Light-emitting element and display device

Abstract

A light-emitting element (1) is provided with: an anode (11) and a cathode (16); a light-emitting layer (14) positioned between the anode (11) and the cathode (16); and a hole transport layer (13) positioned between the light-emitting layer (14) and the anode (11). The maximum value of the thickness of the hole transport layer (13) is 1.15 times or less the average value of the thickness of the hole transport layer (13). The minimum value of the thickness of the hole transport layer (13) is 0.85 times or more the average value of the thickness of the hole transport layer (13).

Description

Light-emitting element and display device

[0001] One aspect of this disclosure relates to a light-emitting element.

[0002] Various technologies have been proposed regarding light-emitting elements. As an example, Patent Document 1 below shows an example of a light-emitting element configuration aimed at improving the light extraction efficiency of the light-emitting element.

[0003] Japanese Patent Publication No. 2017-79146

[0004] An object of one aspect of this disclosure is to reduce variations in brightness in light-emitting elements.

[0005] A light-emitting element according to one aspect of the present disclosure comprises an anode and a cathode, a light-emitting layer located between the anode and the cathode, and a hole transport layer located between the light-emitting layer and the anode, wherein the maximum thickness of the hole transport layer is 1.15 times or less the average thickness of the hole transport layer, and the minimum thickness of the hole transport layer is 0.85 times or more the average thickness of the hole transport layer.

[0006] A light-emitting element according to one aspect of the present disclosure comprises an anode and a cathode, a light-emitting layer located between the anode and the cathode, a hole transport layer located between the light-emitting layer and the anode, and a bank located to the side of the light-emitting layer, wherein the surface of the substrate of the light-emitting element has a recess near the intersection of the slope of the bank and the surface, and the depth of the recess is 2 × Δt or less when the roughness of the upper or lower surface of the hole transport layer is expressed as Δt.

[0007] According to one aspect of this disclosure, variations in brightness in a light-emitting element can be reduced.

[0008] This shows an example configuration of a light-emitting element in Embodiment 1. The overall configuration of the light-emitting element in Embodiment 1 is schematically shown. This shows an example configuration of the light-emitting layer in the light-emitting element. This shows an example of a configuration where an adduct surrounds a quantum dot. This shows another example of a configuration where an adduct surrounds a quantum dot. This shows an example configuration of a light-emitting element as a comparative example. This shows an equivalent circuit model of a light-emitting element according to one aspect of this disclosure. This shows a simplified equivalent circuit model, a simplified version of the equivalent circuit model in Figure 7. This shows an example of voltage-current characteristics in a light-emitting element. This illustrates the variation in hole transport in the hole transport layer of a light-emitting element as a comparative example. This shows an example of the variation in brightness in a light-emitting element as a comparative example. This shows an example of the relationship between Δt, the J deviation, and ΔL. This shows an example of the relationship between t_mean and Δt. This shows an example of the relationship between t_mean and the maximum value of Δt allowed for that t_mean. This is a diagram for explaining an example of a method for manufacturing the light-emitting element of Embodiment 1. This shows an example configuration of a light-emitting element in Embodiment 2. This shows an example configuration of a light-emitting element in Embodiment 3. This shows an example configuration of a display device in Embodiment 4.

[0009] [Embodiment 1] Embodiment 1 will be described below. For the sake of convenience, components having the same function as those described in Embodiment 1 will be denoted by the same reference numerals in subsequent embodiments, and their descriptions will not be repeated. For the sake of simplicity, explanations of known technical matters will be omitted as appropriate.

[0010] In this specification, each component, each material, and each numerical value described herein are merely examples unless otherwise specified. Therefore, for example, unless otherwise specified, the positional relationships of each component are not limited to the examples in each figure. Also, each figure is not necessarily drawn to scale. In this specification, unless otherwise specified, the notation "A to B" for two numbers A and B means "greater than or equal to A and less than or equal to B".

[0011] (Example of the configuration of the light-emitting element 1) Figure 1 shows an example of the configuration of the light-emitting element 1 in Embodiment 1. Figure 1 schematically shows the stacked structure of the light-emitting element 1. In the example of Figure 1, the light-emitting element 1 comprises an anode 11, a hole injection layer 12, a hole transport layer 13, a light-emitting layer 14, an electron transport layer 15, and a cathode 16, in this order from the bottom.

[0012] In the example shown in Figure 1, each part of the light-emitting element 1 is supported by the substrate 90. Therefore, in the example shown in Figure 1, the distance between the anode 11 and the substrate 90 is smaller than the distance between the cathode 16 and the substrate 90. That is, Figure 1 illustrates the case where the anode 11 is the lower electrode and the cathode 16 is the upper electrode. The anode 11 in the example shown in Figure 1 may also be called the substrate-side electrode.

[0013] However, unlike the example in Figure 1, the anode 11 may be the upper electrode and the cathode 16 may be the lower electrode. The anode 11 and the cathode 16 only need to be arranged facing each other.

[0014] The light-emitting element 1 is configured to allow light emitted from the light-emitting layer 14 to be extracted to the outside of the light-emitting element 1. For this reason, in the light-emitting element 1, at least one of the anode 11 and the cathode 16 may be a light-transmitting electrode. Either the anode 11 or the cathode 16 may be a light-reflecting electrode.

[0015] The light-emitting layer 14 only needs to be located between the anode 11 and the cathode 16. In the example in Figure 1, the light-emitting layer 14 is located between the hole transport layer 13 and the electron transport layer 15. The hole transport layer 13 is located above the anode 11. In the example in Figure 1, the hole transport layer 13 is located above the hole injection layer 12. On the other hand, the electron transport layer 15 is located below the cathode 16.

[0016] The light-emitting layer 14 only needs to be configured to emit electro-luminescence (EL) light. Therefore, the light-emitting layer 14 may contain any EL material. For example, the light-emitting layer 14 may contain quantum dots (see also Figure 2 below). Quantum dots are an example of an inorganic EL material.

[0017] The quantum dots in the light-emitting layer 14 emit electroluminescent (EL) light through the recombination of holes supplied from the anode 11 and electrons transported from the cathode 16. Therefore, by applying a forward voltage between the anode 11 and the cathode 16, EL light can be generated in the light-emitting layer 14.

[0018] The hole injection layer 12 may be located between the anode 11 and the hole transport layer 13. The hole injection layer 12 may contain any hole injection material.

[0019] The hole transport layer 13 only needs to be located between the anode 11 and the light-emitting layer 14. In the example shown in Figure 1, the hole transport layer 13 is located between the hole injection layer 12 and the light-emitting layer 14.

