WO2014097878A1

WO2014097878A1 - Light emitting device, and method for producing light emitting device

Info

Publication number: WO2014097878A1
Application number: PCT/JP2013/082425
Authority: WO
Inventors: 村山　浩二
Original assignee: 株式会社村田製作所
Priority date: 2012-12-20
Filing date: 2013-12-03
Publication date: 2014-06-26

Abstract

Nano elements (9) each comprise a core shell construction having a core part (10) and a shell part (11) that protects the core part (10). Quantum dots (5) are each covered by a surfactant (12) that provides the surface of the shell part (11) in the nano element (9) with bulky ligands. In a light emitting layer (6), spaces between the quantum dots (5) are filled with an insulating material comprising a polymer compound. The thickness of the light emitting layer (6) is controlled in such a manner as to be 0.5 to 2 monolayers. A quantum dot dispersion solution obtained by dispersing the quantum dots (5) in a dispersion solvent, and an insulating solution obtained by dissolving the insulating material (14) in an organic solvent are prepared, and the light emitting layer (6) is formed by applying a mixed solution obtained by mixing both of the abovementioned solutions together on a hole transport layer (4). A light emitting device in which the efficiency of injecting a carrier in nano particles is improved, that has good light emission properties, and is efficient at ejecting light to the outside from a light emitting layer is thus achieved. Furthermore, a method for producing the light emitting device is thus achieved.

Description

Light emitting device and method for manufacturing the light emitting device

The present invention relates to a light-emitting device and a method for manufacturing the light-emitting device, and more particularly, a light-emitting device having a quantum dot in which a surface of ultrafine particles is coated with a surfactant and forming a light-emitting layer with the aggregate of the quantum dots The present invention relates to a device and a manufacturing method thereof.

Quantum dots, which are nanoparticles with a particle size of 10 nm or less, have excellent carrier (electron, hole) confinement properties, and can easily generate excitons by electron-hole recombination. Therefore, light emission from free excitons can be expected, and light emission with high emission efficiency and sharp emission spectrum can be realized. Further, since quantum dots can be controlled in a wide wavelength range using the quantum size effect, their application to light emitting devices such as semiconductor lasers and light emitting diodes (LEDs) has attracted attention.

In this type of light emitting device, it is important to confine carriers in quantum dots (nanoparticles) with high efficiency and recombine to increase the light emission efficiency. As a method for producing quantum dots, a method called a self-assembly method is known. This self-assembling method is a method of producing quantum dots by a dry process, and a semiconductor layer is vapor-phase epitaxially grown under a specific condition that causes lattice mismatch to self-form a three-dimensional quantum dot structure.

FIG. 11 is a cross-sectional view schematically showing a quantum dot structure manufactured by this type of self-assembly method.

In the self-assembly method, a plurality of quantum dots 102 are discretely distributed on the surface of the first substrate 101, and a second substrate 103 is formed on the surface of the first substrate 101 so as to cover the quantum dots 102. The That is, in this self-assembling method, distortion is caused by the difference between the lattice constant of the first substrate 101 and the lattice constant of the second substrate 103, and when the epitaxial growth cannot be performed, quantum dots 102 are formed at the locations where the distortion occurs. Is done. The self-assembly method is said to be capable of obtaining a high-quality quantum dot structure in which quantum dots are distributed at a higher density than in the case of artificially performing microfabrication.

However, in such a self-assembly method, since the quantum dots 102 are discretely distributed on the first substrate 101, the quantum dots 102 cannot be integrated on the first substrate 101 with high density. That is, in the self-assembly method, the surface density of the quantum dots 102 is small, and the interval t between the quantum dots 102 is larger than the quantum size of the quantum dots 102. In addition, since the first substrate 101 is bonded to the second substrate 103 at locations other than where the quantum dots 102 are formed, electrons and holes are injected into the quantum dots 102 as indicated by arrows in the figure. Without being transported to the first substrate 101 or the second substrate 103, the luminous efficiency may be reduced.

In the self-assembly method, carriers that are not injected into the quantum dots may be recombined outside the quantum dots 102. If the light is recombined outside the quantum dots 102 to emit light as described above, the emission color purity may be reduced, and if the light is not emitted even after recombination, a leakage current may be caused and the emission efficiency may be reduced. There is.

Therefore, in Patent Document 1, a substrate having a main surface made of a first semiconductor, a plurality of quantum dots distributed discretely on the main surface, and a surface on which the quantum dots are distributed is formed. A covering layer made of the second semiconductor and a band gap of the first and second semiconductors disposed in at least a part of a region where the quantum dots are not arranged in a plane where the quantum dots are distributed. A semiconductor device having a third semiconductor having a larger band gap or a barrier layer formed of an insulating material has been proposed.

In this Patent Document 1, as shown in FIG. 12, a barrier layer 104 having insulating properties is formed by oxidizing a semiconductor such as AlAs having a larger band gap energy than the first substrate 101 and the second substrate 103. As a result, as indicated by arrows in the figure, carriers (electrons and holes) can be easily injected into the quantum dots 102 to improve luminous efficiency by recombination.

