JP2022036685A - Quantum dot light emitting element and display - Google Patents

Abstract

To provide a quantum dot light emitting element with excellent element characteristics by suppressing leakage current caused by the electron injection layer in the quantum dot light emitting element using high color purity quantum dot materials.SOLUTION: A quantum dot light emitting element 10 includes an anode 30, a light emitting layer 50, an electron injection layer 60, and a cathode 70, in that order. The light emitting layer 50 contains quantum dots. The electron injection layer 60 contains nanoparticles and an electron transport material. It is preferred that the nanoparticles comprise nanoparticles containing zinc oxide.SELECTED DRAWING: Figure 1

Description

The present invention relates to a quantum dot light emitting device and a display device.

Color reproducibility is one of the important characteristics required for display devices. In particular, the color system of 4K8K Super Hi-Vision, whose broadcasting service started in 2018, aims to include almost all object colors existing in nature and the color gamut of existing color systems, and displays 4K8K Super Hi-Vision. The display device is required to have color reproducibility in a wide color gamut. Here, in the case of a self-luminous display device, it is necessary to increase the color purity of each of the blue, green, and red light emitting materials.

In recent years, as disclosed in Patent Document 1 and Non-Patent Document 1 below, an electric field light emitting device (quantum dot light emitting device) using quantum dots made of semiconductor nanocrystals as a light emitting material has been proposed. The emission color of the quantum dots can be controlled by changing the crystal grain size, and the half width of the emission spectrum can be reduced by making the particle size distribution uniform. Taking advantage of the small half width of the emission spectrum, the quantum dots may be used as a emission material for a display device having a wide display color gamut. Further, among the electroluminescent devices using quantum dots, there is an example in which a half-value width of 30 nm or less and an external quantum efficiency of about 20% are realized (Non-Patent Document 2).

In order to improve the light emitting performance of the electroluminescent device using the above-mentioned quantum dots, it has been studied to appropriately combine the quantum dots with various materials for transporting and injecting electric charges into a device (Patent Document 2). ). In particular, by combining zinc oxide nanoparticles with quantum dots, high electron injection properties are realized (Non-Patent Documents 2 and 3).

Japanese Patent No. 4948747 Japanese Unexamined Patent Publication No. 2019-33005

Shirasaki et al. (Y. Shirasaki et. Al), Nature Photonics, 7, 13 (2013) X. D. Dai et al., Nature, 515,96 (2014) L. Kian et al. (L. Qian et al.), Nature Photonics, 5,543 (2011).

However, the present inventors have proceeded with a study on a quantum dot light emitting device in which a light emitting layer made of quantum dots as described above and an electron injection layer made of zinc oxide nanoparticles are formed on the cathode side of the light emitting layer. It was found that most of the electrons injected from the electron injection layer reach the hole injection layer located on the anode side of the light emitting layer, resulting in a leakage current that cannot contribute to light emission. If the leakage current that cannot contribute to light emission increases, the light emission efficiency will decrease and unstable operation will occur.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a quantum dot light emitting device having excellent device characteristics by suppressing a leakage current caused by an electron injection layer.
Further, it is a further object of the present invention to provide a display device provided with such a quantum dot light emitting element, capable of displaying a wide color gamut, and having excellent light emitting characteristics.

The present inventors have diligently studied to reduce the leakage current of the quantum dot light emitting element, and found that by mixing an electron transporting material together with nanoparticles in the electron injection layer of the quantum dot light emitting element, when a voltage is applied. We have found that the leakage current can be reduced and have reached the present invention.
The gist structure of the present invention for solving the above problems is as follows.

The quantum dot light emitting device of the present invention includes an anode, a light emitting layer, an electron injection layer, and a cathode in this order.
The light emitting layer contains quantum dots.
The electron injecting layer is characterized by containing nanoparticles and an electron transporting material.
The quantum dot light emitting device of the present invention can reduce leakage current and improve device characteristics.

In a preferred example of the quantum dot light emitting device of the present invention, the nanoparticles are made of nanoparticles containing zinc oxide. In this case, the electron injection property from the cathode is improved.

In another preferred example of the quantum dot light emitting device of the present invention, the electron transport material has a nitrogen-containing heterocycle. In this case, the leakage current can be reduced without impairing the electron transportability of the electron injection layer.

In another preferred example of the quantum dot light emitting device of the present invention, the electron transport material has a phosphine oxide group. In this case, the solubility of the electron transport material in an organic solvent (particularly, an alcohol solvent) is improved, and the formation of the electron injection layer becomes easy.

