Patent ID: 12232345

DESCRIPTION OF EMBODIMENTS

In the following description, the term “in the same layer” refers to that one layer is formed in the same process step (film formation step) as a comparative layer. In addition, the term “in a lower layer than (or under)” refers to that one layer is formed in a process step anterior to a process step of forming a comparative layer. In addition, the term “in a higher layer than (or above)” refers to that one layer is formed in a process step posterior to a process step of forming a comparative layer.

First Embodiment

FIG.1is a schematic sectional view of the configuration of a light-emitting device1according to this embodiment. The light-emitting device1is used in displays of TV sets, smartphones, and other equipment. As illustrated inFIG.1, the light-emitting device1in this embodiment has a plurality of pixels PX arranged on an array substrate10. Each pixel PX includes sub-pixels2B,2G, and2R. In other words, the sub-pixels2B,2G, and2R constitute a single pixel PX by way of example.

A sub-pixel2B (first sub-pixel) emits blue light (first light), which is light having a blue light-emission wavelength. A sub-pixel2G (second sub-pixel, first sub-pixel) emits green light (second light, first light), which is light having a green light-emission wavelength longer than the blue light-emission wavelength. A sub-pixel2R (third sub-pixel, second sub-pixel) emits red light (third light, second light), which is light having a red light-emission wavelength longer than the green light-emission wavelength. By way of example, the sub-pixel2G is adjacent to the sub-pixel2R in a plan view. In addition, the sub-pixel2B is adjacent to the sub-pixel2G in the plan view. It is noted that the order of arrangement of the sub-pixels2R,2G and2B is changeable freely.

It is noted that blue light refers to light having a light-emission center wavelength that falls within a wavelength band of 400 to 500 nm inclusive. It is also noted that green light refers to light having a light-emission center wavelength that falls within a wavelength band greater than 500 nm and equal to or smaller than 600 nm. It is also noted that red light refers to light having a light-emission center wavelength that falls within a wavelength band greater than 600 nm and equal to or smaller than 780 nm.

Light emitters3B,3G, and3R are disposed in a region sectioned by an insulating bank70(pixel restricting layer), which is disposed on the array substrate10. The light emitter3B is included in the sub-pixel2B, the light emitter3G is included in the sub-pixel2G, and the light emitter3R is included in the sub-pixel2R.

The array substrate10is provided with TFTs or thin-film transistors (not shown), which are provided for regulating the emission and non-emission of light from each light emitter3. The array substrate10in this embodiment is composed of a flexible resin layer with TFTs thereon. The resin layer in this embodiment is composed of a resin film (e.g., a polyimide film) with a barrier layer, which is herein an inorganic insulating film (e.g., a silicon oxide film, a silicon nitride film, or a silicon oxide nitride film), stacked thereon. In some embodiments, the array substrate10may be composed of a rigid substrate (e.g., a glass substrate) with TFTs thereon. The array substrate10in this embodiment has an upper surface with an interlayer insulating film20(flattening film) thereon. The interlayer insulating film20is made of a material, such as polyimide or acrylic. The interlayer insulating film20has a plurality of contact holes CH. In the following description, the array substrate10and the interlayer insulating film20, disposed on the upper surface of the array substrate10, are together also referred to as a substrate13.

In this embodiment, the light emitter (first light emitter)3B, included in the sub-pixel2B, has the following by way of example: an anode (first anode)31B; a hole transport layer (first hole transport layer)46B overlapping the anode31B; a light-emitting layer (first light-emitting layer)80B overlapping the hole transport layer46B; an electron transport layer (first electron transport layer)47B overlapping the light-emitting layer80B; and a cathode (first cathode)32B overlapping the electron transport layer47B. In addition, the light emitter (second light emitter, first light emitter)3G, included in the sub-pixel2G, has the following: an anode (second anode, first anode); a hole transport layer (a second hole transport layer, the first hole transport layer)46G overlapping the anode31G; a light-emitting layer (second light-emitting layer, first light-emitting layer)80G overlapping the hole transport layer46G; an electron transport layer (second electron transport layer, first electron transport layer)47G overlapping the light-emitting layer80G; and a cathode (second cathode, first cathode)32G overlapping the electron transport layer47G. In addition, the light emitter (third light emitter, second light emitter)3R, included in the sub-pixel2R, has the following: an anode (third anode, second anode)31R; a hole transport layer (third hole transport layer, second hole transport layer)46R overlapping the anode31R; a light-emitting layer (third light-emitting layer, second light-emitting layer)80R overlapping the hole transport layer46R; an electron transport layer (third electron transport layer, second electron transport layer)47R overlapping the light-emitting layer80R; and a cathode (third cathode, second cathode)32R overlapping the electron transport layer47R.

The anode31B injects holes into the hole transport layer46B. The anode31G injects holes into the hole transport layer46G. The anode31R injects holes into the hole transport layer46R. As illustrated inFIG.1, the anodes31B,31G, and31R in the this embodiment are disposed, on the interlayer insulating film20, in the form of an island in respective regions constituting the sub-pixels2B,2G and2R (i.e., in the respective light emitters3B,3G and3R). The anodes31B,31G, and31R are electrically connected to respective TFTs (not shown) via the contact holes CH, which are bored in the interlayer insulating film20. The anodes31B,31G, and31R are structured in such a manner that, for instance, metal including Al, Cu, Au or Ag, all of which have high reflectivity of visible light, and ITO, IZO, IZO, ZnO, AZO, BZO or GZO, all of which are transparent materials, are stacked on the array substrate10in this order. A first electrode31is formed through sputtering or evaporation for instance.

