Light-emitting element and display device

Abstract

A light-emitting element (ED) according to the present disclosure comprises: an anode (AD); a cathode (CD) that contains at least one element selected from the group consisting of gold, tungsten, molybdenum, chromium, cobalt, nickel, platinum, copper, titanium, and zirconium; a light-emitting layer (Em) that is positioned between the anode (AD) and the cathode (CD) and contains quantum dots (QD); and a charge functional layer (CF) that is positioned between the light-emitting layer (Em) and the cathode (CD), is in contact with the cathode (CD), and contains an n-type semiconductor material.

Description

Light-emitting element and display device

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

[0002] The quantum dot light-emitting device disclosed in Patent Document 1 includes a first electrode layer, a quantum dot light-emitting layer, an electron transport layer, a second electrode layer, and a third electrode layer, which are stacked in this order, and the side of the third electrode layer that is away from the first electrode layer is configured as a light-emitting side, the second electrode layer and the third electrode layer are transparent electrode layers, and the work function of the second electrode layer is larger than the LUMO energy level of the electron transport layer and smaller than the work function of the third electrode layer.

[0003] Japan Special Publication No. 2022-516211

[0004] Patent Document 1 discloses that the disclosed configuration is advantageous for electron injection into the light-emitting layer. However, it is becoming known that an excess of electrons in the light-emitting layer reduces the external quantum efficiency (EQE) of the light-emitting device.

[0005] In order to solve the above problems, a light-emitting element according to one embodiment of the present disclosure includes an anode, a cathode containing at least one selected from the group consisting of gold, tungsten, molybdenum, chromium, cobalt, nickel, platinum, copper, titanium, and zirconium, a light-emitting layer located between the anode and the cathode and containing quantum dots, and a charge functional layer located between the light-emitting layer and the cathode, in contact with the cathode, and containing an n-type semiconductor material.

[0006] In order to solve the above problems, a light-emitting element according to one aspect of the present disclosure includes an anode, a cathode containing titanium oxide or zirconium oxide, a light-emitting layer located between the anode and the cathode and containing quantum dots, and a charge functional layer located between the light-emitting layer and the cathode, in contact with the cathode, and containing an n-type semiconductor material.

[0007] In order to solve the above problems, a display device according to an aspect of the present disclosure includes a light-emitting element according to an aspect of the present disclosure.

[0008] According to a configuration according to one aspect of the present disclosure, the EQE of the light-emitting element can be improved.

[0009] 1 is a schematic diagram illustrating an example of a stacked structure of a light-emitting device according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram illustrating an example of the Fermi level and band gap of materials used in the light-emitting device. FIG. 1 is a schematic diagram illustrating a band structure of a light-emitting device according to an example. FIG. 1 is a schematic diagram illustrating a band structure of a light-emitting device according to an example when a current is passed from the light-emitting layer to the cathode. FIG. 1 is a schematic diagram illustrating a band structure of a light-emitting device according to a comparative example. FIG. 1 is a schematic diagram illustrating a band structure of a light-emitting device according to a comparative example when a current is passed from the light-emitting layer to the cathode. FIG. 2 is a diagram illustrating the results of examining the current density and EQE for a light-emitting device according to a comparative example in which the light-emitting layer contains quantum dots and the work function of the cathode is smaller than the work function of the electron transport layer. FIG. 3 is a diagram illustrating the current density-EQE characteristics of a semiconductor LED. FIG. 4 is a diagram illustrating an abc model of a semiconductor LED. FIG. 5 is a schematic diagram illustrating an example of a stacked structure of a light-emitting device according to an embodiment of the present disclosure. FIG. 6 is a schematic diagram illustrating an example of a band structure of the light-emitting device shown in FIG. 10. FIG. 7 is a schematic diagram illustrating an example of a stacked structure of a light-emitting device according to an embodiment of the present disclosure. FIG. 13 is a schematic diagram illustrating an example of a band structure of the light-emitting element shown in FIG. 12. FIG. 14 is a schematic diagram illustrating an example of a stacked structure of the light-emitting element according to an embodiment of the present disclosure. FIG. 15 is a schematic diagram illustrating an example of a band structure of the light-emitting element shown in FIG. 14. FIG. 16 is a diagram illustrating the results of examining the current density and EQE for a light-emitting element according to a comparative example and a light-emitting element according to an example of the present disclosure. FIG. 17 is a schematic diagram illustrating an example of the configuration of a display device according to one aspect of the present disclosure.

[0010] [Embodiment 1] (Configuration of Light-Emitting Element) Figure 1 is a schematic diagram illustrating an example of a stacked structure of a light-emitting element according to an embodiment of the present disclosure. As shown in Figure 1, the light-emitting element ED according to the present disclosure includes an anode AD, a cathode CD, an emitting layer Em located between the anode AD and the cathode CD and including quantum dots QDs, and a charge functional layer CF located between the emitting layer Em and the cathode CD, in contact with the cathode CD, and including an n-type semiconductor material. The charge functional layer CF may be an electron transport layer ETL. The light-emitting element ED may optionally include a hole injection layer HIL and / or a hole transport layer HTL located between the anode AD and the emitting layer Em.

[0011] The anode AD may include indium tin oxide (InTiO), indium zinc oxide (InZnO), indium gallium zinc oxide (InGaZnO), aluminum (Al), silver (Ag), etc. The anode AD may be a light-reflective electrode.

