Patent Description:
Quantum dots are a nanocrystal semiconductor material having a diameter of about several to several hundreds of nanometers (nm) that provides a large surface area per unit volume, and exhibit quantum confinement effects. Quantum dots generate stronger light in a narrower wavelength region than the commonly used phosphor. Quantum dots emit light while the excited electrons are transited from a conduction band to a valence band and wavelengths are changed depending upon a particle size, even within the same material. The quantum dot particle sizes can be selected to obtain light in a desirable wavelength region.

In other words, the emission layer including quantum dots, and the various types of electronic devices including the same, may generally reduce production cost, compared to the use of organic light emitting diodes having an emission layer including a phosphorescence and/or phosphor material. In addition, a desirable color may be emitted by changing sizes of quantum dots, rather than having to use other organic materials in the emission layer for emitting other colors of light.

The luminous efficiency of the emission layer including quantum dots is determined by the external quantum efficiency (EQE) of quantum dots, which is determined based on a balance of charge carriers, light extraction efficiency, and the like. Accordingly, when an emission layer including a quantum dot is applied as an electro-luminescence layer, improving the luminous efficiency of the emission layer requires adjusting the balance of charge carriers and light extraction efficiency, together with reducing current leakage that can be associated with the use of various charge carrier layers.

<CIT> discloses an electroluminescent device including a first electrode and a second electrode facing each other; an emission layer disposed between the first electrode and the second electrode and including at least two light emitting particles; a hole transport layer disposed between the first electrode and the emission layer; and an electron transport layer disposed between the emission layer and the second electrode, wherein the electron transport layer includes an inorganic layer disposed on the emission layer, the inorganic layer comprising a plurality of inorganic nanoparticles; and an organic layer directly disposed on at least a portion of the inorganic layer on a side opposite the emission layer.

<CIT> discloses an LED in which an emissive layer of quantum dots is between first and second electrodes; a first, inorganic charge transport layer between the first electrode and the emissive layer; and a first interfacial layer, between the emissive layer and the first charge transport layer, to protect quantum dots from charge quenching sites in another device layer.

<CIT> discloses typical particles sizes for inorganic particles as electron transport material in a quantum dot LED.

An electroluminescent device capable of minimizing a leakage current while improving a charge carrier balance and light extraction efficiency of an emission layer is provided.

The electron transport layer may not have electroluminescence.

According to another embodiment, a display device includes the electroluminescent device.

An electroluminescent device having driving characteristics and life-span characteristics by improving a charge carrier balance and light extraction efficiency of an emission layer and simultaneously minimizing a leakage current and a display device including the same may be provided.

These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawing, in which:.

Example embodiments of the present disclosure will hereinafter be described in detail, and may be understood by a person skilled in the art. However, this disclosure may be embodied in many different forms, and is not to be construed as limited to the example embodiments set forth herein. If not defined otherwise, all terms (including technical and scientific terms) in the specification may be defined as commonly understood by one skilled in the art. The terms defined in a generally-used dictionary may not be interpreted ideally or exaggeratedly unless clearly defined. In addition, unless explicitly described to the contrary, the word "comprise" and variations such as "comprises" or "comprising", will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. The term "a combination thereof" is open ended and means including at least one of the listed components, and may include other like components.

Further, the singular includes the plural unless mentioned otherwise.

The term "or" means "and/or. " Expressions such as "at least one of" when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.

In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. Like reference numerals designate like elements throughout the specification. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being "on" another element, it can be directly on the other element or intervening elements may also be present.

It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the present embodiments.

Relative terms, such as "lower" or "bottom" and "upper" or "top," may be used herein to describe one element's relationship to another element as illustrated in the Figures. The exemplary term "lower," can therefore, encompasses both an orientation of "lower" and "upper," depending on the particular orientation of the figure. The exemplary terms "below" or "beneath" can, therefore, encompass both an orientation of above and below.

In an embodiment, for sizes or particle diameters of various particles, although they may be numerized by a measurement to show an average size of a group, the generally used method includes a mode diameter showing the maximum value of the distribution, a median diameter corresponding to the center value of integral distribution curve, a variety of average diameters (numeral average, length average, area average, mass average, volume average, etc.), and the like. Unless particularly mentioning otherwise, average sizes or average particle diameters means to numeral average sizes or numeral average diameters in the present disclosure, and it is obtained by measuring D50 (particle diameters at a position of distribution rate of <NUM> %).

As used herein, "Group" in the term Group III, Group II, and the like refers to a group of Periodic Table.

As used herein, "Group II" refers to Group IIA and Group IIB, and examples of a Group II metal include Cd, Zn, Hg, and Mg, but are not limited thereto.

As used herein, examples of "Group II metal that does not include Cd" refers to a Group II metal except Cd, for example Zn, Hg, Mg, etc..

As used herein, "Group III" refers to Group IIIA and Group IIIB, and examples of Group III metal include Al, In, Ga, and Tl, but are not limited thereto.

As used herein, "Group IV" refers to Group IVA and Group IVB, and examples of a Group IV metal include Si, Ge, and Sn, but are not limited thereto. As used herein, "metal" may include a semi-metal such as Si.

As used herein, "Group I" refers to Group IA and Group IB, and examples include Li, Na, K, Ru, and Cs, but are not limited thereto.

As used herein, "Group V" refers to Group VA, and examples include nitrogen, phosphorus, arsenic, antimony, and bismuth, but are not limited thereto.

As used herein, "Group VI" refers to Group VIA, and examples include sulfur, selenium, and tellurium, but are not limited thereto.

