Light emitting element and method for manufacturing light emitting element

Abstract

Provided is a light-emitting element with high reliability. The light-emitting element includes an anode, a cathode, a light-emitting layer containing a phosphor provided between the anode and the cathode, and at least one intermediate layer containing a metal oxide and at least one of a metal hydroxide provided between the anode and the light-emitting layer.

Description

Technical Field

[0001] This invention relates to light-emitting elements and methods for manufacturing light-emitting elements. Background Technology

[0002] For example, Patent Document 1 discloses a light-emitting element that sequentially comprises an anode, a hole transport layer, a light-emitting layer containing quantum dots, an electron transport layer, and a cathode. The hole mobility of the hole transport layer is less than the electron mobility of the electron transport layer, and the hole transport layer has an electron blocking function.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2009-088276 Summary of the Invention

[0006] The problem the invention aims to solve

[0007] However, the light-emitting element described in Patent Document 1 does not suppress the migration of electrons to the hole transport layer, and the hole transport layer may deteriorate due to electrons. The main objective of this disclosure is to provide a light-emitting element with high reliability, for example, suppressing the deterioration of functional layers such as the hole transport layer.

[0008] Problem-solving methods

[0009] One aspect of the present invention provides a light-emitting element comprising: an anode; a cathode opposite to the anode; a light-emitting layer disposed between the anode and the cathode, comprising a phosphor; and at least one intermediate layer disposed between the anode and the light-emitting layer, comprising at least one of an oxide of a metal and a hydroxide of the metal. Attached Figure Description

[0010] Figure 1 This is a diagram illustrating an example of the stacked structure of the light-emitting element involved in the implementation method.

[0011] Figure 2 This is a diagram illustrating an example of the manufacturing process of a light-emitting element involved in an implementation method.

[0012] Figure 3 This is another example of a stacked structure for a light-emitting element.

[0013] Figure 4 This is a table relating to the results of the embodiments and comparative examples, etc. Detailed Implementation

[0014] The following describes one method of implementing this disclosure.

[0015] Figure 1This is a schematic diagram illustrating an example of the stacked structure of the light-emitting element 100 according to this embodiment.

[0016] like Figure 1 As shown, the light-emitting element 100 includes, for example, a substrate 1, an anode 2, a hole injection layer 3, a hole transport layer 4, an intermediate layer 5, a metal layer 9, a light-emitting layer 6, an electron transport layer 7, and a cathode 8. Furthermore, the light-emitting element 100 may have a structure in which the anode 2, hole injection layer 3, hole transport layer 4, intermediate layer 5, light-emitting layer 6, electron transport layer 7, and cathode 8 are sequentially stacked on the substrate 1.

[0017] The substrate 1 is made of glass or the like and functions as a support for the aforementioned layers. The substrate 1 may be, for example, an array substrate for forming thin-film transistors (TFTs).

[0018] The anode 2 supplies holes to the light-emitting layer 6. In addition, the anode 2 is arranged opposite to the cathode 8.

[0019] The cathode 8 supplies electrons to the light-emitting layer 6.

[0020] Either anode 2 or cathode 8 is made of a light-transmitting material. Alternatively, either anode 2 or cathode 8 may be formed of a light-reflective material. When the light-emitting element 100 is a top-emitting type, for example, a light-transmitting material is formed as the upper cathode 8, and a light-reflective material is formed as the lower anode 2. Conversely, when the light-emitting element 100 is a bottom-emitting type, for example, a light-reflective material is formed as the upper cathode 8, and a light-transmitting material is formed as the lower anode 2. Furthermore, by making either anode 2 or cathode 8 a laminate of a light-transmitting material and a light-reflective material, it can serve as a light-reflective electrode.

[0021] As a light-transmitting material, transparent conductive materials can be used, for example. Specifically, materials such as ITO (indium tin oxide), IZO (indium zinc oxide), SnO2 (tin oxide), AZO (aluminum zinc oxide), and FTO (fluorine-doped tin oxide) can be used. These materials improve the luminous efficiency of the light-emitting element 100 because of their high transmittance of visible light.

