Display device

Abstract

To provide a long-lifetime light-emitting element exhibiting high efficiency.SOLUTION: A display device includes a first pixel, a second pixel, and a third pixel, each having a first light-emitting element, a second light-emitting element, and a third light-emitting element each configured to emit red light, green light, and blue light. Each of the first light-emitting element, the second light-emitting element, and the third light-emitting element includes a pixel electrode, an electron blocking layer on the pixel electrode, a light-emitting layer on the electron blocking layer, a hole blocking layer on the light-emitting layer, and a counter electrode on the hole blocking layer. The third light-emitting element further includes a first buffer layer between the light-emitting layer and the hole blocking layer. The light-emitting layers of the first light-emitting element and the second light-emitting element contain a thermally activated delayed fluorescence material. The light-emitting layer of the third light-emitting element contains a first fluorescent material having a fluorescence lifetime of 1 ps or more and less than 1 ns.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] One embodiment of the present invention relates to a display device having an electroluminescent element. [Background technology]

[0002] In recent years, display devices equipped with organic electroluminescent devices (OLEDs) have been widely used. In addition, organic electroluminescent devices that exhibit thermally activated delayed fluorescence or hyperfluorescence (registered trademark) have attracted attention due to their extremely high luminous efficiency, and vigorous research and development has been conducted (see, for example, Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-048366 [Patent Document 2] Japanese Patent Application Publication No. 2020-013695 [Patent Document 3] Japanese Patent Application Publication No. 2017-222820 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of one embodiment of the present invention is to provide a display device having a new structure, or to provide a display device that exhibits high efficiency and reliability, and a method for manufacturing the display device at low cost. [Means for solving the problem]

[0005] One embodiment of the present invention is a display device. The display device includes a first pixel, a second pixel, and a third pixel, each having a first light-emitting element, a second light-emitting element, and a third light-emitting element configured to emit red, green, and blue light, respectively. Each of the first light-emitting element, the second light-emitting element, and the third light-emitting element has a pixel electrode, an electron blocking layer on the pixel electrode, an emitting layer on the electron blocking layer, a hole blocking layer on the emitting layer, and a counter electrode on the hole blocking layer. The third light-emitting element further has a first buffer layer between the emitting layer and the hole blocking layer. The emitting layers of the first and second light-emitting elements contain a thermally activated delayed fluorescent material. The emitting layer of the third light-emitting element contains a first fluorescent material having a fluorescence lifetime of 1 ps or more but less than 1 ns.

[0006] One embodiment of the present invention is a light-emitting device configured to emit blue light. The light-emitting device includes a pixel electrode, an electron blocking layer on the pixel electrode, a light-emitting layer on the electron blocking layer, a buffer layer on the light-emitting layer, a hole-blocking layer on the buffer layer, and a counter electrode on the hole-blocking layer. The light-emitting layer contains a fluorescent material with a fluorescence lifetime of 1 ps or more and 1 ns or less. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a schematic top view of a display device according to an embodiment of the present invention. [Figure 2] 1 is a schematic top view of a display device according to an embodiment of the present invention. [Figure 3] 1 is a schematic top view of a display device according to an embodiment of the present invention. [Figure 4] FIG. 2 is a schematic end view of a light-emitting element provided in a pixel of a display device according to an embodiment of the present invention. [Figure 5] 1 is a schematic end view of a display device according to an embodiment of the present invention; [Figure 6] FIG. 2 is a schematic end view of a light-emitting element provided in a pixel of a display device according to an embodiment of the present invention. [Figure 7] 1 shows a voltage-current density curve of a red light-emitting element fabricated in an example. [Figure 8]10 is a plot showing the relationship between the thickness of the first buffer layer and the driving voltage of the red light-emitting element fabricated in the example. [Figure 9] 10 is a plot showing the relationship between the thickness of the first buffer layer and the current efficiency of the red light-emitting device fabricated in the example. [Figure 10] 1 shows current density-normalized external quantum efficiency curves of red light-emitting devices fabricated in Examples. [Figure 11] 10 is a voltage-capacity curve of a comparative red light-emitting element. [Figure 12] 1 shows a voltage-capacity curve of a red light-emitting element according to an embodiment. [Figure 13] 1 shows a voltage-current density curve of a blue light-emitting element fabricated in an example. [Figure 14] 10 is a plot showing the relationship between the thickness of the second buffer layer and the driving voltage of a blue light-emitting device fabricated in an example. [Figure 15] 10 is a plot showing the relationship between the thickness of the second buffer layer and the current efficiency of the blue light-emitting element fabricated in the example. [Figure 16] Current density-normalized external quantum efficiency curve of blue light-emitting element fabricated in the examples [Figure 17] 1 shows a voltage-capacity curve of a blue light-emitting element fabricated in an example. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, each embodiment of the present invention will be described with reference to the drawings, etc. However, the present invention can be embodied in various forms without departing from the spirit of the present invention, and should not be construed as being limited to the description of the embodiments exemplified below.

[0009] In order to clarify the description, the drawings may show the width, thickness, shape, etc. of each part schematically compared to the actual embodiment, but these are merely examples and do not limit the interpretation of the present invention. In this specification and each drawing, elements having the same functions as those explained in the previous drawings may be assigned the same reference numerals, and duplicate explanations may be omitted.

[0010] In this specification and claims, when expressing an aspect of placing another structure on top of a certain structure, the term "on top" is used, unless otherwise specified, to include both a case in which another structure is placed directly on top of a certain structure so as to be in contact with the certain structure, and a case in which another structure is placed above a certain structure via yet another structure.

[0011] In this specification and claims, the expression "a structure exposed from another structure" means a state in which a part of a structure is not covered by another structure, and includes a state in which the part not covered by another structure is covered by yet another structure. The state expressed by this expression also includes a state in which a structure is not in contact with another structure.

[0012] In the present invention, when one film is formed across multiple light-emitting elements, this film functions as a functional layer in each of the multiple light-emitting elements. However, since the film is formed simultaneously in the same process, it has substantially the same layer structure, the same material, the same composition, and the same morphology among the multiple light-emitting elements. Therefore, the film is defined as existing in the same layer across the multiple light-emitting elements.

[0013] A display device 100 according to an embodiment of the present invention will be described below.

[0014] 1.Overall structure FIG. 1 shows a schematic top view of a display device 100. As shown in FIG. 1, the display device 100 includes a substrate 102 and an opposing substrate (not shown), with various patterned insulating films, semiconductor films, and conductive films laminated between them. By appropriately laminating these films, a plurality of pixels 120 and drive circuits (scanning line drive circuit 106, signal line drive circuit 108) for driving the pixels 120 are formed on the substrate 102. The substrate 102 and the opposing substrate are fixed with an adhesive (not shown) such as a sealant, thereby sealing and protecting the pixels 120, scanning line drive circuit 106, and signal line drive circuit 108. A plurality of terminals 110 formed of a conductive film are provided on the substrate 102, and the terminals 110 are electrically connected to an external circuit (not shown) via a connector such as a flexible printed circuit (FPC) board. Various signals and power sources for displaying images are supplied from the external circuit to the scanning line drive circuit 106 and the signal line drive circuit 108 via the terminals 110. It should be noted that one or both of the scanning line driving circuit 106 and the signal line driving circuit 108 do not need to be formed directly on the substrate 102, and a driving circuit formed on a substrate (such as a semiconductor substrate) different from the substrate 102 may be provided on the substrate 102 or a connector as the scanning line driving circuit 106 and / or the signal line driving circuit 108.

