Display device

Abstract

Provided is a feature that can contribute to the improvement of light emission performance. This display device 1 comprises a substrate 11, a positive electrode 12, a hole transport layer 15, a light-emitting layer 16, an electron transport layer 17, and a negative electrode 18, in that order. The light-emitting layer 16 is composed of quantum dots 16A, the electron transport layer 17 is composed of metal oxide nanoparticles 17A, and the average particle diameter R1 of the quantum dots 16A is smaller than the average particle diameter R2 of the metal oxide nanoparticles 17A.

Description

display device

[0001] This disclosure relates to a display device.

[0002] In recent years, the development and practical application of self-emissive display devices have been progressing as an alternative to non-self-emissive display devices. Self-emissive display devices are equipped with light-emitting elements such as OLEDs (Organic Light Emitting Diodes) and QLEDs (Quantum Dot Light Emitting Diodes).

[0003] Self-illuminating display devices are provided with two electrodes and a functional layer placed between these electrodes, which includes at least a light-emitting layer containing a light-emitting element. In order to manufacture high-definition display devices inexpensively and easily, attempts have been made to form the functional layer using coating methods such as spin coating and inkjet printing, instead of using vapor deposition. For example, Patent Document 1 describes forming an electron transport layer by inkjet printing. Patent Document 2 describes forming a hole injection layer, a hole transport layer, and an electron transport layer by inkjet printing.

[0004] Japanese Patent Publication No. 2021-116345, International Publication No. 2022 / 070296

[0005] This disclosure aims to provide a technology that can contribute to improving luminescence performance.

[0006] According to one aspect of the present invention, a display device is provided comprising a substrate, an anode, a hole transport layer, a light-emitting layer, an electron transport layer, and a cathode in this order, wherein the light-emitting layer is made of quantum dots, the electron transport layer is made of metal oxide nanoparticles, and the average particle diameter R1 of the quantum dots is smaller than the average particle diameter R2 of the metal oxide nanoparticles.

[0007] According to another aspect of the present invention, a display device is provided in which the ratio R1 / R2 of the average particle diameter R1 of the quantum dots to the average particle diameter R2 of the metal oxide nanoparticles is in the range of 0.0125 or more and less than 1.

[0008] According to yet another aspect of the present invention, a display device is provided relating to any of the above aspects, wherein the average particle size R2 of the metal oxide nanoparticles is in the range of 20 nm to 400 nm.

[0009] According to yet another aspect of the present invention, a display device is provided relating to any of the above aspects, wherein the average particle diameter R1 of the quantum dots is in the range of 5 nm to 20 nm.

[0010] According to yet another aspect of the present invention, a display device relating to any of the above aspects is provided, wherein the thickness of the light-emitting layer is within the range of 5 nm to 60 nm.

[0011] According to yet another aspect of the present invention, a display device according to any of the above aspects is provided, wherein the metal oxide nanoparticles include one or more of a plurality of nanoparticles, each containing bismuth oxide, cobalt oxide, copper oxide, magnesium oxide, nickel oxide, zinc oxide, magnesium-doped zinc oxide, and titanium oxide.

[0012] According to yet another aspect of the present invention, a display device is provided which has an average particle diameter R1 of the quantum dots such that the 10% particle diameter D10 in the volume-based cumulative particle size distribution is in the range of 7 nm to 14 nm, the 50% particle diameter D50 is in the range of 10 nm to 18 nm, and the 95% particle diameter D95 is in the range of 10 nm to 40 nm.

[0013] According to yet another aspect of the present invention, a display device is provided relating to any of the above aspects, wherein the average particle size R2 of the metal oxide nanoparticles is such that the 10% particle size D10 in the volume-based cumulative particle size distribution is in the range of 12 nm to 28 nm, the 50% particle size D50 is in the range of 30 nm to 200 nm, and the 95% particle size D95 is in the range of 50 nm to 400 nm.

[0014] A method for manufacturing a display device is provided, comprising forming a cured coating film made of a first ink composition containing quantum dots on a substrate layer, and forming a cured coating film made of a second ink composition containing metal oxide nanoparticles as an electron transport layer on the cured coating film, wherein the average particle size R1 of the quantum dots is smaller than the average particle size R2 of the metal oxide nanoparticles.

[0015] According to yet another aspect of the present invention, an ink set is provided comprising a first ink composition containing quantum dots and a second ink composition containing metal oxide nanoparticles, wherein the average particle size of the quantum dots is smaller than the average particle size of the metal oxide nanoparticles.

