WO2020261346A1

WO2020261346A1 - Method for producing light-emitting element and light-emitting element

Info

Publication number: WO2020261346A1
Application number: PCT/JP2019/025011
Authority: WO
Inventors: 貴洋土江
Original assignee: シャープ株式会社
Priority date: 2019-06-24
Filing date: 2019-06-24
Publication date: 2020-12-30

Abstract

This method for producing a light-emitting element comprises a positive electrode formation step, a light-emitting layer formation step, an electron transport layer formation step, and a negative electrode formation step. The electron transport layer formation step includes an immersion step for immersing the light-emitting layer surface on which the electron transport layer will be formed in an aqueous alkaline solution containing a zinc source and a sulfur source. In the electron transport layer formation step, a mixed crystal layer which contains ZnS and Zn(OH)2 having Zn-S bonds and Zn-OH bonds is formed as the electron transport layer.

Description

Manufacturing method of light emitting element and light emitting element

The present invention relates to a method for manufacturing a light emitting element and a light emitting element.

Patent Document 1 discloses a quantum dot light emitting diode (QLED: Quantum dot Light Emitting Diode) using an inorganic layer such as ZnO (zinc oxide) in the electron transporting layer (ETL: Electron transporting layer).

In a light emitting element (QLED) that uses quantum dots (QD: Quantum dots) for the light emitting layer in this way, when an organic layer is used for the electron transport layer, the electron transport layer and light emission are the boundaries between the organic layer and the inorganic layer. Defects are likely to occur between the layers and the stability is reduced when the light emitting element is driven. Further, since the organic layer has a slow electron transfer rate and a low electron density, there is a problem that the electron transport efficiency is lower than the hole transport efficiency. By using the electron transport layer as an inorganic layer, the above-mentioned problems can be solved.

Japanese Patent Publication "Special Table 2008-533735"

However, in Patent Document 1, a solution coating process selected from the group consisting of sol-gel coating, spin coating, printing, casting and spraying, or chemical vapor deposition (CVD), sputtering, is used for forming an electron transport layer. A vapor phase coating process selected from the group consisting of electron beam deposition and vacuum deposition is used.

The film forming method by the vacuum method such as sputtering has good film properties, but sputter damage to quantum dots occurs. Further, in such a vapor phase coating process, since a vacuum device is used, the cost is higher than that of the solution coating method.

As described above, Patent Document 1 also discloses that an electron transport layer is formed by a solution coating process. By using the solution coating process, it is possible to reduce the cost and avoid problems such as spatter damage to the quantum dots.

However, when the electron transport layer (ETL) is formed by the solution coating process in this way, the nanoparticles of the separately synthesized ETL material are coated on the light emitting layer.

FIG. 6 is a diagram schematically showing problems when an electron transport layer (ETL) is formed by a solution coating process in a QLED using quantum dot QD as a light emitting layer. In FIG. 6, HTL indicates a hole transport layer. In FIG. 6, the case where the electron transport layer (ETL) is formed by using a solution containing ZnO nanoparticles (ZnO-NP) (hereinafter, referred to as “ZnO nanoparticles solution”) is illustrated as an example. ..

As shown in FIG. 6, when the electron transport layer is formed in QLED by a coating method using a ZnO nanoparticle solution, the contact point at the interface between the quantum dot QD and ZnO-NP is point contact. As a result, the resistance component increases, the contact resistance increases, the applied voltage increases, resulting in problems such as a decrease in power consumption and power luminous efficiency, and a decrease in the quality of the interface between the quantum dot QD and ZnO-NP. Causes an increase in carrier traps.

One aspect of the present disclosure has been made in view of the above problems, and an object thereof is a light emitting device provided with an electron transport layer made of an inorganic layer, which is inexpensive and has excellent coating properties of an electron transport layer. The purpose is to provide a manufacturing method.

In order to solve the above problems, the method for manufacturing a light emitting element according to one aspect of the present disclosure includes an anode forming step of forming an anode on a substrate, a light emitting layer forming step of forming a light emitting layer on the anode, and the like. The electron transport layer forming step comprising an electron transport layer forming step of forming an electron transport layer adjacent to the light emitting layer on the light emitting layer and a cathode forming step of forming a cathode on the electron transport layer. Includes a dipping step of immersing the surface to be formed of the electron transport layer in the light emitting layer in an alkaline aqueous solution containing a zinc source and a sulfur source, and in the electron transport layer forming step, Zn—S bond is used as the electron transport layer. And a mixed crystal layer containing ZnS and Zn (OH) ₂ having a Zn—OH bond is formed.

In order to solve the above problems, the light emitting element according to one aspect of the present disclosure includes an anode, a light emitting layer, an electron transport layer adjacent to the light emitting layer, and a cathode in this order, and the electron transport layer. Is a mixed crystal layer containing ZnS and Zn (OH) ₂ having a Zn—S bond and a Zn—OH bond.

According to one aspect of the present disclosure, it is possible to provide a light emitting device provided with an electron transport layer made of an inorganic layer, which is inexpensive and has excellent coating properties of the electron transport layer, and a method for manufacturing the same.

