JP2022018524A - Quantum dot light-emitting element and display device - Google Patents

Abstract

To provide a quantum dot light-emitting element which is excellent in element characteristics while suppressing a decrease in color purity due to color mixture of light-emitting components other than quantum dots.SOLUTION: A quantum dot light-emitting element 10 includes a cathode 30, a light-emitting layer 50, and an anode 80, the light-emitting layer 50 being positioned between the cathode 30 and the anode 80. The light-emitting layer 50 contains quantum dots and an electron transport material. The electron transport material comprises a compound containing a phosphine oxide group.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 electroluminescent element (quantum dot light emitting element) 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). ).

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

Shirasaki et al. (Y. Shirasaki et. Al), Nature Photonics, 7, 13 (2013) X. Dai et al. (X. Dai et al.), Nature, 515,96 (2014)

In making the quantum dot light emitting device into an element, it is basic to use a material that can easily inject an electric charge into the light emitting layer containing the quantum dots, but the injected electric charge flows out to the counter electrode and cannot contribute to light emission. If such a leakage current increases, it leads to a decrease in luminous efficiency and unstable operation. On the other hand, the present inventors have found that the leakage current when a voltage is applied can be reduced by including the electron transporting material together with the quantum dots in the light emitting layer of the quantum dot light emitting element.

However, when the present inventors further studied a quantum dot light emitting device in which an electron transport material is contained in the light emitting layer as described above, in addition to light emission derived from quantum dots in the electroluminescence (EL) spectrum. It was found that a small amount of luminescent components derived from the electron transport material were generated. Such a mixing of light emitting components leads to a decrease in the color purity of the quantum dot light emitting device.

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 element characteristics while suppressing a decrease in color purity due to color mixing of light emitting components other than quantum dots. do.
Further, it is a further object of the present invention to provide a display device capable of displaying a wide color gamut, which is provided with such a quantum dot light emitting element.

The present inventors have diligently studied an electron-transporting material to be contained in the light-emitting layer of the quantum dot light-emitting element together with the quantum dots. The present invention has been reached by finding that color mixing due to a light emitting component of an electron transporting material can be suppressed.
The gist structure of the present invention for solving the above problems is as follows.

The quantum dot light emitting element of the present invention includes a cathode, a light emitting layer, and an anode, and the light emitting layer is a quantum dot light emitting element located between the cathode and the anode.
The light emitting layer contains quantum dots and an electron transporting material.
The electron transport material is characterized by being composed of a compound containing a phosphine oxide group.
The quantum dot light emitting device of the present invention has excellent device characteristics and can emit light with high color purity.

In a preferred example of the quantum dot light emitting device of the present invention, the compound containing a phosphine oxide group has a nitrogen-containing heterocycle. In this case, the element characteristics are further improved.

In another preferred example of the quantum dot light emitting device of the present invention, the compound containing a phosphine oxide group has a diphenylphosphine oxide group. In this case, the color mixing appearing in the EL spectrum can be further suppressed, and light having a higher color purity can be emitted.

In another preferred example of the quantum dot light emitting device of the present invention, the compound containing the phosphine oxide group is 2,4,6-tris [3- (diphenylphosphinyl) phenyl] -1,3,5-triazine. (PO-T2T). In this case, the element characteristics are further improved, the color mixing appearing in the EL spectrum can be further suppressed, and light having a higher color purity can be emitted.

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 has color reproducibility in a wide color gamut.

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

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-luminance characteristic of the quantum dot light emitting element of Example 1 and Comparative Examples 1 and 2. 5 is an EL spectrum of the quantum dot light emitting device of Example 1 and Comparative Examples 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 a cathode, a light emitting layer, and an anode, and the light emitting layer is a quantum dot light emitting element located between the cathode and the anode, and the light emitting layer is , Quantum dots, and an electron transporting material, wherein the electron transporting material comprises a compound containing a phosphine oxide group.

In the quantum dot light emitting element of the present invention, the light emitting layer includes quantum dots as a light emitting material and further contains an electron transporting 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). Therefore, the quantum dot light emitting device of the present invention has excellent element characteristics because the leakage current is suppressed.
Further, the quantum dot light emitting element containing an electron transporting material containing a phosphine oxide group in the light emitting layer is unlikely to emit light derived from the electron transporting material mainly present in the wavelength range from blue to ultraviolet. Therefore, the quantum dot light emitting device of the present invention can suppress color mixing due to the light emitting component of the electron transport material, and can emit light having high color purity.