[0022] Metallic materials can be used as light-reflecting materials. Specifically, materials such as Al (aluminum), Ag (silver), Cu (copper), and Au (gold) can be used. These materials have high reflectivity for visible light, thus improving luminous efficiency. Furthermore, materials with a Fermi level or a work function close to LUMO (Luminous Induction Motion) of the electron transport layer are suitable as cathode materials, using metals such as Ag and Mg, or ZnO-based compounds such as IZO. When extracting light from the cathode side, to suppress light absorption at the cathode and improve external quantum efficiency, the cathode is preferably thinner than approximately 50 nm.

[0023] The light-emitting layer 6 contains, for example, a phosphor or other light-emitting material. Moreover, the light-emitting layer 6 emits light through the recombination of holes transported from the anode 2 and electrons transported from the cathode 8.

[0024] Quantum dots can be cited as an example of luminescent materials. Quantum dots are semiconductor particles with a particle size of less than 100 nm, and can be crystals of group II-VI semiconductor compounds such as MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, and / or group III-V semiconductor compounds such as GaAs, GaP, InN, In-As, InP, InSb, and / or group IV semiconductor compounds such as Si, Ge. Furthermore, quantum dots can also have a core / shell structure, where the core is a semiconductor crystal with a wide bandgap outer coating. More specifically, the semiconductor microparticles can be those with a core / shell structure consisting of a core made of group II-VI compounds such as CdSe and CdSeTe, group III-V compounds such as InP and InN, or perovskites, and a shell made of ZnS or GaN. Furthermore, to facilitate dispersion of the semiconductor microparticles in the solvent and passivate surface defects, surface modification with common ligands such as ethanolamine and oleic acid is preferred. Organic solvents such as octane and hexadecylamine are used as solvents. These solvents can be used as dispersion media for coating solutions containing semiconductor microparticles.

[0025] Furthermore, the luminescent layer 6 may include, for example, a highly polar liquid. This highly polar liquid can improve the carrier transport properties of electrons, holes, etc., in the luminescent layer 6. Examples of highly polar liquids include carbonate solvents, ethoxy solvents, thiol-carboxyl solvents, thiol-amine solvents, carboxyl-amine solvents, ketone solvents, nitrile solvents, and lactone solvents. Specific examples of highly polar liquids include at least one selected from propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, mercaptopropionic acid, semiamine, and mercaptoacetic acid. Furthermore, when the luminescent material has a polar ligand coordinated on its surface, mercaptopropionic acid, semiamine, and mercaptoacetic acid are preferably used as highly polar liquids. Ethyl carbonate is particularly suitable as a highly polar liquid.

[0026] The light-emitting layer 6 can be formed by various conventional methods, such as coating. In this embodiment, a coating method in which the light-emitting layer 6 is formed by coating a coating liquid containing a light-emitting material and a solvent is particularly preferred. By using the aforementioned highly polar liquid as the solvent when forming the light-emitting layer 6, it is possible to easily incorporate the highly polar liquid into the light-emitting layer 6. The thickness of the light-emitting layer 6 is preferably, for example, around 40 nm.

[0027] Hole transport layer 4 transports holes from anode 2 to light-emitting layer 6. Hole transport layer 4 is disposed between anode 2 and light-emitting layer 6. Hole transport layer 4 contains hole transport material.

[0028] Examples of hole-transporting materials include materials containing one or more oxides, nitrides, or carbides selected from Zn, Cr, Ni, Ti, Nb, Al, Si, Mg, Ta, Hf, Zr, Y, La, and Sr; or 4,4',4'-tris(9-carbazole)triphenylamine (TCTA), 4,4'-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (NPB), zinc phthalocyanine (ZnPC), bis[4-(N,N-dimethylamino)phenyl]cyclohexane (TAPC), and 4,4'-bis(carbazole-9-yl)triphenylamine (TCTA). Materials such as biphenyl (CBP), 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazaphenylene (HATCN), and MoO3, as well as hole-transporting organic materials such as poly(N-vinylcarbazole) (PVK), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-tert-butylphenyl)imino)-1,4-phenylene (TFB), poly(triphenylamine) derivatives (Poly-TPD), and poly(3,4-ethylenedioxythiophene) / poly(4-styrenesulfonic acid) (PEDOT-PSS).

