Light-emitting element, display device, and method for manufacturing light-emitting element

Abstract

This light-emitting element (101) comprises an electron transport layer (4) that is positioned between a light-emitting layer (3) and a cathode (2) and includes an n-type semiconductor. The light-emitting layer (3) has a plurality of quantum dots (5) and a medium material (6) positioned between the plurality of quantum dots (5) and including a p-type semiconductor. The work function of the medium material (6) is greater than the work function of the electron transport layer (4).

Description

Light-emitting element, display device, and method for manufacturing light-emitting element

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

[0002] Patent Document 1 discloses an electroluminescent device having a light-emitting layer including quantum dots and a first hole transport material having a substituted or unsubstituted C4 to C20 alkyl group attached to the main chain.

[0003] Japanese Patent Publication No. 2020-77610

[0004] Conventional light-emitting devices having a light-emitting layer containing a hole-transporting material have a problem in that the efficiency of charge injection into quantum dots is low, resulting in low light-emitting efficiency.

[0005] A light-emitting element according to one aspect of the present disclosure includes an anode and a cathode, a light-emitting layer located between the anode and the cathode, and an electron transport layer located between the light-emitting layer and the cathode and including an n-type semiconductor, wherein the light-emitting layer has a plurality of quantum dots and a medium material located between the plurality of quantum dots and including a p-type semiconductor, and the work function of the medium material is greater than the work function of the electron transport layer.

[0006] According to one aspect of the present disclosure, a light-emitting element with high luminous efficiency can be realized.

[0007] 1 is a cross-sectional view showing a schematic configuration of a light-emitting device according to a first embodiment of the present disclosure; FIG. 2 is a flowchart showing a method for manufacturing a light-emitting device according to a first embodiment of the present disclosure; FIG. 3 is a side view illustrating an example of a method for forming a medium; FIG. 4 is a side view illustrating another example of a method for forming a medium; FIG. 5 is an example of a band gap diagram for a light-emitting layer and an electron transport layer; FIG. 6 shows formulas (1) to (3); FIG. 7 is a band gap diagram for test object A; FIG. 8 is a band gap diagram for test object B; FIG. 9 is a graph showing the efficiency of electron injection into a quantum dot versus the Fermi level difference between a medium and an electron transport layer; FIG. 10 is a graph showing the current density and luminance versus the applied voltage for test objects C to E; FIG. 11 is a graph showing the EQE versus the applied voltage for test objects C to E; FIG. 12 is a graph showing the EQE versus the current density for test objects C to E; FIG. 13 is a graph showing the current density and luminance versus the applied voltage for test objects F to J; 10 is a graph showing EQE versus current density for test objects F to J. FIG 11 is a cross-sectional view showing a schematic configuration of a display device according to a second embodiment of the present disclosure.

[0008] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes an embodiment of the present disclosure. For convenience of explanation, the same reference numerals are used to designate components having the same functions as those previously described, and the description thereof may not be repeated.

[0009] 1 is a cross-sectional view showing a schematic configuration of a light-emitting element 101 according to embodiment 1 of the present disclosure. The light-emitting element 101 includes an anode 1, a cathode 2, a light-emitting layer 3, an electron transport layer 4, a hole injection layer 51, and a hole transport layer 52.

[0010] An example of the material for the anode 1 is ITO (Indium Tin Oxide), and an example of the material for the cathode 2 is Ag.

[0011] The light-emitting layer 3 is located between the anode 1 and the cathode 2. The light-emitting layer 3 includes a plurality of quantum dots 5 and a medium material 6.

[0012] Examples of the material of each of the quantum dots 5 include InP and ZnSeTe.

[0013] The core and shell materials of the quantum dots 5 may each contain materials used for the core and shell materials of conventionally known core / shell quantum dots. The quantum dots 5 may have a core / shell structure such as InP / ZnS, CdSe / ZnS, CdSe / ZnSe, CdSe / CdS, ZnSe / ZnS, or CIGS / ZnS. The shell may be formed of multiple layers containing multiple different materials.