Patent Document 2 discloses a light emitting layer that is formed of quantum dots and emits light by recombination of electrons and holes, an n-type inorganic semiconductor layer that transports the electrons to the light emitting layer, and the holes in the light emitting layer. A transporting p-type inorganic semiconductor layer, a first electrode for injecting the electrons into the n-type inorganic semiconductor layer, and a second electrode for injecting the holes into the p-type inorganic semiconductor layer There has been proposed a light emitting device comprising:

In Patent Document 2, as shown in FIG. 13, a p-type semiconductor layer 111 and an n-type semiconductor layer 112 are formed of an inorganic material, and a light-emitting layer is formed between the p-type semiconductor layer 111 and the n-type semiconductor layer 112. A quantum dot layer 113 is interposed. That is, in Patent Document 2, a potential barrier is formed between the light-emitting layer and the carrier transport layer by forming the p-type semiconductor layer 111 and the n-type semiconductor layer 112 with an inorganic material having a band structure with good carrier transport properties. Even if there is a carrier, carriers are injected into the quantum dot layer 113 by the tunnel effect, thereby improving the injection efficiency of carriers into the quantum dot layer 113.

Moreover, in this patent document 2, the quantum dot layer semiconductor 113 is produced by a wet process, the substrate on which the p-type semiconductor layer 111 is formed is immersed in a colloidal solution containing quantum dots, and the substrate is pulled up. The quantum dot layer 113 is formed by evaporating the colloidal solution.

JP 2002-184970 A (Claim 1, Claim 2, FIG. 1, FIG. 3) JP 2006-185985 (Claim 1, FIG. 1, FIG. 3)

However, in Patent Document 1, although the barrier layer 104 is formed to fill the gap between the quantum dots 102 as shown in FIG. 12 described above, the barrier layer 104 is formed by a dry process. In order to effectively fill the gap with the barrier layer 104, the manufacturing process may be complicated.

That is, in Patent Document 1, the barrier layer 104 is formed after the quantum dot 102 is formed. In this case, the barrier layer on the quantum dot 102 is dry-etched so as not to prevent carrier injection into the quantum dot 102. 104 needs to be removed, resulting in a complicated manufacturing process.

Further, in Patent Document 2, after the substrate is immersed in the colloidal solution, the substrate is pulled up to evaporate the colloidal solution. However, there is a possibility that the ultrafine nanoparticles are aggregated, and the particle size becomes coarse and desired. It is difficult to obtain the quantum dot layer 113 having the above emission characteristics.

And in order to avoid such aggregation of nanoparticles, as shown in FIG. 14, the surface of the nanoparticle 114 is coat | covered with the surfactant 115 which has a ligand, and the surface defect of the nanoparticle 114 is removed. It may be desirable to deactivate.

In this case, if the ligand of the surfactant 115 is low in volume, it is difficult to obtain sufficient dispersibility even if the nano particles 114 are coated with the surfactant 115. Because of its low molecular weight, its melting point and boiling point are low, and many are liquid at room temperature. In addition, such a surfactant that is liquid at normal temperature has a strong molecular motion, and the probability of inactivating the surface defects of the nanoparticles is reduced. Therefore, it is preferable to use a surfactant having a bulky ligand such as hexadecylamine.

However, when the nanoparticles 114 are coated with the surfactant 115 having a bulky ligand, the interval between the nanoparticles 114 is opened, and it is difficult to sufficiently densify the film density. For this reason, in the process of forming the n-type semiconductor layer 112, the n-type semiconductor material 116 may flow between the nanoparticles 114. As a result, the carriers are not injected into the nanoparticles 114 but pass through the surfactant 115 and pass through the gaps between the nanoparticles 114 and the nanoparticles 114, so that the p-type semiconductor layer (hole transport layer) 111 and the n-type semiconductor are used. There is a risk of flowing into the layer (electron transport layer) 112. In addition, as indicated by a in the figure, carriers may recombine in the n-type semiconductor material 116 flowing between the nanoparticles 114, and exciton emission may occur in the n-type semiconductor material 116.

Thus, in Patent Document 2, even if the surface of the nanoparticle 114 is coated with the surfactant 115, if the ligand of the surfactant 115 is bulky, the carrier does not recombine in the nanoparticle 114. The carriers that pass through the outside of the nanoparticles 114 and reach the p-type semiconductor layer 111 and the n-type semiconductor layer 112 or recombine outside the nanoparticles 114 are generated, resulting in a decrease in luminous efficiency and emission color purity. There is a risk of lowering.

On the other hand, when the surfactant 115 having a low ligand volume is used, the dispersibility is lacking as described above, and the surface defects of the nanoparticles 114 may not be sufficiently inactivated.

The present invention has been made in view of such circumstances, and has improved the efficiency of carrier injection into nanoparticles, has good light emission characteristics, and has good light extraction efficiency from the light emitting layer to the outside. It is an object to provide a device and a method for manufacturing the light emitting device.

The present inventor has conducted intensive research to achieve the above object, and as a result, a dense insulating material made of a polymer compound excellent in voltage resistance is filled between ultrafine particles as nanoparticles, and the light-emitting layer is formed. By controlling the thickness to 0.5 to 2 monolayers, it is possible to improve the efficiency of carrier injection into ultrafine particles, thereby improving the light emission characteristics and the efficiency of extracting light to the outside. The knowledge that there is.

The present invention has been made based on such knowledge, and the light-emitting device according to the present invention is a light-emitting device in which a light-emitting layer is formed of an assembly of quantum dots, and the quantum dot has an ultrafine particle surface. The light emitting layer is coated with a surfactant, and the space between the quantum dots is filled with an insulating material made of a polymer compound, and the light emitting layer has a thickness of 0.5 to 2 monolayers. It is a feature.