[0029] Hole injection layer 3 is disposed, for example, between anode 2 and hole transport layer 4. Hole injection layer 3 facilitates the injection of holes from anode 2 into hole transport layer 4. Hole injection layer 3 contains hole transport material. This hole transport material is the same as the hole transport material used in hole transport layer 4 described above. Furthermore, hole injection layer 3 is not necessarily required and can be omitted depending on the desired structure and characteristics of the light-emitting element. In particular, for example, it is acceptable to omit it if the material of hole injection layer is inorganic.

[0030] An intermediate layer 5 is disposed, for example, between the light-emitting layer 6 and the hole transport layer 4. The intermediate layer 5 contains, for example, a metal oxide or hydroxide. In particular, the intermediate layer 5 preferably contains a metal hydroxide. Furthermore, as the metal used for the intermediate layer 5, at least one selected from Mg, Al, Zn, Fe, Ni, Sn, Cu, and Cr can be listed. Among these, Ni is preferred as the metal.

[0031] The intermediate layer 5 is, for example, composed of an oxide or hydroxide of at least one metal selected from Mg, Al, Zn, Fe, Ni, Sn, Cu, and Cr. In particular, the intermediate layer 5 is preferably composed of a hydroxide of the aforementioned metal. Moreover, Ni is preferred as the aforementioned metal.

[0032] Furthermore, examples of hydroxides of the aforementioned metals include Ni(OH)₂, Mg(OH)₂, Al(OH)₃, Zn(OH)₂, Sn(OH)₂, Sn₂(OH)₂, Cr(OH), and Cr(OH)₃. Additionally, examples of oxides of the aforementioned metals include Fe₂O₃, Cu₂O, and Cr₂O₃. Moreover, the oxides and hydroxides of the aforementioned metals may also be peroxides such as NiOOH and SnOOH, complexes of oxides and peroxides containing peroxides such as NiOOH-NiO₂, or complexes of hydroxides containing peroxides.

[0033] Thus, by providing the intermediate layer 5, the degradation of the hole transport layer 4 can be suppressed. This is believed to be because, through the intermediate layer 5, the migration of electrons from the light-emitting layer 6 to the hole transport layer 4 is suppressed, making the hole transport layer 4 less susceptible to electron-induced degradation. In particular, when the hole transport layer 4 is an organic material, degradation can be further reduced.

[0034] The thickness of the intermediate layer 5 is preferably 1 nm or more and 5 nm or less, and more preferably 1 nm or more and 3 nm or less. This can suppress the migration of electrons from the light-emitting layer 6 to the hole transport layer 4, and also suppress the reduction in the efficiency of hole transport from the hole transport layer 4 to the light-emitting layer 6.

[0035] For example, the metal layer 9 is disposed between the intermediate layer 5 and the hole transport layer 4. In addition, the metal layer 9 is disposed adjacent to the intermediate layer 5.

[0036] The metal layer 9 is composed of at least one metal selected from Mg, Al, Zn, Fe, Ni, Sn, Cu, and Cr. Preferably, the metal of the metal layer 9 is the same as the metal used in the aforementioned intermediate layer.

[0037] The thickness of the metal layer 9 is preferably, for example, 5 nm or less. If the thickness of the metal layer 9 exceeds 5 nm, it may sometimes hinder the transport of holes from the hole transport layer 4 to the light-emitting layer 6. Furthermore, the lower limit of the thickness of the metal layer 9 is preferably 1 nm or more, and it is possible to avoid forming discrete islands.

[0038] Electron transport layer 7 transports electrons to light-emitting layer 6. Electron transport layer 7 is disposed between cathode 8 and light-emitting layer 6. Furthermore, electron transport layer 7 may also have the function of hindering hole transport. Moreover, electron transport layer 7 may also have the function of promoting the injection of electrons from cathode 8 to light-emitting layer 6. In addition, light-emitting element 100 may, for example, have a layer between electron transport layer 7 and light-emitting layer 6 that hinders hole transport, and may also have a layer between cathode 8 and electron transport layer 7 that promotes the injection of electrons from cathode 8 to light-emitting layer 6.

