Light-emitting element and display device

Abstract

A light-emitting element (ED) according to the present disclosure comprises: an electron transport layer (ETL) that is positioned between a light-emitting layer (Em) and a cathode (CD), has a film thickness (D) that is 1 / 2 of the emission peak wavelength (λ) of the light-emitting layer (Em) or greater, and includes a low refractive index material having an electron transport property and a refractive index of 2.0 or less; and a protective layer PL that is positioned between the light-emitting layer (Em) and the electron transport layer (ETL) and includes an insulating material.

Description

Light-emitting element and display device

[0001] The present disclosure relates to a light-emitting element and a display device.

[0002] Patent Document 1 discloses a quantum dot device in which an inorganic electron transport layer configured to increase electron transport from the cathode to the quantum dot layer and an inorganic electron adjustment layer configured to decrease electron transport from the cathode to the quantum dot layer are provided between the quantum dot layer and the cathode.

[0003] US 1099899 B2

[0004] There is a demand for improved external quantum efficiency (EQE) of light-emitting devices. EQE is the product of internal quantum efficiency (IQE) and light extraction efficiency (LEE). Excess electrons in the light-emitting layer are one cause of reduced IQE. A high effective refractive index of the light-emitting device is one cause of reduced LEE.

[0005] A light-emitting element according to one aspect of the present disclosure includes an anode and a cathode, an emitting layer located between the anode and the cathode, an electron transport layer located between the emitting layer and the cathode, the electron transport layer having a thickness equal to or greater than half of the emission peak wavelength of the emitting layer and including a low refractive index material having electron transport properties and a refractive index of 2.0 or less, and a protective layer located between the emitting layer and the electron transport layer and including an insulating material.

[0006] A display device according to an aspect of the present disclosure includes a light-emitting element according to an aspect of the present disclosure.

[0007] According to one aspect of the present disclosure, the EQE can be improved by improving both the IQE and the LEE.

[0008] FIG. 1 is a schematic cross-sectional view showing an example of the configuration of a light-emitting device according to an embodiment of the present disclosure. It shows the light transmittance of materials that can be used in an electron transport layer. It shows an example of the band structure of each layer constituting the light-emitting device shown in FIG. 1 when the layers are separated. It shows an example of the band structure of each layer constituting the light-emitting device shown in FIG. 1 when the layers are bonded to each other. It is a schematic cross-sectional view showing an example of the configuration of a light-emitting device according to an embodiment of the present disclosure. It shows an example of the band structure of each layer constituting the light-emitting device shown in FIG. 5 when the layers are bonded to each other. It is a diagram showing an example of the effective refractive index of the electron transport layer shown in FIG. 5 and the light extraction efficiency (LEE) of the light-emitting device when the low refractive index material is gallium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. It is a diagram showing an example of the effective refractive index of the electron transport layer shown in FIG. 5 and the light extraction efficiency (LEE) of the light-emitting device when the low refractive index material is gallium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. 6 is a diagram showing an example of the effective refractive index of the electron transport layer shown in FIG. 5 and the light extraction efficiency (LEE) of the light-emitting element when the low refractive index material is gallium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. FIG. 7 is a diagram showing an example of the effective refractive index of the electron transport layer shown in FIG. 5 and the light extraction efficiency (LEE) of the light-emitting element when the low refractive index material is gallium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. FIG. 8 is a diagram showing an example of the effective refractive index of the electron transport layer shown in FIG. 5 and the LEE of the light-emitting element when the low refractive index material is indium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. FIG. 9 is a diagram showing an example of the effective refractive index of the electron transport layer shown in FIG. 5 and the LEE of the light-emitting element when the low refractive index material is indium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. FIG. 10 is a diagram showing an example of the effective refractive index of the electron transport layer shown in FIG. 5 and the LEE of the light-emitting element when the low refractive index material is indium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. 6 is a diagram showing an example of the effective refractive index of the electron transport layer and the LEE of the light-emitting element shown in Figure 5 when the low refractive index material is indium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. 7 is a diagram showing an example of the effective refractive index of the electron transport layer and the LEE of the light-emitting element shown in Figure 5 when the low refractive index material is indium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. 8 is a diagram showing an example of the effective refractive index of the electron transport layer and the LEE of the light-emitting element shown in Figure 5 when the low refractive index material is indium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. 9 is a schematic cross-sectional view showing an example of the configuration of a light-emitting element according to an embodiment of the present disclosure.16 shows an example of the band structure of each layer constituting the light-emitting device shown in FIG. 15 when the layers are bonded to each other. FIG. 17 shows an example of the effective refractive index of the electron transport layer shown in FIG. 15 when the low refractive index material is gallium oxide and the first high refractive index material is zinc oxide. FIG. 18 is a schematic cross-sectional view showing an example of the configuration of a light-emitting device according to an embodiment of the present disclosure. FIG. 19 shows an example of the band structure of each layer constituting the light-emitting device shown in FIG. 18 when the layers are bonded to each other. FIG. 20 shows an example of the band structure of each layer constituting the light-emitting device according to an embodiment of the present disclosure. FIG. 21 is a schematic cross-sectional view showing an example of the configuration of a display region of a display device according to an embodiment of the present disclosure.

[0009] [Embodiment 1] (Configuration of Light-Emitting Element) Fig. 1 is a schematic cross-sectional view showing an example of the configuration of a light-emitting element according to an embodiment of the present disclosure. As shown in Fig. 1, the light-emitting element ED according to embodiment 1 of the present disclosure includes an anode AD, a cathode CD, an emitting layer Em located between the anode AD and the cathode CD, an electron transport layer ETL located between the emitting layer Em and the cathode CD, the electron transport layer ETL having a mechanical film thickness D equal to or greater than half the emission peak wavelength λ of the emitting layer Em and containing a low refractive index material having electron transport properties and a refractive index of 2.0 or less, and a protective layer PL located between the emitting layer Em and the electron transport layer ETL and containing an insulating material.