[0015] Each pixel 120 has a pixel circuit formed therein, and further has one of the three primary color light-emitting elements (i.e., red, green, and blue light-emitting elements) disposed therein. Based on various signals supplied from external circuits, a scanning line driving circuit 106 and a signal line driving circuit 108 generate signals for operating the pixel circuits and supply the signals to each pixel 120. This causes the light-emitting elements connected to the pixel circuits to emit light, and each pixel 120 functions as the smallest unit for providing color information. As a result, a full-color display is possible. Here, the red light-emitting element, green light-emitting element, and blue light-emitting element refer to elements that exhibit peak emission wavelengths in the ranges of 650 nm to 750 nm, 500 nm to 650 nm, and 400 nm to 500 nm, respectively.

[0016] There are no restrictions on the arrangement of the pixels 120. For example, as shown in FIG. 2, a stripe arrangement may be employed in which a red-emitting pixel 120-1, a green-emitting pixel 120-2, and a blue-emitting pixel 120-3, which respectively emit red, green, and blue light, are arranged in order in the row direction, and pixels 120 emitting the same emitted light color are arranged in the same column. Alternatively, although not shown, various arrangements may be employed, such as a mosaic arrangement in which a red-emitting pixel 120-1, a green-emitting pixel 120-2, and a blue-emitting pixel 120-3 are arranged in order in both the row and column directions, as well as a delta arrangement and a pentile arrangement. Alternatively, as shown in FIG. 3, multiple pixels 120 may be arranged so that one or more red-emitting pixels 120-1 and one or more green-emitting pixels 120-2 are sandwiched between adjacent blue-emitting pixels 120-3. In this case, by arranging the multiple pixels 120 so that the area of ​​the blue light-emitting pixel 120-3, in which the blue light-emitting element with the lowest light-emitting efficiency is arranged, is larger than the area of ​​the other pixels 120, the burden on the blue light-emitting element is reduced and the reliability of the display device 100 can be improved.

[0017] 2. Structure of light-emitting element The light-emitting elements provided in the red-emitting pixel 120-1, green-emitting pixel 120-2, and blue-emitting pixel 120-3 will be described with reference to Figure 4. The light-emitting element 130 provided in each pixel 120 is a so-called organic electroluminescent element, and each includes a pair of electrodes (a pixel electrode 132 and a counter electrode 134) and an electroluminescent layer (hereinafter referred to as an EL layer) 140 provided between the pixel electrode 132 and the counter electrode 134. The EL layer 140 is a laminate of multiple functional layers and includes at least an electron blocking layer 146, a light-emitting layer 150, a hole blocking layer 154, and a second buffer layer 152. In addition to these, the functional layers may further include a hole injection layer 142, a hole transport layer 144, a first buffer layer 148, an electron transport layer 156, an electron injection layer 158, etc.

[0018] (1) Pixel electrode and counter electrode The pixel electrode 132 is provided individually for each pixel 120 and functions as an electrode that injects holes into the EL layer 140 in each pixel 120. When light obtained in the EL layer 140 is extracted through the pixel electrode 132, the pixel electrode 132 is configured to transmit visible light, and therefore the pixel electrode 132 is made of a conductive oxide that transmits visible light, such as indium-tin oxide (ITO) or indium-zinc (IZO). On the other hand, when light is extracted through the counter electrode 134, the pixel electrode 132 is configured to function as a reflective electrode that efficiently reflects light. In this case, the pixel electrode 132 is configured to contain a highly reflective metal such as silver or aluminum, or an alloy thereof. For example, the pixel electrode 132 may have a configuration in which a metal-containing film is covered with or sandwiched between films containing a conductive oxide.

[0019] The counter electrode 134 is an electrode that injects electrons into the EL layer 140. When light obtained in the EL layer 140 is extracted through the pixel electrode 132, the counter electrode 134 also functions as a reflective electrode. Therefore, the counter electrode 134 is configured to contain the above-mentioned metal or alloy (e.g., an alloy of silver and a metal with a low work function, such as magnesium). Conversely, when light obtained in the EL layer 140 is extracted through the counter electrode 134, the counter electrode 134 is configured to contain a conductive oxide that transmits visible light. Alternatively, the counter electrode 134 may be a metal-containing film (e.g., a film containing magnesium, an alloy of magnesium and silver, etc.) having a thickness that allows visible light to transmit (e.g., 5 nm to 20 nm). In the latter case, a conductive oxide film that transmits visible light may be further provided on the metal-containing film. Unlike the pixel electrode 132, the counter electrode 134 is provided so as to be shared by multiple pixels 120 (e.g., all pixels 120 of the display device 100). Therefore, the counter electrode 134 is not divided between the pixels 120 but is continuous.

[0020] (2) Hole injection layer The hole injection layer 142 functions to facilitate hole injection from the pixel electrode 132 to the EL layer 140. The hole injection layer 142 can be made of a compound that easily injects holes, i.e., is easily oxidized (electron-donating). In other words, a compound with a shallow highest occupied molecular orbital (HOMO) level can be used. Examples of suitable materials include aromatic amines such as benzidine derivatives and triarylamines, carbazole derivatives, thiophene derivatives, and phthalocyanine derivatives such as copper phthalocyanine. Alternatively, polymeric materials such as polythiophene, polyaniline, and their derivatives can be used. For example, poly(ethylenedioxythiophene) / poly(styrenesulfonic acid) can be used. A mixture of an electron-donating compound such as the aromatic amine or carbazole derivative or aromatic hydrocarbon and an electron acceptor can also be used. Examples of electron acceptors include transition metal oxides such as vanadium oxide and molybdenum oxide, nitrogen-containing heteroaromatic compounds, and aromatic compounds with strong electron-withdrawing groups such as cyano groups. The hole injection layer 142 can have a single layer structure or can be composed of multiple layers containing different materials. The hole injection layer 142 can also be provided so as to be shared by multiple pixels 120 (for example, all pixels 120 of the display device 100). In this case, the hole injection layer 142 exists in the same layer between the pixels 120 (i.e., the light-emitting elements 130) and is continuous without being divided between the pixels 120 (light-emitting elements 130).

[0021] (3) Hole transport layer The hole transport layer 144 is provided on the hole injection layer 142 so as to be in contact with the hole injection layer 142. The hole transport layer 144 has the function of transporting holes injected into the hole injection layer 142 toward the light-emitting layer 150, and may be made of the same or similar materials as those usable for the hole injection layer 142. For example, a material having a deeper HOMO level than that of the hole injection layer 142, but with a difference of approximately 0.5 eV or less, may be used. Typically, an aromatic amine such as a benzidine derivative may be used. The hole transport layer 144 may also have a single-layer structure or may be made of multiple layers containing different materials.

[0022] The hole transport layer 144 can also be provided so as to be shared by multiple pixels 120 (for example, all pixels 120 of the display device 100). In this case, the hole injection layer 142 exists in the same layer between the pixels 120 (i.e., the light-emitting elements 130) and is continuous without being divided between the pixels 120 (light-emitting elements 130).

