WO2024180583A1

WO2024180583A1 - Nanoparticles, light emitting element, and display device

Info

Publication number: WO2024180583A1
Application number: PCT/JP2023/006972
Authority: WO
Inventors: 久幸内海; 峻之中; 誠野添
Original assignee: シャープディスプレイテクノロジー株式会社
Priority date: 2023-02-27
Filing date: 2023-02-27
Publication date: 2024-09-06

Abstract

This light emitting element (3) comprises an anode (31), a cathode (36), and function layers (32) positioned between the anode and the cathode. At least one of the function layers includes nanoparticles (4). The nanoparticles contain nickel oxide. Moreover, an infrared absorption spectrum obtained through infrared spectroscopy of the nanoparticles has an absorption peak at a wave number corresponding to nitrate ions.

Description

Nanoparticles, Light-emitting devices, Display devices

This disclosure relates to nanoparticles, a light-emitting device that includes the nanoparticles in a functional layer, and a display device that includes the light-emitting device.

Patent Document 1 discloses an example in which nanoparticles are used as a material for functional layers, including a charge transport layer, of a light-emitting element.

Japanese Patent Application Publication No. 2019-522367

When forming a functional layer that contains nanoparticles as a material, a method of forming the film by applying a solution in which the nanoparticles are dispersed is considered. In this case, in order to improve the characteristics of the formed functional layer, it is necessary to ensure the dispersibility of the nanoparticles in the solution and reduce the aggregation of the nanoparticles.

The nanoparticles according to one embodiment of the present disclosure are nanoparticles used in the functional layer of a light-emitting element, contain nickel oxide, and have an absorption peak at a wave number corresponding to nitrate ions in an infrared absorption spectrum obtained by infrared spectroscopy.

A light-emitting element according to another aspect of the present disclosure includes an anode, a cathode, and a functional layer located between the anode and the cathode and including at least a light-emitting layer, and at least one of the functional layers includes nickel oxide and nanoparticles whose infrared absorption spectrum obtained by infrared spectroscopy has an absorption peak at a wave number corresponding to nitrate ions.

To realize nanoparticles with reduced aggregation in solution and light-emitting devices equipped with functional layers containing the nanoparticles.

1 is a schematic side cross-sectional view of a display device according to an embodiment. 1 is a schematic plan view of a display device according to an embodiment.

[Embodiment]
<Display Device>
Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the same components are denoted by the same reference numerals and the description thereof will be omitted.

FIG. 2 is a schematic plan view of the display device 1 according to this embodiment. The display device 1 is a device that can be used, for example, as a display for a television or a smartphone. The display device 1 comprises a display unit DA and a frame unit NA formed on the outer periphery of the display unit DA. The display device 1 performs display on the display unit DA by controlling light emission from each of a number of light-emitting elements (described below) formed in the display unit DA. Drivers and the like for driving each of the multiple light-emitting elements of the display unit DA may be formed in the frame unit NA.

The structure of the display unit DA of the display device 1 according to this embodiment will be described in more detail with reference to FIG. 1. FIG. 1 is a schematic side cross-sectional view of the display device 1 according to this embodiment, and is a cross-sectional view taken along line I-I in FIG. 2. In particular, FIG. 1 is a diagram showing a cross section passing through red sub-pixel SPR, green sub-pixel SPG, and blue sub-pixel SPB, which will be described later, in a plan view of the display device 1 according to this embodiment. In this disclosure, the direction from the substrate 2 to the cathode 36, which will be described later, of the display device 1 may be referred to as "upper", and the opposite direction may be referred to as "lower".

<Substrate>
As shown in Fig. 1, the display device 1 includes a substrate 2. The substrate 2 is formed at a position overlapping the display section DA and the frame section NA in a plan view of the display device 1, in other words, formed across the display section DA and the frame section NA. The upper surface of the substrate 2 may be substantially parallel to the display surface of the display device 1, in other words, the plan view of the substrate 2 may be substantially the same as the plan view of the display device 1. The substrate 2 may be a rigid substrate such as a glass substrate, or may be a flexible substrate including a film substrate including a PET film. In the latter case, the display device 1 may be a flexible device.

The display device 1 includes a red subpixel SPR, a green subpixel SPG, and a blue subpixel SPB at a position overlapping the display section DA in a planar view of the substrate 2. Red light-emitting elements 3R, green light-emitting elements 3G, and blue light-emitting elements 3B, which will be described later, are formed in the red subpixel SPR, green subpixel SPG, and blue subpixel SPB, respectively. The substrate 2 may include a pixel circuit (not shown) including transistors and the like that are connected to the anodes 31, which will be described later, of each of the light-emitting elements described above. The substrate 2 may also include a driver (not shown) that drives each pixel circuit at a position overlapping the frame section NA in a planar view.

The substrate 2 may be manufactured by forming circuits such as transistors and drivers on a glass substrate. Alternatively, the glass substrate may then be peeled off and replaced with a film substrate or the like to form the substrate 2 into a flexible substrate. In addition, in the manufacturing process of the display device 1, a plurality of light-emitting elements 3 (described below) may be formed on a large glass substrate, and then the glass substrate may be cut into a plurality of substrates 2 to manufacture a plurality of display devices 1.