[0010] In the present disclosure, an electron transporting material is defined as a material having a free electron density of 10 １７ cm －３ An insulating material means a material in which both the free electron density and the hole density are 10 １７ cm －３ In this disclosure, dimensions such as distance, length, and thickness are mechanical dimensions unless otherwise specified as "optical" or "optical distance." Optical dimensions are the product of mechanical dimensions and refractive index.

[0011] According to the above configuration, the film thickness D of the electron transport layer ETL is equal to or greater than ½ of the peak emission wavelength λ of the light-emitting layer Em. Therefore, the total film thickness of the light-emitting element ED exceeds ½ of the peak emission wavelength λ, and the light extraction efficiency (LEE) of the light-emitting element ED is improved. On the other hand, in a conventional configuration in which the total film thickness of the light-emitting element is less than ½ of the peak emission wavelength, there was a problem in that the LEE was reduced due to interference.

[0012] According to the above configuration, the electron transport layer ETL contains a low refractive index material. Therefore, compared to conventional techniques in which the electron transport layer does not contain a low refractive index material, the effective refractive index of the light-emitting element ED according to the present disclosure is small, and the LEE is improved. Furthermore, since the film thickness D of the electron transport layer ETL is equal to or greater than half the emission peak wavelength λ, the proportion of the electron transport layer ETL in the total film thickness of the light-emitting element ED is large. This significantly reduces the effective refractive index of the light-emitting element ED, and significantly improves the LEE.

[0013] The effective refractive index of the light-emitting element ED influences the critical angle of total reflection at the interface between the light-emitting element ED and the outside (i.e., air). For example, when the effective refractive index of the light-emitting element is equal to the refractive index of zinc oxide (ZnO), which is 2.2, the critical angle of total reflection is about 27° and the LEE is about 21%. On the other hand, when the effective refractive index of the light-emitting element is equal to the refractive index of gallium oxide (Ga ２ O ３ ), the critical angle for total reflection is about 43.6° and the LEE is about 48%.

[0014] In the present disclosure, the effective refractive index of an element having a multi-layer structure, such as the light-emitting element ED, may be calculated as a weighted average of the refractive indexes of the layers, weighted according to the mechanical thickness of each layer. Note that the reflective electrode is excluded from the calculation of the effective refractive index of the light-emitting element ED.

[0015] According to the above configuration, the protective layer PL containing an insulating material is located between the light-emitting layer Em and the electron transport layer ETL. This can be expected to have the following four effects. First, the protective layer PL protects the light-emitting layer Em during the formation of the electron transport layer ETL, reducing deterioration of the light-emitting layer Em and thereby improving the luminous efficiency of the light-emitting layer Em. Second, the protective layer PL acts as an electron injection barrier and reduces the efficiency of electron injection into the light-emitting layer Em, thereby improving the carrier balance in the light-emitting layer Em and improving the luminous efficiency of the light-emitting layer Em. Third, the protective layer PL exerts a hole trapping effect and reduces the probability of holes penetrating from the light-emitting layer Em to the electron transport layer ETL, thereby improving the carrier balance in the light-emitting layer Em and improving the luminous efficiency of the light-emitting layer Em. Fourth, since the protective layer PL protects the light-emitting layer Em from the electron transport layer ETL, even if the light-emitting layer Em contains quantum dots QDs, materials that may have an adverse effect on the quantum dots QDs, such as gallium oxide, which deactivates the quantum dots QDs, can be used in the electron transport layer ETL.

[0016] The low refractive index material contained in the electron transport layer ETL is, for example, gallium oxide (Ga ２ O ３ ) or indium oxide (In ２ O ３ ). The refractive index of gallium oxide is approximately 1.45, and the refractive index of indium oxide is approximately 2. Gallium oxide may be doped with 20% or less of boron (B) in atomic percentage (at%). In the present disclosure, the compound may or may not be stoichiometric, meaning that the composition is as per the chemical formula. Gallium oxide and indium oxide have increased electron transport properties as the oxygen vacancy concentration increases. Gallium oxide and indium oxide may be doped with Group 14 elements such as silicon (Si), tin (Sn), and germanium (Ge) as donors.

[0017] In the present disclosure, the notation of element group numbers using Roman numerals is based on the old IUPAC (International Union of Pure and Applied Chemistry) system or the old CAS (Chemical Abstracts Service) system, and the notation of element group numbers using Arabic numerals is based on the new IUPAC system.

[0018] When the zinc oxide thin film has a thickness of about 30 nm, －３ cm ２ / Vs and an electron mobility of 10 ２０ ~10 ２１ / cm ３ When the thickness of a gallium oxide thin film is greater than 150 nm, the electron density is about 100 cm ２ / Vs and an electron mobility of 10 １５ ~10 １９ / cm ３ It has an electron density of about 1000 keV.

[0019] FIG. 2 shows the light transmittance of materials that can be used for the electron transport layer. The vertical axis of FIG. 2 shows the light transmittance, and the horizontal axis shows the wavelength of the transmitted light. Of the three graphs shown in FIG. 2, the solid line shows the results for a 60 nm thick continuous film made of gallium oxide, the dashed line shows the results for a 140 nm thick continuous film made of gallium oxide, and the dashed-dotted line shows the results for a 60 nm thick continuous film made of zinc oxide. Zinc oxide (ZnO) is a material that has been used for conventional electron transport layers. On the other hand, gallium oxide (Ga ２ O ３ ) are materials that may be used in the electron transport layer ETL according to the present disclosure.

