KR20210063032A - Quantum dot light emitting diode - Google Patents

Abstract

The present application relates to a quantum dot light emitting diode. The quantum dot light emitting diode of the present application may improve current efficiency and light emitting luminance by smooth hole transport. The quantum dot light emitting diode includes: a board; a lower electrode; a hole injection layer; a first hole transport layer; a second hole transport layer; a light emitting layer comprising quantum dots; an electron transport layer; and an upper electrode.

Description

QUANTUM DOT LIGHT EMITTING DIODE

This application relates to a quantum dot light emitting diode.

As electronic engineering and information technology are improved, a technology in the field of a display for processing and displaying a large amount of information is also rapidly developing. Accordingly, various flat panel display devices have been developed to replace the conventional cathode ray tube (CRT). Among flat panel display devices, organic light emitting diode (OLED) display devices and quantum dot light emitting diode (QLED) display devices can have a thin structure and consume less power, making it easier to use liquid crystal displays (LCDs). It is attracting attention as a next-generation display device to replace it.

These light emitting diodes are devices that emit light while electrons and holes are paired and then extinguished when electric charges are injected into the light emitting layer formed between the electron injection electrode (cathode) and the hole injection electrode (anode). The device can be formed on a flexible transparent substrate such as plastic, as well as being able to be driven at a low voltage (10 V or less), power consumption is relatively low, and color purity is excellent.

The quantum dot light emitting diode has an anode and a cathode facing each other, an emitting layer (EML) positioned between the anode and the cathode, and a hole injection layer (HIL) positioned between the anode and the light emitting layer (EML) and It includes a hole transport layer (HTL) and an electron transport layer (ETL) positioned between the cathode and the emission layer (EML).

A quantum dot light emitting diode is a device that emits light while electrons and holes are paired and then extinguished when electric charges are injected into a light emitting layer formed between an electron injection electrode (cathode) and a hole injection electrode (anode). The light emitting layer EML is made of a light emitting material, and holes and electrons respectively injected from the anode and the cathode meet in the light emitting layer EML to form excitons. By this energy, the light emitting material included in the light emitting layer EML is in an excited state. Energy transfer occurs in the light emitting material from the excited state to the ground state, and the generated energy is emitted as light to emit light. do.

In order to inject and transfer holes and electrons to the light emitting layer (EML), each layer must be made of a material having an appropriate bandgap energy. For example, the hole injection layer (HIL) is made of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and the hole transport layer (HTL) is poly(4-butylphenyl-diphenyl-amine) (Poly-TPD) The electron transport layer (ETL) may be made of ZnO, and the electron injection layer (EIL) may be made of aluminum.

The light emitting layer (EML) is made of nano-sized quantum dots (QD), for example, applied on the hole transport layer (HTL) through a process using a solution containing quantum dots (QD) in a solvent, and then the light emitting layer by volatilizing the solvent (EML) is formed. Since the light emitting layer (EML) finally formed by volatilization of the solvent is made of only nano-sized quantum dots (QD), a layer positioned adjacent thereto, for example, an interface between the hole transport layer (HTL) and the electron transport layer (ETL) It is not formed smoothly and has a rough cross-sectional shape. As the interface between the light emitting layers becomes rough, the overall morphology characteristics of the quantum dot light emitting diode are deteriorated, and holes and electrons are not uniformly injected into the light emitting layer EML over the entire area of the quantum dot light emitting diode.

In addition, compared to the Highest Occupied Molecular Orbital (HOMO) energy level of the hole transport material constituting the hole transport layer (HTL), the HOMO energy level of the inorganic light emitting material such as quantum dots constituting the light emitting layer (EML) is higher. The corresponding valence band energy level is very low (deep). In addition, the conduction band energy level corresponding to the LUMO energy level of the inorganic light emitting material constituting the light emitting layer EML is high compared to the lowest unoccupied molecular orbital (LUMO) energy level of the organic light emitting material. . Therefore, when holes are transported from the hole transport layer (HTL) to the light emitting layer (EML) and when electrons are transported from the electron transport layer (ETL) to the light emitting layer (EML), the energy level of the light emitting material constituting the light emitting layer (EML) is adjacent. An energy barrier is formed due to the difference in energy levels between the electron transport layers.