[0016] This disclosure provides a technology that can contribute to improving luminescence performance.

[0017] Figure 1 is a cross-sectional view of a display device according to one embodiment of the present invention. Figure 2 is a cross-sectional view showing a part of a display device according to a comparative example. Figure 3 is a cross-sectional view showing a part of the display device shown in Figure 1.

[0018] Embodiments of the present invention will be described below with reference to the drawings. The embodiments described below are more specific to any of the above aspects. The matters described below can be incorporated into each of the above aspects, individually or in combination.

[0019] Furthermore, the embodiments shown below illustrate configurations for realizing the technical concept of the present invention, and the technical concept of the present invention is not limited by the material, shape, and structure of the components described below. Various modifications can be made to the technical concept of the present invention within the technical scope defined by the claims described in the claims.

[0020] Please note that the drawings are schematic, and the relationships between dimensions in one direction and those in another, as well as the relationships between the dimensions of one component and those of other components, may differ from those in reality.

[0021] <1> Display Device Figure 1 is a cross-sectional view of a display device according to one embodiment of the present invention. The display device 1 shown in Figure 1 employs an active matrix drive method and is a display device capable of displaying color images.

[0022] The display device 1 includes a plurality of pixels arranged in the X and Y directions, as described later. Each pixel includes a first subpixel PXR, a second subpixel PXG, and a third subpixel PXB. Each of the first subpixel PXR, the second subpixel PXG, and the third subpixel PXB includes a light-emitting element and a pixel circuit. Here, the semiconductor included in the light-emitting element is assumed to be an inorganic material.

[0023] The display device 1 includes a substrate 11, an anode 12, a partition layer 13, a hole injection layer 14, a hole transport layer 15, a light-emitting layer 16, an electron transport layer 17, and a cathode 18. The anode 12, the hole injection layer 14, the hole transport layer 15, the light-emitting layer 16, the electron transport layer 17, and the portion of the cathode 18 facing the anode 12 constitute a light-emitting element.

[0024] In Figure 1, the X and Y directions are parallel to the display surface of the display device 1 and intersect each other. For example, the X and Y directions are orthogonal to each other. The Z direction is perpendicular to the X and Y directions, i.e., the thickness direction of the display device 1.

[0025] In one example, the substrate 11 includes an insulating substrate such as a glass substrate and an array portion provided on one of its main surfaces. In another example, the substrate 11 includes a semiconductor substrate such as a silicon substrate and an array portion provided on one of its surface regions. The array portion includes pixel circuits and wiring that supplies signals and power to the pixel circuits. The pixel circuits are arranged in the X and Y directions. Each pixel circuit includes transistors and capacitors as driving elements and switches, and wiring that connects them to each other. The transistors are, for example, field-effect transistors. Here, as an example, the driving element is a p-channel field-effect transistor and the switch is an n-channel field-effect transistor.

[0026] The anode 12 is, here, pixel electrodes arranged in the X and Y directions corresponding to pixel circuits on the substrate 11. Each anode 12 is connected to the drain of a driving element included in the corresponding pixel circuit.

[0027] When the substrate 11 is light-transmissive, the display device 1 may be a top emission type or a bottom emission type. When the substrate 11 is light-shielding, the display device 1 is a top emission type.

[0028] When the display device 1 is a bottom emission type, the anode 12 is a light-transmissive electrode. As the material of the light-transmissive electrode, for example, transparent conductive oxides such as indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide, and fluorine-doped tin oxide (FTO) can be used. The layer made of a transparent conductive oxide can be formed, for example, by a sputtering method.

[0029] When the display device 1 is a top emission type, the anode 12 preferably includes a light reflection layer. The light reflection layer is made of, for example, a single metal such as aluminum and silver or an alloy containing one or more of them. The layer made of a metal such as a single metal and an alloy can be formed, for example, by a vacuum evaporation method.

[0030] The anode 12 including the light reflection layer can further include a light transmission layer on the light reflection layer. As the material of the light transmission layer, for example, those exemplified as the material of the light-transmissive electrode can be used. Note that the material constituting the upper surface of the anode 12 preferably has a large work function.

[0031] The partition layer 13 is provided on the substrate 11 and the anode 12. The partition layer 13 has through holes at the positions of the anodes 12. Each of these through holes has a tapered shape from the upper opening toward the lower opening. Each of the anodes 12 is covered with the peripheral portion by the partition layer 13, and the central portion is exposed in the internal space of the through holes provided in the partition layer 13.