In another preferred example of the quantum dot light emitting device of the present invention, the electron transport material is 2,4,6-tris [3- (diphenylphosphinyl) phenyl] -1,3,5-triazine (PO-). T2T). In this case, the leakage current can be reduced without impairing the electron transportability of the electron injection layer, the solubility of the electron transport material in an organic solvent (particularly, an alcohol solvent) is improved, and the electron injection layer can be easily formed. become.

Further, the display device of the present invention is characterized by including the above-mentioned quantum dot light emitting element. The display device of the present invention is capable of displaying a wide color gamut and has excellent light emission characteristics.

According to the present invention, it is possible to provide a quantum dot light emitting device capable of EL light emission having a small leakage current and excellent device characteristics.
Further, according to the present invention, it is possible to provide a display device having such a quantum dot light emitting element, capable of displaying a wide color gamut, and having excellent light emitting characteristics.

It is a schematic diagram which showed an example of the structure of the quantum dot light emitting device of this invention. It is a schematic diagram which showed an example of the structure of a quantum dot. It is a graph which shows the voltage-current density characteristic of the quantum dot light emitting element of Examples 1 and 2 and the comparative example 1 and 2. It is a graph which shows the voltage-luminance characteristic of the quantum dot light emitting element of Examples 1 and 2 and the comparative example 1 and 2.

Hereinafter, the quantum dot light emitting device and the display device of the present invention will be illustrated and described in detail based on the embodiment thereof.

<< Quantum dot light emitting device >>
The quantum dot light emitting element of the present invention includes an anode, a light emitting layer, an electron injection layer, and a cathode in this order, the light emitting layer contains quantum dots, and the electron injection layer includes nanoparticles and electrons. It is characterized by including, with and from transport material.

In the quantum dot light emitting device of the present invention, the electron injection layer contains nanoparticles having excellent electron injection properties and further contains an electron transport material. The electron transport material has an action of not obstructing the flow of electrons injected into the light emitting layer from the cathode side and preventing holes injected into the light emitting layer from the anode side from passing through to the cathode side (that is, flowing out of the light emitting layer). It has the action of blocking the hole current). Further, the electron transport material prevents nanoparticles from infiltrating into the gaps between the quantum dots of the light emitting layer and coming into contact with a layer on the anode side of the light emitting layer (for example, a hole injection layer or an anode). Therefore, the quantum dot light emitting device of the present invention has suppressed leakage current and is excellent in device characteristics.

Next, one aspect of the quantum dot light emitting device of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic view showing an example of the structure of the quantum dot light emitting device of the present invention. The quantum dot light emitting device 10 shown in FIG. 1 has a configuration in which an anode 30, a hole injection layer 40, a light emitting layer 50, an electron injection layer 60, and a cathode 70 are laminated in this order on a substrate 20. The quantum dot light emitting device 10 shown in FIG. 1 is configured to inject holes from the anode 30 side arranged at the lower part and electrons from the cathode 70 arranged at the upper part. The light emitting element is not limited to this, and may have a structure that is upside down. Further, in the quantum dot light emitting device of the present invention, a hole transport layer (not shown) may be further present between the hole injection layer 40 and the light emitting layer 50, or the light emitting layer 50 and the hole transport layer 50. , An electron transport layer (not shown) may be further present between the electron injection layer 60 and the electron transport layer 60.

<Board>
In the case of a bottom emission type element that extracts light from the substrate 20 side, the substrate 20 is preferably made of a transparent material. Examples of such transparent materials include glass, quartz, and plastic films. Here, examples of the material of the plastic film include polyimide, polyethylene terephthalate, polyethylene naphthalate, polypropylene, cycloolefin polymer, polyamide, polyether sulfone, polymethylmethacrylate, polycarbonate, polyarylate and the like.
On the other hand, in the case of a top emission type element that extracts light from the upper electrode side, the material of the substrate 20 does not necessarily have to be a transparent material. When an opaque substrate is used as the substrate 20, the opaque substrate may be, for example, a colored plastic film substrate, a substrate made of a ceramic material such as alumina, or an oxide film (insulating film) on the surface of a metal plate such as stainless steel. Examples thereof include a substrate on which the above is formed.
Further, when a flexible substrate such as a plastic film is used as the substrate 20 and a quantum dot light emitting element is formed on the flexible substrate, the flexible quantum dot light emitting element capable of easily deforming the image display unit is used. can do.
The average thickness of the substrate 20 is not particularly limited, but is preferably 0.001 to 30 mm, more preferably 0.01 to 1 mm.