It is a figure which shows typically an example of the laminated structure of the light emitting element which concerns on one Embodiment. It is a flowchart which shows an example of the manufacturing method of the light emitting element which concerns on one Embodiment. It is sectional drawing which shows typically the process from step S4 to step S5 shown in FIG. It is sectional drawing which shows typically the state of the interface between a light emitting layer and an electron transport layer in the light emitting element which concerns on one Embodiment. It is a figure which shows the energy band of each layer of the functional layer in the light emitting element which concerns on one Embodiment. It is a figure which shows typically the problem when the electron transport layer is formed by the solution coating process in QLED which used the quantum dot as a light emitting layer.

FIG. 1 is a diagram schematically showing an example of a laminated structure of the light emitting element 1 according to the embodiment. FIG. 2 is a flowchart showing an example of a method for manufacturing the light emitting element 1 according to the embodiment. FIG. 3 is a cross-sectional view schematically showing the steps from step S4 to step S5 shown in FIG.

First, an outline of a laminated structure of the light emitting element 1 and an example of a manufacturing method will be described.

As shown in FIG. 1, the light emitting element 1 includes an electron transport layer 14 and a light emitting layer 13 as functional layers between the cathode 15 (cathode) and the anode 11 (anode), which are counter electrodes arranged vertically. It is a light emitting element. As an example, the light emitting element 1 is provided with an anode 11, a hole transport layer 12 (HTL), a light emitting layer 13, an electron transport layer 14 (ETL), and a cathode 15 on the substrate 10 in this order from the substrate 10 side. .. The electron transport layer 14 is laminated on the light emitting layer 13 adjacent to the light emitting layer 13. The hole transport layer 12 may not be provided. In the present embodiment, the direction from the substrate 10 to the cathode 15 is defined as the upward direction, and the opposite direction is defined as the downward direction. Further, in the present embodiment, the layer formed in the process before the layer to be compared is referred to as "lower layer", and the layer formed in the process after the layer to be compared is referred to as "upper layer". ..

As shown in FIGS. 1 and 2, in the manufacturing process of the light emitting element 1 according to the present embodiment, first, the anode 11 is formed on the substrate 10 (step S1, anode forming step). Next, the hole transport layer 12 is formed on the anode 11 (step S2, hole transport layer forming step). Next, the light emitting layer 13 is formed on the hole transport layer 12 (step S3, light emitting layer forming step). Next, the electron transport layer 14 is formed on the light emitting layer 13 (electron transport layer forming step). In the present embodiment, a chemical solution method called a CBD (Chemical bath deposition) method is used for forming the electron transport layer 14. Therefore, the electron transport layer forming step according to the present embodiment includes a mask step (step S4), a thin film forming step (step S5, dipping step), a drying step (step S6), and a mask removing step (step S6). There is. In the masking step of step S4, as shown in FIG. 3, a part of the surface of the laminate obtained in step S3 is exposed and a part is masked with a mask 21 (mask material). The laminate obtained in step S3 is a laminate including the substrate 10, the anode 11, and the light emitting layer 13, and in the present embodiment, as described above, the anode 11, the hole, and the hole are placed on the substrate 10. A laminated body formed by laminating the transport layer 12 and the light emitting layer 13 in this order is shown. Hereinafter, the laminated body will be referred to as "laminated body 20" for convenience of explanation.

In step S4, specifically, a part of the surface of the light emitting layer 13 which is the surface to be formed of the electron transport layer 14 (in other words, the formed region of the electron transport layer 14) in the light emitting layer 13 of the laminated body 20. (Specifically, the upper surface) is exposed. Then, at least a part of the surface of the laminated body 20 other than the surface to be formed of the electron transport layer 14 (in other words, the electron transport layer non-formed region that does not form the electron transport layer 14) is masked. In addition, in FIG. 3, as an example, the case where the side surface and the back surface of the laminated body 20 are masked is taken as an example.

Next, in the thin film forming step of step S5, as shown in FIG. 3, the laminate 20 in which a part of the light emitting layer 13 is exposed is immersed in the reaction solution 23 in the reaction vessel 22. As the reaction solution 23, an alkaline aqueous solution containing a zinc source and a sulfur source, which is a base material of the electron transport layer 14, is used. As a result, a thin film derived from the alkaline aqueous solution, which becomes the electron transport layer 14, is formed on the light emitting layer 13. Next, in the drying step of step S6, the thin film formed in step S5 is dried. Then, in the mask removing step of step S7, the mask 21 is removed. Next, the cathode 15 is formed on the electron transport layer 14 thus formed (step S8). The light emitting element 1 shown in FIG. 1 is manufactured by the above steps.

Next, the laminated structure and manufacturing method of the light emitting element 1 will be described in more detail.

As shown in FIG. 1, in the light emitting element 1, the substrate 10 is a support substrate that supports the anode 11, the hole transport layer 12, the light emitting layer 13, the electron transport layer 14, and the cathode 15. The light emitting element 1 may be used as a light source of an electronic device such as a display device, for example. When the light emitting element 1 is the light source of the display device, for example, an array substrate on which a thin film transistor is formed is used for the substrate 10, and the anode 11 is electrically connected to the thin film transistor of the array substrate.

For the formation of the anode 11 and the cathode 15, various conventionally known methods such as a sputtering method, a vacuum vapor deposition method, a CVD (chemical vapor deposition) method, a plasma CVD method, and a printing method can be used.