[0023] The hole transport layer 144 may be formed to have the same thickness among the pixels 120, or may be formed to have different thicknesses. In the latter case, it is preferable to provide the hole transport layer 144 so that its thickness increases in the order of the blue-emitting pixel 120-3, the green-emitting pixel 120-2, and the red-emitting pixel 120-1. Light obtained from the light-emitting layer 150 travels isotropically and is extracted from the pixel electrode 132 and / or the counter electrode 134, where it is repeatedly reflected between the pixel electrode 132 and the counter electrode 134. Therefore, the pixel electrode 132 and the counter electrode 134 form a resonator structure. Therefore, by appropriately adjusting the distance between the pixel electrode 132 and the counter electrode 134, the obtained light can be amplified by resonance, thereby increasing the luminance in the front direction of the display device 100. Since the distance (optical distance) required for resonance increases with increasing wavelength, by increasing the thickness of the hole transport layer 144 in the order of blue light-emitting pixel 120-3, green light-emitting pixel 120-2, and red light-emitting pixel 120-1, an appropriate resonator structure can be formed in each pixel.

[0024] Therefore, for example, as shown in FIG. 4 , the first hole transport layer 144-1 is formed with the same thickness so as to be shared by all pixels 120. Furthermore, in the green light-emitting pixel 120-2 and the red light-emitting pixel 120-1, the second hole transport layer 144-2 is formed with the same thickness on the first hole transport layer 144-1 so as to be shared by the green light-emitting pixel 120-2 and the red light-emitting pixel 120-1. Furthermore, in the red light-emitting pixel 120-1, a third hole transport layer 144-3 may be formed on the second hole transport layer 144-2. This allows the thickness of the hole transport layer 144 to increase as the emission wavelength increases. The first hole transport layer 144-1, the second hole transport layer 144-2, and the third hole transport layer 144-3 may have the same or different compositions. However, using the same composition enables more efficient manufacturing of the display device 100.

[0025] (4) Electron blocking layer The electron blocking layer 146 is provided on the hole transport layer 144 so as to be in contact with the hole transport layer 144. The electron blocking layer 146 prevents electrons injected from the counter electrode 134 from passing through the light emitting layer 150 and being injected into the hole transport layer 144 without contributing to recombination within the light emitting layer 150, thereby confining the electrons within the light emitting layer 150, and also prevents excitation energy obtained in the light emitting layer 150 from being transferred to molecules in the hole transport layer 144. This prevents a decrease in luminous efficiency.

[0026] The electron blocking layer 146 is preferably made of a material that has hole transport properties that are higher or equal to those of the electron transport properties, a shallower lowest unoccupied molecular orbital (LUMO) level than the molecules in the light-emitting layer 150, and a larger band gap. Specifically, the difference between the LUMO levels of the molecules in the electron blocking layer 146 and those of the molecules in the light-emitting layer 150 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. The difference between the band gaps of the molecules in the electron blocking layer 146 and those of the molecules in the light-emitting layer 150 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Specifically, aromatic amine derivatives, carbazole derivatives, 9,10-dihydroacridine derivatives, benzofuran derivatives, benzothiophene derivatives, and the like can be used for the electron blocking layer 146. The electron blocking layer 146 may also have a single-layer structure or may be composed of multiple layers containing different materials.

[0027] The electron blocking layer 146 can also be provided so as to be shared by multiple pixels 120 (for example, all pixels 120 of the display device 100). In this case, the electron blocking layer 146 exists in the same layer between the pixels 120 (i.e., the light-emitting elements 130) and is continuous without being divided between the pixels 120 (light-emitting elements 130). Furthermore, the electron blocking layer 146 has the same composition and thickness between the pixels 120 (light-emitting elements 130).

[0028] (5) First buffer layer The first buffer layer 148 can be selectively provided in the red light-emitting element 130-1 disposed in the red light-emitting pixel 120-1. The first buffer layer 148 is a functional layer for adjusting the carrier balance of the red light-emitting element 130-1 and is provided so as to be in direct contact with the electron blocking layer 146 and the light-emitting layer 150. The first buffer layer 148 contains the host material contained in the light-emitting layer 150. In other words, in the red light-emitting element 130-1, the material contained in the first buffer layer 148 is the same as the host material contained in the light-emitting layer 150. Preferably, the first buffer layer 148 is made of the host material and is substantially free of other components. The thickness of the first buffer layer 148 is relatively small, for example, from 2.0 nm to 10 nm, or from 2.0 nm to 8.5 nm. As described below, the use of the first buffer layer 148 enables an excellent carrier balance to be achieved in the red light-emitting element 130-1, thereby achieving high luminous efficiency and low driving voltage. Furthermore, since excellent carrier balance can be obtained even when many functional layers (e.g., hole blocking layer 154, electron blocking layer 146, electron transport layer 156, etc.) are formed so as to be shared among all pixels 120, the number of deposition masks required to manufacture display device 100 can be reduced, and as a result, display devices can be provided at lower cost.

[0029] (6) Light-emitting layer A. Light-emitting layer of blue light-emitting element The light-emitting layer 150 provided in the blue light-emitting element 130-3 contains a host material as a main component and also contains a blue-emitting fluorescent material that emits light. The volume ratio of the host material to the light-emitting material (light-emitting material / host material) may be, for example, 0.01 or more and 0.20 or less. Examples of host materials that can be used include zinc- and aluminum-based metal complexes, as well as oxadiazole derivatives, triazole derivatives, benzimidazole derivatives, quinoxaline derivatives, dibenzoquinoxaline derivatives, dibenzothiophene derivatives, dibenzofuran derivatives, pyrimidine derivatives, triazine derivatives, pyridine derivatives, bipyridine derivatives, phenanthroline derivatives, aromatic amine derivatives, and carbazole derivatives. Here, blue light emission refers to light emission having a maximum emission peak wavelength in the range of 400 nm to 500 nm.

[0030] The light-emitting material is a blue-emitting fluorescent material that does not exhibit thermally activated delayed fluorescence (TADF). Specifically, the maximum emission peak wavelength is in the range of 400 nm to 500 nm, and the fluorescence lifetime is 10 -9 seconds (1ps) or more 10 -6 Fluorescent materials with a luminescence time of less than 1 ns are used, such as anthracene derivatives, stilbene derivatives, and pyrene derivatives.

[0031] A. Light-emitting layers of red and green light-emitting elements Similar to the blue light-emitting element 130-3, the light-emitting layers 150 provided in the red light-emitting element 130-1 and the green light-emitting element 130-2 also contain a host material as a main component and a light-emitting material responsible for emitting light. However, unlike the blue light-emitting element 130-3, the red light-emitting element 130-1 and the green light-emitting element 130-2 use, as their light-emitting materials, materials that exhibit thermally activated delayed fluorescence and emit red and green light (thermally activated delayed fluorescent materials). Furthermore, the concentration of the light-emitting material in the light-emitting layer, i.e., the thermally activated delayed fluorescent material, is relatively high, and the volume ratio of the host material to the light-emitting material (light-emitting material / host material) is set to, for example, 0.30 or more and 0.60 or less. Here, green emission refers to emission with a maximum emission peak wavelength in the range of 500 nm to 650 nm, and red emission refers to emission with a maximum emission peak wavelength in the range of 650 nm to 750 nm. In thermally activated delayed fluorescent materials, the difference between the triplet excitation energy level and the singlet excitation energy level is small, for example, 5 meV to 20 meV. Therefore, the triplet excited state of the luminescent material, which is generated by carrier recombination, can undergo intersystem crossing to the singlet excited state with extremely small thermal energy at room temperature or below. As a result, the rate of thermal deactivation of the triplet excited state is relatively slowed, and radiative deactivation from the singlet excited state is promoted. Due to this mechanism, thermally activated delayed fluorescent materials exhibit emission with a remarkably long lifetime while having a spectrum similar to that of ordinary fluorescence. The fluorescence lifetime of thermally activated delayed fluorescent materials is 10 -6 seconds (1 ns) or more, preferably 10 -3 The probability of generating a triplet excited state caused by the recombination of holes and electrons is about three times that of the singlet excited state, and therefore, by using a thermally activated delayed fluorescent material, the efficiency of the light-emitting element 130 can be dramatically improved.