<Anode>
In the case of a bottom emission type element that extracts light from the substrate 20 side, the anode 30 is preferably made of a transparent and highly conductive material. In this case, as the anode 30, for example, a conductive transparent oxide such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO) can be used.
On the other hand, in the case of a top emission type element that extracts light from the upper electrode side, the material of the anode 30 does not necessarily have to be a transparent material, so that a metal electrode may be used as the anode 30. Here, as the material of the anode 30, a metal having a relatively large work function is preferable. By using a metal having a large work function, the hole injection barrier from the anode 30 to the organic layer can be lowered, and holes can be easily injected. Examples of the metal used for the anode 30 include, but are not limited to, Al, Au, Ag, Pt, Ni, W, Cr, Mo, Fe, Co, Cu and the like.
The average thickness of the anode 30 is not particularly limited, but is preferably 10 to 500 nm, more preferably 50 to 200 nm.

<Hole injection layer>
The hole injection layer 40 is formed to facilitate hole injection from the anode 30. As the material of the hole injection layer 40, either an inorganic material or an organic material can be used. Examples of the inorganic material include molybdenum oxide (MoO ₃ ), vanadium oxide (V ₂ O ₅ ), ruthenium oxide (RuO ₂ ), rhenium oxide, tungsten oxide, manganese oxide and the like. Further, as the organic material, either a low molecular weight material or a high molecular weight material can be used, and examples of the high molecular weight material include PEDOT: PSS and the like. PEDOT indicates poly (3,4-ethylenedioxythiophene), and PSS indicates poly (styrene sulfonic acid). One or more of these can be used for the hole injection layer 40. Among these, PEDOT: PSS or molybdenum oxide is preferable as the material used for the hole injection layer 40 from the viewpoint of hole injection property.
The average thickness of the hole injection layer 40 is not particularly limited, but is preferably 1 to 500 nm, more preferably 3 to 50 nm.

<Light emitting layer>
The light emitting layer 50 includes quantum dots. In the light emitting layer 50, the holes injected from the anode 30 and the electrons injected from the cathode 70 are recombined to bring the quantum dots into an excited state, and light emission is obtained by the energy emitted when the quantum dots return to the ground state. Be done.
The emission color of the light emitting layer 50 can be changed depending on the crystal grain size and the type (material) of the quantum dots contained in the light emitting layer 50. Here, the crystal grain size of the quantum dots can be selected according to the desired emission color, and is preferably 1 to 20 nm, more preferably 2 to 10 nm, for example. The quantum dots are preferably composed of a central portion called a core made of semiconductor fine particles and a surface modifier called a ligand that covers the surface of the core (or the surface of the shell described later).

FIG. 2 shows an example of a quantum dot structure that can be suitably used for the quantum dot light emitting device of the present invention. The quantum dot 1 shown in FIG. 2 includes a core 2, a shell 3 surrounding the core 2, and a ligand (surface modifier) 4 covering the surface of the shell 3. The quantum dots have high chemical stability and are less likely to aggregate. Further, the quantum dot 1 has an advantage that it is easy to prepare as a solution and it is easy to form a film by a spin coating method or the like.

－－Quantum dot core －－
Examples of semiconductors constituting the core portion of the quantum dots include II-VI group compounds, II-V group compounds, III-VI group compounds, III-V group compounds, and IV-VI group compounds. I-III-VI group compounds, II-IV-VI group compounds, and II-IV-V group compounds, such as ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, InN, InP, InAs, InSb, Examples thereof include CuInS and AgInS. In addition, some of the constituent elements may be replaced with other metal elements due to the inclusion of impurities or the like. Among these, as the core of the quantum dot, CdSe, from the viewpoint of ease of synthesis, ease of controlling the particle size and / or particle size distribution for obtaining light emission of a desired wavelength, and quantum efficiency of light emission, InP is preferred. The core may have, for example, an average particle size of 10 nm or less.

--Quantum dot shell ---
The quantum dots may have one or more semiconductor layers called shells so as to surround the core. Here, as the semiconductor constituting the shell portion, a semiconductor having the same composition as the semiconductor constituting the core portion can be used. The shell of the quantum dots is preferably selected according to the semiconductor used for the core to be coated, and the shell preferably uses a semiconductor having a band gap larger than that of the core. In this case, the excitation energy of the core is efficiently confined in the core by the shell. Specifically, for example, when the core is composed of CdSe and InP, it is preferable to use ZnS having a larger band gap for the shell. The thickness of the shell may be in the range of 0.1 nm to 50 nm, particularly in the range of 0.2 nm to 10 nm.

Further, the shell may have a crystal system of the compound semiconductor familiar with the crystal system of the core compound semiconductor, and the lattice constant thereof is the same as or close to the lattice constant of the core compound semiconductor. It may be there. A shell made of a compound semiconductor that is familiar to the crystal system and has a close lattice constant may cover the circumference of the core well. Alternatively, the shell may be amorphous.

