Electroluminescent device, manufacturing method thereof and display device

Abstract

The electroluminescent device (13) includes, in a direction intersecting the stacking direction, first to n-th color electroluminescent layers (n > = 2) that emit light of different colors, and includes a first electroluminescent layer (34) that sandwiches a charge generation layer (40) on the anode layer side in the stacking direction. And a second electroluminescent layer (53) that sandwiches the charge generation layer (40) on the cathode layer side in the lamination direction. The ratio of the thickness of the first electroluminescent layer (34) on the anode layer side to the thickness of the second electroluminescent layer (53) is greater than 2 / 3 and less than 1.

Description

Technical Field

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

[0002] Mass production of displays using organic EL elements (also known as "Organic Light Emitting Diodes" or "OLEDs") has fully commenced, with their initial application in thin displays for smartphones and high-end TVs. Organic EL element displays are extremely popular and are currently considered a core component of thin displays, replacing LCDs.

[0003] Thin-film stacked organic EL devices are known that use aluminum quinoline complexes (Alq3) as both the electron transport layer and the electroluminescent layer. However, in order to further improve the luminous efficiency of organic EL devices, research and development is being carried out in the following order (1) to (3).

[0004] (1) A host-guest system material formed by adding a guest compound (dopant) to a host compound is used as a material for the electroluminescent layer, carrier (electron, hole) transport layer, or carrier (electron, hole) injection layer of an organic EL element.

[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 electroluminescent 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 light-emitting element adopts a tandem structure (for example, see Patent Documents 1 and 2).

[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 Summary of the Invention Problems to be solved by the invention However, when light-emitting layers of respective colors, RGB, are applied 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 to the Problem An electroluminescent device according to one aspect of the present disclosure includes: an anode layer; a cathode layer, which is opposite to the anode layer in a stacking direction; an electroluminescent layer, of which a plurality is arranged between the anode layer and the cathode layer in the stacking direction, and a plurality is arranged in a direction intersecting the stacking direction; and a charge generation layer, which is arranged between two adjacent electroluminescent layers in the stacking direction, wherein the electroluminescent layer is provided with a plurality of electroluminescent layers emitting light of the same color in the stacking direction, and the electroluminescent layer is provided with a first color electroluminescent layer to an nth color electroluminescent layer emitting light of different colors in a direction intersecting the stacking direction, wherein n is an integer ≥ 2, and when two adjacent electroluminescent layers in the stacking direction separated by the charge generation layer are formed into a group of electroluminescent layers, in at least one group of electroluminescent layers, the ratio of the thickness of the electroluminescent layer on the anode layer side in the stacking direction to the thickness of the electroluminescent layer on the cathode layer side in the stacking direction is greater than 2 / 3 and less than 1.

[0013] In addition, 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 involves a method for manufacturing an electroluminescent device for the above-mentioned electroluminescent device, including a process of alternately forming the electroluminescent layer and the charge generation layer on the anode layer or the cathode layer along the stacking direction, and forming the electroluminescent layer by evaporation.

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

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

[0017] Figure 2 It is schematically shown Figure 1 A diagram showing the layer structure of a display device.

[0018] Figure 3It is schematically shown Figure 2 The layer structure shown is a diagram of the layer structure of an electroluminescent element.

[0019] Figure 4 It shows Figure 3 FIG. 1 is a flow chart showing an example of a method for manufacturing the electroluminescent element 5 .

[0020] Figure 5 It is used for Figure 2 FIG. 2 is a diagram illustrating the light emission mechanism of the electroluminescent element of the electroluminescent device shown.

[0021] Figure 6 This is a 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 involved in the embodiment of the present disclosure has an anode layer, a cathode layer, an electroluminescent layer and a charge generation layer. The electroluminescent element of the present disclosure has a plurality of electroluminescent layers overlapping in the stacking direction. In addition, when two adjacent electroluminescent layers separated by the charge generation layer are a group of electroluminescent layers, in at least one group of electroluminescent layers, the thickness of the electroluminescent layer on the cathode layer side is in a specific ratio relative to the electroluminescent layer on the anode layer side. The electroluminescent element of the present disclosure can adopt a known element structure of a light-emitting element within the range of layers that meet these conditions. In the following, the layer structure of the electroluminescent element of the present disclosure is mainly explained by taking OLED as an example. In addition, in the present disclosure, "electroluminescent element" refers to a combination of the arrangement of the electroluminescent layer and various carrier functional layers in the stacking direction. In addition, in the present disclosure, "electroluminescent device" refers to a combination of multiple electroluminescent elements in a direction intersecting the stacking direction.

[0023] [Anode layer] The anode layer is one of a pair of electrodes (anode and cathode). In the present disclosure, it is used to supply holes to the 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 while 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 (In-Sn-O).

[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, In-Sn-O 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 improve the conductivity of the electrode layer or to add a visible light reflection function, a stacked structure with In-Sn-O formed on the surface of a metal material (e.g., In-Sn-O / Ag) 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. While typically composed of a luminescent material, an electroluminescent layer can also be a stacked structure of multiple functional layers, such as immediate and delayed luminescence layers, where two or more functional layers corresponding to two or more electroluminescent functions are overlapped, resulting in the overall electroluminescent function. In the present disclosure, multiple electroluminescent layers are provided between the anode layer and the cathode layer in the stacking direction, and multiple layers are provided 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 driver withstand voltage 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.

[0032] Here, the same color refers to the following: 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 half-value full width among all the luminous peak wavelengths is less than 1.25 times the minimum value of the half-value full width among the remaining luminous peak wavelengths. Basically, when the host material and the guest compound constituting the electroluminescent layer are both the same material or similar materials with the same skeleton, the same color will be 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 changes. Therefore, in the top emission structure, the same color refers to the colors of light that all luminous peak wavelengths are within the range of ±10nm and satisfy the above-mentioned range of the half-value full width.

[0033] Furthermore, in a direction intersecting the stacking direction, the electroluminescent layer is configured with a first color electroluminescent layer to an nth color electroluminescent layer (n being an integer ≥ 2) that emit light of mutually different colors. Here, "different colors" refers to the relationship between the colors of light that are not included in the range of the same color described above. A representative example of a multi-color electroluminescent layer used for full-color display is a red, green, and blue light-emitting layer. Arranging multiple colors of electroluminescent layers in a direction intersecting the stacking direction is called "split coating." Furthermore, the number of electroluminescent layers of each color in the stacking direction is preferably the same for all luminescent colors. The following description of the electroluminescent layer of the present invention primarily uses the three color luminescent layers described above as an example, but the present disclosure is not limited to this. Furthermore, the electroluminescent layers of different colors in this intersecting direction can be located 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 electroluminescent layers of red, green and blue colors, the electroluminescent elements of each color are independently configured 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 viewpoint of improving color purity, it is preferred that only a plurality of electroluminescent layers emitting light of the same color be configured in the stacking direction. From the viewpoint of more easily determining the thickness of the electroluminescent layer described later, it is advantageous to only stack electroluminescent layers of the same color in the stacking direction.

[0035] In an electroluminescent element having three or more electroluminescent layers arranged in the stacking direction, when a charge generation layer is sandwiched between the electroluminescent layers, optimizing the supply of carriers (electrons and / or holes) to the charge generation layer in the electroluminescent layer on one side of the stacking direction may result in a bottleneck in the supply of electrons and / or holes to the electroluminescent layer on the other side, i.e., insufficient supply. Consequently, the carrier balance among the multiple electroluminescent layers may be unbalanced. In the present disclosure, increasing the thickness of the carrier-functional layer on the anode layer side (particularly the hole transport layer closest to the anode layer) increases the probability of hole deactivation, resulting in a smaller hole supply than electron supply in the electroluminescent layer on the anode layer side. Addressing this phenomenon, the thickness of the electroluminescent layer on the anode layer side, where the carrier (hole) supply is bottlenecked, is reduced to optimize the carrier balance of all electroluminescent layers in the stacking direction.

