JP7668101B2 - Organic EL element, organic EL panel, and method for manufacturing organic EL element - Google Patents

Description

This disclosure relates to preventing an increase in driving voltage and improving the lifetime of organic EL elements that use carrier injection materials.

In recent years, display devices that use organic EL elements that utilize the electroluminescence phenomenon of organic materials have become increasingly popular.

Organic EL elements have a basic structure in which a light-emitting layer is disposed between a pair of electrodes, and when a voltage is applied between the electrodes, holes and electrons recombine to cause the light-emitting layer to emit light. More specifically, holes are injected from the anode into the light-emitting layer, and electrons are injected from the cathode into the light-emitting layer, and holes and electrons recombine in the light-emitting layer. On the other hand, in many cases, there is a large difference between the Fermi level of the cathode material and the energy level of the lowest unoccupied molecular orbital (LUMO; Lowest Unoccupied Molecular Orbital) of the light-emitting material contained in the light-emitting layer. Therefore, in order to facilitate the injection of electrons from the cathode into the light-emitting layer, a configuration is used in which a functional layer containing a metal material with a low work function is provided between the cathode and the light-emitting layer (see, for example, Patent Document 1). Similarly, in order to facilitate the injection of holes from the anode into the light-emitting layer, a configuration is used in which a functional layer containing a material having hole injection properties is provided between the anode and the light-emitting layer.

JP 2016-115748 A

However, metal materials with low work functions are highly reactive and are prone to accelerated deterioration due to contact with moisture or oxygen. On the other hand, when the electron injection from the cathode to the light-emitting layer decreases, a high driving voltage is required to make the organic EL element function. Similarly, when the hole injection from the anode to the light-emitting layer decreases, a high driving voltage is also required to make the organic EL element function. Therefore, the lower the carrier injection into the light-emitting layer, the higher the voltage to be applied to the organic EL element becomes, which further accelerates the deterioration of the functional layer and tends to shorten the life of the organic EL element.

The present disclosure has been made in consideration of the above problems, and aims to provide an organic EL element and organic EL panel in which the driving voltage is unlikely to increase even if the carrier injection into the light-emitting layer decreases due to deterioration of the functional layer, and a method for manufacturing the organic EL element.

An organic EL element according to one embodiment of the present disclosure is an organic EL element in which an anode, a plurality of functional layers including an light-emitting layer and an electron injection layer, and a cathode are stacked in this order, and when the effective film thickness of each of the functional layers is defined as the film thickness of the functional layer divided by the effective carrier mobility, which is the sum of the effective electron mobility and effective hole mobility of the functional layer, the effective film thickness of the light-emitting layer is 20% or more and 30% or less of the sum of the effective film thicknesses of all the functional layers, regardless of fluctuations in the electron injection property of the light-emitting layer, and the increase in the applied voltage of the light-emitting layer and the electron injection layer due to fluctuations in the electron injection property of the light-emitting layer is in the range of 1.5 V to 2 V.

According to an organic EL element according to one aspect of the present disclosure, when carrier injection into the light-emitting layer is reduced, the ratio of the voltage (partial voltage) applied to the light-emitting layer to the driving voltage is increased, and the electric field strength in the light-emitting layer is increased. Therefore, even when carrier injection into the light-emitting layer is reduced, the reduction in carrier density in the light-emitting layer is suppressed, and the light-emitting efficiency is increased due to the improvement in carrier mobility, so that the light-emitting luminance is unlikely to decrease and the degree of increase in driving voltage can be reduced. Therefore, even when carrier injection into the light-emitting layer is reduced due to deterioration of the functional layer, the driving voltage is unlikely to increase, and the life of the organic EL element can be extended while suppressing the decrease in luminance.

1 is a cross-sectional view illustrating a schematic configuration of an organic EL element 1 according to an embodiment. FIG. 2 is a simplified schematic diagram showing band diagrams of a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer in the example. 1 is a graph showing the relationship between the electron injection property into the light-emitting layer, the film thickness of the light-emitting layer, and the driving voltage. 1A is a graph showing the relationship between the distribution of electric potential in the film thickness direction and electron injection property for an embodiment, and FIG. 1B is a graph showing the relationship between the electron injection property and the voltage applied to each functional layer for an embodiment. 1A is a graph showing the relationship between the distribution of electric potential in the film thickness direction and electron injection property for a comparative example, and FIG. 1B is a graph showing the relationship between the electron injection property and the voltage applied to each functional layer for a comparative example. 1A is a graph showing the electron density in the film thickness direction for each electron injection property according to an embodiment, and FIG. 1B is a graph showing the electron density in the film thickness direction for each electron injection property according to a comparative example. FIG. 1A is a graph showing the electron density at the interface of the light-emitting layer on the hole transport layer side and the interface of the light-emitting layer on the electron injecting transport layer side for each electron injection property according to Examples; and FIG. 1B is a graph showing the electron density at the interface of the light-emitting layer on the hole transport layer side and the interface of the light-emitting layer on the electron injecting transport layer side for each electron injection property according to Comparative Examples. 1A is a graph showing the current flowing through the light-emitting layer for each electron injection property according to an embodiment, and FIG. 1B is a graph showing the current flowing through the light-emitting layer for each electron injection property according to a comparative example. 1 is a graph showing the relationship between the ratio of the effective film thickness Lef(EML) of the light-emitting layer to the total effective film thickness Lef(ALL) of all functional layers constituting an organic EL element, and the increase in driving voltage when electron injection property is reduced. 1A and 1B are partial cross-sectional views each showing a schematic diagram of 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. 1A is a partial cross-sectional view showing a schematic diagram of 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. 1A is a partial cross-sectional view showing a schematic diagram of a part of a manufacturing process of an organic EL element 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-injecting and transporting layer is formed on the light-emitting layer and the partition layer, and (c) shows a state in which an optical adjustment layer is formed on the electron-injecting and transporting layer. FIG. 11 is a partial cross-sectional view showing a schematic diagram of a part of the manufacturing process of an organic EL element according to an embodiment, in which (a) shows a state in which a counter electrode is formed on an optical adjustment layer, and (b) shows a state in which a sealing layer is formed on the counter electrode. 4 is a flowchart showing a manufacturing process of the organic EL element according to the embodiment. 1 is a block diagram showing a configuration of an organic EL display device including an organic EL element according to an embodiment.

<Mode of Disclosure>
An organic EL element according to one embodiment of the present disclosure is an organic EL element in which an anode, a plurality of functional layers including an emitting layer, and a cathode are stacked in this order, and when the effective film thickness of each of the functional layers is defined as the film thickness of the functional layer divided by the effective carrier mobility, which is the sum of the effective electron mobility and effective hole mobility of the functional layer, the effective film thickness of the emitting layer is 30% or less of the sum of the effective film thicknesses of all of the functional layers.

