thereof, and a display device.

[0002] Mass production of displays incorporating organic light-emitting diodes (OLEDs) has begun in earnest, starting with flat-panel displays for high-end smartphones and TVs. OLED displays have become so widespread that they can be said to have become a central replacement for LCDs as flat-panel displays.

[0003] Aluminum quinolinol complex (Alq ３ Thin-film stacked organic EL elements using a fluorescein-based compound (FQP) as an electron transport layer and emitting layer are known. In contrast, to further improve the luminous efficiency of organic EL elements, development is being advanced in the following order (1) to (3): (1) A host-guest material in which a guest compound serving as a dopant is added to a host compound is used as a material for the emitting layer, carrier (electron or hole) transport layer, or carrier (electron or hole) injection layer of an organic EL element. (2) A fluorescent material, a phosphorescent material, or a fluorescent material and a phosphorescent material are used as a guest compound (dopant) for the emitting layer. (3) An improved guest compound such as a thermally activated delayed fluorescence (TADF) material or a hyperfluorescent material is used as a dopant for the emitting layer.

[0004] Furthermore, from the viewpoint of improving the current efficiency and device life (the period during which desired characteristics can be maintained) of organic EL devices, the following (4) and (5) have been further investigated: (4) Improving the carrier balance by modifying the carrier transport layer or carrier injection layer; (5) Adopting a tandem structure for the device structure of the light-emitting device (see, for example, Patent Document 1).

[0005] Japanese Patent Application Publication No. 2015-32582

[0006] However, when light-emitting elements of two or more colors are arranged regularly in the display area, as in a full-color light-emitting device, and a tandem structure is adopted for the light-emitting elements of at least one color, the advantages of adopting a tandem structure may not be fully realized.

[0007] An object of one embodiment of the present disclosure is to provide a technique that can fully utilize the advantages of employing a tandem structure for light-emitting elements of one or more colors in a light-emitting device that includes light-emitting elements of two or more colors.

[0008] A light-emitting device according to one aspect of the present disclosure is a light-emitting device in which a plurality of light-emitting elements are configured by stacking a first electrode layer, which is one of an anode layer and a cathode layer, a light-emitting layer, and a second electrode layer, which is the other of the anode layer and the cathode layer, in this order in the stacking direction, and are arranged in a direction intersecting the stacking direction, and at least some of the light-emitting elements among the plurality of light-emitting elements include one or more tandem light-emitting units, which are a group of two light-emitting layers that emit light of the same color and are stacked in the stacking direction, and one charge generation layer disposed therebetween, and the thickness of the light-emitting layer on the cathode layer side of the tandem light-emitting unit is thinner than the thickness of the light-emitting layer on the anode layer side.

[0009] A display device according to an aspect of the present disclosure includes the above-described light-emitting device.

[0010] Furthermore, a manufacturing method of a light-emitting device according to one aspect of the present disclosure is a method for manufacturing a light-emitting device in which a plurality of light-emitting elements, each of which is configured by stacking a first electrode layer, which is one of an anode layer and a cathode layer, a light-emitting layer, and a second electrode layer, which is the other of the anode layer and the cathode layer, in this order in a stacking direction, are arranged in a direction intersecting the stacking direction, and the method includes a step of fabricating a tandem light-emitting unit, which is a group of two light-emitting layers that emit light of the same color and that are stacked in the stacking direction, and one charge generation layer disposed therebetween, in at least some of the plurality of light-emitting elements, and in the step of fabricating the tandem light-emitting unit, the light-emitting layer on the cathode layer side is made thinner than the light-emitting layer on the anode layer side by a vapor deposition method or a coating method.

[0011] According to one aspect of the present disclosure, it is possible to fully utilize the advantages of employing a tandem structure for light emitting elements of one or more colors in a light emitting device including light emitting elements of two or more colors.

[0012] FIG. 1 is a plan view schematically showing the configuration of a display device having a frame region and a display region according to embodiment 1 of the present disclosure. FIG. 2 is a view schematically showing the cross-sectional structure of one pixel portion of the display device of FIG. 1. FIG. 3 is a view schematically showing the layer structure of a blue light-emitting element in the layer structure shown in FIG. 2. FIG. 4 is a flowchart showing an example of a method for manufacturing the blue light-emitting element shown in FIG. 3. FIG. 4 is a schematic view showing in more detail the cross-sectional structure of a light-emitting element in the display device according to embodiment 1 of the present disclosure. FIG. 5 is a view for explaining the light-emitting mechanism of the light-emitting element of the light-emitting device shown in FIG. 2. FIG. 6 is a view for explaining the configuration and light-emitting mechanism of the light-emitting element of the light-emitting device according to embodiment 2 of the present disclosure. FIG. 7 is a view schematically showing the layer structure of a light-emitting element of a light-emitting device according to embodiment 4 of the present disclosure.

[0013] [Light-emitting device] In a light-emitting device according to an embodiment of the present disclosure, a plurality of light-emitting elements, each configured by stacking specific layers in a stacking direction, are arranged in a direction intersecting the stacking direction (hereinafter also referred to as a "plane direction"). The light-emitting elements arranged in the plane direction may be the same or different. Furthermore, the light-emitting elements arranged in the plane direction may emit light of the same color or different colors.

[0014] From the viewpoint of realizing desired surface emission by combining light emitted from a specific number of colors, such as a full-color display device, the light-emitting device preferably includes a group of two or more light-emitting elements that emit light in two or more different colors and are arranged in a surface direction. For example, in a full-color display device, three types of light-emitting elements that emit light in three colors, red, green, and blue, are arranged to form one pixel. The pixel is one aspect of a group of two or more light-emitting elements that emit light in two or more different colors. In this case, each light-emitting element is regularly arranged within the display area of ​​the display device, but some of the light-emitting elements may be arranged in the frame area.

[0015] [Light-emitting element] A light-emitting element according to an embodiment of the present disclosure is configured by stacking a first electrode layer, a light-emitting layer, and a second electrode layer in this order in the stacking direction. The term "light-emitting element" refers to a structure (laminated structure) in the stacking direction of the first electrode layer, the second electrode layer, and the light-emitting layer between these layers, as well as various functional layers disposed as needed between the two electrode layers. Furthermore, each layer constituting the light-emitting element may be configured as a layer common to other light-emitting elements adjacent in the surface direction, as long as the effects of the present disclosure can be obtained. For example, the first electrode layer or the second electrode layer may be configured as a single layer common to multiple light-emitting elements arranged in the surface direction. Each layer constituting the light-emitting element will be described below.

[0016] <First Electrode Layer> The first electrode layer is one of a pair of electrode layers, an anode and a cathode. Hereinafter, the anode layer will be described as the first electrode layer, but the second electrode layer may also be the anode layer. The anode layer is an electrode layer for supplying holes to each layer constituting the light-emitting element. The anode layer is conductive. Furthermore, the anode layer has optical properties, for example, reflecting 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.

[0017] To enhance hole injection properties, a material with a relatively high work function (e.g., a material with a work function of 4.5 eV or more) is preferably used as the material for the anode layer. Examples of electrode materials with a high work function include Pt (5.65 eV), Ir (5.25 eV), Ni (5.2 eV), Au (5.15 eV), and Pd (5.15 eV), as well as indium tin oxide (In—Sn—O).

[0018] 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 (such as APC (Ag-Pd-Cu) alloy).

[0019] Examples of electrode materials that transmit visible light include thin films of transparent metal oxides (e.g., indium tin oxide, indium zinc oxide (In—Zn—O), and indium gallium zinc oxide (In—Ga—Zn—O)), thin films made of metal materials such as Al, Mg, and Ag, or alloys of these metal materials (e.g., Mg—Ag alloy), and nanowires (NW) made of these metal materials. Electrode materials that transmit visible light are also used for the cathode layer in top-emission light-emitting devices, which will be described later.

[0020] Among transparent metal oxides, indium tin oxide has a relatively high work function of 4.6 to 5.0 eV and is therefore suitable for use as a material for the anode layer. Furthermore, for the anode layer, a laminate (e.g., indium tin oxide / Ag) in which an indium tin oxide layer is formed on the surface of a metal material can be used for the purpose of improving the conductivity as an electrode layer or adding a function of reflecting visible light.

[0021] <Second Electrode Layer> The second electrode layer is the other electrode layer of a pair of electrode layers, an anode and a cathode. Hereinafter, the cathode layer will be described as the second electrode layer, but the first electrode layer may be the cathode layer. The cathode layer is an electrode layer for supplying electrons to each layer constituting the light-emitting element. 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.

[0022] For example, from the viewpoint of enhancing electron injection properties, a material with a relatively small work function is preferably used as the material for the cathode layer. 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). Examples of alloys include alloys of Mg and Ag, and Al doped with a small amount of Li.

[0023] <Light-emitting layer> The light-emitting layer is a layer that emits light of a predetermined color when an electric field is applied. The light-emitting layer is made of a light-emitting material. The light-emitting layer may have a laminated structure in which two or more light-emitting functional layers corresponding to two or more functions for light emission are stacked, such as an immediate light-emitting layer and a delayed light-emitting layer, and the stacked light-emitting functional layers as a whole exhibit a specific light-emitting function.

[0024] Furthermore, one or more light-emitting layers may be arranged in the stacking direction in the light-emitting element. The two or more light-emitting layers arranged in the stacking direction may be light-emitting layers that emit light of different colors, but from the viewpoint of increasing the color purity of the emitted color, it is preferable that only light-emitting layers that emit light of the same color are arranged in the stacking direction. Having only light-emitting layers of the same color stacked in the stacking direction is advantageous from the viewpoint of more simply determining the thickness of the light-emitting layer, which will be described later. A configuration in which the light-emitting element has two or more light-emitting layers in the stacking direction (tandem light-emitting unit) will be further described later.

[0025] The light-emitting layer may be a light-emitting layer that emits light when excited by an electric field or current. For example, the light-emitting layer may be a light-emitting layer containing quantum dots. The light-emitting layer containing quantum dots is a light-emitting diode (QLED) element that uses quantum dots as the light-emitting layer.

[0026] Quantum dots are semiconductor particles with a particle size of approximately 100 nm or less (e.g., several nm to several tens of nm), and because their composition is derived from semiconductor materials, they are sometimes called semiconductor nanoparticles. Quantum dots can vary their emission wavelength by adjusting the particle size or composition of the particles. The shape of the quantum dots is not limited. For example, the shape of the quantum dots may be a spherical three-dimensional shape (with a circular cross-section), or may be a polygonal three-dimensional shape, a rod-like three-dimensional shape, a branch-like three-dimensional shape, or a three-dimensional shape with an uneven surface, or a combination thereof.

[0027] The quantum dots may be formed of only a core, or may have a core-shell structure including a core and a shell. The shell may be formed as a solid solution on the surface of the core. The quantum dots may also include doped nanoparticles.

[0028] Examples of quantum dot core materials include Si, Ge, CdSe, CdS, CdTe, InP, GaP, InN, ZnSe, ZnS, ZnTe, CdSeTe, GaInP, and ZnSeTe. Examples of quantum dot shell materials include CdS, ZnS, CdSSe, CdTeSe, CdSTe, ZnSSe, ZnSTe, ZnTeSe, and AIP.

[0029] Examples of combinations of quantum dot core and shell materials include CdSe / CdS, InP / ZnS, ZnSe / ZnS, and CIGS / ZnS.

[0030] The quantum dots emit visible light, and by appropriately adjusting the particle size and composition of the quantum dots, it is possible to control the emission wavelength to any wavelength range, for example, from the blue wavelength range to the red wavelength range.

[0031] A ligand may be coordinated to the surface of the quantum dot as long as the effects of the present disclosure are obtained. Various known ligands can be used as the ligand. The ligand may be an organic ligand or an inorganic ligand.

[0032] Furthermore, the quantum dot-containing light-emitting layer may further contain components other than the above-mentioned ligands, as long as the effects of the present disclosure are achieved. For example, the quantum dot-containing light-emitting layer may further contain a binder component that can be interposed between individual quantum dots. When the light-emitting layer contains a binder component, the reliability of the quantum dot-containing light-emitting device is further improved. Furthermore, since the surface flatness of the quantum dot-containing light-emitting layer is improved, subsequent film formation processes such as the electron transport layer can be easily performed with high yield. Furthermore, maintaining the distance between individual quantum dots can prevent concentration quenching.

[0033] The light-emitting layer containing quantum dots can arbitrarily set the emitted color by adjusting the size of the quantum dots. Furthermore, as will be described in detail later, the light-emitting layer containing quantum dots can be easily and precisely fabricated by a coating method, and the thickness of the fabricated light-emitting layer can be sufficiently controlled. Furthermore, layers other than the light-emitting layer containing quantum dots are not thermally damaged by the manufacturing conditions of the light-emitting layer. Therefore, a light-emitting device including a light-emitting layer containing quantum dots is suitable from the viewpoint of increasing the reliability of the light-emitting device.

[0034] The light-emitting layer may also be a light-emitting layer containing a host compound and a guest compound (hereinafter also referred to as a "host-guest light-emitting layer"). A host-guest light-emitting 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 a light-emitting layer doped with a guest compound in this manner, the fluorescence of the host compound is completely lost, and instead, strong light emission that matches the fluorescent 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, the host-guest light-emitting layer can obtain light emission from the guest compound with higher quantum efficiency.

[0035] The host-guest emitting layer may be an emitting layer containing a fluorescent dopant as a guest compound (hereinafter also referred to as a "fluorescent emitting layer"), or an emitting layer containing a phosphorescent dopant as a guest compound (hereinafter also referred to as a "phosphorescent emitting layer"). The light emitting device may include, as the host-guest emitting layer, only a fluorescent emitting layer, only a phosphorescent emitting layer, or both a fluorescent emitting layer and a phosphorescent emitting layer.

[0036] Known materials can be used for the light-emitting materials (host compound and guest compound) of the light-emitting layer. Examples of the host compound include known light-emitting layer materials of each color. The guest compound may be either a fluorescent dopant or a phosphorescent dopant. The guest compound may be a hole-transporting material or an electron-transporting material. By using a hole-transporting material or an electron-transporting material as the guest compound, it becomes easier to adjust the carrier balance by changing the concentration of the guest compound. Examples of the guest compound include fluorescent dopants and phosphorescent dopants, as well as TADF and hyperfluorescent materials.