The surface of the shell may be modified with any compound. When the surface of the shell is an exposed surface of semiconductor nanoparticles having a core-shell structure, the surface is modified with a ligand (surface modifier) to stabilize the nanoparticles and prevent the aggregation or growth of the semiconductor nanoparticles. And / or the dispersibility of the semiconductor nanoparticles in the solvent can be improved.
Examples of the ligand (surface modifying agent) used for modifying the surface include a nitrogen-containing compound having a hydrocarbon group having 4 to 20 carbon atoms, a sulfur-containing compound, an oxygen-containing compound, and the like. Hydrocarbon groups having 4 to 20 carbon atoms include saturated aliphatic hydrocarbons such as n-butyl group, isobutyl group, n-pentyl group, n-hexyl group, octyl group, decyl group, dodecyl group, hexadecyl group and octadecyl group. Hydrogen group; unsaturated aliphatic hydrocarbon group such as oleyl group; alicyclic hydrocarbon group such as cyclopentyl group and cyclohexyl group; aromatic hydrocarbon group such as phenyl group, benzyl group, naphthyl group and naphthylmethyl group, etc. Of these, saturated aliphatic hydrocarbon groups and unsaturated aliphatic hydrocarbon groups are preferable. Examples of the nitrogen-containing compound include amines and amides, examples of the sulfur-containing compound include thiols, and examples of the oxygen-containing compound include fatty acids.
Examples of the surface modifier of the nitrogen-containing compound include alkylamines such as n-butylamine, isobutylamine, n-pentylamine, n-hexylamine, octylamine, decylamine, dodecylamine, hexadecylamine and octadecylamine, and oleylamine and the like. Alkenylamine.
The surface modifier of the sulfur-containing compound is, for example, n-butanethiol, isobutanethiol, n-pentanethiol, n-hexanethiol, octanethiol, decanethiol, dodecanethiol, hexadecanethiol, octadecanethiol and the like.
The surface modifier of the oxygen-containing compound is, for example, oleic acid, stearic acid, lauric acid and the like.

The synthesized quantum dots usually contain impurities such as unreacted raw materials. If the light emitting layer contains an excess raw material component, charge transport is hindered. Therefore, it is preferable to perform a purification treatment to remove the excess raw material component. Examples of the purification method at this time include a method using precipitation / redispersion. This is a method in which a solvent (poor solvent) that does not disperse quantum dots is added to the quantum dot dispersion liquid to precipitate and recover the quantum dots, and the quantum dots are redispersed in the target solvent. For quantum dots dispersed in an organic solvent having a small polarity, a solvent such as alcohol having a large polarity is generally added as a poor solvent to obtain a precipitate.

The light emitting layer 50 may include a charge transport material together with the above-mentioned quantum dots. Since the light emitting layer 50 contains the charge transport material, the transportability of electrons or holes can be controlled, so that the light emission characteristics of the quantum dot light emitting device can be expected to be improved. The charge transport material is, for example, mixed with the quantum dots and exists in the light emitting layer 50 in a form of filling the gaps between the quantum dots. When the charge transport material is used in combination with quantum dots, it is preferable that the charge transport material is a material that dissolves in the dispersion liquid of the quantum dots. As the charge transport material, an organic material having solubility in an organic solvent and having hole transport property or electron transport property, which does not cause color mixing in the EL light emission of the quantum dot light emitting element, is preferably used. Is desirable. Here, examples of the organic material having hole transportability include poly (N-vinylcarbazole) (PVK), and examples of the organic material having electron transportability include electrons used in the electron injection layer 60 described later. Examples include transportation materials.
The amount of the charge transport material added is preferably in the range of 0.01 to 10 in terms of mass ratio with respect to the quantum dots, and more preferably in the range of 0.1 to 2. When the mass ratio of the quantum dots and the charge transport material is within the above range, the effect of improving the light emitting characteristics of the quantum dot light emitting element becomes large.

The film forming method of the light emitting layer 50 is not particularly limited, but a solution in which quantum dots are dissolved in an organic solvent or water can be prepared and formed by a spin coating method, an inkjet method, a printing method or the like. At this time, by finely painting the materials that emit light in red, green, and blue, the pixels of the display device capable of color display can be obtained.
The average thickness of the light emitting layer 50 is not particularly limited, but is preferably 5 to 200 nm, more preferably 10 to 50 nm.