However, compared to the difference (ΔGL) between the LUMO energy level of the electron transport layer (ETL) and the LUMO energy level of the light emitting layer (EML), the difference between the HOMO energy level of the hole transport layer (HTL) and the HOMO energy level of the light emitting layer (EML) ( ΔGH) is much larger (ΔGH > ΔGL). For example, the HOMO energy level of a hole transport material such as Poly-TPD used in the hole transport layer (HTL) is about -5.0 eV, whereas the valence band energy level corresponding to the HOMO energy level of the shell constituting the inorganic light emitting material. is approximately -7.0 eV. Therefore, compared to the movement and injection of electrons into the light emitting layer EML, the movement and injection of holes into the light emitting layer EML are delayed, so positively charged holes and negatively charged electrons cannot be injected into the light emitting layer EML in a balanced way. .

Since holes and electrons are not uniformly injected over the entire region of the light emitting layer (EML) due to the deterioration of the overall morphological characteristics of the quantum dot light emitting diode, the holes and electrons are not emitted and are lost. In addition, as electrons are excessively injected into the light emitting layer (EML) compared to holes due to a difference in energy barrier, a significant portion of the excessively injected electrons recombine with holes and disappear without forming excitons. In addition, as electrons are rapidly injected into the light emitting layer (EML) compared to holes, holes and electrons cannot recombine in the light emitting material constituting the light emitting layer (EML), and recombine at the interface between the light emitting layer (EML) and the hole transport layer (HTL) do. Accordingly, the current efficiency of the quantum dot light emitting diode is lowered, and a high driving voltage is required to realize desired light emission, thereby increasing power consumption. Accordingly, there is a need for a quantum dot light emitting diode capable of improving light emitting luminance as well as current efficiency by smooth hole transport.

An object of the present application is to provide a quantum dot light emitting diode capable of improving current efficiency and light emitting luminance by smooth hole transport.

Hereinafter, the quantum dot light emitting diode of the present application will be described with reference to the accompanying drawings, and the accompanying drawings are exemplary, and the quantum dot light emitting diode of the present application is not limited to the accompanying drawings.

1 is a diagram illustrating an exemplary quantum dot light emitting diode according to an embodiment of the present application. As shown in FIG. 1 , the quantum dot light emitting diode includes a substrate 110 , a lower electrode 120 , a hole injection layer 130 , a first hole transport layer 140 , a second hole transport layer 150 , and a light emitting layer 160 . , the electron transport layer 170 and the upper electrode 180 are sequentially included.

The first hole transport layer includes a hole transport material having a lower absolute value of the HOMO energy level than that of the second hole transport layer. In the present specification, "HOMO energy level" means the distance from the vacuum level to the highest occupied molecular orbital (Highest Occupied Molecular Orbital). When the absolute value of the HOMO energy level is deep, high, or large, it means that the absolute value is large by setting the vacuum level to '0 eV', and the absolute value of the HOMO energy level is shallow, low, or small means that the vacuum level is set to '0 eV'. eV' means that the absolute value is small. Since the first hole transport layer includes a hole transport material having a lower absolute value of the HOMO energy level compared to the second hole transport layer, hole transport may be facilitated, and thus current efficiency and luminance may be improved.

In one example, the hole injection layer may include a hole injection material that facilitates injection of holes. The absolute value of the HOMO energy level of the hole injection material of the hole injection layer may be lower than that of the hole transport material of the first hole transport layer. The hole injection layer includes a hole injection material having a lower absolute value of the HOMO energy level compared to the hole transport material of the first hole transport layer, thereby making it possible to facilitate hole transport, thereby improving current efficiency and light emitting luminance. can

In addition, the absolute value of the HOMO energy level of the hole transport material of the second hole transport layer may be lower than that of the quantum dots of the emission layer. The second hole transport layer includes a hole injection material having a lower absolute value of the HOMO energy level compared to the quantum dots of the light emitting layer, thereby making it possible to facilitate hole transport, thereby improving current efficiency and light emitting luminance.