[0032] Examples of thermally activated delayed fluorescent materials include fullerenes and their derivatives, acridine derivatives such as proflavine, and eosin. Other examples include metal-containing porphyrins containing magnesium, zinc, cadmium, tin, platinum, indium, or palladium. Examples of metal-containing porphyrins include protoporphyrin-tin fluoride complexes, mesoporphyrin-tin fluoride complexes, hematoporphyrin-tin fluoride complexes, coproporphyrin tetramethyl ester-tin fluoride complexes, octaethylporphyrin-tin fluoride complexes, etioporphyrin-tin fluoride complexes, and octaethylporphyrin-platinum chloride complexes.

[0033] Furthermore, a compound in which an electron donor component and an electron acceptor component are linked may be used. Examples of the electron donor component and the electron acceptor component include a π-electron-rich heteroaromatic ring and a π-electron-deficient heteroaromatic ring, respectively. Examples of the basic skeleton of a π-electron-deficient heteroaromatic ring include a pyridine skeleton, a diazine skeleton, and a triazine skeleton. Examples of the basic skeleton of a π-electron-rich heteroaromatic ring include an acridine skeleton, a phenoxazine skeleton, a phenothiazine skeleton, a furan skeleton, a thiophene skeleton, and a pyrrole skeleton. Examples of such compounds include 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine, 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9'-phenyl-9H,9'H-3,3'-bicarbazole, 9-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-9'-phenyl-9H,9'H-3,3'-bicarbazole, and 2-[4-(10H-phenoxazin-10-yl)phenyl]-4,6-diphenyl-1,3,5-triazine.

[0034] One or both of the light-emitting layers 150 provided in the red light-emitting element 130-1 and the green light-emitting element 130-2 may further include, as light-emitting materials, a fluorescent material (hereinafter also referred to as a second fluorescent material) capable of receiving the excited singlet energy of the thermally activated delayed fluorescent material and forming a singlet excited state. The second fluorescent material is selected so that its energy level in the singlet excited state is lower than that of the thermally activated delayed fluorescent material and its band gap is smaller than that of the thermally activated delayed fluorescent material. The second fluorescent material does not exhibit thermally activated delayed fluorescence in the red light-emitting element 130-1 and the green light-emitting element 130-2, and therefore exhibits a relatively short fluorescence lifetime (e.g., 1 ps or more but less than 1 ns). Specific examples of fluorescent materials include coumarin derivatives, pyran derivatives, quinocridone derivatives, tetracene derivatives, pyrene derivatives, anthracene derivatives, and pyran derivatives. Thermally activated delayed fluorescent materials generally exhibit broad emission spectra and low color purity. In contrast, the above-mentioned fluorescent materials provide an emission spectrum with a relatively narrow half-width, enabling emission of light with high color purity. Therefore, by further adding a second fluorescent material to the light-emitting layer 150, it is possible to provide a light-emitting device 130 that not only has high luminous efficiency due to the thermally activated delayed fluorescent material but also has excellent color purity. As a result, it is possible to provide a display device with high color reproducibility.

[0035] (7) Second buffer layer The second buffer layer 152 is not provided in the red-emitting pixel 120-1 or the green-emitting pixel 120-2, but is selectively provided in the blue light-emitting element 130-3 of the blue-emitting pixel 120-3. The second buffer layer 152 is a functional layer for adjusting the carrier balance of the blue light-emitting element 130-3 and is provided so as to be in direct contact with the light-emitting layer 150 and the hole blocking layer 154. The thickness of the second buffer layer 152 is relatively small, preferably 2 nm to 8.5 nm, or 2 nm to 5 nm. The second buffer layer 152 forms an appropriate energy barrier between the hole blocking layer 154 and the light-emitting layer 150, thereby improving the carrier balance in the blue light-emitting element 130-3, thereby increasing the efficiency, reducing the driving voltage, and improving the lifetime of the blue light-emitting element 130-3.

[0036] Specifically, the second buffer layer 152 is configured so that the difference in LUMO level between the second buffer layer 152 and the hole blocking layer 154 is 0.1 eV to 0.3 eV, and the difference in LUMO level between the second buffer layer 152 and the light-emitting layer 150 is 0.1 eV to 0.3 eV. The second buffer layer 152 is also configured so that the difference in HOMO level between the second buffer layer 152 and the hole blocking layer 154 is 0.1 eV to 0.3 eV, and the difference in HOMO level between the second buffer layer 152 and the light-emitting layer 150 is 0.1 eV to 0.3 eV. For example, a host material that can be used for the blue light-emitting element 130-3 described above or a material that can be used for the hole blocking layer 154 described later and that satisfies the above-mentioned relationship between the HOMO level and the LUMO level may be selected.

[0037] Alternatively, the second buffer layer 152 may be composed substantially of the host contained in the light-emitting layer 150 of the blue light-emitting element 130-3, and may contain substantially no other components. In this case, the second buffer layer 152 does not contain a light-emitting material having a smaller band gap than the second buffer layer 152, and therefore has a lower hole and electron injection property than the light-emitting layer 150. As a result, the second buffer layer 152 can function as a resistance component between the hole blocking layer 154 and the light-emitting layer 150.

[0038] (8) Hole Block Layer The hole blocking layer 154 prevents holes injected from the pixel electrode 132 from passing through the light-emitting layer 150 and being injected into the electron transport layer 156 without contributing to recombination, thereby confining the holes within the light-emitting layer 150, and also prevents the excitation energy obtained in the light-emitting layer 150 from being transferred to molecules in the electron transport layer 156. This prevents a decrease in luminous efficiency.

[0039] The hole blocking layer 154 is preferably made of a material that has electron transport properties that are higher or equal to those of the hole transport properties, a deeper HOMO level than the molecules in the light-emitting layer 150, and a larger band gap. Specifically, the difference between the HOMO levels of the molecules in the hole blocking layer 154 and those of the molecules in the light-emitting layer 150 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. The difference between the band gaps of the molecules in the hole blocking layer 154 and those of the molecules in the light-emitting layer 150 is preferably 0.2 eV, 0.3 eV, or 0.5 eV or more. Specific examples of such materials include phenanthroline derivatives, oxadiazole derivatives, triazole derivatives, and metal complexes with a relatively large band gap (e.g., 2.8 eV or more), such as bis(2-methyl-8-quinolinolato)(4-hydroxybiphenylyl)aluminum. The hole blocking layer 154 may also have a single-layer structure or may be composed of multiple layers containing different materials. As described above, in the red light-emitting element 130-1, the first buffer layer 148 is preferably made of the host material contained in the light-emitting layer 150. Therefore, the difference in HOMO level between the first buffer layer 148 and the hole blocking layer 154 is approximately the same as the difference in HOMO level between the light-emitting layer 150 and the hole blocking layer 154, and is preferably 0.1 eV to 0.5 eV, or 0.1 eV to 0.3 eV.

[0040] The hole blocking layer 154 can also be provided so as to be shared by a plurality of pixels 120 (for example, all pixels 120 of the display device 100). In this case, the hole blocking layer 154 exists in the same layer between the pixels 120 (i.e., the light-emitting elements 130) and is continuous without being divided between the pixels 120 (light-emitting elements 130). Furthermore, the hole blocking layer 154 has the same composition and thickness between the pixels 120 (light-emitting elements 130).

