Patent Description:
Quantum dots are a nanocrystal semiconductor material having a diameter of less than or equal to several to several hundreds of nanometers (nm), which exhibits quantum confinement effects. Quantum dots generate stronger, e.g., brighter, light in a narrow wavelength region than phosphors. Quantum dots emit light while the excited electrons are transitioned from a conduction band to a valance band and wavelengths are changed depending upon a particle size even in the same material. As quantum dots emit light of a shorter wavelength with smaller, e.g., decreasing, particle sizes, quantum dots may provide light in a desirable wavelength region by adjusting the sizes.

An emission layer including quantum dots and various types of electronic devices including the same may provide decreased production costs, compared with an organic light emitting diode using an emission layer including a phosphorescent material, fluorescent material, or a combination thereof, and desirable colors may be emitted by changing sizes of quantum dots, without using other organic materials in the emission layer for emitting other color lights.

Luminous efficiency of the emission layer including quantum dots is determined by quantum efficiency, e.g., external quantum efficiency (EQE), of quantum dots, a balance of charge carriers, light extraction efficiency, and the like. For example, in order to improve the quantum efficiency, excitons may be confined in the emission layer, but when the excitons are not confined in the emission layer, for example, due to, a variety of factors, a problem such as exciton quenching may be caused.

Korean Patent Application Publication Number <CIT> presents an electroluminescent device having a hole transport layer, an electron transport layer and first and second emission layers.

United States Patent Application Publication Number <CIT> relates to multilayer polymer-quantum dot light-emitting diodes (PQD-LEDs).

United States Patent Application Publication Number <CIT> presents a light emitting device having a first and optional second light emitting layer, each layer having a quantum dot with a core part and first and second shell parts. The second shell part has a surface coated with two types of hole-transporting and electron-transporting surfactants.

An electroluminescent device having improved life-span characteristics and color purity simultaneously by minimizing exciton quenching and a display device including the same are provided.

According to an aspect of the invention, there is provided an electroluminescent device according to claim <NUM>.

A hole mobility of the first emission layer may be faster than a hole mobility of the second emission layer.

An electron mobility of the second emission layer may be faster than an electron mobility of the first emission layer.

The hydrophilic solvent may include methanol, ethanol, isopropyl alcohol, acetone, butanol, dimethyl formamide, dimethyl sulfoxide, water, acetonitrile, or a combination thereof.

The hydrophobic solvent may include octane, heptane, nonane, hexane, xylene, toluene, chlorobenzene, chloroform, cyclohexane, or a combination thereof.

The first light emitting particle, the second light emitting particle, or a combination thereof may have a core-shell structure.

The first light emitting particle and the second light emitting particle may independently 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 first emission layer and the second emission layer may emit first light belonging to a predetermined wavelength region.

The first light may belong to a first wavelength region of about <NUM> nanometers (nm) to about <NUM>, a second wavelength region of about <NUM> to <NUM>, a third wavelength region of greater than <NUM> to about <NUM>, a fourth wavelength region of about <NUM> to about <NUM>, or a fifth wavelength region of about <NUM> to about <NUM>.

Each of the first emission layer and the second emission layer may have a thickness of about <NUM> to about <NUM>.

The second emission layer may be disposed directly on the first emission layer.

According to an embodiment, an electroluminescent device includes a first electrode; a hole transport layer disposed on the first electrode; a first emission layer disposed on the hole transport layer, the first emission layer and including a first light emitting particle on which a first ligand having a hole transporting property is attached; a second emission layer disposed on the first emission layer, the second emission layer and including a second light emitting particle on which a second ligand having a electron transporting property is attached; a third emission layer disposed between the first emission layer and the second emission layer, the third emission layer including third light emitting particle on which a third ligand having electrical insulating properties is attached; an electron transport layer disposed on the second emission layer; and a second electrode disposed on the electron transport layer, wherein a solubility of the third ligand in a solvent is different than a solubility of the first ligand in the solvent and different than a solubility of the second ligand in the solvent.

The third emission layer may be disposed directly on the first emission layer and the second emission layer may be disposed directly on the third emission layer.

A hole mobility of the third emission layer may be lower than a hole mobility of the first emission layer.

An electron mobility of the third emission layer may be lower than an electron mobility of the second emission layer.

A charge mobility of the third ligand may be about <NUM>-<NUM> square centimeters per volt-second (cm<NUM>/Vs) to about <NUM>-<NUM> cm<NUM>/Vs.

When the third ligand is soluble in a hydrophilic solvent, each of the first ligand and the second ligand may be soluble in a hydrophobic solvent. When the third ligand is soluble in for a hydrophobic solvent, each of the first ligand and the second ligand may be soluble in a hydrophilic solvent.

The third ligand may include a ligand compound derived from a thiol-based compound, a halide-based compound, an alkyl-based compound, an amine-based compound, a carbazole-based compound, a mercapto-based compound, or a combination thereof.

The first emission layer, the second emission layer, and the third emission layer may emit a first light belonging to a predetermined wavelength region.

The third emission layer may have a thickness of about <NUM> to about <NUM>.

A display device includes the electroluminescent device according to an embodiment.

Excitons may be effectively confined to the emission layer, and exciton quenching may be minimized and an electroluminescent device with improved life-span characteristics and color purity may be provided.

Further, a display device including an electroluminescent device with improved life-span characteristics and color purity may be provided as described above.

The above and other advantages and features of this disclosure will become more apparent by describing in further detail exemplary embodiments thereof with reference to the accompanying drawings, in which:.

Example embodiments of the present disclosure will hereinafter be described in detail, and may be performed by a person having an ordinary skill in the related 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.

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.

As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms, including "at least one," unless the content clearly indicates otherwise.

Furthermore, relative terms, such as "lower" and "upper," 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.

"About" as used herein is inclusive of the stated value and means within an acceptable range of deviation for the particular value as determined by one of ordinary skill in the art, considering the measurement in question and the error associated with measurement of the particular quantity (i.e., the limitations of the measurement system).

As used herein, "alkyl" may refer to a linear or branched saturated monovalent hydrocarbon group (methyl, ethyl hexyl, etc.).

As used herein, "alkenyl" may refer to a linear or branched monovalent hydrocarbon group having one or more carbon-carbon double bond.

As used herein, "alkynyl" may refer to a linear or branched monovalent hydrocarbon group having one or more carbon-carbon triple bond.