Hereinafter, a work function (WF), a highest occupied molecular orbital (HOMO) energy level, and a lowest unoccupied molecular orbital (LUMO) energy level can be expressed as an absolute value from a vacuum energy level (i.e., <NUM> electron volts (eV)). In addition, when the work function, HOMO energy level, or LUMO energy level is said to be 'deep,' 'high', or 'large,' the work function or HOMO energy level has a larger absolute value from the vacuum energy level (<NUM> eV), while when the work function, HOMO energy level, or LUMO energy level is 'shallow, 'low' or 'small,' the work function or HOMO energy level has a smaller absolute value from the vacuum energy level (<NUM> eV).

First, referring to <FIG>, a schematic structure of an electroluminescent device according to an embodiment is described.

<FIG> is a schematic cross-sectional view of an electroluminescent device according to an embodiment.

An electroluminescent device <NUM> according to an embodiment includes a first electrode <NUM> and a second electrode <NUM> facing each other, an emission layer <NUM> disposed between the first electrode <NUM> and the second electrode <NUM> and including a plurality of (e.g., at least two) light emitting particles <NUM>, a hole transport layer <NUM> disposed between the first electrode <NUM> and the emission layer <NUM>, and an electron transport layer <NUM> disposed between the emission layer <NUM> and the second electrode <NUM>.

The electroluminescent device <NUM> according to an embodiment supplies current to the emission layer <NUM> including light emitting particles <NUM> through the first electrode <NUM> and the second electrode <NUM> and causes electro-luminescence of the light emitting particles <NUM> to generate light. The electroluminescent device <NUM> may generate light in various wavelength regions according to materials, sizes, or fine structures of the light emitting particles <NUM> of the emission layer <NUM>.

In an embodiment, the first electrode <NUM> may be directly connected to a driving power source so may function to flow current to the emission layer <NUM>. The first electrode <NUM> may include a material having light transmittance in at least a visible light wavelength region but is not limited thereto. The first electrode <NUM> may include a material having light transmittance in an infrared or ultraviolet (UV) wavelength region. For example, the first electrode <NUM> may be made of an optically transparent material.

In an embodiment, the first electrode <NUM> may include molybdenum oxide, tungsten oxide, vanadium oxide, rhenium oxide, niobium oxide, tantalum oxide, titanium oxide, zinc oxide, nickel oxide, copper oxide, cobalt oxide, manganese oxide, chromium oxide, indium oxide, or a combination thereof.

Meanwhile, in an embodiment, the first electrode <NUM> may be disposed on the substrate <NUM> as shown in <FIG>. The substrate <NUM> may be a transparent insulating substrate or may be made of a ductile material. The substrate <NUM> may include a glass or a polymer material in a film having a glass transition temperature (Tg) of greater than about <NUM>. For example, it may include a cycloolefin copolymer (COC) or cycloolefin polymer (COP)-based material.

In an embodiment, the substrate <NUM> supports the hole transport layer <NUM>, the emission layer <NUM>, and the electron transport layer <NUM> sandwiched by the first electrode <NUM> and the second electrode <NUM>. However, the first electrode <NUM> of the electroluminescent device <NUM> according to an embodiment is not necessarily disposed on the substrate <NUM>, and the substrate may be disposed on the second electrode <NUM> or may be omitted as needed.

The second electrode <NUM> includes an optically transparent material and may function as a light-transmitting electrode to transmit light generated in the emission layer <NUM> that will be described later. In an embodiment, the second electrode <NUM> may include at least one selected from silver (Ag), aluminum (Al), copper (Cu), gold (Au), and an alloy thereof, molybdenum oxide, tungsten oxide, vanadium oxide, rhenium oxide, niobium oxide, tantalum oxide, titanium oxide, zinc oxide, nickel oxide, copper oxide, cobalt oxide, manganese oxide, chromium oxide, indium oxide, or a combination thereof.

Each of the first electrode <NUM> and the second electrode <NUM> may be formed by depositing a material for forming an electrode on the substrate <NUM> or an organic layer <NUM> described later by a method such as sputtering.

The emission layer <NUM> includes a plurality of light emitting particles <NUM> (i.e., at least two light emitting particles). The emission layer <NUM> may be formed by applying a resin in which at least two light emitting particles <NUM> are dispersed on a hole transport layer <NUM> (described later) and curing the same.

The emission layer <NUM> is a site where electrons and holes transported by a current supplied from the first electrode <NUM> and the second electrode <NUM>, the electrons and holes are combined in the emission layer <NUM> to generate excitons, and the generated excitons are transited from an excited state to a ground state to emit light in a wavelength corresponding to the size of the light emitting particles <NUM>.

In an embodiment, the light emitting particles <NUM> may include a quantum dot.

The quantum dot has a discontinuous energy bandgap by the quantum confinement effect. That is, when the emission layer <NUM> includes a quantum dot as a light emitting particle <NUM>, the emission layer <NUM> may produce light having excellent color reproducibility and color purity.

In an embodiment, a material of the quantum dot is not particularly limited and known or commercially available quantum dots may be used. For example, the quantum dot may include a Group II-VI compound that does not include Cd, a Group III-V compound, a Group IV-VI compound, a Group IV element or compound, a Group I-III-VI compound, a Group I-II-IV-VI compound that does not include Cd, or a combination thereof. The quantum dot according to an embodiment may be a non-cadmium-based quantum dot. Like this, when the quantum dot comprises or consists of a non-cadmium-cadmium-based material, it has no toxicity compared with a conventional cadmium-based quantum dot and thus is less dangerous and is environmentally-friendly.