Here, the monolayer refers to the average number of particles in the thickness direction of the quantum dots that are closely packed in the light emitting layer.

In the light emitting device of the present invention, the polymer compound is selected from the group consisting of a polyimide resin, an acrylic resin, a polystyrene resin, a polycarbonate resin, a polyester resin, a polyamide resin, and a polyvinyl resin. Preferably it contains more than one species.

Furthermore, in the light-emitting device of the present invention, it is preferable that the ultrafine particles have a core-shell structure including a core portion and a shell portion covering the core portion.

In the light emitting device of the present invention, it is preferable that a hole transport layer is formed on one main surface of the light emitting layer and an electron transport layer is formed on the other main surface of the light emitting layer.

In the light emitting device of the present invention, it is preferable that the ultrafine particles include any one of a compound semiconductor, an oxide, and a single semiconductor.

Furthermore, the light-emitting device of the present invention is preferably an electroluminescence element (hereinafter referred to as “EL element”).

Further, the light emitting layer can be manufactured inexpensively and efficiently without preparing a mixed solution in which a quantum dot dispersion solution and an insulating solution are mixed, without requiring a plurality of complicated film forming processes.

That is, the method for manufacturing a light-emitting device according to the present invention includes a quantum dot dispersion solution preparation step of preparing a quantum dot dispersion solution coated with ultrafine particles with a surfactant, and an insulating material made of a polymer compound dissolved in a solvent. An insulating solution preparation step for preparing an insulating solution, a mixed solution preparation step for preparing a mixed solution by mixing the quantum dot dispersion solution and the insulating solution, and a light emitting layer using the mixed solution. And a light emitting layer manufacturing step to be manufactured.

Moreover, the manufacturing method of the light-emitting device of the present invention includes a first electrode layer forming step of forming a first electrode layer by applying a first conductive material to the surface of the transparent substrate, and the first electrode. Applying a hole transporting material to the surface of the layer to form a hole transporting layer, and forming an electron transporting layer by applying an electron transporting material to the surface of the light emitting layer. An electron transport layer forming step, and a second electrode layer forming step of forming a second electrode layer by applying a second conductive material to the surface of the electron transport layer, the light emitting layer forming step comprising: The light emitting layer is preferably formed by applying the mixed solution to the surface of the hole transport layer.

As a result, even if the adjacent hole transport layer or electron transport layer is formed of either an inorganic material or an organic material, leakage of the carrier to the adjacent layer can be suppressed, and high-quality injection efficiency into the ultrafine particles is good. A light-emitting device can be manufactured inexpensively and with high efficiency.

According to the light emitting device of the present invention, the quantum dots are coated with a surfactant on the surface of the ultrafine particles, and the light emitting layer has a polymer compound (polyimide resin, acrylic resin, polystyrene) between the quantum dots. Resin, polycarbonate resin, polyester resin, polyamide resin, polyvinyl resin, etc.) and the thickness of the light emitting layer is 0.5 to 2 monolayers. By avoiding recombination of holes and electrons outside the ultrafine particles, carriers can be efficiently transported into the ultrafine particles, which can improve the efficiency of carrier injection into the ultrafine particles. Become.

That is, carriers injected into the electrode by voltage application can be efficiently injected into the ultrafine particles without recombination of holes and electrons outside the ultrafine particles, thereby improving the emission characteristics, In addition, a light-emitting device with good light extraction efficiency can be obtained.

In addition, according to the method for manufacturing a light-emitting device of the present invention, a quantum dot dispersion solution preparation step for preparing a quantum dot dispersion solution coated with ultrafine particles with a surfactant, and an insulating material made of a polymer compound dissolved in a solvent An insulating solution preparation step for preparing an insulating solution, a mixed solution preparation step for preparing a mixed solution by mixing the quantum dot dispersion solution and the insulating solution, and a light emitting layer using the mixed solution A light-emitting layer manufacturing step for manufacturing a light-emitting layer, and thus a light-emitting layer can be manufactured in one process without requiring a plurality of complicated film forming processes such as a dry process, and can be manufactured efficiently at low cost. Can do.

It is sectional drawing which shows typically one Embodiment of the light-emitting device which concerns on this invention. It is an expanded sectional view which shows typically the quantum dot used for the light-emitting device of FIG. It is principal part sectional drawing at the time of setting the thickness of a light emitting layer to 3 monolayers. It is a figure which shows typically the preparation procedures of the mixed solution which uses a light emitting layer as a raw material. It is a manufacturing process figure (1/2) which shows the manufacturing method of the light emitting device which concerns on this invention. It is a manufacturing process figure (2/2) which shows the manufacturing method of the light-emitting device which concerns on this invention. It is a figure which shows the TEM image of the sample number 4 in an Example. It is a figure which shows the emission spectrum of the sample number 2 and the sample number 6 in an Example. It is a figure which shows the voltage-luminance characteristic of an Example sample. It is a figure which shows the current density-external quantum efficiency characteristic of an Example sample. It is sectional drawing for demonstrating the general method in the case of producing a quantum dot by the self-assembly method. It is sectional drawing for demonstrating the prior art described in patent document 1. FIG. It is sectional drawing for demonstrating the prior art described in patent document 2. FIG. 10 is a schematic diagram for explaining the problem of Patent Document 2. FIG.