<Electron injection layer>
The electron injection layer 60 is used for the purpose of facilitating electron injection from the cathode 70. The electron injecting layer 60 contains nanoparticles and an electron transporting material and is located between the light emitting layer 50 and the cathode 70. As the material of the electron injection layer 60, nanoparticles of a metal oxide having an electron injecting property and an organic material having an electron transporting property can be used in combination.

As the material of the nanoparticles, a metal oxide is preferable, and specifically, zinc oxide (ZnO), lithium oxide (Li ₂ O), titanium oxide (TIO ₂ ), silicon oxide (SiO ₂ ), tin oxide (SiO 2). SnO ₂ ), tungsten oxide (WO ₃ ), tantalum oxide (Ta ₂ O ₃ ), zinc oxide (ZrO ₂ ), hafnium oxide (HfO ₂ ), aluminum oxide (Al ₂ O ₃ ) and the like can be mentioned. Among these, zinc oxide is particularly preferable from the viewpoint of electron injectability. Further, a metal element such as Mg, Al, or Li can be added to the zinc oxide for use.

As used herein, the nanoparticles refer to particles having a particle size of less than 1 μm. The particle size of the nanoparticles can be measured by TEM or a light scattering method. The particle size of the nanoparticles is preferably 1 nm to 100 nm, more preferably 1 nm to 10 nm. Further, the particle size of the nanoparticles is preferably equal to or less than the thickness of the electron injection layer 60.

By including the electron transport material together with the above-mentioned nanoparticles, the electron injection layer 60 can suppress the leakage current of the quantum dot light emitting element. When the electron transport material is used by mixing with nanoparticles, it is preferable that the electron transport material is a material that dissolves in a dispersion of nanoparticles. As the electron transporting material, a material which is soluble in an organic solvent and has electron transporting property and which does not cause color mixing in the EL light emission of the quantum dot light emitting element can be preferably used. The organic solvent used for preparing the dispersion of nanoparticles is preferably an alcohol solvent, and examples of the alcohol solvent include methanol, ethanol, propanol, butanol and the like.

に示すような含窒素複素環を含む低分子材料あるいは高分子材料が更に好ましい。前記電子輸送材料が、含窒素複素環を有する場合、電子注入層６０の電子輸送性を損なうことなく漏れ電流を低減できる。 The electron-transporting material is preferably an electron-transporting organic material, and the electron-transporting organic material is preferably a small molecule material or a polymer material containing a nitrogen-containing heterocycle, and the following general formula (1):

A small molecule material or a polymer material containing a nitrogen-containing heterocycle as shown in the above is more preferable. When the electron transport material has a nitrogen-containing heterocycle, the leakage current can be reduced without impairing the electron transport property of the electron injection layer 60.

In the above general formula (1), it is shown that the arc portion forms a ring structure together with C and N. Here, the nitrogen-containing heterocycle represented by the general formula (1) includes a triazine ring, a pyridine ring, a pyrimidine ring, a pyrazine ring, a pyridazine ring, an oxazole ring, an oxadiazole ring, a benzoxazole ring, an imidazole ring, and a benzimidazole. Examples thereof include a ring, a quinoline ring, an isoquinoline ring, a quinoxaline ring, a triazole ring, and a phenanthroline ring.

Examples of the electron transporting material include nitrogen-containing heterocyclic compounds such as triazine derivatives, pyridine derivatives, phenanthroline derivatives, imidazole derivatives, and triazole derivatives. Here, examples of the triazine derivative include 2,4,6-tris [3- (diphenylphosphinyl) phenyl] -1,3,5-triazine (PO-T2T) and 2,4,6-tris (biphenyl-). 3-yl) -1,3,5-triazine (T2T), 2,4,6-tris (3'-(pyridine-3-yl) biphenyl-3-yl) -1,3,5-triazine (TmPPPyTz) ) Etc., examples of the pyridine derivative include tris (2,4,6-trimethyl-3- (pyridine-3-yl) phenyl) borane (3TPYMB), and examples of the phenanthroline derivative include 4,7-. Examples thereof include diphenyl-1,10-phenanthroline (Bphenyl), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and examples of the imidazole derivative include 1,3,5-tris (N). -Phenylbenzimidazole-2-yl) benzene (TPBI), as a triazole derivative, 3- (4-tert-butylphenyl) -4-phenyl-5- (4-biphenylyl) -1,2,4-triazole, Examples thereof include 3- (4-tert-butylphenyl) -4- (4-ethylphenyl) -5- (4-biphenylyl) -1,2,4-triazole.

It is also preferable that the electron transport material has a phosphine oxide group (P = O). When the electron transport material has a phosphine oxide group, the solubility in an organic solvent (particularly an alcohol solvent) is improved, and the formation of the electron injection layer 60 is facilitated.