<Light Emitting Device: Overview>
The display device 1 includes a light-emitting element 3. The light-emitting element 3 includes an anode 31 and a cathode 36 in this order on a substrate 2. Furthermore, the light-emitting element 3 includes, as functional layers located between the anode 31 and the cathode 36, a hole injection layer 32, a hole transport layer 33, a light-emitting layer 34, and an electron transport layer 35, in this order from the substrate 2 side. Note that this embodiment is not limited to this, and the light-emitting element 3 may include, in this order from the substrate 2 side, a cathode, an electron transport layer, a light-emitting layer, a hole transport layer, a hole injection layer, and an anode. Furthermore, the light-emitting element 3 may further include an electron injection layer as a functional layer between the electron transport layer 35 and the cathode 36.

The display device 1 further includes a bank BK on the substrate 2. The bank BK includes an insulating resin material, such as polyimide. The bank BK divides the anode 31 and the light-emitting layer 34 of the light-emitting element 3 into red subpixels SPR, green subpixels SPG, and blue subpixels SPB in a plan view of the substrate 2. In this embodiment, the light-emitting layer 34 is particularly divided by the bank BK into a red light-emitting layer 34R of the red subpixel SPR, a green light-emitting layer 34G of the green subpixel SPG, and a blue light-emitting layer 34B of the blue subpixel SPB. On the other hand, although the anode 31 is divided for each subpixel as described above, it may have the same configuration regardless of the subpixels formed.

In this embodiment, the hole injection layer 32, the hole transport layer 33, the electron transport layer 35, and the cathode 36 are formed in common to the multiple sub-pixels described above. However, the hole injection layer 32, the hole transport layer 33, the electron transport layer 35, and the cathode 36 may also be partitioned for each sub-pixel by the bank BK.

In this embodiment, the light-emitting element 3 includes a red light-emitting element 3R located in the red sub-pixel SPR, a green light-emitting element 3G located in the green sub-pixel SPG, and a blue light-emitting element 3B located in the blue sub-pixel SPB. Therefore, the red light-emitting element 3R, the green light-emitting element 3G, and the blue light-emitting element 3B are partially separated from each other by the bank BK.

In particular, the red light-emitting element 3R includes an anode 31, a hole injection layer 32, a hole transport layer 33, a red light-emitting layer 34R, an electron transport layer 35, and a cathode 36 formed at a position overlapping with the red subpixel SPR in a planar view of the substrate 2. The green light-emitting element 3G includes an anode 31, a hole injection layer 32, a hole transport layer 33, a green light-emitting layer 34G, an electron transport layer 35, and a cathode 36 formed at a position overlapping with the green subpixel SPG in a planar view of the substrate 2. The blue light-emitting element 3B includes an anode 31, a hole injection layer 32, a hole transport layer 33, a blue light-emitting layer 34B, an electron transport layer 35, and a cathode 36 formed at a position overlapping with the blue subpixel SPB in a planar view of the substrate 2.

The bank BK may be formed in a position that covers the end of each anode 31. In other words, the bank BK may have an opening in a part that overlaps with each anode 31 in a plan view of the substrate 2. In this case, the bank BK can reduce the effect of electric field concentration at the end of the anode 31 in each light-emitting element on the injection of holes from the anode 31 to the light-emitting layer 34.

<Light-emitting element: anode and cathode>
At least one of the anode 31 and the cathode 36 is a transparent electrode that transmits visible light. As the transparent electrode, for example, an oxide conductor such as ITO, InZnO, SnO ₂ , or FTO, or a semi-transparent metal thin film containing a metal such as Mg or Ag or an alloy thereof may be used. In addition, either the anode 31 or the cathode 36 may be a reflective electrode. The reflective electrode may contain a metal material with a high reflectance of visible light, and the metal material may be, for example, Al, Ag, Cu, or Au alone or an alloy thereof.

In this embodiment, the anode 31 is a reflective electrode, and the cathode 36 is a transparent electrode. In this case, the display device 1 is a top-emission type device in which light from the light-emitting element 3 is extracted from the side opposite the substrate 2.

For example, the anodes 31 are partitioned into sub-pixels by the banks BK as described above. Each anode 31 is electrically connected to a corresponding pixel circuit. Meanwhile, the cathode 36 is formed in common to a plurality of sub-pixels, and a substantially constant voltage is applied to it. Therefore, the display device 1 drives each light-emitting element individually by controlling the voltage applied to each anode 31 via each pixel circuit using a driver. When the anode 31 in each light-emitting element is driven, holes are generated from the anode 31 and electrons are generated from the cathode 36.

The anode 31 and the cathode 36 may each be formed by depositing a thin film of the above-mentioned material by a sputtering method, a vapor deposition method, or the like. In particular, the anode 31 may be formed so as to be electrically connected to the pixel circuit of the substrate 2, and may be patterned for each subpixel.