The Group II-VI compound may be a binary element compound that is ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, MgSe, MgS, or a combination thereof; a ternary element compound that is ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, HgZnS, HgZnSe, HgZnTe, MgZnSe, MgZnS, or a combination thereof; or a quaternary element compound that is HgZnTeS, HgZnSeS, HgZnSeTe, HgZnSTe, or a combination thereof. The Group II-VI compound may further include a Group III metal.

The Group III-V compound may be a binary element compound that is GaN, GaP, GaAs, GaSb, AIN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, or a combination thereof; a ternary element compound that is GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AINP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, InZnP, or a combination thereof; or a quaternary element compound that is GaAlNP, GaAINAs, GaAINSb, GaAlPAs, GaAlPSb, GalnNP, GalnNAs, GalnNSb, GalnPAs, GalnPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, InAlPSb, or a combination thereof. The Group III-V compound may further include a Group II metal (e.g., InZnP).

The Group IV-VI compound may be a binary element compound that is SnS, SnSe, SnTe, PbS, PbSe, PbTe, or a combination thereof; a ternary element compound that is SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, or a combination thereof; or a quaternary element compound that is SnPbSSe, SnPbSeTe, SnPbSTe, or a combination thereof. Examples of the Group I-III-VI compound may be CuInSe<NUM>, CuInS<NUM>, CuInGaSe, and CuInGaS, but are not limited thereto. Examples of the Group I-II-IV-VI compound may be CuZnSnSe and CuZnSnS, but are not limited thereto. Examples of the Group IV compound may be a single substance that is Si, Ge, or a combination thereof; or a binary element compound that is SiC, SiGe, or a combination thereof.

The binary element compound, the ternary element compound, or the quaternary element compound respectively may exist in a uniform concentration in the particle or in partially different concentrations in the same particle.

According to an embodiment, the quantum dot may have a core-shell structure including one semiconductor nanocrystal core and another semiconductor nanocrystal shell surrounding the core. The core and the shell may have a concentration gradient wherein the concentration of the element(s) of the shell decreases toward the core. In addition, the quantum dot may have one semiconductor nanocrystal core and multiple shells surrounding the core. Herein, the multi-layered shell structure has a structure of two or more shells and each layer may have a single composition or an alloy, or may have a concentration gradient.

When the quantum dot according to an embodiment has a core-shell structure, a material composition of the shell has a larger energy bandgap than that of the core, which may exhibit an effective quantum confinement effect. However, the embodiment is not limited thereto. Meanwhile, in the multi-layered shell, a shell that is outside of the core has may have a higher energy bandgap than a shell that is near to the core and the quantum dot may have ultraviolet (UV) to infrared wavelength ranges.

The quantum dot may have an external quantum efficiency (EQE) of greater than or equal to about <NUM>%, for example, greater than or equal to about <NUM>%, greater than or equal to about <NUM>%, greater than or equal to about <NUM>%, greater than or equal to about <NUM>%, greater than or equal to about <NUM>%, or even <NUM>%.

In a display, the quantum dot may have a relatively narrow spectrum so as to improve color purity or color reproducibility. The quantum dot may have for example a full width at half maximum (FWHM) of a wavelength spectrum of less than or equal to about <NUM>, less than or equal to about <NUM>, or less than or equal to or about <NUM>. Within the ranges, color purity or color reproducibility of a device may be improved.

The quantum dot may have an average particle diameter (the longest diameter for a non-spherically shaped particle) of about <NUM> to about <NUM>. For example, the quantum dot may have an average particle diameter of about <NUM> to about <NUM>, for example, about <NUM> (or about <NUM>) to about <NUM>.

In addition, a shape of the quantum dot particle may be general shapes in this art and thus may not be particularly limited. For example, the quantum dot particle may have a spherical, oval, tetrahedral, pyramidal, cuboctahedral, cylindrical, polyhedral, multi-armed, or cube nanoparticle, nanotube, nanowire, nanofiber, nanosheet, or a combination thereof. The quantum dot may have any cross-sectional shape.

The quantum dot may be commercially available or may be synthesized in any method. For example, several nano-sized quantum dots may be synthesized according to a wet chemical process. In the wet chemical process, precursors react in an organic solvent to grow crystal particles, and the organic solvent or a ligand compound may coordinate the surface of the quantum dot, controlling the growth of the crystal. Examples of the organic solvent and the ligand compound are known. The organic solvent coordinated on the surface of the quantum dot may affect stability of a device, and thus excess organic materials that are not coordinated on the surface of the nanocrystals may be removed by pouring it in excessive non-solvent and centrifuging the resulting mixture. Examples of the non-solvent may be acetone, ethanol, methanol, and the like, but are not limited thereto. After the removal of excess organic materials, the amount of the organic materials coordinated on the surface of the quantum dot may be less than or equal to about <NUM>% by weight (wt%), for example, less than or equal to about <NUM> wt%, less than or equal to about <NUM> wt%, or less than or equal to about <NUM> wt% based on a weight of the quantum dot. The organic material may include a ligand compound, an organic solvent, or a combination thereof.

The quantum dot may have for example an organic ligand having a hydrophobic moiety bound to its surface. In an embodiment, the organic ligand having the hydrophobic moiety may be RCOOH, RNH<NUM>, R<NUM>NH, R<NUM>N, RSH, R<NUM>PO, R<NUM>P, ROH, RCOOR', RPO (OH)<NUM>, R<NUM>POOH (wherein, R and R' are independently a C5 to C24 alkyl group, a C5 to C24 alkenyl group, a C5 to C20 alicyclic group, or a C5 to C20 aryl group), a polymeric organic ligand, or a combination thereof. The organic ligand may be a mono-functional group organic ligand and the functional group may be bound to the surface of the quantum dot.

