Electric field light-emitting device, manufacturing method thereof, and display device

Abstract

The host compound (34H) and the host compound (53H) are made of the same material as each other, the guest compound (34U) and the guest compound (53U) are made of the same material as each other, and the concentration of the guest compound (53U) in the second electroluminescent layer (53) is greater than the concentration of the guest compound (34U) in the first electroluminescent layer (34).

Description

Technical Field

[0001] The present disclosure relates to an electroluminescent device, a manufacturing method thereof, and a display device. Background Art

[0002] Mass production of displays equipped with organic EL elements (also known as "Organic Light Emitting Diode," "OLED") has begun in earnest, starting with thin displays for high-end smartphones and TVs. Displays using organic EL elements are now widespread and are becoming a core component of ultra-thin displays that are replacing LCDs.

[0003] Thin-film stacked organic EL devices using an aluminum quinolate complex (Alq3) for both the electron transport layer and the electroluminescent layer are known. However, in order to further improve the luminous efficiency of organic EL devices, development has been carried out in the following order (1) to (3).

[0004] (1) As materials for the electroluminescent layer, carrier (electron, hole) transport layer, or carrier (electron, hole) injection layer of an organic EL element, a host-guest material is used, in which a guest compound serving as a dopant is added to a host compound.

[0005] (2) As the guest compound (dopant) for the electroluminescent layer, a fluorescent material, or a fluorescent material and a phosphorescent material is used.

[0006] (3) Using modified guest compounds such as thermally activated delayed fluorescence (TADF) materials or superfluorescent materials as dopants for the electric field luminescence layer.

[0007] Furthermore, from the viewpoint of improving the current efficiency and device life of the organic EL device, the following (4) and (5) were further studied.

[0008] (4) Improve carrier balance by studying carrier transport layers or carrier injection layers.

[0009] (5) The element structure of the light-emitting element adopts a tandem structure (for example, see Patent Documents 1 to 3).

[0010] Prior art literature Patent Literature Patent Document 1: Japanese Patent Application Laid-Open No. 2007-329054 Patent Document 2: International Publication No. 2010 / 113493 Patent Document 3: Japanese Patent Application Laid-Open No. 2020-4970 Summary of the Invention Problems to be solved by the invention However, when light-emitting layers of respective colors of RGB are applied so as to enable full-color display and a tandem structure is adopted, the advantages of the tandem structure may not be fully realized.

[0011] One aspect of the present disclosure is to provide a technology capable of fully demonstrating the advantages of adopting a tandem structure in an electroluminescent device capable of full-color display.

[0012] Solutions for solving problems An electroluminescent device according to one aspect of the present disclosure includes: an anode layer; a cathode layer disposed opposite to the anode layer in a stacking direction; a plurality of electroluminescent layers, each of which is disposed between the anode layer and the cathode layer in the stacking direction and in a plurality in a direction intersecting the stacking direction, and comprising a host compound and a guest compound; and a charge generation layer disposed between two adjacent electroluminescent layers in the stacking direction, wherein a plurality of electroluminescent layers emitting light of the same color are disposed in the stacking direction, and a first electroluminescent layer emitting light of different colors is disposed in a direction intersecting the stacking direction. The invention relates to an electroluminescent layer having an n-th color to an electroluminescent layer having an n-th color, wherein n is an integer ≥ 2, and in two adjacent electroluminescent layers separated by the charge generation layer in the stacking direction, the host compound of the electroluminescent layer on the anode layer side and the host compound of the electroluminescent layer on the cathode layer side are the same material, the guest compound of the electroluminescent layer on the anode layer side and the guest compound of the electroluminescent layer on the cathode layer side are the same material, and the concentration of the guest compound in the electroluminescent layer on the cathode layer side is higher than the concentration of the guest compound in the electroluminescent layer on the anode layer side.

[0013] One aspect of the present disclosure relates to a display device including the above-mentioned electroluminescent device.

[0014] In addition, one aspect of the present disclosure relates to a method for manufacturing an electroluminescent device, which is the above-mentioned method for manufacturing an electroluminescent device, including the step of alternately stacking the electroluminescent layer and the charge generation layer on the anode layer or the cathode layer, and forming the electroluminescent layer by evaporation.

[0015] Effects of the Invention According to one aspect of the present disclosure, the advantages brought about by adopting a tandem structure can be fully realized in an electroluminescent device capable of full-color display. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a plan view schematically showing the configuration of the display device according to the first embodiment of the present invention.

[0017] Figure 2 It is schematically represented Figure 1 Schematic diagram of the layer structure of a display device.

[0018] Figure 3 It is schematically shown Figure 2 Schematic diagram of the layer structure of the electroluminescent element in the layer structure shown.

[0019] Figure 4 It shows Figure 3 Flowchart of an example of a method for manufacturing an electroluminescent element shown in FIG.

[0020] Figure 5 Is used to illustrate Figure 2 Schematic diagram of the light-emitting mechanism of the electroluminescent element of the electroluminescent device shown.

[0021] Figure 6 This is a schematic diagram for explaining the light emission mechanism of the electroluminescent element of the electroluminescent device according to Embodiment 2 of the present disclosure. DETAILED DESCRIPTION

[0022] [Electroluminescent element] The electroluminescent element of an embodiment of the present invention has an anode layer, a cathode layer, a host compound, a guest compound, an electroluminescent layer and a charge generation layer. The electroluminescent element disclosed in the present invention has a plurality of electroluminescent layers overlapping in the stacking direction. In addition, the two electroluminescent layers adjacent to each other in the stacking direction with the charge generation layer therebetween have the following structure. The host compound of the electroluminescent layer on the anode layer side and the host compound of the electroluminescent layer on the cathode layer side are the same material as each other. The guest compound of the electroluminescent layer on the anode layer side and the guest compound of the electroluminescent layer on the cathode layer side are the same material as each other. The concentration of the guest compound of the electroluminescent layer on the cathode layer side is higher than the concentration of the guest compound of the electroluminescent layer on the anode layer side. The electroluminescent element disclosed in the present invention can adopt a known element structure of a light-emitting element within the range of layers that meet these conditions. Below, the layer structure of the electroluminescent element disclosed in the present invention is mainly explained by taking OLED as an example. It should be noted that in the present invention, "electroluminescent element" refers to a group of arranged electroluminescent layers and various carrier functional layers in the stacking direction. In the present invention, an "electroluminescent device" refers to a group of a plurality of electroluminescent elements in a direction intersecting the stacking direction.

[0023] [Anode Layer] The anode layer is one of a pair of electrode layers, the anode and cathode. In the present disclosure, it is used to supply holes to the various layers that make up the electroluminescent element. The anode layer is conductive. Furthermore, the anode layer has optical properties such as reflecting a portion of visible light and transmitting the remainder. Typically, the anode layer includes both electrode materials that reflect visible light and electrode materials that transmit visible light.

[0024] To improve hole injection, the anode layer preferably uses a material with a relatively high work function (e.g., a material with a work function of 4.5 eV or higher). Examples of electrode materials with a high work function include Pt (5.65 eV), Ir (5.25 eV), Ni (5.2 eV), Au and Pd (5.15 eV), and indium tin oxide (ITO).

[0025] Examples of electrode materials that reflect visible light include metal materials such as Al, Mg, Li, Ag, Pd, and Cu, and alloys of these metal materials (eg, APC (Ag—Pd—Cu) alloy).

[0026] Examples of electrode materials that transmit visible light include thin films of transparent metal oxides (e.g., indium tin oxide (In-Sn-O), indium zinc oxide (In-Zn-O), and indium gallium zinc oxide (In-Ga-Zn-O)), thin films composed of metal materials such as Al, Mg, and Ag, and nanowires (NWs) composed of these metal materials.

[0027] Among transparent metal oxides, ITO has a relatively high work function of 4.6 to 5.0 eV, making it suitable for use as anode layer materials. Furthermore, in order to enhance the conductivity of the electrode layer or to add the ability to reflect visible light, a laminated structure (e.g., ITO) with ITO formed on the surface of a metal material can be used in the anode layer.

[0028] [Cathode Layer] The cathode layer is the other electrode layer in a pair of anode and cathode electrodes. In the present disclosure, it is used to supply electrons to the layers that make up the electroluminescent element. The cathode layer is arranged opposite the anode layer in the stacking direction. The cathode layer has, for example, electrical conductivity and visible light transmittance.

[0029] For example, to improve electron injection, the cathode layer preferably uses a material with a relatively low work function. Examples of electrode materials for 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.

[0030] [Electroluminescent Layer] An electroluminescent layer is a layer that emits light of a specified color when an electric field is applied. An electroluminescent layer is typically composed of a luminescent material, but may also be a stacked structure of multiple functional layers, such as an immediate luminescent layer and a delayed luminescent layer, with two or more functional layers corresponding to two or more functions of electroluminescence in a known electroluminescent layer overlapping each other, so that the entire structure exhibits electroluminescent function. In the present disclosure, multiple electroluminescent layers are disposed between the anode layer and the cathode layer in the stacking direction and in a direction intersecting the stacking direction.

[0031] In the stacking direction, the electroluminescent layer is configured with multiple electroluminescent layers emitting light of the same color. The number of electroluminescent layers emitting light of the same color that overlap in the stacking direction is not limited; however, from the perspective of improving luminous efficiency and extending the life of the device, it is preferably two or more, and more preferably three or more. On the other hand, from the perspective of suppressing a rise in driving voltage, achieving a voltage resistance of the driver corresponding to the driving voltage, and maintaining the flexibility of the electroluminescent device (suppressing an increase in the total thickness of the electroluminescent device), the number of electroluminescent layers emitting light of the same color that overlap in the stacking direction is preferably five or less, and more preferably four or less. A number of three or more electroluminescent layers is preferably five or less.

[0032] Here, the same color refers to the same color of light in which, when there are more than two luminous peak wavelengths, all of the luminous peak wavelengths are within the range of ±5nm, and the maximum value of the full width at half maximum among all the luminous peak wavelengths is less than 1.25 times the minimum value of the full width at half maximum among the remaining luminous peak wavelengths. Basically, when the host material and the guest compound constituting the electric field luminescent layer are both the same material or similar materials with the same skeleton, the same color is emitted. However, in the top emission structure described later, due to the difference in the microcavity structure of each color, the luminous peak wavelength of each color sometimes fluctuates. Therefore, in the top emission structure, the same color refers to the same color of light in which all luminous peak wavelengths are within the range of ±10nm and satisfy the above range of the full width at half maximum. In two electric field luminescent layers adjacent to each other in the stacking direction across the charge generation layer, the difference between the peak wavelength of the luminous spectrum of the electric field luminescent layer on the anode layer side and the peak wavelength of the luminous spectrum of the electric field luminescent layer on the cathode layer side can be less than 10nm. The difference between the maximum value and the minimum value of the peak wavelength of the emission spectrum in the plurality of electroluminescent layers arranged in the stacking direction may be 10 nm or less. The maximum value of the full width at half maximum of the peak wavelength of the emission spectrum in the plurality of electroluminescent layers arranged in the stacking direction may be 1.25 times or less of the minimum value of the full width at half maximum of the peak wavelength of the emission spectrum.