[0032] The partition layer 13 is made of an insulator. According to one example, the partition layer 13 is made of an inorganic insulator. According to another example, the partition layer 13 is made of a cured resin.

[0033] The positive hole injection layer 14 covers the central portion of the anode 12 within the through hole provided in the partition layer 13. The ionization energy of the positive hole injection layer 14 is typically larger compared to the work function of the anode 12.

[0034] The positive hole injection layer 14 is made of a positive hole injection material. The positive hole injection material is, for example, a composite of poly(3,4-ethylenedioxythiophene) and polystyrenesulfonic acid (PEDOT:PSS).

[0035] The thickness of the positive hole injection layer 14 is preferably in the range of 1 nm or more and 200 nm or less, and more preferably in the range of 5 nm or more and 50 nm or less.

[0036] The positive hole transport layer 15 covers the positive hole injection layer 14 within the through hole provided in the partition layer 13. Typically, the ionization energy of the positive hole transport layer 15 is larger compared to the ionization energy of the positive hole injection layer 14.

[0037] The positive hole transport layer 15 is made of a positive hole transport material. The positive hole transport material is, for example, poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4’(N-(4-sec-butylphenyl)diphenylamine)] (TFB) and polyvinylcarbazole (PVK).

[0038] The thickness of the positive hole transport layer 15 is preferably in the range of 1 nm or more and 200 nm or less, and more preferably in the range of 10 nm or more and 50 nm or less.

[0039] The light-emitting layer 16 covers the positive hole transport layer 15 within the through hole provided in the partition layer 13. The light-emitting layer 16 typically has an ionization energy larger compared to the ionization energy of the positive hole transport layer 15 and an electron affinity larger compared to the electron affinity of the positive hole transport layer 15.

[0040] The light-emitting layer 16 is made of a light-emitting material. The first sub-pixel PXR, the second sub-pixel PXG, and the third sub-pixel PXB have different light-emitting materials included in the light-emitting layer 16. For example, a light-emitting material that emits red light, a light-emitting material that emits green light, and a light-emitting material that emits blue light are used for the light-emitting layer 16 of the first sub-pixel PXR, the light-emitting layer 16 of the second sub-pixel PXG, and the light-emitting layer 16 of the third sub-pixel PXB, respectively.

[0041] According to one example, the light-emitting material is a quantum dot. The quantum dot contains, for example, Zn and Se, or Zn, Se, and S as main components. The quantum dot may contain elements other than these elements, but preferably does not contain at least one of Cd and P. Since the organic phosphorus compound is expensive and is easily oxidized in the air, the synthesis becomes unstable, which tends to cause an increase in cost, instability of fluorescence characteristics, and complexity of the manufacturing process. The quantum dot is, for example, a semiconductor particle having a core-shell structure.

[0042] The core is made of a semiconductor responsible for light emission. The emission spectrum of the quantum dot changes by changing the type of the semiconductor constituting the core and the particle diameter of the core. In order to obtain a quantum dot containing ZnSe with a narrower fluorescence half-width, it is preferable to dissolve sulfur (S) in the core. In this case, it is preferable to add thiol in the synthesis of Cu ２ Se. Also, in order to obtain a quantum dot with a narrower fluorescence half-width, it is more preferable to use Se-DDT / OLA m as the Se raw material. The thiol is not particularly limited, but the thiol is, for example, octadecanethiol: C １８ H ３７ SH, hexadecanethiol: C １６ H ３３ SH, tetradecanethiol: C １４ H ２９ SH, dodecanethiol: C １２ H ２５ SH, decanethiol: C １０ H ２１ SH, or octanethiol: C ８ H １７ SH.

[0043] The shell is a thin layer epitaxially grown on the surface of the core, having a thickness of one to four atoms. The shell contributes to improving and stabilizing the luminescence efficiency. The shell may have a single-layer structure or a multi-layer structure.

[0044] For example, if a core made of nanocrystals such as ZnSe or ZnSeS is coated with a shell, the fluorescence quantum yield can be further increased. Here, "nanocrystal" refers to nanoparticles having a particle size of several nanometers to tens of nanometers.