[0020] In the example shown in Figure 1, the hole transport layer 13 is the layer directly above the hole injection layer 12. In the example shown in Figure 1, there are irregularities on the upper surface of the hole injection layer 12. In other words, in the example shown in Figure 1, there are irregularities on the lower surface of the hole transport layer 13. Therefore, in the example shown in Figure 1, the thickness of the hole transport layer 13, which is located directly above the hole injection layer 12, may be uneven. In this specification, the thickness of the hole transport layer 13 is denoted as t.

[0021] In the example in Figure 1, the hole injection layer 12 is an example of a lower layer, which will be described later. During the coating and deposition process of the hole transport layer 13, non-uniform surface conditions of the lower layer can lead to non-uniformity in the thickness of the hole transport layer 13 to be formed above the lower layer. For example, if fine irregularities exist on the surface of the lower layer, the viscosity or anchoring effect of the HTL solution (described later) to be coated on that surface may cause the flow of the HTL solution to become non-uniform. In this case, non-uniformity in the thickness of the hole transport layer 13 may occur in areas where the flow of the HTL solution is stagnant.

[0022] In the light-emitting element 1 of Embodiment 1, the variation in t in the hole transport layer 13 is set to be kept from becoming too large. A method for reducing the variation in t in the hole transport layer 13 will be described later. In this specification, the average value of t in the hole transport layer 13 is denoted as t_mean. The maximum value of t in the hole transport layer 13 is denoted as t_max, and the minimum value of t in the hole transport layer 13 is denoted as t_min.

[0023] Figure 2 schematically shows the overall configuration of the light-emitting element 1. In Figure 2, for the sake of clarity of illustration, the upper and lower surfaces of the hole transport layer 13 are shown as being flat. However, as described above, it should be noted that the upper and lower surfaces of the hole transport layer 13 do not necessarily have to be flat. Also, in Figure 2, for the sake of clarity of illustration, the illustration of each layer of the light-emitting element 1 other than the hole injection layer 12 and the hole transport layer 13 is omitted.

[0024] As shown in Figure 2, the light-emitting element 1 may include a bank BK. The bank BK will be described more specifically in Embodiment 3 below. In Figure 1, the portion of the light-emitting element 1 in Figure 2 that is included in the region REG in Figure 2 is shown.

[0025] As an example, the width of the region REG in Figure 2 (the length in the horizontal direction within the plane of the drawing of the region REG in Figure 2) may be defined as the narrowest width of the opening of the bank BK. Thus, the range in the horizontal direction within the plane of the drawing of the light-emitting element 1 in Figure 1 may be defined by the narrowest width of the opening of the bank BK.

[0026] As another example, the width of the region REG may be defined as the distance between two normal lines of the substrate 90 passing through the intersection of the edge of the bank BK and the substrate 90. Thus, the range in the horizontal direction within the plane of the drawing of the light-emitting element 1 in Figure 1 may also be defined by the distance between the two normal lines.

[0027] Referring to Figure 1 again. As described below, in the light-emitting element 1, the hole transport layer 13 is formed such that t falls within the range of ±15% with respect to t_mean. Therefore, in the light-emitting element 1, the following relationship of formula (1), 0.85×t_mean ≤ t ≤ 1.15×t_mean...(1) holds.

[0028] As described above, the maximum value of t is t_max, and the minimum value of t is t_min. Therefore, when formula (1) holds, the following formulas (2) to (3), t_max ≤ 1.15×t_mean...(2) t_min ≥ 0.85×t_mean...(3) hold.

[0029] The variation in t in the hole transport layer 13 corresponds to the roughness of the upper or lower surface of the hole transport layer 13. Here, a small variation in t in the hole transport layer 13 corresponds to a small roughness of the upper or lower surface of the hole transport layer 13. In this specification, the roughness of the upper or lower surface of the hole transport layer 13 is denoted as Δt.

[0030] As an example, the roughness of the upper surface of the hole transport layer 13 may be defined as the value obtained by subtracting the maximum valley depth on the upper surface from the maximum peak height on the upper surface. Thus, the roughness of the upper surface of the hole transport layer 13 may be defined as the difference between the maximum peak height and the maximum valley depth on the upper surface. As another example, the roughness of the upper surface of the hole transport layer 13 may be defined as the RMS (Root Mean Square) roughness of the upper surface. In the example in Figure 1, the roughness of the upper surface of the hole transport layer 13 is approximately 0.

[0031] Similarly, the roughness of the lower surface of the hole transport layer 13 may be defined as the value obtained by dividing the maximum peak height on the lower surface by the maximum valley depth on the lower surface. As another example, the roughness of the lower surface of the hole transport layer 13 may be defined as the RMS roughness of the lower surface. In the example in Figure 1, the roughness of the lower surface of the hole transport layer 13 is non-zero.

[0032] The hole transport layer 13 may contain any hole transport material. Embodiment 1 mainly illustrates the case where the hole transport material is an organic material. In other words, Embodiment 1 mainly illustrates the case where the material of the hole transport layer 13 is an organic substance. When the material of the hole transport layer 13 is an organic substance, Child's Law, described later, applies.

[0033] The electron transport layer 15 only needs to be located between the light-emitting layer 14 and the cathode 16. The electron transport layer 15 may contain any electron transport material.

[0034] In this specification, "organic matter" or "organic material" refers to a substance in which carbon is the central atom in the bonding. On the other hand, "inorganic matter" or "inorganic material" refers to a substance other than an organic matter. Therefore, a typical example of "inorganic matter" or "inorganic material" in this specification is a substance in which carbon is not included in the atomic bonding.

[0035] However, "inorganic substance" or "inorganic material" as used herein may refer to a substance in which carbon is not included in the center of the atomic bonds. Furthermore, "inorganic substance" or "inorganic material" as used herein may refer to a substance that does not have a carbon chain.

[0036] An emissive layer according to one aspect of this disclosure may contain an organic material or an inorganic material.

[0037] When the light-emitting layer contains organic materials, typically the other layers in the light-emitting element are also formed from organic materials. Therefore, in every layer of the light-emitting element, an organic layer is deposited on top of an organic layer. For this reason, if the polarity and nonpolarity relationship between one organic substance and another, including the solvent, is appropriately selected, the deposition of the light-emitting layer itself is not significantly affected by the flatness of the underlying layer.

[0038] If the light-emitting layer contains an inorganic material, the light-emitting layer may further contain quantum dots. Quantum dots are particles with a much larger surface area / volume ratio compared to the aforementioned organic materials. Therefore, particle condensation is a particularly common problem during layer formation. In condensed areas, charge transport or injection, and quantum mechanical energy transfer of excitons are likely to occur. This significantly affects the device characteristics. The degree of condensation is correlated with the interaction between the light-emitting layer and the layer directly beneath it or the surrounding layer. In particular, depending on the state of the layer directly beneath the light-emitting layer, condensation tends to occur more easily. Therefore, especially when forming the layer directly beneath the light-emitting layer, it is necessary to more strictly control the materials or manufacturing parameters to prevent the aforementioned condensation. The improvement effect of the hole transport layer according to one aspect of this disclosure can also be a means of solving the problem of quantum dot condensation.