The anode 11 and the cathode 15 are connected to a power source 16 (for example, a DC power source), so that a voltage is applied between them. The anode 11 and the cathode 15 contain a conductive material and are electrically connected to the hole transport layer 12 and the electron transport layer 14, respectively. At least one of the anode 11 and the cathode 15 is made of a light transmissive material. Either one of the anode 11 and the cathode 15 may be formed of a light-reflecting material. The light emitting element 1 can extract light from the electrode (specifically, the transparent electrode) side made of a light transmissive material.

When the light emitting element 1 is a top emission type light emitting element, the upper electrode, for example, the cathode 15 is formed of a light transmitting material, and the lower electrode, for example, the anode 11 is formed of a light reflecting material. When the light emitting element 1 is a bottom emission type light emitting element, the upper electrode, for example, the cathode 15 is formed of a light reflecting material, and the anode 11 which is a lower electrode is formed of a light transmitting material.

As the light transmissive material, for example, a transparent conductive film material can be used. Specific examples of the light-transmitting material include ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), ZnO (zinc oxide), AZO (aluminum-doped zinc oxide), and BZO (boron-doped zinc). Oxide), GZO (gallium-doped zinc oxide) and the like can be used. Since these materials have high visible light transmittance, the luminous efficiency is improved.

As the light-reflecting material, a material having a high reflectance of visible light is preferable, and for example, a metal material can be used. Specifically, for example, Al (aluminum), Cu (copper), Au (gold), Ag (silver) and the like can be used as the light-reflecting material. Since these materials have high visible light reflectance, the luminous efficiency is improved. Further, by forming either one of the anode 11 and the cathode 15 as a laminate of a layer made of a light-transmitting material and a layer made of a light-reflecting material, an electrode having light reflectivity may be used.

The hole transport layer 12 is a layer that transports holes from the anode 11 to the light emitting layer 13. The hole transport layer 12 contains a hole transport material having excellent hole transport properties. The hole transport layer 12 may have a function of inhibiting the transport of electrons. Further, the hole transport layer 12 may also have a function as a hole injection layer (HIL) that promotes hole injection from the anode 11 to the light emitting layer 13, and the anode 11 injects holes. It may also have a function as a layer (EIL).

A known hole transporting material can be used for the hole transporting layer 12. The hole transport layer 12 contains, for example, PEDOT: PSS (poly (3,4-ethylenedioxythiophene): poly (4-styrene sulfonic acid)). The hole transport layer 12 may be replaced with PEDOT: PSS, or in addition to PEDOT: PSS, PVK (polyvinylcarbazole), TFB (poly [(9,9-dioctylfluorenyl-2,7-diyl-2,7-diyl)). ) -Co- (4,4'-(N- (4-sec-butylphenyl)) diphenylamine)]), TPD (4,4'-bis [N-phenyl-N- (3''-methylphenyl)) It may contain at least one organic material selected from the group consisting of (amino] biphenyl).

For the formation of the hole transport layer 12, various conventionally known methods such as an inkjet method and a vacuum vapor deposition method can be used.

The electron transport layer 14 is a layer that transports electrons from the cathode 15 to the light emitting layer 13. The electron transport layer 14 contains an electron transport material having excellent electron transport properties. The electron transport layer 14 may have a function of inhibiting the transport of holes. Further, the electron transport layer 14 may also have a function as an electron injection layer (EIL) that promotes electron injection from the cathode 15 to the light emitting layer 13, and the cathode 15 is a hole injection layer (EIL). ) May also have the function.

The electron transport layer 14 is a mixed crystal layer formed of a solid solution of at least ZnS (zinc sulfide) and Zn (OH) ₂ (zinc hydroxide), and has a Zn—S bond and a Zn—OH bond with ZnS. It is formed of mixed crystals containing Zn (OH) ₂ . The mixed crystal (mixed crystal layer) may further contain ZnO having a Zn—O bond, and has Zn—S, Zn—OH, and Zn—O bonds, ZnS, ZnO, and Zn. It may be a mixed crystal (mixed crystal layer) with (OH) ₂ . In other words, the electron transport layer 14 is, for example, a mixed crystal layer containing at least ZnS and Zn (OH) ₂ among ZnS, ZnO, and Zn (OH) ₂ . Hereinafter, a mixed crystal of ZnS and Zn (OH) ₂ having a Zn—S bond and a Zn—OH bond is referred to as Zn (S, OH). Further, a mixed crystal of ZnS, ZnO (zinc oxide) and Zn (OH) ₂ having a bond of Zn—S, Zn—OH and Zn—O is referred to as Zn (S, O, OH). The electron transport layer 14 contains Zn (S, OH). Zn (S, OH) may be Zn (S, O, OH).

As described above, in the present embodiment, the CBD method using an alkaline aqueous solution containing a zinc source and a sulfur source in the reaction solution 23 is used for forming the electron transport layer 14. In the present embodiment, ZnS is synthesized in an aqueous solution to form a thin film to be an electron transport layer 14, so that at least a hydroxyl group (-OH group) and an oxygen group (-O group) among the hydroxyl groups (-OH group) and the oxygen group (-O group) Group) is always contained in the thin film. Normally, when ZnO or ZnS is formed by a vacuum method or a vapor deposition method, Zn (OH) ₂ is not intentionally mixed.

As described above, in the present embodiment, since an alkaline aqueous solution containing a zinc source and a sulfur source is used as the reaction solution 23, a mask having alkali resistance is used as the mask 21 used in step S4. The material of the mask 21 is not particularly limited as long as it has alkali resistance. For the mask 21, for example, a commercially available masking tape can be used.