[0037] In the present disclosure, the term "hole transporting material" refers to a material in which holes exist relatively stably and are transported by an electric field. In the present disclosure, the term "electron transporting material" refers to a material in which electrons exist relatively stably and are transported by an electric field.

[0038] Examples of fluorescent dopants include perylene, BCzVBi, ADN, TBP, DPT, Coumarin6, C545T, PMDFB, quinacridone, rubrene, BTX, ABTX, DCM1, DCM2, DCJTB, DCJMTB, and TDPF. Delayed fluorescent dopants and fluorescent dopants that emit light via delayed fluorescent materials (hyperfluorescence) are also a type (improved type) of fluorescent dopants. Furthermore, examples of luminescent materials that constitute the blue emitting layer include fluorescent dopants such as pyrene-based compounds and anthracene-based compounds.

[0039] Examples of phosphorescent dopants include Ir(ppy) ３ , Ir(thpy) ３ , Ir(t5m-thpy) ３ , Ir(t-5CF ３ -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, αbsn, btp, ppo, C6, pq, β-bsn, and ppz. Examples of light-emitting materials constituting the red and green light-emitting layers include iridium complexes and palladium complexes, which are phosphorescent dopants.

[0040] A fluorescent-emitting layer is preferred from the viewpoint of improving the device life of a light-emitting device. In a fluorescent-emitting layer, a dopant guest compound functions as a carrier (electron or hole) trap in the solid medium of the host compound, becoming a carrier recombination center and directly generating excitons in the solid medium. The process by which the generated excitons relax to the ground state is called a deactivation process. The deactivation process includes a non-radiative process (thermal deactivation) and a radiative process (luminescence). Of these, electroluminescence is the phenomenon in which light is emitted by a radiative process. The guest compound's function as a carrier trap not only improves the quantum efficiency of the emitting layer but also improves the device life due to an increased carrier recombination probability. Thus, a fluorescent-emitting layer is preferred from the viewpoint of improving the luminous efficiency of the emitting layer and the device life of the light-emitting device. In this disclosure, "carrier" refers to a charged particle that carries an electric charge, such as an electron or hole, or both. In this disclosure, a more appropriate form of the carrier, such as the above-mentioned "carrier (electron or hole)," may be listed in parentheses.

[0041] In contrast, phosphorescent layers are preferred from the perspective of increasing the quantum efficiency of light emission. Excitons generated during carrier recombination include singlet and triplet excitons. In fluorescent layers, singlet excitons contribute to light emission, while triplet excitons contribute to light emission in phosphorescent layers. According to spin statistics, the generation rate of singlet excitons is 25%, while the generation rate of triplet excitons is 75%. Therefore, in the "fluorescence" emission process in fluorescent layers, the generation rate of excitons that can contribute to light emission is at most 25%. In contrast, phosphorescent layers can extract light from triplet excitons, thereby increasing the quantum efficiency of phosphorescent layers by approximately three times that of fluorescent layers. Furthermore, in phosphorescent layers, by utilizing intersystem crossing, which is the inversion of spin from singlet excitons to triplet excitons, theoretically all generated excitons can emit "phosphorescence" from triplet excitons. Therefore, the quantum efficiency of the phosphorescent-emitting layer can be increased by up to four times compared to that of the fluorescent-emitting layer.

[0042] However, fluorescent-emitting layers are more advantageous than phosphorescent-emitting layers in terms of cost and supply stability. For example, complexes containing platinum-group elements such as iridium and palladium, which are used as phosphorescent dopants, are very expensive even in small amounts, due to the limited production of platinum-group elements and uneven distribution of their sources, and stable supply may be difficult. Therefore, reducing the amount of phosphorescent dopants using these platinum-group complexes is preferable from the perspectives of cost reduction and economic security.

[0043] In this way, the host-guest light-emitting layer can effectively utilize carriers, and therefore, it is preferable for the light-emitting layer to include a host-guest light-emitting layer from the viewpoint of achieving high luminous efficiency of the light-emitting layer and improving the device life of the light-emitting device.

[0044] In the present disclosure, adjacent light-emitting layers in the stacking direction or in the plane direction may be the same type or different types. Furthermore, the light-emitting layers may be the same type or different types for each emitted color. For example, as described above, a light-emitting device may include a group of a red light-emitting element including a red phosphorescent light-emitting layer containing an iridium complex as a phosphorescent dopant, a green light-emitting element including a green phosphorescent light-emitting layer containing a palladium complex as a phosphorescent dopant, and a blue light-emitting element including a blue fluorescent light-emitting layer containing a pyrene-based compound or an anthracene-based compound as a fluorescent dopant. In this manner, the light-emitting device may include a fluorescent light-emitting layer and a phosphorescent light-emitting layer. Such a light-emitting device including multiple types of light-emitting layers is preferable from the perspective of obtaining the effects of the various light-emitting layers. Furthermore, in the present disclosure, even if the light-emitting layers have functional disadvantages based on their types, the adoption of the tandem light-emitting unit described below is expected to achieve a favorable carrier balance in the light-emitting device.

[0045] The light-emitting device may also include the quantum dot-containing light-emitting layer and a host-guest light-emitting layer. For example, instead of a blue fluorescent light-emitting layer with a relatively low quantum efficiency, the light-emitting device may include a blue quantum dot-containing light-emitting layer with a higher quantum efficiency and red and green phosphorescent light-emitting layers. This embodiment is advantageous from the viewpoint of improving the quantum efficiency of the entire light-emitting device. Furthermore, from the viewpoint of suppressing the increase in cost due to the use of a phosphorescent light-emitting layer, the light-emitting device may include quantum dot-containing light-emitting layers of one or both of red and green colors and a blue fluorescent light-emitting layer. For example, the light-emitting device may include a red host-guest light-emitting layer, a green host-guest light-emitting layer, and a blue quantum dot-containing light-emitting layer. This embodiment is advantageous from the viewpoint of reducing the manufacturing cost for producing the light-emitting layers by forming a second blue light-emitting layer (the blue light-emitting layer on the cathode layer side) as a common layer for all colors, as described below.

[0046] <Charge Generation Layer> The charge generation layer is disposed between the two light-emitting layers in the stacking direction. The charge generation layer is a layer that generates either or both positive and negative charges, and may be a layer that generates either or both electrons and holes. Negative charges generated in the charge generation layer are supplied, with electrons as carriers, and positive charges are supplied, with holes as carriers, to the light-emitting layers located on the anode layer side and the cathode layer side of the charge generation layer in the stacking direction. The charge generation layer may be formed from a known charge-generating material that exhibits the above-mentioned functions.

[0047] The charge generation layer may be composed of an electron generation layer that generates electrons and a hole generation layer that generates holes. Examples of electron generation layers include n-type charge generation layers, and examples of hole generation layers include p-type charge generation layers. 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.

[0048] The p-type charge generation layer can be formed from a material containing an organic hole transport material and an organic electron accepting material (hole supply material) added in a range of 1 to 10%. Examples of organic hole transport materials include known triarylamine organic compounds. Examples of organic electron accepting materials include tetracyanoquinodimethane tetrafluoride (TCNQ-4F). Regarding the materials for the p-type charge generation layer, the p-type charge generation layer formed from the hole transport material and electron accepting material described above is formed entirely using organic materials and can exhibit sufficient hole generation ability as a charge generation layer.

[0049] The n-type charge generation layer can be formed, for example, from an organic electron transport material and a material containing Yb (ytterbium) or Li (lithium), an inorganic metal material added in a range of 5 to 20% and acting as an electron donor material. Examples of organic electron transport materials include oxadiazole-based compounds and phenanthroline-based compounds. Development is also underway to construct both the electron transport layer and the electron donor material in the n-type charge generation layer using organic materials. Examples of organic electron donor materials include BUPH1, BPen, p-MeO-Phen, and p-NMe. ２ -Phen, and p-Pyrrd-Phen.

[0050] It is currently considered difficult to achieve sufficient electron supplying properties in an n-type charge generation layer composed solely of the above-mentioned organic electron supply material. However, in the present disclosure, even when the charge generation layer is composed of an n-type charge generation layer composed solely of the above-mentioned organic electron supply material, it is expected that a suitable carrier balance can be achieved in the light-emitting device by employing the tandem light-emitting unit described below.

[0051] <Tandem Light-Emitting Unit> The light-emitting element in the light-emitting device of the present disclosure includes one or more tandem light-emitting units. At least some of the light-emitting elements in the light-emitting device may include tandem light-emitting units, and all of the light-emitting elements in the light-emitting device may include tandem light-emitting units. Furthermore, the light-emitting element including a tandem light-emitting unit may include at least one tandem light-emitting unit in one light-emitting element, and may include two or more tandem light-emitting units.

[0052] A tandem light-emitting unit is a group of two of the aforementioned light-emitting layers that emit light of the same color and are stacked in the stacking direction, with one charge generation layer disposed between them. A tandem light-emitting unit is counted as one unit when it consists of a first light-emitting layer, a charge generation layer, and a second light-emitting layer in the stacking direction. A tandem light-emitting unit is counted as two units when it consists of a first light-emitting layer, a first charge generation layer, a second light-emitting layer, a second charge generation layer, and a third light-emitting layer.

[0053] Therefore, the number of light-emitting layers in a light-emitting element including a tandem light-emitting unit may be two or more. The number of light-emitting layers is preferably two or more, and more preferably three or more, from the viewpoints of increasing the light-emitting efficiency of the light-emitting layers and extending the element life of the light-emitting element. On the other hand, the number of light-emitting layers is preferably five or less, and more preferably four or less, from the viewpoints of suppressing an increase in the driving voltage of the light-emitting element, realizing a withstand voltage of the driver corresponding to the driving voltage, and maintaining the flexibility of the light-emitting element (suppressing an increase in the total thickness of the light-emitting element).

[0054] In the present disclosure, the term "same color" means, when there are two or more emission peak wavelengths, that all of the emission peak wavelengths are within a range of ±5 nm from each other and that the maximum of the full widths at half maximum of all the emission peak wavelengths is 1.25 times or less the minimum of the full widths at half maximum of the remaining emission peak wavelengths. When the materials constituting the light-emitting layers are the same or similar materials having the same skeleton, those light-emitting layers usually emit light of the same color.

[0055] However, in the top-emission structure described below, the emission peak wavelengths of the respective colors may vary due to differences in the microcavity structures of the respective colors. Therefore, in the top-emission structure, "the same color" refers to light colors whose emission peak wavelengths are all within a range of ±10 nm and whose full widths at half maximum satisfy the above range.

[0056] In the present disclosure, "different colors" refers to light colors that are not included in the above-mentioned same color range. Typical examples of "different colors" include light colors of a red light-emitting layer, a green light-emitting layer, and a blue light-emitting layer, which are known as multi-color light-emitting layers for full-color display.

[0057] Furthermore, in the light-emitting device of the present disclosure, the positions in the stacking direction of light-emitting layers adjacent in the plane direction may be the same position (height) or different positions (heights) (may be offset).

[0058] <Functional Layer> The light-emitting element of the present disclosure may further include another layer between the anode layer or cathode layer and the light-emitting layer, or between the light-emitting layer and the charge generation layer, as long as the effects of the present disclosure are obtained. Examples of other layers include functional layers that contribute to at least one of carrier (electron or hole) injection, migration, and blocking. Examples of functional layers include 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 and the 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, the electron blocking layer, the light-emitting layer, the hole blocking layer, the electron transport layer, and the charge generation layer may be arranged in a stacking direction of the light-emitting element.

[0059] The hole injection layer is disposed adjacent to the anode layer, for example. The hole injection layer may be composed of a hole transport material and an electron acceptor material. For example, the hole injection layer may be composed of a material including an organic hole transport material and an organic electron acceptor material (hole donor material) added in the range of 1 to 10%. Examples of organic hole transport materials include known triarylamine organic compounds. Examples of organic electron acceptor materials include tetracyanoquinodimethane tetrafluoride (TCNQ-4F).

[0060] The electron injection layer is disposed adjacent to the cathode layer, for example. The electron injection layer may be composed 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 described below, the electron injection layer may be composed of an organic electron transport material such as an oxadiazole-based compound or a phenanthroline-based compound, and a metal material (e.g., Li or Yb) doped therein.

[0061] LiF used in the electron injection layer exhibits excellent electron injection properties. On the other hand, the formation of a functional layer containing an inorganic material such as Yb, not limited to LiF, is generally carried out at a higher temperature than the formation of other layers (e.g., a light-emitting layer or other functional layers) made of organic materials due to the high melting point of the inorganic material. Therefore, there is a risk of thermal damage to previously formed layers. From the viewpoint of reducing such thermal damage to other layers, it is preferable that the light-emitting element be composed of layers made only of organic materials.

[0062] From the above viewpoint, organic electron injection materials have been developed that can exhibit sufficient properties 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. Examples of such organic electron injection materials include BUPH1, BPen, p-MeO-Phen, and p-NMe. ２These include p-Pyrrd-Phen and p-Pyrrd-Phen. However, in terms of the electron injection ability of the electron injection layer, inorganic materials such as LiF are superior to the above-mentioned organic electron injection materials. However, in the present disclosure, even when an electron injection layer made of an organic material is disposed adjacent to the cathode layer, it is expected that a suitable carrier balance can be achieved in the light-emitting device by employing a tandem light-emitting unit described below.

[0063] The hole transport layer may be made of an organic hole transport material, such as a triarylamine organic compound.

[0064] The electron blocking layer, like the hole transporting layer, may be made of an organic hole transporting material, and the material of the electron blocking layer may be the same as or different from that of the hole transporting layer.

[0065] The hole-blocking layer may be composed of an organic electron-transporting material, such as an oxadiazole-based compound or a phenanthroline-based compound, and may contain lithium quinoline (Liq) in addition to the organic electron-transporting material.

[0066] Like the hole-blocking layer, the electron-transporting layer may be made of the organic electron-transporting material described above. The material of the electron-transporting layer may be the same as or different from that of the hole-blocking layer.

[0067] In the present disclosure, the thickness of the light-emitting layer on the cathode layer side in a tandem light-emitting unit is thinner than the thickness of the light-emitting layer on the anode layer side. That is, in the present disclosure, the thickness of the light-emitting layer on the cathode layer side of two light-emitting layers adjacent to each other in the stacking direction via a charge generation layer is thinner than the thickness of the light-emitting layer on the anode layer side. "Adjacent" means that two specific layers are adjacent to each other in a specific direction (stacking direction or surface direction). The two specific layers do not have to be adjacent to each other, and another layer may be interposed between the two layers.