[0020] 2, the optical transmittance of the continuous gallium oxide films having thicknesses of 60 nm and 140 nm is significantly higher than that of the continuous zinc oxide film having thickness of 60 nm, exceeding 80%. Therefore, from the viewpoint of optical transmittance, the use of gallium oxide in the electron transport layer ETL is also beneficial for increasing the light extraction efficiency (LEE).

[0021] Referring again to Fig. 1, the insulating material contained in the protective layer PL may be, for example, a halide of an alkali metal, an alkaline earth metal, a lanthanide metal, or a metal element belonging to any of Groups 4, 5, 6, 7, 13, 14, 15, 16, and 17, and silicon oxide (SiO ２ and inorganic oxides such as fluoride, chloride, bromide, and iodide.

[0022] In this disclosure, alkali metals include elements belonging to the second period or later of Group 1, but do not include hydrogen (H). In this disclosure, alkaline earth metals include all elements of Group 2. In this disclosure, non-metallic elements include elements having a band gap width of 4 eV or more. In this disclosure, metallic elements include elements having a band gap width of less than 4 eV. The band gap is the band gap between the valence band (VB) and the conduction band (CB), and the band gap width is the energy difference (absolute value) between the valence band maximum (VBM) and the conduction band minimum (CBM) at absolute zero.

[0023] The mechanical thickness of the protective layer PL may be 1 nm or more and 5 nm or less. Because the thickness of the protective layer PL is 1 nm or more, the protective layer PL can be formed as a continuous film without holes. Because the thickness of the protective layer PL is 5 nm or less, electrons can move from the electron transport layer ETL to the light-emitting layer Em across the protective layer PL by the tunneling effect.

[0024] The protective layer PL may be in contact with the light-emitting layer Em.

[0025] The light-emitting element ED may be an organic light-emitting diode (OLED), and the light-emitting layer Em may include an organic light-emitting material. Alternatively, the light-emitting element ED may be a quantum dot light-emitting diode (QLED), and the light-emitting layer Em may include light-emitting quantum dots (QDs). The light-emitting layer Em may further include a ligand material that coordinates with or can coordinate with the quantum dots (QDs), or a matrix material MX formed between the quantum dots (QDs).

[0026] The matrix material MX refers to a component that contains and holds other substances such as quantum dots QDs, and can be referred to as a substrate, a base material, or a filler. The matrix material MX may be solid at room temperature. The matrix material MX may be filled into the light-emitting layer Em. The matrix material MX may fill regions (spaces) in the light-emitting layer Em other than the regions containing the quantum dots QDs.

[0027] The matrix material MX being filled between a plurality of quantum dots QD means that the matrix material MX fills the region between two adjacent quantum dots QD. The matrix material MX fills at least the space between two adjacent quantum dots QD, and may fill the region surrounded by a straight line (common circumstantial line) circumscribing the outer periphery D of the two quantum dots QD and the opposing outer peripheries of the two quantum dots Q in a cross-sectional view. The matrix material MX may also fill a space surrounded by three or more quantum dots QD.

[0028] The matrix material MX may cover the outer edge (upper and lower surfaces) of the light-emitting layer Em. The outer edge of the light-emitting layer Em does not need to be formed only by the matrix material MX, and some of the quantum dots QD may be exposed from the matrix material MX. The matrix material MX may refer to the portion of the light-emitting layer Em excluding the quantum dots QD.

[0029] The matrix material MX may contain a plurality of quantum dots QDs. The plurality of quantum dots QDs may be embedded in the matrix material MX at intervals. The matrix material MX may be partially or completely filled between the plurality of quantum dots QDs.

[0030] The matrix material MX is 1000 nm in thickness along the surface direction perpendicular to the film thickness direction.２ The continuous film may include a continuous film having an area of ​​at least 1000 nm. A continuous film means a film that is not separated in one plane by any material other than the material that constitutes the continuous film.

[0031] The matrix material MX may be the same material as the shell contained in each of the multiple quantum dots QD. The concentration of the matrix material MX in the light-emitting layer Em may be 0.1% or more and 79.0% or less. This concentration may be measured, for example, from the area ratio in image processing during cross-sectional observation. When the quantum dots QD have a core-shell structure, the concentration of the shell may be 0.1% or more and 39% or less. When the shell and the matrix material MX are the same material (same composition) and the shell and the inorganic matrix material X cannot be distinguished, the concentration of the combined region of the shell and the matrix material MX may be 0.1% or more and 99.9% or less. In this way, when the shell and the matrix material MX cannot be distinguished, the shell may be part of the inorganic matrix material X.

[0032] The material constituting the matrix material MX desirably has a wider band gap than the material constituting the quantum dots QD (for example, the core material). A semiconductor or an insulator can be used as the material constituting the matrix material MX. Examples of the material constituting the matrix material MX include metal sulfides and / or metal oxides. Examples of metal sulfides include zinc sulfide (ZnS), zinc magnesium sulfide (ZnMgS, ZnMgS ２ ), gallium sulfide (GaS, Ga2S3), zinc tellurium sulfide (ZnTeS), magnesium sulfide (MgS), zinc gallium sulfide (ZnGa ２ S ４ ), magnesium sulfide (MgGa2S4). Metal oxides may be zinc oxide (ZnO), titanium oxide (TiO ２ ), tin oxide (SnO ２ ), tungsten oxide (WO ３ ), zirconium oxide (ZrO ２ ) The chemical formulas written in parentheses after the compound names are representative examples. The composition ratios written in the chemical formulas are preferably stoichiometric, so that the composition of the actual compounds is as shown in the chemical formulas, but they do not necessarily have to be stoichiometric.