The bank70covers the contact holes CH. The bank70is formed by, for instance, application of an organic material, such as polyimide or acrylic, onto the array substrate10, followed by patterning through photolithography. The bank70in this embodiment covers the edges of the individual anodes31B,31G and31R, as illustrated inFIG.1. That is, the bank70in this embodiment serves also as an edge cover of each of the anodes31B,31G and31R. Such a configuration can prevent an excessive electric field at the edges of the individual anodes31B,31G and31R.

The hole transport layer46B transports the holes injected from the anode31B, further to the light-emitting layer80B. The hole transport layer46G transports the holes injected from the anode31G, further to the light-emitting layer80G. The hole transport layer46R transports the holes injected from the anode31R, further to the light-emitting layer80R. The hole transport layer46B is disposed between the anode31B and light-emitting layer80B and on the anode31B, and the hole transport layer46B is electrically connected to the anode31B. The hole transport layer46G is disposed between the anode31G and light-emitting layer80G and on the anode31G, and the hole transport layer46G is electrically connected to the anode31G. The hole transport layer46R is disposed between the anode31R and light-emitting layer80R and on the anode31R, and the hole transport layer46R is electrically connected to the anode31R. The hole transport layers46B,46G and46R in this embodiment are disposed in the form of an island in respective regions defining the sub-pixels2B,2G and2R (in other words, in the respective light emitters3B,3G and3R).

The hole transport layers46B,46G, and46R each contain a hole transport material. The hole transport layers46B,46G, and46R may contain, for instance, polyethylenedioxythiophene/polystyrenesulfonate (PEDOT:PSS), poly-N-vinylcarbazole (PVK), poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)diphenylamine))](TFB), or N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine (poly-TPD). Alternatively, the hole transport layers46B,46G, and46R may contain a plurality of materials among these materials. The hole transport layers46B,46G, and46R are formed through, but not limited to, an ink-jet method, evaporation using a mask, or photolithography using a mask. The hole transport layers46B,46G, and46R may contain respective hole transport materials different from each other. By way of example, the hole transport layers46B,46G, and46R in this embodiment contain a hole transport material of the same kind.

The light-emitting layer80B is disposed between the cathode32B and anode31B. To be specific, the light-emitting layer80B in this embodiment is disposed between the hole transport layer46B and electron transport layer47B. Moreover, the light-emitting layer80G is disposed between the cathode32G and anode31G. To be specific, the light-emitting layer80G in this embodiment is disposed between the hole transport layer46G and electron transport layer47G. Moreover, the light-emitting layer80R is disposed between the cathode32R and anode31R. To be specific, the light-emitting layer80R in this embodiment is disposed between the hole transport layer46R and electron transport layer47R. The light-emitting layers80B,80G, and80R are formed through, but not limited to, an ink-jet method, evaporation using a mask, or photolithography using a mask.

The light-emitting layer80B contains a quantum dot (first quantum dot)81B. The light-emitting layer80G contains a quantum dot (second quantum dot, first quantum dot)81G. The light-emitting layer80R contains a quantum dot (third quantum dot, second quantum dot)81R.

The quantum dots81B,81G, and81R each have a valence band level (equal to an ionization potential) and a conduction band level (equal to an electron affinity) and are each a material that emits light upon rejoining of holes at the valence band level and electrons at the conduction band level together. Light emitted from the quantum dots81B,81G and81R has a narrow spectrum due to a quantum confinement effect, and hence the emitted light can have relatively deep chromaticity.

The quantum dot81B emits blue light. The quantum dot81G emits green light, which has a longer light-emission wavelength than blue light. The quantum dot81R emits red light, which has a longer light-emission wavelength than green light.

The quantum dots81B,81G, and81R may each contain CdSe, ZnSe, CdZnSe, or InP for instance. The quantum dots81B,81G, and81R may be each a semiconductor nanoparticle having a core-shell structure with a core and shell. The quantum dots81B,81G and81R in this embodiment have a core-shell structure with CdSe serving as a core, and with ZnS serving as a shell. Further, the perimeter of the shell in the light-emitting layer80B may coordinate with a ligand82B, the perimeter of the shell in the light-emitting layer80G may coordinate with a ligand82G, and the perimeter of the shell in the light-emitting layer80R may coordinate with a ligand82R. The ligands82B,82G, and82R are composed of an organic substance, including thiol and amine.

By way of example, the quantum dots81B,81G, and81R may be configured in the following manner. That is, one of any two of the quantum dots81B,81G and81R has a first core and a first shell covering the first core. In addition, the other quantum dot has a second core and a second shell covering the second core. The first core has composition expressed by General Formula (1) below.
Ax1B1-x1C  (1)

The second core preferably belongs to the same composition group as the first core and has composition expressed by General Formula (2) below.
A′x3B′1-x3C′  (2)

In General Formulas (1) and (2), A and A′ are the same element and are an element selected from among the group 12 elements (e.g., Zn and Cd) or group 13 elements (e.g., In). In addition, B and B′ are the same element and are an element selected from among elements belonging to the same group as A and different from A. In addition, C or C′ is one or more elements selected from among the group 16 elements (e.g., Se or Te) when A and B are group 12 elements, and C or C′ is one or more elements selected from among the group 15 elements (e.g., P) when A and B are group 13 elements. In addition, |X1−X3|≤0.2 is satisfied. In addition, 0≤X1≤1 and 0≤X3≤1 are satisfied.

