Method for manufacturing light-emitting element and display device

Abstract

This method for manufacturing a light-emitting element includes: a step (S1) for forming an anode; a step (S6) for forming a cathode; a step (S4) for forming a light-emitting layer performed between the step (S1) for forming an anode and the step (S6) for forming a cathode; a step (S2) for forming a hole transport layer that contains an organic hole transport material; and a step (S3) for aging the hole transport layer by at least one of heat and light. The step (S2) for forming a hole transport layer is performed between the step (S1) for forming an anode and the step (S4) for forming a light-emitting layer. The step (S3) for aging is performed before one of the step (S1) for forming an anode and the step (S6) for forming a cathode, which is performed later.

Description

Method for manufacturing a light-emitting element and a display device

[0001] This disclosure relates to a method for manufacturing a light-emitting element and a display device.

[0002] In recent years, various display devices equipped with light-emitting elements have been developed, and in particular, display devices equipped with OLEDs (Organic Light Emitting Diodes) or QLEDs (Quantum Dot Light Emitting Diodes) have attracted considerable attention due to their ability to achieve low power consumption, thin design, and high image quality.

[0003] For example, Patent Document 1 describes a light-emitting element equipped with an organic hole transport layer made of polyvinylcarbazole.

[0004] Japanese Patent Application Publication No. 2011-61028

[0005] However, in light-emitting elements equipped with an organic hole transport layer and in display devices equipped with such light-emitting elements, there is a problem in that stable element characteristics cannot be obtained due to degradation specific to organic materials.

[0006] One aspect of this disclosure aims to provide a method for manufacturing a light-emitting element having stable element characteristics and a display device equipped with a light-emitting element having stable element characteristics.

[0007] To solve the above problems, the method for manufacturing a light-emitting element of the present disclosure includes a step of forming an anode, a step of forming a cathode, a step of forming a light-emitting layer performed between the step of forming the anode and the step of forming the cathode, a step of forming a hole transport layer containing an organic hole transport material, and a step of aging the hole transport layer with at least one of heat and light, wherein the step of forming the hole transport layer is performed between the step of forming the anode and the step of forming the light-emitting layer, and the aging step is performed before the step of forming the anode and the step of forming the cathode whichever is performed later.

[0008] To solve the above problems, the present disclosure's method for manufacturing a light-emitting element includes a step of forming an anode, a step of forming a cathode, a step of forming a light-emitting layer performed between the anode formation step and the cathode formation step, a step of forming a hole transport layer containing an organic hole transport material performed between the anode formation step and the light-emitting layer formation step, and a step of thermal aging the hole transport layer, wherein the aging step is performed after the anode formation step, the cathode formation step, the light-emitting layer formation step and the hole transport layer formation step, and the aging step is performed in an inert gas or dry air atmosphere.

[0009] To solve the above problems, the display device of the present disclosure includes a light-emitting element comprising an anode, a cathode, a light-emitting layer provided between the anode and the cathode, and a first hole transport layer provided between the anode and the light-emitting layer that includes a first organic hole transport material; a display area on which the light-emitting element is provided; and a frame area which is an area outside the display area, and includes at least one of the following: a first element provided in the display area, comprising the first hole transport layer, a first electrode in contact with the first hole transport layer but not in contact with the light-emitting layer, and a second electrode in contact with the first hole transport layer but not in contact with the light-emitting layer and the first electrode respectively; and a second element provided in the frame area, comprising a second hole transport layer containing a second organic hole transport material, a third electrode in contact with the second hole transport layer, and a fourth electrode in contact with the second hole transport layer but not in contact with the third electrode.

[0010] According to one aspect of this disclosure, a method for manufacturing a light-emitting element having stable element characteristics and a display device equipped with a light-emitting element having stable element characteristics can be provided.

[0011] This figure illustrates the rapid degradation observed in a light-emitting element equipped with a hole transport layer containing an organic hole transport material. This figure illustrates the slow degradation observed in a light-emitting element equipped with a hole transport layer containing an organic hole transport material as shown in Figure 1. This figure illustrates that the rapid degradation observed in a light-emitting element equipped with a hole transport layer containing an organic hole transport material as shown in Figure 1 is caused by degradation specific to organic materials. This figure shows the electrical properties of a hole transport layer containing an organic hole transport material. This figure shows the change in brightness over time of a light-emitting element of Embodiment 1 shown in Figure 7, manufactured by a method for manufacturing a light-emitting element that includes a step of aging the hole transport layer containing an organic hole transport material, and the change in brightness over time of a light-emitting element of a comparative example manufactured by a method for manufacturing a light-emitting element that does not include a step of aging the hole transport layer containing an organic hole transport material. This figure shows an example of a method for manufacturing a light-emitting element of Embodiment 1. This is a cross-sectional view showing the schematic configuration of a light-emitting element of Embodiment 1. This figure shows another example of a method for manufacturing a light-emitting element of Embodiment 1. This figure shows an example of a method for manufacturing a light-emitting element that is a modified version of Embodiment 1. This is a cross-sectional view showing the schematic configuration of a light-emitting element that is a modified version of Embodiment 1. This figure shows another example of a method for manufacturing a light-emitting element that is a modified version of Embodiment 1. This is a plan view showing the schematic configuration of a display device including multiple light-emitting elements of Embodiment 1 shown in Figure 7. This is a diagram showing an example of a constant-temperature bath capable of performing an aging process for a hole transport layer containing an organic hole transport material. This is a cross-sectional view showing the schematic configuration of a light-emitting element of Embodiment 1 manufactured by adding or coating a radical polymerization initiator to a hole transport layer containing an organic hole transport material. This is a cross-sectional view showing the schematic configuration of a light-emitting element that is a modified example of Embodiment 1 manufactured by adding or coating a radical polymerization initiator to a hole transport layer containing an organic hole transport material. This is a diagram to explain that the film formation properties of a hole transport layer containing an organic hole transport material are improved by using a coupling agent when forming the hole transport layer containing the organic hole transport material on an inorganic film. This is a diagram showing an example of the materials constituting the light-emitting layer provided in the light-emitting element of Embodiment 1. This is a diagram showing another example of the materials constituting the light-emitting layer provided in the light-emitting element of Embodiment 1. This is a diagram showing yet another example of the materials constituting the light-emitting layer provided in the light-emitting element of Embodiment 1. This is a plan view showing the schematic configuration of a display device of Embodiment 2.This is a cross-sectional view showing the schematic configuration of an example of a second element provided in the display device of Embodiment 2. This is a cross-sectional view showing the schematic configuration of an example of a first element provided in the display device of Embodiment 2. This is a cross-sectional view showing the schematic configuration of another example of a first element that can be provided in the display device of Embodiment 2. This is a cross-sectional view showing the schematic configuration of yet another example of a first element that can be provided in the display device of Embodiment 2. This is a cross-sectional view showing the schematic configuration of yet another example of a first element that can be provided in the display device of Embodiment 2.

[0012] The embodiments of this disclosure will be described below with reference to Figures 1 to 25. For the sake of convenience, in the following description, components having the same function as those described in a particular embodiment will be denoted by the same reference numerals, and their descriptions may be omitted.

[0013] [Embodiment 1] Figure 1 is a diagram illustrating the rapid degradation observed in a light-emitting element equipped with a hole transport layer containing an organic hole transport material. Figure 2 is a diagram illustrating the slow degradation observed in a light-emitting element equipped with a hole transport layer containing an organic hole transport material as shown in Figure 1. Figure 3 is a diagram illustrating that the rapid degradation observed in a light-emitting element equipped with a hole transport layer containing an organic hole transport material as shown in Figure 1 is caused by degradation specific to organic materials. Figure 4 is a diagram showing the electrical characteristics of the hole transport layer containing an organic hole transport material. Figure 5 is a diagram showing the change in brightness over time of the light-emitting element 10 of Embodiment 1 shown in Figure 7, which was manufactured by a method for manufacturing a light-emitting element that includes a step of aging the hole transport layer containing an organic hole transport material, and the change in brightness over time of a comparative example light-emitting element manufactured by a method for manufacturing a light-emitting element that does not include a step of aging the hole transport layer containing an organic hole transport material.

[0014] As shown in Figure 2, the time-dependent changes in the voltage and brightness of a light-emitting element, while keeping the current flowing through it constant, reveal that in a light-emitting element equipped with a hole transport layer containing an organic hole transport material, the brightness decreases and the voltage required to maintain that constant current increases as time passes, exhibiting a typical degradation trend seen in light-emitting elements with organic layers. Furthermore, as shown in Figure 2, both brightness and voltage show large changes at the initial stage of energization, i.e., at the start of the constant current flow.

[0015] As can be seen from the voltage change rate and brightness change rate of the light-emitting element equipped with a hole transport layer containing an organic hole transport material shown in Figure 1, the rapid degradation during the rapid degradation period, which is within 10 minutes of the start of power application, has a major impact on the degradation of the element characteristics of the light-emitting element. On the other hand, after the rapid degradation period, which is within 10 minutes of the start of power application, the degradation of the light-emitting element changes to a slower degradation, and thereafter converges to a constant degradation rate.