[0036] That is, in the electroluminescent device of the present disclosure, when two electroluminescent layers adjacent to each other in the stacking direction with the charge generation layer separated therefrom are formed into a group of electroluminescent layers, in at least one group of electroluminescent layers, the ratio of the thickness of the electroluminescent layer on the anode layer side in the stacking direction to the thickness of the electroluminescent layer on the cathode layer side in the stacking direction is greater than 2 / 3 and less than 1. That is, in the present disclosure, the thickness of a specific electroluminescent layer is thinned at the above ratio relative to the thickness of the electroluminescent layer on the adjacent cathode layer side. In particular, since the hole transport layer closest to the anode layer side is thicker, it is preferred that the thickness of the electroluminescent layer closest to the anode layer side be thinner. In addition, the "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 contained in the electroluminescent element. That is, other layers may also be sandwiched between the electroluminescent layers adjacent to each other in the stacking direction.

[0037] By adopting such a configuration, the electroluminescent device of the present disclosure achieves an appropriate carrier balance in each electroluminescent layer in the stacking direction. The thickness of each electroluminescent layer in the stacking direction can be appropriately determined based on various factors, such as the combination of materials for each layer in the electroluminescent element and the purpose of changing the thickness of the electroluminescent layer, as long as the technical concept of the present disclosure is followed.

[0038] Therefore, the effects of a tandem structure with multiple electroluminescent layers in the stacking direction are weakened. On the other hand, if this ratio is too large, the effect of correcting the carrier balance becomes less effective, and excess carriers (electrons) are generated in the electroluminescent layer on the anode side. This results in current that does not contribute to light emission, leading to increased power consumption.

[0039] Furthermore, if a hole-transport layer is formed on the anode side of each adjacent electroluminescent layer in the stacking direction, the probability of hole deactivation within each hole-transport layer may differ. For example, in a top-emission electroluminescent element, the light extraction efficiency is determined by the resonance conditions of the microcavity structure. The resonance conditions are determined by the total thickness of the organic layers in the stacking direction, the emission wavelength, and the refractive index of the organic material. Therefore, if the light-emitting point (electroluminescent layer) is stacked at an appropriate position relative to the total thickness of the organic layers, the light extraction efficiency to the outside is improved; if it is not positioned appropriately, the light extraction efficiency is reduced. Due to this limitation, the position of the electroluminescent layer cannot be arbitrarily set; it must be located at a position that depends on the emission wavelength and the periodicity of the refractive index. However, if the electroluminescent layer is formed too close to the anode layer, the interaction between the metal oxide (In-Sn-O) constituting the anode layer and the luminescent material (host compound and / or guest compound) (also known as the "heavy atom effect" or "plasmon effect") can cause quenching, resulting in a decrease in luminous efficiency. Therefore, for example, the electroluminescent layer closest to the anode layer should be stacked with the anode layer separated from the electroluminescent layer, not at the first peak (the shortest position, for example, 30 nm) based on the resonance condition, but should be located at the next first peak (for example, 100 nm). To achieve such a stacked structure, the organic layers other than the electroluminescent layer (carrier-functioning layers) must be thickened. More specifically, the hole transport layer on the anode layer side must be thickened, and the electroluminescent layer must be stacked at the appropriate position.

[0040] Since the electroluminescent element is formed by stacking various layers, impurities may be mixed in when forming each layer. The influence of such impurities is usually small in layers with the same thickness, but the influence of the difference in the amount of impurities between the hole transport layer adjacent to the above-mentioned anode layer and the hole transport layer formed in other positions, which is formed thicker, and the hole transport layer formed thinner, will be apparent. The inventors have found that in this case, the carrier balance in the electroluminescent layer in the series structure will be broken, but not too seriously. Therefore, from the viewpoint of reducing the influence of the above-mentioned small carrier balance disruption, it is preferred to set the difference in thickness of adjacent electroluminescent layers in the stacking direction to be smaller.

[0041] Therefore, to ensure sufficient luminescence from the electroluminescent layer on the anode side, the above ratio is preferably greater than 2 / 3 (=0.67), more preferably 5 / 7 (=0.71) or greater, and even more preferably 3 / 4 (=0.75) or greater. Furthermore, to suppress excess carrier generation, the above ratio is preferably less than 1, more preferably 9 / 10 or less, and even more preferably 6 / 7 (=0.86) or less. Furthermore, in addition to the above considerations, to facilitate the variation of the thickness of each electroluminescent layer in the stacking direction, the above ratio is more preferably 4 / 5 (=0.8) or greater, and even more preferably 5 / 6 (=0.83) or less.

[0042] By configuring a group of electroluminescent layers, in which the thickness of the electroluminescent layer on the anode layer side varies relative to the thickness of the electroluminescent layer on the cathode layer side according to the above-mentioned ratio, it is possible to reduce the useless thickness portion of each electroluminescent layer that does not contribute to luminescence, and also to reduce the material used in the electroluminescent layer. Therefore, it is possible to reduce the driving voltage and the manufacturing cost. In addition, in the electroluminescent device disclosed in the present invention, the thickness of the electroluminescent layer on the anode layer side, where the supply of electrons in the electroluminescent element is greater than the supply of holes, is thinned. Therefore, even if the supply of holes supplied from the hole transport layer on the anode layer side is small, the carrier balance in the electroluminescent layer on the anode layer side can be made uniform, reducing the excess electrons in the electroluminescent layer on the anode layer side. Thus, it is possible to reduce the current consumption and improve the luminous efficiency. Therefore, compared with the conventional electroluminescent device having an RGB coating type series structure with multiple electroluminescent layers of the same thickness, the electroluminescent device having a series structure with multiple electroluminescent layers disclosed in the present invention can achieve a reduction in driving voltage, a reduction in manufacturing cost and a further improvement in luminous efficiency.

[0043] It should be noted that, within the scope of achieving the effects of the present disclosure, the thickness of each electroluminescent layer may be varied at the aforementioned ratio for only one color of an electroluminescent device having multiple electroluminescent layers in a direction intersecting the stacking direction, or the thicknesses of any two or more colors may be varied uniformly or independently at the aforementioned ratio. Furthermore, the ratio of the thickness of each electroluminescent layer in the stacking direction may be constant for each luminescent color, or may be different ratios within the aforementioned range.

[0044] Furthermore, the thickness of each electroluminescent layer in the stacking direction can be determined to a desired thickness through simulation evaluation based on the carrier injection and / or carrier transport properties between the layers in the electroluminescent element, and verification experiments based on this evaluation. In this case, by setting the multiple electroluminescent layers stacked in the stacking direction to have only the same color, the parameters of carrier injection and carrier transport properties can be made the same between the different electroluminescent layers in the stacking direction. This makes simulation easier, and the thickness of the electroluminescent layer can be determined more simply.

[0045] Preferred electroluminescent layers disclosed herein include host-guest systems comprising a host compound and a guest compound. Host-guest electroluminescent layers contain a trace amount (e.g., 0.1 to several mol%) of a fluorescent dopant, etc., as the guest compound, in a solid medium containing the host compound. In such guest-doped electroluminescent layers, 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 transfers to the guest compound. This excitation energy transfer enables the host-guest electroluminescent layer to produce luminescence from the guest compound with higher quantum efficiency.

[0046] Furthermore, by doping the electroluminescent layer with a fluorescent dopant as a guest compound, the device lifespan of the electroluminescent element is dramatically improved. This is because the dopant, as a guest compound, acts as a carrier (electron or hole) trap within the solid medium of the host compound, 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. Deactivation processes can occur with or without radiation (thermal deactivation) or with radiation (luminescence). The phenomenon of luminescence generated during the radiation process is electroluminescence. Because the guest compound acts as a carrier trap, it not only improves the quantum efficiency of the electroluminescent layer but also increases the device lifespan due to the increased probability of carrier recombination. As a result, the luminous efficiency of the electroluminescent layer is improved and the lifespan of the electroluminescent element is extended.

[0047] As described above, 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 improved device life of the electroluminescent element.

[0048] 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 theorem, 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%.

[0049] In contrast, when a phosphorescent dopant is used as a guest compound, the quantum efficiency can be increased by three times that of a fluorescent dopant because light can be extracted from triplet excitons. 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, when a phosphorescent dopant is used as a guest compound, the quantum efficiency can be increased by up to four times compared to when a fluorescent dopant is used.

[0050] The luminescent material of the electroluminescent layer can be a known material. For example, examples of luminescent materials constituting the blue electroluminescent layer include pyrene compounds and anthracene compounds as fluorescent dopants. Furthermore, examples of luminescent materials constituting the red and green electroluminescent layers include iridium complexes and palladium complexes as phosphorescent dopants.