[0068] In a tandem light-emitting unit, the ratio of the thickness of the light-emitting layer on the cathode side to the thickness of the light-emitting layer on the anode side is preferably 0.1 or more, more preferably 0.3 or more, from the viewpoints of extending the device life and reducing excess carriers (holes) described below. On the other hand, from the viewpoints of improving luminous efficiency and reducing driving voltage (power consumption), this thickness ratio is preferably 0.9 or less, more preferably 0.8 or less. When a light-emitting device includes two or more tandem light-emitting units in the stacking direction, the "light-emitting layer on the anode side" refers to the light-emitting layer closest to the anode layer, and the "light-emitting layer on the cathode side" refers to the light-emitting layer closest to the cathode layer. In this case, the thicknesses of the three or more light-emitting layers in the light-emitting device may be the same for some of the adjacent light-emitting layer combinations, or may gradually decrease from the anode side to the cathode side. Furthermore, the thicknesses of the three or more light-emitting layers in the light-emitting device may decrease linearly from the anode side to the cathode side, or may vary quadratically or in some other manner. Alternatively, the thicknesses of the three or more light-emitting layers in the light-emitting element may be determined depending on factors other than the correlation described above (for example, the function of an intervening charge generating layer). In the present disclosure, "excess carriers" refers to carriers that are in excess and cannot contribute to the formation of electron-hole pairs. Regarding "excess carriers," more appropriate carrier forms may be expressed as "excess holes," "excess electrons," or the above-mentioned "excess carriers (holes)" in parentheses.

[0069] Such a configuration is preferable from the viewpoint of optimizing the carrier balance in each light-emitting layer in the stacking direction. The thickness of each light-emitting layer in the stacking direction can be appropriately determined depending on various factors, such as the combination of materials for each layer in the light-emitting element and the purpose of making the thicknesses of the light-emitting layers different from each other, as long as it is in accordance with the technical concept of the present disclosure.

[0070] The thicknesses of the multiple light-emitting layers in a tandem light-emitting unit can be determined, for example, by the following method. That is, a light-emitting element such as a tandem light-emitting unit having light-emitting layers of the same thickness in the stacking direction is prepared, and its applied voltage, light-emitting efficiency, or device life is measured to obtain a reference value. Meanwhile, a light-emitting element including a tandem light-emitting unit in which the thickness of the light-emitting layer on the cathode layer side is thinner is prepared, and its applied voltage, light-emitting efficiency, or device life is measured to obtain a measured value. Based on the obtained measured values ​​and the reference value, an appropriate thickness for each light-emitting layer in the tandem light-emitting unit is determined depending on the purpose.

[0071] Furthermore, the thickness of each light-emitting layer in the stacking direction can be determined as a thickness appropriate for the purpose by simulating the carrier injection and / or carrier transport properties between layers in the light-emitting device and conducting demonstration experiments based on the simulation results. Specifically, the applied voltage, luminous efficiency, or device life of a tandem light-emitting device having light-emitting layers of the same thickness in the stacking direction is measured and used as a reference value. Meanwhile, as described above, a tandem light-emitting device is fabricated in which the thickness of the light-emitting layer on the cathode layer side is made thinner, and the applied voltage, luminous efficiency, or device life is measured. Based on the obtained measurements and the reference value, the appropriate thickness is determined according to the purpose. In this case, by setting the multiple light-emitting layers stacked in the stacking direction to only light-emitting layers that emit the same color, the carrier injection and carrier transport parameters between different light-emitting layers in the stacking direction can be made identical. This facilitates simulations, making it possible to more easily determine the thickness of each light-emitting layer.

[0072] The light-emitting layers included in the tandem light-emitting unit may all be light-emitting layers that emit light with a wavelength of 430 nm or more and 470 nm or less. Light with this wavelength exhibits blue color, and these light-emitting layers are so-called blue light-emitting layers.

[0073] Currently, phosphorescent dopants with good blue emission properties are still in the development stage and are not readily available, so there is a trend to use fluorescent dopants for the blue light-emitting layer and phosphorescent dopants for the red and green light-emitting layers. In light-emitting devices including these light-emitting layers, the quantum efficiency of the fluorescent dopant is lower than that of the phosphorescent dopant, both theoretically and practically. Therefore, in such light-emitting devices, the amount of current passed through the blue light-emitting layer may be greater than that passed through the red and green light-emitting layers to balance the light emission.

[0074] However, simply increasing the amount of current flowing through the fluorescent blue-emitting layer inevitably shortens the lifespan of the blue-emitting layer (accelerating degradation). To address this issue, it is effective to arrange multiple blue-emitting layers in the stacking direction, i.e., to stack two or more fluorescent layers. By arranging two or more fluorescent blue-emitting layers in a tandem light-emitting unit in the stacking direction, it is possible to substantially double or more the amount of blue light emitted by the light-emitting device without changing the amount of current flowing through each of the light-emitting layers of each color (including all blue-emitting layers stacked in the stacking direction). This makes it possible to suppress degradation of each blue-emitting layer. Furthermore, the charge generation layer in the tandem light-emitting unit supplies holes and electrons to each of the adjacent blue-emitting layers connected in series to the charge generation layer in the stacking direction, causing each layer to emit light. Therefore, even fluorescent layers can emit light with a luminous efficiency substantially equal to or greater than that of a phosphorescent light-emitting layer when viewed as a whole tandem light-emitting unit.

[0075] As described above, the light-emitting device of the present disclosure is advantageous in balancing the lifetime and light emission intensity of the fluorescent and phosphorescent light-emitting layers. Furthermore, the thickness of light-emitting layers with shorter emission wavelengths and their associated functional layers, such as blue light-emitting layers and their associated functional layers, is generally thinner than the thickness of red and green light-emitting layers and their associated functional layers. Therefore, the above-described configuration having multiple blue light-emitting layers of different thicknesses in the stacking direction is advantageous from the perspective of balancing the total layer thickness of a light-emitting element having a blue light-emitting layer with the total layer thickness of light-emitting elements having light-emitting layers of other colors. From this perspective, the total number of fluorescent light-emitting layers stacked in the stacking direction is preferably two or three. If the number of fluorescent light-emitting layers is four or more, the difference in total layer thickness between the light-emitting element having the phosphorescent light-emitting layer becomes too large, resulting in an unbalanced structure of the light-emitting element, which may result in a low manufacturing yield of the light-emitting device.

[0076] <Stack> When a light-emitting element includes a functional layer, the tandem light-emitting unit includes a set of layers including one light-emitting layer and a functional layer, located 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. Such a set of layers is also referred to as a "stack" in the present disclosure. The stack may include the above-mentioned functional layer in addition to one light-emitting layer. A stack is also included in a light-emitting element with a single structure that does not have a tandem structure, and in a tandem light-emitting unit, multiple stacks may be arranged between the anode layer and the cathode layer in the stacking direction. Note that, in the present disclosure, the anode layer, the cathode layer, and the charge generation layer are not included in the stack.

[0077] The thickness of the stack (length in the stacking direction) is determined so that the amount of light emitted from the light-emitting layer is a theoretical value or a value close to it. In the present disclosure, the thickness of each light-emitting layer in the stacking direction can be set within the aforementioned ratio range, and therefore, it is preferable from the viewpoint of improving the light-emitting efficiency of the light-emitting device to set the thickness of the stack according to the thickness of the light-emitting layer set in this manner. From this viewpoint, the ratio of the thickness of the light-emitting 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. The thickness of the stack is calculated as the sum of the thicknesses of the light-emitting layer and functional layer in the stack. In this case, among the light-emitting layer and functional layer, layers with very small thicknesses (for example, layers that may have a thickness of less than 1 nm) may be ignored when calculating the thickness of the stack.

[0078] [Light Emission Mode of Light-Emitting Device] The light-emitting device of the present disclosure may be a top-emission type. The light-emitting device of the present disclosure is particularly suitable for a top-emission type light-emitting device capable of full-color display. In a top-emission type light-emitting device, the microcavity effect is utilized to further increase light extraction efficiency. The light extraction efficiency is determined by the resonance conditions of the microcavity structure. The resonance conditions are determined by the position of the light-emitting point in the stacking direction of the organic layers, the total thickness of the organic layers (= the distance between the electrodes), the emission wavelength, and the refractive index of the organic material. Therefore, if the light-emitting point (light-emitting layer) is stacked at an appropriate position relative to the appropriate total thickness of the organic layers, the light extraction efficiency to the outside will improve; if the position is not appropriate, the light extraction efficiency will decrease. Therefore, the position of the light-emitting layer cannot be set arbitrarily; the light-emitting layer must be positioned at one of the periodic positions depending on the emission wavelength and refractive index. Thus, in a top-emission type light-emitting device, the appropriate position of the light-emitting layer and the distance between the electrode layers differ for each color.

[0079] The light-emitting device of the present disclosure includes a light-emitting element including a tandem light-emitting unit having multiple light-emitting layers that emit the same color in the stacking direction. Therefore, when the tandem light-emitting unit has functional layers, the number of functional layers whose thicknesses can be adjusted increases, making it easier to adjust the distance between electrode layers. Therefore, the present disclosure can realize a top-emission light-emitting device with appropriate resonance conditions.

[0080] [Method for Manufacturing Light-Emitting Device] In manufacturing the light-emitting device of the present disclosure, the thickness of each light-emitting layer is controlled so that the thickness of the light-emitting layer on the cathode layer side is thinner than the thickness of the light-emitting layer on the anode layer side adjacent to the charge generation layer in the stacking direction. Otherwise, the light-emitting device of the present disclosure can be manufactured in the same way as a light-emitting element having multiple light-emitting layers in the stacking direction.

[0081] That is, the method for manufacturing a light-emitting device according to the present disclosure includes manufacturing a light-emitting device in which a plurality of light-emitting elements, each having an anode layer, a light-emitting layer, and a cathode layer stacked in this order in a stacking direction, are arranged in a direction intersecting the stacking direction, and further includes a step of fabricating the above-mentioned tandem light-emitting unit in some of the plurality of light-emitting elements (a tandem light-emitting unit fabrication step).

[0082] In the tandem light-emitting unit fabrication process, the light-emitting layer on the cathode layer side is fabricated thinner than the light-emitting layer on the anode layer side. The method for fabricating the light-emitting layer can be appropriately determined depending on the light-emitting layer to be fabricated, and may be selected from, for example, a vapor deposition method and a coating method. For example, the method for fabricating the host-guest light-emitting layer may be a vapor deposition method. Fabricating the host-guest light-emitting layer by a vapor deposition method is preferred from the viewpoint of precisely controlling the thickness of the light-emitting layer.

[0083] Light-emitting layers prepared by vapor deposition are generally preferred from the viewpoints of achieving high brightness and low-voltage driving. Vapor deposition is also preferred from the viewpoint of realizing high-definition display devices, since it allows for the highly accurate preparation of light-emitting layers of light-emitting elements consisting of fine pixels. Furthermore, host-guest light-emitting layers can be prepared by doping a host compound with a guest compound. Co-evaporation using multiple evaporation sources is more preferred as a vapor deposition method, since it allows for the preparation of host-guest light-emitting layers.

[0084] Because the tandem light-emitting unit includes a repeating structure of specific layers (light-emitting layer, charge-generating layer, and light-emitting layer), it can be manufactured by repeating the fabrication of the specific layer multiple times. In fabricating the tandem light-emitting unit, light-emitting layers of different thicknesses are preferably fabricated by varying only the deposition time of the light-emitting layer material in the vapor deposition method. This fabrication method allows conditions other than the deposition time for fabricating the light-emitting layer to be constant (e.g., 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 defining the pixel shape). Therefore, variations in characteristics due to changes in the conditions for each light-emitting layer in the stacking direction are suppressed, and the effects of differences in the thickness of the light-emitting layer in the stacking direction can be more significantly exhibited.

[0085] Furthermore, for example, a method for producing a quantum dot-containing light-emitting layer may be a coating method. The quantum dot-containing light-emitting layer can be produced by applying an ink in which quantum dots are dispersed in a dispersion medium, drying the coating, and optionally curing the coating. A slit coater or inkjet printer can be used to apply the ink. Therefore, the coating method makes it possible to produce a fine light-emitting layer under normal conditions, such as room temperature, normal pressure, and atmospheric air.

[0086] [Display Device] The display device according to the present disclosure includes the above-described light-emitting device. The display device according to the present disclosure can be configured similarly to a known display device having a known light-emitting device, except for including the above-described light-emitting device. Examples of the display device include a television set and a smartphone.

[0087] The light emitting device of the present disclosure can be applied to a light source of a device that adjusts and outputs two or more emitted colors, in addition to a display device. For example, the light emitting device of the present disclosure can be applied to a lighting device. The light emitting device of the present disclosure is also suitable for a lighting device that can output any light, for example, from warm light to cool light, by appropriately adjusting the output of two or more emitted colors.

[0088] [Explanation of Specific Embodiments] The light-emitting device, its manufacturing method, and display device of the present disclosure will be described in more detail below with reference to the drawings, taking a light-emitting device including an organic light-emitting diode (OLED) as an example. In this specification, configurations of different colors among similar basic configurations are indicated by adding a symbol indicating the color to the symbol of the basic configuration. For example, a symbol R is added to a configuration related to red, a symbol G is added to a configuration related to green, and a symbol B is added to a configuration related to blue. In addition, arranging light-emitting layers of multiple colors in the surface direction is sometimes referred to as "color-separated."

[0089] Hereinafter, the display device and light-emitting device of the present disclosure will be described in more detail, mainly taking as an example an embodiment having the above three color light-emitting layers. In the following embodiments, a red light-emitting element and a green light-emitting element each have one red light-emitting layer and one green light-emitting layer, and a blue light-emitting element has two overlapping blue light-emitting layers, but the present disclosure is not limited thereto.

[0090] [Embodiment 1] <Configuration of Display Device and Light-Emitting Device> Fig. 1 is a plan view schematically illustrating the configuration of a display device 100 according to embodiment 1 of the present disclosure. Fig. 1 illustrates a smartphone, which is an example of a display device. As illustrated 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, and for example, the plurality of pixels PIX are regularly arranged within the display area. Each pixel PIX includes a red sub-pixel RSP, a green sub-pixel GSP, and a blue sub-pixel BSP.