In addition, a method for manufacturing an organic EL element according to one aspect of the present disclosure includes preparing a substrate, forming an anode above the substrate, forming a plurality of functional layers including an emitting layer above the anode, and forming a cathode above the functional layer, and the thickness of the emitting layer is set so that, for each of the functional layers, when the effective thickness of the functional layer is the value obtained by dividing the thickness of the functional layer by the effective carrier mobility, which is the sum of the effective electron mobility and the effective hole mobility of the functional layer, the effective thickness of the emitting layer is 30% or less of the total effective thickness of all the functional layers.

According to an organic EL element or a manufacturing method thereof according to one aspect of the present disclosure, when carrier injection into the light-emitting layer is reduced, the ratio of the voltage (partial voltage) applied to the light-emitting layer to the driving voltage is increased, and the electric field strength in the light-emitting layer is increased. Therefore, even when carrier injection into the light-emitting layer is reduced, the reduction in carrier density in the light-emitting layer is suppressed, and the light-emitting efficiency is increased due to the improvement in carrier mobility, so that the light emission luminance is unlikely to decrease and the degree of increase in the driving voltage can be reduced. Therefore, even if carrier injection into the light-emitting layer is reduced due to deterioration of the functional layer, the driving voltage is unlikely to increase, and the life of the organic EL element can be extended while suppressing the decrease in luminance.

The organic EL element according to one embodiment of the present disclosure may include an electron injection layer containing a metal material as the functional layer between the light-emitting layer and the cathode.

This configuration makes it possible to reduce the degree of increase in driving voltage even if the metal material contained in the electron injection layer deteriorates due to oxidation or other reasons.

In an organic EL element according to one aspect of the present disclosure, the metal material contained in the electron injection layer may be selected from alkali metals, alkaline earth metals, and rare earth metals.

With this configuration, the electron injection layer before deterioration has high electron inflow properties, so the light-emitting efficiency of the organic EL element can be improved, and even if the metal material contained in the electron injection layer deteriorates due to oxidation or the like, the degree of increase in the driving voltage can be reduced.

In an organic EL element according to one aspect of the present disclosure, the anode is a light-reflective electrode, and the organic EL element includes an intermediate layer between the light-emitting layer and the anode as the functional layer, and the thickness of the intermediate layer is 40 nm or less.

This configuration makes it easy to design an optical resonator structure in which the surface of the anode facing the light-emitting layer serves as one of the reflecting surfaces, without making the optical path length from the anode to the light-emitting center excessively long.

In an organic EL element according to one aspect of the present disclosure, the cathode is a semi-transparent electrode, and the organic EL element may include a transparent conductive layer between the light-emitting layer and the cathode as the functional layer.

This configuration makes it easy to design an optical resonator structure in which the surface of the cathode facing the light-emitting layer is one of the reflecting surfaces, after properly designing the optical path length from the cathode to the light-emitting center.

In an organic EL element according to one aspect of the present disclosure, an optical resonator structure is formed between the surface of the anode facing the light-emitting layer and the surface of the cathode facing the light-emitting layer, and the transparent conductive layer may include ITO or IZO.

This configuration makes it possible to design an optical resonator structure with a long optical path length while suppressing an increase in the impedance of the entire organic EL element, thereby increasing the light extraction efficiency.

An organic EL panel according to one aspect of the present disclosure may include a plurality of organic EL elements according to one aspect of the present disclosure on a substrate.

<Embodiment>
Hereinafter, an organic EL element according to an embodiment will be described. Note that the following description is an example for explaining the configuration and the action and effect according to one aspect of the present invention, and the present invention is not limited to the following embodiment except for the essential part.

[1. Structure of organic EL element]
1 is a diagram showing a schematic cross-sectional structure of an organic EL element 1 according to the present embodiment. The organic EL element 1 includes a pixel electrode 13 serving as an anode, a hole injection layer 15, a hole transport layer 16, a light-emitting layer 17, an electron injection transport layer 18, an optical adjustment layer 19, and a counter electrode 20 serving as a cathode.

In the organic EL element 1, the pixel electrode 13 and the counter electrode 20 are arranged opposite each other with their principal surfaces facing each other, and a light-emitting layer 17 is formed between the pixel electrode 13 and the counter electrode 20.

A hole transport layer 16 is formed on the 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 pixel electrode 13.

An electron injection transport layer 18 is formed on the counter electrode 20 side of the light emitting layer 17 in contact with the light emitting layer 17. An optical adjustment layer 19 is formed between the electron injection transport layer 18 and the counter electrode 20.

[1.1 Components of the 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.

In this embodiment, the pixel electrode 13 functions as an anode.

Specific examples of metal materials with light reflectivity include Ag (silver), Al (aluminum), aluminum alloys, 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 composed of a metal layer alone, or may have a laminated structure in which a layer of a metal oxide such as ITO (indium tin oxide) or IZO (indium zinc oxide) is laminated on top of the metal layer.

When the counter electrode 20 is a light-reflective electrode, the pixel electrode 13 may be a light-transmitting electrode. In this case, the pixel electrode 13 includes at least one of a metal layer made of a metal material and a metal oxide layer made of a metal oxide. The film thickness of the pixel electrode 13 is set to a thin thickness of about 1 nm to 50 nm, and is light-transmitting. Although the metal material is a light-reflective material, the light transmittance can be ensured by making the metal layer thin, to 50 nm or less. Therefore, a portion of the light from the light-emitting layer 17 is reflected by the pixel electrode 13, but the remaining portion is transmitted through the pixel electrode 13.

In this case, examples of metal materials forming the metal layer included in the pixel electrode 13 include Ag, a silver alloy mainly composed of Ag, Al, and an Al alloy mainly composed of Al. Examples of Ag alloys include magnesium-silver alloy (MgAg) and indium-silver alloy. Ag basically has a low resistivity, and Ag alloys are preferable in that they have excellent heat resistance and 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 alloys include lithium-magnesium alloy and lithium-indium alloy.

The metal layer included in the pixel electrode 13 may be, for example, a single layer of Ag or MgAg alloy, or may have a laminated structure of Mg and Ag layers (Mg/Ag), or a laminated structure of MgAg alloy and Ag layers (MgAg/Ag).

Metal oxides that form the metal oxide layer contained in the pixel electrode 13 include ITO (indium tin oxide) and IZO (indium zinc oxide).

The pixel electrode 13 may be composed of a metal layer alone or a metal oxide layer alone, but may also 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.