[0012] The cathode CD contains at least one metal element selected from the group consisting of gold (Au), tungsten (W), molybdenum (Mo), chromium (Cr), cobalt (Co), nickel (Ni), platinum (Pt), copper (Cu), titanium (Ti), and zirconium (Zr). The cathode CD may be composed of a single metal containing the selected metal element, or an alloy containing two or more selected metal elements. The cathode CD may be composed of an alloy containing one or more selected metal elements and any one or more metal elements. The alloy may be a solid solution, an intermetallic compound, or a eutectic. Alloys containing at least one metal element selected from the listed metal elements tend to have a relatively large work function. The material of the cathode CD may be selected so that the work function of the cathode CD is greater than that of silver.

[0013] Furthermore, elemental metals such as gold, tungsten, molybdenum, chromium, nickel, platinum, copper, titanium, and zirconium have the advantage of being highly stable and resistant to deterioration over long periods of time. Many alloys containing at least one metal element selected from these metals are similarly resistant to deterioration. This stabilizes the performance of the light-emitting element ED, particularly the external quantum efficiency (EQE), and improves the reliability of the light-emitting element ED. On the other hand, silver and aluminum, which have a smaller work function than silver, are prone to deterioration. Silver is prone to sulfurization, and the sulfide coating appears black, reducing the light transmittance of the silver electrode. Aluminum is prone to oxidation, and the oxide coating scatters light, making it appear white. Many alloys containing silver or aluminum are also prone to deterioration. Therefore, light-emitting elements using silver or aluminum as electrodes experience a decrease in light extraction efficiency and a decrease in EQE over time.

[0014] The cathode CD is made of titanium oxide (TiO ２) and zirconium oxide (ZrO ２ The work function of titanium oxide is about 6.2 eV, and the work function of zirconium oxide is about 5.8 eV, both of which are larger than the work function of silver. Both titanium oxide and zirconium oxide are highly stable and resistant to deterioration.

[0015] The cathode CD may be a light-transmitting electrode. Light can be sufficiently transmitted through a silver thin film having a thickness of 10 nm or less. Therefore, the thickness of the cathode CD is at least 20 nm or less, preferably 10 nm or less, and more preferably equal to or less than the threshold calculated by the following formula (1). The thickness of the cathode CD may be 0.5 nm or more.

[0016] Threshold = 10 nm × extinction coefficient of silver ÷ extinction coefficient of cathode (1) The charge functional layer CF may be configured so that quantum size effects do not occur. The charge transport material contained in the charge functional layer CF may be formed in particulate form, in which case the particle size of the material is preferably greater than twice the exciton Bohr radius of the material. The charge transport material contained in the charge functional layer CF may be formed in thin film form, in which case the size of the material in the in-plane direction perpendicular to the film thickness direction is greater than twice the Bohr radius. It is preferable that the charge transport material be formed in thin film form rather than particulate form.

[0017] The charge functional layer CF may be composed of a II-VI compound semiconductor material composed of a Group II element and a Group VI element. The charge functional layer CF may include, for example, at least one material selected from the group consisting of zinc oxide, zinc magnesium oxide, zinc sulfide, and zinc magnesium sulfide. In this disclosure, the notation of element group numbers using Roman numerals is based on the old IUPAC (International Union of Pure and Applied Chemistry) system or the old CAS (Chemical Abstracts Service) system, and the notation of element group numbers using Arabic numerals is based on the new IUPAC system.

[0018] The thickness of the charge functional layer CF may be 5 nm or more and 40 nm or less.

[0019] Example 1 A plurality of light-emitting elements ED according to Example 1 of the present disclosure were fabricated as follows. Each of the light-emitting elements ED according to Example 1 had the configuration shown in Fig. 1. The quantum dots QD according to Example 1 were core-shell type quantum dots having a core made of InP and a shell made of zinc sulfide (ZnS), and emitted blue light.

[0020] An InTiO wiring layer is formed as an anode AD on a thin film transistor (TFT) substrate, followed by the formation of a bank separating the light-emitting elements ED. A wiring pattern is transferred to a photoresist material using standard photolithography techniques, and an InTiO film is then formed by sputtering or vacuum deposition, so that each anode AD is connected to the drain electrode of the corresponding TFT. Using photolithography or a metal mask, a bank is formed using a resin material such as polyimide.

[0021] Next, a nickel oxide (NiO) film is formed as the hole injection layer (HIL) by coating or inkjet printing a colloidal solution of dispersed NiO nanoparticles. The particle size of the NiO nanoparticles is preferably approximately 6 nm or greater, so that quantum size effects do not occur. Alternatively, the NiO film may be formed by sputtering or vacuum deposition. Using any of the coating, printing, and deposition techniques, a NiO film can be formed to a substantially uniform thickness across the entire panel, including the bank slopes. Next, a crosslinked TFB film is formed as the hole transport layer (HTL) by coating or inkjet printing. Organic materials are generally considered suitable for the hole transport layer (HTL). The highest occupied molecular orbital (HOMO) of the hole transport layer (HTL) is preferably close to the VBM of the quantum dots (QDs) to reduce the hole injection barrier. The lowest unoccupied molecular orbital (LUMO) of the hole transport layer HTL is preferably closer to the vacuum level than the central molecular orbital (CBM) of the quantum dots QDs in order to reduce electron overflow from the quantum dots QDs. Therefore, the material of the hole transport layer HTL is preferably, for example, TFB, PVK, p-TPD, etc.