That is, the quantum dot light emitting diode has an absolute value of the lowest HOMO energy level in the order of the hole injection material of the hole injection layer, the hole transport material of the first hole transport layer, the hole transport material of the second hole transport layer, and the quantum dots of the light emitting layer. can For this reason, it is possible to facilitate hole transport, thereby improving current efficiency and light emission luminance.

In one example, the hole injection layer, the first hole transport layer, the second hole transport layer, and the light emitting layer have a HOMO energy level in the order of the hole injection layer, the first hole transport layer, the second hole transport layer and the light emitting layer within each range to be described later. It may include a material with a low absolute value of .

For example, the hole injection layer may include a hole injection material having an absolute HOMO energy level of 4.8 eV to 5.4 eV. Specifically, the absolute value of the HOMO energy level of the hole injection material included in the hole injection layer may be 4.9 eV to 5.3 eV, 5.0 eV to 5.2 eV, or 5.1 eV to 5.2 eV. Since the absolute value of the HOMO energy level of the hole injection material included in the hole injection layer satisfies the above-described range, hole transfer may be facilitated, and thus current efficiency and luminance may be improved.

Also, the first hole transport layer may include a hole transport material having an absolute HOMO energy level of 5.2 eV to 5.8 eV. Specifically, the absolute value of the HOMO energy level of the hole transport material included in the first hole transport layer may be 5.2 eV to 5.7 eV, 5.3 eV to 5.6 eV, or 5.3 eV to 5.5 eV. Since the absolute value of the HOMO energy level of the hole transport material included in the first hole transport layer satisfies the above-described range, hole transport may be facilitated, and thus current efficiency and luminance may be improved.

Also, the second hole transport layer may include a hole transport material having an absolute HOMO energy level of 5.3 eV to 6.9 eV. Specifically, the absolute value of the HOMO energy level of the hole transport material included in the second hole transport layer may be 5.4 eV to 6.5 eV or 5.6 eV to 6.2 eV. Since the absolute value of the HOMO energy level of the hole transport material included in the second hole transport layer satisfies the above-described range, hole transport may be facilitated, and thus current efficiency and light emitting luminance may be improved.

In addition, the light emitting layer may include quantum dots having an absolute value of the HOMO energy level of 5.7 eV to 7.5 eV. Specifically, the absolute value of the HOMO energy level of the quantum dots included in the emission layer may be 6.0 eV to 7.3 eV, 6.5 eV to 7.2 eV, or 6.8 eV to 7.0 eV. Since the absolute value of the HOMO energy level of the quantum dots included in the light emitting layer satisfies the above-described range, hole transport may be facilitated, and thus current efficiency and light emission luminance may be improved.

The hole transport material included in the first hole transport layer may be a monomolecular material capable of cross-linking. For example, the hole transport material included in the first hole transport layer is (N4,N4'-di(naphthalen-1-yl)-N4,N4'-bis(4-vinylphenyl)biphenyl-4,4' -diamine (VNPB), N,N0-bis(4-(6-((3-ethylosetan-3-yl)methoxy))-hexyloxy)phenyl-N,N0-bis(4-methoxy Phenyl) biphenyl-4,4'-diamine (QUPD), (N,N0-bis(4-(6-(3-ethylosetan-3-yl)methoxy))-hexylphenyl)-N,N0 -diphenyl-4,4'-diamine (OTPD), X-TAPC, 2-NPD, VB-TCTA, PS-TPD-PFCB, (PPZ-VB)2lzPPZ, BCz-VB, TriTCTA-PFCB, TCzll, It may include at least one selected from the group consisting of TPD-BCB and XPTPA-5. When used as a hole transport material of the first hole transport layer, the above-described type of monomolecular material is a polymer material because cross-linking is possible. Compared to the case of using as a hole transport material of the first hole transport layer, current efficiency and light emission luminance may be improved.