[0039] As used herein, the "quantum dot" means, for example, a dot with a maximum width of 100 nm or less. The shape of the quantum dot is not particularly limited as long as it satisfies the above maximum width, and is not limited to a spherical three-dimensional shape (circular cross-sectional shape). For example, the shape of the quantum dot may be a polygonal cross-sectional shape, a rod-shaped three-dimensional shape, a branched three-dimensional shape, or a three-dimensional shape having irregularities on the surface. The shape of the quantum dot may be a combination of the above-described shapes.

[0040] The composition of the quantum dot generally originates from a semiconductor material. Therefore, the quantum dot may be referred to as a semiconductor nanoparticle. In addition, the composition of the quantum dot generally originates from an inorganic material. Therefore, the quantum dot may also be referred to as an inorganic nanoparticle. Furthermore, the quantum dot generally has a specific crystal structure. Therefore, the quantum dot may also be referred to as a nanocrystal.

[0041] FIG. 3 shows a configuration example of the light-emitting layer 14. As shown in FIG. 3, the light-emitting layer 14 may include a plurality of quantum dots Q. And the light-emitting layer 14 may include an additive MX. The additive MX may cover the periphery of at least one quantum dot Q. Therefore, the additive MX may exist around the quantum dot Q. The additive MX may fill the space between at least two quantum dots Q.

[0042] The additive MX may have properties as a semiconductor or an insulator, for example. The additive MX may include at least one of metal oxides, semimetal oxides, and metal sulfides.

[0043] As an example, the additive MX is, as an oxide, silicon oxide (SiO ２ ), titanium oxide (TiO ２ ), aluminum oxide (Al ２ O ３ ), boron oxide (B ２ O ３ ), phosphorus oxide (P ２ O ５ ), germanium oxide (GeO ２ ), hafnium oxide (HfO ２), zinc oxide (ZnO), zirconium oxide (ZrO ２ ), tellurium oxide (TeO ２ ), bismuth oxide (Bi ２ O ３ ), vanadium oxide (V ２ O ５ ), antimony oxide (Sb ２ O ５ It may contain at least one of the following: lead oxide (PbO) and copper oxide (CuO).

[0044] The adduct MX may be formed as a polymer. Therefore, for example, in this specification, "the adduct MX contains silicon oxide as an oxide" includes "the adduct MX is a siloxane compound having a siloxane bond."

[0045] As another example, the adduct MX is composed of zinc sulfide (ZnS), magnesium zinc sulfide (ZnMgS, ZnMgS) as sulfides. ２ ), gallium sulfide (GaS, Ga ２ S ３ ), zinc telluride (ZnTeS), magnesium sulfide (MgS), zinc gallium sulfide (ZnGa ２ S ４ It may contain at least one of the following: ), and magnesium sulfide (MgGa2S4).

[0046] Please note that the chemical formulas representing each of the above compounds are merely representative examples. Therefore, please note that the composition ratios indicated in the chemical formulas do not necessarily have to be stoichiometric.

[0047] Figure 4 shows an example of a configuration in which the adduct MX surrounds the quantum dot Q. In Figure 4, a cross-section of the light-emitting layer 14 passing through the quantum dot Q is shown, illustrating a configuration in which the adduct MX completely surrounds the quantum dot Q.

[0048] However, the form in which the adduct MX surrounds the quantum dot Q is not limited to the example in Figure 4. For example, if the adduct MX occupies more than 90% of the area around the quantum dot Q, the quantum dot Q can be considered to be covered by the adduct MX.

[0049] In the example shown in Figure 4, the adduct MX is positioned in contact with the surface of the quantum dot Q. However, the adduct MX does not necessarily have to be in contact with the surface of the quantum dot Q. Figure 5 shows another example in which the adduct MX surrounds the quantum dot Q. In the example shown in Figure 5, the organic ligand LG is positioned between the adduct MX and the quantum dot Q. Thus, the adduct MX may be physically connected to the quantum dot Q via the organic ligand LG.

[0050] Refer again to Figure 3. If the adduct MX is formed to fill the space between at least two quantum dots Q, the adduct MX is 1000 nm in a direction perpendicular to the film thickness direction at any position in the film thickness direction of the light-emitting layer 14. ２ It may be formed as a continuous film having the above area.

[0051] Within the light-emitting layer 14, the quantum dots Q may be embedded in a continuous film of adduct MX. For example, if "80% or more of the quantum dots Q located inside the light-emitting layer 14 have 60% or more of their surface in contact with a continuous film of adduct MX," then it can be considered that "the quantum dots Q located inside the light-emitting layer 14 are embedded in the adduct MX."

[0052] In this specification, "the adduct MX fills the space between at least two quantum dots Q" does not mean only that the space between the two quantum dots Q is filled with the adduct MX alone. In this specification, "the adduct MX fills the space between at least two quantum dots Q" also means that the space between the two quantum dots Q is filled with a mixture of the adduct MX and other materials.

[0053] Therefore, a material different from the material of the adduct MX may exist inside the adduct MX filling the space between at least two quantum dots Q. For example, a ligand may exist inside the adduct MX filling the space between at least two quantum dots Q. As an example, the ligand may be an organic ligand LG coordinated to the quantum dot Q. As another example, the ligand may be an organic ligand LG that is separated from the quantum dot Q.

[0054] For example, if the luminescent layer 14 contains an organic ligand LG, the weight ratio of the organic ligand LG to the total weight of the luminescent layer 14 may be less than 5%. The weight ratio of the organic ligand to the total weight of the luminescent layer 14 may be measured, for example, using TOF-SIMS (Time of Flight - Secondary Ion Mass Spectrometry).

[0055] When the organic ligand LG is dispersed within the light-emitting layer 14, the organic ligand LG can contribute to the injection of holes and electrons into the quantum dot Q.

[0056] Next, an example of a method for forming the light-emitting layer 14 will be described. First, a quantum dot dispersion is prepared. The quantum dot dispersion is a dispersion in which quantum dots Q and a precursor of adduct MX are dispersed in a solvent. The precursor contains a material that is converted into adduct MX through hydrolysis and dehydration condensation by performing a specific operation (e.g., heating or light irradiation). The precursor may have a coordinating functional group that forms a coordination bond with the surface of the quantum dots Q in the quantum dot dispersion. In this case, the precursor can coordinate to the quantum dots Q in the quantum dot dispersion.