Further, as the electron transport material, a compound having a nitrogen-containing heterocycle and a phosphine oxide group is preferable. In this case, the electron transportability of the electron injection layer 60 is improved, and the formation of the electron injection layer 60 becomes easy. Specific examples of the compound having a nitrogen-containing heterocycle and a phosphine oxide group include 2,4,6-tris [3- (diphenylphosphinyl) phenyl] -1,3,5-triazine (PO-). T2T), {5- [9'H- (9,3': 6', 9 "-telcarbazole) -9'-yl] pyridine-3-yl} diphenylphosphine oxide, 2,4,6-tris [ 3- (Diphenylphosphinyl) phenyl] pyridine, 3,5-bis [3- (diphenylphosphinyl) phenyl] pyridine, 2,5-bis [3- (diphenylphosphinyl) phenyl] -1,3 , 4-Oxaziazole, 3,5-bis [3- (diphenylphosphinyl) phenyl] -1,2,4-triazole, 4,7-bis [3- (diphenylphosphinyl) phenyl] -1 , 10-Phenyltroline and the like, and one or more of these can be used.

Among the above, 2,4,6-tris [3- (diphenylphosphinyl) phenyl] -1,3,5-triazine (PO-T2T) is particularly preferable as the electron transport material used for the electron injection layer 60. .. 2,4,6-Tris [3- (diphenylphosphinyl) phenyl] -1,3,5-triazine (PO-T2T) has excellent electron transport properties and is an organic solvent (particularly an alcohol solvent). Since it is excellent in solubility in, it also has an advantage in manufacturing the electron injection layer 60.

The amount of the electron transport material added is preferably in the range of 0.01 to 10 in terms of mass ratio with respect to the nanoparticles, and more preferably in the range of 0.1 to 2. When the mass ratio of the nanoparticles to the electron transport material is within the above range, the effect of suppressing the leakage current of the quantum dot light emitting element and the effect of improving the element characteristics become large.

The average thickness of the electron injection layer 60 is not particularly limited, but is preferably 1 to 500 nm, more preferably 10 to 100 nm.
The film forming method of the electron injection layer 60 is not particularly limited, but a solution in which nanoparticles and an electron transporting material are dissolved in an organic solvent or alcohol is prepared, and the film is formed by a spin coating method, an inkjet method, a printing method or the like. be able to.

<Cathode>
For the cathode 70, a metal thin film can be used in the case of a bottom emission type element that extracts light from the substrate 20 side. Here, as the material of the cathode 70, a metal having a relatively small work function is preferable. By using a metal having a small work function, the barrier for electron injection from the cathode 70 to the organic layer can be lowered, and electrons can be easily injected. The metal material used for the cathode 70 is not particularly limited, and examples thereof include Al, Mg, Ca, Ba, Li, and Na, and it is preferable to use Al.
When the substrate 20 and the lower anode 30 are not transparent, the cathode 70 as the upper electrode is a transparent electrode. Here, the material of the transparent electrode is not particularly limited, but for example, a conductive transparent oxide such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO) can be used.
The average thickness of the cathode 70 is not particularly limited, but is preferably 10 to 500 nm, more preferably 30 to 150 nm.

The hole injection layer 40 described above may be omitted, and each layer may have a structure in which it has a plurality of roles. For example, one layer can be used as both a hole injection layer and a light emitting layer.

<Method of forming each layer>
The method for forming the anode 30, the hole injection layer 40, the light emitting layer 50, the electron injection layer 60, and the cathode 70 is not particularly limited, and is not particularly limited, and is a vacuum vapor deposition method, an electron beam method, a sputtering method, a spin coating method, and an inkjet. A method such as a method or a printing method can be used. Further, by using these methods, the thicknesses of the anode 30, the hole injection layer 40, the light emitting layer 50, the electron injection layer 60, and the cathode 70 can be appropriately adjusted according to the purpose. Further, these methods are preferably selected according to the characteristics of the material of each layer, and the production method may be different for each layer.

<Use>
The quantum dot light emitting element of the present invention can be used not only for a display device described later, but also for a lighting device, a backlight, an electrophotographic, a lighting light source, an exposure light source, a signboard, a signboard, an interior, and the like.

<< Display device >>
The display device of the present invention is characterized by including the above-mentioned quantum dot light emitting device. Since the display device of the present invention includes a quantum dot light emitting device capable of emitting light having high color purity and excellent element characteristics by suppressing leakage current, it is possible to display a wide color gamut. In addition to the quantum dot light emitting element described above, the display device of the present invention can include other components generally used in the display device.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples.