[0022] Next, quantum dots QD are formed as the light-emitting layer Em by coating or printing a QD-dispersed colloidal solution. The light-emitting layer Em may contain an organic ligand or an inorganic insulating material such as an inorganic semiconductor or silicon oxide. The organic ligand, inorganic semiconductor, or inorganic insulating material protects the quantum dots. From the perspective of the reliability of the light-emitting element ED, a configuration in which the quantum dots are protected with an inorganic substance is more desirable. Conventional methods can be used to remove the organic ligand and protect the quantum dots QD with an inorganic substance. After the light-emitting layer Em is formed, unnecessary portions of the light-emitting layer Em, such as portions between the light-emitting elements ED, are removed using conventional photolithography techniques. When manufacturing a display device using the three primary colors of RGB, this process is repeated for the red light-emitting layer, green light-emitting layer, and blue light-emitting layer.

[0023] Next, a metal oxide such as ZnO or ZnMgO is deposited as the electron transport layer ETL. For example, the electron transport layer ETL is deposited by coating or printing a colloidal solution of ZnO or ZnMgO nanoparticles. As with NiO nanoparticles, the particle size of the ZnO nanoparticles and ZnMgO nanoparticles is preferably approximately 6 nm or greater, so that the quantum size effect does not occur. Alternatively, the metal oxide may be deposited by sputtering or vacuum deposition. With any of the coating, printing, and deposition methods, the metal oxide can be deposited to a substantially uniform thickness across the entire panel, including the bank slopes.

[0024] Subsequently, a nickel (Ni) film was formed as a cathode CD by a common method such as vapor deposition or sputtering, and the entire panel including the plurality of light-emitting elements ED was sealed.

[0025] Generally, when the particle diameter is less than twice the Bohr radius, the quantum size effect becomes significant. Specifically, exciton confinement occurs, forming quantum levels in the valence band (VB) and conduction band (CB). In semiconductor materials used in the light-emitting layer, hole transport layer (HTL), and electron transport layer (ETL), the effective mass of holes is often significantly larger than the effective mass of electrons. Furthermore, the difference between the quantum levels nearest to the CBM in the CB is often larger than the difference between the quantum levels nearest to the VBM in the VB, resulting in a large quantum size effect in the conduction band. The more significant the exciton confinement effect due to the quantum size effect, the higher the probability of trion Auger generation. Therefore, it is desirable that each layer other than the light-emitting layer (Em) be either (1) made of a particulate material with a particle diameter greater than twice the Bohr radius, or (2) be a thin film without using a particulate material.

[0026] Example 2 A plurality of light-emitting elements 100 according to Example 2 were fabricated in substantially the same manner as the light-emitting element ED according to Example 1 described above. Each of the light-emitting elements ED according to Example 2 had the configuration shown in FIG. 1 . Each layer of the light-emitting element ED according to Example 2, except for the electron transport layer ETL, was formed by the same method, with the same material, and to the same film thickness as each layer of the light-emitting element ED according to Example 1, except for the electron transport layer ETL. A metal sulfide such as ZnS or ZnMgS was formed as the electron transport layer ETL according to Example 2, and the electron transport layer ETL according to Example 2 was formed by the same method and to the same film thickness as the electron transport layer ETL according to Example 1.

[0027] (Example 3) A plurality of light-emitting elements 100 according to Example 3 were fabricated in substantially the same manner as the light-emitting element ED according to Example 1 described above. Each of the light-emitting elements ED according to Example 3 had the configuration shown in FIG. 1. Each layer of the light-emitting element ED according to Example 3, except for the electron transport layer ETL, was formed by the same method, using the same material, and to the same film thickness as each layer of the light-emitting element ED according to Example 1, except for the electron transport layer ETL. The electron transport layer ETL according to Example 3 was formed by the same method, using the same material, and to various average film thicknesses ranging from 3 nm to 100 nm as the electron transport layer ETL according to Example 1.

[0028] Comparative Example 1 A plurality of light-emitting elements 100 according to Comparative Example 1 were fabricated in a manner generally similar to that of the light-emitting element ED according to Example 1. The light-emitting element 100 according to Comparative Example 1 included an anode 101, a hole injection layer 102, a hole transport layer 103, an emitting layer including quantum dots 104, an electron transport layer 105, and a cathode 106. Each layer of the light-emitting element 100 according to Comparative Example 1, except for the cathode 106, was formed by the same method, with the same material, and to the same film thickness as each layer of the light-emitting element ED according to Example 1, except for the cathode CD. An aluminum (Al) film was formed as the cathode 106 according to Comparative Example 1, and the cathode 106 according to Comparative Example 1 was formed by the same method and to the same film thickness as the cathode CD according to Example 1. A metal material with a low work function was used as the material for the cathode 106 according to Comparative Example 1, from the viewpoints of facilitating electron injection into the emitting layer or reducing the driving voltage of the light-emitting element 100. Comparative cathode 106 may include, for example, aluminum (Al), silver (Ag), or magnesium (Mg).

[0029] (Band Structure) FIG. 2 is a schematic diagram showing an example of the Fermi level and band gap of a material used in a light-emitting element. In FIG. 2, the energy level values ​​indicate the electron energy level values, with the vacuum energy level being 0 eV. In FIG. 2 and the figures described below, the Fermi level of a metal is shown by a thick solid line, the Fermi levels of a semiconductor and an insulator are shown by a dashed line, and the band shapes of the semiconductor and the insulator are shown by a rectangle (or a modified rectangle). The band gap is the band gap between the valence band (VB) and the conduction band (CB). The bottom side of the rectangle indicates the valence band maximum (VBM), and the top side of the rectangle indicates the conduction band minimum (CBM). The work function corresponds to the energy difference (absolute value) between the Fermi level and the vacuum level at absolute zero, the electron affinity corresponds to the energy difference (absolute value) between the CBM and the vacuum level, and the ionization potential corresponds to the energy difference (absolute value) between the VBM and the vacuum level.