[0051] Furthermore, complexes containing platinum-group elements such as iridium and palladium used as phosphorescent dopants are extremely expensive, especially due to the low production and uneven distribution of platinum-group elements. This makes even trace amounts extremely expensive, and also poses difficulties in maintaining a stable supply. Therefore, reducing the use of these platinum-group complexes as phosphorescent dopants is extremely important from the perspective of cost reduction and economic security.

[0052] 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.

[0053] The materials for the electroluminescent layer of the host-guest system 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 fluorescent and phosphorescent dopants mentioned above, 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.

[0054] 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 larger current through the blue electroluminescent layer than through the red and green electroluminescent layers.

[0055] However, increasing only the amount of current flowing through the blue electroluminescent layer inevitably shortens the lifespan of the blue electroluminescent layer (degradation is accelerated). The blue electroluminescent layer is the fluorescent electroluminescent layer described above, which uses only a fluorescent dopant as the guest compound. To address this issue, the total number of blue electroluminescent layers stacked in the stacking direction can be greater than the total number of phosphorescent electroluminescent layers, i.e., the red and green electroluminescent layers, in which the guest compound is a phosphorescent dopant. In other words, the total number of fluorescent electroluminescent layers can be greater than the total number of phosphorescent electroluminescent layers. This configuration helps balance the lifespan and luminescence output between the fluorescent and phosphorescent electroluminescent layers in the electroluminescent element. The total number of fluorescent electroluminescent layers is preferably one or two more than the total number of phosphorescent electroluminescent layers. If the number of fluorescent electroluminescent layers exceeds the number of phosphorescent electroluminescent layers by three or more, it may be difficult to achieve a balance between the lifespan and luminescence output of the different-colored phosphorescent electroluminescent layers. Alternatively, the balance of the element structure of the electroluminescent device may be deteriorated, thereby reducing the manufacturing yield.

[0056] [Charge Generation Layer] 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.

[0057] 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.

[0058] The p-type charge generation layer can be made of 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. Examples of organic electron accepting materials include tetracyanoquinodimethane tetrafluoride (TCNQ-4F). The p-type charge generation layer utilizes the aforementioned hole transport material and electron accepting material, all of which are organic materials, to achieve sufficient hole generation capability.

[0059] The n-type charge generation layer can use a material containing, for example, an organic electron transport material and an inorganic metal material, Yb (ytterbium) or Li (lithium), added in a range of 5-20% to act as an electron donor. Examples of organic electron transport materials include oxadiazole compounds. Research is ongoing to utilize 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.

[0060] Currently, no organic electron-donating materials with sufficient electron-donating properties for use in n-type charge generation layers have been found. Consequently, no n-type charge generation layer composed entirely of organic materials and exhibiting sufficient properties has been found. According to the present disclosure, by setting the thickness of each electroluminescent layer in the stacking direction as described above, it is possible to optimize the balance (carrier balance) between the electrons and / or holes supplied from the charge generation layer and the holes and / or electrons supplied by each electrode layer. Therefore, the present disclosure suppresses the generation of excess carriers, achieving an electroluminescent device having a tandem structure with multiple electroluminescent layers in the stacking direction, with excellent power consumption.

[0061] [Other components] 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 structures include carrier functional layers such as hole injection layers, electron injection layers, hole transport layers, electron blocking layers, hole blocking layers, and electron transport layers.

[0062] 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.

[0063] 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).

[0064] LiF, used in the electron injection layer, exhibits excellent electron injection properties. On the other hand, the formation of carrier-functional layers containing inorganic materials, such as Yb and not limited to LiF, is typically performed at higher temperatures than the formation of electroluminescent layers and carrier-functional layers composed of organic materials due to the high melting points of these inorganic materials. Consequently, there is a risk of thermal damage to the organic materials formed earlier. Therefore, it is preferable to form electroluminescent elements using only organic materials whenever possible.

[0065] Therefore, organic electron-injecting materials such as BUPH1, BPen, p-MeO-Phen, p-NMe2-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) deposited at relatively low temperatures. 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, the effect of this reduction in electron supply can be mitigated by reducing the thickness of the electroluminescent layer on the cathode layer side compared to the theoretical value, thereby fully realizing the advantages of a tandem structure. To address the imbalance in carrier balance in this situation, an effective approach is to reduce the thickness of the electroluminescent layer on the anode layer side by a specific ratio, making it thinner than the theoretically thinner thickness of the electroluminescent layer on the cathode layer side. Such a structure may be included in the present disclosure.

[0066] The hole transport layer can be composed of an organic hole transport material, for example, a triarylamine organic compound.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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. The hole transport layer, electron blocking layer, electroluminescent layer, hole blocking layer, electron transport layer, and charge generation layer may be repeatedly stacked multiple times in the stacking direction of the electroluminescent element.

[0071] The electroluminescent device disclosed in the present invention is suitable for an electroluminescent device capable of a 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, any layer of the carrier functional layer is sometimes thickened. The electroluminescent device disclosed in the present invention has a so-called series structure in the stacking direction, which includes a plurality of electroluminescent layers and carrier functional layers corresponding to each electroluminescent layer. Therefore, the thickening of the carrier functional layer for adjusting the distance between the electrode layers is suppressed, and the consumption of functionally unnecessary materials can be further suppressed.

[0072] [Stack] A stack is a collection of layers that includes 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 stack may also include a carrier-functional layer. Multiple stacks may also be disposed between the anode layer and the cathode layer in the stacking direction. Furthermore, in the present invention, the stack does not include an anode layer, a cathode layer, or a charge generation layer.

[0073] The thickness of the stack is determined to be a value at which the amount of light emitted from the electroluminescent layer reaches a theoretical value or a value close to the theoretical value. In the present disclosure, since the thickness of each electroluminescent layer in the stacking direction can be set within the range of the above ratio, from the perspective of improving the luminous efficiency of the electroluminescent element, it is preferably set to a thickness of the stack corresponding to the thickness of the electroluminescent layer set in this manner. From this perspective, the ratio of the thickness of the electroluminescent layer in the stacking direction to the thickness of the stack is preferably greater than 0.05 and preferably less than 0.60. In addition, the thickness of the stack is calculated by summing the thicknesses of the electroluminescent layer and the carrier functional layer in the stack, but layers with very small thicknesses (e.g., layers with a thickness of less than 1 nm) in the electroluminescent layer and the carrier functional layer may be ignored when calculating the thickness of the stack.

[0074] [Method for manufacturing electroluminescent device] In manufacturing the electroluminescent device of the present disclosure, the thickness of each electroluminescent layer is controlled so that, in adjacent electroluminescent layers sandwiching the charge generation layer in the stacking direction, the ratio of the thickness of the electroluminescent layer on the anode layer side in the stacking direction to the thickness of the electroluminescent layer on the cathode layer side in the stacking direction is greater than 2 / 3 and less than 1. Alternatively, the electroluminescent device of the present disclosure can be manufactured using a known manufacturing method capable of manufacturing an electroluminescent element having multiple electroluminescent layers (tandem structure). The manufacturing method of the electroluminescent device may be any method that includes alternately forming electroluminescent layers and charge generation layers on the anode layer or cathode layer in the stacking direction. Furthermore, from the perspective of precisely controlling the thickness of each electroluminescent layer in the electroluminescent device of the present disclosure, it is preferred that at least the electroluminescent layer be formed by vapor deposition in this manufacturing method. Electroluminescent layers formed by vapor deposition are generally preferred from the perspectives of achieving higher brightness and lower voltage operation. Furthermore, since electroluminescent elements composed of fine pixels can be formed with high precision, they are also preferred from the perspective of realizing high-definition display devices. Furthermore, the application of a co-evaporation method using a plurality of evaporation sources is more preferable because a host-guest system electroluminescent layer can also be formed.

[0075] Since the electroluminescent device disclosed in the present invention has a specific layer structure, it can be manufactured by repeatedly forming specific layers. In the present disclosure, it is preferred to form the electroluminescent layer having different thicknesses in the stacking direction by simply changing the evaporation time. According to such a manufacturing method, the conditions other than the evaporation time in the formation of the electroluminescent layer (for example, the evaporation rate controlled by the temperature of the crucible (evaporation temperature) or the evaporation temperature, the ratio of the host compound to the guest compound (the doping concentration of the guest compound), and the evaporation mask that specifies the pixel shape, etc.) can be fixed. Therefore, the characteristic deviation caused by the change of the conditions of each electroluminescent layer in the stacking direction can be suppressed, and the effect caused by the difference in the thickness of the electroluminescent layer in the stacking direction can be more significantly manifested.