[0033] In addition, in the direction intersecting the stacking direction, the electric field luminescent layer is configured with an electric field luminescent layer of the first color to an electric field luminescent layer of the nth color (n is an integer of 2) that emits light of different colors from each other. Here, different colors refer to the same color of light that is not included in the range of the above-mentioned same color. As a representative well-known example, as a multi-color electric field luminescent layer for full-color display, three color luminescent layers are known, namely a red luminescent layer, a green luminescent layer, and a blue luminescent layer. In this way, configuring electric field luminescent layers of multiple colors in a direction intersecting the stacking direction is called "split coating". In addition, it is preferred that the number of electric field luminescent layers of each color in the stacking direction is the same for all luminescent colors. In the following, the electric field luminescent layer in the present invention is mainly explained by taking the above-mentioned three-color luminescent layers as an example, but the present disclosure is not limited to this. It should be noted that the electric field luminescent layers of different colors in the intersecting direction can be at the same position (height) or at different positions (heights) (or staggered) in the stacking direction.

[0034] That is, in the present disclosure, when the electroluminescent device includes three electroluminescent layers of red, green, and blue, the electroluminescent elements of each color are independently arranged in a direction intersecting the stacking direction, for example, in a direction orthogonal to the stacking direction. In addition, in the stacking direction, the red electroluminescent layer overlaps with the red electroluminescent layer, the green electroluminescent layer overlaps with the green electroluminescent layer, and the blue electroluminescent layer overlaps with the blue electroluminescent layer. In addition, electroluminescent layers of different colors can be further stacked in the stacking direction, but from the perspective of improving color purity, it is preferred that only multiple electroluminescent layers emitting light of the same color are arranged in the stacking direction. From the perspective of more simply determining the concentration of the guest compound (dopant concentration) contained in each electroluminescent layer, it is also advantageous to only stack electroluminescent layers of the same color in the stacking direction.

[0035] In an EFL element having three or more EFL layers arranged in the stacking direction, if a charge generation layer is interposed between the EFL layers, optimizing the supply of carriers (electrons and / or holes) in the EFL layer on one side of the stacking direction relative to the charge generation layer may result in a rate-limited supply of electrons and / or holes, i.e., an insufficient rate, in the EFL layer on the other side. This results in an imbalance in the carrier balance between the multiple EFL layers. In the present invention, the guest compound concentration of the EFL layer on the anode side, where the injection amount of rate-limiting carriers (here, electrons) is insufficient, is made lower than the guest compound concentration of the EFL layer on the cathode side. Conversely, by making the guest compound concentration of the EFL layer on the cathode side greater than the guest compound concentration of the EFL layer on the anode side, the carrier balance of all EFL layers in the stacking direction is optimized.

[0036] That is, in the electroluminescent device of the present invention, in the two electroluminescent layers adjacent to each other in the stacking direction across the charge generation layer, the host compound of the electroluminescent layer on the anode layer side and the host compound of the electroluminescent layer on the cathode layer side are the same material, the host compound of the electroluminescent layer on the anode layer side and the host compound of the electroluminescent layer on the cathode layer side are the same material, and the concentration of the host compound of the electroluminescent layer on the cathode layer side is greater than the concentration of the host compound of the electroluminescent layer on the anode layer side. In the case where three or more electroluminescent layers are arranged in the stacking direction, preferably, the farther the three or more electroluminescent layers are from the anode layer, the greater the concentration of the guest compound. It should be noted that "adjacent" in the electroluminescent layer refers to the positional relationship of the electroluminescent layers adjacent to each other in the stacking direction in the configuration relationship of the electroluminescent layers included in the electroluminescent element. That is, other layers may also be sandwiched between the electroluminescent layers adjacent in the stacking direction. The concentration of the guest compound is expressed as the mass of the guest compound relative to the sum of the mass of the host compound and the mass of the guest compound. Examples of methods for measuring the concentration of a guest compound include luminescence spectrum measurement, luminescence time resolution characteristic measurement, absorption spectrum measurement, 1 H-NMR or 13 C-NMR, mass spectrometry, and a combination of at least two of these methods.

[0037] By adopting this configuration, the electroluminescent device of the present disclosure achieves an appropriate carrier balance in each electroluminescent layer along the stacking direction. By adhering to the technical concept of the present disclosure, the concentration of the guest compound in each electroluminescent layer can be appropriately determined based on various factors, such as the material combination of each layer in the electroluminescent element and the concentration of the guest compound in the electroluminescent layer.

[0038] Among the electroluminescent layers disclosed herein, a host-guest system comprising a host compound and a guest compound can be used. In a host-guest system, a solid medium containing a host compound contains a trace amount (e.g., 0.1 to several wt%) of a fluorescent dopant, etc., as the guest compound. In an electroluminescent layer doped with a guest compound, the fluorescence of the host compound completely disappears, replaced by intense luminescence consistent with the fluorescence spectrum of the doped guest compound. This is because the excitation energy of the host compound is transferred to the guest compound. This transfer of excitation energy enables the host-guest system to produce luminescence from the guest compound with higher quantum efficiency.

[0039] The concentration of the guest compound in the electric field luminescent layer on the cathode layer side is less than twice the concentration of the guest compound in the electric field luminescent layer on the anode layer side. In the two electric field luminescent layers adjacent to each other in the stacking direction with the charge generation layer therebetween, the guest compound in the electric field luminescent layer on the anode layer side and the guest compound in the electric field luminescent layer on the cathode layer side may be any one of a fluorescent dopant and a phosphorescent dopant. The guest compound in the electric field luminescent layer of the first color may be a fluorescent dopant, and the guest compound in the electric field luminescent layer of the second color may be a phosphorescent dopant. The peak wavelength of the luminescence spectrum of the electric field luminescent layer containing the guest compound may also be greater than or equal to 440 nm and less than or equal to 660 nm. In the two electric field luminescent layers adjacent to each other in the stacking direction with the charge generation layer therebetween, the guest compound in the electric field luminescent layer on the anode layer side and the guest compound in the electric field luminescent layer on the cathode layer side may be a hole transport material. In the two electric field luminescent layers adjacent to each other in the stacking direction with the charge generation layer therebetween, the guest compound in the electric field luminescent layer on the anode layer side and the guest compound in the electric field luminescent layer on the cathode layer side may be an electron transport material. Here, hole-transporting materials refer to materials that exhibit relatively stable hole presence and hole mobility in response to an electric field. Electron-transporting materials, on the other hand, include relatively stable electron presence and electron mobility in response to an electric field. Delayed fluorescent dopants, thermally active delayed fluorescent dopants, and fluorescent dopants that emit light through delayed fluorescent materials (high-fluorescence dopants) also fall within the scope of fluorescent dopants (improved types).

[0040] By setting the peak wavelength of the luminescence spectrum of the electroluminescent layer containing the guest compound to be greater than 440 nm and less than 660 nm, a full-color display with a sufficiently wide color gamut can be achieved. In addition, by using the guest compound of the electroluminescent layer on the anode layer side and the guest compound of the electroluminescent layer on the cathode layer side as hole-transporting materials, it is easier to adjust the balance of carriers by changing the concentration of the guest compound. In addition, by using the guest compound of the electroluminescent layer on the anode layer side and the guest compound of the electroluminescent layer on the cathode layer side as electron-transporting materials, it is easier to adjust the balance of carriers by changing the concentration of the guest compound.

[0041] Furthermore, by doping the EFL layer with a fluorescent dopant as a guest compound, the device lifespan of the EFL element is dramatically improved. This is because the dopant, acting as a guest compound, acts as a carrier (electron or hole) trap within the host compound's solid medium, becoming a carrier recombination center and directly generating excitons within the solid medium. The process by which these generated excitons deactivate to their ground state is called the deactivation process. The deactivation process can involve either a radiative process (thermal deactivation) or a radiative process (luminescence). The phenomenon of luminescence generated during the radiative process is EFL. By having the guest compound act as a carrier trap, not only is the quantum efficiency of the EFL layer improved, but the device lifespan is also improved due to the increased probability of carrier recombination. Consequently, both the luminous efficiency of the EFL layer and the device lifespan of the EFL element are improved.

[0042] Thus, since carriers can be effectively utilized in the host-guest system electroluminescent layer, the electroluminescent device of the present application can achieve high luminous efficiency of the electroluminescent layer and improve the life of the electroluminescent element.

[0043] In addition, the excitons generated when carriers recombine include singlet excitons and triplet excitons. When a fluorescent dopant is used as a guest compound, the singlet exciton contributes to luminescence. Here, when a phosphorescent dopant is used instead of a fluorescent dopant, the triplet exciton contributes to luminescence. According to the spin statistics law, the generation ratio of singlet excitons to triplet excitons is 25% for singlet excitons and 75% for triplet excitons. Therefore, when a fluorescent dopant is used as a guest compound, that is, in the luminescence process of "fluorescence" emitted only from singlet excitons, the probability of exciton generation that can contribute to luminescence is limited to a maximum of 25%.

[0044] In contrast, using a phosphorescent dopant as a guest compound allows light to be extracted from triplet excitons, resulting in a quantum efficiency three times higher than that of a fluorescent dopant. Furthermore, by utilizing intersystem crossing (ISC), a spin reversal from singlet excitons to triplet excitons, all generated excitons can theoretically emit "phosphorescence" from triplet excitons. Therefore, using a phosphorescent dopant as a guest compound can improve quantum efficiency by up to four times compared to using a fluorescent dopant.

[0045] The luminescent materials for the electric field luminescent layer can be known materials. For example, examples of luminescent materials for the blue electric field luminescent layer include pyrene compounds and anthracene compounds as fluorescent dopants. Furthermore, examples of luminescent materials for the red and green electric field luminescent layers include iridium complexes and palladium complexes as phosphorescent dopants.

[0046] Furthermore, complexes containing platinum-group elements such as iridium and palladium used as phosphorescent dopants are often very expensive, even in trace amounts, due to the low and uneven production of platinum-group elements. This makes stable supply difficult. Therefore, reducing the amount of these platinum-group complexes used as phosphorescent dopants is extremely important from the perspective of cost reduction and economic security.

[0047] Thus, a host-guest system electroluminescent layer can be formed by doping a host compound with a guest compound. A host-guest system electroluminescent layer can be formed by a co-evaporation method using multiple evaporation sources.

[0048] The materials for the host-guest electroluminescent layer can be various well-known examples. Examples of host compounds include well-known luminescent layer materials of various colors. Examples of guest compounds, in addition to the aforementioned fluorescent and phosphorescent dopants, also include TADF and superfluorescent materials. Further examples of fluorescent dopants include perylene, DPT, Coumarin 6, PMDFB, quinacridone, rubrene, BTX, ABTX, DCM, and DCJT. Another example of a phosphorescent dopant includes: Ir(ppy)3, Ir(thpy)3, Ir(t5m-thpy)3, Ir(t-5CF3-py)3, Ir(t-5t-py)3, Ir(mt-5mt-py)3, Ir(btpy)3, Ir(tflpy)3, Ir(piq)3, Ir(tiq)3, Ir(fliq)3, FIrpic, FIr6, ppy, tpy, bzq, thp, op, bo, bt, bon, αbsn, btp, ppo, C6, pq, β-bsn and ppz.