At least one of A and B may have a smaller atomic number than Hg.

Examples of the quantum dot81B include ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, Cd0.25Ze0.75Se, GaN, GaP, GaAs, InN, InP, InAs, In0.25Ga0.75N, In0.25Ga0.75P. The quantum dots81G and81R are preferably CdSe when the quantum dot81B is CdSe, and the quantum dots81G and81R are preferably Cd0.2Ze0.75Se when the quantum dot81B is Cd0.25Ze0.75Se.

The first and second shells may be a material that is selected as appropriate in accordance with the material of the first and second cores and is publicly known in the field.

The quantum dots81B,81G, and81R each have a particle diameter of about 3 to 15 nm for instance. The wavelength of light emitted from the quantum dots81B,81G and81R can be regulated by the particle diameter of the quantum dots81B,81G and81R. Accordingly, regulating the particle diameters of the quantum dots81B,81G and81R individually can obtain the respective colors of emitted light. The average particle diameter of a plurality of quantum dots81B contained in the light-emitting layer80B is smaller than the average particle diameter of a plurality of quantum dots81G contained in the light-emitting layer80G. The average particle diameter of a plurality of quantum dots81G contained in the light-emitting layer80G is smaller than the average particle diameter of a plurality of quantum dots81R contained in the light-emitting layer80R.

The quantum dots81B,81G, and81R may contain respective materials different from each other. By way of example, the quantum dots81B,81G and81R in this embodiment contain the same material and have mutually different particle diameters.

In the light-emitting device1according to this embodiment, the thickness, T1, of the light-emitting layer80B is larger than the thickness, T2, of the light-emitting layer80G and the thickness, T3, of the light-emitting layer80R. In addition, the thickness T2of the light-emitting layer80G is larger than the thickness T3of the light-emitting layer80R.

For instance, the thickness T1of the light-emitting layer80B is the thickness of the center of the light-emitting layer80B, the thickness T2of the light-emitting layer80G is the thickness of the center of the light-emitting layer80G, and the thickness T3of the light-emitting layer80R is the thickness of the center of the light-emitting layer80R. The thickness of each light-emitting layer can be determined by, for instance, cutting out the section of a pixel and measuring the pixel section with a scanning electron microscope (SEM), a transmission electron microscopy (TEM/STEM), or other types of microscope. The light-emitting layers80B,80G, and80R will be detailed later on.

The electron transport layer47B transports the electrons injected from the cathode32B, further to the light-emitting layer80B. The electron transport layer47G transports the electrons injected from the cathode32G, further to the light-emitting layer80G. The electron transport layer47R transports the electrons injected from the cathode32R, further to the light-emitting layer80R. The second electron transport layers47B,47G, and47R may have the function of preventing hole transport to the cathodes32B,32G, and32R (i.e., a hole blocking function). The electron transport layer47B is disposed between the cathode32B and light-emitting layer80B and on the light-emitting layer80B. The electron transport layer47G is disposed between the cathode32G and light-emitting layer80G and on the light-emitting layer80G. The electron transport layer47R is disposed between the cathode32R and light-emitting layer80R and on the light-emitting layer80R. The electron transport layers47B,47G and47R in this embodiment are disposed in the form of an island in respective regions defining the sub-pixels2B,2G and2R (in other words, in the respective light emitters3B,3G and3R).

The electron transport layers47B,47G, and47R may each contain ZnO, ZnMgO, or 1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene (TPBi), or the electron transport layers47B,47G, and47R may each contain a plurality of materials among these materials. The electron transport layers47B,47G, and47R are formed through, but not limited to, an ink-jet method, evaporation using a mask, or photolithography using a mask. The electron transport layers47B,47G, and47R may contain respective electron transport materials different from each other. By way of example, the electron transport layers47B,47G and47R in this embodiment contain the same electron transport material.

The cathode32B is disposed on the electron transport layer47B and is electrically connected to the electron transport layer47B. The cathode32G is disposed on the electron transport layer47G and is electrically connected to the electron transport layer47G. The cathode32R is disposed on the electron transport layer47R and is electrically connected to the electron transport layer47R. The cathode32B injects electrons into the electron transport layer47B. The cathode32G injects electrons into the electron transport layer47G. The cathode32R injects electrons into the electron transport layer47R. The cathodes32B,32G and32R in this embodiment are provided as a single continuous layer extending across the plurality of sub-pixels2B,2G and2R (i.e., the light emitters3B,3G and3R).

The cathodes32B,32G, and32R are made of, for instance, metal or transparent material processed into a film that is thin enough to allow light to pass therethrough. Examples of the metal constituting the cathodes32B,32G and32R include metals, including Al, Ag and Mg. Moreover, examples of the transparent material constituting the cathodes32B,32G and32R include ITO, IZO, ZnO, AZO, BZO and GZO. The cathodes32B,32G, and32R are formed through sputtering or evaporation for instance.

Disposed on the cathodes32B,32G and32R is a sealing layer (not shown). The sealing layer includes the following for instance: an inorganic sealing film covering the cathodes32B,32G and32R; an organic layer located in a higher layer than the inorganic sealing film and is composed of an organic buffer layer; and an inorganic sealing film located in a higher layer than the organic buffer layer. The sealing layer prevents foreign substances, such as water and oxygen, from intruding into the inside of the light-emitting device1. The inorganic sealing films are inorganic insulating films. The inorganic sealing films are composed of a silicon oxide film, a silicon nitride film, or a silicon oxide nitride film, all of which are formed through CVD for instance, or are composed of a laminated film of these materials. The organic buffer layer is a light-transparency organic film that flattens a layer or film, and the organic buffer layer can be made of an organic material that can be applied, such as acrylic. Further, a function film (not shown) may be disposed on the sealing layer. The function film may serve as at least one of an optical compensator, a touch sensor and a protector for instance.