<Light-emitting element: hole injection layer: nanoparticles>
The hole injection layer 32 is a layer that injects holes generated in the anode 31 into the light emitting layer 34. The hole injection layer 32 according to this embodiment contains at least one nanoparticle 4 as a hole transport material. The nanoparticle 4 contains nickel oxide (NiO) and further contains nitrate ions (NO ₃ ⁻ ). In this disclosure, the chemical formulas are representative examples. In this disclosure, the composition ratios described in the chemical formulas do not necessarily have to be stoichiometric, in which the composition of the actual compound is exactly as shown in the chemical formula.

A more specific configuration of the nanoparticles 4 according to this embodiment is explained by analysis using an FT-IR (Fourier transform infrared spectrophotometer). An FT-IR is a spectrometer that measures the infrared absorption spectrum of a sample by irradiating the sample with infrared light and measuring the transmitted or reflected infrared light. In general, the wave number (wavelength) of infrared light absorbed by a substance when infrared light is irradiated onto the substance varies depending on the functional groups or ions contained in the substance. Therefore, by identifying the wave number of the absorption peak contained in the infrared absorption spectrum measured by analysis using an FT-IR, the functional groups or ions contained in the sample can be identified, and thus at least a portion of the composition of the sample can be identified.

The infrared absorption spectrum obtained by infrared spectroscopy using FT-IR with the nanoparticles 4 according to this embodiment as a sample has an absorption peak at a wave number corresponding to nitrate ions. For example, the absorption spectrum obtained by analysis using FT-IR with the nanoparticles 4 according to this embodiment as a sample has an absorption peak at a wave number of about 1400 cm ⁻¹ .

<Light-emitting element: hole injection layer: nanoparticle synthesis method>
A method for synthesizing the nanoparticles 4 according to this embodiment will be described below. In the method for synthesizing the nanoparticles 4, first, a precursor aqueous solution is prepared. For example, 0.05 mmol of nickel nitrate hexahydrate (Ni(NO ₃ ) _2.6H ₂ O) and 20 mL of pure water are mixed and stirred to prepare the precursor aqueous solution.

Then, an alkaline aqueous solution is added to the obtained precursor aqueous solution. For example, a 10 mol/L aqueous solution of sodium hydroxide is added to the precursor aqueous solution until the pH becomes 9 to 11, for example 10. As a result, a precipitate, for example a green precipitate, is obtained.

Then, the resulting precipitate is washed. For example, the washing process of adding pure water to the precipitate, centrifuging it, and then removing the supernatant is repeated three times.

Then, the washed precipitate is dried. For example, the washed precipitate is dried at 60°C to 100°C, for example, 80°C. As a result, a powder, for example, a green powder, is obtained.

Then, the obtained powder is fired. For example, the obtained powder is fired at 250°C to 300°C for 2 hours to 6 hours, for example, at 270°C for 2 hours. As a result, a powder of black nanoparticles 4 is obtained. The hole injection layer according to this embodiment may be formed by film formation by a coating method or the like using a solution containing the nanoparticles 4 obtained by the above synthesis process.

In consideration of the synthesis process of nanoparticles 4, it is presumed that the nitrate ions contained in nanoparticles 4 originate from nickel nitrate hexahydrate contained in the precursor aqueous solution. More specifically, it is presumed that the nitrate ions contained in nanoparticles 4 are part of a by-product obtained by the reaction of nickel nitrate hexahydrate with an alkaline aqueous solution.

<Light-emitting element: hole transport layer and electron transport layer>
Returning to the description of each layer of the light-emitting element 3, the hole transport layer 33 is a layer that transports holes injected from the anode 31 into the hole injection layer 32 to the light-emitting layer 34 side. As the material of the hole transport layer 33, an organic or inorganic hole transport material having hole transport properties that has been conventionally adopted in field injection type light-emitting elements and the like can be used. Examples of the hole transport material include poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-4-sec-butylphenyl))diphenylamine)] (abbreviated as "TFB"), poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] (abbreviated as "poly-TPD"), polyvinylcarbazole (abbreviated as "PVK"), and the like. The hole transport material may contain only one type of the above-mentioned materials, or may contain two or more types as appropriate.

The electron transport layer 35 is a layer that transports electrons generated in the cathode 36 to the light-emitting layer 34. The electron transport layer 35 can use, as an electron transport material, organic or inorganic materials having electron transport properties that have been conventionally used in field injection type light-emitting elements and the like. The electron transport material may include zinc oxide (ZnO) and magnesium zinc oxide (MgZnO). The electron transport material may include only one type of the above-mentioned materials, or may include two or more types as appropriate.

The hole transport layer 33 and the electron transport layer 35 may each be formed by, for example, applying a solution in which the above-mentioned materials are dispersed in a solvent by a spin coating method or the like, and then volatilizing the solvent by baking.