In step S4, it is not necessary to mask the entire electron transport layer non-formation region, and the electron transport layer non-formation region of the portion of the electron transport layer non-formation region immersed in the reaction solution 23 in step S5). It is enough if it is masked. The laminate 20 is one of the light emitting layers 13 so that the formed region (in other words, the formed surface) of the electron transport layer 14 in the light emitting layer 13, which is a part of the light emitting layer 13, comes into contact with the reaction solution 23. The part may be immersed in the reaction solution 23. Therefore, the masked portion may be, for example, only the side surface of the laminate 20 including the anode 11, the hole transport layer 12, and the light emitting layer 13 (specifically, a part of the side surface or the entire side surface). It may be a part of the side surface and a part of the back surface of the laminate 20.

However, as shown in FIG. 3, the entire surface of the laminated body 20 other than the formed region of the electron transport layer 14 (that is, the laminated body) is exposed so that only the formed region of the electron transport layer 14 is exposed. It goes without saying that the entire electron transport layer non-forming region in 20) may be masked.

In the thin film forming step (step S5), first, an alkaline aqueous solution containing a zinc source and a sulfur source is prepared as the reaction solution 23. Then, the reaction solution 23 is placed in the reaction vessel 22. Next, the laminate 20 in which a part of the light emitting layer 13 is exposed is immersed in the reaction solution 23 to form a thin film. Then, the laminate 20 on which the thin film is formed is taken out from the reaction solution 23.

The reaction solution 23 is prepared by mixing a zinc source, an alkali source, and a sulfur source so as to have a predetermined concentration. The zinc source and the sulfur source are used as the base material of the electron transport layer 14 as described above.

The zinc ion concentration in the reaction solution 23 is preferably 0.001 to 0.5 mol / L, more preferably 0.01 to 0.1 mol / L. If the zinc ion concentration in the reaction solution 23 is smaller than 0.01 mol / L, the chemical reaction rate is very slow and the film formation time may be prolonged. On the other hand, when the zinc ion concentration in the reaction solution 23 exceeds 0.5 mol / L, the reaction time becomes extremely short and the film thickness controllability deteriorates, and the yield of the thin film with respect to the raw material deteriorates. There is a risk of increase.

Further, the concentration of sulfide ion in the reaction solution 23 is preferably 0.5 to 5 times, more preferably 1 to 3 times, the zinc ion concentration. If the sulfide ion concentration in the reaction solution 23 is smaller than 0.5 times the zinc ion concentration, the Zn—O bond becomes dominant and the Zn—S bond may not be formed. On the other hand, when the sulfide ion concentration in the reaction solution 23 exceeds 5 times the zinc ion concentration, excess S atoms may be introduced into the ZnS thin film as an acceptor. As a result, the ZnS thin film, which is an n-type semiconductor, behaves like a p-type semiconductor and may not function as the electron transport layer 14.

Further, the zinc ion concentration in the reaction solution 23 is preferably smaller than the sulfide ion concentration in the reaction solution 23. When the concentration of zinc ions in the reaction solution 23 is smaller than the concentration of sulfide ions in the reaction solution 23, the Zn—S bond becomes the main component, and ZnS / Zn (S, OH) can be brought close to 1.

As the zinc source, at least one selected from the group consisting of a weak acid salt containing zinc and a strong acid salt containing zinc is used. Examples of the zinc source include zinc sulfate, zinc acetate, zinc nitrate, zinc chloride and the like. Further, as the sulfur source, at least one selected from the group consisting of sulfur, thiouric acid, water-soluble thiol, and water-soluble thiocarbonyl compound is used. The water-soluble thiol does not need to be completely dissolved in water. As the alkali source, for example, salts such as ammonia and NaOH are used. The alkaline aqueous solution is prepared (prepared) by dissolving the alkaline source in water so that a desired pH can be obtained. At this time, the alkaline aqueous solution is preferably prepared so that the pH thereof and the pH of the reaction solution 23 finally prepared are 9 or more.

The temperature of the reaction solution 23 while immersing the laminate 20 in the reaction solution 23 may be in the range of 15 to 90 ° C., but is preferably maintained in the range of 70 to 90 ° C. It is more desirable that the temperature is maintained in the range of 75 to 90 ° C. By setting the temperature of the reaction solution 23 to 70 ° C. or higher, more preferably 75 ° C. or higher during the immersion of the laminate 20, the formation of the Zn—S bond is promoted over the Zn—O bond (in other words, priority is given). Can grow).

The immersion time of the laminate 20 in the reaction solution 23 is the temperature of the reaction solution 23, the concentration of the zinc source and the sulfur source in the reaction solution 23, and the concentration of the alkali source in the reaction solution 23 (in other words, the pH of the reaction solution 23). ) And the like, the thickness of the thin film to be the electron transport layer 14 is appropriately set to be a desired thickness. Therefore, the immersion time is not particularly limited, but is typically 5 minutes or more and 60 minutes or less.

For example, zinc sulfate (ZnSO ₄ ) as a zinc source is mixed at a concentration of 0.16 mol / L, ammonia as an alkali source at 7.5 mol / L, and thiourea as a sulfur source at a concentration of 0.6 mol / L. By doing so, the reaction solution 23 having a pH of 11 is obtained. By making the concentration of zinc ions in the reaction solution 23 smaller than the concentration of sulfide ions in the reaction solution 23 in this way, ZnS / Zn (S, O, OH) is set to 1 as described above. You can get closer. The temperature of the reaction solution 23 is adjusted to 80 ° C., and the laminate 20 is immersed in the reaction solution 23 for 15 minutes to form a Zn (S, O, OH) thin film having a film thickness of 100 nm.