As used herein, an "amine" may have the general formula NRR, wherein each R is independently hydrogen, a C1-C30 alkyl group, a C3-C8 cycloalkyl group a C2-C30 alkenyl group, a C2-C30 alkynyl group, or a C6-C30 aryl group, each of which may be substituted or unsubstituted.

As used herein, "aryl" may refer to a group formed by removal of at least one hydrogen from an aromatic group (e.g., a phenyl or naphthyl group).

As used herein, "hetero" may refer to one including one or more (e.g., <NUM> to <NUM>) heteroatom of N, O, S, Si, P, or a combination thereof.

As used herein, "Group" may refer to a group of the Periodic Table.

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

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

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

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

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

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

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

As used herein, "halide" may refer to a compound in which one of the elements is a halogen.

As used herein, a hydrocarbon group may refer to a group including carbon and hydrogen (e.g., an alkyl, alkenyl, alkynyl, or aryl group). The hydrocarbon group may be a group having a monovalence or greater formed by removal of one or more hydrogen atoms from, alkane, alkene, alkyne, or arene. In the hydrocarbon group, at least one methylene may be replaced by an oxide moiety, a carbonyl moiety, an ester moiety, -NH-, or a combination thereof.

As used herein, "substituted" may refer to a compound or a moiety, wherein at least one of hydrogen atoms thereof is replaced by a substituent, wherein the substituent may be a C1 to C30 alkyl group, a C2 to C30 alkenyl group, a C2 to C30 alkynyl group, a C6 to C30 aryl group, a C7 to C30 alkylaryl group, a C1 to C30 alkoxy group, a C1 to C30 heteroalkyl group, a C3 to C30 heteroalkylaryl group, a C3 to C30 cycloalkyl group, a C3 to C15 cycloalkenyl group, a C6 to C30 cycloalkynyl group, a C2 to C30 heterocycloalkyl group, a halogen (-F, -Cl, -Br, or -I), a hydroxy group (-OH), a nitro group (-NO<NUM>), a cyano group (-CN), an amino group (-NRR' wherein R and R' are independently hydrogen or a C1 to C6 alkyl group), an azido group (-N<NUM>), an amidino group (-C(=NH)NH<NUM>)), a hydrazino group (-NHNH<NUM>), a hydrazono group (=N(NH<NUM>)), an aldehyde group (-C(=O)H), a carbamoyl group (-C(O)NH<NUM>), a thiol group (-SH), an ester group (-C(=O)OR, wherein R is a C1 to C6 alkyl group or a C6 to C12 aryl group), a carboxyl group (-COOH) or a salt thereof (-C(=O)OM, wherein M is an organic or inorganic cation), a sulfonic acid group (-SO<NUM>H) or a salt thereof (-SO<NUM>M, wherein M is an organic or inorganic cation), a phosphoric acid group (-PO<NUM>H<NUM>) or a salt thereof (-PO<NUM>MH or -PO<NUM>M<NUM>, wherein M is an organic or inorganic cation), or a combination thereof.

<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>, a hole transport layer <NUM> disposed on the first electrode <NUM>, a hole injection layer <NUM> disposed between the first electrode <NUM> and the hole transport layer <NUM>, a first emission layer <NUM> disposed on the hole transport layer <NUM> and including a first light emitting particle <NUM>, a second emission layer <NUM> disposed on the first emission layer <NUM> and including a second light emitting particle <NUM>, an electron transport layer <NUM> disposed on the second emission layer <NUM>, and a second electrode <NUM> disposed on the electron transport layer <NUM>.

The electroluminescent device <NUM> has a stack structure wherein the hole injection layer <NUM>, the hole transporting layer <NUM>, the first emission layer <NUM>, the second emission layer <NUM>, and the electron transporting layer <NUM> are disposed between the first electrode <NUM> and the second electrode <NUM> facing each other.

The electroluminescent device <NUM> according to an embodiment supplies a current to the first emission layer <NUM> and the second emission layer <NUM> through the first electrode <NUM> and the second electrode <NUM> and generates light by electroluminescence of the first light emitting particle <NUM> and the second light emitting particle <NUM>. The electroluminescent device <NUM> generates light having various wavelength regions according to materials, sizes, detailed structures, etc. of the first and second light emitting particles <NUM> and <NUM> of the first and second emission layers <NUM> and <NUM>.

In an embodiment, the first electrode <NUM> may be directly connected to a driving power source so that the first electrode <NUM> may function to flow, e.g., supply a current to the first and second emission layers <NUM> and <NUM>. The first electrode <NUM> may include a material having light transmittance in at least 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 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.

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

In an embodiment, the substrate <NUM> may support the hole injection layer <NUM>, the hole transport layer <NUM>, the first emission layer <NUM>, the second emission layer <NUM>, and the electron transport layer <NUM> disposed between the first electrode <NUM> and the second electrode <NUM>. The first electrode <NUM> of the electroluminescent device <NUM> according to an embodiment may not be disposed on the substrate <NUM>, but the substrate <NUM> may be disposed on the second electrode <NUM> or may be omitted, as desired.

The second electrode <NUM> includes an optically transparent material and may function as a light-transmitting electrode to transmit light generated in the first and second emission layers <NUM> and <NUM> which will be described later. In an embodiment, the second electrode <NUM> may include silver (Ag), aluminum (Al), copper (Cu), gold (Au), 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 by a method such as sputtering.

The hole transport layer <NUM> may be disposed between the first electrode <NUM> and the first emission layer <NUM>. The hole transport layer <NUM> serves to supply and transport the holes to the first and second emission layers <NUM> and <NUM>. The hole transport layer <NUM> is formed directly under the first emission layer <NUM> to directly contact the first emission layer <NUM>.

In an embodiment, the hole transport layer <NUM> may be formed of the p-type semiconductor material, or a material doped with a p-type dopant. For example, the hole transport layer <NUM> may include a poly (<NUM>,<NUM>-ethylenedioxythiophene) (PEDOT) derivative, a poly(styrene sulfonate) (PSS) derivative, a poly-N-vinylcarbazole (PVK) derivative, a polyphenylenevinylene derivative, a poly p-phenylene vinylene (PPV) derivative, a polymethacrylate derivative, a poly(<NUM>,<NUM>-dioctylfluorene) derivative, a poly(spiro-fluorene) derivative, N,N'-diphenyl-N,N'-bis(<NUM>-methylphenyl)-(<NUM>,<NUM>'-biphenyl)-<NUM>,<NUM>'-diamine (TPD), N,N'-di(naphthalen-<NUM>-yl)-N-N'-diphenyl-benzidine (NPB), tris(N-<NUM>-methylphenyl-N-phenylamino)-triphenylamine (m-MTDATA), poly(<NUM>,<NUM>-dioctylfluorene-co-N-(<NUM>-butylphenyl)diphenylamine) (TFB), poly(<NUM>,<NUM>-dioctylfluorene)-co-N,N-diphenyl-N,N-di-(p-butylphenyl)-<NUM>,<NUM>-diaminobenzene (PFB), poly-TPD, metal oxide such as NiO or MoO<NUM>, or a combination thereof, but is not limited thereto.