[0033] The structure of the matrix material MX can be determined by observing the cross section of the light-emitting layer Em with a width of about 100 nm, as long as it is clear that the matrix material MX has the above-mentioned configuration, and it is not necessary to observe the above-mentioned configuration throughout the entire light-emitting layer Em. The matrix material MX may contain a substance different from the main material (e.g., an inorganic substance such as an inorganic semiconductor) as, for example, an additive.

[0034] The anode AD may be a transparent electrode and may include any one selected from the group consisting of indium tin oxide (InTiO), indium zinc oxide (InZnO), aluminum-doped zinc oxide (AlZnO), and gallium-doped zinc oxide (GaZnO). The light-emitting element ED may include a light-reflecting layer on the opposite side of the anode AD from the light-emitting layer Em.

[0035] The cathode CD may include any one selected from the group consisting of aluminum (Al), copper (Cu), gold (Au), and silver (Ag).

[0036] The light-emitting element ED may include a hole-injection layer HIL and / or a hole-transport layer HTL between the anode AD and the light-emitting layer Em.

[0037] The hole transport layer HTL includes a hole transporting material, and may include, for example, any one or more selected from the group consisting of hole transporting organic compounds such as PEDOT:PSS (polyethylenedioxythiophene / polystyrene sulfonate), PVK (poly-N-vinylcarbazole), TFB (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)diphenylamine)]), and poly-TPD (N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine).

[0038] The hole injection layer HIL includes a hole transporting material, and may include, for example, any one or more selected from the group consisting of hole transporting organic compounds such as PEDOT:PSS, PVK, TFB, and poly-TPD, and p-type inorganic semiconductor materials such as nickel oxide (NiO).

[0039] (Band Structure of Light-Emitting Element) The electron transport layer ETL may include a first low refractive index layer L1 made of a low refractive index material. The electron transport layer ETL may have a single-layer structure and may include only the first low refractive index layer L1.

[0040] FIG. 3 shows an example of the band structure of each layer when the layers constituting the light-emitting device shown in FIG. 1 are separated. FIG. 4 shows an example of the band structure of each layer when the layers constituting the light-emitting device shown in FIG. 1 are bonded to each other. The material and film thickness of each layer in FIG. 4 are the same as those in FIG. 3. In FIGS. 3 and 4, the Fermi level of the cathode CD is shown by a solid line. The band gaps of the hole transport layer HTL, the quantum dots QD of the light-emitting layer Em, the protective layer PL, and the electron transport layer ETL are shown by rectangles, and their respective Fermi levels are shown by dashed lines. The band gap is the band gap between the valence band (VB) and the conduction band (CB). The bottom side of the rectangle indicates the valence band maximum (VBM), and the top side of the rectangle indicates the conduction band minimum (CBM). The work function corresponds to the energy difference (absolute value) between the Fermi level at absolute zero and the vacuum level, the electron affinity corresponds to the energy difference (absolute value) between the CBM and the vacuum level, and the ionization potential corresponds to the energy difference (absolute value) between the VBM and the vacuum level.

[0041] In the present disclosure, when a VBM, CBM, or Fermi level is close to the vacuum level, the level is said to be shallow. In this case, the corresponding ionization potential, electron affinity, or work function is small. In the present disclosure, when a VBM, CBM, or Fermi level is far from the vacuum level, the level is said to be deep. In this case, the corresponding ionization potential, electron affinity, or work function is large. In the band structure diagrams shown in FIGS. 3 and 4 and described below, the levels located at the top of the diagrams are shallower, and the levels located at the bottom of the diagrams are deeper.

[0042] As shown in FIG. 3 , the Fermi level of the cathode CD is preferably deeper than the Fermi level of the first low-refractive-index layer L1. Because the free electron density of the cathode CD is significantly greater than that of the first low-refractive-index layer L1, the band deformation due to the junction between the cathode CD and the first low-refractive-index layer L1 primarily appears in the first low-refractive-index layer L1. As shown in FIG. 4 , this deformation results in the band of the first low-refractive-index layer L1 rising toward the cathode CD, creating a thin, sharp, spike-like energy barrier for electrons. The energy barrier is depicted as a convex portion of the band structure in the figure. Electrons accumulate on the cathode CD side near the interface between the first low-refractive-index layer L1 and the cathode CD. This reduces the efficiency of electron injection into the light-emitting layer Em, improving the carrier balance in the light-emitting layer Em and improving the light-emitting efficiency of the light-emitting layer Em.

[0043] As shown in FIG. 3 , the Fermi level of the first low-refractive index layer L1 is preferably shallower than that of the protective layer PL. The difference in Fermi levels may be 1 eV or more. Due to the junction between the first low-refractive index layer L1 and the protective layer PL, as shown in FIG. 4 , the CBM of the first low-refractive index layer L1 slopes downward toward the protective layer PL, while the CBM of the protective layer PL slopes upward toward the first low-refractive index layer L1, creating traps into which electrons fall. The traps are depicted as depressions in the band structure in the figure. Electrons accumulate on the side of the first low-refractive index layer L1 near the interface between the protective layer PL and the first low-refractive index layer L1. This reduces the efficiency of electron injection into the light-emitting layer Em, improving the carrier balance in the light-emitting layer Em and the luminous efficiency of the light-emitting layer Em.

[0044] (Method for Manufacturing Light-Emitting Device) The light-emitting device ED according to the present disclosure may be manufactured by any known method. The electron transport layer ETL may be formed by any method such as sputtering, vapor deposition, or coating. At this time, the oxygen vacancy concentration in the electron transport layer ETL may be adjusted.