[0045] To obtain quantum dots containing zinc selenide (ZnSe) as a shell, for example, a copper chalcogenide such as Cu can be used as a precursor. ２ Se / Cu ２ S can be used. For example, by continuously adding S raw material to a reaction vessel containing copper chalcogenide, and then performing a Cu-Zn metal exchange, it is possible to obtain quantum dots containing ZnSe / ZnS as shells.

[0046] While the S raw material is not particularly limited, the following raw materials are representative examples: Octadecanthiol: C １８ H ３７ SH, Hexanedecanethiol:C １６ H ３３ SH, Tetradecanethiol:C １４ H ２９ SH, Dodecanethiol: C １２ H ２５ SH, Decanethiol: C １０ H ２１ SH, Octanethiol: C ８ H １７ SH and benzenethiol: C ６ H ５ A solution of sulfur dissolved in a high-boiling point solvent such as thiols (SH) or long-chain hydrocarbons like octadecene (S-ODE), or a solution of sulfur dissolved in a mixture of oleylamine and dodecanethiol (S-DDT / OLAm). The shell may be omitted.

[0047] One example of a quantum dot is a structure in which a core made of InP is covered with a first shell made of ZnSe, and this is then covered with a second shell made of ZnS. Such quantum dots emit red light when the particle size is large, and green light when the particle size is small.

[0048] Another example of a quantum dot has a structure in which a core made of ZnSeTe is covered with a first shell made of ZnSe, and this is then covered with a second shell made of ZnS. Such quantum dots emit blue light when the particle size is small.

[0049] Quantum dots may have ligands on the surface of their core-shell particles. The ligands are hydrocarbons with functional groups that contribute to improved resistance and prevention of aggregation in dispersions. However, the ligands may be at least partially absent in the display device 1.

[0050] The average particle size R1 of the quantum dots is smaller than the average particle size R2 of the metal oxide nanoparticles contained in the electron transport layer 17.

[0051] The average particle size R1 of the quantum dots is preferably in the range of 5 nm to 20 nm, and more preferably in the range of 5 nm to 15 nm.

[0052] The average particle diameter R1 of quantum dots can be measured, for example, by the following method using a FIB-TEM (Focused Ion Beam-Transmission Electron Microscope). First, the display device 1 is cut in a cross-section parallel to its thickness. The cross-section of the light-emitting layer 16 is obtained by cutting using a focused ion beam. Next, the cross-section of the light-emitting layer 16 is imaged using a transmission electron microscope. Then, the particle diameter of the quantum dots contained in the light-emitting layer 16 is measured in the acquired image. Here, the area of ​​each of the multiple quantum dots is determined, and the diameter of a circle having an area equal to this area, i.e., the equivalent circle diameter, is taken as the particle diameter of the quantum dot. The arithmetic mean of the obtained particle diameters is taken as the average particle diameter R1 of the quantum dots.

[0053] The thickness of the light-emitting layer 16 is preferably in the range of 5 nm to 60 nm, and more preferably in the range of 10 nm to 40 nm. For example, the light-emitting layer 16 has a thickness equivalent to one or two quantum dots.

[0054] The electron transport layer 17 covers the light-emitting layer 16 within through-holes provided in the partition layer 13. Typically, the electron transport layer 17 has a higher ionization energy and electron affinity compared to the light-emitting layer 16.

[0055] The electron transport layer 17 is made of an electron transport material. The electron transport material is made of metal oxide nanoparticles. The metal oxide nanoparticles include, for example, one or more of several types of nanoparticles, each containing bismuth oxide, cobalt oxide, copper oxide, magnesium oxide, nickel oxide, zinc oxide, magnesium-doped zinc oxide, and titanium oxide.

[0056] As mentioned above, the average particle size R2 of metal oxide nanoparticles is larger than the average particle size R1 of quantum dots.

[0057] The average particle size of the metal oxide nanoparticles is preferably in the range of 20 nm to 400 nm, and more preferably in the range of 30 nm to 200 nm. The average particle size of the metal oxide nanoparticles is obtained by the same method as the method for measuring the average particle size of quantum dots described above.

[0058] The ratio R1 / R2 of the average particle diameter R1 of quantum dots to the average particle diameter R2 of metal oxide nanoparticles is preferably in the range of 0.0125 or more and less than 1, and more preferably in the range of 0.025 or more and 0.5 or less.

[0059] The thickness of the electron transport layer 17 is preferably in the range of 1 nm to 200 nm, and more preferably in the range of 10 nm to 50 nm.

[0060] The cathode 18 covers the electron transport layer 17 and the exposed portion of the partition layer 13. In this case, the cathode 18 is a common electrode facing multiple anodes 12.