[0014] The quantum dots 5 have a particle size of about 1 to 100 nm. The wavelength of light emitted from the quantum dots 5 can be controlled by the particle size. In particular, since the quantum dots 5 have a core / shell structure, the wavelength of light emitted from the quantum dots 5 can be controlled by controlling the particle size of the core. Therefore, by controlling the particle size of the quantum dots 5, the wavelength of light emitted by the light-emitting element 101 can be controlled.

[0015] The medium 6 is located between the quantum dots 5. The medium 6 includes a p-type semiconductor. The medium 6 may include an organic semiconductor such as p-TPD, TFB, or TAPC. p-TPD is an abbreviation for poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine]. TFB is an abbreviation for poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl))diphenylamine)]. TAPC is an abbreviation for di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane. Hereinafter, the p-type semiconductor included in the medium 6 may be referred to as semiconductor P.

[0016] The electron transport layer 4 is located between the light-emitting layer 3 and the cathode 2. The electron transport layer 4 includes an n-type semiconductor. An example of a material for the electron transport layer 4 is ZnMgO. Hereinafter, the n-type semiconductor included in the electron transport layer 4 may be referred to as a semiconductor N.

[0017] The work function of the medium 6 is greater than the work function of the electron transport layer 4. The difference between the work function of the electron transport layer 4 and the work function of the cathode 2 may be 0.2 eV or less.

[0018] The work function can be defined as the difference between the vacuum level and the Fermi level. One method for measuring the work function of a semiconductor is to use a Kelvin probe force microscope (KFM). KFM uses a probe with a known work function to measure the difference in work function between the probe and a sample. If the resolution of the work function measured by KFM is approximately 0.1 eV, the work function of the sample can be sufficiently calculated from the difference in the measured work functions. The work function described herein may be measured by KFM on the surface of a single-layer sample prepared under the same conditions. To measure the work function of each layer of the light-emitting element 101, a method is available in which KFM is used on a cross section. By cleaving the light-emitting element 101 using a focused ion beam (FIB) or the like and performing KFM measurement on the cross section, it is possible to distinguish between the medium material 6, the electron transport layer 4, and the quantum dots 5 and measure their respective work functions.

[0019] The medium material 6 may be an organic semiconductor. The medium material 6 may contain a chalcogen element. Examples of chalcogen elements include oxygen and sulfur.

[0020] The difference between the work function of the medium 6 and the work function of the electron transport layer 4 may be 0.2 eV or more, which makes it possible to sufficiently increase the efficiency of electron injection into the quantum dots 5.

[0021] The electron affinity of the medium 6 may be smaller than the electron affinity of any of the quantum dots 5, and the ionization potential of the medium 6 may be larger than the ionization potential of any of the quantum dots 5. This can reduce energy transfer from the quantum dots 5.

[0022] 2 is a flowchart showing a method for manufacturing the light-emitting element 101. The method for manufacturing the light-emitting element 101 includes steps S1 and S2. Step S1 is a step of forming a light-emitting layer 3 having a plurality of quantum dots 5 and a medium material 6 containing a p-type semiconductor and positioned between the plurality of quantum dots 5. Step S2 is a step of forming an electron transport layer 4 containing an n-type semiconductor. The work function of the medium material 6 is larger than the work function of the electron transport layer 4. Either step S1 or step S2 may be performed first.

[0023] FIG. 3 is a side view illustrating an example of a method for forming the medium 6. In the method for manufacturing the light-emitting element 101, the medium 6 may be formed by doping the base material 7. FIG. 4 is a side view illustrating another example of a method for forming the medium 6. In the method for manufacturing the light-emitting element 101, the medium 6 may be formed by exposing the base material 7 to the atmosphere. The material of the base material 7 may be an organic semiconductor such as p-TPD, TFB, or TAPC. The time for exposing the base material 7 to the atmosphere may be 5 minutes or more and 10 minutes or less. The timing for exposing the base material 7 to the atmosphere may be before or after forming the electron transport layer 4.