[0016] To confirm that the rapid degradation during the rapid degradation period, which is the period within 10 minutes of the start of energization as described above, is due to the hole transport layer containing the organic hole transport material, the inventors of this disclosure performed PL (Photoluminescence) measurements on a light-emitting element equipped with a hole transport layer containing the organic hole transport material, in which an anode composed of a light-reflective electrode material, a hole transport layer containing the organic hole transport material, an emissive layer composed of blue light-emitting quantum dots, and a cathode composed of a light-transmitting electrode material were stacked in this order. A single element was identified during the reliability test, and the reliability test of that element was interrupted at predetermined elapsed times to perform PL measurements, and then the reliability test was resumed. This procedure was repeated to evaluate the change in PL characteristics. PL measurements were performed on the light-emitting element immediately after manufacturing (before continuous energization in Figure 3), after a constant current was passed through the element for 1 hour (after 1 hour of continuous energization in Figure 3), and after the constant current was passed through the element for 200 hours (after 200 hours of continuous energization in Figure 3), and then the reliability test was continued. For PL measurement excitation, the hole transport layer (HTL) should be irradiated with light shorter than the wavelength corresponding to the energy difference between LUMO and HOMO (for example, 390 nm). The intensity of the PL light emitted after absorbing the excitation light (PL intensity) was analyzed for each wavelength. The blue light-emitting quantum dots constituting the light-emitting layer of the light-emitting element described above did not degrade even when the constant current was passed for 200 hours. As shown in Figure 3, in the wavelength range of the PL light of the blue quantum dots (the region labeled QD in the figure), there was almost no decrease in PL intensity after passing the constant current for 1 hour (after 1 hour of continuous current in the figure) and after passing the constant current for 200 hours (after 200 hours of continuous current in the figure) compared to immediately after manufacturing (before continuous current in the figure). On the other hand, the hole transport layer containing the organic hole transport material of the light-emitting element described above shows degradation even when a constant current is applied for 1 hour. As shown in Figure 3, in the wavelength region of the PL light of the hole transport layer containing the organic hole transport material (the region labeled HTL in the figure), it can be confirmed that the decrease in PL intensity after applying the constant current for 1 hour (1 hour after continuous energization in the figure) is greater than immediately after manufacturing (before continuous energization in the figure).The PL intensity after applying the constant current for 200 hours (200 hours of continuous energization in the figure) is lower than the PL intensity after applying the constant current for 1 hour (1 hour of continuous energization in the figure). However, the degree of decrease in PL intensity is smaller compared to the degree of decrease in PL intensity that occurs within 1 hour of continuous energization as described above.

[0017] From the above, it is thought that the hole transport layer containing organic hole transport material undergoes the most significant change in its properties during the rapid degradation period, which is within 10 minutes of the start of current application. This leads to a decrease in the hole injection characteristics into the light-emitting layer, resulting in a decrease in the brightness of the light-emitting element and an increase in the voltage required to maintain a constant current.

[0018] Figure 4 shows the electrical characteristics (V-I characteristics) of a first sample (solid line) in which the hole transport layer 3 was aged, a second sample (dotted line) in which the hole transport layer 3 was not aged, and a short-circuited state (dash-dotted line). The inventors of this disclosure, in the method for manufacturing a light-emitting element shown in Figure 6, described later, compared the electrical characteristics (V-I characteristics) of a first sample measured by making contact with the surface of the hole transport layer 3 and the anode 2 with a probe after performing the aging step (S3) of the hole transport layer 3, with the electrical characteristics (V-I characteristics) of a second sample measured by making contact with the surface of the hole transport layer 3 and the anode 2 with a probe immediately after performing the step (S2) of forming the hole transport layer 3 in the method for manufacturing a light-emitting element shown in Figure 6, described later. In the case of the first sample, the hole transport layer 3 was aged in a process (S3) by holding it at 150°C in a nitrogen atmosphere for 5 hours. In the case of the second sample, the electrical properties (V-I properties) were measured immediately after the process (S2) of forming the hole transport layer 3, so no aging was performed. As shown in Figure 4, from the results of the electrical properties (V-I properties) of the first and second samples, it can be seen that the electrical properties (V-I properties) of the second sample are hopping conduction, while the electrical properties (V-I properties) of the first sample are space charge limited current conduction. Furthermore, it was confirmed that as the aging time of the hole transport layer containing the organic hole transport material at 150°C in a nitrogen atmosphere approached from 0 hours to 5 hours, the electrical properties of the hole transport layer containing the organic hole transport material approached space charge limited current conduction from hopping conduction. The electrical properties of a hole transport layer containing an organic hole transport material are described as hopping conduction if the current value I flowing through the hole transport layer containing the organic hole transport material is proportional to the value of the voltage V applied to the hole transport layer containing the organic hole transport material raised to the power of m (3.0 ≤ m), and the electrical properties of a hole transport layer containing an organic hole transport material are described as space charge limited current conduction if the current value I flowing through the hole transport layer containing the organic hole transport material is proportional to the value of the voltage V applied to the hole transport layer containing the organic hole transport material raised to the power of m (1.8 ≤ m ≤ 2.2).Here, as an example, we will explain the process of aging the hole transport layer 3 (S3) by holding it at 150°C in a nitrogen atmosphere for 5 hours. However, we are not limited to this, and if aging is performed at a temperature higher than 150°C, the aging time may be shortened to less than 5 hours, and if aging is performed at a temperature lower than 150°C, the aging time may be extended to more than 5 hours. For example, aging at 120°C for 12 hours can suppress deterioration of the element characteristics of the light-emitting element. Furthermore, here, the hole transport layer containing the organic hole transport material is aged until it exhibits electrical characteristics proportional to the value of the voltage V applied to the hole transport layer raised to the power of m (1.8 ≤ m ≤ 2.2), but we are not limited to this, and the hole transport layer containing the organic hole transport material only needs to be aged in a direction in which its electrical characteristics move from hopping conduction to space charge-limited current conduction.

[0019] Figure 5 shows the results of a test investigating the change in brightness over time between a light-emitting element that underwent the aging process (S3) of the hole transport layer 3 using the manufacturing method described in Figure 6 ("HTL aging performed") and a light-emitting element that did not undergo the aging process (S3) of the hole transport layer 3 ("HTL not aging performed"). In Figure 5, "Brightness" on the vertical axis is set to 1 when time is 0h. From Figure 5, it can be seen that in the case of "HTL not aging," a large decrease in brightness occurs in the early stages of the test before the brightness stabilizes, whereas in the case of "HTL aging performed," the decrease in brightness in the early stages of the test remains small before the brightness stabilizes. This difference is due to the difference in electrical characteristics shown in Figure 4. As shown in Figure 5, the inventors of this disclosure have found that, before using the light-emitting element or a display device including the light-emitting element, by performing an aging process on the hole transport layer including the organic hole transport material described later, the electrical characteristics of the hole transport layer including the organic hole transport material are changed from hopping conduction to approach space charge-limited current conduction, as shown in Figure 4, thereby suppressing a large decrease in brightness during use of the light-emitting element or a display device including the light-emitting element. As shown in Figure 5, according to the method for manufacturing a light-emitting element that includes an aging step (S3) of the hole transport layer 3, a large decrease in brightness during use of the light-emitting element or a display device including the light-emitting element can be suppressed, thereby realizing a light-emitting element with stable element characteristics and a display device equipped with a light-emitting element with stable element characteristics.

[0020] Figure 6 shows an example of a method for manufacturing the light-emitting element 10 of Embodiment 1. Figure 7 is a cross-sectional view showing the schematic configuration of the light-emitting element 10 of Embodiment 1. Figure 8 shows another example of a method for manufacturing the light-emitting element 10 of Embodiment 1. Figure 9 shows an example of a method for manufacturing a light-emitting element 10a, which is a modified example of Embodiment 1. Figure 10 is a cross-sectional view showing the schematic configuration of the light-emitting element 10a, which is a modified example of Embodiment 1. Figure 11 shows another example of a method for manufacturing a light-emitting element 10a, which is a modified example of Embodiment 1.

[0021] A method for manufacturing a light-emitting element, including an aging step for a hole transport layer, includes the steps of: forming an anode; forming a cathode; forming a light-emitting layer between the anode formation step and the cathode formation step; forming a hole transport layer containing an organic hole transport material; and aging the hole transport layer by heat and light, which is performed after the hole transport layer formation step. The hole transport layer formation step is performed between the anode formation step and the light-emitting layer formation step, and the aging step may be performed after the hole transport layer formation step and before the later of the anode formation step and the cathode formation step. In other words, the aging step may be performed before the anode formation step if the anode formation step is performed after the cathode formation step, and before the cathode formation step if the cathode formation step is performed after the anode formation step.

[0022] As shown in Figure 7, the light-emitting element 10 includes an anode 2, a cathode 6, a light-emitting layer 4 provided between the anode 2 and the cathode 6, and a hole transport layer 3 provided between the anode 2 and the light-emitting layer 4. The hole transport layer 3 contains an organic hole transport material, and an electron transport layer 5 is formed between the cathode 6 and the light-emitting layer 4. The manufacturing method of the light-emitting element 10 may be, for example, as shown in Figure 6, performed in the following order: forming the anode 2 (S1), forming the hole transport layer 3 (S2), aging the hole transport layer 3 (S3), forming the light-emitting layer 4 (S4), forming the electron transport layer 5 (S5), and forming the cathode 6 (S6). The manufacturing method may further include forming a hole injection layer between forming the anode 2 (S1) and forming the hole transport layer 3 (S2). Furthermore, the step of forming the electron transport layer 5 (S5), which is performed between the step of forming the light-emitting layer 4 (S4) and the step of forming the cathode 6 (S6), may be omitted as appropriate. Alternatively, the step of forming the electron injection layer may be performed instead of the step of forming the electron transport layer 5 (S5), or the step of forming the electron injection layer may be performed together with the step of forming the electron transport layer 5 (S5) in this order. Here, the case in which the step of aging the hole transport layer 3 (S3) is performed between the step of forming the hole transport layer 3 (S2) and the step of forming the light-emitting layer 4 (S4) has been given as an example, but the process is not limited to this. The step of aging the hole transport layer 3 (S3) may be performed, for example, between the step of forming the light-emitting layer 4 (S4) and the step of forming the electron transport layer 5 (S5), or between the step of forming the electron transport layer 5 (S5) and the step of forming the cathode 6 (S6). If the step of forming the electron injection layer is included after the step of forming the electron transport layer 5 (S5), the step of aging the hole transport layer 3 (S3) may be performed between the step of forming the electron transport layer 5 (S5) and the step of forming the electron injection layer, or between the step of forming the electron injection layer and the step of forming the cathode 6 (S6).