[0057] Quantum dot dispersions may contain halides having halogen atoms. Examples of halides include zinc chloride (ZnCl). ２ One example is the following: By mixing a halide containing halogen atoms into a quantum dot dispersion, the halogen atoms coat the surface of the quantum dots Q. As a result, defects on the surface of the quantum dots Q can be reduced.

[0058] Therefore, the presence of halogen atoms in the adduct MX can increase the coverage of halogen atoms on the surface of the quantum dot Q. As a result, the luminescence characteristics of the light-emitting element 1 can be improved.

[0059] Next, another example of a method for preparing a quantum dot dispersion will be described. As another example, a mixture may be prepared by stirring (i) a dispersion in which quantum dots Q are dispersed and (ii) another dispersion in which a precursor is dispersed. Then, a quantum dot dispersion may be prepared by extracting a predetermined layer from the mixture.

[0060] A dispersion containing quantum dots Q may have an organic ligand that coordinates to the quantum dots Q. In this case, for example, during stirring for the preparation of the mixture, the ligand coordinating to the quantum dots Q may be replaced by a portion of the precursor from the organic ligand.

[0061] Next, we will discuss examples of precursors. For example, if the adduct MX contains silicon oxide and the precursor has a coordination functional group, the precursor may contain 3-(trimethoxysilyl-1-propanechiol (TMSPT)).

[0062] As another example, if the adduct MX contains silicon oxide and the precursor does not have a coordination functional group, the precursor may contain diphenylsilanediol (DPSD). The precursor is barium hydroxide (Ba(OH) ２ ) may further contain as a catalyst.

[0063] As yet another example, if the adduct MX contains zinc sulfide, the precursor may contain zinc xanthogenic acid, zinc thioureate, and zinc dithiocarboxylate, etc.

[0064] Next, an example of a method for forming the adduct MX will be described. First, the quantum dot dispersion described above is applied to the upper surface of the lower layer of the light-emitting layer 14 (e.g., the upper surface of the hole transport layer 13), and the precursor described above is converted into the adduct MX. For example, by heating the applied quantum dot dispersion, the solvent is evaporated and the precursor is converted into the adduct MX. The conversion of the precursor into the adduct MX occurs sequentially around the quantum dots Q in the quantum dot dispersion.

[0065] For example, if the precursor contains diphenylsilanediol (DPSD) and 3-(trimethoxysilyl-1-propanethol (TMSPT), barium hydroxide (Ba(OH)) may form between diphenylsilanediol (DPSD) molecules, between 3-(trimethoxysilyl-1-propanethol (TMSPT) molecules, and between diphenylsilanediol (DPSD) and 3-(trimethoxysilyl-1-propanethol (TMSPT) molecules. ２ Dehydration condensation occurs using catalysts such as ). As a result, silicon oxide is formed as the adduct MX.

[0066] As another example, if the precursor contains zinc xanthogenic acid, the zinc xanthogenic acid decomposes upon heating of the quantum dot dispersion. As a result, zinc sulfide is formed as the adduct MX.

[0067] As described above, by converting the precursor in the quantum dot dispersion into adduct MX, a light-emitting layer 14 (e.g., the light-emitting layer 14 in Figure 3) containing adduct MX and quantum dots Q can be formed.

[0068] (Comparative Example) Figure 6 shows an example configuration of light-emitting element 1R as a comparative example of light-emitting element 1. Figure 6 is a counterpart to Figure 1. In this specification, the hole injection layer of light-emitting element 1R is referred to as the hole injection layer 12R, and the hole transport layer of light-emitting element 1R is referred to as the hole transport layer 13R.

[0069] The roughness of the lower surface of the hole transport layer 13R in Figure 6 is greater than the roughness of the lower surface of the hole transport layer 13 in Figure 1. This indicates that the variation in t in the hole transport layer 13R in Figure 6 is greater than the variation in t in the hole transport layer 13 in Figure 1.

[0070] In this specification, it is assumed that equation (1) above is not satisfied in the hole transport layer 13R. That is, t in the hole transport layer 13R is not within ±15% of t_mean. Therefore, equations (2) to (3) above are not satisfied in the hole transport layer 13R.

[0071] (Examination of the electrical characteristics of the light-emitting element) Figure 7 shows an equivalent circuit model of a light-emitting element according to one embodiment of the present disclosure. In Figure 7, the equivalent circuit model is denoted by the reference numeral MODEL1. MODEL1 includes a diode 701, a diode vacuum tube 702, a resistor element 703, a diode vacuum tube 704, and a resistor element 705.

[0072] Diode 701 equivalently represents the electrical characteristics of the quantum dot and electron transport layer in the light-emitting element. The electrical characteristics of diode 701 are given by equation (3) below. ...It can be expressed as shown in (3). Equation (3) is also called the diode rectification equation.

[0073] In equation (3), I ＤＩＯＤＥ This represents the current flowing through diode 701 (diode current). ＤＩＯＤＥ This represents the voltage applied to diode 701 (diode voltage). ０ q is a predetermined constant, k is the unit charge, k is the Boltzmann constant, and T is the absolute temperature.

[0074] The diode vacuum tube 704 equivalently represents the electrical characteristics of the hole transport layer in the light-emitting element. The electrical characteristics of the diode vacuum tube 704 are given by the following equation (4): ...It can be expressed as shown in (4). Equation (4) is also called the characteristic equation of a two-electrode vacuum tube.

[0075] In equation (4), I ＨＴＬ V represents the current flowing through the 704 diode vacuum tube, which acts as a hole transport layer. ＨＴＬ represents the voltage applied to the diode vacuum tube 704, which acts as a hole transport layer. S is the conductance of the diode vacuum tube 702. m is a predetermined constant.

[0076] As shown in equation (4), in the diode vacuum tube 704, I ＨＴＬ is, V ＨＴＬ It is proportional to the mth power of the current. The electrical characteristic that the current flowing through a component is proportional to the power of the voltage applied to that component is called Child's law. Equation (4) is an example of Child's law. Child's law is known to hold true for organic materials.

[0077] The diode vacuum tube 702 is connected in parallel with the diode 701. The diode vacuum tube 702 equivalently represents the electrical characteristics of the ligand in the light-emitting element. As mentioned above, the ligand in the light-emitting element is typically an organic ligand. Therefore, the electrical characteristics of the diode vacuum tube 702 can also be expressed by a characteristic equation similar to equation (3) above.

[0078] The resistive element 703 is connected in parallel with the diode 701. The resistive element 703 equivalently represents the electrical characteristics of the shunt in the light-emitting element. The resistive element 705 equivalently represents the electrical characteristics of the leakage component in the light-emitting element.