The hole transport layer <NUM> may be formed using a wet coating method such as spin coating. For example, a polymer film such as PEDOT:PSS or TFB is formed on the first electrode <NUM>, a precursor solution including a precursor polymer and a methanol organic solvent is spin-coated on the first electrode <NUM>, and then thermally treated in an inert gas atmosphere, for example, N<NUM> or under vacuum at a curing temperature of about <NUM> to about <NUM> for <NUM> hours to manufacture the hole transport layer <NUM> including, e.g., a polymer thin film.

The hole injection layer <NUM> may be disposed between the first electrode <NUM> and the hole transport layer <NUM>. The hole injection layer <NUM> may supply holes into the first and second emission layers <NUM> and <NUM> together with the hole transporting layer <NUM> and may be omitted considering the thickness and the material of the hole transporting layer <NUM>.

The hole injection layer <NUM> may be formed of a p-type semiconductor material or a material doped with a p-type dopant, like the aforementioned hole transport layer <NUM>. For example, the hole injection layer <NUM> may include poly (<NUM>,<NUM>-ethylenedioxythiophene) (PEDOT) derivative, a poly(styrene sulfonate) (PSS) derivative, a poly-N-vinylcarbazole (PVK) derivative, a polyphenylenevinylene derivative, a poly p-phenylene vinylene (PPV) derivative, a polymethacrylate derivative, a poly (<NUM>,<NUM>-dioctylfluorene) derivative, a poly(spiro-fluorene) derivative, N,N'-diphenyl-N,N'-bis(<NUM>-methylphenyl)-(<NUM>,<NUM>'-biphenyl)-<NUM>,<NUM>'-diamine (TPD), N,N'-di(naphthalen-<NUM>-yl)-N-N'-diphenyl-benzidine (NPB), tris(N-<NUM>-methylphenyl-N-phenylamino)-triphenylamine (m-MTDATA), poly (<NUM>,<NUM>-dioctylfluorene-co-N-(<NUM>-butylphenyl)diphenylamine) (TFB), poly(<NUM>,<NUM>-dioctylfluorene)-co-N,N-diphenyl-N,N-di-(p-butylphenyl)-<NUM>,<NUM>-diaminobenzene (PFB), poly-TPD, a metal oxide such as NiO and MoO<NUM>, or a combination thereof but is not limited thereto.

The first and second emission layers <NUM> and <NUM> includes a plurality of light emitting particles. That is, the first emission layer <NUM> includes a plurality of first light emitting particles <NUM> and the second emission layer <NUM> includes a plurality of second light emitting particles <NUM>.

The first and second emission layers <NUM> and <NUM> are sites 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 first and second emission layers <NUM> and <NUM> to generate excitons and the generated excitons are transitioned from an exited state to a ground state to emit light in wavelengths corresponding to sizes of the first and second light emitting particles <NUM> and <NUM>.

The second emission layer <NUM> may be directly on the first emission layer <NUM>. That is, the upper surface of the first emission layer <NUM> and the lower surface of the second emission layer <NUM> may be in contact with each other. The first emission layer <NUM> and the second emission layer <NUM> may have different electrical characteristics. According to an embodiment, the first emission layer <NUM> has a faster hole transport velocity than the electron transport velocity and may exhibit stronger, e.g., greater, electron transport capability, and the second emission layer <NUM> has a faster electron transport velocity than the hole transport velocity and may exhibit stronger, e.g., greater, hole transport capability.

For example, the hole mobility of the first emission layer <NUM> may be faster than the hole mobility of the second emission layer <NUM>.

For example, the electron mobility of the second emission layer <NUM> may be faster than the electron mobility of the first emission layer <NUM>.

For example, a relationship of charges (holes/electrons) mobility between the first emission layer <NUM> and the second emission layer <NUM> may satisfy at least one of the aforementioned relationships.

The first emission layer <NUM> transports holes toward the second emission layer <NUM> and the second emission layer <NUM> transports electrons toward the first emission layer <NUM> so that holes and electrons are combined on, e.g., at, the interface between the first emission layer <NUM> and the second emission layer <NUM>, in the vicinity of the interface, or a combination thereof.

<FIG> schematically shows a principle of the electroluminescent device driving according to an embodiment, and each constituent element in electroluminescent device <NUM> is shown by an energy band diagram.

Referring to <FIG>, holes move from the hole injection layer <NUM> to the first emission layer <NUM> through the hole transport layer <NUM>. The first emission layer <NUM> according to an embodiment may have hole transport capability, and the holes reaching the first emission layer <NUM> move toward the second emission layer <NUM>. Electrons may move from the electron injection layer to the electron transport layer <NUM>. The second emission layer <NUM> according to an embodiment may have electron transport capability, and the electrons move toward the first emission layer <NUM> even in the second emission layer <NUM>.

The holes and the electrons in the first and second emission layers <NUM> and <NUM> are combined on, e.g., at, the interface between the first emission layer <NUM> and the second emission layer <NUM>, in the vicinity of the interface, or a combination thereof, as shown in <FIG>.

The first emission layer <NUM> and the second emission layer <NUM> may be adjusted so that excitons may be generated on, e.g., at, the interface between the first emission layer <NUM> and the second emission layer <NUM>, in the vicinity of the interface, or a combination thereof using a difference in charge mobility, for example, due to different electrical characteristics.

If the first and second emission layers <NUM> and <NUM> are relatively thick, it may be relatively difficult to adjust an internal carrier balance in the electroluminescent device <NUM>. If the first and second emission layers <NUM> and <NUM> are relatively thin, it may be relatively difficult to provide the aforementioned electron/hole transport capability.

The thickness of each of the first and second emission layers <NUM> and <NUM>, in consideration of the aforementioned electron/hole transport capability and the carrier balance, may be, 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 greater than or equal to about <NUM> and 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>, less than or equal to about <NUM>, less than or equal to about <NUM>, or less than or equal to about <NUM>.