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 structure in which a cathode 30, an electron injection layer 40, a light emitting layer 50, a hole transport layer 60, a hole injection layer 70, and an anode 80 are laminated in this order on a substrate 20. Have. The quantum dot light emitting device 10 shown in FIG. 1 is configured to inject electrons from the cathode 30 side arranged at the lower part and holes from the anode 80 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, an electron transport layer (not shown) may be further present between the electron injection layer 40 and the light emitting layer 50.

<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.

<Cathode>
In the case of a bottom emission type element that extracts light from the substrate 20 side, the cathode 30 is preferably made of a transparent and highly conductive material. In this case, as the cathode 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 cathode 30 does not necessarily have to be a transparent material, so that a metal electrode may be used as the cathode 30. Here, as the material of the cathode 30, 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 30 to the organic layer can be lowered, and electrons can be easily injected. Examples of the metal used for the cathode 30 include, but are not limited to, Al, Mg, Ca, Ba, Li, Na and the like.
The average thickness of the cathode 30 is not particularly limited, but is preferably 10 to 500 nm, more preferably 50 to 200 nm.

<Electron injection layer>
The electron injection layer 40 is formed to facilitate electron injection from the cathode 30. As the material of the electron injection layer 40, either an organic material or an inorganic material can be used. More specifically, as the material of the electron injection layer 40, zinc oxide (ZnO), lithium fluoride (LiF), lithium oxide (Li ₂ O), titanium oxide (TIO ₂ ), silicon oxide (SiO ₂ ), and oxidation Examples thereof include tin (SnO ₂ ), tungsten oxide (WO ₃ ), tantalum oxide (Ta ₂ O ₃ ), zinc oxide (ZrO ₂ ), hafnium oxide (HfO ₂ ), and aluminum oxide (Al ₂ O ₃ ). Among these, zinc oxide is particularly preferable from the viewpoint of electron injectability.

It is preferable to use nanoparticles for forming the electron injection layer 40. The particle size of the nanoparticles is preferably 1 nm to 100 nm, more preferably 1 nm to 10 nm. Preferably, a thin film formed by forming metal oxide nanoparticles such as zinc oxide nanoparticles by a spin coating method can be used as the electron injection layer 40.
The average thickness of the electron injection layer 40 is not particularly limited, but is preferably 5 to 200 nm, more preferably 10 to 100 nm.

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を用いると、陰極３０から注入された電子が効率よく電子輸送層中を移動し、発光層５０の量子ドットに電子が効率よく注入されるため、高効率の発光素子を得ることができる。 <Electron transport layer>
As described above, an electron transport layer may exist between the electron injection layer 40 and the light emitting layer 50. The electron transport layer is used to transport the electrons injected from the cathode 30 to the light emitting layer 50. The electron transport layer may be formed as an independent layer, or may be integrally formed with the light emitting layer 50. Further, the same material as the electron transport material mixed in the light emitting layer 50 may be used. As a material constituting the electron transport layer, the following general formula (1):

When a small molecule material or a polymer material containing a nitrogen-containing heterocycle as shown in is used, the electrons injected from the cathode 30 efficiently move in the electron transport layer, and the electrons efficiently move to the quantum dots of the light emitting layer 50. Since it is injected, a highly efficient light emitting element can be obtained.

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 material constituting the electron transport layer include nitrogen-containing heterocyclic compounds such as a triazine derivative, a pyridine derivative, an oxadiazole derivative, a triazole derivative, and a phenanthroline derivative. Here, examples of the triazine derivative include 2,4,6-tris (biphenyl-3-yl) -1,3,5-triazine (T2T) and 2,4,6-tris (3'-(pyridine-3-3-yl)). Il) Biphenyl-3-yl) -1,3,5-triazine (TmPPPyTz) and the like, and examples of the pyridine derivative include tris (2,4,6-trimethyl-3- (pyridine-3-yl) phenyl). Examples thereof include borane (3TPYMB), and examples of the oxadiazole derivative include 2- (4-biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole and 1,3-bis. [5- (p-tert-butylphenyl) -1,3,4-oxadiazole-2-yl] benzene and the like can be mentioned, and examples of the triazole derivative include 3- (4-tert-butylphenyl) -4-. Phenyl-5- (4-biphenylyl) -1,2,4-triazole, 3- (4-tert-butylphenyl) -4- (4-ethylphenyl) -5- (4-biphenylyl) -1,2, Examples of the phenanthroline derivative include 4-triazole and the like, and examples of the phenanthroline derivative include 4,7-diphenyl-1,10-phenanthroline (Bphenyl), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) and the like. Can be mentioned.