[0024] The semiconductor P forms a pn junction with the semiconductor N. Here, the pn junction refers to the formation of a pn junction-like charge double layer by injecting electrons present in the semiconductor N into the semiconductor P, and the semiconductors P and N do not necessarily have to be epitaxially joined.

[0025] The medium 6 may contain a highly electron-accepting material such as F4-TCNQ and tris(pentafluorophenyl)borane, or a chalcogen element. F4-TCNQ is an abbreviation for 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane. A highly electron-accepting material such as F4-TCNQ and tris(pentafluorophenyl)borane, or a chalcogen element, functions as a dopant that extracts electrons from the base material 7 and generates holes. The presence of a dopant makes it possible to realize a medium 6 with a large work function. The work function of the medium 6 can be interpreted as the work function obtained by changing the work function of the base material 7 due to the presence of a dopant. The concentration of the dopant in the medium 6 is 10 １５ pieces / cm ３ Greater than or equal to 10 ２２ pieces / cm ３ It may be the following:

[0026] The raw material of the medium 6 must be soluble in the quantum dot dispersion liquid in which a large number of quantum dots 5 are dispersed. The medium 6 is optimally an organic semiconductor that does not impair the film-forming properties of the quantum dot dispersion liquid and can densely fill the spaces between the multiple quantum dots 5. NiO nanoparticles, which are inorganic semiconductors that can be applied to form films, cannot densely fill the spaces between the multiple quantum dots 5, and there is a risk that the pn junction between the semiconductor P and the semiconductor N will not be good.

[0027] 5 is an example of a bandgap diagram of the light-emitting layer 3 and the electron transport layer 4. For simplicity, the reference symbols of the components described above are used in the same way as the reference symbols of the bandgaps of the components. The medium material 6 before the pn junction in FIG. 5 can be replaced with a base material 7.

[0028] At the pn junction between the semiconductors P and N, band bending occurs due to the diffusion potential, reducing the injection of electrons 53 from the electron transport layer 4 into the medium 6. If the work function of the electron transport layer 4 and the work function of the cathode 2 are approximately the same, specifically if the difference between them is 0.2 eV or less, the energy level of the medium 6 shifts upward by the amount of band bending. Since the density of electrons 53 on the electron transport layer 4 side is sufficiently larger than the hole density in the medium 6, the work function on the electron transport layer 4 side is fixed and decreases by the work function on the medium 6 side. As a result, the electron injection barrier from the electron transport layer 4 to the medium 6 decreases due to band bending by qV Ｄ The ionization potential of the medium 6 also increases by qV Ｄ Therefore, if the ionization potential of the pn junction becomes too small compared to the ionization potential of the quantum dots 5, the hole injection performance deteriorates. Ｄ The difference between the reduced size and the ionization potential of the quantum dot 5 may be 0.2 eV or less.

[0029] The probability of electron injection into the medium 6 can be estimated by a Boltzmann function. Therefore, the difference between the electron affinity of the medium 6 before the pn junction and the electron affinity of the electron transport layer 4 is defined as ΔE ｃ Then, the injected charge amount n ｐcan be expressed as equation (1) in FIG. ０ is the carrier density in the electron transport layer 4, and k Ｂ T is the product of Boltzmann's constant and temperature. Regarding electron injection into the quantum dot 5, the injection barrier φ Ｂ Therefore, the injected charge amount n ＱＤ With respect to this, the formula (2) in FIG. 6 holds.