[0039] The electron transport layer 7 uses electron transport materials such as zinc oxide (e.g., ZnO), titanium oxide (e.g., TiO2), and strontium titanium oxide (e.g., SrTiO3). Only one of these electron transport materials may be used, or two or more may be used in combination as appropriate.

[0040] The electron transport layer 7 can be formed by various conventional methods, such as vacuum evaporation, sputtering, or coating. The material of the electron transport layer 7 can also be a ZnO-based compound such as IZO or ZAO, an inorganic compound such as IGZO, TiO2 or MoO3, WO3, or LiF, or an organic compound such as tris(8-hydroxyquinoline)aluminum (Alq3).

[0041] The following is for reference Figure 1 , Figure 2 , Figure 3 An example of a method for manufacturing the light-emitting element 100 according to this embodiment will be described. In this manufacturing method, the case where the light-emitting layer has a predetermined pattern will be described.

[0042] First, such as Figure 1 As shown, an anode 2 (S1) is formed on substrate 1. The anode 2 can be formed by various conventional methods, such as sputtering and vacuum evaporation.

[0043] A hole injection layer 3 (S2) is formed on the anode 2. The hole injection layer 3 can be formed by various conventional methods, such as vacuum evaporation, sputtering, or coating.

[0044] A hole transport layer 4 (S3) is formed on the hole injection layer 3. The hole transport layer 4 can be formed by various conventionally known methods, such as vacuum evaporation, sputtering, or coating. More specifically, for example, the hole transport layer 4 can be formed by evaporation or sputtering of commonly used organic or inorganic materials such as PVK, TFB, NiO, MoO3, and WO3, or by coating and baking of precursor solutions of various materials, especially colloidal solutions formed by dispersing nanoparticles of inorganic materials.

[0045] A precursor metal layer (S4) is formed on the hole transport layer 4. The precursor metal layer can be formed by various conventionally known methods, such as vacuum evaporation and sputtering. The precursor metal layer does not have to be a continuous film; for example, it can be discrete islands. The thickness when it is formed as an island refers to the average thickness. When the precursor metal layer is island-shaped, an intermediate layer (non-dynamic film) 5 can also be continuously formed by in-plane lateral diffusion.

[0046] Furthermore, the precursor metal layer is formed from a material that can serve as both the metal layer 9 and the intermediate layer 5, such as at least one metal selected from Mg, Al, Zn, Fe, Ni, Sn, Cu, and Cr. Al is preferably used as the precursor metal layer. Al is known to be a metal that is very easily oxidized, but in OH... - It exists in the surrounding environment and preferentially forms hydroxides. OH - In containing O - It possesses the strongest oxidizing power among the reactive oxygen species; therefore, if easily oxidized Al is present, it is more potent than O. - Preferably combined with Al, Al hydroxide is formed and stabilized as a non-dynamic material. This property is widely used, for example, as a technique to protect Al molded products from oxidation by forming a strong non-dynamic protective film through anti-corrosion aluminum treatment. Similar to anti-corrosion aluminum treatment, the Al in this embodiment is also formed as an intermediate layer (non-dynamic film) 5 by producing Al hydroxide, thus improving the reliability of the light-emitting element.

[0047] A light-emitting layer 6 (S5) is formed on a precursor metal layer. The light-emitting layer 6 is preferably formed, for example, by a coating method. Alternatively, for example, the light-emitting layer 6 can be formed by using a coating liquid comprising a highly polar liquid, a solvent, and a light-emitting material, printing the coating liquid onto the precursor metal layer by coating or inkjet printing, and then performing heat treatment to evaporate the solvent. Furthermore, considering damage to semiconductor particles, the heat treatment temperature is preferably below 150°C. Additionally, during heat treatment, the vapor pressure of the highly polar liquid is extremely low compared to the solvent, approximately 1 / 1000, therefore, it can remain in the light-emitting layer 6 even after heat treatment. Furthermore, it is preferable to coat the precursor metal layer with the coating liquid in such a way that the highly polar liquid remains in the light-emitting layer 6, and then bake it at a temperature below the boiling point of the highly polar liquid. Furthermore, the highly polar liquid in the coating liquid can be adjusted to, for example, 1% or more by mass and 90% or less by mass to form the light-emitting layer 6. Moreover, for example, a coating liquid that is photosensitive to the coating liquid can be used to form the light-emitting layer 6 by photolithography.