<Hole injection layer>
The hole injection layer 15 has a function of promoting the injection of holes from the pixel electrode 13, which is an anode, to the light emitting layer 17. The hole injection layer 15 is, for example, a coating film, and is formed by coating and drying a solution of a hole injection material and a solute. The hole injection layer 15 is made of a conductive polymer material, such as PEDOT:PSS (a mixture of polythiophene and polystyrene sulfonic acid), polyfluorene or its derivative, or polyarylamine or its derivative. In one embodiment, the thickness of the hole injection layer 15 is 10 nm.

Alternatively, the hole injection layer 15 may be formed of a vapor deposition film. The hole injection layer 15 is made of an oxide of, for example, Ag, Mo, chromium (Cr), vanadium (V), tungsten (W), nickel (Ni), iridium (Ir), or the like.

The hole injection layer 15 may also have a laminated structure in which a coating film is formed on top of a vapor deposition film.

<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. In one aspect of the embodiment, the thickness of the hole transport layer 16 is 13 nm. In forming an optical resonator structure, in order not to excessively increase the optical path length between the light emitting center and the pixel electrode 13, it is preferable that the total thickness of the functional layers present between the pixel electrode 13 and the light emitting layer 17, that is, the total thickness of the hole injection layer 15 and the hole transport layer 16, is 40 nm or less.
Alternatively, the hole transport layer 16 may be formed of a vapor-deposited film. For example, a polymer compound such as polyfluorene or a derivative thereof, or polyarylamine or a derivative thereof may be used.

<Light-emitting layer>
The light-emitting layer 17 has a function of emitting light by recombination of holes and electrons. As described later, it is preferable that the impedance of the light-emitting layer 17 is not too large compared to the impedances of the hole injection layer 15, the hole transport layer 16, the electron injection transport layer 18, and the optical adjustment layer 19, and it is preferable to design the light-emitting layer to have a thin thickness. In one aspect of the embodiment, the thickness of the light-emitting layer 17 is 25 nm.

The light-emitting layer 17 is, for example, a coating film, and is formed, for example, by coating and drying a solution containing the material that forms the light-emitting layer and a solute. Alternatively, the light-emitting layer 17 may be formed as a vapor-deposited film.

As the material for forming the light-emitting layer 17, organic materials that are known fluorescent substances can be used. For example, 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, acridine compounds, etc. can be used. The light-emitting material is not limited to fluorescent materials, but may be a known phosphorescent material such as a metal complex that emits phosphorescence, such as tris(2-phenylpyridine)iridium.

The light-emitting layer 17 may be formed by doping a light-emitting material into a host material having high carrier mobility. Here, high carrier mobility refers to high electron mobility and/or high hole mobility. Examples of the host material that can be used include amine compounds, condensed polycyclic aromatic compounds, and heterocyclic compounds. Examples of the amine compounds that can be used include monoamine derivatives, diamine derivatives, triamine derivatives, and tetraamine derivatives. Examples of the 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 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. When the light-emitting layer is formed from a fluorescent material and a host material, in one embodiment, the concentration of the fluorescent material is 1 wt % or more. In one embodiment, the concentration of the fluorescent material is 10 wt % or less. In one embodiment, the concentration of the fluorescent material is 30 wt % or less.

<Electron Injection Transport Layer>
The electron injection transport layer 18 is formed on the light emitting layer 17, and is formed by doping an organic material having electron transport properties with a metal material that improves electron injection properties. Here, doping refers to dispersing metal atoms or metal ions of the metal material approximately evenly in the organic material, and specifically refers to forming a single phase containing the organic material and a trace amount of the metal material. It is preferable that no other phases, particularly a phase consisting only of the metal material such as metal pieces or metal films, or a phase mainly composed of the metal material, are present. In addition, in the single phase containing the organic material and a trace amount of the metal material, it is preferable that the concentration of the metal atoms or metal ions is uniform, and it is preferable that the metal atoms or metal ions are not aggregated. The metal material is preferably selected from alkali metals, alkaline earth metals, and rare earth metals. In addition, the doping amount of the metal material in the electron injection transport layer 18 is preferably 3 to 60 wt %. It is preferable that the doped 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., Alq ₃ , Liq). Examples of doping metals include alkali metals such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr); alkaline earth metals such as calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra); and rare earth metals such as yttrium (Y), samarium (Sm), europium (Eu), and ytterbium (Yb).

Examples of organic materials with electron transport properties include π-electron low-molecular-weight organic materials such as oxadiazole derivatives (OXD), triazole derivatives (TAZ), and phenanthroline derivatives (BCP, Bphen).

In one embodiment, the thickness of the electron injection transport layer 18 is 30 nm.

The electron injection transport layer 18 does not necessarily have to be formed of a single layer, and may be a multi-layer structure including, for example, an electron injection layer including a metal material and an electron transport layer made of an electron transport material. Or, for example, it may be a multi-layer structure including a layer mainly composed of a metal material or a compound thereof and an electron transport layer. Or, for example, it may be a multi-layer structure including a layer mainly composed of a compound of a metal material and a layer that injects electrons into the metal material to exhibit electron injection properties.

<Optical Adjustment Layer>
The optical adjustment layer 19 is made of a semi-light-transmitting conductive material, and is formed on the electron injection transport layer 18 .

The light reflecting surface of the interface between the counter electrode 20 and the optical adjustment layer 19 is paired with the light reflecting surface of the interface between the pixel electrode 13 and the hole injection layer 15 to form a resonator structure. Therefore, when the light emitted from the light emitting layer 17 enters the counter electrode 20 from the optical adjustment layer 19, a part of the light needs to be reflected to the optical adjustment layer 19. Therefore, it is preferable that the refractive index is different between the counter electrode 20 and the optical adjustment layer 19. For example, when the counter electrode 20 is a metal thin film, it is preferable that the optical adjustment layer 19 is an oxide conductor such as ITO or IZO. Also, for example, when the counter electrode 20 is an oxide conductor such as ITO or IZO, it is preferable that the optical adjustment layer 19 is a metal thin film such as Ag or Al. In one aspect of the embodiment, the material of the optical adjustment layer 19 is IZO, and its film thickness is 104 nm.

<Counter electrode>
The counter electrode 20 is made of a semi-light-transmitting conductive material, and is formed on the optical adjustment layer 19. In this embodiment, the counter electrode 20 functions as a cathode.