[0076] [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 known display device including a known light-emitting device. Examples of display devices include televisions and smartphones.

[0077] [Explanation of specific methods] 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, using the accompanying drawings. In this specification, for components within the same basic configuration that are associated with different colors, the reference numerals for those components are further suffixed with the color. For example, the red component is suffixed with R, the green component is suffixed with G, and the blue component is suffixed with B.

[0078] [Implementation Method 1] <Composition> Figure 1 1 is a plan view schematically showing the configuration 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.

[0079] In addition, the pixel configuration of the display device of the present disclosure is not limited to the configuration described above. In the display device of the present disclosure, for example, one 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.

[0080] Figure 2 It is schematically shown 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.

[0081] 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 for controlling the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B.

[0082] 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.

[0083] Here, in Figure 2 2 shows a configuration in which cathode layers 22R, 22G, and 22B are provided in the red, green, and blue electroluminescent elements 10R, 10G, and 10B, respectively. The electroluminescent device 13 of the present disclosure is not limited to this configuration. For example, the cathode layers 22R, 22G, and 22B may be a common electrode layer provided across the red, green, and blue electroluminescent elements 10R, 10G, and 10B.

[0084] 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 is a layer that adds various functions to the display device 100, such as optical control, touch sensor, and surface protection.

[0085] Edge cover film 16 is insulating and covers the edges of each anode layer 21R, 21G, and 21B. Edge cover film 16 is formed by, for example, applying an organic material such as polyimide or acrylic resin and then patterning it using photolithography. The red, green, and blue electroluminescent elements 10R, 10G, and 10B are, for example, organic light-emitting diodes (OLEDs).

[0086] Reference Figure 3 ,right Figure 2 The device structure of the electroluminescent device 10 shown will be described. Figure 3 It is schematically shown Figure 2 The layer structure shown is a diagram of the layer structure of an electroluminescent element.

[0087] like Figure 3As shown, a substrate 11, a buffer layer 12, and a TFT layer 20 are stacked in this order. The electroluminescent element 10 comprises an anode layer 21, a layer assembly 30, a first charge generation layer 40, a layer assembly 50, and a cathode layer 22, which are stacked 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.

[0088] 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 .

[0089] 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, migration and generation of carriers (electrons or holes).

[0090] The electroluminescent element 10 is an electroluminescent element having a so-called tandem structure, in which two electroluminescent layers, namely 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 are stacked in the stacking direction, both emit light of the same color.

[0091] The thickness of the second electroluminescent layer 53 varies depending on the luminescent color. For example, the thickness of the second electroluminescent layer 53 for red and green electroluminescent elements is 25 nm to 50 nm, and the thickness of the second electroluminescent layer for blue electroluminescent elements is 10 nm to 25 nm. The ratio (T1 / T2) of the thickness of the first electroluminescent layer 34 (T1) to the thickness of the second electroluminescent layer 53 (T2) is greater than 2 / 3 and less than 1. More preferably, the ratio of the thickness of the first electroluminescent layer 34 to the thickness of the second electroluminescent layer 53 is greater than 4 / 5 and less than 5 / 6. Both the first electroluminescent layer 34 and the second electroluminescent layer 53 are host-guest electroluminescent layers.

[0092] <Method for Manufacturing Electroluminescent Element> Next, refer 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 .

[0093] 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 In-Sn-O layer are sequentially formed using a sputtering method.

[0094] In step S2, a hole injection layer 31 is formed on the anode layer 21. Specifically, the hole transport material and the 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. A uniform deposition film is formed over the entire surface of the workpiece without using a fine metal mask.

[0095] In addition, based on Figure 4 In the detailed description of the manufacturing method, some carrier functional layers are formed as a common layer across all color electroluminescent elements. However, the manufacturing method disclosed herein 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 varying thicknesses can be formed locally through masked vapor deposition.

[0096] In step S3, a 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 time, so that the hole transport material is deposited to a predetermined thickness. Here, the deposited film is formed without using a fine metal mask.

[0097] 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 rate by adjusting the evaporation temperature and time, so that the hole transport material is deposited to a predetermined 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.

[0098] In step S5, a first electroluminescent layer 34 is formed on the first electron blocking layer 33. Specifically, the deposition temperature and deposition time are adjusted so that the host compound and the guest compound (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. The first electroluminescent layer 34 is deposited so that the ratio of the thickness of the first electroluminescent layer 34 to the thickness of the second electroluminescent layer 53 is greater than 2 / 3 and less than 1. More preferably, the ratio of the thickness of the first electroluminescent layer 34 to the thickness of the second electroluminescent layer 53 is greater than 4 / 5 and less than 5 / 6. Furthermore, in the stacking direction, the first electroluminescent layer 34 is deposited so that the ratio of the thickness of the first electroluminescent layer 34 to the thickness of the layer assembly 30 is greater than 0.05 and less than 0.60.

[0099] 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 rate by adjusting the deposition temperature and time, so that the electron transport material is deposited to a predetermined thickness. Here, the deposited film is formed without using a fine metal mask.

[0100] In step S7, the first electron transport layer 36 is formed on the first hole blocking layer 35. Specifically, the evaporation temperature and evaporation time are adjusted to evaporate the electron transport material at a predetermined evaporation rate, 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 evaporation film is formed without using a fine metal mask.

[0101] 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) serving as an electron donor material 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.

[0102] 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 so that the layers are stacked at predetermined film thicknesses and ratios. Here, the deposited film is formed without using a fine metal mask.

[0103] 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 time, so that the hole transport material is deposited to a predetermined thickness. Here, the deposited film is formed without using a fine metal mask.

[0104] In step S11, a second electron blocking layer 52 is formed on the second hole transport layer 51. Specifically, the evaporation temperature and evaporation time are adjusted to evaporate the hole transport material at a specified evaporation rate so that the hole transport material is stacked with a specified film thickness. Here, a fine metal mask is used to evaporate to a second thickness corresponding to each color. The second thickness can also be the same or different in the electroluminescent elements of each color, similar to the first thickness.

[0105] In step S12, a second electroluminescent layer 53 is formed on the second electron-blocking layer 52. Specifically, the deposition temperature and deposition time are adjusted so that the host compound and the guest compound (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 of each layer and the guest compound concentration for each color. Furthermore, in the stacking direction, the second electroluminescent layer 53 is deposited so that the ratio of its thickness to the thickness of the layer assembly 50 is between 0.05 and 0.60.

[0106] 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 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.

[0107] In step S14, a second electron transport layer 55 is formed on the second hole blocking layer 54. 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. This deposition can also be a co-deposition of the electron transport material and lithium quinoline. In this case, the deposited film is formed without using a fine metal mask.

[0108] 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.

[0109] 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.

[0110] <Lighting Mechanism> Reference Figure 5 , the light emission 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 emission 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.

[0111] 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 5 In the illustrated example, a first electroluminescent layer 34 and a second electroluminescent layer 53 emitting light of the same color are stacked for each of the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B.

[0112] 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. Furthermore, 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.

[0113] 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 sequentially arranged. 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.

[0114] In addition, in the blue electroluminescent element 10B, an anode layer 21B, a first electroluminescent layer 34B, a first charge generation layer 40B, a second electroluminescent layer 53B, and a cathode layer 22B are arranged in this order. The first stack in the blue electroluminescent element 10B is a set 30B of layers including the first electroluminescent layer 34B, arranged 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, arranged between the first charge generation layer 40B and the cathode layer 22B.

[0115] Here, cathode layers 22R, 22G, and 22B are provided in the red EL element 10R, the green EL element 10G, and the blue EL element 10B, respectively. For example, cathode layers 22R, 22G, and 22B may be a common electrode layer provided across the red EL element 10R, the green EL element 10G, and the blue EL element 10B.

[0116] Furthermore, in Figure 5 10B, a sealing layer 14 is shown provided at the uppermost portion of each of the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B to prevent the intrusion of oxygen and moisture. For example, the sealing layer 14 may be a common layer provided across the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B.

[0117] 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.