Next, an embodiment of the present invention will be described in detail.

FIG. 1 is a cross-sectional view schematically showing one embodiment of an organic EL element such as a light emitting diode as a light emitting device according to the present invention.

In this organic EL element, an anode 2 is formed on a glass substrate (transparent substrate) 1, and a hole injection layer 3 and a hole transport layer 4 made of a hole transporting material are sequentially formed on the surface of the anode 2, A light emitting layer 6 made of an assembly of quantum dot layers 5 is formed on the surface of the hole transport layer 4, and an electron transport layer 7 made of an electron transporting material is formed on the surface of the light emitting layer 6. A cathode 8 is formed on the surface of the layer 7.

As shown in FIG. 2, the quantum dot 5 has a core-shell structure in which nanoparticles (ultrafine particles) 9 have a core portion 10 and a shell portion 11 that protects the core portion 10, and the surface of the shell portion 11 is an interface. Coated with activator 12. Then, the holes transported from the hole transport layer 4 and the electrons transported from the electron transport layer 7 are injected into the core part 10 of the nanoparticles 9, and the holes and electrons are injected into the core part 10. Recombines and emits excitons.

Here, the core material for forming the core portion 10 is not particularly limited as long as it is an inorganic semiconductor material having a photoelectric conversion effect, and InP, CdSe, CdS, PbSe, and the like can be used. As a shell material constituting the shell portion 11, an inorganic material having a higher energy level than the core material, for example, ZnS can be used.

As the surfactant 12, as described in the section [Problems to be Solved by the Invention], a bulky polar group is used from the viewpoint of efficiently deactivating dispersibility and surface defects of the nanoparticles 9. It is possible to use an organic compound having a polar group bonded to an alkyl group such as a long-chain amine such as hexadecylamine or octadecylamine, trioctylphosphine, trioctylphosphine oxide, oleic acid or myristic acid. The polar group is coordinated to the surface of the nanoparticle 9 as a ligand.

Moreover, as a hole transport material which forms the hole injection layer 3 and the hole transport layer 4, various organic semiconductor materials of the hole transport property normally used for organic EL elements, for example, poly (3,4) -Ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT: PSS), N, N'-diphenyl-N, N'-bis (3-methylphenyl) -1,1'-biphenyl-4,4 '-Diamine (TPD), 4,4'-bis [N- (1-naphthyl) -N-phenyl-amino] biphenyl (α-NPD), 4,4', 4 "-tris (2-naphthylphenylamino) ) Triphenylamine (2-TNATA), N, N′-7-di (1-naphthyl) -N, N′-diphenyl-4,4′-diaminobiphenyl (Spiro-NPB), 4,4 ′, 4 ″ -Tris (3-methylphenylphenyla Mino) triphenylamine (m-MTDATA), and derivatives thereof can be used.

Examples of the electron transporting material for forming the electron transporting layer 7 include various organic semiconductor materials having electron transporting properties that are usually used for organic EL elements, such as tris (8-hydroxyquinoline) aluminum (Alq3), 2- (4-Biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole (PBD), 2,2 ′, 2 ″-(1,3,5-benzonitrile) -Tris (1-phenyl-1-H-benzimidazole (TPBi), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 3- (benzothiazol-2-yl) -7- ( Diethylamino) -2H-1-benzopyran-2-one (coumarin 6), bis (2-methyl-8-quinolinolato) -4- (phenylphenolato) aluminum (BAlq) 4,4'-bis (9-carbazolyl) -2,2'-dimethyl-biphenyl (CDBP), and you can use these derivatives.

In the light emitting layer 6, the insulating material 13 is filled in the gaps between the quantum dots 5.

In order to recombine carriers in the core portion 10 of the quantum dot 5 and efficiently emit excitons, it is necessary to efficiently inject the carrier transported in the vicinity of the quantum dot 5 into the core portion 10. .

However, since the surface of the nanoparticle 9 is coated with the surfactant 12 having a bulky ligand, the quantum dot 5 is fixed between the nanoparticles 9 even if the surface density of the quantum dot 5 is improved. A gap is created. Accordingly, the carriers are not efficiently injected into the core portion 10 of the nanoparticles 9 and recombined outside the core portion 10 or reach the hole transport layer 4 or the electron transport layer 7, As a result, the emission color purity and the emission efficiency may be reduced.

Therefore, in the present embodiment, the gaps between the quantum dots 5 are filled with a dense insulating material 13 having excellent voltage resistance.

Such an insulating material is preferably a polymer compound. In the case of an inorganic material, a grain boundary is formed, so that the carrier passes through the periphery of the nanoparticle 9 without being injected into the core portion 10 of the nanoparticle 9 in such a form as to travel through the grain boundary, or the grain boundary. In this case, electrons and holes may be recombined, which is not preferable.

Such a polymer compound is not particularly limited, and examples thereof include polyimide resins, acrylic resins such as polymethyl methacrylate (PMMA) and polyacrylate, homopolymerized styrene, and acrylonitrile-styrene monomers. Polystyrene resins such as copolymer (AS) or acrylonitrile-butadiene-styrene copolymer (ABS), polycarbonate resins, polyamide resins such as nylon, polyester resins, polyvinyl acetate and polyvinyl chloride, etc. Various polymer compounds such as a resin can be used. Further, since the light emitting layer 6 is an ultra-thin film as will be described later, even if it is colored, the light emission purity is not affected.