(Example 1)
<Synthesis of quantum dots>
J.League. Quantum dots CdSe / ZnS (core / shell type) exhibiting green emission according to the method disclosed in J. Kwak et al., Nano Letters, 12, 2362-2366 (2012). Quantum dots) were synthesized. The specific synthesis method is shown below.
0.257 g of cadmium oxide, 7.34 g of zinc acetate, 30.13 g of oleic acid and 150 ml of octadecene were added to the flask. After heating at 150 ° C. for 30 minutes under reduced pressure, a leak occurred with argon, and the temperature was raised to 280 ° C. under an argon flow (solution 1). A solution prepared by dissolving 0.079 g of selenium and 1.122 g of sulfur in 20 ml of trioctylphosphine was prepared and added to solution 1. After mixing at 280 ° C. for 10 minutes, the mixture was cooled to room temperature (solution 2). Ethanol was added to this solution 2, and the precipitate was recovered by centrifugation. The recovered material was dispersed in toluene to adjust the concentration to 10 mg / ml.

<Synthesis of zinc oxide nanoparticles>
J.League. Zinc oxide nanoparticles were synthesized according to the method disclosed in J. Kwak et al., Nano Letters, 12, 2362-2366 (2012). The specific method is shown below.
6.7 mmol of zinc acetate was dissolved in 55 ml of methanol to give a 0.12 mol / l solution. 8.6 mmol of potassium hydroxide was dissolved in 25 ml of methanol to give a 0.34 mol / l solution. The zinc acetate-methanol solution was heated to 60 ° C. under a nitrogen atmosphere, and the potassium hydroxide-methanol solution was added dropwise with stirring, and the reaction was carried out for 2 hours. The solution became cloudy. Zinc acetate recovered by centrifugation from the obtained cloudy liquid was washed with methanol and finally redispersed in 1-butanol to obtain a 20 mg / ml zinc oxide nanoparticle dispersion. The particle size of the zinc oxide nanoparticles confirmed by TEM is 5 nm to 20 nm.

で示される構造式を有する、ホスフィンオキシド基及び含窒素複素環を含む電子輸送材料｛２，４，６－トリス［３－（ジフェニルホスフィニル）フェニル］－１，３，５－トリアジン（ＰＯ－Ｔ２Ｔ）、Ｌｕｍｔｅｃ社製｝と、前項にて合成した酸化亜鉛ナノ粒子との混合１－ブタノール溶液をスピンコートすることにより、電子注入層６０（厚さ：４０－５０ｎｍ）を形成した。この際、ＰＯ－Ｔ２Ｔの添加量を酸化亜鉛ナノ粒子に対する質量比で０．５に調整した。形成された電子注入層６０は、窒素雰囲気下、１３０℃にて３０分間加熱した。
次に、基板を真空蒸着装置に入れ、真空蒸着法により、陰極７０としてＡｌを６０ｎｍ成膜した。
なお、図１には示していないが、量子ドット発光素子は、窒素ガスで満たされたグローブボックス中で、封止用ガラスの周縁部に紫外線硬化樹脂を塗布した後、量子ドット発光素子を形成した前記基板の周縁部に貼り合せて、封止を行った。 <Manufacturing of quantum dot light emitting device>
A quantum dot light emitting device according to the present invention having the structure shown in FIG. 1 was manufactured as follows.
First, an anode 30 (thickness: 100 nm) made of ITO was formed on the glass substrate 20, and this was patterned into a plurality of lines.
Next, as the hole injection layer 40, PEDOT: PSS (CLEVIOSPVP AI4083 manufactured by Heraeus) was formed into a film by a spin coating method (thickness: 30 nm) and heated at 180 ° C. for 60 minutes.
Next, the light emitting layer 50 composed of the quantum dots synthesized in the previous section was formed into a film by spin coating (thickness: 20-30 nm) and heated at 130 ° C. for 30 minutes.
Next, the following formula (2):

An electron transport material containing a phosphine oxide group and a nitrogen-containing heterocycle having a structural formula represented by {2,4,6-tris [3- (diphenylphosphinyl) phenyl] -1,3,5-triazine (PO). -T2T), manufactured by Lumtec} and a mixed 1-butanol solution of zinc oxide nanoparticles synthesized in the previous section were spin-coated to form an electron injection layer 60 (thickness: 40-50 nm). At this time, the amount of PO-T2T added was adjusted to 0.5 by mass ratio with respect to the zinc oxide nanoparticles. The formed electron injection layer 60 was heated at 130 ° C. for 30 minutes under a nitrogen atmosphere.
Next, the substrate was placed in a vacuum vapor deposition apparatus, and Al was formed into a 60 nm film as a cathode 70 by a vacuum vapor deposition method.
Although not shown in FIG. 1, the quantum dot light emitting element forms a quantum dot light emitting element after applying an ultraviolet curable resin to the peripheral edge of the sealing glass in a glove box filled with nitrogen gas. It was bonded to the peripheral portion of the substrate and sealed.