[0118] The following further describes the light emission 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 emission mechanism, the suffixes R, G, and B of the colors added to the reference numerals representing the basic configurations are omitted for explanation.

[0119] 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.

[0120] As a result, electrons and holes recombine in the first electroluminescent 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 electroluminescent 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 electroluminescent layer 34). For example, the first electroluminescent layer 34R and the second electroluminescent layer 53R of the red electroluminescent element 10R each emit red light. Similarly, the first electroluminescent layer 34G and the second electroluminescent layer 53G of the green electroluminescent element 10G each emit green light, and the first electroluminescent layer 34B and the second electroluminescent layer 53B of the blue electroluminescent element 10B each emit blue light.

[0121] At this point, 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. Consequently, multiple light-emitting layers of the same color arranged along the stacking direction all emit light at substantially the same efficiency as the theoretical value, enabling the formation of a high-brightness, high-definition, full-color image.

[0122] However, existing electroluminescent elements that do not adopt the electroluminescent layer structure described in the present disclosure, that is, conventional electroluminescent elements having a tandem structure formed by stacking two host-guest electroluminescent layers of the same thickness that emit light of the same color, sometimes do not emit light twice as much as an electroluminescent element having a single electroluminescent layer (i.e., the theoretical value).

[0123] Here, we introduce the results of our verification. First, we produced an electroluminescent element with a single blue electroluminescent layer (also called a single layer; single-layer structure) (single-layer prototype) and an electroluminescent element with a tandem structure having two blue electroluminescent layers (comparative tandem prototype). The single-layer prototype was produced under the following conditions. The comparative tandem prototype was produced under three conditions: conditions i through iii. The single-layer prototype had the same layer structure as the first stacked structure in condition i, between the cathode and anode layers. The comparative tandem prototype had a conventional tandem structure, with the thickness of each electroluminescent layer stacked in the stacking direction being the same. For each prototype, the device life and driving voltage were measured, and the current efficiency was calculated using the Blue Index (unit: cd / A / y, where y is one side of the chromaticity coordinate in CIE 1931).

[0124] (Single-layer trial work) Layer composition: cathode layer / first stack (electron transport layer / hole blocking layer / first blue electroluminescent layer (thickness: 15nm) / electron blocking layer / hole transport layer / hole injection layer) / anode layer Thickness ratio: the ratio of the thickness of the first electroluminescent layer (15 nm) to the thickness of the first stack (165 nm) was 0.091.

[0125] (Condition i) Layer composition: cathode layer / second stack (electron injection layer / electron transport layer / hole blocking layer / second blue electroluminescent layer (thickness: 15nm) / hole transport layer / hole injection layer) / charge generation layer / first stack (electron transport layer / hole blocking layer / first blue electroluminescent layer (thickness: 15nm) / electron blocking layer / hole transport layer / hole injection layer) / anode layer Thickness ratio: the ratio of the thickness of the second electroluminescent layer (15 nm) to the thickness of the second stack (70 nm) is 0.21, and the ratio of the thickness of the first electroluminescent layer (15 nm) to the thickness of the first stack (165 nm) is 0.091.

[0126] (Condition ii) Layer composition: Same as condition i.

[0127] Thickness ratio: Same as condition i, except adjusting the thickness of the carrier functional layer within the second stack layer to a total thickness of 80 nm. The ratio of the thickness of the second electroluminescent layer (15 nm) to the thickness of the second stack layer (80 nm) was 0.188.

[0128] (Condition iii) Layer composition: Same as condition i.

[0129] Thickness ratio: Same as condition i, except adjusting the thickness of the carrier functional layer within the second stack layer to a total thickness of 90 nm. The ratio of the thickness of the second electroluminescent layer (15 nm) to the thickness of the second stack layer (90 nm) was 0.167.

[0130] The results showed that the current efficiency of the single-layer test piece was 206 cd / A / y. In contrast, the current efficiency of the comparative series-connected test piece was: (Condition i) 305 cd / A / y (1.48 times the current efficiency of the single-layer test piece), (Condition ii) 320 cd / A / y (1.55 times the current efficiency of the single-layer test piece), and (Condition iii) 219 cd / A / y (1.1 times the current efficiency of the single-layer test piece). Furthermore, device lifetime measurements showed that the comparative series-connected test piece had a lifetime of 2.01 times (Condition i), 2.12 times (Condition ii), and 1.92 times (Condition iii) compared to the single-layer test piece. Furthermore, driving voltage measurements showed that the comparative series-connected test piece had a driving voltage of 1.9 times (Condition i), 1.9 times (Condition ii), and 2.0 times (Condition iii) compared to the single-layer test piece.

[0131] The above results show that conventional tandem electroluminescent devices with two electroluminescent layers of the same thickness, but with modified conditions for the carrier functional layer other than the electroluminescent layer, have a device lifespan approximately doubled compared to single-layer electroluminescent devices with a single electroluminescent layer. On the other hand, it is found that the increase in current efficiency in conventional tandem electroluminescent devices with two electroluminescent layers of the same thickness, compared to single-layer electroluminescent devices, is less than the increase in driving voltage. Even with modified conditions for the carrier functional layer, even when the driving voltage is approximately doubled, the luminous efficiency is often far less than twice that of single-layer electroluminescent devices.

[0132] Based on these results, in order to achieve twice the luminous efficiency of an electroluminescent element with a conventional two-layer electroluminescent layer (tandem structure) compared to an electroluminescent element with a single-layer structure (i.e., an electroluminescent element with a single electroluminescent layer), it is possible to apply a higher driving voltage and supply more current. However, this would obviously result in increased power consumption and a shortened element lifespan. This shows that further research is needed to fully realize the advantages of adopting a tandem structure over conventional structures. The following reasons are considered based on the research conducted by the present inventors in this regard.

[0133] In organic materials, the mobility of holes is generally higher than that of electrons. As a result, the supply of electrons can easily become a bottleneck in exciton generation. Conversely, holes can easily become oversupplied. In existing electroluminescent devices with a single electroluminescent layer, efforts are made to achieve a uniform carrier supply within the electroluminescent layer, i.e., carrier balance, by optimizing the material selection or thickness of carrier-functional layers outside the electroluminescent layer. Furthermore, conventional approaches to addressing carrier balance within each electroluminescent layer in tandem electroluminescent devices have been similar to those used in single-layer electroluminescent devices: optimizing the surrounding carrier-functional layers outside the electroluminescent layer. However, in electroluminescent devices with multiple electroluminescent layers (tandem structure), although a charge generation layer exists between adjacent electroluminescent layers in the stacking direction, the balance between electrons and holes generated by the charge generation layer is typically not 1:1.

[0134] 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 advancing. 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. Due to these phenomena, it is difficult to ensure uniform electron supply between two adjacent electroluminescent layers separated by the charge generation layer in the stacking direction in electroluminescent elements with a tandem structure.

[0135] In addition, as mentioned above, if you want to use a microcavity structure to improve the light extraction efficiency, the position of the electroluminescent layer cannot be set arbitrarily, but must be set at any position that depends on the emission wavelength and the periodicity of the refractive index. There are restrictions on the position of the electroluminescent layer due to this microcavity condition and another restriction as follows: if the anode layer and the electroluminescent layer are formed close to each other, quenching will occur due to the interaction, so there is a restriction that the thickness of the hole transport layer formed between the anode layer and the electroluminescent layer above it must be set thicker than the thickness of the other hole transport layers. However, when the thickness of the hole transport layer is increased, another phenomenon occurs, that is, the probability of hole deactivation increases compared to other hole transport layers, resulting in a decrease in the amount of holes supplied to the electroluminescent layer formed directly above the anode layer. Due to these phenomena, in an electroluminescent element with a tandem structure using a microcavity structure, it becomes more difficult to make the supply of electrons and holes between adjacent electroluminescent layers in the stacking direction consistent.

[0136] Specifically, as demonstrated by the aforementioned verification results, conventional optimization of the carrier-functional layer surrounding the electroluminescent layer, other than the electroluminescent layer, in electroluminescent elements having a tandem structure may not always be sufficient. Furthermore, there is room for research on solutions to the new issue that the luminescence output of conventional tandem-structured electroluminescent elements having two electroluminescent layers of equal thickness does not reach twice the luminescence output of an electroluminescent element having a single electroluminescent layer.