The light reflecting surface at the interface between the counter electrode 20 and the optical adjustment layer 19 forms a resonator structure in pair with the light reflecting surface at the interface between the pixel electrode 13 and the hole injection layer 15. Therefore, when the light emitted from the light emitting layer 17 enters the counter electrode 20 from the optical adjustment layer 19, a part of the light must be reflected to the optical adjustment layer 19. Therefore, it is preferable that the refractive index differs between the counter electrode 20 and the optical adjustment layer 19. In one aspect of the embodiment, the counter electrode 20 is a thin metal film. To ensure semi-transparency, the metal layer has a thickness of about 1 nm to 50 nm.

Examples of the material for the counter electrode 20 include Ag, a silver alloy mainly composed of Ag, Al, and an Al alloy mainly composed of Al. Examples of Ag alloys include magnesium-silver alloy (MgAg) and indium-silver alloy. Ag basically has a low resistivity, and Ag alloys are preferable in that they have excellent heat resistance and 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. In this embodiment, the counter electrode 20 is a thin film of Ag.

The counter electrode 20 may be composed of a metal layer alone or a metal oxide layer alone, but may also 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.

When the pixel electrode 13 is a light-transmitting electrode, the counter electrode 20 may be a light-reflective electrode. In this case, the counter electrode 20 includes a metal layer made of a light-reflective metal material. Specific examples of metal materials that have light reflectivity include silver, aluminum, aluminum alloys, molybdenum, APC, ARA, MoCr, MoW, and NiCr.

＜Other＞
The organic EL element 1 is formed on a substrate 11. The substrate 11 is made of a base material 111 which is an insulating material. Alternatively, a wiring layer 112 may be formed on the base material 111 which is an insulating material. For example, a glass substrate, a quartz substrate, a silicon substrate, a plastic substrate, or the like may be adopted as the base material 111. As the plastic material, any of thermoplastic resins and thermosetting resins may be used. For example, 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, epoxy resin, unsaturated polyester, silicone resin, polyurethane, etc., or copolymers, blends, polymer alloys, etc. mainly made of these may be used, and a laminate in which one or more of these are laminated may be used. Examples of materials that can be used to form the wiring 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 polyparaphenylenevinylene. A TFT (Thin Film Transistor) layer may be formed by using a combination of these materials.

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 the steps on the upper surface of the TFT layer 112. Examples of the resin material include positive-type photosensitive materials. Examples of such photosensitive materials include acrylic resins, polyimide resins, siloxane resins, and phenol resins. A contact hole is formed in the interlayer insulating layer 12 for each pixel.

When 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, when the TFT layer 112 is present, at least a portion of the area of the TFT layer 112 that is present below the pixel electrode 13 must be light-transmitting.

In addition, a sealing layer 21 is formed on the organic EL element 1. The sealing layer 21 has a function of preventing organic layers such as the hole injection layer 15, the hole transport layer 16, the light emitting layer 17, and the electron injection transport layer 18 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).

When the organic EL display panel 100 is a top emission type, the sealing layer 21 needs to be made of a light-transmitting material. Although not shown in FIG. 1, a color filter or an upper substrate may be bonded onto the sealing layer 21 via a sealing resin. By bonding the upper substrate, the hole injection layer 15, the hole transport layer 16, the light-emitting layer 17, and the electron injection transport layer 18 can be protected from moisture, air, and the like.

[2. Relationship between electron injection property and driving voltage]
The organic EL element 1 according to one embodiment of the present disclosure is characterized in that an increase in driving voltage can be suppressed even if the electron injection from the cathode to the light-emitting layer is reduced. The relationship between the electron injection from the cathode to the light-emitting layer and the driving voltage of the organic EL element 1 will be described below by comparing the organic EL element 1 according to one embodiment of the present disclosure (hereinafter referred to as "Example") with a comparative example. Note that the electrical characteristics of the examples and comparative examples shown in [2.2] and after are calculated using a device model having the energy band structure shown in [2.1] in the SILVACO device simulation TCAD.

[2.1 Energy band structure]
The organic EL element 1 according to the embodiment and the organic EL element according to the comparative example have the same energy band structure. FIG. 2 is a band diagram showing the energy band structures of the organic EL element 1 according to the embodiment and the organic EL element according to the comparative example. For the sake of simplicity, the energy level of an organic material forming a layer is abbreviated as "energy level of a layer". For a layer made of multiple types of materials, the energy level of a representative organic material responsible for transporting electrons and/or holes is expressed as "energy level of a layer".

In FIG. 2, the LUMO energy level (hereinafter referred to as "LUMO level") and the HOMO energy level (hereinafter referred to as "HOMO level") of the hole transport layer 16, the light emitting layer 17, the electron injection transport layer 18, and the optical adjustment layer 19 are shown, and other layers are omitted. Note that the electron vacuum level is not shown in FIG. 2, but the lower the LUMO level and the HOMO level are on the band diagram, the greater the difference from the electron vacuum level and the lower the energy level.

In the examples and comparative examples, the LUMO level 191 of the optical adjustment layer 19, the LUMO level 181 of the electron injection transport layer 18, the LUMO level 171 of the light-emitting layer 17, and the LUMO level 161 of the hole transport layer 16 are -2.5 eV, -2.4 eV, -2.4 eV, and -2.1 eV, respectively. Therefore, the energy barrier Eg(etl) for injecting electrons from the optical adjustment layer 19 to the electron injection transport layer 18, and the energy barrier Eg(eml) for injecting electrons from the electron injection transport layer 18 to the light-emitting layer 17 are both 0.1 eV or less. In addition, the energy barrier for injecting electrons from the light-emitting layer 17 to the hole transport layer 16 is about 0.3 eV.

In the examples and comparative examples, the HOMO level 162 of the hole transport layer 16, the HOMO level 172 of the light emitting layer 17, the HOMO level 182 of the electron injection transport layer 18, and the HOMO level 192 of the optical adjustment layer 19 are -5.3 eV, -5.5 eV, -5.3 eV, and -5.3 eV, respectively. Therefore, the energy barrier Hg(eml) for injecting holes from the hole transport layer 16 to the light emitting layer 17 is about 0.2 eV. The energy barrier Hg(etl) for injecting holes from the light emitting layer 17 to the electron injection transport layer 18, and the energy barrier for injecting holes from the electron injection transport layer 18 to the optical adjustment layer 19 are -0.2 eV and 0.0 eV, respectively.