In an electroluminescent device using a quantum dot as a light emitting body, hole-moving velocity may not be smooth compared with electron-moving velocity, e.g., hole-moving velocity may not be comparable to electron-moving velocity. The first emission layer <NUM> may have the same thickness or a greater thickness than the second emission layer <NUM>.

An embodiment is not limited thereto, but each of the first and second emission layers <NUM> and <NUM> may have a thickness determined by considering each material, a material and a thickness of the hole transport layer <NUM>, the electron transport layer <NUM>, a combination thereof, and the like.

Both of the first and second emission layers <NUM> and <NUM> may emit first light belonging to a predetermined wavelength region. The first light belongs to a visible light region, for example, one of a first wavelength region of about <NUM> to about <NUM>, a second wavelength region of about <NUM> to about <NUM>, a third wavelength region of about <NUM> to about <NUM>, a fourth wavelength region of about <NUM> to about <NUM>, and a fifth wavelength region of about <NUM> to about <NUM>.

An embodiment is not limited thereto, but the first emission layer <NUM> and the second emission layer <NUM> may be set, e.g., configured to emit light at a different wavelength from each other. The electroluminescent device <NUM> may emit light of a mixed color of light emitted from the first emission layer <NUM> with light emitted from the second emission layer <NUM> and a further mixed color of light with different light supplied from an external light source.

In an embodiment, at least either one of the first and second light emitting particles <NUM> and <NUM> may include a quantum dot. The first and second light emitting particles <NUM> and <NUM> may be all quantum dots, or either one of the first and second light emitting particles <NUM> and <NUM> may be a quantum dot, and the other one may be a different light emitting body from the quantum dot, for example, a commercially-available phosphor and the like.

The quantum dots have a discontinuous energy bandgap by the quantum confinement effect and incident light may be converted into light having a particular wavelength and then radiated. Both of the first and second light emitting particles <NUM> and <NUM> may include, e.g., be composed of, quantum dots, and the first and second emission layers <NUM> and <NUM> may generate light having improved color reproducibility and color purity.

In an embodiment, materials of the quantum dot are not particularly limited and commercially available quantum dots may be used. For example, each of the first and second light emitting particles <NUM> and <NUM> may be a quantum dot including 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. That is, each of the first and second light emitting particles <NUM> and <NUM> according to an embodiment may be a non-cadmium-based quantum dot. The first and second light emitting particles <NUM> and <NUM> may include, e.g., consist of, non-cadmium-based materials, the first and second light emitting particles <NUM> and <NUM> may have reduced or no toxicity compared with cadmium-based quantum dots, and the first and second light emitting particles <NUM> and <NUM> may not be dangerous and may be environmentally-friendly.

The Group II-VI compound may be a binary element compound of ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, MgSe, MgS, or a combination thereof; a ternary element compound of ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, HgZnS, HgZnSe, HgZnTe, MgZnSe, MgZnS, or a combination thereof; or a quaternary element compound of ZnSeSTe, 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 of GaN, GaP, GaAs, GaSb, AIN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, or a combination thereof; a ternary element compound of 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 of GaAlNP, GaAINAs, GaAINSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, InAlPSb, or a combination thereof. The Group III-V compound may further include a Group II metal (InZnP).

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

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

According to an embodiment, the quantum dots 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. The quantum dots may have one semiconductor nanocrystal core and multi-shells surrounding the core. The multi-layered shell structure may have a structure of, e.g., including, two or more shells and each layer may have a single composition or an alloy or may have a concentration gradient.

In an embodiment, the first light emitting particle, the second light emitting particle, or a combination thereof may have a core-shell structure. When at least one of the first and second light emitting particles has a core-shell structure, a material composition of the shell has a larger energy bandgap than an energy bandgap of the core, which may exhibit an effective quantum confinement effect. An embodiment is not limited thereto. In the multi-layered shell, a shell that is outside of the core has may have a larger energy bandgap than a shell that is near, e.g., closer or adjacent, to the core and quantum dots may have an ultraviolet (UV) to infrared wavelength ranges.

The quantum dots may have quantum efficiency 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> %, greater than or equal to about <NUM> %, greater than or equal to about <NUM> %, or even <NUM> %.

In a display, the quantum dots may have a relatively narrow spectrum so as to improve color purity or color reproducibility. The quantum dots may have, for example, a full width at half maximum (FWHM) of a photoluminescence 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 dots may have a particle diameter (the longest diameter for a non-spherically shaped particle) of about <NUM> to about <NUM>. For example, the quantum dots may have a particle diameter of about <NUM> to about <NUM>, for example, about <NUM> (or about <NUM>) to about <NUM>.

The shapes of the quantum dots may be any suitable shape and may not be particularly limited. For example, the quantum dots may have a spherical, oval, cubic, tetrahedral, pyramidal, cuboctahedral, cylindrical, polyhedral, or multi-armed nanoparticle, nanotube, nanowire, nanofiber, nanosheet, or a combination thereof. The quantum dots may have any suitable cross-sectional shape.

The quantum dots may be commercially available or may be synthesized in any suitable method. For example, several nano-sized quantum dots may be synthesized according to a wet chemical process. In the wet chemical process, precursor materials react in an organic solvent to grow crystal particles.

According to the claimed invention a first ligand is attached to the surface of the first light emitting particle <NUM> and a second ligand is attached to the second light emitting particle <NUM>, respectively.

According to the claimed invention, the first ligand and the second ligand have different electrical characteristics. According to the claimed invention, the first ligand has faster hole transport velocity than electron transport velocity and may exhibit strong electron transport capability, and the second ligand has faster electron transport velocity than hole transport velocity and may exhibit strong hole transport capability. The first emission layer <NUM> has stronger, e.g., greater, hole transport capability than the second ligand, for example, due to electrical characteristics of the first ligand, and the second emission layer <NUM> has stronger, e.g., greater, electron transport capability than the first ligand, for example, due to electrical characteristics of the second ligand.

An electroluminescent device using a quantum dot as a light-emitting body may not have smooth hole-moving velocity relative to the electron-moving velocity, e.g., the hole-moving velocity may not be comparable to the electron-moving velocity, as described above, and a place where electrons and holes are combined with each other may be shifted toward an interface between an emission layer and an electron transport layer.