[0061] When the display device 1 is of the bottom emission type, it is preferable that the cathode 18 includes a light-reflecting layer. The light-reflecting layer is made of, for example, elemental metals such as aluminum and silver, or an alloy containing one or more of them. Layers made of elemental metals and alloys can be formed, for example, by vacuum deposition.

[0062] When the display device 1 is of the top-emission type, the cathode 18 is a light-transmitting electrode. As the material for the light-transmitting electrode, transparent conductive oxides such as indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide, and fluorine-doped tin oxide (FTO) can be used. The layer made of the transparent conductive oxide can be formed by, for example, a sputtering method.

[0063] The work function of the cathode 18 is typically smaller than that of the anode 12 and larger than that of the electron affinity of the electron transport layer 17. The cathode 18 may include a layer made of a material with a low work function, such as an MgAg alloy or an AlLi alloy, between the layer made of a metal or transparent conductive oxide and the electron transport layer 17.

[0064] The display device 1 may further include one or more other elements. For example, the display device 1 may further include an electron injection layer, such as a LiF layer, between the electron transport layer 17 and the cathode 18. The display device 1 may also further include a sealing film or sealing substrate that seals the light-emitting element.

[0065] Furthermore, the display device 1 employs a forward structure for the light-emitting element. The light-emitting element may also employ an inverse structure in which the stacking order of the layers it contains is reversed.

[0066] <2> Method of manufacturing the display device The display device 1 shown in Figure 1 can be manufactured, for example, by the following method.

[0067] First, a structure (substrate) is prepared that includes a substrate 11, an anode 12, a partition layer 13, a hole injection layer 14, and a hole transport layer 15.

[0068] Next, the light-emitting layer 16 is formed. As described above, the first subpixel PXR, the second subpixel PXG, and the third subpixel PXB have different light-emitting materials in their light-emitting layer 16. Therefore, the light-emitting layer 16 of the first subpixel PXR, the light-emitting layer 16 of the second subpixel PXG, and the light-emitting layer 16 of the third subpixel PXB are formed separately. Each of these light-emitting layers 16 can be formed, for example, by a printing method using a first ink composition containing quantum dots and a dispersion medium. Each of these light-emitting layers 16 can be formed as a printed layer by applying the first ink composition to the above structure (substrate) to form a coating film, and then drying the coating film to cure it. The light-emitting layer 16 can be formed using printing methods such as inkjet printing, spin coating, and slit coating. The particle size of the quantum dots in the first ink composition is the same as the particle size of the quantum dots in the light-emitting layer 16.

[0069] Quantum dots have a 50% particle size (D50) (hereinafter also referred to as "D50") when the volume-based cumulative particle size distribution is measured by dynamic light scattering, preferably within the range of 5 nm to 20 nm, and more preferably within the range of 10 nm to 18 nm. Here, "D50" refers to the particle size when the volume-based cumulative distribution corresponds to 50% when the volume-based cumulative distribution is measured by dynamic light scattering, with the particle size on the x-axis and the volume-based cumulative (%) on the y-axis.

[0070] Quantum dots have a 10% particle size (D10) (hereinafter also referred to as "D10") when the volume-based cumulative particle size distribution is measured by dynamic light scattering, preferably within the range of 5 nm to 15 nm, and more preferably within the range of 7 nm to 14 nm. Here, "D10" refers to the particle size when the volume-based cumulative distribution corresponds to 10% when the volume-based cumulative distribution is measured by dynamic light scattering, with the particle size on the horizontal axis and the volume-based cumulative (%) on the vertical axis.

[0071] Quantum dots have a 95% particle size (D95) (hereinafter also referred to as "D95") when the volume-based cumulative particle size distribution is measured by dynamic light scattering, preferably within the range of 5 nm to 50 nm, and more preferably within the range of 10 nm to 40 nm. Here, "D95" refers to the particle size when the volume-based cumulative distribution corresponds to 95% when the volume-based cumulative distribution is measured by dynamic light scattering, with the particle size on the x-axis and the volume-based cumulative (%) on the y-axis.

[0072] The dispersion medium contained in the first ink composition is, for example, octane. The proportion of quantum dots in the first ink composition is preferably in the range of 0.5% by mass or more and 20% by mass or less, and more preferably in the range of 1% by mass or more and 10% by mass or less.