[0137] To address this issue, the inventors focused on the carrier mobility and carrier injection properties in the stacking direction of an electroluminescent element having multiple electroluminescent layers (tandem structure), and furthermore, on the carrier supply and excess carrier generation within each electroluminescent layer. The inventors discovered that, in multiple electroluminescent layers stacked with a charge generation layer interposed therebetween, when the hole supply in the electroluminescent layer closer to the anode layer becomes a bottleneck, the amount of carriers (holes) supplied to the electroluminescent layer closer to the anode layer becomes less than the amount of carriers (holes) supplied to the electroluminescent layer closer to the cathode layer. Consequently, the supply of electrons available for recombination with holes decreases in the electroluminescent layer closer to the cathode layer, and the amount of electron-hole pairs (excitons) generated by this recombination decreases relatively. In other words, even with the presence of multiple electroluminescent layers (and even if the thickness of the electroluminescent layers is multiplied), the amount of excitons does not increase severalfold. Furthermore, the results showed that, in conventional electroluminescent devices having two electroluminescent layers of equal thickness, the luminescence amount could not be doubled. In addition, the inventors also found that, in conventional electroluminescent devices having a tandem structure, excess electrons were generated in the electroluminescent layer formed closer to the anode layer, which may be the reason why the current efficiency of the electroluminescent device could not be doubled.

[0138] The present inventors have introduced a new design concept for an electroluminescent element with a tandem structure comprising multiple electroluminescent layers. Specifically, as disclosed herein, by varying the composition (thickness) of each electroluminescent layer, carrier balance is achieved between the multiple layers, thereby resolving the issue of imbalanced carrier supply. This approach eliminates the excess carriers found in conventional tandem electroluminescent elements, allowing for the generation of appropriate and balanced excitons in each electroluminescent layer along the stacking direction. Consequently, the present invention achieves an electroluminescent device with a tandem structure that reduces current consumption and driving voltage, while improving luminous efficiency.

[0139] Therefore, in the present disclosure, as described above, because the hole transport layer closer to the anode layer has more deactivated holes than the other hole transport layers, the amount of holes supplied to the electroluminescent layer closer to the anode layer is reduced. Therefore, the thickness of the electroluminescent layer formed on the anode layer side is reduced to be thinner than the thickness of the electroluminescent layer formed on the cathode layer side. As a result, electrons matching the thickness of the electroluminescent layer on the anode layer side are supplied to the electroluminescent layer on the cathode layer side. Therefore, excess holes that cannot recombine with holes to generate electron-hole pairs (excitons) are not generated, and the amount of excitons generated in the electroluminescent layer on the cathode layer side is substantially consistent with the theoretical value. Therefore, any electroluminescent layer overlapping in the stacking direction emits light at a brightness corresponding to its thickness. Furthermore, the generation of excess holes in any electroluminescent layer overlapping in the stacking direction can be prevented, thereby reducing current consumption and, in turn, power consumption.

[0140] Furthermore, particularly in the case of an electroluminescent element having a top-emission structure in which light is extracted from the cathode layer side, if there is even a slight defect in the sealing layer formed on the cathode layer, the cathode layer deteriorates due to the intrusion of oxygen or moisture, reducing the electron injection property. As a result, the amount of electrons supplied to the electroluminescent layer directly below the cathode layer decreases. However, by anticipating such a situation, the thickness of the electroluminescent layer on the cathode layer side is formed to be thinner than the theoretical thickness from the initial design stage, thereby preventing the generation of excess holes in the electroluminescent layer on the cathode layer side. From this point of view, in the present disclosure, the thickness of the electroluminescent layer on the cathode layer side can be set to be thinner than the theoretical thickness within the range that can achieve the effects of the present disclosure.

[0141] Here, we describe simulation evaluations of the examples and comparative examples of the present disclosure for a full-color electroluminescent device having two electroluminescent layers in a tandem structure. The electroluminescent device was constructed with the following conditions: the thickness of the electroluminescent layer on the cathode side was 36 nm for the red and green electroluminescent elements, and 15 nm for the blue electroluminescent element; and the thickness of the electroluminescent layer on the anode side was 27 nm for the red and green electroluminescent elements, and 12 nm for the blue electroluminescent element.

[0142] The electroluminescent device of the embodiment has the layer structure of this embodiment. The ratio of the thickness of the electroluminescent layer on the anode layer side to the thickness of the electroluminescent layer on the cathode layer side is: 3 / 4 for both the red and green electroluminescent layers, and 4 / 5 for the blue electroluminescent layer. In addition, the thickness of each stack of each color is in the range of 40nm to 241nm, and the ratio of the thickness of the electroluminescent layer in each stack to the thickness of the stack is in the range of 0.053 to 0.59. The electroluminescent device of the comparative example has each layer having the same thickness as each layer of the stack on the cathode layer side described above, and each color has a single electroluminescent layer (non-tandem structure) full-color electroluminescent device.

[0143] According to simulation results based on the above conditions, the current efficiency (also called "luminous efficiency") of the full-color electroluminescent device of the above-mentioned embodiment is 1.7 times the current efficiency of the full-color electroluminescent device of the above-mentioned comparative example, the device life is 2.6 times longer, and the driving voltage is 1.7 times higher. These results show that the electroluminescent element of the above-mentioned embodiment achieves an improvement in luminous efficiency commensurate with the increase in driving voltage. Therefore, the electroluminescent device having the tandem structure of the present disclosure can suppress the increase in driving voltage, extend the device life, and achieve an improvement in luminous efficiency commensurate with the increase in driving voltage, compared to conventional full-color electroluminescent devices having a tandem structure.

[0144] 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, the current efficiency was also doubled, and the device life under these conditions was 2.5 times longer. This demonstrates that a full-color electroluminescent device having two electroluminescent layers (tandem structure) in the stacking direction according to the present invention can achieve characteristics twice those of a single-layer full-color electroluminescent device.

[0145] Furthermore, a comparison of the above-described embodiment and comparative example reveals that the current efficiency of the electroluminescent device in the embodiment increased by 12.2%, the device life increased by 13.1%, and the driving voltage decreased by 12.2% compared to the electroluminescent device in the comparative example. These results demonstrate that the electroluminescent device of this embodiment can achieve significant improvements in characteristics. Furthermore, the electroluminescent device of this embodiment can reduce the amount of guest compound (dopant) used compared to conventional electroluminescent devices having a tandem structure with multiple electroluminescent layers of uniform thickness in the stacking direction.

[0146] A verification example related to the present disclosure (confirmation of the influence of impurities) will be described below.

[0147] (Confirmation of the influence of impurities) Models 1 and 2 of electroluminescent elements with single-layer structures (only the first stack) were produced by vapor deposition. The hole transport layer thickness of Model 1 was 70 nm, and that of Model 2 was 120 nm. The deposition conditions for each layer were identical, except for the deposition time of the hole transport layer. This allowed us to examine the effect of increasing hole transport layer thickness on impurities. It should be noted that the hole transport layer thickness of both Models 1 and 2 met the microcavity requirements. Furthermore, it was confirmed that even with the thinner Model 1, quenching did not occur due to the interaction between the anode layer and the electroluminescent layer.

[0148] The luminescence characteristics, device lifespan, and drive voltage of the prototypes 1 and 2 were determined. The results showed that Model 2 had approximately the same characteristics as Model 1: luminescence characteristics 97.5%, device life 99.2%, and drive voltage 103.5%.

[0149] [Implementation Method 2] Reference Figure 6 , embodiment 2 of the present disclosure is described. Figure 6 This figure illustrates the light emission 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.

[0150] 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.

[0151] 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 stacked body. 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 stacked body.

[0152] 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.

[0153] In addition, in the blue electroluminescent element 10AB, a third electroluminescent layer 71B and a second charge generation layer 80B are disposed 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 disposed 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 disposed between the second charge generation layer 80B and the cathode layer 22B constitutes the third stack.

[0154] Furthermore, carrier functional layers (not shown) (electron injection layer, electron transport layer, hole transport layer, hole injection layer, etc.) are disposed in the layer assemblies 30R, 30G, 30B, 50R, 50G, 50B, and 70R, 70G, and 70B included in the electroluminescent elements 10R, 10G, and 10B of each color. In the following description, the third electroluminescent layers 71R, 71G, and 71B included in the electroluminescent elements 10R, 10G, and 10B of each color are collectively referred to as the "third electroluminescent layer 71."