In this way, the place where electrons and holes are combined with each other may be shifted toward an interface between an emission layer and an electron transport layer, and excitons generated on, e.g., at, the interface may not be confined inside the emission layer but quenched on, e.g., an interface with a common layer. Extra electrons not combined with holes on, e.g., at, the interface may move toward the hole transport layer and form excitons in the hole transport layer. According, the exciton quenching and extra electron loss may significantly deteriorate device efficiency.

As for the electroluminescent device <NUM> according to an embodiment, the first emission layer <NUM> may be disposed to closely, e.g., directly, contact the second emission layer <NUM>, and the first emission layer <NUM> may have hole transport capability, for example, due to the first ligand, and the second emission layer <NUM> may have electron transport capability, for example, due to the second ligand.

The first emission layer <NUM> may transport holes from the hole transport layer <NUM> toward the second emission layer <NUM> relatively easily, and the second emission layer <NUM> may easily transport electrons from the electron transport layer <NUM> toward the first emission layer <NUM> relatively easily. As a result, as shown in <FIG>, the place where electrons and holes are combined inside the first and second emission layers <NUM> and <NUM> may be relatively easy to adjust.

The first and second ligands may be respectively attached on each surface of the first and second light emitting particles <NUM> and <NUM> by using a surfactant for attaching the first and second ligands. The first and second ligands may be attached on, e.g., to, the surface of the first and second light emitting particles <NUM> and <NUM> by putting the surfactants for attaching the first and second ligands along with a precursor material for forming a quantum dot and an organic solvent or putting the surfactants for attaching first and second ligands into a mixed solution of a completely-formed quantum dot and an organic solvent.

The first and second ligands according to an embodiment are coordinated on, e.g., bound to, the surface of the first and second light emitting particles <NUM> and <NUM>. The quantum dot may have a core-shell structure, and the first and second ligands may be coordinated on, e.g., bound to, the surface exposed at an outside of the shell.

A remaining organic solvent may be further coordinated on, e.g., bound to, the surface of the quantum dot in addition to the first and second ligands. The organic solvent coordinated on, e.g., bound to, the surface of the quantum dot may affect stability of a device, and excess organic materials that are not coordinated on, e.g., bound to, the surface of the nanocrystals may be removed by pouring the organic solvent in excess non-solvent, and centrifuging the resulting mixture. The non-solvent may be commercially available various materials. After removing the excess organic solvent, an amount of the materials coordinated on, e.g., bound to, the surface of the quantum dot may be less than or equal to about <NUM> weight percent (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 the weight of the quantum dot.

According to the claimed invention, the first ligand and the second ligand have a different solvent selectivity. In particular, with respect to a given solvent, the first ligand has a solubility different than the second ligand. The first ligand may have solubility for, e.g., may be soluble in, a hydrophilic solvent, and the second ligand may have solubility for, e.g., may be soluble in, a hydrophobic solvent. The first ligand may have solubility for, e.g., may be soluble in, a hydrophobic solvent, and the second ligand may have solubility for, e.g., may be soluble in, a hydrophilic solvent.

Examples of the hydrophilic solvent may be methanol, ethanol, isopropyl alcohol, acetone, butanol, dimethyl formamide, dimethyl sulfoxide, water, acetonitrile, or a combination thereof.

Examples of the hydrophobic solvent may be octane, heptane, nonane, hexane, xylene, toluene, chlorobenzene, chloroform, cyclohexane, or a combination thereof.

The first ligand and the second ligand independently include a ligand compound derived from an alkyl-based compound.

In this way, the first ligand and the second ligand have different solvent selectivity, and a composition for forming a first emission layer including first light emitting particles and a composition for forming a second emission layer including second light emitting particles may not be mixed with each other in a liquid state but respectively formed into separate layers.

The first and second emission layers may be formed by not respectively coating and curing the compositions for first and second emission layers but coating the composition for a first emission layer and the composition for a second emission layer directly thereon and then, curing them at once (so-called, a solution process) to rapidly form first and second emission layers.

The compositions for first and second emission layers may have different solvent selectivity each other, and an interface between the first emission layer <NUM> and the second emission layer <NUM> may not be damaged by a solvent but may have a uniform surface morphology during formation of the first and second emission layers <NUM> and <NUM>. Luminous efficiency and reliability of the first and second emission layers <NUM> and <NUM> may be improved.

In an embodiment, the electron transport layer <NUM> is disposed between the second emission layer <NUM> and the second electrode <NUM> and transports electrons into the first and second emission layers <NUM> and <NUM>. The electron transport layer may include an inorganic oxide nanoparticle or may be an organic layer formed by deposition. The electron transport layer may be formed of an n-type semiconductor material or a material doped with an n-type dopant.

In an embodiment, an electron injection layer facilitating injection of electrons may be further disposed between the electron transport layer <NUM> and the second electrode <NUM>. In an embodiment, a hole blocking layer blocking movement of holes may be further disposed between the second electrode <NUM> and the second emission layer <NUM>.

Each thickness of the electron transport layer <NUM>, the electron injection layer, and the hole blocking layer may be desirably selected. For example, a thickness of each layer may be in a range of greater than or equal to about <NUM> and less than or equal to about <NUM> but is not limited thereto. The electron injection layer may be an organic layer formed by deposition, and may be omitted considering a thickness or a material of the electron transport layer <NUM>.

The electron injection layer, the electron transport layer <NUM>, or a combination thereof may include, for example <NUM>,<NUM>,<NUM>,<NUM>-naphthalenetetracarboxylic dianhydride (NTCDA), bathocuproine (BCP), tris[<NUM>-(<NUM>-pyridyl)-mesityl] borane (3TPYMB), LiF, Alq<NUM>, Gaq<NUM>, Inq<NUM>, Znq<NUM>, Zn(BTZ)<NUM>, BeBq<NUM>, (<NUM>-(<NUM>-(<NUM>,<NUM>-di(naphthalen-<NUM>-yl)-<NUM>,<NUM>,<NUM>-triazin-<NUM>-yl)phenyl)quinolone), <NUM>-hydroxyquinolinato lithium (Liq), n-type metal oxide (e.g., ZnO, ZnMgO, HfO<NUM>, etc.), bathophenanthroline (Bphen), a pyrazole-based compound, a phosphonyl phenol-based compound, compounds represented by Chemical Formula <NUM> to Chemical Formula <NUM>, or a combination thereof, but are not limited thereto. <CHM>
<CHM>
<CHM>
<CHM>.