[0045] In metal oxides such as zinc oxide and gallium oxide, the higher the oxygen vacancy concentration, the greater the electron transport property. Furthermore, the shorter the light-emitting element ED's peak emission wavelength λ, the more free electrons and holes it requires. Therefore, when manufacturing multiple light-emitting elements ED with different peak emission wavelengths λ, the light-emitting element ED with a shorter peak emission wavelength λ may have an electron transport layer ETL with a higher oxygen vacancy concentration.

[0046] Furthermore, for one light-emitting element ED, the oxygen vacancy concentration of the electron transport layer ETL in the light-emitting region 28 (see FIG. 22 described later) may be made higher than the oxygen vacancy concentration of the electron transport layer ETL in the non-light-emitting region 30 (see FIG. 22 described later) surrounding the light-emitting region 28 by a method such as UV exposure. For example, the electron transport layer ETL in the light-emitting region 28 is exposed to UV light, and the electron transport layer ETL in the non-light-emitting region 30 is protected from UV light. UV exposure increases the oxygen vacancy concentration of the electron transport layer ETL in the light-emitting region 28. As described above, the electron transport properties of oxide semiconductors such as gallium oxide and indium oxide depend on the oxygen vacancy concentration. This reduces the leakage current passing through the non-light-emitting region 30 and increases the internal quantum efficiency of the light-emitting element ED.

[0047] [Embodiment 2] (Configuration of Light-Emitting Device) Fig. 5 is a schematic cross-sectional view showing an example of the configuration of a light-emitting device according to an embodiment of the present disclosure. As shown in Fig. 5, the electron transport layer ETL according to embodiment 2 of the present disclosure includes a first low-refractive index layer L1 made of a low-refractive index material, and further includes a first high-refractive index layer H1 made of a first high-refractive index material having electron transport properties and a refractive index higher than that of the low-refractive index material. The first low-refractive index layer L1 is located between the light-emitting layer Em and the first high-refractive index layer H1.

[0048] According to the above configuration, since the electron transport layer ETL has a multilayer structure, it is easy to form the electron transport layer ETL so that the mechanical thickness D of the electron transport layer ETL is large. Therefore, it is easy to form the electron transport layer ETL so that the thickness D is equal to or greater than half of the emission peak wavelength λ of the light-emitting layer Em.

[0049] The mechanical thickness of the first low-refractive-index layer L1 is preferably larger than the mechanical thickness of the first high-refractive-index layer H1. The mechanical thickness of the first low-refractive-index layer L1 is preferably equal to or greater than half the emission peak wavelength λ of the light-emitting layer Em. It is advantageous that the effective refractive index of the electron-transporting layer ETL is 2.0 or less.

[0050] The first high refractive index material may include, for example, at least one selected from the group consisting of zinc oxide (ZnO) and TPBi.

[0051] (Band Structure of Light-Emitting Element) FIG. 6 shows an example of the band structure of each layer when the layers constituting the light-emitting element shown in FIG. 5 are bonded to each other.

[0052] 6, it is preferable that (i) traps into which electrons fall are generated on the protective layer PL side of the first low-refractive-index layer L1, (ii) an energy barrier for electrons is generated on the first high-refractive-index layer H1 side of the first low-refractive-index layer L1, (iii) traps into which electrons fall are generated on the first low-refractive-index layer L1 side of the first high-refractive-index layer H1, and (iv) an energy barrier for electrons is generated on the cathode CD side of the first high-refractive-index layer H1. For this reason, it is preferable that the Fermi level of the first low-refractive-index layer L1 is shallower than the Fermi level of the protective layer PL, the Fermi level of the first high-refractive-index layer H1 is shallower than the Fermi level of the first low-refractive-index layer L1, and the Fermi level of the cathode CD is shallower than the Fermi level of the first high-refractive-index layer H1.

[0053] These energy barriers and traps reduce the efficiency of electron injection into the light-emitting layer Em, improving the carrier balance in the light-emitting layer Em and improving the light-emitting efficiency of the light-emitting layer Em.

[0054] (Effective Refractive Index of Electron Transport Layer and Light Extraction Efficiency of Light-Emitting Device) Figures 7 to 10 are diagrams showing an example of the effective refractive index of the electron transport layer shown in Figure 5 and the light extraction efficiency (LEE) of the light-emitting device when the low refractive index material is gallium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. Figures 11 to 14 are diagrams showing an example of the effective refractive index of the electron transport layer shown in Figure 5 and the LEE of the light-emitting device when the low refractive index material is indium oxide, the first high refractive index material is zinc oxide, and the light-emitting layer emits blue light. In Figures 7 to 14, the left vertical axis represents the effective refractive index, the right vertical axis represents the LEE, and the horizontal axis represents the thickness of the first low refractive index layer L1. Figures 7 and 11 show the case where the film thickness of the first high-refractive-index layer H1 is 20 nm, Figures 8 and 12 show the case where the film thickness of the first high-refractive-index layer H1 is 40 nm, Figures 9 and 13 show the case where the film thickness of the first high-refractive-index layer H1 is 60 nm, and Figures 10 and 14 show the case where the film thickness of the first high-refractive-index layer H1 is 80 nm. In the examples shown in Figures 7 to 14, the emission peak wavelength λ of the light-emitting layer Em is 442 nm.