[0091] Note that the pixel configuration of the display device according to the present disclosure is not limited to the above configuration. For example, in the display device according to the present disclosure, one pixel PIX may include subpixels of other colors in addition to the red subpixel RSP, the green subpixel GSP, and the blue subpixel BSP.

[0092] 2 is a cross-sectional view showing a schematic configuration of one pixel PIX of the display device 100 of FIG. 1. The one pixel PIX includes a red light-emitting element 120R constituting the red subpixel RSP, a green light-emitting element 120G constituting the green subpixel GSP, and a blue light-emitting element 120B constituting the blue subpixel BSP. The light-emitting device 110 includes a substrate 11, a buffer layer 12, a TFT (thin film transistor) layer 13 including a pixel circuit, an edge cover film 14, a sealing layer 15, and an external functional layer 16 stacked in this order. The red light-emitting element 120R, the green light-emitting element 120G, and the blue light-emitting element 120B are disposed on the TFT layer 13 and within the edge cover film 14 and sealing layer 15.

[0093] 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 formed of two polyimide films with an inorganic film sandwiched between them. The buffer layer 12 can be formed of an inorganic insulating layer that prevents the intrusion of foreign substances such as water and oxygen. The TFT layer 13 includes pixel circuits that control the light-emitting elements 120 (red light-emitting element 120R, green light-emitting element 120G, and blue light-emitting element 120B).

[0094] In addition, examples of each of the multiple red light-emitting elements 120R, green light-emitting elements 120G, and blue light-emitting elements 120B included in the light-emitting device 110 include organic light-emitting diodes (OLEDs), which will be described in more detail below, and quantum dot light-emitting diode (QLED) elements.

[0095] Light-emitting device 110 includes red light-emitting element 120R, green light-emitting element 120G, and blue light-emitting element 120B. Red light-emitting element 120R includes an anode layer 21R and a cathode layer 22R, with a first red light-emitting layer 34R between these electrode layers. Similarly, green light-emitting element 120G includes an anode layer 21G, a first green light-emitting layer 34G, and a cathode layer 22G. Blue light-emitting element 120B includes an anode layer 21B, a first blue light-emitting layer 34B, a charge generation layer 40B, a second blue light-emitting layer 53B, and a cathode layer 22B. When the light-emitting element 120 is an OLED, the first red light-emitting layer 34R, the first green light-emitting layer 34G, the first blue light-emitting layer 34B, and the second blue light-emitting layer 53B include organic light-emitting layers, and when the light-emitting element 120 is a QLED, the first red light-emitting layer 34R, the first green light-emitting layer 34G, the first blue light-emitting layer 34B, and the second blue light-emitting layer 53B include quantum dots. As described above, quantum dots are semiconductor particles with a diameter (particle size) of several nanometers to several tens of nanometers, and can emit light at a desired wavelength by appropriately selecting the particle size and material.

[0096] When the light-emitting element 120 is an OLED, the first red light-emitting layer 34R of the red light-emitting element 120R and the first green light-emitting layer 34G of the green light-emitting element 120G may contain a phosphorescent dopant as a guest compound, and the first blue light-emitting layer 34B and / or the second blue light-emitting layer 53B of the blue light-emitting element 120B may contain a fluorescent dopant as a guest compound.

[0097] The light-emitting layer containing a fluorescent dopant is not limited to one that emits blue light. The first red light-emitting layer 34R and / or the first green light-emitting layer 34G may contain a fluorescent dopant. Conversely, the first blue light-emitting layer 34B and / or the second blue light-emitting layer 53B may contain a phosphorescent dopant.

[0098] The light-emitting device 110 includes a single-structure red light-emitting element 120R and a green light-emitting element 120G, and a tandem-structure blue light-emitting element 120B. The single-structure light-emitting element is a light-emitting element in which a single light-emitting layer (first light-emitting layer 34) is disposed between the anode layer 21 and the cathode layer 22 in the stacking direction. The tandem-structure light-emitting element is a light-emitting element in which two light-emitting layers, the first light-emitting layer 34 and the second light-emitting layer 53, are disposed between the anode layer 21 and the cathode layer 22 in the stacking direction, and a charge generation layer 40 is fabricated between them. The first light-emitting layer 34 and the second light-emitting layer 53, which are overlapping in the stacking direction, are light-emitting layers that emit light of the same color.

[0099] In the light-emitting device 110, the blue light-emitting element 120B that emits blue light has two layers, a first blue light-emitting layer 34B and a second blue light-emitting layer 53B, each containing a fluorescent dopant. Thus, in the present disclosure, a configuration in which the tandem-structured light-emitting element includes multiple fluorescent light-emitting layers containing a fluorescent dopant is preferred. However, in the present disclosure, the tandem-structured light-emitting element may have two light-emitting layers containing a phosphorescent dopant, and the red light-emitting element 120R and / or the green light-emitting element 120G may have two light-emitting layers.

[0100] 2 shows a configuration in which the red light-emitting element 120R, the green light-emitting element 120G, and the blue light-emitting element 120B are provided with a cathode layer 22R, a cathode layer 22G, and a cathode layer 22B, respectively. The light-emitting device 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 light-emitting element 120R, the green light-emitting element 120G, and the blue light-emitting element 120B. Note that each light-emitting element 120 according to the present disclosure is a top-emission type (a structure in which light is extracted from the upper side, i.e., the cathode layer 22 side). In the light-emitting element 120, the anode layer 21 functions as an anode, and the cathode layer 22 functions as a cathode.

[0101] The edge cover film 14 is insulating and covers the edges of the anode layers 21R, 21G, and 21B to define the regions of the sub-pixels in a plan view. The edge cover film 14 is fabricated by applying an organic material such as polyimide or acrylic resin and then patterning it by photolithography.

[0102] The sealing layer 15 covering the light-emitting device 110 is a layer that prevents foreign substances such as water or oxygen from penetrating into the light-emitting device 110, and can be composed of, for example, two layers of inorganic sealing films and an organic film formed between them. The external functional layer 16 is a layer that adds various functions to the display device 100, such as optical control, a touch sensor, and surface protection.

[0103] (Configuration of Tandem Light-Emitting Element) With reference to Fig. 3, a description will be given of blue light-emitting element 120B having a tandem structure, which is one of the element configurations of light-emitting element 120 shown in Fig. 2. Fig. 3 is a diagram schematically showing the layer configuration of the tandem light-emitting element in display device 100 shown in Fig. 2.

[0104] The blue light-emitting element 120B is configured by stacking an anode layer 21B, a first stack 30B, a first charge generation layer 40B, a second stack 50B, and a cathode layer 22B in this order on a TFT layer 13. Note that, below the TFT layer 13, a substrate 11 and a buffer layer 12 are stacked in this order.

[0105] When the blue light-emitting element 120B is an OLED, the first stack 30B is composed of a hole injection layer 31, a first hole transport layer 32, a first electron blocking layer 33B, a first blue light-emitting layer 34B, a first hole blocking layer 35B, and a first electron transport layer 36B. The second stack 50B is composed of a second hole transport layer 51B, a second electron blocking layer 52B, a second blue light-emitting layer 53B, a second hole blocking layer 54B, a second electron transport layer 55B, and an electron injection layer 56B. The first charge generation layer 40B is composed of an n-type first charge generation layer (electron generation layer) 41B and a p-type first charge generation layer (hole generation layer) 42B. The first stack 30B, the first charge generation layer 40B, and the second stack 50B constitute an organic laminate 23B. Organic laminate 23B includes first blue light-emitting layer 34B, first charge generation layer 40B, and second blue light-emitting layer 53B in this order, and is one embodiment of the tandem light-emitting unit of the present disclosure.

[0106] When the blue light-emitting element 120B is a QLED, the first blue light-emitting layer 34B and the second blue light-emitting layer 53B included in the first stack 30B and the second stack 50B are light-emitting layers including quantum dots.

[0107] The hole injection layer 31, the first hole transport layer 32, the first electron blocking layer 33B, the first hole blocking layer 35B, the first electron transport layer 36B, the n-type first charge generation layer 41B, the p-type first charge generation layer 42B, the second hole transport layer 51B, the second electron blocking layer 52B, the second hole blocking layer 54B, the second electron transport layer 55B, and the electron injection layer 56B are functional layers that contribute to at least one of the injection, movement, and generation of carriers (electrons or holes).

[0108] As described above, the first red light-emitting layer 34R and the first green light-emitting layer 34G are both host-guest light-emitting layers containing a phosphorescent dopant as a guest compound, while the first blue light-emitting layer 34B and the second blue light-emitting layer 53B are host-guest light-emitting layers containing a fluorescent dopant as a guest compound. The thickness of the first light-emitting layer 34 in the light-emitting element 120 varies depending on the emitted color. For example, the thickness of the first light-emitting layer 34 of the red light-emitting element 120R and the green light-emitting element 20G is 25 nm to 50 nm. The thickness of the first blue light-emitting layer 34B of the blue light-emitting element 120B is 10 to 25 nm. The thickness of the second blue light-emitting layer 53B is thinner than the thickness of the first blue light-emitting layer 34B. For example, the thickness of the second blue light-emitting layer 53B is 0.1 to 0.9 times the thickness of the first blue light-emitting layer 34B. Thus, blue light-emitting element 120B includes one tandem light-emitting unit including a first blue light-emitting layer 34B and a second blue light-emitting layer 53B that emit light of the same color, and a first charge generation layer 40B disposed therebetween.

[0109] (Method of Manufacturing Light-Emitting Element) Next, an example of a method of manufacturing a light-emitting element will be described with reference to Fig. 4. Fig. 4 is a flowchart showing an example of a method of manufacturing light-emitting device 110 including blue light-emitting element 120B shown in Fig. 3.

[0110] 4, in step S1, the anode layer 21 is formed on the TFT layer 13. Specifically, an Ag layer and an indium tin oxide layer are formed in this order by sputtering.

[0111] 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-deposited at a predetermined deposition rate by adjusting the deposition temperature and deposition time so that the material is deposited to a predetermined thickness and ratio. Here, a deposited film is formed uniformly over the entire surface of the workpiece without using a fine metal mask.

[0112] In this example, some functional layers are fabricated as layers common to all light-emitting elements of all colors, but the manufacturing method of the present disclosure is not limited to this. The thickness of each functional layer may be different for each light-emitting element of each color, for example. Such functional layers with different thicknesses for each light-emitting element can be fabricated by vapor deposition through a mask.

[0113] In step S3, the first hole transport layer 32 is formed on the hole injection layer 31. Specifically, the hole transport material is vapor-deposited at a predetermined vapor deposition rate by adjusting the vapor deposition temperature and vapor deposition time so that a layer having a predetermined thickness is formed. Here, the vapor-deposited film is formed without using a fine metal mask.

[0114] In step S4, the first electron blocking layer 33 is formed on the first hole transport layer 32. Specifically, the hole transport material is vapor-deposited at a predetermined vapor deposition rate by adjusting the vapor deposition temperature and vapor deposition time so that the hole transport material is deposited to a predetermined thickness. Here, a fine metal mask is used to vapor-deposit the hole transport material to a first thickness corresponding to each color. The first thickness may be the same for each light-emitting element of each color, or may be different.

[0115] In step S5, the first light-emitting layer 34 is formed on the first electron blocking layer 33. Specifically, the co-evaporation of a host compound and a guest compound (dopant) is carried out at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that a layer having a predetermined film thickness and guest compound concentration (dopant concentration) is formed. Here, a fine metal mask is used to evaporate materials corresponding to each color while precisely controlling the thickness of each layer and the concentration of the guest compound.

[0116] The first light-emitting layer 34 may be formed using a resist mask produced by photolithography instead of a fine metal mask.

[0117] In step S6, the first hole blocking layer 35 is formed on the first light-emitting layer 34. Specifically, the electron transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that a layer having a predetermined thickness is formed. In this step, the evaporated film is formed without using a fine metal mask.

[0118] In step S7, the first electron transport layer 36 is formed on the first hole blocking layer 35. Specifically, the electron transport material is vapor deposited at a predetermined vapor deposition rate by adjusting the vapor deposition temperature and vapor deposition time so that a layer having a predetermined thickness is formed. The vapor deposition may involve vapor deposition of only the electron transport material, or co-deposition of the electron transport material and lithium quinoline. In this step, the vapor deposition film is formed without using a fine metal mask.

[0119] 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 electron donor material, Yb or Li, is performed at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layers are laminated to a predetermined thickness and ratio. Here, a fine metal mask is used to form an evaporated film only on the blue light-emitting element 120B for which the second blue light-emitting layer 53B is to be formed.

[0120] In step S9, the 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 evaporation rate by adjusting the evaporation temperature and evaporation time so that the layers are laminated to a predetermined thickness and ratio. Here, a fine metal mask is used to form an evaporated film only on the blue light-emitting element 120B for which the second blue light-emitting layer 53B is to be formed.

[0121] 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 vapor-deposited at a predetermined vapor deposition rate by adjusting the vapor deposition temperature and vapor deposition time so that the hole transport material is deposited to a predetermined thickness. Here, a fine metal mask is used to form a vapor-deposited film only on the blue light-emitting element 120B for which the second blue light-emitting layer 53B is to be formed.

[0122] In step S11, the second electron blocking layer 52 is formed on the second hole transport layer 51. Specifically, the hole transport material is vapor-deposited at a predetermined vapor deposition rate by adjusting the vapor deposition temperature and vapor deposition time so that the hole transport material is deposited to a predetermined thickness. Here, a fine metal mask is used to vapor-deposit the second electron blocking layer 52 to a second thickness only on the blue light-emitting element 120B for which the second blue light-emitting layer 53B is to be formed.

[0123] In step S12, a second blue light-emitting layer 53B is formed on the second electron blocking layer 52. Specifically, a host compound and a guest compound (dopant) are co-deposited at a predetermined deposition rate by adjusting the deposition temperature and deposition time so that a predetermined film thickness and guest compound concentration (dopant concentration) are achieved. Here, a fine metal mask is used to deposit a material appropriate for a light-emitting element of a desired emission color (here, blue light-emitting element 120B) while precisely controlling the thickness and guest compound concentration. The second blue light-emitting layer 53B is deposited so that its thickness is thinner than that of the first blue light-emitting layer 34B.