[0041] (9)Electron transport layer The electron transport layer 156 transports electrons injected from the counter electrode 134 through the electron injection layer 158 to the light-emitting layer 150. The electron transport layer 156 can be made of an easily reduced (electron-accepting) compound. In other words, it can be made of a compound with a shallow LUMO level. Examples of suitable compounds include metal complexes containing a ligand with a benzoquinolinol skeleton, such as tris(8-quinolinolato)aluminum and tris(4-methyl-8-quinolinolato)aluminum, and metal complexes containing a ligand with an oxadiazole or thiazole skeleton. In addition to these metal complexes, compounds with electron-deficient heteroaromatic rings, such as oxadiazole derivatives, thiazole derivatives, triazole derivatives, and phenanthroline derivatives, can also be used. The electron transport layer 156 can also have a single-layer structure or can be made of multiple layers containing different materials.

[0042] Although not shown, similar to the hole transport layer 144, the electron transport layer 156 may be configured so that its thickness increases in the order of the blue light-emitting element 130-3, the green light-emitting element 130-2, and the red light-emitting element 130-1. Therefore, for example, the blue light-emitting element 130-3 may have a single-layer electron transport layer 156, the green light-emitting element 130-2 may have a two-layer electron transport layer 156, and the red light-emitting element 130-1 may have a three-layer electron transport layer. By adopting such a configuration, an appropriate resonator structure can be formed in each light-emitting element 130.

[0043] (10)Electron injection layer The electron injection layer 158 can be made of a compound that promotes electron injection from the counter electrode 134 to the electron transport layer 156. For example, a mixture of a compound that can be used in the electron transport layer 156 and an electron donor such as lithium or magnesium can be used. Alternatively, an inorganic compound such as lithium fluoride or calcium fluoride can be used.

[0044] 3. Pixel structure Fig. 5 shows a schematic diagram of an end surface corresponding to the dashed line AA' in Fig. 2. Fig. 5 shows a schematic diagram of end surfaces of a red light-emitting pixel 120-1, a green light-emitting pixel 120-2, and a blue light-emitting pixel 120-3 that are successively arranged, and the red light-emitting pixel 120-1, the green light-emitting pixel 120-2, and the blue light-emitting pixel 120-3 are provided with the red light-emitting element 130-1, the green light-emitting element 130-2, and the blue light-emitting element 130-3 shown in Fig. 4, respectively.

[0045] As described above, a pixel circuit for driving the light-emitting element 130 is formed in each pixel 120. The configuration of the pixel circuit can be determined arbitrarily, and a known configuration may be applied. In the example shown in FIG. 5, one transistor 160 electrically connected to the light-emitting element 130 and a capacitance element (auxiliary capacitance element) 180 connected to the transistor 160 are shown as parts of the elements constituting the pixel circuit, but the pixel circuit may be composed of multiple transistors and multiple capacitance elements. Also, in this example, light from the light-emitting layer 150 of all the light-emitting elements is extracted through the counter substrate 104, but the display device 100 may be configured so that light from the light-emitting layer 150 is extracted through the substrate 102.

[0046] (1) Substrate and opposing substrate The substrate 102 and the counter substrate 104 are provided to provide physical strength to the display device 100 and to protect the multiple pixels 120, the scanning line driving circuit 106, and the signal line driving circuit 108. The substrate 102 and the counter substrate 104 may be substrates containing inorganic materials such as crystalline semiconductor substrates, glass substrates, or quartz substrates, or may contain polymers such as polyimide, polyamide, or polycarbonate. The substrate 102 and the counter substrate 104 may each be flexible or inflexible. In the former case, the substrate 102 and / or the counter substrate 104 may be flexible enough to allow elastic deformation, or may be highly flexible enough to allow plastic deformation. When light emitted from the light-emitting elements is extracted to the outside through the counter substrate 104, at least the counter substrate 104 is configured to transmit visible light. Conversely, when light emitted from the light-emitting elements is extracted to the outside through the substrate 102, at least the substrate 102 is configured to transmit visible light.

[0047] (2) Pixel circuit As described above, a known configuration can be applied to the pixel circuit, and therefore detailed description thereof will be omitted. In the example shown in FIG. 5 , a transistor 160 functioning as a driving transistor is provided on a substrate 102. The transistor 160 may be provided directly on the substrate 102, or may be formed on the substrate 102 via an undercoat 112 that prevents diffusion of impurities contained in the substrate 102. The transistor 160 shown in FIG. 5 includes a semiconductor film 162, a gate insulating film 164 on the semiconductor film 162, a gate electrode 166 on the gate insulating film 164, an interlayer insulating film 168 on the gate electrode 166, and a pair of terminals 170 and 172 that are provided on the interlayer insulating film 168 and are electrically connected to the semiconductor film 162. Although the transistor 160 shown here is a top-gate transistor, there are no limitations on the structure of the transistor 160. For example, a bottom-gate transistor or a transistor having gate electrodes above and below a semiconductor film may be used as the transistor 160.

[0048] A planarization film 174 is provided on the transistor 160 to provide a flat surface by absorbing unevenness caused by elements such as the transistor 160 included in the pixel circuit. A capacitance electrode 182, a capacitance insulating film 184 on the capacitance electrode 182, and a pixel electrode 132 on the capacitance insulating film 184 can be disposed on the planarization film 174, thereby constituting an auxiliary capacitance element 180. In this structure, the pixel electrode 132 is shared by the light-emitting element 130 and the auxiliary capacitance element 180. An opening exposing the terminal 172 is provided in the planarization film 174, and the pixel electrode 132 is electrically connected to the terminal 172 through this opening, either directly or via a connection electrode 176 covering this opening. A partition wall 178, which is an insulating film, is provided to cover the end of the pixel electrode 132, and the EL layer 140 and the counter electrode 134 are disposed to cover the pixel electrode 132 and the partition wall 178. This electrically insulates adjacent light emitting elements 130 and prevents the EL layer 140 from being cut by the edge of the pixel electrode 132 .

[0049] (3) Other configurations As an optional configuration, one or more cap layers 190 may be provided on the counter electrode 134 to resonate light extracted from the counter electrode 134 and improve color purity and brightness in the front direction. Furthermore, a protective film 200 may be provided on the light-emitting element to prevent impurities such as water and oxygen from penetrating into the EL layer 140. The protective film 200 may be formed, for example, of a film containing a silicon-containing inorganic compound such as silicon nitride, or a layer containing a polymer such as an acrylic resin or an epoxy resin. For example, as shown in FIG. 5, the protective film 200 may be formed of a first layer 202 and a second layer 206 containing silicon nitride, with a layer containing a polymer provided between them.

[0050] Typically, each functional layer constituting the EL layer 140 is formed using a vapor deposition method. Therefore, metal masks are used to selectively position the functional layers in predetermined regions, but increasing the number of metal masks directly increases the manufacturing cost of the display device. However, in the display device 100, all or part of the layers other than the light-emitting layer 150, the first buffer layer 148, and the second buffer layer 152 are shared by all pixels 120 and can be simultaneously formed so as to be continuous across all pixels. Therefore, for example, the hole injection layer 142, the hole blocking layer 154, the electron blocking layer 146, the electron transport layer 156, and the electron injection layer 158 can be formed on all pixels 120 using the same metal mask, eliminating the need for separate coatings using multiple metal masks. Furthermore, the first buffer layer 148 and the light-emitting layer 150 of the red light-emitting element 130-1 can be formed using the same metal mask, and the light-emitting layer 150 and second buffer layer 152 of the blue light-emitting element 130-3 can also be formed using the same metal mask. Therefore, by applying the embodiments of the present invention, it is possible to prevent an increase in the manufacturing cost of the display device, and to provide the display device at a lower cost.