[0055] As mentioned above, the refractive index of zinc oxide (ZnO) is about 2.2, and that of gallium oxide (Ga ２ O ３ ) has a refractive index of about 1.45, and indium oxide (In ２ O ３ The refractive index of the quantum dot QD is approximately 2. The refractive index of the quantum dot QD is also a II-VI compound, so it is approximately the same as the refractive index of ZnO. ２ O ３ The effective refractive index of the electron transport layer ETL / light-emitting layer Em, which has a structure of ZnO / QD, was considered to be equal to the effective refractive index of the entire light-emitting element ED. For simplicity of calculation, the LEE of the light-emitting element ED was calculated based only on the critical angle of total reflection at the interface between the light-emitting element ED and the outside (i.e., air).

[0056] 7 to 10, when the low refractive index material is gallium oxide, the LEE is significantly improved due to its low refractive index. Even when the thickness of the first high refractive index layer H1 is 80 nm (see FIG. 10), the LEE is 40% or more when the thickness of the first low refractive index layer L1 is 1 / 2 the emission peak wavelength λ, i.e., 221 nm.

[0057] As shown in Figures 11 to 14, when the low refractive index material is indium oxide, the LEE reaches approximately 25%. When zinc oxide is used for the entire electron transport layer, the LEE is approximately 21%. Compared to gallium oxide, the Fermi level of indium oxide is shallower. Therefore, by using indium oxide, the electron injection efficiency decreases, and the luminous efficiency of the light-emitting layer Em can be improved.

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[0059] According to the above configuration, since the electron transport layer ETL includes the first low-refractive index layer L1 and the second low-refractive index layer L2, it is easy to form the electron transport layer ETL so that the mechanical thickness D of the electron transport layer ETL is large. Therefore, it is easy to form the electron transport layer ETL so that the thickness D is equal to or greater than half the emission peak wavelength λ of the light-emitting layer Em.

[0060] The total mechanical thickness of the first low-refractive index layer L1 and the second low-refractive index layer L2 is preferably larger than the mechanical thickness of the first high-refractive index layer H1. The total mechanical thickness of the first low-refractive index layer L1 and the second low-refractive index layer L2 is preferably equal to or greater than half the emission peak wavelength λ of the light-emitting layer Em. It is advantageous that the effective refractive index of the electron-transporting layer ETL is 2.0 or less.

[0061] (Band Structure of Light-Emitting Element) FIG. 16 shows an example of the band structure of each layer when the layers constituting the light-emitting element shown in FIG. 15 are bonded to each other.

[0062] 16 , it is preferable that (i) traps into which electrons fall are generated in the first low-refractive-index layer L1 on the protective layer PL side, (ii) an energy barrier for electrons is generated in the first low-refractive-index layer L1 on the first high-refractive-index layer H1 side, (iii) the CBM of the first high-refractive-index layer H1 becomes a quantum well for electrons, and (iv) an energy barrier for electrons is generated in the second low-refractive-index layer L2 on the first high-refractive-index layer H1 side. For this reason, it is preferable that the Fermi level of the first low-refractive-index layer L1 is shallower than the Fermi level of the protective layer PL, the Fermi level of the first high-refractive-index layer H1 is shallower than the Fermi levels of both the first low-refractive-index layer L1 and the second low-refractive-index layer L2, and the Fermi level of the cathode CD is shallower than the Fermi level of the first low-refractive-index layer L1.

[0063] These energy barriers, traps, and quantum wells reduce the efficiency of electron injection into the light-emitting layer Em, improving the carrier balance in the light-emitting layer Em and improving the luminous efficiency of the light-emitting layer Em. The larger the quantum well, i.e., the deeper and wider the quantum well, the greater the expected decrease in electron injection efficiency. The depth of the quantum well corresponds to the energy difference (absolute value) between the CBM of the first low-refractive-index layer L1 and the second low-refractive-index layer L2 and the CBM of the first high-refractive-index layer H1 at absolute zero, and the width of the quantum well corresponds to the thickness of the first high-refractive-index layer H1.

[0064] (Effective Refractive Index of Electron Transport Layer) Figure 17 shows an example of the effective refractive index of the electron transport layer shown in Figure 15 when the low refractive index material is gallium oxide and the first high refractive index material is zinc oxide. The total film thickness of the first low refractive index layer L1 and the second low refractive index layer L2 was set to a given value of 200 nm or more, and the film thickness of the first high refractive index layer H1 was changed. The vertical axis of Figure 17 represents LEE, and the horizontal axis represents the film thickness of the first high refractive index layer H1. Of the three graphs shown in Figure 17, the solid line represents the results for blue light (λ = 442 nm), the dashed line represents the results for green light (λ = 539 nm), and the dashed-dotted line represents the results for red light (λ = 652 nm).

[0065] A typical zinc oxide (Zn) film has a thickness of 40 nm to 60 nm. As shown in Fig. 17, the effective refractive index of the entire electron transport layer ETL can be set to 2.0 or less.

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[0067] The thickness of the first low-refractive-index layer L1 is preferably greater than the total thickness of the first high-refractive-index layer H1 and the second high-refractive-index layer H2. The thickness of the first low-refractive-index layer L1 is preferably at least half the emission peak wavelength λ of the light-emitting layer Em. It is advantageous that the effective refractive index of the electron-transporting layer ETL is 2.0 or less.

[0068] (Band Structure of Light-Emitting Element) FIG. 19 shows an example of the band structure of each layer when the layers constituting the light-emitting element shown in FIG. 18 are bonded to each other.