[0073] Next, an electron transport layer 17 is formed. The electron transport layer 17 can be formed, for example, by a printing method using a second ink composition containing metal oxide nanoparticles and a dispersion medium. As described above, the average particle size R2 of the metal oxide nanoparticles is larger than the average particle size R1 of the quantum dots. The electron transport layer 17 can be formed as a printed layer by applying the second ink composition to the light-emitting layer 16 (substrate) to form a coating film, and then drying the coating film to harden it. In forming the electron transport layer 17, the application of the ink composition to the substrate can be carried out using printing methods such as inkjet printing, spin coating, and slit coating. The particle size of the metal oxide nanoparticles in the second ink composition is the same as the particle size of the metal oxide nanoparticles in the electron transport layer 17.

[0074] The metal oxide nanoparticles have a 50% particle size (D50) measured by dynamic light scattering, which is preferably in the range of 20 nm to 400 nm, and more preferably 30 nm to 200 nm.

[0075] The metal oxide nanoparticles, when their volume-based cumulative particle size distribution is measured by dynamic light scattering, have a 10% particle size (D10) that is preferably in the range of 10 nm to 30 nm, and more preferably 12 nm to 28 nm.

[0076] The metal oxide nanoparticles have a 95% particle size (D95) measured by dynamic light scattering, where the volume-based cumulative particle size distribution is preferably in the range of 30 nm to 500 nm, and more preferably 50 nm to 400 nm.

[0077] When the average particle size R2 of metal oxide nanoparticles is within the above range for each of the 10% particle size (D10), 50% particle size (D50), and 95% particle size (D95) in the volume-based cumulative particle size distribution, the metal oxide nanoparticles become less likely to penetrate between quantum dots. As a result, the electron transport layer 17 is more easily formed on the light-emitting layer 16. This effect is more pronounced when the average particle size R1 of the quantum dots is within the above range for each of the 10% particle size (D10), 50% particle size (D50), and 95% particle size (D95).

[0078] The dispersion medium contained in the second ink composition is, for example, ethanol. The proportion of quantum dots in the second ink composition is preferably in the range of 0.5% by mass or more and 5% by mass or less, and more preferably in the range of 1% by mass or more and 3% by mass or less.

[0079] Next, the cathode 18 is formed. The cathode 18 can be formed by vacuum deposition, sputtering, or a combination thereof. After that, the light-emitting element is sealed as necessary. In this way, the display device 1 shown in Figure 1 is obtained.

[0080] <3> Effect Figure 2 is a cross-sectional view showing a part of the display device according to the comparative example. Figure 2 shows only the hole transport layer 15, the light-emitting layer 16, and the electron transport layer 17. The display device according to the comparative example is the same as the display device 1 described above, except that the average particle size of the quantum dots is larger than the average particle size of the metal oxide nanoparticles contained in the electron transport layer. In the display device 1 according to the comparative example, the gaps in the light-emitting layer 16 are relatively large, so metal oxide nanoparticles 17A can easily enter the light-emitting layer 16. The metal oxide nanoparticles 17A that have entered the light-emitting layer 16 may reach the layer below the light-emitting layer 16, i.e., the hole transport layer 15. In this case, when an electric current is applied, electrons transported by the electron transport layer 17 reach the hole transport layer 15 and recombine with holes at the location of the hole transport layer 15, so no light emission occurs from the quantum dots. For this reason, high brightness cannot be achieved with the display device according to the comparative example.

[0081] Figure 3 is a cross-sectional view showing a part of the display device 1 shown in Figure 1. Figure 3 shows only the hole transport layer 15, the light-emitting layer 16, and the electron transport layer 17. As shown in Figure 3, in the above-described display device 1, the average particle diameter of the quantum dots 16A is smaller than the average particle diameter of the metal oxide nanoparticles 17A, so the gaps in the light-emitting layer 16 are smaller compared to the display device according to the comparative example above. Therefore, it is difficult for metal oxide nanoparticles 17A to penetrate into the light-emitting layer 16. Accordingly, with the above-described display device 1, a decrease in brightness due to the penetration of metal oxide nanoparticles 17A into the light-emitting layer 16 is less likely to occur. Thus, with the above-described display device 1, high brightness can be achieved.

[0082] The tests conducted in connection with the present invention are described below.

[0083] <1> Manufacturing of display devices <1.1> Example 1 First, a substrate equipped with a substrate 11 and an anode 12 as shown in Figure 1 was prepared.