[0049] Furthermore, in currently mainstream electroluminescent elements, fluorescent dopants are used as guest compounds in the blue electroluminescent layer, while phosphorescent dopants are used as guest compounds in the red and green electroluminescent layers. In electroluminescent devices incorporating these elements, the quantum efficiency of fluorescent dopants is lower than that of phosphorescent dopants, both theoretically and practically. Therefore, in these electroluminescent devices, a balance in luminescence is achieved by flowing a higher current through the blue electroluminescent layer than through the red and green electroluminescent layers.

[0050] The charge generation layer is positioned between two adjacent electroluminescent layers in the stacking direction. The charge generation layer generates one or both electrons and holes. The charges (charges or carriers) generated by the charge generation layer are supplied to the electroluminescent layers located on the anode and cathode sides of the stacking direction, respectively. The charge generation layer can employ known charge generation materials that exhibit the aforementioned functions.

[0051] The charge generation layer can be composed of an electron generation layer that generates electrons and a hole generation layer that generates holes. Examples of the electron generation layer include an n-type charge generation layer, and examples of the hole generation layer include a p-type charge generation layer. When holes are supplied from the anode layer and electrons are supplied from the cathode layer, the n-type charge generation layer generates electrons, and the p-type charge generation layer generates holes.

[0052] The p-type charge generation layer can use a material containing an organic hole transport material and an organic electron accepting material (hole supply material) added in an amount ranging from 1% to 10%. The organic hole transport material can be a well-known triarylamine-based organic compound. An example of an organic electron accepting material is tetracyanoquinodimethane tetrafluoride (TCNQ-4F). The p-type charge generation layer uses the aforementioned hole transport material and electron accepting material, all of which are organic materials, to achieve sufficient hole generation capacity.

[0053] For example, the n-type charge generation layer can contain an organic electron transport material and an inorganic metal material (i.e., Yb (ytterbium) or Li (lithium)) added in a range of 5-20% to function as an electron donor. Examples of organic electron transport materials include oxadiazole compounds. Development is also underway to use organic materials for both the electron transport layer and the electron donor material in the n-type charge generation layer. For example, development of organic electron donor materials such as BUPH1, BPen, p-MeO-Phen, p-NMe2-Phen, and p-Pyrrd-Phen is underway.

[0054] Currently, no organic electron-donating materials with sufficient electron-donating properties suitable for use in n-type charge generation layers have been discovered. Consequently, no n-type charge generation layer composed entirely of organic materials and exhibiting sufficient properties has been discovered. According to the present disclosure, by adjusting the concentration of the guest compound in each electroluminescent layer as described above, it is possible to optimize the balance (carrier balance) between electrons and / or holes supplied from the charge generation layer and holes and / or electrons supplied from each electrode layer. Consequently, the present invention suppresses the generation of excess carriers, achieving an electroluminescent device having a tandem structure with multiple electroluminescent layers and excellent power consumption.

[0055] [Other Configurations] The electroluminescent element of the present disclosure may further include other layers within the scope of achieving the effects of the present disclosure. Examples of other configurations include carrier functional layers such as a hole injection layer, an electron injection layer, a hole transport layer, an electron blocking layer, a hole blocking layer, and an electron transport layer.

[0056] The hole injection layer is, for example, disposed adjacent to the anode layer. The hole injection layer may be composed of a hole transport material and an electron accepting material. These materials are the same organic materials as those described for the p-type charge generation layer. The specific material of the hole injection layer in the electroluminescent element may be the same as or different from that of the p-type charge generation layer.

[0057] The electron injection layer is, for example, disposed adjacent to the cathode layer. The electron injection layer can be composed of an electron transport material. Examples of electron transport materials for the electron injection layer include lithium fluoride (LiF), an inorganic material. Similarly to the n-type charge generation layer, the electron injection layer can also be composed of an organic material, such as an oxadiazole compound, doped with a metal material (e.g., Li or Yb).

[0058] LiF used in the electron injection layer exhibits excellent electron injection properties. On the other hand, the film formation of the carrier functional layer containing inorganic materials such as Yb, not limited to LiF, is usually carried out at a higher temperature than the film formation of the electroluminescent layer and the carrier functional layer composed of organic materials due to the high melting point of these inorganic materials. Therefore, there is a possibility of thermal damage to the organic material formed first. Therefore, if possible, it is preferable to form the electroluminescent element only from organic materials.

[0059] Therefore, organic electron-injecting materials such as BUPH1, BPen, p-MeO-Phen, p-NMe-2-Phen, and p-Pyrrd-Phen are being developed to achieve sufficient properties by combining them with a cathode layer composed of, for example, aluminum (Al), which forms a vapor-deposited layer at a relatively low temperature. However, the electron injection capability of electron-injecting layers using organic materials is inferior to that of electron-injecting layers using inorganic materials such as LiF. Therefore, when an electron-injecting layer composed of an organic material is placed adjacent to the cathode layer, the electron supply to the electroluminescent layer formed on the cathode layer side may be reduced. In such cases, by reducing the guest compound concentration (doping concentration) of the electroluminescent layer on the cathode layer side below the theoretical value, the advantages of a tandem structure can be fully realized, thereby reducing the impact of this reduction in electron supply. To address this carrier imbalance, it is effective to reduce the doping concentration of the electroluminescent layer on the anode layer side by a specific ratio relative to the doping concentration of the electroluminescent layer on the cathode layer side, which is lower than the theoretical doping concentration. Such a configuration may be included in the present disclosure.

[0060] The hole transport layer can be composed of an organic hole transport material, for example, a triarylamine organic compound.

[0061] The electron blocking layer may also be composed of an organic hole transport material, similarly to the hole transport layer. The material of the electron blocking layer may be the same as or different from that of the hole transport layer.

[0062] The hole blocking layer may be formed of an organic electron transport material, such as an oxadiazole compound. The material of the hole blocking layer may contain lithium quinoline (Liq) in addition to the electron transport material.

[0063] The electron transport layer, like the hole blocking layer, may also be composed of the above-mentioned organic electron transport material. The material of the electron transport layer may be the same as or different from that of the hole blocking layer.

[0064] The hole injection layer and electron injection layer may be arranged corresponding to the electrode layers, and are typically arranged adjacent to each electrode layer in the stacking direction. Multiple hole transport layers, electron blocking layers, the aforementioned electroluminescent layer, hole blocking layers, electron transport layers, and the aforementioned charge generation layer may be repeatedly arranged in overlapping fashion in the stacking direction of the electroluminescent element.

[0065] The electroluminescent device disclosed in the present invention is suitable for electroluminescent devices capable of top-emitting full-color display. In a top-emitting electroluminescent device, the distance between the electrode layers is adjusted according to the wavelength of light from the light-emitting layers of each color, thereby utilizing the microcavity effect to improve the light extraction efficiency. For this purpose, sometimes any layer of the carrier functional layer is thickened. The electroluminescent device disclosed in the present invention has a so-called tandem structure, which includes a plurality of electroluminescent layers and carrier functional layers corresponding to each electroluminescent layer. Therefore, the thickness of the carrier functional layer used to adjust the distance between the electrode layers is suppressed, which can further suppress the consumption of functionally unnecessary materials.

[0066] [Laminate] A laminate is a collection of layers including an electroluminescent layer disposed between the anode layer and the charge generation layer, between the charge generation layers, and between the charge generation layer and the cathode layer in the stacking direction. In addition to the electroluminescent layer, the laminate may also include a carrier-functional layer. Multiple laminates may also be disposed between the anode layer and the cathode layer in the stacking direction. In the present invention, the laminate does not include an anode layer, a cathode layer, or a charge generation layer.

[0067] The thickness of the laminate is determined so that the amount of light emitted from the electroluminescent layer is a theoretical value or a value close to the theoretical value. In the present disclosure, from the perspective of improving the luminous efficiency of the light-emitting element, it is preferably set to a laminate thickness corresponding to the thickness of the electroluminescent layer set in this manner. In addition, the thickness of the laminate is calculated by the sum of the thicknesses of the electroluminescent layer and the carrier functional layer within the laminate. However, for layers of the electroluminescent layer and the carrier functional layer that are very thin (for example, layers that may be less than 1 nm in thickness) in the calculation of the thickness of the laminate, they can be ignored.

[0068] [Method for manufacturing an electroluminescent device] The method for manufacturing an electroluminescent device disclosed in the present invention is the method for manufacturing an electroluminescent device described above, and includes the steps of alternately stacking an electroluminescent layer and a charge generation layer on an anode layer or a cathode layer. In addition, from the viewpoint of precisely controlling the thickness and concentration of the electroluminescent layer in the electroluminescent device disclosed in the present invention, it is preferred that the electroluminescent layer be formed at least by a vapor deposition method in the manufacturing method. An electroluminescent layer formed by a vapor deposition method is generally preferred from the viewpoints of high brightness and low voltage driving. In addition, since an electroluminescent element composed of fine pixels can be formed with high precision, it is also preferred from the viewpoint of realizing a high-precision display device. Moreover, if a co-evaporation method using multiple evaporation sources is used, an electroluminescent layer of a host-guest system can also be formed, and therefore it is more preferred.

[0069] Because the electroluminescent device disclosed herein has a specific layer structure, it can be manufactured by repeatedly forming specific layers. In the present invention, it is preferred that the evaporation time of the guest compound when forming multiple electroluminescent layers that emit light of the same color in the stacking direction be different for each electroluminescent layer. In the present disclosure, it is more preferred that only the evaporation time of the guest compound when forming multiple electroluminescent layers that emit light of the same color in the stacking direction be different for each electroluminescent layer. According to such a manufacturing method, conditions other than the evaporation time in the formation of the electroluminescent layer (for example, the evaporation rate controlled by the crucible temperature (evaporation temperature), the evaporation temperature, the ratio of the host compound to the guest compound, and the evaporation mask that specifies the pixel shape) can be kept constant. Therefore, it is possible to suppress the variation in characteristics caused by changes in conditions in each electroluminescent layer in the stacking direction, and to more significantly demonstrate the effects caused by the differences in the guest compound concentration (doping concentration) in the electroluminescent layer in the stacking direction. In the present disclosure, when forming a plurality of electroluminescent layers that emit light of the same color in the stacking direction, the vapor deposition temperature of the guest compound may be made different for each electroluminescent layer.

[0070] [Display Device] The display device disclosed herein includes the aforementioned electroluminescent device. In addition to including the aforementioned electroluminescent device, the display device disclosed herein can be configured similarly to a conventional display device including a conventional light-emitting device. Examples of display devices include televisions and smartphones.

[0071] [Specific Description] The following describes the electroluminescent device, its manufacturing method, and display device disclosed herein in more detail using an organic light-emitting diode (OLED) as an example. In this specification, components associated with different colors within the same basic structure are represented by adding a symbol representing the color. For example, components associated with red are further labeled with R, components associated with green are further labeled with G, and components associated with blue are further labeled with B.