[0048] An electron transport layer 7 (S6) is formed on the light-emitting layer 6. The electron transport layer 7 can be formed by various conventional methods, such as vacuum evaporation, sputtering, or coating.

[0049] A cathode 8 (S7) is formed on the electron transport layer 7. The cathode 8 can be formed by various conventional methods, such as sputtering and vacuum evaporation.

[0050] Furthermore, by energizing the anode 2 and cathode 8, an intermediate layer containing oxides and / or hydroxides of the metal contained in the precursor metal layer is formed on the light-emitting layer 6 side of the precursor metal layer or on the hole transport layer 4 side. The reaction of the metal oxides and / or hydroxides of the precursor metal layer is carried out, for example, through an electrochemical reaction.

[0051] For example, if an intermediate layer is formed on the side of the light-emitting layer 6 of the aforementioned precursor metal layer, it is assumed that by applying electricity, the highly polar liquid contained in the light-emitting layer 6 migrates to the interface between the metal layer and the light-emitting layer 6, and by reacting the highly polar liquid with the metal of the precursor metal layer, an oxide and / or hydroxide of the aforementioned metal as the intermediate layer is formed.

[0052] Furthermore, as described above, by adjusting the process so that all precursor metal layers do not react, a laminate of intermediate layer 5 and metal layer 9 can be formed. Additionally, by reacting all precursor metal layers, all precursor metal layers can be converted into intermediate layer 5.

[0053] Furthermore, for example, when an intermediate layer 5 is formed on the hole transport layer 4 side of the aforementioned precursor metal layer, it is assumed that a highly polar liquid is pre-contained in the hole transport layer. By applying an electric current, the highly polar liquid contained in the hole transport layer 4 migrates to the interface between the aforementioned metal layer and the light-emitting layer 6. By reacting the highly polar liquid with the metal of the precursor metal layer, an oxide and / or hydroxide of the aforementioned metal, which serves as the intermediate layer 5, is formed. Similarly, when a laminate of the intermediate layer 5 and the metal layer 9 is formed, the lamination order is reversed compared to when the intermediate layer 5 is on the hole transport layer 4 side and the metal layer 9 is on the light-emitting layer 6 side.

[0054] Through the above methods, it is possible to manufacture the light-emitting element 100.

[0055] According to the light-emitting element of this embodiment, an intermediate layer comprising at least one of a metal oxide and a metal hydroxide is provided between the hole transport layer 4 and the light-emitting layer 6. This intermediate layer suppresses the migration of electrons from the light-emitting layer to the hole transport layer 4. Therefore, it suppresses the degradation of the hole transport layer 4 due to electrons, and improves the reliability of the light-emitting element.

[0056] Furthermore, the above example illustrates the provision of an intermediate layer 5 and a metal layer 9 between the light-emitting layer 6 and the hole transport layer 4, which serves as a functional layer. However, this is not the only example. For instance, by providing an intermediate layer 5 and a metal layer 9 on the side of the light-emitting layer 6 of the functional layer, degradation of the functional layer can be suppressed. For example, the hole injection layer 3 and the anode 2 can also be referred to as functional layers, and an intermediate layer 5 and a metal layer 9 can also be provided on the side of the light-emitting layer 6 of these layers. For example, as... Figure 3 As shown, an intermediate layer 5 and a metal layer 9 can also be provided between the hole transport layer 4 and the hole injection layer 3. This can suppress the degradation of the hole injection layer 3. Furthermore, an intermediate layer 5 and a metal layer 9 can be provided between the light-emitting layer 6 and the hole transport layer 4, and between the hole transport layer 4 and the hole injection layer 3, respectively.