[0079] Figure 8 shows a simplified equivalent circuit model, which is a simplified version of the equivalent circuit model in Figure 7. In Figure 8, this simplified equivalent circuit model is denoted as MODEL 2. The inventors applied MODEL 1 (the detailed equivalent circuit model) of Figure 7 to measured data of the element characteristics of a light-emitting element according to one aspect of the present disclosure and performed an analysis. As a result, it was confirmed that the current flowing through the resistor element 703 and the current flowing through the diode vacuum tube 702 in MODEL 1 of Figure 7 were significantly smaller than the current flowing through the diode 701, the diode vacuum tube 704, and the resistor element 705.

[0080] Specifically, it was confirmed that the current flowing through the resistor 703 and the diode vacuum tube 702 in MODEL 1 of Figure 7 are all less than 1 / 100th the size of the current flowing through the diode 701, the diode vacuum tube 704, and the resistor 705. As can be seen from this, the resistor 703 and the diode vacuum tube 704 in MODEL 1 do not have a significant impact on the overall electrical characteristics of MODEL 1.

[0081] Therefore, MODEL 2 is obtained by removing the resistor element 703 and the diode vacuum tube 704 from MODEL 1. By using MODEL 2, the electrical characteristics of the light-emitting element can be simulated with simpler calculations compared to when using MODEL 1.

[0082] Figure 9 shows an example of the voltage (V)-current (I) characteristics of a light-emitting element. In the graph in Figure 9, the horizontal axis represents voltage and the vertical axis represents current. In Figure 9, reference numeral 910 indicates the diode current in MODEL 2. Reference numeral 920 indicates the measured value in the light-emitting element.

[0083] In Figure 9, reference numeral 930 indicates the fitting result according to MODEL 2. This fitting result is expressed as the sum of the diode current, the organic conduction component, and the leakage component. The organic conduction component is I in MODEL 2. ＨＴＬ This represents the current flowing through the resistive element 705 in MODEL 2.

[0084] In Figure 9, reference numeral 940 indicates an example of the voltage-current characteristics of a light-emitting element with large luminance variation. The example of reference numeral 940 shows that a light-emitting element with large luminance variation has a large leakage component. On the other hand, reference numeral 950 indicates an example of the voltage-current characteristics of a light-emitting element with small luminance variation. The example of reference numeral 950 shows that a light-emitting element with small luminance variation has a small leakage component.

[0085] Figure 10 illustrates the variation in hole transport in the hole transport layer 13R of the light-emitting element 1R. As an example, the current density J generated in a hole transport layer of thickness t is given by the following equation (5): ...It is known that it can be expressed as shown in (5). Equation (5) is also an example of Child's law. In equation (5), ε ０ ε is the permittivity of vacuum, ｒ This is the relative permittivity of the hole transport layer.

[0086] As shown in equation (5), J is inversely proportional to t cubed. In other words, J is proportional to t -3. Therefore, when t is large, J is significantly smaller than when t is small. Consequently, when there is a large variation in t in the hole transport layer, the variation in J in that hole transport layer becomes significantly larger.

[0087] In the example shown in Figure 10, J becomes larger at positions where t is small in the hole transport layer 13R (e.g., positions corresponding to recesses on the lower surface of the hole transport layer 13R). Therefore, a large amount of holes are transported at these positions. Note that the positions corresponding to recesses on the lower surface of the hole transport layer 13R can also be expressed as positions corresponding to convex portions on the upper surface of the hole injection layer 12R.

[0088] On the other hand, at positions in the hole transport layer 13R where t is large (e.g., positions corresponding to the flat portion of the lower surface of the hole transport layer 13R), J becomes small. Therefore, only a small amount of holes are transported at these positions. The positions corresponding to the flat portion of the lower surface of the hole transport layer 13R can also be expressed as positions corresponding to the flat portion of the upper surface of the hole injection layer 12R.

[0089] Figure 11 shows an example of brightness variation in the light-emitting element 1R. In Figure 11, the positions labeled P_DARK are examples of positions in the light-emitting element 1R where the brightness is low. The positions labeled P_DARK correspond to positions in the hole transport layer 13R where the amount of hole transport is small (i.e., positions where J is small). Therefore, the portions labeled P_DARK correspond to positions in the hole transport layer 13R where t is large.

[0090] On the other hand, in Figure 11, the positions labeled P_LIGHT are examples of positions with low brightness in the light-emitting element 1R. The positions labeled P_DARK correspond to positions in the hole transport layer 13R where the amount of hole transport is small (i.e., positions where J is small). Therefore, the portions labeled P_DARK correspond to positions in the hole transport layer 13R where t is large.

[0091] Figure 11 shows that when the variation in t in the hole transport layer is large, the variation in brightness (hereinafter referred to as L) in the light-emitting element having the hole transport layer also increases. Therefore, in order to reduce the variation in L in the light-emitting element, it is preferable to reduce the variation in t in the hole transport layer. In other words, in order to reduce the variation in L, it is preferable to reduce Δt in the hole transport layer.

[0092] As an example, consider the case in the hole transport layer where t_min = t_mean - Δt and t_min = t_mean + Δt. Generally, L is proportional to J. And, according to equation (5) above, J is proportional to t to the power of -3. Therefore, the minimum value of L (hereinafter referred to as L_min) is proportional to the power of -3 of "t - Δt". On the other hand, the maximum value of L (hereinafter referred to as L_max) is proportional to the power of -3 of "t + Δt". From this, it can be said that the variation in L can be reduced by making Δt smaller.

[0093] In this specification, the maximum allowable variation in L in a light-emitting element is denoted as ΔL. ΔL is also referred to as the maximum allowable luminance deviation in a light-emitting element. ΔL represents the deviation from the average value of L in the light-emitting element (hereinafter referred to as L_mean). In the following explanation, for the sake of clarity, it will be assumed that L_min = L_mean - ΔL and that L_max = L_mean + ΔL.

[0094] The inventors further investigated the relationship between Δt and ΔL. In this specification, the variation in J is denoted as ΔJ. ΔJ is also called the J deviation. In this specification, the average value of J is denoted as J_mean, the maximum value of J is denoted as J_max, and the minimum value of J is denoted as J_min.

[0095] In the following explanation, when ΔJ is a positive value, J_min = J_mean - ΔJ and J_max = J_mean + ΔJ can be expressed as follows. J_max corresponds to L_max. Therefore, J_max corresponds to "t + Δt". On the other hand, J_min corresponds to L_min. Therefore, J_min corresponds to "t - Δt".

[0096] Figure 12 shows an example of the relationship between Δt, J deviation, and ΔL derived by the inventors. In the graph of Figure 12, the horizontal axis represents t_mean [nm], and the vertical axis represents J deviation. In the example of Figure 12, Δt is expressed in units of a percentage of t_mean.