[0051] However, it is relatively difficult to establish an appropriate carrier balance in the light-emitting elements 130, particularly in the red light-emitting elements 130-1 and green light-emitting elements 130-2 that contain a thermally activated delayed fluorescent material, and the carrier balance is significantly influenced by the structures of the electron blocking layer 146 and the hole blocking layer 154 that are disposed near the light-emitting layer 150. For this reason, if the electron blocking layer 146 and the hole blocking layer 154 have a common structure across all of the light-emitting elements 130, the carrier balance of some of the light-emitting elements 130 will be disrupted, which is likely to result in a decrease in the efficiency of the light-emitting elements 130, an increase in the driving voltage, and a decrease in the lifespan of the light-emitting elements 130.

[0052] However, as described above, in the display device 100 according to one embodiment of the present invention, the second buffer layer 152 is provided between the light-emitting layer 150 and the hole blocking layer 154 in the blue light-emitting element 130-3. Also, the first buffer layer 148 can be provided between the electron blocking layer 146 and the light-emitting layer 150 in the red light-emitting element 130-1. Therefore, as demonstrated in the examples, an appropriate carrier balance can be achieved in any of the light-emitting elements 130. For example, even if the hole blocking layer 154 and the electron blocking layer 146, which have optimized carrier balance in the green light-emitting element 130-2, are used in the red light-emitting element 130-1 and the blue light-emitting element 130-3, the provision of the first buffer layer 148 and the second buffer layer 152 allows an appropriate carrier balance to be achieved in the red light-emitting element 130-1 and the blue light-emitting element 130-3 as well. As a result, a display device 100 with low power consumption, high efficiency, and high reliability can be provided at low cost.

[0053] 4. Variations In the display device 100, a buffer layer may also be provided in the green light-emitting element 130-2. Specifically, as shown in FIG. 6, in the green light-emitting element 130-2, a buffer layer (third buffer layer) 149 may be disposed between the electron blocking layer 146 and the light-emitting layer 150, in direct contact with them. The third buffer layer 149 contains the host material contained in the light-emitting layer 150. In other words, the material contained in the third buffer layer 149 is the same as the host material contained in the light-emitting layer 150. Preferably, the third buffer layer 149 is made of the host material and does not substantially contain other components. The thickness of the third buffer layer 149 is also, for example, 2.0 nm to 10 nm, or 2.0 nm to 8.5 nm. Adjusting the carrier balance of the green light-emitting element 130-2 using the third buffer layer 149 can improve reliability without impairing the characteristics of the green light-emitting element 130-2. Furthermore, since many functional layers (e.g., the hole blocking layer 154, the electron blocking layer 146, the electron transport layer 156, etc.) can be formed so as to be shared among all the pixels 120, the number of deposition masks can be reduced, and as a result, a display device can be provided at low cost. Note that when the third buffer layer 149 is provided, the first buffer layer 148 does not need to be provided. For example, if the structures of the electron blocking layer 146 and the hole blocking layer 154 are optimized for the red light-emitting element 130-1, which does not have the first buffer layer 148, the first buffer layer 148 may not be provided, and the third buffer layer 149 may be selectively disposed in the green light-emitting element 130-2.

[0054] 5. Display device manufacturing method The display device 100 can be manufactured by sequentially stacking functional layers on a substrate 102 having pixel circuits and pixel electrodes 132 fabricated by a known method. Specifically, a hole injection layer 142 is formed on the pixel electrodes 132 provided on the substrate 102 by spin coating, dip coating, inkjet printing, or vapor deposition. Then, a hole transport layer 144 and an electron blocking layer 146 are sequentially formed by vapor deposition. To form hole transport layers with different thicknesses among the red light-emitting element 130-1, the green light-emitting element 130-2, and the blue light-emitting element 130-3, a first hole transport layer 144-1 may be formed using a metal mask exposing all pixels 120, a second hole transport layer 144-2 may be formed using a metal mask exposing the green light-emitting pixel 120-2 and the red light-emitting pixel 120-1, and a third hole transport layer 144-3 may be formed using a metal mask exposing the red light-emitting pixel 120-1.

[0055] After this, the electron blocking layer 146 is formed by vapor deposition. Since separate coating is not required at this time, a metal mask that exposes all of the pixels 120 may be used, and the electron blocking layer 146 may be formed on all of the pixels 120 at the same time.

[0056] Thereafter, the light-emitting layers of the red light-emitting element 130-1, the green light-emitting element 130-2, and the blue light-emitting element 130-3 are formed by vapor deposition. At this stage, the light-emitting layer 150 is separately painted using a metal mask that exposes the red light-emitting pixel 120-1, the green light-emitting pixel 120-2, or the blue light-emitting pixel 120-3. When forming the first buffer layer 148, the first buffer layer 148 is formed in the red light-emitting pixel 120-1 using a metal mask that exposes the red light-emitting pixel 120-1, and then the light-emitting layer 150 is formed using the same metal mask. Similarly, when forming the third buffer layer 149, the third buffer layer 149 is formed in the green light-emitting pixel 120-2 using a metal mask that exposes the green light-emitting pixel 120-2, and then the light-emitting layer 150 is formed using the same metal mask.

[0057] Thereafter, the second buffer layer 152 is formed by vapor deposition using a metal mask that exposes the blue light-emitting pixel 120-3. Therefore, in the blue light-emitting pixel 120-3, the light-emitting layer 150 and the second buffer layer 152 are continuously formed using the same metal mask, eliminating the need to replace or realign the metal mask.

[0058] Subsequently, the hole blocking layer 154, the electron transport layer 156, and the electron injection layer 158 are formed by vapor deposition. Since separate coating is not required at this time either, a metal mask that exposes all of the pixels 120 may be used, and the hole blocking layer 154, the electron transport layer 156, and the electron injection layer 158 may be formed simultaneously in all of the pixels 120. However, if the electron transport layer 156 is formed so that its thickness varies among the pixels 120, the electron transport layer 156 may be formed by separate coating as appropriate, as in the formation of the hole transport layer 144.

[0059] The counter electrode 134 is formed by vapor deposition or sputtering. For example, the counter electrode 134 containing an alloy of silver and magnesium may be formed by vapor deposition, and the counter electrode 134 containing ITO or IZO may be formed by sputtering.

[0060] As described above, in the manufacturing method of the display device 100 according to one embodiment of the present invention, at least the electron blocking layer 146 and the hole blocking layer 154 can be simultaneously formed in all of the pixels 120, thereby reducing the number of metal masks. Furthermore, as described above, even if the same electron blocking layer 146 and the same hole blocking layer 154 are formed in all of the pixels 120, by appropriately arranging the first to third buffer layers 148 to 149, a good carrier balance can be achieved in all of the pixels 120. For example, if the electron blocking layer 146 and the hole blocking layer 154, which are shared by all of the pixels 120 and continuously and simultaneously formed across all of the pixels 120, are optimized for the green light-emitting element 130-2, a good carrier balance can also be achieved in the red light-emitting element 130-1 and the blue light-emitting element 130-3 by forming the first buffer layer 148 and the second buffer layer 152. Alternatively, when the electron blocking layer 146 and the hole blocking layer 154, which are shared by all pixels 120 and formed continuously and simultaneously across all pixels 120, are optimized for the red light-emitting element 130-1, good carrier balance can also be achieved in the green light-emitting element 130-2 and the blue light-emitting element 130-3 by forming the second buffer layer 152 and the third buffer layer 149. [Example]

[0061] 1. Effect of the first buffer layer on red light-emitting devices Several red light-emitting devices (Examples) with different first buffer layer (BL) thicknesses were fabricated, as well as two red light-emitting devices (Comparative Examples 1 and 2) without a first buffer layer. All light-emitting devices used ITO as the anode and a co-evaporated film of silver and magnesium as the cathode. The size of the light-emitting region was 2.0 mm × 2.0 mm. The light-emitting layer contained a host material and a red-emitting thermally activated delayed fluorescent material, and lithium fluoride was used as the electron injection layer. All light-emitting devices were identical except for the configuration of the electron blocking layer and the thickness of the first buffer layer. The thicknesses of the functional layers constituting the EL layer are shown in Table 1. The light-emitting device of Comparative Example 1 has an electron blocking layer optimized for red light-emitting devices, while the light-emitting devices of Comparative Example 2 and the Example have electron blocking layers used in light-emitting devices containing a green-emitting thermally activated delayed fluorescent material.

