Organic EL element, organic EL display panel, and method for manufacturing organic EL element

Abstract

To improve the luminous efficiency and extend the life in an organic EL element in which a light emitting layer is formed by using a host material and a fluorescent material.SOLUTION: In an organic EL element in which an anode, a light emitting layer, and a cathode are laminated in this order, the light emitting layer includes a fluorescent light emitting material and a host material, and a difference between the lowest empty orbital (LUMO) level of the fluorescent light emitting material and the highest occupied orbital (HOMO) level of the fluorescent light emitting material is less than or equal to a difference between the LUMO level and the HOMO level of the host material, the LUMO level of the fluorescent light emitting material is equal to or higher than the LUMO level of the host material, and the HOMO level of the fluorescent light emitting material is equal to or higher than the HOMO level of the host material, and the difference in energy level is 0.3 eV or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to improvements in luminous efficiency and lifetime in organic EL devices that use fluorescent materials as light-emitting materials. [Background technology]

[0002] In recent years, display devices using organic EL elements have become widespread.

[0003] An organic EL device has a structure in which at least an emitting layer is sandwiched between an anode and a cathode. In the emitting layer, the energy of excitons generated by the recombination of electrons and holes is converted into light. In organic semiconductors, there are two types of excitons (excited states): singlet excitons and triplet excitons, depending on the spin state of the electrons. In so-called fluorescent materials, the energy of singlet excitons is converted into light.

[0004] Conventionally, in order to improve the luminous efficiency of organic EL elements, various efforts have been made, such as adjusting the balance between electrons and holes (see, for example, Patent Document 1) and using phosphorescent materials that emit light due to triplet excitons (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-187205 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-171368 Summary of the Invention [Problem to be solved by the invention]

[0006] In particular, for blue light-emitting devices used in displays and the like, fluorescent materials with narrow emission spectra and excellent color purity are suitable for light emission, and there is a demand for improved efficiency in organic EL devices using fluorescent materials. Therefore, in order to utilize triplet excitons in fluorescent materials for light emission, the use of the TTF (Triplet-Triplet Fusion) phenomenon, in which singlet excitons are generated from triplet excitons, has been investigated. However, conventional organic EL devices using fluorescent materials as light-emitting materials have a structure suitable for generating excitons in the fluorescent material, and the density of triplet excitons is not sufficiently improved, making them unsuitable for improving quantum efficiency using the TTF phenomenon.

[0007] The present disclosure aims to improve the quantum efficiency of an organic EL element in which an emitting layer is formed using a host material and a fluorescent material by promoting the TTF phenomenon, thereby improving the luminous efficiency and extending the lifetime. [Means for solving the problem]

[0008] An organic EL device according to at least one embodiment of the present disclosure is an organic EL device including an anode, an emitting layer, and a cathode stacked in this order, the emitting layer including a fluorescent material and a host material, a difference between a lowest unoccupied molecular orbital (LUMO) level of the fluorescent material and a highest occupied molecular orbital (HOMO) level of the fluorescent material being equal to or less than a difference between the LUMO level and the HOMO level of the host material, and a luminescence intensity I of the emitting layer at an arbitrary time t after current application is interrupted. (t) changes according to the contribution of Triplet-Triplet Fusion (TTF) to the luminous efficiency, RTTF, and the LUMO level of the fluorescent material is equal to or higher than the LUMO level of the host material, and the HOMO level of the fluorescent material is equal to or higher than the HOMO level of the host material. By setting the difference in energy levels ΔHOMO between the HOMO levels of the fluorescent material and the host material to a value less than 0.3 eV, the RTTF can be increased by more than 20%. and 37.5% or less Value 。

[0009] In this specification, a high LUMO level or HOMO level means that the difference between the level and the vacuum level of electrons is small, that is, the potential energy of electrons present at the level is large.

[0010] Furthermore, the LUMO level or HOMO level being equal to or higher than a predetermined reference level (or lower) means that the difference between the LUMO level or HOMO level and the electron vacuum level is the same as the difference between the predetermined reference level and the electron vacuum level, or is smaller (or larger) than the difference between the predetermined reference level and the electron vacuum level. In other words, this means that the potential energy of electrons at the LUMO level or HOMO level is the same as the potential energy of electrons at the predetermined reference level, or is larger (or smaller) than the potential energy of electrons at the predetermined reference level.

[0011] In the examples and the like of this specification, the ionization potential of the thin film was measured using a photoelectron spectrometer (AC-3, manufactured by Riken Keiki Co., Ltd.) and used as the value of the HOMO level of the material. The optical absorption edge of the thin film was measured using a spectrophotometer (SolidSpec-3700, manufactured by Shimadzu Corporation) and used as the value of the singlet excitation level and energy gap of the material. In principle, the LUMO level of a material can be measured and quantified using low-energy photoelectron spectroscopy, etc., but for convenience in this specification, the LUMO level value is determined by subtracting the value of the singlet excitation level measured by the above method from the value of the HOMO level. [Effects of the Invention]

[0012] In an organic EL device according to at least one embodiment of the present disclosure, electrons and holes recombine in a host material with a high density in the light-emitting layer, which facilitates an increase in the density of triplet excitons in the host material and allows for an improvement in light-emitting efficiency using the TTF phenomenon, thereby improving the light-emitting efficiency of the light-emitting layer and potentially extending the life of the organic EL device. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a cross-sectional view schematically illustrating the configuration of an organic EL element 1 according to an embodiment. [Figure 2] FIG. 2 is a simplified schematic diagram showing band diagrams of a hole transport layer, a light emitting layer, an electron injection control layer, and an electron transport layer according to an example. [Figure 3] FIG. 2 is a simplified schematic diagram showing the band diagrams of a hole transport layer, a light emitting layer, an electron injection control layer, and an electron transport layer and the relationship between the recombination positions of electrons and holes according to Examples and Comparative Examples. [Figure 4] (a) is a schematic diagram showing the energy and state transition of excitons in a host material and a fluorescent material, and (b) is a state transition diagram showing the state transition of excitons and the maximum efficiency. [Figure 5] 1A is a table showing the relationship between the ΔHOMO value and the luminous efficiency for examples and comparative examples, and FIG. 1B is a table showing the detailed values ​​of the HOMO level and the LUMO level of each of the host material and the fluorescent material for examples and comparative examples. [Figure 6] 10A is a schematic graph showing the temporal transition of light emission intensity in pulsed light emission, and FIG. 10B is a schematic graph showing an outline of RTTF estimation based on the temporal transition of light emission intensity. [Figure 7] 10(a) is a graph showing the relationship between the ΔHOMO value and the RTTF, and FIG. 10(b) is a graph showing the relationship between the ΔHOMO value and the quantum efficiency. [Figure 8] 1A and 1B are partial cross-sectional views schematically illustrating a part of a manufacturing process of an organic EL element according to an embodiment, in which (a) shows a state in which a TFT layer is formed on a substrate, (b) shows a state in which an interlayer insulating layer is formed on a substrate, (c) shows a state in which a pixel electrode material is formed on the interlayer insulating layer, (d) shows a state in which a pixel electrode is formed, and (e) shows a state in which a partition material layer is formed on the interlayer insulating layer and the pixel electrode. [Figure 9] 1A and 1B are partial cross-sectional views schematically illustrating a part of a manufacturing process of an organic EL element according to an embodiment, in which (a) shows a state in which a partition wall is formed, (b) shows a state in which a hole injection layer is formed on a pixel electrode, and (c) shows a state in which a hole transport layer is formed on the hole injection layer. [Figure 10]1A and 1B are partial cross-sectional views schematically illustrating a part of a manufacturing process of an organic EL device according to an embodiment, in which (a) shows a state in which a light-emitting layer is formed on a hole-transporting layer, (b) shows a state in which an electron injection control layer is formed on the light-emitting layer and the partition layer, and (c) shows a state in which an electron transport layer is formed on the electron injection control layer. [Figure 11] 1A and 1B are partial cross-sectional views schematically illustrating a part of a manufacturing process of an organic EL device according to an embodiment, in which (a) shows a state in which an electron injection layer is formed on an electron transport layer, (b) shows a state in which a counter electrode is formed on the electron injection layer, and (c) shows a state in which a sealing layer is formed on the counter electrode. [Figure 12] 3 is a flowchart showing a manufacturing process of an organic EL element according to an embodiment. [Figure 13] 1 is a block diagram showing a configuration of an organic EL display device including an organic EL element according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] <<How one aspect of the present disclosure was achieved>> To use an organic EL device as a light-emitting element, it is essential to generate excitons, which are the initial state for light emission. Therefore, conventionally, hole injection from the hole transport layer to the light-emitting layer and electron injection from the electron transport layer to the light-emitting layer are enhanced, thereby increasing the carrier density in the light-emitting layer and increasing the probability of electron-hole recombination. To further increase the carrier density in the light-emitting layer, functional layers are selected that adjust the HOMO (Highest Occupied Molecular Orbital) level of the electron transport layer and / or the LUMO (Lowest Unoccupied Molecular Orbital) level of the hole transport layer to suppress hole leakage from the light-emitting layer to the electron transport layer and electron leakage from the light-emitting layer to the hole transport layer. This configuration increases the carrier density in the light-emitting layer and increases the probability of electron-hole recombination.