, 2-TNATA(4,4

,4″?-Tris[2-naphthyl(phenyl)amino]triphenylamine), TPD(N,N'-bis-(3-methylphenyl)-N,N'-bis-(phenyl)-benzidine) 및 TCTA(tris(4-carbazoyl-9-ylphenyl)amine)로 이루어진 군으로부터 선택된 하나 이상을 포함할 수 있다.In addition, the hole transport material included in the second hole transport layer is poly(N-vinylcarbazole) (PVK), poly[2,7-(9,9-di-n-octylfluorene)-co-(1,4) -pheynylene[(4-sec-butylphenyl)imino]-1,4-phenylene)]), NPD(N,N-dinaphthyl-N,N'-diphenyl benzidine), TAPC(1,1-bis[(di- 4-tolylamino)phenyl]cyclohexane), TPBI (2,2', 2

, 2-TNATA(4,4

,4″?-Tris[2-naphthyl(phenyl)amino]triphenylamine), TPD (N,N′-bis-(3-methylphenyl)-N,N′-bis-(phenyl)-benzidine) and TCTA (tris (4-carbazoyl-9-ylphenyl)amine) may include one or more selected from the group consisting of.

In one example, the sum of the thicknesses of the first hole transport layer and the second hole transport layer may be 5 nm to 100 nm. Specifically, the sum of the thicknesses of the first hole transport layer and the second hole transport layer is 10 nm to 95 nm, 15 nm to 90 nm, 20 nm to 85 nm, 25 nm to 80 nm, 30 nm to 75 nm, 35 nm to 70 nm, 40 nm to 65 nm, 45 nm to 60 nm or 50 nm to 55 nm. When the sum of the thicknesses of the first hole transport layer and the second hole transport layer satisfies the above-described range, hole transport may be facilitated, and thus current efficiency and light emission luminance may be improved. On the other hand, if the sum of the thicknesses of the first hole transport layer and the second hole transport layer is less than the above-mentioned range, the occurrence of pinholes and leakage current is high, and the sum of the thicknesses of the first hole transport layer and the second hole transport layer is the above-mentioned If it exceeds one range, the resistance of each of the first hole transport layer and the second hole transport layer may become too large, so that hole transport may not be smooth.

In one example, the lower electrode may be an anode, and the upper electrode may be a cathode.

The anode may be made of a conductor having a relatively high work function to facilitate hole injection, and may be made of, for example, a metal, a conductive metal oxide, or a combination thereof. Specifically, the anode may include a metal such as nickel, platinum, vanadium, chromium, copper, zinc, gold, or an alloy thereof; conductive metal oxides such as zinc oxide, indium oxide, tin oxide, indium tin oxide (ITO), indium zinc oxide (IZO) or fluorine-doped tin oxide; Alternatively, it may be made of a combination of a metal such as ZnO and Al or SnO _{2 and Sb and an oxide, but is not limited thereto.}

The cathode may be made of a conductor having a relatively low work function to facilitate electron injection, and may be made of, for example, a metal, a conductive metal oxide, and/or a conductive polymer. Specifically, the cathode may include a metal such as magnesium, calcium, sodium, potassium, titanium, indium, yttrium, lithium, gadolinium, aluminum, silver, tin, lead, cesium, barium, or an alloy thereof; and a multilayer structure material such as LiF/Al, LiO ₂ /Al, LiF/Ca, Liq/Al, and BaF ₂ /Ca, but is not limited thereto.

In addition, at least one of the anode and the cathode may be a light-transmitting electrode. For example, the light transmitting electrode may include a conductive metal oxide such as zinc oxide, indium oxide, tin oxide, indium tin oxide (ITO), indium zinc oxide (IZO), or fluorine-doped tin oxide, or a single layer or a plurality of thin layers. It can be made of a thin layer of metal. In addition, when one of the anode and the cathode is an opaque electrode, it may be made of an opaque conductor such as aluminum (Al), silver (Ag), or gold (Au).

The lower electrode and the upper electrode may be electrically connected to each other.

The substrate may be included under the lower electrode. As the substrate, a light-transmitting glass may be used.

The hole injection layer is the layer closest to the lower electrode of the quantum dot light emitting diode structure, that is, the anode, and is a layer that allows holes to easily enter the light emitting layer.