[0124] The second blue light-emitting layer 53B may be formed using a resist mask produced by photolithography instead of a fine metal mask.

[0125] In step S13, the second hole-blocking layer 54 is formed on the second blue light-emitting layer 53B. Specifically, the electron-transporting material is vapor-deposited at a predetermined vapor deposition rate by adjusting the vapor deposition temperature and vapor deposition time so that the layer is deposited to a predetermined thickness. Here, a fine metal mask is used to form a vapor-deposited film only on the blue light-emitting element 120B on which the second blue light-emitting layer 53B has been formed.

[0126] In step S14, a second electron transport layer 55 is formed on the second hole blocking layer 54. Specifically, the electron transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that a predetermined film thickness is formed. Only the electron transport material may be evaporated, or the electron transport material and lithium quinoline may be co-evaporated. Here, a fine metal mask is used to form an evaporated film only on the blue light-emitting element 120B on which the second blue light-emitting layer 53B has been formed.

[0127] In step S15, the electron injection layer 56 is formed on the first electron transport layer 36 and the second electron transport layer 55. Specifically, lithium fluoride is vapor deposited at a predetermined vapor deposition rate by adjusting the vapor deposition temperature and vapor deposition time so that a layer having a predetermined film thickness is formed. Here, the vapor deposition film is formed without using a fine metal mask.

[0128] 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 by vapor deposition.

[0129] (Configuration of Full-Color Light-Emitting Device) FIG. 5 shows a schematic layer configuration of a light-emitting device 110 including the light-emitting element manufactured in this manner.

[0130] The light-emitting device 110 combines a single-structure red light-emitting element 120R and a green light-emitting element 120G with a tandem-structure blue light-emitting element 120B. The red light-emitting element 120R and the green light-emitting element 120G each have only a first light-emitting layer 34, while the blue light-emitting element 120B has a first blue light-emitting layer 34B and a second blue light-emitting layer 53B that are arranged in an overlapping manner and emit light of the same color. That is, the blue light-emitting element 120B is an organic EL element having a tandem structure in which two light-emitting layers, the first blue light-emitting layer 34B and the second blue light-emitting layer 53B, that emit light of the same color (blue), are fabricated between the anode layer 21 and the cathode layer 22. A first charge generation layer 40B is disposed between the first blue light-emitting layer 34B and the second blue light-emitting layer 53B.

[0131] The red light-emitting element 120R is configured by stacking an anode layer 21, a hole injection layer 31, a first hole transport layer 32, a first electron blocking layer 33R, a first red light-emitting layer 34R, a first hole blocking layer 35R, a first electron transport layer 36R, an electron injection layer 56R, and a cathode layer 22R in this order. In the red light-emitting element 120R, a set of layers including the first red light-emitting layer 34R from the hole injection layer 31 to the electron injection layer 56R, which are arranged between the anode layer 21R and the cathode layer 22R in the stacking direction, constitutes a first stack 30R.

[0132] Similarly, green light-emitting element 120G is configured by stacking, in this order, an anode layer 21, a hole injection layer 31, a first hole transport layer 32, a first electron blocking layer 33G, a first green light-emitting layer 34G, a first hole blocking layer 35G, a first electron transport layer 36G, an electron injection layer 56G, and a cathode layer 22G. In green light-emitting element 120G, a set of layers including first green light-emitting layer 34G from the hole injection layer 31 to the electron injection layer 56G, which are arranged between anode layer 21G and cathode layer 22G in the stacking direction, constitutes a first stack 30G.

[0133] The cathode layers 22R, 22G, and 22B may be a common electrode layer provided across the red light emitting element 120R, the green light emitting element 120G, and the blue light emitting element 120B.

[0134] <Light Emission Mechanism> Next, the light emission mechanism of the light emitting elements 120 will be described with reference to Fig. 6. In Fig. 6, only the essential parts of the configuration of each light emitting element 120 are shown.

[0135] The sealing layer 15 in FIG. 6 may also be a common layer provided across the red light emitting element 120R, the green light emitting element 120G, and the blue light emitting element 120B.

[0136] When a current flows through the red light-emitting element 120R, holes are supplied from the anode layer 21R to the first red light-emitting layer 34R, and electrons are supplied from the cathode layer 22R to the first red light-emitting layer 34R. In the first red light-emitting layer 34R, electrons and holes recombine to generate electron-hole pairs (excitons), which transition to the ground state, causing light in a predetermined wavelength range (red) to be emitted from the first red light-emitting layer 34R. Note that the "electron-hole pairs" corresponding to excitons may also be written in parentheses, as described above, together with the "excitons."

[0137] Similarly, when a current flows through green light-emitting element 120G, holes are supplied from anode layer 21G to first green light-emitting layer 34G, and electrons are supplied from cathode layer 22G to first green light-emitting layer 34G. In first green light-emitting layer 34G, electrons and holes recombine to generate electron-hole pairs (excitons), which transition to the ground state, causing light in a predetermined wavelength range (green) to be emitted from first green light-emitting layer 34G.

[0138] When a current flows through blue light-emitting element 120B, holes are supplied from anode layer 21B to first blue light-emitting layer 34B, and electrons are supplied from cathode layer 22B to second blue light-emitting layer 53B. Furthermore, when a current flows through blue light-emitting element 120B, electrons generated by n-type first charge generation layer 41B included in first charge generation layer 40B are supplied to first blue light-emitting layer 34B, and holes generated by p-type first charge generation layer 42B included in first charge generation layer 40B are supplied to second blue light-emitting layer 53B.

[0139] As a result, electrons and holes recombine in the first blue light-emitting layer 34B to generate electron-hole pairs (excitons), which transition to the ground state and emit light in a predetermined wavelength range (blue). Similarly, electrons and holes recombine in the second blue light-emitting layer 53B to generate electron-hole pairs, which transition to the ground state and emit light in a predetermined wavelength range (light of the same color as the first blue light-emitting layer 34B). In this way, the first blue light-emitting layer 34B and the second blue light-emitting layer 53B of the blue light-emitting element 120B each emit blue light.

[0140] In this case, both the first blue light-emitting layer 34B and the second blue light-emitting layer 53B emit light due to the same current flowing from the anode layer 21B to the cathode layer 22B. Therefore, the amount of light emitted by the blue light-emitting element 120B increases by the amount of two light-emitting layers in the blue light-emitting element 120B, and its luminous efficiency (current efficiency = amount of light emitted / current) also increases by the amount of two light-emitting layers. This allows for the creation of high-brightness, high-resolution full-color images. The reasons for this are explained further below.

[0141] First, we will explain the carrier imbalance that needs to be corrected in a light-emitting element. In a light-emitting element that includes a cathode layer and an anode layer, the amount of electrons flowing in from the cathode layer is the same as the amount of holes flowing in from the anode layer. In other words, the amount of electrons flowing in from the cathode layer is equal to the amount of electrons flowing out from the anode layer (Kirchhoff's first law (current law)). Here, the amount of charge flowing per unit time (amount of electrons × elementary charge) is the current.

[0142] When the light-emitting layer and functional layers constituting the light-emitting element all have the same carrier injection and transport properties, the current flowing through the light-emitting element should be proportional to the voltage between the anode layer and the cathode layer and inversely proportional to the resistance of the entire light-emitting element (Ohm's law). However, in the case of a light-emitting element using an organic thin film, the current flowing through the light-emitting element is proportional to the square of the voltage between the anode layer and the cathode layer and inversely proportional to the cube of the thickness of the organic thin film (space charge limited current).

[0143] In any case, inside the light-emitting element, the carrier injection and transport properties differ for each layer. Therefore, the amount of current flowing through the light-emitting element is limited by the layer with the lowest injection and transport properties. When observed from the outside of the light-emitting element, the amount of current flowing through the light-emitting element appears to be limited by the smaller of the amount of electrons injected and transported from the cathode layer or the amount of holes injected and transported from the anode layer (in other words, the amount of electrons flowing into the anode layer).

[0144] In light-emitting devices with a single structure (not a tandem structure) that include a single light-emitting layer, in order to further improve light-emitting efficiency by suppressing the generation of unnecessary carriers (electrons or holes), it is common to select the materials and adjust the thickness of the functional layers so that the number of holes supplied to the light-emitting layer from the anode layer is equal to the number of electrons supplied from the cathode layer. Another approach is to optimize the carrier balance by adjusting the thickness of the light-emitting layer, taking into account the concept of space-charge-limited current, so that the current flowing through the light-emitting layer is optimized to maximize the current density of the light-emitting layer. This structural design guideline is sometimes referred to as "achieving carrier balance."

[0145] Even in the case of a tandem-structure light-emitting element having a charge generation layer and multiple light-emitting layers in addition to an anode layer and a cathode layer, the same approach as for the single-structure light-emitting element described above has been taken so far, namely, the structure has been designed so that the amounts of electrons and holes supplied to the light-emitting layer are equalized by selecting the material or adjusting the thickness of the functional layer.

[0146] However, as will be shown in the verification experiments described below, in a light-emitting device with a tandem structure, when the conditions of the functional layers are adjusted so that the supply amounts of electrons and holes are equal, it is possible to achieve a carrier balance in one of the light-emitting layers, but it has been found to be difficult to generate and supply appropriate excitons or prevent the generation of excess carriers in the remaining light-emitting layers. This is because, in a light-emitting device with a tandem structure, at least one of carriers, electrons or holes, is supplied to each light-emitting layer from the charge generation layer, but carrier generation in the charge generation layer cannot be directly controlled electrically, unlike carrier generation in the anode layer and cathode layer, which are directly connected to a power source.

[0147] Furthermore, in a light-emitting device with a tandem structure, multiple light-emitting layers are present. Here, referring to Kirchhoff's current law, the amount of current flowing through multiple light-emitting layers arranged in series between a pair of cathode and anode layers is the same for each. In other words, the current value flowing through each light-emitting layer arranged in the stacking direction is the same. Therefore, even if the thickness of the light-emitting layer is adjusted as in a single-structure light-emitting device, the current (current density) of the multiple light-emitting layers in a tandem-structure light-emitting device will change simultaneously and in the same manner. Therefore, the idea of ​​carrier balance adjustment by adjusting the thickness of the light-emitting layer to maximize current density is effective for a single-structure light-emitting device, but cannot be applied to a tandem-structure light-emitting device, which requires optimizing the carrier balance for each light-emitting layer.

[0148] In a light-emitting element with a tandem structure, there are two possible cases where the carrier supply is unbalanced. One is when the same amount of electrons and holes are supplied to each light-emitting layer, forming electron-hole pairs (excitons) without generating excess carriers, but there is a shortage or excess of excitons in one of the light-emitting layers. The other is when not only there is a shortage or excess of excitons in one of the light-emitting layers, but the balance of electron and hole supply to at least one of the light-emitting layers is disrupted, resulting in the formation of excess carriers other than excitons. These cases are collectively referred to as a carrier imbalance in a light-emitting element with a tandem structure. That is, carrier imbalance in a light-emitting element with a tandem structure can occur when there is insufficient or excessive generation of excitons in one of the light-emitting layers, and also when there is an imbalance in the amount of electrons and / or holes supplied to one of the multiple light-emitting layers formed between a pair of anode and cathode layers.

[0149] Therefore, in a light-emitting element having a tandem structure in which the light-emitting layers have the same thickness, optimizing the supply of carriers (electrons and / or holes) in one of the first light-emitting layer and the second light-emitting layer may result in an excessive supply of carriers (holes and / or electrons) in the other of the first light-emitting layer and the second light-emitting layer, resulting in an imbalance in the carrier balance between the first light-emitting layer and the second light-emitting layer.

[0150] In the light-emitting device 110, the thickness of the second blue light-emitting layer 53B in the blue light-emitting element 120B is smaller than the thickness of the first blue light-emitting layer 34B. Because the thickness of the second blue light-emitting layer 53B, where an insufficient amount of excitons and / or excess carriers are generated, is smaller than the thickness of the first blue light-emitting layer 34B, the carrier balance between the first blue light-emitting layer 34B and the second blue light-emitting layer 53B is optimized. Therefore, in the light-emitting device 110, in a tandem-structure light-emitting element (blue light-emitting element 120B) in which multiple light-emitting layers (the first blue light-emitting layer 34B and the second blue light-emitting layer 53B) between the anode layer 21 and the cathode layer 22 emit light of the same color, the generation of excess carriers that cannot contribute to light emission is suppressed, and an appropriate amount of excitons is generated in both the first blue light-emitting layer 34B and the second blue light-emitting layer 53B. This enables the blue light-emitting element 120B to emit light with high efficiency and brightness, and as a result, the light-emitting device 110 also enables high-efficiency and high-brightness light emission.

[0151] The excess carriers cannot be eliminated by radiative recombination, but generate heat and become deactivated, which can cause degradation of the light-emitting element. Therefore, in the light-emitting device 110, thermal deactivation caused by such excess carriers is also suppressed, thereby improving element reliability.

[0152] <Experimental Verification> As described above, in a conventional tandem-structure light-emitting device in which two host-guest emitting layers emitting the same color are stacked at the same thickness, an imbalance in carrier balance occurs. As a result, the light emission amount of a conventional tandem-structure light-emitting device may not reach twice the light emission amount of a light-emitting device having a single emitting layer (i.e., the theoretical value). On the other hand, in the tandem-structure light-emitting device of the present disclosure, the carrier balance is optimized, thereby achieving a light emission amount of the light-emitting device closer to the theoretical value. Below, an example of an experiment to verify the light emission amount of a tandem-structure light-emitting device and the results thereof will be described.

[0153] (Verification 1: Verification using prototypes) (Experimental details) First, a single-structure light-emitting element (single prototype) having one light-emitting layer (also called a single layer; single structure) that emits blue light, and a tandem-structure light-emitting element (comparative tandem prototype) having two light-emitting layers of the same thickness were fabricated. The layer configurations of the single prototype and comparative tandem prototype, and the conditions for the thickness of the light-emitting layer relative to the stack are as follows. Note that comparative tandem prototypes were prepared under three types of conditions, Condition i to Condition iii, below.

[0154] (Single prototype) Layer structure: cathode layer / first stack (electron transport layer / hole blocking layer / first blue light-emitting layer (thickness: 15 nm) / electron blocking layer / hole transport layer / hole injection layer) / anode layer Thickness ratio: The ratio of the thickness of the first light-emitting layer (15 nm) to the thickness of the first stack (165 nm) was 0.091.