[0015] There are two types of excitons in organic materials, singlet excitons and triplet excitons, depending on the spin state of the electrons. In fluorescent materials, as mentioned above, singlet excitons contribute to light emission, while triplet excitons do not. However, the generation rate of singlet excitons and triplet excitons is approximately 1:3, making it a challenge to improve the density of singlet excitons.

[0016] In fluorescent materials with low luminous efficiency, especially blue-emitting materials with short emission wavelengths, the use of the TTF phenomenon, in which multiple triplet excitons collide to generate singlet excitons, has been investigated as a way to increase the density of singlet excitons. To utilize this TTF, it is necessary to increase the density of triplet excitons.

[0017] Conventionally, when using an emitting layer in which a fluorescent material is dispersed in a host material, the HOMO level of the fluorescent material is generally set higher than that of the host material and / or the LUMO level of the fluorescent material is set lower than that of the host material. By using such a configuration, holes and / or electrons injected into the emitting layer are trapped by the fluorescent material, which promotes recombination in the fluorescent material and generates singlet excitons of the fluorescent material that directly contribute to light emission.

[0018] However, the inventors discovered that the above-mentioned configuration is not optimal for improving the luminous efficiency using TTF. Although triplet excitons are also generated in the fluorescent material when recombination is promoted in the fluorescent material, the fluorescent material has a low spatial density in the light-emitting layer, so the efficiency of generating singlet excitons in the fluorescent material using TTF is low. Furthermore, in a configuration in which the fluorescent material traps carriers (holes or electrons), it is difficult to localize excitons due to carrier localization in the host material, and the density of triplet excitons in the host material cannot be sufficiently increased.

[0019] Therefore, the inventors have investigated a configuration for sufficiently increasing the density of triplet excitons to maximize the use of TTF, and have arrived at the embodiment of the present disclosure.

[0020] <Mode of Disclosure> An organic EL device according to one aspect of the present disclosure is an organic EL device including an anode, an emitting layer, and a cathode stacked in this order, wherein the emitting layer contains a fluorescent material and a host material, wherein the absolute value of the difference between the lowest unoccupied molecular orbital (LUMO) level of the fluorescent material and the highest occupied molecular orbital (HOMO) level of the fluorescent material is equal to or less than the absolute value of the difference between the LUMO level and the HOMO level of the host material, the LUMO level of the fluorescent material is equal to or greater than the LUMO level of the host material, and the HOMO level of the fluorescent material is equal to or greater than the HOMO level of the host material, and the difference in energy levels is 0.3 eV or less.

[0021] A method for manufacturing an organic EL device according to one aspect of the present disclosure includes forming a first electrode on a substrate, forming an emitting layer above the first electrode, and creating a second electrode above the emitting layer, wherein the emitting layer is formed using a fluorescent material and a host material, the absolute value of the difference between the lowest unoccupied molecular orbital (LUMO) level of the fluorescent material and the highest occupied molecular orbital (HOMO) level of the fluorescent material is equal to or less than the absolute value of the difference between the LUMO level and the HOMO level of the host material, the LUMO level of the fluorescent material is equal to or greater than the LUMO level of the host material, and the HOMO level of the fluorescent material is equal to or greater than the HOMO level of the host material, and the difference in energy levels is 0.3 eV or less.

[0022] In an organic EL device according to an embodiment of the present disclosure or an organic EL device manufactured by a method for manufacturing an organic EL device, electrons and holes recombine in a host material with a high density in the light-emitting layer. This facilitates an increase in the density of triplet excitons in the host material, and improves the light-emitting efficiency by utilizing the TTF phenomenon. This improves the light-emitting efficiency of the light-emitting layer, and is expected to extend the life of the organic EL device.

[0023] In the organic EL element according to the aspect of the present disclosure, the distance between the light-emitting center in the light-emitting layer and a surface of the light-emitting layer facing the cathode may be smaller than the distance between the light-emitting center and a surface of the light-emitting layer facing the anode.

[0024] As a result, in an organic EL device in which recombination occurs in a region of the light-emitting layer close to the cathode-side surface of the light-emitting layer, hole trapping by the fluorescent material can be reduced and the density of triplet excitons in the host material can be increased, thereby making it possible to more effectively enjoy the improvement in quantum efficiency due to the TTF phenomenon.

[0025] In the organic EL device according to the embodiment of the present disclosure, the hole mobility of the light-emitting layer may be greater than the electron mobility of the light-emitting layer.

[0026] In organic EL devices where hole mobility is high and electron injection into the light-emitting layer is the rate-limiting process for recombination, this allows for the formation of regions with locally high hole density in the light-emitting layer, thereby increasing the density of triplet excitons and enabling greater benefits to be enjoyed from the improvement in quantum efficiency due to the TTF phenomenon.

[0027] In the organic EL device according to the aspect of the present disclosure, the host material may have singlet exciton energy equal to or greater than the singlet exciton energy of the fluorescent material.

[0028] As a result, the fluorescent material is excited by the energy of the singlet excitons of the host material, and singlet excitons of the fluorescent material that contribute to light emission are more likely to be generated, and the singlet excitons generated in the host material by recombination or TTF efficiently contribute to light emission.

[0029] The device may further include an electron injection control layer and an intermediate layer between the light-emitting layer and the cathode, the electron injection control layer being in contact with both the light-emitting layer and the intermediate layer, the intermediate layer having at least one of an electron injection property and an electron transport property, and the LUMO level of the functional material contained in the electron injection control layer being higher by 0.1 eV or more than the LUMO level of the functional material contained in the intermediate layer and being equal to or higher than the LUMO level of the host material.

[0030] This reduces hole trapping by the fluorescent material in the light-emitting layer and controls the electron injection into the light-emitting layer, thereby making it possible to appropriately control the carrier balance in the light-emitting layer and further improve the quantum efficiency of the organic EL element.

[0031] The organic EL display panel according to one embodiment of the present disclosure may include a plurality of organic EL elements according to one embodiment of the present disclosure on a substrate.

[0032] <Embodiment> The organic EL device according to the embodiment will be described below. Note that the following description is an example for explaining the configuration, operation, and effect of one aspect of the present invention, and the present invention is not limited to the following embodiment except for the essential part.

[0033] [1. Structure of organic EL element] 1 is a diagram schematically illustrating the cross-sectional structure of an organic EL element 1 according to the present embodiment. The organic EL element 1 includes a pixel electrode 13 (anode), a hole injection layer 15, a hole transport layer 16, a light-emitting layer 17, an electron injection control layer 18, an electron transport layer 19, an electron injection layer 20, and a counter electrode 21 (cathode).

[0034] In the organic EL element 1, the pixel electrode 13 and the counter electrode 21 are disposed opposite each other so that their principal surfaces face each other, and a light-emitting layer 17 is formed between the pixel electrode 13 and the counter electrode 21.