For example, the hole injection layer may include a conductive polymer. Specifically, PEDOT (poly(3,4-ethylenedioxythiophene), PSS (poly(styrenesulfonate), polyaniline, phthalocyanine, pentacene, polydiphenylacetylene, poly(t-butyl)diphenylacetylene, poly( Trifluoromethyl)diphenylacetylene, Cu-PC (copper phthalocyanine) poly(bistrifluoromethyl)acetylene, polybis(T-butyldiphenyl)acetylene, poly(trimethylsilyl)diphenylacetylene, poly(carbazole) ) Diphenylacetylene, polydiacetylene, polyphenylacetylene, polypyridineacetylene, polymethoxyphenylacetylene, polymethylphenylacetylene, poly(t-butyl)phenylacetylene, polynitrophenylacetylene, poly(trifluoromethyl)phenylacetylene , poly(trimethylsilyl)phenylacetylene, and conductive polymers such as derivatives thereof may be used in one or a combination of two or more, but is not limited thereto, and for example, a PEDOT:PSS mixture may be used.

Such a conductive polymer material may be coated on the lower electrode using a general coating method, for example, spraying, spin coating, dipping, printing, doctor blading, sputtering, or the like, or using an electrophoresis method, and the thickness thereof may be preferably 5 nm to 2000 nm.

The light emitting layer is a layer that emits light by recombination of electrons and holes. The light emitting layer includes quantum dots.

In one example, the quantum dots are CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeZnTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, CdHgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe; GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, GaNP, GaNAs, GaNSb, GaPAs, GaPSb, InNP, InNAs, InNSb, InPAs, InPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, SnS, SnSe, SnTe, PbS, PbSe, PbTe, SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, SnPbSSe, SnPbSeTe, SnPbSTe, Si, Ge, SiC and SiGe.

In another example, the quantum dots may have a core-shell structure. By stacking a shell on an existing quantum dot to have a core-shell structure, it is possible to improve the emission reduction caused by the quantum dot surface trap, and the shell serves to protect the quantum dot, thereby increasing the stability of the quantum dot. Specifically, the core may include a first semiconductor crystal, and the shell may include a second semiconductor crystal.

For example, the first semiconductor crystal comprises CdSe, CdTe, ZnSe, ZnTe, ZnS, InP, InAs, PbS, PbSe, PbTe, or any combination thereof, and the second semiconductor crystal comprises ZnSe, ZnTe, ZnS, CdS, HgS, GaAs, or any combination thereof.

The electron transport layer is a layer through which electrons injected from the cathode are sent to emit light in the light emitting layer. The electron transport layer may include an organic material, an inorganic material, or an organic/inorganic material.

Specifically, the organic material may be an n-type organic semiconductor. For example, the n-type organic semiconductor may be a monomer or a polymer. Specifically, the n-type organic semiconductor is Alq3(tris-(8-hydroxyquinilone)aluminum), TAZ(3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4 -triazole), TPBi(2,2,2-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), BPhen(4,7-diphenyl-1,10-phenanthroline), etc. The same may include a monomer-based organic material, or a polymer-based organic material such as poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT).

In addition, the inorganic material may be an n-type inorganic semiconductor. For example, the n-type inorganic semiconductor may be an oxide or a non-oxide. Specifically, the n-type inorganic semiconductor may be _{an n-type oxide semiconductor such as TiO 2} , ZnO, ZrO _{2 or} MgO or an alloy thereof, for example, ZnMgO, or an n-type non-oxide semiconductor such as n-GaN.

The electron transport layer may be formed by sol-gel, spray coating, spin coating, blade coating, printing, deposition, or the like. .

In the quantum dot light emitting diode of the present application, the first hole transport layer includes a hole transport material having a lower absolute value of the HOMO energy level than the second hole transport layer, so that hole transport can be facilitated, and thus current efficiency and light emission Brightness can be improved.