[0097] Figure 13 shows an example of the relationship between t_mean and Δt. The graph in Figure 13 is derived based on Figure 12. The horizontal axis of the graph in Figure 13 represents t_mean [nm], and the vertical axis represents the maximum allowable Δt for that t_mean. The vertical axis in the graph in Figure 13 is expressed in units of [nm]. Figure 13 shows examples of the relationship between t_mean and Δt for the cases of ΔJ = ±10%, ΔJ = ±20%, and ΔJ = ±30%. In the examples in Figure 13, ΔJ is expressed in units of [%] relative to J_mean.

[0098] Figure 14 shows an example of the relationship between t_mean and the maximum allowable value of Δt for that t_mean. The graph in Figure 14 is also derived based on Figure 12. In the graph of Figure 14, the horizontal axis represents t_mean [nm], and the vertical axis represents the maximum allowable value of Δt for that t_mean. In the example in Figure 14, Δt is expressed in units of a percentage of t_mean. Figure 14 shows examples of the relationship between t_mean and Δt for the cases of ΔL = 10%, ΔL = 20%, and ΔL = 30%. In the example in Figure 14, ΔL is expressed in units of a percentage of L_mean.

[0099] According to the example in Figure 14, when ΔL = 10%, the maximum allowable value of Δt for t_mean = 50 nm is 3.5%. When ΔL = 20%, the maximum allowable value of Δt for t_mean = 50 nm is 7.7%. When ΔL = 30%, the maximum allowable value of Δt for t_mean = 50 nm is 12.6%.

[0100] Therefore, when t_mean = 50 nm, setting Δt to 12.6% or less allows ΔL to be kept below 30%. Also, when t_mean = 50 nm, setting Δt to 7.7% or less allows ΔL to be kept below 20%. Furthermore, when t_mean = 50 nm, setting Δt to 3.5% or less allows ΔL to be kept below 10%. As described above, by realizing a light-emitting element with a small Δt, the variation in brightness of the light-emitting element can be reduced.

[0101] According to the inventors' studies, by setting Δt to approximately 15% or less within the general range of t_mean, the variation in brightness of the light-emitting element can be reduced to a degree that satisfies general specifications. From this, the inventors have newly created a light-emitting element 1 that satisfies the above-mentioned equation (1). In other words, they have newly created a light-emitting element 1 that satisfies the above-mentioned equations (2) to (3). As described above, according to Embodiment 1, a light-emitting element (e.g., light-emitting element 1) with reduced variation in brightness is realized.

[0102] As mentioned above, Patent Document 1 shows an example of a light-emitting element configuration aimed at improving the light extraction efficiency of the light-emitting element. However, Patent Document 1 does not mention at all that variations in t in the hole transport layer cause variations in L in the light-emitting element having the hole transport layer. From this, it can be said that the light-emitting element 1 in Embodiment 1 could not have been easily created by a person skilled in the art based on known technology (e.g., the technology described in Patent Document 1).

[0103] (Further example of the relationship between t_mean and Δt) By performing numerical interpolation based on the graph in Figure 14 above, we can obtain the following equation (6): Δt = A × t_mean - B …(6). The units of Δt and t_mean in equation (6) are both [nm].

[0104] In equation (5), A and B are, respectively, A = 4 × 10 －５ ×ΔL ２ +0.003×ΔL+0.003…(7) B=8×10 －５ ×ΔL ２ It is given as +0.003 × ΔL + 0.002 …(8). The unit of ΔL in equations (7) to (8) is [%].

[0105] According to equations (6) to (8) above, Δt can be determined according to the design specifications of the light-emitting element 1. First, A and B corresponding to ΔL given as the design specifications are derived according to equations (7) to (8). Next, based on the derived A and B, Δt corresponding to t_mean given as the design specifications can be derived according to equation (6) above.

[0106] By manufacturing a light-emitting element equipped with a hole transport layer having a roughness smaller than Δt derived according to equation (6), a light-emitting element with reduced brightness variation can be realized.

[0107] (An example of a method for manufacturing a light-emitting element 1) Figure 15 is a diagram illustrating an example of a method for manufacturing a light-emitting element 1. Figure 15 shows a hole transport layer 13 and a lower layer UL to be positioned below the hole transport layer 13. In the example of Figure 15, the anode 11 and the hole injection layer 12 are formed prior to the formation of the bank BK. In other words, in the example of Figure 15, the bank BK is formed after the anode 11 and the hole injection layer 12 have been formed. In this case, the lower layer UL in the example of Figure 15 may be the hole injection layer 12 or the anode 11.

[0108] First, as shown by reference numeral 1510 in Figure 15, a lower layer UL is formed (e.g., film deposition). As shown by reference numeral 1510, relatively large roughness may occur on the upper surface of the lower layer UL after film deposition. This roughness can be caused by changes in the material state of the lower layer UL, and changes in the film deposition conditions of the lower layer UL. This roughness can lead to variations in t in the hole transport layer 13 that should be located above the lower layer UL.

[0109] Therefore, as shown by reference numeral 1520 in Figure 15, the upper surface of the lower UL layer after film formation is flattened. For example, the upper surface of the lower UL layer after film formation may be flattened by polishing.

[0110] Next, as shown by reference numeral 1530 in Figure 15, a coupling layer CP may be formed on the upper surface of the lower layer UL. The coupling layer CP is a layer that connects the lower layer UL and the hole transport layer 13. As an example, the coupling layer CP may be formed by applying a coupling agent to the upper surface of the lower layer UL. A silane coupling agent can be given as an example of a coupling agent.

[0111] Silane coupling agents have the function of chemically bonding organic and inorganic materials. In Embodiment 1, the hole transport layer 13 is typically organic. On the other hand, the lower layer UL (e.g., hole injection layer 12 or anode 11) is typically inorganic. Therefore, for example, a coupling layer CP derived from a silane coupling agent is useful for bonding the lower layer UL and the hole transport layer 13.

[0112] The coupling layer CP reduces the risk of discontinuities or cavities occurring between the upper surface of the lower layer UL and the lower surface of the hole transport layer 13. Therefore, providing a coupling layer CP located between the upper surface of the lower layer UL and the lower surface of the hole transport layer 13 is beneficial for reducing variations in t in the light-emitting element 1.

[0113] Next, as shown by reference numeral 1540 in Figure 15, a hole transport layer 13 is formed (e.g., film deposition) above the lower layer UL. In the example of reference numeral 1540, the hole transport layer 13 is deposited on the upper surface of the coupling layer CP.

[0114] In the example shown in Figure 15, the upper surface of the lower layer UL is flattened. This indicates that the roughness of the lower surface of the hole transport layer 13 in the example shown in Figure 15 is small. Therefore, by reducing the roughness of the upper surface of the hole transport layer 13, a hole transport layer 13 with small variation in t can be obtained.