[0030] The charge injection efficiency η into the quantum dot 5 with respect to the medium 6 is defined as in equation (3) in FIG. 6. The charge injected into the medium 6 is ΔE ｃ or qV Ｄ It can be exponentially reduced by increasing ΔE ｃ is determined by the combination of the material of the medium 6 and the material of the electron transport layer 4, but considering factors such as charge mobility, process resistance, and reliability, the number of practical combinations of the material of the medium 6 and the material of the electron transport layer 4 is limited to a small number. Ｄ is a value determined by the difference between the Fermi level of the medium 6 and the Fermi level of the electron transport layer 4, and can be easily controlled by the concentration of the dopant in the medium 6. Ｆ，ｐ indicates the Fermi level of the medium 6, and E Ｆ，ｎ denotes the Fermi level of the electron transport layer 4, and E Ｆ indicates the Fermi level when the light emitting layer 3 and the electron transport layer 4 after joining are viewed as one component.

[0031] Organic semiconductors are intrinsic semiconductors, and the Fermi level exists in the center of the band gap, but the work function can be increased by doping with chalcogen elements, etc. For example, when forming a medium material 6 containing oxygen, the medium material 6 can be easily formed by simply exposing the base material 7 to the atmosphere. After exposure to the atmosphere, the medium material 6 is thought to contain 500 ppm or more of oxygen atoms chemically and physically.

[0032] (Experiment 1) An experiment was conducted by creating two types of test objects, test object A and test object B, which imitate the light-emitting element 101. For simplicity, in the explanation of the experiment, the names and symbols of the components of the test objects and the names and symbols of the components of the light-emitting element 101 will be consistent.

[0033] Specimen A and specimen B were prepared by the following method. TFB was used as the material for the base material 7 in specimen A, and TAPC was used as the material for specimen B. ZnMgO nanocrystals were used as the material for the electron transport layer 4, and ITO was used as the material for the anode 1. A layer containing quantum dots 5 and base material 7, and the electron transport layer 4 were sequentially formed on the anode 1 by spin coating. After the layer containing quantum dots 5 and base material 7, and the electron transport layer 4 were bonded, the base material 7 was exposed to the atmosphere to form a pn junction.

[0034] FIG. 7 is a bandgap diagram of the test object A. FIG. 8 is a bandgap diagram of the test object B. FIGS. 7 and 8 respectively show the bandgap before and after the pn junction. For simplicity of illustration, the symbols of the components previously described are unified with the symbols of the bandgaps of the components. The medium material 6 before the pn junction in FIGS. 7 and 8 can be replaced with a base material 7.

[0035] After the pn junction, the work function of the medium 6 in the specimen A was 4.53 eV, which was about 0.58 eV higher than the work function of the TFB that was not exposed to the atmosphere. The work function of the medium 6 in the specimen B was 4.91 eV, which was about 1.06 eV higher than the work function of the TAPC that was not exposed to the atmosphere. The work function of the electron transport layer 4 was 4.50 eV.

[0036] After the pn junction, the magnitude qV of the band bending in the laminated structure of the medium material 6 and the electron transport layer 4 is estimated to be 0.03 eV in the specimen A and 0.41 eV in the specimen B.

[0037] Before the pn junction, the difference between the electron affinity of the electron transport layer 4 and the electron affinity of the quantum dots 5 was 1.23 eV. Before the pn junction, the difference between the electron affinity of the electron transport layer 4 and the electron affinity of TFB was 1.25 eV. Before the pn junction, the difference between the electron affinity of the electron transport layer 4 and the electron affinity of TAPC was 1.25 eV. The barrier height after the pn junction is estimated to be 1.21 eV for the quantum dots 5, 1.28 eV for the medium material 6 of the test object A, and 1.66 eV for the medium material 6 of the test object B.

[0038] The charge injection efficiency η into the quantum dots 5 relative to the medium 6 based on the above-mentioned formulas (1) to (3) was approximately 0.88 for the test object A and approximately 1.00 for the test object B.

[0039] From the above, it is clear that it is effective for the work function of the medium 6 to be sufficiently larger than the work function of the electron transport layer 4 .