[0023] As shown in Figure 8, the manufacturing method of the light-emitting element 10, which includes an aging step of the hole transport layer 3, comprises the steps of forming the anode 2 (S11), forming the cathode 6 (S15), forming the light-emitting layer 4 (S13) which is performed between the steps of forming the anode 2 (S11) and forming the cathode 6 (S15), forming the hole transport layer 3 containing an organic hole transport material (S12) which is performed between the steps of forming the anode 2 (S11) and forming the light-emitting layer 4 (S13), and forming the light-emitting layer 4 (S13) and forming the cathode 6. The process includes forming an electron transport layer 5 (S14) between the process of forming the anode 2 (S11), forming the hole transport layer 3 (S12), forming the light-emitting layer 4 (S13), forming the electron transport layer 5 (S14), and forming the cathode 6 (S15), and the process of aging the hole transport layer 3 (S16) may be carried out in an inert gas or dry air atmosphere. Between the process of forming the anode 2 (S11) and the process of forming the hole transport layer 3 (S12), the process may further include forming a hole injection layer. Furthermore, the step of forming the electron transport layer 5 (S14), which is performed between the step of forming the light-emitting layer 4 (S13) and the step of forming the cathode 6 (S15), may be omitted as appropriate. Alternatively, the step of forming the electron injection layer may be performed instead of the step of forming the electron transport layer 5 (S14), or the step of forming the electron injection layer may be performed together with the step of forming the electron transport layer 5 (S14) in this order.

[0024] Both the method for manufacturing the light-emitting element 10, which includes an aging step for the hole transport layer 3 as shown in Figure 6, and the method for manufacturing the light-emitting element 10, which includes an aging step for the hole transport layer 3 as shown in Figure 8, yield a light-emitting element 10 with a sequential stacking structure, as shown in Figure 7, in which the anode 2, the hole transport layer 3, the light-emitting layer 4, the electron transport layer 5, and the cathode 6 are stacked in this order.

[0025] The manufacturing method for the light-emitting element 10, which includes an aging step for the hole transport layer 3 as shown in Figure 6, has the advantage that it does not require consideration of damage to the light-emitting layer 4 caused by the aging step (S3), since the aging step (S3) of the hole transport layer 3 is performed before the step (S4) of forming the light-emitting layer 4. Furthermore, in the aging step (S3) of the hole transport layer 3, the cathode 6 side of the hole transport layer 3 is exposed, so not only thermal aging but also optical aging can be suitably used.

[0026] On the other hand, the manufacturing method of the light-emitting element 10, which includes an aging step for the hole transport layer 3 shown in Figure 8, has the advantage that the film formation process can be carried out as a continuous process in the manufacturing process, since the step of aging the hole transport layer 3 (S16) is performed after all the layers of the light-emitting element 10 have been formed. Furthermore, in the step of aging the hole transport layer 3 (S16), since the anode 2 side surface and the cathode 6 side surface of the hole transport layer 3 are covered, thermal aging can be preferably used rather than optical aging.

[0027] Since the hole transport layer 3 contains an organic hole transport material, even if the polymerization or crosslinking reaction of the organic hole transport material appears to have stopped before the aging process (S3 and S16) of the hole transport layer 3, unbonded groups of the organic hole transport material that create defect levels remain to a degree that strongly affects the electrical properties. If the light-emitting element is used in this state, a large decrease in brightness occurs early in the change in brightness of the light-emitting element over time, as shown in Figure 5.

[0028] Therefore, in the method for manufacturing a light-emitting element of this disclosure, an aging process (S3 and S16) of the hole transport layer 3 is performed before using the light-emitting element, that is, during the manufacturing process of the light-emitting element, thereby reducing the number of unbonded groups in the organic hole transport material that create defect levels, and realizing a more stable hole transport layer 3. When a light-emitting element 10 equipped with such a more stable hole transport layer 3 is used, as shown in Figure 5, it is possible to suppress the large decrease in brightness that occurs in the early stages of the change in brightness of the light-emitting element 10 over time.

[0029] The light-emitting element 10 shown in FIG. 7 may be a top-emission type or a bottom-emission type. The light-emitting element 10 shown in FIG. 7 is a light-emitting element having a sequential stack structure in which the cathode 6 is disposed as an upper layer relative to the anode 2. In order to make such a sequentially stacked light-emitting element a top-emission type, the anode 2 may be formed of an electrode material that reflects visible light, and the cathode 6 may be formed of an electrode material that transmits visible light. In order to make such a sequentially stacked light-emitting element a bottom-emission type, the anode 2 may be formed of an electrode material that transmits visible light, and the cathode 6 may be formed of an electrode material that reflects visible light.

[0030] The electrode material that reflects visible light is not particularly limited as long as it can reflect visible light and has conductivity. Examples thereof include metal materials such as Al, Mg, Li, and Ag, alloys of the metal materials, laminates of the metal materials and transparent metal oxides (for example, indium tin oxide, indium zinc oxide, indium gallium zinc oxide, etc.), and laminates of the alloys and the transparent metal oxides.

[0031] On the other hand, the electrode material that transmits visible light is not particularly limited as long as it can transmit visible light and has conductivity. Examples thereof include transparent metal oxides (for example, indium tin oxide, indium zinc oxide, indium gallium zinc oxide, etc.), thin films made of metal materials such as Al and Ag, and nanowires (Nano Wire) made of metal materials such as Al and Ag.

[0032] The organic hole transport material contained in the hole transport layer 3 provided in the light-emitting element 10 shown in FIG. 7 may be, for example, poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)) diphenylamine)] (TFB), N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine (poly-TPD), polyvinylcarbazole (PVK), or the like. In the present embodiment, for example, a solution containing crosslinkable TFB is applied to form the hole transport layer 3, but it is not limited thereto, and the hole transport layer 3 may be formed by using an inkjet method for a solution containing crosslinkable TFB. Generally, organic materials are preferably used as the hole transport material, and the above-described TFB, poly-TPD, PVK, etc. have a HOMO (highest occupied molecular orbital) level close to the upper end of the valence band (VBM) of the light-emitting layer 4 or the upper end of the valence band (VBM) of the quantum dots when the light-emitting layer 4 is composed of quantum dots, so that hole injection can be realized relatively efficiently.

[0033] When the light-emitting element 10 shown in FIG. 7 includes a hole injection layer (HIL), the material used for forming the hole injection layer (HIL) is not particularly limited as long as it is a hole injection material that can stabilize the injection of holes into the light-emitting layer 4. For example, nickel oxide (NiO) nanoparticles, a composite of poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonic acid (PSS) (PEDOT:PSS), or the like can be used.

[0034] The electron transport material contained in the electron transport layer 5 provided in the light-emitting element 10 shown in FIG. 7 may be an organic material such as 2,2',2''-(1,3,5-benzenetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), or may be nanoparticles having electron transport properties such as ZnO particles or particles of an oxide containing Zn and Mg.

[0035] If the light-emitting element 10 shown in Figure 7 is equipped with an electron injection layer (EIL), the material used to form the electron injection layer (EIL) is not particularly limited as long as it is an electron injection material that can stabilize the injection of electrons into the light-emitting layer 4. For example, alkali metals or alkaline earth metals such as aluminum, strontium, calcium, lithium, cesium, magnesium oxide, aluminum oxide, strontium oxide, lithium oxide, lithium fluoride, magnesium fluoride, strontium fluoride, calcium fluoride, barium fluoride, cesium fluoride, polymethyl methacrylate, sodium polystyrene sulfonate, alkali metals or alkaline earth metals, oxides of alkali metals or alkaline earth metals, fluorides of alkali metals or alkaline earth metals, or organic complexes of alkali metals can be used.

[0036] In each embodiment of this disclosure, the light-emitting layer 4 of the light-emitting element is a light-emitting layer containing quantum dots, and the light-emitting element is a QLED (Quantum dot Light Emitting Diode), as an example, but the invention is not limited thereto. For example, the light-emitting element may be an OLED (Organic Light Emitting Diode) having a light-emitting layer containing an organic light-emitting material instead of a light-emitting layer containing quantum dots.

[0037] Quantum dots are semiconductor nanoparticles that emit light through the recombination of electrons and holes as the excitons lose energy, generating excitons from injected electrons and holes. A quantum dot may have a core / shell structure, for example, comprising a core and a shell covering at least a portion of the core's surface. The shell may be a single layer, a multi-layer shell containing different materials, or a giant shell with a thickness of 2 nm or more. In this case, the recombination of electrons and holes in the quantum dot primarily occurs in the core. The core of a quantum dot has a valence band level and a conduction band level, and emits light through the recombination of holes in the valence band level and electrons in the conduction band level. Because the emission from quantum dots has a narrow spectrum due to the quantum confinement effect, it is possible to obtain emission with relatively deep chromaticity. Furthermore, the shell has the function of suppressing the generation of defects or dangling bonds in the core and reducing carrier recombination through the deactivation process. From the viewpoint of efficiently obtaining the quantum confinement effect in quantum dots, the particle size of the quantum dot core may be approximately twice or less the exciton Bohr radius of the core material.