[0030] The materials shown in Fig. 2 are indium phosphide (InP) in the cores of red quantum dots that emit red light, InP in the cores of green quantum dots that emit green light, InP in the cores of blue quantum dots that emit blue light, zinc oxide (ZnO) and magnesium zinc oxide (MgZnO) used in the electron transport layer ETL, and aluminum (Al), silver (Ag), nickel (Ni), and platinum (Pt) used in the electrodes. ０．１５ Zn ０．８５ The work functions of Al and Ag are small, and the work functions of Ni and Pt are large, compared with the work function of O. Organic materials may be used for the electron transport layer ETL and other components.

[0031] The core of the quantum dot QD is an intrinsic semiconductor, and its Fermi level is located exactly halfway between the VBM and the CBM. The electron transport layer ETL uses n-type semiconductors, such as ZnO and MgZnO, which do not have a very high free electron density, and their Fermi level is located slightly closer to the CBM than halfway between the VBM and the CBM. The electron transport layer ETL may be made of an organic material, such as a triazine-based material or Alq3 (aluminum complex), whose difference between the vacuum level and the LUMO is less than 2.5 eV and which has electron transport properties.

[0032] FIG. 3 is a schematic diagram illustrating the band structure of the light-emitting device according to Example 1. The work function of the cathode CD is greater than the work function of the electron transport layer ETL. This results in a deformation of the band shape of the electron transport layer ETL, as shown in FIG. 3. Because the free electron density of the cathode CD is significantly greater than that of the electron transport layer ETL, the deformation of the band shape due to the junction between the cathode CD and the electron transport layer ETL is primarily observed in the electron transport layer ETL. Due to the influence of the junction with the cathode CD, the CBM and VBM of the electron transport layer ETL are farther from the vacuum level after the junction than before the junction. As a result of this deformation, the energy barrier for electrons between the quantum dots QDs and the electron transport layer ETL becomes larger, and the CBM of the electron transport layer ETL sinks toward the quantum dots QDs. Furthermore, a thin, sharp, spike-like energy barrier for electrons is generated between the cathode CD and the electron transport layer ETL.

[0033] 4 is a schematic diagram showing the band structure from the light-emitting layer to the cathode of the light-emitting device according to Example 1 when a current is applied. As shown in FIG. 4, the band shape is further deformed by the application of current, so that the CBM of the electron transport layer ETL drops more strongly toward the quantum dots QD. In addition, the spike-shaped energy barrier for electrons between the cathode CD and the electron transport layer ETL becomes higher, i.e., thinner and sharper.

[0034] FIG. 5 is a schematic diagram illustrating the band structure of a light-emitting device according to Comparative Example 1. The work function of the cathode 106 is smaller than the work function of the electron transport layer 105. This causes the band shape of the electron transport layer 105 to deform, as shown in FIG. 5. As a result of this deformation, the energy barrier for electrons between the quantum dots 104 and the electron transport layer 105 becomes smaller, and the CBM of the electron transport layer ETL rises toward the quantum dots QD. FIG. 6 is a schematic diagram illustrating the band structure of a light-emitting device according to Comparative Example 1 when current is applied from the light-emitting layer to the cathode. As shown in FIG. 6, electrons are more easily injected from the cathode CD into the quantum dots 104.

[0035] As described above, a metal material with a low work function is used as the material for the cathode 106 in Comparative Example 1 from the viewpoint of promoting electron injection into the light-emitting layer or reducing the driving voltage of the light-emitting element 100. However, when the electrical characteristics and light-emitting characteristics of the light-emitting element 100 were analyzed in relation to each other, it was found that the light-emitting element 100 was operating in a state where the light-emitting layer had an excess of electrons, and that the excess electrons injected into the light-emitting layer had a significant effect on the EQE peak value and roll-off (a decrease in the EQE in the high-energy region).

[0036] Comparing Figures 3 and 4 with Figures 5 and 6, in the light-emitting device 100 of Comparative Example 1, electrons injected from the cathode 106 easily overcome the low, gentle energy barrier, resulting in an excess of electrons in the light-emitting layer. On the other hand, in the light-emitting device ED according to the present disclosure, electrons are blocked by the energy barrier between the quantum dots QD and the electron transport layer ETL, and the energy barrier between the cathode CD and the electron transport layer ETL, and remain there. Electrons are accumulated on the electron transport layer ETL side of the interface between the electron transport layer ETL and the light-emitting layer Em, and on the cathode CD side of the interface between the cathode CD and the electron transport layer ETL. The quantum size effect does not appear in the electron transport layer ETL and the cathode CD, and trion Auger electrons are not generated. In the electron transport layer ETL, as in the semiconductor thin film, 10 １８ ~10 １９ cm －３ Auger recombination occurs only when electrons are accumulated at a high density of 10 １５ ~10１６ cm －３ This reduces the probability of trion Auger electron generation and roll-off due to trion Auger electrons in electron injection at high electron densities. Furthermore, the reduction in electron injection into the quantum dots (QDs) improves the carrier balance, improving the EQE and shifting the EQE peak toward the high-density electron injection. This also reduces the degree of roll-off.

[0037] Therefore, according to the configuration of the present disclosure, the EQE of the light-emitting element ED can be improved.