<Light-emitting element: light-emitting layer>
The light-emitting layer 34 includes a light-emitting material that emits light when excited by excitons generated by recombination of holes injected from the anode 31 and electrons injected from the cathode 36. The light-emitting layer 34 includes, for example, a plurality of quantum dots 5 as the light-emitting material. Each quantum dot 5 included in the light-emitting layer 34 may have a core/shell structure including a core that emits light by the above-mentioned excitons and a shell formed around the core to protect the core. In this embodiment, the light-emitting layer 34 may include an organic or inorganic ligand that coordinates to the quantum dot 5 by forming a coordinate bond with the outermost surface of each quantum dot 5.

In this disclosure, "quantum dot" means a dot with a maximum width of 100 nm or less. The shape of the quantum dot 5 may be within a range that satisfies the above maximum width, and is not particularly restricted, and is not limited to a spherical three-dimensional shape (circular cross-sectional shape). The shape of the quantum dot 5 may be, for example, a polygonal cross-sectional shape, a rod-like three-dimensional shape, a branch-like three-dimensional shape, a three-dimensional shape with unevenness on the surface, or a combination of these.

The quantum dot 5 is typically made of a semiconductor. The semiconductor may have a certain band gap. The semiconductor may be any material capable of emitting light, and may include at least the materials described below. The semiconductor may emit blue, green, and red light, respectively. The semiconductor may include at least one selected from the group consisting of II-VI compounds, III-V compounds, chalcogenides, and perovskite compounds. The II-VI compounds refer to compounds containing II and VI elements, and the III-V compounds refer to compounds containing III and V elements. The II elements may include Group 2 and Group 12 elements, the III elements may include Group 3 and Group 13 elements, the V elements may include Group 5 and Group 15 elements, and the VI elements may include Group 6 and Group 16 elements.

The II-VI compound includes, for example, at least one selected from the group consisting of MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, and HgTe.

The III-V compound includes, for example, at least one selected from the group consisting of GaAs, GaP, InN, InAs, InP, and InSb.

Chalcogenides are compounds that contain elements from group VI A(16), such as CdS or CdSe. Chalcogenides may also include mixed crystals of these.

The perovskite compound has a composition represented by the general formula CsPbX ₃ , for example. The constituent element X includes at least one element selected from the group consisting of Cl, Br and I, for example.

Here, the numbering of element groups using Roman numerals is based on the old IUPAC (International Union of Pure and Applied Chemistry) system or the old CAS (Chemical Abstracts Service) system, and the numbering of element groups using Arabic numerals is based on the current IUPAC system.

In this embodiment, the red light-emitting layer 34R includes red quantum dots 5R that emit red light. The green light-emitting layer 34G includes green quantum dots 5G that emit green light. The blue light-emitting layer 34B includes blue quantum dots 5B that emit blue light. The red quantum dots 5R, green quantum dots 5G, and blue quantum dots 5B may each have the same configuration as one another, except for the light-emitting color.

As a result, the display device 1 individually controls the red light emitting element 3R, the green light emitting element 3G, and the blue light emitting element 3B to extract red light from the red subpixel SPR, green light from the green subpixel SPG, and blue light from the blue subpixel SPB. This allows the display device 1 to perform full color display in the display area DA.

In this disclosure, blue light is, for example, light having a central emission wavelength in the wavelength band of 380 nm or more and 500 nm or less. Green light is, for example, light having a central emission wavelength in the wavelength band of more than 500 nm and less than 600 nm. Red light is, for example, light having a central emission wavelength in the wavelength band of more than 600 nm and less than 780 nm.

Note that the light-emitting layer 34 is not limited to the above-mentioned configuration, so long as it contains a light-emitting material that emits light due to holes from the anode 31 and electrons from the cathode 36. For example, the light-emitting layer 34 may contain an organic light-emitting material including an organic fluorescent material or an organic phosphorescent material. In other words, the light-emitting element 3 may be an OLED element (organic EL element), and the display device 1 may be an OLED display.

The light-emitting layer 34 may be formed for each subpixel by patterning using a lift-off method. For example, a photosensitive resin is first formed in common for a plurality of subpixels, and a layer of the photosensitive resin is formed only in the subpixels other than the subpixel for which the light-emitting layer 34 is formed by photolithography or the like. Next, a solution in which quantum dots 5 of a certain light-emitting color are dispersed is formed in common for the plurality of subpixels. Next, the photosensitive resin is removed with an appropriate developer, and the layer containing the quantum dots 5 formed on the layer is removed together with the photosensitive resin layer. In this way, a layer containing a specific quantum dot 5 can be formed only in a specific subpixel. The lift-off method is repeatedly performed for each subpixel to form the light-emitting layer 34. However, the method for forming the light-emitting layer 34 is not limited to the above, and the light-emitting layer 34 may be formed, for example, by applying a solution in which the quantum dots 5 are dispersed by an inkjet method or the like.

<Display device: Supplementary note>
The light-emitting element 3 according to this embodiment can be manufactured by forming, in this order, an anode 31, a hole injection layer 32 containing nanoparticles 4, a hole transport layer 33, a light-emitting layer 34, an electron transport layer 35, and a cathode 36 on a substrate 2. The method for forming each layer of the light-emitting element 3 may be performed by the method described above.