In an embodiment, the hole transport layer <NUM> is disposed between the first electrode <NUM> and the emission layer <NUM>. The hole transport layer <NUM> may transport holes into the emission layer <NUM>.

The electroluminescent device <NUM> according to an embodiment may further include a hole injection layer <NUM> between the hole transport layer <NUM> and the first electrode <NUM>. The hole injection layer <NUM> supplies holes to the hole transport layer <NUM>.

Each of the hole injection layer <NUM> and the hole transport layer <NUM> may include, independently, for example poly(<NUM>,<NUM>-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), poly(<NUM>,<NUM>-dioctyl-fluorene-co-N-(<NUM>-butylphenyl)-diphenylamine) (TFB), a polyarylamine, poly(N-vinylcarbazole), a polyaniline, a polypyrrole, N,N,N',N'-tetrakis(<NUM>-methoxyphenyl)-benzidine (TPD), <NUM>-bis[N-(<NUM>-naphthyl)-N-phenyl-amino]biphenyl (α-NPD), m-MTDATA (<NUM>,<NUM>,<NUM>"-tris[phenyl(m-tolyl)amino]triphenylamine), <NUM>,<NUM>',<NUM>"-tris(N-carbazolyl)-triphenylamine (TCTA), <NUM>,<NUM>-bis[(di-<NUM>-tolylamino)phenyl]cyclohexane (TAPC), or a combination thereof, but is not limited thereto. Various semiconductor materials, or combinations thereof may be applied according to an internal energy level of the electroluminescent device <NUM>.

The hole injection layer <NUM> and the hole transport layer <NUM> according to an embodiment may be formed by coating a solution-type commercially available materials on the first electrode <NUM> and then curing the same but is not limited thereto.

In an embodiment, the electron transport layer <NUM> is disposed between the emission layer <NUM> and the second electrode <NUM> and transports electrons into the emission layer <NUM>.

The electron transport layer <NUM> according to an embodiment may comprise or consist of a non-light-emitting charge transporting material that does not emit light by an electric field. Thereby, internal light emission of the electroluminescent device <NUM> according to an embodiment occurs in the emission layer <NUM> and not in the electron transport layer <NUM>.

The electron transport layer <NUM> according to an embodiment includes an inorganic layer <NUM> formed on the emission layer <NUM> and including a plurality of inorganic nanoparticles <NUM> (i.e., two or more inorganic nanoparticles) and an organic layer <NUM> formed directly on the inorganic layer <NUM> on a side opposite the emission layer <NUM>.

As shown in <FIG>, the inorganic layer <NUM> may directly be formed on the emission layer <NUM>.

The inorganic layer <NUM> includes two or more (i.e., plurality of) inorganic nanoparticles <NUM> and the two or more inorganic nanoparticles <NUM> may be agglomerated with each other to form a cluster layer. In an embodiment, the inorganic layer <NUM> may include a cluster layer composed of two or more inorganic nanoparticles <NUM>.

In the case where the cluster layer composed of two or more inorganic nanoparticles <NUM> is directly on the emission layer <NUM>, the non-emission quenching of the charge exchange excitons generated in the emission layer <NUM> may be reduced or prevented, and the luminance of the emission layer <NUM> may be improved.

An inorganic nanoparticle of the plurality of inorganic nanoparticles (<NUM>) is ZnO, TiO<NUM>, ZrO<NUM>, SnO<NUM>, WO<NUM>, Ta<NUM>O<NUM>, or a combination thereof.

In the case where the cluster layer is made of inorganic oxide nanoparticles, for example, the emission layer <NUM> including a non-cadmium quantum dot generates a large amount of charge exchange excitons due to internal heat generation during driving of the device, and the generated charge exchange exciton may emit auger electrons on the interface with the cluster layer without non-emission quenching by the cluster layer. The emitted auger electrons may emit light by recombination with holes in the emission layer <NUM>, roll-off of the electroluminescent device <NUM> in a high luminance region may be minimized.

On the other hand, an average particle diameter (as for a non-spherically shaped particle, diameter means the longest dimension) of the inorganic nanoparticles <NUM> according to an embodiment may be for example less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, or less than or equal to about <NUM> and for example greater than or equal to about <NUM>, greater than or equal to about <NUM>, greater than or equal to about <NUM>, greater than or equal to about <NUM>, or greater than or equal to about <NUM>.

When the inorganic layer <NUM> is the cluster layer composed of two or more inorganic nanoparticles <NUM> as described above, electron mobility thereof is much higher than that of a general inorganic semiconductor film or an organic semiconductor film. Therefore luminescence stability and the luminance of the non-cadmium quantum dot may be improved through the cluster layer to which the inorganic layer <NUM> is applied.

In addition, an average thickness of the cluster layer according to an embodiment may be for example less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, or less than or equal to about <NUM> and for example greater than or equal to about <NUM>, greater than or equal to about <NUM>, greater than or equal to about <NUM>, greater than or equal to about <NUM>, or greater than or equal to about <NUM>.

When the average particle diameter of the inorganic nanoparticles <NUM> and the average thickness of the cluster layer are within the above ranges, the inorganic layer <NUM> may exhibit excellent electron mobility.

<FIG> is a microscopic image showing the upper surface of the inorganic layer of the electroluminescent devices according to an embodiment and <FIG> is a graph showing an upper surface three-dimensional shape image and a height deviation depending on a position of an inorganic layer and a position of the inorganic layer of the electroluminescent device according to an embodiment measured using a Zygo interferometer.

<FIG> and <FIG> correspond to a case where a cluster layer having a thickness of about <NUM> is formed by agglomerating ZnO nanoparticles having an average particle diameter of <NUM>.