[0069] As shown in Figure 19, it is preferable that (i) traps into which electrons fall are created on the protective layer PL side of the first low refractive index layer L1, (ii) an energy barrier for electrons is created on the first high refractive index layer H1 side of the first low refractive index layer L1, (iii) a trap into which electrons fall is created on the first low refractive index layer L1 side of the first high refractive index layer H1, (iv) an energy barrier for electrons is created on the second high refractive index layer H2 side of the first high refractive index layer H1, (v) a trap into which electrons fall is created on the first high refractive index layer H1 side of the second high refractive index layer H2, and (vi) an energy barrier for electrons is created on the cathode CD side of the second high refractive index layer H2. For this reason, it is preferable that the Fermi level of the first low refractive index layer L1 is shallower than the Fermi level of the protective layer PL, the Fermi level of the first high refractive index layer H1 is shallower than the Fermi level of the first low refractive index layer L1, the Fermi level of the second high refractive index layer H2 is shallower than the Fermi level of the first high refractive index layer H1, and the Fermi level of the cathode CD is shallower than the Fermi level of the second high refractive index layer H2.

[0070] These energy barriers and traps reduce the efficiency of electron injection into the light-emitting layer Em, improving the carrier balance in the light-emitting layer Em and improving the light-emitting efficiency of the light-emitting layer Em.

[0071] [Embodiment 5] (Configuration of Light-Emitting Device) Fig. 20 is a schematic cross-sectional view showing an example of the configuration of a light-emitting device according to an embodiment of the present disclosure. As shown in Fig. 20, the electron transport layer ETL may include a mixed refractive index layer HL in which a low refractive index material and a first high refractive index material having electron transport properties and a refractive index higher than that of the low refractive index material are mixed. The effective refractive index of the mixed refractive index layer HL may be calculated as a weighted average of the refractive indices of the low refractive index material and the first high refractive index material, with weights corresponding to the volume fractions of each material.

[0072] It is preferable that the volume fraction of the low refractive index material in the mixed refractive index layer HL is larger than the volume fraction of the high refractive index material in the mixed refractive index layer HL. It is advantageous that the effective refractive index of the electron transport layer ETL is 2.0 or less.

[0073] For example, the mixed refractive index layer HL may include low-refractive index particles LP containing a low-refractive index material and high-refractive index particles HP containing a first high-refractive index material. The light extraction efficiency of the light-emitting element ED is improved by light scattering between the low-refractive index particles LP and the high-refractive index particles HP, which have different refractive indices. In this case, gallium oxide (Ga ２ O ３ ) may be combined with zinc oxide (ZnO) as the first high refractive index material.

[0074] For example, the mixed refractive index layer HL may include low refractive index particles LP and a high refractive index matrix material formed between the low refractive index particles LP, and the high refractive index matrix material may include a first high refractive index material. The light extraction efficiency of the light-emitting element ED is improved by light scattering between the low refractive index particles LP and the high refractive index matrix material, which have different refractive indices. Alternatively, for example, the mixed refractive index layer HL may include high refractive index particles HP and a low refractive index matrix material formed between the high refractive index particles HP, and the low refractive index matrix material may include a low refractive index material. The light extraction efficiency of the light-emitting element ED is improved by light scattering between the high refractive index particles HP and the low refractive index matrix material, which have different refractive indices. In both of these cases, gallium oxide (Ga ２ O ３ ) may be combined with zinc oxide (ZnO) as the first high refractive index material.

[0075] For example, the mixed refractive index layer HL may include a continuous film in which a first high refractive index material is solid-dissolved in a low refractive index material, or a continuous film in which a low refractive index material is solid-dissolved in a first high refractive index material. The bond length between gallium and oxygen is close to the bond length between zinc and oxygen, and the electronegativity of gallium is close to the electronegativity of zinc. For these reasons, the solid solubility of gallium oxide in zinc oxide is high, and a continuous film of a mixture in which gallium oxide is solid-dissolved in zinc oxide can be formed with few defects. Furthermore, the solid solubility of zinc oxide in gallium oxide is high, and a continuous film of a mixture in which zinc oxide is solid-dissolved in gallium oxide can be formed with few defects. It is preferable to use gallium oxide as the low refractive index material and zinc oxide as the first high refractive index material.

[0076] (Band Structure of Light-Emitting Element) FIG. 21 shows an example of the band structure of each layer when the layers constituting the light-emitting element shown in FIG. 20 are bonded to each other.

[0077] 21 , in the mixed refractive index layer HL, (i) traps into which electrons fall are generated on the protective layer PL side of the high-refractive-index particles HP, (ii) an energy barrier for electrons is generated on the cathode CD side of the high-refractive-index particles HP, and (iii) the CBM of the low-refractive-index particles LP forms a quantum well for electrons. For this reason, it is preferable that the Fermi level of the first high-refractive-index material is shallower than that of the protective layer PL, the Fermi level of the cathode CD is shallower than that of the first high-refractive-index material, and the band gap of the low-refractive-index material is smaller than that of the first high-refractive-index material.

[0078] These energy barriers, traps and quantum wells reduce the efficiency of electron injection into the light-emitting layer Em, improving the carrier balance in the light-emitting layer Em and improving the light-emitting efficiency of the light-emitting layer Em.

[0079] [Embodiment 6] (Configuration of Display Device) Figure 22 is a schematic cross-sectional view showing an example of the configuration of a display area of ​​a display device according to an embodiment of the present disclosure. As shown in Figure 22, a display device DP according to the present disclosure includes a light-emitting element ED according to the present disclosure. When the display device DP according to the present disclosure includes multiple light-emitting elements, at least one of the multiple light-emitting elements is a light-emitting element ED according to the present disclosure. The light-emitting element ED may have a configuration according to any of the above-described embodiments 1 to 5, or a configuration that combines or modifies them.