The hole blocking layer may include, for example, <NUM>,<NUM>,<NUM>,<NUM>-naphthalenetetracarboxylic dianhydride (NTCDA), bathocuproine (BCP), tris[<NUM>-(<NUM>-pyridyl)-mesityl] borane (3TPYMB), LiF, Alq<NUM>, Gaq<NUM>, Inq<NUM>, Znq<NUM>, Zn(BTZ)<NUM>, BeBq<NUM>, or a combination thereof, but is not limited thereto. The hole blocking layer may be omitted considering a thickness, a material, and the like of other constituent elements in the electroluminescent device <NUM>.

As described above, in the electroluminescent device <NUM> according to an embodiment, the other constituent elements except for, e.g., other than, the first and second emission layers <NUM> and <NUM> may be formed by a method such as deposition. Thereby, it may be relatively easy to form the other constituent elements except for, e.g., other than, the first and second emission layers <NUM> and <NUM> as a common layer during forming a display device.

As described above, as for the electroluminescent device <NUM> according to an embodiment, the first and second emission layers <NUM> and <NUM> respectively may have hole transport capability and electron transport capability, for example, due to the first and second ligands. Holes and electrons may be combined on, e.g., at, an interface between the first and second emission layers <NUM> and <NUM>, in the vicinity of the interface, or a combination thereof, and excitons may be confined inside the emission layers relatively easily.

The first and second ligands may have different solvent selectivity, and as for the electroluminescent device <NUM> according to an embodiment, the first and second emission layers <NUM> and <NUM> may be formed relatively easily through a solution process, and the interface between the first and second emission layers <NUM> and <NUM> may have uniform surface morphology.

The electroluminescent device <NUM> according to an embodiment may minimize exciton quenching by adjusting a place where excitons are generated on, e.g., at, the interface between the first and second emission layers, in the vicinity of the interface, or a combination thereof and emit light having relatively high purity and have improved life-span characteristics.

Hereinafter, referring to <FIG>, a schematic structure and a driving principle of an electroluminescent device according to an embodiment are described.

<FIG> is a cross-sectional view schematically showing an electroluminescent device according to an embodiment and <FIG> schematically shows a driving principle of an electroluminescent device according to an embodiment. An electroluminescent device <NUM>' according to an embodiment, may have the same structure as that of the aforementioned electroluminescent device <NUM> according to an embodiment and is not described in detail.

Referring to <FIG>, the electroluminescent device <NUM>' according to an embodiment may further include a third emission layer <NUM> between the first emission layer <NUM> and the second emission layer <NUM> unlike the electroluminescent device <NUM> according to an embodiment.

The third emission layer <NUM> is disposed directly on the first emission layer <NUM>, the second emission layer <NUM> is disposed directly on the third emission layer <NUM>, and the first emission layer <NUM>, the third emission layer <NUM>, and the second emission layer <NUM> may form a sequentially stacked structure.

The first, second, and third emission layers <NUM>, <NUM>, and <NUM> may all emit first light belonging to a predetermined wavelength region but is not limited thereto, but the first emission layer <NUM>, the second emission layer <NUM>, the third emission layer <NUM>, or a combination thereof may be set, e.g., configured, to emit different light, e.g., different colored light, from one another. The electroluminescent device <NUM> may emit a mixed color of light respectively emitted from the first, second, and third emission layers <NUM>, <NUM>, and <NUM> or a more mixed color of the light with light supplied from an external light source.

The third emission layer <NUM> may have different electrical characteristics from the electrical characteristics of the first and second emission layers <NUM> and <NUM>. The third emission layer <NUM> may have an internal electron mobility and hole mobility this is less than or equal to an internal electron mobility and hole mobility of the first and second emission layers <NUM> and <NUM>.

In the electroluminescent device <NUM>' according to an embodiment, the third emission layer <NUM> may have slower hole mobility than a hole mobility of the first emission layer <NUM>.

In an embodiment, the third emission layer <NUM> may have slower hole mobility than a hole mobility of the second emission layer <NUM>. The hole mobility relationship between the third emission layer <NUM> and the second emission layer <NUM> is not necessarily limited thereto, but the third emission layer <NUM> and the second emission layer <NUM> may have the same hole mobility, or the third emission layer <NUM> may have faster hole mobility than a hole mobility of the second emission layer <NUM>.

In the electroluminescent device <NUM>' according to an embodiment, the third emission layer <NUM> may have slower electron mobility than an electron mobility of the second emission layer <NUM>.

In an embodiment, the third emission layer <NUM> may have slower electron mobility than an electron mobility of the first emission layer <NUM>. The electron mobility between the third emission layer <NUM> and the first emission layer <NUM> is not limited thereto, but the third emission layer <NUM> may have the same electron mobility as that of the first emission layer <NUM> or slower electron mobility than an electron mobility of the first emission layer <NUM>.

In this way, a place where electrons and holes are combined in the electroluminescent device <NUM>' according to an embodiment may be on, e.g., at, the interface between the first emission layer <NUM> and the third emission layer <NUM>, on, e.g., at, the interface between the third emission layer <NUM> and the second emission layer <NUM>, in the vicinity of the interfaces in the third emission layer <NUM>, or a combination thereof by adjusting charge (hole/electron) mobility of the first, second, and third emission layers <NUM>, <NUM>, and <NUM>, as shown in <FIG>.

The electroluminescent device <NUM>' according to an embodiment may improve optical confinement about, e.g., around, a carrier and an exciton and exhibit improved light emitting characteristics.

A thickness of the third emission layer <NUM> considering the aforementioned electron/hole transport capability and the carrier balance may be, 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 greater than or equal to about <NUM> and, 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>, less than or equal to about <NUM>, less than or equal to about <NUM>, or less than or equal to about <NUM>.

The third emission layer <NUM> includes a third light emitting particle <NUM>. The third light emitting particle <NUM> may be a quantum dot like the aforementioned first and second light emitting particles <NUM> and <NUM> and have a core-shell structure.

The third light emitting particle <NUM> may have a third ligand having different electrical properties to the first and second light emitting layers <NUM> and <NUM>. In one embodiment, the third ligand exhibits relatively lower charge(hole/electron) transporting properties than the first and second ligands. The third ligand is attached on the surface of the third light emitting particle <NUM>. The third emission layer <NUM> exhibits relatively lower charge(hole/electron) transporting properties than the first and second light emitting layers <NUM> and <NUM> due to the third ligand.