[0057] <Example 1>

[0058] A 30nm anode made of ITO was formed on a glass substrate by sputtering.

[0059] A 20nm hole injection layer is formed by coating PEDOT-PSS onto the anode.

[0060] A 40nm hole transport layer composed of TFB is formed on the aforementioned hole injection layer.

[0061] A 3nm precursor metal layer composed of Ni was formed on the hole transport layer by sputtering.

[0062] A light-emitting layer is formed by coating a precursor metal layer with semiconductor nanoparticles containing a CdSe core / ZnS shell, a mixed solution of ethylene carbonate as a highly polar liquid, and octane as a solvent.

[0063] A colloidal solution containing dispersed ZnO nanoparticles was coated onto the luminescent layer, and the electron transport layer was formed to a thickness of 50 nm.

[0064] Al is deposited to a thickness of 10 nm on the electron transport layer to serve as the cathode.

[0065] Next, an electric current is applied between the anode and the cathode to cause an electrochemical reaction on the light-emitting side of the precursor metal layer, forming an intermediate layer, making the precursor metal layer a stack of the intermediate layer and the metal layer.

[0066] The above steps are used to fabricate the light-emitting element. Furthermore, details regarding the composition and thickness of the intermediate layer are as follows: Figure 4 The table is shown.

[0067] <Comparative Example 1>

[0068] Except that no precursor metal layer is formed, i.e. no intermediate layer and metal layer are formed, the light-emitting element is fabricated in the same manner as in Example 1.

[0069] <Comparative Example 2>

[0070] Except for changing the highly polar liquid in the coating solution for the light-emitting layer to tetrahydrofuran, the light-emitting element was fabricated in the same manner as in Example 1.

[0071] <Examples 2-1 to 2-4>

[0072] like Figure 4 As shown in the table, the light-emitting element was fabricated in the same manner as in Example 1, except that the highly polar liquid in the coating solution for the light-emitting layer was changed to propylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, and diethyl carbonate, respectively.

[0073] <Examples 3-1 to 3-6>

[0074] like Figure 4 As shown in the table, the light-emitting element is fabricated in the same manner as in Example 1, except that a precursor metal layer composed of Cr, Mg, Fe, Zn, Sn or Cu is formed.

[0075] <Example 4>

[0076] As a precursor metal layer, the light-emitting element is fabricated in the same manner as in Example 1, except that a precursor metal layer composed of Al is formed.

[0077] <Evaluation>

[0078] For the light-emitting elements of the embodiments and comparative examples, the current density was adjusted to a constant 20 mA / cm², and the brightness was measured relative to the elapsed time. Furthermore, if the elapsed time until the brightness reached 95% or less of the initial brightness was less than 10,000 hours, it was evaluated as not practically usable (×), and if it was more than 10,000 hours, it was evaluated as practically usable (○). Here, the driving current density was twice the current density calculated based on the pixel size of the assumed display panel, which has a 4K resolution and a diagonal 50-inch size comparable to the typical current value that the TFT circuit driving the display panel can supply. The reason for using twice the current density is not for average driving conditions, but for comparing reliability under more stringent maximum driving conditions. Under these conditions, the current value is approximately 16 mA.

[0079] Furthermore, the light-emitting element of Example 1 retains more than 97% of its initial brightness after 10,000 hours, which is excellent.

[0080] The present invention is not limited to the above embodiments, and can be replaced by a structure that is substantially the same as the structure shown in the above embodiments, a structure that achieves the same effect, or a structure that can achieve the same purpose.

Claims

1. A method for manufacturing a light emitting element, characterized by, The application includes: a step of forming a metal layer composed of a metal on an anode; a step of forming a light-emitting layer or a functional layer by applying a coating liquid containing a highly polar liquid on the metal layer; a step of forming a cathode on the light-emitting layer or the functional layer; and a step of forming an intermediate layer containing an oxide of the metal and / or a hydroxide of the metal on the light-emitting layer side or the functional layer side of the metal layer by applying electric current between the anode and the cathode.

Images