[0062] [Table 1]

[0063] Figure 7 shows the voltage-current density curve of the fabricated light-emitting device. Comparing Comparative Example 1 and Comparative Example 2, it can be seen that changing the material of the electron blocking layer shifts the voltage-current density curve to a higher voltage. This suggests that changing the electron blocking property generates an internal electric field due to carriers (electrons) accumulated at the interface between the electron blocking layer and the light-emitting layer, changing the carrier balance and increasing the driving voltage of the light-emitting device. However, providing a first buffer layer, as in the light-emitting device of the example, causes carrier diffusion. As a result, the voltage-current density curve shifts to a lower voltage, and it was found that voltage-current density characteristics similar to those of Comparative Example 1 can be obtained. This suggests that the first buffer layer improves the carrier balance even when a non-optimized electron blocking layer is used.

[0064] This is also suggested by Figures 8 and 9. Figure 8 is a plot showing the relationship between the drive voltage and the thickness of the first buffer layer for the light-emitting elements of Comparative Example 2 and Example, and it can be seen that the drive voltage decreases as the thickness of the first buffer layer increases. Figure 9 is a plot showing the relationship between the current efficiency and the thickness of the first buffer layer for the light-emitting elements of Comparative Example 2 and Example, and it can be seen that the current efficiency increases as the thickness of the first buffer layer increases. It is also suggested that the effect saturates as the thickness of the first buffer layer approaches 8 nm.

[0065] Figure 10 shows a plot of the current density and normalized external quantum efficiency. As is clear from a comparison between Comparative Example 1 and Comparative Example 2, when the first buffer layer is not optimized, the normalized external quantum efficiency drops significantly, especially in the low current density region. This suggests that when the current density is low, i.e., when the driving voltage is low, the carrier balance is significantly disrupted and the contribution of non-radiative recombination increases. Non-radiative recombination corresponds to, for example, the generation of exciplexes and light absorption by polarons. In contrast, when the first buffer layer is not optimized, 11, by providing a first buffer layer and increasing its thickness, the plot approaches that of Comparative Example 1, and the normalized external quantum efficiency is improved particularly in the low current density region. This also shows that even when a non-optimized electron blocking layer is used, the effect of the first buffer layer can be suppressed, and characteristics similar to or better than those of a light-emitting device including an optimized electron blocking layer can be obtained.

[0066] Figure 11 shows the voltage-capacity plots of the light-emitting devices of Comparative Examples 1 and 2 obtained by impedance spectroscopy. Comparing Comparative Examples 1 and 2 reveals that the capacitance at low voltages increases when the electron blocking layer is not optimized. This indicates that, without an optimized electron blocking layer, carriers accumulate in the EL layer, particularly at the interface between the electron blocking layer and the light-emitting layer, from the early stages of operation, generating a relatively large internal electric field. In contrast, as can be seen from Figure 12, the increase in capacitance in the low-voltage region of the light-emitting device of the example disappears as the thickness of the first buffer layer increases, and the plot approaches that of the light-emitting device of Comparative Example 1. These results also indicate that the first buffer layer improves carrier balance, suppresses the generation of an internal electric field, and reduces the driving voltage. Furthermore, the capacitance in the high-voltage region actually increases. This suggests that the carriers available for recombination increase, improving efficiency. In other words, even if the structure of the electron blocking layer in a red light-emitting device changes, the effect of this change can be suppressed by the first buffer layer.

[0067] 2. Effect of the second buffer layer on blue light-emitting devices Several blue light-emitting devices (Examples) with different second buffer layer thicknesses and a blue light-emitting device without a second buffer layer (Comparative Example 3) were fabricated. All light-emitting devices used ITO as the anode and a co-evaporated film of silver and magnesium as the cathode. The size of the light-emitting region was 2.0 mm × 2.0 mm. The light-emitting layers of the Example and Comparative Example 3 contained a host material and a blue-emitting fluorescent material that did not exhibit thermally activated delayed fluorescence, and lithium fluoride was used as the electron injection layer. The second buffer layer was formed by independently evaporating the host material contained in the light-emitting layer. All light-emitting devices had the same configuration except for the thickness of the second buffer layer. The thicknesses of the functional layers constituting the EL layer are shown in Table 2. The light-emitting device of Comparative Example 3 is a red light-emitting device without a first buffer layer and has a hole-blocking layer optimized for this purpose.

[0068] [Table 2]

[0069] The voltage-current density curve of the fabricated light-emitting device is shown in Figure 13. Compared with Comparative Example 3, when the second buffer layer was approximately 1 nm thick, the voltage-current characteristics were nearly the same as those of Comparative Example 3. However, as the second buffer layer thickness increased from 2 nm, the voltage-current density curve shifted to a lower voltage. This suggests that in Comparative Example 3, an internal electric field was generated by carriers (holes) accumulated at the interface between the hole blocking layer and the light-emitting layer, changing the carrier balance and increasing the driving voltage of the light-emitting device. However, as in the light-emitting device of the example, the provision of a second buffer layer caused carrier diffusion. As a result, the voltage-current density curve shifted to a lower voltage, and it was found that the light-emitting device could be driven at a lower voltage than in Comparative Example 3. This suggests that the second buffer layer improves the carrier balance even when an unoptimized hole blocking layer is used.

[0070] This is also suggested by the relationship between the driving voltage and current efficiency as a function of the thickness of the second buffer layer in the blue light-emitting devices of Comparative Example 3 and Example 1, shown in Figures 14 and 15, respectively. As shown in these figures, as the thickness of the second buffer layer increases, the driving voltage decreases and the current efficiency increases. These results also show that the provision of the second buffer layer improves the carrier balance, and that power consumption decreases significantly, especially when the thickness of the second buffer layer exceeds 3 nm.

[0071] Similar results were also obtained for the relationship between current density and normalized current efficiency of the blue light-emitting devices of Comparative Example 3 and the Examples. As shown in FIG. 16, when Comparative Example 3 or the second buffer layer was thin (e.g., 1 nm), the normalized current efficiency decreased significantly, especially in the low current density region. This suggests that when the current density is low, i.e., when the driving voltage is low, the carrier balance is significantly disrupted, increasing the contribution of non-radiative recombination. Examples of non-radiative recombination include exciplex generation and polaron absorption. In contrast, FIG. 16 shows that providing a buffer layer and increasing its thickness (e.g., 2 nm or more) improves the normalized current efficiency in the low current density region compared to the plot of Comparative Example 3. This also shows that even when a non-optimized hole-blocking layer is used, the effect of the second buffer layer can be suppressed, resulting in characteristics similar to or better than those of a light-emitting device including an optimized hole-blocking layer.