[0038] The quantum dot may include materials used for conventionally known core and shell materials in the core and shell materials, respectively. For example, the quantum dot may have a core / shell structure of a group I-III-V chalcopyrite material / ZnS including InP / ZnS, CdSe / ZnS, CdSe / ZnSe, CdSe / CdS, ZnSe / ZnS, or CuInGaS (CIGS). In addition, the quantum dot may contain InZnP, CdSeTe, or ZnSeTe. Furthermore, the core of the quantum dot may contain CuInZnS, CuInS, CuGaS, AgInS, or ZnAgInS. The shell may be formed from multiple layers containing multiple different materials.

[0039] The particle size of the quantum dots is approximately 1 to 100 nm. The quantum dots may be spherical or non-spherical in shape. In this disclosure, the particle size of the quantum dots may be measured by performing cross-sectional observation of the light-emitting layer 4 in the film thickness direction. In this cross-sectional observation, the particle size of each quantum dot may be considered to be the same as the diameter of a circle having the same area as its cross-sectional area. The particle size of the quantum dots may also be measured by measuring the particle size of 20 quantum dots in the cross-sectional observation and calculating the average. The cross-sectional observation may also be performed by analyzing images obtained by capturing cross-sectional images of each layer using a transmission electron microscope (TEM).

[0040] The light-emitting element 10a shown in Figure 10 includes an anode 2, a cathode 6, a light-emitting layer 4 provided between the anode 2 and the cathode 6, and a hole transport layer 3 provided between the anode 2 and the light-emitting layer 4. The hole transport layer 3 contains an organic hole transport material, and an electron transport layer 5 is formed between the cathode 6 and the light-emitting layer 4. As shown in Figure 9, the manufacturing method of the light-emitting element 10a may be performed in the following order: forming the cathode 6 (S21), forming the electron transport layer 5 (S22), forming the light-emitting layer 4 (S23), forming the hole transport layer 3 (S24), aging the hole transport layer 3 (S25), and forming the anode 2 (S26). Between the aging of the hole transport layer 3 (S25) and the forming of the anode 2 (S26), a step of forming a hole injection layer may be further included. Furthermore, the step of forming the electron transport layer 5 (S22), which is performed between the step of forming the cathode 6 (S21) and the step of forming the light-emitting layer 4 (S23), may be omitted as appropriate. Alternatively, the step of forming the electron injection layer may be performed instead of the step of forming the electron transport layer 5 (S22), or the step of forming the electron transport layer 5 (S22) may be performed together with the step of forming the electron injection layer in this order. Here, the case in which the step of aging the hole transport layer 3 (S25) is performed between the step of forming the hole transport layer 3 (S24) and the step of forming the anode 2 (S26) has been explained as an example, but the process is not limited to this. If the step of forming the hole injection layer is included after the step of forming the hole transport layer 3 (S24), the step of aging the hole transport layer 3 (S25) may be performed between the step of forming the hole transport layer 3 (S24) and the step of forming the hole injection layer, or between the step of forming the hole injection layer and the step of forming the anode 2 (S26).

[0041] As shown in Figure 11, the manufacturing method of the light-emitting element 10a, which includes an aging step for the hole transport layer 3, comprises the steps of forming a cathode 6 (S31), forming an anode 2 (S35), forming a light-emitting layer 4 (S33) which is performed between the steps of forming the cathode 6 (S31) and forming the anode 2 (S35), forming a hole transport layer 3 containing an organic hole transport material (S34) which is performed between the steps of forming the light-emitting layer 4 (S33) and forming the anode 2 (S35), forming a cathode 6 (S31), and forming the light-emitting layer 4. The process includes forming an electron transport layer 5 (S32) between the process of forming the electron transport layer 6 (S33) and the process of forming the anode 2 (S36), wherein the process of aging the hole transport layer 3 (S36) is performed after the process of forming the cathode 6 (S31), the process of forming the electron transport layer 5 (S32), the process of forming the light-emitting layer 4 (S33), the process of forming the hole transport layer 3 (S34), and the process of forming the anode 2 (S35), and the process of aging the hole transport layer 3 (S36) may be performed in an inert gas or dry air atmosphere. Between the process of forming the hole transport layer 3 (S34) and the process of forming the anode 2 (S35), the process may further include the process of forming a hole injection layer. Furthermore, the step of forming the electron transport layer 5 (S32), which is performed between the step of forming the cathode 6 (S31) and the step of forming the light-emitting layer 4 (S33), may be omitted as appropriate. Alternatively, the step of forming the electron injection layer may be performed instead of the step of forming the electron transport layer 5 (S32), or the step of forming the electron transport layer 5 (S32) may be performed together with the step of forming the electron injection layer in this order.

[0042] Both the method for manufacturing the light-emitting element 10a, which includes an aging step for the hole transport layer 3 as shown in Figure 9, and the method for manufacturing the light-emitting element 10a, which includes an aging step for the hole transport layer 3 as shown in Figure 11, yield a light-emitting element 10a with an inverted stack structure in which the cathode 6, electron transport layer 5, light-emitting layer 4, hole transport layer 3, and anode 2 are stacked in this order, as shown in Figure 10.

[0043] In the manufacturing method of the light-emitting element 10a, which includes an aging step for the hole transport layer 3 shown in Figure 9, in the step (S25) for aging the hole transport layer 3, since the anode 2 side of the hole transport layer 3 is exposed, not only thermal aging but also optical aging can be suitably used.

[0044] On the other hand, the manufacturing method of the light-emitting element 10a, which includes an aging step for the hole transport layer 3 shown in Figure 11, has the advantage that the film formation process can be carried out as a continuous process in the manufacturing process, since the aging step (S36) of the hole transport layer 3 is performed after all the layers of the light-emitting element 10a have been formed. Furthermore, in the aging step (S36) of the hole transport layer 3, since the anode 2 side surface and the cathode 6 side surface of the hole transport layer 3 are covered, thermal aging can be preferably used rather than optical aging.

[0045] Since the hole transport layer 3 contains an organic hole transport material, even if the polymerization or crosslinking reaction of the organic hole transport material appears to have stopped before the aging process (S25 and S36) of the hole transport layer 3, unbonded groups of the organic hole transport material that create defect levels remain to a degree that strongly affects the electrical properties. If the light-emitting element is used in this state, a large decrease in brightness occurs early in the change in brightness of the light-emitting element over time, as shown in Figure 5.

[0046] Therefore, in the method for manufacturing a light-emitting element of this disclosure, an aging process (S25, S36) of the hole transport layer 3 is performed before using the light-emitting element, that is, during the manufacturing process of the light-emitting element, thereby reducing the number of unbonded groups in the organic hole transport material that create defect levels, and realizing a more stable hole transport layer 3. When a light-emitting element 10a equipped with such a more stable hole transport layer 3 is used, as shown in Figure 5, it is possible to suppress the large decrease in brightness that occurs in the early stages of the change in brightness of the light-emitting element 10a over time.

[0047] The light-emitting element 10a shown in Figure 10 may be either a top-emission type or a bottom-emission type. The light-emitting element 10a shown in Figure 10 is a light-emitting element with an inverted stack structure in which the anode 2 is positioned above the cathode 6. In order to make such an inverted stack structure light-emitting element a top-emission type, the cathode 6 should be formed from an electrode material that reflects visible light and the anode 2 should be formed from an electrode material that transmits visible light. In order to make such an inverted stack structure light-emitting element a bottom-emission type, the cathode 6 should be formed from an electrode material that transmits visible light and the anode 2 should be formed from an electrode material that reflects visible light.

[0048] Figure 12 is a plan view showing a schematic configuration of a display device 1 that includes a plurality of light-emitting elements 10 as shown in the embodiment 1 in Figure 7.

[0049] As shown in Figure 12, the display device 1 comprises a frame area NDA and a display area DA. The display area DA of the display device 1 is provided with a plurality of pixels PIX, and each pixel PIX includes a red subpixel RSP, a green subpixel GSP, and a blue subpixel BSP. In this embodiment, the case in which one pixel PIX is composed of a red subpixel RSP, a green subpixel GSP, and a blue subpixel BSP is described as an example, but it is not limited to this. For example, one pixel PIX may include subpixels of other colors in addition to the red subpixel RSP, green subpixel GSP, and blue subpixel BSP. In this embodiment, the display device 1 is provided with a red light-emitting element 10 having a red light-emitting layer as its light-emitting layer 4 in its red subpixel RSP, a green light-emitting element 10 having a green light-emitting layer as its light-emitting layer 4 in its green subpixel GSP, and a blue light-emitting element 10 having a blue light-emitting layer as its blue subpixel BSP, as an example, but is not limited to this. The display device 1 may also be provided with a red light-emitting element 10a having a red light-emitting layer as its red subpixel RSP, a green light-emitting element 10a having a green light-emitting layer as its light-emitting layer 4 in its green subpixel GSP, and a blue light-emitting element 10a having a blue light-emitting layer as its blue subpixel BSP.

[0050] Figure 13 shows an example of a constant temperature bath 20 capable of performing the aging process (S3, S16, S25, S36) of the hole transport layer 3 containing the organic hole transport material described above.

[0051] The aging process (S3, S16, S25, S36) of the hole transport layer 3 containing the organic hole transport material described above may be carried out in a constant temperature bath 20 set to a predetermined temperature as shown in Figure 13.