[0035] A hole transport layer 16 is formed on the anode (pixel electrode 13) side of the light emitting layer 17 in contact with the light emitting layer 17. A hole injection layer 15 is formed between the hole transport layer 16 and the anode (pixel electrode 13).

[0036] On the cathode (counter electrode 21) side of the light-emitting layer 17, an electron injection control layer 18 is formed in contact with the light-emitting layer 17, and an electron transport layer 19 is formed in contact with the electron injection control layer 18. An electron injection layer 20 is formed between the electron transport layer 19 and the cathode (counter electrode 21).

[0037] [1.1 Components of an organic EL element] <Pixel electrode> The pixel electrodes 13 are formed on the interlayer insulating layer 12. The pixel electrodes 13 are provided for each pixel, and are electrically connected to the TFT layer 112 through contact holes provided in the interlayer insulating layer 12.

[0038] In this embodiment, the pixel electrode 13 functions as a light-reflective anode.

[0039] The pixel electrode 13 is made of, for example, a metal material having optical reflectivity. Specific examples of metal materials include Ag (silver), Al (aluminum), aluminum alloy, Mo (molybdenum), APC (an alloy of silver, palladium, and copper), ARA (an alloy of silver, rubidium, and gold), MoCr (an alloy of molybdenum and chromium), MoW (an alloy of molybdenum and tungsten), and NiCr (an alloy of nickel and chromium). The pixel electrode 13 may be made of a metal layer alone, or may have a laminated structure in which a layer made of a metal oxide such as ITO (indium tin oxide) or IZO (indium zinc oxide) is laminated on the metal layer.

[0040] When the counter electrode 21 is a light-reflective cathode, the pixel electrode 13 may be a light-transmitting anode. In this case, the pixel electrode 13 includes at least one of a metal layer formed of a metal material and a metal oxide layer formed of a metal oxide. Examples of the metal material for the pixel electrode 13 include Ag, a silver alloy containing Ag as the main component, Al, and an Al alloy containing Al as the main component. Examples of Ag alloys include magnesium-silver alloy (MgAg) and indium-silver alloy. Ag basically has low resistivity, and Ag alloys are preferred because they have excellent heat resistance, corrosion resistance, and can maintain good electrical conductivity for a long period of time. Examples of Al alloys include magnesium-aluminum alloy (MgAl) and lithium-aluminum alloy (LiAl). Other examples of alloys include lithium-magnesium alloy and lithium-indium alloy. The metal layer has a thin thickness of approximately 1 nm to 50 nm, and is light-transmitting. The metal layer included in the pixel electrode 13 may be configured as a single layer of, for example, an Ag layer or an MgAg alloy layer, or may have a laminated structure of an Mg layer and an Ag layer (Mg / Ag), or a laminated structure of an MgAg alloy layer and an Ag layer (MgAg / Ag). Examples of metal oxides that form the metal oxide layer of the pixel electrode 13 include ITO and IZO. The pixel electrode 13 may be configured as a single metal layer or a single metal oxide layer, or may have a laminated structure in which a metal oxide layer is laminated on a metal layer, or a laminated structure in which a metal layer is laminated on a metal oxide layer.

[0041] <Hole injection layer> The hole injection layer 15 has the function of promoting the injection of holes from the anode (pixel electrode 13) to the light-emitting layer 17. The hole injection layer 15 is, for example, a coating film, and is formed by, for example, applying and drying a solution containing a hole injection material and a solute. The hole injection layer 15 may also be formed as a vapor-deposited film. The hole injection layer 15 is made of, for example, a conductive polymer material such as PEDOT:PSS (a mixture of polythiophene and polystyrene sulfonic acid), polyfluorene or its derivatives, or polyarylamine or its derivatives, or an oxide of Ag, Mo, chromium (Cr), vanadium (V), tungsten (W), nickel (Ni), iridium (Ir), or the like.

[0042] <Hole transport layer> The hole transport layer 16 has a function of transporting holes injected from the hole injection layer 15 to the light-emitting layer 17. The hole transport layer 16 is, for example, a coating film, specifically formed by coating and drying a solution containing a hole transport material as a solute. Alternatively, the hole transport layer 16 may be formed as a vapor-deposited film. For example, a polymer compound such as polyfluorene or its derivative, or polyarylamine or its derivative, which does not have a hydrophilic group, can be used.

[0043] <Light-emitting layer> The light-emitting layer 17 has a function of emitting light by recombination of holes and electrons. The light-emitting layer 17 is, for example, a coating film, and is formed by, for example, coating and drying a solution containing a material for forming the light-emitting layer and a solute. Alternatively, the light-emitting layer 17 may be formed as a vapor-deposited film.

[0044] The light-emitting layer 17 is formed by doping a fluorescent material into a host material with high carrier mobility. Here, high carrier mobility refers to high electron mobility and / or high hole mobility. The host material according to one embodiment of the present disclosure has a higher hole mobility than electron mobility. Examples of host materials that can be used include amine compounds, condensed polycyclic aromatic compounds, and heterocyclic compounds. Examples of amine compounds that can be used include monoamine derivatives, diamine derivatives, triamine derivatives, and tetraamine derivatives. Examples of condensed polycyclic aromatic compounds that can be used include anthracene derivatives, naphthalene derivatives, naphthacene derivatives, phenanthrene derivatives, chrysene derivatives, fluoranthene derivatives, triphenylene derivatives, pentacene derivatives, and perylene derivatives. Examples of the heterocyclic compound that can be used include carbazole derivatives, furan derivatives, pyridine derivatives, pyrimidine derivatives, triazine derivatives, imidazole derivatives, pyrazole derivatives, triazole derivatives, oxazole derivatives, oxadiazole derivatives, pyrrole derivatives, indole derivatives, azaindole derivatives, azacarbazole derivatives, pyrazoline derivatives, pyrazolone derivatives, and phthalocyanine derivatives.

[0045] Known organic fluorescent materials can be used as the fluorescent material doped into the light-emitting layer 17. Examples of such materials include oxinoid compounds, perylene compounds, coumarin compounds, azacoumarin compounds, oxazole compounds, oxadiazole compounds, perinone compounds, pyrrolopyrrole compounds, naphthalene compounds, anthracene compounds, fluorene compounds, fluoranthene compounds, tetracene compounds, pyrene compounds, coronene compounds, quinolone compounds and azaquinolone compounds, pyrazoline derivatives and pyrazolone derivatives, rhodamine compounds, chrysene compounds, phenanthrene compounds, cyclopentadiene compounds, stilbene compounds, diphenylquinone compounds, styryl compounds, butadiene compounds, dicyanomethylenepyran compounds, dicyanomethylenethiopyran compounds, fluorescein compounds, pyrylium compounds, thiapyrylium compounds, selenapyrylium compounds, telluropyrylium compounds, aromatic aldadiene compounds, oligophenylene compounds, thioxanthene compounds, cyanine compounds, and acridine compounds. In one embodiment, the concentration of the fluorescent material is 1 wt% or more, 10 wt% or less, or 30 wt% or less.

[0046] The fluorescent material is a so-called blue light-emitting material, and the energy level difference between the LUMO level and the HOMO level (so-called band gap, energy of singlet excitons) is 2.6 eV or more.

[0047] Generally, generated excitons have a tendency to have difficulty transitioning to an energy state with higher energy than the exciton itself. Therefore, in order for excitons recombined on the host molecule to efficiently transition to the excitation level of the fluorescent material, it is preferable that the singlet excitation level of the fluorescent material is lower than that of the host material. At the same time, in order to achieve a configuration that promotes highly efficient carrier recombination on the host material, it is preferable that the fluorescent material does not trap electrons that have flowed into the emitting layer. Therefore, in the emitting layer 17, the fluorescent material and the host material are selected so that the absolute value of the difference between the LUMO level of the fluorescent material and the HOMO level of the fluorescent material is equal to or less than the absolute value of the difference between the LUMO level and the HOMO level of the host material, and the LUMO level of the fluorescent material is equal to or greater than the LUMO level of the host material.