[0038] Typically, the work function of silver is equal to or greater than that of the electron transport layer ETL. Therefore, conductive materials having a work function greater than that of silver, including but not limited to gold, tungsten, molybdenum, chromium, cobalt, nickel, platinum, copper, titanium, and zirconium, may be used for the cathode CD. For example, silicon (Si), tin (Sn), selenium (Se), and iron (Fe) may be used for the cathode CD. Furthermore, the work function of indium tin oxide is greater than that of silver. Therefore, conductive materials having a work function greater than that of indium tin oxide may be used for the cathode CD.

[0039] Furthermore, Examples 1 and 2 are compared. ZnS or ZnMgS according to Example 2 is a II-VI compound, like ZnO or ZnMgO according to Example 1, but oxygen, a Group VI element, is replaced with sulfur, which is one period lower. Generally, the lower the period in the periodic table, the larger the ionic radius of an element, and the lower the bond energy with other elements (e.g., zinc, a Group II element). The bond energy is reflected in the VBM of the compound, and further in the Fermi level of the compound. Therefore, the ionization potential and work function of ZnS or ZnMgS according to Example 2 are smaller than those of ZnO or ZnMgO according to Example 1.

[0040] As a result, in the light-emitting device ED according to Example 2, the difference in Fermi level between the cathode CD and the electron transport layer ETL is increased compared to Example 1. This increases the energy barrier against electrons between the cathode CD and the electron transport layer ETL, increasing the density of electrons accumulated near the interface between the electron transport layer ETL and the light-emitting layer Em. This further reduces electron injection into the light-emitting layer Em, increases the EQE peak value, and reduces the degree of roll-off.

[0041] Furthermore, Examples 1 and 3 are compared. In Example 3, in the light-emitting element ED in which the average film thickness of the electron transport layer ETL was 3 nm, numerous non-emissive regions occurred in the light-emitting element ED, and the average EQE dropped from 8% to 5% or less. When the cross section of the non-emissive region, mainly the electron transport layer ETL, was observed using a transmission electron microscope (TEM), a portion where the cathode CD was in direct contact with the quantum dots QD was present directly below the non-emissive region. This is presumably because, when the electron transport layer ETL was formed by sputtering, unevenness in film thickness occurred, resulting in the electron transport layer ETL being formed in an island shape. It is presumed that the direct contact deactivated excitons, rendering the quantum dots QD non-emissive.

[0042] On the other hand, in the light-emitting element ED in Example 3 in which the average film thickness of the electron transport layer ETL was 100 nm, many irregularities and cracks occurred on the surface of the electron transport layer ETL, resulting in a light-dark distribution within the light-emitting element ED and between the light-emitting elements ED. Furthermore, unlike the case in which the average film thickness was 3 nm, when the average film thickness was 100 nm, a phenomenon in which the areas around the irregularities and cracks emitted light more brightly was observed. It is presumed that this light-dark distribution occurred as a result of localized electric field concentration occurring at the edges around the irregularities and cracks, which increased the injection density of electrons and holes.

[0043] Further observation of light-emitting devices ED with various average thicknesses of the electron transport layer ETL revealed that the thickness of the electron transport layer ETL is preferably 5 nm to 40 nm. When the thickness of the electron transport layer ETL is less than 5 nm, the cathode CD is close to the light-emitting layer Em, which may deactivate excitons in the light-emitting layer Em. When the thickness of the electron transport layer ETL is more than 40 nm, a large voltage is required to transport electrons across the electron transport layer ETL, resulting in a large driving voltage for the light-emitting device ED.

[0044] (Current Density and External Quantum Efficiency) Fig. 7 is a diagram showing the results of examining the current density and external quantum efficiency (EQE) for the light-emitting devices of Comparative Examples 2 to 11, in which the light-emitting layer contains quantum dots and the work function of the cathode is smaller than the work function of the electron transport layer. As shown in Fig. 7, the light-emitting devices 100 of Comparative Examples 2 to 11 exhibit typical current density-EQE characteristics. Specifically, when the current density corresponding to the EQE peak is 10 １５ cm －３ When carriers are injected at a current density exceeding the peak EQE, the EQE drops sharply. This drop is called "roll-off."

[0045] 8 is a graph showing the current density-EQE characteristics of a semiconductor LED. Figure 8 is based on figures disclosed in "Increasing the Light-Emitting Efficiency of Gallium Nitride Micro-LEDs by Five Times at Low Density—A Step Forward to Realizing High-Efficiency, High-Resolution Micro-LED Displays" (National Institute of Advanced Industrial Science and Technology, press release, https: / / www.aist.go.jp / aist_j / press_release / pr2019 / pr20190703 / pr20190703.html, published July 3, 2019) and "Near-Complete Elimination of Size-Dependent Efficiency Decrease in GaN Micro-Light-Emitting Diodes" (Jun Zhu, Tokio Takahashi, Daisuke Ohori, Kazuhiko Endo, Seiji Samukawa, Mitsuaki Shimizu, and Xue-Lun Wang, Phys. Status Solidi A 2019, 1900380). As shown in Figure 8, the current density-EQE characteristics of a semiconductor LED are similar to those shown in Figure 7. Semiconductor LEDs contain bulk inorganic semiconductor materials as the light-emitting material in the light-emitting layer instead of quantum dots, and are sometimes called micro-LEDs. The current density-EQE characteristics of blue semiconductor LEDs have been analyzed in detail both theoretically and empirically.

[0046] Fig. 9 is a diagram showing the abc model of a semiconductor LED. The vertical axis in Fig. 9 is normalized so that the maximum value of EQE is 1. As shown in Fig. 9, the abc model is a model in which the proportional relationship of EQE to current density changes depending on whether the current density is included in the low region RA, the middle region RB, or the high region RC.