[0052] In this embodiment, as shown in Figure 13, the constant temperature bath 20 is described as having not only the ability to set the temperature to a predetermined temperature but also a mechanism 22 for irradiating ultraviolet light. However, if the hole transport layer 3 containing the organic hole transport material described above is not aged by ultraviolet light, the constant temperature bath 20 does not need to have the mechanism 22 for irradiating ultraviolet light. The hole transport layer 3 containing the organic hole transport material described above may be aged by heat alone, by heat and light, or by light alone. In any case, the constant temperature bath 20 shown in Figure 13 can be suitably used, but a constant temperature bath is not required. Furthermore, when aging the hole transport layer 3 containing the organic hole transport material by light alone, an apparatus equipped with a mechanism 22 for irradiating ultraviolet light may be used. The peak wavelength of the ultraviolet light irradiated from the mechanism 22 for irradiating ultraviolet light is preferably 365 nm or less. Furthermore, the constant temperature bath 20 may be filled with a gas 21, such as an inert gas or dry air. If the constant temperature bath 20 is filled with an inert gas (e.g., nitrogen gas) or dry air, the quantum dots can be protected during aging. Also, when dry air is used as the gas 21 filling the constant temperature bath 20, the dew point temperature of the dry air is preferably -56°C or lower, and the residual moisture content in this case is 1 g / m³. ３ It is less than.

[0053] In this embodiment, in the aging step (S3, S16, S25, S36) of the hole transport layer 3 containing the organic hole transport material described above, if the light-emitting element or the display device containing the light-emitting element is not yet in use, the hole transport layer 3 may be aged by, for example, setting it in the constant temperature bath 20 shown in Figure 13, and then maintaining the temperature at 150°C for 5 hours while purging the inside of the constant temperature bath 20 with nitrogen gas, whether in the state of the light-emitting element or the state of the display device containing the light-emitting element. The rate coefficient A in reactions such as polymerization and crosslinking of the organic hole transport material, which is an organic substance, is expressed by the Arrhenius equation (A = c × exp(-Ea / kT), where c is a coefficient, Ea is the activation energy, k is the Boltzmann constant, and T is the temperature), and the higher the temperature, the larger the A, and the faster the reaction such as polymerization and crosslinking proceeds. Also, for example, the relationship between the degree of polymerization n and the time required for polymerization can be expressed using the reaction rate coefficient (t = (n / n300) Ａ (where n300 is the degree of polymerization at 300K) is expressed as a rate coefficient, and the larger the rate coefficient, the faster the reaction proceeds. Therefore, as in the manufacturing method of the light-emitting element shown in Figure 6, if the step of aging the hole transport layer 3 (S3) is performed before the step of forming the light-emitting layer 4 (S4), the temperature of the constant temperature bath 20 may be set to a temperature higher than 150°C to shorten the aging time. On the other hand, as shown in Figures 8, 9 and 11, if the step of aging the hole transport layer 3 is performed after the step of forming the light-emitting layer 4, it is preferable to set the temperature of the constant temperature bath 20 to a temperature below which ligands do not detach from the quantum dots contained in the light-emitting layer 4. In the case of quantum dots, it is known that they can withstand temperatures up to about 150°C in the atmosphere, so when aging is performed in a constant temperature bath 20 filled with dry air or in the atmosphere, it is preferable to perform aging at a temperature of 150°C or lower.

[0054] Figure 14 is a cross-sectional view showing the schematic configuration of a light-emitting element 10b of Embodiment 1, which is manufactured by adding or coating a radical polymerization initiator to a hole transport layer 3 containing an organic hole transport material. The light-emitting element 10b is an element in which a layer 7 containing a radical polymerization initiator is formed on the hole transport layer 3 of the light-emitting element 10 shown in Figure 7. Figure 15 is a cross-sectional view showing the schematic configuration of a light-emitting element 10c, which is a modified example of Embodiment 1, which is manufactured by adding or coating a radical polymerization initiator to a hole transport layer 3 containing an organic hole transport material. The light-emitting element 10c is an element in which a layer 7 containing a radical polymerization initiator is formed on the hole transport layer 3 of the light-emitting element 10a shown in Figure 10.

[0055] For example, in the method for manufacturing a light-emitting element shown in Figure 6, a step of adding or coating a radical polymerization initiator to the hole transport layer 3 may be included between the step of forming the hole transport layer 3 (S2) and the step of aging the hole transport layer 3 (S3). If a step of adding or coating a radical polymerization initiator to the hole transport layer 3 is included between the step of forming the hole transport layer 3 (S2) and the step of aging the hole transport layer 3 (S3), a layer 7 containing the radical polymerization initiator may be formed between the hole transport layer 3 and the light-emitting layer 4, as shown in the light-emitting element 10b in Figure 14. The radical polymerization initiator is preferably one or more selected from 2,2'-azobisbutyronitrile and benzoyl peroxide. Since the decomposition temperature of the radical polymerization initiator, such as 2,2'-azobisbutyronitrile or benzoyl peroxide, is about 100°C, if the hole transport layer 3 contains the radical polymerization initiator, such as 2,2'-azobisbutyronitrile or benzoyl peroxide, it is preferable to perform aging at a temperature of 90°C or lower. In this case, although the aging temperature is relatively low, below 90°C, the aging time can be shortened due to the effect of the radical polymerization initiator.

[0056] Furthermore, for example, in the method for manufacturing a light-emitting element shown in Figure 9, a step of adding or coating a radical polymerization initiator to the hole transport layer 3 may be included between the step of forming the hole transport layer 3 (S24) and the step of aging the hole transport layer 3 (S25). If a step of adding or coating a radical polymerization initiator to the hole transport layer 3 is included between the step of forming the hole transport layer 3 (S24) and the step of aging the hole transport layer 3 (S25), a layer 7 containing the radical polymerization initiator may be formed between the hole transport layer 3 and the anode 2, as shown in the light-emitting element 10c in Figure 15. In this embodiment, the step of adding or coating the radical polymerization initiator to the hole transport layer 3 was performed using a solution of 2,2'-azobisbutyronitrile, which is a radical polymerization initiator, and ethanol, which is a solvent (the concentration of 2,2'-azobisbutyronitrile is 4.96 g / 100 ml). In this case, even after aging at an aging temperature of 90°C for one hour, it was confirmed that the electrical properties (V-I characteristics) of the hole transport layer 3 exhibited space charge-limited current conduction. This demonstrated that the aging time could be significantly reduced despite the relatively low aging temperature of 90°C, thanks to the effect of the radical polymerization initiator.

[0057] Figure 16 illustrates how the film formation properties of a hole transport layer 3 containing an organic hole transport material are improved by using a coupling agent 25 when forming an inorganic film, for example, a hole transport layer 3 containing an organic hole transport material on an anode 2.

[0058] As shown in Figures 6 and 8, the step of forming the hole transport layer 3, which is performed after the step of forming the inorganic film anode 2, is preferably performed immediately after the step of applying the coupling agent 25. Also, as shown in Figures 9 and 11, the step of forming the hole transport layer 3, which is performed after the step of forming the inorganic film light-emitting layer 4, is preferably performed immediately after the step of applying the coupling agent 25. The coupling agent 25 may contain titanium (Ti) or zirconium (Zr). The coupling agent 25 strongly bonds the inorganic film and the organic film, preventing film delamination even if volume changes occur due to polymerization, for example. Furthermore, if the coupling agent 25 contains titanium (Ti) or zirconium (Zr), the deep work function of titanium (Ti) or zirconium (Zr) can bring the energy level of the coupling agent 25 closer to that of the anode 2 and the hole transport layer 3, thereby increasing hole injection. The coupling agent 25 may be a compound in which, for example, a titanium (Ti) element is bonded to four alkoxy groups, as shown in (Chemical Formula 1) to (Chemical Formula 6) below.

[0059] Figure 17 shows an example of the material constituting the light-emitting layer 4 provided in the light-emitting element of Embodiment 1. Figure 18 shows another example of the material constituting the light-emitting layer 4 provided in the light-emitting element of Embodiment 1. Figure 19 shows yet another example of the material constituting the light-emitting layer 4 provided in the light-emitting element of Embodiment 1.

[0060] As shown in FIGS. 17 and 18, the light-emitting layer 4 provided in the light-emitting element of Embodiment 1 may include an additive AD that covers the periphery of at least one quantum dot BQD, or as shown in FIG. 19, a matrix MR as an additive that fills the space between at least two quantum dots BQD. The additive AD or the matrix MR as an additive may have properties as a semiconductor or an insulator, for example. The additive AD or the matrix MR as an additive may include any one of metal oxides, semimetal oxides, and metal sulfides. As shown in FIGS. 17, 18, and 19, the quantum dot BQD includes a core BQDC and a shell BQDS.

[0061] The additive AD or the matrix MR as an additive may include, as an oxide, for example, silicon oxide (SiO [[ID=**4**]] ２ ), titanium oxide (TiO [[ID=**6**]] ２ ), aluminum oxide (Al [[ID=**8**]] ２ O [[ID=**10**]] ３ ), boron oxide (B [[ID=**12**]] ２ O [[ID=**14**]] ３ ), phosphorus oxide (P [[ID=**16**]] ２ O [[ID=**18**]] ５ ), germanium oxide (GeO [[ID=**20**]] ２ ), hafnium oxide (HfO [[ID=**22**]] ２ ), zinc oxide (ZnO), zirconium oxide (ZrO [[ID=**24**]] ２ ), tellurium oxide (TeO [[ID=**26**]] ２ ), bismuth oxide (Bi [[ID=**28**]]<> ２ O [[ID=**30**]] ３ ), vanadium oxide (V [[ID=**><32**]] ２ O [[ID=**><34**]] ５ ), antimony oxide (Sb [[ID=**><36**]] ２ O [[ID=**><38**]] ５ ), lead oxide (PbO), and copper oxide (CuO). Here, the case where the additive AD or the matrix MR as an additive is formed as a polymer may also be included. For example, when the additive AD or the matrix MR as an additive includes silicon oxide as an oxide, the case where the additive AD or the matrix MR as an additive is a siloxane compound having a siloxane bond may be included. Further, the additive AD or the matrix MR as an additive may include, as a sulfide, for example, zinc sulfide (ZnS) and zinc magnesium sulfide (ZnMgS, ZnMgS２ ) may contain at least one of the following. Note that the chemical formula of the compound is a representative example, and the composition ratio indicated in the chemical formula may or may not be stoichiometric.