[0048] The fluorescent material and the host material are selected so that the HOMO level of the fluorescent material is equal to or higher than the HOMO level of the host material, and the difference in energy levels is 0.3 eV or less. This configuration allows the TTF to be fully utilized, as described below, and improves luminous efficiency. Furthermore, it is preferable that the energy of the singlet excitons of the host material is equal to or higher than the energy of the singlet excitons of the fluorescent material. This configuration allows the energy of the singlet excitons of the host material to be transferred to the fluorescent material, generating singlet excitons, thereby increasing the generation efficiency of singlet excitons in the host material and improving luminous efficiency.

[0049] <Electron injection control layer> The electron injection control layer 18 is a layer responsible for injecting electrons into the light-emitting layer and is composed of one or more layers. In the present disclosure, the following layer configuration is used as an example. The electron injection control layer 18 functions to limit the outflow of holes from the light-emitting layer 17 to the electron injection control layer 18 and to control the injection of electrons from the electron transport layer 19 into the light-emitting layer 17. As a result, electrons injected from the electron transport layer 19 accumulate near the interface between the electron injection control layer 18 and the electron transport layer 19, suppressing degradation of the light-emitting material due to electron accumulation near the interface of the light-emitting layer 17 and contributing to a longer device life. The functions of limiting the outflow of holes from the light-emitting layer 17 and controlling the injection of electrons into the light-emitting layer 17 are achieved by designing the energy band structure described below. To stably achieve electron control by the electron injection control layer, it is preferable to design the electron injection control layer thickness so that it can suppress the carrier tunneling effect. In one embodiment, the electron injection control layer 18 has a thickness of 5 nm or more. In another embodiment, the electron injection control layer 18 has a thickness of 10 nm or more. Furthermore, from the viewpoint of reducing the device driving voltage, it is preferable that the electron injection control layer be thin, and in one embodiment, the thickness of the electron injection control layer 18 is 50 nm or less. In one embodiment, the thickness of the electron injection control layer 18 is 30 nm or less. From the viewpoint of electron blocking properties, the electron injection control layer 18 has a higher LUMO level than the electron transport layer 19, and the difference in LUMO levels between the electron injection control layer 18 and the electron transport layer 19 is 0.1 eV or more. On the other hand, a large LUMO barrier can increase the driving voltage and suppress excessive electron injection, so the difference in LUMO levels between the electron injection control layer 18 and the electron transport layer 19 is 0.5 eV or less.

[0050] Furthermore, it is preferable that the material of the electron injection control layer 18 has a singlet exciton energy greater than that of the host material of the light-emitting layer 17. With this configuration, when singlet excitons are generated in the material of the electron injection control layer 18, energy transfer to the host material or fluorescent material of the light-emitting layer 17 facilitates the generation of singlet excitons in the fluorescent material, while energy transfer to the material of the electron injection control layer 18 can be prevented when singlet excitons are generated in the host material or fluorescent material of the light-emitting layer 17. Similarly, it is preferable that the triplet exciton energy in the material of the electron injection control layer 18 is greater than that of the material of the light-emitting layer 17. The electron injection control layer 18 is formed, for example, from a vapor-deposited film.

[0051] Examples of materials for the electron injection control layer 18 include π-electron low molecular weight organic materials such as pyridine derivatives, pyrimidine derivatives, triazine derivatives, imidazole derivatives, oxadiazole derivatives, triazole derivatives, quinazoline derivatives, and phenanthroline derivatives.

[0052] <Electron transport layer> The electron transport layer 19 has a function of transporting electrons from the cathode (counter electrode 21) to the light-emitting layer 17 via the electron injection control layer 18. The electron transport layer 19 is made of an organic material with high electron transport properties. The electron transport layer 19 is formed, for example, of a vapor-deposited film. Examples of organic materials used for the electron transport layer 19 include π-electron low-molecular organic materials such as pyridine derivatives, pyrimidine derivatives, triazine derivatives, imidazole derivatives, oxadiazole derivatives, triazole derivatives, quinazoline derivatives, and phenanthroline derivatives.

[0053] <Electron injection layer> The electron injection layer 20 has a function of injecting electrons supplied from the cathode (counter electrode 21) into the light-emitting layer 17. The electron injection layer 20 is made of, for example, a vapor-deposited film. The electron injection layer 20 is formed, for example, by doping an organic material with high electron transportability with a doping metal selected from alkali metals, alkaline earth metals, lanthanoids, etc. The doping metal is not limited to a simple metal, and may be doped as a compound such as a fluoride (e.g., NaF) or a quinolinium complex (e.g., Alq3, Liq). In this embodiment, Li is doped as Liq. Examples of doping metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr), which are alkali metals; calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), and yttrium (Y), which are alkaline earth metals; and samarium (Sm), europium (Eu), and ytterbium (Yb), which are lanthanoids.

[0054] Examples of organic materials used for the electron injection layer 20 include π-electron low-molecular organic materials such as oxadiazole derivatives (OXD), triazole derivatives (TAZ), and phenanthroline derivatives (BCP, Bphen).

[0055] <Counter electrode> The counter electrode 21 is formed on the electron injection layer 20 .

[0056] In this embodiment, the counter electrode 21 functions as a semi-transparent cathode.

[0057] The counter electrode 21 may be made of at least one of a metal layer made of a metal material and a metal oxide layer made of a metal oxide. Examples of metal materials for forming the metal layer include Ag, a silver alloy containing Ag as the main component, Al, and an Al alloy containing Al as the main component. Examples of Ag alloys include magnesium-silver alloys and indium-silver alloys. Examples of Al alloys include magnesium-aluminum alloys and lithium-aluminum alloys. Other examples of alloys include lithium-magnesium alloys and lithium-indium alloys. The metal layer included in the counter electrode 21 may be, for example, a single Ag layer or an MgAg alloy layer, or may have a laminated structure of an Mg layer and an Ag layer, or a laminated structure of an MgAg alloy layer and an Ag layer. Examples of metal oxides for forming the metal oxide layer included in the counter electrode 21 include ITO and IZO. The counter electrode 21 may be made of a metal layer alone or a metal oxide layer alone, or may have a laminated structure in which a metal oxide layer is laminated on a metal layer, or a laminated structure in which a metal layer is laminated on a metal oxide layer.

[0058] When the pixel electrode 13 is a light-transmitting anode, the counter electrode 21 may be a light-reflective electrode. In this case, the counter electrode 21 includes a metal layer made of a light-reflective metal material. Specific examples of light-reflective metal materials include silver, aluminum, aluminum alloy, molybdenum, APC, ARA, MoCr, MoW, and NiCr.

[0059] <Other> The organic EL element 1 is formed on a substrate 11. The substrate 11 is made of a base material 111 made of an insulating material and a TFT layer 112. The base material 111 can be, for example, a glass substrate, a quartz substrate, a silicon substrate, or a plastic substrate. The plastic material can be either a thermoplastic resin or a thermosetting resin. Examples of the plastic material include polyethylene, polypropylene, polyamide, polyimide (PI), polycarbonate, acrylic resin, polyethylene terephthalate (PET), polybutylene terephthalate, polyacetal, other fluorine-based resins, various thermoplastic elastomers such as styrene-based, polyolefin-based, polyvinyl chloride-based, polyurethane-based, fluorine rubber-based, and chlorinated polyethylene-based thermoplastic elastomers, epoxy resin, unsaturated polyester, silicone resin, polyurethane, etc., as well as copolymers, blends, polymer alloys, etc., primarily made of these materials. A laminate of one or more of these materials can be used. Materials that can be used to form the TFT layer 112 include metal materials such as molybdenum sulfide, copper, zinc, aluminum, stainless steel, magnesium, iron, nickel, gold, and silver, inorganic semiconductor materials such as gallium nitride and gallium arsenide, and organic semiconductor materials such as anthracene, rubrene, and polyparaphenylene vinylene.

[0060] Although not shown, an interlayer insulating layer 12 is formed on the substrate 11. The interlayer insulating layer 12 is made of a resin material and serves to flatten steps on the upper surface of the TFT layer 112. Examples of the resin material include positive photosensitive materials. Examples of such photosensitive materials include acrylic resins, polyimide resins, siloxane resins, and phenolic resins. A contact hole is formed in the interlayer insulating layer 12 for each pixel.