[0072] [Implementation Method 1] <Composition> Figure 1 1 is a plan view schematically showing the structure of the display device 100 according to the first embodiment of the present disclosure. Figure 1 FIG. 4 shows a smartphone as an example of a display device. Figure 1 As shown, the display device 100 includes a frame area NDA and a display area DA. The display area DA of the display device 100 includes a plurality of pixels PIX, each pixel PIX including a red sub-pixel RSP, a green sub-pixel GSP, and a blue sub-pixel BSP.

[0073] In addition, the pixel structure of the display device of the present disclosure is not limited to the above structure. In the display device of the present disclosure, for example, a pixel PIX may include sub-pixels of other colors in addition to the red sub-pixel RSP, the green sub-pixel GSP, and the blue sub-pixel BSP.

[0074] Figure 2 It is a schematic representation Figure 1 FIG. 1 is a diagram showing a layer structure of the display device 100. Figure 2 As shown, in the display device 100 , a substrate 11 , a buffer layer 12 , a TFT (thin film transistor) layer 20 including a pixel circuit, an electroluminescent device 13 , an edge cover film 16 , a sealing layer 14 , and an external functional layer 15 are sequentially stacked.

[0075] The substrate 11 is a glass substrate or a flexible substrate primarily composed of a resin such as polyimide. For example, the substrate 11 may be composed of two layers of polyimide film with an inorganic film sandwiched between them. The barrier layer 12 may be an inorganic insulating layer that prevents the intrusion of foreign matter such as water and oxygen. The TFT layer 20 includes pixel circuits that control the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B.

[0076] The electroluminescent device 13 includes a red electroluminescent element 10R, a green electroluminescent element 10G, and a blue electroluminescent element 10B. The red electroluminescent element 10R includes an anode layer 21R and a cathode layer 22R, with a first electroluminescent layer 34R and a second electroluminescent layer 53R located between these electrode layers. Similarly, the green electroluminescent element 10G includes an anode layer 21G, a first electroluminescent layer 34G, a second electroluminescent layer 53G, and a cathode layer 22G, and the blue electroluminescent element 10B includes an anode layer 21B, a first electroluminescent layer 34B, a second electroluminescent layer 53B, and a cathode layer 22B.

[0077] Here, in Figure 2 , a configuration is shown in which cathode layers 22R, 22G, and 22B are provided on the red, green, and blue electric field light-emitting elements 10R, 10G, and 10B, respectively. The electric field light-emitting device 13 disclosed herein is not limited to this configuration. For example, the cathode layers 22R, 22G, and 22B may also be a common electrode layer provided across the red, green, and blue electric field light-emitting elements 10R, 10G, and 10B.

[0078] The sealing layer 14 covering the electroluminescent device 13 prevents foreign matter such as water and oxygen from penetrating the electroluminescent device 13. For example, it can be composed of two inorganic sealing films and an organic film formed between them. The external functional layer 15 adds various functions to the display device 100, such as optical control, touch sensor, and surface protection.

[0079] The edge cover film 16 is insulating and covers the edges of each anode layer 21R, 21G, and 21B. The edge cover 16 is formed by applying an organic material such as polyimide or acrylic resin, followed by patterning using photolithography. The red, green, and blue electroluminescent elements 10R, 10G, and 10B are, for example, organic light-emitting diodes (OLEDs).

[0080] The first electric field luminescent layer 34R includes a host compound 34HR and a guest compound 34UR. The second electric field luminescent layer 53R includes a host compound 53HR and a guest compound 53UR. The first electric field luminescent layer 34G includes a host compound 34HG and a guest compound 34UG. The second electric field luminescent layer 53G includes a host compound 53HG and a guest compound 53UG. The first electric field luminescent layer 34B includes a host compound 34HB and a guest compound 34UB. The second electric field luminescent layer 53B includes a host compound 53HB and a guest compound 53UB.

[0081] Host compound 34HR and host compound 53HR are made of the same material. Host compound 34HG and host compound 53HG are made of the same material. Host compound 34HB and host compound 53HB are made of the same material.

[0082] Guest compound 34UR and guest compound 53UR are made of the same material. Guest compound 34UG and guest compound 53UG are made of the same material. Guest compound 34UB and guest compound 53UB are made of the same material.

[0083] The concentration of the guest compound 53UR in the second electric field luminescent layer 53R is higher than the concentration of the guest compound 34UR in the first electric field luminescent layer 34R. The concentration of the guest compound 53UG in the second electric field luminescent layer 53G is higher than the concentration of the guest compound 34UG in the first electric field luminescent layer 34G. The concentration of the guest compound 53UB in the second electric field luminescent layer 53B is higher than the concentration of the guest compound 34UB in the first electric field luminescent layer 34B.

[0084] The concentration of the guest compound 53UR in the second electric field luminescent layer 53R may be less than or equal to twice the concentration of the guest compound 34UR in the first electric field luminescent layer 34R. The concentration of the guest compound 53UG in the second electric field luminescent layer 53G may be less than or equal to twice the concentration of the guest compound 34UG in the first electric field luminescent layer 34G. The concentration of the guest compound 53UB in the second electric field luminescent layer 53B may be less than or equal to twice the concentration of the guest compound 34UB in the first electric field luminescent layer 34B. If the concentration is greater than twice, excitons generated by self-extinction in the second electric field luminescent layer on the cathode layer side, where the concentration of the guest compound is higher, may be wasted. Furthermore, sufficient excitons may not be generated in the first electric field luminescent layer on the anode layer side, where the concentration of the guest compound is lower.

[0085] The difference between the peak wavelengths of the emission spectrum of the first electric field luminescent layer 34R and the peak wavelengths of the emission spectrum of the second electric field luminescent layer 53G can be 10 nm or less. The difference between the peak wavelengths of the emission spectrum of the first electric field luminescent layer 34G and the peak wavelengths of the emission spectrum of the second electric field luminescent layer 53G can be 10 nm or less. The difference between the peak wavelengths of the emission spectrum of the first electric field luminescent layer 34B and the peak wavelengths of the emission spectrum of the second electric field luminescent layer 53B can be 10 nm or less.

[0086] The full width at half maximum of the peak wavelength of the emission spectrum of the first electric field luminescent layer 34R is greater than or equal to 0.8 times and less than or equal to 1.25 times the difference between the full width at half maximum of the peak wavelength of the emission spectrum of the second electric field luminescent layer 53G. The full width at half maximum of the peak wavelength of the emission spectrum of the first electric field luminescent layer 34G is greater than or equal to 0.8 times and less than or equal to 1.25 times the difference between the full width at half maximum of the peak wavelength of the emission spectrum of the second electric field luminescent layer 53G. The full width at half maximum of the peak wavelength of the emission spectrum of the first electric field luminescent layer 34B is greater than or equal to 0.8 times and less than or equal to 1.25 times the difference between the full width at half maximum of the peak wavelength of the emission spectrum of the second electric field luminescent layer 53B.

[0087] The guest compounds 34UR and 53UR can be either fluorescent dopants or phosphorescent dopants. The guest compounds 34UG and 53UG can be either fluorescent dopants or phosphorescent dopants. The guest compounds 34UB and 53UB can be either fluorescent dopants or phosphorescent dopants.

[0088] Any one of guest compounds 34UR and 53UR, guest compounds 34UG and 53UG, and guest compounds 34UB and 53UB (guest compounds of the first color electroluminescent layer) is a fluorescent dopant, and the other one (guest compound of the second color electroluminescent layer) can also be a phosphorescent dopant.

[0089] The peak wavelength of the emission spectrum of the first electric field luminescent layer 34R and the second electric field luminescent layer 53R may be greater than or equal to 440 nm and less than or equal to 660 nm. The peak wavelength of the emission spectrum of the first electric field luminescent layer 34G and the second electric field luminescent layer 53G may be greater than or equal to 440 nm and less than or equal to 660 nm. The peak wavelength of the emission spectrum of the first electric field luminescent layer 34B and the second electric field luminescent layer 53B may be greater than or equal to 440 nm and less than or equal to 660 nm.

[0090] The guest compounds 34UR and 53UR may be hole-transporting materials. The guest compounds 34UG and 53UG may be hole-transporting materials. The guest compounds 34UB and 53UB may be hole-transporting materials.

[0091] The guest compounds 34UR and 53UR may be electron-transporting materials. The guest compounds 34UG and 53UG may be electron-transporting materials. The guest compounds 34UB and 53UB may be electron-transporting materials.

[0092] Reference Figure 3 ,right Figure 2 The device structure of the electroluminescent device 10 shown will be described. Figure 3 It indicates schematically Figure 2 The layer structure of the electroluminescent element is shown in the diagram.

[0093] like Figure 3 As shown, the substrate 11, buffer layer 12, and TFT layer 20 are stacked in this order. The electroluminescent element 10 is constructed by stacking an anode layer 21, a layer assembly 30, a first charge generation layer 40, a layer assembly 50, and a cathode layer 22 in this order on the TFT layer 20. The electroluminescent element 10 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). In the electroluminescent element 10, for example, the anode layer 21 functions as an anode, and the cathode layer 22 functions as a cathode.

[0094] The layer set 30 is composed of a hole injection layer 31, a first hole transport layer 32, a first electron blocking layer 33, a first electroluminescent layer 34, a first hole blocking layer 35, and a first electron transport layer 36. The layer set 50 is composed of a second hole transport layer 51, a second electron blocking layer 52, a second electroluminescent layer 53, a second hole blocking layer 54, a second electron transport layer 55, and an electron injection layer 56. A first charge generation layer 40 including an n-type first charge generation layer (electron generation layer) 41 and a p-type first charge generation layer (hole generation layer) 42 is arranged between the layer set 30 and the layer set 50. Figure 3 In the electroluminescent element 10 , the layer assembly 30 is also referred to as a “first stack”, and the layer assembly 50 is also referred to as a “second stack.” The layer assembly 30 , the first charge generation layer 40 , and the layer assembly 50 constitute an organic stack 60 .

[0095] The hole injection layer 31, the first hole transport layer 32, the first electron blocking layer 33, the first hole blocking layer 35, the first electron transport layer 36, the n-type first charge generation layer 41, the p-type first charge generation layer 42, the second hole transport layer 51, the second electron blocking layer 52, the second hole blocking layer 54, the second electron transport layer 55 and the electron injection layer 56 are carrier functional layers that facilitate at least one of the injection, movement and generation of carriers (electrons or holes).

[0096] The electroluminescent element 10 is a so-called tandem structure electroluminescent element in which two electroluminescent layers, a first electroluminescent layer 34 and a second electroluminescent layer 53, are arranged between the anode layer 21 and the cathode layer 22 in the stacking direction. The first electroluminescent layer 34 and the second electroluminescent layer 53, which overlap in the stacking direction, both emit light of the same color.

[0097] The thickness and guest compound concentration (dopant concentration) of the first ELE layer 34 vary depending on the emission color. For example, the thickness of the first ELE layer 34 for red and green ELE elements is 25 nm to 50 nm, with a guest compound concentration of 0.1 to 10 wt%. The thickness of the first ELE layer 34 for blue ELE elements is 10 nm to 25 nm, with a guest compound concentration of 0.1 to 10 wt%. The thickness and guest compound concentration (dopant concentration) of the second ELE layer 53 vary depending on the emission color. For example, the thickness of the second ELE layer 53 for red and green ELE elements is 25 nm to 50 nm, with a guest compound concentration of 0.1 to 15 wt%. The thickness of the second ELE layer 53 for blue ELE elements is 10 nm to 25 nm, with a guest compound concentration of 0.1 to 10 wt%. However, the guest compound concentration in the first ELE layer 34 is set to be greater than the guest compound concentration in the second ELE layer 53.