In this embodiment, the red light-emitting element 3R, the green light-emitting element 3G, and the blue light-emitting element 3B each have the same layered structure except for the light-emitting layer 34, but this is not limited to the above. In this embodiment, it is sufficient that at least one of the red light-emitting element 3R, the green light-emitting element 3G, and the blue light-emitting element 3B has a hole injection layer 32 containing nanoparticles 4. In other words, the red light-emitting element 3R, the green light-emitting element 3G, and the blue light-emitting element 3B may have different layered structures.

The display device 1 may include, on the upper layer of the light-emitting element 3, an organic or inorganic sealing layer for sealing the light-emitting element 3, or a capping layer for improving the efficiency of light extraction from the light-emitting element 3. The display device 1 may also include, on the upper layer of the light-emitting element 3, a touch panel unit for causing the display device 1 to function as a touch panel display.

<Physical property evaluation: synthesis of nanoparticles>
In order to evaluate the physical properties of the nanoparticles 4 according to this embodiment, the nanoparticles according to each of Examples 1 to 6 described below and the nanoparticles according to the Comparative Example were synthesized. Furthermore, light-emitting devices having the nanoparticles according to each of the Examples and the Comparative Example in the hole injection layer were manufactured, and the characteristics were evaluated as follows.

The nanoparticles of Examples 1 to 6 were synthesized according to the synthesis method of nanoparticles 4 described above. In the synthesis process of nanoparticles of Examples 1 to 6, only the total weight of the nanoparticles to be synthesized was different, and the ratio of each material was the same. In particular, in the synthesis process of nanoparticles of Examples 1 to 6, only the weight of the precursor aqueous solution prepared first was different, and the ratio of each material in the precursor aqueous solution was approximately constant regardless of the Example. The nanoparticles of the comparative example are nanoparticles containing nickel oxide that were synthesized by a method different from the synthesis method of nanoparticles 4 described above, in particular, a method that does not use nickel nitrate as a material.

<Physical property evaluation: Analysis of nanoparticles using FT-IR and TG-DTA>
The physical properties of the nanoparticles according to each of the examples and comparative examples were measured by the following methods.

First, infrared spectroscopy was performed using FT-IR on the nanoparticles of each example and comparative example to obtain the infrared absorption spectrum of each nanoparticle.

Next, the nanoparticles of each example and comparative example were analyzed using a TG-DTA (thermogravimetric differential thermal analyzer). The analysis using TG-DTA is explained below.

The TG-DTA is equipped with a furnace for heating the sample and a measuring device for measuring the weight of the sample. For example, in an analysis using the TG-DTA, the TG-DTA is used to increase the ambient temperature of the sample at a predetermined heating rate, thereby heating the sample and measuring the change in the weight of the sample.

Heating a sample may cause some of the material to be released, decomposed, or reduced, resulting in a loss in weight. In general, the ambient temperature at which release, decomposition, or reduction of materials occurs when a sample is heated depends on the type of material being released, decomposed, or reduced, and is roughly constant.

As a result, by measuring the ambient temperature at which the weight of a sample is reduced through analysis using TG-DTA, it is possible to identify the material contained in the sample. Furthermore, by measuring the weight of a sample that is reduced within a specified range of ambient temperatures through analysis using TG-DTA, it is possible to estimate the proportion of a specific material in the sample.

In the analysis using TG-DTA for the nanoparticles of each of the examples and comparative examples, the nanoparticles were heated by raising the ambient temperature from room temperature by 10°C per minute. In the analysis using TG-DTA, the ambient temperature at which nitrate ions are generally released, decomposed, or reduced from a sample containing nitrate ions is generally 309°C to 411°C. Therefore, if the weight of the nanoparticles decreases while the ambient temperature of the nanoparticles being heated in the analysis using TG-DTA is between 309°C and 411°C, it can be assumed that the nanoparticles contain nitrate ions. In addition, the proportion of the weight of the nanoparticles that is reduced while the ambient temperature is between 309°C and 411°C can be estimated by measuring the proportion of the weight of the nanoparticles that is reduced while the ambient temperature is between 309°C and 411°C, based on the weight of the nanoparticles at room temperature.

<Physical property evaluation: Evaluation of nanoparticle dispersibility>
Next, the dispersibility of the nanoparticles according to each Example and Comparative Example was evaluated. In the evaluation of the dispersibility, first, an aqueous solution was prepared by adding 30 mg of nanoparticles to 1 ml of pure water for each Example and Comparative Example. Then, the aqueous solution was irradiated with ultrasonic waves of 45 kHz for 30 minutes.

　Next, the dispersibility of the nanoparticles in each Example and Comparative Example was evaluated by measuring whether or not each nanoparticle passed through a filtration filter under the following conditions.

10 uL of the aqueous solution prepared in each Example and Comparative Example was taken, and 4 mL of pure water was added to the solution, and the absorption spectrum was measured using a spectrophotometer (Hitachi High-Tech Corporation, Model: U-3900). In particular, the absorbance for ultraviolet light with a wavelength of 250 nm was measured.