As can be seen from the above description, the film thickness of the electron transport layer 14 can be controlled by the concentrations of the zinc source, the sulfur source, and the alkali source in the reaction solution 23, the temperature of the reaction solution 23, the immersion time, and the like. ..

The laminate 20 taken out from the reaction solution 23 is dried by, for example, washing the excess reaction solution 23 with water and then allowing it to stand at room temperature. As a result, as shown in FIG. 1, an electron transport layer 14 is formed on the light emitting layer 13.

The electron transport layer 14 thus formed is a mixed crystal layer containing ZnS mainly composed of a Zn—S bond as a main component and partially containing Zn (OH) ₂ having a Zn—OH bond derived from an aqueous solution. It becomes. Further, not only Zn (OH) ₂ but also ZnO having a Zn—O bond derived from an aqueous solution is partially contained.

The chemical bond state and abundance of the elements contained in the electron transport layer 14 are determined by X-rays on a sample exposed to the electron transport layer 14 by the XPS (X-ray photoelectron spectroscopy) method or the UPS (ultraviolet photoelectron spectroscopy) method. Alternatively, it can be specified by irradiating with ultraviolet rays and analyzing the composition. Since the electron transport layer 14 contains Zn (S, O, OH), the binding energies between Zn—O / Zn—S and between O—Zn / OH are different. Therefore, the presence of each bond of Zn—S, Zn—OH, and Zn—O in the electron transport layer 14 can be identified by the XPS method or the UPS method. Further, the Fermi level is different between ZnS produced by the CBD method and ZnS produced by the vacuum vapor deposition method.

The light emitting layer 13 is a layer that emits light due to recombination of holes transported from the anode 11 and electrons transported from the cathode 15. When a forward voltage is applied between the anode 11 and the cathode 15 by the power source 16 (the anode 11 has a higher potential than the cathode 15), the anode 11 goes to the light emitting layer 13 via the hole transport layer 12. And holes are supplied, while electrons are supplied from the cathode 15 to the light emitting layer 13 via the electron transport layer 14. The voltage applied by the power supply 16 may be controlled by a thin film transistor (TFT) (not shown).

FIG. 4 is a cross-sectional view schematically showing the state of the interface between the light emitting layer 13 and the electron transport layer 14 in the light emitting element 1 according to the embodiment. Further, FIG. 5 is a diagram showing energy bands of each layer of the functional layer in the light emitting element 1 according to the embodiment.

As shown in FIG. 4, the light emitting layer 13 contains quantum dots 13QD as a light emitting material.

The quantum dot 13QD used for the light emitting layer 13 in this embodiment is a coating type quantum dot formed by a solution method instead of crystal growth. In the light emitting layer 13, a dispersion liquid in which quantum dots 13QD are dispersed in a solvent is applied to the upper surface of the hole transport layer 12, a coating film containing the quantum dots 13QD is formed, and then the solvent is volatilized to solidify the coating film. It can be formed by forming (curing).

The dispersion liquid is a colloidal solution containing a quantum dot 13QD, a ligand 13R (organic ligand) composed of an organic component, located on the surface of the quantum dot 13QD as a receptor, and a solvent. The surface of the quantum dot 13QD is protected by the ligand 13R, which is a surface modifying group that modifies the surface of the quantum dot 13QD.

The ligand 13R is composed of an adsorbing group that adsorbs (coordinates) the surface of the quantum dot 13QD and an alkyl group that binds to the adsorbing group. Examples of the adsorbing group include an amino group, a phosphine group, a carboxyl group, a hydroxyl group, a thiol group and the like. Moreover, as the above-mentioned alkyl group, an alkyl group having 2 to 50 carbon atoms can be mentioned.

More specifically, the ligand 13R includes, for example, hexadecylamine, oleylamine, octylamine, hexadecanethiol, dodecanethiol, trioctylphosphine, trioctylphosphine oxide, myristic acid, oleic acid and the like.

The ligand 13R has a role of protecting the surface of the quantum dots 13QD and preventing deterioration of the quantum dots 13QD, while suppressing aggregation of the quantum dots 13QD in the dispersion liquid and improving dispersibility. Has a role as.

The solvent may be water, or may be an organic solvent such as methanol, ethanol, propanol, butanol, pentane, hexane, octane, acetone, toluene, xylene, benzene, chloroform, dichloromethane, and chlorobenzene. Often, it may be a combination thereof. A known coating method such as a spin coating method or an inkjet method can be used for coating the dispersion liquid.

As shown in FIG. 4, the light emitting layer 13 thus formed by the solution method includes a spherical quantum dot 13QD and the above-mentioned ligand 13R. Since the shape of the quantum dot 13QD is spherical rather than island-shaped (lens-shaped) as in the case of crystal growth, the polarization characteristic of light emission can be reduced. Further, since the light emitting layer 13 contains the ligand 13R, the aggregation of the quantum dots 13QD can be suppressed and the quantum dots 13QD can be satisfactorily dispersed when the coating film containing the quantum dots 13QD is formed.