The holes injected from the anodes31B,31G and31R are respectively transported through the hole transport layers46B,46G and46R to the light-emitting layers80B,80G and80R. In addition, the electrons injected from the cathodes32B,32G and32R are respectively transported through the electron transport layers47B,47G and47R to the light-emitting layers80B,80G and80R. The holes and electrons transported to the light-emitting layers80B,80G and80R rejoin together within the quantum dots81B,81G and81R, thus generating excitons. The excitons then change from an excitation state back to a ground state, and thus the quantum dots81B,81G and81R emit light. The light-emitting device1in this embodiment is a top-emission type, where light emitted from the light-emitting layers80B,80G and80R is taken out via the opposite side of the array substrate10(i.e., from above inFIG.1). In some embodiments, the light-emitting device1may be a bottom-emission type, where light is taken out via the array substrate10(i.e., from below inFIG.1). In this case, the cathode32B,32G and32R need to be reflective electrodes, and the anodes31B,31G and31R need to be transparent electrodes.

The light-emitting device1in this embodiment includes the following stacked sequentially on the array substrate10: the anodes31B,31G, and31R; the hole transport layers46B,46G, and46R; the light-emitting layers80B,80G, and80R; the electron transport layers47B,47G, and47R; and the cathodes32B,32G, and32R. In some embodiments, the structure of the light-emitting device1may be inverted; that is, the cathodes32B,32G and32R, the electron transport layers47B,47G and47R, the light-emitting layers80B,80G and80R, the hole transport layers46B,46G and46R, and the anodes31B,31G and31R may be stacked sequentially on the array substrate10.

FIG.2illustrates example energy indicating the electron affinity and ionization potential of each of the light-emitting layers80B,80G and80R of the light-emitting device1according to the first embodiment.FIG.2illustrates, from the left to the right, the energy of the light-emitting layer80R, the energy of the light-emitting layer80G, and the energy of the light-emitting layer80B. When each of the quantum dots81B,81G and81R has a core-shell structure inFIG.2,FIG.2illustrates example energy indicating the electron affinities and ionization potentials of the individual cores of the quantum dots81B,81G and81R, which are respectively included in the light-emitting layers80B,80G and80R.FIG.3illustrates example energy indicating the Fermi level or electron affinity and ionization potential of each layer included in the light emitter3R of the light-emitting device1according to the first embodiment.FIG.4illustrates example energy indicating the Fermi level or electron affinity and ionization potential of each layer included in the light emitter3G of the light-emitting device1according to the first embodiment.FIG.5illustrates example energy indicating the Fermi level or electron affinity and ionization potential of each layer included in the light emitter3B of the light-emitting device1according to the first embodiment.FIG.3illustrates, from the left to the right, the energy of the anode31R, the energy of the hole transport layer46R, the energy of the light-emitting layer80R, the energy of the electron transport layer47R, and the energy of the cathode32R.FIG.4illustrates, individually from the left to the right, the energy of the anode31G, the energy of the hole transport layer46G, the energy of the light-emitting layer80G, the energy of the electron transport layer47G, and the energy of the cathode32G.FIG.5illustrates, individually from the left to the right, the energy of the anode31G, the energy of the hole transport layer46G, the energy of the light-emitting layer80G, the energy of the electron transport layer47G, and the energy of the cathode32G.

The Fermi levels of the individual anodes31R,31G and31B and the Fermi levels of the individual cathodes32R,32G and32B are expressed in the unit eV. The ionization potentials of the individual hole transport layers46R,46G and46B, the ionization potentials of the individual light-emitting layers80R,80G and80B, and the ionization potentials of the individual electron transport layers47R,47G and47B are expressed therebelow in the unit eV with reference to a vacuum level. In addition, the electron affinities of the individual hole transport layers46R,46G and46B, the electron affinities of the individual light-emitting layers80R,80G and80B, and the electron affinities of the individual electron transport layers47R,47G and47B are expressed thereabove in the unit eV with reference to a vacuum level.

The Description will hereinafter describe an ionization potential or an electron affinity with reference to a vacuum level when merely addressing them.

The quantum dots81B,81G and81R in this embodiment contain respective cores belonging to the same composition group. The results of measurements conducted by the inventors have demonstrated that the valence band levels (equal to an ionization potential) of the individual cores are substantially the same irrespective of the wavelength of light emitted by the quantum dots81B,81G and81R when the quantum dots81B,81G and81R have their respective cores belonging to the same material group.

The ionization potentials of the light-emitting layers were measured in the following manner.

The measurements were conducted on the assumption that quantum dots had the substantially same composition, and that the ionization potentials of quantum dots having the same particle diameter (herein, with a tolerance of −2 to +2 nm inclusive) were equal to each other. Here, that the ionization potentials are equal to each other includes a tolerance of −0.1 to +0.1 eV inclusive.

The inventors firstly cut the display through laser cutting to expose the sections of the individual light-emitting layers. The inventors then observed the exposed sections by SEM-EDX to identify the composition and particle diameter of the quantum dots. To be specific, the composition of the quantum dots was CdSe. The inventors calculated the particle diameter of the quantum dots by freely selecting about 100 quantum dots within the quantum dot layer that is about 30 nm thick and included in a field of view about 2 to 3 m inclusive, then measuring the area of each selected quantum dot, and then determining the mean value of the diameters of circles having the measured area. The particle diameter of the quantum dots stood at 5 nm.