[0040] Fig. 9 is a graph showing the electron injection efficiency into the quantum dot 5 versus the Fermi level difference (work function difference) between the medium material 6 and the electron transport layer 4. Fig. 9 shows a case where the injection barrier from the electron transport layer 4 to the medium material 6 (base material 7) before the pn junction is 1.25 eV. According to Fig. 9, the Fermi level difference between the medium material 6 and the electron transport layer 4 is preferably 0.2 eV or more. In other words, the difference between the work function of the medium material 6 and the work function of the electron transport layer 4 is preferably 0.2 eV or more.

[0041] (Experiment 2) Three types of test objects, test objects C to E, which imitate the light emitting element 101, were created and an experiment was carried out.

[0042] Each of specimens C to E was prepared by the following method. A hole injection layer 51 was formed on an anode 1, followed by a hole transport layer 52. A light-emitting layer 3 and an electron transport layer 4 were formed on the hole transport layer 52. A cathode 2 was formed on the electron transport layer 4. The material of the anode 1 was ITO. The material of the hole injection layer 51 was NiO. The material of the hole transport layer 52 was TFB. The material of the electron transport layer 4 was ZnMgO nanocrystals. The material of the cathode 2 was Ag.

[0043] The specimen C was prepared by the following method: A light-emitting layer 3 containing a plurality of quantum dots 5 but not containing a medium material 6 or a base material 7 was formed.

[0044] Test specimens D and E were each created by the following method. A layer of multiple quantum dots 5 and a base material 7 was formed on a hole transport layer 52. An electron transport layer 4 was formed on this layer. The base material 7 was exposed to the atmosphere to form a medium material 6. For test specimens D and E, the material of the electron transport layer 4 was ZnMgO nanocrystals. Therefore, even if the base material 7 was exposed to the atmosphere after the formation of the electron transport layer 4, oxygen could be supplied to the base material 7 through the gaps between the multiple ZnMgO nanocrystals. The material of the base material 7 was TFB in test specimen D and TAPC in test specimen E. In test specimens D and E, the weight ratio of the medium material 6 to the total weight of the multiple quantum dots 5 was 10%.

[0045] Fig. 10 is a graph showing current density and luminance versus applied voltage for test objects C to E. Fig. 11 is a graph showing EQE versus applied voltage for test objects C to E. Fig. 12 is a graph showing EQE versus current density for test objects C to E. EQE stands for external quantum efficiency.

[0046] 10 to 12 show that the drive voltage of the test object D is higher than that of the test object C. 10 to 12 also show that in the low-voltage region where the applied voltage is 3 V or less, the current density of the test object D is higher than that of the test object C. These suggest that the current injection into the medium material 6 reduces the efficiency of charge injection into the quantum dots 5.

[0047] 10 to 12, it can be seen that the drive voltage of the test object E is approximately the same as the drive voltage of the test object C. It can also be seen from Figures 10 to 12 that in the low voltage region, the current density of the test object E is lower than the current density of the test object C. This suggests that the pn junction between the semiconductors P and N suppresses the injection of electrons into the medium material 6.

[0048] The reason why the current density of the test object E is lower than that of the test object C in the low voltage region is thought to be as follows: By filling the spaces between the multiple quantum dots 5 with the medium material 6, the leakage current through the surfaces of the quantum dots 5 and the leakage current through the ligands coordinated to the quantum dots 5 are reduced.

[0049] The maximum value of EQE in the test object E is greater than the maximum value of EQE in the test object C, and is greater than the maximum value of EQE in the test object D. This is the result of the improved efficiency of charge injection into the quantum dots 5 in the test object E.

[0050] (Experiment 3) Five types of test objects, test objects F to J, which imitate the light emitting elements 101, were created and an experiment was carried out.