Furthermore, in the present embodiment, the thickness of the light emitting layer 6 is controlled to be an ultrathin film of 0.5 to 2 monolayers. Here, the monolayer refers to the average number of particles in the thickness direction of the quantum dots 5 that are closely packed in the light emitting layer 6. For example, when the average value of the number of particles in the thickness direction of the light emitting layer 6 is “1” in the state in which the quantum dots 5 are converted into close-packed in-plane, one monolayer (monomolecular layer), The case of “5” is 0.5 monolayer, and the case of “1.3” is 1.3 monolayer.

The smaller the surface density of the nanoparticles 9 of the light emitting layer 6 is, the larger the filling effect of the insulating material 13 is. From this viewpoint, the thickness of the light emitting layer 6 needs to be 2 monolayers or less.

FIG. 3 is a cross-sectional view of the main part of the organic EL element when the thickness of the light emitting layer is 3 monolayers.

When the thickness of the light emitting layer 6 exceeds 2 monolayers and becomes a thick film, for example, 3 monolayers, the surface density becomes saturated. For this reason, when the insulating material 13 is filled between the quantum dots 5, the interval between the quantum dots 5 is expanded, and the carrier transportability may be lowered.

In addition, when the light emitting layer 6 has two or more monolayers such as three monolayers, the quantum dots 5 in contact with only one of the hole transport layer 4 and the electron transport layer 7 are included in the light emitting layer 6. As a result, quantum dots 5 that do not contact any of the hole transport layer 4 and the electron transport layer 7 are formed, and it becomes difficult to efficiently inject electrons and holes into the nanoparticles 9 at the same time. That is, the carrier must be injected into the core portion 10 through the dense insulating material having high withstand voltage in addition to the surfactant 12 and the shell portion 11, and the carrier can be efficiently transported. This may make it difficult to recombine electrons and holes in the core 10 and reduce the possibility of exciton light emission.

Therefore, the thickness of the light emitting layer 6 needs to be 2 monolayers or less.

However, the thickness of the light emitting layer 6 must be at least 0.5 monolayer. When the thickness of the light emitting layer 6 is less than 0.5 monolayer, the thickness of the light emitting layer 6 becomes excessively thin, and the distance between the hole transport layer 4 and the electron transport layer 7 becomes extremely small, and near the boundary surface between the two. Thus, there is a possibility that light is totally reflected and light energy cannot be sufficiently extracted outside, particularly in a region where the current density is low.

In the organic EL element formed in this way, carriers are injected into the anode 2 and the cathode 8 when a voltage is applied. The holes injected into the anode 2 and the electrons injected into the cathode 8 are injected into the nanoparticles 9 through the bulk hetero network of the surfactant 12, and the holes and electrons are injected into the core 10. Recombine and emit light.

And in this Embodiment, since a hole and an electron do not flow into the gap | interval of the quantum dot 5, but are inject | poured into the nanoparticle 9 via the surfactant 12, a hole and an electron are in the middle of transport. Thus, the carrier can be transported efficiently without recombination, and photoelectric conversion from an electric signal to an optical signal can be performed with high efficiency.

Next, a method for manufacturing the organic EL element will be described.

As described above, the nanoparticles 9 can be manufactured using various materials. In the following embodiment, the case where InP is used for the core portion 10 and ZnS is used for the shell portion 11 will be described as an example. .

FIG. 4 is a production process diagram showing a procedure for producing a quantum dot dispersion solution.

First, as shown in FIG. 4A, a quantum dot dispersion solution 14 containing InP / ZnS nanoparticles 9 is prepared.

That is, for example, indium acetate, myristic acid and octadecene are mixed in a container and dissolved by stirring in a nitrogen atmosphere, thereby preparing an indium precursor solution. Further, tristrimethylsilylphosphine, octylamine, and octadecene are mixed in a nitrogen atmosphere, thereby preparing a phosphorus precursor solution.

Next, the indium precursor solution is heated to a predetermined temperature (for example, 190 ° C.), and the phosphorus precursor solution is injected into the heated solution. Then, precursors with high activity react with each other at a high temperature, indium and phosphorus combine to form nuclei, and then react with surrounding unreacted components to cause crystal growth, thereby producing InP quantum dots. The

Next, a zinc oxide solution in which zinc oxide is dissolved in stearic acid and a sulfur solution in which sulfur is dissolved in stearic acid are prepared.

Next, a small amount of zinc oxide solution and sulfur solution are alternately added dropwise to an InP quantum dot solution adjusted to a predetermined temperature (for example, 150 ° C.), heated and cooled, and washed to remove excess organic components in the solution. . After that, while adding a surfactant made of an organic compound such as hexadecylamine (hereinafter referred to as “HDA”), it is dispersed in a dispersion solvent, for example, toluene, whereby InP coated with the surfactant. An InP / ZnS dispersion solution having / ZnS as nanoparticles 9, that is, a quantum dot dispersion solution 14 is produced. The concentration of the quantum dot dispersion solution 14 is adjusted according to a desired monolayer of the light emitting layer 6, for example, 1 to 5 mg / mL.

Next, as shown in FIG. 4B, the insulating material 13 is dissolved in an organic solvent of the same type as the dispersion solvent, for example, toluene, to produce an insulating solution 15.