[0062] As shown in Figures 17 and 18, in the case where the adduct AD surrounds at least one quantum dot BQD, if the adduct AD occupies more than 90% of the periphery of the quantum dot BQD in the cross-section of the light-emitting layer 4 passing through the quantum dot BQD, then the quantum dot BQD may be considered to be covered by the adduct AD. The adduct AD is, for example, mainly composed of silicon oxide and may be arranged in contact with the surface of the quantum dot BQD, or with an organic ligand OL between the adduct AD and the quantum dot BQD.

[0063] As shown in Figure 19, when the matrix MR as an adduct is formed to fill the space between at least two quantum dots BQD, the matrix MR as an adduct has a thickness of 1000 nm in a direction perpendicular to the film thickness direction at any position in the film thickness direction of the light-emitting layer 4. ２It may be formed as a continuous film having the above area. In addition, in the light-emitting layer 4, the quantum dots BQD may be embedded in a continuous film of matrix MR as an adduct. For example, if 60% or more of the surface of 80% or more of the quantum dots BQD constituting the light-emitting layer 4 is in contact with a continuous film of matrix MR as an adduct, then the quantum dots BQD contained in the light-emitting layer 4 can be said to be embedded in matrix MR as an adduct. Furthermore, when we say that matrix MR as an adduct fills the space between at least two quantum dots BQD, the space between the two quantum dots BQD may be filled only with matrix MR as an adduct, or it may be filled with matrix MR as an adduct and other materials. In addition to matrix MR as an adduct, a material such as a ligand different from matrix MR as an adduct may be included between the two quantum dots BQD. The material such as ligand may be an organic ligand that coordinates to the quantum dots BQD, or it may be an organic ligand that is separated from the quantum dots BQD. If the light-emitting layer 4 contains an organic ligand, for example, the weight ratio of the organic ligand to the total weight of the light-emitting layer 4 may be less than 5%. The weight ratio of the organic ligand can be measured, for example, using TOF-SIMS (time-of-flight secondary ion mass spectrometry). If the organic ligand is dispersed in the light-emitting layer 4, the organic ligand may contribute to the injection of holes and electrons into the quantum dot BQD.

[0064] In forming the light-emitting layer 4, first, a quantum dot dispersion is prepared. The quantum dot dispersion is a dispersion in which quantum dots BQD and a precursor of adduct AD or a precursor of matrix MR as an adduct are dispersed in a solvent. The precursor includes a material that is converted to adduct AD or matrix MR as an adduct through hydrolysis and dehydration condensation by performing a specific operation such as heating or light irradiation. The precursor may have a coordinating functional group that forms a coordination bond with the surface of the quantum dot BQD in the quantum dot dispersion. In this case, the precursor may coordinate to the quantum dot BQD in the quantum dot dispersion. The quantum dot dispersion may also contain a halide having a halogen atom (for example, zinc chloride (ZnCl)). ２ )) and barium hydroxide (Ba(OH)) as a reaction catalyst ２) may also be included. By mixing a material having halogen atoms into the quantum dot dispersion, the halogen atoms can coat the surface of the quantum dot BQD independently of the adduct or the matrix MR as an adduct, thereby reducing defects on the surface of the quantum dot BQD. Therefore, by having halogen atoms in the adduct AD or the matrix MR as an adduct, each light-emitting device increases the coverage of the quantum dot BQD in the light-emitting layer 4, thereby improving the light-emitting properties.

[0065] A quantum dot dispersion may be prepared, for example, by stirring a dispersion containing quantum dots BQD and a dispersion containing a precursor to prepare a mixture, and then extracting a predetermined layer from the mixture. Here, the dispersion containing quantum dots BQD may contain an organic ligand that coordinates to the quantum dots BQD. In this case, for example, during stirring for the preparation of the mixture, the ligand that coordinates to the quantum dots BQD may be obtained from the organic ligand by 3-trimethoxysilylpropanethiol (3-trimethoxysilylpropanethiol, C ６ H １６ O ３ It may be substituted with a precursor such as SSDi (hereinafter referred to as TMSPT).

[0066] For example, if the adduct AD or the matrix MR as an adduct contains silicon oxide and the precursor has a coordinating functional group, the precursor may contain TMSPT. Also, if the adduct AD or the matrix MR as an adduct contains silicon oxide and the precursor does not have a coordinating functional group, the precursor may contain Diphenylsilanediol (C １２ H １２ O ２ It may also contain Si) (hereinafter referred to as DPSD). Furthermore, for example, if the adduct AD or the matrix MR as an adduct contains zinc sulfide, the precursor may also contain zinc xanthogenic acid, zinc thioureate, and zinc dithiocarboxylate, etc.

[0067] Next, the quantum dot dispersion is applied to the lower layer of the light-emitting layer 4 to convert the precursor into an adduct AD or a matrix MR as an adduct. The conversion of the precursor into an adduct AD or a matrix MR as an adduct occurs, for example, by heating the applied quantum dot dispersion to volatilize the solvent and convert the precursor into an adduct AD or a matrix MR as an adduct. For example, if the precursor contains DPSD and TMSPT, dehydration condensation occurs between DPSDs, between TMSPTs, and between DPSDs and TMSPTs, etc., to form silicon oxide as an adduct AD or a matrix MR as an adduct. Alternatively, for example, if the precursor contains zinc xanthogenic acid, the zinc xanthogenic acid is decomposed to form zinc sulfide as an adduct AD or a matrix MR as an adduct. The conversion of the precursor into an adduct AD or a matrix MR as an adduct occurs sequentially around the quantum dots BQD in the quantum dot dispersion. By this method, a light-emitting layer 4 having an adduct AD or a matrix MR as an adduct can be formed.

[0068] In the manufacturing method of the light-emitting element shown in Figures 6 and 9, the process of aging the hole transport layer 3 is performed before all the layers of the light-emitting element are formed. Therefore, as long as no other layers are formed on the hole transport layer 3, the electrical characteristics (V-I characteristics) of the aged hole transport layer 3 can be checked during the manufacturing process of the light-emitting element. Depending on the state of the electrical characteristics (V-I characteristics) of the aged hole transport layer 3, the aging conditions of the hole transport layer 3 can be changed and additional aging can be performed.

[0069] On the other hand, in the manufacturing method of light-emitting elements shown in Figures 8 and 11, the process of aging the hole transport layer 3 is performed after other layers are formed on the hole transport layer 3. Therefore, if a test element including the hole transport layer 3 is fabricated in advance before the manufacturing process of the light-emitting element, its electrical characteristics (V-I characteristics) can be confirmed.

[0070] Therefore, the inventors of this disclosure have found a configuration that allows for the measurement or estimation of the electrical characteristics (V-I characteristics) of the hole transport layer 3, even when the aging process of the hole transport layer 3 is performed after other layers have been formed on the hole transport layer 3, as will be described later in Embodiment 2.

[0071] [Embodiment 2] Figure 20 is a plan view showing the schematic configuration of the display device 1a of Embodiment 2. Figure 21 is a cross-sectional view showing the schematic configuration of an example of a second element 30b provided in the display device 1a of Embodiment 2. Figure 22 is a cross-sectional view showing the schematic configuration of an example of a first element 40a provided in the display device 1a of Embodiment 2. Figure 23 is a cross-sectional view showing the schematic configuration of an example of a first element 40b that can be provided in the display device 1a of Embodiment 2. Figure 24 is a cross-sectional view showing the schematic configuration of an example of a first element 40c that can be provided in the display device 1a of Embodiment 2. Figure 25 is a cross-sectional view showing the schematic configuration of an example of a first element 40d that can be provided in the display device 1a of Embodiment 2.

[0072] In this embodiment, as shown in Figures 20, 21, and 22, the present invention will be described as an example in which the display device 1a includes a light-emitting element 10d in the display area DA, which includes an anode 2, a cathode 6, a light-emitting layer 4 provided between the anode 2 and the cathode 6, and a hole transport layer (first hole transport layer) 3 containing a first organic hole transport material provided between the anode 2 and the light-emitting layer 4. As shown in Figure 22, the light-emitting element 10d includes a first element 40a, the first element 40a includes a hole transport layer (first hole transport layer) 3, an anode 2 which is a first electrode that is in contact with the hole transport layer (first hole transport layer) 3 but not in contact with the light-emitting layer 4, and a second electrode 41 which is in contact with the hole transport layer (first hole transport layer) 3 but not in contact with the light-emitting layer 4 and the anode 2 which is the first electrode. In the first element 40a shown in Figure 22, an insulating layer 42 is provided to prevent the second electrode 41 from coming into contact with the light-emitting layer 4.

[0073] In this embodiment, among the multiple red subpixels RSP provided in the display area DA of the display device 1a, only one red subpixel is provided with a light-emitting element 10d, which is a red light-emitting element equipped with a red light-emitting layer 4, while each of the remaining red subpixels is provided with a light-emitting element 10, which is a red light-emitting element equipped with a red light-emitting layer 4 as described in Embodiment 1. Furthermore, among the multiple green subpixels GSP provided in the display area DA of the display device 1a, only one green subpixel is provided with a light-emitting element 10d, which is a green light-emitting element equipped with a green light-emitting layer 4, while each of the remaining green subpixels is provided with a light-emitting element 10, which is a green light-emitting element equipped with a green light-emitting layer 4 as described in Embodiment 1. In addition, among the multiple blue subpixels BSP provided in the display area DA of the display device 1a, only one blue subpixel is provided with a light-emitting element 10d, which is a blue light-emitting element equipped with a blue light-emitting layer 4, while each of the remaining blue subpixels is provided with a light-emitting element 10, which is a blue light-emitting element equipped with a blue light-emitting layer 4 as described in Embodiment 1. Furthermore, among the multiple red subpixels RSP, the position of the red subpixel on which the light-emitting element 10d is provided is preferably a red subpixel close to the frame region NDA; among the multiple green subpixels GSP, the position of the green subpixel on which the light-emitting element 10d is provided is preferably a green subpixel close to the frame region NDA; and among the multiple blue subpixels BSP, the position of the blue subpixel on which the light-emitting element 10d is provided is preferably a blue subpixel close to the frame region NDA, but the device is not limited to these positions.