<Light emitting layer>
The light emitting layer 50 includes quantum dots and an electron transporting material. In the light emitting layer 50, the holes injected from the anode 80 and the electrons injected from the cathode 30 are recombinated, the quantum dots are in 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 an organic substance called a ligand that covers the surface of the core (or the surface of the shell described later).

－－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 extra 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.

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 4 covering the surface of the shell 3. The quantum dots 1 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.

The light emitting layer 50 contains an electron transporting material together with the above-mentioned quantum dots, and the light emitting layer 50 contains an electron transporting material, so that the light emitting characteristics of the quantum dot light emitting element can be improved. The electron 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 electron transport material is used in combination with quantum dots, it is preferable that the electron transport material is a material that dissolves in the dispersion liquid of the quantum dots. 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 electron transporting material contained in the light emitting layer 50 is composed of a compound containing a phosphine oxide group (P = O), and is preferably an electron transporting organic material containing a phosphine oxide group. Examples of the electron-transporting organic material include triazine derivatives, pyridine derivatives, phenanthroline derivatives, imidazole derivatives, triazole derivatives and the like.

Further, as the electron transporting material, a small molecule material or a polymer material containing a nitrogen-containing heterocycle as described in the section of the above-mentioned "electron transporting layer" is preferable, and as shown in the above general formula (1). A small molecule material or a polymer material containing a nitrogen-containing heterocycle is more preferable. That is, as the electron transport material used for the light emitting layer 50, a compound having a phosphine oxide group and a nitrogen-containing heterocycle is preferable. In this case, the element characteristics of the quantum dot light emitting device are further improved.

Further, as the electron transport material (compound containing a phosphine oxide group) used for the light emitting layer 50, a diphenylphosphine oxide group [−P (Ph) ₂ (= O), where “Ph” indicates a phenyl group. ] Is preferred. In this case, the color mixing appearing in the EL spectrum can be further suppressed, and light having a higher color purity can be emitted.

Specific examples of the compound containing a phosphinoxide group used as an electron transport material in the light emitting layer 50 include 2,4,6-tris [3- (diphenylphosphinyl) phenyl] -1,3,5-triazine ( PO-T2T), [4'-(9H-carbazole-9-yl) -2,2'-dimethyl- (1,1'-biphenyl) -4-yl] diphenylphosphine oxide, {5- [9'H -(9,3': 6', 9 "-telcarbazole) -9'-yl] pyridine-3-yl} diphenylphosphinoxide, 2- (diphenylphosphinyl) -spiro [9H-fluorolen-9,9" '-Kino [3,2,1-kl] phenoxazine], 2,7-bis (diphenylphosphoryl) -9-phenyl-9H-carbazole, bis [4- (N-carbazolyl) phenyl] phenylphosphine oxide, 2 , 7-Bis (diphenylphosphoryl) -9,9'-spirobifluorene, 3- (diphenylphosphoryl) -9- [4- (diphenylphosphoryl) phenyl] -9H-carbazole, 2,7-bis (diphenylphosphoryl) -9- (4-diphenylamino) phenyl-9'-phenyl-fluorene, 9- [3,5-bis (diphenylphosphoryl) phenyl] -9H-carbazole, 9- [3- (9H-carbazole-9-yl) ) Phenyl] -3- (diphenylphosphoryl) -9H-carbazole, 9- [8- (diphenylphosphoryl) dibenzo [b, d] furan-2-yl] -9H-carbazole, 3,3', 3 "-phos Finilidinetris (9-phenyl-9H-carbazole), 2,4,6-tris [3- (diphenylphosphinyl) phenyl] pyridine, 3,5-bis [3- (diphenylphosphinyl) phenyl] pyridine , 2,5-bis [3- (diphenylphosphinyl) phenyl] -1,3,4-oxadiazole, 3,5-bis [3- (diphenylphosphinyl) phenyl] -1,2,4 -Triazole, 4,7-bis [3- (diphenylphosphinyl) phenyl] -1,10-phenanthroline and the like can be mentioned, and one or more of these can be used. Among these, 2,4,6-tris [3- (diphenylphosphinyl) phenyl] -1,3,5 from the viewpoint of the effect of suppressing color mixing appearing in the EL spectrum of the quantum dot light emitting element and the element characteristics. -Triazine (PO-T2T) is preferred.

The amount of the electron transporting 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 dot and 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 are improved while suppressing the color mixing due to the electron transport material. do.

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.