Then, the quantum dot dispersion solution 14 and the insulating solution 15 are mixed to prepare a mixed solution 16 having a quantum dot concentration of 0.8 to 1.0 mg / mL as shown in FIG.

Note that the organic solvent for dissolving the insulating material 13 can be appropriately selected according to the insulating material 13 to be used.

For example, when a polyimide resin is used for the insulating material 13, a compound having a five-membered lactam structure such as N-methyl-2-pyrrolitone or γ-butyrolactone can be used. When acrylic resin, polycarbonate resin, or polyester resin is used for the insulating material 13, aromatic hydrocarbons such as benzene, toluene, and xylene, chlorine-containing hydrocarbons such as trichlene, chlorobenzene, dichloroethane, and chloroform, and esters Organic organic compounds, ketone organic compounds, and the like can be used. Moreover, when using a polystyrene-type resin for the insulating material 13, acetone, methanol, ethanol, etc. can be used in addition to the above-mentioned aromatic hydrocarbon and chlorine-containing hydrocarbon, and a polyamide-type resin is used. Phenol or methanol can be used. Furthermore, when using a polyvinyl acetate resin for the insulating material 13, in addition to the aromatic hydrocarbon and chlorine-containing hydrocarbon described above, aniline and pyridine can be used, and when using a polyvinyl chloride resin, The chlorine-containing hydrocarbons and acetone described above can be used.

Further, the dispersion solvent of the quantum dot dispersion solution 14 is selected according to the organic solvent dissolved in the polymer compound as described above. For example, when PMMA is used for the insulating material 13, toluene in which PMMA is dissolved can be selected as an organic solvent for the insulating solution 15 and a dispersion solvent for the quantum dot dispersion solution 14.

5 and 6 are manufacturing process diagrams showing the manufacturing method of the light emitting diode.

As shown in FIG. 5 (a), an ITO film is formed on the glass substrate 1 by sputtering and UV ozone treatment is performed to form an anode 2 having a thickness of 100 nm to 150 nm.

Next, a hole injection layer solution is prepared. As the material for the hole injection layer, the same material as the hole transporting material can be used, and for example, PEDOT: PSS or the like can be used.

Then, the hole injection layer solution is applied onto the anode 2 by using a spin coating method or the like to form the hole injection layer 3 having a film thickness of 20 nm to 30 nm as shown in FIG.

Next, a hole transport layer solution having a lower energy of HOMO (Highest Occupied Molecular Orbital) level than the hole injection layer material is prepared. For example, when PEDOT: PSS is used as the material for the hole injection layer, poly-TPD having a HOMO level lower than that of PEDOT: PSS can be used.

Then, the hole transport layer solution is applied onto the positive electrode injection layer 3 by using a spin coat method or the like to form the hole transport layer 4 having a film thickness of 60 nm to 70 nm as shown in FIG.

Since the hole injection layer 3 is provided to improve the hole transportability, the hole transport layer 4 may also be used as the hole injection layer 3. The hole transport layer 4 can be formed of only poly-TPD, and the hole injection layer 3 can be omitted.

Next, the mixed solution 16 described above is prepared.

Then, using a spin coating method or the like, the mixed solution 16 is applied onto the hole transport layer 4 and dried in an N ₂ atmosphere as shown in FIG. The light emitting layer 6 is formed.

Next, using an electron transporting material such as Alq3, as shown in FIG. 6E, an electron transporting layer 7 having a film thickness of 50 nm to 70 nm is formed on the surface of the light emitting layer 6 by a vacuum deposition method.

Then, as shown in FIG. 6 (f), LiF, Al or the like is used to form a cathode 8 having a film thickness of 100 nm to 300 nm by a vacuum vapor deposition method, whereby a light emitting diode is manufactured.

As described above, in this embodiment, the quantum dot dispersion solution 14 in which the nanoparticles 9 are coated with the surfactant 12 is prepared, while the insulating material 13 made of a polymer compound is dissolved in a solvent to form the insulating solution 15. The mixed solution 16 is prepared by mixing the quantum dot dispersion solution 14 and the insulating solution 15, and the light emitting layer 6 is formed by applying the mixed solution 16 to the surface of the hole transport layer 4. Therefore, the light emitting layer 6 can be manufactured in one process without requiring a plurality of complicated film forming processes such as a dry process, and can be manufactured inexpensively and efficiently.

In the organic EL device thus formed, the space between the nanoparticles 9 is filled with the insulating material 13, so that carriers are injected into the nanoparticles 9 with high efficiency and pass through the outside of the nanoparticles 9. Can be suppressed.

That is, carriers injected into the electrode by voltage application can be efficiently injected into the nanoparticles 9 without recombination of holes and electrons outside the ultrafine particles, thereby improving the light emission characteristics. In addition, it is possible to obtain a light-emitting device having good light extraction efficiency to the outside.

In addition, this invention is not limited to the said embodiment. In the above-described embodiment, the nanoparticles 9 have a core-shell structure in which the core portion 10 is covered with one layer of the shell portion 11, but the core portion has a two-layer core-shell structure or no shell portion. The same applies to the case.

In the above embodiment, a compound semiconductor made of InP / ZnS is used as the nanoparticles 9, but it goes without saying that it may be an oxide or a single semiconductor.