Then, the remaining solution of the aqueous solution was filtered using a filter with a pore size of 0.45 μm, 10 uL of the filtrate was collected, and the absorption spectrum of the solution was measured after adding 4 mL of pure water. In this measurement, the absorbance for ultraviolet light with a wavelength of 250 nm was also measured.

The absorbance of the aqueous solution before filtration to ultraviolet light of a wavelength of 250 nm measured in each Example and Comparative Example is designated as A ₁ , and the absorbance of the aqueous solution after filtration to ultraviolet light of a wavelength of 250 nm is designated as A _2. The filtration transmittance, which is the transmittance of the nanoparticles through the filtration filter in each Example and Comparative Example, is designated as _RF . In this embodiment, the filtration transmittance _RF was evaluated based on the following formula.

R _F = A ₂ ÷ A ₁ × 100
In general, when nanoparticles have low dispersibility in water, they may aggregate in an aqueous solution. When multiple nanoparticles aggregate to form a group, the total particle size of the group is approximately 1/100th of that of the individual nanoparticles. Therefore, the dispersibility of nanoparticles in water can be evaluated by whether or not the nanoparticles in the aqueous solution pass through a filter with a predetermined pore size. When the particles pass through a filtration filter, the nanoparticles can be evaluated as having high dispersibility in an aqueous solution.

In this embodiment, a nanoparticle size measuring device manufactured by Microtrac-Bell (model number: Nanotrac Wave II-UT151) was used for particle size distribution measurement (median diameter D50 (nm)). Frequency analysis using dynamic light scattering (DLS) was used to analyze the results of particle size distribution measurement. Particle size distribution measurement (median diameter D50 (nm)) was measured using the heterodyne method.

Then, the nanoparticles that had passed through the filtration filter were added to pure water to obtain an aqueous solution, and a particle size distribution measurement was performed using a particle size distribution measuring device. In this particle size distribution measurement, the median diameter D50 (nm) of the nanoparticles in each sample was measured. For the same reasons as described above, the dispersibility of the nanoparticles in water can be evaluated through the measurement of the particle size distribution of the nanoparticles in the aqueous solution. In particular, when the particle size of the nanoparticles themselves is approximately the same, it can be evaluated that the smaller the median diameter of the nanoparticles in the aqueous solution, the higher the dispersibility of the nanoparticles in the aqueous solution.

As will be described later, the nanoparticles in the comparative example did not pass through the filtration filter, so further evaluations, including particle size distribution measurement, were not performed.

<Physical property evaluation: Evaluation of external quantum efficiency of light-emitting device>
Next, the nanoparticles according to each example that were subjected to particle size distribution measurement were used to manufacture the light-emitting devices according to each example. The light-emitting devices according to each example were manufactured by forming an anode, a hole injection layer containing the nanoparticles according to each example, a hole transport layer, a light-emitting layer, an electron transport layer, and a cathode on a substrate in this order. In particular, in each example, the method of forming each layer of the light-emitting device was the same as the method of forming each layer of the light-emitting device 3 according to this embodiment, except that the material used for the hole injection layer was a solution containing the nanoparticles according to each example. Next, the external quantum efficiency (EQE) of the light-emitting devices according to each example manufactured was measured and evaluated under the following conditions.

In each example, the external quantum efficiency (Nφ _(exe) ) was evaluated as the number of photons (Np) extracted per unit area of a cell fabricated as a light-emitting element for evaluation relative to the number of carriers (Ne) injected into the cell, as shown in the following formula.

Np=λ/hc×P×1/S (1/m ² )
Ne=I/e×1/S (1/m ² )
N _{φ (exe)} = Np/Ne x 100 = (P x λ x e) / (hc x I) x 100 (%)
In the formula, I represents current (A), P represents light intensity (measured light amount (W)), S represents the cell area (element area (m ² )), and λ represents the emission peak wavelength ( m), e represents the elementary electron mass (A·s), h represents Planck's constant (J·s), and c represents the speed of light (m·s ⁻¹ ).

The current (I) was measured using a 2400-type source meter manufactured by Keithley Instruments Inc. The light intensity (P) was measured using a light intensity meter (model number: BM-5A) manufactured by Topcon House Corporation. The cell area was 4×10 ⁻⁶ (m ² ). The emission peak wavelength (λ) was 536 (nm). The Planck constant was 6.626×10 ⁻³⁴ J·s. The elementary electron quantity (e) was 1.602×10 ⁻¹⁹ A·s. The speed of light (c) was 2.998×10 ⁸ (m·s ⁻¹ ).

<Physical property evaluation: Evaluation results>
The above physical property evaluations are summarized in Table 1 below.