[0084] Next, a hole-injection layer 14 was formed. Specifically, first, the composition containing PEDOT:PSS was stirred at room temperature for 30 minutes, and then filtered through a PVDF (polyvinylidene fluoride) filter with a pore size of 0.45 μm. Next, a coating film consisting of 600 μL of the filtered composition was formed on the anode 12 by spin coating. The rotation speed was set to 4000 rpm and the rotation time to 30 seconds. Next, the obtained coating film was dried and cured at 200°C for 10 minutes under atmospheric pressure. After that, it was wiped with methanol. The thickness of the obtained hole-injection layer 14 was 30 nm.

[0085] Next, a hole transport layer 15 was formed. Specifically, first, the composition containing TFB was stirred at room temperature for 30 minutes, and then the composition was filtered through a PTFE (polytetrafluoroethylene) filter having a pore size of 0.45 μm. Next, a coating film consisting of 220 μL of the filtered composition was formed on the hole injection layer 14 by spin coating. The rotation speed was set to 300 rpm and the rotation time to 4 seconds. Next, spin coating was further performed with a rotation speed of 1500 rpm and a rotation time of 30 seconds. Then, the obtained coating film was dried and cured at 200°C for 60 minutes under atmospheric pressure. After that, it was wiped with methanol. The thickness of the obtained hole transport layer 15 was 20 nm.

[0086] Next, an emissive layer 16 was formed on the hole transport layer 15. Specifically, first, a composition containing quantum dots with an average particle size of 16.0 nm that emit blue color was stirred at room temperature for 30 minutes, and then the composition was filtered through a PTFE filter with a pore size of 0.45 μm. Next, a coating film consisting of 300 μL of the filtered composition was formed on the hole transport layer 15 by spin coating. The rotation speed was set to 300 rpm and the rotation time to 4 seconds. Next, spin coating was further performed with a rotation speed of 1500 rpm and a rotation time of 30 seconds. Finally, the obtained coating film was dried and cured in a nitrogen environment at 80°C for 10 minutes. After that, it was wiped with methanol. The thickness of the obtained emissive layer 16 was 20 nm. Furthermore, when the volume-based cumulative particle size distribution of the quantum dots was measured by dynamic light scattering, the D10, D50, and D95 ​​values ​​were 13.4 nm, 15.8 nm, and 20.6 nm, respectively. The quantum dots used contained ZnSeTe as the core, ZnSe as the shell, and 1-dodecanethiol as the ligand.

[0087] Next, an electron transport layer 17 was formed on the light-emitting layer 16. Specifically, first, a composition containing magnesium-doped zinc oxide with a magnesium doping amount of 20% and a dispersion medium was stirred at room temperature for 30 minutes, and then the composition was filtered through a PTFE filter having a pore size of 0.45 μm. As the dispersion medium, a mixture containing 70% by mass and 30% by mass of ethanol and octanol was used. The concentration of magnesium-doped zinc oxide in the above composition was 1.5% by mass. Next, a coating film consisting of 300 μL of the filtered composition was formed on the light-emitting layer 16 by spin coating. The rotation speed was 4000 rpm and the rotation time was 30 seconds. Next, the obtained coating film was dried and cured in a nitrogen environment at 50°C for 5 minutes. After that, it was wiped with ethanol and then with methanol. The thickness of the obtained electron transport layer 17 was 30 nm. Furthermore, when the volume-based cumulative particle size distribution of magnesium-doped zinc oxide was measured by dynamic light scattering, the D10, D50, and D95 ​​values ​​were 26.6 nm, 42.8 nm, and 352.1 nm, respectively.

[0088] Next, a cathode 18 was formed on the electron transport layer 17. In this manner, the display device according to Example 1 was manufactured.

[0089] The thickness of each layer was measured using a stylus-type step gauge (ET4000A, manufactured by Kosaka Laboratory Co., Ltd.). A particle size analyzer (Nanotrac® UPA-EX150, manufactured by Nikkiso Co., Ltd.) was used to measure the cumulative particle size distribution.

[0090] <1.2> Example 2 A display device according to Example 2 was manufactured in the same manner as the method for manufacturing a display device according to Example 1, except that the following composition was used as the composition of the electron transport layer 17. In this example, a composition was used that contained magnesium-doped zinc oxide with a magnesium doping amount of 20% and 1-octanol as a dispersion medium, with a magnesium-doped zinc oxide concentration of 2.5% by mass. This magnesium-doped zinc oxide had an average particle diameter of 22.0 nm, and when the volume-based cumulative particle size distribution was measured by dynamic light scattering, the D10, D50, and D95 ​​values ​​were 14.5 nm, 22.0 nm, and 65.3 nm, respectively.