[0061] If the organic EL display panel 100 is a bottom-emission type, the substrate 111 and the interlayer insulating layer 12 must be made of a light-transmitting material. Furthermore, if the TFT layer 112 is present, at least a part of the region of the TFT layer 112 that is located below the pixel electrode 13 must be light-transmitting.

[0062] In addition, a sealing layer 22 is formed on the organic EL element 1. The sealing layer 22 has the function of preventing organic layers such as the hole injection layer 15, the hole transport layer 16, the light-emitting layer 17, the electron injection control layer 18, the electron transport layer 19, and the electron injection layer 20 from being exposed to moisture or air, and is formed using a light-transmitting material such as silicon nitride (SiN) or silicon oxynitride (SiON). In addition, a sealing resin layer made of a resin material such as an acrylic resin or a silicone resin may be provided on a layer formed using a material such as silicon nitride (SiN) or silicon oxynitride (SiON).

[0063] If the organic EL display panel 100 is a top-emission type, the sealing layer 22 needs to be made of a light-transmitting material. Although not shown in Fig. 1, a color filter or an upper substrate may be attached onto the sealing layer 22 via a sealing resin. By attaching the upper substrate, the hole injection layer 15, the hole transport layer 16, the light-emitting layer 17, the electron injection control layer 18, the electron transport layer 19, and the electron injection layer 20 can be protected from moisture, air, and the like.

[0064] [2. Energy band structure] The organic EL device 1 is characterized by the energy band structure in the light-emitting layer 17. For the light-emitting layer 17, the energy levels of the fluorescent material and the host material are shown, and for the other layers, for the sake of simplicity, the "energy level of a layer" is expressed as the energy level of a single organic material when the layer is made of that organic material, or as the energy level of a representative organic material responsible for transporting electrons and / or holes when the layer is made of multiple organic materials.

[0065] Figure 2 is a band diagram showing the energy band structure of the organic EL element 1. Figure 2 shows the LUMO energy levels (hereinafter referred to as "LUMO levels") and HOMO energy levels (hereinafter referred to as "HOMO levels") of the hole transport layer 16, the light-emitting layer 17, the electron injection control layer 18, and the electron transport layer 19, and does not show other layers. Note that although the electron vacuum level is not shown in Figure 2, the LUMO level and the HOMO level each have a larger difference from the electron vacuum level and a lower energy level as they are located further down in the band diagram.

[0066] [2.1 Emitting layer] The light-emitting layer 17 is formed by doping a fluorescent material into a host material. In the light-emitting layer 17, the host material is responsible for transporting carriers (electrons and / or holes). Therefore, when the energy level of the HOMO level 172d of the fluorescent material is higher than the HOMO level 172b of the host material, the fluorescent material has hole-trapping properties. It is preferable that the difference ΔHOMO between the HOMO level 172d of the fluorescent material and the HOMO level 172b of the host material satisfies the following formula (1):

[0067] ΔHOMO≦0.3eV …Formula (1) Furthermore, it is preferable that the energy of singlet excitons of the host material is transferred to the fluorescent material to generate singlet excitons in the fluorescent material in the light-emitting layer 17. Therefore, it is preferable that the energy S1(emlb) of the singlet excitons of the host material and the energy S1(emld) of the singlet excitons of the fluorescent material satisfy the following formula (2):

[0068] S1(emlb)≧S1(emld) …Formula (2) Furthermore, the hole mobility is higher than the electron mobility in the light-emitting layer 17. When the electron mobility of the light-emitting layer 17 is μe(eml) and the hole mobility is μh(eml), it is preferable that the following formula (3) is satisfied.

[0069] μh(eml)>μe(eml) ...Equation (3) [2.2 Electron injection barrier] An energy barrier for injecting electrons from the cathode (counter electrode 21) side into the light-emitting layer 17 exists at the interface of each layer from the cathode (counter electrode 21) to the light-emitting layer 17. This energy barrier is caused by the difference in LUMO level between the layer on the anode (pixel electrode 13) side of the interface and the layer on the cathode (counter electrode 21) side. Hereinafter, the energy barrier for injecting electrons from the cathode (counter electrode 21) side to the anode (pixel electrode 13) side at the interface between two adjacent layers will be referred to as the "electron injection barrier."

[0070] The electron injection barrier Eg(eicl) from the electron transport layer 19 to the electron injection control layer 18 is determined by the difference between the LUMO level 181 of the organic material of the electron injection control layer 18 and the LUMO level 191 of the organic material of the electron transport layer 19. It is preferable that Eg(eicl) satisfy the following formula (4). It is more preferable that Eg(eicl) satisfy the following formula (5). In this embodiment, the electron injection barrier Eg(eicl) is 0.1 eV.

[0071] Eg(eicl)≧0.1eV …Formula (4) Eg(eicl)≧0.2eV …Formula (5) The electron injection barrier Eg(eml) from the electron injection control layer 18 to the light-emitting layer 17 is determined by the difference between the LUMO level 171b of the host material of the light-emitting layer 17 and the LUMO level 181 of the organic material of the electron injection control layer 18. It is preferable that the LUMO level 171b of the host material of the light-emitting layer 17 is lower than the LUMO level 181 of the organic material of the electron injection control layer 18, and that the LUMO level 171b has an energy level equal to or lower than the LUMO level 181, and that Eg(eml) satisfy the following formula (6). It is more preferable that Eg(eml) satisfy the following formula (7). In this embodiment, the electron injection barrier Eg(eml) is −0.1 eV.

[0072] Eg(eml)≦0…Formula (6) Eg(eml)≦-0.1eV …Equation (7) [2.3 Hole injection barrier] Meanwhile, an energy barrier for injecting holes from the anode (pixel electrode 13) side to the cathode (counter electrode 21) side through the light-emitting layer 17 exists at the interface of each layer from the anode (pixel electrode 13) to the electron injection control layer 18. This energy barrier is caused by the difference in HOMO level between the layer on the cathode (counter electrode 21) side and the layer on the anode (pixel electrode 13) side of the interface. Hereinafter, the energy barrier for injecting holes from the anode (pixel electrode 13) side to the cathode (counter electrode 21) side at the interface between two adjacent layers will be referred to as the "hole injection barrier."

[0073] Hole transport layer 16 The hole injection barrier Hg(eml) from the host material of the light-emitting layer 17 to the light-emitting layer 17 is determined by the difference between the HOMO level 172b of the host material of the light-emitting layer 17 and the HOMO level 162 of the organic material of the hole-transport layer 16. In this embodiment, the hole injection barrier Hg(eml) is 0.1 eV.

[0074] The hole injection barrier Hg(eicl) from the light-emitting layer 17 to the electron injection control layer 18 is determined by the difference between the HOMO level 182 of the organic material of the electron injection control layer and the HOMO level 172b of the host material of the light-emitting layer. Hg(eicl) preferably satisfies the following formula (8). Furthermore, Hg(eicl) more preferably satisfies the following formula (9). In this embodiment, the hole injection barrier Hg(eicl) is 0.3 eV.

[0075] Hg(eicl)>0…Formula (8) Hg(eicl)≧0.3eV …Formula (9) [3. Effect of composition] [3.1 Effects predicted from the design] 3(a) to 3(b), and (c) are simplified schematic diagrams showing band diagrams of the hole transport layer 16, the light-emitting layer 17, the electron injection control layer 18, and the electron transport layer 19 according to the example and the comparative example, respectively, and showing the recombination of electrons and holes.