The quantum dot 13QD as a receptacle used in the present embodiment is a core-shell type quantum dot (core-shell particle), and as shown in FIG. 4, a nano-sized crystal (semiconductor) of a semiconductor material called a semiconductor nanoparticle. It includes a core 13C as a light emitting portion, which is composed of fine particles of crystals), and a shell 13S that covers the core 13C. Therefore, the light emitting layer 13 includes a core-shell type quantum dot 13QD including a core 13C and a shell 13S covering the core 13C, and a ligand 13R coordinated to the shell 13S in the core-shell type quantum dot 13QD. ..

Quantum dot 13QD is, for example, Cd (cadmium), S (sulfur), Te (tellurium), Se (selenium), Zn (zinc), In (indium), N (nitrogen), P (phosphorus), As (arsenic). ), Sb (antimony), Al (aluminum), Ga (gallium), Pb (lead), Si (silicon), Ge (germanium), Mg (magnesium), at least one element selected from the group. It may contain the semiconductor material. For example, nano-sized crystals of the above semiconductor material can be used for the core 13C.

In the present embodiment, the shell 13S preferably contains ZnS, and more preferably consists of ZnS. The red quantum dot 13QD having a CdS (cadmium sulfide) as a core and having a narrow bandgap may have a shell made of, for example, ZnSe. However, even in this case, in order to improve the quantum efficiency, it is preferable that the shell 13S has a double shell structure in which the outermost shell is made of ZnS. Therefore, the nanocrystal material serving as the core 13C is composed of CdSe (cadmium selenide), CdS, and ZnSe (zinc selenide) in which the conduction band level and the valence band level are present in the bandgap energy of ZnS. Nano-sized crystals of at least one semiconductor material selected from the group are preferred.

In such a core-shell type quantum dot 13QD, the wavelength of light emitted by the quantum dot 13QD is proportional to the particle size of the core 13C and does not depend on the particle size of the shell 13S (the outermost particle size of the quantum dot 13QD). ..

The particle size of the core 13C is, for example, about 1 to 10 nm, and the outermost particle size of the quantum dot 13QD, which is the particle size of the shell 13S, is, for example, about 1 to 15 nm. The number of overlapping layers of the quantum dots 13QD in the light emitting layer 13 is, for example, 1 to 5 layers. The film thicknesses of the hole transport layer 12, the light emitting layer 13, and the electron transport layer 14 can be conventionally known, but are, for example, about 1 to 50 nm. The film thickness of the light emitting layer 13 is preferably about several times the outermost particle size of the quantum dot QD.

As shown in FIG. 5, the core 13C has a valence band level and a conduction band level. The valence band level is equal to the ionization potential between the vacuum level and the upper end of the valence band indicated as the light emitting layer valence band in FIG. Further, the conduction band level is equal to the electron affinity between the vacuum level and the lower end of the conductor described as the light emitting layer conduction band in FIG.

A voltage is applied between the anode 11 and the cathode 15, and holes are generated from the valence band level at the lower end of the hole transport layer 12 (HTL) to the conduction band at the upper end of the electron transport layer 14 (ETL). When electrons are injected into the light emitting layer 13 from the level, these holes and electrons flow into the core 13C having a small band gap between the lower end of the conduction band and the upper end of the valence band, and are confined in the core 13C. .. The quantum confinement effect can be obtained by making the core 13C smaller than the Bohr radius. In the core 13C, electrons located in the conduction band (conduction band of the light emitting layer) of the core 13C and holes located in the valence band (valence band of the light emitting layer) of the core are recombined, and the core 13C It emits light having a wavelength corresponding to the particle size (in other words, light having an emission wavelength corresponding to a band gap between the lower end of the conduction band of the light emitting layer and the upper end of the valence band of the light emitting layer). Since the light emission from the quantum dot 13QD has a narrow spectrum due to the quantum confinement effect, it is possible to obtain light emission with a relatively deep chromaticity.

However, at this time, the ligand 13R composed of an organic component and the shell 13S mainly composed of ZnS behave as a blocking layer that reduces the injection efficiency of electrons and holes. As described above, the ligand 13R is coordinated on the surface of the quantum dots 13QD, and suppresses aggregation of the quantum dots 13QD and quantum in the dispersion liquid for forming the light emitting layer 13 in which the quantum dots 13QD are dispersed. It is used to prevent deterioration of the dot 13QD, but acts as an insulating component. Therefore, it is desirable that the ligand 13R does not exist at the interface between the light emitting layer 13 and the electron transport layer 14.

According to the present embodiment, as described above, in step S5, the surface of the light emitting layer 13 on which the electron transport layer 14 is laminated is brought into contact with the alkaline aqueous solution. Therefore, the ligand 13R on the surface of the light emitting layer 13 on which the electron transport layer 14 is laminated is etched by the alkaline aqueous solution.

As a result, according to the present embodiment, as shown in FIG. 4, the average coordination number of the ligand 13R coordinated to the shell 13S of the quantum dot 13QD included in the interface between the light emitting layer 13 and the electron transport layer 14. Can be less than the average coordination number of the ligand 13R coordinated to the shell 13S of the quantum dot 13QD contained in the portion of the light emitting layer 13 other than the boundary (in other words, the bulk of the light emitting layer 13). .. Preferably, as shown in FIGS. 4 and 5, the ligand 13R at the interface between the light emitting layer 13 and the electron transport layer 14 can be removed. Further, by using an alkaline aqueous solution for the reaction solution 23 as described above, the unnecessary shell 13S at the interface between the light emitting layer 13 and the electron transport layer 14 that comes into contact with the reaction solution 23 is also removed together with the ligand 13R. ..