[0051] Each of specimens F to J was prepared by the following method. A hole injection layer 51 was formed on an anode 1, followed by a hole transport layer 52. A light-emitting layer 3 and an electron transport layer 4 were formed on the hole transport layer 52. A cathode 2 was formed on the electron transport layer 4. The material of the anode 1 was ITO. The material of the hole injection layer 51 was NiO. The material of the hole transport layer 52 was TFB. The material of the electron transport layer 4 was ZnMgO nanocrystals. The material of the cathode 2 was Ag.

[0052] The specimen F was prepared by the following method: A light-emitting layer 3 containing a plurality of quantum dots 5 but not containing a medium material 6 or a base material 7 was formed.

[0053] Each of specimens G to I was prepared by the following method. A layer of a plurality of quantum dots 5 and a base material 7 was formed on a hole transport layer 52. An electron transport layer 4 was formed on this layer. The base material 7 was exposed to the atmosphere to form a medium material 6. The material of the base material 7 was TAPC. The weight ratio of TAPC (medium material 6) to the total weight of the plurality of quantum dots 5 was 5% for specimen G, 10% for specimen H, and 20% for specimen I.

[0054] The specimen J was prepared by the following method. A layer of a base material 7 not including a plurality of quantum dots 5 was formed on a hole transport layer 52. An electron transport layer 4 was formed on this layer. The base material 7 was exposed to the atmosphere to form a medium material 6. The material of the base material 7 was TAPC.

[0055] Fig. 13 is a graph showing current density and luminance versus applied voltage for test objects F to J. Fig. 14 is a graph showing EQE versus applied voltage for test objects F to J. Fig. 15 is a graph showing EQE versus current density for test objects F to J.

[0056] 13 to 15, the test objects H and I exhibited the effect of reducing current injection into the medium material 6 due to the pn junction of the semiconductors P and N, and in particular, the maximum value of EQE was the largest for test object I. For test object F, the current density was significantly high in the low voltage region, and it was found that the reduction effect was not fully exerted.

[0057] 13, the characteristics of the current density with respect to the applied voltage in the low voltage region are similar between the test objects F and J. This characteristic is considered to be due to charge recombination occurring at the pn junction between the semiconductors P and N.

[0058] According to FIG. 13, in the test objects H and I, the characteristics thought to be caused by the charge recombination remain in the region where the applied voltage is 2 V or less, and it can be said that even in the region where the applied voltage is 2 V to 3 V, the current due to charge injection into the quantum dot 5 is dominant.

[0059] The reason why the weight of TAPC (medium 6) relative to the total weight of the quantum dots 5 causes the above difference is considered to be as follows: If the weight of TAPC (medium 6) is small, it is difficult to supply enough holes to the electron transport layer 4 to generate a diffusion potential, and band bending does not occur sufficiently.

[0060] From the above results, it is preferable that the weight ratio of TAPC (medium material 6) to the total weight of the plurality of quantum dots 5 is 10% or more.

[0061] The weight ratio of the medium material 6 to the total weight of the plurality of quantum dots 5 may be 10% or more and 20% or less. The volume ratio of the medium material 6 to the total volume of the plurality of quantum dots 5 may be 50% or more and 100% or less. The weight ratio may be measured by TOF-SIMS (time-of-flight secondary ion mass spectrometry), and the volume ratio may be measured by cross-sectional SEM / EDX (scanning electron microscope / energy dispersive X-ray spectroscopy). The cross-sectional area ratio of the quantum dots 5 to the medium material 6 measured by cross-sectional SEM / EDX may be considered to be the volume ratio.

[0062] 16 is a cross-sectional view showing a schematic configuration of a display device 201 according to a second embodiment of the present disclosure. The display device 201 includes a light-emitting element 101.

[0063] The number of light-emitting elements 101 in the display device 201 is not particularly limited, but in Fig. 16, the number of light-emitting elements 101 in the display device 201 is three. In other words, the display device 201 includes a first light-emitting element 8, a second light-emitting element 9, and a third light-emitting element 10, and each of the first light-emitting element 8, the second light-emitting element 9, and the third light-emitting element 10 is a light-emitting element 101.