[2.2 Electron injection properties and driving voltage of the entire light-emitting device]
3 is a graph showing the relationship between the electron injection from the counter electrode 20 (cathode) to the light-emitting layer 17 and the driving voltage of the organic EL element 1 for each film thickness of the light-emitting layer 17. As shown in FIG. 3, the larger the film thickness of the light-emitting layer 17, the higher the driving voltage. The reason is considered to be that the electric resistance of the light-emitting layer 17 increases as the film thickness of the light-emitting layer 17 increases, and the voltage required to pass the same current increases. Also, as shown in FIG. 3, the higher the electron injection from the counter electrode 20 to the light-emitting layer 17, the lower the driving voltage, and the lower the electron injection, the higher the driving voltage. The reason is considered to be that, if the voltage applied to the organic EL element 1 is the same, if the electron injection from the counter electrode 20 to the light-emitting layer 17 is low, the electron density in the light-emitting layer 17 decreases, the efficiency of exciton generation decreases, and the light-emitting efficiency decreases, so the lower the electron injection, the higher the voltage required to obtain the same amount of light emission.

[2.3 Light-emitting layer thickness and voltage distribution]
FIG. 4(a) is a graph showing the electric potential (electric potential) at each position in the film thickness direction for each degree of electron injection from the counter electrode 20 to the light-emitting layer 17 in an organic EL element 1 having a film thickness of 25 nm as an example. FIG. 4(b) is a graph showing the relationship between the degree of electron injection and the voltage (partial voltage) applied to each functional layer when the organic EL element 1 is driven. FIG. 5(a) is a graph showing the electric potential (electric potential) at each position in the film thickness direction for each degree of electron injection from the counter electrode 20 to the light-emitting layer 17 in an organic EL element having a film thickness of 81 nm as a comparative example. FIG. 5(b) is a graph showing the relationship between the degree of electron injection and the voltage (partial voltage) applied to each functional layer when the organic EL element according to the comparative example is driven. In FIG. 4(a) and FIG. 5(a), the degree of electron injection is highest for A, followed by lowering in the order of B, C, D, E, and F, with F being the lowest. 4(a) and 5(a), when the degree of electron injection property is indicated by the same letter, this indicates that the characteristics of the electron injection transport layer 18 are the same. That is, the organic EL element having the degree of electron injection property A in FIG. 4(a) and the organic EL element having the degree of electron injection property A in FIG. 5(a) differ only in the film thickness of the light-emitting layer 17, and there is no difference in the other configurations.

As shown in Figures 4(a) and 5(a), when the electron injection transport layer 18 deteriorates and its electron transport properties decrease, more specifically, when its electron mobility decreases, the impedance of the electron injection transport layer 18 increases, the voltage drop in the electron injection transport layer 18 increases, and as shown in Figures 4(b) and 5(b), the voltage applied to the electron injection transport layer 18 increases. This increase in the impedance of the electron injection transport layer 18 reduces the current that flows relative to the applied voltage.

On the other hand, when comparing FIG. 4(b) and FIG. 5(b), the organic EL element 1 according to the embodiment shown in FIG. 4(b) has a higher degree of increase in the voltage applied to the electron injection transport layer 18 and the voltage applied to the light-emitting layer 17 than the comparative example shown in FIG. 5(b). The reason for this is thought to be that the impedance of the light-emitting layer 17 is different. As shown in FIG. 5(b), in the comparative example, even when the electron injection from the electron injection transport layer 18 to the light-emitting layer 17 is high, the voltage applied to the light-emitting layer 17 is higher than the voltage applied to the electron injection transport layer 18. That is, since the impedance of the light-emitting layer 17 is higher than the impedance of the electron injection transport layer 18, the voltage applied to the light-emitting layer 17 hardly changes even if the impedance of the electron injection transport layer 18 increases due to the decrease in the electron mobility of the electron injection transport layer 18 caused by the deterioration of the electron injection transport layer 18. Therefore, when the electron injection transport layer 18 deteriorates, the voltage applied to the light-emitting layer 17 does not change, and the current decreases due to the decrease in the electron injection. As a result, if the voltage applied to the organic EL element is the same, the luminance is reduced due to the current reduction, and the driving voltage must be increased to compensate for this. In contrast, as shown in FIG. 4B, in the embodiment, the impedance of the light-emitting layer 17 is the same as the impedance of the electron injection transport layer 18, so that the voltage applied to the light-emitting layer 17 and the voltage applied to the electron injection transport layer 18 are the same regardless of the level of electron injection from the electron injection transport layer 18 to the light-emitting layer 17. That is, when the electron injection from the electron injection transport layer 18 to the light-emitting layer 17 is reduced, both the voltage applied to the electron injection transport layer 18 and the voltage applied to the light-emitting layer 17 are increased. Therefore, the degree of decrease in the electron density of the light-emitting layer 17 is reduced, and the luminous efficiency of the light-emitting layer 17 is improved, so that the decrease in the luminance can be compensated for by increasing the applied voltage. As a result, even if the voltage applied to the organic EL element is the same and the electron injection from the electron injection transport layer 18 to the light-emitting layer 17 is reduced, the degree of decrease in the luminance can be reduced, and the degree of increase in the driving voltage can be reduced.

[2.4 Changes in electron density and current]
The relationship between the decrease in electron injection property and the change in current will be described in more detail below.

Figure 6 is a graph showing the electron density at each position in the film thickness direction for each degree of electron injection from the electron injection transport layer 18 to the light emitting layer 17, where Figure 6(a) corresponds to an example and Figure 6(b) corresponds to a comparative example. Note that, as in Figures 4(a) and 5(a), the degree of electron injection is highest at A, decreasing in the order of B, C, D, E, and F, with F being the lowest.

In the comparative example, even if the electron injection from the electron injection transport layer 18 to the light-emitting layer 17 decreases, the voltage (partial voltage) applied to the light-emitting layer 17 hardly changes. Therefore, as shown in FIG. 6B, the electron density in the light-emitting layer 17 decreases due to the decrease in electron injection. On the other hand, in the example, when the electron injection from the electron injection transport layer 18 to the light-emitting layer 17 decreases, the voltage (partial voltage) applied to the light-emitting layer 17 increases. Therefore, in the example, the electron density in the light-emitting layer 17 decreases due to the decrease in electron injection, but the film thickness of the light-emitting layer 17 is smaller than in the comparative example, and the voltage increase is large, so the electric field strength increases, and the mobility of electrons in the light-emitting layer 17 increases, so the degree of decrease in the electron density in the light-emitting layer 17 is smaller.