<Hole transport layer>
The hole transport layer 60 is used to transport the holes injected from the anode 80 to the light emitting layer 50. As the material constituting the hole transport layer 60, a hole transporting inorganic material or an organic material can be used. The material constituting the hole transport layer 60 is preferably a hole transport organic material. As the hole-transporting organic material, either a small molecule material or a polymer material can be used. Examples of the material constituting the hole transport layer 60 include 4,4', 4 "-tris (carbazole-9-yl) triphenylamine (TCTA) and 2,2'-bis (N-carbazole) -9. , 9'-spirobifluorene (CFL), 4,4'-bis (carbazole-9-yl) biphenyl (CBP), 4,4', 4 "-trimethyltriphenylamine, N, N, N', N '-Tetraphenyl-1,1'-biphenyl-4,4'-diamine, N, N'-diphenyl-N, N'-bis (3-methylphenyl) -1,1'-biphenyl-4,4' -Diamine (TPD1), N, N'-diphenyl-N, N'-bis (4-methoxyphenyl) -1,1'-biphenyl-4,4'-diamine (TPD2), N, N, N', N'-tetrakis (4-methoxyphenyl) -1,1'-biphenyl-4,4'-diamine (TPD3), N, N'-di (1-naphthyl) -N, N'-diphenyl-1,1 '-Biphenyl-4,4'-diamine (α-NPD), 4,4', 4 "-tris (N-3-methylphenyl-N-phenylamino) triphenylamine (m-MTDATA) and the like can be mentioned. , One or more of these can be used. Among these, 2,2'-bis (N-carbazole) -9,9'-spirobifluorene (CFL) from the viewpoint of hole transportability. Is preferable.
The average thickness of the hole transport layer 60 is not particularly limited, but is preferably 10 to 500 nm, more preferably 20 to 100 nm.

<Hole injection layer>
The hole injection layer 70 is used for the purpose of facilitating hole injection from the anode 80. As the material of the hole injection layer 70, 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 70. Among these, molybdenum oxide is preferable as the material used for the hole injection layer 70 from the viewpoint of hole injection property.
The average thickness of the hole injection layer 70 is not particularly limited, but is preferably 1 to 500 nm, more preferably 3 to 50 nm.

<Anode>
As the anode 80, 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 anode 80, 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 80 to the organic layer can be lowered, and holes can be easily injected. The metal material used for the anode 80 is not particularly limited, and examples thereof include Al, Au, Pt, Ni, W, Cr, Mo, Fe, Co, and Cu, and it is preferable to use Al.
When the substrate 20 and the lower cathode 30 are not transparent, the anode 80 to be 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 anode 80 is not particularly limited, but is preferably 10 to 500 nm, more preferably 30 to 150 nm.

The electron injection layer 40, the hole transport layer 60, and the hole injection layer 70 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 hole transport layer.

<Method of forming each layer>
The method for forming the cathode 30, the electron injection layer 40, the electron transport layer, the light emitting layer 50, the hole transport layer 60, the hole injection layer 70, and the anode 80 is not particularly limited, and the vacuum vapor deposition method and the electron beam are used. A method such as a method, a sputtering method, a spin coating method, an inkjet method, or a printing method can be used. Further, using these methods, the thicknesses of the cathode 30, the electron injection layer 40, the electron transport layer, the light emitting layer 50, the hole transport layer 60, the hole injection layer 70, and the anode 80 are appropriately adjusted according to the purpose. can do. 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 element capable of emitting light having high color purity, 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. 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.

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、上記で合成した量子ドットとの混合クロロホルム溶液をスピンコートすることにより、電子輸送材料と量子ドットとからなる発光層５０（厚さ：２０－３０ｎｍ）を形成した。この際、電子輸送材料の添加量を量子ドットに対する質量比で０．２に調整した。形成された発光層は、窒素雰囲気下、１００℃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、正孔輸送層６０として、下記式（３）：

で示される構造式を有する材料［２，２’－ビス（Ｎ－カルバゾール）－９，９’－スピロビフルオレン（ＣＦＬ）］を４０ｎｍ、正孔注入層７０として酸化モリブデン（ＭｏＯ₃）を１０ｎｍ、陽極８０としてＡｌを８０ｎｍ、順次成膜した。
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 having the structure shown in FIG. 1 according to the present invention was manufactured as follows.
First, a cathode 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 electron injection layer 40, zinc oxide (ZnO) nanoparticles were formed into a film by spin coating (thickness: 30 nm).
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 the mixed chloroform solution of the quantum dots synthesized above are spin-coated to form a light emitting layer 50 (thickness: 20-30 nm) composed of an electron transport material and quantum dots. did. At this time, the amount of the electron transporting material added was adjusted to 0.2 by mass ratio with respect to the quantum dots. The formed light emitting layer was heated at 100 ° C. for 30 minutes under a nitrogen atmosphere.
Next, the substrate is placed in a vacuum vapor deposition apparatus, and the hole transport layer 60 is formed by the vacuum vapor deposition method according to the following formula (3):

Material having the structural formula represented by (2,2'-bis (N-carbazole) -9,9'-spirobifluorene (CFL)] is 40 nm, and molybdenum oxide (MoO ₃ ) is 10 nm as the hole injection layer 70. As the anode 80, Al was sequentially formed at 80 nm.
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.