[0076] In the organic EL device according to the embodiment, holes are injected from the hole transport layer 16 into the host material of the light-emitting layer 17, as shown in the schematic diagram of FIG. 3( a). Because the difference ΔHOMO between the HOMO level 172b of the fluorescent material in the light-emitting layer 17 and the HOMO level 172d of the host material is small, at 0.3 eV or less, the holes injected into the light-emitting layer 17 are not trapped by the fluorescent material and move to the vicinity of the interface between the light-emitting layer 17 and the electron injection control layer 18. Then, due to the hole injection barrier Hg(e.i.cl) from the light-emitting layer 17 to the electron injection control layer 18, holes accumulate near the interface between the light-emitting layer 17 and the electron injection control layer 18, as shown in the schematic diagram of FIG. 3( b). Meanwhile, due to the electron injection barrier Eg(e.i.cl) from the electron transport layer 19 to the electron injection control layer 18, electron injection from the electron transport layer 19 to the electron injection control layer 18 becomes the rate-determining process for electron injection into the light-emitting layer 17. Therefore, the recombination region of electrons and holes in the light-emitting layer 17 is limited to a narrow region near the interface between the light-emitting layer 17 and the electron injection control layer 18, making it easier to increase the exciton density and utilize the TTF phenomenon.

[0077] 3(c) is a schematic diagram showing a case where the difference ΔHOMO between the HOMO level 172b of the fluorescent material in the light-emitting layer 17 and the HOMO level 172d of the host material is greater than 0.3 eV. In this case, too, electron injection from the electron transport layer 19 to the electron injection control layer 18 is the rate-determining step for electron injection into the light-emitting layer 17. However, because the difference ΔHOMO between the HOMO level 172b of the fluorescent material in the light-emitting layer 17 and the HOMO level 172d of the host material is greater than 0.3 eV, holes injected from the hole transport layer 16 into the light-emitting layer 17 are trapped by the fluorescent material. Therefore, recombination occurs in the fluorescent material that trapped the holes, so the exciton density in the light-emitting layer is not as high as in the examples, and the TTF phenomenon is less likely to occur.

[0078] [3.2 Energy transfer involved in luminescence] Hereinafter, the energy transfer from recombination to light emission will be described when the light-emitting layer 17 contains a host material and a fluorescent material and the energy of the singlet excitons of the host material is equal to or greater than the energy of the singlet excitons of the fluorescent material.

[0079] Figure 4(a) is a schematic band diagram showing the energy states and transitions of electrons in the host material and fluorescent material. In Figure 4(a), the ground level of the host material and the ground level of the fluorescent state are both designated as the S0 level, and the energies of the other levels are shown as relative values ​​based on the S0 level.

[0080] When electrons and holes recombine in the host material, some of the electrons move to the excited level of the host material. At this time, some of the electrons move to the singlet excited level S1_ by the excitation process 351. host The singlet excitons of the host material then transition to the singlet excited level S1_2, where some or all of the singlet excitons are in the ground state, by energy transfer 381 to the fluorescent material. This energy transfer occurs when electrons of the fluorescent material transition from the ground level S0 to the singlet excited level S1_2 using the energy of the electrons of the host material returning to the ground state. dopant 311. Then, some or all of the singlet excitons in the fluorescent material are converted into photons, resulting in the emission of light by a luminescence process 371.

[0081] On the other hand, when electrons and holes recombine in the host material, some of the electrons are excited to the triplet excited level T1_ host 302. Generally, the triplet state transitions to the singlet state. Transitions to the triplet state are forbidden from the viewpoint of electron spin, and energy transfer to singlet excitons is extremely unlikely. However, when the density of triplet excitons is sufficiently high, singlet excitons are generated in the host material by TTF 353, which generates singlet excitons through collisions of multiple triplet excitons. Some or all of the generated singlet excitons in the host material transfer energy 382 to the fluorescent material in its ground state, generating singlet excitons in the fluorescent material. Some or all of the singlet excitons in the fluorescent material are then converted into photons, resulting in the emission of light via luminescence process 372.

[0082] When electrons and holes recombine in the fluorescent material, electrons move to the excited level of the fluorescent material. At this time, some of the electrons move to the singlet excited level S1_ by the excitation process 371. dopant Then, some or all of the singlet excitons in the fluorescent material are converted into photons, and light is emitted in a light emission process 371. Meanwhile, some of the electrons are converted into triplet excited levels T1_ dopant 312. Because the fluorescent material has a low physical density, TTF hardly occurs, and energy is converted to heat or the like by a non-radiative process 373.

[0083] [3.3 Luminous Efficiency] The luminous efficiency will be examined taking into consideration the process from recombination to light emission as described above.

[0084] FIG. 4(b) is a schematic diagram showing the state transition from recombination to light emission and the maximum probability when the light-emitting layer 17 contains a host material and a fluorescent material and the energy of the singlet excitons of the host material is equal to or greater than the energy of the singlet excitons of the fluorescent material.

[0085] Among the excitons generated when electrons and holes combine, the ratio of singlet excitons to triplet excitons is approximately 1:3. Therefore, for every recombination, 25% of singlet excitons are generated, and 75% of triplet excitons are generated. Therefore, assuming that the photoluminescence quantum efficiency (PLQE) of singlet excitons in the host material is 100%, if TTF does not occur, a maximum of 25% of the excitons generated by recombination will contribute to light emission.

[0086] On the other hand, in TTF, one singlet exciton is generated from five triplet excitons. Therefore, when TTF occurs, a maximum of 20% of the number of singlet excitons is generated relative to the number of triplet excitons. In other words, a maximum of 15% of the number of singlet excitons is generated relative to the number of recombinations. Therefore, assuming that the luminescence quantum efficiency (PLQE) of singlet excitons of the host material is 100%, a maximum of 15% of the excitons generated by recombination due to TTF contribute to luminescence. Therefore, the maximum value of the internal quantum efficiency (IQE) is 40%. Hereinafter, the contribution of TTF to luminescence efficiency is referred to as R TTF Denoted by R TTF The larger the value, the more efficiently the TTF phenomenon is utilized, and the maximum value is 37.5% (=15 / 40).

[0087] [3.4 Evaluation Results] The characteristics of the organic EL element according to the embodiment will be described below together with the evaluation results.

[0088] FIG. 5(a) shows the HOMO levels and ΔHOMO values ​​of the host material and fluorescent material in the light-emitting layer 17, and the calculated R TTF and the evaluation values ​​of element efficiency (EQE). The HOMO level, LUMO level, and energy gap values ​​of the host material and fluorescent material of the light-emitting layer 17 are shown in FIG. 5(b). The HOMO level and LUMO level are absolute values ​​of the difference from the vacuum level, and the larger the value, the lower the energy level. The examples and comparative examples differ only in the fluorescent material, and the other constituent elements are the same. The fluorescent materials used in Examples 1, 2, and the comparative example all have the same energy gap, i.e., the same emission peak wavelength, and differ only in the difference in the HOMO level between the host material and the fluorescent material and the difference in the LUMO level between the host material and the fluorescent material.

[0089] In addition, R TTF was calculated in the following way. Figure 6(a) shows the R TTF6(a) and 6(b). In the evaluation test, the organic EL element was made to emit light by a pulse current, and the transition of the luminance of the emitted light was measured immediately after the current was cut off. Since the lifetime of singlet excitons is shorter than that of triplet excitons, the luminance of the emitted light due to the singlet excitons of the fluorescent material generated directly by recombination and the luminance of the emitted light due to the energy transfer of the singlet excitons of the host material generated directly by recombination to the fluorescent material are quenched in an extremely short time after the current is cut off. On the other hand, the luminance of the singlet excitons generated via the TTF process has a long lifetime and continues even after the quenching of the fluorescence due to the singlet excitons not due to TTF. Therefore, from the decay profile of the luminance intensity after the time when the current is cut off (t=0 in Figures 6(a) and (b)), R TTF More specifically, as shown in FIG. 6(b), Emission intensity I(t) and R TTF The relationship can be approximated by the following equation (10).