[0098] <Method for Manufacturing Electroluminescent Element> Next, referring to Figure 4 An example of a method for manufacturing the electroluminescent element 10 will be described. Figure 4 It shows Figure 3 FIG. 1 is a flow chart showing an example of a method for manufacturing the electroluminescent element 5 .

[0099] like Figure 4 As shown, in step S1, the anode layer 21 is formed on the TFT layer 20. Specifically, an Ag layer and an ITO (Indium Tin Oxide) layer are sequentially formed using a sputtering method.

[0100] In step S2, a hole injection layer 31 is formed on the anode layer 21. Specifically, the hole transport and electron accepting materials are co-deposited at a predetermined deposition rate by adjusting the respective deposition temperatures and deposition times to achieve a predetermined film thickness and ratio. A uniform deposition film is formed over the entire surface of the workpiece without using a fine metal mask.

[0101] In addition, based on Figure 4 In the detailed description of the manufacturing method, a portion of the carrier functional layer is formed as a common layer among all the color electroluminescent elements, but the manufacturing method of the present invention is not limited to this. The thickness of each carrier functional layer can be set differently for each color, for example, depending on the electroluminescent layer. In this way, carrier functional layers with locally varying thicknesses can be formed through vapor deposition through a mask.

[0102] In step S3, the first hole transport layer 32 is formed on the hole injection layer 31. Specifically, the hole transport material is deposited at a predetermined rate by adjusting the deposition temperature and deposition time, so that the hole transport material is deposited to a predetermined film thickness. Here, the deposited film is formed without using a fine metal mask.

[0103] In step S4, a first electron blocking layer 33 is formed on the first hole transport layer 32. Specifically, the hole transport material is deposited at a predetermined deposition rate by adjusting the evaporation temperature and deposition time, so that the hole transport material is deposited to a predetermined film thickness. Here, a fine metal mask is used to deposit the first thickness corresponding to each color. The first thickness can be the same or different for each color of the electroluminescent element.

[0104] In step S5, the first electroluminescent layer 34 is formed on the first electron-blocking layer 33. Specifically, the deposition temperature and time are adjusted to ensure that the host compound 34H and the guest compound 34U (dopant) are co-deposited at a predetermined deposition rate, resulting in a layer with a predetermined film thickness and guest compound concentration (dopant concentration). Using a fine metal mask, deposition is performed under conditions that precisely control the thickness and guest compound concentration of each layer for each color.

[0105] In step S6, the first hole-blocking layer 35 is formed on the first electroluminescent layer 34. Specifically, the electron transport material is deposited at a predetermined deposition rate by adjusting the deposition temperature and deposition time, so that the electron transport material is deposited to a predetermined film thickness. Here, the deposited film is formed without using a fine metal mask.

[0106] In step S7, the first electron transport layer 36 is formed on the first hole blocking layer 35. Specifically, the evaporation temperature and plating time are adjusted to evaporate the electron transport material at a specified evaporation rate, so that the electron transport material is stacked to a specified film thickness. The evaporation can also be a co-evaporation of the electron transport material and lithium quinoline. In this case, the evaporated film is formed without using a fine metal mask.

[0107] In step S8, an n-type first charge generation layer 41 is formed on the first electron transport layer 36. Specifically, an organic electron transport material and an inorganic metal material (Yb or Li, which serves as an electron donor) are co-deposited at a predetermined deposition rate by adjusting the respective deposition temperatures and deposition times so that the layers are stacked at predetermined film thicknesses and ratios. Here, the deposited film is formed without using a fine metal mask.

[0108] In step S9, a p-type first charge generation layer 42 is formed on the n-type first charge generation layer 41. Specifically, the organic hole transport material and the organic electron accepting material are co-deposited at a predetermined deposition rate by adjusting the respective deposition temperatures and deposition times to achieve a predetermined film thickness and ratio. Here, the deposited film is formed without using a fine metal mask.

[0109] 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 deposited at a predetermined rate by adjusting the deposition temperature and deposition time, so that the hole transport material is deposited to a predetermined film thickness. Here, the deposited film is formed without using a fine metal mask.

[0110] In step S11, a second electron blocking layer 52 is formed on the second hole transport layer 51. Specifically, the evaporation temperature and the plating time are adjusted to evaporate the hole transport material at a predetermined evaporation rate so that the hole transport material is stacked with a predetermined film thickness. Here, a fine metal mask is used to evaporate the second thickness corresponding to each color. The second thickness can also be the same as the first thickness, and can be the same or different in the electroluminescent element of each color.

[0111] In step S12, the second electroluminescent layer 53 is formed on the second electron-blocking layer 52. Specifically, the evaporation temperature and time are adjusted to ensure that the host compound 53H and the guest compound 53U (dopant) are co-evaporated at a predetermined rate, resulting in a layer with a predetermined film thickness and guest compound concentration (dopant concentration). Using a fine metal mask, the deposition is performed under conditions that precisely control the thickness of each layer and the guest compound concentration for each color.

[0112] In step S13, the second hole-blocking layer 54 is formed on the second electroluminescent layer 53. Specifically, the electron transport material is deposited at a predetermined deposition rate by adjusting the deposition temperature and deposition time, so that the electron transport material is deposited to a predetermined film thickness. Here, the deposited film is formed without using a fine metal mask.

[0113] In step S14, a second electron transport layer 55 is formed on the second hole blocking layer 54. Specifically, the evaporation temperature and deposition time are adjusted to a predetermined evaporation rate to deposit the electron transport material so that the electron transport material is stacked to a predetermined film thickness. This evaporation can also be a co-evaporation of the electron transport material and lithium quinoline. In this case, the deposited film is formed without using a fine metal mask.

[0114] In step S15, the electron injection layer 56 is formed on the second electron transport layer 55. Specifically, lithium fluoride is deposited at a predetermined rate by adjusting the deposition temperature and deposition time, so that the lithium fluoride is deposited to a predetermined film thickness. Here, the deposited film is formed without using a fine metal mask.

[0115] In step S16, the cathode layer 22 is formed on the electron injection layer 56. Specifically, a magnesium-silver alloy thin film is formed by vapor deposition, for example.

[0116] The method for manufacturing the electroluminescent device 13 includes alternately stacking the first electroluminescent layer 34 and the second electroluminescent layer 53 and the first charge generation layer 40 on the anode layer 21 or the cathode layer 22. The first electroluminescent layer 34 and the second electroluminescent layer 53 are each formed by evaporation.

[0117] Focusing on steps S5 and S12, the following can be said. The evaporation times of the guest compounds 34U and 53U when forming the first and second electric field luminescent layers 34 and 53 that emit light of the same color in the stacking direction may be made different for each of the first and second electric field luminescent layers 34 and 53. Alternatively, only the evaporation times of the guest compounds 34U and 53U when forming the first and second electric field luminescent layers 34 and 53 that emit light of the same color in the stacking direction may be made different for each of the first and second electric field luminescent layers 34 and 53. Alternatively, the evaporation temperatures of the guest compounds 34U and 53U when forming the first and second electric field luminescent layers 34 and 53 that emit light of the same color in the stacking direction may be made different for each of the first and second electric field luminescent layers 34 and 53.

[0118] <Light Emitting Mechanism> Figure 5 , the light emitting mechanism of the electroluminescent element 10 of the electroluminescent device 13 capable of full-color display will be described. Figure 5 It is used for Figure 2 FIG1 is a diagram illustrating the light emitting mechanism of the electroluminescent element 10 of the electroluminescent device 13 shown in FIG1. Figure 5 , only the main parts of the structure of the electroluminescent element 10 are shown.

[0119] Figure 5 The electroluminescent element 10 shown is an organic EL element having a series structure of two electroluminescent layers, a first electroluminescent layer 34 and a second electroluminescent layer 53, formed between an anode layer 21 and a cathode layer 22. A first charge generation layer 40 is disposed between the first electroluminescent layer 34 and the second electroluminescent layer 53. Figure 5In the illustrated example, each of the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B includes a stacked first electroluminescent layer 34 and a second electroluminescent layer 53 that emit light of the same color.

[0120] In more detail, Figure 5 As shown, in the red electroluminescent element 10R, an anode layer 21R, a first electroluminescent layer 34R, a first charge generation layer 40R, a second electroluminescent layer 53R, and a cathode layer 22R are sequentially arranged. In the red electroluminescent element 10R, a set 30R of layers including the first electroluminescent layer 34R, arranged between the anode layer 21R and the first charge generation layer 40R in the stacking direction, constitutes a first stack. A set 50R of layers including the second electroluminescent layer 53R, arranged between the first charge generation layer 40R and the cathode layer 22R, constitutes a second stack.

[0121] Similarly, in the green electroluminescent element 10G, an anode layer 21G, a first electroluminescent layer 34G, a first charge generation layer 40G, a second electroluminescent layer 53G, and a cathode layer 22G are arranged in this order. The first stack in the green electroluminescent element 10G is a set 30G of layers including the first electroluminescent layer 34G, arranged between the anode layer 21G and the first charge generation layer 40G in the stacking direction. The second stack in the green electroluminescent element 10G is a set 50G of layers including the second electroluminescent layer 53G, arranged between the first charge generation layer 40G and the cathode layer 22G.

[0122] In the blue electroluminescent element 10B, the anode layer 21B, the first electroluminescent layer 34B, the first charge generation layer 40B, the second electroluminescent layer 53B, and the cathode layer 22B are stacked in this order. The first stack in the blue electroluminescent element 10B is a set 30B of layers including the first electroluminescent layer 34B, disposed between the anode layer 21B and the first charge generation layer 40B in the stacking direction. The second stack in the blue electroluminescent element 10B is a set 50B of layers including the second electroluminescent layer 53B, disposed between the first charge generation layer 40B and the cathode layer 22B.

[0123] Here, the cathode layers 22R, 22G, and 22B are provided in the red electric field light emitting element 10R, the green electric field light emitting element 10G, and the blue electric field light emitting element 10B, respectively. For example, the cathode layers 22R, 22G, and 22B may also be a common electrode layer provided across the red electric field light emitting element 10R, the green electric field light emitting element 10G, and the blue electric field light emitting element 10B.

[0124] Furthermore, in Figure 5, a sealing layer 14 is shown provided on the uppermost portion of each of the red electric field light emitting element 10R, the green electric field light emitting element 10G, and the blue electric field light emitting element 10B to prevent the intrusion of oxygen and moisture. For example, the sealing layer 14 may be a common layer provided across the red electric field light emitting element 10R, the green electric field light emitting element 10G, and the blue electric field light emitting element 10B.