[0115] Furthermore, a viscosity modifier (e.g., a thickener) may be added to the solution (HTL solution) containing the material for the hole transport layer 13. Examples of viscosity modifiers include polyalkyl acrylates and polyalkyl vinyl ethers.

[0116] For example, by increasing the viscosity of the HTL solution by about 20% through the addition of a viscosity modifier, it is possible to create irregularities on the upper surface of the hole transport layer 13 that correspond to the upper surface of the lower layer UL. For this reason, for example, a viscosity modifier of about 0.2% to 0.6% by weight may be added to the HTL solution.

[0117] Then, the HTL solution with the viscosity modifier added may be applied to the upper surface of the coupling layer CP, and the hole transport layer 13 may be formed by heating the HTL solution. The hole transport layer 13 formed in this way may contain a viscosity modifier.

[0118] As described above, by manufacturing the light-emitting element 1 according to the manufacturing method shown in Figure 15, for example, a light-emitting element 1 can be realized in which both the roughness of the upper surface and the roughness of the lower surface of the hole transport layer 13 are small. As a result, for example, a light-emitting element 1 that satisfies equation (1) above can be realized. That is, a light-emitting element 1 that satisfies equations (2) to (3) above can be realized.

[0119] (Another example of a method for manufacturing the light-emitting element 1) Unlike the example in Figure 15, the bank BK may be formed after the anode 11 is formed, and then the hole injection layer 12 may be formed. In this case, typically the lower layer UL is the hole injection layer 12.

[0120] In another example of a method for manufacturing the light-emitting element 1, first an anode 11 is formed (e.g., by film deposition). Then, the upper surface of the anode 11 after film deposition is flattened. For example, the upper surface of the anode 11 after film deposition may be flattened by polishing.

[0121] Next, a bank BK is formed on the upper surface of the flattened anode 11. After forming the bank BK, a hole injection layer 12 is formed on the upper surface of the flattened anode 11 as a lower layer UL. By forming the lower layer UL on the upper surface of the flattened anode 11, it becomes easier to achieve a lower layer UL with flatness.

[0122] [Embodiment 2] Figure 16 shows an example configuration of the light-emitting element 1 in Embodiment 2. Figure 16 is a counterpart to Figure 1. In the example of Figure 16, unlike the example of Figure 1, the roughness of the upper surface and the roughness of the lower surface of the hole transport layer 13 are large.

[0123] However, in the example shown in Figure 16, the irregularities on the upper surface of the hole transport layer 13 correspond to the irregularities on the lower surface of the hole transport layer 13. In other words, in the example shown in Figure 16, the irregularities on the upper surface of the hole transport layer 13 correspond to the irregularities on the upper surface of the hole injection layer 12.

[0124] Therefore, in the example of Figure 16, as in the example of Figure 1, the variation in t in the hole transport layer 13 is small. For this reason, in the example of Figure 16, a light-emitting element 1 that satisfies the above-mentioned equation (1) can be realized. That is, in the example of Figure 16, a light-emitting element 1 that satisfies the above-mentioned equations (2) to (3) can be realized.

[0125] In the light-emitting element 1 shown in Figure 16, relatively large irregularities exist in each of the hole injection layer 12, hole transport layer 13, light-emitting layer 14, electron transport layer 15, and cathode 16. Therefore, with the light-emitting element 1 shown in Figure 16, the light generated in the light-emitting layer 14 can be scattered by the slopes of these irregularities. As a result, the light extraction efficiency of the light-emitting element can be further improved compared to the configuration of the light-emitting element 1 shown in Figure 1.

[0126] When manufacturing the light-emitting element 1 in Embodiment 2, it is not necessarily required to flatten the upper surface of the lower layer UL (see Figure 15 above) after film formation. Alternatively, when manufacturing the light-emitting element 1 in Embodiment 2, the upper surface of the lower layer UL may be processed after film formation to intentionally create irregularities on the upper surface.

[0127] Furthermore, a surface tension modifier (also called a leveling agent) may be added to the HTL solution. Examples of surface tension modifiers include modified polyether macromers or modified polyether silicon macromers.

[0128] For example, a surface tension modifier in an amount of about 0.2% to 1% by weight may be added to the HTL solution. Then, the HTL solution with the added surface tension modifier may be applied to the upper surface of the coupling layer CP, and the hole transport layer 13 may be formed by heating the HTL solution. The hole transport layer 13 formed in this way may contain the surface tension modifier.

[0129] The surface tension modifier has the function of slightly lowering the surface tension of the HTL solution and homogenizing the surface shape of the HTL solution. Therefore, by forming a hole transport layer 13 based on the HTL solution after adding the surface tension modifier, the roughness of the upper surface of the hole transport layer 13 can be reduced.

[0130] As described above, by manufacturing a light-emitting element according to one aspect of the present disclosure, it is also possible to obtain a hole transport layer (e.g., hole transport layer 13 in Figure 16) in which the irregularities on the upper surface of the hole transport layer correspond to the irregularities on the lower surface of the hole transport layer. Therefore, for example, the light-emitting element 1 shown in Figure 16 can also be realized.

[0131] [Embodiment 3] Figure 17 shows an example of the configuration of the light-emitting element 1 in Embodiment 3. In the example in Figure 17, the lower layer UL is the hole injection layer 12. In Figure 17, for the sake of clarity, the upper and lower surfaces of the hole transport layer 13 are shown as flat. However, as can be understood from Embodiments 1 and 2 described above, the upper and lower surfaces of the hole transport layer 13 do not necessarily have to be flat.

[0132] In the example shown in Figure 17, the light-emitting element 1 includes a bank BK located to the side of the light-emitting layer 14. In the display device 100 described later, the bank BK partitions each of the multiple light-emitting elements 1. Therefore, for example, the bank BK may be formed using any light-absorbing material.

[0133] In the example shown in Figure 17, the surface of the substrate 90 has a recess KM. The recess KM is located near the intersection of the slope of the bank BK and the surface of the substrate 90. In the example shown in Figure 17, the anode 11 is located on the flat portion of the surface of the substrate 90, excluding the recess KM.

[0134] If the material of the lower layer UL is fluid, the lower layer UL may be deposited between the banks BK after the banks BK have been formed by coating or inkjet printing. In this example, if the surface of the substrate 90 does not have recesses KM, the deposited lower layer UL may crawl up onto the slope of the banks BK (see, for example, Figure 2 above). As a result, the variation in t in the hole transport layer 13 may increase.

[0135] Therefore, as shown in Figure 17, by providing the recessed portion KM, the risk of the lower layer UL crawling up the slope of the bank BK after film formation can be reduced. Consequently, the risk of the hole transport layer 13 crawling up the slope of the bank BK after film formation can also be reduced.