[0074] For example, in the display area DA of the display device 1a, if each of the light-emitting elements 10, which is a red light-emitting element provided in the red subpixel RSP, the light-emitting element 10, which is a green light-emitting element provided in the green subpixel GSP, and the light-emitting element 10, which is a blue light-emitting element provided in the blue subpixel BSP, is provided with a hole transport layer (first hole transport layer) 3 made of different materials, then, as described above, it is preferable to provide a red light-emitting element 10d in one of the multiple red subpixel RSPs, a green light-emitting element 10d in one of the multiple green subpixel GSPs, and a blue light-emitting element 10d in one of the multiple blue subpixel BSPs.

[0075] On the other hand, for example, in the display area DA of the display device 1a, if each of the light-emitting element 10, which is a red light-emitting element provided in the red subpixel RSP, the light-emitting element 10, which is a green light-emitting element provided in the green subpixel GSP, and the light-emitting element 10, which is a blue light-emitting element provided in the blue subpixel BSP, each has a common hole transport layer (first hole transport layer) 3, then a light-emitting element 10d of a color corresponding to only one of the multiple red subpixel RSPs, multiple green subpixel GSPs, and multiple blue subpixel BSPs may be provided.

[0076] Furthermore, a light-emitting element 10d, which is a red light-emitting element, may be provided in each of the multiple red subpixels RSP provided in the display area DA of the display device 1a, a light-emitting element 10d, which is a green light-emitting element, may be provided in each of the multiple green subpixels GSP, and a light-emitting element 10d, which is a blue light-emitting element, may be provided in each of the multiple blue subpixels BSP. In such a configuration, each of the color subpixels provided in the display area DA of the display device 1a can be provided with a light-emitting element 10d having the same structure.

[0077] As described above, in this embodiment, the case in which a light-emitting element 10d equipped with a first element 40a is provided in the display area DA of the display device 1a has been described as an example, but the embodiment is not limited to this. For example, instead of the light-emitting element 10d equipped with a first element 40a, the display area DA of the display device 1a may be provided with one or more selected from the light-emitting element 10e equipped with a first element 40b shown in Figure 23, the light-emitting element 10f equipped with a first element 40c shown in Figure 24, and the light-emitting element 10g equipped with a first element 40d shown in Figure 25. Furthermore, the display area DA of the display device 1a may be provided with the light-emitting element 10d equipped with a first element 40a, along with one or more selected from the light-emitting element 10e equipped with a first element 40b shown in Figure 23, the light-emitting element 10f equipped with a first element 40c shown in Figure 24, and the light-emitting element 10g equipped with a first element 40d shown in Figure 25.

[0078] Furthermore, in this embodiment, as shown in Figures 20 and 21, the display device 1a has a light-emitting element 10d with a first element 40a in the display area DA, and a second element 30a and a second element 30b in the frame area NDA, which is the area outside the display area DA. However, the embodiment is not limited to this, and the display device 1a only needs to have at least one of the first element and the second element. Therefore, the display device 1a may have, for example, only the second element, in which case the subpixels of each color in the display area DA are provided with the light-emitting element 10 described in Embodiment 1. Also, when the display device 1a has the second element, it only needs to have at least one of the second element 30a and the second element 30b, and it may have three or more second elements. As shown in Figure 21, the second element 30b includes a second hole transport layer 32 containing a second organic hole transport material, a third electrode 31 in contact with the second hole transport layer 32, and a fourth electrode 33 in contact with the second hole transport layer 32 but not with the third electrode 31. Although not shown, the second element 30a has the same configuration as the second element 30b shown in Figure 21, and as shown in Figure 20, in a plan view, the size of the second element 30b is larger than the size of the second element 30a, but is not limited to this.

[0079] As shown in Figure 21, the second element 30b is a laminate in which a third electrode 31, a second hole transport layer 32, and a fourth electrode 33 are stacked in that order. An insulating layer may be provided on the fourth electrode 33 of the second element 30b. Furthermore, in a plan view, it is preferable that the size of each of the second elements 30a and 30b is larger than the light-emitting element 10 and light-emitting element 10d provided in the display area DA of the display device 1a.

[0080] In the case of the first elements 40a, 40b, 40c, and 40d, they include the hole transport layer 3 provided in the light-emitting elements 10d, 10e, 10f, and 10g, and have the advantage that the electrical characteristics (V-I characteristics) of the hole transport layer 3 can be directly measured. On the other hand, in the case of the second elements 30a and 30b, unlike the first elements 40a, 40b, 40c, and 40d, they are provided in the frame region NDA, so they can be made relatively large, making it easier to make contact with a probe and allowing for relatively easy measurement of their electrical characteristics (V-I characteristics). For these reasons, the display device 1a of this embodiment is provided with the first element 40a and two second elements 30a and 30b.

[0081] In this embodiment, we will describe, as an example, the case in which the third electrode 31 provided on the second element 30b shown in Figure 21 is formed of the same material as the anode 2 provided on the light-emitting element 10 and the light-emitting element 10d, the second hole transport layer 32 provided on the second element 30b is formed of the same material as the hole transport layer (first hole transport layer) 3 provided on the light-emitting element 10 and the light-emitting element 10d, and the fourth electrode 33 provided on the second element 30b is formed of the same material as the cathode 6 provided on the light-emitting element 10 and the light-emitting element 10d, but we are not limited to this. If the display device is equipped with an inverted-stack light-emitting element, the third electrode 31 is formed of the same material as the cathode 6 provided on the inverted-stack light-emitting element, and the fourth electrode 33 is formed of the same material as the anode 2 provided on the inverted-stack light-emitting element. The third electrode 31 and the fourth electrode 33 are not particularly limited as long as they are conductive. Furthermore, if the second hole transport layer 32 can reflect the changes in the electrical properties (V-I characteristics) of the hole transport layer (first hole transport layer) 3 due to aging, and if the electrical properties (V-I characteristics) of the hole transport layer (first hole transport layer) 3 can be inferred from the second hole transport layer 32, then the second hole transport layer 32 may be formed from a different material than the hole transport layer (first hole transport layer) 3.

[0082] As described above in Embodiment 1, when a radical polymerization initiator is added to or applied to the hole transport layer (first hole transport layer) 3 provided on the light-emitting element 10 and the light-emitting element 10d, it is preferable to also add or apply a radical polymerization initiator to the second hole transport layer 32 provided on the second element 30a and 30b. In such a case, the radical polymerization initiator is present on the upper surface of the hole transport layer (first hole transport layer) 3 and the upper surface of the second hole transport layer 32.

[0083] As shown in Figures 22, 23, 24, and 25, the first electrodes provided on the first elements 40a, 40b, 40c, and 40d are anodes 2 provided on the light-emitting elements 10d, 10e, 10f, and 10g.

[0084] As shown in Figure 22, the second electrode 41 provided on the first element 40a may be provided so as to be in contact with the cathode 6 side surface of the hole transport layer (first hole transport layer) 3 provided on the light-emitting element 10d, and as shown in Figure 25, the second electrode 41 provided on the first element 40d may be provided so as to be in contact with the cathode 6 side surface of the hole transport layer (first hole transport layer) 3 provided on the light-emitting element 10g. Furthermore, as shown in Figure 23, the second electrode 41 provided on the first element 40b may be provided so as to be in contact with the side surface of the hole transport layer (first hole transport layer) 3 provided on the light-emitting element 10e, and as shown in Figure 24, the second electrode 41 provided on the first element 40c may be provided so as to be in contact with the anode 2 side surface of the hole transport layer (first hole transport layer) 3 provided on the light-emitting element 10f.

[0085] In the first element 40a shown in Figure 22, an insulating layer 42 is provided to prevent the second electrode 41 from coming into contact with the light-emitting layer 4. In the first element 40b shown in Figure 23, an insulating layer 42 is provided to prevent the second electrode 41 from coming into contact with the first electrode, the anode 2. In the first element 40d shown in Figure 25, an insulating layer 42 is provided to prevent the second electrode 41 from coming into contact with the cathode 6.

[0086] In this embodiment, since the display device 1a is equipped with a forward-facing light-emitting element 10 and a light-emitting element 10d, the case in which the first electrode provided on the first element 40a is the anode 2 has been described as an example, but the invention is not limited to this. For example, even when the display device 1a is equipped with an inverted-facing light-emitting element, the first electrode provided on the first element is the cathode 6.

[0087] As shown in Figures 22, 23, 24, and 25, it is preferable that the second electrode 41 is provided in the non-emitting region NEMR, which is outside the light-emitting region EMR of the light-emitting elements 10d, 10e, 10f, and 10g.

[0088] In the display device 1a, it is preferable that the number of first elements 40a provided in the display area DA shown in Figure 20 is less than the number of light-emitting elements 10 and light-emitting elements 10d provided in the display area DA.