[0047] As the current density in the semiconductor LED increases in the low region RA, the number of electrons and holes injected into the light-emitting layer of the semiconductor LED increases. This rapidly increases the probability of the recombination process occurring in the light-emitting layer, and therefore the EQE also rapidly increases. Specifically, when the current density flowing through the semiconductor LED is included in the low region RA, the EQE of the semiconductor LED increases in proportion to the current density.

[0048] As the current density in the semiconductor LED increases in the middle region RB, the concentration of electrons injected into the light-emitting layer becomes higher than the concentration of holes. This is because the mobility of electrons is higher than that of holes, and therefore the efficiency of electron injection from the cathode to the light-emitting layer tends to be higher than the efficiency of hole injection from the anode to the light-emitting layer.

[0049] Therefore, as the current density in the semiconductor LED increases in the middle region RB, the probability of the recombination process in the light-emitting layer saturates, and further, the probability of the process of generating Auger electrons that do not contribute to the light emission of the light-emitting layer increases. As a result, the EQE reaches a maximum once and then gradually begins to decrease. Specifically, when the current density flowing through the semiconductor LED is included in the middle region RB, the EQE of the semiconductor LED changes in proportion to the square of the current density, and the coefficient is negative.

[0050] As the current density in the semiconductor LED increases in the high region RC, the probability of Auger electron generation in the light-emitting layer further increases, resulting in a further decrease in the EQE of the semiconductor LED. Specifically, when the current density flowing through the semiconductor LED is included in the high region RC, the EQE of the semiconductor LED decreases in proportion to the cube of the current density.

[0051] 7 and 8, the current density-EQE characteristics of the light-emitting elements 100 of Comparative Examples 2 to 11 are similar to the current density-EQE characteristics of semiconductor LEDs, as described above. Therefore, the same abc model as for semiconductor LEDs can be applied to the light-emitting elements 100 of Comparative Examples 2 to 11. However, the current densities of the EQE peaks differ significantly between the light-emitting elements 100 of Comparative Examples 2 to 11 and semiconductor LEDs.

[0052] In the 40 μm semiconductor LED shown in FIG. 8, the middle region RB has a current density of approximately 3 A / cm ２ More than 15A / cm ２ The carrier concentration in the light-emitting layer is approximately 10 １８ / cm ３ From 10 １９ / cm ３ In the light-emitting devices 100 of Comparative Examples 2 to 11 shown in FIG. 7, the middle region RB has a current density of approximately 2 mA / cm ２ 6mA / cm or more ２ The carrier concentration in the light-emitting layer is about 10 １５ / cm ３ From 10 １６ / cm ３ There is some degree of it.

[0053] The light-emitting layer Em of the light-emitting element 100 of Comparative Examples 2 to 11 contains quantum dots 104 as a light-emitting material. Therefore, recombination of electrons and holes injected into the light-emitting layer is likely to occur inside the quantum dots 104, which have a high confinement effect. As a result, if an excess of electrons occurs in the light-emitting layer, further electrons may be injected into the quantum dots 104 where recombination of electrons and holes has occurred, and may be confined inside the quantum dots 104. Excess electrons injected into the light-emitting layer are accumulated in the light-emitting layer due to the electron barrier between the light-emitting layer and the hole-transport layer 103, without recombining with holes. Therefore, a distribution of electron density occurs in the thickness direction of the light-emitting layer. When the light-emitting element operates in such a state of electron excess and electron density distribution, the probability of two or more electrons being injected into one quantum dot 104 becomes too large to be ignored.

[0054] In this case, the extra electrons injected into the quantum dots 104 may become Auger electrons by being excited by the energy of excitons generated in the quantum dots. Auger electrons generated by the above process are generally called "trion Auger electrons." The generation of such trion Auger electrons is more likely to occur inside the quantum dots 104, which have the above-mentioned strong carrier confinement effect, and furthermore, occurs even at a relatively low electron concentration. Therefore, in the light-emitting devices 100 of Comparative Examples 2 to 11, compared to semiconductor LEDs, the generation of Auger electrons becomes dominant even at lower current densities, and the middle region RB shrinks at lower current densities.

[0055] In addition to consuming excitons in the generation process, the trion Auger electrons reduce the internal quantum efficiency (IQE) of the light-emitting element 100 by overflowing through the electron barrier between the light-emitting layer and the hole transport layer 103. If the hole transport layer HTL and the hole injection layer HIL are organic, the trion Auger electrons may cause degradation due to a reduction action.

[0056] From the above, in a light-emitting element including quantum dots in the light-emitting layer, the middle region RB can be expanded to a high current density by reducing the electron excess in the light-emitting layer and improving the carrier balance. Furthermore, such expansion is desirable. Furthermore, as a secondary effect of this expansion, the EQE peak value also increases. Therefore, as described above, the configuration according to the present disclosure can improve the EQE of the light-emitting element ED.

[0057] 10 is a schematic diagram illustrating an example of a stacked structure of a light-emitting device according to an embodiment of the present disclosure. As shown in FIG. 10, the charge functional layer CF is an electron injection layer EIL, and the light-emitting device ED includes an electron transport layer ETL located between the light-emitting layer Em and the charge functional layer CF.