Moreover, although the said embodiment demonstrated the organic EL element which formed the positive hole transport layer 4 and the electron carrying layer 7 adjacent to the light emitting layer 6 with the organic compound, it is the same also about the inorganic EL element formed with the inorganic compound. In addition, it is possible to suppress the leakage of carriers to the adjacent layer and to manufacture a high-quality light-emitting device with good injection efficiency into the nanoparticles 9 at low cost and high efficiency.

In the above embodiment, the electron transport layer 7 is performed by a dry process using a vacuum evaporation method, but may be formed by a wet process such as a spin coating method. However, in this case, it is necessary to use a dispersion solvent having the same polarity as the dispersion solution used in the dipping process.

Needless to say, the present invention can be applied to various light emitting devices such as semiconductor lasers and various display devices other than EL elements.

[Sample preparation]
(Sample numbers 1 to 5)
A quantum dot dispersion solution having a concentration of 2 to 5 mg / mL was prepared by coating InP / ZnS nanoparticles formed of InP in the core and ZnS in the shell with HDA as a surfactant and dispersed in a toluene solution. .

Next, PMMA as an insulating material was dissolved in toluene to prepare an insulating solution having a concentration of 0.1 mol / mL. Thereafter, the quantum dot dispersion solution and the insulating solution were mixed to prepare a mixed solution having a quantum dot concentration of 0.8 mg / mL.

Next, an ITO film was formed on the glass substrate by a sputtering method, UV ozone treatment was performed, and an anode with a film thickness of 120 nm was produced.

Next, PEDOT: PSS as a hole injection layer material was dispersed in pure water to prepare a hole injection layer solution. Then, a hole injection layer solution was applied on the anode using a spin coating method to form a 20 nm thick hole injection layer.

Next, poly-TPD as a hole transporting material was dispersed in chlorobenzene to prepare a hole transport layer solution. Then, the hole transport layer solution was applied onto the positive electrode injection layer by using a spin coating method to produce a 65 nm-thick hole transport layer.

Next, using the spin coating method, the above-described mixed solution was applied onto the hole transport layer to form a light emitting layer. That is, the mixed solution was dropped onto the hole transport layer and the glass substrate was rotated at 3000 rpm for 60 seconds to produce a light emitting layer.

Next, Alq3 was used as an electron transporting material, and an electron transport layer having a film thickness of 50 nm was formed on the surface of the light emitting layer by a vacuum deposition method.

Then, using LiF and Al, a cathode having a film thickness of 250 nm having a two-layer structure was formed by a vacuum deposition method, and thereby samples Nos. 1 to 5 were produced.

The light emitting layer was adjusted to have a thickness of 0.3 to 4 monolayer by adjusting the dropping amount of the mixed solution and the concentration of quantum dots in the mixed solution.

Here, the number of monolayers of the light emitting layer was confirmed by observing a cross section of each sample formed up to the cathode with a transmission electron microscope (hereinafter referred to as “TEM”).

For example, for sample number 4, the number of monolayers was confirmed from the TEM image shown in FIG. 7 as follows.

That is, as can be seen from FIG. 7, the average number of particles closely packed in the thickness direction was 2, thereby confirming that the monolayer of sample number 4 was “2”.

Similarly, the number of monolayers was confirmed from the TEM image for each of the sample numbers 1 to 3, and 5.

(Sample No. 6)
In Sample No. 2, instead of using the mixed solution described above, a quantum dot dispersion solution was used, and the quantum dot dispersion solution was applied on the hole transport layer so as to have a thickness of 0.5 monolayer. A layer was formed. Other than that, the sample No. 6 was prepared by the same method and procedure as Sample No. 2.

(Sample evaluation)
Table 1 shows the thickness of the light emitting layer of each sample Nos. 1 to 6 and whether or not the PMMA is filled in the light emitting layer.

Next, for

sample numbers

2 and 6, each sample was placed in an integrating sphere, and a DC voltage was applied using a constant current power source (2400 manufactured by Keithley Instruments Inc.) to cause the sample to emit light. The emitted light was collected by an integrating sphere, and the emission spectrum was measured using a multichannel detector (PMA-11 manufactured by Hamamatsu Photonics).

FIG. 8 shows the measurement results. The horizontal axis indicates the wavelength (nm), the vertical axis indicates the emission spectrum (au), the upper side indicates the measurement result of the sample number 2, and the lower side indicates the measurement result of the sample number 6.

Sample No. 6 had an emission spectrum peaked at a wavelength of around 614 nm, but an emission spectrum appeared even at a wavelength of around 535 nm, and the purity of the emitted color was reduced. This is because Alq3, which is an electron transporting material, flows between quantum dots and there are carriers that recombine within the nanoparticles, while some holes flow into the gaps between the quantum dots in the electron transporting material. It is considered that light was recombined with electrons or recombined with electrons in the electron transport layer adjacent to the light emitting layer, and light was emitted even in the vicinity of 535 nm which is the absorption wavelength region of Alq3.

On the other hand, sample No. 2 is dense and has excellent voltage resistance, and is filled between the quantum dots, so that it emits light only in the vicinity of 614 nm, which is the absorption wavelength region of the quantum dots, and emits light in the electron transport layer. It was confirmed that good emission color purity was obtained.

Next, for each of the samples Nos. 1 to 5, a DC voltage was applied stepwise, and the luminance at that time was measured with the multi-channel detector.