[0064] The band gap of the quantum dots 5 of the first light-emitting element 8 may be 1.8 eV or more and 2.1 eV or less, the band gap of the quantum dots 5 of the second light-emitting element 9 may be 2.1 eV or more and 2.5 eV or less, and the band gap of the quantum dots 5 of the third light-emitting element 10 may be 2.5 eV or more and 3.2 eV or less.

[0065] The electron transport layer 4 of the first light-emitting element 8, the electron transport layer 4 of the second light-emitting element 9, and the electron transport layer 4 of the third light-emitting element 10 may be the same layer and made of the same material. This allows the electron transport layer 4 of the first light-emitting element 8, the electron transport layer 4 of the second light-emitting element 9, and the electron transport layer 4 of the third light-emitting element 10 to be formed collectively, thereby reducing the manufacturing steps of the display device 201. However, it is not essential that the electron transport layer 4 of the first light-emitting element 8, the electron transport layer 4 of the second light-emitting element 9, and the electron transport layer 4 of the third light-emitting element 10 be the same layer or made of the same material. For example, one of the materials of the electron transport layer 4 of the first light-emitting element 8, the electron transport layer 4 of the second light-emitting element 9, and the electron transport layer 4 of the third light-emitting element 10 may be different from the other materials. As a more specific example of the material of the electron transport layer 4, Zn is used in each of the first light-emitting element 8 and the second light-emitting element 9. ０．９０ Mg ０．１０ O is Zn in the third light-emitting element 10. ０．８５ Mg ０．１５ O are listed respectively.

[0066] The band gap of the medium material 6 of the first light-emitting element 8 may be larger than the band gap of the quantum dots 5 of the first light-emitting element 8, the band gap of the medium material 6 of the second light-emitting element 9 may be larger than the band gap of the quantum dots 5 of the second light-emitting element 9, and the band gap of the medium material 6 of the third light-emitting element 10 may be larger than the band gap of the quantum dots 5 of the third light-emitting element 10.

[0067] Three types of display devices 201, namely, display device K to display device M, are shown as examples.

[0068] The materials of each of the display devices K to M are as follows. The material of the anode 1 is ITO. The material of the hole injection layer 51 is NiO. The material of the hole transport layer 52 is TFB. The material of the quantum dots 5 is InP in each of the first light-emitting element 8 and the second light-emitting element 9, and is ZnSeTe in the third light-emitting element 10. The material of the electron transport layer 4 is ZnMgO. The material of the cathode 2 is Ag.

[0069] In display device K, the first light-emitting element 8, the second light-emitting element 9, and the third light-emitting element 10 all include a medium 6 having TAPC as the base material 7. In display device L, the first light-emitting element 8 and the second light-emitting element 9 include a medium 6 having TFB as the base material 7, and the third light-emitting element 10 includes a medium 6 having TAPC as the base material 7. In display device M, the first light-emitting element 8 and the second light-emitting element 9 do not include the medium 6 or the base material 7, and the third light-emitting element 10 includes a medium 6 having TAPC as the base material 7.

[0070] All of the first light-emitting element 8, the second light-emitting element 9, and the third light-emitting element 10 may include the medium material 6. The first light-emitting element 8 and the second light-emitting element 9, which have a small band gap, may be configured to include the medium material 6 having TFB as the base material 7, which has a relatively small effect of reducing electron injection into the medium material 6 due to the pn junction of the semiconductor P and the semiconductor N. In this case, the range of material options for the first light-emitting element 8 and the second light-emitting element 9 is broadened, which is effective in terms of the manufacturing method and reliability of the light-emitting element 101.