[0125] Furthermore, carrier functional layers (electron injection layer, electron transport layer, hole transport layer, hole injection layer, etc.) not shown are provided in the layer assemblies 30R, 30G, 30B, 50R, 50G, and 50B included in the electroluminescent elements 10R, 10G, and 10B of each color.

[0126] The following further describes the light-emitting mechanism of the electroluminescent element 10. In the following description, since the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B all have the same light-emitting mechanism, the color symbols R, G, and B added to the symbols representing each basic structure are omitted for explanation.

[0127] When current flows through the electroluminescent element 10, holes are supplied from the anode layer 21 to the first electroluminescent layer 34, and electrons are supplied from the cathode layer 22 to the second electroluminescent layer 53. Furthermore, when current flows through the electroluminescent element 10, electrons generated by the n-type first charge generation layer 41 are supplied to the first electroluminescent layer 34, and holes generated by the p-type first charge generation layer 42 are supplied to the second electroluminescent layer 53.

[0128] As a result, electrons and holes recombine in the first EFL layer 34 to generate electron / hole pairs (also known as excitons), which then transition to the ground state and emit light in a predetermined wavelength range. Furthermore, electrons and holes recombine in the second EFL layer 53 to generate electron / hole pairs, which then transition to the ground state and emit light in a predetermined wavelength range (light of the same color as the first EFL layer 34). For example, the first EFL layer 34R and the second EFL layer 53R of the red EFL element 10R each emit red light. Similarly, the first EFL layer 34G and the second EFL layer 53G of the green EFL element 10G each emit green light, and the first EFL layer 34B and the second EFL layer 53B of the blue EFL element 10B each emit blue light.

[0129] At this time, both the first electroluminescent layer 34 and the second electroluminescent layer 53 can emit light at a luminous efficiency substantially equal to the theoretical value. Therefore, the multiple light-emitting layers of the same color arranged along the stacking direction can emit light at substantially the same efficiency as the theoretical value, thereby achieving the formation of a high-brightness and high-definition full-color image.

[0130] However, the luminescence intensity of an existing EL element that does not employ an EL layer as described in the present disclosure, that is, an EL element having an existing tandem structure in which two host-guest EL layers emitting light of the same color are stacked at the same guest compound concentration, sometimes cannot reach twice the luminescence intensity of an EL element having a single EL layer (i.e., the theoretical value).

[0131] Here, the results of verification by the present inventors are introduced.

[0132] First, prototypes were produced for an electroluminescent device with a single blue electroluminescent layer (or single layer; single-layer structure) (single-layer prototype) and an electroluminescent device with a two-layer tandem structure (comparative tandem prototype). The comparative tandem prototype had the same guest compound concentration in each electroluminescent layer stacked in the stacking direction, i.e., a conventional tandem structure. The single-layer prototype was manufactured under the following conditions. The comparative tandem prototype was manufactured under three conditions: conditions i through iii. The single-layer prototype had the same layer structure as the first stack in condition i, between the cathode and anode layers. The comparative tandem prototype had the same guest compound concentration (dopant concentration) in each electroluminescent layer stacked in the stacking direction, i.e., a conventional tandem structure. For each prototype, device life and driving voltage were measured, and current efficiency was calculated using the Blue Index (unit: cd / A / y, where y is one side of the CIE 1931 chromaticity coordinate).

[0133] (Single-layer prototype) Layer structure: cathode layer / first stack (electron transport layer / hole blocking layer / first blue electroluminescent layer (guest compound concentration: 0.3 wt%) / electron blocking layer / hole transport layer / hole injection layer) / anode layer.

[0134] (Condition i) Layer structure: cathode layer / second stack (electron injection layer / electron transport layer / hole blocking layer / second blue electric field luminescent layer (guest compound concentration: 0.3 wt%) / hole transport layer / hole injection layer) / charge generation layer / first stack (electron transport layer / hole blocking layer / first blue electric field luminescent layer (guest compound concentration: 0.3 wt%) / electron blocking layer / hole transport layer / hole injection layer) / anode layer.

[0135] Total organic layer thickness: 235nm.

[0136] (Condition ii) Layer structure: Same as Condition i.

[0137] Thickness ratio: Same as condition i, except that the thickness of the carrier functional layer in the second stack is adjusted to set the total thickness to 255 nm.

[0138] (Condition iii) Layer structure: Same as condition i.

[0139] Thickness ratio: Same as condition i, except that the thickness of the carrier functional layer in the second stack was adjusted to set the total thickness to 215 nm.

[0140] The results showed that the current efficiency of the single-layer prototype was 206 cd / A / y. In contrast, the current efficiency of the comparative series-connected prototype was 305 cd / A / y (1.48 times the current efficiency of the single-layer prototype) under (condition i), 320 cd / A / y (1.55 times the current efficiency) under (condition ii), and 219 cd / A / y (1.1 times the current efficiency) under (condition iii). Furthermore, device lifespan measurements showed that the comparative series-connected prototype had a lifespan 2.01 times that of the single-layer prototype under (condition i), 2.12 times under (condition ii), and 1.92 times under (condition iii). Furthermore, driving voltage measurements showed that the comparative series-connected prototype had a driving voltage 1.9 times that of the single-layer prototype under (condition i), 1.9 times under (condition ii), and 2.0 times under (condition iii).

[0141] The above results show that conventional two-layer tandem structures, in which the conditions of the carrier-functional layers other than the luminescent layer were modified, showed an approximately two-fold improvement in device life compared to single-layer luminescent devices. On the other hand, conventional tandem structures with two luminescent layers having the same guest compound concentration showed a smaller increase in current efficiency than single-layer luminescent devices, relative to increases in driving voltage. Even with changes in the carrier-functional layer conditions, the luminous efficiency did not significantly increase to twice that of single-layer luminescent devices, despite a roughly two-fold increase in driving voltage.

[0142] Based on these results, a possible solution to achieving twice the luminous efficiency of a single-layer EFL element with a two-layer tandem structure is to apply a higher driving voltage and supply more current. This, however, would clearly result in increased power consumption and a reduced element lifespan. This suggests that further research is needed to fully realize the advantages of a tandem structure over existing designs. The present inventors' research suggests the following reasons.

[0143] Conventional EL devices with a single-layer EL layer typically optimize the material selection or thickness of carrier-functional layers outside the EL layer to achieve a uniform carrier supply within the EL layer, effectively achieving carrier balance. Furthermore, research has been conducted to address the issue of carrier balance within each EL layer in conventional tandem EL devices. This involves optimizing the carrier-functional layers surrounding the EL layer outside the EL layer, similar to the approach used in EL devices with a single EL layer. However, in tandem EL devices, a charge generation layer exists between adjacent EL layers in the stacking direction, but the ratio of electrons to holes generated by the charge generation layer is not 1:1. Typically, electron injection from the charge generation layer is lower than electron injection from the cathode, resulting in an insufficient supply of electrons when excitons are generated. Conversely, holes are likely to be oversupplied in the first EL layer. Furthermore, the n-type charge generation layer is primarily a hybrid structure of organic and inorganic materials, doping an organic electron transport material with a small amount of metallic material. However, in recent years, to prevent degradation of the organic materials contained in other layers due to high vapor deposition temperatures, the development of an n-type charge generation layer composed solely of organic materials, similar to the p-type charge generation layer, has been promoted. However, currently, the amount of electrons supplied from an n-type charge generation layer composed entirely of organic materials can be significantly reduced compared to the amount of holes supplied from the p-type charge generation layer. Therefore, in a tandem-structured electroluminescent element, it is difficult to ensure that the electron and hole supplies are consistent between two adjacent electroluminescent layers separated by the charge generation layer in the stacking direction.

[0144] Moreover, if one wants to improve the light extraction efficiency by utilizing the microcavity structure as described above, the position of the electroluminescent layer cannot be arbitrarily set, and it needs to be set at any position in the periodic position that depends on the emission wavelength and the refractive index. Due to the constraints of the microcavity condition on the position of the electroluminescent layer, and another phenomenon that extinction occurs due to interaction if the anode layer and the electroluminescent layer are formed close to each other, there is a restriction that the thickness of the hole transport layer formed between the anode layer and the electroluminescent layer thereon must be thicker than the thickness of the hole transport layer outside it. However, if the thickness of the hole transport layer is thickened, the probability of holes deactivating other hole transport layers becomes higher, and as a result, a further phenomenon occurs in which fewer holes are supplied to the electroluminescent layer formed next to the anode layer. Due to these phenomena, in an electroluminescent element having a series structure utilizing a microcavity structure, it becomes more difficult to make the supply of electrons and holes consistent between adjacent electroluminescent layers in the stacking direction.

[0145] Specifically, as demonstrated by the above verification results, conventional optimization of the carrier functional layer surrounding the EL layer, other than the EL layer, in tandem EL devices cannot be considered sufficient. Furthermore, there is room for research to address the new issue that the luminescence intensity of conventional tandem EL devices, which include two EL layers with the same guest compound concentration, does not reach twice the luminescence intensity of EL devices with a single EL layer.

[0146] To address this issue, the inventors focused on the carrier mobility and carrier injection properties in the stacking direction of an electroluminescent element having a series structure with multiple electroluminescent layers, and also focused on the supply of carriers and the amount of residual carriers generated in each electroluminescent layer. As a result, the inventors discovered that, in a plurality of electroluminescent layers stacked via a charge generation layer, when the supply of electrons in the electroluminescent layer on the cathode layer side is faster, the amount of carriers (electrons) supplied to the electroluminescent layer stacked on the anode layer side is less than the amount of carriers (electrons) supplied to the electroluminescent layer stacked on the cathode layer side. Therefore, in the electroluminescent layer stacked closer to the anode side, the supply of electrons that can combine with holes is insufficient, and as a result, the amount of electron-hole pairs (excitons) generated by recombination becomes relatively small. That is, even if there are multiple electroluminescent layers, the amount of excitons will not be this multiple. Furthermore, the inventors found that even if two electroluminescent layers with the same dopant concentration are present, as in conventional electroluminescent elements with a tandem structure, the luminescence intensity does not double. Furthermore, the inventors found that in conventional electroluminescent elements with a tandem structure, excess holes are generated in the electroluminescent layer formed closer to the anode layer, which is why the current efficiency of the electroluminescent element cannot be doubled.

[0147] The present inventors have introduced a novel design concept for an electroluminescent device with multiple electroluminescent layers and tandem compounds, as disclosed herein. This concept involves varying the concentration of a guest compound (dopant) within each electroluminescent layer to achieve carrier balance between the layers and address the issue of uneven carrier supply. This design eliminates the generation of excess carriers (holes) found in conventional tandem electroluminescent devices, allowing for the generation of an appropriate and balanced amount of excitons within each electroluminescent layer. As a result, the present invention achieves an electroluminescent device that reduces current consumption and driving voltage, while improving luminous efficiency.