[0089] Preferably, the hole transport layer (first hole transport layer) 3 provided in the display device 1a has electrical characteristics such that the current value I flowing through the hole transport layer (first hole transport layer) 3 is proportional to the value of the voltage V applied to the hole transport layer (first hole transport layer) 3 raised to the power of m (1.8 ≤ m ≤ 2.2).

[0090] The substrate 9 shown in Figures 21, 22, 23, 24, and 25 includes a thin-film transistor having a drain electrode electrically connected to the anode 2, although this is not shown.

[0091] Furthermore, although not shown in Figures 21, 22, 23, 24, and 25, the first electrode may be extended to a pad electrode not shown, the second electrode 41 may also be extended to a pad electrode not shown, the third electrode 31 may also be extended to a pad electrode not shown, and the fourth electrode 33 may be used as is or extended to a pad electrode not shown. The first elements 40a, 40b, 40c, and 40d function as TEGs (test elementary groups) for measuring the electrical characteristics (V-I characteristics) of the hole transport layer (first hole transport layer) 3, and the second elements 30a and 30b each function as TEGs (test elementary groups) for measuring or estimating the electrical characteristics (V-I characteristics) of the hole transport layer (first hole transport layer) 3.

[0092] Furthermore, when manufacturing a light-emitting element with a forward-stacked structure, bank 8 can be formed before or immediately after the process of forming the anode 2 shown in Figures 6 and 8. For example, it can be formed by coating an organic material such as polyimide or acrylic and then patterning it using photolithography. On the other hand, when manufacturing a light-emitting element with an inverted-stacked structure, bank 8 can be formed before or immediately after the process of forming the cathode 6 shown in Figures 9 and 11.

[0093] The display device 1a described above includes at least one of a first element 40a that functions as a TEG (test elementary group) for measuring the electrical characteristics (V-I characteristics) of the hole transport layer (first hole transport layer) 3, and a second element 30a or 30b that functions as a TEG (test elementary group) for measuring or estimating the electrical characteristics (V-I characteristics) of the hole transport layer (first hole transport layer) 3. Therefore, even if the aging process of the hole transport layer (first hole transport layer) 3 is performed after other layers have been formed on the hole transport layer 3, the electrical characteristics (V-I characteristics) of the hole transport layer 3 can be measured or estimated.

[0094] [Additional Notes] 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 in the technical scope of this disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

[0095] This disclosure can be used in a method for manufacturing a light-emitting element and a display device.

[0096] 1, 1a Display device 2 Anode 3 Hole transport layer (first hole transport layer) 4 Light-emitting layer 5 Electron transport layer 6 Cathode 7 Layer containing radical polymerization initiator 8 Bank 9 Substrate 10, 10a-10g Light-emitting element 20 Constant temperature bath 21 Gas 22 Mechanism for irradiating with ultraviolet light 25 Coupling agent 30a, 30b Second element 31 Third electrode 32 Second hole transport layer 33 Fourth electrode 40a, 40b, 40c, 40d First element 41 Second electrode 42 Insulating layer BQD Quantum dot BQDC Quantum dot core BQDS Quantum dot shell AD Adductor OL Organic ligand MR Matrix NEMR Non-emitting region EMR Light-emitting region PIX Pixel RSP Red subpixel GSP Green subpixel BSP Blue subpixels DA display area NDA frame area

Claims

1. A method for manufacturing an element-emitting element, comprising the steps of: forming an anode; forming a cathode; forming an emissive layer between the steps of forming the anode and forming the cathode; forming a hole transport layer containing an organic hole transport material; and aging the hole transport layer with at least one of heat and light, wherein the step of forming the hole transport layer is performed between the step of forming the anode and forming the emissive layer, and the aging step is performed before any later steps among the steps of forming the anode and forming the cathode.

2. A method for manufacturing an light-emitting element according to claim 1, wherein the steps of forming the anode, forming the hole transport layer, aging, forming the light-emitting layer, and forming the cathode are performed in this order.

3. The method for manufacturing a light-emitting element according to claim 2, further comprising the step of forming a hole injection layer between the step of forming the anode and the step of forming the hole transport layer.

4. A method for manufacturing a light-emitting element according to claim 1, wherein the steps of forming the cathode, forming the light-emitting layer, forming the hole transport layer, aging, and forming the anode are performed in this order.

5. The method for manufacturing a light-emitting element according to claim 4, further comprising the step of forming a hole injection layer between the aging step and the anode formation step.

6. A method for manufacturing a light-emitting element according to any one of claims 1 to 5, comprising a step of forming an electron injection layer or an electron transport layer between the step of forming the light-emitting layer and the step of forming the cathode.

7. A method for manufacturing an optical element, comprising the steps of: forming an anode; forming a cathode; forming an optical light-emitting layer between the steps of forming the anode and the steps of forming the cathode; forming a hole transport layer containing an organic hole transport material between the steps of forming the anode and the steps of forming the optical light-emitting layer; and aging the hole transport layer by heat, wherein the aging step is performed after the steps of forming the anode, forming the cathode, forming the optical light-emitting layer and forming the hole transport layer, and the aging step is performed in an inert gas or dry air atmosphere.

8. The method for manufacturing a light-emitting element according to any one of claims 1 to 7, wherein the aging step is performed in a constant temperature bath set to a predetermined temperature.

9. The method for manufacturing a light-emitting element according to any one of claims 1 to 6, wherein the aging step involves aging the hole transport layer with heat and ultraviolet light, and the aging step is performed in a constant temperature bath equipped with a mechanism for irradiating the hole transport layer with ultraviolet light and set to a predetermined temperature.

10. The method for manufacturing a light-emitting element according to any one of claims 1 to 6, wherein in the aging step, ultraviolet light is irradiated onto the hole transport layer.

11. The method for manufacturing a light-emitting element according to claim 9 or 10, wherein the peak wavelength of the ultraviolet light is 365 nm or less.

12. The method for manufacturing a light-emitting element according to claim 8 or 9, wherein the constant temperature bath is filled with an inert gas.

13. The method for manufacturing a light-emitting element according to claim 8 or 9, wherein the constant temperature bath is filled with dry air.

14. The method for manufacturing a light-emitting element according to claim 13, wherein the dew point temperature of the dry air is -56°C or lower.

15. A method for manufacturing a light-emitting element according to any one of claims 1 to 14, comprising the step of adding or coating a radical polymerization initiator to the hole transport layer between the step of forming the hole transport layer and the step of aging.

16. The method for producing a light-emitting element according to claim 15, wherein the radical polymerization initiator is one or more selected from 2,2'-azobisbutyronitrile and benzoyl peroxide.

17. The method for manufacturing a light-emitting element according to any one of claims 1 to 16, wherein in the aging step, the hole transport layer is aged until it exhibits electrical characteristics in which the current value I flowing through the hole transport layer is proportional to the value of the voltage V applied to the hole transport layer raised to the power of m (1.8 ≤ m ≤ 2.2).

18. The method for manufacturing a light-emitting element according to any one of claims 1 to 17, wherein the step of forming the hole transport layer is performed immediately after the step of applying the coupling agent.

19. The method for manufacturing a light-emitting element according to claim 18, wherein the coupling agent comprises the element titanium (Ti) or the element zirconium (Zr).

20. A display device comprising: a light-emitting element including an anode, a cathode, a light-emitting layer provided between the anode and the cathode, and a first hole transport layer provided between the anode and the light-emitting layer containing a first organic hole transport material; a display area on which the light-emitting element is provided; and a frame area which is an area outside the display area, the first element provided in the display area including the first hole transport layer, a first electrode in contact with the first hole transport layer but not in contact with the light-emitting layer, and a second electrode in contact with the first hole transport layer but not in contact with the light-emitting layer and the first electrode respectively; and a second element provided in the frame area including a second hole transport layer containing a second organic hole transport material, a third electrode in contact with the second hole transport layer, and a fourth electrode in contact with the second hole transport layer but not in contact with the third electrode.

21. The display device according to claim 20, comprising the first element, wherein the first electrode is the anode.

22. The display device according to claim 21, wherein the second electrode is provided so as to be in contact with the cathode-side surface of the first hole transport layer.

23. The display device according to claim 21, wherein the second electrode is provided so as to be in contact with the side surface of the first hole transport layer.

24. The display device according to claim 21, wherein the second electrode is provided so as to be in contact with the anode-side surface of the first hole transport layer.

25. The display device according to any one of claims 22 to 24, wherein the second electrode is provided in a non-emitting region of the light-emitting element.

26. The display device according to any one of claims 20 to 25, wherein a plurality of the light-emitting elements are provided in the display area, and the number of the first elements provided in the display area is less than the number of the light-emitting elements provided in the display area.

27. The display device according to any one of claims 21 to 26, wherein a radical polymerization initiator is present on the upper surface of the first hole transport layer.

28. The display device according to any one of claims 20 to 27, comprising the second element, wherein the second element is a laminate in which the third electrode, the second hole transport layer, and the fourth electrode are stacked in that order.

29. The display device according to claim 28, wherein an insulating layer is provided on the fourth electrode.

30. The display device according to claim 28 or 29, wherein, in a plan view, the size of the second element is larger than that of the light-emitting element.

31. The display device according to any one of claims 28 to 30, wherein the third electrode is formed of the same material as one of the anode and the cathode, the second hole transport layer is formed of the same material as the first hole transport layer, and the fourth electrode is formed of the same material as the other of the anode and the cathode.

32. The display device according to any one of claims 28 to 31, wherein a radical polymerization initiator is present on the upper surface of the first hole transport layer and the upper surface of the second hole transport layer.

33. The display device according to any one of claims 20 to 32, wherein the first hole transport layer has electrical characteristics such that the current value I flowing through the first hole transport layer is proportional to the value of the voltage V applied to the first hole transport layer raised to the power of m (1.8 ≤ m ≤ 2.2).

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