The inventors then produced quantum dots having the identified composition and identified particle diameter. The inventors next dispersed the produced quantum dots into an organic solvent, such as hexane or toluene, and then adjusted the dispersed solution. The inventors next applied the adjusted dispersed solution onto an ITO film on a glass substrate having a main surface with an indium tin oxide (ITO) film (70 nm thick) thereon, and then vaporized the organic solvent to thus form a 30 nm thick light-emitting layer, which functioned as a sample for ionization potential measurement.

The produced sample underwent a photoelectron spectroscopy measurement using a photoemission yield spectroscopy in air (“AC-3”, made by RIKEN KEIKI Co., Ltd.), thus measuring the ionization potential.

To be specific, the inventors fixed input power at such a power level that a peak observed at around 4.8 eV and resulting from the ITO film is not observed substantially, and the inventors measured quantum yield while changing electron volt (eV), to thus determine the relationship between the electron volt and quantum yield. As a result, the inventors specified, as an ionization potential, the electron volt at which the quantum yield rises when the electron volt is increases.

The ionization potentials of the quantum dots81B,81G and81R are equal to each other and stand at 5.4 eV. That the ionization potentials are equal to each other includes a tolerance of −0.1 to +0.1 eV inclusive.

In contrast, the conduction band levels (equal to an electron affinity) of the quantum dots81B,81G and81R change depending on the wavelength of light emitted by the quantum dots81B,81G and81R even when these dots are made of respective materials belonging to the same group. In particular, the conduction band levels of the quantum dots81B,81G and81R have a deeper energy level along with increase in the wavelength of light emitted by the quantum dots81B,81G and81R, and the conduction band levels thereof have a shallower energy level along with decrease in the wavelength of light emitted by the quantum dots81B,81G and81R.

For instance, the light-emitting layers80R,80G and80B in this embodiment have an ionization potential of 5.4 eV, which is substantially the same between different sub-pixels, as illustrated inFIG.2, whereas the light-emitting layers80R,80G and80B in this embodiment have an electron affinity of 3.4 eV, an electron affinity of 3.1 eV, and an electron affinity of 2.7 eV, respectively. The electron affinity of the quantum dot81B is smaller than the electron affinity of the quantum dot81G, as described above. In addition, the electron affinity of the quantum dot81G is smaller than the electron affinity of the quantum dot81R.

As illustrated inFIGS.3to5, the light emitters3R,3G, and3B are configured, for instance, such that the anodes31R,31G, and31B each contain ITO, such that the hole transport layers46R,46G, and46B each have a layer containing PEDOT:PSS and a layer containing TFB, such that the electron transport layers47R,47G, and47B each contain ZnO, and such that the cathodes32R,32G, and32B contain Al.

In this case, each of the anodes (ITO)31R,31G and31B has a Fermi level of 4.8 eV. The PEDOT:PSS-containing layers in the hole transport layers46R,46G and46B each have a Fermi level of 5.4 eV. The TFB-containing layers in the hole transport layers46R,46G and46B each have an ionization potential of 5.4 eV and an electron affinity of 2.4 eV. In this way, the hole transport materials contained in the respective hole transport layers46R,46G and46B have ionization potentials equal to each other. Further, the hole transport materials contained in the respective hole transport layers46R,46G and46B have electron affinities equal to each other. That the ionization potentials are equal to each other includes a tolerance of −0.1 to +0.1 eV inclusive. Further, that the electron affinities are equal to each other includes a tolerance of −0.1 to +0.1 eV inclusive.

The electron transport layers (ZnO)47R,47G, and47B each have an ionization potential of 7.2 eV and an electron affinity of 3.9 eV. The electron transport materials contained in the respective electron transport layers47R,47G and47B have ionization potentials equal to each other. Further, the electron transport materials contained in the respective electron transport layers47R,47G and47B have electron affinities equal to each other. That the ionization potentials are equal to each other includes a tolerance of −0.1 to +0.1 eV inclusive. Further, that the electron affinities are equal to each other includes a tolerance of −0.1 to +0.1 eV inclusive.

Each of the cathode (Al)32R,32G and32B has a Fermi level of 4.3 eV.

How holes and electrons are transferred in each layer of the light emitters3R,3G and3B will be next described with reference toFIGS.3to5.

Upon a potential difference being produced between the anodes31R,31G and31B and the cathodes32R,32G and32B in the light-emitting device1, holes are injected from the anodes31R,31G and31B into the PEDOT:PSS-containing layers of the individual hole transport layers46R,46R and46G, as denoted by an arrow H1inFIGS.3and5. Likewise, as denoted by arrows ER1, EG1and EB1inFIGS.3to5, electrons are injected from the cathodes32R,32G and32B into the electron transport layers (ZnO)47R,47G and47B of the respective light emitters3R,3G and3B.

Here, a barrier to hole transport from a first layer to a second layer, different from the first layer, for instance is indicated by energy obtained by subtracting the ionization potential of the first layer from the ionization potential of the second layer. A barrier to hole injection denoted by the arrow H1thus measures 0.6 eV irrespective of the kind of the light emitters3R,3G and3B.

A barrier to electron transport from the first layer to the second layer, different from the first layer, for instance is indicated by energy obtained by subtracting the electron affinity of the second layer from the electron affinity of the first layer. Barriers to hole injection denoted by the respective arrows ER1, EG1and EB1measure the same value, i.e., 0.4 eV in this embodiment.