As described above, according to the present embodiment, the electron transport layer 14 of a dense thin film can be formed on the spot by a chemical reaction in the aqueous solution, and the interface between the electron transport layer 14 and the light emitting layer 13 can be contacted. The resistance can be reduced, and the ligand 13R and the shell 13S at the interface can be removed to improve the quality of the interface and reduce the resistance component. Therefore, the efficiency of injecting electrons from the electron transport layer 14 into the light emitting layer 13 can be improved.

Further, according to the present embodiment, since the electron transport layer 14 uses the inorganic layer, defects are unlikely to occur between the electron transport layer 14 and the light emitting layer 13. Moreover, by etching the surface of the quantum dot 13QD with the alkaline aqueous solution as described above, the defects on the surface of the quantum dot 13QD are further reduced as compared with the conventional light emitting device using the inorganic layer for the electron transport layer 14. Can be made to. Therefore, it is possible to obtain a light emitting element 1 having a coating property of the electron transport layer 14 that is superior to that of the conventional light emitting device.

Further, according to the present embodiment, the electron transport layer 14 is not formed by laminating nanoparticles of an electron transport material as in the conventional case, but is formed by a zinc source and a sulfur source in an aqueous solution by the CBD method. It is directly formed as a thin film (mixed crystal layer) by a chemical reaction. Moreover, according to the present embodiment, as shown in FIG. 4, the electron transport material permeates into the gap between the adjacent quantum dots 13QD at the interface between the light emitting layer 13 and the electron transport layer 14, and the electron transport material becomes quantum. Dot 13QD is coated. Therefore, the contact area between the light emitting layer 13 and the electron transport layer 14 increases. Therefore, the film property of the electron transport layer 14 can be improved, and the contact resistance between the light emitting layer 13 and the electron transport layer 14 can be reduced. Further, since the vacuum device is not used for forming the electron transport layer 14, the light emitting element 1 can be manufactured at low cost.

Further, the energy level can be adjusted by changing the ratio of Zn (S, O, OH) depending on the composition of the reaction solution 23 or the film forming conditions such as the reaction temperature. Therefore, the band gap of Zn (S, O, OH) can be changed by, for example, changing the composition of the reaction solution 23, but the concentration of ZnO contained in Zn (S, O, OH) is large. Therefore, the band gap and the valence band level are lowered, resulting in a decrease in electron injection efficiency. Therefore, a preferable ratio of ZnS, ZnO, and Zn (OH) ₂ contained in the mixed crystal (in other words, the electron transport layer 14 which is the mixed crystal layer) constituting the electron transport layer 14 is appropriately set.

Specifically, the mixed crystal (Zn (S, OH) or Zn (S, O, OH)) contains ZnS as a main component as described above, and the concentration of ZnS contained in the mixed crystal is as described above. It is desirable that the concentration is higher than the concentration of Zn (OH) ₂ contained in the mixed crystal. When the mixed crystal is Zn (S, O, OH), it is desirable that the concentration of ZnS contained in the mixed crystal is higher than the concentration of ZnO contained in the mixed crystal. When the mixed crystal is Zn (S, O, OH), it is desirable that the concentration of Zn (OH) ₂ contained in the mixed crystal is higher than the concentration of ZnO contained in the mixed crystal. ..

More specifically, it is desirable that the concentration of ZnS contained in the mixed crystal (Zn (S, OH) or Zn (S, O, OH)) is 50 wt% or more.

Further, the smaller the concentration of Zn (OH) ₂ contained in the mixed crystal (Zn (S, OH) or Zn (S, O, OH)), the more desirable (however, it is not zero), and the upper limit thereof. The value is preferably 50 wt% of ZnS contained in the mixed crystal. The concentration of Zn (OH) ₂ is preferably 0.1 wt% or more from the viewpoint of the concentration above the detection limit. Therefore, the concentration of Zn (OH) ₂ contained in the mixed crystal (Zn (S, OH) or Zn (S, O, OH)) is based on the concentration of ZnS in the mixed crystal (100 wt%). It is desirable that the value is 0.1 wt% or more and 50 wt% or less with respect to the above reference value.

When the mixed crystal is Zn (S, O, OH), it is desirable to set the concentration of ZnO contained in the mixed crystal to an appropriate ratio for adjusting the band gap, and the mixed crystal It is desirable that it is 0.1 wt% or more and 46 wt% or less of ZnS contained therein. That is, with the concentration of ZnS in the Zn (S, O, OH) as a reference value (100 wt%), the concentration of ZnO contained in the Zn (S, O, OH) is 0 with respect to the reference value. It is desirable that it is 1 wt% or more and 46 wt% or less.

Further, when the electron transport layer 14 is formed by the CBD method, it is possible to form a film at a lower temperature than the conventional film formation method such as a vapor deposition method or a sputtering method, and the hole transport layer 12 or the like which is the lower layer thereof , It becomes possible to use organic materials that are sensitive to heat. In the vacuum method such as the vapor deposition method and the sputtering method, the hydroxyl group (−OH group) is eliminated as water by heat. Therefore, no hydroxyl group is detected in the electron transport layer formed by a vacuum method such as a vapor deposition method or a sputtering method.