In one example, the quantum dot light emitting diode may have a light emitting luminance of 25.0 Cd/m ^{2 or} more ^{, measured under driving at a current density of 10 mA/cm 2 .} Specifically, the emission luminance of the quantum dot light emitting diode measured under the above-described conditions may be 26.0 Cd/m ² or more, 27.0 Cd/m ² or more, or 28.0 Cd/m ^{2 or} more. In addition, the upper limit of the emission luminance of the quantum dot light emitting diode measured under the above conditions is not particularly limited. For example, in the case of a quantum dot light emitting diode using blue quantum dots, since the luminance may change a lot depending on the emission wavelength band, the upper limit of the emission luminance of the quantum dot light emitting diode is 500 Cd/m ² or less, 400 Cd/m ² or less, 300 Cd/m ² or less, 200 Cd/m ² or less, or 100 Cd/m ² or less.

In addition, the quantum dot light emitting diode may have a current efficiency of 0.25 Cd/A or more, measured under driving at a current density ^{of 10 mA/cm 2 .} Specifically, the current efficiency of the quantum dot light emitting diode measured under the above conditions may be 0.26 Cd/A or more, 0.27 Cd/A or more, or 0.28 Cd/A or more, and the upper limit thereof is not particularly limited, but 0.40 Cd/A or less, 0.35 Cd/A or less, or 0.30 Cd/A or less.

In one example, the quantum dot light emitting diode may further include an encapsulation substrate (not shown). The encapsulation substrate may be a layer that seals the outside of the lower electrode, the hole injection layer, the first hole transport layer, the second hole transport layer, the light emitting layer, the electron transport layer and the upper electrode to prevent penetration of moisture and/or oxygen from the outside. . For example, the encapsulation substrate may be a substrate made of glass, metal, or polymer. The encapsulation substrate may be omitted in some cases.

The quantum dot light emitting diode of the present application may improve current efficiency and light emitting luminance by smooth hole transport.

1 is a view illustrating an exemplary quantum dot light emitting diode according to an embodiment of the present application.
2 is a view illustrating quantum dot light emitting diodes according to Comparative Examples 1 and 3 of the present application.
3 is a diagram illustrating an exemplary quantum dot light emitting diode according to Comparative Example 2 of the present application.

Hereinafter, the present application will be described in detail through Examples, but the scope of the present application is not limited by the Examples below.

Example 1

Fabrication of quantum dot light emitting diodes

ITO was patterned on a glass substrate to form a lower electrode having a thickness of 50 nm, _{and after the lower electrode was treated with UV-O 3} for 15 minutes, the absolute value of the HOMO energy level on the lower electrode was 5.2 eV Phosphorus PEDOT: PSS was spin-coated at 2000 rpm, dried at 180° C. for 30 minutes to form a hole injection layer having a thickness of 50 nm, dissolved in toluene on the hole injection layer, and the absolute value of the HOMO energy level was After spin-coating 5.3 eV of VNPB at 3000 rpm, drying at 200° C. for 10 minutes to form a first hole transport layer having a thickness of 20 nm, dissolved in m-xylene on the first hole transport layer, and HOMO energy After spin-coating PVK having an absolute level of 5.7 eV at 3000 rpm, drying at 170° C. for 30 minutes to form a second hole transport layer having a thickness of 25 nm, the absolute value of the HOMO energy level on the second hole transport layer After spin-coating ZnSe/Zns quantum dots with a core-shell structure having a value of 6.9 eV at 2000 rpm, drying at 70° C. for 60 minutes to form a light emitting layer having a thickness of 20 nm, ZnMgO was spun on the light emitting layer at 2000 rpm The quantum dots shown in FIG. 1 were formed by coating to form an electron transport layer having a thickness of 20 nm, ^{and depositing aluminum on the electron transport layer at 3 × 10 −6} Torr, 4 Å/s to form an upper electrode layer having a thickness of 100 nm. A light emitting diode was manufactured.

Comparative Example 1

Fabrication of Quantum Dot Light Emitting Diodes

VNPB dissolved in toluene and having an absolute HOMO energy level of 5.3 eV on the hole injection layer was spin-coated at 3000 rpm and dried at 200° C. for 10 minutes to form a first hole transport layer having a thickness of 20 nm, , A quantum dot light emitting diode shown in FIG. 2 was manufactured in the same manner as in Example 1, except that a light emitting layer was formed without forming a second hole transport layer on the first hole transport layer.