FIG. 9 is a characteristic diagram showing the relationship between the applied DC voltage and the luminance. The horizontal axis indicates the DC voltage (a.u.), and the vertical axis indicates the luminance (a.u.).

As shown in Sample No. 5, when the thickness of the light emitting layer exceeds 2 monolayers and becomes 4 monolayers, it was confirmed that high luminance cannot be obtained even when a high voltage is applied. Since the light emitting layer is thick, there are many quantum dots that do not touch either the hole transport layer or the electron transport layer. As a result, electrons and holes are efficiently injected into the nanoparticles simultaneously. It seems that the number of nanoparticles that emit excitons is extremely small.

As is apparent from FIG. 9, it can be seen that the thinner the light emitting layer is, the higher the luminance is emitted. However, Sample No. 1 is not preferable because the thickness of the light emitting layer is excessively thin as 0.3 monolayer as described later, and the external quantum efficiency ηext is lowered.

Next, a DC voltage was applied in a stepwise manner for each of the samples Nos. 1 to 5, and the current density at that time was measured with the multi-channel detector. As shown in Equation (1), The external quantum efficiency ηext was calculated by multiplying the internal quantum efficiency ηint and the light extraction efficiency ηout.

ηext = γ · ηint · ηout (1)
γ is a carrier balance factor of holes and electrons.

The internal quantum efficiency ηint indicates the recombination ratio of electrons and holes in the quantum dot, that is, the ratio of photons that contributed to exciton emission, measured the number of emitted photons, and injected electrons calculated from the current density. And the number of emitted photons. The light extraction efficiency ηout indicates the ratio of the light extracted to the outside with respect to the exciton emission.

FIG. 10 is a characteristic diagram showing the relationship between the current density and the external quantum efficiency. The horizontal axis represents the current density (au), and the vertical axis represents the external quantum efficiency (a.u.).

As is apparent from FIG. 10, when the light emitting layer has a thickness of 0.5 monolayer or more, good external quantum efficiency is obtained even at a low current density.

On the other hand, as shown in Sample No. 1, when the thickness of the light emitting layer is less than 0.5 monolayer and excessively thinned to 0.3 monolayer, the external quantum efficiency decreases in the region where the current density is low. Yes. This is because when the thickness of the light emitting layer is reduced to 0.3 monolayer, the distance between the hole transport layer and the electron transport layer becomes extremely small, and light is totally reflected near the boundary surface between them, and the current density is particularly low. This is probably because light energy cannot be sufficiently extracted outside in the region.

Therefore, in consideration of luminance characteristics and external quantum efficiency, it was confirmed that the thickness of the light emitting layer is preferably 0.5 to 2 monolayers.

It is possible to realize a light emitting device such as an organic EL element which has a good light emission efficiency by improving the efficiency of carrier injection into the nanoparticles and a good light extraction efficiency to the outside.

1 Transparent substrate 2 Anode (first electrode layer)
4 hole transport layer 5 quantum dot 6 light emitting layer 7 electron transport layer 8 cathode (second electrode layer)
9 Nanoparticles (ultrafine particles)
DESCRIPTION OF SYMBOLS 10 Core part 11 Shell part 12 Surfactant 13 Insulating material 14 Quantum dot dispersion solution 15 Insulating solution 16 Mixed solution

Claims

In the light emitting device in which the light emitting layer is formed of an assembly of quantum dots,
The quantum dot has a surface of ultrafine particles coated with a surfactant,
The light emitting layer is filled with an insulating material made of a polymer compound between the quantum dots,
A light emitting device having a light emitting layer thickness of 0.5 to 2 monolayers.
The polymer compound includes at least one selected from the group consisting of a polyimide resin, an acrylic resin, a polystyrene resin, a polycarbonate resin, a polyester resin, a polyamide resin, and a polyvinyl resin. The light emitting device according to claim 1.
3. The light emitting device according to claim 1, wherein the ultrafine particles have a core-shell structure including a core part and a shell part covering the core part.
The hole transport layer is formed on one main surface of the light emitting layer, and the electron transport layer is formed on the other main surface of the light emitting layer. A light emitting device according to any one of the above.
5. The light emitting device according to claim 1, wherein the ultrafine particles include any one of a compound semiconductor, an oxide, and a single semiconductor.
The light-emitting device according to claim 1, wherein the light-emitting device is an electroluminescence element.
A quantum dot dispersion solution preparation step of preparing a quantum dot dispersion solution coated with ultrafine particles with a surfactant;
An insulating solution preparation step of preparing an insulating solution by dissolving an insulating material made of a polymer compound in a solvent;
A mixed solution preparation step of mixing the quantum dot dispersion solution and the insulating solution to prepare a mixed solution;
And a light emitting layer manufacturing step of manufacturing a light emitting layer using the mixed solution.
A first electrode layer forming step of forming a first electrode layer by applying a first conductive material to the surface of the transparent substrate;
Applying a hole transporting material to the surface of the first electrode layer, and forming a hole transport layer;
Applying an electron transporting material to the surface of the light emitting layer, and forming an electron transport layer; and
A second electrode layer forming step of forming a second electrode layer by applying a second conductive material to the surface of the electron transport layer,
8. The method of manufacturing a light emitting device according to claim 7, wherein in the light emitting layer forming step, the light emitting layer is formed by applying the mixed solution to a surface of the hole transport layer.