[0148] Therefore, in the present invention, as described above, when the amount of electron injection into the EFL layer on the anode side is low, resulting in insufficient electron supply to the EFL layer closer to the anode, the guest compound concentration (dopant concentration) in the EFL layer formed on the anode side is reduced compared to the EFL layer formed on the cathode side. This allows the supply of holes to the EFL layer on the anode side to match the guest compound concentration (dopant concentration) of the EFL layer. This results in luminescence with a brightness commensurate with the guest compound concentration (dopant concentration), while the injection of excess holes in the EFL layer on the anode side is suppressed, generating a substantially theoretical amount of excitons. Consequently, all EFL layers stacked in the stacking direction emit light at a brightness corresponding to the guest compound concentration (dopant concentration). Furthermore, by preventing the injection (supply) of excess holes in any EFL layer, current consumption and, consequently, power consumption can be reduced. This also reduces the amount of guest compound (dopant) used.

[0149] Furthermore, particularly in the case of an electroluminescent element having a top-emission structure in which light is extracted from the cathode layer, if there are even slight defects in the sealing layer formed on the cathode layer, the cathode layer will degrade due to the intrusion of oxygen or moisture, reducing the electron injection property. As a result, the electron supply to the electroluminescent layer directly below the cathode layer will be insufficient. However, by assuming such a situation, it is theoretically possible to prevent the generation of excess holes in the electroluminescent layer on the cathode layer side by pre-lowering the guest compound concentration (dopant concentration) of the electroluminescent layer on the cathode layer side during the initial design stage. From this perspective, in the present disclosure, it is preferable to pre-lower the guest compound concentration (dopant concentration) of the electroluminescent layer on the cathode layer side to a theoretical value within the scope of obtaining the effects of the present disclosure.

[0150] Here, we describe simulation evaluations of the disclosed embodiments and comparative examples for a full-color electroluminescent device having two electroluminescent layers (tandem structure). The electroluminescent device was constructed under the following conditions: the dopant concentration of the cathode-side electroluminescent layer was 5 wt% for the red and green electroluminescent elements, and 3 wt% for the blue electroluminescent element; and the dopant concentration of the anode-side electroluminescent layer was 3 wt% for the red and green electroluminescent elements, and 2 wt% for the blue electroluminescent element.

[0151] Here, the dopant concentration of the cathode-side electric field light-emitting layer is set to be no more than twice the dopant concentration of the anode-side electric field light-emitting layer. If it is greater than twice, excitons generated by self-extinction in the cathode-side electric field light-emitting layer with a higher dopant concentration are wasted, and sufficient excitons may not be generated in the anode-side electric field light-emitting layer with a lower dopant concentration.

[0152] According to simulation results based on the above conditions, the full-color ELE device of the embodiment achieved a current efficiency (also called "luminous efficiency") 1.8 times that of the full-color ELE device of the comparative example, a device lifespan 2.9 times longer, and a driving voltage 1.8 times lower. These results demonstrate that the ELE device of the embodiment achieved an improvement in luminous efficiency commensurate with the increase in driving voltage. Therefore, the ELE device of the present disclosure can suppress the increase in driving voltage and achieve an improvement in luminous efficiency commensurate with the increase in driving voltage, compared to conventional full-color ELE devices having a tandem structure with two ELE layers having the same guest compound concentration.

[0153] Furthermore, under the conditions used in the simulations of the aforementioned examples, the driving voltage was further increased to further improve the current efficiency (luminous efficiency). As a result, the driving voltage was doubled, and the current efficiency was also doubled. Furthermore, the device lifetime under these conditions was increased by 2.6 times. This demonstrates that the full-color electroluminescent device having two electroluminescent layers (tandem structure) according to the present invention can achieve characteristics twice those of a single-layer full-color electroluminescent device.

[0154] Furthermore, compared with the blue electroluminescent elements in the above-described embodiment and comparative example, the electroluminescent device in the above-described embodiment achieved a 12.5% ​​increase in current efficiency, a 17.1% increase in device life, and a 12.9% reduction in driving voltage relative to the electroluminescent device in the comparative example. These results demonstrate that the electroluminescent device of this embodiment can achieve significantly improved characteristics.

[0155] [Implementation Method 2] The following reference Figure 6 Embodiment 2 of the present disclosure will be described. Figure 6 This figure illustrates the light-emitting mechanism of electroluminescent element 10A in electroluminescent device 13A according to Embodiment 2 of the present disclosure. Electroluminescent device 13A according to Embodiment 2 differs from electroluminescent device 13 described above in that it includes electroluminescent element 10A having three electroluminescent layers in the stacking direction.

[0156] In addition, Figure 6 , only the main components of the electroluminescent element 10A included in the electroluminescent device 13A are shown. Furthermore, for ease of description, in the subsequent embodiments, components having the same functions as those described in Embodiment 1 are denoted by the same reference numerals, and their descriptions are not repeated. Components having the same functions as those described in the preceding embodiments are denoted by the same reference numerals, and their descriptions are not repeated.

[0157] like Figure 6 As shown, in the red electroluminescent element 10AR, a third electroluminescent layer 71R and a second charge generation layer 80R are further arranged between the cathode layer 22R and the second electroluminescent layer 53R. In the red electroluminescent element 10AR, the set of layers 50R including the second electroluminescent layer 53R, arranged between the first charge generation layer 40R and the second charge generation layer 80R in the stacking direction, constitutes the second stack. The set of layers 70R including the third electroluminescent layer 71R, arranged between the second charge generation layer 80R and the cathode layer 22R, constitutes the third stack.

[0158] Similarly, in the green electroluminescent element 10AG, a third electroluminescent layer 71G and a second charge generation layer 80G are further arranged between the cathode layer 22G and the second electroluminescent layer 53G. In the green electroluminescent element 10AG, the set of layers 50G including the second electroluminescent layer 53G, arranged between the first charge generation layer 40G and the second charge generation layer 80G in the stacking direction, constitutes the second stack. Furthermore, the set of layers 70G including the third electroluminescent layer 71G, arranged between the second charge generation layer 80G and the cathode layer 22G, constitutes the third stack.

[0159] In addition, in the blue electroluminescent element 10AB, a third electroluminescent layer 71B and a second charge generation layer 80B are arranged between the cathode layer 22B and the second electroluminescent layer 53B. In the blue electroluminescent element 10AB, the set of layers 50B including the second electroluminescent layer 53B, arranged between the first charge generation layer 40B and the second charge generation layer 80B in the stacking direction, constitutes the second stack. Furthermore, the set of layers 70B including the third electroluminescent layer 71B, arranged between the second charge generation layer 80B and the cathode layer 22B, constitutes the third stack.

[0160] Furthermore, carrier functional layers (not shown) (electron injection layer, electron transport layer, hole transport layer, hole injection layer, etc.) are disposed within the layer assemblies 30R, 30G, 30B, 50R, 50G, 50B, and 70R, 70G, and 70B included in the electroluminescent elements 10AR, 10AG, and 10AB of each color. In the following description, the third electroluminescent layers 71R, 71G, and 71B included in the electroluminescent elements 10AR, 10AG, and 10AB of each color are collectively referred to as the "third electroluminescent layer 71."

[0161] The third electric field luminescent layer 71R includes a host compound 71HR and a guest compound 71UR. The third electric field luminescent layer 71G includes a host compound 71HG and a guest compound 71UG. The third electric field luminescent layer 71B includes a host compound 71HB and a guest compound 71UB.

[0162] The host compound 53HR and the host compound 71HR are made of the same material. The host compound 53HG and the host compound 71HG are made of the same material. The host compound 53HB and the host compound 71HB are made of the same material.

[0163] Guest compound 53UR and guest compound 71UR are made of the same material. Guest compound 53UG and guest compound 71UG are made of the same material. Guest compound 53UB and guest compound 71UB are made of the same material.

[0164] The concentration of the guest compound 71UR in the third electric field luminescent layer 71R is higher than the concentration of the guest compound 53UR in the second electric field luminescent layer 53R. The concentration of the guest compound 71UG in the third electric field luminescent layer 71G is higher than the concentration of the guest compound 53UG in the second electric field luminescent layer 53G. The concentration of the guest compound 71UB in the third electric field luminescent layer 71B is higher than the concentration of the guest compound 53UB in the second electric field luminescent layer 53B.

[0165] A first electric field luminescent layer 34, a second electric field luminescent layer 53 and a third electric field luminescent layer 71 (more than three electric field luminescent layers) are arranged in the stacking direction. The farther away from the anode layer 21 the first electric field luminescent layer 34, the second electric field luminescent layer 53 and the third electric field luminescent layer 71 are, the greater the concentration of the guest compounds 34U, 53U and 71U can be.

[0166] In this embodiment, the number of electroluminescent layers arranged in the stacking direction is 3, but may be 4 or more. On the other hand, the number of electroluminescent layers arranged in the stacking direction is preferably 5 or less (three or more electroluminescent layers means five or less electroluminescent layers).

[0167] The difference between the maximum value and the minimum value of the peak wavelength of the emission spectrum of the first electric field light-emitting layer 34R, the second electric field light-emitting layer 53R, and the third electric field light-emitting layer 71R can be 10 nm or less. The difference between the maximum value and the minimum value of the peak wavelength of the emission spectrum of the first electric field light-emitting layer 34G, the second electric field light-emitting layer 53G, and the third electric field light-emitting layer 71G can be 10 nm or less. The difference between the maximum value and the minimum value of the peak wavelength of the emission spectrum of the first electric field light-emitting layer 34B, the second electric field light-emitting layer 53B, and the third electric field light-emitting layer 71B can be 10 nm or less.

[0168] The maximum value of the full width at half maximum of the emission spectrum at the peak wavelength in the first electric field luminescent layer 34R, the second electric field luminescent layer 53R, and the third electric field luminescent layer 71R may be less than or equal to 1.25 times the minimum value of the full width at half maximum of the emission spectrum at the peak wavelength. The maximum value of the full width at half maximum of the emission spectrum at the peak wavelength in the first electric field luminescent layer 34G, the second electric field luminescent layer 53G, and the third electric field luminescent layer 71G may be less than or equal to 1.25 times the minimum value of the full width at half maximum of the emission spectrum at the peak wavelength. The maximum value of the full width at half maximum of the emission spectrum at the peak wavelength in the first electric field luminescent layer 34B, the second electric field luminescent layer 53B, and the third electric field luminescent layer 71B may be less than or equal to 1.25 times the minimum value of the full width at half maximum of the emission spectrum at the peak wavelength.

[0169] In the electroluminescent device 13A according to embodiment 2, for example, the carrier balance of all electroluminescent layers in the stacking direction is optimized by making the concentration of the guest compound 71U of the third electroluminescent layer 71, which has a large injection amount of rate-limiting carriers (electrons in this case), higher than the concentration of the guest compound 53U of the second electroluminescent layer 53.

[0170] In addition, Figure 6 In the embodiment, the cathode layer 22 may be a common electrode layer provided across the red electric field light emitting element 10AR, the green electric field light emitting element 10AG, and the blue electric field light emitting element 10AB. Similarly, the sealing layer 14 may be a common layer provided across the red electric field light emitting element 10AR, the green electric field light emitting element 10AG, and the blue electric field light emitting element 10AB.

[0171] Figure 6 In the case where the electroluminescent layer in the stacking direction is three layers, Figure 5 Similarly, in the display device 100 shown, any electroluminescent layer emits light with a brightness and light-emitting efficiency substantially equal to theoretical values.