The third ligand may have, for example, charge (hole/electron) mobility of greater than or equal to about <NUM>-<NUM> cm<NUM>/Vs, greater than or equal to about <NUM>-<NUM> cm<NUM>/Vs, or greater than or equal to about <NUM>-<NUM> cm<NUM>/Vs and, for example, less than or equal to about <NUM>-<NUM> cm<NUM>/Vs, less than or equal to about <NUM>-<NUM> cm<NUM>/Vs, or less than or equal to about <NUM>-<NUM> cm<NUM>/Vs.

In the electroluminescent device <NUM>' according to an embodiment, the third ligand has different solvent selectivity from the first and second ligands. The third ligand may have solubility for, e.g., be soluble in, a hydrophilic solvent, and both of the first and second ligands may have solubility for, e.g., be soluble in, a hydrophobic solvent. The third ligand may have solubility for, e.g., be soluble in, a hydrophobic solvent, and both of the first and second ligands may have solubility for, e.g., be soluble in, a hydrophilic solvent.

The third ligand includes a thiol derivative, a halide derivative, an alkyl derivative, an amine derivative, a carbazole derivative, or a combination thereof, like the aforementioned first and second ligands and may be chosen by considering different solvent selectivity from the aforementioned first and second ligands.

In this way, the third ligand may have different solvent selectivity from the first and second ligands, and a composition for a first emission layer including first light emitting particles and a composition for a second emission layer including second light emitting particles may not be mixed with a composition for a third emission layer including third light emitting particles in a liquid state and may form a separate layer therefrom.

Like the electroluminescent device <NUM> according to an embodiment, a multi-emission layer may be formed relatively easily through a solution process of sequentially coating each composition for first, second, and third emission layer and curing them at one time.

The compositions for first and second emission layers may have different solvent selectivity from the composition for a third emission layer, and an interface between the third emission layer <NUM> and the first emission layer <NUM> and an interface between the third emission layer <NUM> and the second emission layer <NUM> may not be respectively damaged by a solvent but may have a uniform surface morphology. Luminous efficiency and reliability of the first, second, and third emission layers <NUM>, <NUM>, and <NUM> may be improved.

As described above, the electroluminescent device <NUM>' according to an embodiment may have the first, second, and third emission layers <NUM>, <NUM>, and <NUM>, respectively having charge (hole/electron) mobility, for example, due to the first, second, and third ligands having different electrical characteristics. Holes and electrons may be combined with each other on, e.g., at, the interfaces between the first emission layer <NUM> and the third emission layer <NUM> and between the third emission layer <NUM> and the second emission layer <NUM>, in the vicinity of the interfaces inside the third emission layer <NUM>, or a combination thereof, and excitons may be confined relatively easily inside the emission layer.

As the first, second, and third ligands have different solvent selectivity, the first, second, and third emission layers <NUM>, <NUM>, and <NUM> may be formed relatively easily by using a solution process in the electroluminescent device <NUM> according to an embodiment, and the interfaces between the first, second, and third emission layers <NUM>, <NUM>, and <NUM> may also have uniform surface morphology.

Furthermore, the aforementioned electroluminescent device <NUM>' according to an embodiment may minimize exciton quenching and emit light having relatively high color purity and improved life-span characteristics like the aforementioned electroluminescent device <NUM> according to the above embodiment.

Hereinafter, a display device including the aforementioned 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 may have the same structure as the aforementioned electroluminescent device <NUM>, but the wavelengths of the lights 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. The first to third electroluminescent devices may be pixels expressing, e.g., emitting, red light, green light, and blue light, respectively, in the display device.

An embodiment is not necessarily limited thereto, but the first to third electroluminescent devices may respectively express, e.g., emit, magenta light, yellow light, cyan light, or may express, e.g., emit, other colors, e.g., other colored light.

One of the first to third electroluminescent devices may be the aforementioned electroluminescent device <NUM>. In this case, the third electroluminescent device displaying at least blue light may be desirably the aforementioned electroluminescent device <NUM>.

In the display device according to an embodiment, a hole injection layer, a hole transporting layer, an electron transporting layer, an electron injection layer, and a hole blocking layer except an emission layer of each pixel may be integrated to form a common layer. An embodiment is not limited thereto. A hole injection layer, a hole transporting layer, an electron transporting layer, an electron injection layer, and a hole blocking layer may be independently formed in each pixel of the display device, or a hole injection layer, a hole transporting layer, an electron transporting layer, an electron injection layer, a hole blocking layer, or a combination thereof may form a common layer and remaining layers may form a separate independent layer.

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, the substrate may include a COC (cycloolefin copolymer) or COP (cycloolefin polymer) based material. All the aforementioned first to third electroluminescent devices are formed on the substrate. That is, a substrate of the display device according to an embodiment provides a common layer.

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 a scan line, a data line, a driving power source line, a common power source line, and the like, or a combination thereof, 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 suitable 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. The display device according to an embodiment may minimize the exciton quenching phenomenon of each device, and may exhibit improved color purity and life-span characteristics even at a relatively low power.

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

An indium-tin oxide (ITO) layer is deposited on a glass substrate, a hole injection layer is formed thereon by spin-coating poly (<NUM>,<NUM>-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) to be <NUM> thick, and a hole transport layer is formed thereon by spin-coating poly(<NUM>,<NUM>-dioctylfluorene-co-N-(<NUM>-butylphenyl)diphenylamine) (TFB) to be <NUM> thick.

Separately from the above, each composition for first and second emission layers is prepared. First, octane is mixed with a blue quantum dot (ZnTeSe) and a surfactant for a first ligand (zinc N,N-diethyldithio carbamate) to prepare the composition for a first emission layer, wherein N,N-diethyldithio carbamate group (a ligand compound derived from thiol-based compound) as the first ligand is attached to the surface of the blue quantum dot (ZnTeSe).

On the other hand, ethanol is mixed with a blue quantum dot (ZnTeSe) and a surfactant for a second ligand (<NUM>-mercaptoundecanol) to prepare the composition for a second emission layer, wherein <NUM>-hydroxy-<NUM>-undecanthiolate (a ligand compound derived from thiol-based compound) as the second ligand is attached to the surface of the blue quantum dot (ZnTeSe).

The composition for a first emission layer is coated on the hole transport layer, the composition for a second emission layer is coated directly on the composition for a first emission layer, and then, both of the compositions are dried at <NUM> ° C for <NUM> minutes under a nitrogen atmosphere to form both first and second emission layers on the hole transport layer. The first emission layer is about <NUM> thick, and the second emission layer is about <NUM> thick.