The present disclosure is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Is also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

1 Light emitting element 10 Substrate 11 Anode 13 Light emitting layer 14 Electron transport layer 13 C core 13QD Quantum dot 13R Ligand 13S Shell 15 Cathode 20 Laminated body 21 Mask 23 Reaction solution (alkaline aqueous solution containing zinc source and sulfur source)

Claims

An anode forming process for forming an anode on a substrate,
A light emitting layer forming step of forming a light emitting layer on the anode and
An electron transport layer forming step of forming an electron transport layer adjacent to the light emitting layer on the light emitting layer,
A cathode forming step of forming a cathode on the electron transport layer is provided.
The electron transport layer forming step includes a dipping step of immersing the formed surface of the electron transport layer in the light emitting layer in an alkaline aqueous solution containing a zinc source and a sulfur source.
In the electron transport layer forming step, the light emitting element is characterized in that a mixed crystal layer containing ZnS and Zn (OH) 2 having a Zn—S bond and a Zn—OH bond is formed as the electron transport layer. Production method.
The method for producing a light emitting element according to claim 1, wherein the zinc source is at least one selected from the group consisting of a weak acid salt containing zinc and a strong acid salt containing zinc.
The production of the light emitting element according to claim 1 or 2, wherein the sulfur source is at least one selected from the group consisting of sulfur, water-soluble thiol, water-soluble thiourea, and water-soluble thiocarbonyl compound. Method.
The method for manufacturing a light emitting device according to any one of claims 1 to 3, wherein the pH of the alkaline aqueous solution is 9 or more.
Any one of claims 1 to 4, wherein in the immersion step, the surface to be formed of the electron transport layer in the light emitting layer is immersed in the alkaline aqueous solution for 5 minutes or more and 60 minutes or less. The method for manufacturing a light emitting element according to.
In the electron transport layer forming step, before the dipping step, the surface to be formed of the electron transport layer in the light emitting layer among the surfaces of the substrate, the anode, and the laminate including the light emitting layer is exposed. The method for manufacturing a light emitting element according to any one of claims 1 to 5, further comprising a step of masking at least a part of the surface of the light emitting layer other than the surface to be formed of the electron transport layer.
The light emitting layer forming step is characterized in that a light emitting layer including a core, a quantum dot covering the core and containing a shell containing ZnS, and a ligand coordinating the shell of the quantum dot is formed. The method for manufacturing a light emitting element according to any one of claims 1 to 6.
The claim is characterized in that, in the immersion step, the temperature of the alkaline aqueous solution is maintained in the range of 15 to 90 ° C., and the surface to be formed of the electron transport layer in the light emitting layer is immersed in the alkaline aqueous solution. The method for manufacturing a light emitting element according to any one of 1 to 7.
The claim is characterized in that in the dipping step, the temperature of the alkaline aqueous solution is maintained in the range of 70 to 90 ° C., and the surface to be formed of the electron transport layer in the light emitting layer is immersed in the alkaline aqueous solution. The method for manufacturing a light emitting element according to any one of 1 to 8.
An anode, a light emitting layer, an electron transporting layer adjacent to the light emitting layer, and a cathode are provided in this order.
The electron transport layer is a light emitting element having a Zn—S bond and a Zn—OH bond and being a mixed crystal layer containing ZnS and Zn (OH) 2 .
The light emitting element according to claim 10, wherein the mixed crystal layer is a mixed crystal layer of ZnS, ZnO, and Zn (OH) 2, and further has a Zn—O bond.
The light emitting device according to claim 11, wherein the concentration of ZnS contained in the mixed crystal layer is higher than the concentration of ZnO contained in the mixed crystal layer.
The light emitting device according to claim 12, wherein the concentration of ZnO contained in the mixed crystal layer is 0.1 wt% or more and 46 wt% or less of ZnS contained in the mixed crystal layer.
The light emitting device according to any one of claims 11 to 13, wherein the concentration of Zn (OH) 2 contained in the mixed crystal layer is higher than the concentration of ZnO contained in the mixed crystal layer.
The light emitting device according to any one of claims 10 to 14, wherein the concentration of ZnS contained in the mixed crystal layer is higher than the concentration of Zn (OH) 2 contained in the mixed crystal layer.
The light emitting device according to claim 15, wherein the concentration of Zn (OH) 2 contained in the mixed crystal layer is 0.1 wt% or more and 50 wt% or less of ZnS contained in the mixed crystal layer.
The light emitting device according to any one of claims 10 to 16, wherein the concentration of ZnS contained in the mixed crystal layer is 50 wt% or more.
The light emitting device according to any one of claims 10 to 17, wherein the concentration of ZnS contained in the mixed crystal layer is higher than the concentration of Zn (OH) 2 .
The light emitting element according to any one of claims 10 to 18, wherein the light emitting layer includes a core and quantum dots that cover the core and include a shell containing ZnS.
The light emitting layer further comprises a ligand that coordinates the shell of the quantum dots.
The average coordination number of the ligands coordinated to the shell of the quantum dots at the interface between the light emitting layer and the electron transport layer is the number of coordination numbers of the quantum dots contained in the bulk of the light emitting layer other than the interface. The light emitting element according to claim 19, wherein the number is less than the average coordination number of the ligands coordinated to the shell.