First, referring to <FIG>, it may be seen that the surface of the cluster layer includes two or more grains composed of inorganic nanoparticles and a grain boundary formed between neighboring grains.

In addition, referring to the graph showing the surface roughness curve of the linear portion indicated in the surface profile image of the cluster layer of <FIG>, a peak to valley (PV) of the surface of the cluster layer is <NUM> and a root mean square roughness (Rq) calculated by a root mean square (rms) method is <NUM>.

Thus, referring now to <FIG>, the surface region 151a of the cluster layer according to an embodiment may exhibit an uneven surface morphology, which may be caused by a non-uniform agglomeration of the inorganic nanoparticles 152a in the surface region 151a, a particle size variation of the inorganic nanoparticles 152a in the surface region 151a, detachment of the inorganic nanoparticles 152a from the cluster layer, a combination thereof, or the like, which results in the existence of grain boundaries in the surface region 151a. In some embodiments, the upper surface of the cluster layer includes the surface region 151a having two or more grains each comprising inorganic nanoparticles, wherein a grain boundary is formed between adjacent grains. The inorganic nanoparticles in the surface region 151a can include inorganic nanoparticles 152a. In other words, the inorganic nanoparticles 152a are inorganic nanoparticles of the plurality of nanoparticles that are not included in the cluster layer.

On the other hand, according to an embodiment, the organic layer <NUM> is formed directly on at least a portion of the inorganic layer <NUM>. The organic layer <NUM> may be formed through a deposition process or the like on the cluster layer of the inorganic nanoparticles <NUM>.

The organic layer <NUM> is a layer composed of organic semiconductor compounds and may include conductive monomolecular molecule, a low molecular organic nano-material having a conjugation structure, or a combination thereof. The organic semiconductor compounds forming the organic layer <NUM> are a quinolone-based compound, a triazine-based compound, a quinoline-based compound, a triazole-based compound, a naphthalene-based compound, or a combination thereof, or a phosphine oxide based compound (NET-<NUM>, Novaled Electron Transport material <NUM>, obtained from Novaled), phosphonyl phenol based compound (NDN-<NUM>, Novaled Dopant n-side material <NUM>, obtained from Novaled), or a combination thereof.

The organic layer <NUM> may be composed of an organic semiconductor compound having a higher work function than that of the inorganic layer <NUM>.

The organic layer <NUM> is an energy barrier against electrons moving from the second electrode <NUM> to the inorganic layer <NUM>.

A LUMO (Lowest Unoccupied Molecular Orbital) energy level of the organic layer <NUM> may be for example about -<NUM> electron Volts (eV) to about -<NUM> eV, about -<NUM> eV to - about <NUM> eV, about -<NUM> eV to about -<NUM> eV, about -<NUM> eV to about -<NUM> eV, - about <NUM> eV to about -<NUM> eV, about -<NUM> eV to about -<NUM> eV, or about -<NUM> eV to about -<NUM> eV.

On the other hand, the organic layer <NUM> may have electron mobility of greater than or equal to about <NUM>-<NUM> square centimeters per Volt-second (cm<NUM>/V·s) or greater than or equal to about <NUM>-<NUM> cm<NUM>/V·s, and for example, less than or equal to about <NUM><NUM>/V·s or less than or equal to about <NUM>-<NUM> cm<NUM>/V·s.

The organic layer <NUM> according to an embodiment fully or completely covers the upper surface of the inorganic layer <NUM>, so that the inorganic layer <NUM> may not be exposed toward the second electrode <NUM> as shown in <FIG>. In other words, the organic layer <NUM> is disposed on and in contact with the entire upper surface of the inorganic layer <NUM>. Accordingly, the organic layer <NUM> fills at least a part or a whole of a grain boundary formed at, on, or in the surface (i.e., in the surface region 151a between the inorganic nanoparticles 152a) of the cluster layer being in contact therewith.

On the other hand, the organic layer <NUM> may fill a part of or all of the cracks or voids generated by detachment or non-uniform agglomeration of the inorganic nanoparticles 152a on the surface of the cluster layer being in contact therewith. In some embodiments, the organic layer <NUM> is disposed on the upper surface of the inorganic layer <NUM> such that substantially all of the cracks, voids, grain boundaries, or the like, at the upper surface are filled with or include the semiconductor organic compounds of the organic layer <NUM>.

A thickness of the organic layer <NUM> may be variously designed depending on electron mobility, a work function, and the like of the organic layer <NUM>. It may be thin enough to transport at least electrons through tunneling to the inorganic layer <NUM> while it may be thick enough to fill a grain boundary, a crack, and a void on the surface of the inorganic layer <NUM> and not expose but cover the inorganic layer <NUM>.

An average thickness of the organic layer <NUM> is less than or equal to about <NUM>, for example less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, less than or equal to about <NUM>, or less than or equal to about <NUM> and greater than or equal to about <NUM>, for example greater than or equal to about <NUM>, or greater than or equal to about <NUM>.

Since the organic layer <NUM> has a higher work function than the inorganic layer <NUM>, a driving voltage for transporting electrons may be increased compared with a case of using the inorganic layer alone as an electron transport layer. However, when the organic layer <NUM> according to an embodiment has a LUMO and an electron mobility within the range and in addition, a thickness adjusted within the range, the driving voltage and an electron mobility rate may be adjusted to have an appropriate level due to band bending and tunneling effects.

On the other hand, the organic layer <NUM> may include at least two different organic semiconductor compounds. The organic layer <NUM> according to an embodiment may be formed, for example, in a method of co-depositing at least two different organic semiconductor compounds, or the like.