[0080] The light-emitting element ED may include a red element RD that emits red light, a green element GD that emits green light, and a blue element BD that emits blue light. The red element RD, the green element GD, and the blue element BD may share an electron transport layer ETL. This sharing can reduce the manufacturing cost of the display device DP. The mechanical film thickness D of the shared electron transport layer ETL is set to be equal to or less than the emission peak wavelength λ of the emission layer Em of the red element RD. Ｒ The wavelength of red light is longer than the wavelengths of green and blue light. Therefore, when the film thickness D of the electron transport layer ETL is set to be equal to or larger than the emission peak wavelength λ of the red element RD,Ｒ When the thickness D of the electron transport layer ETL is equal to or larger than ½ of the emission peak wavelength λ of the green element GD, Ｇ and the emission peak wavelength λ of the blue element BD is equal to or greater than ½ of the wavelength λ Ｂ It is more than half of the above.

[0081] The free electron density in the electron transport layer ETL may vary depending on the position. The shorter the emission peak wavelength λ of the light-emitting element ED, the more free electrons and holes it requires. The free electron density in the electron transport layer ETL in the blue element BD may be greater than the free electron densities in the electron transport layers ETL in the green element GD and the red element RD, and the free electron density in the electron transport layer ETL in the green element GD may be greater than the free electron density in the electron transport layer ETL in the red element RD.

[0082] In metal oxides such as zinc oxide and gallium oxide, the higher the oxygen vacancy concentration, the greater the free electron density and electron transportability. Therefore, the oxygen vacancy concentration of the electron transport layer ETL in the blue element BD may be higher than the oxygen vacancy concentrations of the electron transport layers ETL in the green element GD and the red element RD, and the oxygen vacancy concentration of the electron transport layer ETL in the green element GD may be higher than the oxygen vacancy concentration of the electron transport layer ETL in the red element RD. This eliminates the electron shortage in the blue element BD and resolves the difference in the electron injection amounts required for the red element RD, the green element GD, and the blue element BD. The oxygen vacancy concentration can be controlled by techniques such as UV exposure, as described above.

[0083] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

[0084] AD Anode BD Blue element CD Cathode D Film thickness DP Display device ED Light-emitting element Em Light-emitting layer ETL Electron transport layer GD Green element L1 First low refractive index layer L2 Second low refractive index layer H1 First high refractive index layer H2 Second high refractive index layer HLM Mixed refractive index layer PL Protective layer RD Red element QD Quantum dot λ Emission peak wavelength

Claims

1. A light-emitting element comprising: an anode and a cathode; a light-emitting layer located between the anode and the cathode; an electron transport layer located between the light-emitting layer and the cathode, the electron transport layer having a thickness equal to or greater than half the emission peak wavelength of the light-emitting layer, the electron transport layer containing a low refractive index material having a refractive index of 2.0 or less; and a protective layer located between the light-emitting layer and the electron transport layer containing an insulating material.

2. The light-emitting element according to claim 1, wherein the low refractive index material includes gallium oxide or indium oxide.

3. The light-emitting element according to claim 1 or 2, wherein the insulating material comprises at least one selected from the group consisting of alkali metals, alkaline earth metals, lanthanide metals, halides of metal elements belonging to any of Groups 4, 5, 6, 7, 13, 14, 15, 16 and 17, inorganic oxides of the metal elements, and silicon oxide.

4. The light-emitting device according to any one of claims 1 to 3, wherein the protective layer has a thickness of 1 nm or more and 5 nm or less.

5. The light-emitting device according to any one of claims 1 to 4, wherein the electron transport layer includes a first low refractive index layer made of the low refractive index material.

6. The light-emitting device according to claim 5, wherein the electron transport layer further includes a first high refractive index layer made of a first high refractive index material having electron transport properties and a refractive index greater than that of the low refractive index material, and the first low refractive index layer is located between the light-emitting layer and the first high refractive index layer.

7. The light-emitting device according to claim 6, wherein the electron transport layer further includes a second low refractive index layer made of the low refractive index material, and the first high refractive index layer is located between the first low refractive index layer and the second low refractive index layer.

8. The light-emitting device described in claim 6, wherein the electron transport layer further includes a second high refractive index layer made of a second high refractive index material having electron transport properties and a refractive index greater than that of the first high refractive index material, and the first high refractive index layer is located between the first low refractive index layer and the second high refractive index layer.

9. The light-emitting device according to any one of claims 1 to 4, wherein the electron transport layer includes a mixed refractive index layer in which the low refractive index material is mixed with a first high refractive index material having electron transport properties and a refractive index greater than that of the low refractive index material.

10. The light-emitting device according to any one of claims 6, 7 and 9, wherein the first high refractive index material includes at least one selected from the group consisting of zinc oxide and TPBi.

11. The light-emitting device according to any one of claims 1 to 10, wherein the protective layer is in contact with the light-emitting layer.

12. The light-emitting element according to any one of claims 1 to 11, wherein the light-emitting layer contains an organic light-emitting material.

13. The light-emitting device according to any one of claims 1 to 11, wherein the light-emitting layer contains quantum dots.

14. The light-emitting device according to any one of claims 1 to 13, wherein the oxygen vacancy concentration of the electron transport layer in the light-emitting region is higher than the oxygen vacancy concentration of the electron transport layer in the non-light-emitting region surrounding the light-emitting region.

15. A display device comprising the light-emitting element according to any one of claims 1 to 14.

16. The display device according to claim 15, wherein the light-emitting elements include a red element that emits red light, a green element that emits green light, and a blue element that emits blue light, the red element, the green element, and the blue element share the electron transport layer, and the film thickness of the electron transport layer is at least half the emission peak wavelength of the light-emitting layer of the red element.

17. The display device of claim 16, wherein the oxygen vacancy concentration of the electron transport layer in the blue element is greater than the oxygen vacancy concentration of the electron transport layer in the red element.

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