In Table 1, the column "FT-IR/NO ₃ ^-Presence/ absence" indicates the presence or absence of an absorption peak of a wave number corresponding to nitrate ion in the infrared absorption spectrum obtained by the analysis using FT-IR for the nanoparticles according to each Example and Comparative Example. The column "TG-DTA/Weight reduction rate" indicates the percentage of the weight of the nanoparticles reduced while the ambient temperature was between 309°C and 411°C in the analysis using the above-mentioned TG-DTA, based on the weight of the nanoparticles at room temperature. The column "Filter passage presence/absence" indicates the presence or absence of the nanoparticles according to each Example and Comparative Example passing through the above-mentioned filtration filter. In this column, when the filtration passage rate _RF measured by the above formula is 80% or more, it is indicated as "passed", when the filtration passage rate _RF is less than 80% and 20% or more, it is indicated as "partially passed", and when the filtration passage rate _RF is less than 20%, it is indicated as "not passed". The column "Particle size distribution measurement/D50 [nm]" shows the median diameter D50 measurement results obtained by particle size distribution measurement for the nanoparticle aqueous solution of each Example in the unit of nm. The column "External quantum efficiency [%]" shows the measurement results, expressed as a percentage, of the external quantum efficiency measured by applying a voltage to the light-emitting element of each Example to cause it to emit light.

As shown in Table 1, the infrared absorption spectrum obtained by FT-IR analysis of the nanoparticles according to each example has an absorption peak at a wave number corresponding to nitrate ions. On the other hand, the infrared absorption spectrum obtained by FT-IR analysis of the nanoparticles according to the comparative example did not show this absorption peak.

Furthermore, in the analysis of the nanoparticles according to each Example using TG-DTA, the weight of the nanoparticles decreased by 7.9% to 18.9% based on the weight at room temperature while the ambient temperature was between 309°C and 411°C. On the other hand, in the analysis of the nanoparticles according to the Comparative Example using TG-DTA, there was almost no decrease in the weight of the nanoparticles while the ambient temperature was between 309°C and 411°C.

These results show that the nanoparticles of each example contain nitrate ions, while the nanoparticles of the comparative example contain almost no nitrate ions.

As shown in Table 1, 80% or more of the nanoparticles of each of Examples 1 to 5 passed through the above-mentioned filter. Also, 70% or more of the nanoparticles of Example 6 passed through the filter. On the other hand, less than 20% of the nanoparticles of the Comparative Example passed through the filter. This indicates that the nanoparticles of the Comparative Example have low dispersibility in the aqueous solution, resulting in aggregation of the nanoparticles.

These results show that nanoparticles containing nitrate ions, such as the nanoparticles in each Example, have improved dispersibility in aqueous solution compared to nanoparticles not containing nitrate ions, such as the nanoparticles in the Comparative Examples.

When a functional layer including a hole injection layer is formed from a solution containing the nanoparticles according to each embodiment with improved dispersibility in an aqueous solution, the nanoparticles are also less likely to aggregate in the functional layer. In general, aggregation of nanoparticles in a functional layer containing nanoparticles as a charge transport material or the like leads to deterioration of the properties of the functional layer. Therefore, the nanoparticles according to each embodiment with improved dispersibility in a solution and reduced aggregation can improve the properties of the functional layer containing the nanoparticles. A light-emitting element including a functional layer containing the nanoparticles according to each embodiment has a functional layer with improved properties, and therefore can improve the light-emitting efficiency or improve the reliability of the functional layer to extend the lifespan. A display device including the light-emitting element can improve display quality or achieve power saving.

As shown in Table 1, the median diameter of the nanoparticles according to each of Examples 1 to 5 was 11.12 nm to 29.49 nm, which was below 30 nm. On the other hand, the median diameter of the nanoparticles according to Example 6 was 34.49 nm, which exceeded 30 nm. This indicates that the dispersibility in aqueous solution of the nanoparticles according to each of Examples 1 to 5 is even better than that of the nanoparticles according to Example 6.

From the above results, when nanoparticles 4 are used as a sample and analyzed using TG-DTA with the above method, the weight of nanoparticles 4 may decrease by 8.0% or more based on the weight at room temperature while the ambient temperature is between 309°C and 411°C. Furthermore, the median diameter of nanoparticles 4 in the aqueous solution may be 30 nm or less. In this case, the dispersibility of nanoparticles 4 in the aqueous solution is improved.

As shown in Table 1, the external quantum efficiency of the light-emitting devices containing the nanoparticles of Examples 1 to 4 in the hole injection layer was 8.5% to 10.5%, which was 8.0% or higher. On the other hand, the external quantum efficiency of the light-emitting devices containing the nanoparticles of Examples 5 and 6 in the hole injection layer was 5.8% and 4.8%, respectively, which was below 6.0%.

As described above, the external quantum efficiency of the light-emitting devices according to Examples 1 to 4 exceeded the external quantum efficiency of the light-emitting device according to Example 5. This is believed to be because the proportion of nitrate ions contained in the nanoparticles in the hole injection layer of each of the light-emitting devices according to Examples 1 to 4 was low, reducing the inhibition of hole transport from the anode to the light-emitting layer by the nitrate ions.

On the other hand, the external quantum efficiency of the light-emitting devices according to Examples 1 to 4 exceeded the external quantum efficiency of the light-emitting device according to Example 6. This is believed to be because the nitrate ions contained in the nanoparticles according to Examples 1 to 4 increased the dispersibility of the nanoparticles in an aqueous solution, reduced the aggregation of nanoparticles in the hole injection layer, and improved the hole transport properties of the hole injection layer.