For example, when either one of the organic semiconductor compounds is used, a plurality of island-shaped intermediates is formed due to uneven morphology on the surface of the inorganic layer <NUM> during the formation of the organic semiconductor compound, and accordingly, the organic layer <NUM> may have a large thickness deviation. On the other hand, when at least two different organic semiconductor compounds are used, the organic layer <NUM> may have a relatively small thickness deviation.

In addition, the organic layer <NUM> may include two different organic semiconductor compounds (a first organic semiconductor compound and a second organic semiconductor compound).

Herein, the first organic semiconductor compound and the second organic semiconductor compound in the organic layer <NUM> may be used in various weight ratios depending on materials of each organic semiconductor compound, wherein the weight ratio may be adjusted to minimize formation of the island-shaped intermediate.

In an embodiment, the first organic semiconductor and the second organic semiconductor in the organic layer <NUM> may, for example, have a weight ratio of about <NUM>:<NUM> to about <NUM>:<NUM>, about <NUM>:<NUM> to about <NUM>:<NUM>, about <NUM>:<NUM> to about <NUM>:<NUM>, or about <NUM>:<NUM>.

On the other hand, additional units including the inorganic layer <NUM> and the organic layer <NUM> may be alternately stacked on the emission layer <NUM>. In other words, the electron transport layer <NUM> may include plurality of stacked units, such as at least two stacked units, with each unit including the inorganic layer <NUM> and the organic layer <NUM>. In this way, the number of alternately stacking units each including the inorganic layer <NUM> and the organic layer <NUM> may be adjusted to appropriately maintain a charge carrier balance in an electroluminescent device by considering each energy level of the first electrode <NUM>, the second electrode <NUM>, the hole injection layer <NUM>, the hole transport layer <NUM>, and the emission layer <NUM>, a difference of their energy differences, and the like.

On the other hand, when the cluster layer composed of the inorganic nanoparticles is applied to an emission layer including quantum dots, device efficiency at high luminance may be secured, but since a crack, a void, a grain boundary, or the like on the surface of the cluster layer may for example result in a leakage path of a current, a leakage current from the leakage path may deteriorate device efficiency at a low voltage and in a low luminance region.

However, the electroluminescent device <NUM> according to an embodiment includes the organic layer <NUM> filling the crack, the void, the grain boundary, or the like, on the surface of the cluster layer and thus may minimize the generation of the leakage current due to the cluster layer.

The electroluminescent device <NUM> according to an embodiment may further include an electron injection layer (not shown) between the second electrode <NUM> and the electron transport layer <NUM> and may further include an electron blocking layer (not shown) between the second electrode <NUM> and the electron injection layer (not shown) or between the electron injection layer (not shown) and the electron transport layer <NUM>. However, the embodiment is not limited thereto and the electron injection layer (not shown) and the electron blocking layer (not shown) may be omitted in order to maintain charge carrier balance of the electroluminescent device <NUM> to be an appropriate level.

Hereinafter, referring to <FIG>, a driving principle of an electroluminescent device according to an embodiment is explained.

<FIG> is an energy band diagram showing an electroluminescent device according to an embodiment.

The electroluminescent device <NUM> according to an embodiment includes quantum dots as the light emitting particles <NUM>, and accordingly, the emission layer <NUM> formed thereof has a different energy level from that of a general organic light emitting diode.

In particular, the general electroluminescent device sequentially transports electrons along with a LUMO energy level from an electrode through an electron transport layer toward an emission layer, but the electroluminescent device <NUM> according to an embodiment has the organic layer <NUM> having a higher LUMO energy level than that of the inorganic layer <NUM> and thus working as a high energy barrier. Accordingly, the electron transport layer <NUM> according to an embodiment has a hybrid structure consisting of the inorganic layer <NUM> as a cluster layer formed of inorganic nanoparticles and the organic layer <NUM> covering the inorganic layer <NUM>.

In an embodiment, when the cluster layer formed of inorganic nanoparticles is formed on the surface of the emission layer <NUM> including quantum dots, this cluster layer has very high electron mobility relative to hole mobility between the hole injection layer (HIL) <NUM> and the hole transport layer (HTL) <NUM>, and a leakage current may be generated due to uneven surface morphology of the cluster layer.

Accordingly, in an embodiment, the organic layer <NUM> having a higher work function than that of the cluster layer may be formed to cover the surface of the cluster layer to minimize the leakage current generated from the cluster layer. In addition, the organic layer <NUM>, as shown in <FIG>, simultaneously works as a kind of an energy barrier and thus may adjust entire electron mobility of the electron transport layer <NUM> into an appropriate level and thus improve a charge carrier balance.

As described above, the electroluminescent device <NUM> according to an embodiment includes the inorganic layer <NUM> having very high electron mobility and improving luminous efficiency of the emission layer <NUM> including quantum dots and the organic layer <NUM> minimizing the leakage current due to the surface morphology of the inorganic layer <NUM> and simultaneously, adjusting entire electron mobility of the electron transport layer <NUM> into an appropriate level, and accordingly, the electron transport layer <NUM> has a hybrid stacking structure of the inorganic layer <NUM>/the organic layer <NUM>.

Accordingly, the electroluminescent device <NUM> according to an embodiment may improve a charge carrier balance and light extraction efficiency of an emission layer and also, minimize a leakage current and thus show excellent driving characteristics and life-span characteristics.

A display device according to an embodiment including the electroluminescent device <NUM> is described.

A display device according to an embodiment includes a substrate, a driving circuit formed on the substrate, and a first electroluminescent device, a second electroluminescent device, and a third electroluminescent device spaced apart from each other in a predetermined interval and disposed on the driving circuit.