[0155] In the stacking direction, if the thickness of the second electroluminescent layer 53 is set to 1, then, for example, the thickness of the third electroluminescent layer 71 is 1, and the thickness of the first electroluminescent layer 34 is 4 / 5. In this way, the thickness of the first electroluminescent layer 34, which is arranged closest to the anode layer, is greater than 2 / 3 and less than 1 of the thickness of the second electroluminescent layer 53 and the third electroluminescent layer 71.

[0156] Furthermore, the thickness of each set of layers 30, 50, and 70 for each color is within a range of 40 nm to 241 nm, and the ratio of the thickness of each set of layers 30, 50, and 70 to the thickness of each electroluminescent layer 34, 53, and 71 is within a range of 0.053 to 0.59. Within these ranges, the thickness of the electroluminescent layers 34, 53, and 71 included in the electroluminescent element 10A is such that the first electroluminescent layer 34, which is located closest to the anode layer 21, is thinner, while the second electroluminescent layer 53 and the third electroluminescent layer 71, which are located closer to the cathode layer 22 than the first electroluminescent layer 34, have the same thickness.

[0157] In the electroluminescent device 13A involved in embodiment 2, for example, it is assumed that the probability of hole deactivation increases due to the increase in thickness of the carrier functional layer, especially the hole transport layer, included in the set 30 of layers arranged closest to the anode layer 21. In this case, the supply of holes will become a bottleneck in the first electroluminescent layer 34 arranged closest to the anode layer 21. Therefore, the thickness of the first electroluminescent layer 34 is set to be thinner than the thickness of the second electroluminescent layer. In addition, the thickness of the third electroluminescent layer 71 can be set to be thicker or thinner than the thickness of the second electroluminescent layer 53. In addition, the electroluminescent layer to be thinned should be determined based on the sets 30, 50, and 70 of the layers on the anode layer 21 side that include the thicker carrier functional layers, especially the hole transport layer.

[0158] In addition, Figure 6 In the embodiment, the cathode layer 22 may be a common electrode layer provided across the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B. Similarly, the sealing layer 14 may be a common layer provided across the red electroluminescent element 10R, the green electroluminescent element 10G, and the blue electroluminescent element 10B.

[0159] Figure 6 In the case where the electroluminescent layer in the stacking direction is three layers, Figure 5 The display device 100 shown similarly prevents excessive carrier (electron or hole) supply in the electroluminescent layer on the cathode layer side, and both electroluminescent layers emit light with a brightness and luminous efficiency substantially in accordance with theoretical values.

[0160] As described above, in this embodiment, in two adjacent stacks among the three stacks, the carrier balance is adjusted by a method other than controlling the thickness of the electroluminescent layer disclosed in the present invention. As a result, when a good carrier balance cannot be obtained in the remaining stack, it is preferred to adjust the carrier balance in the remaining stack. Examples of the configuration of such an electroluminescent device include: in an electroluminescent device comprising one or more configurations selected from the following combinations, the ratio of the thickness of the electroluminescent layer between the second stack and the third stack is the ratio of the thickness of the electroluminescent layer described in the present invention, wherein the above combination includes: a configuration in which the thickness of the corresponding carrier functional layer in the first stack is different from that in the second stack, a configuration in which the corresponding carrier functional layer in the first stack is made of different materials, and a configuration in which the first electroluminescent layer contained in the first stack is made of different materials from the second electroluminescent layer contained in the second stack.

[0161] This embodiment has the same effects as those of the first electroluminescent layer 34 and the second electroluminescent layer 53 that are adjacent to each other in the stacking direction in the first embodiment, and can produce the same effects.

[0162] [Implementation Method 3] This embodiment differs from the aforementioned embodiment 2 in that, among the three electroluminescent layers in embodiment 2, the ratio of the thickness of the electroluminescent layer on the anode layer side to the thickness of the electroluminescent layer on the cathode layer side gradually becomes the above-mentioned specific ratio, and other than that, it is the same as the aforementioned embodiment 2. This embodiment can be realized by Figure 6 The thicknesses of electroluminescent layer 71, electroluminescent layer 53, and electroluminescent layer 34 are modified to gradually decrease within the above-mentioned specific ratio range. In this embodiment, when two electroluminescent layers adjacent to each other in the stacking direction in embodiment 2 are grouped together, any group can produce the same effects as embodiment 1.

[0163] 〔Main effects〕 In the electroluminescent device disclosed herein, multiple electroluminescent layers are formed overlappingly in the stacking direction according to the emission color. Furthermore, in at least one pair of adjacent electroluminescent layers in the stacking direction, the ratio of the thickness of the electroluminescent layer on the anode layer side in the stacking direction to the thickness of the electroluminescent layer on the cathode layer side in the stacking direction is greater than 2 / 3 and less than 1. This configuration allows for a uniform carrier balance in the electroluminescent layer on the anode layer side, even when the amount of holes supplied to the electroluminescent layer on the anode layer side is low, thereby reducing excess electrons in the electroluminescent layer on the anode layer side. This reduces current consumption and improves luminous efficiency.

[0164] Furthermore, in the electroluminescent device of the present disclosure, since the thickness of the electroluminescent layer on the anode side is thinner than that on the cathode side, the total thickness of the electroluminescent layer from the anode to the cathode layer is reduced compared to a case where the thickness of each electroluminescent layer in the stacking direction is the same. Therefore, while maintaining the same electric field (unit: V / m) applied to each electroluminescent layer, the driving voltage (unit: V) can be reduced. Furthermore, since the generation of excess carriers in each electroluminescent layer is suppressed, current consumption can be reduced. Consequently, power consumption (unit: W = V × A) can also be reduced.

[0165] Therefore, compared to conventional electroluminescent devices having a tandem structure in which the thickness of each electroluminescent layer is the same in the stacking direction, the electroluminescent device of the present disclosure achieves higher brightness emission, low voltage drive, and low power consumption. Therefore, according to the present disclosure, in an electroluminescent device capable of full-color display without using color filters, etc., it is possible to achieve high brightness and / or power-saving emission, which are advantages of the tandem structure.

[0166] By configuring a set of electroluminescent layers with the above-mentioned specific ratio as: a first electroluminescent layer closest to the anode layer side in the stacking direction, and a second electroluminescent layer on the cathode layer side adjacent to the first electroluminescent layer, even if the carrier functional layer, especially the hole transport layer, arranged closest to the anode layer side becomes thicker, it is advantageous from the perspective of reducing the influence of slight carrier balance disruption caused by the influence of impurities, etc.

[0167] In addition, by using the electroluminescent layer as the electroluminescent layer of the host-guest system, it is possible to achieve a high luminous efficiency and an improvement in the life of the device. However, according to the present disclosure, it is possible to further improve the carrier balance (balance of electrons and holes) in each electroluminescent layer. Therefore, from the perspective of being able to give full play to the performance of each electroluminescent layer of the host-guest system with high luminous efficiency and excellent life, it is advantageous. In addition, it is also advantageous from the perspective of being able to reduce the amount of the expensive and rare phosphorescent dopant used as the guest compound.

[0168] Furthermore, when the electroluminescent layer includes a fluorescent electroluminescent layer containing a fluorescent dopant and a phosphorescent electroluminescent layer containing a phosphorescent dopant, the number of fluorescent electroluminescent layers in the stacking direction may be greater than the number of phosphorescent electroluminescent layers, for example, greater than 1 or 2. This configuration is more effective from the perspective of optimizing the balance between the lifetime and the amount of light emitted between the fluorescent electroluminescent layer and the phosphorescent electroluminescent layer.

[0169] Furthermore, by ensuring that the multiple electroluminescent layers stacked along the stacking direction are of the same color, the ability to separate the RGB colors for full-color display is advantageous from the perspective of achieving improved color purity. Furthermore, when the same material emitting the same color is stacked on each electroluminescent layer, simulations facilitate the evaluation of carrier injection and carrier transport properties between the electroluminescent layers. Thus, the above configuration is more effective from the perspective of more simply realizing an electroluminescent device capable of full-color display, including electroluminescent elements with desired characteristics.

[0170] The number of electroluminescent layers in the stacking direction may be 2 or more and 5 or less. This configuration is more effective from the viewpoint of achieving improved luminous efficiency, longer life, and lower driving voltage.