7 is a graph showing the relationship between the electron density at the interface of the light-emitting layer 17 on the hole transport layer 16 side and the interface of the light-emitting layer 17 on the electron injection transport layer 18 side, and the electron injection from the counter electrode 20 to the light-emitting layer 17, where FIG. 7(a) corresponds to an example and FIG. 7(b) corresponds to a comparative example. As shown in FIGS. 7(a) and (b), when the electron injection from the electron injection transport layer 18 to the light-emitting layer 17 decreases, the electron density at the interface of the light-emitting layer 17 on the hole transport layer 16 side decreases in both the comparative example and the example, but does not change significantly. The reason for this is that the electron density at the interface of the light-emitting layer 17 on the hole transport layer 16 side is largely influenced by the electron injection barrier from the light-emitting layer 17 to the hole transport layer 16. On the other hand, the electron density at the interface of the light-emitting layer 17 on the electron injection transport layer 18 side decreases in both the example and the comparative example. The reason for this is thought to be that the electron density at the interface of the light-emitting layer 17 on the electron injection transport layer 18 side is directly affected by the degree of electron injection from the electron injection transport layer 18 to the light-emitting layer 17. However, in the example, the electron density at the interface of the light-emitting layer 17 on the hole transport layer 16 side is higher than in the comparative example. The reason for this is thought to be that, as described above, in the example, the voltage gradient between the electron injection transport layer 18 and the light-emitting layer 17 is large and the electric field strength is large, which increases the electron mobility in the electron injection transport layer 18 and the light-emitting layer 17, and the number of electrons injected into the light-emitting layer 17 is greater than in the comparative example.

Figure 8 is a graph showing the relationship between the current flowing through the light-emitting layer 17 and the electron injection from the electron injection transport layer 18 to the light-emitting layer 17, where Figure 8(a) corresponds to an embodiment and Figure 8(b) corresponds to a comparative example. As shown in Figures 8(a) and (b), when the electron injection from the electron injection transport layer 18 to the light-emitting layer 17 decreases, the current flowing through the light-emitting layer 17 decreases. However, in the embodiment, the decrease in the current flowing through the light-emitting layer 17 is smaller than in the comparative example, even if the electron injection from the counter electrode 20 to the light-emitting layer 17 decreases. The reason for this is that, as described above, in the embodiment, the voltage gradient of the electron injection transport layer 18 is larger and the electric field strength is larger than in the comparative example, so that the mobility of electrons is higher and the number of electrons injected into the light-emitting layer 17 is larger. In addition, when the mobility of electrons in the light-emitting layer 17 increases, the recombination coefficient (recombination probability) of electrons and holes in the light-emitting layer 17 also increases, and the luminous efficiency in the light-emitting layer 17 also increases. Therefore, in the embodiment, even if the electron injection from the counter electrode 20 to the light-emitting layer 17 is reduced, the degree of decrease in current is reduced compared to the comparative example, and the degree of decrease in the light-emitting efficiency of the organic EL element 1 can be reduced by improving the light-emitting efficiency of the light-emitting layer 17.

[2.5 Relationship between the thickness of the light-emitting layer and the thickness of other functional layers]
As described above, when the electron injection from the cathode to the light-emitting layer is reduced, the degree of increase in the driving voltage can be reduced by improving the ratio of the voltage (partial voltage) applied to the light-emitting layer to the driving voltage of the organic EL element. Below, the conditions for determining this configuration are examined in detail.

The ratio of the voltage (partial voltage) applied to the light-emitting layer to the driving voltage of the organic EL element can be determined by the relationship between the total impedance of the organic EL element and the impedance of the light-emitting layer. Here, the factors that determine the impedance of the functional layer include the film thickness of the functional layer, the carrier mobility in the functional layer, the electric field strength in the functional layer, and the carrier density in the functional layer. Here, the electric field strength in the functional layer is determined by the partial pressure of the functional layer and the film thickness of the functional layer, and the carrier density in the functional layer depends on the carrier mobility and the electric field strength in the functional layer, so the film thickness of the functional layer and the carrier mobility in the functional layer are considered as the basis. In general, the larger the film thickness of the functional layer, the higher the electrical resistance, that is, the higher the impedance. On the other hand, the higher the carrier mobility in the functional layer, the smoother the carriers move and the larger the current value, that is, the lower the impedance. Therefore, the effective film thickness Lef, which is the film thickness L of the functional layer divided by the carrier mobility μ of the functional layer, is specified as an index indicating the impedance of the functional layer. That is, the effective film thickness Lef of the functional layer, where μe is the electron mobility, μh is the hole mobility, and L is the film thickness, is defined as follows.

Lef=L/(μe+μh)
Then, the relationship between the ratio of the effective film thickness Lef(EML) of the light-emitting layer to the total effective film thickness Lef(ALL) of all functional layers and the increase in driving voltage was investigated. Figure 9 is a graph showing the relationship between the ratio of the effective film thickness Lef(EML) of the light-emitting layer to the total effective film thickness Lef(ALL) of all functional layers and the increase in driving voltage ΔV when the electron injection from the cathode to the light-emitting layer is reduced. Here, the total effective film thickness Lef(ALL) of all functional layers is the total value of the effective film thickness Lef of each of the hole injection layer 15, the hole transport layer 16, the light-emitting layer 17, the electron injection transport layer 18, and the optical adjustment layer 19.

As shown in FIG. 9, the driving voltage increases as the ratio of the effective film thickness Lef (EML) of the light-emitting layer to the total effective film thickness Lef (ALL) of all functional layers increases, and the driving voltage decreases as the ratio decreases. The reason is as follows. The ratio of the voltage (partial voltage) applied to the light-emitting layer to the driving voltage of the organic EL element increases as the ratio of the effective film thickness Lef (EML) of the light-emitting layer to the total effective film thickness Lef (ALL) of all functional layers increases, so even if the electron injection from the cathode to the light-emitting layer decreases, the voltage (partial voltage) applied to the light-emitting layer to the driving voltage of the organic EL element does not increase. Therefore, the electron injection from the cathode to the light-emitting layer decreases, and the electron density in the light-emitting layer decreases, and the luminous efficiency decreases, so it becomes necessary to increase the driving voltage. On the other hand, the lower the ratio of the effective film thickness Lef (EML) of the light-emitting layer to the total effective film thickness Lef (ALL) of all functional layers, the lower the ratio of the voltage (partial voltage) applied to the light-emitting layer to the driving voltage of the organic EL element, so that when the electron injection from the cathode to the light-emitting layer is reduced, the voltage (partial voltage) applied to the light-emitting layer to the driving voltage of the organic EL element is likely to increase. Therefore, even if the electron injection from the cathode to the light-emitting layer is reduced, the electron density in the light-emitting layer is compensated by the increase in the voltage (partial voltage) applied to the light-emitting layer, so the degree of decrease in the light-emitting efficiency is low and the degree of increase in the driving voltage is low. In other words, the lower the ratio of the effective film thickness Lef (EML) of the light-emitting layer to the total effective film thickness Lef (ALL) of all functional layers, the better. For example, by making this ratio 30% (0.3) or less, the increase in the driving voltage can be suppressed to 2V or less. Generally, in an organic EL panel equipped with an organic EL element, the voltage margin for the increase in the drive voltage of the organic EL element is about 2V, so the above configuration makes it possible to prevent a decrease in brightness even when the organic EL panel is driven for a long time. The voltage margin is a voltage designed as a surplus to prepare for an increase in the drive voltage when the output voltage of the drive circuit is allocated to the drive voltage of the organic EL element, the drive voltage of circuits other than the organic EL element (e.g., TFT), the voltage drop due to wiring, etc. For example, if the drive voltage of the organic EL element before deterioration is 11V, the drive voltage of the TFT is 10V, and the voltage drop due to wiring is 0.5V, and the output voltage of the drive circuit is about 20 to 30V, the voltage margin for the increase in the drive voltage of the organic EL element is upper limit of about 1 to 2V.