As denoted by an arrow H2inFIGS.3to5, the barrier to hole transport from the PEDOT:PSS-containing layers of the respective hole transport layers46R,46G and46B to the TFB-containing layers of the respective hole transport layers46R,46G and46B measures 0 eV. Further as illustrated in an arrow H3, the barrier to hole transport from the TFB-containing layers of the respective hole transport layers46R,46G and46B to the light-emitting layers80R,80G and80B measures 0 eV.

As denoted by arrows ER2, EG2and EB2inFIGS.3to5, the light emitters3R,3G and3B are configured such that the electrons injected into the respective electron transport layers (ZnO)47R,47G and47B are transferred to the respective light-emitting layers80R,80G and80B. Here, the barrier to electron transport denoted by the arrow ER2measures 0.5 eV, the barrier to electron transport denoted by the arrow EG2measures 0.8 eV, and the barrier to electron transport denoted by the arrow EB2measures 1.2 eV. As such, among the barriers used for electron transport from the electron transport layers (ZnO)47R,47G and47B to the light-emitting layers80R,80G and80B, the barrier to transport from the electron transport layer47R to the light-emitting layer80R is the smallest, the barrier to transport from the electron transport layer47G to the light-emitting layer80G is the second smallest, and the barrier to transport from the electron transport layer47B to the light-emitting layer80B is the largest.

The holes and electrons transported to the light-emitting layers80R,80G and80B in this way rejoin together within the quantum dots81R,81G and81B, and thus each of the quantum dots81R,81G and81B emits light.

Here, a balance (carrier balance) needs to be achieved between the number of holes injected into each of the quantum dots81R,81G and81B and the number of electrons injected into each of the quantum dots81R,81G and811, in order for each of the quantum dots81R,81G and81B to emit light at high efficiency.

The light-emitting device1according to this embodiment is configured such that the thickness T1of the light-emitting layer80B is larger than the thickness T2of the light-emitting layer80G, as shown inFIG.1. This configuration can improve the carrier balance within the light-emitting layer80G, thus improving the internal quantum efficiency of the light-emitting layer80G. In addition, the thickness T2of the light-emitting layer80G is larger than the thickness T3of the light-emitting layer80R.

Such a configuration, i.e., the thickness T1of the light-emitting layer80B>the thickness T2of the light-emitting layer80G>the thickness T3of the light-emitting layer80R, can improve the carrier balance in each of the light-emitting layers80B and80G, thereby enhancing the internal quantum efficiency in each of the light-emitting layers80B and80G. The following describes the details.

That is, as described with reference toFIGS.2to5, the valence band levels of the respective light-emitting layers80R,80G and80B are equal to each other irrespective of the wavelength of light emitted therefrom. As illustrated inFIGS.2to5in contrast, the conduction band levels of the respective light-emitting layers80R,80G and80B are ranked in the order of their bandgaps; that is, the light-emitting layer80R has the deepest conduction band level of the three, the light-emitting layer80G has a shallower conduction band level than the light-emitting layer80R, and the light-emitting layer80B has the shallowest conduction band level of the three. The light-emitting layer80R is hence most likely to receive electrons of the three light-emitting layers80R,80G and80B. The light-emitting layer80G is less likely to receive electrons than the light-emitting layer80R, and furthermore, the light-emitting layer80B is less likely to receive electrons than the light-emitting layer80G. The number of electrons with respect to the number of holes within a layer hence tends to be smaller in the light-emitting layer80G than in the light-emitting layer80R. Furthermore, the number of electrons with respect to the number of holes within a layer tends to be smaller in the light-emitting layer80B than in the light-emitting layer80G.

FIG.6is a graph showing carrier distribution within the light-emitting layer80R.FIG.7is a graph showing carrier distribution within the light-emitting layer80B. In bothFIGS.6and7, the lateral axes indicate the thicknesses of the respective light-emitting layers, and the longitudinal axes indicate the number of carriers (holes or electrons).

Referring to carrier (holes and electrons) mobility within the corresponding light-emitting layer, the mobility of holes is lower than the mobility of electrons, as shown inFIGS.6and7. In other words, the holes have a shorter diffusion length in the direction of thickness within the light-emitting layer than the electrons. The holes injected into each light-emitting layer hence exhibit such a distribution that the number of holes decreases exponentially along with distance in the thickness direction of the light-emitting layer from the hole transport layer toward the electron transport layer (i.e., along with approach to the right inFIGS.6and7). The electrons injected into each light-emitting layer in contrast exhibit an approximately constant distribution in the thickness direction of the light-emitting layer. However, the number of electrons with respect to the number of holes within a layer tends to be smaller in the light-emitting layer80B than in the light-emitting layer80R, as illustrated inFIGS.3and5. In addition, the thickness T1of the light-emitting layer80B that minimizes electrons that do not contribute to light emission among the electrons injected into the light-emitting layer80B (i.e., the thickness T1with the carrier balance improved) is larger than the thickness T3of the light-emitting layer80R that minimizes electrons that do not contribute to light emission among the electrons injected into the light-emitting layer80R, as shown inFIGS.6and7. Referring to the light-emitting layer80G likewise, its thickness T2that minimizes electrons that do not contribute to light emission (i.e., the thickness T2with the carrier balance improved) is larger than the thickness T3and is smaller than the thickness T1.