Comparative Example 2

Fabrication of quantum dot light emitting diodes

Without forming the first hole transport layer on the hole injection layer, PVK dissolved in m-xylene and having an absolute HOMO energy level of 5.7 eV was spin-coated at 3000 rpm, and then dried at 170° C. for 30 minutes, A quantum dot light emitting diode shown in FIG. 3 was manufactured in the same manner as in Example 1, except that a second hole transport layer having a thickness of 25 nm was formed.

Comparative Example 3

Fabrication of quantum dot light emitting diodes

Poly-TPD dissolved in chlorobenzene and having an absolute HOMO energy level of 5.4 eV on the hole injection layer was spin-coated at 2000 rpm, dried at 110° C. for 20 minutes, and a first hole transport layer having a thickness of 25 nm The quantum dot light emitting diode shown in FIG. 2 was manufactured in the same manner as in Example 1 except that a light emitting layer was formed without forming a second hole transport layer on the first hole transport layer.

Comparative Example 4

Fabrication of quantum dot light emitting diodes

Poly-TPD dissolved in chlorobenzene and having an absolute HOMO energy level of 5.4 eV on the hole injection layer was spin-coated at 2000 rpm, dried at 110° C. for 20 minutes, and a first hole transport layer having a thickness of 25 nm After spin coating at 3000 rpm with PVK dissolved in m-xylene and having an absolute HOMO energy level of 5.7 eV on the first hole transport layer at 3000 rpm, dried at 170° C. for 30 minutes, 25 nm thick A quantum dot light emitting diode was manufactured in the same manner as in Example 1, except that a second hole transport layer having

Evaluation Example 1. Current Efficiency Evaluation

Current efficiency was measured using a CCD spectrometer (Spectroradiometer, CS-2000, Konica Minolta) when ^{10 mA/cm 2} current density was injected into the quantum dot light emitting diodes prepared in Examples and Comparative Examples using a source meter unit (2400, Keithley). evaluated, and the results are shown in Table 1 below.

Evaluation Example 2. Driving Voltage Evaluation

Using a source meter unit (2400, Keithley) in the quantum dot light emitting diodes prepared in Examples and Comparative Examples, 10 mA/cm ^{2 When the} current density was injected, the driving voltage was evaluated, and the results are shown in Table 1 below.

Evaluation Example 3. Emission luminance evaluation

^{A current density of 10 mA/cm 2} was injected into the quantum dot light emitting diodes prepared in Examples and Comparative Examples using a source meter unit (2400, Keithley), and a CCD spectrometer (Spectroradiometer, CS-2000, Konica Minolta) was used. Emission luminance was evaluated, and the results are shown in Table 1 below.

Current Efficiency (Cd/A) Driving voltage (V) Luminance of emission (Cd/m ² ) Example 1 0.28 6.1 28.1 Comparative Example 1 0.21 6.7 21.5 Comparative Example 2 0.23 7.1 23.2 Comparative Example 3 0.13 10.7 13.4 Comparative Example 4 0.13 10.7 13.4

As shown in Table 1, it was confirmed that the quantum dot light emitting diodes prepared in Example 1 had higher current efficiency than the quantum dot light emitting diodes prepared in Comparative Examples 1 to 4 and had excellent light emitting luminance according to the driving voltage.

However, the quantum dot light emitting diode prepared in Example 1 uses VNPB, a monomolecular material capable of cross-linking as the first hole transport layer, so that in Comparative Examples 3 and 4 using Poly-TPD, a polymer material, as the first hole transport layer It was confirmed that the current efficiency was higher than the manufactured quantum dot light emitting diode, and the light emitting luminance according to the driving voltage was excellent.