[0172] In this embodiment, the same effects as those of the first embodiment are achieved in the first electroluminescent layer 34 and the second electroluminescent layer 53 that are adjacent to each other in the stacking direction.

[0173] [Main Effects] Currently, OLED electroluminescent layers utilize a host material doped with a guest material (dopant) at a concentration of approximately 0.05 to several wt% (mass concentration). This host-guest electroluminescent layer can achieve quantum efficiencies approaching the theoretical value.

[0174] Organic EL devices using a tandem structure, also known as a multiphoton type, theoretically generate multiple excitons from a single charge. Therefore, for example, a two-layer tandem structure should double the current efficiency. However, in conventional tandem OLEDs with multiple stacked electroluminescent layers, the total luminescence of the multiple layers does not reach multiples of the luminescence of a single layer.

[0175] Generally speaking, holes have higher mobility than electrons in organic materials. This means that exciton generation depends on the amount of injected electrons. To improve this, electron injection layers have been studied in single-layer OLEDs. However, in tandem OLEDs, the stacked electroluminescent layers are separated by a charge generation layer (CGL) rather than an electrode layer. Consequently, electron injection from the CGL is lower than from the cathode layer. Furthermore, the dopant materials contained in the electron transport layer may have low electron transport properties.

[0176] For these reasons, even if excitons generated by electron / hole combination are generated ideally (for example, 100) in an electroluminescent layer of one layer having a certain doping dose, in an electroluminescent layer of another layer having the same structure (same doping dose), the supply of charges (electrons) that become the rate limit is insufficient, and therefore, the generation of excitons is also less than 100.

[0177] As a result, the present inventors have found that the current efficiency (luminescence amount) of a tandem OLED including two electroluminescent layers having the same doping amount is not twice that of an OLED including a single electroluminescent layer.

[0178] By coating RGB separately, a tandem OLED can be constructed with only electric field light-emitting layers of the same color stacked in the stacking direction. The present disclosure focuses on the issue of injection from the CGL and the issue of carrier transport properties of the dopant material itself. That is, when the hole transport properties of the dopant material are higher than the electron transport properties, exciton generation is not necessarily limited by holes, but may be limited by electrons depending on the composition of the organic layers other than the electric field light-emitting layer, the electrode composition, etc. In addition, when the electron injection from the CGL is lower than the electron injection from the cathode layer, exciton generation is also limited by electrons. As a result, the exciton generation efficiency of the first electric field light-emitting layer on the anode layer side becomes low. In other words, when the electron transport properties of the dopant material are lower than the hole transport properties, and the electron injection properties from the CGL are lower than the electron injection from the cathode layer, the amount of dopant that does not contribute to light emission in the first electric field light-emitting layer (anode side electric field light-emitting layer) is larger than that in the second electric field light-emitting layer (cathode side electric field light-emitting layer).

[0179] Therefore, in the present invention, to address these issues, the dopant concentration in the first luminescent layer, where the rate-limiting charge (electrons) supply is lower, is lower than the dopant concentration in the second luminescent layer. Specifically, the dopant concentration in the luminescent layer on the anode side is lower than the dopant concentration in the luminescent layer on the cathode side. When stacking three or more layers, the dopant concentration increases in order from the anode side toward the cathode side.

[0180] As can be seen from the above discussion, according to the present disclosure, an electroluminescent device having multiple electroluminescent layers (tandem structure) with high luminous efficiency, low driving voltage, and thus low power consumption and long life can be realized.

[0181] The electroluminescent device and display device disclosed herein can be expected to enhance sustainability by, for example, achieving Goal 9.4 of the Sustainable Development Goals (SDGs) promoted by the United Nations: "Improving infrastructure and industrial processes through increased resource efficiency and the introduction and expansion of clean and environmentally friendly technologies and industrial processes."

[0182] The present invention is not limited to the above-described embodiments and may be modified in various ways within the scope of the claims. Embodiments obtained by appropriately combining the technical methods disclosed in different embodiments are also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical methods disclosed in various embodiments.

[0183] Description of Reference Numerals 10, 10A: Electroluminescent element 10R, 10AR: Red electroluminescent element 10G, 10AG: Green electroluminescent element 10B, 10AB: blue electroluminescent element 11:Substrate 12: Buffer layer 13, 13A: Electroluminescent device 14: Sealing layer 15: External functional layer 16:Edge covering film 20: TFT layer 21, 21R, 21G, 21B: anode layer 22, 22R, 22G, 22B: cathode layer 30, 30R, 30G, 30B, 50, 50R, 50G, 50B, 70R, 70G, 70B: a collection of layers 31: hole injection layer 32: first hole transport layer 33: First electron blocking layer 34, 34R, 34G, 34B: first electroluminescent layer 34H, 34HR, 34HG, 34HB: main compounds 34U, 34UR, 34UG, 34UB: guest compounds 35: first hole blocking layer 36: first electron transport layer 40, 40R, 40G, 40B: first charge generation layer 41: n-type first charge generation layer 42: p-type first charge generation layer 51: Second hole transport layer 52: Second electron blocking layer 53, 53R, 53G, 53B: second electroluminescent layer 53H, 53HR, 53HG, 53HB: main compounds 53U, 53UR, 53UG, 53UB: guest compounds 54: Second hole blocking layer 55: Second electron transport layer 56: electron injection layer 60: Organic laminate 71, 71R, 71G, 71B: third electroluminescent layer 71H, 71HR, 71HG, 71HB: main compounds 71U, 71UR, 71UG, 71UB: Guest compounds 80R, 80G, 80B: second charge generation layer 100, 100A: Display device DA: Display Area NDA: Border Area PIX: pixel.

Claims

1. An electroluminescent device, characterized in that: include: Anode layer; a cathode layer, arranged opposite to the anode layer in a stacking direction; a plurality of electroluminescent layers, each of which is disposed between the anode layer and the cathode layer in the stacking direction and in a direction intersecting the stacking direction, and comprises a host compound and a guest compound; as well as The charge generation layer is arranged between two adjacent electroluminescent layers in the stacking direction. In the electroluminescent layer, a plurality of electroluminescent layers emitting light of the same color are arranged in the stacking direction, and in a direction intersecting the stacking direction, electroluminescent layers of a first color to an nth color emitting light of different colors are arranged, where n is an integer ≥ 2. In the two electroluminescent layers adjacent to each other in the stacking direction with the charge generation layer interposed therebetween, The host compound of the electroluminescent layer on the anode layer side and the host compound of the electroluminescent layer on the cathode layer side are the same material. The guest compound of the electroluminescent layer on the anode layer side and the guest compound of the electroluminescent layer on the cathode layer side are the same material. The concentration of the guest compound in the electroluminescent layer on the cathode layer side is higher than the concentration of the guest compound in the electroluminescent layer on the anode layer side.

2. The electroluminescent device according to claim 1, wherein: The concentration of the guest compound in the electroluminescent layer on the cathode layer side is twice or less the concentration of the guest compound in the electroluminescent layer on the anode layer side.

3. The electroluminescent device according to claim 1 or 2, characterized in that: In the two electroluminescent layers adjacent to each other in the stacking direction with the charge generation layer interposed therebetween, the difference between the peak wavelength of the emission spectrum of the electroluminescent layer on the anode layer side and the peak wavelength of the emission spectrum of the electroluminescent layer on the cathode layer side is 10 nm or less.

4. The electroluminescent device according to any one of claims 1 to 3, characterized in that: In the two electroluminescent layers adjacent to each other in the stacking direction separated by the charge generation layer, the full width at half maximum of the peak wavelength of the luminescence spectrum of the electroluminescent layer on the anode layer side is greater than or equal to 0.8 times and less than or equal to 1.25 times the full width at half maximum of the peak wavelength of the luminescence spectrum of the electroluminescent layer on the cathode layer side.

5. The electroluminescent device according to any one of claims 1 to 4, characterized in that: In the two electroluminescent layers adjacent to each other in the stacking direction with the charge generation layer interposed therebetween, the guest compound of the electroluminescent layer on the anode layer side and the guest compound of the electroluminescent layer on the cathode layer side are either fluorescent dopants or phosphorescent dopants.

6. The electroluminescent device according to any one of claims 1 to 5, characterized in that: The guest compound of the first color electroluminescent layer is a fluorescent dopant, and the guest compound of the second color electroluminescent layer is a phosphorescent dopant.

7. The electroluminescent device according to claim 5 or 6, characterized in that: The peak wavelength of the light emitting spectrum of the electroluminescent layer containing the guest compound is 440 nm to 660 nm.

8. The electroluminescent device according to any one of claims 1 to 7, characterized in that: In the two electroluminescent layers adjacent to each other in the stacking direction with the charge generation layer interposed therebetween, the guest compound of the electroluminescent layer on the anode layer side and the guest compound of the electroluminescent layer on the cathode layer side are hole transporting materials.

9. The electroluminescent device according to any one of claims 1 to 7, characterized in that: In the two electroluminescent layers adjacent to each other in the stacking direction with the charge generation layer interposed therebetween, the guest compound in the electroluminescent layer on the anode layer side and the guest compound in the electroluminescent layer on the cathode layer side are electron transporting materials.

10. The electroluminescent device according to any one of claims 1 to 9, characterized in that: The electroluminescent device is a top-emitting type.

11. The electroluminescent device according to any one of claims 1 to 10, characterized in that: Three or more electroluminescent layers are arranged in the stacking direction, The farther the three or more electroluminescent layers are from the anode layer, the greater the concentration of the guest compound.

12. The electroluminescent device according to claim 11, wherein: The electroluminescent layer having more than three layers may have less than five layers.

13. The electroluminescent device according to any one of claims 1 to 12, characterized in that: In the plurality of electroluminescent layers arranged in the stacking direction, a difference between a maximum value of a peak wavelength of a light emission spectrum and a minimum value of a peak wavelength of a light emission spectrum is 10 nm or less.

14. The electroluminescent device according to any one of claims 1 to 13, characterized in that: In the plurality of electroluminescent layers arranged in the stacking direction, the maximum full width at half maximum of the emission spectrum at the peak wavelength is not more than 1.25 times the minimum full width at half maximum of the emission spectrum at the peak wavelength.

15. A display device, characterized in that: It comprises the electroluminescent device according to any one of claims 1 to 14.

16. A method for manufacturing an electroluminescent device, the method for manufacturing an electroluminescent device according to any one of claims 1 to 14, characterized in that: The method comprises the steps of alternately stacking the electroluminescent layer and the charge generation layer on the anode layer or the cathode layer, The electroluminescent layer is formed by an evaporation method.

17. The method for manufacturing an electroluminescent device according to claim 16, wherein: When forming a plurality of the electroluminescent layers emitting light of the same color in the stacking direction, the deposition time of the guest compound is made different for each of the electroluminescent layers.

18. The method for manufacturing an electroluminescent device according to claim 17, wherein: When forming a plurality of electroluminescent layers emitting light of the same color in the stacking direction, the deposition time of the guest compound is made different for each electroluminescent layer.

19. The method for manufacturing an electroluminescent device according to claim 16, wherein: When forming a plurality of the electroluminescent layers emitting light of the same color in the stacking direction, the vapor deposition temperature of the guest compound is made different for each of the electroluminescent layers.

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