3. Summary
As described above, in the organic EL element according to one embodiment of the present disclosure, when the effective film thickness Lef of the functional layer is obtained by dividing the film thickness L of the functional layer by the carrier mobility μ of the functional layer, the effective film thickness Lef (EML) of the light-emitting layer is 30% or less of the total effective film thickness Lef (ALL) of all the functional layers. Here, the carrier mobility μ of the functional layer is the sum of the electron mobility μe of the functional layer and the hole mobility μh of the functional layer. In addition, all the functional layers refer to all layers present between the cathode and the anode, including the light-emitting layer. According to this configuration, when the electron injection property from the cathode to the light-emitting layer is reduced, the ratio of the voltage (partial voltage) applied to the light-emitting layer to the driving voltage increases, so that the electric field strength in the light-emitting layer is strengthened and the degree of reduction in the current is reduced. Furthermore, when the electric field strength in the light-emitting layer is strengthened, the mobility of carriers increases, and the recombination coefficient increases, so that the light-emitting efficiency of the light-emitting layer 17 is improved, and the degree of reduction in the light-emitting efficiency of the organic EL element 1 can be reduced. Therefore, in the organic EL element according to one embodiment of the present disclosure, even if the electron injection property from the cathode to the light-emitting layer is reduced, the degree of increase in the driving voltage is reduced, and the acceleration of deterioration of the organic EL element can be suppressed.

[4. Manufacturing method of organic EL element]
The method for manufacturing an organic EL element will be described with reference to the drawings. Figures 10(a) to 13(b) 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 14 is a flowchart showing the method for manufacturing an organic EL display panel including an organic EL element.

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.

10A, 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.

Next, as shown in FIG. 10(b), an interlayer insulating layer 12 is formed on the substrate 11 (step S20). The interlayer insulating layer 12 can be formed by stacking using, for example, a plasma CVD method or a sputtering method.

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

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, for example, by a sputtering method, and after a metal film is formed, it is patterned by using a photolithography method and a wet etching method.

(2) Formation of pixel electrodes 13 Next, as shown in Fig. 10(c), a pixel electrode material layer 130 is formed on the interlayer insulating layer 12 (step S31). The pixel electrode material layer 130 can be formed, for example, by vacuum deposition or sputtering. Next, as shown in Fig. 10(d), the pixel electrode material layer 130 is patterned by etching to form a plurality of pixel electrodes 13 partitioned for each subpixel (step S32). The pixel electrodes 13 function as the anodes of the respective organic EL elements.

The method of forming the pixel electrode 13 is not limited to the above-mentioned method. For example, the pixel electrode 13 and the hole injection layer 15 may be formed together by forming a hole injection material layer 150 on the pixel electrode material layer 130 and patterning the pixel electrode material layer 130 and the hole injection material layer 150 by etching.

(3) Formation of the partition wall 14 Next, as shown in FIG. 10(e), a partition wall resin, which is a material of the partition wall 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 phenol resin, which is a partition wall resin, dissolved in a solvent (e.g., 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, the partition wall material layer 140 is subjected to pattern exposure and development to form the partition wall 14 (FIG. 11(a), step S42), and the partition wall 14 is baked. As a result, an opening 14a, which is a region for forming the light-emitting layer 17, is defined. The partition wall 14 is baked, for example, at a temperature of 150° C. to 210° C. for 60 minutes.

In addition, in the process of forming the partition 14, the surface of the partition 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 done for the purpose of adjusting the contact angle of the partition 14 with the ink (solution) applied to the opening 14a, or for the purpose of imparting water repellency to the surface.

(4) Formation of Hole Injection Layer 15 Next, as shown in FIG. 11(b), ink containing a constituent material of the hole injection layer 15 is ejected from the nozzles of an 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).

(5) Formation of Hole Transport Layer 16 Next, as shown in FIG. 11(c), an 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 coat the hole injection layer 15 in the opening 14a, and then baked (dried) to form the hole transport layer 16 (step S60).

(6) Formation of Light-Emitting Layer 17 Next, as shown in FIG. 12(a), ink containing a constituent material of the light-emitting layer 17 is ejected from the nozzles of the ink-jet head 403 to apply it onto the hole transport layer 16 in the opening 14a, and then baking (drying) is performed to form the light-emitting layer 17 (step S70).

12B, the electron injecting and transporting layer 18 is formed on the light-emitting layer 17 and the partition 14 (step S80). The electron injecting and transporting layer 18 is formed by, for example, forming a film of an organic compound having an electron transporting property and a metal material having an electron injecting property by a co-evaporation method in common to each subpixel.

12C, the optical adjustment layer 19 is formed on the electron injection transport layer 18 (step S90). The optical adjustment layer 19 is formed by, for example, forming a film of an oxide conductor such as ITO or IZO by a sputtering method in common to each subpixel.

13A, the counter electrode 20 is formed on the optical adjustment layer 19 (step S100). The counter electrode 20 is formed by depositing silver, aluminum, or the like by a sputtering method or a vacuum deposition method. The counter electrode 20 functions as a cathode for each organic EL element.

13B, the sealing layer 21 is formed on the counter electrode 20 (step S110). The sealing layer 21 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 applied on the inorganic film of SiON, SiN, or the like, and then baked to form the sealing layer.

A color filter or an upper substrate may be placed on top of the sealing layer 21 and bonded.

[5. Overall configuration of organic EL display device]
Fig. 15 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. 15, 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.

Note that 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.