From the above results, when the nanoparticles 4 are used as a sample and analyzed using TG-DTA by the above method, the weight of the nanoparticles 4 may decrease by 8.0% to 18.0% based on the weight at room temperature while the ambient temperature is between 309°C and 411°C. In this case, the external quantum efficiency of the light-emitting element 3 containing the nanoparticles 4 in the hole injection layer 32 can be improved.

<Addendum>
The light-emitting element 3 according to this embodiment contains nanoparticles 4 containing nickel oxide as a material of the hole injection layer 32. Therefore, in the light-emitting element 3, the nitrate ions contained in the nanoparticles 4 reduce the aggregation of the nanoparticles 4 in the hole injection layer 32, while the nickel oxide contained in the nanoparticles 4 improves the efficiency of hole injection into the light-emitting layer 34.

However, the light-emitting element 3 according to this embodiment is not limited to a configuration in which the nanoparticles 4 are included as a material of the hole injection layer 32. For example, the nanoparticles 4 may be used in at least one functional layer of the light-emitting element 3, including the hole transport layer 33, the light-emitting layer 34, and the electron transport layer 35. Even in this case, the nanoparticles 4 are unlikely to aggregate in the functional layer, and therefore the characteristics of the functional layer can be improved.

In this embodiment, the light-emitting layer 34 contains quantum dots 5 as the light-emitting material. In general, light-emitting elements that use quantum dots as the light-emitting material of the light-emitting layer have a low hole concentration in the light-emitting layer, and tend to have an excess of electrons in the light-emitting layer. An excess of electrons in the light-emitting layer not only reduces the light-emitting efficiency of the light-emitting element, but also increases the occurrence of the Auger electron generation process in the light-emitting layer, which may reduce the reliability of the light-emitting element.

Therefore, the light-emitting element 3 according to this embodiment, which uses quantum dots 5 as the light-emitting material of the light-emitting layer 34, is provided with a hole injection layer 32 containing nanoparticles 4, and thus the light-emitting element 3 more efficiently reduces the excess of electrons in the light-emitting layer 34. As a result, the nanoparticles 4 according to this embodiment more efficiently improve the light-emitting efficiency and reliability of the light-emitting element 3. However, the nanoparticles 4 according to this embodiment may also be used as a material for the functional layer of a light-emitting element that contains a material other than the quantum dots 5 as the light-emitting material of the light-emitting layer, such as an organic fluorescent material or an organic phosphorescent material.

This disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. The technical scope of this disclosure also includes embodiments obtained by appropriately combining different technical means disclosed in the respective embodiments. Furthermore, new technical features can be formed by combining the technical means disclosed in the respective embodiments.

REFERENCE SIGNS LIST 1 Display device 2 Substrate 3R Red light emitting element 3G Green light emitting element 3B Blue light emitting element 4 Nanoparticles 5 Quantum dots 31 Anode 32 Hole injection layer (functional layer)
34 Light-emitting layer (functional layer)
36 Cathode

Claims

Nanoparticles used in the functional layer of a light-emitting element, which contain nickel oxide and have an infrared absorption spectrum obtained by infrared spectroscopy that has an absorption peak at a wave number corresponding to nitrate ions.
The nanoparticles according to claim 1, which lose weight by 8.0% to 18.0% based on the weight at room temperature while the ambient temperature is between 309°C and 411°C when heated by increasing the ambient temperature from room temperature at a rate of 10°C per minute.
The nanoparticles according to claim 1 or 2, in which 30 mg of the nanoparticles are added to 1 ml of pure water, an aqueous solution is synthesized, and then irradiated with 45 kHz ultrasonic waves for 30 minutes. When the aqueous solution is then filtered through a filter with a pore size of 0.45 μm, 80% or more of the nanoparticles pass through the filter.
The nanoparticles according to claim 3, which have a median diameter of 30 nm or less as measured by particle size distribution measurement of an aqueous solution added to pure water after passing through the filtration filter.
An anode, a cathode, and a functional layer located between the anode and the cathode, the functional layer including at least a light-emitting layer;
A light-emitting device, wherein at least one of the functional layers contains nickel oxide and nanoparticles whose infrared absorption spectrum obtained by infrared spectroscopy has an absorption peak at a wave number corresponding to nitrate ions.
The light-emitting device according to claim 5, wherein when the nanoparticles are heated by increasing the ambient temperature from room temperature at 10°C per minute, the weight of the nanoparticles decreases by 8.0% to 18.0% based on the weight at room temperature while the ambient temperature is between 309°C and 411°C.
The light-emitting element according to claim 5 or 6, wherein the functional layer containing the nanoparticles includes a hole injection layer located between the anode and the light-emitting layer.
The light-emitting element according to any one of claims 5 to 7, wherein the light-emitting layer contains a plurality of quantum dots as a light-emitting material.
A substrate is provided.
a red light emitting element, a green light emitting element, and a blue light emitting element are provided on the substrate;
A display device, wherein at least one of the red light emitting element, the green light emitting element, and the blue light emitting element is the light emitting element according to claim 5 .