For instance, the ratio of thickness between T3, T2and T1preferably stands at 1.0:2.0:3.7. For instance, the thickness T3preferably ranges from 10 to 20 nm inclusive. Further, the thickness T2preferably ranges from 25 to 35 nm inclusive. Further, the thickness T1preferably ranges from 50 to 60 nm inclusive.

That is, the following relational expression (Expression 3) is preferably established between the thicknesses T1and T2, for instance.
1.3≤T1/T2≤2.4  (Expression 3)

Further, the following relational expression (Expression 4) is preferably established between the thicknesses T1and T3, for instance.
2.5≤T1/T3≤6.0  (Expression 4)

These relations improve the carrier balance within each of the light-emitting layers80R,80G and80B, thus achieving enhancement in their internal quantum efficiency. By way of example, the thickness T3can be set at 15 nm; the thickness T2, at 30 nm; and the thickness T3, at 56 nm.

FIG.8is a sectional view of a light-emitting device1B according to a first modification of the first embodiment. The light-emitting device1B shown inFIG.8is different from the light-emitting device1shown inFIG.1in that the electron transport layers47B,47G and47R are replaced with an electron transport layer47. The other configuration of the light-emitting device1B is similar to that of the light-emitting device1.

The electron transport layer47of the light-emitting device1B is composed of the electron transport layers47B,47G and47R joined together, to thus constitute a single layer extending continuously across the light emitters3B,3G and3R. The electron transport layer47can be made of the same material as the electron transport layers47B,47G and47R. The light-emitting device1B may be configured in this manner.

FIG.9is a sectional view of a light-emitting device1C according to a second modification of the first embodiment. The light-emitting device1C shown inFIG.9is different from the light-emitting device8shown inFIG.1in that the hole transport layers46B,46G and46R are replaced with a hole transport layer46. The other configuration of the light-emitting device1C is similar to that of the light-emitting device1B.

The hole transport layer46of the light-emitting device1C is composed of the hole transport layers46B,46G and46R joined together, to thus constitute a single layer extending continuously across the light emitters3B,3G and3R. The hole transport layer46can be made of the same material as the hole transport layers46B,46G and46R. The light-emitting device1C may be configured in this manner.

Second Embodiment

The following describes a second embodiment. Differences between the first and second embodiments will be mainly described, and redundancies between the first and second embodiments will not be described.

FIG.10illustrates the middle region, R1, of a light-emitting device1E according to the second embodiment. For instance, let the light-emitting device1E be a portable information terminal, such as a smartphone, and let the light-emitting device1E have a display panel with pixels PX are arranged thereon, and a battery, which is a power source that supplies power to the display panel, on the backside of the display panel. When the light-emitting device1E is used in this case, the middle region R1of the display panel gets hot as a result of an increase in the battery's temperature, whereas a peripheral region R2, located outside the middle region R1, gets cooler than the middle region R1.

For instance, let the light-emitting device1E be a TV set, and let the light-emitting device1E have a display panel with pixels PX arranged thereon, a circuit substrate with a power source circuit, and a casing surrounding the perimeters of the display panel and circuit substrate. In addition, let the casing have an exhaust hole for dissipating heat generated from the circuit substrate. When the light-emitting device1E is used in this case, the heat generated from the circuit substrate is dissipated from the exhaust hole of the casing. Temperature hence tends to get higher near the middle region R1of the display panel than near the peripheral region R2of the display panel. As such, the temperature of the display panel of the light-emitting device1E tends to get higher in the middle region R1than in the peripheral region R2.

Here, if the temperature of a light emitter rises, the electrical resistance of ITO, which is a semiconductor, gets low, whereas the electrical resistance of Al, which is a metal, gets high, for instance. In addition, if the temperature of the light emitter rises, electrons injected from its electron transport layer into its light-emitting layer go over the barrier between the light-emitting layer and hole transport layer, thus easily causing leakage current. Hence if the temperature of the light emitter rises, the light-emitting layer tends to involve an excess of holes and a shortage of electrons in carrier balance, thus causing reduction in internal quantum efficiency.

FIG.11is a schematic plan view of a display panel1Ea of the light-emitting device1E according to the second embodiment and of a peripheral circuit of the same. As illustrated inFIG.11, the light-emitting device1E has the display panel1Ea, a gate driver GD, a source driver SD, and a display control circuit101.

The display panel1Ea has an image display region with pixels PX arranged in matrix therein. The display panel1Ea has the following: a plurality of data lines Sn (n is an integer ranging from 1 to N) provided for supplying a source signal (data signal) to the pixels PX; and a plurality of gate lines Gm (m is an integer ranging from 1 to M) provided for supplying a gate signal to the pixels PX. The plurality of data lines Sn include one to n number of data lines arranged sequentially in one direction, where n is an integer equal to or greater than three. The plurality of gate lines Gm include one to m number of gate lines arranged sequentially in one direction, where m is an integer equal to or greater than three. The plurality of pixels PX are arranged at points where the plurality of data lines Sn intersect with the plurality of gate lines Gm.

Each of the plurality of data lines Sn has one end connected to the source electrode of the TFT in a corresponding one of the pixels PX and has the other end connected to the source driver SD. Each of the plurality of gate lines Gm has one end connected to the gate electrode of the TFT in a corresponding one of the pixels PX and has the other end connected to the gate driver GD.