[0071] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

[0072] REFERENCE SIGNS LIST 1 anode 2 cathode 3 light-emitting layer 4 electron transport layer 5 quantum dots 6 medium material 7 base material 8 first light-emitting element 9 second light-emitting element 10 third light-emitting element 51 hole injection layer 52 hole transport layer 53 electrons 101 light-emitting element 201 display device

Claims

1. A light-emitting device comprising: an anode and a cathode; a light-emitting layer located between the anode and the cathode; and an electron transport layer located between the light-emitting layer and the cathode and including an n-type semiconductor, wherein the light-emitting layer has a plurality of quantum dots and a medium material located between the plurality of quantum dots and including a p-type semiconductor, and the work function of the medium material is greater than the work function of the electron transport layer.

2. The light-emitting device according to claim 1, wherein the difference between the work function of said electron transport layer and the work function of said cathode is 0.2 eV or less.

3. The light-emitting element according to claim 1 or 2, wherein the medium material is an organic semiconductor.

4. The light-emitting element according to any one of claims 1 to 3, wherein the medium material contains a chalcogen element.

5. The light-emitting device according to any one of claims 1 to 4, wherein the difference between the work function of the medium material and the work function of the electron transport layer is 0.2 eV or more.

6. A light-emitting element according to any one of claims 1 to 5, wherein the electron affinity of the medium material is smaller than the electron affinity of any of the plurality of quantum dots, and the ionization potential of the medium material is larger than the ionization potential of any of the plurality of quantum dots.

7. The light-emitting device according to any one of claims 1 to 6, wherein the weight ratio of the medium material to the total weight of the plurality of quantum dots is 10% or more and 20% or less.

8. The light-emitting device according to any one of claims 1 to 7, wherein the volume ratio of the medium material to the total volume of the plurality of quantum dots is 50% or more and 100% or less.

9. The light-emitting element of claim 1, wherein the difference between the work function of the electron transport layer and the work function of the cathode is 0.2 eV or less, the medium material is an organic semiconductor, the medium material contains a chalcogen element, the difference between the work function of the medium material and the work function of the electron transport layer is 0.2 eV or more, the electron affinity of the medium material is smaller than the electron affinity of any of the plurality of quantum dots, and the ionization potential of the medium material is larger than the ionization potential of any of the plurality of quantum dots.

10. A display device comprising a light-emitting element according to any one of claims 1 to 9.

11. A display device comprising a first light-emitting element, a second light-emitting element, and a third light-emitting element, each of which is a light-emitting element according to any one of claims 1 to 9, wherein the band gap of the quantum dots of the first light-emitting element is 1.8 eV or more and 2.1 eV or less, the band gap of the quantum dots of the second light-emitting element is 2.1 eV or more and 2.5 eV or less, and the band gap of the quantum dots of the third light-emitting element is 2.5 eV or more and 3.2 eV or less.

12. The display device according to claim 11, wherein the electron transport layer of the first light-emitting element, the electron transport layer of the second light-emitting element, and the electron transport layer of the third light-emitting element are the same layer and made of the same material.

13. A display device as described in claim 11 or 12, wherein the band gap of the medium material of the first light-emitting element is larger than the band gap of the quantum dots of the first light-emitting element, the band gap of the medium material of the second light-emitting element is larger than the band gap of the quantum dots of the second light-emitting element, and the band gap of the medium material of the third light-emitting element is larger than the band gap of the quantum dots of the third light-emitting element.

14. A method for manufacturing a light-emitting device, comprising: forming a light-emitting layer having a plurality of quantum dots and a medium material located between the plurality of quantum dots and containing a p-type semiconductor; and forming an electron transport layer containing an n-type semiconductor, wherein the work function of the medium material is greater than the work function of the electron transport layer.

15. The method for producing a light-emitting device according to claim 14, wherein the medium material is formed by doping a base material.

16. The method for manufacturing a light-emitting element according to claim 14, wherein the medium material is formed by exposing a base material to the atmosphere.

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