[0090]

number

[0091] In Fig. 5(a), the relative EQE value is obtained when the applied current is 10 mA / cm 2 Calculations are based on measurements taken at

[0092] [3.5 Luminous Center] Here, the luminescence center in the light-emitting layer will be described. The luminescence center refers to a representative position of the emission peak, as described below. The position of the emission peak is the position where excitons of the host material are concentrated, and is generally either the cathode-side interface of the light-emitting layer or the anode-side interface of the light-emitting layer, or both. When the hole mobility in the light-emitting layer is sufficiently higher than the electron mobility, holes migrate to the cathode-side interface of the light-emitting layer, while electrons are consumed by recombination near the cathode-side interface of the light-emitting layer. As a result, excitons are generated intensively near the cathode-side interface of the light-emitting layer. On the other hand, when the electron mobility in the light-emitting layer is sufficiently higher than the hole mobility, electrons migrate to the anode-side interface of the light-emitting layer, while holes are consumed by recombination near the anode-side interface of the light-emitting layer. As a result, excitons are generated intensively near the anode-side interface of the light-emitting layer. Furthermore, depending on the relationship between the hole mobility and the electron mobility in the light-emitting layer, excitons may be generated intensively both near the cathode-side interface of the light-emitting layer and near the anode-side interface of the light-emitting layer. In general, the position where excitons are generated intensively is the position of the emission peak.

[0093] If the host material has a high exciton diffusion property and a long exciton lifetime, the position where excitons are generated intensively may not necessarily coincide with the position of the emission peak due to exciton diffusion. In this case, the emission center is not the position where excitons are generated intensively, but the position where transition from exciton energy to photon energy occurs intensively.

[0094] [4. Summary] As described above, in the organic EL device according to one embodiment of the present disclosure, the light-emitting layer 17 includes a host material and a fluorescent material, and the HOMO level of the fluorescent material is equal to or higher than the HOMO level of the host material, with the difference in energy levels being 0.3 eV or less. Therefore, the exciton density can be increased by reducing holes trapped by the fluorescent material and increasing the hole density in a narrow region near the cathode-side surface of the light-emitting layer 17. Furthermore, increasing the exciton density increases the TTF generation efficiency, thereby increasing the quantum efficiency of the organic EL device, improving the luminous efficiency, and thereby extending the device's lifetime.

[0095] Furthermore, the organic EL device according to one embodiment of the present disclosure includes an electron injection transport layer between the electron transport layer and the light-emitting layer, and the LUMO level of the electron injection transport layer is higher than the LUMO level of the electron transport layer by at least 0.1 eV and is higher than the LUMO level of the host material of the light-emitting layer. This prevents recombination on the cathode side of the light-emitting layer 17, thereby preventing a decrease in luminous efficiency and deterioration of the electron transport layer. Furthermore, by appropriately controlling electron injection into the light-emitting layer 17, the carrier balance can be maintained favorably, thereby improving the recombination probability.

[0096] [5. Manufacturing method of organic EL element] The method for manufacturing an organic EL element will be described with reference to the drawings. Figures 8(a) to 11(c) are schematic cross-sectional views showing the states at each step in the manufacturing of an organic EL display panel including an organic EL element. Figure 12 is a flowchart showing the method for manufacturing an organic EL display panel including an organic EL element.

[0097] In an organic EL display panel, the pixel electrode (lower electrode) functions as the anode of the organic EL element, and the counter electrode (upper electrode, common electrode) functions as the cathode of the organic EL element.

[0098] (1) Formation of the substrate 11 8(a), the TFT layer 112 is formed on the base material 111 to form the substrate 11 (step S10). The TFT layer 112 can be formed by a known TFT manufacturing method.

[0099] 8(b), an interlayer insulating layer 12 is formed on the substrate 11 (step S20). The interlayer insulating layer 12 can be formed by laminating layers using, for example, a plasma CVD method, a sputtering method, or the like.

[0100] Next, a contact hole is formed by dry etching at a location above the source electrode of the TFT layer in the interlayer insulating layer 12. The contact hole is formed so that the surface of the source electrode is exposed at the bottom thereof.

[0101] Next, a connection electrode layer is formed along the inner wall of the contact hole. A portion of the upper part of the connection electrode layer is disposed on the interlayer insulating layer 12. The connection electrode layer can be formed by, for example, sputtering, and after depositing a metal film, it is patterned by photolithography and wet etching.

[0102] (2) Formation of pixel electrode 13 8(c), the pixel electrode material layer 130 is formed on the interlayer insulating layer 12 (step S31). The pixel electrode material layer 130 can be formed by using, for example, a vacuum deposition method, a sputtering method, or the like. 8(d), the pixel electrode material layer 130 is patterned by etching to form a plurality of pixel electrodes 13 partitioned into subpixels (step S32). The pixel electrodes 13 function as the anodes of the respective organic EL elements.

[0103] The method for forming the pixel electrode 13 is not limited to the above-described method. For example, a layered structure of the pixel electrode 13 and the hole injection layer 15 may be formed by forming the hole injection material layer 150 on the pixel electrode material layer 130 and performing a single patterning step on the layered structure of the pixel electrode material layer 130 and the hole injection material layer 150.

[0104] (3) Formation of partition wall 14 Next, as shown in FIG. 8(e), a partition wall resin, which is a material for the partition walls 14, is applied onto the pixel electrodes 13 and the interlayer insulating layer 12 to form a partition wall material layer 140. The partition wall material layer 140 is formed by uniformly applying a solution of a phenolic resin, which is the partition wall resin, dissolved in a solvent (for example, a mixed solvent of ethyl lactate and GBL), onto the pixel electrodes 13 and the interlayer insulating layer 12 by a spin coating method or the like (step S41). Then, as shown in FIG. 9(a), the partition wall material layer 140 is subjected to pattern exposure and development to form the partition walls 14 (step S42), and the partition walls 14 are baked. This defines openings 14a, which are regions for forming the light-emitting layers 17. The partition walls 14 are baked, for example, at a temperature of 150° C. to 210° C. for 60 minutes.

[0105] Furthermore, in the process of forming the partition walls 14, the surfaces of the partition walls 14 may be further treated with a predetermined alkaline solution, water, an organic solvent, or the like, or may be subjected to plasma treatment. This is performed for the purpose of adjusting the contact angle of the partition walls 14 with respect to the ink (solution) to be applied to the openings 14a, or for the purpose of imparting water repellency to the surfaces.

[0106] (4) Formation of the hole injection layer 15 Next, as shown in FIG. 9(b), ink containing a constituent material of the hole injection layer 15 is ejected from the nozzle of the inkjet head 401 into the opening 14a defined by the partition 14 to apply it onto the pixel electrode 13 in the opening 14a, and then baked (dried) to form the hole injection layer 15 (step S50).

[0107] (5) Formation of the hole transport layer 16 Next, as shown in FIG. 9(c), ink containing a constituent material of the hole transport layer 16 is ejected from the nozzle of the inkjet head 402 into the opening 14a defined by the partition 14 to apply it onto the hole injection layer 15 in the opening 14a, and then baked (dried) to form the hole transport layer 16 (step S60).

[0108] (6) Formation of the light-emitting layer 17 Next, as shown in Figure 10(a), ink containing a host material and a fluorescent material, which are constituent materials of the light-emitting layer 17, is ejected from the nozzle of the inkjet head 403 and applied onto the hole transport layer 16 in the opening 14a, and then baked (dried) to form the light-emitting layer 17 (step S70).

[0109] (7) Formation of the electron injection control layer 18 10(b), the electron injection control layer 18 is formed on the light-emitting layer 17 and the partition wall 14 (step S80). The electron injection control layer 18 is formed, for example, by depositing an organic compound, which is the material of the electron injection control layer 18, in common to all sub-pixels by a vapor deposition method.

[0110] (8) Formation of electron transport layer 19 10(c), the electron transport layer 19 is formed on the electron injection control layer 18 (step S80). The electron transport layer 19 is formed, for example, by depositing an organic compound, which is the material of the electron transport layer 19, in a film common to all sub-pixels by a vapor deposition method.

[0111] (9) Formation of the electron injection layer 20 11(a), the electron injection layer 20 is formed on the electron transport layer 19 (step S100). The electron injection layer 20 is formed, for example, by co-evaporating an electron-transporting organic material and a doped metal or a compound thereof to form a film common to each subpixel.

[0112] (10) Formation of the counter electrode 21 11(b), a counter electrode 21 is formed on the electron injection layer 20 (step S110). The counter electrode 21 is formed by depositing ITO, IZO, silver, aluminum, or the like by sputtering or vacuum deposition. The counter electrode 21 functions as a cathode for each organic EL element.