[0155] (Comparative tandem prototype) (Condition i) Layer structure: cathode layer / second stack (electron injection layer / electron transport layer / hole blocking layer / second blue light-emitting layer (thickness: 15 nm) / hole transport layer / hole injection layer) / charge generation layer / first stack (electron transport layer / hole blocking layer / first blue light-emitting layer (thickness: 15 nm) / electron blocking layer / hole transport layer / hole injection layer) / anode layer Thickness ratio: The ratio of the thickness of the second light-emitting layer (15 nm) to the thickness of the second stack (70 nm) was 0.21, and the ratio of the thickness of the first light-emitting layer (15 nm) to the thickness of the first stack (165 nm) was 0.091. (Condition ii) Layer structure: same as condition i. Thickness ratio: same as condition i except that the thickness of the functional layer in the second stack layer was adjusted to a total thickness of 80 nm. The ratio of the thickness of the second light-emitting 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 functional layer in the second stack layer was adjusted to a total thickness of 90 nm. The ratio of the thickness of the second light-emitting layer (15 nm) to the thickness of the second stack layer (90 nm) was 0.167.

[0156] The single prototype has the same layer configuration between the cathode layer and the anode layer as the first stack of condition i.

[0157] The device lifetime and driving voltage were measured for each of the single prototype, comparative tandem prototype (condition i), comparative tandem prototype (condition ii), and comparative tandem prototype (condition iii). The current efficiency was also calculated using the Blue Index (unit: cd / A / y, where y is one of the chromaticity coordinates in CIE 1931).

[0158] With regard to element life, the element life of the comparative tandem prototype (condition i) was 2.01 times that of the single prototype, the element life of the comparative tandem prototype (condition ii) was 2.12 times, and the element life of the comparative tandem prototype (condition iii) was 1.92 times.

[0159] With regard to the drive voltage, the drive voltage of the comparative tandem prototype (condition i) was 1.9 times that of the single prototype, the drive voltage of the comparative tandem prototype (condition ii) was 1.9 times, and the drive voltage of the comparative tandem prototype (condition iii) was 2.0 times.

[0160] The current efficiency of the single prototype was 206 cd / A / y. In contrast, the current efficiency of the comparative tandem prototype (condition i) was 305 cd / A / y, which was 1.48 times that of the single prototype. The current efficiency of the comparative tandem prototype (condition ii) was 320 cd / A / y, which was 1.55 times that of the single prototype. Furthermore, the current efficiency of the comparative tandem prototype (condition iii) was 219 cd / A / y, which was 1.1 times that of the single prototype.

[0161] (Discussion) From the above, it can be seen that the element lifetime of a conventional tandem-structure light-emitting element having two light-emitting layers of the same thickness is improved by approximately twice as much as that of a single-structure light-emitting element. On the other hand, the increase in current efficiency of a conventional tandem-structure light-emitting element relative to the increase in driving voltage is smaller than that of a single-structure light-emitting element. Furthermore, it can be seen that in a conventional tandem-structure light-emitting element, even if the conditions of the functional layers are changed, the current efficiency (which can also be called "light-emitting efficiency") may not reach twice that of a single-structure light-emitting element, by a large margin.

[0162] Based on these results, applying a higher driving voltage and supplying more current would be a possible solution to double the luminous efficiency of conventional tandem-structure light-emitting devices compared to single-structure light-emitting devices. However, adopting such a solution would clearly result in increased power consumption and a shortened device lifespan.

[0163] As described above, conventional tandem-structure light-emitting devices are unable to fully realize the advantages of adopting a tandem structure, and there is room for further study. In other words, as shown by the above-mentioned verification results, it can be said that the effects of tandem-structure light-emitting devices cannot be fully realized by simply optimizing the functional layers around the light-emitting layer, as has been done conventionally. Furthermore, there is also room for study on a new issue: the light emission output of conventional tandem-structure light-emitting devices sometimes does not reach twice the light emission output of light-emitting devices with a single structure.

[0164] Therefore, when the cause of the above phenomenon is examined by focusing on the carrier mobility and carrier injection properties in the stacking direction of the tandem-structure light-emitting element, and the amount of carriers supplied to each of the multiple light-emitting layers of the tandem-structure light-emitting element and the amount of excess carriers generated, the reason why the current efficiency and light emission amount of conventional tandem-structure light-emitting elements do not improve as expected is thought to be as follows.

[0165] Generally, in organic materials, the mobility of holes is often higher than the mobility of electrons. In this case, the electron supply is likely to be insufficient when generating excitons necessary for radiative recombination in the light-emitting layer. In other words, the hole supply is likely to be oversupplied in the light-emitting layer. In single-structure light-emitting devices, efforts are made to match the amounts of electrons and holes (carrier supply) supplied to the light-emitting layer, i.e., to achieve carrier balance, by optimizing the thickness of the light-emitting layer, the material selection of functional layers other than the light-emitting layer, or the thickness of the functional layers. As mentioned above, similar efforts have been made in conventional tandem-structure light-emitting devices.

[0166] However, tandem-structure light-emitting elements have a charge generation layer in addition to the anode and cathode layers, which are directly connected to the TFTs including the pixel circuits and can directly control the potential for carrier generation and injection. The charge generation layer is not directly connected to the TFTs, etc. Therefore, the amount of carriers (electrons and holes) generated by the charge generation layer is likely to vary quantitatively due to differences in the carrier generation and supply capabilities between the electron generation layer and the hole generation layer. Furthermore, the only way to change the potential applied to the charge generation layer is to indirectly control it by changing the potential between the anode and cathode layers. As a result, it is thought that the balance of carriers supplied to each light-emitting layer may not be 1:1.

[0167] Furthermore, the electron generation layer constituting the charge generation layer is typically an organic-inorganic hybrid structure in which an organic electron transport material is doped with a small amount of metal material. In recent years, to prevent deterioration of the organic materials contained in other layers due to high deposition temperatures, development has been underway for electron generation layers that contain only organic materials, similar to the hole generation layer. However, at present, the amount of electrons supplied from an electron generation layer made entirely of organic materials can be significantly smaller than the amount of holes supplied from the hole generation layer. In this case, in a tandem-structure light-emitting element, it becomes even more difficult to match the amount of electrons and holes supplied between two light-emitting layers adjacent to each other in the stacking direction via a charge generation layer.

[0168] From the above, it is considered that in a light-emitting element with a tandem structure, in addition to matching the amount of electrons and holes supplied in each of the multiple light-emitting layers, i.e., the carrier balance, it is necessary to consider the difference in the amount of exciton generation and the amount of carrier supply between the light-emitting layers, which did not need to be considered in a light-emitting element with a single structure.

[0169] Furthermore, it is believed that in light-emitting layers stacked via a charge generation layer, the amount of carriers (electrons) supplied to the light-emitting layer stacked closer to the cathode layer may be smaller than the amount of carriers (electrons) supplied to the light-emitting layer stacked closer to the anode layer. In this case, the amount of electron-hole pairs (excitons) generated in the light-emitting layer stacked closer to the cathode layer decreases, and excess carriers (holes) are generated in the light-emitting layer closer to the cathode layer.

[0170] Due to the factors mentioned above, in conventional light-emitting elements with a tandem structure, even if there are multiple light-emitting layers (even if the volume of the light-emitting layers is multiple times larger), the amount of excitons does not increase multiple times. As a result, it is thought that the current efficiency of light-emitting elements with a conventional tandem structure decreases, and the current efficiency may not double that of a single light-emitting element.

[0171] From the above, it is believed that an effective measure to substantially double the current efficiency of a tandem-structure light-emitting device compared to a single-structure light-emitting device is to balance the supply of electrons and holes to each light-emitting layer, as well as to balance the carriers in each of the multiple light-emitting layers and between them. Furthermore, from the viewpoint of the generation of the above-mentioned excitons and excess carriers, it is believed that one of the above-mentioned effective measures is to make the thickness of the light-emitting layer on the cathode layer side thinner than the thickness of the light-emitting layer on the anode layer side. Therefore, the effectiveness of this measure was verified as follows.

[0172] (Verification 2: Verification by Simulation) (Experimental Content) The current efficiency, element life, and driving voltage were determined by computer simulation for a full-color light-emitting device of an example having a blue light-emitting element according to the present disclosure and a full-color light-emitting device of a comparative example consisting only of a single-structure light-emitting element. The current efficiency, element life, and driving voltage of only the blue light-emitting element in the full-color light-emitting device of the example and the full-color light-emitting device of the comparative example were also determined by computer simulation.

[0173] The full-color light-emitting device of the example includes a blue light-emitting element with a tandem structure, and red and green light-emitting elements with single structures. The tandem blue light-emitting element has the same layer structure as the comparative tandem prototype (condition i) described above, except that the blue light-emitting layer on the cathode layer side is 10 nm thick and the blue light-emitting layer on the anode layer side is 15 nm thick. The single red and green light-emitting elements each have a 35 nm thick light-emitting layer and the same layer structure as the single prototype described above (however, the layer thickness of the functional layer was optimized for each color).

[0174] The full-color light-emitting device of the comparative example includes a single-structure red light-emitting element, a green light-emitting element, and a blue light-emitting element. The layer structures of the red light-emitting element and the green light-emitting element are the same as those of the full-color light-emitting device of the example. The layer structure of the blue light-emitting element is the same as that of the red light-emitting element and the green light-emitting element, except that the blue light-emitting layer material is the same as that of the full-color light-emitting device of the example.

[0175] Simulation results showed that the current efficiency of the full-color light-emitting device of the example was 1.6 times that of the full-color light-emitting device of the comparative example. The element life of the full-color light-emitting device of the example was 2.5 times that of the full-color light-emitting device of the comparative example. Furthermore, the driving voltage of the full-color light-emitting device of the example was 1.5 times that of the full-color light-emitting device of the comparative example.

[0176] Furthermore, when the driving voltage in the above simulation was doubled for the full-color light-emitting device of the example, the current efficiency also doubled and the device life increased by 2.4 times.

[0177] Furthermore, when comparing the blue light-emitting elements, the current efficiency of the blue light-emitting element of the full-color light-emitting device of the Example was 12.5% ​​higher than that of the blue light-emitting element of the full-color light-emitting device of the Comparative Example, the element life of the blue light-emitting element of the full-color light-emitting device of the Example was 13.6% higher than that of the blue light-emitting element of the Comparative Example, and the driving voltage of the blue light-emitting element of the full-color light-emitting device of the Example was 12.5% ​​lower than that of the blue light-emitting element of the Comparative Example.

[0178] (Discussion) From the above results, it can be seen that the full-color light-emitting device of the example (including the tandem structure) can achieve characteristics more than twice those of the full-color light-emitting device of the comparative example (consisting of a single structure). Therefore, it can be seen that the full-color light-emitting device of the example can suppress an increase in driving voltage, extend the element life, and achieve an improvement in light-emitting efficiency commensurate with the increase in driving voltage, compared to a conventional full-color light-emitting device with a tandem structure. In this way, it can be seen that the light-emitting device of the present disclosure can achieve a significant improvement in characteristics.

[0179] The light emitting device of the present disclosure can also achieve the effect of reducing the amount of guest compound (dopant) used, compared to a conventional light emitting device including a tandem structure.

[0180] In this way, fabricating each light-emitting layer in a tandem-structured light-emitting device so that each layer has a different thickness can achieve carrier balance among the multiple light-emitting layers and solve the problem of imbalance in carrier supply. As a result, excess carriers are not generated as in conventional tandem-structured light-emitting devices, and an appropriate amount and balance of excitons can be generated in each light-emitting layer in the stacking direction. Therefore, it can be seen that the present disclosure provides a light-emitting device including a tandem-structured light-emitting device that can reduce current consumption and driving voltage, and improve light-emitting efficiency and reliability.

[0181] Specifically, in the present disclosure, the thickness of the light-emitting layer fabricated on the cathode layer side is thinner than that of the light-emitting layer fabricated on the anode layer side. As a result, holes are supplied to the light-emitting layer on the cathode layer side in an amount commensurate with the thickness of the light-emitting layer. Therefore, electron-hole pairs (excitons) are generated without the generation of excess holes that cannot combine with electrons, and the amount of electron-hole pairs (excitons) generated in the light-emitting layer on the anode layer side is substantially the theoretical value. Therefore, each of the light-emitting layers stacked in the stacking direction emits light with a brightness corresponding to its thickness. Furthermore, since the generation of excess holes can be prevented in each of the light-emitting layers, current consumption can be reduced, and therefore power consumption can also be reduced.

[0182] In particular, in the case of a top-emission light-emitting device in which light is emitted from the cathode layer side, even a slight 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, resulting in a decrease in electron injection properties and, as a result, a decrease in the amount of electrons supplied to the light-emitting layer directly below the cathode layer. However, by anticipating such a case and setting the thickness of the light-emitting 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 light-emitting layer on the cathode layer side due to use over time. From this perspective, in the present disclosure, the thickness of the light-emitting layer on the cathode layer side may be thinner than the theoretical thickness as long as the effects of the present disclosure are obtained.

[0183] [Embodiment 2] Embodiment 2 of the present disclosure will be described below with reference to Fig. 7. Fig. 7 is a diagram for explaining the configuration and light-emitting mechanism of a light-emitting element 220 of a display device 200 (light-emitting device 210) according to Embodiment 2 of the present disclosure. The light-emitting device 210 according to Embodiment 2 differs from the light-emitting device 110 described above in that it includes a blue light-emitting element 220B having three light-emitting layers in the stacking direction. Fig. 7 shows only the essential configuration of the light-emitting element 220 included in the light-emitting device 210.

[0184] In the following description of the embodiments, for the sake of convenience, the same explanations as those in the above-described embodiments will not be repeated, and components having the same functions as those described in the above-described embodiments will be denoted by the same reference numerals, and their explanations will not be repeated.

[0185] 7, in the red light emitting element 220R, a first red light emitting layer 34R is disposed between the anode layer 21R and the cathode layer 22R. Similarly, in the green light emitting element 220G, a first green light emitting layer 34G is disposed between the anode layer 21G and the cathode layer 22G.