[6. Other Modifications]
(1) In the above embodiment, the case where the electron injection from the cathode to the light-emitting layer is reduced due to the deterioration of the metal material having electron injection properties has been described, but the degree of increase in the driving voltage can be reduced by a similar design even when the electron injection from the cathode to the light-emitting layer is reduced due to the deterioration of the organic material having electron injection properties. The reason for this is that, as described above, by setting the effective film thickness of the light-emitting layer to a predetermined ratio or less with respect to the total effective film thickness of all functional layers, the impedance of the light-emitting layer can be prevented from being excessively large relative to the impedance of the functional layer having electron injection properties. With this configuration, when the electron injection from the cathode to the light-emitting layer is reduced, the applied voltage (partial voltage) ratio to the driving voltage can be increased for the light-emitting layer and the electron injection layer, so that the effect of the reduction in the electron injection to the light-emitting layer can be counteracted by the increase in electron mobility due to the increase in the electric field strength in the light-emitting layer and the electron injection layer. The same applies to the case where the electron injection from the cathode to the light-emitting layer is reduced due to the deterioration of the organic material having electron transport properties.

Similarly, when the hole injection from the anode to the light-emitting layer is reduced due to deterioration of the metal material or organic material having hole injection properties, the degree of increase in the driving voltage can be reduced by a similar design. The reason for this is that, as described above, by setting the effective film thickness of the light-emitting layer to a predetermined ratio or less of the total effective film thickness of all functional layers, the impedance of the light-emitting layer can be prevented from being excessively large relative to the impedance of the functional layer having hole injection properties. With this configuration, when the hole injection from the anode to the light-emitting layer is reduced, the applied voltage (partial voltage) ratio to the driving voltage can be increased for the light-emitting layer and the hole injection layer, so that the effect of the reduction in hole injection to the light-emitting layer can be countered by the increase in hole mobility due to the increase in the electric field strength in the light-emitting layer and the hole injection layer. The same applies to the case where the hole injection from the anode to the light-emitting layer is reduced due to deterioration of the organic material having hole transport properties.

(2) In the above embodiment, the hole injection layer 15 and the hole transport layer 16 are essential components, but this is not limited to the above. For example, the organic EL element may not have the hole transport layer 16. Also, for example, instead of the hole injection layer 15 and the hole transport layer 16, a single layer hole injection transport layer may be provided.

In addition, in the above embodiment, a single electron injection transport layer 18 is provided, but the electron injection layer and the electron transport layer may be provided separately.

(3) In the above embodiment, the optical adjustment layer 19 is an essential component, but this is not limited to the above. In the above embodiment, since the film thickness of the light-emitting layer 17 is small, an optical resonator structure in which the optical path length is minimized using zero-order interference may be formed by designing the film thickness of other functional layers. In addition, when an optical adjustment layer is provided, the optical adjustment layer does not need to be a single layer or be located in the position shown in the embodiment. For example, the optical adjustment layer may be provided between the light-emitting layer and the electron injection transport layer.

(4) In the above embodiment, the organic EL display panel has a top emission configuration, but it may have a bottom emission configuration by using a light-transmitting electrode for the pixel electrode and a light-reflective electrode for the counter electrode.

In addition, 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.

(5) In the above embodiment, the panel having the organic EL element is an organic EL display panel, but this is not limited to this. For example, as long as the panel has the organic EL element as a light-emitting element, it may be used for purposes other than display, such as a light-emitting panel as part of a lighting device, a backlight panel for a liquid crystal panel, or a multi-function panel that can be used selectively as a light-emitting panel or a display panel.

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-mentioned embodiments and modifications. The present invention also includes forms obtained by applying various modifications to the above-mentioned embodiments and modifications that would come to mind by 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.

The present invention is useful for manufacturing long-life organic EL elements and organic EL panels incorporating the same.

1 Organic EL element 11 Substrate 12 Interlayer insulating layer 13 Pixel electrode (anode)
14 Partition wall 15 Hole injection layer 16 Hole transport layer 17 Light emitting layer 18 Electron injection transport layer 19 Optical adjustment layer 20 Counter electrode (cathode)
21 Sealing layer 100 Organic EL display panel 200 Drive control section 210 to 240 Drive circuit 250 Control circuit 1000 Organic EL display device

Claims (8)

An organic EL element comprising an anode, a plurality of functional layers including an emitting layer and an electron injection layer, and a cathode laminated in this order,
an effective film thickness of each of the functional layers obtained by dividing the film thickness of the functional layer by the effective carrier mobility, which is the sum of the effective electron mobility and effective hole mobility of the functional layer, the effective film thickness of the light-emitting layer is 20% or more and 30% or less of the sum of the effective film thicknesses of all the functional layers, regardless of fluctuations in the electron injection property of the light-emitting layer, and the increase in the applied voltage to the light-emitting layer and the electron injection layer due to fluctuations in the electron injection property of the light-emitting layer is in the range of 1.5 V to 2 V. 2. The organic electroluminescence element according to claim 1, further comprising an electron injection layer containing a metal material as the functional layer between the light emitting layer and the cathode, the effective thickness of the electron injection layer being approximately the same as that of the light emitting layer. 3. The organic electroluminescence element according to claim 2, wherein the metal material contained in the electron injection layer is selected from the group consisting of alkali metals, alkaline earth metals, and rare earth metals, the thickness of the electron injection layer is 30 nm, and the thickness of the light emitting layer is 25 nm. the anode is a light-reflective electrode,
the organic EL element includes an intermediate layer between the light-emitting layer and the anode as the functional layer,
The organic electroluminescence element according to claim 1 , wherein the intermediate layer has a thickness of 40 nm or less. The cathode is a semi-transparent electrode,
The organic EL element according to claim 1 , further comprising a transparent conductive layer between the light emitting layer and the cathode as the functional layer. an optical resonator structure is formed between a surface of the anode facing the light-emitting layer and a surface of the cathode facing the light-emitting layer;
The organic electroluminescence element according to claim 5 , wherein the transparent conductive layer contains ITO or IZO. An organic EL panel in which a plurality of organic EL elements according to any one of claims 1 to 6 are formed on a substrate. Prepare the substrate,
forming an anode above the substrate;
forming a plurality of functional layers including a light-emitting layer and an electron injection layer above the anode;
A method for manufacturing an organic electroluminescence element, comprising forming a cathode above the functional layer,
a method for manufacturing an organic EL element, wherein the thickness of the light-emitting layer is set so that, when the effective thickness of each of the functional layers is defined as the thickness of the functional layer divided by the effective carrier mobility, which is the sum of the effective electron mobility and effective hole mobility of the functional layer, the effective thickness of the light-emitting layer is 20% or more and 30% or less of the sum of the effective thicknesses of all the functional layers, regardless of fluctuations in the electron injection property of the light-emitting layer, and the increase in the applied voltage to the light-emitting layer and the electron injection layer due to fluctuations in the electron injection property of the light-emitting layer is in the range of 1.5 V to 2 V.

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