[0113] (11) Formation of sealing layer 22 11(c), a sealing layer 22 is formed on the counter electrode 21 (step S120). The sealing layer 22 can be formed by depositing SiON, SiN, or the like by a sputtering method, a CVD method, or the like. Note that a sealing resin layer may be further formed on the inorganic film of SiON, SiN, or the like by coating and baking, or the like.

[0114] A color filter or an upper substrate may be placed on the sealing layer 22 and bonded thereto.

[0115] [6. Overall configuration of organic EL display device] 13 is a schematic block diagram showing the configuration of an organic EL display device 1000 equipped with an organic EL display panel 100. As shown in Fig. 13, the organic EL display device 1000 includes the organic EL display panel 100 and a drive control unit 200 connected thereto. The drive control unit 200 is made up of four drive circuits 210 to 240 and a control circuit 250.

[0116] In an actual organic EL display device 1000, the arrangement of the drive control unit 200 relative to the organic EL display panel 100 is not limited to this.

[0117] [7. Other Modifications] (1) In the above embodiment, the host material of the light-emitting layer 17 has hole-transporting properties higher than electron-transporting properties. However, a material with electron-transporting properties higher than hole-transporting properties may be used as the host material of the light-emitting layer 17. In this case, hole injection from the anode side to the light-emitting layer 17 is the rate-limiting process for recombination in the light-emitting layer 17, so it is preferable that electron trapping by the fluorescent material does not occur. Therefore, it is preferable that the difference between the LUMO levels of the host material and the fluorescent material is small. In this case, it is also preferable to provide a hole injection control layer between the light-emitting layer 17 and the hole-transport layer 16, and it is preferable that the HOMO level of the hole injection transport layer is 0.1 eV or more lower than the HOMO level of the hole-transport layer and lower than the HOMO level of the host material of the light-emitting layer. This configuration suppresses recombination on the anode side of the light-emitting layer 17, thereby suppressing a decrease in luminous efficiency and deterioration of the hole-transport layer. Furthermore, by appropriately controlling hole injection into the light-emitting layer 17, it is possible to maintain a favorable carrier balance and improve the recombination probability.

[0118] (2) In the above embodiment, the organic EL element includes an electron injecting and transporting layer, but this is not a required configuration. Even in an organic EL element without an electron injecting and transporting layer, the HOMO level of the fluorescent material is equal to or higher than the HOMO level of the host material, and the difference in energy levels is 0.3 eV or less. This reduces hole trapping by the fluorescent material and improves the exciton density of the light-emitting layer 17.

[0119] (3) In the above embodiment, the hole injection layer 15 and the hole transport layer 16 are essential components, but this is not limiting. For example, the organic EL element may not have the hole transport layer 16. Furthermore, for example, instead of the hole injection layer 15 and the hole transport layer 16, a single hole injection transport layer may be provided. Furthermore, in the above embodiment, the electron injection layer 20 is provided separately from the electron transport layer 19, but the electron transport layer 19 may also serve as an electron injection layer.

[0120] (4) In the above embodiment, the organic EL display panel has a top emission structure. However, it may have a bottom emission structure by using a light-transmitting electrode as the anode and a light-reflecting electrode as the cathode.

[0121] Furthermore, in the above embodiment, the anode is the pixel electrode and the cathode is the counter electrode, but the cathode may be the pixel electrode and the anode may be the counter electrode.

[0122] The organic EL element and organic EL panel according to the present disclosure have been described above based on the embodiments and modifications, but the present invention is not limited to the above-described embodiments and modifications. The present invention also includes forms obtained by applying various modifications to the above-described embodiments and modifications that would occur to a person skilled in the art, and forms realized by arbitrarily combining the components and functions of the embodiments and modifications within the scope of the present invention. [Industrial Applicability]

[0123] INDUSTRIAL APPLICABILITY The present invention is useful for producing a highly efficient, long-life organic EL element, and an organic EL display panel and a display device including the same. [Explanation of symbols]

[0124] 1. Organic EL element 11 Circuit Board 111 Base material 112 TFT layers 12 Interlayer insulation layer 13 Pixel electrode (anode) 14 Bulkhead 15 Hole injection layer 16 Hole transport layer 17 Light-emitting layer 18 Electron injection control layer 19 Electron transport layer 20 Electron injection layer 21 Counter electrode (cathode) 22 Sealing layer 100 Organic EL display panel 1000 organic EL display device

Claims

1. An organic EL element having an anode, a light-emitting layer, and a cathode laminated in this order, the light-emitting layer includes a fluorescent light-emitting material and a host material; a difference between a lowest unoccupied molecular orbital (LUMO) level of the fluorescent light-emitting material and a highest occupied molecular orbital (HOMO) level of the fluorescent light-emitting material is equal to or less than a difference between the LUMO level and the HOMO level of the host material; The luminous intensity I(t) of the emitting layer at any time t after the current application is cut off is the contribution R of Triplet-Triplet Fusion (TTF) to the luminous efficiency. TTF It changes depending on the LUMO level of the fluorescent material is equal to or higher than the LUMO level of the host material; The HOMO level of the fluorescent light-emitting material is equal to or higher than the HMO level of the host material, and the energy level difference ΔHOMO between the HMO levels of the fluorescent light-emitting material and the host material is set to a value less than 0.3 eV, thereby TTF Set the value to be above 20% and below 37.5% Organic EL element.

2. an electron injection control layer and an intermediate layer between the light-emitting layer and the cathode; the hole mobility of the light-emitting layer is greater than the electron mobility of the light-emitting layer; an energy gap Eg(eicl) when injecting electrons from the electron transport layer to the electron injection control layer is set to a first threshold value or more; an energy gap Eg(eml) when injecting electrons from the electron injection control layer to the light emitting layer is equal to or less than a second threshold; the first threshold is a threshold for accumulating holes near the interface between the light-emitting layer and the electron injection control layer, The second threshold is a threshold that causes the recombination region of electrons and holes in the light-emitting layer to be near the interface between the light-emitting layer and the electron injection control layer. The organic EL device according to claim 1 .

3. the energy of singlet excitons of the host material is equal to or greater than the energy of singlet excitons of the fluorescent light-emitting material; A hole injection layer and a hole transport layer are provided between the anode and the light emitting layer, The ΔHOMO adjusts the trapping by the fluorescent emitting material of holes injected from the hole transport layer into the emitting layer by the fluorescent emitting material. The organic EL device according to claim 1 .

4. an electron injection control layer and an intermediate layer are further provided between the light-emitting layer and the cathode; the electron injection control layer is in contact with both the light emitting layer and the intermediate layer, the intermediate layer has at least one of an electron injection property and an electron transport property, The LUMO level of the functional material contained in the electron injection control layer is higher than the LUMO level of the functional material contained in the intermediate layer by 0.1 eV or more and is equal to or higher than the LUMO level of the host material. The organic EL device according to claim 1 .

5. An organic EL panel comprising a plurality of organic EL elements according to claim 1 on a substrate.

6. forming a first electrode on a substrate; forming a light-emitting layer above the first electrode; A method for manufacturing an organic EL element, comprising: forming a second electrode above the light-emitting layer; In forming the light-emitting layer, a fluorescent material and a host material are used as materials for the light-emitting layer; a difference between a lowest unoccupied molecular orbital (LUMO) level of the fluorescent light-emitting material and a highest occupied molecular orbital (HOMO) level of the fluorescent light-emitting material is equal to or less than a difference between the LUMO level and the HOMO level of the host material; The luminous intensity I(t) of the emitting layer at any time t after the current application is cut off is the contribution R of Triplet-Triplet Fusion (TTF) to the luminous efficiency. TTF It changes depending on the LUMO level of the fluorescent material is equal to or higher than the LUMO level of the host material; The HOMO level of the fluorescent light-emitting material is equal to or higher than the HMO level of the host material, and the energy level difference ΔHOMO between the HMO levels of the fluorescent light-emitting material and the host material is set to a value less than 0.3 eV, thereby TTF Set the value to be above 20% and below 37.5% A method for manufacturing an organic EL element.

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