[0186] In the blue light-emitting element 220B, a first stack 30B, a first charge generation layer 40B, a second stack 50B, a second charge generation layer 60B, and a third stack 70B are arranged in this order in the stacking direction between the cathode layers 22B. The first stack 30B includes a first blue light-emitting layer 34B and a functional layer, the second stack 50B includes a second blue light-emitting layer 53B and a functional layer, and the third stack 70B includes a third blue light-emitting layer 71B and a functional layer. The functional layers in each stack are the same as those in the first embodiment. The second charge generation layer 60B has a layer configuration similar to that of the first charge generation layer 40B.

[0187] In the stacking direction, when the thickness of the first blue light-emitting layer 34B is defined as 1, the thickness of the second blue light-emitting layer 53B is, for example, 0.1 to 0.9, more preferably 0.3 to 0.8. Furthermore, when the thickness of the second blue light-emitting layer 53B is defined as 1, the thickness of the third blue light-emitting layer 71B is, for example, 0.1 to 0.9, more preferably 0.3 to 0.8. Within the above ranges, the thicknesses of the first blue light-emitting layer 34B, second blue light-emitting layer 53B, and third blue light-emitting layer 71B included in the blue light-emitting element 220B are thicker on the anode layer 21B side and thinner on the cathode layer 22B side.

[0188] The blue light-emitting element 220B is manufactured, for example, by a manufacturing method that, after step S14 described above, performs steps similar to steps 8 to 14 to fabricate the second charge generation layer and the third stack.

[0189] 6, in the display device 200, the supply of excess carriers (electrons or holes) is prevented in the light-emitting layer closer to the cathode layer, and all light-emitting layers emit light with brightness and luminous efficiency substantially equal to the theoretical values. Thus, in the display device 200, like the display device 100, an increase in driving voltage can be suppressed, the element life can be extended, and the luminous efficiency can be improved to compensate for the increase in driving voltage.

[0190] [Embodiment 3] This embodiment is similar to the above-described embodiment 2, except that, of the three light-emitting layers in the above-described embodiment 2, the light-emitting layer on the cathode layer side is thinner than the light-emitting layer on the anode layer side for two adjacent light-emitting layers in the stacking direction. Such an embodiment can be illustrated, for example, by a diagram modified from FIG. 7 in which the first blue light-emitting layer 34B and the second blue light-emitting layer 53B have the same thickness and the third blue light-emitting layer 71B is thinner than the second blue light-emitting layer 53B.

[0191] As described above, this embodiment is suitable for adjusting the carrier balance in one of the three stacks when, as a result of adjusting the carrier balance in two adjacent stacks by a method other than controlling the thickness of the light-emitting layer according to the present disclosure, a good carrier balance cannot be obtained in the remaining stack. Examples of such light-emitting device configurations include a configuration in which the thicknesses of corresponding functional layers in the first stack and the second stack are different, a configuration in which the materials of corresponding functional layers in the first stack and the second stack are different, and a configuration in which the material of the first light-emitting layer included in the first stack is different from the material of the second light-emitting layer included in the second stack, and in which the ratio of the thicknesses of the light-emitting layers between the second stack and the third stack is the ratio of the thicknesses of the light-emitting layers described above according to the present disclosure.

[0192] The display device of this embodiment, like display device 100 and display device 200, can also suppress an increase in driving voltage, extend the element life, and achieve an improvement in luminous efficiency commensurate with the increase in driving voltage.

[0193] [Embodiment 4] The main layer configuration of a light-emitting device according to embodiment 4 of the present disclosure is shown in Fig. 8. The light-emitting device according to embodiment 4 differs from light-emitting device 110 according to embodiment 1 mainly in that layers other than the anode layer and the light-emitting layer on the anode layer side are layers common to light-emitting elements of each color.

[0194] That is, as shown in FIG. 8, the display device 300 has a light emitting device 310 that includes a red light emitting element 320R, a green light emitting element 320G, and a blue light emitting element 320B.

[0195] The red light emitting element 320R, the green light emitting element 320G, and the blue light emitting element 320B have independent anode layers 21R, 21G, and 21B, respectively, and have a hole injection layer 31 and a first hole transport layer 32 as common layers.

[0196] The red light-emitting element 320R has a first electron blocking layer 33R and a first red light-emitting layer 34R on the first hole transport layer 32. The green light-emitting element 320G has a first electron blocking layer 33G and a first green light-emitting layer 34G on the first hole transport layer 32. The blue light-emitting element 320B has a first electron blocking layer 33B and a first blue light-emitting layer 34B on the first hole transport layer 32.

[0197] The red light-emitting element 320R, the green light-emitting element 320G, and the blue light-emitting element 320B have, on the first light-emitting layer 34 of each color, a first hole blocking layer 35, a first electron transport layer 36, a charge generation layer 40, a second hole transport layer 51, a second electron blocking layer 52, a second blue light-emitting layer 53, a second hole blocking layer 54, a second electron transport layer 55, an electron injection layer 56, and a cathode layer 22 as common layers.

[0198] The thicknesses of the first blue light-emitting layer 34B and the second blue light-emitting layer 53 can be determined in the same manner as those in the above-described embodiment 1. In this case, the thicknesses and materials of the light-emitting layers and various functional layers in the red light-emitting element 320R and the green light-emitting element 320G can be appropriately determined within a range in which light of a desired color and intensity can be obtained in each of the red light-emitting element 320R and the green light-emitting element 320G, depending on the electrical characteristics of the functional layers on the light-emitting layers and the second blue light-emitting layer 53 during operation.

[0199] On the cathode layer 22, a sealing layer 15 is formed as a layer common to the light-emitting elements of each color.

[0200] The light emitting device 310 can be manufactured by performing the above-described steps S4 and S5 for each light emitting element of each color, and performing each process from step S8 to step S14 as vapor deposition without a mask.

[0201] In the red light-emitting element 320R and the green light-emitting element 320G, the light-emitting layers of each color function, including the driving of the various functional layers and the second blue light-emitting layer 53, to achieve the desired light emission of each color. Therefore, although the red light-emitting element 320R and the green light-emitting element 320G each emit a slightly bluish color, the light-emitting materials contained in the first red light-emitting layer 34R and the first green light-emitting layer 34G can be selected taking this into consideration to achieve the desired light emission. In the blue light-emitting element 320B, as in embodiment 1, the carrier balance between the first blue light-emitting layer 34B and the second blue light-emitting layer 53 is optimized, preventing an imbalance in the carrier supply amount. Therefore, like the display device 100, the display device 300 can also suppress an increase in drive voltage, extend the element life, and achieve an improvement in light-emitting efficiency commensurate with the increase in drive voltage.

[0202] Furthermore, as described above, the light-emitting element of the light-emitting device 310 is fabricated such that all layers other than the first light-emitting layer and the first electron blocking layer are common to all colors. Therefore, in this embodiment, compared to the previously described embodiments, the initial cost and operating cost for the mask, such as the preparation of the mask itself in mask-based vapor deposition and cleaning associated with its use, are reduced. Therefore, this embodiment is advantageous from the perspective of reducing the cost of light-emitting devices and display devices, compared to the previously described embodiments.

[0203] In red light-emitting element 320R, the desired red light is emitted by the light emitted by first red light-emitting layer 34R and second blue light-emitting layer 53, and in green light-emitting element 320G, the desired green light is emitted by the light emitted by first green light-emitting layer 34G and second blue light-emitting layer 53. In this case, red light-emitting element 320R and green light-emitting element 320G each emit light with an amount of light that is substantially equivalent to that of one light-emitting layer. A layer configuration including a light-emitting layer and a charge generation layer that realizes emission of a desired color by such color mixing does not fall under the category of the tandem light-emitting unit described above.

[0204] [Modifications] In the present disclosure, the light-emitting element including the tandem light-emitting unit does not have to be a blue light-emitting element, but may be a red light-emitting element or a green light-emitting element in the above-described embodiments. Also, in the present disclosure, the light-emitting element including the tandem light-emitting unit does not have to be one, but may be two light-emitting elements, a blue light-emitting element and a green light-emitting element, or two light-emitting elements, a green light-emitting element and a red light-emitting element, or may be light-emitting elements of all colors, red, green, and blue.

[0205] In the present disclosure, it is sufficient that the thickness of the light-emitting layer on the cathode layer side in the tandem light-emitting unit is thinner than the thickness of the light-emitting layer on the anode layer side, and the first electrode layer may be the cathode layer and the second electrode layer may be the anode layer. Furthermore, the light-emitting device according to the present disclosure may be fabricated from either the first electrode layer or the second electrode layer, as long as the thickness of the light-emitting layer on the cathode layer side in the tandem light-emitting unit is thinner than the thickness of the light-emitting layer on the anode layer side.

[0206] In the present disclosure, the charge generating layer may be composed of two or more layers that generate one or both of positive and negative charges, or may be a single layer that generates one or both of positive and negative charges.

[0207] In the present disclosure, a device equipped with a light-emitting device according to the present disclosure may be a full-color display device including light-emitting elements of four or more colors, or may be a surface-emitting device (illumination device) equipped with light-emitting elements of two or more colors and capable of changing color tones based on white.

[0208] The light-emitting layer of the light-emitting device according to the present disclosure may include a quantum dot-containing light-emitting layer, which is preferable from the viewpoint of enhancing the thermal stability of the light-emitting device, due to the advantages in the manufacturing process described below.

[0209] The quantum dot luminescent layer can be fabricated by a coating method that applies an ink in which quantum dots are dispersed. In the above-described embodiment, if the blue luminescent layer is a quantum dot luminescent layer, steps S5 and S12 described above can be replaced with processes of applying a blue quantum dot ink to the electron blocking layer using a slit coater or inkjet device, and then solidifying or drying the ink to fabricate the blue luminescent layer. The quantum dot luminescent layer can be fabricated with high precision, for example, by a coating method using a slit coater or inkjet device. Therefore, including a quantum dot luminescent layer in a light-emitting element can reduce the costs of manufacturing equipment and masks compared to vapor deposition methods using fine metal masks, and is advantageous from the perspective of reducing initial and operating costs in manufacturing.

[0210] In the present disclosure, the host-guest emitting layer may be a combination other than a blue fluorescent emitting layer and red and green phosphorescent emitting layers. For example, the emitting layer of the light-emitting device in the present disclosure may be only a fluorescent emitting layer, only a phosphorescent emitting layer, or only a quantum dot emitting layer. Furthermore, the combination of emitting layers in the light-emitting element or light-emitting device in the present disclosure may be a combination of a host-guest emitting layer and a quantum dot emitting layer.

[0211] As described above, the light emitting device according to the present disclosure is suitable for a top emission type light emitting device, but may also be a bottom emission type light emitting device.

[0212] [Major Effects] As described above, in a light-emitting element having two or more light-emitting layers arranged in the stacking direction, when a charge generation layer is interposed between the light-emitting layers, optimizing the supply amount of carriers (electrons and / or holes) in the light-emitting layer on one side of the charge generation layer in the stacking direction may result in the supply amount of carriers (holes and / or electrons) in the light-emitting layer on the other side deviating from the optimal value. As a result, an imbalance in the carrier balance may occur among the multiple light-emitting layers included in the same light-emitting element.

[0213] In a light-emitting device having multiple light-emitting layers stacked between a pair of cathode and anode layers, the amount of current flowing through each light-emitting layer is the same according to Kirchhoff's current law. That is, in a light-emitting device with stacked light-emitting layers, even if the thicknesses of the light-emitting layers are different, the amount of current flowing through each light-emitting layer remains the same. Therefore, in such a light-emitting device with stacked light-emitting layers, if an imbalance in the carrier balance occurs and the amount of carriers (electrons and / or holes) supplied to one of the light-emitting layers decreases, not only does the amount of current flowing through that light-emitting layer decrease, but the amount of current flowing through all light-emitting layers arranged between the pair of cathodes and anodes, including other light-emitting layers in the stacking direction, also decreases uniformly. In contrast, maximizing the amount of current cannot prevent the generation of excess carriers.

[0214] In the present disclosure, by appropriately reducing the thickness of the light-emitting layer on the cathode layer side, where the rate of carrier (electron) supply is rate-determining, the carrier balance is optimized without generating excess carriers in all light-emitting layers in the stacking direction of the light-emitting element including the tandem light-emitting unit, without making the values ​​of currents flowing through the light-emitting layers on the cathode layer side and the anode layer side different from each other.

[0215] Therefore, for example, as in the case where an electron injection layer made of the above-mentioned organic material is employed, even when the amount of electrons supplied to the light-emitting layer formed on the cathode layer side is insufficient, the carrier balance in all the light-emitting layers in the stacking direction can be optimized.

[0216] Similarly, even when the amount of electrons supplied to the light-emitting layer on the anode layer side is insufficient, as in the case of employing the aforementioned organic material-based n-type charge generation layer, by setting the thickness of each light-emitting 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, in the present disclosure, even in a light-emitting device including a tandem light-emitting unit having charge generation layers formed entirely of organic materials, generation of excess carriers (holes) is suppressed, and a light-emitting device with reduced power consumption and excellent reliability can be realized.

[0217] More specifically, the light-emitting device of the present disclosure has a tandem light-emitting unit formed by stacking multiple light-emitting layers of at least one of the emitted colors in the stacking direction, and the thickness of the light-emitting layers in the unit is 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 light-emitting layer in the stacking direction, and light emission with nearly theoretical brightness and efficiency can be achieved.

[0218] Furthermore, since the luminous efficiency (ratio of luminance to applied voltage) is theoretically the same for any light-emitting layer in the stacking direction, compared to a case where the thickness of each light-emitting layer in the stacking direction is constant, the total thickness of the light-emitting layer from the anode layer to the cathode layer is reduced by the thickness of the light-emitting layer on the cathode layer side. Therefore, when the electric field (unit: V / m) applied to each light-emitting layer in the stacking direction is the same, a reduction in driving voltage (unit: V) can be achieved. Furthermore, since the generation of excess carriers (positive holes) in each stacked light-emitting layer is suppressed, current consumption is reduced. Therefore, a reduction in power consumption (unit: W = V x A) is also achieved.