The first to third electroluminescent devices have the same structure as the electroluminescent device <NUM> and but the wavelengths of the light emitted from each of the quantum dots may be different from each other.

In an embodiment, the first electroluminescent device is a red device emitting red light, the second electroluminescent device is a green device emitting green light, and the third electroluminescent device is a blue device emitting blue light. In other words, the first to third electroluminescent devices may be pixels expressing (i.e., emitting) red, green, or blue colored light, respectively, in the display device.

However, an embodiment is not necessarily limited thereto, and the first to third electroluminescent devices may respectively express magenta, yellow, or cyan, or may express other colors.

One or more of the first to third electroluminescent devices may be the electroluminescent device <NUM>. For example, an electroluminescent device displaying blue in the display device is the electroluminescent device <NUM> and electroluminescent devices displaying red and green may include an electron transport layer that consists of an organic layer or inorganic layer or that includes both organic layer and inorganic layer, provided that an organic layer is formed directly on the emission layer. Alternatively, one of the first to third electroluminescent devices may be the electroluminescent device <NUM> and the rest may be electroluminescent devices including a fluorescent material or a phosphor material instead of the quantum dot as the light emitting particle.

The substrate may be a transparent insulating substrate or may be made of a ductile material. The substrate may include glass or a polymer material in a film having a glass transition temperature (Tg) of greater than about <NUM>. For example, it includes a cycloolefin copolymer (COC) or cycloolefin polymer (COP) based material.

The driving circuit is disposed on the substrate and is independently connected to each of the first to third electroluminescent devices. The driving circuit may include at least one set of lines including a scan line, a data line, a driving power source line, a common power source line, and the like, at least two of thin film transistors (TFT) connected to the wire and corresponding to one organic light emitting diode, and at least one capacitor, or the like. The driving circuit may have a variety of the known structures.

As described above, a display device according to an embodiment may display images having improved color purity and color reproducibility without separate light source such as a backlight unit, and particularly may exhibit improved driving characteristics and life-span characteristics.

However, these examples are exemplary, and the present disclosure is not limited thereto.

An indium time oxide (ITO) layer is deposited on a glass substrate as a first electrode (an anode), and a PEDOT:PSS layer (hole injection layer) and a poly(<NUM>,<NUM>-dioctyl-fluorene-co-N-(<NUM>-butylphenyl)-diphenylamine) (TFB) polymer layer (hole transport layer) are respectively and sequentially formed thereon by using a solution process. Subsequently, a blue emission layer is formed thereon by coating blue quantum dots (ZnTeSe) dispersed in an organic solvent and heat-treating it under a nitrogen atmosphere at <NUM> for <NUM> minutes.

On the other hand, ZnO particles having an average particle diameter of <NUM> are three times washed and then, formed into a ZnO cluster layer (inorganic layer) having a thickness of about <NUM> on the blue emission layer.

Subsequently, on the surface of the ZnO cluster layer, an organic compound <NUM> layer which includes an organic compound <NUM> (phosphine oxide based compound, NET-<NUM>, Novaled Electron Transport material <NUM>, obtained from Novaled) layer (organic layer) is formed to be <NUM> to <NUM> thick at a deposition rate of <NUM>Å/s to <NUM>Å/s, and an Al layer is deposited thereon to manufacture an electroluminescent device according to Example <NUM>.

An electroluminescent device is manufactured according to the same method as Example <NUM> except that the organic compound <NUM> and an organic compound <NUM> (phosphonyl phenol based compound, Novaled Co. , NDN-<NUM>, Novaled Dopant n-side material <NUM>) in a weight ratio of <NUM>:<NUM> are co-deposited to form an organic compound <NUM>: organic compound <NUM> blend layer instead of the organic layer.

An electroluminescent device is manufactured according to the same method as Example <NUM>, except that the ZnO cluster layer is formed to have a thickness of <NUM>.

An electroluminescent device is manufactured according to the same method as Example <NUM>, except that the Al layer is formed directly on the ZnO cluster without forming the organic compound <NUM> layer.

An electroluminescent device is manufactured according to the same method as Example <NUM>, except that the organic compound <NUM>: organic compound <NUM> blend layer is formed on the blue emission layer by twice to three times washing the ZnO particles having an average particle diameter of <NUM> and then mixing them with the organic compound <NUM> in a weight ratio of <NUM>:<NUM>, and then, the Al layer is deposited thereon.

An electroluminescent device is manufactured according to the same method as Comparative Example <NUM>, except that the ZnO: organic compound <NUM> blend layer is formed by mixing ZnO and the organic compound <NUM> in a weight ratio of <NUM>:<NUM> instead of the organic compound <NUM>.

First, I-V-L characteristics of the electroluminescent devices according to Example <NUM> and Comparative Examples <NUM> to <NUM> are respectively shown in <FIG>, and J-V characteristics thereof are shown in <FIG>.

<FIG> are graphs showing I-V-L characteristics of the electroluminescent devices according to Example <NUM> and Comparative Examples <NUM> to <NUM>; <FIG> shows voltage-current density, <FIG> shows voltage-luminance, and <FIG> shows luminance-external quantum efficiency (EQE), respectively.

Referring to <FIG> and <FIG>, Example <NUM> shows a lower current density at the same voltage and a lower luminance at the same voltage compared with Comparative Examples <NUM> and <NUM>, and the reason is that the organic compound <NUM> layer works as an energy barrier and decreases current density at a lower voltage.