110: substrate
120: lower electrode
130: hole injection layer
140: first hole transport layer
150: second hole transport layer
160: light emitting layer
170: electron transport layer
180: upper electrode

Claims (15)

Board; lower electrode; hole injection layer; a first hole transport layer; a second hole transport layer; a light emitting layer including quantum dots; electron transport layer; and an upper electrode sequentially,
The first hole transport layer is a quantum dot light emitting diode including a hole transport material having a lower absolute value of the HOMO energy level than the second hole transport layer. The quantum dot light emitting diode of claim 1, wherein the hole injection layer includes a hole injection material, and the hole injection material of the hole injection layer has a lower absolute value of the HOMO energy level than the hole transport material of the first hole transport layer. The quantum dot light emitting diode of claim 1, wherein the hole transport material of the second hole transport layer has a lower absolute value of the HOMO energy level than the quantum dots of the light emitting layer. The method of claim 1, wherein the hole transport material included in the first hole transport layer is (N4,N4'-di(naphthalen-1-yl)-N4,N4'-bis(4-vinylphenyl)biphenyl-4; 4'-diamine (VNPB), (N,N0-bis(4-(6-((3-ethylosetan-3-yl)methoxy))-hexyloxy)phenyl-N,N0-bis(4 -Methoxyphenyl)biphenyl-4,4'-diamine (QUPD), (N,N0-bis(4-(6-(3-ethylosetan-3-yl)methoxy))-hexylphenyl)- N,N0-diphenyl-4,4'-diamine (OTPD), X-TAPC, 2-NPD, VB-TCTA, PS-TPD-PFCB, (PPZ-VB)2lzPPZ, BCz-VB, TriTCTA-PFCB, A quantum dot light emitting diode comprising at least one selected from the group consisting of TCz ll, TPD-BCB and XPTPA-5. The quantum dot light emitting diode of claim 1, wherein the hole transport material included in the second hole transport layer comprises at least one selected from the group consisting of PVK, TFB, NPD, TAPC, TPBI, 2-TNATA, TPD, and TCTA. The quantum dot light emitting diode of claim 1, wherein the sum of the thicknesses of the first hole transport layer and the second hole transport layer is 5 nm to 100 nm. The quantum dot light emitting diode of claim 1, wherein the lower electrode is an anode and the upper electrode is a cathode. The quantum dot light emitting diode of claim 1, wherein the hole injection layer comprises a conductive polymer. According to claim 1, wherein the quantum dots are CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeZnS, ZnSTe, HgSeS, HgSeTe, HgSeTe , CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, CdHgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe; GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, GaNP, GaNAs, GaNSb, GaPAs, GaPSb, InNP, InNAs, InNSb, InPAs, InPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, SnS, SnSe, A quantum dot light emitting diode selected from the group consisting of SnTe, PbS, PbSe, PbTe, SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, SnPbSSe, SnPbSeTe, SnPbSTe, Si, Ge, SiC and SiGe The quantum dot light emitting diode of claim 1, wherein the quantum dot has a core-shell structure including a core including a first semiconductor crystal and a shell including a second semiconductor crystal. 11. The method of claim 10, wherein the first semiconductor crystal comprises CdSe, CdTe, ZnSe, ZnTe, ZnS, InP, InAs, PbS, PbSe, PbTe, or any combination thereof, and the second semiconductor crystal comprises ZnSe, ZnTe, A quantum dot light emitting diode comprising ZnS, CdS, HgS, GaAs, or any combination thereof. The quantum dot light emitting diode of claim 1, wherein the electron transport layer includes an organic material, an inorganic material, or an organic/inorganic material. The quantum dot light emitting diode of claim 1, wherein the light emitting luminance measured under a current density driving of ^{10 mA/cm 2 is 25.0 Cd/m 2 or more.} The quantum dot light emitting diode according to claim 1, wherein the current efficiency measured under a current density driving of ^{10 mA/cm 2 is 0.25 Cd/A or more.} The quantum dot light emitting diode of claim 1, further comprising an encapsulation substrate, wherein the encapsulation substrate encapsulates the outside of the lower electrode, the hole injection layer, the first hole transport layer, the second hole transport layer, the light emitting layer, the electron transport layer, and the upper electrode. .

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