[0091] <1.3> Comparative Example A display device according to the comparative example was manufactured in the same manner as the method for manufacturing a display device according to Example 1, except that the following composition was used as the composition of the electron transport layer 17. In this example, a composition containing zinc oxide and ethanol as a dispersion medium was used. The zinc oxide had an average particle size of 3.5 nm, and the D10, D50, and D95 ​​values ​​when the volume-based cumulative particle size distribution was measured by dynamic light scattering were 2.4 nm, 3.4 nm, and 6.2 nm, respectively.

[0092] <2> For each of the display devices according to Evaluation Example 1, Example 2, and Comparative Example 1, the maximum luminance value obtained when an electric charge was applied and the light-emitting element was made to emit light was measured. A luminance chromaticity uniformity meter (UA-1000, manufactured by Topcon Techno House Co., Ltd.) was used for the measurement.

[0093] The results of the above evaluation are shown in Table 1 below.

[0094]

[0095] As shown in Table 1, the display devices according to Examples 1 and 2 had higher brightness compared to the display device according to the comparative example. Furthermore, in the display devices according to Examples 1 and 2, the difference between the average particle size of the quantum dots contained in the light-emitting layer 16 and the average particle size of the metal oxide nanoparticles contained in the electron transport layer 17 could be observed using a transmission electron microscope (HT7800, manufactured by Hitachi High-Tech Corporation).

[0096] 1...Display device, 11...Substrate, 12...Anode, 13...Block layer, 14...Hole injection layer, 15...Hole transport layer, 16...Light-emitting layer, 16A...Quantum dot, 17...Electron transport layer, 17A...Metal oxide nanoparticle, 18...Cathode, PXB...Third subpixel, PXG...Second subpixel, PXR...First subpixel.

Claims

1. A display device comprising a substrate, an anode, a hole transport layer, a light-emitting layer, an electron transport layer, and a cathode in this order, wherein the light-emitting layer is made of quantum dots, the electron transport layer is made of metal oxide nanoparticles, and the average particle diameter R1 of the quantum dots is smaller than the average particle diameter R2 of the metal oxide nanoparticles.

2. The display device according to claim 1, wherein the ratio R1 / R2 of the average particle diameter R1 of the quantum dots to the average particle diameter R2 of the metal oxide nanoparticles is in the range of 0.0125 or more and less than 1.

3. The display device according to claim 1 or 2, wherein the average particle size R2 of the metal oxide nanoparticles is in the range of 20 nm to 400 nm.

4. The display device according to any one of claims 1 to 3, wherein the average particle diameter R1 of the quantum dots is in the range of 5 nm or more and 20 nm or less.

5. The display device according to any one of claims 1 to 4, wherein the thickness of the light-emitting layer is in the range of 5 nm or more and 60 nm or less.

6. The display device according to any one of claims 1 to 5, wherein the metal oxide nanoparticles comprise one or more of a plurality of nanoparticles, each containing bismuth oxide, cobalt oxide, copper oxide, magnesium oxide, nickel oxide, zinc oxide, magnesium-doped zinc oxide, and titanium oxide.

7. The display device according to any one of claims 1 to 6, wherein the average particle diameter R1 of the quantum dots is such that the 10% particle diameter D10 in the volume-based cumulative particle size distribution is in the range of 7 nm to 14 nm, the 50% particle diameter D50 is in the range of 10 nm to 18 nm, and the 95% particle diameter D95 is in the range of 10 nm to 40 nm.

8. The display device according to any one of claims 1 to 7, wherein the average particle size R2 of the metal oxide nanoparticles is such that the 10% particle size D10 in the volume-based cumulative particle size distribution is in the range of 12 nm to 28 nm, the 50% particle size D50 is in the range of 30 nm to 200 nm, and the 95% particle size D95 is in the range of 50 nm to 400 nm.

9. A method for manufacturing a display device, comprising: forming a cured coating film made of a first ink composition containing quantum dots on a substrate layer; and forming a cured coating film made of a second ink composition containing metal oxide nanoparticles as an electron transport layer on the cured coating film, wherein the average particle diameter R1 of the quantum dots is smaller than the average particle diameter R2 of the metal oxide nanoparticles.

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