However, referring to <FIG>, Example <NUM> shows very excellent external quantum efficiency at luminance of less than or equal to about <NUM> cd/m<NUM> compared with the Comparative Examples. In particular, Example <NUM> shows the best external quantum efficiency (about <NUM> %) at a luminance of about <NUM> cd/m<NUM>. In addition, Example <NUM> shows a smaller external quantum efficiency decrease at a higher voltage in a higher luminance region compared with the Comparative Examples, and thus provides stable driving characteristics.

In other words, since the organic compound <NUM> layer is further formed on the ZnO cluster layer and thus works as an energy barrier against electrons moving from the Al layer to the ZnO cluster layer, Example <NUM> has a slight decrease in current density and luminance but shows excellent luminous efficiency at low luminance.

On the other hand, Comparative Example <NUM> shows lower current density at the same voltage, lower luminance at the same voltage, and lower external quantum efficiency at the same luminance compared with Example <NUM> as well as Comparative Example <NUM>, and these results are expected based on a difference of organic semiconductor materials.

<FIG> is a graph showing a voltage-current density relationship of the electroluminescent devices according to Example <NUM> and Comparative Examples <NUM> to <NUM>. <FIG> is a graph showing the current density in the y-axis on a logarithmic scale.

Referring to <FIG>, Example <NUM> has an inorganic-organic hybrid stacking structure by further forming the organic compound <NUM> layer on the surface of the ZnO cluster layer and thus shows about <NUM>,<NUM> times less current leakage compared with Comparative Example <NUM> including only the ZnO cluster layer.

On the other hand, when electroluminescence intensity relative to a wavelength is examined regarding the electroluminescent device of Example <NUM>, the electroluminescent device of Example <NUM> shows a narrow full width at half maximum (FWHM) of about <NUM>.

The electroluminescent devices according to Example <NUM> and Comparative Examples <NUM> and <NUM> are measured regarding a luminance decrease depending on time, and the results are shown in <FIG>.

<FIG> is a time-luminance graph of the electroluminescent devices according to Example <NUM>, Comparative Example <NUM>, and Comparative Example <NUM>.

Referring to <FIG>, the electroluminescent device of Example <NUM> shows a slower rate of luminance decrease compared with those of Comparative Examples <NUM> and <NUM>, and when T<NUM> is defined as a time when an electroluminescent device shows <NUM> % luminance relative to initial luminance, Example <NUM> shows the longest T<NUM> of <NUM> hours, Comparative Example <NUM> shows T<NUM> of <NUM> hours, and Comparative Example <NUM> shows T<NUM> of <NUM> hours.

The life-span characteristics decrease as a leakage path due to surface morphology of the ZnO cluster layer increases, and accordingly, Example <NUM> including the organic compound <NUM>: organic compound <NUM> blend layer covering and minimizing the leakage path of the ZnO cluster layer shows excellent life-span characteristics compared with the Comparative Examples.

On the other hand, I-V-L characteristics of the electroluminescent devices of Example <NUM> and Comparative Example <NUM> are respectively shown in <FIG>, and J-V characteristics are shown in <FIG>.

<FIG> are graphs showing I-V-L characteristics of the electroluminescent devices according to Example <NUM> and Comparative Example <NUM>; <FIG> shows voltage-current density, <FIG> shows voltage-luminance, and <FIG> shows luminance-external quantum efficiency, respectively.

Referring to <FIG>, Example <NUM> shows a slightly lower current density at the same voltage and a slightly lower luminance at the same voltage, since the organic compound <NUM>: organic compound <NUM> blend layer works as an energy barrier against electrons moving from the Al layer to the ZnO cluster, and also a very high luminance-external quantum efficiency of about <NUM> % at a lower luminance of less than or equal to about <NUM> cd/m<NUM> and particularly, less than or equal to about <NUM> cd/m<NUM>.

<FIG> is a voltage-current density graph of the electroluminescent devices according to Example <NUM> and Comparative Example <NUM>. <FIG> is a graph showing the current density in the y-axis on a logarithmic scale.

Referring to <FIG>, Example <NUM> further having the organic compound <NUM>: organic compound <NUM> blend layer on the surface of the ZnO cluster layer shows an excellent reduction in leakage current compared with Comparative Example <NUM> having only the ZnO cluster layer.

Claim 1:
An electroluminescent device (<NUM>), comprising:
a first electrode (<NUM>) and a second electrode (<NUM>) facing each other;
an emission layer (<NUM>) disposed between the first electrode and the second electrode and comprising a plurality of light emitting particles (<NUM>);
a hole transport layer (<NUM>) disposed between the first electrode and the emission layer; and
an electron transport layer (<NUM>) disposed between the emission layer and the second electrode;
wherein the electron transport layer comprises:
an inorganic layer (<NUM>) disposed on the emission layer, the inorganic layer comprising a plurality of inorganic nanoparticles (<NUM>); and
an organic layer (<NUM>) directly disposed on at least a portion of an upper surface of the inorganic layer on a side opposite the emission layer,
wherein an inorganic nanoparticle of the plurality of inorganic nanoparticles (<NUM>) is ZnO, TiO<NUM>, ZrO<NUM>, SnO<NUM>, WO<NUM>, Ta<NUM>O<NUM>, or a combination thereof,
wherein the organic layer (<NUM>) comprises an organic semiconductor compound that is a quinolone-based compound, a triazine-based compound, a quinoline-based compound, a triazole-based compound, a naphthalene-based compound, or a combination thereof, or a phosphine oxide based compound, a phosphonyl phenol based compound, or a combination thereof,
wherein the organic layer is an energy barrier against electrons moving from the second electrode to the inorganic layer, and
wherein an average thickness of the organic layer is <NUM> nanometers to <NUM> nanometers.