Subsequently, an electron transport layer is formed on the emission layer by co-depositing a compound represented by Chemical Formula <NUM> and a pyrazole-based compound (Novaled Dopant n-side material <NUM>, obtained from Novaled) in a weight ratio of <NUM>:<NUM> under vacuum condition. The formed electron transport layer is about <NUM> thick.

Then, an about <NUM>-thick aluminum (Al) layer is deposited on the electron transport layer to ultimately manufacture an electroluminescent device including the double emission layer (the first and second emission layers) of Example <NUM>.

An electroluminescent device of Example <NUM> including a double emission layer (first and second emission layers) is manufactured according to the same method as Example <NUM> except that oleic acid is used instead of the zinc N,N-diethyldithiocarbamate as a first ligand.

An electroluminescent device of Comparative Example <NUM> including a single emission layer instead of the double emission layer of the first and second emission layers is manufactured according to the same method as Example <NUM> except that the composition for a first emission layer wherein zinc N,N-diethyldithiocarbamate is attached to a blue quantum dot is used to form a single emission layer.

An electroluminescent device of Comparative Example <NUM> including a single emission layer is manufactured according to the same method as Comparative Example <NUM> except that the composition for a second emission layer wherein <NUM>-mercaptoundecanol as a a surfactant for a second ligand to form the single emission layer.

An electroluminescent device of Comparative Example <NUM> including a single emission layer is manufactured according to the same method as Comparative Example <NUM> except that oleic acid is used instead of the zinc N,N-diethyldithiocarbamate as a first ligand.

Light emitting characteristics of the electroluminescent devices of Example <NUM> and Comparative Examples <NUM> and <NUM> are evaluated. <FIG> shows a luminance change depending on a voltage, and <FIG> shows an external quantum efficiency change depending on luminance. <FIG> shows an intensity change of each electroluminescent device of Example <NUM> and Comparative Example <NUM> depending on a wavelength.

Referring to <FIG>, the electroluminescent device of Example <NUM> exhibits relatively high luminance compared with the electroluminescent devices of Comparative Examples <NUM> and <NUM>. The electroluminescent device including a double emission layer of Example <NUM> may exhibit relatively high luminance within a voltage range that may be used in practice.

Referring to <FIG>, the electroluminescent device of Example <NUM> exhibits improved external quantum efficiency at less than or equal to about <NUM>,<NUM> nits (candelas per square meter), less than or equal to about <NUM>,<NUM> nits, less than or equal to about <NUM>,<NUM> nits, less than or equal to about <NUM>,<NUM> nits, less than or equal to about <NUM>,<NUM> nits, less than or equal to about <NUM>,<NUM> nits, less than or equal to about <NUM>,<NUM> nits, or less than or equal to about <NUM> nits compared with the electroluminescent devices of Comparative Examples <NUM> and <NUM>. An electroluminescent device manufactured by applying a double emission layer as shown in Example <NUM> exhibits improved luminous efficiency and, for example, improved luminous efficiency at less than or equal to about <NUM>,<NUM> nits compared with a single layer electroluminescent device.

Referring to <FIG>, the electroluminescent device of Comparative Example <NUM> has a profile in a graph of normalized intensity (a. ) versus wavelength (nm) that protrudes convexly toward the left (i.e., in a direction of decreasing wavelength) in a wavelength region of about <NUM> to <NUM>. Without being bound by theory, the reason is that holes are slowly transported from the hole transport layer (TFB layer) to a single emission layer of the electroluminescent device of Comparative Example <NUM>.

The electroluminescent device of Example <NUM> includes a second emission layer to which <NUM>-mercaptoundecanol as a second ligand is attached but has a profile in a graph of normalized intensity (a. ) versus wavelength (nm) that does not protrude but increases in the corresponding wavelength region. Without being bound by theory, the reason is that the first emission layer contacting the hole transport layer (TFB layer) is believed to help hole transportation to the second emission layer.

An electroluminescent device manufactured by applying a double emission layer as shown in Example <NUM> may exhibit improved color purity compared with a single layer electroluminescent device.

Life-span characteristics of each electroluminescent device of Examples <NUM> and <NUM> and Comparative Examples <NUM>-<NUM> are evaluated. First, a luminance change of the electroluminescent device depending on driving time is measured by radiating light having luminance of <NUM> nits (candelas per square meter) in a blue wavelength region of <NUM> to <NUM>, and the result is shown in <FIG>.

T95, the time at which initial luminance becomes <NUM> %, and T50, the time at which initial luminance becomes <NUM> %, in <FIG> are respectively shown and provided in Table <NUM>.

Referring to <FIG> and Table <NUM>, the electroluminescent devices of Examples <NUM> and <NUM> exhibit improved life-span characteristics compared with the electroluminescent devices of Comparative Examples <NUM>-<NUM>.

Regarding the electroluminescent devices of Example <NUM> and Comparative Examples <NUM>-<NUM>, each initial driving voltage, each driving voltage at T50, and each driving voltage difference between the initial driving voltage and the driving voltage at T50 are calculated and shown in Table <NUM>.

Referring to Table <NUM>, the electroluminescent device of Example <NUM> exhibits a driving voltage decrease at T50 compared with the electroluminescent devices of Comparative Examples <NUM> and <NUM>. Referring to <FIG>, Table <NUM>, and Table <NUM> together, the electroluminescent device of Example <NUM> exhibits improved life-span compared with the electroluminescent devices of Comparative Examples <NUM>-<NUM>.

While this disclosure has been described in connection with what is presently considered to be practical example embodiments, it is to be understood that the invention is not limited to the disclosed embodiments.

Claim 1:
An electroluminescent device (<NUM>), comprising a first electrode (<NUM>);
a hole transport layer (<NUM>) disposed on the first electrode;
a first emission layer (<NUM>) disposed on the hole transport layer, the first emission layer comprising a first light emitting particle (<NUM>) on which a first ligand having a hole transporting property is attached;
a second emission layer (<NUM>) disposed on the first emission layer, the second emission layer comprising a second light emitting particle (<NUM>) on which a second ligand having a electron transporting property is attached;
an electron transport layer (<NUM>) disposed on the second emission layer; and
a second electrode (<NUM>) disposed on the electron transport layer,
wherein a solubility of the first ligand in a solvent is different than a solubility of the second ligand in the solvent,
and characterized in that at least one of the first ligand or the second ligand independently comprises a ligand compound derived from an alkyl-based compound, and wherein the first ligand comprises an oleic acid.