[0072] Figure 17 shows the voltage-capacity plots of the blue light-emitting devices of Comparative Example 3 and the Example obtained by impedance spectroscopy. When the thickness of the second buffer layer in Comparative Example 3 or the Example was small (e.g., 2 nm), the capacitance increased, and this tendency was particularly pronounced at low voltages. This indicates that when the hole blocking layer is not optimized, carriers accumulate in the EL layer, especially at the interface between the hole blocking layer and the light-emitting layer, from the early stage of operation, generating a relatively large internal electric field. In contrast, in the Example light-emitting device, the increase in capacitance in the low-voltage region is eliminated as the buffer layer thickness increases. These results also indicate that the buffer layer improves the carrier balance, suppresses the generation of an internal electric field, and reduces the operating voltage. Furthermore, the capacitance in the high-voltage region also decreases. This suggests that the carriers available for recombination increase, further improving the carrier balance and allowing for more efficient recombination. This result also indicates that even if the structure of the hole blocking layer in the blue light-emitting device changes, the effect of this change can be suppressed by the second buffer layer.

[0073] As described above, light-emitting elements containing thermally activated delayed fluorescent materials can achieve extremely high luminous efficiency, but achieving an optimal carrier balance is not necessarily easy, and they are particularly susceptible to the influence of carrier blocking layers (hole blocking layer and electron blocking layer). Therefore, when light-emitting elements containing thermally activated delayed fluorescent materials are used in full-color display devices that require the three primary colors, it is necessary to individually adjust optimal carrier blocking layers for the red, green, and blue light-emitting elements. This increases the number of deposition processes that require so-called separate coating, resulting in a significant increase in the manufacturing cost of the display device.

[0074] However, in a display device according to one embodiment of the present invention, even if the carrier block layers of all light-emitting elements arranged in all pixels emitting different colors have the same structure, an appropriate carrier balance can be achieved in all light-emitting elements by providing the second buffer layer 152 or both the second buffer layer 152 and the first buffer layer 148. This makes it possible to prevent an increase in the vapor deposition process that requires separate coating, and to provide a display device with low power consumption and high reliability at low cost.

[0075] The above-described embodiments of the present invention can be combined as appropriate as long as they are not mutually inconsistent. Furthermore, even if a person skilled in the art appropriately adds or deletes components or modifies the design of a display device of each embodiment, or adds or omits processes or modifies conditions, such a display device is included in the scope of the present invention as long as it includes the gist of the present invention.

[0076] Even if there are other effects and advantages different from those brought about by the aspects of each of the above-mentioned embodiments, those that are clear from the description in this specification or that can be easily predicted by a person skilled in the art are naturally understood to be brought about by the present invention. [Explanation of symbols]

[0077] 100: display device, 102: substrate, 104: counter substrate, 106: scanning line drive circuit, 108: signal line drive circuit, 110: terminal, 112: undercoat, 120: pixel, 120-1: red light-emitting pixel, 120-2: green light-emitting pixel, 120-3: blue light-emitting pixel, 130: light-emitting element, 130-1: red light-emitting element, 130-2: green light-emitting element, 130-3: blue light-emitting element, 132: pixel electrode, 134: counter electrode, 140: EL layer, 142: hole injection layer, 144: hole transport layer, 144-1: first hole transport layer, 144-2: second hole transport layer, 144-3: third hole hole transport layer, 146: electron blocking layer, 148: first buffer layer, 149: third buffer layer, 150: light emitting layer, 152: second buffer layer, 154: hole blocking layer, 156: electron transport layer, 158: electron injection layer, 160: transistor, 162: semiconductor film, 164: gate insulating film, 166: gate electrode, 168: interlayer insulating film, 170: terminal, 172: terminal, 174: planarizing film, 176: connection electrode, 178: partition wall, 180: auxiliary capacitance element, 182: capacitance electrode, 184: capacitance insulating film, 190: cap layer, 200: protective film, 202: first layer, 206: second layer

Claims

1. a first pixel, a second pixel, and a third pixel, each having a first light-emitting element, a second light-emitting element, and a third light-emitting element configured to emit red light, green light, and blue light, respectively; Each of the first light-emitting element, the second light-emitting element, and the third light-emitting element is pixel electrodes, an electron blocking layer on the pixel electrode; a light-emitting layer on the electron blocking layer; a hole-blocking layer on the light-emitting layer; and a counter electrode on the hole blocking layer, the third light-emitting element further includes a first buffer layer between the light-emitting layer and the hole-blocking layer; the light-emitting layers of the first light-emitting element and the second light-emitting element contain a thermally activated delayed fluorescent material; The display device, wherein the light-emitting layer of the third light-emitting element contains a first fluorescent material having a fluorescence lifetime of 1 ps or more and less than 1 ns.

2. The display device according to claim 1 , wherein the electron blocking layer and the hole blocking layer are present in the same layer across the first light-emitting element, the second light-emitting element, and the third light-emitting element.

3. The display device according to claim 1 , wherein the thermally activated delayed fluorescent material has a fluorescence lifetime of 1 ns or more.

4. 2. The display device according to claim 1, wherein at least one of the light-emitting layers of the first light-emitting element and the second light-emitting element further contains a second fluorescent material having a fluorescence lifetime of 1 ps or more and less than 1 ns.

5. The display device according to claim 1 , wherein the first light-emitting element further comprises a second buffer layer between the electron blocking layer and the light-emitting layer.

6. The display device according to claim 1 , wherein the second light-emitting element further comprises a third buffer layer between the electron blocking layer and the light-emitting layer.

7. the light-emitting layer of the first light-emitting element further comprises a first host material; The display device according to claim 5 , wherein the second buffer layer is made of the first host material.

8. the light-emitting layer of the second light-emitting element further comprises a second host material; The display device according to claim 6 , wherein the third buffer layer is made of the second host material.

9. the light-emitting layer of the third light-emitting element further contains a third host material; The display device according to claim 1 , wherein the first buffer layer is made of the third host material.

10. 2. The display device according to claim 1, wherein in the third light-emitting element, a difference in lowest unoccupied molecular orbital level between the light-emitting layer and the first buffer layer is 0.1 eV or more and 0.3 eV or less.

11. 2. The display device according to claim 1, wherein in the third light-emitting element, a difference in lowest unoccupied molecular orbital level between the first buffer layer and the electron blocking layer is 0.1 eV or more and 0.3 eV or less.

12. 2. The display device according to claim 1, wherein in the third light-emitting element, a difference in highest occupied molecular orbital level between the light-emitting layer and the first buffer layer is 0.1 eV or more and 0.3 eV or less.

13. 2. The display device according to claim 1, wherein in the third light-emitting element, a difference in highest occupied molecular orbital level between the first buffer layer and the electron blocking layer is 0.1 eV or more and 0.3 eV or less.

14. The display device according to claim 1 , wherein the first buffer layer has a thickness of 2 nm to 8.5 nm.

15. In the first light-emitting element, 6. The display device according to claim 5, wherein a difference in highest occupied molecular orbital level between the second buffer layer and the hole blocking layer is 0.1 eV or more and 0.3 eV or less.

16. The display device according to claim 5 , wherein the second buffer layer has a thickness of 1 nm or more and 7 nm or less.

17. The display device according to claim 8 , wherein the third buffer layer has a thickness of 1 nm or more and 7 nm or less.

Images