(Comparative Example 1)
Using the quantum dot CdSe / ZnS synthesized in Example 1, a quantum dot light emitting element was produced in the same manner as in Example 1 except that the light emitting layer 50 did not contain an electron transport material (addition of electron transport material). Amount: 0).

で示される構造式を有する材料［トリス（２，４，６－トリメチル－３－（ピリジン－３－イル）フェニル）ボラン（３ＴＰＹＭＢ）］を用いたこと以外は、実施例１と同様にして、量子ドット発光素子を作製した（電子輸送材料添加量（量子ドットに対する質量比）：０．２）。 (Comparative Example 2)
Using the quantum dots CdSe / ZnS synthesized in Example 1, the following formula (4): is used as the electron transport material contained in the light emitting layer 50.

In the same manner as in Example 1, except that a material having the structural formula shown in (2,4,6-trimethyl-3- (pyridin-3-yl) phenyl) borane (3TPYMB)] was used. A quantum dot light emitting device was manufactured (electron transport material addition amount (mass ratio to quantum dots): 0.2).

<Characteristic evaluation of quantum dot light emitting device>
The voltage-brightness characteristics were measured by applying a voltage so as to be negative on the ITO cathode 30 side and positive on the Al anode 80 side of each of the quantum dot light emitting elements of Example 1, Comparative Example 1 and Comparative Example 2 described above. , Electroluminescence (EL) spectrum was observed. The voltage-luminance characteristics are shown in FIG. 3 and the EL spectrum is shown in FIG. In FIG. 4, the EL spectrum of each quantum dot light emitting device is standardized by the peak intensity.

The element of Example 1 shows the same voltage-luminance rise as the element of Comparative Example 1 without electron transport material mixing, and the reached luminance is much higher than that of Comparative Example 1. It can be determined that even if the electron transport material (PO-T2T) is mixed with the light emitting layer, the device characteristics can be improved without hindering the electron transport property. The voltage-luminance characteristic of Comparative Example 2 has also been confirmed to have the effect of increasing the ultimate luminance as in Example 1, but the voltage increase is further suppressed in Example 1.

Next, the EL spectra of the quantum dot light emitting devices of Example 1, Comparative Example 1, and Comparative Example 2 are compared. The EL spectra from 460 nm to 700 nm are derived from quantum dots and are in agreement. On the other hand, at a shorter wavelength of around 400 nm, only Comparative Example 2 showed a higher value, and a color mixture of light emission derived from the electron transport material was confirmed, whereas in Example 1, a comparative example containing no electron transport material was confirmed. It was suppressed to the same low level as No. 1 (similar to the noise level).

The emission characteristics of Example 1, Comparative Example 1, and Comparative Example 2 are summarized in Table 1. In Table 1, "EL intensity ratio at a wavelength of 400 nm" indicates the ratio of the emission intensity at a wavelength of 400 nm to the emission intensity at the peak of the EL spectrum.

From the results of Example 1 above, in the quantum dot light emitting device according to the present invention, by including the electron transporting material having a phosphine oxide group together with the quantum dots in the light emitting layer, the color purity of EL light emission is improved and the color purity is improved. It was shown that the emission characteristics can be improved.

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 device 20: Substrate 30: Cathode 40: Electron injection layer 50: Light emitting layer 60: Hole transport layer 70: Hole injection layer 80: Anode

Claims (5)

A quantum dot light emitting device comprising a cathode, a light emitting layer, and an anode, wherein the light emitting layer is located between the cathode and the anode.
The light emitting layer contains quantum dots and an electron transporting material.
A quantum dot light emitting device, wherein the electron transport material is made of a compound containing a phosphine oxide group. The quantum dot light emitting device according to claim 1, wherein the compound containing a phosphine oxide group has a nitrogen-containing heterocycle. The quantum dot light emitting device according to claim 1 or 2, wherein the compound containing a phosphine oxide group has a diphenylphosphine oxide group. Any of claims 1 to 3, wherein the compound containing a phosphine oxide group is 2,4,6-tris [3- (diphenylphosphinyl) phenyl] -1,3,5-triazine (PO-T2T). The quantum dot light emitting element according to item 1. A display device comprising the quantum dot light emitting device according to any one of claims 1 to 4.

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