[0171] Furthermore, by setting the ratio of the thickness of the stack to the thickness of the electroluminescent layer within the stack to be greater than 0.05 and less than 0.60, the thickness of the carrier-functional layer can be optimized, further alleviating concerns about insufficient carrier supply to the electroluminescent layer. Therefore, this structure is more effective from the perspective of achieving theoretical brightness.

[0172] Furthermore, the electroluminescent device of the present disclosure may be a top emission type. This structure is more effective from the viewpoint of optimizing the thickness of the carrier function layer.

[0173] Furthermore, the display device disclosed herein includes the aforementioned electroluminescent device. Therefore, the ratio of the thickness of the electroluminescent layer on the anode layer side in the stacking direction to the thickness of the electroluminescent layer on the cathode layer side in the stacking direction can be made greater than 2 / 3 and less than 1. Therefore, this structure is advantageous from the perspective of preventing the generation of excess carriers in each electroluminescent layer in the stacking direction, thereby allowing the injected current to contribute to luminescence more efficiently. Therefore, according to the present disclosure, high-brightness, energy-saving luminescence is achieved in the display device, enabling the advantages of adopting a series structure in the electroluminescent device to be fully demonstrated.

[0174] In addition, in the electroluminescent device disclosed herein, at least the electroluminescent layer is formed by evaporation. Compared to the case where the electroluminescent layer is formed by other methods, this is advantageous in realizing an electroluminescent device capable of full-color display because it can form an electroluminescent layer of a host-guest system and separate coating of a highly precise luminescent layer. Moreover, it is advantageous to achieve high-precision control of the thickness of each electroluminescent layer. In particular, in the formation of the electroluminescent layer by a coating method using an inkjet device or the like, it is difficult to form the electroluminescent layer while doping the host compound with the guest compound and to control the thickness of the electroluminescent layer to the order of several nanometers. Therefore, it is believed that the coating method is difficult to form the electroluminescent device capable of full-color display of the present disclosure.

[0175] Furthermore, in the manufacture of the electroluminescent device of the present invention, electroluminescent layers having different thicknesses in the stacking direction can be formed by simply changing the vapor deposition time. This configuration allows electroluminescent layers of varying thicknesses to be formed using the same manufacturing equipment while applying the same conditions except for the time. This is more effective from the perspective of reducing manufacturing costs and improving productivity.

[0176] Furthermore, in electroluminescent devices comprising electroluminescent elements having multiple electroluminescent layers (tandem structure), the thickness of the electroluminescent layers is typically constant along the stacking direction. On the other hand, there are sporadic technologies in which the thickness of the electroluminescent layer varies along the stacking direction (e.g., Japanese Patent Laid-Open No. 2007-329054, International Publication No. 2010 / 113493, Japanese Patent Laid-Open No. 2015-153587, and Japanese Patent Laid-Open No. 2015-32582). Some of these technologies include technologies in which the cathode side has a thicker electroluminescent layer and the anode side has a thinner electroluminescent layer (e.g., Japanese Patent Laid-Open No. 2007-329054 and International Publication No. 2010 / 113493). However, these prior arts do not clearly explain the reason for varying the thickness of the electroluminescent layers along the stacking direction, and the aforementioned publications do not disclose achieving full-color display by applying each of the RGB colors separately.

[0177] According to the present disclosure, in an electroluminescent display device, by controlling the thickness of each electroluminescent layer in the stacking direction, it is possible to achieve further improved luminous efficiency, high brightness, and / or energy savings. Therefore, the electroluminescent device and display device of the present disclosure can enhance sustainability, for example, by achieving Goal 9.4 of the United Nations Sustainable Development Goals (SDGs): "Improving infrastructure and industry through increased resource efficiency and the introduction and expansion of clean and environmentally friendly technologies and industrial processes."

[0178] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical methods disclosed in different embodiments are also included in the technical scope of the present disclosure. Moreover, new technical features can be formed by combining the technical methods disclosed in various embodiments.

[0179] Description of Reference Numerals 10, 10A electroluminescent element 11 base plate 12 Buffer layer 13 Electroluminescent device 14 Encapsulation layer 15 External functional layer 16 Edge Covering Film 20 TFT layers 21 Anode layer 22 cathode layer 30, 50, 70-layer collections 31 hole injection layer 32. First hole transport layer 33 First electron blocking layer 34. First electroluminescent layer 35. First hole blocking layer 36 First electron transport layer 40 first charge generation layer 41 n-type first charge generation layer 42 p-type first charge generation layer 51 Second hole transport layer 52 Second electron blocking layer 53 second electroluminescent layer 54 second hole blocking layer 55 Second electron transport layer 56 Electron injection layer 60 organic laminates 71 third electroluminescent layer 80 Second charge generation layer 100, 100A display device DA display area NDA border area PIX pixels.

Claims

1. An electroluminescent device, characterized in that include: Anode layer; a cathode layer, which is opposite to the anode layer in a stacking direction; a plurality of electroluminescent layers arranged between the anode layer and the cathode layer in the stacking direction, and further arranged in a direction intersecting the stacking direction; and a charge generation layer disposed between two adjacent electroluminescent layers in the stacking direction; The electroluminescent layer is configured with a plurality of electroluminescent layers emitting light of the same color in the stacking direction. The electroluminescent layer is provided with a first color electroluminescent layer to an nth color electroluminescent layer (n is an integer ≥ 2) that emit light of different colors in a direction intersecting the stacking direction, and When two electroluminescent layers adjacent to each other in the stacking direction with the charge generation layer separated therefrom form a group of electroluminescent layers, in at least one group of electroluminescent layers, the ratio of the thickness of the electroluminescent layer on the anode layer side in the stacking direction to the thickness of the electroluminescent layer on the cathode layer side in the stacking direction is greater than 2 / 3 and less than 1.

2. The electroluminescent device according to claim 1, wherein The group of electroluminescent layers includes a first electroluminescent layer and a second electroluminescent layer, wherein the first electroluminescent layer is the electroluminescent layer that is closest to the anode layer in the stacking direction, and the second electroluminescent layer is the electroluminescent layer that is adjacent to the first electroluminescent layer in the stacking direction across the charge generation layer and is on the cathode layer side.

3. The electroluminescent device according to claim 1 or 2, characterized in that The electroluminescent layers are all electroluminescent layers of a host-guest system comprising a host compound and a guest compound.

4. The electroluminescent device according to claim 3, wherein: The electroluminescent layer includes a fluorescent electroluminescent layer in which the guest compound is a fluorescent dopant and a phosphorescent electroluminescent layer in which the guest compound is a phosphorescent dopant, according to the luminescent color. In the stacking direction, the number of the fluorescent electroluminescent layers is greater than the number of the phosphorescent electroluminescent layers.

5. The electroluminescent device according to claim 4, characterized in that In the stacking direction, the number of the fluorescent electroluminescent layers is one or two more than the number of the phosphorescent electroluminescent layers.

6. The electroluminescent device according to any one of claims 1 to 5, characterized in that In the stacking direction, only a plurality of electroluminescent layers emitting light of the same color are arranged.

7. The electroluminescent device according to any one of claims 1 to 6, characterized in that In the stacking direction, the number of the electroluminescent layers is 2 or more and 5 or less.

8. The electroluminescent device according to any one of claims 1 to 7, characterized in that When a set of layers including one electroluminescent layer arranged between the anode layer and the charge generation layer, between the charge generation layers, and between the charge generation layer and the cathode layer in the stacking direction is considered as a stacked body, In the stacking direction, a ratio of a thickness of the electroluminescent layer in the stacked body to a thickness of the stacked body is 0.05 or more and 0.60 or less.

9. The electroluminescent device according to any one of claims 1 to 8, characterized in that The electroluminescent device is of top emission type.

10. A display device, characterized in that: An electroluminescent device comprising the electroluminescent device according to any one of claims 1 to 9.

11. A method for manufacturing an electroluminescent device, for manufacturing the electroluminescent device according to any one of claims 1 to 9, comprising the steps of alternately forming the electroluminescent layer and the charge generation layer on the anode layer or the cathode layer along the stacking direction, wherein: include: The electroluminescent layer is formed by an evaporation method.

12. The method for manufacturing an electroluminescent device according to claim 11, wherein: The electroluminescent layer is formed to have different thicknesses in the stacking direction by simply changing the evaporation time.

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