The source driver SD may include, but not limited to, a serial-parallel conversion and latch circuit, a DA conversion circuit, an AD conversion circuit, and an input-output buffer circuit. The source driver SD generates a source signal on the basis of an input signal from the display control circuit101and supplies the generated source signal to each data line Sn at a predetermined timing. The gate driver GD generates a gate signal on the basis of an input signal from the display control circuit101and supplies the generated gate signal to each gate line Gm at a predetermined timing. The TFT of each pixel PX is driven on the basis of the gate signal supplied from the gate line Gm, and the source signal supplied from the data line Sn.

Among the plurality of pixels PX arranged on the display panel1Ea, a pixel PX included in the middle region R1will be referred to as a pixel PX1(first pixel), and a pixel PX included in the peripheral region R2, which is around the middle region R1, will be referred to as a pixel PX2(second pixel).

For instance, the pixel PX1is disposed on at least one of the plurality of data lines Sn included in 0.4n or greater and 0.6n or smaller. The pixel PX1may be disposed on all the plurality of data lines Sn included in 0.4n or greater and 0.6n or smaller. In n=100 for instance, the pixel PX1is disposed on at least one of the 40th to 60th data lines Sn counted in one direction (for instance, counted from the left to the right). That is, the pixel PX1can be also described as being disposed on at least one of 20% data lines Sn arranged in the middle of the plurality of data lines.

For instance, the pixel PX2is disposed on at least one of the plurality of data lines Sn excluding the plurality of data lines Sn included in 0.4n or greater and 0.6n or smaller. The pixel PX2may be disposed on all the plurality of data lines Sn excluding the plurality of data lines Sn included in 0.4n or greater and 0.6n or smaller. In n=100 for instance, the pixel PX2is disposed on at least one of the 1st to 39th and 61th to 100th data lines Sn counted in one direction (for instance, counted from the left to the right). That is, the pixel PX2can be also described as being disposed on at least one of 40% data lines Sn arranged at both ends of the plurality of data lines Sn.

FIG.12is a sectional view of the pixel PX1in the middle region R1of the light-emitting device1E according to the second embodiment.FIG.13is a sectional view of the pixel PX2in the peripheral region R2of the light-emitting device1E according to the second embodiment. As illustrated inFIG.12, the pixel PX1of the light-emitting device1E includes the following: the light-emitting layer80B having a thickness T11; the light-emitting layer80G having a thickness T21; and the light-emitting layer80R having a thickness T31. As illustrated inFIG.13, the pixel PX2of the light-emitting device1E includes the following: the light-emitting layer80B having a thickness T12; the light-emitting layer80G having a thickness T22; and the light-emitting layer80R having a thickness T32. As illustrated inFIGS.12and13, the light-emitting device1E is configured such that the thickness T12of the light-emitting layer80B of the pixel PX2is smaller than the thickness T11of the light-emitting layer80B of the pixel PX1. This configuration can prevent a shortage of electrons within the light-emitting layer80B of the pixel PX1even when the temperature of the pixel PX1gets higher than the temperature of the pixel PX2, thereby preventing an increase in the difference between the carrier balance of the light-emitting layer80B of the pixel PX1and the carrier balance of the light-emitting layer80B of the pixel PX2. That is, the foregoing configuration can prevent an increase in the difference between the internal quantum efficiency of the light-emitting layer80B of the pixel PX1and the internal quantum efficiency of the light-emitting layer80B of the pixel PX2. This can prevent in-plane brightness unevenness in the display panel1Ea even when the temperature of the middle region R1gets higher than the temperature of the peripheral region R2.

The light-emitting device1E may be configured such that the thickness T22of the light-emitting layer80G of the pixel PX2is smaller than the thickness T21of the light-emitting layer80G of the pixel PX1. The light-emitting device1E may be also configured such that the thickness T32of the light-emitting layer80R of the pixel PX2is smaller than the thickness T31of the light-emitting layer80R of the pixel PX1. This configuration can further prevent in-plane brightness unevenness in the display panel1Ea.

FIG.14is a sectional view of the pixel PX2in the peripheral region R2of the light-emitting device1E according to a modification of the second embodiment. The pixel PX2may be configured such that the thickness T12of the light-emitting layer80B, the thickness T22of the light-emitting layer80G, and the thickness T32of the light-emitting layer80R are equal to each other, as illustrated inFIG.14. For instance, the thicknesses T12, T22and T32may be set at about 15 nm.

Here, the light-emitting layers80B,80G and80R are formed by applying a solution with quantum dots dispersed therein onto a substrate, followed by heating the substrate with a heater to thus heat the solvent of the dispersed solution to a temperature higher than its boiling point to thus vaporize the solvent. Due to heating unevenness in the heater unfortunately, the temperature of the pixels included in the peripheral region is hard to rise, and the solvent is hard to vaporize. The temperature of the pixels included in the peripheral region is particularly hard to rise when these pixels are close to the end of the substrate, like a display having a narrow frame for instance. The dispersed solution, constituting the forgoing light-emitting layers, is hence hard to vaporize in the peripheral region, thus easily generating thickness unevenness in the light-emitting layers formed.

One method to address this problem is reducing the thicknesses T12, T22and T32of the respective light-emitting layers80B,80G and80R included in the peripheral region R2. This method includes reducing the amount of application to the pixel PX2, which is included in the peripheral region R2, in the amount of application of the dispersed solution, which constitutes the light-emitting layers80B,80G and80R. This method can prevent thickness unevenness in each of the light-emitting layers80B,80G and80R formed in the pixel PX2in the peripheral region R2. This can prevent in-plane brightness unevenness in the display panel1Ea as well.

The components appeared in the forgoing embodiments and modifications may be combined as appropriate unless otherwise contradicted.