[0219] Therefore, the light-emitting device of the present disclosure achieves higher brightness, lower voltage operation, and lower power consumption than a light-emitting device having a conventional tandem structure in which the thickness of each light-emitting layer is the same in the stacking direction. Therefore, according to the present disclosure, it is possible to fully utilize the advantages of adopting a tandem structure for one or more light-emitting elements of a color in a light-emitting device including light-emitting elements of two or more colors. For example, according to the present disclosure, in a light-emitting device capable of full-color display without using a color filter or the like, it is possible to achieve the advantages of a tandem structure, such as high brightness and / or low power consumption.

[0220] In addition, by gradually reducing the thickness of the light-emitting layer from the anode layer side toward the cathode layer side in the stacking direction, the above-mentioned effects of improving brightness (light-emitting efficiency) and reducing power consumption can be achieved throughout the stacking direction of the light-emitting element.

[0221] In the light-emitting device according to the present disclosure, when the charge generation layer includes an electron generation layer and a hole generation layer, positive charges and negative charges are supplied to the light-emitting layer on the anode side and the light-emitting layer on the cathode side of the charge generation layer in amounts corresponding to the respective configurations (materials, etc.) of the electron generation layer and the hole generation layer. Therefore, including an electron generation layer and a hole generation layer in the charge generation layer is even more effective from the viewpoint of achieving an appropriate carrier balance in the tandem light-emitting unit.

[0222] The light-emitting device according to the present disclosure is suitable for optimizing the carrier balance between light-emitting elements having different carrier characteristics, and is suitable for light-emitting devices including a group of two or more light-emitting elements that emit light of two or more different colors and are arranged in a planar direction, such as a light-emitting device for a full-color display device.

[0223] Furthermore, by using only the same color for the multiple light-emitting layers stacked in the stacking direction, it is advantageous in terms of improving color purity in a light-emitting device that enables full-color display by painting each of the RGB colors separately. Furthermore, when the same material that emits the same color is stacked in each light-emitting layer, it becomes easier to evaluate the carrier injection property and carrier transport property between each light-emitting layer by simulation. Thus, the above configuration is even more effective in terms of more easily realizing a light-emitting device that can display full colors and includes light-emitting elements having desired characteristics.

[0224] As described above, in the present disclosure, a preferred embodiment is one in which a tandem light-emitting unit is formed from a fluorescent-emitting layer used in combination with a phosphorescent-emitting layer. Since a fluorescent-emitting layer is currently more suitable as a blue-emitting layer, it is more effective to have all of the light-emitting layers included in the tandem light-emitting unit be light-emitting layers that emit light with a wavelength of 430 nm or more and 470 nm or less, i.e., blue-emitting layers, in order to improve the carrier balance in a light-emitting device in which a blue fluorescent-emitting layer is used in combination with a phosphorescent-emitting layer of another color, thereby achieving a highly efficient light-emitting device.

[0225] Furthermore, by using a host-guest emitting layer as the emitting layer, high luminous efficiency and improved device lifetime can be achieved. According to the present disclosure, the carrier balance (balance between electrons and holes) in each emitting layer can be further improved. Therefore, it is advantageous from the viewpoint of being able to fully utilize the performance of each host-guest emitting layer, which has high luminous efficiency and excellent lifetime. It is also advantageous from the viewpoint of being able to reduce the amount of expensive and rare phosphorescent dopants used as guest compounds.

[0226] The fluorescent-emitting layer is preferable from the viewpoint of realizing an improvement in the luminous efficiency of the emitting layer and the device life of the light-emitting element, and the phosphorescent-emitting layer is preferable from the viewpoint of cost and stable supply, while the phosphorescent-emitting layer is preferable from the viewpoint of increasing the quantum efficiency of light emission. From these viewpoints, the light-emitting device may include one or both of a fluorescent-emitting layer and a phosphorescent-emitting layer. For example, each light-emitting element of the light-emitting device may have a fluorescent-emitting layer or a phosphorescent-emitting layer, or a single light-emitting element (e.g., the above-mentioned tandem light-emitting unit) may have both a fluorescent-emitting layer and a phosphorescent-emitting layer. In the former case, by configuring the tandem light-emitting unit using fluorescent layers with two or more fluorescent-emitting layers in the stacking direction, it is possible to further optimize the balance between the life and the light-emitting amount between the fluorescent-emitting layer of the light-emitting element including the tandem light-emitting unit and the phosphorescent-emitting layers of the other light-emitting elements.

[0227] Similarly, a light-emitting device may include both a quantum dot-containing light-emitting layer and a host-guest light-emitting layer, which is advantageous from the viewpoint of obtaining the respective advantages of both types of light-emitting layers.

[0228] Furthermore, in a tandem light-emitting unit, it is even more effective to set the ratio of the thickness of the light-emitting layer on the cathode layer side to the thickness of the light-emitting layer on the anode layer side to be 0.1 or more and 0.9 or less from the viewpoints of achieving a longer life, improved light-emitting efficiency, and reduced driving voltage.

[0229] Furthermore, the light emitting device of the present disclosure being of a top emission type is even more effective in terms of optimizing the balance of the total layer thickness of the tandem light emitting element and the single light emitting element.

[0230] Incidentally, it is more effective to have the number of light-emitting layers in the stacking direction be 2 or more and 5 or less from the viewpoint of improving the light-emitting efficiency, extending the life span, and reducing the driving voltage.

[0231] The display device of the present disclosure also includes the light-emitting device described above. Therefore, the thickness of the light-emitting layer on the cathode layer side can be reduced by the thickness of the portion that does not contribute to light emission. Therefore, compared to display devices including conventional light-emitting devices in which the thickness of the portion that does not contribute to light emission is not reduced, this display device is advantageous in terms of realizing high-brightness, high-resolution image display and power saving. Therefore, according to the present disclosure, high-brightness, low-power light emission can be realized in the display device, and the advantages of adopting a tandem structure in the light-emitting device can be fully realized.

[0232] The present disclosure also provides a method for manufacturing a light-emitting device, which includes a plurality of light-emitting elements, each of which is configured by stacking a first electrode layer (one of an anode layer and a cathode layer), a light-emitting layer, and a second electrode layer (the other of the anode and cathode layers) in this order in the stacking direction, and which are arranged in a direction intersecting the stacking direction, and which includes a step of fabricating a tandem light-emitting unit, which is a group of two light-emitting layers that emit light of the same color and are stacked in the stacking direction, and one charge generation layer disposed therebetween, in at least some of the plurality of light-emitting elements, and in the step of fabricating the tandem light-emitting unit, the light-emitting layer on the cathode layer side is fabricated by a vapor deposition method or a coating method so as to be thinner than the light-emitting layer on the anode layer side. This manufacturing method can provide a light-emitting device that can fully utilize the advantages of employing a tandem structure for light-emitting elements of one or more colors in a light-emitting device that includes light-emitting elements of two or more colors.

[0233] Furthermore, in the light-emitting device of the present disclosure, forming the host-guest emitting layer by vapor deposition is advantageous in realizing a light-emitting device capable of full-color display, since it allows for the preparation of the host-guest emitting layer and the application of highly precise color separation to the emitting layer, compared to the case of forming the emitting layer by other methods. In addition, it is advantageous in realizing highly accurate control of the thickness of each emitting layer.

[0234] It is difficult to prepare a host-guest emitting layer by a coating method using an inkjet device or the like, because the emitting layer must be prepared while doping the guest compound into the host compound.

[0235] Furthermore, in the manufacture of the light-emitting device of the present disclosure, light-emitting layers of different thicknesses in the stacking direction may be manufactured by changing only the deposition time. This configuration allows light-emitting layers of different thicknesses to be manufactured using the same manufacturing equipment under the same conditions except for the deposition time, which is even more effective in terms of reducing manufacturing costs and improving productivity.

[0236] In addition, light-emitting devices including light-emitting elements having multiple light-emitting layers (tandem structure) typically have a constant thickness of the light-emitting layers that emit the same color in the stacking direction. On the other hand, there are also technologies in which the thickness of the light-emitting layers varies in the stacking direction (e.g., JP 2007-329054 A, JP 5180369 A, JP 2015-153587 A, and JP 2015-32582 A). Some of these technologies include a thicker light-emitting layer on the anode layer side and a thinner light-emitting layer on the cathode layer side (e.g., JP 2015-153587 A and JP 2015-32582 A). However, these conventional technologies do not clarify the reason for varying the thickness of each light-emitting layer in the stacking direction, and the above publications do not disclose the realization of a full-color display by painting each RGB color separately.

[0237] As is clear from the above description, the thickness of the light-emitting layer in the stacking direction in the tandem light-emitting unit of the present disclosure can be determined from the perspective of improving carrier balance, depending on various factors, such as the light-emitting element itself, the light-emitting layer material in the light-emitting device including the light-emitting element, and the electrical characteristics of the light-emitting layer and functional layer. These various factors may also vary depending on the application of the light-emitting device. It is impossible to generally specify in words that the conditions for the thickness of the light-emitting layer in the stacking direction that result in improved carrier balance are specified by the various conditions described above. Furthermore, while it may be possible to specify a desired structure that involves various specific factors other than the thickness of the light-emitting layer by comparing and considering numerous and diverse conditions, this requires enormous time and cost, and is largely impractical in light of the nature of patent applications, which require speed, etc. Therefore, it can be said that the present disclosure is essentially different from the technology described above that simply discloses different light-emitting layer thicknesses in the stacking direction.

[0238] According to the present disclosure, in an electroluminescent display device, it is possible to achieve even higher luminous efficiency, higher brightness, and / or power saving by controlling the thickness of each light-emitting layer in the stacking direction. Therefore, the light-emitting 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 adoption of clean technologies and environmentally friendly technologies and industrial processes."

[0239] The present 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 different embodiments are also included in the technical scope of the present disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

[0240] REFERENCE SIGNS LIST 11 Substrate 12 Buffer layer 13 TFT layer 14 Edge cover film 15 Sealing layer 16 External functional layer 21 Anode layer (first electrode layer) 22 Cathode layer (second electrode layer) 23 Organic laminate (tandem light-emitting unit) 30 First stack 31 Hole injection layer 32 First hole transport layer 33 First electron blocking layer 34 First light-emitting layer 35 First hole blocking layer 36 First electron transport layer 40 First charge generation layer 41 n-type first charge generation layer 42 p-type first charge generation layer 50 Second stack 51 Second hole transport layer 52 Second electron blocking layer 53 Second light-emitting layer 54 Second hole blocking layer 55 Second electron transport layer 56 Electron injection layer 60 Second charge generation layer 70 Third stack 71 Third light-emitting layer 100, 200, 300 Display device 110, 210, 310 Light-emitting device 120, 220, 320 Light-emitting element DA Display area NDA Frame area PIX Pixel RSP Red sub-pixel GSP Green sub-pixel BSP Blue sub-pixel

Claims

1. A light-emitting device in which a plurality of light-emitting elements, each configured by stacking a first electrode layer which is one of an anode layer and a cathode layer, a light-emitting layer, and a second electrode layer which is the other of the anode layer and cathode layer, in this order in the stacking direction, are arranged in a direction intersecting the stacking direction, wherein at least some of the light-emitting elements among the plurality of light-emitting elements include one or more tandem light-emitting units which are a group of two light-emitting layers which emit light of the same color and which are stacked in the stacking direction, and one charge generation layer disposed therebetween, and wherein the thickness of the light-emitting layer on the cathode layer side in the tandem light-emitting unit is thinner than the thickness of the light-emitting layer on the anode layer side.

2. The light-emitting device according to claim 1, wherein the charge generation layer includes an electron generation layer that generates electrons and a hole generation layer that generates holes.

3. The light emitting device according to claim 1 or 2, comprising two or more groups of the light emitting elements that emit light of two or more different colors and are arranged in a direction intersecting the stacking direction.

4. The light emitting device according to any one of claims 1 to 3, wherein the light emitting layer comprises quantum dots.

5. The light-emitting device according to any one of claims 1 to 3, wherein the light-emitting layer contains a host compound and a guest compound.

6. The light-emitting device according to any one of claims 1 to 3, wherein the light-emitting layer comprises a light-emitting layer containing quantum dots and a light-emitting layer containing a host compound and a guest compound.

7. The light-emitting device according to claim 5 or 6, wherein the light-emitting layer comprises one or both of a fluorescent light-emitting layer containing a fluorescent dopant as the guest compound and a phosphorescent light-emitting layer containing a phosphorescent dopant as the guest compound.

8. The light emitting device according to any one of claims 1 to 7, wherein the light emitting layers included in the tandem light emitting unit all emit light with a wavelength of 430 nm or more and 470 nm or less.

9. The light-emitting device according to any one of claims 1 to 8, wherein the ratio of the thickness of the light-emitting layer on the cathode layer side to the thickness of the light-emitting layer on the anode layer side in the tandem light-emitting unit is 0.1 or more and 0.9 or less.

10. The light emitting device according to any one of claims 1 to 9, which is a top emission type.

11. A display device comprising the light-emitting device according to any one of claims 1 to 10.

12. A method for manufacturing a light-emitting device in which a plurality of light-emitting elements, each configured by stacking a first electrode layer, which is one of an anode layer and a cathode layer, a light-emitting layer, and a second electrode layer, which is the other of the anode layer and the cathode layer, in this order in the stacking direction, are arranged in a direction intersecting the stacking direction, the method comprising: a step of fabricating a tandem light-emitting unit, which is a group of two light-emitting layers that emit light of the same color and that are stacked in the stacking direction, and one charge generation layer disposed therebetween, in at least some of the plurality of light-emitting elements; and in the step of fabricating the tandem light-emitting unit, fabricating the light-emitting layer on the cathode layer side by a vapor deposition method or a coating method so as to be thinner than the light-emitting layer on the anode layer side.

13. A method for manufacturing a light-emitting device according to claim 12, wherein in the step of manufacturing the tandem light-emitting unit, a host-guest light-emitting layer containing a host compound and a guest compound, which is one or both of the light-emitting layer on the anode layer side and the light-emitting layer on the cathode layer side, is manufactured by a vapor deposition method.

14. The method for manufacturing a light-emitting device according to claim 13, wherein both the light-emitting layer on the anode layer side and the light-emitting layer on the cathode layer side are formed by a vapor deposition method in which only the vapor deposition time is changed.

15. A method for manufacturing a light-emitting device according to claim 12, wherein in the step of manufacturing the tandem light-emitting unit, the light-emitting layer containing quantum dots, which is one or both of the light-emitting layer on the anode layer side and the light-emitting layer on the cathode layer side, is manufactured by a coating method.

Images