[0136] However, if the depth of the depression KM (ΔD_sub in Figure 17) is too large, the presence of the depression KM may cause a large variation in t in the hole transport layer 13. For this reason, it is preferable to set ΔD_sub to a value that is not too large.

[0137] As an example, ΔD_sub may be set to 2 × Δt or less. When ΔD_sub is set in this way, even if a depression KM exists, the variation in t in the hole transport layer 13 will not be so large. Therefore, even in the example of Figure 17, a light-emitting element 1 that satisfies the above-mentioned equation (1) can be realized. That is, even in the example of Figure 17, a light-emitting element 1 that satisfies the above-mentioned equations (2) to (3) can be realized.

[0138] [Embodiment 4] Figure 18 shows an example configuration of the display device 100 in Embodiment 4. The display device 100 only needs to be equipped with a light-emitting element (e.g., light-emitting element 1) according to one aspect of the present disclosure. In the example of Figure 18, the display device 100 has a first light-emitting element 1_1, a second light-emitting element 1_2, and a third light-emitting element 1_3 as the light-emitting element 1.

[0139] For example, the first light-emitting element 1_1, the second light-emitting element 1_2, and the third light-emitting element 1_3 may each emit light of a different color. For instance, the first light-emitting element 1_1 may be a red light-emitting element that emits red light, the second light-emitting element 1_2 may be a green light-emitting element that emits green light, and the third light-emitting element 1_3 may be a blue light-emitting element that emits blue light.

[0140] However, the first to third light-emitting elements 1_1 to 1_3 in Embodiment 4 are not limited to the above example. For example, the first to third light-emitting elements 1_1 to 1_3 may be light-emitting elements that emit light of the same color.

[0141] The display device 100 may include a display unit DA containing a plurality of sub-pixels SP, a first driver X1 and a second driver X2 for driving the plurality of sub-pixels SP, and a display control unit DC for controlling the first driver X1 and the second driver X2. Each sub-pixel SP may have a light-emitting element 1 and a pixel circuit PC connected to the light-emitting element 1.

[0142] The pixel circuit PC may be connected to the scan signal line GL, the data signal line DL, and the light emission control line ELL. For example, the scan signal line GL and the light emission control line ELL may be connected to the first driver X1. The data signal line DL may be connected to the second driver X2.

[0143] In this specification, the standard deviation of the thickness of all hole transport layers 13 in the display device 100 is denoted as σ. As an example, it is preferable that the following equation (9), Δt ≤ 6 × σ … (9) holds true for all hole transport layers 13 in the display device 100.

[0144] If equation (9) holds true in all hole transport layers 13 of the display device 100, a display device 100 with excellent brightness uniformity is realized. As an example, according to the inventors' studies, if equation (9) holds true in all hole transport layers 13 of the display device 100, a display device 100 with a brightness defect pixel count of 0.0003% or less is realized.

[0145] [Additional Notes] One aspect of this disclosure is not limited to the embodiments described above, 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 within the technical scope of one aspect of this disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

[0146] 1. Light-emitting element 1_1 First light-emitting element (light-emitting element) 1_2 Second light-emitting element (light-emitting element) 1_3 Third light-emitting element (light-emitting element) 11. Anode 12. Hole injection layer 13. Hole transport layer 14. Light-emitting layer 16. Cathode 90. Substrate 100. Display device Q. Quantum dot MX. Additive BK. Bank KM. Recess UL. Lower layer CP. Coupling layer

Claims

1. A light-emitting element comprising an anode and a cathode, a light-emitting layer located between the anode and the cathode, and a hole transport layer located between the light-emitting layer and the anode, wherein the maximum thickness of the hole transport layer is 1.15 times or less the average thickness of the hole transport layer, and the minimum thickness of the hole transport layer is 0.85 times or more the average thickness of the hole transport layer.

2. The light-emitting element according to claim 1, further comprising a hole injection layer located between the hole transport layer and the anode.

3. The light-emitting element according to claim 1 or 2, wherein the material of the hole transport layer is an organic substance.

4. The light-emitting element according to any one of claims 1 to 3, wherein the light-emitting layer includes quantum dots.

5. The light-emitting element according to claim 4, wherein an adduct exists around the quantum dot.

6. The light-emitting element according to claim 5, wherein a ligand is present inside the adduct filling the space between at least two of the quantum dots.

7. The light-emitting element according to any one of claims 1 to 6, further comprising a coupling layer located between the lower surface of the hole transport layer and the upper surface of a lower layer located below the hole transport layer.

8. The light-emitting element according to any one of claims 1 to 7, wherein the hole transport layer contains a viscosity modifier.

9. The light-emitting element according to claim 8, wherein the viscosity modifier is a polyalkyl acrylate or a polyalkyl vinyl ether.

10. The light-emitting element according to any one of claims 1 to 7, wherein the hole transport layer contains a surface tension modifier.

11. The light-emitting element according to claim 10, wherein the surface tension modifier is a modified polyether macromer or a modified polyether silicon macromer.

12. When the roughness of the upper or lower surface of the hole transport layer is expressed as Δt [nm], the average thickness of the hole transport layer is expressed as t_mean [nm], and the maximum allowable luminance deviation in the light-emitting element is expressed as ΔL [%], then A is determined by the following formula (1) and B is determined by the following formula (2): A = 4 × 10 －５ ×ΔL ２ +0.003×ΔL+0.003…(1) B=8×10 －５ ×ΔL ２ +0.003 × ΔL + 0.002 …(2) The light-emitting element according to any one of claims 1 to 11, wherein the following equation (3), Δt = A × t_mean - B …(3) is true.

13. The light-emitting element according to any one of claims 1 to 12, wherein the light-emitting element comprises a bank located to the side of the light-emitting layer, and the surface of the substrate of the light-emitting element has a recess near the intersection of the slope of the bank and the surface.

14. The light-emitting element according to claim 13, wherein, when the roughness of the upper or lower surface of the hole transport layer is expressed as Δt, the depth of the recess is 2 × Δt or less.

15. A light-emitting element comprising: an anode and a cathode; a light-emitting layer located between the anode and the cathode; a hole transport layer located between the light-emitting layer and the anode; and a bank located to the side of the light-emitting layer, wherein the surface of the substrate of the light-emitting element has a recess near the intersection of the slope of the bank and the surface, and the depth of the recess is 2 × Δt or less, when the roughness of the upper or lower surface of the hole transport layer is expressed as Δt.

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

17. The display device according to claim 16, wherein the standard deviation of the thickness of all the hole transport layers in the display device is represented as σ, and the roughness of the upper or lower surface of the hole transport layer is represented as Δt, and for all the hole transport layers, Δt ≤ 6 × σ.

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