で示される構造式を有する、ポリ（Ｎ－ビニルカルバゾール）（ＰＶＫ）と量子ドットからなる発光層５０をスピンコートにより成膜し（厚さ：３０－４０ｎｍ）、１３０℃にて３０分間加熱した。この際、ＰＶＫの添加量を量子ドットに対する質量比で０．５に調整した。それ以外は、実施例１と同様にして、量子ドット発光素子を作製した。 (Example 2)
Quantum dots and zinc oxide nanoparticles were synthesized in the same manner as in Example 1. In the production of the quantum dot light emitting device, when the light emitting layer 50 is formed, the following equation (3):

A light emitting layer 50 composed of poly (N-vinylcarbazole) (PVK) and quantum dots having the structural formula shown by is formed into a film by spin coating (thickness: 30-40 nm) and heated at 130 ° C. for 30 minutes. .. At this time, the amount of PVK added was adjusted to 0.5 in terms of the mass ratio to the quantum dots. Other than that, a quantum dot light emitting device was manufactured in the same manner as in Example 1.

(Comparative Example 1)
A quantum dot light emitting device was produced in the same manner as in Example 1 except that the electron injection layer 60 was formed using only the zinc oxide nanoparticles synthesized in Example 1.

(Comparative Example 2)
A quantum dot light emitting device was produced in the same manner as in Example 2 except that the electron injection layer 60 was formed using only the zinc oxide nanoparticles synthesized in Example 1.

<Characteristic evaluation of quantum dot light emitting device>
Voltage-current is applied to the ITO anode 30 side of each of the quantum dot light emitting elements of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 so as to be positive and negative on the Al cathode 70 side. Density characteristics and voltage-luminance characteristics were measured. The voltage-current density characteristic is shown in FIG. 3, and the voltage-luminance characteristic is shown in FIG.

In the elements of Examples 1 and 2, the current density in the low voltage region below the emission start voltage is suppressed to be small as compared with the elements of Comparative Examples 1 and 2, and the ultimate brightness is greatly exceeded.
In the phenomenon that the current leaks in the low voltage region below the emission start voltage, the electron injection layer 60 made of zinc oxide nanoparticles coated and formed penetrates into the gap of the light emission layer 50 and comes into contact with the hole injection layer 40 in the lower layer. It is caused by doing. It can be determined that by mixing the electron transport material (PO-T2T) with the zinc oxide nanoparticles, it was possible to suppress the zinc oxide nanoparticles from coming into contact with the hole injection layer 40 without interfering with the electron transport property. By suppressing the leakage of current, it became possible to effectively convert the current into light emission, and it was possible to increase the ultimate brightness.
Table 1 summarizes the element characteristics of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 as described above.

From the results of the above examples, in the quantum dot light emitting device according to the present invention, by including the electron transporting material together with the nanoparticles in the electron injection layer, the leakage current at the time of EL light emission is suppressed, and thereby the light emission characteristics. Was shown to be able to improve.

The quantum dot light emitting element of the present invention can be applied to various devices and products that require high color purity light emission, and can be applied to display devices, lighting devices, backlights, electrographs, lighting light sources, exposed light sources, and the like. It can be suitably used for signs, signboards, interiors, etc.

1: Quantum dot 2: Core 3: Shell 4: Ligand 10: Quantum dot light emitting element 20: Substrate 30: Anode 40: Hole injection layer 50: Light emitting layer 60: Electron injection layer 70: Cathode

Claims (6)

An anode, a light emitting layer, an electron injection layer, and a cathode are provided in this order.
The light emitting layer contains quantum dots.
A quantum dot light emitting device, wherein the electron injection layer contains nanoparticles and an electron transporting material. The quantum dot light emitting device according to claim 1, wherein the nanoparticles are made of nanoparticles containing zinc oxide. The quantum dot light emitting device according to claim 1 or 2, wherein the electron transport material has a nitrogen-containing heterocycle. The quantum dot light emitting device according to any one of claims 1 to 3, wherein the electron transport material has a phosphine oxide group. Any one of claims 1 to 4, wherein the electron transport material is 2,4,6-tris [3- (diphenylphosphinyl) phenyl] -1,3,5-triazine (PO-T2T). The quantum dot light emitting device described in 1. A display device comprising the quantum dot light emitting device according to any one of claims 1 to 5.

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