[0058] FIG. 11 is a schematic diagram illustrating an example of the band structure of the light-emitting element shown in FIG. 10 . The work function of the cathode CD is larger than the work function of the electron injection layer EIL. This deforms the band shape of the electron injection layer EIL as shown in FIG. 11 . According to the configuration of the second embodiment, electrons also accumulate on the electron injection layer EIL side of the interface between the electron injection layer EIL and the electron transport layer ETL, and the accumulated electrons reduce the efficiency of electron injection from the electron injection layer EIL. Therefore, the efficiency of electron injection into the light-emitting layer Em can be further reduced, and the carrier balance in the light-emitting layer Em can be further improved. Therefore, the configuration of the fourth embodiment can improve the EQE of the light-emitting element ED more than the configuration of the first embodiment.

[0059] [Embodiment 3] (Configuration of Light-Emitting Element) Fig. 12 is a schematic diagram illustrating an example of a stacked structure of a light-emitting element according to an embodiment of the present disclosure. As shown in Fig. 12, the light-emitting element ED according to the present disclosure may include a first conductive layer C1 in contact with a cathode CD. The cathode CD is located between the first conductive layer C1 and a charge functional layer CF.

[0060] The first conductive layer C1 includes a compound containing indium, such as an indium-based metal oxide. The indium-based metal oxide includes a light-transmitting conductive material. The first conductive layer C1 may be made of a conductive material having a work function smaller than that of the cathode CD.

[0061] The first conductive layer C1 is preferably transparent and has a refractive index smaller than that of the cathode CD. This reduces the effective refractive index of the entire light-emitting element ED, thereby increasing the light extraction efficiency. Furthermore, the first conductive layer C1 affects interference due to multiple reflections of the light-emitting element ED, so the film thickness of the first conductive layer C1 may be adjusted appropriately.

[0062] Fig. 13 is a schematic diagram illustrating an example of a band structure of the light-emitting device shown in Fig. 12. As shown in Fig. 13, the work function of the first conductive layer C1 is smaller than the work function of the cathode CD. The first conductive layer C1 does not substantially contribute to electron injection from the cathode CD to the charge functional layer CF.

[0063] The addition of the first conductive layer C1 essentially constitutes a multi-layered cathode CD. The improvement in light extraction efficiency due to the multi-layered structure raises the overall EQE characteristics, regardless of the injection current. Therefore, the configuration according to the third embodiment can improve the EQE of the light-emitting element ED more than the configuration according to the first embodiment. The configuration according to the third embodiment can be combined with the configuration according to the second embodiment.

[0064] [Embodiment 4] (Configuration of Light-Emitting Element) Fig. 14 is a schematic diagram illustrating an example of a stacked structure of a light-emitting element according to an embodiment of the present disclosure. As shown in Fig. 14, the light-emitting element ED according to the present disclosure may further include a light-transmitting second conductive layer C2. A first conductive layer C1 is located between the second conductive layer C2 and the cathode CD.

[0065] The second conductive layer C2 includes at least one selected from the group consisting of gold, tungsten, molybdenum, chromium, cobalt, nickel, platinum, copper, titanium, and zirconium. The second conductive layer C2 may be composed of a single metal including one selected metal, or an alloy including two or more selected metals. The material of the second conductive layer C2 may be selected so that the work function of the second conductive layer C2 is equal to or less than the work function of the cathode CD. Alternatively, the second conductive layer C2 may include silver or aluminum, and the work function of the second conductive layer C2 may be greater than the work function of the cathode CD.

[0066] Fig. 15 is a schematic diagram illustrating an example of a band structure of the light-emitting device shown in Fig. 14. As shown in Fig. 15, the work function of the second conductive layer C2 is equal to or less than the work function of the cathode CD. The second conductive layer C2 does not substantially contribute to electron injection from the cathode CD to the quantum dots QD.

[0067] The cathode CD and the second conductive layer C2 may comprise the same metal material, and the first conductive layer C1 may have a thickness ten times or more that of the second conductive layer. The light transmittance per thickness of the metal material is smaller than the light transmittance per thickness of the light-transmitting metal oxide. The refractive index per thickness of the metal material is larger than the refractive index per thickness of the light-transmitting metal oxide, thereby making the effective refractive index from the cathode CD to the second conductive layer C2 smaller than the refractive index of the cathode CD. Therefore, the light extraction efficiency of the light-emitting element ED can be increased. The thicknesses of the first conductive layer C1 and the second conductive layer C2 may be adjusted to enhance the microcavity effect in the light-emitting element ED. The microcavity effect can also increase the light extraction efficiency of the light-emitting element ED.

[0068] The addition of the first conductive layer C1 and the second conductive layer C2 essentially constitutes a multi-layered cathode CD. The improvement in light extraction efficiency due to the multi-layered structure raises the overall EQE characteristics, regardless of the injection current. Therefore, the configuration according to the fourth embodiment can improve the EQE of the light-emitting element ED more than the configuration according to the first embodiment. The configuration according to the fourth embodiment can be combined with the configuration according to the second embodiment.

[0069] Example 4 A light-emitting element ED according to Example 4 of the present disclosure was fabricated as follows. The light-emitting element ED according to Example 2 had the configuration shown in FIG. 14 . The anode AD to the electron transport layer ETL according to Example 4 were formed using the same method, materials, and film thicknesses as the anode AD to the electron transport layer ETL according to Example 1. The cathode CD and second conductive layer C2 according to Example 4 were each formed of nickel (Ni) with a film thickness of 5 nm, and the first conductive layer C1 was formed of indium tin oxide with various film thicknesses.

[0070] As a result of comparing first conductive layers C1 with various thicknesses, a good light extraction efficiency was obtained when the thickness of the first conductive layer C1 was 100 nm. While the light extraction efficiency of the light-emitting elements ED according to Examples 1 and 3 was up to about 20%, the light extraction efficiency of the light-emitting element ED according to Example 4 when the thickness of the first conductive layer C1 was 100 nm was 26.6%. This is an improvement of about 1.3 times.

[0071] 16 is a diagram showing the results of examining the current density and EQE for the light-emitting element according to Comparative Example 1 and the light-emitting elements according to Examples 1 and 4 of the present disclosure. In Fig. 16, the dashed line P0 indicates the results for Comparative Example 1, the solid line P1 indicates the results for Example 1, and the solid line P4 indicates the results for Example 4.

[0072] As shown in FIG. 16, when Example 1 is compared with Comparative Example 1, first, the current density at the EQE peak is 2 mA / cm ２ 12mA / cm from the vicinity ２ . This is the result of an increase in electron-hole pairs that can radiatively recombine per unit time. Secondly, the value of the EQE peak increases by approximately 2%. This is the result of an improvement in the carrier balance in the light-emitting layer Em. Thirdly, the slope of the approximation line in the high region exceeding the current density of the EQE peak decreases. In other words, the degree of roll-off decreases. This is the result of a decrease in excess electrons in the light-emitting layer Em and a decrease in trion Auger. Furthermore, the middle region RB occupied only the vicinity of the EQE peak in Comparative Example 1, while in Example 1, it was 20 mA / cm at a current density higher than the EQE peak. ２ Therefore, the light-emitting element ED according to Example 1 is suitable for use in a wide range exceeding the peak EQE.

[0073] Furthermore, comparing Example 2 with Example 1, the EQE peak value increases by about 2% without substantial changes in the current density and roll-off of the EQE peak, which is a result of increased light extraction efficiency of the emitting layer Em.

[0074] [Embodiment 5] (Configuration of Display Device) Fig. 17 is a schematic diagram showing a configuration example of a display device according to one aspect of the present disclosure. As shown in Fig. 17, a display device DP according to the present disclosure includes a light-emitting element ED according to the present disclosure. For example, the display device DP includes a display area DA in which a plurality of sub-pixels PX are provided, and a frame area NA in which a drive circuit DC that drives the display area DA is provided, and at least one of the plurality of sub-pixels PX includes a light-emitting element ED.

[0075] The light emitting element ED may have a configuration according to any one of the above-described first to fourth embodiments, or may have a configuration that is a combination or modification of these embodiments.

[0076] 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.

[0077] AD Anode CD Cathode CF Charge functional layer C1 First conductive layer C2 Second conductive layer DP Display device ED Light-emitting element EM Light-emitting layer QD Quantum dot

Claims

1. A light-emitting element comprising: an anode; a cathode containing at least one metal element selected from the group consisting of gold, tungsten, molybdenum, chromium, cobalt, nickel, platinum, copper, titanium, and zirconium; a light-emitting layer located between the anode and the cathode and containing quantum dots; and a charge functional layer located between the light-emitting layer and the cathode, in contact with the cathode, and containing an n-type semiconductor material.

2. The light-emitting device according to claim 1, wherein the cathode comprises an alloy and an intermetallic compound containing the at least one metallic element.

3. The light-emitting device according to claim 1 or 2, wherein the work function of the cathode is greater than the work function of silver.

4. A light-emitting element comprising: an anode; a cathode containing titanium oxide or zirconium oxide; a light-emitting layer located between the anode and the cathode and containing quantum dots; and a charge functional layer located between the light-emitting layer and the cathode, in contact with the cathode, and containing an n-type semiconductor material.

5. The light-emitting device according to any one of claims 1 to 4, wherein the thickness of the cathode is 20 nm or less.

6. The light-emitting device according to any one of claims 1 to 5, wherein the thickness of the cathode is equal to or less than a threshold calculated by the following formula (1): Threshold = 10 [nm] × extinction coefficient of silver ÷ extinction coefficient of cathode (1) 7. The light-emitting device according to any one of claims 1 to 6, wherein the thickness of the cathode is 0.5 nm or more.

8. The light-emitting element according to any one of claims 1 to 7, further comprising a first conductive layer in contact with the cathode and containing an indium-containing compound, the cathode being located between the first conductive layer and the charge functional layer.

9. The light-emitting element according to claim 8, wherein the first conductive layer is light-transmitting and has a refractive index smaller than that of the cathode.

10. The light-emitting device according to claim 9, wherein the work function of the first conductive layer is less than the work function of the cathode.

11. The light-emitting element according to any one of claims 8 to 10, further comprising a light-transmitting second conductive layer containing at least one metal element selected from the group consisting of gold, tungsten, molybdenum, chromium, cobalt, nickel, platinum, copper, titanium, and zirconium, and the first conductive layer is located between the second conductive layer and the cathode.

12. The light-emitting device according to claim 11, wherein the work function of the second conductive layer is equal to or less than the work function of the cathode.

13. The light-emitting element according to claim 11 or 12, wherein the cathode and the second conductive layer comprise the same metal material, and the first conductive layer has a thickness at least ten times that of the second conductive layer.

14. The light-emitting device according to any one of claims 1 to 13, wherein the charge functional layer is made of a II-VI compound semiconductor material.

15. The light-emitting device according to claim 14, wherein the charge functional layer comprises at least one material selected from the group consisting of zinc oxide, zinc magnesium oxide, zinc sulfide, and zinc magnesium sulfide.

16. The light-emitting element according to any one of claims 1 to 15, wherein the thickness of the charge functional layer is 5 nm or more and 40 nm or less.

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

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